Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10), high current density (1-10 μA·μm), and good field-effect transistor mobility (∼30 cm·V·s) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

Thin films of contact metals, specifically Au, Ir, Cr, and Sc, are deposited on exfoliated, bulk MoS2 using electron beam deposition under two different reactor base pressures to determine the contact metal–MoS2 interface chemistry and its dependence on the reactor ambient. The high work function metal Au does not react with MoS2 regardless of reactor ambient. In contrast, interfacial reactions between MoS2 and another high work function metal, Ir, are observed when it is deposited under both high vacuum (HV, ∼ 1 × 10–6 mbar) and ultrahigh vacuum (UHV, ∼ 1 × 10–9 mbar). Interfacial reactions occur between metals with low work functions (Cr, Sc) near the electron affinity of MoS2 when the contact metal is deposited under UHV conditions. In addition, Sc is rapidly oxidized on the MoS2 surface, whereas Cr is only partially oxidized when deposited under HV conditions. This indicates that deposition chamber ambient can affect the contact metal chemistry in addition to the chemistry present at the contact metal–MoS2 interface. These results elucidate the true chemistry of some contact metal–MoS2 interfaces and its dependence on the deposition ambient, and highlight the need to consider the chemical states present at the interface and their impact on contact resistance with MoS2.

The formation of the Ti-MoS2 interface, which is heavily utilized in nanoelectronic device research, is studied by X-ray photoelectron spectroscopy. It is found that, if deposition under high vacuum (∼1 × 10(-6) mbar) as opposed to ultrahigh vacuum (∼1 × 10(-9) mbar) conditions are used, TiO2 forms at the interface rather than Ti. The high vacuum deposition results in an interface free of any detectable reaction between the semiconductor and the deposited contact. In contrast, when metallic titanium is successfully deposited by carrying out depositions in ultrahigh vacuum, the titanium reacts with MoS2 forming Ti(x)S(y) and metallic Mo at the interface. These results have far reaching implications as many prior studies assuming Ti contacts may have actually used TiO2 due to the nature of the deposition tools used.

Contact metals (Au, Ir, and Cr) are deposited on bulk WSe 2 under ultra-high vacuum (UHV, 1 × 10 −9 mbar) and high vacuum (HV, 5 × 10 −6 mbar) conditions and subsequently characterized with x-ray photoelectron spectroscopy (XPS) to elucidate the effects of reactor base pressure on resulting interface chemistry, contact chemistry, and band alignment. Au forms a van der Waals interface with WSe 2 regardless of deposition chamber ambient. In contrast, Ir and Cr form a covalent interface by reducing WSe 2 to form interfacial metal selenides. When Cr is deposited under HV conditions, significant oxygen incorporation is observed resulting in the thermodynamically favorable formation of tungsten oxyselenide and a substantial concentration of Cr x O y . Regardless of contact metal, WO x (2.63 < x < 2.92) forms during deposition under HV conditions which may positively affect interface transport properties. Cr and Ir form unexpectedly large electron and hole Schottky barriers, respectively, when deposited under UHV conditions due to interfacial reactions that contribute to anomalous band alignment. These results reveal the true interface chemistry formed between metals and WSe 2 under UHV and HV conditions and demonstrate the impact on the Fermi level position following contact formation on WSe 2 . PAPEROriginal content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.

Semiconducting MoTe2 is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe2 transistors with the highest performance to date, including high saturation current (>400 µA/µm at 80 K and >200 µA/µm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ•µm from 80 to 300 K), achieved with Ag contacts and AlOx encapsulation. We also investigate other contact metals, extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer h-BN between MoTe2 and Sc. These metal-insulator-semiconductor (MIS) contacts partly de-pin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.