Filament Switching Research Articles

Analog and Digital Mode α‐In2Se3 Memristive Devices for Neuromorphic and Memory Applications

AbstractThe emergence of 2D ferroelectrics offers a promising path to implement next‐generation information technology, including digital memory and analog computing. Here, it is demonstrated that digital or analog memory can be achieved in a single crossbar type two‐terminal (Au‐Ti) α‐In2Se3 device depending on the driving voltage. The analog operation is enabled by ferroelectric polarization‐modulated Schottky barrier, while resistive filament switching drives the digital operation. By tuning ferroelectric properties, multiple analog conductance states can be obtained for mimicking various synaptic behaviors like excitatory postsynaptic current, inhibitory postsynaptic current, potentiation/depression and spiking‐timing‐dependent plasticity. A simulated neural network built from these synaptic devices shows good on‐line learning accuracy (≈93.2%) for digital recognition of handwriting. Based on filament formation/rupture mechanism, α‐In2Se3 devices also exhibits excellent digital memory performance with large on/off ratio (>103), high on‐current density (105 A cm−2) and fast switching speed (≈10 ns). The combination of analog and digital memory modes in two‐terminal α‐In2Se3 devices is useful in highly dense and complex electronics.

Modeling spatiotemporally varying protein-protein interactions in CyLaKS, the Cytoskeleton Lattice-based Kinetic Simulator.

Interaction of cytoskeletal filaments, motor proteins, and crosslinking proteins drives important cellular processes such as cell division and cell movement. Cytoskeletal networks also exhibit nonequilibrium self-assembly in reconstituted systems. An emerging problem in cytoskeletal modeling and simulation is spatiotemporal alteration of the dynamics of filaments, motors, and associated proteins. This can occur due to motor crowding, obstacles along the filament, motor interactions and direction switching, and changes, defects, or heterogeneity in the filament binding lattice. How such spatiotemporally varying cytoskeletal filaments and motor interactions affect their collective properties is not fully understood. We developed the Cytoskeleton Lattice-based Kinetic Simulator (CyLaKS) to investigate such problems. The simulation model builds on previous work by incorporating motor mechanochemistry into a simulation with many interacting motors and/or associated proteins on a discretized lattice. CyLaKS also includes detailed balance in binding kinetics, movement, and lattice heterogeneity. The simulation framework is flexible and extensible for future modeling work and is available on GitHub for others to freely use or build upon. Here we illustrate the use of CyLaKS to study long-range motor interactions, microtubule lattice heterogeneity, motion of a heterodimeric motor, and how changing crosslinker number affects filament separation.

Transition between bipolar and abnormal bipolar resistive switching in amorphous oxides with a mobility edge

Resistive switching is an important phenomenon for future memory devices such as resistance random access memories or neuronal networks. While there are different types of resistive switching, such as filament or interface switching, this work focuses on bulk switching in amorphous, binary oxides. Bulk switching was found experimentally in different oxides, for example in amorphous gallium oxide. The forms of the observed current–voltage curves differ, however, fundamentally. Even within the same material, both abnormal bipolar and normal bipolar resistive switching were found. Here, we use a new drift–diffusion model to theoretically investigate bulk switching in amorphous oxides where the electronic conductivity can be described by Mott’s concept of a mobility edge. We show not only that a strong, non-linear dependence of the electronic conductivity on the oxygen content is necessary for bulk switching but also that changing the geometry of the memristive device causes the transition between abnormal and normal bipolar switching.

Influence of oxygen ion elementary diffusion jumps on the electron current through the conductive filament in yttria stabilized zirconia nanometer-sized memristor

