Forced convection induced thermal fluctuations at the solid-liquid interface and its effect on the radial alloy distribution in vertical Bridgman grown Ga1−xInxSb bulk crystals

The mode of redistributing the excess solute species at the melt-solid interface accumulated during the growth of bulk ternary semiconductor crystals is a key factor dictating the crystalline quality and spatial alloy composition. Since the solidification temperatures vary with alloy composition, a control over both heat and mass transport during crystal growth is necessary for achieving spatial compositional homogeneity in the grown crystals. The effects of forced and natural convections, as well as diffusion (in the liquid) on the radial alloy distribution in vertical Bridgman growth Ga1−xInxSb bulk polycrystals (20-50 mm diameter) have been investigated. In the absence of forced convection, the shape of the solid-liquid interface has been found to be highly curved under growth conditions necessary for avoiding constitutional supercooling (high axial temperature gradients and low growth rates). Lowering the axial temperature gradient does flatten the interface shape, but extremely slow growth rate (below 0.1 mm/h) is necessary for avoiding constitutional supercooling. Forced convection helps in accelerating the removal and re-distributing the excess solute from the growth interface to the entire melt volume thus leading to a 4-5 fold increase in the growth rate without constitutional supercooling and flattening the solid-liquid interface necessary for uniform radial composition in the crystal. However, the mechanism of forced convection coupled with axial temperature gradient in the melt leads to thermal fluctuation at the growth interface. As a consequence, fluctuation in radial alloy composition is observed. A comparative study of various melt stirring schemes with different axial temperature gradients and the resulting alloy distribution in the crystals has been presented in this paper.