Formation Of Metallic Filaments Research Articles

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.

Functional bipolar resistive switching in AlN/Ni–Mn–In based magnetoelectric heterostructure

This report explores the influence of temperature on resistive switching characteristics in the AlN/Ni–Mn–In magnetoelectric (ME) heterostructure-based resistive random access memory (ReRAM) device. The fabricated Cu/AlN/Ni–Mn–In/Si device exhibits a sharp transition from a high resistance state (HRS) to low resistance state (LRS) at a SET voltage. The rupture of the filament from its weakest point at a RESET voltage turn the device back to its HRS. The stable bipolar resistive switching behavior is described by the current–voltage (I–V) characteristic. The HRS and LRS are explained by the trap-controlled space charge limited conduction mechanism and a well-known Ohmic conduction mechanism, respectively. The temperature-dependent resistance has been observed to further confirm the conduction mechanism in HRS and LRS. The current conduction in LRS is explained by an analytical model based on copper metallic filament formation via Cu+ migration from the top to the bottom electrode. A significant change in the SET voltage has been observed with the decrease in temperature. This variation in the SET voltage is explained via strain-mediated coupling in interfacially connected AlN/Ni–Mn–In ME heterostructure. The fabricated device displays an appreciable OFF/ON ratio of the order ∼3 × 103 with good endurance and retention of ∼1000 cycles and ∼900 s, respectively. A slight variation (<40%) in SET and RESET voltages has been observed for total endurance cycles. This study demonstrates the importance of ME heterostructure for futuristic tuneable ReRAM applications.

Two-Dimensional MXene Synapse for Brain-Inspired Neuromorphic Computing.

MXenes, an emerging class of two-dimensional (2D) transition metal carbides and nitrides, have attracted wide attention because of their fascinating properties required in functional electronics. Here, an atomic-switch-type artificial synapse fabricated on Ti3 C2 Tx MXene nanosheets with lots of surface functional groups, which successfully mimics the dynamics of biological synapses, is reported. Through in-depth analysis by X-ray photoelectron spectroscopy, transmission electron microscopy, and energy dispersive X-ray spectroscopy, it is found that the synaptic dynamics originated from the gradual formation and annihilation of the conductive metallic filaments on the MXene surface with distributed functional groups. Subsequently, via training and inference tasks using a convolutional neural network for the Canadian-Institute-For-Advanced-Research-10 dataset, the applicability of the artificial MXene synapse to hardware neural networks is demonstrated.

Inelastic Electron Tunneling Spectroscopic Analysis of Bias-Induced Structural Changes in a Solid-State Protein Junction.

A central issue in protein electronics is how far the structural stability of the protein is preserved under the very high electrical field that it will experience once a bias voltage is applied. This question is studied on the redox protein Azurin in the solid-state Au/protein/Au junction by monitoring protein vibrations during current transport under applied bias, up to ≈1 GV m-1 , by electrical detection of inelastic electron transport effects. Characteristic vibrational modes, such as CH stretching, amide (NH) bending, and AuS (of the bonds that connect the protein to an Au electrode), are not found to change noticeably up to 1.0V. At >1.0V, the NH bending and CH stretching inelastic features have disappeared, while the AuS features persist till ≈2 V,i.e., the proteins remain Au bound. Three possible causes for the disappearance of the NH and CH inelastic features at high bias, namely, i) resonance transport, ii) metallic filament formation, and iii) bond rupture leading to structural changes in the protein are proposed and tested. The results support the last option and indicate that spectrally resolved inelastic features can serve to monitor in operando structural stability of biological macromolecules while they serve as electronic current conduit.

Memristive and artificial synapse performance by using TiOx/Al2O3 interface engineering in MoS2-based metallic filament memory

The MoS2 as a switching material has recently shown promising resistive switching characteristics. In this work, we demonstrate the impact of TiOx/Al2O3 interfacial layer on memristive/artificial synapse characteristics using MoS2-based Al/Cu/TiOx (or)Al2O3/MoS2/TiN structure. Owing to its barrier properties over Cu migration, the memristor with 2 nm-thick TiOx layer shows significant improvement in resistive switching as compared to both Al2O3 interfacial layer and 4 nm-thick MoS2. The memristor shows uniform long P/E endurance of 2 × 109 cycles under a low operation current of 200 μA and the high switching speed of 100 ns is applied. The ex-situ transmission electron microscope image reveals the formation of Cu metallic filament in MoS2 layer after long P/E endurance. At high operation current (1 mA), both the TiOx and Al2O3 interfacial layers show long P/E endurance of >109 cycles. In addition, the TiOx based memory device shows excellent RESET voltage controlled gradual RESET behavior without memory loss, enhanced uniform artificial synapse behavior with a maximum long-term potentiation/depression states of 55/500 at a small energy consumption of 5.7 pJ for future memory in computing application.

