Phonon Scattering in Bi₂Te₃/Sb₂Te₃ Superlattice

Thermoelectric materials are characterized with the figure of merit, ZT = S²σT/κ, where T is the temperature, S the Seebeck coefficient, σ the electrical conductivity and κ the thermal conductivity. Many researches have been focused on reducing lattice thermal conductivity through increasing phonon scattering at interfaces. Thin-film superlattices are one of the promising candidates for high ZT thermoelectric materials. Several theoretical models have been used to explain the large ZT in superlattice. One comes from the extra scattering channels at interfaces introduced by the hetero-structure. Another is a result of quantum confinement effect which reduces the phonon group velocity propagating perpendicularly through the superlattice layers through flattening the dispersion curve of acoustic phonons. In this work, ultrafast time-resolved measurements were conducted on Bi₂Te₃, Sb₂Te₃ and Bi₂Te₃/Sb₂Te₃ superlattice (SL) films to detect coherent acoustic phonons in these materials. Scattering of these phonons is revealed in the Bi₂Te₃/Sb₂Te₃ SLs, which comes from the interfaces of the hetero-structure in SL. Also, a decrease of acoustic phonon velocity resulted from folding and flattening of phonons branches is observed. Results show that both interface scattering and the reduced phonon velocity contribute to suppressing the heat transfer process.