Optical properties of recent chalcogenide-alkali metal Se80Te8(NaCl)12 thin film

Abstract

Recent chalcogenide-alkali metal Se80Te8(NaCl)12 semiconducting thin films of 750 nm thickness are deposited onto glass substrates by thermal evaporation method. The effect of the thermal annealing on structure and optical properties are investigated. The amorphous-to-crystalline phase transformation in the powdered sample is achieved by differential scanning calorimetry (DSC) heat treatment. The X-ray diffraction studies show that the as-prepared film is amorphous while the annealed films exhibit crystalline phases. The crystallite grain size decreases as the annealing temperature increases. The transmission spectra are measured in the wavelength range of the incident photons from 250 to 2500 nm. The films show highly transparence reaching about 90% in the visible and infrared regions. The refractive index of the films is calculated using the Swanepoel method based on the generation of the envelopes of the interference maxima and minima of the optical transmission spectrum. It progressively decreases with the incident wavelength representing the behavior of the normal dispersion of the film material. The effect of the annealing temperature on the refractive index is studied. The absorption mechanism of the films referred to indirect allowed transition. Optical band gap ranging from 1.60 to 1.45 eV is favorable for photovoltaics applications. With increasing the annealing temperature, both the optical band gap and the single-oscillator energy decrease while the energy of localized state increase.