Controlling The Growth Direction of ZnO Nanowires on c -Plane Sapphire
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Controlling The Growth Direction of ZnO Nanowires on c-Plane Sapphire Babak Nikoobakht 1, Albert Davydov and Stephan J. Stranick 1 Surface and Microanalysis Science Division, CSTL National Institute of Standards and Technology, Gaithersburg, MD 20899 ABSTRACT Well oriented vertical ZnO nanowires (NWs) are grown on c-plane sapphire via a vaporphase transport process using an Au thin film as a catalyst. This new finding is unexpected due to the fact that the lattice mismatch between the zinc oxide and the underlying substrate is 18%. Xray diffraction (XRD) analysis shows that single-crystal, wurtzite NWs grow in the [0001] direction normal to the basal sapphire plane, which proves that a-plane sapphire is not essential for growth of vertical ZnO NWs, as has been previously stated.[1] We have found that by controlling the thickness of the Au-film and pre-growth annealing of the Au/sapphire substrate NWs can be grown either tilted or vertical. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) studies on Au films with thicknesses ranging from 1 to 10 nm show that in the absence of film annealing, NWs can be grown 32º tilted from the surface normal, whereas pre-annealed Au films result in growth of NWs in the surface normal direction. We attribute the formation of the normal and tilted growth directions to the surface concentration of O and Al ions on sapphire. INTRODUCTION In the past two years there has been a renewal of interest in ZnO sparked in part by the reports on synthesis of ZnO NWs as quasi one-dimensional materials [1] and by their potential applications as blue light emitters, electron field emitters and sensors. Due to its wide band gap (3.37eV) and large exciton binding energy ZnO is an excellent alternative to the UV and bluegreen GaN-based light sources. [2] In general, in designing NW-based light emitting devices, there are fabrication issues such as controlling the doping level of NWs, ideal connection of NWs to the conducting electrodes and the geometry of NWs in the device that ought to be addressed in order to have an efficient device. This study focuses on the latter issue. Our goal is to control the growth direction of ZnO NWs on sapphire and other suitable substrates to prepare arrays of NWs with preferred orientation. c-plane sapphire (0001) has been the most popular substrate in the epitaxial growth of IIIV and II-VI semiconductors due to its high crystalline perfection, stability at high temperature and transparency. The growth of ZnO films on this substrate is accompanied by a large number of defects at the substrate/film interface, which in part is due to the significant lattice mismatch between ZnO (a = 3.2495 Å, c=5.2069 Å) and sapphire (a =4.758 Å, c=12.990 Å). [3, 4] In device fabrication, this mismatch is significant and ultimately results in lower charge mobility, defect related carrier concentrations [3] and has been one major obstacle to the use of ZnO films. Recently, Fons et al. [4] have reported that ZnO films can be grown on a-plane sapphire with lower
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