Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P37: Focus Session: Complex Oxide Thin Films -- Conductivity and Metal-Insulator Transition I |
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Sponsoring Units: DMP GMAG Chair: Nicola Spaldin, University of California, Santa Barbara Room: E147-E148 |
Wednesday, March 17, 2010 8:00AM - 8:12AM |
P37.00001: Novel Resistive Switching in MgO with Nitrogen Doping Cheng-Han Yang, Xin Jiang, Mahesh Samant, Brian Hughes, Li Gao, Andrew Kellock, Stuart Parkin Resistive switching in oxide thin films has been extensively explored as a candidate for the next generation nonvolatile memory. The oxide layer is formed from a dielectric material, normally insulating oxides or wide-bandgap semiconductors. The mechanisms of the resistive switching have been proposed to be the formation of a conducting filament by defect drift or metal migration. However, there is no experimental observation of such a switching in the alkali earth metal oxide due to its strong ionic bonding which leads to a relatively low concentration of natural defects such as oxygen vacancies. In this report, we demonstrate resistive switching in Nitrogen doped MgO. The ratio of the resistance change in N-doped MgO can be varied between 1-4 orders of magnitude by varying the nitrogen content by just a few percent. The measured RESET current is as low as few tenths of $\mu $A indicating a low power consuming device. We also demonstrate an ultra-fast switching with use of SET/RESET voltage pulses in the range of 1ns/100ns. Moreover, we show that multi-level resistance switching can also be achieved by controlling the nitrogen doping level and/or the RESET voltage. [Preview Abstract] |
Wednesday, March 17, 2010 8:12AM - 8:24AM |
P37.00002: Magnetic and transport properties of EuO films fabricated by oxidation of Eu metal films S. Schlotter, T. Brenner, C. Carter, B. Colwell, A. Kinsey, B. Schuster, M. Eblen-Zayas EuO holds potential for spintronics applications, but is also of interest due to the similarities between its magnetic and transport properties and those of the manganites. EuO is most commonly grown by reactive deposition of Eu in an oxygen partial pressure, but we have grown polycrystalline EuO films by deposition of metallic Eu films followed by oxidation. Although Eu inclusions are often present in films grown by this method, the samples exhibit the insulator-metal transition associated with the onset of ferromagnetism and the colossal magnetoresistance response seen in samples grown by reactive evaporation. We will report on how the transport and magnetization responses of EuO films grown by oxidation of metallic Eu films depend on several growth parameters. We have found that variations in substrate heating during the growth process can produce films with Curie temperatures ranging from 69 K to 200K, with a corresponding variation in the resistivity response of the films. [Preview Abstract] |
Wednesday, March 17, 2010 8:24AM - 8:36AM |
P37.00003: \textit{In Situ} Angle-Resolved Photoemission Study of Gd-Doped EuO Thin Films Daniel Shai, Alexander Melville, Dawei Shen, Eric Monkman, John Harter, Darrell Schlom, Kyle Shen We present an \textit{in situ} angle-resolved photoemission study of electron-doped Eu$_{1-x}$Gd$_{x}$O thin films grown by molecular beam epitaxy. Stoichiometric EuO orders ferromagnetically below T$_{C}$ = 69 K, and with the addition of excess electrons, T$_{C}$ can be increased to greater than 120 K. Additionally, in the electron-doped compound, a semiconductor to metal transition accompanies the paramagnetic to ferromagnetic transition. We show that the Gd dopants introduce electron pockets, which compose a Fermi surface that is clearly observable by photoemission. As the Gd concentration is increased, we observe that the Fermi surface maintains the same topology, but grows in volume. Additionally, by tracking the position and structure of the valence band as a function of doping, we show that with increasing Gd concentration, the chemical potential shifts toward the valence band in a manner that is entirely inconsistent with a rigid band shift. The relationship between these observations and density functional calculations will be discussed. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 9:12AM |
P37.00004: Materials Design using Correlated Materials -- Where do we stand? Invited Speaker: Theory providing guidance for the fabrication of new materials or devices with given electronic properties, such is the dream of modern electronic structure theory. Where do we stand, and what are the possibilities and limitations nowadays? In this talk, we will discuss some examples of materials where strong electronic Coulomb interactions invalidate a simple band picture. We will review electronic structure techniques based on dynamical mean field theory, designed to cure such shortcomings. Results on transition metal oxides and pnictides will be used for illustration. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P37.00005: Optically induced metal-insulator transition in gold::vanadium dioxide hybrid structures Davon W. Ferrara, Evan R. MacQuarrie, Joyeeta Nag, Anthony Kaye, Richard F. Haglund, Jr. Vanadium dioxide (VO$_{2})$ is a strongly-correlated electron material with a well-known semi-conducting to metallic phase transition that can be induced thermally, optically, or electrically. By coating lithographically prepared arrays of gold nanoparticles (NPs) of diameters up to 200 nm with 60 nm thick films of VO$_{2}$ via pulsed laser deposition, hybrid Au::VO$_{2}$ structures were created. Due to the sensitivity of the Au particle-plasmon resonance (PPR), a temperature dependent shift in the PPR can be generated by switching the VO$_{2}$ from one phase to another, creating a tunable plasmonic metamaterial. To study the low-power switching characteristics of these structures, transient absorption measurements were made using a chopped 780 nm pump laser, corresponding to the PPR resonance of the Au NPs, and 1550 nm CW probe. Additionally, pump-probe measurements were conducted on the structures using a Ti:sapphire oscillator with 100-fs pulses. Results show that the presence of Au NPs lowers the threshold laser power required to induce the phase transition. Finite element modeling was performed to better understand the complex thermodynamics of the structure. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P37.00006: Energetics of Metal-Insulator Transition Field-Effect Transistors using Vanadium Oxide Thin Films Sahand Hormoz, Shriram Ramanathan There is growing interest in exploring the use of metal-oxide materials as an alternative to traditional semiconductors in field effect transistors (FET), as current $Si$ FET technology inevitably encounters intrinsic scaling limitations. We discuss the prospect of a thin film Vanadium Oxide Mott metal-insulator transition (MIT) field- effect transistor as an electronic logic switch. Focusing on the intrinsic material properties of $VO_2$ and the underlying physical mechanisms of its MIT, we estimate the energy dissipation and time delay per switching operation. The device- independent power-delay plane of $VO_2$ Mott transistors is presented and its scaling limits compared to that of $Si$. Our simple model predicts an intrinsic $VO_2$ material lower bound switching time of the order of $0.5 \ ps$ at a power transfer of $0.1 \ \mu W$. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P37.00007: Pulsed Laser Deposition of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ Epitaxial Films on SrTiO$_{3}$ Buffered Silicon Grace Yong, Vera Smolyaninova, Sanjay Adhikari, Benjamin Hofmann, Rajeswari Kolagani, Yong Liang Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ is a photo-responsive material. Upon illumination with visible light, the resistivity of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ epitaxial thin films on oxide substrates decreases significantly in a wide temperature range due to the destruction of charge ordering, with the resistivity ($\rho )$ recovering upon subsequent blocking of the light. We demonstrate that Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ can be grown epitaxially (by PLD) on SrTiO$_{3}$ buffered Si(001). (The Si was buffered with $\sim $ 100{\AA} epitaxial SrTiO$_{3}$ grown via a Motorola Molecular Beam Epitaxy process). In general, epitaxy on silicon is needed for integration of a detector component with complementary MOS readout. Epitaxial growth on Si also opens up the possibility of fabricating a free-standing, strain-free Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ membrane via standard Si micromachining techniques. Such a free standing film may be expected to have properties similar to that of the bulk single crystal which exhibit permanent photoinduced reflectivity changes attractive for photonic device application. \newline [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P37.00008: Investigation of the impact of hydrogen on the forming behavior of Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ RF-sputtered thin films exhibiting Electric Pulse Induced Resistance change Mihir Tendulkar, John Jameson, Peter Griffin, Yoshio Nishi Flash scaling limits have generated interest in finding an alternative non-volatile memory. Resistance-change RAM (RRAM) is particularly attractive for its high density, high speed, and low power. Transition metal oxides that exhibit Electrical Pulse Induced Resistance (EPIR) change are ideal for RRAM. Thin films of NiO, SrTiO$_{3}$, TiO$_{2}$, Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (PCMO), and others have been studied extensively as promising memory candidates. However, these materials require an initial electrical forming step to induce soft breakdown into a conductive state before their EPIR properties can be accessed. Proposed models claim that forming propagates a filament of oxygen vacancies through the film, but little experimental attention has been devoted to tuning or eliminating this step. In this work, we study the forming step in RF-sputtered PCMO films. We show that forming changes dramatically with short, low-temperature anneals, and we link that result to hydrogen. We then investigate the effects of: 1) adding hydrogen directly into the sputtering plasma; 2) exposing PCMO thin films to water during patterning; and 3) alternating vacuum- and hydrogen-containing anneals. Our results indicate that hydrogen contamination plays a significant role in device variability and forming. [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P37.00009: Light-induced Resistance Changes in Epitaxial Thin Films of Hole Doped Rare Earth Manganites Rajeswari Kolagani, Grace Yong, Vera Smolyaninova, David Schaefer, Dou Dou Qian, Gilles Dongmo-Momo, Christie Nelson, Chuhee Kwon We observe light induced resistance changes in oxygen deficient La$_{0.67}$Ba$_{0.33}$MnO$_{3-y}$(LBMO) thin films and compressively strained La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) thin films grown by Pulsed Laser Deposition. LBMO thin films grown under low oxygen pressures are insulating down to 140 K where we observe a hump in resistivity reminiscent of transition to a ferromagnetic metallic state, immediately followed by an upturn in resistivity. Illumination of these films by continuous Argon laser causes a pronounced decrease in resistivity over a broad temperature range. In compressively strained LCMO thin films