Effect of biaxial strain and composition on vacancy mediated diffusion in random binary alloys: A first principles study of the SiGe system

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.