Current status of resistive nonvolatile memories

Several emerging nonvolatile memories (NVMs) such as ferroelectric memory, magnetoresistive rams and ovonic universal memory are being developed for possible applications. Resistive random access memory (RRAM) is another interesting competitor in the class of NVMs. The RRAM is based on a large change in electrical resistance when the memory film is exposed to voltage or current pulses, and can keep high or low resistance states without any power. The ideal RRAM should have the superior properties of reversible switching, long retention time, multilevel switching, simple structure, small size, and low operating voltage. Perovskite oxides, transition metal oxides, and molecular materials were found to have resistive memory properties. This presentation reviews the ongoing research and development activities on future resistance NVMs technologies incorporating these new memory materials. The possible basic mechanisms for their bistable resistance switching are described. The effect of processing, composition, and structure on the properties of resistive memory materials and consequently the devices are discussed.     

Design rules for threshold switches based on a field triggered thermal runaway mechanism

We investigate a new type of threshold switching devices, which is based on a purely electronic phenomena. These threshold switches are polarity independent and switch abruptly from a high resistive state to a low resistive state at a threshold voltage. The device stays in this low resistive state as long as a high voltage drops over the device. When the voltage is reduced, the low resistive state is lost and the device switches back to the initial high resistive state. This makes these threshold switches highly interesting as selector elements for resistive switching memory concepts, based on device arrays, which are the prerequisite for new applications like logic-in-memory concepts. The threshold switching considered here is based on a combination of a Poole–Frenkel conduction mechanism and Joule heating. Hence, it is not strongly restricted to specific materials rather it is connected to the physical quantities of the Poole–Frenkel conduction mechanism and the thermal conductance. This enables to design the threshold switch to its application requirements by adjusting the relevant physical material properties or designing the device geometry. Here we present a theoretical study, which tackles the influence of several material properties and the device design. From this simulation model the impact on technical important figures of merits is determined, such as the threshold switching voltage and the selectivity.     

Investigation on Resistive Memory Switching Mechanism of NiO

ABSTRACT

Experimental investigations on the resistive memory switching in sub-micron sized NiO memory cell are presented to elucidate the resistive memory switching mechanism. The voltage or current-biased I-V measurements show that the resistive switching transitions can be regarded as the combination of a voltage-controlled negative differential resistance phenomenon and a current-controlled negative differential resistance phenomenon. Along with experimental observations of multiple resistance states, these indicate that the memory switching in NiO would come from the percolative formation and rupture of filamentary conducting paths. Pulse experiments further suggest that the memory switching would come from local domains inside filaments.     

Modeling resistive switching materials and devices across scales

Resistance switching devices based on electrochemical processes have attractive significant attention in the field of nanoelectronics due to the possibility of switching in nanosecond timescales, miniaturization to tens of nanometer and multi-bit storage. Their deceptively simple structures (metal-insulator-metal stack) hide a set of complex, coupled, processes that govern their operation, from electrochemical reactions at interfaces, diffusion and aggregation of ionic species, to electron and hole trapping and Joule heating. A combination of experiments and modeling efforts are contributing to a fundamental understanding of these devices, and progress towards a predictive understanding of their operation is opening the possibility for the rational optimization. In this paper we review recent progress in modeling resistive switching devices at multiple scales; we briefly describe simulation tools appropriate at each scale and the key insight that has been derived from them. Starting with ab initio electronic structure simulations that provide an understanding of the mechanisms of operation of valence change devices pointing to the importance of the aggregation of oxygen vacancies in resistance switching and how dopants affect performance. At slightly larger scales we describe reactive molecular dynamics simulations of the operation of electrochemical metallization cells. Here the dynamical simulations provide an atomic picture of the mechanisms behind the electrochemical formation and stabilization of conductive metallic filaments that provide a low-resistance path for electronic conduction. Kinetic Monte Carlo simulations are one step higher in the multiscale ladder and enable larger scale simulations and longer times, enabling, for example, the study of variability in switching speed and resistance. Finally, we discuss physics-based simulations that accurately capture subtleties of device behavior and that can be incorporated in circuit simulations.     

