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Laser Guide Star (LGS) systems are essential for adaptive optics in ground-based astronomical observation. This dissertation demonstrates the feasibility of semiconductor-based LGS systems using the membrane external-cavity surface-emitting laser (MECSEL) platform, employing multiple quantum wells. Various laser cavity configurations were analyzed through simulations and experiments. The in-well pumping method was explored to reduce the quantum defect and address thermal limitations. Multi-pass pumping schemes were designed with Zemax modeling and demonstrated experimentally. To simplify multi-pass pumping, the hybrid-MECSEL (H-MECSEL) design was introduced. COMSOL modeling studied thermal management and thermal lensing effect. The H-MECSEL achieved approximately 30 W of output power at 1178 nm, frequency-doubled to over 10 W at 589.2 nm with TEM00 beam profile and 7 MHz linewidth. Hyperfine transitions within sodium D-lines were resolved using saturated absorption spectroscopy and dither-locking was implemented, stabilizing the H-MECSEL to the sodium D2a line over an hour with watt-level output power. Allan deviation calculations confirmed the stabilization efficacy. more...

Renewable energy sources, such as solar power, are rapidly expanding worldwide to meet increased energy demands. However, these sources are exposed to challenging environmental stressors, such as extreme wind, heavy snow loads, and hailstorms that can cause irreversible damage to the crystalline semiconductor solar cells. Today, single crystalline silicon solar cells dominate the market, and our work is focused on improving their reliability against environmental stressors. One of the main degradation mechanisms caused by environmental stressors is cell cracks. These microcracks can go virtually undetected and lead to reduced power output from solar modules over time. To solve this engineering problem, we have investigated the use of low-cost carbon nanotubes as an ingredient in silver metal matrix composite paste (MMC). In place of conventional commercial silver paste, the composite paste is used as mechanical reinforcement of the front side metallization of Passivated Emitter and Rear Contact (PERC) cells. The resulting metal contacts, also known as gridlines or fingers on silicon solar cells, show enhanced fracture toughness (> 7x), improved ductility where the critical strain increases by > 7x, and electrical gap-bridging of cell cracks (> 50 um gaps). In addition, the composite gridlines lead to improved cell efficiency by promoting silver particle sintering and lowering the contact resistance between gridlines and underlying silicon. Recent mini-module testing demonstrates that we can increase the module lifetime by > 1.5x against cyclic mechanical stress caused by environmental stressors. Our solution simultaneously improves module durability and efficiency. This dissertation work ranges from materials engineering, modeling more...

The Internet of Things (IoT) has become commonplace in society, but it has been demonstrated that many IoT systems are vulnerable to significant security exploits. This necessitates the need for a closer examination of IoT security. IoT design prerequisites are low power consumption and an emphasis on smaller die areas for increased production yield. Security on the software level typically provides adequate protection but there are hardware-level exploits that are difficult or impossible to counteract. Booting attacks, eavesdropping and interference, and Side-Channel Attacks (SCA) are exploits deployed against IoT devices on the hardware level. To combat these vulnerabilities, several lightweight hardware encryption techniques such as PRESENT-80, in particular, are developed for their lower power usage and smaller device footprint. However, studies show that PRESENT-80 is still vulnerable to SCA or power analysis attacks. Literature on improved PRESENT-80 circuits provides SCA resistance but the trade-offs are often an increase in device area and distinguishable power usage patterns. A recently developed adiabatic circuit technique introduced as Energy Efficient Positive Feedback Adiabatic Logic (EE-SPFAL) shows promise of resistance against SCA and consumes lower power compared to traditional CMOS digital circuits. However, the simulation results are solely presented through the schematic layer without the inclusion of post-layout simulation. This work aims to fill the gap by providing the layout design of EE-SPFAL standard cells and a prototype of 1 Round PRESENT-80 S-box using EE-SPFAL. Post-layout Correlation Power Analysis (CPA) attacks and energy consumption analysis are conducted and compared with the conventional CMOS circuit. In addition, this work aims to show the effects of parasitic capacitance within inter-logic cell routing on SCA resistance and highlight the limitations of schematic-only simulations against CPA attacks. This work is designed using open-source CAD tools from Efabless/Google with Skywater 130nm technology. more...

Neuromorphic computing mimics the functional components and structure of the human brain to achieve highly efficient computing with minimal resources and power consumption. Creating neuromorphic systems in Complementary Metal-Oxide-Semiconductor (CMOS) technology offers an alternative computing paradigm to Von neumann computing. However, implementing these systems on an CMOS Integrated Circuit (IC) poses major challenges. These challenges include implementing synaptic weight multiplication and weight tuning operation that conserves energy and occupies minimal area. Additionally, designing a network-on-chip architecture that is reconfigurable and offers a full-connectivity design space is crucial. Furthermore, implementing a complete architecture for nonlinear data processing and, specifically, online learning on-chip is another significant challenge. Memristors are emerging nano devices that exhibit behavior similar to synapse in the brain with few of the memristor types offering extremely small footprint and non-volatile resistance on-chip. These properties lead to small area and low-power IC designs. The device models for these emerging devices are crucial for designing circuits and systems that utilize them. In this work, we address the integration of memristors with the circuit simulation environment by modeling crucial features of these devices in a SPICE-compatible model. The presented model offers a very low 2-3% error in LRS tuning when compared to average experimental data while converging well in simulation. Further, we also present a reconfigurable and scalable network architecture that allows full connectivity while minimizing resource overhead for spike communication between proximal neurons by ensuring "spikes-as-spikes" communication. For spike communication between neurons farther away on-chip from each other, AER is utilized. Thus, a Hierarchical Circuit-Packet Hybrid approach for scalable neuromorphic systems is proposed. This technique leads to up to 48% savings in area and 90% savings in power for the neurons placed physically close to each other on-chip. Lastly, we present resource-efficient hardware architecture for online learning using the reservoir computing approach for non-linear signal processing. The application of wireless channel equalization is considered which includes reflection, inter-symbol interference, and signal attenuation and also has a memoryless non-linearity of a second order. The proposed approach leverages the circuit-switched Network-on-Chip (NoC), SpiCS-Net, as a robust "spikes-as-spikes" routing mechanism for handling input and output spikes within the reservoir. This strategy enables resource optimization by strategically reducing the number of output neurons, while simultaneously preserving the system’s flexibility to designate any neuron within the reservoir as an output neuron. The proposed approach offers upto 30% savings in area of the output layer in IBM CMOS10LPE 65 nm technology. more...

