A key challenge in the development of two-dimensional (2D) devices is the fabrication of metal–semiconductor junctions with minimal contact resistance and depinned energy levels. An ideal solution for practical applications is to make contacts between 2D van der Waals semiconductors and 2D van der Waals metals. Here we report the wafer-scale production of patterned layers of metallic transition metal ditellurides on different substrates. Our tungsten ditelluride and molybdenum ditelluride layers, which are grown using a tellurization process applied to a precursor transition metal layer, have an electronic performance comparable to that of mechanically exfoliated flakes and can be combined with the 2D semiconductor molybdenum disulfide. The resulting metal–semiconductor junctions are free from significant disorder effects and Fermi-level pinning, and are used to create monolayer molybdenum disulfide field-effect transistors. The Schottky barrier heights of the devices also largely follow the trend of the Schottky–Mott limit. Two-dimensional metallic WTe2 and MoTe2 layers can be combined with a semiconducting MoS2 monolayer to create metal–semiconductor junctions that are free from substantial disorder effects and Fermi-level pinning.

Wafer-scale production of patterned transition metal ditelluride layers for two-dimensional metal–semiconductor contacts at the Schottky–Mott limit

Two-dimensional crystals of semi-metallic van der Waals materials hold much potential for the realization of novel phases, as exemplified by the recent discoveries of a polar metal in few-layer 1T′-WTe2 and of a quantum spin Hall state in monolayers of the same material. Understanding these phases is particularly challenging because little is known from experiments about the momentum space electronic structure of ultrathin crystals. Here, we report direct electronic structure measurements of exfoliated mono-, bi-, and few-layer 1T′-WTe2 by laser-based microfocus angle-resolved photoemission. This is achieved by encapsulating with monolayer graphene a flake of WTe2 comprising regions of different thickness. Our data support the recent identification of a quantum spin Hall state in monolayer 1T′-WTe2 and reveal strong signatures of the broken inversion symmetry in the bilayer. We finally discuss the sensitivity of encapsulated samples to contaminants following exposure to ambient atmosphere.

/pdf/microfocus-laser-angle-resolved-photoemission-on-13d8is79yr.pdf

Microfocus Laser-Angle-Resolved Photoemission on Encapsulated Mono-, Bi-, and Few-Layer 1T'-WTe2.

Tungsten ditelluride (WTe2) has attracted increasing attention due to its large magnetoresistance and pressure-induced superconductivity. In this work, we investigate the thermal conductivity (κ) of monolayer WTe2 by performing first-principles calculations, and find strong anisotropic κ with predicted room-temperature values of 9 and 20 W m–1 K−1 along two principal lattice directions, respectively. Such strong anisotropy suggests the importance of orientation when engineering thermal-related applications based on WTe2. The anisotropy of κ is attributed to the in-plane linear acoustic phonon branches, while the out-of-plane quadratic acoustic phonon branch is almost isotropic. The size dependence of κ shows that the size effect can persists up to 10 μm, and the anisotropy decreases with decreasing sample size due to the suppression of low-frequency anisotropic phonons by boundary scattering.

Strong anisotropic thermal conductivity of monolayer WTe 2

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe2, in a van der Waals hybrid structure obtained by mechanically transferring NbSe2 onto various thicknesses of WTe2. When the WTe2 thickness (tWTe2) reaches 21 nm, the superconducting transition occurs around the critical temperature (Tc) of NbSe2 with a gap amplitude (Δp) of 0.38 meV and an unexpected ultralong proximity length (lp) up to 7 μm. With the thicker 42 nm WTe2 layer, however, the proxi...

Inducing Strong Superconductivity in WTe2 by a Proximity Effect

As the elements of integrated circuits are downsized to the nanoscale, the current Cu-based interconnects are facing limitations due to increased resistivity and decreased current-carrying capacity because of scaling. Here, the bottom-up synthesis of single-crystalline WTe2 nanobelts and low- and high-field electrical characterization of nanoscale interconnect test structures in various ambient conditions are reported. Unlike exfoliated flakes obtained by the top-down approach, the bottom-up growth mode of WTe2 nanobelts allows systemic characterization of the electrical properties of WTe2 single crystals as a function of channel dimensions. Using a 1D heat transport model and a power law, it is determined that the breakdown of WTe2 devices under vacuum and with AlO x capping layer follows an ideal pattern for Joule heating, far from edge scattering. High-field electrical measurements and self-heating modeling demonstrate that the WTe2 nanobelts have a breakdown current density approaching ≈100 MA cm-2, remarkably higher than those of conventional metals and other transition-metal chalcogenides, and sustain the highest electrical power per channel length (≈16.4 W cm-1) among the interconnect candidates. The results suggest superior robustness of WTe2 against high-bias sweep and its possible applicability in future nanoelectronics.

/pdf/electrically-robust-single-crystalline-wte2-nanobelts-for-2jqk2tmdbl.pdf

Electrically Robust Single-Crystalline WTe2 Nanobelts for Nanoscale Electrical Interconnects.

Atomically thin layers of transition-metal dicalcogenides (TMDCs) are often known to be metastable in the ambient atmosphere. Understanding the mechanism of degradation is essential for their future applications in nanoelectronics, and thus has attracted intensive interest recently. Here, we demonstrate a systematic study of atomically thin WTe$_{2}$ in its low temperature quantum electronic transport properties. Strikingly, while the temperature dependence of few layered WTe$_{2}$ showed clear metallic tendency in the fresh state, degraded devices first exhibited a re-entrant insulating behavior, and finally entered a fully insulating state. Correspondingly, a crossover from parabolic to linear magnetoresistance, and finally to weak anti-localization was seen. Real-time Raman scattering measurement, together with transmission electron microscopy studies done before and after air degradation of atomically thin WTe$_{2}$ further confirmed that the material gradually form amorphous islands. It thus leads to localized electronic states and explains the low temperature Coulomb gap observed in transport measurements. Our study reveals for the first time the correlation between the unusual magnetotransport and disorder in few-layered WTe$_{2}$, which is indispensable in providing guidance on its future devices application.

Effect of aging-induced disorder on the quantum transport properties of atomically thin WTe$_{2}$

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Effect of aging-induced disorder on the quantum transport properties of atomically thin WTe$_{2}$