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
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 Show Abstract
1 MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands
Two-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 LaAlO3
 and LaAlO3
 systems. In this contribution, the work on the LaAlO3
system is extended to heterostructures with two n
interfaces. We will focus on the heteroepitaxial growth, transport properties as well as the dipole model, explaining the doping behaviour.Heterostructures of LaAlO3
were fabricated using pulsed laser deposition. A second LaO//TiO2
interface was created using a monolayer of LaTiO3
. A series of samples was grown where the interface separation was varied. XRD measurements indicated that the films were coherently grown on the SrTiO3
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
heterostructures depends on the interface separation. However, the trend is opposite that of the np
heterostructures published in literature . 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
//SrO interface is nominally p
-doped and acts as a sink for electrons . 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 LaTiO3
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 LaAlO3
system . 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 Show Abstract
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
The 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 Show Abstract
1 IFF-6, FZ Jülich, Jülich Germany, 2 , Ernst Ruska-Centre, Jülich Germany
Interfaces 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 Show Abstract
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
The 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 Show Abstract
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The 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 Show Abstract
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The 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 Show Abstract
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
A 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 Show Abstract
1 Microscopy and Microanalysis, Chalmers University of Technology, Gothenburg Sweden, 2 Department of Microtechnology and Nanoscience, Chalmers University of Technology, Gothenburg Sweden
The 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.  A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004) S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider and J. Mannhart, Science 313, 1935 (2006) M. Huijben, A. Brinkman, G. Koster, G. Rijnders, H. Hilgenkamp and D.H.A. Blank, Advanced Materials 21, 1 (2009)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 Show Abstract
1 Physics and Astronomy, Rutgers University, Piscataway, New Jersey, United States
In 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
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 Show Abstract
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
Perovskite 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 Show Abstract
1 Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Considerable 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 Show Abstract
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
It 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 Show Abstract
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
Complex 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 Show Abstract
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
In 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 Show Abstract
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
In 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 Show Abstract
1 ICMAB, CSIC, Bellaterra, catalunya, Spain, 2 ICMM, CSIC, Madrid Spain
Chemical 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 . 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. 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 Show Abstract
1 MESA+ institute of nanotechnology, University of Twente, Enschede Netherlands
Perovskite-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 Show Abstract
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
Conductivity 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. 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. 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. 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. 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 Show Abstract
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
Using 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
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 Show Abstract
1 Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu Taiwan
We 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 Show Abstract
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
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 Show Abstract
1 , Moscow State Institute of Radioengineering, Electronics and Automation (Technical University), MIREA, Moscow Russian Federation, 2 , Institute of Crystallography, Moscow Russian Federation
Electrical 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, asy