Optical characterization of filtered vacuum arc deposited zinc oxide thin films

ZnO thin films, 100-250 nm thick, were deposited on microscope glass slides and UV fused silica (UVFS) substrates using a filtered vacuum arc deposition (FVAD) system, operating at room temperature (RT) and 200 A for 60 s. The cathode was prepared from 99.9% pure Zn metal and the oxygen background pressure during deposition was in the range 0.67-0.93 Pa. The films were characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), optical transmission and spectroscopic ellipsometry. As-deposited ZnO films were found to be polycrystalline with c-axis orientation. The atomic concentration ratio of Zn to O in the film as determined by XPS analysis was stoichiometric. Film transmission in the visible was 70-90%. The maximum and minimum values of the refractive index n and the extinction coefficient k in the visible, for all samples, were 2.23 to 1.90 and 0.6 to approximately 0, respectively. The type of inter-band electron transition was found to be direct transition with optical band gap in the range of 3.25-3.42 eV.