Complex perovskite materials, including Ba(Zn1/3Ta2/3)O3 (BZT), are commonly used to make resonators and filters in communication systems because of their low dielectric loss and high-quality factors (Q). Transition metal additives are introduced (i.e., Ni2+, Co2+, Mn2+) to act as sintering agents and tune their temperature coefficient to zero or near-zero. However, losses in these commercial dielectric materials at cryogenic temperatures increase markedly due to spin-excitation resulting from the presence of paramagnetic defects. Applying a large magnetic field (e.g., 5 Tesla) quenches these losses and has allowed the study of other loss mechanisms present at low temperatures. Work was performed on Fe3+ doped LaAlO3. At high magnetic fields, the residual losses versus temperature plots exhibit Debye peaks at ~40 K, ~75 K, and ~215 K temperature and can be tentatively associated with defect reactions O_i^x+V_O^x→O_i^'+V_O^•, Fe_Al^x+V_Al^"→Fe_Al^'+V_Al^' and Al_i^x+Al_i^(••)→〖2Al〗_i^•, respectively. Peaks in the loss tangent versus temperature graph of Zn-deficient BZT indicate a higher concentration of defects and appear to result from conduction losses.Guided by the knowledge gained from this study, a systematic study to develop high-performance microwave materials for ultra-high performance at cryogenic temperatures was performed. To this end, the production and characterization of perovskite materials that were either undoped or contained non-paramagnetic additives were carried out. Synthesis of BZT ceramic with over 98% theoretical density was obtained using B2O3 or BaZrO3 additives. At 4 K, the highest Q x f product of 283,000 GHz was recorded for 5% BaZrO3 doped BZT. A portable, inexpensive open-air spectrometer was designed, built, and tested to make the electron paramagnetic resonance (EPR) technique more accessible for high-school and university lab instruction. In this design, the sample is placed near a dielectric resonator and does not need to be enclosed in a cavity, as is used in commercial EPR spectrometers. Permanent magnets used produce fields up to 1500 G, enabling EPR measurements up to 3 GHz.

Electron Paramagnetic Resonance (EPR) has facilitated great scientific advancements in many fields, like material science, engineering, medicine, biology, and health. EPR provided the ability to investigate samples on molecular level to detect chemical composition and identify harmful substances like free radicals. This thesis aims to explore current health and diagnostics EPR research and investigate the free radical content in related paramagnetic centers. Examining paramagnetic diagnostic markers of Cancer, Sicklecell disease, oxidative stress, and food oxidation. After exploring current literature on EPR, an experiment is designed and conducted to test seven different coffee samples (Turkish coffee, Espresso Coffee, European Coffee, Ground Arabic Coffee, American Coffee, Roasted Arabic Coffee, and Green Arabic Coffee), using Bruker ELEXSYS E580 spectrometer at x-band and under both room temperature (298 K) and low temperature (106 -113 K). Several microwave powers (1, mW, 0.25 mW, 0.16 mW, 0.06 mW, 0.04 mW) and different modulation frequency (10 G, 5 G, 3 G) are used. The results revealed average g-value was 2.009, highest linewidth was 16.312. Espresso coffee had the highest concentration of radicals, and green Arabic coffee beans had the lowest. Obtained spectra showed signals of Reactive Oxygen Species (ROS) radicals; believed to be result of natural oxidation process, as well as trace amounts of Fe3+ and other transition metals impurities, likely to be naturally found in coffee or resulting from the process of coffee production.

GaAs thermophotovoltaic (TPV) devices with a patterned dielectric back contact (PDBC) architecture, featuring a dielectric spacer between the semiconductor and back metal contact over most of the back surface for high reflectance, and metal point contacts over a smaller area for electrical conduction were demonstrated. In the TPV application, high sub-bandgap reflectance is needed to reflect unused sub-bandgap photons to the thermal emitter to minimize energy losses in this portion of the thermal spectrum. Different PDBC fabrication processes with SU-8 and SiO2 dielectric spacer layers to maximize sub-bandgap reflectance while minimizing series resistance to increase TPV conversion efficiency was explored. GaAs SU-8 PDBC TPV devices with 2200°C blackbody-weighted sub-bandgap reflectance of 94.9% and 96.5% with and without a front metal grid, respectively were demonstrated. This was 0.7% and 2.3% (absolute) higher than the mean sub-bandgap reflectance of 94.2% for GaAs baseline TPV devices with 100% Au back contact with a front metal grid. Lower sub-bandgap reflectance in TPV devices with front grids indicated the front grid induced light scattering led to additional parasitic absorption in the TPV device. For higher contact coverage fractions, the PDBC reflectance cannot, in general, be treated by linear interpolation of the mirror and point-contact areas using simple 1D transfer matrix method modeling and should be treated instead as a diffraction grating by solving Maxwell's equations in 3D. GaAs PDBC TPV device with series resistance less than 10 mΩ·cm2 was demonstrated. Finally, GaAs PDBC TPV device with 22.8% TPV efficiency measured in a thermophotovoltaic test platform with the thermal emitter at 2100℃ was demonstrated

