Anyone who ever took an electronics laboratory class will be familiar with the fundamental passive circuit elements: the resistor, the capacitor and the inductor. However, in 1971 Leon Chua reasoned from symmetry arguments that there should be a fourth fundamental element, which he called a memristor (short for memory resistor). Although he showed that such an element has many interesting and valuable circuit properties, until now no one has presented either a useful physical model or an example of a memristor. Here we show, using a simple analytical example, that memristance arises naturally in nanoscale systems in which solid-state electronic and ionic transport are coupled under an external bias voltage. These results serve as the foundation for understanding a wide range of hysteretic current-voltage behaviour observed in many nanoscale electronic devices that involve the motion of charged atomic or molecular species, in particular certain titanium dioxide cross-point switches.

/pdf/the-missing-memristor-found-55upit5xe7.pdf

The missing memristor found

Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

http://chialvo.org/Curso/UBACurso/DIA7/Papers/Aono_NatMat_07_Ionic%20Switching%20Memory.pdf

Nanoionics-based resistive switching memories

Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

Memristive devices are electrical resistance switches that can retain a state of internal resistance based on the history of applied voltage and current. These devices can store and process information, and offer several key performance characteristics that exceed conventional integrated circuit technology. An important class of memristive devices are two-terminal resistance switches based on ionic motion, which are built from a simple conductor/insulator/conductor thin-film stack. These devices were originally conceived in the late 1960s and recent progress has led to fast, low-energy, high-endurance devices that can be scaled down to less than 10 nm and stacked in three dimensions. However, the underlying device mechanisms remain unclear, which is a significant barrier to their widespread application. Here, we review recent progress in the development and understanding of memristive devices. We also examine the performance requirements for computing with memristive devices and detail how the outstanding challenges could be met.

Memristive devices for computing

Nanoscale metal/oxide/metal switches have the potential to transform the market for nonvolatile memory and could lead to novel forms of computing. However, progress has been delayed by difficulties in understanding and controlling the coupled electronic and ionic phenomena that dominate the behaviour of nanoscale oxide devices. An analytic theory of the ‘memristor’ (memory-resistor) was first developed from fundamental symmetry arguments in 1971, and we recently showed that memristor behaviour can naturally explain such coupled electron–ion dynamics. Here we provide experimental evidence to support this general model of memristive electrical switching in oxide systems. We have built micro- and nanoscale TiO2 junction devices with platinum electrodes that exhibit fast bipolar nonvolatile switching. We demonstrate that switching involves changes to the electronic barrier at the Pt/TiO2 interface due to the drift of positively charged oxygen vacancies under an applied electric field. Vacancy drift towards the interface creates conducting channels that shunt, or short-circuit, the electronic barrier to switch ON. The drift of vacancies away from the interface annilihilates such channels, recovering the electronic barrier to switch OFF. Using this model we have built TiO2 crosspoints with engineered oxygen vacancy profiles that predictively control the switching polarity and conductance. Nanoscale metal/oxide/metal devices that are capable of fast non-volatile switching have been built from platinum and titanium dioxide. The devices could have applications in ultrahigh density memory cells and novel forms of computing.

Memristive switching mechanism for metal/oxide/metal nanodevices.

Resistive switching in transition metal oxides

We have characterized the vertical transport properties of epitaxial layered structures composed of Pr0.7Ca0.3MnO3(PCMO) sandwiched between SrRuO3(SRO) bottom electrode and several kinds of top electrodes such as SRO, Pt, Au, Ag, and Ti. Among the layered structures, Ti∕PCMO∕SRO is distinct due to a rectifying I–V characteristic with a large hysteresis. Corresponding to the hysteresis of the I–V characteristics, the contact resistance of the Ti∕PCMO interface reversibly switches between two stable states by applying pulsed voltage stress. We propose a model for the resistance switching at the Ti∕PCMO interface, in which the width and/or height of a Schottky-like barrier are altered by trapped charge carriers in the interface states.

