The shape- and polarity-control of ZnO nanostructures on the faceted GaN films and related optical properties

Abstract

With the rapid developments in nanotechnology, there has been tremendous interest in ZnO-related nanomaterials because ZnO is a wide band-gap semiconductor and a piezoelectric oxide. For ZnO at the nanometer scale, 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 {1100} and {112 0} side surfaces and stacked pyramid-structured ZnO nanorods terminated mainly with O-polar {101 1} and (0001 ) surfaces planes 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. 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 discussed.