Influence of induced defects on transport properties of the Bridgman-grown Bi2Se3-based single crystals

The impact of induced defects such as Bi intercalants, Bi antisites, and Se vacancies on the transport properties of the Bridgman-grown Bi2Se3-based single crystals in the ab plane were investigated by means of temperature-dependent electrical resistivity, Seebeck coefficient, and thermal conductivity measurements. We found that the Bi2Se3-based crystals can be grown either along the c-axis or perpendicular to it, depending on the different Bi-Se ratios in the starting material. All grown crystals showed a weak metallic behavior with a predominant electron-phonon scattering governing their electrical transport. The absolute value of the Seebeck coefficient of the n-type crystals grown along the c-axis is higher than those grown perpendicular to the c-axis (along the ab plane) at room temperature. The Fermi energy estimated from the Seebeck coefficient data is in the range of 0.20-0.43 eV. The thermal conductivity measurement showed that the lattice phonons dominate thermal transport in these Bi2Se3-based crystals. Analyses of lattice thermal conductivity data of the crystals by the Debye-Callaway approximation revealed that both boundary and point-defect scattering of phonons have a significant effect on the size and shape of the low-temperature phonon peak. These present findings indicate that, in addition to the change in composition, the physical properties of the Bi2Se3-based crystals are also affected significantly by the induced defects. Finally, the near-stoichiometric Bi2.04Se2.99 compound has a room-temperature thermoelectric figure of merit (ZT) of ∼0.12.