This study investigates the thermal performance of fins with perforations inspired by the Sierpinski carpet fractal pattern under natural convection conditions in both vertical and horizontal orientations. The objective is to analyze the effects of fractal iteration levels on three primary performance metrics: efficiency, effectiveness, and effectiveness per unit mass. Experimental evaluations were conducted for fractal fins from iterations 0 through 4, utilizing aluminum material, with each fin heated to controlled conditions. The results reveal that efficiency generally decreases with higher fractal iterations, reflecting increased thermal resistance associated with greater porosity and complex geometry. However, effectiveness per unit mass improves markedly with fractal iteration, highlighting the enhanced heat transfer capability per unit of material, especially in the vertical orientation where natural convection is supported by gravitational forces. The study demonstrates the potential for Sierpinski carpet fractal fins to serve as lightweight, high-performance solutions for thermal management applications. These findings provide a foundation for the development of optimized fractal designs in engineering applications requiring efficient heat dissipation and weight efficiency. In the vertical orientation, effectiveness begins at 60.56, decreases to about 48.95 by the second iteration, and then recovers to 60.81 at the highest iteration. The horizontal orientation starts at 60.31, drops to around 47.37 at the second iteration, and then rises to approximately 60.55 at the highest iteration. In the vertical orientation, effectiveness starts at about 60.56 but drops to around 48.95 by the second iteration, then recovers to approximately 60.81 at the highest iteration. The horizontal orientation shows a similar pattern, beginning at 60.31, dipping to around 47.37 by the second iteration, then increasing to about 60.55 at the highest iteration. more...

Nanoelectronic devices based on graphene are a promising technology that combines the sensitivity and specificity of ion and element recognition with the accuracy and precision of electronics. The detection principle is based on the interaction of the target molecules with the nanodevice surface, which generates a measurable electrical signal. In the current study, graphene and its derivatives, synthesis and fabrication of high quality, good uniformity, and low defects graphene have been investigated as they are critical for high-performance and highly sensitive devices. Among many synthesis methods, chemical vapor deposition (CVD), have been used that needs to be transferred from the metal substrates on which it is grown onto insulating substrates for practical applications. A novel complete pathway for fabrication of graphene-based devices is studied and comprehensively stated that addresses the challenges the separation and transferring of graphene onto insulating substrates i.e., glass slides, for a uniform single atomic layer without damaging or affecting graphene's structures and properties. Polymethyl Methacrylate (PMMA) assisted graphene transferring approach, fabrication of graphene devices, and electrical signal detection were demonstrated. To attend to the cleaning issue of residual PMMA from PMMA assisted graphene transfer, acetone vapor acetone cleaning protocol has been devised and demonstrated. RAMAN Spectroscopy and Atomic Force Microscopy (AFM) have been applied to characterize and ensure the quality of monolayer graphene. The method is shown to be a practical approach for transferring clean and residue-free graphene while preserving the underlying graphene lattice onto an insulating substrate for electronics or sensing applications. more...

The payload capacity of spacecraft is constrained by the weight of the craft itself, including fuel and electronic systems. The protective measures used to shield onboard electronics from the harsh space environment, characterized by high-energy particles and significant temperature fluctuations, can further diminish the available payload capacity. This thesis explores the potential of naturally radiation-hard alternatives to commonly used electronic materials, such as Silicon, to reduce the need for shielding and other protective measures, thereby decreasing the weight and cost of space missions. III-V semiconductor materials, such as Gallium Nitride (GaN), are known for their inherent resilience to temperature swings and lattice damage. Despite their successful application on Earth, GaN-based electronics have not been widely adopted in space missions. Most evaluations of GaN materials’ resilience to radiation and temperature have been conducted terrestrially using accelerated lifetime testing approaches. However, there is a lack of studies evaluating GaN materials in actual space environments over timeframes consistent with space missions. This thesis describes a CubeSat experiment designed to investigate the effects of radiation and temperature on 24 GaN Light Emitting Diodes (LEDs) housed inside a 3U cube satellite in low earth orbit at 500km, as part of West Virginia’s STF-1 mission. Using a custom-designed Low-powered Optoelectronic Characterizer for CubeSat (LOCC), periodic current and voltage measurements were taken to construct the current/voltage (IV) curves of each LED. Additionally, the luminosity and color of the LEDs (designed for 465nm emission) were measured. Data collected since 2019 has been processed, revealing a likely outcome that the LEDs are still functioning with minimal damage. The data from the MaZET MTCSiCF MCDC04 Analog to Digital Converter (ADC) was interpreted for plotting in a Commission Internationale de l’éclairage (CIE) 1931 X Y Z color space. A correction matrix K was calculated, where K = () * ; T is a reference matrix consisting of reference measurements, and S is the Signal matrix interpreted from the ADC values. The X, Y, and Z ADC values were multiplied by K to obtain the X, Y, and Z CIE colorimetric plot values, which were then plotted on a two-dimensional CIE graph. This study provides valuable insights into the performance of GaN LEDs in space environments, contributing to the development of more efficient and cost-effective space missions. more...

