High-Performance n-type and p-type WSe$_{2}$ Field Effect Transistors with Ionic-Liquid Gated Graphene Electrodes

We report the application of graphene as a work-function-tunable electrode material for few-layer WSe$_{2}$ field-effect transistors (FETs). By tuning the carrier density of graphene at the graphene/WSe$_{2}$ contacts using an extremely-large-capacitance ionic liquid gate, we have successfully achieved low resistance Ohmic contacts and high ON-current for both holes and electrons in WSe$_{2}$ FETs. The extrinsic electron and hole mobility values increase with decreasing temperature reaching $\approx $ 300 cm$^{2}$V$^{-1}$s$^{-1}$ at 77 K when the graphene contacts are highly n- and p-doped by large positive and negative ionic-liquid gate voltages, respectively, indicating that the intrinsic phonon-limited mobility is approached for both electrons and holes in graphene contacted few-layer WSe$_{2}$. We attribute the enhanced device performance to the drastic reduction of the Schottky barrier height via tuning the work function of graphene electrodes to align with the conduction and valence band edges of WSe$_{2}$ by an ionic liquid gate. This work was supported by NSF (DMR-1308436).