Exploring the Cutting-Edge Frontiers of Electrochemical Random Access Memories (ECRAMs) for Neuromorphic Computing: Revolutionary Advances in Material-to-Device Engineering.

Advanced materials and device engineering has played a crucial role in improving the performance of electrochemical random access memory (ECRAM) devices. ECRAM technology has been identified as a promising candidate for implementing artificial synapses in neuromorphic computing systems due to its ability to store analog values and its ease of programmability. ECRAM devices consist of an electrolyte and a channel material sandwiched between two electrodes, and the performance of these devices depends on the properties of the materials used. This review provides a comprehensive overview of material engineering strategies to optimize the electrolyte and channel materials' ionic conductivity, stability, and ionic diffusivity to improve the performance and reliability of ECRAM devices. Device engineering and scaling strategies are further discussed to enhance ECRAM performance. Last, perspectives on the current challenges and future directions in developing ECRAM-based artificial synapses in neuromorphic computing systems are provided.

Enhancement of NbO2-based oscillator neuron device performance via cryogenic operation.

The Niobium Dioxide (NbO2) oscillator neuron has garnered significant interest because of its simple structure compared to conventional CMOS-based circuits. However, the limited on/off resistance ratio narrows the range of series resistances that satisfy the self-oscillation conditions and limits its use in large-scale synaptic arrays. In this study, we report the possibility of improving the performance of NbO2-based oscillator neuron devices through cryogenic operation. The study emphasizes two crucial parameters: the on/off resistance ratio and the oscillation amplitude, both of which are essential for accurate weighted sum classification. The data suggest that these parameters can be effectively enhanced under cryogenic conditions. In addition, we revealed that 120 K is the optimal temperature for cryogenic operation, as it represents the temperature where the on/off resistance ratio ceases to increase. As a result, we revealed that the series resistance range satisfying the self-oscillation condition in a single oscillator increases from 20 to 126 kΩ. The research also probes the maximum possible array size at each temperature. At 300 K, representation is only possible for a 5 × 5 array, but at 120 K, a 30 × 30 array can be represented as a frequency. The evidence implies that the 120 K conditions not only broaden the range of series resistors that can be connected to a single oscillator but also increases the array size, thereby representing different weighted sum currents as frequencies. The research indicates that using carefully optimized cryogenic operation could be a viable method to enhance the necessary NbO2properties for an oscillator neuron device.

High-performance resistive random access memory using two-dimensional electron gas electrode and its switching mechanism analysis.

In this study, a two-dimensional electron gas (2DEG), which is a conductive layer formed at the interface of Al2O3and TiO2, was used as an electrode for resistive random access memory (RRAM) and implemented in a cell size down to 30 nm. For an RRAM device comprising W/2DEG/TiO2/W, we confirmed that the dominant switching mechanism changed from interfacial to filamentary as the cell size decreased from 500 nm to 30 nm. Through analyses of changes in forming characteristics and conduction mechanisms in the low resistive state depending on the cell size, it was identified that the 2DEG acted as an oxygen-scavenging layer of TiO2during the resistive switching process. By comparing the switching characteristics of RRAM devices with and without 2DEG for a 30 nm cell size, we confirmed that a high-performance 2DEG RRAM was realized, with highly uniform current-voltage characteristics, a low operating voltage (∼1 V), and a high on/off ratio (>102). Finally, the applicability of the proposed device to a crossbar array was validated by evaluating 1S1R operation with an NbO2-based selector. Considering the improved switching uniformity, the 2DEG RRAM shows promise for high-density memory applications.

Improvement of endurance and switching speed in Hf1-xZrxO2thin films using a nanolaminate structure.

To improve the endurance and polarization switching speed of Hf1-xZrxO2(HZO) ferroelectric films, we designed a 10 nm Hf0.5Zr0.5O2 + ZrO2(HZZ) nanolaminate structure. Three films with different ZrO2interlayers thicknesses were compared to find the optimal condition to implement the effect of the topological domain wall which was proposed recently. The HZZ film were deposited by repeatedly stacking ten HZO (∼0.92 nm) and six ZrO2(∼0.53 nm) layers; they exhibited a dramatic reduction of coercive field without an effective loss of remnant polarization. The endurance at operation voltage increased by more than 100 times compared with that of the solid solution HZO film, and the switching speed was increased by more than two times. The formation of the tetragonal phase-like spacer between the ferroelectric polar regions appears to be the main factor associated with the reduction of the switching barrier and leads to the acceleration of the switching propagation over multiple domains.

