Thin films of carbon nitride have been realized in a downstream RF plasma beam discharge generated in argon/nitrogen with carbon electrodes. The deposition has been performed on the top of a-Si:H or a-SiC:H layers previously deposited in diode type CVD reactor from methane and silane. The surface topography produced by the deposition procedures has been studied by atomic force microscopy (AFM) technique. The obtained samples have been investigated by secondary ions mass spectrometry (SIMS), Fourier transform infrared spectroscopy (FTIR) and UV-Vis optical absorption. It is shown that the formation of SiC and SiCN interlayers occurs, which is promoted by the reactions of plasma C, N, and CN gaseous species with the silicon atoms from the base film.