An open-shell version of the modified intermediate neglected diffusional overlap molecular orbital method was used to deduce the equilibrium positions of H, H+ and H2 in a 32-atom cyclic Si cluster. Two shells of Si neighbors around the defect were allowed to relax and to reconstruct, while the potential surface for H was mapped. The absolute minimum, for both H and H+ was found at the bond-centered interstitial site. The H or H+ broke the lattice bond only if lattice relaxation was allowed. The energy of the H atoms, relative to the lattice, was 0.1eV lower at this point than its energy in a H2 molecule located at a tetrahedral interstitial site. An additional local minimum for atomic H was found at a distance of 0.042nm from the tetrahedral interstitial site, in the [111] direction towards the next Si atom (the anti-bonding site). This site was 0.92eV higher in energy than the bond-centered interstitial site. Saddle-point calculations showed that the activation energy for H diffusion was about 0.8eV from the bond-centered interstitial site and was about 0.4eV for diffusion between anti-bonding sites. No gap levels arose with the H atom on the bond-centered interstitial site. A donor level emerged above the valence band edge when the H atom moved from one bond-centered interstitial site to another. This could be ionized in p-type material. The ionized system exhibited the same diffusion path for H.

P.Deak, L.C.Snyder, J.W.Corbett: Physical Review B, 1988, 37[12], 6887-92