Symposium Organizers
Arthur P. Baddorf Oak Ridge National Laboratory
Ulrike Diebold Tulane University
Dietrich Hesse Max-Planck-Inst. of Microstructure Physics
Andrew Rappe University of Pennsylvania
Naoya Shibata The University of Tokyo
M1: Two-dimensional Electron Gases in Oxide Heterostructures I
Session Chairs
Tuesday PM, April 06, 2010
Room 2003 (Moscone West)
9:30 AM - M1.1
Interface-to-Interface Doping Mechanism in LaAlO3/SrTiO3 Heterostructures.
Gerwin Hassink 1 , Hans Boschker 1 , Gertjan Koster 1 , Guus Rijnders 1 , Dave Blank 1
1 MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands
Show AbstractTwo-dimensional electron layers have been a subject of scientific research and commercial application. Doping of such layers, either chemically or electrically, enhances their usefulness for both. Recently, such doping has been observed in the LaAlO
3/LaVO
3 [1] and LaAlO
3/SrTiO
3 [2] systems. In this contribution, the work on the LaAlO
3/SrTiO
3 system is extended to heterostructures with two
n-type LaO//TiO
2 interfaces. We will focus on the heteroepitaxial growth, transport properties as well as the dipole model, explaining the doping behaviour.Heterostructures of LaAlO
3 and SrTiO
3 were fabricated using pulsed laser deposition. A second LaO//TiO
2 interface was created using a monolayer of LaTiO
3. A series of samples was grown where the interface separation was varied. XRD measurements indicated that the films were coherently grown on the SrTiO
3 substrates. AFM scans of the surface of the films revealed step-and-terrace structures.Hall measurements showed that the electron density of these double
n-type
nn heterostructures depends on the interface separation. However, the trend is opposite that of the
np heterostructures published in literature [2]. Where for the
np heterostructures the electron density increases with increasing interface separation, for the
nn heterostructures it decreases with increasing interface separation.Qualitatively, this can easily be understood. The
p-type AlO
2//SrO interface is nominally
p-doped and acts as a sink for electrons [3]. Oppositely, the
n-type interface acts as a source of electrons. For
np heterostructures at small interface separation electrons from the
n-type interface transfer to the
p-type interface, reducing the electron density. As the interface separation increases, this doping becomes more energetically unfavourable and the electron density increases. For the
nn heterostructures the LaTiO
3 interface acts as a reservoir of electrons that can be doped into the other interface. Again the transfer is reduced for larger interface separations, leading to a decrease of the electron density.More quantitatively, this behaviour can be modelled using a simple dipole model to describe the polar discontinuity inherent to the LaAlO
3/SrTiO
3 system [4]. From this model the binding energy of the transferred electrons at the donor/receiver interface can be determined. It is positive for the
np heterostructures, reflecting the electron trapping. For the
nn heterostructures it was found to be negative, indicating that electron transfer lowers the energy of the system. Band schematics representing the two mechanisms can then be drawn.
- M. Takizawa et al., Phys. Rev. Lett. 102, 236401 (2009).
- M. Huijben et al., Nat. Mat. 5. 556-560 (2006).
- N. Nakagawa, H.Y. Hwang & D.A. Muller, Nat. Mat. 5, 204-209 (2006).
- W.-J. Son et al., Phys. Rev. B 79, 245411 (2009).
9:45 AM - M1.2
Instability and Intermixing at the LaAlO3/SrTiO3(001) Interface.
Scott Chambers 1 , Tim Droubay 1 , Liang Qiao 1 , Mark Engelhard 1 , Weilin Jiang 1 , V. Shutthanandan 1 , Peter Sushko 3 , Tian Feng 2 , Hang Dong Lee 2 , Torgny Gustafsson 2 , Eric Garfunkel 2 , Hiroki Sato 4 , Yasuyuki Hikita 4 , Chris Bell 4 , Harold Hwang 4
1 Fundamental and Computational Sciences, Pacific Northwest National Laboratory, Richland, Washington, United States, 3 Physics, University College London, London United Kingdom, 2 Physics, Rutgers University, Piscataway, New Jersey, United States, 4 Advanced Materials Science, University of Tokyo, Chiba Japan
Show AbstractThe LaAlO3/SrTiO3(001) heterojunction exhibits novel electronic properties that have been widely explored. Data interpretation is typically based on an idealized model involving stoichiometric LaAlO3 and a rather abrupt interface. We show that LaAlO3/SrTiO3(001) interfaces grown by pulsed laser deposition from three different laboratories undergo extensive intermixing upon formation due to thermodynamic instability. This conclusion is drawn from data obtained with three different techniques, as well as first-principles calculations on the energetics of intermixing. LAO films were grown on TiO2-terminated STO(001) by on-axis PLD at the University of Tokyo and the University of Augsburg, as well as by off-axis PLD at PNNL. Specimens were characterized at PNNL and Rutgers using angle-resolved x-ray photoelectron spectroscopy and medium energy ion scattering, respectively, as well as high-resolution Rutherford backscattering spectrometry at National Electrostatics Corporation. The LAO/STO system prepared by pulsed laser deposition is most properly characterized by a complex quaternary oxide with concentration gradients normal to the interface not previously considered in connection with this much-studied interface. These results have significant implications for the mechanism of electrical conductivity at the LAO/STO interface. Charge transfer from LAO to STO, as postulated to occur in order to alleviate the ‘polar catastrophe’ stemming from the growth of a polar film (LAO) on a nonpolar substrate (STO), may be occurring. However, the impact on electronic structure created by the extensive intermixing that occurs cannot be ignored in constructing realistic physical models of the interface.
10:00 AM - M1.3
High Temperature Conductivity Measurements of LaAlO3/SrTiO3 Heterostructures.
Felix Gunkel 1 , Susanne Hoffmann-Eifert 1 , Regina Dittmann 1 , Shaobo Mi 2 , Chunlin Jia 2 , Paul Meuffels 1 , Rainer Waser 1
1 IFF-6, FZ Jülich, Jülich Germany, 2 , Ernst Ruska-Centre, Jülich Germany
Show AbstractInterfaces in perovskite-type oxides exhibit significantly different electrical properties compared to the pure bulk material. Interface engineering with respect to grain boundary doping and the creation of heterointerfaces is performed in order to achieve new functionalities. An interesting and prominent example is the highly conducting interface in epitaxial heterostructures of LaAlO3 on SrTiO3 grown by PLD methods. A strong effect of the oxygen atmosphere during PLD growth has been observed, but no studies of the conductivity of the interface under equilibrium oxygen atmospheres have been reported so far.By means of the point defect chemistry model effects of doping elements and of exchange reactions with the ambient oxygen atmosphere on the conduction behavior of SrTiO3 single crystals can be explained. The model is proved by high temperature conductivity measurements in controlled oxygen atmospheres. Using a special measurement set-up consisting of a pO2 pumping system and a four point characterization technique we were able to collect for the first time electrical conductivity data of a conducting LaAlO3/SrTiO3 heterostructure under controlled oxygen partial pressure. In order to reach an equilibrium state the temperature range was set between 550 °C and 700°C while the pO2 values were controlled between 10-23 bar to 1 bar.The high temperature conductivity data of the conducting heterostructures are compared with the pure bulk single crystals of SrTiO3 and LaAlO3. Significant differences could be observed in a certain temperature-oxygen pressure regime. Thereby, it can be excluded that the electronic conductivity of the interface originates from mobile oxygen vacancies. Instead we observed clear hints on localized donor states at the interface. According to the interdiffusion observed by HRTEM at the LaAlO3/SrTiO3 interface, these donor states might be identified with incorporated lanthanum.
10:15 AM - M1.4
Two-dimensional Electron Gas at LaAlO3/SrTiO3 Heterointerfaces on Silicon.
Jae-Wan Park 1 , Daniela Bogorin 2 , Cheng Cen 2 , David Felker 3 , Christofer Nelson 4 , Yi Zhang 4 , Chung Wung Bark 1 , Chad Folkman 1 , Xiaoqing Pan 4 , Mark Rzchowski 3 , Jeremy Levy 2 , Chang-Beom Eom 1
1 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 3 Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin, United States, 4 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe heterointerfaces between different oxide layers can display remarkable electrical properties that differ from either constituent, such as two-dimensional electron gas (2DEG) and interfacial superconductivity. Reversible nanoscale control over the metal-insulator transition in a 2DEG, formed at the heterointerface between LaAlO3 and SrTiO3, raises the possibility to develop ultrahigh-density oxide nanoelectronics. Prerequisites to the development of new technologies are integration with existing semiconductor electronics platforms and scaling to a commercially available large wafer process. Here, we demonstrate the room temperature conductivity switching of 2DEG nanowires formed at LaAlO3/SrTiO3 heterointerfaces grown directly on (001) Si substrates. The electrical transport properties of LaAlO3/SrTiO3 heterointerface on Si are comparable to those on SrTiO3 bulk single crystal. The ability to form reversible conducting nanostructures below ~10 nm-scales highlights the viability of this materials synthesis route for commercial device applications. Atomic-scale control of the surfaces of quasi-single-crystal STO templates on Si substrates also inspires the development of nanoelectronics using novel oxide interfacial phenomena.
11:00 AM - **M1.5
Two-dimensional Multichannel Conduction and Interface Charge Screening in Oxide Superlattices.
Ho Nyung Lee 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe discovery of high mobility, two-dimensional (2D) electron gases formed at the interface of two insulators opened a door to a fascinating new approach for realizing oxide electronics. However, the charge carrier transport at electronically reconstructed 2D interfaces is not fully understood so far. Here we report the transport properties of atomic-scale LaTiO3/SrTiO3 superlattices comprised of unit-cell-thin layers. We have found that the resulting electronically reconstructed interface generates a very high-density electron gas, i.e. 0.5e per interface unit (~3×1014 cm-2), but the carrier transport with high mobility is rather limited due to the high density carriers. This transport bottleneck, however, can be alleviated by spatial redistribution of charge carriers, i.e. multichannel conduction, resulting in highly increased carrier mobility. The transport properties of another type of superlattices composed of polar LaAlO3 and nonpolar SrTiO3 will be also presented. In this case, we have found that the thickness of constituent layers greatly influences the conducting states of superlattices. This originates from the polarization screening of interface charges generated by p- and n-type interfaces. A similar phenomenon found in ferroelectric heterostructures will be comparatively shown in order to explain the underlying mechanism.*Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy.
11:30 AM - M1.6
Tunable Conducting States in SrTiO3-based Heterostructures Grown by Pulsed Laser Deposition.
Sung Seok Seo 1 , Zsolt Marton 1 , Ho Nyung Lee 1
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe interfacial metallic state with high-mobility is a scientifically interesting phenomenon with potential in many technical applications. Recently, observation of the metallic transport in a heterointerface between two insulators of LaAlO3 (LAO) and SrTiO3 (STO) has attracted a lot of attention due to its high-mobility (>103 cm2V-1s-1) at low temperatures. Following studies also have revealed a number of interesting features such as electric-field controllable superconductivity at the LAO/STO heterointerface while concerns about the oxygen off-stoichiometry as its origin have been under debate. In this presentation, we discuss the transport properties in STO-based oxide heterostructures grown by pulsed laser deposition. In LAO/STO heterostructures, we find experimental evidences showing that there are multiple types of conducting carriers with different mobilities by optical spectroscopy and conventional dc-Hall measurements. Since the optical spectroscopy is an ac-transport measurement technique, its spectral analysis can provide us with both qualitative and quantitative information on the nature of conducting carriers. When multiple types of conducting carriers exist, however, the optical method dominantly reflects the contribution from the high-density carriers whereas dc-transport measurement may exaggerate the contribution of the high-mobility carriers even though their density is very low. By comparing the physical quantities extracted from the optical spectroscopic measurements with the results of conventional dc-transport, we suggest that only a small fraction of the carriers is responsible for the high-mobility observed in the LAO/STO heterostructure while the majority of conducting carriers have low-mobility of around 10 cm2V-1s-1. We extend this approach to various STO-based oxide heterostructures and discuss about the fact that their conducting states can be influenced by the energetic process of pulsed laser deposition.Research sponsored by the Division of Materials Sciences and Engineering, US Department of Energy.
11:45 AM - M1.7
Quantifying the Effect of Intermixing in SrTiO3/LaMnO3/La(Al0.5Sc0.5)O3 Superlattices.
Hyun-Sik Kim 1 , Michael Biegalski 2 , Hans Christen 1 2
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractA fascinating range of properties emerge in perovskite heterostructures and can be attributed to electronic effects at atomically sharp interfaces. However, rather similar properties could also be found if the interfaces were chemically broad, leading to ultrathin layers of magnetic, metallic, or superconducting solid-solutions. Pulsed-laser deposition (PLD) readily allows us to intentionally form such solid-solutions (including meta-stable structures). This makes it possible to quantitatively determine the possible effects of intermixing, and thus to clearly identify interfacial effects that cannot possibly be explained by chemical broadening of an interface. This study examines the effect of intermixing in trilayer supperlattices composed of SrTiO3/LaMnO3/La(Al0.5Sc0.5)O3, in which a strongly enhanced magnetization is observed. By studying the alloys of (LaMnO3)1-x(SrTiO3)x (i.e. La1-xSrxMn1-xTixO3) and (LaMnO3)1-x(LaAl0.5Sc0.5O3)x (i.e. LaMn1-xAl0.5xSc0.5xO3), we demonstrate that interdiffusion explains only a small fraction of the enhanced magnetization. However, these alloys by themselves present interesting properties: In fact, isovalent substitutions in both LaMn1-xScxO3 and LaMn1-xAlxO3 result in an insulating, ferromagnetic phase. In our epitaxial films, the saturation ferromagnetic moments increase with x from a background magnetization of 0.27 μB/Mn for LaMnO3 (x = 0) to 1.842 and 1.905 μB/Mn with Al and Sc, respectively. These results can be compared to earlier studies of isovalently-substituted manganites in order to understand the origin of the magnetism in these materials. This presentation will thus address the mechanisms of enhanced magnetization both in the superlattices and in the solid-solution films. Research sponsored by the Division of Materials Science and Engineering (HSK, HMC) and the Division of Scientific User Facilities (MDB), US Department of Energy.
