CITY UNIVERSITY OF HONG KONG 香港城市大學

Growth and Secondary Engineering of II-VI Semiconductor Nanostructures and Their

Optical Properties II-VI族半導體納米結構的生長、修飾及其

光學特性的研究

Submitted to Department of Physics and Materials Science

物理及材料科學系 in Partial Fulfillment of the Requirements

for the Degree of Doctor of Philosophy 哲學博士學位

by

Li Yanqing 李艷青

February 2008 二零零八年二月

GROWTH AND SECONDARY ENGINEERING OF II-VI

SEMICONDUCTOR NANOSTRUCRURES AND THEIR

OPTICAL PROPERTIES

LI YANQING

DOCTOR OF PHILOSOPHY

CITY UNIVERSITY OF HONG KONG

FEBRUARY 2008

LI YA

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PhD

2008

CityU

i

Abstract

The rational growth of well-defined 1D nanostructured materials is at the heart of

building blocks for future nanodevices, since it is crucial to control over the material

physical properties. Meanwhile, the capability of controlling the structural complexity of

nanomaterials and their physical properties is presently limited in the one-step “thermal

evaporation and condensation” growth. The secondary engineering can provide

conceptually a new dimension to the rational design of nanostructured materials. In this

work, a series of systematic studies towards the rational growth and engineering of 1D

nanostructured II-VI compound semicondutors are introduced, e.g., ZnS and CdS in the

morphology of nanowire and nanoribbon are presented because of their unique optical

properties and wide potential applications in nanoscale photonic devices. In addition, a

range of advanced techniques are adopted to characterize the intriguing physical

properties of the resulting semiconductor nanostructures. Some efforts are also made

towards the possibilities in the rational assembly of nanostructured materials into

electrically-driven photonic devices based on nanostructured II-VI semiconductors.

Firstly, the growth approach for the synthesis of hexagonal wurtzite 2H structured

single-crystal nanowires and nanoribbons of group II-VI compound semiconductors like

ZnS and CdS will be addressed by the one-step “thermal evaporation and condensation”

growth mechanism. Particularly, heteroepitaxial growth of single-crystal ZnS nanowire

arrays on CdS nanoribbon substrates has been demonstrated by the metal-catalyzed

vapor-liquid-solid growth method. ZnS nanowire arrays are obtained, which are vertically

or crosswise aligned to the surface of CdS nanoribbon substrates. The orientation

ii

dependence of ZnS nanowire arrays on the substrate indicates the importance of

crystalline substrates on the epitaxial growth process of 1D nanostructures.

Secondly, a conceptually new method is introduced for enabling the structural

modification of pre-synthesized nanomaterials with highly defined hierarchical

nanoarchitectures. Nanocantilever arrays are formed on the edge of the ±(001) planes of

pre-synthesized ZnS nanoribbons via catalyst-assisted post-annealing treatment, which is

associated with orientation-dependent chemical etching of the ±(001) polar surfaces of

ZnS nanoribbons.

Thirdly, the feasibility of doping nanostructured materials is demonstrated by post-

annealing treatment via thermal solid-state diffusion. Mn doping of ZnS nanoribbons is

achieved by annealing the host in MnS powder, where Mn dopants occupy the Zn2+

cation sites producing deep-lying states in the band gap of ZnS. In contrast to doping by

ion implantation where high-density defects are invariably induced, Mn doping by

thermal annealing appears to have negligible deleterious effect on the crystal structure

and luminescence properties.

Fourthly, the optical properties of pure and doped nanomaterials are investigated.

The intrinsic near-bandgap emission of ZnS nanostructures is around 335 nm, and the

defect related luminescence is around 500 nm. Notably, room-temperature lasing

emission from ZnS nanowire arrays is revealed according to a superlinear increase of

emission intensity and the peak narrowing when the excitation power density is above a

threshold value. Temperature-dependent optical properties of Mn-doped ZnS nanorribons

are also discussed. A photoluminescence peak at 585 nm independent of the measuring

temperature and excitation power is demonstrated in Mn-doped ZnS nanoribbons that is

iii

attributed to Mn2+ ion incorporated in ZnS, leading to an indirect excitation mechanism

upon photo-excitation.

