Thesis (Ph. D.)--Physics, Georgia Institute of Technology, 2008. / Committee Chair: James Gole; Committee Member: Ahmet Erbil; Committee Member: Alexei Marchenkov; Committee Member: Meilin Liu; Committee Member: Peter Hesketh.

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The photoluminescence (PL) emitted by porous silicon has been investigated under different conditions of excitation using a pulsed nitrogen laser source, and the continuous tunable DV synchrotron source at Daresbury Laboratory. The project involved sample preparation, and PL measurements using a custom-built optical laser-based system for lifetime measurements. This in itself necessitated software and hardware development to enable interfacing and data-logging using an IBM-compatible PC. The equipment development formed a major part of the project.

This thesis describes the fabrication and modification of porous silicon and titania structures for the purposes of energy storage and conversion. The first chapter provides the reader with background information on porous silicon, batteries, and photocatalysis. The second chapter describes porous silicon fabrication methods and the equipment used in these studies. The third and fourth chapters are journal articles which describe the results of efforts to produce a porous silicon electrode for lithium ion batteries. The fifth chapter is a journal article detailing the fabrication of a thin, free-standing porous silicon film which can be activated for possible photovoltaic and microreactor applications. The last chapter describes the formation of novel silver/silver oxide seed structures for titania photocatalyst nanostructures to be prepared for deposition on a porous silicon support interface.

No description available.

Acoustic filters for signal filtering are used in wireless technologies operating at gigahertz frequencies for communication systems such as next generation cell phones. Multilayered porous silicon structures have been fabricated from silicon wafers to create the Bragg mirror section of a bulk acoustic wave filter. These porous silicon multilayers have been designed for use from 500 MHz – 20 GHz with primary focus on frequencies at 1 GHz. The porous silicon multilayers consist of alternating layers of high and low acoustic impedance layers on a bulk silicon substrate. They are fabricated using electrochemical etching where the current density during the etch determines the porosity and hence acoustic impedance of each layer. Bragg mirrors, FabryPerot filters, microcavities and rugate filters can be produced in this way due to the control of the tuneable porosity profile throughout the structure. The porosity of the layer modifies the elastic constants of the layer such as the Young’s modulus and hence the velocity of the bulk acoustic waves travelling through it. The behaviour of bulk acoustic waves through silicon is known but in order to fabricate porous silicon acoustic filters, the dependence of the longitudinal wave velocity as a function of porosity must also be known. This has been studied using acoustic transmission measurements on single porous silicon layers and then extended to multilayered structures. Rugate filters are single frequency filters that have not previously been studied for acoustic applications. In this study the first acoustic rugate filters have been fabricated using porous silicon material that exhibit only one stopband near 1 GHz. Bragg mirrors have been made with acoustic transmission measurements showing the locations of the stopbands. Porous silicon microcavities have also been fabricated along with filters that have apodisation functions. This work could form the basis of future efforts to produce and incorporate allSi multilayers into acoustic filters that are easily fabricated at a high level of quality and reliability that will serve to be efficient and cost effective.

When porous silicon is plated with a catalytic metal, the two materials can act together as a single entity whose electrical properties are sensitive to its environment – the sensing component of an electrochemical gas sensor. Etching pores into silicon is an electrochemical process; and which type of doped silicon used is one of its key parameters. For nearly all reported porous silicon gas sensors, the silicon has been of the p-doped variety – because p-doped porous etching is better understood and the layers that result from it are more predictable – despite n-doped silicon having potentially significant benefits in ease of fabrication and being more conducive to plating by a catalyst. This experiment is an attempt at creating a palladium plated n-doped porous silicon layer, and an examination into what differentiates this fabrication process and the layers that result from the traditional p-doped type. The porous layers to be plated are to be the same and would ideally have properties that are a close approximation to what a functional gas sensor would require. This experiment defined a process that fabricated this “ideal” layer out of N-type, , double polished silicon wafers with a resistance of 20 Ω cm. The wafers were subjected to the anodic etching method with an HF/ethanol mixture as the electrolyte; and only two (of among many) fabrication parameters were varied: HF concentration of the electrolyte and total etching time. We find that a concentration of 12% HF (by volume) and an etching time of 6 hours result in layers most appropriate to carry into plating. The anodization current density is 15 mA cm-2. Deposition of the catalyst, palladium, is done using the electroless method by immersing the porous layer in a .001M PdCl2 aqueous bath. Characterization of this Pd/Porous Silicon layer was done by measuring resistivity by four point probe and imaging through Scanning Electron Microscopy. It was found that layers of a maximum average of 63 ± 6% porosity were created using our fabrication method. There is evidence of palladium deposition, but it is spotty and irregular and is of no improvement despite the n-doping wafer makeup. Resistivity in well-plated regions was measured to be 7-10 Ωcm, while resistivity in regions not well-plated was measured to be 70-140 Ω cm. This is comparable to previous literature values, indicating n-silicon porous silicon can be fabricated and still have potential as a catalytic layer, should metal deposition methods improve.

