Laterally and vertically grown ZnO nanostructures on sapphire

Lateral growth of ZnO nanowall arrays with subsequent growth of vertical nanowires using a two-step vapour phase transport method on a-plane sapphire are reported. X-ray diffraction and scanning electron microscopy data show that the nanostructures are aligned with c-axis normal to the substrate. Photoluminescence data demonstrate the exceptionally high optical quality of these structures, with intense emission and narrow bound exciton linewidths. We observe high energy excitonic emission at low temperatures close to the band-edge which we assign to the surface exciton in ZnO at ~3.366 eV. This assignment is consistent with the large surface to volume ratio of the nanowire systems and indicates that this large ratio has a significant effect on the luminescence even at low temperatures. The band-edge intensity decays rapidly with increasing temperature compared to bulk single crystal material, indicating a strong temperature-activated non-radiative mechanism peculiar to the nanostructures. No evidence is seen of the free exciton emission due to exciton delocalisation in the nanostructures with increased temperature, unlike the behaviour in bulk material. The use of such nanostructures in room temperature optoelectronic devices appears to be dependent on the control or elimination of such surface effects.