The Effect of Annealing Above Glass Transition Temperature on the Optical Properties of Se85Te10Bi5 Thin Films

Se85Te10Bi5 films have been deposited using the thermal evaporation technique. Films with different thicknesses in the thickness range 590.2-273.9 nm were annealed at different annealing temperatures above the glass transition temperature for 120 min. The structure of the annealed films was checked by x-ray diffraction analysis, which indicated a polycrystalline nature for all annealed films, and that the degree of crystallinity increased with increasing annealing temperature. From the reflectance (R) and transmittance (T) measurements, the values of the optical absorption coefficient (alpha) for the annealed films were estimated to be in the wavelength range of 500-2500 nm. Analysis of the absorption coefficient data reveals allowed indirect transitions and the values of optical band gap (E (g)). The values of (E (g)) were found to be obeying the Tauc's relation and decreasing with increasing annealing temperature. This behavior is discussed as due to thermal disordering with the structural changes upon annealing. Optical parameters such as lattice and the infinite frequency dielectric constant as (epsilon (L) and epsilon (a)), plasma frequency (omega (p)), carrier concentration to the effective mass ratio (N/m*), single- oscillator and dispersion energies (E (o) and E (d)) were found. The dependence of the optical parameters on the annealing temperature was studied and discussed.