Dynamics of Si self-interstitial clustering using the fast-diffusing I₃ cluster

Recent ab initio molecular-dynamics simulations have shown that the di-interstitial (I₂) and the stable tri-interstitial (I₃^a) clusters in Si are highly mobile. In these complexes, the two (or three) self-interstitials share a single bond-centered site, a feature which greatly facilitates exchange processes and is responsible for the rapid diffusion. In this contribution, we consider the possibility that the precipitation of self-interstitials results from the clustering of I₃ 'units' rather than isolated Is. We study the interactions I₃+I₃→I₆ and I₆+I₃→I₉ by bringing an I₃ toward either another I₃ or I₆ along various crystalline directions in 216 host atoms supercells. The calculations show that these reactions occur at a substantial gain in energy and that the stacking along some directions is energetically preferred over others. The I₆+I₂→I₈ reaction leads to a very stable cluster. The results suggest that precipitation mechanisms involving rapidly moving self-interstitial clusters could play an important role in the formation of extended defects.