The long-standing technological pillars for computing systems evolution, namely Moore's law and Von Neumann architecture, are breaking down under the pressure of meeting the capacity and energy efficiency demands of computing and communication architectures that are designed to process modern data-centric applications related to Artificial Intelligence (AI), Big Data, and Internet-of-Things (IoT). In response, both industry and academia have turned to 'more-than-Moore' technologies for realizing hardware architectures for communication and computing. Fortunately, Silicon Photonics (SiPh) has emerged as one highly promising ‘more-than-Moore’ technology. Recent progress has enabled SiPh-based interconnects to outperform traditional electrical interconnects, offering advantages like high bandwidth density, near-light speed data transfer, distance-independent bitrate, and low energy consumption. Furthermore, SiPh-based electro-optic (E-O) computing circuits have exhibited up to two orders of magnitude improvements in performance and energy efficiency compared to their electronic counterparts. Thus, SiPh stands out as a compelling solution for creating high-performance and energy-efficient hardware for communication and computing applications. Despite their advantages, SiPh-based interconnects face various design challenges that hamper their reliability, scalability, performance, and energy efficiency. These include limited optical power budget (OPB), high static power dissipation, crosstalk noise, fabrication and on-chip temperature variations, and limited spectral bandwidth for multiplexing. Similarly, SiPh-based E-O computing circuits also face several challenges. Firstly, the E-O circuits for simple logic functions lack the all-electrical input handling, raising hardware area and complexity. Secondly, the E-O arithmetic circuits occupy vast areas (at least 100x) while hardly achieving more than 60% hardware utilization, versus CMOS implementations, leading to high idle times, and non-amortizable area and static power overheads. Thirdly, the high area overhead of E-O circuits hinders them from achieving high spatial parallelism on-chip. This is because the high area overhead limits the count of E-O circuits that can be implemented on a reticle-size limited chip. My research offers significant contributions to address the aforementioned challenges. For SiPh-based interconnects, my contributions focus on enhancing OPB by mitigating crosstalk noise, addressing the optical non-linearity-related issues through the development of Silicon-on-Sapphire-based photonic interconnects, exploring multi-level signaling, and evaluating various device-level design pathways. This enables the design of high throughput (>1Tbps) and energy-efficient ( more...

This study presents a flexible sensor/harvester device to be used in both electromagnetic sensing and energy harvesting applications for smart grids. When a current passes through a wire, the sensor detects the magnetic field created by that current. The sensor magnet interacts with the wire magnetic field resulting in a transfer of energy through the piezoelectric cantilever. Piezoelectric, conductive, magnetic, and magnetostrictive composite thin films were prepared to fabricate this device. Initially, the magnet of the cantilever was optimized considering its shape, thickness, length, taper angle etc. via both simulations and experiments. Peak to peak voltage versus cantilever position graph was drawn for several different magnets. The triangular shape showed the widest sensing range around 18 mm. Experiments have been verified by COMSOL Multi physics software simulations constructing the magnetic flux density-position graphs. After the geometry optimization of the magnet, numerous 0-3 composite thin films were fabricated using piezoelectric, conductive, magnetic, and magnetostrictive powder in different polymers for comparison. After blending the materials, the spin coating method was used with various spin speeds to obtain different film thicknesses. Mainly, lead zirconate titanate (PZT), silver (Ag), neodymium (NdFeB), and Terfenol-D particles were used to create these functional films. After demonstrating that the functional thin films fabricated in this research can be successfully used to fabricate multilayered devices such as a micro-scale capacitor or a thin film energy harvester, thin film sensor/harvester versions of the bulk devices were fabricated and tested successfully. The broadest sensitivity region was obtained again from the piezoelectric thin film that has triangular thin film magnet positioned in such a way that its tip faces the free end of the cantilever. 40 wt.% concentration Ag-PI composites were used as electrodes. Finally, magnetostrictive thin films having various ball-milled Terfenol-D particle concentrations were fabricated to be used as a single layer sensor instead of using multiple layers of thin films. The thin film sensors were tested using a Helmholtz coil and their response were determined in terms of tip deflection and current gained. more...

Enhancing physical human-robot interaction in modern robotics relies on refining the tactile perception of robot skin sensors. This research focuses on crucial aspects of the development process, including fabrication techniques, miniaturization, and integration for a more efficient collaborative human-robot interface. The fabrication process of robot skin sensors, designed to mimic human skin, is explored both within and outside cleanroom environments. An enhanced technique is presented to increase fabrication yield and create more miniaturized sensor designs with feature sizes in the tens of microns. These sensors function as piezoresistive arrays using organic polymers like PEDOT: PSS as the pressure-sensing medium. Various deposition techniques, such as cleanroom spin coating and direct-write inkjet printing with Aerosol inkjet printers, are discussed. A NeXus microfabrication platform is introduced to eliminate errors, simplify the cleanroom process, and reduce production time for sensor arrays. This platform is employed for the prototyping of tactile strain gauges, integrating an Aerosol jet printer station for patterning sensor electrodes on flexible substrates and a piezo-electric fluid dispenser for PEDOT:PSS deposition, bypassing cleanroom photolithography. The post-processing phase is detailed, highlighting the sintering of patterned silver traces using an oven or intense pulse light (IPL). The curing process determines the resistance and conductivity of printed samples, with IPL offering flexibility and efficiency compared to traditional ovens. Cured samples undergo testing on a specialized testbench equipped with an indenter, force feedback control, motorized stage, and computer vision functionality. LabVIEW Programs synchronize testing components, producing tangible results for each tactile sensor test. Test quality influences the integration of tactile sensors with a robotic arm. A novel tactile fingerprint design, realizable in the NeXus, is proposed and characterized based on performance and reliability. Sensitivity, indentation cycles, and spatial resolution studies contribute to a comprehensive understanding of the proposed design. The research's ultimate goal is to integrate tactile sensors, including commercially available options like Flexiforce sensors and robot skin sensor patches, with a robot to enhance direct interaction. The effective use of the Robot Operating System (ROS) and local area connectivity to implement the robot's response to physical touch on the skin sensors marks a significant stride in advancing human-robot interaction. The abstract encompasses the critical elements of improved fabrication, miniaturization, and integration, making strides toward more effective and adaptable physical human-robot collaboration. more...

