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

Abstract

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/MoS 2 interface chemistry and band alignment, Ni and Ag metal contacts are deposited on MoS 2 bulk and chemical vapor deposition bilayer MoS 2 (2L-MoS 2 ) 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/MoS 2 stacks. Ni forms covalent contact on MoS 2 bulk and 2L-MoS 2 film by reducing MoS 2 to form interfacial metal sulfides. In contrast, van der Waals gaps form at the Ag/MoS 2 bulk and Ag/ 2L-MoS 2 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/MoS 2 systems studied in this work, Ag shows potential to form an Ohmic contact on MoS 2 bulk regardless of the deposition ambient. Fermi levels (E F 's) are pinned near the intrinsic E F of the 2L-MoS 2 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.

Reduced Fermi Level Pinning at Physisorptive Sites of Moire-MoS2/Metal Schottky Barriers

“…The SBH slope for the stable chemisorptive sites is only S = 0.24, the green line in Figure , indicating a strong pinning effect. This is consistent with the previous calculations and experimental observations. − On the other hand, the SBH value for the physisorptive site in the Moire geometry has a much larger slope of S = 0.37, indicating the Fermi level depinning by the physisorptive Moire sites. Note that these Moire sites lead to the overall larger interlayer distance (see Figure ), that is, the MIGS decay length into the semiconductor in eq is decreased.…”

“…The other case is silver. The Fermi energy of Ag contacts lies quite close the conduction band edge of MoS 2 so the n-SBH is very small (∼0.2 eV) for this geometry, consistent with the small energy difference seen experimentally by photoemission by Wang et al 14 Although Ag is a noble metal, its work function is surprisingly low for a noble metal. It is interesting to compare the SBHs of Ag and Au.…”

“…This achievement of unpinned Schottky barriers is a remarkable demonstration, but this method might be difficult to replicate for realistic devices. Recently, Wang et al 4 used In–Au alloys to achieve low-resistance contacts with excellent performance, while Wang et al 14 showed by photoemission that Ag has low resistance n-type contacts. Here, we explain why this unusual choice of metals can be expected to have favorable contact properties.…”

“…The interfaces of layered dichalcogenide semiconductors such as MoS 2 on metals were calculated to have a slope factor of S ∼ 0.3 for the defect-free case and an experimental value of S ∼ 0.1 for typical defective MoS 2 contacts. − Recently, Liu et al achieved a truly weakly bonded metal/MoS 2 interface and a Schottky-like slope by using a metal film mounted on a polydimethylsiloxane layer, mechanically transferred onto an exfoliated MoS 2 film, and avoiding any energetic deposition or disordering processes of the metal. They obtained a van der Waals (vdW) interfacial distance of ∼3.0 Å for contacts of noble or precious metals Ag, Cu, Au, and Pt.…”

Emergent second-harmonic generation in van der Waals heterostructure of bilayer MoS 2 and monolayer graphene

“…With the interlayer coupling in 2LM/1LG, the electrons in 1LG would partially transfer to 2LM because 1LG has a work function (~4.5 eV) ( 34 ) lower than that of 2LM (~5.1 eV) ( 35 ). When 1LG is in contact with 2LM, the Fermi level will shift to the same level, leading to the accumulation of negative charge in 2LM and positive charge in 1LG ( 36 ), as schematically shown in Fig.…”