An optical study of vacuum evaporated amorphous Ge20Te80−x Bi x thin films using transmission and reflection spectra

Abstract

The present paper reports the effect of Bi addition on the optical behavior (optical band gap and refractive index) of Ge20Te80−xBix (where x=0, 1.5, 2.5, 5.0) glassy alloys by analyzing the transmission and reflection spectra of their thin films in the 900–2400 nm range. Films are deposited on glass substrate using a thermal evaporation technique under vacuum. Various optical parameters viz. refractive index, extinction coefficient, absorption coefficient, optical band gap, etc. are determined and the effect of Bi incorporation on these parameters is studied. The refractive index has been found to increase with increasing Bi content over the entire spectral range and this behavior is due to the increased polarizability of the larger Bi atomic radius (1.46 A) compared to Te atomic radius (1.36 A). Dispersion energy, Ed, average energy gap, E0 and static refractive index, n0 is calculated using Wemple–DiDomenico model. Optical band gap is estimated using Tauc’s extrapolation and is found to decrease from 0.86 to 0.73 eV with the Bi addition. This behavior of the optical band gap is interpreted in terms of the electronegativity difference of the atoms involved and the cohesive energy of the system.