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Halide perovskites with naturally coupled electron‐ion dynamics hold great potential for nonvolatile memory applications. Self‐rectifying memristors are promising as they can avoid sneak currents and simplify device configuration. Here we report a self‐rectifying memristor firstly achieved in a single perovskite (NHCINH3)3PbI5 (abbreviated as (IFA)3PbI5), which is sandwiched by Ag and ITO electrodes as the simplest cell in a crossbar array device configuration. The iodide ions of (IFA)3PbI5 can be easily activated, of which the migration in the bulk contributes to the resistance hysteresis and the reaction with Ag at the interface contributes to the spontaneous formation of AgI. The perfect combination of n‐type AgI and p‐type (IFA)3PbI5 gives rise to the rectification function like a p–n diode. Such a self‐rectifying memristor exhibits the record‐low set power consumption and voltage. This work emphasizes that the multifunction of ions in perovskites can simplify the fabrication procedure, decrease the programming power, and increase the integration density of future memory devices. [ABSTRACT FROM AUTHOR]

Herein we report on a facile sol-gel spin-coating technique to fabricate ZnO thin films that grow preferentially along the (002) plane on FTO substrates. By employing the magnetron sputtering technique to deposit a tungsten (W) top metal electrode onto these ZnO thin films, we successfully realize a W/ZnO/FTO memory device that exhibits self-rectifying and forming-free resistive switching characteristics. Notably, the as-prepared device demonstrates impressive nonvolatile and bipolar resistive switching behavior, with a high resistance ratio (RHRS/RLRS) exceeding two orders of magnitude at a reading voltage of 0.1 V. Moreover, it exhibits ultralow set and reset voltages of approximately +0.5 V and −1 V, respectively, along with exceptional durability. In terms of carrier transport properties, the low resistance state of the device is dominated by ohmic conduction, whereas the high resistance state is characterized by trap-controlled space-charge-limited current conduction. This work highlights the potential of the ZnO-based W/ZnO/FTO memory device as a promising candidate for future high-density nonvolatile memory applications. [ABSTRACT FROM AUTHOR]

Abstract Halide perovskites with naturally coupled electron‐ion dynamics hold great potential for nonvolatile memory applications. Self‐rectifying memristors are promising as they can avoid sneak currents and simplify device configuration. Here we report a self‐rectifying memristor firstly achieved in a single perovskite (NHCINH3)3PbI5 (abbreviated as (IFA)3PbI5), which is sandwiched by Ag and ITO electrodes as the simplest cell in a crossbar array device configuration. The iodide ions of (IFA)3PbI5 can be easily activated, of which the migration in the bulk contributes to the resistance hysteresis and the reaction with Ag at the interface contributes to the spontaneous formation of AgI. The perfect combination of n‐type AgI and p‐type (IFA)3PbI5 gives rise to the rectification function like a p–n diode. Such a self‐rectifying memristor exhibits the record‐low set power consumption and voltage. This work emphasizes that the multifunction of ions in perovskites can simplify the fabrication procedure, decrease the programming power, and increase the integration density of future memory devices.

A broad compositional range of Nb-Ti anodic memristors with volatile and self-rectifying behaviour was studied using a combinatorial screening approach. A Nb-Ti thin-film combinatorial library was co-deposited by sputtering, serving as the bottom electrode for the memristive devices. The library, with a compositional spread ranging between 22 and 64 at.% Ti was anodically oxidised, the mixed oxide being the active layer in MIM-type structures completed by Pt discreet top electrode patterning. By studying I–U sweeps, memristors with self-rectifying and volatile behaviour were identified. Moreover, all the analysed memristors demonstrated multilevel properties. The best-performing memristors showed HRS/LRS (high resistive state/low resistive state) ratios between 4 and 6 × 105 and very good retention up to 106 successive readings. The anodic memristors grown along the compositional spread showed very good endurance up to 106 switching cycles, excluding those grown from alloys containing between 31 and 39 at.% Ti, which withstood only 10 switching cycles. Taking into consideration all the parameters studied, the Nb-46 at.% Ti composition was screened as the parent metal alloy composition, leading to the best-performing anodic memristor in this alloy system. The results obtained suggest that memristive behaviour is based on an interfacial non-filamentary type of resistive switching, which is consistent with the performed cross-sectional TEM structural and chemical characterisation. [ABSTRACT FROM AUTHOR]

A mixed‐dimensional heterostructure comprising nanomaterials with varying dimensions provides a promising structure for an artificial synapse for reconfigurable neuromorphic functions. In this study, an 8 × 8 memristor crossbar array based on a mixed‐dimensional heterostructure comprising Cs1−xFAxPbBr3 (0.00 ≤ x ≤ 0.15) quantum dots (QDs) and different dimensional interfacial nanomaterial layers between the Al and ITO electrodes is designed and fabricated. This array device exhibits a high yield and reliable self‐rectifying analog switching characteristics with low synaptic‐coupling (SC, up to 5.19 × 10−5) and light emission, facilitating stimuli response visualization and preventing undesired pathways in the network array. Furthermore, because the formamidinium (FA) concentration alters the QD size, thereby engineering interfacial band alignment in the heterostructure, the essential synaptic properties such as dynamic range, SC, and nonlinearity can be improved. Especially, as x increases from 0 to 0.11, the recognition accuracy for the MNIST patterns increases significantly, from 68.97% to 89.08%, even for single‐layer ANNs. The energy consumption required for a specific accuracy level is reduced by a factor of 25.15. The utilization of mixed‐dimensional perovskite QD‐based heterostructures in neural networks may provide desirable neuromorphic electronic functions with enhanced learning capability and energy efficiency, while preventing unwanted neural signals. [ABSTRACT FROM AUTHOR]

