Photoinduced structural instability around the Si-H bond in undoped hydrogenated amorphous silicon

When hydrogenated amorphous silicon (a-Si:H) is illuminated at low temperature by a modulated band-gap light, a photoinduced bleaching band, which is superimposed on a usually observed photoinduced absorption spectrum, can be observed around 2000 cm-1. The observation of such a photomodulated ir-absorption band shows that a real-time change in the absorption strength of the Si-H stretching vibration takes place in accordance with the modulated excitation. Since the Si-H stretching vibration is not affected directly by the band-gap light and is strongly localized in a-Si:H, the observed change in the vibronic absorption should be caused indirectly by some structural instability of local environment of the Si-H bond. Therefore, the localized Si-H oscillator can serve as a detector of a dynamic structural change in its vicinity. Preliminary measurements of the photomodulated vibrational absorption have been performed with undoped a-Si:H prepared at different substrate temperatures (T-S), in which bonded hydrogen takes on two different configurations, namely, monohydride (Si-H) and dihydride (Si-H-2). Irrespective of T-S, it is found that the modulated vibrational absorption band is dominated by the 2000-cm-1 component due to the Si-H vibration. This disclosure includes even those cases where the 2100-cm-1 component due to the Si-H-2 vibration dominates the usual ir-absorption band of the Si-H stretching mode. It follows that the structural instability takes place more effectively in the neighborhood of the Si-H bond than it does in the neighborhood of the Si-H-2 bond. It will be argued that such a site-dependent structural instability is concerned to the nonuniformity of the local strain in the disordered a-Si:H network.