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Edge Contact for Carrier Injection and Transport in MoS2 Field-Effect Transistors.

The contact properties of van der Waals (vdW) layered semiconducting materials are not adequately understood, particularly for edge contact. Edge contact is extremely helpful in the case of graphene, for producing efficient contacts to vertical heterostructures as well as for improving the contact resistance through strong covalent bonding. Herein, we report on edge contacts to MoS2 of various thicknesses. The carrier-type conversion is robustly controlled by changing the flake thickness and metal workfunctions. Regarding the ambipolar behaviour, we suggest that the carrier injection is segregated in relatively thick MoS2 channel, i.e., electrons are in the uppermost layers, and holes are in the inner layers. Calculations reveal that the strength of the Fermi-level pinning (FLP) varies layer-by-layer, owing to the inhomogeneous carrier concentration, and particularly, there is negligible FLP in the inner layer, supporting the hole injection. The contact resistance is large despite the significantly reduced contact resistivity normalized by the contact area, which is attributed to the current-crowding effect arising from the narrow contact area.