Germanium diffusion mechanisms in silicon from first principles

We present an extensive numerical study of the basic mechanisms that describe germanium diffusion in silicon mediated by point defects. This diffusion can be created by vacancies, interstitial atoms, or fourfold coordinated defects. All energies and elementary barriers have been precisely determined by ab initio calculations. The results for vacancies are compared with recently published values. The complex interstitial landscape is systematized and the key role of the hexagonal location is stressed as a halfway stable state between two, more stable, dumbbell [110] states. Finally, the mechanism of a concerted exchange linking two fourfold coordinated defects is fully calculated. Its activation energy is higher than for interstitial or vacancy mediated movements.