Thesis (Ph. D.)--Auburn University, 2009. / Abstract. Vita. Includes bibliographical references (p. 110-118).

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 64-70). Also available via the Internet.

Thesis (Ph.D.)--Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 2005. / Also available in print.

Thesis (Ph. D.) University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 3, 2007) Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2007. / Title from first page of PDF file. Includes bibliographical references (p. 142-153).

Neste trabalho foram investigadas estruturas de diodos Schottky de carbeto de silício (SiC) com potencial uso em detectores de energia de partículas carregadas. Para tanto, foram fabricados diodos Schottky de SiC do tipo Metal-Isolador-Semicondutor (MIS). Uma estrutura MIS é considerada uma vez que o SiC sempre forma em sua superfície uma fina camada de oxicarbeto de silício (SiCxOy) nativo, de difícil remoção por ataques químicos. Foi desenvolvido um modelo modificado da teoria de Emissão Termiônica (TE), de modo a levar em conta o óxido nativo e/ou finas camadas dielétricas inseridas entre metal e semicondutor nas estruturas de diodos Schottky. Foram fabricadas estruturas alumínio/dielétrico/silício para caracterização dos dielétricos utilizados. Foram depositados os dielétricos de SiO2, TiO2, HfO2 e Al2O3 entre o metal Ni e o semicondutor de SiC, variando as espessuras de 1 a 8 nm. As espessuras depositadas foram confirmadas por Elipsometria espectral e Reflectometria de raio X, anteriormente à deposição por sputtering do contato Schottky de Ni. Após a deposição e o tratamento térmico do Ni, as estruturas de diodos Schottky foram caracterizadas eletricamente por meio de medidas de Corrente-Tensão (I-V) e Capacitância-Tensão (C-V), variando a temperatura de medida. Foi observado que a presença de uma fina camada dielétrica entre metal e semicondutor aumenta artificialmente a Altura da Barreira Schottky (SBH), diminuindo a corrente reversa quando o diodo é polarizado reversamente. Por meio do modelo modificado da TE, foi calculada uma espessura variando de 0.18 – 0.20 nm para o oxicarbeto de silício presente nos diodos estudados. As SBH reais foram extraídas por meio das medidas de I-V, variando-se a temperatura. Foram obtidos os valores da SBH de 1.39, 1.32 e 1.26 V, para os dielétricos TiO2, Al2O3, HfO2 e com 1 nm de espessura nominal cada, respectivamente. Para esses, o fator de idealidade calculado ficou próximo de 1. Espessuras de dielétricos acima de 4 nm começam a apresentar características de capacitores Metal-Óxido-Semicondutor e não de diodos Schottky. Por fim, reportamos as estruturas de Ni/Al2O3/4H-SiC/Ni e Ni/HfO2/4H-SiC/Ni, com 1 nm de dielétrico depositado, para uso como detector de partículas alfa no experimento de Espectrometria de Retroespalhamento Rutherford (RBS). Ambos os detectores apresentaram corrente reversa menor que 70 nA.cm-2 e resolução em energia de 76 keV, para polarização reversa de 40 V. / In the present work, silicon carbide (SiC) Schottky diodes with potential use in energy particle detectors were investigated. Metal-Insulator-Semiconductor (MIS) SiC Schottky diodes were fabricated. The MIS structures are considered because SiC always forms a thin native silicon oxycarbide (SiCxOy) layer in its surface that is difficult to remove by chemical means. A modified Thermionic Emission theory (TE) was developed to take into account the native oxide and/or thin dielectric layers present between metal and semiconductor in Schottky diodes. Aluminum/dielectric/silicon structures were fabricated for the dielectric characterization. SiO2, TiO2, HfO2 and Al2O3 dielectrics were deposited between Ni and SiC, with thicknesses varying from 1 to 8 nm. The deposited dielectrics layers thicknesses were confirmed by Ellipsometry spectra and X ray reflectometry before deposition of Ni Schottky contacts by sputtering. After Ni deposition and annealing, the Schottky diodes were electrically characterized by Current-Voltage (I-V) and Capacitance-Voltage measurements, varying the temperature. A thin dielectric layer present between metal and semiconductor artificially augments the Schottky Barrier Height (SBH) and lowers the reverse current when the diodes are reverse biased. A 0.18 – 0.20 nm of SiCxOy layer was inferred for the diodes using the modified TE. The real SBH was extracted from the I-V measurements and presented values of 1.39, 1.32 and 1.26 V for the diodes with 1 nm of TiO2, Al2O3 and HfO2, respectively. For these, an ideality factor close to 1 was calculated. Diodes with thicker (>4 nm) dielectrics layers shows Metal-Oxide-Semiconductor capacitors behavior. Ni/Al2O3/4H-SiC/Ni and Ni/HfO2/4H-SiC/Ni structures with 1 nm of dielectric layer thickness were used in Rutherford Backscattering Spectrometry experiments. Both detectors presented reverse current lower than 70 nA.cm-2 and energy resolution of 76 keV, when applied 40 V reverse bias.