Advancements in microscale actuating technologies has substantially expanded the possibilities of interacting with the surrounding environment. Microstructures that deflect in response to mechanical forces are one of the largest application areas of microelectromechanical systems (MEMS). MEMS devices, functioning as sensors, actuators, and support structures, find applications in inertial sensors, pressure sensors, chemical sensors, and robotics, among others. Driven by the critical role of catalytic membrane reactors, this dissertation aims to evaluate enzyme activity on polymeric membranes and explore how fabrication methods from the field of Electrical and Computer Engineering (ECE) can incorporate sensing and actuation into these porous surfaces. Toward better understanding of conditions in flowing systems, this dissertation investigates how MEMS devices perform in flows, demonstrating a set of thin-film out of plane cantilevers that deflect in the flow velocity range of 0.5 to 5.7 mm/s in high viscosity solution (glycerol). We show with the same processing methods, MEMS devices can be developed for actuation over angles 0 to 90 degrees at speeds in the millisecond range and for resistive temperature sensing (temperature range 20 to 500 °C). Finally, this dissertation presents an innovative packaging approach that employs mechanical tangling, allowing the integration of MEMS microgrippers with fibrous materials commonly used in wearables, soft robotics, and applications requiring large deformation. more...

Silicon Carbide (SiC) is a rather new material that possesses unparalleled properties when compared to Silicon. Due to its larger band gap alongside other thermal properties, SiC can survive in hotter, more radiation intensive environments, whether that be within the crust of the earth or in the reaches of space. As a desirable semiconductor for these applications, CMOS is an especially important device due to its low power consumption. However, creating a good contact between the metal and semiconductor optimally requires two different metals for the n -type and the p-type semiconductor. This greatly increases the processing time, as separate masks must be produced to allow for each metal to go to the n doped or p doped part of the semiconductor. This thesis explores the use of one metal stack for both n-type and t-type semiconductors. A discussion on the differences between Silicon and SiC will be introduced, as well as the benefits of wide bandgap semiconductors. Additionally, explanations as to why most metals are incompatible with the simultaneous formation of ohmic contacts for n-type and p-type will be discussed. Electrical and material characterizations used in the thesis will also be defined and how they are used. After the separate fabrications for CTLM and TLM will be explained in depth, along with the equipment used in the fabrication process. After, samples that were used in this study will be defined, and the electrical and material characterization of each set of samples will be explained. Finally, discussion of results between the different metals on n-type and p-type will be discussed, as well as future works. more...

This dissertation will show successful development and characterization of amorphous boron carbide-amorphous silicon heterojunction device with potential for neutron detection. The amorphous hydrogenated boron carbide (a-BC:H) has been extensively researched as a semiconductor for neutron voltaic device fabrication. Naturally occurring boron contains 19.8% of boron isotope B10 that has a high absorption cross section of thermal neutrons at lower energies, and boron carbide contains 14.7% of that B10 isotope. Therefore, as a semiconductor compound of boron a-BC:H has the ability to absorb radiation, generate charge carriers, and collect those carriers. Previous work on a-BC:H devices investigated the fabrication of homojunction, heterojunction and heteroisomeric devices from the polymeric precursors ortho-carborane (p-type) and meta-carborane (n-type) using plasma enhanced chemical vapor deposition (PECVD). However, the metal contact formation with a-BC:H has not been previously studied with respect to its possible effects on device performance. The metal/a-BC:H contact investigation was performed, producing contact resistance for an Ohmic contact formation of Ti on n-type a-BC:H. The resistivity of the n-type a-BC:H in the direction of the device current flow was also investigated. However, a metal that forms an Ohmic contact the p-type a-BC:H has not been identified. The p-type a-BC:H made from ortho-carborane has high resistivity and doping limitations, so p-type single crystal silicon with n-type a-BC:H grown from meta- carborane has been previously studied and shown to produce the most optimal device performance compared to different a-BC:H device structures. As single crystal silicon has well known electrical and material properties, with X-Ray Photoelectron Spectroscopy (XPS) and Spectroscopic Ellipsometry measurements, electronic properties at the heterojunction interface of a-BC:H/c-Si is obtained by calculating valence band offset. However, as single crystal silicon degrades over time due to radiation induced damage to its crystalline structure, p+ -type hydrogenated amorphous silicon (a-Si:H) is investigated as a potential layer in the formation of the a-BC:H heterojunction device. From the characterization of a-BC:H/c-Si and a-BC:H/a-Si:H devices, the a-BC:H/a-Si:H device shows potential in fabricating a novel neutron voltaic device. Advisor: Natale Ianno more...

Solid-state lighting has achieved significant success over the past two decades, but the low quantum efficiency of green LEDs (i.e., the “green gap”) remains a barrier to full red-green-blue (RGB) displays in numerous applications. Combating efficiency reduction in longer-wavelength LEDs requires understanding the relative roles of intrinsic effects (e.g., wave-function overlap, carrier-current density relationship, phase-space filling (PSF)) vs. extrinsic effects (e.g., material degradation due to increased defect density, compositional inhomogeneities, etc.). A systematic study of the carrier dynamics in InGaN/GaN LEDs is very important for understanding the origin of the green gap and for providing solutions to improve the efficiency of the LEDs. In this dissertation, several techniques, such as small-signal electroluminescence (SSEL), deep-level optical spectroscopy (DLOS) and lighted capacitance-voltage (LCV), were applied to LEDs grown under state-of-the-art growth conditions using metal-organic chemical vapor deposition (MOCVD). The effect of indium composition was examined to study the origin of the lower quantum efficiency in green LEDs compared with blue and cyan LEDs. The impact of the deep-level defect density on various non-radiative recombination mechanisms was also investigated, and the role of each recombination mechanism was analyzed separately. Furthermore, the effect of quantum well (QW) thickness was studied to determine the optimized value and tradeoffs between various design parameters. Finally, a novel multiple carrier lifetime model was developed to study carrier dynamics in InGaN/GaN multiple-quantum-well (MQW) LEDs with non-uniform carrier distribution. This model can also be applied to micro-LEDs and other LEDs with similar carrier dynamics behavior. The studies mentioned above will contribute to a better understanding of carrier dynamics in green InGaN/GaN LEDs and inform approaches for improving quantum efficiency in LEDs that already possess state-of-the-art growth quality. more...