Multiscale modeling of oxide RRAM devices for memory applications: from material properties to device performance

RRAM devices have been subjected to intense research efforts and are proposed for nonvolatile memory and neuromorphic applications. In this paper we describe a multiscale modeling platform connecting the microscopic properties of the resistive switching material to the electrical characteristics and operation of RRAM devices. The platform allows self-consistently modeling the charge and ion transport and the material structural modifications occurring during RRAM operations and reliability, i.e., conductive filament creation and partial disruption. It allows describing the electrical behavior (current, forming, switching, cycling, reliability tests) of RRAM devices in static and transient conditions and their dependence on external conditions (e.g., temperature). Thanks to the kinetic Monte Carlo approach, the inherent variability of physical processes is properly accounted for. Simulation results can be used both to investigate material properties (including atomic defect distributions) and to optimize stack and bias pulses for optimum device performances and reliability.     

Operating mechanism and resistive switching characteristics of two- and three-terminal atomic switches using a thin metal oxide layer

Atomic switches are nanoionic devices that are operated by controlling redox reactions and the local migration of metal ions in solids. The essential mechanism is the growth and shrinkage of a metal filament formed between two electrodes, resulting in repeatable resistive switching between high-resistance and low-resistance states, which can be used for next-generation nonvolatile memories. This review focuses on the operating mechanism and resistive switching characteristics of two- and three-terminal atomic switches using a thin metal oxide layer as an ion-conducting matrix. First, we describe the operating mechanism of a two-terminal atomic switch based on nucleation theory and present the results of temperature dependence and switching speeds to determine the validity of our switching model. Then, we discuss the effects that moisture absorption in the oxide matrix has on the fundamental processes and switching behavior in order to elucidate the importance of the porosity of the oxide matrix. Finally, we demonstrate a three-terminal atomic switch and describe the impact of the anode material or metal-ion species. These findings will contribute to the development of next-generation logic circuits with low-voltage operation and low-power consumption.     

Resistive random access memory (RRAM) technology: From material,device, selector, 3D integration to bottom-up fabrication

Emerging non-volatile memory technologies are promising due to their anticipated capacity benefits, non-volatility, and zero idle energy. One of the most promising candidates is resistive random access memory (RRAM) based on resistive switching (RS). This paper reviews the development of RS device technology including the fundamental physics, material engineering, three-dimension (3D) integration, and bottom-up fabrication. The device operation, physical mechanisms for resistive switching, reliability metrics, and memory cell selector candidates are summarized from the recent advancement in both industry and academia. Options for 3D memory array architectures are presented for the mass storage application. Finally, the potential application of bottom-up fabrication approaches for effective manufacturing is introduced.     

Review of mechanisms proposed for redox based resistive switching structures

Mechanisms proposed for redox-based memristors are reviewed. Emphasis is given on MIM (metal/insulator/metal) devices of the type MOM where the insulator is an oxide. The oxide conducts oxygen via oxygen vacancies. MOM devices in which the insulator conducts intercalated cations are analogous to the ones with mobile oxygen vacancies. Switching, memory and short term hysteresis are three independent phenomena governed by different mechanisms. A necessary condition for memory is presented. Electroforming, filament formation and alteration and I-V curve crossing are discussed. A new mechanism for unipolar switching is suggested. The metal electrodes are sorted into four types according to the nature of their oxygen transfer. The effect of humidity in the ambient is discussed.     

Resistive switching effect in metal/insulator/metal heterostructures and its application for non‐volatile memory

This paper presents two types of resistive switching effects observed in metal/insulator/metal heterostructures: one is the forced‐switch type, and the other is the memory‐switch type. The former was observed in metal/semi‐insulating GaAs hybrid structures. The nonlinear resistive switch showed a strong magnetic field dependence at room temperature. The latter, described in this report, appears in the Pt/Co oxide/Pt capacitor structures. The resistive switch showed bistability, which is certainly a phenomenon with great promise of application to nonvolatile solid state electronic memory. For both cases, the recent experimental evidences show that the electronic state of the interface between the metal electrode and the insulator plays a crucial role in the resistive switching effect. © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Physics-based modeling approaches of resistive switching devices for memory and in-memory computing applications