Color perception has been widely studied and well modeled with respect to combining visible electromagnetic frequencies, yet new technology provides the means to better explore and test novel temporal frequency characteristics of color perception. Experiment 1 tests how reliably participants categorize static spectral rainbow colors, which can be a useful tool for efficiently identifying those with functional dichromacy, trichromacy, and tetrachromacy. The findings confirm that all individuals discern the four principal opponent process colors, red, yellow, green, and blue, with normal and potential tetrachromats seeing more distinct colors than color blind individuals. Experiment 2 tests the moving flicker fusion rate of the central electromagnetic frequencies within each color category found in Experiment 1 as a test of the Where system. It then compares this to the maximum temporal processing rate for discriminating direction of hue change with colors displayed serially as a test of the What system. The findings confirm respective processing thresholds of about 20 Hz for Where and 2-7 Hz for What processing systems. Experiment 3 tests conditions that optimize false colors based on the spinning Benham’s Top illusion. Findings indicate the same four principal colors emerge as in Experiment 1, but at low saturation levels for trichromats that diminish further for dichromats. Taken together, the three experiments provide an overview of the common categorical boundaries and temporal processing limits of human color vision.

Realization of efficient, high-bandgap photovoltaic cells produced using economically viable methods is a technological advance that could change the way we generate and use energy, and thereby accelerate the development of human civilization. There is a need to engineer a semiconductor material for solar cells, particularly multijunction cells, that has high (1.6-2.0 eV) bandgap, has relatively inactive defects, is thermodynamically stable under normal operating conditions with the potential for cost-effective thin-film growth in mass production.This work focuses on a material system made of gallium, indium, and phosphorus – the ternary semiconductor GaInP. GaInP based photovoltaic cells in single-crystal form have demonstrated excellent power conversion efficiency, however, growth of single-crystal GaInP is prohibitively expensive. While growth of polycrystalline GaInP is expected to lower production costs, polycrystalline GaInP is also expected to have a high density of electronically active defects, about which little is reported in scientific literature. This work presents the first study of synthesis, and structural and optoelectronic characterization of polycrystalline GaInP thin films. In addition, this work models the best performance of polycrystalline solar cells achievable with a given grain size with grain-boundary/surface recombination velocity as a variable parameter. The effects of defect characteristics at the surface and layer properties such as doping and thickness on interface recombination velocity are also modeled. Recombination velocities at the free surface of single-crystal GaInP and after deposition of various dielectric layers on GaInP are determined experimentally using time-resolved photoluminescence decay measurements. In addition, experimental values of bulk lifetime and surface recombination velocity in well-passivated single crystal AlInP-GaInP based double heterostructures are also measured for comparison to polycrystalline material systems. A novel passivation method – aluminum-assisted post-deposition treatment or Al-PDT – was developed which shows promise as a general passivation and material improvement technique for polycrystalline thin films. In the GaInP system, this aluminum post-deposition treatment has demonstrated improvement in the minority carrier lifetime to 44 ns at 80 K. During development of the passivation process, aluminum diffusivity in GaInP was measured using TEM-EDS line scans. Introduction, development, and refinement of this novel passivation mechanism in polycrystalline GaInP could initiate the development of a new family of passivation treatments, potentially improving the optoelectronic response of other polycrystalline compound semiconductors as well.

