Electrochemical Metallization Memory Research Articles

Demonstration of bipolar resistive memory fabricated using an ultra-thin BaTiOx resistive switching layer with a thickness of ∼5 nm

In this study, a high-performance non-volatile bipolar resistive random-access memory (RRAM), which was fabricated using an ultra-thin barium titanate (BaTiOx, BTO) film as a resistive switching (RS) layer, was demonstrated. The BTO RS layers, whose thicknesses were only about 5 nm, were prepared using radio-frequency sputtering under different oxygen flow rates. Introduction of oxygen was used to modify the chemical compositions of the BTO films. Copper was used as the top electrode material to realize a Cu/BTO/n+-Si electrochemical metallization memory, where the resistance switching was triggered by electrochemical reaction of Cu electrode. The RRAM could repeatedly and consistently switch between a high-resistance state and a low-resistance state over 300 cycles. A large memory window of 105 and a long data retention time of >1.5 × 104 s both at room temperature and 85 °C were observed. Moreover, the Cu/BTO/n+-Si RRAM showed high switching speeds of 60–70 ns.

Variability-aware modeling of electrochemical metallization memory cells

Abstract Resistively switching electrochemical metallization memory cells are gaining huge interest since they are seen as promising candidates and basic building blocks for future computation-in-memory applications. However, especially filamentary-based memristive devices suffer from inherent variability, originating from their stochastic switching behavior. A variability-aware compact model of electrochemical metallization memory cells is presented in this study and verified by showing a fit to experimental data. It is an extension of the deterministic model. Since this extension consists of several different features allowing for a realistic variability-aware fit, it depicts a unique model comprising physics-based, stochastically and experimentally originating variabilities and reproduces them well. In addition, a physics-based model parameter study is executed, which enables a comprehensive view into the device physics and presents guidelines for the compact model fitting procedure.

Fractional Marcus-Hush-Chidsey-Yakopcic current-voltage model for redox-based resistive memory devices.

We propose a circuit-level model combining the Marcus-Hush-Chidsey electron current equation and the Yakopcic equation for the state variable for describing resistive switching memory devices of the structure metal-ionic conductor-metal. We extend the dynamics of the state variable originally described by a first-order time derivative by introducing a fractional derivative with an arbitrary order between zero and one. We show that the extended model fits with great fidelity the current-voltage characteristic data obtained on a Si electrochemical metallization memory device with Ag-Cu alloy.

Resistive Switching: Physics, Devices and Applications

Resistive Switching: Physics, Devices and Applications

SET Kinetics of Ag/HfO2-Based Diffusive Memristors under Various Counter-Electrode Materials

The counter-electrode (CE) material in electrochemical metallization memory (ECM) cells plays a crucial role in the switching process by affecting the reactions at the CE/electrolyte interface. This is due to the different electrocatalytic activity of the CE material towards reduction-oxidation reactions, which determines the metal ion concentration in the electrolyte and ultimately impacts the switching kinetics. In this study, the focus is laid on Pt, TiN, and W, which are relevant in standard chip technology. For these, the influence of CE metal on the switching kinetics of Ag/HfO2-based volatile ECM cells is investigated. Rectangular voltage pulses of different amplitudes were applied, and the SET times were analyzed from the transient curves. The results show that CE material has a significant effect on the SET kinetics, with differences being observed depending on the voltage regime. The formation of interfacial oxides at the CE/electrolyte interface, particularly for non-noble metals, is also discussed in relation to the findings. Overall, this work highlights the important role of the CE material in the switching process of Ag/HfO2-based diffusive memristors and the importance of considering interfacial oxide formation in the design of these devices.

Whisky‐Born Memristor

It is recently shown that when a drop of Glenlivet whisky evaporates, it leaves behind a uniform deposit [Phys. Rev. Lett. 116, 124501 (2016)]. This fascinating finding is utilized in the fabrication of electrochemical metallization memory (ECM) cells. The top (Ag) and bottom (Co) electrodes in the proposed structure are separated by a layer of Glenlivet whisky deposit (an insulator). Measurements show the device response is typical of ECM cells that involve threshold‐type switching, pinched hysteresis loops, and a large difference between the high‐ and low‐resistance states. The surface coating process used in these experiments simplifies the device fabrication and results in a biodegradable insulating layer, which may facilitate the recovery of recyclable materials at the end of the device's use.

Experimental and Modeling Study of Metal-Insulator Interfaces to Control the Electronic Transport in Single Nanowire Memristive Devices.

