Effect of dehydrogenation on optical constants of silicon nitride thin films

Abstract

This study presents annealing experiments conducted on near-stoichiometric hydrogenated amorphous silicon nitride (a-SiNx:H) thin films deposited via Plasma Enhanced Chemical Vapor Deposition. The heat treatment was carried out under an inert atmosphere within a temperature range of 400–1000 °C, with increments of 100 °C. After each annealing step, a-SiNx:H films were subjected to characterization using visible and FTIR spectroscopies, evaluating their optical and structural properties, as well as hydrogen concentration. The infrared analysis reveals a significant reduction in Si–H bond concentration within the temperature range of 400–700 °C, becoming virtually undetectable at higher annealing temperatures. Conversely, the N–H bond concentration primarily decreases at elevated temperatures, persisting at 23% of its initial value at 1000 °C. Optical constants were determined from transmittance spectra using a dispersion model that combines two unbounded Tauc-Lorentz oscillators. The refractive index experiences an increase up to 600 °C due to material densification, followed by a decrease at higher temperatures attributed to optical gap widening. Dispersion curves of the extinction coefficient reveal a wide sub-gap absorption band, which vanishes after a 900 °C annealing step, rendering the material suitable for waveguide applications.