HighHigh--temperature optical constants, bandtemperature optical constants, band--gap gap energies and inenergies and in--situ growth monitoring of ZnOsitu growth monitoring of ZnOthin filmsthin films

N. AshkenovN. Ashkenov##,, M. SchubertM. Schubert, M. Lorenz, H. , M. Lorenz, H. HochmuthHochmuth,,M. M. GrundmannGrundmannInstitute for Experimental Physics II, Faculty of Physics and Geosciences, University of Leipzig, Germany

#E-mail: ashkenov@physik.uni-leipzig.deTuP D01

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PLD chamber setup

Pulsed-Laser-Deposition

� Growth of wurtzite-type ZnO and Zn1-x(Cd,Mg)xO compound films on a-, c-, and r- plane Al2O3 � KrF excimer laser ablation� Optical ports at AOI of 70°

� polarizer-sample-rotating-compensator-analyzer� spectral range: 0.75-3.345 eV� diode-array detector� ex-situ mode possible� typical measurement time: 0.5 … 2s

In-situ Ellipsometer setups

PLD: temperature calibration

PLD growth of (0001) ZnO thin film on (0001) Al2O3

High-temperature ZnO band gap

Roll-coater setup, Solarion GmbH, Leipzig

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Ion-Beam-Deposition

� Growth of Cu1-x(In,Ga)xSe2-based solar-cells on flexible polymer sheets� Optical access ports at AOI of 43° and 67°

3rd International Conference on Spectroscopic Ellipsometry, July 7-11, Vienna, Austria

Goals: in-situ Ellipsometry control of novel flexible solar cells

2.1 2.4 2.7 3.0 3.3

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973 K

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Photon Energy [eV]

ε 1

Sik et al.1

present work

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<e1>

Photon Energy [eV]

Experiment Model Fit

� SE-spectra of Si-wafer at different sample-heater power values � Use of known temperature-dependent

Si-optical constants1

� Calibration of the heater power to adjust the actual sample temperature

1 J. Šik et al., J. Appl. Phys.84, 6291 (1998).

� Temperature-dependent ex-situ SE spectra of a bulk (0001) ZnO from 300 K to 1154 K

� Strong red-shift and splitting of the E0

A,B,C band-gap energies with temperature:dE0

A/dT= – (7±1)×10-4 eV/K dE0

B/dT = – (6±1)×10-4 eV/KdE0

C/dT = – (5±1)×10-4 eV/K � Linear increase of the optical constants

In-situ monitoring and feed-back control of CuInSe2 (Cu(In,Ga)Se2) and ZnO (Zn(Cd,Mg)O ) thin film growth

Funded by the BMBF within the Wachstumskern INNOCIS

In Summary

In-situ: Virtual-Interface-ApproachStep 1. Nucleation-zone thickness: 15 nm

Growth-rate increases Step 2. Growth-rate decreases

Optical constants changeTotal film thickness: 630 nm

Linear-growth-rate analysis fails because the growth rates changes with time

Ex-situ-analysisNucleation zone is not detectableTotal film thickness: 634 nmSurface roughness: 4.7 nmFilm-thickness-non-uniformity: 7.3%

ConclusionsEx-situ effective thickness in good agreement with actual thickness, but gradient due to growth-rate-change and nucleation zone cannot be resolved

In-situ ellipsometry is a versatile technique for optical monitoring of PLD-grown ZnO thin films.

Sample temperature calibration versus heater power can be easily done.

We report the high-temperature band-gap energies and optical constants of ZnO.

The ZnO growth rate depends on deposition time, causing vertical gradients of optical constants and likely changes in the thin film microstructure. This gradient cannot be resolved anymore from a post-growth ex-situ ellipsometry experiment.

Here: ZnO thin films� Attractive for developing short-wavelength optical devices:

wide band gap (~3.37 eV) and high exciton binding energiesband gap engineering: Zn1-x(Cd,Mg)xO compound thin films

� Applications as transparent conducting oxide:front contacts for flexible solar cells

(0001) Sapphire

(0001) ZnO

M-2000VI in-situ Ellipsometer (J.A.Woollam Co.)

Si ε1 at elevated temperatures

Sample temperature vs. heater power ZnO band-gap energies

ZnO <ε1> at elevated temperatures

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-10

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Del

ta [d

eg.]

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2.355 eV

Two-step-growth-process2

In-situ data analysis: Virtual-Interface-Approach3

In-situ data analysis results

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Photon Energy [eV]

16 min. 29 min. 39 min.

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k

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Model Fit 3.317 eV 2.355 eV 1.817 eV 1.252 eV

Step 1

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ta [d

eg.]

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Model Fit 3.317 eV 2.355 eV 1.817 eV 1.252 eV

Step 2

Ex-situ data analysis

Growth Conditions

2 E. M. Kaidashev et al., Appl. Phys. Lett. 82, 3901 (2003).

O2-Pressure = 3*10-4 mbar400 Laser pulses /1 Hz/600 mJ

Step 1

O2- Pressure = 1*10-2 mbar16000 Laser pulses /10 Hz/600 mJ

Step 2

3 D. E. Aspnes et al., Appl. Phys. Lett. 57, 2707 (1990).

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T = A*Pn/(kn+Pn)+B

T - Temperature [K]P - Heater-Power [W]A-,B-,n - Calibration Coefficients

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ture

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Power [W]

Si Wafer Model Fit

400 600 800 1000 1200

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3.4 Eg

BE(T)=Eg(0)-β*θ/[exp(θ/T)-1]

Ene

rgy

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T [K]

E0

A

E0

B

E0

C

Bose-Einstein Model Fit

T=1053 K

www.solarion.de

Future task: CuInSe2/ZnO heterostructures � CIGS absorber layers for high-efficient solar cells:

New generation of thin film solar cell structureson flexible polymer sheetsBand gap engineering: Cu1-x(In,Ga)xSe2 compound thin films