2, Osaka Prefecture University, Sakai, Osaka, , Japan
3, National Institute for Materials Science, Tsukuba, Ibaraki, , Japan
4, Saitama University, Saitama, , Japan
Two-dimensional (2D) semiconductors such as transition metal dichalcogenides have great potential as post-silicon channel materials that could suppress short channel effects. However, 2D semiconductors still suffer from the thermal limit for the subthreshold swing, as long as conventional metal-insulator-semiconductor (MIS) structures are employed. To overcome the fundamental limit, we introduce the metal-insulator transition material VO2 as a contact electrode in an atomically thin WSe2-based MIS transistor. Polycrystalline VO2 films with thicknesses of ~ 50 nm were grown on Al2O3 by the pulsed layer deposition method and observed to show the metal-insulator transition near 340 K. The VO2 films were etched down to ~ 1 μm in width. Then, few-layer WSe2 was transferred directly onto the VO2 wire so that the VO2 served as one of the contact electrodes. After defining another electrode by depositing Ti/Au on WSe2, a gate dielectric of hexagonal boron nitride was transferred, followed by the definition of the gate electrode. The WSe2 transistor is on the off-state, with the VO2 electrode being in the insulating phase at room temperature. However, the drain current shows abrupt and discontinuous increase at a given gate voltage, suggesting that the insulator-to-metal transition in VO2 is induced thermally by Joule heating. Since the on-off switching is governed by the phase-transition in the VO2 electrode, the subthreshold slope is, in principle, expected not to be limited by the thermal voltage. We observe a relatively small value of 150 mV/decade for the subthreshold slope in this transistor, but this is still larger than the thermal limit of 60 mV/decade. We envision that the subthreshold slope could be further improved by employing single crystalline VO2 as well as a high-k gate dielectric such as HfO2. This work is an important first step in demonstrating steep-slope transistors based on 2D semiconductors.