Lastly, various device structures are explored, including crossed nanowire p-n

junction and nanowire-thin film hybrid structure, in order to realize the electrically-driven

nanophotonic devices based on nanostructured II-VI semiconductors by taking advantage

of their direct bandgap character. It is demonstrated that the contacting point (or area) of

the heterojunction formation is the obstacle to carrier injection in these structures.

vii

Table of Contents

Abstract i

Certification of Approval by the Panel of Examiners iv

Acknowledgements v

Table of Contents vii

List of Figures xi

List of Tables xix

List of Symbols and Abbreviations xx

Chapter 1 Introduction 1

1.1 Introductory background 1

1.2 Growth strategies for 1D nanostructured materials 11

1.3 Unique properties of 1D nanomaterials 18

1.4 Applications of 1D nanomaterials 22

1.5 Overview of the thesis 27

References 29

Chapter 2 1D Nanomaterials: Synthesis & Characterization 36

2.1 Synthesis methods of 1D nanostructures 36

2.1.1 Physical vapor deposition: Evaporation, sputtering, e-beam and laser

ablation

38

2.1.2 Chemical vapor deposition: thermal, plasma and metalorganic 39

2.2 Characterization techniques of nanostructured materials 42

viii

2.2.1 X-ray diffraction 43

2.2.2 Scanning electron microscopy 45

2.2.3 Transmission electron microscopy 50

2.2.4 Optical spectroscopy 55

2.3 Experimental setup and methodology 58

References 63

Chapter 3 Growth & Characterization of ZnS and CdS Nanowires and

Nanoribbons

64

3.1 Overview 64

3.2 Crystal structures of ZnS and CdS 66

3.3 Growth of ZnS nanowires and nanoribbons 66

3.4 Growth of CdS nanoribbons 75

3.5 Summary 82

References 83

Chapter 4 Epitaxial Growth & Optical Properties of ZnS Nanowire Arrays

on CdS Nanoribbons

88

4.1 Overview 88

4.2 Heteroepitaxial growth of ZnS nanowire arrays on CdS nanoribbons 92

4.3 Optical properties of ZnS nanowire arrays on CdS nanoribbons 103

4.4 Summary 106

References 108

ix

Chapter 5 Catalyst-Assisted Formation of Nanocantilever Arrays on ZnS

Nanoribbons by Post-Annealing Treatment

111

5.1 Overview 111

5.2 Nanocantilever array formation via catalyst-assisted post-annealing

treatment

115

5.3 Proposed mechanism of nanocantilever array formation 124

5.4 Summary 127

References 129

Chapter 6 Manganese Doping and Optical Properties of ZnS Nanoribbons by

Post-Annealing

131

6.1 Overview 131

6.2 Structural analysis of Mn-doped ZnS nanoribbons 134

6.3 Optical properties of Mn-doped ZnS nanoribbons 138

6.4 Manganese doping and thermal diffusion 144

6.5 Summary 147

References 148

Chapter 7 Primary Exploration of Single Nanowires as Building Blocks for

Nanophotonics 150

7.1 Overview 150

7.2 Cross p-n heterojunction nano-device 153

7.3 Nanowire-thin film hybrid nano-device 163

7.4 Summary 167

x

References 169

Chapter 8 Concluding Remarks and Future Prospects 171

8.1 Concluding Remarks 171

8.2 Future prospects 173

References 176

Appendix: Publication List 178

xi

List of Figures

FIG. 1.1. Human civilizations designated by the level of materials development (Stone

Age, Bronze Age, Iron Age).

FIG. 1.2. Length scale in materials science and technology. The scale of physical

measurements extends over a range of 1042.

FIG. 1.3. Development of Intel® microprocessors complexity from 1971 to 2006. Insets

are the first transistor invented in 1947 (top) and the latest Intel® Itanium 2 processor

(bottom). As predicted by Moore’s Law, the number of transistors on a chip is roughly

doubled every two years.

FIG. 1.4. Scale down of transistor dimensions in microprocessors in the past decade

referenced Intel’s products.

FIG. 1.5. Evolution of dimensions of materials and quantum size effects on density of

states. (a) bulk material (3D structure), (b) quantum well (2D structure), (c) quantum wire

(1D structure), and (d) quantum dot (0D structure).

FIG. 1.6. Structural schematics (left column) and representative transmission electron

microscopy (TEM) images of 1D nanomaterials with homogeneous structure, axial

heterostructure, and radial (core/shell, core/multi-shell) heterostructure (right column).

FIG. 1.7. Schematic illustration of six different approaches to achieving the growth of 1D

nanostructures: (a) catalyst-assisted vapor-liquid-solid process (VLS); (b) catalyst-free

vapor-solid (or surface diffusion) process driven by anisotropic crystallization rate of

different crystal directions; (c) selective control provided by a capping reagent; (d)

confinement through the use of a template; (e) self-assembly of 0D nanostructures; and (f)

cutting of macroscopic materials to 1D nanostructures.

xii

FIG. 1.8. Schematics of representative nanowire-based electronic devices. (a1) Lateral

and (a2) vertical nanowire MOSFET; (b) Bio- or chemical nanosensor for detecting

targeted molecules in gaseous or solution environment; and LEDs by using core/shell

structured nanowires (c1), crossing p- and n-type nanowires (c2), or connecting nanowire

with semiconducting films (c3).

