Silicon (oxy)nitride thin films were deposited on (100) 4″ silicon substrates in a conventional LPCVD reactor at 812°C and 230 mTorr, using mixtures of SiCl2H2 -NH3-N2O of variable composition in order to evaluate their optical properties for potential applications in SOI based Si technology. The depositions were carried out keeping the flow of SiCl2H2 and NH3 constant at 20 and 60 sccm respectively while that of N2O was varying between 0 and 180 sccm. Optical reflection measurements within the range 190 to 860 nm, performed on our films were analyzed within the Forouhi-Bloomer model (Phys. Rev. B34, 7018 (1986)) for amorphous semiconductors. From the analysis of the optical spectra, the constants of the model were derived. It was shown that the refractive index of the films varies between the limits set by those of silicon nitride and silicon dioxide, the increase of the N2O flow implying a decrease of the refractive index of the films. The film thicknesses were also derived from the analysis of the optical spectra. Capacitance - voltage measurements made on Metal-Insulator-Semiconductor (MIS) structures using our films as the insulator, have shown that, the electric charge density within the dielectric initially increases and then decreases with increasing the amount of oxygen into the deposition ambiance. Our films are suitable for optoelectronic applications in SOI structures compatible with C-MOS technology.