GROWTH AND CHARACTERISATION OF Cu(In, Ga)Se, THIN FILMS FOR SOLAR CELL APPLICATIONS

A thesis presentedfor the degreeof DOCTOR OF PHILOSOPHY in the University of Salford

By

E0 Ahmed jaz

Departmentof Electronic and Electrical Engineering University of Salford, U.K.

1995

Abstract

The developmentof low cost, efficient photovoltaic devices is a major technological demands suitable materials and fabrication processes.Thin film challenge which heterojunctionsolar cells appearto be most appropriatewith respectto polycrystalline cost and ease of manufacture, and it is anticipated that the next generation of photovoltaic devices will be basedentirely on thin film technologies.

Copper based ternary and multinary compounds are well establishedas exceptional in the fields of solar cells for both terrestrial semiconductors with potential applications and spaceapplications,infra-red detectors,light emitting diodes etc. The chalcopyrite forms of these compounds have large absorption coefficients and exhibit superior Among thesecompounds,CuInSe. (CIS) and CuIn,..GaSe2 (CIGS) radiation resistance. , have raised the most interest and recent thin film heterojunction photovoltaic devices based on these materials have achieved efficiencies of the order 15.5% and 16.9%

The in CIGS baseddevicesis due to the fact respectively. higher efficiencies realised that the bandgapof the materialcanbe adjusted towardsthe optimumvalue (1.45eV) by the partial substitution gallium for indium. of

In this work, thin films of both CIS andCIGSwere deposited by onto glasssubstratesflash evaporation of the respective pre-reacted source materials. The substrate Two types of evaporation temperaturewas varied betweenroom temperatureand 20011C.

twin chimneywereused.'Me effectof sources, flat tungsten a anda molybdenum stripthe growth conditions on the film propertieswas observed.The structural,compositional

and electro-optical propertieswere studiedusing a variety of analyticaltechniques

including x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive analysis with x-ray (EDAX), x-ray fluorescence (XRF), Rutherford backscattering spectroscopy (RBS), four point and thennal probe techniques, (PC) and photoacousticspectroscopy(PAS). photoconductivity

The as-grown films were found to have a columnar structure and a strong preferred Resultsfrom EDAX, XRF and the planeparallel to the substrate. orientationwith RBS indicated that the as-grownfilms were slightly deficient in selenium,otherwisethe compositionwas comparablewith that of the starting polycrystalline material. Electrical both n- and p-type conductivities with resistivity values in the measurements revealed rangeto 106 gCM. 101

Ile

as-grown films were subsequentlyprocessedunder several sets of conditions

including vacuum,selenium,inert andforming gasambientsat different temperatureand times. A two stagepost-depositionheattreatmentof the films was developedto improve

It the composition crystalstructureandto optimisethe electro-optical and properties. was observedthat the first annealingstage (in a seleniumambient)producedan improvement the composition thefilm. An increase the film grainsize in in excellent of (to > 2pm)was observed in a forming gas the films weresubsequently when annealed in the opticalproperties. ambient.Significantimprovements were alsoobserved

The as-grownand annealed films were analysedusing the PAS techniquewhich revealed

donorandacceptor the existence several of states originatingfrom intrinsicdefectlevels. The resultswerecompared PhotoconductivitY from singlecrystals. with thoseobtained thin films. measurements alsoperformed the as-grown were on

Table of contentsList of figures ............................................... List of tables ............................................... List of symbols .............................................. i vii ix

Acknowledgments

...........................................

X

Chapter 1 1.1 1.2 1.3 1.4

Introduction Introduction ...................................... Photovoltaic materials ............................... Aims and objectives ................................ Preview of the thesis ...............................

9 10

Chapter 2

Literature survey of Cu(In,Ga)Se2 thin films photovoltaic materials and their preparationtechniques Introduction ..................................... CuInSe2 ........................................ 2.2.1 Structural properties .......................... 13 14 18

2.1 2.2

2.2.2 Electricalproperties ........................... 2.2.3 Opticalproperties ............................ 2.2.4 Heterojunctions .............................. CuGaSe2 ....................................... 2.3.1 Structural properties ..........................

2123 24 25 28

2.3

2.3.2 Opticalproperties ............................

2930 30 31 32 33 34 34 35 35 37 38 38 39 39 40 40 41 42

2.4

2.5 2.6

2.3.3 Electrical properties ........................... 2-3A Heterojunctions .............................. 2.3.5 Photoconductivity ............................ CuInl..Ga.Se2 .......... 0 ......................... 2.4.1 Structural properties .......................... 2.4.2 Electrical properties ........................... 2.4.3 Optical properties ............................ 2.4.4 Heterojunctions .............................. Post deposition processing ........................... Thin film deposition techniques ....................... 2.6.1 Physical methods * .......... .......... 0 ...... i) Flash evaporation ....................... Single/double sourceevaporation U) ............ Co-evaporationtechnique iii) .................. iv) Electron beam evaporation ................. Laser beam evaporation V) ................... Molecular beam epitaxy (MBE) vi) ... ****... Sputtering process vii) .......................

0a0

2.6.2

2.7

Chemical methods ............................ Chemical vapour deposition (CVD) i) ........... ii) Electrodeposition ....................... iii) Chemical spray pyrolysis .................. iv) Screen printing ......................... Conclusions .....................................

42 43 44 45 45 46

Chapter 3 3.1 3.2 3.3

Experimental 48 Introduction ..................................... 48 Preparationof stoichiometric mixture .................... 51 Flash evaporationsystem o. .......................... 51 3.3.1 Vacuum system o .......... . ................. 53 3.3.2 Working chamber ............................ 53 3.3.3 Control unit 0 ................ .. 0..... 00..... 55 3.3.4 Evaporationsources .... o ............ ........ 55 3.3.5 Improvementsmade in the system ... ........... 61 Preparationof thin films o o ..... .... o.. o............. 61 Post deposition heat treatments o .... o ..... ............. 63 X-ray diffractometer o.. o.. oo........... .o.......... 66 Raman scatteringspectroscopy o ........ o. o...... ...... 69 Rutherford backscatteringspectroscopy 0 ..... ...... o ..... 71 Energy dispersivex-ray analysis (EDAX) 0 ...... ... 0...... 74 X-ray fluorescence ........... 0000....... 0 ......... 75 Dektak measurements o .o.... o ...................... 76 Resistivity and carrier type measurements ... o ............ 81 Photoconductivity system ...... o .... o ....... o 86 Photoacousticspectroscopy o .................. 88 Conclusions ........... o ...... o ...... o ...

3.4 3.5 3.6 3.7 18 3.9 3.10 3.11 3.12 3.13 3.14 3.15

Chapter 4 4.1 4.2

Effects of deposition parameters the propertiesof as-grown on thin films Introduction ..................................... Investigation the startingmaterial of .................... 4.2.1 CuInO. 75GaO25Se2 ............................. 4.2.2 CuInSe. ................................... Effect of source temperature ......................... Sourcegeometry ................................. Effect of substrate temperature................. 4.5.1 X-ray diffractionanalysis ...................... 4.5.2 Ramanspectroscopy ......................... 4.5.3 Scanning electronmicroscopy ................... Depositions underoptimisedconditions ................. 4.6.1 Structural properties ......................... XRD analysis .............................. 89 90 91 97 102 113 119 120 124 130 134 135 135

4.3 4.4 4.5.

*. 00..

4.6

4.7 4.8

ScanningElectron Microscopy .................. 4.6.2 Compositional analysis ....................... X-ray fluorescencespectroscopy ................. Energy dispersivex-ray analysis ................ Rutherford backscatteringspectroscopy ............ 4.6.3 Comparisonof XRF, EDAX and RBS results ........ Electrical Properties ............................... Conclusions ....................................

138 140 140 142 147 153 154 157

Chapter 5 Effect of post-depositionannealingon the film properties 5.1 5.2 Introduction .................................... Structural properties ............................... 5.2.1 X-ray diffraction analysis ...................... 159 160 160

CIGS

5.3

cis ,**,, * ... ****, ........... 5.2.2 Scanningelectron microscopy ................... 5.2.3 Raman spectroscopy ......................... Compositionalproperties ...........................5.3.1 Energy dispersive x-ray analysis .................

....................................

160* 167 170 176 181181

5.4 5.5

5.3.2 Rutherford backscattering analysis ................ Electrical properties ............................... Conclusions ....................................

CIGS .................................... CIS .....................................

181 183185 191 197

Chapter 6 6.1 6.2

Photoacoustic spectroscopy Photoconductivity analysis and Introduction .................................... (PAS) Photoacoustic Spectroscopy .................... 6.2.1 The RG theory ............................. 199 200 201206 209 215 223 223 231 242 250 254 257 271

6.3

6.4 6.5 6.6 6.7

6.2.2 Determination of the absorptioncoefficient a ........ 6.2.3 Modifications to the standardPAS theories - applications to this work ............................... PAS experimentalanalysisand results .................. 6.3.1 The effect of annealingon the observeddefect levels .. Selenium anneal ............................ Two stageanneal ........................... Laser anneal ............................... Effect of ion-implantation ........................... TransmissionPAS analysis .......................... Photoconductivity analysis .......................... Conclusions ....................................

Chapter 7 7.1 7.2

Summary and future work recommendations Conclusions .................................... Future recommendations ........................... 273 276 280 300 301 308

References ............................................... Appendices Appendix A ......................................... Appendix B ......................................... Appendix C .........................................

iList of figures Chapter 2 Figure 2.1: The tetragonalchalcopyritestructureMustratedfor ternary and multinary compounds.[351. The pseudobinary phase diagram of the Cu2Se-In2Se3 SYSteM

Figure 2.2:

Figure 2.3:

[1151. The pseudobinaryphasediagram of the Cu2Se-Ga2Se 3system ,

Chapter 3 Figure 3.1: Figure 3.2: Figure 3.3: Figure 3A Schematicarrangement the vacuum system. of Internal arrangement the vacuum chamber. of Design of the molybdenumtwin chimney evaporationsource. Design of the substrateholder/heater

Figure3.5: Figure3.6:Figure 3.7:

diin films. for post-deposition Pyrex ampoule of annealing used Schematic diagramof the x-ray diffractometer.Block diagram of the Raman spectroscopyexperimental.

