Effect of rotation on radial dopant segregation in microgravity floating-zone crystal growth

Abstract

Computer simulation was conducted to study the effect of rotation on radial dopant segregation in floating-zone crystal growth under microgravity. Heat transfer, fluid flow and mass transfer in the melt zone were considered simultaneously, with the surface tension gradients along the free surface of the melt and the centrifugal force induced by rotation as the driving forces for fluid flow. The growth front, feed front and free surface were calculated, these interfaces being nonisothermal and having nonuniform dopant concentrations. The most common case where the dopant has a segregation ratio k 0<1 was considered. In the absence of thermocapillary convection, dopant segregation can be easily reversed from toward the crystal surface to toward the crystal axis when the feed rod alone is rotated during crystal growth. As the rotation speed is increased further, dopant segregation is still toward the crystal axis but reduced significantly due to mixing. Dopant segregation, however, is not reversed when the crystal alone is rotated or when the crystal and the feed rod are counterrotated. A nearly uniform dopant distribution can be produced by counterrotation at sufficient speeds. Thermocapillary convection tends to cause dopant segregation toward the crystal axis. In the presence of thermocapillary convection, crystal rotation and counterrotation both cause the location of the maximum dopant concentration to shift radially outward from the crystal axis, whereas feed-rod rotation does not. Again, a nearly uniform dopant distribution can be produced by counterrotation at sufficient speeds. The stronger the thermocapillary convection, the higher these counterrotation speeds become.