The high temperature reactor (HTR) requires a completely new fuel design as it operates at around 1000°C in normal conditions and can reach up to 1600°C in case of an accident. The fuel and its cladding consist fully of ceramic materials, which precludes the possibility of a core meltdown and thus ensures inherent safety. The integral part of all HTR core designs is the tristructural-isotropic (TRISO) particle, which encapsulates the fissionable materials in succeeding coatings of pyrolytic carbon and silicon carbide (SiC). An exceptional mechanical integrity of the silicon carbide layer in all conditions is required to ensure full fission product retention. Within this work simulated TRISO fuel has been fabricated by fluidized bed chemical vapour deposition and was annealed in protective atmosphere up to 2200°C for short durations. Subsequent mechanical tests showed only minor reductions in the fracture strength of the SiC up to 2000°C. Substantial weight loss and crystal growth were observed after annealing at 2100°C and above. Raman spectroscopy identified the formation of a multi-layered graphene film covering the SiC grains after annealing and scanning electron microscopy revealed significant porosity formation within the coating from 1800°C onwards. These observations were attributed towards an evaporation-precipitation mechanism of SiC at very elevated temperatures that only slightly diminishes the hardness, elastic modulus or fracture strength, but might still be problematic in respect to fission product retention of the SiC layer. The new technique of high temperature nanoindentation was applied to measure the elastic modulus and hardness of SiC in-situ up to 500°C in argon atmosphere. The elastic modulus was found to be only slightly reduced over the measurement range, while the hardness showed a significant drop. Investigations of the deformation zone beneath the indenter tip executed by transmission electron microscopy showed slip and deformation twinning. On specimens that had been subject to neutron irradiation an irradiation hardening effect was noted. The elastic modulus showed only a minor increase compared with the non-irradiated samples. Oxidation experiments were carried out in air up to 1500°C. Analysis of the oxidation layer showed the formation of amorphous silica and cristobalite for the highest temperatures.

