Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session A10: Focus Session: Ferroelectrics I |
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Sponsoring Units: DMP Chair: Matthew Dawber, Stonybrook University Room: 304 |
Monday, March 16, 2009 8:00AM - 8:36AM |
A10.00001: First-principles modeling of closure domains in ferroelectric ultrathin films Invited Speaker: Capacitors based on ferroelectric perovskites are potentially attractive for nanoelectronic devices. However, in many cases their use depends on the stability of a ferroelectric state with an out-of-plane polarization. The interplay between mechanical and electrical boundary conditions, and the local chemistry at the surface or interface might give rise to exotic patterns of the polarization, especially in the thin film regime. The screening of the interfacial polarization charges by real metallic electrodes has been extensively discussed in the literature. We have carried out first-principles computations on two other screening mechanisms in ultrathin capacitors made of a few unit cells of BaTiO$_3$ with metallic SrRuO$_3$ electrodes. First, the simulation of the energetic, structural, and electronic properties of ferroelectric domains in short circuit. \footnote{P. Aguado-Puente and J. Junquera, Phys. Rev. Lett. {\bf 100}, 177601 (2008).} The domains are stabilized down to two unit cells, adopting the form of a domain of closure, common in ferromagnetic thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, that behaves more like SrO in highly polarizable SrTiO$_3$ than in metallic SrRuO$_3$. Even if small, these lateral displacements are essential to stabilize the domains, and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in films fatigue. The closure domain structure, predicted also by other phenomenological and effective Hamiltonian models, is more general than expected. Second, the possibility of screening by a surface state or metallization of the first few layers of the ferroelectric without a top electrode is also explored. [Preview Abstract] |
Monday, March 16, 2009 8:36AM - 8:48AM |
A10.00002: Polarization Switching at the Intrinsic Coercive Field of PbTiO$_{3}$ by Changing Oxygen Partial Pressure M. Highland, T.T. Fister, M.-I. Richard, D.D. Fong, J.A. Eastman, S.K. Streiffer, P.H. Fouss, G.B. Stephenson, Carol Thompson Previously we have found that changing the partial pressure of oxygen in the gas above ultrathin PbTiO$_{3}$ films on SrRuO$_{3}$ can induce inversion in the sign of the polarization. Here we present x-ray measurements that allow us to determine the polarization magnitude and domain distribution during switching. For films of thickness above $\sim $5 nm, switching occurs by the usual mechanism of nucleation and growth of 180\r{ } domains having the same polarization magnitude but opposing signs. However, in thinner films switching of the polarization occurs without nucleation; the polarization magnitude decreases to zero and changes sign uniformly without domain formation, indicating that the intrinsic coercive field (E$_{IC})$ is reached. Since E$_{IC}$ has never been reached in oxide ferroelectrics using applied electric field, our results suggest that the barrier to nucleation is large during chemical switching. Work supported by the Dept. of Energy under Contract DE-AC02-06CH11357. [Preview Abstract] |
Monday, March 16, 2009 8:48AM - 9:00AM |
A10.00003: Understanding the surface reconstruction during chemical switching of ultrathin PbTiO$_3$ films from density functional theory Jun He, Brian Stephenson, Serge Nakhmanson First-principles calculations are used to understand the structure and energetics of the newly discovered 4$\times$1 surface reconstruction that forms under reducing conditions during chemical switching of the ferroelectric polarization in ultrathin films of PbTiO$_3$ with SrRuO$_3$ bottom electrodes coherently strained to SrTiO$_3$ (001). Relaxed surface structures are obtained for polar films with various oxygen stoichiometries in the outermost PbO layer. To model the behavior of many-unit-cell thick films, which are observed to have polarizations near the bulk value, the lowest unit cell(s) of the PbTiO$_3$ film are forced to be polar. The observed surface reconstructions are compared with experimental synchrotron x-ray measurements. [Preview Abstract] |
