The rapid development of information technology has led to urgent requirements for high efficiency and ultralow power consumption. In the past few decades, neuromorphic computing has drawn extensive attention due to its promising capability in processing massive data with extremely low power consumption. Here, we offer a comprehensive review on emerging artificial neuromorphic devices and their applications. In light of the inner physical processes, we classify the devices into nine major categories and discuss their respective strengths and weaknesses. We will show that anion/cation migration-based memristive devices, phase change, and spintronic synapses have been quite mature and possess excellent stability as a memory device, yet they still suffer from challenges in weight updating linearity and symmetry. Meanwhile, the recently developed electrolyte-gated synaptic transistors have demonstrated outstanding energy efficiency, linearity, and symmetry, but their stability and scalability still need to be optimized. Other emerging synaptic structures, such as ferroelectric, metal–insulator transition based, photonic, and purely electronic devices also have limitations in some aspects, therefore leading to the need for further developing high-performance synaptic devices. Additional efforts are also demanded to enhance the functionality of artificial neurons while maintaining a relatively low cost in area and power, and it will be of significance to explore the intrinsic neuronal stochasticity in computing and optimize their driving capability, etc. Finally, by looking into the correlations between the operation mechanisms, material systems, device structures, and performance, we provide clues to future material selections, device designs, and integrations for artificial synapses and neurons.

A comprehensive review on emerging artificial neuromorphic devices

In the advancement of the semiconductor device technology, ZnO could be a prospective alternative than the other metal oxides for its versatility and huge applications in different aspects. In this review, a thorough overview on ZnO for the application of resistive switching memory (RRAM) devices has been conducted. Various efforts that have been made to investigate and modulate the switching characteristics of ZnO-based switching memory devices are discussed. The use of ZnO layer in different structure, the different types of filament formation, and the different types of switching including complementary switching are reported. By considering the huge interest of transparent devices, this review gives the concrete overview of the present status and prospects of transparent RRAM devices based on ZnO. ZnO-based RRAM can be used for flexible memory devices, which is also covered here. Another challenge in ZnO-based RRAM is that the realization of ultra-thin and low power devices. Nevertheless, ZnO not only offers decent memory properties but also has a unique potential to be used as multifunctional nonvolatile memory devices. The impact of electrode materials, metal doping, stack structures, transparency, and flexibility on resistive switching properties and switching parameters of ZnO-based resistive switching memory devices are briefly compared. This review also covers the different nanostructured-based emerging resistive switching memory devices for low power scalable devices. It may give a valuable insight on developing ZnO-based RRAM and also should encourage researchers to overcome the challenges.

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Status and Prospects of ZnO-Based Resistive Switching Memory Devices

The control of magnetization via the application of an electric field, known as magnetoelectric coupling, is among the most fascinating and active research areas today. In addition to fundamental scientific interest, magnetoelectric effects may lead to new device concepts for data storage and processing. There are several known mechanisms for magnetoelectric coupling that include intrinsic effects in single-phase materials, strain-induced coupling in two-phase composites, and electronically driven effects at interfaces. Here we explore a different type of magnetoelectric effect at a ferromagnetic-ferroelectric interface: magnetic reconstruction induced by switching of electric polarization. We demonstrate this effect using first-principles calculations of a ${\\text{La}}_{1\\ensuremath{-}x}{A}_{x}{\\text{MnO}}_{3}/{\\text{BaTiO}}_{3}$ (001) interface, where $A$ is a divalent cation. By choosing the doping level $x$ to be near a transition between magnetic phases we show that the reversal of the ferroelectric polarization of ${\\text{BaTiO}}_{3}$ leads to a change in the magnetic order at the interface from ferromagnetic to antiferromagnetic. This predicted electrically induced magnetic reconstruction at the interface represents a substantial interfacial magnetoelectric effect.

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Electrically-induced magnetic reconstruction at the manganite/ferroelectric interface

This article reviews the resistive random-access memory (ReRAM) technology initialization back in the 1960s and its heavily focused research and development from the early 2000s. This review goes through various oxygen/oxygen vacancy and metal-ion-based ReRAM devices and their operation mechanisms. This review also benchmarks the performance of various oxygen/oxygen vacancy and metal-ion-based ReRAM devices with general trend drawn. Being a semiconductor memory and storage technology, the commercialization attempts for both stand-alone mass storage/storage-class memory and embedded nonvolatile memory are also reviewed. Looking toward the coming era, the potential of using ReRAM technology to improve machine learning efficiency is discussed.