The primary research aim in this thesis is to demonstrate the versatility of porous silicon based nanomaterials for biomaterial and photonic applications. In chapter 2 of this thesis, the suitability of porous silicon as a biomaterial was investigated by performing different surface modifications on the porous silicon films and evaluating biocompatibility of these surfaces in vitro. The porous silicon surfaces were characteriszed by means of atomic force microscopy (AFM), scanning electron microscopy (SEM), diffuse reflectance infrared spectroscopy (DRIFT) and interferometric reflectance spectroscopy (IRS). Cell attachment and growth was studied using fluorescence microscopy and cell viability assays. Both fabrication of the porous silicon films and subsequent surface modifications were demonstrated. Polyethylene glycol functionalised porous silicon prevented cell attachment, whilst collagen or fetal bovine serum coating encouraged cell attachment. Surface modifications were also performed on porous silicon films with different pore sizes and the influence of pore size and surface modification on primary hepatocyte growth was recorded over a course of 2 weeks by means of laser scanning confocal microscopy (LSCM), toxicity and metabolic assays. On collagen-coated surfaces with average pore sizes of 30 nm, multilayer cells stacks were formed. This stacking behaviour was not observed on samples with smaller pore sizes (10 nm), or in the absence of collagen. Hepatocytes remained viable and functional (judging by a metabolic assay) for 6 days, after which they generally underwent apoptosis. Collagen-coated porous silicon films showed later onset of apoptosis than porous silicon films not coated with collagen or collagen-coated flat silicon.. In chapter 3 of this thesis, the nitrogen laser of a laser desorption/ionization (LDI) mass spectrometer was used to selectively ablate regions on porous silicon films that had been functionalised with a non-fouling polyethylene oxide layer, affording a microscale patterning of the surface. Surface characterization was performed by means of AFM, SEM, LDI mass spectrometry, DRIFT and IRS. This approach allowed the confinement of mammalian cell attachment exclusively on the laser-ablated regions. By using the more intense and focussed laser of a microdissection microscope, trenches in a porous silicon film were produced of up to 50 micron depth, which allowed the construction of cell multilayers within these trenches, mimicking the organization of liver cords in vivo. Fluorescent staining and LSCM was used to study cell multilayer organization. To gain a better understanding of how surface topography influences cell attachment and behaviour, porous silicon films were fabricated containing a gradient of pore sizes by means of asymmetric anodisation (chapter 4). These gradients allowed the investigation of the effect of subtle changes of pore size on cell behaviour on a single sample. Analysis by means of LSCM and SEM showed that pore size can dictate cell size and area as well as cell density. In addition, a region of pore size where cell attachment and proliferation was strongly discouraged was also identified. This information can prove to be useful for designing non-biofouling surface topographies. Using the same asymmetric anodisation setup, photonic mirrors gradients were produced and overlaid over one another to produce multidirectional lateral photonic mirror gradients that display a series of roving spectral features (photonic stop-bands) from each gradient layer (chapter 4). These multidirectional photonic gradients have the potential to serve as optical barcodes or contributing to the development of graded refractive index devices such as lenses for high quality image relay and graded-index optical fibers.

Graduation date: 1996

High performance inductors are playing an increasing role in modern communication systems. Despite the superior performance offered by discrete components, parasitic capacitances from bond pads, board traces and packaging leads reduce the high frequency performance and contribute to the urgency of an integrated solution. Embedded inductors have the potential for significant increase in reliability and performance of the IC. Due to the driving force of CMOS integration and low costs of silicon-based IC fabrication, these inductors lie on a low resistivity silicon substrate, which is a major source of energy loss and limits the frequency response. Therefore, the quality factor of inductors fabricated on silicon continues to be low. The research presented in this thesis investigates amorphous Si and porous Si to improve the resistivity of Si substrates and explores amorphous Si as a structural material for low temperature MEMS fabrication. Planar inductors are built-on undoped amorphous Si in a novel application and a 56% increase in quality factor was measured. Planar inductors are also built-on a porous Si and amorphous Si bilayer and showed 47% improvement. Amorphous Si is also proposed as a low temperature alternative to polysilicon for MEMS devices. Tunable RF MEMS inductors and transformers are fabricated based on an amorphous Si and aluminum bimorph coil that is suspended and warps in a controllable manner. The 3-D displacement is accurately predicted by thermomechanical simulations. The tuning of the devices is achieved by applying a DC voltage and due to joule heating the air gap can be adjusted. A tunable inductor with a 32% tuning range from 5.6 to 8.2 nH and a peak Q of 15 was measured. A transformer with a suspended coil demonstrated a 24% tuning range of the mutual coupling between two stacked windings. The main limitation posed by post-CMOS integration is a strict thermal budget which cannot exceed a critical temperature where impurities can diffuse and materials properties can change. The research carried out in this work accommodates this temperature restriction by limiting the RF fabrication processes to 150°C to facilitate system integration on silicon.