<p>The heteroepitaxial growth of InAs self-assembled quantum wires on vicinal substrates is investigated. InGaAlAs lattice-matched to InP was first deposited onto an InP(001) substrate with and without a 0.9 degree off-cut toward the (110) direction, followed by the deposition of a strained layer of InAs. Dense InAs quantum wires were successfully grown on both nominally flat and vicinal substrates in order to observe the effect of the presence of atomic steps. The off-cut angle was chosen based on the wire spacing on a flat substrate to serve as a template for their nucleation and improve their size distribution for use as 1.55 um wavelength lasers required by the telecommunications industry. The results have shown a modest but statistically significant improvement in the width of their size distribution. In addition, a kinetic Monte Carlo simulation including full strain calculations was developed to further understand the nucleation process. The model developed here disproves the idea that InAs quantum wires are aligned towards the (-110) direction due to diffusion anisotropy. The simulation of the formation of quantum wires similar to those observed experimentally has been achieved and the Stranski-Krastanow growth mode is demonstrated.</p> / Master of Materials Science and Engineering (MMatSE)

<p>Light emitting diode (LED)-based solid state displays (SSD) have attracted growing interest due to their advantages in terms of contrast ratio, brightness, viewing angle, and response time compared to liquid crystal displays. GaN based III-nitride thin film materials are suitable materials for SSD due to their wide and tunable bandgaps. However, the large size and costly manufacturing process of commercially available GaN-based LED chips limit the potential uses of LEDs as the pixels of SSD.</p> <p>In this work, tiny single crystal beta-phase (111) oriented SiC whiskers 2 microns in diameter and 18 microns in length are proposed as the substrates for GaN growth due to their small lattice constant mismatch (3%) with GaN, their conductive nature and their small size for potential use in SSD pixels. Aligned SiC whiskers with (111) planes exposed in an alumina matrix prepared by a precise manipulation and alignment method of SiC whiskers including a series of steps was developed in this work. The alignment degree of whiskers achieved in this work is higher than conventional extrusion methods, and a sintering approach capable of forming an aligned alumina/SiC composite was developed and understood using a self-limiting oxidation reaction mechanism.</p> <p>To take advantage of the potential versatility, scalability and cost effectiveness of sputtering for SSD manufacturing, a reactive sputtering system was built for a detailed investigation of GaN thin film growth nucleation and subsequent growth behavior on SiC. 6H-SiC single crystal substrates were chosen as a reference substrate for SiC whiskers. An XRRC indicates that a high quality single crystalline GaN thin film was successfully grown epitaxially on 6H-SiC by sputtering. Two-dimensional X-ray diffraction and scanning transmission electron microscopy results demonstrated that the epitaxial growth of GaN thin films relies on the short range order and/or crystalline area of the native oxide layer in GaN/SiC interface for the first time.</p> / Doctor of Philosophy (PhD)

Microfluidic device construction uses certain critical structures throughout many different applications. The valve structure remains one of the primary structures that present a barrier to miniaturization and portability. The extensive support devices required to power common microfluidic valves remove a significant amount of freedom from microfluidic device design. Moving to electrostatic methods of actuation could reduce the overall footprint of the microfluidic valve. This thesis covers three concept prototypes. Concept I presents an attempt at inlaying gold electrodes into polydimethylsiloxane substrates. Concept II attempts to use liquid silver injected into channels as electrode materials. Concept III uses aluminum sputtering to fabricate valve electrodes. Each device encountered complications during fabrication which led to improved fabrication guidelines for future devices. Designing and fabricating these concept devices required the development of several new processes in the clean room, including RIE Plasma bonding, PDMS sputtering techniques, and multilayer PDMS thin film fabrication. The PDMS sputtering technique in particular allows profilometry measurement of PDMS surfaces without risk of damaging the profilometer tip, a development that could allow for much more control over PDMS film thicknesses in future projects.

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.

Conjugated polymer based electronics, a type of flexible electronic devices, can be produced from solution by traditional printing and coating processes in a roll-to-roll format such as papers and graphic films. This shows great promise for the emerging energy generation and conversion. The device performance of polymer electronics is largely dependent of crystalline structures and morphology of photoactive layers. However, the solution crystallization kinetics of conjugated polymers in the presence of electron acceptor nanoparticles has not been fully understood yet. In this study, solution crystallization kinetics of poly (3-hexylthiophene) in the presence of carbon nanotubes and graphene oxide has been investigated by using UV-visible absorption spectroscopy and transmission electron microscope. Various kinetics parameters such as crystallization temperature, polymer solution concentration and nanoparticle loading will be discussed. The crystallization rate law and fold surface free energy will be addressed by using polymer crystallization theory of heterogeneous nucleation. This fundamental study will provide a foundation of fabricating high efficiency polymer based electronics.