FIG. 2.1. Schematic relationship of typical material synthesis methods, including physical

vapor deposition (PVD) and chemical vapor deposition (CVD), which are categorized by

the vaporization approaches.

FIG. 2.2. Schematic of chemical vapor deposition (CVD) process based on diffusion

model, showing the adsorption and desorption of the precursor molecules, surface

diffusion, nucleation and growth, and desorption of reaction products.

FIG. 2.3. (a) Principle of X-ray diffraction as indicated by Bragg’s law. Constructive

interference occurs when the distance traveled by the rays reflected from successive

planes differs by a complete number n of wavelengths. (b) Schematic diagram of a

XRD instrument.

FIG. 2.4. Schematic of diffraction pattern obtained by XRD for (a) perfect crystals, (b)

polycrystalline materials, and (c) disordered materials.

FIG. 2.5. Schematic of electron beam interaction with solids. The primary electron beam

spreads in a teardrop-shaped volume, producing secondary, backscattered, and Auger

electrons, X-ray, and cathodoluminescence.

FIG. 2.6. (a) Schematic of a transmission electron microscope, and simplified electron

optics of an electron microscope operating in the (b) and (c) conventional (diffraction

contrast) and (d) high-resolution (phase contrast) modes.

xiii

FIG. 2.7. (a) and (b) Schematic of electron diffraction in TEM, where electron beam

strongly diffract only from planes of atoms almost being parallel to the beam. (c) Typical

electron diffraction patterns obtained from amorphous, polycrystalline and single-crystal

materials.

FIG. 2.8. Schematic of a simplified luminescence process. The material is excited by

light or the beam of electrons or ions.

FIG. 2.9. Schematic of (a) thermal and (b) laser based nanomaterial growth apparatus.

FIG. 2.10. Schematic of the instrument of PL spectroscopy.

FIG. 3.1. SEM images of (a) ZnS nanowire and (b,c) nanoribbons. A large amount of

nanomateials were produced on the Si substrate. Inset is an EDX spectrum showing the

existence of sulfur and zinc atoms in the specimen, with little oxygen signal coming from

the adsorption.

FIG. 3.2. XRD patterns of (a) ZnS nanoribbons together with (b) hexagonal wurtzite-2H

structure and (c) cubic zinc blende structure of bulk ZnS.

FIG. 3.3. (a) TEM bright-light image of ZnS nanoribbons, and (b,c) HRTEM images and

SAED pattern of the [100] zone axis (inset) of a nanoribbon with wurtzite-2H structure

and [120] growth direction.

FIG. 3.4. Room-temperature photoluminescence spectrum of ZnS nanoribbons, showing

the deconvolution of the main emission peak.

FIG. 3.5. SEM images of (a) CdS nanoribbons, (b) nanowires, (c) nanocomb, and (d)

nanorods on the Si substrate. Insets are the corresponding high-magnification images.

FIG. 3.6. (a) TEM bright-light image of CdS nanoribbons, and (b,c) the corresponding

HRTEM images showing wurtzite-2H structure and [120] growth direction.

xiv

FIG. 3.7. (a) Wurtzite hexagonal crystal structure of CdS single crystal. (b)-(d) theoretical

simulations of electron diffractions of hexagonal structured CdS along different beam

direction. (e)-(f) measured electron diffractions of CdS nanoribbons along different zone

axes. z: zone axis; B: electron beam direction; x: forbidden diffraction spots.

FIG. 4.1. Schematic illustrating device configurations of (a) vertical nanowire array-

polymer solar cells and (b) vertical nanowire metal-oxide-semiconductor field effect

transistor (MOSFET).

FIG. 4.2. SEM images of arrays of ZnS nanowires vertically grown on CdS nanoribbons.

FIG. 4.3. SEM images of arrays of ZnS nanowires crossed grown on the top surface of

CdS nanoribbons.

FIG. 4.4. SEM images of (a) vertical growth and (b) crossed growth of ZnS nanowire

arrays on CdS nanoribbons at the initial growth stage.

FIG. 4.5. (a) TEM images of as grown CdS nanoribbons and (b) corresponding HRTEM

image, revealing the growth direction of CdS nanoribbons along [120]. Insets: electron

diffraction patterns along [100] and [001] zone axes, respectively.

