Integrated one diode-one resistor architecture in nanopillar SiOx resistive switching memory by nanosphere lithography

Constructing van der Waals heterostructures by dry-transfer assembly for novel optoelectronic device

Since the first successful exfoliation of graphene, the superior physical and chemical properties of two-dimensional (2D) materials, such as atomic thickness, strong in-plane bonding energy and weak inter-layer van der Waals (vdW) force have attracted wide attention. Meanwhile, there is a surge of interest in novel physics which is absent in bulk materials. Thus, vertical stacking of 2D materials could be critical to discover such physics and develop novel optoelectronic applications. Although vdW heterostructures have been grown by chemical vapor deposition, the available choices of materials for stacking is limited and the device yield is yet to be improved. Another approach to build vdW heterostructure relies on wet/dry transfer techniques like stacking Lego bricks. Although previous reviews have surveyed various wet transfer techniques, novel dry transfer techniques have been recently been demonstrated, featuring clean and sharp interfaces, which also gets rid of contamination, wrinkles, bubbles formed during wet transfer. This review summarizes the optimized dry transfer methods, which paves the way towards high-quality 2D material heterostructures with optimized interfaces. Such transfer techniques also lead to new physical phenomena while enable novel optoelectronic applications on artificial vdW heterostructures, which are discussed in the last part of this review.

Electrochemistry of the Silicon Oxide Dielectric Layer: Principles, Electrochemical Reactions, and Perspectives

Electrochemistry of the silicon oxide dielectric layer, a notable insulator often used as a gate oxide, is counterintuitive, but addresses fundamental questions to yield novel scientific discoveries. In this minireview, the fundamental electron transfer mechanism of silicon oxide in the electrolyte solution is elucidated. The possible electrochemical reactions to date are discussed in detail, providing numerous potential areas of application which are elaborated and justified. This minireview not only provides background but also guides future research.

Chemical Nature of Electrode and the Switching Response of RF-Sputtered NbOx Films

In this study, the dominant role of the top electrode is presented for Nb₂O₅-based devices to demonstrate either the resistive switching or threshold characteristics. These Nb₂O₅-based devices may exhibit different characteristics depending on the selection of electrode. The use of the inert electrode (Au) initiates resistive switching characteristics in the Au/Nb₂O₅/Pt device. Alternatively, threshold characteristics are induced by using reactive electrodes (W and Nb). The X-ray photoelectron spectroscopy analysis confirms the presence of oxide layers of WO_y and NbO_x at interfaces for W and Nb as top electrodes. However, no interface layer between the top electrode and active layer is detected in X-ray photoelectron spectroscopy for Au as the top electrode. Moreover, the dominant phase is Nb₂O₅ for Au and NbO₂ for W and Nb. The threshold characteristics are attributed to the reduction of Nb₂O₅ phase to NbO₂ due to the interfacial oxide layer formation between the reactive top electrode and Nb₂O₅. Additionally, reliability tests for both resistive switching and threshold characteristics are also performed to confirm switching stabilities.

A Novel Resistive Switching Identification Method through Relaxation Characteristics for Sneak-path-constrained Selectorless RRAM application

Resistive random access memory (RRAM) is a leading candidate in the race towards emerging nonvolatile memory technologies. The sneak path current (SPC) problem is one of the main difficulties in crossbar memory configurations. RRAM devices with desirable properties such as a selectorless, 1R-only architecture with self-rectifying behavior are potential SPC solutions. In this work, the intrinsic nonlinear (NL) characteristics and relaxation characteristics of bilayer high-k/low-k stacked RRAMs are presented. The intrinsic nonlinearity reliability of bilayer selectorless 1R-only RRAM without additional switches has been studied for their ability to effectively suppress SPC in RRAM arrays. The relaxation properties with resistive switching identification method by utilizing the activation energy (Ea) extraction methodology is demonstrated, which provides insights and design guidance for non-uniform bilayer selectorless 1R-only RRAM array applications.

Self-selective van der Waals heterostructures for large scale memory array

The large-scale crossbar array is a promising architecture for hardware-amenable energy efficient three-dimensional memory and neuromorphic computing systems. While accessing a memory cell with negligible sneak currents remains a fundamental issue in the crossbar array architecture, up-to-date memory cells for large-scale crossbar arrays suffer from process and device integration (one selector one resistor) or destructive read operation (complementary resistive switching). Here, we introduce a self-selective memory cell based on hexagonal boron nitride and graphene in a vertical heterostructure. Combining non-volatile and volatile memory operations in the two hexagonal boron nitride layers, we demonstrate a self-selectivity of 10¹⁰ with an on/off resistance ratio larger than 10³. The graphene layer efficiently blocks the diffusion of volatile silver filaments to integrate the volatile and non-volatile kinetics in a novel way. Our self-selective memory minimizes sneak currents on large-scale memory operation, thereby achieving a practical readout margin for terabit-scale and energy-efficient memory integration.

