Time-dependent polarization relaxation behaviors induced by a depolarization field E d were investigated on high-quality ultrathin SrRuO3/BaTiO3/SrRuO3 capacitors. The E d values were determined experimentally from an applied external field to stop the net polarization relaxation. These values agree with those from the electrostatic calculations, demonstrating that a large E d inside the ultrathin ferroelectric layer could cause severe polarization relaxation. For numerous ferroelectric devices of capacitor configuration, this effect will set a stricter size limit than the critical thickness issue.PACS numbers: 77.22. Ej, 77.22.Gm, 77.80.Dj, 77.55.+f With recent breakthroughs in fabricating high-quality oxide films [1,2,3], ultrathin ferroelectric (FE) films have attracted much attention from the scientific as well as application points of view. As the FE film thickness d approaches tens of unit cell length, the FE films often show significantly different physical properties from those of bulk FE materials. Some extrinsic effects, especially coming from FE film surfaces and/or interfaces with other materials, could be very important [4]. For some other cases, intrinsic physical quantities could play vital roles in determining the unique properties of ultrathin films.Many FE-based electronic devices have the capacitor configuration, where a FE layer is inserted between two conducting electrodes. Then, polarization bound charges will be induced at the surfaces of the FE layer, but compensated by free charge carriers in the conducting electrodes. In real conducting electrodes, however, the compensating charges will be induced with a finite extent, called the screening length λ. This will result in an incomplete compensation of the polarization charges. Such an incomplete charge compensation should induce a depolarization field E d inside the FE layer, with a direction opposite to that of the FE polarization P [5]. Therefore, E d will appear in every FE capacitor, and its effects will becomes larger with the decrease of d [5]. (For a FE film without electrodes, there is no compensation for the polarization bound charge, so the value of E d will become even larger than that of the FE capacitor case.) E d has been known to be important in determining the critical thickness [6] and domain structure of ultrathin FE films [7,8,9], and reliability problems of numerous FE devices [10,11].Recently, using a first principles calculation, Junquera and Ghosez investigated the critical thickness of BaTiO 3 (BTO) layers in SrRuO 3 (SRO)/BTO/SRO capacitor [6]. For calculations, they assumed that all of the BTO and SRO layers were fully strained with the SrTiO 3 substrate. By taking the real SRO/BTO interfaces into account properly, they showed that E d could make the ferroelectricity vanish for the BTO films thinner than 6 unit cells, i.e. 2.4 nm [6]. More recently, using pulsed laser deposition with a reflection high energy electron diffraction monitoring system, we fabricated high-quality fully-strained SRO/BTO/SRO capacitors...

We investigated domain kinetics by measuring the polarization switching behaviors of polycrystalline Pb(Zr,Ti)O3 films, which are widely used in ferroelectric memory devices. Their switching behaviors at various electric fields and temperatures could be explained by assuming the Lorentzian distribution of domain switching times. We viewed the switching process under an electric field as a motion of the ferroelectric domain through a random medium, and we showed that the local field variation due to dipole defects at domain pinning sites could explain the intriguing distribution.PACS numbers: 77.80. Fm, 77.80.Dj, 77.84.Dy Domain switching kinetics in ferroelectrics (FEs) under an external electric field E ext have been extensively investigated for several decades [1,2,3,4,5,6,7,8,9]. The traditional approach to explain the FE switching kinetics, often called the Kolmogorov-Avrami-Ishibashi (KAI) model, is based on the classical statistical theory of nucleation and unrestricted domain growth [10,11]. For a uniformly polarized FE sample under E ext , the KAI model gives the time (t)-dependent change in polarization ∆P (t) aswhere n and t 0 are the effective dimension and characteristic switching time for the domain growth, respectively, and P s is spontaneous polarization. When the nuclei are appearing in time with the same probability, n = 3 for bulk samples and n = 2 for thin films [12]. In addition, t 0 is proportional to the average distance between the nuclei, divided by the domain wall speed. Several studies have used the KAI model successfully to explain the ∆P (t) behaviors of FE single crystals and epitaxial thin films [2]. Recently, FE thin films have been intensively investigated for FE random access memory (FeRAM) [1]. Most commercial FeRAM use polycrystalline Pb(Zr,Ti)O 3 (poly-PZT) films, and their ∆P (t) behaviors determine the reading and writing speeds of the FeRAM. In such non-epitaxial FE films, a domain cannot propagate indefinitely due to pinning caused by numerous defects, so the KAI model cannot be applied. Therefore, it is important both scientifically and technologically to clarify the domain switching kinetics of polycrystalline FE films.Numerous studies have examined the ∆P (t) behaviors of polycrystalline FE films, and the reported results vary markedly [3,4,5,6,7]. Lohse et al. measured the polarization switching currents of poly-PZT films, and showed that ∆P (t) slowed significantly compared to Eq. (1) [3]. Tagantsev et al. observed similar phenomena for poly-PZT films. To explain these behaviors, they developed the nucleation-limited-switching (NLS) model. They assumed that films consist of several areas that have independent switching kinetics:where F (log t 0 ) is the distribution function for log t 0 [4]. They assumed a very broad mesa-like function for F (log t 0 ), and could explain their ∆P (t) data. The same

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O 3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at ferroelectric/electrode interfaces.

We investigated the ferroelectric domain wall propagation in epitaxial Pb(Zr,Ti)O3 thin films over a wide temperature range (3 -300 K). We measured the domain wall velocity under various electric fields and found that the velocity data is strongly nonlinear with electric fields, especially at low temperature. We found that, as one of surface growth problems, our domain wall velocity data from ferroelectric epitaxial film could be classified into the creep, depinning, and flow regimes due to competition between disorder and elasticity. The measured values of velocity and dynamical exponents indicate that the ferroelectric domain walls in the epitaxial films are fractal and pinned by a disorder-induced local field.

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