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

Domains in ferroelectrics were considered to be well understood by the middle of the last century: They were generally rectilinear, and their walls were Ising-like. Their simplicity stood in stark contrast to the more complex Bloch walls or N\'eel walls in magnets. Only within the past decade and with the introduction of atomic-resolution studies via transmission electron microscopy, electron holography, and atomic force microscopy with polarization sensitivity has their real complexity been revealed. Additional phenomena appear in recent studies, especially of magnetoelectric materials, where functional properties inside domain walls are being directly measured. In this paper these studies are reviewed, focusing attention on ferroelectrics and multiferroics but making comparisons where possible with magnetic domains and domain walls. An important part of this review will concern device applications, with the spotlight on a new paradigm of ferroic devices where the domain walls, rather than the domains, are the active element. Here magnetic wall microelectronics is already in full swing, owing largely to the work of Cowburn and of Parkin and their colleagues. These devices exploit the high domain wall mobilities in magnets and their resulting high velocities, which can be supersonic, as shown by Kreines' and co-workers 30 years ago. By comparison, nanoelectronic devices employing ferroelectric domain walls often have slower domain wall speeds, but may exploit their smaller size as well as their different functional properties. These include domain wall conductivity (metallic or even superconducting in bulk insulating or semiconducting oxides) and the fact that domain walls can be ferromagnetic while the surrounding domains are not.

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Domain wall nanoelectronics

The resistance switching behaviour of several materials has recently attracted considerable attention for its application in non-volatile memory (NVM) devices, popularly described as resistive random access memories (RRAMs). RRAM is a type of NVM that uses a material(s) that changes the resistance when a voltage is applied. Resistive switching phenomena have been observed in many oxides: (i) binary transition metal oxides (TMOs), e.g. TiO(2), Cr(2)O(3), FeO(x) and NiO; (ii) perovskite-type complex TMOs that are variously functional, paraelectric, ferroelectric, multiferroic and magnetic, e.g. (Ba,Sr)TiO(3), Pb(Zr(x) Ti(1-x))O(3), BiFeO(3) and Pr(x)Ca(1-x)MnO(3); (iii) large band gap high-k dielectrics, e.g. Al(2)O(3) and Gd(2)O(3); (iv) graphene oxides. In the non-oxide category, higher chalcogenides are front runners, e.g. In(2)Se(3) and In(2)Te(3). Hence, the number of materials showing this technologically interesting behaviour for information storage is enormous. Resistive switching in these materials can form the basis for the next generation of NVM, i.e. RRAM, when current semiconductor memory technology reaches its limit in terms of density. RRAMs may be the high-density and low-cost NVMs of the future. A review on this topic is of importance to focus concentration on the most promising materials to accelerate application into the semiconductor industry. This review is a small effort to realize the ambitious goal of RRAMs. Its basic focus is on resistive switching in various materials with particular emphasis on binary TMOs. It also addresses the current understanding of resistive switching behaviour. Moreover, a brief comparison between RRAMs and memristors is included. The review ends with the current status of RRAMs in terms of stability, scalability and switching speed, which are three important aspects of integration onto semiconductors.

https://iopscience.iop.org/article/10.1088/0034-4885/75/7/076502/pdf

Emerging memories: resistive switching mechanisms and current status.

Computer memory based on ferroelectric polarization is a promising alternative to technologies based, for example, on magnetism. Here, Garcia and Bibes review how ferroelectric tunnel junctions, where ferroelectric polarization controls electrical resistance, could improve the performance of these devices.

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Ferroelectric tunnel junctions for information storage and processing

Multiferroic bismuth ferrite-based materials for multifunctional applications: Ceramic bulks, thin films and nanostructures

A ferroelectric-resistive random access memory consisting of a conductive BiFeO3 epitaxial thin film with a unipolar diode current modulated by electric polarization orientation is reported. This device has a memory that lasts for months, a sufficiently high on current and on/ off ratio to permit ordinary sense amplifiers to measure "1" or " 0", and is fully compatible with complementary metal- oxide semiconductor processing.

A Resistive Memory in Semiconducting BiFeO3 Thin‐Film Capacitors

Reduced graphene oxide thin films were fabricated on quartz by spray coating method using a stable dispersion of reduced graphene oxide in N,N-Dimethylformamide. The dispersion was produced by chemical reduction of graphene oxide, and the film thickness was controlled with the amount of spray volume. AFM measurements revealed that the thin films have near-atomically flat surface. The chemical and structural parameters of the samples were analyzed by Raman and XPS studies. It was found that the thin films show electrical conductivity with good optical transparency in the visible to near infrared region. The sheet resistance of the films can be significantly reduced by annealing in vacuum and reach 58 kΩ with a light transmittance of 68.69% at 550 nm. The conductive transparent properties of the reduced graphene oxide thin films would be useful to develop flexible electronics.

Transparent conductive reduced graphene oxide thin films produced by spray coating

Mechanisms for the enhancement of the lateral photovoltage in perovskite heterostructures

By a Monte Carlo simulation of the deposition process and a mean-field theory model of the surface structure, we have successfully reproduced experimental oblique-incidence optical reflectance difference signals from laser molecular beam epitaxy of Nb:SrTiO3 on a SrTiO3 substrate in ambient oxygen. We find that during deposition the concentration of oxygen vacancies close to step edges is larger than that on flat terraces and remains stable. We suggest that oxygen vacancies diffusing on the surface tend to accumulate near step edges due to their slow in-diffusion rate there, and that this in-diffusion dominates the oxidation of as-deposited film.

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Kui Juan Jin

Papers

A Resistive Memory in Semiconducting BiFeO3 Thin‐Film Capacitors

Transparent conductive reduced graphene oxide thin films produced by spray coating

Mechanisms for the enhancement of the lateral photovoltage in perovskite heterostructures

In-diffusion of oxygen vacancies near step edges dominates the oxidation of perovskite films