Thesis (Ph.D.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Graham, Samuel; Committee Co-Chair: Nair, Sankar; Committee Member: Grover - Gallivan, Martha; Committee Member: McDowell, David; Committee Member: Schelling, Patrick; Committee Member: Zhang, Zhuomin

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 191-206).

Cette thèse s’inscrit dans le cadre du développement d’une filière de transistors de puissance à base de nitrure de Gallium au CEA-LETI. Ces transistors, en particulier les HEMT utilisant l’hétérostructure AlGaN/GaN, présentent des propriétés très utiles pour les applications de puissance. L’essor de cette technologie passe notamment par le développement de contacts ohmiques peu résistifs. Cette thèse a pour objectif d’approfondir la compréhension des mécanismes de formation du contact ohmique sur une structure AlGaN/GaN. Dans un premier temps, une étude thermodynamique sur une dizaine de métaux de transition utilisables comme base de l’empilement métallique du contact a été menée, ce qui a permis de retenir une métallisation Ti/Al. Puis, les différentes réactions physico-chimiques de cet empilement avec des substrats nitrurés ont été étudiées en faisant varier la composition et les températures de recuit de formation du contact ohmique. Enfin, plusieurs études sur structure AlGaN/GaN couplant caractérisations électriques et physico-chimiques ont permis d’identifier des paramètres décisifs pour la réalisation d’un contact ohmique, peu résistif et nécessitant une faible température de recuit. / This PhD is part of the development of Gallium nitride based power transistors at the CEA-LETI. These transistors, especially those based on AlGaN/GaN heterostructure, are very promising for power electronics applications. The goal of this PhD is to increase the knowledge of the mechanisms responsible for the ohmic contact formation on a AlGaN/GaN structure. First, a thermodynamic study of several transition metals has been performed, leading us to select Ti/Al metallization. Then, the multiple physico-chemical reactions of this stack with nitride substrates have been studied depending on the stack composition and the annealing temperature. Finally, several studies on AlGaN/GaN structure coupling both physico-chemical and electrical characterizations reveal different decisive parameters for the formation of an ohmic contact with a low-resistance and a low annealing temperature.

Mélanger un oxyde de gallium avec une zéolithe H-ZSM-5 donne les mêmes résultats catalytiques en craquage du n-hexane, déshydrogénation du cyclohexane et en aromatisation du propane qu'un catalyseur préparer par échange cationique avec un sel de gallium. En fait, le véritable catalyseur est synthétisé lors du prétraitement sous hydrogène pendant lequel le suboxyde de gallium (Ga2O) issu de la réduction de Ga2O3 réagit avec les sites de Brønsted de la zéolithe pour donner des hydrures de gallium. La réaction de déshydrogénation des alcanes fait intervenir un site catalytique bifonctionnel composé d'un site de Lewis du Ga et d'un site basique généré par l'oxygène de la charpente zéolithique. L'activation du propane se produit sur un hydrure de gallium via un mécanisme de type alkyle. Les aluminosilicates dopés avec Ga sont plus performants que les gallosilicates, ce qui signifie que les espèces de gallium sont plus actives en extra-réseau que dans le réseau de la zéolithe.Le coke généré lors de l'aromatisation du propane est très polyaromatique avec plus de quinze noyaux benzéniques, localisé dans les micropores il s'avère très toxique. La création de mésopores intracristallins sans modifier les propriétés acides de la zéolithe (nombre et force des sites acides) est possible par un traitement alcalin. Leur présence permet de limiter les réactions de transfert d'hydrogène mais est peu efficace pour contrôler la croissance du coke, les mésopores sont mêmes négatifs pour la réaction de déshydrogénation rendant les catalyseurs bifonctionnels hiérarchisés inefficaces en aromatisation du propane ; l'étape cinétiquement limitante pour cette réaction étant la déshydrogénation. / The mixing Ga2O3 with the H-ZSM-5 zeolite yields to the same catalytic performance in n-hexane cracking, cyclohexane dehydrogenation and propane aromatization than a bifunctional catalyst prepared by cationic exchange. The real catalyst appears upon hydrogen pretreatment in which gallium (Ga2O) suboxide that results from Ga2O3 reduction, reacts with the zeolite Brønsted sites to yield to gallium hydrides.The dehydrogenation reaction of alkanes involves a bifunctional catalytic site constituted of a Lewis site (Ga species) and basic site (an oxygen of the zeolite framework). The aluminosilicate catalysts loaded with Ga are more efficient than the gallosilicate catalysts, therefore extraframework gallium species is more active than the framework gallium species.The coke formed during the propane aromatization is very polyaromatic with more than fifteen benzenic rings, is very toxic. The creation of intracrystalline mesopores by alkaline treatment.preserves the acidic properties of the zeolite (number and strength of acidic sites). The mesopores allow limiting the hydrogen transfer reactions but is not very effective for impeding the growth of the coke, the presence of mesopores are even negative for the dehydrogenation reaction making inefficient the hierarchical bifunctional catalysts in propane aromatization; the kinetically limiting step for this reaction being dehydrogenation.

