Off-state Resistance Research Articles

Multistate Ferroelectric Diodes with High Electroresistance Based on van der Waals Heterostructures.

Some van der Waals (vdW) materials exhibit ferroelectricity, making them promising for novel nonvolatile memories (NVMs) such as ferroelectric diodes (FeDs). CuInP2S6 (CIPS) is a well-known vdW ferroelectric that has been integrated with graphene for memory devices. Here we demonstrate FeDs with self-rectifying, hysteretic current-voltage characteristics based on vertical heterostructures of 10 nm thick CIPS and graphene. By using vdW indium-cobalt top electrodes and graphene bottom electrodes, we achieve a high electroresistance (on- and off-state resistance ratios) of ∼106, an on-state rectification ratio of 2500 for read/write voltages of 2 V/0.5 V, and a maximum output current density of 100 A/cm2. These metrics compare favorably with state-of-the-art FeDs. Piezoresponse force microscopy measurements show that stabilization of intermediate net polarization states in CIPS leads to stable multibit data retention at room temperature. The combination of two-terminal design, multibit memory, and low-power operation in CIPS-based FeDs is potentially interesting for compute-in-memory and neuromorphic computing applications.

Energy-efficient design of quaternary logic gates and arithmetic circuits using hybrid CNTFET-RRAM technology

Multi-valued logic (MVL) extends binary logic by providing a framework to represent complex systems with more than two truth values. MVL was introduced to confront the enormous interconnect issue associated with the binary logic in implementing the presnt day complex nanoelectronic architectures. This paper delves into the circuit design, computational aspects, and practical applications of the quaternary logic system, which is a type of MVL with four truth values. The multi-threshold property of carbon nanotube field-effect-transistors (CNTFETs), combined with the ability of resistive random-access memory (RRAM) to store multiple resistance values, has enabled the design of quaternary logic gates and arithmetic circuits. A new CNTFET-based design architecture has been proposed to implement the quaternary logic compatible with the existing technologies. Quaternary logic gates such as inverter, NAND, and NOR, and quaternary arithmetic circuits including decoder, half adder, and multiplier have been designed. The power-delay-product (PDP) of the proposed quaternary inverter, NAND, NOR, half adder, and multiplier is 62.38%, 93.4%, 80.29%, 14.79%, and 20% less than the least PDP of the quaternary designs under consideration. The static power reduction due to the effecciency of the design architecture and high OFF state resistance offered by integrating RRAM into the logic design was explored.The proposed circuits have been subject to various types of parameter variations to validate thir proper functionality in presence of these variations.

Nondefective Vacancy Enhanced Resistive Switching Reliability in Emergent van der Waals Metal Phosphorus Trisulfide-Based Memristive In-Memory Computing Hardware.

Two-dimensional-material-based memristors are emerging as promising enablers of new computing systems beyond von Neumann computers. However, the most studied anion-vacancy-enabled transition metal dichalcogenide memristors show many undesirable performances, e.g., high leakage currents, limited memory windows, high programming currents, and limited endurance. Here, we demonstrate that the emergent van der Waals metal phosphorus trisulfides with unconventional nondefective vacancy provide a promising paradigm for high-performance memristors. The different vacancy types (i.e., defective and nondefective vacancies) induced memristive discrepancies are uncovered. The nondefective vacancies can provide an ultralow diffusion barrier and good memristive structure stability giving rise to many desirable memristive performances, including high off-state resistance of 1012 Ω, pA-level programming currents, large memory window up to 109, more than 7-bit conductance states, and good endurance. Furthermore, a high-yield (94%) memristor crossbar array is fabricated and implements multiple image processing successfully, manifesting the potential for in-memory computing hardware.

