Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S9: Vanadium Oxides, Resistive Switching, and Interfaces with Oxides |
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Sponsoring Units: FIAP Chair: Rick Mengyan, Texas Tech University Room: 006D |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S9.00001: Vanadium Dioxide Phase Change Switches Mark Field, Christopher Hillman, Philip Stupar, Jonathan Hacker, Zachary Griffith, Kang-Jin Lee We have built RF switches using vanadium dioxide thin films fabricated within a section of inverted transmission line with integrated on chip heaters to provide local thermal control. On heating the films above the metal insulator transition we obtain record low switch insertion loss of -0.13 dB at 50 GHz and -0.5 dB at 110 GHz. We investigate the device physics of these switches including the effect of a deposited insulator on the VO$_{2}$ switching characteristics, the self-latching of the devices under high RF powers and the effect of resistance change with temperature on the device linearity. Finally we show how these devices can be integrated with silicon germanium RF circuits to produce a field programmable device where the RF signal routing can be selected under external control. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S9.00002: Interstitial hydrogen in vanadium dioxide W. B. Fowler, M. Stavola, Weikai Yin, Ying Qin, L. A. Boatner It has long been recognized that VO$_{\mathrm{2}}$ has a monoclinic-to-rutile phase transition as the temperature increases through 340K, accompanied by a remarkable increase in conductivity from insulator to metal[1,2]. Recently it has been found that interstitial hydrogen can modify both the structural and the electronic phase transition[3]. The way hydrogen does this has only recently begun to be studied[4]. We are using infrared spectroscopy to study the properties of H$_{\mathrm{i}}$, and are also using the CRYSTAL06 code[5] with hybridized DFT Hamiltonian to determine equilibrium positions and vibrational frequencies. IR spectroscopy finds several OH vibrational lines in hydrogenated VO$_{\mathrm{2}}$ samples. Within the monoclinic structure of VO$_{\mathrm{2}}$ there are four inequivalent sites for H$_{\mathrm{i}}$ which may lead to distinct spectroscopic signatures. [1] F. J. Morin, Phys. Rev. Lett. \textbf{3}, 34 (1959). [2] A. Zylbersztejn and N. F. Mott, Phys. Rev. B \textbf{11}, 4383 (1975). [3] J. Wei \textit{et al.}, Nature Nanotechnology \textbf{7}, 357 (2012). [4] K. H. Warnick \textit{et al.}, Appl. Phys. Lett. \textbf{104}, 101913 (2014). [5] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual }(University of Torino, Torino, 2006). [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S9.00003: Magnetism in Bulk Vanadium Dioxide Compounds P. W. Mengyan, R. L. Lichti, B. B. Baker, G. Jayarathna Vanadium dioxide (VO$_2$) compounds show a metal-semiconductor transition (MST) near room temperature (stoichiometric VO$_2$ is metallic $T > T_{MST}\approx 340$~K and semiconducting $T < T_{MST}$) that is accompanied by a structural transition; both can be triggered by thermal, optical, electrical or barometric means. This ultrafast (sub ms) transition has been studied extensively and there is still considerable disagreement regarding the mechanism responsible for these transitions. Incorporation of a few atomic percent of H in VO$_2$ stabilizes the metallic phase down to 200~K. Impurities such as W, Ti, Au, Cr or F lower or raise $T_{MST}$ without significantly modifying other properties. Some effects that dopants have on the material are well known, however, the role dopants play in modifying them is far from understood. This contribution presents results of the first muon spin rotation and relaxation (MuSR) measurements on bulk VO$_2$ compounds where we find and characterize a low temperature magnetic phase that has not yet been reported. The introduction of 2.4 at\% of W or 5 at\% of Ti raise the onset of the magnetic phase from 35~K to nearly 170~K. MuSR probes the local magnetic environment and hence provides a direct measure of the local field properties. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S9.00004: Voltage switching of a VO$_2$ memory metasurface using ionic gel M.D. Goldflam, M.K. Liu, B.C. Chapler, H.T. Stinson, A.J. Sternbach, A.S. McLeod, J.D. Zhang, K. Geng, M. Royal, Bong-Jun Kim, R.D. Averitt, N.M. Jokerst, D.R. Smith, Hyun-Tak Kim, D.N. Basov We have demonstrated large area, low voltage, non-volatile tuning of an electrolyte-based vanadium dioxide (VO$_2$) THz memory metasurface. Using ionic gel gating, voltage is applied to drive the insulator-to-metal transition in an underlying VO$_2$ layer. Through application of positive and negative voltages, the metasurface resonance can be switched into the ``off'' or ``on'' state by driving VO$_2$ into a more conductive or insulating regime, respectively. As compared to our graphene-based control devices, the longer saturation time of resonance modification in VO$_2$-based devices suggests that this voltage-induced switching originates primarily from electrochemical effects resulting from oxygen migration across the electrolyte-VO$_2$ interface. