Optical Properties of MacEtch-Fabricated Porous Silicon Nanowire Arrays

The increasing demand for complex devices that utilize unique, three-dimensional nanostructures has spurred the development of controllable and versatile semiconductor fabrication techniques. However, there exists a need to refine such methodologies to overcome existing processing constraints that compromise device performance and evolution. Conventional wet etching techniques (e.g., crystallographic KOH etching of Si) successfully generate textured Si structures with smooth sidewalls but lack the capabilities of controllably producing high aspectratio structures. Alternatively, dry etching techniques (e.g., reactive-ion etching), while highly controllable and capable of generating vertically aligned, high aspect-ratio structures for IC technologies, introduce considerable sidewall and lattice damage as a result of high-energy ion bombardment that may compromise device performance. Metal-assisted chemical etching (MacEtch) provides an alternative process that is capable of anisotropically generating high aspect-ratio micro and nanostructures using a room temperature, solution-based technique. This fabrication process employs an appropriate metal catalyst (e.g., Au, Ag, Pt, Pd) to induce etching in several semiconducting materials (e.g., Si, GaAs) submerged in a solution containing an oxidant and an etchant. The MacEtch process resembles a galvanic cell such that cathodic and anodic half reactions take place at the catalyst/solution interface and catalyst/substrate interface, respectively. At the cathode, the metal catalyzes the reduction of the oxidant resulting in the generation and accumulation of charge carriers (e.g., holes, h+) that are subsequently injected into the underlying substrate at the anode. This results in the formation of oxide species that are preferentially dissolved by the etchant. Thus, MacEtch provides a tunable, top-down, catalytic fabrication technique enabling greater process control and versatility for generating high aspect-ratio semiconductor structures. In this thesis, Au and Au/Pd catalyzed MacEtch is used to generate ultradeep Si micropillar structures, and porous SiNW (p-SiNW) arrays with enhanced optical properties. Using a combination of Au-MacEtch and a crystallographic KOH etch, Si micropillars with ~100 μm height were fabricated with up to 70 μm clearance between pillars to allow efficient fluid flow for optical detection of viral particles. Alternatively, porous SiNW arrays fabricated via AuPd- MacEtch demonstrated broadband absorption ≥ 90% from 200 – 900 nm and were shown to outperform RCWA-simulated SiNW arrays with similar morphologies. Additionally, photoluminescence (PL) spectra collected from as prepared p-SiNW showed significant enhancement in intensity centered near 650 nm as etch depth increased from 30 μm to 100 μm, attributed to an increase in the porous volume. Using atomic layer deposition (ALD) the p-SiNW were passivated using alumina (Al2O3) and hafnia (HfO2) thin films in addition to ITO thin films deposited via sputtering. PL intensity also increased after ALD passivation, attributed to a quenching effect on non-radiative SRH recombination sites on the NW surfaces, with a red shift in the peak wavelength as ALD film thickness increased from 10 nm to 50 nm, resulting from strain effects acting on the NW themselves. These results show promise in such micropillar and coated and uncoated p-SiNW structures towards applications in microfluidic devices, and indoor light-harvesting and outdoor solar-based technologies

Optical properties of p-type porous GaAs

Samples of p-type porous GaAs was obtained by electrochemical anodization of (100) oriented p-type GaAs. The formation of porous structure has been confirmed by Raman spectroscopy and scanning electron microscopy investigations. The low-frequency Raman shift of the peaks conditioned by the main optical phonons was observed in the Raman spectra of the porous GaAs. Estimation of the size of nanocryslallites in porous GaAs both by Raman shift and scanning electron microscopy gives approximately the same values and was about 10-20 nm. Photoluminescence investigations of porous GaAs exhibit the presence of two infrared and one visible bands

