Local Electronic Structures and Polarity of ZnO Nanorods Grown on GaN Substrates

The polarization and surface reactivity of the ZnO nanorods (NRs) grown on the (0001̅) N-polar GaN substrate and the (0001) Ga-polar GaN substrate by hydrothermal process were studied. We found that the polarization direction of the ZnO NRs grown on the GaN substrates depends on the surface polarity of the GaN growth substrates. The surface planes of ZnO NRs on the Ga-polar GaN substrate and the N-polar GaN substrate are the Zn-polar (positive-charged surface) and the O-polar (negative-charged surface) ZnO surface plane, respectively. The O-polar ZnO plane is more chemically stable than the Zn-polar ZnO plane. After the water dissolution process, the Zn-polar ZnO NRs transformed to the tube-like nanostructure and the O-polar ZnO NRs formed hexagonal facets on the top of ZnO NRs. X-ray absorption spectroscopy (XAS) was used to analyze the electronic structures of the growth surface of the ZnO NRs on the two GaN growth substrates. The absorption peaks in the XAS spectral reveal the density of the dangling bonds on the terminal surface of ZnO NRs and the occupancy probability of the hybridized orbitals of the terminal surface of ZnO NRs. With the information on the electronic structure of the surface planes of the ZnO NRs, the surface plane of the ZnO NRs on the different GaN substrates can be verified.