Quantitative Cathodoluminescence Opens New Areas of Investigation in Semiconductor Research and Production

The increasing demand for new opto-electronics devices such as solar cells, laser diodes (LD), and high-brightness light-emitting diodes (HBLED), combined with the economic necessity to achieve lower energy consumption levels and higher device yields, is motivating researchers to develop new materials. The semiconductor industry is actively looking for alternatives to silicon, for example, to address new niche market applications in power devices. Constant efforts employed to reduce production costs are leading manufacturers to grow GaN on silicon substrate, creating new technical challenges, especially regarding the control of defect density on wafer. For all these reasons many studies are being initiated to improve understanding of the fundamental physical properties and behavior of compound semiconductor materials used in quantum wells, quantum dots and nanowire-like structures. Cathodoluminescence (CL) is a spectroscopy method that can generate reliable, quantitative, and stable data for research as well as prepare a basis for quality control during productio

Analyser l’habileté motrice comme une « forme » : étude de cas en gymnastique

Cette étude visait à analyser l’activité motrice d’étudiants en STAPS réalisant le « saut de main » en gymnastique au sol. Le cadre de la Gestalttheorie a été convoqué pour mettre en correspondance les formes de corps observables au cours de la réalisation de l’habileté, c’est-à-dire des aspects du mouvement corporel, avec les significations vécues par l’acteur en cours d’action. Les analyses se sont appuyées d’une part sur une description de type morphologique des formes de corps produites par les participants, et d’autre part sur une analyse psycho-phénoménologique de leur expérience. Les deux types de données ont été articulés pour comprendre la cohérence des organisations motrices correspondant aux différents niveaux d’élaboration de l’habileté gymnique considérée. Les résultats ont ainsi permis de mettre en évidence les organisations motrices typiques de quatre niveaux de réalisation du saut de main

Arbitrer en EPS : une expérience de responsabilisation

Cette étude s’intéresse à l’expérience d’arbitrage d’élèves dans des leçons d’EPS. Centrée sur des données d’expérience vécue, cette étude les enrichit avec des données relatives à la motivation des élèves dans le but de mettre à jour leurs stratégies pour occuper le rôle d’arbitre et expérimenter la responsabilité inhérente à ce rôle. Les données d’expérience ont été recueillies et traitées dans le cadre théorique et méthodologique du cours d’action (Theureau, 2004, 2006). Les données motivationnelles ont été recueillies et traitées dans le cadre théorique de la motivation auto-déterminée (Deci & Ryan, 2002). Les résultats font apparaitre (1) trois préoccupations typiques (tenir le rôle d’arbitre, s’aider des autres, se délester du rôle d’arbitre), et (2) des formes d’amotivation, de régulation externe et de motivation intrinsèque à la connaissance et à l’accomplissement, ainsi qu’un besoin d’autonomie plus faible que ceux de compétence et d’affiliation. Sur cette base sont (1) discutés le repérage de stratégies d’alternance entre ces préoccupations typiques caractérisant chez les élèves leur façon d’assumer des responsabilités, et (2) proposées des aides à l’intervention pour concevoir des environnements encourageant les élèves à occuper le rôle d’arbitre à partir de gestalts.This research investigates students’ refereeing experience in physical education (PE) classes. Focused on lived experience datas, this study enhances them with students’ motivational datas in order to highlight their strategies to assume the role of referee and experiment with the specific responsabilities of this role. Experience datas were collected and processed in the theoretical framework of course-of-action (Theureau, 2004, 2006). Motivational datas were collected and processed in the theoretical framework of self-determination motivation (Deci & Ryan, 2002). Results showed (1) three typical concerns (fulfilling the role, getting help, and occasionally dropping out of the role), and (2) that students mainly expressed amotivation, external regulation and intrinsic motivation toward knowledge and accomplishment, with a need of autonomy lower that their needs of affiliation and competence. On this basis, were (1) discussed the tracking of alternation strategies between these typical concerns characterizing how students assume specific responsibilities and (2) proposed aid to intervention to design environments encouraging students’ to assume the role of referee from identification of gestalts

Engineering the spatial confinement of exciton-polaritons in semiconductors

We demonstrate the spatial confinement of electronic excitations in a solid state system, within novel artificial structures that can be designed having arbitrary dimensionality and shape. The excitations under study are exciton-polaritons in a planar semiconductor microcavity. They are confined within a micron-sized region through lateral trapping of their photon component. Striking signatures of confined states of lower and upper polaritons are found in angle-resolved light emission spectra, where a discrete energy spectrum and broad angular patterns are present. A theoretical model supports unambiguously our observations

