Surface State Transport and Ambipolar Electric Field Effect in Bi2Se3 Nanodevices

Abstract: Electronic transport experiments involving the topologically protected states found at the surface of Bi2Se3 and other topological insulators require fine control over carrier density, which is challenging with existing bulk-doped material. Here we report on electronic transport measurements on thin (<100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel surface and bulk contributions. Moreover, the conductance dependence on top-gate voltage is ambipolar, consistent with tuning between electrons and hole carriers at the surface.

• The paper reveals that ambipolar electric field effect effectively modulates carrier density in Bi₂Se₃ nanodevices, enabling a switch between electron and hole conduction.
• It uses top-gate and back-gate configurations to differentiate surface and bulk contributions, reporting mobilities of ~1000 cm²/Vs for surface and ~1700 cm²/Vs for bulk carriers.
• The study’s two-carrier model clarifies the parallel conduction channels, providing insights for designing high-performance topological quantum and spintronic devices.

An Analysis of Surface State Transport and Ambipolar Electric Field Effect in Bi$_2$Se$_3$ Nanodevices

This paper presents a comprehensive study on the electronic transport phenomena in topological insulator Bi$_2$Se$_3$ nanodevices, specifically elucidating the contributions of surface and bulk states and their modulation through electric field effects. Notably, the research highlights the ambipolar electric field effect as a means to control the carrier density in these systems, which resolves a fundamental challenge in utilizing topological insulators for practical applications.

Key Findings and Measurements

The authors employed exfoliated Bi$_2$Se$_3$ nanodevices, measuring electronic transport to delineate the contributions of surface and bulk states to conductance. Top-gate and back-gate electrodes enabled tuning of these contributions, revealing the ambipolar nature of conductance at the top surface. This ambipolar behavior, a switch between electron and hole conduction states, is crucial as it is consistent with the gapless band structure characteristic of topological insulator surface states.

The study measured temperature-dependent conductance to ascertain the mobilities and densities associated with bulk and surface states. The findings established mobilities of about 1700 cm$^2$/Vs for bulk and 1000 cm$^2$/Vs for surface carriers. Moreover, the surface state contribution was predominantly measurable on the bottom surface, which interacts weakly with the SiO$_2$ substrate, contrasting with the top surface interfacing strongly with high-k dielectrics.

Surface and Bulk Conductance Interplay

The authors adopted a two-carrier model to analyze Hall coefficient $R_H$ and its dependence on device thickness. A fitted model permitted the estimation of surface and bulk densities and mobilities across nanodevices, although with inherent limitations due to the narrow range of thickness examined.

The paper reports that the conductance exhibits parallel channels from surface and bulk states, elucidating that surface conduction significantly contributes to overall conductance and can be modulated by electric fields. This modulation was essential in identifying the ambipolar behavior signifying effective electronic tunability in these ultra-thin topological systems.

Implications and Future Directions

The insights gained from this work suggest potential applications in topological quantum devices where precise carrier density control is imperative. The demonstrated ability to modulate surface state density via gating signifies a significant step toward integrating topological insulators in electronic and spintronic devices. Practically, these findings suggest a route for designing ultra-thin high-performance devices with surface-dominant electronic characteristics, unlocking avenues for further exploration into topological quantum computing and advanced material science applications.

In future research, deeper investigations should include a broader thickness range to refine mobility and density estimations and develop more complex theoretical models to account for simultaneous charging effects on surface and bulk states. Furthermore, exploration into alternate materials and synthetization techniques may yield insights into optimizing interfacial effects and carrier modulation, promising further innovation in this domain.