Perfectly c-axis oriented epitaxial lead titanate thin film deposited by a hydrothermal method for a data storage medium
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Perfectly c-axis oriented epitaxial lead titanate thin film deposited by a hydrothermal method for a data storage medium Takeshi MORITA and Yasuo CHO Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi, 980-8577, Japan ABSTRACT A hydrothermal method is a unique method to deposit ferroelectric thin films utilizing chemical reaction in solutions. In addition to the low reaction temperature below 200o C, its simple process procedure, automatically aligned polarization and a three-dimensional deposition are advantages. In this study, a lead titanate epitaxial thin film was obtained on a strontium ruthenate bottom electrode sputterd on strontium titanate (100). The highresolution X-ray diffraction mapping showed the film was perfectly c-axis oriented. The transmission electron microscope observation revealed that the film had no lattice dislocation at the interface between lead titanate and strontium ruthenate. A remanent polarization of 96.5µC/cm2 was measured with the single crystal-like DE hysteresis curve. The observation of a scanning nonlinear dielectric microscopy indicated that this film did not contain any defect such as an a-domain and a grain boundary even on the nano scale. With various pulse parameters, nano-domain dots were patterned and the minimum dot radius of inverted domain was 12 nm corresponding to data storage density of 1Tbit/inch2 . INTRODUCTION Epitaxial ferroelectric thin films, particularly lead zirconate titanate (PZT) and lead titanate (PbTiO3 ) films, have been studied due to their large piezoelectric displacement and permanent polarization .1−3) 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) A hydrothermal method7−15) enables the deposition of a perovskite thin film in aqueous solution at very low temperatures, typically 100-200o C. The high purity film attributed to the absence of a phase changes during deposition at this low temperature of crystallization. In other deposition methods using a high temperature process, a lead defect must be taken into consideration, because the vapor pressure of lead oxide is high therefore an excess of lead must compensate. Such lead evaporation does not occur in the hydrothermal method. In addition to the low reaction temperature, the hydrothermal method is versatile in that it is a simple process procedure, polarization is aligned automatically and it achieves a threedimensional deposition. Barium titanate (BaTiO3 )7−9) , PZT10−14) , and PbTiO3 15) were previously reported to be deposited by the hydrothermal method. As reported already14) , based on Chien’s reaction condision15) , we improved the surface morphology of epitaxial PZT thin films and succeeded in DE hysteresis measurements. However, the DE hysteresis curve of the PZT thin film indicated a large imprint electrical field; this film could not be utilized for a data storage medium14) . For this PZT thin
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