Site-dependent structural instability at the surroundings of the Si-H bond in hydrogenated amorphous silicon based wide-gap alloys

A real-time change in the absorption strength of the Si-H stretching vibration in hydrogenated amorphous silicon (a-Si:H), so-called modulated IR-absorption, is observable when a-Si:H is illuminated with modulated band-gap light at low-temperature. This IR-absorption change should reflect a change in a structural configuration at the local surroundings of the Si-H bond, since the band-gap light does not directly affect the localized vibrational mode. We have utilized this phenomenon as a sensor for detecting the real-time structural instability in the vicinity of the bonded hydrogen. Following a previous study on a difference in the structural instability between the surroundings of Si-H and Si-H-2 bonds in updoped a-Si:H [ H. Oheda, Phys. Rev. B 68, 085206 (2003) ], the present study extends the investigation to a-Si:H-based wide-gap alloys, such as a-SiN:H and a-SiO:H. In particular, it is concerned with how the structural instability around the Si:H bond is affected by a change in the structural configuration at the local surroundings of the Si:H bond due to neighboring of alloy atoms. It is found that the magnitude of the modulated IR-absorption, or the structural instability, is reduced as the number of neighboring alloy atoms increases. An examination of the spectral difference between the modulated IR-absorption and the corresponding normal IR-absorption shows that the structural instability takes place more effectively at the specific site where the Si-H bond is surrounded by two or three nearest-neighbor Si atoms. The change in strength of the local strain around the Si-H bond, due to the neighboring of alloy atoms, is a major cause of the site-dependent structural instability in the wide-gap alloys.