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

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 mu A/mu m, which is the highest value achieved in any monolayer transition-metal dichalcogenide- (TMD) based FET to date. An electron mobility of 142 cm(2)/V.s (with an ON/OFF current ratio exceeding 10(6)) 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-kappa dielectric environment, electron mobility of Ag-WSe2 FET reached similar to 202 cm(2)/V-s with an ON/OFF ratio of over 10(6) and a high ON-current of 205 mu A/mu m. In tandem with a recent report of p-type monolayer WSe2FET (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.