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New aspects of electronic interactions of keV ions with matter
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Applied Nuclear Physics. Institute of Experimental Physics - AOP, Johannes Kepler University Linz, Linz, Austria..ORCID iD: 0000-0003-2208-6581
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

Low- and medium-energy ion scattering are powerful techniques to perform high-resolution depth profiling with sub-nanometer resolution. Typically, ions with primary energies between a few keV and a few hundred keV are used to probe the sample and backscattered projectiles are detected. To obtain highly accurate composition profiles, knowledge on physical processes governing ion-matter interaction is crucial. Apart from the main (back-)scattering process, which yields a detectable signal, the projectile loses energy in interactions with both electrons and nuclei (stopping) along its path in matter. In all these interactions, also the charge state of the probing particle can be altered. Information on this multitude of interaction mechanisms can be deduced from two different experimental approaches: either in backscattering or transmission geometry. Especially towards lower primary energies, available experimental data are found more scarce. This situation is particularly true for more complex targets, i.e. reactive transition metals and their compounds. This absence of quantitative information on energy loss or charge exchange processes hampers in many cases the quality of characterization despite the high technological relevance of these materials.

To contribute to an improvement of this status quo, this thesis focuses on (i) an analysis of sources of uncertainties in the evaluation of electronic energy loss, (ii) experiments to obtain stopping data for protons and He ions in different reactive samples and (iii) studies of charge exchange between projectile and target.

The first part presents a discussion of two possible sources of systematic errors, i.e. the composition of the investigated sample (thin films of the reactive transition metals often have low Z impurities like H, C, N and O), and deficiencies in the available models for the scattering potential. Concerning impurities in the films, it is shown that a correction according to Bragg's rule yields good agreement with data obtained from clean samples, even for energies down to a few keV, as long as the concentration levels of the impurities are low. In the second part experimentally deduced electronic energy loss data for transition metal nitrides as well as self-supporting Au and W-foils are presented. In the latter study a comparative approach using backscattering and transmission experiments is performed with measurements in both geometries conducted on the same sample, and in the same scattering chamber with only the position of the detector varied. In the final section the influence of surface oxygen on the energy spectra of backscattered ions at primary energies ≤ 5 keV is investigated. Depending on the host material O is found to enhance or suppress sub-surface signals. Additionally, also the change in neutralization efficiency for surface oxides in comparison to clean metal surfaces is studied for single crystalline Al(111) and Ta(111).

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. , p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1876
Keywords [en]
ToF-MEIS, LEIS, electronic stopping, charge exchange, scattering potential
National Category
Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:uu:diva-395952ISBN: 978-91-513-0803-6 (print)OAI: oai:DiVA.org:uu-395952DiVA, id: diva2:1366084
Public defence
2019-12-06, Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2019-11-08 Created: 2019-10-28 Last updated: 2019-11-08
List of papers
1. A note on extracting electronic stopping from energy spectra of backscattered slow ions applying Bragg's rule
Open this publication in new window or tab >>A note on extracting electronic stopping from energy spectra of backscattered slow ions applying Bragg's rule
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2018 (English)In: Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, ISSN 0168-583X, E-ISSN 1872-9584, Vol. 423, p. 82-86Article in journal (Refereed) Published
Abstract [en]

