Evolution of optical phonon modes and thermoelectric properties in doped Bi2Te3: A temperature-dependent Raman spectroscopy study

Due to its three-dimensional bulk-insulating nature and better room-temperature thermoelectric properties, Bi2Te3 is widely explored as potential thermoelectric material and topological insulator. Here, we report the effect of smaller atoms (Ge, Se) doping on the evolution of the temperature-dependent optical phonon modes in polycrystalline single-phase Bi2Te3. While undoped Bi2Te3 exhibits crystal inversion symmetry lowering in the investigated temperature range, the doped Bi2Te3 retains the inversion symmetry below 270 K. As a result, the IR-active A21u phonon mode, which arises due to the lowering of crystal inversion symmetry, disappears in the doped Bi2Te3 below 270 K. The increase in the full width at half maxima of Raman peaks and reduction in the phonon lifetime with an increase in temperature confirm the optical phonon decay in doped Bi2Te3, and hence lattice thermal conductivity is expected to decrease at higher temperatures. The lattice thermal conductivity estimated from the optical phonon modes is in line with the experimentally measured value, which affirms that optical phonon vibrations dominate the lattice thermal conductivity of Bi2Te3. At low temperature, the Ge-Se codoped Bi2Te3 shows a fourfold enhancement of the Seebeck coefficient compared to that of undoped Bi2Te3. Our results give an insight into the optical phonon decay mechanism in the doped Bi2Te3 and corroborate that optical phonons play a vital role in the lattice thermal conductivity of polycrystalline Bi2Te3. Also, it is found that Se-Ge codoped Bi2Te3 can be used as an efficient near-room temperature thermoelectric material.