Single-phase, single-crystal, epitaxial Ge1-xCx films were deposited on Si(100) substrates with C concentration as high as 9.0 at. %. The film composition was measured by energy dispersive x-ray spectroscopy. The x-ray diffraction analysis, in combination with Vegard's law, shows a different composition than what is obtained by the energy dispersive x-ray spectroscopy, indicating that Ge1-xCx alloys show a positive deviation from Vegard's law. Both the film thickness and the deposition temperature had a strong influence on the structure of the film. At low temperatures, an amorphous film was obtained. The crystalline structure pf the films was also strongly dependent on the thickness of the film. Films thicker than 60 nm were polycrystalline. At temperatures higher than 800 degreesC and for thicknesses around 50 nm, good-quality single-crystal, epitaxial films were obtained. An energetic condensation process was crucial for incorporating such a high C concentration in Ge. Near-edge x-ray absorption fine structure analysis results showed that the majority of C was in the diamond-like state, which was direct evidence that the C is situated on substitutional sites.