Monday, March 16, 2009 9:00AM - 9:12AM |
A10.00004: First-principles calculations of structural energetics of Pb-O divacancies in PbTiO$_3$ and their role in up-down asymmetry of thin films. O. Paz, S. P. Beckman, D. Vanderbilt, K. M. Rabe Defects have been proposed as an important influence on the performance of technologically relevant ferroelectric (FE) materials, even at low concentrations. In PbTiO$_3$, $V_\textrm{Pb}$-$V_\textrm{O}$ vacancy pairs (VPs) are neutral defects, with a local electric dipole moment that can couple to the bulk polarization. To investigate the role that these defects might play in imprint behavior (i.e., a history-dependent up-down asymmetry), we carried out first-principles supercell calculations of VPs in PbTiO$_3$. While previous works considered a cubic host,% \footnote{S. P\"oykk\"o and D. J. Chadi, Appl. Phys. Lett. \textbf{76}, 499 (2000).}$^,$% \footnote{E. Cockayne and B. P. Burton, Phys. Rev. B \textbf{69}, 144116 (2004).} we study the tetragonal FE case for periodic arrays of 1$^\textrm{st}$, 2$^\textrm{nd}$ and 3$^\textrm{rd}$-neighbor VPs at densities as low as 3\%. Our lowest energy VP is a 2$^\textrm{nd}$-neighbor one oriented so as to break the bulk symmetry between the up and down polarization states. Other VP arrangements are higher in energy by at least 0.2 eV. Atomic relaxation is of particular importance in lowering the energy of the 2$^\textrm{nd}$ below that of the 1$^\textrm{st}$-neighbor VP. Berry-phase polarization calculations reveal that the total polarization is only slightly modified for VPs whose dipoles are aligned with the polarization, but is significantly suppressed by antialigned VPs. [Preview Abstract] |
Monday, March 16, 2009 9:12AM - 9:24AM |
A10.00005: Study on the electro-optic effect in Pb(Zr,Ti)O$_{3}$ (001) film using spectroscopic ellipsometry Tae Dong Kang, Xiao Bo, Vitaliy Avrutin, \"Umit \"Ozg\"ur, Hadis Morko\c{c}, Jun Woo Park, Ho Suk Lee, Hosun Lee, Xiaoyu Wang, David Smith Spectroscopic ellipsometry was applied to study electro-optic effect in lead zirconate titanate$^{ }$(PZT) thin films grown epitaxially on Nb-doped SrTiO$_{3}$(001) substrates by$^{ }$RF magnetron sputtering. Multilayer model analysis was applied$^{ }$to extract the ordinary and extraordinary refractive indices of the PZT thin film with electric$^{ }$field applied along the (001) direction. The effective linear and$^{ }$quadratic coefficients at a wavelength of 632.8 nm were estimated to be -134.6$\times $10$^{-12}$~m/V and 8.5$\times $10$^{-18}$~m$^{2}$/V$^{2}$, respectively, while the individual linear electro-optic$^{ }$coefficients $r_{33}$ and $r_{13}$ were -157.1 and 22 pm/V, respectively. We attributed existence of the linear electro-optic effect in unpoled PZT$^{ }$films to the presence of a built-in polarization$^{ }$and simultaneous poling during ellipsometric measurements. [Preview Abstract] |
Monday, March 16, 2009 9:24AM - 9:36AM |
A10.00006: A first-principles study of enhancement of polarization with electric field in tetragonal PbTiO$_3$ Anindya Roy, Massimiliano Stengel, David Vanderbilt We present first-principles calculations of the electric polarization and $c/a$ ratio of ferroelectric tetragonal PbTiO$_3$ as an external electric field is applied parallel to the polarization direction. The work is motivated in part by experimental observations\footnote{ A. Grigoviev {\it et al.}, Phys.\ Rev.\ Lett.\ \textbf{100}, 027604, (2008). } showing an anomalous enhancement (i.e., nonlinear piezoelectric response) in the $c/a$ ratio for electric fields above $\sim$200\,MV/m in PbZr$_{0.2}$Ti$_{0.8}$O$_3$. Working here with pure PbTiO$_3$ for reasons of computational convenience, we calculate the strain response to an electric field applied parallel to the spontaneous polarization. We focus mainly on the case of fixed in-plane lattice constants, which appear most suited to the experimental conditions, but we also study the case in which all lattice constants are relaxed. In addition explore the effects of artificial negative pressure, with and without the in-plane epitaxial constraint. Two ab-initio computer codes are used for this purpose, and the results are compared to each other and to the experiment. [Preview Abstract] |
