The Influence of Annealing in Nitrogen Atmosphere on the Electrical, Optical and Structural Properties of Spray-Deposited ZnO Thin Films

Large area and highly uniform polycrystalline ZnO thin films have been produced by a spray pyrolysis (SP) technique resorting to a customized system (spraying) on glass substrates at temperature T-s = 450 degrees C. This study deals with the related investigation about the influence of heat treatment (in nitrogen atmosphere) on the resulting properties (electrical, optical and structural) of such films. Properties are analyzed by means of I-V plots, transmittance curves, X-Ray diffractograms (XRD) and scanning electron microscope (SEM) micrographs. Results show that the resistivity of the films decreases from about 200 W. cm for the as-deposited films to about 95 W. cm for annealed films. XRD diffractograms reveal that the films are polycrystalline with a hexagonal wurtzite structure. The preferential orientation (002) does not change with annealing, but some very weak lines appear, as witnessed by the XRD patterns. According to such patterns and SEM images there is no noticeable increase in the grain size. The transmittance of the films increases as a result of annealing beside a slight increase in the bandgap energy. From the analysis of the Urbach tail at the absorption edge, the width of the tail of localized states E-e is 1.084 eV for the as-deposited film and 1.126 eV for the annealed film. The large values of E-e depend on localized states resulting from oxygen defects that increase with annealing.