Compliance Current Research Articles

Toward Simultaneous Achievement of Outstanding Durability and Photoelectrochemical Reaction in Cu2O Photocathodes via Electrochemically Designed Resistive Switching

AbstractPhotoelectrochemical (PEC) cells using Cu2O, semiconductor photoabsorbers passivated by protection layers, show a trade‐off between high photocurrent and stability because of the thickness of the energy band transport along the conduction band. Based on nanofilaments with non‐volatile metal‐like current flow characteristics in resistance‐change memory devices, a strategically advanced conducting filament transport mechanism for vigorous and robust PEC operation is proposed. The breakdown‐like electrochemical forming behavior effectively occurs with a rapid increase in current at ≈2 V (vs RHE). The fundamental properties of filaments, such as diameter, density, and conductivity, are controlled by varying the artificial compliance currents. This process does not require any top electrodes that obstruct light‐harvesting and the injection of photo‐charges into electrolytes or individual forming process with point‐by‐point sweeping, and provides electrochemical forming sites with homogeneous and dense distribution. Additionally, some photocorrosive sites that induce photocurrent degradation are passivated by the preferential photoelectrodeposition of co‐catalysts. From the electrochemical filament forming process and selective Pt‐photoelectrodeposition on filaments, the Cu2O/AZO/TiO2 photocathodes exhibit an unprecedented photocurrent density of ≈11.9 mA cm−2 and open‐circuit potential of 0.73 V and produce vigorous hydrogen and oxygen evolutions for over 100 h, even when the TiO2 passivation film exceeds 100 nm in thickness.

Clarifying the switching layer transformation through analysis of an abnormal I–V curves with increasing set compliance current in oxide-based resistive random access memory

This work investigates an abnormal rising current in high resistance state (HRS) during the reset process that occurs when increasing the sweeping set compliance current (SCC). When the SCC is increased, the HRS current read at a lower voltage (−0.1 V) remains similar, yet exhibits a gradual rise when read at a higher voltage (−0.8 V). In order to clarify this abnormal phenomenon, the current fittings were performed to obtain the corresponding switching layer (SL) properties after set operations with different SCCs. Furthermore, COMSOL electric field simulations with different SL properties were executed to verify the mechanism.

Effect of conducting filament radius on local temperature and activation power of ON-state ReRAM device

The formation and disruption of conducting filament (CF) are responsible for the SET/RESET switching of resistive random access memory (ReRAM). The ReRAM enters into a low resistive state soon after the complete formation of CF and the process is followed by the radial growth of CF during the ON-state. The present investigation aims to develop a numerical and analytical model to realize the effect of CF radius on local temperature rise and activation power, combining the thermal reaction model (TRM) and power signature model. The numerical investigation is carried out using finite element method to obtain the local temperature rise and temperature distribution across the device. The numerical results show a similar increment pattern of local temperature as of analytical TRM i.e. the local temperature increases linearly with a rapid growth rate of ∼63% in initial CF radii and then gradually attains a steady-state temperature with ∼10% increment in higher CF radii. The relationship between resistance and compliance current is calculated which obeys the same reported trend in other switching oxides like TiO X , CuO, NiO. Additionally, the activation power is determined where the resistance decreases (101.87–0.25 KΩ) due to the CF radial growth, which results in four orders of exponential increment in activation power from ∼0.04 µW to 500 ± 100 µW. The power signature of numerical TRM is validated with that analytical TRM where the device attends a steady-state temperature after a certain CF radius (>25 nm) along with the activation power.

Enhancing the Switching Performance of CH3NH3PbI3 Memristors by the Control of Size and Characterization Parameters

AbstractThis paper analyzes the effects of scaling and measurement conditions on the performance of hybrid organic inorganic perovskite (HOIP) devices having bipolar resistive switching characteristics. Electroforming voltage, SET current, and ON/OFF ratios are found to be area dependent. Linear voltage sweep and pulse voltage characterization showed that parameters such as compliance current, voltage scan rate, pulse width, amplitude, and frequency significantly alter the device switching parameters. The CH3NH3PbI3 memristors fabricated has FORMING, SET, and RESET voltages of 1.18, 0.17, and −0.13 V, respectively, along with an ON/OFF ratio of 1.3 × 103. It has been found that, both the RESET voltage and current significantly increase with scan rate, whereas the ON/OFF ratio increases significantly with compliance current. In case of voltage pulse characterization, the ON/OFF ratio can be enhanced by increasing the pulse amplitude and width. Additionally, the experimental data fitting to a SPICE based analytical model establishes that the HOIP memristor current conduction is dominated by tunneling process in the OFF state and is ohmic in the ON state. Based on the results, the authors arrive at a general protocol that can be used to characterize memristors made of HOIPs and other related materials such that they can operate optimally.

