Thermal conductivity of Si-Ge superlattices

The thermal conductivity of Si-Ge superlattices with superlattice periods 30<L<300 Å, and a Si_0.85Ge_0.15 thin film alloy is measured using the 3ω method. The alloy film shows a conductivity comparable to bulk SiGe alloys while the superlattice samples have a thermal conductivity that is smaller than the alloy. For 30<L<70 Å, the thermal conductivity decreases with decreasing L; these data provide a lower limit to the interface thermal conductance G of epitaxial Si-Ge interfaces: G> 2 × 10 W m^-2 KT^-1 at 200 K. Superlattices with relatively longer periods, L> 130 Å, have smaller thermal conductivities than the short-period samples. This unexpected result is attributed to a strong disruption of the lattice vibrations by extended defects produced during lattice-mismatched growth.