Resistive random-access memory has emerged as a promising non-volatile memory technology, receiving substantial attention due to its potential for high operational performance, low power consumption, temperature robustness, and scalability. Two-dimensional nanostructured materials play a pivotal role in RRAM devices, offering enhanced electrical properties and physical attributes, which contribute to overall device improvement. In this study, the self-rectifying switching behavior in RRAM devices is analyzed based on molybdenum disulfide nanocomposites decorated with Pd on SiO2/Si substrates. The switching layer integration of Pd and MoS2 at the nanoscale effectively mitigates leakage currents decreasing from cross-talk in the RRAM array, eliminating the need for a separate selector device. The successful demonstration of the expected RRAM switching operation and low switching dispersion follows the application of a Pd nanoparticle embedding method. The switching channel layer is presented as an independent (Pd nanoparticle coating and MoS2 nanosheet) nanocomposite. The switching layer length (4000 μm) and width (7000 μm) play an important role in a lateral-conductive-filament-based RRAM device. Through the bipolar switching behavior extraction of RRAM, the formation of the conductive bridges via electronic migration is explained. The fabricated Pd-MoS2 synaptic RRAM device results in a high resistive current ratio for a forward/reverse current higher than 60 at a low resistance state and observes a memory on/off ratio of 103, exhibiting stable resistance switching behavior. [ABSTRACT FROM AUTHOR]

The immense increase of unstructured data require novel computing systems that can process the input data with low power and parallel processing. This functionality is similar to that of human brains that are composed of numerous neurons, synapses, and their complex connections. To mimic the functionality of the human brain with an electronic device, the resistive switching device and crossbar array has attracted considerable attention for artificial synaptic devices and integrated systems, respectively. For this purpose, the self‐rectifying resistive switching cell based on the Si:ZrOx thin film is developed and its reliability characteristics are tested. Four achievements are highlighted in this study. 1) The retention characteristic is improved by the adoption of TaOx thin film as an oxygen reservoir layer. 2) The asymmetric electrodes can make the self‐rectifying resistive cell (SRC) have sufficient rectifying characteristic. 3) The linearity of conductance update has a dominant effect on the inference performance compared to that of the conductance range variation. 4) The device of the interface‐type resistive switching shows a high enough device yield in the crossbar array device and exhibits reliable multiply‐and‐accumulate operations in the crossbar array to mimic the human brain‐inspired computing system. [ABSTRACT FROM AUTHOR]

Abstract The immense increase of unstructured data require novel computing systems that can process the input data with low power and parallel processing. This functionality is similar to that of human brains that are composed of numerous neurons, synapses, and their complex connections. To mimic the functionality of the human brain with an electronic device, the resistive switching device and crossbar array has attracted considerable attention for artificial synaptic devices and integrated systems, respectively. For this purpose, the self‐rectifying resistive switching cell based on the Si:ZrOx thin film is developed and its reliability characteristics are tested. Four achievements are highlighted in this study. 1) The retention characteristic is improved by the adoption of TaOx thin film as an oxygen reservoir layer. 2) The asymmetric electrodes can make the self‐rectifying resistive cell (SRC) have sufficient rectifying characteristic. 3) The linearity of conductance update has a dominant effect on the inference performance compared to that of the conductance range variation. 4) The device of the interface‐type resistive switching shows a high enough device yield in the crossbar array device and exhibits reliable multiply‐and‐accumulate operations in the crossbar array to mimic the human brain‐inspired computing system.

A broad compositional range of Nb-Ti anodic memristors with volatile and self-rectifying behaviour was studied using a combinatorial screening approach. A Nb-Ti thin-film combinatorial library was co-deposited by sputtering, serving as the bottom electrode for the memristive devices. The library, with a compositional spread ranging between 22 and 64 at.% Ti was anodically oxidised, the mixed oxide being the active layer in MIM-type structures completed by Pt discreet top electrode patterning. By studying I–U sweeps, memristors with self-rectifying and volatile behaviour were identified. Moreover, all the analysed memristors demonstrated multilevel properties. The best-performing memristors showed HRS/LRS (high resistive state/low resistive state) ratios between 4 and 6 × 105 and very good retention up to 106 successive readings. The anodic memristors grown along the compositional spread showed very good endurance up to 106 switching cycles, excluding those grown from alloys containing between 31 and 39 at.% Ti, which withstood only 10 switching cycles. Taking into consideration all the parameters studied, the Nb-46 at.% Ti composition was screened as the parent metal alloy composition, leading to the best-performing anodic memristor in this alloy system. The results obtained suggest that memristive behaviour is based on an interfacial non-filamentary type of resistive switching, which is consistent with the performed cross-sectional TEM structural and chemical characterisation.

