Electron Transport through Metal/MoS₂ Interfaces: Edge- or Area-Dependent Process?

Abstract

In ultra-thin 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 MoS 2 , both short (≤30 nm) and long (≥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 MoS 2 monolayer, for example TiO 2 in case of titanium electrodes, favors an area-dependent 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/MoS 2 interfaces and provide a framework to design future devices with lower contact resistances. 1 Keywords: 2-D materials, metal-semiconductor interfaces, contact physics, transfer length, Fermi level pinning, ab initio device simulations Transistors made of novel two-dimensional (2-D) materials beyond graphene such as single-layer MoS 2 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 ultra-scaled 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. 11-13Before MoS 2 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.

Impact of Device Scaling on the Electrical Properties of MoS2 Field-effect Transistors

“…3a shows that Ion (@ ns = 10 13 cm -2 ) does not degrade as Lcont is scaled down to 13nm. This agrees with TCAD simulations 10,17,18 that predict contact edge injection of carriers for 1-3 layers of MoS2 channel. This observation holds true for three different Lch (30 nm, 100 nm, nm) over a wide range of Lcont (500nm to 13nm) and for varying lateral field (VDS = 0.05 V, V).…”

“…Therefore, injection is only allowed from the edge of the metal contact directly into the carrier-rich channel, which is also predicted in other work 18,19 . For thicker MoS2 (more than 5 ML), the MoS2 region underneath the contact is no longer depleted and a longer section of the contact contributes to carrier injection 20,21,22 .…”

One-Dimensional Edge Contacts to a Monolayer Semiconductor

“…It was constructed according to the upscaling technique described in Ref. 10 for the Ti-MoS 2 edge contact structure depicted schematically in Fig. S10a.…”

Layered Semiconducting 2D Materials for Future Transistor Applications

“…However, the transfer length of top contact, which decides the "effective" contact area, is still controversial. [65][66][67] Fundamental understanding of carrier injection from the outof-plane top contact to the in-plane 2D material is essential. Edge contact is immune from contact length scaling, but a lower specific contact resistivity (sub 10 À9 Ω cm 2 ) than that of Si technology to reach comparable contact resistance is needed.…”