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The direct epitaxial growth of multilayer BN by atomic layer deposition is of critical significance forfo two-dimensional device applications. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) demonstrate layer-by-layer BN epitaxy on two different substrates. One substrate was a monolayer of RuO2(110) formed on a Ru(0001) substrate, the other was an atomically clean Ni(111) single crystal. Growth was accomplished atomic layer deposition (ALD) cycles of BCl3/NH3 at 600 K substrate temperature and subsequent annealing in ultrahigh vacuum (UHV). This yielded stoichiometric BN layers, and an average BN film thickness linearly proportional to the number of BCl3/NH3 cycles. The BN(0001)/RuO2(110) interface had negligible charge transfer or band bending as indicated by XPS and LEED data indicate a 30° rotation between the coincident BN and oxide lattices. The atomic layer epitaxy of BN on an oxide surface suggests new routes to the direct growth and integration of graphene and BN with industrially important substrates, including Si(100). XPS and LEED indicated epitaxial deposition of h-BN(0001) on the Ni(111) single crystal by ALD, and subsequent epitaxially aligned graphene was deposited by chemical vapor deposition (CVD) of ethylene at 1000 K. Direct multilayer, in situ growth of h-BN on magnetic substrates such as Ni is important for spintronic device applications. Solid-state electrolytes (SSEs) are of significant interest for their promise as lithium-ion conducting materials but are prone to degradation due to lithium carbonate formation on the surface upon exposure to atmosphere, adversely impacting Li ion conduction. In situ XPS monitored changes in the composition of the SSE Li garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTaO) upon annealing in UHV and upon Ar+ ion sputtering. Trends in core level spectra demonstrate that binding energy (BE) calibration of the Li 1s at 56.4 eV, yields a more consistent interpretation of results than the more commonly used standard of the adventitious C 1s at 284.8 eV. Annealing one ambient-exposed sample to >1000 K in UHV effectively reduced surface carbonate and oxygen, leaving significant amounts of carbon in lower oxidation states. A second ambient-exposed sample was subjected to 3 keV Ar+ ion sputtering at 500 K in UHV, which eliminated all surface carbon, and reduced the O 1s intensity and BE. These methods present alternative approaches to lithium carbonate removal than heating or polishing in inert atmospheres and are compatible with fundamental surface science studies. In particular, the data show that sputtering at mildly elevated temperatures yields facile elimination of carbonate and other forms of surface carbon. This is in contrast to annealing in either UHV or in noble gas environments, which result in carbonate reduction, but with significant remnant coverages of other forms of carbon.

To design high pressure fuel system components that resist wear and scuffing failure when operated in low viscosity fuels, a comprehensive study on the tribological performance of various existing coating materials is necessary. This thesis aims to provide the relative performance of a variety of coating materials across different fuel environments by testing them in conditions that model those experienced in fuel pumps. The relative performance of these coatings are then indexed across a variety of material properties, including hardness, elastic modulus, wettability, and the interaction between the surface and the various types of fuel molecules.

Two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are of great interest for electronic and optoelectronic applications. However, a major challenge for both fundamental studies and practical applications of TMD-based devices lies in the difficulty of establishing low-resistance, reliable electrical contacts. Conventional direct metal evaporation methods often bcauses physical damage at the metal-TMD interface due to the high kinetic energy of the metalatoms. This results in the formation of chemical defects at the contact interface, leading to Fermi level pinning and high contact resistance. While strategies like using low-melting-point metals or transferred metal contacts have shown promise in forming van der Waals contacts and reducing contact resistance, most prior research has focused on TMDs grown by chemical vapor deposition (CVD) method and devices built on silicon dioxide/silicon (SiO₂/Si) substrates. In these cases, atomic defects in the TMDs and surface charges from the substrate can significantly degrade device performance. Our lab has addressed these limitations by growing ultra-high-quality TMD crystals using a two-step flux growth method and minimizing extrinsic disorder in the TMD devices through hexagonal boron nitride (hBN) encapsulation. This thesis aims to study the high-performance electrical contacts to ultra-high-purity monolayer TMDs encapsulated within hBN. Chapter 1 introduces the fundamentals of 2D TMDs and the challenges associated with achieving low-resistance contacts. Chapter 2 describes high-quality TMD crystals and hBN encapsulation technique developed to minimize disorder in 2D TMD devices. Chapter 3 examines low-melting-point bismuth (Bi) semimetal contacts to ultra-clean MoSe₂ monolayers, demonstrating that Bi forms damage- and strain-free van der Waals contacts to high-purity MoSe₂. To characterize these devices, contact-front and contact-end measurements were combined to unambiguously extract key metal-semiconductor junction parameters that could not be accurately determined using the standard transfer length method (TLM). Additionally, pre-etched hBN was employed as a self-aligned mask, which mechanically stabilizes the weakly coupled van der Waals contacts. This technique enables the scaling of contact lengths from the micron scale down to tens of nanometers. In these deeply scaled contacts, both two-probe resistance and end resistance increased with the characteristic length 𝐿_𝑡 , confirming the calculated transfer length. Chapter 4 explores a novel in situ via contact technique for air-sensitive monolayer TMDs. This method uses plasma etching and metal deposition to create ’vias’ in the hBN with graphene forming an atomically thin etch-stop. The resulting partially fluorinated graphene (PFG) protects the underlying device layer from air-induced degradation and damage during metal deposition. Using the in situ via technique, an ambipolar contact to air-sensitive monolayer 2H-molybdenum ditelluride (MoTe₂) was achieved with more than one order of magnitude improvement in on-current density compared to previous literature. The complete encapsulation provides high reproducibility and long-term stability. This contact technique was also extended to other air-sensitive materials as well as air-stable materials, offering highly competitive device performance. In summary, these studies lay a solid foundation for future research on high-quality devices based on TMD monolayers, which are critical for both practical applications and the exploration of their intrinsic properties.