ReRAM: History, Status, and Future

The memristor is the fundamental nonlinear circuit element, with uses in computing and computer memory. Resistive Random Access Memory (ReRAM) is a resistive switching memory proposed as a non-volatile memory. In this review we shall summarize the state of the art for these closely-related fields, concentrating on titanium dioxide, the well-utilized and archetypal material for both. We shall cover material properties, switching mechanisms and models to demonstrate what ReRAM and memristor scientists can learn from each other and examine the outlook for these technologies.

https://iopscience.iop.org/article/10.1088/0268-1242/29/10/104004/pdf

TiO2-based memristors and ReRAM: materials, mechanisms and models (a review)

The SrTiO3 (STO) thin fi lms on aP t/Ti/SiO2/Si substrate were synthesized using a sol–gel method to form a metal–insulator–metal structure. This device shows the bipolar resistance switching (BRS) behavior for a compliance current Icc of less than 0.1 mA but exhibits soft breakdown at a higher level of compliance current. A transition from the BRS behavior to the stable unipolar resistive switching behavior (URS) was also observed. We found that the BRS behavior may be controlled by the structure interface while the URS behavior is likely bulk controlled. Our study indicates that the external compliance current is a key factor in resistance switching phenomenon of STO thin films. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0268-1242/26/7/075019/pdf

Bipolar and unipolar resistive switching behaviors of sol-gel-derived SrTiO 3 thin films with different compliance currents

Rare earth element La-doped ZnO polycrystalline films are prepared on Pt/Ti/SiO 2 /Si substrate and p -Si substrate by chemical solution deposition (CSD) method. High R OFF / R ON ratios, low operation voltages within 150 switching cycles of test and long retention measurement are obtained, which indicate that the two different structure devices exhibit reversible, controllable and remarkable reliability unipolar resistive switching (RS) behaviors. The RS mechanism is related to the different substrate of the samples. The filament theory and the interface effect are suggested to be responsible for the RS phenomenon for the Pt/Ti/SiO 2 /Si substrate and p -Si substrate, respectively.

Resistive switching behavior of La-doped ZnO films for nonvolatile memory applications

The interfacial magnetoelectric (ME) effect in ferromagnetic-ferroelectric (FM-FE) tunneling junctions was investigated in this work. We found that the tunneling magnetoresistance (TMR) changes with the reversing of the electric polarization of the FE barrier. The theoretical results also indicated that TMR is strongly dependent on the electric polarization, the exchange splitting energy, the screening lengths in the electrodes, and the dielectric constant of the FE barrier layer. These results may provide some insights into switching magnetization electrically for spintronics and presenting independent tunneling states in a single junction for multi-value storage memory applications.

Magnetoelectric effect in ferromagnetic-ferroelectric tunneling junctions

The evolution of the fatigue behaviors in Bi3.5Nd0.5Ti3O12 (BNT) ferroelectric thin films deposited on Pt(111)/Ti/SiO2/Si(100) substrates by chemical solution deposition (CSD) method effected by amplitude, frequency and profile of the driving electric field were reported. It is found that the switching with lower frequency and higher amplitude of the external voltages resulted in higher fatigue rates and only bipolar waveform type voltage can result in fatigue, whereas a unipolar voltage cannot. An empirical function with N/f is proposed in the frequency-dependence of polarization fatigue, where N is the number of switching cycles and f is the frequency of driving. It is indicated that injected charges from electrodes into films, the trapped charges, and suppression of the seeds of opposite domain nucleation are the main mechanism of fatigue in ferroelectric BNT thin films.

S. B. Yang

Papers

Bipolar and unipolar resistive switching behaviors of sol-gel-derived SrTiO 3 thin films with different compliance currents

Resistive switching behavior of La-doped ZnO films for nonvolatile memory applications

Magnetoelectric effect in ferromagnetic-ferroelectric tunneling junctions

POLARIZATION FATIGUE BEHAVIORS IN Bi3.5Nd0.5Ti3O12 FERROELECTRIC THIN FILMS