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

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

This is the first time we report that simply air plasma treatment can also enhances the optical absorbance and absorption region of titanium oxide (TiO2) films, while keeping them transparent. TiO2 thin films having moderate doping of Fe and Co exhibit significant enhancement in the aforementioned optical properties upon air plasma treatment. The moderate doping could facilitate the formation of charge trap centers or avoid the formation of charge recombination centers. Variation in surface species viz. Ti3+, Ti4+, O2−, oxygen vacancies, OH group and optical properties was studied using X-ray photon spectroscopy (XPS) and UV-Vis spectroscopy. The air plasma treatment caused enhanced optical absorbance and optical absorption region as revealed by the formation of Ti3+ and oxygen vacancies in the band gap of TiO2 films. The samples were treated in plasma with varying treatment time from 0 to 60 seconds. With the increasing treatment time, Ti3+ and oxygen vacancies increased in the Fe and Co doped TiO2 films leading to increased absorbance; however, the increase in optical absorption region/red shift (from 3.22 to 3.00 eV) was observed in Fe doped TiO2 films, on the contrary Co doped TiO2 films exhibited blue shift (from 3.36 to 3.62 eV) due to Burstein Moss shift.

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Formation of oxygen vacancies and Ti 3+ state in TiO 2 thin film and enhanced optical properties by air plasma treatment

In this paper, we review the recent progress in the resistive random access memory (ReRAM) technology, one of the most promising emerging nonvolatile memories, in which both electronic and electrochemical effects play important roles in the nonvolatile functionalities. First, we provide a brief historical overview of the research in this field. We also provide a technological overview and the epoch-making achievements, followed by an account of the current understanding of both bipolar and unipolar ReRAM operations. Finally, we summarize the challenges facing the ReRAM technology as it moves toward the beyond-2X-nm generation of nonvolatile memories and the so-called beyond complementary metal-oxide-semiconductor (CMOS) device.

Resistive Random Access Memory (ReRAM) Based on Metal Oxides

Through a simple industrialized technique which was completely fulfilled at room temperature, we have developed a kind of promising nonvolatile resistive switching memory consisting of Ag/ZnO:Mn/Pt with ultrafast programming speed of 5 ns, an ultrahigh R(OFF)/R(ON) ratio of 10(7), long retention time of more than 10(7) s, good endurance, and high reliability at elevated temperatures. Furthermore, we have successfully captured clear visualization of nanoscale Ag bridges penetrating through the storage medium, which could account for the high conductivity in the ON-state device. A model concerning redox reaction mediated formation and rupture of Ag bridges is therefore suggested to explain the memory effect. The Ag/ZnO:Mn/Pt device represents an ultrafast and highly scalable (down to sub-100-nm range) memory element for developing next generation nonvolatile memories.

Fully room-temperature-fabricated nonvolatile resistive memory for ultrafast and high-density memory application.

The characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt-based resistance random access memory devices with stable and reproducible nanosecond bipolar switching behavior were studied. The dependencies of memory behavior on cell area, operating temperature, and frequency indicate that the conduction mechanism in low-resistance states is due to electrons hopping through filament paths. We also identify that the set process is essentially equivalent to a soft dielectric breakdown associated with a polarization effect caused by the migration of space charges under a low electric field stress. The generation/recovery of oxygen vacancies and nonlattice oxygen ions play a critical role in resistance switching.

Characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt resistance switching random-access memories

Magnetic skyrmion, a nanosized spin texture with topological property, has become an area of significant interest due to the scientific insight that it can provide and also its potential impact on applications such as ultra-low-energy and ultra-high-density logic gates. In the quest for the reconfiguration of single logic device and the implementation of the complete logic functions, a novel reconfigurable skyrmion logic (RSL) is proposed and verified by micromagnetic simulations. Logic functions including AND, OR, NOT, NAND, NOR, XOR, and XNOR are implemented in the ferromagnetic (FM) nanotrack by virtue of various effects including spin orbit torque, skyrmion Hall effect, skyrmion–edge repulsions, and skyrmion–skyrmion collision. Different logic functions can be selected in an RSL by applying voltage to specific region(s) of the device, changing the local anisotropy energy of FM film. Material properties and geometrical scaling studies suggest RSL gates fit for energy-efficient computing as well as prov...

Reconfigurable Skyrmion Logic Gates

This paper presents a unified physical model to elucidate the resistive switching behavior of metal-oxide-based resistive random access memory (RRAM) devices using the ion-transport-recombination model. In this model, the rupture of conductive filaments is caused by recombination of oxygen ions and electron-low-occupied oxygen vacancies. The transport equations of interstitial oxygen ions in the oxide matrix are introduced to quantitatively investigate the reset speed and other properties such as uniformity, endurance, and reset current. The proposed mechanism was verified by experiments.

Oxide-based RRAM switching mechanism: A new ion-transport-recombination model

Highly stable bipolar resistive switching behaviors of TiN/ZnO/Pt devices were demonstrated for the first time. The excellent memory characteristics including fast switching speed (<20 ns for set and <60 ns for reset), long retention (in the order of 105 s) and non-electroforming process were demonstrated. The bipolar switching behaviors can be explained by formation and rupture of the filamentary conductive path consisting of oxygen vacancies. The excellent bipolar switching behavior can be attributed to the significant amount of oxygen vacancies in ZnO film and the effect of TiN layer serving as an oxygen reservoir.

https://iopscience.iop.org/article/10.1088/0268-1242/23/7/075019/pdf

Bipolar switching behavior in TiN/ZnO/Pt resistive nonvolatile memory with fast switching and long retention

Al/CeOx/Pt devices with nonstoichiometric CeOx (1.5<x<2) films were fabricated. The unique resistive switching (RS) behaviors for resistive random access memory applications, including stable and sharp bipolar RS processes and a multilevel and self-stop set process without current compliance and without excessive requirement on a high-voltage electroforming process, were demonstrated. A multifilament switching model based on the distribution characteristics of oxygen vacancies in CeOx films is proposed to explain the observed RS behaviors.

Nuo Xu

Papers

Characteristics and mechanism of conduction/set process in TiN∕ZnO∕Pt resistance switching random-access memories

Reconfigurable Skyrmion Logic Gates

Oxide-based RRAM switching mechanism: A new ion-transport-recombination model

Bipolar switching behavior in TiN/ZnO/Pt resistive nonvolatile memory with fast switching and long retention

Resistive Switching in $\hbox{CeO}_{x}$ Films for Nonvolatile Memory Application