Effects of BiFeO3 Thickness on the Write‐Once‐Read‐Many‐Times Resistive Switching Behavior of Pt/BiFeO3/LaNiO3 Heterostructure

Thickness of the resistive switching materials is an essential factor to determine the integration density of resistive switching devices. Herein, the effect of the thickness of BFO on the write‐once‐read‐many‐times (WORM) resistive switching behavior of Pt/BFO/LNO‐based devices is investigated. The thicknesses of BFO thin films are controlled in the range of 70–220 nm. All the devices exhibit WORM resistive switching behavior with high ON/OFF ratio (≈10−2–10−4), long‐term data retention (>3600 s), and reliable endurance (>1000 cycles). The set voltages (Vset) of the devices exhibit an approximately linear relation to the BFO thicknesses, while the highest ON/OFF ratio appears in the device with BFO thickness of 150 nm. The thickness‐dependent resistive switching characteristic is attributed to the variation of oxygen vacancies and OFF state resistances with the increase of BFO thickness. The results underline the importance of the thickness of resistive switching materials for the future device applications.

A Numerical Design of High-Resistance and Energy-Efficient HTS Switch Based on Dynamic Resistance

Dynamic resistance effect introduces dissipative loss to the type-II high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> c superconductors, which is initially considered as a disadvantage for superconductors when it was first discovered. However, in some cases “dynamic resistance”, a unique effect, can be effectively exploited. Based on the dynamic resistance effect, the AC-magnetic-field-actuated high temperature superconducting (HTS) power switch is developed. It is the key component in the HTS transformer-rectifier flux pump (TRFP). The operating characteristics of power switch are vital to the performance and reliability of the transformer rectifier flux pump. In this paper, we compare three designs of HTS power switch with different stabilizer structures: striated stabilizer layers with wide and narrow strips as well as original non-striated stabilizer layers. The performance of HTS power switch is evaluated by several criteria, including off-state voltage and resistance, temperature rise, power loss, and energy conversion efficiency. This study will help design high-performance, low-loss, and energy-efficient HTS power switch based on “dynamic resistance” effect.

Novel four-port RF phase change switches based on GeTe thin film

An indirect-heated phase-change switch (PCS) using germanium telluride (GeTe) has been fabricated using thermal actuation driven by thin film heater on the model. Switches require a low ON-state resistance and a high OFF-state resistance with OFF/ON resistance ratio of 105. The finite element analysis simulation is applied to simulate the temperature of individual node GeTe with different microwave heating pulses. Finally, in order to reduce the phase-change time and increase the switching speed of indirectly heated switching structures, a new four-port indirectly heated phase change switching structure is proposed. In this paper, the heat dissipation of the switch is increased by etching deep grooves on the back of the switch. This structure obviously reduces the phase change time compared to conventional indirectly heated phase change switches the time between ON-state and OFF-state is reduced by more than 19% and the total process is reduced by more than 47%. The GeTe PCSs with etched grooves not only significantly increases the switching speed, but also reduces the risk of recrystallization of the phase change material.

Gate modulation drive and modeling of IGBT in MMC converters

The energy surplus caused by the alternating current (AC) system’s failure results in a severe overvoltage problem in Bai hetan-Jiangsu ±800 kV hybrid cascade direct current (DC) project. However, the insulated gate bipolar transistor’s (IGBT) off-state voltage is the crucial reason for the insufficient voltage capacity of modular multilevel converter (MMC). Therefore, analysis and suppression of the off-state overvoltage are of vital significance. The off-state model of IGBT in an MMC converter is established, and the process of IGBT’s turning off is analyzed. Then, based on the characteristics of IGBT’s off-state and traditional resistance switching strategy, the gate modulation drive strategy is proposed, and the equivalent topology of the MMC submodule with the strategy is established. Finally, MMC submodule models with gate modulation drive and resistance switching strategy, respectively, are built in simulation software, and two groups of data are compared under multiple groups of the control cycle and reference current slope given. The simulation results show that the gate modulation drive supresses the off-state overvoltage on IGBT better than the resistance switching strategy. Moreover, the shorter the control cycle and the less reference current slope, the better the overvoltage suppression.

