Photoelectron Diffraction Patterns Research Articles

Structural analysis of Fe/Ni(001) films by photoelectron diffraction

The structure of Fe films, epitaxially grown on Ni(001), has been studied in the 0--14 ML coverage range by means of photoelectron diffraction (PD) in the forward scattering regime. Quantitative analysis by a multiple scattering approach has been performed on Fe films at a coverage of 3 and 7 ML. Analysis of the 3-ML data showed that growth was not layer-by-layer but rather occurred through islands nucleation and that transition from the pseudomorphic fcc to the bcc phase was located in this early stage of growth. In fact, best fit was obtained by calculations on a 2 ML bcc(110)/3 ML fcc(001) Fe film with the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ in-plane orientation. Interlayer spacings of $2.05\ifmmode\pm\else\textpm\fi{}0.06\AA{},$ $2.01\ifmmode\pm\else\textpm\fi{}0.03\AA{},$ and $1.85\ifmmode\pm\else\textpm\fi{}0.03\AA{}$ were found in the bcc region, between bcc and fcc layers and in the fcc region, respectively. Best-fit in-plane nearest-neighbors (n-n) distance was $2.49\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ in registry with that of the Ni substrate. To analyze the 7-ML data a 4 ML bcc(110)/3 ML fcc(001) film was employed, varying the fitting parameters in the bcc region only. Best fit was obtained for an interlayer spacing of $2.04\ifmmode\pm\else\textpm\fi{}0.04\AA{}$ and in plane n-n distance of $2.47\ifmmode\pm\else\textpm\fi{}0.01\AA{}.$ At 14 ML the PD pattern collected over a 94\ifmmode^\circ\else\textdegree\fi{} azimuthal range displayed symmetry around the [110] substrate direction, which was explained by the equipopulation of the 4 bcc(110) domains satisfying the $\mathrm{bcc}〈111〉\ensuremath{\parallel}\mathrm{fcc}〈110〉$ alignment.

DETERMINATION OF TiO2(110) SURFACE RELAXATION BY VARIABLE POLARIZATION PHOTOELECTRON DIFFRACTION

A new method to determine the surface relaxation by means 0 of photoelectron diffraction (PED) from an s-core level has been applied to the (1 × 1)- TiO 2(110) surface. Complete 2π solid angle PED patterns for both oxygen 1s and titanium 3s core level peak intensities have been measured at the ALOISA beamline (Elettra Synchrotron, Italy) in two different photon polarization/electron detection geometrical configurations: one with the photon polarization vector parallel to the surface normal (TM polarization) and the other with the photon polarization vector perpendicular to the surface normal (TE polarization). In the latter case the PED sensitivity to the vertical spacing is enhanced. Because forward scattering is suppressed on the node of the primary wave along the surface normal, the relative weight of the distance-sensitive higher order interference fringes is enhanced. The opposite occurs for the TM polarization, which enhances the PED sensitivity to lateral spacing at grazing emission angles. The experimental PED patterns are compared with the theoretical simulations calculated for the relaxed and unrelaxed TiO 2(110) surface. The results prove the feasibility of the method and the adequacy of the PED technique to the studies of relaxation phenomena at crystal surfaces.

STRUCTURE OF Si(001)-(4×3)-In SURFACE STUDIED BY X-RAY PHOTOELECTRON DIFFRACTION

X-ray photoelectron diffraction (XPD) patterns of In 3d core levels have been measured for the Si(001)-(4×3)-In surface. An R factor analysis with single-scattering and multiple-scattering simulations of In 3d XPD patterns was performed for three structural models proposed so far. Only the model proposed by surface X-ray diffraction [Appl. Surf. Sci.123/124, 104 (1998)] appeared to give a reasonably small R factor when the geometric parameters were modified from the original ones.

