Dilute Cu2Te-alloying enables extraordinary performance of r-GeTe thermoelectrics

Abstract

Thermoelectric GeTe intrinsically comes with a p-type conduction and a carrier concentration (∼1021 cm−3) significantly higher than needed (∼1020 cm−3) because of the existence of high-concentration cation vacancies (∼3%). Its rhombohedral phase (r-GeTe) has recently been found to be very interesting because of the overall high valence band degeneracy enabled by the rhombohedral distortion from its cubic structure. Existing efforts on advancing GeTe thermoelectrics usually involve a very high concentration of impurities, which usually leads to a significant simultaneous change in the band structure and scattering of both electrons and phonons. In this study, we focus on r-GeTe and illustrate Cu2Te as a particularly effective dopant, enabling carrier concentration to be optimized at a very low overall impurity concentration for minimizing the changes in the band structure and scattering. This work reveals an inherently high mobility (∼140 cm2/V-s) that has never been realized in the literature (≤90 cm2/V-s) and demonstrates the degree of rhombohedral distortion as a core descriptor for the valence band structure. A further PbTe-alloying for a reduction in lattice thermal conductivity successfully realizes an extraordinary thermoelectric performance, demonstrating the superior thermoelectric potential inherent to r-GeTe.