基于I-V特性的阻变存储器的阻变机制研究

随着器件尺寸的缩小,阻变存储器(RRAM)具有取代现有主流Flash存储器成为下一代新型存储器的潜力。但对RRAM器件电阻转变机制的研究在认识上依然存在很大的分歧,直接制约了RRAM的研发与应用。通过介绍阻变存储器的基本工作原理、不同的阻变机制以及基于阻变存储器所表现出的不同I-V特性,研究了器件的阻变特性;详细分析了阻变存储器的五种阻变物理机制,即导电细丝(filament)、空间电荷限制电流效应(SCLC)、缺陷能级的电荷俘获和释放、肖特基发射效应(Schottky emission)以及普尔-法兰克效应(Pool-Frenkel);同时,对RRAM器件的研究发展趋势以及面临的挑战进行了展望。     

Preface to the Special Issue on Beyond Moore:Resistive Switching Devices for Emerging Memory and Neuromorphic Computing

Traditional charge-based memories,such as dynamic random-access memory(DRAM)and flash,are approaching their scaling limits.A variety of resistance-based memories,such as phase-change memory(PCM),magnetic random-access memory(MRAM)and resistive random-access memory(RRAM),have been long considered for emerging memory applications thanks to their non-volatility,fast speed,low power,and compact size for potentially high-density integration.     

两类阻变机理及性能改善方法的研究

在新型非易失性存储领域,结构简单、高速低耗的阻变存储器具有巨大优势和很强的竞争力.简要介绍了阻变存储器的结构及其两个电阻转变行为.总结了两类阻变机理,探讨了阻变存储器性能优化的方法,以及优化方法在阻变性能与器件的可靠性和稳定性之间如何取得平衡统一的问题,并展望了其前景.     

A Parallel Circuit Model for Multi‐State Resistive‐Switching Random Access Memory

Large, rapidly growing literature is available on bipolar resistive‐switching random access memories (RRAM) made of myriad of simple and advanced materials. Many of them exhibit similar resistance switching behavior but, until now, no unifying model can allow quantification of their voltage and time responses. Using a simple parallel circuit model, these responses of a newly discovered RRAM made of a thin‐film random material are successfully analyzed. The analysis clearly reveals a large population of intermediate states with remarkably similar switching characteristics. Such modeling framework based on simple circuit constructs also appears applicable to several RRAM made of other materials. This simple approach to analyze data write/rewrite and memory retention in RRAM may aid their further understanding and development.     

Current-Sweep Operation on Nonlinear Selectorless RRAM for Multilevel Cell Applications

Bilayer selectorless resistive random-access memories (RRAM) have been demonstrated by utilizing the intrinsic nonlinear resistive switching (RS) characteristics, without additional transistor or a selector integration. The bilayer structures, i.e. high-k layer/low-k layer stacks, are highly scalable while suppressing the sneak path currents (SPC) and reading error in the crossbar RRAM array. The nonlinearity (NL) modulation is also investigated by different operating schemes, and a multilevel cell application is demonstrated with the current-sweep method. The results provide additional insights into the development and optimization of bilayer selectorless RRAMs with high nonlinearity, good memory window, and low switching energy (∼ 40 pJ/bit), which enable the high-density storage and low-power crossbar array memory applications.     

Low-Energy Oxygen Plasma Injection of 2D Bi2Se3 Realizes Highly Controllable Resistive Random Access Memory

Resistive random access memory (RRAM) based on ultrathin 2D materials is considered to be a very feasible solution for future data storage and neuromorphic computing technologies. However, controllability and stability are the problems that need to be solved for practical applications. Here, by introducing a damage-less ion implantation technology using ultralow-energy plasma, the transport mechanisms of space charge limited current and Schottky emission are successfully realized and controlled in RRAM based on 2D Bi₂Se₃ nanosheets. The memristors exhibit stable resistive switching behavior with a high resistive switching ratio (>10⁴), excellent cycling endurances (300 cycles), and great retention performance (>10⁴ s). The reliability and controllability of Bi₂Se₃ memory endowed by oxygen plasma injection demonstrate the great potential of this ultralow-energy ion implantation technology in the application of 2D RRAM.     

