Tuning the Band Structure and Electronic Properties of (Bi1-xSbx)2Te3 Films

Topological insulators, such as BiSe-, BiTe- and SbTe-based materials, are an exciting class of quantum materials possessing a bulk band gap and gapless topological surface states. Bi2Te3 and Sb2Te3 are of particular interest for spintronics due to their native electronic properties. For example, Te anti-site defects lead to n-type conduction in Bi2Te3. Sb vacancies and Sb anti-site defects lead to p-type conduction in Sb2Te3. However, the Fermi level of Bi2Te3 (Sb2Te3) is pinned within the bulk conduction (valence) band, leading to native n-type (p-type) conduction. Furthermore, the Dirac point is pinned within the valence band for Bi2Te3. It is of current interest to bring the Dirac point to a position within the insulating gap, which effectively reduces bulk conduction while making surface electrons accessible for transport.<br/>In this work, a series of Bi2Te3-Sb2Te3 alloy films were grown by MBE on Al2O3 (0001) substrates. The film thicknesses, Bi:Sb flux ratios, and post-growth annealing in a Te flux were utilized to tune the Fermi level across the band gap. Scanning tunneling microscopy (STM) was performed to probe the local band structure of the films at room temperature. For all samples, the effective band gaps range from 0.15 to 0.22 ± 0.10 eV. Interestingly, as the film thickness is increased from 6 nm to 30 nm, the Fermi level shifts from the valence band edge (VBE) towards the conduction band edge (CBE), indicating a p-type to n-type transition. Furthermore, as the Sb composition of film is increased from x = 0.58 to x = 0.64, the Fermi level shifts from CBE to VBE, indicating a n-type to p-type transition.<br/><br/>Cross-sectional high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images reveal distinct differences at atomic scale between the 6-nm and 30-nm-thick Bi2Te3-Sb2Te3 alloy films. The 6-nm-thick films consist of a stack of five quintuple layers abruptly interfaced with the Al2O3 substrate. In some regions, the quintuple layer stacks appear misaligned in [10-10] and [11-20] cross-sectional HAADF-STEM images, indicating the presence of surface-terminating screw dislocations. On the other hand, the 30-nm-thick films consist of multi-stacks of quintuple layers with an SbxTe1-x layer at the film/substrate interface. Both of the 6 nm and 30 nm films contain 60° twin boundaries, either within or between quintuple layers. Interestingly, the density of the twin boundary in 30 nm film is higher than in 6 nm film. We hypothesize free carriers can be created at the twin boundary, resulting in different electronic properties. Magneto-transport measurements and multi-channel analysis will also be discussed to further understand conductivity contributions of different carrier species from different carrier channels.