Structural and electrical conductivity measurement of zinc oxide thin film prepared by sol-gel dip coating method

Abstract

Zinc oxide (ZnO) exhibits a hexagonal wurtzitic structure with a wide direct energy band gap of 3.37 eV. Furthermore, the ZnO thin films have low resistivity and high transparency in visible region and thus can be used as the fundamental layer in the fabrication of optoelectronic devices and as the transparent electrodes in solar cell panel. Undoped ZnO thin films were deposited onto corning glass substrates via the sol-gel dip coating method with annealing temperature and solution concentration being varied. The films were prepared under the annealing temperatures and solution concentrations ranging from 350 to 550 oC and 0.10 M to 0.30 M respectively. The effects of lowering the annealing temperature and solution concentration onto the characteristics of ZnO films were studied. The prepared sol-gel ZnO films can be used as a part of fundamental platform for the synthesis of novel ZnO nanostructures such as nanorods (1D) and nanowires (2D). The starting material in the form of zinc acetate dehydrates was dissolved into a mixing solution consisting of isopropanol and diethanolamine which acts as the solvent and stabilizer respectively. The thickness of the films has been measured by a surface profilometer. The structural properties of sol-gel ZnO films were characterized by X-Ray diffractometer (XRD) and atomic force microscope (AFM). Meanwhile, the film conductivity level was determined by Van der Pauw resistivity measurement method. The XRD spectra revealed that the prepared sol-gel ZnO films were polycrystalline with the hexagonal wurtzite structure and had the preferential growth along (002) c-axis orientation. The ZnO films prepared at 0.30 M concentration and annealed at 450 °C showed the highest crystallinity among all the samples. The grain size and the root mean square roughness of the ZnO films increased with the increase of annealing temperature and solution concentration. Furthermore, the biggest grain size of 66.60 nm was observed for ZnO films prepared with 0.30 M concentration and annealed at 550 °C. The thickness of ZnO films annealed at 450 °C increases from 63 to 260 nm as the solution concentration increases from 0.10 to 0.30 M. The ZnO films prepared with 0.30 M concentration and annealed at 350 °C showed the lowest conductivity value of 2.49 × 10-4 S/cm with the activation energy Ea = 27 meV. Meanwhile, the highest film conductivity level was observed for the ZnO film prepared at 0.30 M concentration and annealed at 450 °C with value of 1.29 ×10-2 S/cm with Ea = 95 meV.