rf Quantum Capacitance of the Topological Insulator Bi2Se3 in the Bulk Depleted Regime for Field-Effect Transistors

A metal-dielectric topological-insulator capacitor device based on hexagonal-boron-nitrate-(h-BN) encapsulated CVD-grown Bi 2 Se 3 is realized and investigated in the radio-frequency regime. The rf quantum capacitance and device resistance are extracted for frequencies as high as 10 GHz and studied as a function of the applied gate voltage. The superior quality h-BN gate dielectric combined with the optimized transport characteristics of CVD-grown Bi 2 Se 3 (n ∼ 10 18 cm −3 in 8 nm) on h-BN allow us to attain a bulk depleted regime by dielectric gating. A quantum-capacitance minimum and a linear variation of the capacitance with the chemical potential are observed revealing a Dirac regime. The topological surface state in proximity to the gate is seen to reach charge neutrality, but the bottom surface state remains charged and capacitively coupled to the top via the insulating bulk. Our work paves the way toward implementation of topological materials in rf devices.