# 摘要

In ultrathin two-dimensional (2-D) materials, the formation of ohmic contacts with top metallic layers is a challenging task that involves different processes than in bulk-like structures. Besides the Schottky barrier height, the transfer length of electrons between metals and 2-D monolayers is a highly relevant parameter. For MoS2, both short ($\le 30 nm$) and long ($\ge 0.5 μm$) values have been reported, corresponding to either an abrupt carrier injection at the contact edge or a more gradual transfer of electrons over a large contact area. Here we use ab initio quantum transport simulations to demonstrate that the presence of an oxide layer between a metallic contact and a MoS2 monolayer, for example, TiO2 in the case of titanium electrodes, favors an areadependent process with a long transfer length, while a perfectly clean metal−semiconductor interface would lead to an edge process. These findings reconcile several theories that have been postulated about the physics of metal/MoS2 interfaces and provide a framework to design future devices with lower contact resistances.

# 前言

Transistors made of novel two-dimensional (2-D) materials beyond graphene such as single-layer MoS2[1] have generated considerable excitement among the scientific community for their potential as active components of future integrated circuits. Transition metal dichalcogenides (TMDs),[2] black phosphorus,[3,4] and hundreds of other presumably exfoliable 2-D monolayers[5] appear as excellent candidates to outperform Si FinFETs, the current workhorse of the semiconductor industry, for next-generation ultrascaled logic switches.[6] The advantages of 2-D materials over competing technologies reside in their naturally passivated surfaces, their planar geometry providing an excellent electrostatic control,[7] their exceptionally high carrier mobilities as compared to 3-D compounds with the same subnanometer thickness,[8−10] and the possibility of stacking them on top of each other to form van der Waals heterostructures.[11−13]

Before MoS2 field-effect transistors (FETs) with a monolayer channel can reach their full potential and deliver the expected performance,[14] several key challenges remain to be solved. The source and drain contact resistances represent one of the main limiting factors as they usually lie in the $\rm{k\Omega \cdot \mu m}$ range,[15,16] instead of $150 \sim 200\ \rm{\Omega \cdot \mu m}$ as in conventional Si transistors. Lower values have been reported with metalized 1T MoS2[17] or nickel-etched graphene[18] electrodes, in the order of $200\ \rm{\Omega \cdot \mu m}$, but for multilayer MoS2. While top contacts are the most widely used variants due to their ease of fabrication, side contacts have started to emerge as a promising alternative,[19−22] motivated by theoretical studies that predict a stronger orbital overlap and shorter tunneling distances between metals and MoS2 in lateral configurations.[23,24] Apart from the electrode geometry, other well-known techniques have been applied to reduce the contact resistance of MoS2 FETs, among them the usage of different metals,[25,26] the introduction of an interfacial layer between the metal and semiconductor,[27−29] or the doping of MoS2.[30,31] Despite significant progresses made over the past few years, metal/MoS2 interfaces have not yet revealed all their secrets, hindering the development of future electronic components based on 2-D materials.

# 结论

In conclusion, we have used ab initio simulations to demonstrate that the injection of electrons from a top metallic contact into an underlying 2-D material can occur either at the edge or through the metal−semiconductor overlap area, depending on the presence or not of an interfacial layer. In this paper, Ti electrodes deposited on a MoS2 layer, with and without an intermediate TiO2 oxide, have served as an example to illustrate the physics at play. This finding can in principle be generalized to any blocking layer placed at the interface between a top contact and a 2-D monolayer, intentionally or not. Such a layer can hinder the penetration of the wave function originating from the metal into the band gap of the semiconductor, thus enabling an area-dependent transfer process. It can be envisioned that by engineering the properties of the interfacial layer the contact resistance of FETs based on 2-D semiconductors could be reduced, for example by selecting a material with a conduction band edge well-aligned with that of the 2-D crystal. Mobile electrons could then be directly injected into the transistor channel, without tunneling. At the same time, the charges pinning the Fermi level would still be stopped by the interfacial layer.