Memristive devices relying on redox-based resistive switching mechanisms represent promising candidates for the development of novel computing paradigms beyond von Neumann architecture. Recent advancements in understanding physicochemical phenomena underlying resistive switching have shed new light on the importance of an appropriate selection of material properties required to optimize the performance of devices. However, despite great attention has been devoted to unveiling the role of doping concentration, impurity type, adsorbed moisture, and catalytic activity at the interfaces, specific studies concerning the effect of the counter electrode in regulating the electronic flow in memristive cells are scarce. In this work, the influence of the metal-insulator Schottky interfaces in electrochemical metallization memory (ECM) memristive cell model systems based on single-crystalline ZnO nanowires (NWs) is investigated following a combined experimental and modeling approach. By comparing and simulating the electrical characteristics of single NW devices with different contact configurations and by considering Ag and Pt electrodes as representative of electrochemically active and inert electrodes, respectively, we highlight the importance of an appropriate choice of electrode materials by taking into account the Schottky barrier height and interface chemistry at the metal-insulator interfaces. In particular, we show that a clever choice of metal-insulator interfaces allows to reshape the hysteretic conduction characteristics of the device and to increase the device performance by tuning its resistance window. These results obtained from single NW-based devices provide new insights into the selection criteria for materials and interfaces in connection with the design of advanced ECM cells.

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Rainer Waser – A Pioneer of Fundamentals of Resistive Switching Memories

Reconfigurable multivalued memristor FPGA model for digital recognition

AbstractCompared with the traditional memristor, it's more significant to research the multivalued memristor in improving the stability and reliability of memristive neural networks. Developing some multivalued memristor emulators is highly attractive since the fabrication and integration of the memristor are not mature at present. This work presents a behavioral‐level model of a general multivalued memristor FPGA that exhibits continuous or discrete behavior, similar to electrochemical metallization memories. The proposed solution has been successfully synthesized and verified on a Xilinx ZYNQ‐7000 FPGA XQ7Z020 with less than 1% hardware utilization. Additionally, to test the synaptic function of the memristor model in artificial neural networks, this paper constructs a quantized artificial neural network based on eight‐valued memristors in FPGA. In the training of the neural network, the pixel values of the images and the network weights are quantized and then mapped to the input voltage and conductance respectively of the memristor during the forward propagation. The experimental results show that the memristive network circuit achieves 92.8% recognition accuracy for 10,000 MNIST images.

Photocatalysis‐Induced Nanopores toward Highly Reliable Organic Electrochemical Metallization Memory

AbstractOrganic electrochemical metallization (ECM) memory that possesses high reliable switching performance is in great demand for the future smart wearable and flexible electronics. The resistive switching (RS) behavior of organic ECM memory is determined by the micro‐morphology evolution of metal conductive filaments (CFs) during the operating process. However, the morphology controllability of CFs is generally deteriorated by their random distribution and unexpected overgrowth. Herein, a kind of nanoporous PVC/TiO2 nanocomposite is developed using photocatalytic method for improving the RS reliability of organic ECM memory. The introduction of engineered nanopores can effectively simplify the CFs morphology to obtain excellent uniformity, good retention, and low cycling degradation. In addition, taking advantage of the multilevel RS behavior and 200 × 200 memristive array artificial neural network (ANN), alphabetic data storage and image pattern recognition are successfully realized. The proposed approach is expected to provide novel platforms for the advance of highly reliable flexible electronics and ANN for intelligence applications.

Cu x S thin films for printed memory cells and temperature sensors

Printed electronics require a multitude of various inks for different applications which leads to compatibility issues for their integration. We present a procedure to deposit a thin layer of Cu x S via inkjet printing of Na2S on a thermally grown or inkjet-printed Cu surface that provides applications in electrochemical metallization memory cells (ECMs) or temperature sensors. The nanosized transformation from Cu to Cu x S is investigated via confocal microscopy, scanning electron microscopy (SEM), as well as energy-dispersive x-ray spectroscopy (EDX). We analyze individual responses from the sensor and memory and evaluate their respective potential in printed electronics. The negative temperature coefficient of the semiconducting Cu x S is determined to be K. Resistive switching is observed for a current compliance between 0.1 and 1000 µA, with a resistance ratio R/R up to 105. The use of the same inks and processes for the memory and sensor components paves the way for new and customized designs for smart logistics applications where temperature monitoring is required.

