Isotope effect on the phonon-drag component of the thermoelectric power of germanium

We have measured the thermoelectric power of several germanium single crystals with different isotopic compositions: natural, 96.3%, and 99.99% enriched ${}^{70}\mathrm{Ge}.$ N- and p-type Ge samples with the net concentration of electrically active impurities $<2\mathrm{}\times {10}^{13}{\mathrm{cm}}^3$ were studied. Our experimental data show a decrease of thermopower with an increase in isotopic mass disorder. This isotope effect is evident at temperatures below approximately 70 K where a phonon-drag part of the thermopower dominates. At high temperatures, in the diffusion regime, the thermopower does not correlate with the isotopic composition within experimental error. The observed isotope effect can be explained with a special phonon-drag contribution induced by a momentum transfer from thermal phonons to low-energy phonons in the enriched samples.