Alloy enhanced anisotropy in the thermal conductivity of SixGe1−x nanowires

Abstract

A strong dependence on crystalline orientation with a most conductive ⟨001⟩ growth direction is theoretically predicted for the thermal conductivity (κ) of SixGe1−x nanowires, even above room temperature and above 1 μm thickness. For instance, the room temperature κ of a 100 nm thick Si0.6Ge0.4 nanowire in the ⟨001⟩ direction is 16% higher than in the ⟨111⟩ direction. In contrast, the dependence for Si or Ge nanowires of the same diameter is much weaker at the same temperature, and ⟨111⟩ direction can be the most conductive one at relatively thin nanowires. In the low temperature limit, the anisotropy of κ can reach 87% at any alloy concentration. The anisotropy arises from the phonon focusing effect on the long mean free path phonons. The low frequency phonons focus in the ⟨001⟩ direction, while the intermediate frequency phonons focus in the ⟨111⟩ direction. The relative contribution to κ from the low frequency phonons is largely enhanced by alloying. Moreover, alloying eliminates the anisotropy caused by the intermediate frequency phonons. Our results imply that orientation has to be taken into account when engineering alloyed nanowire devices, such as thermoelectric modules, even if the corresponding bulk material is completely isotropic.