Figure3.8: Figure3.9:

Rutherford backscattering setup. spectroscopy Ilermal probeapparatus measure conductivitytype of the sample. to the

Figure 3.10: Four point probe apparatus measurethe resistivity of the sample. to

Figure3.11: Correction divisor for probeson a thin film (non-conducting substrate) Figure3.12: Photoconductivity experimental setup.Figure 3.13: Cross-sectional view of the cryostat used for low temperature photoconductivity measurements. Chapter 4 Figure 4.1: XRD spectrumof the polycrystalline CuIno. starting 75GaozSe2pre-reacted material.

iiFigure 4.2: EDAX spectrum of the polycrystalline, CuInO. pre-reacted 75GaO. 25Se2 starting material. XRD spectrum of the polycrystalline CuInSe2 pre-reacted starting material. EDAX spectrum of the polycrystalline CuInSe2 pre-reacted starting material. Effect of sourcetemperatureon the compositionof polycrystalline CIGS thin films. Different regions of indium, gallium and selenium curves exhibiting the behaviour of polycrystalline CIGS film composition with respect to sourcetemperature. Vapour pressuredata of copper, indium, gallium and selenium [1961. Normalised concentration of copper, indium, gallium and selenium in CIGS thin films against the normalised temperature (Normalisation temperaturefor, copper - 1150"C; indium - 1300*C; gallium - 1200"C and selenium - 1200*0 to indicate the dilution effect.

Figure 4.3:

Figure 4A

Figure 4.5:

Figure 4.6:

Figure 4.7: Figure 4.8:

Figure4.9:

(Ra=214A) polycrystalline Surface CIGS thin film sample roughness of by prepared flat strip tungsten source.

Figure 4.10: Surface roughness(Ra=12A) of polycrystalline CIGS thin film sample preparedby twin chimney molybdenum source. Figure 4.11: Thickness profile of the polycrystalline, CIGS thin film measuredby Dektak.

Figure4.12: X-ray diffraction spectra CIGS films deposited different substrate of at temperatures. Figure4.13: Raman CIGSthin films deposited 200T (a), at NOT (b), spectra of at (c) CIGS singlecrystal (d). at room temperature andstandardFigure 4.14: Effect of film composition on the Raman spectraof CIGS samples. Figure 4.15: Scanning electron micrographs of CIGS films deposited at substrate temperatureof 200*C (a), at NOT (b) and at Room temperature(c). Figure 4.16: Nucleation and physical film growth process. Figure 4.17: Representative x-ray diffraction spectraof three CIGS thin films prepared under optimised deposition conditions.

iiiFigure 4.18: Representative diffraction spectraof three CIS thin films prepared x-ray under optimised deposition conditions. Figure 4.19: Scanning electron micrographs of CIGS (a) and CIS (b) thin films preparedunder optimised deposition conditions. bY Figure 4.20: Weight percentof elementsin polycrystalline CIGS films asmeasured XRF technique. by Figure 4.21: Weight percentof elementsin polycrystalline CIGS films asmeasured EDAX technique. Figure 4.22: Compositionaltriangle of CIGS and CIS thin films measuredby EDAX technique; filled circles (CIGS thin films), filled square (CIGS source (CIS thin films) and empty square (CIS source material), empty circles material). Figure 4.23: EDAX spectrumof the polycrystalline CIGS thin film. Figure 4.24: EDAX spectrumof the polycrystalline CIS thin film. Figure 4.25: Measuredand calculatedRBS spectraof CIGS thin film.

Figure4.26: Effect of thickness the RBS spectrum CIGSthin films. on ofFigure 4.27: Effect of thicknesson the RBS spectnm of CIS thin films. Chapter 5

Figure5.1:Figure 5.2:

XRD spectra CIGSthin films; (a) vacuumannealed 3000C one for of at hour, (b) the as-grown.XRD spectraof CIGS thin films; (a) annealedin selenium at 3000Cfor H2 two hours, (b) N2: annealedat 300*C for 2 hours and (c) the as-grown.

Figure5.3:

XRD spectra CIGSthin films; (a)selenium of at annealed 300"Cfor two hours+ argonannealed 300T for half hour, (b) selenium at at annealed 300*Cfor two hours+ N2: H2annealed MOT for half hour and W the at as-grown. XRD spectra CIS thin films; (a) selenium of at annealed 300"Cfor two hours+ N2: annealed 300"Cfor two hoursand (b) the as-grown. H2 at Scanning in electronmicrographs CIGSthin films annealed selenium of a) at 400*C,b) at 300"Candc) at 2500Cfor two hours.

Figure5A Figure5.5:

ivFigure 5.6: Scanningelectronmicrographsof CIGS thin films; (a) annealed under 9:1 N2: ambient at 300"C for two hours and (b) at 3000Cfor H2 mixture of one hour. Scanningelectronmicrographsof CIGS thin films; (a) seleniumannealed 300*C for two hours + N2: annealedat 300'C for two hours in a H2 at infra-red heater, (b) in a constantresistive heaterat 300"C for two hours (c) the as-grown. and Scanningelectron micrographsof CIS thin films; (a) selenium annealed 300'C for two hours + N2: annealedat 300"C for two hours, (b) H2 at 300*C for two hours and (c) the as-grown. selenium annealedat Raman spectraof CIGS thin films; (a) selenium annealedat NOT for two hours and (b) the as-grown.Raman spectra of CIGS thin films; (a) selenium at 300"C for two hours + N. M. annealed at 300*C for two hours, (b) the as-grown and (c) a standard CIGS single crystal.

Figure 5.7:

Figure 5.8:

Figure 5.9:

Figure 5.10:

Figure 5.11: Ramanspectraof CIGS thin films; (a) N2: H2annealed at 200*C for two hours, (b) selenium annealedat 300"C for two hours + N2: annealed H2 NOT for two hours and (c) a standardCIGS single crystal. at Figure 5.12: Compositionaltriangle for CIGS thin films; (a) the as-grown,(b) vacuum annealedat 300"C for one hour, (c) annealedunder selenium ambient at 300T for two hours, (d) two stageannealedat 300'C for two hours and (e) annealedunder N2: ambient at NOT for two hours. H2

Figure5.13: RBSspectra CIGSthin films beforeandaftervacuumanneal 300"C of at for onehour.Figure 5.14: RBS spectraof CIGS thin films; the as-grown (full line), annealedunder 9:1 mixture of N,: H2 (dashedline) and annealedunder selenium ambient (dotted line) at 300*C for two hours.

Figure5.15: RBS spectraof CIGS thin films annealed under seleniumambientat NOT for two hours followed by an annealin 9:1 mixture of N,:H2 (dottedline) or argon(dashed line) at 300T for half an hour.Figure 5.16: RBS spectraof CIS thin films annealed under seleniumambient at 300*C for two hours followed by an annealin N2: H2ambient at 300"C for one hour. Figure 5.17: Resistivity against the annealing temperature for CIGS film annealed under selenium ambient.

V

Figure 5.18: Log of resistivity against the annealing temperature for CIGS film annealedunder selenium ambient Chapter 6 Figure 6.1: Figure 6.2: Schematicof a standardphotoacousticspectrometercell [2331. Dependence of the absorption coefficient a on the normalised [240] and oQ; m- present work, photoacousticamplitude signal [2411. The effect of interference from a two layer structure (CIGS thin film depositedonto molybdenumcoated glass substrate). Comparative room temperature amplitudesignal normalisedphotoacoustic from p-type,CIGS thin film and a specimenof un-coatedglass slide.

Figure 6.3:

Figure 6A

Figure6.5:Figure 6.6:

Normalised for photoacoustic spectra threerepresentative polycrystalline CIGSthin films. as-grownSemi logarithmic plot of the absorption coefficient as a function of photon energy. Plot of (ahV)2againstthe photon energyto calculatethe band gap of the as-grownpolycrystalline CIGS thin films.

Figure 6.7:

Figure6.8:

Photoacoustic spectra the as-grown of at and seleniumannealed 3000C for two hoursCIGSthin films. Photoacoustic spectraof p-type single crystal and seleniumannealed thin film of CIGS. polycrystalline

Figure6.9:

Figure 6.10: Semi logarithmic plot of the absorption coefficient as a function of (0.9 eV to 1.3 eV) for a selenium annealedCIGS thin photon energy film.

Figure6.11: Spectral response the absorption of coefficientin the tail of the spectra (0.7 eV to 1.0 eV) exhibitingthe deeper statetransitions.Figure 6.12: Plot of (ahv)' againstthe photon energyto calculatethe band gap of the selenium annealedpolycrystalline CIGS thin films. Figure 6.13: Comparativeplots of the Spectral distribution of CIGS thin films annealed H, under selenium and N2: (FG) ambient at different temperatures. Figure 6.14: Comparativeplots of the spectraldistribution of CIGS thin films annealed under selenium and argon ambient at different temperatures.

viFigure 6.15: Comparative plots of the absorption coefficient of CIGS thin films annealedunder selenium and argon ambient at different temperatures. Figure 6.16: Plot of (ahV)2 against the photon energy to calculate the band gap of polycrystalline CIGS thin films. selenium and argon annealed CIGS thin film Figure 6.17: Normalisedphotoacoustic spectrumof seleniumannealed in the photon energy range between0.7 eV to 3.1 eV. Figure 6.18: Comparativeplot of the normalised photoacousticamplitude signal for polycrystalline,CIS thin films before and after laser annealing. Figure 6.19: Comparative plot of the absorption coefficient of a single crystal (a), CIS thin film before Q and after laser annealing (b). polycrystalline Figure 6.20: plot Of (CdV)2 the photon energyto calculatethe band gap of CIS against thin films before and after laser annealing. Figure 6.21: Comparativeplots of the spectraldistribution of CIGS thin films annealed under N,: H2 (FG) ambient at 400*C and H* implanted. Figure 6.22: Comparativeplots of the spectraldistribution of CIGS thin films annealed under argon ambient at 400*C,and H' implanted. Figure 6.23: Transmissionphotoacousticspectraof un-coatedglass slide and the asgrown CIGS thin film. Figure 6.24: Photoconductivity spectrumof n-type CIS single crystal.

