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Recently, it has been shown that the thermoelectric figure of merit is strongly enhanced in quantum wells and superlattices due to two-dimensional carrier confinement. We predict that the figure of merit can increase even further in quantum well structures with free-surface or rigid boundaries. This additional increase is due to spatial confinement of acoustic phonons and corresponding modification of their group velocities. The latter leads to an increase of the phonon relaxation rates and thus, a significant drop in the lattice thermal conductivity.
Balandin et al. (Tue,) studied this question.