Crystal Truncation Rod Measurements Research Articles

Determination of the α-Ai2O3(0001) Surface Relaxation and Termination by Measurements of Crystal Truncation Rods

We have investigated the unreconstructed (0001) surface structure of sapphire (α- Al 2 O 3) by grazing incidence X-ray scattering. Modulations along the crystal truncation rods were analyzed in order to determine the chemical nature of the terminating plane, and the structural relaxations of the first few atomic planes below the surfoce. The most likely model yields a single Al layer termination with relaxations of the first four planes of -51%, +16%, -29% and +20% respectively. These results compare well with the most recent theoretical calculations on this surface.

X-Ray Characterization of Buried δ Layers

A combination of measurements of crystal truncation roda and X-ray standing waves has been used for a detailed characterization of buried Ge δ layers on Si(001). Measurements of crystal truncation rods reveal the interface roughness, the δ layer lattice constant and the δ layer concentration. From measurements with X-ray standing waves the dopant lattice position and crystallinity of the δ layer are determined. We find a linear dependence of the local tetragonal distortion of the Ge bonding in the δ layer on the Ge concentration in the layer.

High concentration Bi δ-doping layers on Si(001)

Medium energy ion scattering, X-ray standing waves and measurements of crystal truncation rods were used to show that it is possible to prepare spatially well confined Bi doping layers (δ-doping layers) on Si(001) with a Bi doping level of 3 × 1021cm−3 by a combination of Bi molecular beam epitaxy and low temperature deposition of a Si top layer with subsequent annealing (solid phase epitaxy). The Bi concentration exceeds the equilibrium Bi solubility by more than three orders of magnitude. The Bi atoms are incorporated into the Si host lattice on substitutional sites. The Bi doping profile exponentially decays into the top Si layer with an attenuation length ranging from 40 to 6Åand a fraction of Bi atoms in substitutional lattice sites of up to 96%, depending on the annealing conditions.

Strain induced interface roughness of Si1−xCx δ layers on Si(001)

Ultra thin buried Si1−xCx films (δ layers) of monolayer thickness and high carbon concentration (x>0.1) in a Si matrix have been prepared by molecular beam epitaxy and were structurally characterized by high resolution x-ray diffraction, i.e., measurements of crystal truncation rods. The average interface roughness of the δ layers is in the order of 6–10 Å. A larger carbon deposit results in the formation of thicker Si1−xCx δ layers with lower carbon concentrations and smoother interfaces. This effect is attributed to a reduction of the strain in the δ layer due to the lower carbon concentration.

A comparison of surface roughness as measured by atomic force microscopy and x-ray scattering

We compare measurements of the roughness of silicon(001) wafers cleaned by several methods. The roughness values were obtained using crystal truncation rod (CTR) scattering and atomic force microscopy. Although they do not yield identical results, both methods show the same relative roughness for the different cleans. CTR scattering is sensitive to roughness on lateral length scales down to atomic dimensions. The quantitative differences in roughness can be explained by the different wavelength spectrum of roughness probed by the two techniques. CTR measurements were also performed after a 60 Å thermal oxide was grown on the wafers. The roughness trends are the same after oxidation, but we also find that the oxidation process has significantly reduced the interfacial roughness.

High resolution x-ray diffraction and scattering measurement of the interfacial structure of ZnTe/GaSb epilayers

The surface and interface structures of ZnTe epilayers grown by molecular beam epitaxy on GaSb (001) substrates under different conditions have been investigated by high resolution x-ray diffraction and grazing incidence scattering. Reciprocal space mapping around the symmetrical diffraction reciprocal point 004 and asymmetrical diffraction point 1̄1̄5 showed that the ZnTe epilayers, in the samples investigated, were fully strained to the substrate. The crystalline quality of the ZnTe epilayer grown on a substrate annealed in a Zn flux was very good, while evidence for an interfacial layer, of thickness varying from 2–20 nm, was found when the substrate was annealed in a Te flux prior to growth. This is attributed to Ga2Te3 formation at the interface. The interfacial layer roughens the interface and surface, and both crystal truncation rod measurements and grazing incidence x-ray reflectivity show the surface roughness to be about 4 nm. Such a rough surface and interface is also inferred from the broader distribution along the transverse direction in reciprocal space maps. A shorter lateral correlation length is found for the roughness of the sample containing the interfacial layer. The disappearance of interference fringes is attributed to nonuniformity of the interfacial layer.

Electrochemical deposition of copper onto Pt(111) in the presence of (bi)sulfate anions

Using in situ x-ray diffraction we have studied the underpotential deposition (UPD) of copper onto a Pt(111) electrode in sulfuric acid. Deposition occurs in a two-stage process; close to a full monolayer of Cu being preceded by a $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ structure that consists of both Cu and (bi)sulfate anions and is similar to the structure observed on the Au(111) electrode. Utilizing anomalous scattering methods we can accurately determine the Cu coverage during UPD and propose structural models based on crystal truncation rod measurements.

