The growth of amorphous hydrogenated germanium carbide (a-GeCx:H) alloys was performed with high deposition rates by radiolysis chemical vapor deposition (X-ray) of germane/allene (GeH4/C3H4, 70/30 %) mixtures at different irradiation times. The experimental deposition parameters were correlated to the composition, the structural features, and the optical coefficients of the films, as studied by different spectroscopic techniques, namely, IR, Raman, and UV-Vis. It was observed that the increase of irradiation time yields a more hydrogenated and more disordered material, with abundant formation of sp(3) CH2 groups, characterized by high band-gap values. In addition, we report the effects of thermal annealing on bonding structures and optical properties of the amorphous germanium carbon alloys. The decrease of hydrogen extent, together with the enhancement of sp(2) C bonds present and amorphous-to-crystalline germanium phase transition, contribute to a larger structural order of the material and to the reduction of the optical gap at higher temperatures.