A memristive crossbar array (MCA) is an ideal platform for emerging memory and neuromorphic hardware due to its high bitwise density capability. A charge trap memristor (CTM) is an attractive candidate for the memristor cell of the MCA, because the embodied rectifying characteristic frees it from the sneak current issue. Although the potential of the CTM devices has been suggested, their practical viability needs to be further proved. Here, a Pt/Ta2O5/Nb2O5‐x/Al2O3‐y/Ti CTM stack exhibiting high retention and array‐level uniformity is proposed, allowing a highly reliable selector‐less MCA. It shows high self‐rectifying and nonlinear current‐voltage characteristics below 1 µA of programming current with a continuous analog switching behavior. Also, its retention is longer than 105 s at 150 °C, suggesting the device is highly stable for non‐volatile analog applications. A plausible band diagram model is proposed based on the electronic spectroscopy results and conduction mechanism analysis. The self‐rectifying and nonlinear characteristics allow reducing the on‐chip training energy consumption by 71% for the MNIST dataset training task with an optimized programming scheme. [ABSTRACT FROM AUTHOR]

Charge-trap based resistive switching (RS) has attracted attention in the resistive random-access memory (RRAM) industry due to its gradual RS behavior for multi-level and synaptic applications. In this work, in order to lower the operating current level closely related to device’s degradation, we applied a hydrogen passivation to Zr3N2 based RRAM devices and investigated the correlation between current level and trap density, such as an interface trap density ( $\text{N}_{\mathrm {it}}$ ) at the Zr $_{3}\text{N}_{2}/p$ -Si layer and nitride trap density ( $\text{N}_{\mathrm {nt}}$ ) within Zr3N2 films, for memory cells annealed in conventional N2 gas as well as H2 gas. Compared to the N2-annealed sample, after H2 annealing, $\text{N}_{\mathrm {it}}$ is lowered by the hydrogen passivation effect, which results in a reduction of both current level at high resistive state (HRS) and variation of HRS and low resistive state (LRS). As a result, in the H2 annealed Zr3N2 RRAM cell, we observed a lower operation voltage/current, longer endurance, and larger read margin due to the hydrogen passivation effect.

Abstract A memristive crossbar array (MCA) is an ideal platform for emerging memory and neuromorphic hardware due to its high bitwise density capability. A charge trap memristor (CTM) is an attractive candidate for the memristor cell of the MCA, because the embodied rectifying characteristic frees it from the sneak current issue. Although the potential of the CTM devices has been suggested, their practical viability needs to be further proved. Here, a Pt/Ta2O5/Nb2O5‐x/Al2O3‐y/Ti CTM stack exhibiting high retention and array‐level uniformity is proposed, allowing a highly reliable selector‐less MCA. It shows high self‐rectifying and nonlinear current‐voltage characteristics below 1 µA of programming current with a continuous analog switching behavior. Also, its retention is longer than 105 s at 150 °C, suggesting the device is highly stable for non‐volatile analog applications. A plausible band diagram model is proposed based on the electronic spectroscopy results and conduction mechanism analysis. The self‐rectifying and nonlinear characteristics allow reducing the on‐chip training energy consumption by 71% for the MNIST dataset training task with an optimized programming scheme.

Storage‐class memory and advanced neuromorphic‐related applications place an urgent requirement on large‐scale, highly compacted memristor array. However, crosstalk issue constrains the addressing accuracy and effective data scale of current array‐level memristor, which is detrimental to the industrial realization. Herein, scalability and physical processes of Al/AlN/W self‐rectifying memristor are investigated. The devices exhibit improved rectification ratio from 94 to 2600, switching ratio from 286 to 6099, higher uniformity, reliability, and nonvolatility by scaling the size from 10 to 5 µm. The application of aluminum nitride as active layer material not only makes the electrical properties more sensitive to the external electric field, but also dominates the switching process by constructing nitrogen vacancy conducting filaments, as demonstrated by a combination of scalability and mechanistic analysis. The high‐scaled AlN‐based self‐rectifying memristor exhibits a maximum effective array size of N = 14 372, which is 53 times enhancement compared to 10 µm size cells. The Al/AlN/W self‐rectifying memristor is confirmed to have a strong ability to suppress sneak currents and has the potential to serve large‐scale array‐level applications. [ABSTRACT FROM AUTHOR]

This study introduces a Ta 2 O 5 -based self-rectifying memristor (SRM) with an Al 2 O 3 interfacial layer adopted to improve switching uniformity, read voltage margin, and long-term retention. The Pt/Ta 2 O 5 /Al 2 O 3 /TiN (PTAT) device exhibits a 10 5 rectification ratio, 10 4 on/off ratio, 2 × 10 6 endurance, and retention of 10 4 s at 150 °C. A 3-layer 4 × 4 vertical resistive random access memory structure exhibits uniform switching parameters. The coefficient of variation (CV) for device-to-device measurements is 0.23 for the low resistance state (LRS) and 0.22 for the high resistance state (HRS), while for cycle-to-cycle measurements, the CV is 0.38 for the LRS and 0.11 for the HRS. Finally, the present study demonstrates the superior performance of the PTAT devices in the context of hardware-aware training for a fully connected neural network implementation. These advancements position the PTAT device as a promising candidate for high-density three-dimensional storage class memory and low-power neural networks, offering the consistent performance and reliability necessary for future high-density storage applications.

