Numerical Simulation Of PVT Growth Of Large Silicon Carbide Crystals

SiC is a promising semiconductor material for devices operating at extreme conditions such as high voltage, high temperature and high current density. PVT method is most commonly used for SiC single crystal growth. The growth process includes complex physical and chemical changes which is difficult to control the thermal field precisely. It is, therefore, important to accurately understand the thermal phenomenon occurs in the growth system in order to effectively control the growth of silicon carbide crystals. Comparing the experimental knowledge, numerical simulations can provide additional information, which will be helpful to improve the SiC growth system and optimize operational conditions.In this thesis, the thermal and stress field problems in growth process have been investigated. The simulation of SiC growth process by PVT method has been carried out by using the VR-PVT code based on the finite element method that incorporates the models of heat and mass transportation, growth kinetics, thermal stress and dislocation.First, the temperature distribution of the growth system is calculated. The effects of seed temperature, the axial temperature gradient, and cavity pressure on the growth rate have been investigated. The transient thermal field distribution and the powder evolution are analyzed. Second, thermal stress distribution and the effect of different seed attachments to the crucible lid on the stress and dislocation density are studied. Finally, the effects of the equipment parameters including the crucible wall thickness, position and top windows of insulation are discussed.The results are summarized as follows: the temperature distribution in the growth chamber is non-uniform. The growth rate increases with increasing seed temperature and gradient, and deceases with increasing pressure. The growth rate reduced gradually with the growth process, and the powder near the wall of crucible is sublimated, while the recrystallization occurs at the surface of the powder. Different seed attachments lead to different thermostress field and dislocation density. It is important to choose the proper attachment method to reduce the thermostress in the as- grown crystal. If the crucible wall thickness is larger than 2 mm, the temperature of seed surface deceases as the thickness of the crucible wall increasing. However, the growth rate increases as the thickness of the crucible wall increasing. Different crucible positions will lead to different thermal field distribution. The closer to the upper hole, the greater the axial gradient, growth rate and the thermal stress increases. It can control the radiation by changing the size of the upper hole to change the temperature distribution. With the diameter of top windows increasing, the growth rate, axial temperature gradient and stress generated in the as-grown crystal increase.It provides theoretical guidance to optimize the growth process of silicon carbide and to produce high-quality large-size silicon carbide crystals by studying these key issues.