A grease for domain walls motion in HfO2-based ferroelectrics.

A large coercive fieldECof HfO2based ferroelectric devices poses critical performance issues in their applications as ferroelectric memories and ferroelectric field effect transistors. A new design to reduceECby fabricating nanolaminate Hf0.5Zr0.5O2/ZrO2(HZZ) thin films is used, followed by an ensuing annealing process at a comparatively high temperature 700 °C. High-resolution electron microscopy imaging detects tetragonal-like domain walls between orthorhombic polar regions. These walls decrease the potential barrier of polarization reversal in HfO2based films compared to the conventional domain walls with a single non-polar spacer, causing about a 40% decrease inEC. Capacitance versus electric field measurements on HZZ thin film uncovered a substantial increase of dielectric permittivity near theECcompared to the conventional Hf0.5Zr0.5O2thin film, justifying the higher mobility of domain walls in the developed HZZ film. The tetragonal-like regions served as grease easing the movement of the domain wall and reducingEC.

Synergy effect of microwave annealing and high-pressure hydrogen annealing on Poly-Si thin-film transistor.

Grain boundary (GB) is a significant factor that deteriorates the transfer characteristics of poly-Si thin-film transistors (TFTs). In this study, we utilized the synergistic effect of microwave annealing (MWA) and high-pressure hydrogen annealing (HPHA) to effectively reduce grain boundary trap (GBT) density, resulting in improved field-effect mobility (µ) and subthreshold swing (SS). To investigate the synergistic effect of MWA and HPHA, the transfer characteristics of rapid thermal annealing and forming gas annealing devices were compared and analyzed as control devices. Furthermore, the mechanism of SS and mobility enhancement can be quantitatively understood by lowering the GB barrier height. In addition, Raman spectroscopy proved that poly-Si crystallinity was improved during MWA. Our results showed that MWA and HPHA play a vital role in reducing GBT density and improving poly-Si TFT characteristics.

Single-Atom Quantum-Point Contact Switch Using Atomically Thin Hexagonal Boron Nitride.

The first report of a quantized conductance atomic threshold switch (QCATS) using an atomically-thin hexagonal boron nitride (hBN) layer is provided. This QCATS has applications in memory and logic devices. The QCATS device shows a stable and reproducible conductance quantization state at 1·G0 by forming single-atom point contact through a monoatomic boron defect in an hBN layer. An atomistic switching mechanism in hBN-QCATS is confirmed by in situ visualization of mono-atomic conductive filaments. Atomic defects in hBN are the key factor that affects the switching characteristic. The hBN-QCATS has excellent switching characteristics such as low operation voltage of 0.3 V, low "off" current of 1 pA, fast switching of 50 ns, and high endurance > 107 cycles. The variability of switching characteristics, which are the major problems of switching device, can be solved by reducing the area and thickness of the switching region to form single-atom point contact. The switching layer thickness is scaled down to the single-atom (≈0.33 nm) h-BN layer, and the switching area is limited to single-atom defects. By implementing excellent switching characteristics using single-layer hBN, the possibility of implementing stable and uniform atomic-switching devices for future memory and logic applications is confirmed.

Ionic Sieving Through One-Atom-Thick 2D Material Enables Analog Nonvolatile Memory for Neuromorphic Computing.

The first report on ion transport through atomic sieves of atomically thin 2D material is provided to solve critical limitations of electrochemical random-access memory (ECRAM) devices. Conventional ECRAMs have random and localized ion migration paths; as a result, the analog switching efficiency is inadequate to perform in-memory logic operations. Herein ion transport path scaled down to the one-atom-thick (≈0.33 nm) hexagonal boron nitride (hBN), and the ionic transport area is confined to a small pore (≈0.3 nm2 ) at the single-hexagonal ring. One-atom-thick hBN has ion-permeable pores at the center of each hexagonal ring due to weakened electron cloud and highly polarized B-N bond. The experimental evidence indicates that the activation energy barrier for H+ ion transport through single-layer hBN is ≈0.51 eV. Benefiting from the controlled ionic sieving through single-layer hBN, the ECRAMs exhibit superior nonvolatile analog switching with good memory retention and high endurance. The proposed approach enables atomically thin 2D material as an ion transport layer to regulate the switching of various ECRAM devices for artificial synaptic electronics.