Figure6.25: Photoconductivity spectrum n-type CIS singlecrystal. of Figure6.26: Photoconductivity of spectrurn p-typeCIS singlecrystal.Figure 6.27: Photoconductivity spectrumof p-type CIS thin film.

Figure6.28: Photoconductivity at spectrum p-typeCIGSthin film measured room of temperature. Figure6.29: Photoconductivity at spectrum p-typeCIGSthin film measured 250'K. ofFigure 6.30: Photoconductivityspectrumof p-type CIGS thin film measuredat 150"K. Figure 6.31: Photoconductivity spectrumof p-type CIGS thin film measuredat 77*K. Figure 6.32: Photoconductivity spectral distribution of CIGS single crystal using the photoacousticspectrometer.

viiList of tables

Table 2.1:

Physical and electro-optical properties of CuInSe2 semicon ucting compound.Properties of CuGaSe. semiconducting compound.

Table 2.2:

Table 3.1:

A calibration table to obtain an accurate estimate of the evaporation temperature.

Table 4.1: Table 4.2: Table 4.3: Table 4A Table 4.5: Table 4.6:

X-ray powder diffraction data of CIGS pre-reactedstarting material. EDAX compositional analysisof CIGS pre-reactedstarting material. X-ray powder diffraction data of CIS pre-reactedstarting material. EDAX compositional analysisof CIS pre-reactedstarting material. Enthalpies and Gibb's free energies[1971. Comparisonof the surfaceroughnessof CIGS thin films preparedby flat strip type tungstensourceand twin chimney type molybdenum source. Comparative analysis of the structural properties of Cu(In,Ga)Sezthin films using XRD technique.

Table 4.7:

Table4.8:

Comparative compositional analysisof CIGS thin films using EDAX, XRF andRBS techniques. Electricalproperties flash evaporated CIGS and CIS thin films. of

Table4.9:

Table 5.1:

Comparisonof the x-ray powder diffraction data of the polycrystalline pre-reactedstarting material and CIGS thin films. Comparisonof the x-ray powder diffraction data of the polycrystalline, pre-reactedstarting material and CIS thin films.

Table 5.2:

Table5.3:

Compositional analysis theas-grown, ambient of annealed underselenium thin andtwo stageannealed films of CIS.

viiiTable 5A Electrical properties of CIS thin films before and after annealingunder selenium ambient at 300*C for two hours followed by an annealunder N2: 112ambientat 300*C for two hours.

Table 6.1:

The gap energy and the ionisation energiesof the defect levels of the asgrown and annealed(under different ambient and temperatures)CIGS thin films. Energy band gap values of annealed (under different ambient and temperatures) thin films of CIGS at different modulating frequencies. 'Me fundamental energy band gap E. and different defect ionization energies (E, - E) of p-type CIS single crystal and thin films as determinedby photoacousticspectroscopy.

Table 6.2:

Table 6.3:

ixList of symbols

10 a: h: V: hv 1% f: (0 T. is Is lb Qj Cj j Bj xj

PS 9: b: R: Rb q: 11

: Incident monochromaticlight flux Absorption coefficient Planck's constant Frequencyof light : Photon energy : Wavelengthof the incident light Chopping/Modulatingfrequency : Radian frequency : Ambient temperature : Thicknessof the sample : Thicknessof the gas column : Thicknessof the backing material : Density of material j : Specific heat of material j : Temperatureof medium j : Thermal diffusivity of material j : Thermal conductivity of material j : Thermal diffusion coefficient of material J : Thermal diffusion length of material j : Photoacousticamplitude signal Ratio of thermal conductivity & diffusivity of gas and sample Ratio of thermal conductivity & diffusivity of backing material & sample Reflectancefrom the gas to the sample : Reflectancefrom the sampleto the backing material Normalised.photoacousticamplitude signal : Non-radiative quantum efficiency

xAcknowledgments

The author wishes to thank Dr. A. E. Hill for his invaluable guidance and supervision throughout this work. Special thanks are due to Dr. P.D. Tomlinson and Dr. A. given Zegadi for their helpful suggestions, advice, help and encouragements.

My sincere thanks goes to Dr. R.D. Pilkington for his great help and moral support during the period of this work. Many thanks are due to ProfessorH. Neumannfor his informative discussionson my researchtopic, and to Professor S Leppdvuori of the University of Oulu, Finland for organising the Raman analysis.

It is a great pleasureto acknowledge valuabletechnicaladvice, assistance the andfriendship that I have had from Mr. JJ. Smith, Mr. E. Brimble, Mr. K. Bullock and Mr. P. Cardwell.

I am also extremely grateful to the Ml*Mstryof Education, Governmentof the Islamic Republic of Pakistanfor awardingme the Central Overseas Training scholarshipand the BahauddinZakariya University, Multan, Pakistanfor granting me study leave. Without

their supportnoneof this work would havebeenpossible.

Chapter I

Introduction

1

1.1

Introduction

Shortly after the discovery of photovoltaic effect in single crystal Silicon p-n junction [11 and CdS-Cu schottky barriers [21 in mid 1950's, the replacementof fossil energy feasible andhighly attractive [3]. Considerable resources seemed researchefforts started highly worldwide and led to a large numberof relevantpublications.It thereforeappears likely that major advanceswill be made by the use of new advancedmaterials and A processes. number of materialssuch as copper (I) sulphide, silicon, gallium arsenide, cadmium telluride and copper indium, gallium diselenide, are being studied for the efficient harnessingof solar energy through solid state solar cells in an cost effective

manner

The aim of this chapteris to describebriefly the historical developmentof photovoltaic materials. The most promising of these materials are compared in terms of their

The aims and objectivesand a brief introductionof this propertiesand applications. thesisareoutlinedin the last sectionof this chapter.

1.2

Photovoltaicmaterials

The sun has alwaysbeenthe main sourceof energyfor life on earth. However, it is only during the last two decades that man has learnedto convert solar radiationsdirectly into degreeof efficiency. The efforts that went into space electrical energywith a reasonable

technologyled to an intensifiedand systematic development photovoltaicpower of

2generation.Ile presenttechnologicallevel of this type of power generationreflects the high requirements of space technology. The ever increasing scope of space-craft further development. In addition, the missions, satellite power and life, necessitates terrestrial applications of solar energy have become increasingly more important throughout the world which has given further impetus to researchand developmentin this area.

The limited resources the earthhasforced man to searchfor and developnew sources of of energy. Photovoltaic power generationis one attractive alternative way to produce energy.A major advantage solar energyis that it is renewableand, unlike fossil fuels, of

will not run out. Solarenergyis alsoa muchcleaner non-pollutingsource power and of by-products. Sincethe amount energythatreaches asit doesnot haveany dangerous ofthe earth surface is approximately 10,000 times than that of it's total energy consumption,the global potential for solar energy is large and rapidly growing due to

it our ability to harness efficiently.

Photovoltaicconversion systems requirespecialforms of semiconductor materialsorcompoundswhich can be fabricatedto contain a junction or interfacewith the necessary

In into the material, it is electroniccharacteristics. general,when light penetrates free As drift across junction andgenerates electricalcharges. thesecharges the aabsorbed they arecollectedby contacts appliedto the exteriorsurfaceof the semiconductor andthus produce a current. By combining a number of solar cells in an appropriateway it is possi e to develop power sources of various capacities. The generation of photovoltaic power dependsupon the employment of solar photons for the production

3holes. This can be most readily done by the absorption of solar of electrons and in a suitable semiconductingmaterial. Since the solar photons have specific radiation the efficient creation of electron-holepair is thus highly dependent the type on energies of materials employed and their properties.

A material is consideredto be suitable for use as an absorber layer in photovoltaic devices if it has the following properties:

direct band gap high absorptioncoefficient few and inactive grain boundaries

goodmobility andminority carrierlife timelong term stability i. e. radiation resistant

For practicalpurposes availabilityof suitable for the with materials largeareadeposition high throughputand high material yield on inexpensivesubstrates must also be Many materialsand processes being studiedin order to fulfil these considered. are importantobjectives[3].

Solar cells have been mainly developed in either single crystal or thin film form. Although single crystal solar cells of Silicon (Si) and Gallium Arsenide (GaAs) have achievedhigh conversionefficiencies,their production costs are still very high and with

The effectsof it existing technologies is difficult to achievefurther cost reductions.radiation [4,51 and ambient temperature [61 on the device properties showed these

4to be highly susceptible their conversionefficiencies degradingwith time. materials with Alternatively, thin film technologiesoffer an important option for low cost devicesand have recently become the focus of increasedattention [7]. Considerableprogresshas been made with respect to high conversion efficiencies and low cost m acturing.

Furthermore, thin film solar cells have exceptionally high radiation toleranceswhen to conventional single crystal cells [4]. compared

General analysis shows that there is a range (1.15 eV to 1.4 eV) of optimum energy band gaps for photovoltaic conversion [8]. Although, the band gap of silicon lies at 1.12eV,just below the optimum band gapsrange.However, a large amountof research done on this material has resulted in a large conversionefficiencies [9]. A major work

disadvantage photovoltaic is that the bandgap in silicon is indirect To excite for usean electronfrom the valenceband of silicon to the conductionband both the energy and momentum must be changed. Since sunlight supplies only the required energy the

Due from the latticevibrations(phonons). to theneed momentum change mustoriginate for both a photon and phononto be involved, the absorptionof light in silicon is therefore thanin directbandgapsemiconductor muchweaker suchasgallium materials [101. arsenide

Thin film polycrystalline silicon is usedfor low cost solar cells. However there is a need for further developmentin order to improve it's stability and conversion efficiencies. The main advantages thin film solar cells comesfrom the lower production costs,the of potential for higher conversionefficiencies and a reduction in the material requirements comparedto single crystal baseddevices [7]. The use of polycrystalline silicon for solar

5the potential for high perfonnance in conjunction with low cost cells offers However, both theoretical and experimental investigations [11] have manufacturing. identified issuesof grain size and film thicknessas limiting factors and further work on these and back-surfacepassivation is necessaryto raise this technology to the next technological level.

