Local Si-H bonding environment in hydrogenated amorphous silicon films in relation to structural inhomogeneities

The hypothesis of the relation between the local environment of polyhydride $\left[\right({\mathrm{SiH}}_2{)}_n]$ groups and structural inhomogeneities (due to the presence of voids) of an amorphous silicon network has been tested in hydrogenated amorphous silicon films prepared by rf magnetron sputtering (RF MS). Comparative infrared (IR) absorption, Raman spectroscopy and optical transmission measurements have been performed on samples deposited under the same plasma conditions at various substrate temperature (T). The results of the IR studies show that all samples exhibit the same hydrogen content incorporated as isolated monohydride (SiH) groups responsible for the absorption band at 2004 ${\mathrm{cm}}^{\mathrm{-}1}$. As a consequence, the gradual decrease in the total bonded hydrogen content as T increases is essentially due to the decrease in the amount of hydrogen bonded as polyhydride $\left[\right({\mathrm{SiH}}_2{)}_n]$ groups which absorb around 2086 ${\mathrm{cm}}^{\mathrm{-}1}$. A direct relationship is established between the changes in the IR spectra and the variation of structural inhomogeneities observed by Raman spectroscopy. The linewidth in the low-frequency side of the transversal acoustic (TA) -like band in the Raman spectra decreases monotonously as T increases. This decrease is attributed to a decrease in density of quasilocalized vibrational modes in structural inhomogeneities (disordered domains around the surface of voids) in the amorphous network. The good correlation between the IR and Raman results indicates that the $({\mathrm{SiH}}_2{)}_n$ groups are located in structural inhomogeneities due to the presence of voids in the amorphous network where the formation of these complexes is favorable. This interpretation is corroborated by the variation of both the static refractive index, which gives information about the density of material, and the dispersion energy, which measures the mean coordination number of Si atoms and the effective number of valence electron per Si atom in the amorphous network.