Burstein-moss effect leads to an unusual suppression of bipolar conduction with shrinking bandgap

Abstract

Thermoelectrics have shown promising applications in cooling, temperature management and waste heat recovery. However, the thermoelectric performance deteriorates when bipolar conduction intensifies, making it necessary to suppress this adverse effect for efficient thermoelectrics. Generally, increasing the bandgap (E_g) to inhibit intrinsic excitation through doping is widely accepted as an effective way to suppress bipolar conduction. However, we experimentally observed that intrinsic excitation and bipolar conduction are unusually suppressed as E_g 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 actual energy required for carrier excitation in degenerate semiconductors is the sum of E_g and the Fermi level shift (ΔE_shift) rather than E_g alone, as we observed that after indium doping, E_g narrows but E_g + ΔE_shift 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 conduction, it is more appropriate to consider E_g + ΔE_shift rather than E_g alone as E_g + ΔE_shift is the actual energy required for intrinsic excitation. This physical law is important to thermoelectric materials as the Burstein–Moss effect is strongly intensified in narrow-bandgap degenerate semiconductors, which is precisely a common characteristic of typical thermoelectric materials.