Effects of water adsorption on conductive filaments of a Ta2O5 atomic switch investigated by nondestructive electrical measurements

Atomic switches based on a metal/oxide/metal structure have attracted considerable attention for application in nonvolatile switching memory devices. In atomic switches, the formation and rupture of atomic-scale conductive metal filaments can be controlled via an applied voltage, which is central to the optimization of the resistive switching of such switches. Because the high-resistance (OFF) state is determined by the previous rupture (RESET) process and affects the subsequent formation process, it is important to know the filament structure and the electronic states in the OFF state. However, direct observation of the structure and electronic states is exceedingly difficult in the case of filaments embedded in the oxide matrix. In this paper, we propose a nondestructive methodology to evaluate the filament structure and electronic states in the OFF state based on current–voltage measurements. Using this method, the OFF states of a Ag/Ta2O5/Pt atomic switch were characterized, and changes in the size and potential barrier of a ruptured filament were estimated according to the introduction and evacuation of water molecules. The results clarify the effects of moisture, which are important for the operation of oxide-based resistive memories under ambient conditions.

Investigating the origins of ultra-short relaxation times of silver filaments in forming-free SiO2-based conductive bridge memristors

The threshold switching effect is considered of outmost importance for a variety of applications ranging from the reliable operation of crossbar architectures to emulating neuromorphic properties with artificial neural networks. This property is strongly believed to be associated with the rich inherit dynamics of a metallic conductive filament (CF) formation and its respective relaxation processes. Understanding the origin of these dynamics is very important in order to control the degree of volatility and design novel electronic devices. Here, we present a synergistic numerical and experimental approach in order to deal with that issue. The distribution of relaxation time is addressed through time-resolved pulse measurements whereas the entire switching behavior is modeled through a 2D dynamical model by taking into account the destructive interference of the drift/diffusion transport mechanisms and the Soret diffusion flux due to the intense local Joule heating. The proposed mechanism interprets successfully both the threshold to bipolar switching transition as well as the self-rectifying effects in SiO2-based memories. The model incorporates the effect of electrode materials on the switching pattern and provides a different perception of the ionic transport processes, shading light into the ultra-small lifetimes of the CF and explaining the different behavior of the silver or copper active materials in a conductive bridge random access memory architecture.

Effects of Resistance States on the Magnetoresistance in Ni/Al2O3/Ni by Resistive Switching

Formation and rupture of conductive filaments is one of the main mechanisms for unipolar resistive switching (URS), which can be manifested by the observation of magnetoresistance (MR) effect if ferromagnetic metallic filaments are formed. In this work, ferromagnetic metallic Ni is used as top and bottom electrodes on Al2O3 film, and stable URS behavior is observed after a forming process. By inserting a series resistor in the circuit during the set process, the resistance value at low resistance state (LRS) increases with increasing resistance of the series resistor. The URS behavior can be attributed to the formation and rupture of metallic Ni filaments penetrating through the thin Al2O3 layer, which has been confirmed by the observation of tunneling magnetoresistance. However, the MR value decreases continuously with increasing resistance in LRS, which is due to that the diameter of Ni conductive filaments becomes smaller. Thus, the nonmagnetic scattering at the surfaces of filaments increases, leading to the decrease of MR.

A non-oxidizing fabrication method for lithographic break junctions of sensitive metals.

Electrochemically active metals offer advanced functionalities with respect to the well-established gold electrode arrangements in various electronic transport experiments on atomic scale objects. Such functionalities can arise from stronger interactions with the leads which provide better coupling to specific molecules and may also facilitate metallic filament formation in atomic switches. However, the higher reactivity of the electrode metal also imposes challenges in the fabrication and reliability of nanometer scale platforms, limiting the number of reported applications. Here we present a high-yield lithographic fabrication procedure suitable to extend the experimental toolkit with mechanically controllable break junctions of oxygen sensitive metallic electrodes. We fabricate and characterize silver break junctions exhibiting single-atomic conductance and superior mechanical and electrical stability at room temperature. As a proof-of-principle application, we demonstrate resistive switching between metastable few-atom configurations at finite voltage bias.

