Amorphous silicon-carbon alloys: a promising but complex and very diversified series of materials

Amorphous silicon-carbon alloys, with widely different proportions of silicon and carbon, can be prepared in various ways, in particular by chemical vapour deposition (CVD) or by pyrolysis of silicated organic polymers. The literature on disordered SiC is very abundant with an extremely wide range of reported properties. This diversity is due to the admixture, in variable proportions, of sp(2) and sp(3) local structures of carbon. This is rather unfortunate because-if the sp(3) structure (diamond-like) tends indeed to give a wide gap material-the admixture of sp(2) bonds (graphite-like, with a zero optical gap) results in a material with an overall gap which is often too small for optoelectronic applications in the visible. One notable exception is that of amorphous Si-C alloys prepared by CVD at low power. In this regime, the power delivered to the plasma is below the threshold of primary decomposition of the CH4 precursor. Carbon is, therefore, incorporated as methyl groups -CH3, thus forcing spa hybridisation in the solid. The optical gap can then reach high values, allowing electroluminescent devices emitting visible light. However, since the Si-C network is not allowed to relax to the more stable sp(2)-sp(3) admixture, this material is more strained than the low-gap standard material. (C) 2001 Elsevier Science B.V. All rights reserved.