Mesoporous Germanium Layer Formation by Electrochemical Etching

Weight reduction of multi-junction III-V semiconductor solar cells is an important budget issue for space applications. Typically, space solar cells are epitaxially formed on a Ge or GaAs substrate wafer. The substrate material determines the lattice constant of the stack, provides mechanical stability during the cell process, and serves as bottom cell. The substrate wafer is typically more than 100 µm thick for reasons of mechanical stability during cell processing, whereas a few µm thickness are sufficient for the bottom cell to match the photogenerated currents in the top and middle cells and not to be current limiting. Unnecessarily heavy substrate wafers hence reduce the available payload for satellite missions. There are several techniques that permit the production of very-thin lightweight highly-efficient space solar cells. Ge or GaAs substrates are commonly removed by chemical wet etching, which reduces weight but has the disadvantage that the substrate wafer is lost for further use. Separating the electrically active solar cells from their substrates by a lift-off process, could save the substrate and reduce costs. The application of a layer transfer process for multi-junction III-V semiconductor space solar cells is hence of main interest for all space agencies. Lift-off processes based on epitaxial growth of the absorber layer onto a porous etched substrate already exist for the fabrication of monocrystalline silicon solar cells. Brendel demonstrated the so-called Porous Silicon (PSI) process for the production of monocrystalline thin-film Si solar cells. This method uses a double layer of mesoporous Si formed by means of electrochemical etching: A mesoporous layer with low porosity at the surface of the substrate is used as a seed layer for the Si epitaxy, while a buried high porosity layer is used as a pre-determined breaking-point. The formation of porous germanium (PGe) has been not intensively studied. This doctoral work focuses on the fabrication and characterization of porous germanium layers by means of electrochemical etching. This thesis evaluates the potential applications of porous Ge layers for the fabrication of very-thin space solar cells. Additionally, the formation of mesoporous GaAs and mesoporous Si layers with miscut orientations is investigated

Porous silicon as a platform for gradient refractive index photonics

Porous silicon (PSi) is a versatile optical material that is formed by electrochemically etching bulk Silicon (Si). The refractive index of PSi is readily modulated by the electrochemical current density, making PSi inherently applicable to gradient refractive index (GRIN) applications. A GRIN broadly refers to a spatially-varying refractive index, whether discrete or continuous in nature, which offers a means for strategically controlling the flow of electromagnetic radiation. As such, GRINs are useful in different fields such as photonic crystals (PhCs) and transformation optics (TO) for applications including—but not limited to—light sources, imaging, optical communication, and solar energy conversion. This dissertation focuses on utilizing PSi as a platform for GRIN photonics. A modified transfer-printing method was developed to modularly assembly hybrid PSi microcavities (MCs) comprised of a foreign, light-emitting cavity material sandwiched between PSi 1D PhC reflectors formed from flat Si wafers. These hybrid light-emitting MCs were imparted with tunability by the introduction of a PSi cavity coupling layer. Next, Si wafers were patterned with conventional microfabrication techniques to provide a shape-defined path for PSi formation. The shape-defined process has realized light-focusing GRIN square micro-columns with potential on-chip applicability, as well as cylindrical GRIN microlens arrays that could be useful for integration with detector pixels or light-sheet microscopes. Finally, work was conducted on utilizing PSi templates to create visibly transparent GRIN photonic elements. This concept is demonstrated by a combination of thermal oxidation, to create transparent porous silicon dioxide (PSiO2), and infiltration with titanium dioxide (TiO2) by atomic layer deposition, forming optically tunable discrete and continuous PSiO2/TiO2 composite GRINs

Ga-based III-V semiconductor photoanodes for solar fuels and novel techniques to investigate their photocorrosion.

