Effect of Bi addition on the optical properties of Ge30Se70-xBix thin films

Optical and structural properties of vacuum evaporated Ge30Se70-xBix (x = 0, 5, 10, 15 and 20) thin films are discussed in the present report. The physical properties such as coordination number (<r>), average heat of atomisation (H-s), average bond energy (H-s/<r>), energy gap (E-g), and lone pair electrons (L) for different Bi concentration were calculated theoretically. The structural property in the film is not disturbed as probed by X-ray diffraction method. The SEM and EDAX were done for the composition and microstructural characterization. The optical transmission is measured by the Fourier Transform Infrared spectroscopy in the spectral range of 500-1200 nm. The mechanism of the optical absorption follows the rule of allowed non-direct transition. It was found that, the optical band gap E-g decreases while the width of localized states (Urbach energy) E-c increases by increasing Bi content. The relationship between Eg and chemical composition of the Ge30Se70-xBix system were discussed in terms of cohesive energy (CE), the average heat of atomisation Hs, and the average coordination number N-r. The variation in the optical behavior for different thin films is explained on the basis of defect states and the decrease in average bond energy of the system. The Raman spectroscopy was done to correlate the optical property change with Bi addition. (C) 2017 Elsevier B.V. All rights reserved.