Light from the sun is an attractive source of energy for its renewability, supply, scalability, and cost. Silicon solar cells are the dominant technology of choice for harnessing solar energy in the form of electricity, but the designs are approaching their practical efficiency limits. New multijunction designs which use the tunable properties of the more expensive III-V semiconductors have historically been relegated to space applications where absolute power conversion efficiency, resilience to radiation, and weight are more important considerations than cost. Some of the more recent developments in the field of semiconductor materials are the so-called III-nitride materials which mainly use either indium, aluminum or gallium in combination with nitrogen. Indium gallium nitride (InGaN) is one of these III-nitride semiconductor alloys that can be tailored to span the vast majority of the solar spectrum. While InGaN growth traditionally requires expensive substrate materials such as sapphire, three-dimensional nanowire growth modes enable high quality lattice mismatched growth of InGaN directly on silicon without a metamorphic buffer layer. The absorption and electronic properties of InGaN can also be tuned by incorporating it into quantum confined regions in a GaN host material. This opens up a route towards cost-effective, high efficiency devices such as light emitted diodes and solar cells which can operate over a large range of wavelengths. The combination of the two material systems of InGaN/GaN and silicon can marry the low cost of silicon wafers with the desirable optoelectronic properties of III-nitride semiconductors. This thesis investigates the potential for highly nanostructured InGaN/GaN based devices using quantum-dot-in-nanowire designs as novel solar cells which can enable intermediate band absorption effects and multiple junctions within a single nanowire to absorb more of the solar spectrum and operating more efficiently. Such semiconductor nanostructures can in principle reach power conversion efficiencies of over 40\% on silicon, with a cost closer to conventional silicon solar cells as opposed to methods which use non-silicon substrates. In the primary strategy, the nanowires contain InGaN quantum dots which act as photon absorption/carrier generation centres to sequentially excite photons within the large band gap semiconductor. By using this intermediate band of states, large operating voltages between contacts can be maintained without sacrificing the collection of long wavelength solar photons. In this work, we characterize the properties of such nanowires and experimentally demonstrate sub-bandgap current generation in a large area InGaN/GaN dot-in-nanowire solar cell. Experimental characterization of InGaN / GaN quantum dots in nanowires as both LEDs and solar cells is performed to determine the nanowire material parameters to understand how they relate to the nanowire device performance. Multiple microscopy techniques are performed to determine the nanowire morphology and contact effectiveness. Optical characterization of bare and fabricated nanowires is used to determine the anti-reflection properties of nanowire arrays. Photoluminescence and electroluminescence spectroscopy are performed. Illuminated current-voltage characteristics and quantum efficiencies are determined. Specular and diffuse reflectivities are measured as a function of wavelength. Technology computer-aided design (TCAD) software is used to simulate the performance of the overall nanowire device. The contribution from quantum dots or quantum wells is simulated by solving for the carrier wavefunctions and density of states with the quantum structures. The discretized density of states from the quantum dots is modelled and used in a complete drift-diffusion device simulation to reproduce electroluminescence results. The carrier transport properties are modified to demonstrate effects on the overall device performance. An alternate design is also proposed which uses an InGaN nanowire subcell on top of a silicon bottom subcell. The dual-junction design allows a broader absorption of the solar spectrum, increasing the operating voltage through monolithically grown series-connected, current-matched subcells. The performance of such a cell is simulated through drift-diffusion simulations of a dual-junction InGaN/Si solar cell. The effects of switching to a nanowire subcell based on the nanowires studied in this thesis is discussed.

