Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J21: Focus Session: Oxide Superlattices, Interfaces and Growth |
Hide Abstracts |
Sponsoring Units: DMP Chair: Hena Das, Cornell University Room: 323 |
Tuesday, March 19, 2013 2:30PM - 2:42PM |
J21.00001: Oxygen Octahedral Rotations in BaTiO$_{3}$/CaTiO$_{3}$ Superlattices Margaret Cosgriff, Pice Chen, Nathaniel Corey, Xifan Wu, Apurva Mehta, Hiroo Tajiri, Ho Nyung Lee, Paul Evans Complex oxide superlattices have a wide range of electronic and magnetic properties, which are affected by the structure of the interfaces between different components of the superlattice. The magnitude, coherence, and electric field response of oxygen displacements in three different BaTiO$_{3}$/CaTiO$_{3}$ superlattice compositions are measured using x-ray diffraction. The displacements are greater in compositions with more consecutive CaTiO$_{3}$ layers. The pattern of layer-by-layer alternating displacements is coherent over less than two superlattice unit cells. The net in-phase rotation of the oxygen octahedra gives rise to an x-ray reflection at (3/2 1/2 1). Density functional theory calculations for a 2(BaTiO$_{3})$/4(CaTiO$_{3})$ composition predict a decrease in displacements of oxygen octahedra between barium and calcium layers when an electric field is applied, causing an intensity increase in this reflection. We found the intensity of this reflection for this composition increases by 1.6{\%} when a 12.5 V pulse is applied, a weaker response than the 11{\%} increase predicted. When a 20 V pulse is applied, the reflection intensity actually decreases by 3{\%}, indicating a more complicated response. [Preview Abstract] |
Tuesday, March 19, 2013 2:42PM - 2:54PM |
J21.00002: Coherent X-ray Diffraction from Striped Nanodomain in a PbTiO3/SrTiO3 Superlattice Qingteng Zhang, Pice Chen, Zhonghou Cai, Matthew Dawber, Sara Callori, Paul Evans Polarization striped domains in ferroelectric/dielectric superlattices reflect the coupling between the polarization and a lattice distortion in each component layer. We have used coherent x-ray diffraction to study the variation of the striped domain pattern in a PbTiO3/SrTiO3 ferroelectric/dielectric superlattice over lateral length scales of hundreds of nanometers to microns. A coherent beam of synchrotron x-rays with a photon energy of 10 keV was focused to a spot with a diameter of approximately 200 nm. The arrangement of domains produces a speckle pattern of intensity in reciprocal space that varies according to the detailed arrangement of domains within the focal spot. When the focal spot is moved across the sample, it is found that the intensity of the total diffuse scattering remains constant while the positions of speckles vary in reciprocal space. This provides additional spatial information about the speckles which leads to better understandings of the configurations of the striped domains in ferroelectric/dielectric superlattices. This work is supported by US DOE under Grant No. DE-FG02-10ER46147. [Preview Abstract] |
Tuesday, March 19, 2013 2:54PM - 3:06PM |
J21.00003: Component-Layer-Dependent Distortion of Striped Domains in PbTiO3/SrTiO3 Superlattices Pice Chen, Margaret Cosgriff, Qingteng Zhang, Sara Callori, Bernhard Adams, Eric Dufresne, Matthew Dawber, Paul Evans Weakly-coupled ferroelectric/dielectric superlattices show novel ferroelectric properties that are not accessible in compositionally uniform ferroelectrics. Nanoscale polarization striped domains are formed as a result of the minimization of the energy associated with depolarization fields. The dielectric layers are polarized, however, with a magnitude that is much smaller than in the ferroelectric layers. The unequal distribution of polarization has been predicted to induce layer-dependent dynamics of the polarization switching of striped domains. Here we experimentally test this prediction in a PbTiO3/SrTiO3 superlattice with time-resolved x-ray diffraction under electric fields up to 2.38 MV/cm. The intensities of x-ray reflections arising from striped domains decrease at a nanosecond timescale, as the polarization switching occurs. The relative magnitude of the intensity change depends on the indices of reflections. We compared the observed intensity changes of domain reflections with a kinematic x-ray simulation. The measurement agrees with a model in which the average polarization of striped domains in dielectric SrTiO3 layers is slightly increased under applied electric fields, and the ferroelectric PbTiO3 layers are unchanged. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:42PM |
