Tight-binding molecular dynamics and density-functional simulations of Si seeded with a di-interstitial revealed its detailed diffusion mechanisms. The lowest-energy di-interstitial performs a translation/rotation diffusion-step with a barrier of 0.3eV and a prefactor of 11THz followed by a reorientation diffusion step with a 90meV barrier and a 2300THz prefactor. The intermediate reorientation steps allow di-interstitials to diffuse isotropically along all possible <111> bond directions in the diamond lattice. The dominating diffusion barrier of 0.3eV was not inconsistent with the experimental value of 0.6eV. In addition, this lowest energy di-interstitial may diffuse to neighbouring sites through an intermediate structure which was the bound state of two single interstitials. The process in which migrating single interstitials combine into a di-interstitial was exothermic with almost zero energy barrier.

Diffusion Mechanisms for Silicon Di-Interstitials. Y.A.Du, R.G.Hennig, J.W.Wilkins: Physical Review B, 2006, 73[24], 245203 (5pp)