Impurity doping and Fermi-Level Tuning in Bi₂Te₃-related Alloys

Fermi level (E_F) tuning of Bi₂Te₃-related alloys is very critical for their applications to the thermoelectrics (TEs) or topological insulators (TIs). Bi₂Te₃ and its alloys are known as good TE materials at near room temperature. The room temperature thermoelectric performance is very large in these materials. However due to the narrow band gap nature and the large bipolar effect, the thermoelectric performance is rapidly vanishing with temperature. Recent studies revealed that the high doping concentration of ~ 6-8 x 10¹⁹ cm^-3 can prevent the bipolar effect. Meanwhile, Bi₂Te₃ is known as the TI material. For the use of the topological surface states for future device, the E_F should be positioned at the mid gap to reduce the bulk transport while maintaining surface transport..
In this work, by performing the density functional calculations, we investigated the impurity doping for Fermi level tuning in Bi₂Te₃-based alloys. We considered the alkali and halogen point impurities and calculated their formation energies and band structures to understand their electrical properties. Based on the results, we will discuss the dopability of impurity dopants and the Fermi-level tuning in Bi₂Te₃-based alloys.