Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures III |
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Sponsoring Units: DMP Chair: Yoshihiro Asai, National Institute of Advanced Industrial Science and Technology Room: Mile High Ballroom 2C |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W53.00001: Modeling the Operation of Resistive Switching Memory Devices Invited Speaker: Blanka Magyari-Kope Resistance change based nonvolatile memory devices are currently considered as leading candidates for future memory modules. To assess the scalability, retention and endurance properties of these devices, however, a detailed understanding of the underlying resistive switching mechanism is imperative. Filamentary arrangements of oxygen vacancies in transition metal oxides under applied electric field were investigated theoretically and recently detected experimentally. Generally, the process of forming in these systems may include a mechanism by which oxygen vacancies can cluster into filaments and/or the diffusion of oxygen atoms away from the oxide region to form a thin interfacial reduced oxide. During electroforming, oxygen vacancies and/or ions drift due to the applied bias, trap electrons or holes and facilitate the formation of vacancy ordered domains. We review the implications on the electronic structure and energetics of conductive filament channels formation corresponding to the ``ON'' state and discuss the interplay between the ionic and electronic transport mechanisms. We show that charge trapping effects play a significant role in the switching process under applied electrical field affecting the atomistic pathways of the filament rupturing/dissolution process from the ``ON'' into the ``OFF'' state. Furthermore, in order to improve on the device characteristics, favorable effects and ``ON''-``OFF'' transition process control can be achieved with preferential impurity doping. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W53.00002: Metal oxide resistive switching: evolution of the density of states across the metal insulator transition Alireza Mottaghizadeh, Qian Yu, Alexandre Zimmers, Herve Aubin Memristive devices have attracted considerable attention since the recognition that two-terminal resistive switching elements represents an example of a memristive element. In oxide materials such as SrTiO$_{3}$ (STO), oxygen vacancies are doping sites that can be displaced by an electric field. This allows for electric-field manipulation of doping as exploited in memristive devices. In this work, we present the study of metal-semiconductor-metal junctions formed on STO, where we demonstrate that the junction characteristics can be fine-tuned through electric field migration of oxygen vacancies at very low temperature (T $\sim$ 260 mK). At very low dopant concentration, the junction displays characteristic signatures of discrete dopants levels. As the dopant concentration increases, the semiconductor band gap fills in but a soft Coulomb gap remains, at even higher doping, a transition to a metallic state occurs where the density of states at the Fermi level becomes finite and Altschuler-Aharonov correction to the density of states is observed. This work demonstrates that electric field induced migration of dopants can be used to tackle open questions on the physics of correlated electron systems. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W53.00003: \textit{In Situ} TEM of Conductive Bridge Formation in Nanoscale Resistive Memory Devices William A. Hubbard, Edward R. White, Jared Lodico, B.C. Regan We observe formation and dissolution of conductive filaments in nanoscale conductive bridge memory (CBRAM) devices \textit{in situ} by scanning and conventional TEM. Horizontally separated CBRAM devices are fabricated on electron-transparent membranes, and the solid electrolyte layer is deposited between the inactive and active metal layers via atomic layer deposition. An additional ALD film caps the entire device. This geometry allows for unambiguous determination of active filaments and precludes filament formation by surface migration of the active metal, ensuring that conductive paths form within the solid electrolyte. In this study the inactive metal, active metal, and solid electrolyte are platinum, copper or silver, and alumina, respectively. Devices exhibit repeatable switching between high and low resistance states and the impact of filament number, size, and geometry on device switching parameters is discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W53.00004: Memristor Physics Driven by Joule Heating Harold Hjalmarson, Michael McLain, Denis Mamaluy, Xujiao Gao Switching in bipolar memristive devices involves the growth of conductive filaments following the application of a voltage pulse that causes heating. This Joule heating by the electric field is a large contributor to the migration of atoms and vacancies. In this talk, the results of continuum calculations will be used to describe the switching of tantalum oxide devices. The continuum calculations include the effects of Joule heating, chemical species migration, ionizing radiation and chemical reactions. These calculations will be focused on the temporal evolution of a conductive filament in a simple structure. