Low Current Compliance Research Articles

Controllable Electrical Properties of ZrO2/BiFeO3 Bilayer Memristor from Synaptic Mimic to TRNG Circuit Application by Modulating Compliance Currents

AbstractApart from simulating biological synapses, memristors can also be used in the secure encryption by exploiting their inherent random resistive switching (RS) properties. In this work, nonvolatile Ta/BiFeO3/ZrO2/Pt memristor is fabricated with 2 nm inserting BiFeO3 layer. At a high compliance current of 10 mA, it presents gradual RS characteristics during the set process, which resulting in 30 conductance states. Synaptic behavior can be successfully mimicked by precise conductance modulating under pulse electrical stimulation. Whereas under a low compliance current of 500 µA, it exhibits abrupt RS behavior, and the set voltage ranged from 0.3–0.8 V, which can be effectively as an entropy source for true random number generator (TRNG). Due to its Shannon entropy of 300 bits is 0.99987, Hamming weights and intra‐Hamming distances tend to be 50%. Furthermore, the software simulation of encryption and decryption of cat image is achieved by combining the key generated by TRNG. The controllable electrical properties of the Ta/BiFeO3/ZrO2/Pt device can be modulated by the compliance current, thus meeting the computing or safety requirements, which provides a solid foundation for the development of integrating computation and security at the hardware level.

Nanoscale Ni/Mo/MoO3/Ni memristor for synaptic applications

AbstractFor the first time, a physics‐based modelling of a nanoscale Ni/Mo/MoO3/Ni memristor is presented in this letter by inserting a ‘Mo:Capping layer’ between the top electrode (Ni) and the insulating layer (MoO3). The proposed memristor has stable hysteresis I–V characteristics as well as a significant reduction in ‘Forming voltage’ (VFORM) to 0.75 V. The simulated resistive switching responses using the COMSOL Multiphysics package demonstrate consistently low values of coefficient of variability (CV) with 14.31% and 14.85% for the SET and RESET modules, respectively, during cycle‐to‐cycle variations along with a low compliance current (ICC) of 193 µA. In addition to observing synaptic plasticity behaviour, it also examines how ramp‐rates impact ‘Potentiation’ and ‘Depression’ as memristor conductance (G) is closely related to synaptic weights.

Versatile GeS-based CBRAM with compliance-current-controlled threshold and bipolar resistive switching for electronic synapses

Chalcogenide materials have promising physical and electrical characteristics for use in advanced memory and electronic synaptic devices. However, limited research has been conducted on chalcogenide materials for highly advanced and scalable memory devices. In this study, we investigate compliance current (CC)-controlled resistive switching (RS) in a monochalcogenide GeS-based conductive bridge random access memory (CBRAM) device for use in advanced memory and electronic synapses. A CBRAM device with a structure stacking sequence of Ag/GeS/Pt/Ti/SiO2 is fabricated by depositing a 13 nm GeS layer using a sputtering process. The GeS-based CBRAM device exhibits the reversible CC-controlled transition between threshold resistive switching (T-RS) and bipolar resistive switching (B-RS). Under a high CC (1 mA and above), the device exhibits B-RS behavior with excellent retention and highly reproducible endurance characteristics, illustrated by a very high ION/IOFF ratio of almost ∼2 × 108. Under a low CC (100 µA and below), the device exhibits T-RS, with the automatic transition from a low resistance state (LRS) to a high resistance state (HRS) when sweeping the voltage in the reverse direction. Under all CC conditions, the device has a very low HRS current of around ∼1 × 10−12 A. A model based on the formation and rupture of conductive filaments (CFs) is proposed to explain the coexistence of T-RS and B-RS in GeS-based CBRAM devices. In particular, the change in the size and geometry of the CFs in accordance with the applied CC is speculated to be the main reason for the coexistence of T-RS and B-RS. The retention characteristics of the device in both T-RS and B-RS mode are investigated for possible application to case-sensitive hardware-based data security and the disposal of trusted and untrusted information. The ability of the CBRAM device in T-RS mode to perform important biological synaptic functions for potential use in brain-inspired neuromorphic systems is also investigated.

Conductance Quantization Behavior in Pt/SiN/TaN RRAM Device for Multilevel Cell

In this work, we fabricated a Pt/SiN/TaN memristor device and characterized its resistive switching by controlling the compliance current and switching polarity. The chemical and material properties of SiN and TaN were investigated by X-ray photoelectron spectroscopy. Compared with the case of a high compliance current (5 mA), the resistive switching was more gradual in the set and reset processes when a low compliance current (1 mA) was applied by DC sweep and pulse train. In particular, low-power resistive switching was demonstrated in the first reset process, and was achieved by employing the negative differential resistance effect. Furthermore, conductance quantization was observed in the reset process upon decreasing the DC sweep speed. These results have the potential for multilevel cell (MLC) operation. Additionally, the conduction mechanism of the memristor device was investigated by I-V fitting.

