Numerical Analysis of Homogeneous and Stratified Turbulence under Horizontal Shear via Lagrangian Stochastic Model: Richardson Number Effect

The present investigation is carried out to reveal Richardson number (Ri) effects on an homogeneous and stratified turbulence under horizontal shear. The problem is simulated via Lagrangian Stochastic model (LSM). Hence, the method of Runge Kutta with fourth order is adopted for the numerical integration of three differential systems under non linear initial conditions of Jacobitz (2002) and Jacobitz et al. (1998). This study is performed for Ri ranging from 0.2 to 3.0. It has been found that computational results by the adopted model (LSM) gave same findings than that of preceding works. It has been shown a global tendency of different parameters governing the problem to equilibrium asymptotic states for various values of Ri. The comparative study between the computations of the present LSM and direct numerical simulation of Jacobitz demonstrates a good agreement for both methods for the ratios of; potential energy Kθ/E and kinetic energy K/E toward the total energy E and the principal component of anisotropy b12 It has been found that Ri is the most important parameter affecting the thermal and dynamic fields of the flow. Hence, increase Ri conduct to increase the uniform stable stratification and decrease for the uniform mean shear S. It can be concluded that Ri is a main non-dimensional parameter which enable us to understand physical phenomenons produced inside stratified shear flows

Photoluminescence and time-resolved photoluminescence studies of lateral carriers transfer among InAs/GaAs quantum dots

International audienceWe report on the lateral transfer and thermal escape of carriers in InAs quantum dots (QDs) grown on a GaAs substrate by solid source molecular beam epitaxy by mean of photoluminescence (PL) and time-resolved PL measurements. The temperature-dependent PL spectra are discussed in terms of the inhomogeneous size distribution of the QDs and the carrier tunneling process from small to large QDs. The dependence of the photoluminescence decay time on the emission-wavelength is attributed to lateral carriers’ transfer within QDs with an interdot carrier tunneling time of 910 ps under low excitation conditions

Ordered arrays of Au catalysts by FIB assisted heterogeneous dewetting

International audienceSynthesizing Au 0.8 Si 0.2 nanocatalysts that are homogeneous in size and have controlled position is becoming a challenging and crucial prequisite for the fabrication of ordered semiconductor nanowires. In this study, Au 0.8 Si 0.2 nanocatalysts are synthesized via dewetting of Au layers on Si(111) during thermal annealing in an ultra-high vacuum. In the first part of the paper, the mechanism of homogeneous dewetting is analyzed as a function of the Au-deposited thickness ( h Au ). We distinguish three different dewetting regimes: (I) for a low thickness ( ##IMG## [http://ej.iop.org/images/0957-4484/26/50/505602/nano514960ieqn1.gif] \h_\\rmAu\\leqslant 0.4\\;\\mathrmnm\ ), a submonolyer coverage of Au is stabilized and there is no dewetting. (II) For an intermediate thickness ( ##IMG## [http://ej.iop.org/images/0957-4484/26/50/505602/nano514960ieqn2.gif] \0.4\\;\\mathrmnm\\lt h_\\mathrmAu\\leqslant 5\\;\\mathrmnm\ ), there is both dewetting and Au 0.8 Si 0.2 phase formation. The size and density of the Au 0.8 Si 0.2 clusters are directly related to h Au . When cooling down to room temperature, the clusters decompose and reject the Si at the Au/Si substrate interface. (III) For a large thickness ( ##IMG## [http://ej.iop.org/images/0957-4484/26/50/505602/nano514960ieqn3.gif] \h_\\rmAu\\gt 5\\;\\mathrmnm\ ), only dewetting takes place, without forming AuSi clusters. In this regime, the dewetting is kinetically controlled by the self-diffusion of Au (activation energy ∼0.43 eV) without evidence of an Si-alloying effect. As a practical consequence, when relying solely on the homogeneous dewetting of Au/Si(111) to form the Au 0.8 Si 0.2 catalysts (without a supply of Si atoms from vapor), regime II should be used to obtain good size and density control. In the second part of the paper, a process for ordering the catalysts using focused ion beam-(FIB) assisted dewetting (heterogeneous dewetting) is developed. We show that no matter what the FIB milling conditions and the Au nominal thickness are, dewetting is promoted by ion beam irradiation and is accompanied by the formation of Au 0.8 Si 0.2 droplets. The droplets preferentially form on the patterned areas, while in similar annealing conditions, they do not form on the unpatterned areas. This behavior is attributed to the larger Au-Si interdiffusion in the patterned areas, which results from the Si amorphization induced by the FIB. A systematic analysis of the position of the nanodroplets shows their preferential nucleation inside the patterns, while thicker platelets of almost pure Au are observed between the patterns. The evolutions of the size homogeneity and the occupancy rate of the patterns are quantified as a function of the FIB dose and annealing temperature. Nice arrays of perfectly ordered AuSi catalysts are obtained after optimizing the FIB and dewetting conditions

Ge condensation for the fabrication of fully strained and defect-free planar Si0.5Ge0.5 layers on silicon

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New strategies for producing strain engineered SiGe nanomembranes

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