In this study, we present a correlation between the resistive switching (RS) characteristics and the traps of Zr 3 N 2 , which is based resistive random-access memory (RRAM) with a TiN barrier electrode, in order to identify the RS mechanism of the trap-based RRAM devices. First, gradual asymmetrical RS properties were discovered in the RS test, which included a current ratio of >2.51 × 102 and a large read margin of 480. As a result of analyzing the trap, the RS characteristics were deeply related to the nitride trap density (N nt) in the Zr 3 N 2 films as opposed to the interface trap density (N it) at Zr 3 N 2 / p -Si layer. In addition, in the trap evaluation for four different resistance states, we observed that the N nt was consistently increased with a decreasing resistance. These results indicate that the resistance state of the proposed TiN/Zr 3 N 2 / p -Si memory cell is determined by the N nt , which means that controlling the N nt is important in order to achieve a stable resistance state. [ABSTRACT FROM AUTHOR]

In this work, we report the resistive switching behavior of an amorphous La 2 Ti 2 O 7 (LTO) film as sandwiched between two Pt electrodes. The resistive switching is forming-free and highly uniform. Furthermore, it exhibits self-rectifying resistive switching behaviors owing to the Schottky contact and quasi-ohmic contact formed at the top and bottom interfaces, respectively. The mechanism of switching behavior in the device is attributed to the trapping/detrapping-mediated electronic bipolar resistance switching. By fitting the current-voltage characteristics, it indicates the coexistent conduction mechanisms of Schottky emission and space-charge-limited-conduction (SCLC), while the Schottky barrier modified by electron trapping/detrapping plays a dominating role in the resistive switching process. [ABSTRACT FROM AUTHOR]

Sneak path current phenomenon in a memristor array based on cross-point structure could lead to crosstalk, which limits its application in high-density large-scale arrays. Self-rectifying devices are the best choice for suppressing leakage currents without compromising the original performance of the device. It eliminates the need for selectors and effectively reduces process costs. In this work, a self-rectifying memristor based on Au/TaOx/Al2O3/TiN is proposed. Nonlinearity of ~236.76 with a sneak current below 1 nA with a tunable conductance under $1 \boldsymbol {\mu } \text{A}$ compliance current by introducing a rectifying switching mode after constructing the TaOx/Al2O3 different stoichiometric stack. Detailed analysis suggested that the interface barrier and the vacancy-related defects in Al2O3 are responsible for the special behavior. The effective array scale up to ~400 by readout margin calculation further confirms that the memristor can restrain the sneak current. Our self-rectifying memristor simplifies the device design of subsequent arrays, paving the way for addressing applications in highly compact scenes. [ABSTRACT FROM AUTHOR]

A physical transient memristor synapse with self-rectifying and analogue switching behaviors based on Mo/MgO/AZO (Al2O3 2 wt %, ZnO 98 wt %)/W is presented. By modulating thickness of MgO layer, the device with 5 nm MgO insulator layer shows stable resistive switching memory with a rectification ratio up to 102, and precise tuning synaptic functions. Meanwhile, the formation of Schottky barrier and oxygen vacancy conductive filament is proposed to further explain self-rectifying and analogue switching behaviors. Additionally, the electrical characteristics of the Mo/MgO/AZO/W devices degraded after being immersed in deionized water (DI) for 3 min, which demonstrated the physically transient features successfully. The physical transient self-rectifying memristors have great potential for applications in secure neuromorphic computing systems, and bio-integrated electronics. [ABSTRACT FROM AUTHOR]

Sneak path current generated by adjacent cells in three-dimensional (3D) memristor arrays must be curbed while securing the multi-bit storage capability of each cell to aid in the cost-effective increase in array size. For this purpose, a 3D stackable TaOx/HfO2-based selectorless memristor has been proposed and optimized via capacitance-dependent voltage division analysis. The proposed device utilizes the formation or rupture of conductive filaments for self-rectifying resistive switching operation, in contrast to nonfilamentary devices that often exploit the change in the charge state of the electron trap. This approach enables the reduction of the trapped charge leakage through the interface between the resistive switching and metal layers effectively, giving rise to excellent retention properties (>5 × 105 s). Furthermore, the proposed device exhibits a sufficiently high on/off ratio (~1.35 × 103), rectification ratio (~2.3 × 103), endurance (1.5 × 102 cycles), and low resistance variation (standard deviation

In this study, we present a self-rectifying resistive switching property observed in Ti/Zr 3 N 2 / p- Si and Ti/Zr 2 N/ p- Si metal-insulator-semiconductor (MIS) capacitors for array applications. Compared to the Zr 2 N film, the Zr 3 N 2 film has a higher trap density due to weaker bonding energy and a higher trap generation rate. In the experimental results for the Zr 3 N 2 sample, we observed a higher on/off ratio, a larger read margin, more stable retention, and more stable DC cycling. In addition, both the trap concentrations increased at the LRS compared to of the common HRS, which was a result of analyzing the nitride trap density (N nt) within the Zr x N y film and an interface trap (N it) between the Zr x N y and the p- Si layers. Particularly, it is found in the Zr 3 N 2 film that the N nt is rapidly increased, which results in an increased current ratio and an increased read margin. [ABSTRACT FROM AUTHOR]

