Influence of annealing on optical properties and surface structure of ZnO thin films

Thermal annealing effects on optical properties and surface structure of zinc oxide (ZnO) thin films prepared by filtered cathodic vacuum arc technique were investigated by spectroscopic ellipsometry (SE) and atomic force microscopy (AFM). In the SE study, Cauchy model was used to extract the optical constants of ZnO thin films for photon energies below the band gap while point-by-point fitting was used to determine the optical constants of the films for photon energies above the band gap. The influence of annealing on the optical properties, in the photon energy ranging from 1.1 to 5 eV, has been demonstrated. It was found that the values of the refractive index, the reflectance and the real part of the complex dielectric function decrease with increasing annealing time. On the other hand, tapping mode AFM was used to study the surface structure and topography of ZnO thin films. AFM study revealed that annealing roughened the surface of the films and increased the size of grains on the surface. It was observed that the changes in the optical properties were correlated to the changes in the surface structure as a result of annealing.