Dielectric and optical properties of nanometric nickel silicides from valence electrons energy-loss spectroscopy experiments.

Valence and Core Electron Energy Loss Spectroscopy (VEELS and CEELS) experiments are performed from nanocrystallized nickel silicide thin films. Three different silicide compounds are identified in the films. Their chemical compositions are determined from Ni-L(2,3) to Si-K core edges quantification. The results obtained are coherent within less than 2% error with the pure Ni2Si, NiSi and NiSi2 phases. The analysis of the shape and energy position of Ni-L(2,3) near edge structures and volume plasmon peaks indicates that both are reliable signatures to identify unambiguously each compound. Nickel silicides low-loss spectra have been submitted as references to the EELS database (www.cemes.fr~eelsdb). Low-loss spectra are processed to extract single scattering spectra and determine the dielectric function. The results show that nickel silicides dielectric functions deduced from VEELS are in quite good agreement with epsilon1 and epsilon2 deduced from ellipsometry experiments. The optical properties (refractive index (n), absorption coefficient (k), reflectivity (R%) and resistivity (rho(opt))), calculated from VEELS dielectric function are then compared in details with the data resulting from others techniques available in the literature. We show that, except some minor divergences, the nickel silicides optical properties are generally well reproduced. This indicates that VEELS is a relevant technique for accessing reliably to physical properties and can be a successful alternative to conventional techniques when high spatial resolution is needed.

Comparison of Si and Ge low-loss spectra to interpret the Ge contrast in EFTEM images of Si(1-x) Ge(x) nanostructures.

Recently, an EFTEM imaging method, exploiting the inelastically scattered electrons in the 60-90eV energy range, was proposed to visualise Ge in SiGe alloys [Pantel, R., Jullian, S., Delille, D., Dutartre, D., Chantre, A., Kermarrec, O., Campidelli, Y., Kwakman, L.F.T.Z., 2003. Inelastic electron scattering observation using Energy Filtered Transmission Electron Microscopy for silicon-germanium nanostructures imaging. Micron 34, 239-247]. This method was proven to be highly more efficient in terms of noise, drift and exposure time than the imaging of the weak and delayed ionization GeL2,3 edge at 1236eV. However, the physical phenomenon behind this Ge contrast was not clearly identified. In this work, we explain the origin of this Ge contrast, by comparing in details EELS low-loss spectra (<100eV) recorded from pure Si and Ge crystals. High resolved low-loss experiments are performed using analytical Field Emission Gun Transmission Electron Microscopes fitted or not with a monochromator. Low-loss spectra (LLS) are then deconvoluted from elastic/quasi-elastic and plural scattering effects. The deconvolution procedure is established from Si spectra recorded with the monochromated machine. The absence of second plasmon and the measurement of a band gap (1.12eV) on the Si single scattering distribution (SSD) spectrum allowed us to control the accuracy of the deconvolution procedure at high and low energy and to state that it could be reliably applied to Ge spectra. We show that the Ge-M4,5 ionisation edge located at 29eV, which is shadowed by the high second plasmon in the unprocessed Ge spectrum, can be clearly separated in the single scattering spectrum. We also show that the front edge of Ge-M4,5 is rather sharp which generates a high intensity post edge tail on several tens of eV. Due to this tail, the Si and Ge EELS signals in the 60 to 100eV energy window are very different and the monitoring of this signal gives information about the Ge concentration inside SiGe alloys. It is now evident that the EFTEM imaging technique proposed to quantify Ge (90eV/60eV image ratio) in Si-Ge nanostructures is valid and is a relevant way of exploiting the Ge-M4-5 ionisation edge.

Improved comparison of low energy loss spectra with band structure calculations: the example of BN filaments.

Electron energy loss spectra have been recorded from BN filaments for energy losses between 2 and 50eV. They compare well with other BN shapes (nanotubes, sheets, etc). The interpretation of all the peaks in the spectra has been made via an ab initio calculation using the FLAPW code WIEN97 (A full potential linearized augmened plane wave package for calculating crystal properties, Karlheinz Schwarz, Techn. Universität Wien, Austria, 1999. ISBN 3-9501031-0-4). In order to fully simulate the observed spectra, the geometry particular to the EELS experiment has been taken into account making use of Wessjohann's relativistic formula for thin anisotropic slabs (Phys. Stat. Sol. B 77 (1976) 535). Furthermore, the effect of the convergent beam has been introduced into the simulation and has been proven to be an important parameter in obtaining the corresponding peak intensities.

Role of alveolar macrophages in the dissolution of two different industrial uranium oxides.

This study was aimed at assessing and understanding some mechanisms involved in the intracellular particle transformation of two uranium oxides (U3O8 and UO2 + Umetal) produced by a new isotopic enrichment plant using laser technology. Instillations were conducted on rats with both uranium compounds and alveolar macrophages were harvested at different dates and prepared in order to be studied using transmission electron microscopy and electron energy loss spectrometry (EELS). The presence of particles in the cells was observed from the first day after instillation, and crystalline needles of uranyl phosphate appeared in the cytoplasm of the cells. These needles were more numerous after instillation with the mixture UO2 + Umetal than after administration of U3O8 and may be correlated with the higher solubility of UO2 + Umetal observed in vitro. The formation of insoluble needles in lysosomes is consistent with the insolubilisation of uranium observed after phagocytosis by alveolar macrophages.