Stopping of Light Ions at the Threshold

The electronic stopping force exerted on ions moving in a medium is a ubiquitous quantity in all kinds of ion- matter interaction like solid state physics, plasma physics, fusion research, dosimetry and ion beam analysis. and therefore of fundamental importance. The underlying physical processes are well understood for metals, semiconductors and gases as stopping medium. For insulators with a large band gap, it is not yet clear, which are the physical processes responsible for the quite effective stopping force exerted on slow ions in large band gap materials. Therefore, this proposal aims at a thorough understanding of the underlying interaction mechanisms. In detail, we aim at finding answers to the following open questions: (1) Which is the dominant mechanism to excite electron-hole pairs in ionic and covalent insulators? (2) Is there a finite threshold in electronic stopping for a semiconductor like Si? (3) Is there a systematic difference in the stopping force, deduced from evaporated layers and a single crystal of the same type of insulator, due to electronic defects? To this aim, the following strategy will be followed: on the one hand, thin insulator layers will be deposited in situ by electron beam induced evaporation and electronic stopping will be studied in backscattering geometry by means of time-of-flight spectroscopy (TOF-LEIS). Evaluation of the electronic stopping force from the energy spectrum of backscattered ions requires the comparison of measurements to Monte-Carlo simulations. On the other hand, single crystals will be used as samples. With single crystals, ion scattering experiments can be performed either with the crystal lattice aligned with the ion beam or the detection direction (channelling/blocking) or in "random" geometry. Measurements in channelling/blocking geometry will be sensitive to the outermost atomic layer(s) only and therefore best suited to look for single electron-hole pair excitation losses. For analysis of single crystalline samples in random direction, the information corresponds to "amorphous" samples. In this case, the evaluation of the electronic stopping force from the backscattering spectrum requires - as for evaporated layers - comparison of measurements to Monte-Carlo simulations. To summarise, the main aim of this proposal is to establish a physical understanding of electronic stopping of ions in materials with finite minimum excitation energy, like semiconductors and insulators. Most important information is expected at very low ion velocities, where the ions cannot excite electron-hole pairs via binary collisions anymore, i.e. in the excitation-threshold regime.

The electronic stopping force exerted on ions moving in a medium is a ubiquitous quantity in all kinds of ion- matter interaction like solid state physics, plasma physics, fusion research, dosimetry and ion beam analysis. and therefore of fundamental importance. The underlying physical processes are well understood for metals, semiconductors and gases as stopping medium. For insulators with a large band gap, it is not yet clear, which are the physical processes responsible for the quite effective stopping force exerted on slow ions in large band gap materials. Therefore, this proposal aims at a thorough understanding of the underlying interaction mechanisms. In detail, we aim at finding answers to the following open questions: 1. Which is the dominant mechanism to excite electron-hole pairs in ionic and covalent insulators? 2. Is there a finite threshold in electronic stopping for a semiconductor like Si? 3. Is there a systematic difference in the stopping force, deduced from evaporated layers and a single crystal of the same type of insulator, due to electronic defects? To this aim, the following strategy will be followed: on the one hand, thin insulator layers will be deposited in situ by electron beam induced evaporation and electronic stopping will be studied in backscattering geometry by means of time-of-flight spectroscopy (TOF-LEIS). Evaluation of the electronic stopping force from the energy spectrum of backscattered ions requires the comparison of measurements to Monte-Carlo simulations. On the other hand, single crystals will be used as samples. With single crystals, ion scattering experiments can be performed either with the crystal lattice aligned with the ion beam or the detection direction (channelling/blocking) or in "random" geometry. Measurements in channelling/blocking geometry will be sensitive to the outermost atomic layer(s) only and therefore best suited to look for single electron-hole pair excitation losses. For analysis of single crystalline samples in random direction, the information corresponds to "amorphous" samples. In this case, the evaluation of the electronic stopping force from the backscattering spectrum requires - as for evaporated layers - comparison of measurements to Monte-Carlo simulations. To summarise, the main aim of this proposal is to establish a physical understanding of electronic stopping of ions in materials with finite minimum excitation energy, like semiconductors and insulators. Most important information is expected at very low ion velocities, where the ions cannot excite electron-hole pairs via binary collisions anymore, i.e. in the excitation-threshold regime.