Solar energy is one of the most abundant renewable energy sources. However, the diurnal variation of the sun as well as seasonal and weather effects, limits the widespread global implementation of solar energy. Thus, Cost-effective energy storage is critical to overcome the intermittent nature of solar energy available on the earth. Photoelectrolysis of water to oxygen and hydrogen fuel is a promising large-scale solution to store intermittent solar energy in a dense and portable form. Photoelectrochemical (PEC) water-splitting, or artificial photosynthesis, research strives to develop a semiconductor photoelectrode with both high efficiency and long-term stability. Semiconductors of the III–V class are among the most promising materials for high efficiency solar fuels applications. However, they suffer from severe instability in acidic and alkaline electrolyte and fundamental understanding of the corrosion mechanism of III-V semiconductors is of significant importance for the solar fuels community. This dissertation is focused on study of photocorrosion of Gallium based III-V semiconductors. A thorough review of important in-situ analytical techniques for the investigation of materials stability is given. The review explains some of the main in-situ electrochemical characterization techniques, briefly explaining the principle of operation and the necessary modifications for in-situ operation, and highlighting key relevant work in applying the method for the investigation of material stability and interfacial properties for electrocatalysts and photoelectrode materials. Next, in this dissertation, the corrosion of n-GaP, a promising III–V material for tandem top subcells, was investigated in strongly acidic electrolyte using an in-situ UV-Vis spectroscopy technique to monitor dissolved Ga and P species as a function of applied bias and time. The changing faradaic efficiency of the electrochemical GaP oxidation reaction was calculated from this data and used to interpret the corrosion process in conjunction with SEM and XPS characterization. In addition, corrosion measurements were made with thin conformal coatings of TiO2 as a protective barrier layer on the GaP surface. Although the protective coating slowed the rate of GaP dissolution, the TiO2 layers produced herein contributed significant charge-transfer resistance and still showed similar trends in the corrosion faradaic efficiency vs. time as the bare n-GaP. Further, photocorrosion of n-GaAs, one of the most well-developed and efficient III-V semiconductors was studied in strongly acidic electrolyte. Three type of Ir, OER co-catalyst, were tested to investigate their affect on photocorrosion of n-GaAs. In-situ UV-Vis spectroscopy was utilized to monitor the corrosion faradaic efficiency and the results showed decreased dissolution faradaic efficiency to a small degree over the first 15 minutes for samples with thin layers of Ir. SEM and XPS characterization have also been used to understand the photocorrosion mechanism. To develop high efficiency and stable water splitting systems new semiconductor materials with appropriate band gap, band edge positions, charge carrier mobility and chemical stability are demanded. Synthesis of ternary III-V alloys enable us to tune the band gap of III-V semiconductor with changing the compositions according to the requirements of PEC systems. Herein, optical and electrical properties of a novel III-V ternary alloy GaSbxP(1-x), synthesized in Conn Center for Renewable Energy Research by Halide Vapor Phase Epitaxy (HVPE) is reported. The effect of Sb addition on the band gap of the semiconductor was studied utilizing diffuse reflectance spectroscopy and photoluminescence spectroscopy. Band gap of HVPE-grown GaSbxP(1-x) film, with x=0.03-0.06 is decreased due to Sb incorporation to the lattice of GaP indicating that it can be a promising photoabsorber for PEC systems. In addition, incorporation of Sb to the lattice of GaP was estimated using Vegard’s law and X-ray diffraction spectrum of samples. Finally, resistivity and Hall effect measurements were performed to study the electrical properties of GaSbxP(1-x) films

Mechanism and Catalyst Stability of Metal-Assisted Chemical Etching of Silicon

Ph.DDOCTOR OF PHILOSOPH

Photonic Bandgap Analysis and Fabrication of Macroporous Silicon by Electrochemical Etching

