This review article attempts to provide a comprehensive review of the recent progress in the so-called resistive random access memories (RRAMs) First, a brief introduction is presented to describe the construction and development of RRAMs, their potential for broad applications in the fields of nonvolatile memory, unconventional computing and logic devices, and the focus of research concerning RRAMs over the past decade Second, both inorganic and organic materials used in RRAMs are summarized, and their respective advantages and shortcomings are discussed Third, the important switching mechanisms are discussed in depth and are classified into ion migration, charge trapping/de-trapping, thermochemical reaction, exclusive mechanisms in inorganics, and exclusive mechanisms in organics Fourth, attention is given to the application of RRAMs for data storage, including their current performance, methods for performance enhancement, sneak-path issue and possible solutions, and demonstrations of 2-D and 3-D crossbar arrays Fifth, prospective applications of RRAMs in unconventional computing, as well as logic devices and multi-functionalization of RRAMs, are comprehensively summarized and thoroughly discussed The present review article ends with a short discussion concerning the challenges and future prospects of the RRAMs

Recent progress in resistive random access memories: Materials, switching mechanisms, and performance

Memristors are novel electrical devices used for a variety of applications, including memory, logic circuits, and neuromorphic systems. Memristive technologies are attractive due to their nonvolatility, scalability, and compatibility with CMOS. Numerous physical experiments have shown the existence of a threshold voltage in some physical memristors. Additionally, as shown in this brief, some applications require voltage-controlled memristors to operate properly. In this brief, a Voltage ThrEshold Adaptive Memristor (VTEAM) model is proposed to describe the behavior of voltage-controlled memristors. The VTEAM model extends the previously proposed ThrEshold Adaptive Memristor (TEAM) model, which describes current-controlled memristors. The VTEAM model has similar advantages as the TEAM model, i.e., it is simple, general, and flexible, and can characterize different voltage-controlled memristors. The VTEAM model is accurate (below 1.5% in terms of the relative root-mean-square error) and computationally efficient as compared with existing memristor models and experimental results describing different memristive technologies.

VTEAM: A General Model for Voltage-Controlled Memristors

Threshold Switching of Ag or Cu in Dielectrics: Materials, Mechanism, and Applications

In this paper, we present a synapse function using analog resistive-switching behaviors in a SiNx-based memristor with a complementary metal-oxide-semiconductor compatibility and expandability to three-dimensional crossbar array architecture. A progressive conductance change is attainable as a result of the gradual growth and dissolution of the conducting path, and the series resistance of the AlOy layer in the Ni/SiNx/AlOy/TiN memristor device enhances analog switching performance by reducing current overshoot. A continuous and smooth gradual reset switching transition can be observed with a compliance current limit (>100 μA), and is highly suitable for demonstrating synaptic characteristics. Long-term potentiation and long-term depression are obtained by means of identical pulse responses. Moreover, symmetric and linear synaptic behaviors are significantly improved by optimizing pulse response conditions, which is verified by a neural network simulation. Finally, we display the spike-timing-dependent pl...

Analog Synaptic Behavior of a Silicon Nitride Memristor

Memristors have been intensively studied in recent years as potential building blocks for the construction of versatile neuromorphic architectures. The prevalent developments focus on nanoionic devices due to their ease of reversible regulation, non-volatility, scalability down to the atomic level, low-energy consumption, nanosecond response times, sufficient yield and reliability, as well as their homologous regulatory mechanisms with biological synapses. However, despite their desirable structures and excellent properties, the previously reported properties of nanoionic devices are rather diverse and dissatisfactory for the delivery of reliable analog properties, which is the precondition for the imitation of brain-like functions. Here, the general requirements of neuromorphic engineering in terms of device structure, characterization parameters, synaptic functions and device engineering are introduced. A critical overview of the proposed nanoionic mechanisms for memristive switching is given, focusing particularly on providing fundamental insights into the strategies for regulating the adaptive memristive characteristics of devices that resemble the behaviors of biological synapses, which is an element of neural networks. In addition, the research progress in active materials (e.g., oxides, polymers, small molecules, bio-macromolecules and dopants), especially regarding the correlation among materials, is elaborated and a rational approach to provide insight into new design strategies is considered, and how to understand the memristor mechanisms and further achieve excellent memristive devices is discussed. Finally, the current challenges and several possible future research directions in this area are discussed.

