Reversible chemical and structural changes induced by ionic motion and reaction in response to electrical stimuli leads to resistive switching effects in metal‐insulator‐metal structures. Filamentary ...switching based on the formation and rupture of nanoscale conductive filament has been applied in non‐volatile memory and volatile selector devices with low power consumption and fast switching speeds. Before the mass production of resistive switching devices, great efforts are still required to enable stable and reliable switching performances. The conductive filament, a bridge of microscopic metal‐insulator‐metal structure and macroscopic resistance states, plays an irreplaceable part in resistive switching behavior, as unreliable performance often originates from unstable filament behavior. In this Review, departing from the filamentary switching mechanism and the existing issues, recent advances of the switching performance improvement through the conductive filament modulation are discussed, in the sequence of material modulation, device structure design and switching operation scheme optimization. In particular, two‐dimensional (2D) nanomaterials with excellent properties including and beyond graphene, are discussed with emphasis on performance improvement by their active roles as the switching layer, insertion layer, thin electrode, patterned electrode, and edge electrode, etc.
Ionic motion and reaction in response to electrical stimuli may lead to resistive switching effects in metal‐insulator‐metal nanometer devices. The various achievements of RS device performance improvement are discussed with a focus on the utilization of two‐dimensional materials that show the potential for high performance memory applications.
Abstract
The resistive switching effect in memristors typically stems from the formation and rupture of localized conductive filament paths, and HfO
2
has been accepted as one of the most promising ...resistive switching materials. However, the dynamic changes in the resistive switching process, including the composition and structure of conductive filaments, and especially the evolution of conductive filament surroundings, remain controversial in HfO
2
-based memristors. Here, the conductive filament system in the amorphous HfO
2
-based memristors with various top electrodes is revealed to be with a quasi-core-shell structure consisting of metallic hexagonal-Hf
6
O and its crystalline surroundings (monoclinic or tetragonal HfO
x
). The phase of the HfO
x
shell varies with the oxygen reservation capability of the top electrode. According to extensive high-resolution transmission electron microscopy observations and ab initio calculations, the phase transition of the conductive filament shell between monoclinic and tetragonal HfO
2
is proposed to depend on the comprehensive effects of Joule heat from the conductive filament current and the concentration of oxygen vacancies. The quasi-core-shell conductive filament system with an intrinsic barrier, which prohibits conductive filament oxidation, ensures the extreme scalability of resistive switching memristors. This study renovates the understanding of the conductive filament evolution in HfO
2
-based memristors and provides potential inspirations to improve oxide memristors for nonvolatile storage-class memory applications.
Memory devices with high speed and high density are highly desired to address the 'memory wall' issue. Here we demonstrated a highly scalable, three-dimensional stackable ferroelectric diode, with ...its rectifying polarity modulated by the polarization reversal of Hf
Zr
O
films. By visualizing the hafnium/zirconium lattice order and oxygen lattice order with atomic-resolution spherical aberration-corrected STEM, we revealed the correlation between the spontaneous polarization of Hf
Zr
O
film and the displacement of oxygen atom, thus unambiguously identified the non-centrosymmetric Pca2
orthorhombic phase in Hf
Zr
O
film. We further implemented this ferroelectric diode in an 8 layers 3D array. Operation speed as high as 20 ns and robust endurance of more than 10
were demonstrated. The built-in nonlinearity of more than 100 guarantees its self-selective property that eliminates the need for external selectors to suppress the leakage current in large array. This work opens up new opportunities for future memory hierarchy evolution.
Seeking an effective electronic synapse to emulate biological synaptic behavior is fundamental for building brain-inspired computers. An emerging two-terminal memristor, in which the conductance can ...be gradually modulated by external electrical stimuli, is widely considered as the strongest competitor of the electronic synapse. Here, we show the capability of TiOx/Al2O3-based memristor devices to imitate synaptic behaviors. Along with analog resistive switching performances, the devices replicate the bio-synapse behaviors of potentiation/depression, short-term-plasticity, and long-term-potentiation, which show that TiOx/Al2O3-based memristors may be useful as electronic synapses. The essential memorizing capabilities of the brain are dependent on the connection strength between neurons, and the memory types change from short-term memory to long-term memory. In the TiOx/Al2O3-based electronic synaptic junction, the memorizing levels can change their state via a standard rehearsal process and also via newly introduced process called "impact of event" i.e. the impact of pulse amplitude, and the width of the input pulse. The devices show a short-term to long-term memory effect with the introduction of intermediate mezzanine memory. The experimental achievements using the TiOx/Al2O3 electronic synapses are finally psychologically modeled by considering the mezzanine level. It is highly recommended that similar phenomena should be investigated for other memristor systems to check the authenticity of this model.
Negative‐SET behavior is observed in various cation‐based memories, which degrades the device reliability. Transmission electron microscopy results demonstrate the behavior is caused by the ...overgrowth of the conductive filament (CF) into the Pt electrode. The CF overgrowth phenomenon is suppressed and the negative‐SET behavior is eliminated by inserting an impermeable graphene layer. The graphene‐based devices show high reliability and satisfying performance.