Diffusive Memristive Switching on the Nanoscale, from Individual Nanoparticles towards Scalable Nanocomposite Devices

Nanoscale memristive phenomena are of great interest not only to miniaturize devices and improve their performance but also to understand the details of the underlying mechanism. Herein, we utilize conductive atomic force microscopy (C-AFM) as a non-invasive method to examine the nanoscale memristive properties of individual noble metal alloy nanoparticles that are sparsely encapsulated in a thin SiO2 dielectric matrix. The measurement of current-voltage hysteresis loops at the level of individual nanoparticles, enabled by the nanoscopic contact area of the C-AFM tip, indicates reliable memristive switching for several hours of continuous operations. Alongside the electrical characterization on the nanoscale, the method of C-AFM offers the potential for in situ monitoring of long term operation induced morphological alterations and device failure, which is demonstrated at the example of nanoparticle-based devices with additional Cr wetting layer. The application of alloy nanoparticles as reservoir for mobile silver species effectively limits the formation of stable metallic filaments and results in reproducible diffusive switching characteristics. Notably, similar behaviour is encountered on macroscopic nanocomposite devices, which incorporate multiple stacks of nanoparticles and offer a high design versatility to tune switching properties and engineer scalable memristive devices with diffusive switching characteristics. No additional forming step is required for the operation of the presented alloy nanoparticle based memristive devices, which renders them very attractive for applications.

Volatile Resistive Switching Memory Based on Ag Ion Drift/Diffusion—Part II: Compact Modeling

Resistive-switching random access memory (RRAM) devices based on filamentary switching are attracting widespread interest for their unique properties, such as high ON–OFF ratio, ultrasteep slope, good endurance, and low-current operation. Recently, volatile RRAMs based on Ag drift and diffusion were also demonstrated for possible applications such as crosspoint array selectors and neuromorphic computing. However, the mechanism of the volatile switching, namely, the spontaneous dissolution of the Ag filament, is still not clear. A deep understanding of the metallic filament formation and spontaneous disruption would strongly help the engineering of the device for optimized performances. Here, we present a numerical physics-based drift/diffusion modeling framework to describe the threshold switching, ${I}$ – ${V}$ characteristics, and morphological evolution of the metallic filament. The model can support TCAD-type device simulations for scaling, reliability, and variability studies.

Resistive switching in FeNi/Al2O3/NiO/Pt structure with various Al2O3 layer thicknesses

Abstract Noble metals are generally used as electrodes in resistive switching devices. In this work, ferromagnetic metallic NiFe is used as top electrode on NiO films, and stable unipolar resistive switching (URS) behavior has been observed after a forming process. By inserting a thin Al2O3 layer between NiFe and NiO, URS persists with increasing the thickness of Al2O3 layer up to 20 nm. The URS behavior can be attributed to the formation and rupture of metallic Ni filaments in NiO layer, which penetrate through the thin Al2O3 layer, and have been confirmed by the observation of anisotropic magnetoresistance. With further increasing the thickness of Al2O3 layer up to 30 nm, forming free bipolar resistive switching (BRS) behavior is observed, which is due to the oxygen vacancy accumulation and dissipation at the interface because the thick Al2O3 layer blocks the penetration of Ni filaments. By inserting a thin Al2O3 layer with thickness below 20 nm, the distribution of the reset voltages in URS is significantly narrowed.

Ultra compact electrochemical metallization cells offering reproducible atomic scale memristive switching

Here we show electrochemical metallization cells with compact dimensions, excellent electrical performance, and reproducible characteristics. An advanced technology platform has been developed to obtain Ag/SiO2/Pt devices with ultra-scaled footprints (15 × 15 nm2), inter-electrode distances down to 1 nm, and a transition from the OFF to ON resistance state relying on the relocation of only few atoms. This technology permits a well-controlled metallic filament formation in a highly confined field at the apex of an atomic scale tip. As a consequence of this miniaturization process, we achieve set voltages around 100 mV, ultra-fast switching times of 7.5 ns, and write energies of 18 fJ. Furthermore, we demonstrate very good cell-to-cell uniformity and a resistance extinction ratio as high as 6 · 105. Combined ab-initio quantum transport simulations and experiments suggest that the manufactured structures exhibit reduced self-heating effects due to their lower dimensions, making them very promising candidates as next-generation (non-)volatile memory components.

Lead-Free All-Inorganic Cesium Tin Iodide Perovskite for Filamentary and Interface-Type Resistive Switching toward Environment-Friendly and Temperature-Tolerant Nonvolatile Memories.

