GROWTH AND STRUCTURAL CHARACTERIZATION OF EPITAXIAL CuInS2 THIN FILMS ON Si(001)

AND ON GaP(001)

Name: Reynaldo Magdadaro Vequizo

Doctoral Program in Advanced Materials Science and Technology Graduate School of Science and Technology

Niigata University

ABSTRACT

CuInS2 as an active absorber layer for future cost-effective, environment-friendly and highly efficient thin film solar cells is facing a problem with low efficiency, a problem not yet understood. Hence, it is necessary to investigate the fundamentals of such devices by studying the crystal quality of the absorber layer. To do this, single crystalline chalcopyrite CuInS2 has to be produced first.

However, single crystalline chalcopyrite structure CuInS2 is hard to obtain in thin films even by molecular beam epitaxy since other competing phases such as Cu-Au and sphalerite structures usually coexist. For the first time, in this thesis the successful growth of single crystalline chalcopyrite structure of CuInS2 films is presented. Relaxed epitaxial CuInS2 thin films of various compositions are grown on Si(001) and on GaP(001) at substrate temperatures of 500 (for Si and GaP), 570 and 600°C for GaP alone using a homebuilt three-source vacuum evaporation system at about 10-5 Torr pressure. The Cu and In source temperature is varied from 980 to 1080°C and 650 to 750°C, respectively, to produce various film compositions of 0.70≤[Cu]/[In]≤2.9.

Films grown on Si(001) are epitaxial chalcopyrite, Cu-Au, and/or sphalerite, and some unknown phases of CuInS2. Single crystalline chalcopyrite structure is not successfully grown on Si(001) at 500°C. Improved epitaxial films of CuInS2 are grown on GaP(001) at the same substrate temperature. Although, no rings in the RHEED patterns are observed on the films on GaP, still single crystalline chalcopyrite structure is not grown.

Single crystalline c-axis orientated chalcopyrite ordering is successfully grown on GaP(001) at elevated substrate temperature 570°C in a narrow range of 1.28≤[Cu]/[In]≤1.41 ratios. For the first time, the photoluminescence emissions of the film grown at 570°C with only c-axis chalcopyrite ordering exhibited near band edge emission around 820nm (1.51eV) assigned to exciton bound to ionized donor. Little improvements are observed on films grown at 600°C. Unique surface morphology of CuInS2 films on Si and GaP are observed depending on substrate and substrate temperature. On both Si and GaP substrates, the CuInS2 crystal grain size increases with [Cu]/[In]; however, less dense grains are observed on Si. Dense and large grain size absorber layer is well suited for solar cell application. The In-rich and some stoichiometric films on GaP exhibit pyramidal and domeshaped island grains that correspond to spinel CuIn5S8. The growth of these island grains seems to obey the Stranski-Krastanow (SK) growth mode.

GROWTH AND STRUCTURAL CHARACTERIZATION OF EPITAXIAL CuInS2 THIN FILMS ON Si(001)

AND ON GaP(001)

_________________________

A DISSERTATION

Presented to The Graduate School of Science and Technology

Niigata University Niigata City, Japan

_________________________

In Partial Fulfillment Of the Requirements for the Degree of Doctor of Philosophy in Engineering

In Advanced Materials Science and Technology

_________________________

Reynaldo Magdadaro Vequizo March, 2007

ACKNOWLEDGMENTS Throughout the course of the study, many people and institutions have stood by me; without their support this research endeavor would not have been a rewarding and a successful one. I would like to express my heartfelt gratitude to the MSU-Iligan Institute of Technology (MSU-IIT) through the Chancellor and the Vice Chancellor of Academic Affairs for the financial support and for granting me a study leave to finish my doctorate in Niigata University, Japan, and to the Monbukagakusho of the Ministry of Education in Japan for the scholarship. I would also like to thank: Prof. Drs. Satoshi Kobayashi, my very kind and witty main thesis adviser, for sharing his expertise and close supervision on my studies and experiments, Nozomu Tsuboi, my ever active assistant supervisor, who work in tandem with my adviser for his critical and loafty ideas, Masaki Goda, my another assistant supervisor, for the guidance and critical reading of the manuscript, Futao Kaneko, a research collaborator for his encouraging and helpful comments on the manuscript, Koichiro Oishi, a research collaborator, for the friendship and the healthy discussions on chalcopyrites, and Angelina M. Bacala, the dean of the College of Science and Mathematics in MSU-IIT, for the moral support and encouragement. I am further indebted to Kozakai and Sagayama-san, my labmates for being good tutors and good friends, Mr. M. Kobayashi, the EPMA technician, for the patience in teaching me the important techniques in EPMA and SEM, T. Kawakami sensei in Kaneko laboratory, who worked actively with Hiruta and Kamijou in sharing the knowledge and the passion in doing AES and XPS, Hokuto Kikuchi, a student in the Department of Electrical Engineering in Niigata University, for being a nice friend and for helping me in my problems with SEM and many Japanese translations, to Sugaya san who helped me a lot in AFM, and to the very cool and fun-loving Kobayashi-Tsuboi laboratory members since 2003 to the present especially to my Raman Spectroscopy partner Masuda-san and to my batchmates Takashi Yamaguchi and Yoshitaka Kikuchi, for the support and kindness in many things, and of course, the fun that made me feel at home while doing my studies. I wish to thank the Department of Physics in MSU-IIT through the Chairman for the constant support most especially to Ms. Febe Macasero, Profs. Rosario Reserva, Editha Jacosalem and Lolita Ungui.

How thankful I am for the brotherly and sisterly kindness of Ma’ams Sheila, Edith,

iv

Bing, Daisy, Gie and Sir Gerry, even to Cha-cha, Vesna and Jong Kap, Gene, Poypoy, Lee, Sarah, Ate Miks, Motoko, Nobou Sakamoto, Rudy, the Niigata Cellgroup members (Cindy, Nadina, Ben, Victor, Yui, etc). I am also thankful to Kim, Tun, Rahman, Riva and Alex, my classmates and friends in Autumn Term Nihongo Class (Oct. 2003-March 2004), Dong Hei, Allister, Jennifer, Tina, Ruth, Arnold, Lando, the rest of the Filipino family in Niigata City since 2000 to 2007, Evelyn Ng, Saiful, Lee Speakman, Tetsuya Saito, Winaya, Gilang, Katia and Vanna, for the friendship.

I am very grateful to Ikarashi Church through Ptr. Myura, to Jesus Our Banner (JOB) church through Ptras. Ana and Evelyn, and Grace Chapel through Ptrs. Tom, Tsuchiya and Tony, for the spiritual guidance, blessings and prayers, to my ever supportive Mama Rose, Manoy Erick, Inday Rosemarie, Jeffrey, Junie, Tata, Dodong Christian, Inday Christine, Ate Ester, Dymel, Michael and Jennelyn, for the love and happiness you have given me as always.

To all the people and friends whom I forgot to mention, thank you so much for everything.

I thank the Almighty GOD for giving you all to me. Reynaldo M. Vequizo Niigata University Niigata City, Japan, 2007

CONTENTS Page ABSTRACT ------------------------------------------------------------------------------------ TITLE PAGE ----------------------------------------------------------------------------------- ACKNOWLEDGMENTS ----------------------------------------------------------------------- TABLE OF CONTENTS ----------------------------------------------------------------------- LIST OF FIGURES ---------------------------------------------------------------------------- CHAPTER

1 INTRODUCTION --------------------------------------------------------------- Objectives ------------------------------------------------------------------------- Significance of the study -------------------------------------------------------- Scope and Limitation ------------------------------------------------------------ Thesis Organization -------------------------------------------------------------

2 REVIEW OF RELATED LITERATURE -------------------------------------

2.1 Characteristics of CuInS2 ---------------------------------------------------- 2.2 Alloying ------------------------------------------------------------------------ 2.3 Existence of other phases ---------------------------------------------------

3 EXPERIMENTAL DETAILS ----------------------------------------------------

3.1 Pre-evaporation preparation ------------------------------------------------- 3.2 Three-source evaporation ---------------------------------------------------- 3.3 Composition, structural ordering and orientation ------------------------ 3.3.1 X-ray diffraction -------------------------------------------------------- 3.3.2 Reflection high energy electron diffraction ------------------------- 3.3.3 Electron probe microanalysis ------------------------------------------

4 STRUCTURAL CHARACTERISTICS OF FILMS ON Si

AND GaP AT 500°C ------------------------------------------------------------ 4.1 Characteristic structures of films on Si at 500°C -------------------------- 4.2 Improved structures of films on GaP at 500°C -------------------------

i ii iii v vii 1 3 3 3 4 5 5 8 9 10101013131520 222231

vi

5 GROWTH OF CHALCOPYRITE STRUCTURE ------------------------------ 5.1 Ordering and orientation of films on GaP at Tsub=570°C ---------------- 5.2 Structures of films on GaP at Tsub=600°C ----------------------------------

6 CHARACTERISTIC SURFACE MORPHOLOGY OF FILMS ------------- 6.1 Distinct SEM images of films ----------------------------------------------- 6.2 AFM and compositional analyses -------------------------------------------

7 SUMMARY AND CONCLUSION --------------------------------------------- APPENDIX A ------------------------------------------------------------------------------------- APPENDIX B ------------------------------------------------------------------------------------- APPENDIX C ------------------------------------------------------------------------------------- REFERENCES ------------------------------------------------------------------------------------

434356 636369 74 76777879

LIST OF FIGURES Figure No. Caption Page

2-1 Comparison between (a) the zinc blende represented by ZnS structure and (b) the tetragonal chalcopyrite CuInS2 structures --------------

2-2 Schematic diagram of cross-substitution that starts with Group IV atoms (e.g. Si or Ge) to derive I-III-VI2 compound (e.g. CuInS2) ---------------------

2-3 The Cu-III-VI2 chalcopyrites that are candidates for alloying to produce new and quality quaternary absorbers --------------------

2-4 The Cu-Au and the chalcopyrite ordered structures of CuInS2 ----------------- 3-1 The schematic diagram for the main components in the

three-source evaporation system which is mainly composed of the exhaust system, vacuum chamber, vacuum gauge, temperature control unit, and power supply -------------------------------------

3-2 The schematic setup of the components of the vacuum chamber (a) and the cell (b) in the three-source evaporation system ---------------------

3-3 The Ewald sphere construction where hkl are the Miller indices of the point on the Ewald sphere ---------------------------------------------------

3-4 Theoretical RHEED patterns of sphalerite structure taken at (a) ⟩⟨010 and (b) ⟩⟨ 101 directions ------------------------------------------------------------

3-5 Theoretical RHEED patterns of chalcopyrite structure if taken at ⟩⟨010 direction for (a) a-axis orientated (a-axis⊥substrate) and

(b) c-axis oriented (c-axis⊥substrate)--------------------------------------------- 3-6 Theoretical RHEED patterns of Cu-Au structure taken at (a) to

(c) ⟩⟨010 and (d) ⟩⟨ 101 directions. In here, (a) and (b) are the two variants of a-axis growth Cu-Au structure while (c) for c-axis growth --------

3-7 Theoretical RHEED patterns of spinel structure taken at (a) ⟩⟨010 and (b) ⟩⟨ 101 directions --------------------------------------------------

4-1 XRD spectra of CuInS2 films with (a) [Cu]/[In]=0.9, (b) [Cu]/[In]=1.4, and (c) [Cu]/[In]=2.0, grown on Si(001) at 500°C ------------------------------

4-2 Composite curves A and B of the observed XRD pattern (solid line) fitted using Lorentzian distribution. ----------------------------------------------

4-3 RHEED patterns for CuInS2 with [Cu]/[In]=0.9 on Si(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams

6 6 8 9 11 12 15 18 18 19 20 23 24

viii

(c) and (d) --------------------------------------------------------------------------------- 4-4 RHEED patterns for CuInS2 with [Cu]/[In]=1.4 on Si(001) taken at

(a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d) --------------------------------------------------------------------------------

4-5 RHEED patterns for CuInS2 with [Cu]/[In]=2.0 on Si(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d) --------------------------------------------------------------------------------

4-6 Photoluminescence spectra of CuInS2 for (a) bulk, stoichiometric, and (b) the film with [Cu]/[In]=1.4 on Si(001) taken at 23K. The excitation wavelength is 532nm -------------------------------------------------

4-7 XRD spectra of films grown on GaP (001) with (a) [Cu]/[In]=0.8, (b) [Cu]/[In]=1.27, [Cu]/[In]=1.5 and (c) [Cu]/[In]=1.7 at 10°≤2θ≤90 ----------------------------------------------------------------------------

4-8 XRD spectra of films grown on GaP (001) with (a) [Cu]/[In]=0.8, (b)[Cu]/[In]=1.27, [Cu]/[In]=1.5 and (c)[Cu]/[In]=1.7

at 31°≤2θ≤35°------------------------------------------------------------------------------ 4-9 XRD spectra of films grown on Si(001) with (a)[Cu]/[In]=0.9,

(b)[Cu]/[In] =1.4 and (c)[Cu]/[In]=2.0 at 31°≤2θ≤35°. The calculated positions are indicated as c-CH, a-CH, c-CA, a-CA and SP -------------------------------------------------------------------------------

4-10 RHEED patterns for CuInS2 with [Cu]/[In]=0.8 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d) --------------------------------------------------------------------

4-11 RHEED patterns for CuInS2 with [Cu]/[In]=1.27 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d) -------------------------------------------------------------------------------

4-12 RHEED patterns for CuInS2 with [Cu]/[In]=1.5 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d) -------------------------------------------------------------------------------

4-13 RHEED patterns for CuInS2 with [Cu]/[In]=1.7 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d)--------------------------------------------------------------------------------

4-14 Photoluminescence spectra of CuInS2 for (a) the stoichiometric bulk, and (b)the film with [Cu]/[In]=1.7 on GaP(001) taken at around 23K. The excitation wavelength is 532nm --------------------------------------------

5-1 XRD spectra of films grown on GaP(001) with (a) [Cu]/[In]=1.01,

26 26 27 30 32 33 35 37 38 39 40 42

ix

(b)[Cu]/[In]=1.25, (c)[Cu]/[In]=1.28 and (c)[Cu]/[In]=1.4 at 10≤2θ≤90° at Tsub=570°C -------------------------------------------------------------

5-2 XRD spectra of films grown on GaP (001) with (a)[Cu]/[In]=1.01, (b)[Cu]/[In]=1.25 and (c)[Cu]/[In]=1.41 at 31≤2θ≤35°. In c-CH, a-CH, c-CA, a-CA and SP, c and a are the lattice parameters of chalcopyrite (CH), Cu-Au (CA) and sphalerite (SP) structures normal to the substrate----------------------------------------------------------------

5-3 XRD spectra at 10≤2θ≤90° of films grown on GaP(001) with (a) [Cu]/[In]=1.41, (b)[Cu]/[In]=1.64, (c)[Cu]/[In]=1.92 and (d)[Cu]/[In]=2.55 at Tsub=570°C.----------------------------------------------

5-4 XRD spectra at 31≤2θ≤35° of films grown on GaP(001) with [Cu]/[In]=1.41, 1.64, 1.92 and 2.55 grown at Tsub=570°C. In c-CH, a-CH, c-CA, a-CA and SP, c and a are the lattice parameters of chalcopyrite (CH), Cu-Au (CA) and sphalerite (SP) structures normal to the substrate.---------------------------------------------------

5-5 RHEED patterns for CuInS2 with [Cu]/[In]=1.01 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d)-----------------------------------------------------------------------------

5-6 RHEED patterns for CuInS2 with [Cu]/[In]=1.25 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d)-----------------------------------------------------------------------------

5-7 RHEED patterns for CuInS2 with [Cu]/[In]=1.28 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d) ---------------------------------------------------------------------------

5-8 RHEED patterns for CuInS2 with [Cu]/[In]=1.41 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d). -----------------------------------------------------------------

5-9 RHEED patterns for CuInS2 with [Cu]/[In]=1.64 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).---------------------------------------------------------------

5-10 RHEED patterns for CuInS2 with [Cu]/[In]=1.92 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).----------------------------------------------------------------------------

5-11 RHEED patterns for CuInS2 with [Cu]/[In]=2.55 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).----------------------------------------------------------------

44 45 46 47 49 50 50 51 51 52 52

x

5-12 AES composition depth profile of the film with composition of [Cu]/[In]=1.01, 1.28 and 1.41 measured by EPMA. The respective surface compositions are [Cu]/[In]=1.06, 1.31 and 1.28).--------

5-13 AES composition depth profile of the film with composition of [Cu]/[In]=1.64, 1.92 and 2.55 measured by EPMA. The respective surface compositions are [Cu]/[In]=1.17, 0.95 and 1.41). --------

5-14 Photoluminescence spectra of CuInS2 for (a) bulk at 23K, stoichiometric, and (b) the film with [Cu]/[In]=1.28 on GaP(001) taken at 21K. The excitation wavelength is 532nm--------------------------------------------

