Electronic interactions of ions with matter are of importance in applied science as well as in fundamental research. Many techniques used for analysis or modification of material properties make use of ion beams and thus require quantitative information on the energy loss of ions in solids. At high energies good qualitative understanding and a quantitative prediction of the deceleration force acting on the ion (called stopping power) is available. For low energy ions, however, the interaction mechanisms are not yet well understood. Recent results indicate a strong dependence of the stopping power on the electronic structure of the target that is observed at low ion velocities. Moreover, for different classes of materials – metals, semiconductors and insulators – the observed dependence of the electronic energy loss on characteristic excitation thresholds can not be explained by one single model. Thus, an experimental study together with theoretical support is planned to find a reasonable classification and eventually an adequate modelling of the interaction mechanisms. For that purpose, Time-Of-Flight Low-Energy Ion Scattering (TOF-LEIS) experiments will be performed at very low ion velocities for different materials with complex electronic structure. Amorphous and single crystalline samples of selected metals, intrinsic and doped semiconductors and insulators with different band gap width will be investigated. The results together with theoretical considerations will help to answer the following questions:
(1) Why are the threshold velocities for electronic stopping in insulators so low, compared to the corresponding values for metals or semiconductors?
(2) Is it possible to understand how the velocity threshold in electronic stopping is related to the band gap energy?
(3) Can the electronic interactions of ionic and covalent insulators and semiconductors be explained on the same footing?
(4) Is there a noticeable influence of doping on electronic stopping in semiconductors in the threshold regime?
(5) Does electronic stopping significantly depend on the defect density in the material – in other words, is there a distinct difference in electronic stopping in single crystalline samples and polycrystalline thin films, especially in the threshold regime?
The results obtained in the planned project thus will represent a major step forward in the physical understanding of ion solid interaction. Furthermore, this will be of interest for manifold technological applications like ion implantation and fusion research, where the electronic interaction of ions with solids is of relevance.
This project is funded by the Austrian Science Fund under contract number P22587-N20.