Convection and segregation in horizontal bridgman growth of GaAs single crystal

Abstract

An important factor of single crystal quality is the distribution of impurity in the crystal. In order to investigate the solute distribution in single crystal growth, numerical simulation of transport phenomena(momentum, heat, solute) in the horizontal Bridgman crystal growth process has been performed. As the driving force for natural convection increases, we investigated the flow structure and time dependent behavire in the melt only system. Constructed pseudo-steady-state and transient model for crystal growth of GaAs which include the free- or moving-interface. Since the moving melt/solid interface changes the liquid volume, we investigated the effect of natural convection and liquid volume to the solute distribution. In 2-dimensional PSSM, the shape of interface is flat with adiabatic thermal boundary condition, but with completely conducting thermal boundary. In 3-dimensional PSSM, the interface is less curved than 2-dimensional case and the flow intensity is weaker than 2-dimensional case. In a 2-dimensional transient problem, at Gr=0, the concentration boundary layer behaves as a diffusion controlled crystal growth process. At Gr=170, the concentration profile is distorted, and the maximum solute concentration appears near the bottom of interface. At Gr=1,700, the flow intensity is so strong that the minimum solute concentration appears near interface, which appeared at the left boundary of melt for the case of lower flow intensity. As solidification proceeds, interface deflection increases steadily, and transverse segregation increases until mixing by flow becomes steady. The axial segregation increases with the progress of solidification, however, at Gr=1700, the mixing is so strong that the axial segregation is nearly uniform after 5% solidified. Interface velovity does not reach moving velocity of furnace because of low Bi. At small and large Gr, the result showed a good agreement with the prediction from Smith and Scheil equation.