Abstract
The theory of valleytronics as a material design tool for engineering both thermal and electrical transport properties is presented. It is shown that the interplay among the valleytronics parameters such as the degeneracy of the band, intervalley transitions, effective mass, scattering exponent, and the Fermi energy may deteriorate or ameliorate any or all of the main thermoelectric properties. A flowchart classifying the different paths through which the valleytronics can influence the thermoelectric figure-of-merit ZT is derived and discussed in detail. To exemplify the application of the flowchart, valleytronics in four different semiconductors, Mg2Si, Si0.8Ge0.2, AlxGa1−xAs and clathrate Si46-VIII were studied, which showed different trends. Therefore, a degenerate multivalley bandstructure, which is typically anticipated for a good thermoelectric material, cannot be a general design rule for ZT enhancement and a detailed transport study is required to engineer the optimum bandstructure.
Highlights
The theory of valleytronics as a material design tool for engineering both thermal and electrical transport properties is presented
Valleytronics can be considered as a material design tool through which a multivalley bandstructure is engineered for improving the thermoelectric power factor
The efficiency of a thermoelectric convertor is a function of the dimensionless thermoelectric figure-of-merit ZT which is quantified as ZT = S2σ T/κ, in which S, σ, T, and κ are the Seebeck coefficient, electrical conductivity, temperature and total thermal conductivity, respectively
Summary
The multivalley band structure affects directly and indirectly the interdependent thermoelectric variables S, σ , κ e, and κ l It can directly affect the Seebeck coefficient through the density of states effective mass mdos and intervalley scattering, carrier mobility through intervalley scattering, and electronic thermal conductivity through intervalley scattering. The first set includes the scattering mechanisms that affect the electrical properties such as charge mobility, electronic thermal conductivity and Seebeck coefficient. In this step, we calculated the ionized impurity, electron-phonon (acoustic and optical), and intervalley scatterings[33]. The multivalley band structure indirectly affects the scattering mechanisms by changing the energy of carriers and the density of states effective mass. Scattering type Acoustic Phonons Intervalley Defect Carrier− carrier Piezoelectric Ionized impurity Polar LO phonons
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