Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering

The optical properties of tungsten trioxide thin films prepared by DC magnetron sputtering, with different oxygen vacancy (Vo) concentration, have been studied by spectrophotometry and photoluminescence (PL) emission spectroscopy. Absorption and PL spectra show that the films exhibit similar band gap energies, Eg ≈ 2.9 eV. The absorption spectra of the films show two pronounced absorption bands in the near-infrared region. One peak (P1) is located at approximately 0.7 eV, independent of Vo concentration. A second peak (P2) shifts from 0.96 eV to 1.16 eV with decreasing Vo concentration. Peak P1 is assigned to polaron absorption due to transitions between tungsten sites (W5+ → W6+), or an optical transition from a neutral vacancy state to the conduction band, Vo0 → W6+. The origin of peak P2 is more uncertain but may involve +1 and +2 charged vacancy sites. The PL spectra show several emission bands in the range 2.07 to 3.10 eV in the more sub-stoichiometric and 2.40 to 3.02 eV in the less sub-stoichiometric films. The low energy emission bands agree well with calculated optical transition energies of oxygen vacancy sites, with dominant contribution from neutral and singly charged vacancies in the less sub-stoichiometric films, and additional contributions from doubly charged vacancy sites in the more sub-stoichiometric films.