Graphene/h-BN Heterostructures for Vertical Architecture of RRAM Design

Quantum Dots for Resistive Switching Memory and Artificial Synapse

Memristor devices for resistive-switching memory and artificial synapses have emerged as promising solutions for overcoming the technological challenges associated with the von Neumann bottleneck. Recently, due to their unique optoelectronic properties, solution processability, fast switching speeds, and low operating voltages, quantum dots (QDs) have drawn substantial research attention as candidate materials for memristors and artificial synapses. This review covers recent advancements in QD-based resistive random-access memory (RRAM) for resistive memory devices and artificial synapses. Following a brief introduction to QDs, the fundamental principles of the switching mechanism in RRAM are introduced. Then, the RRAM materials, synthesis techniques, and device performance are summarized for a relative comparison of RRAM materials. Finally, we introduce QD-based RRAM and discuss the challenges associated with its implementation in memristors and artificial synapses.

Amorphous BN-Based Synaptic Device with High Performance in Neuromorphic Computing

The von Neumann architecture has faced challenges requiring high-fulfillment levels due to the performance gap between its processor and memory. Among the numerous resistive-switching random-access memories, the properties of hexagonal boron nitride (BN) have been extensively reported, but those of amorphous BN have been insufficiently explored for memory applications. Herein, we fabricated a Pt/BN/TiN device utilizing the resistive switching mechanism to achieve synaptic characteristics in a neuromorphic system. The switching mechanism is investigated based on the I-V curves. Utilizing these characteristics, we optimize the potentiation and depression to mimic the biological synapse. In artificial neural networks, high-recognition rates are achieved using linear conductance updates in a memristor device. The short-term memory characteristics are investigated in depression by controlling the conductance level and time interval.

Effect of Hydrogen Annealing on Performances of BN-Based RRAM

BN-based resistive random-access memory (RRAM) has emerged as a potential candidate for non-volatile memory (NVM) in aerospace applications, offering high thermal conductivity, excellent mechanical, and chemical stability, low power consumption, high density, and reliability. However, the presence of defects and trap states in BN-based RRAM can limit its performance and reliability in aerospace applications. As a result, higher set voltages of 1.4 and 1.23 V were obtained for non-annealed and nitrogen-annealed BN-based RRAM, respectively, but lower set voltages of 1.06 V were obtained for hydrogen-annealed BN-based RRAM. In addition, the hydrogen-annealed BN-based RRAM showed an on/off ratio of 100, which is 10 times higher than the non-annealed BN-based RRAM. We observed that the LRS changed to the HRS state before 10,000 s for both the non-annealed and nitrogen-annealed BN-based RRAMs. In contrast, the hydrogen-annealed BN-based RRAM showed excellent retention characteristics, with data retained up to 10,000 s.

Research progress in architecture and application of RRAM with computing-in-memory

The development of new technologies has led to an explosion of data, while the computation ability of traditional computers is approaching its upper limit. The dominant system architecture is the von Neumann architecture, with the processing and storage units working independently. The data migrate between them via buses, reducing computing speed and increasing energy loss. Research is underway to increase computing power, such as developing new chips and adopting new system architectures. Computing-in-memory (CIM) technology allows data to be computed directly on the memory, changing the current computation-centric architecture and designing a new storage-centric architecture. Resistive random access memory (RRAM) is one of the advanced memories which has appeared in recent years. RRAM can change its resistance with electrical signals at both ends, and the state will be preserved after power-down. It has potential in logic computing, neural networks, brain-like computing, and fused technology of sense-storage-computing. These advanced technologies promise to break the performance bottleneck of traditional architectures and dramatically increase computing power. This paper introduces the basic concepts of computing-in-memory technology and the principle and applications of RRAM and finally gives a conclusion about these new technologies.

