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Thèse doctorat : Physique : Strasbourg 1 : 2006. / Titre provenant de l'écran-titre. Bibliogr. 5 p.

Thèse de doctorat : Automatique, Traitement du Signal et Génie informatique : Nancy 1 : 2008. / Titre provenant de l'écran-titre. Bibliogr.

Reproduction de : Thèse de doctorat : Automatique et informatique industrielle : Villeneuve d'Ascq, Ecole centrale de Lille : 2006. / Titre provenant de la page de titre du document numérisé. Bibliogr. p. 143-153.

The fabrication processes development for on III-nitride (III-N) heterojunction bipolar transistors (HBTs), heterojunction field-effect transistors (HFETs) and metal-insulator-semiconductor field-effect transistors (MISFETs) were performed. D.c, microwave and quasi-static I-V and C-V measurements were carried out to characterize the fabricated III-N transistors and diodes. The GaN/InGaN direct-growth HBTs (DG-HBTs) grown on free-standing GaN (FS-GaN) substrates demonstrated a high current gain (hfe) > 110, high current density (JC) > 141 kA/cm2, and high power density (Pdc) > 3 MW/cm2. The first III-N DG-HBT showing fT > 8 GHz and fmax > 1.3 GHz were also demonstrated on sapphire substrates. Recessed-gate AlGaN/AlN/GaN HFETs demonstrated Vth = 0 V with 0.17 V deviation across the sample. Baliga's figure of merit is 240 MW/cm2 was achieved. Current collapse was eliminated and the dynamic on-resistance was reduced by 67% after using a remote-oxygen-plasma treatment. Normally-off recessed-gate AlGaN/AlN/GaN MISFETs with Vth = 0.9 V were also fabricated with the remote-oxygen-plasma treatment. Low leakage current (< 1 pA/mm), high on-off ratio (> 2.2E11) are achieved. These achievements suggest that high-performance III-N transistors are very promising for high-power switching and microwave amplification. Findings concerning remaining process issues and implications for future research are also discussed.

An advancement of the two stage growth recipe for the fabrication of CIGS solar cells was developed. The developed advancement was inconsistent in producing samples of similar stoichiometry. This was a huge barrier for up scaling the process as the behavior of devices would be different due to variation in stoichiometry. Samples with reproducible stoichiometry were obtained once the heating rate of elements, selenium in particular was better understood. This is mainly attributed to the exponential increase of selenium flux after its evaporation temperature. Monitoring the selenium flux was vital in getting constant selenium fluxes. Few changes to the growth recipe were induced to optimize the amount of selenium being used. Depositions were done using constant selenium to metal flux ratio of 5. Elemental tradeoffs were observed as a result of the growth recipe change. These tradeoffs are in favor of the two stage growth recipe. The solar cells were fabricated on a soda lime glass substrate with a molybdenum back contact. Improper sample cleaning and storage were found to affect the deposition outcome of the molybdenum back contact. This also had a cascading effect on the absorber layer. Residual precipitates during deposition of CdS were avoided by increasing the spinner speed which increased the reaction rate. This is attributed to the growth of CdS either by cluster-by-cluster growth or by ion-by-ion growth. SEM, EDS were some important tools used to characterize the devices. EDS in particular, was used extensively at different stages throughout the growth process to ensure that we were heading in the right direction. Current-voltage (I-V) measurements were done to study the solar cell performance under light and dark.

Laser Sintering, a method of additive manufacturing, is used in the production of concept models, functional prototypes, and end-use production parts. As the technology has transitioned from a product development tool to an accepted production technique, functional qualities have become increasingly important. Tension properties reported for popular polyamide sintering materials are comparable to the molded properties with the exception of elongation. Reported strains for laser sintered polyamide are in the 15-30% range with 200-400% strains reported for molding. (CES Edupack n.d.) The primary contributors to poor mechanical properties in polyamide materials used during Selective Laser Sintering® are studied. Methods to quantify decreased mechanical properties are compared against each other and against mechanical properties of components fabricated using multiple process parameters. Of primary interest are Ultimate Tensile Strength (UTS) and Elongation at Break (EOB) of tensile specimens fabricated under conditions that produce varying degrees of ductile and brittle fracture. / text

Solar cells are at the frontier of renewable energy technologies. Photovoltaic energy is clean, reusable, can be used anywhere in our solar system and can be very well integrated with power distribution grids and advanced technological systems. Thin film solar cells are a class of solar cells that offer low material cost, efficient fabrication process and compatibility with advanced electronics. However, as of now, the conversion efficiency of thin film solar cells is inferior to that of thick crystalline cells. Research efforts to improve the performance bottlenecks of thin film solar cells are highly motivated. A class of techniques towards this goal is called light trapping methods, which aims at improving the spectral absorptivity of a thin film cell by using surface texturing. The precise mathematical and physical characterization of these techniques is very challenging. This dissertation proposes a numerical and computational framework to optimize, design, and fabricate efficient light trapping structures in thin film solar cells, as well as methods to verify the fabricated designs. The numerical framework is based on the important "inverse optimization" technique, which is very is widely applicable to engineering design problems. An overview of the state-of-the-art thin film technology and light trapping techniques is presented in this thesis. The inverse problem is described in details with numerous examples in engineering applications, and is then applied to light trapping optimization. The proposed designs are studied for sensitivity analysis and fabrication error, as other aspects of the proposed computational framework. At the end, reports of fabrication, measurement and verification of some of the proposed designs are presented. / text

The modification of spontaneous emission, i.e. the Purcell effect, with optical cavities has been highly studied over the past 20 years as one of the most important goals for cavity quantum electrodynamics (cQED). The recent development of using surface plasmon resonances to concentrate optical field into sub-wavelength scale further extended cQED research of into a new regime. However, although metallic reflectors are used in some of the earliest demonstrations of cQED, the use of metals is not preferable in high Q optical cavities due to the lossy nature of metals. The presence of metals near an optical emitter also strongly alters its radiation dynamics. As a result, the development of plasmonic cavities brings not only new opportunities but also new problems and challenges. In this thesis we describe four different plasmonic cavity designs along with optical simulations and measurements on them to demonstrate: large spontaneous emission enhancement, controlled mode tuning, and control of the plasmonic band-gap and resonances of high-Q plasmonic cavities for coupling to specific emitters. We hope that our work can guide and inspire researchers who are moving from traditional cavity designs to novel plasmonic devices, helping them to establish design concepts, fabrication criteria, and baselines for characterizing these devices. / Engineering and Applied Sciences

In an effort to gain a more comprehensive and complete understanding of the thermal behaviors occurring during the selective laser sintering process, external temperature measurements were taken during the build process. To accomplish this, an infrared camera was aimed directly through a viewport on the front of the sinterstation. The temperature was monitored during the heating process which showed slightly non-uniform heating of the part bed surface. Temperatures were also recorded while the laser was sintering each layer and the subsequent cooling of the entire machine following the build. By directly capturing infrared images of the part bed’s surface, it is clearer how the temperature gradients behave and the impact such variables have on part build efficiency. / text