Modelling the intermixing effects in highly strained asymmetric InGaAs/GaAs quantum well

In this work, we have theoretically investigated the intermixing effect in highly strained In$_{0.3}$Ga$_{0.7}$As/GaAs quantum well (QW) taking into consideration the composition profile change resulting from in-situ indium surface segregation. To study the impact of the segregation effects on the postgrowth intermixing, one dimensional steady state Schrodinger equation and Fick's second law of diffusion have been numerically solved by using the finite difference methods. The impact of the In/Ga interdiffusion on the QW emission energy is considered for different In segregation coefficients. Our results show that the intermixed QW emission energy is strongly dependent on the segregation effects. The interdiffusion enhanced energy shift is found to be considerably reduced for higher segregation coefficients. This work adds a considerable insight into the understanding and modelling of the effects of interdiffusion in semiconductor nanostructures.Comment: 6 pages, 4 figure

Sensors data fusion for the absolute localisation of a mobile robot by a possibility theory based method. Comparison with Kalman filter

In order to improve autonomous system, it is necessary to determine accurately its position . In this paper, a method based on possibility theory has been developed in the experimental framework of the localisation of a miniature mobile robot from odometry reading and exteroceptive sensors into an environment equipped with beacons . The data are modelled in the setting of the possibility theory which provides interesting tools of representing imprecision and uncertainty . A comparison with a classical method (Kalman filter) taken as a reference is realised . Basically, the fusion procedure by Kalman filtering method can be seen as a weighted average by the information uncertainties . Its principle is to favour the information with low uncertainty (i.e. low variance) and to realise a fusion by "minimisation of variance" . On the other hand, the adaptive combination rule used in the possibilistic method takes into account the level of conflict between the sources and favour the redundancy of the information which are in agreement . Then, it is rather a fusion by "agreement" . In spite of these fundamental discrepancies, the outcomes of simulation and/or of experiences on the real robot, obtained by both methods are satisfactory and quite close .Pour améliorer l'autonomie d'un système mobile, il nécessaire de pouvoir déterminer précisément sa position. Dans cet article, une méthode basée sur la théorie des possibilités a été développée dans le cadre applicatif de la localisation d'un robot mobile miniature à partir de données odométriques et extéroceptives dans un environnement balisé. La modélisation des données est réalisée dans le cadre de la théorie des possibilités qui fournit des outils intéressants de représentation de l'imprécision et de l'incertitude. De plus, les contraintes souples qui régissent cette théorie permettent un choix étendu pour la combinaison des informations issues des différentes sources, allant du mode conjonctif au mode disjonctif en passant par le mode compromis. Nous nous sommes particulièrement intéressés ici à la règle de combinaison adaptative proposée par Dubois et Prade. Une chaîne complète de fusion des données odométriques et extéroceptives sous forme de distributions de possibilité est ainsi proposée et mise en oeuvre pour la localisation aussi bien en simulation que par des expériences réelles. Cette méthode fait l'objet d'une comparaison détaillée avec la méthode du filtrage de Kalman. Alors que la méthode de Kalman réalise une fusion par moyenne pondérée par l'incertitude liée aux informations (fusion par minimisation de la variance) la méthode possibiliste privilégie, elle, la redondance des informations fournies par les sources (fusion par concordance). Malgré ces différences fondamentales, les résultats obtenus par les deux approches sont assez voisines et la méthode possibiliste possède a priori une meilleure robustesse pour traiter des données conflictuelles

Time resolved and temperature dependence of the radiative properties of thiol-capped CdS nanoparticles films

In this work, we present the temperature-dependence and time-resolved photoluminescence (PL) of CdS nanoparticles capped independently with three different ligands thiophenol, thioglycerol, and l-cysteine over a broad temperature range from 10 to 300 K. The respective nanoparticles sizes in the three systems studied in this work are 1.5, 4, and 2 nm as determined from X-ray diffraction (XRD). From the analysis of AFM images, it was found that the lateral particle sizes of capped CdS nanoparticles are greater than those deduced from XRD or optical absorption measurements. The aim of this study is the investigation of the impact of the organic ligands on the radiative recombination dynamics in organically capped CdS nanoparticles. From the PL study and based on the temperature-dependence and time-resolved emission spectroscopy, we conclude that the emission of CdS QDs film originates from recombination of the delocalized carriers in the internal core states with a small contribution of the localized carriers at the interface. The PL decay reveals a biexponential behavior for the entire three samples at all temperatures. One of the two exponential components decays rapidly with a time τ(1) in the range 0.5–0.8 ns, whereas the other decays much more slowly, with a time τ(2) in the range 1–3 ns. The weak activation energy (32–37 meV) deduced from the temperature dependence of the PL intensity suggests the involvement of shallow traps. The analysis of the experimental results reveals a relatively narrow size distribution, an efficient surface passivation, and a satisfactory thermal stability of CdS nanocrystals

