Download - Diss Paez-sierra Raman
Raman Spectroscopy of Metal/Organic/Inorganic
Heterostructures and Pentacene-Based OFETs
von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz
genehmigte Dissertation zur Erlangung des akademischen Grades
doctor rerum naturalium (Dr. rer. nat.)
vorgelegt von MSc. Phys. Beynor Antonio Paez-Sierra
geboren am 24. September 1971 in Florián-Santander
eingereicht am 06. März 2007
Gutachter:
Prof. Dr. Dr. h.c. Dietrich R. T. Zahn
Prof. Dr. Veit Wagner
Prof. Dr. Michael Hietschold
Tag der Verteidigung: 20. Dezember 2007
http://archiv.tu-chemnitz.de/pub/2008
B. A. Paez-Sierra, Dedication ii
Dedication
This thesis is dedicated to my wife Viktoriia
B. A. Paez-Sierra, Bibliografische Beschreibung iii
Bibliografische Beschreibung
MSc. Phys. Beynor Antonio Paez Sierra Raman Spectroscopy of Metal/Organic/Inorganic Heterostructures and Pentacene-Based OFETs Technische Universität Chemnitz Dissertation (in englische Sprache), 2007 Im Rahmen dieser Arbeit wurden die Wechselwirkung von Indium (In) und Magnesium (Mg) als Topelektroden auf zwei Perylen-Derivativen, 3,4,9,10-Perylentetracarbonsäure Dianhydrid (PTCDA) und Dimethyl-3,4,9,10-Perylentetracarbonsäure Diimid (DiMe-PTCDI) untersucht. Die Metal/organische Schichten wurden auf S-passivierten GaAs(100):2x1-Substraten hergestellt und unter Ultrahochvakuum (UHV)-Bedingungens aufgedampft. Als Hauptcharakterisierungsmethode wird die Raman-Spektroskopie eingesetzt, die eine nicht-destruktive Methode ist, und auch in situ Untersuchungen des Wachstumsprozesses ermöglicht. Die experimentell Ergebnisse haben gezeigt, dass alle aufgedampft Metallen auf die organische Schichten von PTCDA und DiMe-PTCDI eine Verstärkung des aktive Raman Signals von interne Schwingungsmoden fördern, begleitet durch die Aktivierung von normalerweise Infrarotaktivemoden. Diesem Phänomen als Oberflächenverstärkte Raman-Spektroskopie (SERS) genannt ist. Die Verstärkungsfaktoren sind in bereich von 101 zu 102 für die chemische Verstärkung und von 101 zu 104 für die Elektromagnetische Verstärkung. Interessant war, dass das In Wachstum auf PTCDA und DiMe-PTCDI nur eine Einfluss auf der Molekülgeometrie hat, und mit eine starker Diffusion durch die PTCDA- als in DiMePTCDI- Schichten. Das Mg Wachstum auf beiden Molekularstrukturen wurde durch die viel niedrigere Diffusion des Metalls in die organischen Molekülen im Vergleich zum Indium, es war durch die Bewahrung des von externe molekulare Schwingungsmoden nach das Metallswachstum, und in ersten Mal in einem Ramanexperiment beobachtet. Die PTCDA/Mg Strukturen formen sich durch zwei Stufen des Metallwachstum, die erste gehört zu einer neuen molekularen Struktur für eine Mg Schicht dünner als 2.8 nm, wo das PTCDA Molekühl des Sauerstoff-Atoms von die dianhydride Gruppe verliert. Die zweite gehört zu das SERS Spektrum von die vorherige Struktur. Im Fall von Mg/DiMe-PTCDI Heterostrukturen, den Molekühl wird gut bewahrt, wo die Raman Verschiebung an der diimide Gruppe wird nicht modifiziert. Auch von dieser Struktur eine interessante Eigenschaft wurde durch die Kopplung zwischen diskret Moleküleigenschwingungen am 221 cm-1, 1291 cm-1 und 1606 cm-1 des organischen Materials und den elektronischen Kontinuum-Zuständen des Mg-Metallkontakts. Ihre entsprechenden Energieliniengestalten werden gut durch die Breit-Wigner-Fano-Funktion beschrieben. Die Untersuchungen auf dem vorherigen Heterostrukturen half, die Kanalbildung von Pentacen-basierten organische Feldeffekt-Transistoren (OFETs) experimentell zu analysieren, und in ersten Mal in einem Ramanexperiment durchgeführt. Der organische Kanal war gebildet durch die organische Molekularstrahldeposition (OMBD) unter UHV-Bedingungens der Pentacen Moleküle, und es war mit eine Evaporationsrate von ca. 0.65 Å/min aufgedampft. Nach jede Aufdampfung von ca. 0.1 nm des organische Moleküle, den Strom und den Ramansignal in den Kanal wurden in situ gemessen. Die minimale nominelle Dicke des organischen Materials erforderlich für den effizienten Ladungstransport durch den OFET Kanal wurde um ungefähr 1.5 nm nomineller Einschluss oder 1.1 Monolagen (ML) zu sein. Eigenschaften der ersten Monolagen werden gut im Vergleich mit dickeren Schichten definiert, wo die 1.1 ML eine gestrecktes Natur wegen seines direkten Kontakts mit dem Gate-Isolator präsentieren. Es wurde gefunden, dass der leitende organische Kanal bzw. -organische erhöhende Schicht (OBL)- eine Druckdeformierung hat. Dieses Phänomen durch die rote Verschiebung der Ramanbanden beobachtet war. Das Ausgangskennlinienfeld des OFETs wurden nach die letzte aufgedampft organische Schicht gemessen. Es wurde gefunden, dass der Drain-Strom einem Relaxationsprozesse mit zwei Zeitkonstanten hat, wo eine in der Ordnung von 101 min ist und die zweite unter 102 min. Ein ähnliches Experiment mit der Beleuchtung des Kanals mit einer 676.4 nm Laserquelle, es erhöht der Drain-Strom und lässt ummodifiziert die Zeitkonstanten. In der Ergänzung, die OFET-Strukturen waren ex situ durch Landungstransientspektroskopie (QTS) unstersucht. Die QTS Spektren zeigten positive und negative Banden zum Gesamtsignal der relaxierte Ladung in Bezug auf die einzigartige Biaspulsepolarität. Wir haben dieses Phänomen als ,,anomales Verhalten des QTS-Signals“ genannt, und in ersten Mal in einem QTS-Experiment beobachtet. Bei Wiederholung der QTS-Messung innerhalb ca. 100 min, die QTS-Spektre eine langsame Relaxationsprozesse von Störstellen am s5 μ in bereich ca. 63 min < 102 min hat. Die Einfangsquerschnitten sind Zeitabhängig, es bedeutet, dass die Störstellendichte nicht Konstant im Lauf der Betriebs des OFET bleibt. Dafür des Drain-Strom verändert sich und die Beweglichkeit unabhängige des elektrisches Feld ist. Experimentell Untersuchungen auf dem OFETs mit der Kombination der Ramanspektroskopie und elektrischen Felder zeigten eine Erhöhung des Ramanseinfangsquerschnitt in endliche Bereich als die chemische SERS-Verstärkung von In bzw. Mg auf die Perylen-Derivativen PTCDA und DiMe-PTCDI. Nach den Ausschaltung des elektrisches Felds den Ramansignal des Pentacen-basierten OFET eine Relaxationsprozesse mit Zeitkonstant von ca. 94 min hat. Deshalb ist die Summe von Störstellensdichte wegen dieser am organische/anorganische Grenze plus dieser dass die elektrisches Felds am die organische Halbleiter induziert. Schlagwörter Organische Moleküle, Organische Molekularstrahldeposition (OMBD), In, Mg, GaAs(100), Grenzflächen, Ramanspektroskopie, Oberflächenverstärkte Raman-Spektroskopie (SERS), Fanoresonanz, organische Feld-Effekt Transistor (OFET), Landungstransientspektroskopie (QTS), Störstellen, organische erhöhende Schicht (OBL), Beweglichkeit, Modellbildung, Aktivierungsenergie, Ladungsdichte, Stromsrelaxation.
B. A. Paez-Sierra, Table of contents iv
Table of contents
Bibliografische Beschreibung……………………………………………………………….…..…... ii
Dedication……………………………………………………………………………….....………... iii
Table of contents……………………………………………………………………………….....…. iv
List of abbreviations…………………………………………………………………………...……. viii
Chapter 1. Introduction……………………………………………………………………...……. 1.1
1.1. A brief historic review of organic materials …………………………………………...….. 1.1
1.2. Investigation of organic/ anorganic heterostructures …………………………………....... 1.2
References……………………………………………………………………………..………... 1.4
Chapter 2. Fundamentals of molecular structures and Raman spectroscopy …………………. 2.1
2.1 Aromatic molecules……………………………………………………………………….… 2.1
2.2. The Benzene ring and π-electron delocalization……………………………………..…… 2.2
2.3. Organic semiconductor solid…………………………………………………………...….. 2.2
2.4. Transport in organic materials………………………………………………………..….… 2.3
2.4.1. Some input parameters for the coupled electron-phonon system………………..…... 2.4
2.4.2. Field effect mobility………………………………………………………..………... 2.5
2.5. Light-matter interaction……………………………………………………………..……... 2.7
2.5.1. Raman spectroscopy…………………………………………………………………. 2.9
2.5.2. Basic theoretical background on Raman spectroscopy…………………………...…. 2.9
2.5.2.1. Classical description of the Raman effect……………………………..…….. 2.9
2.5.2.2. Quantum mechanical description of the Raman effect…………………...….. 2.10
2.5.3. Surface-enhanced Raman scattering (SERS)…………………………………..….…. 2.14
2.5.4. Combined Raman spectroscopy with electric fields…………………………...…….. 2.16
References………………………………………………………………………………..……... 2.17
Chapter 3. Experimental techniques, materials, and algorithms…………………………...….. 3.1
3.1. Combined Raman Spectroscopy and electrical characterization setups……………..…….. 3.1
3.2. Sulphur passivation of GaAs(100)………………………………………………..………... 3.3
3.3. Materials and structures……………………………………………………………………. 3.3
3.3.1. Structures I. Perylene derivatives thin films capped by metallic overlayers of
Indium and Magnesium……………………………………..……………………………....
3.4
3.3.1.1. Molecular beam deposition (MBD) and metal evaporation…………..……... 3.4
3.3.2 Structures II. Pentacene and C60 organic molecules as active layers in field effect
B. A. Paez-Sierra, Table of contents v
devices………………………………………………………………………………..…….. 3.6
3.3.2.1. Molecular structures: Pentacene and C60…………………………………...... 3.6
3.3.2.2. Field effect structures……………………………………………...………… 3.8
3.3.2.3. Molecular beam deposition: pentacene and C60…..…………….....………… 3.8
3.4. Charge transient spectroscopy (QTS)…………………………………………...………..... 3.9
3.4.1. Shallow and deep level states in semiconductors……………………………...…….. 3.9
3.4.2. The charge transient spectroscopy (QTS) technique………………………...……….. 3.12
3.5. Algorithms for data evaluation and simulations………………………………………..….. 3.14
3.5.1. Gauss-Legendre quadrature……………………………………………………..….... 3.14
3.5.2. QTS spectra line-profile……………………………….………………………...…... 3.15
3.5.3. Correlated fitting algorithm…………………………………………………….…..... 3.15
References…………………………………………………………………………...………….. 3.16
Chapter 4. Metal / Organic interface formation investigated by in situ surface-enhanced
Raman spectroscopy (SERS)……………………………………………………………………...
4.1
4.1. Introduction………………………………………………………………………………… 4.2
4.2. Interaction of metals with perylene derivatives…………………………………………..... 4.3
4.3. Morphology of the metal film…………………………………………………………….... 4.6
4.4. Phonons and interface structural properties…………………………………………….…. 4.10
4.5. Mg/DiMe-PTCDI structures and discrete-molecular coupling with continuum electronic
metal states……………………………………………………………………………………....
4.13
4.6.1. Chemistry, metal film morphology and metal indiffusion……………….………….. 4.13
4.6.2. Coupling of vibrational modes and electronic excitations…………………….…….. 4.15
Conclusions……………………………………………………………………………….….… 4.18
References……………………………………………………………………………………..... 4.18
Chapter 5. Organic Field Effect Transistors (OFETs)………………………………………..…. 5.1
5.1. Introduction………………………………………………………………………………… 5.1
5.2. Statistical mechanics of charge carriers………………………………………………….... 5.3
5.2.1. Density of states……………………………………………………………………… 5.3
5.3.2 Charge carriers density…………………………………………………………….…. 5.3
5.3.3. Fermi integral argument……………………………………………………………... 5.4
5.4. Charge carrier density of organic materials………………………………………………... 5.5
5.5. The field effect transistor (FET)..................................................................................….…. 5.8
5.5.1. Energy band structure of an OFET..........................................................................…. 5.9
5.6. Output characteristics of the OFET………………………………………………………... 5.10
5.6.1. ,,Linear” regime……………………………………………………………………… 5.10
5.6.2. Saturation regime…………………………………………………………………….. 5.12
B. A. Paez-Sierra, Table of contents vi
5.6.3. Field effect mobility effμ .......................................................................................…. 5.13
5.7. Threshold voltage shift and field dependence……..………………………………………. 5.16
Conclusions………………………..………………………………………………………….… 5.18
References………………………….………………………………………………………..….. 5.18
Chapter 6. Combined Raman spectroscopy and electrical characterization of the conductive
channel in OFETs…….………………………………………………….…………………………
6.1
6.1. Introduction………………………………………………………………………………… 6.2
6.2. Simultaneous in situ I - V characterization and molecular vibration measurements of
OFETs………………………………..…………………………………………………….……
6.3
6.3. Organic boosting layer (OBL) in OFETs….……………………………………………..... 6.7
6.4. Characteristic regions of the organic layer in OFETs…….…………………………..…… 6.9
6.5. Organic – Insulator electrodynamics…………………….……………………………..….. 6.10
6.6. Vibrational bands profiling of the active layer………….……………………………..…... 6.12
6.7. Bias-stress effects and multi-exponential current relaxation………………………………. 6.15
Conclusions…………………………………………………………………………………...… 6.18
References………………………………………………………………………………………. 6.19
Chapter 7. Influence of electric fields and illumination in OFETs…………………………..…. 7.1
7.1. Introduction…………………………………………………………………………….…... 7.1
7.2. Raman bands and external electric fields………………..………………………… 7.2
7.2.1. Band gap modification by external electric fields………………………….………... 7.3
7.2.2. Raman bands alteration by external electric fields……………………………..……. 7.5
7.2.2.1. Pentacene…………………………………………………………………...... 7.5
7.2.3. The C60 fullerene………………………………………………………………..….… 7.9
7.3. Illumination and charge transport in OFETs……………………………………………..... 7.11
7.3.1. Persistent effects and multi-exponential kinetics……………………………………. 7.11
Conclusions…………………………………………………………………………………...… 7.13
References…………………………………………………………………………………….… 7.14
Chapter 8. Trap distribution in OFETs and anomalous QTS…………………………….…….. 8.1
8.1. Introduction……………………………………………………………………….………... 8.1
8.2. Traps and charge density distribution…………………………………………….………... 8.3
8.2.1. Effect of the electric field………………………………………………….………… 8.3
8.3. Anomalous behavior of the QTS signal…………………………………….……………… 8.7
8.3.1. Advantage of floating gate configuration in QTS measurements……….…………… 8.7
8.3.2. OFET devices with interdigitated source-drain electrodes…………….…………….. 8.8
8.3.3. Single channel OFET devices……………………………………………..…………. 8.10
8.4. Approach to modeling the ,,anomalous QTS signal”……………………………….….… 8.13
B. A. Paez-Sierra, Table of contents vii
8.5. Current collapse……………………………………………………………………………. 8.17
8.5.1. Negative conductance and photodetachment………………………….……………... 8.17
8.5.1.1. Charge conservation and photodetachment…………………….……………. 8.19
8.6. Photodetachment………………………………………………………..………………….. 8.19
Conclusions………………………………………………………………..……………………. 8.20
References………………………………………………………………………………………. 8.21
Chapter 9. Summary………………………………………………………………………………. 9.1
List of figures……………………………………………………………………………………….. 10.1
List of tables……………………………………………………………………………………….... 10.7
Erklärung............................................................................................................................................. 10.8
Curriculum Vitae................................................................................................................................. 10.9
List of publications………………………………………………………………………………….. 10.13
Acknowledgements………………………………………………………………………………….. 10.16
B. A. Paez-Sierra, List of abbreviations viii
List of abbreviations
AFM Atomic Force Microscopy
B3LYP Becke’s three parameter hybrid functional
BWF Breit-Wigner-Fano
cf. Confer
CCD Charge-Coupled Device
CT Charge Transfer
DFG Deutschen Forschungsgemeinschaft
DFT Density Functional Theory
DH4T Dihexylquaterthiophene
DLTS Deep Level Transient Spectroscopy
DiMe-PTCDI N-N’-dimethyl-3,4,9,10- perylene tetracarboxylic diimide
DIODE Designing Inorganic/Organic Devices
EFMPM Electrostatic Force Microscopy Phase Mode
FET(s) Field Effect Transistor(s)
gIRSE generalized Infrared Spectroscopy Ellipsometry
HOMO Highest Occupied Molecular Orbital
IPES Inverse Photoemission Spectroscopy
IRAV Infrared Active Vibrational
IR Infrared
I-V Current-Voltage
LED(s) Light Emitting Diode(s)
LO Longitudinal Optical
LUMO Lowest Unoccupied Molecular Orbital
MBD Molecular Beam Deposition
ML Mono Layer
MML Multi Mono Layers
MOSFET Metal-Oxide-Semiconductor Field Effect Transistor
NEXAFS Near Edge X-Ray Absorption Fine Structure
OBL Organic Boosting Layer
OFET(s) Organic Field Effect Transistor(s)
OLED(s) Organic Light Emitting Diode(s)
B. A. Paez-Sierra, List of abbreviations ix P3HT Poly(3-hexylthiophene)
PABA Para-Amino-Benzoic-Acid
PCBM [6,6]-Penyl-C61-butyric acid methyl ester
PEI Polyethylenimine
PTCDA 3,4,9,10-perylene tetracarboxylic dianhydride
QTS Charge Transient Spectroscopy
RS Raman Spectroscopy
RSS Raman Scanning Spectroscopy
(S)FWHM (Semi) Full Width at Half Maximum
SERS Surface-Enhanced Raman Scattering
SFG Sum Frequency Generation
SML Submonolayers
MESFET(s) Metal Semiconductor Field Effect Transistor(s)
MIS Metal Insulator Semiconductor
PPM Pentagonal Pinch Mode
RFID Radio Frequency Identification
SKPM Scanning Kelvin Probe Microscopy
SWCNT(s) Single-Wall Carbon Nanotube(s)
TFT(s) Thin Film Transistor(s)
TPD Thermal-Programmed Desorption
UPS Ultraviolet Photoemission Spectroscopy
UHV Ultra High Vacuum
UV-vis Ultraviolet-visible Spectrophotometry
VHR Variable Hopping Range
XRD X-Ray Diffraction
α -T6 α -sexithiophene
Chapter 1. B. A. Paez-Sierra, Introduction 1.1
Chapter 1
Introduction
1.1. A brief historic review of organic materials Carbon-based compounds are considered the fundamental core of organic materials. These molecules
provide a rich variety of properties, attractive for fundamental investigations in physics, chemistry,
biology, engineering, and material sciences [Reic2005]. Therefore, special interest has been focused
on the structural, reactive, and transport properties of such materials.
Through careful design and manipulation of a wide range of carbon-based structures, organic materials
have been implemented as hybrid structures, capable of acting as electronic conductors,
semiconductors, and insulators. In the electronic field, pioneering research based on organic materials
dates back to the early 1900s, when the aim was to identify changes in the electrical conductivity of an
alcoholic solution of eosin due to its fluorescence [Nich1904, Regn1903], and, in a similar manner,
with anthracene [Howe1910]. Later on, in the late 1940s, organic molecular films were produced
under vacuum conditions and revealed semiconducting properties [Pope1999].
Parallel to the experimental activity, numerous theoretical studies related to the ground state energy in
benzene, azulene, naphthalene, anthracene, tetracene (naphthacene), and pentacene were carried out
[Mann1949, Pari1956]. Also, electrical conductivity measurements on metal-free (mf) phthalocyanine
(Pc) [Fiel1957, Heil1962], and on several metal Pcs (mPcs) with central metal atom like Cu, Ni, Co,
Mn, and Fe, were performed, where a thermoelectric power of about C/V50 °μ+ for mf-Pc and
CuPc was measured, proving in this way the dominant p-type character of the molecules [Fiel1957].
An interesting complement related to electronic properties of organic solids and based on mf-Pc was
summarized in a publication issued in 1960 [Toll1960]. On the other hand, the n-type organic
counterpart dates back to a work reported in 1961 by Kommandeur and Hall [Komm1961]. The
research was performed on pyrene-iodine and perylene-iodine samples, where a high electronic
conductivity in these organic molecules was recorded. Another important issue is that of charge
trapping phenomena, with a pioneering investigation into Pc single crystals reported by Barbe et al.
[Barb1970].
Chapter 1. B. A. Paez-Sierra, Introduction 1.2 Particularly in the last 40 years, markedly growing research activity in the field of organic materials
has been registered, with greater emphasis on two well-linked aspects, namely fundamental research
and technology, during the past decade. Currently, research on large or small organic molecules for
organic-based electronics is huge [Klau2006]. Applications range from single organic-based devices,
i.e., diodes, transistors, light emitting devices (LEDs) [Kali2005], and gas sensors, towards more
sophisticated structures, such RF identity tags, large area solar cells, passive and active-matrix light-
emitting displays [Hadz2000, Kafa2005, Klau2006, Some2005].
Consequently, many investigations are targeted at understanding the electronic and chemical structure
of organic/inorganic interfaces and bulk-organic materials in order to tune the device performance
[Kahn2003, Scot2003, Zahn2005-2006]. In general, the physical and chemical properties of small
conjugated molecules and semiconducting or metallic polymers [Brüt2004, Heeg2001] make them
challenging candidates to solve drawbacks of the silicon-based devices, i.e., temperature processing,
the ability to be structured on plastic substrates and low-cost production.
The investigated aromatic molecules, namely the perylene derivative 3,4,9,10-perylenetetracarboxylic
dianhydride (PTCDA), N,N’-dimethylperylene 3,4,9,10-dicarboximide (DiMe-PTCDI), C60, and
pentacene, have served as prototype molecules to assemble different organic-based structures.
Experiments have probed n-channel transistors based on PTCDA [Ostr1997, Xue2004], n-alkyl
perylene diimides [Ches2004], and C60 [Koba2003]; while p-channel OFETs have been realized with
pentacene and many other organic molecules [Klau2006]. Still, the efficiency of p-channel OFETs is
well above the most efficient n-channel organic-based transistor. Other devices, named diodes, have
been demonstrated with In/PTCDA/Ag [Hude2002-2003] and Ag/DiMe-PTCDI/GaAs
heterostructures [Thur2005]. 1.2. Investigation of organic/ anorganic heterostructures Several devices based on organic molecules have demonstrated the reliability of a new electronic era.
Particularly, the organic field effect transistor (OFET) has been shown to be one of the most promising
structures to be coupled as a key building block in complex technological applications [Bao2006,
Hadz2000, Klau2006, Some2005]. Although the field effect transistor (FET) device has been known
since the previous century and was proposed by Lilienfeld between 1926 and 1933 [Klei1998], there
are still several challenges to be examined in order to understand the OFET version.
Several factors can influence the OFETs' performance, i.e., substrate handling, electrodes patterning,
organic material purification, environment, and formation of interfaces, among others. Within the
framework of the present research, special interest was addressed to the strategic features presented in
Figure 1.1; they consist of an inorganic semiconductor/organic semiconductor/metal heterostructure.
Chapter 1. B. A. Paez-Sierra, Introduction 1.3 The project in which the present research activity was included was called, "Organische Feldeffekt-
Transistoren: strukturelle und dynamische Eigenschaften“, or in its English version, “Organic Field
Effect Transistors: Structural and Dynamical Properties”. This was a six-year project, started in 2001
and supported by a grant from the ",Deutsche Forschungsgemeinschaft,DFG” (German Research
Society). The project was coordinated by Prof. Dr. Christoff Wöll of Ruhr Universität Bochum. More
than 20 academic institutions, mainly from Germany, participated in this research [ofet].
Figure 1.1. Organic materials forming different interfaces, i.e.metal / organic semiconductor/ inorganic
semiconductor heterostructure.
The contribution of the present work is the investigation, using combined Raman spectroscopy (RS)
and electrical characterization of OFETs, as well as in situ RS of organic/metal interface formation of
perylene derivatives PTCDA and DiMe-PTCDI with top metal contacts indium and magnesium (cf.
Figure 1.1). Concerning the latter, previous studies were devoted to the same perylene derivatives,
with Ag as top electrode [Paez2003, Salv2003, Zahn2004].
This thesis is organized as follows: Chapter 2 outlines an introduction to the fundamental aspects of
aromatic molecules and their role in building up the molecular solid. Additionally, the basic theoretical
background of Raman spectroscopy (RS), surface-enhanced RS (SERS) and the combined external-
applied electric fields with RS, important to characterize the heterostructures and OFET devices, is
presented.
In chapter 3, the experimental setups, i.e., RS, charge transient spectroscopy (QTS) and current-
voltage, are discussed. Afterwards, the procedures related to substrate preparation, molecular beam
deposition and metal evaporation (In, Mg), and the algorithms developed to analyze the measured data
are described. In chapter 4, the in situ measurements of RS and SERS of the interface formation
Chapter 1. B. A. Paez-Sierra, Introduction 1.4 between the metals indium and magnesium onto the perylene derivatives PTCDA and DiMe-PTCDI
deposited on S-GaAs substrates are analyzed.
The field effect transistor theory and its drawbacks, together with further improvements probed by
experimental measurements, are addressed in chapter 5. In chapter 6, the in situ combined Raman
spectroscopy and electrical characterization of the conductive channel in OFETs are presented. It is
demonstrated that the charge transport is developed in a two-dimensional regime, proving that the
minimum organic material needed to form the conductive channel scales well below a 10 nm organic
layer thickness. Additionally, the Raman bands assignments are based on density functional theory
(DFT) calculations. The chapter is concluded with a proposed model to describe the drain current
relaxation during operation of the OFET device.
In chapter 7, the influence of electric fields and illumination on the structural and transport properties
of the OFET channel is discussed. The results are complemented with DFT calculations. In chapter 8,
the intrinsic and field induced traps at the organic channel of OFET devices are investigated by QTS
measurements. Additionally, the anomalous behavior of the QTS signal is experimentally
demonstrated and analyzed by a proposed theoretical model. Finally, concluding remarks and a
summary of this research are given in chapter 9.
References
[Bao2006] “Organic field-effect transistors V”, edited by Z. Bao and D. J. Gundlach, Proc. SPIE 6336, Washington 2006. [Barb1970] D. F. Barbe and C. R. Westgate, “Bulk trapping states in β -phthalocyanine single crystals”, J. Chem. Phys.
52, 4046-4054 (1970). [Barb1970] D. F. Barbe and C. R. Westgate, “Surface state parameters of metal-free phthalocyanine single crystals”, Phys.
Chem. Solids 31, 2679-2687 (1970). [Brüt2004] “Physics of organic semiconductors”, edited by W. Brütting, phys. stat. sol. (a) 201, 1037-1371 (2004). [Ches2004] R. J. Chesterfield, J. C. McKeen, Ch. R. Newman, P. C. Ewbank, D. A. da Silva Filho, J. L. Brédas, L. L.
Miller, K. R. Mann, and C. D. Frisbie, “Organic thin film transistors based on N-alkyl perylene diimides: charge transport kinetics as a function of gate voltage and temperature”, J. Phys. Chem. 108, 19281-19292 (2004).
[Fiel1957] P. E. Fielding and F. Gutman, “Electrical properties of phthalocyanines”, The J. Chem. Phys. 26, 411-419 (1957).
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[ofet] Schwerpunktprogramm 1121: Organische Feldeffekt-Transistoren: strukturelle und dynamische Eigenschaften www.ofet.de.Coordinator Prof. Dr. Christof Wöll. Activity in Chemnitz coordinated by Prof. Dr. D.r. h.c. D. R. T. Zahn and Dr. R. Scholz under the reference Za 146/4-2 as part of SPP 1121: Organic field effect transistors: Structural and dynamical properties.
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Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.1
Chapter 2
Fundamentals of molecular structures and Raman
spectroscopy
2.1 Aromatic molecules
The π conjugation is what makes many organic molecules attractive for electronic applications, i.e.,
organic light emitting diodes (OLEDs) [Kafa2005], solar cells [Kafa2005], and OFETs [Bao2006].
The present work is based on a particular branch of π -conjugated aromatic structures named arenes,
with the benzene ring as their structural building block. Most worked out molecules in the arene
family are acenes, consisting of planar arrangements of benzene rings and sharing two carbons
between them.
Pentacene is a prototype molecule of the acene group (cf. Figure 2.1(a)). The acene family can be
described by the formula 4n22n4 HC ++ , n being the number of benzene rings constituting the molecule.
Another important arene sub-group is the perylene derivatives, with perylene, PTCDA, and DiMe-
PTCDI some representative molecular structures of this sub-family (cf. Figure 2.1(b)).
(a)
(b)
Figure 2.1. Some representative molecules of the arene family. (a) APentacene and (b) Perylene
derivatives. (*) Main structures investigated in the present research.
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.2 2.2. The benzene ring and π-electron delocalization
As an example of π orbitals conjugation, the benzene ring is considered. Its carbon skeleton is
arranged in a regular hexagon with bond lengths of 1.4 Å being halfway between the typical 22 spsp − single-bond distance of 1.46 Å and the 22 spsp = double-bond distance of 1.34 Å. Each
carbon atom is involved in 2sp hybrid atomic orbitals bonding to the hydrogen atom and the two
adjacent carbon atoms, thus describing the σ bonds presented in Figure 2.2(a). The fourth electron
takes up a zp orbital, which is for the most part situated at right angles to the plane of the molecule
(cf. Figure 2.2(b)). These zp orbitals are fairly extended in space and overlap to form a pair of π -
electron clouds on each side of the plane of the molecule, as illustrated in Figure 2.2(c) [Bowe2002,
Care2004]
(a) (b) (c)
Figure 2.2. σ−hybrids and π−molecular orbitals in benzene. a) Localized σ−orbitals, b) pz atomic
orbital, and c) delocalised π−orbitals with highest densities above and below the plane ring. (Thanks to
G. Gavrila).
The delocalized π -electrons can move freely inside the molecule and are, therefore, sometimes called
the mobile electrons or the unsaturated electrons, because they result from unsaturated bonds. Through
such mobile π−electrons, electrical perturbations are easily transmitted from one part of the molecule
to another.
2.3. Organic semiconductor solid
Organic materials are distinguished from inorganic solids basically due to the weak electronic
interaction between nearer molecules.
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.3 Thus, in a conglomerate of molecules forming the organic solid, the weak electron-electron interaction
takes the form of a kind of band structure through the π orbitals [Ishii1999, Pire1974-1984], cf. Figure
2.3. The effective potential well of an electron is formed by the atomic nuclei and other electrons. The
wells of the nuclei are merged in the upper part to form a broad well. Deep atomic orbitals are still
localized in the atomic potential well (core levels), but the upper ones interact to form delocalised
molecular orbitals (MOs). The outermost horizontal part of the potential well is the vacuum level
(VL). The energy separations of the highest occupied MO (HOMO) and lowest unoccupied (LUMO)
from the vacuum level (VL) are the ionisation potential (IP) and the electron affinity (χ) of the
molecular solid, respectively. Since the molecules interact only by the weak Van der Waals
interaction, the top part of the occupied valence states and the lower unoccupied states are usually
localised in each molecule, with narrow intermolecular bandwidths (typically lower than 0.1eV)
[Gutm1967, Kao1981]. Thus, the electronic structure of an organic solid largely preserves that of a
molecule and the validity of usual band theory (which assumes itinerant electrons) is often limited
[Duke1980]. The electronic structure in an organic solid is usually simplified and presented as the one
depicted on the right side of Figure 2.2. The Fermi level is also indicated there, since electrons fill the
energy levels following the Fermi statistics [Ménd2006].
Figure 2.3. Molecular states of an
organic solid [Ishii1999, Ménd2006,
Pire1974-1984].
2.4. Transport in organic materials
A charge transport in the organic semiconductor is developed through overlapping of π neighboring
molecular orbitals. The transfer of charge from site to site in the organic solid is assisted by internal
and external or libronic phonons. The electronic conductivity of organic materials does not fully
follow the classical diffusion models, and the mobility, together with other kinetic coefficients, is
thermally activated due to the phonon-assisted process. Accordingly, there is a need to introduce a
model able to describe the hopping process.
The dynamics of charge carrier-phonon interaction in an organic solid is given by the Hamiltonian of a
coupled fermion-boson system [Cruz1992, Well1996]:
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.4
( ) ( )( )( )∑
∑∑∑
σ
+σ−
+
σσ
+σσ
σσ
+σσ
+σ
+−ωε+
++ω+μ−+−=
,iiiiphn
iii2
1
,i,i,i,i
)j,i(,i,j,j,io
bb1n
bbcccccctH
h
h
, (2.1)
where ot− denotes the transfer amplitude (attractive interaction) between nearest neighbor pairs ( ji, ).
This quantity is given by the product between the ionization potential and the overlap integral. The
operator )(,ic +σ annihilates (creates) an electron at the site i with spin projection σ , σμ ,i is the
chemical potential and takes into account the mean number of particles at a given temperature, ω
denotes the bare phonon frequency, )(ib + is the phonon annihilation (creation) operator,
↓↑σ += iii nnn is the number operator (electrons), and phn−ε is the energy of the electron-phonon
coupling.
The Hamiltonian given in eq. (2.1) is the modern idea of the pioneering version of the coupling
between the electron and the lattice vibrations, leading to the formation of a composite particle named
polaron and introduced by Fröhlich [Fröh1950-1954], and later on by Lee [Lee1952], and in a
variational form with path integrals by Feynmann [Feyn1955]. From tight binding theory, it is found
that the band gap is approximately four times the transfer amplitude. Based on this result, in chapter 5
the charge carrier density for a two dimensional charge carrier gas is determined. An interesting
feature of the Hamiltonian in eq.(2.1) is that the last term also accounts for trap occupation, and the
coupling between the excess charge carriers with the phonons.
It is known that
( ) i
2/1ph
ii qm2
bb ⎟⎟⎠
⎞⎜⎜⎝
⎛ ω=++
h, (2.2)
phm being the phonon effective mass and iq the phononic-spatial coordinate. From eq.(2.2) and the
last term of the previous Hamiltonian (eq.(2.1)), one finds the strength between the phonon and the
excess charge carriers given by
phnphphn2
ph A2m2 −− εε=εω=ξ , (2.3)
with A the maximum amplitude of the normal mode with energy phε .
2.4.1. Some input parameters for the coupled electron-phonon system
In the present work it has been theoretically and experimentally proved by combined Raman
spectroscopy and applied electric fields that the vibrations of pentacene layers forming the channel in
FETs (chapter 7) are affected, in particular those where the ring and C-H vibrations are involved. The
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.5 representative Raman region is between 1120 cm-1 - 1210 cm-1, which amounts to energies ranging
between 139 meV and 150 meV. The affected vibrations basically correspond to gA modes, having
in-plane vibrations of C-H bonds at the outer rings, and vibrations of C-H bonds parallel to the main
axis of the molecule.
Table 1 summarizes some input parameters for the Hamiltonian in eq.(2.1). The quoted values are the
force constants K determined from density functional theory (DFT) at the B3LYP/3-21G level in
Gaussian 98 [Gaus1998]. These values are extracted after geometry optimization and calculations of
the vibrational Raman and infrared spectra of the pentacene molecule.
The next quantity in the table is the average energy of the local phonons >ε< ph , which are affected
by the external electric field; these quantities were extracted from experimental measurements of the
Raman spectra in pentacene-based FETs, discussed in chapter 7. Afterwards, the third column of the
table lists the maximum amplitude (A) of the normal modes with average energy >ε< ph . The next
column is the energy of the electron-phonon coupling >ε< −phn ; here the >ε< −phn is assumed to be
approximately the experimental activation energies actε described in chapter 8. Finally, the last
column is the electron-phonon coupling (ξ ) computed by means of the relation given in eq.(2.3).
