EFFECTS OF BUOYANCY-DRIVEN FLOW AND THERMAL-BOUNDARY CONDITIONS ON PHYSICAL VAPOR TRANSPORT

Crystal growth by physical vapor transport (PVT) in closed ampoules, due to its experimental simplicity and minimal need for process control, is an attractive technique for materials preparation in low gravity environments. In order to ascertain if reduced gravity conditions are beneficial to PVT and to determine its tolerance limits to residual accelerations, we developed a steady-state two-dimensional numerical model. This was solved using the finite volume code PHOENICS. Reduction of gravitational accelerations to less than 0.1 g0 was found to be sufficient to suppress buoyancy-driven convection to an extent that diffusion was the dominant transport mode, whence a greater uniformity in the growth rate could be obtained. Further, we found that the convection is usually solutally-driven for any significant disparity in the molecular weights of the crystal components and the inert gas. We show that a uniform temperature gradient on the ampoule walls will cause the vapor to be supersaturated throughout the ampoule, potentially resulting in undesirable nucleation at the walls. A "hump" in the wall temperature profile can be used to avoid this. The prevailing transport conditions determine the size of the hump needed.