Germanium carbon (Ge1-xCx) films were prepared by radio frequency (RF) reactive magnetron sputtering of a pure Ge (1 1 1) target in a CH4 + H-2 + Ar mixture and their composition, chemical bonding, optical properties were investigated as a function of substrate temperature and gas flow ratio of H-2/(CH4 + Ar). The results show that the deposition rate decreased almost linearly with increasing the substrate temperature and gas flow ratio. And the optical gap of the Ge1-xCx films decreased from 1.5 to 0.9 eV due to the drop in the carbon content of the films as the substrate temperature was increased with the relative content of Ge-C bonds decreasing. The optical gap first decreased and then increased as the gas flow ratio was increased from 0 to 1.125. Through the analysis of XPS spectrum, it is found that the formation of Ge-C bonds in the films was promoted both by low substrate temperature and by high atomic hydrogen content in the working gas mixture near the growth surface. Especially in high H-2 concentration, the optical gap shows an abnormal increase due to the increase of the formation of sp(3)-hybridized C and high concentration of C-H bonds. (C) 2012 Elsevier B.V. All rights reserved.