Les travaux réalisés dans le cadre de cette thèse ont porté sur la réalisation de matériaux composites nanostructurés à base de nanotubes de carbone verticalement alignés (NTC alignés) et de polymères π-conjugués en vue de leur utilisation en tant que matériaux d’électrodes dans des dispositifs de stockage d’énergie de type supercondensateurs. Dans une première partie, les travaux se sont focalisés sur la croissance par CVD d’aérosol de NTC sur des substrats d’acier inoxydable via le dépôt préalable d’une sous-couche céramique SiOx. Grâce à l’optimisation de ce procédé, des tapis de NTC longs, denses et alignés pouvant directement servir de supports à l’électrodépôt de polymères π-conjugués ont pu être obtenus. Dans une seconde partie, les travaux se sont concentrés sur l’électrodépôt de poly(3-méthylthiophène) (P3MT) en milieu liquide ionique EMITFSI sur les tapis de NTC alignés à partir d’une méthode chronopotentiométrique « séquencée » permettant de réaliser des dépôts homogènes dans la profondeur des tapis. Une composition massique optimale de 70 % de P3MT permettant d’atteindre des capacitances spécifiques de 170 F.g-1 de polymère tout en conservant des cinétiques de charge-décharges élevées, comparativement à des composites NTC/P3MT enchevêtrés, a pu être déterminée. A partir des matériaux composites optimisés, des dispositifs symétriques NTC/P3MT // P3MT/NTC et hybrides CA // P3MT/NTC ont été assemblés. Le dispositif hybride à notamment permis d’atteindre une tension de 2,7 V et une capacitance de système de 26 F.g-1 en milieu EMITFSI à 25 °C. Par ailleurs, une énergie maximale de 23 Wh.kg-1 et une puissance maximale de 6,9 kW.kg-1 ont été obtenues avec une perte de seulement 7 % après 4000 cycles. Pour finir, l’électrodépôt de polypyrrole (Ppy) a été étudié dans différents milieux liquides ioniques protiques et aprotiques. Après des études réalisées par microbalance à cristal de quartz permettant de mieux comprendre les mécanismes d’insertion des espèces ioniques lors de la croissance du polymère conjugué et lors de son dopage positif réversible, des dépôt de Ppy ont été réalisés et optimisés dans la profondeur des tapis de NTC alignés. Des nanocomposites NTC alignés/Ppy présentant des capacitances spécifiques comprises entre 100 et 130 F.g-1 ont ainsi pu être obtenus. / This thesis focused on the elaboration of nanostructured composite materials based on vertically aligned carbon nanotubes (aligned CNT) and π-conjugated polymers and their use as electrode materials in supercapacitor-type energy storage devices. The first part focused on aligned CNT growth by aerosol-assisted CVD on stainless steel substrates and the deposition of a SiOx ceramic sublayer. Thanks to the optimization of this growth process, long, dense, and aligned CNT carpets which can directly act as support for the electrodeposition of π-conjugated polymers were obtained. The second part focused on the electrodeposition of poly (3-methylthiophene) (P3MT) in EMITFSI ionic liquid medium on aligned CNT carpets using a “pulsed” chronopotentiometric method to produce homogeneous deposits in the depth of the carpets. An optimal P3MT mass composition of 70 %, which helped achieve a specific capacitance of 170 F.g-1 of polymer while maintaining high charge-discharge kinetics, compared with NTC/P3MT entangled composites, was determined. NTC/P3MT // P3MT/NTC symmetrical devices and CA // P3MT/NTC hybrid devices were assembled using the optimized composite materials. The hybrid device reached a voltage of 2.7 V and a system capacitance of 26 F.g-1 in EMITFSI at 25 ° C. Furthermore, a maximum energy of 23 Wh.kg-1 and a maximum power of 6.9 kW.kg-1 were obtained with only a 7 % loss after 4000 cycles. Finally, the electrodeposition of polypyrrole (Ppy) was investigated in different protic and aprotic ionic liquids. After quartz crystal microbalance studies in order to better understand the insertion mechanisms of ionic species during conjugated polymer growth and during its reversible positive doping, the electrodeposition of Ppy within the deepness of the aligned CNT carpets was optimized. Aligned CNT/Ppy nanocomposites with specific capacitances ranging between 100 and 130 F.g-1 were obtained.

