Investigation of melt-grown dilute GaAsN and GaInAsN nanostructures for photovoltaics

AbstractThe present work demonstrates the possibility to use liquid phase epitaxy to incorporate nitrogen in epitaxial GaAsN/GaAs and GaInAsN/GaAs heterostructures, including nanoscaled ones. The structures are grown from Ga - and GaIn - melts containing polycrystalline GaN as a nitrogen source. The red shift of the absorption spectra corresponds to nitrogen content in the epitaxial layers near or less than 0.2 at %. Photoluminescence spectra of dilute nitride GaAsN and GaInAsN show emission from localized nitrogen states - N-nanoclusters of more than two N atoms. These studies show that the melt grown dilute GaAsN and GaInAsN nanostructures can be used for solar cells with extended long wavelength edge

Rewriting System for Profile-Guided Data Layout Transformations on Binaries

International audienceCareful data layout design is crucial for achieving high performance. However exploring data layouts is time-consuming and error-prone, and assessing the impact of a layout transformation on performance is difficult without performing it. We propose to guide application programmers through data layout restructuring by providing a comprehensive multidimensional description of the initial layout, built from trace analysis, and then by giving a performance evaluation of the transformations tested and an expression of each transformed layout. The programmer can limit the exploration to layouts matching some patterns. We apply this method to two multithreaded applications. The performance prediction of multiple transformations matches within 5% the performance of hand-transformed layout code

From Driving Simulation to Virtual Reality

Driving simulation from the very beginning of the advent of VR technology uses the very same technology for visualization and similar technology for head movement tracking and high end 3D vision. They also share the same or similar difficulties in rendering movements of the observer in the virtual environments. The visual-vestibular conflict, due to the discrepancies perceived by the human visual and vestibular systems, induce the so-called simulation sickness, when driving or displacing using a control device (ex. Joystick). Another cause for simulation sickness is the transport delay, the delay between the action and the corresponding rendering cues. Another similarity between driving simulation and VR is need for correct scale 1:1 perception. Correct perception of speed and acceleration in driving simulation is crucial for automotive experiments for Advances Driver Aid System (ADAS) as vehicle behavior has to be simulated correctly and anywhere where the correct mental workload is an issue as real immersion and driver attention is depending on it. Correct perception of distances and object size is crucial using HMDs or CAVEs, especially as their use is frequently involving digital mockup validation for design, architecture or interior and exterior lighting. Today, the advents of high resolution 4K digital display technology allows near eye resolution stereoscopic 3D walls and integrate them in high performance CAVEs. High performance CAVEs now can be used for vehicle ergonomics, styling, interior lighting and perceived quality. The first CAVE in France, built in 2001 at Arts et Metiers ParisTech, is a 4 sided CAVE with a modifiable geometry with now traditional display technology. The latest one is Renault’s 70M 3D pixel 5 sides CAVE with 4K x 4K walls and floor and with a cluster of 20 PCs. Another equipment recently designed at Renault is the motion based CARDS driving simulator with CAVE like 4 sides display system providing full 3D immersion for the driver. The separation between driving simulation and digital mockup design review is now fading though different uses will require different simulation configurations. New application domains, such as automotive AR design, will bring combined features of VR and driving simulation technics, including CAVE like display system equipped driving simulators

