Effect of free surface and gravity on silicon dissolution in germanium melt

Silicon germanium (SiGe) is an important emerging compound semiconductor material. To produce quality bulk material by solution or melt techniques, a good understanding of the transport behavior is necessary. For melt growth, the large miscibility gap of the material requires that the melt be replenished with silicon as the crystal is pulled. Transport of silicon to this interface is of prime importance. In solution growth utilizing germanium as the solvent, transport of the silicon through the solution determines growth velocity.The dissolution behavior of silicon in a germanium melt has been experimentally investigated. The effect of free surface on mixing was examined. The effect, while small, did show a trend of higher mixing with a free surface on the melt. The effect of the direction of gravity was also examined. One set of experiments placed the silicon dissolution interface on top of the melt and the others the bottom of the melt. The chosen orientation with respect to gravity had a profound effect on dissolution. Far more dissolution was realized with the dissolution interface at the bottom of the melt. Silicon transport was aided by its buoyancy in the germanium melt. The melt, in these cases, remained inhomogeneous. This is likely due to very fast dissolution and strong convective currents.The results indicate that careful consideration of system geometry is needed when silicon is to be dissolved into germanium melt. Silicon dissolution from the surface of the melt will be limited by diffusion time scales. Silicon dissolution from the bottom of the melt will occur much faster but be subject to instability and melt inhomogeneity.