We propose self-rectifying resistive random access memory (RRAM) synapse to prevent reverse leakage current problem which occurs when RRAM is integrated with integrate and fire (IF) circuit in spiking neural network (SNN). Ni/W/SiNx/n-Si RRAM was fabricated by varying the bottom electrode (BE) doping concentration and their rectifying characteristics were analyzed. Low BE doping concentration device showed self-rectifying characteristics without any additional selector or diode device. Furthermore, hardware-based system-level simulation was conducted to evaluate the effect of self-rectifying RRAM synapse on MNIST classification accuracy. About 93.34% accuracy was obtained using the proposed RRAM. [ABSTRACT FROM AUTHOR]

[Display omitted] • Self-rectifying optoelectronic synaptic memristor on Cu 3 SnS 4 -MoO 3 is innovatively developed. • Excellent rectification ratio (>4000) for self-inhibition of stealth current in crossbar arrays application. • Excellent RS performance of concentrated Set/Reset voltage distribution, On/Off ratio (>103) and retention (>104 s). • Mimic of diverse electrical-/light-stimulated synaptic plasticity (EPSC, PPF, LTP/LTD, STDP). • Potential for realizing neuromorphic architectures in brain-like computing system. Self-rectifying memristor with integrated excellent bio-synaptic behaviors are great potential to realize high-density memristor neuromorphic networks with self-inhibition of stealth current effect, but it's still inferior and challenging yet achievable. Here, optoelectronic memristor in accordance with p-n heterostructured Cu 3 SnS 4 -MoO 3 (CTS-MoO 3) is developed. The Ag/CTS-MoO 3 /Mo memristor exhibits stable nonvolatile resistive switching with excellent spatial uniformity and high self-rectifying characteristics (rectification ratio >4000), which is beneficial to implement crossbar memristive synapse architectures. The memristor demonstrates not only concentrated Set/Reset voltage distribution (variation < 0.04 V/0.01 V), high On/Off ratio (>103) and long retention time (>104 s), but also continuously modulable conductance by applying electric pulses (triangular and square-wave) or various light (470–808 nm) stimulus. This behavior makes such memristor the ability to emulate vital bio-synaptic functionalities including excitatory and inhibitory, short-/long-term plasticity, spike-timing-dependent plasticity, as well as the learning-forgetting-learning process and Ebbinghaus forgetting rule. Moreover, the recognition rate for MNIST handwritten digits in such memristor based artificial neural network model is verified to be 89.3 % for neuromorphic simulations. The results dramatically facilitate the development of self-rectifying optoelectronic artificial synapse for future neuromorphic applications. [ABSTRACT FROM AUTHOR]

Abstract A retention behavior model for self-rectifying TaO/HfO x - and TaO/AlO x -based resistive random-access memory (RRAM) is proposed. Trapping-type RRAM can have a high resistance state (HRS) and a low resistance state (LRS); the degradation in a LRS is usually more severe than that in a HRS, because the LRS during the SET process is limited by the internal resistor layer. However, if TaO/AlO x elements are stacked in layers, the LRS retention can be improved. The LRS retention time estimated by extrapolation method is more than 5 years at room temperature. Both TaO/HfO x - and TaO/AlO x -based RRAM structures have the same capping layer of TaO, and the activation energy levels of both types of structures are 0.38 eV. Moreover, the additional AlO x switching layer of a TaO/AlO x structure creates a higher O diffusion barrier that can substantially enhance retention, and the TaO/AlO x structure also shows a quite stable LRS under biased conditions.

Self-rectifying using a pure HfO2 layer with asymmetric structure was realized in Pt/HfO2/n+-Si resistive random access memory (RRAM). The device exhibits repeatable self-rectifying behavior with high rectifying ratio (>105). The electrostatic effect induced by the asymmetric structure of the HfO2 layer is responsible for the rectifying behavior. This kind of rectifying paves a new way for resolving the sneak current issue in RRAM arrays. [ABSTRACT FROM AUTHOR]

To effectively solve the crosstalk issue in high-density crossbar array (CBA), high rectifying characteristics should be introduced in the resistance random-access memory (ReRAM) device, and in-depth understanding of the affecting factors on rectifying properties is essential for the large-scale application of ReRAM. In this paper, a high-performance self-rectifying device with CMOS compatible Pd/HfO2/TaOx/Ta structure was demonstrated in a 1-kb CBA. Forming-free, self-compliance, and high uniformity characteristics were successfully achieved. By modulating the thickness of the HfO2 rectifying layer, the rectifying ratio of device could be achieved as high as $\sim 2\times 10^{\textsf {3}}$ under ±3 V at low-resistance state (LRS). It was also experimentally confirmed that the selected unit cell in high-resistance state (logically the “ OFF” state) was stably readable when it was surrounded by unselected LRS (logically the “ ON” state) cells, in an array of up to $32 \times 32$ cells. Furthermore, a model based on interfacial barrier modulation and defects trapping/detrapping was proposed to elucidate the impact of the dielectric thickness on the self-rectifying characteristics of the device. The results presented in this paper provide a great potential for selector-free high-density memory applications. [ABSTRACT FROM AUTHOR]

