Guy Terwagne

Stoichiometric characterization of YBaCuO superconductors with nuclear probes

Abstract Nuclear probes using energetic (MeV range) charged particle irradiation and prompt detection of atomic and nuclear signals are powerful tools to determine quickly and accurately the chemical constituants of YBaCuO superconductors. PIXE in milliprobe geometry is used to determine the actual concentration ratios of Y/Ba and Cu/Ba, with an accuracy of 3% or better. Calculated matrix effects in the detection of characteristic K X-rays allow accurate determinations even when the composition of samples are very different from reference materials. RBS of 2 MeV α-particles is used to certify the homogeneity of the Y-Ba-Cu composition without reference material to a thickness of 250 μg/cm2 under the surface. Deuteron microbeam irradiation leads to simultaneous composition maps of all the elements (YBaCuO) in volume of 5 × 5 × 5 μm using X-rays and protons as identification signals.

Precise measurement of the differential cross section from the O16(α,α)O16 elastic reaction at 165° and 170° between 2.4 and 6.0MeV

Non-Rutherford cross section for elastic scattering of α particles from oxygen has been measured for energies between 2.4 and 6.0MeV at commonly used backscattering angles of 165° and 170°. High precision in energy has been obtained with a 2MV Tandetron accelerator, with precise and stable energy calibration. The choice of a thin Ta2O5∕Ta∕C standard increased the sensitivity and reduced systematic errors due to the geometry. Precise data for energy, width, and ratio to Rutherford of the resonances have been extracted from nuclear shell models, and excitation levels of Ne20 have been deduced. The well-known resonance ER=3031.7±0.5keV and the narrow and intense resonance ER=5375.5±0.5keV, not included in cross-section libraries, have been applied for surface oxygen analysis in thin WCN∕SiO2∕Si multilayer samples, with improved depth resolution by optimizing the measuring geometry.

Cross-section measurements of the N14(α,p)O17 and N14(α,α)N14 reactions between 3.5 and 6 MeV

The cross-section of the N14(α,p0)O17 reaction at angles of 90°, 135°, and 165° was measured for incident energies between 3.5 and 6.0 MeV simultaneously with the cross-section of the N14(α,α)N14 reaction at 165°. Interference between these two reactions at the angle of 165° and around 3.9 MeV was taken into account. The technique used is very powerful, thanks to the Ta450 nm/C target being implanted with a high dose of nitrogen. The N14(α,p0)O17 reaction exhibits some resonances allowing traces of nitrogen to be quantified. This reaction also offers an alternative to the N14(d,α)C12 and N14(H3e,p)O16 nuclear reactions for profiling nitrogen in the first few microns below the surface. Moreover, by using α-particles, Rutherford backscattering spectroscopy can be performed simultaneously with a good mass resolution to depth profile high Z elements in the sample. The sensitivity of these reactions (0.1%) has been tested by measuring the nitrogen traces in a titanium oxide film deposited on silicon. Depth pro...

Influence of nitrogen on the growth and luminescence of silicon nanocrystals embedded in silica

Silicon nanocrystals (Si-ncs) have been produced by implantation of Si+ in excess into SiO2 followed by both annealing and passivation using argon or nitrogen. Nitrogen increases the photoluminescence (PL) emission and shifts the spectra toward the blue. The measured Si-nc diameter is 4.3 and 3.8 nm after annealing performed under Ar and N2, respectively. A significant quantity of nitrogen atoms has been detected in all samples by resonant nuclear reaction analysis (RNRA). The nitrogen concentration is significantly higher when the annealing and passivation are performed in a nitrogen environment, in agreement with a larger Si–N vibration signal on the Raman spectra. The depth profiles of nitrogen are very similar to those of Si-nc, suggesting that the N2 molecules may diffuse in the SiO2 during the annealing and then are trapped in proximity to the Si-nc. In addition to Si+, the implantation of N2+ to concentrations of 3 and 6 at. % produced a decrease in the PL intensity (accentuated at the higher conce...

Method for fabricating third generation photovoltaic cells based on Si quantum dots using ion implantation into SiO2

In this paper, we report on the synthesis of silicon quantum dots for photovoltaic applications by means of ion implantation followed by annealing. Nucleation was achieved by implanting Si+ ions into SiO2 thin films, previously thermally grown on a Si(100) substrate, and annealing to 1100 °C. Passivation was used for photoluminescence (PL) measurements. The thickness of the oxide layer, the stoichiometry of the implanted layer, and the depth profiles of the implanted ions were determined for all samples by both Rutherford backscattering spectroscopy (RBS) and ellipsometry techniques. Characterization by transmission electron microscopy (TEM) indicates that the diameter of the silicon quantum dots (Si-QDs) varies from 2 to 4 nm, which is less than the Bohr radius of bulk crystalline Si(∼5 nm). Optical and electrical properties have been investigated by PL and I-V measurements. When passivated silicon nanocrystals (Si-nc) embedded into SiO2 are excited using a 450 nm diode laser, they exhibit a strong PL emission in the range of 650-1000 nm. Based on these investigations, p-type Si-QDs/n-type c-Si junctions were fabricated and electrically characterized in the dark as well as under an AM1.5G terrestrial solar spectrum for nonimplanted, as-implanted, and implanted-annealed samples for different implantation fluences. The electrical curves of the structures under illumination demonstrate the photovoltaic behavior of the Si-QDs. Despite the weak light conversion of these devices, these results remain very promising and offer potentially unprecedented, vast improvements to third generation solar cells.

