Contact Engineering n-Type Molybdenum Ditelluride Transistors

Atomically-thin transistors based on transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 have demonstrated significant current modulation, moderate carrier mobilities, and high drive currents down to nanometer-scale gate lengths. TMDs are thus becoming a candidate for scaling CMOS to ultra-short, sub-10 nm channels. Of the TMDs, MoTe2 is still largely unexplored, despite that in monolayer thickness (~0.7 nm) it demonstrates a direct optical band gap of ~1.1 eV, nearly identical to that of bulk silicon. This moderate band gap suggests that MoTe2 may be a suitable candidate for low-power CMOS.

However, it is presently difficult to produce n- and p-type transistors from the same type of TMD, which limits their practical applications. This is partly due to lack of stable doping and partly due to strong Femi level pinning at the TMD-metal contacts.

In this work, we investigate contacts to MoTe¬2 transistors with seven different metals from low to high workfunction (Sc, Ag, Ti, Cr, Au, Ni, and Pt) and extract their respective Schottky barriers in a consistent fashion for the first time. We demonstrate that while all bare metals form some type of ambipolar contact to MoTe2, metal-insulator-semiconductor (MIS) contacts with a single layer of hexagonal boron nitride (h-BN) de-pin the Fermi level of the metal contact, allowing us to achieve the first reported unipolar electron current in MoTe2.

To gain insight into the origin of ambipolar contact pinning to MoTe2, we investigate Ag- and Sc-MoTe2 interfaces. We performed X-ray Photoelectron Spectroscopy (XPS) to uncover that AgxTey and ScxTey compounds were forming at the respective interfaces. These interfacial compounds may largely determine the band alignment.

To prevent chemical reactions at the contact and to de-pin the Fermi-level, we formed MIS contacts by transferring a single layer of h-BN between Sc metal and the MoTe2. Sc was chosen due to its low workfunction (~3.5 eV) to provide good alignment to the MoTe2 conduction band. Thus, we achieved unipolar transistor behavior in a 170 nm channel length MoTe2 device, with ~80 μA/µm electron current and ~105 ON/OFF current ratio at 1 V drain bias.

This is an important result because it demonstrates that monolayer h-BN de-pins the Fermi level and affords unipolar behavior in our MIS-contacted MoTe2 transistors. This study furthers our understanding of metal-TMD contacts and demonstrates a practical route for de-pinning the Fermi level and preventing surface reactions, towards selective, unipolar TMD contacts.