Table 2.1. Parameters of the electron-phonon
coupling in pentacene-based FETs.
K
/ eV- Å-2
>ε< ph
/ meV
A
/ Å actphn ε≈ε −
/ meV
ξ
/ eV- Å-1
172.45 143.0 0.04 25.0 2.9
167.1 7.60
127.25 146.3 0.05 25.0 2.51
167.1 6.49
2.4.2. Field effect mobility
The organic-semiconducting solid is characterized by a charge transport with low mobilities in
conjunction with the monocrystalline or polycrystalline inorganic semiconductor. Usually, in most of
the cases, the mobility is lower than that of the organic-crystal form. The charge transport is
accompanied by thermally activated and electric-field dependent processes. Therefore, the charge
transport model given by the coupled fermion-boson system in eq. (2.1) is well complemented by
descriptions taking into account the explicit thermally activated and field dependent processes, i.e.,
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.6 Poole-Frenkel [Gill1972], disorder formalism [Bäss1981-2000], and multiple trapping and release
processes [Nool1977].
A model that might summarize most of the features stated in the previous descriptions is the “variable
hopping range” (VHR) by Vissenberg and Matters [Viss1998]. The VHR model predicts a thermal-
activated and gate voltage dependent mobility of the OFET device. 1
orgoB
2s
11c
3
3o
eff Tk2)VC(
)1()1(B)~2(e
−ςς
−−⎥⎥⎦
⎤
⎢⎢⎣
⎡
ε⎥⎦
⎤⎢⎣
⎡ς+Γς−Γα
ςπσ=μ g , (2.4)
here oσ is a prefactor, e is the electron charge, α~ is the effective overlap between hopping sites, cB
gives the percolation criterion of the organic layer, T/To=ς , ∫∞ −−=Γ0
x1x dxet)x( is the gamma
function. The thermal energy TkB is below the width oBTk of the exponential distribution of the
DOS, and the terms sC , gV , orgε correspond to the gate-dielectric capacitance per unit area, gate
voltage, and the dielectric constant of the organic material, respectively.
Considering a drop potential through the insulating dielectric layer and leaving eq.(2.4) as a simpler
power in dependence with the gate voltage, the drop potential yields [Meij2002]
( )kTgoeff VV −μ=μ , (2.5)
with oμ the none-field dependent mobility, TV the threshold voltage, and k an exponent given by )1( −ς in eq.(2.4). A typical value of the mobility in a-Si is about 1.1 cm2 V-1 s-1 comparable with the
hole mobility in several thin film organic-based transistors.
The thermally-activated field effect mobility, based on the Bolttzmann statistics and with a single trap
state following an exponential distribution, is proportional to [Ches2004]
⎟⎟⎠
⎞⎜⎜⎝
⎛ ε−≈μ
TkexpS
B
acteff , (2.6)
the factor S a fitting parameter.
If the material is free of traps, then the field effect mobility is described as meff T −∝μ being 1≤ m≤4.
This power dependence is produced via scattering of charge carriers with phonons. When the
temperature increases, the exponent m has been found to increase for holes and decrease for electrons.
Experimental results have shown larger mobilities for holes than for electrons. On the other hand, if
the organic solid has a relative trap concentration c, the mobility is given by [Wolf1997] 1
B
teffeff Tk
expc1)T,0c()T,c(−
⎥⎦
⎤⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛ ε+=μ=μ , (2.7)
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.7 the first factor accounts for the mobility without traps, and the second one is the trapping factor.
Results presented in eqs.(2.4-2.7) are a fairly good estimate of the mobility. Those models are lacking
a description of the field dependence of threshold voltage ( TV ). This has been experimentally
evidenced by Gomes et al. [Gome2004], and in this work has been proved by the correlated fitting of
the output characteristics in OFETs (chapter 5), and by calculation of the time-dependent capture cross
section based on QTS measurements (chapter 8).
Additionally, field-dependent QTS measurements on OFETs with open gate configuration (chapter 8)
proved that the trapping mechanism has several components which depend on the way the organic
material is bias-pulsed. Although the gate-dielectric/organic interface is affected by traps, the applied
electric field induces additional trapping sites in the organic material (chapters 5-8). Therefore, the
conductive channel itself and likewise its boundaries, i.e., electrodes, bulk organic material, should not
be excluded from the trapping phenomena.
2.5. Light-matter interaction
2.5.1. Raman spectroscopy
The interaction between photons and matter has gained an important position, fueling pure research
and applications at different industrial levels. Among the optical techniques, Raman spectroscopy
plays a very important role in determining the vibrational properties of matter. The vibronic spectrum
is a fingerprint of each substance. Indeed, any physical or chemical alteration of the investigated
structure modifies its vibrational spectrum. The previous section addressed the importance of phonon-
assisted processes of charge transport in organic semiconductors.
Historically, the Raman effect was predicted by Smekal [Smek1923] in 1923 and the first experiment
was performed by Sir C. V. Raman in 1928 [Rama1928]. Since then, innumerable experiments in
diverse research fields, i.e., physics, chemistry, astronomy, biology, and many others, have been
realised. Two years after the experimental discovery by C. V. Raman, an article by D. H. Andrew
about the Raman spectra in organic molecules and the relation between the involved atoms and the
formed bond nature [Andr1930] was reported, which can be considered as pioneering research in the
vibrational molecular branch.
The usefulness of the method is mainly due to its sensitivity and non-destructiveness. It is capable of
providing information on chemical identity [Gloc1943, Schm2006, Kuzm1988], charge states
[Auss1986, Fleu1967, , Shan1972], processes at interfaces [Mile2006, Paez2003-2004, Zahn2001],
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.8 and structural order in organic materials [Colo2003], among many other interesting molecular
features.
Raman spectroscopy [Berm1988, Brig1980, Kuzm1988, Schü2006, Sing2005] is based on the
inelastic interaction between light and matter. When light is scattered by any form of matter, the
energies of the majority of the photons are unchanged by the process, corresponding to elastic or
Rayleigh scattering. A minor number of photons, approximately one per million, are involved in the
inelastic scattering process of creation (Stokes process) or annihilation (anti-Stokes process) of
excitations within the medium. Therefore, the scattered photons will have an energy lower (Stokes) or
higher (anti-Stokes) than that of the incident light. Most routine Raman experiments use the red-
shifted Stokes peaks, because they are more intense at room temperature.
Figure 2.4. Spectrum of scattered light showing the Raman Stokes, Rayleigh, and Raman anti-Stokes
bands.
Figure 2.4 shows the Stokes, Rayleigh, and anti-Stokes processes, where a photon with energy excωh
and momentum exck is scattered by the creation or annihilation of an elementary excitation with
energy iωh and momentum iq . The scattered photon has an energy ω′h and momentum exck ′ . This
means each scattered photon in the Stokes component is associated with a gain in energy iωh by the
sample. Similarly, the sample loses energy iωh for each scattered photon in the anti-Stokes
component.
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.9 The conservation energy reads:
iexc ω±ω=ω′ hhh , (2.8)
where the minus (plus) sign is for the Stokes (anti-Stokes) process. Conservation of momentum
requires
iexc qkk ±=′ , (2.9)
Here, the plus (minus) sign is for the Stokes (anti-Stokes) process. Schematically, a Raman spectrum
with energy iωh looks like that displayed in Figure 2.4. There is a very strong component due to
elastic scattered photons, i.e., with energy excωh .
In chapter 3, the experimental Raman setup used during this investigation to characterize the
vibrational bands of organic / inorganic interfaces (chapter 4) and OFETs (chapters 6-8) is discussed.
2.5.2. Basic theoretical background on Raman spectroscopy
2.5.2.1. Classical description of the Raman effect
Classically, Raman scattering can be explained by molecular polarizability (α ) modulation. When a
molecule is subjected to the electric field ∑ ω=l
l )tjexp(oEE of a multi-energetic ( lhω )
electromagnetic beam, its dipole momentμ is given by
Εαμμ += )0( , (2.10)
with )0(μ the permanent dipole and Εαμ =)ind( the induced dipole moment by the electric field Ε ,
and α the polarizability, which can be expressed by the superposition due to elastic (Rayleigh) and
inelastic (Raman) contributions )Raman()Rayleigh( ααα += , (2.11)
with the first term on the right hand side being the induced-Rayleigh polarizability and the second the
Raman induced polarizability.
The matrix form of eq.(2.10) in Cartesian coordinate is written as follows:
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
ααα
ααα
ααα
+⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
μ
μ
μ
=⎥⎥⎥
⎦
⎤
⎢⎢⎢
⎣
⎡
μ
μμ
z
y
x
zzyzxz
zyyyxy
zxyxxx
)0(z
)0(y
)0(x
z
y
x
E
EE
. (2.12)
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.10 The character of the polarizability tensor depends on the symmetry of the molecule. On the other hand,
it must be noted that the dipole moment and the electric field do not necessarily share similar unit
vectors (sect. 8.8).
Since Ε is a frequency dependent electric field, the coordinates of the atoms might be affected.
Therefore, instead of specifically using the atom position, the normal coordinates )t(qi are
considered. The number of normal coordinates equals the total number (N) of vibrational modes. For
small vibrations, )t(q i can be approximated by
)tcos(q)t(q ioi ω= , (2.13)
where oq is the maximum amplitude and iω is the vibrational frequency of i-th normal mode.
Assuming a monochromatic light source ( 1=l ) to excite the sample, and expanding the polarizability
in terms of the normal coordinates, the total dipole moment is then given by:
, (2.14)
where the different contributions to the induced dipole moment are commented on in the equation, the
term O(3) meaning third order derivatives. For the present research, only terms up to first order
derivatives are considered. If 0q 0i
=⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂α , then the corresponding -i-th- mode will not appear in the
Raman spectrum. A similar situation occurs for the infrared spectrum if 0q 0i
=⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂μ . It has been proved
that if the molecule has an inversion center of symmetry, the Raman active modes are not observed in
infrared, and those active in infrared will be absent from the Raman spectrum.
2.5.2.2. Quantum mechanical description of the Raman effect
The classical theory correctly predicts the frequency dependence for Rayleigh scattering and
vibrational Raman scattering. It also shows correctly the dependence of the Rayleigh scattering tensor
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.11 ( )Rayleigh(α ). On the other hand, the classical description has some limitations. It cannot be applied to
molecular rotations. The result for the induced polarizability ( )Raman(α ) of the Raman scattering tensor
is only partly correct.
The classical picture does not provide information about how the )Raman(α is related to the properties
of the molecule, in particular its characteristic transition frequencies, or to the frequency of the
incident radiation. Fortunately, the quantum mechanical model provides this information and bridges
the gap from the classical description to a complete treatment of all aspects of Raman scattering
[Long2002].
The electric dipole moment (μ ) given in eq.(2.10) can vary, due to the dynamics of the atomic motifs
comprising the molecule; consequently, vibronic transitions at any electronic state are expected. As an
example, in Figure 2.5(a) the vibronic energy levels of a diatomic molecule are depicted with the wave
functions on the intemolecular Morse potential, and are distributed for each electronic state with
energy Ee and the associated quantum numbers, i.e., principal (n), angular (j), orbital (l) and magnetic
( m~ ). Each vibrotional state is characterized by a frequency iω and normal coordinate modes iq . The
likely vibronic transitions at this electronic level are summarized by the infrared transitions shown in
Figure 2.5(b).
Another process is developed when the electronic state remains unmodified, thus the transitions are
developed between tne vibrational levels at the electronic ground state. If the scattered photon has the
same energy as the incoming radiation, the process is named Rayleigh scattering (cf. Figure 2.7(a)).
(a) (b)
Figure 2.5. Molecular Morse potential of the (a) ground electronic and bound vibrational states. (b)
Infrared activity described by transitions between vibrational states at a given electronic state
mljneE .
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.12 Otherwise, the normal Raman scattering is achieved and described by the Stokes (cf. Figure 2.6(b))
and anti-Stokes (cf. Figure 2.6(c)) scattering. There is an extension of the previous description when
the excitation energy is comparable to that of the first excited electronic state or above; then similar
scattering phenomena are developed, where the excitation attains the manifold electronic states and
consequently the resonant Raman effect is achieved as described in Figure 2.7.
It can be deduced from Figure 2.7 that the Raman bands (transitions) are most intense when the wave
function of the upper vibronic state resembles that of the ground state of the vibrational wave function.
The amplitudes of these vibronic transitions are proportional to the square of the probability of the
corresponding initial ( gor;nj − ) and final ( eor;mj −′ ) electronic excited vibronic states [Fran2000,
Joha2005].
(a)
(b)
(c)
Figure 2.6. Non-resonant Raman effect involving (a)
elastic light scattering or Rayleigh scattering, and the
inelastic processes: (b) Stokes scattering with the
scattered photon energy higher than the incoming one,
and (c) anti-Stokes where the scattered photon
possesses lower energy than that of the excitation
source.
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.13 In this scheme the vibronic wave function ( )r( on,jψ ) of the Morse potential is described by
)(L)2/exp()1nj2()nj(!n2njr)r( n2j2
nnj
oon,j ξξξ−+−Γ−β
==ψ −− , (2.15)
with )rexp()1j2( oβ−+=ξ , )(L n2j2n ξ− the associated Legendre polynomials, and the bonding
energy of the state nj expressed as 222n )nj)(m~2/(E −β−= h . The amplitude is proportional to
the transition 2
eogo mj;rnj;r ′−− , which can be readily integrated to determine the overlap
between vibronic states. Note that as long as the difference of the minimum position between the non-
excited and excited states is small, the vibrational transition probability increases. This is attributed to
allowed dipole transitions.
Figure 2.7. Resonant Raman
effect (a) vibronic transitions
pointing to the Raman-Stokes,
Raman-anti-Stokes, and Rayleigh
scattering.
For a molecular structure, those states are mainly between HOMO and LUMO. The intensity of the
Raman scattered light with polarization ρ of the excitation light is given by [Garo1976, Kürt1991]
24foII σρσρ αω∝ , (2.16)
where σoI is the intensity of the incoming light with polarizability σ , and inelastically scattered with
frequency fω . The polarizability in this frame is described in the form [Hass2004]:
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.14
∑ ⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
Γ+ω+−+
Γ−ω−−=α
ρσσρ
σρf~ f~excff~f~excgf~ i)EE(
gˆf~f~ˆf
i)EE(
gˆf~f~ˆf
hhhh
μμμμ, (2.17)
where σρμ is the electric dipole operator and takes into account the electronic and nuclear
contributions. The states f , f~ , and g denote the final, intermediate, and initial states,
respectively. The corresponding energies are fE , f~E , and g~E . The factor f~Γ is a damping
coefficient. If the energy of the incident radiation ( excωh ) approaches the molecular transition energy
)EE( gf~ − , then the interaction will be in resonance.
The efficiency of the Raman signal is measured by the photon-scattering cross section [Joha2005] 2
n totgevibexc42
o
exc3
4oRS in
)n,0(f)n,1(fc6
p ∑ Γ+Ω−ω−ωπεωω′
=σhhhh
, (2.18)
with
vibexc ω−ω=ω′ , (2.19)
2/2/ 0,radvibphtot γ+γ+γ=Γ , (2.20)
0,radγ : interaction with the vacuum fluctuations expressed by Fermi’s golden rule
0,radγ : dephasing rate
geΩh : energy level separation
dipo lep = : dipole with elementary charge e and length dipl
vibΩ corresponds to the pure vibrational frequencies of the associated harmonic Hamiltonian
The typical order of RSσ is about 229 cm10− . The next section describes the procedure for enhancement
of the Raman signal. It involves the magnification of the cross section.
2.5.3. Surface-enhanced Raman scattering (SERS)
In the previous section, it was pointed out how small the Raman signal is in comparison to the
intensity of the incoming excitation laser energy. In order to enhance the detected Raman spectrum
intensity, several technological efforts have been targeted at circumventing this detection limit.
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.15 An interesting phenomenon which developed locally in the tested sample is observed when the
material is embedded in a metal-matrix environment. Although a perfect metal surface behaves as a
mirror for the electromagnetic radiation, this can be avoided by texturing the surface in a way that the
patterned layer will no longer be a quasi or perfect reflector for the light. Indeed, the modified surface
geometry substantially enhances the local electric field. The resulting local field produced in the
clusters is much higher, then the material in the vicinity is electronically and structurally altered by the
field, therefore, parallel signatures are induced, organic electric dipole ( )ind(μ ) coupling with the
metallic cluster plasmon, molecular polarizability ( )()Raman( ωα ), and charge transfer (CT) (cf. Figure
2.8) among others. The Raman signal is enhanced, usually by several orders of magnitude. The
phenomenon is referred to as surface-enhanced Raman spectroscopy SERS [Aroc2006, Mosk2002].
Figure 2.8. Examples of SERS from a (a) pentacene (30 nm)-based OFET, and a (b) In (15 nm)/
PTCDA (15 nm)/S-GaAs heterostructure.
The SERS signal is divided into two principal contributions, one from the interface formed between
the first molecular layer and the metal, called chemical enhancement or first layer effect. The other
one is a long-range contribution mediated by the huge local electric field, referred to as
electromagnetic enhancement.
The magnification factors (based on this research) are estimated to be 101-102 for the chemical
contribution or first monolayer effect [Pers2006], and 102 or higher for the electromagnetic
contribution. Therefore, the cross sections are magnified from ca. 10-29 cm2 to 10-17-10-14 cm2
[Joha2005, Knei1997, Otto2001]. Due to this pronounced enhancement of the photon cross section,
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.16 SERS is widely used in analytical science and technology [Aroc2006, Cao2005, Mosk2002, Vo2004].
Detailed theoretical descriptions about SERS are found elsewhere [Aroc2006, Joha05, Knei1997,
Otto2001, Pers2006]. As an example, figs. 2.9(a) and 2.9(b) illustrate the Raman spectra of a
pentacene (30 nm) layer forming the channel in an OFET structure. It should be noted that the Raman
signal of molecules on the gate dielectric is much lower that that produced by molecules deposited on
the Au electrodes; the enhancement factor for this experiment was about 31.
The panel given in Figure 2.8(b) consists of the Raman spectra from bare PTCDA (15 nm) deposited
on S-GaAs and an In layer of 15 nm deposited on the previous structure; the Raman bands are
enhanced by a factor of about 102. The metallic overlayer produces a breakdown of the molecular
symmetry, since the activated modes were originally infrared active modes of the bare molecular
layer. Additionally, a well-defined charge transfer (CT) is developed. It can be observed that both
structures, pentacene (30 nm)-based OFET and In/PTCDA, have a markedly different base line,
depending on whether the molecular layer is on the inorganic substrate or is interfacing with the metal
electrode.
2.5.4. Combined Raman spectroscopy with electric fields
The application of an electric field affects the optical absorption and the scattering of electromagnetic
(EM) radiation by optical phonons; both have been observed in inorganic semiconductors and
aromatic molecules [Auss1986, Fleu1967, Shan1972]. These changes result from several effects of the
modulating field: First it lifts the degeneracies of both the phonon and the electronic states, and second
it affects the symmetries of the corresponding wave functions. As a consequence, the Raman-active
phonon modes can shift energetically, and, due to changes of the selection rules, new scattering
transitions can contribute to the Raman signal. When an external electric field is applied to a molecule,
the electronic charges are redistributed between different atomic sites, resulting in induced dipole
moments; therefore, the energy potential described in section 2.5.2.1 is distorted. Thus the Frank-
Condon transitions are subjected to the new molecular organization, provoking changes in the Raman
cross section.
In chapter 7, the experimental results of the Raman signal with applied electric fields in organic-based
field effect transistors (OFETs) are described. Moreover, it is theoretically described how the applied
field (E) in a given direction produces modifications of the molecular dipole moment, resulting in
effective contributions that in general do not follow the direction of the field (eq. (2.12)).
The nature of the applied field can be a time dependent or a static perturbation. Both fields were
applied to the OFETs (chapter 6-8). In addition, in chapter 7, the experimental and theoretical
measurements of combined Raman spectroscopy with electric fields and the effect on the structural
properties of the pentacene layer in OFETs are described. Furthermore, after the removal of the
Chapter 2 B. A. Paez-Sierra, Fundamentals of molecular structures… 2.17 applied bias, the molecular layer follows a relaxation process, which was demonstrated by the Raman
bands relaxation.
The decay time was confirmed by charge transient spectroscopy (QTS), revealing novel phenomena
never seen in pentacene-based FETs [Thur2006], addressed by the authors as “anomalous QTS”. On
the other hand, the calculated cross sections from QTS are comparable with the photon cross section,
due to chemical enhancement in SERS. From both techniques, it has been inferred that the application
of the electric field induces electric dipole states at the organic material.
The theoretical simulations of the vibronic properties in the present investigation were done within the
framework of the density functional theory (DFT), using the package Gaussian 98 [Gaus1998] at the
B3LYP level [Beck1993] with 3-31G basis sets having Gaussian-like wave functions [Helg2000]. The
fundamentals of the DFT are reported in detail elsewhere [Koch2002, Parr1989].
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Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.1
Chapter 3
Experimental techniques, materials, and algorithms
In this chapter, the investigated molecular structures and the main experimental characterization
techniques involved to scope the organic/inorganic structures are discussed. The most frequently used
experimental methods to characterize the samples during this research were Raman spectroscopy,
scanning Raman spectroscopy, current voltage (I-V) characteristics, and charge transient spectroscopy
(QTS).
The molecular structures consist of perylene derivatives named 3,4,9,10-perylene tetracarboxylic acid
dianhydride (PTCDA) and 3,4,9,10- N-N’-dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMe-
PTCDI). These organic molecules were evaporated by molecular beam deposition (MBD) under UHV
conditions and deposited on S-GaAs substrates. Indium or magnesium were deposited on these organic
molecules and they were characterized in situ by surface-enhanced Raman spectroscopy (SERS)
experiments.
The organic molecules to form the active layer of the FET structures were pentacene and C60; minor
work was done on the C60–based devices. The data evaluation was carried out primarily in special
routines written in MatLab [MATL2003], with a particular code based on a consistent recursivity of
initial boundary conditions developed to achieve efficient data evaluation. Furthermore, in some
theoretical modeling involving numerical integration, a routine based on the Gauss quadrature was
constructed. As a result, high accuracy and convergence speed in comparison with standard
geometrical integration methods were achieved.
3.1. Combined Raman spectroscopy and electrical characterization setups
In chapter 2, the fundamentals of Raman spectroscopy were given. In this section, the experimental
setup used for this research is outlined.
The Raman spectrometer employed during this work was a triple monochromator Dilor XY 800 model
from Dilor. The system is equipped with a Peltier-cooled CCD (256x1024 pixels) camera for
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.2 multichannel detection in a back scattering configuration geometry. This allows the setup to be used in
macro- and micro-configuration.
The samples can be excited with different monochromatic laser lines, provided by Ar+, Kr+ or HeCd
gas-based lasers. During the macro-Raman experiments, the laser power density on the samples was
set to 25.4 Wcm-2 for C60 (to avoid photopolymerization), 70 Wcm-2 for pentacene active layers in
FETs, and 140 Wcm-2 for PTCDA, DiMe-PTCDI and related interlayers.
The spectral resolution ranged from 2 cm-1 to ~ 3.5 cm-1
as determined from the FWHM of the
Rayleigh line.
For the in situ measurements the angle between the incident and scattered light is fixed by the position
of the corresponding UHV windows. The sample surface was always oriented parallel to the collection
window, i.e. the scattered light was analysed in the direction parallel to the normal of the sample. The
incidence angle was 30°. The diameter of laser light focused on the sample was ~ 300 μm.
Additionally, in this research the polarization of the incoming and scattered beams is given according
to the Porto coding, which takes into account the propagation of the incoming and scattered beams
together with the polarization, i.e., z(xy)z’: means the incident light is propagating along the z axis and
then back scattered (z’) in the same axes; xy in the brackets labels the electric field polarizations with
respect to a fixed reference plane holding the sample.
Figure 3.1. Experimental setup for combined Raman spectroscopy, scanning Raman spectroscopy, and
current-voltage (I-V) characteristics measurements.
The incoming electric field is polarized in the “x” direction and the scattered electric field in the “y”
direction. The codes z(yy)z’, x(zx)y, x(zz)y, etc., are set up in a similar manner.
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.3 The samples investigated in situ by Raman spectroscopy or in combination with electric fields were
placed in an analysis chamber with a base pressure of 2x10-10 mbar and with the capability of
performing organic beam deposition, metal evaporation from Knudsen cells, on-line spectroscopy and
electrical characterization of the structure formation, as sketched in Figure 3.1. The current-voltage
characteristics were obtained with a Keithley 238 high current source/meter unit controlled by a
computer with a code written in LabView [LabV2004] or MATLAB [MATL2003], as depicted in
Figure 3.1.
3.2. Sulphur passivation of GaAs(100)
The substrates were Si doped n-GaAs(100) wafers acquired from Freiberger Compound Materials
GmbH (FCM) with a doping concentration of ~ 318 cm108.1 −× estimated by infrared spectroscopy
measurements.
The passivation procedure of GaAs(100) surfaces was done in a S2Cl2 solution [Salv2003]. The
procedure is described in Figure 3.2.
Figure 3.2. Substrate passivation:
ex situ chemical treatment and in
situ annealing and material
deposition.
3.3. Materials and structures
The structures are divided into two types, one series intended for metal organic interface formation
investigations (structures I), the others consisting of organic molecules deposited on field effect
devices to form organic-based field effect transistors (OFETs) and referred to as structures II.
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.4 3.3.1. Structures I: perylene derivatives thin films capped by metallic
overlayers of indium and magnesium
The molecular structures are described in fig 3.3. Additionally, Figure 3.3 indicates the group
symmetry, together with the character of the vibrational bands, i.e., Raman active, infrared active or
silent modes. In order to estimate the appropriate working energy for the resonant Raman experiments,
the absorption spectra of both molecules are shown in Figure3.3(c). Further description on crystal
structures and the molecular alignment on similar substrates are reported elsewhere [Ferg2006,
Frie2003, Gavr2006, Kobi2004, Ménd2006, Salv2003]. Details on the organic/inorganic interface
formation and the in situ investigation by Raman spectroscopy are presented in chapter 4.
3.3.1.1. Molecular beam deposition (MBD) and metal evaporation
The molecular and metallic materials were thoroughly degassed one by one in the vacuum chamber
for at least 24 h prior to deposition and were evaporated from two separate Knudsen cells at 280 °C
and 270 °C for PTCDA and DiMe-PTCDI, respectively. These parameters led to a growth rate of
about 0.3 nm/min for both molecules. The metals were evaporated at 830 °C for indium and 360 °C
for magnesium, with an evaporation rate of 2 nm/min.
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.5
PPTTCCDDAA
3,4,9,10- PeryleneTetraCarboxylic DiAnhydride
E0-0 = 2.21 eV
CC2244HH88OO66
Symmetry D2h
Raman active: 19Ag+18B1g+10B2g+7B3g
IR active: +10B1u+18B2u+18B3u
Silent: + 8Au
108 internal vibrations
(a)
DDiiMMee--PPTTCCDDII
3,4,9,10- PeryleneTetraCarboxylic DiImide
E0-0 = 2.14 eV
CC2266HH1144OO44NN22
Symmetry C2h
Raman active:
44Ag+22Bg
IR active:
+23Au+43Bu
132 internal vibrations
(b)
(c)
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.6 Figure 3.3. Molecular structure of perylene derivatives with the associated symmetry group of (a)
PTCDA and (b) DiMe-PTCDI [Kobi2004, Salv2003]. (c) UV-vis absorption spectra of organic layers
deposited on quartz.
The film thickness was monitored in situ using a quartz crystal microbalance positioned near the
substrate. Ex situ thickness calibration was performed by atomic force microscopy (AFM).
Reevaporation of the molecules during metal deposition and laser exposure during the Raman
spectroscopy measurements was not achieved. The stepwise deposition of submonolayer metal
coverage was performed in short intervals and with a significant off-time of several minutes between
subsequent depositions. Furthermore, the substrate temperature was continuously monitored. In
complement, the double, or, in some cases, triple collection of the vibronic spectrum confirmed the
structure integrity.
This complies with recent published results related to experimental investigations of thermal
desorption (TPD) of PTCDA deposited on Ag(111) by Umbach and colleagues [Zou2006], who
revealed evaporation temperatures of the organic layer from metallic substrates at (538 ± 30) K for the
first monolayer in intimate contact with the metallic surface, while for the second monolayer it was
about (494 ± 30) K, as could be deduced from their publication. This also proves the physical
properties of the structures discussed in this work.
3.3.2 Structures II: pentacene and C60 organic molecules as active layers in
field effect devices
3.3.2.1. Molecular structures: pentacene and C60
The model molecules to construct organic-based field effect transistors were pentacene and C60. A
summary of the symmetry group, vibrational activity, UV-vis absorption spectra [Kolo2005], and
working energies for resonant Raman experiments is provided in Figure 3.4.
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.7
Pentacene C22H14
E0-0 = 1.84 eV
Symmetry D2h
102 internal vibrations
Raman active: g3g2g1g1 B17B7B9A18 +++
IR active: u3u2u1 B9B17B17 ++
Silent: uA8
(a)
(c)
CC6600
E0-0 = 2.02 eV
Symmetry : Icosahedron
Raman active: )eneracy(degA2H8 gg +
IR active: )eneracy(degF4 u1
Silent:
uuu2ugg2g1 H73G63F5AG6F4F3 ++++++
176 Internal modes most of them with degeneracy
(b)
(d)
Figure 3.4. Molecular structure of (a) pentacene (C22H14) and its 102 internal vibrational modes divided
into Raman active, IR active, and silent bands, belonging to the D2h symmetry group [Ross2002], (b)
Fullerene C60, which belongs to the symmetry group of the truncated icosahedron [Kost1994]. (c), (d)
Absorption spectrum of a 30 nm pentacene and 30 nm C60 film deposited on quartz, respectively
[Kolo2005]. The excitation energies for resonant Raman spectroscopy measurements are indicated on the
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.8 spectra.
3.3.2.2. Field effect structures
The field effect devices were provided by Prof. H. von Seggern from TU Darmstadt. The field effect
substrates shown in Figure 3.5(a) were fabricated using heavily doped n-type silicon substrates (3–5
Ωcm resistivity) covered by a a layer of high quality thermally grown SiO2 about 285 nm thick, acting
as gate electrode and gate oxide, respectively. A thin chromium adhesive layer was deposited on the
entire oxide surface, before a 50 nm Au layer was deposited.
The Au source and drain electrodes were photolithographically structured. They were configured as
interdigitated fingers with a channel length of 5 μm and a channel width of 2 cm [Hepp2003]. Similar
substrates with single channels consisting of two parallel contacts for drain and source were provided
by C. Pannemann from the group of Prof. U. Hilleringmann in Paderborn University. A detailed
description of the substrates is presented elsewhere [Pann2004].
Width = 5 μm
Length = 2 cm
dSiO2 = 285 nm
(a)
(b)
Figure 3.5. Field effect structures used for the fabrication of organic-based field effect transistors
(OFETs). (a) Interdigitated structures and (b) single channel structures.
3.3.2.3. Molecular beam deposition: pentacene and C60
Prior to the molecular beam deposition, the material pentacene was purified, while C60 from Tokyo
Kasei Kogyo Co. Ltd. was acquired with a purity as high as 99.9%. The materials were degassed
during 24 h in Knudsen cells in the analysis chamber. Before the organic deposition, the cleanliness of
the FET substrates was electrically tested by measuring the current-voltage characteristics in order to
detect possible leakage currents. Additionally, the Raman bands of the prominent spectral region of
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.9 the involved organic material was measured. During independent experiments under UHV conditions,
pentacene and C60 were evaporated at 155 oC and at 400 oC, respectively; the respective evaporation
rates were 0.65 Å / min and 1.7 Å / min. The layers were deposited onto field effect structures as
depicted in Figure 3.6. After each molecular coverage, the Raman bands and the channel current were
measured (chapter 6).
Figure 3.6. Field effect structure and formation of
the channel by organic molecules of pentacene or
C60. The polarity of the Vg and Vd depends on the
charge carrier type i.e. n or p (chapter 5).
The resulting monolayers (below 5 nm) deposited on the gate dielectric built up the organic channel.
Though further molecular coverage up to about 30 nm did not significantly contribute to the drain
current (chapter 6) [Paez2005], it helped to improve the protection of the working device against
atmospheric conditions (chapters 6-8).
3.4. Charge transient spectroscopy (QTS)
The interface formation between organic/inorganic or organic1/organic2 plays a decisive role in charge
injection into and transport through the device. Particularly the metal/organic interfaces differ
significantly from their inorganic counterparts, since the depletion depth is larger than the organic
layer thickness. Therefore, the Fermi level in the organic material and the charge injection barriers
basically depends on the interface offset. These factors might make it difficult to investigate charge
states at the interface by means of capacitance-based spectroscopies.
3.4.1. Shallow and deep level states in semiconductors
Shallow states in a semiconductor are located in the band gap and are close to the band edges of the
valence (HOMO) or conduction (LUMO) band in an inorganic (organic) semiconductor. These states
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.10 introduce minor perturbations in the lattice and are usually ionized at room temperature, since their
energy is comparable to kT ( meV25≈ ) or much lower in reference to the band gap of the material.
A procedure to investigate shallow traps entails tuning the Fermi level towards the ionized shallow
level. Thus the trap center becomes neutral and can be ionized with a suitable bias voltage (sect.
3.4.3). Another type of trap level is referred to as a deep level. These states are deeper in the band gap
and away from the band edges, which makes them more localized.
In Figure 3.7, the trapping/detrapping process for a single deep level is summarized. For simplicity,
the recombination processes are excluded. The first process (a) is a capture of an electron and
described by the capture coefficient nc . Electrons in the conduction state might migrate to the ionized
trap. The second process (b) consists of emission of the electron from the trap center towards the
conduction band and is described by the coefficient ne .
It should be noted that in the situation described by (a) there is a density of carriers that can fill the
trap, while the emission involves only one electron with a spin up or down (double degeneracy of the
trap). A similar situation is developed for hole-related traps where the capture (c) and emission (d) are
described by the coefficients pc and pe , respectively.
Figure 3.7. Band diagram for a semiconductor
with a single deep level trap (recombination
processes between HOMO-LUMO or intermediate
states are excluded).
The deep level state can switch between a filled or an occupied state. When it is occupied by an
electron or a hole, the state is named Tn (donors) or Tp (acceptors), respectively. The total number
of deep levels occupied by electrons and holes is
TTT pnN += . (3.1.)
A trap is only occupied by an electron or a hole and will be in a neutral state. The opposite happens
when an external electric field induces dipole sites in the structure; then anion-dipole or cation-dipole
related traps are formed. This was experimentally demonstrated with pentacene-based field effect
transistors in the present work and is discussed in chapter 8.
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.11
The free charge carriers, either for the electron density (n) or hole density (p), can be increased by the
released carriers from the trap centers or decreased due to the capture process. Therefore, the electron
or hole time rate process is described by
⎥⎥⎦
⎤
⎢⎢⎣
⎡
+−−+
+−−+=⎥
⎦
⎤⎢⎣
⎡−−
=⎥⎥
⎤⎢⎢
⎡
TpnTTnp
TnpTTpn
TpTp
TnTn
p)enc()pN)(epc(
n)epc()nN)(enc(pncpenpcne
pn
tdd
. (3.2)
In general, all terms in eq.(3.2) are time-dependent variables. If one type of deep level is dominant,
i.e., Tn or Tp , then the electron or rate balance equations are solved independently. In addition, if n
and p are time independent quantities, the solutions of the balance equations given in eq.(3.2) yield
⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛
τ−−
+++
++⎟
⎠⎞
⎜⎝⎛
τ−
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛
τ−−
+++
++⎟
⎠⎞
⎜⎝⎛
τ−
=⎥⎦
⎤⎢⎣
⎡
texp1pcence
N)epc(texp)0(p
texp1pcence
N)enc(texp)0(n
)t(p)t(n
ppnn
TnpT
ppnn
TpnT
T
T , (3.3)
where )0(n T and )0(pT are the initial situation (t = 0) of trap densities occupied by electrons or
holes, respectively, and the time constant τ is given by
pcence1
ppnn +++=τ , (3.4)
here the capture coefficients p,nc depend on the capture cross section p,nσ of the deep level and the
charge carrier thermal velocity ( thv ). Therefore,
thp,np,c vc σ= . (3.5)
Assuming one type of deep level for electrons ( pn ee >> ) or holes ( np ee >> ), the balance time
process given in eq.(3.3) is reduced to
⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛
τ−
⎟⎟⎠
⎞⎜⎜⎝
⎛τ
−
≈⎥⎦
⎤⎢⎣
⎡
pT
nT
T
T
texpN
texpN
)t(p)t(n
. (3.6)
nτ and pτ being the relaxation time constants of electron- and hole-related traps.