FIG. 4.6. (a) TEM images of vertically aligned ZnS nanowire arrays grown on CdS

nanoribbon substrates, and (b) corresponding HRTEM image, revealing the growth

direction along [100]. Insets: electron diffraction pattern along [001] zone axis.

FIG. 4.7. (a) TEM images of cross-aligned ZnS nanowire arrays grown on CdS

nanoribbon substrates, and (b) corresponding HRTEM image, revealing the growth

direction along [210]. Insets: electron diffraction pattern along [100] zone axis.

FIG. 4.8. Schematic of the proposed growth of arrays of ZnS nanowires on CdS

nanoribbons with different orientations. (c) Schematic of the basic cell of hexagonal

xv

structure revealing the crystallographic relations of the lattice planes and directions

involved in epitaxial growth.

FIG. 4.9. (a) SEM and (b) CL images of ZnS nanowire arrays on CdS nanoribbons, (c)

the corresponding CL spectrum.

FIG. 4.10. PL spectra and PL intensity (inset) of epitaxially grown nanostructures excited

by different excitation power density.

FIG. 5.1. Schematic illustrating the progression of nanomaterials from homogenous

structures, to structures modulated in the axial (supperlattice) and radial (core/shell)

directions, or to multi-branched nanoarchitectures via multistep growth. Distinct colors

are shown to indicate variations in material composition.

FIG. 5.2. Schematic illustrating selective etching of Si wafer. It is orientation-dependent,

where a specific crystal plane of single-crystal Si is dissolved much faster than other

planes.

FIG. 5.3. (a) SEM image of as-synthesized ZnS nanoribbons. (b)-(e) SEM images of

representative nanocantilever arrays formed on the edge of ZnS nanoribbons dispersed on

Si substrate by annealing at 600 °C for 30 mins.

FIG. 5.4. XRD patterns of (a) as-synthesized ZnS nanorbbons, (b) annealed ZnS

nanoribbons, and (c) ZnS nanoribbons annealed in an oxygen environment, together with

hexagonal wurtzite-2H structure of bulk ZnS and ZnO.

FIG. 5.5. TEM images of ZnS nanocantilever structures formed on ZnS nanoribbons. The

inset in (a) is the corresponding SAED pattern recorded on the entire structure along the

[2110] zone axis.

xvi

FIG. 5.6. (c) HRTEM image of the tip region of a nanocantilever. (d) HRTEM image of

the interspace region between two nanocantilevers.

FIG. 5.7. (a) SEM image of ZnS nanoribbons annealed in O2 ambient at 600 °C. The

insets are the EDX spectra of two selected regions as indicated. (b) and (c) TEM images

of annealed ZnS nanoribbons showing numerous nanograins.

FIG. 5.8. SEM images of ZnS nanoribbons annealed at 600°C with a mixture of (a) Si

powder and (b) SiO powder, respectively.

FIG. 5.9. (a) Side and (b) top views of atomic models for Zn-terminated (001) polar

surface in a wurtzite hexagonal ZnS crystal, where (001) is S-terminated. STM images of

(c) flat and (d) vicinal (001)-Zn surface, where terraces are covered by the triangular

islands and holes that are separated by single layer height steps. (e) and (f) STM images

of (001) -O surface, adapted from Dulub et al.

FIG. 6.1. (a) Diffusion and (b) ion-implantation techniques for impurity doping into the

materials, together with the profile of the dopant distribution.

FIG. 6.2. (a) SEM image of MnS-doped ZnS nanoribbons, (b) a typical HRTEM image

and SAED pattern of the [100] zone axis (inset) of a doped nanoribbon with a growth

direction of [120]. The doping process was carried out at 600 °C.

FIG. 6.3. XRD patterns of as grown and Mn-doped ZnS nanoribbons with respect to the

annealing temperature ranging from 300 °C to 700 °C.

FIG. 6.4. Normalized room-temperature PL spectra of as-synthesized and Mn-doped ZnS

nanoribbons annealed at various temperatures.

FIG. 6.5. PL spectra of ZnS nanoribbons annealed at 600 °C, (a) alone without MnS, (b)

with MnS powder in proximity but untouched, and (c) mixed with MnS powder.

xvii

FIG. 6.6. PL spectra of 600 °C-annealed MnS-doped ZnS nanoribbons recorded at

different temperatures excited by the same power. The inset plots the temperature

dependence of the FWHM of the PL peak.

FIG. 6.7. Schematic representation of the energy transfer process in a system without

impurity (a) or with impurity (b).

FIG. 7.1. Schematics showing a crossed nanowire junction formed between a n-type

nanowire and a p-type nanowire.

