Ultrafast van der Waals diode using graphene quantum capacitance and Fermi-level depinning

Graphene, with superior electrical tunabilities, has arisen as a multifunctional insertion layer in vertically stacked devices. Although the role of graphene inserted in metal-semiconductor junctions has been well investigated in quasi-static charge transport regime, the implication of graphene insertion at ultrahigh frequencies has rarely been considered. Here, we demonstrate the diode operation of vertical Pt/n-MoSe2/graphene/Au assemblies at similar to 200-GHz cutoff frequency (f(C)). The electric charge modulation by the inserted graphene becomes essentially frozen above a few GHz frequencies due to graphene quantum capacitance-induced delay, so that the Ohmic graphene/MoSe2 junction may be transformed to a pinning-free Schottky junction. Our diodes exhibit much lower total capacitance than devices without graphene insertion, deriving an order of magnitude higher f(C), which clearly demonstrates the merit of graphene at high frequencies.

Automated summary

Graphene has been widely used as an insertion layer in vertically stacked devices due to its excellent electrical tunabilities. However, its implications at ultrahigh frequencies have been rarely explored. In this study, diode operation of vertical Pt/n-MoSe2/graphene/Au assemblies at a cutoff frequency of about 200 GHz was demonstrated. The electric charge modulation by the inserted graphene becomes essentially frozen above a few GHz frequencies, transforming the Ohmic graphene/MoSe2 junction into a pinning-free Schottky junction. The diodes with graphene insertion exhibit much lower total capacitance and an order of magnitude higher cutoff frequency compared to devices without graphene, highlighting the significance of graphene at high frequencies.