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

Abstract

Due to its three-dimensional bulk-insulating nature and better room-temperature thermoelectric properties, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ 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 ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. While undoped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ exhibits crystal inversion symmetry lowering in the investigated temperature range, the doped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ retains the inversion symmetry below 270 K. As a result, the IR-active ${A}_{1u}^{2}$ phonon mode, which arises due to the lowering of crystal inversion symmetry, disappears in the doped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ 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 ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, 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 ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. At low temperature, the Ge-Se codoped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ shows a fourfold enhancement of the Seebeck coefficient compared to that of undoped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. Our results give an insight into the optical phonon decay mechanism in the doped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and corroborate that optical phonons play a vital role in the lattice thermal conductivity of polycrystalline ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. Also, it is found that Se-Ge codoped ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ can be used as an efficient near-room temperature thermoelectric material.