SOLID STATE ELECTROCHEMICAL STUDIES AND ASSESSMENT OF III-V COMPOUND SEMICONDUCTOR SYSTEMS By SUDHANSHU MISRA A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULHLLMENT OFTHE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 1998 ACKNOWLEDGEMENTS I would like to thank my research supervisor. Dr. Timothy J. Anderson, for his guidance and support all throughout this doctoral research. I am also grateful to Dr. Ursula R. Kattner at National Institute of Standards and Technology, Gaithersburg, for her valuable input during the optimizations and calculations performed in this study. It was a pleasure and a learning experience working on experiments on Sb-Te System with Dr. Yves Feutelais associated with Laboratoire de Chimie Minerale II, Faculte de Pharmacie at Chatenay-Malabry in France. I am thankful to Dr. Ranga Narayanan and Dr. Mark E. Orazem for their constant interest and involvement in the experiments conducted during this research. 11 1 TABLE OF CONTENTS page ACKNOWLEDGEMENTS iii ABSTRACT ^ CHAPTERS INTRODUCTION 1 1 Phase Diagrams and Their Importance to Compound Semiconductors 1 Typical Phase Diagrams ofIII-V Systems 3 Thermodynamic Information From Phase Diagrams 8 Solid State Activity Measurements 10 Solid Electrolytes 1 Principle ofElectrochemical Cells 12 2 EXPERIMENTAL STUDIES 16 Description ofApparatus and Measurement Techniques 16 Experimental Results in Binary Systems 25 Ga-As System 25 Standard Gibbs Energy ofFormation ofGaAs (s) 28 In-As System 33 Activity ofIn Along the In-rich Liquidus 40 Standard Gibbs Energy ofFormation ofInAs (s) 41 Sb-Te System 49 Activity ofSb in Sb-Te Melts 53 Experimental Results in Ternary Systems 61 Activity ofGa in In-rich Ternary Liquid GaUnyAs^x-y Solutions 62 Activity ofGaAs (s) in the Pseudobinary Section GaAs-InAs 66 111 3 ASSESSMENT STUDIES 73 Assessment Procedure (Lukas Program) 73 Assessment ofBinary Systems 77 Ga-As Assessment 77 In-As Assessment 83 In-Sb Assessment ]Q5 Ga-Sb Assessment J24 Assessment ofTernary Systems 133 . Ga-Sb-As Assessment j33 In-Sb-As Assessment 39 1 4 CONCLUSIONS 15 ] REFERENCES BIOGRAPHICAL SKETCH 171 IV Abstract ofDissertation Presented to the Graduate School ofthe University ofFlorida in Partial Fulfillment ofthe Requirements for the Degree ofDoctor ofPhilosophy SOLID STATE ELECTROCHEMICAL STUDIES AND ASSESSMENT OF ffl-V COMPOUND SEMICONDUCTOR SYSTEMS By Sudhanshu Misra December 1998 Chairman: Dr. Timothy J. Anderson Major Department: Chemical Engineering Thermodynamic properties have been determined for compound semiconductors by solid state electrochemical technique and used successfully in making assessments to the available data on these systems. The measurement technique used a solid electrolyte and two custom designed unique cell configurations to maintain stable open circuit electromotive force (emf) values, and minimize volatile V-component (-As, -Sb) loss due to vaporization during the experimental stage. The experimental data on Gibbs energy data for the binary systems Ga-As, In-As and Sb-Te are the first of their kind and are very important for a good understanding of their thermodynamic properties and their alloys. In addition to these systems, In-Sb and Ga-Sb systems were also assessed. The assessments of the III-V binary systems were used as start points for thermodynamic predictions into the two ternary systems, namely V Ga-Sb-As and In-Sb-As. Solid state emf measurements were also conducted on the ternary Ga-In-As to understand the thermodynamic characteristics in the pseudobinary section of the GaAs-InAs phase diagram. The experiments confirmed the presence of a miscibility gap in the GaxIni.xAs solid phase. VI CHAPTER 1 INTRODUCTION Phase Diagrams and Their Importance to Compound Semiconductors Phase diagrams are graphical representations of the relations between state parameters and the delineation of phase fields within this parameter space. Phase diagrams are useful in selecting initial compositions and subsequent processing conditions for thermal processes including the separation and purification of materials since phase diagrams are a graphical representation of the conditions of phase equilibria. Extensive thermodynamic information can be extracted from them. A thermodynamic description of a phase is related to the interaction of the species and the structure of the phase at the molecular level. Thus phase diagrams can be calculated from known thermodynamic phase functions and, conversely, thermodynamic properties and phase structure characteristics can be obtained fromphase equilibria data. Since temperature, pressure and concentration are typically measurable and controllable state parameters in many processes; the Gibbs energy is the important thermodynamic function, the value of this function being a minimum at equilibrium for constant temperature, pressure and overall composition. Graphically, the compositions of two phases in equilibrium at a certain temperature and pressure is determined by the common tangent to the two curves representing the concentration-dependent Gibbs energy ofeach phase. This is a graphical interpretation of the equilibrium condition that 1 2 the chemical potentials of the species in the coexisting phases are equal. It is noted that the pressure dependence in solid-liquid phase equilibria problems is often assumed negligible for pressures less