Los cristales fotónicos son materiales creados artificialmente, que pueden hacer con los fotones lo que los semiconductores ordinarios hacen con los electrones: es decir, pueden mostrar una banda fotónica prohibida (PBG), situación en la cual fotones con determinadas energías no pueden propagarse dentro del cristal independientemente de la polarización y la dirección de propagación. Por lo tanto, la banda prohibida para los fotones puede ser el verdadero análogo óptico de la banda prohibida fundamental en los semiconductores. Desde su invento en 1987, los cristales fotónicos han atraído un interés considerable debido a sus propiedades ópticas inusuales. Las propiedades únicas de los cristales fotónicos también han llevado al reconocimiento de su estudio como un nuevo y principal campo de la optoelectrónica.El silicio macroporoso con su elevada constante dieléctrica, sus altas relaciones de aspecto y su total compatibilidad con la industria microelectrónica es un modelo excelente para estudiar las propiedades ópticas de cristales fotónicos bidimensionales y asimismo tridimensionales. Adicionalmente, se ha demostrado que el silicio macroporoso tiene varias aplicaciones únicas en muchos otros campos, como la electrónica, el micromecanizado, la detección de gases y la biotecnología. La investigación del silicio macroporoso crece continuamente debido a su enorme potencial de aplicaciones.El trabajo presentado en esta tesis trata dos temas: simulación de la estructura de bandas fotónicas y análisis de cristales fotónicos bidimensionales, y la fabricación de estructuras bidimensionales basadas en silicio macroporoso para aplicaciones como cristales fotónicos en el espectro infrarrojo. Debido a que muchas posibles aplicaciones de los cristales fotónicos están basadasen sus bandas fotónicas prohibidas, es interesante diseñar cristales fotónicos con una bandaprohibida absoluta, que sea tan grande como es posible. En esta tesis describimos el método para alargar la banda fotónica absoluta, mostrando el papel de la simetría en el diseño de estructuras fotónicas óptimas. Hemos estudiado como reduciendo la simetría mediante incorporación de elementos adicionales en la celda unitaria o mediante cambio de la forma de los "átomos" afecta la relación de dispersión de los dos modos de polarización (TM y TE) en cristales fotónicos bidimensionales. Nuestro objetivo ha sido optimizar la magnitud de la banda fotónica absoluta, reduciendo la simetría de las celdas cuadrada y triangular y construir de este modo estructuras nuevas, llamadas celdas híbridas. Usando el método de las deferencias finitas en el dominio de tiempo (FDTD) hemos realizado un detallado análisis numérico de la relación de dispersión en celdas híbridas bidimensionales que consisten en columnas de aire en silicio.En el caso de celda cuadrada, la reducción de la simetría ha sido aplicada con éxito para maximizar la magnitud de la banda prohibida. Para la celda cuadrada que consiste en columnas cilíndricas de aire, la incorporación de una columna adicional aumenta tres veces la magnitud de la PBG absoluta. En el caso de celda cuadrada de columnas cuadradas de aire, la rotación de las columnas juega un papel crítico en la creación de la PBG absoluta.Si las columnas cuadradas no están rotadas no existe una PBG absoluta. La magnitud de la PBG absoluta se ha mejorado considerablemente a través de la combinación de incorporación de una columna adicional y rotación de las columnas cuadradas. Además, se genera una nueva PBG absoluta que se encuentra para un amplio rango de ángulos de rotación y dimensiones de las columnas, que están lejos de la condición de empaquetamiento (cuando las columnas se tocan). Esto favorece la fabricación de los cristales fotónicos.La PBG absoluta es de mayor magnitud para la celda triangular formada por columnas cilíndricas de aire. Los resultados de las simulaciones demuestran que modificando la estructura triangular mediante incorporación de columnas adicionales o mediante columnas cuadradas (aunque las columnas estén rotadas) no mejora la PBG absoluta, por lo menos en el caso estudiado de estructura aire/silicio. La adición de columnas adicionales en la celda triangular reduce la magnitud de la PBG absoluta.Hemos realizado un detallado análisis cuantitativo de las PBG absolutas para 2D celdas triangulares y hexagonales, considerando que entre las columnas y la matriz dieléctrica hay una capa superficial de otro material dieléctrico. Esta capa superficial puede ser indeseada (resultado del proceso de fabricación) o puede ser creada intencionadamente.Las propiedades de las bandas fotónicas se ven afectadas del grosor y también de la constante dieléctrica de la capa superficial. Los resultados de las simulaciones demuestran que para estructuras que están formadas por columnas de aire en un material dieléctrico la existencia de una capa superficial reduce la magnitud de la PBG absoluta. Por otro lado, para estructuras formadas de columnas dieléctricas en aire la capa superficial puede mejorar la PBG cuando la constante dieléctrica de la capa es mayor que la de las columnas.Esto proporciona mayor flexibilidad en la realización práctica de estos 2D cristales fotónicos. Por ejemplo, en ciertas ocasiones es imposible obtener pilares dieléctricos con un diámetro determinado o de un material concreto por limitaciones tecnológicas. Sin embargo, los pilares se pueden fabricar de un material con menor constante dieléctrica para el cual existe una técnica bien desarrollada. Después los pilares se pueden cubrir con el material deseado mediante deposición, obteniendo las mismas propiedades como en el caso de la estructura sin capa superficial.Hemos desarrollado un equipo de ataque electroquímico para fabricación de 2D estructuras periódicas basadas en la formación de silicio macroporoso. Asimismo, hemos realizado un estudio de la influencia de los parámetros del ataque electroquímico sobre la morfología de los poros. Crecimiento estable de macroporos se puede obtener sólo si todos los parámetros del proceso de ataque (resistividad del substrato, concentración de HF, corriente de ataque, potencial anódico, temperatura, etc.) están ajustados apropiadamente.Las condiciones óptimas ocupan una pequeña parte de todos los posibles parámetros del proceso. Por ejemplo, concentraciones de HF mayores de 10 wt.%, que se usan generalmente para crecer películas micro- y mesoporosas, no son apropiadas para crecer macroporos con una profundidad grande y una forma cilíndrica. Potenciales relativamente altos (para nuestras muestras mayores de 2 V) aumentan inevitablemente la formación de "breakdown-type" poros. Por otro lado, potenciales relativamente bajos (menores de 1 V) generalmente producen un crecimiento inestable de los poros que están parcial o totalmente recubiertos de silicio microporoso.La corriente aplicada es el parámetro más crítico del proceso. Densidades de la corriente mayors de la densidad crítica Jps, que depende de la temperatura y de la concentración de HF, situaría el proceso en la región de electropulido. El control de la corriente durante el proceso es una tarea clave. Mantener la corriente de ataque constante durante todo el proceso es insuficiente para el crecimiento estable de macroporos cilíndricos. Se han identificado dos efectos que influyen la forma de los poros en profundidad. Primero, la concentración de HF disminuye cerca de la punta de los poros debido a las limitaciones por difusión en poros estrechos y hondos. Este efecto produce un incremento del diámetro del poro cerca de la punta. Segundo, la superficie interna de los poros aumenta para prolongados tiempos de ataque, provocando un incremento de la corriente de oscuridad y por lo tanto la formación de poros cónicos. Su diámetro decrece en profundidad. El incremento de la corriente de ataque de manera adecuada, tal que se produzca crecimiento de poros con forma cónica inversa, es un método para compensar la conicidad inicial de los poros. Si el ataque se realiza a temperaturas más bajas y burbujeando el electrolito con nitrógeno se puede reducir la corriente de oscuridad, formando poros menos cónicos. Otro método efectivo es el uso de un surfactante apropiado. Los surfactantes se usan por lo general para prevenir degradación causada por las burbujas de hidrógeno que se pegan en la superficie de la muestra. Hemos probado dos diferentes tipos de surfactants (TritonX-100 no iónico y SDS aniónico). Hemos observado que la adición de surfactantes no iónicos aumenta la corriente de oscuridad y la formación de poros cónicos. Por otro lado, el uso de surfactantes aniónicos reduce considerablemente la corriente de oscuridad y poros cilíndricos se pueden producir casi sin dificultad.Aplicando las reglas explicadas arriba se han obtenido matrices altamente uniformes de macroporos con diferente distribución y dimensiones.Por último, también se presentan algunos resultados preliminares sobre aplicaciones novedosas de silicio macroporoso. Las características estructurales de las matrices de macroporos se han utilizado para fabricar pilares de óxido de silicio que podrían encontrar aplicaciones en la biotecnología como plataformas tridimensionales para detección de reconocimiento de moléculas o como matrices de microjeringas. También se ha fabricado un filtro que consiste en membranas de silicio macroporoso y se han medido sus características ópticas. Este filtro se comporta como pasabajas cuando la luz incidente es paralela a los poros. Los resultados obtenidos son solamente cuantitativos y sugieren una futura optimización del proceso de ataque para fabricar muestras de alta calidad.Asimismo se ha introducido modulación periódica del diámetro de los poros en profundidad y se han fabricado matrices de "ratchet-type" macroporos, los cuales podrían tener aplicaciones como dispositivos para separación de partículas. Se ha demostrado que mediante unos pocos pasos adicionales las matrices de macroporos modulados se pueden convertir en microestructuras tridimensionales de huecos interconectados. Esta técnica se puede aplicar para la fabricación de cristales fotónicos tridimensionales.Photonic crystals