FIG. 7.2. SEM images of a crossed CdS/GaN nanowire junction with Au contacts.

FIG. 7.3. SEM images of a crossed CdS/GaN nanowire junction with Au contacts.

FIG. 7.4. Electrical behavior of a single CdS nanowire. Inset is the corresponding SEM

image of a CdS nanowire positioned onto pre-patterned Au contacts.

FIG. 7.5. I-V behavior of a single ZnO nanowire measured between different pins. Inset:

SEM image of a ZnO nanowire patterned with Ti/Au contacts.

FIG. 7.6. Typical SEM images of a crossed n-ZnO/p-Si nanowire junction with Ti/Au

contacts to ZnO nanowire and tungsten tips to Si nanowire before (a) and after (b)

electrical measurement. The dashed circle marks the crossing point, where Si nanowire

disappeared after applying a voltage drop.

FIG. 7.7. Crossed n-ZnO/p-Si nanowire junction and electrical properties. (a)-(b) Typical

SEM images of the crossed junction before (a) and after (b) electrical measurement,

where Si nanowire was etched by HF before the manipulation. The dashed circle marks

the crossing point, where Si nanowire remained in the initial area after the electrical

measurement. (c) I-V behavior of the crossed junction by contacting a pair of tungsten

tips contacting the Si nanowire directly and the Ti/Au electrode, respectively.

xviii

FIG. 7.8. Schematic description of the procedure for fabricating nanowire-thin film

hybrid junction. (a, b) Photoresist pattern on the Si substrate by UV exposure; (c, d)

Deposition of an AlN insulating film and removal of the exposed photoresist; (e)

Dispersion of ZnO nanowires on the substrate; (f, g) Second-time photolithography

process using a shrinking mask; (h, i) Ti/Au contact deposition with an electron-beam

evaporator and lift-off process to form a patterned metal contact.

FIG. 7.9. Optical images of the finished hybrid structure formed between a single n-ZnO

nanowire and p-Si wafer. The metal contact is electrically insulated from the p-Si

substrate by the thin layer of AlN film.

xix

List of Tables

Table 1.1 1D nanomaterials with homogeneous structure, axial heterostructure, and radial

(core/shell, core/multi-shell) heterostructure.

Table 3.1 Physical parameters of ZnS and CdS bulk crystals (d: density; Tm: melting

point; Ttr: transition temperature; ε: dielectric constant; µ: mobility for electrons µe or

holes µh).

xx

List of Symbols and Abbreviations

AAO anodic aluminum oxide

AEI absorb electron image

ALE atomic layer epitaxy

APCVD atmospheric chemical vapor deposition

BEI backscattered electron image

BNM bulk nanocrystalline metal

CBE chemical beam epitaxy

CCD charge coupled device

CCM charge collection microscopy

CL cathodoluminescence

CNT carbon nanotube

CRT cathode ray tube

CTEM conventional transmission electron microscopy

CVD chemical vapor deposition

EBIC electron beam induced current

EBSD electron backscattered diffraction

EDS energy dispersive spectroscopy

xxi

EELS electron energy loss spectroscopy

EL electroluminescence

FET field effect transistor

FWHM full width at half maximum

HOT holographic optical trap

HREM high resolution electron microscopy

IR infrared

JCPDS joint committee on powder diffraction standards

LED light emitting diode

LPCVD low pressure chemical vapor deposition

MBE molecular beam epitaxy

MOCVD metalorganic chemical vapor deposition

MOSFET metal oxide semiconductor field effect transistor

MOVPE metalorganic vapor phase epitaxy

MPCVD microwave plasma assistant chemical vapor deposition

NIR near infrared

OAG oxide assisted growth

OIM orientation imaging microscopy

OMCVD organometallic chemical vapor deposition

PECVD plasma enhanced chemical vapor deposition

xxii

PL photoluminescence

PMMA poly(methylmethacrylate)

PVD physical vapor deposition

RF radio frequency

RPECVD remote plasma enhanced chemical vapor deposition

RT room temperature

SAD selected area diffraction

SAED selected area electron diffraction

SDLTS scanning deep level transient spectroscopy

SEI secondary electron image

SEM scanning electron microscopy

SEMPA scanning electron microscopy with polarization

analysis

STM scanning tunneling microscopy

TED transmitted electron diffraction

TEM transmission electron microscopy

UHVCVD ultrahigh vacuum chemical vapor deposition

UV ultraviolet

VFET vertical field effect transistor

VLS vapor liquid solid

xxiii

VS vapor solid

WDS wavelength dispersive spectroscopy

XRD x-ray diffraction

0D zero dimension

1D one dimension

2D two dimension