Detection of defects in cobalt doped danocrystalline ZnO 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 reveal stronge c-axis orientated wurtzite structure. However with the increase in Co doping concentration ( at 5%), an inclination from (0 0 2) to (1 0 1) plane has been observed. This may be due to 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. Self-assembly of particles are clearly displayed in AFM images. 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. In addition to the band edge, absorption bands are also observed for all Co doped samples at 573nm, 606nm and 666 nm, which are attributed to d-d transitions of tetrahedrally coordinate Co 2+ . This is particularly important because Co 2+ doping in ZnO produce a significant red shift in band gap energy and, most importantly, the additional optical absorption bands could extend to the visible range of solar energy.