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[en] In order to improve the accuracy of laser atom probe analyses, it is important to understand all the physical processes induced by the combination of the high electrical field and the femtosecond laser beam during field evaporation. New information can be accessed from the energy of evaporated surface atoms or field-ionised atoms of an imaging gas. In order to study the ions energy, we combine La-APT and FIM analyses in a new experimental setup equipped with electrostatic lenses. We report measurements for semiconductors and oxides and we study the influence of the illumination conditions (laser power and wavelength), the evaporation rate, the sample geometry and the tip preparation processes. The results are discussed taking into account the resistive properties of non-metallic samples and the photo-stimulated conductivity. This work clarifies the role of the laser and DC field in the energy deficit of field evaporated ions. - Highlights: • Energy from field emitted ion was studied. • Ion energy is influence by ohmic effect in atom probe and Field Ion Microscopy. • Ion energy deficit can be reduce by laser illumination and cancelled metallisation. • Tip electrical resistance is strongly dependant of the tip preparation.
[en] Full text: Magnetic rare-earth (R.E.)/transition metal (T.M.) multilayered films have been extensively investigated during the last decade because they may exhibit strong perpendicular magnetic anisotropy, a large Kerr rotation angle and a high coercive field. The origin of these unusual properties is attributed to the artificial nanostructure that enhances the phenomena previously observed in rare earth-transition metal amorphous alloys. Our approach is based on the coupling of tomographic atom probe (T.A.P.) analysis with magnetometery measurements, in order to correlate the structural properties of the interfaces with the magnetic anisotropy properties. The samples are prepared by evaporation under ultrahigh vacuum, and studied by various investigation techniques: Moessbauer spectrometry, x-ray diffraction and reflectometry, SQUID magnetometery and T.A.P. The high resolution of the tomographic atom probe enables to get a 3D reconstruction of the analyzed material, and therefore to determine a concentration profile through the multilayer. The goal is then to explain the variation of the perpendicular anisotropy, as a function of preparation conditions, via the tomographic atom probe analysis. Refs. 3 (author)
[en] Three-dimensional atom-probe (3DAP) is the only analytical microscope able to map out the distribution of elements in 3D at the atomic scale. 3DAP provides quantitative measurements of local chemical composition in a small selected volume. A new generation of instrument, namely, a laser assisted tomographic atom probe (laser assisted wide angle atom probe LaWaTAP) has recently been designed in order to open the instrument to bad electric conductors. The use of ultra-fast laser pulses rather than of high-voltage pulses to field evaporate surface atoms makes the analysis of semiconductors or oxides that are key materials in microelectronics possible. This article is focussed on methodology and applications to boron-doped silicon. Depth profiles related to boron in various samples (boron deltas, SiGe Maya layers, boron-implanted silicon, ...) will be discussed and compared to SIMS results. Spatial resolution and sensitivities will be compared.
[en] The irradiation damage in the target window of a demonstrator of an Accelerator Driven System (ADS) consists of atomic displacements (dpa) and spallation element production that will affect the in-service properties of the structural material of the target. The atomic displacements (about 100 dpa/year) will promote the formation of point defect clusters, dislocation loops and the precipitation of various phases that contribute to hardening and embrittlement of the structural material. As an example, the Ca and Ti production should harden the material via precipitation, in parallel to embrittlement due to P and S segregation. The purpose of this work was to simulate the spallation element loading, via ion implantation (using the IRMA implanter at CSNSM) and to study at the atomic scale, with the 3D atom probe, their evolution in the 9Cr-1Mo reference martensitic steel. In order to realize this, specific experiments, performed at 300 deg C, were carried out using low energy ions (Ca, Ti or S) implanted in the extremely small atom probe specimens (needles of 100 nanometers thickness). (authors)
[en] Powders of Al68.5Ni31.5 alloy have been produced by gas atomisation and sieved in different grain size families. The resulting families have been analysed by combined neutron and X-ray diffraction in order to investigate the structure and identify the existing phases at the surface and in the bulk of the grains. The weight fraction of the identified phases (Al3Ni2, Al3Ni and Al) has been estimated from a profile refinement with the FULLPROF computer codes. An additional phase was observed but could not be identified in the diffraction patterns. Starting from grains less than 5 μm in diameter, samples have been shaped by annular focused ion beam into needles that were suitable for atom probe investigations. The structure and morphologies observed by different techniques are compared and discussed. It has also been possible to estimate the crystallite sizes and the strains corresponding to the different phases present in the powders from the refinement of the ND patterns. In addition to Al3Ni2 and Al3Ni, a phase of composition close to the nominal one of the alloy was observed in the atom probe measurements. This phase could be one of the decagonal ones referred to in the literature. Small particles of composition close to Al82Ni18 are attributed to the metastable Al9Ni2 phase. The achieved conclusions demonstrate the complementarity of X-ray and neutron diffraction techniques and atom probe tomography to analyse complex structures.
[en] Atomic scale chemistry of polycrystalline Cu(In,Ga)Se2 (CIGSe) thin film has been characterized at key points of the 3-stage process using atom probe tomography. 3D atom distributions have been reconstructed when the layer is Cu-poor ([Cu]/([Ga] + [In]) < 1), Cu-rich ([Cu]/([Ga] + [In]) > 1), and at the end of the process. Particular attention has been devoted to grain boundary composition and Na atomic distribution within the CIGSe layer. Significant variation of composition is highlighted during the growing process, providing fundamental information helping the understanding of high efficiency CIGSe formation.
