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The filament growth processes, crucial to the performance of nanodevices like resistive switching memories, have been widely investigated to realize the device optimization. With the combination of kinetic Monte Carlo (KMC) simulations and the restrictive percolation model, three different growth modes in electrochemical metallization (ECM) cells were dynamically reproduced, and an important parameter, the relative nucleation distance, was theoretically defined to measure different growth modes quantitatively; hence their transition can be well described. In our KMC simulations, the inhomogeneity of storage medium is realized through introducing evolutionary void versus non-void sites within it to mimic the real nucleation during filament growth. Finally, the renormalization group method was used in the percolation model to analytically illustrate void-concentration-dependent growth mode transition, fitting KMC simulation results quite well. Our study found that the nanostructure of the medium can dominate the filament growth dynamics, as the simulation images as well as the analytical results are consistent with experiments results. Our study spotlights a vital and intrinsic factor, void concentration (relative to defects, grains, or nanopores) of a storage medium, in inducing filament growth mode transition within ECM cells. This theoretically proves a mechanism to tune performance of ECM systems that controlling microstructures of the storage media can dominate the filament growth dynamics, indicating an accessible strategy, nanostructure processing, for device optimization of ECM memristors.

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Memristors are drawing attention as neuromorphic hardware components because of their non-volatility and analog programmability. In particular, electrochemical metallization (ECM) memristors are extensively researched because of their linear conductance controllability. Two-dimensional materials as switching medium of ECM memristors give advantages of fast speed, low power consumption, and high switching uniformity. However, the multistate retention in the switching conductance range for the long-term reliable neuromorphic system has not been achieved using two-dimensional materials-based ECM memristors. In this study, the copper migration-controlled ECM memristor showing excellent multistate retention characteristics in the switching conductance range using molybdenum disulfide (MoS2 ) and aluminum oxide (Al2 O3 ) is proposed. The fabricated device exhibits gradual resistive switching with low switching voltage (<0.5 V), uniform switching (σ/µ âˆ¼ 0.07), and a wide switching range (>12). Importantly, excellent reliabilities with robustness to cycling stress and retention over 104 s for more than 5-bit states in the switching conductance range are achieved. Moreover, the contribution of the Al2 O3 layer to the retention characteristic is investigated through filament morphology observation using transmission electron microscopy (TEM) and copper migration component analysis. This study provides a practical approach to developing highly reliable memristors with exceptional switching performance.

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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.

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Conductive bridge random access memory devices such as Cu/SiO2/W are promising candidates for applications in neuromorphic computing due to their fast, low-voltage switching, multiple-conductance states, scalability, low off-current, and full compatibility with advanced Si CMOS technologies. The conductance states, which can be quantized, originate from the formation of a Cu filament in the SiO2electrolyte due to cation-migration-based electrochemical processes. A major challenge related to the filamentary nature is the strong variability of the voltage required to switch the device to its conducting state. Here, based on a statistical analysis of more than hundred fifty Cu/SiO2/W devices, we point to the key role of the activation energy distribution for copper ion diffusion in the amorphous SiO2. The cycle-to-cycle variability is modeled well when considering the theoretical energy landscape for Cu diffusion paths to grow the filament. Perspectives of this work point to developing strategies to narrow the distribution of activation energies in amorphous SiO2.

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Electric field control of exchange bias (EB) plays an important role in spintronics due to its attractive merit of lower energy consumption. Here, we propose a novel method for electrically tunable EB at room temperature in a device with the stack of Si/SiO2/Ta/Pt/Ag/Mn-doped ZnO (MZO)/Pt/FeMn/Co/ITO by resistive switching (RS) via electrochemical metallization (ECM). The device shows enhanced and weakened EB when set at high-resistance state (HRS) and low-resistance state (LRS), respectively. For the device at LRS, the aberration-corrected scanning transmission electron microscopy (STEM) characterizations unambiguously reveal that the Ag filaments grow initially from the Ag anode and then elongate toward the ITO cathode. It is inferred that at LRS, a small portion of Ag filaments have passed through the MZO and the intervening thin Pt layer and extended into the FeMn layer. After applying reverse voltage, these Ag filaments are electrochemically dissolved and ruptured near the MZO/Pt interface. This is considered to be the main mechanism responsible for RS and switchable EB as well. This work presents a new strategy for designing low-power, nonvolatile magnetoelectric random access memory devices.