Recently, organometallic and all-inorganic halide perovskites (HPs) have become promising materials for resistive switching (RS) nonvolatile memory devices with low power consumption because they show current-voltage hysteresis caused by fast ion migration. However, the toxicity and environmental pollution potential of lead, a common constituent of HPs, has limited the commercial applications of HP-based devices. Here, RS memory devices based on lead-free all-inorganic cesium tin iodide (CsSnI3) perovskites with temperature tolerance are successfully fabricated. The devices exhibit reproducible and reliable bipolar RS characteristics in both Ag and Au top electrodes (TEs) with different switching mechanisms. The Ag TE devices show filamentary RS behavior with ultralow operating voltages (<0.15 V). In contrast, the Au TE devices have interface-type RS behavior with gradual resistance changes. This suggests that the RS characteristics are attributed to either the formation of metal filaments or the ion migration of defects in HPs under applied electric fields. These distinct mechanisms may permit the opportunity to design devices for specific purposes. This work will pave the way for lead-free all-inorganic HP-based nonvolatile memory for commercial application in HP-based devices.

Theoretical investigation of the Ag filament morphology in conductive bridge random access memories

Conductive bridge random access memories (CBRAMs) usually involve active Ag or Cu metals, where the formation of metal filaments accounts for the low resistance state. For the application of neuromorphic computation, it is highly desirable to develop artificial neurons and synapses, which utilize the complicated volatile or nonvolatile resistive switching phenomena, respectively. This can be achieved by controlling the morphology and stability of the filaments, which requires a deep understanding of the filament formation and disruption mechanisms. Using ab initio calculations, we explored the physical mechanism behind various Ag filament morphologies and growth modes, using GeSe, ZrO2, SiO2, and a-Si as the examples. The roles of Ag and Ag+ stability inside the dielectric, the migration barrier of Ag+, and the Ag+ solvation effect have been investigated in detail. A comprehensive model has been proposed, which in particular could explain the diverse Ag filament morphology experimentally observed in sputtered SiO2 and PECVD SiO2. Our theoretical approach can serve as a pre-screening method in designing new solid-state electrolyte materials of CBRAM, aiming at new functionalities in neuromorphic computation or in-memory logic computing.

On the Electrochemistry of Lipon Breakdown

Solid electrolytes are considered a necessary step for Li-ion ions to surpass the practical limit of 800Wh/L. Next to safety aspects, solid-state electrolytes could enable more compact batteries thus increasing their volumetric storage capacity. A main driver, however, is the potential for solid-electrolytes to increase the electrochemical window to accommodate high voltage positive electrodes (5V cathodes) and stability against metallic lithium as negative electrode. As such, solid-state batteries could enable high energy density through increased cell voltage. Numerous Li-ion conducting solids are known today, however most do not have the broad electrochemical window and stability against lithium yet. Nitrogen doped lithium phosphate glass or LiPON is a well known solid electrolyte material attributed with excellent electrochemical stability above 5V and stable against metallic lithium [1]. However, due to its limited ionic conductivity (~10-6 S/cm) it is only applicable in thin-film batteries for use for medical implants and various sensor systems. It has also been evaluated as solid-electrolyte coatings on electrode particles as buffer layer between active material and the bulk electrolyte. In a thin-film stack, it is an ideal model system for electrochemical study. In this paper, we report on the electrochemistry of RF-sputtered LiPON thin films and studied the out plating of lithium until break down. The LiPON thin-films had ionic conductivities ranging between 1x10-7 and 1x10-6 S/cm [2]. The decomposition potential was calculated from basic thermodynamic quantities and was linked to current-voltage (I-V) measurements with great consistency. The reported I-V measurements were conducted on metal-electrolyte-metal or MEM structures. The decomposition of LiPON was shown to occur at a potential of 4.8 V and proceeds in a diffusion limited way. It could be shown that LiPON breakdown is rather determined by the time needed for the decomposition reactions to complete, rather than being solely driven by the electric field. From the diffusion limited Li-ion current, the Li+ diffusion coefficient was extracted and found to be 2x10-11 cm2/s. This value is in excellent agreement with literature reports [3]. Ultimately hard breakdown was shown by TOF-SIMS imaging experiments and occurs through the formation of metallic lithium filaments. In the present work, we have for the first time formulated a breakdown mechanism for LiPON layers based on experimental measurements and thermodynamic considerations. The presented work provides a detailed evaluation of a solid electrolyte's breakdown and stability using I-V measurements. As such these measurements allow determination of ion transport phenomena as well as of the electrochemical stability window. It can also provide insight in the breakdown mechanism of RRAM devices.

