Infrared spectroscopy of amorphous hydrogenated GaAs: Evidence for H bridges

The infrared absorption spectra of hydrogenated amorphous GaAs show two prominent hydrogen-related bands at 530 and 1460 cm^-1. These bands are very broad and they amount to most of the hydrogen-induced infrared absorption. The remaining structures are a number of comparatively sharp lines which we interpret as Ga-H and As-H modes in partial agreement with earlier investigations. We argue that the broad bands arise from near-stretching (1460 cm^-1) and from wagging (530 cm^-1) vibrations of H atoms situated in bridging positions between two Ga atoms. This assignment is supported by similar bands in Al-H polymers, a-GaP: H, and a-GaSb: H. A model calculation of the mode frequencies is also presented.