5-15 XRD spectra of films grown on GaP(001) at Tsub=600°C with (a)[Cu]/[In]=0.70, (b)[Cu]/[In]=1.01, (c)[Cu]/[In]=1.26 and (c)[Cu]/[In]=2.08 at 10≤2θ≤90°-------------------------------------------------------

5-16 XRD spectra of films grown on GaP (001) with (a)[Cu]/[In]=0.70, (b)[Cu]/[In]=1.01 and (c)[Cu]/[In]=1.26 at 31≤2θ≤35°. ------------------------

5-17 XRD spectra of the film grown on GaP (001) with [Cu]/[In]=2.08 at 31≤2θ≤35°. ---------------------------------------------------------------------------

5-18 RHEED patterns for CuInS2 with [Cu]/[In]=0.70 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d). ----------------------------------------------------------------

5-19 RHEED patterns for CuInS2 with [Cu]/[In]=1.01 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d). ----------------------------------------------------------------

5-20 RHEED patterns for CuInS2 with [Cu]/[In]=1.26 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d). ----------------------------------------------------------------

5-21 RHEED patterns for CuInS2 with [Cu]/[In]=2.08 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d). ----------------------------------------------------------------

6-1 SEM micrographs of films grown on Si(001) at Tsub=500°C with [Cu]/[In] ratio of (a)0.85, (b)0.91 , (c)1.24, (d) 1.5, (e)2.24, and (d)2.79. ----------------------------------------------------------------

6-2 Scanning electron micrographs of films grown on GaP(001) at Tsub=500°C with [Cu]/[In] ratio of (a)0.65, (b)1.02 , (c)1.27, (d)1.97, (e)2.44, and (f)2.93. -------------------------------------------------------

6-3 Scanning electron micrographs of films grown on GaP(001) at Tsub=570°C with [Cu]/[In] ratio of (a)0.89, (b)1.01 , (c)1.25

53 53 55 57 58 59 60 61 62 62 64 65

xi

and (d)1.28. ------------------------------------------------------------------------------- 6-4 Scanning electron micrographs of films grown on GaP(001) at

Tsub=570°C with [Cu]/[In] ratio of (a)1.41, (b)1.64, (c) 1.92 and (c) 2.55. ------------------------------------------------------------------------------

6-5 Scanning electron micrographs of films grown on GaP(001) at Tsub=600°C with [Cu]/[In] ratio of (a)0.70, (b)1.01 , (c)1.26, and (d)2.08. ----------------------------------------------------------------------

6-6 Atomic force micrographs of In-rich and stoichiometric films grown on GaP(001) with [Cu]/[In] of (a)0.65 and (b)1.02 at Tsub=500°C, (c)0.89 and (d)1.01 at Tsub=570°C, and (e)0.70 and (f)1.01 at Tsub=600°C. --------------------------------------------------------------------

6-7 (a) Scanning electron micrograph and the EPMA compositional mapping of the surface of the film with bulk composition [Cu]/[In]=0.70 for (b) Cu, (c) In and (d) S. This film was grown on GaP at 600oC. The same film used in Fig. 4-33 (e) -------------------------------

6-8 AES spectra of the wetting layer with less island grains for (a) non-etched and (b) 2 sec Ar ion etching. The same film in Fig. 6-7 is used ---------------------------------------------------------------------------

6-9 AES spectra of the island grain region for (a) non-etched and (b) 2 sec Ar ion etching. The same film in Fig. 6-7 is used -------------------------

66 67 68 70 71 72 73

CHAPTER 1

INTRODUCTION

Due to the world’s issues on environment and energy, interests on the development of renewable energy resources such as the solar energy by fabricating environment-friendly, stable, and promising photovoltaic (PV) materials are increasing. Solar photovoltaic energy conversion is a one-step conversion process which generates electrical energy from sunlight. Since time a memorial, the sun is the almost non-exhaustive source of power and energy which is for free and is safe.

Being the pioneer material in the PV technology, crystalline silicon (Si) dominates the PV market nowadays. However, Si has low optical absorption (~103 cm-1) due to an indirect transition.1) Hence, Si-based PV technology requires large amount of Si raw material and complex manufacturing leading to high installation cost. In this light, Si-based PV power generators are not competitive in areas where conventional power generators are available. Thus, alternative, if not a replacement of Si, active absorber materials such as the direct gap I-III-VI2 ternaries for example CuInX2 (X=S or Se) are now explored.

CuInS2 is one of the promising absorbers for thin film solar cells due to its direct fundamental energy bandgap of ~1.5 eV that matches well to the solar spectral region and its absorption coefficient (105cm-1) is 100 times higher than that of Si. To date the reported efficiency of CuInS2-based solar cells is about 12% in laboratory scale2), while in mini-module scale it is about 9.2%, which are still low compared to its selenide counterpart CuInSe2 that has an energy bandgap different from that of the optimum value.2, 3) Many approaches have been tried to produce highly efficient CuInS2-based thin film solar cells but it seems difficult to obtain it at present. Hence, it is necessary to investigate the fundamentals of such devices by studying the crystal quality of the absorber layer which is one of the important aspects to be considered in the fabrication of solar cell devices. CuInS2, in bulk, generally crystallizes to chalcopyrite structure. But in thin films, it seems hard to produce chalcopyrite single phase alone. Coexistence of Cu-Au and chalcopyrite structures was confirmed in the CuInS2 films grown on Si(001) by molecular beam epitaxy (MBE) and it was found that the difference between their formation energies is small as predicted by Su and Wei from first principles calculations.4) On the other hand, Metzner et al. reported the growth of Cu-Au structure

2

with sphalerite structure for the films grown on Si(001) prepared by MBE.5). One approach to improve the crystallinity of the epitaxial films is selecting the suitable substrate. In this thesis, two substrates, the monocrystalline Si and GaP oriented at (001), are used as templates in growing epitaxial CuInS2 thin films. Although, there were already reports on the growth of CuInS2 thin films on Si(001), it is deemed wise to note that the film growth varies with growth conditions and methods of preparation. In this work, the homebuilt three-source evaporation system is used in growing the films samples at substrate temperatures of 500, 570, and 600oC.

Gallium phosphide (GaP) is used as substrate also considering the lattice mismatch between the substrate and the CuInS2 grown film. The lattice mismatch (ε) for the epitaxial growth of chalcopyrite structure towards c-axis on GaP (εCH/GaP=1.3%) is lower than that for both chalcopyrite and Cu-Au structures towards c-axis on Si (εCH/Si=1.6% and εCA/Si=1.5%). Moreover, GaP is bipolar that may help in suppressing the randomness of the adsorbing initial atoms on GaP. Gallium phosphide has been used as host lattice for the growth of CuInS2 and zinc copper indium sulphide (Zn2x-2xCuxInxS2, 0.78≤x≤1) by pulse laser deposition, and CuGaS2 by multi-source evaporation method.6-8) So far, no reports have been presented about the crystal growth of CuInS2 on GaP(001) grown by evaporation and molecular beam deposition.

It is deemed wise to note that for the fundamental research, epitaxial films in many aspects are better suited than bulk single crystals because of the following reasons:9)

(a) The growth kinetics of the polycrystalline absorber layers deposited by co-evaporation that is sometimes called multi-source evaporation is similar to the kinetics of epitaxial growth. Thus, there are similarities in the structural and microstructural properties between the bulk single crystals and the epitaxial films.

(b) The composition of epitaxial layers is easily controlled by adjusting the evaporation flux of the elements during the evaporation, the capability that is difficult to carry out in the growth of bulk crystals with controlled composition.

3

Objectives

This thesis generally aims to grow and to characterize monocrystalline chalcopyrite CuInS2 thin films using a homebuilt evaporation system, the X-ray and electron diffractions, microscopic imaging, spectroscopic, and analytical techniques.

Specifically, this work aims: (a) To grow CuInS2 thin films using three-source evaporation technique, (b) To study the effect of Si(001) and GaP(001) substrates and substrate

temperatures on the epitaxial growth of CuInS2 thin films, (c) To investigate the orderings and orientations of the CuInS2 domains

composing the films at various compositions by using mainly X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED) and electron probe microanalysis (EPMA).

(d) To clarify the crystal quality of the films by photoluminescence (PL), and (e) To elucidate the surface morphologies of the CuInS2 thin films by using

scanning electron microscopy (SEM) and atomic force microscopy (AFM).

Significance of the Study

Tailored structural ordering, c-axis orientated chalcopyrite for instance, via heteroepitaxy may help open avenues to gain deeper insight on the problems of the low conversion efficiencies of the recent CuInS2-based thin film solar cells. Besides, considering the potent properties of CuInS2 as a good photon absorber, its technological applications other than for PV’s that have not yet explored nowadays such as radiation sensors for photo detection, may also be realizable.

Scope and Limitation

This study focuses mainly on the growth and on the structural elucidations of the ordering, orientation and morphology of CuInS2 films on (001) oriented Si and GaP substrates fabricated using a homebuilt three-source evaporation growth system in Kobayashi-Tsuboi Laboratory in the Faculty of Engineering, Niigata University, Japan. The details on the film growth thermodynamics, molecular dynamics, and the electrical and optical properties of the produced films are beyond the scope of this study.

4

Thesis Organization

As for the structure of this work, this is divided into seven (7) main chapters. It starts with the motivations in Chapter 1, where the main objectives and focus of the study are presented. This is followed by a brief review of related literature in Chapter 2 about chalcopyrites and their properties mainly on CuInS2. The main experimental methods and characterization techniques used in this study such as the three-source evaporation, X-ray diffraction (XRD), reflection high energy electron diffraction (RHEED) and electron probe microanalysis (EPMA), are briefly discussed in Chapter 3. Microscopic and spectroscopic characterization techniques used in this study are discussed whenever necessary in the discussion of the results in scanning electron microscopy (SEM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and photoluminescence (PL). For the results and discussion, Chapters 4 to 6 present the details of these. In these chapters, the results and discussion on the ordering and orientation and photoluminescence investigations are arranged with increasing substrate temperature which is then followed by the discussions on the morphology of the film samples. The brief summary and some concluding remarks are presented in Chapter 7.

CHAPTER 2

REVIEW OF RELATED LITERATURE

2.1 Characteristics of CuInS2

CuInS2, a chalcogenide material, belongs to a group of ternary semiconductors with ABX2 molecular formula. The ABX2 group is composed of two major groups: the chalcopyrites (AIBIIIC2

VI) and the pnictides (AIIBIVC2VI). CuInS2 is in the chalcopyrite

group with A, B and C atoms corresponding to Cu, In, and S, respectively. Chalcopyrite materials are the ternary isoelectronic analogues of II-VI binary compounds. The analogue binary compound for a given chalcopyrite material is obtained by taking the cation located in the Periodic Table between the A and B atoms. For CuInS2, the corresponding binary analogue is Zn0.5Cd0.5S. Despite the similarities between the ternary compounds and their analogues, difference in their structural and electronic properties exists. Take for instance, the ternary materials exhibit bandgap anomaly wherein their energy bandgaps are much lower than their analogues.10)

In chalcopyrites, the atomic distributions are given as follows. 11) equivalent position, 000; 1/2 1/2 1/2, 4A positions, 000; 0 1/2 1/4, 4B positions 00 1/2; 0 1/2 1/4, and 8C positions x 1/4 1/8; -x 3/4 1/8; 3/4 x 7/8; 1/3 –x 7/8.

The crystal structure of the ternary chalcopyrite belongs to the space group dI 24 (D2

12), with eight atoms per primitive unit cell. Chalcopyrite structure may be thought of as a superlattice of the zincblende structure (Td

2). Since there are two different types of atoms in the unit cell, the volume of the chalcopyrite unit cell is four times larger than the zincblende unit cell. In the chalcopyrite cell, each anion C is tetrahedrally coordinated by two atoms A and two atoms B, while each cation is coordinated by four anions C. Differences between the chalcopyrite and the zinblende structures are featured in Fig. 2-1. There are three significant differences between the chalcopyrite and the zincblende structures:

1) the existence of two cation sublattices, 2) the tetrahedral distortion, and 3) the anion displacement. The presence of two different cations in the structure leads to two different

6

chemical bonds A-C and B-C, and in general, the bond lengths are different (RAC≠RBC). As a result of the two types of cations, the tetragonal cell is distorted, and it is commonly defined the tetragonal distortion (η) as c/2a, where a and c are the tetragonal cell lattice parameters.12)

Cu

S

In

( a)Zincblende Structur ( b)Chalcopyrite Structure

Zn

S

Fig. 2-1 Comparison between (a) the zinc blende represented by ZnS structure and (b) the tetragonal chalcopyrite CuInS2 structures.

Fig 2-2 Schematic diagram of cross-substitution that starts with Group IV atoms (e.g. Si or Ge) to derive I-III-VI2 compound (e.g. CuInS2).

Group Ⅴ

Group Ⅵ

Group Ⅳ

Group Ⅱ

Group Ⅲ

Group ⅢGroup Ⅰ Group（Ⅵ）

ZnS

Example

Si,Ge

CuInS

GaAs

2 2

7

Table 2-1 Material parameters of some semiconductors. Unless stated otherwise these parameters have been measured at 300K. The parameters of IV , III-V and II-VI are taken from reference [13], while the ternary I-III-VI2 chalcopyrites are from reference [1] and if from other references it is otherwise stated in the superscript. D – direct and I – indirect energy band gap.

Semi

conductor

Band Gap

(eV)

Electron

Mobility

(cm2/V.s)

Hole

Mobility

(cm2/V.s)

Lattice

Constant

(Å)

Density

(g/cm3)

Melting

Point

(K)

Coefficient

of Thermal

Expansion

(10-6 K-1)

Si 1.12 (I) 1400 470 5.43095 2.328 1685 2.92 1)

Ge 0.67 (I,D) 3900 1900 5.64613 5.327 1231 5.90 1)

GaAs 1.42 (D) 8000 340 5.6533 5.32 1510 5.734 1)

GaP 2.26 (I) 350 100 5.4512 4.13 1750 4.65 1)

ZnS 3.66 (D) 165 5 5.410 4.079 2100 6.7 1)

ZnO 3.35 (D) 200 180 a=3.252

c=5.213

5.66 2242 1) 4.75c⊥1)

2.92 c‖

CuInS2 1.53 (D) 165 499 a=5.52

c=11.08

4.74 1320 11.7 c⊥14)

9.6c‖

CuInSe2 1.01 (D) 6 3.1 a=5.78

c=11.55

5.77 1260 10.6 c⊥14)

7.9c‖

CuInTe2 0.98 (D) 189 50.6 a=6.17

c=12.34

6.10 1050 10.9 c⊥14)

7.0c‖

CuGaS2 2.43 (D) ------- 15 a=5.35

c=10.46

4.38 1550 11.2 c⊥15)

6.9c‖

CuGaSe2 1.68 (D) ------- 3.12 a=5.61

c=11.00

5.57 1310 11.1 c⊥15)

5.2c‖

CuGaTe2 1.23 (D) ------- 50 a=6.00

c=11.93

5.95 1140 10.1 c⊥15)

7.5c‖

The semiconductor compounds of I-III-VI2 and II-IV-V2 are known to be the

ternary analogs of II-VI and III-V binary semiconductor compounds, respectively. By ordered substitution of other atomic groups by maintaining an electron-to-atom ratio of 4, the ternary compounds can be derived. The schematic illustration of this ordered substitution starting from Si and Ge covalent crystals is shown in Fig. 2-2 and the selected properties of some IV, III-V, II-VI and I-III-VI2 semiconductors at room temperature are listed in Table 2-1.

Copper indium disulphide (CIS) is considered as one of the most promising

8

chalcopyrite absorbers for photovoltaic application compared to other polycrystalline thin film materials such as copper indium diselenide (CISe). In CIS, S replaces Se in CuInX2 (X=Se or S) and as a result the bandgap increases from 1.1 to 1.5eV. Thus, the open circuit voltage of the CIS-based solar cell increases also. Furthermore, this higher bandgap allows CuInS2-based solar cell devices to withstand higher temperatures and it also improves the blue spectrum response of the cell.

2.2 Alloying

In addition to the intrinsic properties of I-III-VI2 and II-IV-VI2 chalcopyrite compounds, alloying to form quaternary materials is also possible. This is a good property of I-III-VI2 and II-IV-VI2 chalcopyrite compounds to be explored for structural and bandgap engineering. The possible candidate quaternary compounds are shown in Fig. 2-3. Among the family of chalcopyrite materials, the most widely used compound is the Cu-based Cu-III-VI2 chalcopyrites that can possibly be alloyed to produce quaternary materials such as Cu(In,Ga)Se2 or in short CIGSe that has proved to show better and more promising results.16)

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

CuInSe2

CuInTe2

CuGaTe2

CuInS2

CuGaSe2

CuAlTe2

CuGaS2

CuAlSe2

Energy B

andgap

(eV

)

Energ

y B

andga

p (

eV

)

Fig 2-3 The Cu-III-VI2 chalcopyrites that are candidates for alloying to produce new and quality quaternary absorbers.16)

9

2.3 Existence of other phases

Experimental results demonstrate that the only ternary compounds which are stable at room temperature are the chalcopyrite ordered structure of CuInS2 and the cubic spinel CuIn5S8.17 ) Unlike the Cu-In-Se system, the ordered vacancy (OVC) such as CuIn3Se5 and CuIn2Se3.5 are not reported.18) In addition to these reported equilibrium phases of CuInS2, it also shows an order-disorder transition, that is from chalcopyrite to sphalerite transition, at temperature 1253K 19). It is worth noting that this order-disorder transition temperature is below the melting point (1320 or 1364K) of CuInS2.14, 20) A list of the order-disordered transition structures is provided by Wei et al. in their results in first principles calculations.21) The coexistence of the metastable crystal domains such as Cu-Au ordered domains was reported for Cu-In-S system since the difference of the formation energy of the ordered chalcopyrite and the Cu-Au crystal domains is very small.22) The illustration of the Cu-Au and the chalcopyrite structures of CuInS2 is shown in Fig. 2-4 below. As can be seen in this figure, the difference between the chalcopyrite and Cu-Au orderings is the alternation of Cu and In atoms in the unit cell in chalcopyrite structure which is not observed in Cu-Au structure.

