The optical properties of tungsten trioxide thin films prepared by DC magnetron sputtering, withdifferent oxygen vacancy (Vo) concentration, have been studied by spectrophotometry andphotoluminescence (PL) emission spectroscopy. Absorption and PL spectra show that the filmsexhibit similar band gap energies, Eg 2.9 eV. The absorption spectra of the films show twopronounced absorption bands in the near-infrared region. One peak (P1) is located atapproximately 0.7 eV, independent of Vo concentration. A second peak (P2) shifts from 0.96 eV to1.16 eV with decreasing Vo concentration. Peak P1 is assigned to polaron absorption due totransitions between tungsten sites (W5þ!W6þ), or an optical transition from a neutral vacancystate to the conduction band, Vo0!W6þ. The origin of peak P2 is more uncertain but may involveþ1 and þ2 charged vacancy sites. The PL spectra show several emission bands in the range 2.07 to3.10 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 oxygenvacancy sites, with dominant contribution from neutral and singly charged vacancies in the lesssub-stoichiometric films, and additional contributions from doubly charged vacancy sites in themore sub-stoichiometric films.
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