Reactive sputtered ZnO thin films: Influence of the O2/Ar flow ratio on the oxygen vacancies and paramagnetic active sites

Zinc oxide thin films were prepared under oxygen-deficient and oxygen-rich conditions by changing the oxygen to argon ratio (O-2/Ar) during the reactive sputtering deposition at room temperature. The effects of increasing the partial pressure of oxygen in the sputtering gas from 20 to 70% O-2/Ar on the thin film composition, crystallinity and defects that can act as active sites for gas reactions were studied using X-rays diffraction, X-rays photoelectron spectroscopy (XPS), Raman scattering and electron paramagnetic resonance (EPR). All the films exhibited a textured (0002) wurtzite phase and a crystallite size that increases as the partial pressure of oxygen in the sputtering gas increases. The XPS analysis showed that the number of oxygen vacancies decreases as the oxygen percent in the sputtering gas increases. The Raman spectra of the samples contained a band related to a hydroxide, OH, bond in addition to the vibrational modes associated with the wurtzite structure. A strong EPR signal, consistent with the OH acting as a paramagnetic center, was detected in all the films. An additional, but very weak EPR peak, was observed in the film grown at 20% O-2/Ar, which was assigned to singly ionized oxygen vacancies located in the crystallite lattice. These paramagnetic centers are highly reactive because of their unpaired electrons and their formation will have important effects on the physical and chemical properties of the thin films.