Novel Candidate of c-axis-oriented BLSF Thin Films for High-Capacitance Condenser
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Novel Candidate of c-axis-oriented BLSF Thin Films for High-Capacitance Condenser Takashi Kojima, Yukio Sakashita1, Takayuki Watanabe, Kazumi Kato2, and Hiroshi Funakubo Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan 1 TDK Corporation, 570-2, Matsugashita, Minamihatori, Narita-shi, Chiba 286-8588 Japan 2 National Institute of Advanced Industrial Science and Technology, 2266-98 Anagohara, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
ABSTRACT c-axis-oriented SrBi2Ta2O9 thin films with various thickness ranging from 20–170 nm were epitaxially grown on (100)SrRuO3 (SrRuO3 is give for the pseudo-cubic unit cell)//(100)SrTiO3 substrates by metalorganic chemical vapor deposition (MOCVD). The relative dielectric constants of these films kept a constant value of about 55 with decreasing film thickness down to 20 nm. The capacitances of these films were almost independent of the applied electric field; change of capacitance for these films on applied electric field from 0 kV/cm to 100 kV/cm was within 0.017 % and tanδ value was within 1.3 %. The leakage current densities were constant against the film thickness on the order of 10-8 A/cm2 at 150 kV/cm. Surface flatness of these films were also almost the same irrespective of the film thickness. These characteristics are very attractive for high-capacitance condenser application. Therefore, c-axis-oriented BLSF thin films are novel candidates for high-capacitance condenser application having both bias-free and thickness independent characteristics together with the good surface smoothness.
INTRODUCTION Perovskite materials such as (Ba, Sr)TiO3 (BST), Pb(Zr,Ti)O3, (PZT), and SrTiO3 (STO) are the leading candidates for the dielectric in advanced dynamic random access memory (DRAM) technologies and integrated capacitor application because of their high dielectric constant. However, various problems have been remained for such materials, especially, drop of the dielectric constant with the decrease in film thickness and the large bias-voltage dependency [1]. On the other hand, lead (Pb)-free bismuth layer-structured ferroelectrics (BLSF) with a general formula of (Bi2O2)2+(Am-1BmO3m+1)2- (m: BO6 octahedron number) have been investigated for a ferroelectric random access memory (FeRAM) application because of its good ferroelectric properties, such as excellent fatigue endurance [2]. We already reported that the c-axis-oriented epitaxial Bi4Ti3O12 thin films had several superior properties such as small leakage current density and the independence of the dielectric constant on the film thickness together with the surface smoothness [3]. On the basis of these results, we propose novel candidate of c-axis-oriented BLSF films having even number of m as a high-capacitance condenser because of the nonferroelectricity along c-axis [2]. This is because, c-axis-oriented BLSF film having even number of m is expected to have the following excellent properties. Firstly, good surface smooth
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