Enhancement of Field Emission Current from ZnO Nanorods Fabricated by Two Step Chemical Vapor Deposition with Laser Abla
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Enhancement of Field Emission Current from ZnO Nanorods Fabricated by Two Step Chemical Vapor Deposition with Laser Ablation of ZnO Takashi Hirate, Takashi Kimpara, Kazumoto Takizawa, and Tomomasa Satoh Electronics and Informatics Frontiers, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama, 2218686, Japan ABSTRACT We fabricated aligned ZnO nanorods with thin diameter by two-step chemical vapor deposition method combined with laser ablation of a sinterd ZnO target. Firstly, aligned ZnO nanorods with thick diameter of about 110 nm are grown on an n-Si (111) wafer. Next, a thin ZnO nanorod with about 30 nm diameter is grown on the center of the flat tip of each aligned and thick ZnO nanorod by controlling a flow rate of oxygen. Although thick ZnO nanorods do not emit a recognizable field emission current, thin ZnO nanorods with 1500 nm length show a field emission current of 500 µA under an electric field of 31 V/µm. INTRODUCTION Field emission (FE) is a unique quantum-mechanical effect where electrons tunnel from condensed matter into a vacuum under a strong electric field. FE is of great commercial interest in flat panel displays, x-ray sources and other vacuum microelectronic devices. In past decades, research in this area mainly focused on carbon nanotubes (CNTs) [1, 2] which were particularly thought to be good candidates for FE because of their good conductivity and high aspect ratio that enhanced the electric field on the sharp end of their structures. However, it has been pointed out that CNT can operate either as conductors or semiconductors depending on how they have been rolled [2]. This results in varied voltage drops across CNTs and, thus, in varied effective electric fields on the emission tips of CNTs. Zinc oxide (ZnO), a wide-direct-bandgap (3.37 eV) semiconductor, is an attractive II-VI compound semiconductor material for various optoelectronic devices. Recently, growth of various nanostructures of ZnO such as nanorod, nanobelt, nanowall, etc. has been reported [3-13], and ZnO has been considered as a promising material for nanodevices. Among the nanostructural zinc oxides, a ZnO nanorod is a potential alternative for FE because it has structural similarity to the CNT, thermal stability and no ‘rolling’ problem. In this paper we studied to develop vertically aligned ZnO nanorods with thin diameter to enhance the FE current, and succeeded in fabrication of vertically-aligned ZnO nanorods with 30 nm tip diameter by controlling the O2 flow rate in two-step chemical vapor deposition (CVD) method. FE current from ZnO nanorods is enhanced by thinning the diameter.
EXPERIMENTAL DETAILS ZnO nanorods were grown on an n-Si (111) substrate by CVD method combined with laser ablation that was developed by us [3, 10]. The growth equipment is almost same as that reported before [3, 10]. This is basically a low-pressure thermal CVD system. The precursors to synthesize ZnO are metal Zn vapor and O2 that are introduced into a deposition chamber through each orifice whose diameter is 1 mm. N
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