Matching Items (27)
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- All Subjects: Materials Science
- All Subjects: Characterization
- Creators: Goryll, Michael
Description
The past two decades have been monumental in the advancement of microchips designed for a diverse range of medical applications and bio-analysis. Owing to the remarkable progress in micro-fabrication technology, complex chemical and electro-mechanical features can now be integrated into chip-scale devices for use in biosensing and physiological measurements. Some of these devices have made enormous contributions in the study of complex biochemical processes occurring at the molecular and cellular levels while others overcame the challenges of replicating various functions of human organs as implant systems. This thesis presents test data and analysis of two such systems. First, an ISFET based pH sensor is characterized for its performance in a continuous pH monitoring application. Many of the basic properties of ISFETs including I-V characteristics, pH sensitivity and more importantly, its long term drift behavior have been investigated. A new theory based on frequent switching of electric field across the gate oxide to decrease the rate of current drift has been successfully implemented with the help of an automated data acquisition and switching system. The system was further tested for a range of duty cycles in order to accurately determine the minimum length of time required to fully reset the drift. Second, a microfluidic based vestibular implant system was tested for its underlying characteristics as a light sensor. A computer controlled tilt platform was then implemented to further test its sensitivity to inclinations and thus it‟s more important role as a tilt sensor. The sensor operates through means of optoelectronics and relies on the signals generated from photodiode arrays as a result of light being incident on them. ISFET results show a significant drop in the overall drift and good linear characteristics. The drift was seen to reset at less than an hour. The photodiodes show ideal I-V comparison between photoconductive and photovoltaic modes of operation with maximum responsivity at 400nm and a shunt resistance of 394 MΩ. Additionally, post-processing of the tilt sensor to incorporate the sensing fluids is outlined. Based on several test and fabrication results, a possible method of sealing the open cavity of the chip using a UV curable epoxy has been discussed.
ContributorsMamun, Samiha (Author) / Christen, Jennifer Blain (Thesis advisor) / Goryll, Michael (Committee member) / Yu, Hongyu (Committee member) / Arizona State University (Publisher)
Created2011
Description
Carrier lifetime is one of the few parameters which can give information about the low defect densities in today's semiconductors. In principle there is no lower limit to the defect density determined by lifetime measurements. No other technique can easily detect defect densities as low as 10-9 - 10-10 cm-3 in a simple, contactless room temperature measurement. However in practice, recombination lifetime τr measurements such as photoconductance decay (PCD) and surface photovoltage (SPV) that are widely used for characterization of bulk wafers face serious limitations when applied to thin epitaxial layers, where the layer thickness is smaller than the minority carrier diffusion length Ln. Other methods such as microwave photoconductance decay (µ-PCD), photoluminescence (PL), and frequency-dependent SPV, where the generated excess carriers are confined to the epitaxial layer width by using short excitation wavelengths, require complicated configuration and extensive surface passivation processes that make them time-consuming and not suitable for process screening purposes. Generation lifetime τg, typically measured with pulsed MOS capacitors (MOS-C) as test structures, has been shown to be an eminently suitable technique for characterization of thin epitaxial layers. It is for these reasons that the IC community, largely concerned with unipolar MOS devices, uses lifetime measurements as a "process cleanliness monitor." However when dealing with ultraclean epitaxial wafers, the classic MOS-C technique measures an effective generation lifetime τg eff which is dominated by the surface generation and hence cannot be used for screening impurity densities. I have developed a modified pulsed MOS technique for measuring generation lifetime in ultraclean thin p/p+ epitaxial layers which can be used to detect metallic impurities with densities as low as 10-10 cm-3. The widely used classic version has been shown to be unable to effectively detect such low impurity densities due to the domination of surface generation; whereas, the modified version can be used suitably as a metallic impurity density monitoring tool for such cases.