Electronic stopping measurements in chemically reactive targets, e.g., transition and rare earth metals are challenging. These metals often contain low Z impurities, which contribute to electronic stopping. In this article, we present two ways how one can correct for the presence of impurities in the evaluation of proton and He stopping in Ni for primary energies between 1 and 100 keV, either considering or ignoring the contribution of the low Z impurities to multiple scattering. We find, that for protons either method leads to concordant results, but for heavier projectiles, e.g. He ions, the influence on multiple scattering must not be neglected.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2018
Keywords
Low energy ion scattering, Electronic stopping, Bragg's rule
National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:uu:diva-356188 (URN)10.1016/j.nimb.2018.02.005 (DOI)000430901400013 ()
Conference
22nd International Workshop on Inelastic Ion-Surface Collisions, SEP 17-22, 2017, Dresden, GERMANY
Available from: 2018-07-31 Created: 2018-07-31 Last updated: 2019-10-28Bibliographically approved
2. On the influence of uncertainties in scattering potentials on quantitative analysis using keV ions
Open this publication in new window or tab >>On the influence of uncertainties in scattering potentials on quantitative analysis using keV ions
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(English)In: Article in journal (Refereed) Submitted
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-395223 (URN)
Available from: 2019-10-15 Created: 2019-10-15 Last updated: 2019-10-28
3. Electronic interaction of light, keV ions in transition metal nitrides
Open this publication in new window or tab >>Electronic interaction of light, keV ions in transition metal nitrides
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(English)In: Article in journal (Refereed) Submitted
Keywords
electronic stopping, low and medium energy ion scattering, transition metal nitrides, HfN, ZrN, TiN, VN, CrN
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-395284 (URN)
Available from: 2019-10-16 Created: 2019-10-16 Last updated: 2019-10-28
4. Impact of the experimental approach on the observed electronic energy loss for keV ions in self-supporting, thin films
Open this publication in new window or tab >>Impact of the experimental approach on the observed electronic energy loss for keV ions in self-supporting, thin films
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-395951 (URN)
Available from: 2019-10-25 Created: 2019-10-25 Last updated: 2019-10-30
5. The impact of surface oxidation on energy spectra of keV ions scattered from transition metals
Open this publication in new window or tab >>The impact of surface oxidation on energy spectra of keV ions scattered from transition metals
2019 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 479, p. 1287-1292Article in journal (Refereed) Published
Abstract [en]

Studying the initial stages of surface oxidation is of great relevance to understand how oxygen alters the physical and chemical properties at the interface of the host material to the environment and is therefore, crucial for improvement in manifold technological applications. We investigated the influence of surface oxygen on ion spectra recorded for keV noble gas ions backscattered from metal surfaces in low energy ion scattering (LEIS). Initially pure Zn and Ta surfaces, chosen for their well-characterized properties in ion-neutralization in LEIS, have been oxidized and ion spectra for pure and oxidized surfaces have been compared. Oxygen on the surface significantly influences shape and intensity of the backscattered ion spectrum at all energies: for both metal systems, the surface scattered ion yield of the metal is drastically decreasing under oxygen presence. The observed decrease, however, cannot be explained by the reduction in the surface areal density of the metal constituents exclusively. At least for Zn an additional significant change in charge exchange behavior is necessary to explain the observations. In contrast to the generally observed decrease in the yield of ions scattered from the outermost surface, the change in shape and intensity of the reionization background are found to show opposing trends and different energy dependencies for Zn and Ta.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE BV, 2019
Keywords
Surface oxides, Transition metals, Low energy ion scattering, Ion yield
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-382807 (URN)10.1016/j.apsusc.2018.12.210 (DOI)000464931800152 ()
Available from: 2019-05-07 Created: 2019-05-07 Last updated: 2019-10-28Bibliographically approved
6. Neutralization of slow helium ions scattered from single crystalline aluminum and tantalum surfaces and their oxides
Open this publication in new window or tab >>Neutralization of slow helium ions scattered from single crystalline aluminum and tantalum surfaces and their oxides
2020 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 691, article id 121491Article in journal (Refereed) Published
Abstract [en]

We investigated the impact of surface oxygen on the ion yield for He+ ions scattered from different single crystalline surfaces in low-energy ion scattering. Initially clean Al(111) and Ta(111) were exposed to molecular oxygen and ion spectra for different oxidation stages and different primary energies were recorded. A comparison of ion yields normalized to the differential scattering cross section as well as experimental factors allows obtaining information about the influence of oxygen on charge exchange processes. The decrease in the ion yield of both metals with exposure cannot be explained by different surface coverages exclusively, but requires the neutralization efficiency to be dependent on the chemical structure of the surface. For Ta, additionally, a different energy dependency of the ion yield obtained in the metal and oxide occurs. The ion yield for O shows in both surfaces a significantly weaker energy dependency than the investigated metals.

Keywords
Low energy ion scattering, Ion yields, Charge exchange, Oxygen exposure, Al(111), Ta(111)
National Category
Materials Chemistry
Identifiers
urn:nbn:se:uu:diva-394735 (URN)10.1016/j.susc.2019.121491 (DOI)
Available from: 2019-10-09 Created: 2019-10-09 Last updated: 2019-10-28Bibliographically approved

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