Initial research [121 reported that amorphous silicon, when deposited by vacuum evaporationor sputtering,showeda high degreeof conformationaldisorderwith a large number of unsaturateddangling bonds which affect the electronic properties due to inactive dopants.However, in 1972, Spear [131 used hydrogen during the electrically deposition process, saturating most of the dangling bonds producing hydrogenated

amorphous silicon with good electronicproperties.The effective band gap of thismaterial can be varied, dependingon the depositionparameters, within a rangeof about 1.6 - 1.7 eV. It can further be tailored, by the addition of carbon to raise, germanium or tin to reduce,the band gap. However, the resultantperfonnanceof the devicesbased

is to on these materials poorcompared thatof purea-Si.Bandgapengineering thus was fabricated. dateglow-discharge have To the key to optimisingthe devices techniques 13% [141. the produced mostefficient devices with conversion efficiencies about of

A significantdifficulty with a-Sisolarcell technology that it suffersfrom an intrinsic isdegradation effect [4]. Mie first generation a-Si modules experienced about 20% degradationin peak power over two years of exposureto light [4]. The most recent a-Si

solar cells are more stable,but still experience 10-15%loss of performance aFurther improvements and better understandingof the physics of these devices are

6in orderto reducethis degradation effect. necessary

For many years copper (I) sulphide was known to be one of the most suitable material for hetero-junctionsolar cells [3]. From the late 1950's, for almosttwo decades, only the thin film solar cells were p-Cu2S/n-CdS[3]. The Cu2Swas preparedwith available all reasonableelectronic propertiesusing simple preparationtechnologiessuch as thermal havefailed However, all theseprocesses evaporation,sputtering,sintering, spraying,etc. to achievethe desiredconversionefficiencies. Only the topotaxial processes[31 based ion exchangereactionswere successfulin the production of Cu2Slayers with high on cell efficiencies. Simple heat processing in various atmospheresand several other treatmentshad also resulted in significant improvementsin the performance of these

devices. Furtherdetailsandthe historicaldevelopments this type of hetero-junction of devices be found in variousarticles[3,15,161. can

During the development of Cu2S/CdS thin film solar cells, a number of degradation

[31were observed to the complexnatureof this type of device.These due problems included:

(i)

The interfacebetweentwo materialswith different electronaffinities, bandgapsandcrystalstructures.

(ii)

'Me lattice mismatch and the inter-diffusion of components,in particular copper, resulting in defect statesat or near the interface.

(iii)

A variety of Cu2S phases room temperature. atInterface roughness,grain boundariesand random crystal orientation.

7Due to limited efficiency poor reproducibility and long term instability, minimal work on the developmentof this type of device has been carried out.

Thin film solar cell devicesbasedon 11-VIand I-III-VI2compounds are currently being [171 and have the following advantageous properties: studied extensively

Direct band gap High optical absorptioncoefficient. Moderate surfacerecombinationvelocities. Radiation hardness. Low cost.

For example, CdTe as an absorber material in thin film photovoltaic devices offers due to its Optimal direct band gap value of 1.45 eV. Since this significant advantages

materialcan be dopedp- or n-typethereis the possibilityof forming homo-junctions. Howeverhetero-junctions generally for solarcells.A wide variety of thin film are used depositiontechniques including vacuumprocesses, screenprinting, chemicalvapour deposition electrodeposition beenutilisedfor the preparation this material have of and [171.All of theseprocesses typically requirea high temperature at 400450"C [181 stepto achievethe desired material properties.

in preparing Of considerable interestaretwo problems which areparticularlyrelevant thesedevicesi.e. CdTedopingandsuitableohmic contacts[191.Varioussolutionstothe doping problem have beenproposedincluding ion assisteddoping by PVD, arsenic

8CVD and diffusion of copper [191. The ohmic contact problem can be overcome by using semi-metalHgTe as a contact material or by consideringmixed ternary alloys of HgCdTe for which preparationof ohmic contactsis easierthan CdTe [4].

Other materials which are being extensively studied for use as absorberlayers in thin film photovoltaic devices are copper basedternary and multinary compounds[7].

CopperIndium Diselenide (CuInSe) is a memberof the I-IH-VI, semiconductorfamily direct energy band gap of approximately 1.02 eV, with a chalcopyrite structure and a 300"K [201. Its excellent thermal stability, radiation hardness and high optical at it an ideal candidatematerial in efficient and low cost thin absorptioncoefficient make

film solarcellsfor both singleandtandem junctions[211.Solarcells based vacuum onevaporatedCIS films have been fabricated with a conversion efficiency of more than 16% [221.The main limits to CIS baseddevice efficiency are reproducibility, large area uniformity and the slightly less than optimum band gap. However, research is in

to progress improvethe efficienciesof the devicesby improving the uniformity and developing betterdeposition techniques.

Copper gallium diselenide (CuGaSe2, CGS) is another member of the

I'III-VI2

family andhasvery similar properties CIS in termsof structuraland to semiconductorelectrical behaviour. It has a direct band gap of 1.68 eV [231, slightly higher than the optimum value. Cell efficiencies better than 6% have been reachedfor CuGaSej(Cd Zn)S structures[241.

9Thin film polycrystalline solar cells basedon the ternary chalcopyrite compoundsCIS CGS have shown good photovoltaic properties.However, the fundamentallimit on and is the small open circuit voltage due to the the efficiency of CuInSe2based solar cells band gap value (1.02 eV) of the absorberlayer, the optimum value being - 1.5 eV. 'Me band gap of CIS can be adjustedtowards the optimum value by the partial substitution Gallium for Indium. The band gap of the resultant CuInl-.Ga.Se2 (CIGS) can be of increasedcontinuously from 1.02 eV (for pure CIS) to 1.68 eV (for pure CGS) by Y between zero and one [25]. Hetero-junctionswith (Zn,Cd)S as a window varying material have yielded more than 16% efficient cells [221.

1.3

Aims and objectives

Although, the I-III-VI2 family of semiconductingcompound appearto be a promising

for devices, therearesomemembers this family, such candidate usein optoelectronic of is Ga. for the as CuInl.. Se2, which the infortnationconceming electro-optical propegies scarce.In addition, these propertiesare highly dependent their structure and on Iherefore, to composition, which in turn arestronglyrelated thefilm growthparameters. films with is very importantto obtaincompound the choiceof the processing approachgood properties.A number of depositiontechniqueshave beenused in order to produce thin films of these materials. Whilst these have generateda great deal of scientific information none of the technologies employed have been widely accepted due to

Initial problems associated stoichiometric non-uniformities poorreproducibility. with andstudies have shown flash evaporationto be a technique with potential for producing

10

films of CIS and has been the subject of this investigation. stoichiometric

The objectives of the presentwork can be summarisedas follows:

1.

To investigateand developthe flash evaporationsystemto deposit reproducibly good quality CIS and CIGS films suitable for use in solar cells.

2.

To obtain a better fundamental understanding of the effect of deposition parameterson the propertiesof thin films.

3.

To characterise fully the compositional, structural and electro-optical properties

using a variety of techniques.To improve the reproducibility by optimising the deposition conditions.

5.

To study and optimisethe properties thin films by the use of variouspost of deposition processes.

6.

To study the defectchemistryusing optical techniquessuchasphotoconductivity

andphotoacoustic spectroscopy. 7. To assess and CIGS film properties CIS for the future fabricationof required solarcells.

1.4

Preview of the thesis

Many advancesin the technology of semiconductor devices were made possible by fundamental researchconcernedwith both qualitative and quantitative descriptionsof the physical and chemical phenomena in the semiconducting materials for their

11

The use of materials, such as Cu(In,Ga)Se2,in device photovoltaic applications. technology provide motivation as well as a basic frame work for the investigation Of both in Extensivebasic and appliedresearchon Cu(In,Ga)Se2, many researchproblems. thin film and single crystal form, have shown a considerablepotential for success. However, there is a need for further investigations in order to gain a better understandingof the fundamentalproperties.

Chapter2 highlights existing researchliterature of Cu(In,Ga)Se.ternary and quaternarylk IC .,

a opyrite semiconductingcompoundsfollowed by the review of some of the more I.

frequently employed deposition techniquesused for the growth of this material in thin film form. There have beena number of perspectives employedby different researchers towards the ultimate objective of understandingthe properties of this material. This chapter reviews the researchby dividing the discussioninto structural, electrical and the materials in device preparation. Thin film optical properties and application of deposition technologies are consideredwith respect to their use for the deposition of

ternaryandquaternary copperbased compounds.

Chapter3 gives a survey of the experimental techniquesfor the measurement of different propertiesof thin films along with the preparation pre-reacted starting ofthin films. material and the experimentalapparatusfor the deposition of Cu(In,Ga)Se2 The modifications made in the experimental system have also been considered.The analyzing techniquesare explained in terms of their working principle precededby a brief introduction about the technique.

12

A major consideration most depositionprocesses the effects of deposition for isparameterson the film properties.lberefore, the effects of deposition parameters such as source temperature,substratetemperature,source to substratedistance and source design,on the propertiesof depositedthin films are consideredin chapter4. r1beresults obtained are explained in view of the existing literature.

In many casesthin film propertiescan be tailored to achievethe required specifications for solar cells and this therefore merits investigation. Chapter 5 discusses effect of the post depositionheat treatmentson the propertiesof thin films. The film's characteristics are comparedwith respectto the different ambientsunder which post deposition heat treatmentsare carried out. Results are explainedby considering different mechanisms.

Chapter6 is devotedto the photoconductivity and photoacousticstudieson both the asgrown and annealedthin films. 7he mathematical analysis for the determination of carrier life time in photoconductivity mea=ements and the absorption coefficient and

bandgap calculations is usedin photoacoustic analysis alsoconsidered.

