B. Carrière

The early stages of oxygen adsorption on silicon surfaces as seen by electron spectroscopy

Abstract The oxidation of silicon surfaces has been studied by AES, XPS and LEED. Attention was focussed on the modifications induced in the AES spectra by oxygen adsorption. It has been shown that the fine structure of the silicon LW transition is changed in the early stages of oxidation. Several steps are found: the first and most stable is a SiO x layer where silicon is probably covalently bonded to oxygen. The second is a silica-like overlayer. The study of the oxygen KLL transition which can be either delocalized or quasi-atomic favors these assignments.

Shapes and shifts in the oxygen Auger spectra

Abstract The oxidation of silicon and platinum single crystal faces, of polycrystalline supported catalysts and of some alloy surfaces has been studied by AES and as far as possible by LEED. A comparison of the oxygen Auger spectra obtained during the oxidation process with those found on oxides has been made; it shows that the modification of the fine structure of the oxygen Auger peaks gives some information about the binding state of oxygen. Two different structures, which compete one with the other, are described. In one case, a spectrum where three lines dominate is obtained; in the other case, a “quasi-atomic” spectrum characterized by five features is observed: multiplet splitting in the two-hole final state is predominant. Besides these differences in the fine structure of the Auger spectra one can notice shifts of several eV for the main feature. They have been correlated with the various observed LEED patterns. Physisorption, chemisorption, solution of oxygen in the metal lattice, growth of ordered or amorphous oxides are the different possibilities which are discussed.

Oligothiophene nanorings as electron resonators for whispering gallery modes.

Structural and electronic properties of oligothiophene nanowires and rings synthesized on a Au(111) surface are investigated by scanning tunneling microscopy. The spectroscopic data of the linear and cyclic oligomers show remarkable differences which, to a first approximation, can be accounted by considering electronic state confinement to one-dimensional boxes having, respectively, fixed and periodic boundary conditions. A more detailed analysis shows that polythiophene must be treated as a ribbon (i.e., having an effective width) rather than a purely 1D structure. A fascinating consequence is that the molecular nanorings act as whispering gallery mode resonators for electrons, opening the way for new applications in quantum electronics.

Formation of a surface alloy by annealing of Pt/Cu(11

The dissolution of Pt/Cu(111) and the kinetic blocking of the dissolution process which results in the formation of a surface alloy have been studied. We use AES, PES, XPD, and LEED to characterise the non-classical dissolution behaviour. Chemical shifts on the Pt 4f72 and the Cu 3p core levels are measured during dissolution. The top layer of the surface alloy is Cu3Pt for an annealing temperature of 315°C. For the higher annealing temperature of 350°C the Pt concentration is smaller. The alloy composition is therefore temperature dependent and the formation of a relatively stable alloy in local equilibrium over the surface layers is determined by the segregation and ordering behaviour. The chemical ordering of the surface alloy and its electronic structure remains to be verified.

Growth of Pt/Cu(111) characterised by Auger electron spectroscopy, core level photoemission and X-ray photoelectron diffraction

Abstract Auger data shows a layer by layer growth of Pt/Cu(111) for the first three monolayers at room temperature. Surface core level photoemission spectroscopy using synchrotron radiation confirms the Auger analysis. Furthermore decomposition of the Pt4f 7/2 level permits quantification of the emission from the surface and interface atoms. Surface mobility of the Pt may also be inferred, coalescence of the surface layer occurs for coverages greater than 0.5 ML. XPD results confirm that the Pt grows with an fcc structure. An analysis of the anisotropies of the forward scattering peaks confirms the layer by layer growth model.

LEED analysis of ultra-thin cobalt layers grown on Cr(100)

Abstract Cobalt overlayers grown at room temperature on a chromium (100) surface have been studied with low energy electron diffraction (LEED) up to a thickness of four atomic layers. A quantitative analysis via dynamical LEED calculations is given for zero and two cobalt layers grown on Cr(100). A metastable bcc phase of cobalt is evidenced. The influence of nitrogen contamination of the films is discussed.

Evidence of epitaxial cobalt on platinum (100) surfaces

Abstract The investigation of the initial stages of growth of cobalt on Pt(100) has been undertaken to test the possibilities of obtaining modulated metallic magnetic multilayers having new interesting magnetic properties. Using surface physics methods like LEED, Auger and photoemission (synchrotron radiation), we established the growth mode to find that at room temperature no epitaxy occurs between the layers even if the growth is homogeneous. As far as magnetism is concerned, we evidenced an induced magnetic Pt state at the interface. Annealing the cobalt films in the temperature range 200–410°C leads to order the interface and bring up fcc LEED and RHEED patterns related to an epitaxial alloy overlayer.

Evidence of epitaxial growth of bcc Co on Cr(100)

Abstract We report a LEED/Auger study of the growth of cobalt ultra-thin films on Cr(100) surfaces. We demonstrate that Co can be grown epitaxially at room temperature, probably in a metastable bcc phase on this chromium surface. A 1 × 1 LEED pattern is observed at least up to 20 cobalt monolayers. The Auger data are consistent with an abrupt interface and a layer-by-layer growth mode.

Growth of cobalt ultra-thin films deposited on Pt(100) surfaces: An Auger electron spectroscopy study

Abstract To investigate the possibility of building Co/Pt modulated multilayers, attention has been paid to the early stages of interface formation between cobalt and platinum. The growth of cobalt layers less than 10 monolayers thick on Pt(100) surfaces has been studied by Auger electron spectroscopy. Growth kinetics obtained by AES show that two different models of interface formation can be possible: the Volmer-Weber case (cobalt islands) or an interdiffusion process between the two metals (similar to suicide formation). Looking at the fine structure of the low-energy platinum Auger transitions suggests that there is a strong interaction between cobalt and platinum as soon as the equivalent of a cobalt 2 monolayer coverage is deposited. This would favor the interdiffusion process model.

Preparation and characterization of Co() surfaces

We present a detailed characterization of the Co(1010) single crystal surface. In particular, we report a pre-martinsitic structural surface transition and a new interpretation of the EELS spectra of Co. Through an original kinematic approach of the LEED patterns, we discuss both the in-plane and out-of-plane surface parameters, and we show that the shorter surface termination is preferred.