Determination of optical properties of a-SiOxNy thin films by ellipsometric and UV-visible spectroscopies

Optical properties of silicon oxynitride thin films of variable composition were investigated. Their refractive index was found to decrease linearly from 1.94 to 1.46 when their composition varies between silicon nitride and silicon oxide. Spectroscopic ellipsometry showed that all samples were transparent over the visible range and that the layer structure also influences their optical properties. Using UV-visible spectroscopy, it was found that the film absorption decreases with increasing the oxygen amount in their composition. The optical gap determined from the transmission spectra showed high values increasing with the oxygen concentration. The Urbach energy was also estimated and linked to the film structure and composition.