Oxide Resistive Random Access Memory Research Articles

Filament-Induced Anisotropic Oxygen Vacancy Diffusion and Charge Trapping Effects in Hafnium Oxide RRAM

The impacts of charge trapping and filament-induced anisotropy on the resistive switching mechanism in hafnia are formulated and quantified. Using ab initio calculations, we find the energetics and kinetics of oxygen vacancies in hafnium oxide resistive random access memory are strongly influenced by charge trapping at vacancy sites as well as the local crystal structure and presence of nearby conductive filaments.

Technological Exploration of RRAM Crossbar Array for Matrix-Vector Multiplication

Matrix-vector multiplication is the key operation for many computationally intensive algorithms. The emerging metal oxide resistive switching random access memory (RRAM) device and RRAM crossbar array have demonstrated a promising hardware realization of the analog matrix-vector multiplication with ultra-high energy efficiency. In this paper, we analyze the impact of both device level and circuit level non-ideal factors, including the nonlinear current-voltage relationship of RRAM devices, the variation of device fabrication and write operation, and the interconnect resistance as well as other crossbar array parameters. On top of that, we propose a technological exploration flow for device parameter configuration to overcome the impact of non-ideal factors and achieve a better trade-off among performance, energy, and reliability for each specific application. Our simulation results of a support vector machine (SVM) and Mixed National Institute of Standards and Technology (MNIST) pattern recognition dataset show that RRAM crossbar array based SVM is robust to input signal fluctuation but sensitive to tunneling gap deviation. A further resistance resolution test presents that a 6-bit RRAM device is able to realize a recognition accuracy around 90 %, indicating the physical feasibility of RRAM crossbar array based SVM. In addition, the proposed technological exploration flow is able to achieve 10.98 % improvement of recognition accuracy on the MNIST dataset and 26.4 % energy savings compared with previous work. Experimental results also show that more than 84.4 % power saving can be achieved at the cost of little accuracy reduction.

Impact of Post-Oxide Deposition Annealing on Resistive Switching in HfO<sub>2</sub>-Based Oxide RRAM and Conductive-Bridge RAM Devices

In this letter, we report the impact of post-oxide deposition annealing on the performance of HfO2-based resistive random access memory (RRAM) devices, namely, oxygen-ion-based oxide RRAM (OxRRAM) and Cu-ion-based conductive-bridge RAM (CBRAM) devices. Considerable degradation of the ON/OFF ratio and switching properties was observed in the OxRRAM devices after high-temperature annealing in vacuum, which is attributed to a considerable amount of oxygen vacancies created in the switching layer. However, the substantial improvement in device performance, such as stable switching, high switching cycles, decreased set/reset voltages, and near-100% device yield, were obtained in the CBRAM devices during the post-HfO2 deposition annealing in vacuum.

Effects of Varied Negative Stop Voltages on Current Self-Compliance in Indium Tin Oxide Resistance Random Access Memory

We have previously investigated the automatic current compliance property for indium tin oxide (ITO) resistance random access memory (RRAM). Traditionally, for the purpose of protecting RRAM, it is necessary to set equipment current compliance during the set and forming processes of RRAM devices. ITO RRAM devices, however, have an intrinsic capability to limit their current. This letter examines this ITO RRAM current compliance in depth by applying a varied stop-voltage measurement method, where different negative stop voltages were adopted to manipulate oxygen ions. Combined with material analysis and conduction current fitting, a model was established.

Bipolar resistive switching behaviours of perovskite CaBi4Ti4O15 thin films

The perovskite CaBi4Ti4O15 thin film for resistive random access memory device application has been investigated in this work. Experimentally, the CaBi4Ti4O15 thin films were deposited on the indium-tin oxide/glass substrates by radio-frequency (RF) magnetron sputtering and then annealed by rapid thermal annealing. The annealing effects on the bipolar resistive switching behaviours and the conduction mechanisms of the Al/CaBi4Ti4O15/indium-tin oxide resistive random access memory structure with a compliance current of 0·1 A were discussed in this work. The results show a stable bipolar resistive switching with a retention time of over 1 × 104 s and a high resistance state/low resistance state ratio larger than 5 × 102.