ContributorsElhami Khorasani, Arash (Author) / Alford, Terry (Thesis advisor) / Goryll, Michael (Committee member) / Bertoni, Mariana (Committee member) / Arizona State University (Publisher)
Created2013
Description
Photodetectors in the 1.7 to 4.0 μm range are being commercially developed on InP substrates to meet the needs of longer wavelength applications such as thermal and medical sensing. Currently, these devices utilize high indium content metamorphic Ga1-xInxAs (x > 0.53) layers to extend the wavelength range beyond the 1.7 μm achievable using lattice matched GaInAs. The large lattice mismatch required to reach the extended wavelengths results in photodetector materials that contain a large number of misfit dislocations. The low quality of these materials results in a large nonradiative Shockley Read Hall generation/recombination rate that is manifested as an undesirable large thermal noise level in these photodetectors. This work focuses on utilizing the different band structure engineering methods to design more efficient devices on InP substrates. One prospective way to improve photodetector performance at the extended wavelengths is to utilize lattice matched GaInAs/GaAsSb structures that have a type-II band alignment, where the ground state transition energy of the superlattice is smaller than the bandgap of either constituent material. Over the extended wavelength range of 2 to 3 μm this superlattice structure has an optimal period thickness of 3.4 to 5.2 nm and a wavefunction overlap of 0.8 to 0.4, respectively. In using a type-II superlattice to extend the cutoff wavelength there is a tradeoff between the wavelength reached and the electron-hole wavefunction overlap realized, and hence absorption coefficient achieved. This tradeoff and the subsequent reduction in performance can be overcome by two methods: adding bismuth to this type-II material system; applying strain on both layers in the system to attain strain-balanced condition. These allow the valance band alignment and hence the wavefunction overlap to be tuned independently of the wavelength cutoff. Adding 3% bismuth to the GaInAs constituent material, the resulting lattice matched Ga0.516In0.484As0.970Bi0.030/GaAs0.511Sb0.489superlattice realizes a 50% larger absorption coefficient. While as, similar results can be achieved with strain-balanced condition with strain limited to 1.9% on either layer. The optimal design rules derived from the different possibilities make it feasible to extract superlattice period thickness with the best absorption coefficient for any cutoff wavelength in the range.
ContributorsSharma, Ankur R (Author) / Johnson, Shane (Thesis advisor) / Goryll, Michael (Committee member) / Roedel, Ronald (Committee member) / Arizona State University (Publisher)
Created2013
Description
Electrospun nanofibers can be prepared from various kinds of inorganic substances by electro-spinning techniques. They have great potential in many applications including super capacitors, lithium ion batteries, filtration, catalyst and enzyme carriers, and sensors [1]. The traditional way to produce electrospun nanofibers is needle based electro-spinning [1]. However, electrospun nanofibers have not been widely used in practice because of low nanofiber production rates. One way to largely increase the electro-spinning productivity is needleless electro-spinning. In 2005, Jirsak et al. patented a rotating roller fiber generator for the mass production of nanofibers [2]. Elmarco Corporation commercialized this technique to manufacture nanofiber equipment for the production of all sorts of organic and inorganic nanofibers, and named it "NanospiderTM". For this project, my goal is to build a needleless electro-spinner to produce nanofibers as the separator of lithium ion batteries. The model of this project is based on the design of rotating roller fiber generator, and is adapted from a project at North Dakota State University in 2011 [3].
ContributorsQiao, Guanhao (Author) / Yu, Hongyu (Thesis director) / Jiang, Hanqing (Committee member) / Goryll, Michael (Committee member) / Barrett, The Honors College (Contributor) / Ira A. Fulton School of Engineering (Contributor)
Created2012-12
Description
The Metal Semiconductor Field Effect Transistor (MESFET) has high potential to enter analog and RF applications due to their high breakdown voltage and switching frequency characteristics. These MESFET devices could allow for high voltage analog circuits to be integrated with low voltage digital circuits on a single chip in an extremely cost effective way. Higher integration leads to electronics with increased functionality and a smaller finished product. The MESFETs are designed in-house by the research group led by Dr. Trevor Thornton. The layouts are then sent to multi-project wafer (MPW) integrated circuit foundry companies, such as the Metal Oxide Semiconductor Implementation Service (MOSIS) to be fabricated. Once returned, the electrical characteristics of the devices are measured. The MESFET has been implemented in various applications by the research group, including the low dropout linear regulator (LDO) and RF power amplifier. An advantage of the MESFET is that it can function in extreme environments such as space, allowing for complex electrical systems to continue functioning properly where traditional transistors would fail.