• Nanometer-sized virtual memristors based on an yttria stabilized zirconia thin film. • Low-frequency flicker noise in the electron current through the conducting filament. • The random motion (drift/diffusion) of oxygen ions/vacancies effect in this current. • Estimates of the root-mean-square value of the current jumps caused by this motion. • The determination of elementary electronic processes in memristor systems. The structure of the electron current through an individual filament of a nanometer-sized virtual memristor consisting of a contact of a conductive atomic force microscope probe to an yttria stabilized zirconia (YSZ) thin film deposited on a conductive substrate is investigated. Usually, such investigation is performed by the analysis of the waveform of this current with the aim to extract the random telegraph noise (RTN). Here, we suggest a new indirect method, which is based on the measurement of the spectrum of the low-frequency flicker noise in this current without extracting the RTN, taking into account the geometrical parameters of the filament. We propose that the flicker noise is caused by the motion (drift/diffusion) of oxygen ions via oxygen vacancies within and around the filament. This allows us to estimate the root mean square magnitude i 0 of the current jumps, which are caused by random jumps of oxygen ions, and the number M of these ions. This is fundamental for understanding the elementary mechanisms of electron current flowing through the filament and resistive switching in YSZ–based memristor devices.

Transparent and Unipolar Nonvolatile Memory Using 2D Vertically Stacked Layered Double Hydroxide

AbstractVarious 2D materials have received considerable attention as emerging nanoscale materials for low‐power and high‐performance electronic and optoelectronic device applications. Among these, layered double hydroxide (LDH)‐based nanocomposites are promising materials because of their structural diversity and electronic functionality, which are suitable for photocatalysts, catalytic supports, and charge storage. Here, three Al‐based LDHs using different divalent cations (Zn2+, Ni2+, and Co2+) are synthesized, and their electrical characteristics are investigated in the form of a two‐terminal Pt/Al‐based LDHs/fluorine‐doped tin oxide junction structure. Only the ZnAl‐LDH junction exhibits a distinct unipolar switching behavior with an ON–OFF ratio of ≈103 and transmittance of ≈87%; the other junctions (NiAl‐ and CoAl‐LDHs) do not exhibit switching and possess relatively low transparency. This difference is attributed to the relatively vertically stacked ZnAl‐LDH layer, which enables the formation of a switching filament through the vertical 2D layer and enhances transparency. The ZnAl‐LDH junction has a relatively low trap energy (Et) of ≈0.1 eV that can decrease the SET voltage as the temperature increases, which can be understood by trap‐assisted space‐charge‐limited conduction with thermal‐assisted electron excitation. This study sheds light on the potential use of transparent and self‐organized vertical stacked ZnAl‐LDH materials as resistive switching devices.

Near-Atomic Structure of the 10S form of Myosin II: Implications for Inhibition, Activation and Disease

Myosin II is the molecular motor that powers contractility in muscle and nonmuscle cells. The two-headed molecule can exist in two conformations: compact (10S sedimentation coefficient) and extended (6S). The extended conformation assembles into filaments, which pull on actin to generate muscle contraction and cell motility. In the 10S structure, the tail is folded into three segments and the heads bend back and interact with each other and the tail, switching off ATPase activity, actin-activation and filament assembly. This energy-conserving, storage form can be activated by phosphorylation of its regulatory light chains (RLC) to form functional filaments as needed. We have solved the structure of smooth muscle 10S myosin to 4.3 Å resolution by cryo-EM, revealing near-atomic insights into its structure and inhibitory function (Yang et al., Nature, in press). The resulting refined atomic model shows clear secondary structure (α-helices, coiled-coil), with side-chain detail in some regions. The motor domains have the same (ADP.Pi) conformation, but their regulatory domains differ, allowing the two heads to meet asymmetrically. The coiled-coil shows considerable variations in pitch along its length. The regulatory domain backbone remains mostly helical as it joins the tail, with little apparent melting. The model reveals that the heads are prevented from binding to actin by steric clashes involving the tail and the other head, while their ATPase activity is impeded by constraints on movement of their converter domains. The positively charged N-terminal region of the RLC interacts with negative charge on the tail near the RLC phosphorylation site, suggesting a simple mechanism for activation. Disease-causing mutations in smooth and nonmuscle myosin II occur close to sites of head interaction with the tail, suggesting that interference with the 10S structure may underlie such diseases.

Configurable switching behavior in polymer-based resistive memories by adopting unique electrode/electrolyte arrangement.

