SiGe quanrum rings (QRs) grown at 500 o C and 600 o C were observed on SiGe quantum dots (QDs) capped with Si. Average depth and diameter are 9 nm and 185 nm, respectively, for QRs at 500 o C, while those are 0.9 nm and 84 nm for QRs at 600 o C. Ge out-diffusion mechanism is proposed to be responsible for nanorings formation at 500 o C, and Si surface diffusion toward strain-free edges is proposed to be responsible for nanorings formation at 600 o C. Raman spectroscopy suggests that formation of QRs at 600 o C is closely correlated with a strain-driven process. QRs grown at 600 o C are the metastable states and can be only observed in very limited conditions. Both thick cap and high thermal budget can destroy SiGe nanorings structures.

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The evolution of the thermal conductivity of silicon‐germanium superlattices upon thermal treatment was investigated experimentally and theoretically by Rastelli and co‐workers (pp. http://doi.wiley.com/10.1002/pssa.201532468). 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.

Surface orientation effects on SiGe quantum dots and nanorings have been investigated in this work. The base shapes of SiGe QDs and nanorings can be controlled by different surface orientation. Moreover, the hydrogen desorption rate is different in three kinds of surface orientation. The surface diffusion, which plays an important role in Ge out-diffusion mechanism, is dominated by the hydrogen desorption rate, and thus the formation rate of SiGe nanorings becomes different.