The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states1, 2. Although the STI surface state has been studied spectroscopically, for example, by photoemission3, 4, 5 and scanned probes6, 7, 8, 9, 10, transport experiments11, 12, 13, 14, 15, 16, 17 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 cm−3) 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 a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region18, 19, 20. A theory of charge disorder in a Dirac band19, 20, 21 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 (0.4×1012 to 4×1012 cm−2). From the measured carrier mobilities 320–1,500 cm2 V−1 s−1, the charged impurity densities 0.5×1013 to 2.3×1013 cm−2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1×1013 to 3×1013 cm−2), identifying dopants as the charged impurities.
At a glance
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