CONSTRUCTING AND OPTIMIZING A SINGLE WAFER SOLAR CELL ARRAY IN THE MICROFABRICATION LAB AT CALIFORNIA POLYTECHNIC STATE UNIVERSITY AT SAN LUIS OBISPO Solar cells are more and more becoming a significant source of energy in the world today. They are used to power entire buildings as well as small devices and everything in between, and are utilized all around the world. Smaller solar devices, such as hearing aid battery chargers, cost a lot of money relative to the monetary wealth in third-world countries. For this purpose, a less expensive, more efficient solar cell array should be developed. This study contains research that details all aspects of how solar cells work. It also details three years’ worth of studies at California Polytechnic State University (Cal Poly) that attempt to fabricate a solar cell array on a single wafer. Two tests were carried out that will help determine the optimal attributes of the solar cells. The first compared a solar cell made on a 10 µm thick silicon on insulator (SOI) wafer to solar cells made with the exact same masks on a 500 µm thick wafer. The thicker solar cell had 2.5 times the maximum power as the SOI solar cell. Aspects of the solar cell that would need to be improved are: increase thickness to between 70-100 µm from the SOI thickness, texture the front surface, add a passivation layer on the front surface, decrease the contact resistance for the metal electrodes, and add in a rear reflector. The next test was all about analyzing the metal contacts and interconnects. Ten gold-silver filled epoxy-gold bonds were constructed and measured ten times each, giving a grand mean between 10 and 11 Ω. Another short test was run with a commercial solar cell to characterize the change in power based on the series resistance. It was discovered that the both the epoxy and the gold add too much to the resistance. To fix this, a silver solder-like paste and a thicker contact metal should be used. There is also a derivation that details the design of a top contact layer that optimizes the finger spacing and finger width based on other solar cell factors. With the materials available at Cal Poly, a solar cell array can be fabricated on a single wafer. When accounting for the materials and processes available to the scientific community as a whole, a very effective and efficient solar cell can be fabricated.

Electrostatically actuated silicon membranes were designed, modeled, fabricated, and characterized. The intended application was for use in a microspeaker. Fabrication issues necessitated the use of thick diaphragms with a large gap between the electrodes. The devices did not function as speakers but did show actuation with a high DC voltage. Device dimensions were chosen by examining membrane mechanics, testing the processing steps required for device fabrication, and modeling with COMSOL. Several adhesives were researched to fabricate the device sidewalls, including BCB, PMMA, and TRA-Bond F112. A method for patterning PMMA through photolithography was found using a scanning electron microscope. Masks were designed in AutoCAD to create the electrostatically actuated devices and a microfabrication process was developed to produce diaphragms that could be characterized. Twenty micron thick diaphragms were fabricated by etching an SOI wafer in 25% TMAH and the etch depth was measured with a profilometer. Glass slides were coated with gold and patterned with positive photoresist to create counter-electrodes. The diaphragms were bonded to the glass slides using a forty micron thick layer of patterned SU-8 as sidewalls. Bonding was successful in the initial fabrication testing but not successful for the final devices. The final fabrication run resulted in eight devices that were partially bonded. Three devices were chosen to test the membrane actuation and the data analyzed for statistical significance. A DC voltage was applied to the electrodes with a MEMS driver and the change in force measured with a micro-force displacement system. Data analysis showed device actuation at high voltages (300V) for the medium and large devices.

