Silicon oxynitride films were deposited by plasma-enhanced chemical vapor deposition at low temperatures using nitrous oxide (N2O) and silane (SiH4) as gas precursors. The influence of the N2O/SiH4 flow ratio (varied froth 0.25 up to 5) and the thickness of the films on the optical and structural properties of the material was analyzed. The films were characterized by ellipsometry, Fourier-transform infrared spectroscopy, Rutherford backscattering spectroscopy and optical absorption. Two distinct types of material were obtained, silicon dioxide-like oxynitrides SiO2-xNy and silicon-rich oxynitrides SiOxNy (x+y<2). The results demonstrate that in silicon dioxide-like material, the nitrogen concentration can be adequately controlled (within the range 0-15 at.%) with total hydrogen incorporation below 5 at.% and no appreciable Si-H bonds. It is also shown that the composition remains uniform through the entire thickness of the films. Furthermore, a linear relation between the refractive index and the nitrogen concentration is observed, which makes this material very attractive for optoelectronic applications. On the other hand, silicon-rich material is similar to amorphous silicon, and presents an increasing concentration of Si-H bonds, increasing refractive index and decreasing optical gap, which makes it promising for applications in light-emitting devices.