L. Vincent

Multistep damage evolution process in cubic zirconia irradiated with MeV ions

This work reports the study, via the combination of Rutherford backscattering spectrometry and channeling, x-ray diffraction, and transmission electron microscopy experiments, of the damage formation in cubic yttria-stabilized zirconia single crystals irradiated with medium-energy (4 MeV) heavy (Au) ions. The damage buildup, which is accounted for in the framework of the multistep damage accumulation model, occurs in three steps. The first step at low fluences (up to 1015u2002cm−2), characterized by a regular increase in both the damage yield and the elastic strain, is related to the formation of small defect clusters. The second step in the intermediate fluence range (from 1015 to 5×1015u2002cm−2) leads to a sharp increase in the damage yield and to a large drop of the strain due to the formation of dislocation loops which collapse into a network of tangled dislocations. The third step at high fluences (above 5×1015u2002cm−2) exhibits a surprising decrease in the damage yield, which may be attributed to the reorgani...

Damage induced by electronic excitation in ion-irradiated yttria-stabilized zirconia

This article presents a study of the damage production in yttria-stabilized cubic zirconia single crystals irradiated with swift heavy ions. The combination of techniques which probe the material at different spatial scales (Rutherford backscattering spectrometry in channeling geometry, x-ray diffraction, transmission electron microscopy, and atomic force microscopy) was used in order to gain information about the damage depth distribution, the disordering buildup, the nature of radiation defects, and the occurrence of microstructural modifications. The damage results from the formation of tracks, due to the huge electronic excitations induced in the wake of incident ions. The melting of the material in the core of tracks, via a thermal spike mechanism, leads to the creation of large hillocks at the surface of the crystals. The overlapping of ion tracks at high fluence (above similar to 10(12) cm(-2)) induces a severe transformation of the microstructure of the material. Nanodomains slightly disoriented from the main crystallographic direction are formed, with a size decreasing with increasing irradiation fluence. These results may be used to predict the damage evolution in other nonamorphizable ceramics irradiated with swift heavy ions.

Novel heterostructured Ge nanowires based on polytype transformation.

We report on a strain-induced phase transformation in Ge nanowires under external shear stresses. The resulted polytype heterostructure may have great potential for photonics and thermoelectric applications. ⟨111⟩-oriented Ge nanowires with standard diamond structure (3C) undergo a phase transformation toward the hexagonal diamond phase referred as the 2H-allotrope. The phase transformation occurs heterogeneously on shear bands along the length of the nanowire. The structure meets the common phenomenological criteria of a martensitic phase transformation. This point is discussed to initiate an on going debate on the transformation mechanisms. The process results in unprecedented quasiperiodic heterostructures 3C/2H along the Ge nanowire. The thermal stability of those 2H domains is also studied under annealing up to 650 °C by in situ TEM.

7Be Recoil Implantation for Ultra-Thin-Layer Activation of Medical Grade Polyethylene. Effect on Wear Resistance.

Abstract Wear of ultra-high-molecular-weight-polyethylene (UHMWPE) is usually measured by gravimetric methods making laboratory wear tests a time consuming exercise. Methods for the determination of polyethylene wear with a higher sensitivity would reduce test times and costs. One of these alternative methods is ultra-thin-layer-activation (UTLA), which relies on recoil implantation of heavy radioactive nuclei, such as 7 Be, by using light mass particle beams. However, the possibility of damages within the polyethylene surface, which would have consequences on its wear behavior, cannot be excluded. In this work the effect of an implantation of 7 Be on wear of a medical grade UHMWPE was studied using a block-on-cylinder screening wear tester. The results show that the implantation of UHMWPE with 7 Be recoils under the implantation conditions chosen does not alter the tribological behavior of medical grade UHMWPE.

High current density GaAs/Si rectifying heterojunction by defect free Epitaxial Lateral overgrowth on Tunnel Oxide from nano-seed

Interest in the heteroepitaxy of GaAs on Si has never failed in the last years due to the potential for monolithic integration of GaAs-based devices with Si integrated circuits. But in spite of this effort, devices fabricated from them still use homo-epitaxy only. Here we present an epitaxial technique based on the epitaxial lateral overgrowth of micrometer scale GaAs crystals on a thin SiO2 layer from nanoscale Si seeds. This method permits the integration of high quality and defect-free crystalline GaAs on Si substrate and provides active GaAs/Si heterojunctions with efficient carrier transport through the thin SiO2 layer. The nucleation from small width openings avoids the emission of misfit dislocations and the formation of antiphase domains. With this method, we have experimentally demonstrated for the first time a monolithically integrated GaAs/Si diode with high current densities of 10u2009kA.cm−2 for a forward bias of 3.7u2009V. This epitaxial technique paves the way to hybrid III–V/Si devices that are free from lattice-matching restrictions, and where silicon not only behaves as a substrate but also as an active medium.

