First-principles calculations of self-interstitial defect structures and diffusion paths in silicon

A first-principles pseudopotential study of neutral self-interstitial defects in silicon is reported, together with calculations for Pandey’s concerted exchange mechanism for selfdiffusion. The energies and structures of the fully relaxed hexagonal, tetrahedral, split-〈110〉, ‘caged’ (Clark S J and Ackland G J 1997 Phys. Rev. B 56 47), split-〈100〉, and bond-centred interstitials are calculated using supercells with up to 128 + 1 atoms. We present results obtained using the local density approximation (LDA) and the PW91 generalized gradient approximation (GGA) for the exchange–correlation energy. Both the LDA and PW91–GGA functionals give the hexagonal and split-〈110〉 defects as the lowest-energy self-interstitials. The hexagonal and split〈110〉 defects are essentially degenerate in energy with formation energies of about 3.3 eV (LDA) and 3.80 eV (PW91–GGA). Energy barriers are studied by calculating saddle-point structures using a simple ‘ridge-walking’ method. The energy barrier for a diffusive jump between the hexagonal and split-〈110〉 interstitial sites is calculated to be 0.15 eV (LDA) and 0.20 eV (PW91–GGA) and the barrier between neighbouring hexagonal sites is 0.03 eV (LDA) and 0.18 eV (PW91–GGA), but we have not found a low-energy path between split-〈110〉 interstitial sites. The results suggest that self-interstitial diffusion in silicon occurs via diffusive jumps between the hexagonal sites and between hexagonal and split-〈110〉 defects.