Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 138-143). Also available on the Internet.

This thesis reports observations of kink and tearing modes in the High Beta Tokamak - ExtendedPulse (HBT-EP) experiment. When unstable, these modes could limit the operation of tokamaks used for fusion power by terminating the plasma discharge and causing rapid loss of plasma energy. The aim of this work is to characterize the sudden transition after a sawtooth crash of coupled 2/1-3/1 tearing-kink modes into a sustained and disruptive 2/1 tearing mode. The following diagnostic techniques are used. Kink and tearing modes in HBT-EP distort the plasma edge, measured by a large array of Mirnov sensors, and perturb the interior of the plasma, observed routinely with Extreme Ultraviolet (EUV) detector arrays. Two arrays, with different transmission filters, are located with tangential views to estimate the time evolution of the plasma temperature profile. Four EUV arrays, with 16 detectors each, are positioned with different poloidal views for poloidal Extreme Ultraviolet (pEUV) emission tomography. The 2D emissive structures producing the pEUV signals are reconstructed with tomographic inversion using a pixel basis and fixed weighting smoothness regularization. Spatial and temporal correlations across these independent diagnostics are used to measure the evolution and structure of coupled modes using a technique called multidiagostic Singular Value Decomposition (mdSVD). In mdSVD, orthogonal modes are identified within any fixed time window with their unique spatial and temporal characteristics. The technique uncovers: coherent behavior of coupled (𝑚/𝑛) = (2/1) and (3/1) tearing-kink modes and rapid changes in plasma structure associated with sawtooth crashes which trigger disruptive and nondisruptive tearing modes. HBT-EP’s unique radially movable wall is found to significantly influence sawtooth triggering of disruptive tearing modes. The onset of sawtooth-triggered modes depends both on the plasma-wall separation, or wall coupling, and on the value of edge safety factor qₐ. We confirm that the condition for sawtooth triggering of disruptive (𝑚/𝑛) = (2/1) tearing modes does not correspond to the mode’s single-helicity stability condition Δ′₂/₁. We identify a dependency of the sawtooth period 𝝉_𝑠𝑡 on the wall position and qa as a candidate to explain the onset of the saturated tearing mode. This thesis motivates future efforts to model the influence of a nearby resistive wall on sawtooth triggering of tearing modes.

Aluminum Gallium Nitride (AlGaN) / Gallium Nitride (GaN) based deep ultraviolet (DUV) light emitting didoes (LEDs) with emission wavelengths between 200-280 nm enable key emerging technologies such as water/air purification and sterilization, covert communications and portable bio-agent detection/identification systems for homeland security, and surface and medical device sterilization. These devices produce a large amount of undesired heat due to low quantum efficiencies in converting electrical input to optical output. These low efficiencies are attributed to difficulties in the growth&doping of AlₓGa₁₋ₓN materials and UV absorbing substrates leading to excessive joule heating, which leads to device degradation and a spectral shift in the emission wavelength. With this regard, effective thermal management in these devices depends on the removal of this heat and reduction of the junction temperature. This is achieved by decreasing the package thermal resistance from junction-to-air with cost-effective solutions. The use of heat sinks, thermal interface materials, and high conductivity heat spreaders is instrumental in the reduction of the overall junction-to-air thermal resistance. This thesis work focuses on thermal modeling of flip-chip packaged deep UV LEDs to gain a better understanding of the heat propagation through these devices as well as the package parameters that have the biggest contributions to reducing the overall thermal resistance. A parametric study focusing on components of a lead frame package is presented to ascertain the thermal impacts of various package layers including contact metallizations, thermal spreading sub-mounts, and thermal interface materials. In addition the use of alternative thermal interface materials such as phase change materials and liquid metals is investigated experimentally.

