C. Jaouen

High electronic excitations and ion beam mixing effects in high energy ion irradiated Fe/Si multilayers

Mossbauer spectroscopy (57Fe) shows evidence for mixing effects induced by electronic energy deposition in nanoscale Fe/Si multilayers irradiated with swift heavy ions. A decrease in the mixing efficiency with electronic stopping power is reported; a threshold is found, under which iron environment modifications no longer occur. The kinetics of Fe–Si phase formation after irradiation suggests the existence of three regimes: (i) for high excitation levels, a magnetic amorphous phase is formed directly in the wake of the incoming ion and an almost complete mixing is reached at low fluence (1013 U/cm2); (ii) for low excitation levels, a paramagnetic Si-rich amorphous phase is favored at the interface while crystalline iron subsists at high fluences; (iii) for intermediate excitation levels, saturation effects are observed and the formation rate of both magnetic and paramagnetic phases points to direct mixing in the ion wake but with a reduced track length in comparison to U irradiation. The measured interfac...

Nanoscaled composite TiN/Cu multilayer thin films deposited by dual ion beam sputtering: growth and structural characterisation

Abstract We present a detailed structural characterisation of TiN/Cu multilayers, with bilayer period ranging from 5 to 20 nm, deposited by dual ion beam sputtering on Si(001) substrates. Low-angle X-ray diffraction scans exhibit Bragg reflections up to the 15th order, indicative of a well-defined periodicity and non-cumulative roughness. High-angle X-ray diffraction shows that the multilayers adopt a (001) TiN/(001) Cu texture and are polycrystalline in the plane, the mosaic spread for the TiN and Cu grains being of ∼6° and more than 9°, respectively. High Resolution Transmission Electron Microscopy (HRTEM) observations confirm the cube on cube epitaxial growth, but show the presence of several interfacial and growth defects, introduced to relieve the huge misfit (15.9%) between the two lattices. The HRTEM images also reveal facetted islands growth morphology of Cu, leading to lateral roughness at the TiN/Cu interface as well as fluctuations in the interplanar spacings, which could explain the lack of superlattice reflections in the high-angle X-ray diffraction.

Ion-induced phase formation in Ni-Al and Fe-Al thin films: Role of chemical disordering energy on amorphization

Phase formation in the metallic systems Ni/Al and Fe/Al by ion beam mixing is studied at low temperature when chemical diffusion is prevented. Chemical disorder or/and topological disorder are therefore observed. Thermodynamic analysis based on free energy diagrams shows that stored energy in antisite defects in intermetallic compounds plays a determining role. Chemical disorder, by raising the energy of the crystal lattice, induces the transformation towards the amorphous state. The dissimilar results obtained for equiatomic ordered compounds FeAl and NiAl can be easily understood in this regard. Moreover a high mixing energy in the twc phases regions can explain the preferential stability of the topological disordered state.

Stress field in sputtered thin films: Ion irradiation as a tool to induce relaxation and investigate the origin of growth stress

The stress state of sputtered Mo thin films was studied, and a detailed analysis of elastic strains, using x-ray diffraction and the “sin2 Ψ method,” was performed. The evolution of the lattice parameter under ion irradiation showed that the usual assumption of a biaxial stress state is not adequate to determine the true stress-free lattice parameter a0 of the film. An original stress model, including a hydrostatic component linked to volume distortions induced by point defects, is required. This model, which describes a triaxial stress field, allows a reliable determination of a0. Furthermore, ion irradiation was shown to be a powerful method for stress relaxation, providing a stress-free lattice parameter solely linked to chemical effects.

Low-temperature mixing in Cu/W superlattices irradiated with light and heavy ions

Ion irradiation effects, induced at low temperature in Cu/W superlattices prepared by ion sputtering, were studied and compared using in situ resistivity measurements and x‐ray diffraction. The influence of the particle mass was particularly investigated in order to measure the role of spike effects in this immiscible system. It is shown that a more important mixing can be obtained with light‐ion irradiation. Nevertheless, the contribution of ‘‘thermally’’ activated jumps opposing ballistic effects is significant even when small and weakly energetic cascades are formed. A dependence of the microstructural state and of the compositional substructure on the irradiating particle is also evidenced. Light‐ion irradiation favors grain growth, restores grain texture, and preserves the composition modulation.