Preparation of epitaxial ultrathin RuO 2–TiO 2(110) films by decomposition of Ru 3(CO) 12

Ru 3(CO) 12 has been used as a source for depositing metallic ruthenium particles and RuO 2 films over a TiO 2(110) surface. At 300°C in ultrahigh vacuum the precursor is completely decomposed, and the deposited metal particles show some residual carbon contamination. Such a deposit is disordered, both in the short and long range, and the high binding energy found for the metal suggests the presence of nano-clusters on the surface. The Ru–TiO 2 interface seems to be non-reactive, in the sense that the titanium atoms of the substrate are not reduced by reaction with the incoming metal atoms. On the other hand, RuO 2 ultrathin epitaxial films are easily obtained either by heating the metal deposit at 300°C or by directly decomposing the organometallic precursor in an oxygen atmosphere (2×10 −6 mbar). These films grow in a three-dimensional cluster-like way and are epitaxial, as demonstrated by the presence of well-defined X-ray photoelectron diffraction patterns. The new RuO 2–TiO 2 interface is, on the contrary, reactive and there is some evidence for the presence of some charge transfer from the electron-rich RuO 2 toward the electron-poor TiO 2.

EPITAXIAL GROWTH OF SrF2 ON Ge(111)-c(2×8) AS STUDIED BY PHOTOELECTRON DIFFRACTION AND HOLOGRAPHY

We have investigated the photoelectron diffraction (PED) from a SrF 2 epitaxial thin film on Ge(111) from the viewpoint of holographic inversion for imaging the atomic structure. The holographic interference fringes were retrieved from the PED pattern including a wide variety of diffraction features via averaging the pattern around each forward-scattering peak. Furthermore, another method for enhancing holographic fringes by subtracting one photoelectron hologram from another at slightly different kinetic energies was theoretically investigated.

Disappearance of element-specific Kikuchi bands from fluoride surfaces

We measured scanned-angle x-ray photoelectron diffraction (XPED) patterns from both SrF2(111) and MgF2(001) surfaces, which exhibit complex element-specific characteristics: (1) A set of Kikuchi bands due to Bragg reflection of photoelectrons by {111} planes are detectable for Sr 3p3/2 emission, but not for F 1s emission from SrF2(111). (2) By contrast, {111} bands are observed for F 1s emission, but not for Mg 2s emission from MgF2(001). (3) {110} bands are weak for F emission and strong for Mg emission from MgF2(001). We have used the simple two-beam model of the Kikuchi-band formation to interpret the phenomena and found out that the two kinds of element-specific extinction of the Kikuchi bands occur in completely different ways; the extinction mechanisms have been confirmed by multiple-scattering cluster calculation, another more accurate approach to simulating XPED patterns.

Photoelectron diffraction study of the surfaces of Si(111)√3×√3–Al and –In with Mo Mζ and Cr Lα lines

Abstract Laboratory X-ray sources of Mo M ζ and Cr L α lines were used for a photoelectron diffraction (PED) study of the well-documented surfaces of Si(111)√3×√3–Al and Si(111)√3×√3–In. Azimuthal angle PED patterns of Al 2p and In 3d core levels were measured and analyzed by a multiple scattering formalism. The determined geometric parameters showed good agreement with those already established. This indicates the good potential of a PED study with laboratory sources.

Elastic scattering of Si 2p photoelectrons in silicon oxide

The cross-sections for elastic and inelastic scattering of Si 2p photoelectrons in silicon oxide were uniquely determined from the analysis of oxidation-induced changes in Si 2p photoelectron diffraction patterns arising from single crystalline Si substrate under the assumption that the structure of hydrogen-terminated Si substrate does not change by the oxidation. It was found from the Monte Carlo calculation of path of elastically and inelastically scattered Si 2p photoelectrons for the simulation of oxidation-induced changes in photoelectron diffraction patterns that the total elastic scattering cross-section and the inelastic scattering cross-section are 1.54×10 −20 and 1.26×10 −20 m 2, respectively.

Holographic atomic images from surface and bulk W(110) photoelectron diffraction data

Photoelectron diffraction data can, in principle, be regarded as enabling the experimental recording of electron diffraction phases (relative to a direct reference wave), as well as intensities, thus also permitting the holographic reconstruction of atomic positions. Such holographic photoelectron diffraction patterns have been measured for surface and bulk core-level-shifted W $4f$ photoemission from W(110), yielding a data set of unprecedented size and quality. To test the role of experimental uncertainties and of multiple-scattering effects, we have also performed corresponding theoretical calculations at the single- and multiple-scattering levels. The surface and bulk holograms so obtained have been analyzed so as to provide the first parallel comparison of the three-dimensional atomic images that can be directly obtained via the five principal reconstruction algorithms proposed to date. The advantages and disadvantages of each of these methods are discussed. The prospects and limitations of atomic photoelectron holography as an ab initio technique for determining local-surface structures are also explored.

