Photoluminescence and optical properties of plasma-deposited amorphous Si_<i>x</i>C_1–<i>x</i> alloys

Abstract

Abstract Results on the photoluminescence, optical properties and photoconductivity of amorphous Si : C : H alloys prepared from the plasma decomposition of silane and ethylene are presented. Films deposited at substrate temperatures of 30 and 300°C have been studied with compositions ranging from 100 to 10% silicon. The photoluminescence spectra measured at 77 and 300 K show considerable discrepancies with respect to previously reported results. The emission is in the form of a single band which broadens and shifts towards higher photon energies as the carbon concentration is increased. The highest emission-peak energy of 2·05 eV is achieved for samples with 90 at.% C. The temperature quenching of the luminescence is reduced with increasing carbon concentration. For 90% C specimens there is less than a factor of 2 reduction in emission intensity between 77 and 300 K. As a consequence visible (yellow–orange) room-temperature luminescence can be clearly observed with the naked eye. The luminescence efficiency is minimum in films of composition near Si0·5C0·5 but increased for both carbon- and silicon-rich alloys. The photoconductivity in hydrogenated a-SiC decreases rapidly as the carbon concentration is increased and there is also a pronounced broadening of the absorption edge indicative of a wider range of localized tail states. The results are discussed in terms of models proposed for a-Si : H. The relative influence of phonon-coupling and disorder on the shift and line-width of the luminescence spectra is considered. It is suggested that the increase in the threshold quenching temperature observed in C-rich alloys is predominantly a consequence of a stronger Coulomb interaction.