Energetics and vibrational frequencies of interstitial H2 molecules in semiconductors

We report a comprehensive investigation of the incorporation of H2 molecules in semiconductors. We consider five different materials (Si, GaAs, InAs, GaP and GaN), and use a density-functional-pseudopotential approach. Combined with calculations for H2 in vacuum, these results enable us to examine trends and develop an understanding of the physics of incorporation of a strongly bound molecule in a semiconducting environment. We find that the vibrational frequency of the interstitial molecules is shifted down significantly compared to the free molecule. We also investigate anharmonicity in the molecular vibrations. The results confirm a recent assignment of Raman lines to interstitial H2 in GaAs, but contradict an assignment claiming that the frequency for H2 in Si is close to the free-molecule value. Finally, we calculate and discuss diffusion of H2 molecules through a Si crystal.