Resonant levels in bulk thermoelectric semiconductors

TLDR

Distortions of the electronic density of states can raise the thermopower of thermoelectric semiconductors, and resonant levels have been observed experimentally in a range of III–V, II–VI, IV–VI, and V2–VI3 compound semiconductors. This review examines band‑structure engineering strategies—resonant impurity levels, dilute Kondo effects, and hybridization in strongly correlated systems—to enhance thermoelectric performance. The enhancement arises via two pathways: an added density of states that boosts thermopower nearly independently of temperature, and resonant scattering that provides strong electron‑energy filtering, especially at cryogenic temperatures where electron–phonon interactions are weaker. Electronic‑structure calculations for Tl:PbTe and Ti:PbTe show that the thermopower increase in Tl:PbTe originates from a resonant impurity level, and the analysis identifies optimal delocalization of impurity states for maximizing DOS distortions.

Abstract

Distortions of the electronic density of states (DOS) are a potent mechanism to increase the thermopower of thermoelectric semiconductors, thereby increasing their power factor. We review band-structure engineering approaches that have been used to achieve this, resonant impurity levels, dilute Kondo effects, and hybridization effects in strongly correlated electron systems. These can increase the thermoelectric power of metals and semiconductors through two mechanisms: (1) the added density of states increases the thermopower in a nearly temperature-independent way; (2) resonant scattering results in a strong electron energy filtering effect that increases the thermopower at cryogenic temperatures where the electron–phonon interactions are weaker. Electronic structure calculation results for Tl:PbTe and Ti:PbTe are contrasted and identify the origin of the thermopower enhancement in Tl:PbTe. This leads to a discussion of the conditions for DOS distortions to produce thermopower enhancements and illustrates the existence of an optimal degree of delocalization of the impurity states. The experimentally observed resonant levels in several III–V, II–VI, IV–VI and V2-VI3 compound semiconductor systems are reviewed.