D. Eyidi

Influence of stoichiometry and structure on the optical properties of AlNxOy films

The AlNxOy system offers the possibility to obtain a wide range of responses, by tailoring the properties between Al, AlN and Al2O3, opening a significant number of possible applications. The aim of this work is to correlate the optical properties of AlNxOy thin films with their composition and structural features, taking as reference the binary systems AlNx and AlOy. In the AlNx system, the increase in the nitrogen content induced a wide variation in the optical properties, ranging from the typical profile of a polycrystalline Al-type film towards nearly constant reflectance values as low as 5%, as well as a smooth increase in samples transparency as the ratio N/Al approached unit. In the case of the AlOy system, the reflectance also decreased as the oxygen content increased; however, the transition to transparent films (Al2O3-like) was more abrupt. The ternary system AlNxOy, revealed optical responses that ranged from a typical profile of a polycrystalline Al-type film towards low and constant reflectance values in a wide range of x and y coefficients, ending up as semi-transparent when Al2O3-like films were formed. The unusual low optical reflectance of some films reveals some potential applications in solar power systems and sensors.

Copper coverage effect on tungsten crystallites texture development in W/Cu nanocomposite thin films

Morphological and crystallographic structures of multilayered W/Cu nanocomposite thin films elaborated by physical vapor deposition were studied by varying copper and tungsten thicknesses. Sample examinations were performed by x-ray diffraction (XRD), grazing incidence small-angle x-ray scattering and transmission electron microscopy (TEM). Samples were found to be composed of copper nanoparticles, homogeneously dispersed in planes parallel to the film-substrate interface and periodically separated by tungsten layers along the growth direction. Our observations revealed an original texture development of the tungsten matrix from a mixture of unexpected α-W⟨111⟩ and α-W⟨110⟩ components to unique α-W⟨110⟩ component as the copper coverage passes a thickness threshold of 0.6 nm. Local TEM texture stereology investigations revealed simultaneous columnar growth of both preferential orientations posterior to polycrystalline development while XRD reveals strong compressive residual stresses in both texture compon...

Controlled nanostructuration of polycrystalline tungsten thin films

Nanostructured tungsten thin films have been obtained by ion beam sputtering technique stopping periodically the growing. The total thickness was maintained constant while nanostructure control was obtained using different stopping periods in order to induce film stratification. The effect of tungsten sublayers thicknesses on film composition, residual stresses, and crystalline texture evolution has been established. Our study reveals that tungsten crystallizes in both stable α- and metastable β-phases and that volume proportions evolve with deposited sublayers thicknesses. α-W phase shows original fiber texture development with two major preferential crystallographic orientations, namely, α-W⟨110⟩ and unexpectedly α-W⟨111⟩ texture components. The partial pressure of oxygen and presence of carbon have been identified as critical parameters for the growth of metastable β-W phase. Moreover, the texture development of α-W phase with two texture components is shown to be the result of a competition between ...

Solid-phase epitaxial regrowth of amorphous silicon containing helium bubbles

Transmission electron microscopy has been used to study processes occurring when a layer of amorphous silicon (a-Si) containing helium-filled cavities buried in crystalline silicon (c-Si) recrystallizes by solid phase epitaxial growth (SPEG). The buried layer was formed in (100) silicon by means of bombardment with 150 keV Li ions with the bubbles resulting from subsequent implantation of 80 keV He ions; the energies being chosen to ensure that the resulting bubble distribution was entirely contained within the amorphous layer. The presence of bubbles in a-Si undergoing SPEG at a (100) interface with c-Si has previously been observed to give rise to the formation of microtwin lamellas, assumed to nucleate at the bubble surfaces; however, the present work indicates clearly that, in fact during SPEG, many microtwins nucleate remote from the bubbles. There is also an apparent interaction between the amorphous-crystalline (a-c) interface and the bubbles, in which the bubbles seem to be swept by the moving interface. The paper will discuss possible mechanisms for both phenomena, in terms of interstitial defects for the nucleation of microtwins and in terms of enhanced bubble mobility in a-Si for the apparent interaction between the a-c interface and the bubbles.

Kinetics of the 3C-6H polytypic transition in 3C-SiC single crystals: A diffuse X-ray scattering study

In this work, the kinetics of the 3C-6H polytypic transition in 3C-SiC single crystals are studied in details by means of diffuse x-ray scattering (DXS) coupled with numerical simulations and transmission electron microscopy and optical birefringence microscopy. Upon high-temperature annealing, spatially correlated stacking faults (SFs), lying in the {111} planes, are generated within the crystal and tend to form bands of partially transformed SiC. It is shown that the numerical simulation of the DXS curves allows to unambiguously deduce the transformation level within these bands, as well as the volume fraction corresponding to these bands. Increasing annealing time results (1) in the growth of the partially transformed regions by the glide of the partial dislocations bounding the SFs and (2) in the generation of new SFs within the crystal by means of a double-cross slip motion. The kinetics of each of these mechanisms are presented and discussed with respect to the annealing temperature, the initial SF density and crystalline quality.

Size effects on the Mechanical Behavior of Nanometric W/Cu Multilayers

The mechanical behavior of nanostructured stratified W/Cu composites prepared by ion beam sputtering has been investigated using a method combining X-ray diffraction and tensile testing. Tests were performed on a synchrotron light source to analyze the elastic response of the tungsten phase. Three different microstructures have been analyzed: the specimen composed of the thinner tungsten layers reveals an elastic behavior different from the one expected assuming bulk elastic constants. However, Transmission Electron Microscopy (TEM) and Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) measurements reveal discontinuities in the copper layers. As the strain in the related copper clusters as well grains boundary contributions are not experimentally accessible, atomistic calculation are of utmost importance. Polycrystalline materials have already been constructed through the Voronoi method and thanks to TEM observations. Atomistic simulation and calculation are underway to validation.

Fe-implanted 6H-SiC: Direct evidence of Fe3Si nanoparticles observed by atom probe tomography and 57Fe Mössbauer spectroscopy

In order to understand ferromagnetic ordering in SiC-based diluted magnetic semiconductors, Fe-implanted 6H-SiC subsequently annealed was studied by Atom Probe Tomography, 57Fe Mossbauer spectroscopy and SQUID magnetometry. Thanks to its 3D imaging capabilities at the atomic scale, Atom Probe Tomography appears as the most suitable technique to investigate the Fe distribution in the 6H-SiC host semiconductor and to evidence secondary phases. This study definitely evidences the formation of Fe3Si nano-sized clusters after annealing. These clusters are unambiguously responsible for the main part of the magnetic properties observed in the annealed samples.

A Revisitation of Some Problems of Dislocations in Silicon

Dislocation microstructures obtained following plastic deformation close to the Brittle to Ductile Transition temperature have been investigated using multiscale imaging techniques. TEM investigations show a multiplicity in the dislocation core configurations which can appear as dissociated of as perfect segments. The evidence, after etching, of trails of point defect behind dislocations is also characteristic of these deformation conditions. These observations are discussed in the light of the possible core structures of dislocation proposed in silicon.

Fe implantation effect in the 6H-SiC semiconductor investigated by Mössbauer spectrometry

P-doped 6H-SiC substrates were implanted with 57Fe ions at 380u2009°C or 550u2009°C to produce a diluted magnetic semiconductor with an Fe homogeneous concentration of about 100u2009nm thickness. The magnetic properties were studied with 57Fe Conversion Electron Mossbauer Spectrometry at room temperature (RT). Results obtained by this technique on annealed samples prove that ferromagnetism in 57Fe-implanted SiC for Fe concentrations close to 2% and 4% is mostly due to Fe atoms diluted in the matrix. In contrast, for Fe concentrations close to 6%, it also comes from Fe in magnetic phase nano-clusters. This study allows quantifying the Fe amount in the interstitial and substitutional sites and the nanoparticles and shows that the majority of the diluted Fe atoms are substituted on Si sites inducing ferromagnetism up to RT.P-doped 6H-SiC substrates were implanted with 57Fe ions at 380u2009°C or 550u2009°C to produce a diluted magnetic semiconductor with an Fe homogeneous concentration of about 100u2009nm thickness. The magnetic properties were studied with 57Fe Conversion Electron Mossbauer Spectrometry at room temperature (RT). Results obtained by this technique on annealed samples prove that ferromagnetism in 57Fe-implanted SiC for Fe concentrations close to 2% and 4% is mostly due to Fe atoms diluted in the matrix. In contrast, for Fe concentrations close to 6%, it also comes from Fe in magnetic phase nano-clusters. This study allows quantifying the Fe amount in the interstitial and substitutional sites and the nanoparticles and shows that the majority of the diluted Fe atoms are substituted on Si sites inducing ferromagnetism up to RT.

Dislocations in (Hg, Re)Ba2Ca2Cu3O8+δ high-temperature superconducting ceramics plastically deformed at room temperature

Dislocations in (Hg,u2009Re)Ba2Ca2Cu3O8+ δ superconducting ceramics plastically deformed at room temperature have been characterized by transmission electron microscopy (TEM). Samples were submitted to strain levels up to 8, 18 and 63% by uniaxial compression under a confining pressure. Dislocation densities within grains were found to be in the range 109–1012u2009cm−2. The average grain size decreases from about 20u2009μm in the as-grown material to about 0.5u2009μm for the sample deformed up to 63%. Evidence for (001)⟨ 100⟩, (001)⟨ 110⟩ and glide systems was obtained by TEM. Dislocations are characterized by long straight screw segments and shorter edge parts. These features are interpreted as resulting from high Peierls valleys along the ⟨ 100⟩ and ⟨ 110⟩ screw directions. Cross-slip as well as climb dissociation of ⟨ 110⟩ dislocations occur. The shape of deformation-induced dislocations, their evolution during deformation and the interaction between dislocations in the (001) ⟨ 110⟩ and { }⟨ 110⟩ glide systems are discussed.