Das elektronische Bremsvermögen von Stickstoff und Sauerstoff für niederenergetische Protonen

The electronic stopping power of molecular oxygen and nitrogen for protons with energies between 1 keV and 30 keV has been measured using a differentially pumped stopping cell. Our results give a surprisingly good confirmation of the Lindhard-Scharff statistical theory which predicts a linear velocity dependence of the electronic stopping power at low projectile energies. Moreover our data are in fair agreement with earlier high energy (E ≧ 20 keV) measurements in other laboratories. The combination of the present measurements with theoretically calculated nuclear stopping powers yields an estimate of the atomic stopping power. This estimate leads to substantially lower atomic stopping powers at low energies compared to values derived from range measurements. This result may have interesting implications on auroral hydrogen emissions.     

Non-Gaussian range profiles in amorphous solids

Range profiles of boron in amorphous silicon exhibit pronounced deviations from Gaussian at energies above about 40 keV due to increasing electronic stopping. A detailed comparison with computed profiles allows a semiempirical determination of the electronic stopping cross section (S_e ≈ E^0.4).     

Excitation of Autoionization States and Electronic Stopping Powers at Collisions of Slow Ions with a Solid

It has been shown that the excitation of autoionization states at collisions of keV ions with a solid is decisive for inelastic energy loss and, correspondingly, the electronic stopping power dE/dx. It has been proposed to estimate the electronic stopping power dE/dx using the relation of cross sections for the excitation of autoionization states to ionization cross sections. When ionization cross sections are unknown, scaling is used to calculate ionization cross sections at the excitation of the L and M shells. A threshold dependence of the electronic stopping power dE/dx on the energy of bombarding ions has been predicted.     

Range and range straggling of oxygen implanted into silicon at energies between 2 and 20 MeV

Oxygen profiles in silicon implanted with energies between 2 and 20 MeV by means of a Tandem accelerator have been investigated with a microprobe after bevelling the sample surface. It is shown that the measured profiles correspond to the implantation profiles when the microprobe is operated with a well focussed 2 keV electron beam. The projected ion ranges and the profiles thus obtained are compared with theoretical profiles which have been calculated by a Monte Carlo simulation of the stopping procedure. Takingk=1.30k _LSS for the electronic stopping coefficient in the LSS region up to 2.55 MeV and a constant value of 162 eV/Å for the electronic stopping at higher energies the calculation yields satisfactory range estimates, whereas the range straggling is systematically too small up to 13% in comparison with the experimental results.     

Effects of Kr and Xe ion irradiation on the structure of Y2O3 nanoprecipitates in YBCO thin film conductors

ABSTRACT

The response of Y₂O₃ nanoprecipitates in a 1-µm YBa₂Cu₃O_7-x layer from a superconducting wire Ag/YBCO/buffer metal oxides/Hastelloy to 107?MeV Kr and 167?MeV Xe ion irradiation was investigated using a combination of transmission electron microscopy, diffraction and X-ray energy-dispersive spectrometry. The direct observation of the radiation-induced tracks in Y₂O₃ nanocrystals is reported for the first time to the authors’ best knowledge. Structureless damaged regions of 5–9?nm (average 8?nm) in diameter were observed in Y₂O₃ nanocrystals when the electronic stopping power S_e was about or higher than 4.7 keV/nm. This value of S_e is the upper estimate of the minimum electronic stopping power to create damage in yttria nanocrystals. The electron diffraction patterns, high-resolution transmission electron microscopy, high-resolution scanning transmission electron microscopy, Fourier transform patterns from areas extending a few nanometres around the tracks show that yttria and YBCO keep their respective cubic and orthorhombic pristine structures.     

Reflection of keV heavy-ion beams from solid targets. Dependence on energy and angle of incidence

Recent experimental and theoretical studies of the reflection of keV heavy-ion beams have been extended to higher energies and to non-perpendicular incidence. The reflection coefficient for Na⁺ and K⁺ ions backscattered from polycrystalline gold and silver targets has been obtained for perpendicular incidence at energies of 100–500 keV. The dependence on angle of incidence has been investigated at 30 keV for the same combinations of targets and projectiles. Effects of electronic stopping have been included in the theoretical calculations. Good agreement between the experimental results and the theoretical calculations is found.     

Experimental determinations of electron stopping power at low energies

Abstract

An accurate knowledge of electron stopping power is important for calculations and simulations of electron beam interactions with solids especially in the low energy region (< 10 keV). This paper describes a simplified and rapid experimental procedure using based on electron energy loss spectroscopy which permits accurate stopping power determinations to be made from any material which can be observed in a transmission electron microscope. It is demonstrated that the stopping power and oscillator strength data so generated is in good agreement with other measurements and theoretical predictions. The technique is now being applied to a wide variety of materials of semiconductor interest.     

