A SENSITIVE SPECTROPOLARIMETER FOR THE MEASUREMENT OF CIRCULAR POLARIZATION OF LUMINESCENCE A Thesis Presented to The Faculty of the College of Engineering and Technology Ohio University In Partial Fulfillment of the Requirements for the Degree Master of Science By Ziad I. AI-Akir, June, 1990 DlHtft7~~·"E:RsaTY LI~~RY ACKNOWLEDGEMENT With gratitude and constancy, I praise the Almighty Allah for the grace and favors that He bestowed on me. Without His guidance and blessing, I would not be able to achieve any good deed in this life. I wish to extend my genuine appreciation to my advisor Dr. Henryk Lozykowski, for his teachings, assistance, encouragement and helpful suggestions. A special appreciation goes to Mr. V. K. Shastri and Mr. T. Lee for their assistance and valuable help during the preparation of this thesis. Finally, I would like to thank my brothers: Mohammed EI-Gamal, Amer AI-shawa, Abdulbaset AI-Abadleh and Rabah Odeh for their encouragement and all the brothers who helped me without knowing it. DEDICATION This thesis is dedicated to my family in Palestine and Kuwait, who have been a great source of blessing, motivation and encouragement. CONTENTS CHAPTER ONE Introduction 1 1.1 Circular Polarization of Luminescence 2 1.2 SPC in Luminescence Measurements 3 1.3 Objectives........... 5 CHAPTER TWO Literature Review......... 8 2.1 The Nature ofLight 8 2.2 Light in Matter 12 2.3 Semiconductor Materials 13 2.3.1 Intrinsic and Extrinsic Semiconductors. 15 2.3.2 Direct and Indirect Semiconductors ............... 16 2.4 Photoluminescence in Semiconductors...................... 18 2.5 Polarization 22 2.5.1 Linear Polarization 22 2.5.2 Circular Polarization 24 2.5.3 Elliptical Polarization................................... 24 2.5.4 Birefringence. 26 2.5.5 Degree of Polarization....... 27 2.6 Polarized Emission in Semiconductors 28 CHAPTER THREE Circular Polarization of Luminescence - Experimental Techniques - 31 3.1 Overview 31 3.2 Instru.mentation.................................................. 35 3.2.1 Excitation Source 35 3.2.2 Monochromator 39 3.2.3 Sam.ples.......................... 39 3.2.4 Polarizers and Retarders..... 39 3.2.5 Photoelastic Modulator .; 40 3.2.6 Detection ................................................ 41 3.2. 7 Data Acquisition 44 3.4 <:aJibraticn1 Metl1CHis 45 CHAPTER FOUR Experimental Results and Conclusion 51 4.1 Single Quantum Well ZnSelZnl_xCdxSe 51 4.2 PL spectra of InP: Yb3+ .••••..•••••••••••••••.•.•••.•.•.•.•. 59 4.3 Conclusion . 61 REFERENCES 63 APPENDIX I 65 APPENDIX II 75 1 CHAPTER ONE INTRODUCTION Circular polarization of luminescence (CPL) refers to the differential (spontaneous) emission of left- and right-circularly polarized light by luminescent materials. CPL provides a probe of the configuration and structure of the material in its electronic excited states. It is a useful and sensitive indicator of the carriers' spin state and its change under the influence of external factors and relaxation processes which helps in determining the kinetics ofthe nonequilibrium carriers. Although the techniques for detecting and measuring CPL have been used to study all types of luminescent materials, in this research we shall be entirely concern with the CPL measurements for solid-state semiconductor materials. Over the past few years, various investigations have reported different techniques to measure CPL. Although, many of these reports were concerned with chemical solutions, but the techniques of measurement can beapplied to all luminescent materials. The general technique for analyzing circularly polarized luminescence is to phase shift it to linearly polarized light and then to analyze that for direction ofpolarization. For the detection, the most widely used method has been single photon counting (SPC) technique. It is a powerful method that can be used in the measurement of luminescence at all intensities [11-16] and it is very sensitive with high linear dynamic range. In a SPC system, a photon striking the sensitive area of a photodetector (usually a photomultiplier tube) will remove an electron. Using a fast counter coupled to a pulse hight discriminator, the photons are counted by collecting the electrons removed by the photons and measuring their current In this thesis, SPC method is used in the measurement ofCircular Polarization of Luminescence (CPL). An apparatus built and used to obtain the CPL data is described. 