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<p> Periodic arrays of Au patterns (dots and lines) were produced via electron beam lithography (EBL). GaAs mesas were produced by using the Au structures as a mask and wet etching the GaAs (lll)B substrates, leaving Au resting above GaAs pillars. Annealing experiments at typical nanowire growth temperatures (550°C) were performed on both mesa-supported samples and a control sample without mesas, and were later characterized by scanning electron microscopy (SEM). From SEM images, a model is proposed to describe the evolution of the Au seed particle during exposure to typical growth conditions. The Au particle is subject to not only a melting process but is also modified by a volume increase due to incorporating Ga atoms and a subsequent crystal structure change. Palpable discrepancies between the mesa-supported and control samples were observed after annealing experiments, suggesting the mesas were effective in confining the migration of the Au. NW s were then grown via gas source molecular beam epitaxy (GS-MBE). Discemable variation amongst the results was evident when a comparison between annealed samples and the grown counterpart was made. The inconsistency is ascribed to the NW growth process beginning only after supersaturation at the growth interface. This saturation took place only after 2-D film growth on the substrate surpassed the height of the mesas rendering the structures less functional. </p> / Thesis / Master of Applied Science (MASc)

La croissance localisée de SiC dopé p par un mécanisme Vapeur-Liquide-Solide (VLS) a été effectuée sur substrats SiC-4H (0001) 8°off et diamant (100). Pour ce faire, des motifs constitués d'un empilement silicium-aluminium sont fondus puis alimentés en propane. Dans le cas de l'homoépitaxie de SiC-4H, il a été démontré que la quantité limitée de phase liquide initiale entraine une évolution constante des paramètres de croissance en raison de l'appauvrissement graduel en silicium. Il est toutefois possible de trouver des conditions de croissance satisfaisantes (alliage contenant 40 at% Si, 1100 °C) résultant en un dépôt conforme sur l'ensemble des motifs avec une morphologie step-bunchée. A partir de tels dépôts, des contacts ohmiques de très faible résistivité (jusqu'à 1,3.10-6 Ω.cm2) ont été mesurés et des diodes PiN ont été fabriquées et caractérisées. Dans le cas de la croissance de SiC sur diamant, la forte réactivité entre l'alliage Si-Al liquide et le substrat diamant conduit à la formation d'un dépôt dense et polycristallin de SiC-3C par un mécanisme de dissolution-précipitation. Nous avons montré que la formation préalable d'une couche tampon nanométrique de SiC par siliciuration du substrat de diamant (réaction solide-solide entre une couche de Si et le diamant) permet d'obtenir une croissance épitaxiale de SiC-3C en ilots, avec les relations [110] SiC // [110] diamant et (100) SiC // (100) diamant. Il n'a cependant pas été possible de former une couche complète et épitaxiale de SiC sur diamant par VLS localisée. Nous avons toutefois montré que cela est réalisable par dépôt chimique en phase vapeur (CVD) en utilisant la même étape de siliciuration / The localized growth of p-doped SiC by Vapor-Liquid-Solid (VLS) mechanism was made on (0001) 8°off 4H-SiC and (100) diamond substrates. A silicon-aluminium stacking, localized on top of the substrate, is used after melting as the liquid phase for the growth, carbon being brought by the propane of the gas phase. Regarding the homoepitaxial growth of 4H-SiC, the limited amount of liquid phase leads to a significant consumption of silicon during the growth which is responsible for a continuous variation of the growth parameters. Satisfying growth conditions can therefore be found (40 at% Si alloy, 1100 °C) leading to the formation of a step-bunched layer on the initial Si-Al patterns. Very Low resistivity ohmic contacts (as low as 1.3x10-6 Ω.cm2) and PiN diodes were successfully fabricated from these deposits. Regarding the SiC growth on diamond, the high reactivity between the Si-Al liquid alloy and the diamond substrate leads to the polycrystalline growth of 3C-SiC by a dissolution-precipitation mechanism. It is thus necessary to use a SiC buffer layer in order to achieve an epitaxial growth. This buffer layer, grown by a solid-solid reaction between silicon (deposited by CVD) and the diamond called silicidation, favors the epitaxial growth of 3C-SiC ([110] SiC // [110] diamond and (100) SiC // (100) diamond) during the later VLS growth. Considering the 3D growth mechanism that takes place the formation of a single-crystalline layer from these epitaxial islands seems difficult. Such single-crystalline layer can be achieved using chemical vapor deposition (CVD) after the silicidation step of the diamond substrates

This diploma thesis deals with the growth of semiconductor nanowires on Ge(111) surface. The nanowires were prepared by means of PVD (physical vapor deposition). The growth was calatyzed by Au colloidal nanoparticles. An impact of different growth conditions on nanowire morfology is presented. It is demonstrated that Ge nanowires grow preferentially along axis. Ge wires with orientation were observed as well.

