Photoinduced structural instability in a hydrogenated amorphous silicon-germanium alloy with high structural inhomogeneity

We studied photoinduced structural instability around the Si-H bond in hydrogenated amorphous silicon-germanium alloy (a-SiGe:H) using modulated-infrared (IR) absorption, which is a real-time change in the absorption strength of the Si-H stretching vibration following a modulated band-gap excitation. We found in the modulated IR-absorption spectrum, that a band expected around 2000 cm(-1), based on the absorption-coefficient spectrum of the Si-H stretching vibration, is missing. An intense modulated IR-absorption band is present at about 2000 cm(-1) in the spectra of materials such as undoped a-Si:H and its alloys with N or O, which are characterized by a continuously connected, disordered network with a built-in local strain. Thus, the absence of the 2000-cm(-1) spectral component is interpreted as a sign of the absence of structural instability in the vicinity of the isolated Si-H bond in a-SiGe:H that has a fragmented network structure. The modulated IR-absorption band that is still observed in a-SiGe:H is found to be closely related with the 2030-cm(-1) constituent component of the IR-absorption band of the Si-H stretching mode in this alloy. Studies on the 2030-cm(-1) component present in the IR-absorption band of the Si-H stretching mode in polymorph- and microcrystalline-Si make us infer that structural instability in a-SiGe:H takes place in the vicinity of the H-induced platelets.