Demonstration of Threshold Switching and Bipolar Resistive Switching in Ag/SnOx/TiN Memory Device

In this work, we observed the duality of threshold switching and non-volatile memory switching of Ag/SnOx/TiN memory devices by controlling the compliance current (CC) or pulse amplitude. The insulator thickness and chemical analysis of the device stack were confirmed by transmission electron microscope (TEM) images of the Ag/SnOx/TiN stack and X-ray photoelectron spectroscopy (XPS) of the SnOx film. The threshold switching was achieved at low CC (50 μA), showing volatile resistive switching. Optimal CC (5 mA) for bipolar resistive switching conditions with a gradual transition was also found. An unstable low-resistance state (LRS) and negative-set behavior were observed at CCs of 1 mA and 30 mA, respectively. We also demonstrated the pulse operation for volatile switching, set, reset processes, and negative-set behaviors by controlling pulse amplitude and polarity. Finally, the potentiation and depression characteristics were mimicked by multiple pulses, and MNIST pattern recognition was calculated using a neural network, including the conductance update for a hardware-based neuromorphic system.

Thermal-assisted electroforming enables performance improvement by suppressing the overshoot current in amorphous carbon-based electrochemical metallization memory

A thermal-assisted electroforming (TAE) method is proposed to address the current overshoot issue and improve the resistive switching (RS) performance of electrochemical metallization (ECM) memory with a Cu/amorphous carbon (a-C)/Pt structure. In the initial electroforming process, thermal treatment can promote the electrochemical formation of metallic conductive filament (CF). The required electroforming voltage is reduced, and the undesirable overshoot current is suppressed. As a result, the RS performance of the cell is improved, including reduced RS parameter fluctuations, enlarged off/on ratio, and enhanced cycling endurance. The cell can be operated with a low compliance current (ICC) of 50 μA, which is among the lowest value of reported a-C-based ECM memories. Benefit from the low ICC, multilevel memory with five nonvolatile resistance states in a single cell is obtained. The main role of the TAE method is interpreted as to avoid the extreme high-temperature caused by the overshoot current. Conductive-atomic force microscopy mapping implies that the suppression of overshoot current can avoid over-injection of Cu cations into the a-C layer, facilitating CF with a simple structure and low randomness. The present work offers a feasible approach for addressing the current overshoot issue and improving the performance of ECM memory.

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.

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.

Existence of bipolar and unipolar resistive switching in CaZrO3 thin film device

Resistive Random Memory (RRAM) effect has been investigated in Pt/CaZrO3/Pt/Si thin-film device, which is demonstrated bipolar resistive switching (BRS) and unipolar resistive switching (URS) phenomenon. BRS has a wide voltage window to SET and RESET the device during the repetition of the cycle. In the URS measurement, the device was SET at a lower voltage window and RESET to a higher voltage window. The resistance ratio (RHRS/RLRS) is ≈102 in BRS and 103 in URS, respectively. It revealed a good retention time ≈103 s and ≈104 s in BRS and URS, respectively, to retain the data. During URS measurement, the device was SET and RESET at a compliance current of 5 μA and 1 mA, respectively. Poole-Frenkel (P-F) and Schottky emission (S-E) conduction mechanisms provide unrealistic dielectric constant. SCLC conduction mechanism has been dominated in the phenomenon of resistive switching. Formation and rupturing of thin conducting filament depend upon the migration of oxygen vacancies and oxygen ions. This device illustrates BRS and URS phenomenon and low compliance current, which may be suitable for nonvolatile memories.

Localized Probing of Dielectric Breakdown in Multilayer Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) has emerged as a promising 2D/layered dielectric owing to its successful integration with graphene and other 2D materials, although a coherent picture of the overall dielectric breakdown mechanism in h-BN is yet to emerge. Here, we have carried out a systematic study using conduction atomic force microscopy to provide insights into the process of defect generation and dielectric degradation in the progressive breakdown (PBD) and hard breakdown (HBD) stages in 2-5 nm thick chemical vapor deposition (CVD)-grown multilayer h-BN films. The PBD and HBD regimes show different behaviors. Under electrical stress in the PBD stage, defects are generated progressively in the h-BN, leading to a gradual reduction of the effective barrier resistance and continuous soft breakdowns (SBDs) of the dielectric material. Random telegraph noise nano-spectroscopy shows that low frequency noise becomes dominant after an SBD event due to the creation of additional defects around the percolation path. We also observe a wide variation in the current-voltage (I-V) breakdown plots in the PBD stage, giving rise to non-Weibull statistical distribution of the breakdown voltage. We attribute this observation to the significant thickness inhomogeneity in the CVD films. At HBD, h-BN materials are always physically removed from the film, leading to the formation of pits at the breakdown location. Interestingly, pit formation is also occasionally observed in the PBD stage under very low current compliances, suggesting that breakdown may proceed by a mixture of defect generation and material removal in h-BN CVD films.

