Thermochromic VO 2 Films Heteroepitaxially Grown on ZnO Coated Glass by RF Sputtering
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Thermochromic VO2 Films Heteroepitaxially Grown on ZnO Coated Glass by RF Sputtering Kazuhiro Kato, Pung Keun Song, Yuzo Shigesato and Hidehumi Odaka1 College of Science and Engineering, Aoyama Gakuin University, Chitosedai, Setagaya-ku, Tokyo, 157-8572, Japan 1 Research Center, Asahi Glass Co., Ltd., Hazawa, Kanagawa-ku, Yokohama 221-0863, Japan ABSTRACT Vanadium dioxide (VO2) is one of the most attractive thermochromic materials, which shows large changes in optical and electrical properties at around 68 oC, nearly room temperature. This thermochromic behavior has been explained in terms of the Mott-Hubbard transition from a high-temperature rutile structure (metal phase) to a low-temperature monoclinic structure (semiconductor phase). We already reported that rf magnetron sputtering using V2O3 or V2O5 targets enable us to deposit polycrystalline thermochromic VO2 films with high reproducibility by introduction of oxygen gas (O2/(Ar+O2)=1~1.5%) or hydrogen gas (H2/(Ar+H2)=2.5~10%), respectively, as reactive gases [see ref.1]. In this study, ZnO polycrystalline films were deposited as a buffer layer between the VO2 film and glass substrate also by rf magnetron sputtering, which have been known to exhibit preferred orientation in the wide range of the deposition conditions. Very thin thermochromic VO2 films with thickness of 50nm were successfully deposited on the ZnO coated glass substrate because of the heteroepitaxial relationship of VO2(010)[100]//ZnO(001)[100],[010],[ 1 1 0]. INTRODUCTION Thin film vanadium oxide (VOx) has been reported to show abrupt changes in optical and electrical properties with varying temperature, which originates in semiconductor-metal phase transitions [2,3]. Of all the different structured VOx, monoclinic-tetragonal VO2 has been considered to have the largest potential for various applications, such as thermal sensors or thermochromic smart windows, because its transition temperature is around room temperature (68 oC for non doped VO2) [4,5]. The metallic high-temperature phase of VO2 is tetragonal (rutile; P42/mnm), whereas its semiconducting low-temperature phase is monoclinic (P21/C), characterized by tilted V-V pairs slightly off the monoclinic a axis. The semiconductor-metal transitions in VO2 have been theoretically interpreted in terms of a Mott-Hubbard-like transition [6] or electron trapping in a homopolar bond [7,8]. The electrical and optical properties of VOx films through the transition were reported to depend considerably on the stoichiometry [3], dopants [9,10] and microstructure [11,12]. Various deposition methods have been reported for the thermochromic VO2 films. Among those deposition methods, magnetron sputtering should be one of the most promising techniques for large area uniform coatings with high packing density and strong adhesion. Thermochromic VO2 films have been deposited with high reproducibility by rf magnetron sputtering using V2O3 or V2O5 targets [1]. However, the thickness of the VO2 films should be larger than 400nm because stoichiometry x of the VOx
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