An optical study of vacuum evaporated Se 85−xTe 15Bi x chalcogenide thin films

Abstract

Thin films of Se 85− x Te 15Bi x ( x=0, 1, 2, 3, 4, 5) glassy alloys prepared by melt quenching technique, are deposited on glass substrate using thermal evaporation technique under vacuum. The analysis of transmission spectra, measured at normal incidence, in the spectral range 400–1500 nm helphelps us in the optical characterization of thin films under study. Well -known Swanepoel's method is employed to determine the refractive index ( n) and film thickness ( d). The increase in n with increasing Bi content over the entire spectral range is related to the increased polarizability of the larger Bi atom (atomic radius 1.46 Å) compared with the Se atom (atomic radius 1.16 Å). Dispersion energy ( E d ), average energy gap ( E 0) and static refractive index ( n 0) isare calculated using Wemple–DiDomenico model (WDD). The value of absorption coefficient ( α) and hence extinction coefficient ( k) hashave been determined from transmission spectra. Optical band gap ( E g ) is estimated using Tauc's extrapolation and is found to decrease from 1.46 to 1.24 eV with the Bi addition. This behavior of optical band gap is interpreted in terms of electronegativity difference of the atoms involved and cohesive energy of the system.