are artificially created materials that can do to photons what an ordinary semiconductor does to electrons: that is to say, they can exhibit a photonic band gap, a situation in which photons with certain energies cannot propagate inside the crystal, regardless of polarization and propagation direction. The photonic band gap is therefore likely to be the true optical analog of the fundamental gap of a semiconductor. Since their invention in 1987, photonic crystals have triggered considerable interest because of their unusual optical properties. The unique properties of photonic crystals also led to their study being recognized as a new and major field of optoelectronics.Macroporous silicon, with its high dielectric contrast, very high aspect ratios and full compatibility with the silicon microelectronic industry is an excellent model system for studying the optical properties of two-dimensional and even three-dimensional photonic crystals. Besides, macroporous silicon has been shown to have several unique uses in many other fields, like electronics, micromachining, gas sensing and biotechnology. Research into macroporous silicon is continuously growing, prompted by its enormous potential for applications.The work presented in this thesis deals with two subjects: photonic band structure simulations and analysis of 2D photonic crystals, and the fabrication of macroporous silicon structures suitable for application as 2D infrared photonic crystals.Since many potential applications of photonic crystals are based on their photonic band gaps, it is of interest to design photonic crystals with an absolute band gap that is as large as possible. In this thesis we describe a way to enlarge the absolute photonic band gap, showing the role that symmetry plays in designing optimal photonic structures. We have examined how reducing symmetry by inserting additional elements into the lattice unit cell or by changing the shape of the scatterers alters the dispersion behavior of the TMand TE-polarization modes in 2D photonic crystals. Our goal was to maximize the absolute PBG width by breaking the symmetry of the simple square and triangular lattices and thus to construct new structures, the so-called hybrid lattices. Using the FDTD method for photonic band structure calculations, we performed a detailed numerical analysis of the photonic dispersion relation in 2D hybrid lattices that consist of air holes drilled in silicon.For square lattices, the symmetry reduction approach has been successfully applied to maximize the absolute PBG width. In the case of square lattices of circular air rods, the inclusion of an additional rod increases the absolute PBG threefold. For the case of square lattices of square air rods, the rotation of the rods plays a critical role in the opening of an absolute PBG. No absolute PBG was found if the square rods were not rotated. The size of the absolute PBG is improved most significantly by a combination of the inclusion of an additional rod and the rotation of square rods. Moreover, a new absolute PBG is generated that persists over a wide range of rotation angles and filling fractions, which are far from the closed-packed condition. This greatly favors the fabrication of photonic crystals.The largest absolute PBG is the one for the triangular lattice of circular air rods.Our results have shown that modifying the triangular structure by adding interstitial rods or using square rods (even though the rods are rotated) is not a good way of achieving a larger absolute PBG, at least for the special case of air/silicon structures. Adding more rods to the lattice unit cell cannot further enlarge the absolute PBG width.We have made a detailed quantitative analysis of the absolute PBGs in 2D triangular and honeycomb lattices considering that there is an interfacial (shell) layer between the rods and the background dielectric matrix. This interfacial layer may be the unwanted result of the fabrication process itself or created intentionally. The properties of the photonic gaps are strongly affected by the thickness and the dielectric constant of the shell layer. The results of band structure simulations show that for structures consisting of air rods embedded in a dielectric background this layer reduces the absolute photonic gap.For structures consisting of dielectric rods in air, however, an interfacial layer can yield larger photonic gaps if the dielectric constant of the layer is greater than that of the rods.This provides further flexibility in the practical realization of such 2D photonic crystals.For example, in certain cases we may not be able to obtain dielectric rods of the required diameter or of the particular material we need because of technological limitations.However, we are enabled to grow the rods of materials with lower dielectric constants, for which a well-developed technology exists. The rods can then be covered with the required dielectric by deposition, thus achieving almost the same gap properties as those of the ideal shell-less structure.We have developed an electrochemical etching set-up for fabricating 2D periodic structures based on macroporous silicon formation. We have also made a detailed study of how the electrochemical etching parameters influence the pore morphology. Straight and stable macropores can only be etched if all parameters of the etching process (doping level, HF concentration, etching current, anodic potential, temperature, etc.) are properly adjusted. The optimal conditions are only a very tiny part of the total parametric space, which requires a fine control of the process. For example, HF concentrations higher than 10 wt.