Finally, conclusionsand future work recommendations consideredin chapter 7. are

Chapter 2

Literature survey of Cu(In, Ga)Se2 thin films photovoltaic materials and their preparation techniques00

132.1 Introduction

The potential of the ternary andmultinary family of compoundsemiconductor materials, such as CIS, CGS and CIGS, for use in various electronic devices continuesto attract an increasingamount of interest. This has led to an extensivestudy of the growth and thesematerialsboth in thin film and single crystal forms. Compound characterisation of havebecomeleadingcandidates semiconductors amongstphotovoltaic materialsbecause a wide range of physical, optical and electrical propertiescan be achievedcomparedto elemental materials. However, for cost effectivenessand high throughput required for large scaleand large areaproduction,the preparationof thesematerialsin tin film form is necessary. is generally accepted, It therefore,that the next generationof high quality,

low costphotovoltaic devices largelyon currentandfuture developments will be based in thin film technologies.

The purpose of this chapter is to review relevant existing literature on copper based

importantwork on the The ternary and multinary compounds. first sectiondescribes structural,electricaland opticalproperties CIS, CGS and CIGS thin film materials of in the in The techniques alongwith their applications heterojunctions. deposition used in preparation thesethin film photovoltaicmaterialsare also considered the last of section.

142.2 CuInSe2

CIS, one of many I-III-Vl2 ternary semiconductingcompounds,was first reported by Hahn et al. [261 and later studied by Tomlinson [27,28]. This material crystallisesin a [291 which has a diamond like lattice with a face centred chalcopyrite structure tetragonal unit cell (spacegroup I42d). The chalcopyrite structure is a super-structure (see figure 2.1), which arises from the ordered substitution of the of sphalerite lattice, resulting in the doubling of the c-axis [291. The lattice structure of zincblende chalcopyrite CIS is shown in figure 2.1. The detailed calculations of the lattice gave of a=5.782A and c= 11.621A [301. values

In general, vacuumevaporation manycompounds, of alloys andmixturesis a difficultprocessrequiring a careful considerationof factors effecting their dissociation, which doesnot necessarilyhave can take placebelow their melting points. Ilus the condensate the same composition as that of the source material. Many workers [31,321 have

in this observed dissociation the deposition CIS.Partialvapourpressures In2Se and of of

Se2in temperature 900-1160"K have the measured a spectrometer been range using mass[331.'Mennal dissociation [341canthus occurvia the following route: 4CuInSe2-21n. Se(Vapor) (Vapor) +3Se, +4Cu(Solia)

The pseudobinaryphasediagram [351 of Cu-In-Se is shown in figure 2.2. This shows

that CIS melts congruentlyat 986"C. Above 810"C, CIS lies in the 8-phaseandcrystallizes with the sphaleritestructure,while below 810"C, it lies in the y-phaseand

from sphalerite Duringthe ft-m&ormation to ,,ryStallises with the chalcopyrite structure.

15

Cc=2a

01

via -

Figure 2.1:

The tetragonal chalcopyrite structure illustrated for ternary and multinary comounds.

16

Cu In Se, 2 I1000

800

cD

600400

E-4

20001

30Cu2Se

40Mole

50% In2Se3

60

70

Figure 2.2:

The pseudobinary phase diagram of the CU2Se-ln2Se3 system [351.

17 CuInSe2 336.28 (00 (00 (gm cm-') (A) 986 810 5.77 a=5.782 c=11.620 Sphalerite Chalcopyrite (7.89-8.32)10 (at 2730K) ('K) 207 221.9 0.73 Me Holes Electrons 0.09 me acceptor (meV) donor 85 400 10 225 [411 [421 [431 [431 [441 [451 [451 [451 [361 [361 [461 [361 [371 [371 [381 [301 [301 [391 [361 [401

Formula Molecular weight Melting point Phase transition Density Lattice parameters Symmetry

Thermal expansion coeff icient Deby temperature Effective mass Ionization energy

Dielectric constant Refractive index Then-nal conductivity Electron affinity Mobility (mW/Cm-K)

High freq. 8.1 Low freq. 13.6 2.7-3.0

86

[471

(eV) (cm'/V. s)

3.97 electrons 800 20 holes both n- and p-type

[361 [481 [491 [501

Conductivity type Band gap (eV)

[49,51,52-541 1.02 0.01 1.03 n-type crystal [551 [561 1.04 n-type film

Table 2.1:

Physical and electro-optical properties of CuInSe2 semiconducting compound.

18high cooling rates lead to micro-cracking due to thermal stresses and/or chalcopyrite, different expansion coefficient. Table 2.1 summarisessome measuredand estimated properties of CIS.

2.2.1

Structural properties

Researchers Salford University [571reportedthe preparationof the first CIS thin films at by the flash evaporationmethod.Sincethen the techniquehasbeenusedwidely [58-611. A systematicstudy of the effect of sourcetemperatureon the composition of CuInSe2 thin films has been carried out [62]. These films were copper deficient at source

below 1200"C.Comparable film composition that of bulk material to temperatures thintemperaturesgreaterthan 1300T. This was later confirmed by was realised at source Neumannet al. [63] and Hanemannet al. [64]. Ile effect of substratetemperature(27500"C) on the elementalcompositionof flash evaporatedthin films was examined[651.

Nearly stoichiometric films were obtainedat 447*C. Both lower and higher substrate in depletedand enrichedcoppercontentrespectively.Copper temperatures resulted deficiencycould be due to the reportedsourcetemperature under 1300T. This just is on the bottomedgeof the sourcetemperature temperature reportedby range value in Tomlinson et al. [621.On the other hand, higher substrate temperatures resulted(deficient in indium and selenium) which could be due to the enriched copper samples re-evaporationof In-Se compound.

Deposition of the first amorphous in thin films with thicknesses the range of 200-1000A

19in 1980 [661. A vibrating hopper and resistively heatedtantalum source was reported X-ray analysis in order to deposit thin films onto glass substratesat 77111C. were used the amorphousnature of the depositedfilms since no preferred crystalline confirmed direction was observed.Rutherford backscatteringspectroscopy(RBS) confirmed the films to be copper deficient.

Dissociation and decompositionof compoundsand alloy evaporantsmade the single technique very difficult for thin film deposition of Cu(In,Ga)Se2 source evaporation [67,681.However, someresearchers [69,701have reported successfulgrowth of CIS by this technique. Single phasefilms were obtained at a substratetemperatureof 500*K. X-ray and electron diffraction analysis of the films showed a preferred orientation

[69]. 7lie lossof selenium(themost dependent substrate temperature thickness on andfor volatile element in CIS) was compensated either by using an additional selenium [711 or by using excesselementalselenium in the source material [72,731. source

Co-evaporation copper,indium andselenium a mostwidely usedtechnique is is and of [36,48,74].It has also been usedby to be the most successful generallyconsideredDhere et al. [751. Indium, rich samples showed a sphalerite structure with

Don et al. [761reported the orientation a smallproportion the chalcopyrite and of phase.growdi of CIS layers,with different compositions,onto glasssubstrates GaAs single and in copper rich films deposited crystals. Additional phasessuch as Cu2Se were observed A on glass substrates. secondphase,identified as CGS, due to out-diffusion of gallium

from GaAssubstrates observed Cu-richfilms. Grain size,crystallinephaseand in wascrystalline orientation have beenfound [771to be strongly dependenton the eposition

20

heatingmethodswere When electronbombardment temperature. rate and substratefor copper and indium and with selenium being heatedusing a coil heater, employed effects were observed [78]. However, the substrate (either gold coated glass similar slides or gold metallized polycrystalline alumina discs) temperaturerange was varied between 200-500"C. The films were amorphous below 300"C and chalcopyrite at higher than 420"C. substratetemperatures

Thin films have also been deposited [79-81] by r.f sputtering from CIS targets. However, sputtering from compound targets is reported to be limited in terms of reproducibility, due to difficulties in reliably fabricating large area targets [821. Nearstoichiometric but amorphousfilms were produced [80] at low substratetemperatures.

It wasobserved r.f voltageandr.f powerandsubstrate havesignificant that temperatureeffect on the film structure. Chalcopyrite structure was observed at substrate temperatures greater than 673*K. Further temperature increases showed a rise in crystallite size at the expenseof film quality. Piekoszewski et al. [83,791 howevm

that to reported the composition their films wasinsensitive r.f voltage,argonpressure of distance.A fine powder producedIn rich and Se-deficient and target to substrate films whereas films resultedfrom targets multiphase single phaseand stoichiometric madeof coarse powder.

CIS has been depositedonto US substrates beam epitaxy (MBE) [84using molecular 861. A minimum substratetemperatureof about 573"K was found to be necessaryto achieveepitaxial growth. The ratio of arrival rates of Cu and In are critical in goveming layer stoichiometry [861. The use of an ion gauge beam flux monitor, to control the

21 [871. fluxes,in an MBE system resultedin a goodlayer stoichiometry has elemental

Many workers [88-901have reportedthe use of the Chemical Spray Pyrolysis technique in the preparationof CIS films. Initial investigations [88,891showedeither a sphaleriteor chalcopyrite structure along with secondary phases such as In203 in the as sprayed

films. Higher substratetemperatures (250-350*C) revealed only sphalerite,structure However, Bates et al. [901 observedthe conversion of without any additional phases. sphalerite structure to the chalcopyrite,when annealedfor a short time in nitrogen at temperatures 400-600"C. It was later reported [911that the chalcopyrite phasein the of

films resulted from increase pH of the original solution. in as-sprayed

Several other methods such as screen printing [921, electro-deposition[931,layers by thermal [94] and laser annealing [951, synthesizationof stacked elemental close spacechemical vapour trunsport (CSCVT) [961and metal oxide chemical vapour deposition (MOCVD) [97,98] have also been used with varying degree of successto

depositCIS thin films.

2.2.2

Electrical properties

P-type polycrystalline films (600-800 Qcm) were deposited at substratetemperatures above 7001% [57,99], whereas,highly resistive n-type deposits were obtained at low temperatures(520-620"K). The resistivity of the first flash evaporatedamorphousCIS films [661,measuredby Van der Pauw's technique,was in the range 2.2-8.5X106 gCM.

