Modeling and design of PVT growth of silicon carbide crystals.

摘要

Physical vapor transport method (PVT) is an important technique for growing SiC bulk crystals, which is a promising semiconductor material for electrical and optoelectronic applications in the areas of high power, high temperature, high frequency and strong radiation. The ever-increasing demand for SiC substrates of high quality and large diameter has motivated extensive research effort on the growth of SiC boule using PVT method. The PVT growth process involves highly complex physics and elaborate system that significantly affect the rate of growth, growth area and defect density. This dissertation is aimed at developing a fundamental understanding of the growth process and identifying the foremost process conditions and parameters that affect crystal productivity and quality. To achieve this goal, we have developed a comprehensive model that involves major physical mechanisms of PVT growth, i.e. , transport of energy and vapor species, chemical reaction, growth kinetics, and anisotropic thermal stresses. Moreover, the multiplication of dislocation is integrated into this model to correlate thermal stresses to dislocation distribution. Through this work a relationship is established between the transport phenomena at the macroscale and defect development at the microscale. Finite volume method with adaptive non-orthogonal grid has been used for the thermal and mechanical calculations in the complex geometry.; Using this integrated model, we have carried out numerical simulation of SiC growth process to predict the global temperature distribution in the furnace, the rate of growth and the shape of the as-grown crystals. In addition, the thermal stresses in the growing crystal and the dislocation distribution are also calculated. It is found that the temperature distribution in the induction-heated growth chamber is quite non-uniform. Under the growth temperatures, thermal radiation is the dominant heat transfer mode and accurate modeling is essential. The rate of growth and the shape of growth interface are closely related to the process parameters such as temperature distribution and pressure. For the system used in this thesis, the magnitude of the shear stress acting on the basal plane exceeds the estimated critical resolved shear stress in some portions of the crystal, which means thermal stress is a main cause of dislocations present in SiC bulk crystals. The dislocation distributions predicted using CRSS and A-H model are consistent. More importantly, the growth is a transient process; the geometry of the growing crystal and the temperature across the growth interface change appreciably, varying the growth rate, magnitude of thermal stress and the density of dislocation during the process. (Abstract shortened by UMI.)