Matching Items (5)
Mass spectrometric analysis requires that atoms from the sample be ionized in the gas phase. Secondary ion mass spectrometry achieves this by sputtering samples with an energetic primary ion beam. Several investigations of the sputtering and ionization process have been conducted. Oxygen is commonly used in secondary ion mass spectrometry (SIMS) to increase ion yields, but also can complicate the interpretation of SIMS analyses. An 18O implant in silicon has been used to quantify the oxygen concentration at the surface of sputtered silicon in order to study the dependence on oxygen of several sputtering and depth profile phenomena. The ion yield dependence of trace elements in silicon on the surface oxygen concentration is a function of the ionization potential of the element. The ion yield is high and unaffected by oxygen for elements with low ionization potential and ranges over several orders of magnitude for elements with high ionization potential. Depth resolution in sputter profiles has been shown to be degraded by the presence of oxygen, the mechanism of this effect has been investigated using an 18O implant to quantify oxygen levels and it is shown that the process does not appear to be a consequence of surface oxide formation. Molecular ions are a source of mass interference in SIMS analysis, and multiply charged atomic ion signals might be interference-free due to the possible instability of multiply-charged molecular ions. Sputtered SiH2+, AlH2+, BeH2+, Mo22+ and Mg22+ ions have been observed and appear surprisingly stable. The formation mechanism of some of these species has been explored.
Volcanic devolatilization is one of the major processes in the global nitrogen cycle. Past studies have often estimated the magnitude of this flux using volcanic emission measurements, which are limited to currently active systems and sensitive to atmospheric contamination. A different methodological approach requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry (SIMS), which have not yet been established. To this end, we analyze various ion implanted basaltic and rhyolitic glasses by SIMS. We demonstrate that water content significantly affects the ion yields of 14N+ and 14N16O−, as well as the background intensity of 14N+ and 12C+. Application of implant-derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions. These measurements are from samples from the Bishop Tuff in California, the Huckleberry Ridge Tuff of the Yellowstone Volcanic Center, and material from the Okaia and Oruanui eruptions in the Taupo Volcanic Center. In all studied material, we find maximum nitrogen contents of less than 45 ppm and that nitrogen concentration varies positively with CO2 concentration, which is interpreted to reflect partial degassing trend. Using the maximum measured nitrogen contents for each eruption, we find that the Bishop released >3.6 x 1013 g of nitrogen, the Huckleberry Ridge released >1.3 x 1014 g, the Okaia released >1.1 x 1011 g of nitrogen, the Oruanui released >4.7 x 1013 g of nitrogen. Simple calculations suggest that with concentrations such as these, rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere, but are insignificant compared to the 4 x 1021 g of nitrogen stored in the atmosphere.
Oxygen fugacity (ƒO2) is a thermodynamic variable used to represent the redox state of a material or a system. It is equivalent to the partial pressure of oxygen in a particular environment corrected for the non-ideal behavior of the gas. ƒO2 is often used to indicate the potential for iron to occur in a more oxidized or reduced state at a particular temperature and pressure in a natural system. Secondary ion mass spectrometry (SIMS) is a powerful analytical instrument that can be used to analyze elemental and isotopic compositional information about microscopic features within solid materials. SIMS analyses of the secondary ion energy distribution of semi-pure metals demonstrate that the energy spectrum of individual mass lines can provide information about alterations in its surface environment.
The application of high-resolution (see Appendix C) energy spectrum calibrations to natural ilmenite led to the investigation of zirconium (90Zr+) and niobium (93Nb+) as potential indicators of sample ƒO2. Energy spectrum measurements were performed on an array of ilmenite crystals from the earth’s upper mantle retrieved from kimberlites and from a reduced meteorite. In all studied materials, variability in the peak shape and width of the energy spectra has been correlated with inferred sample ƒO2. The best descriptor of this relationship is the full-width at half-maximum (FWHM; see Appendix C) of the energy spectra for each sample. It has been estimated that a 1eV change in the FWHM of 93Nb+ energy spectra is roughly equivalent to 1 log unit ƒO2. Simple estimates of precision suggest the FWHM values can be trusted to 1eV and sample ƒO2 can be predicted to ±1 log unit, assuming the temperature of formation is known.