Thermal quenching of far-red Fe3+ thermoluminescence of volcanic K-Feldspars

International audienceStrong thermal quenching is observed from 77 to 550 K in the far-red luminescence of K-feldspars. This far-red emission, next to the emission in the UV-to-blue spectrum recorded for thermoluminescence (TL), is reported in most alkaline feldspars with a characteristic peak centered on 710 nm with a width of 100 nm. This emission was observed by cathodoluminescence (CL) at room temperature (RT) for more than 30 K-feldspars, ranging from volcanic sanidines to granitic microclines and sediments and it is attributed to an Fe3+ impurity. Contrary to ‘blue’ emission in volcanic feldspars, the far-red emission displays very low anomalous fading (AF). This makes it attractive for dating purposes; however, it has weak natural TL intensity, even at saturation, which competes with the black-body emission of the heater plate. This is in contrast to an intense tunneling afterglow at liquid nitrogen temperature (LNT). Further observations show that the disadvantage of weak TL can be overcome. Photoluminescence (PL) under UV shows a very strong thermal quenching effect of the far-red emission from 77 to 550 K, which accounts for the above contrast. Near the LNT, the far-red Fe3+ photoluminescence is at a maximum and is dominant over other emissions in the spectrum. However, as the temperature increases, the efficiency decreases, falling to well below one percent, whereas the ‘blue’ emissions remain stable. This thermal quenching effect in photoluminescence is paralleled in TL. After irradiation and during storage at RT, whereas the ‘blue’ emission in volcanic feldspars is affected by ‘fast’ anomalous fading, charge trapped at Fe3+ centers as latent far-red emission is almost stable. As the TL evolves and the extant ‘blue’ emission is emitted, more and more of the trapped charge associated with far-red emission recombines non-radiatively, resulting in an efficiency for emission in natural TL that is less than one percent. A modified band model, which calls for 'hopping' conductivity during the storage, accounts for the anomalous fading. Trap emptying at lower temperatures should lead to better use of the stable latent far-red stored charge for the dating of volcanic deposits

Visible luminescence of Al2O3 nanoparticles embedded in silica glass host matrix

International audienceThis paper deals with the sol-gel elaboration and defects photoluminescence (PL) examination of Al2O3 nanocrystallites (size similar to 30 nm) confined in glass based on silica aerogel. Aluminium oxide aerogels were synthesized using esterification reaction for hydrolysis of the precursor and supercritical conditions of ethyl alcohol for drying. The obtained nanopowder was incorporated in SiO2 host matrix. After heating under natural atmosphere at 1150 degrees C for 2h, the composite Al2O3/SiO2 (AS) exhibited a strong PL bands at 400-600 and 700-900 nm in 78-300 K temperature range. PL excitation (PLE) measurements show different origins of the emission. It was suggested that OH-related radiative centres and non-bridging oxygen hole centres (NBOHCs) were responsible for the bands at 400-600 and 700-900 nm, respectively. (c) 2006 Elsevier Ltd. All rights reserved

EFFECTS ON PRESSURE ON THE SPECTRA, LIFETIMES AND ENERGY TRANSFER OF Eu P5O14, Tb P5O14 AND Eu1-xTbxP5O14

L'effet de la pression sur la fluorescence des ions Tb3+ et Eu3+ dans des cristaux mixtes Tbx Eu1-x P5O14 est étudié jusqu'à 210 kbars. Un fort transfert d'énergie Tb3+ → Eu3+ est observé et on montre que l'effet de la pression sur ce transfert est gouverné par une relaxation diffusion-limité. En utilisant la spectroscopie résolue dans le temps sous haute pression on trouve que ce transfert est plus rapide avec la pression.The effect of pressure (up to 210 kbars) on the fluorescence of Tb3+ and Eu3+ ions in the mixed single crystals Tbx Eu1-x P5O14 is investigated. A strong Tb3+ → Eu3+ energy transfer is observed and the pressure dependence of this transfer is shown to be governed by diffusion-limited relaxation. Using time-resolved spectroscopy under high pressure this energy transfer is found to be faster with pressure

Luminescence of composites based on oxide aerogels incorporated in silica glass host matrix

5th Maghreb/Europe Meeting on Materials and Their Applications for Devices and Physical, Chemical and Biological Sensors (MADICA 2006), Mahdia, TUNISIA, OCT 30-NOV 01, 2006International audienceThis paper deals with the elaboration and photoluminescence (PL) examination of oxide powder (OP = ZnO, Al2O3, SnO2, TiO2) incorporated in silica host matrix. OP, in the first step, was prepared by a sol-gel method using supercritical drying of ethyl alcohol. The obtained powder, in a second step, was incorporated in silica aerogel monolith using tetraethylorthosilicate as SiO2 precursor and ethyl alcohol supercritical conditions for drying. In the third step, silica aerogel containing oxide powders were heated under natural atmosphere at 1200 degrees C for 2 h to form the composites that exhibited a strong PL bands at Vis-IR range. Photoluminescence excitation (PLE) measurements show different origins of the emission. It was suggested that OH-related radiative centres and non-bridging oxygen hole centres (NBOHCs) defects resulting from thermal treatment and crystallization at the interfaces between OP and silica host matrix, were responsible for the bands at 400-600 and 700-900 nm respectively. (c) 2007 Elsevier B.V. All rights reserved