Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3

TL;DR

This study demonstrates surface conduction of Dirac electrons in thin, low-doped Bi2Se3 topological insulator crystals, showing ambipolar transport and charge disorder effects, confirming the topological surface state in transport experiments.

Contribution

It provides the first clear transport evidence of topological surface state conduction in bulk-insulating Bi2Se3, with detailed analysis of charge disorder and impurity effects.

Findings

Surface band conduction observed with linear Hall resistivity

Ambipolar field effect demonstrated in thin Bi2Se3

Charge disorder theory explains minimum conductivity and residual carriers

Abstract

The newly-discovered three-dimensional strong topological insulators (STIs) exhibit topologically-protected Dirac surface states. While the STI surface state has been studied spectroscopically by e.g. photoemission and scanned probes, transport experiments have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (<10 nm), low-doped Bi2Se3 (\approx10^17/cm3) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with linear Hall resistivity and well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region. A theory of charge disorder in a Dirac band explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e^2/h per surface) and the residual (puddle) carrier density (0.4 x 10^12 cm^-2 to 4 x 10^12 cm^-2). From the measured carrier mobilities 320 cm^2/Vs to 1,500 cm^2/Vs, the charged impurity densities 0.5 x 10^13 cm^-2 to 2.3 x 10^13 cm^-2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1 x 10^13 cm^-2 to 3 x 10^13 cm^-2), identifying dopants as the charged impurities.