Study of Tauc gap, optical density and penetration depth of vacuum evaporated Pb15Se85−xGex (x = 0, 3, 6 at. %) thin films supported by chemical bond approach and physical parameters

Abstract

The present study deliberates the optical properties of vacuum evaporated Pb15Se85−xGex (x = 0, 3, 6 at. %) thin films. The absence of sharp peaks in XRD hints about the non-crystalline nature. Transmission spectra (at normal incidence) are acquired in the spectral region of 400–2400 nm. A blue-shift in the transmission spectra hints about the increase in the band-gap. Experimentally, the band-gap is calculated using Tauc-extrapolation method. The band-gap transition is in-direct and is found to increase with the substitution of Se by Ge in the glassy alloy. The optical density and penetration depth are calculated using the absorption coefficient data. The penetration depth increases with the increase in Ge-content and the cut-off wavelength shows a blue shift. The obtained optical properties are correlated with the physical parameters. Considering the topological model, it is revealed that the system shows a transition from floppy to the intermediate region. Lone pair electron value is greater than 3 and that favours the glass formation. The distribution of bonds in the present system is also examined using chemical bond approach (CBA) model and the system’s cohesive energy (CE) is also calculated. The increase in cohesive energy shows that the average stabilisation energy of the system is increasing. This also reflects the increased value of experimentally calculated optical band-gap. The average heat of atomization also shows an increase with Ge addition. Conduction band potential seems to shift away from the Fermi level. The results are discussed based on the change in bond energy, bond length, total system energy and topological models applied on the system.