Hydrogenated amorphous silicon-germanium-carbide (a-SiGeC:H) thin films have been fabricated by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) using monomethyl germane (MMG, GeH(3)CH(3)) as the germanium source. It was found that C incorporation into the films was considerably suppressed under high hydrogen dilution (R(H)) and substrate temperature (T(S)). Under high R(H) and T(S), we were able to narrow the optical band gap (E(opt)) of a-SiGeC:H thin films to 1.39 eV, whereas under low R(H) and T(S), E(opt) increased up to 1.9 eV with increasing MMG. The best electrical properties were obtained for a sample of a-Si(0.68)Ge(0.29)C(0.03):H, whose E(opt) and photosensitivity were 1.58 eV and 10(5), respectively. When this film was used as an absorber layer in a p-i-n structured solar cell, significant enhancements were observed in its quantum efficiency for long wavelengths. (C) 2010 Elsevier B.V. All rights reserved.