The effect of annealing on filtered vacuum arc deposited ZnO thin films

ZnO thin films were deposited on UV fused silica (UVFS) substrates using filtered vacuum arc deposition (FVAD). During deposition, the substrates were at room temperature (RT). As-deposited films were annealed at 400 and 600 °C in Ar atmosphere for 50 min. The structure, surface morphology and composition were determined as function of annealing temperature using X-ray diffraction (XRD), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The XRD patterns of as-deposited ZnO films showed a strong c-axis orientation with a (002) peak, and the diffraction line intensity increased with annealing temperature. The average transmission of as-deposited and annealed films in the visible range was 85% to 90%, and affected by interference. The films' optical constants in the 250-989 nm wavelength range were determined by variable angle spectroscopic ellipsometry (VASE). The refractive indexes of as-deposited, and 400 and 600 °C annealed ZnO thin films decreased with the annealing temperature in the visible spectrum, and as function of wavelength, varied from 1.93 to 2.16, 1.89 to 2.09, and 1.77 to 1.96, respectively. The extinction coefficients of as-deposited and annealed films were approximately zero at wavelengths > 400 nm, and increased at lower wavelengths with the annealing temperature up to 0.9 for 600 °C at ∼ 350 nm. The optical band gap (E_g), determined by the dependence of the absorption coefficient on the photon energy at short wavelengths, was in the range 3.21-3.27 eV. The lowest electrical resistivity was 1.1 × 10^{- 2} Ω cm, obtained on as-deposited films. The annealing greatly increased the resistivity.