Cu

S

In

( a)Cu-Au Structure ( b)Chalcopyrite Structure

Cu

S

In

Fig. 2-4 The Cu-Au and the chalcopyrite ordered structures of CuInS2.

CHAPTER 3

EXPERIMENTAL DETAILS

The methods used in preparing and characterizing the CuInS2 film samples are presented in this chapter. There are three main steps followed in preparing, characterizing and investigating the samples. These are given as follows: (a) the pre-evaporation preparation that includes cleaning and etching of the elemental Cu, In and the substrates, (b) the three-source evaporation; and (c) the structural and optical properties elucidations, investigations and analyses in comparison to compositional analyses.

3.1 Pre-evaporation preparation

The high purity elemental copper (Cu) chips (5N Grade, 99.999%), indium (In) shots (6N Grade, 99.9999%) and sulfur (S) powders (99.9999%) are the main materials that are processed to produce the CuInS2 thin films. The sulfur powder is used as received. Wet chemical cleaning is done only to the metallic elements, the 001-oriented (625µm) Si and the (250 & 400µm) GaP substrates before using for evaporation. The Cu chips are treated with aqueous solution of nitric acid (HNO3) with deionized water to HNO3 ratio of 2:1 for 1 minute while the In is cleaned by using HCl with deionized water to HCl ratio of 2:1 for 5 min. The Cu chips and the In shots are then rinsed with deionized water. The Si (1.5mm × 30mm), on the other hand, is precleaned first with Semicoclean 23 solution for 5 min, followed by etching with aqueous solution of HF with deionized water to HF ratio of 10:1 for another 5 min. As for the GaP substrate of the same dimension as that of Si, it is immersed first with ethanol for 10 min with or without supersonic agitation, rinse with deionized water, then etch with aqueous solution of HCl and HNO3 with deionized water to HCl:HNO3 ratio of 1:1:2. These elemental sources and the substrates are dried under N2 gas before using.

3.2 Three-source evaporation

Figure 3-1 shows the schematic diagram of the three-source evaporation set-up used in this experiment. The evaporation system is mainly composed of the exhaust system, vacuum chamber, vacuum gauge, temperature control unit, and power supply.

11

This setup is equipped with a cold trap by using liquid nitrogen trap (ULVAC:LCT baffle-type), an oil diffusion pump (ULVAC:ULK-04A) and a rotary pump (ULVAC:D-950K). The vacuum evaporation chamber is made of stainless steel. To minimize backstreaming of oil into the vacuum chamber, the cold trap is used. In this manner, the inner part of the chamber, the substrate in particular, is protected from unwanted contaminants of condensed oil from the oil diffusion pump. Moreover, this cold trap on the liquid nitrogen cooled stage between the chamber and the diffusion pump avoids the condensation of the sulphur vapor at the pumps. In the vacuum chamber shown in Fig. 3-2, three crucibles, water-cooled shroud assembly, the substrate support stand, the shutter, and the substrate heaters, have been installed. These crucibles are made of high purity alumina tubes (Al2O3:95.4%). The water-cooled shroud separates the Cu and the In deposition sources; thus, minimizing the effects of the temperature gradient of each heat source during heating. The substrate support stand and the chamber itself are designed to withstand high temperature. The crucible brim to substrate distance is about 25 cm.

Exhaust System

LN Cooled Stage

Water Cooling

Vacuum Chamber

Vacuum Gauge

Temperature Control

Unit (×4)

SSR

DP Power Supply (×4)

oC

Torr

RP

VR IT

Fig. 3-1 The schematic diagram for the main components in the three-source evaporation system which is mainly composed of the exhaust system, vacuum chamber, vacuum gauge, temperature control unit, and power supply.

12

Sources

Exhaust (DP + RP)

Shroud (Water Cooled)

Shutter

Substrate (GaP or Si)

Water Cooling

Substrate Heater The Vacuum Chamber

Cu In S

(a)

Fig. 3-2 The schematic setup of the components of (a) the vacuum chamber and (b) the cell in the three-source evaporation system.

Alumina Crucible

Mo Heating Wire

Alumina Tube (heat insulator)

Thermal Shield

Thermocouple

(b)

13

Each crucible in a heater (of zigzag wound Mo wires 0.5mm diameter that are separated with alumina tubes of 1mm inner diameter) is covered with Mo case. The temperature is measured with the thermocouple at the bottom of the crucible during resistive heating. This crucible-heater-Mo case-thermocouple assembly is then called a cell. The material used for the thermocouple of each cell is given as follows. The Cu, In, and S cells use, respectively, the W-Re5%/W-Re26%, Pt/Pt-Rh13%, and the sheath thermocouples. The supplied electricity is controlled by a solid state relay (SSR) system to synchronize with the temperature controller (CHINO DB series) system. Prior to growth, the evaporation system is air-evacuated to approximately 10-6Torr. During film growth, the temperature of the S source is fixed to 150°C. Either the Cu or the In source temperature is varied during evaporation to obtain various compositions. The Cu source is changed from 980 to 1080°C and the In source temperature is varied from 650 to 750°C. The base gas pressure during film deposition is about 10-5 Torr that gives the approximate vaporants mean free pathlength of 102 cm that assures the vaporants to reach the substrates. The film samples are grown at three different substrate temperatures, first at 500°C for Si and GaP substrates then at 570 and 600°C for films on GaP substrate only. The film growth rate is between 17 to 200nm/hr. The composition of the films in terms of [Cu]/[In] ranges from 0.7 to 2.9.

3.3 Composition, structural ordering and orientation

To investigate the structural ordering, orientation, morphology, and epitaxial growth of the film samples, X-ray diffraction (XRD) and reflection high energy electron diffraction (RHEED) are carried out and compared to the composition obtained by electron probe microanalysis (EPMA). Although initial investigations on the optical and electronic properties of the films are done by performing photoluminescence (PL) measurements, and the scanning electron microscopy (SEM) and atomic force microscopy (AFM) are carried out, the details are usually presented before the main discussion on the results. Only the main structural and compositional characterization techniques (XRD, RHEED and EPMA) are presented in this chapter.

3.3.1 X-ray diffraction (XRD)

X-ray diffraction is a powerful non-destructive tool for material characterization, by which the crystal structure, orientation, and grain size can be determined. The

14

characterization is usually done with a typical X-ray wavelength comparable to the interatomic distance in a crystal (～0.1nm). When X-rays impinge on a crystal, the individual atoms act as secondary radiation emitters and cause interference analogous to the diffraction of light from a grating. Constructive interferences give the diffraction peaks according to Bragg’s law, 2dhkl sinθhkl = nλ, (3.1) which relates the real space distance dhkl in a crystal, between a set of planes indicated by the Miller indices hkl, and the angle θhkl, at which these planes will diffract X-rays of a particular wavelength λ with n as the order of diffraction. Chalcopyrite structures, such as CuInS2, are characterized by its symmetry space group ( dI 24 ). The five reflection conditions, as found in the crystallographic tables, for the space group dI 24 , are summarized as23)

1. (hkl): h+k+l=2n, 2. (00l):l=4n, 3. (h00): h=2n, 4. (hh0): h=2n, and 5. (hhl):2h+l=4n. (3.2)

In Eq. (3.2), n is a positive integer different from the n in Eq. (3.1). Thus, whichever is used in the text it is specified.

The first rule in Eq. (3.2) arises from the fact that the crystal unit cell is body-centered that leads to the reflection condition (hkl):h+k+l=2n. Furthermore, screw dyads along the crystallographic axis require additional conditions for the reflections (h00) and (00l). The glide planes normal to the {110} directions are responsible for the reflection condition involving the (hhl) planes.

In general, the conditions for systematic absences the chalcopyrite structure presented in Eq. (3.2), predict the appearance of the 011, 112 (the most intense for bulk chalcopyrite), 013, 004, 020, and others including their higher reflections. For the tetragonal CuInS2 to be epitaxially grown on the (001) or (100) plane of the substrate, 200 (or 020) for a-axis growth orientation and 004 for c-axis growth, and their higher order reflections, appear in the XRD results. The angular positions (2θ) for a specific reflection can be calculated from the famous Bragg’s law in Eq. (3.1) which is re-arranged for convenience as

= −

21sin

hklhkl d

nλθ with 2

2222

2

)/(1a

lcakhdhkl

++= and

15

+

+

+

+

++

=

2

22

2

22

22

2

21

2

21

21

221

22121

cos

cl

akh

cl

akh

cll

akkhh

ϕ (3.3)

where λ, (hkl), φ, a and c are the wavelength of the X-ray source which is 1.5418Å for CuKα, the reflecting plane denoted by the Miller indices hkl, the angle between the reflecting plane normals (h1k1l1) and (h2k2l2), and the lattice parameters of the crystal

samples, respectively. For cubic structure where a=b=c, the 1/dhkl in Eq. (3.3) is

2

222

2

1a

lkhdhkl

++= . (3.4).

In this study, the X-ray diffraction (XRD) is done following the Bragg-Brentano configuration utilizing Cu-Kα radiation (1.5418Å) using an X-ray diffractometer equipped with a graphite monochromator. The diffraction lines of the substrates are used as standards in analyzing the film overgrowth layers. The photograph of the XRD facility used in this study is presented in Appendix A.

3.3.2 Reflection high energy electron diffraction (RHEED)

Although X-ray diffraction (XRD) is the most widely used technique for studying

k

k'

reciprocal lattice

Ewaldsphere

q2

000

ghkl

hkl

θ

Fig. 3-3 The Ewald sphere construction where hkl are the Miller indices of the point on the Ewald sphere.

16

the crystal structure of a specimen, a complementary method is necessary to elucidate the surface crystal structure considering that XRD is a volumetric structure identification. In this study, the reflection high energy electron diffraction (RHEED) is used as a complementing method to analyze the surface structure orderings and orientation of the produced film samples. In carrying out the RHEED measurements, the incident electron beam is accelerated at 50kV at a grazing angle of about 5°, which is almost parallel to the film surface along <010> and <ī10> directions of the substrates. Photographic plates are used to record electron diffraction patterns. In Appendix B the photograph of the RHEED facility is shown.

Reflection high energy electron diffraction (RHEED) is another non-destructive technique of determining surface structures. Usually, a relatively high electron beam (5-100keV), electron mean free pathlength (20-100Å) is used. The electron beam is directed towards the sample at a very grazing angle of incidence. This grazing incidence arrangement means that RHEED is sensitive to surface roughness and imperfections on an atomic scale. The presence of protuberances on the surface of the sample under study may lead to the appearance of the diffraction spots, instead of streaks which are characteristics of a smooth surface.

To describe briefly the mechanism in electron diffraction such as that in RHEED, it should be noted that X-rays (in XRD) are scattered by the electrons in atoms while the incident electrons (in the case of RHEED) are primarily scattered by the protons in the nuclei making the scattering amplitude being proportional to the atomic number Z. The electrons in the sample actually shield the nuclei from the incident electrons by an amount proportional to the X-ray atomic scattering factor, f. Consequently, the electron atomic scattering amplitude, fe, is proportional to (Z-f). The electron atomic scattering amplitude is normally expressed as a scattering length and not as a number like f. Thus, the electron scattering intensities are expressed as areas or atomic scattering cross-sections that require dynamical interactions discussion of which geometrical construction; the Ewald reflecting sphere construction for instance, would help amplify the understanding of the mechanics of electron diffraction. Consider the Ewald sphere shown in Fig. 3-3. The lattice shown in Fig. 3-3 is the reciprocal lattice. The significance of Ewald sphere is that it maps the magnitude of the incident wave vector k onto the reciprocal lattice and by conservation of momentum, no diffraction events can occur outside the sphere. If any of the reciprocal lattice points are intersected by the Ewald sphere, then the condition for elastic scattering is satisfied i.e. the momentum of the electron beam changes but not its energy, with the scattered beam having a wavevector k’. By conservation of momentum,

17

k = k’ + ghkl , (3.5) where the change in momentum on scattering is represented by the reciprocal lattice

vector ghkl. For elastic scattering, 'kk = and from Fig. 3-3, it can be seen that it will

lead to Bragg’s law presented in Eq. (3.1).24)

The scattering produced by the crystal as a whole depend on the number of atoms in each unit cell, and on their position relative to one another. This is summarized by the structure factor Fhkl= Fg given as

∑∑=

++

=

• ===N

i

lzkyhxii

N

i

iighkl

iiiihkl efefFF1

)(2

1

πrg , (3.6)

where fi and r are the atomic factor and the position of the ith atom in the unit cell of the

crystal, and ∗∗∗ ++= cbag lkhhkl is the reciprocal lattice vector expressed in terms of

a*, b* and c* unit cell vectors that are related to the real space unit cell vectors a, b and c by

)(,

)(,

)(***

bacbac

acbacb

cbacba

×⋅×

=×⋅×

=×⋅×

= (3.7).

The intensity Ihkl of the diffracted peaks is

∗∝ hklhklhkl FFI . . (3.8)

With the inclusion of the systematic absences of the crystal in the analysis of the positions of the diffracted beams, information about the crystal lattice size and symmetry are obtained. Depending on structural ordering and orientation of the specimen, it demonstrates various electron diffraction patterns. The expected structural orderings of CuInS2 are the chalcopyrite, Cu-Au and the sphalerite orderings. Should the In-rich film is composed of the cubic spinel CuIn5S8, its structure could also be observed in the RHEED patterns. Figures 3-4, 3-5, 3-6 and 3-7 show respectively the expected RHEED patterns for the sphalerite, chalcopyrite, Cu-Au and the spinel structures at the specified directions in each figure. The theoretical RHEED patterns of the chalcopyrite structure in <ī10> direction are not shown since the diffraction intensities locations are similar to that of the sphalerite structure. From Figs. 3-4 to 3-7, the sphalerite RHEED patterns are somewhat common in all other structures in both directions <010> and <ī10>. The c-axis orientated chalcopyrite specific reflections are 101, 103, 105 and 107 while 301 and 501 reflections for a-axis orientation that has the selection rule l=4n±1 and h+k=2n±1.4) The perfectly ordered and chalcopyrite structure cannot give rise to 00l

18

reflections where l=odd since these reflections violate the selection rule numbers 1 and 5 (for h=k) in Eq. (3.2). However, Cu-Au structure that has a space group of 24mP allows the diffraction intensities at 00l (l=odd) as depicted in Figs. 3-6(c) and 3-6(d). In the reflections 002, 110, 222 and 114 in <010> (Fig. 3-6(a)) and <ī10> (Fig. 3-6(d)) diffraction patterns, the 0,0,0 and 1/2,1/2,0 sites in the tetragonal unit cell are preferentially occupied with Cu atoms and the 1/2,0,1/4 and 0,1/2,1/4 sites preferentially occupied with In atoms.22) Two variants of a-axis orientation of Cu-Au ordering are featured in Figs. 3-6(a) and (b) at <010> direction that is when c is parallel

Fig 3-5 Theoretical RHEED patterns of chalcopyrite structure if taken at ⟩⟨010 direction for (a) a-axis orientated (a-axis⊥substrate) and (b) c-axis oriented (c-axis⊥substrate).

(b) (a)

105

200

000

400

204

101

204

301301

103 101 103 105

105004

000

008

204

101

204

301301

103

107

101

103

105

107

c <001>

a <100>

b <010>

c <001>

a <100>b

<010>

Fig. 3-4 Theoretical RHEED patterns of sphalerite structure taken at (a) ⟩⟨010 and (b) ⟩⟨ 101 directions.

000

(a)

400

200 202

a <100>

c<001> b

<010> (b)

c<001>

a<110> b

⟩⟨ 101

002

004

111

000

19

to <010> (for Fig. 3-6(a)) and to <ī00> (for Fig. 3-6(b)) directions. Due to the increase

number of In atoms in cubic spinel CuIn5S8 (of space group )(34 2dTmF and lattice

parameter 6734.1085=SCuIna Å) it exhibits unique RHEED patterns as shown in

Figs.3-7(a) and 3-7(b). It should be noted that if the crystal specimen is a mixture of a-axis orientated Cu-Au with c parallel to <010> (Fig. 3-6(a)) and spinel CuIn5S8 (Fig. 3-7(a)), overlapping of diffraction intensities (110 and 310 of a-axis Cu-Au phase, and 202 and 206 of the spinel CuIn5S8) is expected.