Semiconductor nanowire synthesis provides a promising route to engineer novel nanoscale materials for applications in energy conversion, electronics, and photonics. The addition of methylgermane (GeH₃CH₃) to standard GeH₄/H₂ chemistry is demonstrated to induce a transition from <111> to <110> oriented growth during the vapor-liquid-solid synthesis of Ge nanowires. This hydride-based chemistry is subsequently leveraged to rationally fabricate kinking superstructures based on combinations of <111> and <110> segments with user defined angles and segment lengths. The addition of GeH₃CH₃ also eliminates sidewall tapering and enables Ge nanowire growth at temperatures exceeding 475 °C, which greatly expands the process window. User-programmable diameter modulation is demonstrated without kinking using tetramethyltin (Sn(CH₃)₄) or trimethylsilane (SiH(CH₃)₃) reacting directly on the sidewalls of growing nanowires to either block or allow conformal deposition. Catalyst modification with tetramethyltin is demonstrated to tune growth kinetics and provides further control over nanowire design. Morphological markers, generated via user-defined changes to diameter along the nanowire axial direction, enable a new approach to rapid, accurate, and facile extraction of growth rate information from electron microscopy images. The ability to engineer nanowire structure by tuning chemistry either at the nucleation point or on the sidewall is demonstrated in this work, thus enabling the rational fabrication of complex superstructures.

O objetivo deste trabalho é conduzir um estudo experimental em regime transônico para verificar a influência aerodinâmica da variação do ângulo de "boat-tail" de um veículo lançador de satélite com geometria do tipo "Hammer-Head", ou em português "Cabeça de Martelo". Este tipo de veículo lançador recebe este nome por possuir um diâmetro maior na região da parte frontal, cuja finalidade é transportar uma carga útil maior que o diâmetro do corpo do veículo, como no caso do Veículo Lançador de Satélites brasileiro (VLS-1). A região estudada é a parte frontal do veículo, que é caracterizada por quatro partes distintas: uma ponta arredondada seguida por um tronco de cone, um compartimento de diâmetro constante na qual é alojada a carga útil, um setor com ângulo de decaimento denominado "boat-tail", e uma parte do corpo do foguete. Estudos computacionais em um software semi-empírico e experimentais no Túnel Transônico Piloto (TTP) do Instituto de Aeronáutica e Espaço (IAE) foram realizados para determinar os coeficientes de arrasto e distribuição de pressão sobre o modelo variando-se o ângulo de "boat-tail". Para este fim, utilizou-se uma balança de esforços para medir o arrasto e a técnica de Tinta Sensível à Pressão, do inglês Pressure Sensitive Paint (PSP), para a obtenção de medidas globais de pressão sobre a superfície do modelo. Também foi utilizada a técnica Schlieren de visualização de escoamentos para obtenção de informações detalhadas sobre o padrão de formação das ondas de choque sobre o modelo. O estudo realizado permitiu verificar que a variação do ângulo de decaimento apresentou um maior arrasto para os ângulos maiores e que o ângulo original do "boat-tail" do VLS-1 de 8, possui o menor coeficiente de arrasto em relação às demais configurações testadas.

The focus of the present research is to find the relative thermodynamic stability of ?-boron and ?-boron via heat capacity measurements. Efforts to synthesize ?-boron through the application of vapour-liquid-solid theory resulted in the discovery of a new chemical vapour deposition approach. The heat capacities of both synthesized ?-boron and commercial (99.5%) ?-boron were determined using relaxation calorimetry over the temperature range 0.2 K to 400 K. These data, in combination with literature information, allowed the calculation of the Gibbs energy of the ?-boron to ?-boron transition from 0 K to 1985 K. It was found that the transition from ?-boron to ?-boron was thermodynamically favourable at all temperatures up to 1985 K with a value of ?Gt(T = 300 K) = -10 kJ mol-1 ± 1 kJ mol-1 and ?Gt(T = 1985 K) = -15 kJ mol-1 ± 1 kJ mol-1.