The work of this thesis also explores the applicability of this technique beyond analysis of semi-pure metals and ilmenite crystals from kimberlites. This technique was applied to titanium oxides experimentally formed at known ƒO2 as well as an ilmenite crystal that showed compositional variations across the grain (i.e., core to rim chemical variations). Analyses of titanium oxides formed at known ƒO2 agree with the estimation that 1 eV change in the FWHM of 93Nb+ is equivalent to ~1 log unit ƒO2 (in all cases but one); this is also true for analyses of a natural ilmenite crystal with compositional variations across the grain.
A novel technique for measuring heavy trace elements in geologic materials with secondary ion mass spectrometry (SIMS) is presented. This technique combines moderate levels of mass resolving power (MRP) with energy filtering in order to remove molecular ion interferences while maintaining enough sensitivity to measure trace elements. The technique was evaluated by measuring a set of heavy chalcophilic elements in two sets of doped glasses similar in composition to rhyolites and basalts, respectively. The normalized count rates of Cu, As, Se, Br, and Te were plotted against concentrations to test that the signal increased linearly with concentration. The signal from any residual molecular ion interferences (e.g. ²⁹Si³⁰Si¹⁶O on ⁷⁵As) represented apparent concentrations ≤ 1 μg/g for most of the chalcophiles in rhyolitic matrices and between 1 and 10 μg/g in basaltic compositions. This technique was then applied to two suites of melt inclusions from the Bandelier Tuff: Ti-rich, primitive and Ti-poor, evolved rhyolitic compositions. The results showed that Ti-rich inclusions contained ~30 μg/g Cu and ~3 μg/g As while the Ti-poor inclusions contained near background Cu and ~6 μg/g As. Additionally, two of the Ti-rich inclusions contained > 5 μg/g of Sb and Te, well above background. Other elements were at or near background. This suggests certain chalcophilic elements may be helpful in unraveling processes relating to diversity of magma sources in large eruptions. Additionally, an unrelated experiment is presented demonstrating changes in the matrix effect on SIMS counts when normalizing against ³⁰Si⁺ versus ²⁸Si²⁺. If one uses doubly charged silicon as a reference, (common when using large-geometry SIMS instruments to study the light elements Li - C) it is important that the standards closely match the major element chemistry of the unknown.
The availability of bulk gallium nitride (GaN) substrates has generated great interest in the development of vertical GaN-on-GaN power devices. The vertical devices made of GaN have not been able to reach their true potential due to material growth related issues. Power devices typically have patterned p-n, and p-i junctions in lateral, and vertical direction relative to the substrate. Identifying the variations from the intended layer design is crucial for failure analysis of the devices. A most commonly used dopant profiling technique, secondary ion mass spectroscopy (SIMS), does not have the spatial resolution to identify the dopant distribution in patterned devices. The possibility of quantitative dopant profiling at a sub-micron scale for GaN in a scanning electron microscope (SEM) is discussed. The total electron yield in an SEM is shown to be a function of dopant concentration which can potentially be used for quantitative dopant profiling.
Etch-and-regrowth is a commonly employed strategy to generate the desired patterned p-n and p-i junctions. The devices involving etch-and-regrowth have poor performance characteristics like high leakage currents, and lower breakdown voltages. This is due to damage induced by the dry etching process, and the nature of the regrowth interface, which is important to understand in order to address the key issue of leakage currents in etched and regrown devices. Electron holography is used for electrostatic potential profiling across the regrowth interfaces to identify the charges introduced by the etching process. SIMS is used to identify the impurities introduced at the interfaces due to etch-and-regrowth process.