Role of Surface-Segregation-Driven Intermixing on the Thermal Transport through Planar Si/Ge Superlattices

It has been highly debated whether the thermal conductivity kappa of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to kappa values not only lower than the alloy's, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that kappa does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower kappa below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface-to alloylike values at intermediate phonon frequencies of similar to 3 THz. Our calculated kappa's quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.