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.

Ion tracks formed in amorphous Ge by swift heavy-ion irradiation have been identified with experiment and modeling to yield unambiguous evidence of tracks in an amorphous semiconductor. Their underdense core and overdense shell result from quenched-in radially outward material flow. Following a solid-toliquid phase transformation, the volume contraction necessary to accommodate the high-density molten phase produces voids, potentially the precursors to porosity, along the ion direction. Their bow-tie shape, reproduced by simulation, results from radially inward resolidification. DOI: 10.1103/PhysRevLett.110.245502 PACS numbers: 61.80.Jh, 61.43.Dq, 61.43.Bn, 61.05.cf Swift heavy-ion irradiation (SHII) has many applications, spanning geochronological dating [1] to nanostructure fabrication [2]. Though this approach has found industrial application [3], the fundamental nature of ionsolid interactions at very high ion energies remains poorly understood. Such interactions are dominated by inelastic processes (electronic stopping) resulting in the excitation and ionization of substrate atoms while, in contrast, the elastic processes (nuclear stopping) that lead to ballistic atomic displacements at much lower energies are negligible in the SHII regime. The efficiency with which energy deposited in the electronic subsystem is subsequently transferred to the lattice is governed by the electronphonon coupling parameter g where typically g amorphous > g crystalline due to a reduced electron mean free path in the former. When the lattice temperature exceeds that required for melting, a narrow cylinder of molten material is formed along the ion path. The ensuing rapid resolidification of this transient liquid phase can yield remnant structural modifications within the substrate in the form of an ion track.Crystalline Ge (c-Ge) is relatively insensitive to SHII such that ion-track production necessitates very high electronic stopping S e values. Discontinuous tracks follow single-ion irradiation (S e ¼ 35 keV=nm) [4,5] while cluster-ion irradiation (S e ¼ 37-51 keV=nm) yields tracks of diameter 5-15 nm [5]. In contrast, amorphous Ge (a-Ge) is rendered porous under SHII with S e > $10 keV=nm [6] while ion hammering results for S e > $12 keV=nm [6], the latter manifested as a nonzero deformation yield [7]. These observations are consistent with g amorphous > g crystalline and ion-track formation has been suggested as the origin of these two phenomena [6,7]. A recent molecular dynamics (MD) study of irradiated a-Ge [8] suggested voids originate from outgoing shock waves resulting from rapid heating and expansion of the ion-track core. The sole report of ion tracks in a-Ge is that of Furuno et al. [9] who reported recrystallization of tracks in a 5-nm a-Ge layer following SHII in a grazing-incidence orientation, a geometry that can lead to significant reductions in threshold S e values for ion-track formation [10]. The proximity of the surface could also perturb resolidification and enable recrystallization given the molten i...

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 :H and SiO :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 :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 :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.

We present experimental evidence for the formation of ion tracks in amorphous Si induced by swift heavy-ion irradiation. An underlying core-shell structure consistent with remnants of a high-density liquid structure was revealed by small-angle x-ray scattering and molecular dynamics simulations. Ion track dimensions differ for as-implanted and relaxed Si as attributed to different microstructures and melting temperatures. The identification and characterization of ion tracks in amorphous Si yields new insight into mechanisms of damage formation due to swift heavy-ion irradiation in amorphous semiconductors.

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