Hydrogenated amorphous silicon carbon alloys (a-Si1-xCx:H) with different carbon contents are deposited by a plasma enhanced chemical vapor deposition (PECVD) system with different hydrogen diluted ethylene (C2H4) concentrations at two power densities of 30 and 90 mW/cm(2). First, the carbon and hydrogen configurations and their relative concentrations in these films are investigated in details by IR spectroscopy. Then, the ultraviolet/visible transmittance spectroscopy and a relevant characterization software are used for finding out the effects of both the carbon content and power density on the refractive index, optical gap and Urbach parameter. This work shows that the refractive index and optical gap may be modulated on purpose by carbon content within the context of hydrogen dilution. However, beyond a critical rf power density, an inhomogencity may occur along the radial direction of the plasma electrode, rendering such a-Si1-xCx:H films useless in practical applications. Moreover, both the evaluated Urbach and slope parameters carry out an increased disorder for carbon rich a-Si1-xCx:H films at high power. (c) 2007 Elsevier B.V. All rights reserved.