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S9.00005: Effect of doping and chemical ordering on the optoelectronic properties of complex oxide semiconductors Iffat Nayyar, Chamberlin Sara, Tiffany Kaspar, Niranjan Govind, Scott Chambers, Peter Sushko Transition metal oxide hematite, $\alpha $-Fe$_{2}$O$_{3}$, is of interest in photovoltaic and photoelectrochemical applications due to its natural abundance, narrow band gap and electrochemical stability. Doping of $\alpha $-Fe$_{2}$O$_{3}$ may lead to conductivity enhancement and band-gap reduction. In this work, we have studied the electronic and optical properties of $\alpha $-(Fe$_{\mathrm{1-x}}$V$_{\mathrm{x}})_{2}$O$_{3}$(0 $\le $ x $\le$ $\sim$0.5) solid-solution epitaxial thin films using advanced theoretical models employing embedded cluster approach and time-dependent density functional theory. We observe that V doping results in localized, occupied V \textit{3d} states which are hybridized with Fe \textit{3d} and are located in the midgap of pure $\alpha $-Fe$_{2}$O$_{3}$. The lowest energy transitions for $\alpha $-(Fe$_{\mathrm{1-x}}$V$_{\mathrm{x}}$)$_{2}$O$_{3}$ films are the electronic excitations from these levels to the unoccupied Fe \textit{3d}$^{\ast}$ orbitals, reducing the onset of $\alpha $-Fe$_{2}$O$_{3} $photoconductivity by nearly 1.2 eV. Our calculated optical absorption spectra are in good agreement with the experiment. This insight into the atomic, electronic and spin ordering provides guiding principles for the design of new oxide semiconducting materials for efficient visible light harvesting, thus enabling the technological growth of alternate energy sources for solving the renewable solar energy and photo-chemical organic waste remediation problems. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S9.00006: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S9.00007: Oxygen controlled bipolar switching in NiO memristor Zhong Sun, Yonggang Zhao, Diyang Zhao As Leon Chua demonstrated, both unipolar and bipolar resistance switching devices are memristors. Over the past decade, metal/oxide/metal structure with NiO as a ReRAM functional layer has been investigated widely and in depth, due to its intrinsic unipolar resistance switching, which is attributed to the connection and rupture of nickel filament in NiO. Recently, several papers studying NiO nanowires or NiO films with C-AFM infer that bipolar switching mechanism may govern the NiO memristors on the nanoscale. However, a systematic research on the mechanism of bipolar switching in NiO memristor on the nanoscale is still lacking. Especially, the role of oxygen in a NiO memristor has never been explored. Here we carry out a comprehensive study of the mechanism of bipolar switching in NiO memristor, and uncover the dominant role of oxygen. NiO/Pt structures were measured by C-AFM equipped with 20 nm conductive tips. By controlling the inherent oxygen concentration of NiO film, film thickness, and chamber oxygen pressure, we demonstrate that it is the inner oxygen distribution, related to electric field-induced ion drift and oxygen exchange between NiO film and ambient, that acts as the state variable, whose response to applied bias results in the bipolar switching in NiO memristor. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S9.00008: Understanding Electroforming and Resistive Switching in Silicon Dioxide Resistive Memory Devices Yao-Feng Chang, Burt Fowler, Fei Zhou, Kwangsub Byun, Jack Lee Electroforming and resistive switching in SiO2 materials are investigated by anneal temperature, etch time and operating ambient. Thermal anneal in reducing ambient lowers electroforming voltage to small than 10 V. Conductive filaments form within 4 nm of sidewall surfaces in devices with an etched SiO2 layer, whereas most filaments are large than 10 nm from the electrode edge. Switching unpassivated devices fails at 1 atm air and pure O2/N2, with recovery of vacuum switching at about 4.6 V after switching attempts in O2/N2 and at about 9.5 V after switching attempts in air. Incorporating a hermetic passivation layer enables switching in 1 atm air. Discussions of defect energetics and electrochemical reactions lead to a localized switching model describing device switching dynamics. Low-frequency noise data are consistent with charge transport through electron-trapping defects. Low-resistance-state current is modeled by hopping conduction at bias small than 1.5 V. A current overshoot phenomenon starting near 1.6 V is modeled as electron tunneling. Results demonstrate that SiO2-based resistive memory (RM) devices provide a good experimental platform to study SiO2 defects. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S9.00009: Bio-Inspired Learning Demonstration with Synaptic Resistive Memory Devices Seyoung Kim, Tayfun Gokmen, Mark Ritter Simple two-terminal semiconductor memory devices which can mimic the functions of biological synapses has recently opened up exciting opportunities for enabling native implementation of brain-inspired computing. Here, we demonstrate in hardware that a biologically-inspired architecture employing a novel resistive memory technology used to implement a simple learning algorithm that imitates some features of the sensorimotor stage of cognitive development of a newborn baby. Actuation and sensing were indicated by spiking neurons, and the associations between neurons were learned in a HfOx-based resistive memory array using a local rule. The system essentially realize a biased random walk algorithm using a fully connected spiking sensorimotor network and can be generalized to solve other optimization problems by redefining the input and output functions of the neurons. This demonstrates a new approach to solve classical problems with resilient, adaptive, and fault-tolerant non-Von Neumann architecture. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S9.00010: Ab initio study of the epitaxial ZrO2/Si interface Mehmet Dogan, Divine Kumah, Charles Ahn, Frederick Walker, Sohrab Ismail-Beigi Growing thin films of crystalline metal oxides on semiconductors has been of much scientific interest because of the potential applications of such systems in electronic devices. One particular research goal is to achieve ferroelectricity in a crystalline and thin oxide film that is epitaxial on a semiconductor. This would enable one to realize non-volatile field-effect transistors where the state of the system is encoded in the polarization direction of the oxide. In this work, we study oxides that are not ferroelectric in the bulk but become ferroelectric as an ultrathin film on a semiconductor such as silicon. Recent developments in epitaxial growth methods also permit fabrication of such systems. Here, we use density functional theory to study the interface between ZrO$_{2}$ and Si. When the oxide is only 1 monolayer thick, we find a set of stable structures with a variety of positive and negative out-of-plane ferroelectric polarizations. We present an analysis of these structures as a function of oxide thickness and the size of interface unit cell. Furthermore, the ZrO$_{2}$ can be used as a buffer layer to induce ferroelectricity in ultrathin perovskite oxides such as SrTiO$_{3}$ on Si which can couple the oxide polarization to the silicon carrier density. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S9.00011: Band alignment in metal-oxide interface Al/SiO$_{2}$ and Cu/SiO$_{2}$ in presence of defect Jianqiu Huang, Celine Hin Semiconductors have wide use especially in electronic devices. Technological development tends to decrease the physical size of the electronic devices into the nanoscale. Dielectric Breakdown was an old problem has never been fully resolved, which exponentially related to the size of electronic devices. It causes a severe and irreversible degradation on integrated circuits. Therefore, the motivation and main purpose of this research is to explore the dominant factors that will cause dielectric breakdown to occur in order to develop techniques to effectively prevent this occurrence. Density functional theory has been used to study the~interface of Al/SiO$_{2}$ and Cu/SiO$_{2}$. Results on the investigation of atomic and charge vacancies at the interface will be presented and compared with the defect free models. The band alignment has been constructed in order to describe the behavior of the conduction band in the depletion layer. From the comparison and band alignment, a row conclusion that causes the breakdown occurrence was made. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S9.00012: Carbon impurities in oxide dielectrics Hiral D. Tailor, John L. Lyons, Minseok Choi, Anderson Janotti, Chris G. Van de Walle The high-k oxides ZrO$_{\mathrm{2}}$, and LaAlO$_{\mathrm{3}}$ can be used as dielectrics in metal-oxide-semiconductor (MOS) devices. Dielectrics are commonly grown with atomic layer deposition (ALD), often leading to unintentional incorporation of impurities such as carbon from the metal-organic precursors. Experiments indicated that carbon can be a significant cause of leakage current. We investigate this problem using density functional theory with a hybrid functional. Our results show that carbon substituting on the cation site undergoes an off-site displacement and forms close \textit{sp} bonds with three oxygen atoms in ZrO$_{\mathrm{2}}$ and LaAlO$_{\mathrm{3}}$. We calculate the corresponding defect levels, and in order to determine the impact on MOS devices we align the band structures of the dielectrics with those of the semiconductor channel materials (including GaN, Si, and GaAs) . We find that carbon incorporation leads to defect levels near the conduction-band minimum of the channel materials, proving potentially detrimental for $n$-type devices. Intriguingly, we find that the defect levels of these carbon centers in a variety of oxides and semiconductors are aligned at roughly -3.5 eV below the vacuum level. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S9.00013: Understanding the Oxygen Vacancy in Tungsten Trioxide Wennie Wang, Anderson Janotti, Chris G. Van de Walle Tungsten trioxide (WO$_{3}$) has a variety of applications in gas sensors, photocatalysis, and smart windows. As an electrochromic BO$_{3}$ perovskite, WO$_{3}$ turns from transparent to blue upon doping. This color change is correlated with a drop in transmittance of near-IR radiation, and is used in smart windows for energy efficiency. In addition to monovalent species doping that modulates optical properties, oxygen deficiencies have been found to have a similar electrochromic effect. The influence of oxygen vacancies on electronic structure and how it corresponds to electrochromic behavior remains a topic of debate. In this work, we examine the oxygen vacancy in monoclinic WO$_{3}$ and its influence on electronic structure using density functional theory with a hybrid functional. We investigate the relative stability of different charge states and its implications for electrical properties, such as conductivity and electrochromism. We find oxygen vacancies to be shallow donors, and explore similarities and differences with monovalent species doping. Finally, we compare our theoretical findings with experiment to elucidate how vacancies may contribute to electrochromic behavior. This work is supported by DOE and NSF. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S9.00014: Muonium Diffusion in In$_{2}$O$_{3}$ Brittany Baker, Roger Lichti, Y. Gurkan Celebi, Patrick Mengyan Indium oxide (In$_{2}$O$_{3})$ is a transparent conducting oxide (TCO) commonly found in mixtures used as windows and transparent electrodes in optical semiconductor devices (i.e. LEDs and solar cells). Hydrogen diffusion in the TCO layer and across the interface between the TCO and the semiconductor device plays an important role in the degradation of the transparency of TCO windows or electrodes. Theoretical calculations show positive H as the only stable, interstitial H charge state above the neutral H ionization temperature. Muon Spin Relaxation measurements were performed to investigate positive muon (Mu$^{+})$ diffusion which are an experimentally accessible analog to H$^{+}$. Three distinct Mu$^{+}$ states are identified between 2 K and 1000 K; a static low temperature state, a dynamic state above room temperature, and a trapping state from 400 K to 800 K. The trap component creates complex dynamics and has been modeled assuming the Mu$^{+}$ transfers between the dynamic state and the trapping state. Fits of the model to the data provide information about capture and release rates and energy barriers into and out of the trap state. Here we present and discuss results from these fits, possible site locations for each state and likely diffusion paths. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S9.00015: Group-IV Impurity Defect Levels in beta-Gallium Oxide Stefan Badescu Beta-Gallium Oxide ($\beta $-Ga$_{\mathrm{2}}$O$_{\mathrm{3}})$ is a wide-bandgap semiconductor with a significant potential as a native substrate for electronic devices. One avenue for tuning its carrier concentration and electronic properties is doping with group--IV impurity atoms. This work presents a first-principles understanding of the effects of C, Si, Ge and Sn dopants at Ga sites. C is found to act like a bistable center whereas the other dopants preserve the symmetry of the Ga site. Hybrid functionals are used to describe accurately the effects that occur mainly in the conduction band. A Brillouin zone unfolding is used that enables a direct comparison to possible spectroscopy experiments. We delineate the effects on bandgap modulation induced by charge density on the one hand, and by conduction band resonances and effective masses on the other hand. [Preview Abstract] |
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