Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L41: Focus Session: Local Ionic Dynamics and Domain Walls in Oxides |
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Sponsoring Units: DMP Chair: Lane Martin, University of Illinois Room: Mile High Ballroom 3C |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L41.00001: Probing Local Ionic Dynamics in Functional Oxides: From Nanometer to Atomic Scale Invited Speaker: Sergei Kalinin Vacancy-mediated electrochemical reactions in oxides underpin multiple applications ranging from electroresistive memories, to chemical sensors to energy conversion systems such as fuel cells. Understanding the functionality in these systems requires probing reversible (oxygen reduction/evolution reaction) and irreversible (cathode degradation and activation, formation of conductive filaments) electrochemical processes. In this talk, I summarize recent advances in probing and controlling these transformations locally on nanometer level using scanning probe microscopy. The localized tip concentrates the electric field in the nanometer scale volume of material, inducing local transition. Measured simultaneously electromechanical response (piezoresponse) or current (conductive AFM) provides the information on the bias-induced changes in material. Here, I illustrate how these methods can be extended to study local electrochemical transformations, including vacancy dynamics in oxides such as titanates, La$_{x}$Sr$_{\mathrm{1-x}}$CoO$_{3}$, BiFeO$_{3}$, and Y$_{x}$Zr$_{\mathrm{1-x}}$O$_{2}$. The formation of electromechanical hysteresis loops and their bias-, temperature- and environment dependences provide insight into local electrochemical mechanisms. In materials such as lanthanum-strontium cobaltite, mapping both reversible vacancy motion and vacancy ordering and static deformation is possible, and can be corroborated by \textit{post mortem} STEM/EELS studies. In ceria, a broad gamut of electrochemical behaviors is observed as a function of temperature and humidity. The possible strategies for elucidation ionic motion at the electroactive interfaces in oxides using high-resolution electron microscopy and combined ex-situ and in-situ STEM-SPM studies are discussed. In the second part of the talk, probing electrochemical phenomena on in-situ grown surfaces with atomic resolution is illustrated. I present an approach based on the multivariate statistical analysis of the coordination spheres of individual atoms to reveal preferential structures and symmetries. The relevant statistical techniques including k-means clustering, principal component analysis, and Baesian unmixing are briefly intriduced. This approach is illustrated for several systems, including chemical phase identification, mapping ferroic variants, and probing topological and structural defects, and provides real space view on surface atomic processes. Research supported (SVK) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and partially performed at the Center for Nanophase Materials Sciences (AK, SJ), a DOE-BES user facility. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L41.00002: Charge carrier trapping into mobile, ionic defects in nanoporous ultra-low-k dielectric materials Joel Plawsky, Juan Borja, Toh-Ming Lu, William Gill Reliability and robustness of low-k materials for advanced interconnects has become a major challenge for the continuous down-scaling of silicon semiconductor devices. Metal catalyzed time dependent breakdown (TDDB) is a major force preventing the integration of sub-32nm process technology nodes. We investigate how ionic species can become trapping centers (mobile defects) for charge carriers. A mechanism for describing and quantifying the trapping of charge carriers into mobile ions under bias and temperature stress is presented and experimentally investigated. The dynamics of trapping into ionic centers are severely impacted by temperature and species mass transport. After extended bias and temperature stress, the magnitude of charge trapping into ionic centers decreases asymptotically. Various processes such as the reduction of ionic species, moisture outgassing, and the inhibition of ionic drift via the distortion of local fields were investigated as possible cause for the reduction in charge trapping. Simulations suggest that built-in fields reduce the effect of an externally applied field in directing ionic drift, which can lead to the inhibition of the trapping mechanism. In addition, conduction mechanisms are investigated for reactive and inert electrodes. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L41.00003: Resistive Switching and Temperature-dependent Transport in HfOx-based Resistive Memory Devices Seyoung Kim, Chiyui Ahn, Tayfun Gokmen, Oliver Dial, Mark Ritter Resistive switching phenomenon in transition metal oxide materials has been studied intensively as a candidate technology for future non-volatile memory applications and electronic synapse devices. Here, we demonstrate an HfOx-based resistive memory device with rare earth metal contact in which the device resistance can be modulated with applied voltage and current. Repeatable and self-compliance switching as well as high yield and device-to-device uniformity are achieved in our devices. To understand the conduction mechanism, we perform transport measurement in multiple devices at different resistance states (initial, low and high resistance states) by probing current as a function of applied voltage at temperatures from 40K to 350K. We find that temperature insensitive tunneling conduction dominates at low temperature, while thermally activated conduction is observed at high temperature. Trap-assisted tunneling and Poole-Frenkel mechanisms are accounted for the characteristics found in different regimes. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L41.00004: Study of the Filament Evolution in TiO$_2$ Resistive Switching Device by Schottky Junction Analysis Trithep Devakul, Badih A. Assaf, Pegah M. Hosseinpour, Laura H. Lewis, Don Heiman Resistive switching in TiO$_2$-based metal-dielectric-metal devices is thought to be driven by the dynamics and evolution of oxygen deficient conducting filaments through the bulk of TiO$_2$ [1]. We present an analysis of the resistive switching characteristics of Ti/TiO$_2$/Au devices grown by anodizing Ti on Si and Ti foil. The device is SET and RESET by increasing the switching voltage in small steps. Current-voltage data is obtained at low voltages at each step and analyzed using the Simmons model for thermionic emission of electrons over an energy barrier. The energy barrier consists of an insulating TiO$_2$ barrier sandwiched between an electrode and an oxygen deficient TiO$_x$ filament. The IV fits yield information about the height and width of the energy barrier. In the low resistance state, we find that the barrier width becomes wider, but this is overcome by a lower barrier height. The observed results can be explained by a model in which a field-driven migration of oxygen vacancies [2] modulates the Schottky barrier height and width. \\[4pt][1] D-H. Kwon et al, \emph{Nature Nanotechnology} {\bf 5}, 148-153 (2010) \\[0pt][2] K. J. Yoon et al, \emph{Nanotechnology} {\bf 23}, 185202 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L41.00005: Surface potential modification of molecular beam epitaxially grown SrTiO$_{3-\delta}$/Si(001) measured by Kelvin Force Probe Microscopy Ryan Cottier, Alexander Currie, Nikoleta Theodoropoulou SrTiO$_{3}$ (STO) films have been grown by molecular beam epitaxy on p-Si(001), n-Si(001), and STO(001) substrates. The STO/Si films were of high crystalline quality as determined by x-ray diffraction (XRD) and TEM and ranged in thickness from 3.6 to 60 nm as measured by x-ray reflectivity (XRR). The partial pressure of oxygen (O$_{2})$ was varied during growth to induce oxygen vacancies within the STO structure. Through additional XRD and magnetotransport studies, we estimate that for the lowest O$_{2}$ pressure the oxygen deficiency is $\delta =$0.02. The surface potential of the films was modified through the use of a conducting atomic force microscopy (AFM) tip by scanning regions of the STO surface in contact mode with a DC bias on the tip (referred to as `writing'). Regions were written with either positive or negative voltage and then analyzed by Kelvin Force Probe Microscopy (KFPM). Following this writing mode, KFPM revealed a retained surface potential of the same polarity used in writing. The ability of the films to be written and read through this method depended on the growth O$_{2}$ partial pressure with higher O$_{2}$ pressures demonstrating weaker surface potential modification. The results agree with other studies regarding the drift and diffusion of charged O$_{2}$ vacancies in STO. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L41.00006: Effect of antisite-like defect in ferroelectricity of SrTiO$_{3}$ films Fang Yang, Zhenzhong Yang, Lin Gu, Jiandong Guo Ferroelectricity in thin films of nominally nonferroelectric materials such as SrTiO$_{3}$ has been widely studied. It is known that some extrinsic factors such as strain [M. P. Warusawithana et al. Science 324, 367 (2009)] and defect [H. W. Jang et al., Phys. Rev. Lett. 104, 197601 (2010), M. Choi et al., Phys. Rev. Lett. 103, 185502 (2009)] can result in the ferroelectricity of SrTiO$_{3}$ thin films. The SrTiO$_{3}$ thin films with ferroelectricity were prepared on Si (001) substrates by oxide molecular beam epitaxy. The energy dispersive x-ray spectroscopy (EDX) mapping measurement results demonstrate Sr diffuses to the interface of SrTiO$_{3}$ and Si. The cross sectional high-resolution transmission electron microscopy (HRTEM) results show that there are interstitial Ti atoms in the unit cells. The off-centered Ti from the Sr site along [100] or [110] direction can be regarded as a polar defect pair composed of a Sr vacancy and an interstitial Ti. It is predicted that Ti antisitelike defects in SrTiO$_{3}$ are responsible for the ferroelectricity . Such antisitelike defects observed in SrTiO$_{3}$ films are considered as the origin of the ferroelectricity. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L41.00007: Main photorefractive Fe center in KNbO$_{3}$ Sergey Basun, Dean Evans KNbO$_{3}$ crystals have been attracting a lot of interest in nonlinear optics. In iron-doped samples, a dramatic increase in photorefractive sensitivity and speed can be achieved. Despite the variety of Fe centers known in KNbO$_{3}$:Fe, the Fe center responsible for the photorefractive effect has not been previously identified. Correlated EPR and optical studies of the as-grown and reduced samples shed light on the nature of the main photorefractive center in KNbO$_{3}$:Fe -- it is Fe[Nb]-V$_{\mathrm{O}}$, Fe on the Nb site next to oxygen vacancy. Fe[Nb] centers that are commonly considered as the cause of photorefraction in KNbO$_{3}$:Fe are only of secondary significance. Free electrons are provided through photoionization of Fe$^{2+}$[Nb]-V$_{\mathrm{O}}$ with photon energies higher than 0.85 eV, Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$ centers serve as electron traps. Concentration of the Fe[Nb]-V$_{\mathrm{O}}$ centers is quite comparable to that of Fe[Nb], but in the as-grown samples they are only present in the form of Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$. Reduction of the samples takes almost no effect on Fe$^{3+}$[Nb], but it appreciably converts Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$ to Fe$^{2+}$[Nb]-V$_{\mathrm{O}}$, which gives rise to the concentration of the photo-electron donors and dramatically improves photorefractive performance. Locations of the Fe$^{2+/3+}$[Nb]-V$_{\mathrm{O}}$ and Fe$^{3+/4+}$[Nb] centers in the bandgap will be presented. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L41.00008: Structure and switching of in-plane ferroelectric nano-domains in strained Pb(x)Sr(1-x)TiO(3) thin films Sylvia Matzen, Oleksiy Nesterov, Gijsbert Rispens, Jeroen Heuver, Beatriz Noheda, Michael Biegalski, Hans Christen Understanding and controlling domain formation in nanoscale ferroelectrics is interesting from a fundamental point of view and of great technological importance. Increasing miniaturization allows creating complex domain structures, offering novel functionalities that could be particularly useful for the development of nanoelectronic devices. While most studies in thin films focus on domain patterns with up/down polarization for ferroelectric memories, domain structures with purely in-plane polarization have not been much investigated. However, such structures are potentially useful in optical devices or to avoid depolarization fields in ultra-thin films, as long as the domains can be addressed and switched. We use a combination of compositional substitutions and epitaxial growth on a substrate in order to tune the domain configuration. The substitution of Pb by Sr in Pb$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_{\mathrm{3}}$ thin films grown epitaxially on (110)-DyScO$_{\mathrm{3}}$, stabilizes a domain structure with purely in-plane polarization. In this work, we show that it is possible to stabilize and control a complex domain architecture at two different length scales, yielding periodic ferroelectric nano-domains with purely in-plane polarization. Most importantly, these in-plane domains can be switched by a scanning probe. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L41.00009: Complex structural behaviors of domain walls revealed from first principles Jorge Iniguez, Jacek C. Wojdel, Oswaldo Dieguez We have used a variety of theoretical techniques, including accurate first-principles methods and model potentials allowing for large-scale simulations, to investigate the structural and lattice-dynamical behavior of the domain walls in several ferroic perovskite oxides. In this talk I will review the most striking effects our work has revealed, which range from emerging orders confined at the walls to novel forms of polytypism. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:48AM |
L41.00010: Charged domain walls in ferroelectrics Invited Speaker: Tomas Sluka Solid interfaces including compositionally homogeneous ferroic domain walls (DWs) display uniquely distorted electronic structures and ionic displacements. Their intrinsic properties may therefore be fundamentally different from those of their parent matrices. Indeed, phenomena like semiconductor-metal transition, the quantum Hall effect, magnetoresistance and superconductivity were discovered at hetero-interfaces between transition metal oxides and elevated photoactivity and conductivity were reported at (multi-) ferroic DWs. Unlike hetero-interfaces, the DWs provide ``perfect'' structure by nature and can be written, displaced, and erased inside a material monolith of functioning devices. Theory predicts the existence of charged DWs which seemingly violate electrostatic compatibility due to head-to-head and tail-to-tail polarization discontinuity, but are stable because bound polarization charge is compensated by mobile charge carriers including quasi-two-dimensional electron gas. This talk will introduce current theory, engineering, control and characteristics of charged DWs, which are mobile, extremely wide and exhibit steady metallic-like conductivity up to $10^9$ times that of the insulating bulk. [Preview Abstract] |
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