A CMOS-compatible morphotropic phase boundary.

Morphotropic phase boundaries (MPBs) show substantial piezoelectric and dielectric responses, which have practical applications. The predicted existence of MPB in HfO2-ZrO2solid solution thin film has provided a new way to increase the dielectric properties of a silicon-compatible device. Here, we present a new fabrication design by which the density of MPBρMPBand consequently the dielectric constantϵrof HfO2-ZrO2thin film was considerably increased. TheρMPBwas controlled by fabrication of a 10 nm [1 nm Hf0.5Zr0.5O2(ferroelectric)/1 nm ZrO2(antiferroelectric)] nanolaminate followed by an appropriate annealing process. The coexistence of orthorhombic and tetragonal structures, which are the origins of ferroelectric (FE) and antiferroelectric (AFE) behaviors, respectively, was structurally confirmed, and a double hysteresis loop that originates from AFE ordering, with some remnant polarization that originates from FE ordering, was observed inP-Ecurve. A remarkable increase inϵrcompared to the conventional HfO2-ZrO2thin film was achieved by controlling the FE-AFE ratio. The fabrication process was performed at low temperature (250 °C) and the device is compatible with silicon technology, so the new design yields a device that has possible applications in near-future electronics.

A new approach to achieving strong ferroelectric properties in TiN/Hf0.5Zr0.5O2/TiN devices.

In this paper, we propose a method to improve the performance of TiN/Hf0.5Zr0.5O2 (HZO)/TiN Nano-capacitors used in memory devices. Instead of direct fabrication of the TiN/HZO/TiN device, our method involves an intermediate step in which W metal is used as a capping material to induce a large in-plane tensile strain during rapid thermal annealing, resulting in a total suppression of the monoclinic phase and the appearance of the ferroelectric phase. Consequently, after removing the W capping electrode through an etching process and the post-deposition of a TiN top electrode at room temperature, a high remnant polarization of approximately 40 µC cm-2 and a 65% increase of coercive field were obtained. Moreover, the leakage current was reduced by an order of magnitude compared to the normal TiN/HZO/TiN capacitor; this result is attributed to the presence (absence) of the W/HZO (TiN/HZO) top interface during thermal annealing. The formation of a TiO x interfacial layer at elevated temperatures, which pulls oxygen from the HZO layer, resulting in the formation of oxygen vacancies, is the main cause of the high leakage current through the TiN/HZO/TiN stacks. It was confirmed that the re-capped TiN/HZO/TiN capacitor has a comparable endurance to a normal capacitor. Our results offer the re-capping process as a promising approach to fabricating HfO2-based ferroelectric memory devices with various electrode materials.

Effects of high pressure oxygen annealing on Hf0.5Zr0.5O2ferroelectric device.

We report a high-pressure oxygen annealing (HPOA) process to improve the performance of TiN/Hf0.5Zr0.5O2(HZO)/TiN devices by controlling the number of oxygen vacancies and carbon contaminants. The ferroelectric properties of HZO film after HPOA at 250 °C for 30 min under different oxygen pressures from 0 to 80 bar were evaluated by electrical and structural characterizations. We found that a sample treated with an oxygen pressure at 40 bar exhibited large switchable polarization (2Pr) of approximately 38 and 47µC cm-2in its pristine and wake-up states, respectively. Compared to a control sample, an approximately 40% reduction in the wake-up effect was achieved after HPOA at 40 bar. Improved ferroelectric properties of HZO film can be explained by the appropriate amount of oxygen vacancies and reduced carbon contaminants after HPOA.

High polarization and wake-up free ferroelectric characteristics in ultrathin Hf0.5Zr0.5O2devices by control of oxygen-deficient layer.

