Scott A. McHugo

Microscale investigation into the geochemistry of arsenic, selenium, and iron in soil developed in pyritic shale materials

Abstract In this study, we report on the distribution and mineralogy of micron-sized mineral aggregates formed in the top horizon of an acid sulfate soil. The distribution and oxidation state of arsenic (As) and selenium (Se) were also determined. The soil used in this study was formed from pyritic shale parent materials on the east side of the California Coast Range. Synchrotron-based X-ray fluorescence microprobe (μ-XRF) was used to generate elemental distribution maps of soil thin sections. Using the elemental distribution maps and optical micrographs, distinct mineral aggregates of iron oxide and iron sulfate were identified throughout the top horizon of the soil. These aggregates range in size from 10 to 100 μm in diameter and can be found only a few micrometers apart. The As and Se concentrations in the iron oxide aggregates were 5–10 times the concentrations in the iron sulfate aggregates and the weathered shale matrix. This suggests that the As and Se become preferentially associated with iron oxides during the weathering process. Using a focused micron-sized beam, Fe, As, and Se X-ray absorption spectroscopy (XAS) data were collected from the sub-millimeter soil aggregates. The micro-extended X-ray absorption fine structure (μ-EXAFS) spectrum collected from the iron oxide aggregate revealed that its mineralogy was a combination of ferrihydrite (>50%) and goethite. The μ-EXAFS spectra from the iron sulfate region suggest that these aggregates contain jarosite. Using micro-X-ray absorption near edge spectroscopy (μ-XANES), oxidation states of the As and Se were determined. Arsenic was present in the iron oxide aggregate as As(V). Selenium was present in the soil as both Se(IV) and Se(VI), with a higher percentage of Se(VI) in the jarosite aggregate than the iron oxide aggregate. These results provide direct evidence of the distribution, oxidation states, and speciation of As and Se in the solid phase of an unaltered native soil. Information on the weathering and geochemistry of the pyritic materials, and the associated arsenic and selenium is useful for predicting the pedogenic processes of acid sulfate soils and the long-term fate of newly exposed pyritic materials (e.g., mine tailings and drained wetlands).

Direct correlation of transition metal impurities and minority carrier recombination in multicrystalline silicon

Impurity and minority carrier lifetime distributions were studied in as-grown multicrystalline silicon used for terrestrial-based solar cells. Synchrotron-based x-ray fluorescence and the light beam induced current technique were used to measure impurity and lifetime distributions, respectively. The purpose of this work was to determine the spatial relation between transition metal impurities and minority carrier recombination in multicrystalline silicon solar cells. Our results reveal a direct correlation between chromium, iron, and nickel impurity precipitates with regions of high minority carrier recombination. The impurity concentration was typically 5×1016 atoms/cm2, indicating the impurity-rich regions possess nanometer-scale precipitates. These results provide the first direct evidence that transition metal agglomerates play a significant role in solar cell performance.

Nanometer-scale metal precipitates in multicrystalline silicon solar cells

In this study, we have utilized characterization methods to identify the nature of metal impurityprecipitates in low performance regions of multicrystalline silicon solar cells. Specifically, we ha ...

Diffusion, solubility and gettering of copper in silicon

Abstract The feasibility of quantitative predictive modeling of gettering of Cu in silicon, which requires quantitative understanding of its diffusivity and precipitation behavior, is discussed. Investigations of diffusion of Cu at low temperatures enabled us to determine the pairing constants of copper with boron, re-evaluate its diffusivity at room temperature in p-Si, and to predict its diffusivity in p + -Si substrates. We demonstrate that copper may either precipitate in the bulk of the wafer or diffuse to its surface, depending on the position of Fermi level in the sample. It is suggested that the Fermi level position determines the sign and magnitude of the electrostatic charge on the growing copper precipitates, and thus enhances or suppresses precipitation of interstitial copper ions. Modeling of p/p + segregation gettering of copper shows that while the copper can be gettered in p + layer during or after high-temperature anneals, it eventually will be released and will precipitate in the device region within the first few months of operation, unless more stable gettering (precipitation) sites for copper are utilized. An n-type layer is predicted to be an effective gettering site.