Fig. 3-6 Theoretical RHEED patterns of Cu-Au structure taken at (a)-(c) ⟩⟨010 and (d) ⟩⟨ 101 directions. In here, (a) and (b) are the two variants of a-axis growth Cu-Au structure while (c) for c-axis growth.

000

001

003

201

202

203

204

012

022

032

042

(c)

c <001>

a <100>b

<010>

110

220

310

(a)

000

a<100>

b<010> c

<010>

200

400 140 240

120

000 c < 001 >

a <100>

b <010 >

200

400

220

111

114

113

112

(d)

c<001>

a<110> b

⟩⟨ 101

000

001

002

003

004

221

222

223

(b)

20

3.3.3 Electron probe microanalysis (EPMA)

To examine the composition of the film samples, the wavelength dispersive spectrometer (WDS) electron probe microanalysis (EPMA) facility shown in Appendix C in the Department of Dentistry of Niigata University, Asahimachi Campus, is used. This elemental analyzer is a product of Shimadzu (Model EPMA-8705). In performing the EPMA analysis, the sample set on the sample holder is first coated with carbon. To avoid charging of the sample during prolong bombardment with electrons, carbon paste and carbon tapes are used to provide pathways of electrons on the sample to the conductive EPMA sample holder. Although the main elements present in the sample are approximately known considering that the CuInS2 film grown on Si or on GaP substrate is done in vacuum, in the first stage of the analysis, qualitative analysis is performed to identify the elements present including the unknown elements. The compositions of main elements to be determined are Cu, In and S. Since the film samples are thin (about 0.10 to 1.2µm), the contribution of the Si or the GaP substrates are also noted. The standards used are copper (Cu), indium (In), cadmium sulfide (CdS), gallium phosphide (GaP), and silicon (Si) each of high purity. The stable compounds CdS and GaP are used as standards for sulfur and phosphorus, respectively, since pure S and P are not stable during electron bombardment. The elemental analysis by EPMA is performed by measuring the energy and intensity distribution of the X-ray signal generated by a focused electron beam on the target that is the sample. This X-ray photons emerging from the sample have energies

c <001>

a <100>b

<010>

004

008

202

206

404

408

000

(a)

022

062

008

004

c<001>

<110> b

⟩⟨ 101

115

333 222

335

113

117

000

448

(b)

Fig. 3-7 Theoretical RHEED patterns of spinel CuIn5S8 structure taken at (a) ⟩⟨010 and (b) ⟩⟨ 101 directions.

21

specific to the elements in the sample; these are the characteristic X-rays that provide the analytical capabilities of scanning electron microscope (SEM). The primary generated X-ray intensities are roughly proportional to the respective mass fractions of the emitting element. If other contributions to X-ray generation are very small, the measured intensity ratios between sample and standard are approximately equal to the mass or weight fractions of the emitting element. This is often applied to X-ray quantification used in Castaing’s first approximation to quantitative analysis given in symbols as

ii

i

i

i kII

CC

==)()(

, (3.9)

where Ci and C(i) are the composition in weight (mass) concentration of ith element in the sample and in the standard, respectively. The continuum background is subtracted and peak overlaps are then accounted for accordingly. The ratio of the measured unknown-to-standard intensities, Ii / I(i), is also known as “k ratio”. However, in mixtures of elements, the elastic and inelastic scattering processes and in the propagation of X-rays through the sample to reach the detector are different. So, matrix effects usually arise in mixtures of elements. These effects are divided into atomic number Zi, X-ray absorption Ai, and X-ray fluorescence Fi, effects. To account for these effects, correction factor ZAF, is introduced to Eq. (3.9) to have

iii

ii

i

i kZAFII

ZAFCC

][][)()(

=⋅= . (3.10)

In this equation, Ci is the weight fraction of the element of interest in the sample and C(i) is the weight fraction of the element i in the standard. Equation (3.10) is the basis for X-ray microanalysis in the SEM/EPMA.19)

CHAPTER 4

STRUCTURAL CHARACTERISTICS OF FILMS ON Si AND GaP AT 500°C

The epitaxial growth of CuInS2 thin films with specific orderings and orientations

may vary with the growth conditions such as the choice of substrate as host lattice, the

substrate temperatures and the composition of the films. These variations are revealed in

the XRD and RHEED results in comparison to the compositional and morphological

analyses of the films. Representative PL results of selected films are also presented.

The evolution of the growth of epitaxial Cu-Au, chalcopyrite, sphalerite and/or

spinel orderings and orientations is evident in the XRD and RHEED results of the CuInS2

thin films. Depending on composition, substrate and growth temperature, the CuInS2 thin

films exhibit specific structural ordering.

4.1 Characteristic structures of films on Si at 500°C

Films with thickness 0.15-1.2µm (which is greater than the approximate critical thickness of 11-25nm) of various compositions grown on Si(001) at 500°C show specific structural ordering and orientation and optical quality as presented in the XRD, RHEED and PL results.

X-ray diffraction (XRD)

Figure 4-1 shows the XRD patterns for the samples of [Cu]/[In]=0.9, 1.4 and 2.0. The vertical axes are logarithmically scaled to search for a small amount of extra phases. Miller indices hklcubic are denoted assuming cubic structure. For the chalcopyrite structure, l must be doubled, i.e. hk2lCH. For films of [Cu]/[In]≤1.4, all diffraction peaks except for the peaks from Si substrate can be assigned to CuInS2 as reported by Hahn et al.26) The peaks at 001 and 003 are forbidden for both the chalcopyrite and sphalerite structures, but allowed for the Cu-Au structure. As was confirmed in films prepared at Tsub= 400°C, the existence of the Cu-Au structure is quite probable.27) Although a very weak extra diffraction peak is noticeable and is marked in the figure by (×), the degree of orientation is fairly well.

23

In the XRD pattern for the film with [Cu]/[In]=2.0, the number of peaks corresponding to extra phases increases compared to films with 0.9≤[Cu]/[In]≤1.4, but the intensities are extremely weak. The same situation is observed for other films with 1.5≤[Cu]/[In] ≤2.9. From the Cu2S-In2S3 phase tie line, the Cu2S-CuInS2 system can be assumed for the films with [Cu]/[In]≥1 and the films may be mixtures of the solid solutions of Cu2S and CuInS2. In the case of film with [Cu]/[In]=2.0, it is considered that most Cu2S are amorphous since the trace of Cu2S is not clear in the XRD patterns. In the latter case, the solid solutions must preserve the structure and the lattice parameters of CuInS2 with large amounts of defects such as In and S vacancies, Cu interstitials and antisited Cu and so on and so forth. Although the existence of CuS was

20 40 60 80

10 1

10 2

10 3

× ××

××

004

Si004

003

001

002

111

2θ [deg .]

X-r

ay In

tens

ity [a

rb.u

nit]

10 1

10 2

10 3

×

004

Si004

003

001

002

10 1

10 2

10 3

×

004

Si004

003

002

001

(b ) [C u ]/ [In ]=1 .4

(c ) [C u ]/ [In ]=2 .0

(a ) [C u ]/ [In ]=0 .9

Fig. 4-1 XRD spectra of CuInS2 films with (a)[Cu]/[In]=0.9, (b)[Cu]/[In]=1.4, and (c) [Cu]/[In]=2.0, grown on Si(001) at 500°C.

24

also reported,28) the effort to reveal the substances of extra phases is not continued, since the amount is thought to be very small, thus negligible. Since the XRD peaks near 2θ=32.5° seem to be asymmetrical and should have been composed of some peaks with different angle, intensity and full-width at half maximum (FWHM), Lorentzian curve fitting is used to examine the two superimposed curves so that the composite curve could reproduce the observed diffraction pattern. The typical composite curve for a film with [Cu]/[In]=1.4 is shown in Fig. 4-2 together with the graphs of the two components. For convenience, the component peaked at lower

angle is denoted as A, the higher peak as B, and the composite curve as C. The peak position (2θ) of curve A, B and C is 32.31°, 32.51° and 32.48°, respectively. Curve A is tentatively assumed that it corresponds to the 004 diffraction of the chalcopyrite structure. As for curve B in Fig. 4-2, four possibilities are considered: (1) the 002 diffraction of the Cu-Au structure, (2) the 200 diffraction of the Cu-Au structure, (3) the 002 diffraction of the sphalerite structure and (4) the 200 diffraction of the chalcopyrite

31.0 31.5 32.0 32.5 33.0 33.5 34.0

0.0

0.2

0.4

0.6

0.8

1.0

Si 002

[Cu]/[In]=1.40 A B C

X-r

ay In

tens

ity [a

rb.u

nit]

2θ [deg.]Fig. 4-2 Composite curves A and B of the observed XRD pattern (solid line) fitted using Lorentzian distribution. In here, A and B curves once superimposed will reproduce the observed peak as patterned by curve C.

25

structure. The first case is probable considering the fact that the 001 diffraction of the Cu-Au structure is observed (Fig. 4-1) and it was confirmed in the films grown at Tsub=400°C.27) However, the 002 diffraction intensity (I002 Cu-Au) must be small comparing to the intensity of the curve B (IB ) : the ratio I002 Cu-Au / IB is estimated to be approximately 0.11 using the observed 001 peak intensity with the thickness correction in the XRD measurement system. The intensity of the 200 diffraction of the Cu-Au structure must be weak, because the Cu-Au phase predominantly grows with c-axis normal to the substrate as will be revealed from the RHEED analysis later. These discussions imply that the contribution of Cu-Au structure to curve B is small, although it is supposed that the curve B is composed of four peaks as enumerated in the preceding paragraph. Now, suppose the sphalerite and chalcopyrite structures are considered. The lattice parameter of the sphalerite structure is tentatively known to be aSP=5.49Å (2θ=32.60°). It should be noted that the diffraction angle (2θ=32.59°) observed on the film sample with 73% sphalerite and 27% Cu-Au structures cited in Ref. 27 agrees well with identified 002 diffraction angle for the sphalerite structure, as the fraction of the Cu-Au structure to the total diffraction intensity is estimated to be only 10%. On the other hand, the 200 diffraction angle of the chalcopyrite structure is estimated to be 2θ=32.44° from the lattice parameters a and c, that is if the curve A is the chalcopyrite 004 diffraction. The peak position of curve B fall almost between these two angles (32.44 and 32.59°) described above. This implies that both 002 diffraction of the sphalerite structure and 200 diffraction of the chalcopyrite structure contribute to curve B.

The observed diffraction patterns at 2θ=32.5° is reproduced by superposing two curves for all films. However, the calculated fraction of the 004 diffraction of the chalcopyrite structure is now the problem to be solved by referring to the RHEED analyses that follow.

Reflection High Energy Electron Diffraction (RHEED)

Three typical RHEED patterns (taken along the <010> and <11 0> directions of the Si substrate) of films with [Cu]/[In]=0.9,1.4 and 2.0 are shown in Figs. 4-3, 4-4 and 4-5, respectively.

26

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) Cu-Au (a⊥substrate)

(a)

(c)

(b)

(d)

Fig. 4-3 RHEED patterns for CuInS2 with [Cu]/[In]=0.9 on Si(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) chalcopyrite (c⊥substrate) Cu-Au (a⊥substrate) chalcopyrite (c⊥substrate)

twin (common) unknown

(b)

(d) (c)

(a)

Fig. 4-4 RHEED patterns for CuInS2 with [Cu]/[In]=1.4 on Si(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

27

In the diagrams, the observed diffraction spots are classified by open circles, squares, and open or shaded diamond figures for clarification. The large circles are common for the sphalerite, Cu-Au and chalcopyrite structures. Moreover, the small squares represent the spots due to twins. Shaded vertical and horizontal diamond figures denote respectively the c- and a-axes orientated chalcopyrite structure while the open vertical and horizontal diamond marks are for c- and a-axes orientated Cu-Au structure. The small triangles denote the unknown spots. These notations are used all throughout the text for RHEED diagrams of the observed RHEED patterns. To simplify discussions, the growth with c-axis and a-axis normal to the substrate are called as c-axis and a-axis orientations, respectively. The characteristic spots of the Cu-Au structure with both a- and c-axes orientations are observed but the spots of the a-axis are weak as shown in Fig. 4-3(a). This means that the c-axis orientation is predominant over a-axis orientation. The characteristic spots of the chalcopyrite structure are not observed in a film with [Cu]/[In]=0.9 as shown in Figs. 4-3(a) and 4-3(b). As the spots of the sphalerite structure are fully included in the spots of the Cu-Au structure, it can not be known

Fig. 4-5 RHEED patterns for CuInS2 with [Cu]/[In]=2.0 on Si(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) chalcopyrite (c⊥substrate) Cu-Au (a⊥substrate) chalcopyrite (c⊥substrate)

twin (common) unknown twin (Cu-Au)

(c)

(a) (b)

(d)

28

exactly whether the sphalerite structure coexists or not by means of RHEED. The resolution of the XRD pattern as examined in Fig. 4-2 implies the coexistence of the sphalerite structure.

The characteristic spots of the chalcopyrite structure are observed in the films with [Cu]/[In]=1.4 and 2.0 as shown in Figs. 4-4(a) and 4-4(b), and Figs. 4-5(a) and 4-5(b), respectively. It is also found that the Cu-Au structure with two growth directions (a-axis and c-axis orientations) coexists. The degree of the preferential c-axis orientation seemed to be weak compared to the film with [Cu]/[In]=0.9; it weakens with the increase of [Cu]/[In]. The coexistence of the sphalerite structure is clear in the RHEED patterns, but it is probable in the XRD analyses. As mentioned before, the existence of the chalcopyrite structure with two growth directions was assumed through the analysis of XRD pattern. The RHEED patterns agree well with the interpretation for the XRD, although the spots for the c-axis orientation are weak in a film with [Cu]/[In]=1.4. The spots for this orientation are clearer for the film with [Cu]/[In]=2.0.

The twin structure is not recognized in the film with [Cu]/[In]=0.9 as shown in Figs. 4-3(c) and 4-3(d). However, it is obvious in films with larger [Cu]/[In] as depicted in Figs. 4-4(c), 4-4(d), 4-5(c) and 4-6(d). The spots attributed to twins are more intense in Fig. 4-5(c) than in Fig. 4-4(c). The characteristic twin spots of the Cu-Au structure are observed in a film [Cu]/[In]=2.0 as featured in Figs. 4-5(c) and 4-5(d) although they are very weak.

The rings which represent the existence of polycrystalline are weak in films with [Cu]/[In]=1.4 and 2.0, but fairly strong in a film [Cu]/[In]=0.9. This implies that the surface condition of the latter is poor, as the misorientation is not remarkable in the XRD pattern.

Photoluminescence

Photoluminescence (PL) emission of films is investigated using a green laser (532nm) which is pumped by a semiconductor laser diode. The PL emission detection is performed using a photocounting system with a monochromator (46cm) and a photomultiplier which is sensitive up to 900nm at 23 K.

The photoluminescence (PL) properties often reflect the crystalinity of materials. The PL properties of the epitaxial CuInS2 films grown on the CaF2(111) were reported but no reports have been known for films grown on the Si(001) wafers.29) In this section, the typical PL spectrum of a film with [Cu]/[In]=1.4 together with a bulk crystal as reference are presented.

29

Figure 4-6 shows the typical PL spectra measured at 23 K for a film and a bulk crystal grown from the melt. The bulk CuInS2 shows two sharp emissions near 810nm (A and B) and two broad emission bands near 860nm (C) and 880nm (D) as depicted in Fig.4-6(a). Peak A is the free exciton emission and peak B is interpreted as the bound exciton emission.30) This is consistent with the above interpretation. Referring to the works of Binsma et al.,31) emission bands C and D are interpreted as the donor-acceptor pair emission and its phonon replica, respectively. Töpper et al. interpreted in the same manner on the polycrystalline films and proposed that the donor is S vacancy and the acceptor is Cu vacancy. 32, 33) In Fig.4-6(b) the PL spectrum of the film is shown and no structure can be observed near 810nm where the sharp exciton emissions are observed in the bulk sample. However, the weak broad emission band E is observed over the range of 810~840nm. Moreover, the emission band centered at 855nm (F) and the shoulder near 875nm (G) are noticeable. First we suppose that the band edge is indefinite as the film is composed of chalcopyrite, Cu-Au and sphalerite structures. Su et al. calculated that the forbidden gap energy of the Cu-Au structure is 30meV lower than CuInS2.34) The position of the exciton emission of the Cu-Au structure is estimated to be 827nm if the exciton binding energy is the same as that of the chalcopyrite structure. Although the forbidden gap energy of the sphalerite structure is unknown, it is natural to suppose that it slightly differs from that of the chalcopyrite structure. The weak broad emission band over 810~840nm can be interpreted as the superposition of the exciton emissions of various phases.35) The peak position of F and G emission bands (855nm and 875nm, respectively) is 5nm shorter (8-9meV higher) than the bulk and the thin film deposited on CaF2.29) However, the analogy between two spectra shown in Fig. 4-6 implies that F band corresponds to the donor-acceptor pair emission and G band is its phonon replica. However, it is not clear whether the acceptor is Cu vacancy, as the In and S vacancies are probable considering the composition ([Cu]/[In]=1.4). The shift of these emissions toward shorter wavelength implies that the concentration of the defects in the film is higher than that of the bulk. Although the change of the band structure may affect these emissions, the observed peak shift is opposite considering the shift of the exciton emission. Other films with [Cu]/[In]=0.9 and 2.0 did not show the emission. Judging from the works of Metzner et al. who also could observe the PL on the Cu rich film ([Cu]/[In]=1.2), 29) the slightly Cu rich films may show the intense emission.

