An anomalously high Seebeck coefficient and power factor in ultrathin Bi2Te3 film: Spin–orbit interaction

The present study reports a strong thickness-dependence and anomalously large enhancement in the values of the Seebeck coefficient and electrical conductivity in Bi2Te3 films at ultralow thickness. An opposite sign of the Hall coefficient (negative) and Seebeck coefficient (positive) is observed in an ultrathin Bi2Te3 film (65 nm) as compared to the normally observed identical sign in the case of Bi2Te3 thin films (520 nm). A simultaneous enhancement in the values of electrical conductivity and the Seebeck coefficient results in a giant enhancement in the value of power factor from 1.86 mW/m K2 to 18.0 mW/m K2 at 416 K, with a reduction in thickness. X-ray photoelectron spectroscopy investigation reveals the absence of any significant change in stoichiometry and chemical bonding upon reduction of thickness. Magnetoresistance vs magnetic field data show a sharp dip at the lower magnetic field values, indicating a weak antilocalization effect in the case of the ultrathin film sample suggesting the role of strong spin–orbit coupling toward the carrier filtering effect resulting in enhancement of thermoelectric properties. Observation of the large Seebeck coefficient and the power factor at lower thickness values and its relationship with spin–orbit coupling is an important result, both for practical applications and for better understanding of the thermoelectric properties.