B. Murphy

Microcrystallography with an X-ray waveguide

A waveguide microdiffraction setup is described for an undulator beamline at the European Synchrotron Radiation Facility. The composite optics consists of a waveguide, which confines the beam vertically, and a horizontally focusing multilayer mirror. A beam size of about 0.1 × 3u2005µm (vertical × horizontal) at λ = 0.095u2005nm has been obtained. The sample stage comprises a three-axis gantry with micrometre precision and a three-axis piezo-scanner with about 0.1u2005µm repeatability. Diffraction experiments are demonstrated for selected inorganic and polymeric samples. Possibilities for scanning diffractometry and small-angle scattering experiments are discussed.

A NIXSW and NEXAFS investigation of thiophene on Cu(111)

The local registry and geometry of thiophene at sub-monolayer coverages on Cu(111) has been probed with normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS). Thiophene was found to adopt a flat geometry on the surface with the sulphur atom in an atop position. The relatively short data collection times required for the NIXSW technique were found to be beneficial when looking at thiophene overlayers which are sensitive to X-ray induced damage.

The interaction of bismuth with the GaAs(111)B surface

Abstract We present a core level photoemission and low energy electron diffraction study of the interaction of Bi with the GaAs(111)B-(2×2) surface. Results for room temperature growth are consistent with islanding where the islands contain Bi in two distinct chemical forms. Some 30% of the clean surface As trimers are removed by the deposition of Bi. A c(4×2) structure is stable in the 110–350°C temperature range and its formation coincides with the complete removal of the clean surface As trimers and with all of the Bi being bonded to As of the bulk-terminating layer. A structural model for the Bi terminated surface is proposed.

In-situ direct measurement of activation energies for the generation of misfit dislocations in the InGaAs/GaAs(001) system

Abstract In-situ X-ray topography (XRT) studies of misfit dislocation generation and movement in epitaxial InGaAs strained-layer structures on (001) GaAs are described. Examination of the changes in dislocation structure during a series of successive post-growth in-vacuo sample anneals has, for the first time, yielded activation energies of 0.7 and 0.8 eV for the formation of α-and β-misfit dislocations (MDs) by the initial glide of substrate threading dislocations (TDs) in the InGaAs epilayer. The introduction of MDs by this method is supplemented by the presence of an additional MD generation process. The activation energy for this is found to be comparable to that required to initiate the glide of a TD. The XRT studies have also confirmed the existence of MD cross-slip events, where α to β cross-slip was found to have an activation energy of 1.2 eV and to be much more common than the reverse β-α cross-slip process.

The Ni(100)(2×2)p4g–N reconstruction determined by surface X-ray diffraction

The ‘clock’ reconstruction of the Ni(100)(2×2)p4g–N system has been measured using surface X-ray diffraction. An in-plane displacement of the surface layer Ni atoms of dxy=0.30±0.01 A was found. This value is smaller than that measured for this system using photoelectron diffraction (PED) and surface extended X-ray absorption fine structure (SEXAFS). Possible reasons for this are discussed.

Investigating surface dynamics with inelastic x-ray scattering

A brief overview of application of the grazing incidence inelastic x-ray scattering experimental method will be presented. Inelastic x-ray scattering in grazing incidence conditions provides a new tool for selectively studying both surface and bulk lattice dynamics in a single experiment. It is possible to study acoustic and optical surface phonon modes currently with a 3 meV resolution over a wide range of momentum space and make a direct comparison between surface and bulk dispersion. In particular the case of 2H-NbSe2 will be discussed. For this material a Kohn anomaly has been previously reported. Our data demonstrate that the softening of the Kohn anomaly at the 2H-NbSe2 surface is significantly greater than that reported for the bulk. This is an indication of a relaxation in the topmost layers of the crystal.

Core-level photoemission study of the BiGaAs(111)A interface

Abstract Core-level photoemission spectra of the As 3d, Ga 3d and Bi 5d core levels were recorded for the Biue5f8GaAs (111)A-(2×2) surface. From the early stages of Bi growth, there are two distinct chemical environments for Bi. Surface Bi is bonded to both As and Ga atoms. The deposition of approximately 1 ML of Bi removes the dangling bonds from the surface As and Ga atoms, which are associated with the (2×2) vacancy-buckling structure. Annealing to 350°C reverses this process and surface dangling bonds for Ga reappear. When the Bi had fully desorbed at 425°C, photoemission and LEED results showed that the surface recovers the (2×2) vacancy structure but with a larger degree of surface disorder than that found before the deposition of Bi.

Reciprocal-space mapping of synthetic and natural diamond

High-resolution reciprocal-space maps of synthetic and natural diamonds have been recorded for the first time on the new six-circle diffractometer at the Synchrotron Radiation Source of the Daresbury Laboratory, UK. Variations in lattice spacing (typically 10 ppm) have been measured within specific growth sectors of synthetic diamonds as well as lattice tilts (typically 1 arcmin). Similar measurements have been made on diamonds containing strain-producing metallic inclusions. Platelet radii in a natural diamond have also been estimated, giving results (such as 140 A) agreeing with measurements from other techniques.

SRS Station 16.3: description and utilization of a high-resolution diffraction facility

In this paper we give a brief description of SRS Station 16.3, and outline a novel approach to instrument control software. Operation of the Station is illustrated with three examples: resonant magnetic x-ray diffraction, high-resolution measurements of Kossel lines, and observations of resonant peak shifts in dynamical diffraction.

Probing the adsorption structure of a multifunctional organic molecule: a NIXSW and NEXAFS study of 3-chlorothiophene on Cu(111)

Abstract The adsorption structure of 3-chlorothiophene on Cu(111) has been investigated using a combination of normal incidence X-ray standing wavefield absorption (NIXSW) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. For coverages up to saturation of the chemisorbed layer, the 3-chlorothiophene bonds through the S atom. The S atom adsorbs in an atop site with a Cu–S distance identical, within experimental error, to that observed for thiophene on the same substrate. From a combination of NEXAFS and NIXSW, thiophene was found to adsorb with the aromatic ring flat lying. From NIXSW measurements the S–Cl axis was found to be inclined by 12±2° from the surface. Whilst NEXAFS data suggested an orientation of 23±8° for the aromatic ring of 3-chlorothiophene. The Cl atom interacted only weakly with the substrate, with a Cl–surface distance longer than the Cu–Cl van der Waal separation.