An overview of resistive random access memory devices

With recent progress in material science, resistive random access memory (RRAM) devices have attracted interest for nonvolatile, low-power, nondestructive readout, and high-density memories. Relevant performance parameters of RRAM devices include operating voltage, operation speed, resistance ratio, endurance, retention time, device yield, and multilevel storage. Numerous resistive-switching mechanisms, such as conductive filament, space-charge-limited conduction, trap charging and discharging, Schottky Emission, and Pool-Frenkel emission, have been proposed to explain the resistive switching of RRAM devices. In addition to a discussion of these mechanisms, the effects of electrode materials, doped oxide materials, and different configuration devices on the resistive-switching characteristics in nonvolatile memory applications, are reviewed. Finally, suggestions for future research, as well as the challenges awaiting RRAM devices, are given.     

过渡性金属氧化物薄膜阻变存储材料进展概况

随着半导体技术和集成电路的进步,器件的集成度也不断提高,器件的特征尺寸不断减小,基于电荷存储的传统非易失性随机存储器面临着物理和技术上极限的挑战。阻变式存储器（RRAM）作为新一代的存储器件,因其器件具有结构简单、制备简便、存储密度高、擦写速度快、写入电流小等优势引起了人们广泛的研究。本文就目前基于过度氧化物薄膜的RRAM研究概况,从RRAM的基本工作原理、材料体系、存储机理和器件应用所面临的各种困难等方面对RRAM进行了简要评述。     

An overview of the switching parameter variation of RRAM

Resistive random access memory（RRAM） has been considered as one of the most promising candidates for next-generation nonvolatile memory, due to its advantages of simple device structure, excellent scalability, fast operation speed and low power consumption. Deeply understanding the physical mechanism and effectively controlling the statistical variation of switching parameters are the basis of fostering RRAM into commercial application. In this paper, based on the deep understanding on the mechanism of the formation and rupture of conductive filament, we summarize the methods of analyzing and modeling the statistics of switching parameters such as SET/RESET voltage, current, speed or time. Then, we analyze the distributions of switching parameters and the influencing factors. Additionally, we also sum up the analytical model of resistive switching statistics composed of the cell-based percolation model and SET/RESET switching dynamics. The results of the model can successfully explain the experimental distributions of switching parameters of the Ni O- and Hf O2-based RRAM devices. The model also provides theoretical guide on how to improve the uniformity and reliability such as disturb immunity. Finally, some experimental approaches to improve the uniformity of switching parameters are discussed.     

Enhanced resistive switching properties of HfAlOx/ZrO2- based RRAM devices

Resistive Random-Access Memory (RRAM) devices are recognized as potential candidates for next-generation memory devices, due to their smallest cell size, high write/erase speed, and endurance. Particularly, the resistive switching (RS) characteristics in oxide materials have offered new opportunities for developing CMOS-compatible high-density RRAM devices. In this study, the RS behavior of HfAlO_x/ZrO₂ thin films sandwiched structure between TiN bottom electrode and Au top electrodes were investigated. It was found that Au/HfAlO_x/ZrO₂/TiN stacks were superior in terms of RS performance when compare to Au/HfAlO_x/TiN memory stacks. The devices demonstrated a good resistance ratio of high resistance state and low resistance state ～10³ for Au/HfAlO_x/TiN and ～10⁵ for Au/HfAlO_x/ZrO₂/TiN stacks, respectively. Both stacks showed good retention characteristics (>10⁴ ?s) and endurance (>10³ cycles). The experimental current-voltage characteristics fitted with different conducting mechanisms, the linear lower bias region is dominated by ohmic conductivity, whereas the non-linear higher bias region was dominated by space-charge limited current conduction mechanism.     

一种阻变存储单元Hspice模型设计

提出一种满足新型双通道阻变存储器读写操作要求的Hspice模型.这种模型基于新的机理,即通过改变一块1 Mb阻变存储阵列的一个单元中2种可重配置的稳定电阻存储模式实现＂RESET态＂和＂SET态＂之间的转换,它可以通过一个模拟电流-电压特性的分立器件模型来验证.与传统阻变存储器模型相比,利用这种模型,可以用较少的器件准...     

NiO_x阻变存储器性能增强方法及相关机理研究

通过精确控制在Pt衬底上制备NiOx薄膜的工艺过程,制备出阻值窗口增大5倍以上,高低阻态稳定的TiN/NiOx/Pt结构阻变存储器.研究发现,NiOx薄膜的多晶态结晶结构和化学组分,尤其是Ni元素的化学态,是影响NiOx阻变存储器阻值窗口和稳定性的主要因素.X射线光电子能谱和X射线多晶体衍射测试结果表明,当NiOx薄膜中间隙氧或Ni2+空位增多时,Ni2+会被氧化成为Ni3+以保持电中性,Ni3+离子在材料中引入空穴导致P型氧化物NiO的漏电流增大.基于此机理,提出通过提高淀积温度、降低氧气分压的方法抑制NiOx薄膜中间隙氧或Ni2+空位的产生,降低TiN/NiOx/Pt结构阻变存储器关态漏电流,增大阻值窗口.这种基于工艺的性能增强方法,在NiOx阻变存储器实际应用中有良好前景.     

