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INVESTIGATION OF OPTOELECTRONIC PROPERTIES IN THIN-FILM AND CRYSTALLINE CADMIUM SULFIDE Mithun Bhowmick A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE August 2007 Committee: Bruno Ullrich, Advisor Lewis P. Fulcher Robert I. Boughton ii ABSTRACT Bruno Ullrich, Advisor Photocurrent (PC), transmission and photoluminescence (PL) properties were measured in cadmium sulfide (CdS). A thin-film (G69) and a single crystal of CdS were used for the measurements. These measured spectra were compared to theoretical values using two different models. In each of the comparisons, a theoretical absorption spectrum was used for the analysis. Along with these experiments, reflectivity of the material was measured to find the thickness of the film. The PL spectrum was used to determine crystal orientation. Based on these measurements, efficiency of the model as well as the material was verified. iii To my parents, Dinesh Chandra Bhowmick and Renukana Bhowmick. iv ACKNOWLEDGEMENTS I would like to express sincere gratitude to my advisor, Dr. Bruno Ullrich, for guiding me throughout the entire project. I have learned numerous technical skills from him. His ideas, enthusiasm, and encouragement have made significant changes to this thesis and I am grateful for that. I would like to thank Dr. Lewis Fulcher and Dr. Robert Boughton for evaluating this thesis as committee members. Their suggestions have made important additions to the thesis and their comments made during discussions were very helpful for me to understand my research. I feel indebted to Mr. Marco Nordone for his assistance in Mathematica programming. Additionally, I would like to thank Mr. Chinthaka Liyanage and Mr. Krishna Acharya for their valuable suggestions and help. I am thankful to Dr. John Laird and the faculty and staff of the Department of Physics and Astronomy at Bowling Green State University for their continuous support to my graduate education and research. I am also grateful to Mrs. Kimberly Spallinger of ESL department whose valuable comments made this thesis more organized. Finally, I am grateful to my family and friends for supporting me throughout the academic career. v TABLE OF CONTENTS Page CHAPTER 1 INTRODUCTION ……………………………………………………...........1 1.1 History of semiconductors …………………………………………………………...1 1.2 Semiconductors : fundamental concepts ………………………………………..........2 1.3 Purpose of the project ……………………………………………………………......3 1.4 Future of semiconductor devices …………………………………………………….4 CHAPER 2 STRUCTURE AND PROPERTIES OF THE SAMPLE USED…………........6 2.1 Structure and properties of CdS…………………………………………………........6 2.2 An overview of the samples used ……………………………………………………9 CHAPTER 3 INVESTIGATION OF PHOTOCURRENT ……………………………….13 3.1 Theory of photocurrent generation in a photoconductor ……………………………13 3.2 Experimental set-up …………………………………………………………………16 3.3 Results: photocurrent spectra ……………………………………………………......19 a] Measurements with CdS thin-film (G69) ………………………………………...19 b] Measurements with CdS single crystal …………………………………………...22 CHAPTER 4 INVESTIGATION OF TRANSMITTANCE ………………………………25 4.1 Theoretical overview of transmittance and absorption coefficient ………………….25 4.2 Experimental setup for the transmittance measurement …………………………….26 4.3 Experimental results …………………………………………………………………27 CHAPTER 5 INVESTIGATION OF PHOTOLUMINESCENCE FROM SINGLE CRYSTAL ……………………………………………………………………….30 vi Page 5.1 Theory of photoluminescence: Van Roosbroeck-Shockley equation ……………......30 5.2 Experimental set-up for measuring photoluminescence ………………………….....30 5.3 Results ……………………………………………………………………………….31 CHAPTER 6 COMPARISON OF PHOTOCURRENT, TRANSMISSION, AND PHOTOLUMINESCENCE MEASUREMENTS…………………………………...…...34 6.1 Comparison of calculated and measured photocurrent from thin-film……….….......34 6.2 Comparison of calculated and measured transmittance obtained from thin-film……35 6.3 Comparison of measured and calculated photoluminescence data using Van Roosbroeck-Shockley relation …………………………………………………………..37 6.4 Comparison of photocurrents measured from the thin-film and single crystal….......38 6.5 Comparison of Transmission coefficients from thin-film and single crystal………..38 6.6 Comparison of absorption coefficients calculated from the photoluminescence and photocurrent measurements through the single crystal …………………………….39 6.7 The photocurrent fit obtained for the crystal ………………………………………..40 CHAPTER 7 ANALYSIS AND RELATED DISCUSSIONS ……………………………42 7.1 Discussions on photocurrent data collected and fitted for the thin-film …………….42 7.2 Discussions on the transmittance data collected and compared with theory ………..42 7.3 Analysis of the theoretical and measured photoluminescence from CdS crystal .......42 7.4 Discussion on the photocurrent measurements from thin-film and crystal …………43 7.5 Analysis of the plot showing thin-film and crystal transmission measurements ……43 7.6 Calculation of effective thickness of thin-film using transmittance data ……….......43 7.7 Comparing photoluminescence and photocurrent of crystal through absorption …...44 vii Page 7.8 Discussion on photocurrent fit found for the crystal ……………………………......44 7.9 Determination of the crystal orientation from photoluminescence data …………….44 CHAPTER 8 CONCLUSION……………………………………………………………...46 APPENDIX…………………………………………………………………………………47 BIBLIOGRAPHY…………………………………………………………………………..49 REFERENCES……………………………………………………………..