Band structure of carbonated amorphous silicon studied by optical, photoelectron, and x-ray spectroscopy

Hydrogenated silicon-carbon films prepared by glow discharge from silane-methane mixtures at low power density are used as a model system for the analysis of optical properties and band structure of amorphous tetrahedral semiconductors. Between 0 and 20 at. % of carbon in the solid, the optical gap and the static refractive index can be varied over a wide range without changing the chemical structure of the solid and its good semiconducting properties. The optical constants are well described by a simple two-band model of optical transitions, and the information on the band structure in the solid is condensed into one parameter, the average gap EM, which corresponds to the energy difference between the centers of gravity of the valence and conduction bands. This value is very close to the energy spacing of the maxima in the distributions of the conduction and valence band deduced from soft-x-ray spectra, and exhibits a similar increase with carbon content. Beyond a carbon concentration of about 20 at. %, we observe a change in the nature of the material. The average gap EM increases much more rapidly than the optical gap; x-ray photoelectron spectroscopy indicates an incorporation of carbon in the form of Si-C-Si units in a tetrahedral network, whereas for concentrations smaller than 20 at. % the carbon is mainly incorporated as methyl groups CH₃.