12:00 PM - M1.8
The Effect of Oxygen Pressure on the Intermixing at the Interface Between Epitaxial LaAlO3 Thin Films on (001) SrTiO3 Substrates.
Nikolina Ljustina 1 , Alexey Kalabukhov 2 , Johan Borjesson 1 , Dag Winkler 2 , Eva Olsson 1
1 Microscopy and Microanalysis, Chalmers University of Technology, Gothenburg Sweden, 2 Department of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg Sweden
Show AbstractThe conducting region at the interface between epitaxial LaAlO3 (LAO) thin films and SrTiO3 (STO) substrate is proposed to be due to oxygen vacancies and/or the polar discontinuity at the LAO/STO interface [1-4]. The oxygen pressure during the thin film growth as well as the thickness of the LAO thin film are crucial parameters that determine the electrical transport properties. This work concerns the structure of the interface between epitaxial LAO thin films, deposited on Ti terminated (001) STO substrates. Films grown at two different oxygen pressures, 10-2 mbar (non-conducting interface) and 10-4 mbar (conducting interface) have been investigated. The interfacial structure of the LAO/STO has characterized using high-resolution analytical transmission electron microscopy (TEM) of cross-sectional and plan-view samples. Details of the atomic structure have been determined using the atomic number contrast in high angle annular dark field scanning TEM (HAADF STEM) images and also electron energy loss spectroscopy (EELS). Intermixing was observed and there was a significant difference in the extent of La/Sr compared to Al/Ti intermixing. The correlation between the intermixing of material at the interface, oxygen pressure during the film growth and properties will be discussed. [1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004)[2] S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider and J. Mannhart, Science 313, 1935 (2006)[3] M. Huijben, A. Brinkman, G. Koster, G. Rijnders, H. Hilgenkamp and D.H.A. Blank, Advanced Materials 21, 1 (2009)[4]A. Kalabukhov, R. Gunnarsson, J. Börjesson, E. Olsson, T. Claeson and D. Winkler, Phys. Rev. B 75, 121404 (2007).
12:15 PM - **M1.9
Novel Phase Boundaries and Functional Properties in Perovskite Oxide Superlattices from First Principles.
Karin Rabe 1 , Jun Hee Lee 1 , Carl-Johan Eklund 1 , Lucia Palova 1
1 Physics and Astronomy, Rutgers University, Piscataway, New Jersey, United States
Show AbstractIn perovskite oxides, a wide variety of distorted equilibrium structures can be realized, including ferroelectric, antiferroelectric, antiferrodistortive, and mixed-character structures. For a particular system, the ground state structure can be understood to be determined by the coupling and competition of various lattice instabilities of the ideal perovskite high-symmetry reference structure. In many cases, there may be one or more distinct alternative structures which have very low energy relative to the ground state but are not manifest in observations of the equilibrium phase. These structures can, however, be favored by the symmetry breaking and atomic rearrangements at interfaces and the high strains accessible in ultra-short-period layered superlattices. Near the resulting structural and magnetic phase boundaries, the responses to external perturbations can be enhanced, yielding desirable functional behavior.In this talk, we develop and illustrate these ideas by presenting first-principles results for systems that exhibit novel phase transitions with epitaxial strain and artificial superlattice structuring, drawing primarily on recent studies of titanate and manganate perovskites.
M2: The Surface Chemistry of Perovskite Oxides
Session Chairs
Tuesday PM, April 06, 2010
Room 2003 (Moscone West)
2:30 PM - M2.1
Correlations of Structural, Chemical and Electronic State on La0.8Sr0.2MnO3 Dense Thin-film Surfaces.
Khabiboulakh Katsiev 1 , Bilge Yildiz 1 , Stefan Krause 2 , Clemens Heske 2 , Hui Du 3 , Paul Salvador 3
1 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Chemistry, Univesity of Nevada, Las Vegas, Nevada, United States, 3 Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractPerovskite type ionic-electronic conductor oxides are used as Solid Oxide Fuel Cell (SOFC) cathodes. A major limitation in cathode performance is the slow kinetics of oxygen exchange on the perovskite oxide surfaces, particularly at lower temperatures (T<700 C) desired for material durability. A fundamental understanding of the surface structure, electronic and chemical state and its relation to the oxygen reduction at the atomistic level is essential for the development of cathodes with enhanced electrocatalytic activity. The objective here is to identify correlations of the surface structure, electron tunneling properties and chemical characteristics on dense thin-film model cathodes, particularly La0.8Sr0.2MnO3 (LSM). We deployed a new in situ approach combining surface sensitive probes of electronic structure and chemical state on the dense thin film cathodes – scanning tunneling microscopy and spectroscopy (STM/STS), Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy (XPS). Utilizing these surface probes, particularly the STM/STS, at high temperature and non-UHV conditions is unique, and makes it possible to relate the chemical and electronic state of the model cathode surfaces closely to the reacting environment of SOFC cathodes.Two coupled investigations are reported here. First focuses on the correlation between the surface chemistry and the electron transfer characteristics of LSM. For this, Sr-enrichment and Mn-depletion were found on the surface at high temperature, accompanied by a reduction in tunneling conductance in STS. This suggests that the Mn-terminated surfaces are more active for electron transfer in oxygen reduction compared to the (La,Sr)-terminated surfaces.Second focuses on the role of the structural inhomogeneities on the electronic properties of the surface. High resolution mapping of the tunneling spectra onto the surface topography of polycrystalline dense-thin film LSM surfaces showed a higher electron tunneling rate at select grain boundaries compared to grain surfaces. Furthermore, regardless of the grain boundaries, a broad distribution of the STS spectra was observed on the grain surfaces – for example, on a 50nm-thick LSM film on ZrO2 substrate, the band gap varied from 1.9 to 3.6 eV at room temperature. We are investigating the reasons behind this phenomenon at smaller spatial scales, on individual terraces of single crystal epitaxial LSM film surfaces using the STM. Post-growth annealing of the epitaxial films in oxygen revealed formation of distinct regions on the terraces, evolving with temperature. Different electron tunneling rates were consistently correlated to the dissimilar surface regions identified. The differences in the surface structure and electron tunneling are attributed to partial oxidation of the terrace surface. Higher resolution investigations to probe the atomic scale explanation for these correlations are ongoing using STM/STS and XPS in controlled environment.
2:45 PM - M2.2
Investigation of High Temperature Catalytic Activity of Solid Oxide Fuel Cell Cathode Using Surface Engineered Thin Films.
Lu Yan 1 , Balasubramaniam Kavaipatti 1 , Shanling Wang 1 , Hui Du 1 , Paul Salvador 1
1 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractConsiderable effort has been expended to reduce cathode losses in solid oxide fuel cells (SOFCs) over the past few decades and, in spite of considerable progress, it is becoming increasingly clear that further improvement of cell performance is likely to come from enhancements of the surface properties of cathode materials. To help build correlations between surface characteristics and properties, we investigated the relationships between the surface chemistry / microstructure and the oxygen reduction reaction for thin films of cathode materials. Two distinct types of perovskite thin films having smooth surface morphologies were fabricated by pulsed laser deposition (PLD): the first were epitaxial single crystal films deposited on perovskite substrates, and the second were epitaxial multi-variant films deposited on fluorite substrates. Epitaxial single crystal films of (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3 films were deposited on (100), (110), and (111) orientations on SrTiO3 and/or NdGaO3 substrates, and (100) and (110) oriented multi-variant films were deposited on (100) and (111) oriented fluorite substrates. Electrical conductivity relaxation (ECR) and transient Kelvin probe (TKP) measurements were carried out at elevated temperatures over a range of oxygen pressures to determine the oxygen surface exchange coefficient and contact potential variation with oxygen pressure. The values of the oxygen exchange coefficient, activation energy, work function change during reaction, and the effect of crystallographic orientation and film thickness, will be presented quantitatively. For example, the surface exchange coefficient determined from ECR varies by 50% as a function of orientation, though this anisotropy value is temperature dependent. These values will also be discussed with respect to mechanisms of oxygen exchange between the two systems, which differ in their defect chemistry.
3:00 PM - **M2.3
Surface Chemistry of Ferroelectric Oxides.
Yang Yun 1 2 , Matthew Herdiech 1 2 , Harry Moenig 1 2 , Kevin Garrity 1 3 , Alexie Kolpak 1 3 , Sohrab Ismail-Beigi 1 3 , Eric Altman 1 2
1 Department of Chemical Engineering, Yale University, New Haven, Connecticut, United States, 2 Center for Interfacial Structure and Phenomena, Yale University, New Haven, Connecticut, United States, 3 Department of Applied Physics, Yale University, New Haven, Connecticut, United States
Show AbstractIt has long been recognized that the polar and switchable nature of ferroelectric surfaces can potentially lead to polarization direction-dependent surface chemistry that may be exploited to create switchable catalysts and chemical sensors. Therefore, we have been studying the polarization dependence of the structure and chemistry of ferroelectric oxides. Despite the expectation that the bare polar surfaces that result from ferroelectric polarization would reconstruct, we only observed (1x1) surface diffraction patterns for both positively and negatively poled LiNbO3 (0001) surfaces. Ion scattering and photoelectron spectroscopies indicated that the surfaces were predominantly oxygen-terminated, also independent of the polarization direction. Despite the structural and spectroscopic similarities between the positively and negatively poled LiNbO3 surfaces, it was found that the polar molecules acetic acid and 1-propanol adsorbed more strongly on the positively poled surface, while non-polar dodecane was insensitive to the polarization direction. Although the differences in adsorption energies were not large, 11 kJ/mol for 2-propanol, they were still comparable to the energy barrier required to switch the polarization of ~10 nm thick films suggesting that chemical switching of ferroelectric thin films is possible. In an effort to enhance the sensitivity of the surface chemistry to the polarization direction, we explored the impact of ferroelectric polarization on the properties of supported catalytic metals. For Pd on LiNbO3 it will be shown that the Pd tends to cluster into particles on the LiNbO3 surfaces even at coverages as low as 0.1 ML and temperatures as low as 200 K. CO adsorption on these Pd particles was similar to CO adsorption on Pd on inert supports and was independent of the polarization direction. It was concluded that the Pd clusters were too thick for their surfaces to be influenced by the polarization of the underlying ferroelectric. Alternate approaches to increasing the reactivity of ferroelectric surfaces will be presented including controlling the surface termination of titanate ferroelectrics and epitaxial growth of reactive oxides.
4:00 PM - M2.4
Measurements of Surface Diffusivity and Coarsening During Pulsed Laser Deposition of SrTiO3.
John Ferguson 1 4 , Darren Dale 3 , Arthur Woll 3 , Joel Brock 1 2 4
1 Department of Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 4 Cornell Center for Materials Research, Cornell University, Ithaca, New York, United States, 3 Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, United States, 2 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States
Show AbstractComplex oxide interfaces exhibit novel electronic and magnetic properties not seen in the bulk. Furthermore, for these properties to be realized, the interface must often be atomically sharp. While pulsed laser deposition (PLD) is often the deposition technique of choice for these materials systems, the physical mechanisms responsible for smooth growth remain poorly understood. Here, a model system, homoepitaxial SrTiO3 <001>, was studied with in-situ x-ray specular reflectivity and surface diffuse x-ray scattering. Using these techniques, we measure the time-dependent surface length scale and time constant, obtaining the surface diffusivity for both inter- and intra-layer transport during deposition. Our results explicitly limit the possible role of island break-up as a smoothening mechanism, demonstrate the key roles played by nucleation and coarsening in pulsed deposition, provide new insight into possible hyper-thermal mechanisms in PLD, and place an upper bound on the Ehrlich Schwoebel barrier for downhill diffusion. We believe this experimental technique may be advantageously applied to other growth techniques such as chemical vapor deposition and molecular beam epitaxy, while the PLD-specific analysis described here can be applied to other epitaxial systems, including heteroepitaxial growth.
4:15 PM - M2.5
Zwitterion Molecular Adsorption on the Ferroelectric Perovskite Lithium Niobate (LiNbO3).
