Effects of individual layer thickness on the microstructure and optoelectronic properties of sol-gel-derived zinc oxide thin films

Zinc oxide (ZnO) thin films were prepared under different conditions on glass substrates using a sol-gel process. The microstructure of ZnO films was investigated by means of diffraction analysis, and plan-view and cross-sectional scanning electron microscopy. It was found that the preparation conditions strongly affected the structure and the optoelectronic properties of the films. A structural evolution in morphology from spherical to columnar growth was observed. The crystallinity of the films was improved and columnar film growth became more dominant as the zinc concentration and the substrate withdrawal speed decreased. The individual layer thickness for layer-by-layer homoepitaxy growth that resulted in columnar grains was < 20 nm. The grain columns are grown through the entire film with a nearly unchanged lateral dimension through the full film thickness. The columnar ZnO grains are c-axis oriented perpendicular to the interface and possess a polycrystalline structure. Optical transmittance up to 90% in the visible range and electrical resistivity as low as 6.8 x 10(-3).Omega.cm were obtained under optimal deposition conditions.