Cation substitution induced blue-shift of optical band gap in nanocrystalline Zn(1-x)Cax thin films deposited by sol-gel dip coating technique

Transparent nanocrystalline thin films were deposited on glass substrates by sol-gel dip coating method. The X-ray diffraction (XRD) pattern revealed the polycrystalline nature of the films with hexagonal wurtzite structure and confirmed the non-existence of the secondary phase corresponding to CaO indicating the monophasic nature of the deposited films. The crystallinity of the films deteriorated with higher dopant concentration due to the segregation or separation of dopant ions in grain boundaries. The lattice parameters and the unit cell volume increased to a higher Ca-dopant concentration. This was due to the successful incorporation of ions with larger ionic radius in the host zinc oxide (ZnO) lattice. The optical transmittance spectra of the samples showed transmittances above 60% in the visible spectral range and the absorption edge in the near ultra-violet region got blue-shifted with cation substitution. The estimated optical energy gaps confirmed the band gap widening with increase in Ca-dopant concentration. The calculated values increased from 3.30 eV for undoped ZnO to 3.73 eV for thin films giving 13.03% enhancement in the energy gap value due to the electronic perturbation caused by cation substitution as well as deterioration in crystallinity.