This work initiates a series of reports aimed at a construction of rugate optical filters based on silicon rich materials of alternating gradients of refractive index n with the help of plasma enhanced chemical vapor deposition (PECVD) technique, The idea is to start deposition of high refractive index SixNy type of material using hexamethyldisilazane (HMDSN) vapor and nitrogen rich atmosphere, and then to gradually replace nitrogen with oxygen in that atmosphere in order to lower n down to a minimum characteristic of SixOy type of material. A return to initial gas composition should increase the index back to its maximum. In the present work, thin SixNyCz films were synthesized from a mixture of HMDSN vapor with gaseous NH3 and N2. The effect of NH3/N2 ratio on the coating morphology, its elemental composition, chemical bonding and optical properties was studied using scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, ultra-violet absorption spectroscopy and variable angle spectroscopic ellipsometry. The results show that films of the highest index of refraction and the lowest extinction coefficient have been deposited from the gas mixture containing 90 % of ammonia. These coatings are also characterized by the lowest carbon and the highest nitrogen contents.