The “Acoustic Local” Mode of H, D, and F in Amorphous Germanium and Silicon

The quasi‐local mode of H in a‐Ge and a‐Si associated with the TA‐phonon branch is studied as a function of temperature of measurement and annealing. Isotopic substitution experiments are also performed. The results confirm the mass defect model recently proposed for this local mode. A similar local mode is observed for a‐Si:F, without hydrogen. It is much stronger than the corresponding mode of a‐Si:H for the same dopant concentration. Likewise in a‐Si:F the TO phonon band, as observed in ir absorption, is stronger than in pure and hydrogenated a‐Si and appears shifted to 510 cm−1. This shift, which is not seen in the Raman spectrum, is attributed to the strong polarity of the Si‐F bond.