Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm² V⁻¹s⁻¹. The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (V_Zn) with one or more oxygen vacancies (V_O) in larger structural entities. Such V_O-V_Zn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.