Fast Diffusion Mechanism of Silicon Tri-interstitial Defects

Abstract

We reveal the microscopic self-diffusion process of compact tri-interstitials in silicon using a combination of molecular dynamics and nudged elastic band methods. We find that the compact tri-interstitial moves by a collective displacement, involving both translation and rotation, of five atoms in a screw-like motion along $[111]$ directions. The elucidation of this pathway demonstrates the utility of combining tight-binding molecular dynamics with \textit{ab initio} density functional calculations to probe diffusion mechanisms. Using density functional theory to obtain diffusion barriers and the prefactor, we calculate a diffusion constant of $4\cdot 10^{-5}\exp (-0.49\mathrm{eV}/k_{B}T)\mathrm{c}{\mathrm{m}}^2/s$. Because of the low diffusion barrier, $I_3^b$ diffusion may be an important process under conditions such as ion implantation that creates excess interstitials, hence favoring formation of interstitial clusters.