Compositional dependence of the optical parameters for Bi5GexSe65−x (30 ≤ x ≤ 45) films

Different compositions of Bi5GexSe95−x (x = 30, 35, 40 and 45 at %) thin films were deposited onto cleaned glass substrates by thermal evaporation method. The structural characterization revealed that, the as-prepared films of x = 30, 35 and 40 at. % are in amorphous state but there are few tiny crystalline peaks of relatively low intensity for the film with x = 45 at. %. The chemical composition of the as-prepared Bi5GexSe65−x films has been checked using energy dispersive X-ray spectroscopy (EDX). The optical properties for the as-deposited Bi5GexSe65−x thin films have been studied. The additions of Ge content were found to affect the optical constants (refractive index, n and the extinction coefficient, k). Tauc’s relation for the allowed indirect transition is successfully describing the mechanism of the optical absorption. It was found that, the optical energy gap (Eg) decreases with the increase in Ge content. These obtained results were discussed in terms of the chemical bond approach proposed by Bicermo and Ovshinsky. The composition dependence of the refractive index was discussed in terms of the single oscillator model.

► Present work deals the incorporation the optical properties of Bi5GexSe95−x thin films.
► The allowed indirect electronic transitions are responsible for photon absorption.
► The optical band gap Eg was found decrease with the increase Ge contents.
► It was found that E0 decrease while Ed and ɛL increase with increase in Ge content.
► This trend was discussed in terms of the chemical bond approach.