Composition-dependent topological-insulator properties of epitaxial (Bi_1-xSb_x)₂(Te_1-ySe_y)₃ thin films

Due to strong spin-orbit coupling (SOC), topological insulators (TIs) have attracted much interest in developing novel energy-efficient electronic devices. (Bi_1-xSb_x)₂(Te_1-ySe_y)₃ (BSTS) has been regarded as a prototypical TI because of the dominance of the surface channel in carrier transport. Nevertheless, the effect of the composition of BSTS on SOC has not been explored, which is important for the application in novel electronic devices. In this work, BSTS thin films with systematically varying compositions are examined to release composition-dependent maps for the bulk carrier density, the dominance of the surface states in carrier transport, and the strength of SOC. The maps show that the regions for optimizing each property are located in separate positions, with a narrow overlap between them. These results imply that a stringent control in composition of the BSTS thin film is required in order to take advantage of the spin-momentum-locked surface channel of TI in novel electronic devices.