The difference in resistive switching characteristics by modifying the device configuration provides a unique operating principle, which is essential for both fundamental studies and the development of future memory devices. Here, we demonstrate the poly(methyl methacrylate) (PMMA)-based resistive switching characteristics using four different combinations of electrode/electrolyte arrangement in the device geometry. From the current–voltage (I–V) measurements, all the PMMA-based devices revealed nonvolatile memory behavior with a higher ON/OFF resistance ratio (∼105–107). Significantly, the current conduction in the filament and resistive switching behavior depend majorly on the presence of Al electrode and electrochemically active silver (Ag) element in the PMMA matrix. A trap-controlled space charge limited conduction (SCLC) mechanism constitutes the resistive switching in the Al/PMMA/Al device, whereas the current conduction governed by ohmic behavior influences the resistive switching in the Ag-including devices. The depth-profiling X-ray photoelectron spectroscopy (XPS) study evidences the conducting filament formation processes in the PMMA-based devices. These results with different conduction mechanisms provide further insights into the understanding of the resistive switching behavior in the polymer-based devices by simply rearranging the device configuration.

Self‐Assembled NiO Nanocrystal Arrays as Memristive Elements

AbstractMemristive switching, a nonlinear current–voltage (I–V) characteristic, has seen a tremendous surge in interest as an approach to achieve implementation of synaptic functions. The memristive switching behavior of self‐assembled NiO nanocrystals is investigated via scanning probe microscopy, based on first‐order reversal curve current–voltage spectroscopy. Synaptic switching is clearly observed as a direct consequence of filament growth (i.e., gradually increased conductance) in the nanocrystals. A spatial dependency of the conduction in the nanocrystals suggests that there is a localization of the switching filament. The current understanding of this localization ignores features related to local lateral variation current, which can generate an excessive local heat and temperature such as electrical dissipation. The observation of low electrical dissipation at the edge of the nanocrystals shows that less energy is wasted as heat such that the bias applied can be utilized more efficiently to assist the nucleation of the filament and thus reduces the power consumption. Electrical power dissipation is also found to scale with nanocrystal height and has spatial dependence within the nanocrystals. The combination of synaptic switching and high density of the nanocrystals demonstrate that it is feasible to exploit them to create a basic architecture for neuromorphic memory devices.

Optical rectification in a reconfigurable resistive switching filament

We demonstrate optical rectification in a reconfigurable and relatively simple nanoscopic tunneling junction formed via resistive switching. In optical rectification, electrons must keep up with the rapid oscillations of an illuminating optical field and harness the nonlinearities of a tunneling contact to produce the desired DC field. Among the intrinsic requirements for such devices are tunneling junctions with an exceedingly small capacitance and surface area. In contrast to tunneling junctions formed by different methods, the resistive switching approach explored here allows the system to be tuned, set, and reset via the application of DC electric fields. This makes it ideally suitable for exploring optical rectification phenomena under different tunneling conditions and for dynamically tuning the device's responsivity. This “on-the-go” tunability opens the possibility for adaptive devices, such as ultrafast photon detectors, wireless power transmitters, and energy harvesting systems.

Newly developed process integration technologies for highly reliable 40 nm ReRAM

We have developed a 40 nm resistive random access memory (ReRAM) technology embedded in a foundry-standard CMOS process for low-power applications. Excellent reliability was achieved in 8 Mbit, 40 nm ReRAM: 100 k cycles and 10 year retention at 85 °C after 10 k cycles were demonstrated for resistive switching element (RSE) process technologies and filament characterization methods. Regarding process technologies for 40 nm ReRAM, we tackled four forms of integration. First, we experimented with RSE etching, to optimize damage-less etching of RSE, metal and dielectric. Second, we tried to maintain the back end of line process/parameter compatibility with the standard 40 nm CMOS process. Third, we developed our own techniques to form RSE cells that are protected against extra free oxygen to realize design flexibility. Fourth, we optimized the components of the high-resistivity layer (Ta2O5), specifically film density, and confirmed that it is possible to achieve both ease of occurrence of formation and reduced non-uniformity of the resistance values corresponding to the high resistive state.