The semiconductor industry is currently challenged by the emergence of Internet of Things, Big data, and deep-learning techniques to enable object recognition and inference in portable computers. These revolutions demand new technologies for memory and computation going beyond the standard CMOS-based platform. In this scenario, resistive switching memory (RRAM) is extremely promising in the frame of storage technology, memory devices, and in-memory computing circuits, such as memristive logic or neuromorphic machines. To serve as enabling technology for these new fields, however, there is still a lack of industrial tools to predict the device behavior under certain operation schemes and to allow for optimization of the device properties based on materials and stack engineering. This work provides an overview of modeling approaches for RRAM simulation, at the level of technology computer aided design and high-level compact models for circuit simulations. Finite element method modeling, kinetic Monte Carlo models, and physics-based analytical models will be reviewed. The adaptation of modeling schemes to various RRAM concepts, such as filamentary switching and interface switching, will be discussed. Finally, application cases of compact modeling to simulate simple RRAM circuits for computing will be shown.     

The Modeling and Calculation of One Transistor Memory Devices

Metal/Ferroelectrics/Metal/Insulator/Si (MFMIS) and Metal/Ferroelectrics/ Insulator/Si (MFIS) one-transistor devices are being proposed for non-volatile memory applications. In order to determine the basic theory of one-transistor memory devices, we use equivalent circuits to model and calculate the basic properties of one-transistor memory devices. The capacitance of MFMIS and MFIS capacitors, memory windows, operation voltages, threshold voltages, and both switching and retention properties of one-transistor memory devices have also been calculated. According to the modeling and calculation, the ferroelectric materials with lower dielectric constant (?), low polarization (P_R), appropriate coercive field and square hysteresis loop are required for one-transistor memory devices and low voltage applications. High dielectric constant insulator materials may improve the performance of one-transistor memory devices. In addition, the effects of depolarization fields, leakage current, and defect density on the switching and retention properties of one-transistor memory devices are also calculated. Based on the modeling and calculation, the retention problem dealing with depolarization fields and leakage current is a big challenge for one-transistor memory devices.     

$${ SIM}^2{ RRAM}$$: a physical model for RRAM devices simulation

In the last few years, resistive random access memory (RRAM) has been proposed as one of the most promising candidates to overcome the current Flash technology in the market of non-volatile memories. These devices have the ability to change their resistance state in a reversible and controlled way applying an external voltage. In this way, the resulting high- and low-resistance states allow the electrical representation of the binary states “0” and “1” without storing charge. Many physical models have been developed with the aim of understanding the mechanisms that control the resistive switching. In this work, we have compiled the main theories accepted as well as their corresponding models for the conduction characteristics. In addition, simulation tools play a very important role in the task of checking these theories and understanding these mechanisms. For this reason, the simulation tool called \(\hbox {SIM}^{2}\hbox {RRAM}\) has been presented. This simulator is capable of replicating the global behavior of RRAM cell based on \(\hbox {HfO}_{x}\).     

Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory

Gallium doped ZnO (GZO) top electrode thickness dependence of resistive switching characteristic of GZO/ZnO₂/ZnO/ITO transparent valence change memory device is investigated. The thickness of the GZO top electrode modulates the resistance of the pristine device. Devices made with thicker GZO layer have higher leakage current; thus, require higher current compliance. An excessively high current compliance leads to a device breakdown upon reset process. Conversely, a very low current compliance may form a tiny conducting filament and is difficult to rejuvenate after the rupture; thus, its cycle-to-cycle characteristic shows a decaying behavior. Nevertheless, transparent valence change devices with a stable endurance and sufficient memory window that operate at a moderate level of current compliance are successfully fabricated by employing an appropriate thickness of the top electrode. We suggest that a good switch-ability of transparent valence change memory devices are strongly affected by the thickness of the top electrode.

     

Stochastic modeling of bipolar resistive switching in metal-oxide based memory by Monte Carlo technique

A stochastic model of the resistive switching mechanism in bipolar metal-oxide based resistive random access memory (RRAM) is presented. The distribution of electron occupation probabilities obtained is in agreement with previous work. In particular, a low occupation region is formed near the cathode. Our simulations of the temperature dependence of the electron occupation probability near the anode and the cathode demonstrate a high robustness of the low occupation region. This result indicates that a decrease of the switching time with increasing temperature cannot be explained only by reduced occupations of the vacancies in the low occupation region, but is related to an increase of the mobility of the oxide ions. A hysteresis cycle of RRAM switching simulated with the stochastic model including the ion dynamics is in good agreement with experimental results.     