X-ray photoelectron diffraction from (3×3) and (√3×√3)R 30° (0001)Si 6H–SiC surfaces

High-resolution (±1°) x-ray photoelectron diffraction (XPD) patterns were obtained along high symmetry azimuths of the (3×3) and (√3×√3)R30° reconstructed (0001)Si 6H–SiC surfaces. The data were compared to XPD patterns obtained from (7×7) Si (111) as well as to models proposed for the (3×3) and (√3×√3)R30° 6H–SiC reconstructions. Forward scattering features similar to those observed from the (7×7) Si (111) were also observed from the (√3×√3)R30° 6H–SiC (0001)Si surface. Additional structures were found and attributed to the substitution of carbon atoms for silicon. Unlike (1×1) and (7×7) Si (111) surfaces, the XPD patterns of (3×3) and (√3×√3)R30° SiC (0001)Si surfaces are different which is due to the presence of an incomplete bilayer of Si on the (3×3) surface. The most significant difference with the Si system is the equivalence of the [101̄0] and [011̄0] azimuths in the (3×3) structure. These results are consistent with a faulted Si bilayer stacking sequence which was proposed based on scanning tunneling microscopy observations.

The Overlayer Structure on the Si(001)-(2×3)-Ag Surface Determined by X-ray Photoelectron Diffraction

X-ray photoelectron diffraction (XPD) patterns of Ag 3d electrons from a single domain Si(001)-(2×3)-Ag surface were examined. Single scattering cluster and multiple scattering cluster simulations of the Ag 3d XPD patterns indicate an overlayer that contains four Ag atoms aligned almost linearly along the threefold direction in the unit cell on the Si(001) surface. The four-Ag-atom arrangement is discussed in the light of other information obtained by STM, LEEM and photoemission studies, finding it very feasible.

Polarization effects to enhance surface sensitivity of angle-scanned X-ray photoelectron diffraction in synchrotron-radiation-based experiments

We show that the surface sensitivity of angle-scanned photoelectron diffraction applied to bulk crystals or epitaxial multilayers can be enhanced by exploiting the angular anisotropy of the photoelectron primary wave excited by linearly polarized light, combined with the possibility of performing measurements in two different light polarization/electron detection geometries. The obtained relative photoelectron diffraction patterns of the form I 1( k )/I 2( k ), where 1 and 2 denote the two different geometric set-ups discussed in the present paper, show an enhanced structural surface sensitivity, which should allow the determination of the presence and the extent of surface relaxation by comparing the experimental data to suitable theoretical simulations. In a set of preliminary model calculations, the best results are obtained for emission from an s-initial state. High angular resolution of the electron detectors is an additional requirement.

New diffraction technique may improve understanding of mineral surfaces

Measurement of the photoelectron diffraction patterns of all major constituents of mineral surfaces by means of X ray photoelectron diffraction (XPD), a new and largely undeveloped technique, may lead to a much better understanding of those surfaces, research suggests [Fadley, 1992; Chambers, 1992; Osterwald et al., 1995; Fadley et al., 1997]. The technique allows scientists to characterize clean surfaces of minerals and investigate interactions between such surfaces and gases, liquids, and solids. It also promises accurate three‐dimensional imaging of the atomic environment around each type of atom near a mineral surface. In a specific experiment, the X ray photoelectron diffraction (XPD) patterns were measured for all major chemical constituents of a clean phlogopite (001) surface prepared in ultrahigh vacuum. Such patterns have been shown to provide direct and unique information, at the atomic scale, of the location and local bonding geometry for each element in the mineral surface.