Here, we propose a Pt/HfO2/TaOx/TiN artificial synaptic device that is an excellent candidate for artificial synapses. First, XPS analysis is conducted to provide the dielectric (HfO2/TaOx/TiN) information deposited by DC sputtering and atomic layer deposition (ALD). The self-rectifying resistive switching characteristics are achieved by the asymmetric device stack, which is an advantage of the current suppression in the crossbar array structure. The results show that the programmed data are lost over time and that the decay rate, which is verified from the retention test, can be adjusted by controlling the compliance current (CC). Based on these properties, we emulate bio-synaptic characteristics, such as short-term plasticity (STP), long-term plasticity (LTP), and paired-pulse facilitation (PPF), in the self-rectifying I–V characteristics of the Pt/HfO2/TaOx/TiN bilayer memristor device. The PPF characteristics are mimicked by replacing the bio-stimulation with the interval time of paired pulse inputs. The typical potentiation and depression are also implemented by optimizing the set and reset pulse. Finally, we demonstrate the natural depression by varying the interval time between pulse inputs.

To efficiently increase the capacity of resistive switching random-access memory (RRAM) while maintaining the same area, a vertical structure similar to a vertical NAND flash structure is needed. In addition, the sneak-path current through the half-selected neighboring memory cell should be mitigated by integrating a selector device with each RRAM cell. In this study, an integrated vertical-type RRAM cell and selector device was fabricated and characterized. TaO as the switching layer and TaON as the selector layer were used to preliminarily study the feasibility of such an integrated device. To make the side contact of the bottom electrode with active layers, a thick AlO insulating layer was placed between the Pt bottom electrode and the TaO/TaON stacks. Resistive switching phenomena were observed under relatively low currents (below 10 μA) in this vertical-type RRAM device. The TaON layer acted as a nonlinear resistor with moderate nonlinearity. Its low-resistance-state and high-resistance-state were well retained up to 1000 s. [ABSTRACT FROM AUTHOR]

A retention behavior model for self-rectifying TaO/HfO- and TaO/AlO-based resistive random-access memory (RRAM) is proposed. Trapping-type RRAM can have a high resistance state (HRS) and a low resistance state (LRS); the degradation in a LRS is usually more severe than that in a HRS, because the LRS during the SET process is limited by the internal resistor layer. However, if TaO/AlO elements are stacked in layers, the LRS retention can be improved. The LRS retention time estimated by extrapolation method is more than 5 years at room temperature. Both TaO/HfO- and TaO/AlO-based RRAM structures have the same capping layer of TaO, and the activation energy levels of both types of structures are 0.38 eV. Moreover, the additional AlO switching layer of a TaO/AlO structure creates a higher O diffusion barrier that can substantially enhance retention, and the TaO/AlO structure also shows a quite stable LRS under biased conditions. [ABSTRACT FROM AUTHOR]

The n-ZnO/p-NiO junctions have been fabricated by sol-gel method using Al as top electrodes and ITO as bottom electrodes for applications in resistive switching devices. Such devices exhibit homogenous and filamentary characteristics depending on the amplitude of applied bias. The two switching types show different switching polarities and transport mechanisms. Under a higher bias, the filamentary behavior is dominated by Ohmic conduction at low resistance state and trap related Poole-Frenkel conduction at high resistance state, while under a lower bias the homogenous switching exhibits diode conduction at high resistance state and space charge limited current at low resistance state. The homogenous switching shows self-rectifying effect with a good endurance. It may open up a simple route to suppress the sneak current in a p-oxide/n-oxide device while maintaining reasonable good resistive switching and self-rectifying properties. Graphical Abstract: [ABSTRACT FROM AUTHOR]

• The compatible fabrication strategy combines the typical memristor with the self-rectifying memristor. • Adjustable memristor can be realized by changing the doping ratio and voltage frequency. • Self-rectifying memristor was realized by using the green material sodium alginate (SA) + ZnO NPs. • Stable and reliable memristor function exists in the self-rectifying memristor. In this paper, a fabrication strategy that is compatible with the typical memristor (TM) and the self-rectifying memristor (SRM) is proposed. By modulating the concentration of ZnO NPs in SA (sodium alginate) based memristor, the device will transition from TM to SRM. The Ag/SA + ZnO NPs/ITO device exhibits a great rectification effect under a low operating voltage, with a large ON/OFF parameter and very good self-rectifying properties. At the same time, by changing the concentration of ZnO NPs and voltage frequency, the adjustable effect of memristors can be achieved in the device. These results will be helpful to promote the development of flexible electronics, intelligent computing, and bionics. [ABSTRACT FROM AUTHOR]

Memristor-based gas sensors (gasistors) have been considered as the most promising candidate for detecting NO gas suitable for neural network (NN) analysis. In this work, in order to solve an overfitting issue arising from the training data when using a single gasistor, which degrades the accuracy of NN, we here propose a metal-insulator-silicon (MIS)-structured Zr 3 N 4 -based gasistor array that results in an improvement in both the accuracy of the NN analysis and the efficiency of the operating power. As a result, the proposed gasistor array showed a decrease of epoch and a 2.5% improvement of prediction accuracy at room temperature compared to single cells with metal/insulator/metal (MIM) and MIS structures. These results imply that an array structure based on MIS can efficiently solve the overfitting issue by receiving multiple responses at once, compared to a single gas sensor that obtains one response per sensing.

