Showing papers by "G. Le Lay published in 1994"

Growth mode and electronic structure of the epitaxial C60(111)/GeS(001) interface.

Abstract: The heteroepitaxial growth of ${\mathrm{C}}_{60}$ on GeS(001) has been studied using low-energy electron diffraction, selected-area electron diffraction, high-resolution electron microscopy, x-ray diffraction, and x-ray and ultraviolet-photoelectron spectroscopy (UPS). The simultaneous observation of diffraction spots characteristic of the substrate and the ${\mathrm{C}}_{60}$(111) overlayer allows us to specify the geometry of the epitaxy. The shape of the intensity curves of the C 1s and Ge 3d photoemission lines strongly suggests a layer-by-layer-type growth, confirmed by the observation in the synchrotron x-ray diffraction spectrum of finite-size oscillations on the (111) Bragg reflection peak of a thin ${\mathrm{C}}_{60}$(111) film. From a theoretical simulation of the C 1s and Ge 3d line-intensity curves, the mean free path of a C 1s and Ge 3d photoelectron in solid ${\mathrm{C}}_{60}$ is estimated to about 15.4 and 17 \AA{}, respectively. The plot of the film thickness versus deposition time shows evidence for a small difference in sticking coefficient between the first monolayer and the upper ones. A detailed analysis of the C 1s line shapes for normal and grazing emission suggests the existence of inequivalent carbon sites at the interface. The first valence-band feature of the substrate presents a downward band bending of about 200 meV with increasing ${\mathrm{C}}_{60}$ coverage. From the shift of the cutoff in the UPS spectra we deduce a work function increase of about 100 meV upon monolayer adsorption. The characteristic spectral features of ${\mathrm{C}}_{60}$ observed in the UPS spectra for bulk fullerite are slightly broadened and shifted to lower binding energies at submonolayer coverages and show no direct evidence for significant hybridization, indicating that the ${\mathrm{C}}_{60}$-substrate interaction is mainly dominated by van der Waals bonding. All these observations can be explained by a positive effective dipole of about 8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}31}$ C m induced on the ${\mathrm{C}}_{60}$ molecule upon adsorption onto the GeS substrate.

Alkali-Metal-Induced Highest Fermi-Level Pinning Position above Semiconductor Conduction Band Minimum

Abstract: The room temperature deposition of small amounts of Cs on the InAs(110) surface induces the highest Fermi-level pinning position (~ 0.6 eV) above the conduction band minimum ever met for any semiconductor. The Fermi-level movement is monitored by core level photoemission spectroscopy using synchrotron radiation. This striking behaviour is explained in terms of donor-type surface states induced by few Cs atoms present on InAs(110) and suggests the existence of a two-dimensional electron gas at the surface.

Cs‐induced highest EF jump above InAs(110) conduction‐band minimum

Abstract: Upon room‐temperature deposition of minute amounts of Cs on InAs(110) surfaces, one induces probably the highest Fermi‐level pinning position (≊0.6 eV) for a semiconductor above the conduction‐band minimum. Synchrotron‐radiation core‐level photoemission spectroscopy was used to follow the Fermi‐level movement from the shift of the In 4d and As 3d core levels as a function of Cs coverages at room temperature. Already at very low coverages the Fermi level reaches an extremely high maximum above the conduction‐band minimum. The maximum of the Fermi‐level position correlates fairly well with the ionization energy of the individual atoms, as expected in the framework of the theory of donor‐type surface states induced by metallic adatoms. We thus consider that individual Cs adatoms produce donor‐type surface states placed at ≊0.6 eV above the conduction‐band minimum. This induces a very strong downward band bending which suggests the existence of a two‐dimensional electron gas at the open, nearly clean InAs(110) surface.

Schottky limit on ideally H-terminated unpinned silicon(111) surfaces

Abstract: Lead overlayers, condensed in situ on ideally H-terminated Si(111) surfaces, prepared by wet chemical procedure, have been studied by high resolution synchrotron radiation photoemission spectroscopy. Flat band conditions are initially fulfilled for both n- and p-type samples. Starting with these unpinned surfaces, the simple Schottky limit is obtained for a prototypical, inert system: Pb/Si(111) 1×1-H prepared under UHV. These results are unique compared to most of the known metal-silicon interfaces.