Weak Antilocalization and Anisotropic Magnetoresistance in Topological Bi₂Te_xSe_3-x Thin Films

Topological materials, such as the quintessential topological insulators in the Bi₂X₃ family (X = O, S, Se, Te), are extremely promising for beyond Moore’s Law computing applications where alternative state variables and energy efficiency are prized. It is essential to understand how the topological nature of these materials changes with growth conditions and, more specifically, chalcogen content. In this study, we investigate the evolution of the magnetoresistance of Bi₂Se_3-xTe_x for varying chalcogen ratios as a function of both temperature and angle of applied field. Weak antilocalization (WAL), an indicator of topological surface states, is observed to weaken with Te substitution. We also demonstrate that the anisotropy of the WAL follows the Tkachov-Hankiewicz model of magnetoconductance in topological insulators rather than a more trivial sinθ dependence. This model, which is a generalization of the Hikami-Larkin-Nagaoka model, allows for measurement of both coherence length and skin depth of the conducting surface states. These results show the surface states in Bi₂Se₃ to be most isolated from the bulk states, with quality degrading through Te substitution.