Neutralization of Noble Gas Ions at Oxide Surfaces in Low-Energy Ion Scattering

Charge exchange of ions with solid surfaces is 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 charge exchange processes acting on the ion. Whereas low energy ion scattering (LEIS) is well established as a very surface sensitive method for surface composition analysis, the underlying charge exchange mechanisms are not yet well understood, e.g. the fact that matrix effects are absent in the majority of applications. Recent results indicate a strong dependence of charge exchange on details of the electronic structure of the target. It is well known that the presence of oxygen in a surface leads to pronounced changes in its electronic properties. Nevertheless, information on the influence of oxygen on charge exchange processes of He projectiles is scarce. This is a surprise insofar as oxygen is an important constituent of technical surfaces. Therefore, an experimental study on charge exchange of He projectiles at surfaces containing oxygen is proposed in this project. To gain a thorough understanding it is planned to model the results in cooperation with theoreticians. ESA- and TOF-LEIS-experiments will be performed for different materials with complex electronic structure. Amorphous and single crystalline samples of selected metals and semiconductors will be exposed to oxygen, charge exchange processes will be studied at adsorbate structures and at surface oxides. These experiments and theoretical considerations will help to answer the following questions: 1. How do the adsorption of oxygen at a surface or the formation of a surface oxide influence the charge exchange between He projectiles and surface atoms? 2. Which are the relevant charge exchange processes between He+ projectiles and oxygen in the adsorbate phase or in the surface oxide, respectively? 3. How large is the information depth in a LEIS experiment when surfaces containing oxygen are analyzed? 4. Which are favourable conditions to determine the concentrations of oxygen and of surface atoms quantitatively? The expected results will represent a major step forward in the physical understanding of ion solid interaction. A deeper comprehension, which leads to quantitative predictions of charge exchange probabilities, is highly desirable in LEIS-experiments. 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.

The aim of this project was to better understand the electronic interactions of energetic ions in solids and at solid surfaces. We investigated, for instance, the slowing down process of ions in compounds that contain nitrogen or oxygen atoms. A key quantity in this context is the stopping power (stopping force), i.e., the energy loss per travelled path. As a main result of this research, electronic stopping of slow hydrogen and helium ions in oxides is virtually entirely due to those electrons that are localized at oxygen atoms. This is in contrast to compounds containing nitrogen, where the valence electrons also found around the metal ion cores, due to the more covalent character of the chemical bonds. In cooperation with Prof. Primetzhofer at Uppsala University and his group, stopping power measurements were performed also using heavier ions (boron, nitrogen, neon, aluminum) in titanium nitride. As the primary result, the state of the art model to describe electronic stopping of heavy ions in metals is incomplete and therefore unrealistic. Consequently, it has to be thoroughly modified to permit realistic predictions for the stopping power of ions in modern high tech materials that are currently being developed. This information is important for ion-beam based analytical techniques in materials research, where accurate knowledge of the stopping power is indispensable for determination of the thickness of nanometer films. We succeeded in publishing our results in high impact journals, since our experiments yielded novel insights and breakthroughs that are relevant not only in the fundamental understanding of the underlying physical processes, but also for applied research like materials science based on ion beam techniques. As a second focus of our investigations within this project we studied thoroughly and systematically how neutralization und ionization of slow helium projectiles at metal surfaces are influenced by the presence of oxygen atoms. For quantitative surface analysis by means of Low Energy Ion Scattering (LEIS) it is important to know which fraction amongst the projectiles that backscattered from the outermost surface are positive He ions. Up to now, only empirical models and heuristic theoretical approaches are available for this ion fraction. The understanding of the underlying processes is, however, still limited. Therefore, we studied different aspects of quantification by LEIS, e.g., the effective information depth. This is important information, since for many applications it is essential to know whether the measured ion yield originates from the outermost atomic layer only or whether also subsurface atomic layers contribute to the ion signal. In a thorough investigation we studied how the presence of oxygen at metal surfaces modifies the complex interplay of the relevant charge exchange mechanisms and, consequently, the ion fractions for backscattering from metal atoms and from oxygen. This research activity is still ongoing at Uppsala University by Prof. Primetzhofer and coworkers.