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Achieving low contact resistance (RC) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate‐voltages (VTG and VBG). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG, in sharp contrast to Ti contacts that only modulate RC by varying VBG. The anomalous behavior is attributed to the strongly modulated pseudo‐junction resistance (Rjun) by VTG, resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG. Technology computer aided design simulations further confirm the contribution of VTG to Rjun, which improves overall RC of Sb‐contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual‐gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG. The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals. [ABSTRACT FROM AUTHOR]

Even though atomically thin 2D semiconductors have shown great potential for next‐generation electronics, the low carrier mobility caused by poor metal–semiconductor contacts and the inherently high density of impurity scatterings remains a critical issue. Herein, high‐mobility field‐effect transistors (FETs) by introducing few‐layer PdSe2 flakes as channels is achieved, via directly depositing semimetal antimony (Sb) as drain–source electrodes. The formation of clean and defect‐free van der Waals (vdW) stackings at the Sb–PdSe2 heterointerfaces boosts the room temperature transport characteristics, including low contact resistance down to 0.55 kΩ µm, high on‐current density reaching 96 µA µm−1, and high electron mobility of 383 cm2 V−1 s−1. Furthermore, metal–insulator transition (MIT) is observed in the PdSe2 FETs with and without hexagonal boron nitride (h–BN) as buffer layers. However, the layered h–BN/PdSe2 vdW stacking eliminates the interference of interfacial disorders, and thus the corresponding device exhibits a lower MIT crossing point, larger mobility exponent of γ ∼ 1.73, significantly decreased hopping parameter of T0, and ultrahigh electron mobility of 2,184 cm2 V−1 s−1 at 10 K. These findings are expected to be significant for developing high mobility 2D‐based quantum devices. [ABSTRACT FROM AUTHOR]

Recently, 2D materials have been intensively investigated for their novel nanoelectronic applications; among these materials, tungsten diselenide (WSe2) is attracting substantial research interest due to its high mobility, sizable bandgap, and ambipolar characteristics. However, Fermi‐level pinning (FLP) at the metal–semiconductor contact is a critical issue preventing further integration of WSe2 to complementary metal–oxide–semiconductor (CMOS) technology. In this study, a facile doping method of oxygen (O2) plasma treatment and an aging effect to overcome the FLP of WSe2 field‐effect transistors (FETs) are utilized. After aging, a reduction is observed in FLP on oxidized WSe2 FETs, along with a decrease in pinning factor (S) for holes from −0.06 to −0.36. Further, the field‐effect mobility of high‐ (Pd) and low‐ (In) work‐function contacted WSe2 devices indicates the presence of more improvement in high‐work‐function metal‐contacted p‐type WSe2 FETs, which further strengthens the Fermi level de‐pinning behavior attributed to the O2 plasma and aging processes. The existence of different tunneling behaviors of Pd and In devices also confirms the effect of O2 plasma doping into WSe2 FETs. Ultimately, this work demonstrates a simple and efficient method for achieving the de‐pinning of Fermi‐levels and modulating FLP of 2D FETs. [ABSTRACT FROM AUTHOR]

Surface charge transfer doping (SCTD) using oxygen plasma to form a p‐type dopant oxide layer on transition metal dichalcogenide (TMDs) is a promising doping technique for 2D TMDs field‐effect transistors (FETs). However, patternability of SCTD is a key challenge to effectively switch FETs. Herein, a simple method to selectively pattern degenerately p‐type (p+)‐doped WSe2 FETs via electron beam (e‐beam) irradiation is reported. The effect of the selective e‐beam irradiation is confirmed by the gate‐tunable optical responses of seamless lateral p+–p diodes. The OFF state of the devices by inducing trapped charges via selective e‐beam irradiation onto a desired channel area in p+‐doped WSe2, which is in sharp contrast to globally p+‐doped WSe2 FETs, is realized. Selective e‐beam irradiation of the PMMA‐passivated p+‐WSe2 enables accurate control of the threshold voltage (Vth) of WSe2 devices by varying the pattern size and e‐beam dose, while preserving the low contact resistance. By utilizing hBN as the gate dielectric, high‐performance WSe2 p‐FETs with a saturation current of −280 µA µm−1 and on/off ratio of 109 are achieved. This study's technique demonstrates a facile approach to obtain high‐performance TMD p‐FETs by e‐beam irradiation, enabling efficient switching and patternability toward various junction devices. [ABSTRACT FROM AUTHOR]

Localized trap density (Dt) at the 2D channel–gate dielectric interface and its relative strength to carrier–carrier interactions depending on the thickness of the 2D channel can determine the nature of a metal–insulator transition (MIT) in 2D materials. Here, the MIT occurring in WSe2 devices is systematically analyzed by varying the WSe2 thickness from ≈20 nm to monolayer to explore the effects of Dt on MIT. The corresponding critical carrier density increases from ≈8.30 × 1011 to 9.45 × 1012 cm–2 and Dt from ≈6.02 × 1011 to 1.13 × 1013 cm–2 eV–1 as WSe2 thickness decreases from ≈20 nm to monolayer. These large increments in Dt with decreasing thickness of WSe2 induce a strong potential fluctuation in the band of WSe2, causing charge density inhomogeneity in the system, which attributed to tuning the MIT. The critical percolation exponent is strongly dependent on WSe2 thickness with an excellent agreement between the transport data and percolation theory achieved from thinner WSe2 devices, while the transport data measured from multilayer WSe2 devices does not obey the percolation theory. These results suggest that the nature of MIT strongly depends on the WSe2 channel thickness and corresponding unscreened charge impurity and strength of Dt at the interface. [ABSTRACT FROM AUTHOR]

