Radek Holeňák: Close encounters: electronic excitations by keV ions in single crystals

Abstract

This thesis focuses on the study of ion-solid interactions, particularly at ion energies of several tens of keV. In this regime, ion beams hold technological potential for high-resolution depth profiling as well as for increased precision in ion implantation, which is paramount for applications in semiconductor manufacturing or quantum computing. The understanding of the exact nature of energy dissipation processes active during ion penetration is crucial for technological advancement but simultaneously presents a fundamental physics problem worth investigating. A widely accepted picture describing keV ion-solid interactions is the one of electronic excitations in an electron gas. Nevertheless, the experimentally accessible observables, such as energy or charge transfer, show unexpectedly clear signs of an atomic signature in the individual interaction. The origin of these processes is currently under investigation both in dedicated experiments and using novel computational approaches.

The research employs ion transmission experiments through thin self-supporting single-crystalline membranes which allow for a confinement of interaction distances and thus processes. Ions experiencing close collisions with target atoms displays energy losses beyond those expected in the electron gas picture. Correlating these observations with the charge state distributions of the transmitted projectiles underscores the role of close collisions facilitating the formation of molecular orbitals. A quantitative analysis reveals this atomistic nature of the interaction being the dominant energy dissipation channel for slow, heavy ions.

The Time-of-Flight Medium-Energy Ion Scattering system (ToF-MEIS) at Uppsala University was used as a primary tool for the investigations. The present work further developed this instrumentation as well as established new ion-based analytical techniques in-situ. Control over the surface composition is mandatory to yield reliable empirical data in the investigations presented in this thesis. Elastic recoil detection was shown capable of quantitative analysis of light surface contaminants, both on bulk and transmission samples. Ion-induced surface sputtering and desorption were employed on self-supporting membranes shedding light on the underlying mechanism for the desorption process. Finally, the construction and commissioning of an advanced UHV preparation chamber opened up for in-situ synthesis and modification of materials, which in combination with ToF-MEIS enables precise compositional and structural analysis at a nanometre scale.