Fig. 3.

(A) Contribution of the most preferable modes to the phonon drag Seebeck coefficient at different reduced thermal conductivity values and (B) the enhancement of zT achieved by selecting preferable modes at 300 K for n-type silicon. In A, for lightly doped silicon, the thermal conductivity can be reduced to 30 W/(mK) before observing significant diminishment of the phonon drag effect. Dashed lines represent the diffusive Seebeck coefficient for heavily doped silicon at different temperatures. For heavily doped silicon, the phonon drag part is still nonnegligible and becomes larger compared with the diffusion part when the temperature is decreased. In calculating zT, the experimental data are used for the electrical conductivity as a function of doping concentration for n-type silicon (52).