Jie Xiao 1 , Zhengzheng Zhang 1 , Dong Wu 2 , Lucie Routaboul 3 , Pierre Braunstein 3 , Bernard Doudin 4 , Orhan Kizilkaya 5 , Alexi Gruverman 1 , Peter Dowben 1
1 Physics, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Physics, North Carolina State University, Raleigh, North Carolina, United States, 3 Laboratoire de Chimie de Coordination, Institut de Chimie, Université Louis Pasteur Strasbourg, Strasbourg France, 4 Institut de Physique, Applique de Physique et Chimie des Matériaux de Strasbourg, Université Louis Pasteur Strasbourg, Strasbourg France, 5 The J. Bennett Johnston Sr. Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States
Show AbstractIn this work we studied the interactions of a molecular zwitterionic p-benzoquinonemonoimine-type system, with a large intrinsic dipole of 10 Debyes, and periodically poled lithium niobate (PPLN) substrates. The PPLN substrates were used as ferroelectric templates with periodic domain pattern period of ~28 µm. As a result, domain patterns containing stripes with their dipoles oriented either positive or negative along the surface normal were produced. Domain patterns were visualized in ambient environment by means of piezoresponse force microscopy (PFM). However, from the atomic force microscopy (AFM) topographical mode images, the resulting PPLN surfaces were smooth with no features that can be associated with the ferroelectric domain stripes. By the infrared (IR) spectra-microscopy mapping, we find that the p-benzoquinonemonoimine zwitterion preferentially adsorbed, from solution, on one favored dipole-oriented domain orientation. This selective deposition is not a result of the change in bulk composition, as PPLN surface is compositionally uniform. Niobium K-edge spatially resolved X-ray adsorption near edge spectroscopy (μ-XANES) experiments with an X-ray beam size much smaller than the ferroelectric domain also were performed to detect any bulk composition differences. Little variation was observed in the μ-XANES spectra with changing sample position, indicating that the PPLN is compositionally uniform and the bulk composition is not altered by the ferroelectric poling. Thus the selective deposition of this molecule is a result of variations in the surface interactions at the PPLN surface. However, the possibility of differences in the surface chemistry (caused by ferroelectric poling) resulting in spatial chemical localization issues cannot be excluded. The IR absorption spectra of this zwitterion on PPLN show only two major absorption lines, much less than the IR lines of this molecule on Au and in solid-state phase, indicative of very selective selection rules: the p-benzoquinonemonoimine zwitterion must be aligned placing its dipole parallel with the lithium niobate dipole direction (along the surface normal).
4:30 PM - **M2.6
Effects of Chemical Boundary Conditions and Substrate Polarity on the Behavior of Ferroelectric PbTiO3 Thin Films.
Dillon Fong 1 , M. Highland 1 , G. Stephenson 1 2 , P. Fuoss 1 , S. Streiffer 3 , J. Eastman 1 , T. Fister 1 , Carol Thompson 4
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 Energy Sciences and Engineering, Argonne National Laboratory, Argonne, Illinois, United States, 4 Department of Physics, Northern Illinois University, DeKalb, Illinois, United States
Show AbstractIn this presentation we describe in-situ synchrotron x-ray studies showing that chemical boundary conditions have important effects on the behavior of ferroelectric PbTiO3 thin films. We find that polarization switching can be induced through control of the chemical environment above the film surface. In particular, our experiments have found that changing the partial pressure of oxygen (pO2) in the gas above (001)-oriented epitaxial PbTiO3 films can reversibly induce inversion in the sign of the polarization. Monodomain films are grown in-situ by metal-organic chemical vapor deposition onto conducting SrRuO3 layers on SrTiO3 substrates. X-ray scattering measurements allow us to determine the polarization magnitude and domain distribution as a function of pO2 during switching. Under many conditions we find that switching occurs by the usual mechanism of nucleation and growth of 180° domains having approximately the same polarization magnitude but opposite sign. Interestingly, we find that as film thickness decreases and/or the observation temperature increases, polarization switching may become continuous; i.e., the polarization magnitude decreases to zero and changes sign uniformly without domain formation. This behavior is in quantitative agreement with the predictions of Landau theory for the case in which nucleation is suppressed and switching occurs at the intrinsic coercive field. We also have investigated the effects of substrate surface polarity on PbTiO3 film domain structure and chemical environment-induced switching behavior. The polar nature of substrates such as (110) DyScO3 is expected to impose a different electrical boundary condition than would arise at a traditional non-polar substrate surface such as SrTiO3 (001). We find, for example, that for a 5-nm-thick PbTiO3 film on (110) DyScO3, weak polarization persists even above the ~450°C ferroelectric transition temperature (Tc) of the film. Below Tc the film is polydomain, but transforms from having a weak net polarization in the direction of the substrate at T > ~150°C to having a strong net polarization in the opposite direction below this temperature. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357.
5:00 PM - M2.7
Tunable Atomic Termination and Self-nanostructuration of SrTiO3 (001) Surfaces.
Romain Bachelet 1 , Florencio Sanchez 1 , F. Javier Palomares 2 , Carmen Ocal 1 , Josep Fontcuberta 1
1 ICMAB, CSIC, Bellaterra, catalunya, Spain, 2 ICMM, CSIC, Madrid Spain
Show AbstractChemical stability of the substrates used for high-temperature epitaxial thin film growth is an issue of major impact for understanding the physics of interfaces and the emerging properties related to it. SrTiO3 is probably the most common single-crystalline substrate for epitaxial growth of oxides and its TiO2 terminated surface is the most used one. Here we show that atomically-flat single SrO-terminated SrTiO3(001) substrates can be obtained through simple high-temperature treatment. Amplitude-modulation atomic force microscopy with phase-lag analysis and x-ray photoelectron spectroscopy, have been used to demonstrate that the ratio between the two chemical terminations can be tailored by choosing the annealing time [1]. This finding raises some concerns on actual surface termination of the employed SrTiO3(001) substrates. Moreover, the progressive SrO surface enrichment up to 100% is accompanied by a self-assembly process which results in the spatial separation at the nanoscale of both chemical terminations. We further demonstrate that this opens an interesting avenue for selective chemical reaction and growth of oxide nanostructures.[1] R. Bachelet, F. Sánchez, F.J. Palomares, C. Ocal, and J. Fontcuberta, Appl. Phys. Lett. 95, 141915 (2009)
5:15 PM - M2.8
Single Terminated DyScO3 (110) Surfaces Through Selective Wet Etching.
Gertjan Koster 1 , Josee Kleibeuker 1 , David Dubbink 1 , Bouwe Kuiper 1 , Jeroen Blok 1 , Andre tenElshof 1 , Dave Blank 1 , Guus Rijnders 1
1 MESA+ institute of nanotechnology, University of Twente, Enschede Netherlands
Show AbstractPerovskite-type oxides, ABO3, are of high interest as they exhibit diverse physical properties and, as their structure is determined by the oxygen octahedra, heterostructures of high complexity can be formed. The ABO3 (001) is often represented by a stack of alternating layers, AO and BO2. After creating a surface, both layers are expected at the surface as the difference in surface energy is negligible. However, for atomically controlled growth of heteroepitaxial structures, it is essential to start with single terminated, ordered and crystalline substrate surfaces. The typical surface treatment, high temperature annealing, results in ordered and crystalline surfaces, but no single termination. By introducing a selective wet etching step, removing the AO or BO2 layer at the surface, single terminated SrTiO3 (001) [Science 266, 1540 (1994); APL 73, 2920 (1998)], SrTiO3 (111) [APL 92, 152920 (2008)] and NdGaO3 (110) [V. Leca, PhD dissertation, University of Twente (2003)] can currently be obtained. To be able to, e.g., examine strain-effects in heterostructures, the number of single terminated perovskite-type substrates has to be expanded. The orthorombic DyScO3 (110) (DSO), apseudocubic: 3.944 Å, is often applied to create strained heterostructures. However, surface treatments for DSO have hardly been addressed in literature. We present a surface treatment for DSO (110), consisting of a thermal and a wet etching step, resulting in ScO2 terminated, ordered and crystalline surfaces. The atomic smoothness is confirmed by 2D growth of SrRuO3 (SRO). The actual surface termination was inferred from reflective high energy electron diffraction analysis of the SRO growth without direct chemical analysis.
5:30 PM - M2.9
Correlation of Transport Properties and Valence Band Spectral Characteristics in Iron Perovskites as a Function of Temperature and Substitution Parameter: What We Can Learn From Bulk Sensitive Conductivity and Surface Sensitive X-ray Spectroscopy Studies.
Artur Braun 1 , Selma Erat 1 2 , Zhi Liu 4 , Sam Mao 3 , Yun Sun 5 , Hans Grimmer 6 , Kazimierz Conder 6 , Elena Pomjakushina 6 , Bongjin Mun 7 , Mehmet Ari 8 , Ludwig Gauckler 2 , Thomas Graule 1 9
1 Laboratory for High Performance Ceramics, EMPA, Dübendorf Switzerland, 2 Non-metalic Inorganic Materials, ETH Zürich, Zürich, Zurich, Switzerland, 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Environmental Energy Technolgies Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 5 , Stanford Synchrotron Radiation Laboratory, Stanford, California, United States, 6 Laboratory for Development and Methods, Paul Scherrer Institut, Villigen Switzerland, 7 Applied Physics, Hanyang University, Ansan Korea (the Republic of), 8 Physics, Erciyes University, Erciyes Turkey, 9 , TU Bergakademie Freiberg, Freiberg Germany
Show AbstractConductivity and structure of the hole-doped polaron conductor La0.9Sr0.1FeO3-δ are reported for high temperatures, which are rrelevant for solid oxide fuel and electrolyser cells and sensors. The conductivity increases exponentially with temperature to a maximum and decreases for T > 700 K following a power law, accompanied by a shift of spectral weight in the photoemission valence band. The conductivity decrease is accompanied by a phase transformation, due to the reduction of Fe, as evidenced by x-ray absorption spectra (XAS). Additional fine structures in the conductivity are correlated with a strong decrease in valence band intensity near the Fermi energy, and with the onset of a corresponding structural transition.Reversible and irreversible discontinuities at around 573 K and 823 K in the electric conductivity of a strained 175 nm thin film of (La0.8Sr0.2)0.95Ni0.2Fe0.8O3-δ grown by pulsed laser deposition on SrTiO3 (110) are reflected by valence band changes as monitored in photoemission and oxygen K-edge XAS. The irreversible jump at 823 K is attributed to depletion of doped electron holes concomitant with reduction of Fe3+ towards Fe2+, as evidenced by oxygen and iron core level soft XAS, and possibly of a chemical origin, whereas the reversible jump at 573 K possibly originates from structural changes.The eg↑ / (t2g↓+ eg↓) band ratio in cation-substituted La-Fe-oxides is identified in O (1s) XAS as a linear spectral indicator for conducting electron holes. The t2g↓ and eg↓ bands act as a conductivity inhibitor by ferromagnetic double exchange coupling on the eg↑ electron. Disorder induced by substitution appears to modulate the hole conduction such that an exponential relation is found between the conductivity and the eg↑ / (t2g↓+ eg↓) ratio and hole concentration. The quantitative correlation of conductivity and x-ray absorption spectra from heterovalent substituted LaFeO3, lets substitution driven metal insulator transitions appear in a new light.[1] S. Erat et al., Correlation of O(1s) and Fe(2p) NEXAFS spectra and electrical conductivity of La1-xSrxFe0.75Ni0.25O3-δ, Appl. Phys. Lett., 95(17), 174108, 2009.[2] A Braun et al., Correlation of high temperature X-ray photoemission valence band spectra and conductivity in strained LaSrFeNi-oxide on SrTiO3(110), Applied Physics Letters, 95, 022107, 2009.[3] A Braun et al., Pre-edges in oxygen (1s) x-ray absorption spectra: A spectral indicator for electron hole depletion and transport blocking in iron perovskites, Applied Physics Letters 94 (20), 202102, 2009.[4] A Braun et al., Electron hole–phonon interaction, correlation of structure, and conductivity in single crystal La0.9Sr0.1FeO3, Applied Physics Letters 93, 262103, 2008.
5:45 PM - M2.10
Tunneling Electroresistance Effect in Ferroelectric Tunnel Junctions at the Nanoscale.
Haidong Lu 1 , Alexander Stamm 1 , Dong Wu 2 , Yi Wang 1 , Ho Jang 3 , Chad Folkman 3 , David Felker 3 , Chang Eom 3 , Evgeny Tsymbal 1 , Alexei Gruverman 1
1 Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 , North Carolina State University, Raleigh, North Carolina, United States, 3 , University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractUsing a set of scanning probe microscopy techniques for nanoscale polarization detection (Piezoresponse Force Microscopy, or PFM) and spatially-resolved local conductance measurements (Conducting Atomic Force Microscopy, or C-AFM) we demonstrate the reproducible tunneling electroresistance effect in nanometer-thick epitaxial BaTiO3 single-crystalline films on SrRuO3 bottom electrodes. Epitaxial BaTiO3 films have been fabricated by atomic layer controlled growth with in-situ monitoring using reflection high-energy electron diffraction (RHEED). Correlation between ferroelectric and electronic transport properties is established by direct nanoscale PFM visualization and control of polarization and C-AFM detection of tunneling current in BaTiO3 films. The obtained results show a change in resistance by about two orders of magnitude upon polarization reversal on a lateral scale of 20 nm at room temperature. Furthermore, a pulse switching PUND approach in conjunction with PFM has been used to study polarization stability and switching in SrRuO3/BaTiO3/SrRuO3 structures. Polarization retention is not affected by conductance measurements thus allowing multiple nondestructive polarization readouts and opening a possibility for application as non-charge based logical switches in nonvolatile memory devices.
M3: Poster Session: Structure-Function Relations at Perovskite Surfaces and Interfaces
Session Chairs
Tuesday PM, April 06, 2010
Exhibition Hall (Moscone West)
6:00 PM - M3.1
Correlating Phonon Frequency Shift With Magnetoelectric Effect in the PbTiO3-CoFe2O4 Multiferroic System Due to Interfacial Stress.
