Interface Chemistry and Band Alignment Study of Ni and Ag Contacts on MoS2.

High contact resistance of transition-metal dichalcogenide (TMD)-based devices is one of the bottlenecks that limit the application of TMDs in various domains. Contact resistance of TMD-based devices is strongly related to the interface chemistry and band alignment at the contact metal/TMD interfaces. To understand the metal/MoS2 interface chemistry and band alignment, Ni and Ag metal contacts are deposited on MoS2 bulk and chemical vapor deposition bilayer MoS2 (2L-MoS2) film samples under ultrahigh vacuum (∼3 × 10-11 mbar) and high vacuum (∼3 × 10-6 mbar) conditions. X-ray photoelectron spectroscopy is used to characterize the interface chemistry and band alignment of the metal/MoS2 stacks. Ni forms covalent contact on MoS2 bulk and 2L-MoS2 film by reducing MoS2 to form interfacial metal sulfides. In contrast, van der Waals gaps form at the Ag/MoS2 bulk and Ag/2L-MoS2 film interfaces, proved by the absence of an additional metal sulfide chemical state and the detection of Ag islands on the surface. Different from other metal/MoS2 systems studied in this work, Ag shows potential to form an Ohmic contact on MoS2 bulk regardless of the deposition ambient. Fermi levels (EF's) are pinned near the intrinsic EF of the 2L-MoS2 film with high defect density regardless of the work function of the metal, which highlights the impact of substrate defect density on the EF pinning effect and contact resistance.