Dispersion and optical properties of thermally deposited Se91 − xTe9Px (x = 0, 10) films

Se91Te9 and Se81Te9P10 bulk glasses have been prepared by melt-quench technique. The amorphous nature of the as-prepared alloys is confirmed by using X-ray diffraction patterns. Thin films of Se91Te9 and Se81Te9P10 are prepared on chemically cleaned glass substrates by thermal evaporation under vacuum. Differential scanning calorimetery revealed a higher glass transition temperature for Se91Te9 in comparison to Se81Te9P10 bulk glasses. Transmission spectra of the as-prepared films are measured at normal incidence in the wavelength range of 300-2500 nm. Refractive index and the film thickness were determined from optical transmittance data using Swanepoel envelope method. It was found that the refractive index dispersion data obeys the single oscillator Wemple and DiDomenico model from which the dispersion parameters were determined. The single oscillator energy, the dispersion energy, the high frequency dielectric constant and the ratio of free charge carrier concentration to the effective mass are then estimated as a function of alloy composition. Analysis of the spectral behavior of the absorption coefficient revealed an allowed indirect optical band gap in this alloy system. With phosphorus incorporation, the optical band gap is found to decrease along with a subsequent increase in the refractive index. The ensemble of these results is interpreted in terms of the chemical bond approach, electronegativity difference of the atoms, cohesive energy and overall mean bond energy.