P-type Ohmic contact to monolayer WSe₂ field-effect transistors using high electron affinity amorphous MoO₃

Monolayer tungsten diselenide (1L-WSe₂) has recently received attention because of its favorable band structure for probing novel correlated phenomena of p-type carriers, such as interaction-driven and topological insulating phases and superconductivity in twisted bilayers, and Bose-Einstein condensation of excitons in double-layer heterostructures. However, electrical transport studies have been impeded by the lack of a reliable method to realize Ohmic hole-conducting contacts to 1L-WSe₂ especially at low carrier densities and low temperatures. Here, we present low-temperature p-type Ohmic contact to 1L-WSe₂ field-effect transistors at carrier densities (n) below n = 1 × 10¹² cm⁻². Monolayer WSe₂ was sandwiched by hexagonal boron nitride flakes by a dry transfer technique and electrically contacted with 20 nm thick amorphous MoO₃ followed by Pd metal. The finding of p-type Ohmic contact is supported by linear current-voltage characteristics down to a temperature of 10 K and carrier densities from n = 7.7 × 10¹¹ cm⁻² to below the threshold, temperature-independent output curves up to room temperature, and negligible contact barrier down to subthreshold regime. Furthermore, the contact resistivity of MoO₃-contacted 1L-WSe₂ FET is 30.2–64.8 kΩ·μm at n = 1.5 × 10¹² cm⁻², which is the lowest reported for 1L-WSe₂ FETs at such low carrier density. Achieving p-type Ohmic contact in 1L-WSe₂ FETs will enable direct electronic measurements of quantum transport in correlated phases in the valence bands of monolayer semiconductors.