22 films of 1.2-1.6[unthicknesswere p-type with a hole concentrationof Flash evaporated 2xIO` Cm-3 the chargecontained20% excessselenium.For low, and sometimes when for high, percentagesof excess selenium, n-type conductivity was observed [731. A temperatureof 523"K was found to be the boundary between low and high substrate films with p-type conductivity. KazmersId et al. [691 observedn to p-type resistivity Hanemanet films were annealedin Ar/H2Se atmosphere. changein conductivity when [641however, noticed p to n-type conductivity changein electro-deposited films by al. annealingat 350*K in the presenceof indium, with an increasein resistivity.

Electrical properties of co-evaporated films were dependenton Cu:In ratios [751. thin P-type films with resistivities in the range 0.02-27 Qcm with a variation of 3 atm.%

In andCu aroundstoichiometry Highly resistivefilms wererealised of werereported.the indium. content was 5 atm.% higher than the stoichiometric value. Similar when dependenceof electrical properties on the Cu:In ratios was observed in CIS layers depositedby MBE [871.Ille seleniumarrival rate was also found to be important as low

selenium ratesalwaysproduced n-typelayers.

It has been reportedthat films were p-type when seleniumwas more than 45 wt"/o with

to n-type for lower values [1001.However,films deficient in copperwere a changethough they containedexcessof selenium [101]. The resistivity of always n-type even the sputtered [801 and screen printed films [92] produced from starting materials contamimig extra selenium was found to be lower than those made from stoichiometric

CIS materials.

23 2.2.3 Optical properties

The highest absorption co-efficient (6xlO' cnf') for films of thickness nearly 2000A by the three sourcetechniquehas been reported by Kazmerski et al. [1021.An grown improvement in the optical properties after heat treatments in argon, nitrogen and oxygen was also noted. High vacuum annealing, however, produced selenium deficienciesnear the film surfacewhich in turn degradedthe absorptioncharacteristics.

A shift of the characteristic peak from the near absorption edge towards shorter wavelengdiswas observedas the percentage excessseleniumwas raisedin the source of [731. For a near stoichiometric film an energy gap of 1.04 eV was found, material whereasfilms with Cu:In ratio J. 25 showedthe presenceof an additional phasewith an energy gap of 1.23 eV, which was attributed to the Cu2Se.

A detailed study [1031of optical propertiesof co-evaporatedCIS films has been made

in relationto the composition deposition Ile and process parameters. films showeda direct optical transitiongap with an energynear 1.0 eV. For somefilms therewas a forbidden direct transition with an energy of 1.2 eV and strong dependencies of refractiveindex,absorption co-efficientandopticalgapson Cu:In ratio. The measured absorption coefficientwasin the range1-6xl04 CM-1

for differentatomicpercentages of

constituent elements [1041.The energy gaps for thesefilms ranged from 0.940.01 to 1.020.01eV. A mild dependency band gap on compositionwas found only in films of depositedat 723"K.

24

in films [851showed increase absorption Molecularbeamdeposited an epitaxialp-typebelow the edgesas well as shoulderwhich can be attributedto the coppervacancyband. films which A higher band gap of 1.10 eV was formed in the as-grownelectro-deposited [64]. Optical band gapsof relatively after post depositionheattreatmentswas decreased films of CIS were examined using photothermal deflection rough polycrystalline spectroscopy[1051.A slight shift in band gap towards lower energiesoccurred for air

annealed samples.

2.2.4

Heterojunctions

The fabricationof solarcells utilizing CIS as an absorber by layer hasbeenreported a [106-108,791. Among the many possiblematerials,cadmium numberof researcherssulphide is promising as high band gap window material, due to the fact that the lattice mismatch is very small, especially with ZnCdS. Ihe fabrication of p-CuInSejn-CdS

heterodiodes 70%quantum 550 between to 1250 with efficiencyin thewavelength range in nm has beenreported[1061.Theseheterodiodes solar cell configuration a reached powerconversion efficiencyof 12%.

A 5% efficient solarcell was produced depositing CIS by US films onto rf-sputteredfilm deposited on Au metallized alumina substrates [791. Heterojunctions made by molecular beam epitaxy of CIS layers onto single crystal US have also shown a

5% [85]. An improvedefficiencyof 8.7%hasbeen maximumconversion efficiencyof[109] for solar cells made entirely by screenprinting. reported

25 The photocurrentof a thin film CIS solar cell was improved by 25% when a wide band layer of ZnO on top of an un-doped US layer over CIS was gap conducting window [1101. A post deposition heat treatment in air at 200"C was shown to be employed to increasethe efficiency of a CdS/ClS solar cell [1111. necessary

2.3

CuGaSe2

Copper gallium diselenide (CGS), is a member of I-IH-VI2 group of semiconducting compounds,and in thin film form it has not beeninvestigatedin detail. CGS has shown less promise in stand alone devices, with only 6% efficient solar cells having been

fabricated [24]. However, it is an ideal high bandgappartner for copper indiumdiselenidein tandemstructures, which the theoreticalmaximum achievableefficiency for is as high as 33% [1121.

Like CIS, this is also a direct bandgap materialof 1.68eV [1131with a chalcopyrite [291andbelongs thetetragonal (space Crystallographic 142d). to structure system group parameters a=5.612A, c= 11.032Aand c/a = 1.966for CGShavebeenreported of[1141. Various sections of the phase diagram of I-III-VI, ternary system have been published by PlaMik and Belova [1151.A part of the pseudobinaryphasediagram with in referenceto the binary compoundsCu2 and G%Se3, the concentrationrange 40-100

iSshownin figure 2.3.Themeltingpoint hasbeenestablished 1070"C mol% Ga2Se3,, as[1161.UnUe CIS, CGS can only be madewith p-type conductivity [117,1181, therefore it can only be used as an absorber layer with certain n-type window materials in

26

L110C

Y+ L

41 looc

le. o 11.1

11

E-4

90040 20 40 60 soluu Ga2Se3

Mole-% Ga2Se3

Figure 2-3:

Ile pseudobinaryphasediagram of the Cu2Se-Ga2Se3 system

27

Compotind properties Melting Point ("C) Density (gm cnf3) Lattice Parameters Carrier Type Hole Concentration(cm) Electro-conductivity (9-cm) Mobility(CM2/V. S)

CuGaSe2 1040 4.35 [1161 [1191

a=5.635 c=11.035p-type only 4x1O` as-grown R1016 annealed 1-10-5 2-5 1.66 1.69

[1201 [1201[1211 [1221 [1221 [1171 [1231 [1231 R

Energy Gap (eV)

Table 2.2:

Propertiesof CuGaSe2semiconducting compoundL

28 heterojunctions.Fundamentalpropertiesof CGS are listed in table 2.2.

2.3.1

Structural properties

A number of techniques,such as flash evaporation [1251, electron beam evaporation [1131, laser beam evaporation [1261, elemental constituent evaporation [1271p [1281,etc. have been used for the deposition of CGS thin films. Epitaxial selenization layerswere preparedby flash evaporationonto oriented GaAs and GaPsubstrates [1291.The orientation of the epitaxial layer was different for different types of substrate. For example, for GaP the c-axis of the deposit is parallel to the substratesurface. On

is The the contrary, c-axisof thedeposit perpendicular the GaAssubstrate the to surface.temperaturerange for epitaxial growth was larger for GaP (795-8951K) comparedto GaAs (870-895"K).

The structureof sputtered CGSfilms [1241was amorphous binary phases low at with temperatures becamesingle phasewith (112) preferredorientationat substrate and temperatures more than 573"K. Ile measurement granularstructurewith average of 5000 to 7000 A was only possiblein the films grown at about 623"K grain size ofwhereastemperatureslower than 573"K showed no surface structure. The application of electron beam evaporation in the temperature range of 673 to 723"K for the deposition of CGS thin films resultedin a changeof sphaleritestructure to chalcopyrite [1201.

29To study the composition and substrate effect on the structure of CGS films, samples on four different were prepared substrates by the co-evaporation of elemental

[1301. 'Me resultant film thickness was in the range of 1.0-2.0 Pm. 'Me 20 constituents for the peak was found to increase with increasing molecularity for A1203 value decreasewas observed for Mo-coated substrates. Dittrich et and glass substrate while a [1311 introduced a new selenization method to produce single phase chalcopyrite thin al. films. Elemental layers of Cu, Ga and Se were deposited onto molybdenum coated glass by sequential thermal vacuum evaporation and were subsequently heat treated substrates in a quartz tube flooded with argon. ne films depends surface morphology of metal

the sequence of deposition. For example, Cu on Ga films showed a granular strongly on the Cu rich phase on the liquid Ga. On the other hand Ga on Cu films agglomeration of exhibited polycrystalline appearance.

2.3.2

Optical properties

The optical absorption CGSthin films [1201showed dataof electronbeamevaporated band splitting and copper3d level three energygapscorresponding fundamental, to transitionsat 1.66,1.76 and 2.45 eV respectively.The dependence absorption of temperature with coefficient on substrate also observedand found to decrease was [1261 laser for increasing Similarobservations beenreported have temperature. substrate CGSthin films. evaporated

A study [231of the optical propertiesof CGS thin films near and abovethe fundamental

30for the flash evaporated films, depositedonto glass substrates 920"K at absorptionedge was carried out by Horig et al. The characteristic energy gaps at 1.67,1.78 eV are similar to those observedlater by Reddy et al [120,1261.However, transition at 2.8 eV is higher and attributed to the p-d hybridization of the valence band predicted in a theoretical model [1321.

2.3.3

Electrical properties

A change in resistivity from 10' to 1 Qcm correspondingto the variation in growth temperaturewas observedin r.f. sputteredfilms of CGS [121] due both to changesin

The compositional structural and properties. conductivityof thefilms wasalwaysp-type Hall films for with no measurable effect.Similar effectswereobserved co-evaporatedwith a correspondingchangein resistivity from 106to 0.1 Qcm [1341.