Considering the Fermi statistics for the trap density distribution and from the equilibrium conditions
(no external fields) in eq. (3.3), one finds the relation between the emission rate and the trap energy
level,
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.12
⎟⎠⎞
⎜⎝⎛ −−
σ=
τ=
⎟⎠⎞
⎜⎝⎛ −−
σ=
τ=
kTEEexp
gv N
1 e
kTEEexp
gv N 1 e
HTthpL
pp
TLthnL
nn
. (3.7)
Here LN is the effective density of states in the LUMO, g is the degeneracy factor of the deep level,
and kT is the thermal energy.
Assuming the constant effective mass approximation, eqs.(3.7) are simplified by
⎟⎟⎠
⎞⎜⎜⎝
⎛ Δ
σγ=⎟
⎠⎞
⎜⎝⎛ −
σγ=τ
⎟⎠⎞
⎜⎝⎛ Δ
σγ=⎟
⎠⎞
⎜⎝⎛ −
σγ=τ
−
−
kTE
exp1kT
EEexp1 T
kTEexp1
kTEEexp1 T
pT
pp
HT
pp
2p
nT
nn
TL
nn
2n
, (3.8)
To determine the trap energy TEΔ level, it is necessary to perform the experiment at different
temperatures, and the slope of the plot )Tln( 2p,eτ as a function of T/1 will give the )p,n(TE −Δ
value. The intercept on the )Tln( 2p,eτ axis gives the information related to the capture cross section
( p,nσ ) through ))/(1ln( p,np,n σγ .
3.4.2. The charge transient spectroscopy (QTS) technique
This section describes the spectroscopy of defects in the channel of pentacene based OFETs via bias
pulse excitation of the organic semiconductor between source and drain electrodes, followed by
processing of the transient charge Q(t) by means of a filter of time constants (rate window concept).
The rate window concept is based on the assumption of first order (exponential) kinetics of either
capacitance or charge decay after the excitation.
Here a transient charge decay is assumed as Q(t) = Q0 exp(-t/τ), where Q0 is the full released charge
and τ the time constant of the relaxation (eq.(3.7)). Application of the rate window concept means
designing a filter with a response peaking at a specified (programmed) delay ti that is related to τ
through a constant. To transform the exponential decay to a peak on the time scale ti three samples of
Q(t) are used at delays ti, 2ti, 4ti and combined in order to obtain a weighted sum [Thur1994, 2005-
2006],
)t4(Q2/1)t2(Q2/3)t(Q)t(Q 111 +−=Δ . (3.9)
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.13 For comparison, the double and triple boxcar integrators are compared in Figure 3.8 to measure deep
levels, figs. 3.8(a) and 3.8(b), respectively. The accuracy of the latter method is higher than that of the
double boxcar, since it samples the emptying signal at three different times.
Actually, to obtain a peak on the time scale ti (cf. Figure 3.8 (b)), it is sufficient to combine and weight
two sampled values only, as demonstrated by Kirov and Radev [Kirov1981], and Farmer et al.
[Farm1981]. After adding the third sample (eq. (3.9)), an improvement in selectivity as well as
immunity against any constant or linear component in Q(t) is achieved [Thur1994]. When illustrating
the performance of the filter defined by eq. (3.9), it is convenient to set t1 = ti and replace the time axis
by x = log10(t1) – Figure 3.9. Three QTS spectra belonging to three different time constants are shown
while scanning the delay t1.
(a) (b)
(c) (d)
Figure 3.8. Charge transient spectroscopy (QTS) based on the rate window concept. (a) Sample
wiring, and (b) applied bias pulse. (c) Triple boxcar integrator and; (d) output signal displayed as a
function of rate window time, the QTS maximum coincides with the relaxation time constant τ of the
trap (see sample spectra below for three different time constants) [Thur1994, 2005-2006].
It must be noted that both the FWHM and the peak height ΔQm are invariant against τ; the response
depicted by circles corresponds to simulating the QTS signal ΔQ(x) by an amplitude Gaussian with
variance w*. It is evident that the signal is peaking when the condition t1m ≈ τ is fulfilled with sufficient
accuracy; the peak height ΔQm corresponds to approximately 0.17Q0.
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.14 Throughout the present study, only the isothermal regime, i. e., scanning of the delay t1 at a constant
temperature T, will be considered. As to the pulse duration (pulse), two modes were alternatively used:
i) pulse = 1 ms; ii) pulse = 4t1 (both pulse and t1 scanned). In other words, the respective repetition
periods of excitation are (10-3 s + 4t1) and 8t1 for the two modes. For a correct understanding of what
follows, a sign convention is necessary, namely that signal ΔQ is of the same sign as ΔU.
Figure 3.9. Normalized QTS
responses to three exponential
decays with different time
constants τ are peaking when the
processing starts at t1 = τ. It should
be noted that both the height ΔQm
and the FWHM are invariant
against τ; approximation of the
fastest charge relaxation by a
Gaussian is shown by circles; w*
stands for the variance of the
Gaussian.
3.5. Algorithms for data evaluation and simulations
Most of the data evaluations were carried out in MatLab [MATL2003]. In some calculations that
involved numerical integration (Fermi-Dirac integrals of arbitrary order), a routine based on the
Gaussian quadrature and described below was used.
3.5.1. Gauss-Legendre quadrature
In order to evaluate some Fermi integrals, an algorithm based on the Gauss-Legendre quadrature
[Pres2002] was developed. The scheme is as follows; given a smooth function f(x) in an interval (a,b),
its integration can be approached by the summation of the weighted function in the interval (-1,1)
)(R)(g)(w2
d)(g2
xdf(x) kn
n
1kkk
1
1
ζ+ζζ≈ζζ= ∑∫∫=−
a-ba-bb
a
, (3.10)
with )(w kζ the weighting coefficients evaluated at the zeros ( kζ ) of the Legendre polynomial ( nP )
and defined by
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.15
( )[ ]2kn2k
k )(P12)(w
ζ′ζ−=ζ . (3.11)
nP′ is the derivative of the Legendre polynomial, and
( )[ ] )(g)!n2(1n2
)!n(2)(R )n2(3
41n2
kn ζ+
=ζ+
, (3.12)
is the rest of the sum.
The implemented algorithm allows the integrals to be evaluated with an accuracy of less than 1 %,
compared with the Simpson’s method; obviously, this depends on the order (n) of the Legendre
polynomial. The evaluation of the Fermi Dirac integral in this work is an example of the advantage of
the implemented routine.
3.5.2. QTS spectra line-profile
The QTS spectra were fitted by means of the filtering formula described by eq.(3.9), since it is
electronically implemented in the QTS setup to sense the sample signal. It is obvious that this method
of data evaluation gives greater confidence than the approximation given by the Gaussian profile
counterpart.
3.5.3. Correlated fitting algorithm
The fitting of the Raman spectra, current-voltage characteristics, QTS spectra and some other
measured quantities was carried out in a program written in MatLab [MATL2003]. The fitting
algorithm was based on the “Levenberg Marquardt” method. Although MatLab has a function to
perform the fitting at a given tolerance, it is not totally optimized and the convergence is quite
dependent on the initial boundary conditions.
To overcome this limitation, a routine with a correlated self-consistency algorithm was developed (cf.
Figure 3.10). The written program allows the boundary conditions to be optimized in such a way that,
after each iteration, the initial conditions are self-adjusted until the set tolerance is reached. Also, it is
possible to carry out back and forth fittings in order to determine a common set of boundary
conditions for two consecutive spectra. Figure 3.10 shows the flux diagram of the correlated fitting
algorithm.
The recursivity of the algorithm considerably reduces the computational time (by ca. 85 %). As an
example, the spectra shown in Figure 6.1 and the corresponding parameters presented in figs. 6.3(a)-
Chapter 3 B. A. Paez-Sierra, Experimental techniques… 3.16 6.3(b) were fitted in an Intel Pentium 1.6 GHz (512 RAM) after 7.5 h, while with the non-optimized
routine and with lower correlation factors (used earlier) the fitting took approximately 48 h.
Figure 3.10. Flux diagram of the correlated fitting algorithm.
References
[Farm1981] J. W. Farmer, C. D. Lamp, and J. M. Meese, “Charge transient spectroscopy”, Appl. Phys. Lett. 41, 1063-1065 (1981).
[Ferg2006] A. J. Ferguson, T. S. Jones, “Photophysics of PTCDA and Me-PTCDI thin films: Effects of growth temperature”, J. Phys. Chem. B 110, 6891-6898 (2006).
[Frie2003] M. Friedrich, G. Gavrila, C. Himcinschi, T. U. Kampen, A. Yu Kobitski, H. Méndez, G. Salvan, I. Cerillo, J. Méndez, N. Nicoara, A. M. Baró, and D. R. T. Zahn, “Optical properties and molecular orientation in organic thin films”, Phys. Condens. Matters 15, S2699-S2718 (2003).
[Gavr2006] G. N. Gavrila, “Electronic properties and chemistry of metal / organic semiconductor / S- GaAs(100) heterostructures”, PhD thesis http://archiv.tu-chemnitz.de/pub/2006/0004/index.html TU Chemnitz (2006).
[Hepp2003] A. Hepp, H. Heil,W. Weise, M. Ahles, R. Schmechel, and H. von Seggern, “Light-emitting field-effect transistor based on a tetracene thin film”, Phys. Rev. Lett. 91,157406 1-4 (2003).
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prepared and characterized with UV-vis by MSc. Phys. Viktoriia Kolotovska, TU Chemnitz (2005). [Kobi2004] A. Yu Kobitski, R. Scholz, D. R. T. Zahn, “Theoretical studies of the vibrational properties of the 3,4,9,10-
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chemnitz.de/pub/2006/0124/index.html TU Chemnitz (2006). [Paez2005] B.A. Paez S, I. Thurzo, G. Salvan, R. Scholz, Dietrich R. T. Zahn, and H. von Seggern, “Combined Raman
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[Pann2004] Ch. Pannemann, T. Diekmann, and U. Hilleringmann, “Degradation of organic field-effect transistors made of Pentacene”, J. Mater. Res. 19, 1999-2002 (2004).
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[Zou2006] Y. Zou, L. Kilian, A. Schöll, Th. Schmidt, R. Fink, and E. Umbach, “Chemical bonding of PTCDA on Ag surfaces and the formation of interface states”, Surf. Sci. 600, 1240-1251 (2006).
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.1
Chapter 4
Metal / organic interface formation investigated by in situ
surface-enhanced Raman spectroscopy (SERS)
In this chapter, the in situ investigation by resonant Raman spectroscopy of organic/metal
heterostructures prepared under UHV conditions is described. The heterostructures consist of indium
and magnesium deposited onto two perylene derivatives, 3,4,9,10-perylene tetra-carboxylic
dianhydride (PTCDA) and N, N' dimethyl-3,4,9,10-perylene tetracarboxylic diimide (DiMe-
PTCDI). The organic/metal structures were assembled on sulphur passivated Si-doped GasAs(100)
substrates (S-GaAs). The experimental results proved that all metals deposited onto the organic layers PTCDA or DiMe-
PTCDI promote enhancement of the Raman-active internal vibrational mode intensities, accompanied
by the activation of normally infrared-active modes. The developed phenomenon is called surface-
enhanced Raman spectroscopy (SERS). It is shown that metal coverages of several nanometer
thickness about 40 nm or above still allow the identification of vibrational bands, probing the
roughness of the metallic layers. In this research, the enhancement factors are estimated to be 101 for
the chemical contribution or first monolayer effect, and 103 for the electromagnetic contribution.
The comparison between organic monolayers capped by tiny amounts of metal, and thicker organic
films covered with similar metallic layers is used to extract information about chemical reactions at
the interface, diffusion of the metal into the organic material, and morphology of the metal film.
Indium deposition onto PTCDA and DiMePTCDI revealed molecular distortion, along with a
remarkable in-diffusion of In into In/ PTCDA structures, as demonstrated for the metal coverage on
organic monolayers and onto thicker PTCDA (15 nm) films. The phenomenon was much less apparent
in DiMe-PTCDI thin films.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.2
The deposition of Mg on both perylene derivatives revealed much less diffusion of the metal into the
organic layers, as demonstrated by the preservation of the external modes upon metal coverage. The
Mg/PTCDA structures undergo modification in two stages. The first one is the formation of a new
molecular structure at the interface and continued until there is a nominal metal coverage of about 2.8
nm, attributed to the removal of the oxygen atom from the anhydride groups. The second phenomenon
consists of the surface-enhancement of Raman signal of the former structure by further depositions of
Mg.
In the case of Mg/DiMe-PTCDI, it was found that the molecule exhibits a breakdown of selection
rules, and no formation of new molecular species compared to the Mg/PTCDA interfaces was
observed. This structure is characterized by the outstanding coupling between discrete molecular states
of the organic DiMe-PTCDI material and the electronic continuum of electronic states at the Mg metal
contact. The phenomenon is evidenced through the asymmetrical broadening of the Raman bands at
221 cm-1, 1291 cm-1 and 1606 cm-1 subsequent to the metal deposition. The line-shape of these bands
is well described by the Breit-Wigner-Fano function [Brow2001].
4.1. Introduction
The intimate contact of the organic/inorganic or organic1/organic2 junctions has been theoretically and
experimentally proven to be decisive in the further growth of the organic material, [Forr2003,
Hein2006, Otto2001-2005, Paez2005a, Pers2006, Schr2004, Wagn2003, Witt2004, Zahn2006,
Zou2006]. Therefore, the combined organic/inorganic or so called hybrid systems are of great interest
in different applications like organic-based field effect transistors (OFETs) [Paez2003a-b-2005b,
Scho2005, Thur2006, Xue2004] or organic modified diodes [Ménd2006, Park2002].
Several preparation methods to produce these types of structures are of current interest [Möll2003,
Nadi2005]. In particular, scientific reports concerning optical properties and molecular ordering
[Frie2003], electronic properties of PTCDA, PTCDI (3,4,9,10-perylenetetracarboxylic-diimide) and
DiMe-PTCDI investigated by surface sensitive PES (photoemission spectroscopy), IPES (inverse
photoemission spectroscopy) and NEXAFS (near edge x-ray absorption fine structure) are reported by
Gavrila [Gavr2006]. Extensions in conjunction with phthalocyanines “Pcs” (H2Pc, CuPc, F4Pc, F16Pc)
have been recently reviewed by Zahn and colleagues [Zahn2006]. The vibrational investigations based
on Raman spectroscopy and infrared spectroscopy have been a major focus of the group, which has
pursued investigations of metal/organic contact formation of PTCDA and DiMe-PTCDI with silver as
well [Salv2003].
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.3 The organic/metal contact heterostructures discussed in this thesis and OFET structures are
summarized in various publications [Paez2003a-c-2004a-b,-2005a-b, Salv02004a-b-2005, Zahn2004-
2005]. Also, an extensive effort has been directed at the organic-based devices and particularly
summarized in the projects “Designing Inorganic/Organic Devices” (DIODE) [diod2001], and the
“Organic Field Effect Transistors” (OFET). The latter project was carried out under the auspices of
the -,Deutsche Forschungsgemeinschaft, DFG, (German Research Promotion Society) [ofet], and
included this investigation (further details were given in chapter 1). Previous research, based on
electrical characterization of organic modification of metal/semiconductor Schottky contacts by
PTCDA [Park2002] and DiMe-PTCDI [Ménd2006, Thur2005a-b] has also been done by the group.
The perylene derivatives are known to be n-type materials, as demonstrated by the realization of n-
channel FETs [Ostr1997, Xue2004]. Complex structures based on coevaporated PTCDA and ZnPc (p-
type) [Dero2004] and bilayer structures formed by PTCDA/CuPc [Heut2005] have proved to be suited
for solar cell applications.
Likewise, p-n junctions between Pc and DiMe-PTCDI have provided power-conversion efficiencies of
approx. 1 % [McKe1998]. Recently, novel combinations of polyethyleneimine (PEI) with PTCDA
have been realized to produce functionalized structures where the PTCDA derivative molecules act as
the nanotube wall of the PEI-based backbone [Tian2006], hence leading to inexpensive and bendable
electronics.
The experimental results described in this chapter mainly relate to the resonant surface enhanced
Raman spectroscopy (SERS). Some authors have estimated the enhancement factors to be 102-106 for
the chemical contribution or first monolayer effect [Pers2006], and 1010 [Joha2005] for the
electromagnetic contribution. The magnified Raman cross section is about 10-17-10-14 cm2 [Joha2005,
Knei1997].
4.2. Interaction of metals with perylene derivatives
In Figure 4.1, the Raman spectra of 15 nm films of PTCDA are shown for metal coverages of 5 nm In,
4.5 nm Ag, and 5 nm Mg. The spectra in the low-frequency windows are normalized to the height of
the molecular breathing mode at 233 cm-1. The normalization in the high-frequency region is
performed with respect to the C=C stretch mode (1572 cm-1).
The deposition of Mg or In onto PTCDA leads to the appearance of the B1u band at 1243 cm-1 and the
increase in the relative intensity of the B3g mode at 1338 cm-1. These bands are strongly enhanced
compared to the Ag modes during the Mg deposition onto a monolayer of PTCDA on S-GaAs
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.4 [Zahn2004]. Therefore, they are considered to be a signature of the “first layer effect”, i.e. of the
molecules having direct contact with Ag [Zahn2004-2005].
The band at 1338 cm-1 was identified in reference [Kobi2002] as a B3g band on the basis of its
frequency and intensity in the crystal spectra, while a band 1292 cm-1 is likely to be a shifted variant of
the C-H deformation Ag mode at 1303 cm-1 in the single crystal [Salv2004b]. The other bands
correspond to modes that normally show infrared activity.
Figure 4.1. Raman spectra of In (5nm), Ag (4.5 nm) and Mg (5 nm) coverages on 15 nm thick PTCDA
films, compared with the spectrum of the bare PTCDA film in the spectral region of the internal
breathing mode (left) and in the spectral region of HC− deformation and C=C stretching modes
(right). (The Raman spectra involving Ag do not belong to this work, and are addressed elsewhere
[Salv2003]. They are presented here for comparison of metal contact formation on similar molecular
structures).
These modes are also activated in the spectra of PTCDA monolayers covered with indium, only with
higher intensities relative to those of the normally Raman active modes [Zahn2004]. In reference
[Zahn2004], the observed breakdown of the Raman-infrared selection rules was proposed to originate
from a weak charge transfer between the molecules and the metal surface mediated by molecular
vibrations. The Raman spectra of the (5 nm) Mg / (15 nm) PTCDA system also exhibit the break-
down of selection rules, with the occurrence of the modes observed in the other two metal/organic
heterostructures (cf. Figure4.1).
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.5 In addition, several modes with significant intensity appear at 307 cm-1, 502 cm-1, 598 cm-1, 696 cm-1,
1088 cm-1 and at 1225 cm-1, and 1433 cm-1. The assignment of these modes cannot yet be done
unambiguously. The frequency of the mode at 598 cm-1 is very close to the calculated value of 592 cm-
1 for a B3g mode of an isolated PTCDA molecule [Kobi2004].
Frequency calculations performed with the same basis set and density functional method in
Gaussian’98 as in the reference [Kobi2004] but for a modified PTCDA molecule, in which the central
O atom in the anhydride group is removed, deliver several frequencies that may be candidates for the
assignment of the experimentally observed modes 308 cm-1, 500 cm-1, 581 cm-1, 702 cm-1, 1090 cm-1.
Raman active modes in MgO microcrystals were observed at 595 cm-1, 719 cm-1 and 1096 cm-1
[Böck1974]. Thus the modes observed in the present work at 598 cm-1, 696 cm-1 and 1088 cm-1 may
also indicate the formation of MgO as a result of the interaction between Mg and PTCDA.
No modes of PTCDA or the modified molecule are found in the vicinity of 1225 cm-1. Whatever the
final assignment of the new modes is, they are not activated in the molecules in contact with either Ag
or In. Therefore, it can be concluded that the model of weak charge transfer is not sufficient to
describe the interaction involved at the Mg/PTCDA interface. Indeed, recent PES measurements
performed during the Mg deposition onto PTCDA show that the C1s and O1s core levels undergo
dramatic changes which can only be explained by the breaking of the OC− bonds in the PTCDA
molecule with formation of MgO [Gavr2006, Paez2003c-2004b].
In Figure 4.2, the spectra of 15 nm DiMe-PTCDI films for metal coverages of 5 nm In, 4,5 nm Ag and
5 nm Mg are shown. The spectra in the low-frequency windows are normalized to the height of the
breathing mode at 221 cm−1. The normalization in the high-frequency region is performed with respect
to the CC− stretch modes (1570 cm−1). In the case of DiMe-PTCDI, all the investigated metals, i.e.,
Ag, In and Mg, lead to the breakdown of selection rules with the occurrence of normally infrared
active modes at 1246 cm−1 and 1606 cm−1. The breathing mode at 221 cm−1 survives with increasing
metal coverage.
Consequently, a chemical reaction between these metals and the O atoms of DiMe-PTCDI molecules
can be ruled out [Zahn2004]. Interestingly, the features potentially assigned to MgO phonons do not
appear in the spectra, even for higher coverages of Mg. It can thus be concluded that the imide-methyl
group in the DiMe-PTCDI is less reactive compared to the O atoms in the anhydride group of PTCDA.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.6
Figure 4.2. Raman spectra of In (5nm), Ag (4.5 nm) and Mg (6 nm) coverages on 15 nm thick DiMe-
PTCDI films, compared with the spectrum of the bare DiMe-PTCDI film.
4.3. Morphology of the metal film Besides the occurrence of internal modes related to molecules in direct contact with the metal, the
totally symmetric modes are also enhanced (as shown by the normalization factors in Figure 4.1 and
Figure 4.2) in the spectra of Ag, In and Mg on 15 nm thick PTCDA and DiMe-PTCDI films. The latter
effect originates from the coupling of the incident and scattered radiation with localized and/or
collective plasmon resonances in the rough metal film. Accordingly, the intensity of the totally
symmetric Ag modes is very sensitive to the morphology of the metal film.
For a quantitative determination of the enhancement factors, curve fitting of each set of spectra
recorded during silver, indium and magnesium deposition onto PTCDA and DiMe-PTCDI was
performed using Lorentzian peaks. The dependence of relative area on metal coverage is plotted in
Figure 4.3 for a representative totally symmetric mode and for a normally infrared active mode of each
organic material. The relative intensities of a given metal coverage relate to the intensities of the
spectrum where the mode occurs for the first time. For example, the reference spectrum for the totally
symmetric Raman band is that of the pure organic film, while the reference spectrum for the normally
infrared active band is that taken after the first metal deposition.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.7
(a)
(b)
Figure 4.3. Enhancement factors of the Bu mode (1243 cm-1 in PTCDA and 1246 cm-1 in DiMe-
PTCDI) and of the C-C stretch Ag mode (1572 cm-1 in PTCDA and 1570 cm-1 in DiMe-PTCDI) for
PTCDA (a), and DiMe-PTCDI (right) as a function of the metal coverage (b).
The intensities of the Ag modes initially increase upon Ag and In deposition, reflecting an increase in
number and size of metal clusters as their plasmon energy approaches the energy of the laser field.
When Mg is deposited onto PTCDA, the intensities initially decrease, reflecting a reduction in the
number of Raman active PTCDA molecules. This corresponds to the conclusion drawn in the previous
section regarding the disruption of the PTCDA molecular structure upon reaction with Mg.
Above 2.8 nm Mg nominal coverage, however, the Ag Raman modes start to be enhanced, indicating
the formation of metallic clusters. Interestingly, the enhancement of the DiMe-PTCDI modes occurs
only above 20 nm nominal Mg thickness. The large difference in nominal thickness for which the
metallic character of Mg clusters is formed on PTCDA and DiMe-PTCDI is probably related to the
different morphology of the underlying organic layer. The DiMe-PTCDI films have very large empty
spaces between the organic islands, while the PTCDA films are much more compact [Frie2003].
The maximum enhancement of PTCDA modes for the Ag/PTCDA (15 nm) system is observed around
11 nm nominal Ag coverage (cf. Figure 4.3). This corresponds to the optimum cluster size for the
dipolar plasmon resonance.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.8 The In film thickness yielding the maximum enhancement for PTCDA and DiMe-PTCDI films is 26
nm and 5 nm, respectively. A further increase in the metal thickness leads to increasing size of the
metal clusters associated with screening of the inelastic-scattered radiation. Furthermore, the
absorption in the metal film also plays an important role in decreasing the Raman signal for higher
nominal coverages, when the clusters start to percolate. The signal from PTCDA and DiMe-PTCDI
internal modes remains visible even for a metal coverage of 43 nm, with higher intensity compared to
the pure organic film.
For Ag deposition onto DiMe-PTCDI, no saturation of the signal intensity was observed up to a
coverage of 263 nm. Considering that oI is the intensity of the light incident on the sample, d the
nominal thickness of the metal coverage and δ~
is the light penetration depth in the metal, then the
light intensity I scattered by the sample can be described by
)~d2exp(II o δ
−∝ . (4.1)
A summary of the values obtained from the fitting of the experimental decay of the enhancement
factors for the totally symmetric C=C stretching mode in all investigated heterostructures is given in
table 4.1. The obtained values are much larger compared to the penetration depth of 488 nm light into
smooth closed metal films. This is a clear indication that In and Ag films grown on PTCDA and
DiMe-PTCDI are not closed and have a high degree of roughness.
The apparent light penetration depth in Mg films grown on PTCDA and DiMe-PTCDI estimated from
the decrease in intensity of the C=C stretching mode has values comparable with the penetration depth
in a closed smooth Mg film. This indicates that the Mg film is smoother and that the efficiency of the
488 nm radiation in exciting dipolar resonances is lower for Mg.
Figure 4.4. AFM topographic
images of a 30 nm thick In film
on PTCDA. (a) (right part
showing PTCDA covered by In
clusters) and of a 113 nm thick
Mg film on PTCDA (b).
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.9 The AFM topographic images in Figure 4.4 confirm the higher roughness of In compared to that of
Mg films. It is recalled that the Ag modes are enhanced via the long range electromagnetic effect,
while the activation of Bu modes is characteristic for the molecules in intimate contact with or in the
very near vicinity of the metal surface. Therefore, the intensity of the Bu modes relative to that of the
Ag modes will be considered in the following to extract the metal diffusion depth into the organic
films.
Table 4.1. Skin depth of smooth metallic films, apparent penetration
depth of 488 nm light in In, Ag and Mg films grown on DiMe-PTCDI and
PTCDA layers
In Mg Ag
)filmsmoothdepthSkin(nm488=λΔ 8 nm 14 nm 2.5 nm
nm488=λδ (PTCDA) 49 nm 15 nm 50 nm
nm488=λδ (DiMe-PTCDI) 98 nm 15 nm --
Roughness (AFM) for a 15 nm
metal coverage on PTCDA
41 nm 28 nm --
In the case of Ag/PTCDA and Ag/DiMe-PTCDI, the intensity of Bu modes is low, indicating that only
a few molecules have intimate contact with Ag. This leads to the conclusion that the Ag atoms diffuse
very little into the PTCDA grains. On the other hand, the Bu bands are stronger compared to the Ag
modes in the spectra of In/PTCDA. This suggests that a large number of PTCDA molecules are in
direct contact with the metal, indicating a strong diffusion of In into the PTCDA islands. In/DiMe-
PTCDI represents an intermediate case between Ag/PTCDA and In/PTCDA..
The ratio between the area of the Bu mode at 1243 cm-1 (1246 cm-1) and that of the Ag mode at 1572
cm-1(1570 cm-1) in PTCDA (DiMe-PTCDI) is shown as a function of metal thickness in Figure 4.3. In
the case of Ag/DiMe-PTCDI, the maximum value of the ratio is observed for the first Ag deposition,
i.e., 0.4 nm Ag, whereas for PTCDA it increases up to the 1.4 nm nominal coverage of Ag. For In
deposition onto both organics, this ratio shows a saturation tendency only above 15 nm nominal In
coverage, but its value is lower for In/DiMe-PTCDI.
It is proposed that a maximum in the ratio defined above can be directly related to the metal diffusion
length in the organic film. Thus the Ag atoms arriving at the organic film surface diffuse into the
PTCDA or DiMe-PTCDI islands up to a nominal Ag coverage of 1.4 nm and 0.4 nm, respectively. An
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.10 exception is observed in the case of Mg/DiMe-PTCDI. Here the maximum in the defined ratio occurs
at the nominal metal coverage where the Ag modes start to be enhanced, i.e., around 20 nm.
The conclusions regarding the structural properties of the metal/organic interfaces drawn from the
enhancement factors of the internal modes are further confirmed by the spectral changes in the region
of external modes below 125 cm-1 that are discussed in detail in the reference [Salv2004a]. Whereas
the Ag and Mg deposition causes a broadening and a slight decrease in intensity of the phonon bands,
these smear out completely in the In/PTCDA system. This proves that indium diffuses much more
easily, as compared to Ag or Mg, into the organic islands, provoking the disruption of their crystalline
structure.
4.4. Phonons and interface structural properties
In Figure 4.5, the Raman spectra of 15 nm films of PTCDA in the region of external modes are shown
for 0.4 nm of Ag, In, and Mg. The spectra were recorded in crossed polarization configuration, i.e., the
incident electric field vector of the scattered light is perpendicular to that of the analyzed light or in the
Porto notation z(xy)z’ (see sect. 3.1).
Figure 4.5. Spectra of external Raman modes
from 15 nm thick PTCDA films capped with
0.4 nm metal layers, i.e., Ag, In, and Mg. The
spectral Raman shift between 25 cm-1 and 125
cm-1 corresponds to the libronic or collective
modes of the interacting molecules in the unit
cell [Salv2003].
In Figure 4.6, the spectra of the external modes are shown following stepwise metal deposition onto 15
nm PTCDA films.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.11
(a)
(b)
(c)
Figure 4.6. Raman monitoring in the external
mode region upon metal deposition: (a) Ag, (b)
Mg, (c) In. The experimental spectra are shown by
open symbols and the fitted spectra by red lines.
The Lorentzian functions used for the fitting of
the Raman spectrum of the pure PTCDA film are
shown by lines in the lower parts of the figures.
The spectra of Ag/PTCDA are normalized for a
better resolution of the phonons.
It can be seen in Figure 4.6(b) that for Mg overlayers, the external modes are still visible at 12 nm
coverage, whereas they are almost completely smeared out at 1.3 nm Ag. This is a clear indication that
the crystalline structure of the organic layers is less affected by the Mg deposition compared to Ag.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.12 However, it should be noted that the fitting of the spectra in the case of Ag/PTCDA is complicated by
the significant increase in the low-frequency background (see normalization factors in Figure4.6(a)).
The background evolution reflects an increasing degree of roughness, which is consistent with an
increasing number of metallic clusters that diffusely scatter the light. A strong increase in the low-
frequency background is also observed in the case of In deposition onto PTCDA, while it hardly
affects the spectra of Mg/PTCDA, supporting that the roughening due to Mg is lower compared to that
of the Ag and In films.
While the external molecular modes already disappear in the first deposition stages for In / PTCDA,
two new modes develop at 33 cm-1 and 112 cm-1 above an In coverage of 2.8 nm. They may
correspond to the transversal acoustic and longitudinal acoustic phonon peaks located at 33 cm-1 and
112 cm-1, respectively, in bulk indium [Flei2003].
This observation, corroborated by the concomitant increase in the low-frequency background,
indicates the formation of metallic In clusters. Moreover, the enhancement of the internal modes also
increases dramatically above 2.8 nm In, supporting the conclusion about the formation of metallic
clusters.
For a quantitative evaluation, the spectra of Ag/PTCDA and Mg/PTCDA were fitted using Lorentzian
functions. The evolution of the FWHM as a function of Ag and Mg thickness is plotted in Figure 4.7
for the external mode at 41 cm-1. This mode is fairly well separated from its neighbors and
consequently, the fitting parameters of the corresponding Lorentzian function are less correlated. As
the metal thickness increases, the FWHM of the external modes increases faster in Ag/PTCDA.
Figure 4.7. Evolution of the
FWHM of the external mode at
41 cm-1 as a function of the metal
coverage relative to the initial
values before the metal (Mg, Ag)
deposition. The dashed lines are
visual guides.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.13 4.5. Mg/DiMe-PTCDI structures and discrete molecular coupling with
continuum electronic metal states
In the previous sections, it has been found that In and Ag diffuses into PTCDA and DiMe-PTCDI to a
greater extent, as compared to Mg. On the other hand, it has been proven that at the Mg/PTCDA
interface a modified molecular structure is developed, possibly due to the withdrawal of the oxygen
atoms joining the anhydride groups of PTCDA.
In the case of DiMe-PTCDI, where the anhydride groups are replaced by imide groups, the Mg
overlayer on the organic layers undergoes interaction with the oxygen of the carboxylic groups and no
oxygen withdrawal has been evidenced by complementary PES measurements [Gavr2006]. An
interesting signature of the Mg/DiMe-PTCDI structure is the discrete molecular coupling with the
continuum electronic states of the metallic atoms, referred to as Breit-Wigner-Fano (BWF) resonance
[Brow2001, Cohe1992, Fano1961, Paez2005a, Zhou1993].
It has been found that when Mg is deposited onto a 15 nm DiMe-PTCDI layer the external molecular
modes are preserved up to large metal coverage. Since the external molecular modes are a fingerprint
for the molecular crystal, their preservation indicates a low diffusion of Mg into the DiMe-PTCDI
layer. Concerning the internal molecular modes, the Mg deposition induces a breakdown of selection
rules which is proposed to originate from a dynamic charge transfer between the DiMe-PTCDI
molecules and the metal. The line shape of the molecular breathing mode at 221 cm-1 becomes
asymmetrical at its high-frequency side above 0.3 nm nominal Mg thickness.
A similar effect is observed for the bands that occur at 1291 cm-1 and 1606 cm-1, but the asymmetry
appears on the low-frequency side. This line-shape asymmetry is likely to be related to a Fano
resonant coupling between the molecular vibration modes and the electronic continuum of states of
metallic clusters formed above 0.3 nm Mg coverage.
4.5.1. Chemistry, metal film morphology and metal indiffusion
Figure 4.8 shows the Raman spectrum of a bare 15 nm DiMe-PTCDI film and its evolution with step-
wise deposition of magnesium.
The first 0.3 nm Mg deposition leads to the occurrence of a band at 1252 cm-1. Upon further Mg
deposition, other modes develop at: 1291 cm-1, 1460cm-1 and 1606 cm-1. As shown above in Figure
4.8, all of these modes can be assigned to shifted normal modes of the DiMe-PTCDI molecule that are
only infrared active in the unperturbed molecule.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.14
50 100 150 200 250 300 350
GaAs
modesExternal
154/ 4
/ 4.4
/ 3.3
/ 0.8
/ 4.1
/ 0.8
/ 0.4
/ 0.4/ 0.4
/ 0.7
34
1x10-2
21
12258
12
6.48
In
tens
ity /
cts.
mW
-1s-1
Raman shift / cm-1
Mg thickness / nm
00.32.2
z(xy)-z
}*
(a)
1200 1280 1360 1440 1520 1600 1680
*154
/ 7.3/ 7.6
/ 2.7
/ 0.49.6
/ 4.7
/ 0.6
/ 6.4
/ 0.8/ 0.45
/ 0.4/ 0.4/ 0.6
34
1x10-1
21
12250
12
6.48
In
tens
ity /
cts.
mW
-1s-1
Raman shift / cm-1
Mg thickness / nm
00.32.2
28
*
z(xy)-z
(b)
Figure 4.8. Raman spectra of Mg/DiMePTCDI in the region of: (a) external modes and the breathing
molecular vibration mode. (b) C-C and C-H modes. The spectra in (a) and (b) are normalized with
respect to the intensity of the breathing mode and to that of the C-C stretching mode at 1570 cm-1,
respectively. An asymmetric broadening develops for the three modes marked with stars upon the Mg
deposition.
