Observation by infrared transmission spectroscopy and infrared ellipsometry of a new hydrogen bond during light-soaking of a-si: H

Evidence for participation of atomic hydrogen in the light-induced metastability mechanism of hydrogenated amorphous silicon (a-Si: H) has been observed by low-temperature infrared transmission spectroscopy and room-temperature infrared phase-modulated ellipsometry. A band at 1730cm^-1accompanied by an increase of the amplitude of the bending mode or a new broad band at - 870 cm^-1was observed after 1 h of intense illumination of an annealed a-Si: H sample. A shift to lower frequencies of this band is observed with prolonged light-soaldng time and the band disappears after - 4 h of illumination. At the same time the density of atomic hydrogen bonded to silicon decreases, as inferred from the area under the broad band at - 640 cm^-1. Our results can be interpreted by a hydrogen diffusion model in which the hydrogen atom of a Si-H bond next to a weak Si-Si bond diffuses to form a three-centre bond at intermediate illumination times, then diffuses further to form a stable Si-H bond at longer times.