Nanoionics-Enabled Memristive Devices: Strategies and Materials for Neuromorphic Applications

We developed an analog memristor based on the thickening/thinning of Ag nanofilaments in amorphous La1–xSrxMnO3 (a-LSMO) thin films. The Ag/a-LSMO/Pt memristor exhibited excellent pinched hysteresis loops under high-excitation frequency, and the areas enclosed by the pinched hysteresis loops shrank with increasing excitation frequency, which is a characteristic typical of a memristor. The memristor also showed continuously tunable synapselike resistance and stable endurance. The a-LSMO thin films in the memristor acted as a solid electrolyte for Ag+ cations, and only the Ag/a-LSMO/Pt memristor electroformed with a larger current compliance easily exhibited high-frequency pinched hysteresis loops. On the basis of the electrochemical metallization (ECM) theory and electrical transport models of quantum wires and nanowires, we concluded that the memristance is ultimately determined by the amount of charge supplied by the external current. The state equations of the memristor were established, and charge was ...

Analog memristors based on thickening/thinning of Ag nanofilaments in amorphous manganite thin films.

Amorphous Sr-doped LaMnO3 (a-LSMO) thin films were deposited on Pt/Ti/SiO2/Si substrate by radio frequency magnetron sputtering. The Ag/a-LSMO/Pt device exhibited reversible bipolar resistive switching over 100 cycles with a resistance ratio (high resistance state to low resistance state) of over 4 orders of magnitude and stable retention for over 104 s at room temperature. Analysis indicates that the resistive switching originates from the formation/rupture of Ag nanofilaments in the a-LSMO thin films acting as solid electrolytes. The device showed potential for multibit storage as well as low power consumption applications.

Nonvolatile bipolar resistive switching in amorphous Sr-doped LaMnO3 thin films deposited by radio frequency magnetron sputtering

Amorphous Sr-doped LaMnO3 (a-LSMO) thin films can exhibit diode-like volatile resistive switching (RS) properties under lower compliance current (CC). The Ag/a-LSMO/Pt cell exhibits stable volatile RS cycles up to 100 times with rectification ratio above 102. The volatility depends strongly on the temperature as well as the CC-controlled dimension of the Ag nanofilament forming in a-LSMO. The conductive atomic force microscopy current-mapping images confirm the instability of conducting nanofilaments forming under lower CC. The volatile RS behaviors could be explained by the Rayleigh instability of the Ag nanofilament, together with the diffusion of Ag atoms to the a-LSMO matrix. The diode-like volatile RS properties have great application potential in the beyond von-Neumann computers.

Diode-like volatile resistive switching properties in amorphous Sr-doped LaMnO3 thin films under lower current compliance

Programmable metallization cells based on amorphous La0.79Sr0.21MnO3 thin films for memory applications

Nanoscale electrochemical metallization (ECM) memories based on amorphous La1−xSrxMnO3 (a-LSMO) were fabricated using ultrathin porous alumina masks. The ultrathin alumina masks, with thicknesses of about 200 nm and pore diameters of about 80 nm, were fabricated through a typical two-step anodization electrochemical procedure and transferred onto conductive Pt/Ti/SiO2/Si substrates. Resistive switching (RS) properties of the individual Ag/a-LSMO/Pt ECM cell were directly measured using a conductive atomic force microscope. The cells exhibited typical RS characteristics and the OFF/ON resistance ratio is as high as 102. Reproducible RS behaviours on the same ECM cell and the I–V cycles obtained from different ECM cells ensured that the RS properties in nanoscale Ag/a-LSMO/Pt cells are reproducible and reliable. This work provides an effective approach for the preparation of nanostructured large-scale ordered ECM memories or memristors.

https://iopscience.iop.org/article/10.1088/0022-3727/47/8/085108/pdf

Xuan Zhu

Papers

Analog memristors based on thickening/thinning of Ag nanofilaments in amorphous manganite thin films.

Nonvolatile bipolar resistive switching in amorphous Sr-doped LaMnO3 thin films deposited by radio frequency magnetron sputtering

Diode-like volatile resistive switching properties in amorphous Sr-doped LaMnO3 thin films under lower current compliance

Programmable metallization cells based on amorphous La0.79Sr0.21MnO3 thin films for memory applications

Nanoscale electrochemical metallization memories based on amorphous (La, Sr)MnO3 using ultrathin porous alumina masks