A technique to form the edge contact in two-dimensional (2D) field-effect transistors (FETs) has been intensively studied for the purpose of achieving high mobility and also recently overcoming Fermi-level pinning (FLP). However, most of the previous studies on the effects of edge contact have been conducted mainly on graphene and molybdenum disulfide (MoS2) until now. Here, we report an anomalous electrical transport of edge-contacted WSe2 FETs that is different from the typical Fermi-level depinning behavior in MoS2 FETs reported previously. Such WSe2 FETs showed a consistent p-type behavior regardless of contacting metals including low work-function chromium and indium and high work-function palladium, with a small pinning factor of 0.04. It suggests a strong FLP near the valence band edge of WSe2. The cross-sectional high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy analyses revealed the formation of an oxide after plasma etching at the interfacing edge near the metallic contact, which was found to be responsible for the strong FLP. The temperature dependent electrical characteristics of the device indicated the occurrence of Fowler–Nordheim tunneling with high electrical biases, further supporting the presence of an oxide at the contact edge of WSe2. Interestingly, the oxide was removed upon wet treatment in potassium hydroxide, which could be an effective way to control carrier polarity by suppressing the FLP. This work provides a new insight into the impact of edge contacts on surface sensitive 2D materials. [ABSTRACT FROM AUTHOR]

Tungsten diselenide (WSe2) is attracting attention because of its superior electronic and optoelectronic properties. In recent years, the number of research works related to the WSe2‐based field‐effect transistors (FETs) has increased dramatically. Nonetheless, the performance of 2D WSe2 is influenced sensitively by metal–semiconductor (MS) interface states, where Fermi‐level pinning is substantial. This research explores Fermi‐level depinning by doping with an n‐type polymer. In this work, spin‐coated polyvinyl alcohol (PVA) is used as an n‐type dopant for achieving low‐contact‐resistance WSe2 FETs in cases of both high‐work‐function (Pd) and low‐work‐function (In) metals. Interestingly, the increase in the Schottky barrier height resulting from the application of PVA gives rise to Fowler–Nordheim tunneling for a doped Pd‐WSe2 contact. By contrast, only direct tunneling is observed for an In‐WSe2 contact irrespective of whether the dopant is used. The barrier‐height modification after doping reveals that the improvement of the contact resistance is correlated to the enhancement of tunneling current after doping, which is consistent with the measurement results. This work suggests a practical direction for contact engineering of future WSe2‐based electronic devices and expands the current understanding of charge transport at the MS contact when a polymeric n‐type dopant is applied. [ABSTRACT FROM AUTHOR]

Recently, researchers have been investigating artificial ferroelectricity, which arises when inversion symmetry is broken in certain R-stacked, i.e., zero-degree twisted, van der Waals (vdW) bilayers. Here, the study reports the twist-controlled ferroelectricity in tungsten diselenide (WSe 2 ) bilayers. The findings show noticeable room temperature ferroelectricity that decreases with twist angle within the range 0° < θ < 3°, and disappears completely for θ ≥ 4°. This variation aligns with moiré length scale-controlled ferroelectric dynamics (0° < θ < 3°), while loss beyond 4° may relate to twist-controlled commensurate to non-commensurate transitions. This twist-controlled ferroelectricity serves as a spectroscopic tool for detecting transitions between commensurate and incommensurate moiré patterns. At 5.5 K, 3° twisted WSe 2 exhibits ferroelectric and correlation-driven ferromagnetic ordering, indicating twist-controlled multiferroic behavior. The study offers insights into twist-controlled coexisting ferro-ordering and serves as valuable spectroscopic tools., (© 2024 Wiley‐VCH GmbH.)

The surface oxidation of 2D transition metal dichalcogenides (TMDs) has recently gained tremendous technological and fundamental interest owing to the multi-functional properties that the surface oxidized layer opens up. In particular, when integrated into other 2D materials in the form of van der Waals heterostructures, oxidized TMDs enable designer properties, including novel electronic states, engineered light-matter interactions, and exceptional-point singularities, among many others. Here, the evolving landscapes of the state-of-the-art surface engineering technologies that enable controlled oxidation of TMDs down to the monolayer-by-monolayer limit are reviewed. Next, the use of oxidized TMDs in van der Waals heterostructures for different electronic and photonic device platforms, materials growth processes, engineering concepts, and synthesizing new condensed matter phenomena is discussed. Finally, challenges and outlook for future impact of oxidized TMDs in driving rapid advancements across various application fronts is discussed., (© 2024 The Author(s). Advanced Science published by Wiley‐VCH GmbH.)

Two-dimensional transition metal dichalcogenides (TMDs) are promising materials for semiconductor nanodevices owing to their flexibility, transparency, and appropriate band gaps. A variety of optoelectronic and electronic devices based on TMDs p-n diodes have been extensively investigated due to their unique advantages. However, improving their performance is challenging for commercial applications. In this study, we propose a facile and doping-free approach based on the contact engineering of a few-layer tungsten di-selenide to form a lateral p-n homojunction photovoltaic. By combining surface and edge contacts for p-n diode fabrication, the photovoltaic effect is achieved with a high fill factor of ≈0.64, a power conversion efficiency of up to ≈4.5%, and the highest external quantum efficiency with a value of ≈67.6%. The photoresponsivity reaches 283 mA/W, indicating excellent photodiode performance. These results demonstrate that our technique has great potential for application in next-generation optoelectronic devices.