Thermal transport through short-period SiGe nanodot superlattices

The cross-plane thermal conductivity kappa of multilayers of SiGe nanodots separated either by Si or SiGe can be decreased by reducing the period length or by increasing the nanodot density. It is, however, not clear how far kappa can be reduced by using these strategies. In addition, the role of SiGe nanodots on the reduction of kappa is still not fully understood. In this work, we addressed these issues by studying experimentally the cross-plane kappa of Ge/Si superlattices with period lengths down to 1.5 nm. Although kappa tends to preserve the decreasing trend with reducing the period length, for periods shorter than 2 nm we observed a drastic drop of the average thermal resistance per period. This finding indicates a weakening of the effect of the interfaces on phonon scattering and implies a lower limit for kappa. To assess the role played by the nanodots in the reduction of kappa we studied Ge/Si superlattices with nanodot densities varying from 0 to similar to 8 x 10(10) cm(-2) and a fixed Si spacer thickness of 2.7 nm. The experimental results suggest that SiGe nanodots with "pyramid"-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Finally, the comparison of superlattices with nanodots separated by Si1-xGex (with x from 0 to 0.2) shows that spacer alloying is beneficial in reducing the kappa by similar to 20%. The results presented in this work are expected to be relevant to micro/nanoscale energy conversion which requires minimizing the thermal conductivity of superlattice-based thin film thermoelectrics. (C) 2014 AIP Publishing LLC.