Designing large-scale crossbar arrays for energy efficient neuromorphic computing systems remains a challenge. Here, the authors propose Van der Waals (h-BN/graphene/h-BN) self-selective memory design able to combine, in the same cell, non-volatile and volatile behaviors with negligible sneak current.

Silicon Oxide (SiOx ): A Promising Material for Resistance Switching?

Interest in resistance switching is currently growing apace. The promise of novel high-density, low-power, high-speed nonvolatile memory devices is appealing enough, but beyond that there are exciting future possibilities for applications in hardware acceleration for machine learning and artificial intelligence, and for neuromorphic computing. A very wide range of material systems exhibit resistance switching, a number of which-primarily transition metal oxides-are currently being investigated as complementary metal-oxide-semiconductor (CMOS)-compatible technologies. Here, the case is made for silicon oxide, perhaps the most CMOS-compatible dielectric, yet one that has had comparatively little attention as a resistance-switching material. Herein, a taxonomy of switching mechanisms in silicon oxide is presented, and the current state of the art in modeling, understanding fundamental switching mechanisms, and exciting device applications is summarized. In conclusion, silicon oxide is an excellent choice for resistance-switching technologies, offering a number of compelling advantages over competing material systems.

Electrochemistry at single molecule occupancy in nanopore-confined recessed ring-disk electrode arrays

Electrochemical reactions at nanoscale structures possess unique characteristics, e.g. fast mass transport, high signal-to-noise ratio at low concentration, and insignificant ohmic losses even at low electrolyte concentrations. These properties motivate the fabrication of high density, laterally ordered arrays of nanopores, embedding vertically stacked metal-insulator-metal electrode structures and exhibiting precisely controlled pore size and interpore spacing for use in redox cycling. These nanoscale recessed ring-disk electrode (RRDE) arrays exhibit current amplification factors, AF_RC, as large as 55-fold with Ru(NH₃)₆^2/3+, indicative of capture efficiencies at the top and bottom electrodes, Φ_t,b, exceeding 99%. Finite element simulations performed to investigate the concentration distribution of redox species and to assess operating characteristics are in excellent agreement with experiment. AF_RC increases as the pore diameter, at constant pore spacing, increases in the range 200-500 nm and as the pore spacing, at constant pore diameter, decreases in the range 1000-460 nm. Optimized nanoscale RRDE arrays exhibit a linear current response with concentration ranging from 0.1 μM to 10 mM and a small capacitive current with scan rate up to 100 V s^-1. At the lowest concentrations, the average pore occupancy is 〈n〉 ∼ 0.13 molecule establishing productive electrochemical signals at occupancies at and below the single molecule level in these nanoscale RRDE arrays.

Scaling Effect on Silicon Nitride Memristor with Highly Doped Si Substrate

A feasible approach is reported to reduce the switching current and increase the nonlinearity in a complementary metal-oxide-semiconductor (CMOS)-compatible Ti/SiN_x /p⁺ -Si memristor by simply reducing the cell size down to sub-100 nm. Even though the switching voltages gradually increase with decreasing device size, the reset current is reduced because of the reduced current overshoot effect. The scaled devices (sub-100 nm) exhibit gradual reset switching driven by the electric field, whereas that of the large devices (≥1 µm) is driven by Joule heating. For the scaled cell (60 nm), the current levels are tunable by adjusting the reset stop voltage for multilevel cells. It is revealed that the nonlinearity in the low-resistance state is attributed to Fowler-Nordheim tunneling dominating in the high-voltage regime (≥1 V) for the scaled cells. The experimental findings demonstrate that the scaled metal-nitride-silicon memristor device paves the way to realize CMOS-compatible high-density crosspoint array applications.

Analog Synaptic Behavior of a Silicon Nitride Memristor

In this paper, we present a synapse function using analog resistive-switching behaviors in a SiN_x-based memristor with a complementary metal-oxide-semiconductor compatibility and expandability to three-dimensional crossbar array architecture. A progressive conductance change is attainable as a result of the gradual growth and dissolution of the conducting path, and the series resistance of the AlO_y layer in the Ni/SiN_x/AlO_y/TiN memristor device enhances analog switching performance by reducing current overshoot. A continuous and smooth gradual reset switching transition can be observed with a compliance current limit (>100 μA), and is highly suitable for demonstrating synaptic characteristics. Long-term potentiation and long-term depression are obtained by means of identical pulse responses. Moreover, symmetric and linear synaptic behaviors are significantly improved by optimizing pulse response conditions, which is verified by a neural network simulation. Finally, we display the spike-timing-dependent plasticity with the multipulse scheme. This work provides a possible way to mimic biological synapse function for energy-efficient neuromorphic systems by using a conventional passive SiN_x layer as an active dielectric.