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W53.00005: \emph{Ab initio} Calculations for Hydrogen-Doped HfO$_{2-x}$ RRAM Dan Duncan, Blanka Magyari-Kope, Yoshio Nishi Hydrogen impurities are shown to have significant effects on the mechanism of electronic conduction in HfO$_2$-based resistance change memory (RRAM) devices, and to affect the ionic transport during the forming, set, and reset processes. Using density functional theory and employing the LDA+\emph{U} formalism, the diffusion of oxygen ions in hydrogen-doped HfO$_{2-x}$ was examined and its implications on the electronic structure are determined. Results indicate that hydrogen can have multiple substantial effects on device operation, and has a strong potential to improve device switching and uniformity. These hydrogen-doped devices make promising candidates for low-voltage and forming-free memory schemes, as well as for electronic synapses in neuromorphic systems. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W53.00006: Theoretical Investigation of the Hafnia-Hafnium Interface in RRAM Devices Andrew O'Hara, Gennadi Bersuker, Alexander Demkov Oxide based resistive-switching memory devices (RRAM) utilizing hafnia (HfO$_{\mathrm{2}})$ as the dielectric serve as an attractive option for embedded non-volatile memory systems. Successful operation requires a degree of oxygen deficiency caused by application of a forming voltage. A recent approach to help facilitate this has been the use of an oxygen gettering layer overlaying hafnia. Using density functional theory (DFT) in the local density approximation (LDA), we construct and study a hafnia-hafnium interface to understand the reducing and gettering properties. With this interface, we compare two routes to the creation of substoichiometric hafnia: formation of oxygen vacancies that leave hafnium unoxidized and migration of oxygen to hafnium to form an extended Frenkel pair (FP). Our work shows that the presence of the interface lowers the vacancy formation energy by 1.1 eV from the bulk value of 7.5 eV. Using the nudged-elastic band method, we show that not only is the formation energy lower for an extended FP, but that the barrier to formation of the shortest such FP is only 1.3 eV implying the favorability of such defects. Finally, we study the diffusion of oxygen in bulk hafnium to learn how the defect would behave after disassociation of the FP. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W53.00007: Physical Description and Experimental Characterization of the Resistive Switching Filament Andrew Lohn, Patrick Mickel, Conrad James, Matthew Marinella We derive an analytical, steady state solution for resistive switching from the heat equation. Fitting our equation to a single hysteresis loop (the most fundamental experiment in the field), provides experimental determination of the filament radius, conductivity, temperature and the thermal conductivity of the surroundings. These parameters are determined continuously and show excellent agreement with the detailed experimental work to date. This approach enables every researcher with a current-voltage sourcemeter to experimentally characterize their filament. The analytical nature of our equation also elucidates the relation between materials, design parameters, and performance metrics. We generalize the empirical relation for uniting resistive switches and we show that our steady state solution is valid over all relevant timescales by fitting to a hysteresis loop taken within 10 nanoseconds. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W53.00008: Multidimensional information storage in configurational changes of resistive switching filaments Patrick Mickel, Andrew Lohn, Conrad James, Matthew Marinella We present a new methodology which enables direct control of the geometry and radial composition of nanoscale filamentary resistive switches, and demonstrate the ability of this technique to store multidimensional information in a single device. Using bi-polar, power limited switching (as opposed to the common voltage or current sourcing), we demonstrate individual control over both the radius and the conductivity of the nanoscale conducting filaments which control resistive switching elements. Using this control, we show that degenerate resistances states may be composed of alternate radius/conductivity pairs which require distinct power thresholds to thermally activate resistive switching (thereby constituting 2D storage: R and P, or radius and conductivity). Finally, by implementing a series of alternate polarity power pulses, we show that the radial composition profile within the nanoscale filament may be precisely tuned leading to designed trajectories through P-R space and a third dimension of information storage. Using this technique we estimate that a single resistive switch may realistically supplant as many as 10 digital devices. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W53.00009: Transient doping in atomic chains -- a case study in time-resolved STM Paul Snijders, Stefan Polei, Steve Erwin, Franz Himpsel, Karl-Heinz Meiwes-Broer, Ingo Barke Doping one-dimensional (1D) systems is notoriously difficult due to the structural disorder created by the dopants. The Si(553)-Au surface features an array of step edges with 1D chains of dangling bonds. These chains have a 1x3 ordered ground state [1]. Using a scanning tunneling microscope we inject electrons from the tip into these step-edge chains and we observe that the periodicity of the atomic chains changes from the 1x3 ordered ground state to a 1x2 ordered excited state with increasing tunneling current. The threshold current for this transition is reduced at lower temperatures. In conjunction with first principles density-functional calculations we conclude that the 1x2 phase is created by transient doping of the atom chains [2]. Random telegraph fluctuations between two levels of the tunneling current provide direct access to the dynamics of the phase transition, revealing a monostable state, and lifetimes in the millisecond range. Our method provides a possible avenue to map out a doping-dependent phase diagram in cases where conventional impurity doping is problematic. \\[4pt] [1] S.C. Erwin, F.J. Himpsel, Nat. Comm. 1:58 (2010).