The ability to precisely control the thermal conductivity (κ) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of κ of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low κ are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as ∼ 15 nm. Single-barrier thermal resistances between 2 and 4×10⁻⁹ m² K W⁻¹ were attained, resulting in a room-temperature κ down to about 0.9 W m⁻¹ K⁻¹, in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Green’s function simulations.

We demonstrate optical orientation in $\mathrm{Ge}/\mathrm{SiGe}$ quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding $\sim 37\%$ and $\sim 85\%$ for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be $\sim 0.5\mathrm{ps}$ and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.

A comprehensive study of the biaxial strain-induced shift of the ${\text{Si}}_{1-x}{\text{Ge}}_x$ Raman active phonon modes is presented. High-resolution Raman measurements of ${\text{Si}}_{1-x}{\text{Ge}}_x/\text{Si}$ heterostructures have been compared to x-ray diffraction data. Our approach, unlike previous works, is effective to decouple and quantify separately the effect of strain and composition on the phonon frequencies, yielding an accurate determination of the phonon strain shift coefficients in the entire composition range. Our results show that the strain shift coefficients are independent of the composition, a result which is in good agreement with theoretical calculations, performed within the framework of valence force-field theory.

Large-scale integration of MoS ₂ in electronic devices requires the development of reliable and cost-effective deposition processes, leading to uniform MoS ₂ layers on a wafer scale. Here we report on the detailed study of the heterogeneous vapor–solid reaction between a pre-deposited molybdenum solid film and sulfur vapor, thus resulting in a controlled growth of MoS ₂ films onto SiO ₂ /Si substrates with a tunable thickness and cm ² -scale uniformity. Based on Raman spectroscopy and photoluminescence, we show that the degree of crystallinity in the MoS ₂ layers is dictated by the deposition temperature and thickness. In particular, the MoS ₂ structural disorder observed at low temperature (<750 °C) and low thickness (two layers) evolves to a more ordered crystalline structure at high temperature (1000 °C) and high thickness (four layers). From an atomic force microscopy investigation prior to and after sulfurization, this parametrical dependence is associated with the inherent granularity of the MoS ₂ nanosheet that is inherited by the pristine morphology of the pre-deposited Mo film. This work paves the way to a closer control of the synthesis of wafer-scale and atomically thin MoS ₂ , potentially extendable to other transition metal dichalcogenides and hence targeting massive and high-volume production for electronic device manufacturing.