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Contents Page No.

Acknowledgement Overview

…………………………………………………………….. i …………………………………………………………….. ii

Chapter One: Introduction 1.1 Introduction ……………………………………………………………….... 1.2 Thin film semiconductors ……………………………………………….…. 1.3 Ternary compound semiconductors ……………………………….……….. 1.4 AgGaSe2 as a thin film solar cell absorber material……………….……….. 1.5 Objectives of the present work …………………………………..…………. References………………………………………………………….……………

2 5 7 10 10 11

Chapter Two: Growth of AgGaSe2 Thin Films 2.1 Introduction ……………………………………………………….………... 14 2.2 Theoretical consideration 2.2.1 Transmittance, reflectance and absorption coefficient ………………...……….. 14 2.2.2 Fundamental absorption in semiconductor …………………............................... 16 2.3 Experimental details 2.3.1 Deposition system ……………………………………………………...………... 20 2.3.2 Deposition of AgGaSe2 thin films ……………………………………...………... 20 2.3.3 Measurements …………………………………………………………...………. 23 2.4 Results and discussion 2.4.1 Effect of sequence and stage of deposition to grow AGS thin films…………...… 24 2.4.2 Effect of post-deposition annealing temperature to grow AGS thin films.............. 30 2.4.3 Effect of post-deposition annealing duration to grow AGS thin films ……….…. 31 2.5 Summary…………………………………………………………….……… 34 References …………………………………………………………….………… 34

Chapter Three: Compositional and Structural Characterization 3.1 Introduction ………………………………………………………………… 37 3.2 Theoretical consideration 3.2.1 Energy dispersive analysis of x-ray (EDAX) …………………………….............. 37 3.2.2 X-ray diffraction (XRD)………………………………………………….............. 39 3.2.3 Crystallite size …………………………………………………………………… 40 3.3 Experimental details 3.3.1 Compositional measurements …………………………………………………… 42 3.3.2 Structural measurements ………………………………………………...………. 42 3.4 Results and discussion 3.4.1 Compositional ……………………………………………………………………………. 45 3.4.2 Structural ……………………………………………………………………….... 48 3.5 Summary ……………………………………………………………............ 61 References ………………………………………………………………............ 61

Page No.

Chapter Four: Optical Characterization 4.1 Introduction ………………………………………………………………. 64 4.2 Theoretical consideration 4.2.1 Determination of absorption coefficient ………………………………………. 64 4.2.2 Optical band gap energy ………………………………………………………. 68 4.3 Optical measurements 4.3.1 Transmittance, reflectance and thickness ……………………………………... 70 4.3.2 Absorption coefficient and band gap energy …………………………………... 74 4.4 Results and discussion 4.4.1 Effect of compositional variation on the optical properties.…………………… 74 4.4.2 Effect of post-deposition annealing temperature on the optical properties. 81 4.4.3 Effect of thickness on the optical properties..………………………………...... 84 4.5 Summary ………………………………………………………...………... 87 References …………………………………………………………...………... 88

Chapter Five: Thermoelectrical and Electrical Characterization 5.1 Introduction ………………………………………………………………. 91 5.2 Theoretical consideration 5.2.1 Thermoelectric power ……………………………………………………...….. 91 5.2.2 Electrical conductivity …………………………………………………………. 92 5.2.3 Hall effect ……………………………………………………………...………. 94 5.3 Experimental details 5.3.1 Thermoelectrical measurements …………………………………………….… 97 5.3.2 Electrical measurements …………………………………………………….… 97 5.4 Results and discussion 5.4.1 Thermoelectrical properties ………………………………………………….... 101 5.4.2 Effect of annealing temperature on the electrical properties …......................... 101 5.4.3 Effect of compositional variation on the electrical properties …………………. 105 5.5 Summary ………………………………………………………………….. 108 References ……………………………………………………………………... 109

Chapter Six: Conclusions and Scope of Further Work 6.1 Summary of results 112 112 112 113 6.2 Conclusions …………………………………………………………….... 114 6.3 Scope of further work …………………………………….……...………. 115 Publications ………………………………………………………………..….. 115 6.1.1 6.1.2 6.1.3 6.1.4

Growth ……………………………………………………….…….… Compositional and structural ………………………………………... Optical …………………………………….…………………….…… Thermoelectrical and electrical ……………..……………………….

