Thermal annealing–dependent linear and nonlinear optical parameters in Ge₂₀Se₇₀S₁₀ chalcogenide thin films

In this work, amorphous Ge₂₀Se₇₀S₁₀ thin films were deposited by thermal evaporation and annealed at 373, 423, and 473 K. The morphology of the sample was examined using scanning electron microscopy (SEM) and the presence of component elements in the films under study was confirmed by energy-dispersive X-ray analysis (EDX). Additionally, the structural study was done by X-ray diffraction and Raman spectroscopy. The optical parameters were calculated from transmittance spectra measured in the 200–2500 nm range. Optical transmission spectra showed a blue shift of the absorption edge with increasing annealing temperature. Tauc optical gap increased from 1.88 to 1.96 eV, while Urbach energy decreased from 0.14 to 0.12 eV, reflecting reduced structural disorder. Refractive index analysis using the Wemple–DiDomenico and Sellmeier models demonstrated a decrease in refractive index and dielectric constant with annealing. Third-order nonlinear susceptibility and nonlinear refractive index were calculated using semi-empirical relations. The third-order nonlinear susceptibility decreased from 2.609 × 10^–12 to 1.287 × 10^–12 esu, also the nonlinear refractive index decreased from 4.221 × 10^–11 to 2.237 × 10^–11 esu with annealing. These results highlight the strong influence of thermal processing on the linear and nonlinear optical behaviour of Ge–Se–S thin films, paving the way for their optimiation in optical photonic applications.