The effects of thermal annealing in vacuum on the bonding structures, optical and mechanical properties for germanium carbide (Ge-1 C-x(x)) thin films, deposited by radio frequency (RF) reactive sputtering of pure Ge(1 1 1) target in a CH4/Ar mixture discharge, are investigated. We find that there are no significant changes in the bonding structure of the films annealed below 300 degrees C. The fraction of Ge-H bonds for the film annealed at temperatures (T-a) above 300 degrees C decreases, whereas that of C-H bonds show a decrease only when T-a exceeds 400 degrees C. The out-diffusion of hydrogen promotes the formation of Ge-C bonds at T-a above 400 degrees C and thus leads to a substantial increase in the compressive stress and hardness for the film. The refractive indices and optical gaps for Ge-1 C-x(x) films are almost constant against T-a, which can be ascribed to the unchanged ratios of Ge/C and sp(2)-C/sp(3)-C concentrations. Furthermore, we also find that the excellent optical transmission for an antireflection Ge-1 C-x(x) double-layer film on ZnS substrate is still maintained after annealing at 700 degrees C. (C) 2008 Elsevier B.V. All rights reserved.