We demonstrate the growth of twin-free Bi 2 Te 3 and Sb 2 Te 3 topological insulators by molecular beam epitaxy, and a sizable reduction of the twin density in Bi 2 Se 3 , on latticematched BaF 2 (111) substrates. Using x-ray diffraction, electron diffraction and atomic force microscopy, we systematically investigate the parameters influencing the formation of twin domains and the morphology of the films, and show that Se-and Tebased alloys differ by their growth mechanism. Optimum growth parameters are shown to result in intrinsically low-doped films, as probed by angle-resolved photoelectron spectroscopy. In contrast to previous approaches, in which twin-free Bi 2 Se 3 films are achieved by increasing the substrate roughness, the quality of our Bi 2 Te 3 is superior on the flattest BaF 2 substrates. This finding indicates that, during nucleation, the films not only interact with the topmost atomic substrate layer but also with buried layers that provide the necessary stacking information to promote a single twin, an observation that is supported by ab-initio calculations. Bismuth and antimony chalcogenides, Bi 2 Te 3 , Sb 2 Te 3 and Bi 2 Se 3 , are prototypical topological insulators (TIs), a new class of bulk insulators with conductive surface states. [1-3] Their surface hosts Dirac fermions protected by time-reversal symmetry and characterized by spinmomentum locking, a property that prevents backscattering and that is highly attractive for spintronics and fault-tolerant quantum computation. [3,4] However, bismuth and antimony chalcogenides are narrow gap semiconductors that are very sensitive to doping, and the surface currents are typically overwhelmed by bulk currents. In order to favour surface transport, it is necessary to suppress the bulk conductivity, which requires the growth of lowdefect thin films, a task that has proven to be very challenging. [5-9] Due to doping by impurities or crystalline defects, the Fermi level of Bi 2 Se 3 and Bi 2 Te 3 is typically shifted to the conduction band. [5,8,10-14] Although counter-doping or electrostatic gating may be used to tune the Fermi energy, structural defects impact negatively the transport properties by coupling surface and bulk channels, by reducing the carrier mobility or by suppressing the surface states in the case of strain localized at grain boundaries. [15] Some common imperfections are Se/Te vacancies, dislocations, mosaïcity, 30° rotational domains and twin domains. [5,7,15-18] In particular, mirror-symmetric twin domains are of special importance owing to the extended planar nature of their boundaries (see Figure 1a). They are known to introduce electron scattering, strain and doping in various semiconductors such as GaAs [19] or HgCdTe. [20] In TIs, it has been demonstrated that twin boundaries hold a spontaneous polarization responsible for self-doping of the surface states, which may reach several hundreds of meV. [21] TIs are usually grown on conventional substrates such as Al 2 O 3 (0001) [7,10,13,22] and Si(111), [8,13,14,23-26] despite l...
