Efficiency of thermoelectric materials is generally discussed in terms of the dimensionless figure-of-merit, ZT
= S2σT/κ, Many researchers have found that it is possible to reduce the lattice thermal conductivity by
incorporating nanostructures (i.e. nanoparticles or heterobarriers) into materials, thereby scattering phonons.
At the same time, it has been theoretically predicted and experimentally demonstrated that barriers can be
used to "filter" the distribution of carriers which contribute to conduction. By doing so, it is possible to
significantly increase the Seebeck coefficient while only modestly decreasing the electrical conductivity. As a
result of this energy-dependent scattering of carriers, the thermoelectric power factor is increased. We present
theoretical and experimental results for metal/semiconductor nanocomposites consisting of metallic rareearth-
group V nanoparticles within III-V semiconductors (e.g. ErAs:InGaAlAs) demonstrating both an
increase in thermoelectric power factor and a decrease in thermal conductivity, resulting in a large figure of
merit. We also discuss metal/semiconductor superlattices made of lattice-matched nitride materials for
electron filtering and the prospects of these materials for efficient thermoelectrics, especially at high
temperatures. Finally, we will discuss both various synthesis techniques for these materials, including the
prospects for bulk growth, and also devices fabricated from these materials.