Numerical Study On Sapphire Crystal Growth By Heat Exchanger Method

Due to its unique lattice structure, excellent optical performance, thermal stability, mechanical strength and chemical resistance, Sapphire crystal has been widely used in many areas, such as substrate material for growing GaN-based blue LED device, and window materials for infrared and laser devices, The primary trend of the artificial sapphire production is to produce large-size and high-quality crystals by two main methods: Kyropoulos method(KY) and heat exchanger method(HEM).HEM has the advantages of easily producing large size crystals with low dislocation density, by changing helium flux in heat exchanger and heating power to control the temperature gradient, which has made it the important method for high-quality,large-size sapphire crystal growth. Due to the high temperature of above 2000℃ in sapphire crystal growth, in situ observation is very difficult, numerical simulation becomes the most important method to study sapphire crystal growth.Global modeling of sapphire crystals grown by HEM is performed by CGSim to study the characteristics in sapphire growth and the temperature and flow fields and thermal stresses at different growth stages. The predicted crystal sizes and the heating power are compared with the experimental values, and good correspondence are obtained.On the basis of the understanding of the characteristics of different growth stages, the influences of heat exchanger area and inner tube height of heat exchanger,and top insulation structure on sapphire crystal growth are also investigated.The simulation results show that during the sapphire crystal growth, large temperature gradient region with condensed isotherm are located near the crucible wall, the front of melt-crystal interface and the seed region on the bottom of crucible.During the initial stage of crystal growth,the solid-liquid interface shape is semi ellipsoidal; at the middle stage, crystal is equal-diameter approximately, and the interface becomes flat gradually without contact with crucible wall; at the final growth stage, the axial growth rate near the center axis increases, and the center crystal begins to emerge from the melt free surface. With the increase of crystal height, the melt convection changes from two vortexes at the initial stage into one vortex at the equal-diameter stage; the maximum convective velocity is on the order of 10-3 m/s. The maximum thermal stress distributes on the bottom of the crystal, and the thermal stress is distributed as W-shape.When the heat exchanger area increases, the crystallization rate decreases, and the melt-crystal interface becomes flat at final growth stage. By lowering the inner tube height of heat exchanger, the thermal stress at the crystal bottom will be reduced significantly. When the inner tube height is too low, small vortexes appear,which influence the temperature field. When the upper furnace insulation gets thicker, crystallization rate at the top of the crystal decreases and the melt-crystal interface becomes flat at final growth stage, which will benefit the removal of bubble impurities in the top of crystal.Through this study, we try to clarify the basic rules for temperature and flow fields and the thermal stresses in HEM sapphire crystal growth at different stages,and the influences of heat exchanger area and inner tube height of heat exchanger,and top insulation structure on sapphire crystal growth.