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 su bstrate by thermal evaporation under vacuum. The optical gaps (E(g)) a re determined from the absorbance and transmittance measurements in th e visible and near IR spectral range (500-1100 nm), and are found to d ecrease with the tellurium concentration. After annealing at different elevated temperatures (423 K and 473 K), the values of optical gaps o f Se0.8Te0.2 films, as a representative example, are also found to inc rease 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 mod el 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 h eat treatment. The valence band density of states of Se1-xTex films is calculated from the optical absorption data.