Photoassisted scanning tunneling microscopy investigation on the ZnO(0001)-Zn surface treated by alkaline solution

In this study, the surface geometric structures of epitaxial (0001) ZnO films treated by NaOH solution are investigated using photoassisted scanning tunneling microscopy (STM). By illuminating ultraviolet (UV) light on the epitaxial (0001) ZnO film, the tunneling current can be significantly enhanced to construct the well-defined STM images. Polarity identification of the epitaxial (0001) ZnO film by convergent-beam electron diffraction indicates that the epitaxial (0001) ZnO film exhibits the Zn-polar surface. Two types of topographic features, i.e., hexagonal pyramid and flat plane, are observed in the AFM images of the as-grown epitaxial (0001) ZnO film. UV-assisted STM images reveal the anisotropic etching behaviors of the epitaxial (0001) ZnO films in NaOH solution. The faceted and symmetrically layered hexagonal-pyramid feature gets asymmetrical and rounded with increasing etching time. On the other hand, few small hexagonal pits on the as-grown flat ZnO(0001)-Zn surface are developed to asymmetrically hexagonal cavities with flat terraces and steps after NaOH treatments. In addition, triangular reconstruction of the NaOH-treated ZnO(0001)-Zn surface and evidently layer-stacking feature on a faceted ZnO surface with a step height resolved in the atomic scale are also demonstrated in ambience using the photoassisted STM.