Physical models of size-dependent nanofilament formation and rupture in NiO resistive switching memories

NiO films display unipolar resistance switching characteristics, due to the electrically induced formation and rupture of nanofilaments. While the applicative interest for possible use in highly dense resistance switching memory (RRAM) is extremely high, switching phenomena pose strong fundamental challenges in understanding the physical mechanisms and models. This work addresses the set and reset mechanisms for the formation and rupture of nanofilaments in NiO RRAM devices. Reset is described in terms of thermally-accelerated diffusion and oxidation processes, and its resistance dependence is explained by size-dependent Joule heating and oxidation. The filament is described as a region with locally-enhanced doping, resulting in an insulator-metal transition driven by structural and chemical defects. The set mechanism is explained by a threshold switching effect, triggering chemical reduction and a consequent local increase of metallic doping. The possible use of the observed resistance-dependent reset and set parameters to improve the memory array operation and variability is finally discussed.     

Resistive switching in copper oxide nanorods: a bottom up approach applicable for enhanced scalability

Reversible, stable and reproducible resistive switching in a parallel network of Cu2O nanorods, observed in the present study, highlights the advantages of using nanorods in comparison to normally used thin films. Unipolar and symmetric current-voltage characteristics of the metal/insulator/metal structure consisting of Hg top contact/Copper oxide (Cu2O) nanorods/Ag bottom contact in a sandwich configuration shows electroforming at about 11 V, reproducible reset and set points at 0.53 +/- 0.03 and 4.2 +/- 0.02 V and a high OFF/ON resistance ratio > 10(3). Slope of current-voltage characteristics and current contrast in CAFM mapping indicate that filamentary conduction mechanism is responsible for resistive switching. This study sets the foundation for fabricating a nanorods based resistive random access memory device and thus a manifold increase in the device scalability.     

Memristive tri-stable resistive switching at ruptured conducting filaments of a Pt/TiO₂/Pt cell

A tri-stable memristive switching was demonstrated on a Pt/TiO?/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO?/TiO(2-x)/Pt cell.     

Resistive switching effects in oxide sandwiched structures

Resistive switching (RS) behaviors have attracted great interest due to their promising potential for the data storage. Among various materials, oxide-based devices appear to be more advantageous considering their handy fabrication and compatibility with CMOS technology, though the underlying mechanism is still controversial due to the diversity of RS behaviors. In this review, we focus on the oxide-based RS memories, in which the working mechanism can be understood basically according to a so-called filament model. The filaments formation/rupture processes, approaches developed to detect and characterize filaments, several effective attempts to improve the performances of RS and the quantum conductance behaviors in oxide-based resistive random access memory (RRAM) devices are addressed, respectively.     

Physical Mechanism and Performance Factors of Metal Oxide Based Resistive Switching Memory:A Review

This review summarizes the mechanism and performance of metal oxide based resistive switching memory The origin of resistive switching(RS) behavior can be roughly classified into the conducting filament type and the interface type. Here,we adopt the filament type to study the metal oxide based resistive switching memory,which considers the migration of metallic cations and oxygen vacancies,as well as discuss two main mechanisms including the electrochemical metallization effect(ECM) and valence change memory effect(VCM). At the light of the influence of the electrode materials and switching layers on the RS characteristics,an overview has also been given on the performance parameters including the uniformity endurance,the retention,and the multi-layer storage. Especially,we mentioned ITO(indium tin oxide electrode and discussed the novel RS characteristics related with ITO. Finally,the challenges resistive random access memory(RRAM) device is facing,as well as the future development trend,are expressed.     

Multilevel resistive switching and synaptic plasticity of nanoparticulated cobaltite oxide memristive device

Multilevel resistive switching(RS)is a key property to embrace the full potential of memristive devices for non-volatile memory and neuromorphic computing applications.In this study,we employed nanopar-ticulated cobaltite oxide(Co3O4)as a model material to demonstrate the multilevel RS and synaptic learning capabilities because of its multiple and stable redox state properties.The Pt/Co3O4/Pt memris-tive device exhibited tunable RS properties with respect to different voltages and compliance currents(CC)without the electroforming process.That is,the device showed voltage-dependent RS at a higher CC whereas CC-dependent RS was observed at lower CC.The device showed four different resistance states during endurance and retention measurements and non-volatile memory results indicated that the CC-based measurement had less variation.Besides,we investigated the basic and complex synap-tic plasticity properties using the analog current-voltage characteristics of the Pt/Co3O4/Pt device.In particular,we mimicked the potentiation-depression and four-spike time-dependent plasticity(STDP)rules such as asymmetric Hebbian,asymmetric anti-Hebbian,symmetric Hebbian,and symmetric anti-Hebbian learning rules.The results of the present work indicate that the cobaltite oxide is an excellent nanomaterial for both multilevel RS and neuromorphic computing applications.     

