The diffusivity of the vacancy in silicon: Is it fast or slow?

Vacancies in silicon are known to be highly mobile both at high temperatures (just below the melting point) and at cryogenic temperatures. Contrary to this, however, vacancy diffusivity near 800 °C — as deduced from the radiation-enhanced self-diffusion coefficient Dsd — was reported to be surprisingly low. An apparent explanation of this contradiction is that the defect concentrations (and accordingly Dsd) are reduced by an impurity-mediated recombination of vacancies and self-interstitials. This effect however is shown to be insufficient to account for such a low Dsd. A suggested solution to the puzzle is that self-interstitials (and vacancies as well) exist in two structural forms, a localized one and an extended one, of strongly differing diffusivities. A low diffusivity manifested in radiation-enhanced self-diffusion is likely to correspond to a low apparent diffusivity of self-interstitials (averaged over the two forms) rather than that of vacancies. The fast and slow forms of vacancies are concluded to be both of a high diffusivity at elevated temperatures while it is most likely that one of them (the extended one) becomes practically immobile at low temperatures.