The IR spectra depicted in Figure 4.9, do not cover the spectral region below 650 cm-1; therefore, any
assignment of this band is difficult. However, from density functional theory calculations, this band
might belong to a normal mode with Au symmetry. The activation of modes with lower symmetry is
again observed, leading to the well known effect for molecules in contact with metal surfaces
[Otto2001-2005]. It can be induced by either molecular deformation in the vicinity of an interface or
by charge transfer processes from the molecule into the metal or vice-versa.
In addition to the spectral changes discussed above, the deposition of Mg up to the coverage of 21 nm
leads to a decrease in the overall signal, as reflected by the normalization factors in Figure 4.8. The
signal decrease is caused by the light attenuation in the metal overlayer. When the Mg coverage is
increased above 21 nm, however, the intensity of all bands again increases. The maximum
enhancement factor relative to the intensity in the spectrum of the bare organic film is observed for the
C-C stretching mode at 1570 cm-1 and it amounts to 7.6 for the Mg coverage of 122 nm. An increase
of the Raman signal of the organic layer upon metal deposition has also been observed for In and Ag.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.15 This effect occurs due to the enhancement of the electric field of the incoming and scattered radiation
induced by plasmons that are resonantly excited by the electromagnetic radiation in the metal clusters.
As a consequence, the enhancement of the Raman signal provides clear proof of a high degree of
roughness of the metal film.
900 950 1000 1050 1100 1150
Inte
nsity
/ ct
s.m
W -1
s -1
Raman shift / cm-1
DiMePTCDI (15nm)/ Mg (2.8 nm)
(a)
1200 1300 1400 1500 1600 1700 1800 1900 2000
Raman shift / cm -1
Mg/ DiMePTCDI
IR DiMePTCDI
Inte
nsity
/ ct
s.m
W -1
s -1
DiMePTCDI
DiMePTCDI (15nm) / Mg (2.8 nm)
(b)
Figure 4.9. Comparison between the Raman spectra of bare DiMe-PTCDI, Mg (2.8 nm) / DiMe-
PTCDI and the IR for the organic.
Upon metal deposition up to 21 nm, the external modes (phonons) below 120 cm–1 remain unchanged.
Considering that the external modes are a signature of the organic layer crystallinity, their preservation
indicates that the Mg atoms do not diffuse into the organic crystalline islands as much as In and Ag do
on both perylene derivatives.
4.5.2. Coupling of vibrational modes and electronic excitations
Another modification induced in the Raman spectra by the metal deposition is an asymmetric
broadening of the breathing mode at 221 cm-1, and the modes at 1291 cm-1 and 1600 cm-1. Similar
asymmetric line-shapes were observed for metal-doped fullerene films [Brow2001, Knei2001] and in
the Raman spectra of metallic carbon nanotubes [Zhou1993]. There, it has been suggested that the
asymmetric line-shape can be fitted with a Breit-Wigner-Fano (BWF) function that accounts for a
coupling between discrete states and an electron states continuum, so-called Fano resonance. The
phenomenon has been observed in the Raman spectra of inorganic systems such as highly doped Si
[Card1983, Cerd1973, Chan1978] and semiconductor superlattices [Pan1996, Kanz2000].
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.16 The BWF function reads:
( )
( ),
/)(1)/()(1
)( 2
2
Γω−ω+
Γω−ω+=ω
BWF
BWFo
qII (4.2)
This expression can be rewritten as: ( ) ,1qI~)(I 2
2
o ε+ε+
=ω where Γω−ω
=ε BWF is the reduced energy,
with BWFω being the Breit-Wigner-Fano frequency and Γ the resonance width related to the phonon
self-energy when the interaction with the electronic continuum takes place. q is the asymmetry
parameter and (1/q) is proportional to the degree of coupling.
Figure 4.10. Fitted Raman spectra of Mg / DiMe-PTCDI: from bottom to top: bare 15 nm DiMe-PTCDI
covered with 34 nm Mg and 122 nm Mg. The peaks fitted with the BWF function are represented by
thick black lines.
Here the BWF function is employed for fitting the modes at 221 cm-1, 1291 cm-1 and 1606 cm-1. All
other modes are fitted using Lorentzian functions. Figure 4.10 shows, from bottom to top, the
experimental Raman spectra along with the fitting curves for bare DiMePTCDI (15nm), coverage of
34 nm Mg, and 122 nm Mg thickness. The absolute values of q-1 range from 0.07 to 0.16.
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.17 For comparison, q-1 ranges from 0.18 to 0.27 for a band at 1543 cm-1 in the case of metallic carbon
nanotubes. The value of q-1 is influenced by the choice of the background. Therefore, it is difficult to
draw conclusions regarding changes in the degree of coupling with the increase in metal coverage. For
an excitation energy of 2.54 eV, the energy of the photons scattered by the molecular vibrations at 221
cm-1, 1291cm-1, and 1606 cm-1 amounts to 2.51 eV, 2.38 eV and 2.34 eV, respectively. It must be
noted that the asymmetry parameter of the band at 1606 cm-1 band is negative, while the other two
BWF line-profiles are characterized by positive asymmetry parameters.
According to reference [Zhou1993] a negative/positive value of the coupling parameter indicates that
the central frequency of the continuum lies below/above the discrete mode frequency. This would
imply that in this case the central frequency with the continuum is located between 160 meV and 200
meV above the molecular ground state. On the other hand, the band asymmetry occurs at metal
coverages for which the enhancement of the Raman signal due to dipolar plasmon resonances is also
observed.
Consequently, it might be proposed that the origin of the observed BFW line-shapes is a coupling
between the molecular electronic levels and the plasmons in the Mg clusters modulated by the
molecular vibrations. Interestingly, the band at 221 cm-1 stems from a breathing vibration of the whole
molecule and the band at 1606 cm-1 stems from a stretching vibration. Thus both involve the breathing
of the carbon rings.
In order to summarize the evaluated Fano resonance energies, Figure 4.11 illustrates the energy level
alignment of the DiMe-PTCDI/Mg interface determined by NEXAFS spectroscopy [Gavr2006]. The
Fano resonances distributions were determined from the enhancement factors of the SERS spectra
documented in Figure 4.8.
Figure 4.11. Energy level
alignment of the DiMe-PTCDI /
Mg heterostructure. The Fano
resonances indicated in the band
diagram were obtained from the
resonant Raman measurements,
while the other energy levels
were quoted from NEXAFS
spectroscopy measurements on a
similar sample [Gavr2006].
Chapter 4 B. A. Paez-Sierra, Metal / organic interface formation… 4.18 Conclusions
Ag, In and Mg deposition onto 15 nm thick PTCDA and DiMe-PTCDI films on S-GaAs(100):2x1
have been characterized in situ by Raman spectroscopy. The breakdown of selection rules in the
spectra of Ag/organic and In/organic heterostructures originates from a dynamic fractional charge
transfer process modulated by molecular vibrations. In the spectra recorded during the Mg deposition
onto PTCDA, bands assigned to MgO develop, indicating a reaction of Mg with PTCDA molecules
that causes the loss of the central O atom of the anhydride groups.
Upon Ag and In deposition, the totally symmetric modes are initially strongly enhanced.
Subsequently, the signal is attenuated exponentially with an exponent that is much smaller than the
penetration depth of the incident radiation in a smooth closed metal film, reflecting a high level of
roughness of the metal overlayer. The intensity of the normally infrared active modes relative to the
Raman active modes provides information on the metal diffusion depth in the organic films.
Complementary information on the metal diffusion depth into the organic layers is provided by the
attenuation rate of the external mode intensities as a function of the metal coverage. While Mg and Ag
form abrupt interfaces, In strongly diffuses into the organic layers. For Mg, however, additional
features are observed in the Raman spectra compared to those induced by Ag and In, clearly indicating
the reactive nature of the interface between Mg and PTCDA.
Additionally, in the DiMe-PTCDI/Mg structure, the observed Raman spectral bands at 221 cm-1, 1291
cm-1 and 1606 cm-1 amount to energy gap states above the HOMO of 30 meV, 160 meV and 200 meV,
respectively. These modes are broadened asymmetrically upon the metal deposition and their line-
shape is well described by the Breit-Wigner-Fano function. This effect is accompanied by an
enhancement of the Raman signal intensity, due to plasmon excitations in the metallic clusters. It is
proposed that the BWF line-shapes originate in a coupling between the molecular electronic levels and
the plasmons in the Mg clusters modulated by the molecular vibrations.
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Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.1
Chapter 5
Organic field effect transistors
(OFETs)
In this chapter, the basic concepts of the organic field effect transistor (OFET) are presented. The main
topics discussed include the statistics of charge carriers, estimation of charge carrier density and
molecular density per cm2. Before this, a previous discussion concerning the Fermi integrals is
addressed.
After the statistics and transport description, the chapter focuses on the comparison between
experimental and theoretical OFET output characteristics. The analysis is done in order to identify
some of the drawbacks of the transistor equations. The electric field dependence of the mobility
( effμ )the threshold voltage ( TV ), the dynamics of boundary conditions, i.e., time dependence of traps
developing at interfaces and promoted in the organic material, structural relaxation, among others,
probe the necessity of reformulating the OFET modeling.
Special attention is dedicated to an analysis of existing formulations to describe the drain current in
different regimes, and some proposals are made in order to pursue a more reliable analysis of the
experimental data. As an example, a single channel device is considered. An accurate analysis of the
output characteristics data has shown that the field effect mobility ( effμ ) does not remain constant and
its relationship with the applied voltage is not limited to a simple electric gate-field relation. Although
models based on field-effect mobilities as a function of electric fields, i.e., the Frenkel-Poole
description, are available, these are limited to the sub-linear regime, meaning that an analysis of the
transistor as a switching device is not covered.
5.1. Introduction
The field effect transistor is a three-terminal device, where two leads serve to drive charge carriers
through a medium behaving as a channel and joining both contacts. A third terminal, isolated from the
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.2 other two, modulates the amount of charge density by an electric field. The first two terminals are
referred to as source and drain and the last one is called the gate terminal (cf. Figure 5.1.).
Further and novel details on the OFET device are
given in the present and next chapters. Concerning the
pioneering ideas about FETs, proposals date from
1926-1933 with Lilienfeld´s patented works about
"method and apparatus for controlling electric
currents” (1930 US Patent N. 1,745,175), "amplifier
for electric currents”, (1932 US Patent 1,877,140),
and "device for controlling electric current” (1933 US
Patent 1,900,018) [Klei1998]. The modern description
of an inorganic-based FET is found elsewhere
[Kaga2003, Kwok1995, Oda2006, Paul1994,
Raza2001, Shoc1952].
In the previous chapter, the metal contact formation on
Figure 5.1. Scheme of a field effect
transistor. The drain and source terminals
serve to drive the modulated current
through the channel shown in blue color.
The charge in the channel is modulated by
the capacitive effect of the third (isolated)
terminal named gate.
organic films was discussed. The metal/organic junctions on perylene derivatives have served as
model systems to investigate the physics developed at the organic/inorganic interface [Forr2003,
Paez2004, Xue2004, Zahn2004]. Vibrational spectroscopy has demonstrated great potential when
investigations of homo- and hetero-structure formation are required.
In the context of interface formation, the transistor is an effective means by which to probe the
formation of different boundaries, being one of the most appropriate configurations for the
organic/inorganic or organic1/organic2 scenarios to study several phenomena developed in the active
layer and its surroundings. The transistor geometry, electrodes configuration, active layer and many
other features make it a well-suited and almost closed system to identify not only charge transport
properties but many other combined properties that improve the knowledgement of basic signatures of
the organic channel.
After the experimental realization of the first organic field effect transistor based on polythiophene
[Tsum1986] and later on with small molecules, i.e., LuPc2 /ZnPc [Madr1987], numerous innovations
and intense research have been initiated. Applications in active matrices, such as flexible displays
driven by transistors with a polymer semiconductor [Bao2006, Bric2006, Huit2001, Kalu2006,
Some2005], printed pentacene and oligothiophene for RFID tags [Subr2005], and gas sensors
[Guil1998, Tane2005] are some of myriad examples involving the OFET device.
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.3 5.2. Statistical mechanics of charge carriers
5.2.1. Density of states
In this section, some preliminary ideas about the density of states ( εεΩ d)( ) and charge carriers
distribution are presented.
The density of states εεΩ d)( accounts for the number of cells per volume in the phase space,
h)()(with,dkdkdkV)(~zyx ε=ε==εΩ kpp , (5.1)
where V is the volume given by the configuration space, p the momentum, k the wave number and
h Planck´s constant divided by π2 .
,V
)(~2d)( εΩ=εεΩ (5.2)
Here the factor 2 accounts for the spin contribution. The accessibility of particles or quasiparticles is
determined according to the Fermi-Dirac or Bose-Einstein distributions ( )(f ε ) for fermions or bosons,
respectively. If traps are present in the material, then these are added to this expression.
Accordingly, the density of states given by eq.(5.2) is written as follows
∑=
ε−εδΛ+εΩ=εΩN
1ii )()()( , (5.3)
with )( iε−εδ being the Dirac delta function describing the discrete trap with energy iε and Λ a
scaling factor to keep the corresponding units.The total number of particles on per volume is given by
the probability of finding them in a given energy interval
εεεΩ= ∫∞
∞−d)(f)(n o , (5.4)
5.3.2 Charge carrier density
The charge carrier density is determined by means of eq.(5.4). Therefore, it is necessary to define or
measure the density of states ( εεΩ d)( ) in order to determine the charge density of the organic
material. In polyacenes, a quasi-constant density of states has been theoretically predicted [Rosa1991].
Then, the electron and hole carrier densities n and p are given through the equations
ε+−εΩ
= ∫∞
−ε d)exp(1)E(n
LUMO fETkE
LUMO , (5.5)
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.4
ε+
ε−Ω= ∫ ∞− ε− d
)exp(1)E(p HOMO
f
E
TkE
HOMO , (5.6)
with Ef Fermi level.
The expressions given by eqs. (5.5, 5.6) can be written in terms of a reduced energy using the
following substitutions
TkE~ LUMO−ε
=ε , (5.7)
TkEE LUMOF −=η , (5.8)
TkE~ HOMO ε−
=ε , (5.9)
TkEE FHOMO −
=η , (5.10)
where ε~ is the reduced energy and η the reduced chemical potential.
Another consideration is to replace the density of states by a power law of energy; a special case is the
electron gas in a metal, where the relation between energy and density of states obeys a square root
law ( 2/1d)( ε∝εεΩ ). In general, if there are constrictions of the electron gas, then the density of
states will hold those signatures and will involve modifications of the particle energy. Therefore, the
charge carrier density defined by eqs.(5.5, 5.6) is proportional to
εη−ε+
ε+Γ
=η ∫∞ ~d
)~exp(1
~
)j1(1)(F
0
j
j , (5.11)
this integral form is called the Fermi-Dirac integral and its order is defined by the power j .
The next section summarizes some values of the arguments and helps to identify whether the
molecular structure is highly-, middle- or non-degenerate. In this work, a MatLab [MATL2003] code
to evaluate the Fermi integral of any order was developed.
5.3.3. Fermi integral argument
In this section, some values of the quoted Fermi integral are shown; its complete evaluation is
performed in a developed code based on the Gauss-Legendre quadrature already described in chapter
3. Table 5.1 presents some values of the reduced (η ) and increment ( ηΔ ) of the chemical potential
along with the order (j) of the integral [Blak1987].
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.5
Table 5.1. Parameters reported for the Fermi-Dirac integral
[Blak1987]
η ηΔ j
]4,4[− 1.0 4;;3;;2;;1;;;1; 27
25
23
21
21
23 −−−
]10,4[ 2.0 4;;3;;2;;1;;;1; 27
25
23
21
21
23 −−−
As an example, Figure 5.2 shows the argument of some Fermi-Dirac integrals as a function of the
reduced energy ( ε~ ) and reduced chemical potential (η ).
(a) (b) (c)
Figure 5.2. Argument of the Fermi-Dirac integral as a function of the reduced energy ε , and the
reduced chemical potential η (evaluated in MatLab [MATL2003]). Orders (j) of the arguments
(a) 21 , (b) 2
1− , and (c) 0.
5.4. Charge carrier density of organic materials
In section 5.3.2, the integral expressions for the charge carrier density of organic materials were given.
A particular feature of aromatic molecules is that the band gap is approximately four times the transfer
integral ( ζ~
), as determined from tight binding calculations. Therefore, the interval for computing the
Fermi-Dirac integral indicated in eq. (5.5) is reduced to the interval ]~4,~4[ ζ−ζ− , and the charge
density can be determined as a consequence.
It has been demonstrated experimentally that the charge transport in OFETs is restricted to a two-
dimensional charge carrier gas [Muck2004, Paez2005]. Accordingly, the charge carrier density reads
⎥⎦
⎤⎢⎣
⎡
ζ+ηζ−ηζπ
=)~22/cosh()~22/cosh(ln
~m4pr
r2
r*
D2h
(5.12)
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.6 with Tk/~~
Br ζ=ζ the reduced transfer integral. The reduced chemical potential for pentacene is
about 34≈η at room temperature.
The charge density given in eq.(5.12) differs from that one proposed by Daraktchiev and colleagues
[Dara2005]. Presumably they considered further assumptions before obtaining their charge density
calculations.
As an illustration of the two-dimensional charge density given in eq.(5.12), the charge density as a
function of the reduced chemical potential and the reduced transfer integral is shown in Figure 5.3.
The estimated density values are scaled with respect to 2* /m4 hπ , *m being the effective mass of the
charge carriers; for pentacene o* m7.1m = and o
* m5.5m = [Wijs2003].
Figure 5.3. Charge density distribution as
a function of the reduced chemical
potential and reduced transfer integral.
The effective masses for pentacene are
o* m7.1m = and o
* m5.5m =
[Wijs2003]
Assuming a two-dimensional molecular packing and the experimental lattice constants of pentacene
determined by XRD measurements [Ruiz2004], then the number of molecules per 2cm is estimated
and given in table 5.2. The lattice constants a and b are parallel to the substrate while the molecule is
perpendicular and aligned on the diagonal of the rectangle formed by a and b (cf. Figure 5.4).
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.7
Figure 5.4. Triclinic crystal
structure to estimate the number
of molecules per 2cm .
Table 5.2. Lattice constants of the triclinic cell for pentacene
with cell parameters °≈α 978.76 , °≈β 136.88 , °=γ 415.84 ,
and density of molecules per 2cm .
The molecular densities summarized in table 5.2 are useful to
determine the number of charge carriers in the organic material.
More recent results have revealed a modified triclinic structure of pentacene submonolayers (SML)
and multimonolayers (MML) grown on native SiO2 [Ruiz2004]. Consequently, a scheme similar to the
one used in the former triclinic structure is employed for the modified crystal cell, and the molecular
densities are summarized in table 5.3.
Table 5.3. Lattice constants of the triclinic cell for SML and
MML Pentacene, with cell parameters °≈α 978.76 ,
°≈β 136.88 , °=γ 415.84 . The last column indicates the
density of pentacene molecules per 2cm .
a /Å b / /Å 2cm
P~
−
SML 7.62 * 5.90 * 141044.4 ×
MML 7.58 * 5.91 * 141046.4 ×
Pentacene
(distance H-H
central ring)
4.99
7.62
5.90
4.99
141078.6 × 141026.5 ×
*Lattice parameters reported in the literature [Ruiz2004]
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.8 5.5. The field effect transistor (FET)
A modified version of the FET architecture given in chapter 3 is presented here and illustrated in
Figure 5.5. The geometrical arrangement of the constituent elements is as follows: there is a substrate
on which an isolating layer of thickness id is deposited, on top of this layer there are patterned
contacts and in between a semiconducting material is deposited. Experimentally, the organic material
is deposited on the whole gate dielectric region, including the patterned contacts (cf. Figure 3.6). Here,
the addressed scheme shown in Figure 5.5 helps to strategically distinguish betweeen the different
boundaries and domains assembling the organic field effect structure.
During the experiments under UHV conditions, and prior to the deposition of the semiconducting
material, the current through any configuration of two contacts was tested to assure negligible leakage
currents, but this did not guarantee cleanliness of the gate dielectric between the top contacts on the
isolating layer. Therefore, organic-free paths on the dielectric were probed by Raman spectroscopy
measurements.
Before the deposition of the active layer, it had to be ensured that there was no current flowing
between the gate terminal and any of the top contacts.
The organic layer bridging the contacts is called channel and has length L, width W, thickness d, and
an effective thickness δ where the two dimensional charge transport mainly is developed (cf. Figure
5.5). A further discussion of the channel formation is presented in chapter 6.
Figure 5.5. Organic field effect transistor
To assure the device functionality, it is aplied
a voltage to the gate therminal, thereby an
excess charge density is induced in the
channel by the capacitive effect developed at
the isolating layer. The effect of the induced
charge is extended mainly to an effective
thickness (δ ) of the organic film (discussed in
chapter 6). Until this stage, the charge density
has only been modulated and no charge
transport between the electrodes joint by the
organic material is observed
The next step is to ground one of these contacts and set the other to a suitable potential driving the
gate-induced charge through the channel. The applied voltages depend on the majority charge carrier
of the organic layer and the work function at the metal organic interface. Finally, the grounded and
biased contacts forming part of the channel are named drain and source, respectively.
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.9 5.5.1. Energy band structure of an OFET
The energy band diagram of the organic material forming the different interfaces in the channel is
outlined in Figure 5.6. In the case of OFETs, the conduction and valence bands of the organic material
are replaced by the lowest unoccupied and highest occupied molecular orbitals LUMO and HOMO,
respectively.
(a) (b) (c)
(d)
Figure 5.6. Energy band diagram between the gate and the organic film with an isolating interlayer, (a)
under equilibrium conditions. (b) Accumulation mode of the organic field energy bands for negative
and (c) positive (c) gate voltages ( gV ) respectively. (d) Band diagram between the organic and the
contacts drain and source (UDS = 0 V).
The diagrams indicated in Figure 5.6 (a, b, c) correspond to the organic/insulator/gate (heavily doped
Si) structure; the first scheme (cf. Figure 5.6 (a)) indicates equilibrium conditions, while the second
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.10 and third diagrams depict the energy accumulation developed in the organic material when a negative
or positive gate voltage is applied. The different gate fields induce charge on the organic material. The
accumulated charge density per monolayer plane and parallel to the gate dielectrics is inversely
dependent on the square distance between them, i.e., monolayers away from the gate have lower
charge density in comparison with those close to the dielectric substrate (chapter 6).
The next energy diagram is the one between the molecular layer and the contacts source and drain (cf.
Figure 5.6 (d)). In this work, the metal contacts of the OFETs were gold. Therefore, a band diagram of
the interface Au/pentacene based on ultraviolet photoemission spectroscopy (UPS) and inverse
photoemission spectroscopy (IPES) measurements [Kahn2003] is documented in Figure 5.6(d). It
should be observed that the Fermi level (EF) alignment between Au/pentacene shows a more favorable
p type transport in the organic layer, but there is still the probability of having an n-type transport as
probed by charge transient spectroscopy (QTS) measurements.
5.6. Output characteristics of the OFET
5.6.1. "Linear” regime
As an example of the OFET output characteristics, Figure 5.7 shows an ideal and experimental drain
current -(Id) vs. drain-voltage (Vd) characteristics. The rising part of the dd VI − corresponds to the so
called "linear” regime, where the applied drain voltage does not exceed the potential at the organic
insulator interface induced by the applied gate bias. This means that the condition or the drain
potential )VV(V Td −< g must be fulfilled, with TV a threshold voltage or drop potential across the
organic/isolator/ interface.
The physical stationary (time-independent) description of a prototypic field effect transistor (FET) can
be found elsewhere [Kaga2003, Kwok1995, Oda2006, Paul1994, Raza2001, Shoc1952]. In the
isolated gate geometry, such as that shown in Figure 5.1, the field effect is produced by a capacitive
effect leading to an induced charge Q in the organic material, given by
)VV(CQ T−= g , (5.13)
with gV the gate potential, and TV the threshold voltage which takes into account the field free-
carrier concentration as well as details of the internal charge distribution and trapping at the interfaces
[Libs1993].
The induced charge density forms a region called an accumulation layer, with length L , width W ,
and thickness δ . As long as no other external field different from the gate bias is applied, the charge
density is confined to the volume δ××WL .
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.11 Otherwise, when a drain voltage is applied, there is a superposed electric field parallel to the channel.
Then the effective electric field is the contribution due to gate and drain, and the volume where the
charge density is distributed is modified. For drain voltages below the potential drop )VV( T−g , the
drain current density transported through the channel is proportional to the drain-source field and
given by
EJ σ=d , (5.14)
where σ is the effective conductivity of the channel, expressed as (in the one dimensional model)
eneffμ=σ , (5.15)
with effμ the effective field effect mobility, n the charge density and e the magnitude of the electron
charge.
Considering only the charge transport from source to drain and combining eqs.(5.13-5.15), the drain
current intensity ( dI ) is as follows
( )xdVd
V)VV(WLW
CenI dTeffeffd −−⎟⎟⎠
⎞⎜⎜⎝
⎛μ=μ= g , (5.16)
with io d/AC εε= the capacitance of the insulating gate material of thickness id , area LWA = and
dielectric constant ε .
Next step is to integrate eq. (5.16) from source to drain
( )∫∫ −−μ=dV
0Tseff
L
0d VdV)VV(WCxdI g , (5.17)
Td
2d
dTseff
d VVVwith2
VV)VV(
LWC
I −<⎟⎟⎠
⎞⎜⎜⎝
⎛−−
μ= gg , (5.18)
ios d/C εε= is the capacitance per unit area.
As mentioned above, eq.(5.18) counts only for the drain current intensity. This means that the charges
driven by the drain field are those induced solely by the gate field in the channel.
An additional contribution is the bulk current, which is due to the drain voltage without any applied
gate bias. The drain current given in eq.(5.18) has a parabolic dependence on the drain voltage dV ,
and it is linearly proportional to the drain field only if the condition Td VVV −<< g is satisfied.
The restriction on the drain voltage has some consequences. The first one is that the dynamics in the
channel do not fall in the event that the channel is pinched off. Second, the dd VI − is well described
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.12 by linear superposition of two contributions of the drain voltage; when )VV(V Td −<< g , then the
first-order contribution of dV to the drain current is dominant. In this region, the behavior of the drain
current with the drain voltage is linear.
The opposite happens when the quadratic component of dV is significant (eq. (5.18)). Then it is clear
that the latter situation no longer belongs to the linear regime. For this reason, the quotation marks ""
are used. The linearity of the drain current with the drain voltage holds if and only if
)VV(V Td −<< g .
5.6.2. Saturation regime
This regime occurs when the drain voltage cannot drive more charge to increase the intensity of the
drain current. The saturated drain current starts at the maximum drain value reached in the "linear”
(parabolic) regime. Thus from eq. (5.18)
0VdId
d
d = , (5.19)
this yields the maximum drain potential in the "linear” regime
Tmaxd VVV −=− g , (5.20)
2T
seffd )VV(
L2WCI −
μ= g . (5.21)
The dependence of the drain current on the drain voltage is depicted in Figure 5.7(a); the drain onset
potential joining both linear and saturation regions is clearly identified. In Figure 5.7(b), experimental
data of an organic field effect transistor (OFET) are shown, together with the simulated equations. The
first message of this inset is that the developed ideas are only approaches to the real data. Further in
this chapter, additional physical phenomena that can improve the theoretical model are considered.
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.13
(a)
(b)
Figure 5.7. Output characteristics of a field-effect transistor and maximum drain current where the
saturation starts. (a) Simulation of an ideal FET and (b) experimental drain current - drain voltage of a
pentacene (30 nm)-based FET; the solid lines correspond to the fitted output characteristics.
5.6.3. Field effect mobility effμ
An important quantity, not only for OFETs but also in other systems involving charge transport, is the
mobility, which gives information about the capability to drive charges in a well-defined medium
under internal and external fields.
The relations obtained in the previous section for the drain current involve the mobility; by means of
them, the field effect mobility in OFETs can be roughly estimated [Roic2004]
⎪⎪⎪
⎩
⎪⎪⎪
⎨
⎧
−≥−
−<
⎟⎟⎠
⎞⎜⎜⎝
⎛−−
=μ
Td2Ts
d
Td2d
dTs
d
eff
VVVwithregimesaturation,
2)VV(
LWC
I
VVVwithregime,,,
2VV)VV(
LWC
I
gg
g
g
linear"
(5.22)
A reference quantitity that shows how small changes in the gate field can modify the drain current in
the channel is
gm
g
gVI
lim d
0V ΔΔ
=→Δ
, (5.23)
the previous expression is usually presented in the literature as
Tddseff
.constV
dm VVVif,V
LWC
VI
−<<μ
=⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
==
gg
d
g , (5.24)
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.14 from this expression, a value for the field effect mobility ( effμ ) in the so called "linear regime” is
obtained, when the quadratic contribution to the drain current is negligible, i.e, Td VVV −<< g . This
approximation is referred to as "gradual channel approximation”. Therefore, the field effect mobility
is found to be a constant.
In accordance with the parameters extracted from the fitted output characteristics of the experimental
drain current - drain voltage characteristics shown in Figure 5.7(b), the field effect mobility is not
constant and presents a field dependence as can be inferred from Figure 5.8. There is a dependence on
the applied gate voltage with a saturation voltage at about –15 V. Similar results have been reported in
n-type OFETs with n-alkyl perylene diimides as the active layer [Ches2004].
In chapter 7, the influence of electric fields on the molecular structure of the OFET channel is
discussed. It is experimentally demonstrated that the Raman signal increases with the applied gate
field and saturates at about –22 V. Therefore, charge carrier – vibration interaction mediated by
electrical polarization of the organic material is expected.
Figure 5.8. Field effect mobility in a pentacene
(30 nm)-based FET. The values (half filled
circles) were extracted from the output
characteristics, while the solid curve corresponds
to a power law with the applied gate field.
Withinn the framework of the previous approximation, some authors have also considered the
relationship between the transconductance ( mg ) and conductance ( dg ) [Horo1999, Kaga2003]
TdTseff
.constV
dd VVVif),VV(
LWC
VI
−<<−μ
=⎟⎟⎠
⎞⎜⎜⎝
⎛∂∂
==
ggd
g
g . (5.25)
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.15 Combining eqs.(5.24) and (5.25) yields [Horo1999]
Td2Ts
deff VVVif,
)VV(CV
WL
−<<−
==μ ggm
2d
gg
. (5.26)
A first conclusion from this result is that no effect of the bias gate voltage on the effμ is considered (a
constant value is assumed), contrary to what is found from the experimental results displayed in Figure
5.8. The quoted formulae derived under ideal conditions can lead to values of effμ with an uncertainty
of at least 100% [Roic2004]. These observations push to look for different procedures to determine
more realistic field-effect mobility values. One way is by fitting the output characteristics by a
correlated algorithm, i.e. all drain-current – drain voltages are fitted in a common set of simultaneous
equations.
For the previous fitting (cf. Figure 5.8), it can be determined that the mobility as a function of the
applied gate field follows a power law:
2k
1eff c)VV(csat
++−=μ g , (5.27)
with 62sat
61 105.6cand,81.1k,V03.16V,1003.4c −− ×==−=×= . The corresponding fitting is
presented in Figure 5.8 by the solid red line.
Actually, the mobility of pentacene-based transistors is about 112 sVcm5 −− [Kell2003]. The field
effect mobility can be improved in some extent if an appropriate surface treatment of the gate
dielectric is realized [Bao2006]. This subject was not investigated during the present work.
One should be aware of the measurement conditions, i.e., exposure to atmosphere, voltage-sweep
either the drain or the gate voltage. It has been observed that consecutive measurements of the output
characteristics makes the drain current lower; likewise, its monitoring over a period of several hours
has resulted in a decrease composed of multiple Debye-like decays (chapter 7-8) [Paez2005,
Thur2006, Gu2006]. It is a question whether the mobility is determined for the sub-linear regime, i.e.,
very small voltages, or derived from a flash measurement taking no longer than a few minutes.
It must be noted that eq.(5.27) differs from the expression for the mobility deduced from the "variable
hopping range” (VHR) described in chapter 2 eq.(2.5). Particularly here, the voltage satV does not
correspond to the threshold voltage. Additionally, the mobility dependence on the charge density
should be taken into account, since it involves natural and artificial trapps (chapter 8). The latter
trapping phenomenon is promoted by the applied fields leading to formation of dipoles which behave
as trapping centers for the charge carriers (chapter 8).
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.16 5.7. Threshold voltage shift and field dependence
The shift of the threshold voltage makes an additional contribution to the line-shape of the drain
current. For very low drain voltages, where the mobility has been shown to be dependent on the
applied field, the threshold shift is considered to stem from trapping of charge carriers at the
channel/isolator interface, accompanied by an extra trap-density developed in the conductive channel.
This is referred to as the defect-pool model originally applied to a-Si:H TFTs [Foma2005, Powe1987-
1992] and recently to organic based devices after having observed non-exponential drain current
relaxation [Stal2004].
Consequently, in the case of very low drain voltages, the current decay within the linear region can be
well described by a threshold voltage shift function, in which the effect of the gate voltage is involved
[Chia1998, Foma2005, Libs1993] and described by
⎪⎭
⎪⎬⎫
⎪⎩
⎪⎨⎧
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎠⎞
⎜⎝⎛τ
−−Δ=Δβtexp1VV oT , (5.28)
with )kT/Eexp(o ττ=τ being the characteristic trapping time of carriers, and oVΔ approximately
the initial voltage drop value across the insulator towards the organic channel. In this model, the
thermal activation energy is given by β= τEEa , with β being the stretched-exponential exponent;
β=τ /EE a is interpreted as the average effective energy barrier that carriers in the a-Si:H channel
need to overcome before they can enter the insulator, and oτ is the thermal prefactor for emission
over the barrier.
The trap dynamics and threshold voltage shift in OFETs have been recently demonstrated for several
organic and polymer-channel materials, such as α -sexithiophene (α -T6), dihexylquaterthiophene
(DH4T), poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)
[Gome2004-2005, Sall2003, Stal2004, Stre2003]. In the inorganic counterpart, such as hydrogenated
amorphous silicon [Powe1987-1992], the authors have found similar results.
During the present research, an interesting dynamic and anomalous behavior of the QTS spectra on
pentacene-based devices, with interdigitated source and drain contacts, and on single channel
structures were found. Part of the results are published elsewhere [Thur2006] and a further discussion
is addressed in chapter 8. The QTS results indicate the involvement of dipolar features, and deep
levels of minor charge carriers.
For some time the defect-pool model has been partially available in some commercial packages for
device simulation, but these tools are still lacking with regard to giving reasonable values in
comparison with those obtained from experimental measurements. The latest forecast for improving
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.17 organic-based device simulation involves the implementation of routines based on an extended
Frenkel-Poole [Gill1972] description and the Holstein [Well1996] model. These will obviously
improve the simulations of organic materials, but a dynamics of both boundary and initial conditions is
desired, phenomena proved in the present research by vibronic characterization of metal/organic
interface formation, and the combined Raman spectroscopy with electrical characterization of OFETs.
In the present investigation it has been experimentally demonstrated that the current
relaxation is related to the dynamics of the capture cross section (chap. 8).
As was stated in eq.(5.28), the threshold voltage probes the developed dynamics of the device
[Gomez2004]. Following the discussion of the experimental output characteristics described in Figure
5.8, the threshold voltage was determined and given in Figure 5.9. Assuming only the induced
capacitive charge, the threshold voltage can be considered as a quantity proportional to the trapped
charge at the organic/isolator interface. Therefore, with an organic channel of about 1.5 nm thick and a
surface capacitance of 2cm/nF23 , the trap density is about 218 cm10 − . The quoted value is so huge
that pure interfacial effects developed at the OBL/insulator interface can hardly reach or give a
comparable number of traps. The divergence between the computed trap density and the one resulting
from interfacial states suggests that additional trap sources in the device should be taken into account.
