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 z 103 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. UVeViseNIR 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 104 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. |
