The strange behavior of interstitial H 2 molecules Si and GaAs

Abstract

Interstitial H 2 molecules have been observed in GaAs and in Si and behave very differently in the two hosts. In GaAs, H 2 has rotational symmetry, a stretch frequency close to the free-molecule value, and the Raman spectrum exhibits the expected ortho/para splitting. In Si, the symmetry is C 1, the stretch frequency is about 550 cm −1 lower than that of free H 2, and no ortho/para splitting is seen. In this paper, we use ab-initio molecular-dynamics simulations (SIESTA) to calculate the vibrational frequencies of H 2 using linear response theory as well as the velocity–velocity autocorrelation function. We also study the interactions between H 2 and O i as well as paramagnetic H in Si. Based on these calculations and arguments about the formation process of H 2, we argue that rapid H 2⇌H T 0 interactions are a possible candidate to explain the absence of ortho-H 2 in Si.