Monday, March 16, 2009 9:36AM - 9:48AM |
A10.00007: Polarization reversal and backswitching kinetics in epitaxial ferroic thin films Jianheng Li, Bruce Wessels Polarization reversal and backswitching kinetics were investigated in epitaxial ferroelectric BaTiO3 thin films. Using the electro-optic effect to monitor domain dynamics, the dynamic response was measured as a function of bias pulse magnitude and temperature. The dynamics followed a Kohlrausch-Williams-Watts (KWW) stretched exponential function in time. From the measurements the activation field for polarization reversal and the activation energy for domain motion were determined. The measured activation energy of 6-12 kJ/mol is in good agreement with prior experiments on bulk material and recent theoretical calculations using molecular dynamics simulations. [Preview Abstract] |
Monday, March 16, 2009 9:48AM - 10:00AM |
A10.00008: Thickness dependent ferroelectric properties of ultrathin BaTiO$_{3}$ thin films D.A. Felker, H.W. Jang, C.B. Eom, M.S. Rzchowski The thickness dependence of the coercive field and the spontaneous polarization were studied for epitaxial trilayer heterostructures with SrRuO$_{3}$ electrodes and ultrathin (001) BaTiO$_{3}$ ferroelectric layers grown on TiO$_{2}$ terminated (001) SrTiO$_{3}$ substrates. The BaTiO$_{3}$ thickness ranged from 2.4 (6 unit cells) to 50 nm. The 3.2 nm (8 unit cells) sample provides the thinnest direct electrical measurement of ferroelectricity in BaTiO$_{3}$ in a device structure, showing the thickness dependence of ferroelectric properties down into the ultrathin regime and providing an experimental upper bound on the critical thickness. The coercive field increases dramatically in thinner samples consistent with the Kay-Dunn model of domain nucleation and propagation. Below 10 nm the spontaneous polarization decreases with decreasing thickness due to decreasing screening of the depolarization field by the electrodes. For barriers thicker than 10 nm the polarization decreases due to strain relaxation in the ferroelectric barrier. We discuss these measurements, as well as the temperature dependence of the coercive fields and hysteresis loops. [Preview Abstract] |
Monday, March 16, 2009 10:00AM - 10:12AM |
A10.00009: Temperature dependence of the dielectric properties of strained barium strontium titanate films for tunable microwave applications Lisa Alldredge, Wontae Chang, Steven Kirchoefer, Jeffrey Pond Understanding strain effects is critical to achieve desirable dielectric properties in ferroelectric films, which are of interest for tunable microwave applications. Sputter-deposited Ba$_{1-x}$Sr$_{x}$TiO$_{3}$ films on (001) MgO were studied in various strain states: in-plane or out-of-plane tetragonal lattice distortions. The optimal system calibration for microwave measurements changes greatly with temperature, requiring frequent recalibration. A temperature-dependent interpolation calibration technique was developed to increase the efficiency of measurements taken as a function of temperature. The films showed significant differences in the ferroelectric phase transition due to lattice distortions, with a strong temperature dependence of the in-plane dielectric behavior for films under tensile strain and a weak temperature dependence for films under compressive strain. We believe that films under tensile strain have polarizations aligned parallel to the applied electric field and so the in-plane dielectric properties are strongly coupled with the field, while films under compressive strain have polarizations perpendicular to the field, resulting in minimal influence on the in-plane dielectric behavior. [Preview Abstract] |
Monday, March 16, 2009 10:12AM - 10:24AM |
