  Atomically thin two-dimensional (2D) materials are promising candidates for
sub-10 nm transistor channels due to their ultrathin body thickness, which
results in strong electrostatic gate control. Properly scaling a transistor
technology requires reducing both the channel length (distance from source to
drain) and the contact length (distance that source and drain interface with
semiconducting channel). Contact length scaling remains an unresolved epidemic
for transistor scaling, affecting devices from all semiconductors, from silicon
to 2D materials. Here, we show that clean edge contacts to 2D MoS2 provide
immunity to the contact-scaling problem, with performance that is independent
of contact length down to the 20 nm regime. Using a directional ion beam, in
situ edge contacts of various metal-MoS2 interfaces are studied.
Characterization of the intricate edge interface using cross-sectional electron
microscopy reveals distinct morphological effects on the MoS2 depending on its
thickness, from monolayer to few-layer films. Chromium is found to outperform
other metals in the edge contact scheme, which is attributed to the shorter
Cr-MoS2 bond length. Compared to scaled top contacts with 20 nm contact length,
in situ edge contacts yield better performance with an effective contact length
of ~ 1 nm and 18 times higher carrier injection efficiency. The in situ edge
contacts also exhibit ~8 times higher performance compared to the best-reported
edge contacts. Our work provides experimental evidence for a solution to
contact scaling in transistors, using 2D materials with clean edge contact
interfaces, opening a new way of designing devices with 2D materials.