Quantum dots are semiconductor nanocrystals that have tunable emission through changes in their size. Producing bright, efficient quantum dots with stable fluorescence is important for using them in applications in lighting, photovoltaics, and biological imaging. This study aimed to optimize the process for coating CdSe quantum dots (which are colloidally suspended in octadecene) with a ZnS shell through the pyrolysis of organometallic precursors to increase their fluorescence and stability. This process was optimized by determining the ZnS shell thickness between 0.53 and 5.47 monolayers and the Zn:S ratio in the precursor solution between 0.23:1 and 1.6:1 that maximized the relative photoluminescence quantum yield (PLQY) while maintaining a small size dispersion and minimizing the shift in the center wavelength (CWL) of the fluorescence curve. The process that was developed introduced a greater amount of control in the coating procedure than previously available at Cal Poly. Quantum yield was observed to increase with increasing shell thickness until 3 monolayers, after which quantum yield decreased and the likelihood of flocculation of the colloid increased. The quantum yield also increased with increasing Zn:S ratio, possibly indicating that zinc atoms may substitute for missing cadmium atoms at the CdSe surface. The full-width at half-maximum (FWHM) of the fluorescence spectrum did not change more than ±5 nm due to the coating process, indicating that a small size dispersion was maintained. The center wavelength (CWL) of the fluorescence spectrum red shifted less than 35 nm on average, with CWL shifts tending to decrease with increasing Zn:S ratio and larger CdSe particle size. The highest quantum yield was achieved by using a Zn:S ratio of 1.37:1 in the precursor solution and a ZnS shell thickness of approximately 3 monolayers, which had a red shift of less than 30 nm and a change in FWHM of ±3 nm. Photostability increased with ZnS coating as well. Intense UV irradiation over 12 hours caused dissolution of CdSe samples, while ZnS coated samples flocculated but remained fluorescent. Atomic absorption spectroscopy was investigated as a method for determining the thickness of the ZnS shell, and it was concluded that improved sample preparation techniques, such as further purification and complete removal of unreacted precursors, could make this testing method viable for obtaining quantitative results in conjunction with other methods. However, the ZnS coating process is subject to variations due to factors that were not controlled, such as slight variations in temperature, injection speed, and rate and degree of precursor decomposition, resulting in standard deviations in quantum yield of up to half of the mean and flocculation of some samples, indicating a need for as much process control as possible.

<p>The energy diagram for organic photovoltaics (OPV), involving the bulk heterojunction (BHJ), on which the device analysis is usually based, has long been a subject of debate. The widely used Metal-insulator-Metal model and P-type Schottky Junction model, both of which are based on inappropriate assumptions, could be incorrect to explain the working principle of BHJ OPV.</p> <p>To further explore the controversy, we start the investigation from the opposite direction, to the usually asymmetrical OPV, involving electron and hole passages, by introducing a pair of symmetric electrodes to a BHJ, to form a completely symmetrical device structure, which, in theory, would produce zero output.</p> <p>Surprisingly, it is found that such a symmetrical device exhibits asymmetrical I-V curves. In particular, it produces a non-zero open-circuit voltage, and a finite short-circuit current. The cause of the output was the asymmetrical charge carrier distribution due to the asymmetrical illumination. To explain the operational mechanism of the symmetrical device, the equivalent circuit including a pair of inverse-parallel diodes and a new model for the BHJ energy diagram are introduced. Those findings would certainly improve the understanding of the device physics of OPV, especially the working principle for BHJ.</p> / Master of Materials Science and Engineering (MMatSE)

<p>Please email me at kdunn@celccocontrols.com to confirm receipt of my thesis.</p> <p>Thanks,</p> <p>Kayne</p> / <p>In current microelectronic interconnect technology, significant delay is incurred due to capacitances in the intermediate and global interconnect layers. To avoid capacitive effects optical interconnects can be used; however conventional technologies are expensive to manufacture. One method to address these issues is to make use of quantum confinement effects and states lying within the bandgap of the material to enhance luminescence in a CMOS compatible silicon based system. Thin SiCxNy films appear to be suitable to work as luminescent silicon based films due to their lower direct bandgap and chemical stability but have not yet been studied in great detail.</p> <p>This thesis is an exploratory work aiming to assess the suitability of SiCxNy films for the above applications and to identify future research areas. The films analyzed in this thesis were manufactured on the inductively coupled plasma-chemical vapour deposition reactor (ICP-CVD) at McMaster University. The ICP-CVD produces films of high uniformity by using a remote RF plasma and an arrangement of high vacuum pumps to attain a vacuum on the order of 10-7Torr.</p> <p>Several experimental techniques have been used to analyse the films. The complex index of refraction has been determined through the use of ellipsometry giving results typical of that of a-SiNx:H. The photoluminescence spectroscopy results show a large broad emission peak with at least one shoulder at higher energies. The precise luminescence mechanism(s) could not be identified though a strong relationship with the bonding state of nitrogen has been found. The composition and structure of the films, as determined through ion beam measurements, infrared absorption measurements, and transmission electron microscopy measurements demonstrate the formation of a two phase structure consisting of carbon rich clusters surrounded by a mostly silicon nitride matrix. These carbon rich regions have some graphitic character and act to dampen the luminescence.</p> / Master of Applied Science (MASc)