Stateful characterization of resistive switching TiO2 with electron beam induced currents

Metal oxide resistive switches are increasingly important as possible artificial synapses in next-generation neuromorphic networks. Nevertheless, there is still no codified set of tools for studying properties of the devices. To this end, we demonstrate electron beam-induced current measurements as a powerful method to monitor the development of local resistive switching in TiO2-based devices. By comparing beam energy-dependent electron beam-induced currents with Monte Carlo simulations of the energy absorption in different device layers, it is possible to deconstruct the origins of filament image formation and relate this to both morphological changes and the state of the switch. By clarifying the contrast mechanisms in electron beam-induced current microscopy, it is possible to gain new insights into the scaling of the resistive switching phenomenon and observe the formation of a current leakage region around the switching filament. Additionally, analysis of symmetric device structures reveals propagating polarization domains.

A structural model of flagellar filament switching across multiple bacterial species

The bacterial flagellar filament has long been studied to understand how a polymer composed of a single protein can switch between different supercoiled states with high cooperativity. Here we present near-atomic resolution cryo-EM structures for flagellar filaments from both Gram-positive Bacillus subtilis and Gram-negative Pseudomonas aeruginosa. Seven mutant flagellar filaments in B. subtilis and two in P. aeruginosa capture two different states of the filament. These reliable atomic models of both states reveal conserved molecular interactions in the interior of the filament among B. subtilis, P. aeruginosa and Salmonella enterica. Using the detailed information about the molecular interactions in two filament states, we successfully predict point mutations that shift the equilibrium between those two states. Further, we observe the dimerization of P. aeruginosa outer domains without any perturbation of the conserved interior of the filament. Our results give new insights into how the flagellin sequence has been “tuned” over evolution.

Thermodynamics of Phase Transitions and Bipolar Filamentary Switching in Resistive Random-Access Memory

We present a phenomenological theory of filamentary resistive random access memory (RRAM) describing the commonly observed features of their current-voltage characteristics. Our approach follows the approach of thermodynamic theory developed earlier for chalcogenide memory and threshold switches and largely independent of their microscopic details. It explains, without adjustable parameters, such features as the domains of filament formation and switching, voltage independent current in SET and current independent voltage in RESET regimes, the relation between the set and reset voltages, filament resistance independent of its length, etc. Furthermore, it expresses the observed features through the material and circuitry parameters thus paving a way to device improvements.

Driven transport on open filaments with interfilament switching processes.

We study a two-filament driven lattice gas model with oppositely directed species of particles moving on two parallel filaments with filament-switching processes and particle inflow and outflow at filament ends. The filament-switching process is correlated with the occupation number of the adjacent site such that particles switch filaments with finite probability only when oppositely directed particles meet on the same filament. This model mimics some of the coarse-grained features observed in context of microtubule-(MT) based intracellular transport, wherein cellular cargo loaded and off-loaded at filament ends are transported on multiple parallel MT filaments and can switch between the parallel microtubule filaments. We focus on a regime where the filaments are weakly coupled, such that filament-switching rate of particles scale inversely as the length of the filament. We find that the interplay of (off-) loading processes at the boundaries and the filament-switching process of particles leads to some distinctive features of the system. These features includes occurrence of a variety of phases in the system with inhomogeneous density profiles including localized density shocks, density difference across the filaments, and bidirectional current flows in the system. We analyze the system by developing a mean field (MF) theory and comparing the results obtained from the MF theory with the Monte Carlo (MC) simulations of the dynamics of the system. We find that the steady-state density and current profiles of particles and the phase diagram obtained within the MF picture matches quite well with MC simulation results. These findings maybe useful for studying multifilament intracellular transport.

Kinetic factors determining conducting filament formation in solid polymer electrolyte based planar devices.

Resistive switching characteristics and conducting filament formation dynamics in solid polymer electrolyte (SPE) based planar-type atomic switches, with opposing active Ag and inert Pt electrodes, have been investigated by optimizing the device configuration and experimental parameters such as the gap distance between the electrodes, the salt inclusion in the polymer matrix, and the compliance current applied in current-voltage measurements. The high ionic conductivities of SPE enabled us to make scanning electron microscopy observations of the filament formation processes in the sub-micrometer to micrometer ranges. It was found that switching behaviour and filament growth morphology depend strongly on several kinetic factors, such as the redox reaction rate at the electrode-polymer interfaces, ion mobility in the polymer matrix, electric field strength, and the reduction sites for precipitation. Different filament formations, resulting from unidirectional and dendritic growth behaviours, can be controlled by tuning specified parameters, which in turn improves the stability and performance of SPE-based devices.

