First-principles molecular dynamics simulations of intrinsic defects and H in crystalline material revealed an unexpected process. The vacancy and the self-interstitial, which were both rapid diffusers, could dissociate interstitial H2 molecules, with a substantial gain in energy, according to: V + H2 → {V,H,H} + 4.0eV and I + H2 → {I,H,H} + 1.7eV. The dissociation of H2 was caused by the lattice strain which was associated with the defect, and occurred whenever H2 molecules were in the vicinity of strained Si-Si bonds. After dissociation, the two H atoms could either bind to the defect that caused the strain, or diffuse away from it. The calculated Frenkel-pair formation energy was 8.2eV. The reaction, H2 + H2 → {V,H4} + I, released less than 0.1eV; thus suggesting that H molecules in otherwise perfect Si would not generate intrinsic defects.

S.K.Estreicher, J.L.Hastings, P.A.Fedders: Physical Review B, 1998, 57[20], R12663-5