M. Tolan

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.

Thin polymer films on rough surfaces

Abstract X-ray diffraction measurements in the region of small incidence and exit angles on thin polystyrene (PS) films deposited on laterally structured surfaces are performed. From fits of the data we obtain how the Fourier-components of the substrates are damped by the adsorbed films. The results are compared with theoretical predictions for liquid films. It turns out that PS-films of low molecular weight (⩽300k) behave like frozen liquids. Within films of a molecular weight of 1000k viscoelastic forces dominate over the substrate—adsorbate van der Waals-interactions so that even a 680 A thick film is not totally flat on top of a grating with a height of 130 A.

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.

Roughness of surfaces and interfaces

Abstract X-ray reflectivity is now a common tool for investigating density profiles of thin films and multilayers in a nondestructive manner. In contrast to the specularly reflected beam the nonspecular diffuse intensity is sensitive to the lateral structure of rough interfaces. The most reliable results are obtained from simultaneous fits of all available data (i.e. reflectivity together with all diffuse scans) with a single set of parameters. It turns out that diffuse scattering experiments from a large class of systems can be described by correlation functions of self-affine fractal surfaces. Two layer systems serve as test cases: (1) A CoSi2/Si-layer system grown by molecular beam epitaxy (MBE) and (2) Langmuir—Blodgett films. The analysis within the distorted-wave Born approximation (DWBA) of the scattering from the silicide sample gives a large amount of information, e.g. strong vertical correlations. For the topmost surface a comparison between scanning tunneling microscopy (STM) and X-ray results leads to very good agreement. Nine and eleven monolayer Langmuir—Blodgett films with high defect concentrations also can be successfully analyzed with the fractal model. The lateral lengths obtained from the diffuse scattering and grazing-incidence diffraction (GID) agree.

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.

X-ray diffraction from mesoscopic systems: thin films on 'rough' surfaces

In this work X-ray diffraction measurements on thin polystyrene films deposited on laterally structured surfaces are reported. The experiments were performed in the region of small incidence and exit angles. The X-ray data are compared with the results of atomic force microscopy investigations, both being performed to obtain the morphology of the polymer film on top of the surface grating. Our results do not confirm existing theoretical predictions assuming pure van der Waals interactions between the substrate and the film.

Coherence length determination of X-rays with laterally structured surfaces

Abstract It is investigated if the spatial extent of the coherently illuminated area of a sample surface is completely defined by the q x -resolution of the used diffractometer. For this purpose, X-ray diffraction experiments with laterally structured GaAs-surfaces have been performed.

X-ray scattering from polymer films

Abstract The effect of confinement on the surface structure of thin polymer films is investigated by specular and diffuse X-ray scattering. It will be shown to what extent information of the film surfaces and the substrate/film interactions can be obtained. In the case of polymer and liquid films predictions for correlation functions exist which may be quantitatively compared with scattering data.