L’augmentation du nombre d’interfaces dans une matrice céramique permet d’améliorer sa ténacité. L’étude de la structure feuilletée de la nacre a démontré que cette ténacité pouvait être accrue par la présence de pontages entre les couches. Dans la première partie, le modèle de décohésion proposé par Pompidou et al. a été utilisé pour choisir un bicouche dont l’interface est naturellement favorable aux décohésions. Compte tenu du contexte de l’étude, cette analyse a permis de choisir le couple SiC/pyC comme bi-couche de base pour l’étude des interfaces. Par la suite, des matrices multicouches modèles (SiC/pyC)n (SiC, carbure de silicium issu du mélange CH3SiCl3/H2 – pyC, pyrocarbone à partir du propane) ont été élaborées par dépôt chimique en phase vapeur (CVD). Deux voies de pontage ont été abordées. La première met en œuvre une discontinuité entre les couches : les conditions d’élaboration ont été optimisées de façon à contrôler la croissance de couches minces massives et le développement de particules de surface (submicroniques) faisant office de pontage. La deuxième est basée sur un gradient de composition entre les couches de SiC grâce au développement d’une couche de SiC riche en co-dépôt de carbone, une interphase mixte est créée. Le pontage est assuré par la présence simultanée dans les couches à gradient de composition de grains de SiC et d’une phase carbonée. Les propriétés physico-chimiques et structurales des différents éléments des matrices ont été analysées et les différents comportements des fissures dans chacune des matrices ont été observés à la suite d’essais mécaniques. / The improvement of ceramic matrix toughness may be achieved through the presence of interfaces. Moreover, studies on a mother of pearl structure have shown the usefulness of mineral bridges between the layers. On the first part of this work, the Pompidou model was used for the selection of a bi-layered ceramic with an interface which is naturally favorable to crack deflection. SiC/pyC was taken as basic material for the interfaces study. Then, multilayered ceramic matrices (SiC/pyC)n (silicon carbide from CH3SiCl3/H2 mixture – pyC from propane) were fabricated using chemical vapor deposition (CVD). In the study, two bypass ways were proposed. On the one hand, a physical discontinuity exists between the different layers: elaboration parameters were optimized in order to develop both bulk layers and submicronic surface particles, acting as ceramic bypass. On the other hand, composition gradient films were developed between each SiC layers: by realizing carbon rich SiC layers, a mixed interphase was created. The presence of both SiC grains and carbon phases ensures the bypass structure. Physico-chemical and structural properties of multilayered ceramic matrices were analyzed and the crack propagation in each of them was observed following mechanical tests.

The possibility of using methylamines instead of ammonia as a nitrogen precursor for the CVD of nitrides is studied using quantum chemical computations of reaction energies: reaction electronic energy (Δ𝑟𝐸𝑒𝑙𝑒𝑐) reaction enthalpy (Δ𝑟𝐻) and reaction free energy (Δ𝑟𝐺). The reaction energies were calculated for three types of reactions: Uni- and bimolecular decomposition to more reactive nitrogen species, adduct forming with trimethylgallium (TMG) and trimethylaluminum (TMA) followed by a release of methane or ethane and surface adsorption to gallium nitride for both the unreacted ammonia or methylamines or the decomposition products. The calculations for the reaction entropy and free energy were made at both STP and CVD conditions (300°C-1300°C and 50 mbar). The ab inito Gaussian 4 (G4) theory were used for the calculations of the decomposition and adduct reactions while the surface adsorptions were calculated using the Density Functional Theory method B3LYP. From the reactions energies it can be concluded that the decomposition was facilitated by the increasing number of methyl groups on the nitrogen. The adducts with mono- and dimethylamine were more favorable than ammonia and trimethylamine. 𝑁𝐻2 was found to be most readily to adsorb to 𝐺𝑎𝑁 while the undecomposed ammonia and methylamines was not willingly to adsorb.

