Efficient Dual-Gate Tuning of Fermi Level in Thin-Film Topological Insulator*

Experimental studies of novel quantum phenomena predicted for three-dimensional (3D) topological insulators (TIs) often require tuning of the Fermi level across the Dirac point. Both back gating and top gating techniques have been successfully applied to 3D TI thin films, however, with a single gate the chemical potential of only one surface can be controlled effectively, which is not sufficient for many applications. Recently, we have succeeded in developing a comprehensive method for fabricating dual-gated devices on TI thin films grown by molecular beam epitaxy. The method combines 1) the transfer of a high-quality bulk-insulating (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ thin film grown on sapphire onto a Si/SiO$_{2}$ wafer, which serves as a back gate, and 2) the fabrication of a top gate by using a low-temperature deposition of SiN$_{x}$. We demonstrate that the dual gating allows effective tuning of the chemical potentials of the top and bottom surfaces across the Dirac point, which is manifested in a large peak of the sheet resistance accompanied by a sharp sign change of R$_{yx}$ upon sweeping the top and bottom gate voltages. This dual-gating method opens exciting opportunities for realization of various novel phenomena expected for 3D TIs.