We report a room temperature study of the direct band gap photoluminescence of tensile-strained Ge/Si0.13Ge0.87 multiple quantum wells grown on Si-based germanium virtual substrates by ultrahigh vacuum chemical vapor deposition. Blueshifts of the luminescence peak energy from the Ge quantum wells in comparison with the Ge virtual substrate are in good agreement with the theoretical prediction when we attribute the luminescence from the quantum well to the cΓ1-HH1 direct band transition. The reduction in direct band gap in the tensile strained Ge epilayer and the quantum confinement effect in the Ge/Si0.13Ge0.87 quantum wells are directly demonstrated by room temperature photoluminescence.

We directly demonstrate quantum-confined direct band transitions in the tensile strained Ge/SiGe multiple quantum wells grown on silicon substrates by room temperature photoluminescence. The tensile strained Ge/SiGe multiple quantum wells with various thicknesses of Ge well layers are grown on silicon substrates with a low temperature Ge buffer layer by ultrahigh vacuum chemical vapor deposition. The strain status, crystallographic, and surface morphology are systematically characterized by high-resolution transmission electron microscopy, atomic force microscopy, x-ray diffraction, and Raman spectroscopy. It is indicated that the photoluminescence peak energy of the tensile strained Ge/SiGe quantum wells shifts to higher energy with the reduction in thickness of Ge well layers. This blue shift of the luminescence peak energy can be quantitatively explained by the direct band transitions due to the quantum confinement effect at the Gamma point of the conduction band.

Rate-limiting step, as well as self-limited oxidation of SiGe alloys is so far under controversy. Contrasting to the monoparabolic growth mode for oxidation of Si, a parabolic growth mode and self-limited oxidation of SiGe alloys at different temperature are clearly observed depending on the oxidation time. With modified Deal-Grove model, we extract the parabolic rate constants related to the oxygen diffusion at different temperature and the activation energy of oxygen diffusivity finding that oxygen diffusion is still the rate-limiting step. We attribute this oxidation behavior to the strain effects associated with the volume change in converting Si/SiGe to SiO(2)/mixed oxide at different oxidation stages. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3191382

An enhancement of the direct bandgap photoluminescence from Ge layer on silicon with boron or phosphorous δ-doping SiGe layers at room temperature is reported. The n-type δ-doping SiGe layer is proposed to transfer extra electrons to L valley in Ge, which decreases the possibility of the excited electrons in the Γ valley to be scattered to the L valley, and improve the photoluminescence of the direct band transition in the Ge layer. Additionally, 2.5 fold enhancement of luminescence from the strained Ge layer on a silicon-on-insulator substrate is demonstrated due to the resonant effect. This investigation is very promising for efficient Si-based Ge light emitting diodes compatible with silicon technology.

A novel microencapsulated phase change material was prepared by sol-gel method using lauric acid (LA) as core material and titanium dioxide (TiO2) as shell material. The composites were characterized by field emission scanning electron microscope (FE-SEM), Fourier transformation infrared spectrometer (FT-IR), differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA). The results of the FE-SEM and FT-IR indicated that LA was well coated in the shell of TiO2 and no chemical reaction occurred between them. The results of the DSC denoted that the composites with 43.5 % encapsulation ratio of the LA melted at 44.39 ℃ with the melting enthalpy of 67.54 J/g and solidified at 43.64 ℃ with the solidification enthalpy of 65.17 J/g. The results of the TGA signified that the thermal stability and fire resistance of the microcapsules were greatly enhanced under the protection barrier of TiO2. The thermal decomposition of the microencapsulated LA with TiO2 shell hardly occurred under 200 ℃. That is to say, the composites possess good thermal stability in the operating temperature range. In conclusion, the prepared microencapsulated composites can be incorporated into solar energy storage like solar air heaters and into electronic devices as heat sinks.

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A novel microencapsulated phase change materials for cold energy storage was synthesised through sol-gel means using decanol as phase change material and titanium dioxide (TiO2) as encapsulated material. The micromorphology and composition of microcapsules were observed by field emission scanning electron microscope (FE-SEM), Fourier transformation infrared spectrometer (FT-IR).Using differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA) thermal properties of microcapsules were characterized. Results of FE-SEM and FT-IR indicated that micro sized decanol droplets were encapsulated with TiO2 to form the well-developed core-shell structure, which was only physical coating between them. Furthermore, the chemical and thermal stability of the microcapsules were improved and the inflammability of the microcapsules was lowered using TiO2 as shell material. The DSC result of the desirable ones melt at 3.87 ℃ with a latent melting enthalpy of 61.12 J·g-1 and solidified at – 1.32 ℃ with a latent solidification enthalpy of 59.54 J·g-1. In general, the prepared microcapsules have potential for cold energy storage.

A novel form-stable composite phase change materials for cold energy storage were prepared using physical blending adsorption method. In the shape-stabilized composites, lauryl alcohol (LA) and caprylic acid (CA) were employed as phase change materials, which were blended together at specific mass ratio based on theoretical calculations. Activated charcoal (AC) was selected as supporting material due to its advantages like large specific surface area and high thermal conductivity. The composites were characterized by field emission scanning electron microscope (FE-SEM), Fourier transform infrared spectrometer (FT-IR), differential scanning calorimeter (DSC) and thermogravimetric analyzer (TGA). The results of FE-SEM and FT-IR displayed that the eutectics of LA and CA was well absorbed and dispersed homogeneously into the porous network structure of the AC and the melted eutectics was not easy to leak from the reticular structure. Moreover, there was only physical absorption between the eutectic mixture and AC. The results of DSC and TGA indicated that phase change temperature and latent heat of the prepared composites increased with the increase of the binary eutectics mass ratio and AC can enhance the thermal stability of composites. The composites with the mass ratio 60% of the eutectics melted at – 0.21 ℃ with a latent heat enthalpy of 28.08 J/g and solidified at – 2.33 ℃ with a latent solidification enthalpy of 29.70 J/g. The prepared composites will contribute to cold energy storage of low temperature range.