Optical and other physical characteristics of Ge–Se–Cd thin films

Amorphous Ge₂₀Se_80−xCd_x thin films with different compositions (x = 0, 2.5, 5, 7.5 and 10 at.%) were deposited onto glass substrates by thermal evaporation. The reflection spectra, R(λ), of the films at normal incidence were obtained in the spectral region from 400 to 2500 nm. Based on the use of the maxima and minima of the interference fringes, a straightforward analysis proposed by Minkov has been applied to derive the optical constants and the film thickness for the Ge₂₀Se_80−xCd_x thin films. The dispersion of the refractive index is discussed in terms of the single-oscillator Wemple and DiDomenico model. Tauc relation for the allowed non-direct transition describes the optical transition in the studied films. With increasing cadmium content the refractive index increases while the optical band gap decreases. The optical band gap decreases from 2 to 1.5 eV with increasing cadmium content from 0 to 10 at.%. The chemical-bond approach has been applied successfully to obtain the excess of Se–Se homopolar bonds and the cohesive energy of the Ge₂₀Se_80−xCd_x system.