First-principles Study of Phonon Drag Effect in SiGe Alloys*

Phonon drag is the effect that Seebeck coefficients of semiconductors are often greatly augmented at low temperatures. Recent works have shown that it is important in many materials’ thermoelectric (TE) performance even at room temperature, and the major phonons contributing to phonon drag are with longer mean free path (MFP) and lower frequency than those carrying heat. Meanwhile, the point defects in alloys tend to scatter phonons with short MFP and high frequency. Combining phonon drag with alloying might lead to better low-temperature TE materials.
We perform the first-principles calculations on the zT value of n-type SiGe alloys with different doping levels at different temperatures. We found that phonon drag contributes significantly to the Seebeck coefficient, especially at lower temperatures. The Seebeck coefficient peaks around the composition of Si_0.15Ge_0.85 due to band convergence, which agrees with reported experimental data. Our first-principles calculations are able to provide guidelines for the design of better low-temperature TE materials.