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 (formula presented) Contrary to the popular belief that diffusion is via successive detachment and recombination (a two-step process), we find that (formula presented) 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 (formula presented) pair the neighboring Si atoms on each side of the (formula presented) must move inward simultaneously to form the stable (formula presented) configuration, and then a third neighboring Si atom hops inward to leads to the (formula presented) state whose energy is almost equivalent to that of two separated monovacancies (formula presented) 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.