In this letter, effects of top electrodes (TEs) on ferroelectric properties of Hf 0.5 Zr 0.5 O 2 (HZO) thin films are examined systematically. The remnant polarization (P r ) of HZO thin films ...increases by altering TEs with lower thermal expansions coefficient (<inline-formula> <tex-math notation="LaTeX">\alpha </tex-math></inline-formula>). The largest 2P r value of 38.72 <inline-formula> <tex-math notation="LaTeX">\mu \text{C} </tex-math></inline-formula>/cm 2 is observed for W TE with <inline-formula> <tex-math notation="LaTeX">\alpha = 4.5\times 10^{\mathsf {-6}} </tex-math></inline-formula>/K, while the 2P r value is only <inline-formula> <tex-math notation="LaTeX">22.83~\mu \text{C} </tex-math></inline-formula>/cm 2 for Au TE with <inline-formula> <tex-math notation="LaTeX">\alpha = 14.2\times 10^{\mathsf {-6}} </tex-math></inline-formula>/K. Meanwhile, coercive field (E c ) shifts along the electric field axis and the offset is found to be dependent on the difference of workfunctions (WFs) between TE and TiN bottom electrode (BE). E c shifts toward negative/positive direction, when the WF of TE is larger/smaller (Pt, Pd, Au/W, Al, Ta) than TiN BE. This letter provides an effective way to modulate HfO 2 -based device performance for different requirements in actual application.
Volatile threshold switching (TS) and non‐volatile memory switching (MS) are two typical resistive switching (RS) phenomena in oxides, which could form the basis for memory, analog circuits, and ...neuromorphic applications. Interestingly, TS and MS can be coexistent and converted in a single device under the suitable external excitation. However, the origin of the transition from TS to MS is still unclear due to the lack of direct experimental evidence. Here, conversion between TS and MS induced by conductive filament (CF) morphology in Ag/SiO2/Pt device is directly observed using scanning electron microscopy and high‐resolution transmission electron microscopy. The MS mechanism is related to the formation and dissolution of CF consisting of continuous Ag nanocrystals. The TS originates from discontinuous CF with isolated Ag nanocrystals. The results of current–voltage fitting and Kelvin probe force microscopy further indicate that the TS mechanism is related to the modulation of the tunneling barrier between Ag nanocrystals in CF. This work provides clearly experimental evidence to deepen understanding of the mechanism for RS in oxide‐electrolyte‐based resistive switching memory, contributing to better control of the two RS behaviors to establish high‐performance emerging devices.
The coexistence of volatile threshold and non‐volatile memory switching phenomena is observed in oxide‐electrolyte‐based resistive random access memory (RRAM). The switching behaviors of the device transform from threshold to memory switching when increasing the compliance current in electroforming. Analysis reveals that the threshold and memory switching behaviors correspond to the conductive filament consisting of isolated Ag nanocrystals and continuous Ag nanocrystals, respectively.
Evolution of growth/dissolution conductive filaments (CFs) in oxide‐electrolyte‐based resistive switching memories are studied by in situ transmission electron microscopy. Contrary to what is ...commonly believed, CFs are found to start growing from the anode (Ag or Cu) rather than having to reach the cathode (Pt) and grow backwards. A new mechanism based on local redox reactions inside the oxide‐electrolyte is proposed.
Spiking neural networks (SNNs) sharing large similarity with biological nervous systems are promising to process spatiotemporal information and can provide highly time‐ and energy‐efficient ...computational paradigms for the Internet‐of‐Things and edge computing. Nonvolatile electrolyte‐gated transistors (EGTs) provide prominent analog switching performance, the most critical feature of synaptic element, and have been recently demonstrated as a promising synaptic device. However, high performance, large‐scale EGT arrays, and EGT application for spatiotemporal information processing in an SNN are yet to be demonstrated. Here, an oxide‐based EGT employing amorphous Nb2O5 and LixSiO2 is introduced as the channel and electrolyte gate materials, respectively, and integrated into a 32 × 32 EGT array. The engineered EGTs show a quasi‐linear update, good endurance (106) and retention, a high switching speed of 100 ns, ultralow readout conductance (<100 nS), and ultralow areal switching energy density (20 fJ µm−2). The prominent analog switching performance is leveraged for hardware implementation of an SNN with the capability of spatiotemporal information processing, where spike sequences with different timings are able to be efficiently learned and recognized by the EGT array. Finally, this EGT‐based spatiotemporal information processing is deployed to detect moving orientation in a tactile sensing system. These results provide an insight into oxide‐based EGT devices for energy‐efficient neuromorphic computing to support edge application.
An oxide‐based electrolyte‐gated transistor (EGT) with prominent analog switching performance is introduced and integrated into a 32 × 32 array. The EGT array is leveraged for hardware implementation of spiking neural networks to process spatiotemporal information and is further deployed to detect moving orientation in a tactile sensing system. These results provide an insight into oxide‐based EGT devices for energy‐efficient neuromorphic computing to support edge application.
Resistive memory (ReRAM) based on a solid-electrolyte insulator is a promising nanoscale device and has great potentials in nonvolatile memory, analog circuits, and neuromorphic applications. The ...underlying resistive switching (RS) mechanism of ReRAM is suggested to be the formation and rupture of nanoscale conductive filament (CF) inside the solid-electrolyte layer. However, the random nature of the nucleation and growth of the CF makes their formation difficult to control, which is a major obstacle for ReRAM performance improvement. Here, we report a novel approach to resolve this challenge by adopting a metal nanocrystal (NC) covered bottom electrode (BE) to replace the conventional ReRAM BE. As a demonstration vehicle, a Ag/ZrO2/Cu NC/Pt structure is prepared and the Cu NC covered Pt BE can control CF nucleation and growth to provide superior uniformity of RS properties. The controllable growth of nanoscale CF bridges between Cu NC and Ag top electrode has been vividly observed by transmission electron microscopy (TEM). On the basis of energy-dispersive X-ray spectroscopy (EDS) and elemental mapping analyses, we further confirm that the chemical contents of the CF are mainly Ag atoms. These testing/metrology results are consistent with the simulation results of electric-field distribution, showing that the electric field will enhance and concentrate on the NC sites and control location and orientation of Ag CFs.
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