Coexistence of volatile and non-volatile resistive switching in Ni/SiO2/Pt memristor device controlled from different current compliances

We fabricated a Ni/SiO2/Pt resistive random-access memory device referred to as a memristor, which demonstrates both volatile and non-volatile switching. A reversible transition between volatile and non-volatile switching is achieved by applying different current compliances (CCs). Volatile switching was observed when the CC is 50 nA, whereas volatile switching changed to non-volatile switching when the CC is 100 nA. The switching mechanism model can be explained by the formation and rupture of conducting filaments by electrochemical metallization effects, which are vital in resistive switching. We demonstrate a simple pulse measurement method for both switching behaviors. In a single Ni/SiO2/Pt memristor device, short-term memory for volatile switching and a long-term memory synaptic function for non-volatile switching is observed under repeated stimuli. Owing to the distinctive characteristics of the Ni/SiO2/Pt device, the short-term synaptic function, i.e. paired-pulse facilitation and depression for volatile switching, is successfully mimicked by the memristor. These electrical results indicate that the Ni/SiO2/Pt memristor device with synaptic behavior has potential for use in brain-inspired computing systems.

Research on Improving the Working Current of NbOx-Based Selector by Inserting a Ti Layer

To achieve the highest possible integration storage density in the V-point structure, the working current of the selector in the one-selection one-resistance (1S1R) structure should match with the resistance random access memory (RRAM). In this study, a selector device is designed with a Ti/NbOx/Ti/Pt structure through the magnetron sputtering method and achieves excellent performance of threshold switching under ultra-large compliance current (CC) up to 100 mA. Furthermore, both the switching voltages and the OFF-state resistance of the device demonstrate excellent stability even when CC is increased to a milliampere level, attributed from the existence of metallic NbO in the switching layer. This study provides evidence that a Ti/NbOx/Ti/Pt device has a great potential to drive RRAM in the V-point structure.

Reliable Resistive Switching and Synaptic Behaviors Based on a TiOx‐Doped N Memristor for Information Storage and Neuromorphic Computing

Bipolar resistive switching (RS) and synaptic behaviors of resistive random access memory (RRAM) based on TiO x are demonstrated. RS uniformity is improved by introducing nitrogen into the RS layer using radio frequency sputtering in the reactive Ar/N2 ambient. The conductive mechanism is in good agreement with the space−charge‐limited conduction model. The activation energy fit by the Arrhenius equation and conductive atomic force microscopy results indicate that the conductive filaments are formed by oxygen vacancies. More importantly, reliable multilevel RRAM can be achieved by tuning the compliance current, which enables the achievement of distinguishable resistance states. Furthermore, multilevel RRAM enables the simulation of synaptic functions, such as learning−forgetting−relearning, habituation, and spike‐timing‐dependent plasticity (STDP). Image pattern recognition based on STDP learning rules using a digital memristor is demonstrated. The findings may offer a route to the development of future storage and neuromorphic computing.

SecureMem: efficient flexible Pt/GO/Cu memristor for true random number generation

This work reports on flexible cost-effective memristor device synthesis, using Pt/GO/Cu structure. The memristor is fabricated on flexible substrate and it preserves its switching ability at a bending angle of up to 60°. Compared to the state of the art, in our fabrication method, the graphene oxide (GO) film is deposited directly to the device using spin coating. The fabricated memristor stack exhibits tunable volatility based on the used compliance current, which expands its application horizon for memory, computing and security schemes. In this work, the non-volatile regime along with the natural stochasticity of the switching behavior of the device, named SecureMem, are utilized for efficient true random number generation. A prototype of the full system interfacing with microcontroller is built to get SecureMem ready for integration with other circuits and systems. The randomness of the data generated by SecureMem prototype is confirmed by passing all National Institute of Standards and Technology tests without any post-processing or hardware overhead. SecureMem is considered a great asset as it provides new insights for highly efficient cost-effective hardware security in smart wearable devices.

