Interstitial H2 molecules have been observed in GaAs and in Si and behave very differently in the two hosts. In GaAs, H2 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 C1, the stretch frequency is about 550cm-1 lower than that of free H2, and no ortho/para splitting is seen. In this paper, we use ab-initio molecular-dynamics simulations (SIESTA) to calculate the vibrational frequencies of H2 using linear response theory as well as the velocity-velocity autocorrelation function. We also study the interactions between H2 and Oi as well as paramagnetic H in Si. Based on these calculations and arguments about the formation process of H2, we argue that rapid H2⇌HT0 interactions are a possible candidate to explain the absence of ortho-H2 in Si.
- Gallium arsenide
- Molecular dynamics