Physical Properties of Vertically-Aligned ZnO Nanorods Grown on Metal Buffer Layers
- Physical Properties of Vertically-Aligned ZnO Nanorods Grown on Metal Buffer Layers
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- One-dimensional (1-D) nanostructures, including nanowires, nanorods and nanotubes, have potential applications as basic units for construction of nano-scale electronics and photonics. as well as fundamental research area. ZnO 1-D nanostructures have a wide band gap of 3.3 eV and exciton binding energy of 60 meV, so they have been investigated for their practical applications to ultraviolet light emitting diodes, sensors and solar cells. ZnO nanorods also have excellent piezoelectric properties and thermal stability. To achieve practical applications of ZnO nanorods, vertically-aligned ZnO nanorods must be fabricated on various substrates. However, the quality of ZnO nanorods is affected by the substrate conditions. Because of these effects, fabrication of high-quality, vertically-aligned ZnO nanorods on various substrates can be difficult.
In this research, vertically well-aligned ZnO nanorods were fabricated on an Al2O3 (sapphire) substrate with a ZnO homobuffer layer and a GaN interlayer by using a catalyst-free metal organic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) measurements demonstrated that ZnO nanorods grown on the ZnO homo-buffer layer had more structural disorders than did, nanorods grown on the GaN interlayer. Field emission transmission electron microscopy (FE-TEM) of the interfaces of the nanorods and the substrates also revealed structural disorders in the bottom part of the ZnO nanorods grown on the Al2O3 substrate and on the ZnO homo-buffer layer, whereas no distinguishable disorder was observed at the ZnO/GaN interface. However, extended X-ray absorption fine structure (EXAFS) at the Zn K-edge revealed a small but distinguishable amount of structural disorder in the Zn-O pairs at the origin of the nanorods？ growth. This result suggested that structural strain due to the surface roughness of the ZnO homo buffer layer and to the lattice mismatch between the ZnO and the GaN mainly contributed to island growth in the initial stage of the ZnO nanorod growth.
ZnO nanorods grown on homo-buffer layer with surface roughness < 1.0 nm were well aligned along the c-axis and on the ab-plane. When the nanorods grew on rough surface films, they had growth directions of 28？, 62？, and 90？ to the interface. The slant of 62？ corresponds to the angle between the ZnO(0001) and (101 ̅
1) planes. The initial growth direction difference caused structural disorder at the interface between the ZnO nanorods and the buffer layer, and prevented epitaxial growth and alignment of nanorods.
To improve the electrical transporting properties, vertically-aligned ZnO nanorods were synthesized on various metal buffer layers. When ZnO nanorods were grown on a Ti buffer layer, they were well-aligned in the c-axis and on the ab-plane. FE-TEM showed that the Ti buffer layer was amorphous and that Ti had interdiffused into the ZnO nanorods. Energy-dispersive spectroscopy (EDS) analysis revealed the Ti buffer layers to be slightly oxidized. EXAFS confirmed the TEM and EDS results. Current-voltage (I-V) measurements showed that the Ti buffer layer increased current density by a factor of 20, suggesting excellent electrical contact between the Ti buffer layer and ZnO nanorods. Vertically-aligned ZnO nanorods were also synthesized on an Al buffer layer. XRD and TEM measurements demonstrated that the Al buffer layer had locally-crystallized structure with large disorder and amorphous properties. I-V measurement showed that the Al buffer layer increased the Ohmic behavior of the ZnO nanorods, similar to the Ti buffer layer.
ZnO nanorods were grown vertically on a Ni buffer layer which was entirely transformed to the Ni-O phase during MOCVD. Electrically, NiO has larger band gap energy than ZnO, which can be acted as the barrier to prevernt the carrier transport through the interface. FE-TEM showed that the Ni buffer layer was bent because this transformation. Both of the structural deformation with large surface roughness and the electrical semiconducting property caused degradation of the I-V characteristics. When the identical growing process was used on Au thin films and Ti/Pt bilayers, conical ZnO nanostructures and nanowires were synthesized, respectively.
N+ ions with energies of 10-50 keV and beam fluxes of 1013 - 1016 particles/cm2 were implanted on vertically-aligned ZnO nanorods. EDS showed that N+ ions were spread uniformly throughout the nanorods. EXAFS revealed that the implanted N+ ions had partially substituted for the oxygen sites in the nanorods. The I-V characteristic curves showed that the nanorods with N+ ions were n-type. Moreover, annealing of N+-implanted nanorods at 800？C increased the charge carrier density in the nanorods to ten times greater than in the N+-implanted ZnO nanorods that were not annealed.
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