The optical parameters of γ-irradiated and annealed thin films of Ge₁₅Se₅₀Te₃₅

The influence of annealing and γ-irradiation on the optical and structural parameters of Ge₁₅Se₅₀Te₃₅ films synthesized by thermal evaporation technique was investigated. The samples were characterized using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), and Energy Dispersive X-ray Analysis (EDX) techniques. XRD analysis showed that the as-prepared films are amorphous, while the annealed ones are polycrystalline. The presence of the crystalline particles in the annealed films is attributed to Se and GeSe₂ phases, which have a monoclinic and an orthorhombic lattice structure, respectively. The observation of glass transition, at 465 ± 1 K, and crystallization peaks, at 625 ± 1 K, in DSC curve confirms the glassy as well as the amorphous nature of the studied composition. On the other side, the analysis of the absorption spectra in the wavelength range (200–900 nm) of annealed and γ-irradiated Ge₁₅Se₅₀Te₃₅ thin films reveals that the allowed indirect transitions are responsible for inter-band transitions. The indirect band gap of Ge₁₅Se₅₀Te₃₅ films was found to decrease from 1.44 to 1.21 eV and from 1.44 to 1.27 eV with increasing the annealing temperature and irradiation dose, respectively. Other optical parameters such as the extinction coefficient and the refractive index were calculated, which depend on thermal annealing and γ-irradiation. These results are interpreted in terms of the structure defects formed in the investigated annealed and irradiated films. - Highlights: • The Ge₁₅Se₅₀Te₃₅ thin films were prepared by thermal evaporation technique. • The amorphous state of investigated thin films was confirmed by XRD. • XRD and SEM analysis of annealed films reveal the appearance of crystallites. • The heat treatment as well as the irradiation affected the optical parameters. • The γ-irradiation resulted in a decrease in the band gap of the studied films