Growth of Ge quantum dot superlattices for thermoelectric applications

We report on the thermal conductivity measurement of Ge quantum dot superlattices. The samples used were grown using molecular beam epitaxy. The typical dot sizes were determined by transmission electron microscopy measurements to be 75 nm in base and 7 nm in height. A differential 3ω method was employed to characterize the thermal conductivity of the samples. At room temperature, thermal conductivity was determined to be 6.2 and 30.5 W/mK in the cross-plane and in-plane direction, respectively. Temperature-dependent measurements showed that cross-plane thermal conductivity monotonously decreased while in-plane thermal conductivity showed a peak as the temperature decreased from 300 to 80 K. The results were well explained using a model based on the Boltzmann transport equation for cross-plane conductivity and based on the relaxation time approximation by including phonon scattering by quantum dots for in-plane thermal conductivity.