30

Fig. 4-6 Photoluminescence spectra of CuInS2 for (a) bulk, stoichiometric, and (b) the film with [Cu]/[In]=1.4 on Si(001) taken at 23K. The excitation wavelength is 532nm.

780 800 820 840 860 880 900

G

F

E

PL In

tens

ity [a

rb.u

nit]

Wavelength [nm]

film[Cu]/[In]=1.4

DC

BA

bulk(a)

(b)

31

4.2 Improved structures of films on GaP at Tsub=500°C

The weak undesirable 111 reflection (cubic notation) and the unknown peaks observed in X-ray diffraction (XRD) results of CuInS2 films grown on Si presented in section 4.1. Moreover, the weak unknown spots, twin spots and some rings indicating the existence of polycrystals were observed in the reflection high energy electron diffraction (RHEED) patterns of these films. Thus, the crystallinity is considered not satisfactory. In this part, the compound substrate gallium phosphide (GaP) with fairly smaller lattice mismatch with CuInS2 is considered. As presented in Chapter 1, gallium phosphide (GaP) is used as substrate considering that the lattice mismatch (ε) for the epitaxial growth of chalcopyrite structure towards c-axis on GaP (εCH/GaP=1.3%) is lower than that for both chalcopyrite and Cu-Au structures towards c-axis on Si (εCH/Si=1.6% and εCA/Si=1.5%). GaP is also bipolar that may help suppress the randomness of the adsorbing initial atoms on GaP.

Films of various compositions are grown on GaP(001) at 500°C. These films of typical thickness 0.16-0.20µm (which is larger than the approximate critical thickness 13-40nm) exhibit different growth tendency compared to films grown on Si substrate as featured in the XRD and RHEED results that follow.

X-ray diffraction (XRD)

The XRD spectra at θ-2θ scan (10°≤2θ≤90°) of the samples with [Cu]/[In]=0.8, 1.27, 1.5, and 1.7 ratios are shown in Fig. 4-7. Logarithmic scaled vertical axes are again chosen to show the existing phases including minor competing phases. The peaks belonging to CuInS2 are labeled as A, B, C and D (with E for film with [Cu]/[In]=1.27) and the peaks originated from the extra phases and the substrate, are denoted using their respective chemical formula. The peaks A, B and E are forbidden for chalcopyrite and sphalerite orderings but are allowed for Cu-Au ordering. These peaks are assigned to 001, 003 and 005 reflections of Cu-Au ordering, respectively.26) The very small unknown peaks observed at about 27.8° and 30.4° for films on Si are completely suppressed. Furthermore, the 111 reflection observed for fairly Cu-rich films on Si was not observed in any films on GaP even in the highly Cu-rich films of [Cu]/[In]=2.93.

However, for the film with [Cu]/[In]=0.8, in addition to the peaks of CuInS2, the peaks assigned to 004 and 008 reflections of spinel CuIn5S8 are evident. No traces of 111, 311, 511 and 440 reflections of CuIn5S8 are noticeable that were observed for the

32

In-rich films grown on Si (001) at 400°C27). The coexistence of CuIn5S8 is observed even on the nearly stoichiometric film that differed from the film on Si.

As shown in Fig. 4-7, the highest intensity of 001 and 003 reflections are observed for film with [Cu]/[In]=1.27 together with the noticeable appearance of a peak around 89° that is assigned to 005, the higher order Cu-Au reflection. Except for the peaks of GaP substrate a peak around 68.3° for this film is evident. This angle corresponds to the Cu-Au 004 angle estimated from the observed 003 Cu-Au XRD angle, indicating that the observed XRD peak is due to 004 reflection of Cu-Au structure. But the sphalerite 004 reflection could also slightly contribute to the observed XRD peak because the sphalerite 004 reflection is also allowed around the Cu-Au 004 reflection angle. The

Fig. 4-7 XRD spectra of films grown on GaP (001) with (a) [Cu]/[In]=0.8, (b) [Cu]/[In]=1.27, [Cu]/[In]=1.5 and (c) [Cu]/[In]=1.7 at 10°≤2θ≤90°.

10 20 30 40 50 60 70 80 90

101

102

103

104

105

DG a P

C

G a P

BA

2θ [deg.]

101

102

103

104

105

?

D

G a P

?C

G a P

BA

(d) [Cu]/[In] = 1.70

(c) [Cu]/[In] = 1.50

101

102

103

104

105

E

DG a P

C

G a P

B

A

X-r

ay In

tens

ity (a

rb.u

nits

)

(b) [Cu]/[In] = 1.27

(a) [Cu]/[In] = 0.80

101

102

103

104

105

C u I n 5 S

8

0 0 8

G a P

C u I n 5 S

8

0 0 4

G a P

D

B

C

A

33

chalcopyrite structure is not considered to coexist in the film because of the absence of the peculiar spots of chalcopyrite structure in its RHEED pattern as described the

RHEED section. This slightly Cu-rich film is considered to be highly c-axis oriented Cu-Au structure.

The peaks corresponding to CuIn5S8 are absent on the Cu-rich films, and unknown peaks appear at about 33.9° and 71.4° shown by question marks (?) as depicted in Fig. 4-7(c). The peak at 71.4° is considered as the higher order reflection of the peak at 33.9°. Since both peaks were not observed for films grown on Si, this may be due to some reactions that had occurred at the Cu-rich film and the GaP substrate interface region

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0

102

103

104

105

SP

c -C

H

c -C

Aa

-C

Ha

-C

A

(c) [C u]/[I n]=1.7 G a P

X-r

ay In

tens

ity (a

rb.u

nits

)

2θ [deg.]

102

103

104

105

?

(b) [C u]/[I n]=1.5 G a P

102

103

104

105

(a) [C u]/[I n]=0.8 C u I n 5 S

8

0 0 4

G a P

68 70

102

103

104

105

c -C

H

c -C

A

a -C

H

a -C

Ac -

CH

G a P

X-r

ay In

tens

ity

2θ [deg.]

Fig. 4-8 XRD spectra of films grown on GaP (001) with (a) [Cu]/[In]=0.8, (b) [Cu]/[In]=1.5 and (c) [Cu]/[In]=1.7 at 31°≤2θ≤35°.

34

during film growth. To make clear the structural ordering, the detailed XRD spectra around 32° (i.e. peak C) are shown in Fig. 4-8 for films with comparable compositions to films grown on Si at the same substrate temperature (Tsub=500°C). For comparison, the corresponding spectra of films on Si are also shown in Fig. 4-9. Although the spectra at higher angle positions (higher order reflections) are favorable, the lower angle XRD spectra are adopted since the interference of the Si substrate is minimized because of the extinction rule of the 200 reflection. For the analysis, we used the lattice constants cCH=11.122Å, aCH=5.520Å, cCA=5.51Å and aSP=5.49Å for chalcopyrite, Cu-Au and sphalerite structural orderings.26,36) These parameters yield reflections at 32.19°, 32.44°, 32.50° and 32.62°, respectively. For the Cu-Au ordering, we used aCA=5.54Å obtained by dividing the value of the cited cCA=5.51Å by the ratio cCA/aCA=0.994 which was estimated from the twin spots analysis of the RHEED pattern.27) The aCA value gives a diffraction angle of 32.32°. However, the 2θ position of 200 reflection of Cu-Au structure (a-CA) is still tentative because the lattice constant 5.54Å differs from 5.47Å reported by Hahn et al.20) Moreover, the lattice parameter aSP cited above was estimated from TEM observation near the interface region. Considering the reported error limit, the diffraction angle of the sphalerite ordering is expected to extend over the 2θ range between 32.38° and 32.87°. For the film with [Cu]/[In]=0.8, the shoulder is observed on the lower skirt of

GaP reflections as shown in Fig. 4-8(a). Although the specific structure cannot be

ascertained directly, the a- and the c-axes orientations of Cu-Au ordering are probable

since the shoulder lies near 2θ=32.32° and 32.50°. The peak position of the film grown on

Si with [Cu]/[In]=0.9 shown in Fig. 4-9(a) lies at slightly lower angle of c-axis orientated

Cu-Au ordering. This is consistent with our interpretation for the film on GaP described

above that a- and c-axes orientations of Cu-Au ordering coexist. In fact, this was evident

from the RHEED pattern shown in Fig. 4-10. Since the present film contains CuIn5S8

phase with [Cu]/[In]=0.2, the [Cu]/[In] ratio of the remaining CuInS2 phase must be

larger than 0.8. It may exceed unity (Cu-rich) and the chalcopyrite phase may exist in

this Cu-rich area as demonstrated by the film with [Cu]/[In]=1.5. The sphalerite

ordering may also exist in this film considering the error limit of the lattice parameter

aSP.

35

The film with [Cu]/[In]=1.5 shows peaks that span over a broader range of 2θ

values from 32.1 to 32.6° featured in Fig. 4-8(b). Referring to the calculated diffraction

angles, this sample is considered to compose of both the a- and c-axes orientations of

chalcopyrite, and the a- and c-axes orientations of Cu-Au orderings. The inclusion of

sphalerite structural ordering of cations is also likely to exist. It is worth noticing that

001 and the 003 reflections (i.e. peak A and B) of Cu-Au that correspond to the c-axis

normal to the substrate are weak and the intensity of peak C is almost the same as the film

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0

102

103

104

105

SP

c -C

A

a -C

Ha

-C

A

c -C

H

S i0 0 2

(c) [C u]/[I n]=2.0

X-r

ay In

tens

ity (a

rb.u

nits

)

2θ [deg.]

102

103

104

105 S i0 0 2

(b) [C u]/[I n]=1.4

102

103

104

105 S i0 0 2

(a) [C u]/[I n]= 0.9

Fig. 4-9 XRD spectra of films grown on Si(001) with (a)[Cu]/[In]=0.9, (b)[Cu]/[In] =1.4 and (c)[Cu]/[In]=2.0 at 31°≤2θ≤35°. The calculated positions are indicated as c-CH, a-CH, c-CA, a-CA and SP.

36

with [Cu]/[In]=0.8 as shown in Fig. 4-8(b). Thus, we consider the contribution of c-axis

orientated Cu-Au ordered structure to be low. Comparing these results to the XRD

spectrum of CuInS2 on Si with [Cu]/[In]=1.4 the fraction of chalcopyrite phase is high

and the Cu-Au structures are suppressed. These results suggest an improved quality of the

present samples grown on GaP substrate.

In the XRD spectrum shown in Fig. 4-8(c) for fairly Cu-rich film with

[Cu]/[In]=1.7, c-axis orientation of Cu-Au ordering is dominant. The sphalerite ordering

may also exist in this film. Referring to the reflection at higher 2θ positions (peak D)

shown in the inset of Fig. 4-8 (c), the presence of a fraction of other orderings such as

a-axis orientation of chalcopyrite ordering seems to be probable. However, the c-axis

orientation of chalcopyrite ordering decreased. It is slightly similar to the film with

[Cu]/[In]=2.0 grown on Si whose fraction of c-axis chalcopyrite orientation is lower

than that of the film with [Cu]/[In]=1.4.

The improvement of the quality of the films on GaP is evident compared to the films grown on Si. It was reported that the fraction of chalcopyrite ordering of Cu-rich sample is larger than that of the In-rich sample for films on Si(111) and this film was composed of both chalcopyrite and Cu-Au orderings.29,37) It is worth noting that in section 4.1.1 for the CuInS2 grown on Si(001), the same result was observed, but the fraction of chalcopyrite slightly decreased on the Cu-rich film as demonstrated in Figs. 4-8(b) and 4-8(c). However, considering the analyses on other films with various compositions, the appearance of chalcopyrite ordering on GaP seems to be restricted in narrower range of [Cu]/[In] ratio than that of films on Si. These improvements of CuInS2 film growth on GaP is seen clearly in the RHEED results that follow.

Reflection high energy electron diffraction (RHEED)

Typical RHEED patterns of film samples with [Cu]/[In]=0.8, 1.27, 1.5 and 1.7

ratios are featured in Figs. 4-10 to 4-13. Each figure shows clear patterns unique for

specific [Cu]/[In] ratio without any evidence of rings that indicate random direction

which were observed in films on Si regardless of the intensity. All diffracted spots

correspond to the CuIn5S8 and/or the CuInS2 of various orderings. The unknown spots

evident in Cu-rich films on Si are not observed on films on GaP.

In each figure, the observed patterns for the electron beam incident along <010>

37

and <ī10> directions of the GaP substrate, are shown in (a) and (c), respectively. For analysis, the observed diffraction spots are illustrated in the diagrams (b) and (d) corresponding to (a) and (c), respectively. In these diagrams, we mark and classify the observed diffraction spots by circles and squares. The large open circles are common for the sphalerite, the Cu-Au, the chalcopyrite, and the spinel structures while the larger squares correspond uniquely to the characteristic spots of the spinel structure. On the other hand, the medium-sized shaded and the open circles denote respectively the characteristic spots for Cu-Au ordering of a- and c-axes orientations normal to the

substrate. The smallest shaded and the open circles correspond to the a- and the c-axes orientated chalcopyrite ordering.

In Figs. 4-10(a) and 4-10(c), we present the RHEED patterns of film with [Cu]/[In]=0.8. The spots of a-axis orientated spinel structure (CuIn5S8) exist together with the spots attributed to a- and c-axes orientated Cu-Au ordering. This is consistent with the interpretation of the XRD spectrum presented in Figs. 4-7(a) and 4-8(a). The

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) spinel Cu-Au (a⊥substrate)

(c)

(a) (b)

(d)

Fig. 4-10 RHEED patterns for CuInS2 with [Cu]/[In]=0.8 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d).

38

spinel structure was not observed on slightly In-rich film with [Cu]/[In]=0.9 grown on Si (001) at 500°C.35) The evidence of chalcopyrite ordering is not noticeable. Thus, the surface region of this film is composed dominantly of spinel structure and a- and c-axes orientated Cu-Au structures. The CuInS2 film with [Cu]/[In]=1.27 exhibits Cu-Au ordering with c-axis normal to the substrate as demonstrated by the RHEED pattern shown in Figs. 4-11(a) and 4-11(b). The typical RHEED spots peculiar for chalcopyrite structure is not visible and the twins of Cu-Au structure are not recognizable in the pattern of <ī10> direction featured in Figs. 4-11(c) and 4-11(d). These results are consistent with the analyses of the XRD spectrum of this film (Fig. 4-7) that it is composed of monocrystalline Cu-Au ordering highly oriented along the c-axis normal to the substrate. Streaky RHEED patterns are exhibited by the film with [Cu]/[In]=1.5 suggesting a smooth film surface. Spots of Cu-Au ordering of c-axis orientation and chalcopyrite

ordering of a-axis orientation are clearly featured in Fig. 4-12(a). Despite the weak intensities of a-axis orientated Cu-Au and the c-axis orientated chalcopyrite orderings, both variants are observed. All observed spots are depicted in the schematic diagram shown in Fig. 4-12(b). These results confirm the interpretation of the broad peak C in

Fig. 4-11 RHEED patterns for CuInS2 with [Cu]/[In]=1.27 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d).

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate)

(a)

(c) (d)

39

the XRD spectra featured in Figs. 4-7(c) and 4-8(b) that span through the 2θ positions corresponding to the various variants of chalcopyrite and Cu-Au orderings. Very weak spots due to twins common to chalcopyrite, Cu-Au and sphalerite are also observed as shown by the small open squares in Fig. 5(d). These spots were also observed on CuInS2 film grown on Si with [Cu]/[In]=1.4 together with the presence of some

unknown spots. However, for the film on GaP, the number of observed twin spots decreased because their intensities are so weak to detect and the unknown spots are not observed. Moreover, no spots corresponding to the unknown phase observed in the XRD spectrum shown in Fig. 4-8(b) are recognizable. This agrees with the interpretation that the unknown phase exists in the interface between the film and the substrate. The RHEED patterns shown in Figs. 4-13(a) and 4-13(c) reveal the existence of

dominantly c-axis orientated Cu-Au structure for the film with [Cu]/[In]=1.7 that

confirms the observed XRD spectra shown in Fig. 4-7(d) and Fig. 4-8(c). Compared to

common for chalcopyrite, Cu-Au and sphalerite structures Cu-Au (c⊥substrate) chalcopyrite (c⊥substrate) Cu-Au (a⊥substrate) chalcopyrite (a⊥substrate) twin (common)

(a)

(c)

(b)

(b)

Fig. 4-12 RHEED patterns for CuInS2 with [Cu]/[In]=1.5 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

40

the film with [Cu]/[In]=1.27 (Fig. 4-11) where dominant c-axis Cu-Au orientation is

evident, this film is composed of less amount of Cu-Au ordering. It should be noted that

this film ([Cu]/[In]=1.7 has lower intensity in its XRD spectrum shown in Fig. 4-7 than

the film with [Cu]/[In]=1.27. It cannot be denied that the sphalerite ordering coexist

because its spots almost completely overlap with that of Cu-Au ordering. The presence

of other orderings such as a-axis orientated chalcopyrite suggested from the XRD

spectrum, is not confirmed in the RHEED patterns. This may be due to the difference of

the detection volume; RHEED occurs at the surface while XRD in the whole spatial

region. The composition difference between the surface and the bulk is probable for this

film.38) Smooth film surface is suggested by these streaky patterns.