Atomic mixing of Ni2O3/SiO2, NiO/SiO2, and Ni/SiO2 interfaces induced by swift heavy ion irradiation

We have investigated the interface mixing of Ni₂O₃/SiO₂, NiO/SiO₂, and Ni/SiO₂ induced by the irradiation with Ar, Kr and Xe ions of energies ranging from 90 MeV to 260 MeV. Since these energies are in the electronic stopping regime, atomic transport processes will not be directly initiated by elastic ion–target collisions, but need to be excited by secondary processes like electron–phonon coupling or Coulomb explosion. Nevertheless, we have observed a strong mixing effect in the ceramic systems if the electronic energy loss exceeds a certain threshold value. Estimation of an effective diffusion constant indicates that diffusion takes place in the molten ion track. In contrast to the ceramics, the metallic Ni layer is still insensitive even for the highest electronic stopping power used (S_e=28 keV/nm) and does not exhibit mixing with its SiO₂ substrate. In addition, NiO/SiO₂ and Ni/SiO₂ were irradiated in the nuclear stopping regime with 600 keV Kr and 900 keV Xe–ions. Here the intermixing effect is in good agreement with the assumption of ballistic atomic transport. Received: 5 February 2002 / Accepted: 11 February 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +49-711/685-3866, E-mail: bolse@ifs.physik.uni-stuttgart.de     

Inelastic energy loss in solids

The dependence on the bombarding energy of the number of ejected electrons from a policrystalline Ag target is determined. Statistical methods are used in the calculation of the inelastic energy loss due to binary atomic collisions and in that of the electronic stopping cross-section. The calculation is executed for the case of Se⁺ bombarding ion in the 10–200 keV energy range.     

Experimental and theoretical determination of the stopping power of ZrO<Subscript>2</Subscript> films for protons and <Emphasis Type="Italic">α</Emphasis>-particles

We report the results of an experimental-theoretical study on the stopping power of ZrO₂ films for swift H and He ion beams. The experiments, using the Rutherford Backscattering technique, were done for protons with incident energies in the range 200–1500 keV and for α-particle beams with energies in the range 160–3000 keV. The theoretical calculations were done in the framework of the dielectric formalism using the MELF-GOS model to account for the ZrO₂ target electronic response. It is shown that for both ion beams, the agreement between theory and experiment is quite remarkable.     

Electronic stopping power calculation method for molecular dynamics simulations using local Firsov and free electron-gas models

Molecular dynamics simulations have proven to be accurate in predicting depth distributions of low-energy ions implanted in materials. Free parameters adjusted for every ion-target combination are conventionally used to obtain depth profiles in accordance with the experimental ones. We have previously developed a model for predicting depth profiles in crystalline Si without free parameters. The electronic stopping power was calculated using local total electron density. The model underestimated the stopping in the ?1 1 0? channeling direction. We have now taken a new approach to calculate the electronic stopping power. We use the local valence (3p²) electron density to account for the electronic energy loss between collisions and the Firsov model to account for the electronic energy loss during collision. The lowest electron densities are adjusted with a parametrization that is same for all ions in all implanting directions to correct the problems in the ?1 1 0? channeling direction.     

Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of ²³⁵U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. AΔE-E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE-E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ($\text{E}$) and the standard deviation (σ_E) of the energy spectra of different light charged particles with incident neutron energy was observed, although $\text{E}{}_{\mathrm{\alpha }}$ was seen to have a slightly higher value beyond E_n = 230 keV. The yield of α-particles in fission induced by neutrons of E_n ～ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.     

Dependence of the space distribution of particles sputtered from monocrystalline copper on the ion incidence angle

The dependence of the angular distribution of atoms sputtered from the (001) monocrystalline copper face on the incidence angle of 22 keV neon ions has been studied. Use is made of well known ideas defining the observed distribution as a result of superposition of the directed atomic yield along the close-packed axes and of the random yield along all directions. The values obtained for the relative contribution of the directed and random sputterings indicate that the dependence of these values on the ion incidence angle is distinctly anisotropic. The yield of the directed sputtering had a minimum value when the incidence directions of the bombarding ions coincided with the low-index crystallographic axes. In addition, the width of (011) and [001] pattern spots was also found to be an anisotropic function of the ion incidence angle. The spots have the smallest width when the direction of ion incidence coincides with the low-index axes. The observed dependences are discussed on the basis of the focusing mechanism of the sputtered atom yield.     