Chih-Ya Tsai 1 , Tung-Ching Huang 1 , Wen-Feng Hsieh 1
1 Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan
Show AbstractWe report on the correlation between local behavior of interfacial phonon and ferromagnetic properties in three multiferroic consisting of different geometric shapes of ferromagnetic CoFe2O4 (CFO) embedded in the ferroelectric matrix of PbTiO3 (PTO) by using micro-Raman spectroscopy and SQUID. The three multiferroic films on Pt/Si substrates fabricated by chemical solution method were respectively CFO particles distributed in PTO matrix (0-3 type), CFO and PTO multi-layered structure (2-2 type), and CFO discs embedded in PTO matrix (disk-3 type). Among all the multiferroics, the phonon frequencies of A1(2TO) and A1(3TO) modes of PTO in the disk-3 type shift the most from 345 cm^-1 to 325 cm^-1 and 625 cm^-1 to 580 cm^-1. In addition, the magnetic properties of CFO in this self-assembled disk-3 type film showed the lowest coercive field Hc of 0.2 kOe and absence of saturation magnetization Ms. Furthermore, the Hc for the negative field was not equal to the one for the positive field. The nonsymmetrical Hc was again found the largest in the disk-3 type (34%). The above-mentioned results are due to the interfacial stress between the CFO and PTO phases. The self-assembled film, in which CFO is compressed to a strain of -0.301 %, illustrates the strongest elastic interaction than other films that is also the key parameter to enhance the coupling of magnetoelectric (ME) effect. In a word, the tendency of phonon frequencies shift and variations in ferromagnetic properties of different film types depend on the effective transmitted stress at the interfacial boundary consistent with the theoretical prediction on the magnitude of ME effect in different types [Phys. Rev. Lett. 94, 197203 (2005)].
6:00 PM - M3.11
Metallic or Insulating Interfaces in Epitaxial SrTiO3/RO (R = La, Pr, Nd, Sm, Y) Monolayer/SrTiO3 Heterostructures.
Ho Won Jang 1 5 , David A. Felker 1 , Karolina Janicka 2 , Chris T. Nelson 3 , Zhang Yi 3 , Chad M. Folkman 1 , Chung-Wung Bark 1 , Seung-Hyup Baek 1 , Sanghan Lee 1 , Yimei Zhu 4 , Xiaqing Pan 3 , Evgeny Y. Tsymbal 2 , Mark S. Rzchowski 1 , Chang-Beom Eom 1
1 , University of Madison, Madison, Wisconsin, United States, 5 Thin Film Materials Research Center, KIST, Seoul Korea (the Republic of), 2 , University of Nebraska, Lincoln, Nebraska, United States, 3 , University of Michigan, Ann Arbor, Michigan, United States, 4 , Brookhaven National Laboratory, Upton, New York, United States
Show Abstract The formation of 2DEG at the interface between two insulating oxides including LaAlO3/SrTiO3, LaTiO3/SrTiO3, and LaVO3/SrTiO3 heterostructures could be explained by electronic reconstruction. A half of electron per unit cell transfers from LaAlO3 to SrTiO3 in order to avoid the polar catastrophe by the diverging electric potential in LaAlO3 consisting of alternatively charged (LaO)+ and (AlO2)- planes. In the same principle, one can easily imagine that 2DEG can be formed at the interfaces between charged rare-earth oxide and neutral TiO2 planes, (RO)+/(TiO2)0. However, except for the LaO/TiO2 interface, none of experimental and theoretical works has reported the formation of 2DEG at the RO/TiO2 interfaces and their electrical properties. In this report, we investigate the formation of 2DEG at various RO (R = La, Pr, Nd, Sm, Y)/TiO2 interfaces and clarify whether the electrical properties of the 2DEG has a dependence on the R ion. Our major finding is that LaO/TiO2, PrO/TiO2, and NdO/TiO2 interfaces are conducting with 2DEG, but SmO/TiO2 and YO/TiO2 interfaces are surprisingly insulating. The concentration of mobile carriers in the NdO-based heterostructures, is less than half that of the other conducting LaO-based and PrO-based heterostructures. This trend is consistent with that in bulk RTiO3 where the electron correlation strength increases as R moves from La to Y. We propose that the Ti-3d band filling due to an extra electron introduced in SrTiO3 by the inserted RO monolayer produces an effect on transport properties reminiscent to bulk RTiO3 Mott-Hubbard insulators where the same amount of hole doping may or may not produce a metal-insulator transition depending on the type of the R ion. Our finding demonstrates the crucial role of electron correlations in the formation of 2DEG at oxide heterointerfaces.
6:00 PM - M3.12
Effect of Annealing Temperature on Microstructure Evolution and Electrical Properties of Sol-gel PZT Thin Films.
Alexander Sigov 1 , Konstantin Vorotilov 1 , Olga Zhigalina 2 , Yury Podgorny 1 , Dmitry Seregin 1
1 , Moscow State Institute of Radioengineering, Electronics and Automation (Technical University), MIREA, Moscow Russian Federation, 2 , Institute of Crystallography, Moscow Russian Federation
Show AbstractElectrical properties of PZT films are governed by their microstructure. Key issues are phase composition, texture, grain size, residual pores, crystal dislocations, film/substrate interface. Sol-gel process involves deposition of metallorganic precursors (oxoalkoxide comlexes in the case of true sol-gel process) with subsequent drying and annealing steps. This heat treatment leads to sintering (mainly by a viscous flow) and crystallization. Nucleation of a crystalline phase from an amorphous one generally requires overcoming large energy barrier, it can proceed mainly homogeneously or heterogeneously. Pt (111) substrate presents a site for heterogeneous nucleation which proceed directly or via some intermediate phase.The purpose of the research is an experimental observation of PZT fine crystalline structure during heat treatment process, as well as its correlation with electrical properties. PZT precursor solution (Zr/Ti=48/52) is prepared by dissolution in methoxyethanol of zirconium isopropylate monosolvate, titanium tetraisopropoxide, and water-free lead acetate. PZT thin films are deposited on 200 mm Si-SiO2-TiO2-Pt substrates. Annealing temperature is varied from 550 to 7500C. The samples are examined by XRD, SEM, TEM, HREM structural analysis, as well as by polarization-voltage, dielectric constant and loss frequency dependences, CV and IV measurements.After annealing at 5500C large (111) perovskite grains (up to 120 nm) are growing directly on (111) platinum grains, whereas 2-15 nm pyrochlore particles are observed throughout in the film body. Formation of column perovskite structure is not completed: grains are growing at the half thickness of the film. Pyrochlore upper layer that leads to high leakage current hinders as well polarization reversal in this structure. Formation of column structure is completed at 6000C. Pyrochlore inclusions at the grain boundaries and into perovskite grain body are decrease with the increase of annealing temperature and completely diminished at the annealing at 7000C. Increasing of annealing temperature causes intensification of Ti diffusion processes through platinum boundaries. Appearance of rutile at the interface leads to growing perovskite grains with (100) texture. This structural transformation causes alteration of electrical properties, for example polarization value, asymmetry and squareness of hysteresis loop are decrease with the increase of annealing temperature.
6:00 PM - M3.14
Superconducting Cuprates Films on Single Crystalline Manganite Substrates.
N. Haberkorn 1 , J. Guimpel 1 , G. Nieva 1
1 , Centro Atómico Bariloche. Comisión Nacional de Energía Atómica, Bariloche Argentina
Show AbstractWe present a study on the superconducting and structural properties of GdBa2Cu3O7-δ thin films grown on Gd0.7Ca0.3MnO3 bulk single crystals. The superconducting films were grown by sputtering on the (220) surface of the manganite. The films show epitaxial growth along the (103) direction and a critical temperature of 87K. Magnetization measurements evidence that the superconducting properties of the films are influenced by the magnetic state of the ferrimagnetic substrate
6:00 PM - M3.15
Structure and Electronic Properties of the Manganite Cuprate Interface Pr1-xCaxMnO3 / YBa2Cu3O7-δ.
J. Norpoth 1 , C. Jooss 1 , H. Inada 2 , Y. Zhu 3
1 Institute of Materials Physics, University of Goettingen, Goettingen Germany, 2 , Hitachi High Technologies Corp., Ibaraki Japan, 3 Materials Science Department, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractInterfaces between highly correlated electron systems may exhibit novel electronic properties that are absent in the isolated materials. Especially, complex oxide interfaces often feature nontrivial electronic behaviour, the understanding of which needs a careful analysis of the relation between interface structure and electronic properties at the atomic scale.In this work we study the interface between the high-Tc superconductor YBa2Cu3O7-δ and the hole-doped perovskite Pr1-xCaxMnO3. Thin film multilayers fabricated with pulsed laser deposition on epitaxial substrates exhibit atomically sharp interfaces as was demonstrated by high-resolution transmission electron microscopy. Electron energy loss spectroscopy indicates electron transfer across the interface from PCMO to YBCO according to a band bending scenario from the difference in the materials` workfunctions. Furthermore, short-range diffusion of Ca cations across the interface is observed. These charge transfer processes establish doping gradients in the interfacial region capable of affecting both the polaronic transport in the manganite and the characteristics of the superconductivity.Electronic transport investigations are performed at various combinations of layer numbers and thicknesses to distinguish between bulk-like behaviour and additional contributions of the interfaces. Different measurement geometries either probe primarily the in-plane conductivity or charge transport across the interfaces.
6:00 PM - M3.16
Dielectric Spectroscopy and Maxwell-Wagner Relaxation Studies of Ferroelectric/Ferromagnetic Heterostructures.
Ricardo Martinez 1 , Kumar Ashok 1 , Palai Ratnakar 1 2 , Katiyar Ram S. 1 2
1 Department of Physics, University of Puerto Rico, San Juan , Puerto Rico, United States, 2 Institute for Functional Nanomaterials, University of Puerto Rico, San Juan , Puerto Rico, United States
Show AbstractArtificially engineered multiferroic heterostructures using a ferromagnetic La0.7Sr0.3MnO3 (LSMO) and ferroelectric Ba0.7Sr0.3TiO3 (BST) were grown on (111) Pt/TiO2/SiO2/Si and MgO substrates by pulsed laser deposition (PLD) with various periodicities. We have studied the frequency and temperature dependence permittivity and impedance properties of hetrostructure in the ranges of 100Hz to 1–1MHz and 100K to 500K. The Dielectric dispersion of the heterostructures was dominated by the interfaces via inter-layer coupling that is explained in term of the Maxwell-Wagner capacitor model. Since the heterostructures grown on platinized silicon substrate and MgO are polycrystalline and single crystalline in nature respectively. The grain and the grain boundary contributions are used to explain the dielectric enhancement in polycrystalline thin films compare to epitaxial films. It was seen that the dielectric constant in polycrystalline at 1 KHz and 300K is up to 2 orders of magnitude greater that their epitaxial counterpart, while that dielectric loss remained almost constant. The epitaxial films showed good ferroelectric and ferromagnetic properties at room temperature. The presence of electrical and magnetic order parameter at room temperature gave us an extra degree of freedom for potential application of these heterostructures for magneto-electric applications.Keywords: heterostructure, ferroelectric, ferromagnetic, Maxwell-Wagner Model.
6:00 PM - M3.17
(100) Oriented Sputter-deposited Pt Thin Films on MgO/Fe Composite Bilayers With Addition of O2 During Pt Sputtering.
Hiroshi Nakano 1 , Tatsuro Matsumoto 1 , Yuji Murashima 2 , Kazuki Komaki 2 , Shigeki Nakagawa 1
1 Department of Physical Electronics, Tokyo Institute of Technology, Tokyo Japan, 2 , Panasonic Electronic Devices Japan Co., Ltd., Osaka Japan
Show AbstractPerovskite type oxide thin films have attracted a lot of attention, because they are essential materials which will be used for various electric devices such as ferroelectric random access memory (FeRAM) and tunable filter devices. When the materials are used for such capacitive devices, bottom electrode layers for oxide films are very important, since they significantly affect the crystallinity of the oxide films. Platinum (Pt) is one of the well known bottom electrode materials used for the oxide thin films. Pt provides also better nucleation sites for such perovskite materials due to small lattice misfit. Since dielectric properties of ferroelectric films are originated from the displacement of ions in a crystal along the c-axis direction, c-axis oriented ferroelectric thin films are required to attain better dielectric properties. (100) oriented Pt layers are required to attain c-axis oriented perovskite type ferroelectric films. In our previous report, we succeeded in preparing (100)-oriented Pt thin films with thickness of 20 nm on SiO2/Si substrate at substrate temperature Ts above 400 °C using MgO (100) buffer layers which deposited on Fe (100) seed layers. However, the growth of Pt(111) texture appeared when the thickness was increased from 20 nm to 100 nm, since (100) texture has relatively higher surface energy than (111) closely packed texture for Pt surface. It suggested that surface energy of the films changed during the deposition. In order to keep the surface energy, addition of O2 gas was performed during Pt deposition. Pt thin films with (100) preferred orientation with thickness above 100 nm were deposited on the (100) oriented MgO layer prepared on very thin seed Fe layers deposited on SiO2/Si substrates at Ts of 500 °C by facing-targets sputtering. It was also succeeded to attain (100) oriented perovskite oxide layer when they were deposited on the Pt(100)/Mg(100)/Fe underlayer.
6:00 PM - M3.18
Tensile Strain in Relaxed BiFeO3 Film on MgAl2O4.