J21.00004: Static and Dynamic Properties of Ferroelectric Nanodomains Invited Speaker: Pavlo Zubko The performance of ferroelectric devices is closely connected with the structure and dynamics of ferroelectric domains. In ultrathin ferroelectrics, very dense domain structures can arise naturally in response to the presence of a depolarizing field and are expected to exhibit unusual static and dynamic behavior. Superlattices composed of ultrathin ferroelectric blocks sandwiched between paraelectric layers offer an ideal system for investigating the structure and functional properties of such nanodomains. The electrostatic coupling between the ferroelectric layers can be controlled by modifying the thicknesses of the paraelectric layers and the domain structure can be tailored by exploiting Kittel's law. X-ray diffraction and transmission electron microscopy combined with electron energy loss spectroscopy were used to study the electrostatic interactions in PbTiO$_3$-SrTiO$_3$ superlattices, revealing highly inhomogeneous polarization and structural profiles that arise due to the presence of ferroelectric nanodomains. The superlattice geometry is also ideal for studying the response of the ultrathin ferroelectric layers to applied fields. Large uniform electric fields can be supported without significant leakage, while changes in the domain structure can be observed simultaneously using X-ray diffraction. The tiny, reversible displacements of the nanodomain walls were found to contribute to a large enhancement of the effective dielectric response that persists over a broad range of temperatures and exhibits low losses. The static and dynamic properties of nanodomains in PbTiO$_3$-SrTiO$_3$ superlattices will be discussed and compared with those of isolated thin films. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J21.00005: Photo-induced stabilization and enhancement of the ferroelectric polarization in Ba$_{0.1}$Sr$_{0.9}$TiO$_{3}$/La$_{0.7}$Ca(Sr)$_{0.3}$MnO$_{3}$ thin film heterostructures Y.M. Sheu, S.A. Trugman, L. Yan, Q.X. Jia, A.J. Taylor, R.P. Prasankumar We demonstrate that optically pumping carriers across the interface between ferroelectric Ba$_{0.1}$Sr$_{0.9}$TiO$_{3}$ and ferromagnetic La$_{0.7}$Ca(Sr)$_{0.3}$MnO$_{3}$ thin films can not only stabilize but also enhance (``write'') the remanent polarization, which breaks crystal inversion symmetry, generating an optical second-harmonic signal that we ``read.'' The new photo-induced (``written'') ferroelectric state remains stable at low temperatures for over one day after removing the laser pulse. By optically decoupling the energy of the internal electric field from the ferroelectric double potential wells, we show that the displacement of the Ti atom increases, leading to a larger, more stable polarization state that may be suitable for applications in data storage (using similar writing and reading processes) as well as energy storage (e.g., solar nanocapacitors). [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J21.00006: Low dielectric loss in electric field-tunable Ba$_{x}$Sr$_{1-x}$TiO$_3$ thin films grown by hybrid molecular beam epitaxy Adam Kajdos, Evgeny Mikheev, Adam Hauser, Susanne Stemmer Ba$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_3$ (BST) is an electric field-tunable dielectric that exhibits both low dielectric loss and high tunability, making it a system of particular interest for microwave device applications. In this presentation we report on the dielectric properties of paraelectric BST films (x $=$ 0.19 - 0.46) grown by hybrid molecular beam epitaxy (MBE) on epitaxial Pt bottom electrodes. Using the hybrid MBE technique to achieve unprecedented stoichiometry control and low defect densities, we demonstrate dielectric quality factors ($Q =$1/tan $\delta $, where tan $\delta $ is the dielectric loss tangent) exceeding 1000, an order of magnitude greater than any previously reported BST thin film. These low-loss films also exhibit high electric field tunability, with the relative tunability, $n$(E) $= \varepsilon $(0)/$\varepsilon $(E), i.e. the ratio of the dielectric permittivity under zero and positive applied field, respectively, exceeding $n =$ 5. The high quality of these BST films enables the investigation of intrinsic dielectric loss mechanisms, such as quasi-Debye loss. We will discuss the effect of point defect densities, stoichiometry and microstructure on the dielectric properties of these BST thin films. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J21.00007: Tunable dielectric properties of Barium Magnesium Niobate (BMN) doped Barium Strontium Titanate (BST) thin films by magnetron sputtering Fikadu Alema, Aaron Reinholz, Konstantin Pokhodnya We report on the tunable dielectric properties of Mg and Nb co-doped Ba$_{0.45}$Sr$_{0.55}$TiO$_{3}$ (BST) thin film prepared by the magnetron sputtering using BST target (pure and doped with BaMg$_{0.33}$Nb$_{0.67}$O$_{3}$ (BMN)) on Pt/TiO$_{2}$/SiO$_{2}$/Al$_{2}$O$_{3}$ 4'' wafers at 700 $^{\circ}$C under oxygen atmosphere. The electrical measurements are conducted on 2432 metal-ferroelectric-metal capacitors using Pt as the top and bottom electrode. The crystalline structure, microstructure, and surface morphology of the films are analyzed and correlated to the films dielectric properties. The BMN doped and undoped BST films have shown tunabilities of 48{\%} and 52{\%}; and leakage current densities of 2.2x10$^{-6}$ A/cm$^{2}$ and 3.7x10$^{-5}$ A/cm$^{2}$, respectively at 0.5 MV/cm bias field. The results indicate that the BMN doped film exhibits a lower leakage current with no significant decrease in tunability. Due to similar electronegativity and ionic radii, it was suggested that both Mg$^{2+}$ (accepter-type) and Nb$^{5+}$ (donor-type) dopants substitutTi$^{4+}$ ion in BST. The improvement in the film dielectric losses and leakage current with insignificant loss of tunability is attributed to the adversary effects of Mg$^{2+}$ and Nb$^{5+}$ in BST. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J21.00008: Atomic Layer-by-Layer Growth of Homo-epitaxial SrTiO$_{3}$ Films by Laser MBE Maryam Golalikhani, Qingyu Lei, Guozhen Liu, Ke Chen, Suilin Shi, Fuqiang Huang, Xiaoxing Xi A precise customization of oxide hetero-structures at the atomic layer level became possible with layer-by-layer growth of oxide thin films by laser MBE from separate oxide targets. In situ characterization during growth helps to optimize the composition of these superlattices. In this work we focused on the reflection high energy electron diffraction (RHEED) spot analysis for in situ growth control of stoichiometric SrTiO$_{3}$ thin films in an atomic layer-by-layer manner from separate SrO and TiO$_{2}$ targets. We have shown that both stoichiometry and full monolayer dose can be controlled using RHEED diffraction spot intensity oscillations. Observations of a single sharp peak in x-ray diffraction spectra confirm the same composition of the films as that of the stoichiometric SrTiO$_{3}$ substrate. We have successfully demonstrated that this new approach of laser MBE can achieve the same precise stoichiometry control as shown by reactive MBE. [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J21.00009: In-situ x-ray diffraction studies of the epitaxial growth of BaTiO$_{3}$ on SrTiO$_{3}$ S.J. Callori, J. Sinsheimer, B. Bein, P.V. Chinta, A. Ashrafi, R. Headrick, M. Dawber When BaTiO$_{3}$ is grown on SrTiO$_{3}$ it is subject to a large epitaxial compressive strain, which means that it can actually be ferroelectric during growth. Therefore, screening provided by a bottom electrode is important in realizing fully strained BaTiO$_{3}$ thin films. To fully understand the role of strain, electrical boundary conditions and deposition technique in forming highly strained ferroelectric thin films, we grew thin films of BaTiO$_{3}$ on SrTiO$_{3}$, both with and without SrRuO$_{3}$ bottom electrodes, using both off axis RF magnetron sputtering and pulsed laser deposition, while the growth was monitored by in-situ x-ray diffraction at X21 at the National Synchrotron Light Source. Out-of-plane and in-plane lattice parameters and x-ray reflectivity were measured during growth, allowing changes in strain and tetragonality of the films to be correlated with changes in growth modes. The presence or absence of an electrode impacted the relaxation and tetragonality of the films differently for the two different growth techniques. Information gained by these synchrotron experiments provides important guidance for the growth of high quality ferroelectric thin films and superlattices. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J21.00010: Polar structure evolution of ultrathin BaTiO$_3$ films: in-situ LEED I-V Junsoo Shin, Von Braun Nascimento, Diogo Duarte dos Reis, Lina Chen, E. Ward Plummer, Jiandi Zhang Understanding the fundamental physics of ferroelectricity in ultrathin films is a key issue of a critical size, of which a strong debate has erupted over the existence for decades. Ferroelectricity has previously been observed experimentally down to a few unit cells, depending on a complex interplay of electrostatic depolarization energy, domain formation, and so on. Using in situ Low Energy Electron Diffraction (LEED) I-V, we have systematically examined the structure evolution of ultrathin fully strained BaTiO$_{3}$ films (1-12 ML) on Nb-doped SrTiO$_{3}$. Comparison of observed diffraction intensities for 10 ML films at 300 K with calculated intensities reveals a vertical displacement of the central Ti, corresponding to a single-domain upward polar state. To investigate the polar structure evolution of ultrathin films, we have calculated all R-factors between two sets of experimental curves from 1 ML to 12 ML with 10 ML polar structure curve as a reference. As a result, we demonstrate that 8-12 ML thick BaTiO$_{3}$ films have very similar polar structures, whereas thinner films (1-7 ML) have continuously evolved from uncorrelated to correlated polar structures. