Growth Structure And Lattice Dynamics Of Rare Earth Silicide Nanostructures

Zusammenfassung: In the present thesis the epitaxial growth, crystal structure, stoichiometry, thermal stability and lattice dynamics of self-organized EuSi2 and DySi2 films, nanoislands and nanowires are investigated. The rare earth silicide (RESi) nanostructures have attracted considerable interest due to their high conductivity, very low Schottky barrier heights, remarkable chemical stability, self-organization in high area density and defects-free nano-objects with tunable size and shape, and the direct integration into the Si technology. The extensive research is driven by the continuous downscaling of the CMOS electronics that require new approaches in the devices architecture and circuits interconnects. Although RESi nanostructures attracted a lot of interest already several years ago and a lot of research has been done in this field, the lattice dynamics of these materials are still unknown. Recent developments at third generation synchrotron radiation sources have brought their performance to a stage where phonon spectroscopy of nanostructures and thin layers became feasible using nuclear inelastic X-ray scattering. This novel experimental technique is based on the process of phonon-assisted nuclear resonant absorption/emission of X-rays from the nuclei of Mössbauer-active isotopes. The method provides direct access to the phonon density of states (DOS) of the investigated element. Together with the ab inito calculations it was possible to get a comprehensive understanding of the lattice dynamics.EuSi2 films and nanoislands and DySi2 films, nanoislands and nanowires have been grown on the vicinal Si(001) surface by molecular beam epitaxy. While DySi2 was grown following known growth procedures, the growth conditions for EuSi2 had to be established first. EuSi2 was grown at two different growth conditions to study the influence of crystal structure and morphology upon different growth temperatures. The structure has been characterized by in situ and ex situ diffraction techniques revealing the tetragonal I41/amd crystal structure for both films and nanoislands at both growth temperatures, while the surface morphology changes significantly depending on the growth temperature. The thermal stability study performed by in situ high temperature GISAXS experiments demonstrated that EuSi2 nanoislands break down upon annealing at 1050 °C due to silicide decomposition followed by desorption of Eu atoms from the Si(001) surface. In contrast, DySi2 nanoislands increase their sizes upon annealing at 1050°C due to Ostwald ripening. The DySi2 nanowires are shrinking drastically because they only stabilize in a narrow temperature range. For the first time the lattice dynamics of EuSi2 and DySi2 was studied applying in situ nuclear inelastic scattering and ab initio theory. The phonon DOS was obtained and the related thermodynamic and elastic properties were calculated. A significant change of the phonon density of states upon the transition from bulk-like films to nanoislands and nanowires has been observed. With decreasing the sizes of the nanostructures properties like vibrational amplitude, vibrational entropy and the lattice specific heat are drastically increased, while the mean force constant is decreased. All values received from the experiments are in very good agreement with the values from the ab inito calculations for the EuSi2 system, while the calculations for the DySi2 system are still in progress