Ultrasonically sprayed ZnO:Co thin films: Growth and characterization

Abstract In this work, undoped and cobalt-doped ZnO thin films were deposited at 275 ± 5 °C on glass substrates by the ultrasonic spray pyrolysis technique. The structural, electrical, optical and surface characterization of the films as a function of the cobalt concentration in the spraying solution were studied by means of x-ray diffractometer, current–voltage characteristics, UV–vis spectrophotometer and atomic force microscope, respectively. X-ray diffraction reveals that the films are polycrystalline in nature with preferred orientations of (0 0 2) for the ZnO:Co (0, 2, 4 at.%) and (1 0 0) for the ZnO:Co (6 at.%). The optical transmittance of all films was studied as a function of wavelength in the range of 300–1100 nm. They exhibit high transparency in the visible wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical band gap and Urbach energy values of the films were found in the range of 3.250–3.301 eV and 90–230 meV, respectively. The electrical studies for all films were carried out by using conductivity-temperature measurements and it was seen that the electrical conductivity of ZnO films decreases slightly depending on the increasing of Co doping. Also, Co doping increases both energies of donor-like traps and activation energy for ZnO films. The surface morphology was analyzed by atomic force microscope and a strong dependence on the cobalt incorporation was found.