Optical constants of vacuum annealed radio frequency (RF) magnetron sputtered zinc oxide thin films

Zinc oxide thin films were deposited by radio frequency magnetron sputtering. The films were annealed in vacuum at temperatures of 400, 600, and 800 °C. The influence of annealing on the structural, chemical, and optical properties of the films was investigated. From a structural point of view, the films were highly oriented, with columnar microstructure. Chemical analysis indicated that the films were sub-stoichiometric, and that the concentration of oxygen vacancies was enhanced upon annealing. The films were highly transparent in the visible and near-infrared spectral regions. Transparency was reduced as the annealing temperature was increased. The refractive index and extinction coefficient, in the transparent regions of the films, were derived from transmittance measurements. The refractive index manifested variation that was affected by crystallite size, roughness and defect concentration. The extinction coefficient of the as-deposited films and those annealed at 400 °C was negligible. However, the films annealed at 600 or 800 °C had much larger values of the extinction coefficient due to increased absorption or scattering. The absorption coefficient and optical band gap of the films were derived from spectrophotometric measurements. The absorption coefficient showed progressive increase with the annealing temperature. However, the band gap did not show significant variation.