Acknowledgement It gives me much pleasure to express my deepest sense of gratitude to Dr. S.M. Firoz Hasan, Chief Scientific Officer and Head, Experimental Physics Division, Atomic Energy Centre Dhaka (AECD), the supervision of this work. I would like to thank the Director, AECD, for allowing me to use the laboratory facilities in Experimental Physics Division (EPD). I am grateful to Islamic University authority for granting me a study-leave to carry out this research work. I am thankful to Dr. Hayati Mamur, Dr. Momtazul Islam, Mr. Shajahan Ali, Mr. Nazibul Haque, Mr. Monjurul Haque and Mr. Mahbubar Rahman for their wise suggestions during this work. I would also like to thank Mr. Syed Ayubur Rahman, Mrs. Jahanara Parvin, Mr. Albert Joydhar and Mr. Syed Ahmed of EPD for their active cooperation in carrying out this work. I would like to express my sincere appreciation to Mrs. Latifa Quadir, Dr. Dilip Kumar Saha and Professor A.S.M.A. Haseeb for helping me to carry out optical, XRD and EDAX measurements. I would also like to thanks Mr. Atiqur Rahman, Mr. Ahsan-ul-Ambia, Mr. Parash Chandra Barman for their kind help during this work. I offer my deepest gratitude to my family for the encouragement and moral support. I am indebted to my father for his blessing. I gratefully acknowledge the University Grants Commission of Bangladesh for providing me a Ph.D. research fellowship.

Author M. Ruhul Amin Bhuiyan

i

Overview Thin films of silver gallium di-selenide (AgGaSe2) were formed onto chemically and ultrasonically cleaned glass substrates by Stacked Elemental Layer (SEL) deposition method. In the SEL method, pure silver, gallium and selenium films were deposited insitu, successively and sequentially in vacuum onto a substrate by thermal evaporation of the individual elements to make a stack, using an oil diffusion pump. The stack of layers, thus formed, was then annealed insitu at different temperatures in the range from 100 to 350oC and for different durations between 5 and 20 minutes to form the solid solution of AgGaSe2 (AGS) having wide range of compositions. The thickness of the individual elements to be deposited was calculated previously to prepare films of desired stoichiometries. The thickness was measured insitu by quartz crystal thickness monitor. The same monitor was also used to record the rate of evaporation of the elements. The grown films were then characterized compositionally, structurally, optically, thermoelectrically and electrically. The atomic compositions of the grown films have been determined by Energy Dispersive Analysis of X-ray (EDAX) method using an attachment to the scanning electron microscope. The structural properties of the films were ascertained by X-Ray Diffraction (XRD) method utilizing a diffractometer. The optical properties were measured in the photon wavelength ranging between 300 and 2500 nm by a UV-VISNIR spectrophotometer. Hot-prove method was employed to determine the type of electrical conduction in the films. The electrical conductivity and Hall coefficient have been measured as a function of temperature, from 103 to 373 K, by standard dc method using a cryostat. The AGS thin films grown by SEL deposition method having the three-stage, glassgallium-selenium-silver sequence and annealed at 300°C for 15 minutes were yield the favorable characteristics. To check the homogeneity of the films, EDAX data at several different locations for each film were noted. The variation of composition from one location to another did not exceed 2%, which ascertains the reasonable homogeneity of the films. The XRD patterns revealed that the films were polycrystalline in nature. The films exhibit tetragonal chalcopyrite structure with average lattice parameters, a § 6.00 Å and c § 10.92 Å. The lattice parameters are independent of post-deposition annealing temperature, annealing duration and non-molecularity, ΔX of the films. The grain size of the films varies directly whereas strain and dislocation density vary indirectly with post-deposition annealing temperature and ΔX. The increase of grain size with ii