[pt] O objetivo deste projeto foi a montagem de um sistema que permitisse a caracterização de novos materiais em forma de filmes finos para aplicações em sistemas com detectores de UV, concentrando a nossa pesquisa na caracterização da radiação emitida por alguns filmes finos irradiados com um feixe de elétrons. A utilização de uma Micro-Channel Plate (MCP) nos direcionou para a escolha dos materiais catodoluminescentes que deveriam ser depositados como filmes. Este tipo de detector é sensível a fótons na faixa do ultravioleta de vácuo (VUV) e dos raios-X e, portanto, a escolha deveria recair em materiais cuja catodoluminescência ocorresse nessas regiões do espectro eletromagnético. Um sistema foi montado em uma câmara de vácuo para a realização do estudo destes materiais através da incidência de elétrons (produzidos por um canhão de elétrons com energia variável de 0 a 400 eV) nos mesmos. Os materiais, uma vez irradiados com elétrons, emitem fótons na região do VUV, os quais são, por sua vez, detectados pela MCP. Paralelamente, foram realizadas deposições de filmes baseados em óxidos puros ou dopados (ZnO, ZnO:Eu, ZnO:Er) e filmes de LiYF4:Er+, nunca antes depositado. Para tanto foram utilizadas diferentes técnicas de deposição como a de feixe de elétrons e a de spin-coating. Foram realizadas duas séries de medidas destas amostras na câmara de vácuo, para detecção de emissão de fótons por transmissão e por reflexão a 90º. Percebe-se, através dos resultados obtidos, que os materiais dopados com Érbio possuem um forte pico de emissão em torno de 120 eV. Estes resultados preliminares indicam que é possível utilizar filmes dopados com Érbio como materiais catodoluminescentes na faixa do VUV. / [en] Our project was to build a system that would allow the characterization of new materials in the form of thin films for UV detectors applications. The main objective of our research was the characterization of the radiation emitted by electron-beam irradiated thin films. The use of a Micro Channel Plate detector (MCP) leaded us to choose certain cathodoluminescent materials that should be deposited as films. Since this kind of detector is sensitive to photons in the vacuum ultraviolet (VUV) and X-rays regions of the electromagnetic spectrum, our investigation was restricted to the materials which present cathodoluminescence in this region. Thin films based in pure or doped oxides (ZnO, ZnO:Eu, ZnO:Er) and LiYF4:Er+ were deposited. To accomplish this, different kinds of techniques were used, such as the electron-beam and the spincoating depositions. Once irradiated with electrons, these materials emit VUV photons, which are in turn detected by the MCP. For these purposes it was necessary to build an electron gun, with energy ranging from 0 to 400 eV, placed in a vacuum chamber, used for these experiments. Two series of measurements of the produced samples were performed in this vacuum chamber, and the photonic emission was detected by using transmission and reflection geometries. Our data show that, at the available incident energy range, the Erbium-doped materials have a strong emission peak around 120 eV. These preliminary results show that it is possible to use Erbium-doped films as VUV- cathodoluminescent materials

Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1−xN UV/IR dual-band detectors are reported. The presence of shallow Si-donor, deep Si-donor, and C-donor/N-vacancy defect states were found to significantly alter the electrical characteristics of the detectors. The barrier Al fraction was found to change the position of the interface defect states relative to the Fermi level. The sample with Al fraction of 0.1 shows a distinct capacitance-step and hysteresis, which is attributed to C-donor/N-vacancy electron trap states located above the Fermi level (200 meV) at the heterointerface; whereas, the sample with Al fraction of 0.026 shows negative capacitance and dispersion, indicating C-donor/N-vacancy and deep Si-donor defect states located below the Fermi level (88 meV). When an i-GaN buffer layer was added to the structure, an anomalous high-frequency capacitance peak was observed and attributed to resonance scattering due to hybridization of localized Si-donor states in the band gap with conduction band states at the i-GaN/n+-GaN interface.

In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ∼ 6.25, ∼ 10.5 and ∼ 23.3 µm. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 µm operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 µm. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 µm, and intraband responses with zero response thresholds at 70 and 32 µm, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 µm region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation.