Non-Thermionic Switching in an Atomically Thin WSe₂ Transistor with the Phase-Change Material VO₂ Contact

By using the metal-insulator transition material VO₂ (T_C ~ 340 K) as a contact electrode, we demonstrate an atomically thin WSe₂ transistor whose subthreshold swing (SS) is limited not by the Boltzmann limit but by the abruptness of the phase transition of VO₂. Polycrystalline VO₂ films wtith thicknesses of 50 nm were grown on Al₂O₃ by the pulsed layer deposition method and etched to 1 μm in width. Few-layer WSe₂ was transferred onto the VO₂ wire so that the VO₂ served as one of the contact electrodes. After defining another electrode with Ti/Au, a gate dielectric of hBN was transferred. The transistor is on the off-state, with the VO₂ electrode being in the insulating phase at room temperature. However, the drain current shows abrupt increase at a given gate voltage, suggesting that the pahse transition in VO₂ is induced by Joule heating. We observe a relatively small value of 150 mV/dec. for the SS, but this is still larger than the thermal limit of 60 mV/dec. We expect that the SS could be further improved by employing single crystalline VO₂ that shows an abrupt transition.