Development of ferroelectric oxides based resistive switching materials

Resistive random access memory (RRAM) is one of the most promising candidates that satisfies the requirements of new generation non-volatile memories, as a consequence of its high density, outstanding scalability, and low power consumption. The review is based on a summary of recent studies in ferroelectric oxides based resistive switching (RS) materials and devices. It highlights the various ferroelectric oxide materials with RS behaviour and the underlying mechanisms including filament-type and interface-type mechanism. In the end, the challenge in current RRAM for future high-density data storage applications is addressed.     

Resistive switching mechanisms in random access memory devices incorporating transition metal oxides: TiO2, NiO and Pr0.7Ca0.3MnO3

Resistance change random access memory (RRAM) cells, typically built as MIM capacitor structures, consist of insulating layers I sandwiched between metal layers M, where the insulator performs the resistance switching operation. These devices can be electrically switched between two or more stable resistance states at a speed of nanoseconds, with long retention times, high switching endurance, low read voltage, and large switching windows. They are attractive candidates for next-generation non-volatile memory, particularly as a flash successor, as the material properties can be scaled to the nanometer regime. Several resistance switching models have been suggested so far for transition metal oxide based devices, such as charge trapping, conductive filament formation, Schottky barrier modulation, and electrochemical migration of point defects. The underlying fundamental principles of the switching mechanism still lack a detailed understanding, i.e. how to control and modulate the electrical characteristics of devices incorporating defects and impurities, such as oxygen vacancies, metal interstitials, hydrogen, and other metallic atoms acting as dopants. In this paper, state of the art ab initio theoretical methods are employed to understand the effects that filamentary types of stable oxygen vacancy configurations in TiO(2) and NiO have on the electronic conduction. It is shown that strong electronic interactions between metal ions adjacent to oxygen vacancy sites results in the formation of a conductive path and thus can explain the 'ON' site conduction in these materials. Implication of hydrogen doping on electroforming is discussed for Pr(0.7)Ca(0.3)MnO(3) devices based on electrical characterization and FTIR measurements.     

Studies on switching mechanisms in Pd-nanodot embedded Nb2O5 memristors using scanning tunneling microscopy

Current imaging tunneling spectrum obtained from scanning tunneling microscopy has been used to probe the formation and/or rupture of conductive filaments responsible for bipolar switching in Pd nano-dots embedded Nb₂O₅ memristors. Filamentary conduction mechanism has been confirmed by scanning tunneling microscopy study using a Pt-Ir tip that enabled performing electroforming and reset operations at the nanoscale. The back and forth transition between the fully oxidized and metallic sub-oxide states of niobium under applied bias, as observed from X-ray photoelectron spectroscopy, is believed to be the source of bipolar switching in Nb₂O₅ memristors. The incorporation of Pd nanodots in Nb₂O₅ matrix plays a critical role by acting as an oxygen ion reservoir and/or by polarizing a large volume of oxygen vacancies. The formation and/or rupture of the conducting filaments through trapping-detrapping phenomena are found to boost the memristive switching performance.     

Memory and threshold resistance switching in Ni/NiO core-shell nanowires

We report on the first controlled alternation between memory and threshold resistance switching (RS) in single Ni/NiO core-shell nanowires by setting the compliance current (I(CC)) at room temperature. The memory RS is triggered by a high I(CC), while the threshold RS appears by setting a low I(CC), and the Reset process is achieved without setting a I(CC). In combination with first-principles calculations, the physical mechanisms for the memory and threshold RS are fully discussed and attributed to the formation of an oxygen vacancy (Vo) chain conductive filament and the electrical field induced breakdown without forming a conductive filament, respectively. Migration of oxygen vacancies can be activated by appropriate Joule heating, and it is energetically favorable to form conductive chains rather than random distributions due to the Vo-Vo interaction, which results in the nonvolatile switching from the off- to the on-state. For the Reset process, large Joule heating reorders the oxygen vacancies by breaking the Vo-Vo interactions and thus rupturing the conductive filaments, which are responsible for the switching from on- to off-states. This deeper understanding of the driving mechanisms responsible for the threshold and memory RS provides guidelines for the scaling, reliability, and reproducibility of NiO-based nonvolatile memory devices.     

