Optical and thermal band gap energy of chemically deposited bismuth(III) selenide thin films

The band gap energy of bismuth(III) selenide in thin-film form was determined using the optical and thermal methods. The optical band gap energy of 0.35 eV was calculated on the basis of the recorded optical spectra in the near-infrared region, within the framework of a parabolic approximation for the dispersion relation, using the equations which arise from Fermi's golden rule for electronic transitions from valence to conduction band. From the temperature dependence of the dark electrical resistance of the bismuth(III) selenide thin films in the region of intrinsic and extrinsic conduction, a thermal band gap energy of 0.37 eV and an ionization energy of the donor impurity level of 0.13 eV were calculated. The thermal, as well as the optical band gap energy are in excellent agreement with a literature value for bulk bismuth(III) selenide. On the basis of these data, several conclusions on the film microstructure (nanocrystalline versus glassy) are derived and also an estimation of the higher bound to the Bohr's excitonic radius for bulk Bi2Se3 is given