Electrical resistivity and photoluminescence of zinc oxide films prepared by ultrasonic spray pyrolysis

Zinc oxide (ZnO) thin films have been prepared by ultrasonic Spray pyrolysis (USP) technique using zinc acetate dihydrate dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, UV-visible, electrical resistivity, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. The evolution of the preferred crystalline orientations in the ZnO films was systematically investigated. The PL measurements indicated that the as-grown ZnO thin films showed UV and green emission bands when they were excited by a Hg arc lamp using 313 nm as the excitation source. A red-shift in the near band edge (NBE) has been observed with the increase in the substrate temperature and has been attributed to the compressive intrinsic stress present in the films. It is confirmed that oxygen vacancy (V-O) is the most important factor that causes the broad visible emission. Furthermore, the visible emission and electrical resistivity of ZnO thin films are found to be a function of porosity. Also, it has been interestingly found that the intensity of green emission at similar to 2.5 eV remarkably increased when the obtained ZnO films were deposited at 320 degrees C. The reason might be the V-O, intrinsic stress, surface-to-volume ratio and porosity in the sample increased at low substrate temperature. The resistivity follows the same behavior as the intensity of the green emission. We propose a new luminescence mechanism based on the recombination related to oxygen vacancies in Zn-rich or stoichiometric conditions. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim