Low-resistance metal contacts to encapsulated semiconductor monolayers with long transfer length

Two-dimensional semiconductors such as transition metal dichalcogenides are of potential use in electronic and optoelectronic devices due to their high mobility, direct optical bandgap and mechanical flexibility. These semiconductors are often encapsulated with hexagonal boron nitride to minimize extrinsic disorder and improve performance, but it is challenging to make high-quality contacts to encapsulated high-purity monolayers. Here we show that metal contacts embedded within hexagonal boron nitride can be transferred onto clean transition metal dichalcogenide monolayers, in an approach that reduces doping, strain and interfacial roughness compared with evaporated metal contacts. Contacts to encapsulated ultraclean tungsten diselenide monolayers created using this technique exhibit a room-temperature contact resistance of around 5 kΩ μm, and provide transistors with zero hysteresis and room-temperature mobility of 655 cm2 V−1 s−1. The contacts also exhibit a transfer length of 1 μm, which is several orders of magnitude larger than the channel thickness.