A journey towards reliability improvement of TiO2 based Resistive Random Access Memory: A review

A Resistive Random Access Memory (RRAM), where the memory performance principally originated from ‘resistive’ change rather than ‘capacitive’ one (the case with conventional CMOS memory devices), has attracted researchers across the globe, owing to its unique features and advantages meeting the demands of future generation high-speed, ultra low power, nano dimensional memory devices. A large family of semiconducting oxides have been investigated as insulator for Resistive Random Access Memory (RRAM), amongst which TiO₂ is one of the potential candidate, principally owing to some of its remarkable advantages e.g. wide band gap, high temperature stability and high dielectric constant with flexibility to offer both unipolar and bipolar switching, which are essential for RRAM device applications. In this review article, we tried to represent the long voyage of TiO₂ based RRAM, towards the improvement of the reliability aspects of the device performance in a comprehensive manner. Starting with the key factors like oxygen vacancies, Ti interstitials and electroforming, which are responsible for resistive switching phenomenon, various material preparation techniques for RRAM development have been discussed with emphasis on relative merits and bottlenecks of the process. The factors like electrode material and geometry, device structuring, doping, compliance current, annealing effect etc., which play the pivotal role in determining the switching performance of the device, have been reviewed critically. Finally, the article concludes with the comparison of different TiO₂ based RRAM devices followed by the prediction of possible future research trends.     

The resistive switching characteristics of a Ti/Gd2O3/Pt RRAM device

We successfully fabricated the Gd₂O₃ film for the application of resistive random access memory (RRAM). The resistive switching behavior of the Ti/Gd₂O₃/Pt capacitor structure could be both operated under positive or negative bias. However, there was a significant difference on the switching properties. The switching behavior under positive bias operation was more stable, had less voltage and resistance fluctuation, and had longer endurance than that of the negative one. We propose that the anode electrode plays an important role in the switching characteristics and may be the cause of the asymmetry of the I-V curves between positive and negative operation.     

新型阻变存储器内的电压解决方案

相对于现在流行的FLASH型存储器,新型阻变存储器(resistive-RAM,RRAM)有很多优势,比如较高的存储密度和较快的读写速度。而针对RRAM的读写操作特性,提出了一种适用于新型阻变存储器的提供操作电压的电路。该方案解决了新型存储器需要外部提供高于电源电压的操作电压的问题,使得阻变存储器能应用于嵌入式设备。同时,对工艺波动和温度波动进行补偿,从而降低了阻变存储器的读写操作在较差的工艺和温度环境下的失败概率,具有很强的实际应用意义。该设计采用0.13μm标准CMOS 6层金属工艺在中芯国际(SMIC)流片实现,测试结果表明,采用此电路的RRAM能正确地进行数据编程和擦除等操作,测试结果达到设计要求。     

Thickness effect on the bipolar switching mechanism for nonvolatile resistive memory devices based on CeO2 thin films

Impact of switching layer thickness on the bipolar resistive memory performance, stability and uniformity has been investigated in Ti/CeO₂/Pt devices. XRD and FTIR analyses demonstrate polycrystalline nature of CeO₂ films and the formation of a TiO interface layer. The bipolar switching characteristics like HRS and LRS dispersion are found to be dependent on the thickness of CeO₂ layer. As it is noted that forming as well as SET voltages gradually increase with increasing CeO₂ layer thickness however RESET voltages are slightly affected. Oxygen gettering ability of Ti causes the formation of TiO layer, which not only extracts oxygen ions from the ceria film but also acts as ion reservoir, hence plays a key role in stable functioning of the memory devices. Current transport behavior is based upon Ohmic and interface modified space charge limited conduction. Based on unique distribution characteristics of oxygen vacancies in CeO₂ films, a possible mechanism of resistive switching in CeO₂ RRAM devices has been discussed.     

