Contact-induced Strain for Enhanced Performance in Monolayer WS₂ Transistors

Beyond-silicon electronics require good contact resistance to approach their fundamental limits of operation. This is particularly the case for today’s 2D semiconductors – for example, Ni contacts are often used with WS₂, but their operation is not well understood because the work function alignment of Ni and n-type WS₂ is suboptimal. Here, we investigate the effects of contact size on nanoscale Ni-WS₂ devices and conclude that the stress imparted by the Ni contacts substantially affects the contact resistance (R_c) to WS₂ devices. We uncover that electron-beam deposited Ni applies tensile stresses to the WS₂ channel and contact region, leading to 64% lower R_c in long (1 µm) contacts compared to short (100 nm) contacts at the same gate voltage overdrive. Because annealing is integral to device fabrication and can substantially vary the strain profile in thin films, we further investigate the effect of annealing on the Ni-WS₂ devices through charge transport measurements, Raman and photoluminescence spectroscopy, and in-plane x-ray diffraction. We demonstrate that annealing can cause strain relaxation in the WS₂ channel, increasing R_c from 1.7 kΩ⋅μm to 8.5 kΩ⋅μm and lowering the strain-enhanced mobility in the channel of long contact devices. This work provides a deeper fundamental understanding of the thermal and mechanical phenomena impacting the metal-2D semiconductor interface in high-performance transistor applications.