Afin de poursuivre la réduction d'échelle des transistors MOS, l'industrie des semiconducteurs a su anticiper les limitations de la miniaturisation par l'introduction de nouveaux matériaux ou de nouvelles architectures. L'avènement des structures à triples grilles (FinFET) a permis de maitriser les effets canaux courts et poursuivre les efforts de miniaturisation (nœud technologique 14 nm en 2014). Le cas ultime pour le contrôle électrostatique de la grille sur le canal est donné par une grille entourant totalement le canal du dispositif. A cet effet, un transistor à nanofil à grille entourante est considéré comme la structure la plus adaptée pour les nœuds technologiques en dessous de 7 nm. Au cours de cette thèse, un procédé de réalisation large échelle de transistors MOSFET miniaturisés à base de nanofils verticaux en silicium a été développé. Tout d'abord, les nanofils verticaux ont été réalisés par une approche descendante via le transfert par gravure d'un masque de résine en Hydrogène Silsesquioxane (HSQ), réalisé par lithographie électronique à basse tension d'accélération. Une stratégie de dessin inédite dite "en étoile " a été développée pour définir des nanofils parfaitement circulaires. Les nanofils en Si sont obtenus par gravure plasma puis amincis par oxydation humide sacrificielle. Ce procédé permet d'obtenir des nanofils verticaux en Si avec des parois parfaitement anisotropes, une parfaite reproductibilité et un rendement maximal. L'implémentation des MOSFETs sur les réseaux nanofils a été effectuée par l'ingénierie successive de couches minces nanométriques (conductrices et diélectriques). Dans ce cadre, un procédé innovant de réalisation de couches d'isolations en HSQ par gravure chimique contrôlée a démontré une excellente planéité associée à une rugosité de surface inférieure à 2 nm. Enfin, un procédé utilisant la photolithographie UV conventionnelle a été développé pour réaliser le transistor de longueur de grille nanométrique. Ces dispositifs ont démontré d'excellentes performances électriques avec des courants de conduction supérieurs à 600 µA/µm et une excellente maîtrise des effets de canaux courts (pente sous le seuil de 95 mV/dec et DIBL à 25 mV/V) malgré l'extrême miniaturisation de la longueur de grille (15 nm). Enfin, nous présentons une première preuve de concept d'un inverseur CMOS à base de cette technologie à nanofils verticaux. / In order to further downscaling of the MOS transistors, the semiconductor industry has anticipated the limitations of miniaturization by the introduction of new materials and new architectures. The advent of triple gate structures (FinFET) allowed mastering the short channel effects and further miniaturization efforts (14 nm technology node in 2014). The ultimate case to the electrostatic control of the gate on the channel is given by a gate completely surrounding the device channel. For this purpose, Gate All Around (GAA) nanowire transistor is considered as the most suitable structure for technology nodes below 7 nm. In this thesis, a large scale process for the realization of miniaturized MOSFETs based on vertical silicon nanowires has been developed. Firstly, the vertical nanowires were made by a top down approach by the transfer by etching of hard mask made of Hydrogen silsesquioxane (HSQ) resist created at low voltage electron beam lithography. An original design strategy called "star" was developed to define perfectly circular nanowires. Si nanowires are obtained by plasma etching then thinned by sacrificial wet oxidation. This method allows obtaining vertical Si nanowires with perfectly anisotropic walls, a perfect reproducibility and a maximum yield. The implementation of the MOSFETs on the nanowire network was done by successive engineering of nanoscale thin films (conductive and dielectric). In this context, an innovative process for producing insulation layers in HSQ by controlled chemical etching showed excellent flatness associated with surface roughness of less than 2 nm. Finally, a method using conventional UV photolithography has been developed to achieve the nanometer gate length transistor. These devices have demonstrated excellent electrical performances with conduction currents superior than 600 µA/µm and excellent control of short channel effects (subthreshold slope of 95 mV/dec and DIBL of 25 mV/V) despite extreme miniaturization of the gate length (15 nm). Finally, we present a first proof of concept of a CMOS inverter based on vertical nanowires technology.