Four-Bits-Per-Cell Operation in an HfO2 -Based Resistive Switching Device

The quadruple-level cell technology is demonstrated in an Au/Al₂ O₃ /HfO₂ /TiN resistance switching memory device using the industry-standard incremental step pulse programming (ISPP) and error checking/correction (ECC) methods. With the highly optimistic properties of the tested device, such as self-compliance and gradual set-switching behaviors, the device shows 6σ reliability up to 16 states with a state current gap value of 400 nA for the total allowable programmed current range from 2 to 11 µA. It is demonstrated that the conventional ISPP/ECC can be applied to such resistance switching memory, which may greatly contribute to the commercialization of the device, especially competitively with NAND flash. A relatively minor improvement in the material and circuitry may enable even a five-bits-per-cell technology, which can hardly be imagined in NAND flash, whose state-of-the-art multiple-cell technology is only at three-level (eight states) to this day.

Three-dimensional crossbar arrays of self-rectifying Si/SiO2/Si memristors

Memristors are promising building blocks for the next-generation memory and neuromorphic computing systems. Most memristors use materials that are incompatible with the silicon dominant complementary metal-oxide-semiconductor technology, and require external selectors in order for large memristor arrays to function properly. Here we demonstrate a fully foundry-compatible, all-silicon-based and self-rectifying memristor that negates the need for external selectors in large arrays. With a p-Si/SiO₂/n-Si structure, our memristor exhibits repeatable unipolar resistance switching behaviour (10⁵ rectifying ratio, 10⁴ ON/OFF) and excellent retention at 300 °C. We further build three-dimensinal crossbar arrays (up to five layers of 100 nm memristors) using fluid-supported silicon membranes, and experimentally confirm the successful suppression of both intra- and inter-layer sneak path currents through the built-in diodes. The current work opens up opportunities for low-cost mass production of three-dimensional memristor arrays on large silicon and flexible substrates without increasing circuit complexity.

Self-Compliant Bipolar Resistive Switching in SiN-Based Resistive Switching Memory

Here, we present evidence of self-compliant and self-rectifying bipolar resistive switching behavior in Ni/SiN_x/n⁺ Si and Ni/SiN_x/n⁺⁺ Si resistive-switching random access memory devices. The Ni/SiN_x/n⁺⁺ Si device’s Si bottom electrode had a higher dopant concentration (As ion > 10¹⁹ cm⁻³) than the Ni/SiN_x/n⁺ Si device; both unipolar and bipolar resistive switching behaviors were observed for the higher dopant concentration device owing to a large current overshoot. Conversely, for the device with the lower dopant concentration (As ion < 10¹⁸ cm⁻³), self-rectification and self-compliance were achieved owing to the series resistance of the Si bottom electrode.

Nano-cone resistive memory for ultralow power operation

SiN _x -based nano-structure resistive memory is fabricated by fully silicon CMOS compatible process integration including particularly designed anisotropic etching for the construction of a nano-cone silicon bottom electrode (BE). Bipolar resistive switching characteristics have significantly reduced switching current and voltage and are demonstrated in a nano-cone BE structure, as compared with those in a flat BE one. We have verified by systematic device simulations that the main cause of reduction in the performance parameters is the high electric field being more effectively concentrated at the tip of the cone-shaped BE. The greatly improved nonlinearity of the nano-cone resistive memory cell will be beneficial in the ultra-high-density crossbar array.

Modulation of nonlinear resistive switching behavior of a TaOx-based resistive device through interface engineering

A resistive switching device with inherent nonlinear characteristics through a delicately engineered interfacial layer is an ideal component to be integrated into passive crossbar arrays for the suppression of sneaking current, especially in ultra-dense 3D integration. In this paper, we demonstrated a TaO_x-based bipolar resistive switching device with a nearly symmetrical bi-directional nonlinear feature through interface engineering. This was accomplished by introducing an ultra-thin interfacial layer (SiO_2-x) with unique features, including a large band gap and a certain level of negative heat of oxide formation between the top electrode (TiN) and resistive layer (TaO_x). The devices exhibit excellent nonlinear property under both positive and negative bias. Modulation of the inherent nonlinearity as well as the resistive switching mechanism are comprehensively studied by scrutinizing the results of the experimental control groups and the extensive characterizations including detailed compositional analysis, which suggests that the underlying mechanism of the nonlinear behavior is associatively governed by the serially connected metallic conductive filament and Flower-Nordheim tunneling barrier formed by the SiO_2-x interface layer. The proposed device in this work has great potential to be implemented in future massive storage memory applications of high-density selector-free crossbar structure.