Investigation of deposition technique and thickness effect of HfO2 film in bilayer InWZnO-based conductive bridge random access memory

In recent research on memristor of brain-mimicking integrated circuit (IC) architecture, the electrical performance of electrochemical metallization memory can be significantly improved by applying a bilayer switching structure. In this work, a novel amorphous semiconductor InWZnO (IWZO) is used as the main switching layer, and a bilayer structure with HfO2 is fabricated by different deposition technique. By surface and chemical composition analysis, the effect of deposition process and thickness of HfO2 have been fully explored. The bilayer memory inserted with 3 nm HfO2 formed by atomic layer deposition process exhibits excellent electrical properties, such as high endurance cycle (more than 2 × 103), better retention time (up to 2 × 104 s, 85 °C), relatively uniform resistance state distribution and low compliance current operation. Consequently, the low power electrochemical metallization memory with optimized HfO2 layer has great potential for next generation memory in pixel and three-dimensional brain-mimicking IC technology.

Memristive devices based on single ZnO nanowires—from material synthesis to neuromorphic functionalities

Memristive and resistive switching devices are considered promising building blocks for the realization of artificial neural networks and neuromorphic systems. Besides conventional top-down memristive devices based on thin films, resistive switching devices based on nanowires (NWs) have attracted great attention, not only for the possibility of going beyond current scaling limitations of the top-down approach, but also as model systems for the localization and investigation of the physical mechanism of switching. This work reports on the fabrication of memristive devices based on ZnO NWs, from NW synthesis to single NW-based memristive cell fabrication and characterization. The bottom-up synthesis of ZnO NWs was performed by low-pressure chemical vapor deposition according to a self-seeding vapor-solid (VS) mechanism on a Pt substrate over large scale (∼cm2), without the requirement of previous seed deposition. The grown ZnO NWs are single crystalline with wurtzite crystal structure and are vertically aligned respect to the growth substrate. Single NWs were then contacted by means of asymmetric contacts, with an electrochemically active and an electrochemically inert electrode, to form NW-based electrochemical metallization memory cells that show reproducible resistive switching behaviour and neuromorphic functionalities including short-term synaptic plasticity and paired pulse facilitation. Besides representing building blocks for NW-based memristive and neuromorphic systems, these single crystalline devices can be exploited as model systems to study physicochemical processing underlaying memristive functionalities thanks to the high localization of switching events on the ZnO crystalline surface.

Design of Materials Configuration for Optimizing Redox-Based Resistive Switching Memories.

Redox-based resistive random access memories (ReRAMs) are based on electrochemical processes of oxidation and reduction within the devices. The selection of materials and material combinations strongly influence the related nanoscale processes, playing a crucial role in resistive switching properties and functionalities. To date, however, comprehensive studies on device design accounting for a combination of factors such as electrodes, electrolytes, and capping layer materials related to their thicknesses and interactions are scarce. In this work, the impact of materials' configuration on interfacial redox reactions in HfO2 -based electrochemical metallization memory (ECM) and valence-change memory (VCM) systems is reported. The redox processes are studied by cyclic voltammetry, and the corresponding resistive switching characteristics are investigated. In ECM cells, the overall cell resistance depends on the electrocatalytic activity of the counter electrode. Nonetheless, the capping layer material further influences the cell resistance and the SET and RESET voltages. In VCM systems, the influence of the electrode material configuration is also pronounced, and is capable of modulating the active resistive switching interface. For both types of memory cells, the switching behavior changes significantly with variation of the oxide thickness. The results present important materials selection criteria for rationale design of ReRAM cells for various memristive applications.

Thermal-assisted electroforming enables performance improvement by suppressing the overshoot current in amorphous carbon-based electrochemical metallization memory

A thermal-assisted electroforming (TAE) method is proposed to address the current overshoot issue and improve the resistive switching (RS) performance of electrochemical metallization (ECM) memory with a Cu/amorphous carbon (a-C)/Pt structure. In the initial electroforming process, thermal treatment can promote the electrochemical formation of metallic conductive filament (CF). The required electroforming voltage is reduced, and the undesirable overshoot current is suppressed. As a result, the RS performance of the cell is improved, including reduced RS parameter fluctuations, enlarged off/on ratio, and enhanced cycling endurance. The cell can be operated with a low compliance current (ICC) of 50 μA, which is among the lowest value of reported a-C-based ECM memories. Benefit from the low ICC, multilevel memory with five nonvolatile resistance states in a single cell is obtained. The main role of the TAE method is interpreted as to avoid the extreme high-temperature caused by the overshoot current. Conductive-atomic force microscopy mapping implies that the suppression of overshoot current can avoid over-injection of Cu cations into the a-C layer, facilitating CF with a simple structure and low randomness. The present work offers a feasible approach for addressing the current overshoot issue and improving the performance of ECM memory.