Resistive switching and transport characteristics of an all-carbon memristor

We describe the structural and electrical characteristics of a resistive switching device fabricated from amorphous carbon. The device consisted of two low resistance sp2-rich contacts separated by a resistive layer with mixed sp2 and sp3 hybridisation states deposited using energetic deposition. Systematic bipolar switching between high- and low-resistance states (HRS/LRS) was observed. The ratio between the HRS and LRS exceeded 100:1 and the device provided stable operation up to at least 10 k cycles. Unlike conventional metal-oxide memristors, this device does not rely on the formation and breakage of conductive metal filaments. A mechanism based on the formation of conductive pathways caused by transitions between sp2 and sp3 hybridisation states is proposed. The current-voltage characteristics were modelled assuming a conducting pathway formed either at the interface regions where barriers could be present or within the interior of the resistive layer where Poole-Frenkel conduction occurred. The switching was attributed to reversible electric field induced alteration of the hybridisation states of carbon atoms. The ability to fabricate these devices entirely from carbon suggests the possibility of a large scale manufacture of durable, biocompatible memristors.

Probing the Mechanism for Bipolar Resistive Switching in Annealed Graphene Oxide Thin Films.

The bipolar resistive switching (BRS) between a metallic low resistance state (LRS) and an insulating high resistance state (HRS) is demonstrated for annealed graphene oxide (GO) thin film-based device structures with aluminum (Al) as one of the contact electrodes. An optimal switching of ∼104 order is recorded for Al/GO (200 °C)/indium tin oxide (ITO) among the device structures in metal (M2)/GO (T)/metal (M1) configurations (M1 = Al, Au, or ITO and M2 = Au or Al), fabricated using GO (T)/metal (M1), annealed at different temperatures, T = 100, 200, 300, and 400 °C. The initial Ohmic conduction for electronic transport and the presence of metal contents through GO thin films in the X-ray photoelectron spectroscopy support the physical evidence of Al filament formation between the two electrodes as imaged by the high-resolution transmission electron microscopy. The speculated mechanism for BRS in repeated voltage sweep cycles is attributed to the current triggered breaking of metal filaments because of the combined effect of Joule's heating and Peltier heat generation at LRS → HRS transition, and electric field induced migration of metal atoms, leading to the formation of metal filaments through the GO film at the HRS → LRS transition. The higher switching ratio exhibited in the current study could be translated to engineer simple and low-cost resistive memory devices.

Effect of AlN layer on the resistive switching properties of TiO2 based ReRAM memory devices

The present study reports the resistive switching behaviour in Titanium Dioxide (TiO2) material, with possible implementations in non volatile memory device. The Cu/TiO2/Pt memory device exhibit uniform and stable bipolar resistive switching behaviour. The current-voltage (I-V) analysis shows two discrete resistance states, the High Resistance State (HRS) and the Low Resistance State (LRS). The effect of an additional AlN layer in the resistive memory cell is also investigated. The Cu/TiO2/AlN/Pt device shows a multilevel (tri-state) resistive switching. Multilevel switching is facilitated by ionic and metallic filament formation, and the nature of the formed filaments is confirmed by performing a resistance vs. temperature measurement. The bilayer device shows improved reliability over the single layer device. The formation of high thermal conductive interfacial oxy-nitride (AlON) layer is the main reasons for the enhancement of resistive switching properties in Cu/TiO2/AlN/Pt cell. The performance of device was measured in terms of endurance and retention, which exhibits good endurance over 105 cycles and long retention time of 105 s at 125 °C. The above result suggests the feasibility of Cu/TiO2/AlN/Pt devices for multilevel non volatile ReRAM application.

A Redox‐Based Resistive Switching Memory Device Consisting of Organic–Inorganic Hybrid Perovskite/Polymer Composite Thin Film

AbstractThis study describes the first perovskite‐based redox resistive switching memory using CH3NH3PbBr3 nanoparticles (NPs) dispersed in an insulating solid polymer electrolyte, poly(ethylene oxide) (PEO), and scrutinizes it in detail. Herein, PEO is chosen not only to perform a matrix function due to its ionic conductivity but also to support a preservative material surrounding the CH3NH3PbBr3 NPs to improve their stability. Further, it is revealed that PEO can serve as the chelating agent to coordinate with PbBr2/CH3NH3PbBr3 NPs in consequence of the direct interaction between Pb2+ cations and electron pairs of ether oxygen on the PEO chain to provide a host medium for the Pb2+ cations on both amorphous and crystalline phases. Consequently, it facilitates the associated redox‐based reactions to result in the metallic filament formation in the derived device, leading to the write‐once‐read‐many times resistive switching behavior. The field‐effect scanning electron microscopy and X‐ray photoelectron spectroscopy analyses are conducted to ascertain the detailed mechanism. It is unveiled that a stable dendritic‐like filament is grown in the CH3NH3PbBr3 NPs:PEO hybrid film, which is thus proposed to be the origin of the stable low resistive state and recovery of the conductive path during the reverse bias scan. This study presents a new perspective on the perovskite‐based resistive memory devices.