退火温度对TiO2基电阻开关器件性能的影响

采用直流磁控溅射法在n+-Si上制备了TiO2薄膜,采用电子束蒸发镀膜仪在TiO2薄膜上沉积Au电极,获得了Au/TiO2/n+-Si结构的器件.研究了退火温度对薄膜结晶性能及器件电阻开关特性的影响.Au/TiO2/n+-Si结构的器件具有单极性电阻开关特性,置位(set)电压,复位(reset)电压、reset电流及功率的大小随退火温度的不同而不同,并基于灯丝理论对器件的电阻开关效应的工作机理进行了探讨.研究结果表明,500℃退火的器件具有良好的非易失性.器件高低阻态的阻值比大于103,其信息保持特性可达10年之久.在读写次数为100次时,器件仍具有电阻开关效应.     

Mechanisms of current conduction in Pt/BaTiO3/Pt resistive switching cell

The 80-nm-thickness BaTiO₃ (BT) thin film was prepared on the Pt/Ti/SiO₂/Si substrate by the RF magnetron sputtering technique. The Pt/BT/Pt/Ti/SiO₂/Si structure was investigated using X-ray diffraction and scanning electron microscopy. The current-voltage characteristic measurements were performed. The bipolar resistive switching behavior was found in the Pt/BT/Pt cell. The current-voltage curves were well fitted in different voltage regions at the high resistance state (HRS) and the low resistance state (LRS), respectively. The conduction mechanisms are concluded to be Ohmic conduction and Schottky emission at the LRS, while space-charge-limited conduction and Poole-Frenkel emission at the HRS. The electroforming and switching processes were explained in terms of the valence change mechanism, in which oxygen vacancies play a key role in forming conducting paths.     

A study of gold migration in Au-Y2O3-Y junctions by secondary ion mass spectrometry

Secondary ion mass spectrometry has been used to detect the migration of gold from the electrode into the oxide layer of Au-Y₂O₃-Y junctions during the electroforming process. The depth profiles obtained showed an increase of gold migration after the development of conductivity and again after the appearance of a voltage-controlled negative resistance when a forming voltage was applied. The results are discussed in terms of localized conduction at defects which are observed on the junction surface.     

Gas sensing applications of 1D-nanostructured zinc oxide: Insights from density functional theory calculations

Gas sensor devices have traditionally comprised thin films of metal oxides, with tin oxide, zinc oxide and indium oxide being some of the most common materials employed. With the recent discovery of novel metal oxide nanostructures, sensors comprising nano-arrays or single nanostructures have shown improved performance over the thin films. The improved response of the nanostructures to different gases has been primarily attributed to the highly single crystalline surfaces as well as large surface area of the nanostructures. In this paper the properties of clean and defected quasi one-dimensional ZnO nanostructures, including hexagonal and triangular nanowires, nanotubes and facetted nanotubes are reviewed. The adsorption of atoms and molecules on the ZnO nanostructures are also reviewed and the findings are compared to studies examining similar reactions on nanostructured metal oxide surfaces for sensing purposes. While both experimental and theoretical approaches have been employed to examine gas sensor reactions, this review focuses on studies that employ electronic structure calculations, which primarily concentrate on using density functional theory. Computational studies have been useful in elucidating the reaction mechanism, binding strength, charge transfer as well as other electronic and structural properties of the nanomaterials and the gas-sensor interaction. Despite these studies there are still significant areas of research that need to be pursued that will assist in the link between theoretical and experimental findings, as well as advancing the current chemical and physical understanding of these novel materials. A summary and outlook for future directions of this exciting area of research is also provided.     

