Tin-vacancy complexes in silicon

The structure and electrical properties of of SnV₂, Sn₂V, and Sn₂V₂ complexes in Si are investigated using first-principles cluster and supercell methods. The formation of SnV₂ and Sn₂V₂ is found to be energetically favorable, in agreement with the experimental results. All the tin-vacancy defects are found to possess deep donor and acceptor levels, although the number of the gap states decreases with increasing size of the defect. The diffusion of tin in silicon is considered and the mechanism found to be distinct from the diffusion of group V shallow donors. In contrast with these, the Sn-V interaction is found to extend only to the third nearest neighbor distance. This implies that the activation energy for Sn diffusion via vacancies should be nearly the same as self-diffusion by this mechanism. We find an activation energy of 3.5 eV which is close to some experimental findings but considerably less than given by others.