Impact of S-Vacancies on the Charge Injection Barrier at the Electrical Contact with the MoS2 Monolayer

Making electrical contacts to semiconducting transition metal dichalcogenides (TMDCs) represents a major bottleneck for high device performance, often manifesting as strong Fermi level pinning and high contact resistance. Despite intense ongoing research, the mechanism by which lattice defects in TMDCs impact the transport properties across the contact-TMDC interface remains unsettled. Here we study the impact of S-vacancies on the electronic properties at a MoS2 monolayer interfaced with graphite by photoemission spectroscopy, where the defect density is selectively controlled by Ar sputtering. A clear reduction of the MoS2 core level and valence band binding energies is observed as the defect density increases. The experimental results are explained in terms of (i) gap states' energy distribution and (ii) S-vacancies' electrostatic disorder effect. Our model indicates that the Fermi level pinning at deep S-vacancy gap states is the origin of the commonly reported large electron injection barrier (similar to 0.5 eV) at the MoS2 ML interface with low work function metals. At the contact with high work function electrodes, S-vacancies do not significantly affect the hole injection barrier, which is intrinsically favored by Fermi level pinning at shallow occupied gap states. Our results clarify the importance of S-vacancies and electrostatic disorder in TMDC-based electronic devices, which could lead to strategies for optimizing device performance and production.