Redox-based Resistive Random Access Memory Research Articles

Oxygen Diffusion in Platinum Electrodes: A Molecular Dynamics Study of the Role of Extended Defects

AbstractPlatinum serves as a model electrode in solid‐state electrochemistry and as the inert electrode in redox‐based resistive random‐access memory (ReRAM) technology. Experimental work has proposed that oxygen may diffuse faster along platinum's extended defects, but quantitative, unambiguous transport data do not exist. In this study, the diffusion of oxygen atoms in crystalline platinum and along its extended defects is studied as a function of temperature by means of molecular dynamics (MD) simulations with the ReaxFF interatomic potentials. The MD simulations indicate that platinum vacancies trap oxygen atoms, inhibiting their diffusion through the platinum lattice and leading to a high activation enthalpy of diffusion of around 3 eV. This picture of trapping is supported by static density‐functional‐theory calculations. MD simulations of selected dislocations and selected grain boundaries indicate that oxygen diffusion is much faster along these extended defects than through the Pt lattice at temperatures below 1400 K, exhibiting a much lower activation enthalpy of ≈0.7 eV for all extended defects examined. Producing specific electrode microstructures with controlled densities and types of extended defects thus offers a new avenue to improve the performance of ReRAM devices and to prevent device failure.

A nanoscale analysis method to reveal oxygen exchange between environment, oxide, and electrodes in ReRAM devices

The limited sensitivity of existing analysis techniques at the nanometer scale makes it challenging to systematically examine the complex interactions in redox-based resistive random access memory (ReRAM) devices. To test models of oxygen movement in ReRAM devices beyond what has previously been possible, we present a new nanoscale analysis method. Harnessing the power of secondary ion mass spectrometry, the most sensitive surface analysis technique, for the first time, we observe the movement of 16O across electrically biased SiOx ReRAM stacks. We can therefore measure bulk concentration changes in a continuous profile with unprecedented sensitivity. This reveals the nanoscale details of the reversible field-driven exchange of oxygen across the ReRAM stack. Both the reservoir-like behavior of a Mo electrode and the injection of oxygen into the surface of SiOx from the ambient are observed within one profile. The injection of oxygen is controllable through changing the porosity of the SiOx layer. Modeling of the electric fields in the ReRAM stacks is carried out which, for the first time, uses real measurements of both the interface roughness and electrode porosity. This supports our findings helping to explain how and where oxygen from ambient moisture enters devices during operation.

Neuromorphic Low-Power Inference on Memristive Crossbars With On-Chip Offset Calibration

Monolithic integration of silicon with nano-sized Redox-based resistive Random-Access Memory (ReRAM) devices opened the door to the creation of dense synaptic connections for bio-inspired neuromorphic circuits. One drawback of OxRAM based neuromorphic systems is the relatively low ON resistance of OxRAM synapses (in the range of just a few kilo-ohms). This requires relatively large currents (many micro amperes per synapse), and therefore imposes strong driving capability demands on peripheral circuitry, limiting scalability and low power operation. After learning, however, a read inference can be made low-power by applying very small amplitude read pulses, which require much smaller driving currents per synapse. Here we propose and experimentally demonstrate a technique to reduce the amplitude of read inference pulses in monolithic neuromorphic CMOS OxRAM-synaptic crossbar systems. Unfortunately, applying tiny read pulses is non-trivial due to the presence of random DC offset voltages. To overcome this, we propose finely calibrating DC offset voltages using a bulk-based three-stage on-chip calibration technique. In this work, we demonstrate spiking pattern recognition using STDP learning on a small 4×4 proof-of-concept memristive crossbar, where on-chip offset calibration is implemented and inference pulse amplitude could be made as small as 2mV. A chip with pre-synaptic calibrated input neuron drivers and a 4×4 1T1R synapse crossbar was designed and fabricated in the CEA-LETI MAD200 technology, which uses monolithic integration of OxRAMs above ST130nm CMOS. Custom-made PCBs hosting the post-synaptic circuits and control FPGAs were used to test the chip in different experiments, including synapse characterization, template matching, and pattern recognition using STDP learning, and to demonstrate the use of on-chip offset-calibrated low-power amplifiers. According to our experiments, the minimum possible inference pulse amplitude is limited by offset voltage drifts and noise. We conclude the paper with some suggestions for future work in this direction.