基于带符号位的浮点数运算的多位宽3D RRAM设计

本文介绍了卷积神经网络（convolutional neutral network,CNN）系统中具有多位存储的三维阻变式存储器（three-dimensional resistive random-access memory,3D RRAM）的带符号位的浮点数运算. 与其他类型存储器相比,3D RRAM可以在存储器内部进行运算,且具有更高的读取速率和更低的能耗,为解决冯诺依曼架构的瓶颈问题提供新方案. 单个RRAM单元的最大和最小电阻分别达到10 GΩ和10 MΩ,可在多级电阻状态下稳定,以存储多比特位宽的数据. 测试结果表明,带符号位的浮点数的卷积运算系统的精度可以达到99.8%,测试中3D RRAM模型的峰值读取速度为0.529 MHz.      

Advancements in organic nonvolatile memory devices

As one of the most promising candidates for next generation storage media, organic memory devices have aroused worldwide research interest in both academia and industry. In recent years, organic memories have experienced rapid progress. We review the development of organic resistive switching memories in terms of structure, characteristics, materials used, and integration. Some basic concepts are discussed, as well as the obstacles hindering the development and possible commercialization of organic memory d...     

浮栅型有机非易失性存储器的研究

有机柔性电子器件具有低制造成本、大面积、可柔性折叠等优点，是近年来国内外学术界和工业界的研究热点。有机非易失存储器是一种重要的有机柔性电子器件。本文阐述了浮栅型有机非易失性存储器件的工作原理；系统介绍了当然国内外学术界对浮栅型有机非易失性存储器的研究进展，存在的一些问题以及一些可能的解决对策。     

Improving the electrical performance of resistive switching memory using doping technology

In this paper, improvements of resistive random access memory (RRAM) using doping technology are summarized and analyzed. Based on a Cu/ZrO2/Pt device, three doping technologies with Ti ions, Cu, and Cu nanocrystal, respectively, are adopted in the experiments. Compared to an undoped device, improvements focus on four points: eliminating the electroforming process, reducing operation voltage, improving electrical uniformity, and increasing device yield. In addition, thermal stability of the high resistance state and better retention are also achieved by the doping technology. We demonstrate that doping technology is an effective way of improving the electrical performance of RRAM.     

过渡金属氧化物自旋和电荷相关信息存储

信息技术的快速发展在某种程度上要求有高速度和大容量的非易失存储器.然而,随着晶体管尺度达到其量子极限,传统硅半导体器件的继续集成化发展遇到了瓶颈.因此,人们提出了一系列有潜力成为下一代更具功能性的存储器原型器件,并引起了广泛而持续的研究热潮.本文介绍3种基于新材料和新结构的新型存储原型器件：阻变开关器件、有机自旋阀和多铁隧道结.我们发现通过改变界面态,可将阻变式开关器件的反应速度提高数个量级,达到5ns;在实验上确认了超精细相互作用对自旋阀效应的影响;利用多铁隧道结实现了室温下的四重阻态存储.基于自旋、电荷相关信息存储的原理和实验结果,我们对这3种过渡金属氧化物器件目前还存在的问题及未来的应用前景进行了分析和讨论.     

基于非易失性存储器的存储系统技术研究进展

 非易失性存储器（NVM）主要包括两类,即适用于外存的、块寻址的闪存和适用于内存的、字节寻址的持久性内存。相比于传统磁盘,闪存具有性能高、能耗低和体积小等优势;相比于DRAM（动态随机存储器）,持久性内存如PCM（相变存储器）、RRAM（阻变存储器）等,具有非易失、存储密度高以及同等面积/内存插槽下能给多核环境的CPU 提供更多的数据等优点,这些都为存储系统的高效构建带来了巨大的机遇。然而,传统存储系统的构建方式不适用于非易失性存储器,阻碍了其优势的发挥。为此,分析了基于非易失性存储器构建存储系统的挑战,从闪存、持久性内存两个层次分别综述了它们在存储体系结构、系统软件以及分布式协议方面的变革,总结了基于非易失性存储器构建存储系统的主要研究方向。     

聚酰亚胺基电双稳态信息存储材料研究进展

在简介聚合物基电双稳态信息存储材料基本概念的基础上,总结了近年来制备的可用于信息存储领域的功能性聚酰亚胺。针对以聚酰亚胺为活性层所构造出的存储器件,详细描述了其易失和非易失型的存储性能。通过阐述电双稳态信息存储的机理,据此提出调控聚酰亚胺材料信息存储行为的方法,并探讨了该领域亟待解决的一些问题。最后展望了聚酰亚胺材料和器件在信息存储材料领域的发展趋势。     

Approaches for improving the performance of filament-type resistive switching memory

Resistive random access memory (RRAM) has received significant research interest because of its promising potential in terms of down-scaling, high density, high speed and low power. However, its endurance, retention and uniformity are still imperfect. In this article, the physical mechanisms of filament-type RRAM and the approaches for improving the switching performance, including doping, process optimization and interface engineering, are introduced.     