A Pt/NbOx/TiOy/NbOx/TiN stack integrated on a 30 nm contact via shows a programming current as low as 10 nA and 1 pA for the set and reset switching, respectively, and a self-rectifying ratio as high as ∼105, which are suitable characteristics for low-power memristor applications. It also shows a forming-free characteristic. A charge-trap-associated switching model is proposed to account for this self-rectifying memrisive behavior. In addition, an asymmetric voltage scheme (AVS) to decrease the write power consumption by utilizing this self-rectifying memristor is also described. When the device is used in a 1000 × 1000 crossbar array with the AVS, the programming power can be decreased to 8.0% of the power consumption of a conventional biasing scheme. If the AVS is combined with a nonlinear selector, a power consumption reduction to 0.31% of the reference value is possible. [ABSTRACT FROM AUTHOR]

To resolve the sneak leakage problem and reduce the power consumption in crossbar RRAM arrays, a Cu/Al2O3/aSi/Ta cell with self-rectifying characteristics is developed. The cell exhibits low operating current ( $\sim $ nA), high ON/OFF ratios ( > 100\times $ ), and pronounced nonlinearity. The use of low-programming-current RRAM elements avoids the current-driving capability bottleneck of selectors, while the integrated rectifying layer improves the RRAM operation reliability. Endurance of over 500 cycles with \sim 100$ ON/OFF ratio was achieved without external current compliance. Even at such low programming levels, retention over s at 100 °C can still be obtained. [ABSTRACT FROM AUTHOR]

In this work, a self-rectifying property of silicon nitride (Si 3 N 4 )/silicon (Si)-based one diode type resistive random access memory (RRAM) device with silicon oxide (SiO 2 ) tunnel barrier is demonstrated. The RRAM devices, switching layers consisted of Si 3 N 4 /SiO 2 /Si, are fabricated by a low-pressure chemical vapor deposition and dry oxidation, and revealed intrinsic diode property, so as to remove unwanted sneak path currents from an RRAM cross-bar array without extra switching devices. In addition, compared to Pt/Si 3 N 4 /Ti RRAM cells, whole operating current levels in proposed cells have been lowered about ∼10 3 since introduced Si bottom electrode and SiO 2 tunnel barrier efficiently suppress the current in both low and high resistive states. Consequently, these results show that the Si 3 N 4 -based one diode type RRAM cells warrant the realization of selector-free RRAM cell in the high density crossbar array. [ABSTRACT FROM AUTHOR]

[Display omitted] • The flower-like δ-MnO 2 was prepared on FTO substrate by chemical bath deposition. • Analog-to-digital and self-rectifying resistive switching behavior are achieved in the device. • Multi-level digital resistive switching behavior is realized by regulating compliance current. • This work opens up a new way for facilitating development of analog–digital circuits. Memristors can be divided into two types, analog and digital, according to the characteristics of current gradients and sudden changes. It is particularly important to realize analog and digital switching behaviors in the same device to facilitate the development of analog–digital circuits. In this work, the flower-like δ-MnO 2 was prepared on fluorine doped tin oxide (FTO) substrate by chemical bath deposition method, and it can be used as dielectric layer of memristor. Analog and digital types of resistive switching behaviors were achieved in the same device with Ag/δ-MnO 2 /FTO structure by adjusting the applied voltage range on top electrode of memristor. Moreover, the digital type of resistive switching behavior with multi-level and self-rectifying properties was realized in this device. The charge transport and resistive switching mechanism of the device with analog to multi-level self-rectifying digital types was explained by analyze its current–voltage (I-V) curve. Therefore, this new type of memristive device with both analog and digital properties can simplify the circuit complexity, and the multilevel and self-rectification behavior can greatly improve the storage density and solve the misreading problem of memristive devices. [ABSTRACT FROM AUTHOR]

In this work, we have implemented self-rectifying TaO x /TiO 2 RRAM in a selector-less 6 × 6 crossbar array with various desiring features, including: (1) simple fabrication using only three masks, (2) high self-rectifying ratio up to 10 3 for sneak current suppression, (3) stable bipolar resistive-switching characteristics without the need for electro-forming and current compliance, (4) data retention time over 10 4 s, and (5) robust READ and WRITE disturb immunity. Finally, an achievable array size of 1 Mb was simulated using an All-LPU read scheme and a V/3 write scheme. [ABSTRACT FROM AUTHOR]

We reported that the resistive switching of Ag/In-Ga-Zn-O/Pt cells exhibited self-rectifying performance at low-resistance state (LRS). The self-rectifying behavior with reliability was dynamic at elevated temperature from 303 to 393 K. The Schottky barrier originated from the interface between Ag electrode and In-Ga-Zn-O films, identified by replacing Ag electrode with Cu and Ti metals. The reverse current at 1.2 V of LRS is strongly suppressed and more than three orders of magnitude lower than the forward current. The Schottky barrier height was calculated as approximately 0.32 eV, and the electron injection process and resistive switching mechanism were discussed. [ABSTRACT FROM AUTHOR]

