Correlation of structural and optical properties in as-prepared and annealed Bi2Se3 thin films

Bi2Se3 bulk alloys were synthesized using a standard mono-temperature melting process and used as source materials to deposit thin films on non-conductive super cleaned glass substrates by a vacuum thermal evaporation technique. Both bulk and thin-film samples were polycrystalline as confirmed by X-ray diffraction patterns. The film samples were subjected to the annealing process in air atmosphere at 250 and 300 ℃ for 3 h, aiming to boost the optical properties, in particular, to enhance the light transmission. The surface roughness, morphology and granular structure were investigated by atomic force microscope and scanning/transmission electron microscopes. The correlation between structural changes due to annealing and optical properties was observed. For a higher annealing temperature of 300 ℃, the coefficient of transmittance increased reaching 80-90% in the IR range. Consequently, the absorption coefficient greatly decreased by annealing, especially for low photon energies. The optical band gap corresponds to the direct transition and increased notably by annealing from 1.51 to 1.83 eV. The spectra of optical parameters including the refraction index, coefficient of extinction and complex-valued dielectric function were deduced from the transmittance/reflection measurements to evaluate the effect of annealing. The energy loss functions were also estimated demonstrating that the volume and surface energy losses are of the same order.