Zinc oxide films prepared by sol-gel spin-coating

Abstract The d.c. electrical conductivity and optical properties of undoped zinc oxide films prepared by the sol-gel process using a spin-coating technique were investigated. The ZnO films were obtained by 10 cycle spin-coated and dried zinc acetate films followed by annealing in air at 500–575°C. The films deposited on the Pyrex glass substrate were polycrystalline and c-axis oriented. A minimum film resistivity gave 28.2 Ω cm at an annealing temperature of 525°C. The temperature dependence of the conductivity indicated electron transport in the conduction band due to thermal excitation of donor electrons for temperatures from 250 to 300 K. The grain boundary scattering effect due to thermionic emission was confirmed. For temperatures below 250 K nearest-neighbor hopping conduction was dominant in the films. The films were transparent in the visible range above 400 nm and had sharp ultraviolet absorption edges at 380 nm. The absorption edge analysis revealed that the optical band gap energy for the films was 3.20–3.21 eV and the electronic transition was of the direct transition type. From the Urbach tail analysis the width of the localized state Ee=0.07–0.08 eV.