This dissertation investigates the growth of high quality GaN and InN thin films by plasma assisted molecular beam epitaxy (PAMBE). It also explores the growth of self-seeded GaN branching nanowires and p-type doping of InN, two topics of particular interest at present. The growth of high quality III-Nitride semiconductor thin films have been shown to be dependent on the group-III (metal) to nitrogen ratio. A metal-rich growth environment enhances the diffusion of the group-III adatoms through the formation of a group-III adlayer. By using a metal-rich growth environment, determined by growth rate studies using laser reflection interferometry or RHEED analysis of the surface, both GaN and InN films have been grown with a smooth surface morphology. Additionally the smooth surface morphology has beneficial effects on the electrical and optical properties of both materials. However, with the growth using a metal-rich environment, group-III droplets are present on all film surfaces, which can be an issue for device fabrication, as they produce facets in the crystal structure due to enhanced growth rates. MBE growth of GaN nanowires via the vapour liquid solid (VLS) and vapour solid (VS) growth techniques have so far been based on the N-rich growth regime. However, we have shown that the Ga-rich growth regime can be used to grow self-seeded one dimensional and hierarchical GaN nanowires. 7 µm long hierarchical GaN nanowires with at least three branches were grown and shown to have a high crystalline quality. The suggested growth mechanism is a self-seeding VLS process driven by liquid phase epitaxy at the nanoscale, while the branching growth was nucleated due to the Ga-rich growth regime by excess Ga droplets forming on the trunk during growth. The growth of vertical GaN nanowires has also been achieved using the same self-seeding process and the critical parameter seems to be the Ga to N ratio. Also, the growth rate of the Ga-rich grown GaN nanowires can supersede the growth rates reported from N-rich grown GaN nanowires by at least a factor of two. The fabrication of vertical and planar GaN nanowire devices has been demonstrated in this study. Two point and three point contacts were fabricated to the branching GaN nanowires in the planar direction with resistive measurements ranging from 200 - 900 kΩ, similar to chemical vapour deposition and MBE grown GaN nanowires. The nonlinear current-voltage characteristics from the three point contacts may lead to unique nano-devices. The planar nanowires have also shown to have potential as UV detectors. Schottky diodes were fabricated on the vertical nanowires, with values for the barrier heights consistent with bulk diodes. Mg and Zn doping studies of InN were also performed. Both InN:Mg and InN:Zn have strong photoluminescence only at low doping concentrations. However, the InN:Mg films have reduced mobilities with increased Mg content, whereas the mobility determined from the InN:Zn films is independent of Zn. When the InN:Zn film quality was improved by growing under the In-rich growth regime, electrochemical capacitance-voltage results suggest n{type conductivity, and strong photoluminescence was obtained from all of the films with four features seen at 0.719 eV, 0.668 eV, 0.602 eV and 0.547 eV. The features at 0.719 eV and 0.668 eV are possibly due to a near band edge to valence band or shallow acceptor transition, while the 0.547 eV has an activation energy of 60 meV suggesting a deep level acceptor.

abstract: Zinc telluride (ZnTe) is an attractive II-VI compound semiconductor with a direct bandgap of 2.26 eV that is used in many applications in optoelectronic devices. Compared to the two dimensional (2D) thin-film semiconductors, one-dimensional (1D) nanowires can have different electronic properties for potential novel applications. In this work, we present the study of ZnTe nanowires (NWs) that are synthesized through a simple vapor-liquid-solid (VLS) method. By controlling the presence or the absence of Au catalysts and controlling the growth parameters such as growth temperature, various growth morphologies of ZnTe, such as thin films and nanowires can be obtained. The characterization of the ZnTe nanostructures and films was performed using scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), high- resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), photoluminescence (PL), Raman spectroscopy and light scattering measurement. After confirming the crystal purity of ZnTe, two-terminal diodes and three-terminal transistors were fabricated with both nanowire and planar nano-sheet configurations, in order to correlate the nanostructure geometry to device performance including field effect mobility, Schottky barrier characteristics, and turn-on characteristics. Additionally, optoelectronic properties such as photoconductive gain and responsivity were compared against morphology. Finally, ZnTe was explored in conjunction with ZnO in order to form type-II band alignment in a core-shell nanostructure. Various characterization techniques including scanning electron microscopy, energy-dispersive X-ray spectroscopy , x-ray diffraction, Raman spectroscopy, UV-vis reflectance spectra and photoluminescence were used to investigate the modification of ZnO/ZnTe core/shell structure properties. In PL spectra, the eliminated PL intensity of ZnO wires is primarily attributed to the efficient charge transfer process occurring between ZnO and ZnTe, due to the band alignment in the core/shell structure. Moreover, the result of UV-vis reflectance spectra corresponds to the band gap energy of ZnO and ZnTe, respectively, which confirm that the sample consists of ZnO/ZnTe core/shell structure of good quality. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017