2.3.4

Heterojunctions

Gopalswamy and Reddy [123] reported the fabrication of an all thin filmZnO/ZnCdS/CGSheterojunction, in which a window layer of ZnO was depositedby spray pyrolysis and a CGS absorberlayer by electron beam evaporation.A thin film of ZnCdS was used asthe buffer layer to reducethe lattice mismatchbetweenwindow and absorberfilms. Ile measuredopen circuit voltage was 400 mV and the short circuit current was 10.5 mA/cm, giving an electrical conversion efficiency of about 2%.

31A p-type,thin film of CGS was laser evaporatedonto vacuum depositedn-tyrpeUS films at a temperatureof 643"K to prepare a thin film heterojunction [122]. This was for then heat treatedin Se-atmosphere compensate the loss of seleniumin the films. to From the dark IN measurements these junctions at different temperaturesit was on found that tunnelling is the predominant current transport mechanism below room temperature,while recombinationof chargecarriers is observedabove 303*K.

A significant increasein efficiency of a solar cell consisting of CGS as the absorber layer was found [1341when CGS layers were chemically etchedin Br2-MeOH and KcN solutions, before the deposition of window layers. The cell propertieswere investigated by quantum efficiency, electron beam induced current and capacitancemeasurements.

A significantincrease fill factor (FF) up to 56% for a CGS/(Cd, in Zn)S cell has also been reported[1351.Measurements conductivity and carrier density showedthe ofremoval of secondaryphases,by KCN dip, from Cu-rich films.

2.3.5

Photoconductivity

Laserevaporated films [1361showeda dependence photoconductivity thickness on ofand a maximum photoresponseat 750 nm was observed for a thickness of 1.2 pm. Annealing in selenium ambient showed no changein the position of the peak but the photoconductivity increased sharply. Analysis of the shoulder observed on the high energy side at 720 nm revealed that the transition originated from the split p-state. A variation of photocurrentwith applied bias voltage and temperature(303 to 373% can

32also occur. Highly resistive, p-type copper deficient films deposited by the coevaporationtechniquewere used in the photoconductivity experimentsto measurethe for mobility life time products [1371.Ibe mobility life time product a=6x10-6cm2/V holes and 2.6xlO-9cm/V for electronswere found at an illumination of about 0.25 AML Much improved photoresponsedue to the decreasein effective doping levels and increasedeffective minority carrier diffusion lengths on chemical and electrochemical etched CGS fla films was revealed [1381.

2.4

Culn,, Ga,, Se2

CopperIndium gallium diselenideis anothermemberof I-HI-VI2 family of chalcopyrite [29]. This material has received an increasing amount of attention semiconductors recently due to the fact that it's band gap is direct and can be varied from 1.02 to 1.68 eV [671,the range of maximum efficiency for photovoltaic conversion,by varying

The lattice parameters a=5.696, c= 11.322for 50% gallium the composition. are [1391. Other values of lattice parameters, to corresponding different values of x has by composition, alsobeenreported the sameauthor[1401.

Except for someinvestigationsabout the lattice parameters, dependence of composition energy band gaps, preparation in the form of single crystal and thin film and some characterizationof the electrical and optical properties, only a very small amount of information is available in the literature about the properties of CuInl, Ga.Se2.

332.4.1 Structural properties

Ile

deposition of CuInD3Gao. diin films by flash evaporation onto glass substrates 5Se2

30-400"C has been demonstrated[1391.A molybdenum boat source was heatedto at about 1500"C for the evaporationof pre-synthesized powder. Films were amorphousat lower than 200"C. Above 200"C, single phaseand polycrystalline, temperatures substrate films with strong orientation were observed.However, the XRD clearlY showed the authorsfailed to identify. For chalcopyrite structure a small peak at about 2311 which there should not be any peaks other than and between 20 values of 20 30*. Hence, it appearsthat this peak was due to the presenceof secondaryphases and such as Cu.Se. EDAX analysis of the films revealed that film composition depends

temperature. critically on the deposition

The preparationof thin CuIn.Gal-.Se2films onto mica substrate(with x=30 and 507o)

by single sourceevaporation flash evaporation has techniques beeninvestigated and [681.The temperature the substrates variedbetween 200-300*C 200-250*C of and wasrespectively. The flash evaporation method was found to be better in terms of reproducibility. Similar conclusion was drawn by Romeo et al. [67]. 7hey used a

tantalumsourcetemperature greaterthan 1500"Cto depositCIGS films. Mica sheetsheatedat temperatures between473-523"K Successfulgrowth in were usedassubstratesof CUh'0.5Gao. 5Se2

thin films by the single sourceevaporation methodhas also been

reported [141]. In order to compensatefor the loss of selenium during film growth polycrystalline powder with 3 mol.% excessselenium source material was used in a

boat at about 1650"K The substrates resistiveheatedmolybdenum usedwere glass .

34temperaturein the range between 303-700*K. slides, maintained at constant

2.4.2

Electrical properties

Romeo et al. [67] reported that their films were always p-type with resistivities in the 1-100 9-cm. Hole mobility was found to be 1-2 cm/V. s. P-type films with range of resistivities rangedfrom 0.1-10 Qcm were obtained [68,142] by flash evaporation.'Me higher evaporation rate on the film resistivities was also observed. The effect of influence of copper diffusion from the film to the ZnSe single crystal was enhancedat

[1421. temperatures substrate elevated

The effects of depositiontemperatureand film thicknesson the electrical resistivity and films havebeeninvestigated[1381. Hall mobility of flash evaporated CuIno. thin 5GaO. 5Se2 An increase in film thickness and substrate temperature resulted in a decreasein resistivity, from 104to 102 However, Hall mobility changedinversely with substrate .

temperature directly with film thickness. and

2.4.3

Optical properties

Optical absorption studies [143] on flash evaporated P-CUInO. thin filMS 75GaO25Se2

revealedthree energygapsof 1.16,1.22 and 1.38 eV, due to the fundamentaledge,band

splitting by crystal field and spin-orbit effects, respectively.7he forbidden gap of

35from absorptionspectrawas reportedas 1.29 eV for CuGa.1n, Se2films [681,measured -. 1.36 eV for x=0.7. Determinationof the optical constantsof CIGS thin films x--0.5 and [1411by transmittanceand reflectancemeasurements the wavelengthrange from 0.4 in to 1.2 pm revealedthree characteristicenergy gaps of 1.30,1.55 and 2.46 eV.

2.4.4

Heterojunctions

by elementalvacuum Thin film polycrystalline ZnCdS/CuInl-. Se2solar Ga. cells produced have beenreported [1441.The best cell, madefrom the selenidefilms with evaporation 23% Ga and anti-reflection coating, showed a total area efficiency of 10.7%.

Improvements opencircuit voltageand fill factor after 5 minutesbake in oxygen in havealsobeenobserved. deposited withoutanti-reflection As enviromnent cells coatingsshowed an efficiency of 9% which was increasedup to 10.06% after post deposition [1451.Heterojunctionswith nearly zero mismatch between flash evaporated annealing

CIGS thin films andAl-dopedZnSesinglecrystalhavebeenreported[681.

2.5

Post depositionprocessing

Post deposition treatmentscan considerablyimprove the properties of absorberlayers and hencethe parameters photovoltaic devices [1461.The literature containsa number of of reports on post deposition heat treatments of Cu(In,Ga)Se2compound material preparedby different deposition techniques.However, they are mostly concernedwith

36the structural properties.

Two stage processingof compound t1iin.films involves two distinct processingsteps. During the first stageof the processa precursorfilm which contains all or some of the constituentelementsof the compoundis depositedon a substratewhich may or may not be heated.During the secondstageof the processthe constituentsare reactedwith each other and/or with other species introduced from a reactive atmosphereto form a

film of CIS. continuous compactcompound and

Rapid thermal annealingof stackedelementallayers (SEL) in different gas ambientsat

50-600*Cfor a rangeof annealing time (from a few seconds two hours)havebeen toreported by Ournouset al. [94]. Reaction mechanismstudies [147] showed that, prior to the fon-nationof CIS, the material goes through different phasessuch as Cu.Se and

B-In2Se3, Sphalerite chalcopyrite temperatures or phase wasonly observed annealing at of > 30011C.

Effects of thermal and chemicalpost depositiontreatments the compositionand on by structure one-step thin of electro-deposited films of CIS havebeenreported Guillenand Herrero [1481.In this process,co-depositedcopper, indium and seleniumwere first heat treated at or above400"C to eliminate the Cu.Se and InSephases,followed by a KCN treatment to remove the remaining secondaryphases.An increasein subsequent grain size of about 50nm has also been observed.Ile properties of treated thin films showeda strong dependence the sequence annealingand chemicaltreatmentsused. on of

Laudeet al. [1491 havedemonstrated synthesis CISby laserirradiationof vacuum the of

37indium and seleniumin an atomic ratio of evaporated multilayer sandwichesof copper, 1:1:2. Electron diffraction analysisrevealedthat CIS was formed, apparentlyin isolated areas.However, no detailed optical and electrical data was presented.

Another approach for the preparation of good absorber layers for photovoltaic

applicationsis selenization.Copper and indium/gallium layers preparedby co[1501are selenizedin a seleniumenvironment to form the required CIS of evaporation chalcopyrite structure.This approach,however, has a problem due to the use of toxic including annealing H. Se.To avoid this problem, severalrouteshave beendemonstrated layers of Cu/(In or Ga)/Se in an inert gas environment [151,152,941and of elemental

[1531. indium alloy layersin the presence selenium annealing copper, of vapour of

2.6

Thin film deposition techniques

To a largeextent,the choiceof anyoneof the manyavailable for growingthin methodsfilms of the I-HI-VI2 compoundsis dependentupon the application which dictates the

desired material propertiessuch as crystalline order, perfection and the impurityconcentrationin the film.

