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 (≈1017/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 e2/h per surface) and the residual (puddle) carrier density (0.4 x 1012 cm-2 to 4 x 1012 cm-2). From the measured carrier mobilities 320 cm2/Vs to 1500 cm2/Vs, the charged impurity densities 0.5 cm-2 to 2.3 x 1013 cm-2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1 x 1013 cm-2 to 3 x 1013 cm-2), identifying dopants as the charged impurities.
Citation: Nature Physics
Pub Type: Journals
topological insulator, Hall effect, dopants, Dirac band structure, surface states