Evolution of thermal, structural, and optical properties of SiGe superlattices upon thermal treatment (Phys. Status Solidi A 3∕2016)

The evolution of the thermal conductivity of silicon-germanium superlattices upon thermal treatment was investigated experimentally and theoretically by Rastelli and co-workers (pp. 533–540). Several complementary techniques ranging from transmission-electron microscopy and X-ray diffraction to photoluminescence spectroscopy were employed to gain insight into the structural evolution. Silicon–germanium intermixing occurring during annealing produces a progressive smearing of the interfaces up to complete alloying. Using the structural information as input it was possible to reproduce theoretically the experimental results. The theoretical analysis shows that intermixing produces a significant reduction of scattering of phonons with mid–low frequency, accompanied by a modest increase of scattering of high-frequency phonons. The result is a gradual increase of thermal conductivity as the annealing temperature is increased. Effects of phonon scattering at the superlattice/substrate interface are also discussed. The work shows that while superlattices can display thermal conductivity values well below the alloy limit, their temperature stability is limited. This result is important also in view of potential applications of Si–Ge superlattices in thin-film thermoelectric devices. The cover image shows the evolution of the thermal conductivity of a superlattice annealed at increasing temperature. The calculations are able to reproduce the experimental results and show that phonon scattering at the thin film/substrate interface becomes important as annealing proceeds (see article for details). Examples of cross-sectional TEM images of the superlattice annealed at different temperatures are also shown.