OPTICAL-PARAMETER STUDIES OF AS-DEPOSITED AND ANNEALED SE1-XTEX FILMS

Thin films of Se1-xTex (x = 0.2, 0.4, 0.6) are deposited on a glass substrate by thermal evaporation under vacuum. The optical gaps (E(g)) are determined from the absorbance and transmittance measurements in the visible and near IR spectral range (500-1100 nm), and are found to decrease with the tellurium concentration. After annealing at different elevated temperatures (423 K and 473 K), the values of optical gaps of Se0.8Te0.2 films, as a representative example, are also found to increase with temperature from 1.91 eV at room temperature to 1.97 eV at 473 K. This effect is interpreted in terms of the density-of-state model of Mott and Davis. The optical constants (extinction coefficient k, and refractive index n) were also determined. It was found that both n and k depend markedly on composition as well as the temperature of heat treatment. The valence band density of states of Se1-xTex films is calculated from the optical absorption data.