Strain Induced by Evaporated-Metal Contacts on Monolayer MoS2 Transistors

Electron-beam evaporation is commonly used to form metal contacts on two-dimensional (2D) materials. Many evaporated metals contain high levels of stress, but the effect of this stress on 2D device performance has yet to be determined. Here, we investigate the impact of tensile-stressed nickel evaporated onto gold contacts of monolayer MoS2 transistors. Optical measurements reveal a distribution of tensile strain along the MoS2 channel between stressed contacts, up to ~0.8% near the contact edges. Further, we show that stressed contacts can substantially influence device performance, leading to negative threshold voltage shifts and increased transconductance. In the limit of short (50 nm) channels with large (<inline-formula> <tex-math notation="LaTeX">$2~\mu $ </tex-math></inline-formula> m) contact stressors, we find that this can cause an on-state current increase up to 2.5x. These results show that contact-induced strain must be closely examined in emerging technologies, and this approach could be used to improve future device performance.