Hysteretic current–voltage characteristics and resistance switching at a rectifying Ti∕Pr0.7Ca0.3MnO3 interface

Transport properties have been studied for a perovskite heterojunction consisting of SrRuO3 (SRO) film epitaxially grown on SrTi0.99Nb0.01O3 (Nb:STO) substrate. The SRO/Nb:STO interface exhibits rectifying current–voltage (I–V) characteristics agreeing with those of a Schottky junction composed of a deep work-function metal (SRO) and an n-type semiconductor (Nb:STO). A hysteresis appears in the I–V characteristics, where high resistance and low resistance states are induced by reverse and forward bias stresses, respectively. The resistance switching is also triggered by applying short voltage pulses of 1μs–10ms duration.

Hysteretic current–voltage characteristics and resistance switching at an epitaxial oxide Schottky junction SrRuO3∕SrTi0.99Nb0.01O3

We investigated the electrical properties of heteroepitaxial oxide Schottky junctions, $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}∕\mathrm{Sr}{\mathrm{Ti}}_{1\ensuremath{-}x}{\mathrm{Nb}}_{x}{\mathrm{O}}_{3}$ $(0.0002\ensuremath{\leqslant}x\ensuremath{\leqslant}0.02)$. The overall features agree with those of a conventional semiconductor Schottky junction, as exemplified by the rectifying current $(I)$-voltage $(V)$ characteristics with linear $\mathrm{log}\phantom{\rule{0.2em}{0ex}}I\text{\ensuremath{-}}V$ relationship under forward bias and the capacitance $(C)\text{\ensuremath{-}}V$ characteristics with linear $1∕{C}^{2}\text{\ensuremath{-}}V$ relationship under reverse bias. The $x$ dependence of the junction parameters, such as barrier height, built-in potential, and depletion layer width, can be analyzed by taking into account the band-gap narrowing due to degeneration, as well as the bias-dependent dielectric constant of depleted $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$. All junctions, except for the most heavily doped $(x=0.02)$ one, show hysteretic $I\text{\ensuremath{-}}V$ characteristics with a colossal electroresistance (CER) effect, where forward (reverse) bias stress reduces (enhances) the junction resistance. The $x$ dependence of the CER effect and the absence of hysteresis in the $C\text{\ensuremath{-}}V$ relationship suggest that the resistance switching in Schottky junctions comes from the change in conductance through additional tunneling paths rather than the change in barrier potential profile. Electron charging in or discharging from a self-trap depending on the bias polarity may account for the nonvolatility of the CER effect. This model is supported by the fact that the CER effect is completely suppressed in interface-engineered junctions with a $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_{3}∕2\text{\ensuremath{-}}\mathrm{nm}$-thick $X∕\mathrm{Sr}{\mathrm{Ti}}_{0.99}{\mathrm{Nb}}_{0.01}{\mathrm{O}}_{3}$ structure, where $X$ is either pristine $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ or very heavily electron-doped ${\mathrm{La}}_{0.25}{\mathrm{Sr}}_{0.75}\mathrm{Ti}{\mathrm{O}}_{3}$.

Electrical properties and colossal electroresistance of heteroepitaxial Sr Ru O 3 ∕ Sr Ti 1 − x Nb x O 3 ( 0.0002 ⩽ x ⩽ 0.02 ) Schottky junctions

We have studied the transport and resistance switching properties of Ti∕Sm0.7Ca0.3MnO3 (n unit cells)/La0.7Sr0.3MnO3 [Ti∕SCMO(n)∕LSMO] layered structures. The metal-to-metal contact of the Ti/LSMO junction (n=0) does not exhibit resistance switching effect, while the insertion of a very thin insulating SCMO layer (n⩾1) induces resistance switching effect. As the SCMO layer thickness (n) increases, the resistance switching amplitude grows and the response gets faster. This indicates that the SCMO layer as thin as several u.c. adjacent to the interface works as an active source for the resistance switching effect.

Akihito Sawa

Papers

Resistive switching in transition metal oxides

Hysteretic current–voltage characteristics and resistance switching at a rectifying Ti∕Pr0.7Ca0.3MnO3 interface

Hysteretic current–voltage characteristics and resistance switching at an epitaxial oxide Schottky junction SrRuO3∕SrTi0.99Nb0.01O3

Electrical properties and colossal electroresistance of heteroepitaxial Sr Ru O 3 ∕ Sr Ti 1 − x Nb x O 3 ( 0.0002 ⩽ x ⩽ 0.02 ) Schottky junctions

Interface resistance switching at a few nanometer thick perovskite manganite active layers