Role of Metal Contacts in Designing High-Performance Monolayer n‑Type WSe₂ Field Effect Transistors

Abstract

This work presents a systematic study toward the design and first demonstration of high-performance n-type monolayer tungsten diselenide (WSe₂) field effect transistors (FET) by selecting the contact metal based on understanding the physics of contact between metal and monolayer WSe₂. Device measurements supported by ab initio density functional theory (DFT) calculations indicate that the d-orbitals of the contact metal play a key role in forming low resistance ohmic contacts with monolayer WSe₂. On the basis of this understanding, indium (In) leads to small ohmic contact resistance with WSe₂ and consequently, back-gated In–WSe₂ FETs attained a record ON-current of 210 μA/μm, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm²/V·s (with an ON/OFF current ratio exceeding 10⁶) is also achieved with In–WSe₂ FETs at room temperature. This is the highest electron mobility reported for any back gated monolayer TMD material till date. The performance of n-type monolayer WSe₂ FET was further improved by Al₂O₃ deposition on top of WSe₂ to suppress the Coulomb scattering. Under the high-κ dielectric environment, electron mobility of Ag–WSe₂ FET reached ∼202 cm²/V·s with an ON/OFF ratio of over 10⁶ and a high ON-current of 205 μA/μm. In tandem with a recent report of p-type monolayer WSe₂ FET (Fang, H. et al. Nano Lett. 2012, 12, (7), 3788−3792), this demonstration of a high-performance n-type monolayer WSe₂ FET corroborates the superb potential of WSe₂ for complementary digital logic applications