The formation of an interfacial layer is believed to affect the ferroelectric properties in HfO2based ferroelectric devices. The atomic layer deposited devices continue suffering from a poor bottom interfacial condition, since the formation of bottom interface is severely affected by atomic layer deposition and annealing process. Herein, the formation of bottom interfacial layer was controlled through deposition of different bottom electrodes (BE) in device structure W/HZO/BE. The transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy analyses done on devices W/HZO/W and W/HZO/IrOxsuggest the strong effect of IrOxin controlling bottom interfacial layer formation while W/HZO/W badly suffers from interfacial layer formation. W/HZO/IrOxdevices show high remnant polarization (2Pr) âˆ¼ 53µC cm-2, wake-up free endurance cycling characteristics, low leakage current with demonstration of low annealing temperature requirement as low as 350 °C, valuable for back-end-of-line integration. Further, sub-5 nm HZO thicknesses-based W/HZO/IrOxdevices demonstrate high 2Prand wake-up free ferroelectric characteristics, which can be promising for low power and high-density memory applications. 2.2 nm, 3 nm, and 4 nm HZO based W/HZO/IrOxdevices show 2Prvalues 13.54, 22.4, 38.23µC cm-2at 4 MV cm-1and 19.96, 30.17, 48.34µC cm-2at 5 MV cm-1, respectively, with demonstration of wake-up free ferroelectric characteristics.

Improved synaptic functionalities of Li-based nano-ionic synaptic transistor with ultralow conductance enabled by Al2O3barrier layer.

In this study, we investigated the effect of an Al2O3barrier layer in an all-solid-state inorganic Li-based nano-ionic synaptic transistor (LST) with Li3PO4electrolyte/WOxchannel structure. Near-ideal synaptic behavior in the ultralow conductance range (∼50 nS) was obtained by controlling the abrupt ion migration through the introduction of a sputter-deposited thin (∼3 nm) Al2O3interfacial layer. A trade-off relationship between the weight update linearity and on/off ratio with varying Al2O3layer thickness was also observed. To determine the origin of the Al2O3barrier layer effects, cyclic voltammetry analysis was conducted, and the optimal ionic diffusivity and mobility were found to be key parameters in achieving ideal synaptic behavior. Owing to the controlled ion migration, the retention characteristics were considerably improved by the Al2O3barrier. Finally, a highly improved pattern recognition accuracy (83.13%) was achieved using the LST with an Al2O3barrier of optimal thickness.

Sodium-based nano-ionic synaptic transistor with improved retention characteristics.

We propose an all-solid-state Na ion-based synaptic transistor (NST) to overcome the low retention problem of the Li ion-based synaptic transistor (LST). Through our analysis, it was found that the retention instability in an ionic synaptic transistor originated from its high ionic diffusivity. As confirmed by cyclic voltammetry analysis, Na ions have a lower ionic diffusivity than Li ions in the WOx layer. The state retention of NST was found to be improved to 20 times that of LST. Furthermore, near-ideal synaptic behaviors, such as linear weight update and linear I-V characteristics, were also obtained by material engineering.

Excellent synaptic behavior of lithium-based nano-ionic transistor based on optimal WO2.7 stoichiometry with high ion diffusivity.

In this study, we introduce a lithium (Li) ion-based three-terminal (3-T) synapse device using WO x as a channel. Our study reveals a key stoichiometry of WO2.7 for excellent synaptic characteristics that is related to Li-ion diffusivity. The open-lattice structure formed by oxygen deficiency promoted Li-ion injection and diffusion. The optimized stoichiometry and improved Li-ion diffusivity were confirmed by x-ray photoelectron spectroscopy analysis and cyclic voltammetry, respectively. Furthermore, the transient conductance change that inevitably occurs in ion-based synaptic transistors was resolved by applying a two-step voltage pulse scheme. As a result, we achieved a symmetric and linear weight-update characteristic with reduced program/erase operation time.

One transistor-two resistive RAM device for realizing bidirectional and analog neuromorphic synapse devices.

