R. L. Johnson

Adsorbate registry and subsurface relaxation of the 3x3 reconstructions

We have performed a surface X-ray diffraction study of submonolayer √3 structures of Sn and Pb on Ge(111). The structure factor analysis of fractional-order Bragg reflections shows a significant relaxation in the Ge substrate induced by the Sn/Pb adatoms. The registry of the adatoms is determined from the analysis of the integer-order intensities. The adatoms are situated on top of the second layer of Ge atoms. The intensity profile normal to the surface of a subset of the fractional-order Bragg rods were measured. They display a pronounced intensity variation for both Sn and Pb, which can be explained only by a subsurface relaxation extending at least four atomic layers into the bulk.

X-ray determination of the GaSb(111)2×2 surface structure

We have examined the GaSb(111)2×2 surface prepared by sputtering and annealing using grazing-incidence X-ray diffraction. Our structure factor analysis shows the similarity between this surface and the vacancy-buckling model for GaAs, GaP and InSb(111): it contains one Ga vacancy per unit cell and an adjustment of bond angles associated with valence change of the remaining surface atoms. Analysis of the integer-order intensities unambiguously determines the mode of attachment (registry) of the surface layer with the bulk crystal.

Optimisation of grazing-incidence monochromators

Abstract The general characteristics of existing grazing-incidence monochromators are described and the present technological limitations discussed. A total system design approach is emphasized whereby the characteristics of a particular synchrotron light source can be optimally matched to an experimental technique. As an example FLIPPER II, an ultrahigh vacuum grazing-incidence monochromator for photoemission experiments built at the Max-Planck-Institut Stuttgart for use at DESY, Hamburg is briefly described.

Surface X-Ray Diffraction: the Ge(00l)2×1 Reconstruction and Surface Relaxation

The (001) surfaces of Si and Ge exhibit a two domain 2×l reconstruction. Despite the variety of experiments performed on both these surfaces, the detailed atomic geometry of the 2×l unit cell is not known. Of the many models proposed, a dimer model is supported by many experiments, including STM /1/, LEED /2/, and ion scattering /3/. In a pioneering experiment, EISENBERGER and MARRA /4/ measured five Bragg reflections from the Ge(OOl) 2×l surface by synchrotron x-ray diffraction. The data set is too small to perform detailed crystallographic analysis, but it supports a dimer model and suggests second layer motion.

Surface X-Ray Diffraction: The Atomic Geometry of the Ge(111)7×7-Sn and Ge(111)5×5-Sn Reconstructions

The Ge(111) surface exhibits 7×7 and 5×5 reconstructions after deposition of submonolayer coverages of Sn and subsequent annealing. We have examined these structures by surface x-ray diffraction and collected 90 non-equivalent fractional-order reflections for the 7×7 structure, and 115 for the 5×5. The diffraction pattern for both structures show considerable similarity with the pattern for Si(111)7×7, and it is shown that the DAS (Dimer Adatom Stacking fault) model for Si(111)7×7 is also applicable to the Sn-induced Ge(111)7×7 and 5×5 structures. Furthermore, we have identified the adatoms to be Sn. For the 5×5 structure a partial substitution of Ge with Sn was found at the dimer positions closest to the corner vacancies.

Ge(111) √3 x √3-Pb: The atomic geometry

Abstract We have performed surface X-ray diffraction experiments on submonolayer coverages of Pb on Ge(111) using the wiggler beamline at the Hamburger Synchrotronstrahlungslabor (HASY-LAB). Two 3 × 3 R30° Pb structures at different coverages have been identified. In the dilute structure there is only one atom per unit cell, the dense structure is a distorted close packed overlayer. Relating the scattering from the Pb overlayers with that from the underlying Ge crystal provides a novel method for determining the registry of the overlayers. We observe a difference in chemisorption site between the dilute and the dense phase.

X-Ray Diffraction from the (3 × 3) Reconstructed \bar 1\bar 1\bar 1B Surface of InSb

X-ray diffraction measurements have been perfgrmed with synchrotron radition under UHV conditions on the Sb rich ((bar 1bar 1bar 1)) surface of InSb. This InSb. ((bar 1bar 1bar 1)) B surface has a (3 × 3) reconstruction [49.1]. The surface was prepared by argon-ion bombardment and annealing at 400°C and was characterized using LEED and high-resolution photoemission. Using X-rays incident at the critical angle for total reflection (0.31°) we have measured the intensities of fractional-order Bragg rods corresponding to the (3 × 3) reconstruction on the B surface.

Grazing Incidence Gratings And Mirrors - Case Studies

The performance of gratings and mirrors in the VUV is given for specific systems at HASYLAB. Metal mirrors and bent glass optics are considered first. The photon fluxes for a Seya-Namioka monochromator, a toroidal grating monochromator, and the FLIPPER plane grating monochromator are given.