Faceting mechanisms of Si nanowires and gold spreading

We report detailed structural analysis ofxa0〈111〉xa0oriented silicon nanowires (NWs) grown by UHV–CVD using the VLS process with a gold catalyst. STEM-HAADF observations have revealed an unexpected inhomogeneous distribution of gold nanoclusters on the NW surface. Gold is mainly distributed on three sides among the six {112}-sidewalls and is anchored on upward {111} facets. This original observation brought us a new comprehension of the faceting mechanisms. The stability of thexa0〈111〉xa0growth direction needs the formation of facets on {112}-sidewalls with energetically favorable planes. We demonstrate that the initial formation of covered facets with a three-fold symmetry is driven by the formation of {111} Au/Si interfaces between the nucleated Si NW and the Au droplet.

Gold anchoring on Si sawtooth faceted nanowires

This paper reports on the sawtooth faceting and the related gold coverage of silicon nanowires (NWs). 111-oriented Si NWs were grown on Si(111) substrates by ultra high vacuum chemical vapor deposition using the vapor-liquid-solid mechanism with a gold catalyst. We observed that gold nanoclusters are unequally distributed on the NW surface. They are mainly distributed on only three non-consecutive sidewalls corresponding to the (), () and () planes among the six available crystallographic {112} surfaces. In addition they are anchored on upward {111} facets. This original observation brings enhanced knowledge on the faceting mechanisms. The threefold symmetry of the facet formation and gold anchoring is supported by criteria of minimal Au/Si interfacial energy. Moreover results evidence that the selective presence of gold on the NW sidewalls affects the overall morphology due to an increased radial growth on the covered sidewalls.

Simplified methodology of the ultra-thin layer activation technique

Abstract Recoil implantation of 56 Co radioactive nuclei is used to measure the nanometric corrosion rate of steam generator tubes of nuclear power plants. The results show a constant corrosion rate of nickel alloy pipe (≈4 nm/month), after a short running-in period. To simplify the use of this technique and to extend its application field, a Monte Carlo simulation program (Implantation of Recoil Ions Simulation) has been developed to calculate the depth profile of the implanted nuclei. It is associated with SRIM software for the transport of particles in matter. The validity and the interest of the IRIS computer program are demonstrated throughout the mentioned application.

Ultra thin layer activation by implantation of recoil radioactive nuclei: Experiments and simulations

Abstract The ultra thin layer activation (UTLA) by recoil implantation of heavy radioactive nuclei has a great potential in wear or corrosion rate measurements, because of its nanometric sensibility. In regard of the complexity of its calibration, a Monte Carlo simulation program (Implantation of Recoil Ions Simulation) was developed to calculate the depth profile of implanted nuclei. It is associated to SRIM software for the transport of particles in matter. In this paper, the different simulation parameters and the potentiality of IRIS program are described. Implantation experiments with the 56Fe(p,n)56Co nuclear reaction have been performed at 22.8 MeV protons to measure the angular and depth distributions of 56Co nuclei emerging from the iron target. In comparison with experiments, the simulated angular distribution has a good magnitude of implanted activity, but presents an excess of nuclei in high angles of implantation. This excess comes from too great emission probability around the critical angle. For depth profile, the simulated spectrum of transmitted particles is too energetic.

Composition and local strain mapping in Au-catalyzed axial Si/Ge nanowires

For most applications, heterostructures in nanowires (NWs) with lattice mismatched materials are required and promise certain advantages thanks to lateral strain relaxation. The formation of Si/Ge axial heterojunctions is a challenging task to obtain straight, defect free and extended NWs. And the control of the interface will determine the future device properties. This paper reports the growth and analysis of NWs consisting of an axial Si/Ge heterostructure grown by a vapor-liquid-solid process. The composition gradient and the strain distribution at the heterointerface were measured by advanced quantitative electron microscopy methods with a resolution at the nanometer scale. The transition from pure Ge to pure Si shows an exponential slope with a transition width of 21 nm for a NW diameter of 31 nm. Although diffuse, the heterointerface makes possible strain engineering along the axis of the NW. The interface is dislocation-free and a tensile out-of-plane strain is noticeable in the Ge section of the NW, indicating a lattice accommodation. Experimental results were compared to finite element calculations.