Migration of di- and tri-interstitials in silicon

Small self-interstitial clusters play an important role in the evolution of radiation damage in Si during post-implantation annealing. These small clusters can be formed by two different processes: (i) fast relaxation of highly disordered cascade regions immediately after ion impact and (ii) clustering of diffusing self-interstitials in the first stage of the annealing process.
The structure and energetics of the small self-interstitial clusters have been investigated by different theoretical methods. However, there are only few data on their mobility and migration mechanisms. They have been mainly obtained by static potential energy calculations. This work presents results of a systematic molecular dynamics study of the migration of di- and tri-interstitials. A classical potential approach is employed since it allows the investigation of defect migration under relatively realistic conditions, by considering large computational cells, long periods and different initial conditions. The di- and tri-interstitial configurations with the lowest formation energies are determined, and the results are compared with literature data. Then, the migration of di- and tri-interstitials is followed for 10 – 30 ns, at temperatures between 800 and 1600 K. Defect diffusivity and self-diffusion coefficient per defect are obtained by the Wigner-Seitz cell analysis and the Guinan method as well as from the mean square displacements of all atoms. The effective migration energies are determined and compared with the data from literature. The elementary migration mechanisms are discussed in detail by the analysis of movies and snapshots of defect trajectories and atomic rearrangements.