Electrical properties of zinc oxide thin films deposited using high-energy H 2 O generated from a catalytic reaction on
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Electrical properties of zinc oxide thin films deposited using high-energy H2O generated from a catalytic reaction on platinum nanoparticles Kanji Yasui, Naoya Yamaguchi, Eichi Nagatomi, Souichi Satomoto, and Takahiro Kato Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan ABSTRACT Zinc oxide (ZnO) with excellent crystallinity and large electron mobility was grown on aplane (11-20) sapphire (a-Al2O3) substrates by a new chemical vapor deposition method via the reaction between dimethylzinc (DMZn) and high-energy H2O produced by a Pt-catalyzed H2-O2 reaction. The electron mobility at room temperature increased from 30 cm2/Vs to 189 cm2/Vs with increasing film thickness from 0.1 μm to approximately 3 μm. Electron mobility increased significantly with decreasing temperature to approximately 110 – 150 K, but decreased at temperatures less than 100 K for films greater than 500 nm in thickness. On the other hand, the mobility hardly changed with temperature for films lesser than 500 nm in thickness. Based on the dependence of the electrical properties on the film thickness, the ZnO films grown on a-Al2O3 substrates are considered to consist of an interfacial layer with a high defect density (degenerate layer) generated due to a large lattice mismatch between ZnO and Al2O3 substrates and an upper layer with a low defect density. INTRODUCTION Zinc oxide (ZnO) is a useful material for many applications such as surface acoustic wave devices [1], gas sensors [2], photoconductive devices [3], and transparent electrodes [4]. Due to its large bandgap (3.37 eV at RT) and large exciton binding energy (60 meV) [5], its application to optoelectronic devices such as light emitting diodes and laser diodes operating in the ultraviolet region has been intensively investigated [6-12]. Many growth techniques, including molecular beam epitaxy (MBE) [8, 9], pulsed laser deposition (PLD) [7, 10], laser MBE (LMBE) [6], and metal-organic chemical vapor deposition (MOCVD) [11, 12], have been used to prepare ZnO thin films. Although MOCVD has many advantages for industrial applications, such as a high growth rate on large surface substrates and a wide selection of metalorganic and oxygen source gases, ZnO film growth by conventional MOCVD requires high electric power to react the source gases and raise the substrate temperature. To overcome this, a more efficient means for reacting oxygen and metalorganic source gases is needed. In addition to the low reaction efficiency, conventional CVD methods yield low-quality ZnO films (exhibiting small electron mobility) compared to those prepared by MBE and PLD, due to incomplete reaction between metalorganic and oxygen source gases in the gas phase. However, if thermally excited water is used to hydrolyze the metalorganic source gases, high-energy ZnO precursors are produced in the gas phase, thus allowing the growth of ZnO films in a manner similar to PLD and MBE. In a previous paper [13], we reported a new growth method for ZnO films using the reaction between dimethy
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