Improvement of Ferroelectric Properties of Lead Zirconate Titanate Thin Films by Ion-substitution using Rare-earth Catio
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Improvement of Ferroelectric Properties of Lead Zirconate Titanate Thin Films by Ion-substitution using Rare-earth Cations Hiroshi Nakaki1, Hiroshi Uchida1, Shoji Okamoto2, Shintaro Yokoyama2, Hiroshi Funakubo2 and Seiichiro Koda1 1 Department of Chemistry, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo, 102-8554, Japan 2 Department of Innovative and Engineered Materials, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan
ABSTRACT Rare-earth-substituted tetragonal lead zirconate titanate thin films were synthesized for improving the ferroelectric property of conventional lead zirconate titanate. Thin films of Pb1.00REx(Zr0.40Ti0.60)1-(3x/4)O3 (x = 0.02, RE = Y, Dy, Er and Yb) were deposited on (111)Pt/Ti/SiO2/(100)Si substrates by a chemical solution deposition (CSD). B-site substitution using rare-earth cations described above enhanced the crystal anisotropy, i.e., ratio of PZT lattice parameters c/a. Remanent polarization (Pr) of PZT film was enhanced by Y3+-, Dy3+- and Er3+-substitution from 20 µC/cm2 up to 26, 25 and 26 µC/cm2 respectively, while ion substitution using Yb3+ degraded the Pr value down to 16 µC/cm2. These films had similar coercive fields (Ec) of around 100 kV/cm. Improving the ferroelectric property of PZT film by rare-earth-substitution would be ascribed to the enhancement of the crystal anisotropy. We concluded that ion substitution using some rare-earth cations, such as Y3+, Dy3+ or Er3+, is one of promising technique for improving the ferroelectric property of PZT film.
INTRODUCTION Lead zirconate titanate [Pb(Zr,Ti)O3; PZT] is an attractive candidate for the component material of nonvolatile random access memory (NvRAM), microelectromechanical system (MEMS), electro-optic devices, etc., which is due to its excellent ferroelectric properties, i.e., large polarization and small coercive electric field, etc. Recently, many techniques for enhancing the spontaneous polarization of PZT thin films have been investigated because larger polarization is required for further integration of NvRAM in next generation. Some researchers revealed that ferroelectric properties of Pb-free bismuth layer structured ferroelectrics, such as strontium bismuth tantalate (SrBi2Ta2O9; SBT) and bismuth titanate (Bi4Ti3O12; BIT), can be improved by the ion substitution. For example, the remanent polarization of polycrystalline SBT film was enhanced by Nb5+-substitution [1,2]. La3+-substituted BIT thin films were reported to have large remanent polarization (24 µC/cm2) [3]. V5+-substituted BIT thin films were reported to show the improvement of the remanent polarization (20 µC/cm2) [4]. Also, larger remanent polarization comparable with PZT films (~ 30 µC/cm2) was realized at Nd3+-substituted BIT thin films [5,6]. On the other hand, only a few successful cases were reported for improving the ferroelectric properties of Pb-based ferroelectric thin films, such as lead titanate [PbTiO3; PT] and PZT, by ion substitution although ion-doping tech
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