There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

The great variability in the electrical properties of multinary oxide materials, ranging from insulating, through semiconducting to metallic behaviour, has given rise to the idea of modulating the electronic properties on a nanometre scale for high-density electronic memory devices. A particularly promising aspect seems to be the ability of perovskites to provide bistable switching of the conductance between non-metallic and metallic behaviour by the application of an appropriate electric field. Here we demonstrate that the switching behaviour is an intrinsic feature of naturally occurring dislocations in single crystals of a prototypical ternary oxide, SrTiO(3). The phenomenon is shown to originate from local modulations of the oxygen content and to be related to the self-doping capability of the early transition metal oxides. Our results show that extended defects, such as dislocations, can act as bistable nanowires and hold technological promise for terabit memory devices.

Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3

The occurrence of metallic conductivity in SrTiO3 single crystals is reported for reduction under low partial pressure of oxygen at 800 degrees C. This transition is shown to result from the formation of a high concentration of vacancy defects along a network of extended defects within the skin region. A self-healing phenomenon is observed for progressive reduction which causes the concentration of initially introduced defects to decrease in the course of heat treatment and leads to a breakdown of the metallic conductivity as well as a substantial loss of optical subgap absorption.

/pdf/localized-metallic-conductivity-and-self-healing-during-1nlrnsmpww.pdf

Localized metallic conductivity and self-healing during thermal reduction of SrTiO3.

Redox-based memristive switching has been observed in many binary transition metal oxides and related compounds. Since, on the one hand, many recent reports utilize TiO2 for their studies of the memristive phenomenon and, on the other hand, there is a long history of the electronic structure and the crystallographic structure of TiO2 under the impact of reduction and oxidation processes, we selected this material as a prototypical material to provide deeper insight into the mechanisms behind memristive switching. In part I, we briefly outline the results of the historical and recent studies of electroforming and resistive switching of TiO2-based cells. We describe the (tiny) stoichiometrical range for TiO2 − x as a homogeneous compound, the aggregation of point defects (oxygen vacancies) into extended defects, and the formation of the various Magneli phases. Furthermore, we discuss the driving forces for these solid-state reactions from the thermodynamical point of view. In part II, we provide new experimental details about the hierarchical transformation of TiO2 single crystals into Magneli phases, and vice versa, under the influence of chemical, electrical and thermal gradients, on the basis of the macroscopic and nanoscopic measurements. Those include thermogravimetry, high-temperature x-ray diffraction (XRD), high-temperature conductivity measurements, as well as low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and LC-AFM (atomic force microscope equipped with a conducting tip) studies. Conclusions are drawn concerning the relevant parameters that need to be controlled in order to tailor the memristive properties.

https://iopscience.iop.org/article/10.1088/0957-4484/22/25/254001/pdf

TiO2?a prototypical memristive material

The local conductivity of SrTiO3 thin films epitaxially grown on SrRuO3-buffered SrTiO3 single crystals has been investigated in detail with an atomic force microscope equipped with a conducting tip (LC-AFM). These experiments demonstrate that the conductivity of SrTiO3 thin films originates from nanoscale well-conducting filaments connecting the surface to the SrRuO3 bottom electrode. The electrical conduction of the filaments is shown to be reversibly modulated over several orders of magnitude by application of an appropriate electrical field. We analyze the resistive switching by addressing individual filaments with the AFM tip as well as by scanning areas up to the µm scale. Temperature dependent measurements reveal that resistive switching on a macroscopic scale can be traced down to the insulator-to-metal transition of the independently switchable filaments. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Krzysztof Szot

Papers

Switching the electrical resistance of individual dislocations in single-crystalline SrTiO3

Localized metallic conductivity and self-healing during thermal reduction of SrTiO3.

TiO2?a prototypical memristive material

Nanoscale resistive switching in SrTiO3 thin films

Formation of micro-crystals on the (100) surface of SrTiO3 at elevated temperatures