S. M. Heald

Synchrotron-based investigations of the nature and impact of iron contamination in multicrystalline silicon solar cells

Synchrotron-based microprobe techniques were used to obtain systematic information about the size distribution, spatial distribution, shape, electrical activity, chemical states, and origins of iron-rich impurity clusters in multicrystalline silicon (mc-Si) materials used for cost-effective solar cells. Two distinct groups of iron-rich cluster have been identified in both materials: (a) the occasional large (diameter ⩾1μm) particles, either oxidized and/or present with multiple other metal species reminiscent of stainless steels or ceramics, which are believed to originate from a foreign source such as the growth surfaces, production equipment, or feedstock, and (b) the more numerous, homogeneously distributed, and smaller iron silicide precipitates (diameter ⩽800nm, often ⩽100nm), originating from a variety of possible formation mechanisms involving atomically dissolved iron in the melt or in the crystal. It was found that iron silicide nanoprecipitates account for bulk Fe concentrations as high as 1014–...

Interactions Between Metals and Different Grain Boundary Types and Their Impact on Multicrystalline Silicon Device Performance

The mechanical and electrical properties of polycrystalline solids, such as metals, ceramics, and photovoltaic-grade multicrystalline silicon (mc-Si), are strongly regulated by the interactions between impurities and grain boundaries. In this broader context, we combine synchrotron-based X-ray fluorescence microscopy (mu-XRF), SEM-based electron back-scatter diffraction (EBSD), and conventional techniques to correlate metal precipitation behavior with grain boundary character (type), electrical activity, and microstructure in commercial multicrystalline silicon (mc-Si) materials. It is directly observed that metals tend to form precipitates selectively at higher-Sigma coincidence site lattice (CSL) boundaries and non-CSL boundaries, while largely avoiding precipitation at Sigma3 boundaries, and to a lesser extent, Sigma9. The electrical impacts of this behavior differ, depending on surrounding intragranular quality. A discussion of mc-Si grain boundary engineering ensues

Spatiotemporally separating electron and phonon thermal transport in L10 FePt films for heat assisted magnetic recording

We report the spatio-temporal separation of electron and phonon thermal transports in nanostructured magnetic L10 FePt films at the nanometer length scale and the time domain of tens of picosecond, when heated with a pulsed laser. We demonstrate that lattice dynamics measured using the picosecond time-resolved laser pump/X-ray probe method on the FePt (002) and Ag (002) Bragg reflections from different layers provided the information of nanoscale thermal transport between the layers. We also describe how the electron and phonon thermal transports in nanostructured magnetic thin films were separated.

Investigation of Heat-Assisted Magnetic Recording Media Films in Four Dimensions

Heat-assisted magnetic recording is one of the most promising approaches to achieve extremely high density magnetic recording. A laser is used to temporarily heat the recording media film to render its switching field to become lower than the magnetic field of the writing head. After writing, the film material is cooled down to room temperature for data storage. In this dynamic writing process, the challenge is to tailor and optimize the time resolved structural/thermal/magnetic properties of the media films and their correlations during/after the laser heating. In this paper, the four dimensions of current progress in HAMR media research, i.e., structure, magnetic properties, thermal properties and time, are briefly overviewed and discussed. Their correlations are also emphasized.

Investigation of spin and orbital moments of L10 FePtRh thin films

Fe50Pt50−xRhx films have attracted much interest recently due to their potential usage in heat assisted magnetic recording media and the ferromagnetic (FM) to antiferromagnetic (AFM) transition as Rh concentration is increased. In this study, we present an investigation of the spin moment (ms) and orbital moment (ml) of Fe in Fe50Pt50−xRhx films using x-ray magnetic circular dichroism. The measurements were made in the soft x-ray region along the film normal, which corresponds to the easy axis of the (001) textured L10 FePt with uni-axial magnetic anisotropy. They show that the magnetic moment of Fe mainly contributes to the whole magnetization of the films. The decrease of ml with Rh doping concentration implies a decrease of magneto-crystalline anisotropy, whereas the significant decrease of ms is consistent with the FM to AFM transition at above 15 at. % of Rh.

Mineralogic Residence and Desorption Rates of Sorbed 90Sr in Contaminated Subsurface Sediments: Implications to Future Behavior and In-Ground Stability

The project is investigating the adsorption/desorption process of 90Sr in coarse-textured pristine and contaminated Hanford sediment with the goal to define a generalized reaction-based model for use in reactive transport calculations. While it is known that sorbed 90Sr exists in an ion exchangeable state, the mass action relationships that control the solid-liquid distribution and the mineral phases responsible for adsorption have not been defined. Many coarse-textured Hanford sediment display significant sorptivity for 90Sr, but contain few if any fines that may harbor phyllosilicates with permanent negative charge and associated cation exchange capacity. Moreover, it is not known whether the adsorption-desorption process exhibits time dependence within context of transport, and if so, the causes for kinetic behavior.