Determination of Carrier Polarity in Fowler-Nordheim Tunneling and Evidence of Fermi Level Pinning at the Hexagonal Boron Nitride/Metal Interface

2D Memory Selectors with Giant Nonlinearity Enabled by Van der Waals Heterostructures

The integration of one-selector-one-resistor crossbar arrays requires the selectors featured with high nonlinearity and bipolarity to prevent leakage currents and any crosstalk among distinct cells. However, a selector with sufficient nonlinearity especially in the frame of device miniaturization remains scarce, restricting the advance of high-density storage devices. Herein, a high-performance memory selector is reported by constructing a graphene/hBN/WSe2 heterostructure. Within the temperature range of 300-80 K, the nonlinearity of this selector varies from ≈103 - ≈104 under forward bias, and increases from ≈300 - ≈105 under reverse bias, the highest reported nonlinearity among 2D selectors. This improvement is ascribed to direct tunneling at low bias and Fowler-Nordheim tunneling at high bias. The tunneling current versus voltage curves exhibit excellent bipolarity behavior because of the comparable hole and electron tunneling barriers, and the charge transport polarity can be effectively tuned from N-type or P-type to bipolar by simply changing source-drain bias. In addition, the conceptual memory selector exhibits no sign of deterioration after 70 000 switching cycles, paving the way for assembling 2D selectors into modern memory devices.

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Ultimate low leakage and EOT of high-κdielectric using transferred metal electrode

The increase of gate leakage current when the gate dielectric layer is thinned is a key issue for device scalability. For scaling down the integrated circuits, a thin gate dielectric layer with a low leakage current is essential. Currently, changing the dielectric layer material or enhancing the surface contact between the gate dielectric and the channel material is the most common way to reduce gate leakage current in devices. Herein, we report a technique of enhancing the surface contact between the gate dielectric and the metal electrode, that is constructing an Au/Al₂O₃/Si metal-oxide-semiconductor device by replacing the typical evaporated electrode/dielectric layer contact with a transferred electrode/high-κdielectric layer contact. The contact with a mild, non-invasive interface can ensure the intrinsic insulation of the dielectric layer. By applying 2-40 nm Al₂O₃as the dielectric layer, the current density-electrical field (J-E) measurement reveals that the dielectric leakage generated by the transferred electrode is less than that obtained by the typical evaporated electrode with a ratio of 0.3 × 10¹ ∼ 5 × 10⁶atV_bias = 1 V. Furthermore, atJ = 1 mA cm^-2, the withstand voltage can be raised by 10⁰-10²times over that of an evaporated electrode. The capacitance-voltage (C-V) test shows that the transferred metal electrode can efficiently scale the equivalent oxide layer thickness (EOT) to 1.58 nm, which is a relatively smaller value than the overall reported Si-based device's EOT. This finding successfully illustrates that the transferred electrode/dielectric layer's mild contact can balance the scaling of the gate dielectric layer with a minimal leakage current and constantly reduce the EOT. Our enhanced electrode/dielectric contact approach provides a straightforward and effective pathway for further scaling of devices in integrated circuits and significantly decreases the overall integrated circuit's static power consumption (ICs).

Ultrafast Operation of 2D Heterostructured Nonvolatile Memory Devices Provided by the Strong Short-Time Dielectric Breakdown Strength of h-BN

Recently, the ultrafast operation (∼20 ns) of a two-dimensional (2D) heterostructured nonvolatile memory (NVM) device was demonstrated, attracting considerable attention. However, there is no consensus on its physical origin. In this study, various 2D NVM device structures are compared. First, we reveal that the hole injection at the metal/MoS₂ interface is the speed-limiting path in the NVM device with the access region. Therefore, MoS₂ NVM devices with a direct tunneling path between source/drain electrodes and the floating gate are fabricated by removing the access region. Indeed, a 50 ns program/erase operation is successfully achieved for devices with metal source/drain electrodes as well as graphite source/drain electrodes. This controlled experiment proves that an atomically sharp interface is not necessary for ultrafast operation, which is contrary to the previous literature. Finally, the dielectric breakdown strength (E_BD) of h-BN under short voltage pulses is examined. Since a high dielectric breakdown strength allows a large tunneling current, ultrafast operations can be achieved. Surprisingly, an E_BD = 26.1 MV/cm for h-BN is realized under short voltage pulses, largely exceeding the E_BD = ∼12 MV/cm from the direct current (DC) measurement. This suggests that the high E_BD of h-BN can be the physical origin of the ultrafast operation.

Deep-ultraviolet electroluminescence and photocurrent generation in graphene/hBN/graphene heterostructures

Hexagonal boron nitride (hBN) is a van der Waals semiconductor with a wide bandgap of ~ 5.96 eV. Despite the indirect bandgap characteristics of hBN, charge carriers excited by high energy electrons or photons efficiently emit luminescence at deep-ultraviolet (DUV) frequencies via strong electron-phonon interaction, suggesting potential DUV light emitting device applications. However, electroluminescence from hBN has not been demonstrated at DUV frequencies so far. In this study, we report DUV electroluminescence and photocurrent generation in graphene/hBN/graphene heterostructures at room temperature. Tunneling carrier injection from graphene electrodes into the band edges of hBN enables prominent electroluminescence at DUV frequencies. On the other hand, under DUV laser illumination and external bias voltage, graphene electrodes efficiently collect photo-excited carriers in hBN, which generates high photocurrent. Laser excitation micro-spectroscopy shows that the radiative recombination and photocarrier excitation processes in the heterostructures mainly originate from the pristine structure and the stacking faults in hBN. Our work provides a pathway toward efficient DUV light emitting and detection devices based on hBN.

Visualization of a hexagonal born nitride monolayer on an ultra-thin gold film via reflected light microscopy

Hexagonal boron nitride (h-BN) is an important insulating layered material for two-dimensional heterostructure devices. Among many applications, few-layer h-BN films have been employed as superior tunneling barrier films. However, it is difficult to construct a heterostructure with ultra-thin h-BN owing to the poor visibility of flakes on substrates, especially on a metallic surface substrate. Since reflectance from a metallic surface is generally high, a h-BN film on a metallic surface does not largely influence reflection spectra. In the present study, a thin Au layer with a thickness of ∼10 nm deposited on a Si substrate with a thermally grown SiO₂was used for visualizing h-BN flakes. The thin Au layer possesses conductivity and transparency. Thus, the Au/SiO₂/Si structure serves as an electrode and contributes to the visualization of an ultra-thin film according to optical interference. As a demonstration, the wavelength-dependent contrast of exfoliated few-layer h-BN flakes on the substrate was investigated under a quasi-monochromatic light using an optical microscope. A monolayer h-BN film was recognized in the image taken by a standard digital camera using a narrow band-pass filter of 490 nm, providing maximum contrast. Since the contrast increases linearly with the number of layers, the appropriate number of layers is identified from the contrast. Furthermore, the insulating property of a h-BN flake is examined using a conductive atomic force microscope to confirm whether the thin Au layer serves as an electrode. The tunneling current through the h-BN flake is consistent with the number of layers estimated from the contrast.

Material and Device Structure Designs for 2D Memory Devices Based on the Floating Gate Voltage Trajectory

Two-dimensional heterostructures have been extensively investigated as next-generation nonvolatile memory (NVM) devices. In the past decade, drastic performance improvements and further advanced functionalities have been demonstrated. However, this progress is not sufficiently supported by the understanding of their operations, obscuring the material and device structure design policy. Here, detailed operation mechanisms are elucidated by exploiting the floating gate (FG) voltage measurements. Systematic comparisons of MoTe₂, WSe₂, and MoS₂ channel devices revealed that the tunneling behavior between the channel and FG is controlled by three kinds of current-limiting paths, i.e., tunneling barrier, 2D/metal contact, and p-n junction in the channel. Furthermore, the control experiment indicated that the access region in the device structure is required to achieve 2D channel/FG tunneling by preventing electrode/FG tunneling. The present understanding suggests that the ambipolar 2D-based FG-type NVM device with the access region is suitable for further realizing potentially high electrical reliability.

Understanding the Memory Window Overestimation of 2D Materials Based Floating Gate Type Memory Devices by Measuring Floating Gate Voltage

The memory window of floating gate (FG) type non-volatile memory (NVM) devices is a fundamental figure of merit used not only to evaluate the performance, such as retention and endurance, but also to discuss the feasibility of advanced functional memory devices. However, the memory window of 2D materials based NVM devices is historically determined from round sweep transfer curves, while that of conventional Si NVM devices is determined from high and low threshold voltages (V_th s), which are measured by single sweep transfer curves. Here, it is elucidated that the memory window of 2D NVM devices determined from round sweep transfer curves is overestimated compared with that determined from single sweep transfer curves. The floating gate voltage measurement proposed in this study clarifies that the V_th s in round sweep are controlled not only by the number of charges stored in floating gate but also by capacitive coupling between floating gate and back gate. The present finding on the overestimation of memory window enables to appropriately evaluate the potential of 2D NVM devices.

Combinatorial Large-Area MoS2/Anatase-TiO2 Interface: A Pathway to Emergent Optical and Optoelectronic Functionalities

The interface of transition-metal dichalcogenides (TMDCs) and high-k dielectric transition-metal oxides (TMOs) had triggered umpteen discourses because of the indubitable impact of TMOs in reducing the contact resistances and restraining the Fermi-level pinning for the metal-TMDC contacts. In the present work, we focus on the unresolved tumults of large-area TMDC/TMO interfaces, grown by adopting different techniques. Here, on a pulsed laser-deposited MoS₂ thin film, a layer of TiO₂ is grown by atomic layer deposition (ALD) and pulsed laser deposition (PLD). These two different techniques emanate the layer of TiO₂ with different crystallinities, thicknesses, and interfacial morphologies, subsequently influencing the electronic and optical properties of the interfaces. Contrasting the earlier reports of n-type doping at the exfoliated MoS₂/TiO₂ interfaces, the large-area MoS₂/anatase-TiO₂ films had realized a p-type doping of the underneath MoS₂, manifesting a boost in the extent of p-type doping with increasing thickness of TiO₂, as emerged from the X-ray photoelectron spectra. Density functional analysis of the MoS₂/anatase-TiO₂ interfaces, with pristine and interfacial defect configurations, could correlate the interdependence of doping and the terminating atomic surface of TiO₂ on MoS₂. The optical properties of the interface, encompassing photoluminescence, transient absorption and z-scan two-photon absorption, indicate the presence of defect-induced localized midgap levels in MoS₂/TiO₂ (PLD) and a relatively defect-free interface in MoS₂/TiO₂ (ALD), corroborating nicely with the corresponding theoretical analysis. From the investigation of optical properties, we indicate that the MoS₂/TiO₂ (PLD) interface may act as a promising saturable absorber, having a significant nonlinear response for the sub-band-gap excitations. Moreover, the MoS₂/TiO₂ (PLD) interface had exemplified better phototransport properties. A potential application of MoS₂/TiO₂ (PLD) is demonstrated by the fabrication of a p-type phototransistor with the ionic-gel top gate. This endeavor to analyze and perceive the MoS₂/TiO₂ interface establishes the prospectives of large-area interfaces in the field of optics and optoelectronics.

Modulating Charge Separation with Hexagonal Boron Nitride Mediation in Vertical Van der Waals Heterostructures

Tuning the optical and electrical properties by stacking different layers of two-dimensional (2D) materials enables us to create unusual physical phenomena. Here, we demonstrate an alternative approach to enhance charge separation and alter physical properties in van der Waals heterojunctions with type-II band alignment by using thin dielectric spacers. To illustrate our working principle, we implement a hexagonal boron nitride (h-BN) sieve layer in between an InSe/GeS heterojunction. The optical transitions at the junctions studied by photoluminescence and the ultrafast pump-probe technique show quenching of emission without h-BN layers exhibiting an indirect recombination process. This quenching effect due to strong interlayer coupling was confirmed with Raman spectroscopic studies. In contrast, h-BN layers in between InSe and GeS show strong enhancement in emission, giving another degree of freedom to tune the heterojunction property. The two-terminal photoresponse study supports the argument by showing a large photocurrent density for an InSe/h-BN/GeS device by avoiding interlayer charge recombination. The enhanced charge separation with h-BN mediation manifests a photoresponsivity and detectivity of 9 × 10² A W^-1 and 3.4 × 10¹⁴ Jones, respectively. Moreover, a photogain of 1.7 × 10³ shows a high detection of electrons for the incident photons. Interestingly, the photovoltaic short-circuit current is switched from positive to negative, whereas the open-circuit voltage changes from negative to positive. Our proposed enhancement of charge separation with 2D-insulator mediation, therefore, provides a useful route to manipulate the physical properties of heterostructures and for the future development of high-performance optoelectronic devices.

Layer-Dependent Interfacial Transport and Optoelectrical Properties of MoS2 on Ultraflat Metals

Layered materials based on transition-metal dichalcogenides (TMDs) are promising for a wide range of electronic and optoelectronic devices. Realizing such practical applications often requires metal-TMD connections or contacts. Hence, a complete understanding of electronic band alignments and potential barrier heights governing the transport through metal-TMD junctions is critical. However, it is presently unclear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few-layered (1 to 5 layers) MoS₂ immobilized on ultraflat conducting Au surfaces [root-mean-square (rms) surface roughness < 0.2 nm] and indium-tin oxide (ITO) substrates (rms surface roughness < 0.7 nm) forming a vertical metal (CAFM tip)-semiconductor-metal device. We have observed that the current increases with the number of layers up to five layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed how this barrier decreases as the number of layers increases. Using density functional theory calculations, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating TMDs on a transparent ultraflat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of the TMD-metal junction, which depends on the numbers of TMD layers and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.

Recent Progress in the Fabrication, Properties, and Devices of Heterostructures Based on 2D Materials

With a large number of researches being conducted on two-dimensional (2D) materials, their unique properties in optics, electrics, mechanics, and magnetics have attracted increasing attention. Accordingly, the idea of combining distinct functional 2D materials into heterostructures naturally emerged that provides unprecedented platforms for exploring new physics that are not accessible in a single 2D material or 3D heterostructures. Along with the rapid development of controllable, scalable, and programmed synthesis techniques of high-quality 2D heterostructures, various heterostructure devices with extraordinary performance have been designed and fabricated, including tunneling transistors, photodetectors, and spintronic devices. In this review, we present a summary of the latest progresses in fabrications, properties, and applications of different types of 2D heterostructures, followed by the discussions on present challenges and perspectives of further investigations.