High performance 1D-2D CuO/MoS2 photodetectors enhanced by femtosecond laser-induced contact engineering

The integration of 2D materials with other dimensional materials opens up rich possibilities for both fundamental physics and exotic nanodevices. However, current mixed-dimensional heterostructures often suffer from interfacial contact issues and environment-induced degradation, which severely limits their performance in electronics/optoelectronics. Herein, we demonstrate a novel BN-encapsulated CuO/MoS₂ 2D-1D van der Waals heterostructure photodetector with an ultrahigh photoresponsivity which is 10-fold higher than its previous 2D-1D counterparts. The interfacial contact state and photodetection capabilities of 2D-1D heterojunctions are significantly improved via femtosecond laser irradiation induced MoS₂ wrapping and contamination removal. These h-BN protected devices show highly sensitive, gate-tunable and robust photoelectronic properties. By controlling the gate and bias voltages, the device can achieve a photoresponsivity as high as 2500 A W^-1 in the forward bias mode, or achieve a high detectivity of 6.5 × 10¹¹ Jones and a typical rise time of 2.5 ms at reverse bias. Moreover, h-BN encapsulation effectively protects the mixed-dimensional photodetector from electrical depletion by gas molecules such as O₂ and H₂O during fs laser treatment or the operation process, thus greatly improving the stability and service life in harsh environments. This work provides a new way for the further development of high performance, low cost, and robust mixed-dimensional heterostructure photodetectors by femtosecond laser contact engineering.

Dynamic and Static Switching in ITO/SnOx/ITO and Its Synaptic Application

The attempts to devise networks that resemble human minds are steadily progressing through the development and diversification of neural networks (NN), such as artificial NN (ANN), convolution NN (CNN), and recurrent NN (RNN). Meanwhile, memory devices applied on the networks are also being studied together, and RRAM is the one of the most promising candidates. The fabricated ITO/SnOX/TaN device showed two forms of current–voltage (I-V) curves, classified as dynamic and static. It was triggered from the forming process, and the difference between the two curves resulted from the data retention measured at room temperature for 10³ s. The dynamic curve shows a time-dependent change in the data, and the cause of the data preservation period was considered through X-ray photoelectron spectroscopy (XPS) and linear fitting in conduction mechanisms. To confirm whether the memory performance of the device may be implemented on the synapse, the change in the plasticity was confirmed using a rectangular-shaped pulse. Paired-pulse facilitation (PPF) was implemented, and the change from short-term potentiation (STP) to long-term potentiation (LTP) was achieved.

Insights and Implications of Intricate Surface Charge Transfer and sp3-Defects in Graphene/Metal Oxide Interfaces

Adherence of metal oxides to graphene is of fundamental significance to graphene nanoelectronic and spintronic interfaces. Titanium oxide and aluminum oxide are two widely used tunnel barriers in such devices, which offer optimum interface resistance and distinct interface conditions that govern transport parameters and device performance. Here, we reveal a fundamental difference in how these metal oxides interface with graphene through electrical transport measurements and Raman and photoelectron spectroscopies, combined with ab initio electronic structure calculations of such interfaces. While both oxide layers cause surface charge transfer induced p-type doping in graphene, in sharp contrast to TiO_x, the AlO_x/graphene interface shows the presence of appreciable sp³ defects. Electronic structure calculations disclose that significant p-type doping occurs due to a combination of sp³ bonds formed between C and O atoms at the interface and possible slightly off-stoichiometric defects of the aluminum oxide layer. Furthermore, the sp³ hybridization at the AlO_x/graphene interface leads to distinct magnetic moments of unsaturated bonds, which not only explicates the widely observed low spin-lifetimes in AlO_x barrier graphene spintronic devices but also suggests possibilities for new hybrid resistive switching and spin valves.

Self-Rectified Graphite-Based Reprogrammable One-Time Programmable (RS-OTP) Memory for Embedded Applications

A graphite-based RRAM device with a self-rectifying characteristic named “non-linearity (NL)” is developed for a high-density crossbar array for in-memory computing with low power and high scalability. Meanwhile, the reprogrammable functions are presented in self-selected RRAM as a promising candidate for one-time programmable (OTP) in the emerging memory-embedded applications such as security, system-on-chip (SoC), and Internet of Things (IoT).

