Hans Hofsäss

We perform Low-energy Ion Scattering with He ions ranging in energy from 1 keV to 3 keV on CaSiO 3 using a commercial electrostatic detector system and determine the charge fraction of scattered ions from comparison with binary collision approximation simulations. The simulations take composition changes due to surface cleaning Xe sputtering into account. Scattered He particles are separated in the simulation into single, dual, and multiple scattering trajectories. We find that the charge fraction of single and dual scattered He is about ten times higher than the one for multiple collisions. Our results show that the charge fraction of scattered He can be reliably extracted for different scattering regimes and that for a given He incidence energy, the entire scattered He spectrum can be modeled with only few constant parameters.

Ultra-low energy ion implantation has become an attractive method for doping of two-dimensional materials and ultra-thin films. The new dynamic Monte Carlo program IMINTDYN based on the binary collision approximation allows a reliable prediction of low energy implantation profiles and target compositional changes, as well as efficient simulation of high energy light ion scattering. To demonstrate the quality of these predictions and simulations, we present a model case experiment where we implanted W ions into tetrahedral amorphous carbon with low (10 keV) and ultra-low (20 eV) ion energies and analyzed the W implantation profiles with high resolution Rutherford backscattering spectrometry (HR-RBS). This experiment is compared with a complete simulation of all aspects of ion-solid-interactions of the experiment using the new IMINTDYN program. A unique novel simulation option, also relevant for implantation into 2D materials, is the inclusion of the vacancy as target species with dynamic vacancy generation and annihilation. Whereas simulations neglecting vacancy formation cannot reproduce the measured implantation profiles, we find excellent agreement between simulated and measured HR-RBS spectra. We also demonstrate the important role of simultaneous weak collisions in the binary collision approximation at low projectile energies.

In this paper, the effectiveness of ultra-low-energy ion implantation as a means of defect engineering in graphene was explored through the measurement of Scanning Kelvin Probe Microscopy (SKPM) and Raman spectroscopy, with boron (B) and helium (He) ions being implanted into monolayer graphene samples. We used electrostatic masks to create a doped and non-doped region in one single implantation step. For verification we measured the surface potential profile along the sample and proved the feasibility of lateral controllable doping. In another experiment, a voltage gradient was applied across the graphene layer in order to implant helium at different energies and thus perform an ion-energy-dependent investigation of the implantation damage of the graphene. For this purpose Raman measurements were performed, which show the different damage due to the various ion energies. Finally, ion implantation simulations were conducted to evaluate damage formation.

In this work, we report on the hyperfine parameters of the foreign ¹⁸¹Ta probe in the rutile structure of the single crystal TiO₂ using the e−γ and γ−γ time differential perturbed angular correlation (TDPAC) technique. We implanted ¹⁸¹Hf ions into a sample of single crystal rutile TiO₂ in the Bonn Isotope Separator. The implanted sample was then thermally annealed at a temperature of 873 K for 315 min in a vacuum. The ¹⁸¹Hf radioisotopes decayed by β⁻ emission, followed by a cascade to the ground of γ rays or conversion electrons into a stable state ¹⁸¹Ta. The ¹⁸¹Ta probe substitutes the Ti lattice site with a unique nuclear quadrupole interaction, allowing for the precise measurement of the largest electric field gradient (V_zz) and asymmetry parameter (η). The hyperfine parameters obtained from the e−γ TDPAC spectroscopy agree with those of the γ−γ TDPAC spectroscopy at room temperature, apart from a calibration factor, both from our experiments and the literature. This suggests that the electronic recombination following the internal conversion of the L shell electron takes less time (ps) than the intermediate lifetime of the metastable ¹⁸¹Ta state (ns).

Solid-state physics research has long employed radioactive isotopes to investigate the crystallographic, electric and magnetic properties of nanostructures. Ion implantation (1–100 keV) is the method of choice for incorporating radioactive nuclei into the crystal structure. However, the enormous scientific interest in 2D materials, multiferroics and their interfaces of the last decades has lead to more stringent demands for isotope incorporation. Ultra-low energy (ULE) ion implantation (10–100 eV) provides the ability to precisely tune the depth of the implanted radioactive probes, even in the case of atomically thin 2D materials. To unlock this potential and expand the experimental capabilities of the ISOLDE collaboration in CERN, the apparatus for surface physics and interfaces at CERN (ASPIC), an experienced ultra-high vacuum chamber dedicated to surface characterization and modification, is refurbished and upgraded with a new component: the ASPIC’s ion implantation (ASCII) chamber, designed for ULE ion implantation of radioactive probes. This paper describes the scientific context, design and application of these vacuum chambers.

Dot and ripple nanopatterns on Si surfaces with defined symmetry and characteristic dot spacings of 50-70 nm were created by 1 keV Ar ion irradiation at normal incidence and simultaneous co-deposition of Fe atoms at grazing incidence. Fe was continuously supplied from different sputter targets surrounding the Si substrate, leading to a steady-state Fe content in the near-surface region of the substrates. The pattern formation is self-organized, most probably caused by ion-induced phase separation. Patterns were analyzed with atomic force microscopy and the Fe content in the irradiated layer was measured with Rutherford backscattering. The symmetries of the produced patterns are isotropic, four-fold symmetric, three-fold symmetric and various types of two-fold symmetric patterns, depending on the geometrical arrangement of the sputter targets. Pattern formation was studied for a steady-state coverage of Fe between 0.5 and 3.3 × 10(15) Fe cm(-2). The threshold coverage for the onset of pattern formation is about 0.5-1 × 10(15) Fe cm(-2). The coherence length of the patterns is comparable to the average dot spacing. Nevertheless, the autocorrelation analysis reveals a residual long-range periodicity of some patterns. The dot spacing can be adjusted between about 20 nm and several hundred nm depending on the ion species and ion energy.