The influence of the substitution of Se for Sn on the thermal, optical and dispersion properties of Ge14Se86−xSnx thin films

Abstract

Bulk glasses of Ge14Se86−xSnx (x=0, 6, 10 and 22) were prepared by melt-quench technique. Glass transition temperature (Tg) and thermal stability of the bulk samples were found to decrease with Sn content. Thin films of Ge14Se86−xSnx were deposited onto chemically cleaned glass substrates by thermal evaporation. X-ray diffraction (XRD) patterns of the as-prepared films confirmed its amorphous nature except for the film with x=22at% which is polycrystalline. These results have been supported by scanning electron microscopy (SEM) and atomic force microscope (AFM) investigations. Transmittance and reflectance spectra of the as-prepared films were measured at normal incidence in the wavelength range of 350–2500nm. Refractive index and the film thickness were determined from optical transmittance data using Swanepoel’s 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 as a function of alloy composition. Analysis of the spectral behavior of the absorption coefficient revealed an allowed indirect optical band gap in the investigated films. The optical band gap was found to decrease along with a corresponding increase in the refractive index with the incorporation of Sn. These results have been discussed on bases of the chemical bond approach, electronegativity difference of the atoms, cohesive energy and heat of atomization.