A new target design is presented to model high-energy radiative accretion shocks in polars. In this paper, we present the experimental results obtained on the GEKKO XII laser facility for the POLAR project. The experimental results are compared with 2D FCI2 simulations to characterize the dynamics and the structure of plasma flow before and after the collision. The good agreement between simulations and experimental data confirms the formation of a reverse shock where cooling losses start modifying the post-shock region. With the multi-material structure of the target, a hydrodynamic collimation is exhibited and a radiative structure coupled with the reverse shock is highlighted in both experimental data and simulations. The flexibility of the laser energy produced on GEKKO XII allowed us to produce high-velocity flows and study new and interesting radiation hydrodynamic regimes between those obtained on the LULI2000 and Orion laser facilities.

We have investigated the structural defects formed in Mg-doped GaN and AlGaN epitaxial layers grown by metal organic chemical vapor deposition. These defects have an inverse pyramidal shape and appear when the Mg concentration [Mg] is higher than ∼4×1019/cm3. The density of the defects increases as [Mg] increases, but the size of the defects becomes smaller as [Mg] increases. The density of the defects also has a strong correlation with the hydrogen concentration in the epitaxial layers. Transmission electron microscope analysis reveals that the defects have an inversion operation to the matrix and that their boundaries are Mg-rich. We also propose a model for defect formation.

The Vapor - Liquid - Solid (VLS ) technique allows the growth of high aspect ratio Si wires. The Si nanowires formed by this technique can be thinned down by oxidation. This approach allows the formation of very thin Si cores which may be used to research the properties of Si nanostructures. In this work the growth and oxidation of these wires is characterized.

In the growth a very thin layer of Au is deposited on a Si (111) surface, silane gas is introduced into the chamber as the Si source gas and the temperature is raised to 300 – 600°C. Initially a catalytically active Au surface phase leads to the growth of a defective epitaxial Si layer. As Au / Si molten alloy balls nucleate and grow in size to approach the threshold size for VLS wire growth, which is determined by the Gibbs - Thomson effect, the epitaxial layer growth rate decreases and a transition to Si nanowire growth occurs. The morphology and width of the wires is strongly dependent on the growth temperature and pressure. At low pressure and high temperature relatively thick well-formed wires grow straight up from the substrate surface along the [111] direction. As the temperature is decreased and the pressure is increased thinner wires (as thin as 10 nm ) grow which tend to exhibit growth defects. A light oxidation yields Si cores which are of the order of 5 nm in diameter.

Discontinuities in nanoindentation loading curves are frequently observed in ceramic materials. These normally occur at fairly random loads and displacements, probably due to the random distance of pre-existing defects from the indent location. Here we report the observation of reproducible, sudden indent-depth changes which occur over a very narrow distribution of depths and loads, for GaAs and a range of related epitaxial layer systems. The surface preparation and material defect density have a significant influence. Different tip geometries have been used to gain insights into the deformation processes which cause these discontinuities.

Step bunching during epitaxial growth results in the transformation of a vicinal surface into a periodic array of micro-facets. Molecular beam epitaxial growth on the vicinal GaAs (110) surface exhibits this phenomenon which has primarily been characterized by electron microscopy. GaAs quantum wells with AlAs barriers were grown on GaAs(110) substrates vicinal 0.5-2· towards [010]. The faceting on the vicinal surface creates two distinct quantum well thicknesses. While most of the quantum well is 96Å thick, it broadens at the faceted regions. This thickness modulation produces two distinct luminescence peaks. By using temperature dependent photoluminescence, we have observed trends in exciton mobility. The exciton mobility decreases at low temperatures for the 1.0° and 2.0° samples, indicating a scattering mechanism. We will discuss interface roughness and other sources of scattering.

The cavities of molecular sieves offer an opportunity for synthesis of periodically ordered semiconductor nanoclusters. Ge clusters were synthesized in zeolite Y by thermal decomposition of GeH4 absorbed in the zeolite cages. Upon inclusion of germanium in proton exchanged zeolite Y (HY), the X-ray diffraction (XRD) pattern shows a small change in line positions and considerable change in intensities. No extra reflections were detected in either XRD or transmission electron diffraction (TED) for the samples synthesized at 300°C. These observations are in accordance with the energy dispersive X-ray (EDX) spectroscopy data which reveal the presence of germanium within the HY in concentrations higher than 5x1019 cm-3. At annealing temperatures higher than 500°C, small Ge crystallites (∼0.2μm) form on the exterior surface of the HY, as documented by TED, TEM and absorption spectroscopy. The results from the absorption and photoluminescence spectroscopy of the Ge loaded HY will be discussed.