The effect of post-deposition annealing on the optical properties of filtered vacuum arc deposited ZnO-SnO₂

Zinc stannate (ZnO-SnO₂) thin films were deposited on ultraviolet fused silica (UVFS) substrates using filtered vacuum arc deposition (FVAD). During deposition, the substrates were at 200 and 400 °C. As-deposited films were annealed at 500 and 600 °C in Ar for 50min. The structure was determined before and after annealing using x-ray diffraction (XRD). The XRD patterns of all ZnO-SnO₂ thin films had an amorphous structure. The average optical transmission of the film in the visible spectrum was>80% and was affected by annealing. The films' optical constants in the 250-989nm wavelength range were determined by variable angle spectroscopic ellipsometry (VASE). The refractive indexes of as-deposited and annealed films were in the ranges 1.95-2.35 and 2.0-2.32, respectively. The extinction coefficients of as-deposited annealed films were in the same range, approximately 0-0.5. However, in the UV range (<450nm) the extinction coefficient values decreased significantly for annealed films. The optical energy band gap (E_g) was determined by the dependence of the absorption coefficient on the photon energy at short wavelengths. It varied between 3.65 and 3.72eV for annealed films as a function of deposition pressure. Although the lowest electrical resistivity of zinc stannate films obtained for as-deposited films on 400 °C heated substrates, using 0.93Pa oxygen pressure, was 1.08 × 10^-2Ωcm, highly resistive films (>10⁵Ωcm) were obtained by annealing.