In a topological insulator (TI) the bulk electronic band structure behaves like an ordinary band insulator. However, at the surface topologically protected surface states occur, which give rise to a spin-locked, dissipation-less electronic transport. Hence, topological insulator materials, such as Sb₂Te₃ or Bi₂Te₃, are of great interest for spintronic devices or quantum computing. In order to achieve the dissipation-less transport, the Fermi level has to be exactly tuned within the Dirac cone like band structure at the surface.  To date intrinsic doping by vacancies or antisite defects renders Sb₂Te₃ and Bi₂Te₃ p- and n-type, respectively, which results in hole or electron transport in the bulk. Recently we solved this problem by growing p-n junctions made of Sb₂Te₃ and Bi₂Te₃ [1]. In this approach the carrier concentration at the surface is reduced by formation of the space charge layer at the buried heterointerface.

Structure-wise the X₂Te₃ (X=Bi, Sb) rhombohedral unit cell consists of three Te-X-Te-X-Te quintuple layers, which are linked by van der Waals forces.  In order to achieve layers of high structural perfection on Si(111) substrates  careful control of the growth parameters of the molecular beam epitaxy is required. In particular, we could demonstrate that the suppression of twin domains, which are the most prominent structural defects, is possible by van der Waals epitaxy [2].

Here we report on advanced scanning transmission electron microscopy and energy dispersive X-ray studies on MBE grown Sb₂Te₃/Bi₂Te₃ heterostructures. Figure 1 displays STEM bright-field and dark-field images, where the quintuple layers within the heterostructures and a highly perfect interface to the Si substrate are atomically resolved. The EDX measurement in Figure 2 reveals, that at the heterointerface a Sb and Bi gradient extends over about 4 nm, effectively introducing a ternary compound as interlayer. Corresponding electrical transport measurements demonstrate the tunability of the intrinsic carrier concentration by variation of the thickness of the individual films [3].

[1]   Eschbach M, Młyńczak E, Kellner J, Kampmeier, J, Lanius, M, Neumann, E., Weyrich C, Gehlmann M, Gospodaric P, Döring S, Demarina N, Luysberg M, Biehlmayer G, Schäpers, T,  Plucinski L, Blügel S, Morgenstern M, Schneider C M, Grützmacher D. Nature Communications. 2015;6(May):8816. doi:10.1038/ncomms9816.

[2]   Kampmeier J, Borisova S, Plucinski L, Luysberg M, Mussler G, Grützmacher D. Crystal Growth and Design. 2015;15(1):390-394. doi:10.1021/cg501471z.

[3]   Lanius, M,  Mussler, G.,  Kampmeier, J,  Weyrich, C.,  Schall, M.  Kölling, S.,  Schüffelgen, P., Neumann, E., Luysberg, M.,  Koenraad, P.  Schaepers, T.,  Grützmacher, D.  accepted for publication in Crystal Growth and Design. doi: 10.1021/acs.cgd.5b01717

Figures:

STEM bright-field (a) and dark-field (b) images of 15 nm Sb2Te3 on 6 nm Bi2Te3. The interface to the Si substrate showing bright contrast in (a) is perfectly crystalline. The Bi2Te3 layer is identified by bright atom positions in (b). Some of the Bi, Sb, and Te layers are marked with red, blue, and green arrows, respectively.

a) Line profile of the calculated mass percentage from the EDX signal across the heterostructure. The maxima of Bi and Sb EDX signal correspond to the quintuple layers in the darkfield (DF) image (b) Normalized line profile of the gradient regime.

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EMC Abstracts - https://emc-proceedings.com/abstract/topological-insulator-sb2te3bi2te3-heterostructures-structural-properties/