Local vibrational modes of H₂and H₂^*complexes in crystalline Si

We study the local vibrational properties of H2 molecules in crystalline Si using a first-principles pseudopotential method within the local-density-functional approximation and the generalized gradient approximation. The dynamical matrix is calculated using a supercell geometry, so that all the normal vibrational modes are identified for the [100], [110] and [111] orientations of the H2 molecule at or near a tetrahedral site. For the orientations considered here, a new local mode is found at 650-700 cm-1, which lies above the bulk phonon band, while the vibrational frequencies of the stretch mode are in the range of 3556-3643 cm-1, close to the experimentally measured value of 3618 cm-1. However, considering anharmonic effects, the calculated frequencies for the stretch mode are expected to be lowered by about 200 cm-1. We also examine the vibrational frequencies for an H2* complex, and find the stretch and wagging modes to be in good agreement with experiments.