Effect of Co doping on structural, morphological, electrical and optical properties of nanocrystalline zinc oxide films

We report the effect of cobalt doping on the structural, morphological, electrical and optical properties of nanocrystalline ZnO films febricated by chemical spray pyrolysis technique (CSPT). The structural studies reveals that films have strongly c-axis oriented wurtzite structure. However with the increase (at 5%) in Co doping percentage, an inclination from (0 0 2) to (1 0 1) orientation has been observed. This may be due to the fact that, in addition to substitutional sites, Co may start to occupy interstitial sites due to the deformation of lattice structure caused by substitution of nearby Zn atoms by other Co atoms. This will also result in an increase of lattice constant. The films appear to be homogeneous and single-phase material, where cobalt enters the ZnO structure as Co 2+ rather than forming metallic clusters. Self-assembly of particles are clearly displayed in Atomic Force Microscope (AFM) micrograph. The particle size and roughness of the films are found to decrease with the increase in Co doping percentage. Optical transmittance spectra showed red shift at higher doping. A sharp increase in transmittance occurs at 375 nm for all films, which correspond to band edge absorption. In addition to the band edge, absorption bands are also observed for all Co doped samples at 573, 606 and 666 nm, which are attributed to d-d transitions of tetrahedrally coordinate Co 2+ . The absolute strength of these absorption bands increases almost linearly with the increase in Co concentrations. Band gap values are found in the interval between 3.26 to 3.16 eV. Resistance are found to decrease at 15% of Cobalt doping.