Write-once-read-many-times characteristics of CuO layer with Ag conductive bridges

This paper demonstrates write-once-read-many-times (WORM) behavior of an Ag/CuO/n+-Si memory device. The CuO resistive switching layer with a thickness of 9.7 nm was prepared using a sol–gel process. The device shows a low conduction current of 10−10 A at a read voltage of 1 V. A sudden current increase is observed when the voltage increases to ∼3 V, corresponding to a resistance switching from a high-resistance state (HRS) to a low-resistance state (LRS). After that, the device remains in the LRS. Degradation of the HRS and the LRS is not observed for 104 read cycles. The HRS/LRS resistance ratio is 106, and the data retention time is over 105 s. The abrupt current increase cannot be triggered by a negative bias. The formation of Ag conductive bridges in the CuO layer is responsible for the resistance switching. Statistical analyses are performed to study the distributions of the set voltages and the resistances in the HRS and LRS. The effect of the compliance current on the operating current of the Ag/CuO/n+-Si WORM memory is also investigated. The carrier transport mechanisms are found to be dominated by Schottky emission, Ohmic, and SCLC conductions. The energy barrier is 1.066 eV, which is extracted by Schottky emission analysis.

Emulation of multiple-functional synapses using V2C memristors with coexistence of resistive and threshold switching

Mxenes, an emerging class of two-dimensional materials, have attracted extensive attention due to their unique electronic properties. One of the MXenes that has been successfully synthesized recently is vanadium carbide MXene. In this work, this new member of MXenes, V2C, was applied as the active layers of memristors involving Ag and W electrodes. The transition from threshold switching to resistive switching was found in the V2C-based memristors which could be achieved via tuning compliance current. The essence of the switching mechanisms was assumed to be the diffusion process of Ag ions. This process can functionally resemble Ca2+ dynamics in synaptic transmission. Together with a positive analog behavior observed, the coexistence of the threshold and resistive switching mechanisms eventually allowed the V2C-based memristors to emulate various synaptic functions. Furthermore, the study of intrinsic frequency dependence of the V2C memristor also indicated that both spike-timing-dependent plasticity (STDP) and spike-rate-dependent plasticity (SRDP) could be achieved on a single V2C memristor without complex pulse engineering or spike overlapping, which may dramatically reduce the complexity of both circuit and algorithm design. The V2C-based memristors thus show great potential to promote the efficiency and scalability of brain-inspired computing systems in the future by simplifying the artificial synapses design.

Flexible Nonvolatile Bioresistive Random Access Memory with an Adjustable Memory Mode Capable of Realizing Logic Functions.

In this study, a flexible bioresistive memory with an aluminum/tussah hemolymph/indium tin oxide/polyethylene terephthalate structure is fabricated by using a natural biological material, tussah hemolymph (TH), as the active layer. When different compliance currents (Icc) are applied to the device, it exhibits different resistance characteristics. When 1 mA is applied in the positive voltage range and 100 mA is applied in the negative voltage range, the device exhibits bipolar resistive switching behavior. Additionally, when 1 mA is applied in both the positive- and negative-voltage ranges, the device exhibits write-once-read-many-times (WORM) characteristics. The device has good endurance, with a retention time exceeding 104 s. After 104 bending cycles, the electrical characteristics remain constant. This memory device can be applied for “AND” and “OR” logic operations in programmable logic circuits. The prepared flexible and transparent biomemristor made of pure natural TH provides a promising new approach for realizing environmentally friendly and biocompatible flexible memory, nerve synapses, and wearable electronic devices.