A conversion electron Mössbauer spectroscopy and transmission electron microscopy study of monocrystalline iron implanted with nitrogen

Monocrystalline films of α‐iron were implanted at room temperature with nitrogen to different doses. The transmission electron microscopy and conversion electron Mossbauer spectroscopy measurements have shown clearly that the two techniques are complementary in determining the nitride phases present in the implanted films. At low doses (lower than 5×1016 ions/cm2) the α’‐martensite is formed with the implanted nitrogen occupying octahedral sites. At higher doses (greater than 5×1016 ions/cm2) two phases which are more compact and progressively richer in nitrogen are observed, e‐Fe3N and e‐Fe2+xN.

Nanocavities and germanium nanocrystals produced by Ge ion implantation in fused silica

High-resolution SEM images of germanium nanocrystals (Ge-nc) synthesized by ion implantation in fused silica samples annealed at temperatures below and above the melting point of Ge show a strong size-selective depth-distribution of nanostructures, as evidenced by the correlation between the dimension of the observed objects and the local concentration of implanted Ge measured by Rutherford backscattering spectroscopy (RBS). Whereas the Ge-nc nucleation seems to obey the Ostwald ripening process in samples annealed below 900 °C, Ge desorption effects, non-uniform in depth, in conjunction with the formation of large and spherical nanocavities, become dominant for annealing performed above the solid-liquid phase transition of Ge. Measurements for different annealing times at 1050 °C show two distinct processes in the Ge desorption dynamics: the first is related to direct Ge outgassing effects during the nucleation of Ge-nc, which occurs within the first minutes of the thermal annealing, while the second is due to the release of Ge from Ge-nc, associated with the formation of nanocavities. The formation rate of these nanocavities is more efficient at greater depth than in the vicinity of the sample surface. It appears to be strongly dependent on the local concentration of defects, responsible for the reduction of the Ge diffusion, and to be related to the breaking of Ge-O and Si-Ge bonds at the Ge-nc/SiO(2) interface.

Control of the Ge nanocrystal synthesis by co-implantation of Si+

The synthesis of Ge nanocrystals (Ge-nc) prepared by 74Ge+ implantation into fused silica followed by co-implantation of Si+ has been investigated for annealing temperatures varying between 850 and 1150 °C. By limiting the thermal diffusion of Ge, co-implanting Si reduces the Ge desorption and affects the growth of Ge-nc, through a Ge trapping mechanism involving the formation of Ge-Si chemical bonds. This is supported by Raman analysis, providing information regarding the material composition for a large variety of fabrication parameters, as well as high resolution scanning electron microscopy imaging, indicating that the average dimension of the synthesized Ge-nc decreases for increasing doses of co-implanted Si. From the spectral analysis of Raman measurements, a systematic evolution of the Ge-Ge, Ge-Si, and Si-Si bond concentrations is characterized as a function of the co-implantation fluences. Two different regimes are clearly identified for each annealing temperature. The first is associated with a...

An implanted He-4 target for experiments with radioactive beams

Solid He-4 targets were prepared by implantation of He ions of various energies into thin Al foils. The targets were tested using the proton Rutherford backscattering (RBS) technique. The rests showed that a considerable amount of the implanted He atoms stays in the host foils for a long time after the implantation. A possibility of using the He-implanted targets in experiments with radioactive beams is demonstrated. Further steps to increase He amount in the implanted targets are discussed

Influence of silicon dangling bonds on germanium thermal diffusion within SiO2 glass

We study the influence of silicon dangling bonds on germanium thermal diffusion within silicon oxide and fused silica substrates heated to high temperatures. By using scanning electron microscopy and Rutherford backscattering spectroscopy, we determine that the lower mobility of Ge found within SiO2/Si films can be associated with the presence of unsaturated SiOx chemical bonds. Comparative measurements obtained by x-ray photoelectron spectroscopy show that 10% of silicon dangling bonds can reduce Ge desorption by 80%. Thus, the decrease of the silicon oxidation state yields a greater thermal stability of Ge inside SiO2 glass, which could enable to considerably extend the performance of Ge-based devices above 1300 K.