A combined experimental and modeling study for the growth of SixGe₁-x single crystals by liquid phase diffusion (LPD)
Date
2008-04-10T06:02:39Z
Authors
Yildiz, Mehmet, Ph. D.
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Abstract
Si,Ge,-, alloy is an emerging semiconductor material with many important potential applications in the electronic industry due to its adjustable physical, electronic and optical properties. It has been scrutinized for the fabrication of high-speed micro electronics (e.g., SiGe heterojunction bipolar transistors (HBT) and high electron mobility field effect transistors) and thermo-photovoltaics (e.g., photodetectors, solar cells, thermoelectric power generators and temperature sensor). Other applications of Si,Ge,-, include tuneable neutron and x-ray monochromators and y-ray detectors. In these applications, SixGel-, alloy is generally used in the form of epilayers that have to be deposited on a lattice-matched substrate (wafer). Therefore, SixGel-, bulk single crystals with a specific composition (x) are needed for the extraction of such wafers. LPEE (Liquid Phase Electroepitaxy) was considered as a technique of choice for the growth of single crystals. However, LPEE growth process needs a single crystal seed with the same composition as the crystal to be grown. Yet, such a seed substrate with particularly higher composition is not commercially available. In order to address this important issue in LPEE, a crystal growth technique, which is named "Liquid Phase Diffusion" (LPD), was developed and used to produce the needed seed substrate materials. This was the main motivation of the present research. This thesis presents a combined experimental and modelling study for LPD growth of compositionally graded, germanium-rich single crystals of 25 rnrn in diameter for use as lattice-matched seed substrates. The experimental part focuses on the design and development of a complete LPD grow system. The experimental set-up was tested by growing ten Si,Ge,-, single crystals. Grown crystals were characterized by macroscopic and microscopic examinations after chemical etching for delineation of the degree of . . . Abstract 111 single crystallinity and growth striations. Compositional mapping of selected crystals were performed by using Electron Probe Micro Analysis (EPMA) as well as Energy Dispersive X-ray analysis (EDX). It was shown that the LPD technique can be successfully utilized to obtain Si,Ge,-, single crystals up to 6-8 % at.Si with uniform radial composition distribution. The modeling part presents a rational continuum mixture model developed to study transport phenomena (heat and mass transfer, fluid flow) occurring during the LPD growth of Si,Ge,-, . Based on the continuum model developed, two and three-dimensional transient numerical simulations were carried out. The numerical simulation models presented account for some important physical features of the LPD growth process ofSi,Ge,-, , namely (1) a growth zone design on the thermal field, (2) the structure of the buoyancy induced convective flow and its effect on the growth and transport mechanisms, (3) the shape and evolution of the initial and progressing growth interfaces, and (4) the spatial and time variation of the crystal growth velocity. It was numerically shown that, as the name LPD implies, the growth of Si,Ge,-, by LPD is mainly a diffusion driven process except the initial stages of the growth process during which the natural convection in the solution zone is prominent and has significant effects on the composition of the grown crystal. The simulated evolution of the growth interface agrees with experimental observations. In addition, the numerical growth velocities are in good agreement with those of experiments. The numerical model developed can be used to study other crystal growth processes such as LPEE, Traveling Heater Method THM, and vertical Bridgman with slight modifications.