Persistent photocurrent characteristics of ZnO polycrystalline films prepared by RF magnetron sputtering

We study zinc oxide (ZnO) polycrystalline films highly aligned in the hexagonal c-axis direction 〈0002〉 deposited by RF magnetron sputtering. Photoconductive decay current characteristics are investigated. The decay current shows a long decay tail called persistent current. This temporal feature is analyzed with regard to the inhomogeneous amorphous semiconductor structure. We conclude that the photo-excited carriers of electrons and holes recombine between spatially separated states near the thermo-activated conduction band edge and those near the valence band edge. Although the activation energy of the recombination process is dependent on the ultraviolet irradiation intensity, under weak values an activation potential of about 0.15 eV is obtained, which clarifies the carrier transport in this polycrystalline system, and may give rise to future applications of this system, e.g., catalytic processes.