Thermoelectric Materials and Devices - Hierarchical Nanostructures for Energy Devices - page 114

d n 3 r 4 n g | 7

Figure 6.4 (a) Schematic of energy filtering effect. Energy filtering increases

E ave E F , so it can enhance the Seebeck coecient dramatically. Re-
printed with permission from Ref. 100. Copyright r 2002 American
Institute of Physics. (b) Figure of merit (left axis) and optimal barrier
height (right axis) versus Fermi level for a metallic superlattice with
conserved and non-conserved lateral momentum. 31,32 Reprinted with
permission from Ref. 31. Copyright r 2004 American Institute of
Physics.

order to achieve the filtering effect. A large number of scatterings within the
layers may force the filtered electron distribution back to the equilibrium, so
that the filtering effect is relaxed. This filtering effect can enhance the See-
beck coecient dramatically without decreasing the electrical conductivity
too much because it can yield a large E ave E F (Figure 6.4(a)). Using multiple
layers of thermionic emission was suggested to achieve this electron filtering
effect. 31,33 Figure 6.4(b) shows figure of merit (left axis) and optimal barrier
height (right axis) versus Fermi level for a metallic superlattice with con-
served and non-conserved lateral (toward the barrier) momentum. It has
been reported 23 that if the Fermi energy is aligned at an appropriate level
with a suciently large carrier density, and if the lateral momentum con-
servation is broken at the interfaces, ZT could be enhanced beyond 7.
However, Kim et al. 34 recently pointed out that the enhancement due to
the lateral momentum non-conservation could be modest because the
smallest number of modes in the well and barrier layers limits the emission
current over the barrier. Later, Bahk et al. 35 proposed the use of distributed
resonant scatterings to partially realize the non-planar electron energy fil-
tering effect in bulk materials, which is not limited by the lateral momentum
conservation as in the planar filtering.

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6.2.2.3 Band Engineering (Resonant State, Band
Convergence etc)
Recently, there have been many remarkable studies on the enhancement of
the power factor. 26,36 Band engineering by resonant impurities is one of the
most remarkable results. 26 Figure 6.5(a) shows a schematic diagram of the
distortion of the electronic density of states by Tl impurities in PbTe.