In-situ Tracer Diffusion in Solids

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One of the most important analytical methods for the experimental determination of diffusivities, activation energies and diffusion mechanisms in solids is based on the use of stable tracers. A rare stable isotope of an element is deposited on the surface of the material under investigation. After diffusion annealing at elevated temperature the tracer is redistributed. After cooling down and subsequent isotope depth profile analysis by e. g. Secondary Ion Mass Spectrometry tracer self-diffusivities can be determined.During the last years the method of neutron reflectometry was established in this field of research. The method allows to determine diffusion length < 1 nm and diffusivities down to 10-26 m2/s on thin films and bulk materials. The drawback of this - and all other tracer methods – is that the diffusion processes cannot be monitored in-situ, directly during annealing. There is always a cool down of the sample to room temperature necessary. For various modern metastable and nanoscale materials systems an in-situ detection of diffusivities during the actual diffusion process is of big advantage.Due to the actual advancement of the Neutron Reflectometry method in form of the “Focusing Reflectometry”, higher neutron intensities can be produced at the sample, which will allow to record a reflectivity pattern in less than 5 min. This will enable an in-situ detection of self-diffusion in solids during annealing for the first time. A quasi-continuous detection of diffusivities as a function of time becomes possible. In addition, the error of a single measurement is significantly reduced and activation energies can be determined more precisely. The aim of the present research project is the quantification of self-diffusivities in-situ during annealing with neutron reflectometry using “amorphous Germanium” as a model system. The experiments should be done to test the method on a broad base and to give the necessary conditions for future development. Experiments for a quasi-continuous detection of time-dependent diffusivities during structural relaxation and crystallization will be done. The work should also help to establish this method in the field of diffusion science.In the framework of the prolongation proposal present here, a method for the determination of activation energies in a single annealing experiment (isochronic method) developed during the first period should be tested and refined. In addition, systematic investigations on the diffusion in amorphous Si1-xGex should be carried out, while the advantages of the in-situ method (high sample throughput, time dependence of diffusivities) will be used.