Breaking the Current‐Retention Dilemma in Cation‐Based Resistive Switching Devices Utilizing Graphene with Controlled Defects

Cation‐based resistive switching (RS) devices, dominated by conductive filaments (CF) formation/dissolution, are widely considered for the ultrahigh density nonvolatile memory application. However, the current‐retention dilemma that the CF stability deteriorates greatly with decreasing compliance current makes it hard to decrease operating current for memory application and increase driving current for selector application. By centralizing/decentralizing the CF distribution, this current‐retention dilemma of cation‐based RS devices is broken for the first time. Utilizing the graphene impermeability, the cation injecting path to the RS layer can be well modulated by structure‐defective graphene, leading to control of the CF quantity and size. By graphene defect engineering, a low operating current (≈1 µA) memory and a high driving current (≈1 mA) selector are successfully realized in the same material system. Based on systematically materials analysis, the diameter of CF, modulated by graphene defect size, is the major factor for CF stability. Breakthrough in addressing the current‐retention dilemma will instruct the future implementation of high‐density 3D integration of RS memory immune to crosstalk issues.     

Improvement of resistive switching in Cu/ZnO/Pt sandwiches by weakening the randomicity of the formation/rupture of Cu filaments

We report an improvement in minimizing the dispersion of resistive switching (RS) parameters such as ON/OFF state resistances and switching voltages of Cu/ZnO/Pt structures in which ZnO films have been deposited at elevated temperature with N doping. This deposition process can enlarge the ZnO grain size and lessen grain boundaries while maintaining a high initial resistance since ZnO naturally shows n-type conductivity and N is a p-type dopant but with a low solubility. Cu filaments with a diameter of 15?nm are found to form at the ZnO grain boundaries. Therefore, fewer grain boundaries could depress the randomicity of the formation/rupture of Cu filaments and result in more stable RS performances. Such memory devices show a fast programming speed of 10?ns.     

Graphene oxide thin films for flexible nonvolatile memory applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.     

Controllable Organic Resistive Switching Achieved by One‐Step Integration of Cone‐Shaped Contact

Conductive filaments (CFs)‐based resistive random access memory possesses the ability of scaling down to sub‐nanoscale with high‐density integration architecture, making it the most promising nanoelectronic technology for reclaiming Moore's law. Compared with the extensive study in inorganic switching medium, the scientific challenge now is to understand the growth kinetics of nanoscale CFs in organic polymers, aiming to achieve controllable switching characteristics toward flexible and reliable nonvolatile organic memory. Here, this paper systematically investigates the resistive switching (RS) behaviors based on a widely adopted vertical architecture of Al/organic/indium‐tin‐oxide (ITO), with poly(9‐vinylcarbazole) as the case study. A nanoscale Al filament with a dynamic‐gap zone (DGZ) is directly observed using in situ scanning transmission electron microscopy (STEM) , which demonstrates that the RS behaviors are related to the random formation of spliced filaments consisting of Al and oxygen vacancy dual conductive channels growing through carbazole groups. The randomicity of the filament formation can be depressed by introducing a cone‐shaped contact via a one‐step integration method. The conical electrode can effectively shorten the DGZ and enhance the localized electric field, thus reducing the switching voltage and improving the RS uniformity. This study provides a deeper insight of the multiple filamentary mechanisms for organic RS effect.     

反应合成银氧化锡电接触材料抗熔蚀性研究

采用反应合成技术和传统粉末冶金技术制备银氧化锡(AgSnO2)电接触材料，利用千瓦级CO2激光器模仿电弧作用在试样表面产生局部熔化。对AgSnO2块体材料进行抗熔蚀性测试，对块体材料及冷拉拔的AgSnO2线材进行显微组织分析(扫描电镜、透射电镜)。研究结果表明，采用反应合成技术可以在银基体中合成尺寸细小、界面新鲜的SnO2颗粒，制备AgSnO2电接触材料；用反应合成法制备的AgSnO2材料中，微米级的SnO2颗粒系由纳米级的SnO2颗粒聚集而成；用反应合成法制备的AgSnO2电接触材料由于改变了银和SnO2的结合状态使材料的抗熔蚀性得到改善和提高。     

Physical Aspects of Process Control in Selective Laser Sintering of Metals

Direct selective laser sintering (SLS) is a layered manufacturing technique that can produce fully dense, functional components in high performance metals. In this review paper, a first step is taken towards identifying and understanding some of the important physical mechanisms in direct SLS. This study not only provides an insight into phenomena observed during direct SLS processing of a variety of metallic materials but also helps in selecting those materials that are most amenable to direct SLS processing. The physical mechanisms discussed include oxidation, non‐equilibrium wetting, epitaxial solidification, metal vaporization, and oxide purification. Understanding these mechanisms is crucial for the design of direct SLS machines, process development, and process control.