X-ray reflectivity (XRR) is a technique for studying the detailed surface properties of materials. Specifically, x-rays are used to probe the electron density perpendicular to the surface and thereby obtain information about the surface roughness, thin film thickness and density.
The technique involves measuring the reflected X-ray intensity as a function of incidence angle over a range of angles close to the critical angle for total reflection. Above this critical angle the specularly reflected intensity (i.e. with symmetric incident and reflected angles) decreases, with a form that is dependent on the structural properties of the interface. A typical reflectivity curve is essentially a combination of the Fresnel reflectivity (with a 1/Qz4 dependence) and an interference pattern (Kiessig fringes) from the scattering at different layers (for example, in a thin film or multilayer).
Further, closely related to the complementary techniques of GIXD and GISAXS, the off-specular or diffuse part of the reflectivity (i.e. with non-symmetric incident and reflected angles) offers information about lateral correlations at the surface.
Above the critical angle the reflectivity falls off rapidly with Qz, (momentum transfer perpendicular to the surface). A high incident intensity is required to reach high Qz, so a synchrotron can provide a relatively wide dynamic range. The high incident intensity also enables the detection of diffuse scattering to high angles. Further, due to the weak interaction with matter, x-ray reflectivity is one of the few techniques available for structural investigation of buried interfaces (e.g. solid-liquid and liquid-liquid interfaces). Compared to neutrons, for example, x-ray reflectivity also offers relatively low-backgrounds and high resolution.
XRR has a wide range of applications and is often used in combination with related techniques. Examples of research areas include the study of thin films or multilayers of metals and semiconductors or soft matter (polymers, surfactants, biological systems, etc) at both solid and liquid interfaces.
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