Ab Initio Studies of Hydrogen Molecules in Silicon

Abstract

The results of first principles calculations of the structure and vibrational modes of H₂ molecules in Si located at i) isolated interstitial sites, ii) bound to oxygen impurities in the silicon and iii) trapped in voids in the lattice are reported. These results are compared with recent experimental investigations. The isolated molecule is found to lie at a T _d interstitial site, oriented along [Oil] and is infra-red active. The rotational barrier is at least 0.17 eV. The molecular frequency is a sensitive function of cage size and increases to the gas value for cages about 40% larger than the T _d site. We find that it is possible for H₂ molecules to cause an upward shift in the antisymmetric stretch mode of O_i; when H is replaced by D, which could explain the anomalous shift in the 1075 cm⁻¹ O-H related local vibrational mode. It is suggested that Raman active modes around 4158 cm⁻¹ are due to molecules within voids.