As electrical devices shrink to the atomic scale, it is expected that Moore's law will soon be obsolete for semiconductor devices. In 1974, Avriam and Ratner predicted that organic devices could replace semiconductor technology, leading to extensive research on molecular-based organic devices. This dissertation delves into the theoretical frameworks used to examine the transport in molecular junctions and aims to enhance our comprehension of charge transport and conduction properties. The studies presented in this thesis illustrates that a molecule's alteration by just a single atom can change it from an insulator to a conductor, and also that, by fine-tuning the molecule-electrode coupling strength and the tunneling distance in a molecular junction, the mechanism of charge transport across molecular wires can be switched between the normal and Inverted Marcus regions. The dissertation also presents molecular devices that function as reliable electrical switches, both static and dynamic. The findings of this research provide evidence of the feasibility of organic devices, including rectifiers and switches, with applications ranging from traditional semiconductor device replacement to neuromorphic computing. more...

Specialized and optimized low pressure direct oxidation technique have been implemented to synthesize high quality VO2 thin films on various substrates (sapphire, SiO2/Si, AT-cut quartz, GaN/AlGaN/GaN/Si and muscovite). Structural and surface characterization methods such as X-ray diffraction, Raman spectroscopy and atomic force microscopy have been administered on the grown VO2 films which indicate their material quality. Transition of characteristics of the VO2 films are caused by semiconductor metal transition (SMT). This phenomenon is attributed as the change maker in transition of resistivity and transmitted optical power through the VO2 films. Apart the substrates mentioned, metal doped VO2 films have been synthesized and characterized on quartz and muscovite, which are found to have transition temperature near the room temperature level, indicating to the possibility of high sensitive VO2 based sensors to be manufactured. Also, the VO2 films on flexible muscovite displays the direct effect of mechanical strain on the electrical and optical characteristics of the films. After metal finger patterning and releasing the VO2 membranes from the synthesized thin films, combination of all these characteristics of VO2 thin film is the reason of transmitted infrared beam through the VO2 membrane being modulated periodically due to implementation of external electric field at different frequencies. more...

Silicon heterojunction solar cell of Heterojunction with Thin Intrinsic Layer (HIT) structure is a commercially available technology, and its market share will significantly increase by the next decade. With such a significant market share, any minor improvement in the device’s overall efficiency can be beneficial three folds - customer return on investment, industry revenue, and the overall carbon footprint (from manufacturing to recycling/ disposing of the device). Conventionally, device optimization for solar cells has been achieved using a hit & trial approach where multiple experiments are done to evaluate the best process conditions and device parameters. This approach has some inherent disadvantages, especially, because it is expensive in terms of resources, time, and manpower required. In the past couple of decades, simulation techniques are also being utilized in addition to the conventional approaches and very recently use of data science-based techniques has become popular in research and is gaining some traction in the photovoltaics industry. In this doctoral research, an innovative approach is presented, where device simulations for solar cells are designed and calibrated to match the performance of an industrially manufactured device using a minimal set of measurement data. Then a digital twin for the numerical simulations is developed by training Machine Learning (ML) models using simulation and measurement data. ML methods are also used for the calibration of simulation models in addition to being used for the digital twin. The use of machine learning helps significantly reduce computational time to prototype any design changes in the device. In our work, apart from providing mean predictions, we are also providing a measure of uncertainty with every prediction, using Gaussian Process Regression (GPR). more...

Micro-electromechanical systems (MEMS)-based sensors have gained significant attention due to their ability to sense, measure, and process various physical, chemical, and biological parameters. The small size of MEMS sensors provides numerous advantages, including low power consumption, high sensitivity, and rapid response time, making them suitable for various applications in healthcare, automotive, aerospace, and consumer electronics. In the past few years, AlGaN/GaN MEMS devices have been found to offer several advantages over silicon-based MEMS devices. One of the main advantages of AlGaN/GaN MEMS is their high sensitivity to surface stresses and forces due to their high piezoelectric coefficients. This sensitivity allows them to detect small changes in mass or mechanical properties of samples, making them ideal for applications requiring high sensitivity and resolution. Additionally, the high mechanical strength of AlGaN/GaN materials enables these devices to withstand harsh environments, including high temperatures, radiation, and corrosive substances. Moreover, the wide bandgap of AlGaN/GaN materials allows them to operate at higher temperatures and higher power levels, making them ideal for high-temperature sensing and power electronics applications. These advantages make AlGaN/GaN MEMS devices a promising option for various sensing and measurement applications, offering higher sensitivity, resolution, and durability than silicon-based MEMS devices. Microcantilevers are MEMS-based sensors consisting of a thin beam or plate attached to a fixed base at one end, with the other free to move. These sensors can measure various physical, chemical, and biological parameters such as force, mass, pressure, and chemical reactions. Triangular microcantilevers have emerged as a promising platform for sensing applications due to their high surface area-to-volume ratio and ability to detect subtle changes in their electrical properties. This work focuses on developing and characterizing a novel UV photodetector structure using dual AlGaN/GaN heterostructure-based two-dimensional electron gas (2DEG) channels and a semi-insulating GaN layer in a triangular microcantilever shape. Using a semi-insulating interchannel layer helped reduce the dark current and achieve a high photo-to-dark current ratio. A unique challenge is presented by the volatile organic compounds (VOCs) in air quality monitoring. VOCs emitted from various sources and mixtures are complex to monitor. We demonstrate a technique for real-time detection of VOCs with AlGaN/GaN dual-channel triangular microcantilevers. These cantilevers were exposed to various VOCs, and corresponding changes in electrical characteristics were recorded and monitored. A single-channel triangular microcantilever-based airflow sensor is also demonstrated in this work. This study also details the formation of indium tin oxide (ITO)-based transparent ohmic contacts for bulk n-GaN and HVPE-grown n-GaN thin films on c-face sapphire, with improved contact characteristics following SiCl4 plasma treatment. Overall, this work presents the design, fabrication, and characterization of AlGaN/GaN triangular microcantilevers for UV, VOCs, and airflow detection. This work contributes to advancing photodetector technology and sensor system development for various applications using III-Nitride triangular microcantilevers. more...

