Based on ab initio calculations with a 216-atom super-cell, mechanisms were found for the diffusion and dissociation of the neutral-state divacancy (V20). Contrary to the popular belief that diffusion was via successive detachment and recombination (a 2-step process), it was found that V20 diffusion follows predominantly a one-step hopping mechanism; that is, two adjacent vacancies move together. The calculated activation energy of 1.35eV was in excellent agreement with experiment (about 1.3eV). This suggested that, to dissociate the V-V pair, the neighboring Si atoms on each side of the V20 had to move inwards simultaneously to form the stable V-Si-Si-V configuration, and then a third neighboring Si atom hopped inwards to lead to the V-Si-Si-Si-V state whose energy was almost equivalent to that of 2 separated monovacancies (2 V20) of 6.96eV. The formation energy of the vacancy-vacancy complex was also presented for various relative positions, thus providing insight into the vacancy-vacancy interaction.

Diffusion and Dissociation of Neutral Divacancies in Crystalline Silicon. G.S.Hwang, W.A.Goddard: Physical Review B, 2002, 65[23], 233205