Surface Extended X-ray Absorption Fine Structure Data Research Articles

GROWTH OF ULTRATHIN Mn FILMS ON Ag(100)

Mn films with a thickness of one monolayer have been deposited, in ultrahigh vacuum, on a Ag (100) single crystal at room temperature, and analyzed by X-ray photoelectron diffraction (XPD) and surface extended X-ray absorption fine structure (SEXAFS). The strong enhancement of the X-ray photoelectron Mn2p3/2 signal along the Ag[1100] direction indicates that a substantial fraction of the Mn atoms is located within the second topmost atomic layer. The SEXAFS data demonstrate that the Mn – Ag and Mn – Mn first neighbor distances are the same as the Ag – Ag distances in the Ag lattice; as a result, the atoms occupy Ag sites on the fcc lattice. Comparison of the experimental data with FEFF calculations indicates that a superficial Mn – Ag alloy is formed. If the film is mildly (330 K) annealed, the contrast in the XPD Mn 2p 3/2 modulations versus polar angle increases and reaches a maximum value of 60%. As confirmed by single scattering simulations, these results mean that atomic place exchange occurs between Mn and Ag atoms, and eventually the second atomic plane of the sample is constituted mainly by Mn atoms, whereas the first atomic layer is a pure Ag plane, i.e. a buried atomic Mn monolayer is formed.

Interfacial structure of two-dimensional epitaxial Er silicide on Si(111).

Auger-electron diffraction (AED) and surface-extended x-ray-absorption fine structure (SEXAFS) have been used to obtain a complete description of the atomic structure of a two-dimensional epitaxial Er silicide layer on Si(111). AED reveals that a monolayer of Er is located underneath a buckled Si double layer. The relevant Er-Si interlayer spacings are determined by means of single scattering cluster simulations and a R-factor analysis to be 1.92\ifmmode\pm\else\textpm\fi{}0.05 \AA{} to the first and 2.70\ifmmode\pm\else\textpm\fi{}0.05 \AA{} to the second Si top layer. Er near-neighbor bond lengths and coordination numbers are obtained independently from polarization-dependent SEXAFS. The SEXAFS data, when combined with the Si top-layer geometry inferred from AED, permit the determination of the atomic positions at the silicide/Si(111) interface. The Er is found to reside in relaxed ${\mathit{T}}_{4}$ sites of Si(111) with a single Er-Si distance of 3.09\ifmmode\pm\else\textpm\fi{}0.04 \AA{} to the first- and second-layer Si atoms of the substrate.