Burstein-Moss Effect Leads to an Unusual Suppression of Bipolar Conduction with Shrinking Bandgap

Abstract

Thermoelectrics have shown promising applications in temperature management and waste heat recovery. However, thermoelectric performance deteriorates when a distinct bipolar conduction occurs, making it necessary to inhibit bipolar conduction for efficient thermoelectrics. Generally, increasing bandgap (Eg) through doping is widely adopted to suppress bipolar conduction by inhibiting intrinsic excitation. However, we experimentally observed that intrinsic excitation and bipolar conduction are unusually suppressed as the bandgap narrows in indium-doped Bi2Te2Se. We attribute this unusual suppression to the Burstein-Moss effect through angle-resolved photoemission spectroscopy (ARPES) and transport tests. The Burstein-Moss effect suggests that the energy required for carrier excitation in degenerate semiconductors is the sum of Eg and the Fermi level shift (Eshift) rather than Eg alone, as we observed that after indium doping, Eg narrows, but Eg + Eshift expands, accompanied by the suppression of intrinsic excitation and bipolar conduction. Our work emphasizes that when assessing and predicting the impact of a dopant on bipolar transport, it is more appropriate to consider Eg + Eshift rather than Eg alone, as Eg + Eshift is the actual energy required for intrinsic excitation. This physical principle is essential to thermoelectric materials, as Burstein-Moss effect is significantly intensified in narrow-bandgap degenerate semiconductors, a commonness of thermoelectric materials.