Thin films of amorphous germanium-nitrogen (a-GeNx) alloys were prepared by the reactive sputtering method. The germanium target was sputtered with argon and nitrogen gases in radio-frequency (rf) magnetron sputtering equipment. Changing the composition of the sputtering gas can adjust the composition, x, of the film in the range from 0 to 1.5 and the corresponding optical energy gap in the range from 0.68 to 2.7 eV. The structures of the film are discussed on the basis of measured X-ray photoelectron spectroscopy and Fourier transform infrared spectrometry. The film prepared at an rf power of 200 W includes more defects and structural disorder compared to film prepared at an rf power of 20 W.