Investigation of Domain Wall Velocity and Nucleation Rate in Polarization Switching of Epitaxial Pb(Zr,Ti)O 3 Thin Films
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Investigation of Domain Wall Velocity and Nucleation Rate in Polarization Switching of Epitaxial Pb(Zr,Ti)O3 Thin Films Using Piezoresponse Scanning Force Microscopy H. Fujisawa, T. Yagi, M. Shimizu and H. Niu Department of Electrical Engineering and Computer Sciences, Graduate School of Engineering, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-2201, Japan. ABSTRACT Domain wall velocity and nucleation rate in 250nm-thick epitaxial Pb(Zr,Ti)O3 thin films were studied by piezoresponse scanning force microscopy (PFM). Domain growth observed after applying switching pulses shorter than the switching time can be described using the Ishibashi theory. At a pulse voltage of +5V, experimental results indicated that new nucleation occurred during the switching period, which corresponded to the Category I in the Ishibashi theory. Switching time, domain wall velocity and nucleation rate at +5V can be obtained as 70ns, 8.3m/s and 43µs-1·µm-2, respectively. As compared with experimental results reported in BaTiO3 single crystals, domain wall velocity was much smaller but nucleation rate was much larger than those in single crystals. INTRODUCTION Polarization switching processes in ferroelectric materials have been extensively studied by many researchers. For example, W. J. Merz studied the nucleation and growth of ferroelectric domains in BaTiO3 as a function of applied electric field and temperature [1]. Recently, V. Gopalan et al. reported in-situ observation of domain switching in LiTaO3 single crystals by electro-optic imaging microscopy [2]. Polarization switching was also theoretically understood by phenomenal theories based on the Avrami theory [3], such as the Ishibashi theory [4]. In ferroelectric thin films, understandings of polarization switching processes are important especially for nonvolatile memory applications because the polarization switching is utilized as read and write operations. However, there have been few reports which described the domain wall velocity and nucleation rate in polarization switching of ferroelectric thin films [5, 6]. This is because it has been quite difficult with thin films to directly observe ferroelectric domains and their growth or motion. Therefore, polarization switching processes in thin films have been investigated by measuring switching currents [5-7]. On the other hand, recently developed piezoresponse scanning force microscopy (PFM) has the capability to observe domain structures through the piezoelectric effects of ferroelectrics [8-12]. In our previous study, using this PFM technique, ex-situ observations of domain motions of ferroelectric capacitors were achieved [13,
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14]. In this study, polarization switching processes of epitaxial Pb(Zr,Ti)O3 thin films are investigated by PFM observations of domain growth after applying switching pulses shorter than the switching time. Sideward domain wall velocity and nucleation rate are also evaluated by applying the Ishibashi theory to experimental results. EXPERIMENTAL PROCEDURE 250nm-thick tetra
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