Electrical and structural properties of Co-doped ZnO thin films

The temperature dependent conductivity behavior of 15% Co and 25% Co-doped ZnO nanocrystalline thin films prepared by spin-coating method was examined. It was found that the conductivity shows a change when the Co concentration varies from 15% to 25%. The observed increase of conductivity with increasing Co concentration was interpreted through the grain boundary conduction model. The temperature dependent of conductivity of the films was analyzed in term of formulas in consistence with grain boundary conduction model. With rising temperature for 25% Co doped ZnO, a transition from the region in which crystallites are only partially depleted to the region in which the crystallites are entirely depleted was observed around 375 K. Some important electrical parameters were determined for the films. XRD and XPS studies evidenced that Co entered in ZnO lattice as Co2+. Co doping determined an increase in lattice parameter and grain size. The optical band gaps of the films have been determined by evaluating the fundamental absorption coefficient from transmittance spectra. It was found that the band gap energy of films decreases with increasing Co concentration.