………………50 viii LIST OF FIGURES Figure Page Figure 1.1: Typical Energy band diagram of a semiconductor …………………………...2 Figure 2.1: A typical zinc blende unit cell …………………………………………..........6 Figure 2.2: A typical wurtzite unit cell …………………………………………………...7 Figure 2.3: Hexagonal CdS crystal structure ……………………………………………..7 Figure 2.4: Thin film of CdS …………………………………….....................................11 Figure 2.5: CdS crystals used for the measurements …………………………………....12 Figure 3.1: Photocurrent generation in a semiconductor ………………………………..14 Figure 3.2: Top view of a diffractive monochromator ……………………………….....16 Figure 3.3: Path of light rays in a diffractive monochromator ……………………….....17 Figure 3.4: The front panel of lock-in-amplifier (SR 530) ………………………….......18 Figure 3.5: Internal circuitry of a Lock-in Amplifier …………………………………...18 Figure 3.6: Block diagram of the photocurrent setup …………………………………...19 Figure 3.7: Photocurrent plot from CdS thin-film before correction ……………............20 Figure 3.8: Monochromator output plotted for different wavelengths …………….........21 Figure 3.9: Responsivity curve of the calibrated diode ……………………...…….........21 Figure 3.10: Corrected photocurrent from CdS thin-film ……………………………….22 Figure 3.11: Photocurrent from the crystal before correction ……………………….......23 Figure 3.12: Photocurrent from the crystal after correction ……………………….........23 Figure 3.13: Magnified part of the plot around the band-gap …………………………...24 Figure 4.1: A schematic diagram of the process inside the spectrometer ……………….26 ix Page Figure 4.2: Experimental set up for transmission measurement from crystal …………..27 Figure 4.3: Transmittance plot of thin-film CdS (G69) …………………………………27 Figure 4.4: Transmittance of the CdS crystal …………………………………………...28 Figure 4.5: Experimental set-up for the measurement of reflection coefficient R ……...29 Figure 5.1: Photoluminescence measurement setup …………………………………….31 Figure 5.2 Plot showing the total output data for the PL measurement ………………....32 Figure 5.3 The polynomial which corresponds to the laser line, particularly in the PL region ……………………………………………………………………………………32 Figure 5.4 Subtracted function for the total output spectrum …………………………...33 Figure 5.5 Corrected photoluminescence output after subtraction and magnification ….33 Figure 6.1: Theoretical fit found using Mathematica for thin-film photocurrent ……….35 Figure 6.2: Theoretical absorption spectra used to fit measured PC ……………………36 Figure 6.3: Comparison of theoretical and experimental transmittance ………………...36 Figure 6.4: Normalized theoretical and measured photoluminescence spectra …………37 Figure 6.5: Normalized photocurrent plots for thin-film and single crystal …………….38 Figure 6.6: Normalized transmission plots for thin-film and single crystal …………….39 Figure 6.7: Comparison of absorption coefficients calculated from the PC and PL measurements in a normalized form …………………………………………………….40 Figure 6.8: Photocurrent fit obtained for the measurement using single crystal ………..41 x LIST OF TABLES Table Page Table 2.1 Important physical properties of CdS ……………………………...8 Table 4.1 Results from reflectivity measurement ……………………………29 CHAPTER ONE INTRODUCTION This chapter deals with the fundamental concepts of semiconductors and the significance of a project involving measurement of optoelectronic properties of semiconductors: 1.1 History of semiconductors According to their ability to conduct electricity, solids are primarily divided into three groups, i.e., as insulators, metals, and semiconductors. Semiconductors are materials having conductivities in between metals (conductors) and insulators (bad or non conductors). The discovery of selenium photoconductivity in 1870 started the trend which now is called semiconductor device exploration. Even though, the development almost stopped due to widespread use of electronic tubes. The situation changed just before World War II, when investigators started to look for new crystal devices with smaller inter electrode capacitances than electron tubes. In 1949, amplification