N. Jedrecy

The hexagonal polar ZnO(0001)-(1×1) surfaces: structural features as stemming from X-ray diffraction

The ZnO surfaces, apart from their role in catalytic processes, raise fundamental questions regarding the key parameters, which stabilise ionic material surfaces. The ZnO structure consists of hexagonal planes stacked with the sequence aBbAaB… The polar (0001) surfaces should be unstable because of the normal dipole moment. However, a long-range (1×1) order is obtained after several Ar+-800°C cycles. We investigated, by grazing incidence X-ray diffraction (XRD), both the O- and Zn-terminated faces, issued from a single substrate. In both cases, the coherent domain width was found between 180 and 100 A. This is the average size of the terraces, limited by bilayer steps, and thus either a or b terminated. Several intensity rods (along the surface normal) were analysed, attesting that the surface is close to bulk truncation (no stacking fault). The effects of relaxation or non-stoichiometry are strongly damped by the contribution of the two types of terraces. In the Zn case, the atomic displacements are not significant (outward relaxation of +0.05 A), whereas in the O case, four layers have to be displaced (inward relaxation by −0.3 A). The fits are greatly improved allowing for partial occupancy in the topmost planes: 0.75 for the external plane in the Zn case, a result which fits well with the electrostatic arguments for the cancellation of the dipole moment, and 1.3, 0.7, for the two topmost planes on the O surface. Impurity or Zn atoms substituting the O atoms could explain these latter values.

Structural approach of the Fe-Si(1 1 1) annealed interface

The Fe-Si(1 1 1) surface phase diagram has been the object of several updates in the recent years and even for epitaxial FeSi2, three different structures have been proposed on the basis of electron diffraction data: the room temperature stable β-FeSi2, the metastable ψ phase (flourite type) and a cubic CsCl type with a random distribution of Fe vacancies. We present here X-ray diffraction experiments performed both under ultra-high vacuum on 2 × 2 reconstructed annealed interfaces in the monolayer range and on encapsulated silicide epitaxial films. Fe deposits annealed above 540°C lead to β-FeSi2 and evidence for the six variants expected for a B type epitaxy is given. Annealing a 10 monolayers Fe deposit at 500°C leads to a metastable cubic phase derived from the tetragonal α phase (stable at high temperature) and not to the random defect CsCl structure. For ultrathin deposits, the correlation between the 2 × 2 reconstruction, primarily due to Si adatoms, and the structure of the underlying silicide-like layer is discussed. Anomalous dispersion at the Fek edge has been used to improve the accuracy of the Fe site assignment.

Coupling between spatial and angular variables in surface X-ray diffraction: effects on the line shapes and integrated intensities

The intensity line shape, as provided by a rocking scan, and the derivation of the structure factor from the integrated intensity are reviewed in the context of surface X-ray diffraction, focusing on the z-axis geometry. In a first step, under the assumption of a Dirac-like rod and a point-like sample, the effects of the detector slit settings on the scan width and on the integrated rod height are described. In a second step, it is shown that it is incorrect to treat the integrated intensity as being proportional to the active area A of the surface, defined as the sample area that is illuminated by the incident beam and viewed by the detector. Indeed, one must take account of the changes in the scattering direction that occur during the θ scan, and define at every θ the surface fraction A(θ) that scatters into the detector. In a third step, a rod with finite width is considered, and the spilling and travelling of the diffracted spot, arising from the centre of the sample, over the detector window is described. The spots emerging from any other sample position are then considered. By coupling spatial and angular variables, the scan line shape can be simulated quantitatively, by means of the in-plane intensity distribution of the rod. The resulting integrated intensity provides the correction factor to be applied to the raw data for the derivation of the structure-factor amplitude. This correction factor is compared to the usual correction (ALΔl), where A is assumed to be constant, L is the Lorentz factor and Δl is the l range as integrated during the scan (in the context of the Dirac-like rod). Significant differences occur at large l values when using grazing-incidence conditions.

High resolution X-ray scattering investigation of Pt/LaF3/Si(1 1 1) structures

Recently developed high resolution X-ray methods have been used to characterize the structure of Pt/LaF 3 /Si samples. The asymptotic Bragg di⁄raction is used for the study of the LaF 3 /Si interface and the grazing-incidence X-ray di⁄raction (GIXD) for the study of the LaF 3 texture. The LaF 3 /Si interface was found to be thin and smooth with the interface thickness value 0.55

Anomalous Scattering Applied to Co/Si(111) Interface Structure

0.1 nm. The essential extra-broadening of the main peak in di⁄raction experiments for samples with the top platinum layer is explained by the small angle scattering of the X-ray beam on the rough Pt film. The LaF 3 -film domain structure is derived from GIXD data. ( 1998 Elsevier Science B.V. All rights reserved.

Geometrical and compositional structure at metal-oxide interfaces: MgO on Fe(001).

The anomalous scattering effect is used in a grazing incidence X-ray diffraction (GIXD) experiment to perform structural analysis of the Co/Si(111) 7×7 reconstructed interface. Data have been collected at different wavelengths, on both sides of the Co absorption K edge. The 7×7 ordering of the Co chemisorbed atoms is confirmed by variations in intensity of fractional order Bragg reflexions when changing X-ray energy.