%, which are commonly used for growing micro- and mesoporous films, are not suitable for growing deep, straight macropores. Relatively high anodic potentials (e.g. even higher than 2 V for our samples) inevitably enhance the formation of spiking breakdowntype pores on macropore walls. On the other hand, low anodic potentials (less than 1 V) usually lead to unstable pore growth with macropores that are partially or fully filled with microporous silicon.Of all etching parameters the applied etching current is the most critical. Current densities greater than the critical current density Jps, which depends on the temperature and electrolyte concentration, will move the system into the electropolishing regime.Controlling the etching current during the process is a key issue. Keeping the etching current constant was found not to be sufficient to grow deep, straight macropores. Two effects that influence the pore shape in depth were identified. First, the decrease in HF concentration towards the pore tips because of diffusional limitations leads to an increase of the pore diameter close to the tip. Second, the pore surface area increases for long anodization times, which leads to an increase in the dark current density and yields conical pores, the diameter of which decreases with depth. Increasing the etching current accordingly, which means to etch pores with the reverse conical shape is one of the methods to reduce the pore conicity. Performing the etching at lower temperatures and bubbling the electrolyte with nitrogen can reduce the dark current and produce less conical pores. Another effective way is to use appropriate surfactants. Surfactants are commonly used to prevent degeneration caused by bubbles sticking to the sample surface. Two surfactants of different types (nonionic TritonX-100 and anionic SDS) were tested. We found that the addition of nonionic surfactants increases the dark current contribution and thus enhances the formation of conical pores. On the other hand, the use of anionic surfactants considerably reduces the dark current and straight pores can be formed almost without difficulty. Highly uniform macropore arrays with different arrangements and dimensions were obtained by applying these "compensation" rules.Finally, we have also presented some preliminary results of our work on novel applications of macroporous silicon. The structural features of the etched macropore arrays have been exploited to fabricate high-aspect-ratio silicon dioxide pillars, which may have applications in biotechnology as a 3D sensor platform for molecular recognition detections or as dense arrays of microsyringes for fluid delivery or precise chemical reaction stimulation. We have also fabricated a macroporous filter consisting of through-wafer pores and measured its optical characteristics. For light incidence parallel to the pores, a shortpass spectral behavior has been observed. The obtained results are only qualitative and suggest that further optimization of the etching process is needed in order to produce higher quality samples. We were also able to introduce periodic modulations of the pore diameter in depth and to fabricate ratchet-type macropore arrays, which have been envisioned for applications as ratchet devices for large-scale particle separation. We have shown that by a few post-etching steps the modulated macropore arrays can be converted into microstructures consisting of interconnected voids in all three dimensions. The technique used can be exploited for the fabrication of fully 3D photonic crystals

Selective area epitaxy of III-V nanowires: toward nanowire-on-silicon tandem solar cells

Nanowires grown via the selective area epitaxy technique (SAE-NWs) are of great research interest for use in next-generation electronic and electro-optic devices. As compared to other nanowire growth techniques commonly studied, SAE-NW is a highly controllable process due to the use of a lithographically defined growth mask, and the lack of need for a catalytic seed particle results in impurity-free material with nearly atomically flat sidewalls formed on low index crystal facets. In this thesis, the SAE-NW growth technique is examined and progress in the field is reviewed. A study of the geometric evolution of SAE-NWs during growth is presented, followed by results of efforts to work towards fabrication of a tunnel diode for use in a nanowire-on-silicon solar cell. Finally, future directions for the continued study of SAE-NWs are outlined