Diffusion mechanisms for silicon di-interstitials

Tight-binding molecular dynamics and density-functional simulations on silicon seeded with a di-interstitial reveal its detailed diffusion mechanisms. The lowest-energy di-interstitial performs a translation/rotation diffusion-step with a barrier of 0.3 eV and a prefactor of 11 THz followed by a reorientation diffusion step with a 90 meV barrier and a 2300 THz 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 0.3 eV is not inconsistent with the experimental value of 0.6±0.2 eV. In addition, this lowest energy di-interstitial may diffuse to neighboring sites through an intermediate structure which is the bound state of two single interstitials. The process in which migrating single interstitials combine into a di-interstitial is exothermic with almost zero energy barrier.