RADIAL DOPANT SEGREGATION IN ZERO-GRAVITY FLOATING-ZONE CRYSTAL-GROWTH

A computer model was developed to study radial dopant segregation in floating-zone crystal growth under zero-gravity. Heat transfer, fluid flow and mass transfer in the melt zone were considered simultaneously. The growth front, feed front and free surface were calculated, these interfaces being nonisothermal and having nonuniform dopant concentrations. For a dopant with a segregation ratio k0 < 1, the model showed the following trends. A convex growth front causes the dopant to segregate toward the crystal surface whereas a concave one toward the crystal axis, the former being consistent with the experiment of Levenstam et al. The effect of the growth front shape is quickly overshadowed as thermocapillary convection begins to increase in strength. Thermocapillary convection causes the dopant to segregate toward the crystal axis. However, as thermocapillary convection grows stronger, dopant segregation toward the crystal axis first increases and then decreases due to improved mixing. The direction of dopant segregation is reversed when k0 > 1. Finally, the shape of the growth front can be affected by radial dopant segregation.