Effect of Bi addition on optical properties of Se–Te–Bi films

Abstract

The present paper reports the optical properties of Se80−xTe20Bix (0 ≤ x ≤ 12) films. Melt quenching method has been employed to prepare amorphous bulk samples, and thermal evaporation method has been used to deposit films of the samples under the vacuum of 10−5 torr. The structural characterization of films was carried out using X-ray diffraction and transmission electron microscopy. The X-ray diffraction studies showed that samples are amorphous in nature, while surface morphology results indicated the formation of nanorods of diameter 20–100 nm. The absorption and transmission spectra were recorded in the wavelength range 400–2500 nm, and data were used to determine refractive index, thickness of film, extinction coefficient and band gap of film samples. It was found that refractive index increases from 1.76 to 1.94 on increase in Bi wt% in the samples. Extinction coefficient also shows the same trend. This is due to increased polarizability of the larger Bi atoms. Further, the results show that energy band gap decreases on increase in Bi concentration in Se–Te–Bi nanostructured system. The value of Eg for x = 0 is 1.43 eV, while for x = 12, it decreases to 0.27 eV. The decrease in band gap is due to increase in the number of localized states, which eventually narrows the optical band gap. The values of band tail width (Ee) were also found to increase from 0.064 to 0.460 on Bi addition to Se–Te system. The small value of band gap and extinction coefficient makes these nanostructured films suitable for nano-electronic devices and anti-reflection multi layers.