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After the invention of the high-efficiency blue light-emitting diode (LED) at the end of the twentieth century, a new generation of light-emitting devices based on III-nitrides emerged, showcasing the capabilities of this semiconductor family. Despite the current limitations in the fabrication of III-nitrides, their optical and electronic properties still place them as some of the most promising semiconductors to continue the development of optoelectronic devices. To take full advantage of the versatility offered by these materials, the fabrication of novel III-nitride-based devices demands rigorous control of all of its stages. From the initial deposition of the materials, which involves controlling the composition and size of often complex heterostructures, up to the microfabrication processing used to create a final device, any deficiency occurring will negatively impact the performance of the device. Most of the time, these deficiencies reflect in microscopic defects, hindering their detection and identification of their origin. Without such knowledge, the deficiencies cannot be fixed, stalling the improvement of the device fabrication process and, consequently, its performance. This dissertation presents a variety of methodological approaches to characterize, from a microstructural point of view, different properties of novel III-nitride-based heterostructures and devices. The characterizations include studying the structure, interface, composition, and crystalline defects of different heterostructures and evaluating the microfabrication quality of microscopic LEDs. The results of the different characterizations contributed to developing novel LED and photocatalytic devices, for example, a single-quantum-well InGaN-based red LED with a high color purity, a monolithic phosphor-free white LED, microscopic green LEDs with a size smaller than 5×5 μm$^2$, and metal oxide/GaN-based photocatalysts with improved resilience to photocorrosion. The analyses and results presented in this dissertation strongly relied on the analytical capabilities offered by transmission electron microscopy, which proved to be a convenient and versatile tool for the characterization of many aspects related to the fabrication of III-nitride-based optoelectronic devices.

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One-dimensional vertical nanostructures, nanowire arrays, are investigated for applications in photovoltaics. Specifically, III-V core-shell p-i-n nanowire arrays are grown by molecular beam epitaxy on silicon substrates, using the self-assisted vapour-liquid-solid growth method. GaAs1-xPx nanowires are grown with an optimized composition to maximize the potential efficiency of a GaAsP nanowire-on-silicon tandem solar cell under AM1.5G illumination. Photovoltaic devices are fabricated and assessed by optical and electrical characterization techniques, to identify areas for refinement of device design and processing. Combining the unique properties of nanowire arrays, the quality and tunability of III-V materials, and the economics and infrastructure of silicon-based device fabrication, this work examines a novel approach to affordable renewable energy. Methods of substrate removal via etching are investigated for optical characterization of nanowire arrays, and an improved technique for electrical characterization of ITO contacts is explored. The first nanowire-on-silicon tandem device utilizing a radial p-n junction nanowire structure is reported, achieving an open circuit voltage of 1.2 V, a short circuit current density of 7.6 mA/cm2, a fill factor of 40%, and an efficiency of 3.5%. Finally, projects for future improvements to the work described herein are suggested. / Thesis / Master of Applied Science (MASc)

En esta tesis se investigó el crecimiento de AlN sobre sus-tratos de zafiro por el método MOVPE para su posterior utilización como pseudo-substrato en la fabricación de tran-sistores de alta movilidad de electrones (HEMT). Los experi-mentos de crecimiento se llevaron a cabo a 50 − 100 hPa y una temperatura 950 − 1200 ◦C. Como precursores del grupo III y grupo V y gas portador se utilizaron TMAl, NH3, N2 y/o H2, respectivamente. La evaluación de la calidad de las películas se realizó ópticamente por reflectometría in-situ, estructuralmente mediante mediciones de difracción de ra-yos-X y morfológicamente por mediciones AFM y SEM. Se estudiaron principalmente dos tipos de estrategia de creci-miento, en una etapa y en dos etapas. Para ambas estrate-gias de crecimiento se estudió la influencia de la configuración de entrada (convencional - invertida) de los gases (precur-sores) al reactor. El impacto de la presión parcial de los pre-cursores del grupo III y grupo V a V/III constante se estudió para el caso de la estrategia de crecimiento en una etapa. En el caso del crecimiento de AlN en dos etapas se investigó el efecto del tiempo de nucleación a temperatura constante de 950 ◦C y la temperatura de crecimiento en el rango de 1140 − 1200 ◦C en el crecimiento de las películas de AlN. De los resultados obtenidos de las distintas experiencias se puede concluir en forma general y sólo para el rango de los valores utilizados para los distintos parámetros de crecimiento de AlN, que no es posible optimizar la calidad cristalina y la suavidad de la superficie de las películas al mismo tiempo con sólo variar un parámetro de crecimiento. / In this thesis the growth of AlN on sapphire substrates by the method MOVPE for later use as pseudo-substrate in the manufacture of transistors high electron mobility tran-sistors (HEMT) was investigated. Growth experiments were carried out at 50 − 100 hPa and a temperature 950 − 1200 ◦C. As precursors of group III and group V and carrier gas, TMAl, NH3, and N2/H2, were used, respectively. The evaluation of the quality of the films was carried out optically by in-situ reflectometry, structurally by diffraction of X-ray (XRD) and morphologically by AFM and SEM measurements. Principally, two types of growth strate-gies were studied, one stage and two stages. For both growth strategies the influence of the gas(precursors) input configuration (conventional - inverted) to reactor was investigated. The impact of the partial pressure of group III and group V precursors at V/III constant was studied for the case of the one stage growth strategy. In the case of AlN growth, by means of the two stages growth strategy, the effect of nucleation time at a constant temperature of 950 ◦C and growth temperature in the range of 1140 − 1200 ◦C in the growth of AlN films was investi-gated. From the results obtained from the diverse experien-ces, it can be concluded, in general and only for the range of values used for the various parameters of growth of AlN, that is not possible to optimize the crystalline quality and surface smoothness of the films at the same time with the only variation of one growth parameter.

Arsenic sorbs strongly to the surfaces of Fe(III) (hydr)oxides. Under aerobic conditions, oxygen acts as the terminal electron acceptor in microbial respiration and Fe(III) (hydr)oxides are highly insoluble, thus arsenic remains associated with Fe(III) (hydr)oxide phases. However, under anaerobic conditions Fe(III)-reducing microorganisms can couple the reduction of solid phase Fe(III) (hydr)oxides with the oxidation of organic carbon. When ferric iron is reduced to ferrous iron, arsenic is mobilized into groundwater. Although this process has been documented in a variety of pristine and contaminated environments, minimal information exists on the mechanisms causing this arsenic mobilization. Arsenic mobilization was studied by conducting controlled microcosm experiments containing an arsenic-bearing ferrihydrite and an Fe(III)-reducing microorganism, Geobacter metallireducens. Results show that arsenic mobility is strongly controlled by microbially-mediated disaggregation of arsenic-bearing iron nanoparticles. The most likely controlling mechanism of this disaggregation of iron oxide nanoparticles is a change in mineral phase from ferrihydrite to magnetite, a mixed Fe(III) and Fe(II) mineral, due to the microbially-mediated reduction of Fe(III). Although arsenic remained associated with the iron oxide nanoparticles and was not released as a hydrated oxyanion, the arsenic-bearing nanoparticles could be readily mobilized in aquifers. These results have significant implications for understanding arsenic behavior in aquifers with Fe(III) reducing conditions, and may aid in improving remediation of arsenic-contaminated waters. / Master of Science

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