Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T20: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures - Resistive Switching Phenomena |
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Sponsoring Units: DMP Chair: Luca Larcher, Universita di Modena e Reggio Emilia Room: 322 |
Thursday, March 21, 2013 8:00AM - 8:12AM |
T20.00001: X-ray Irradiation Induced Colossal Resistance Change in Pt/TiO2/Pt cellss Seo Hyoung Chang, Jungho Kim, Seong Keun Kim, Cheol Seong Hwang, Kenneth D'Aquila, Jeffrey A. Eastman, Jiyoon Kim, Seungbum Hong Interaction between x-ray and matters has been drawing much attention due to its scientific interests as well as technological applications. In particular, synchrotron-based x-ray has been used as a powerful diagnostic tool to unveil nanoscale phenomena in functional materials. However, understanding of how the functional materials respond to the brilliant x-ray is far from complete. Here we report the x-ray-induced colossal resistance change in 40 nm thick TiO$_{2}$ films sandwiched by Pt top and bottom electrodes. We observe that the resistance level is modulated in a few orders of magnitude by the intensity of impinging x-ray. In addition, this photovoltaic-like effect can trigger an irreversible resistance change by another few orders of magnitude. We will discuss the physical mechanism behind the emergent phenomenon. Work at the APS, Argonne is supported by a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 21, 2013 8:12AM - 8:24AM |
T20.00002: Effect of metallic buffer at electrode-oxide interface on current-voltage characteristics of resistive random access memories (ReRAMs): A first-principles study Takehide Miyazaki, Hisao Nakamura, Kengo Nishio, Hisashi Shima, Hiroyuki Akinaga, Yoshihiro Asai We present the electric current ($I$)-voltage ($V$) characteristics (-1.0 eV $<$ $V$ $<$ +1.0 eV) for a model of ReRAM devices with metal-oxide-metal structures, based on first principles nonequilibrium Green's function (NEGF) theory [1]. We choose TiN and hafnia (HfO$_2$) for the electrode and oxide materials, respectively, because this combination has been widely known in literature. We investigate the $I$-$V$ characteristics for two different compositions of the TiN/HfO$_2$ interface, (a) with and (b) without the Ta buffer layer between TiN and HfO$_2$. We assume cubic HfO$_2$ layers for simplicity. For case (a), a clear distinction between the ``ON" and ``OFF" states appears depending on the occurrence and absence of the oxygen vacancies (V$_{\rm O}$s), respectively. For case (b), however, little electric current flows even when the V$_{\rm O}$s exist in hafnia. In the latter, the O atoms abstracted from hafnia are strongly bound to N, leading to substantial separation of TiN from HfO$_2$. In contrast, in the former, the Ta buffer not only absorbs the O atoms but also bridges TiN and HfO$_2$ to secure the occurrence of the ``ON" state. [1] H. Nakamura et al., J. Phys. Chem. C \underline{115}, 19931 (2011). [Preview Abstract] |
Thursday, March 21, 2013 8:24AM - 8:36AM |
T20.00003: Identifying and Measuring the State Variables in TaOx Memristors Patrick Mickel, Matthew Marinella, Conrad James We present evidence of the identification and characterization of new state variables in TaOx memristors. Thus far, the state variable controlling the resistive switching has been believed to be the oxygen concentration in the conducting Ta filament. However, using voltage pulse measurements sensitive to small changes in resistance, we identify three distinct switching regimes governed by three unique state variables. Oxygen concentration in the Ta filament is shown to control the memristor resistance for low resistances, after which we observe a clear crossover to the area state variable dominated resistance range, and finally a large non-linear resistance range governed by the thickness of a developing insulating layer. The amplitude and time-scale of the applied tuning voltage pulses is investigated, providing insight into thermal properties of the device during switching. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T20.00004: Atomic Level Design Rule for Ta-based Resistive Switching devices Seo Hyoung Chang, S. Hong, M.-J. Lee, Y.-B. Kim, S. Chattopadhyay, T. Shibata, B. Magyari-Kope, J.A. Kaduk, J.A. Eastman, J. Kim Understanding resistive switching phenomena is a prerequisite to realizing the next generation of information storage systems. Ta-based resistive switching devices have been extensively investigated due to their fast switching and reliable endurance among other materials. Despite extensive recent interests, there is still a lack of fundamental understanding of electronic structure and local structure of the Ta-based device. Here, we investigated Ta$_{2}$O$_{5}$ powder, Ta$_{2}$O$_{5-\delta }$ and TaO$_{x}$ thin films and devices using synchrotron x-ray studies at the Advanced Photon Source, combining resonant x-ray inelastic scattering (RIXS), extended x-ray absorption spectroscopy (EXAFS) and density functional theory based \textit{ab initio} calculations. We found that there are strong correlations between critical values of band gap energies and local atomic environments around Ta atoms. These studies can provide vast possibilities to create new materials based on atomic level design rather than the traditional trial-error methods. Work at the APS, Argonne is supported by a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T20.00005: First-principles modeling of the