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Vertically aligned single-crystal ZnO nanorods have been successfully fabricated on semiconducting GaN, Al0.5Ga0.5N, and AlN substrates through a vapor-liquid-solid process. Near-perfect alignment was observed for all substrates without lateral growth. Room-temperature photoluminescence measurements revealed a strong luminescence peak at approximately 378 nm. This work demonstrates the possibility of growing heterojunction arrays of ZnO nanorods on AlxGa1-xN, which has a tunable band gap from 3.44 to 6.20 eV by changing the Al composition from 0 to 1, and opens a new channel for building vertically aligned heterojunction device arrays with tunable optical properties and the realization of a new class of nanoheterojunction devices.

An effective, low cost, simple, and mask-free pathway is demonstrated for achieving density control of the aligned ZnO nanowires grown for large-scale applications. By a slight variation of the thickness of the thermally evaporated gold catalyst film, a significant change in the density of aligned ZnO nanowires has been controlled. The growth processes of the nanowires on an Al 0.5 Ga 0.5 N substrate has been studied based on the wetting behavior of gold catalyst with or without source vapor, and the results classify the growth processes into three categories: separated dots initiated growth, continuous layer initiated growth, and scattered particle initiated growth. This study presents an approach for growing aligned nanowire arrays on a ceramic substrate with the simultaneous formation of a continuous conducting electrode at the roots, which is important for device applications, such as field emission.One-dimensional (1D) ZnO nanostructures are considered to be one of the most important semiconducting nanomaterials for fabricating nanodevices with applications in optics, electronics, mechanics, and biomedical sciences. 1,2 The attraction of ZnO is the result of its high purity, high crystallinity, wide direct energy band gap (3.37 eV), large excitation binding energy (60 meV), piezoelectricity, and biocompatibility, as well as the divisive nanostructures. 3 Since the successful growth of aligned ZnO nanowires on a single-crystal substrate, 4-6 a system that may be very useful for vertical device fabrication has been found. As a result, great interest in acquiring more control over the alignments, including supporting substrates, distribution of nanowires, and density of nanowires, to maximally meet the requirements of nanodevices has been inspired.The vapor-liquid-solid (VLS) process is the most widely used technique for growing nanowires because of its relatively low cost and simple procedure. [4][5][6][7][8] Metal-organic chemical vapor deposition (MOCVD) has also been proven as an alternative method for aligned nanowires but with a much higher cost. 9,10 A wet chemistry process has recently been demonstrated as a powerful technique for growing aligned nanowires at a very low cost and over a very large surface area. 11-13 However, the density of nanowires grown on the surface still cannot be controlled unless a catalyst pattern created by a mask or lithography is applied.From the application point of view, the density of the aligned nanowires is very important since it is directly related to how the nanowires interact with each other optically, electronically, and mechanically. In field emission, 14,15 for example, an array of densely packed nanowires greatly reduces the field enhancement effect at the nanowire tip to a level not much different from a flat metal plate, while too loosely distributed nanowires cannot meet the desired requirement of high-emitting points. The ability to systematically control the density of the aligned nanowires so that optimal performance can be achieved by adjusting the space ...

The electrical characteristics of metallization contacts to a thin film N-polar n-type GaN layer fabricated by a laser lift-off process combined with a dry etching are investigated. It is shown that for Pt Schottky contacts, the Schottky barrier height of the N-polar GaN is 1.27eV, which is larger than that (1.23eV) of reference Ga-polar GaN. Ti∕Al Ohmic contacts to the N-polar GaN experience thermal degradation even at 400°C. Such annealing-induced degradation is explained in terms of the presence of the complex surface states of the N-polar GaN, which consists of impurities and process-induced donorlike and acceptorlike defects.

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