than lO’ MPa. Of interest to the present study are the class of semiconductors constituted by the elements from the Groups III and V columns of the periodic table, and termed III-V compound semiconductors. These compounds exhibit extended miscible substitutional solid solutions when mixed on the same sublattice, thereby allowing the semiconductor device designer control of the lattice parameter and electrical or optical characteristics of the material to conform to the required device specification. The increased number of elements in III-V compounds as opposed to the elemental nature of Si, however, make their property characterization and processing technology more difficult. The attractive characteristics of compound semiconductors include the possibility of direct bandgap energy and the concomitant high electron mobility. As a result, these materials are important in high speed electronic and optoelectronic device applications. Furthermore, these materials offer a broad selection of optical and electrical properties that are often superior to those ofelemental Si. Group III-V compounds have been used in a wide range of applications as the host semiconductor material, including high-speed, optoelectronic and integrated optical devices [Hei78, Sew75 and Yos75]. In particular, (In, Ga) Sb and (In, Ga) As form important constituents in the solid solutions used for long wavelength photodetectors, light emitting diodes, diode lasers and heterogeneous bipolar transistors. Moreover, the semiconductor InSb has the lowest bandgap energy (Eg = 0.18 eV) and highest room temperature electron mobility (p.„ = 10^ cmW-s) of any III- V compound semiconductor [Wel52, Wel53 and Bre54]. \ \ 3 The processes for fabrication of compound semiconductor devices often require near equilibrium contact between solid and liquid phases (e.g., bulk crystal growth, liquid phase epitaxy) or between solid and vapor phases (e.g., chemical vapor deposition, rapid « thermal annealing). Phase diagram ai\d thermodynamic property functions are important in specifying the necessary boundary conditions in the analysis of such device processing steps. It is observed that the optical and electrical properties of compound semiconductors are sensitive to the processing conditions. As an example, variations in the melt stoichiometry during bulk crystal growth or in the vapor phase IIW input molar CVD ratio during significantly changes the electrical properties of the grown material. This implies that extremely accurate thermodynamic properties and phase relations are sometimes necessary for their meaningful application to process analysis. V Typical Phase Diagrams ofIII- Systems A V typical III- binary phase diagram is shown in Figure 1-1. It is noted that pure solid elements show negligible solubility of the other element and each half of the phase diagram is termed simple eutectic. In this phase diagram, the eutectic towards the Group Ill-rich region ofthe phase diagram is shown as being nearly degenerate, which means that the eutectic composition is so close to pure Group III component that it cannot be depicted in the figure. The eutectic composition is determined in the first order by the differences between the compound and pure component melting temperatures, and modified by the liquid solution behavior and component thermodynamic properties. For example, the relatively close melting temperatures of GaSb and Sb produces a eutectic composition that is relatively rich in Ga. The eutectic compositions in the Group V-rich 4 portion of the III-As or III-P phase diagrams, however, can be experimentally difficult to determine because ofthe experimental problems encountered by high vapor pressures. The upper curve in Figure 1-1 stretching across the limits of the phase diagram is the liquidus curve which is a boundary of the two-phase region (liquid + AB) and separates the pure liquid phase from the two-phase region. The vertical line drawn at the composition 0.5 atom fraction of the phase diagram represents the position of the essentially stoichiometric compound AB (i.e., a line compound). In actuality the compound AB in III-V systems is a solid solution phase with negligible extent of solubility, and, when comparing this boundary to other phase boundaries, AB is treated as a line compound. Although the solid solution extent of the line compound has negligible influence on the other boundary positions, the extent is of utmost importance to the semiconducting properties. Any deviation in the ideal stoichiometry of the III-V compound A0.5 B0.5 in this region ofsolid solution (typically ofthe order 10'^ to 10'^ mole fraction) is the result of native point defects. Many of these defects are ionic and can significantly change the impurity behavior and electrical and optical properties of these materials. Thus, by being able to accurately describe the thermodynamics of these solid solutions it is possible to predict bulk phase equilibria as well as the stability and extent of solid solution. Figure 1-2(a) shows a typical III-III-V ternary phase diagram, where A and B are the group III elements. The vertical plane through the elements A and C is the binary III- V limit, while a similar plane through the points B and C is the binary IIl'-V limit. Both the binary phase diagrams are ofthe typejust discussed and as shown in Figure 1-1. The