Xiaolan Zhou 1 , Ludi Miao 1 , Ilan Stern 1 , Punam Silwal 1 , Dae ho Kim 1
1 Physics and Engineering Physics, Tulane University, New Orleans, Louisiana, United States
Show AbstractBiFeO3 is attracting strong attention as a unique multiferroic material due to large remnant polarization and other strain-induced properties. So far, research has been mostly focused on compressive strain of BiFeO3 grown on the SrTiO3 substrate, which has a smaller lattice constant than BiFeO3. The effect of tensile strain on the film properties is not yet known. We have grown BiFeO3 films on MgAl2O4 substrates with (001) orientation by pulsed laser deposition. MgAl2O4 has larger lattice constant than BiFeO3 with high crystalline quality made by Czochralski method. During the growth the temperature and the oxygen pressure were kept at 7000C and 20mTorr, respectively.X-ray diffraction reveals strong peaks from (00l) reflections in all the films. Out of plane lattice constants of the films with varying thicknesses show similar tensile strain up to 500nm. The relatively large thickness with persistent tensile strain makes good contrast to the case of previously reported films on SrTiO3 (001) substrates where compressive strains relaxes around 100nm of thickness. Close investigation of reciprocal space maps of (113) and (103) reflections of BiFeO3 films to calculate in-plane and out-of-plane lattice constants at various temperature shows that tensile strain originates from the difference in thermal expansion behavior between the film and the substrate. The BiFeO3 film with larger thermal expansion coefficient experiences tensile strain as it cools from the high growth temperatures due to the relative increase of the substrate’s lattice with smaller expansion coefficient. Furthermore, we have found a direct correlation between the tensile strain and the Poisson ratio of the film as well as the difference in thermal expansion coefficient.
6:00 PM - M3.19
Local Electromechanical Properties of CaCu3Ti4O12 Ceramics.
R. Tararam 1 , I. Bdikin 2 , P. Bueno 1 , J. Varela 1 , Andrei Kholkin 3
1 Depto de Fisico-Quimica, Universidade Estadual Paulista (UNESP), Araraquara Brazil, 2 Dept. of Mechanical Engineering, TEMA, University of Aveiro, Aveiro Portugal, 3 Dept. of Ceramics and Glass Engineering, CICECO, University of Aveiro, Aveiro Portugal
Show AbstractHigh dielectric constant materials are technologically important due to their potential impact on microelectronic device applications. Recently, the perovskite related body-centered cubic (bcc) material CaCu3Ti4O12 (CCTO) was reported to have an extraordinarily high dielectric constant of 105 at room temperatures [1]. Moreover, when the material is doped with iron (III) on a Ti site, the dielectric constant reaches values up to 150000 [2]. The high dielectric constant of CCTO can be understood based on a barrier layer mechanism [3]. The exceptionally high dielectric constant for crystals of CCTO indicates that the barriers to conductivity are not between crystallites but rather intrinsic for the material. In order to characterize the grain and boundary structures, piezoresponse force microscopy (PFM) has been used to map the local electromethanical properties of polished surface of CCTO ceramics. For obtaining PFM images high ac voltage (~30-45 V) was used. Local natural and bias induced charge states in CCTO ceramics were detected. Piezoelectric contrast was observed on the polished surfaces in both vertical (out-of-plane) and lateral (in-plane) modes and depended on the grain orientation varying in the sign and amplitude. These bias induced states display also a ferroelectric-like hysteresis with piezoelectric contrast reversed by the applied field. The lifetime of these non-equilibrium states exceeds 10 hours. The observed PFM signal is roughly uniform within the grains and but differs from grain to grain by magnitude (value of the corresponding effective piezoelectric coefficient) and by direction (phase of the signal). These images are almost identical to typical PFM contrast of ferroelectric polycrystalline materials [4] with single domain grains, being much smaller in magnitude. Flexoelectric effect (strain-gradient-induced polarization) due to surface relaxation was invoked to explain the observed contrast. Current and potential images has been observed for CCTO samples, too. We unambiguously identify the conduction path in CCTO by local spreading resistance microscopy. Grain distribution of surface potential and grain boundary effect was correlated with I-V curves and piezoelectic behavior of the CCTO.1. M. A. Subramanian, D. Li, N. Duan, B. A. Resner, and A. W. Sleight, J. Solid State Chem. 151, 323-325 (2000).2. G. Chiodelli, V. Massarotti, D. Capsoni, M. Bini, C.B. Azzoni, M.C. Mozzati, and P. Lupotto, Solid State Commun. 132, 241–246 (2004).3. J. Li, A.W. Sleight, and M.A. Subramanian, Solid State Commun. 135, 260–262 (2005). 4. N. Balke, I. Bdikin, S. V. Kalinin, and A. L. Kholkin, J. Am. Ceram. Soc. 92 (8), 1629 - 1647 (2009).
6:00 PM - M3.2
The Concept of Optical Doping. A Contribution to Address the Oxygen Problem at Oxide Interfaces.
Hanns-Ulrich Habermeier 1 , Soltan Soltan 1
1 Technology, MPI-FKF, Stuttgart Germany
Show AbstractThe physics of complex oxide interfaces is rich in intriguing phenomena ranging from band bending effects, charge transfer to orbital reconstruction. In all these analyses so far, the problem of oxygen stoichiometry at the interfaces even of structurally identical materials are barely addressed. In this contribution, the concept of optical doping is introduced as a means to study the role of oxygen vacancies at the interface of YBCO-LCMO interfaces. The principle is based on the doping effect in oxygen deficient YBCO leading to persistent photoconductivity, where photo-generated electrons reside at the oxygen vacancy sites and the remaining holes contribute to the doping of the CuO2 planes.The results of the first measurements using YBCO-LCMO superlattices will be presented
6:00 PM - M3.20
High Resolution Mapping of Ferroelectric Properties of the BaTiO3 Surface Using Elevated Temperature PFM.
Maxim Nikiforov 1 , William King 2 , Stephen Jesse 1 , Sergei Kalinin 1 , Roger Proksch 3
1 CNMS, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 3 , Asylum Research Co., Santa Barbara, California, United States
Show AbstractTemperature dependence piezoelectric properties and polarization dynamics in ferroelectric materials contain a wealth of information on intrinsic switching mechanisms, including activation energies for domain nucleation and wall motion and pinning, as well as polarization-mediated chemical reactivity. To date, multiple studies have explored the temperature-dependent polarization dynamics in ferroelectrics using piezo-response force microscopy experiments, in which sample was heated uniformly. Here, we show recent preliminary results of dynamic thermal PFM, using heated cantilevers which both locally heat and measure the electromechanical PFM signal.We measured a change in piezo-response contrast on BaTiO3 (100) single crystal as a function of tip heater power. We estimate that the local temperature of the tip changed from 20°C to ~300°C. Temperature calibration is complicated in these measurements by the relatively high thermal conductivity of the ferroelectric ceramics. We observed the deterioration of the contrast at local temperature of the tip ~160°C. The vanishing of PFM contrast is attributed to the phase transition of the surface layer of BaTiO3. The bulk BaTiO3 has a phase transition at TC = 120°C, at temperatures lower than TC crystal has tetragonal structure (ferroelectric) at temperatures higher than TC crystal has cubic structure (non-ferroelectric). The image formation mechanisms in dynamic heating PFM is analyzed and relative contribution of heated and biased volumes are compared. Results will be compared to similar studies on PZT samples. The future potential of this methodology onto study the change in ferroelectric properties of the materials at different temperatures, as well as development of novel dynamic modes, is discussed.A portion of this research at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. A portion of this research was sponsored by Asylum Research Co.
6:00 PM - M3.21
The Improvement of Dielectric Properties of CaCu3Ti4O12 Films on Pt/TiO2/SiO2/Si Substrates With CaTiO3 Insulating Inter-layer Fabricated by Pulsed-laser Deposition.
Sung Yun Lee 1 , Soon Mi Choi 1 , Jie Hye Lee 2 , William Jo 2 , Sang Im Yoo 1
1 Material science and Engineering, Seoul National University, Seoul, Korea, Seoul Korea (the Republic of), 2 Department of Physics, Ewha Womans University, Seoul Korea (the Republic of)
Show AbstractWe report that polycrystalline CaCu3Ti4O12 (CCTO) thin films on the 60nm thickness of insulating inter-layers of CaTiO3 (CTO) exhibit remarkably reduced dielectric losses and leakage current in the multi-layered capacitor structure of Ag/CCTO/CTO/Pt/TiO2/SiO2/Si. The thickness of CCTO films fabricated by using Nd-YAG (λ=355nm) laser was varied from 400nm to 1.3 micron with a deposition temperature of 800°C and laser energy density of 1.5J/cm2. The dielectric constant of CCTO films was increased from 1100 to 6000 at 10 kHz with the film thickness of 400nm and 1.3 micron, respectively. However, the dielectric constant was seriously decreased when the 60nm-CTO layer was introduced. (εr=770 and 4500 at 10 kHz with the CCTO thickness of 400nm and 1.3 micron, respectively.) Compared to the degraded dielectric constant properties, the dielectric loss and leakage current properties of all CCTO films with CTO inter-layer under AC and DC field, respectively, were highly improved. The highest dielectric loss drop (tanδ~0.137 to.0.016 at 10 kHz) was observed when the CTO was introduced in 800nm-thick CCTO film. Especially, the leakage current density of 400nm-CCTO film was extremely reduced from 2.6x10-3 to 5.1x10-6 A/cm2 at the applied DC voltage of 0.5V. In this presentation, we will suggest the reason for this unprecedently high dielectric loss drop and reduced leakage current density for CCTO dielectrics in multi-layered structure. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government. (MEST) (No. 2009-0083-793)
6:00 PM - M3.22
Dielectric Properties of Epitaxial CaCu3Ti4O12 Thin Films on SrRuO3 – Buffered SrTiO3 Substrates by Pulsed-laser Deposition.
Sung Yun Lee 1 , Young Hwan Kim 2 , Kyoung-Jin Choi 3 , Sang Im Yoo 1
1 Material science and Engineering, Seoul National University, Seoul, Korea, Seoul Korea (the Republic of), 2 Nano-Device Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Nano-Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show AbstractEpitaxial CaCu3Ti4O12 (CCTO) films were successfully fabricated on the bottom SrRuO3 (SRO) electrode layer pre-deposited on single crystal SrTiO3 (STO) (100) substrates by using KrF-Excimer laser (λ= 248 nm). The laser energy density of 1.6 J/cm2 with a repetition rate of 3 Hz was used for fabricating 70nm-thickness SRO layer. The high resolution X-ray diffraction pattern of θ-2θ scans and rocking curve measurement revealed that as grown SRO films were highly c-axis oriented and the full width at half maximum (FWHM) showed only 0.23°, indicating that high quality epitaxial SRO layer was successfully deposited on STO substrates. The resistivity of 200 μΩ-cm was measured for the SRO film at room temperature using four point probe measurement. The epitaxial CCTO films with various thicknesses from 100 to 800 nm were fabricated in the oxygen partial pressure of 100 mTorr at 725°C with a laser repetition rate of 5 Hz. Because of the large lattice misfit between CCTO and SRO (~6 %), only (00l) reflections of CCTO were not sustained over the thickness of 500nm but polycrystalline peaks and various second phase peaks started to appear. The leakage current density was decreased with increased CCTO film thickness using LCR source meter ranging from 0 to 10 V. In this presentation, we will identify the detailed leakage current mechanism and the relation between crystallinity and dielectric properties. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government. (MEST) (No. 2009-0083-793)
6:00 PM - M3.3
Thickness Dependent Structural Phase Transition of Ultrathin SrRuO3 Films.
Seo Hyoung Chang 1 , Young Jun Chang 2 , Da Woon Jeong 1 , Chang-Uk Jung 3 , Tae Won Noh 1 , Jin-Seok Chung 4
1 ReCOE & FPRD, Department of Physics and Astronomy, Seoul National University, Seoul Korea (the Republic of), 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Physics, Hankuk University of Foreign Studies, Yongin, Gyeonggi, Korea (the Republic of), 4 Department of Physics, Soongsil University, Seoul Korea (the Republic of)
Show AbstractRecently, Oxide electronics have been attracting a lot of attention because of their novel properties and its potential application. SrRuO3, which is a unique itinerant ferromagnetic system in 4d transition metal oxides, has been utilized for good electrode in multiferroic and ferroelectric capacitors due to lots of advantages, e.g., its good thermal/electrical conductivity and surface stability. One of the important and fundamental questions for future microelectronic devices is how ultrathin SrRuO3 film can maintain its own physical properties in comparison to that of bulk SrRuO3. We investigated structure of ultrathin SrRuO3 films with systematically varying thickness. We fabricated ultrathin films of SrRuO3 on TiO2 terminated SrTiO3 by pulsed laser deposition with high-pressure reflection high energy electron diffraction. Using x-ray reciprocal space mapping around the SrTiO3 (-204) peak, we confirmed that bulk SrRuO3/or thick SrRuO3 films on SrTiO3 (001) substrate exhibited an orthorhombic structure at room temperature. Interestingly, we found that ultrathin SrRuO3 films, which were below a certain critical thickness, showed reduce the orthorhombic distortion and show structural phase transition from orthorhombic to tetragonal structure. To get the further insight, we carried out their corresponding spectroscopic ellipsometry measurements, x-ray photoemission spectroscopy measurements, and the first principles calculations of the ultrathin films on SrTiO3 substrate.