\\[0pt] [2] S. Polei et al., PRL 111, 156801 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W53.00010: Local Gating of Carbon Nanotube inside TEM Li-Ying Chen, Yen-Song Chen, Chia-Seng Chang We report a new method of fabricating ultra-clean and hysteresis-free multi-wall carbon nanotube field-effect transistor (CNFET) inside an ultra-high vacuum transmission electron microscopy (TEM) equipped with a movable Au tip as a local gate. Local gating of CNFET is demonstrated concurrently with atomic-scale imaging. The development of the ambipolar characteristic of CNFET, the Vds effect on CNFET as well as the localized characteristics of CNFET have been investigated. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W53.00011: Equilibrium charge fluctuations of a charge detector and its effect on a nearby quantum dot David Ruiz-Tijerina, Edson Vernek, Sergio Ulloa We study the Kondo state of a spin-1/2 quantum dot (QD), in close proximity to a quantum point contact (QPC) charge detector near the conductance regime of the 0.7 anomaly. The electrostatic coupling between the QD and QPC introduces a remote gate on the QD level, which varies with the QPC gate voltage. Furthermore, models for the 0.7 anomaly [Y. Meir et al., PRL 89,196802(2002)] suggest that the QPC lodges a Kondo-screened level with charge-correlated hybridization, which may be also affected by capacitive coupling to the QD, giving rise to a competition between the two Kondo ground states. We model the QD-QPC system as two capacitively-coupled Kondo impurities, and explore the zero-bias transport of both the QD and the QPC for different local gate voltages and coupling strengths, using the numerical renormalization group and variational methods. We find that the capacitive coupling produces a remote gating effect, non-monotonic in the gate voltages, which reduces the gate voltage window for Kondo screening in either impurity, and which can also drive a quantum phase transition out of the Kondo regime. Our study is carried out for intermediate coupling strengths, and as such is highly relevant to experiments; particularly, to recent studies of decoherence effects on QDs. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W53.00012: Magnetosymmetries of nonlinear transport in dissipative conductors Salil Bedkihal, Dvira Segal We demonstrate with numerically exact simulations that nonlinear transport coefficients obey certain magnetic field symmetries. Our model includes a two terminal Aharonov-Bohm interferometer with a quantum dot located at each of its arms. One quantum dot is interacting electrostatically with a reservoir, a fermionic environment made of a quantum dot coupled to one or more leads. We study the dynamics and steady state properties of this many-body out of equilibrium setup, by using a numerically exact influence functional path integral technique (Phys. Rev.B 82, 205323 (2010)). We show that, in agreement with phenomenological treatments of dephasing and mean field approaches, even (odd) conductance terms obey odd (even) symmetry with threading magnetic flux, as long as system acquires spatial inversion symmetry. When spatial asymmetry is introduced, magnetic field symmetries are broken, but more general symmetries with respect to left-right interchange are obeyed. Finally we also numerically demonstrate that double quantum dot Aharonov-Bohm interferometer coupled electrostatically to a fermionic environment can act as a charge current rectifier when two conditions are met simultaneously (I)broken time reversal and (II) many body effects. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W53.00013: Theoretical study of carbon-nanotube-based molecular sensors Yan Li, Miroslav Hodak, Jerry Bernholc Carbon Nanotubes (CNTs) are highly promising for chemical and biological sensing applications, owing to their high chemical and mechanical stabilities, high surface areas as well as unique electronic properties. We report results of theoretical studies of detection abilities of several small analyte molecules, such as ammonia and nitrogen dioxide. We use density functional theory (DFT) and Keldysh non-equilibrium Green's function (NEGF) formalism to investigate differences in transmission coefficients and current due to interactions between the CNT and analyte molecules. For nitrogen dioxide, which chemisorbs on the CNT, we show that its attachment produces significant differences in both transmission and Current-Voltage (I-V) curve. For ammonia, we find that it can be either physisorbed or chemisorbed on the CNT depending on its position relative to the metalic leads. The chemisorbed case shows detectable differences in transmission and I-V curve. We also investigate sensing mechanisms of CNTs functionalized with receptor molecules for specific analyte molecules. [Preview Abstract] |
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