Structure, Properties and Applications of Two-Dimensional Hexagonal Boron Nitride

Hexagonal boron nitride (h-BN) has emerged as a strong candidate for two-dimensional (2D) material owing to its exciting optoelectrical properties combined with mechanical robustness, thermal stability, and chemical inertness. Super-thin h-BN layers have gained significant attention from the scientific community for many applications, including nanoelectronics, photonics, biomedical, anti-corrosion, and catalysis, among others. This review provides a systematic elaboration of the structural, electrical, mechanical, optical, and thermal properties of h-BN followed by a comprehensive account of state-of-the-art synthesis strategies for 2D h-BN, including chemical exfoliation, chemical, and physical vapor deposition, and other methods that have been successfully developed in recent years. It further elaborates a wide variety of processing routes developed for doping, substitution, functionalization, and combination with other materials to form heterostructures. Based on the extraordinary properties and thermal-mechanical-chemical stability of 2D h-BN, various potential applications of these structures are described.

Interface engineering of two-dimensional transition metal dichalcogenides towards next-generation electronic devices: recent advances and challenges

Over the past decade, two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted tremendous research interest for future electronics owing to their atomically thin thickness, compelling properties and various potential applications. However, interface engineering including contact optimization and channel modulations for 2D TMDCs represents fundamental challenges in ultimate performance of ultrathin electronics. This article provides a comprehensive overview of the basic understanding of contacts and channel engineering of 2D TMDCs and emerging electronics benefiting from these varying approaches. In particular, we elucidate multifarious contact engineering approaches such as edge contact, phase engineering and metal transfer to suppress the Fermi level pinning effect at the metal/TMDC interface, various channel treatment avenues such as van der Waals heterostructures, surface charge transfer doping to modulate the device properties, and as well the novel electronics constructed by interface engineering such as diodes, circuits and memories. Finally, we conclude this review by addressing the current challenges facing 2D TMDCs towards next-generation electronics and offering our insights into future directions of this field.

Self-Compliance and High Performance Pt/HfOx/Ti RRAM Achieved through Annealing

A self-compliance resistive random access memory (RRAM) achieved through thermal annealing of a Pt/HfO_x/Ti structure. The electrical characteristic measurements show that the forming voltage of the device annealing at 500 °C decreased, and the switching ratio and uniformity improved. Tests on the device’s cycling endurance and data retention characteristics found that the device had over 1000 erase/write endurance and over 10⁵ s of lifetime (85 °C). The switching mechanisms of the devices before and after annealing were also discussed.

Solution-processed black phosphorus nanoflakes for integrating nonvolatile resistive random access memory and the mechanism unveiled

In this study, we demonstrated the integration of black phosphorus (BP) nanoflakes in a resistive random access memory (RRAM) with a facile and complementary metal-oxide-semiconductor-compatible process. The solution-processed BP nanoflakes embedded in polystyrene (PS) as an active layer were sandwiched between aluminum electrodes (Al/BP:PS/Al). The device shows a figure of merit with typical bipolar behavior and forming-free characteristics as well as excellent memory performances such as nonvolatile, low operation voltage (1.75 V) and high ON/OFF ratio (>10²) as well as the long retention time (>1500 s). The improved device performances were attributed to the formation of effective trap sites from the hybrid structure of the active layer (BP:PS), especially the BP nanoflakes and the partly oxidized species (P _x O _y ). Moreover, the extrinsic aluminum oxide layer was observed after the device operation. The mechanism of switching behavior was further unveiled through the carrier transport models, which confirms the conductive mechanisms of space-charge-limited current and Ohmic conductance at high resistance state (HRS) and low resistance state, respectively. Additionally, in the high electric field at HRS, the transfer curve was well fitted with the Poole-Frenkel emission model, which could be attributed to the formation of the aluminum oxide layer. Accordingly, both the trapping/de-trapping of carriers and the formation/rupture of conductive filaments were introduced as transport mechanisms in our devices. Although the partial P _x O _y species on BP were inevitable during the liquid phase exfoliation process, which was regarded as the disadvantages for various applications, it turns to a key point for improving performances in memory devices. The proposed approach to integrating BP nanoflakes in the active layer of the RRAM device could pave the way for next-generation memory devices.