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When designing the gate-dielectric of a floating-gate-transistor, one must make a tradeoff between the necessity of providing an ultra-small leakage current behavior for long state retention, and a moderate to high tunneling-rate for fast programming speed. Here we report on a memristively programmable transistor that overcomes this tradeoff. The operation principle is comparable to floating-gate-transistors, but the advantage of the analyzed concept is that ions instead of electrons are used for programming. Since the mass of ions is significantly larger than the effective mass of electrons, gate-dielectrics with higher leakage current levels can be used. We demonstrate the practical feasibility of the device using a proof-of-concept study based on a micrometer-sized thin-film transistor and LT-Spice simulations of 32 nm transistors. Memristively programmable transistors have the potential of high programming endurance and retention times, fast programming speeds, and high scalability.

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Atomic switch-based selectors, which utilize the formation of conductive filaments by the migration of ions, are researched for cross-point array architecture due to their simple structure and high selectivity. However, the difficulty in controlling the formation of filaments causes uniformity and reliability issues. Here, a multilayer selector with Pt/Ag-doped ZnO/ZnO/Ag-doped ZnO/Pt structure by the sputtering process is presented. A multilayer structure enables control of the filament formation by preventing excessive influx of Ag ions. The multilayer selector device exhibits a high on-current density of 2 MA cm-2 , which can provide sufficient current for the operation with the memory device. Also, the device exhibits high selectivity of 1010 and a low off-current of 10-13 A. The threshold voltage of selector devices can be controlled by modulating the thickness of the ZnO layer. By connecting a multilayer selector device to a resistive switching memory, the leakage current of the memory device can be reduced. These results demonstrate that a multilayer structure can be used in a selector device to improve selectivity and reliability for use in high-density memory devices.

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In this work, native GaOx is positioned between bulk gallium and degenerately doped p-type silicon (p+-Si) to form Ga/GaOx/SiOx/p+-Si junctions. These junctions show memristive behavior, exhibiting large current-voltage hysteresis. When cycled between -2.5 and 2.5 V, an abrupt insulator-metal transition is observed that is reversible when the polarity is reversed. The ON/OFF ratio between the high and low resistive states in these junctions can reach values on the order of 108 and retain the ON and OFF resistive states for up to 105 s with an endurance exceeding 100 cycles. The presence of a nanoscale layer of gallium oxide is critical to achieving reversible resistive switching by formation and dissolution of the gallium filament across the switching layer.

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The switching parameters and device performance of memristors are predominately determined by their mobile species and matrix materials. Devices with oxygen or oxygen vacancies as the mobile species usually exhibit a great retention but also need a relatively high switching current (e.g., >30 µA), while devices with Ag or Cu as cation mobile species do not require a high switching current but usually show a poor retention. Here, Ru is studied as a new type of mobile species for memristors to achieve low switching current, fast speed, good reliability, scalability, and analog switching property simultaneously. An electrochemical metallization-like memristor with a stack of Pt/Ta2 O5 /Ru is developed. Migration of Ru ions is revealed by energy-dispersive X-ray spectroscopy mapping and in situ transmission electron microscopy within a sub-10 nm active device area before and after switching. The results open up a new avenue to engineer memristors for desired properties.

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Electrical performance of self-directed channel (SDC) ion-conducting memristors which use Ag and Cu as the mobile ion source are compared over the temperature range of 6 K to 300 K. The Cu-based SDC memristors operate at temperatures as low as 6 K, whereas Ag-based SDC memristors are damaged if operated below 125 K. It is also observed that Cu reversibly diffuses into the active Ge2Se3 layer during normal device shelf-life, thus changing the state of a Cu-based memristor over time. This was not observed for the Ag-based SDC devices. The response of each device type to sinusoidal excitation is provided and shows that the Cu-based devices exhibit hysteresis lobe collapse at lower frequencies than the Ag-based devices. In addition, the pulsed response of the device types is presented.

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Electrochemical metallization (ECM) threshold switches are in great demand for various applications such as next-generation logic technology, future memory, and neuromorphic computing. However, the instability of operation due to inherent filamentary randomness is a severe problem that is yet to be solved. Here, we propose a specially treated hafnium oxide (HfO x:N)-based ECM threshold switch with high reliability, low-voltage operation (0.2 V), high ON/OFF ratio (5 × 108), great endurance (106), and fast switching speed (1.5 µs at 2 V). The nitrogen ions in the HfO x:N layer assist confining the path of the metallic filament, which significantly suppresses filament randomness as well as reduces power consumption and alternating current response time. The feasibility of ECM threshold switches to logic applications, AND and OR, is first introduced. The ECM threshold switch has great potential to be utilized in complementary logic circuits because of its ultralow operation power consumption, high integrability using an array structure (4 F2), and fast switching characteristics. Furthermore, we have successfully verified its applicability to flexible electronics on polyethylene naphthalate films that can retain stable operation under considerable mechanical stress. We believe that this research paves the way to fabricate highly reliable ECM threshold switches for flexible complementary logic circuits with ultralow power consumption.