For the RHEED patterns of the film on Si with [Cu]/[In]=2.0 shown in Fig. 4-5 in

section 4.1, both a- and c-axes orientations of Cu-Au and chalcopyrite orderings and

the strong twin spots of these orderings were observed. The RHEED pattern of the

Fig. 4-13 RHEED patterns for CuInS2 with [Cu]/[In]=1.7 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate)

(a)

(c)

(b)

41

present film on GaP, however, is simpler with only monocrystalline c-axis orientated

Cu-Au structure (overlapping the sphalerite pattern). Neither twins nor rings can be

recognized in this film. Even the unknown spots observed on the RHEED patterns for

the film on Si are not observed.

These results suggest that improved crystal quality have been grown on GaP(001)

compared to Si(001). Although the growth of chalcopyrite ordering was observed by

using GaP substrate, it is not sufficient considering that the chalcopyrite exist together

with the variants of Cu-Au crystal domains, and the growth of films with only

chalcopyrite ordered structure seems not possible at 500°C substrate temperature on

both GaP and Si substrate since Cu-Au are almost always present in the films studied.

The option is to proceed to evaluate the growth of chalcopyrite structures at elevated

(>500°C) substrate temperatures. Before considering the results at elevated substrate

temperature, the photoluminescence of the representative film grown at 500°C is

discussed as follows.

Photoluminescence (PL)

Figure 4-14 shows the photoluminescence (PL) spectra of the film with

[Cu]/[In]=1.7 and the bulk sample used in Fig. 4-7 (in section 4.1). The spectrum of the

film is broader than that of Si featured in Fig. 4-7 (b). Comparing to the PL spectra

observed for the bulk CuInS2 sample, this broad spectrum seems to span over the peaks

that include A (exciton), B (bound exciton), C (donor-acceptor pair) and D (phonon

replica).31) The position of the estimated exciton emission of Cu-Au structure (827nm)

is also included in the broad peak. This result implies that the film with [Cu]/[In]=1.7 is

composed of chalcopyrite and Cu-Au ordered structures, which is consistent with the

RHEED analyses of this film (Fig. 4-13). It should be noted that the PL emission of the

sphalerite structure is not yet known. Although not so evident peaks due to the exciton

emission of chalcopyrite structure are observed, it can be deduced from these results

that the films grown on GaP at 500°C are of better quality compared to films grown on

Si at the same substrate temperature.

42

800 820 840 860 880 900

Cu-A

u

(827nm

)

(b) Film

PL Inte

nsi

ty [

arb.

unit]

Wavelength (nm)

DC

BA (a) Bulk

Fig. 4-14 Photoluminescence spectra of CuInS2 for (a) the stoichiometric bulk, and (b) the film with [Cu]/[In]=1.7 on GaP(001) taken at around 23K. The excitation wavelength is 532nm.

43

CHAPTER 5

GROWTH OF CHALCOPYRITE STRUCTURE

High quality CuInS2 films grown on GaP at 570 and 600°C are presented here compared to the films grown on Si and on GaP at 500°C. Depending on composition, the relaxed CuInS2 thin films of thickness 0.10-0.170µm exhibit specific structural ordering.

5.1 Ordering and orientation of films on GaP at Tsub=570°C

The evolution of the growth of epitaxial chalcopyrite ordering and orientation is evident in the XRD and RHEED results of the CuInS2 thin films grown on GaP at 570°C.

X-ray diffraction (XRD)

Figure 5-1 shows the θ-2θ scan (10≤2θ≤90°) XRD spectra of the representative film samples with compositions [Cu]/[In]=1.01, 1.25, 1.28 and 1.41. The almost stoichiometric film shows the coexistence of epitaxial spinel structure CuIn5S8 around 33.5 and 70.6°. It is worth mentioning that the epitaxial growth of CuIn5S8 is always observed on In-rich films.

In Fig. 4-15, the peaks are labeled A, B, C, D, E and F; each marks a certain structural ordering. The positions of XRD peaks A, C, B and D shown in Fig. 5-1(a) and 5-1(b) for films with [Cu]/[In]=1.01 and 1.25 are found respectively at about 16, 32.5, 49.7, and 68.6°; all are forbidden for chalcopyrite ordering which is characterized by its symmetry space group ( dI 24 ) that requires (00l):l=4n and (h00):h=2n reflections due to screw dyads along the crystallographic axis in addition to (hkl):h+k+l=2n, (hh0):h=2n and (hhl):2h+l=4n.23) These 00l reflections with l=odd is allowed in Cu-Au ordering of cations with a space group 24mP . The A, C, B, and D peaks respectively correspond to 001, 002, 003, and 004 reflections of Cu-Au. Thus, the Cu-Au ordered structures are said to be oriented with c-axis normal to the (001) plane of GaP substrate. If the XRD spectra will be viewed around 2θ=32.5° for the film with [Cu]/[In]=1.01 as shown in Fig. 5-1(a), a broad skirt can be seen in the range 32.2≤2θ≤32.6° indicative of the presence of a-axis orientated Cu-Au and sphalerite cationic orderings. A broader spectral range (32≤2θ≤32.6°) can be observed for film with [Cu]/[In]=1.25 which can be attributed to the existence of a-axis ordering of Cu-Au, c-axis Cu-Au, and both a- and c-axes orientated chalcopyrite orderings.

44

However, for the film with [Cu]/[In]=1.28 and 1.41 shown in Figs. 5-1(c) and 5-1(d), the structure is only dominated by 004 (32.15°) and 008 (67.3°) chalcopyrite reflections denoted respectively by peaks E and F. Despite of the logarithmic scale, the positions of chalcopyrite structure is dominant while the positions corresponding to the Cu-Au structure are somewhat depressed. Thus, the contribution of Cu-Au structure if present is considered minimal. This is confirmed in the RHEED patterns shown in the RHEED section of these films. Since the film with [Cu]/[In]=1.28 has the same XRD

10 20 30 40 50 60 70 80 90

101

102

103

104

105G a PG a P

FE

X-ra

y In

tensi

ty (ar

b.units)

2θ [deg.]

101

102

103

104

105

?

G a PG a PFE

G a P

(c) [Cu]/[In] = 1.28

(d) [Cu]/[In] = 1.41

101

102

103

104

105

G a PG a P

DC

BA

(b) [Cu]/[In] = 1.25

101

102

103

104

105

106

C D

G a P

C u I n 5 S

8

0 0 8

BA

C u I n 5 S

8

0 0 4

(a) [Cu]/[In] = 1.01

Fig. 5-1 XRD spectra of films grown on GaP(001) with (a) [Cu]/[In]=1.01, (b)[Cu]/[In]=1.25, (c)[Cu]/[In]=1.28 and (d)[Cu]/[In]=1.41 at 10≤2θ≤90° at Tsub=570°C.

45

pattern with the film with [Cu]/[In]=1.41, plotting only one in the 31≤2θ≤35° region is sufficient. The peaks at 32.15 and 67.3° in Figs. 5-1(c) and (d), and 5-2(d), indicate that the c-axis of chalcopyrite domains are highly oriented normal to the (001) plane of GaP substrate. The XRD results for the film with [Cu]/[In]=1.28 are reported elsewhere.39). The difference between these two films is the presence of the peaks around 34.5° and its higher order reflection at 68.5° for the film with [Cu]/[In]=1.28. These peaks denoted by question marks (?) are considered unknown. These are not observed in the film with [Cu]/[In]=1.41 and if present these are considered amorphous or negligible. These unknown peaks may be due to the substrate-film overgrowth interface reaction. In the XRD results for films with 1.41<[Cu]/[In]≤2.55 shown in Figs. 5-3(a) through 5-3(d) where the peaks are labeled with A, C, D and question mark, the c-axis orientated Cu-Au ordering (peak A) is somewhat depressed and are considered negligible. These results were not observed in films grown on both Si and GaP at 500°C. Looking closely at the XRD results around peak C in the region 31≤2θ≤35°, the structural ordering

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0100

101

102

103

104

105

SP

c -C

H

c -C

Aa

-C

Ha

-C

A

(c) [C u]/[I n]=1.41 G a P

X-r

ay In

tens

ity (a

rb.u

nits

)

2θ [deg.]

100

101

102

103

104

105(b) [C u]/[I n]=1.25 G a P

100

101

102

103

104

105

(a) [C u]/[I n]=1.01 C u I n

5 S

8

0 0 4

G a P

Fig. 5-2 XRD spectra of films grown on GaP (001) with (a)[Cu]/[In]=1.01, (b)[Cu]/[In]=1.25 and (c)[Cu]/[In]=1.41 at 31≤2θ≤35°. In c-CH, a-CH, c-CA, a-CA and SP, c and a are the lattice parameters of chalcopyrite (CH), Cu-Au (CA) and sphalerite (SP) structures normal to the substrate.

46

transformation is noticeable. In Figs. 5-3 and 5-4, the XRD spectrum of the film with (i.e. [Cu]/[In]=1.41) monocrystalline chalcopyrite ordering only is also shown for comparison. With increasing [Cu]/[In] ratio (Fig. 5-4), the trend of ordering transformation is: chalcopyrite to Cu-Au then to sphalerite orderings transformation. It should be noted that all the films in the composition range 1.64≤[Cu]/[In]≤2.55 exhibit the growth of unknown peak labeled with question marks (?). This appearance is somewhat regular and normal for all the highly Cu-rich films grown at 570°C. Thus, there is a reason to believe that these unknown peaks are usually present in highly Cu-rich films.

10 20 30 40 50 60 70 80 90

101102103104105

(d)

D??

C

A

Inte

nsity

(arb

. uni

ts)

2θ (deg.)

[Cu]/[In]=2.55

101102103104105

(c)

D??

C

A

[Cu]/[In]=1.92

101102103104105

(b)

D??

C

A

[Cu]/[In]=1.64101102103104105

(a) Tsub

=570oC

DC

[Cu]/[In]=1.41

Fig. 5-3 XRD spectra at 10≤2θ≤90° of films grown on GaP(001) with (a) [Cu]/[In]=1.41, (b)[Cu]/[In]=1.64, (c)[Cu]/[In]=1.92 and (d)[Cu]/[In]=2.55 at Tsub=570°C.

47

Reflection high energy electron diffraction (RHEED) The RHEED patterns of the films in the range of 1.01≤[Cu]/[In]≤2.55 ratios are

shown in Figs. 5-5 to 5-11. Figures 5-5(a) and 5-5(b) for almost stoichiometric film show spots due to spinel

ordering coexisting with both a- and c-axes orientated Cu-Au ordering. Together with the intensified spots of the two domain variants of a-axis Cu-Au and the c-axis Cu-Au ordering orientations, the appearance of the two orientations (a- and c-axes) of chalcopyrite ordering becomes apparent as depicted in Figs. 5-5(a) and (b). For the films with 1.28≤[Cu]/[In]≤1.41 represented by Figs. 5-7(a) and (b), the prominent c-axis orientated chalcopyrite ordering is observed and the disappearance of the spots for Cu-Au ordering is evident. In Fig. 5-8(b), c-axis Cu-Au seems to be present but its contribution is negligible considering its absence in the XRD result depicted in Fig. 5-1(c).

Most of the RHEED results agree well with XRD results except for the highly Cu-rich films. No diffraction spots can be attributed to the unknown peaks in the XRD

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0

0

2000

4000

6000

8000

10000

12000

14000T

sub=570oC

GaP

SP

c-C

Aa-

CH

a-C

Ac-

CH

In

tensi

ty (

arb. unit)

2θ (deg.)

[Cu]/[In]= 1.41 1.64 1.92 2.55

Fig. 5-4 XRD spectra at 31≤2θ≤35° of films grown on GaP(001) with [Cu]/[In]=1.41, 1.64, 1.92 and 2.55 grown at Tsub=570°C. In c-CH, a-CH, c-CA, a-CA and SP, c and a are the lattice parameters of chalcopyrite (CH), Cu-Au (CA) and sphalerite (SP) structures normal to the substrate.

48

results for the film with [Cu]/[In]=1.28 (Fig. 5-7). Since RHEED is sensitive only at the surface area and only small buried area can be detected, the unknown (?) peaks observed in Fig. 5-1(c) may be due to the reaction at the substrate-film epilayer interface. The representative AES depth profiles of the films within 1.01≤[Cu]/[In]≤1.41 are shown in Fig. 5-12.

The AES measurements are done using the product of JEOL (JAMP-7100) that has energy accuracy and resolution of 0.5%. The accelerating voltage used is 10 kV with probe current that ranges from 2.62 to 5.13x10-8A. Surface analyses are done in 5µm probe diameter of the film samples at about 10-8 Torr environment. The AES data acquisition is done first on the surface then subsequent Ar＋ ion sputtering to etch this region is performed. The respective sensitivity factors used in the elemental analyses are 4.674, 1.295, 1.043, 6.480 and 1.212 for Cu, In, S, Ga and P. These sensitivity factors are measured with respect to the standard samples of pure silver, single crystalline iodine-vapor grown CuInS2 of composition Cu:In:S=1:1:2, and single crystal GaP.

Going back to Fig. 5-12, it features the somewhat homogenous compositional distribution of Cu, In and S throughout the volume of the film. The representative films ([Cu]/[In]=1.01, 1.28 and 1.41 via EPMA) depict surface compositions 1.06, 1.31 and 1.28, respectively. The AES depth profile analyses for these films show bulk [Cu]/[In] ratios 1.12, 1.29 and 1.40. Within the experimental error, these values agree well with the EPMA ([Cu]/[In]=1.01, 1.28 and 1.41) results.

The growth of c-axis orientated chalcopyrite structure reduces as observed in films with 1.64<[Cu]/[In]≤2.55 ratios shown in Figs. 5-9 through 5-11. However, as depicted in Figs. 5-9 and 5-10, the spinel structure seems to co-exist with the chalcopyrite and/or sphalerite structures. Going back to the XRD results for these films in Figs. 5-3 and 5-4, the peaks due to spinel (CuIn5S8) were not detected. To explain these inconsistencies with XRD and RHEED results for these films ([Cu]/[In]=1.64 and 1.92), AES compositional depth profile analyses were done also.

Figures 5-13 features the depth profile AES of the films with [Cu]/[In]=1.64, 1.92 and 2.55. From this figure, it can be deduced that Cu and S are homogenously distributed throughout the volume of the film (which is the same in Fig. 5-12 for stoichiometric and slightly Cu-rich films) but not In. In Fig. 5-13, the indium composition decreases with the sputtering time (i.e throughout the thickness of the films from the surface to the film-substrate interface). The surface compositions are 1.17 and 0.95 for the films with [Cu]/[In]=1.64 and 1.92, respectively, and become more Cu-rich down to the film-substrate interface. Note that in the RHEED and XRD results of most of the stoichiometric films grown on GaP at 500 to 600°C exhibited the coexistence of Cu-Au

49

and spinel ordered structures. Judging from the XRD, RHEED and AES results of these films it can be suggested that:

(a) The surface of the film with [Cu]/[In]=1.64 (EPMA) is composed of Cu-Au and spinel structure of minimal chalcopyrite structures with the Cu-rich region at the bulk of the film to be dominantly chalcopyrite structure; and

(b) The film with [Cu]/[In]=1.92 (EPMA) has a surface that is composed of spinel, c-axis Cu-Au and sphalerite structures with bulk which is dominantly c-axis CH and/or sphalerite ordered structures.

The highly Cu-rich film ([Cu]/[In]=2.55) shown in Fig. 5-11 that exhibits rings, however, demonstrates inconsistency also with the XRD results since the spots due to c-axis chalcopyrite structure is noticeable which is not the case in the XRD results depicted in Figs. 5-3 and 5-4. However, from the AES depth profile of this film presented in Fig. 5-13, the surface composition is less Cu-rich (about [Cu]/[In]=1.40) and the volume of the film is highly Cu-rich. Thus, it is possible for the chalcopyrite ordered structure to grow on the surface on top of the sphalerite structure that dominates the volume of the film. From the results, despite that Cu-Au structure seems to infect most of the films, monocrystalline chalcopyrite ordering highly oriented along c-axis normal to the substrate seems to grow at the narrow range 1.28≤[Cu]/[In]≤1.41 ratios.