Tin Oxide Nanorod Array-Based Photonic Memristors with Multilevel Resistance States Driven by Optoelectronic Stimuli

One-dimensional (1D) metal oxide-based photonic memristors, combining information storage and optical response, have shown great potential for the design and development of high-density and high-efficient computing systems beyond the era of von-Neumann architecture and Moore's law. Here, the functional memristive devices based on SnOx slanted nanorod arrays are demonstrated; wherein both the optical and electrical stimuli have been used to modulate the switching characteristics to achieve multilevel cell operations. The switching characteristics of Al/SnOx/FTO devices include low operating voltages (0.7 V/-0.6 V), moderate ON/OFF ratio (>10), and longer endurance (>102 cycles) and retention (>103 s) with a self-compliance effect in the dark. Under illumination, ranging from ultraviolet (254 and 365 nm) to visible light (405 and 533 nm), an unusual negative photo response with an enlarged ON/OFF ratio of >107 and a faster response time of <8 ms is observed. Additionally, multiple low and high resistance states have been achieved by modulating the programming current and the optical stimulus, respectively. The optoelectronic resistive memory behavior is attributed to the electric field-induced formation and light-stimulated dissolution of oxygen vacancies. Comprehensively, the results suggest that the optical illumination reduces the oxygen ion migration barrier, leading to the dissolution of conductive filaments and thereby locally increasing the OFF state resistance. The fabricated photonic memristors demonstrate the potential applications of metal oxide-based 1D nanostructures for artificial visual memory and optoelectronic applications.

Thermal Field Effect in Resistive Random Access Memory With Sidewall Structures of Different Thermal Conductivity

The effect of different pulse rise time in resistive random access memory (RRAM) is presented in this study. The result shows that the length of pulse rise time will lead to variations in RRAM OFF-state resistance that occur due to which of two distinct effects governs the reaction, either the electric field or the thermal field. Different sidewall materials are adopted in RRAM devices to verify these phenomena, and a physical model supplemented with COMSOL simulation is proposed.

Neutron flat-panel detector using In-Ga-Zn-O thin-film transistor.

Neutron imaging is a powerful tool for observing the internal structure of an object without destroying the object. Neutron imaging (neutron radiography) is a prominent application of neutrons but still requires significant improvements, for example, in sensitivity, resolution, radiation hardness, and handling of neutron imaging detectors. This paper presents the development and the first neutron imaging results of a neutron flat-panel detector (nFPD) based on an In-Ga-Zn-O (IGZO) thin-film transistor (TFT)/photodiode array coupled with a LiF/ZnS scintillator sheet. Direct photo-coupling to the scintillator increases the light collection efficiency. Moreover, unlike lens-coupled neutron cameras, the proposed detector is compact and easy to handle. Owing to the high off-state resistance of IGZO TFTs, their leakage current is lower than that of conventional TFTs, enabling the IGZO TFTs to hold an accumulated charge for a longer period of time and allowing longer exposure times for imaging. This would be a powerful feature for imaging at compact neutron sources with limited flux. This paper reports on the first neutron imaging results with an IGZO nFPD, its performance evaluation, and a demonstration of three-dimensional computed tomography with neutrons.

Resistive Switching in CdTe/CdSe Core–Shell Quantum Dots Embedded Chitosan-Based Memory Devices

In this paper, we report on the resistive switching (RS) and conduction mechanisms in devices consisting of CdTe/CdSe core–shell quantum dots embedded chitosan composites active layer. Two devices with active layers sandwiched between (1) Al and Ag, and (2) ITO and Ag electrodes were studied. Both devices exhibited bipolar memory behavior with [Formula: see text] V and [Formula: see text][Formula: see text]V, for the Al-based device, while [Formula: see text] V and [Formula: see text][Formula: see text]V were observed for the ITO-based device, enabling both devices to be operated at low powers. However, the switching mechanisms of both devices were different, i.e., RS in Al device was attributed to conductive bridge mechanism, while space-charge-limited driven conduction filament attributed the switching mechanism of the ITO device. Additionally, the Al-based device showed long retention ([Formula: see text][Formula: see text]s) and a reasonable large ([Formula: see text]) ON/OFF ratio. Additionally, for this device, we also observed sweeping cycle-induced reversal of voltage polarity of the [Formula: see text] and [Formula: see text]. In contrast, we observed that increasing sweeping cycles resulted in an exponential decrease of the OFF-state resistance of the ITO-based device.

