Optical properties of Se80Te20 thin films during amorphous-crystalline transition

Thin films of amorphous Se80Te20 alloys with thicknesses ranging from 200 to 300 nm were prepared by vacuum deposition. The as-grown films were annealed in argon atmosphere at different temperatures for different periods of time. The transition from amorphous to crystalline state has been characterized through different degrees of crystallinity using techniques of x/ray and electron diffractions. The optical constants for all films, viz., refractive index (n), extinction coefficient (k), absorption coefficient (alpha) and band gap (E-g) were calculated from the transmission spectra recorded in the wavelength region 400-1900 nm. For the amorphous films, the refractive index increases in the U-V region, but it remains almost constant in the annealed films. The extinction coefficient for the amorphous films remains constant in the transparent region, then decreases slightly in the visible region and then increases towards the U-V region. In the annealed thin films, the extinction coefficient decreases towards the smaller wavelength. The absorption coefficient increases towards the smaller wavelength in the amorphous thins films, whereas in annealed thin films, it decreases. The variation in physical properties seems to indicate applications in switching devices.