W. Press

X‐ray diffraction from laterally structured surfaces: Crystal truncation rods

X‐ray‐diffraction measurements from single‐crystalline GaAs(001) surface gratings are presented. The experiments were performed using a three‐crystal diffractometer. The vicinity of the (004) Bragg reflection for five samples was investigated in detail. Model calculations based on the kinematical crystal truncation rod theory are able to explain all measurements quantitatively. Mesoscopic grating parameters as well as microscopic surface roughnesses of the samples were obtained from fits of the data. For three samples scanning electron microscope pictures were taken. The analysis of these pictures leads to the same mesoscopic parameters as obtained from x‐ray Bragg diffraction.

X-Ray Diffraction from Laterally Structured Surfaces: Total External Reflection and Grating Truncation Rods

A high-resolution study of x-ray scattering from a laterally structured surface is reported. Total external reflection displays a distinct dip in the reflectivity for incident anglesxa0xa0<xa0c (critical angle) and around a Bragg peak a system of truncation rods is found. Generalized Fresnel equations in conjunction with a kinematical theory provide a quantitative explanation of both findings. The method is well suited for characterising modulations on a mesoscopic length scale.

The interface structure of thin liquid hexane films

Abstract A series of thin liquid hexane films adsorbed on a rough silicon wafer was studied with specular and off-specular X-ray scattering. Assuming an interfacial profile at the liquid–solid interface which does not depend on film thickness and modeling the density tail at the liquid–vapor interface by an error-function, we find an interfacial profile which explains the reflectivity data of all the measured 13 hexane films of thicknesses 25–350xa0A. The interfacial region near the substrate extends to 40xa0A from the solid–liquid interface and is in good agreement with calculated density profiles of Lennard–Jones fluids near a hard wall. For films thicker than 50xa0A the rms-roughness found for the liquid surface accounts well to predictions based on standard continuum capillary-wave theory. The surface roughness of thinner films is drastically reduced. Above 200xa0A a power law of the form qη−2r describes the diffusely scattered intensity.

Setup for in situ surface investigations of the liquid/glass transition with (coherent) x rays

A dedicated setup comprising an efficient cryogenic device for the in situ preparation of large surface areas of prototypical organic glass formers in a wide temperature range (170–340 K) is presented. This setup provides the necessary temperature and vibrational stability for surface x-ray and neutron scattering experiments, including the extremely sensitive technique of x-ray photon correlation spectroscopy (XPCS). XPCS is an emerging method which is made possible by the high coherent photon flux produced by third-generation synchrotrons. We demonstrate that microscopic motion at the surface can be studied in a direct way in the liquid and supercooled state using XPCS. In addition, we have used a charge-coupled-device detector to record two-dimensional images of static speckle patterns forming on surfaces in the glassy state.

An examination of a multiple-twinned periodic approximant of the decagonal phase Al70Co15Ni15

High-resolution x-ray scattering was performed on a decaprismatic sample of the nominal composition Al70Co15Ni15. The overall diffraction pattern has tenfold symmetry within experimental resolution. But many low-indexed reflections reveal a splitting, which is evidence for twinned high-order periodic approximants of the decagonal phase, rather than for a truly decagonal quasicrystal. To our knowledge. We report the clearest separation of this kind of splitting to date. For the orthorhombic approximant, the space group Ccmm with lattice parameters a=61.024(5) AA, b=83.99(1) AA and c=4.0799(5) AA was derived. Deviations from the diffraction pattern predicted by the twinning model were observed. We point out the close similarity between the x-ray diffraction of the real decagonal quasicrystal and that of our twinned sample.

Diffuse X-Ray Scattering in Specular Direction: Analysis of a Wetting Film

X-ray diffraction measurements within the region of total external reflection from a thin wetting film of liquid CCl4 on a Si/SiO2 substrate are presented. The in-plane structure of the liquid surface yields a peak in the diffuse scattering at the specular position. Since there is no way to separate specular and off-specular contributions, the data were analysed simultaneously, which leads to a consistent set of interface parameters. Although the obtained power spectral density function of the liquid surface is very similar to that used in the case of free capillary waves, a modified cut-off has to be introduced. The difference results from the constrained thin-film geometry and is therefore caused by the interaction with the underlying substrate.

Orientational order and rotational dynamics of the amide ions in potassium amide. I. Neutron diffraction

Potassium amide (KNH2/KND2) is an ionic molecular crystal. The amide ions (NH2−/ND2−)—isoelectronic with the water molecule OH2—display orientational disorder in the cubic and tetragonal high–temperature phases. In the monoclinic low–temperature phase they are orientationally ordered. We investigated the deuterated compound KND2 in all three phases using high resolution neutron powder diffraction and determined the time–averaged probability density function of the deuterons in detail. The structure of the intermediate tetragonal phase was solved, and deviations of the reorientations of the amide ions in the cubic phase from free rotational diffusion were quantified. From the structural properties of the different phases of KND2 and the structural changes at the phase transitions conclusions on the respective dynamics of the amide ions can be drawn.

Surface and interface topography of amorphous SiO2/crystalline Si(100) studied by X-ray diffraction

Several samples of a-SiO2/c-Si(100) with different oxide layer thicknesses and different silicon surface qualities were examined by X-ray diffraction. The layer thickness as well as the surface roughness and the interface roughness were studied by measuring the reflectivity of the samples near total external reflection. A separate and detailed determination of the interface roughness is obtained by analysing the intensity near the Si(400) reciprocal lattice point. There is a pronounced diffuse scattering parallel to the normal of the interface, the frequently called crystal truncation rods. Both the reflectivity and the isointensity contours around the (400) reflection demonstrate the presence of an intermediate layer between a-SiO2 and c-Si in a crystalline state.

X-ray characterization of buried allotaxially grown CoSi2 layers in Si(100)

An x-ray study of the interface morphology and lattice parameters of buried expitaxial CoSi2 layers in Si(100) is presented. Specular reflectivity, diffuse and crystal truncation rod scattering, together with grazing incidence diffraction yield detailed information about the interface quality and lattice mismatch. It turns out that the CoSi2 interfaces are considerably smoothened by an annealing step at 1150u2009°C. Also the in-plane correlation length of the roughness increases yielding laterally smoother interfaces. While the perpendicular lattice parameter is between that of a free relaxed and a pseudomorphic structure and a linear contraction as function of the annealing temperature is obtained, grazing incidence diffraction reveals the opposite effect for the in-plane lattice mismatch.

The structure of hexane/perfluorohexane thin liquid films

Thin films of the binary liquid mixture hexane/perfluorohexane were prepared on silicon wafers. The film thicknesses were between 140 A and 350 A; this range was chosen in order to achieve strong confinement of the mixture. The vertical structure of these films was investigated by means of x-ray reflectivity measurements. The data show that the component with the lower surface tension (perfluorohexane) segregates in the surface region of the films in agreement with theoretical predictions. Furthermore, no complete miscibility of the two components was observed in regions of the phase diagram where a one-component bulk system is expected. Preliminary results are given for the internal hexane/perfluorohexane interface.