Short-circuit and open-circuit faults of an inverter’s power device often lead to catastrophic failure of the entire system if not detected and acted upon within a few microseconds, particularly for emerging wide bandgap (WGB) power semiconductors. While a significant amount of research has been done on the fast and accurate protection and detection of short-circuit faults, there has been less success corresponding to the research on open-circuit faults. Common downfalls include protection and detection that are too application-specific, take longer than a couple of microseconds, and are not cost-efficient. This study proposes a new open-circuit fault protection and detection system integrated with a pre-existing short-circuit system called desaturation protection. First, a literature review is conducted to confirm the necessity of the new protection and detection scheme. Second, the operation principle of the newly proposed protection and detection circuitry is discussed, and design considerations are given. Third, a comprehensive case study revolving around implementing the new protection and detection system is conducted using Synopsys/Saber simulation software. Fourth, an experiment is devised and constructed to showcase the protection and detection scheme’s success, effectiveness, and adaptability in a real-world environment. Fifth, concluding remarks are given, summarizing all the work presented in this study. The results of testing the proposed system illustrate the success and reliability of the new fault protection and detection system. more...

A dispenser cathode’s ability to thermionically emit electrons is highly dependent on its material properties, especially those of the surface. Understanding the relationship between surface properties and electron emission, therefore, is vital to reach the next generation of the many vacuum electron devices (VEDs) that rely on the physics of electron emission. In the past century, many techniques have been developed to characterize material surfaces and quantify thermionic emission. These techniques are based on a wide range of different physical phenomena, including measuring photoemission via the photoelectric effect, measuring the electrostatic potential between metals in electrical contact, and current collection via positively biased anodes in a diode configuration. They do not necessarily have one-to-one correlation with each other and they are often incapable of studying cathodes in their operational conditions of high temperatures and in a vacuum. An investigation into the correlation between surface properties and electron emission in such an environment is needed to move the field forward. As such, the Cathode Characterization Chamber (CCC) has been assembled at the University of Kentucky to function as one testing space to utilize many diverse surface and emissions characterization techniques simultaneously. In this work, multiple methods of improved implementations of the aforementioned techniques are described, including lesser-known applications such as measurement of contact potential difference at high temperatures and the collection of electron emission using a Kelvin probe in an Ultra-High Vacuum (UHV). Additionally, an investigation into dispenser cathode design using this knowledge is described here, wherein a multi-element alloy cathode coating is explored as a candidate for improved emission performance when compared to current standard cathode coatings. A key parameter for evaluating thermionic emission performance that will receive significant attention throughout here is the work function. And the full testing capabilities of the CCC are shown to be a valuable toolset for evaluating the work function under many environmental conditions that would not be possible without this unique system. Ultimately, the results show a path toward multiple improved implementations of cathode characterization and the use of those methods to propose improved cathode design. more...

Convolutional Neural Networks (CNNs) have proven to be highly effective in various fields related to Artificial Intelligence (AI) and Machine Learning (ML). However, the significant computational and memory requirements of CNNs make their processing highly compute and memory-intensive. In particular, the multiply-accumulate (MAC) operation, which is a fundamental building block of CNNs, requires enormous arithmetic operations. As the input dataset size increases, the traditional processor-centric von-Neumann computing architecture becomes ill-suited for CNN-based applications. This results in exponentially higher latency and energy costs, making the processing of CNNs highly challenging. To overcome these challenges, researchers have explored the Processing-In Memory (PIM) technique, which involves placing the processing unit inside or near the memory unit. This approach reduces data migration length and utilizes the internal memory bandwidth at the memory chip level. However, developing a reliable PIM-based system with minimal hardware modifications and design complexity remains a significant challenge. The proposed solution in the report suggests utilizing different memory technologies, such as Dynamic RAM (DRAM) and phase change memory (PCM), with Stochastic arithmetic and minimal add-on logic. Stochastic computing is a technique that uses random numbers to perform arithmetic operations instead of traditional binary representation. This technique reduces hardware requirements for CNN's arithmetic operations, making it possible to implement them with minimal add-on logic. The report details the workflow for performing arithmetical operations used by CNNs, including MAC, activation, and floating-point functions. The proposed solution includes designs for scalable Stochastic Number Generator (SNG), DRAM CNN accelerator, non-volatile memory (NVM) class PCRAM-based CNN accelerator, and DRAM-based stochastic to binary conversion (StoB) for in-situ deep learning. These designs utilize stochastic computing to reduce the hardware requirements for CNN's arithmetic operations and enable energy and time-efficient processing of CNNs. The report also identifies future research directions for the proposed designs, including in-situ PCRAM-based SNG, ODIN (A Bit-Parallel Stochastic Arithmetic Based Accelerator for In-Situ Neural Network Processing in Phase Change RAM), ATRIA (Bit-Parallel Stochastic Arithmetic Based Accelerator for In-DRAM CNN Processing), and AGNI (In-Situ, Iso-Latency Stochastic-to-Binary Number Conversion for In-DRAM Deep Learning), and presents initial findings for these ideas. In summary, the proposed solution in the report offers a comprehensive approach to address the challenges of processing CNNs, and the proposed designs have the potential to improve the energy and time efficiency of CNNs significantly. Using Stochastic Computing and different memory technologies enables the development of reliable PIM-based systems with minimal hardware modifications and design complexity, providing a promising path for the future of CNN-based applications. more...

