M. Bertolo

Spectromicroscopy beamline at ELETTRA: Performances achieved at the end of commissioning

In the course of 1998, the Spectromicroscopy beamline at ELETTRA completed commissioning and succeeded in performing its first test experiments. The beamline is designed to perform photoemission experiments with high spatial resolution, which is obtained by focusing the radiation in a small spot on the sample by means of a multilayer-coated Schwarzschild Objective. Three objectives are currently available; these operate at photon energies of 74, 95, and 110 eV. A review is presented of the performances achieved together with an outlook on the future upgrades of the microscope. The smallest achievable spot size is currently 0.5 mu m. At present, the limit to the spatial resolution is due to aberrations caused by figure errors of the objective. Typical counting rates in photoemission spectra, for example, on the Au 5d peak, are of the order of 10(4)-10(5) counts per second with an energy resolution of the order of 100-200 meV. Among the first experiments in which p- and n-type GaAs layers of 0.25 mu m thickness were imaged

Writing submicrometric metallic patterns by ultraviolet synchrotron irradiation of nanostructured carbon and TiOx–carbon films

Nanostructured carbon and carbon–TiOx films produced by supersonic cluster beam deposition have been characterized by spatially resolved (0.5 μm lateral resolution) photoemission spectroscopy with synchrotron light. We have found that pure and TiOx-containing nanostructured carbon is modified under high flux UV exposure showing the onset of a metallic character near the Fermi level. We have studied the spectroscopic features, the spatial confinement, and the kinetics of this metallization process of the irradiated regions. This effect allows one to write metallic patterns in the TiOx-containing films with a submicrometric resolution and with a fast kinetic.

Surface analyses of In–V oxide films aged electrochemically by Li insertion reactions

The surface of a Li-insertion In–V mixed-oxide electrode and the evolution of its composition and morphology upon prolonged intercalation/deintercalation cycles have been studied by a combination of the following bulk and surface-sensitive analytical techniques: Electrochemical analysis (cyclic voltammetry, impedance spectroscopy), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and scanning photoelectron spectromicroscopy (SPEM). The In–V oxide film, obtained by rf sputtering as a thin film on conductive glass for use as a transparent electrode in electrochromic windows, is a Li-insertion material with good reversibility and high charge capacity, having a surface morphology characterised by a low initial roughness. XPS results showed that Li+-insertion brought about the reduction of V5+ to V4+ and V3+, with the onset of new structures in the valence band, related to the formation of Li2CO3 on the electrode surface. Such chemical changes were largely reversed upon Li-deinsertion and only a barely detectable effect on the surface morphology was seen after up to ten cycles. Extensive Li-charge–discharge cycling induced larger changes in the electrode morphology (increase in grain size and roughness), in the surface composition and microscopic aspect, with a partial passivation effect on the film electrochemical behaviour. The formation of surface deposits with a characteristic, elongated shape has been revealed to occur after 500 Li charge–discharge cycles. Submicron chemical analysis of such deposits onto aged oxide electrodes has been performed by SPEM, which enabled the location and assignment of the surface composition.

Cross sectional studies of buried semiconductor interfaces by means of photoemission microscopy

An important application of photoemission spectromicroscopy would be to measure heterostructures and semiconductor devices in cross section to directly determine band offsets and band bending. We present here studies of p-n GaAs homojunctions and Al/GaAs Schottky junctions fabricated by molecular-beam epitaxy. Our results suggest that a minimum experimental uncertainty of about 0.15 eV will effect band offset determination. In general, useful quantitative information on the junction electrostatics can be obtained provided that the experimental data are analyzed to substract the diffuse photon background and take into account the intensity profile of the photon spot.

Highly sensitive detection technique of buried defects in extreme ultraviolet masks using at-wavelength scanning dark-field microscopy

A technique to probe defects buried inside extreme ultraviolet (EUV) masks has been implemented using a dark-field microscopy detection setup. Specific samples have been fabricated to evaluate the sensitivity of this technique. They consist of silicon oxide gratings of a few nanometers height, coated with 40 layer pairs of molybdenum–silicon. We observed images with a good contrast on samples with defects as low as 3nm. However, the imaging mechanism of scanning dark-field microscopy is not linear and can produce image distortions. Conditions of correct imaging have been analyzed, and simulations have been performed that show good agreement with the experimental data. This work opens the way for a better understanding of the capability of at-wavelength inspection technique for EUV mask.

A microspectroscopic study of the electronic homogeneity of ordered and disordered Sr2FeMoO6

Disordered Sr2FeMoO6 shows a drastic reduction in saturation magnetization compared to highly ordered samples, moreover magnetization as a function of the temperature for different disordered samples shows qualitatively different behaviours. We investigate the origin of such diversity by performing spatially resolved photoemission spectroscopy on various disordered samples. Our results establish that extensive electronic inhomogeneity, arising most probably from an underlying chemical inhomogeneity in disordered samples, is responsible for the observed magnetic inhomogeneity. It is further pointed out that these inhomogeneities are connected with composition fluctuations of the type Sr2Fe1+xMo1-xO6 with Fe-rich (x > 0) and Mo-rich (x < 0) regions.

PHOTOEMISSION MICROSCOPY INVESTIGATION OF BURIED p–n GaAs HOMOJUNCTIONS AND Al/n-GaAs SCHOTTKY BARRIERS

A natural application of the emerging technique of photoemission microscopy to the study of semiconductor interfaces involves measuring a device in cross section to directly determine heterojunction parameters. We present here results on p–n GaAs homojunctions, which served as a prototype system to demonstrate the applicability of this technique to buried semiconductor interfaces. We also describe preliminary measurements of the electrostatic potential profile across Al/GaAs Schottky junctions.

Cross-Sectional Photoemission Spectromicroscopy of Semiconductor Heterostructures

A natural application of the emerging technique of photoemission microscopy to the study of semiconductor interfaces is direct determination of heterojunction parameters by measuring the device in cross section. We present here results on p-n GaAs homojunctions, which served as a prototype system to demonstrate the applicability of this novel technique to buried semiconductor interfaces. We also describe preliminary measurements of the electrostatic potential profile across Al/GaAs Schottky junctions.

RECENT ADVANCES IN IMAGING WITH SPECTROSCOPIC ANALYSIS AT ELETTRA

The paper is a brief overview of the efforts and achievements at Elettra in development of instrumentation for imaging and spectroscopic analysis using different spectral range of the synchrotron radiation. Selected results will illustrate the potential of these techniques for multi-disciplinary applications.

Photoemission spectromicroscopy study of a Bi

Using photoemission spectromicroscopy at sub-micron lateral resolution we studied two superconductor systems of high fundamental and practical importance, such as Bi2Sr2CaCu2O8+delta and MgB2. Our findings demonstrate the importance of considering, for this class of systems, the possible presence of spatial inhomogeneities in interpreting the results of conventional photoemission experiments, which typically probe an area of the order of 1 mm(2). In particular, in the case of MgB2, we report a measured density of states directly comparable to the theoretical predictions, thus rejecting the claim for the existence of strong correlation effects proposed by previous spatially averaged measurements.