INFLUENCE DU PROCESSUS D'ELABORATION SUR LES DEFAUTS CRISTALLOGRAPHIQUES DANS LES COUCHES DE GaAs EPITAXIEES PAR JETS MOLECULAIRES

Nous présentons une étude sur Les défauts cristallographiques présents dans les couches d'AsGa épitaxiées par la technique des jets moléculaires. La principale technique de caractérisation utilisée est l'attaque chimique sélective, assistée de techniques complémentaires telles que microscopie électronique à transmission et microsonde électronique. Cette étude a été réalisée en vue de mettre en évidence l'effet des dégâts mécaniques, de la contamination et des défauts liés aux impuretés sur la présence de dislocations, de fautes d'empilement et de défauts macroscopiques affectant la morphologie de surface de la couche. En effet pour chaque défaut considéré, une étude est faite pour clarifier son origine et l'influence du choix du substrat, de sa préparation et des conditions de croissance, est prise en compte. Cela est réalisé par le biais d'épitaxies simultanées réalisées sur substrats différents et de l'analyse d'un grand nombre de couches. Enfin, sur la base des tendances observées, des moyens pour réduire la densité des défauts sont proposés.A study of crystallographic defects in MBE GaAs Layers is presented. The main technique used for this purpose was selective chemical etching assisted by others techniques such as transmission electron microscopy and electron microprobe when necessary. This study has been done in order to show the effect of mechanical damages, contamination and impurity related defects on the presence of dislocations, stacking faults and macroscopic defects affecting the surface morphology of the layer. For each defect considered, an attempt is made to clarify its origin and the influence of substrate choice, substrate preparation and growth conditions is taken into account. This is done in the light of the experimental results obtained on a large number of layers and on some particular results obtained by simultaneous epitaxy on different substrates. Finally, on the basis of the trends observed, means for defect density reduction are proposed

Self-Powered Adaptive Switched Architecture Storage

International audienceAmbient energy harvesting coupled to storage is a way to improve the autonomy of wireless sensors networks. Moreover, in some applications with harsh environment or when a long service lifetime is required, the use of batteries is prohibited. Ultra-capacitors provide in this case a good alternative for energy storage. Such storage must comply with the following requirements: a sufficient voltage during the initial charge must be rapidly reached, a significant amount of energy should be stored and the unemployed residual energy must be minimised at discharge. To answer these apparently contradictory criteria, we propose a self-adaptive switched architecture consisting of a matrix of switched ultra-capacitors. We present the results of a self-powered adaptive prototype that shows the improvement in terms of charge time constant, energy utilization rate and then energy autonomy

Under Voltage Lock-Out Design Rules for Proper Start-Up of Energy Autonomous Systems Powered by Supercapacitors

International audienceThis paper deals with the issue of the initial start-up of an autonomous and battery-free system powered by an energy harvester associated with a storage subsystem based on supercapacitors initially discharged. A review of different low power Under Voltage Lock-Out (UVLO) solutions used to delay the load start-up and to avoid a useless discharge of supercapacitors is presented and discussed

ESD failure signature in capacitive RF MEMS switches

RF MEMS are commonly known as electrostatic devices using high electric field for their actuation. They can be exposed to transient voltages in any environment, and are very sensitive. According to this point of view, it is necessary to understand and analyze the degradations and failure criteria that can make them useless or reduce their lifetime. This paper deals with the investigation of ESD failure signature in capacitive RF MEMS. ESD experiments were carried out using a transmission line pulsing technique. It has been observed that electrical discharges give rise to sparks or electrical arcing and induced DC parameter shift, which can directly lead to changes in RF metrics. The contact-less dielectric charging effects of ESD pulses have been reported in this paper. It has been found that induced charges are predominant compared to injected ones through the trend of slope of the shift in the voltage corresponding to the minimum of capacitance. © 2008 Elsevier Ltd. All rights reserved

Self-Powered energy harvester strain sensing device for structural health monitoring

International audienceThis paper presents an envisaged autonomous strain sensor device, which is dedicated to structural health monitoring applications. The paper introduces the ASIC approach that replaces the discrete approach of some of the main modules