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

Abstract

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 (∼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.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

“…More recently, Bussolotti et al have indicated that the Fermi level can also be pinned at the deep sulfur-vacancy gap state in defective MoS 2 controlled by argon sputtering, measured by using X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy. [40] Figure 4d shows that the location of the FLP is tuned toward the valence band edge with an increased concertation of deep sulfur vacancies, therefore limiting electron injection to MoS 2 with a considerable barrier. Such limitations are mostly absent for hole injection as the shallow sulfur-vacancy gap states pin Fermi level close to the valence band minimum (VBM).…”

“…Reproduced with permission. [ 40 ] Copyright 2021, American Chemical Society. SV and DV in (c) stand for single line vacancy and double line vacancy, respectively.…”

“…Generation of defects by c) electron irradiation and d) Ar + plasma treatment. Reproduced with permission [40]. Copyright 2021, American Chemical Society.…”

Unexpected Electron Transport Suppression in a Heterostructured Graphene–MoS2Multiple Field-Effect Transistor Architecture

“…Using DFT, we made an ab initio analysis of the electronic states of graphene–MoS 2 in the presence of sulfur vacancies: These have an energy that falls in the gap of MoS 2 and are located at a distance of few Angstroms from graphene, so their effect is hard to evaluate without a first-principles approach (see the “Methods” section and the Supporting Information for further details). Numerical calculations were performed using a density of S-vacancies of ρ v ≈ 1.8 × 10 13 cm –2 .…”

In situ establishment of Co/MoS2 heterostructures onto inverse opal‐structured N,S‐doped carbon hollow nanospheres: Interfacial and architectural dual engineering for efficient hydrogen evolution reaction

“…To further elucidate the enhanced catalytic efficacies of the Co/MoS 2 @N,S-CHNSs, DFT calculations are performed to investigate the HER process at the atomic level. [54][55][56] The computational model of Co/MoS 2 heterojunction is built by coupling the Co(111) facet with the MoS 2 (002) facet (Figure 6A). The optimized geometry structure of Co/MoS 2 indicates that metallic Co strongly connects with MoS 2 through the S-edge atoms at the interface.…”