Electrical characterization and modeling of pulse-based forming techniques in RRAM arrays

The forming process, which corresponds to the activation of the switching filament in Resistive Random Access Memory (RRAM) arrays, has a strong impact on the cells’ performances. In this paper we characterize and compare different pulse forming techniques in terms of forming time, yield and cell-to-cell variability on 4kbits RRAM arrays. Moreover, post-forming modeling during Reset operation of correctly working and over formed cells has been performed. An incremental form and verify technique, based on a sequence of trapezoidal waveforms with increasing voltages followed by a verify operation that terminates when the expected switching behavior has been achieved, showed the best results. This procedure narrows the post-forming current distribution whereas reducing the Reset switching voltage and the operative current. These advantages materialize in a better control of the cell-to-cell variability and in an overall time and energy saving at the system level.

Spectromicroscopic insights for rational design of redox-based memristive devices.

The demand for highly scalable, low-power devices for data storage and logic operations is strongly stimulating research into resistive switching as a novel concept for future non-volatile memory devices. To meet technological requirements, it is imperative to have a set of material design rules based on fundamental material physics, but deriving such rules is proving challenging. Here, we elucidate both switching mechanism and failure mechanism in the valence-change model material SrTiO3, and on this basis we derive a design rule for failure-resistant devices. Spectromicroscopy reveals that the resistance change during device operation and failure is indeed caused by nanoscale oxygen migration resulting in localized valence changes between Ti4+ and Ti3+. While fast reoxidation typically results in retention failure in SrTiO3, local phase separation within the switching filament stabilizes the retention. Mimicking this phase separation by intentionally introducing retention-stabilization layers with slow oxygen transport improves retention times considerably.

Electrical and thermal analysis of frequency dependent filamentary switching in printed rectifying diodes

Filamentary conduction and switching properties are studied in printed rectifying diodes with a poly(triaryl amine) (PTAA) semiconductor layer sandwiched between Cu and Ag electrodes. Formation of conductive filaments caused defective operation of the rectifier at low frequencies. In contrast, the normal operation was restored at high frequencies. Reversible switching was observed between the low and high frequency states. Therefore, it is clear that the operational frequency has a significant effect on the filament formation and switching characteristics. The filamentary conduction was confirmed by lock-in IR thermography and physical defect analysis. The results reveal the existence of filamentary operation in p-type rectifying diodes and clearly demonstrate the effect of the device operation frequency on the switching properties. This has far-reaching implications on the switching properties in similar devices in literature.

Discussion on device structures and hermetic encapsulation for SiOx random access memory operation in air

An edge-free structure and hermetic encapsulation technique are presented that enable SiOx-based resistive random-access memory (RRAM) operation in air. A controlled etch study indicates that the switching filament is close to the SiOx surface in devices with an exposed SiOx edge. Electrical test of encapsulated, edge-free devices in 1 atmosphere air indicates stable switching characteristics, unlike devices with an edge. This work demonstrates that SiOx RRAM is able to operate in air with proper encapsulation and an edge-free structure. The resistive switching failure mechanism when operating in air is explained by the oxidation of hydrogen-complexed defects in the switching filament.

Thickness independent reduced forming voltage in oxygen engineered HfO2 based resistive switching memories

The conducting filament forming voltage of stoichiometric hafnium oxide based resistive switching layers increases linearly with layer thickness. Using strongly reduced oxygen deficient hafnium oxide thin films grown on polycrystalline TiN/Si(001) substrates, the thickness dependence of the forming voltage is strongly suppressed. Instead, an almost constant forming voltage of about 3 V is observed up to 200 nm layer thickness. This effect suggests that filament formation and switching occurs for all samples in an oxidized HfO2 surface layer of a few nanometer thickness while the highly oxygen deficient thin film itself merely serves as a oxygen vacancy reservoir.