Characterisation of silicon oxynitride thin films and their response to swift heavy-ion irradiation

Silicon oxynitrides (a-SiOxNy) are materials whose composition ranges between two binary materials: a-SiO2 and a-Si3N4. In this work, we present a systematic study of the fine structure of the damaged regions produced by swift heavy-ions (SHIs), or ‘ion-tracks’ and quantify the density variation profiles with respect to composition. Thin films were deposited by plasma-enhanced chemical vapor deposition (CVD), where thickness, density, stoichiometry and bond configuration were initially determined. The fine structure and radial size of the ion tracks was determined using small angle X-ray scattering. The tracks exhibit a core-shell cylindrical geometry, with an under-dense core surrounded by an over-dense shell with a smooth transition between the two regions. We observed two trends with composition: a constant increasing ion track radius is observed when the O/Si ratio is below one (0≤x≤1). And saturation of the radial dimensions above this value, being similar to a-SiO2. The IR spectra allowed to quantify the bond configuration and its evolution with fluence. After irradiation, the energy deposited by the SHI irradiation leads to a preferential damage of Si-N bonds. IR spectroscopy also showed the formation of new Si-H bonds with increasing fluences and resulting in a rather complex ion-induced structural modification of the a-SiOxNy network.

Reconciling Anomalously Fast Heating Rate in Ion Tracks with Low Electron-Phonon Coupling

Formation of swift heavy ion tracks requires extremely fast energy transfer between excited electrons and a lattice. However, electron-phonon energy exchange is too slow, as known from laser-irradiation experiments and calculations. We resolve this contradiction noticing that electron-phonon coupling is not the sole mechanism of energy exchange between electrons and ions: heating of electrons also alters potential energy surface of atoms, accelerating them and increasing their kinetic energy.

Ion tracks in silicon formed by much lower energy deposition than the track formation threshold

Damaged regions of cylindrical shapes called ion tracks, typically in nano-meters wide and tens micro-meters long, are formed along the ion trajectories in many insulators, when high energy ions in the electronic stopping regime are injected. In most cases, the ion tracks were assumed as consequences of dense electronic energy deposition from the high energy ions, except some cases where the synergy effect with the nuclear energy deposition plays an important role. In crystalline Si (c-Si), no tracks have been observed with any monomer ions up to GeV. Tracks are formed in c-Si under 40 MeV fullerene (C60) cluster ion irradiation, which provides much higher energy deposition than monomer ions. The track diameter decreases with decreasing the ion energy until they disappear at an extrapolated value of ~ 17 MeV. However, here we report the track formation of 10 nm in diameter under C60 ion irradiation of 6 MeV, i.e., much lower than the extrapolated threshold. The diameters of 10 nm were comparable to those under 40 MeV C60 irradiation. Furthermore, the tracks formed by 6 MeV C60 irradiation consisted of damaged crystalline, while those formed by 40 MeV C60 irradiation were amorphous. The track formation was observed down to 1 MeV and probably lower with decreasing the track diameters. The track lengths were much shorter than those expected from the drop of Se below the threshold. These track formations at such low energies cannot be explained by the conventional purely electronic energy deposition mechanism, indicating another origin, e.g., the synergy effect between the electronic and nuclear energy depositions, or dual transitions of transient melting and boiling.

Ion track etching of polycarbonate membranes monitored by in situ small angle X-ray scattering

In situ small angle X-ray scattering (SAXS) measurements of ion track etching of polycarbonate foils are used to directly monitor the selective dissolution of ion tracks with high precision, including...

Multi-scale Investigation of Heterogeneous Swift Heavy Ion Tracks in Stannate Pyrochlore

Er2Sn2O7 pyrochlore was irradiated with swift heavy Au ions (2.2 GeV), and the induced structural modifications were systematically examined using complementary characterization techniques including transmission electron microscopy (TEM), X-ray diffraction...

Comprehensive Understanding of Hillocks and Ion Tracks in Ceramics Irradiated with Swift Heavy Ions

Amorphizable ceramics (LiNbO3, ZrSiO4, and Gd3Ga5O12) were irradiated with 200 MeV Au ions at an oblique incidence angle, and the as-irradiated samples were observed by transmission electron microscopy (TEM). Ion tracks in amorphizable ceramics are confirmed to be homogenous along the ion paths. Magnified TEM images show the formation of bell-shaped hillocks. The ion track diameter and hillock diameter are similar for all the amorphizable ceramics, while there is a tendency for the hillocks to be slightly bigger than the ion tracks. For SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored, 200 MeV Au ion irradiation and TEM observation were also performed. The ion track diameters in these materials are found to be markedly smaller than the hillock diameters. The ion tracks in these materials exhibit inhomogeneity, which is similar to that reported for non-amorphizable ceramics. On the other hand, the hillocks appear to be amorphous, and the amorphous feature is in contrast to the crystalline feature of hillocks observed in non-amorphizable ceramics. No marked difference is recognized between the nanostructures in STO and those in Nb-STO. The material dependence of the nanostructure formation is explained in terms of the intricate recrystallization process.