6:00 PM - M3.5
Universal Behavior in Structural and Ferroelectric Properties for RE-substituted BiFeO3 Thin Films.
Daisuke Kan 1 , Lucia Palova 2 , Anbusathaiah Anbusathaiah 3 , Ching Cheng 3 , Fujino Fujino 1 , Valanoor Nagarajan 3 , Karin Rabe 2 , Ichiro Takeuchi 1
1 , Department of Materials Science and Engineering, College Park, Maryland, United States, 2 , Rutgers University, Piscataway, New Jersey, United States, 3 , University of New South Wales, Sydney, New South Wales , Austria
Show AbstractWe have recently discovered substantial enhancement in dielectric/ferroelectric/piezoelectric properties at a rhombohedral to pseudo-orthorhombic structural boundary in (Bi,Sm)FeO3.[1] In this study, we have performed systematic investigations on structural and ferroelectric properties of BiFeO3 thin films doped with rare-earth (RE) elements of Sm, Gd and Dy in a combinatorial manner. Thin film composition spreads of (Bi,RE)FeO3 were fabricated by combinatorial pulsed laser deposition on SrTiO3 (100) substrates with SrRuO3 buffer layers. Scanning x-ray diffraction reveals that a rhombohedral to an orthorhombic structural transitions are universally observed for all RE elements studied here and that the structural properties can be described as a function of average ionic radius of A-site. This indicates that the primary cause of the transition is the chemical pressure effect due to the substitutions. We also found that, at the transition boundary, a single ferroelectric hysteresis loop on the undoped BFO side undergoes a transition to a double hysteresis loop, with an enhancement of dielectric constant ε33 and piezoelectric coefficient d33. In this presentation, we will also discuss the possible origin of the double hysteresis loop based on results of from the first principles calculations. This work is supported by DMR 0520471, NSF DMR 0603644, ARO W911NF-07-1-0410 and the W. M. Keck Foundation.[1] S. Fujino, M. Murakami, V. Anbusathaiah, S.-H. Lim, V. Nagarajan, C. J. Fennie, M. Wuttig, L. Salamanca-Riba, and I. Takeuchi Appl. Phys. Lett. 92, 202904 (2008).
6:00 PM - M3.7
In situ Nanoscale Investigation of Piezoelectric Thin Films Voltage Generation.
Sharath Sriram 1 , Madhu Bhaskaran 1 , Simon Ruffell 2 , Arnan Mitchell 1
1 Microelectronics and Materials Technology Centre, RMIT University, Melbourne, Victoria, Australia, 2 Research School of Physical Sciences and Engineering, Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractThe ability of piezoelectric materials to convert mechanical input into electrical energy and vice versa is a well documented and accepted phenomenon. While this property has been explored in detail for ceramics and more recently for thin films and nanostructures, most studies have been limited to the macro-scale or micro-scale. This work will report on the in situ electrical measurements carried out during nanoscale mechanical indentation, shedding light on the nanoscale electromechanics of polycrystalline piezoelectric thin films.Small forces from μN to mN were applied using a nanoindenter, which was configured to perform electrical measurements using a conductive tip (doped diamond Berkovich tip). Forces were typically restricted to regimes which caused reversible deformation by indentation. Voltage generation from thin films of strontium-doped lead zirconate titanate (PSZT), which is a piezoelectric with a relaxor ferroelectric composition, was studied. A variety of force curves, with variables of force amplitude and frequency were utilised, with the resulting piezoelectric voltage generation studied. Dependence of voltage generation on the area of the piezoelectric under study was investigated using electron-beam lithography patterned islands of PSZT with diameters of 200-400 nm. In addition to studies of voltage generation, the magnitude and dependence of transient currents generated by changes in mechanical indentation were determined.These results from this investigation reinforce the potential for use of piezoelectric thin films for low frequency energy scavenging. Moreover, results from area dependence of voltage and transient current generation show promise for use of micro-fabricated piezoelectric thin films structures in energy harvesting devices.
6:00 PM - M3.8
Prediction of Giant Electromechanical Response in PZT Bilayers.
Reza Mahjoub 1 , Nagarajan Valanoor 1 , Pamir Alpay 2
1 School of Material Science and Engineering, University of New South Wales, Sydney, New South Wales, Australia, 2 University of Connecticut, Institute of Material Science, Storrs, Connecticut, United States
Show AbstractThe free energies of mono- and polydomain structures in a heteroepitaxial bilayer structure consisting of (001) PbZrxTi1-xO3 (x<0.5) and (001) PbZr1-xTixO3 (x>0.5) films on thick (001) substrates are calculated taking into account the electrostatic and elaastic coupling between layers as well as the energies of the microstrains associated with polytwin structures. The results show that the interaction energy at the interface of the layers has a significant contribution to the stability of ferroelastic domains in the tetragonal layer. Moreover, when the free energy is expanded to express the nonlinear potentials explicitly and to take into account the electrical interactions at the interface, it is found that not only does the electrostatic coupling between the active ferroelectric layers induce a shift in the phase diagram of single layer constituents and particularly the tricritical transition points, but also gives rise to small signal piezoelectric coefficient several times larger than the comparable single layer constituents for certain misfit strain regimes. Particularly, the model predicts small signal as large as 600 pm/V not only at phase transition points but also all through certain ranges of experimentally achievable misfit strains. This giant electromechanical response can be attributed to the interaction between tetragonal (c/a) ferroelastic polydomains in the PbZrxTi1-xO3 (x<0.5) layer and subjacent monoclinic (r1/r2) domains in the PbZr1-xTixO3 (x>0.5) film. It is also shown that this effect can be fine tuned by changing the film compositions or varying the thickness ratio of the active layers. These predictions support previously reported giant electromechanical response in bilayered ferroelectric films [Varatharajan et al , Advanced Materials 2009] and thereby show innovative routes towards enhanced electromechanical properties in pervoskite ferroelectrics.The work at UNSW was supported by an ARC Discovery Project and Faculty of Science Researcg grant. S.P. Alpay acknowledges the support from U.S. Army Research Office through Grants W911NF-05-1-0528 and W911NF-08-C-0124
6:00 PM - M3.9
Misfit-strain Film-thickness Phase Diagrams and Related Electromechanical Properties of Ultra-thin Perovskite Films.
Qiao Qiu 1 , Nagarajan Valanoor 1 , Pamir Alpay 2
1 School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales, Australia, 2 Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States
Show AbstractThe phase stability of ultra-thin (001) oriented ferroelectric PbTiO3, PbZr1-xTixO3 (PZT) and BaTiO3 epitaxial thin films as a function of the film composition, film thickness, and the misfit strain is analyzed using a non-linear Landau-Ginzburg-Devonshire thermodynamic model taking into account the electrical and mechanical boundary conditions. (Qiu et al, Phys Rev B 2008) The novelty of our method is that we take into account not only the (an)isotropic misfit strain relaxation through the formation of interfacial dislocations and the formation of ferroelastic polydomains but also electrical boundary conditions. Particularly, as the films are made thinner such that the in-plane strains reach the pseudomorphic limit, we show that the competition between the depolarization fields arising from the incomplete compensation of the polarization charges (even under the presence of real electrodes) and the termination of the ferroelectric layer and the microstrain energies for polydomains leads to phase stability conditions and domain patterns that are quite different from prior predictions. Film thickness-misfit strain phase diagrams are developed for all three systems as a function of the film thickness. The results show that the so-called rotational r-phase appears in a very narrow range of misfit strain and thickness of the film, although we also found that the range of stability for rotational monodomain phases is markedly increased under anisotropic strains in comparison to isotropic substrates. Furthermore, the in-plane and out-of-plane dielectric permittivities ε11 and ε33 as well as the out-of-plane piezoelectric coefficients d33 for the thin films are computed as a function of misfit strain taking into substrate induced clamping. The model reveals that previously predicted ultrahigh piezoelectric coefficients due to misfit strain induced phase transitions are practically achievable only in an extremely narrow range of film thickness, composition and misfit strain parameter space. (Qiu et al Acta Mater, in press) We also show that the dielectric and piezoelectric properties of epitaxial ferroelectric films can be tailored through anisotropic strain engineering and microstructural optimization. (Qiu et al, Phys Rev B submitted)
Symposium Organizers
Arthur P. Baddorf Oak Ridge National Laboratory
Ulrike Diebold Tulane University
Dietrich Hesse Max-Planck-Inst. of Microstructure Physics
Andrew Rappe University of Pennsylvania
Naoya Shibata The University of Tokyo
M4: Advances in Electron Microscopy for Oxide Interfaces
Session Chairs
Wednesday AM, April 07, 2010
Room 2003 (Moscone West)
9:30 AM - **M4.1
Investigation of Interfaces in Perovskite Heterostructures by Atomic Resolution Electron Microscopy.
Knut Urban 1 , Chunlin Jia 1
1 ER-C / IFF-8, Research Centre Juelich, Juelich, NRW, Germany
Show AbstractThe demands with respect to interfaces in perovskite heterostructures can be quite different; neutral interfaces are the goal in simple substrate-epitaxy applications and functional interfaces are the goal in heterostructures where the particular electronic properties of the interface are to be exploited. In either case interface engineering should be precise down to the atomic scale, and the interfaces should be free of lattice defects. On the other hand, structure-property relations concerning real interfaces in nominally neutral interfaces and for engineered functional interfaces require as a pre-requisite measurement of individual atomic positions. Such measurements have become possible in recent years with the advent of ultra-high resolution aberration-corrected transmission electron microscopy. In contrast to earlier rather loose definitions of “atomic resolution” this means proven single atom lateral resolution, the possibility to measure single atomic-column occu¬pancies and the possibility to measure lateral atomic shifts with an accuracy down to fractions of the Rayleigh resolution limit, i.e. a few picometres. This allows for the first time to determine physically relevant parameters on the basis of ultra-high precision measurement of individual atom shifts. Since this type of ultra-high resolution studies is exploiting quantum mechanics intuitive image interpretations are inadequate. As a consequence this new technique requires to invert the non-linear imaging process numeri¬cally on a computer. This in done in two steps. The first is to calculate backward from an interferometric image series the quantum mechanical exit-plane electron wave function. In the second step the atomic structure is obtained in an iterative process in which the Dirac equation is solved numerically on the computer starting from a first guess model. In each iteration the atomic positions are adjusted on the picometres scale until a best fit to the experimental exit-plane wave function is obtained.We have applied this technique to measure the individual atomic shifts across ferroelectric 180° domain walls in Pb(Zr0.2Ti0.8)O3 (PZT) on SrTiO3 (STO) substrates. The O-atom shifts are found to be about 30 pm, the Zr/Ti-atom shifts 10 pm with an error of only a few picometres. In an investigation of the effect of individual dislocations on the local ferroelectric polarization in the PZT-STO interface we could measure the individual atomic shifts of the order of a few 10 pm and the decay of the polarization induced by the dislocation strain field. A study of the functional LaAlO3 (LAO) to STO interface, whose electronic properties have recently been the subject of numerous investigations, we could detect strong interdiffusion and a ferroelectric-like atom arrangement in the interface as a result of oxygen-octahedron rotation.
10:00 AM - M4.2
Direct Evidence for Cation Non-stoichiometry and Cottrell Atmospheres Around Dislocation Cores in Functional Perovskite Oxide Interfaces.
Miryam Arredondo 1 , Quentin Ramasse 2 , Matthew Weyland 3 , Reza Mahjoub 1 , Ionela Vrejoiu 4 , Dietrich Hesse 4 , Nigel Browning 5 6 , Marin Alexe 4 , Paul Munroe 1 , Valanoor Nagarajan 1
1 Materials Science and Engineering, The University of New South Wales, Sydney, New South Wales, Australia, 2 National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Monash Centre for Electron Microscopy, Monash University, Melbourne, Victoria, Australia, 4 , Max Planck Institute of Microstructure Physics, Weinberg 2 D-06120, Halle (Saale), Germany, 5 Department of Chemical Engineering and Materials Science, University of California-Davis, Davis, California, United States, 6 Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractExploiting the electronic properties of nanostructured oxides in functional devices requires their deposition on an underlying substrate, often with an unavoidable lattice mismatch. We consider the chemical implications in the accommodation of this mismatch by misfit dislocations, and the attendant long range strain fields, by the characterization of one such lattice mismatched system: an epitaxial (001) PbZr0.52Ti0.48O3 ferroelectric film deposited on a SrRuO3 oxide electrode-buffered SrTiO3 substrate. Energy-filtered transmission electron microscopy (EFTEM) and x-ray chemical maps (EDX) demonstrate distinct intermixing of Pb cation in and around dislocation cores. A combination of aberration corrected (Cs) atomic-resolution Z-contrast imaging and nanoscale chemical mapping techniques is exploited to yield evidence for cation excess within a single dislocation core present at the interface between the ferroelectric and the underlying electrode. Multislice image simulations from a model structure where the dislocation core has excess cations reproduces fairly well the observed image contrast. Nanoscale energy dispersive X-ray maps reveal Pb and Sr interdifussion, while electron energy loss spectroscopy (EELS) scans at the core reveal that it is oxygen-deficient compared to the defect-free lattice. Quantitative geometric phase analysis in conjunction with theoretical elastic modeling reveals that the local strain around the dislocation core is sufficient to create stress-assisted diffusion of cations, i.e. Cottrell atmospheres around the core. The results presented here provide insights into the link between the local chemistry, physical structure and observed functional behavior in the ferroelectric at the proximity of a dislocation core.
