Bandstructure Effects in Electronic Stopping (BEES)

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.

Energetic atomic particles (ions, electrons) are of importance in many distinct areas, as in the cosmic background radiation, which represents an important part of natural radioactivity, or in fusion reactors, which are intensively investigated as potential future sources of energy, and last but not least in radiotherapy using particle beams, e.g. in the irradiation facility MedAustron which is being installed in Wr. Neustadt. Since ions belong to the ionizing radiation, they can transmit energy to electrons when ions interact with atoms and are thereby decelerated. For fast ions (moving with a velocity higher than about 10 % of the speed of light) the processes of energy transfer are well understood for a long time; for slow ions (with a velocity less than about 1 % of the speed of light) it is not yet clear, which processes are important in the deceleration process. Aim of the present project was to contribute to finding an answer to this open question. A question of special interest was, to which extent the deceleration process of the lightest ions protons and helium ions depends on the properties of the conduction electrons in metals or of the valence electrons in semiconductors and insulators. For ions which move with a speed similar as that of the fastest conduction electrons it was already known before that the deceleration is governed by the effective density of the conduction electrons. Furthermore it was found that for noble metals the energy distribution of the conduction electrons (band structure), which is also responsible for the color of copper and gold, influences the deceleration process of protons and helium ions. Therefore, the project BEES (Band-structure Effects in Electronic Stopping) investigated the influence of the band structure of metals and semiconductors on the deceleration of protons and helium ions. The stopping power for protons and helium ions was measured in metallic samples with different band structures (nickel, zinc, indium, tantalum, platinum), in elemental semiconductors (silicon, germanium), in binary semiconductors (zinc oxide, zinc sulfide and vanadium dioxide) and in binary insulators (hafnium dioxide, tantalum pentoxide). For the metals it was discovered that the d-electrons, which are responsible for the color of copper and gold, can be excited by slow helium ions but not by slow protons. The results for the semiconductors cannot be interpreted in a straight forward way and require a cooperation with theoreticans (e.g. Prof. Artacho, San Sebastian) to understand the underlying processes. Due to delays caused by experimental troubles the measurements on vanadium dioxide can be performed only after this projects end. It is a key experiment since this material changes from being a semiconductor to a metal at a temperature of 65? C. Thus, experiments on this material at room temperature and at elevated temperatures (at 70? C) will help to decide whether there are fundamental differences between the interactions of slow light ions with metals and with semiconductors.