Increasing local field by interfacial coupling in nanobowl arrays

An increased local field is crucial to create hotspots when applied in detections, which usually means the fabrication of nanostructure arrays with strong electromagnetic couplings.

Localized dielectric breakdown and antireflection coating in metal-oxide-semiconductor photoelectrodes

Silicon-based photoelectrodes for solar fuel production have attracted great interest over the past decade, with the major challenge being silicon's vulnerability to corrosion. A metal-insulator-semiconductor architecture, in which an insulator film serves as a protection layer, can prevent corrosion but must also allow low-resistance carrier transport, generally leading to a trade-off between stability and efficiency. In this work, we propose and demonstrate a general method to decouple the two roles of the insulator by employing localized dielectric breakdown. This approach allows the insulator to be thick, which enhances stability, while enabling low-resistance carrier transport as required for efficiency. This method can be applied to various oxides, such as SiO₂ and Al₂O₃. In addition, it is suitable for silicon, III-V compounds, and other optical absorbers for both photocathodes and photoanodes. Finally, the thick metal-oxide layer can serve as a thin-film antireflection coating, which increases light absorption efficiency.

Understanding rectifying and nonlinear bipolar resistive switching characteristics in Ni/SiNx/p-Si memory devices

Two resistive memory devices were prepared with different doping concentrations in the silicon bottom electrodes to explore the self-rectifying and nonlinear resistive switching characteristics of Ni/SiN_x/p-Si devices.

Ultralow power switching in a silicon-rich SiNy/SiNx double-layer resistive memory device

Here we demonstrate low-power resistive switching in a Ni/SiN_y/SiN_x/p⁺⁺-Si device by proposing a double-layered structure (SiN_y/SiN_x), where the two SiN layers have different trap densities.

Flexible Nanoporous WO3-x Nonvolatile Memory Device

Flexible resistive random access memory (RRAM) devices have attracted great interest for future nonvolatile memories. However, making active layer films at high temperature can be a hindrance to RRAM device fabrication on flexible substrates. Here, we introduced a flexible nanoporous (NP) WO3-x RRAM device using anodic treatment in a room-temperature process. The flexible NP WO3-x RRAM device showed bipolar switching characteristics and a high ION/IOFF ratio of ∼10(5). The device also showed stable retention time over 5 × 10(5) s, outstanding cell-to-cell uniformity, and bending endurance over 10(3) cycles when measured in both the flat and the maximum bending conditions.

A flexible organic resistance memory device for wearable biomedical applications

Parylene is a Food and Drug Administration (FDA)-approved material which can be safely used within the human body and it is also offers chemically inert and flexible merits. Here, we present a flexible parylene-based organic resistive random access memory (RRAM) device suitable for wearable biomedical application. The proposed device is fabricated through standard lithography and pattern processes at room temperature, exhibiting the feasibility of integration with CMOS circuits. This organic RRAM device offers a high storage window (>10(4)), superior retention ability and immunity to disturbing. In addition, brilliant mechanical and electrical stabilities of this device are demonstrated when under harsh bending (bending cycle >500, bending radius <10 mm). Finally, the underlying mechanism for resistance switching of this kind of device is discussed, and metallic conducting filament formation and annihilation related to oxidization/redox of Al and Al anions migrating in the parylene layer can be attributed to resistance switching in this device. These advantages reveal the significant potential of parylene-based flexible RRAM devices for wearable biomedical applications.

A synaptic device built in one diode–one resistor (1D–1R) architecture with intrinsic SiOx-based resistive switching memory

We realize a device with biological synaptic behaviors by integrating silicon oxide (SiOx) resistive switching memory with Si diodes to further minimize total synaptic power consumption due to sneak-path currents and demonstrate the capability for spike-induced synaptic behaviors, representing critical milestones for the use of SiO2-based materials in future neuromorphic computing applications. Biological synaptic behaviors such as long-term potentiation, long-term depression, and spike-timing dependent plasticity are demonstrated systemically with comprehensive investigation of spike waveform analyses and represent a potential application for SiOx-based resistive switching materials. The resistive switching SET transition is modeled as hydrogen (proton) release from the (SiH)2 defect to generate the hydrogenbridge defect, and the RESET transition is modeled as an electrochemical reaction (proton capture) that re-forms (SiH)2. The experimental results suggest a simple, robust approach to realize programmable neuromorphic chips compatible with largescale complementary metal-oxide semiconductor manufacturing technology.

