Possible Rashba band splitting and thermoelectric properties in CuI-doped Bi2Te2.7Se0.3 bulk crystals

We investigated thermoelectric properties of the CuI-doped (CuI)xBi2Te2.7Se0.3 bulk crystals grown by the Bridgman method. From the formation energy calculation and lattice parameter expansion along the c-axis, we confirm that the Cu-atom intercalates at the van der Waals layer while iodine substitutes at the Te(2) site which donates two electrons per CuI doping. The band structure calculation in iodine doped bismuth telluride monolayer shows band splitting along the momentum direction implying the Rashba type band splitting near the Fermi level. The Seebeck coefficient as a function of Hall carrier concentration on the compounds does not follow the Pisarenko's relation, whereas it follows the Rashba type Seebeck coefficient. The theoretical fitting with the Rashba type Seebeck coefficient indicates that the Rashba energy is increased with increasing CuI doping concentration. Owing to the enhancement of power factor near room temperature and reduction of lattice thermal conductivity by atomic scattering of phonon, the CuI doped compound (CuI)xBi2Te2.7Se0.3 (x= 0.3 mol.%) exhibited high ZT value over a wide temperature range.