Diffusion and dissociation of neutral divacancies in crystalline silicon

Based on ab initio calculations with a 216-atom supercell, we find mechanisms for the diffusion and dissociation of the neutral-state divacancy (V₂⁰). Contrary to the popular belief that diffusion is via successive detachment and recombination (a two-step process), we find that V₂⁰ diffusion follows predominantly a one-step hopping mechanism; that is, two adjacent vacancies move together. The calculated activation energy of 1.35 eV is in excellent agreement with experiment (≈1.3 eV). This work suggests that to dissociate the V-V pair the neighboring Si atoms on each side of the V₂⁰ must move inward simultaneously to form the stable V-Si-Si-V configuration, and then a third neighboring Si atom hops inward to leads to the V-Si-Si-Si-V state whose energy is almost equivalent to that of two separated monovacancies (2 V₁¹) of 6.96 eV. We also present the formation energy of the vacancy-vacancy complex for different relative positions, providing insight into the vacancy-vacancy interaction.