Redox-based Resistive Switching Devices Research Articles

Ultrahigh-Speed In-Memory Electronics Enabled by Proximity-Oxidation-Evolved Metal Oxide Redox Transistors.

The pursuit of a universal device that combines nonvolatile multilevel storage, ultrafast writing/erasing speed, nondestructive readout, and embedded processing with low power consumption demands the development of innovative architectures. Although thin-film transistors and redox-based resistive-switching devices have independently been proven to be ideal building blocks for data processing and storage, it is still difficult to achieve both well-controlled multilevel memory and high-precision ultrafast processing in a single unit, even though this is essential for the large-scale hardware implementation of in-memory computing. In this work, an ultrafast (≈42ns) and programable redox thin-film transistor (ReTFT) memory made of a proximity-oxidation-grown TiO2 layer is developed, which has on/off ratio of 105 , nonvolatile multilevel analog storage with a long retention time, strong durability, and high reliability. Utilizing the proof-of-concept ReTFTs, circuits capable of performing fundamental NOT, AND, and OR operations with reconfigurable logic-in-memory processing are developed. Further, on-demand signal memory-processing operations, like multi-terminal addressable memory, learning, pattern recognition, and classification, are explored for prospective application in neuromorphic hardware. This device, which operates on a fundamentally different mechanism, presents an alternate solution to the problems associated with the creation of high-performing in-memory processing technology.

Selection of Switching Layer Materials for Memristive Devices: from Traditional Oxide to 2D Materials

Redox-based resistive switching devices (ReRAM) provide new hardware concepts which make it possible to break the von Neumann bottleneck and build a new computing system in the information. However, the materials for switching layers are various and mechanisms are quite different, these will block the further exploration for practical applications. This review tends to demonstrate different kinds of memristors fabricated with various materials, such as oxide, nitride and 2D materials. The electrical properties of those based on different materials are compared and the advantages of each are listed. It would give a guidance to the selection of materials of memristors.

Studying the switching variability in redox-based resistive switching devices

The variability of switching parameters in redox-based resistive switching RAM (ReRAM) devices is investigated by a 3D kinetic Monte Carlo approach. This physics-based model can simulate the filamentary resistive switching in the electroforming, SET and RESET processes and captures their key features. It allows to predict the impact of the forming and switching conditions on the fluctuations of key parameters like the current and resistance levels of the cell in on and off states. The origin of the variability of the switching parameters was investigated in terms of the involved physical processes. The simulations also confirm the multilevel cell operation capabilities of ReRAM devices.

Analyses of a 1-layer neuromorphic network using memristive devices with non-continuous resistance levels

The emerging nonvolatile memory technology of redox-based resistive switching (RS) devices is not only a promising candidate for future high density memories but also for computational and neuromorphic applications. In neuromorphic as well as in memory applications, RS devices are configured in nanocrossbar arrays, which are controlled by CMOS circuits. With those hybrid systems, brain-inspired artificial neural networks can be built up and trained by using a learning algorithm. First works on hardware implementation using relatively large and high current level RS devices are already published. In this work, the influence of small and low current level devices showing noncontinuous resistance levels on neuromorphic networks is studied. To this end, a well-established physical-based Verilog A model is modified to offer continuous and discrete conduction. With this model, a simple one-layer neuromorphic network is simulated to get a first insight and understanding of this problem using a backpropagation algorithm based on the steepest descent method.

The ultimate switching speed limit of redox-based resistive switching devices.

In contrast to classical charge-based memories, the binary information in redox-based resistive switching devices is decoded by a change of the atomic configuration rather than changing the amount of stored electrons. This offers in principle a higher scaling potential as ions are not prone to tunneling and the information is not lost by tunneling. The switching speed, however, is potentially smaller since the ionic mass is much higher than the electron mass. In this work, the ultimate switching speed limit of redox-based resistive switching devices is discussed. Based on a theoretical analysis of the underlying physical processes, it is derived that the switching speed is limited by the phonon frequency. This limit is identical when considering the acceleration of the underlying processes by local Joule heating or by high electric fields. Electro-thermal simulations show that a small filamentary volume can be heated up in picoseconds. Likewise, the characteristic charging time of a nanocrossbar device can be even below ps. In principle, temperature and voltage can be brought fast enough to the device to reach the ultimate switching limit. In addition, the experimental route and the challenges towards reaching the ultimate switching speed limit are discussed. So far, the experimental switching speed is limited by the measurement setup.

KMC Simulation of the Electroforming, Set and Reset Processes in Redox-Based Resistive Switching Devices

This paper presents a physical model to investigate the electroforming, set and reset processes in Redox-based resistive switching RAM based on the valence change mechanism. The model uses a kinetic Monte Carlo code in a three-dimensional structure. It is based on the formation and dissolution of an oxygen-deficient/oxygen-vacancy-rich filament in the resistive switching oxide material. In contrast to other proposed models, oxygen vacancies only form at the anode/oxide boundary due to an oxygen exchange reaction. The generated oxygen vacancies are mobile and move away from the interface allowing for further oxygen vacancy generation. The model includes electric field, temperature and temperature gradient as driving forces for the electroforming, set and reset transition of these devices. It is demonstrated that this alternative model could successfully reproduce $I-V$ characteristics observed in experimental results.

Smooth Interfacial Scavenging for Resistive Switching Oxide via the Formation of Highly Uniform Layers of Amorphous TaOx.

We demonstrate that the inclusion of a Ta interfacial layer is a remarkably effective strategy for forming interfacial oxygen defects at metal/oxide junctions. The insertion of an interfacial layer of a reactive metal, that is, a "scavenging" layer, has been recently proposed as a way to create a high concentration of oxygen defects at an interface in redox-based resistive switching devices, and growing interest has been given to the underlying mechanism. Through structural and chemical analyses of Pt/metal/SrTiO3/Pt structures, we reveal that the rate and amount of oxygen scavenging are not directly determined by the formation free energies in the oxidation reactions of the scavenging metal and unveil the important roles of oxygen diffusibility. Active oxygen scavenging and highly uniform oxidation via scavenging are revealed for a Ta interfacial layer with high oxygen diffusibility. In addition, the Ta scavenging layer is shown to exhibit a highly uniform structure and to form a very flat interface with SrTiO3, which are advantageous for the fabrication of a steep metal/oxide contact.

A Complementary Resistive Switch-Based Crossbar Array Adder

Redox-based resistive switching devices (ReRAM) are an emerging class of non-volatile storage elements suited for nanoscale memory applications. In terms of logic operations, ReRAM devices were suggested to be used as programmable interconnects, large-scale look-up tables or for sequential logic operations. However, without additional selector devices these approaches are not suited for use in large scale nanocrossbar memory arrays, which is the preferred architecture for ReRAM devices due to the minimum area consumption. To overcome this issue for the sequential logic approach, we recently introduced a novel concept, which is suited for passive crossbar arrays using complementary resistive switches (CRSs). CRS cells offer two high resistive storage states, and thus, parasitic sneak currents are efficiently avoided. However, until now the CRS-based logic-in-memory approach was only shown to be able to perform basic Boolean logic operations using a single CRS cell. In this paper, we introduce two multi-bit adder schemes using the CRS-based logic-in-memory approach. We proof the concepts by means of SPICE simulations using a dynamical memristive device model of a ReRAM cell. Finally, we show the advantages of our novel adder concept in terms of step count and number of devices in comparison to a recently published adder approach, which applies the conventional ReRAM-based sequential logic concept introduced by Borghetti et al.