Monolithic semiconductor lasers including edge-emitting lasers (EELs) and vertical-cavity surface-emitting lasers (VCSELs) fail to realize simultaneous achievement of both high-power high-quality beams due to intrinsic limitations of their resonant cavity. Photonic crystal surface emitting lasers (PCSELs) address these structural limitations through employment of a two-dimensional photonic crystal (PC) cavity that creates feedback at a single wavelength for laterally scalable devices. This function of the PC allows PCSELs to power scale with area while maintaining single-mode emission required for diffraction-limited beams. In this way, PCSELs fulfill an industry demand for high-power high-quality lasing from a single chip. Early PCSELs were fabricated by way of wafer fusion where two wafers, the first containing the active layer and the second containing the PC, are joined together via heat treatment in a liquid phase epitaxy furnace. This technique produces a high density of light absorbing defects at the bonded interface. Modern PCSEL fabrication forgoes wafer bonding in favor of processes that maintain crystalline order throughout the entire structure. This work demonstrates PCSEL fabrication by epitaxial regrowth and single- epitaxy methods to illustrate their respective advantages and engineering challenges. While specific properties of epitaxial regrowth are beneficial for total output power, a single-epitaxy process simplifies fabrication and provides a more scalable and wavelength versatile platform. PCSELs with InGaAs multiple-quantum-well (MQW) active regions are constructed by molecular beam epitaxy (MBE) using both regrowth and single-epitaxy methods. Total output power of epitaxially regrown PCSELs is dramatically improved by implementation of a flip-chip bonding process and a metal- organic chemical vapor deposition (MOCVD) regrowth step. By affecting stable large- area coherent lasing, the fabricated PCSELs described in this work address the shortcomings presented by traditional EEL and VCSEL devices. Ultimately, both epitaxial regrowth and single-epitaxy PCSEL fabrication present advantages to satisfy different ends and are critical for future PCSEL development. more...

An original design and photolithographic fabrication process for poly(3-hexylthiophene-2, 5-diyl) (P3HT) based organic thin-film transistors (OTFTs) is presented. The structure of the transistors was based on the bottom gate bottom contact OTFT. The fabrication process was efficient, cost-effective, and relatively straightforward to implement. Current–voltage (I-V) measurements were performed to characterize the primary electronic properties of the transistors. The measured mobility of these transistors was significantly higher than most results reported in the literature for other similar bottom gate bottom contact P3HT OTFTs. The higher mobility is explained primarily by the effectiveness of the fabrication process in keeping the interfacial layers free from contamination, as well as the proper annealing of the P3HT. An interface engineering method is investigated to further enhance the performance of the OTFTs. Three interfacial materials were used for this purpose: graphene oxide (GO), poly(oligo (ethylene glycol) methyl ether methacrylate- glycidyl methacrylate- lauryl methacrylate) (P(OEGMA-GMA-LMA)) or POGL, and a composite of GO and P(OEGMA-GMA-LMA) or GO-POGL. The OTFTs with a GO interfacial layer were observed to have a higher drain current and field-effect mobility than the OTFTs with no interfacial layer. The enhanced drain current and mobility are associated with the particular structure of the P3HT layer on the dielectric surface and the reduction in the contact resistance between the GO-covered electrodes and the P3HT. The OTFTs with a POGL interfacial layer were observed to have a smaller threshold voltage than the OTFTs with no interfacial layer. The POGL OTFTs were also observed to have much more ideal drain current saturation characteristics with very small I-V curve slope. This is explained by the deep trap states on the POGL surface and the reduction of the contact resistance at the electrode/organic semiconductor interface. The OTFTs with a GO-POGL composite layer were observed to have a higher drain current and mobility, and a smaller threshold voltage than the OTFTs without an interfacial layer, which is the optimum case for these two device parameters. The higher drain current and field-effect mobility are attributed to the larger interconnecting grains of the P3HT that is deposited onto the GO-POGL surface and the smaller interfacial tension between the GO-POGL and the P3HT. The smaller threshold voltage is attributed to the deep trap states on the GO-POGL layer and the smaller contact resistance between the GO-POGL modified electrodes and the P3HT. Furthermore, experiments that could be performed to advance this research work and enhance the performance of the OTFTs even further are proposed. more...

Polycrystalline VO2 thin films synthesized on two piezoelectric substrates (AT-cut quartz and GaN/AlGaN/GaN/Si) using low pressure direct oxidation technique have been characterized and compared to VO2 grown on traditional non-piezoelectric substrates sapphire and SiO2/Si. X-ray diffraction and atomic force microscopy characterization performed on the as grown films confirmed high quality of the VO2 films grown on both the piezoelectric and non-piezoelectric substrates. Changes in material properties associated with the semiconductor metal transition (SMT) of the VO2 films were investigated through resistivity and transmitted optical power changes measured across the SMT. It was observed that the VO2 films grown on the piezoelectric substrates are of high quality, and their electrical and optical properties changes are quite comparable to the best reported values on films synthesized on various substrates using different synthesis processes. more...

