Algorithm portfolios for noisy optimization

Noisy optimization is the optimization of objective functions corrupted by noise. A portfolio of solvers is a set of solvers equipped with an algorithm selection tool for distributing the computational power among them. Portfolios are widely and successfully used in combinatorial optimization. In this work, we study portfolios of noisy optimization solvers. We obtain mathematically proved performance (in the sense that the portfolio performs nearly as well as the best of its solvers) by an ad hoc portfolio algorithm dedicated to noisy optimization. A somehow surprising result is that it is better to compare solvers with some lag, i.e., propose the current recommendation of best solver based on their performance earlier in the run. An additional finding is a principled method for distributing the computational power among solvers in the portfolio.     

Effect of growth parameters on the heteroepitaxy of 3C-SiC on 6H-SiC substrate by chemical vapor deposition

Epitaxial 3C-SiC(1 1 1) films were grown on 6H-SiC(0 0 0 1) Si face on axis substrates by chemical vapor deposition under H₂, SiH₄ and C₃H₈ in a cold wall vertical reactor. Two temperatures were studied (1450 and 1700 °C) with various C/Si ratio and deposition time. It was found that under conditions giving high lateral growth (low C/Si and/or high temperature), homoepitaxial growth occurred even at temperatures as low as 1450 °C. For other conditions, the 3C-SiC polytype was detected and always together with the formation of double positioning boundaries whose density was found to depend on the growth conditions but not on the initial surface reconstruction. Single domain enlargement was observed when growth was performed at 1700 °C over a nucleation layer grown at 1450 °C.     

Effect of nitrogen impurity on the stabilization of 3C–SiC polytype during heteroepitaxial growth by vapor–liquid–solid mechanism on 6H–SiC substrates

This work reports on the stabilization of 3C–SiC polytype during heteroepitaxial growth by vapor–liquid–solid (VLS) on on-axis and 2° off-axis 6H–SiC(0001) substrates using Si–Ge as liquid phase. It was found that, depending on growth conditions (mainly temperature or nitrogen amount in the reactor), the deposit could be either a complete 3C or 6H–SiC layer or even a mixture of both polytypes. The proportion of 3C inside the deposit increases when 1) nitrogen amount in the reactor increases or 2) temperature is decreased. Though the effect of temperature could be explained in terms of 3C–SiC initial island dissolution, the influence of nitrogen is less obvious but it is shown to be effective at the early stage of growth. Several hypotheses are proposed such as SiC lattice modification by N incorporation or surface effects during the early stage of growth.     

Crystal structure,band structure and electrical properties of κ-(BEDT-TTF)2SbF6 grown on a Si(0 0 1) electrode

The new κ-(BEDT-TTF)₂SbF₆ phase was grown by electrodeposition on a Si(0 0 1) electrode. This new phase was characterized by X-ray diffraction (XRD) and electronic conductivity measurements, accompanied by calculations of electronic band structures and Fermi surfaces. Below T = 120 K, a decrease in the electronic conductivity suggests a phase transition, attributed to anion ordering.     

Deposition of nanocrystalline translucent h-BN films by chemical vapor deposition at high temperature

h-BN layers were deposited on α-SiC and sapphire substrates by chemical vapor deposition at high temperature (1500-1900 °C) using B₂H₆ and NH₃ diluted in Ar. Growth rates were in the 6-10 μm/h range. In all the conditions studied, the as deposited BN layers were found to be translucent to light, some having a light whitish aspect and other a more yellowish one. It was also observed that the deposit was not always adhesive. Characterizations showed that the layers were nano-crystalline with crystallite size < 10 nm. The growth rate was found to be temperature and N/B ratio dependent due to an N limited growth regime which is more pronounced above 1700 °C.     

Nucleation of 3C–SiC on 6H–SiC from a liquid phase

The aim of this work is to elucidate the mechanism involved in the 3C–SiC formation during growth by a vapor–liquid–solid mechanism on 6H–SiC substrate. Polytype selection is shown to occur at the first stage of the experiments, before propane injection into the reactor. The contact of the seed with a Si–Ge melt during the initial heating ramp causes the formation of 3C–SiC islands on the seed surface, probably below 1200 °C. The proposed mechanism first involves a partial dissolution of the seed in a Ge-rich liquid which becomes C-supersaturated. Then the Si content of the liquid rapidly increases, which provokes the precipitation of the dissolved carbon in the form of 3C–SiC islands. When growth starts upon propane injection, these islands enlarge and coalesce to form a continuous 3C–SiC layer. If the growth temperature is too high (1550 °C), the initial 3C–SiC islands are dissolved and homoepitaxial layers are obtained.     

Study of the interaction between graphite and Al-Si melts for the growth of crystalline silicon carbide

The chemical interaction between Al-Si melts of different compositions and graphite was investigated in order to clarify the mechanism of spontaneous growth of silicon carbide crystals from these melts. Calibrated graphite small rods were used as carbon source to facilitate comparison between experiments. For a temperature set to 1100°C, the reaction time and Si content of the melt were varied from 1 to 48 hours and 20 to 40 at.%, respectively. It has been found that in a first stage the liquid reacts at a relatively slow rate to form a microcrystalline SiC layer around the graphite rod. When this SiC layer has reached a certain thickness, a violent attack follows in some specific sites by rapid dissolution of the rod. Radial liquid channels progress from the surface of the rod up to its centre and then total conversion of graphite into SiC rapidly occurs. The local Si content of the melt, which controls the carbon solubility in the liquid, governs the overall mechanism. To form faceted -SiC crystals, the growth mechanism should involve carbon dissolution in one place and supersaturation in another place in relation with local changes of the Si content in the melt.