Interfacial Oxidation Unlocks Degenerate n-Doping in Monolayer MoS2

Recent advances in interface engineering have overcome high contact resistance in 2D semiconductor devices by modifying band alignment and carrier density of transition metal dichalcogenide contacts, including through group-V semimetals and charge-transfer doping. Combining photoelectron spectroscopy, diffraction, and microscopy, we demonstrate that placing single-layer MoS2 in contact with a pristine Bi layer results only in weak n-doping and no detectable conduction-band occupation, whereas oxidation of the Bi layer produces a pronounced occupation of the MoS2 conduction band with an electron density on the order of 1013 cm-2. We attribute this strong n-type charge transfer to the formation of an ultrathin β-Bi2O3-like layer with an unusually low effective work function, consistent with our measurements and calculations for an ideal Bi-terminated β-Bi2O3(201) surface. These results establish interfacial oxidation as a means to engineer strong n-type charge transfer at transition metal dichalcogenide interfaces and motivate future transport studies of oxide-mediated contact architectures.