The influence of solvent and number of layers on the homogeneity and properties of ZnO thin films obtained by spin-coating method

Homogeneous semiconducting ZnO thin films were successfully produced using spin-coating method. The influence of structure and morphological properties of three-layered and five-layered ZnO films, made with different solvents (methanol and 2-methoxyethanol), on the band gap width was investigated. Scanning electron microscopy and atomic force microscopy analyses clearly showed the wrinkled surface of the ZnO thin films grown from the methanol solution, while the films grown from the 2-methoxyethanol solution were much more homogeneous. X-ray diffraction analysis revealed the hexagonal wurtzite structure of the obtained ZnO films, with preferential crystal growth along the c-axis. As the number of layers increased, the diffraction peaks became sharper and more intense. The crystallite size was calculated to be a few nanometers and gradually increased with the number of layers regardless of the solvent type, although it was slightly smaller for the 2-methoxyethanol solvent. UV-Vis spectra analysis, using the Tauc formula, showed that as the number of layers increased, the band gap width decreased regardless of the solvent choice.