Apparent velocity threshold in the electronic stopping of slow hydrogen ions in LiF

The electronic energy loss of hydrogen ions (protons and deuterons) in thin supported films of LiF has been studied in backscattering geometry for specific energies from 700 eV/u to 700 keV/u, using Rutherford backscattering spectroscopy and time-of-flight low-energy ion scattering spectroscopy. For specific energies below 8 keV/u, our data confirm velocity proportionality for the stopping cross section epsilon (like in a metal) down to 3.8 keV/u, as observed previously for protons and antiprotons despite the large band gap (14 eV) of LiF. Below 3.8 keV/u, the present results indicate an apparent velocity threshold at about 0.1 a.u. for the onset of electronic stopping.     

Low-energy antimony implantation in silicon

Abstract

Antimony is known to be a donor in silicon, Low-energy implantations of Sb in Si produce very shallow profiles which have many device applications. Gibbons et al. ¹ calulated the projected ranges of Sb ion-implanted in Si, using the LSS (Lindhard, Scharff, and Schiott) method. Oetzmann et al. ² measured projected ranges and range straggling for several heavy ions in Si, Al, and Ge, using high-resolution backscattering; in the energy region of interest to us, e = 10^?2 to 10^?1, their results were about 30% higher than those reported by Gibbons et al. In the study reported here, we implanted 5 × 10¹⁴ Sb/cm² in Si at 5–60 keV, measured the resulting depth distribution by secondary ion mass spectrometry, and checked the measurements by backscattering. Our results showed the experimental projected ranges to be about halfway between those reported in the earlier studies. The discrepancies between theoretical calculations and experimental results are due not to the electronic stopping cross section, which is negligible in the range of interest here, but to the nuclear stopping power. Using a modified nuclear scattering potential given by Wilson et al.,³ we calculated the projected range distribution according to the method described by Winterbon.⁴ Our results are in very good agreement with the experimental measurements.     

The dependence of sputtering efficiency on ion energy and angle of incidence

Abstract

Earlier measurements of sputtering efficiency of polycrystalline targets (fraction of impinging ion energy leaving the target through sputtering and backscattering) have been extended to higher energies. Lead and copper targets were bombarded with several different projectiles with energies between 80 and 1200 keV. The sputtering efficiency decreases with increasing energy. This decrease is ascribed to the combined influence of changes in the scattering cross section with energy, and to electronic stopping. The results may be described as a function of the mass ratio M ₂/M ₁ and the reduced energy ? only.

The sputtering efficiency was measured as a function of angle of incidence of the bombarding ions. To ensure complete collection of sputtered and backscattered particles, it was possible to cover only the region of incidence angle from 0° to 45°. Targets of copper, silver, and lead were investigated with 17 different ion-target combinations. The sputtering efficiency increased with angle of incidence. This increase is described well by a simple interpolation formula by Sigmund.     

Stopping power of low-energy deuterons in 3He gas

The stopping power of atomic and molecular deuterons in ³He gas was measured over the range E _d = 10 to 100 keV using the ³He pressure dependence of the ³He(d,p) ⁴He reaction yield. At energies above 30 keV, the observed stopping power values are in good agreement with a standard compilation. However, near 18 keV the experimental values drop by a factor 50 below the extrapolated values of the compilation. In a simple model, the behavior is due to the minimum 1s↦2s electron excitation of the He target atoms (= 19.8 eV, corresponding to E _d = 18.2 keV), i.e. it is a quantum effect, by which the atoms become nearly transparent for the ions.     

Analysis of the electronic structure of ZrO2 by Compton spectroscopy

The electronic structure of ZrO₂ is studied using the Compton scattering technique. The first-ever Compton profile measurement on polycrystalline ZrO₂ was obtained using 59.54 keV gamma-rays emanating from the ²⁴¹Am radioisotope. To explain the experimental data, we compute theoretical Compton profile values using the method of linear combination of atomic orbitals in the framework of density functional theory. The correlation scheme proposed by Perdew-Burke-Ernzerhof and the exchange scheme of Becke are considered. The ionic-model-based calculations for a number of configurations, i.e., Zr^+x (O^?x/2)₂ (0 ≤ x ≤ 2), are also performed to estimate the charge transfer on compound formation, and the study supports transfer of 1.5 electrons from Zr to O atoms.     

High resolution measurements of energy spectra of protons scattered from silicon crystals in the case of planar channelling

The energy spectrum of backscattered protons in the case of incidence along several planar directions shows a fine structure near the high-energy edge. This structure, an oscillatory dependence of the probability of backscattering vs. depth in the crystal, offers a possibility to study the proton trajectory in the lattice and also to obtain the stopping power of protons near planes in silicon.

Application of a simple model for the proton trajectory yields a stopping power near the planes 4 to 5 times higher than for random incidence. These effects have been observed using primary energies in the range 40–140 keV and for incidence along (110). (111), (100) and (112) planes.