Energy enables basic and innovative services to reach a seemingly ever-growing population and when its generation costs are reduced or when its usage is optimized it has the greatest impact on the reduction of poverty. Furthermore, there is a pressing need to decouple energy generation from non-renewable and carbon-heavy sources which has led mayor economies to increase research efforts in these areas. This thesis discusses research on water oxidation using nanostructured iron oxide electrodes and current-induced magnetic domain wall motion in nickel/cobalt bi-segmented nanowires. These two fields may seem disparate at first glance, but are linked by such common theme: materials for energy, and more precisely, materials for energy conversion and economy. The work presented in this document aims also to reflect this theme by using widely available materials like iron and aluminum, and optimizing the methods to produce the final samples using the least resources possible. All samples were prepared by electroplating metals (iron, cobalt and nickel) into anodized alumina templates fabricated inhouse. For water oxidation, iron nanorods were integrated into an electrode and annealed in air, while nickel/cobalt nanowires were isolated and contacted individually to test for spintronics-related effects. Spintronic-based devices aim to reduce energy usage in nowadays microelectronic devices. The nanostructured iron oxide electrode showed its usefulness for water oxidation in a laboratory environment, making it an appropriate complement to other electrodes specially designed for water reduction in a photoelectrochemical cell. This two-electrode design, allows for hydrogen and oxygen to be produced at each electrode and therefore eases their separate collection for, e.g., fuel or fertilizers. On the other hand, this work presents one of the first experimental demonstration of current-induced domain wall motion in soft/hard cylindrical magnetic nanowires at zero applied external magnetic field. These kinds of experiments are expected to be the first of many which will allow researchers in the field to test for spintronic-relevant properties and interactions in cylindrical magnetic nanowires.

Self-catalyzed GaAs nanowires (NWs) are grown epitaxially on Si(111) substrates using molecular beam epitaxy (MBE). The dynamics of the droplet are examined to improve NW yield and to control NW morphology. Control and understanding of the NW diameter via droplet dynamics is applied to NW photovoltaics and to novel corrugated NW distributed Bragg reflectors (DBRs). At the beginning of the MBE growth, a Ga pre-deposition step, between 0 s and 500 s in duration, is introduced to improve the yield of the NW arrays. The effect of the pre-deposition time was examined for five different hole diameters and yield was increased to nearly 100% for the appropriate combination of hole diameter and pre-deposition time. Two models were used to model the NW growth progression under different atomic flux ratios. The first model considers the contributions from direct and diffusion fluxes to the droplet and solves coupled equations for the droplet contact angle and the NW radius. The second model treats the contact angle as constant. Both models explained the accompanying experimental observations. Both models could be used to model future NW growths and the choice between the two would depend on the availability of contact angle data and whether the crystal phase must be considered. Absorption in NWs is determined by the diameter and the HE1n modes. The effectiveness of a linearly tapered inverted conical NW is demonstrated using finite element simulations. The photocurrent of an optimized inverted conical NW array is found and shown to be similar to that achieved by optical nanocones and nanowires. Diameter modulations can also be introduced into NW structures periodically to produce corrugated NW distributed Bragg reflectors (DBRs). The tunability of the reflectance peaks is demonstrated and explained by changes to the effective refractive index of the structure. / Thesis / Doctor of Philosophy (PhD) / This thesis seeks to understand the growth processes behind self-catalyzed nanowire growth. Nanowires (NWs) are very thin, vertical columns of semiconducting material. Self-catalyzed growth is a method of producing these structures that uses a droplet at the top of the structure to add material to the structure over time. These structures have numerous applications. This thesis focuses on solar cells and distributed Bragg reflectors (DBRs). Experiments show how control over the droplet can improve NW yield and give significant control of the NW diameter. These experiments are supported by mathematical models. Control over the diameter is important for the applications discussed. Using numerical simulations, it is shown how control over the diameter of the structure can lead to improvements in light absorption in NW solar cells. Additionally, periodic changes to the diameter can be used to create novel NW structures such as DBRs, which is a promising new application.