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J21.00011: Epitaxial growth of BaTiO$_{3}$ on Ge Patrick Ponath, Agham Posadas, Kurt Fredrickson, Alexander Kvit, Alex Demkov Germanium with its higher hole and electron mobility than silicon, in conjunction with a ferroelectric material like barium titanate (BTO) might be a potential candidate for a Ferroelectric RAM in the future. We report the epitaxial growth of BTO directly on a germanium (100) substrate. First, 0.5 monolayer of strontium metal is deposited on the cleaned Ge surface as a passivation layer at 600C. Molecular oxygen to a pressure of 5x10$^{-6}$ torr is then introduced and barium and titanium are alternately deposited on the substrate at the same temperature. The BTO film is crystalline as-deposited and remains so throughout the growth as monitored by in situ reflection high energy electron diffraction. X-ray diffraction measurements of BTO films show only substrate peaks and ($h$00) peaks of BTO, indicating an in-plane ferroelectric polarization. This is expected due to the thermal expansion mismatch of BTO and Ge. We will report on efforts to induce out of plane polarization of BTO films grown on Ge. We have also measured the valence band offset between BTO and Ge using x-ray photoelectron spectroscopy (XPS) and found it to be 2.6 eV, resulting in a zero conduction band offset. We compare this value to density functional calculations of the offset. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J21.00012: Theoretical study of growth of Pt on BaTiO$_{3}$ slabs Kurt Fredrickson, Agham Posadas, Catherine Dubourdieu, John Bruley, Alexander Demkov BaTiO$_{3}$ (BTO) is a well-known ferroelectric perovskite, which is tetragonal at room temperature. As BTO has potential as a component of a ferroelectric field-effect transistor (Matthews \textit{et al.}, Science \textbf{276}, 238 (1997)), development of metal electrodes is of crucial importance. We investigate Pt deposition on (001) BTO. The metal is deposited in ultra-high vacuum in the molecular beam epitaxy reactor and characterized using transmission electron microscopy and in situ photoemission. Using density functional theory we calculate the surface energies of Pt(001), (011), and (111), and BTO (001) and investigate the wetting conditions of Pt(001) and (011) on TiO$_{2}$-terminated BTO(001). We use transmission electron microscopy to examine the interfaces of epitaxially grown Pt on BTO and compare the sizes of the islands with the sizes predicted by theory. In addition, we examine the dependence of ferroelectricity on the sample thickness. Results of our photoemission experiments are compared with first principles spectra. [Preview Abstract] |
Tuesday, March 19, 2013 5:18PM - 5:30PM |
J21.00013: Dielectric properties of perovskite oxynitride epitaxial thin films Daichi Oka, Yasushi Hirose, Hideyuki Kamisaka, Tomoteru Fukumura, Tetsuya Hasegawa, Seiji Ito, Akira Morita, Hiroyuki Matsuzaki, Katsuyuki Fukutani, Satoshi Ishii, Kimikazu Sasa, Daiichiro Sekiba Perovskite oxynitrides with the formula \textit{AB}O$_{2}$N are expected to show unique electric properties hardly accessible by conventional oxides. For example, N-2$p$ orbitals tend to form a shallow band at the top of the oxygen-nature valence band. This enables us to develop narrow-bandgap ferroelectric materials with $d^{0}$ configuration, which is applicable to ferroelectrics-based photovoltaic cells. In this study, we fabricated (001)-oriented epitaxial thin films of SrTaO$_{2}$N by nitrogen-plasma assisted pulsed laser deposition on (Nb-doped) SrTiO$_{3}$ substrate. X-ray diffraction measurements revealed large lattice distortion (c/a of 1.015-1.03) due to compressive strain from substrate (mismatch of -3.2 {\%}), though it is partially relaxed. The films had yellow color with a bandgap of about 600 nm. Ferroelectric behavior was observed at room temperature by piezoresponse force microscopy. As far as we know, this is the first experimental observation for ferroelectricity in perovskite oxynitrides. First principles calculations suggested that the ferroelectricity originates from \textit{trans}-type nitrogen ordering, which can be driven by compressive strain. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700