2 It involves an effective square-wave modulation and a digital photon counting system. The instrument is very sensitive, selective, and precise. A sophisticated computer software supporting the digital photon counter makes it possible to detect small degrees ofcircular polarization to study poorly fluorescing compounds. In fact, this optical setup can be used to measure and study the optical properties of semiconductors and many other compounds. It has been used to perform photoluminescence and polarization measurements on different semiconductor samples. Some examples [7,8] of these measurements are reported to show the performance ofthe device. The construction details and performance characteristics of the instrument are described. The system utilizes a photoelastic light modulator, a photomultiplier tube, a monochromator to scan the signal, and a digital photon counter on line with a computer for data acquisition and analysis. 1 .1 CIRCULAR POLARIZATION OF LUMINESCENCE When a photon is absorped by a semiconductor, an electron in the conduction band and a hole in the valence band are generated. Photons of 0+ (right) or 0"- (left) circularly polarized light have a projection of the angular momentum on the direction of the wave vector equal to +1 and -1, respectively [9]. The photoexcited carriers live some time 1" before recombination. During this time, the spin orientation of carriers, which is equal to the angular momentum of the photon absorped, decreases due to different relaxation processes. If the orientation has not entirely disappeared by the time of recombination, the recombination radiation will be partially circularly polarized. The degree of circular polarization of the recombination radiation serves as a useful and sensitive indicator of the carriers' spin state and its changes under the influence of external factors and relaxation processes. It determines the kinetics of the nonequilibrium carriers in a semiconductor. 3 As a result of this importance, during the past few years many efforts have been made to develope a standard technique to detect and measure the circular polarization of luminescence. Not many studies, however, have been reported on the study and applications of CPL untill recently, probably due to commercially unavailable instrumentations. Generally, the measurement of luminescence in a particular region of the electromagnetic spectrum requires a source with a continuous spectral emission in that region, a monochromator to select the wavelength, and a detector to measure the transmitted monochromatic light CPL measurement is no exception, it basically uses the same configuration with some modification and introduction of other optical components to meet different applications' requirements [28] . 1 .2 SPC IN LUMINESCENCE MEASUREMENTS To understand spectral lines and spectrum, it is important to understand how the spectrum is recorded and what are the instrumens needed to do that Single photon counting is a good technique to be used when the intensity of luminescence is low. The method measures a light signal by making use of the inherent digital or discrete nature of light [11,12]. In a simple photon counting system, the output pulse caused by a photon striking the photocathod of a photomultiplier (PMT) is amplified by a fast preamplifier and compared to a reference voltage by a pulse-hieght discriminator. If the pulse amplitude exceeds the discriminator's level, the discriminator generates a standard electrical pulse then the pulses are counted by a photon counter. In the absance of light, the photomultiplier in a photon counting system should not give out a pulse. Fig. 1.1 shows a block diagram of a simple photon counting system. Most recently, computers have been used to display the data by directly plotting the number of pulses in a particular measurement time interval versus the wavelength. Obviously, the 4 DIGITAL O~ PHOTONS --...I...----H3> .1 DISC HCOUNTERI PHOTOMULTIPLIER Fig. 1.1 Block diagram of a simple photon counting system (SPC) SAMPLE .. .. .. r .- r MONOCHROMATOR PMT .. DISC H COUNTER MONOCHROMATIC UGHf Fig. 1.2 Single photon counting method in luminescence measurement 5 more pulses we have in an interval, the higher the intensity of the incident light. Fig. 1.2 illustrates the use of single photon counting system in the detection and measurement of luminescence. Monochromatic light from a source falls on a sample cell and luminescence is detected by a monochromator-photomultiplier combination. The photomultiplier is considered to be one of the most critical components in a SPC system. Different limiting factors such as spectral range, transit time, rise time and fall time must be taken in consideration when choosing a PMT for a specific measurement application. 1.3 OBJECTIVES The purpose ofthis thesis is to : 1- Review the different techniques reported on the measurement of CPL and describe in details an optical setup based on single photon counting method (SPC) used for the same purpose. 2- Example measurements ofphotoluminescence (PL) and circular polarization of luminescence (CPL) spectra for some semiconductor