Understanding the Impact of Contact-Induced Strain on the Electrical Performance of Monolayer WS₂ Transistors

Two-dimensional (2D) electronics require low contact resistance (R_C) to approach their fundamental limits. WS₂ is a promising 2D semiconductor that is often paired with Ni contacts, but their operation is not well understood considering the nonideal alignment between the Ni work function and the WS₂ conduction band. Here, we investigate the effects of contact size on nanoscale monolayer WS₂ transistors and uncover that Ni contacts impart stress, which affects the WS₂ device performance. The strain applied to the WS₂ depends on contact size, where long (1 μm) contacts (R_C ≈ 1.7 kΩ·μm) show a 78% reduction in R_C compared to shorter (0.1 μm) contacts (R_C ≈ 7.8 kΩ·μm). We also find that thermal annealing can relax the WS₂ strain in long-contact devices, increasing R_C to 8.5 kΩ·μm. These results reveal that thermo-mechanical phenomena can significantly influence 2D semiconductor-metal contacts, presenting opportunities to optimize device performance through nanofabrication and thermal budget.