Electron Band Engineering for Thermoelectric Semiconductors

Thermoelectric semiconductors typically have complex electronic band structures as it leads to improved performance. This complexity arises from orbital chemistry as well as structure. Band inversion and warping can also lead to complex band structures and unusual electronic properties beyond thermoelectrics such as topological insulating behavior.<br/>The DOS and transport effective masses are commonly used to describe electronic transport in semiconductors using a classical analogy to the kinetic theory of gasses. The Seebeck coefficient can be used conductivity and Hall measurements to characterize the DOS mass [1] and DOS weighted mobility [2] to provide direct experimental characterization of DOS and mobility at room temperature and above. In materials with complex electronic structures the DOS and inertial masses can vary significantly. For example, in many valley electronic structures such as PbTe, with <i>N_V</i>isolated valleys in the Fermi surface, the DOS mass can be high and inertial mass low for high thermoelectric quality factor. High valley degeneracy has been found in many high <i>zT</i> systems such as Bi₂Te₃, Mg₂(Si,Sn), CoSb₃ skutterudites and some Half Heuslers.<br/>In some systems the Fermi surface pockets can be anisotropic and even non-ellipsoidal, such as the tubes of Fermi surface found in SrTiO₃. Tubes or sheets of Fermi surface rather than ellipsoidal pockets leads to a density of states like 1D or 2D low dimensional materials. By engineering the orbital chemistry complex behavior and unusual properties can be devised even in 3D materials.