Structural and cathodoluminescent properties of surface polarity and shape-controlled ZnO nano structures on the GaN thin films

Abstract

The unique and fascinating properties of one-dimensional (1D) nanostructures have attracted tremendous research interests to explore their physical properties and possibilities for future electronic and photonic device applications. For the nanoscale materials, the surface to volume ratio increases significantly with size reduction and thus, the effect of the surface, especially the surface polarity, plays an important role in determining the structures as well as the chemical and physical properties of the ZnO nanostructures. Therefore the control of the surface is a crucial factor in designing nanomaterials for various applications. Here we report shape-controlled synthesis of vertically aligned ZnO nanostructures on GaN thin films using catalyst-free metalorganic vapor phase epitaxy (MOVPE). By adjusting the synthesis parameters, smooth-surfaced ZnO nanorod-nanowall networks with nonpolar {1 00} and {11 0} side surfaces (Figs. 1a and b) and stacked pyramid-structured ZnO nanorods terminated mainly with O-polar {10 1} and (000 ) surfaces planes (Figs. 1c and d) were obtained. We suggest that the morphologies of ZnO nanostructures are dominated by the path of mass transport of Zn adatoms from side facets of GaN hillocks to adjacent ZnO nanorods according to growth conditions. Low temperature CL spectra measured at 80 K exhibits the dominant NBE emission peaks at 3.30 eV for smooth-surfaced, non-polar ZnO nanostructures, and 3.45 eV for stacked pyramid-structured ZnO nanorods with O-polar surfaces. The emission shift (~15 meV) is consistent with the energy difference between neutral donor bound exciton and free exciton peaks, thus indicating that free excitons dominate over bound excitons in stacked pyramid-structured ZnO nanorods. In addition, as indicated by the arrow in Fig. 1e, a longitudinal optical phonon replica is also observed in stacked pyramid-structured ZnO nanorods. These results indicate that stacked pyramid-structured ZnO nanorods have good optical qualities, and passivation of the surfaces with chemically inert O-polar planes may further enhance their luminescent properties. Detailed growth mechanism of two kinds of nanostructures and correlated properties will be discussed.