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 (E_F's) are pinned near the intrinsic E_F 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 E_F pinning effect and contact resistance.