Transformation of threshold volatile switching to quantum point contact originated nonvolatile switching in graphene interface controlled memory devices

Resistive switching devices based on binary transition metal oxides have been widely investigated. However, these devices invariably manifest threshold switching characteristics when the active metal electrode is silver, the dielectric layer is hafnium oxide and platinum is used as the bottom electrode, and have a relatively low compliance current (<100 μA). Here we developed a way to transform an Ag-based hafnium oxide selector into quantum-contact originated memory with a low compliance current, in which a graphene interface barrier layer is inserted between the silver electrode and hafnium oxide layer. Devices with structure Ag/HfO x /Pt acts as a bipolar selector with a high selectivity of >108 and sub-threshold swing of ∼1 mV dec-1. After introducing a graphene interface barrier, high stress dependent (forming at +3 V) formation of localized conducting filaments embodies stable nonvolatile memory characteristics with low set/reset voltages (<±1.0 V), low reset power (6 μW) and multi-level potential. Grain boundaries of the graphene interface control the type of switching in the devices. A good barrier can switch the Ag-based volatile selector into Ag-based nonvolatile memory.

Nonvolatile Memories Based on Graphene and Related 2D Materials

The pervasiveness of information technologies is generating an impressive amount of data, which need to be accessed very quickly. Nonvolatile memories (NVMs) are making inroads into high-capacity storage to replace hard disk drives, fuelling the expansion of the global storage memory market. As silicon-based flash memories are approaching their fundamental limit, vertical stacking of multiple memory cell layers, innovative device concepts, and novel materials are being investigated. In this context, emerging 2D materials, such as graphene, transition metal dichalcogenides, and black phosphorous, offer a host of physical and chemical properties, which could both improve existing memory technologies and enable the next generation of low-cost, flexible, and wearable storage devices. Herein, an overview of graphene and related 2D materials (GRMs) in different types of NVM cells is provided, including resistive random-access, flash, magnetic and phase-change memories. The physical and chemical mechanisms underlying the switching of GRM-based memory devices studied in the last decade are discussed. Although at this stage most of the proof-of-concept devices investigated do not compete with state-of-the-art devices, a number of promising technological advancements have emerged. Here, the most relevant material properties and device structures are analyzed, emphasizing opportunities and challenges toward the realization of practical NVM devices.

Efficient Self-Driven Photodetectors Featuring a Mixed-Dimensional van der Waals Heterojunction Formed from a CdS Nanowire and a MoTe2 Flake

Heterojunctions formed from low-dimensional materials can result in photovoltaic and photodetection devices displaying exceptional physical properties and excellent performance. Herein, a mixed-dimensional van der Waals (vdW) heterojunction comprising a 1D n-type Ga-doped CdS nanowire and a 2D p-type MoTe₂ flake is demonstrated; the corresponding photovoltaic device exhibits an outstanding conversion efficiency of 15.01% under illumination with white light at 650 µW cm^-2 . A potential difference of 80 meV measured, using Kelvin probe force microscopy, at the CdS-MoTe₂ interface confirms the separation and accumulation of photoexcited carriers upon illumination. Moreover, the photodetection characteristics of the vdW heterojunction device at zero bias reveal a rapid response time (<50 ms) and a photoresponsivity that are linearly proportional to the power density of the light. Interestingly, the response of the vdW heterojunction device is negligible when illuminated at 580 nm; this exceptional behavior is presumably due to the rapid rate of recombination of the photoexcited carriers of MoTe₂ . Such mixed-dimensional vdW heterojunctions appear to be novel design elements for efficient photovoltaic and self-driven photodetection devices.