Approaches for improving the performance of filament-type resistive switching memory

Resistive random access memory (RRAM) has received significant research interest because of its promising potential in terms of down-scaling,high density,high speed and low power. However,its endurance,retention and uniformity are still imperfect. In this article,the physical mechanisms of filament-type RRAM and the approaches for improving the switching performance,including doping,process optimization and interface engineering,are introduced.     

新一代存储技术:阻变存储器

阻变存储器具有存储单元结构简单、工作速度快、功耗低、有利于提高集成密度等诸多优点,受到广泛的关注。作者论述了 RRAM 的基本结构和工作原理, 并介绍了三维集成和多值存储等 RRAM 新型技术。     

Nonvolatile resistive switching memories-characteristics,mechanisms and challenges

<正>This review presents a summary of current understanding of the resistive switching materials and devices which have inspired extraordinary interest all over the world.Although various switching behaviors and different conductive mechanisms are involved in the field,the resistive switching effects can be roughly classified into filament type and interface type according to their conducting behavior in low resistance state.For those filament based systems,the migration of metallic cations and oxygen vacancies, characterization of the filament as well as the role of Joule heating effects are discussed in detail.As to the interface based system, we describe the methods of modulating interface barrier height such as using different electrodes,inserting a tunnel layer.It is demonstrated that the switching mechanism can transform from one to another along the change of some specific conditions.We also give an overview on the latest developments in multilevel storage and the resistive switching in organic materials.In this paper,the solutions to address the sneak current problems in crossbar structure are discussed.     

Ion transport-related resistive switching in film sandwich structures

Resistive switching memories based on ion transport and related electrochemical reactions have been extensively studied for years. To utilize the resistive switching memories for high-performance information storage applications, a thorough understanding of the key information of ion transport process, including the mobile ion species, the ion transport paths, as well as the electrochemical reaction behaviors of these ions should be provided for material and device optimization. Moreover, ion transport is usually accompanied by processes of microstructure modification, phase transition, and energy band structure variation that lead to further modulation of other physical properties, e.g., magnetism, optical emission/absorbance, etc., in the resistive switching materials. Therefore, novel resistive switching memories that are controlled through additional means of magnetic or optical stimulus, or demonstrate extra functionalities beyond information storage, can be made possible via well-defined ion transportation in various switching materials and devices. In this contribution, the mechanism of ion transport and related resistive switching phenomena in thin film sandwich structures is discussed first, followed by a glanceat the recent progress in the development of high-performance and multifunctional resistive switching memories. A brief perspective of the ion transport-based resistive switching memories is addressed at the end of this review.     

相变存储器单元皮秒测试信号完整性研究

针对相变存储器皮秒测试系统由于矩阵开关电路分布参数以及信号通道中阻抗突变所引起的反射,严重影响了皮秒脉冲信号的完整性,使施加在相变单元上的皮秒脉冲信号严重失真,为了消除失真,改善信号完整性,从2个方面提出了解决方案：一是采用性能优良的高频继电器构成矩阵开关;二是减小反射,采取了源端阻抗匹配、负载分支端阻抗补偿、菊花链双线拓扑结构等措施,使得反射的影响减小到最小.理论分析与仿真结果表明：皮秒脉冲信号波形质量良好,满足测试要求.本测试系统为研究皮秒编程脉冲作用下相变存储器的存储机理、速度、可靠性等提供了良好的平台,也可作为电阻式随机擦写存储器单元测试系统.     

Investigation of the synaptic device based on the resistive switching behavior in hafnium oxide

Metal-oxide based electronics synapse is promising for future neuromorphic computation application due to its simple structure and fab-friendly materials. HfOx resistive switching memory has been demonstrated superior performance such as high speed, low voltage, robust reliability, excellent repeatability, and so on. In this work, the HfOx synaptic device was investigated based on its resistive switching phenomenon. HfOx resistive switching device with different electrodes and dopants were fabricated. TiN/Gd:HfOx/Pt stack exhibited the best synaptic performance, including controllable multilevel ability and low training energy consumption. The training schemes for memory and forgetting were developed.