A10.00010: Density functional study of ferroelectric-electrode interfacial effects on the stability of ferroelectricity in thin-films Wissam A. Al-Saidi , Alexie Kolpak, Ilya Grinberg, Andrew Rappe Ferroelectric (FE) thin-films are very promising materials for various technological applications. The continuous demand of miniaturization of devices based on FE thin-films by the micro-electronic industry demands an understanding of the critical thickness of ferroelectricity in thin films. Using an ab initio density-functional approach, we study the properties of several capacitor-like structures which are based on PbTiO3 and BaTiO3 ferroelectric materials. Different electrodes are used in our study to gain a thorough understanding of the electrode-ferroelectric interfaces, and the role of the interfacial chemical bonding and charge transfer in stabilizing the FE polar phase. We finally used our ab initio results to develop a phenomenological predictive model based on a Landau-Ginzburg-Devonshire functional. [Preview Abstract] |
Monday, March 16, 2009 10:24AM - 10:36AM |
A10.00011: Boundary conditions on ferroelectricity in ultrathin SrTiO$_3$ films on silicon Alexie Kolpak, Fred Walker, James Reiner, Charles Ahn, Sohrab Ismail-Beigi The properties of SrTiO$_3$ films expitaxially grown on Si(001) are strongly influenced by the electronic structure of the interface. Using density functional theory, we demonstrate the presence of an intrinsic interface dipole, the direction of which is independent of the particular combination of Sr, Ti, O, and Si atoms at the interface, and therefore independent of growth conditions. As a result of this intrinsic dipole, a local, positive polarization is induced in the SrTiO$_3$ interfacial region, fixing the electrostatic boundary conditions at the interface and preventing the formation of a negatively polarized state with a single domain. We suggest ways in which this constraint on the ferroelectric behavior can be overcome by interfacial cation doping, allowing for the integration of ferroelectricity with traditional silicon-based devices. [Preview Abstract] |
Monday, March 16, 2009 10:36AM - 10:48AM |
A10.00012: Strain Effect in the Problem of Critical Thickness for Ferroelectric Memory A.M. Bratkovsky, A.P. Levanyuk We account for inhomogeneous strains while calculating \emph {two} characteristic thicknesses arising in the problem of critical thickness for ferroelectric memory. One of them marks the stability limit of metastable single domain state under zero external voltage with respect to small fluctuations (spinodal point of the single domain state.) The second one appears when free energies of the single and multidomain state become equal while the latter is considered within one- sinusoidal approximation [1]. At this thickness the single domain state remains metastable, but one may hope that the lifetime of this state becomes suitable for the memory applications[2]. We use the Landau approach for elastically isotropic solid with a single electrostriction constant to illustrate general behavior. It is found that the effect of the elastic strains is qualitatively different for free-standing films versus films on substrates.\\[3pt] [1] A.M. Bratkovsky and A.P. Levanyuk, arXiv: 0801.1669.\\[0pt] [2] A.M. Bratkovsky and A.P. Levanyuk, Phys. Rev. Lett. {\bf 100}, 149701 (2008). [Preview Abstract] |
Monday, March 16, 2009 10:48AM - 11:00AM |
A10.00013: Unusual polarization patterns in flat epitaxial ferroelectric nanoparticles Ivan Naumov, Alexandr Bratkovsky We investigate the effects of a lattice misfit strain on a ground state and polarization patterns in flat perovskite nanoparticles (nanoislands of BaTiO$_{3}$ and PbZr$_{0.5}$Ti$_{0.5}$O$_{3}$) with the use of an \textit{ab-initio} derived effective Hamiltonian. We show that the strain strongly controls the balance between the depolarizing field and the polarization anizotropy in determining the equilibrium polarization patterns. Compressive strain favors 180 $^0$ stripe/tweed domains while a tensile strain leads to in-plane vortex formation, with the unusual intermediate phase (s) where both ordering motifs coexist [1]. The results may allow to explain contradictions in recent experimental data for ferroelectric nanoparticles. [1] Ivan Naumov and Alexander M. Bratkovsky, Phys. Rev. Lett. 101, 107601 (2008). [Preview Abstract] |
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