Determination of the ion track structure in amorphous SiO2 using small angle x-ray scattering

In amorphous SiO2 (a-SiO2), a technologically most important material, average structural properties of ion tracks have been inferred from macroscopic measurements, yet details of the track structure and inner morphology are difficult to retrieve due to the lack of sufficient contrast inherent with most techniques. Here we report on measurements of a fine structure in ion tracks in a-SiO2 using small angle x-ray scattering (SAXS) [1]. The ion tracks were produced in 2-μm thick a-SiO2, thermally grown on Si(100) substrates, by irradiation with 1.4 and 0.6 GeV Xe ions at the UNILAC accelerator. The fluence was limited to 3×10 ions/cm to avoid significant track overlap. Thin SiO2 layers were utilized to achieve a uniform energy loss over the extent of the layer and hence uniformity of the tracks along the ion paths. The track structure was studied using synchrotron SAXS in transmission geometry performed at beamline 15ID-D of the Advanced Photon Source, Argonne National Laboratories, USA. The x-ray wavelength was 1.1 Å (~10.27 keV) and the distance between the sample and the CCD detector was 555 and 1894 mm. All measurements were performed with the sample surface aligned normal to the x-ray beam, i.e. parallel to the ion tracks. The resulting isotropic images were radially integrated around the beam center. Scattering from an unirradiated SiO2 standard was subtracted from all spectra. Figure 1 shows the scattering intensities of the samples.