PHOTOLUMINESCENCE OF TETRAHEDRALLY COORDINATED A-SI1-XCX-H

We have measured photoluminescence spectra of a series of hydrogenated amorphous silicon-carbon alloys a-Si1-xCx:H (0<x<0.4) prepared by plasma-enhanced chemical-vapor deposition from SiH4/CH4 mixtures. The power delivered to the plasma during the depositions was below the threshold of primary decomposition of CH4 (''low power regime''). Carbon in the samples is mostly in the form of -CH3 groups, keeping its sp(3) hybridization from the gas in the solid. These samples are tetrahedrally coordinated in the sense that they do not have sp(2) carbon. They have higher gap and are more strained than ordinary ''high-power'' alloys with corresponding carbon contents. The results indicate that for low carbon concentrations (including pure a-Si:H) the photoluminescence spectra are determined by static disorder only, electron-phonon effects being negligible. The effective disorder for radiative recombination is higher than the disorder probed by optical absorption. For higher carbon contents, high room-temperature luminescence efficiences (of the order of that of a-Si:H) with very small temperature dependence are found. This is interpreted as due to the enhancement of a fast excitonlike recombination process.