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 [(SiH2)(n)] groups and structural inhomogeneities (due to the presence of voids) of an amorphous silicon network has been tested in hydrogenated amor phous silicon films prepared by rf magnetron sputtering (RF MS). Comparativ e infrared (IR) absorption, Raman spectroscopy and optical transmission mea surements have been performed on samples deposited under the same plasma co nditions at various substrate temperature (T). The results of the IX studie s show that all samples exhibit the same hydrogen content incorporated as i solated monohydride (SiH) groups responsible for the absorption band at 200 4 cm(-1). As a consequence, the gradual decrease in the total bonded hydrog en content as T increases is essentially due to the decrease in the amount of hydrogen bonded as polyhydride [(SiH2)(n)] groups which absorb around 20 86 cm(-1). A direct relationship is established between the changes in the IR spectra and the variation of structural inhomogeneities observed by Rama n spectroscopy. The linewidth in the low-frequency side of the transversal acoustic (TA) -like band in the Raman spectra decreases monotonously as T i ncreases. This decrease is attributed to a decrease in density of quasiloca lized vibrational modes in structural inhomogeneities (disordered domains a round the surface of voids) in the amorphous network. The good correlation between the IR and Raman results indicates that the (SiH2)(n) groups are lo cated in structural inhomogeneities due to the presence of voids in the amo rphous network where the formation of these complexes is favorable. This in terpretation is corroborated by the variation of both the static refractive index, which gives information about the density of material, and the disp ersion energy, which measures the mean coordination number of Si atoms and the effective number of valence electron per Si atom in the amorphous netwo rk. [S0163-1829(99)02608-9].