Experiments presented in this work are a part of an extended study that examines the possibility of fabrication of oxynitride layers for future Si and SiC MOS structures by nitrogen implantation from RF plasma and subsequent plasma oxidation process. In order to avoid analysis of more complex Sic MOS devices, at this stage, the experiments were performed using silicon substrates. The obtained layers were characterized by means of ellipsometry, XPS and SIMS. The results of electrical characterization of MOS test structures fabricated with investigated layers used as gate dielectric, are also discussed.