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The aim of this thesis is to comprehensively analyse presolar silicon carbide (SiC) grains from several primitive meteorites in order to investigate their complicated history. During their residence in the interstellar medium, presolar grains are predicted to be affected by many processes which may modify their original elemental and isotopic composition. Presolar SiC grains from three acid residues and two polished meteorite sections were analysed for their carbon, nitrogen and silicon isotope ratios with high spatial resolution, in order to compare the distribution of 14N/15N ratios compared to those found in the literature. As a result of this work, isotopic fractionation effects caused by the distortion of the electric field around the grain topography were identified. These effects have the potential to cause differential transmission of atomic and molecular secondary ions, particularly when small slits and apertures are selected during NanoSIMS analyses. The measured 14N/15N ratios of the presolar SiC grains analysed in this work match well with existing literature data, although many grains cluster at relatively low 14N/15N values. These low ratios do not appear to be the result of either terrestrial contamination or isotopic dilution, and may instead represent real differences between the SiC grain populations of different meteorites. The majority of mainstream SiC grains analysed in this work lie on a slope with a gradient of ~1.3 on a Si 3-isotope plot, in agreement with literature data. SiC grains from the JAMM and JA-MM2 acid residues appear to lie on shallower slopes, although these samples show significant scatter in the data. Neither terrestrial contamination nor isotopic dilution can explain the apparent fractionation of silicon isotopes in these samples. It is possible that these ratios may represent a difference in the Si ratios of grain populations of different meteorites, although fractionation during the sample preparation phase cannot be excluded. Ten presolar SiC grains from the KJG and JA-MM2 acid residues are comprehensively analysed for their trace element compositions using Time-of-Flight Secondary Ion Mass Spectrometry. The majority of analyses are significantly affected by the proximity of neighbouring grains, leading to high background counts which prevent the reliable determination of elemental abundances for many elements. Depth profiles of several elements are determined for two grains from the KJG residue. Each of the measured elements displays approximately homogeneous profiles through the grains, with abundances in agreement with existing literature data. The uniform depth profiles may represent formation in a stellar envelope with a stable composition, although homogenisation by secondary alteration processes cannot be ruled out.

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DoD EO/IR applications continue to push imaging technology. Next generation advances require new material systems, novel device structures, as well as the improvement and development of read out integrated circuits. The common methodology used to assess devices and materials is to process test chip structures and focal plane arrays. Test chips have advantages over focal plane arrays; such as, variable area and stand alone devices and can be measured as a function of bias and temperature; however, focal plane arrays are the heart of the imager, but are measured through the read out integrated circuits, which can add complexity to the measurement and to the extraction of properties. Differences between a test chip and a focal plane array make it difficult to relate test chip characterization with focal plane array performance and the interaction of defects and the impact they have to the image. Therefore, it is vital to be able to characterize the detector array directly from 77 to 300 K. This thesis presents the design for a transparent cryogenic probe card based on a silicon carbide substrate. An inspection into the design for the bonding printed circuit board and its compliant structures is included. The design for the silicon carbide probe die is explored. The design and process for fabricating tungsten probe tips is discussed. Results on research efforts are presented showing the validity of the design based upon fabrication outcomes. It is recommended that future work explore additional tungsten tip fabrication methods.

Intermetallic composites of molybdenum disilicide reinforced with various amounts of silicon carbide particulate were produced by combustion synthesis from their elemental powders. Elemental powders were mixed stoichiometrically then ball-milled. The coldpressed mixture was then chemically ignited at one end under vacuum at approximately 700°C. The combustion temperature of the process was approximately 1600°C which was lower than the melting point of molybdenum disilicide. This processing technique allowed the fabrication of the composites at 700°C within a few seconds, instead of sintering at temperatures greater than 1200°C for many hours. The end product was a porous composite, which was densified to >97% ofthe theoretical density by hot pressing. The grains ofthe matrix were 8-14 μm in size surrounded by SiC reinforcement of 1-5 μm. The morphology and structure of the products were studied by x-ray diffraction and scanning electron microscopy (SEM). Samples were prepared for hardness, fracture strength, and toughness testing at room temperature. There were improvements in the mechanical properties of the composites with increasing SiC reinforcement. The hardness of the materials increased from 10.1 ± 0.1 GPa (959 ± 13 kg/mm2) to 11.7 ± 0.6 GPa (1102 ± 52 kg/mm2) to 12.7 ± 0.4 GPa (1199 ± 36 kg/mm2) with the 10 vol% and 20 vol% SiC reinforcement, respectively. The strength increased from 195±39 MPa to 237±39 MPa with 10 vol% and to 299 ± 43.2 MPa with a 20 vol% SiC reinforcement. The fracture toughness increased from 2.79 ± 0.36 MPa.m1/2 to 3.31± 0.41 MPa.m1/2 with 10 vol% SiC and to 4.08± 0.30 MPa.m1/2 with 20 vol% SiC. The increase in hardness and flexural strength is due to the effective load transfer across the strong interface in the composites. The main toughening mechanism is crack deflection by the residual stress in the materials, induced by the differences in the thermal expansion coefficients and the elastic moduli ofthe matrix and reinforcement. / Thesis / Master of Engineering (ME)