Rapid microwave annealing of CH3NH3PbI3 with controllable crystallization for enhancing the resistive-switching performance

High switching fluctuation induced by the random formation and rupture of conductive filaments (CFs) is a critical issue for the development of organic-inorganic perovskite-based resistive switching (RS) memory devices. In this study, a rapid microwave-annealing method is developed to reduce grain boundaries (GBs) of CH3NH3PbI3 (MAPbI3) film and improve the switching reliability of the Au/MAPbI3/FTO device. The crystal grain size of MAPbI3 is increased and the corresponding GBs number is reduced with microwave irradiation time ranging from 40 to 120 s. In the optimized case, the dispersion of the RS parameters of the Au/MAPbI3/FTO cell is greatly reduced. Specifically, the fluctuations of the high/low resistance states and SET/RESET voltages dropped from 70.4%/69.5% and 40.5%/40.3% to 20.1%/26.7% and 16.0%/11.9%, respectively. The device can be operated with reduced compliance current. These results demonstrate that reducing the number of GBs is a feasible strategy to reduce the randomness of the CFs, and thus achieve more stable RS performance in MAPbI3-based memory devices.

Volatile Resistive Switching Characteristics of Pt/HfO2/TaOx/TiN Short-Term Memory Device

In this work, we study the threshold switching and short-term memory plasticity of a Pt/HfO2/TaOx/TiN resistive memory device for a neuromorphic system. First, we verify the thickness and elemental characterization of the device stack through transmission electron microscopy (TEM) and an energy-dispersive X-ray spectroscopy (EDS) line scan. Volatile resistive switching with low compliance current is observed under the DC sweep in a positive bias. Uniform cell-to-cell and cycle-to-cycle DC I-V curves are achieved by means of a repetitive sweep. The mechanism of volatile switching is explained by the temporal generation of traps. Next, we initiate the accumulation of the conductance and a natural decrease in the current by controlling the interval time of the pulses. Finally, we conduct a neuromorphic simulation to calculate the pattern recognition accuracy. These results can be applicable to short-term memory applications such as temporal learning in a neuromorphic system.

Gradually Modified Conductance in the Self-Compliance Region of an Atomic-Layer-Deposited Pt/TiO2/HfAlOx/TiN RRAM Device

This study presents conductance modulation in a Pt/TiO2/HfAlOx/TiN resistive memory device in the compliance region for neuromorphic system applications. First, the chemical and material characteristics of the atomic-layer-deposited films were verified by X-ray photoelectron spectroscopy depth profiling. The low-resistance state was effectively controlled by the compliance current, and the high-resistance state was adjusted by the reset stop voltage. Stable endurance and retention in bipolar resistive switching were achieved. When a compliance current of 1 mA was imposed, only gradual switching was observed in the reset process. Self-compliance was used after an abrupt set transition to achieve a gradual set process. Finally, 10 cycles of long-term potentiation and depression were obtained in the compliance current region for neuromorphic system applications.

Short-term and long-term synaptic plasticity in Ag/HfO2/SiO2/Si stack by controlling conducting filament strength

This article presents diverse resistive switching properties in Ag/HfO2/SiO2/Si memristor device for neuromorphic computing. We demonstrate that two characteristics of long term and short-term memory can be implemented in a single device depending on the strength of the applied signal. The elemental analysis, chemical characterization, and accurate film thickness of the device stack is confirmed by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The volatile resistive switching and non-volatile switching are dependent on the strength of the conducting filament. The short-term memory with a current decay is conducted by the low compliance current (CC) and a pulse with small voltage amplitude. On the other hand, the long-term memory with non-volatile properties is obtained by high CC and a pulse with large voltage amplitude. Potentiation and depression characteristics with multi-states are achieved for long-term plasticity, which is verified by the retention.

Gradual conductance modulation of Ti/WOx/Pt memristor with self-rectification for a neuromorphic system

In this study, we present an analysis of the gradually modulated conductance of the Ti/WOx/Pt memristor. The deposited material layers were verified by transmission electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The results revealed that the current level and the rectification behavior differed depending on the presence or absence of the compliance current (CC). If the current is controlled low at a CC of 100 μA, the current is suppressed at the positive voltage bias. It was verified through array simulation that this can mitigate the sneak current in the crossbar array structure. Finally, we conduct the potentiation and depression characteristics for an in-mode and a self-compliance mode and evaluate the pattern recognition accuracy of Modified National Institute of Standards and Technology database through neuromorphic simulation. The synaptic device with a self-rectifying behavior has considerable potential for the synapse array structure in a neuromorphic system.

