Regensburg 2007 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 38: Diffusion and point defects III
MM 38.2: Talk
Thursday, March 29, 2007, 15:00–15:15, H6
How to Measure Diffusion Lengths in the Sub-nanometer Range? — •Harald Schmidt1,2, Thomas Gutberlet2, and Michael Bruns3 — 1Institut für Metallurgie, AG Materialphysik, Technische Universität Clausthal, 38678 Clausthal-Zellerfeld — 2Laboratorium für Neutronstreuung, ETH Zürich und Paul Scherrer Institut, 5232 Villigen, Schweiz — 3Institut für Materialforschung III, Forschungszentrum Karlsruhe, 76021 Karlsruhe
The study of self-diffusion in metastable solids like glasses or nano-materials necessitates the detection of extremely short diffusion lengths in order to prevent crystallization or growth processes during the measurement. This is especially true for covalently bound amorphous materials with their low atomic mobility. We demonstrate that it is possible to detect diffusion lengths down to 70 Å by neutron reflectometry. Such small values cannot be achieved by conventional methods of diffusivity determination (radiotracer technique, SIMS, NMR, QENS). The reflectivity measurements were carried out on magnetron sputtered amorphous Si14Nx/Si15Nx isotope multilayers which were used as a model system. Due to the periodically modulated structure of the multilayers and the different neutron scattering lengths of the nitrogen isotopes, Bragg peaks occur in the reflectivity pattern. Self-diffusivities down to 5 x 10−25 m2/s were determined from the decay of the Bragg peaks due to interdiffusion of the two nitrogen isotopes. The ability of the method to resolve a time dependence of the diffusivities due to short-time structural relaxation is demonstrated.