Ferroelectric property of an epitaxial lead zirconate titanate thin film deposited by a hydrothermal method Takeshi Morita a) and Yasuo Wagatsuma Research Institute of Electrical Communication, Tohoku University, Miyagi 980-8577, Japan Hitoshi Morioka and Hiroshi Funakubo Department of Innovative and Engineered Materials, Tokyo Institute of Technology, Yokohama 226-8502, Japan Nava Setter Ceramics Laboratory, Faculty of Engineering, Material Institute, Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland Yasuo Cho Research Institute of Electrical Communication, Tohoku University, Miyagi 980-8577, Japan (Received 7 January 2004; accepted 25 March 2004) Deposition of thin films via hydrothermal method has various advantages: low deposition temperature, high purity, deposition on a three-dimensional structure, and a large thickness. Although an epitaxial lead zirconate titanate (PZT) thin-film deposition has been reported, the ferroelectric measurement has not been conducted due to the peel-off morphology of the film. The current paper investigates the improvement of an epitaxial PZT thin film deposited via a hydrothermal method. By adjusting the position at which the substrate was suspended in the solution, smooth morphology surface was successfully obtained. As a bottom electrode, a 200-nm SrRuO 3 thin film was deposited on SrTiO 3 single crystals, and the PZT thin film was deposited on SrRuO 3 . The remanent polarization 2Pr and coercive electric field for PZT on SrRuO 3 /SrTiO 3 (001) were 17.1 C/cm 2 and 36 kV/cm, respectively, and those of PZT on SrRuO 3 /SrTiO 3 (111) were 32.7 C/cm 2 and 59 kV/cm, respectively. The reason for large imprint electrical field, 91 kV/cm and 40 kV/cm for each film, was unclear at this stage, although it is associated with self–alignment poling direction. This self–alignment poling direction was confirmed via scanning nonlinear dielectric microscopy and is thought to have been related to the deposition mechanisms. I. INTRODUCTION Epitaxial ferroelectric thin films, particularly lead zir- conate titanate (PZT) epitaxial thin films, have inten- sively been studied for use in smart microelectrical me- chanical systems (MEMS) and a ferroelectric random access memory (FeRAM) due to their large piezoelectric displacement and permanent polarization. 1–3 Using a single-crystal substrate with a lattice constant close to that of PZT, the deposited film can be orientated accord- ing to the properties of the substrate. In an effort to obtain such a film, various film deposition processes have been explored, including sol-gel methods, sputtering methods, and chemical-vapor-deposition methods. 4–6 However, with these methods, a high-temperature crystallization process (above 600 °C) is required to achieve a PZT film. The advantage of the hydrothermal method is low re- action temperature, less than 200 °C. It is important to note that this temperature is below the Curie temperature of PZT and more than 400 °C below the reaction tem- perature required of other methods. Generally, a high- temperature procedure results in lead evaporation. This lead vacancy causes the generation of a subphase, such as a pyrochlore phase. Excess strain remains in the structure due to the phase change that occurs at the Curie tempera- ture and the difference in the thermal expansion coeffi- cient between PZT and the substrate. In some of the alternative deposition methods, a chemical reaction is carried out on the substrate; hence relict by-products might be observed in the ferroelectric films. On the other hand, using the hydrothermal method, the PZT thin film is synthesized directly as an ionic reaction in solution, a) Address all correspondence to this author. e-mail: tmorita@ieee.org DOI: 10.1557/JMR.2004.0243 J. Mater. Res., Vol. 19, No. 6, Jun 2004 © 2004 Materials Research Society 1862 https://doi.org/10.1557/JMR.2004.0243 Downloaded from https:/www.cambridge.org/core. University of Basel Library, on 30 May 2017 at 22:09:15, subject to the Cambridge Core terms of use, available at https:/www.cambridge.org/core/terms.