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 Si${\mathrm{H}}_4$ 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, Si${\mathrm{H}}_2$, Si${\mathrm{H}}_3$, and coupled Si${\mathrm{H}}_2$ or ${\left(\mathrm{Si}{\mathrm{H}}_2\right)}_n$ units. On the basis of these identifications, it is found that samples produced on high-temperature (above 200°C) substrates have SiH, Si${\mathrm{H}}_2$, and ${\left(\mathrm{Si}{\mathrm{H}}_2\right)}_n$ groups with very little Si${\mathrm{H}}_3$. In contrast, the ir and Raman spectra of samples produced on room-temperature or cooled substrates are dominated by vibrational modes of Si${\mathrm{H}}_3$ and ${\left(\mathrm{Si}{\mathrm{H}}_2\right)}_n$. The relative concentrations of these hydrogen-containing groups are not simply proportional to the total hydrogen concentration in a given sample.