Band-gap engineering, conduction and valence band positions of thermally evaporated amorphous Ge15-x Sbx Se50 Te35 thin films: Influences of Sb upon some optical characterizations and physical parameters

Quaternary chalcogenide Ge15-xSbxSe50Te35, (0.0 <= x <= 15.0, at. %) thin films are prepared using the thermal evaporation process under the residual vacuum pressure of approximate to 10(-3) Pa. The rate of deposition and thicknesses of the film samples are kept constant at 10 nm/s and 200 nm, respectively. X-ray diffraction is employed to examine the amorphous or crystalline structure of films. The absence of sharp and distinct peaks confirms the amorphous nature. The compositional element ratio of all prepared samples is analysed by energy-dispersion X-ray spectroscopy technique. UV-Vis-NIR absorption spectra are used to study the optical parameters of thin films. The values of the absorption coefficient for all film samples are found to be higher than 10(4) cm(-1). The optical band-gap energy is determined both experimentally and theoretically. The band gap is due to the indirect electronic transitions, and the band gap values are found to decrease from 1.047 eV to 0.864 eV with the increase in Sb-ratio. The band-tail width increases from 0.103 eV to 0.258 eV. The optical density, skin effect and extinction coefficient increase with an increase in Sb-content. The positions of the conduction and valence bands, as well as the conduction potentials of all samples, are also evaluated. The Fermi level shifts closer to the valence band and away from the conduction band with increasing Sb content. The chemical bond approach model is applied to study the cohesive energy and average-coordination number of these a-Ge-Sb-Se-Te films. The total number of constraints, floppy modes, cross-linking density, the number of the valence electrons and the number of lone-pair electrons are also investigated. All discussed parameters are found to be strongly dependent on the Sb-content. (C) 2019 Elsevier B.V. All rights reserved.