R. Cortenraad

A round robin experiment of elemental sensitivity factors in low-energy ion scattering

In a round robin experiment a set of five polycrystalline, metallic samples is studied by low-energy ion scattering (LEIS) in five different laboratories, The energy range is 0.6-3.5 keV and He and Ne ions are used. Even though different experimental setups are used the evaluated elemental sensitivity factors agree within +/-20%. Reproducibility within single laboratories is better than 10%. In an additional study carried out in three laboratories the surface composition of an alloy, Cu55Pd45, was determined, using in situ calibration standards, These surface composition measurements agreed within +/-3 at% demonstrating that quantitative composition determination is possible using this procedure

Influence of analyser transmission and detection efficiency on the energy dependence of low‐energy ion scattering signals

The signal intensity in low-energy ion scattering depends, among others, on the kinetic energy of the scattered ions. The measured energy dependence is a combination of the intrinsic energy dependence, determined by the ion- sample interaction, and the energy dependence of the instrumental response function. The intrinsic energy dependence is often exploited to obtain information on the charge transfer processes taking place between the ions and the sample surface. The resulting information is of relevance both for quantitative surface analysis as well as for obtaining a fundamental understanding of charge transfer processes. For a correct determination of the intrinsic energy dependence it is crucial that the energy dependencies of the analyser transmission and the ion detection efficiency of the instrument are accurately known. In this paper we outline how these energy dependencies may be obtained, and we present the results for the cylindrical mirror analyser and microchannelplate-based detector in our apparatus. The discussions about the energy dependencies are not specific for our apparatus, but can be applied directly to other cylindrical mirror analysers and channelplate-based detectors. Therefore, the similarities and differences with other types of analysers are briefly discussed. The implications of uncertainties in these energy dependencies for charge-exchange studies and quantitative compositional surface analysis are treated in detail.

Quantitative LEIS analysis of thermionic dispenser cathodes

An UHV LEIS setup has been converted into a dedicated apparatus to study the surface composition, structure and dynamics of real dispenser cathodes and cathode model systems based on W single crystals. LEIS, AES and LEED are available to investigate the surface characteristics, and the cathode emission properties are derived in situ from a close-spaced diode configuration. In this paper, the focus is on the quantitative surface composition of B-type and M-type dispenser cathodes by LEIS. A straightforward quantification is hampered by the influence of the cathode workfunction on the neutralisation of the ions. It is shown that the ion fraction decreases as the workfunction of the cathode decreases. The Ba surface density is observed to increase with decreasing workfunction. However, before an accurate quantitative surface analysis can be performed a validation of the model used to correct for the influence of the ion fraction has to be performed.

In situ surface analysis by low energy ion scattering

We discuss the possibilities to apply low energy ion scattering (LEIS) for in situ surface analysis of devices and functional materials. First we discuss the limitations imposed by the technique such as pressure and spatial resolution. Next, we present a new method to suppress backgrounds in LEIS spectra due to the presence of light elements by using a combined electrostatic and flight-time analysis. Examples of in situ analysis on thermionic cathodes are presented which shed light on the surface structure and composition of these cathodes, and also allow us to study the resistivity of such cathodes to ion bombardement. Finally, a design is presented for a pressure cell for in situ surface analysis of catalysts during catalytic reaction.

Dynamic behavior of thermionic dispenser cathodes under ion bombardment

We have investigated the surface coverage and electron emission of thermionic dispenser cathodes during 3 keV Ar+ ion bombardment, thereby simulating the bombardment of the cathodes by residual gases that takes place in cathode-ray tubes as used in television sets. During the ion bombardment at the operating temperature of 1030 °C, a dynamic equilibrium is established between the sputter removal and resupply mechanisms of the Ba and O atoms that form the dipole layer on the cathode substrate. We demonstrated that the performance of the cathodes under ion bombardment is governed by the O removal and resupply rates. It was found that the Ba resupply rate is almost an order of magnitude higher than the O resupply rate, but that the Ba can only be present on the surface bound to O atoms. Therefore, the Ba/O ratio is approximately equal to unity during the ion bombardment. Based on the investigations of the removal and resupply processes, we proposed a model that accurately describes the surface coverage and e...

Growth and characterization of monocrystalline tungsten substrates

High-purity tungsten single crystals of different crystallographic orientations were prepared by electron beam floating zone melting as well as by a strain-annealing technique. The substructure of these substrates was examined by SEM, optical microscopy and X-ray diffraction methods. Samples were prepared with different crystallographic planes and with substructures free from small-angle boundaries. The samples had excellent structure parameters, as deduced from the full-width at half-maximum of the rocking curves which was less than 0.1° for the substrates prepared by strain annealing. The substrates were subsequently cleaned in UHV using a newly designed compact electron beam gun for heating to 2500°C. The surface cleaning procedures were studied using AES, LEIS and LEED. Clean and well-ordered surfaces were obtained.

Cleaning procedure for single-crystal tungsten substrates

Single-crystal tungsten substrates produced by electron-beam melting were cleaned by annealing in an oxygen atmosphere (∼10–3Pa, ≃1500°C) for a few hours to remove adsorbed carbon and by high-temperature ultrahigh-vacuum annealing (≃2 × 10–8Pa, ≥2500°C) for a few minutes to remove residual oxygen. The process was followed by low-energy ion scattering, Auger electron spectroscopy, and low-energy electron diffraction. The carbon and oxygen adsorbed on the surface of single-crystal tungsten were found to form ordered, oriented structures. The resultant high-purity, structurally perfect W substrates were used to study electron emission in the Ba/W and Re/W systems.