post-deposition annealing temperature and ΔX may be attributed to the decrease of lattice imperfection due to decrease of internal microstrain within the film. The film annealed at 300oC for 15 minutes seems to have better structural perfection as evident from the XRD spectra. Near-stoichiometric (NS) film shows almost perfect semiconducting nature ascertained from the optical transmittance spectrum and has lower sub-band gap (SBG) absorption, indicating smaller imperfect states within the band gap. Most of the films have high absorption coefficient (∼105 cm-1) above the fundamental absorption region. The optical absorption behaviour of the films above the fundamental absorption edge can be interpreted by considering the existence of two types of optical transitions (band gap energy). The first band gap energy is direct allowed and the subsequent one is direct forbidden. The direct allowed band gap energies Eg1 are found to have general tendency to decrease with the increases or decrease of Ag/Ga ratios from unity. The direct forbidden band gap energies Eg2, however, do not show any correlation with Ag/Ga ratios. The value of Eg1 was found to be 1.75 eV for NS film. The films have vanishing crystal field splitting, Δcf. The value of spin-orbit splitting, Δso adheres to the single-crystal value for NS film. The Se-p and the Ag-d levels are hybridized. The extent of p-d hybridization for the stoichiometric films annealed at 300oC for 15 minutes is 13%. The p-d hybridization varies directly while Δso varies inversely with post-deposition annealing temperature for stoichiometric films. The value of Eg1 was found to decrease with increasing film thickness. The refractive index varies inversely with incident photon energy. The thermoelectric power was found to be positive for all the films ranging from 0.026 to 0.113 mV/0C, which are indicates the presence of p-type majority carriers. The electrical conductivity of the films (at room temperature) having different annealing temperatures and various ΔX has been found to vary from 2.07x10-3 to 8.92x10-3 (Ω-cm)-1 and 1.36x10-6 to 4.93x10-3 (Ω-cm)-1, respectively. The conductivity varies directly with both annealing temperature and ΔX. The variation shows thermally activated process. The activation energy varies from 43.60 to 94.70 meV and 21.61 to 60.64 meV as annealing temperature varies between 100 and 350oC and ΔX varies between –0.798 and 1.710, respectively. The activation energies are found to have a general tendency to increase as the annealing temperature increases or decreases from 250oC. The films annealed at 100 and 250oC have acceptor-like levels, which may be attributed to the Se-interstitials and, Ag- and iii

Ga-vacancies, respectively. The probable identity of the origin of acceptor levels for the films annealed at 200, 300 and 350oC is Ga-vacancy and Ag in Ga-sites. Stoichiometric or slightly Ag-rich film shows minimum activation energy. The dominance of grain boundary effect has been ascertained applying Seto’s model. The carrier-concentration varies directly and Hall mobility varies indirectly with ΔX. The increase of carrier-concentration with ΔX is more rapid than the decrease of mobility with ΔX, which in turn increases the conductivity with ΔX. The carrier-concentration and Hall mobility of the films have been found to vary from 1012 to 1017 cm-3 and 5 to 2 cm2/V-Sec, respectively as ΔX varies between –0.798 and 1.710. Analyzing the results of various types of characterizations, the growth parameters of the thin films of AGS, grown by SEL method, have been optimized and it has been possible to produce films having reasonably acceptable properties for using this material as efficient solar energy absorber.

iv

Introduction 1.1 Introduction ……………………………………………………………….... 1.2 Thin film semiconductors ……………………………………………….…. 1.3 Ternary compound semiconductors ……………………………….……….. 1.4 AgGaSe2 as a thin film solar cell absorber material……………….……….. 1.5 Objectives of the present work …………………………………..…………. References………………………………………………………….……………

2 5 7 10 10 11

1.1 Introduction The AIBIIIX2VI (A ≡ Ag, Cu; B ≡ In, Ga, Al; X ≡ S, Se, Te) compounds are direct band gap semiconductors having chalcopyrite structure [1]. These materials are drawing increasing attention because of their fascinating optical properties. The formation of ternary compound of semiconductors depends on balanced valences and the formation of closed shells of eight electrons as for the binary compounds [2]. Ternary compounds of AIBIIIX2VI group are semiconductors, for A atoms are monovalent and the B atoms are trivalent, the four electrons thus provided, together with twelve from X2, making two closed groups of eight. The chalcopyrites can be viewed as a super lattice on that of zinc-blende structure, arising from the ordered substitution of zinc-blende metal of valence z by two metals having the same average valence z. Thus, AIBIIIX2VI semiconductors are the ternary analogs of II-VI semiconductors (ZnS, ZnSe, CdS etc). This substitution leads to an approximate doubling of the size of the unit cell. Chalcopyrite has a tetragonal structure with c axis doubled compared to a axis. Metal atoms alternate with one another normal to the doubled c axis. In the chalcopyrite structure, each anion, X serves as the center of a tetrahedron of two A and B cations. In turn, each cation is surrounded by a tetrahedron of 4 anions. The chalcopyrites are different from zinc-blende in two ways: chemically and structurally. Zinc-blende structure is formed by a single cation whereas chalcopyrites are formed by two dissimilar cations. This implies the difference of chemical properties. The structural difference may be analyzed as follows: (i) there are two cation sub lattices, which give two near-neighbour chemical bonds A-X and B-X. The bond-lengths are generally unequal RA–X ≠ RB-X. (ii) the c-axis is almost doubled but c ≠ 2a; i.e., c>2a or c