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.

Effect of oxygen stoichiometry on the threshold switching of RF-sputtered NbOx (x = 2.0–2.5) films

In this study, we report the Poole-Frenkel induced threshold switching characteristics of niobium dioxide (NbO2) films by tuning oxygen stoichiometry. Similar to correlated oxides, NbO2 is also found sensitive to oxygen related defects. The oxygen stoichiometry of NbO2 films was varied by changing the oxygen flux during its synthesis in reactive RF-sputter. Nb2O5 dominant phase of as-grown films was detected during Raman spectroscopy. For the reduction of Nb2O5 films, we chose tungsten as a top electrode due to its considerable oxygen affinity. After the required forming process, stable threshold switching was observed which is attributed to the reduction of Nb2O5 phase to NbO2 due to interfacial WOx layer formation between top electrode and Nb2O5. Increase of O2 flux during deposition leads to an increase in the selectivity, threshold voltage and off-state resistance. These devices showed good thermal stability up to 160 °C making them useful for RRAM and neuromorphic computing applications.

Effects of heavy ion irradiation on Cu/Al2O3/Pt CBRAM devices

In this work, the effects of heavy ion irradiation on atomic switches with Cu/Al2O3/Pt structure are investigated. The initial device is prone to hard breakdown after forming, while the device after irradiation exhibits good performance such as stable operating voltage, non-volatile switching (a retention characteristic of 105 s at room temperature) and good endurance (2000 switching cycles). Compared with the device after low-dose irradiation, the device under higher-dose irradiation shows more uniform off-state resistances but no relevant changes were observed on distributions of on-state resistances, set and reset voltages.

Steep-Switching Fully Depleted Silicon-on-Insulator (FDSOI) Phase-Transition Field-Effect Transistor With Optimized HfO₂/Al₂O₃-Multilayer-Based Threshold Switching Device

The OFF-state resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> ) and threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) of a HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based threshold-switching (TS) device for phase-transition fully depleted siliconon-insulator (FDSOI) device on silicon substrate can be improved, e.g., ROFF is increased by implementing a multilayered HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (rather than HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer only). The HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> layer with the atomic layer deposition (ALD) cycle ratio of 3:1 (i.e., Hf:Al = 3:1) shows approximately 3.47 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> Ω of average ROFF and approximately 2.12 V of average VT. Next, VT can be modulated by adjusting the work function of the bottom electrode. We demonstrate that a high work function of the bottom electrode results in stronger built-in electric-field and tunneling, decreasing VT and ROFF. The Hf <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al1-based TS device with platinum electrode shows approximately 0.88 V of average VT and approximately 2.81 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> Ω of average ROFF. The phase-transition FDSOI device used in this work is fabricated by connecting the optimized TS device in series to the drain electrode of a baseline FDSOI device, whose channel length is 130 nm. The phase-transition FDSOI device shows the minimum subthreshold slope of 9 mV/decade owing to the abrupt resistive switching of the TS device. Additionally, it exhibits an increase in the ON/OFF-state current ratio by three times because of high R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> .