配电网低压反孤岛装置设计原理及参数计算

针对分布式光伏发电接入配电网发生孤岛效应对电力检修人员现场安全作业的影响,设计了一种应用于220 V/380 V配电网中的低压反孤岛装置。装置基于光伏发电的孤岛运行机理和防孤岛保护策略,通过破坏分布式光伏发电孤岛运行的条件,实现反孤岛功能。提出了3种类型(阻性、感性、容性)低压反孤岛装置的设计原理,结合工程实际,计算了系列化低压反孤岛装置设计参数。通过试验对设计原理进行了验证和分析,通过比较不同类型低压反孤岛装置的试验结果和负载特性,建议低压反孤岛装置优先选用电阻型扰动负载。     

Studies on Al-doped ZnO thin-film transistors with Pb(Zr0.3Ti0.7)O3 gate insulator

We report the fabrication of Al-doped ZnO thin-film transistors (FeFETs) on the ferroelectric Pb(Zr_0.3Ti_0.7)O₃ (PZT) gate insulator for the application of nonvolatile random access memory. The results demonstrate the basic principle of Al-doped ZnO resistive switching between the high and low resistive states upon the polarization switching of ferroelectric layer. Own to the good ferroelectric property and high reliability of PZT, such as fatigue, high speed of signal reading and writing, low coercive electric field, etc., this device has an excellent electrical performance. The memory device exhibits a source-drain current modulation with an ON/OFF current ratio close 10³.     

SiC MOSFET与SiC SBD换流单元瞬态模型

相较于硅(Si)器件,碳化硅(SiC)器件所具有的高开关速度与低通态电阻特性增加了其瞬态波形的非理想特性与对杂散参数影响的敏感性,对其瞬态建模的精度提出更高的要求。通过功率开关器件瞬态过程的时间分段、机理解耦与参数解耦,突出器件开关特性,弱化物理机理,简化瞬态过程分析,建立基于SiC MOSFET与SiC SBD的换流单元瞬态模型。理论计算结果与实验结果对比表明,该模型能够较为精细地体现SiC MOSFET开关瞬态波形且能够较为准确地计算SiC MOSFET开关损耗。该模型参数可全部由数据手册提取,有较强的实用性。     

Resistive Switching and Current Conduction Mechanisms in Amorphous LaLuO3 Thin Films Grown by Pulsed Laser Deposition

The unipolar resistive switching characteristics of the amorphous LaLuO₃ thin films deposited by pulsed laser deposition have been studied. Reliable and repeatable nonvolatile switching of the resistance of LaLuO₃ films was obtained between two well defined states of low and high resistance with nearly constant resistance ratio ～10⁷ and non-overlapping switching voltages in the range of 0.66-0.83 V and 1.9-2.7 V respectively. The temperature dependent measurement revealed metallic and semiconducting behavior in low and high resistance states respectively. The switching between low and high resistance states was attributed to the change in the separation between oxygen vacancies in light of the correlated barrier hopping theory. The current conduction mechanism of the device in high-resistance state followed the Poole's law, whereas the conduction in low-resistance state was found to be dominated by percolation. The resistance of low and high resistance states of the film showed no obvious degradation for up to ～10⁴ seconds indicating good retention. The achieved characteristics of the resistive switching in LaLuO₃ thin films seem to be promising for futuristic nonvolatile memory applications.     

GaN基功率电子器件及其应用专辑特邀主编述评

GaN基功率电子器件因其具有开关速度快、开关频率高、工作结温高、通态电阻小、开关损耗低等优势,适合应用于新型高效、大功率等的电力电子系统。国内关于GaN基功率电子器件的研究已经取得了较为明显的进展,但与发达国家相比仍有一定的差距。为此,《电源学报》特别推出了"GaN基功率电子器件及其应用"专辑,基本涵盖GaN功率电子器件及其应用研究的热点问题,展示了不同研究机构和企业在该领域的研发现状,具有良好的学术研究和应用参考价值。