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) spinel Cu-Au (a⊥substrate)

(c)

(b) (a)

Fig. 5-5 RHEED patterns for CuInS2 with [Cu]/[In]=1.01 on GaP(001) taken at (a)<010> and (c)<ī10> directions and their respective diagrams (b) and (d).

(d)

50

Fig. 5-6 RHEED patterns for CuInS2 with [Cu]/[In]=1.25 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).

common for chalcopyrite, Cu-Au and sphalerite structures Cu-Au (c⊥substrate) chalcopyrite (c⊥substrate) Cu-Au (a⊥substrate) chalcopyrite (c⊥substrate)

(a)

(c)

(b)

(d)

Fig. 5-7 RHEED patterns for CuInS2 with [Cu]/[In]=1.28 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).

common for chalcopyrite and sphalerite structures chalcopyrite (c⊥substrate)

(a)

(c)

(b)

(d)

51

common for chalcopyrite and sphalerite structures chalcopyrite (c⊥substrate)

(a)

(c)

(b)

(d)

Fig. 5-8 RHEED patterns for CuInS2 with [Cu]/[In]=1.41 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).

Fig. 5-9 RHEED patterns for CuInS2 with [Cu]/[In]=1.64 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).

(a) (b)

(c) (d) common for chalcopyrite, Cu-Au, spinel and sphalerite

structures Cu-Au (c⊥substrate) chalcopyrite (c⊥substrate) Cu-Au (a⊥substrate)

52

Fig. 5-10 RHEED patterns for CuInS2 with [Cu]/[In]=1.92 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).

common for Cu-Au, spinel and sphalerite structures Cu-Au (c⊥substrate) Cu-Au (a⊥substrate)

(a) (b)

(d) (c)

Fig. 5-11 RHEED patterns for CuInS2 with [Cu]/[In]=2.55 on GaP(001) taken at (a) <010> and (c)<ī10> directions and their respective diagrams (b) and (d).

common for chalcopyrite, and sphalerite structures chalcopyrite (c⊥substrate)

(a) (b)

(c) (d)

53

Fig. 5-12 AES composition depth profile of the film with composition of [Cu]/[In]=1.01, 1.28 and 1.41 measured by EPMA. The respective surface compositions are [Cu]/[In]=1.06, 1.31 and 1.28).

-200 0 200 400 600 800 1000 1200 1400 1600 1800 2000

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

surface

(EPMA), [Cu]/[In]= Black :1.01 Red :1.28 Blue : 1.41

(circles) :Cu (squares):In (triangle):S

Rel

ativ

e C

ompo

sitio

n (%

)

Sputtering Time (s)

Fig. 5-13 AES composition depth profile of the film with composition of [Cu]/[In]=1.64, 1.92 and 2.55 measured by EPMA. The respective surface compositions are [Cu]/[In]=1.17, 0.95 and 1.41).

-200 0 200 400 600 800 1000 1200 1400 1600 1800

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

surface

(EPMA), [Cu]/[In]= Black :1.64 Red :1.92 Blue : 2.55

(circles) :Cu (squares):In (triangle):S

Rel

ativ

e C

ompo

sitio

n (%

)

Sputtering Time (s)

54

Photoluminescence (PL)

Figure 5-14 features the photoluminescence spectra of the CuInS2 film. The PL emissions of the bulk CuInS2 crystal shown in Fig. 5-14(a) are also plotted for comparison. In the photoluminescence (PL) measurements, the same PL setup and conditions are used for the films grown at 500°C on Si and on GaP i.e. at around 20K using a green (532nm) laser pumped by a semiconductor diode. The PL emission detection system uses a 46cm-monochromator and a photomultiplier which is sensitive up to 900nm.

To simplify the discussion, the peaks in the figure are labeled A, B, C, D, E, F, and G. In Fig. 5-14(b) for the CuInS2 film with [Cu]/[In]=1.28, exhibits roughly three emissions (E, F and G). The broad peak E at the skirt of peak F corresponds to the exciton (peak A) and the bound exciton (peak B) emissions of the bulk crystal. The clear peak (F) at around 820nm (1.51eV) is not observed in the PL emissions of the bulk CuInS2 crystal, but and even on the epitaxial CuInS2 film grown on GaP(112) by liquid phase epitaxy.40) It should be noted that from the results of Binsma et al. on melt-grown bulk CuInS2 samples, highly resolvable peaks at 1.52eV (815.7nm), 1.5185eV (816.5nm), 1.5165eV (817.5nm) for donor-to-valence band emission, and 1.513eV (819.4nm) and 1.509eV (821.6nm) for bound exciton emissions EX3, EX4, EX5, and EX6, respectively, were observed.30,31) Referring to these results, peak F centered at 820nm (1.51eV) for the present film sample corresponds to the exciton EX6 bound to ionized donor. Moreover, this near band edge peak F which is 7nm lower than the Cu-Au band edge emission at 827nm(1.50eV) cannot be assigned to Cu-Au band edge emission considering that it is not observed in the XRD and RHEED patterns (Figs. 5-1(c) and Fig. 5-5) of this sample. Peak G which spans over 830 to 890nm includes the donor-acceptor pair emissions (peaks C and D). Since the film is slightly Cu and S-rich, the probable intrinsic defects of this film are Cu or S-interstitials, CuIn substitution, and/or In vacancies. The In vacancy acts as a trivalent acceptor while Cu atom substituting for an In atom acts as a divalent acceptor. But the dominant type of defects cannot be easily known since this will be determined by both the defects’ concentrations and the defects’s formation energies that the details are beyond the scope of this study.41)

55

Fig. 5-15 Photoluminescence spectra of CuInS2 for (a) bulk at 23K, stoichiometric, and (b) the film with [Cu]/[In]=1.28 on GaP(001) taken at 21K. The excitation wavelength is 532nm.

800 820 840 860 880 900

(b) Film

GFE

Inte

nsi

ty [

arb. unit]

Wavelength (nm)

(a) Bulk

DC

BA

56

5.2 Structures of films on GaP at Tsub=600°C

Since the improvement of epitaxial CuInS2 films have been observed in films with Tsub=570°C in section 5.1, films of thickness 0.10-0.12µm (which still larger than the critical thickness 13-40nm) on GaP(001) at Tsub=600°C are also grown to investigate if further improvement of the structural ordering and orientations can be seen. From the XRD and RHEED results below, evident growth of epitaxial chalcopyrite ordering and orientation of the CuInS2 thin films are noticeable depending also on composition.

X-ray diffraction (XRD)

The typical θ-2θ scan (10≤2θ≤90°) of the XRD spectra of the samples grown at Tsub=600°C are shown in Figure 5-15. The composition of these samples are [Cu]/[In]=0.70, 1.01, 1.26 and 2.08.

The peaks in Fig. 5-8, are labeled A, B, C, D, E and F to mark a specific structural ordering. Using the same analyses of XRD peaks used in sections 5.1 peaks A (16°), C (32.5°), B (49.7°) and D (68.6°) shown in Fig. 5-16(a) and 5-15(b) for films with [Cu]/[In]=0.70, 1.01 and 1.26 are attributed to Cu-Au reflections for these positions are not allowed for chalcopyrite ordering. Peaks A and B specifically assigned to 001 and 003 are signatures of c-axis orientated Cu-Au ordered structures. It should be noted here that these peaks seem to decrease with increasing [Cu]/[In] ratio. Although the same tendency were observed for the films grown on GaP(001) at 500 and 570°C substrate temperatures, the intensity of peaks A and B are lower even for In-rich and stoichiometric films. This implies that at 600°C substrate temperature, a mass reduction of Cu-Au amount had occurred in the films. In Figs. 5-15(c) and 5-15(d), peaks E and F correspond to the reflections of the chalcopyrite structure. The Cu-Au reflections are almost not noticeable in Cu-rich films.

Looking closely at the peaks around 2θ=32.5° as shown in Figs. 5-16 and 5-17 , evident reduction of Cu-Au ordering is apparent. In Fig. 5-16(a) for the film with [Cu]/[In]=0.70, the film is dominated by the epitaxial c-axis orientated spinel CuIn5S8 as typified by the 004 reflection at around 2θ=33.6°. Notice that the intensity of the peak corresponding to the a- and c-axes Cu-Au (the a- and c-CA, respectively) are roughly the same for films with [Cu]/[In]=0.70 and 1.01. However, the intensity of the spinel CuIn5S8 in the almost stoichiometric film is lower than that of the In-rich film. Indicative of the reduction of the spinel structures in the stoichiometric film. The same behavior was observed in films grown on GaP at 570°C but not at 500°C. The somewhat

57

broad peak that spans over 32.2 to 32.6° is an evident that the a- and c-axes chalcopyrite structures are also probable in the In-rich and the stoichiometric films.

For the film with [Cu]/[In]=1.26 shown in Fig. 5-16(c), the epitaxial chalcopyrite ordering highly oriented along the c-axis is dominant. If other orderings are present, they are considered minimal. Figure 5-17 for the film with [Cu]/[In]=2.08 is plotted separately for emphasis that will be discussed later in the RHEED section of this film.

10 20 30 40 50 60 70 80 90

101

102

103

104

105

??

G a PG a PF

E

X-ra

y In

tensi

ty (ar

b.units)

2θ [deg.]

101

102

103

104

105

?

A

?

G a PG a P

FE

G a P

(c) [Cu]/[In] = 1.26

(d) [Cu]/[In] = 2.08

101

102

103

104

105

C u I n 5 S

8

0 0 8C u I n

5 S

8

0 0 4

G a PG a P

DC

A

(b) [Cu]/[In] = 1.01

101

102

103

104

105

106

C D

G a P

C u I n 5 S

8

0 0 8

BA

C u I n 5 S

8

0 0 4

(a) [Cu]/[In] = 0.70

Fig. 5-15 XRD spectra of films grown on GaP(001) Tsub=600°C with (a) [Cu]/[In]=0.70, (b)[Cu]/[In]=1.01, (c)[Cu]/[In]=1.26 and (c)[Cu]/[In]=2.08 at 10≤2θ≤90°.

58

In this figure, the highest intensity corresponds to either the c-axis oriented Cu-Au or the sphalerite structure and the chalcopyrite structure is somewhat not probable. The XRD patterns of the Cu-rich films often show the unknown peaks denoted by the question mark (?). Note that these unknown peaks were only observed in Cu-rich films in any GaP substrate temperature. However, these peaks were seldom observed in Cu-rich films grown on GaP at 500 and 570°C substrate temperatures. In this stage now, there is a reason to believe that these unknown peaks will most likely to occur in large amount in Cu-rich films on GaP at Tsub>570°C. These unknown peaks may be due to the substrate-epilayer film interface reaction that the thorough investigation is beyond the scope of this study.

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0100

101

102

103

104

105

SP

c -C

H

c -C

Aa

-C

Ha

-C

A

(c) [C u]/[I n]=1.26 G a P

X-r

ay In

tens

ity (a

rb.u

nits

)

2θ [deg.]

100

101

102

103

104

105

C u I n 5 S

8

0 0 4

(b) [C u]/[I n]=1.01 G a P

100

101

102

103

104

105

(a) [C u]/[I n]=0.70 C u I n 5 S

8

0 0 4

G a P

Fig. 5-16 XRD spectra of films grown on GaP (001) with (a)[Cu]/[In]=0.70, (b)[Cu]/[In]=1.01 and (c)[Cu]/[In]=1.26 at 31≤2θ≤35°. In c-CH, a-CH, c-CA, a-CA and SP, c and a are the lattice parameters of chalcopyrite (CH), Cu-Au (CA) and sphalerite (SP) structures orthogonal to the substrate.

59

Reflection high energy electron diffraction diffraction (RHEED)

Typical RHEED patterns of the films with [Cu]/[In]=0.70, 1.01, 1.26, and 2.08 are featured in Figs. 5-18 to 5-21.

The In-rich film represented by the film with [Cu]/[In]=0.70 in Fig. 5-18(a) exhibits highly intense spots of the spinel CuIn5S8 and the a- and c-axes orientation

Cu-Au orderings are noticeable, however, weak. But these spots of a- and c-axes Cu-Au ordered structrure seem to be more intense (Fig. 5-19(a) and 5-19(b)) than in In-rich film. This implies that less spinel structure have been grown on the surface of the stoichiometric film. This agrees well with the interpretation of the XRD results in Figs. 5-15(a) and 5-15(b). For these films no rings and twins (Figs. 5-18(c) and 5-18(d)) are observed indicative of a good crystallinity of the grown film.

Figure 5-20(a) shows that the surface of film with [Cu]/[In]=1.26 is smooth as indicated by the streaky RHEED patterns. Neither rings nor twins are evident in this film as depicted in Figs. 5-20(c) and 5-20(d). This signifies that this film is composed of single

31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0 35.5 36.0101

102

103

104

105

?

Ga P [Cu]/[In]=2.08

SP

c-C

A

a-C

H a-C

A

c-C

H

X-ra

y In

tensi

ty (ar

b.units)

2θ [deg.]

Fig. 5-17 XRD spectra of the film grown on GaP (001) with [Cu]/[In]=2.08 at 31≤2θ≤35°. The c-CH, a-CH, c-CA, a-CA and SP, c and a are the lattice parameters of chalcopyrite (CH), Cu-Au (CA) and sphalerite (SP) structures normal to the substrate.

60

crystalline chalcopyrite ordering with c-axis highly oriented normal to the GaP substrate as was observed also for the film grown on GaP at 570°C (with [Cu]/[In]=1.28 and 1.41).

The same streaky RHEED pattern and c-axis chalcopyrite ordering is observed in the film with [Cu]/[In]=2.08 shown in Fig. 5-21(a) and 5-21(b). However, this result does not agree with the XRD result of this film (Figs. 5-15 and 5-17). This discrepancy can only be explained if it is noted that RHEED can only detect the surface structure but not the bulk volume of the crystals considered, and that the depth structures of the film are mainly Cu-Au and/or sphalerite structures. This case could be that initially during film growth, the surface and the bulk volume of the film is mainly composed of the metastable Cu-Au structure and/or sphalerite structure, however, the thermal energy is enough to allow rearrangements of the surface adatoms to approach the equilibrium and stable structure, which is the chalcopyrite structure. It is worth noting here that the equilibrium structures of Cu-In-S system are the chalcopyrite CuInS2 and the spinel CuIn5S8 structures at room temperature. As shown in Figs. 5-21(c) and 5-21(d), this Cu-rich film demonstrates twin-growth tendency. From the presented results and interpretations, the growth of the Cu-Au ordering at

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) spinel Cu-Au (a⊥substrate)

Fig. 5-18 RHEED patterns for CuInS2 with [Cu]/[In]=0.70 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d).

(c)

(a) (b)

(d)

61

600°C in slightly Cu-rich film is suppressed and the chalcopyrite growth is more favorable with increasing density of twins and unknown structures that may be due to the film-substrate interface reaction. Thus, films grown on GaP at 570°C are still of better quality than the films at 600°C at the same substrate material.

Fig. 5-19 RHEED patterns for CuInS2 with [Cu]/[In]=1.01 on GaP(001) taken at (a)<010> and (b)<ī10> directions and their respective diagrams (c) and (d).

common for Cu-Au and sphalerite structures Cu-Au (c⊥substrate) spinel Cu-Au (a⊥substrate)

(b) (a)

(d) (c)

62

(a) (b)

(c) (d)

common for chalcopyrite and sphalerite structures chalcopyrite (c⊥substrate) twin (common)

Fig. 5-21 RHEED patterns for CuInS2 with [Cu]/[In]=2.08 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

(c)

(a) (b)

(d)

common for chalcopyrite and sphalerite structures chalcopyrite (c⊥substrate)

Fig. 5-20 RHEED patterns for CuInS2 with [Cu]/[In]=1.26 on GaP(001) taken at (a) <010> and (b)<ī10> directions and their respective diagrams (c) and (d).

CHAPTER 6

CHARACTERISTIC SURFACE MORPHOLOGY OF FILMS

Depending on composition and substrate temperatures, distinct surface morphologies are demonstrated by the obtained films as shown in SEM and AFM images.