NiO-based memristor with three resistive switching modes

In this research, solution-processed NiO-based memristors have been demonstrated with three resistive switching modes, including analog resistive switching, volatile threshold resistive switching and nonvolatile digital resistive switching, where the specific behavior is determined by the applied bias voltage and compliance current. The analog resistive switching is achieved via local oxygen ion migration at the NiO/Ag interface under low voltage stress. Based on this resistive switching mode, several synaptic functions are mimicked. When the device is stressed using a high voltage and a low compliance current, the device behavior changes to volatile threshold resistive switching. This behavior allows spiking neuron functions to be simulated. Furthermore, under application of a high compliance current, the device behavior converts to nonvolatile digital resistive switching. Ag filament formation and rupture processes are considered to be the mechanism behind the volatile threshold and nonvolatile resistive switching behavior. This multifunctional NiO-based memristor will provide a basis for fabrication of memory devices, analog circuits and artificial neural networks.

Resistive switching memory and artificial synapse by using Ti/MoS2 based conductive bridging cross-points

The Ti/MoS2 based 9 × 9 conductive bridge cross-points in Al/Cu/Ti/MoS2/Pt structure and its suitability in brain-inspired neuromorphic application have been investigated. Repeatable forming-free multilevel switching cycles with the low current compliances (CCs) of 10 μA–300 μA and long program/erase (P/E) endurance of >7 × 108 with low P/E current of 10 μA/100 μA at a high-speed of 100 ns have been obtained than pure MoS2 based devices. Copper reduction-oxidation (redox) reaction associated resistive switching mechanism through cyclic-voltammetry is also investigated. Conductivity modulated potentiation and depression results as well as LTP/LTD characteristics have been explored. Significantly low energy consumption in between 17 fJ to 398 fJ has been obtained.

(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.

Reversible transition of volatile to non-volatile resistive switching and compliance current-dependent multistate switching in IGZO/MnO RRAM devices

A controllable and reversible transition of volatile and non-volatile resistive switching is presented in Ag/indium-gallium-zinc oxide (IGZO)/manganese oxide (MnO)/Pt bilayer resistive memory devices. The coexistence of volatile and non-volatile switching characteristics was demonstrated by controlling the current compliance during the SET process. With lower current compliance (<50 μA), the formation of an unstable conducting filament presented typical diode-like non-volatile switching. A reversible transition from volatile to non-volatile switching could be obtained by applying a higher current compliance (≥50 μA). Moreover, highly uniform multistate memory characteristics were achieved by modulating the current compliance in the non-volatile switching region. The coexistence of volatile and multistate non-volatile resistive switching behaviors with a reversible transition demonstrates the capability of developing a selector element for crossbar arrays and the application for next generation multistate high-density storage with the same Ag/IGZO/MnO/Pt device.

Set compliance current induced resistive memory characteristics of W/Hf/HfOx/TiN devices

In this paper, the authors have investigated the effect of current compliance during the set process on the resistive memory characteristics and switching mechanism of W/Hf/HfOx/TiN devices. The presence of an Hf thin cap layer enables the stable and uniform bipolar resistive switching behavior. Compliance current can modify the barrier height at the oxide-electrode interface by increasing or reducing the oxygen vacancies and induce different switching mechanisms. Low compliance current (50 μA) based switching confirms the Schottky conduction mechanism due to the interfacial effects, while high compliance current (500 μA) involves the ohmic conduction mechanism, signifying the formation of a conductive filament. No significant dispersion of reset current and reset voltage has been found for each set compliance current varying from 50 to 500 μA, indicating uniform performance of the devices. The devices also exhibited a read endurance up to 2000 cycles.