Structure‐Dependent Influence of Moisture on Resistive Switching Behavior of ZnO Thin Films

AbstractResistive switching mechanisms underlying memristive devices are widely investigated, and the importance as well as influence of ambient conditions on the electrical performances of memristive cells are already recognized. However, detailed understanding of the ambient effect on the switching mechanism still remains a challenge. This work presents an experimental investigation on the effect of moisture on resistive switching performances of ZnO‐based electrochemical metallization memory cells. ZnO thin films are grown by chemical vapor deposition (CVD) and radio frequency sputtering. Water molecules are observed to influence electrical resistance of ZnO by affecting the electronic conduction mechanism and by providing additional species for ionic conduction. By influencing dissolution and migration of ionic species underlying resistive switching events, moisture is reported to tune resistive switching parameters. In particular, the presence of H2O is responsible for a decrease of the forming and SET voltages and an increase of the ON/OFF resistance ratio in both CVD and sputtered films. The effect of moisture on resistive switching performance is found to be more pronounced in case of sputtered films where the reduced grain size is responsible for an increased adsorption of water molecules and an increased amount of possible pathways for ion migration.

Dual Buffer Layers for Developing Electrochemical Metallization Memory With Low Current and High Endurance

In this letter, the dual buffer layers of AgInSbTe (AIST) and monolayer defective graphene (DG) are introduced into amorphous carbon (a-C)-based electrochemical metallization (ECM) type memristor. The advantages of promoting Ag dissolution and localizing cation diffusion region of AIST and DG layers can effectively suppress the well-known current overshoot issue and break the current-retention dilemma. Compared with its counterpart without buffer layer (Ag/a-C/Pt), the device with dual buffer layers (Ag/AIST/DG/a-C/Pt) presents excellent resistive switching (RS) performance, such as lower forming voltage with reduced overshoot current, smaller parameter fluctuations, and better cycling endurance. More importantly, the nonvolatile RS with low operation current of 10 μA and remarkable endurance (> 3.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cycles) is realized, which is among the best of reported a-C-based ECM memories. Benefit from the low operation current, the device also shows the capacity of multilevel memory with six nonvolatile resistance states through regulating the compliance current.

Neutron irradiation-induced effects on the reliability performance of electrochemical metallization memory devices

In this work, electrochemical metallization memory (ECM) devices with an Ag/AgInSbTe (AIST)/amorphous carbon (a-C)/Pt structure were irradiated with 14 MeV neutrons. The switching reliability performance before and after neutron irradiation was compared and analyzed in detail. The results show that the irradiated memory cells functioned properly, and the initial resistance, the resistance at the low-resistance state (LRS), the RESET voltage and the data retention performance showed little degradation even when the total neutron fluence was as high as 2.5 × 1011 n/cm2. Other switching characteristics such as the forming voltage, the resistance at the high-resistance state (HRS), and the SET voltage were also studied, all of which merely showed a slight parameter drift. Irradiation-induced Ag ions doping of the a-C layer is proposed to explain the damaging effects of neutron irradiation. The excellent hard characteristics of these Ag/AIST/a-C/Pt-based ECM devices suggest potential beneficial applications in the aerospace and nuclear industries.

Electric field induced charge migration and formation of conducting filament during resistive switching in electrochemical metallization (ECM) memory cells

Although it has been commonly accepted that resistive random access memories (ReRAMs) operate by exploiting the electric field to form or break the conducting filaments (CFs), the formation of CF still remains elusive and so is the correlation of physiochemical properties with electrical responses. Using the phase field model, we show the complete correlation between the physiochemical process driven CF formation and the electrical responses in ReRAM systems, specifically in the electrochemical metallization (ECM) memory cells. We could qualitatively validate all the unique I–V characteristics (associated with CF formation) available in the experimental literature for inorganic as well as hybrid organic–inorganic ReRAM systems. We believe that the universal description and the obtained qualitative validation of the model could shape the future ReRAM research where the formation of CF plays the dominant role.

An Experimental Proof that Resistance‐Switching Memory Cells are not Memristors

AbstractIt has been suggested that all resistive‐switching memory cells are memristors. The latter are hypothetical, ideal devices whose resistance, as originally formulated, depends only on the net charge that traverses them. Recently, an unambiguous test has been proposed to determine whether a given physical system is indeed a memristor or not. Here, such a test is experimentally applied to both in‐house fabricated Cu‐SiO2 and commercially available electrochemical metallization cells. The results unambiguously show that electrochemical metallization memory cells are not memristors. Since the particular resistance‐switching memories employed in the study share similar features with many other memory cells, the findings refute the claim that all resistance‐switching memories are memristors. They also cast doubts on the existence of ideal memristors as actual physical devices that can be fabricated experimentally. The results then lead to formulate two memristor impossibility conjectures regarding the impossibility of building a model of physical resistance‐switching memories based on the memristor model.