Coadsorption of Sulfate/Bisulfate Anions with Hg Cations during Hg Underpotential Deposition on Au(111): An in Situ X-ray Diffraction Study

The first stage of mercury underpotential deposition on Au(111) electrodes in 0.10 M H2SO4 containing 1.0 mM Hg2+ has been studied by synchrotron X-ray scattering techniques including grazing incidence X-ray diffraction and specular crystal truncation rod measurements. An ordered coadsorbed structure of sulfate/bisulfate anions and Hg cations was found at potentials between the first and second Hg UPD peaks (+0.80 V ≤ E ≤ +0.88 V vs Ag/AgCl(3 M KCl)). The coadsorption structure was found to consist of a compressed Hg honeycomb lattice with the honeycomb centers occupied by sulfate or bisulfate anions. The compression of the lattice is likely due to the formation of mercurous (Hg22+) ions which have a much shorter Hg−Hg distance than that in frozen bulk Hg crystals. The net charge transferred under the first Hg UPD peak suggests that the chemical state of the species in the coadsorbed structure is likely Hg2SO4. Our results indicate that both the chemical state of the mercury cations and the nature of the anions are important in the resulting electrodeposited structures.

Stress reduction and interface quality of buried Sb δ doping layers on Si(001)

We have investigated the width dependence of Sb delta (δ) doping layers grown by Si solid phase epitaxy (SPE) on the Sb surface reconstruction prior to Si deposition. Depending on the Sb adsorption conditions a 2×1 and a 2×n surface reconstruction is observed. Measurements of crystal truncation rods and x-ray standing waves show a drastically reduced interface roughness and a better crystal quality for δ layers grown on Sb:Si(001)−2×n substrates in comparison to Sb:Si(001)-2×1, which we attribute to reduced surface stress of the Sb:Si(001)-2×n reconstruction.

Towards perfect Ge δ layers on Si(001)

The effect of different methods of growth on the interface quality of extremely thin buried GexSi1−x films (δ layers) was studied by means of in situ spot profile analysis low-energy electron diffraction and extensive postgrowth x-ray characterization by measurements of crystal truncation rods and x-ray standing waves. In comparison to molecular beam epitaxy, solid phase epitaxy and surfactant mediated growth result in a significant reduction of Ge–Si intermixing and interface roughness.

The calcite (10l̄4) cleavage surface in water: Early results of a crystal truncation rod study

This paper introduces the application of the crystal truncation rod (CTR) technique to the study of mineral-fluid interfaces. The CTR of the calcite (10l̄4) cleavage surface in contact with a calcite-saturated aqueous solution was measured as a function of time by using synchrotron X-ray radiation. These results are consistent with the presence of steps of monomolecular height (3.04 Å high) separated by terrace areas, as observed in previous atomic force microscope studies of the calcite surface. We found that the step density decreased with time of exposure to solution, implying that the surface healed by migration and annihilation of monomolecular steps. An atomic model of the CTR measurements of the healed surface indicates that the atomic structure of the cleavage surface is not measurably different than that of (10l̄4) crystallographic planes within the crystal and the root mean square roughness of the terrace areas between steps is 0.35 ± 0.1 Å. Higher resolution CTR measurements could reveal exact atomic positions of the calcite surface and the structure of aqueous fluids in the interfacial region.

Surface scattering of x rays from InP (001) wafers

We have studied the surface scattering of x rays from mechanical-chemical polished InP (001) wafers with sulfur and/or iron doping. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model was proposed to explain the experimental data. The results were also compared with those obtained from crystal truncation rod measurements.

Determination of surface roughness of InP (001) wafers by x-ray scattering

The surface roughness of polished InP (001) wafers were examined by x-ray reflectivity and crystal truncation rod (CTR) measurements. The root-mean-square roughness and the lateral correlation scale were obtained by both methods. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model of surface faceting is proposed to explain the experimental data. The sensitivities of the two methods to the surface structure and the role of the resolution functions in the CTR measurements are discussed.

Synchrotron X-ray Scattering Studies at Mineral-Water Interfaces

AbstractSynchrotron X-ray scattering techniques provide a powerful tool for the in situ study of atomic scale processes occurring at solid-liquid interfaces. We have applied these techniques to characterize and study reactions at mineral-water interfaces. Here we present two examples. The first is the characterization of the calcite (CaCO3) (1014) cleavage surface, in equilibrium with deionized water, by crystal truncation rod measurements. The second is the in situ study of the heteroepitaxial growth of otavite (CdCO3) on the calcite (1014) cleavage surface. The results of such studies will lead to significant progress in understanding mineral-water interface geochemistry.

New insight into the structure and growth of CaF2/Si(111)

We have used transmission electron microscopy and x-ray crystal truncation rod measurements to investigate thin (<50 Å) CaF2 films grown on Si(111) substrates by molecular beam epitaxy. The results indicate that CaF2/Si can be structurally as perfect as NiSi2/Si and CoSi2/Si, and that a reconstructed layer is present at the CaF2/Si(111) interface.