Effect of thickness on the structure and properties of ZnO thin films prepared by pulsed laser deposition

ZnO thin films with various thicknesses were prepared onto glass substrates by pulsed laser deposition. The crystallinity, microstructure and surface morphology of ZnO thin films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and scanning probe microscopy (SPM). As the thickness of the films increased, the grain size and the surface roughness increased as well the tensile stress decreased. Also, to study the optical-electrical properties of the films, we tested the carrier concentration, mobility, resistivity, impedance spectroscopy and optical transmission spectra. The results showed that the carrier concentration gradually increased as the thickness increased from 20 to 106 nm, thereafter decreased as the thickness further increased to 216 nm. The main contribution to the conduction of the films with thickness from 20 to 106 nm gradually converted from grain boundaries to grains, and converted into grain boundaries again for the films with a thickness of 216 urn. As the thickness of the films decreased, it was found that the blue shift of absorption edge of optical transmission spectra, Effects of the thicknesses on the optical and electrical properties for ZnO films were related to their grain size, stress and carrier concentration.