In chapter 8, the formation of additional traps -called "artificial traps” and generated by the applied
fields - is discussed.
Figure 5.9. Threshold voltage of a single
channel pentacene (30 nm)-based FET. The
values were extracted after fitting the output
characteristics depicted in Figure 5.8(b)
The threshold shift is also responsible for the
observed negative conductance in OFETs
(chapter 8) and the drain current collapse as
probed in inorganic-based FETs [Klei2003].
Although several scientific reports about OFETs
present this phenomenon in the output
characteristics, they are lacking a discussion or
explanation of its origins [Hepp2003,
Rost2004].
In chapter 6 the experimental and theoretical
descriptions of the Id-Vd characteristics are
given. Multi-exponential kinetics [Paez2005]
are assumed, where three drain current terms are
added together to reproduce the measured data.
One is independent of the trapping process,
while the other two are dependent on the
relaxation time constants of the OFET device.
Chapter 5 B. A. Paez-Sierra, Organic field effect transistors 5.18 Conclusions
A basic formula to model the OFET output characteristics was described. Signposts determined after
the confrontation with experimental data and the simulated results were established.
It was found that the mobility is strongly affected by the applied fields and its determination in the
sub-linear regime produces errors of about 100% in comparison with values obtained in the saturation
regime. In order to eliminate this substantial uncertainty, the set of equations describing the output
characteristics was considered in a self-consistent way, and all drain current–voltage measurements
were correlated by means of a back and forth procedure until the desired convergence with the
experimental data was achieved. The computation was performed by further extensions of the
correlated fitting algorithm discussed in chapter 3.
The detailed data analysis revealed that the extracted field-effect mobility follows a power law
proportional to the applied gate voltage. The mobility law presents a saturation at a given gate field;
afterwards it decreases, probably due to traps induced by the applied field.
Similar to the mobility, a dependence between the threshold voltage (VT) and the applied electric
fields was also determined. The VT is proportional to the trapped charge at the organic/isolator
interface; therefore, the extracted threshold-voltage values are strong indications that trapping effects
in the device should be considered. On the other hand, assuming only a charge induced by the
capacitive-gate structure, it was found that the amount of charge trapped at the interface is only a small
fraction of the huge relaxed charge observed, a finding prompting new concepts that will be discussed
in chapter 8.
Although there are models based on field effect mobilities and threshold voltage shifts as a function of
electric fields, they are limited to the sub-linear output regime of the OFET, meaning that the analysis
of the transistor as a switching device is not covered. There is a need to include temporal and field
dependences between the total drain current and the initial conditions. This would produce much
better correspondence between the experimental and simulated output characteristics.
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amorphous silicon thin film transistors”, J. Appl. Phys. 51, 1242-1243 (1987). [Powe1992] M. J. Powell, C. van Berkel, A. R. Franklin, S. C. Deane, and W. I. Milne, “Defect Pool in amorphous-silicon
thin-film transistors”, Phys. Rev. B 45, 4160-4170, (1992). [Raza2001] B. Razavi, “Design of analog CMOS Integrated circuits”, McGrawHill, Boston, 2001. [Roic2004] Y. Roichman, Y. Preezant, and N. Tessler, “Analysis and modelling of organic devices”, phys. Stat. Sol. (a)
201, 1246-1262 (2004). [Rosa1991] A. L. S. da Rosa and C. P. de Melo, “Electronic properties of polyacene”, Phys. Rev. B 43, 2183-2186 (1991). [Rost2004] C. Rost, D. J. Gundlach, S. Karg, and W. Rieß, “Ambipolar organic field-effect transistor based on an organic
heterostructure”, J. Appl. Phys. 95, 5782-5787 (2004). [Ruiz2004] R. Ruiz, A. C. Mayer, G. G. Malliaras, B. Nickel, G. Scoles, A. KazimirovH. Kim, R. L. Headrick and Z.
Islam, “Structure of Pentacene thin films”, Appl. Phys. Lett., 85, 4926-4928 (2004). [Sall2003] A. Salleo and R. A. Street, "Light-induced bias stress reversal in polyfluorene thin-film transistors”, J. Appl.
Phys. 94, 471-479 (2003).
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[Shoc1952] W. Schockley, “A unipolar field effect transistor”, Proc. Of the I.R.E. 1365-1376 (1952). [Some2005] T. Someya, Y. Kato, S. Iba, Y. Noguchi, T. Sekitani, H. Kawaguchi, and T. Sakurai, “Integration of organic
fets with organic photodiodes for a large area, flexible, and lightweight sheet image scanners”, IEEE TRANSACTIONS ON ELECTRON DEVICES 52, 2502-2511 (2005).
[Stal2004] P. Stallinga, H. L. Gomes, F. Biscarini, M. Murgia, and D. M. de Leeuw, “Electronic transport in field-effect transistors of sexithiophene”, J. Appl. Phys. 96, 5277 (2004).
[Stre2003] R. A. Street, A. Salleo, and M. Chabinyc, “Bipolaron mechanism for bias-stress effects in polymer transistors”, Phys. Rev. B 68, 085316 1-7 (2003).
[Subr2005]
V. Subramanian, J. M. J. Frechet, P. C. Chang, D. C. Huang, J. B. Lee, S. E. Molesa, A. R. Murphy, D. R. Redinger, and S. K. Volkman, “Progress toward development of all-printed RFID tags: Materials, processes, and devices”, Proc. IEEE 93, 1330-1338 (2005).
[Tane2005] M. C. Tanese, D. Fine, A. Dodabalapur b, L. Torsi, “Interface and gate bias dependence responses of sensing organic thin-film transistors”, Biosensors and Bioelectronics 21, 782–788 (2005).
[Thur2006] I. Thurzo, B. Paez, H. Méndez, R. Scholz, and D. R. T. Zahn, “Anomalous charge relaxation in channels of Pentacene-based organic field-effect transistors: a charge transient spectroscopy study”, phys. stat. sol. (a) 203, 2326-2340 (2006).
[Tsum1986] A. Tsumura, H. Koezuka, and T. Ando, “Macromolecular electronic device: Field-effect transistor with a polythiophene thin film”, App. Phys. Lett. 49, 1210-1212 (1986).
[Well1996] G. Wellein, H. Röder, and H. Fehske, “Polarons and bipolarons in strongly interacting electron-phonon systems”, Phys. Rev. B 53, 9666–9675 (1996).
[Wijs2003] G. A. de Wijsa, C. C. Mattheusa, R. A. de Groota, T. T.M. Palstraa, “Anisotropy of the mobility of Pentacene from frustration”, Synthetic Metals 139, 109-114(2003).
[Xue2004] J. Xue and S. R. Forrest, “Bipolar doping between a molecular organic donor-acceptor couple”, Phys. Rev. B 69, 245322 1-8 (2004).
[Zahn2004] D. R. T. Zahn, G. Salvan, B.A. Paez, R. Scholz, “Interaction between metals and organic semiconductors studied by Raman spectroscopy”, J. Vac. Sci. Technol. A 22, 1482-1487 (2004).
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.1
Chapter 6
Combined Raman spectroscopy and electrical characterization
of the conductive channel in OFETs
In this chapter, the combined vibrational and electrical measurements of the organic layer in OFETs
are discussed. During the deposition of pentacene on a Si-SiO2 gate structure with Au bottom contacts
for source and drain, the film growth was monitored with simultaneous in situ macro Raman
spectroscopy and drain current (Id) measurements of the OFET. The deposition of the active layer was
carried out under UHV conditions at a growth rate of 0.6 Å/min.
The aim of the in situ characterization was to determine the minimum nominal thickness of the
pentacene layer required for efficient charge transport through the OFET circuit. It was found that at a
thickness around 1.5 nm nominal coverage, the first percolation paths through the first organic
monolayer develop, resulting in a sharp rise of the drain current. Up to a nominal film thickness of
about 5 nm, a subsequent slower increase of the drain current can be observed, revealing that the
percolation of pristine monolayers continues at a slower pace up to rather thick organic layers
contributing to a very minor extent to the overall device drain current.
Throughout the chapter, the conductive channel is referred to as an "organic boosting layer (OBL)”.
The interpretation of the OBL is based on some ideas used for inorganic transistors, in which the
active layer for particular configurations is divided into two arrangements.
The first set of monolayers closest to the gated dielectric is composed of a strained material
[Abst1985, Cell2005, Coll2003, Drak2003, Gámi2002, Lamm2003, Wu2004]. The second one, on top
of the first, is the normal layer with the corresponding bulk signatures. It is observed that the charge is
flowing mainly through the strained material and a much lower amount is transported through the
bulk material. Electrical and structural properties of both films might reflect differences between their
physical properties, i.e., the effective mass of the charge carriers, mobility, lattice constants, and/or
vibronic features.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.2
Within the framework of this research, the OBL is outlined by two properties: (i) the minimal number
of monolayers forming the conducting channel where the maximum percentage of charge carriers are
transported; and (ii) by its strained signature evidenced from vibronic features when the OBL forms
different interfaces in the OFET. It is shown that the OBL behaves as a two- dimensional confinement
for the charge carriers [Paez2005a]. Obviously, the OBL concept might be modified by the
configuration of the electrodes.
6.1. Introduction
As was already discussed in chapters 1 and 2, organic-based electronics has become very attractive for
technological applications. The delocalized carriers in the π molecular orbitals make organic
molecules suitable for charge transport. Devices based on organic materials are cheaper and easier to
manufacture than their inorganic semiconductor-based counterparts.
Some prototype organic materials can be printed out or sublimated on bendable substrates [Ridl1999,
Sun2005], thus providing a new trend in the electronic market. Indeed, several applications of organic-
based electronic devices are found as price tags, plastic electronics, transparent displays, and non-
volatile plastic memories with gate insulators based on polymer ferroelectric materials [Brab2001,
Kim2005, Liab2005, Rees2004, Yoo2001].
These interesting features, among many others, have been the focus of several experimental and
theoretical investigations in order to understand the organic/inorganic or organic1/organic2 interface
formation. In particular, the optimization of device performance is of great interest [Liab2005,
Sche2005, Schr2004, Stre2005]. In this work, the investigation of charge transport and the conductive
channel formation in organic field effect transistors is carried out by means of vibronic and electrical
characterization techniques. The experimental research was performed by in situ drain current-voltage
(Id -V) and Raman spectroscopy measurements.
The vibrational spectrum is a characteristic fingerprint of the investigated system. Therefore, Raman
spectroscopy facilitates the identification of the molecular vibrations and thereby provides information
about normal mode frequencies of the molecule under different conditions. Accordingly, information
concerning charge state processes at interfaces, or structural order in organic materials, can be easily
distinguished by using this non-destructive spectroscopic technique [Chen2003, Colo2003,
Paez2005a-b, Salv2005, Zahn2001-2005].
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.3
In addition, the drain current monitoring provides evidence of the minimal amount of the organic
material required to reach the charge transport in the OFET channel. Both techniques were employed
during the molecular beam deposition of the pentacene molecules. Therefore, correlations between the
pentacene film thickness with vibrational properties and the drain current were outlined. The vibronic
and drain current monitoring was achieved on sub-monolayer coverage of the organic material with
rather thicker films of approx. 30 nm.
6.2. Simultaneous in situ I - V characterization and molecular vibration
measurements of OFETs
Prior to the molecular evaporation, the basic structures were electrically tested for possible leakage
currents. Additionally, the Raman spectrum was taken to assure the absence of organic contaminants
on the substrate before starting the organic deposition. Along with the step-wise molecular beam
deposition, the substrate was biased with a V10V −=d and V6V −=g . This allowed the continuous
monitoring of the drain current as a function of the organic film thickness.
The 3D plot shown in Figure 6.1 presents the evolution of the pentacene Raman bands in accordance
with the film thickness. On the right hand side, the vertical plane depicts the drain current as a function
of the organic layer thickness. Some intermediate Raman spectra were intentionally skipped for better
presentation clarity.
From this figure, it can be clearly seen that with pristine organic molecule depositions, i.e., below 1.5
nm nominal thickness, the Raman bands start to appear while the drain current is approximately zero.
This indicates the formation of molecular clusters which do not percolate to form the conducting
channel. At a thickness of about 1.5 nm, the drain current increases abruptly and with further organic
molecule deposition there is an asymptotic increase of the Id. From this result, it is deduced that the
transport takes place in layers with intimate contact or very close to the gated dielectric.
The first monolayers form the OFET channel and behave as an organic boosting layer (OBL), since a
high percentage of the nominal drain current is transported. A similar effect has been observed in
strained-Ge buried-channel MOSFET structures [Shan2006] with a thickness of about 4 nm.
In situ experiments on mobility as a function of the molecular channel thickness in
dihexylquaterthiophene (DH4T)-based FETs were conducted by the group of Prof. Wagner and
colleagues [Muck2004] It has been experimentally demonstrated that the main charge transport for
this structure is developed in the first two monolayers. Similarly, experiments on sexithienylbased
FETs revealed that the charge carrier mobility rapidly increases with increasing coverage and saturates
at a coverage of about two monolayers [Dine2004].
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.4
Figure 6.1. Simultaneous monitoring of
Raman bands (black spectra) and drain
current (red curve on the right hand side of
the vertical plane) during the pentacene
deposition. The pointed Raman bands
correspond to the in-plane C-C ring and in-
plane C-H vibrations.
Further analysis of the present results (sec. 6.3) shows that the sub-monolayer pentacene deposition of
about 1.5 nm is significantly affected by the substrate. Molecules on the electrodes present a splitting
of the band at 1157 cm-1, while those on the gate dielectric are much less affected. This indicates that
the organic/inorganic interface is decisive for the OBL formation.
Additionally, detailed analysis of the molecular growth and Raman bands were extracted by fitting
each spectrum. The data evaluation was performed by using the correlated fitting algorithm described
in chapter 3. In Figure 6.2(a), some extracted Raman spectra at organic thicknesses of about 0.2, 1.5,
5.4, 10.2 and 33.3 nm, respectively, are illustrated from bottom to top. The spectra are normalized
with respect to the band at 1179 cm-1.
In order to have a better comparison, each experimental spectrum is plotted together with its fitting
and the corresponding line-shapes used for the data evaluation. Results indicate a red shift of the Ag
band at 1158 cm-1 for organic coverages above 10 nm. This is mediated by the bulky pentacene
formation. Additionally, in the same figure, the vibration symmetries of the considered line-shapes are
highlighted.
The peak position and symmetry assignments are based on density functional calculations carried out
with the three parameter hybrid functional B3LYP and the 3-21G basis set in the Gaussian 98 package
[Gaus1998]. The theoretical evaluation was done for a single pentacene molecule.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.5
1140 1160 1180 1200
Pentacene based OFETsRaman spectra at different thickness
Raman shift / cm-1
Inte
nsity
/ a.
u
A g 11
79.2
cm-1
? 118
0.4 cm
-1
? 11
77.8
cm-1
B 1g 11
63.8
cm-1
A g 11
55 cm
-1
5x10-2 mW. cts-1. s-1 A g
1158
cm-1
λ = 647.1 nm130 W cm-2
0.2 nm/ 1
1.5 nm/ 7
5.4 nm/ 7.2
10.2 nm/ 11
33.3 nm/ 108.4
(a)
(b)
Figure 6.2. Extracted intermediate Raman spectra of pentacene based OFETs at different organic layer
thicknesses, together with the corresponding fitting line-shapes (a); and comparison between the area
of the Raman band at 1179 cm-1 and the drain current as a function of the organic film thickness (b).
The drain current as a function of the organic film thickness is shown in Figure 6.2(b); the dependence
of the Raman intensity on the band at 1179 cm-1 is depicted here as well. The combined Figure 6.2(b)
shows a strong correlation between the electric current intensity and the molecular coverage below 1.5
nm, indicating the transition between separate molecular clusters and their percolation onset to form
molecular layers, and hence the channel for the corresponding charge transport.
Analyzing the total organic deposition of approximately 33 nm and the abrupt increase of the drain
current, it is concluded that the charge transport is mainly developed below 5 nm, for which thickness
approximately 70 % of the total charge is driven. Additionally, the development of the Raman
intensity of the band at 1179 cm-1 provides an indication that basically six molecular arrangements
should be considered, as follows:
• Overlayers below 1.5 nm: These are governed by the formation of molecular clusters which
seem to be isolated, since no drain current is registered. A pentacene layer of about 1.5 nm can
be distinguished as the threshold thickness between the isolated molecular clusters and the
layer percolation. This is suggested by the dramatic increase in the drain current.
• Coverages between 1.5 nm and 10 nm: This interlayer is likely to be characterized by the
formation of more compact films. Therefore, the higher percentage of the drain current
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.6
confirms the nature of the OBL in comparison with that obtained at 33 nm pentacene film
thickness.
Considering molecules in upright position and from the geometry optimization performed
with density functional theory, it is found that the height of a single pentacene molecule is
about 1.41 nm. Therefore, the conductive channel ranges between 1.1 ML and 7 ML of
pentacene.
Measurements with AFM of pentacene growth on several substrates have shown a terrace-like
growth [Kell2006, Ruiz2004, Tiba2003] where a sub-monolayer coverage of 0.5 nm
pentacene on SiO2 exhibited a lateral size of approximately 0.5 μm [Ruiz2004]. It is
interesting to note that when 5.7 nm of pentacene is deposited on Co substrates at 360 K, six
different height levels of terracing are revealed [Tiba2003].
• Between 10 nm and 20 nm: This regime is also manifested by a kink in the Raman bands
intensity as a function of the film thickness. Formation of substantial molecular islands and a
higher number of molecules in contact with the drain and source electrodes are likely. This
intermediate thickness contributes much less to the total drain current. Roughly this can be
explained by the square inverse dependence of the distance between the molecular plane
position and the gate dielectric interface (sec. 6.5).
• Between 20 nm and 25 nm thickness, the indicated Raman band intensities in Figure 6.2 (b)
saturate, forming a plateau, while the broadening of the pristine clusters is increased.
• Above 25 nm: the current does not increase significantly, showing an asymptotic dependence
on the molecular film thickness.
Figure 6.3(a) shows the plotting of the remaining Raman band intensities as a function of the
molecular layer thickness. The trend of these bands is similar to that given by the profile of the band at
1179 cm-1 as a function of the layer thickness and addressed in Figure 6.2(b). This indicates that each
of the Raman bands develops in a similar way as the organic molecules are deposited.
Additionally, the FWHM of the fitted bands in relation to pentacene deposition are illustrated in
Figure 6.3(b). It is found that there is a nearly linear dependence of the FWHM tendency on the
organic film thickness, with a slope of about 0.045 cm-1 /nm. The broadening of the features is
attributed to the modified molecular scenario. Therefore, a larger scattering of the FWHM is expected.
The behavior of the band at 1163 cm-1 is surprising; the FWHM decreases with molecular deposition.
This band - as can be seen from the inset of Figure 6.3(b) - has the highest broadening and makes a
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.7
tiny contribution to the overall spectral region in comparison to other bands. A detailed observation of
the 1163 cm-1 band reveals that the linear dependence is modulated by oscillations whose maxima and
minima correspond fairly well to the characteristic domains already discussed in the previous
paragraph.
0 5 10 15 20 25 30
Pentacene based OFETsband-area in dependenceof organic thickness
1155 cm-1
1157 cm-1
1163 cm-1
1178 cm-1
1179 cm-1
Pentacene thickness / nm
Pea
k ar
ea /
a.u
(a)
0 5 10 15 20 25 30
2
4
6
8
10
12FW
HM
/ cm
-1
Pentacene thickness / nm
Pentacene based OFETs FWHMin dependence of organic thickness
1155 cm-1
1157 cm-1
1163 cm-1
1178 cm-1
1179 cm-1
1180 cm-1
(b)
Figure 6.3. Fitting parameters in dependence on the molecular layer thickness in pentacene-based
OFETs (a) Raman band intensities and (b) FWHM broadening.
Although a low nominal coverage of about 10 nm is enough to have charge transport in the OFET, it is
not sufficient to protect the channel from deterioration when it is exposed to atmospheric conditions.
Therefore, a thicker layer of the organic material or a capping material is required in order to obtain
working devices under environmental conditions.
6.3. Organic boosting layer (OBL) in OFETs
In this section, the vibrational analysis of the sub-monolayer coverage is addressed. The Raman bands
of 1.5 nm pentacene growth on SiO2 and on Au/SiO2 substrates are illustrated in Figure 6.4. The
difference between the spectra indicates that the substrate significantly modifies the way the pentacene
grows. This can be inferred from the different line-shape and the splitting of the band at 1157 cm-1 for
molecule growth on Au. Molecules deposited on SiO2 substrates do not present selection rules in the
Raman spectrum.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.8
Results reported by Ruiz et al. on grazing incidence, x-ray diffraction, x-ray reflectivity and atomic
force microscopy have proved that the crystal cell parameters of sub-monolayer (0.5 nm) pentacene
differ from those for bulky layers (19 nm). A common signature of both regimes, sub-monolayer and
multiplayer, is that the volume of the unit cell is nearly unmodified [Ruiz2004-2005].
1140 1160 1180 1200
Raman shift / cm-1
Inte
nsity
/ a.
u
Splitting
A g 11
79.5
cm-1
? 118
0.9 cm
-1
? 11
77.1
cm-1
B 1g 11
65.4
cm-1
A g 11
56.6
cm-1
Pentacene(1.5 nm) forming different interfaces in FETs
2x10-2 mW. cts-1. s-1
A g 11
59.4
cm-1
λ = 647.1 nm130 W cm-2
on SiO2/ 1
z(yy)z' = z(xy)z'
z(yy)z'
z(xy)z' on Au / SiO2
/ 29
on Au / SiO2/ 76
Figure 6.4. In situ Raman band measurements of pentacene (1.5 nm) forming different interfaces when
deposited on Au and SiO2 substrates.
Table 6.1 summarizes the vibratioal bands of ML and bulk pentacene. For comparison, reported
vibrational values and those measured in the present work are presented.
The Raman band at 1163 cm-1 and measured in the z(xy)z´ geometry for the sub-monolayer (1.5 nm)
coverage is difficult to detect when the sample is exposed to the atmosphere. Additionally, this band
coincides with a broader feature observed in thicker pentacene layers (30 nm). The sub-monolayer
regime is governed by the Davydov splitting, whereas for the layers the bulky effect of the film
contributes, thereby hiding the Davydov splitting. This band has been assigned to the B2u symmetry
[Ross2002]. Experimental measurements based on generalized infrared spectroscopy ellipsometry
gIRSE have delivered the same symmetry [Schu2004].
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.9
The last column in table 6.1 shows the alteration of the Raman shift, indicating the deformation of
pentacene monolayers in comparison with bulk pentacene. An interesting feature of the OBL is that it
exhibits a compressive deformation, demonstrating the phenomenon of strain due to the pristine
coverage in closest contact to the inorganic substrate (Au or SiO2).
For the spectral region indicated in table 6.1, the elastic potential energy per molecule is modified,
presumably leading to an increase in the number of molecules per unit volume. Therefore, a higher
layer compactness in conjunction with an increase of the molecular orbital overlapping is likely.
Table 6.1. Raman shift of monolayer and bulk pentacene thin films
Symmetry Calculated
/ cm-1
Reported
/ cm-1
Symmetry
/ cm-1
ML
(Exp.)
/ cm-1
Bulk
(Exp.)
Elasticity
(ML-Bulk)
Ag 996a 996.4 995.7 compressive
B1g 1125a
Ag 1156.6 1155 compressive
Ag 1158a Ag 1159.4 1157 compressive
Davidov 1159b (pellets)
B1g (IR B2u
c?)
1173 1163a,c B1g 1165.4 1163 compressive
? 11778 1177.8 compressive
Ag 1178a Ag 1179.5 1179 compressive
1180.9 1180 compressivea Experimental values of Matteheus [Matt2002] on 1.54 nm pentacene bMeasured in pellets IR and theoretical calculation based on mean-field theory and HF/6-31G(d)
B3LYP / 631G(d) [Ross2002] cMeasured in pentacene thin films [Schu2004]
6.4. Characteristic regions of the organic layer in OFETs
The analysis of the experimental Raman bands in the previous sections revealed a stratified type of
pentacene growth. Submonolayer Raman features below 5 nm are different than the thicker overlayers,
demonstrating a coexistence of more than one crystal structure. Similar behavior has been observed in
other organic layers, such as VOPc and perfluorinated -VOPc [Hash1999a-b]; the different layer
arrangement might be due to the molecular dipole moment.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.10
The organic layer building up the channel can be divided into three main regions (cf. inset Figure
6.5(a)). The first one is described by the metal/organic interface, where pentacene layers are
influenced by the Au contact to an extent of 10 nm [Amy2005]. The second region is the effective
OBL, with a thickness below 10 nm. Raman measurements have proved that the OBL is formed by the
strained layer of about 1.5 nm [Paez2005a-b].
In addition, reported HREELS measurements by S. D. Wang and colleagues of pentacene growth on
SiO2 [Wang2005] demonstrated two stages of the 4.2 nm pentacene film. The first one was composed
of a layer with a thickness of ∼2.6 nm, while the second one was ∼1.6 nm thick.
A similar finding was observed by Ruiz et al. [Ruiz2005, Maye2004] through in situ synchrotron x-ray
scattering. Further investigations evidenced that interactions between the organic molecules and the
substrate during the deposition have dramatic effects on the crystallinity of the thin film and thus on
the resulting electronic properties [Vere2004]. The third region can be referred to as the bulk material
that contributes, to a minor extent, to the drain current.
Figure 6.5. Principal regions of the
organic layer forming the channel in
OFETs
6.5. Organic – insulator electrodynamics
Assuming that the organic material is already deposited onto the gated dielectric, a first consideration
is to assume the molecular arrangement as a domain composed of a compact layer (cf. Figure 6.6(a))
and with a nominal thickness below 10 nm as determined by the combined in situ Raman and
electrical measurements. The dynamic molecular arrangement of this layer is also given by
intermediate states. This means that molecules in closest contact with the substrate initially form
clusters (below 1.5 nm), then islands, and finally percolates, forming a compact layer.
From the point of view of charge transport, the organic/insulator interface acquires a capacitive charge
which in first approximation and under steady state conditions follows the Mott equation [Mott1938],
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.11
indicating an inverse square root law of the capacitive induced charge density in the channel and the
applied field through the isolator. This sequence of ideas leads to the charge density given by
⎟⎟⎠
⎞⎜⎜⎝
⎛ ϕ−ϕ=′=ρ
Tk)(q
expnqnq sss , (6.1)
where
erfacetheatdensityechnpotentialerface
s
s
intarg:int:ϕ
The charge density is determined by solving the Poisson equation
ερ−=ϕ∇ /2 , (6.2)
with ϕ the electrostatic potential and related to the electric field )( E perpendicular to the channel
plane
ϕ−∇=E , (6.3)
the induced charge density n is determined as a function of the applied gate voltage gV and the
location x of each monolayer parallel to the channel, 2
Ds
2i
s xL2
q1Tk2
)V(Cn
−
⎟⎟⎠
⎞⎜⎜⎝
⎛+
ε=′ g , (6.4)
with iC the gate capacitance per unit area and DL the Debye´s length. The surface charge density of
each layer as a function of its location parallel to the gated dielectric is depicted in Figure 6.6(b).
In section 6.2, it was experimentally determined that 1.1 ML are enough to have charge transport in
the channel. Below this coverage, the drain current is negligible. Within the framework of the induced
charge density by the gate voltage, it is also plausible to assume that the clusters are affected by the
applied gate field. Therefore, for the steady state conditions and for a channel thickness of about 7
ML, the charge dependence illustrated in Figure 6.6(b) gives, as a first approximation, an estimate of
the two- dimensional charge carrier gas density. One must be careful with eq. (6.4), since the two-
dimensional charge density for layers thicker than 7 ML exhibit a terrace-like morphology (suggested
in sect. 6.2).
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.12
Monolayersorganic
0Gateddielectric
X a b
OBL
1 2 3 4 5 6 7 8 9 10
Cha
rge
dens
ity /
a.u.
monolayer position / x
Gate voltage -1 V -2 V -3 V -4 V -5 V
Figure 6.6. Charge density distribution induced in the organic layers by applying different gate
voltages. (a) Monolayer stratification and (b) dependence of the charge density as a function of the
layer location parallel to the gated dielectric with the gate voltage as parameter.
6.6. Vibrational band profiling of the active layer
In situ Raman scanning spectroscopy (RSS) measurements between the organic/Au and organic/SiO2
structures were carried out. The addressed experiment allowed the influence of the substrates and
interfaces on the organic channel formation to be identified.
The sample was RSS scanned as shown in Figure 6.7(a), where the laser spot on the organic/inorganic
interface is sketched. A detailed behavior of the measured Raman bands is shown in Figure 6.7(b),
where the spatial scale is basically divided into two regions. The first one corresponds to pentacene
molecules on the gold substrate, ranging from 0 μm to 900 μm. The second one corresponds to
pentacene on the SiO2 substrate, starting from 900 μm to 1100 μm. For better recognition of the
organic/inorganic interface, it was subtracted from the 3D plot; the intensity of the mode at 1179 cm-1,
which corresponds to the C-H outer ring molecular in plane vibration, it is shown as middle up half-
filled circles in Figure 6.7(c). Additionally, on this plot, to highlight the metal / SiO2 boundary
position, the derivative of the signal was taken, which is shown as the middle down half filled circles,
resulting in an asymmetric Gaussian like profile. The derivative permits better localization of the
metal/insulator interface. From this profile, the semi halfwidth at full maximum (SFWHM) of the
Raman signal from molecules on Au was found to be 41.1 μm, and for those on SiO2 34.7 μm.
Deconvoluting these SFWHMs with the laser line profile, one finds the effective SFWHMs of the
Raman signal from pentacene on Au and on SiO2 of about 14.6 μm and 8.2 μm, respectively.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.13
Excitation energy1.91 eV
Phonon profiling
SiO2
Metalcontact
Organiclayer
n-Si
(a)
(b)
0.0 0.2 0.4 0.6 0.8 1.00
2
4
6
8
10
-120
-100
-80
-60
-40
-20
0
dA / dxλ = 647.1 nm
130 mW.cm-2
Pentacene(30 mn) / Au / SiO2
Spot position / mm
Pen
tace
ne(3
0 m
n) /
SiO
2
Area of the bandat 1179 cm-1
Ram
an A
rea
Figure 6.7. Raman spectra of the
organic/inorganic interface. (a) Sketch of
the swept interface in pentacene-based
OFETs, (b) 3D plot of the Raman signal as a
function of the vibronic bands and spatial
position in the organic/inorganic interface,
and (c) Profile of the Raman intensity
(middle up half filled circles) and derivative
(middle down filled circles) as a function of
scanning position.
The determined quantities indicate the influence of the boundary electrode upon molecules deposited
on Au and those on SiO2.
It must be noted that the reported values are limited by the lateral scanning resolution of the setup.
Another effect exhibited by the molecules on the metal contact is the enhancement of the Raman
signal, promoted by the roughness of the metallic surface. As a consequence, the local electric field at
the metal/organic interface is tremendously enhanced about 31 times, leading to the surface-enhanced
Raman spectroscopy (SERS) effect (described in chapter 2).
The difference between the two SFWHMs indicates that the organic layer on the metal is more
influenced by the Au contact boundary than those molecules on the SiO2 substrate. The discrepancy is
also an indication that the way the organic layer grows in the vicinity of the metal / gate insulator
interface is not uniform [Kell2006]. Complementary measurements based on scanning Kelvin probe
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.14
microscopy (SKPM) have shown a polynomial dependence of the electric potential through the
channel [Scho2005]. Therefore, a non-constant electric field parallel to the gated region was
determined, which might additionally cause variation of the charge density distribution along the
channel.
It is important to consider that the non-steady state potential behavior can be affected by technical and
physical factors, such as the tip apex, cone cantilever and the limited spatial resolution governed by
the long-range property of the Coulomb force [Jaco1998]. In this respect, experimental measurements
based on electrostatic force microscopy phase mode (EFMPM) would give more accurate values of the
local electrostatic surface potential [Lei2004].
Performing a correlated fitting of the spectra by using the algorithm introduced in chapter 3, one
obtains the resulting relative FWHM shown in Figure 6.8 with reference to molecules on the SiO2
substrate and away from the metal interface. From Figure 6.8(a), it can be seen that for molecules on
the metal, the relative change of the FWHM remains constant for each band, while those on the
isolator present higher deviations. An interesting and non-simple dependence of the FWHM close to
the metal/dielectric interface is observed.
0.0 0.2 0.4 0.6 0.8 1.0
-10
-5
0
5
10
15
1155 cm-1
1158 cm-1
1162 cm-1
1177 cm-1
1179 cm-1
1180 cm-1
Pen
tace
ne(3
0 m
n) /
SiO
2
OFET: FWHM of the Raman bandsas a function of the scanning position
Pentacene(30 mn) / Au / SiO2
Spot position / mm
Rel
ativ
e FW
HM
(a)
0.6 0.8 1.0-8
-6
-4
-2
0
2
OFET: FWHM of the Raman bandsas a function of the scanning position
1155 cm-1
1158 cm-1
1162 cm-1
1177 cm-1
1179 cm-1
1180 cm-1
Pent
acen
e(30
mn)
/ Si
O2
Pentacene(30 mn) / Au / SiO2
Spot position / mm
Rel
ativ
e FW
HM
(b)
Figure 6.8. Fitted FWHM of the Raman bands at different spot scanning positions (a), and inset of the
FWHM beneath the organic/inorganic interface (b).
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.15
6.7. Bias-stress effects and multi-exponential current relaxation
Within the framework of electrical characterization, in situ measurements of the current voltage
characteristics ( dd V)t(I − ) were carried out. The investigation was performed in two situations, in
darkness and under illumination. A series of successive drain current-voltage ( dd V)t(I − )
characteristics measurements were done for a fixed gate voltage.
The resulting experimental data are plotted in Figure 6.9. The dd V)t(I − characteristics taken in
darkness (cf. Figure 6.9(a)) and under illumination (cf. Figure 6.9(b)) exhibit a decrease of the Id
current over a long period of time. Consequently, this non-steady state behavior of the dd V)t(I −
characteristics might point to a time-dependent charge carrier density. On this occasion, the
drain current as a timedependent quantity prompted an estimate of the dominant time scales to
reach the steady state condition of the drain current. Due to the tendency of numerous
excitation-initiated natural phenomena to relax via exponential decays, the Id is considered to
be a linear combination of single-exponential (Debye) functions.
Given a well-behaved function in a certain interval (continuous and differentiable), in general it can be
linearly decomposed to an exponential basis set. Following the idea formulated above, from the
)t(V)t(I dd − characteristics shown in Figure 6.9, two time constants scaling the dynamic behavior of
the drain current were determined.
(a) (b)
Figure 6.9. Effect of bias stress on the dd V)t(I − characteristics of OFETs (a) in darkness and (b) under
illumination. The red contour lines indicate the time profile behavior of the Id current for a fixed Vd voltage.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.16
A first attempt to describe the dd V)t(I − characteristics is given by [Paez2005a]
)/texp(A)/texp(AII 2211od τ−+τ−+= , (6.5)
with )V,t(I do the steady-state contribution to the current and 1A and 2A the coefficients weighting
the exponential decay with time constants 1τ and 2τ , respectively. These time decays are related to
relaxation phenomena developed in the OFET device, as confirmed by combined Raman spectroscopy
and QTS measurements (chapters 7-8).
The variable t (time) is assumed to have a discrete value for each )t(V)t(I dd − characteristic. The
devices were swept at 10 V/min; accordingly, the drain current intensity given in eq. (6.5) can be
correlated with the sweep drain voltage by td/dVV dd =& , and by replacing
K&
,3,2,1jwithVV
jtd
d =⎟⎟⎠
⎞⎜⎜⎝
⎛= , (6.6)
where j represents the number of repetitions of the )t(V)t(I dd − measurement. It should be noted
that when 0Vd →& then dI is reduced to the expressions already discussed in the previous chapter.
Under stationary conditions, one obtains the drain current in the form
21od AAI)0t(I ++== . (6.7)
This expression is reduced to the non-trap formulation given in chapter 5. The addition of the
coefficients gives the initial drain current of the device.
