Examining the crystal growth that influences the electronic device output from vertical arrays of ZnO nanowires
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Examining the crystal growth that influences the electronic device output from vertical arrays of ZnO nanowires Alex M. Lord1, Michael B. Ward2, Alex S. Walton3†, Jonathan Evans1, Nathan Smith4, Thierry G. Maffeis1 and Steve P. Wilks4 1 Centre for Nanohealth, College of Engineering, University of Swansea, Singleton Park, SA2 8PP, United Kingdom 2 Institute for Materials Research, University of Leeds, Leeds, LS2 9JT, United Kingdom 3 School of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, United Kingdom 4 Multidisciplinary Nanotechnology Centre, College of Engineering, College of Science, University of Swansea, Singleton Park, SA2 8PP, United Kingdom 5 Multidisciplinary Nanotechnology Centre, Department of Physics, College of Science, University of Swansea, Singleton Park, SA2 8PP, United Kingdom
† Present address: Interdisciplinary Nanoscience Center (iNano), Aarhus University, Gustav, Wieds Vej 14, DK-8000 Aarhus C, Denmark. ABSTRACT ZnO nanowire (NW) arrays were examined with Transmission Electron Microscopy (TEM) in cross-section after preparation by Focused Ion Beam (FIB) milling. This technique revealed that ZnO nanowires grown using a Au catalyzed vapor technique typically have Au particles at the NW tips, and also randomly dispersed across the base crystal growth that joins adjacent NWs. It is shown the adjacent NWs and the combined base growth is one crystal structure which can be used as a back electrical contact making fabrication of vertical array devices possible. However, the base growth displays detrimental features such as embedded Au particles and lattice defects which can affect the electrical output through depletion regions and scattering centers. In an effort to overcome these problems we investigate a growth method that is nucleated through a minor alteration of the a-plane sapphire surface roughness via a weak chemical etch. Observations of various stages of the growth show the growth nucleates as separate nanoislands that grow in c-plane alignment with Sapphire (1-210), and as growth continues these islands meet and form a polycrystalline film. Further growth initiates nanowire growth and the formation of a single crystal base layer and NW structure that can cover several square millimeter’s. This allows high quality arrays that are relatively free from defects to be formed without any metals contamination and ready for further device processing. INTRODUCTION Recent advances in analytical techniques have allowed researchers to establish that ZnO nanowires are high quality single crystals with controllable electrical properties.[1] Various growth methods enable the application of ZnO nanostructures in a diverse range of electronic devices such as those used for lasing[2] or energy harvesting[3]. Many of these devices use nanowires in aligned vertical arrays grown on a flat substrate. When used in this configuration
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numerous reports show a large range of measured current or rectifying characteristics making controllable device design a difficult proposition.[3]–[6] Most arr
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