Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3

Journal name:
Nature Physics
Volume:
8,
Pages:
459–463
Year published:
DOI:
doi:10.1038/nphys2286
Received
Accepted
Published online

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 (~ 10nm), low-doped Bi2Se3 (1017cm−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 4×1012cm−2). From the measured carrier mobilities 320–1,500cm2V−1s−1, the charged impurity densities 0.5×1013 to 2.3×1013cm−2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1×1013 to 3×1013cm−2), identifying dopants as the charged impurities.

At a glance

Figures

  1. Bi2Se3 thin-film device.
    Figure 1: Bi2Se3 thin-film device.

    a,b, Schematics of the p-type doping scheme and gate configuration for charge transfer doping with F4TCNQ organic molecules (a) and polymer electrolyte (PEO+LiClO4) (b) top gating. c,d, Longitudinal resistivity ρxx (c) and  sheet carrier density (d) determined from Hall measurement as a function of back-gate voltage for device 4 (F4TCNQ-doped) at various temperatures from 2 to 50K, as indicated in the figure. The inset of d shows an optical micrograph of the device. The scale bar is 2μm.

  2. Single band conduction in the topological insulator regime.
    Figure 2: Single band conduction in the topological insulator regime.

    a, Hall resistivity ρxy of device 4 as a function of magnetic field B at a temperature of 2K at different carrier densities tuned by the back-gate electrode. b, Polar plot of the normalized longitudinal resistivity ρxx of the dual-gated Bi2Se3 thin-film device as a function of the total magnitude of displacement field (Dtotal)and the gating asymmetry factor α (defined in the text).

  3. Transport properties of the Bi2Se3 surface state.
    Figure 3: Transport properties of the Bi2Se3 surface state.

    a, The conductivity per surface versus carrier density per surface σ(n) at zero magnetic field for five different devices. Devices 1–3 are electrolyte-gated and devices 4 and 5 are F4TCNQ-doped. The inset shows σ(n) near the Dirac point. Dashed lines are fits to equation (1a). Transport data outside the topological regime (n>nbulk=5×1012cm−2) are denoted as dotted curves. b, Hall carrier density per surface versus carrier density measured at the same conditions as in a. Dashed lines show the residual carrier density n* (defined in the text) for different devices. c, Variation of field effect mobility as a function of carrier density. Dashed curves indicate the region |n|<n* within which electron and hole puddles dominate transport.

  4. Charge inhomogeneity and minimum conductivity versus disorder strength.
    Figure 4: Charge inhomogeneity and minimum conductivity versus disorder strength.

    a, Residual carrier density n*versus inverse field effect mobility (1/μFE). b, Minimum conductivity σmin versus 1/μFE. Shaded areas indicate the expectations of the self-consistent theory of ref. 20, open symbols are experimental data.

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Author information

  1. These authors contributed equally to this work

    • Dohun Kim &
    • Sungjae Cho

Affiliations

  1. Center for Nanophysics and Advanced Materials, Department of Physics, University of Maryland, College Park, Maryland 20742-4111, USA

    • Dohun Kim,
    • Sungjae Cho,
    • Nicholas P. Butch,
    • Paul Syers,
    • Kevin Kirshenbaum,
    • Johnpierre Paglione &
    • Michael S. Fuhrer
  2. Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-6202, USA

    • Shaffique Adam
  3. Present address: Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801-2902, USA

    • Sungjae Cho
  4. Present address: Condensed Matter and Materials Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA

    • Nicholas P. Butch

Contributions

D.K. conceived the p-type doping schemes. D.K. and S.C. fabricated devices, performed the electrical measurements with K.K. and analysed the data. N.P.B., P.S. and J.P. prepared single crystal Bi2Se3 starting material. S.A. assisted with the theoretical analysis. D.K., S.C. and M.S.F. wrote the manuscript.

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The authors declare no competing financial interests.

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