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With the advent of artificial intelligence (AI), memristors have received significant interest as a synaptic building block for neuromorphic systems, where each synaptic memristor should operate in an analog fashion, exhibiting multilevel accessible conductance states. Here, we demonstrate that the transition of the operation mode in poly(1,3,5-trivinyl-1,3,5-trimethyl cyclotrisiloxane) (pV3D3)-based flexible memristor from conventional binary to synaptic analog switching can be achieved simply by reducing the size of the formed filament. With the quantized conductance states observed in the flexible pV3D3 memristor, analog potentiation and depression characteristics of the memristive synapse are obtained through the growth of atomically thin Cu filament and lateral dissolution of the filament via dominant electric field effect, respectively. The face classification capability of our memristor is evaluated via simulation using an artificial neural network consisting of pV3D3 memristor synapses. These results will encourage the development of soft neuromorphic intelligent systems.

Asunto(s)

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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.

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Graphene has been broadcasted as a promising choice of electrode and substrate for flexible electronics. To be truly useful in this regime, graphene has to prove its capability in ordering the growth of overlayers at an atomic scale, commonly known as epitaxy. Meanwhile, graphene as a diffusion barrier against atoms and ions has been shown in some metal-graphene-dielectric configurations for integrated circuits. Guided by these two points, this work explores a new direction of using graphene as a bifunctional material in an electrochemical metallization memory, where graphene is shown to (i) order the growth of a low-ionicity semiconductor ZnS single-crystalline film and (ii) regulate the ion migration in the resistive switching device made of Cu/ZnS/graphene/Cu structures. The ZnS film is confirmed to be van der Waals epitaxially grown on single-crystal graphene with X-ray structural analysis and Raman spectroscopy. Charge transport studies with controlled kinetic parameters reveal superior ion regulating characteristic of graphene in this ZnS-based resistive switching device. The demonstration of the first graphene-directed epitaxial wide band gap semiconductor resistive switching suggests a possible and promising route toward flexible memristors.

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Bernal- and rhombohedral-stacked trilayer graphene (B- and r-TLG) on nickel (Ni) and iridium (Ir) films acting as bottom electrodes (BEs) of silver electrochemical metallization cells (Ag-EMCs) have been investigated in this study. Prior to the fabrication of the EMC devices, Raman mapping and atomic force microscopy are applied to identify the B- and r-TLG sheets, with the latter revealing a significant D peak and a rough surface for the Ir film. The Ag-EMCs with the stacked BE of r-TLG on the Ir film show a conductive mechanism of Schottky emission at the positive top electrode bias for both high- and low-resistance states that can be examined by the resistance change with the device area and are modulated by pulse bias operation. Thus, an effective electron barrier height of 0.262 eV at the r-TLG and Ir interface is obtained because of the conspicuous energy gap of r-TLG on the Ir film and the van der Waals (vdW) gap between the r-TLG and Ir contact metal. With the use of Ni instead of Ir contact metal, the Ag-EMCs with TLG BE demonstrate +0.3 V/-0.75 V operation voltages, more than 104 s data retention at 115 °C and 250 times endurance testing, making the TLG sheets suitable for low-power nonvolatile memory applications on flexible substrates.

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Under a layer of 0.1 M HCl in isopropanol, soft ultraviolet (UV) (365 nm) photolysis of the thiol-on-gold self-assembled monolayer (SAM) derived from the lipoic acid ester of α-hydroxy-1-acetylpyrene results in the expected removal of the acetylpyrene protecting group. When photolyzing through a mask, this can be used to produce a patterned surface and, at a controlled electrochemical potential, it is then possible to selectively and reversibly electrodeposit copper on the photolyzed regions. Rather surprisingly, under these photolysis conditions, there is not only the expected photodeprotection of the ester but also partial removal of the lipoic acid layer which has been formed. In further studies, it is shown that this type of acid-catalyzed photoremoval of SAM layers by soft UV is a rather general phenomenon and results in the partial removal of the thiol-on-gold SAM layers derived from other ω-thiolated carboxylic acids. However, this phenomenon is chain-length dependent. Under conditions in which there is a ∼60% reduction in the thickness of the SAM derived from dithiobutyric acid, the SAM derived from mercaptoundecanoic acid is almost unaffected. The process by which the shorter-chain SAM layers are partially removed is not fully understood because these compounds do not absorb significantly in the 365 nm region of the spectrum! Significantly, this study shows that acid catalysis photolysis of thiol-on-gold SAMs needs to be used with caution.