abstract: The state of the solar industry has reached a point where significant advancements in efficiency will require new materials and device concepts. The material class broadly known as the III-N's have a rich history as a commercially successful semiconductor. Since discovery in 2003 these materials have shown promise for the field of photovoltaic solar technologies. However, inherent material issues in crystal growth and the subsequent effects on device performance have hindered their development. This thesis explores new growth techniques for III-N materials in tandem with new device concepts that will either work around the previous hindrances or open pathways to device technologies with higher theoretical limits than much of current photovoltaics. These include a novel crystal growth reactor, efforts in production of better quality material at faster rates, and development of advanced photovoltaic devices: an inversion junction solar cell, material work for hot carrier solar cell, ground work for a selective carrier contact, and finally a refractory solar cell for operation at several hundred degrees Celsius. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2016

Made available in DSpace on 2014-10-09T12:24:10Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:22Z (GMT). No. of bitstreams: 1 00931.pdf: 2473042 bytes, checksum: d56b30414065c63c73322142142d66e6 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2016-02-03T12:08:21Z No. of bitstreams: 0 / Made available in DSpace on 2016-02-03T12:08:21Z (GMT). No. of bitstreams: 0 / Tese (Doutorado em Tecnologia Nuclear) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Orientador: Maria José Santos Pompeu Brasil / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física "Gleb Wataghin" / Made available in DSpace on 2018-08-17T05:37:13Z (GMT). No. of bitstreams: 1 GonzalezBalanta_MiguelAngel_M.pdf: 3522641 bytes, checksum: 4f38baba120bb574a54a03d7c1ebc335 (MD5) Previous issue date: 2010 / Resumo: Nesta dissertação investigamos os efeitos dos íons de Mn na dinâmica do spin de elétron em poços quânticos de InGaAs/GaAs. Os poços têm um gás de buracos gerado por dopagens em suas barreiras, sendo uma dopagem tipo delta de Mn numa das barreiras e uma dopagem tipo delta de C, na outra. A densidade de buracos foi determinada mediante medidas de transporte e são consistentes com as densidades obtidas das energias de Stokeshift. Utilizamos diversas técnicas ópticas, como a fotoluminescência no modo contínuo (PL-CW) e resolvida no tempo (PL-RT), a fotoluminescência de excitação (PLE-CW), e o efeito Hanle óptico, sempre usando luz circularmente polarizada para excitação e analisando a polarização circular da luz emitida. Comparamos os tempos de vida ( ? ) e de relaxação do spin ( ? s) dos elétrons obtidos através destas técnicas e discutimos as diferenças intrínsecas destes métodos e o significado físico dos parâmetros fornecidos por eles. Analisamos também o efeito da presença dos íons de Mn, que são íons magnéticos, sobre os tempos vida e de spin dos elétrons em uma série de amostras com diferentes quantidades de Mn incluindo a amostra de referencia sem Mn. Os resultados encontrados revelam um limite para a concentração de Mn, para a qual ambos, ? e ? s, apresentam uma queda abruta. Surpreendentemente, esta queda não afeita o grau de polarização CW, pois a razão ?/? s que determina este parâmetro permanece basicamente constante para todas as amostras / Abstract: We have studied the effect of Mn ions on the spin-relaxation of electrons in a InGaAs/GaAs quantum well (QW). The QW has a two-dimensional hole-gas generated by doping the barriers, whereas one of the barriers has a Mn-planar layer and the other one, a C planarlayer. The hole densities were determined by Shubnikov-de-Haas oscillations and are consistent with the Stokes-shift energies obtained by optical measurements. We have performed continuouswave photoluminescence measurements (CW-PL), excitation photoluminescence (CW-PLE), timeresolved (TR-PL), and Hanle effect with circularly polarized excitation and detection. We compare the lifetime ( ? ) and the spin relaxation time ( ? s) obtained using those techniques and we discuss the differences between the various techniques and the physical meaning of those parameters. We also analyze the effect of Mn ions on ? s and ? for the series of samples with different Mn concentrations, including a reference sample with no Mn doping. The results revealed a threshold of Mn concentration at which both, and ? s, show a strong and abrupt fall. Surprisingly, this fall does not affect the CW effective polarization degree, since the ratio s that determines this parameter remains basically constant for all samples / Mestrado / Física da Matéria Condensada / Mestre em Física

Made available in DSpace on 2014-10-09T12:24:10Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:22Z (GMT). No. of bitstreams: 1 00931.pdf: 2473042 bytes, checksum: d56b30414065c63c73322142142d66e6 (MD5) / Dissertacao (Mestrado) / IEA/D / Instituto de Fisica, Universidade de Sao Paulo - IF/USP