S. Ferrer

Atomic structure of the CdTe(001) C(2×2) reconstructed surface: A grazing incidence x‐ray diffraction study

We present a grazing incidence x‐ray diffraction surface structure determination on a II–VI compound, namely, the CdTe(001) C(2×2) reconstructed surface, grown by molecular beam epitaxy. The structural analysis leads to a model with cadmium bridges corresponding to a coverage of 0.5 ML Cd, as expected from previous studies. This surface arrangement is accompanied by a significant relaxation of the underlying substrate down to the sixth atomic layer. Moreover, a strong anisotropy of the reconstructed domain dimensions is observed and quantified. This findings may explain the anisotropic behavior observed during homoepitaxial and heteroepitaxial growth on CdTe.

Incomplete Melting of the Si(001) Surface: A Photoelectron Diffraction Study

A high-energy photoelectron diffraction study of Si(001) at different temperatures reveals that at about 1400 K a surface phase transition occurs. It has been interpreted as being an incomplete melting. The thickness of the liquid film is estimated to be about 2 A (or two atomic layers) on the basis of single-scattering cluster calculations.

A structural study of the K adsorption site on a Si(001)2 × 1 surface: Dimer, caves or both

Abstract The atomic structure of the clean Si(100) and K covered surfaces has been investigated by Auger electron diffraction (AED) monitoring the intensities along polar scans. This technique is sensitive to the asymmetric-dimer nature of the 2 × 1 reconstruction of the Si(001) surface. Data taken at room temperature for submonolayer coverages are consistent with adsorption of K on the troughs (cave position) existing between two consecutive dimer chains along the [110] direction. At 110 K both dimer and cave sites are occupied. A mild annealing to 300 K produces an overlayer redistribution in favor of the “cave” site further indicating that this site is energetically favoured as found in some recent calculations.

Surface science done at third generation synchrotron radiation facilities

Abstract In this paper we present a few examples of surface science done at third generation synchrotron facilities. As explained in the introduction, third generation sources are characterised by a gain in brightness of three or four orders of magnitude. This allows performing experiments which were difficult or impossible before. The first part of the paper is dealing with experiments on magnetic materials and shows how dichroism and surface diffraction can bring new information. In the second part, we discuss two examples related to catalysis: the elementally resolved imaging of chemical waves and the structure of chemisorbs layers on a nickel surface at atmospheric pressure. How do atoms assemble in monatomic liquids? Do they form clusters? This question has been without answer for many years and it is only recently that an X-ray experiment has solved the problem. The fourth part of the paper describes recent results on the electronic properties of high Tc superconductors and heavy fermions, studied by high resolution photoemission. Finally, we present a prospect of a few experiments that could be done in the near future.

Structural characterisation and homoepitaxial growth on Cu(111)

Abstract A comprehensive study of the homoepitaxial MBE growth of Cu on Cu(111) is presented. This system displays a wealth of features and a large accumulation of morphological and structural defects. It is demonstrated that all of them can be ascribed to two basic characteristics of fcc-(111) faces: the presence of two threefold adsorption sites at the surface, which allows the formation of stacking faults, and the existence of high Ehrlich–Schwoebel barriers at steps, hindering interlayer diffusion. This behaviour, therefore, must be common during growth on compact metallic faces, and could have important implications for the preparation of low-dimensional heterostructures.

The structure of the Ge(001)-(2 × 1) reconstruction investigated with X-ray diffraction

Abstract The atomic structure of the room temperature (2 × 1) reconstruction of Ge(001) has been investigated with X-ray diffraction measurements by collecting an extensive and accurate set of diffracted intensities. The structural model that best agrees with the data consists of a buckled array of disordered dimers: there is a 0.5 probability of finding one of the two dimer orientations (positive and negative tilt angles) in any unit cell. The dimer tilt angle is found to be 15.6 ± 0.6°, its bond length is expanded by 4% compared to the bulk bond length, the reconstruction is found to extend to eight atomic layers and the maximum distortion of the bonds has been found to be less than 6%. The model is compared with similar models, and the dynamical and ordered models and the dynamics of the dimer flipping are discussed.

Electron loss spectroscopy study of the growth by laser ablation of ultra-thin diamond-like films on Si(100)

Abstract Carbon films with thicknesses up to 10 monolayers (ML) have been grown on Si(100) substrates by means of laser ablation of graphite under ultra-high vacuum (UHV) conditions. The early stages of the growth have been characterized by Auger-electron (AES), electron-energy-loss (EELS) and ion-scattering (ISS) spectroscopies. EELS and AES can be used to qualitatively distinguish between the graphitic or diamond-like character of the films. The effect of submonolayer coverages on the surface electronic density of the silicon substrate has also been investigated. Carbon does not diffuse into silicon for room temperature depositions. Annealing at 950 °C causes graphitization and the formation of silicon carbide together with an intermixing of C and Si.

Helium scattering study of the growth mechanism and phase transitions of Pb overlayers on Cu(100)

We present the first results of thermal energy He scattering by an heteroepitaxial system, namely Pb on Cu(100). The intensity of the specularly reflected He beam as a function of the coverage of Pb is sensitive to disordered adsorption, to the ordering of the overlayer, and to three‐dimensional cluster growth. Quantitative information, such as the number of random vacancies in the as‐deposited monolayer, can be obtained. The large differences in the cross sections for diffuse scattering between Pb adatoms on the Cu terraces and on the steps have been used to determine the heat of two‐dimensional vaporization of Pb atoms from step to terrace sites.

The early stages of growth of crystalline, diamond-like films on Si(100) by pulsed laser evaporation of graphite

Abstract Pulsed laser evaporation of graphite targets in an ultra high vacuum environment has been utilized to deposit ultra thin carbon films on Si(100) substrates. Analysis of the fine structure of the carbon Auger line reveals significant sp 3 bonding. STM images show that the films are crystalline and that the surface lattice is hexagonal.

Evaluation of the anticlastic curvature of elastically bent crystals for X-ray focusing optics

Abstract An analytical derivation of the anticlastic radius R a of a thin plate bent to a principal R p and champed from two opposite sides is performed by minimization of the elastic free energy. This calculation is an improvement of a previously published one [M. Krisch et al., Nucl. Instr. and Meth. A305 (1991) 208] and it reinforces the general conclusion indicating that with suitable crystal dimensions, the anticlastic effect which would strongly degrade the optical performances of a sagitally focusing X-ray monochromator is small, and for low divergence beams the anticlastic effect is not important except at high energies.