The decomposition of an dd V)t(I − curve using eq. (6.5) is shown in Figure 6.10. The time
dependence of the exponential functions is closely related to the physical properties of the organic
layer, i.e., charge carrier interaction, charge density distribution, mobility, and structural relaxation.
The average time constants in darkness and under illumination were quite similar min591 =>τ< and
min62 =>τ< differing by less than 5%. The time constants determined from the dd V)t(I −
measurement taken in darkness and under illumination indicate that for this excitation photon energy
the trap density is preserved but not the charge carrier density as expected. The electrical and
vibrational experimental observations suggest that a modified model to describe the output
characteristics in OFETs must be considered.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.17
Figure 6.10. Simulation of one of the dd V)t(I − characteristics shown in Figure 6.9. Experimental
curves are plotted with circles. The quantities Io, A1, A2, τ1, τ2, were determined from the red
lineprofiles shown in Figure 6.9. To reproduce the experimental curve here (uppermost curve), the
time t related to the drain sweep rate of 10 V/min.
The continuous decay of the output characteristics indicates electrical instability of the device. This is
still an issue concerning not only small organic molecules but also polymers and a-Si based devices. In
the field of pentacene-based transistors, the phenomenon has been attributed to shifts in the threshold
voltage (VT) which might cause the dynamic behavior of the drain current (not only decay).
The issue is addressed in more detail in chapters 7 and 8, where traps induced by the applied electric
fields in the organic layer are considered. It should be noted that at last two types of degradation
processes should be considered, one related to the natural aging of the structure, and the other one
when the device is in operation. In most cases, the former phenomenon is irreversible, while the latter
can be reversed after leaving the structure un-biased for a certain time. The mobility of this structure
under darkness conditions was determined to be between 1123 sVcm100.1 −−−× and
1123 sVcm109.3 −−−× for the sublinear and saturation regions, respectively. After long bias stressing,
the mobility is reduced about two orders of magnitude
Time scales lower than >τ< 1 can be measured by means of more refined experimental techniques. In
order to achieve this requirement, charge transient spectroscopy (QTS) (chapter 3) was found to be
suitable for measuring in the range between s1 μ and ms10 . Several time constants were registered
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.18
in this range and a relaxation envelope of about min63=>τ< was estimated after the data
evaluation (chapter 8).
Conclusions
Pentacene-based field effect transistors were characterized by in situ electrical and Raman
spectroscopy measurements. It was demonstrated that for monolayer coverages between 1.1 ML and
∼7 ML with a nominal thickness of about 1.5 nm and 10 nm, respectively, a noticeable increase in the
drain current starting at the pentacene coverage of 1.1 ML indicates that the charge transport in the
channel is governed by a two-dimensional charge carrier gas.
This result, in conjunction with the compressive signature of the Raman bands (table 6.1), proves that
the conductive channel of the OFET behaves in a manner similar to the boosting channel of strained
FETs. In addition, it has been observed that this layer thickness is not enough to produce stable
devices under normal atmospheric conditions. Therefore, either a thicker layer or a capping material is
required.
The profiling of the Raman bands indicates a strong effect of the electrode edges upon the
organic/inorganic interface formation, provoking a non-uniform growth of the active layer. This effect
is quite likely to promote an electric field gradient in the channel accompanied by a spatially
dependent distribution of the charge carrier density in a given molecular plane of the organic active
layer.
Additionally, drain current – drain-voltage ( dd V)t(I − ) characteristics as a function of time showed
that there are relaxation phenomena promoting the reduction of the drain current intensity and the
mobility. The phenomena have an influence on both threshold voltage and on field effect mobility.
The mobility under darkness conditions was estimated to be between 1123 sVcm100.1 −−−× and
1123 sVcm109.3 −−−× for the sublinear and saturation regions, respectively; while for long term bias
stressing, it is reduced by a factor of 210 .
Complementary dynamic dd V)t(I − measurements were carried out in two situations, in darkness and
under illumination (1.916 eV). From both experiments, similar sets of (two) time constants were
determined, indicating that for this excitation photon energy the charge accumulation kinetics may be
preserved, while the injected charge carrier density is enhanced under the illumination.
Chapter 6 B. A. Paez-Sierra, Combined Raman spectroscopy and electrical characterization…
6.19
The dd V)t(I − characteristics were modeled considering a bias-dependent steady-state current
combined with a linear superposition of two exponential current decays. Regarding the tremendous
amount of the total transient charge (in mC range), it is important to consider not only trap states of
charge carriers at the organic/inorganic interface but also traps induced into the organic material by the
externally applied field.
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Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.1
Chapter 7
Influence of electric fields and illumination in OFETs
In this chapter, the modification of the vibronic-spectrum in molecular systems under external electric
fields is discussed. The resulting combined Raman measurements with applied electric fields gave
valuable information about the molecular structure alteration, and the correlation between charge
carriers scattering and the relaxation phenomena developed at intra- and inter-molecular levels.
The investigated structures revealed a structural relaxation with a time constant of approximately 94 s.
This was estimated from the Raman spectroscopy measurements performed after switching off the
external electric field. Therefore, an extended recovery time for the molecular structure of about 9 h
was estimated. The induced molecular modification partially explains the drain current decrease when
the OFETs are continuously operated.
7.1. Introduction The interaction of matter with external electric fields leads to alterations at electron energy levels,
changes in the molecular dipole moment, structural configuration [Bish1990, Feyn1963] and
molecular redistribution [Ren2006]. The electric field induces charge displacement of the electrons in
the outer shell of the atom, then seen as induced dipole promoting shifts of the Raman bands and/or
modifications of the mode intensity [Auss1986, Jeon2003, Paez2003, Ralp1990].
Alteration of the normal molecular absorption and distortion in the scattered light under optical
excitation is likewise a phenomenon able to demonstrate the interaction between matter and the
applied fields. The electronic or geometric changes produced by the electric field are understood as a
breakdown of the molecular symmetry. Despite these issues, some theoretical calculations have been
carried out on organic structures and assuming periodic boundary conditions [Tóbi2004].
In a fashion similar to that of inorganic semiconductors, it is expected to have intraband and interband
transitions reflected in the Fröhlich and Franz-Keldysh type interaction matrix elements, respectively.
The first type of interaction involves intermediate excitation states promoted by the non-local
molecular polarized discrete and continuous electron-hole pair states [Shan1972]; the second one is a
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.2 consequence of the Fröhlich interaction. On the other hand, the probability of the resonant Raman
transition is affected by the field. Therefore, the established rules by the non-perturbed Franck-Condon
effect are modified as a consequence. Further detailed analysis shows the influence of the electric
field on the Franz-Condon effect.
Investigations of combined Raman and applied external electric fields cover a broad area. By using
Pico-Raman spectroscopy, the drift velocity in heavily degenerated semiconductors [Gran1994] has
been proved. An interesting and apparent antagonistic feature of the Raman effect is that in spite of the
fact that the scattering cross section is inversely proportional to the square of the effective mass, it
selectively proves electron distribution functions even if holes are present. This experimental
procedure works nicely for high charge carrier concentrations.
As shown in the previous chapter, the conductive channel thickness extends up to a few nanometers
(approx. three monolayers), indicating that the enhanced charge density of the organic layer is
confined to a two-dimensional gas [Ando1982]. Recent novel investigations using surface-sensitive
IR, visible sum frequency generation (SFG), nonlinear optical spectroscopy on interfaces of OFETs
during operation [Ye2006], and absorption infrared active vibrational (IRAV) modes spectroscopy
restate the capability of vibrational techniques to scope the depleted organic material by gate fields in
OFETs [Li2006]. In order to undertake the combined vibronic-electrical research of the active
material, one aspect to be considered is the geometrical configuration of the leads in the OFET. The
bottom contacts arrangement gives greater advantages over other geometrical contact patterns.
7.2. Raman bands and external electric fields
From the phenomenological point of view, first-order IR absorption and the first-order Raman
scattering tensor arise from conditions in the electric dipole moment vector (per unit cell) μ . The
Raman line intensities are determined by differentiating the classical polarizability tensor (α ) with
respect to the normal molecular coordinate ( iQ ).
It should be noted that α contains elements equal to the derivatives of the μ components with respect
to the applied electric field (Ε ) components. Therefore, rewriting the expressions found in chapter 2,
one obtains Raman intensities related to
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.3
oij
i2
oji
i2
k
ij
QQQ ⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂μ∂
=⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂μ∂
=∂
α∂
EE, (7.1)
therefore, depending on the molecular structure orientation and for different applied external fields,
the polarizability scales as the inverse of the square of the molecular band gap [Kozi2006].
7.2.1. Band gap modification by external electric fields
As an example, consider the benzene molecule fixed in a coordinate system and exposed to external
electric fields, as illustrated in Figure 7.1. For the applied electric fields, the HOMO and LUMO states
were evaluated at the B3LYP / 3-21G level in Gaussian 98 [Gaus1998] and the resulting molecular
orbitals were plotted in ChemOffice. The picture indicates the changes due to the excitation field.In
the same figure, the band gap alterations in comparison with the relaxed structure are summarized. It is
observed that the highest modification of the band gap occurs when the electric field is applied along
the z axis; the peculiarity of this direction in comparison with the other two is that it involves the
highest delocalization of the electronic cloud.
Figure 7.1. Benzene molecule under different external electric fields. (a) Molecular structure, (b) chart
summarizing the band gap modification by the applied fields, and (c) HOMO and LUMO
configuration in response to the applied electric fields. (The unit H in the table means Hartree = 27.2
eV).
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.4 In a similar manner, the interaction of the external electric field with pentacene was simulated. It has
been found that the main electronic alteration is along the y axis according to the orientation depicted
in Figure 7.2. It was also observed that the “y” axis corresponds to the direction of the active layer
thickness in the OFET.
In chapter 5, the molecular arrangement of the pentacene layers on the gate-dielectric substrate was
discussed. Moreover, it was estimated that the molecules are in an upright position. This means that
for molecules oriented in this direction, the influence of the gate field is decisive for the charge
carriers transport.
For isolated molecules or interlayer structures without electrodes, the polarized charge density is given
by
α⋅−∇=ρpol , (7.2a)
or, in the event that the spin electron contribution is considered, eq.(7.2a) reads [Park1986],
α⋅∇−ΨΨ−=ρ +epol , (7.2b)
where α is given through eq.(7.1) andΨ is the spinor describing the quantum state of the electron. It
must be noted that this expression keeps the proportionality between the Raman signal and the charge
induced by the external field. Then theoretically, as expected, any external field will produce
modification of the Raman bands.
A particular feature of the pentacene molecule and similar acenes is the higher number of C-H bonds
than C-C bonds. This makes the induced polarization in the direction of the hydrogen atoms more
favorable and delocalization of the molecular orbitals broader.
Figure 7.2. DFT simulation of the influence of an applied electric field on the molecular band gap and
HOMO-LUMO states in pentacene. (The unit H in the table means Hartree = 27.2 eV).
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.5 7.2.2. Raman bands alteration by external electric fields
7.2.2.1. Pentacene
Similar calculations with electric fields were made to investigate the Raman shifts. In Figure 7.3, the
orientation of the pentacene molecule and the relative Raman shift of the electrically excited molecule
are shown. In order to quantify the effect of the applied electric field on the Raman bands of the
molecule, the Raman shifts of the excited structure were subtracted from those of the free molecule
(cf. Figure 7.3(b)). The experiment was primarily focused on the shaded region indicated in Figure 7.3,
where the in-plane C-H vibrations are involved. The discrepancy between the experimental Raman
shifts upon applied electric fields and those calculated by DFT, proved the necessity to include the
electron-electron correlation and delocalized basis sets in the calculations.
The non-shaded spectral regions were experimentally monitored under the influence of the electric
field as well, and minor or absent structural modifications were detected. Therefore, attention was
mainly targeted to the shaded zone depicted in Figure 7.3(b).
In chapter 6, the characteristics of the organic boosting layer, which holds most of the gate induced
charge density for the charge transport in the OFET, was discussed. In analogy to investigations of the
depletion region in highly doped ( 319 cm105.2 −× ) inorganic semiconductors with Raman
spectroscopy [Fuka1988], the method appears very sensitive to comparable doping modifications
where the electronic susceptibility is assumed to be modulated by the atomic relative displacement and
the macroscopic electric field associated with the LO phonons propagating along the inter-atomic
sites.
Organic materials are characterized as having a much lower charge carrier density. This makes the
analysis of the accumulation region of the OFET more challenging, since lower charge density
distributions for both inorganic and organic semiconductors are more complex.
Other factors involved are the conductive channel thickness and the photon excitation energy. The
OBL maintains the field-modulated charge density with non-degenerated distribution of charge
carriers. Therefore, despite the long penetration depth λ relative to the accumulation region width, the
contribution to the Raman spectrum of surface phonons is dominant compared to the bulk
contribution.
It was shown in chapter 6 that the accumulated charge density scales as the square inverse of the OBL
thickness, being higher for molecular planes closer to the gate dielectric. This has advantages for the
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.6 experiment combining Raman measurements and electric fields. Applying drain-source fields, the
contribution to the Raman cross section is negligible or at least below the setup resolution.
(a)
900 1050 1200 1350 1500-40
-30
-20
-10
0
10
20
30
40
Freq
uenc
y sh
ift /
cm-1
Frequency / cm-1
Ex - Eo
Ey - Eo
Ez - Eo
Influence of an applied electricfield on the vibronic bands
experiment
B3LYP / 321 - G
(b)
Figure 7.3. Calculated vibrational states of a pentacene molecule under the influence of external electric
fields. (a) Orientation of the pentacene molecule, and (b) variation of the frequency assignment as a
function of the vibrations without an applied electric field.
The experimental results of Raman spectroscopy with electric fields are shown in Figure 7.4. For the
investigations, the drain and source contacts were grounded, and step-wise voltages were applied to
the gate contact. The gate voltages were varied between 0 V and –24 V.
The most significant modification of the Raman bands was found to occur at the molecular in-plane
C-C ring and C-H vibrations with Ag symmetry. For the experiment, the Raman intensity followed a
square law dependence with the applied gate field; this is summarized in the correlated fitting
illustrated in Figure 7.5. Additionally, no changes in bands positions were observed.
The Raman signal is basically dependent on the polarizability modulation (chapter 2) of the
investigated media Consequently, one expected signature is the change of the Raman intensity (cf.
Figure 7.4 and 7.5 (a)).
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.7
Figure 7.4. Experimental measurements of the Raman bands in pentacene (30 nm) for different applied
fields. The involved modes of this spectral region belong to in-plane vibrations of C-H bonds of the
outer ring and antiphase vibrations of the C-H bonds parallel to the main axis of the molecule.
The gradual enhancement of the Raman bands in response to the increase of the gate voltage provides
an indication as to the formation of the conductive channel in the pentacene-based FET. The increase
of the Raman intensity as a function of the electric field (cf. Figure 7.5) is developed up to a critical
value; afterwards a saturation region appears, followed by a decrease in the intensity.
The first part to be considered is the charge state formation at the OBL. The saturation corresponds to
the influence of molecular polarizability inducing dipole-anion sites trapping the induced charge by
the gate field. A further increase of the electric field enhances the dipole-anion sites population, while
a decrease of the Raman cross section is observed.
Combining the equations (7.1, 7.2), one can describe the proportionality between the induced charge
density and the Raman intensity as
enIRaman l∝ , (7.3)
with ln the charge density per layer and e the electron charge. It should be noted that by
accumulating charge, the Raman intensity becomes proportional to the square of gV as shown
eq.(6.4) and experimentally indicated in Figure 7.5. Nevertheless, additional considerations should be
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.8 taken into account, since the higher the electric field the higher the probability of generating artificial
flaws (chap. 8) in the organic layer.
0 -5 -10 -15 -20 -250
15
30
45
60
75
90
Voltage / V
λ = 676.4 nm / 130 W cm-2
Raman intensiy in dependence
of the applied electric field
Pentacene(30 nm) based FET
Ram
an In
tens
ity (a
rea)
/ c
ts.m
W-1.s
-1.c
m-1
1155.5 cm-1
1158.6 cm-1
1164.0 cm-1
1176.5 cm-1
1179.0 cm-1
1180.2 cm-1
Figure 7.5. Correlated fitting of the Raman intensity as a function of the applied gate voltage (the
digits behind the floating point were delivered after the correlated fitting procedure).
The enhancement of the Raman signal is an indication that the Raman cross section ( RSσ ) is
increased. Typical values of RSσ are between 10-27 and 10-14 cm2, where the lower RSσ is the normal
value of isolated molecules interacting with the photon beam, while the higher one scales with the
typical values quoted for surface enhancement Raman spectroscopy and mediated by electromagnetic
fields (Chapters 2 and 4). On the other hand, by combining the Raman cross section and the charge
carrier capture cross section determined from complementary QTS measurements (chap. 8), it is
possible to evaluate the charge density appearing in eq. (7.3), if a coupled electron-phonon system
with impurities is assumed [Itai1992]; hence a value of about 1012 cm-2 is obtained.
The experiment was then carried out by switching off the gate field and in situ monitoring of
the vibrational bands relaxation, as illustrated in Figure 7.6. The time decay constant was
found to be about 94 min.
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.9
0 30 60 90 120 150 18020
22
24
26
28
30
32
Relaxation of the bandat 1179 cm-1 after switching offthe electric field (Vg = - 24 V )
Inte
nsity
/ ct
s. m
W -1
s-1
time / min
Decay time: 94 min
λ = 676.4 nm / 130 W cm-2
Pentacene(30 nm) based FET
(b)
Figure 7.6. In situ measurements of the Raman bands relaxation after switching off the electric field, and
(b) time decay of the Ag band at 1179 cm-1 after switching off the electric field.
7.2.3. The C60 fullerene
In this work, the charge induced in C60 was produced by changing the gate voltage in the OFET
structure. In Figure 7.7, the effect of the electric field on the Raman spectra is presented. Theoretical
calculations performed with Gaussian 98 [Gaus1998] at B3LYP [Beck1993, Koch2002, Salo2002]
level and the 6-21G basis set reveal that, of the 46 symmetry operations, many of the vibrational
modes are two-fold and even five-fold degenerated, giving 174 vibrational modes. The most
representative Raman modes of the isolated C60 molecule theoretically evaluated are: squeezing (271
cm−1), breathing (491 cm−1), and pentagonal pinch (1495 cm−1) modes.
In the experiment, the in situ macro-Raman signal was measured at very low power density (25.4
W/cm-2 and laser power of 2.0 mW), approximately 50 times lower than that used to investigate the
perylene derivatives (chapter 4) and about five times lower in comparison with the micro-Raman
experiments performed on bias-stressed pentacene FETs. The obtained Raman intensity is about 100
times lower than that registered from pentacene-based OFETs. The light source and intensity were set
to such low values in order to avoid additional features in the spectrum induced by the photo-
polymerization phenomenon. The investigated spectral region was selected where the pentagonal
pinch mode is located, being the only region where signal detection was possible for such thin film.
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.10 The electrical charging basically leads to two results. The first one is symmetry breakdown, which is
reflected in the splitting of the degenerate modes, and the second one is the significant change of the
pentagonal pinch mode, i.e., the change in the Raman intensity and the center of mass of this peak, is
shifted to lower frequencies, in agreement with the theoretical results. Experimentally, the advantage
of the OFET structure is utilized to induce charge in the C60 molecules.
The differential Raman spectra, in addition to the spectra with and without an electric field, are shown
in Figure 5. There are clear effects of the voltage on the Raman signal which are fully reversible. One
is the change of the FWHM and the other is the decrease of the Raman signal, producing quenching of
the resonance inter-band transitions. Consequently, on the basis of the Franck-Condon effect, it is
more probable that higher stretching in and out of the pentagonal pinch mode will be promoted. Then
attenuation of the Raman signal is expected, as experimentally observed and shown in Figure 7.7.
Figure 7.7. Raman spectra (upper plot) and differential Raman spectra (lower plot) of 3 nm C60 layer,
the latter obtained by subtracting the spectra without an electric field from the ones under the influence
of the gate voltage.
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.11 7.3. Illumination and charge transport in OFETs
The samples were also illuminated by different wavelengths, for which the maximum drain current in
two situations was monitored, one taking into account the saturation drain-gate voltages of the
saturation regime (illumination), and the second one with zero gate voltage (cf. Figure 7.8). Both
situations illustrated in Figure 7.8 show a similar line-profile with a difference in the enhancement
drain-current intensity. The photon-molecule interaction promotes an increase of free charge carriers,
due to ionization of neutral traps and photodetachment of free excess carriers at anion-dipole sites. The
first mechanism is strongly dependent on the excitation photon energy, while the second one is
understood as the emission of trapped charge carriers at molecular dipole domains. Therefore, these
two effects have an impact on the measured drain current.
The measured Id-max as a function of the excitation photon energy looks like a mirror image of the
absorption spectrum of pentacene (30 nm) deposited on quartz. This result corresponds to a large
extent with the vibronic characterization at energies close to the first absorption peak.
Figure 7.8. Photocurrent of pentacene (30
nm) based FET. The spectra were taken for
a drain voltage of –40 V and gate voltage of
0 V (blue) and –10 V (green). The red
spectrum corresponds to the absorption of
30 nm pentacene deposited on quartz.
7.3.1. Persistent effects and multi-exponential kinetics
A complementary investigation, consisting of measurement of the drain current after suppressing the
photon source, is displayed in Figure 7.9. The drain current relaxes monotonously, obeying
exponential kinetics of at least two relaxation times. Similar dynamics have been addressed elsewhere
[Dutt2003, Palm1984, Quei1985, Schi1995]. The quoted relaxation time constants indicate the
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.12 evolution of two charge carrier scattering mechanisms, one scenario with s771 =τ and the other as a
collective phenomenon with a time constant of s3372 =τ . Both time constants were stimated with an
accuracy better than 3 %.
0 100 200 300 400 500 600
-6.5
-7.0
-7.5
-8.0
-8.5
-9.0
-9.5
-10.0
Vg = -12 V Vds = -40 V
I ds /
nA
t / s
Off: λ = 676.4 nm 0.6 W.cm-2
τ1 = 77 s
τ2 = 337 s
Pentacene (30 nm) based FET
Figure 7.9. Persistent behavior of the
drain current in pentacene (30 nm)-
based FET after switching off the
photon source. The decay follows
multi-exponential kinetics.
A similar photocurrent experiment was performed by switching on and off the coherent photon source
(Kr+ laser). The dynamics during and after illumination are well described by multi-exponential
kinetics. It is important to note that the first effect of light exposure is the recovery of the molecules.
Subsequent light doses induce an envelope function for the drain current with sample illumination and
a second envelope develops following the minima of the drain current under dark conditions. The time
constant of the upper envelope is about 209 days (ON state), while the lower envelope has a relaxation
constant of approximately 96 days (OFF state). In both situations (light ON and OFF), the device
operates under quasi-stationary conditions.
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.13
Figure 7.10. Photocurrent and
dark current in 30 nm pentacene
FET. The sample was illuminated
by coherent light of 676.4 nm.
0 1000 2000 3000 40000
-20
-40
-60
-80
-100
-120
ON OFF OFFON
Non first order exponential process
I d / n
A
t / s
Induced Photocurrent inPentacene OFETsd x W x L = 16 μm x 100 μm
λ = 676.4 nmVg = -12 VVds = -40 V
Conclusions
The influence of electric fields on molecular properties has been investigated by density functional
theory (DFT), and experimentally with a combination of Raman spectroscopy and electrical
characterization. The theoretical calculations showed an alteration of the molecular band-gap, vibronic
and electronic states and removal of degeneracy when the investigated structure had a multiplicity of
states at a given energy level.
The experimental results of Raman spectroscopy with electric fields in OFETs demonstrated a
proportionality between the Raman signal and the applied field, which for pentacene (30 nm)-based
FETs increases until applied voltages of about –20 V, followed by an attenuation for higher fields. The
behavior suggests considering competing phenomena between induced charge density and the creation
of dipole-anion sites induced by the external field. The enhanced Raman signal in pentacene-based
FETs indicates that the cross section of inelastic scattered photons is enhanced as well.
The subsequent vibronic measurements after switching-off the electric field indicate a dynamic
relaxation of the Raman cross section with a time constant of about 94 min. The experiment predicted
a long recovery time for the device (confirmed indirectly in chap. 8 by QTS measurements).
The pentacene FETs were illuminated with different photon energies, resulting in selection rules for
the enhanced drain current. The photocurrent followed a mirror-like profile of the absorption
Chapter 7 B. A. Paez-Sierra, Influence of electric fields… 7.14 spectrum. Further experiments with chopped photon light gave additional hints about the charge
relaxation transport phenomena with temporal envelope profiles for the drain current during sample
illumination and darkness conditions. After illumination, the drain current evolves in a persistent
multi-exponential decay, described by two time constants of about 77 s and 337 s.
The second model system was a C60 (3 ML)-based FET which had to be carefully handled in order to
avoid additional spectral features promoted by the photon source. Therefore, an optimum set of
experimental parameters, such as photon energy, power density and layer thickness, were tuned to skip
artifacts during the measurments. Under these conditions, the Raman intensity was approximately 100
times lower than that obtained from pentacene (30 nm)-based FETs. The C60 molecule is characterized
as being totally symmetric and the first consequence of the applied electric field is the removal of this
degeneracy, as demonstrated experimentally and theoretically.
The experimental Raman measurements of the C60 films were delimited to the pentagonal pinch mode
(PPM) region, where the signal was significant for extracting the phenomena induced by the applied
field. In this molecule, the Raman band of the PPM had a decrement of its intensity together with a
blue shift. In the framework of the Franck-Condon effect, this experimental behavior is likely to occur,
since the PPM can be decomposed as a combination of breathing modes; consequently, the
internuclear distances are modified, producing a reduction of the Raman transition probability.
Another factor is the splitting of some bands close to the PPM; due to the setup resolution, however, it
was not possible to observe this.
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Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.1
Chapter 8
Trap distribution in OFETs and anomalous QTS
In this chapter, anomalies of the charge-transient spectroscopy (QTS) signal in pentacene-based
OFETs are presented. The anomalous phenomenon is evidenced by deviation of the QTS spectra from
those which are in close accordance with the polarities of the applied bias and the relaxed signal. The
anomalous QTS spectrum results with combination of peaks with positive and negative components.
This phenomenon demosntrates the presence of trap centers for minority and majority charge carriers.
The unusual behavior is attributed to intrinsic and extrinsic trap sources of the OFET device. The first
trap source is developed at the boundaries of the organic material, while the second one comes from
dipole sites in the organic layer prompted by the external electric field. In addition, the negative
transconductance, or so-called "drain current collapse", provides strong indications of trapping from
both types of charge carriers at the pentacene film. A description of the anomalous QTS measurements
is complemented by a theoretical model. For the simulation, two relaxation processes are assumed,
one dielectric and the other governed by decay of the induced dipoles in the local field.
It is shown that under illumination the transconductance can be switched from negative to positive
values. Thus, the trapped charge can be ionized by well-defined photon energies. Therefore, a
photodetachment process at the (anion, cation)-dipole sites might be developed.
Furthermore, subsequent light doses increase the mobile charge density, while the removal of the
photon source has a persistent effect, a phenomenon registered by the slow decay of the drain current.
Experiments revealed that the drain current measurements, either with illuminated channels or in
darkness, follow time-dependent multiexponential kinetics [Paez2005].
8.1. Introduction
In organic electronics, the organic/inorganic interface formation is the main object responsible for
several interactions affecting the device performance. The most widely accepted conclusions of these
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.2 interface studies is that metal/organic contacts generally do not follow the Schottky-Mott model and
exhibit large dipole barriers [Amy2005]. Here it is assumed that the local dynamics of charge
distribution contributes to the electric dipole moment of the total active layer in spite of not being in
direct contact with the inorganic material.
In order to identify the charge carrier interaction with traps, ex situ charge transient spectroscopy
(QTS) measurements were performed on the OFETs. These results provide valuable information about
the influence of dipolar traps on the majority and minority charge carrier energetic distributions
[Thur2004].
The anomalous behavior of the QTS signal observed in pentacene-based OFETs is similar to results
that have been reported for GaAs MESFETs [Zhao1990], current DLTS in AlGaAs/GaAs FETs
[Cava2003], and capacitance-DLTS measurements in pentacene MIS structures [Yang2002].
Experimental measurements of drain current relaxation for single active layers of pentacene, CuPc,
and PT FETs as a gas sensor like device [Tane2005] show minority charge carrier trapping and dipole-
anion formation (a phenomenon not discussed in the publication by Tanese et al. [Tane2005]). In a
similar context of minority charge carrier trapping, the capacitance as a function of the applied bias in
polymer light-emitting diodes has proved this phenomenon of sign inversion maintaining the same
pulse polarity [Shro2005].
In the present investigation, transient current Id(t) measurements were performed to determine the
condition required to achieve steady state of charge transport at the conductive channel, and to
determine the relaxation time constants. These measurements revealed a non-steady state behavior of
the )t(V)t(I dd − characteristics for short time scales, i.e., less than 5 min. This means that there is a
strong influence of the stress biases Vg and Vd, respectively, on the charge transport.
A first attempt to simulate these transient effects was made by considering a model based on two
exponential contributions to the Id-Vd characteristics. The mathematical formalism relies upon the
basis of the fractional calculus [Hilf2004]. Although the present discussion is not devoted to an
analysis of the well-known transport equations, fractional calculus is considered the expedient
mathematical tool to model the multiexponential behavior of the )t(V)t(I dd − characteristics.
Therefore, the relaxation process is expressed as a linear combination of Debye exponential functions
rather than a single stretched exponential [Schi1993].
Non-exponential relaxation has been observed in the persistent behavior of the photo-induced Id
current after the photon source is switched off [Dutt2003]. Moreover, persistent photoconductivity in
inorganic semiconductors [Quei19886, Theo1982], meta-stable transport effects in pentacene single
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.3 crystals due to bias-dependent generation and the quenching of defects [Chi2004] have been
experimentally confirmed; drain current relaxation of printed RFID tags based on pentacene and
oligothiophenes [Subr2005] have proved the phenomenon as well. However, these developments are
lacking of discusions in terms of multiexponential kinetics.
8.2. Traps and charge density distribution
Previous analyses of photo-enhanced current in organic-based devices have been founded on the
assumption of Gaussian-like trap distributions [Godl2001]. Further experimental evidence suggests a
combination of such distribution and the time dependencies of the traps, i. e., they might be transitory
and/or static, as has been found in the present investigation.
The first kind of trapping appears when the device is stressed gradually by an external field. When the
device is continuously switched on and off, it preserves part of the previous history (memory effects),
in this way modifying the physical conditions of the traps and thereby acquiring a series of quasi-
equilibrium states.
A model to augment the comprehension of the trapping phenomena in organic electronics is based on
the extension of earlier theoretical investigations by Fermi and Teller in 1947 [Ferm1947]. The idea
was to determine the minimum electric dipole moment required to form a dipole-anion state.
8.2.1. Effect of the electric field
A specificity of weakly bound complexes is that they can undergo large deformations, as has been
experimentally evidenced on para-amino-benzoic-acid dimers (PABA dimers) [Vasc1999]. Because of
the vibrational motion. Despite the molecular dipole being zero in equilibrium, the average of the
square dipole moment for the dimer is different from zero, which leads to a high electric susceptibility.
To verify this, the vibration-induced dipole moment in the gas phase of a molecular complex was
determined [Comp2002]. Therefore, the total dipole moment, with all likely contributions ( iμ ), is
∑μi
i . (8.1)
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.4 In chapter 2, the nature of the probability of having a sort of band structure in organic solids was
discussed. Earlier experiments on several molecules investigated by means of x-ray excited
photoelectron valence band spectroscopy have demonstrated this dispersion relation for the organic
solid [Pire1984].
The quasi-band diagram depicted in Figure8.1 as a modification of a similar band diagram reported
elsewhere [Pire1984], shows the normal organic solid with intercalated dipole states. Consequently,
the charge carrier has three possibilities: to be driven, transmitted, reflected or trapped (bound) by the
dipolar center. It should be noted that the possibility of back-scattering is not rejected; this might play
a fundamental role at the metal/organic interface. Thus, The transmission is understood as tunneling
through the dipole-wall potential, and the charge trapping is seen as a quantum confinement. For the
last situation, a detailed description of the dipole-bound anion with spherical symmetry can be found
elsewhere [Rone2003].
Then it is also quite plausible to think about a molecular cluster arrangement with zero dipole, which
in an external field will produce sides with zero and nonzero dipole moments (cf. Figure 8.1). This
means that not all molecules would contribute to the charge transport, hence, the free charge carriers
face the presence of dipoles induced by the electric field, and will tend to be trapped (bonded),
forming dipolar anions. This is what is labeled as artificial traps in this research.
The total number of flaw states in the organic material is higher than that for isolated systems.
Therefore, it is convenient to consider a time-dependent total dipole moment following a linear
combination of exponential relaxations
))/texp(1())/texp(1()t( 22o11o τ−−μ+τ−−μ=μ −− , (8.2)
with 1o−μ and 2o−μ the expected maxima of dipole moments for the time constants 1τ and 2τ ,
respectively.
Figure 8.1. Schematic view of combined
artificial flaws or traps induced by the
external electric field dipole, and
intermolecular potential acting on the two-
dimensional charge carrier gas. The z
coordinate gives the strength of the
potentials, while the xy plane correponds
to spatial coordinates
The number of dipoles that can bond the free charge carriers is
)t()t()t( Ts μ=μ′+μ , (8.3)
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.5
with )t(sμ the static dipole moment and )t(μ′ the dipole moment, including finite effects and degree
of freedom (rotation, vibration). In this way, the number of (anion, cation)-dipole sites arN can be
calculated as follows
minsmin
sar D
)t(D
)t(N
′μ′
+μ
=−
, (8.4)
with sminD − the minimum static dipole moment and minD′ the minimum dipole moment, including
internal coordinates. Both quantities are required in order to form a dipole-anion bond configuration.
The calculation of both dipoles is done on the basis of solving the Schrödinger equation for a quasi-
free particle subjected to a dipolar potential,
⎟⎠⎞
⎜⎝⎛
′′+
′−+∇−=
r1
r1q
m2H 22
2h. (8.5)
The time-independent Schrödinger equation is
−
h2
2m∇2 + eq −
1′ r +
1′ ′ r
⎛ ⎝ ⎜
⎞ ⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥ ψ = εψ ; (8.6)
by introducing the variables indicated in Figure 8.2,
the situation becomes azimuthally symmetric, where
the particle is confined in two dimensions. The
numerical solutions of eq. (8.6) thus result in
D625.1D smin =− [Byer1967, Ferm1947, Garr1971-
1982, Lapi1980, Turn1968-1977], approximately
50 % lower than the classical estimation, and
D]5.2;0.2[Dmin =′ .
Figure 8.2. Particle subjected to electric
dipole potential.
In the framework of the present discussion, the applied external electric field that can promote dipole-
anion bond states will effect an artificial trap (artificial flaw) distribution for the charge carriers.
Therefore, the total density of flaws is given by
tarT N)t(NN += , (8.7)
with Nar (t) the artificial flaw density determined by eq. (8.4).
Measurements of the current as a function of time provide important information in order to estimate Nar (t) ; this issue is discussed further in the chapter.
For this simple model, dipole-dipole bonds are not assumed. The same holds true for the transition rate
contribution between Nar (t) and Nt . Otherwise, the creation and annihilation of "particles" at the site
should be considered.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.6 The external electric field produces two effects: one is the polarization of the organic molecules and
the other is the driving of free charge carriers. If the molecular solid is not affected by the field, then
the traps are independent of that contribution, at least for moderate applied fields. What it is observed
is the appearance of "anomalous features”, related to creation of dipoles in the organic structure.
The dipoles act as scattering centers for the charge carriers. One special case is when the magnitude of
the dipole moment equals or surpasses a certain critical value [Ferm1947], increasing the probability
of dealing with electron-dipole bound states, responsible for the induced artificial flaw density.
Therefore, it is expected that within the framework of the standard trapping and detrapping modeling
the estimated trap densities will exhibit strong deviations from those expected from non-stressed
device.
Some consequences of the dipole-electron bound states on charge transport are
- negative differential conductance,
- anomalous contributions to the QTS or DLTS signal: enhancement of minority-charge carrier
trapping,
- reduction of the current,
- dynamics of the capture cross section, and
- threshold voltage dynamics.
