Elaboration et caracterisation des couches minces d’oxyde de zinc et sulfure de zinc preparees par spray ultrasonique

Abstract

The present work deals with the preparation and the characterization of zinc oxide (ZnO) and zinc sulfide (ZnS) thin films by ultrasonic spray. Our goal is to carry a comprehensive study on the effect of the deposition parameters (substrate temperature and the nature of the precursor solution (zinc acetate and zinc nitrate) for ZnO, molarity and the nature of the precursor solution (zinc nitrate and zinc chloride) for ZnS) on the physical properties of zinc oxide and zinc sulfide thin films. X-ray diffraction (XRD ) analysis reveals that at low temperature the films are composed of nanocrystallites with a privileged orientation depending on the precursor nature;these orientations are along the plane (100) for the films deposited with zinc acetate and along (002) for films prepared with zinc nitrate precursor. However, with increasing deposition temperature, the privileged orientation disappears. The scanning electron microscopy (SEM) observations suggest that the liquid droplets react once they reach the substrate surface when zinc acetate precursor is used. However, in the case of zinc nitrate solution, the droplets spread first on the substrate surface followed by the subsequent surface reaction. The surface tension of the starting solution controls the shape of the droplet impact and thereafter the film microstructure. The ZnS thin films have an amorphous structure at 300°C. The growth rate is closely related to the sulfide molarity and to the nature of the starting solution. It increases with sulfide molarity for both used solutions. MEB and optical microscope surface observations reveal that the increasing in sulphide molarity yields to denser and rougher films regardless the used precursors. The RMS value increase with an increase in sulfide molarity for the two precursors. The analysis of the transmittance spectra in the visible range allows us to deduce the optical band gap and disorder of ZnS thin films. The transparency in our films was more than 70% for the two precursors and absorption edge shifts towards blue with the decreasing of the molarity of sulfide from 0,05 to 0.02mol/l is shown for Zinc chloride. The electrical conductivity of zinc oxide and zinc sulfide thin films was deduced from transport measurement using two probes coplanar structure.