Ch. Ziethen

Recent progress in photoemission microscopy with emphasis on chemical and magnetic sensitivity

Abstract With the improved access to synchrotron radiation sources photoemission electron microscopy is developing into a versatile analytical tool in surface and materials science. The broad spectral range and the well-defined polarization characteristics of synchrotron light permit a unique combination of topographic, chemical, and even magnetic investigations down to a mesoscopic scale. The potentiality of photoemission electron microscopy is demonstrated by several experiments on surfaces and microstructured thin film systems, which have been carried out with a newly designed instrument. We discuss its different modes of operation with respect to both microscopy and spectroscopy. A combination of elemental selectivity and magnetic sensitivity is achieved by using circularly polarized soft X-rays and exploiting the effect of magnetic circular dichroism. This way one obtains information about the magnetic state of individual chemical components within the sample.

Nanoelectron spectroscopy for chemical analysis : a novel energy filter for imaging x-ray photoemission spectroscopy

An ovel instrument for imaging ESCA is described. It is based on a tandem arrangement of two hemispherical energy analysers used as an imaging energy filter. The main spherical aberration (α 2 -term) of the analyser is corrected by the antisymmetry of the tandem configuration. The kinetic energy range useable for imaging extends up to 1.6 keV; this is compatible with Mg and Al Kα laboratory x-ray sources. First experiments on the chemical surface composition of a Cu0.98Bi0.02 polycrystal, a GaAs/AlGaAs heterostructure and Ag crystallites on Si(111) have been performed using synchrotron radiation. The results reveal an energy resolutio no f190 meV and a lateral resolution (edge resolution) of 120 nm. Besides elimination of the analyser’s spherical aberration, the tandem arrangement largely retains the time structure of the electron signal, unlike a singl eh emispherical analyser.

Microspectroscopy and spectromicroscopy with photoemission electron microscopy using a new kind of imaging energy filter

The use of an imaging retarding field analyser attached to the FOCUS IS-PEEM is described. This kind of energy filter is a simple, powerful tool to obtain microspectra from areas of down to about 1 μm using (V)UV and X-ray excitation sources. First results of microspectroscopy measured by excitation with a laboratory as well as a synchrotron X-ray source are presented.

Fast elemental mapping and magnetic imaging with high lateral resolution using a novel photoemission microscope

Abstract Using tunable soft X-ray synchrotron radiation and a new-generation photoemission electron microscope with integral sample stage and microarea selector, elemental images and local XANES spectra have been measured. Given the present conditions (PM3 at BESSY), the lateral resolution was in the range of 130 nm with the potential of considerable improvement with high-brilliance sources (a base resolution of 25 nm was obtained in threshold photoemission). Measurements at the oxygen K-edge demonstrate that differences in the local chemical environment of the emitter atom are clearly revealed and can thus be used as a fingerprint technique for its chemical state and geometrical surroundings. By exploiting the magnetic circular dichroism effect it was possible to view magnetic domains and domain walls.

Element-Selective Magnetic Imaging in Exchange-Coupled Systems by Magnetic Photoemission Microscopy

We have used a photoemission microscope to obtain element-resolved magnetic contrast in stacked magnetic thin film systems. Magnetic information is thereby provided by X-ray magnetic circular dichroism. Elemental sensitivity, which is crucial for studying magnetic coupling phenomena in systems with several different layers, is achieved by tuning the energy of the illuminating photons to atomic absorption edges. We present measurements of a Ni-coated Co micropattern on Cu(001), and a wedged Co/Cr/Fe(001) sample. In the former sample the Ni magnetization is seen to follow the magnetization of the Co pattern, thereby changing from an out-of-plane easy axis in areas without underlying Co to in-plane on top of the Co microstructures. In the latter a reversal of the exchange coupling of the Co layer to the Fe magnetization is observed when the Cr layer thickness exceeds approximately two monolayers. A small net magnetic moment is also observed in the Cr spacer layer, which follows in sign the Co magnetization at the reversal of the exchange coupling. This finding is discussed in terms of interface roughness or interdiffusion.

