D.O Boerma

Oxidation and annealing of thin FeTi layers covered with Pd

Abstract The hydrogen storage material FeTi has the disadvantage to lose its sorption capacity in contact with impurities such as O 2 and H 2 O. A possibility to overcome this problem is to coat it with an anti-corrosive layer which is permeable for hydrogen. In this study we prepared FeTi layers covered with a (4 or 20 nm) thin Pd layer. We used ion beam and sputter profiling techniques, X-ray photoelectron spectrometry and scanning probe techniques to investigate the response of these bi-layers upon annealing up to 300°C in vacuum, air and 10 −5 mbar O 2 . The layered structure remains intact up to 150°C. At 200°C in air and O 2 , Fe and (some) Ti move towards the Pd surface where they form oxide regions. At higher temperatures thicker oxide regions, presumably along the Pd grains, are formed. These processes are more pronounced for the case of 4 nm Pd. A model is presented to explain the observed phenomena. We conclude that up to 150°C 4 nm of Pd is sufficient to act as a protective layer. For a temperature of 200°C, 20 nm Pd may still provide sufficient protection against oxidation.

Low-temperature nitriding of iron through a thin nickel layer

We present a new method for nitriding iron at low temperatures. First, iron is coated with a thin layer of nickel (∼36 nm), after which it is exposed to an NH3u2002atmosphere at temperatures below 300u2009°C. Underneath the nickel layer e-Fe3−xN is formed at temperatures as low as 225u2009°C, while uncovered iron samples show a large uptake of oxygen after the same treatment. The nickel layer prevents the oxidation of iron by impurities in the NH3u2002gas, and acts as a catalyst for the decomposition of NH3. After decomposition the atomic nitrogen diffuses through the nickel layer towards the iron. With the process described, pore-free iron nitrides can be formed at low temperatures.

Nitrogen diffusion and nitride formation during and after implantation in Ni/Fe bilayers

Abstract At relatively low temperatures (room temperature and 200 °C) nitrogen was introduced into an iron layer underneath a top layer of nickel. This was done by implanting the nitrogen into the Ni layer. It was observed that subsequently part of the N diffuses into the Fe layer. The concentration depth profiles of N in the Ni/Fe bilayers were recorded with the nuclear reaction analysis technique using the 15 N(p,α) 12 C reaction at E p = 1.02 MeV. From these depth profiles, measured as a function of implantation dose, implantation temperature and anneal temperature, it was suggested that nitrides are formed in iron. To our knowledge, nitride formation has not been reported before at such low temperatures in the absence of radiation damage.

Nitrogen uptake and rate-limiting step in low-temperature nitriding of iron

Recently, a method to nitride iron in NH3 ambients at low temperature (225–350u200a°C) has been developed. In this method, the Fe is covered with a thin (∼40 nm) Ni layer, which acts as a catalyst for the nitriding process. From experiments, in which the amount of nitrogen uptake is measured as a function of nitriding time, it is concluded that the decomposition of NH3 at the Ni surface contains the rate-limiting step in this low-temperature nitriding process. From a model calculation, it is further concluded that the reaction step NH3→NH2+H at the Ni surface is the rate-limiting step with an activation energy of ∼1.5 eV.

Simulation of low-energy ion scattering

Abstract A new simulation program ‘MATCH’ has been developed for a detailed analysis of low-energy ion scattering (LEIS) and recoiling data. Instead of performing the full calculation of the three-dimensional trajectories through the sample from the ion source towards the detector, incoming trajectories as well as reversed-time outgoing trajectories are calculated, separately. Finally, these trajectories are matched to obtain the yield. The program has been tested for spectra and azimuthal scans of scattering and recoiling events of various sample species in different scattering geometries.

Bending MeV proton beams in graded composition Si1−xGex/Si layers

Abstract This paper characterises the ability of graded composition Si 1− x Ge x /Si bilayers to bend MeV proton beams through small angles. The graded germanium epilayer composition results in off-normal lattice directions of the epilayer being gradually bent, so channeled protons are gently deflected away from their initial direction. The relationship between the incident beam energy, bilayer tilt angle, emergent angle and bending efficiency for 3, 5 and 10 MeV protons are simulated using a Monte Carlo channeling code and studied experimentally using the Utrecht magnetic spectrometer. A maximum bending efficiency of 35% of those protons, which are channeled at the front surface of the epilayer is measured with the bilayer in a reverse geometry, and good agreement with simulated results is demonstrated. These results are used to predict the bending behaviour at higher beam energies and the relevant optimum layer characteristics.

Charge-state distribution in close collisions of 3 MeV C2+ ions with Ag and Au atoms

Abstract The charge-state distributions of 3 MeV carbon ions scattered over angles of 40° and 60° from sub-monolayers of Ag and Au atoms evaporated on a substrate and from thick layers of Ag and Au have been measured. A close similarity of the charge distributions in all cases is interpreted as a consequence of achieving an equilibrium-like charge-state distribution in a single ion-atom collision.

Use of an ultra-high resolution magnetic spectrograph for materials research

A brief description is given of a magnetic spectrograph for RBS and ERD analysis with MeV beams, delivered by a Tandem accelerator. With a number of examples of thin layer analysis it is shown that the spectrograph is uniquely suited for the measurement of concentration depth profiles up to a depth of about 50 nm. The positions of light and heavy atoms in epitaxial layers can be determined by using blocking, or channeling/blocking geometries in ERD or RBS measurements. Such determinations are especially interesting for samples with shallow interfaces and delta-doped layers. It is also shown that the impact-parameter dependence of energy loss and charge exchange in ion-solid interactions can be determined from measurements at samples with submonolayer coverage of elements at the surface, single crystalline samples, or (in transmission geometries) very thin layers. Some features of this spectrograph are compared with those of a selection of other high-resolution analyzers.

Study of the higher-order corrections to the impact-parameter dependence of energy loss

A method is proposed to describe the impact-parameter dependence of energy loss in ion‐atom collisions, DEObU, which is based on the local plasma frequency (LPF) approach. In this method, a linear response approach is combined with an explicit description of the energy loss to a free electron. This results in a general scheme of calculation where both the Barkas and Bloch corrections are presented. The calculated stopping cross-sections are in satisfactory agreement with the relevant experimental results. However, a serious disagreement was found by comparing the calculations with our recent results for energy losses in a thin gold foil, measured as a function of exit angle. This data reflect the impact-parameter dependence of energy loss in single ion‐atom collisions. To explain this disagreement we address the problem of proper conversion of the impact parameter scale to the angle of deflection in single ion‐atom collisions. It is argued that, due to specific quantum eAects, the deflection angle for a given impact parameter depends significantly on the eventual energy loss. Under the conditions used in the experiment this can lead to an increase by a factor of two of the variation of the energy loss in a foil as a function of exit angle. ” 2000 Elsevier Science B.V. All rights reserved.

IMPACT-PARAMETER DEPENDENCE OF ENERGY LOSS OF SWIFT LIGHT IONS IN GOLD

Abstract In this paper we present the results of measurement of the mean energy losses of 0.5 and 1 MeV/amu H + , D + , He ++ and Li 3+ ions transmitted through a gold foil of 24 nm thickness as a function of exit angle. The simultaneous manifestation of the higher-order corrections in energy loss and the non-equilibrium stopping in a thin foil has been observed.