Defect-Assisted Contact Property Enhancement in a Molybdenum Disulfide Monolayer

Contact engineering for two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for realizing high-performance 2D TMDC devices, and most studies on contact properties of 2D TMDCs have mainly focused on Fermi level unpinning. Here, we investigated electrical and photoelectrical properties of chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) monolayer devices depending on metal contacts, Ti/Pt, Ti/Au, Ti, and Ag, and particularly demonstrated the essential role of defects in MoS₂ in contact properties. Remarkably, MoS₂ devices with Ag contacts show a field-effect mobility of 12.2 cm² V^-1 s^-1, an on/off current ratio of 7 × 10⁷, and a photoresponsivity of 1020 A W^-1, which are outstanding compared to similar devices with other metal contacts. These improvements are attributed to a reduced Schottky barrier height, thanks to the small work function of Ag and Ag-MoS₂ orbital hybridization at the interface, which facilitates efficient charge transfer between MoS₂ and Ag. Interestingly, X-ray photoelectron spectroscopic analysis reveals that Ag₂S was formed in our defect-containing CVD-grown MoS₂ monolayer, but such orbital hybridization is not observed in a nearly defect-free exfoliated MoS₂. This distinction shows that defects existing in MoS₂ enable Ag to effectively couple to MoS₂ and correspondingly enhance multiple electrical and photoelectrical properties.