Optoelectronic characterization of morphology-controlled zinc oxide nanowires
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Optoelectronic characterization of morphology-controlled zinc oxide nanowires Shou-Yi Kuo1,2, Fang-I Lai3, Chun-Chieh Wang1 and Woei-Tyng Lin3, 1 Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan Tao-Yuan 333, Taiwan 2 Green Technology Research Center, Chang Gung University, 259 Wen-Hwa 1st Road, KweiShan Tao-Yuan 333, Taiwan 3 Department of Photonics Engineering, Yuan-Ze University, 135 Yuan-Tung Road, Chung-Li 320, Taiwan ABSTRACT In this paper, we report the characterization of vertically aligned ZnO nanowire (NW) arrays synthesized by metal-catalyzed chemical vapor deposition. The growth mechanism of ZnO NWs may be related to vapor-solid-nucleation. Morphological, structural, optical and field emission characteristics can be modified by varying the growth time. For growth time reaches 120 min, the length and the diameter of ZnO NWs are 1.5 μm and 350 nm, and they also show preferential growth orientation along the c-axis. Moreover, strong alignment and uniform distribution of ZnO NWs can effectively enhance the antireflection to reach the average reflectance of 5.7% in the visible region as well. Field emission measurement indicated that the growth time play an important role in density- and morphology-controlled ZnO NWs, and thus ZnO NWs are expected to be used in versatile optoelectronic devices. INTRODUCTION ZnO is one of the most important optoelectronic materials with a wide and direct band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature [1,2]. Because of their remarkable physical and chemical properties, one dimensional (1D) ZnO nanostructures, such as nanorods , nanowires, nanoneedles, and nanotubes have attracted increasing attention in recent years. Therefore, vertically aligned ZnO nanowires are regarded as a promising candidate for applications in blue-UV light emitters, field emission transistors, nanodevice , photonic, biosensor, field emission devices and dye-sensitized solar cells (DSSCs) [3-5]. So far, many methods have been developed to synthesize the high quality and well alignment ZnO nanowires and nanorods such as vapor-phase-transport, metalorganic vaporphase epitaxy, pulsed laser deposition, and various wet-chemistry methods [6-9]. In most cases, the vapor-liquid-solid (VLS) process and the vapor-solid (VS) process was claimed as the growth mechanism. Because of metal catalysts caused serious contaminations since they may be incorporated into nanowires creating deep-level traps. Therefore, we report a simple synthesis of ZnO nanowires on an silicon substrate by using the VS process at a relatively low growth temperature (550 °C) and adjusted the growth time to investigated the optoelectronic properties of ZnO nanowires. EXPERIMENT
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The ZnO nanowires were fabricated via a simple vapor-phase transport process in a horizontal tube furnace. In the first set of experiments, the Si(100) substrates were coated with thick Au films as buffer layer by DC sputter. Then zinc powder (99.99%) was loaded into a aluminum oxide
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