Models for convection and segregation in the growth of HgCdTe by the vertical bridgman method

Abstract

Convection driven by both temperature and concentration gradients, melt/crystal interface morphology and solute segragation in growth of HgCdTe by the vertical Bridgman technique are studied by numerical analysis of an idealized, quasi-steady-state model for the system. Convection is driven by the radial temperature gradients due to the mismatch of thermal conductivities between the melt, crystal and ampoule. A strong cellular motion exists near the interface, but flow away from it is damped by the stabilizing dependence of the melt density on the heavier component HgTe of the pseudo-binary which is rejected at the interface. Increasing the composition of CdTe lowers the melt density far from the interface and further damps the melt motion, but leads to formation of additional vertically stacked cells in the melt. This transition in the flow structure is linked to the sideways diffusive instability identified by Hart (1972) for thermosolutal convection in the presence of a vertically stabilizing solute gradient. Multiple steady-state flows and flow hysteresis are predicted. Large radial segregation caused by incomplete solute mixing adjacent to the interface is predicted, as has been observed in several experiments. The prediction of solute mixing only in a region adjacent to the interface suggests a model for axial segregation that couples a small well-mixed region to diffusion-controlled axial transport elsewhere. The axial segregation predicted by this model is qualitatively similar to the results for only diffusion and agrees with experimental observation.