Physical properties of zinc doped tin oxide films prepared by spray pyrolysis technique

The structural and optical properties of pure and zinc doped tin oxide (SnO2) thin films have been studied in detail. Thin films of pure and zinc doped tin oxide (Zn : SnO2) were prepared on Si(100) and quartz substrates by the spray pyrolysis technique at a substrate temperature of 400 degrees C. The zinc dopant concentration was varied from 0 to 25 wt%. The films were systematically characterized by different methods to understand their structural and optical property variations, and the results were correlated. The x-ray diffraction (XRD) method shows that pure SnO2 films possess tetragonal crystalline structure with the preferred (110) orientation. Upon increasing the zinc concentration the preferred orientation changes from the (110) plane to the (200) plane, and at the same time the crystalline quality was found to be deteriorated. The Raman measurements also confirm the tetragonal structure of the films for the entire range of Zn doping. High resolution scanning electron microscopy measurements reveal that upon increasing the Zn concentration, the surface morphology of the films changes continuously and the grains also deteriorate. The elemental analysis of the films measured by energy dispersive XRD spectroscopy shows that the Zn concentration in the solid film is slightly less than that of the starting solution. Optical transmittance measurements of the films reveal that the films are fully transparent in the visible region. Upon increasing the Zn concentration, the band gap of the films decreases from 3.85 to 3.57 eV. Thus, the structural study performed by XRD and Raman spectroscopy clearly indicates the incorporation of Zn ion into SnO2 lattice, and the change in the optical properties of the films was directly attributed to the effect of Zn ion incorporation into SnO2 lattice.