[en] The reason why so-called wide-bandgap CuIn1−xGaxSe2 (CIGSe with x > 0.4) based solar cells show hindered performance compared with theoretical expectations is still a matter of debate. In the present Letter, atom probe tomography studies of CuIn1−xGaxSe2 polycrystalline thin films with x varying from 0 to 1 are reported. These investigations confirm that the grain boundaries (GBs) of low gallium containing (x < 0.4) CIGSe layers are Cu-depleted compared with grains interior (GI). In contrast, it is observed that the GBs of widest band gap CIGSe films (x > 0.8) are Cu-enriched compared with GI. For intermediate gallium contents (0.4 < x < 0.8), both types of GBs are detected. This threshold value of 0.4 surprisingly coincides with solar cells output voltage deviation from theoretical expectations, which suggests modifications of GBs properties could participate in the loss of photovoltaic performance.
[en] In the presently envisaged Accelerator Driven System (ADS), the spallation source window material (containing the molten Pb-Bi target) will be a steel containing 9 wt.% chromium (EM10 and T91). The material will be subjected to both high damage rates (some 100 dpa per year at full power) and to the production of significant concentrations (typically 1000 appm) of a large spectrum of impurity elements produced by spallation of the window constituents. We have experimentally simulated the effects due to two of the more deleterious spallation products - Ca and S - by ion implantation at (or near) the expected ADS window operating temperature (500 deg C). Transmission Electron Microscopy (TEM) observations were carried out either after the high temperature implantations (ex-situ) or during the latter (in situ) using the CSNSM) on-line TEM coupled to an ion implanter. Two compromises were made (besides the limitation in the number of implanted species) which differ from the true ADS operating conditions: (i) for practical reasons the implantation beam flux was typically a factor 102 higher than the expected spallation product generation rate; (ii) in order to maximize the possibility of observing precipitation of these presumably insoluble elements, Ca and S implant concentrations reached 1-2 at.%. The displacement levels corresponding to such implantations were 20-25 dpa. As expected, TEM revealed that the defect structure evolved from dislocation loops at low fluence to the formation of a dislocation network as the fluence was increased; this did not depend on the implanted element. No other significant changes occurred in the steel (including in the size and composition of its inclusions): no new phases were detected in the TEM experiments. On the other hand, small (<2 nm) Ca precipitates were indeed detected in 3D atom probe measurements. Their small size accounts for our TEM results. (authors)
[en] Nanometer scale layered systems are well suited to investigate atomic transport processes induced by high-energy electronic excitations in materials, through the characterization of the interface transformation. In this study, we used the atom probe technique to determine the distribution of the different elements in a (amorphous-Fe2Tb5nm/hcp-Co3nm)20 multilayer before and after irradiation with Pb ions in the electronic stopping power regime. Atom probe tomography is based on reconstruction of a small volume of a sharp tip evaporated by field effect. It has unique capabilities to characterize internal interfaces and layer chemistry with sub-nanometer scale resolution in three dimensions. Depth composition profiles and 3D element mapping have been determined, evidencing for asymmetric interfaces in the as-deposited sample, and very efficient Fe-Co intermixing after irradiation at the fluence 7x1012 ion cm-2. Estimation of effective atomic diffusion coefficients after irradiation suggests that mixing results from interdiffusion in a molten track across the interface in agreement with the thermal spike model.
[en] Significant efforts have been made into understanding the dissolution of silicate glasses and minerals, but there is still debate about the formation processes and the properties of surface layers. Here, we investigate glass coupons of ISG glass - a 6 oxide borosilicate glass of nuclear interest - altered at 90 C in conditions close to saturation and for durations ranging from 1 to 875 days. Altered glass coupons were characterized from atomic to macroscopic levels to better understand how surface layers become protective. With this approach, it was shown that a rough interface, whose physical characteristics have been modeled, formed in a few days and then propagated into the pristine material at a rate controlled by the reactive transport of water within the growing alteration layer. Several observations such as stiff interfacial B, Na, and Ca profiles and damped profiles within the rest of the alteration layer are not consistent with the classical inter-diffusion model, or with the interfacial dissolution-precipitation model. A new paradigm is proposed to explain these features. Inter-diffusion, a process based on water ingress into the glass and ion-exchange, may only explain the formation of the rough interface in the early stage of glass corrosion. A thin layer of altered glass is formed by this process, and as the layer grows, the accessibility of water to the reactive interface becomes rate-limiting. As a consequence, only the most easily accessible species are dissolved. The others remain undissolved in the alteration layer, probably fixed in highly hydrolysis resistant clusters. A new estimation of water diffusivity in the glass when covered by the passivating layer was determined from the shift between B and H profiles, and was 10-23 m2.s-1, i.e. approximately 3 orders of magnitude lower than water diffusivity in the pristine material. Overall, in the absence of secondary crystalline phases that could consume the major components of the alteration layer (Si, Al), it is assumed that the glass dissolution rate continuously decreases due to the growth of the transport limiting alteration layer, in good agreement with residual rates reported in the literature for this glass. According to our results it can be expected that new kinetic models should emerge from an accurate time dependent budget of water within the nano-porous alteration layer. (authors)