The effect of vacancies on the optical properties and plasmonic states of zinc oxide: A first-principle study

We study structural and optical properties of ZnO film prepared by pulsed laser deposition. The film was characterized by x-ray diffraction and UV–visible spectroscopies. We find that the as-grown film is mostly amorphous. After hydrogen-annealing treatment, the film is crystallized with lattice parameters of a = 3.241 Å and c = 5.203 Å. From UV–visible spectra, the low absorption edges of 1.44 and 1.43 eV are observed in the as-grown and annealed films, respectively, suggested to be promoted by some vacancies. Then, the generalized gradient approximation method is used to calculate electronic and optical properties of ZnO0.94, and Zn0.94O systems as the possible models of the annealed film. Properties ZnO is also calculated as the reference. Bandgaps of 0.75 and 1.73 eV are obtained for ZnO and ZnO0.94, respectively. The larger bandgap of ZnO0.94 is caused by the increase of Fermi level induced by Zn 4s electrons, leading to the n-type semiconducting behaviour. On the other hand, Zn0.94O exhibits the p-type metallic behaviour caused by the decrease of Fermi level induced by O 2p electrons with a minimum interband transition (ΔE) of 0.95 eV. Then, a shift of ΔE is applied in the optical properties calculation for approaching the experimental results. From the imaginary part of dielectric function (ε2(E)) for xy plane, ΔE of ZnO and ZnO0.94 systems are 3.30 and 4.10 eV, respectively. The optical dichroism of ZnO0.94 is smaller than that of ZnO. On the other hand, ΔE of Zn0.94O is 1.80 eV based on ε2(E) for z axis indicating the optical dichroism flip by the Zn vacancy in ZnO. The low absorption edge of the annealed film is promoted by the Zn vacancy. Furthermore, plasmonic-state energy levels in ZnO can be tuned by the O or Zn vacancies. This study shows the essential properties of ZnO for potential high-energy plasmonic device applications.