Effect of hydrogen (H-2) dilution of the Silane (SiH4), acetylene (C2H2) gas mixture during the deposition of hydrogenated amorphous silicon carbon alloy (a-SiC:H) filins by Cat-CVD process shows that the H-2 dilution induced additional carbon incorporation, leading to an increase of the carbon content in the films from 52% to 70% for the maximum H-2 dilution employed. A slight increase in graphitic carbon in the films deposited with H-2 dilution is also observed. A drastic increase in the optical band gap E-g from 2.5 eV for zero dilution to 3.5 eV is observed for a H-2 dilution of 10 seem. Raman spectra for the films deposited with increasing H-2 dilution indicate structural changes in the amorphous network associated with increasing graphitic carbon. (c) 2005 Elsevier B.V. All rights reserved.