SiGe layer thickness effect on the structural and optical properties of well-organized SiGe/SiO2 multilayers.

In this work, we report on the production of regular (SiGe/SiO2)20 multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO2 thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (t SiGe) from 3.5-2 nm results in a transition from continuous SiGe crystalline layer (t SiGe ∼ 3.5 nm) to layers consisting of isolated nanocrystals (t SiGe ∼ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ∼3-8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for t SiGe ∼ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO2 nanostructures show an emission band located between 0.7-0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.

High-Performance µ-Thermoelectric Device Based on Bi2Te3/Sb2Te3 p-n Junctions.

A flexible and ultralight planar thermoelectric generator based on 15 thermocouples composed of n-type bismuth telluride (Bi2Te3) and p-type antimony telluride (Sb2Te3) legs (each with 400 nm thick) connected in series, on 25 µm thick Kapton substrate, was fabricated with impressive power factor values of 2.7 and 0.8 mW K-2 m-1 (at 298 K) for Bi2Te3 and Sb2Te3 films, respectively. The p-n junction thermoelectric device can generate a maximum open-circuit voltage and output power of 210 mV and 0.7 µW (3.3 mW cm-2), respectively, for a temperature difference of 35 K, which is higher than the one observed for a conventional thermoelectric device with metallic contacts for p-n junctions. The results were combined with numerical simulations, showing a good match between the experimental and the numerical results. The current density versus voltage (J-V) characteristics of the fabricated p-n junctions revealed a diode behavior with a turn-on voltage of ≈0.3 V and an impressive rectifying ratio (I+1V/I-1V) of ≈2 × 104.