Energetics and Vibrational Frequencies of InterstitialH2Molecules in Semiconductors

The incorporation of ${\mathrm{H}}_2$ molecules on interstitial sites in various semiconductors is investigated using a density-functional-pseudopotential approach. The vibrational frequency of the interstitial molecules is shifted down compared to the free molecule. The results confirm a recent assignment of Raman lines to interstitial ${\mathrm{H}}_2$ in GaAs [Vetterhöffer et al., Phys. Rev. Lett. 77, 5409 (1996)], but contradict the conclusion of Murakami et al. [Phys. Rev. Lett. 77, 3161 (1996)] that the frequency for ${\mathrm{H}}_2$ in Si is close to the free-molecule value. The interaction between interstitial molecules and the host is elucidated, and the significant weakening of the H-H bond is correlated with the semiconductor host charge density near the interstitial site.