10:15 AM - M4.3
Atomic Resolution Direct Imaging of O Distortions Across LaMnO3/SrTiO3 Interfaces.
Maria Varela 1 2 , Stephen Pennycook 1 3 , Timothy Pennycook 3 , Weidong Luo 3 1 , Sokrates Pantelides 3 1 , Javier Garcia-Barriocanal 2 , Flavio Bruno 2 , Carlos Leon 2 , Jacobo Santamaria 2
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Universidad Complutense de Madrid, Madrid Spain, 3 , Vanderbilt University, Nashville, Tennessee, United States
Show AbstractMany of the interesting properties displayed by complex oxide interfaces are determined by the oxygen-metal bonds. This is especially the case in perovskite systems, where a transition metal is surrounded by O atoms in octahedral coordination. The near-neighbor bond angles and lengths determine fundamental properties such as bandgaps, bandwidths, etc, and hence the macroscopic properties of the system (e.g., ferroelectricity, magnetism). Oxide interfaces show unexpected behaviors due to factors such as low dimensionality, proximity, charge transfer or novel electronic phenomena. Understanding their properties, however, relies on a careful quantification of the atomic structure including the light O species, which is not always possible through standard (average) diffraction methods. Aberration corrected scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) have demonstrated the imaging of such light atoms in real space and with atomic resolution. Here we will apply these techniques to the imaging of subtle O displacements across LaMnO3/SrTiO3 (LMO/STO) interfaces. The LMO/STO relative layer thickness ratio changes the degree of epitaxial strain within the layers and dramatically affects the physical properties of the system, which can be tuned from insulating, mild ferromagnetic, to metallic ferromagnets [1]. While STO is a cubic perovskite, LMO is characterized by a strong Jahn-Teller distortion and orbital ordering at low temperatures. We will show how the octahedral distortions of relaxed LMO layers can be tuned through different degrees of epitaxial strain and affect the O sublattice of ultrathin STO layers. These results will be discussed and combined with density functional theory, in connection with the magnetotransport properties. Research sponsored by the Division of Materials Science and Engineering US Department of Energy.[1] J. Garcia-Barriocanal et al., Advanced Materials, in press (2009)
11:00 AM - **M4.4
Ultrafast X-ray Diffraction Studies of Nano-layered Perovskites.
Matias Bargheer 1 2 3
1 Institute of Physics and Astronomy, University of Potsdam, Potsdam Germany, 2 , Helmholtz-Zentrum Berlin, Berlin Germany, 3 Colloids and Interfaces, Max-Planck-Institut, Potsdam-Golm Germany
Show AbstractUltrafast x-ray diffraction (UXRD) is a novel technique that allows for taking snapshots of the atomic structure of crystals and nano-layers on the timescale of optical phonon periods (~100fs). The structural rearrangements induced by an ultrafast stimulus (charge carriers excited or heat deposited by a laser pulse) can be recorded in real time. This allows the direct measurement of physical properties such as the electron phonon coupling. The timescale, amplitude and phase of the collective atomic motion can be determined with high accuracy, even when the induced amplitude is smaller than thermal fluctuations.We show examples for semiconductor multilayers and for nano-layered perovskite structures such as SrTiO3, SrRuO3, Pb(Zr,Ti)O3, including the direct observation of an indirect magneto-electric coupling. We discuss the possibility to study the primary events and couplings involved in phase-transitions.
11:30 AM - M4.5
Atomic-scale Characterization of Surfaces and Interfaces in Oxides by Aberration-corrected Scanning Transmission Electron Microscopy.
Naoya Shibata 1 2 , Scott Findlay 1 , Teruyasu Mizoguchi 1 , Katsuyuki Matsunaga 3 , Takahisa Yamamoto 1 4 , Yuichi Ikuhara 1 4 5
1 , The University of Tokyo, Tokyo Japan, 2 , PRESTO, Japan Science and Technology Agency, Saitama Japan, 3 , Kyoto University, Kyoto Japan, 4 , Nanostructures Research Laboratory, Japan Fine Ceramic Center, Nagoya Japan, 5 , WPI Advanced Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractUnderstanding the atomic-scale structures of surfaces and interfaces is essential to control the functional properties of many advanced oxide materials. Recent advanced electron microscopy techniques such as aberration-corrected scanning transmission electron microscopy (STEM) and high-voltage electron microscopy (HVEM) have become capable of direct characterization of localized atomic structures inside materials, and open the possibility for understanding the atomic-scale mechanism of surface and interface related properties in oxides. In the present talk we will show our recent studies on the structural characterization of interfaces and surfaces in many kinds of oxide materials using aberration-corrected STEM and HVEM [1-3]. We find that aberration-corrected STEM is a very powerful method to directly determine the complex structures of surfaces and interfaces in oxide materials. We will also discuss the prospects for the new imaging modes in STEM such as annular bright-field imaging[4], which utilize unique geometry of STEM detectors.[1] N. Shibata et al., Phys. Rev. Lett., 102, 136015 (2009).[2] N. Shibata et al., Science 322,570(2008).[3] N. Shibata et al., Nature Mater.,8,654(2009).[4] S.D. Findlay et al., Appl. Phys. Lett. in press.
11:45 AM - M4.6
Properties of Multiferroic Interfaces from Atomic Displacements by STEM.
Hye Jung Chang 1 , Anna Morozovska 2 , Mark Huijben 3 4 , Ying-Hao Chu 5 , Pu Yu 3 , Ramamoorthy Ramesh 3 , Evgeny Eliseev 2 , G. Svechnikov 2 , Sergei Kalinin 6 , Stephen Pennycook 1 , Albina Borisevich 1
1 Materials Science and Technilogy Division, Oak Ridge National laboratory, Oak Ridge, Tennessee, United States, 2 , V. Lashkarev Institute of Semiconductor Physics, NAS of Ukraine, Kiev Ukraine, 3 Department of Physics, University of California, Berkeley, California, United States, 4 Science & Technology, University of Twente, Enschede Netherlands, 5 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan, 6 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractWe present an approach for direct studies of space charge layers at oxide heterointerfaces using the atomic position mapping by high-resolution Scanning Transmission electron microscopy. Using a set of model metal-metal (LSMO-SRO), metal-incipient ferroelectric (LSMO-STO) and metal-ferroelectric (LSMO-BFO for both polarization orientations) interfaces, we explore the interplay between polarization and intrinsic interface charge. Aberration-corrected microscopy is uniquely suited for imaging sub-angstrom structural distortions such as polarization-related displacements, making it a powerful tool for studies of ferroelectric materials [1]. Aberration-corrected Z-contrast scanning transmission electron microscopy (STEM) can be used to quantify the mechanical strain and polarization-induced displacements across the domain walls and interfaces between ferroelectric and the electrode material. Epitaxial BiFeO3 (BFO) film was grown using molecular-beam epitaxy on single-crystal SrTiO3 (STO) substrates. A thin layer of epitaxial (La,Sr)MnO3 (LSMO) was used as both an electrical contact and heteroepitaxial growth buffer. STEM images were collected using a VG Microscopes HB603U operated at 300 kV, equipped with Nion® aberration corrector. Bi positions can be used to quantify local strain, while analysis of Bi and Fe positions gives local ferroelectric polarization. Bi position analysis at the BFO-LSMO interfaces for both thick (50nm) and ultrathin (3.2 nm) BFO films shows that the first 4 atomic layers of BFO exhibit expansive strain perpendicular to the interface. Ferroelectric polarization at the interface shows different behavior depending on film thickness and/or polarization direction. In ultrathin 3.2 nm film, polarization is approximately constant inside the film and drops off quickly at the interface. Interestingly, the first 6 layers of LSMO also show small off-center displacements of Mn, indicating induced polarization in the half-metal electrode. In the 50 nm film, on the contrary, polarization falls off gradually at the interface. However, the displacements inside LSMO have the same magnitude and sign, even for the opposite direction of polarization in BFO. This led us to conclude that displacements in LSMO are not driven directly by polarization but can be affected by other parameters of the system, such as trapped interface charge or different charge compensation mechanisms at the interface. The results of a corresponding Ginsburg-Landau calculation will be presented. The results for LSMO-SRO and LSMO-STO system are also discussed.The research is sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, the Office of the Scientific User Facilities of the U.S. Department of Energy, and by appointment (H.J.C.) to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE.[1]C. L. Jia et al., Nature Materials 6, 64 (2007).
12:00 PM - M4.7
Origin of Modified Interface Dielectric Properties in BFO Thin Films.
Albina Borisevich 1 , Hye Jung Chang 1 , Mark Huijben 2 3 , Mark Oxley 1 , Satoshi Okamoto 1 , Manish Niranjan 5 , John Burton 5 , Evgeny Tsymbal 5 , Ying-Hao Chu 4 , Pu Yu 2 , Ramamoorthy Ramesh 2 , Sergei Kalinin 6 , Stephen Pennycook 1
1 Materials Science and Technology Division, Oak Ridge National laboratory, Oak Ridge, Tennessee, United States, 2 Department of Physics, University of California, Berkeley, California, United States, 3 Science & Technology, University of Twente, Enschede Netherlands, 5 Department of Physics and Astronomy, Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska, United States, 4 Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu Taiwan, 6 Center for Nanophase Materials Sciences, Oak Ridge National laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe emergence of novel properties at the interfaces between materials with dissimilar electronic properties has become the fundamental paradigm of modern technology. The details of the interactions between spin, charge, structural, and lattice degrees of freedom at interfaces between strongly correlated oxides has remained enigmatic. Here we study BiFeO3 - La0.7Sr0.3MnO3 (BFO-LSMO) interface using the direct structural mapping by high-resolution scanning transmission electron microscopy (STEM) to determine lattice parameters and oxygen octahedral rotations synergistically combined with electronic structure and dielectric property imaging by electron energy loss spectroscopy (EELS). The combination of these techniques allows us to directly correlate the atomic structure, polarization, strain fields and dielectric behavior locally on the atomic level. The images and the EELS data were acquired using a VG Microscopes HB603U operated at 300 kV equipped with Nion® aberration correctors and Gatan Enfina® spectrometer. The EELS compositional mapping at the interfaces was performed using principal component analysis combined with neural network interpolation.Depending on the specific energy range used for the analysis, the resulting compositional maps had a slightly different appearance. Region with low-lying core-loss edges produces a well resolved compositional map. Notably, the map for the energy range of plasmonic excitations (5 to 30 eV) shows that a region at the LSMO/BFO interface cannot be identified as any of the three phases. This feature is located on the BFO side of the interface, suggesting that first 2 nm of BFO have modified dielectric properties.STEM examination of the interfaces also revealed that the out-of-plane lattice parameter is increased in the first few layers of BFO and then saturates at a lower value. This behavior cannot be explained via Poisson ratio effect as there are no observable defects in the film to account for the eventual decrease of strain. We propose that this behavior is caused by local suppression of the octahedral tilts in the vicinity of the interface, which is also observed by STEM. Using first principles theoretical calculations, we demonstrate that suppression of octahedral tilts results in decreasing band gap for BFO, thus explaining the modified dielectric properties observed by EELS. The combination of STEM and DFT data reveals the formation of a mesoscopic interface –induced phase transition at the BFO-LSMO interface. A thin layer of BFO transforms into a phase with reduced band gap or possibly even metallic behavior. The research is sponsored by the Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, the Office of the Scientific User Facilities of the U.S. Department of Energy, and by appointment (H.J.C.) to the ORNL Postdoctoral Research Program administered jointly by ORNL and ORISE.
M5: Two-dimensional Electron Gases in Oxide Heterostructures II
Session Chairs
Wednesday PM, April 07, 2010
Room 2003 (Moscone West)
2:30 PM - M5.1
Electronic Reconstruction at LaMnO3/SrTiO3 Interfaces.
J. Garcia-Barriocanal 2 1 , Flavio Y. Bruno 1 , Maria Varela 3 , Alberto Rivera-Calzada 1 , Norbert Nemes 1 , Zouhair Sefrioui 1 , Carlos Leon 1 , Mar Garcia-Hernandez 4 , Stephen Pennycook 3 , Jacobo Santamaria 1
2 , SpLine Spanish CRG Beamline at the ESRF, BP 220-38043 Grenoble France, 1 , U. Complutense, Madrid Spain, 3 Materials Science and Technology Division, Oak Ridge National Laboratory , Oak Ridge, TN 37831-603, Tennessee, United States, 4 , Instituto de Ciencia de Materiales de Madrid-CSIC, Madrid 28049 Spain
Show AbstractEpitaxial interfaces between complex oxides are becoming the focus of increasing interest due to the promise of the possibility of tailoring their electronic structure to display novel behaviours and functionalities. A large part of the thrust of this field has been provided by the metallic state at the interface between band insulators SrTiO3 (STO) and LaAlO3 (LAO). In a very related context, the charge leakage resulting from the electronic reconstruction occurring at the interface between a Mott and a band insulator is also a very interesting phenomenon. In this communication we examine charge leakage at the interface between two oxide materials with different B- site cations: SrTiO3 (STO), a band insulator, and LaMnO3 (LMO), a Mott insulator, which is of special relevance in practical spintronic devices. This system incorporates a polar discontinuity at the interface, as in LAO/STO, such that similar interesting physics may be playing a role in these multilayers. Using combined atomic column resolution imaging and spectroscopy we provide direct evidence for charge leakage. Due to the discontinuous BO6 network, an unambiguous change of the Ti oxidation state from 4+ to 3+ at the interface is resolved, resulting from the transfer of electrons from the manganite into the titanate layer. Moreover, we show that the direction of the charge leakage across at the STO/LMO interface can be modified and even reversed by changing the STO/LMO thickness ratio. When this ratio is high, the manganite is relaxed and doped with holes, but it is doped with electrons when thin, and under epitaxial strain, which deeply modifies magnetism and transport. This finding highlights the importance of relative layer thicknesses and/or epitaxial strain in controlling doping at the interface and may trace future avenues for the design of new device concepts. Work at UCM and UPM supported by Spanish Ministry for Science and Innovation grant MAT2008 6517. Research at ORNL (MV and SJP) sponsored by the Division of Materials Sciences and Engineering of the US Department of Energy.[1] J. Garcia-Barriocanal et al., Advanced Materials, in print, (2009)
2:45 PM - **M5.2
Electric Field Modulation of Thermopower for SrTiO3.