Using on-board medium voltage (MV) dc distribution system has been a megatrend for next-generation electric aircraft systems due to its ability to enable a significant system mass reduction. In addition, it makes electric propulsion more feasible using MV power electronic converters. To develop high-performance high-density MV power converters, the emerging silicon carbide (SiC) devices are more attractive than their silicon (Si) counterparts, since the fast switch frequency brought by the SiC can effectively reduce the volume and weight of the filter components and thus increase the converter power density. From the converter topology perspective, with the MV dc distribution, the state-of-the-art two-level converters are no longer suitable for next-generation electric aircraft system due to the excessive dv/dt and high voltage stress across the power devices.To address these issues while still maintaining cost-effectiveness, this work demonstrates a megawatt-scale MV seven-level (7-L) Si/SiC hybrid converter prototype implemented by active-neutral-point-clamped (ANPC) converter and H-bridges which is called ANPC-H converter in this work, and a MV five-level (5-L) Si/SiC hybrid ANPC converter prototype, which are hybrid multilevel converters with internal cascaded and paralleled structures, respectively. Using multilevel circuit topology, the voltage stress across the devices and converter output voltage dv/dt are reduced. The tradeoff between the system cost and efficiency was addressed by the adoption of the Si/SiC hybrid configuration with optimized modulation strategies. Comprehensive design and evaluation of the full-scale prototypes are elaborated, including the low-inductance busbar designs, power converter architecture optimization and system integration. To control the 7-L Si/SiC hybrid ANPC-H converter prototype, a low computational burden space-vector-modulation (SVM) with common-mode voltage reduction feature is proposed to fully exploit the benefits of 7-L Si/SiC hybrid ANPC-H converter. To further reduce the converter losses and simplify control algorithm, an active hybrid modulation is proposed in this work by applying low frequency modulation in Si cells and high frequency modulation in SiC cells, thus the control framework is simplified from the 7-L SVM to a three-level SVM. To control the 5-L Si/SiC hybrid ANPC converter prototype to overall loss minimization, the low frequency modulation and high frequency modulation are also adopted for Si cells and SiC cells respectively in 5-L Si/SiC hybrid ANPC converter prototype. Compared to the SVM-based hybrid modulation in 7-L ANPC-H converter, the hybrid modulation for 5-L hybrid ANPC adopts a simpler carrier-phase-shifted pulse width modulation for its inner-paralleled high frequency SiC cells, which extensively suppresses harmonics caused by high frequency switching. With the proposed modulation strategies, extensive simulation and experimental results are provided to evaluate the performance of each power stage and the full converter assembly in both the steady-state operation and variable frequency operations of the demonstrated hybrid converters. more...

In the photovoltaic (PV) generation system, two types of grid-tied inverter systems are usually deployed: the single-stage grid-tied inverter system and the two-stage grid-tied inverter system. In the single-stage grid-tied inverter system, the input of the inverter is directly connected to the PV arrays, while an additional dc-dc stage is inserted between the PV arrays and the dc-ac inverter in the two-stage design. The additional dc-dc stage could provide a stable dc-link voltage to the inverter, which also enables new design possibilities, including the multi-MPPT operation and solar-plus-storage application. Thus, the two-stage grid-tied inverter has been widely used in the PV generation system.As the core component of the PV generation system, the reliability of the grid-tied inverter determines the overall robustness of the system. The two-stage grid-tied inverter system includes three parts: the dc-dc stage, dc-link capacitor, and dc-ac inverter. Thus, the reliability of the two-stage grid-tied inverter relies on the reliability of each part. The dc-dc stage is used to provide a stable dc-link voltage to the inverter. However, when the inverter stage provides constant power to the grid, the load of the dc-dc stage becomes the constant power load (CPL), which will deteriorate the stability of the dc-dc stage. The dc-link capacitor is used to attenuate the voltage ripple on the dc-link and balance the transient power mismatch between the dc-dc stage and the dc-ac stage. However, during the operation of the inverter system, the degradation of the capacitor will reduce the converter reliability, and even result in system failure. The inverter stage is connected to the grid through the output filter, and the LCL type filter has been commonly used due to its superior performance. The resonance of the LCL filter must be properly damped to enhance the inverter stability. However, the grid-side impedance will lead to the resonant frequency drifting of the LCL filter, which will worsen the stability margin of the inverter. Thus, the control design of the two-stage grid-tied inverter system must consider those reliability challenges. In this work, three control algorithms are proposed to solve the reliability challenges. For the dc-dc stage, an uncertainty and disturbance estimator (UDE) based robust voltage control scheme is proposed. The proposed voltage control scheme can actively estimate and compensate for the disturbance of the dc-dc stage. Both the disturbance rejection performance and the stability margin of the dc-dc stage, especially under the CPL, could be enhanced. For the dc-link capacitor, a high-frequency (HF) signal injection based capacitance estimation scheme is proposed. The proposed estimation scheme can monitor the actual dc-link capacitance in real-time. For the inverter stage, an adaptive extremum seeking control (AESC) based LCL filter resonant frequency estimation scheme is proposed. The AESC-based estimation scheme can estimate the resonant frequency of the LCL filter online. All the proposed reliability enhancing control algorithms could enhance the reliability of the two-stage grid-tied inverter system. Detailed theoretical analysis, simulation studies, and comprehensive experimental studies have been performed to validate the effectiveness. more...

Since power converters shape a wide range of power electronic applications, the 'heart' of these conversions, such as solid-state switches, is crucial for achieving higher power density, lower loss, and higher efficiency operations. Although the limitations imposed by silicon-based power devices have been reduced by introducing novel devices such as IGBTs and super junction MOSFETs to allow for broader applications, solutions beyond silicon have presented a continuous challenge to the device engineers. Driving these converters at a higher switching frequency allows for higher power density. However, to support a higher frequency and operating temperature, superior materials, e.g., wide bandgap materials, are also needed. Two such materials, namely silicon carbide (SiC) and gallium nitride (GaN), thanks to their excellent material properties like higher mobility, higher breakdown electric field, and lower leakage, are paving the way to improved efficiency and high power density with broader functionality. Although SiC plays a vital role in a wider range of power electronics applications, the use of SiC outside of standard operation regions, e.g., cryogenic temperature ( more...