ContributorsKam, Jason (Author) / Thornton, Trevor (Thesis director) / Goryll, Michael (Committee member) / Barrett, The Honors College (Contributor) / Electrical Engineering Program (Contributor)
Created2015-05
Description
This research emphasizes the use of low energy and low temperature post processing to improve the performance and lifetime of thin films and thin film transistors, by applying the fundamentals of interaction of materials with conductive heating and electromagnetic radiation. Single frequency microwave anneal is used to rapidly recrystallize the damage induced during ion implantation in Si substrates. Volumetric heating of the sample in the presence of the microwave field facilitates quick absorption of radiation to promote recrystallization at the amorphous-crystalline interface, apart from electrical activation of the dopants due to relocation to the substitutional sites. Structural and electrical characterization confirm recrystallization of heavily implanted Si within 40 seconds anneal time with minimum dopant diffusion compared to rapid thermal annealed samples. The use of microwave anneal to improve performance of multilayer thin film devices, e.g. thin film transistors (TFTs) requires extensive study of interaction of individual layers with electromagnetic radiation. This issue has been addressed by developing detail understanding of thin films and interfaces in TFTs by studying reliability and failure mechanisms upon extensive stress test. Electrical and ambient stresses such as illumination, thermal, and mechanical stresses are inflicted on the mixed oxide based thin film transistors, which are explored due to high mobilities of the mixed oxide (indium zinc oxide, indium gallium zinc oxide) channel layer material. Semiconductor parameter analyzer is employed to extract transfer characteristics, useful to derive mobility, subthreshold, and threshold voltage parameters of the transistors. Low temperature post processing anneals compatible with polymer substrates are performed in several ambients (oxygen, forming gas and vacuum) at 150 °C as a preliminary step. The analysis of the results pre and post low temperature anneals using device physics fundamentals assists in categorizing defects leading to failure/degradation as: oxygen vacancies, thermally activated defects within the bandgap, channel-dielectric interface defects, and acceptor-like or donor-like trap states. Microwave anneal has been confirmed to enhance the quality of thin films, however future work entails extending the use of electromagnetic radiation in controlled ambient to facilitate quick post fabrication anneal to improve the functionality and lifetime of these low temperature fabricated TFTs.
ContributorsVemuri, Rajitha (Author) / Alford, Terry L. (Thesis advisor) / Theodore, N David (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2013
Description
In-field characterization of photovoltaics is crucial to understanding performance and degradation mechanisms, subsequently improving overall reliability and lifespans. Current outdoor characterization is often limited by logistical difficulties, variable weather, and requirements to measure during peak production hours. It becomes a challenge to find a characterization technique that is affordable with a low impact on system performance while still providing useful device parameters. For added complexity, this characterization technique must have the ability to scale for implementation in large powerplant applications. This dissertation addresses some of the challenges of outdoor characterization by expanding the knowledge of a well-known indoor technique referred to as Suns-VOC. Suns-VOC provides a pseudo current-voltage curve that is free of any effects from series resistance. Device parameters can be extracted from this pseudo I-V curve, allowing for subsequent degradation analysis. This work introduces how to use Suns-VOC outdoors while normalizing results based on the different effects of environmental conditions. This technique is validated on single-cells, modules, and small arrays with accuracies capable of measuring yearly degradation. An adaptation to Suns-VOC, referred to as Suns-Voltage-Resistor (Suns-VR), is also introduced to complement the results from Suns-VOC. This work can potentially be used to provide a diagnostic tool for outdoor characterization in various applications, including residential, commercial, and industrial PV systems.
