Effect of cobalt doping on microstructural and optical properties of nickel oxide thin films

In this study, transparent thin films of cobalt-doped nickel oxide (CoxNi1−xO; x=0, 0.005, 0.01, 0.05, 0.075 and 0.15) were deposited on to microscopic glass substrates using a spray pyrolysis technique. The effect of cobalt doping on structural, morphological and optical properties was investigated. XRD studies reveal that all the films are polycrystalline with cubic structure and exhibit (111) preferential orientation. Co is well incorporated in the host lattice, i.e. octahedrally coordinated on the Ni site without altering the structure. The effect of Co doping was observed to have a strong influence on the surface morphology of NiO films. An interesting correlation between the optical transmittance and the RMS roughness was observed. All the coatings retain high transparency throughout the visible spectral regime. The optical band gap varies from 3.44 eV to 3.26 eV due to the presence of Ni vacancies and/or oxygen defects. The optical reflectance spectra along with refractive index and extinction coefficients of the prepared films have also been discussed. Among all the Co-doped NiO thin films in this study, films doped with 1 at% Co concentration exhibited the best properties, namely improved crystallinity, smooth and compact surface morphology, lowest RMS roughness value of 2.26 nm and highest transmittance of ~85% in the visible region.