6.1 Distinct SEM images of films

The surface profiles of all films show specific and unique grain habits depending on substrates, composition and substrate temperatures. Surface morphology investigation of the films is done using mainly scanning electron microscopy (SEM). No voids, dense and large grain size are characteristics of good surface morphology of absorbers for PV applications. Figures 6-1(a)-(f) shows the surface SEM micrographs of CuInS2 films grown on Si(001) at Tsub=500oC with increasing [Cu]/[In] ratio. The In-rich film show smaller crystal grains that are densely packed. The films with Cu/In=0.91, 1.24 and 1.5 show crystal grains size that increases with Cu/In ratio, however, some voids are noticeable indicative of the loosely packed films at this substrate temperature. The highly Cu-rich films ([Cu]/[In]=2.24 and 2.79) lost the mat-knitted habit. This mat-knitted crystal habit may be due to the unipolarity of the Si substrate wherein the adatoms during evaporation can rearrange themselves in semi-random orientation at the growth temperature. This coarseness of the grains at the surface of the films is the reason why the RHEED patterns showed diffused spots in the RHEED results. Different crystal grain habit can be observed for films grown on GaP at various temperature and compositions. Figures 6-2(a)-(f) depicts the shape and orientation of the crystal grains of films within 0.65≤[Cu]/[In]≤2.93 compositions grown at Tsub=500oC. Surface segregated and rectangular-based grains are observed in In-rich ([Cu]/[In]=0.65) sample. The densely packed nature of In-rich film is evident only below the rectangular grains. Although the grain size increases with [Cu]/[In] ratio within 1.27≤[Cu]/[In]≤2.44, the film with [Cu]/[In]=1.27 exhibits a peculiar semi-well-aligned orientation. It is worth noting that this film is dominantly c-axis orientated Cu-Au ordered structure basing on the XRD and RHEED results of this film. Although without voids, the grains of the highly Cu-rich film with [Cu]/[In]=2.93 are semi-loosely held together, but still denser than the highly Cu-rich films grown on Si(001) at the same temperature. This increase of grain size with [Cu]/[In]

64

ratio is attributed to the capability of the Cu atoms to promote grain growth.42) The SEM micrographs of the films grown on GaP(001) at Tsub=570oC are shown in Figs. 6-3 and 6-4. From Fig. 6-3(a), the surface segregated grains are still observed in the In-rich film with more defined rectangular-based shape and these are not observed in near

Fig. 6-1 SEM micrographs of films grown on Si(001) at Tsub=500°C with [Cu]/[In] ratio of (a)0.85, (b)0.91 , (c)1.24, (d) 1.5, (e)2.24, and (d)2.79.

(c) [Cu]/[In]=1.24 (d) [Cu]/[In]=1.5

(e) [Cu]/[In]=2.24 (f) [Cu]/[In]=2.79

(a) [Cu]/[In]=0.85 (b) [Cu]/[In]=0.91 Tsub=500oC

65

stoichiometric film as depicted in Fig. 6-3(b). However, the number of grains seems to be lesser than the In-rich film grown on GaP at 500oC. In Figs. 6-3(c) and 6-4(a), the films with [Cu]/[In]=1.25 and 1.41 (with spotty RHEED patterns in Figs. 5-6 and 5-8) show somewhat rougher surface than the films with [Cu]/[In]=1.28 and 1.64 shown in Figs.

Fig. 6-2 Scanning electron micrographs of films grown on GaP(001) at Tsub=500°C with [Cu]/[In] ratio of (a)0.65, (b)1.02 , (c)1.27, (d)1.97, (e)2.44, and (f)2.93.

(a) [Cu]/[In]=0.65 (b) [Cu]/[In]=1.02 Tsub=500oC

(c) [Cu]/[In]=1.27 (d) [Cu]/[In]=1.97

(e) [Cu]/[In]=2.44 (f) [Cu]/[In]=2.93

5μm

66

6-3(d) and 6-4(b) that show well-aligned densely packed grains giving a smooth motif that leads to the streaky RHEED pattern of this film in Fig.5-7.39) This grain alignment is clearer and more defined than the films grown on GaP at 500oC. This well-alignment of dense grains may be attributed to the bipolarity of the GaP substrate for the first adatoms on GaP during growth provides a template for the subsequent overgrowth layers. The Cu-rich film with [Cu]/[In]=2.55, on the other hand, exhibits more dense packing of grains than the highly Cu-rich film at 500oC.

As demonstrated in Figs. 6-5(a)-(c) for films grown on GaP(001) at 600oC, more or less the same grain habit is observed on films grown at 570oC substrate temperature. The main differences between the films grown on GaP at 500, 570 and at 600 oC are the following: (a) the segregated rectangular-based crystal grains on the surface of the In-rich films are most defined and the largest in films grown at 600 oC, (b) the nearly stoichiometric film grown at 600 oC exhibits the segregated grains on the surface which are

(a) [Cu]/[In]=0.89 (b) [Cu]/[In]=1.01 Tsub= 570oC

(c) [Cu]/[In]=1.25 (d) [Cu]/[In]=1.28

5μm

Fig. 6-3 Scanning electron micrographs of films grown on GaP(001) at Tsub=570°C with [Cu]/[In] ratio of (a) 0.89, (b) 1.01, (c) 1.25 and (c) 1.28

67

not observed in films grown at 500 and 570 oC, and (c) the well-aligned and densely packed grains are most defined in the highest substrate temperature. The differences (a) and (b) are related and the discussion is expounded in the AFM results in section 6.2. For (c), in addition to the template provided by the bipolar GaP substrate to follow preferential growth, the high substrate temperature and the Cu-excess adatoms promote coalescence of grains to produce well-aligned and densely packed grains.43)

(d) [Cu]/[In]=2.55

5μm

(b) [Cu]/[In]=1.64

(c) [Cu]/[In]=1.92

(a) [Cu]/[In]=1.41

Fig. 6-4 Scanning electron micrographs of films grown on GaP(001) at Tsub=570°C with [Cu]/[In] ratio of (a)1.41, (b)1.64, (c) 1.92 and (c) 2.55.

Tsub= 570oC

68

Fig. 6-5 Scanning electron micrographs of films grown on GaP(001) at Tsub=600°C with [Cu]/[In] ratio of (a)0.70, (b)1.01 , (c)1.26, and (d)2.08.

(a) [Cu]/[In]=0.70 (b) [Cu]/[In]=1.01 Tsub=600oC

(c) [Cu]/[In]=1.26 (d) [Cu]/[In]=2.08

5μm

69

6.2 AFM and compositional analyses

In order to clarify some aspects of the rectangular-based grains observed on In-rich and stoichiometric films grown on GaP substrates, the tapping mode atomic force microscopy (AFM) is carried out in the surface of 5x5µm2 area for each film. Indeed, unique features are exhibited by the films at different substrate temperatures. The height of the grains ranges from 40-160nm. The AFM images of the In-rich and stoichiometric films are shown in Fig. 6-6 (a)-(f). The film with [Cu]/[In]=0.65 grown at 500oC shows not well-distinct structures of domes and semi-pyramidal islands located side by side with mixed semi-pyramidal and dome-shaped smaller islands. Hence, the observed segregated rectangular-based grains in SEM are semi-pyramidal island grains with square or rectangular bases. The shapes are not well-defined since Ostwald ripening could have occurred on many of the islands at the specified growth temperature. The islands are prone to Ostwald ripening considering the small distance between the islands during growth in addition to the continuous supply of adatoms at a certain substrate temperature. Ostwald ripening model is the detachment of atoms from surrounding small islands and diffusion to the large islands leading to coarsening of an island size distribution and this model includes the driving force that depends on the inverse radius of curvature of the island.44) In Fig. 6-6(c), the typical feature of the island grains is dome-shape while the film in Fig. 6-6(e) shows more pronounced large pyramidal island grains. Ostwald ripening during growth might be the reason of this large area pyramidal island grains. Most of the stoichiometric films show island grain growth on top of wetting film and only the film grown at 500 oC does not exhibit this feature. The reason may be the low growth temperature because at higher growth temperature, the adsorbing adatoms diffuse faster on the surface increasing the probability of arriving atoms diffusing to existing islands instead of clustering to form a new nucleated island grain. This faster surface diffusion of adsorbing atoms leads to lower island density on the films. This behavior was observed on the growth of three-dimensional germanium islands on Si(001).45) The crystal growth on a lattice-mismatched substrate often proceeds via formation of essentially three-dimensional islands on the bare substrate surface for Volmer-Weber (VW) growth mode or on the wetted surface for coherent Stranski-Krastanow (SK) growth mode. The three-dimensional islands formation leads in SK growth mode to a reduction of the strain energy which is proportional to the volume of the island and to an increase of the surface energy which is proportional to the surface area.46) The observed pyramidal island grains in In-rich and some near

70

stoichiometric films seems to follow the SK growth mode.

(a) [Cu]/[In]=0.65

Tsub=500oC

(b) [Cu]/[In]=1.02

Tsub=570oC

(d) [Cu]/[In]=1.01 (c) [Cu]/[In]=0.89

Tsub=600oC

(f) [Cu]/[In]=1.01 (e) [Cu]/[In]=0.70

Fig. 6-6 Atomic force micrographs of In-rich and stoichiometric films grown on GaP(001) with [Cu]/[In] of (a)0.65 and (b)1.02 at Tsub=500°C, (c)0.89 and (d)1.01 at Tsub=570°C, and (e)0.70 and (f)1.01 at Tsub=600°C.

71

In order to clarify what crystals compose the island grains that have grown on the surface of most In-rich and the stoichiometric films, EPMA elemental compositional mapping and AES analyses are done.

Due to the limitation of the magnification of the SEM/EPMA system as can be seen in Fig. 6-7(a), only the film with large island grains (i.e. the film grown at 600oC with [Cu]/[In]=0.70) exhibits noticeable difference in intensity for each element of interest. The discussion then is focused on this film. Figures 6-7(b) to (d) feature the elemental compositional maps of this film. In Fig. 6-7(b), it is evident that the grains show almost the same composition of Cu with the wetting film layer as can be noticed in the homogenous intensity of Cu in the surface of the film. The high intensity regions in Figs. 6-7(c) and 6-7(d) for In and S, respectively, correspond well to the island grains in Fig. 6-7(a). Thus, it can be deduced that the island grains of this film are In- and S-rich with minimal Cu. It can be said therefore that the only candidate material is spinel CuIn5S8. However, the quantitative elemental composition of the islands is still

(a)

(c)

(b)

(d)

Fig. 6-7 (a) Scanning electron micrograph and the EPMA compositional mapping of the surface of the film with bulk composition [Cu]/[In]=0.70 for (b) Cu, (c) In and (d) S. This film was grown on GaP at 600oC. The same film used in Fig. 6-6 (e).

72

necessary in order to clarify that these islands are indeed CuIn5S8. The complementing method used to clarify the elemental composition of the island grains is Auger electron spectroscopy (AES) since it can render information essentially on the elemental composition of the first 2-10 atomic layers.47)

The AES measurements are done in Niigata University, Ikarashi Campus using the product of JEOL (JAMP-7100). This system has energy accuracy and resolution of 0.5%. In this measurement, the accelerating voltage used is 10 kV with probe current that ranges from 2.62 to 5.13x10-8A. Surface analyses are done in probe diameter of 300nm of the film samples in vacuum (10-8 Torr) environment. This AES system can analyze up to 50nm probe diameter. For the film with composition of [Cu]/[In]=0.70 that showed the well pronounced pyramidal island grains (in Fig. 6-6(e)), AES is done first on the surface of the wetting layer without island grains (as confirmed in SEM/AES) followed by 2 seconds Ar+ ion sputtering to etch this region, then on the surface of the island-grain populated region which is subsequently Ar+ ion-etched for 2 seconds. It should be noted that the rate of Ar＋ ion etching is 3.125 Å/s. The sensitivity factors used in the elemental analyses are 4.674, 1.295, 1.043, 6.480 and 1.212 for Cu,

Fig. 6-8 AES spectra of the wetting layer with less island grains for (a) non-etched and (b) 2 sec Ar ion etched film. The same film in Fig. 6-7 is used.

0 100 200 300 400 500 600 700 800 900 1000-10000

-5000

0

5000

10000 (b)

S

In

In

O CuCu

2 sec Ar+ ion etching [Cu]/[In]=0.44

Inte

nsi

ty (

a.u.)

Kinetic Energy (eV)

-10000

-5000

0

5000

10000 (a)

SIn

In O CuCu

Wetting layer surface Tsub

=600oC[Cu]/[In]=0.44Non-etched

73

In, S, Ga and P, respectively. These are measured with respect to the standard samples of pure silver, single crystalline iodine vapor grown CuInS2 of composition Cu:In:S=1:1:2, and single crystal GaP.

Typical differentiated AES spectra of the surface of the film with [Cu]/[In]=0.70 (using EPMA) are shown in Figs. 6-8 and 6-9. The non-etched region of the wetting layer less populated with island grains shown in Fig. 6-8(a) has the same [Cu]/[In] ratio of 0.44 with the 2-second etched region in Fig. 6-8(b) but less than [Cu]/[In]=0.70. On the other hand, the dense region of island grains for non-etched and the 2-seconds etched in Fig. 6-9 shows that once etched it has a composition of Cu:In:S = 1:5.67:7.18 ([Cu]/[In]=0.18), a composition very near to the spinel CuIn5S8 ([Cu]/[In]=0.20). Thus, to a certain error limit, the pyramidal island grains are of spinel structure. These results indicate that the spinel structures occupy the surface of the film wetting layer and that the claim of SK growth mode is further confirmed. This surface segregation was reported (but without the details of the island grain structures) by Morell et al.48)

0 100 200 300 400 500 600 700 800 900 1000-10000

-5000

0

5000

10000(b)

O

In

In

S

CuCu

2 sec Ar+ ion etching

Island grains surface

[Cu]/[In]=0.18

Inte

nsi

ty (

a.u.)

Kinetic Energy (eV)

-10000

-5000

0

5000

10000 (a)

CuCuO

In

In

S

Tsub

=600oC[Cu]/[In]=0.28Non-etched

Fig. 6-9 AES spectra of the island grain region for (a) non-etched and (b) 2 sec Ar ion etched film. The same film in Fig. 6-7 is used.

CHAPTER 7

SUMMARY AND CONCLUSION

Epitaxial CuInS2 thin films of various compositions had been successfully grown on Si(001) and GaP(001) at substrate temperatures 500, 570, and 600 oC using the conventional vacuum evaporation system with three temperature controlled sources of elemental Cu, In, and S. The Cu source temperature is changed from 980 to 1080°C and the In source temperature is varied from 650 to 750°C to produce various film compositions ranging from 0.85≤[Cu]/[In] ≤2.9 and 0.65≤[Cu]/[In] ≤2.93 for films on Si and on GaP, respectively. The base gas pressure during film deposition is about 10-5

Torr. From the XRD and RHEED results, the films grown on Si(001) were epitaxial

chalcopyrite, Cu-Au, and/or sphalerite, and some unknown phases of CuInS2. Monocrystalline chalcopyrite structure was not successfully grown on Si(001) at 500°C. Improved epitaxial films of CuInS2 were grown on GaP(001) at 500°C. No rings in the RHEED patterns were observed on the films. Only few films showed unknown phase and compared to the films grown on Si, the unknown phase is minimal. The In-rich films exhibit the coexistence of the spinel ordered structure of CuIn5S8 and the a- and c-axes orientated Cu-Au ordering of CuInS2. Monocrystalline c-axis Cu-Au structure without twins was obtained for the fairly Cu-rich sample. The observed intensities of twins were weak compared to the films grown on Si. Although the two variants of structural domains of chalcopyrite were obtained, inclusion of Cu-Au ordered structured were almost always present, and monocrystalline chalcopyrite structure was not obtained. The films on both Si and GaP substrates showed broad PL emissions that span through the donor-acceptor region (860nm). However, at elevated substrate temperature of 570°C, monocrystalline epitaxial c-axis orientated chalcopyrite CuInS2 thin films were successfully grown on GaP(001) in a narrow range of 1.28≤[Cu]/[In]≤1.41 ratios. For the first time, the PL emissions of the film with only c-axis orientated chalcopyrite ordering exhibited near band edge emission around 820nm (1.51eV) attributed to exciton bound to ionized donor. The presence of this bound exciton indicates high crystal quality of CuInS2. Less improvements of the film quality was observed on films grown on GaP at 600°C. For the first time, the unique surface crystal habit and surface morphology of CuInS2 films on GaP were revealed. The crystal grain size on Si and on GaP were found

75

to increase with [Cu]/[In], however, less dense grains were observed on Si. The mat-knitted habit of the films on Si demonstrates the semi-random adsorption of adatoms during growth. Preferential growth orientation was exhibited by the slightly Cu-rich films on GaP. Dense and large grains are preferred for highly efficient solar cell absorbers. The In-rich and some stoichiometric films on GaP exhibit pyramidal and domeshaped island grains that correspond to spinel CuIn5S8. The growth of these island grains seems to obey the Stranski-Krastanow (SK) growth mode. Thus, this thesis presents the growth and structural characterization of epitaxial CuInS2 thin films with c-axis oriented single crystalline chalcopyrite ordering on GaP(001) for the first time. The presented photoluminescence properties and surface morphologies of the films are also original.

76

APPENDIX A

Fig. 8-A The Faculty of Engineering, Niigata University XRD facility which is used in this study. This shows the Bragg-Brentano configuration.

film sample

77

APPENDIX B

Fig. 8-B The Faculty of Engineering, Niigata University RHEED facility used in this study. This shows the control bench and the specimen compartment.

View port of specimen compartment

78

APPENDIX C

Fig. 8-C EPMA/SEM facility used in this study. This is located in the Department of Dentistry, Asahimachi Campus, Niigata Unversity, Japan.

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