Thermal induced depletion of cationic vacancies in NiO thin films evidenced by x-ray absorption spectroscopy at the O 1s threshold

The effects of thermal annealing on the concentration of cationic vacancies in p-type semiconducting NiO thin films have been studied by x-ray absorption spectroscopy at the O 1s threshold. This technique proves to be very sensitive to the amount of Ni vacancies through the intensity of a prepeak observed below the absorption threshold, associated with Ni ions in a high oxidation state. Samples with different vacancy concentrations were obtained by radio frequency magnetron sputtering with different O₂/Ar ratios in the plasma. Thermal effects have been studied both during thin film growth and after postprocessing annealing. In both cases, the observed effects were very similar, showing a depletion of cationic vacancies with temperature. By changing the surface sensibility of the x-ray absorption spectroscopy measurements, the authors could find out that the transition to stoichiometric NiO begins at the surface.