Structure and dynamics of the diarsenic complex in crystalline silicon

Based on first-principles density functional theory (DFT) calculations within the generalized gradient approximation (GGA), we propose a structural model for a diarsenic (Ass-Asi) complex in Si and a mechanism for its reorientation, diffusion and dissociation. We find that the lowest-energy structure of Ass-Asi consists of two As atoms bonded together in a single lattice site with a bond axis which is tilted slightly from the [110] direction. Our study demonstrates the Ass-Asi pair may undergo reorientations within a lattice site and diffusion, with energy barriers of 1.05 and 1.33eV, respectively. Dissociation of an Ass-Asi pair is likely to occur predominantly by the liberation of an As-Sii pair to leave behind a substititional As atom. The dissociation barrier is predicted to be 1.33eV, and the Ass-Asi binding energy is calculated to be 1.00eV relative to dissociation products As0 and AsSii0. We also discuss the role of Ass-Asi pairs in As transient enhanced diffusion and clustering.