Effect of Bi Content on Optical Properties of Se-Sb-Bi Chalcogenide Amorphous Thin Films

Se90-xSb10Bix (x=0, 2, 4, 6, 8 & 10) chalcogenide glasses were prepared by well-established melt quenching technique. The glassy nature was verified by X-ray diffraction (XRD). Thin films of these samples were deposited on glass substrate using thermal evaporation technique at room temperature. The transmission spectra of thin films have been taken using UV-VIS-NIR spectrophotometer (Varian Cary 500) in the wavelength range 200 nm to 1500 nm. The refractive index and film thickness are calculated by using envelope method proposed by Swanepoel. The results indicate that n increases with the increasing Bi content which is related to the increased polarizability of the larger Bi atomic radius 1.46 Å compared with the Se atomic radius 1.16 Å. The value of absorption coefficient (α) and hence extinction coefficient (k) has been determined from transmission spectra. Optical band gap (Eg) is estimated using Tauc's extrapolation and is found to decrease from 1.46eV 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. The variation of optical band gap with Bi content has been studied. This study is aiming to examine such structures if they are employed as photonic devices such as photo-detectors, LED's and optical switches.