Mechanisms of self-diffusion in stoichiometric and substoichiometric amorphous silicon dioxide

We have investigated the Si and O self-diffusion in stoichiometric and substoichiometric amorphous SiO2 by means of molecular-dynamics simulations. The diffusivity and the migration energies at different Si concentrations are reported and the results in qualitative agreement with previous experimental and theoretical ab initio results. We prove that the diffusion of Si and O occurs through steplike events. In particular, we identify three mechanisms, associated with coordination and “local stoichiometry” defects, responsible for the diffusion. The migration energy and pre-exponential factor, as well as the relative relevance of these mechanisms, is computed as a function of the Si concentration and temperature. A model for interpreting our results is proposed and discussed.