a-SiC:H films with energy gap in the range 2.00–2.65 eV have been grown by plasma enhanced chemical vapor deposition in undiluted andH2dilutedSiH4+CH4gas mixtures, by making use of optimized deposition conditions. A complete picture of structural, compositional, optoelectronic, and defective properties for high quality films has been drawn for the first time. We show that the addition ofH2to the gas mixture leads to a different chemical composition of the deposited films; in particular, carbon incorporation is enhanced and a carbon fraction in the solid matrix up to C/(C+Si)≈0.45 can be obtained. These films have a higher mass density, a reduced microvoid and carbon cluster concentration, a better structural connectivity, and improved optoelectronic properties. For samples with optical gap below 2.4 eV, the reduced defect concentration ofH2diluted films results in an increase of the photoconductivity gain and the steady-state(&eegr;&mgr;&tgr;)ssvalues up to two orders of magnitude. ©1997 American Institute of Physics.