In this paper, we propose a one transistor-two resistive RAM (RRAM) (1T2R) device to overcome the non-ideal switching behavior of artificial synapse devices, such as the unidirectional and abrupt change in the conductance. Our findings reveal that the 1T2R device can exhibit bidirectional conductance changes using unidirectional switching RRAMs. Thus, we introduce a unidirectional but analog switching Cu-based RRAM device (Cu/Cu2-X S/WO3-X /W) having an internal voltage suppressor (Cu2-X S) to realize a bidirectional and analog 1T2R synapse device. The synaptic behaviors of the 1T2R device are calculated using the subthreshold region of an NMOSFET. In addition, we improve the on/off conductance ratio and conductance change linearity owing to the nonlinear current transition characteristics of the subthreshold region. Finally, we demonstrate that an ideal synaptic behavior can be achieved through the 1T2R device even when non-ideal switching RRAM elements are used.

Understanding of proton induced synaptic behaviors in three-terminal synapse device for neuromorphic systems.

In this study, we investigate a proton-based three-terminal (3-T) synapse device to realize linear weight-update and I-V linearity characteristics for neuromorphic systems. The conductance states of the 3-T synapse device can be controlled by modulating the proton concentration in the WOx channel. Therefore, we estimate the dynamic change of proton concentration in the channel region, which directly affects synaptic behaviors. Our findings indicate that the supply of an excess number of protons from the SiO2-H electrolyte and low proton diffusivity in the WOx channel result in asymmetric and non-linear weight-update characteristics. In addition, though the linear I-V characteristics can be obtained using non-stoichiometric WOx, we observe that significant oxygen deficiency in the channel region increases the operating current levels. Thus, based on this information, we introduce optimized conditions of each component in the 3-T synapse device and shape of the gate voltage pulses. As a result, an excellent classification accuracy is achieved using linear weight-update and I-V linearity characteristics under optimized device and pulse conditions.

Microstructural engineering in interface-type synapse device for enhancing linear and symmetric conductance changes.

The origins of the nonlinear and asymmetric synaptic characteristics of TiO x -based synapse devices were investigated. Based on the origins, a microstructural electrode was utilized to improve the synaptic characteristics. Under an identical pulse bias, a TiO x -based synapse device exhibited saturated conductance changes, which led to nonlinear and asymmetric synaptic characteristics. The formation of an interfacial layer between the electrode and TiO x layer, which can limit consecutive oxygen migration and chemical reactions, was considered as the main origin of the conductance saturation behavior. To achieve consecutive oxygen migration and chemical reactions, structural engineering was utilized. The resultant microstructural electrode noticeably improved the synaptic characteristics, including the unsaturated, linear, and symmetric conductance changes. These synaptic characteristics resulted in the recognition accuracy significantly increasing from 38% to 90% in a neural network-based pattern recognition simulation.

Effect of cation amount in the electrolyte on characteristics of Ag/TiO2 based threshold switching devices.

In this study, we investigate the effect of cation amount in an electrolyte on Ag/TiO2 based threshold switching (TS) devices, based on field-induced nucleation theory. For this purpose, normal Ag/TiO2, annealed Ag/TiO2, and AgTe/TiO2 based TS devices are prepared, which have different cation amounts in their electrolytes during the switching process. First, we find that all of the prepared TS devices follow the field-induced nucleation theory with different nucleation barrier energy (W0), by investigating the delay-time dependency at various voltages and temperatures. Based on the investigation, we reveal that the amount of cations in the electrolyte during the switching process is the control parameter that affects the W0 values, which are found to be inversely proportional to the turn-off speed of the TS devices. This implies that the turn-off speed of the TS devices can be modulated by controlling the amount of cations in the matrix.

A C-Te-based binary OTS device exhibiting excellent performance and high thermal stability for selector application.

In this letter, we demonstrate a new binary ovonic threshold switching (OTS) selector device scalable down to ø30 nm based on C-Te. Our proposed selector device exhibits outstanding performance such as a high switching ratio (Ion/Ioff > 105), an extremely low off-current (∼1 nA), an extremely fast operating speed of <10 ns (transition time of <2 ns and delay time of <8 ns), high endurance (109), and high thermal stability (>450 °C). The observed high thermal stability is caused by the relatively small atomic size of C, compared to Te, which can effectively suppress the segregation and crystallization of Te in the OTS film. Furthermore, to confirm the functionality of the selector in a crossbar array, we evaluated a 1S-1R device by integrating our OTS device with a ReRAM (resistive random access memory) device. The 1S-1R integrated device exhibits a successful suppression of leakage current at the half-selected cell and shows an excellent read-out margin (>212 word lines) in a fast read operation.