阻变存储器及其集成技术研究进展

在各种新型非挥发性存储器中,阻变存储器(RRAM)具有成为下一代存储器的潜力.介绍了RRAM器件的基本结构,分类总结了常用的材料以及制备工艺,对RRAM阵列的集成方案进行了比较,并讨论了目前存在的问题;最后,对RRAM的研究趋势进行了展望.     

Resistive Random Access Memory Cells with a Bilayer TiO2/SiOX Insulating Stack for Simultaneous Filamentary and Distributed Resistive Switching

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO₂, SiO_X, and n⁺⁺Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle‐to‐cycle endurance (>1000 cycles), and low device‐to‐device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.     

超薄氧化铝层的电场调制效应

研究了基于高k介质材料的阻变存储器的写入/擦除 (SET/RESET) 特性和物理机制.研究发现基于NbAlO材料的阻变存储器SET/RESET电压具有较大波动性, 通过结构优化, 在Al2O3/NbAlO/Al2O3纳米薄片堆垛结构器件中获得高度稳定性的可重复的阻变特性.基于电场调制效应, 提出了一种统一的电阻开关模型去模拟阻变存储器的SET/RESET行为, 并探讨了单层阻变薄膜的阻变存储器中由导电单元形成和湮灭的巨大随机性引起的阻变特性分布.当在NbAlO基阻变存储器中嵌入超薄Al2O3膜后, 阻变存储器的SET/RESET电压稳定性将显著提升, 其原因在于采用堆垛结构的阻变器件中各介质层中的电场重新分布并精确可控, 因此导电细丝的导通/断裂通过电场调制作用稳定均匀地在发生在具有高电场的薄缓冲层介质层中.     

UV‐Induced Multilevel Current Amplification Memory Effect in Zinc Oxide Rods Resistive Switching Devices

Zinc oxide (ZnO) devices represent an alternative in the semiconductor technology for their application in resistive switching memory devices and ultraviolet (UV) photodetectors due to their chemical and electrical properties. The multilevel current amplification of ZnO rods RRAM devices induced by UV light illumination is reported here for the first time. The resistive switching mechanism underlying in this type of devices is attributed to the formation of conductive filaments composed of oxygen vacancies. The analysis of the photodecay processes carried out on the devices fabricated with different electrodes shows that the type of interface (Ag/ZnO and Au/ZnO) affects the surface barrier height, which influences the photodecay rate. It is shown that by applying UV light, higher relaxation constants (slower photodecay rates) are obtained and lead to multilevel current amplification behavior.     

Realization of the Switching Mechanism in Resistance Random Access Memory™ Devices: Structural and Electronic Properties Affecting Electron Conductivity in a Hafnium Oxide–Electrode System Through First-Principles Calculations

The resistance random access memory (RRAM?) device, with its electrically induced nanoscale resistive switching capacity, has attracted considerable attention as a future nonvolatile memory device. Here, we propose a mechanism of switching based on an oxygen vacancy migration-driven change in the electronic properties of the transition-metal oxide film stimulated by set pulse voltages. We used density functional theory-based calculations to account for the effect of oxygen vacancies and their migration on the electronic properties of HfO₂ and Ta/HfO₂ systems, thereby providing a complete explanation of the RRAM? switching mechanism. Furthermore, computational results on the activation energy barrier for oxygen vacancy migration were found to be consistent with the set and reset pulse voltage obtained from experiments. Understanding this mechanism will be beneficial to effectively realizing the materials design in these devices.     

Control of Switching Modes and Conductance Quantization in Oxygen Engineered HfOx based Memristive Devices