Irradiation effects in Cu/W multilayers: Ion beam mixing and structural evolution

Irradiation effects in the immiscible Cu/W system are investigated for helium 70 keV and krypton 340 keV irradiations at liquid-nitrogen temperature. Evolution of the [(3.1 nm Cu/4.2 nm W)×12] multilayer was characterized by x-ray diffraction measurements. Limits of solubility were observed in both matrixes for both irradiations. As expected, a strong thermal spike effect is evidenced in copper layers under krypton irradiation where almost no solubility is observed. For helium irradiations very similar limits of solubility are observed in both matrixes. These latter results can be analyzed by a model with two atomic fluxes: one due to ballistic displacement and the other due to chemically driven thermally activated transport. A structural study shows a transition to an amorphous state for heavy ion irradiation in the large cascades above a critical solubility threshold.

X-ray diffraction study of residual stress modification in Cu/W superlattices irradiated by light and heavy ions

Abstract The effect of low temperature ion irradiation on the residual stress state was studied as a function of the ion fluence in Cu/W superlatices prepared by ion beam sputtering. The residual stress tensor in tungsten layers is completely determined from X-ray diffraction data using the “sin2ψ method”. In the as-prepared state, the Cu/W superlattices are strongly strained, and we find in-plane compressive stresses as high as 6.4 GPa in tungsten layers. Relaxation of the stress state is observed after low temperature ion irradiation with increasing dose. This phenomenon is almost complete for doses as low as 0.1–0.2 dpa, and appears related to atomic rearrangement in the elemental layers rather than interfacial mixing. The role of the incident particle mass is also evidenced. Per dpa, heavy ion irradiation (Kr) induces the strain relaxation more quickly than light ions (He).

Phase transformation in ion irradiated NiAl and FeAl

Single crystals of NiAl and FeAl B2 type ordered intermetalhc compounds were irradiated at 90 K with xenon ions and the radiation damage studied by in situ Rutherford backscattering and channeling experiments. After a first stage (up to 5 x 1013 Xe cm-−2) where a strong increase in the dechanneling rate of the analyzing ions is obtained, an unexpected reordering effect (i.e. a decrease in the channeling rate) is observed in both alloys. This process is discussed in relation with defect rearrangements or induced crystalline structural phase transformation. Finally for irradiation fluences higher than 2.5 x 1015 Xe CM−2 complete dechanneling occurs, confirming that total amorphization takes place in NiAl, while the disorder level remains at a very low and constant value in FeAl.

Strains and stresses in an epitaxial Ni(111)/Mo(110) multilayer grown by direct ion beam sputtering

Abstract An epitaxial Ni(111)/Mo(110) multilayer was grown on a (1 1 2 0) oriented monocrystalline sapphire substrate in a high vacuum sputtering deposition system. The strain and stress state of the layers has been measured with X-ray diffraction in symmetric and asymmetric geometries. Non-equal biaxial coherency stresses due to the Nishiyama–Wassermann epitaxial relation between both lattices are clearly evidenced. The values of the stress-free lattice parameters of molybdenum and nickel sublayers, deduced from the global stress analysis, supports the hypothesis of an interfacial alloying effect between layers with diffusion of nickel in the Mo layers and, to a lesser degree, of molybdenum in Ni layers. On the other hand, the Ni layers appear nearly fully relaxed. A detailed analysis of the stresses in the film shows a strong contribution of a compressive stress field due to possible post-growth atomic rearrangements occurring inside the multilayered film.

Track formation in amorphous Fe0.55Zr0.45 alloys irradiated by MeV C60 ions: Influence of intrinsic stress on induced surface deformations

Abstract Amorphous Fe0.55Zr0.45 films, having thickness of 400 nm, were grown on silicon substrates by co-deposition using ion beam sputtering. Limited surface roughness makes this system particularly suitable for fine-scale scanning force microscopy analysis and nano-indentation. The samples were irradiated with MeV C60 clusters, and the surface morphology of single impacts was found to have a “doughnut” shape, i.e. hillocks having a central crater. Quantitative evaluation of the deformation was achieved by measuring their height and diameter. When C60 projectiles deviate from normal incidence, a tail emerges along the direction of the incident beam. The height of the hillock and length of the tail are increasing with the incidence angle, and the magnitude of the deformation indicates that the damage mainly occurs due to a radial coherent mass transport outwards from the track core by a compression shockwave-like mechanism. The residual compressive in-plane stress, ∼−0.4 GPa for the as-deposited films, was found to notably influence the C60 induced plastic deformations. Indeed, stress relaxation results in a marked decrease in height combined with a significant widening of the surface features. This “flat” surface morphology is attributed to an enhanced radial efficiency of the pressure pulse, owing to a significant reduction of the hardness of the amorphous film after stress relaxation. The overall picture outlined from our observations suggests that the surface damage induced by single MeV C60 ions possibly is the signature of plastic deformation induced at large distances by an energetic radial pressure pulse. This unsteady shockwave allows the energy transfer outwards from the localised region along the ion path that experiences a sudden transient heating.