Fully Transparent and Sensitivity-Programmable Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistor-Based Biosensor Platforms with Resistive Switching Memories

This paper presents a fully transparent and sensitivity-programmable biosensor based on an amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) with embedded resistive switching memories (ReRAMs). The sensor comprises a control gate (CG) and a sensing gate (SG), each with a resistive switching (RS) memory connected, and a floating gate (FG) that modulates the channel conductance of the a-IGZO TFT. The resistive coupling between the RS memories connected to the CG and SG produces sensitivity properties that considerably exceed the limit of conventional ion-sensitive field-effect transistor (ISFET)-based sensors. The resistances of the embedded RS memories were determined by applying a voltage to the CG–FG and SG–FG structures independently and adjusting the compliance current. Sensors constructed using RS memories with different resistance ratios yielded a pH sensitivity of 50.5 mV/pH (RCG:RSG = 1:1), 105.2 mV/pH (RCG:RSG = 2:1), and 161.9 mV/pH (RCG:RSG = 3:1). Moreover, when the RCG:RSG = 3:1, the hysteresis voltage width (VH) and drift rate were 54.4 mV and 32.9 mV/h, respectively. As the increases in VH and drift rate are lower than the amplified sensitivity, the sensor performs capably. The proposed device is viable as a versatile sensing device capable of detecting various substances, such as cells, antigens, DNA, and gases.

Alloyed High-k-Based Resistive Switching Memory in Contact Hole Structures

Resistive random-access memory (RRAM) devices are noticeable next generation memory devices. However, only few studies have been conducted regarding RRAM devices made of alloy. In this paper, we investigate the resistive switching behaviors of an Au/Ti/HfTiOx/p-Si memory device. The bipolar switching is characterized depending on compliance current under DC sweep mode. Good retention in the low-resistance state and high-resistance state is attained for nonvolatile memory and long-term memory in a synapse device. For practical switching operation, the pulse transient characteristics are studied for set and reset processes. Moreover, a synaptic weight change is achieved by a moderate pulse input for the potentiation and depression characteristics of the synaptic device. We reveal that the high-resistance state and low-resistance state are dominated by Schottky emissions.

Pulse and Compliance Current-Dependent Negative Differential Resistance in BaTiO3/Nb:SrTiO3 Heterojunctions

The negative differential resistance (NDR) is closely related to the pulse and compliance current (CC) but not ferroelectric polarization in BaTiO3/Nb:SrTiO3 heterojunctions. The NDR feature shows an opposite trend on the positive (+VP) and negative pulse (−VP) and the same trend on pulse amplitude and width while maintaining the same polarity. When the applied pulse was small or short, the NDR characteristics are almost not influenced. With increase of the +Vp (−Vp) amplitude or width, NDR enhances (reduces) while shifting to a smaller (bigger) negative bias voltage (−Vb), and the low resistance state (LRS) reduces (enhances) while the high resistance state (HRS) almost remains unchanged. Furthermore, as CC increases, the NDR enhances markedly and barely shifts, while the LRS and HRS almost remain unchanged. Interestingly, the NDR location and LRS exhibit the same dependence on the amplitude/width of ±Vp and CC. The above behaviors of NDR can be attributed to the variance of oxygen vacancy (Vo) concentration and depletion layer width. It is beneficial for the development of NDR devices.

Any-polar resistive switching behavior in Ti-intercalated Pt/Ti/HfO2/Ti/Pt device* *Project supported by the National Natural Science Foundation of China (Grant Nos. 62004087, 61474081, and 61534005), the Natural Science Foundation of Fujian Province, China (Grant No. 2020J01815), the Natural Science Foundation of Zhangzhou, China (Grant No. ZZ2020J32), and the Natural Science Foundation of Jiangxi Province, China (Grant No. 20192ACBL20048).

The special any-polar resistive switching mode includes the coexistence and stable conversion between the unipolar and the bipolar resistive switching mode under the same compliance current. In the present work, the any-polar resistive switching mode is demonstrated when thin Ti intercalations are introduced into both sides of Pt/HfO2/Pt RRAM device. The role of the Ti intercalations contributes to the fulfillment of the any-polar resistive switching working mechanism, which lies in the filament constructed by the oxygen vacancies and the effective storage of the oxygen ion at both sides of the electrode interface.