Abstract
In this review on contacts with MoS2, we consider reports on both interface chemistry and device characteristics. We show that there is considerable disagreement between reported properties, at least some of which may be explained by variability in the properties of geological MoS2. Furthermore, we highlight that while early experiments using photoemission to study the interface behavior of metal-MoS2 showed a lack of Fermi-level pinning, device measurements repeatedly confirm that the interface is indeed pinned. Here we suggest that a parallel conduction mechanism enabled by metallic defects in the MoS2 materials may explain both results. We note that processing conditions during metal depositions on MoS2 can play a critical role in the interface chemistry, with differences between high vacuum and ultra-high vacuum being particularly important for low work function metals. This can be used to engineer the interfaces by using thin metal-oxide interlayers to protect the MoS2 from reactions with the metals. We also report on the changes in the interfaces that can occur at high temperature which include enhanced reactions between Ti or Cr and MoS2, diffusion of Ag into MoS2, and delamination of Fe. What is clear is that there is a dearth of experimental work that investigates both the interface chemistry and device properties in parallel.
Highlights
The last 15 years have seen a renewed interest in van der Waals solids with a new focus on their potential in nanoelectronic applications
Given the absence of dangling bonds on the surface of transition metal dichalcogenide (TMDC), they were believed to be chemically inert exhibiting minimal interactions with a metal overlayer [28]. This is in contrast with conventional semiconductors, like Si or group III-V materials such GaAs, which have surface dangling bonds that result in the formation of defect-induced or metal-induced gap states that pin the Fermi level [65,66] Gong et al [62] suggest that in metal-MoS2 contacts, dipoles formed at the interface modify the metal work function, and that the S-Mo bonding is weakened by the adsorbed metal leading to the formation of states in the band gap of MoS2
A substantial volume of recent work in the literature is focused on the synthesis and characterization of 2D materials and on the fabrication and characterization of devices using 2D materials
Summary
The last 15 years have seen a renewed interest in van der Waals solids with a new focus on their potential in nanoelectronic applications These materials have a long history of use as dry lubricants [1]. Since the isolation of monolayer graphene and the demonstration of its electronic properties [8,9,10], the interest in 2D materials beyond graphene has been increasing. These 2D materials beyond graphene include hexagonal boron nitride (hBN), transition metal dichalcogenides, Silicene/Germanene/Stanene, as well as group III and group IV metal chalcogenides such as GaSe or SnS2 [11,12,13]. - Heating above 526 ◦ C restored binding energies to pre-anneal positions likely due to the diffusion of Ag into MoSx or the formation of AgMoSx
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