Defect-induced dissociation of H2 in silicon

Ab initio molecular-dynamics simulations of intrinsic defects and hydrogen in crystalline silicon reveal an unexpected process with considerable implications. The vacancy (V) and the self-interstitial (I), both rapid diffusers in c-Si, dissociate interstitial H-2 molecules with a substantial gain in energy: V+H-2-->(V,H,H) + 4.0 eV and I+ H-2-->{I,H,H}+ 1.7 eV. The dissociation of H-2 is caused by the lattice strain associated with the defect, and occurs whenever H-2 molecules are in the vicinity of strained Si-Si bonds. After the dissociation, the two H's may either bind to the defect that caused the strain or diffuse away from it. The calculated Frenkel pair formation energy is 8.2 eV. The reaction H-2+H-2-->{V,H-4}+I releases less than 0.1 eV, suggesting that H-2's in otherwise perfect Si will not generate intrinsic defects.