  We present the results of a systematic study using the density functional
theory (within the local density approximation) of the effects of biaxial
strain and composition on the self-diffusion of Si and Ge in SiGe alloys
diffusing by a vacancy mechanism. The biaxial strain dependence of the vacancy
formation energy was reconfirmed with previous calculations. The effect of
biaxial strain on the interaction potential energy between a substitutional Ge
atom and a vacancy was calculated. These calculations were used to estimate the
change in the activation energy (due to biaxial strain) for the self-diffusion
of Si and Ge in Si by a vacancy mechanism. The composition dependence of the
vacancy formation energy was calculated. A database of ab initio migration
energy barriers for vacancy migration in different local environments was
systematically developed by considering the effect of the first nearest
neighbor sites explicitly and the effect of the other sites by a mean field
approximation. A kinetic Monte Carlo simulation based on the migration energy
barrier database was performed to determine the dependence (on the
composition)of the activation energy for the diffusion of Si and Ge in SiGe. A
detailed study of the variation of the correlation factor with composition and
temperature in SiGe was performed using the results of the KMC simulation.
These analyses constitute essential building blocks to understand the mechanism
of vacancy mediated diffusion processes at the microscopic level.