A wide variety of depositiontechniquesare available.For the fonnation of high quality

thin films for photovoltaicapplications, preferred the techniques generallyvacuum aredeposition processes.'Mese can be classified into two categories, Physical Vapour

Deposition such as evaporation,sputtering, laser, electron and molecular beam

38evaporation, and Chemical Vapour Deposition such as laser or plasma assisted deposition, spray pyrolysis, screenprinting, etc. 7he following section discussessome of the thin film deposition techniques.

2.6.1

Physical methods

Physical methods have been widely used to deposit thin films by condensationof vapours in high vacuum, 10' to 10' mbar, atomistically at the substratesurface.71lie techniqueis extremely versatile and covers a wide range of variants including thermal

evaporation,activated reactive evaporation,ion-beam sputtering and ion-plating. Virtually anymetal,alloy,ceramic, inter-metallic some and and polymerictypematerialstheir mixtures can be easily depositedon to virtually any substratematerial which are stable at operating temperaturesin vacuum.

I)

Flash evaporation

Thismethod involves (powder) rapidevaporation multi-component of or compound alloy by dropping it continuouslyonto a source,heatedat a temperature high enoughto Historically,flashevaporation evaporate thedisassociation all products thecompound. of in wasdescribed 1948for thefirst time, in connection of with the evaporation brassand Sincethen,this method beenappliedto the deposition otheralloy metalfilms [1541. has of a large numberof alloys and compounds such as copperternary and multinary compounds.

39u) Single/double source evaporation

Single sourceevaporationis the simplestmethodfor the evaporationof requiredmaterial in thin film form. The suitability of this technique is limited for single element evaporation,because,in compoundand alloy evaporantsthe constituentelementshave different vapour pressuresand may evaporateat different rates with respectto the rate

of increase of source temperature,which makes it very difficult to producestoichiometric films. However, the successfulgrowth of CIS thin films by single source evaporation method has been reported [69]. Ile disadvantages disassociationand of

decomposition leading to the loss of the most volatile components(selenium) can be overcome by using another source in the system [671, known as double source

technique. evaporation

iii)

Co-evapomtion technique

Multi-component compounds or alloys of thin films with precisely controlled

by composition be prepared evaporating from a separate can eachcomponent source. This techniquehas beendeveloped[1551and usedmost successfully workers at by BoeingAerospace Company[1561, Universityof Stuttgart[1571andSwedish Institute [1581. The temperatures the sources be controlledby using of Microelectronics of can boat. an appropriatethermocouple spot welded onto the base of each evaporationThickness of the deposited films can be monitored by frequency controlled quartz

crystals.

40 Three sourceevaporationtechniquehas been greatly successfulin producing large areaCu(In, Ga)Se. thin film solar cells modules, and devices with efficiencies of more than

16% have been fabricated [221.

iv)

Electron beam evaporation

The use of a focused beam of electrons to evaporatethe material under vacuum has broken many barriers in the field of thin film deposition. Materials can be evaporated by focusing a beam of high voltage electrons,provided by an electron gun, onto a small areaof the material. The energyof the electronswhich strike this areaheatsthe material directly, causing it to evaporate.The advantageof this technique is that crucibles or

boats,which might contaminate reactwith the materials, not required. or are

V)

Laser beam evaporation

Ile useof pulsedlaserdeposition(PLD) wasfirst reported[1591in the mid 1960'stoinvestigate the deposition of thin films of a variety of materials including [ dielectrics,organometallicsandchalcogenides 1601. However, it is only semiconductors, relatively recently with the successof the techniquefor the deposition of an important

class of materialscalled high temperatur-e that it has been widelY superconductors,by accepted industry as a viable processtechnologyfor industrial use.The techniquehas

the ability to depositalmost any material and preservethe stoichiometryof multi-

41 Recently it has beenused successfullyto deposit Cu(In,Ga)Se2 componentcompounds. thin film semiconductingcompounds[161-1631.

A laser pulse entersthe vacuum chamberthrough a window and impinges on the target wide with an energy of material to be deposited.The pulse is about 20-30 nanoseconds density of approximately 1-10 J/cm. It vaporises: target material in the fonn of a the ionic and molecular species,which have a kinetic plasma plume containing neutral, few electron volts which travel towards the substrateand deposit to form energy of a

a coating.

Vi)

Molecular beam epitaxy (MBE)

MBE is defined as epitaxial growth onto a substrateresulting from the condensation of directed beamsof moleculesor atoms in a vacuum system.This is basically a vacuum

which assembly evaporationsystemconsistingof effusion cells in a source-shroudprovides water cooling around the cells and a liquid nitrogen shroud to separatethe

beams,aroundthe entire assembly. beam of moleculesproducedby 'Me emergent by using heating materialcontained anevaporation aremonitored in the periodically cell Molecularbeamepitaxyhasbeenusedto deposita variety of fla a massspectrometer.films of semiconductingcompoundsincluding Cu(In,Ga)Se2[1651.

42vii) Sputtering process

Sputteringis a processwhich involves the ejection of atomsfrom the surfaceof a target material by bombardmentwith energeticparticles and condensationof ejectedmaterial to form a thin film. The fast moving atomic sized particles knock out onto a substrate the surfaceatoms of the target material by the transfer of energy.The atomsthat leave the target surface are able to travel in straight lines to condenseon a substrate,because both the target and substrateare under a vacuum. Ille large number of atomic sized bombardingparticles can be obtainedby putting the target in a plasma and by applying negativehigh voltage to the target surface.This negative voltage attractsthe ions from

due The the plasma the targetsurface. ejectionof atoms to positiveion-bombardment to is known as cathodicsputtering donein the presence mixtureof of sputtering whereasinert and reactive gasesis referred to as reactive sputtering.RF [165], DC [71] and ion beam sputtering [601 have all been used in the preparationof CIS thin films.

Ile deposition rate, system pressure, powerdensity,targetsizeandtargetto substrateseparation are some of the parameterswhich can affect the quality of films. rMe

is advantage sputtering that it can alsobe usedfor the etchingof oxide layersfrom of the surface a metaltargetandpuremetalcanbe readily sputtered of onto the substrate.

2.6.2

Chemical methods

Chemical processes utilise volatile componentsof coating materials which are

43chemically decomposed reactedwith the substrateto form a coating atomistically on or the hot substrate.The chemical reactions,generally,take place in the temperaturerange of 150 to 2200"C at pressures ranging from a few mbar to atmosphericpressure.rMese methods are highly versatile and flexible in producing a wide range of thin films with excellent adhesion.

i)

Chemical vapour deposition (CVD)

CVD is the formation of thin films from the decompositionof chemical precursorsonto heatedsubstrate.The occurrenceof a chemical reaction is an essentialpart of the CVD method. Mostly, the depositionreaction at the surfaceof the substrateis heterogeneous, but sometimes homogeneousreactions (which usually affect the composition of the gaseousphase)can produce powdery or flaky deposits,which should be avoided.Ile following is a sequence eventswhich occur during the deposition process. of

1.

Transport of precursorsto the substratesurface.

Adsorptionof precursors onto substrate surface.I 4. 5. De-composition of precursorsto solid films and gaseousby-products. Desorption of gaseousproducts and un-reactedprecursor molecules. Transportof un-reactedand gaseous by-productsaway from the substratesurface into main gas stream.

In any CVD set-up,the reactorplays an important role. The physical and chemical

44 the systemdeterminethe reactor geometryemployedfor a particular characteristics of 7lie provision of sufficient energyto decompose chemicalprecursors the at the process. (substrate is essentialin all types of reactors,and this can be site of reaction surface) in different ways. 7he most frequently and successfullyused method is accomplished by heating.7le reactionat the surfaceof the substrate be enhanced the use can resistive CVD) or by r-f/d-c laserbeam (in laserassisted CVD), an ion beam (in ion assisted of a (in plasmaCVD) etc. Recently,thin films of polycrystallinechalcopyriteCIS havebeen grown successfully by using plasma enhancedand metal organic chemical vapour depositiontechniques[166,167].

ii)

Electrodeposition

Electrodeposition a processof depositingfilms onto an electrode,with the chemical is in changes a solution.Metalsin solutionsmay be either in cationic or anionicfon'ns,and ions move with individual mobilities toward the electrodesunder the influence of an electric charge.Since the mobilities of various ions are different, it follows that, in a given solution, more current may be carried by cations than by anionsor vice versa. Different factors like pH, current density, bath composition etc. can influence the [1681first madeelectrodeposited propertiesof electrodeposited material. Bhattacharya for CIS thin film in 1984.This technique,although,at early stagesin its development CIS, has considerable potential for coating large areasubstrates.

45 iii) Chemical spray pyrolysis

Spray pyrolysis is a chemical processin which metal compoundsare dissolved in asolvent and atomized into fine droplets which are then blown onto the hot surface of the

substrate with the aid of a carrier gas, where they react and form the desiredchemical compound.Chemical reactantsare selectedsuch that all the products other than the desiredcompound,arevolatile. This methodwas described ChamberlinandSkarman by [1691for the productionof CdS films onto heatedsubstrate following a patentby Hill Chamberlin[170]. 7be preparationof copperternary compoundfilms by chemical and spray depositionwas first reportedby Pamplin and Feigelson[391.

71e sprayingsolutionsnormally contain CuCl or CuCl2as the Cu source,sulphur and selenium in the form of dimethyl thiourea and dimethyl selenourea respectivelyand trichloride sourcesfor gallium and indium. Ile major advantage spray pyrolysis is of its ability to producethin films from a simple and low cost apparatus comparedto as vacuum depositiontechniques.A disadvantage this method is the wastageof high of cost materialduring sprayingdueto the deflectionof gasflow out of the coatingregion and the vaporization of significant amount of small droplets before reaching the substrate.

iv)

Screen printing

Screenprinting is a cheapand scalablethin film fabrication techniquewhich includes

46 the printing of semiconductingmaterial (pastes)through a screen onto a substrate (in the followed by sintering.'Me elementalpowdersor pre-reacted compoundmaterials form of powder) can be mixed with an appropriateamountof glycol (which acts as a binder) to form a screenprintable paste.The screenprinted films then need sintering which can be achievedin an inert atmosphere over a period of time at an appropriate temperature.This method can also be used in making