HEAT-TRANSFER, FLUID-FLOW AND INTERFACE SHAPES IN FLOATING-ZONE CRYSTAL-GROWTH

Computer simulation was carried out to study heat transfer and fluid flow in the melt zone in floating-zone crystal growth. A high Prandtl-number material, i.e., NaNO3, and a low Prandtl-number material, i.e., Si, were considered. The unknown shapes of the melt/gas, melt/crystal and melt/feed interfaces were calculated for each of the following three cases: (1) conduction, (2) natural convection and (3) thermocapillary and natural convections. The effects of the growth rate, gravity and feed/crystal diameter ratio were demonstrated. It was observed that in both NaNO3 and Si thermocapillary convection dominates over natural convection, at least for the conditions examined in the present study. This thermocapillary convection tends to reduce the stability of the melt zone, increase the convexity of the melt/crystal interface and reduce the maximum surface temperature. The effect of thermocapillary convection is significantly more pronounced in NaNO3 than in Si, even though thermocapillary convection in Si is far stronger. The dynamic effect of convection on the shape of the melt/gas interface is very small.