Atomistic study of the mobility of small interstitial clusters in silicon

In this work a comprehensive atomistic study is performed in order to get a better understanding of the mechanisms of di- and tri-interstitial diffusion at different temperatures and to obtain more data for their diffusivities and migration barriers. A classical potential approach is employed since it allows the investigation of defect migration under relatively realistic conditions, by considering a large computational cell, a very long simulation time, and different initial conditions. In order to test the potential employed, at first the structure and energetics of di- and tri-interstitials are determined and the results are compared with the data obtained by tight-binding and density-functional-theory calculations.
The migration of mono-, di- and tri-interstitials is investigated for temperatures between 800 and 1600 K. The defect diffusivity, the self-diffusion coefficient per defect and the corresponding effective migration barriers are calculated. Compared to the mono-interstitial, the di-interstitial migrates faster, whereas the tri-interstitial diffuses slower. The mobility of the di- and the mono-interstitial is higher than the mobility of the lattice atoms during the diffusion of these defects. On the other hand, the tri-interstitial mobility is lower than the corresponding atomic mobility. The migration mechanism of the di-interstitial shows a pronounced dependence on the temperature. At low temperature a high mobility on zigzag-like lines along a <110> axis within a {110} plane is found, whereas the change between equivalent <110> directions or equivalent {110} planes occurs seldom and requires a long time. At high temperature a frequent change between equivalent <110> directions or {110} planes is observed. During the diffusion within {110} planes the di-interstitial moves like a wave packet so that the atomic mobility is lower than that of the defect. On the other hand, the change between equivalent {110} migration planes is characterized by frequent atomic rearrangements. The visual analysis of the tri-interstitial diffusion reveals complex migration mechanisms and a high atomic mobility. The implications of the present results for the explanation of experimental data on defect evolution and migration are discussed.

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