Iron solubility in highly boron-doped silicon

We have directly measured the solubility of iron in high and low boron-doped silicon using instrumental neutron activation analysis. Iron solubilities were measured at 800, 900, 1000, and 1100 °C in silicon doped with either 1.5×1019 or 6.5×1014 boron atoms/cm3. We have measured a greater iron solubility in high boron-doped silicon as compared to low boron-doped silicon, however, the degree of enhancement is lower than anticipated at temperatures >800 °C. The decreased enhancement is explained by a shift in the iron donor energy level towards the valence band at elevated temperatures. Based on this data, we have calculated the position of the iron donor level in the silicon band gap at elevated temperatures. We incorporate the iron energy level shift in calculations of iron solubility in silicon over a wide range of temperatures and boron-doping levels, providing a means to accurately predict iron segregation between high and low boron-doped silicon.

Scanning room-temperature photoluminescence in polycrystalline silicon

Photoluminescence (PL) mapping was performed on polycrystalline silicon wafers at room temperature. Two PL bands are observed: (1) a band-to-band emission with a maximum at 1.09 eV, and (2) a deep “defect” luminescence at about 0.8 eV. PL mapping of 10 cm×10 cm wafers revealed inhomogeneity of the band-to-band PL intensity which could be correlated to the distribution of minority carrier diffusion length in the wafer bulk. We have also observed that the intensity of the 0.8 eV band is strongest along those grain boundaries where the band-to-band PL is suppressed as well as minority carrier diffusion length. The origin of the 0.8 eV luminescence band is discussed.

Gettering of iron by oxygen precipitates

In order to better understand and model internal gettering of iron in silicon, a quantitative investigation of iron precipitation in silicon containing different oxygen precipitate densities was performed. The number of iron precipitation sites was obtained from the iron precipitation kinetics using Ham’s Law. At low temperatures, the iron precipitate density corresponded to the oxygen precipitate density. A strong temperature dependence of the iron precipitate density was observed for the samples with larger oxygen precipitate densities. These data were used to simulate iron precipitation during a slow cool. From those simulations, optimal cooling rates were obtained for different silicon materials assuming various iron precipitation site densities in the epitaxial layer.

Transient ion drift detection of low level copper contamination in silicon

The transient ion drift (TID) method was used to measure quenched interstitial copper concentrations in both copper plated and copper implanted silicon. Comparison with existing literature data allows one to conclude that, contrary to the general expectation, it is possible to quench in most of the Cu dissolved at temperatures of 600 °C and below. This result suggests that the TID technique could be an excellent means to detect copper contamination in p-type silicon. The expected detection limit, on the order of 1011 cm−3, makes the method a potentially interesting tool to use in gettering or in-diffusion barrier studies.

Release of metal impurities from structural defects in polycrystalline silicon

Metal impurity release from structural defects in polycrystalline silicon was studied following thermal treatments, and, in addition, a correlation between impurity distributions and structural defects was established. Impurities were mapped with synchrotron-based x-ray fluorescence in the as-grown state, after rapid thermal annealing and following aluminum gettering treatments. The goal of this work was to determine if impurity release from structural defects limits gettering of metal impurities. The results reveal that nickel and copper metal impurities are primarily found at dislocations in as-grown crystals, and the release of these impurities from defects occurs rapidly with no apparent barrier to dissolution. Gettering treatments dissolved metal impurity precipitates to <2–5 nm in radii; however, the material performance was not greatly enhanced.

Efficiency-limiting defects in silicon solar cell material

The precipitation rate of intentionally introduced iron during low-temperature heating is studied among a variety of single-crystal and polycrystalline silicon solar cell materials. A correlation exists between the iron precipitation rate and the carrier recombination rate in dislocation-free as-grown material, suggesting that diffusion-length-limiting defects in as-grown material are structural defects which accelerate iron precipitation. Phosphorous diffusion gettering was found to be particularly ineffective at improving diffusion length after intentional iron contamination in materials with high iron precipitation rates. We propose that intragranular structural defects in solar cell silicon greatly enhance transition metal precipitation during cooling from the melt and become highly recombination-active when decorated with these impurities. The defects then greatly impair diffusion length improvement during phosphorus gettering and limit carrier lifetimes in as-grown material.