Electroplating of ZnO Nanowires Using Nanohole Arrays of Anodized Aluminum Oxide and Effects of Thermal Annealing
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Electroplating of ZnO Nanowires Using Nanohole Arrays of Anodized Aluminum Oxide and Effects of Thermal Annealing Ken-ichi Ogata1, Shoso Shingubara1, Hiromi Yorozu2, and Tadahiko Nakanishi2 1 Kansai University, Suita, 564-8680, Japan 2 Shin-chuo Kogyo, Higashi-hiroshima, 739-0145, Japan
ABSTRACT Electroplating of ZnO nanowires was conducted using gold embedded anodized aluminum oxide (AAO) films on Si substrates. For electroplating, insulating layers at the bottom of the AAO nanohole structures needed to be removed. After electroplating, hexagonal structures of vertical ZnO nanowires were observed, however subsequent thermal annealing resulted in broken and knocked over nanowires. Photoluminescence (PL) investigations show that post annealed ZnO nanowires have nitrogen atoms incorporated as an acceptor. INTRODUCTION Zinc oxide (ZnO) is expected to explore novel devices based on its multifunctional properties, such as large exciton binding energy of 60 meV and transparent conductivity [1]. In addition, a variety of fabrication methods, from high quality epitaxial growth under ultra high vacuum conditions [2] to low-cost deposition via aqueous solutions [3], is a great advantage for developing functional devices. It should also be mentioned that bulk crystals as large as 2 inch [4] are available, and that the surface stability of ZnO makes exciton transfer in organic/ inorganic hybrid structures possible [5]. One dimensional nanostructures of ZnO have become the focus of intensive research owing to their unique properties and potential applications in the fabrication of nanodevices. Among several growth techniques, the electroplating method [6] can be applied, which is preferable for low-cost and selective deposition for LSI and MEMS technologies. For the fabrication of nanostructures, anodized aluminum oxide (AAO) membranes, where ordered nanohole structures are formed and the size of nanoholes can be controlled by properly choosing the acid solution and applied voltage [7,8] were utilized as templates. In this paper, ZnO nanostructures were electroplated in nano-sized gold embedded AAO films on Si substrates, instead of a few µm thickness of AAO membranes removed from Al foils [9]. Effects of postannealing on the structural and optical properties of the nanowires were also investigated. EXPERIMENT Preparation of AAO templates [10] Aluminum films were deposited by sputtering with a high purity 5 N Al target on highly conductive n-type Si (100) substrates (resistivity < 0.02 Ωcm). Prior to deposition, the Si substrates were treated with a dilute HF solution. Deposition of Al was performed in Ar pressure of 0.5Pa, where the base pressure was 2×10-3 Pa. The deposition rate was 4.5 nm/sec. and the
thickness of the Al film on Si was 1µm. Al films were anodized using 0.3M of oxalic acid (H2C2O4) solution kept at 5 oC with DC 40V. To facilitate electroplating by eliminating the aluminum oxide barrier layer at the bottom of AAO, AAO films were immersed in 5 wt% phosphoric acid (H3PO4) solution kept at 30 oC.
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