Demonstration of Synaptic Behaviors and Resistive Switching Characterizations by Proton Exchange Reactions in Silicon Oxide

We realize a device with biological synaptic behaviors by integrating silicon oxide (SiO(x)) resistive switching memory with Si diodes. Minimal synaptic power consumption due to sneak-path current is achieved and the capability for spike-induced synaptic behaviors is demonstrated, representing critical milestones for the use of SiO2-based materials in future neuromorphic computing applications. Biological synaptic behaviors such as long-term potentiation (LTP), long-term depression (LTD) and spike-timing dependent plasticity (STDP) are demonstrated systematically using a comprehensive analysis of spike-induced waveforms, and represent interesting potential applications for SiO(x)-based resistive switching materials. The resistive switching SET transition is modeled as hydrogen (proton) release from (SiH)2 to generate the hydrogen bridge defect, and the RESET transition is modeled as an electrochemical reaction (proton capture) that re-forms (SiH)2. The experimental results suggest a simple, robust approach to realize programmable neuromorphic chips compatible with large-scale CMOS manufacturing technology.

Preparing non-volatile resistive switching memories by tuning the content of Au@air@TiO2-h yolk-shell microspheres in a poly(3-hexylthiophene) layer

Crystalline hybrid microspheres, encapsulating a Au nanocore in the hollow cavity of a hairy semiconductor TiO2 shell (Au@air@TiO2-h microspheres) were prepared using template-assisted synthesis methods. The as-prepared microspheres are dispersed into a poly(3-hexylthiophene) (P3HT) matrix and used as a memory active layer. The electrical rewritable memory effects of Al/[Au@air@TiO2-h + P3HT]/ITO sandwich devices can be effectively and exactly controlled by tuning the microsphere content in the electroactive layer. To clarify the switching mechanism, different components in the device, such as P3HT and the microspheres, have been investigated. And it was determined that the switching mechanism can be attributed to the formation and rupture of oxygen vacancy filaments. These results suggest that the Au@air@TiO2-h microspheres are potentially capable of high density data storage. In addition, this finding could provide important guidelines for the reproducibility of nanocomposite-based memory devices and is helpful to demonstrate the switching mechanism of these devices.

Study of Carbon Nanotubes as Etching Masks and Related Applications in the Surface Modification of GaAs-based Light-Emitting Diodes

The surface modification of LEDs based on GaAs is realized by super-aligned multiwalled carbon nanotube (SACNT) networks as etching masks. The surface morphology of SACNT networks is transferred to the GaAs. It is found that the light output power of LEDs based on GaAs with a nanostructured surface morphology is greatly enhanced with the electrical power unchanged.

Physical and chemical mechanisms in oxide-based resistance random access memory

In this review, we provide an overview of our work in resistive switching mechanisms on oxide-based resistance random access memory (RRAM) devices. Based on the investigation of physical and chemical mechanisms, we focus on its materials, device structures, and treatment methods so as to provide an in-depth perspective of state-of-the-art oxide-based RRAM. The critical voltage and constant reaction energy properties were found, which can be used to prospectively modulate voltage and operation time to control RRAM device working performance and forecast material composition. The quantized switching phenomena in RRAM devices were demonstrated at ultra-cryogenic temperature (4K), which is attributed to the atomic-level reaction in metallic filament. In the aspect of chemical mechanisms, we use the Coulomb Faraday theorem to investigate the chemical reaction equations of RRAM for the first time. We can clearly observe that the first-order reaction series is the basis for chemical reaction during reset process in the study. Furthermore, the activation energy of chemical reactions can be extracted by changing temperature during the reset process, from which the oxygen ion reaction process can be found in the RRAM device. As for its materials, silicon oxide is compatible to semiconductor fabrication lines. It is especially promising for the silicon oxide-doped metal technology to be introduced into the industry. Based on that, double-ended graphene oxide-doped silicon oxide based via-structure RRAM with filament self-aligning formation, and self-current limiting operation ability is demonstrated. The outstanding device characteristics are attributed to the oxidation and reduction of graphene oxide flakes formed during the sputter process. Besides, we have also adopted a new concept of supercritical CO2 fluid treatment to efficiently reduce the operation current of RRAM devices for portable electronic applications.

Electroforming and resistive switching in silicon dioxide resistive memory devices

Electroforming and resistive switching data are presented and models are given addressing the unusual operating features of SiO₂ resistive memory.