An excess electron can be bound to many molecules and into a very diffusive molecular orbital as a
result of the long-range contributions of the molecular multipole electrostatic fields. In this context, further theoretical analysis requires diffuse molecular orbitals, in order to model the delocalization of
the electronic charge density.
An electron interacting with a molecule or molecular cluster which possesses a large dipole moment
may bond to the molecule or cluster to form a dipolar anion. Compagnon and colleagues reported the
first electric deflection measurement on the PABA dimer. It is characterized as being a weakly bound
molecular complex. The PABA molecule has a strong permanent dipole moment (3:9 D [Vasc1999])
but the dimer has a symmetrical structure, which is bound by a pair of hydrogen bonds [Meij1990],
and the two dipoles cancel each other at equilibrium.
A specificity of weakly bound complexes is that they can undergo large deformations. Because of the
vibrational motion, the average of the square of the dipole moment is non-zero, which leads to a high
electric susceptibility. This is the first observation of a vibration induced dipole moment of a
molecular complex in gas phase. The spectacular effect was predicted by Whitehouse [Whit1993], and
never observed for an isolated system.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.7 The results are interpreted by a simple and general model within the framework of the linear response
theory. It constitutes an original example of a system with an electric susceptibility that does not
follow the well-known Curie law. This proves the probability of observing electric-field induced
modification in the Raman spectra of structures with zero or negligible dipole moment under
equilibrium conditions.
8.3. Anomalous behavior of the QTS signal
In chapter 3, the description of the QTS method was addressed on the basis of interfacial phenomena.
In general, the applied electric field produces modifications not only in the charge carriers and or
trapped particles densities, but also in the molecular structure properties [Paez2003].
In terms of relaxations, the fast processes are achieved by those charge carriers with high mobility,
while for carriers with lower mobility, the relaxation time constant can be hundreds of times higher in
comparison with those filling up the faster capture process. Each time the electric field is applied the
"reservoir” (molecules) is modified and the new state is a quasi-equilibrium state.
8.3.1. Advantage of floating gate configuration in QTS measurements
The experimental spectra presented in this section were obtained by applying a bias UDS between the
source and drain, while keeping the gate electrode floating. It may be clear that the goal is to
characterize the properties of the channel material between the source and drain of an operating OFET.
Yet it is really important to explain why the floating gate is profitable. If one applies a dc bias between
the gate and source, the excitation pulse ΔU would appear across the gate oxide with the following
consequences:
– transient charging of the relatively high oxide capacitance through the Si gate electrode,
– populating Si/SiO2 interface states,
– populating SiO2/organics interface states,
- transient charging of the oxide capacitance through the organics (dielectric relaxation) [Agar1974],
and ion movements in the oxide.
Under such conditions, it is difficult to resolve the transient charge across the tiny capacitance of the
channel. Applying a bias pulse ΔU (cf. Figure 3.8(b)), there is a net bias UDS + ΔU during the pulse; in
most experiments, the reference bias potential was set to zero. After the pulse, the three-channel
correlator was activated to provide a ΔQ signal for each discrete delay t1, further details of the
experimental procedure are given in sect. 3.4.2.
The respective results for high- and low-power architectures will be presented separately.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.8
8.3.2. OFET devices with interdigitated source-drain electrodes
The QTS spectra measured in the interdigitated source-drain electrodes OFETs already mentioned in
chapter 3 are partially shown in Figure 8.3. The stressing pulse 1U (ΔU) was set between the drain and
source in the floating gate configuration. The pulses were applied according to the scheme depicted in
Figure 8.3(a), starting with –1.5 V and being gradually increased to –10 V.
For the first pulse amplitude of –1.5 V, an inverted satellite feature appears, peaking at around
s10 25.3− in the logarithmic time scale. This is attributed to the minority charge carrier trapping
[Zhao1990].
In order to confront bulky organic and the organic/metal interface contributions to the QTS signal, an
analysis of the QTS peak sign is given in the next section. The trap filling by minority charge carriers
is considered to be one of the mechanisms responsible for allowing the inversion of the QTS intensity
against the stressing potential.
The dynamic monitoring of the QTS signal during the experiment was achieved by recording the
average time per measurement. Therefore, the extracted 1τ determined by the correlated fitting
algorithm (Chap. 3) allows the temporal 1τ dependence illustrated in Figure 8.3(b) to be evaluated.
Each point of the panel (b) corresponds to the fitted spectra partially described in Figure 8.3(a) and
using the relationship defined in eq. (3.7). There is a fluctuating behavior of 1τ around the quasi-
equilibrium profile described by the red exponential decay.
-5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5
0
3
6
9
12
15
Pulse U1
-9.0
-7.5
-5.5
-5.0
-4.5-4.0
-3.0
-10.0
-1.5
-3.0
-2.5-2.5-2.0-1.5
log10( t / s)
Transitory QTS Spectrain Pentacene (30 nm) FETs
QTS
sig
nal Δ
Q /
pC
(a)
0 20 40 60 80 1000
10
20
30
40
50Transitory QTS trappingparameter ( τ
1 ) in
Pentacene (30 nm) FETs
τ1 /
μ s
< τ = 63 min >
time / min (b)
Figure 8.3. Transitory QTS spectra (a), and
evolution of the trapping time constant of
pentacene (30 nm) FETs (b).
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.9
The time constant 1τ is independent of the stressing pulse. The advantage of gradually increasing the
pulse amplitude is that the temporal QTS evolution can be identified more precisely. The average time
relaxation constant of about min63=>τ< (cf. Figure 8.3(a) ) indicates that 1τ develops towards
lower values and indicates the gradual ionization from shallow to deeper traps as long as the device is
pulsed. Therefore, one consequence in the working OFET device is the dynamics of the threshold
voltage [Gome2004].
This memory phenomenon influences the Id-Vd characteristics, promoting a correlated hierarchical
scattering of charge carriers. The first signature of the 1τ dynamics is the decrease of the effective
charge carrier density, inducing the so-called current collapse (sect. 8.4), where the channel
conductance becomes negative due to the charge-carrier optical phonon interaction [Pop2005].
Afterwards, the previous dynamics is followed by a continuous decrease in the drain current (sect. 5.5)
with positive channel conductance.
In order to recover the pristine QTS signals, the sample should be relaxed. This means it should be
without any stress bias potential, since polarity inversion of the former applied pulse does not fully
reverse the structure to the initial state.
Using the experimental data points of Figure 8.3(b) for 1τ , and the reported effective mass for
pentacene [Gill2003] of o* m7.1m = and o
* m5.5m = , the dynamics of the capture cross section
( σ ) can be estimated, as is effectively described in Figure 8.4. The results coincide with typical values
reported in similar molecular structures [Ménd2006, Thur2005a-b].
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.10
0 100 200 300 4000
3
6
9
12
15
effective mass m* = 1.7 mo
m* = 5.5 mo
σ
x 10
-17 c
m 2
time / min
Dynamics of the capture crosssection in Pentacene OFETs
Figure 8.4. Temporal evolution of the capture
cross section ( σ ) in bias-stressed OFETs with
floating gate configuration. The estimate of σ
was carried out by fitting the capture probability
1/1 τ shown in Figure 8.3 and assuming
reported charge carrier effective masses of
o* m7.1m = and o
* m5.5m = [Gill2003].
On the other hand, the time dependence of σ is confirmed by the drain current collapse and drain
current relaxation in OFETs. Another important feature of this finding is the scattering of the charge
carriers, which is an important factor in the differential conductance of the conductive-organic
channel.
The time constant >τ< found by the transitory QTS measurement coincides in order of magnitude
with those determined from dd V)t(I − measurements (chap. 5), and from the relaxation of the
Raman band after removing the externally applied electric field. Therefore, the underlying mechanism
is the structural modification of the molecular solid by external fields [Paez2003].
8.3.3. Single-channel OFET devices
In order to skip multi-interfacial configurations, the QTS measurements of the single channel OFET
gives more direct information of the organic/inorganic interface. Here the investigated structures have
a channel geometry with width (W) and length (L)a of m16m100LW μ×μ=× . Previous
investigations into similar structures with current-voltage measurements and monitored during a
period of about 9 months, revealed a dynamics of the threshold voltage with a decrease in the mobility
by about 3 orders [Pann2004].
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.11 In contrast to the interdigitated geometry, here the amplification of the transitory QTS discussed in the
previous section is lost; the advantage of this geometry is the improvement in probing more local
charge relaxations in the organic layer. The experiment was carried out between 140 K and 350 K at
intervals of 10 K ± 0.5 K; the resulting measurements for easier recognition are partially addressed in
Figure 8.5(a).
The QTS spectra basically exhibit two main features: one is related to the expected peak polarity in
agreement with the applied negative pulse (negative peaks), while the other feature corresponds to the
positive peaks. Here again, the "anomalous behavior of the QTS signal" is revealed. Mechanisms
promoting the inverted peak sign contribution come from trap filling by minority charge carriers and
induced artificial flaws, i.e., local dipole distribution in the organic material. The first contribution is
due to higher mobility distribution of the minor charge carriers, while the second one is mediated by
the induced electric dipoles promoted by the applied field. In chapters 6 and 7, it was demonstrated by
time-dependent current-voltage and vibrational relaxation experiments, respectively, that longer
relaxation time scales for the organic based device are also possible.
Previous investigations by capacitance DLTS in pentacene organic films assembled as an interlayer in
MIS capacitors with a n+-Si/SiO2/pentacene/Au structure revealed positive and negative C-DLTS
features. These indicate a contribution of majority and minority trapping centers [Yang2002].
The anomalous QTS signal is indirectly observable not only for small molecules, but has also been
proved for long polymer chains, despite the reported literature lacks of discussion about the
multiexponential dynamics and dipole bound states [Tane2005]. Particular signatures of the relaxing
drain current - not discussed in the publication - are its anomalous nature and substantial decay time
constants [Tane2005].
A simple transformation of the QTS spectra displayed in Figure 8.5(b) can restore the relaxed charge
signal as a function of time. In this way, the time dependence of the charge dynamics resembles the
behavior of the drain current as a function of time in organic-gas-sensor FETs [Tane2005].
In previous chapters, the long time scale relaxation processes in organic-based FETS have been
experimentally demonstrated. Measurements of the drain current with and without illumination, and
Raman bands after switching off the external field in OFETs have yielded time scales ranging from
seconds to several hours. These time scales are a strong indication to devise new setups with a
modified rate window concept in order to undertake DLTS characterization on a broader time scale.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.12
(a)
(b)
Figure 8.5. Anomalous behavior of
the QTS signal in pentacene-based
OFETs. (a) QTS measurements
performed on pentacene (30 nm)-
based FET. The sample was
stressed by a negative pulse of -
10V between the source and drain
in the floating gate configuration.
Similar anomalies have been
observed in resistance-DLTS (R-
DLTS) measurements on GaAs-
based MESFETs [Zhao1990], and
from capacitive-DLTS (C-DLTS)
in organic interlayers of MIS
capacitors with structure n+-
Si/SiO2/pentacene/Au [Yang2002].
(b) Charge relaxation QTS
spectrum measured at 293 K.
Figure 8.6 shows the activation energies obtained after fitting the anomalous QTS spectra presented in
Figure 8.5. Results indicate a distributed trap of states and some of them are already ionized at room
temperature. It can be observed that the tiny feature appearing at room temperature resembles the one
found in the interdigitated geometry. The other peaks indicate the organic/metal interface.
For a given applied pulse, a change in the QTS signal preserving the sign of the stressing bias is
expected. The measured spectra show apparently additional components with inverted signs,
indicating the influence of the minority charge carriers, which have lower effective mass and higher
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.13 mobility. Therefore, they are the most probable to fill/escape from the occupied traps after removing
the applied pulse.
Figure 8.6. Activation energies of
pentacene (30 nm) based FETs
determined from the anomalous
QTS spectra sketched in Figure 8.5
8.4. Approach to modeling of the "anomalous QTS signal"
What has been found is that in transitory conditions the probability of minor charge carriers being
trapped is higher; therefore, the anomalous peak appears in the QTS spectra. Another finding is the
formation of anion-dipole sites, which become dominant when the minimum dipole required to bind a
charge carrier is reached.
Let’s consider the charge dynamics in the form
∑ τ−=i
ii )/texp()t(Q a , (8.8)
the coefficients ia are related to the sign of the net charge involved in the process with time constant
iτ .
As an example, a charge dynamics phenomenon composed of three processes with 031 >= aa and
02 <a is assumed. A likely temporal dependence of the charge Q is shown in Figure 8.7(a); in a
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.14 similar manner, the current and capacitance can be obtained. It is noted that )t(Q should not be
strictly positive; it is allowed negative values or even intervals with 0)t(Q = . The QTS spectra are
displayed in Figure 8.7 (b), where the time constants are marked by arrows and correspond to the peak
maximum for positive components and to the minimum for negative contributions. The charge
dynamics described in Figure 8.5(b) were determined with similar multiexponential kinetics.
Figure 8.7. Simulation of multiexponential kinetics of charge relaxation (a) and the corresponding
QTS spectrum indicating the relaxation time constants (b).
A partial experimental and theoretical description of the anomalous QTS signal was published by us
[Thur2006]. Some key features and extended insights into the model were given in the previous
sections and complemented in this paragraph.
As soon as an electric field ( )t(E ) is applied, the initial number of dipoles is modified. In linear
approximation, then,
)t(E)T,t()T,t(td
)T,t(dsidip
i χ=μ+τμ
, (8.9)
here the term )T,t(iμ means the average value of dipoles oriented along the "i" direction, T is
temperature, and )T,t(χ is the molecular susceptibility.
Introducing tx = , )T,t(y iμ= , 1dip)x(p −τ= , and )t(E)x(s sχ= , one has to solve a differential
equation of the type
( ) )x(syxp'y =+ . (8.10)
The initial internal electric field oE)t(E = is determined from the space charge sheet oσ and is
defined as roo / εεσ . The relaxation of the local field is assumed to be described as
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.15
)/xexp(E)x(E Do τ−= , with Dτ the dielectric relaxation time constant. Then the solution to eq.
(8.9) takes the form
( )⎥⎥⎦
⎤
⎢⎢⎣
⎡+
τχ
τ−= Caxexpa
E)/xexp(y
dip
0sdip , (8.11)
with )(a 1D
1dip
−− τ−τ= and C being a constant.
Considering the initial condition 0~)0(y i =>μ=< due to the in-plane anisotropy (parallel to the gate
dielectric substrate) molecular distribution, then a/EC dipos τχ−= . The DFT calculations
demonstrate that the single pentacene molecule has a non-zero electric dipole moment in equilibrium
conditions, as depicted in Figure 8.8.
Figure 8.8. Effective electric dipole
in pentacene. The calculations were
performed in Gaussian 98
[Gaus1998] and at the B3LYP/3-21G
level with and without an electric
field (dipole in D).
In this way, one obtains the time-dependent dipole moment )t(μ
( ) ( ) ( )[ ]dipDDdip
os /texp(/texp/1
Et τ−−τ−
ττ−χ
=μ , (8.12)
when simulating the QTS response component. Due to the polarization, the susceptibility is replaced
by the Langevin-Debye formula [Comp2002] and multiplied by the initial local electric field oE ;
therefore, the saturated polarization is given by
0e
2dip
os EkT3
~NE~
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛α+
μ=χ=μ
∞, (8.13)
where k Boltzmann’s constant (1.38x10-23 JK-1), ]m[N 3dip
− is the concentration of dipoles, eα is the
static electronic polarizability, and the dipole moment is in C-m.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.16
For non-interacting harmonic oscillators and expanding 2~μ in terms of the normal coordinates, one
finds a linear proportionality between 2~μ and the temperature. Finally, eq.(8.12) reads
( ) ( ) ( )[ ]dipDedip
Ddip
o /texp(/texpk3
NB/1
Et τ−−τ−⎟⎟
⎠
⎞⎜⎜⎝
⎛α+
ττ−=μ , (8.14)
with B a constant to be determined.
In order to recover the charge dynamics given in eq.(8.8), one can readily describe the dipole in terms
of a center of dipole and then use the expression for the QTS signal addressed in chapter 3. The
simulation of the dipolar contribution to the QTS spectrum is described in Figure 8.9.
Figure 8.9. Simulation of the dipolar
contribution to the QTS spectrum for
three different dielectric relaxation
constants and one dipolar relaxation.
The dielectric relaxation of the local field was assumed to be a single Debye-like decay. As has been
seen in complementary current-voltage and Raman measurements, there are still contributions by
much lower time-relaxation rates. Despite the QTS measurements in a set-up with a limited rate
window range, the temporal envelope decay described in Figure8.3(b) predicts the existence of
features outside the minimum available rate window. Another interesting result is given by eq.(8.9),
which represents a key link between the vibronic and QTS measurements, predicting electrical and
vibronic signal detection in the lower and higher time-relaxation scales.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.17
8.5. Current collapse
8.5.1. Negative conductance and photodetachment
The I-V measurements done on previously unstressed samples exhibit negative behavior in the
channel transconductance (cf. Figure 8.8) This has been evidenced by the character of the temporal
variation of the Id-Vd characteristics, also known as current collapse [Klei2003]. The effect has been
observed in similar organic-based structures but not discussed [Hepp2003, Panz2005]; reports on
inorganic-based structures discussed these sorts of characteristics in more detail, indicating
phenomena involving a thermal-electric effect and leading to the power loss of the device [Paul1994].
Some authors argue a reversibility when the incident photon energy
exceeds the AlGaAs band gap near 1.8 eV [Klei2003], contrary to OFETs, where the effect of
illumination plays a selection role (sec. 6.3), i.e., the negative conductance remains even if the channel
is illuminated by some photon energies higher than the band-gap. For comparison, the drain current
characteristics for some intermediate photon energies are displayed in Figure 8.10(b). It can be
observed that the most efficient photodetachment process is achieved with photon energies of about
2.40 eV (cf. Figure 8.10(c)).
Recent investigations of molecular quantum dots with metallic contact electrodes and suspended
single-wall carbon nanotubes (SWCNTs) have shown a highly negative differential conductance
[Peng2006, Pop2005]. For the first system [Peng2006], the authors considered the Schottky barrier
formation at the metal/carbon nanotube to be responsible for the negative differential conductance,
while for the molecular quantum dots, polaronic effects mediated by the electron-phonon coupling
were identified. The latter proposal applies better to the output characteristics described in Figure 8.10
(a), since the vibronic-charge carrier interaction has been proved to be one of the signatures of current
relaxation. This permits determination of the loss of power of the device as proposed elsewhere for
metal-oxide FETs (MOSFETs) [Paul1994]. The Id(t)-Vd(t) measurements indicate that the condition
0d <g is transitory and this state remains for no longer than 5 minutes, despite the continuous decrease
of the drain current at a given drain voltage.
The time scale of the first Id-Vd during the experiments coincides with the time of measuring the QTS
bottom-most spectrum in Figure 8.3(a), taken at U1 = -1.5 V. This peak, presenting a maximum
distribution at 3.3)s/t(log10 −≈ ( )Tk/Eexp( BtΔ∝τ ), did not appear when the device was biased
again by the same pulse. The phenomenon is associated with a fast trap filling due to high mobility of
minority charge carriers.
Additionally, one has to take into account the artificial traps mediated by dipole-anion states which
can relax slower than the detrapping and trapping dynamics, thus hampering the restoration of the
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.18 initial condition stated for the bottom-most spectrum of Figure 8.3(a). In order to restore the initial
condition prior to the next pulsed biasing, the device was left to relax for about 8 h.
(a) (c)
Figure 8.10. Experimental evidence of "drain
current collapse" in OFETs, and negative
differential conductance ( 0g d < ) in OFETs.
Measurements in (a) darkness, and (b) under
illumination. (c) Drain current intensity as a
function of the excitation photon energy. The
photodetachment is more efficient with photon
energy of about 2.40 eV.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.19
8.5.1.1. Charge conservation and photodetachment
After applying a bias to the structures, the drain current drops temporarily, the reduction being an
indication of relaxation effects in the structure. Usually the current density is given as the transported
charge per unit time (I = dq/dt). If the stressing electric field and the sample conditions remain the
same, then the current intensity will remain constant and then the charge flow is not modified.
Therefore, in a given time interval, the charge in a control volume is zero. Now the variation of charge
in terms of the current density can be defined:
∫ Δ=Δ→Δ
tdIlimQ0t
. (8.15)
This means that the current is evaluated twice and the difference will tell us about the modification of
charge flow in the control volume. In ideal situations 0Q =Δ .
The modification of QΔ does not by any means imply that the charge conservation is violated. On
the contrary, within the framework of this research, the artificial flaw formation was considered by
applying external fields. Accordingly, if there are dipoles in the control volume, there is a probability
that at the output the number of particles is lower, leading to 0Q ≠Δ .
8.6. Photodetachment
The electron affinities of dipole-bound anions can also be inferred by detaching the excess electron via
application of an external electric field [Comp2001, Desf1994-1996, Hamm2003] or by
photodetachment [Cher2005, Dike2004, Hamm2004, Rau1971, Sind2004] i.e., by applying well-
defined photon energies to the structure Figure 8.7(c). During illumination, matrix elements are
formed which contribute to the photodetachment cross section [Cher2005, Rau1971, Sind2004] and
are proportional to the square matrix element of the dipole operator μ~ .
The differential cross section for an electron transition from the bound state χηλ to k , due to
interaction with a photon with frequency ω and electrically polarized along ε is
( ) ( ) kkk Ω⋅χηλπ
ω=ω→χηλσε dˆ~
c2km;d
2*
εμh
, (8.16)
with kΩ the solid angle subtended by the scattered electrons of effective mass *m . Following
Chernov [Cher2005] and doing additional algebra on the cross section, one can readily find the
effective cross section for the artificial traps in organic electronic devices by complementing with the
trap time constant
h1
1 Nk=τ − . (8.17)
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.20 As far as the Id recovery after illumination is concerned, it is merely a first order effect. This does not
mean that the interaction of photons with the traps in organic-based transistorsresults in the total
recovery of the charge transported in the channel, as stated by some authors for organic solids
[Stre2004] and inorganic-based transistors [Klei2003]. In the work of Street and colleagues
[Stre2004], long-term recovery of the devices in about two days was observed. The bipolaron model
can be extended, considering an additional charge trapping effect. The organic layer is divided into
two regions, one connected with the channel OBL and the second with the rest of the organic material.
This view can be utilized when admitting a polaronic interaction between the two regions, leading to
the phenomena already observed: bi-exponential decay of the drain current (chap. 6), negative
conductance, and the dynamics of the capture cross section. The effect can be explained in the
framework of slow carrier trapping, implying states with small and/or varying capture cross sections,
as experimentally found in the previous sections. The short time scales below 10-1 s, in which the QTS
measurements were done, might be explained by considering faster effects like first-order dipole
formation in the organic material. Additionally, both vibrational (Raman) and electrical
characterization techniques (Id(t)-V and QTS) give different time scales, prompting us to consider a
polaronic distribution in either time or energy.
Conclusions
The pentacene molecule possesses a natural tendency to form artificial traps induced by external
applied fields, in this way forming (cation, anion)-dipole sites with an identity similar to polaronic
systems, reducing the number of charge carriers transported through the channel. These traps can be
partially removed by applying well-defined photon energies.
The bulky features are considerably enhanced in OFETs with interdigitated source-drain electrodes,
while the interfacial contributions are much better observed in single channel devices. The
measurements performed in OFETs with interdigitated source-drain contacts considerably hinder the
QTS signal coming from the monolayers in contact with or close to (less than 10 nm as determined by
RSS) the metal electrode. For this reason, the device is more suitable to enhance bulky contributions
and the minority charge carrier dynamics participating in the detrapping and trapping filling process.
The complementary measurements on the single channel device allow for more scoping of the metal /
organic interface than the bulk contribution in comparison with the former device. This conclusion is
based on a simple picture of the metal/organic band diagram with two electrodes. It is found that the
difference in enhancement is about 103 which becomes appreciable when sensing the QTS signal in
the range from 10-6 s to 0.1 s.
Chapter 8 B. A. Paez-Sierra, Trap distribution in OFETs… 8.21 The computed capture cross sections explain the threshold voltage shift of the working device.
The complementary photodetachment measurements with selected photon energies highlight the fact
that the channel conductance can be modulated to enable freezing or the dynamics of the charge
carrier capture cross section. Finally, in table 8.1, the main time scales of the organic channel in
OFETs involving vibrational-, charge transport, and trap-dynamics are summarized.
Table 8.1. Time-dependent phenomena in OFETs determined by complementary experimental
techniques (based on the setups used in this research).
Raman Current-Voltage DLTS
Time scale 20 min – days 1s – 59 min or days s1 μ - s1 and temporal averages
of hours
Involved
molecular and/or
transport
property
Structural, slow
polaronic effects.
Structural, internal molecular
domains. Current collapse,
negative differential
conductance
Traps, fast polaronic effect,
influence of free charge carriers
and dipoles (anomalous
response), charge carriers hot
optical phonons scattering
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Chapter 9 B. A. Paez-Sierra, Summary
9.1
Chapter 9
Summary
Within the framework of the present research, the formation of top metal contacts, consisting of
indium and magnesium deposited onto two Perylene derivatives, 3,4,9,10-perylene tetra-carboxylic
dianhydride (PTCDA) and N, N' dimethyl-3,4,9,10-perylene tetracarboxylic diimide (DiMe-
PTCDI), were investigated in situ and under UHV conditions by resonant Raman spectroscopy. The
metal/organic structures were assembled on sulphur-passivated Si-doped GasAs(100) substrates (S-
GaAs). The experimental results have proved that all metals deposited onto the organic layers of
PTCDA or DiMe-PTCDI promote enhancement of the normally Raman-active internal vibrational
mode intensities, accompanied by the activation of normally infrared-active modes. The promotion
phenomenon is called surface enhancement Raman spectroscopy (SERS).
The magnification of the scattering Raman cross section is basically due to two factors. The first one
is associated with the close proximity of the molecules and the metal atoms, realizing in this way
activation of former non-Raman active modes assisted by the metal-induced molecular distortion. This
signature is also referred to as chemical enhancement, because an interface formation at the level of
organic-metallic mono-layers is achieved. The second factor is the enhancement achieved by the huge
local electric field developed at the metal clusters and extending towards the organic film.
Consequently, further electronic and nuclear distortions appear on the molecular side, leading to
enhancement of the molecular dipole and alterations of the polarizability. In this context, the
phenomenon is understood as a long-range interaction. It has been shown that metal coverage of
several nanometers of about 40 nm or above still allow the identification of vibrational bands, taking
into account the roughness of the metallic layer. This contribution to the SERS effect is referred to as
electromagnetic enhancement. The enhancement factors are estimated to be 101-102 for the chemical
contribution or first monolayer effect and 104 for the electromagnetic contribution. Contrary to
standard SERS investigations, where the molecules are deposited on metal substrates, or single
molecules embedded in metal-like solvent media, here the metal atoms were deposited onto the
organic structure, resulting in a more efficient enhancement of the related vibrational features at the
interface. The break-down of the selection rules is characteristic of molecules in the very near vicinity
of a metal surface and can be induced by several mechanisms: structural deformation of the molecule,
charge transfer from the molecule into the metal or vice versa, or formation of new chemical bonds.
Thus the spectral changes induced by SERS can be used to extract information about the chemical
reactions at the interface, as well as the morphology of the metal film.
Chapter 9 B. A. Paez-Sierra, Summary
9.2
Indium deposition onto PTCDA and DiMe-PTCDI caused only molecular distortion, with a
remarkable metal diffusion into In/PTCDA structures, as demonstrated for the metal coverage on
organic monolayers and onto thicker (15 nm) films. The phenomenon was much less obvious in
DiMe-PTCDI thin films.
The deposition of Mg on both perylene derivatives was accompanied by much lower diffusion of the
metal into the organic layers compared to In, as demonstrated by the preservation of the external
modes upon metal coverage. The Mg/PTCDA structures are formed in two stages; one is the formation
of a new molecular structure at the interface, which continues until a nominal metal coverage of about
2.8 nm is reached, which is attributed to the removal of the oxygen atom from in the anhydride groups,
and the second one is surface-enhanced Raman spectra of the former structure by further depositions
of Mg.
In the case of Mg/DiMe-PTCDI, it has been found that the molecule exhibits a break-down of
selection rules as well, and there is no formation of new molecular species, contrary to the case with
Mg/PTCDA interfaces. This structure is characterized by the coupling between discrete molecular
states of the organic DiMe-PTCDI material and the electronic continuum states of the Mg metal
contact. The phenomenon was observed by the asymmetric broadening of the modes at 221 cm-1, 1291
cm-1 and 1606 cm-1 upon the metal deposition. These features amount to energy gap states above the
HOMO of 30 meV, 160 meV and 200 meV, respectively. Their corresponding energy line-shapes are
well described by the Breit-Wigner-Fano function. The structure is well preserved by identification of
the vibrational band of the imide groups.
The investigations on the previous heterostructures helped to experimentally analyze the channel
formation of pentacene-based field effect transistors. The organic channel was formed gradually by
molecular beam deposition of the organic molecules under UHV conditions, and at an evaporation rate
of 0.65 Å/min. After each deposition of the organic molecule (82 times), the resulting interlayer was
vibrationally and electrically characterized in situ in order to determine the minimum nominal
thickness of the organic material required for efficient charge transport through the OFET channel.
At a thickness of around 1.5 nm nominal coverage (1.1 ML), the first percolation paths through the
first organic monolayer develop, resulting in a sharp rise in the drain current. Up to a nominal film
thickness of about 32 nm, a subsequent slower increase of the drain current can be observed,
revealing that the percolation of the first monolayer continues at a slower pace up to thick organic
layers. The extracted areas of the decomposed vibrational spectra peaks as functions of the layer
thickness undergo a four-stage process with a well-defined staircase-like tendency, where the first step
is extending to about 10 nm, the second one going on up to 20 nm, the third one to 25 and the last one
to 32 nm or above.
Chapter 9 B. A. Paez-Sierra, Summary
9.3
This thickness dependence is closely related to the layer compactness; therefore, the thicker the layer
the smaller the terrace-like feature. The earlier organic coverages below 1.5 nm (1.1 ML) are seen to
be composed of isolated molecular structures, since no drain current was observed, while for the
thicker layers below 5 nm the channel becomes compact and the highest contribution to the
transported charge through the channel is reached. Above the quoted first-stage thickness, the
contribution of the overlayers to the current is not significant and their compactness is reduced.
These two correlations indicate that the charge induced by the gate voltage is initially confined to the
first compact channel with a thickness of about 1.5 nm.
Signatures of the first monolayers are well defined in comparison to thicker films, where the 1.1 ML
exhibits a strained nature due to its direct contact with the gate insulator, resembling in this way
inorganic-based transistors with a channel formed by a strained material at the gated dielectric and
followed by a graded active material towards outer layers away from the insulator, with recovery of
the bulk-structural nature.
It was found that the conductive channel, here referred to as –the Organic Boosting Layer (OBL)
exhibits a compressive deformation, demonstrating the phenomenon of strain due to the pristine
coverage in closest contact to the inorganic substrate. This type of deformation is related to an increase
of the atomic bond strength, as was proved by comparison of the 1.1 ML coverage (1.5 nm) with the
thicker pentacene films (32 nm).
The output characteristics of the OFETs were measured after the final coverage. It was found that the
drain current undergoes a relaxation process with two decay time constants, one in the order of 101
min and the other below 102 min. A similar experiment, involving illumination of the channel with a
676.4 nm laser source, enhances the drain current density, leaving the relaxation time constants
unmodified. Additionally, the devices were characterized ex situ by charge deep-level transient
spectroscopy QTS. The spectra showed positive and negative peaks of the relaxed charge with respect
to the unique bias pulse polarity. The phenomenon has been called "anomalous behavior of the Q-
DLTS signal" and seen for the first time in OFETs. Consecutive QTS measurements of the device over
a period of several hours showed a slow relaxation of the anomalous detrapping time constant
around s5 μ ; its temporal envelope function decayed with a time constant of ca. 63 min < 102 min.
Based on these results and assuming the effective masses o* m7.1m = and o
* m5.5m = for
pentacene, the capture cross sections were 217 cm10]15;1[ −× and 217 cm10]5.1;1[ −× for the smaller
and bigger effective masses, respectively. Additionally, calculations of the two-dimensional charge
density were performed in an algorithm based on the Gauss quadrature and developed in the frame of
this thesis. This computing procedure is of great advantage to solve the Fermi-Dirac integrals without
significant approximations.
Chapter 9 B. A. Paez-Sierra, Summary
9.4
Further investigations of the OFETs with a combination of Raman spectroscopy and applied electric
fields revealed magnification of the Raman cross section comparable to the chemical enhancement in
experiments based on SERS upon In or Mg deposition onto the perylene derivatives PTCDA and
DiMe-PTCDI. The Raman intensity was found to be proportional to the applied field followed by a
saturation and then by a decrease. The Raman bands were monitored after switching off the electric
field, pointing to a structural relaxation time constant of about 94 min.
Based on the time-dependent current-voltage characteristics and QTS measurements, the phenomenon
is interpreted as artificial generation of dipoles acting as traps for the charge carriers. Therefore, the
total number of traps is due to the existing defects at the interface and those induced by the applied
electric field in the organic material. This formulation was extensively explored and theoretically
substantiated in this work.
The reported experiments on the formation of organic/metal heterostructures and the combined Raman
spectroscopy with electrical characterization of OFETs suggest that refinements of the transport
models in organic-based structures should be pursued.
B. A. Paez-Sierra, List of figures 10.1
List of figures Figure 1.1. Organic materials forming different interfaces, i.e.metal / organic semiconductor/
inorganic semiconductor heterostructure.
1.3
Figure 2.1. Some representative molecules of the arene family. (a) APentacene and (b) Perylene
derivatives. (*) Main structures investigated in the present research.
2.1
Figure 2.2. σ−hybrids and π−molecular orbitals in benzene. a) Localized σ−orbitals, b) pz atomic
orbital, and c) delocalised π−orbitals with highest densities above and below the plane ring.
(Thanks to G. Gavrila).
2.2
Figure 2.3. Molecular states of an organic solid [Ishii1999, Ménd2006, Pire1974-1984]. 2.3
Figure 2.4. Spectrum of scattered light showing the Raman Stokes, Rayleigh, and Raman anti-
Stokes bands.
2.8
Figure 2.5. Molecular Morse potential of the (a) ground electronic and bound vibrational states.
(b) Infrared activity described by transitions between vibrational states at a given electronic state
mljneE .
2.11
Figure 2.6. Non-resonant Raman effect involving (a) elastic light scattering or Rayleigh
scattering, and the inelastic processes: (b) Stokes scattering with the scattered photon energy
higher than the incoming one, and (c) anti-Stokes where the scattered photon possesses lower
energy than that of the excitation source.
2.12
Figure 2.7. Resonant Raman effect (a) vibronic transitions pointing to the Raman-Stokes,
Raman-anti-Stokes, and Rayleigh scattering.
2.13
Figure 2.8. Examples of SERS from a (a) pentacene (30 nm)-based OFET, and a (b) In (15 nm)/
PTCDA (15 nm)/S-GaAs heterostructure.
2.15
Figure 3.1. Experimental setup for combined Raman spectroscopy, scanning Raman
spectroscopy, and current-voltage (I-V) characteristics measurements.
3.2
Figure 3.2. Substrate passivation: ex situ chemical treatment and in situ annealing and material
deposition.
3.3
Figure 3.3. Molecular structure of perylene derivatives with the associated symmetry group of
(a) PTCDA and (b) DiMe-PTCDI [Kobi2004, Salv2003]. (c) UV-vis absorption spectra of
organic layers deposited on quartz.
3.5
Figure 3.4. Molecular structure of (a) pentacene (C22H14) and its 102 internal vibrational modes
divided into Raman active, IR active, and silent bands, belonging to the D2h symmetry group
[Ross2002], (b) Fullerene C60, which belongs to the symmetry group of the truncated
icosahedron [Kost1994]. (c), (d) Absorption spectrum of a 30 nm pentacene and 30 nm C60 film
deposited on quartz, respectively [Kolo2005]. The excitation energies for resonant Raman
spectroscopy measurements are indicated on the spectra.