Soft X-ray photoelectron microscopy used for the characterization of diamond, a-C and CNx, thin films

Abstract This article gives an overview about the application of X-ray photoemission electron microscopy (X-PEEM) used for the analysis of carbon thin films. We present the results of an X-ray absorption near edge structure (XANES) study of CVD diamond, a-C and CNx films on Si (100) as well as a defect analysis of a hard disc scratch test. The sp2/sp3 ratio of the carbon films was determined and mapped in the electron micrographs, which show localized defects in the surface.

Peculiarities of imaging one- and two-dimensional structures in an emission electron microscope. 1. Theory

Local changes in work function cause deviations of the electrical microfield near a sample surface as a result of the uniform accelerating field distribution between the sample (cathode) and the extractor electrode (anode). This results in a change in the electron trajectories. As a consequence, the microscope image shows remarkable changes in position, size, intensity and lateral resolution of distinct details, which can be quantitatively described by the calculations presented here. Analysing these effects in the image gives an opportunity to determine the real lateral size of the observed structures and the distribution of local contact potentials.

Size dependence of magnetic domain patterns in exchange-biased Permalloy/NiO microstructures

The magnetic domain structure in Permalloy (Ni81Fe19) micropatterns (10?100??m) on NiO has been investigated by means of soft x-ray photoemission electron microscopy. The exchange anisotropy between the Ni81Fe19 patterns and the NiO layer results in the formation of complex domain structures which markedly differ from the simple Landau?Lifshitz configurations. The domain structures reflect the competition between the exchange anisotropy and the dipole?dipole interaction in a weakly coupled system. The observed domain structures change with the feature size, as the domain patterns lose complexity in the smaller structures.

CHEMICAL MICROANALYSIS BY SELECTED-AREA ESCA USING AN ELECTRON ENERGY FILTER IN A PHOTOEMISSION MICROSCOPE

Abstract We present a new and simple device for microspectroscopy being independent of the mode of electron-excitation. Micro-X-ray photoelectron spectroscopy, electron-induced Auger-spectroscopy, as well as local energy-loss spectroscopy were used to investigate metal-adsorption on silicon. This new approach employs a non-imaging electron energy analyser attached to a new-generation photoemission electron microscope with integral microarea selector. Photoelectron microspectroscopy was performed using the direct beam of an undulator (U2 at BESSY) being monochromatised and focused by means of multilayer optics at hv = 95 eV. Similarly, local Auger-electron and EELS spectra have been taken using a simple electron gun for the excitation. The chemical compositions of inhomogenities in thin layers of indium on silicon and the local state of oxidation of a structured Pt Co multilayer have been determined.

IMAGING OF DICHROISM IN PHOTOEMISSION ELECTRON MICROSCOPY AT NONMAGNETIC MATERIALS USING CIRCULARLY POLARIZED SOFT X-RAYS

A new approach for investigations of circular dichroism in the angular distribution of photoelectrons (CDAD) is presented. The image contrast using a photoemission line of a certain material is combined with imaging of the angular distribution pattern using a photoemission electron microscope (PEEM). CDAD can be used to investigate pure scattering information by means of the same instrument in microscopically selected regions on a surface. The experiment combines angle-resolved XPS imaging with the indirect mapping of the local environment of atoms by means of CDAD holography. In a conventional photoelectron diffraction or photoelectron holography experiment, it is necessary to move the sample and/or the detector; for example, a rotatable electron analyzer is used to map a full angular pattern. In the present approach, the diffraction plane is mapped for a certain kinetic energy of electrons in the backfocal plane. An imaging high-pass energy filter has been used in combination with the PEEM in order to acquire surface images and/or diffraction pattern maps. Direct photoemission from the W-4f core levels was observed at different photon polarization to obtain two images and two diffraction patterns of the W-4f emission line using light of opposite helicity. The agreement of calculations with the experimental result is satisfactory. This constitutes a very encouraging starting point for the use of the CDAD holography together with the PEEM technique providing mesoscopic and atomic as well as chemical resolution in microselected areas on solid surfaces.