ContributorsKillam, Alexander Cameron (Author) / Bowden, Stuart G (Thesis advisor) / Goryll, Michael (Committee member) / Augusto, Andre (Committee member) / Rand, James (Committee member) / Arizona State University (Publisher)
Created2022
Description
Highly reflective back surfaces are critical for reaching the detailed balance efficiency limits of photovoltaics. In addition to being highly reflective, the back surface and contact of the cell must have low resistance. A traditional approach to balance reflectance and contact resistance has been to use point contact geometries, which are process intensive. This work considers using a transparent conductive oxide and metal mirror, which, due to being two planar layers, can be fabricated much more easily. To study the tradeoff between resistance and absorptance for this contact, the oxide doping concentration is varied. Test structures to measure the doping concentration, contact resistance, and parasitic absorptance were fabricated. Using measured parameters, the performance of high-quality GaAs photonic power converters is modeled. Measurements show that although the contact resistance is comparatively high, it can be controlled through doping in the oxide and semiconductor composition. Furthermore, modeling shows the contact resistance is not prohibitively high for one-sun or lower illumination level devices. The hemispheric reflectance of the experimental oxide/metal back contact is modeled to be 96.7%, which is quite high considering that it is a conductive back contact. Although the oxide/metal contact structure does not perform electrically or optically as well as more complex point contact structures, this work indicates the advantages of the planar transparent conductive oxide/metal contact structure near one-sun equivalent current densities for solar cells and photonic power converters, where it is desirable to avoid the device fabrication costs of back contact patterning.
ContributorsGregory, Christopher Thomas (Author) / King, Richard R (Thesis advisor) / Goryll, Michael (Committee member) / Rolston, Nicholas (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2024
Description
The silicon-based solar cell has been extensively deployed in photovoltaic industry and plays an important role in renewable energy industries. A more energy-efficient, environment-harmless and eco-friendly silicon production technique is required for price-competitive solar energy harvesting. Silicon electrorefining in molten salt is promising for the ultrapure solar-grade Si production. To avoid using highly corrosive fluoride salt, CaCl2-based salt is widely employed for silicon electroreduction. For Si electroreduction in CaCl2-based salt, CaO is usually added to enhance the solubility of SiO2. However, the existence of oxygen in molten salt could result in system corrosion, anode passivation and the co-deposition of secondary phases such as CaSiO3 and SiO2 at the cathode. This research focuses on the development of reusable oxygen-free CaCl2-based molten salt for solar-grade silicon electrorefining. A new multi-potential electropurification process has been proposed and proven to be more effective in impurities removal. The as-received salt and the salt after electrorefining have been electropurified. The inductively-coupled plasma mass spectrometry and cyclic voltammetry have been utilized to determine the impurities removal of electropurification. The salt after silicon electrorefining has been regenerated to its original purity level before by the multi-potential electropurification process, demonstrating the feasibility of a reusable salt by electropurification. In an oxygen-free CaCl2-based salt without silicon precursor, the silicon dissolved from the silicon anode can be successfully deposited at the cathode. The silicon anode has been operated for more than 50 hours without passivation in the oxygen-free system. Silicon ions start to be deposited after 0.17 g of silicon has been dissolved into the salt from the silicon anode. A 180 µm deposit with a silver-luster surface was obtained at the cathode. The main impurities in the silicon anode such as aluminum, iron and titanium were not found in the silicon deposits. No oxygen-containing secondary phases are detected in the silicon deposits. These results confirm the feasibility of silicon electrorefining in the oxygen-free CaCl2-based salt.
ContributorsTseng, Mao-Feng (Author) / Tao, Meng (Thesis advisor) / Kannan, Arunachala Mada (Committee member) / Mu, Linqin (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2023
Description
In this project, current-voltage (I-V) and Deep Level Transient Spectroscopy (DLTS) measurements are used to (a) characterize the electrical properties of Nb/p-type Si Schottky barriers, (b) identify the concentration and physical character of the electrically active defects present in the depletion region, and (c) use thermal processing to reduce the concentration or eliminate the defects. Barrier height determinations using temperature-dependent I-V measurements indicate that the barrier height decreases from 0.50 eV to 0.48 eV for anneals above 200 C. The electrically-active defect concentration measured using DLTS (deep level transient spectroscopy) drops markedly after anneals at 250 C.
A significant increase in leakage currents is almost always observed in near-ideal devices upon annealing. In contrast, non-ideal devices dominated by leakage currents annealed at 150 C to 250 C exhibit a significant decrease in such currents.
A significant increase in leakage currents is almost always observed in near-ideal devices upon annealing. In contrast, non-ideal devices dominated by leakage currents annealed at 150 C to 250 C exhibit a significant decrease in such currents.
ContributorsKrishna Murthy, Madhu (Author) / Newman, Nathan (Thesis advisor) / Goryll, Michael (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2018