Electronic properties of zinc oxide thin films prepared by RF magnetron sputtering for varistor applications

ZnO thin films for varistor applications have been prepared by RF magnetron sputtering at a power of 0-1 kW using an argon pressure of 05 Pa. The films were polycrystalline with mean grain size typically 13 nm, and were preferentially oriented in the [002] direction. Optical absorption studies revealed a fundamental absorption edge at a wavelength of approximately 360 nm with a direct bandgap of 3.36 eV. van der Pauw measurements showed a decrease in resistivity from approximately 12ωm at thickness 50 nm to 0-5 ωm at thickness greater than 500 nm. Below 350 K the resistivity was essentially constant, while at higher temperatures an activation energy of approximately 0-2eV was observed, which was attributed to the effects of oxygen vacancies.