Influence of thermal annealing on optical and electrical properties of ZnO films prepared by electron beam evaporation

We have investigated the influence of post-deposition annealing on the optical and electrical properties of c-axis oriented zinc oxide films prepared on sapphire substrates by electron beam evaporation. The ZnO films as-deposited and annealed in air were colourless and transparent in visible range and had sharp ultraviolet absorption edges. It is found that the optical bandgap energy of the films lies in the range of similar to3.27 to similar to3.30 eV depending on the annealing regime. From the analysis of the Urbach tail at the absorption edge, the width of the tail of localized states extending into the bandgap was obtained and a value of 44 meV can be achieved by annealing in air.