Substitutional C fraction and the influence of C on Si dimer diffusion in Si_1-yC_y alloys grown on (001) and (118) Si

The dependence of substitutional C fraction on growth temperature and substrate orientation is measured for Si_1-yC_y. alloy films grown on (001) and (118) Si by molecular-beam epitaxy. Secondary ion mass spectrometry and high-resolution x-ray diffraction were used to measure the total C and the substitutional C concentrations, respectively, in several samples prepared at temperatures between 450 and 650°C. The substitutional C fraction decreased rapidly with increasing temperature in this range, regardless of orientation, and was slightly lower for growth on (118) Si. Cross-sectional transmission electron microscopy on (118)-oriented samples revealed a tendency for C to concentrate periodically on (001) facets which formed immediately after initiation of Si_1-yC_y growth. A kinetic Monte Carlo simulation based upon enhanced diffusion of Si dimers in the presence of subsurface C predicted a step instability leading to step bunching and the formation of periodic surface features, as well as the accumulation of high C concentrations on nearly (001) planes.