In this paper, recent progress of binary metal-oxide resistive switching random access memory (RRAM) is reviewed. The physical mechanism, material properties, and electrical characteristics of a variety of binary metal-oxide RRAM are discussed, with a focus on the use of RRAM for nonvolatile memory application. A review of recent development of large-scale RRAM arrays is given. Issues such as uniformity, endurance, retention, multibit operation, and scaling trends are discussed.

Metal–Oxide RRAM

The electronic properties of graphene, a two-dimensional crystal of carbon atoms, are exceptionally novel. For instance, the low-energy quasiparticles in graphene behave as massless chiral Dirac fermions which has led to the experimental observation of many interesting effects similar to those predicted in the relativistic regime. Graphene also has immense potential to be a key ingredient of new devices, such as single molecule gas sensors, ballistic transistors and spintronic devices. Bilayer graphene, which consists of two stacked monolayers and where the quasiparticles are massive chiral fermions, has a quadratic low-energy band structure which generates very different scattering properties from those of the monolayer. It also presents the unique property that a tunable band gap can be opened and controlled easily by a top gate. These properties have made bilayer graphene a subject of intense interest. In this review, we provide an in-depth description of the physics of monolayer and bilayer graphene f...

Properties of graphene: a theoretical perspective

With the explosive growth of digital data in the era of the Internet of Things (IoT), fast and scalable memory technologies are being researched for data storage and data-driven computation. Among the emerging memories, resistive switching memory (RRAM) raises strong interest due to its high speed, high density as a result of its simple two-terminal structure, and low cost of fabrication. The scaling projection of RRAM, however, requires a detailed understanding of switching mechanisms and there are potential reliability concerns regarding small device sizes. This work provides an overview of the current understanding of bipolar-switching RRAM operation, reliability and scaling. After reviewing the phenomenological and microscopic descriptions of the switching processes, the stability of the low- and high-resistance states will be discussed in terms of conductance fluctuations and evolution in 1D filaments containing only a few atoms. The scaling potential of RRAM will finally be addressed by reviewing the recent breakthroughs in multilevel operation and 3D architecture, making RRAM a strong competitor among future high-density memory solutions.

https://iopscience.iop.org/article/10.1088/0268-1242/31/6/063002/pdf

Resistive switching memories based on metal oxides: mechanisms, reliability and scaling

Although several types of architecture combining memory cells and transistors have been used to demonstrate artificial synaptic arrays, they usually present limited scalability and high power consumption. Transistor-free analog switching devices may overcome these limitations, yet the typical switching process they rely on—formation of filaments in an amorphous medium—is not easily controlled and hence hampers the spatial and temporal reproducibility of the performance. Here, we demonstrate analog resistive switching devices that possess desired characteristics for neuromorphic computing networks with minimal performance variations using a single-crystalline SiGe layer epitaxially grown on Si as a switching medium. Such epitaxial random access memories utilize threading dislocations in SiGe to confine metal filaments in a defined, one-dimensional channel. This confinement results in drastically enhanced switching uniformity and long retention/high endurance with a high analog on/off ratio. Simulations using the MNIST handwritten recognition data set prove that epitaxial random access memories can operate with an online learning accuracy of 95.1%. Controlled widening of threading dislocations in SiGe layers epitaxially grown on Si allows the realization of resistive switching devices with enhanced uniformity, high on/off ratio and long retention times.

SiGe epitaxial memory for neuromorphic computing with reproducible high performance based on engineered dislocations

Resistive switching effect in transition metal oxide (TMO) based material is often associated with the valence change mechanism (VCM). Typical modeling of valence change resistive switching memory consists of three closely related phenomena, i.e., conductive filament (CF) geometry evolution, conduction mechanism and temperature dynamic evolution. It is widely agreed that the electrochemical reduction-oxidation (redox) process and oxygen vacancies migration plays an essential role in the CF forming and rupture process. However, the conduction mechanism of resistive switching memory varies considerably depending on the material used in the dielectric layer and selection of electrodes. Among the popular observations are the Poole-Frenkel emission, Schottky emission, space-charge-limited conduction (SCLC), trap-assisted tunneling (TAT) and hopping conduction. In this article, we will conduct a survey on several published valence change resistive switching memories with a particular interest in the I-V characteristic and the corresponding conduction mechanism.

