RF compressibility of topological surface and interface states in metal–hBN–Bi₂Se₃ capacitors

Abstract

The topological state that emerges at the surface of a topological insulator (TI) and at the TI-substrate interface are studied in metal-hBN-Bi 2 Se 3 capacitors. By measuring the RF admittance of the capacitors versus gate voltage, we extract the compressibility of the Dirac state located at a gated TI surface. We show that even in the presence of an ungated surface that hosts a trivial electron accumulation layer, the other gated surface always exhibits an ambipolar effect in the quantum capacitance. We succeed in determining the velocity of surface Dirac fermions in two devices, one with a passivated surface and the other with a free surface that hosts trivial states. Our results demonstrate the potential of RF quantum capacitance techniques to probe surface states of systems in the presence of a parasitic density-of-states.

Microwave surface transport in narrow-bandgap PdSe2 -MOSFETs

“…Here we use an in-plane variant of admittance spectrocopy for a long channel transistor geometry, taking into account the Schottky contact impedance. This variant is similar to previous capacitor modeling that has been introduced for gate capacitance spectroscopy to study graphene electronic compressibility [22] and used to characterize high-mobility hBN-encapsulated graphene devices (including plasma-resonant capacitors) [30], the topological Dirac states at the surface of Bi2Se3tetradymites [33,34], the interface states of topological heterojunctions between normal and inverted HgTe/CdHgTe semiconductors [31], as well as the edge states of strained HgTe quantum wells [32]. The complex gate-source admittance is measured over a broad frequency range allowing for a combined in-situ characterization of gate capacitance C G (V G ) and channel conductivity σ(V G ).…”

Ultra-long wavelength Dirac plasmons in graphene capacitors

“…The admittance spectrum is reminiscent of the evanescent wave response reported in Refs. [16][17][18]. Fingerprints of a resonant behavior become perceptible for Q ≃ 0.5 with a shallow minimum of ℑ(Y ) at f ≃ f 0 (Fig.…”