Electrical and optical properties of zinc oxide films post-annealed in H2 after fabrication by sol–gel process

Undoped ZnO films were post-annealed in hydrogen at 350 °C for 3 h after sol–gel spin-coating film fabrication followed by annealing from 500 to 575 °C, and their electrical and optical properties were investigated. The post-annealed films indicated c-axis oriented polycrystalline zinc oxide including metallic zinc as interstitial atom in the lattice as well as those before H2 annealing. The films were n-type semiconductors. The minimum resistivity of the films was 0.22 Ω cm. Donor levels for the films Ed=0.04–0.08 eV were estimated from the temperature dependence of the conductivity above 250 K. The conduction mechanism of the post-annealed ZnO films was shifted from band conduction including ionized impurity scattering and grain boundary scattering at temperatures between 250 and 300 K to variable-range hopping conduction below 250 K. The films were transparent in the visible range above 400 nm and had sharp ultraviolet absorption edges at 380 nm. The optical band gap energy was evaluated to be Eopt=3.20–3.21 eV, and the width of the localized state Ee was obtained to be Ee=0.08–0.09 eV from the Urbach tail analysis.