Neutral Oxygen Beam Treated ZnO-Based Resistive Switching Memory Device

The room-temperature oxidation process allows irradiation with neutral oxygen particles onto the resistive layer that leads to the absorption of oxygen by the surface of the ZnO layer. The irradiation is effective in controlling the defect concentrations; thus, the ON and OFF resistances of devices can be significantly increased. These characteristics promote the occurrence of resistive switching at much lower current compliance as well as induce switching behavior in very thin ZnO films with thicknesses of 14–42 nm. The thickness dependence of the transformation from filamentary to homogeneous switching was also studied using the neutral beam technique, and the underlying mechanism is discussed.

Parasitic engineering for RRAM control

The inevitable current overshoot which follows forming in filamentary RRAM devices is often perceived as a source of variability that should be minimized. This sentiment has led to efforts to curtail the overshoot by decreasing the parasitic capacitance using highly integrated 1T-1R or 1R-1R device structures. While this is readily achievable in single device test structures, it poses an intricate design constraint for memory array designs. Several papers (Degraeve et al., 2010, 2014; Fantini et al., 2013; Raghavan et al., 2013; Padovani et al., 2015) suggest that there is insufficient current to form stable filaments for small parasitic capacitances and/or low current compliance levels. Thus, the relationship between minimizing overshoot current and improved filament stability is tenuous. In this study, we utilize the forming energy-based understanding of filamentary forming to reveal that the parasitic capacitance should be optimized, rather than minimized for better filament control.

Influence of the Chalcogen Element on the Filament Stability in CuIn(Te,Se,S)2/Al2O3 Filamentary Switching Devices.

In this paper, we report on the use of CuInX2 (X = Te, Se, S) as a cation supply layer in filamentary switching applications. Being used as absorber layers in solar cells, we take advantage of the reported Cu ionic conductivity of these materials to investigate the effect of the chalcogen element on filament stability. In situ X-ray diffraction showed material stability attractive for back-end-of-line in semiconductor industry. When integrated in 580 μm diameter memory cells, more volatile switching was found at low compliance current using CuInS2 and CuInSe2 compared to CuInTe2, which is ascribed to the natural tendency for Cu to diffuse back from the switching layer to the cation supply layer because of the larger difference in electrochemical potential using Se or S. Low-current and scaled behavior was also confirmed using conductive atomic force microscopy. Hence, by varying the chalcogen element, a method is presented to modulate the filament stability.

Ultralow Power Consumption Flexible Biomemristors.

Low power consumption is the important requirement in memory devices for saving energy. In particular, improved energy efficiency is essential in implantable electronic devices for operation under a limited power supply. Here, we demonstrate the use of κ-carrageenan (κ-car) as the resistive switching layer to achieve memory that has low power consumption. A carboxymethyl (CM) group is introduced to the κ-car to increase its ionic conductivity. Ag was doped in CM:κ-car to improve the resistive switching properties of the devices. Memory devices based on Ag-doped CM:κ-car showed electroforming-free resistive switching. This device exhibited low reset voltage (∼0.05 V), fast switching speed (50 ns), and high on/off ratio (>103) under low compliance current (10-5 A). Its power consumption (∼0.35 μW) is much lower than those of the previously reported biomemristors. The resistive switching may be a result of an electrochemical redox process and Ag filament formation in the CM:κ-car under an electric field. This biopolymer memory can also be fabricated on flexible substrate. This study verifies the feasibility of using biopolymers for applications to future implantable and biocompatible nanoelectronics.

Printing an ITO-free flexible poly (4-vinylphenol) resistive switching device

Resistive switching in a sandwich structure of silver (Ag)/Polyvinyl phenol (PVP)/carbon nanotube (CNTs)-silver nanowires (AgNWs) coated on a flexible PET substrate is reported in this work. Densely populated networks of one dimensional nano materials (1DNM), CNTs-AgNWs have been used as the conductive bottom electrode with the prominent features of high flexibility and low sheet resistance of 90 Ω/sq. Thin, yet uniform active layer of PVP was deposited on top of the spin coated 1DNM thin film through state of the art printing technique of electrohydrodynamic atomization (EHDA) with an average thickness of 170 ± 28 nm. Ag dots with an active area of ∼0.1 mm2 were deposited through roll to plate printing system as the top electrodes to complete the device fabrication of flexible memory device. Our memory device exhibited suitable electrical characteristics with OFF/ON ratio of 100:1, retention time of 60 min and electrical endurance for 100 voltage sweeps without any noticeable decay in performance. The resistive switching characteristics at a low current compliance of 3 nA were also evaluated for the application of low power consumption. This memory device is flexible and can sustain more than 100 bending cycles at a bending diameter of 2 cm with stable HRS and LRS values. Our proposed device shows promise to be used as a future potential nonvolatile memory device in flexible electronics.