3.7
B. A. Paez-Sierra, List of figures 10.2 Figure 3.5. Field effect structures used for the fabrication of organic-based field effect
transistors (OFETs). (a) Interdigitated structures and (b) single channel structures.
3.8
Figure 3.6. Field effect structure and formation of the channel by organic molecules of
pentacene or C60. The polarity of the Vg and Vd depends on the charge carrier type i.e. n or p
(chapter 5).
3.9
Figure 3.7. Band diagram for a semiconductor with a single deep level trap (recombination
processes between HOMO-LUMO or intermediate states are excluded).
3.10
Figure 3.8. Charge transient spectroscopy (QTS) based on the rate window concept. (a) Sample
wiring, and (b) applied bias pulse. (c) Triple boxcar integrator and; (d) output signal displayed
as a function of rate window time, the QTS maximum coincides with the relaxation time
constant τ of the trap (see sample spectra below for three different time constants) [Thur1994,
2005-2006].
3.13
Figure 3.9. Normalized QTS responses to three exponential decays with different time constants
τ are peaking when the processing starts at t1 = τ. It should be noted that both the height ΔQm
and the FWHM are invariant against τ; approximation of the fastest charge relaxation by a
Gaussian is shown by circles; w* stands for the variance of the Gaussian.
3.14
Figure 3.10. Flux diagram of the correlated fitting algorithm. 3.16
Figure 4.1. Raman spectra of In (5nm), Ag (4.5 nm) and Mg (5 nm) coverages on 15 nm thick
PTCDA films, compared with the spectrum of the bare PTCDA film in the spectral region of the
internal breathing mode (left) and in the spectral region of HC− deformation and C=C
stretching modes (right). (The Raman spectra involving Ag do not belong to this work, and are
addressed elsewhere [Salv2003]. They are presented here for comparison of metal contact
formation on similar molecular structures).
4.4
Figure 4.2. Raman spectra of In (5nm), Ag (4.5 nm) and Mg (6 nm) coverages on 15 nm thick
DiMe-PTCDI films, compared with the spectrum of the bare DiMe-PTCDI film.
4.6
Figure 4.3. Enhancement factors of the Bu mode (1243 cm-1 in PTCDA and 1246 cm-1 in DiMe-
PTCDI) and of the C-C stretch Ag mode (1572 cm-1 in PTCDA and 1570 cm-1 in DiMe-PTCDI)
for PTCDA (a), and DiMe-PTCDI (right) as a function of the metal coverage (b).
4.7
Figure 4.4. AFM topographic images of a 30 nm thick In film on PTCDA. (a) (right part
showing PTCDA covered by In clusters) and of a 113 nm thick Mg film on PTCDA (b).
4.8
Figure 4.5. Spectra of external Raman modes from 15 nm thick PTCDA films capped with 0.4
nm metal layers, i.e., Ag, In, and Mg. The spectral Raman shift between 25 cm-1 and 125 cm-1
corresponds to the libronic or collective modes of the interacting molecules in the unit cell
[Salv2003].
4.10
B. A. Paez-Sierra, List of figures 10.3 Figure 4.6. Raman monitoring in the external mode region upon metal deposition: (a) Ag, (b)
Mg, (c) In. The experimental spectra are shown by open symbols and the fitted spectra by red
lines. The Lorentzian functions used for the fitting of the Raman spectrum of the pure PTCDA
film are shown by lines in the lower parts of the figures. The spectra of Ag/PTCDA are
normalized for a better resolution of the phonons.
4.11
Figure 4.7. Evolution of the FWHM of the external mode at
41 cm-1 as a function of the metal coverage relative to the initial values before the metal (Mg,
Ag) deposition. The dashed lines are visual guides.
4.12
Figure 4.8. Raman spectra of Mg/DiMePTCDI in the region of: (a) external modes and the
breathing molecular vibration mode. (b) C-C and C-H modes. The spectra in (a) and (b) are
normalized with respect to the intensity of the breathing mode and to that of the C-C stretching
mode at 1570 cm-1, respectively. An asymmetric broadening develops for the three modes
marked with stars upon the Mg deposition.
4.14
Figure 4.9. Comparison between the Raman spectra of bare DiMe-PTCDI, Mg (2.8 nm) / DiMe-
PTCDI and the IR for the organic
4.15
Figure 4.10. Fitted Raman spectra of Mg / DiMe-PTCDI: from bottom to top: bare 15 nm
DiMe-PTCDI covered with 34 nm Mg and 122 nm Mg. The peaks fitted with the BWF function
are represented by thick black lines.
4.16
Figure 4.11. Energy level alignment of the DiMe-PTCDI / Mg heterostructure. The Fano
resonances indicated in the band diagram were obtained from the resonant Raman
measurements, while the other energy levels were quoted from NEXAFS spectroscopy
measurements on a similar sample [Gavr2006].
4.17
Figure 5.1. Scheme of a field effect transistor. The drain and source terminals serve to drive the
modulated current through the channel shown in blue color. The charge in the channel is
modulated by the capacitive effect of the third (isolated) terminal named gate.
5.2
Figure 5.2. Argument of the Fermi-Dirac integral as a function of the reduced energy ε , and the
reduced chemical potential η (evaluated in MatLab [MATL2003]). Orders (j) of the arguments
(a) 21 , (b) 2
1− , and (c) 0.
5.5
Figure 5.3. Charge density distribution as a function of the reduced chemical potential and
reduced transfer integral. The effective masses for pentacene are o* m7.1m = and
o* m5.5m = [Wijs2003]
5.6
Figure 5.4. Triclinic crystal structure to estimate the number of molecules per 2cm . 5.7
Figure 5.5. Organic field effect transistor 5.8
B. A. Paez-Sierra, List of figures 10.4 Figure 5.6. Energy band diagram between the gate and the organic film with an isolating
interlayer, (a) under equilibrium conditions. (b) Accumulation mode of the organic field energy
bands for negative and (c) positive (c) gate voltages ( gV ) respectively. (d) Band diagram
between the organic and the contacts drain and source (UDS = 0 V).
5.9
Figure 5.7. Output characteristics of a field-effect transistor and maximum drain current where
the saturation starts. (a) Simulation of an ideal FET and (b) experimental drain current - drain
voltage of a pentacene (30 nm)-based FET; the solid lines correspond to the fitted output
characteristics.
5.13
Figure 5.8. Field effect mobility in a pentacene (30 nm)-based FET. The values (half filled
circles) were extracted from the output characteristics, while the solid curve corresponds to a
power law with the applied gate field.
5.14
Figure 5.9. Threshold voltage of a single channel pentacene (30 nm)-based FET. The values
were extracted after fitting the output characteristics depicted in Figure 5.8(b)
5.17
Figure 6.1. Simultaneous monitoring of Raman bands (black spectra) and drain current (red
curve on the right hand side of the vertical plane) during the pentacene deposition. The pointed
Raman bands correspond to the in-plane C-C ring and in-plane C-H vibrations.
6.4
Figure 6.2. Extracted intermediate Raman spectra of pentacene based OFETs at different
organic layer thicknesses, together with the corresponding fitting line-shapes (a); and
comparison between the area of the Raman band at 1179 cm-1 and the drain current as a function
of the organic film thickness (b).
6.5
Figure 6.3. Fitting parameters in dependence on the molecular layer thickness in pentacene-
based OFETs (a) Raman band intensities and (b) FWHM broadening.
6.7
Figure 6.4. In situ Raman band measurements of pentacene (1.5 nm) forming different
interfaces when deposited on Au and SiO2 substrates.
6.8
Figure 6.5. Principal regions of the organic layer forming the channel in OFETs 6.10
Figure 6.6. Charge density distribution induced in the organic layers by applying different gate
voltages. (a) Monolayer stratification and (b) dependence of the charge density as a function of
the layer location parallel to the gated dielectric with the gate voltage as parameter.
6.12
Figure 6.7. Raman spectra of the organic/inorganic interface. (a) Sketch of the swept interface in
pentacene-based OFETs, (b) 3D plot of the Raman signal as a function of the vibronic bands
and spatial position in the organic/inorganic interface, and (c) Profile of the Raman intensity
(middle up half filled circles) and derivative (middle down filled circles) as a function of
scanning position.
6.13
Figure 6.8. Fitted FWHM of the Raman bands at different spot scanning positions (a), and inset
of the FWHM beneath the organic/inorganic interface (b).
6.14
B. A. Paez-Sierra, List of figures 10.5 Figure 6.9. Effect of bias stress on the dd V)t(I − characteristics of OFETs (a) in darkness and
(b) under illumination. The red contour lines indicate the time profile behavior of the Id current
for a fixed Vd voltage.
6.15
Figure 6.10. Simulation of one of the dd V)t(I − characteristics shown in Figure 6.9.
Experimental curves are plotted with circles. The quantities Io, A1, A2, τ1, τ2, were determined
from the red lineprofiles shown in Figure 6.9. To reproduce the experimental curve here
(uppermost curve), the time t related to the drain sweep rate of 10 V/min.
6.17
Figure 7.1. Benzene molecule under different external electric fields. (a) Molecular structure,
(b) chart summarizing the band gap modification by the applied fields, and (c) HOMO and
LUMO configuration in response to the applied electric fields. (The unit H in the table means
Hartree = 27.2 eV).
7.3
Figure 7.2. DFT simulation of the influence of an applied electric field on the molecular band
gap and HOMO-LUMO states in pentacene. (The unit H in the table means Hartree = 27.2 eV).
7.4
Figure 7.3. Calculated vibrational states of a pentacene molecule under the influence of external
electric fields. (a) Orientation of the pentacene molecule, and (b) variation of the frequency
assignment as a function of the vibrations without an applied electric field.
7.6
Figure 7.4. Experimental measurements of the Raman bands in pentacene (30 nm) for different
applied fields. The involved modes of this spectral region belong to in-plane vibrations of C-H
bonds of the outer ring and antiphase vibrations of the C-H bonds parallel to the main axis of
the molecule.
7.7
Figure 7.5. Correlated fitting of the Raman intensity as a function of the applied gate voltage
(the digits behind the floating point were delivered after the correlated fitting procedure).
7.8
Figure 7.6. In situ measurements of the Raman bands relaxation after switching off the electric
field, and (b) time decay of the Ag band at 1179 cm-1 after switching off the electric field.
7.9
Figure 7.7. Raman spectra (upper plot) and differential Raman spectra (lower plot) of 3 nm C60
layer, the latter obtained by subtracting the spectra without an electric field from the ones under
the influence of the gate voltage.
7.10
Figure 7.8. Photocurrent of pentacene (30 nm) based FET. The spectra were taken for a drain
voltage of –40 V and gate voltage of 0 V (blue) and –10 V (green). The red spectrum
corresponds to the absorption of 30 nm pentacene deposited on quartz.
7.11
Figure 7.9. Persistent behavior of the drain current in pentacene (30 nm)-based FET after
switching off the photon source. The decay follows multi-exponential kinetics.
7.12
Figure 7.10. Photocurrent and dark current in 30 nm pentacene FET. The sample was
illuminated by coherent light of 676.4 nm.
7.13
B. A. Paez-Sierra, List of figures 10.6 Figure 8.1. Schematic view of combined artificial flaws or traps induced by the external electric
field dipole, and intermolecular potential acting on the two-dimensional charge carrier gas. The
z coordinate gives the strength of the potentials, while the xy plane correponds to spatial
coordinates
8.4
Figure 8.2. Particle subjected to electric dipole potential. 8.5
Figure 8.3. Transitory QTS spectra (a), and evolution of the trapping time constant of pentacene
(30 nm) FETs (b).
8.8
Figure 8.4. Temporal evolution of the capture cross section (σ ) in bias-stressed OFETs with
floating gate configuration. The estimate of σ was carried out by fitting the capture probability
1/1 τ shown in Figure 8.3 and assuming reported charge carrier effective masses of
o* m7.1m = and o
* m5.5m = [Gill2003].
8.10
Figure 8.5. Anomalous behavior of the QTS signal in pentacene-based OFETs. (a) QTS
measurements performed on pentacene (30 nm)-based FET. The sample was stressed by a
negative pulse of -10V between the source and drain in the floating gate configuration. Similar
anomalies have been observed in resistance-DLTS (R-DLTS) measurements on GaAs-based
MESFETs [Zhao1990], and from capacitive-DLTS (C-DLTS) in organic interlayers of MIS
capacitors with structure n+-Si/SiO2/pentacene/Au [Yang2002].
(b) Charge relaxation QTS spectrum measured at 293 K.
8.12
Figure 8.6. Activation energies of pentacene (30 nm) based FETs determined from the
anomalous QTS spectra sketched in Figure 8.5
8.13
Figure 8.7. Simulation of multiexponential kinetics of charge relaxation (a) and the
corresponding QTS spectrum indicating the relaxation time constants (b).
8.14
Figure 8.8. Effective electric dipole in pentacene. The calculations were performed in Gaussian
98 [Gaus1998] and at the B3LYP/3-21G level with and without an electric field (dipole in D).
8.15
Figure 8.9. Simulation of the dipolar contribution to the QTS spectrum for three different
dielectric relaxation constants and one dipolar relaxation.
8.16
Figure 8.10. Experimental evidence of "drain current collapse" in OFETs, and negative
differential conductance ( 0g d < ) in OFETs. Measurements in (a) darkness, and (b) under
illumination. (c) Drain current intensity as a function of the excitation photon energy. The
photodetachment is more efficient with photon energy of about 2.40 eV.
8.18
B. A. Paez-Sierra, List of tables 10.7
List of tables Table 2.1. Parameters of the electron-phonon coupling in pentacene-based FETs. 2.5
Table 4.1. Skin depth of smooth metallic films, apparent penetration depth of 488 nm light in In,
Ag and Mg films grown on DiMe-PTCDI and PTCDA layers
4.9
Table 5.1. Parameters reported for the Fermi-Dirac integral [Blak1987] 5.5
Table 5.2. Lattice constants of the triclinic cell for pentacene with cell parameters °≈α 978.76 ,
°≈β 136.88 , °=γ 415.84 , and density of molecules per 2cm .
5.7
Table 5.3. Lattice constants of the triclinic cell for SML and MML Pentacene, with cell
parameters °≈α 978.76 , °≈β 136.88 , °=γ 415.84 . The last column indicates the density of
pentacene molecules per 2cm .
5.7
Table 6.1. Raman shift of monolayer and bulk pentacene thin films 6.9
Table 8.1. Time-dependent phenomena in OFETs determined by complementary experimental
techniques (based on the setups used in this research).
8.21
B. A. Paez-Sierra, Erklärung 10.8
Erklärung
Ich erkläre, dass ich die vorliegende Arbeit selbständig und nur unter Verwendung der
angegebenen Literatur und Hilfsmittel angefertigt habe.
06. March 2007
MSc. Phys. Beynor Antonio Paez-Sierra
B. A. Paez-Sierra, Curriculum Vitae 10.9
Curriculum Vitae
Beynor Antonio Paez-Sierra Date of birth Place of birth Nationality Gender Marital status Languages
24 September 1971 Florián - Santander, Colombia Colombian Male Married Spanish (native), German, English, Russian, French
EDUCATION AND AWARDS
1984 – 1989 High School: Colegio Vecinal Policarpa Salavarrieta Suba, Bogota, Colombia 1990 - 1996
B. Sc. in Physics Thesis work awarded with Merit Mention Thesis title: Thermoelectric power in SnO2 thin films Universidad Nacional de Colombia, Bogotá Advisor: Prof. Gerardo Gordillo
1997 - 1999 Msc. in Physics Thesis title: Transport properties of CdTe thin films Universidad Nacional de Colombia, Bogotá Advisor: Prof. Dr. Gerardo Gordillo-Guzmán
2001 14. May, Award for my teaching labor, Science Faculty, Pontificia Universidad Javeriana, Bogotá – Colombia
2002 - 2005 Phd in Physics Thesis title: ,,Raman Spectroscopy of Metal/Organic/Inorganic Heterostructures and Pentacene-Based Organic Field Effect Transistors” Semiconductors department, Technical University of Chemnitz, Germany Prof. Dr. Dr. h.c. Dietrich R. T. Zahn
WORK AND RESEARCH EXPERIENCE
1997 - 2002 As a physics teacher and as an assistantship researcher, physics department, Pontificia Universidad Javeriana. Bogotá – Colombia
1994 - 1999 Characterization of thin films for solar cells (SnO2, CdTe, ZnO) through thermoelectric power, Hall effect and electrical conductivity measurements to study transport properties in semiconductors, (Prof. Dr. Rer Nat. Gerardo Gordillo Guzmán), Universidad Nacional de Colombia
1998 19. January - 3. April, stay research. Work : Thermoelectric power measurements in quasicrystals, (Prof. Dr. Roberto Escudero Derat). Universidad Nacional Autónoma de México, UNAM, México D.F. - México
B. A. Paez-Sierra, Curriculum Vitae 10.10 1998 - 1999 Design and construction of an equipment to measure electrical conductivity.
Universidad Javeriana, Bogotá – Colombia
2000 - 2001 Design and construction of an equipment to measure thermoelectric power in a wide range of temperature. Pontificia Universidad Javeriana, Bogotá – Colombia
At the semiconductors department Technical University of Chemnitz, Germany
20.05.2002 05.11.2005
PhD student. Experimental and theoretical investigations of: • Metal/organic interface formation by vibrational spectroscopies (Raman and
infrared) • Combined vibrational and electrical techniques to characterize organic field
effect transistors This research was supported by the Deutsche Forschungsgemeinschaft (DFG) within the project Za 146/4-2 as part of SPP 1121: Organic field effect transistors: Structural and dynamical properties
2002 Investigation of GaN structures by Raman spectroscopy and surface enhancement Raman spectroscopy. In cooperation with Dr. Elena Konenkova, St. Petersburg, Russia
2003 Summer semester. Auger spectroscopy Practical training for the students in Physics VI semester
2002 - 2003 Co-orientation of a diploma work (Matthias Bartzsch)
2003 - 2004 Winter semester. Micro Raman spectroscopy. Practical training for the students in Physics VI semester
2003 Summer semester. Advanced laboratory practice: Photoluminescence in Organic materials. Practical training for the students in Physics VIII semester
2004 Raman Spectroscopy. Practical training for the PhD students of the ACCUMOL program
2004 Summer semester. Auger spectroscopy. Practical training for the students in Physics VI semester
2004 September. Experimental and theoretical spectroscopic investigations of the interfaces Si/Au/Tetracene/Au and Si/Tetracene/Au. Dipl. Phys. Aline Hepp Darmstadt. Cooperation with the group of Prof.Dr. Heinz von Seggern, Fachbereich Material- und Geowissenschaften, Technische Universität Darmstadt
2004 - 2005
Winter semester. Micro Raman spectroscopy. Practical training for the students in Physics VI semester
2005 January. Investigation of GaN nanotubes by Raman spectroscopy and surface enhancement Raman spectroscopy. In cooperation with Dr. Alexander Milekhin, Noborsibirsk – Russia
2005 Micro Raman spectroscopy. Practical training for the students in Physics VI semester
B. A. Paez-Sierra, Curriculum Vitae 10.11
SEMINARS AND CONGRESSES1
1994 August. First Workshop in Semiconductor Physics, Universidad Nacional de Bogotá Colombia, Bogotá
1995 August. Seminar “Photovoltaic Systems”, Universidad Nacional de Colombia, Bogotá
1995 July. XVI National Congress of Physics, Santiago de Cali - Colombia
1997 June. XVII National Congress of Physics, Medellín - Colombia
1998 11.-16. January. XIV Latin American Congress of Solid State Physics, Oaxaca - México
1998 21.-25. September. Third School of Condensed Matter Physics, Armenia - Colombia
1999 June. XVIII National Congress of Physics, Bogotá, D.C - Colombia
1999 1.-5. November. XV Latin American Symposium on Solid State Physics “SLAFES - XV”, Cartagena de Indias – Colombia
2001 3.-6. July. X CLACSA Latinamerican Congress of Surface Science and their applications, San José - Costa Rica
2001 24.-28. September. XIX National Congress of Physics, Manizales – Colombia,
2003 16.-18. January, XI International Workshop on Computational Physics and Material Science: Total Energy and Force Methods, Trieste – Italy,
2003
4.-22 February , Winter College on "Numerical Methods in Electronic Structure Theory" Trieste - Italy
2003 24.-28. March, Deutsche Physikalische Gesellschaft Tagung, Dresden - Germany
2003 1.-3. April. VIII Holzhau group Meeting, Carlovi Bary - Czech Republic
2003 3.-8. August. Organic Field Effect Transistors II, San Diego - USA
2003 1.-5. September. XX National Congress of Physics, Cali - Colombia
2003 15.-19. September, 9th International Conference on the Formation of Semiconductor Interfaces (ICFSI-9) Madrid - Spain
2003 14.-16. September. International Workshop on Semiconductor Surface Passivation, SSP, Ustrón - Poland
2003 1.-2. October. Kolloquium des DFG Schwerpunktprogramms Organische
1 Where I directly participated or where part of my research activity was involved
B. A. Paez-Sierra, Curriculum Vitae 10.12
Feldeffekt-Transistoren: strukturelle und dynamische Eigenschaften, Bremen 2003 2.-7. November. AVS 50th International Symposium, Baltimore - USA
2004 18.-22. January. 31st conference on the physics and chemistry of semiconductor
interfaces, Kailua-Kona - Hawaii
2004 8.-12. March. Deutsche Physikalische Gesellschaft Tagung, Regensburg - Germany
2004 22.-25. March. IX Holzhau group Meeting, Breslau – Poland
2004 19.-22. May. Bad Honnef - Germany
2004 7.-8. July. Kolloquium des DFG Schwerpunktprogramms Organische Feldeffekt-Transistoren: strukturelle und dynamische Eigenschaften in Kaiserslautern - Deutschland
2004 9-13 August. 14th "International Conference on Crystal Growth" 12th "International Conference on Vapor Growth and Epitaxy", Alpes Congrès, Grenoble – France
2004
16.-17. December. Kolloquium des DFG Schwerpunktprogramms Organische Feldeffekt-Transistoren:
Berichtskolloquiums in Braunschweig - Deutschland
2005 6.-9. March, Materials for Advanced Metallization (MAM), Dresden - Germany
2005 04.-9. March, Deutsche Physikalische Gesellschaft Tagung, , Berlin - Germany
2005 17. May. Semiconductors Department, Technical University of Chemnitz
2005 26.29. June. 13 Tagung Festkörperanalytik, Technical University of Chemnitz
2005 3.-8, July. 10th International Conference on the Formation of Semiconductor Interfaces (ICFSI-10) Aix-en-Provence, France
2005 31. July -4. August. Optics & Photonics, San Diego, California USA
B. A. Paez, List of publications 10.13
List of publications
2006 [39] L. C. Jimenez B., H. A. Méndez P., B. A. Paez S., M. E. Ramírez, and H. Rodríguez H, ,, Production and Characterization of Indium Oxide and Indium Nitride”, Br. J. Phys., 36, 1017-1020 (2006).
[38] A.G. Milekhin, R. Meijers,T. Richter, R. Calarco, H. Lüth, B. A. Paez Sierra, and D.R.T. Zahn, “Surface enhanced Raman scattering by GaN nanocolumns”, phys. stat. sol. (c) 3, 2065-2068 (2006).
[37] A.G. Milekhin, R. Meijers,T. Richter, R. Calarco, S. Montanari, H. Lüth, B. A. Paez Sierra, and D.R.T. Zahn, “Surface enhanced Raman scattering by GaN nanostructures obtained by bottom-up and top-down approach”, J.Phys.Cond.Matt. 18, 5825-5834 (2006).
[36] I. Thurzo, B. Paez, H. Méndez, R. Scholz, and D. R. T. Zahn, ,,Anomalous charge relaxation in channels of Pentacene-based organic field-effect transistors: a charge transient spectroscopy study”, phys. stat. sol. (a) 204, 1-15 (2006).
2005 [35] B. A. Paez S, I. Thurzo, G. Salvan, R. Scholz, Dietrich R. T. Zahn, and H. von Seggern, “Combined Raman spectroscopic and electrical characterization of the conductive channel in pentacene based OFETs”, Proc. of SPIE 5940, 59400F 1-9 (2005).
[34] R. Scholz, A.-D. Müller, F. Müller, I. Thurzo, B. A. Paez, L. Mancera, D. R. T. Zahn, “Comparison between the charge carrier mobilities in pentacene OFET structures as obtained from electrical characterization and potentiometry”, Proc. of SPIE 5940, 59400I (2005).
[33] B. A. Paez, G. Salvan, R. Scholz and D. R.T. Zahn, “Interface formation of Mg with DiMePTCDIsStudied by Raman spectroscopy”, phys. stat. sol. (c) 2, 4048-4052 (2005).
[32] G. Salvan, B. A. Paez¸ S. Silaghi, and D. R. T. Zahn, “Deposition of silver, indium, and magnesium onto organic semiconductor layers: Reactivity, indiffusion, and metal morphology”, Microelectronic Engineering, 82, 228-235 (2005).
[31] Salvan, S. Silaghi, B. Paez, G. Baumann, T.U. Kampen, R. Scholz, and D. R. T. Zahn, “Structural and morphological properties of N,N0-dimethyl-3,4,9,10-terylenetetracarboxylic diimide films on passivated GaAs(1 0 0) substrates”, Journal of Crystal Growth 275, e1155–e1162 (2005).
[30] D.R.T. Zahn, G. Salvan, G. Gavrila, and B. A. Paez, “Chemistry and Morphological Properties of Metal Interfaces to Organic Semiconductors”, Solid State Phys. 45, 299–310 (2005).
2004 [29] G. Salvan, S. Silaghi, B. Paez, T.U. Kampen, D. R. T. Zahn, “Structural and Morphological Properties of 3,4,9,10-PeryleneTetraCarboxylic Dianhydride Films on Passivated GaAs(100) Substrates”, ICSM-2004 special issue of Synthetic Metals.
[28] B. A. Paez, G. Salvan, S. Silaghi, R. Scholz, T. U. Kampen and D. R. T. Zahn, “Raman Monitoring of In and Ag Growth on PTCDA and DiMe-PTCDI Thin Films”, Appl. Surf. Sci. Vol 234/1-4 pp 168-172 (2004).
[27] D.R.T. Zahn, G. Salvan, B. A. Paez, R. Scholz, “Interaction between metals and organic semiconductors studied by Raman spectroscopy”, J. Vac. Sci. Technol. A 22, 1482-1487 (2004).
[26] G. Salvan, D.R.T. Zahn, and B. Paez, “Surface enhanced Raman scattering in organic thin films covered with silver, indium and magnesium”, Journal of luminescence 110, 296 (2004).
[25] V. N. Bessolov, E. V. Konenkova, Yu. V. Zhilyaev, B. A. Paez Sierra, D. R. T. Zahn, “The effect of Ag-coated of surface on Raman spectra of GaN nanocrystals”, Appl. Surf. Sci. 235, 274–278 (2004).
B. A. Paez, List of publications 10.14 [24] G. Salvan, S. Silaghi, B. Paez, T.U. Kampen, D. R. T. Zahn, “Modification of
GaAs(100) Surfaces Upon Adsorption of Perylene Derivatives”, Appl. Surf. Sci. 234, 178-184 (2004).
2003 [23] B. A. Paez, N. C. Forero, and L. Castañeda, Electron-positron plasma under an external electromagnetic field, Rev. Col. Fís. 35-2, 222 (2003).
[22] B. A. Paez, G. Salvan, R. Scholz, T. U. Kampen, and D. R. T. Zahn, “Interaction of metals with perylene derivatives as a model system for contact formation in OFET structures”, Proc. SPIE Int. Soc. Opt. Eng. 5217, 210-217 (2003).
[21] B. A. Paez, M. Bartzsch, G. Salvan, R. Scholz, T. U. Kampen, and D. R. T. Zahn, “Combined Electrical and Raman characterization of C60 based organic field effect transistors”, Proc. SPIE Int. Soc. Opt. Eng. 5217, 63 (2003).
2002 [20] L.C. Jiménez, H. Méndez, B. A. Paez and I. Morales, “Thermoelectric power measurements in indium oxide thin Films”, Rev. Col. Fís. 34 (2), 434 (2002) .
[19] B. A. Paez and J.I. Barrera, “Resonance and phase curves in oscillators forced by multi-harmonic signals”, Rev. Col. Fís. 34 (1), (2002) (in spanish).
[18] J.I. Barrera and B. A. Paez, “Impedance determination of a damped harmonic oscillator”, Rev. Col. Fís. 34 (1), (2002) (in spanish).
[17] B. A. Paez, H. Méndez and L.C. Jimenez, “Design and construction of an equipment to measure the thermoelectric power, α, in bulk and thin films materials”, Rev. Col. Fís. 33 (2), 89 (2002).
[16] B. A. Paez, Nanotech, “Current state and perspectives”, Quantum 2 (2002) (in Spanish).
[15] B. A. Paez, A. Moreno and H. Méndez, “Solution of the Quantum Boltzmann Equation in Linear Transport”, Surf. Rev. Lett. 9, 1761 (2002).
[14] B. A. Paez, H. Méndez and J.C. Giraldo, “Thermoelectric Power Coefficient, α, in a Quantum Well”, Surf. Rev. Lett. 9, 1765 (2002).
2001 [13] B. A. Paez, A. Moreno and H. Méndez, “Dirac delta function and step function, a study with oscillators”, Rev. Col. Fís. 33 (2), 89 (2001) (in spanish).
2000 [12] B. A. Paez, “Thermoelectric power and Hall effect measurements in polycrystalline CdTe thin films”, phys. stat. sol. (b), 220 (1), 233 (2000).
[11] B. A. Paez, J.M Flórez, C.E. Jácome and G. Gordillo, “Thermoelectric characterization of CdTe thin films”, Rev. Col. Fís. 32 (1), 29 (2000) (in spanish).
[10] L.C. Jiménez, H. Méndez and B. A. Paez, “Tin oxide thin films deposited by reactive sputtering”, Rev. Col. Fís. 32 (1), 63 (2000) (in Spanish).
[9] H. Méndez, H. Rodriguez and B. A . Paez, “Design and construction of an equipment for electrical characterization”, Rev. Col. Fís. 31 (2), 353 (1999) (in Spanish).
1999 [8] B. A. Paez and H. Méndez, “A pedagogical view of the Boltzmann transport equation”, Rev. Col. Fís. 31 (2), 345 (1999) (in Spanish).
[7] C.E. Jácome, J.M. Flórez, and Y.G. Gurevich, B. A. Paez and G. Gordillo, “Study of transport properties in semiconducting CdS thin films”, Rev. Col. Fís. 31 (2) 243 (1999). (in Spanish).
[6] G. Gordillo, B. A. Paez, C.E. Jácome and J.M. Flórez, “Thermoelectric power in SnO2 thin films”, Thin Solid Films 342, 160-166 (1999).
1998 [5] B. A. Paez, C.E. Jácome, J.M. Flórez and G. Gordillo, “Thermoelectric power measurements in CdTe thin films”, Rev. Mex. Fís.44 S3, 74 (1998) (in Spanish)
[4] B. A. Paez, C.E. Jácome, J.M. Flórez and G. Gordillo, “Thermoelectric power in SnO2 thin films”, Rev. Col. Fís. 30 (1), 97 (1998). (in Spanish).
1997 [3] G. Gordillo, B. A. Paez, C.E. Jácome, L.C. Hernández, J.M. Florez, and H. Méndez, “Characterization of SnO2 thin films through thermoelectric power measurements”, Conference Record Of The 26th IEEE Photovoltaic Specialists Conference, Anaheim CA. PSC, 26, 519 (1997).
[2] C.E. Jácome, B. A. Paez, J.M. Flórez and G. Gordillo, “Theoretical investigation of the thermoelectric power in SnO2 thin films. Rev. Col. Fís. 29 (2) 115 (1997)
B. A. Paez, List of publications 10.15
(in Spanish). 1995 [1] A. Ortiz, B. Paez, F. Fajardo, J. Quiñones and G. Gordillo, Study of
optoelectronic properties in ZnO thin films deposited by reactive and DC sputtering. Revista Colombiana de Física (Rev. Col. Fís). 27(1) 359 (1995) (in Spanish).
ESSAYS AND BOOKS
[1] Beynor A. Paez S., “Phasorial Algebra” Physics Department, Pontificia
Universidad Javeriana; Bogotá, D.C, August 1999 (http://newton.javeriana.edu.co) (essay in Spanish)
[2] Beynor A. Paez S., “Physics V.1, Mechanics”, Physics Department, Pontificia Universidad Javeriana, (2001) (Book in Spanish) http://unicornio.javeriana.edu.co/
B. A. Paez-Sierra, Acknowledgements 10.16
Acknowledgements
To the invaluable and tremendous moral and scientific support of my wife “Viktoriia”, for being along with me during those almost endless experiments, and actively joining this scientific venture despite she was running her own magneto-optical experiments. Special thanks to the Deutsche Forschungsgemeinschaft (DFG) in the frame of the “Schwerpunktprogramm 1121: Organische Feldeffekt-Transistoren: strukturelle und dynamische Eigenschaften”, for the grant which has financially supported me to conduct this research in TU Chemnitz at the group of Prof. Dr. Dr. h.c. Dietrich R. T. Zahn. Several cooperations grew up during the evolution of this work. In particular I want to mention the cooperation with Christoph Pannemann at the group of Prof. Dr.-Ing. Ulrich Hilleringmann, who provided me the single channel structures; with Aline Hepp at the group of Prof. Dr. H. von Seggern, from them we got the interdigitated structures. These interchanges seeded mutual benefits ranging from substrates, organic molecules, DFT calculations, Raman spectroscopy, QTS experiments; and to what is important, the collective work to improve our comprehension about nature. Other substrates for transistors were processed at the center for microtechnologies in cooperation with Dr. Christian Kaufmann in the group of Prof. Dr. Dr. Prof. h.c. mult. T. Gessner. Thanks to Matthias Bartzsch at the pmTUC in the group of Prof. Dr. -Ing. A. C. Hübler, who provided me some flexible substrates for plastic electronic applications (polymer-based transistors), where we run some experiments with small organic molecules. Many other cooperations different from the OFET project streamed during my research. In particular those about GaN nanocolums with Alexander Milekhin (Novosibirsk), in GaN nanocrystals with Elena V. Konenkova (St. Petersburg), on quantum dots of CdSe/ZnS in cooperation with Prof. Dr. C. von Borczyskowski (TU Chemnitz). Specially those experiments and the involved scientists help me to understand much better the nanoscience and nanotechnology that I was doing with the SERS effect in organic molecules. Although no one of my colleagues at the semiconductors group (may 2002-Nov. 2005) was directly involved in the OFET project, just let me say that I got any message from all of you, I learned something to take with. Specially, I want to thank Georgeta Salvan for her orientation on some SERS experiments in PTCDA and DiMe-PTCDI. I also want to thank Simona Silaghi for her patience when I asked her to give me more time to use the Raman lab during her experiments. Concerning the theoretical part, I want to thank the people at the “International Center for Theoretical Physics (ICTP), Trieste” in particular Prof. Dr. S. Baroni whom allow me to join the Institue (16-January. 4-February 2003) to learn about “Numerical Methods in Electronic Structure Theory”. After this wondeful course, I appreciate the openning of an account at the super computer by Reinhard Scholz in the group of Prof. M. Schreiber. I perfromed there several DFT calculations that helped me to understand much better the influence of electric fileds on vibrational properties of organic molecules, and the relation of dipole-bound states on the QTS signal. I want to express my gratitude to Ilja Thurzo with whom we came to the conclusion on the “anomalous QTS in organic field effect transistors” that we condensed in a very nice publication. I am also indebted with Ilja, and my wife, who independetly gave me very nice highlights to improve this thesis. To my friends Henry and Luis, with who not only interesting experiments were realized, specially those about synthesizing Mg with PTCDA, but also for the mutual support and help to spread the hispanic culture at the TU Chemnitz. Special gratitude to the members of the spanish club (http://www.tu-chemnitz.de/stud/club/hispano/), who kindly cooperate for the sucessfull meetings at the “Club der Kulturen (CdK)” Finally, the last but not least to my marvelous family in Colombia: Gotita, Segundiales, Peluca, and co.