Charge-Transfer Contact to a High-Mobility Monolayer Semiconductor*

Two-dimensional transition metal dichalcogenide (TMD) semiconductors have demonstrated tremendous potential for the development of highly tunable quantum devices. Nevertheless, realizing the potential of these materials requires low-resistance electrical contacts that perform well at the low temperatures and low densities where quantum properties are relevant. Here we present a new contact architecture for monolayer semiconductors that combines a charge-transfer layer and a contact metal to reach contact resistances as low as 20 kW mm at cryogenic temperatures. When applied to monolayer samples derived from high-purity bulk crystals, the polymer-free heterostructure fabrication allows for significantly higher mobility compared to previous generations of devices. The advances in contact quality and channel mobility enable the electronic transport study of strongly interacting quantum phases in monolayer semiconductors. Using these contacts in a monolayer WSe₂ device, we observe a metal-to-insulator transition at low carrier density and transport signatures of the fractional quantum Hall effect under high magnetic fields.