Neste trabalho foram investigadas estruturas de diodos Schottky de carbeto de silício (SiC) com potencial uso em detectores de energia de partículas carregadas. Para tanto, foram fabricados diodos Schottky de SiC do tipo Metal-Isolador-Semicondutor (MIS). Uma estrutura MIS é considerada uma vez que o SiC sempre forma em sua superfície uma fina camada de oxicarbeto de silício (SiCxOy) nativo, de difícil remoção por ataques químicos. Foi desenvolvido um modelo modificado da teoria de Emissão Termiônica (TE), de modo a levar em conta o óxido nativo e/ou finas camadas dielétricas inseridas entre metal e semicondutor nas estruturas de diodos Schottky. Foram fabricadas estruturas alumínio/dielétrico/silício para caracterização dos dielétricos utilizados. Foram depositados os dielétricos de SiO2, TiO2, HfO2 e Al2O3 entre o metal Ni e o semicondutor de SiC, variando as espessuras de 1 a 8 nm. As espessuras depositadas foram confirmadas por Elipsometria espectral e Reflectometria de raio X, anteriormente à deposição por sputtering do contato Schottky de Ni. Após a deposição e o tratamento térmico do Ni, as estruturas de diodos Schottky foram caracterizadas eletricamente por meio de medidas de Corrente-Tensão (I-V) e Capacitância-Tensão (C-V), variando a temperatura de medida. Foi observado que a presença de uma fina camada dielétrica entre metal e semicondutor aumenta artificialmente a Altura da Barreira Schottky (SBH), diminuindo a corrente reversa quando o diodo é polarizado reversamente. Por meio do modelo modificado da TE, foi calculada uma espessura variando de 0.18 – 0.20 nm para o oxicarbeto de silício presente nos diodos estudados. As SBH reais foram extraídas por meio das medidas de I-V, variando-se a temperatura. Foram obtidos os valores da SBH de 1.39, 1.32 e 1.26 V, para os dielétricos TiO2, Al2O3, HfO2 e com 1 nm de espessura nominal cada, respectivamente. Para esses, o fator de idealidade calculado ficou próximo de 1. Espessuras de dielétricos acima de 4 nm começam a apresentar características de capacitores Metal-Óxido-Semicondutor e não de diodos Schottky. Por fim, reportamos as estruturas de Ni/Al2O3/4H-SiC/Ni e Ni/HfO2/4H-SiC/Ni, com 1 nm de dielétrico depositado, para uso como detector de partículas alfa no experimento de Espectrometria de Retroespalhamento Rutherford (RBS). Ambos os detectores apresentaram corrente reversa menor que 70 nA.cm-2 e resolução em energia de 76 keV, para polarização reversa de 40 V. / In the present work, silicon carbide (SiC) Schottky diodes with potential use in energy particle detectors were investigated. Metal-Insulator-Semiconductor (MIS) SiC Schottky diodes were fabricated. The MIS structures are considered because SiC always forms a thin native silicon oxycarbide (SiCxOy) layer in its surface that is difficult to remove by chemical means. A modified Thermionic Emission theory (TE) was developed to take into account the native oxide and/or thin dielectric layers present between metal and semiconductor in Schottky diodes. Aluminum/dielectric/silicon structures were fabricated for the dielectric characterization. SiO2, TiO2, HfO2 and Al2O3 dielectrics were deposited between Ni and SiC, with thicknesses varying from 1 to 8 nm. The deposited dielectrics layers thicknesses were confirmed by Ellipsometry spectra and X ray reflectometry before deposition of Ni Schottky contacts by sputtering. After Ni deposition and annealing, the Schottky diodes were electrically characterized by Current-Voltage (I-V) and Capacitance-Voltage measurements, varying the temperature. A thin dielectric layer present between metal and semiconductor artificially augments the Schottky Barrier Height (SBH) and lowers the reverse current when the diodes are reverse biased. A 0.18 – 0.20 nm of SiCxOy layer was inferred for the diodes using the modified TE. The real SBH was extracted from the I-V measurements and presented values of 1.39, 1.32 and 1.26 V for the diodes with 1 nm of TiO2, Al2O3 and HfO2, respectively. For these, an ideality factor close to 1 was calculated. Diodes with thicker (>4 nm) dielectrics layers shows Metal-Oxide-Semiconductor capacitors behavior. Ni/Al2O3/4H-SiC/Ni and Ni/HfO2/4H-SiC/Ni structures with 1 nm of dielectric layer thickness were used in Rutherford Backscattering Spectrometry experiments. Both detectors presented reverse current lower than 70 nA.cm-2 and energy resolution of 76 keV, when applied 40 V reverse bias.

M.Sc. (Physics) / Please refer to full text to view abstract