Improved Switching Stability and the Effect of an Internal Series Resistor in HfO2/TiO<italic>x</italic> Bilayer ReRAM Cells

Bipolar redox-based resistive random-access memory cells are intensively studied for new storage class memory and beyond von Neumann computing applications. However, the considerable variability of the resistance values in ON and OFF state as well as of the SET voltage remains challenging. In this paper, we discuss the physical origin of the significant reduction in the switching variability of HfO2-based devices achieved by the insertion of a thin TiO x layer between the HfO2 layer and the oxygen exchange metal layer. Typically, HfO2 single layer cells exhibit an abrupt SET process, which is difficult to control. In contrast, self-compliance effects in the HfO2/TiO x bilayer devices lead to an increased stability of SET voltages and OFF-state resistances. The SET process is gradual and the RESET becomes abrupt for higher switching currents. Comparison of the experimental data with simulation results achieved from a physics-based compact model for the full description of the switching behavior of the single layer and bilayer devices clearly reveal three major effects. The TiO x layer affects the temperature distribution during switching (by modifying the heat dissipation), forms an additional series resistance and changes the current conduction mechanism in the OFF state of the bilayer device compared to the single layer device.

Correlation between the transport mechanisms in conductive filaments inside Ta2O5-based resistive switching devices and in substoichiometric TaOx thin films

Conductive filaments play a key role in redox-based resistive random access memory (ReRAM) devices based on the valence change mechanism, where the change of the resistance is ascribed to the modulation of the oxygen content in a local region of these conductive filaments. However, a deep understanding of the filaments' composition and structure is still a matter of debate. We approached the problem by comparing the electronic transport, at temperatures from 300 K down to 2 K, in the filaments and in TaOx films exhibiting a substoichiometric oxygen content. The filaments were created in Ta (15 nm)/Ta2O5 (5 nm)/Pt crossbar ReRAM structures. In the TaOx thin films with various oxygen contents, the in-plane transport was studied. There is a close similarity between the electrical properties of the conductive filaments in the ReRAM devices and of the TaOx films with x ∼ 1, evidencing also no dimensionality difference for the electrical transport. More specifically, for both systems there are two different conduction processes: one in the higher temperature range (from 50 K up to ∼300 K), where the conductivity follows a T dependence, and one at lower temperatures (<50 K), where the conductivity follows the exp(−1/T) dependence. This suggests a strong similarity between the material composition and structure of the filaments and those of the substoichiometric TaOx films. We also discuss the temperature dependence of the conductivity in the framework of possible transport mechanisms, mainly of those normally observed for granular metals.

Competition between Metallic and Vacancy Defect Conductive Filaments in a CH3NH3PbI3-Based Memory Device

Ion migration, which can be classified into cation migration and anion migration, is at the heart of redox-based resistive random access memory. However, the coexistence of these two types of ion migration and the resultant conductive filaments (CFs) have not been experimentally demonstrated in a single memory cell. Here we investigate the competition between metallic and vacancy defect CFs in a Ag/CH3NH3PbI3/Pt structure, where Ag and CH3NH3PbI3 serve as the top electrode and memory medium, respectively. When the medium layer thickness is hundreds of nanometers, the formation/diffusion of iodine vacancy (VI) CFs dominates the resistive switching behaviors. The VI-based CFs provide a unique opportunity for the electrical-write and optical-erase operation in a memory cell. The Ag CFs emerge and coexist with VI ones as the medium layer thickness is reduced to ∼90 nm. Our work not only enriches the mechanisms of the resistive switching but also would advance the multifunctionalization of resistive random acc...