electron and ion transport in TiO$_{2}$ ReRAM Liang Zhao, Blanka Magyari-Kope, Yoshio Nishi Transition metal oxide ReRAM is a promising candidate for next generation non-volatile memories. One of the key challenges in modeling ReRAM operations is the prediction of conduction behaviors. The conduction mechanism was found to vary from metallic in ON state, to quantum tunneling/hopping in OFF state. Since resistive switching is a gradual transition between the two, quantitative prediction of I-V characteristics through arbitrary oxygen vacancy (V$_{\mathrm{O}})$ configuration is desirable. Here we systematically calculated the electron transport properties of pristine and defective TiO$_{2}$, by introducing isolated and clustered V$_{\mathrm{O}}$ in a TiN/TiO$_{2}$/TiN device structure. The relaxed atomic structures were obtained from density functional theory (DFT) calculations, and the transport behaviors were calculated by DFT-based non-equilibrium Green's function (NEGF) approach. The I-V characteristics of both ON and OFF states can be well reproduced. It was also found that oxygen diffusion into to the vacancy sites is strongly affected by the interface with metal electrodes. Based on the results of transport calculations, a 3D analytical model is parameterized to allow the detailed prediction of device characteristics. [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:36AM |
T20.00006: Nanoionic Memristive Switches -- From Fundamentals to Applications Invited Speaker: Rainer Waser A potential leap beyond the limits of Flash (with respect to write speed, write energies) and DRAM (with respect to scalability, retention times) emerges from nanoionic redox-based switching effects encountered in metal oxides (ReRAM). A range of systems exist in which highly complex ionic transport and redox reactions on the nanoscale provide the essential mechanisms for memristive switching. One class relies on mobile cations which are easily created by electrochemical oxidation of the corresponding electrode metal, transported in the insulating layer, and reduced at the inert counterelectrode (so-called electrochemical metallization memories, ECM, also called CBRAM). Another important class operates through the migration of anions, typically oxygen ions, towards the anode, and the reduction of the cation valences in the cation sublattice locally providing metallic or semiconducting phases (so-called valence change memories, VCM). The electrochemical nature of these memristive effects triggers a bipolar memory operation. In yet another class, the thermochemical effects dominate over the electrochemical effects in metal oxides (so-called thermochemical memories, TCM) which leads to a unipolar switching as known from the phase-change memories. In all systems, the defect structure turned out to be crucial for the switching process. The presentation will cover fundamental principles in terms of microscopic processes, switching kinetics and retention times, and device reliability of bipolar ReRAM variants. Passive memory arrays of ReRAM cells open up the paths towards ultradense and 3-D stackable memory and logic gate arrays. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 10:12AM |
T20.00007: Nanoionic switching in metal oxide nanostructures Invited Speaker: Daniele Ielmini Ion migration in oxide nanostructures is a key process in information storage technologies, where the logic data are stored as nanoscale conductive filaments [1]. Due to the inherently nanoscale size of the ionic switching location (few cubic nanometers), the local electric field and current density induce extremely high temperatures as a result of Joule heating [2,3]. To develop and design advanced nanoionic materials and devices with improved performance and reliability, the ion migration phenomena in metal oxides must be carefully understood and modeled. This talk will address the modeling of ionic migration and the consequent switching in HfO$_{\mathrm{x}}$ layers of RRAM devices [4]. The model solves drift/diffusion equations for thermally-activated hopping of positive ion, such as oxygen vacancies (V$_{\mathrm{O}}^{+})$ and metal cations (Hf$^{+})$, in presence of intense Joule heating and electric field. The impact of the ion distribution on the local conductivity is described physics-based models of defect-assisted electronic conduction in semiconductors [5,6]. Microscopic parameters, such as the energy barrier for ion hopping, are directly inferred from the experimental switching kinetics at variable voltages. The simulation results picture the filament growth/depletion with time and account for the observed switching characteristics, such as the progressive opening of a depleted gap and the possibility of electrode-to-electrode migration of ions. Finally, new phenomena, such as switching variability at atomic-size filaments and stress-induced symmetric switching, will be discussed.\\[4pt] [1] R. Waser, et al., Adv. Mater. 21, 2632 (2009).\\[0pt] [2] D. Ielmini, et al., Nanotechnology 22, 254022 (2011).\\[0pt] [3] S. Menzel, et al., Adv. Funct. Mater. 21, 4487 (2011).\\[0pt] [4] S. Larentis, et al., IEEE Trans. Electron Devices 59, 2468 (2012).\\[0pt] [5] H. D. Lee, et al., Phys. Rev. B 81, 193202 (2010).