Resistance random access memory (ReRAM) has received attention as next-generation memory because of its excellent operating properties and high density integration capability as a crossbar array. However, the application of the existing ReRAM as a crossbar array may lead to crosstalk between adjacent cells due to its symmetric I- V characteristics. In this study, the self-rectifying effect of contact between amorphous In-Ga-Zn-O (a-IGZO) and TaO was examined in a Pt/a-IGZO/TaO/AlO/W structure. The experimental results show not only self-rectifying behavior but also forming-free characteristics. During the deposition of a-IGZO on the TaO, an oxygen-rich TaO interfacial layer was formed. The rectifying effect was observed regardless of the interface formation and is believed to be associated with Schottky contact formation between a-IGZO and TaO. The current level remained unchanged despite repeated DC sweep cycles. The low resistance state/high resistance state ratio was about 10 at a read voltage of −0.5 V, and the rectifying ratio was about 10 at ±2 V. [ABSTRACT FROM AUTHOR]

A bipolar RRAM device based on /2/n+- structure with self-rectifying characteristics is demonstrated for high density cross-bar memory application. Experimental results indicate that conductive filament generated at LRS plays an important role in resistive switching, resulting in the formation of a Schottky junction at the -CF/n+- interface which determines the self-rectifying behavior at LRS. These results are very important from the point of view of understanding the self-rectifying switching mechanism and improving the resistive switching characteristics of self-rectifying RRAM devices. [ABSTRACT FROM AUTHOR]

Although experimental implementations of memristive crossbar arrays have indicated the potential of these networks for in-memory computing, their performance is generally limited by an intrinsic variability on the device level as a result of the stochastic formation of conducting filaments. A tunnel-type memristive device typically exhibits small switching variations, owing to the relatively uniform interface effect. However, the low mobility of oxygen ions and large depolarization field result in slow operation speed and poor retention. Here, we demonstrate a quantum-tunneling memory with Ag-doped percolating systems, which possesses desired characteristics for large-scale artificial neural networks. The percolating layer suppresses the random formation of conductive filaments, and the nonvolatile modulation of the Fowler-Nordheim tunneling current is enabled by the collective movement of active Ag nanocrystals with high mobility and a minimal depolarization field. Such devices simultaneously possess electroforming-free characteristics, record low switching variabilities (temporal and spatial variation down to 1.6 and 2.1%, respectively), nanosecond operation speed, and long data retention (>10 4 s at 85 °C). Simulations prove that passive arrays with our analog memory of large current-voltage nonlinearity achieve a high write and recognition accuracy. Thus, our discovery of the unique tunnel memory contributes to an important step toward realizing neuromorphic circuits.

Ferroelectric field-effect transistors (Fe-FET) are promising candidates for future information devices. However, they suffer from low endurance and short retention time, which retards the application of processing memory in the same physical processes. Here, inspired by the ferroelectric proximity effects, we design a reconfigurable two-dimensional (2D) MoS 2 transistor featuring with asymmetric ferroelectric gate, exhibiting high memory and logic ability with a program/erase ratio of over 10 6 and a self-rectifying ratio of 10 3 . Interestingly, the robust electric and optic cycling are obtained with a large switching ratio of 10 6 and nine distinct resistance states upon optical excitation with excellent nonvolatile characteristics. Meanwhile, the operation of memory mimics the synapse behavior in response to light spikes with different intensity and number. This design realizes an integration of robust processing memory in one single device, which demonstrates a considerable potential of an asymmetric ferroelectric gate in the development of Fe-FETs for logic processing and nonvolatile memory applications.

Turbines for wave energy conversion have a special feature to be taken into account in the study of the tip leakage flow: These turbines are self-rectifying, which work inside a cyclically bidirectional flow alternatively as an inflow/outflow turbine. The phenomena at the blade tip will be different in these two situations. Moreover, it is necessary to take into account the tip clearance of the guide vanes because it has a significant influence on the rotor performance. A previously developed numerical model has been used for this study. The geometry proposed by Setoguchi et al. (2002, "A Performance Study of a Radial Impulse Turbine for Wave Energy Conversion," Journal of Power and Energy, 216, pp. 15-22) is used in the model. Three different tip clearance sizes have been simulated to compare the influence of the tip clearance size on the performance. Results show that changing the size of the tip clearance from 0% to 4% of the blade span reduces the turbine maximum efficiency by up to 8%. However, the efficiency reduction is more pronounced when the turbine works as an inflow turbine because the tip clearance effect is more important in the inner part of the rotor, since flow velocities are higher and the relative casing motion is lower. This study achieves its main aim, which is to improve knowledge about the phenomena related to the tip clearance and its influence on the performance of radial impulse turbines. [ABSTRACT FROM AUTHOR]

Resistive random access memory (RRAM) with crossbar structure is receiving widespread attentions due to its simple structure, high density, and feasibility of three-dimensional (3D) stack. It is an extremely promising solution for high density storage. However, a major issue of crosstalk restricts its development and application. In this paper, we will first introduce the integration methods of RRAM device and the existing crosstalk phenomenon in passive crossbar array, and then focus on the 1D1R (one diode and one resistor) structure and self-rectifying 1R (one resistor) structure which can restrain crosstalk and avoid misreading for the passive crossbar array. The test methods of crossbar array are also presented to evaluate the performances of passive crossbar array to achieve its commercial application in comparison with the active array consisting of one transistor and one RRAM cell (1T1R) structure. Finally, the future research direction of rectifying-based RRAM passive crossbar array is discussed. [ABSTRACT FROM AUTHOR]