which are insulating, we observe a similar suppression of resistivity under illumination at temperatures below 250 K (the temperature range of the metal insulator transition in the unstrained films). Both lattice mismatch strain and oxygen deficiency are known to suppress the insulator-metal transition and enhance charge and orbital ordering in manganite materials. Radiation over a broad range of wavelength is known to destroy charge/orbital ordering. We will discuss the origin of the observed photoconductivity and the possible role of charge/orbital ordering . [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P37.00010: Reversible Resistive Switching in (La,Pr,Ca)MnO$_{3}$; Cryogenic nonvolatile RAM Hee Taek Yi, Taekjib Choi, Sang-Wook Cheong Cryogenic-temperature electronics technologies are a practical promise for continuing demand for high performance electronics. By utilizing the unique hysteretic behavior of perovskite (La,Pr,Ca)MnO$_{3}$ in the variation of temperature and applied electric fields, we have discovered that two electronically-distinct phases, with a huge difference in resistance ($>$10$^{5})$, can be repeatedly switched by applying various voltage pulses at cryogenic temperatures (e.g., 2 K), and the magnitude of resistance of each phase is highly stable with time. A multilevel memory effect for storing multiple bits was also found. We believe that the non-volatile cryo-PRAM utilizing our findings is an excellent candidate for memory devices for low-temperature electronic technologies such as quantum computers, Superconducting Rapid Single Flux Quantum (RSFQ) technology, low temperature detectors. [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P37.00011: \textit{In situ} study of emerging metallicity on ion-milled SrTiO$_{3}$ Namrata Bansal, Heiko Gross, Yong Seung Kim, Carlos Chaparro, Seongshik Oh Here we show how metallic states emerge on the surface of SrTiO$_{3}$ (STO) single crystals through Ar-ion-milling process. It is well known that ion-milling creates oxygen vacancies on STO, creating a metallic surface state. So far, however, detailed studies of how ion-milling process induces metallic states on STO are lacking. In order to answer this question, we performed systematic \textit{in situ} conductance measurements on STO crystals in various conditions inside a high vacuum chamber. Ion-milling temperature was a crucial factor in determining the properties of the conducting state. At cryogenic temperatures, thermal diffusion is suppressed and ion-milling creates conducting states only at the very top surface. However, near or above room temperature, the conducting state diffuses deeply into the bulk. Higher temperature also fostered vacancy clustering. Because clustered vacancies trap electron carriers, the clustering causes electrical conductivity to drop. We observed up to four times reduction in conductivity due to cluster formation. [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P37.00012: Peering inside a functioning Pt/TiO$_{2}$/Pt bipolar resistance switching device John Paul Strachan, Matthew Pickett, Jianhua Yang, Julien Borghetti, A. L. D. Kilcoyne, Andreas Scholl, Shaul Aloni, Gilberto Medeiros-Ribeiro, R. Stanley Williams Many transition-metal oxides exhibit electrical polarization-dependent resistance changes. Direct observation of the physical changes induced during this switching has been limited, due to challenges in observing subtle material changes occurring in a small volume. We use x-ray absorption spectromicroscopy and transmission electron microscopy (TEM) in order to determine the chemical and structural identity of the switching region in a functioning Pt/TiO$_{2}$/Pt device, fabricated on a thin transparent window. These transmission techniques allow probing of the physical properties of the switching layer between/during the application of an electrical bias. We directly observed the formation of a conductive channel following an electroforming process. The conductive channel is identified to be a nanoscale metallic suboxide (TiO$_{2-X})$, a phase generated within the TiO$_{2}$ matrix by the creation and ordering of oxygen vacancies. Besides shedding light on long-standing questions regarding the physical changes, we show how this work has informed new device engineering. [Preview Abstract] |
Wednesday, March 17, 2010 10:48AM - 11:00AM |
P37.00013: Metal-insulator transitions in hole- and electron-doped Sm$_{1-x}$A$_{x}$NiO$_{3}$ thin films P.-H. Xiang, H. Yamada, I.H. Inoue, A. Sawa, H. Akoh We present a study of the transport properties of hole- and electron-doped Sm$_{1-x}$A$_{x}$NiO$_{3}$ (A = Ca$^{2+}$, Ce$^{4+}$, 0 $\le \quad x \quad \le $ 0.1) thin films deposited on LaAlO$_{3}$(001) substrates by pulsed-laser deposition method. The temperature-driven metal-insulator (MI) transition of the Sm$_{1-x}$A$_{x}$NiO$_{3}$ films is progressively suppressed by either hole or electron doping. The Sm$_{1-x}$A$_{x}$NiO$_{3}$ ($x$ = 0.1) films show metallic conductivity over measured temperature range (5-350K). The effect of hole doping on the MI transition has been investigated in detail. The MI transition temperature ($T_{MI})$ can be tuned around room temperature by 3{\%} Ca doping. In addition, we observe an anomaly in the resistivity below $T_{MI}$, which may correspond to the N\'eel temperature ($T_{N})$ for SmNiO$_{3}$ system [1]. Based on the results of the transport properties, a possible phase diagram for the hole-doped Sm$_{1-x}$A$_{x}$NiO$_{3}$ thin films has been deduced. \\[4pt] [1] J. P\'erez-Cacho, J. Blasco, J. Garc\'ia, M. Castro and J Stankiewicz, J. Phys.: Condens. Matter. \textbf{11 }(1999) 405. [Preview Abstract] |
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