Conduction Mechanism of Valence Change Resistive Switching Memory: A Survey

The conduction mechanism of metal oxide resistive switching memory is debated in the literature. We measured the I-V characteristics below the switching voltages through TiN/HfOx/Pt memory stack and found the conduction cannot be described by the commonly used Poole-Frenkel model, because the fitted dielectric constant and the trap energy are unreasonable as compared to their known values. Therefore, we provide an alternate viewpoint based on a trap-assisted-tunneling model. Agreement of the bias polarity/temperature/resistance state-dependent conduction behavior was achieved between this model and experimental data. And insights for the multilevel capability due to the control of tunneling distance were obtained.

Conduction mechanism of TiN/HfOx/Pt resistive switching memory: A trap-assisted-tunneling model

The variation of switching parameters is one of the major challenges to both the scaling and volume production of metal-oxide-based resistive random-access memories (RRAMs). In this two-part paper, the source of such parameter variation is analyzed by a physics-based simulator, which is equipped with the capability to simulate a large number ( ~1000) of cyclic SET-RESET operations. By comparing the simulation results with experimental data, it is found that the random current fluctuation experimentally observed in the RESET processes is caused by the competition between trap generation and recombination, whereas the variation of the high resistance states and the tail bits are directly correlated to the randomness of the trap dynamics. A combined strategy with a bilayer dielectric material and a write-verification technique is proposed to minimize the resistance variation. We describe the simulation methodology and discuss the dc results in Part I. The corroboration of the model and the device optimization strategy will be discussed in Part II.

On the Switching Parameter Variation of Metal-Oxide RRAM—Part I: Physical Modeling and Simulation Methodology

A SPICE compact model is developed for metal-oxide-based resistive random access memory (RRAM). The model includes the critical impact of temperature change and temporal variation. Using experimental data from HfOx-based RRAM, the model reproduces both the voltage-time relationship and the cycle-to-cycle variation in the RESET of the memory cells.

A SPICE Compact Model of Metal Oxide Resistive Switching Memory With Variations

The origin of switching parameter variations in metal oxide resistive switching random access memory (RRAM) is studied. The stochastic formation/rupture of the conductive filaments (CFs) is modeled and incorporated with a trap-assisted-tunneling (TAT) current solver. The experimental DC I–V characteristics and pulse transient waveform featuring the current fluctuation during the reset process are reproduced by Monte Carlo simulations. It is found that the wide spread of high resistance states (HRS) are due to the variation of tunneling gap distances, and the tail bits of the HRS are due to the newly generated traps near the electrode at the end of the reset process. To solve the over-reset and tail bits problems, a device structure with active/buffer bi-layer oxides combined with the reset-verify technique is proposed. Our model is corroborated by measured experimental data of HfO x based RRAM.

On the stochastic nature of resistive switching in metal oxide RRAM: Physical modeling, monte carlo simulation, and experimental characterization

Using the model developed in Part I of this two-part paper, the simulated dc sweep and pulse transient characteristics of a metal oxide resistive random access memory cell are corroborated with the experimental data of HfOx memory. Key switching features such as the abrupt SET process, gradual RESET process, current fluctuation in the RESET process, and multilevel resistance state distributions are captured by the simulation. The current fluctuation in the RESET process is caused by the competition between the simultaneous oxygen vacancy recombination and generation processes. The origin of the high-resistance state variation and the tail bit problem are attributed to the variation of the tunneling gap distances and the stochastic nature of new Vo generation in the tunneling gap region, respectively. The use of the write-verify technique and a bilayer oxide structure are proposed to achieve a tighter resistance distribution.

Ximeng Guan

Papers

Conduction mechanism of TiN/HfOx/Pt resistive switching memory: A trap-assisted-tunneling model

On the Switching Parameter Variation of Metal-Oxide RRAM—Part I: Physical Modeling and Simulation Methodology

A SPICE Compact Model of Metal Oxide Resistive Switching Memory With Variations

On the stochastic nature of resistive switching in metal oxide RRAM: Physical modeling, monte carlo simulation, and experimental characterization

On the Switching Parameter Variation of Metal Oxide RRAM—Part II: Model Corroboration and Device Design Strategy