The effect of surface defects on the optical and electrical properties of ZnO nanorods

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This thesis reports on the effect of native point defects on both the NH₃ gas sensing properties and the surface electronic structure of ZnO nanorods. Low temperature hydrothermal growth at 90°C was utilised to synthesize ZnO nanorods approximately 55 ± 5 nm in diameter. The type and density of intrinsic surface defects on the ZnO nanorod was controlled using post growth annealing in O₂ gas and Zn vapor environments. Low voltage cathodoluminescence (CL) spectroscopy confirmed that the heat treatment process produced different surface defect structures. The as-grown, O₂, and Zn annealed nanorods exhibited broad CL peaks centered at 1.90 eV (YL), 1.70 eV (RL), and 2.44 eV (GL), which were attributed to O interstitials or LiZn deep acceptors, acceptor-like VZn complexes, and donor-like VO related centers, respectively. The first part of this thesis focuses on the influence of deep level surface defects on NH₃ gas response of ZnO nanorods. Electrical and gas sensing measurements revealed that the NH₃ gas sensitivity was 4.1 for the as-grown (YL), 22.6 for O₂ anneal (RL), and 1.4 for Zn vapor anneal (GL) samples. Hydrogen plasma treatment quenched the RL and inverted the ammonia electrical response due to the incorporation of shallow hydrogen donors. Changes to the gas sensing response were attributed to a shift in the ZnO Fermi level position relative to the ammonia gas chemical potential due to the formation near surface donor or acceptor centers. In the second part of this thesis the effect of native point defects on the surface electronic structure of all ZnO nanorod samples was studies utilizing X-ray photoemission spectroscopy (XPS), X-ray Absorption Near-Edge Structure (XANES), electron spin resonance (ESR), low voltage CL spectroscopy, sub band gap photoluminescence (PL) spectroscopy and electrical I-V measurements. Correlative characterization measurements established that the degree of surface band bending and surface conductivity is controlled by the amount of adsorbed ambient gas species (O₂, H₂O and OH), which is mediated by type and concentration of intrinsic surface point defects. The last part of this thesis concentrates on the chemical origin of RL peak and its optical properties. A wide range of analysis techniques, including PL and CL spectroscopy, XANES and ESR was used to comprehensively characterize the RL emission. From these data, the RL has been assigned to highly lattice coupled VZn-related acceptor-like center. No correlation was found between the RL and nitrogen impurities.
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