Structural interpretation of the vibrational spectra ofa-Si: H alloys

The ir and Raman spectra of a-Si: H alloys produced by plasma decomposition of SiH₄ are studied for a wide range of deposition conditions. The vibrational spectra display modes which can be characterized as predominantly hydrogen motions. Analysis of these modes shows four types of local Si-H bonding environments which are identified as SiH, SiH₂, SiH₃, and coupled SiH₂ or (SiH₂)ₙ units. On the basis of these identifications, it is found that samples produced on high-temperature (above 200^∘{}C) substrates have SiH, SiH₂, and (SiH₂)ₙ groups with very little SiH₃. In contrast, the ir and Raman spectra of samples produced on room-temperature or cooled substrates are dominated by vibrational modes of SiH₃ and (SiH₂)ₙ. The relative concentrations of these hydrogen-containing groups are not simply proportional to the total hydrogen concentration in a given sample.