The influence of increase in flow rate of CH4 in a source gas mixture of SiH4 + CH4 + H-2 for the preparation of high-quality wide band gap a-SiC : H film by the RF PECVD method has been studied. We have been able to increase the optical gap of the film by increasing CH, flow rate under appropriate deposition conditions. These films are structurally better which also shows good opto-electronic properties. This has been achieved mainly by using CHn (where n = 3, 2 or 1) precursors in the plasma as the etchant for weak bonds on the growing surface of a-SiC : H films.