Initial Growth Behavior of Ultra-Thin c -Axis-Oriented Epitaxial SrBi 2 Ta 2 O 9 films on SrTiO 3
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0902-T03-39.1
Initial Growth Behavior of Ultra-Thin c-Axis-Oriented Epitaxial SrBi2Ta2O9 films on SrTiO3 Kenji Takahashi1, Muneyasu Suzuki1, Mamoru Yoshimoto2 and Hiroshi Funakubo1,3 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, J2-1508, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, JAPAN 2 Materials and Structures Laboratory, Tokyo Institute of Technology, R3-6, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, JAPAN 3PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honmachi, Kawaguchi, Saitama 332-0012, JAPAN 1
ABSTRACT c-axis-oriented epitaxial SrBi2Ta2O9 ultra-thin films were grown by pulse-gas-introduced metalorganic chemical vapor deposition (pulsed-MOCVD) on (100)SrTiO3 single crystal substrates with atomic scale step structure and their growth behavior was investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Minimum growth unit was found to be “half-unit-cell” of SrBi2Ta2O9. Height of steps and width of terraces observed at SrBi2Ta2O9 film surface were in good agreement with those at SrTiO3 substrate surface. This shape transfer was induced by lattice displacement of SrBi2Ta2O9 along c-direction formed at atomic step on SrTiO3 substrate. In-plane growth of half-unit-cell SrBi2Ta2O9 2D-islands striding across the step walls was observed. It was considered to be a special phenomenon for c-axis-oriented films of layer-structured compounds due to their large crystal anisotropy and/or several times larger half-unit-cell height than single step one of SrTiO3.
INTRODUCTION Perovskite-structured oxides such as (Ba,Sr)TiO3 and SrTiO3 are the leading candidates for the dielectrics in the thin film capacitor application because of their high dielectric constant [1-3]. However, various problems still remained, such as the decrease of the dielectric constant and the increase of the leakage current with the decrease of the film thickness [2-5]. The former was widely known as “size-effect”. Although conductive oxide electrodes such as SrRuO3 and IrO2 were reported to be effective to suppress the size-effect [6], this is not an ultimate solution because others have reported conflicting data. For example, Sinnamon et al. reported the size-effect of epitaxial (Ba0.5Sr0.5)TiO3 films grown on SrRuO3 bottom electrodes [5]. Takemura et al. also reported the size-effect of (Ba0.5Sr0.5)TiO3 films deposited on the bottom electrode of RuO2 whose crystal structure is the same as IrO2 [7]. To make matters worse, (Ba0.5Sr0.5)TiO3 films deposited on RuO2 conductive oxide bottom electrodes were reported to lead to increased leakage current [8] which causes increased power consumption and exothermic heat in devices. Hence, there has been a strong desire to discover size-effect-free materials with superior electric insulation. In the previous study, we proposed thin films of bismuth layer-structured dielectrics (BLDs), as typified by SrBi2Ta2O9 and SrBi4Ti4O15, having c-axis-orientation as novel candidates to overcome these problems [9-13]. They show not
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