Lateral programmable metallization cells (PMC) utilize the properties of electrodeposits grown over a solid electrolyte channel. Such devices have an active anode and an inert cathode separated by a long electrodeposit channel in a coplanar arrangement. The ability to transport large amount of metallic mass across the channel makes these devices attractive for various More-Than-Moore applications. Existing literature lacks a comprehensive study of electrodeposit growth kinetics in lateral PMCs. Moreover, the morphology of electrodeposit growth in larger, planar devices is also not understood. Despite the variety of applications, lateral PMCs are not embraced by the semiconductor industry due to incompatible materials and high operating voltages needed for such devices. In this work, a numerical model based on the basic processes in PMCs – cation drift and redox reactions – is proposed, and the effect of various materials parameters on the electrodeposit growth kinetics is reported. The morphology of the electrodeposit growth and kinetics of the electrodeposition process are also studied in devices based on Ag-Ge30Se70 materials system. It was observed that the electrodeposition process mainly consists of two regimes of growth – cation drift limited regime and mixed regime. The electrodeposition starts in cation drift limited regime at low electric fields and transitions into mixed regime as the field increases. The onset of mixed regime can be controlled by applied voltage which also affects the morphology of electrodeposit growth. The numerical model was then used to successfully predict the device kinetics and onset of mixed regime. The problem of materials incompatibility with semiconductor manufacturing was solved by proposing a novel device structure. A bilayer structure using semiconductor foundry friendly materials was suggested as a candidate for solid electrolyte. The bilayer structure consists of a low resistivity oxide shunt layer on top of a high resistivity ion carrying oxide layer. Devices using Cu2O as the low resistivity shunt on top of Cu doped WO3 oxide were fabricated. The bilayer devices provided orders of magnitude improvement in device performance in the context of operating voltage and switching time. Electrical and materials characterization revealed the structure of bilayers and the mechanism of electrodeposition in these devices.

Losses in commercial microwave dielectrics arise from spin excitations in paramagnetic transition metal dopants, at least at reduced temperatures. The magnitude of the loss tangent can be altered by orders of magnitude through the application of an external magnetic field. The goal of this thesis is to produce “smart” dielectrics that can be switched “on” or “off” at small magnetic fields while investigating the influence of transition metal dopants on the dielectric, magnetic, and structural properties.

A proof of principle demonstration of a resonator that can switch from a high-Q “on state” to a low-Q “off state” at reduced temperatures is demonstrated in (Al1-xFex)2O3 and La(Al1-xFex)O3. The Fe3+ ions are in a high spin state (S=5/2) and undergo electron paramagnetic resonance absorption transitions that increase the microwave loss of the system. Transitions occur between mJ states with a corresponding change in the angular momentum, J, by ±ħ (i.e., ΔmJ=±1) at small magnetic fields. The paramagnetic ions also have an influence on the dielectric and magnetic properties, which I explore in these systems along with another low loss complex perovskite material, Ca[(Al1-xFex)1/2Nb1/2]O3. I describe what constitutes an optimal microwave loss switchable material induced from EPR transitions and the mechanisms associated with the key properties.

As a first step to modeling the properties of high-performance microwave host lattices and ultimately their performance at microwave frequencies, a first-principles approach is used to determine the structural phase stability of various complex perovskites with a range of tolerance factors at 0 K and finite temperatures. By understanding the correct structural phases of these complex perovskites, the temperature coefficient of resonant frequency can be better predicted.

A strong understanding of these parameters is expected to open the possibility to produce new types of high-performance switchable filters, time domain MIMO’s, multiplexers, and demultiplexers.

This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors.

The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.

The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.

The chemical, structural, and electrical properties of niobium-silicon, niobium-germanium, and YBCO-dielectric interfaces are characterized. Reduction in the concentration of interfacial defects in these structures can improve the performance of (i) many devices including low-loss coplanar, microstrip, and stripline microwave resonators used in next-generation cryogenic communication, sensor, and quantum information technologies and (ii) layers used in device isolation, inter-wiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication.

Methods were developed to synthesize amorphous-Ge (a-Ge) and homoepitaxial-Si dielectric thin-films with loss tangents of 1–2×10 -6 and 0.6–2×10 -5 at near single-photon powers and sub-Kelvin temperatures (≈40 mK), making them potentially a better choice over undoped silicon and sapphire substrates used in quantum devices. The Nb/Ge interface has 20 nm of chemical intermixing, which is reduced by a factor of four using 10 nm Ta diffusion layers. Niobium coplanar resonators using this structure exhibit reduced microwave losses.

The nature and concentration of defects near Nb-Si interfaces prepared with commonly-used Si surface treatments were characterized. All samples have H, C, O, F, and Cl in the Si within 50 nm of the interface, and electrically active defects with activation energies of 0.147, 0.194, 0.247, 0.339, and 0.556 eV above the valence band maximum (E vbm ), with concentrations dominated by a hole trap at E vbm +0.556 eV (presumably Nb Si ). The optimum surface treatment is an HF etch followed by an in-situ 100 eV Ar ion mill. RCA etches, and higher energy ion milling processes increase the concentration of electrically active defects.

A thin SrTiO 3 buffer layer used in YBa 2 Cu 3 O 7-δ superconductor/high-performance Ba(Zn 1/3 Ta 2/3 )O 3 and Ba(Cd 1/3 Ta 2/3 )O 3 microwave dielectric trilayers improves the structural quality of the layers and results in 90 K superconductor critical temperatures. This advance enables the production of more compact high-temperature superconductor capacitors, inductors, and microwave microstrip and stripline devices.