RHEED digital image analysis system for in-situ growth rate and alloy composition measurements of GaAs-based nanostructures

Monitoring the intensity of the reflected spot in a RHEED image is the most important method used to control the growth of semiconductors in MBE. The accurate control of both layer thickness and alloy composition is particularly important for the growth of high quality heterostructures. Indeed, under such conditions, extremely uniform and high quality epitaxial devices become possible. RHEED intensity oscillation can be used as accurate, quick and direct measure of the growth rates and alloy compositions as well. Although analog signal could be obtained by using an optical fiber associated to a photo-detector PM tube and a plotter, this method is troublesome and limited. In some application, the availability of the intensity signal as a digital voltage is useful to realize further advanced analysis, and achieve feedback between growth dynamic and the external parameters, such as the cells temperatures and the synchronization of the shutters. In this paper we describe the advantages gained with the upgrade of our analog system into a digital package using CCD camera, frame grabber and a home made software. Its main purpose is to track RHEED intensity changes and measures the rate of oscillation. A state-of-the-art RHEED digital image analysis system gives us the necessary tools to gain insight into the thin film growth process and optimize material quality

Optical anisotropy and photoluminescence temperature dependence for self-assembled InAs quantum islands grown on vicinal (001) InP substrates

International audienceIn this paper, we report on a detailed investigation of the effect of misorientated InP(OOl) substrates on the optical properties of InAs quantum islands grown by molecular beam epitaxy in the Stranski-Krastanow regime. Temperature-dependent photoluminescence and polarization of photoluminescence (PPL) are studied. PPL shows a high degree of linear polarization, near 40%, for the sample grown on the substrate with 2°off miscut angle towards [110] direction (2°F) and only 16% for the sample grown on the substrate with 2°off miscut angle towards [010] direction (2°B). This result pointing out the growth ofInAs quantum wires (QWr) on 2°F substrate and of quasi-isotropic InAs quantum dots (QD) on 2°B substrate. The luminescence remains strong at 300 K as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP(OOl)

Optical properties of self-assembled InAs quantum islands grown on InP(001) vicinal substrates

International audienceWe have investigated the effect of misorientated InP001 substrates on the optical properties of InAs quantum islands QIs grown by molecular-beam epitaxy in the Stranski-Krastanow regime. Detailed temperature-dependent photoluminescence PL, excitation density PL, and polarization of photoluminescence PPL are studied. PPL shows a high degree of linear polarization near 40% for the nominally oriented substrate n and for the substrate with 2° off miscut angle toward the 110 direction (2° F), while it is near 15% for the substrate with 2° off miscut angle towards 010 direction (2° B), indicating the growth of InAs quantum wires on nominal and 2° F substrates and of InAs quantum dots on 2° B substrate. These island shapes are confirmed by morphological investigations performed by atomic force microscopy. The integrated PL intensity remains very strong at room temperature, as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP001

RETRATO SIN IDENTIFICAR [Material gráfico]

Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte, 201

Etude de contacts métal-InP(n) clivé : barrière de Schottky et états d'interface

Au, Ag, AI and Pd-InP (n type) interfaces have been obtained by UHV cleavage and $in situ$ metallization. These interfaces have been characterized by electrical methods : $I- V$, $C- V$, SCS (Shottky Capacitance Spectroscopy). In spite of great variations in the reactivities of the deposited metals on InP, the Schottky barrier heights are quite similar: 0.37 eV (AI) ; 0.41 eV (An, Pd) ; 0.54 eV (Ag). The study of interface states by SCs shows the presence of two characteristic states whatever the metal localized near $E_c - 0,25$ eV and $E_c - 0,37$ eV. The latter could play a significant role in the Fermi level pinning.Des interfaces Au, Ag, AI et Pd-InP de type n ont été réalisées par clivage et métallisation $in situ$ sous ultra-vide. Les caractérisations ont été effectuées par des méthodes électriques $I-V$, $C-V$ et SCS (Spectroscopie de Capacité Schottky). Malgré les importantes différences de réactivité entre les métaux utilisés et InP les barrières de Schottky varient peu: 0,37 eV pour AI, 0,41 eV pour Au et Pd et 0,54 eV pour Ag. L'analyse des états d'interface par SCS met en évidence deux états communs à toutes les interfaces et localisés au voisinage de $E_c - 0,25$ eV et $E_c - 0,37$ eV. Ce dernier pourrait jouer un rôle important dans l'ancrage du niveau de Fermi