Effect of annealing on the morphology and optoelectrical characteristics of ZnO thin films grown by plasma-assisted molecular beam epitaxy

The dependence of characteristics of plasma-assisted molecular beam epitaxy-grown ZnO thin films on different postgrowth annealing conditions was investigated. It was found that, under oxygen atmosphere, annealing temperature can profoundly affect the morphological, electrical, and optical properties of ZnO thin films. In particular, the surface morphology changed from a relatively smooth surface before annealing to various island morphologies after annealing above 800°C for samples grown directly on sapphire without a buffer layer. It is speculated that intrinsic stress due to lattice mismatch drives the island formation and the high temperature provides the energy needed for this surface rearrangement. Single-field Hall-effect measurement showed that the carrier concentration improved by an order of magnitude and the mobility increased from about 30 cm2/Vs to ∼70 cm2/Vs by annealing at 750°C. Variable-field Hall effect shows that a model with two carriers, one a degenerate low-mobility electron and the other a higher mobility non-degenerate electron, is needed to explain the transport properties of the thin film. Analysis indicates that annealing at 750°C decreased the carrier concentration and increased the mobility for the high-mobility carrier. Annealing also led to a significant improvement in photoluminescence, with temperatures of ∼750–850°C yielding the best results.