Characterization of a Highly Bulk-Resistive Topological Insulator and Carrier Control by External Gating*

On the surface of a topological insulator, kinds of novel phenomena are expected, owing to the exotic band structure with the Dirac fermion. An observation of transport properties on surfaces of a topological insulator is generally difficult because massive bulk carriers covers up the surface signals. In order to suppress the bulk transport, several new materials with high bulk resistivity have been suggested. Among them, Sn-doped BiSbTe₂S (Sn-BSTS) is a promising material with a particularly large activation energy, which even reaches 150 meV. This is more than twice that of formerly-known highly-resistive ones, e.g. BiSbTeSe₂. We synthesized single crystals of Sn-BSTS by the modified Bridgman method and characterized their electric transport properties. We thinned the crystals and made it in Hall devices. In the thinned crystal, the top and the bottom surfaces form transport channels, which we controlled individually by applying gate voltage. We used a thermal oxidation layer of SiO₂ on the Si substrate as the insulating layer for the back gate and used an ionic liquid for the top gate. Successful control of carrier types on the surfaces will be presented.