Effect of annealing on the optical properties of amorphous Se₇₉Te₁₀Sb₄Bi₇ thin films

Thin films of Se₇₉Te₁₀Sb₄Bi₇, were prepared by Electron beam deposition technique. The structure of the as-prepared and annealed films has been studied by X-ray diffraction and the surface morphology by the scanning electron microscope (SEM). These studies show that there is a gradual change in structure and the formation of some polycrystalline structures in the amorphous phases is observed when the Se₇₉Te₁₀Sb₄Bi₇ film is annealed in the temperature range of 333–393 K. The optical transmission of these films has been studied as a function of photon wavelength in the range 300–2500 nm. It has been found that the optical band gap E_g^opt decreased with increasing annealing temperature in the range 333–393 K. The Urbach energy (E_u), optical conductivity (σ_opt), imaginary (ε_i), and real (ε_r) parts of the complex dielectric constant (ε) and lattice dielectric constant (ε_L) were also determined. The changes noticed in optical parameters with increasing annealing temperature were explained on the basis of structural relaxation as well as change in defect states and density of localized states due to amorphous-crystalline transformation.