Performance improvement of amorphous indium–gallium–zinc oxide ReRAM with SiO2 inserting layer

In this study, the resistive switching performance of amorphous indium–gallium–zinc oxide (a-IGZO) resistive switching random-access memory (ReRAM) was improved by inserting a thin silicon oxide layer between silver (Ag) top electrode and a-IGZO resistive switching layer. Compared with the single a-IGZO layer structure, the SiO2/a-IGZO bi-layer structure exhibits the higher On/Off resistance ratio larger than 103, and the lower operation power using a smaller SET compliance current. In addition, good endurance and excellent retention characteristics were achieved. Furthermore, multilevel resistance states are obtained through adjusting SET compliance current and RESET stop voltage, which shows a promise for high-performance nonvolatile multilevel memory application.

Cycling-Induced SET-Disturb Failure Time Degradation in a Resistive Switching Memory

A new degradation mode with respect to write-disturb failure time due to SET/RESET cycling in a tungsten oxide resistive random access memory is reported. In a crossbar array memory, we find that a write-disturb failure time in high resistance state reduces suddenly by several orders of magnitude after certain SET/RESET cycles. This abrupt degradation is believed due to the creation of a new soft breakdown path in a switching dielectric by cycling stress. Although a memory window still remains after the degradation, the occurrence probability of over-SET state increases significantly. This cycling-induced degradation mode imposes a serious constraint on the number of SET-disturb pulses and thus an endurance cycle number in a resistive switching memory.

A novel method of identifying the carrier transport path in metal oxide resistive random access memory

Characterization of defect energy levels is of crucial importance to understand the carrier transport and conduction mechanism of a conducting filament. Currently, it is difficult to probe the defect energy level of a conducting filament in random access memory (RRAM) by experiment. Based on the activation energy of carrier transport from the first-principles calculations, we present a physical model correlating macroscopic I–V characteristics with material microstructure to analyze the defect energy level of a conducting filament in metal oxide RRAM. The carrier transport path in the conducting filament can be specially extracted using the defect energy level.

Role of Anode on Resistance Switching Phenomenon of Metal Oxide Resistive Random Access Memory

ABSTRACTAlthough the presence of oxygen reservoir is assumed in many theoretical models which explain resistive switching of ReRAM with an electrode/metal oxide (MO)/electrode structure, the location of oxygen reservoir is not clear. We have previously reported a method for preparing an extremely small ReRAM cell which has removable bottom electrode (BE), by using AFM cantilever. In this study, we used this cell structure to specify the location of oxygen reservoir. Since an anode is assumed to work as an oxygen reservoir in most models, we investigated the effect of changing anodes for the same filament on the presence or absence of the occurrence of reset switching. It was revealed that reset occurred independently of catalytic ability and Gibbs free energy (ΔG) of anode material. However, reset was caused by repairing oxygen vacancies of which filament consists when metals with high ΔG is used as an anode, whereas by oxidizing an anode when metals with low ΔG is used as an anode. This result suggests that the MO film works as an oxygen reservoir for anode with high ΔG, whereas an anode works as an oxygen reservoir for anode with low ΔG.

Highly reliable switching via phase transition using hydrogen peroxide in homogeneous and multi-layered GaZnO(x)-based resistive random access memory devices.

Here, we propose an effective method for improving the resistive switching characteristics of solution-processed gallium-doped zinc oxide (GaZnO(x)) resistive random access memory (RRAM) devices using hydrogen peroxide. Our results imply that solution processed GaZnO(x) RRAM devices could be one of the candidates for the development of low cost RRAM.

Controllable Set Voltage in Bilayer ZnO:SiO<sub>2</sub>/ZnO<sub><italic>x</italic></sub> Resistance Random Access Memory by Oxygen Concentration Gradient Manipulation

In this letter, we investigated oxygen ion concentration gradient method, which can manipulate the set voltage of zinc oxide-doped silicon oxide resistance random access memory. To analyze this method, the ITO/ZnO:SiO 2 /ZnOx/TiN bilayer structure was proposed and discussed. On the basis of the oxygen ions migration effect, the set voltage of the oxide-based resistive memory can be altered after a bias stress at the TiN electrode. The physical mechanism of the special resistive switching characteristics were depicted by the interaction between [O 2- ] gradient driving force and electrical force.

Microscopic and spectroscopic analysis of the nature of conductivity changes during resistive switching in silicon‐rich silicon oxide

AbstractRedox‐based resistive random access memory (RRAM) has the scope to greatly improve upon current methods of data storage, despite incomplete understandings of material switching mechanisms. We make use of atomic force microscopy (AFM), conductive atomic force microscopy (cAFM) and X‐ray photoelectron spectroscopy (XPS) to characterise the physical processes occurring in the changes in conductance state in silicon‐rich silicon oxide RRAM. Surface analyses of the insulating oxide layer of our devices are employed to establish the chemical and structural properties of pristine and switched states. The removal of oxygen from the active layer is observed to be concomitant with the appearance of varying degrees of surface distortion and regions of high conductivity in an otherwise‐insulating material. These results support the currently‐recognised model of a resistive switching mechanism that is reliant upon the migration of oxygen ions under an electrical bias. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mechanism Analysis of Nonvolatile Graphene Oxide Based Reram with Laterally Structured Device

There has been an increase in demand for novel nonvolatile-memory technology for low-power, high speed and high scalability because of development of digital technology. With this background, various types of memory are now enthusiastically developed to overcome the limitations of traditional Si-based memory which is difficulty to miniaturize the device and to advance high performance. Resistive random access memory (ReRAM) is a good candidate for next generation devices due to its high speed, low voltage consumption and high scalability. Recently flexible electronic devices such as wearable electronics and optoelectronics have attracted interest, so flexible ReRAM devices are also required. Metal oxide thin film based memory in ReRAM have hitherto been studied mainly as next generation nonvolatile memory devices and have shown good properties such as retention, endurance, and programming characteristics. However, ReRAM based on metal oxide thin film has limitations of fabrication process for flexible device due to their material properties. ReRAM using graphene oxide (GO) is the most promising candidate for the flexible device due to graphene oxide’s great electrical and mechanical properties. Graphene oxide ReRAM can be fabricated at room temperature on flexible substrates where metal oxides cannot be.Despite of its excellent properties, the resistive switching mechanisms remain unclear. It is generally assumed that filamentary theory and oxygen migration theory are the major mechanisms of resistive switching phenomena. In this work, we demonstrate the resistive switching mechanisms of ReRAM with laterally structured devices, which have two-terminal electrodes in the same plane. The laterally structured graphene oxide ReRAM devices are fabricated by simple spin-coating method on silicon dioxide substrates. The aluminum (Al) electrodes are deposited on spin-coated grapheme oxide layer by thermal evaporation as shown in Figure 1a. These Al/GO/Al lateral devices show memory properties similar to vertical structure as shown in Figure 1b. Conventional memory devices with vertical structure consist of a MIM (metal / insulator / metal) stack are hard to demonstrate the switching properties because the conducting paths are formed inside of the active layer, graphene oxide. In case of lateral structure device, conducting paths are formed on the surface of the active layer, where it’s easily observed the conducting paths directly using TEM or XPS analysis. Graphene oxide sheets have chemically reactive oxygen functional groups such as carboxyl, epoxy and hydrogen groups decorating the edges and the basal plane. It is assumed that these oxygen functional groups acting as the electron trap site influence the performance of the device. With functional group modification which can change the properties of graphene oxide, we can infer the role of oxygen functional groups in switching mechanisms.So, we can demonstrate the mechanism of ReRAM more clearly by lateral memory device.

Resistive Switching Modification by Ultraviolet Illumination in Transparent Electrode Resistive Random Access Memory

Photosensitivity to ultraviolet (UV) light for zinc oxide (ZnO) resistance random access memory (RRAM) with transparent electrode was investigated and characterized in this paper. The resistive switching properties were affected severely through oxygen manipulation by UV light irradiation. To clarify the switching mechanism, conduction current fitting was applied and meanwhile a reaction model was proposed to explain the origin of drastic current variation. UV light-assisted oxygen manipulation in ZnO RRAM is an efficient method to modify device switching behavior, and this investigation also unveils the interesting phenomenon of transparent electrode RRAM under UV light illumination condition.

Tri-Resistive Switching Behavior of Hydrogen Induced Resistance Random Access Memory

In this letter, the special role of hydrogen ions in hafnium doped silicon oxide resistive random access memory (RRAM) is presented. In addition to the more typical oxygen ion-dominated resistive switching, hydrogen ions were also observed to trigger a resistance transformation phenomenon, producing a tri-resistive device. Unlike a normal RRAM device, a hydrogen plasma-treated device is operated with a reversed voltage polarity, and the direction of hydrogen ion migration results in the chemical bonds breaking and repairing. By changing the voltage polarity and stop voltage, this tri-resistive behavior can be achieved. This particular hydrogen-induced switching behavior suggests a different RRAM switching mechanism and is finally explained by our model.

