Epitaxial growth of silicon-germanium single crystals

The problems associated with obtaining single crystals of silicon-germanium with comparable concentrations of the two elements, are discussed. Epitaxial growth from the vapour phase was found to be much more suitable than growth from the melt, which yielded crystals with gross inhomogeneities, even when only 3% of Ge was included. By using a modified epitaxial system which is described, Si-Ge single crystal layers of up to 32% Ge were grown succesfully. It was found that growth temperature must be reduced with increased Ge content for optimum results. The measured growth rate was a function of both the temperature and the molar ratio of Si to Ge in the gas phase. The Ge/Si ratio in the solid phase was found to be about twice as high as in the gas phase for low Ge content, but the ratios became nearly the same for high Ge content, and decreased for increased temperature. The crystal structure and composition of the layers were measured by X-ray diffraction and an electron microprobe respectively. The lattice constant was found to vary approximately linearly with the composition, though stresses due to mismatch between layer and substrate cause this measurement to vary slightly with the choice of the crystal planes. The same stresses account for a slight spherical curvature of the layers. A pattern of ordered slip lines is found in the layers, which becomes more discernible with higher Ge content, and accounts for surface roughness depending on the composition.