Chemical bond approach to optical properties of some flash evaporated Se_100−XSb_X chalcogenide alloys

¹ Department of Physics, University of Botswana, P/Bag UB-0022 Gaborone, Botswana
² Department of Physics, University of Nairobi, PO Box 30197-00100 Nairobi, Kenya
³ African Materials Science and Engineering Network (AMSEN), a Carnegie IAS-RISE Network, UB-Node, P/Bag UB-0022, Gaborone, Botswana

^a e-mail: sathiara@mopipi.ub.bw

Received: 2 March 2012
Revised: 5 June 2012
Accepted: 6 June 2012
Published online: 11 July 2012

Abstract

Amorphous thin films of Se_{100 − X}Sb_X (X = 1, 5, 10, 15 and 20) were synthesized by flash evaporation of the premelt quenched bulk samples. The optical properties were investigated from spectrophotometric measurements in the UV-VIS-NIR spectral region using Swanepoel’s standard envelope method and related techniques. The optical band gap energy (E_g^opt) was evaluated from the Wemple-Didomenico (WDD) single oscillator model and Tauc’s extrapolation method in the region where the absorption coefficient, α ≥ 10⁴ cm^–1. The observed values of E_g^opt and oscillator energy E_o were found to decrease monotonously with increasing Sb additive. The complex dielectric constant (ε), Urbach energy (E_u), optical conductivity (σ), plasma frequency (ω_p) and lattice dielectric constant (ε_L) were deduced for each alloy composition. The complex refractive index (n) fitted well to Sellmeier function which can allow extrapolation of n outside the measured spectral range. The observed changes in optical parameters with Sb content were explained on the basis of increased defect states and changes in cohesive energy indicators (average heat of atomization (H_s), mean coordination number 〈Z〉 and average single bond energy (Hs/〈Z〉).