Ambipolar field effect in the ternary topological insulator (Bi_<i>x</i>Sb_1-<i>x</i>)₂Te₃ by composition tuning

Topological insulators exhibit a bulk energy gap and spin-polarized surface states that lead to unique electronic properties1–9, with potential applications in spintronics and quantum information processing. However, transport measurements have typically been dominated by residual bulk charge carriers originating from crystal defects or environmental doping10–12, and these mask the contribution of surface carriers to charge transport in these materials. Controlling bulk carriers in current topological insulator materials, such as the binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3, has been explored extensively by means of material doping8,9,11 and electrical gating13–16, but limited progress has been made to achieve nanostructures with low bulk conductivity for electronic device applications. Here we demonstrate that the ternary sesquichalcogenide (BixSb1–x)2Te3 is a tunable topological insulator system. By tuning the ratio of bismuth to antimony, we are able to reduce the bulk carrier density by over two orders of magnitude, while maintaining the topological insulator properties. As a result, we observe a clear ambipolar gating effect in (BixSb1–x)2Te3 nanoplate field-effect transistor devices, similar to that observed in graphene field-effect transistor devices17. The manipulation of carrier type and density in topological insulator nanostructures demonstrated here paves the way for the implementation of topological insulators in nanoelectronics and spintronics. The binary sesquichalcogenides Bi2Te3, Sb2Te3 and Bi2Se3 have recently been identified as three-dimensional topological insulators with robust surface states consisting of a single Dirac cone in the band spectra6–8. In these materials, topological surface states have been experimentally confirmed using surface-sensitive probes such as angle-resolved photoemission spectroscopy (ARPES)7,8,18 and scanning tunnelling microscopy/spectroscopy (STM/STS)19,20. Several recent magnetotransport experiments have also revealed charge carriers originating from surface states11,21–23. However, despite substantial efforts in relation to material doping8,9,11 and electric gating13–16, manipulating and suppressing the bulk carriers of these materials, especially in nanostructures, is still challenging because of the impurities formed during synthesis as well as extrinsic doping from exposure to the ambient environment10–12. Here, we propose ternary sesquichalcogenide (BixSb1–x)2Te3 as a tunable three-dimensional topological insulator system in which we are able to engineer the bulk properties by means of the bismuth/antimony composition ratio. (BixSb1–x)2Te3 is a non-stoichiometric alloy that has a similar tetradymite structure to its parent compounds Bi2Te3 and Sb2Te3 (Fig. 1a). To verify the topological nature of (BixSb1–x)2Te3, we performed ARPESmeasurements on the (0001) plane of (BixSb1–x)2Te3 bulk single crystals with a variety of compositions, yielding experimental Fermi surface topology maps and band dispersions (Fig. 1b). Together with the broad electronic spectra originating from the bulk states, the single Dirac cone that forms the topological surface state band (SSB) is revealed around the G point for all ternary compositions, indicated by the hexagram Fermi surfaces (top row) and the sharp linear dispersion in the band spectra (bottom row). The parent compounds, as-grown Bi2Te3 (n type) and Sb2Te3 (p type), are highly metallic, with the Fermi energy EF located deep inside the bulk conduction band (BCB) and bulk valence band (BVB), respectively, due to excessive carriers arising from crystal defects and vacancies8. With increasing antimony concentration, EF systematically shifts downward (Fig. 1b, bottom row). In particular, at a bismuth/antimony ratio of 1:1, that is, (Bi0.50Sb0.50)2Te3, the bulk states completely disappear at EF with a vanished bulk pocket in the Fermi surface map and band dispersion. The surface Dirac cone of (BixSb1–x)2Te3 (Fig. 1c) noticeably exhibits clear hexagonal warping, similar to that of pure Bi2Te3 (refs 8,19,24). The experimentally observed ARPES measurements are qualitatively reproduced by ab initio band structure calculations (Fig. 1d) in which the linear SSB dispersion around the G point in all compositions confirms their topological non-triviality. This is not surprising, because the spin–orbit coupling strength (critical for the formation of topological insualtors) and the bulk energy gap in ternary (BixSb1–x)2Te3, with varying bismuth/antimony ratios, are comparable to those of the parent compounds Bi2Te3 and Sb2Te3. Consequently, a quantum phase transition to an ordinary insulator does not occur when varying the bismuth/antimony ratios, which is complementary to a recent study on the topological trivial/non-trivial phase transition in the BiTl(S1–dSed)2 system 25. This non-triviality across the entire compositional range in ternary (BixSb1–x)2Te3 compounds demonstrates a rich material assemblage of topological insulators based on an alloy approach, which is an attractive avenue to search for material candidates with improved properties. The compositional engineering of the bulk properties of topological insulators can also be applied to nanostructures. Single-crystalline (BixSb1–x)2Te3 nanoplates were synthesized using a catalyst-free vapour–solid growth method using a mixture of Bi2Te3 and Sb2Te3 powders as precursors. The growth method was established in our previous work26. Figure 2a shows a typical optical microscopy image of as-grown (BixSb1–x)2Te3 nanoplates on an oxidized silicon substrate (300 nm SiO2/silicon) with thicknesses of a few