Esta pesquisa engloba o estudo e desenvolvimento de novas heteroestruturas semicondutoras, tomando como base as estruturas SOI (Silicon-On-Insulator - silício sobre isolante) obtidas pela técnica Smart Cut, estudadas nestes últimos anos no Departamento de Engenharia de Sistemas Eletrônicos da Escola Politécnica da Universidade de São Paulo (EPUSP). Esta técnica combina a solda direta para a união de lâminas e a implantação iônica (I/I) de íons leves para a separação de camadas especificadas. São essenciais na preparação destas estruturas SOI, processos de I/I, limpeza e ativação das superfícies das lâminas e recozimentos em fornos a temperaturas moderadas. Estudamos também, diferentes métodos para a obtenção de novas heteroestruturas, basicamente combinando as técnicas de fabricação da estrutura SOI e os métodos de formação do carbeto de silício (SiC), que chamaremos de heteroestruturas SiCOI (Silicon Carbide-On-Insulator). O método usado para a formação do SiC depende, em cada caso, das características desejadas para o filme que, ao mesmo tempo, estão relacionadas com a aplicação à qual estará destinado. Analisamos três métodos de obtenção do material SiC com características específicas diferentes. A metodologia proposta aborda as seguintes tarefas: Tarefa 1: Obtenção de estruturas SOI pelo método convencional utilizado em trabalhos anteriores e melhoramento das características superficiais da estrutura resultante. Tarefa 2: partindo de uma lâmina de Si previamente coberta por uma camada isolante, fabricar a heteroestrutura SiC/isolante/Si, onde a camada de SiC é crescida pelo método de deposição química de vapor assistida por plasma (PECVD). O filme obtido por deposição PECVD é amorfo e portanto são necessárias etapas de cristalização posteriores ao crescimento. Tarefa 3: partindo de uma estrutura SOI, fabricar a heteroestrutura SiC/SiO2/Si, onde a camada de SiC é obtida por implantação de íons de carbono (C+) na camada ativa de Si da estrutura SOI para sua transformação em SiC. Tarefa 4: partindo de uma estrutura SOI, fabricar a heteroestrutura SiC/SiO2/Si, onde a camada de SiC é obtida por conversão direta da camada ativa de Si da estrutura SOI em SiC como resultado da carbonização do Si usando exposição a ambiente de hidrocarbonetos. Como resultado deste trabalho foram obtidas estruturas SOI Smart Cut com valor médio de rugosidade superficial dentro dos valores esperados segundo a bibliografia consultada. Durante o desenvolvimento de heteroestruturas SiC/isolante/Si obtidas utilizando a técnica de PECVD obtivemos filmes com boas características estruturais. Os recozimentos feitos em ambiente de N2 aparentemente trazem resultados satisfatórios, conduzindo à completa cristalização dos filmes. Nas análises feitas para a fabricação de heteroestruturas SiC/isolante/Si utilizando I/I de carbono confirma-se a formação de c-SiC depois de realizado o recozimento térmico. / In this work we study new semiconductors heterostructures, based on SOI (Silicon-On- Insulator) structures obtained by \"Smart-Cut\" process, that were studied in the last years at Departamento de Engenharia de Sistemas Eletrônicos da Escola Politécnica da Universidade de São Paulo (EPUSP). This technique combines high-dose hydrogen ion implantation (I/I) and direct wafer bonding. To produce SOI structures some processes are essential: I/I process, cleaning and activation of the surfaces, and conventional thermal treatments at moderated temperatures. We also investigate different methods to obtain new heterostructures, basically combining SOI technologies and silicon carbide (SiC) growth processes, which will be called as SiCOI (Silicon Carbide-On-Insulator) heterostructures. The utilized methods to obtain the SiC are related, in each case, with the desired film\'s characteristics, which at the same time are associated with the final application. We analyze three methods to obtain SiC material with specific different characteristics. The proposed methodology approaches the following tasks: Task 1: Fabrication of SOI structures by the conventional technology previously used by us, and the improvement of superficial characteristic of the final structure. Task 2: Fabrication of SiC/insulator/Si heterostructures from Si substrate previously covered with an insulator capping layer, where the SiC layer is deposited by plasma enhanced chemical vapor deposition (PECVD). The PECVD film is amorphous and therefore, a thermal annealing step is necessary for crystallization. Task 3: Fabrication of SiC/SiO2/Si heterostructures from SOI structure, where the SiC layer is synthesized through a high dose carbon implantation into the thin silicon overlayer of a SOI wafer. Task 4: Fabrication of SiC/SiO2/Si heterostructures from SOI structure, where the SiC layer is achieved by direct carbonization conversion of the silicon overlayer of a SOI wafer In this work we have obtained Smart Cut SOI structures with surface roughness similar to the previous reported. We also obtained SiC/insulator/Si heterostructures with good structural characteristics using PECVD technique. The investigated N2 thermal annealing appears to be suitable for the crystallization of all the amorphous films deposited by PECVD. We have shown the possibility of using carbon ion implantation and subsequent thermal annealing to form c-SiC for SiC/insulator/Si heterostructures.

Les matériaux (Si)GeSn sont très prometteurs pour les composants optiques sur puce fonctionnant dans le Moyen Infra-Rouge (MIR). Lors de cette thèse de doctorat, j’ai étudié le Dépôt Chimique en Phase Vapeur d’alliages GeSn. L’épitaxie basse température de Ge pur, de Ge dopé phosphore et d’alliages GeSi a tout d’abord été explorée. L’utilisation du digermane (Ge2H6) au lieu du germane (GeH4) nous a permis d’augmenter considérablement la vitesse de croissance du germanium à des températures en dessous de425 °C. Des concentrations très importantes en atome de P électriquement actifs ont été atteintes à 350 °C, 100 Torr en chimie Ge2H6 + PH3 (au maximum 7.5x1019 cm-3). Nous avons par la suite combiné le Ge2H6 avec le disilane (Si2H6) ou le dichlorosilane (SiH2Cl2) afin d’étudier la cinétique de croissance du GeSi à 475 °C, 100 Torr. Des concentrations de Ge définitivement plus élevées (77-82%) et une meilleure qualité de surface ont été obtenues avec le SiH2Cl2. Finalement, la croissance basse température d’alliages GeSn a été étudiée dans notre bâti d’épitaxie industriel 200 mm. Le digermane (Ge2H6) et le tétrachlorure d'étain (SnCl4) ont été utilisés pour explorer la cinétique de croissance et les mécanismes de relaxation des contraintes du GeSn. Une large gamme de concentrations en Sn, i.e. 6-16%, a été sondée et ces points de fonctionnement utilisés pour épitaxier des couches épaisses de GeSn partiellement relaxées. Nous avons ainsi mis en évidence l’intérêt d’utiliser une structure dite en escalier, en termes de qualité cristalline et de morphologie de surface. Un tel empilement, avec 16% de Sn dans sa partie supérieure, a montré une structure de bande directe et a conduit à une émission laser (dans des micro-disques) à une longueur d’onde de 3.1 µm. Ce laser a fonctionné jusqu’à 180 K et a un seuil de 377 kW/cm² à 25K. / (Si)GeSn is very promising for use in Mid Infra-Red (MIR) group-IV optical components on chip. During this PhD, I have studied the Reduced Pressure Chemical Vapor Deposition of GeSn alloys. The very low temperature epitaxy of pure Ge, heavily phosphorous doped Ge and Ge-rich SiGe alloys have first of all been investigated. Using digermane (Ge2H6) instead of germane (GeH4) enabled us to dramatically increase the Ge growth rate at temperatures 425 °C and lower. Very high electrically active P concentrations were obtained at 350 °C, 100 Torr with a Ge2H6 + PH3 chemistry (at most 7.5x1019 cm-3). We have then combined digermane with disilane (Si2H6) or dichlorosilane (SiH2Cl2) in order to study the GeSi growth kinetics at 475 °C, 100 Torr. Definitely higher Ge concentrations (77-82%) and smoother surfaces have been obtained with SiH2Cl2. We have then explored the low temperature epitaxy of high Sn content GeSn alloys in our 200 mm industrial RP-CVD tool. Digermane (Ge2H6) and tin tetrachloride (SnCl4) were used to investigate the GeSn growth kinetics and strain relaxation mechanisms. Large range of Sn concentrations, i.e. in the 6-16% range, was probed and data points used to grow thick, partially relaxed GeSn layers. The benefits of using Step-Graded structures, in terms of crystalline quality and surface morphology, was conclusively demonstrated for thick GeSn layers with high Sn contents. Such a stack, with 16% of Sn in the top part, was direct bandgap and led to a laser operation (in micro-disks) up to 180 K at an emission wavelength of 3.1 µm and with a lasing threshold of 377 kW/cm² at 25K.