Carbon nitride thin films have been grown by the microwave plasma-enhanced chemical vapor deposition (,MW-PECVD) technique. Gas mixtures es containing CH4, H-2 and NH3 at various ratios were tested as precursors, and Si (100) wafers were used as substrates. X-ray photoelectron spectroscopy (SPS), Auger electron spectroscopy (AES), electron microscopy (both SEM and TEM), and Raman spectroscopy have been employed to characterize the resultant films. The phase contents in the films were found to be strongly dependent on the substrate temperature. The incorporation of significant amounts of Si into the film was observed when the substrate temperature exceeded 1000 degrees C. However, the presence of Si along with high substrate temperature also promotes the formation of large crystallites. XPS analyses of C(1s) and N(1s) core levels suggest a multiple bonding structure between carbon and nitrogen atoms. Microscopic investigations of the films reveal the coexistence of large grain (> 10 mu m) and fine grain (< 1 mu m) crystals. Preliminary structural studies suggest the presence of a crystalline carbon nitride compound corresponding to a hypothetical alpha-C3N4 phase (isomorphic to alpha-Si3N4), which may also be a stable hard material. Furthermore, we propose that some of the Si has be en incorporated as a substitutional element for the C site in the new phase. The Raman spectra exhibit man sharp lines, of which the most distinct ones mimic those of the alpha-Si3N4 structure,