Hetero-epitaxial Growth of (1, 0, m+1) One Axis-oriented Bismuth Layered Structured Ferroelectrics Thin Films Directly C
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Hetero-epitaxial Growth of (1, 0, m+1) One Axis-oriented Bismuth Layered Structured Ferroelectrics Thin Films Directly Crystallized by MOCVD Norimasa Nukaga, Takayuki Watanabe, Tomohiro Sakai, Toshimasa Suzuki1, Yuji Nishi1, Masayuki Fujimoto1 and Hiroshi Funakubo Department of Innovative and Engineered Materials, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan 1 Taiyo Yuden Co., Ltd., 5607-2 Nakamuroda, Haruna-machi, Gunma 370-3347, Japan ABSTRACT Bismuth layered structured ferroelectrics (BLSF) thin films with different number of octahedron number (m-number) were prepared by MOCVD and directly crystallized on the substrates. Directly-crystallized SrBi2Ta2O9 (SBT) (m=2) films on a (111) Pt/Ti/SiO2/Si substrate were ascertained to have a strong (103) one-axis orientation by the X-ray reciprocal space mapping and to be hetero-epitaxially grown on the (111) Pt grains by the TEM observation. Moreover, directly crystallized Bi2VO5.5 (m=1) and Bi4Ti3O12 (m=3) films deposited on the same substrate showed (102) and (104) one-axis preferred orientations, respectively. These orientations are basically the equal ones with SBT (103) orientation because the tilting angle of c-axis from the substrate surface is also about 55˚. Therefore, the direct crystallization is one of the important key techniques for orientation control of BLSF films. Moreover, the directly crystallized SBT film deposited on a (111) Ir/TiOx/SiO2/Si substrate at 570 ˚C by ECR-MOCVD exhibited (103) one-axis orientation, which also originated from the local epitaxial growth on (111)-oriented Ir grains. The remanent polarization (2Pr) , and the coercive field (Ec) of this film were 16.1 µC/cm2 and 83 kV/cm at an applied electric field of 360kV/cm, respectively. This Pr value is about 88% of the expected value of (103)-oriented SBT film from both the Pr values of the (116) and (001)-oriented epitaxial films and detailed crystal analysis. INTRODUCTION Bismuth layer structured ferroelectric (BLSF) thin films have been extensively investigated for nonvolatile ferroelectric random access memory (FeRAM) applications due to their fatigue-free property. The orientation control of BLSF films, especially one-axis preferred orientation, is essential to obtain uniform ferroelectricity in each small-area cells because it strongly depends on the film orientation. However, the control method of preferred orientation has been hardly reported on polycrystalline substrate. SBT thin film that is one of the BLSF has been prepared by many deposition techniques, such as metalorganic deposition (MOD), pulsed laser deposition (PLD), sputtering method and metalorganic chemical vapor deposition (MOCVD). Among them, the MOCVD process is suitable for high-density memory applications because of its superior step coverage, high deposition rate, good control of the film composition and thickness, and composition uniformity over a large area. In most of the previous studies, the SBT phase was
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