Characteristics of nitrogen plasma immersion ion implantation treatment on gadolinium oxide resistive switching random access memory

The effect of nitrogen plasma immersion ion implantation (PIII) treatment on the thin films of gadolinium oxide (Gd x O y ) for use in resistive random access memory (RRAM) applications is reported. From the X-ray photoelectron spectroscopic analysis, the formation of Gd-N bond was evidenced by a shift in the peak Gd 4d peaks towards lower binding energy. It was observed that the nitrogen PIII treatment enhanced the product yield of the Gd x O y RRAM up to 77%. With the nitrogen PIII treatment, the switching mechanism of the Gd x O y RRAMs changed from Schottky emission to space-charge-limited-current conduction, as determined by the current-voltage fitting and device-area-dependent measurements. The nitrogen PIII-treated Gd x O y RRAMs prepared in this study could provide a high resistance ratio of 10 4 , suitable for future application as non-volatile memory.

Performance improvement of gadolinium oxide resistive random access memory treated by hydrogen plasma immersion ion implantation

Characteristics improvement of gadolinium oxide (GdxOy) resistive random access memories (RRAMs) treated by hydrogen plasma immersion ion implantation (PIII) was investigated. With the hydrogen PIII treatment, the GdxOy RRAMs exhibited low set/reset voltages and a high resistance ratio, which were attributed to the enhanced movement of oxygen ions within the GdxOy films and the increased Schottky barrier height at Pt/GdxOy interface, respectively. The resistive switching mechanism of GdxOy RRAMs was dominated by Schottky emission, as proved by the area dependence of the resistance in the low resistance state. After the hydrogen PIII treatment, a retention time of more than 104 s was achieved at an elevated measurement temperature. In addition, a stable cycling endurance with the resistance ratio of more than three orders of magnitude of the GdxOy RRAMs can be obtained.

Effects of sidewall etching on electrical properties of SiOx resistive random access memory

The electroforming voltages (Vef) of silicon oxide resistive random access memory devices with oxide sidewall etched to different degrees are compared. The results show that the Vef is significantly reduced when more sidewall area is formed, and Vef of around 17 V is achieved in devices with maximum sidewall area. Plausible electroforming and state switching mechanisms are discussed using a filament-gap model. Endurance measurements up to 107 pulse cycles are compared for different device types. An external series resistance may be helpful for decreasing voltage stress during pulsed cycling to help enable device survival beyond 107 pulse cycles.

Characterization of Resistive Switching Behaviors of RF Sputtered Si Oxide Resistive Random Access Memories with Ti-Based Electrodes

We have fabricated metal–insulator–metal (MIM) diodes with a radio frequency (RF) sputtered Si oxide (SiOx) dielectric layer and studied the impact of three kinds of top electrodes (Ti, TiN, and Pt) on the resistive switching characteristics of the MIM diodes by current–voltage (I–V) measurements. For the MIM diodes with Ti-based electrodes, a significant increase in the initial current level and a decrease in the ON/OFF resistance rate were observed as compared to those of the reference MIM diodes with the Pt top electrodes. To gain a better understanding of the changes in the current levels with the top electrodes, the chemical bonding features in the region near the interface between the top electrode and SiOx layer were investigated using hard X-ray photoemission spectroscopy (HAXPES) under synchrotron radiation (hν= 7939 eV). From HAXPES analyses, it was found that the SiOx surface partially reacted with the Ti and TiN top electrodes during the deposition. Due to the formation of a TiOx barrier layer at the interface between the TiN top electrode and SiOx layer, distinct bi-polar type resistive switching with lower operation voltages below 2.0 V has been achieved.

Dynamics of percolative breakdown mechanism in tantalum oxide resistive switching

Switching dynamics are studied for tantalum oxide resistive random access memory subjected to long-duration constant current pulses for both SET and RESET transitions. The processes draw parallels to the widely studied percolation model for dielectric breakdown. The RESET transition is shown to consist of changes to critical local conduction sites and their effect on performance parameters such as switching speed and energy are discussed. Additionally, the SET transition shows an unexpected minimum stable resistance state. When driven below that state the device is found to increase resistance, returning to the stable state.