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Conductive-bridge random access memory (CBRAM) is considered a strong contender of the next-generation nonvolatile memory technology. Resistive switching (RS) behavior in CBRAM is decided by the formation/dissolution of nanoscale conductive filament (CF) inside RS layer based on the cation injection from active electrode and their electrochemical reactions. Remarkably, RS is actually a localized behavior, however, cation injects from the whole area of active electrode into RS layer supplying excessive cation beyond the requirement of CF formation, leading to deterioration of device uniformity and reliability. Here, an effective method is proposed to localize cation injection into RS layer through the nanohole of inserted ion barrier between active electrode and RS layer. Taking an impermeable monolayer graphene as ion barrier, conductive atomic force microscopy results directly confirm that CF formation is confined through the nanohole of graphene due to the localized cation injection. Compared with the typical Cu/HfO2 /Pt CBRAM device, the novel Cu/nanohole-graphene/HfO2 /Pt device shows improvement of uniformity, endurance, and retention characteristics, because the cation injection is limited by the nanohole graphene. Scaling the nanohole of ion barrier down to several nanometers, the single-CF-based CBRAM device with high performance is expected to achieve by confining the cation injection at the atomic scale.

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As an alternative to transistor-based flash memories, redox reactions mediated resistive switches are considered as the most promising next-generation nonvolatile memories that combine the advantages of a simple metal/solid electrolyte (insulator)/metal structure, high scalability, low power consumption, and fast processing. For cation-based memories, the unavailability of in-built mobile cations in many solid electrolytes/insulators (e.g., Ta2O5, SiO2, etc.) instigates the essential role of absorbed water in films to keep electroneutrality for redox reactions at counter electrodes. Herein, we demonstrate electrochemical characteristics (oxidation/reduction reactions) of active electrodes (Ag and Cu) at the electrode/electrolyte interface and their subsequent ions transportation in Fe3O4 film by means of cyclic voltammetry measurements. By posing positive potentials on Ag/Cu active electrodes, Ag preferentially oxidized to Ag+, while Cu prefers to oxidize into Cu2+ first, followed by Cu/Cu+ oxidation. By sweeping the reverse potential, the oxidized ions can be subsequently reduced at the counter electrode. The results presented here provide a detailed understanding of the resistive switching phenomenon in Fe3O4-based memory cells. The results were further discussed on the basis of electrochemically assisted cations diffusions in the presence of absorbed surface water molecules in the film.

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Metal oxide-based electrochemical metallization memory (ECM) shows promising performance for next generation non-volatile memory. The negative-SET behavior has been observed in various oxide-based ECM devices. But the underlying mechanism of this behavior remains unaddressed and the role of the metal cation and oxygen vacancy in this behavior is unclear. In this work, we have observed two kinds of negative-SET (labeled as N-SET1 and N-SET2) behaviors in our Cu/ZrO2/Pt devices. Both the two behaviors can result in hard breakdown due to the high compliance current in reset process. The I-V characteristic shows that the two negative-SET behaviors have an obvious difference in operation voltage. Using four-probe resistance measurement method, the resistance-temperature characteristics of the ON-state after various negative-SET behaviors have been studied. The temperature dependence results demonstrate that the N-SET1 behavior is dominated by Cu conductive filament (CF) reformation caused by the Cu CF overgrowth phenomenon while the N-SET2 is related to the formation of oxygen vacancy CF. This work may provide a comprehensive understanding of the switching mechanism in oxide-based ECM devices.

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By modification of the electrode-solid-electrolyte interface with graphene, transit from valence change memories (VCM) to electrochemical metallization memories (ECM) in the cell Ta(C)/Ta2 O5 /Pt is demonstrated, thus, bridging both mechanisms. The ECM operation is discussed in the light of Ta-cation mobility in TaOx . The crucial role of electrochemical processes and moisture in the resistive switching process is also highlighted.