Concurrent ionic migration and electronic effects at the memristive TiOx/La1/3Ca2/3MnO3−x interface

The development of reliable redox-based resistive random-access memory devices requires understanding and disentangling concurrent effects present at memristive interfaces. We report on the fabrication and electrical characterization of TiOx/La1/3Ca2/3MnO3−x microstructured interfaces and on the modeling of their memristive behavior. We show that a careful tuning of the applied external electrical stimuli allows controlling the redox process between both layers, obtaining multilevel non-volatile resistance states. We simulate the oxygen vacancies dynamics at the interface between both oxides, and successfully reproduce the experimental electrical behavior after the inclusion of an electronic effect, related to the presence of an n-p diode at the interface. The formation of the diode is due to the n- and p-character of TiOx and La1/3Ca2/3MnO3−x, respectively. Our analysis indicates that oxygen vacancies migration between both layers is triggered after the diode is polarized either in forward mode or in reverse mode above breakdown. Electrical measurements at different temperatures suggest that the diode can be characterized as Zener-type. The advantages of our junctions for their implementation in RRAM devices are finally discussed.

Resistive Switching in Aqueous Nanopores by Shock Electrodeposition

Solid-state programmable metallization cells have attracted considerable attention as memristive elements for Redox-based Resistive Random Access Memory (ReRAM) for low-power and low-voltage applications. In principle, liquid-state metallization cells could offer the same advantages for aqueous systems, such as biomedical lab-on-a-chip devices, but robust resistive switching has not yet been achieved in liquid electrolytes, where electrodeposition is notoriously unstable to the formation of fractal dendrites. Here, the recently discovered physics of shock electrodeposition are harnessed to stabilize aqueous copper growth in polycarbonate nanopores, whose surfaces are modified with charged polymers. Stable bipolar resistive switching is demonstrated for 500 cycles with <10s retention times, prior to any optimization of the geometry or materials.

Impact of oxygen exchange reaction at the ohmic interface in Ta2O5-based ReRAM devices.

Interface reactions constitute essential aspects of the switching mechanism in redox-based resistive random access memory (ReRAM). For example, the modulation of the electronic barrier height at the Schottky interface is considered to be responsible for the toggling of the resistance states. On the other hand, the role of the ohmic interface in the resistive switching behavior is still ambigious. In this paper, the impact of different ohmic metal-electrode (M) materials, namely W, Ta, Ti, and Hf on the characteristics of Ta2O5 ReRAM is investigated. These materials are chosen with respect to their free energy for metal oxide formation and, associated, their impact on the formation energy of oxygen vacancy defects at the M/Ta2O5 interface. The resistive switching devices with Ti and Hf electrodes that have a negative defect formation energy, show an early RESET failure during the switching cycles. This failure process with Ti and Hf electrode is attributed to the accumulation of oxygen vacancies in the Ta2O5 layer, which leads to permanent breakdown of the metal-oxide to a low resistive state. In contrast, the defect formation energy in the Ta2O5 with respect to Ta and W electrodes is positive and for those highly stable resistive switching behavior is observed. During the quasi-static and transient-pulse characterization, the ReRAM devices with the W electrode consistently show an increased high resistance state (HRS) than with the Ta electrode for all RESET stop voltages. This effect is attributed to the faster oxygen exchange reaction at the W-electrode interface during the RESET process in accordance to lower stability of WO3 than Ta2O5. Based on these findings, an advanced resistive switching model, wherein also the oxygen exchange reaction at the ohmic M-electrode interface plays a vital role in determining of the resistance states, is presented.

Phase-Change and Redox-Based Resistive Switching Memories

This paper addresses the two main resistive switching (RS) memory technologies: phase-change memory (PCM) and redox-based resistive random access memory (ReRAM). It will review the basic concepts, the initial promises, and current state of the art, with focus on possible scaling pathways for low-power operation and dense, true 3-D memory. Recent physical insights and new potential concepts will be discussed.