Influence of thermal annealing on optical and structural properties change in Bi-doped Ge30Se70 thin films

The effect of annealing on the structural and optical properties of thermally evaporated Ge30Se70 and Ge30Se60Bi10 thin films is reported in this paper. The prepared films were thermally annealed at 250 degrees C to optimize the optical properties which can be used for the optical device fabrication. X-ray diffraction study revealed no structural transformation whereas the surface morphology changed as observed from scanning electron microscopy. The optical properties of the deposited and annealed films have been investigated by using a UV-VIS-NIR spectrophotometer in the wavelength range of 400-1100 nm. The optical band gap of the Ge30Se70 annealed film is found to be increased while the energy gap of the Bi-doped annealed Ge30Se60Bi10 thin film decreased which is explained by the chemical disorderness, defect states and density of localized states in the mobility gap. The Tauc parameter and Urbach energy which measure the degree of disorder changed with the annealing process. The transmittivity increases and the absorption power decreases in the Ge30Se70 annealed film, whereas the reverse effect is noticed for the annealed Ge30Se60Bi10 thin film. The irreversible nature of this change can be useful for optical recording purposes.