Hiromichi Ohta 1 2
1 , Nagoya University, Nagoya Japan, 2 , JST-PRESTO, Kawaguchi Japan
Show AbstractThermoelectric energy conversion (TE) technology attracts great attention to convert the waste heat into electricity by utilizing the Seebeck effect, in addition to refrigerate various devices by means of the Peltier effect. The performance of thermoelectric materials is evaluated in terms of a dimensionless figure of merit, ZT = S2σTκ-1, where Z, T, S, σ and κ are, respectively, a figure of merit, the absolute temperature, the thermopower, the electrical conductivity, and the thermal conductivity. Recently, we found that high-density (~1021 cm-3) two-dimensional (2D) electron layers in SrTiO3 crystal exhibits giant S, which is ~5 times larger than that of simple electron doped bulk, when the layer thickness is as thin as one unit cell (0.3905 nm) [1]. The 2D electron layer is realized by superlattices of SrTiO3/SrTi0.8Nb0.2O3 or TiO2/SrTiO3 heterointerface. To realize 2DEG in a SrTiO3 crystal, field effect transistor (FET) would be another good way because applying gate voltage accumulates high density conduction electrons within an extremely thin layer at the gate insulator/SrTiO3 heterointerface. Moreover, FET structure on single-crystalline materials would be powerful to explore thermoelectric materials because it provides charge carrier dependence of both S and σ simultaneously. Here we report transistor characteristics and field-modulated S for single crystal SrTiO3-based FETs [2]. We used 150-nm-thick amorphous 12CaO7Al2O3 glass as the gate insulator of the SrTiO3-FET. The resultant SrTiO3-FET exhibits excellent transistor characteristics at room temperature: on-to-off current ratio >106, threshold gate voltage +1.1 V, sub-threshold swing ~0.3 Vdecade-1, and effective mobility ~2 cm2V-1s-1. Field-modulated S-value of the SrTiO3-FET varied from -900 to -580 μVK-1 with electric field up to +2 MVcm1 demonstrating that the effectiveness of FET structure for the exploration of thermoelectric materials.[1] H. Ohta et al., Nature Mater. 6, 129 (2007).[2] H. Ohta et al., Appl. Phys. Lett. 95, 113505 (2009).
3:15 PM - M5.3
Evidence for Band-bending Across LaAlO3 in LaAlO3/SrTiO3 Heterostructures.
Guneeta Singh Bhalla 1 2 3 , Christopher Bell 3 , Wolter Siemons 1 , Jayakanth Ravichandran 1 , Arthur Hebard 4 , Yasuyuki Hikita 3 , Harold Hwang 3 , Ramamoorthy Ramesh 2 1
1 Department of Physics, University of California, Berkeley, California, United States, 2 Department of Materials Science, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8651, Japan, 4 Department of Physics, University of Florida, Gainesville, Florida, United States
Show AbstractAs of today, a complete understanding of the origin and properties of the electron gas at the LaAlO3/SrTiO3 interface remains elusive. Virtually all transport studies so far have focused on in–plane measurements where the properties of the SrTiO3 dominate the system. However the LaAlO3 is also a crucial component of the system and much information about it can be gained by measuring electronic transport across LaAlO3, perpendicular to the film plane. Current-voltage and capacitance-voltagemeasurements can provide a wealth of information about the properties of the system. Rectifying current-voltage curves are observed for various metal electrodes evaporated onto the LaAlO3 surface. Capacitance-voltage measurements demonstrate that the LaAlO3 is a robust insulator over asignificant voltage range, but reversibly breaks down at a voltage concomitant with breakdown measured from the current-voltage curves. Our data agree well with a model that incorporates a built-in polarization and associated electric field across polar LaAlO3. We discuss the implications of our results in terms of polarization induced band bending at the LaAlO3/SrTiO3 interface.
4:00 PM - **M5.4
Two-dimensional Electron Liquid State at Oxide Interfaces.
Martin Breitschaft 1 , Veronika Tinkl 1 , Natalja Pavlenko 2 , Stefan Paetel 1 , Christoph Richter 1 , John Kirtley 1 , Yang Liao 1 , German Hammerl 1 , Volker Eyert 1 , Thilo Kopp 1 , Jochen Mannhart 1
1 , University of Augsburg, Augsburg Germany, 2 , Institute for Condensed Matter Physics, Lviv Ukraine
Show AbstractTwo-dimensional electron gases based on conventional semiconductors such as Si or GaAs have played a pivotal role in fundamental science and technology. The high mobilities achieved enabled the discovery of the integer and fractional quantum Hall effects and are exploited in high electron mobility transistors. Recent work has shown that two-dimensional electron systems can also exist at oxide interfaces [1]. These electron systems are characterized by properties that fundamental differ from those shown by semiconductors.In the presentation I will provide an overview of our studies of the properties of these unusual electronic systems (see, e.g., [2-4]) and explore the potential for electron liquids at oxide interfaces for the use in nanoscale electronic devices. [1] A. Ohtomo et al., Nature 419, 378 (2002)[2] S. Thiel et al., Science 313, 1942 (2006)[3] N. Reyren et al., Science 317, 1196 (2007)[4] C. Cen et al., Nature Materials 7, 298 (2008)
4:30 PM - M5.5
Two-dimensional Electron Gas at LAO/STO Heterointerfaces on SrTiO3 Template.
Chung Wung Bark 1 , David Felker 2 , Ho Won Jang 1 , Chad Folkman 1 , Sueng Hyub Baek 1 , Mark Rzchowski 2 , Chang-Beom Eom 1 , Daniela Bogorin 3 , Jeremy Levy 3 , Michael Biegalski 4
1 Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 Department of Physics, University of Wisconsin-Madison, Madison, Wisconsin, United States, 3 Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania, United States, 4 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe recent discovery of a two-dimensional electron gas (2DEG) at SrTiO3/LaAlO3 heterointerfaces has been made possible by advances in growth techniques that allow the creation of atomically-abrupt interfaces between complex oxide materials. Extensive research suggests that the conducting layer can be localized within a few nanometers of the interface, and that an electronic reconstruction is responsible for the emergence of this new interfacial material. We have grown LaAlO3 on epitaxial (001) SrTiO3 templates on (001) (La,Sr)(Al,Ta)O3 (LSAT) substrate by pulsed laser deposition with in situ high pressure RHEED. The full width at half maximum of the (001) rocking curve of the SrTiO3 template was 0.004, much narrower than SrTiO3 bulk single crystals. We observe an abrupt conductance increase as the SrTiO3 template layer becomes thicker than 5 unit cells. We also report on the temperature dependence of the sheet resistance, carrier concentration and mobility. We will discuss the ability to form reversible conducting nanostructures by conductive AFM tips to explore oxide nanoelectronic devices on a scalable substrate platform.
4:45 PM - M5.6
Doping SrCuO2 Monolayer by Charge Transfer in Coupled Interfaces.
Mark Huijben 1 , Hajo Molegraaf 1 , Alexander Brinkman 1 , Sander Wenderich 1 , Maarten van Zalk 1 , Josee Kleibeuker 1 , Jeroen Huijben 1 , Gertjan Koster 1 , Hans Hilgenkamp 1 , Guus Rijnders 1 , Dave Blank 1
1 , University of Twente, Enschede Netherlands
Show AbstractPrevious studies have shown that charge transfer can take place within the SrTiO3-LaAlO3 system, in which the polarity of LaAlO3 is crucial in understanding the conductivity at the interfaces [1]. For every unit cell of LaAlO3, electric potential is built up, albeit partly screened by counteracting ionic displacements within the charged layers [2]. After a critical number of LaAlO3 unit cells, electronic reconstruction is thought to take place, in which charge is transferred from the LaAlO3 surface to the SrTiO3-LaAlO3 interface. Apart from the critical role of induced oxygen vacancies in the SrTiO3 [3], also reconstruction at the LaAlO3 surface can be expected to have a large influence on the interface electronic reconstruction. We have recently extended our work on coupled complementary interfaces from 2006 [4] by repeating the experiments at higher oxygen deposition pressures and variable LaAlO3 and SrTiO3 capping layer thicknesses. In these studies we have observed an insulator to metal transition for a very thin (~2 unit cells) LaAlO3 layer on SrTiO3 when a single unit cell of SrTiO3 is used as capping [5]. Combined scanning tunneling spectroscopy and magneto transport measurements give evidence for parallel electron-hole bilayer conductivity, in which the hole conductivity in the surface SrTiO3 is separated from the electron conductivity at the bottom LaAlO3-SrTiO3 interface by about 1 nm. In this contribution we will show our results for SrTiO3-LaAlO3-SrCuO2-SrTiO3 heterostructures, in which a single SrCuO2 layer is placed at the top interface where we have observed hole conductivity in previous studies. RHEED analysis will be given to show the highly controlled growth as well as magneto transport measurement, from 50 mK to 300 K and 0 to 30 Tesla, will be presented to demonstrate the doping of a single SrCuO2 layer by charge transfer from the underlying SrTiO3-LaAlO3 interface.[1] A. Ohtomo and H.Y. Hwang, Nature 427, 423 (2004).[2] R. Pentcheva and W.E. Pickett, Phys. Rev. Lett. 102, 107602 (2009).[3] M. Huijben et al., Adv. Mater. 21, 1665 (2009).[4] M. Huijben et al., Nature Mater. 5, 556 (2006).[5] Work done in collaboration with R. Pentcheva and K. Otte (University of Munich) and W.E. Pickett (University of California at Davis)
5:00 PM - M5.7
Atomic Structure and Electronic Properties of Interfaces Between Rare-earth Oxides and SrTiO3.
Xiaoqing Pan 1 , Christopher Nelson 1 , Yi Zhang 1 , Hon-Won Jang 2 , Jae-Wan Park 2 , Chang-Beom Eom 2 , Dong Su 3 , Yimei Zhu 3
1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States, 3 , 3Brookhaven National Laboratory, Upton, New York, United States
Show AbstractThe interfaces in perovskite oxide heterostructures can display remarkably different properties from either constituent. The discovery of a two-dimensional electron gas (2DEG) at oxide heterointerfaces has attracted a number of experimental and theoretical investigations of the origins and properties of this novel material system. We have recently studied the properties of a single layer of ReTiO3 (Re = La and Sm) inserted into an epitaxial SrTiO3 layer and found that the structure consisting of La results in conducting 2DEGs, but one with Sm is insulating. We also found that the electronic properties of the interface between LaAlO3 and SrTiO3 grown on (001) Si strongly depend on the terminating atomic layer of SrTiO3 and on the post-deposition annealing. In this paper we report the atomic structure and electronic characteristics of the inserting rare-earth oxide layers and the interfaces between ReTiO3 and SrTiO3 by atomic resolution imaging and electron energy-loss spectroscopy using spherical aberration-corrected transmission electron microscopes. The relationships between atomic structure and electronic properties of interfaces will be addressed.
5:15 PM - M5.8
Electron-lattice Instabilities Supress Cuprate-like Fermi Surfaces at Oxide Interfaces.
James Rondinelli 1 , Nicola Spaldin 1
1 Materials Department, UC Santa Barbara, Santa Barbara, California, United States
Show AbstractProgress in the layer-by-layer growth of transition metal oxide films motivated the intriguing recent suggestion that oxide heterostructures could be engineered to have band structures close to those of the high-Tc cuprates. While theoretical candidate materials have been identified, experimental realization has not yet been achieved. In this work, using first-principles density functional theory calculations, we explore the behavior of thin layers of metallic, orbitally degenerate, d4 SrFeO3 confined between the d0 dielectric SrTiO3 in a superlattice geometry. We show that the conventional heteroepitaxial constraint, which requires the film and substrate to have identical in-plane lattice constants, imposes an additional crystal field splitting on the electronic structure; the splitting of the electronic degeneracy combines with the two-dimensionality of the superlattice to produce a metallic cuprate-like band structure. We find, however, that the band structure is drastically changed by the existence of lattice instabilities which occur in the superlattice, yet are stable in each bulk component. We show that these enhanced electron-lattice instabilities are strongly sensitive to the superlattice periodicity and compete with the formation of a cuprate-like Fermi surface: in ultra thin films a metastable metallic structure transforms into a fully insulating superlattice by condensation of a cooperative Jahn-Teller distortion. We suggest our results provide a plausible explanation for the absence of metallic behavior in ultra thin orbitally degenerate oxide superlattices that are predicted to be superconducting. We summarize by outlining materials design criteria that may circumvent the strong electron-lattice couplings to promote metallic behavior in oxide superlattices.