X-Ray Photoelectron Diffraction in the Backscattering Geometry: A Key to Adsorption Sites and Bond Lengths at Surfaces

For a well-characterized s2 3 1d-1O adsorbate phase of oxygen on rhodium (111) we have identified photoelectron interference fringes in the backscattering geometry for O 1s adsorbate emission at 723 eV kinetic energy. Although these features are weak, they show up as characteristic rings covering large regions of the full hemispherical photoelectron diffraction pattern. They represent the situation of a simple two-atom electron interferometer and yield directly the most relevant information on the adsorbate geometry such as the adsorption site, the adsorbate-substrate bond length, and the bond angle. [S0031-9007(98)06936-1]

Theoretical Studies of Element-Specific Kikuchi-Band Effects in X-Ray Photoelectron Diffraction

A violation of the extinction rule for Bragg reflection has been recently observed as the anomalous extinction of Kikuchi bands with nonzero structure factors in the X-ray photoelectron diffraction pattern of F1s emission from a CaF2(111) surface. To explain this violation, we derived the Kikuchi-band intensity formula and found that this formula leads to a novel extinction rule of Kikuchi bands. In addition, on the basis of the formula, we set up a scheme of element-specific Kikuchi-band analysis for determining structure factors.

Direct structure analysis of W(110)-(1×1)-O by full solid-angle X-ray photoelectron diffraction with chemical-state resolution

The atomic structure of the W(110)-(1×1)-O surface with an additional (1×12) periodicity has been determined using full solid-angle X-ray photoelectron diffraction (XPD) patterns with chemical-state resolution. The W 4f binding energy of atoms directly bonded to oxygen is 0.73 eV higher than that of “bulk” W atoms. The full solid-angle XPD data for this shifted peak show strong forward focusing peaks, making it possible to estimate the adsorbate position directly. The oxygen is found to be in a triply-coordinated site, with a height of about 1.1 Å above the first-layer W atoms, and a lateral distance between the oxygen atom and the top of the triangle of W atoms of l=1.5 Å, which is shifted from the centered position of l=1.67 Å. This position of the oxygen atoms was then used as a starting point for a more refined structure analysis by fitting theoretically calculated patterns to the observed XPD data. This gives a more precise position of z=0.84 Å and l=1.52 Å, in good agreement with the XPD estimate.

The influence of the photoelectron elastic scattering on the photoelectron diffraction patterns, in-depth profiling and thin film thickness determination

The previously obtained analytical expressions for the account for the elastic photoelectron scattering are checked by diffraction patterns are considered. A simple analytical expression connecting the overlayer thickness and the change in diffraction patterns is proposed.

Surface structure of thin CaO layers formed on CaF 2(111) studied by photoelectron diffraction

Surface structure of thin CaO layers formed on the CaF 2(111) surface was investigated by two types of photoelectron diffraction methods. High angular resolved X-ray photoelectron diffraction (XPED) patterns indicate that two CaO domains. CaO(111) and CaO(-1-1-1), epitaxially grew and the lattice expansion occurred parallel to the surface because of the large mismatch. The change of dominant domain orientation was observed as increasing the thickness of films. We found that 8% lateral expansion and no vertical contraction occurred in 8A˚CaO layers, compared with Ca2p scanned energy photoelectron diffraction patterns and those calculated for model structures.

Surfactant mediated heteroepitaxial growth of Ge/Si(111) probed by X-ray photoelectron diffraction

X-ray photoelectron diffraction (XPD) is used as a tool to investigate the growth of Ge on Si(111) with and without surfactant (Sb) mediation. Structural information is extracted directly from the forward scattering maxima of recorded photoelectron diffraction patterns. The result of a quantitative analysis of the patterns taken for Ge layers of different coverage are used for a structural growth model of Ge on Si(111). Without Sb, a compression of the top Si and the first Ge double layers is found followed by a relaxation of the Ge structure. Under the presence of Sb an intermixed interface is formed followed by compressed Ge double layers and relaxed distances between double layers.

Photoelectron diffraction as a tool for the study of a buried interface during heteroepitaxial growth of Sb/Ge/Si(111)

X-ray photoelectron diffraction is used to analyze the structure of a buried Ge Si interface below a covering Sb layer. The Si(111) crystalline structure below Ge and Sb adsorbate layers is clearly seen in the diffraction pattern. The epitaxial growth of 2–10 Ge layers on Si(111) is mediated by a monolayer of Sb terminating the interface to the vacuum. The Sb photoelectron diffraction pattern shows no intense forward scattering maximum, indicating no Sb below the surface. The Ge film grows to form an epitaxial double layer at the interface, followed by layer-by-layer growth. For two Ge layers, the average vertical lattice constant of Si(111) is compressed due to adsorbate-induced stress. Also, the average vertical Ge lattice constant is compressed.