ICT2014 Abstracts

The figure-of-merit, ZT, is a function of the number of valleys near the Fermi energy. This dependency occurs through the effect of multivalley band structure on all three quantities of the Seebeck coefficient, the charge carrier mobility, and the lattice thermal conductivity. The degeneracy of a valley in the bandstructure represents the k points with the same energy extrema around Fermi energy. It is widely accepted that a good thermoelectric material often has degenerate multivalley bandstructure. However, as we will discuss, the presence of the intervalley charge carrier scattering through acoustic phonons in such materials can significantly affect the figure-of-merit through modification of both the charge and thermal transport properties. We employed a multiband Boltzmann transport theory and reported the results of our calculations for several materials with multivalley bandstructures including Al­_xGa_1-xAs, Si_0.8Ge_0.2, and type-VIII silicon clathrate. In case of Al­_xGa_1-xAs, the beneficial effect of a multivalley band structure is remarkably reduced due to the deteriorating effect of intervalley scattering on charge carrier mobility. However, in case of Si_0.8Ge_0.2 and type-VIII silicon clathrate the beneficial effects of the larger degeneracy of the valleys dominates and enhances the ZT. It is shown that the multivalley bandstructure has two effects on the Seebeck coefficient. The degeneracy of the valley increases the density of states effective mass; hence, the Seebeck coefficient. However, the intervalley scattering reduces the Seebeck coefficient. It is concluded that the increase of the degeneracy of the band extrema cannot be a general design rule for ZT enhancement and a detailed transport study is required to engineer the bandstructure for good thermoelectric properties.