Annealing effects on the bonding structures, optical and mechanical properties for radio frequency reactive sputtered germanium carbide films

The effects of thermal annealing in vacuum on the bonding structures, optical and mechanical properties for germanium carbide (Ge1−xCx) 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 °C. The fraction of Ge-H bonds for the film annealed at temperatures (Ta) above 300 °C decreases, whereas that of C-H bonds show a decrease only when Ta exceeds 400 °C. The out-diffusion of hydrogen promotes the formation of Ge-C bonds at Ta above 400 °C and thus leads to a substantial increase in the compressive stress and hardness for the film. The refractive indices and optical gaps for Ge1−xCx films are almost constant against Ta, which can be ascribed to the unchanged ratios of Ge/C and sp2-C/sp3-C concentrations. Furthermore, we also find that the excellent optical transmission for an antireflection Ge1−xCx double-layer film on ZnS substrate is still maintained after annealing at 700 °C.