Photoluminescence of tetrahedrally coordinateda-Si1−xCx:H

Abstract

We have measured photoluminescence spectra of a series of hydrogenated amorphous silicon-carbon alloys a-${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{C}}_{\mathit{x}}$:H (0<x<0.4) prepared by plasma-enhanced chemical-vapor deposition from ${\mathrm{SiH}}_4$/${\mathrm{CH}}_4$ mixtures. The power delivered to the plasma during the depositions was below the threshold of primary decomposition of ${\mathrm{CH}}_4$ (‘‘low power regime’’). Carbon in the samples is mostly in the form of -${\mathrm{CH}}_3$ groups, keeping its ${\mathit{sp}}^3$ hybridization from the gas in the solid. These samples are tetrahedrally coordinated in the sense that they do not have ${\mathit{sp}}^2$ 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.