Effect of thermal annealing on the optical properties of Ge₂₀Se₆₅S₁₅ thin films

Abstract

The present work reports the effect of heat treatment under vacuum on the optical properties of Ge₂₀Se₆₅S₁₅ thin films prepared by the thermal evaporation technique. The structural units and the surface morphology of the films were characterized using Raman spectroscopy and scanning electron microscopy (SEM), respectively. The optical parameters of as-deposited and annealed films were calculated using the Swanepoel's method from the optical transmission spectra $T\left( \lambda \right)$ in the wavelength range of 300–2500 nm. The variations of Urbach energy E_U and optical energy gap $E_{\text{g}}^{\text{Opt}}$ with the annealing temperature are discussed in terms of the density states model in amorphous solids. The refractive index dispersion is analyzed and discussed in terms of Sellmeier and Wemple–DiDomenico models. The nonlinear susceptibility $\chi^{\left( 3 \right)}$ and nonlinear refractive index n⁽²⁾ were evaluated based on the generalized Miller’s rule as a function of annealing temperature. Lastly, the two-photon absorption coefficient (β_c) was obtained theoretically via $E_{\text{g}}^{\text{Opt3}}$ scaling law.