Silicon oxynitride thin films are of a great interest for technological applications in the optical domain due to the possibility of tailoring their properties by changing the stoichiometry of the deposit. Indeed, these materials are transparent in the visible range and their refractive index can be varied from one of silicon oxide (1.46) to that of silicon nitride (2.02) by only acting on the oxygen and nitrogen concentrations.
The aim of this work is to determine the optical properties (refractive index, extinction coefficient, optical gap and Urbach energy) of amorphous silicon oxynitride thin films (a-SiOxNy) and study their variation as function of the film composition and structure. Layers with composition varying between those of Si3N4 and SiO2 were obtained by sputtering a silicon target under different argon-oxygen-nitrogen gas mixtures. Ellipsometry measurements (λ = 632.8 nm) showed that the refractive index varies quite linearly with the film composition following the equation: n = 1.94 – 0.56 [O/(O+N)] where O and N are respectively the oxygen and nitrogen concentration in the deposit. Spectroscopic ellipsometry analysis revealed a constant refractive index and a negligible extinction coefficient over all the visible range. The ellipsometric characteristics Is and Ic fitted using the Tauc-Lorentz dispersion model, combined with structural analysis, showed a surface oxidation of all the samples and an interface inhomogeniety of layers with intermediate composition. Also, the optical gap and Urbach tails were determined by UV-visible spectroscopy and were both found to increase with increasing the oxygen content in the deposits.
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