The wide band-gap semiconductor silicon carbide has tremendous potential for use in high power, high temperature, and high frequency electronic devices. One of the more important design factors for these devices is the epitaxial layer. It is desirable that this thin film have uniform polytype, thickness, and impurity concentration, as well as be defect free. One method used for SiC to ensure epitaxial layers with homogenous polytype is to cut wafers from a boule that has been tilted towards a specific crystallographic face at a fixed angle (known as ?off cut?). The purpose of this thesis was to investigate the growth mechanisms of alternative boule tilting directions with 6H-SiC. Four alternative crystallographic tilting faces were chosen: <1230>, <1340>, <2130>, and <3140>. A lightly doped 1um-thick layer was grown on samples representing the four alternative off-cut directions and, as references, commercially available substrates off cut towards the traditional direction <1120>. The physical and electrical properties of the layers were characterized by means of optical microscopy, Fourier Transform Infrared Reflectance Spectroscopy, Atomic Force Microscopy, capacitance vs. voltage, and current vs. voltage. Three facts were observed: 1) the alternative off-cut directions affected the growth mechanisms and surface morphology, 2) the quality of the substrate affects the morphology of the epitaxy layer, and 3) the relative differences between the surface roughness attributed to the different off-cut directions affected the observed electrical characteristics of Schottky barrier diodes fabricated on the epi layers. The samples cut towards the <31-40> and <13-40> directions showed to the most promising alternative off-axis tilting direction.

4H-SiC n-channel power MESFETs with nitrogen-doped epitaxially grown channel and nitrogen n+-implanted source/drain ohmic contact regions, with and without p-buffer layer fabricated on semi-insulating substrates exhibited hysteresis in the drain I-V characteristics of both types of devices at 300 K and 480 K due to traps. However, thermal spectroscopic measurements could detect the traps only in the devices without p-buffer. In this study the two-dimensional device simulator, MediciTM, and optical admittance spectroscopy (OAS) measurements are used to help resolve the discrepancy in the initial experimental characterization results and interpret the results. Device simulations also showed hysteresis in the drain I-V curves of both types of devices at 300 K and 480 K. Simulations suggest that, in addition to the SI substrate traps, which are known to be major cause of hysteresis in MESFET drain I-V characteristics, acceptor traps due to source/drain residual implant lattice damage could also contribute to the hysteresis observed in the drain I-V characteristics of the experimental MESFETs. Although surface traps are known to cause hysteresis in the I-V curves of MESFETs, their presence was not observed in the experimental devices. The results of the OAS measurements showed several peaks in the spectra of the devices without p-buffer, while in the spectra of the devices with p-buffer the peaks were generally non-existent or reduced. This demonstrates that the peaks observed in the OAS spectra are largely due to substrate traps and that the p-buffer layer is effective in isolating the channel from the substrate. A peak centered around 1.51 eV below the conduction band, which has also been observed in the literature after He+-implantation, is consistently observed in the spectra of both types of devices although it appears reduced in the spectra of the devices with buffer. In this dissertation it is shown that it is likely the traps responsible for this peak could contribute to the hysteresis observed at 300 K and could be solely responsible for the hysteresis observed at high temperatures such as 480 K, since simulations suggest that hysteresis due to semi-insulating substrate traps disappear at high temperatures such as 480 K.

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