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Commercial success had been achieved with electroluminescent phosphor powders, particularly zinc sulfide activated with copper. The applications of AC Powder EL (ACPEL) are mainly limited to backlighting and lamp applications that require low brightness. This includes low illumination environments, such as nightlights and backlighting for LCDs and keypads in portable electronics and home electronics. By using nanowires as nanoelectrodes, a novel and self-supported nanostructured powder EL device has been developed in this thesis. The novel structure contains a single layer of ZnS:Cu powder phosphor which is embedded in a polymer matrix with one surface exposed. A dilute layer of conductive nanowires directly contacts the phosphor layer and works as rear electrodes. A highly intensified electric field can be induced in the phosphor region by the metal nanowires if a specific voltage is applied to the device. Simulations of the electric field by using commercial software show that the localized electric field can be at least one order of magnitude higher than the average field depending on the dimensions of the nanowires. As a result, electrons can be injected into the phosphor lattice by high-field-assisted tunneling, hence inducing electron avalanching. The electrons finally are trapped at the donors. When the external field is reversed, the electrons recombine with the holes that are previously injected by the same process and trapped at the acceptors. Therefore, visible light is produced by the recombination of the electron-hole pairs through the donor-acceptor pairs. The indium nanowires, with diameters of 300 nm and lengths of several microns, have been fabricated by using anodic aluminum oxide (AAO) templates which are known as self-organized porous structures formed by anodization of aluminum in an appropriate acid solution. A hydraulic pressure injection method has been applied to inject molten indium metal into the nanopores of the AAO template and form nanowires. By dissolution of the template, a large number of free indium nanowires is obtained. The nanowires are transferred onto a ZnS-embedded substrate by a wet-coating method. Finally, the entire device is completed by deposition of Au rectangular electrodes on the top of the indium nanowires. The indium nanowires have been characterized by using SEM and XRD. The tests of the dependence of luminance on voltage at various frequencies for a nanowire contact EL device sample are performed. A peak luminance of 25 cd/m2 has been achieved for the device driven at frequency of 8.2 kHz and a voltage of 425 V. The EL performance of the nanowire contact EL device is not as good as traditional powder EL devices so far, however, the novel structures have the potential for a lower operating voltage with simultaneous long lifetime and high luminance to overcome limitations of traditional powder EL. / Thesis / Doctor of Philosophy (PhD)

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The use of nanowires, nano and micro needles in biomedical applications has markedly increased in the past years, mainly due to attractive properties such as biocompatibility and simple fabrication. Specifically, these structures have shown promise in applications including cell separation, tumor cell capture, intracellular delivery, cell therapy, cancer treatment and as cell growth scaffolds. The work proposed here aims to study two platforms for different applications: a vertical magnetic nanowire array for mesenchymal stem cell differentiation and a micro needle platform for intracellular delivery. First, a thorough evaluation of the cytotoxicity of nanowires was done in order to understand how a biological system interacts with high aspect ratio structures. Nanowires were fabricated through pulsed electrodeposition and characterized by electron microscopy, vibrating sample magnetometry and energy dispersive X-ray spectroscopy. Studies of biocompatibility, cell death, cell membrane integrity, nanowire internalization and intracellular dissolution were all performed in order to characterize the cell response. Results showed a variable biocompatibility depending on nanowire concentration and incubation time, with cell death resulting from an apoptotic pathway arising after internalization. A vertical array of nanowires was then used as a scaffold for the differentiation of human mesenchymal stem cells. Using fluorescence and electron microscopy, the interactions between the dense array of nanowires and the cells were analyzed, as well as the biocompatibility of the array and its effects on cell differentiation. A magnetic field was additionally applied on the substrate to observe a possible differentiation. Stem cells grown on this scaffold showed a cytoskeleton and focal adhesion reorganization, and later expressed the osteogenic marker osteopontin. The application of a magnetic field counteracted this outcome. Lastly, a micro needle platform was fabricated through lithography and electrodeposition, characterized using the previously mentioned techniques and then evaluated as a vector for intracellular delivery. Fluorescence and electron microscopy imaging were first performed to assess the biocompatibility, cell spreading and the interface of the cells and the needles. Intracellular delivery of a fluorescent dye was achieved via inductive heating of the needles, with the results showing a dependency of delivery and cell survivability on the exposure time.

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