Correlation between changes in nanoscale structural and optical properties upon swift heavy ion irradiation of SiNx thin films

Abstract

This work reports the correlation of changes in structural and optical properties of a-SiNx:H thin films irradiated with 100 MeV Au8+ at various fluences. Cross-sectional TEM studies show the evolution of discontinuous ion tracks with a radius of 3 nm as the nanoscale structural changes in films. Spectroscopic ellipsometry was used for the extraction of different optical parameters, viz., refractive index, extinction coefficient, Urbach tail energy, and bandgap along with thickness and roughness by fitting the experimental data and modeling the dielectric function with the Cody–Lorentz model. An increase in the Si-N phase was evident, with an increase in the bandgap. Change in the Urbach energy and broadening of oscillator parameters suggest an increased disorder and introduction of defect states upon irradiation. The Wemple–Dedomenico dispersion model was used to deduce dispersion energy to establish a change in thin films' density upon irradiation. The change in the density of thin films is further correlated to the increase in the refractive index. The evolution of various parameters reveals the correlation between changes in optical properties with structural properties down to nanoscale upon swift heavy ion irradiation on a-SiNx:H thin films.