Thomas Bierschenk

SAXS investigations of the morphology of swift heavy ion tracks in α-quartz

The morphology of swift heavy ion tracks in crystalline α-quartz was investigated using small angle x-ray scattering (SAXS), molecular dynamics (MD) simulations and transmission electron microscopy. Tracks were generated by irradiation with heavy ions with energies between 27 MeV and 2.2 GeV. The analysis of the SAXS data indicates a density change of the tracks of ~2 ± 1% compared to the surrounding quartz matrix for all irradiation conditions. The track radii only show a weak dependence on the electronic energy loss at values above 17 keV nm(-1), in contrast to values previously reported from Rutherford backscattering spectrometry measurements and expectations from the inelastic thermal spike model. The MD simulations are in good agreement at low energy losses, yet predict larger radii than SAXS at high ion energies. The observed discrepancies are discussed with respect to the formation of a defective halo around an amorphous track core, the existence of high stresses and/or the possible presence of a boiling phase in quartz predicted by the inelastic thermal spike model.

Nano-porosity in GaSb induced by swift heavy ion irradiation

Nano-porous structures form in GaSb after ion irradiation with 185 MeV Au ions. The porous layer formation is governed by the dominant electronic energy loss at this energy regime. The porous layer morphology differs significantly from that previously reported for low-energy, ion-irradiated GaSb. Prior to the onset of porosity, positron annihilation lifetime spectroscopy indicates the formation of small vacancy clusters in single ion impacts, while transmission electron microscopy reveals fragmentation of the GaSb into nanocrystallites embedded in an amorphous matrix. Following this fragmentation process, macroscopic porosity forms, presumably within the amorphous phase.

Porosity as a function of stoichiometry and implantation temperature in Ge/Si1−xGex alloys

The development of porosity in single-crystal germanium and silicon-germanium alloys (c-Si1−x Gex) of (100) orientation was studied under bombardment with 140 keV Ge− ions over a wide range of temperatures (−180 to 400 °C) and ion fluences up to 1 × 1018 ions/cm2. The surface swelling and morphology were investigated using multi-characterization techniques including optical profilometry, transmission electron microscopy, and scanning electron microscopy. The initiation of porosity and the evolution of the near-surface microstructure strongly depend on the ion fluence, the irradiation temperature, and the stoichiometry of the substrate. Significant results and new findings include: (i) the fact that, over the entire temperature and stoichiometry range, porosity is only developed once the substrate is rendered amorphous; (ii) with increasing Si content in the alloy, the onset of porosity is pushed to higher fluences; (iii) porosity is observed for Si contents in the alloy up to 23% but not higher under the ...

Shape manipulation of ion irradiated Ag nanoparticles embedded in lithium niobate

Spherical silver nanoparticles were prepared by means of ion beam synthesis in lithium niobate. The embedded nanoparticles were then irradiated with energetic (84)Kr and (197)Au ions, resulting in different electronic energy losses between 8.1 and 27.5 keV nm(-1) in the top layer of the samples. Due to the high electronic energy losses of the irradiating ions, molten ion tracks are formed inside the lithium niobate in which the elongated Ag nanoparticles are formed. This process is strongly dependent on the initial particle size and leads to a broad aspect ratio distribution. Extinction spectra of the samples feature the extinction maximum with shoulders on either side. While the maximum is caused by numerous remaining spherical nanoparticles, the shoulders can be attributed to elongated particles. The latter could be verified by COMSOL simulations. The extinction spectra are thus a superposition of the spectra of all individual particles.

Direct observation of substitutional Ga after ion implantation in Ge by means of extended x-ray absorption fine structure

We acknowledge the support from the Research Foundation Flanders, the epi-team from imec, the KU Leuven GOA 09/06 project, the IUAP program P6/42 and the Australian Research Council. S.C. acknowledges support from OCAS NV by an OCAS-endowed chair at Ghent University.

Nanoscale density variations induced by high energy heavy ions in amorphous silicon nitride and silicon dioxide

The cylindrical nanoscale density variations resulting from the interaction of 185 MeV and 2.2 GeV Au ions with 1.0 μm thick amorphous SiN x :H and SiO x :H layers are determined using small angle x-ray scattering measurements. The resulting density profiles resembles an under-dense core surrounded by an over-dense shell with a smooth transition between the two regions, consistent with molecular-dynamics simulations. For amorphous SiN x :H, the density variations show a radius of 4.2 nm with a relative density change three times larger than the value determined for amorphous SiO x :H, with a radius of 5.5 nm. Complementary infrared spectroscopy measurements exhibit a damage cross-section comparable to the core dimensions. The morphology of the density variations results from freezing in the local viscous flow arising from the non-uniform temperature profile in the radial direction of the ion path. The concomitant drop in viscosity mediated by the thermal conductivity appears to be the main driving force rather than the presence of a density anomaly.