A thermodynamic and kinetic study of the vapour transport of some semiconductors

Abstract

Detailed models of crystal growth have, for the most part, been limited to simple sublimation systems. These systems have little relevance to the complex chemical vapour transport systems used to produce modern semiconductors such as the III-V compounds, and the development of these systems has frequently been by trial and error. The work described in this thesis has attempted to bridge the gap between the crystal growth theories and technologically important growth systems. Crystal growth is a sequential process in which reactants are first transported to the surface of the growing crystal and then undergo a series of surface reactions. A major difficulty in investigating heterogeneous reaction kinetics is separating the two processes. In chapter 2 a formalism is developed which enables transport and surface kinetics to be uncoupled. The practical application of this is demonstrated in chapters 3 and 4 in which the sublimation of red phosphorus and the GaAs/HBr reaction are investigated using the modified entrainment method (MEM). The choice of experimental systems, and the depth to which they could be studied, was dictated largely by the interests of British Telecom. Consequently, the experimental work could only demonstrate the potential of MEM as a kinetic probe. More detailed examination of the dependence of the GaAs/HCl and GaAs/HBr reactions on reactant pressures has still to be carried out. The later chapters are concerned with the growth of the mixed semiconductor systems GalnAs and GalnAsP. These materials are required for the new generation of optical communications systems currently under development. Not only were the surface kinetics for these systems unknown, but even the transport equations for these systems had not been developed. Chapters 5-7 describe the development of a vapour phase system for the growth of these ternary and quaternary compounds. The basic principles are discussed for InP growth in chapter 5. Chapter 6 then develops thermodynamic and transport models for the mixed compounds and chapter 7 discusses the results obtained with practical growth system. Although many features of growth can be explained by the models developed in chapter 6, it is clear that a complete understanding of growth requires close study of the various processes occurring on the surface.