\\[0pt] [6] D. Ielmini, Phys. Rev. B 78, 035308 (2008). [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T20.00008: Finite Element Modeling of Ag Transport and Reactions in Chalcogenide Glass Resistive Memory Hugh Barnaby, Arthur Edwards, David Oleksy, Michael Kozicki Silver-based electrochemical memories show potential for non-volatile applications. While several groups have made significant strides in device development and process integration, challenges remain to improve function and reliability. The central problem is the large variability of operational parameters and programmed resistance. To understand these variabilities, we need to grasp the physics of conducting filament formation and dissolution. In this paper the mechanisms of Ag transport and reactions are modeled using a finite element device simulator. The ChG film is modeled as a wide-bandgap semiconductor with material constants (e.g., bandgap, permittivity, electron affinity) extracted from data reported in literature and the results of first principles density functional theory calculations. Active and inert electrodes are modeled as ideal metals with specified workfunctions. The code solves carrier statistics and transport equations (continuity, drift-diffusion, and Poisson) and, simultaneously, performs ion transport and reaction calculations. The chemistry captured by the simulator are the reduction/oxidation (RedOx) reactions, incorporated as generation (G) and recombination (R) terms in the continuity equations for both ionic and neutral Ag species in the ChG film. The results show how neutral Ag builds up in the film under applied bias. The simulations also reveal that the neutral Ag density is left unchanged once the bias is removed, which enables memristive action. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T20.00009: Electronic Structure of Cu$_{2}$N, a Thin-film Insulating Surface Sareh Heidari, Andrew J. Fisher Thin-film insulators on metals have been used extensively as substrates when studying single molecule magnets (e.g. DyPc$_{2}$) and magnetic atoms (e.g. Co) using inelastic tunneling spectroscopy (IETS). They decouple the states of the adsorbed molecule from the underlying metallic bulk, which is necessary for IETS measurements [C. F. Hirjibehedin \textit{et al}., \textit{Science} 312, 1021, (2006)] and also leads to higher resolution imaging of molecular states [J. Repp \textit{et al}., \textit{Phys. Rev. Lett.} 9, 026803, (2005)]. The Cu$_{2}$N-Cu(100) surface has been shown by STM measurements to have insulating character, however the origin of the insulating behaviour has not been determined. By using Density Functional Theory calculations, we investigate the electronic structure of this surface. We show that the apparent insulating behaviour arises from a strong suppression of the Cu 4s density of states near the Fermi energy in the Cu$_{2}$N thin film. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T20.00010: Giant piezoresistive response in SmSe thin films under uniaxial strain Marcelo Kuroda, Matt Copel, Thomas Shaw, Xiao Hu Liu, Glenn Martyna, Dennis Newns Mixed valence compound SmSe shows a continuous insulator to metal transition which has been widely studied in bulk materials during the 1970's and 1980's. Here we report that the effect remains observable experimentally in SmSe films as thin as 12 nm. Our results indicate that the resistivity of film (when subject to uniaxial strain) reduces by about three orders of magnitude under a 4\% change volume. This piezoresistive response in SmSe thin films is nearly half of that reported for bulk crystals [Jayamaran et al., PRL \textbf{25}, 1430, (1970)]. The experiments are quantified using a combination of finite-element and first-principles (FP-LAWP) calculations. We compare the cases of isotropic and uniaxial strain along specific directions in SmSe crystals and discuss its impact in electronic transport. The results demonstrate the potential of rare-earth monochalcogenides as promising materials for new generation of electronic switches and MEMs [Newns et al., Adv. Mat. \textbf{24}, 3672 (2012)]. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T20.00011: Resistive and Capacitive Memory Effects in Oxide Insulator/ Oxide Conductor Hetero-Structures Rene Meyer, Maosheng Miao, Jian Wu, Christophe Chevallier We report resistive and capacitive memory effects observed in oxide insulator/ oxide conductor hetero-structures. Electronic transport properties of Pt/ZrO$_{\mathrm{2}}$/PCMO/Pt structures with ZrO$_{\mathrm{2}}$ thicknesses ranging from 20A to 40A are studied before and after applying short voltage pulses of positive and negative polarity for set and reset operation. As processed devices display a non-linear IV characteristic which we attribute to trap assisted tunneling through the ZrO$_{\mathrm{2}}$ tunnel oxide. Current scaling with electrode area and tunnel oxide thickness confirms uniform conduction. The set/reset operation cause an up/down shift of the IV characteristic indicating that the conduction mechanism of both states is still dominated by tunneling. A change in the resistance is associated with a capacitance change of the device. An exponential relation between program voltages and set times is found. A model based on electric field mediated non-linear transport of oxygen ions across the ZrO$_{\mathrm{2}}$/PCMO interface is proposed. The change in the tunnel current is explained by ionic charge transfer between tunnel oxide and conductive metal oxide changing both tunnel barrier height and PCMO conductivity. DFT techniques are employed to explain the conductivity change in the PCMO interfacial layer observed through capacitance measurements. [Preview Abstract] |
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