(212b) Simulation of Heat Transfer and Convection During Sapphire Crystal Growth by the Heat Exchanger Method

Sapphire single crystals are an important substrate for the fabrication of gallium-nitride light-emitting-diodes (LEDs) that emit white light.  Theses solid-state devices promise great energy savings over both incandescent and compact fluorescent lighting technologies, and their market is expanding at double-digit growth rates.  Key to continuing cost reduction for such LED-based lighting is reducing the cost of sapphire without compromising quality.  The heat exchanger method (HEM) represents a possible yield improvement over existing growth technologies (primarily Kyropoulos growth) and is the subject of the study presented here. 

We have performed quasi-steady state analysis using the commercial code CrysMAS to investigate the effects of furnace geometry and processing parameters on the temperature distribution, convection field, and melt-crystal interface shape during the crystal growth of sapphire by heat exchanger method (HEM).  The temperature profile in the system is calculated to have lower temperatures both at the base of crucible and at the center top of the melt; however, the presence of convection flow arising from Marangoni force and buoyancy can ameliorate the latter effect.  The interface shape is also predicted to change from flat to convex as crystal growth proceeds.  Changes in geometry of furnace, such as segmentation of heater, changing the crucible position relative to heater center, and altering the width of crucible support, are shown to affect the temperature profile significantly. Based upon such results, optimization of furnace design will be presented.