Showing papers by "Karsten Horn published in 2000"

Quasicrystalline valence bands in decagonal AlNiCo

Abstract: Quasicrystals are metallic alloys that possess perfect long-range structural order, in spite of the fact that their rotational symmetries are incompatible with long-range periodicity. The exotic structural properties of this class of materials1 are accompanied by physical properties that are unexpected for metallic alloys. Considerable progress in resolving the geometric structures of quasicrystals has been made using X-ray and neutron diffraction, and concepts such as the quasi-unit-cell model2 have provided theoretical insights. But the basic properties of the valence electronic states—whether they are extended as in periodic crystals or localized as in amorphous materials—are still largely unresolved3. Here we investigate the electronic bandstructure of quasicrystals through angle-resolved photoemission experiments on decagonal Al71.8Ni14.8Co13.4. We find that the s-p and d states exhibit band-like behaviour with the symmetry of the quasiperiodic lattice, and that the Fermi level is crossed by dispersing d-bands. The observation of free-electron-like bands, distributed in momentum space according to the surface diffraction pattern, suggests that the electronic states are not dominated by localization.

Symmetric versus nonsymmetric structure of the phosphorus vacancy on InP(110)

Abstract: The atomic and electronic structure of positively charged P vacancies on InP(110) surfaces is determined by combining scanning tunneling microscopy, photoelectron spectroscopy, and density-functional theory calculations. The vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge transfer level 0.75+/-0.1 eV above the valence band maximum. The scanning tunneling microscopy (STM) images show only a time average of two degenerate geometries, due to a thermal flip motion between the mirror configurations. This leads to an apparently symmetric STM image, although the ground state atomic structure is nonsymmetric.

Ethylene adsorption on Si(100)2×1: A high-resolution photoemission study

Abstract: The adsorption of ethylene on $\mathrm{Si}(100)2\ifmmode\times\else\texttimes\fi{}1$ has been investigated at room temperature by high-resolution synchrotron radiation photoemission in the exposure range: 1\char21{}1000 L. A consistent picture in favor of molecularly di-\ensuremath{\sigma} bonded ethylene is obtained from the analysis of the photoemission spectral features, which shows the progressive decreasing of the surface states in the valence band and the Si $2p$ features related to the surface dimers upon adsorption. By careful fitting of the Si $2p$ complex envelope, the appearance of a component in the Si $2p$ core-level spectrum is evidenced already after 1 L exposure to ethylene. This component, which grows with exposure, is assigned to the formation of two Si-C bonds per dimer as a result of ethylene adsorption. The line-shape analysis by model functions permits us to follow the symmetrization of Si-Si dimers, which are known to be asymmetric on the clean surface, and the growth of a Si-C related spectral component upon adsorption. Relative quantitative analysis of the various peak components suggests that the intensity increase of the new component almost completely parallels the progressive decrease of the surface dimer components. No evidence for carbide formation has been found from the C $1s$ spectrum.

Gold and silver Schottky barriers on ZnS(110)

Abstract: The evolution of the Schottky barrier between Au and Ag metal films and ZnS(110) has been studied using photoemission. Clean and well-ordered ZnS(110) surfaces were prepared by molecular beam epitaxy on cleaved GaP(110) surfaces. Chemical reaction and/or intermixing between the metal and substrate were not observed upon room temperature deposition. Substrate Zn 3d attenuation plots indicate that an initial layer-by-layer growth is followed by island growth at higher depositions. The Schottky barrier heights were found to be φBAu=2.19 and φBAg=1.81 eV, indicating a considerable dependence on metal work function. This observation agrees well with predictions of Schottky barrier heights based on the concept of metal-induced gap states and the influence of charge transfer based on electronegativities, and discussed in the light of current concepts of Schottky barrier characteristics.

Photoemission results on intralayer insertion at III-V/III-V junctions: A critical appraisal of the different interpretations

Abstract: Several researchers have proposed that band discontinuities at semiconductor heterojunctions may be “tuned” by inserting very thin layers of foreign atoms at the interface which are thought to induce an “interface dipole.” Modifications of the apparent valence-band offset, as measured by photoelectron spectroscopy (PES), have been indeed observed upon Si insertion at GaAs–AlAs interfaces, and they have been generally interpreted as real band-offset changes. However, there is an alternative explanation of the photoemission results in terms of band-bending effects. Here, we present results of PES experiments designed to test the two opposing interpretations. We have examined the effect of Si insertion at polar (100) and nonpolar (110) interfaces, and we have studied the insertion of Si (n-type) and Be (p-type) intralayers. Similar results are obtained for polar and nonpolar interfaces, and effects of opposite sign are observed for Si and Be intralayers. These results can be readily interpreted in terms of a...

Photon-excited collective modes in a surface alloy

Abstract: We investigate the influence of modifications of the surface charge-density profile on photon-excited collective excitations of valence electrons, through a comparison of the angleand energy-resolved photoyield of clean Al and the Al/Na ‘‘surface alloy.’’ Although the charge-density profile is considerably different for the clean and surface alloy systems, the Al multipole plasmon energy is essentially similar, in contrast to what one might expect from simple charge-density arguments. A Na multipole plasmon is observed only when excess Na is deposited over the surface alloy. We base a qualitative explanation for these observations on results of surface electronic structure calculations using total-energy density-functional theory.