Rapid threshold switching dynamics of co-sputtered chalcogenide Ge15Te85 device for selector application

Threshold switching is a unique characteristic feature in amorphous chalcogenide materials that establishes stable and fast switching between a high resistance OFF state and a conductive ON state in the amorphous phase, envisaging the electronic nature of two-terminal ovonic threshold switch (OTS) selectors in vertically stackable cross-point memory arrays. In this paper, we demonstrate voltage-dependent nanosecond threshold switching dynamics and stable OFF–ON transitions of co-sputtered thin Ge15Te85 film devices using ultrafast time-resolved current–voltage measurements. The time-resolved measurement of device current upon the application of voltage pulse reveals a stable threshold switching and OFF–ON transient characteristics of OTS devices and the measured delay time is found to decrease to few nanoseconds upon increasing the amplitude of the applied voltage pulse and such OTS characteristics are found to be stable even above 60% of the high value of threshold voltage. These experimental results found to be consistent with analytical solutions and also demonstrate a systematic trend in the voltage dependent switching properties enabling ultrafast threshold switching characteristics suitable towards designing reliable and stable OTS selector devices.

Observation of resistance switching in Vanadyl-phthalocyanine thin films

Herein, we report the first observation of Negative differential resistance (NDR) associated with resistance switching in Vanadyl-phthalocyanine (VOPc) based devices with the configuration: ITO/F4TCNQ/VOPc/MoO3/Al. It was observed that the devices were initially at low resistance ON state (LS) and were switched to high resistance OFF state (HS) at sufficient applied bias. The NDR behaviour was observed during the initial sweep for each device (often referred to as the writing process). The ON/OFF state transition was attributed to the formation and neutralization of interface dipoles at the ITO/VOPc interface. Finally, the observed non-volatile RS switching behaviour was demonstrated employing impedance spectroscopic studies. This study opens up the potential applications of VOPc Resistance Switching devices in security and data protection applications.

Steady-State Parametric Optimization and Transient Characterization of Heat Flow Regulation With Binary Diffusion

Thermal regulators and switches are nonlinear devices that can greatly aid in the management of transient and/or spatially varying heating events. Various physical mechanisms have been harnessed to achieve nonlinear and switchable thermal behavior, though device characteristics are often evaluated in terms of steady-state performance only. Accurately capturing the transient behavior of these devices is a crucial missing link required for assessing the effectiveness of implementation in real-world scenarios. Here, we explore the physics of binary vapor diffusion through a noncondensable gas cavity as a promising mechanism for high-resistance contrast thermal regulation. Through a parametric steady-state optimization, we present a roadmap for future designs that can potentially reach switching ratios of up to 14.5. In addition, we perform a transient investigation to characterize the thermal response time of the device. The device possesses an extremely desirable attribute under certain transient heat loads, where the effective switching ratio appears significantly greater than the steady-state ratio due to a temporary increase in the OFF-state resistance immediately after switching. This work informs future experimental efforts for design optimization of binary-diffusion-based regulators and establishes simple modeling schemes to approximate device performance in system-level regulation scenarios.

Hybrid CMOS-PCM temperature sensor

In this paper, we propose a hybrid CMOS and phase-change memory (PCM)-relaxation-oscillator based circuit for temperature-sensing applications. Unlike conventional CMOS temperature sensors based on ring- or relaxation-oscillators, the proposed sensor does not require any curvature calibration technique for linearity improvement of the thermal response. The presented system explores the temperature dependence of Ovonic-threshold-switching voltage (Vth) and DC OFF state resistance (ROFF) of a PCM device. The proposed temperature sensor exhibits a resolution of ∼0.04 °C (for the 0 °C to 80 °C temperature range) with 0.51 nJ energy consumption per conversion in simulations.

Cryogenic behavior of NbO2 based threshold switching devices as oscillation neurons

This Letter investigates the cryogenic behavior of NbO2 threshold switching devices. Pt/NbO2/Pt devices are demonstrated to be well functional as threshold switching devices at ultra-low temperature (4 K). When the temperature decreases, the OFF-state resistance of NbO2 increases and the switching voltage increases. With the extracted characteristics of NbO2 ranging from 4 K to 300 K, we continue to study the neuromorphic system using the crossbar array with resistive memories as resistive synapses and NbO2 as oscillation neurons at different temperatures through SPICE simulation. The simulation results show that the oscillation systems could still work properly at 4 K. The oscillation amplitude decreases as temperature increases. The oscillation frequency depends on both the temperature and the input voltage.