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

Abstract

The unique and fascinating properties of one-dimensional (1D) nanostructures have attracted tremendous research interests to explore their novel 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. We report shapecontrolled synthesis of vertically aligned ZnO nanostructures on GaN thin films using catalyst-free metal-organic vapor phase epitaxy (MOVPE). By adjusting the synthesis parameters, smooth-surfaced ZnO nanorod-nanowall networks with nonpolar side surfaces and stacked pyramid-structured ZnO nanorods terminated mainly with O-polar surfaces planes were obtained. 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. CL measurements reveal strong excitonic emission peaks with extremely low deep level emission for both type of the nanostructures, providing a significant opportunity to understand the surface polarity and shape-dependent luminescent properties of nanoscale materials. Detailed growth mechanism of two kinds of nanostructures and correlated optical properties will be presented.