Hafnium oxide (HfO_x)‐based memristive devices have tremendous potential as nonvolatile resistive random access memory (RRAM) and in neuromorphic electronics. Despite its seemingly simple two‐terminal structure, a myriad of RRAM devices reported in the rapidly growing literature exhibit rather complex resistive switching behaviors. Using Pt/HfO_x/TiN‐based metal–insulator–metal structures as model systems, it is shown that a well‐controlled oxygen stoichiometry governs the filament formation and the occurrence of multiple switching modes. The oxygen vacancy concentration is found to be the key factor in manipulating the balance between electric field and Joule heating during formation, rupture (reset), and reformation (set) of the conductive filaments in the dielectric. In addition, the engineering of oxygen vacancies stabilizes atomic size filament constrictions exhibiting integer and half‐integer conductance quantization at room temperature during set and reset. Identifying the materials conditions of different switching modes and conductance quantization contributes to a unified switching model correlating structural and functional properties of RRAM materials. The possibility to engineer the oxygen stoichiometry in HfO_x will allow creating quantum point contacts with multiple conductance quanta as a first step toward multilevel memristive quantum devices.     

The study of lithographic variation in resistive random access memory

Reducing the process variation is a significant concern for resistive random access memory (RRAM). Due to its ultra-high integration density, RRAM arrays are prone to lithographic variation during the lithography process, introducing electrical variation among different RRAM devices. In this work, an optical physical verification methodology for the RRAM array is developed, and the effects of different layout parameters on important electrical characteristics are systematically investigated. The results indicate that the RRAM devices can be categorized into three clusters according to their locations and lithography environments. The read resistance is more sensitive to the locations in the array (~30%) than SET/RESET voltage (<10%). The increase in the RRAM device length and the application of the optical proximity correction technique can help to reduce the variation to less than 10%, whereas it reduces RRAM read resistance by 4×, resulting in a higher power and area consumption. As such, we provide design guidelines to minimize the electrical variation of RRAM arrays due to the lithography process.     

Biocompatible and Flexible Chitosan‐Based Resistive Switching Memory with Magnesium Electrodes

A flexible and transparent resistive switching memory based on a natural organic polymer for future flexible electronics is reported. The device has a coplanar structure of Mg/Ag‐doped chitosan/Mg on plastic substrate, which shows promising nonvolatile memory characteristics for flexible memory applications. It can be easily fabricated using solution processes on flexible substrates at room temperature and indicates reliable memory operations. The elucidated origin of the bipolar resistive switching behavior is attributed to trap‐related space‐charge‐limited conduction in high resistance state and filamentary conduction in low resistance state. The fabricated devices exhibit memory characteristics such as low power operation and long data retention. The proposed biocompatible memory device with transient electrodes is based on naturally abundant materials and is a promising candidate for low‐cost memory applications. Devices with natural substrates such as chitosan and rice paper are also fabricated for fully biodegradable resistive switching memory. This work provides an important step toward developing a flexible resistive switching memory with natural polymer films for application in flexible and biodegradable nanoelectronic devices.     

Electric-pulse-induced resistive switching and possible superconductivity in the Mott insulator GaTa4Se8

There is accumulated evidence today that an electric pulse can drastically modify the physical properties of correlated materials. An electric pulse was shown for example to induce an insulator-to-metal transition in manganites or in organic Mott insulators. We report here the first experimental evidence of a non-volatile electric pulse-induced insulator-to-metal transition and possible superconductivity in the Mott insulator GaTa₄Se₈. This resistive switching is concomitant to an electronic phase separation induced by the pulse. This phenomena most probably differs from the thermal, electronic injection or ionic diffusion processes explaining the resistive switching in materials foreseen for non-volatile memory (RRAM) applications.     

缺陷相互作用对HfO2基阻变存储器件均匀性的影响：第一性原理研究

The physical mechanism of doping effects on switching uniformity and operation voltage in Al-doped HfO₂ resistive random access memory（RRAM） devices is proposed from another perspective:defects interactions, based on first principle calculations.In doped HfO₂,dopant is proved to have a localized effect on the formation of defects and the interactions between them.In addition,both effects cause oxygen vacancies（V_O） to have a tendency to form clusters and these clusters are easy to form around the dopant.It is proved that this process can improve the performance of material through projected density of states（PDOS） analysis.For V_O filament-type RRAM devices, these clusters are concluded to be helpful for the controllability of the switching process in which oxygen vacancy filaments form and break.Therefore,improved uniformity and operation voltage of Al-doped HfjO₂ RRAM devices is achieved.