Nanoindentation cracking in gallium arsenide: Part II. TEM investigation

The nanoindentation fracture behavior of gallium arsenide (GaAs) is examined from two perspectives in two parent papers. In the first paper (part I), we address the morphology of the crack field induced by different types of indenters by means of in situ nanoindentation inside a scanning electron microscope (SEM) and of cleavage cross-sectioning techniques. In the present paper (part II), we investigate the early stage of crack nucleation under wedge nanoindentation through cathodoluminescence and transmission electron microscopy. We find that the apex angle of the wedge indenter influences the dislocation microstructure and, as a consequence, the mechanism of crack nucleation under nanoindentation. The formation of microtwins depends on both the orientation of the indenter with respect to the orientation of the GaAs crystal and on the apex angle of the indenter. For dicing applications of GaAs wafers, it is desirable to have an opening angle of the indenter smaller than 70° to facilitate the formation of precursor crack

Picosecond Time-Resolved Cathodoluminescence to Probe Exciton Dynamics in α-Plane (Al,Ga)N/GaN Quantum Wells

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7-August 11, 201

Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

An original time resolved cathodoluminescence set up has been used to investigate the optical properties and the carrier transport in quantum structures located in InGaAs/AlGaAs tetrahedral pyramids. An InGaAs quantum dot formed just below the top of the pyramid is connected to four types of low-dimensional barriers: InGaAs quantum wires on the edges of the pyramid, InGaAs quantum wells on the (111)A facets and segregated AlGaAs vertical quantum wire and AlGaAs vertical quantum wells formed at the centre and at the pyramid edges. Experiments were performed at a temperature of 92K, an accelerating voltage of 10kV and a beam probe current of 10pA. The cathodoluminescence spectrum shows five luminescence peaks. Rise and decay times for the different emission wavelengths provide a clear confirmation of the peak attribution (previously done with other techniques) to the different nanostructures grown in a pyramid. Moreover, experimental results suggest a scenario where carriers diffuse from the lateral quantum structures towards the central structures (the InGaAs quantum dot and the segregated AlGaAs vertical quantum wire) via the InGaAs quantum wires on the edges of the pyramid. According to this hypothesis, we have modeled the carrier diffusion along these quantum wires. An ambipolar carrier mobility of 1400cm2/V s allows to obtain a good fit to all temporal dependence

Revisiting the transatlantic divergence over GMOs: Toward a cultural-political analysis

'Revisiting the transatlantic divergence over GMOs: towards a cultural-political analysis' applies a constructivist perspective to the persistent transatlantic divergence over the regulation of GM foods and crops. Political economy and institutionalism have so far dominated the literature. Notwithstanding their crucial insights, this article argues that to achieve a better understanding of the nature and depth of transatlantic regulatory divergence, one should also study prevalent cultural values and identity-related public concerns regarding food and agriculture. These factors can be identified in public opinion trends and have fuelled Europeans' resistance, while contributing to relative regulatory stability in the US. By conceptualizing cultural contexts as catalytic structures, the article also differs from more explicitly discursive accounts of political mobilization. The cultural politics of agricultural biotechnology (agbiotech) relies both on pre-existing values and identities, on the one hand, and on the strategies (and material or other power resources) of political agents

Exciton recombination dynamics in a-plane (Al,Ga)N/GaN quantum wells probed by picosecond photo and cathodoluminescence.

International audienceWe present a combined low-temperature time-resolved cathodoluminescence and photoluminescence study of exciton recombination mechanisms in a 3.8 nm thick a-plane (Al,Ga)N/GaN quantum well (QW). We observe the luminescence from QW excitons and from excitons localized on basal stacking faults (BSFs) crossing the QW plane, forming quantum wires (QWRs) at the intersection. We show that the dynamics of QW excitons is dominated by their capture on QWRs, with characteristic decay times ranging from 50 to 350 ps, depending on whether the local density of BSFs is large or small. We therefore relate the multiexponential behavior generally observed by time-resolved photoluminescence in non-polar (Al,Ga)/GaN QW to the spatial dependence of QW exciton dynamics on the local BSF density. QWR exciton decay time is independent of the local density in BSFs and its temperature evolution exhibits a zero-dimensional behavior below 60 K. We propose that QWR exciton localization along the wire axis is induced by well-width fluctuation, reproducing in a one-dimensional system the localization processes usually observed in QWs