reflectivity

X-ray reflectivity is a powerful technique for investigating the structure of surfaces, thin solid films and multilayered structures. The technique can return information about layer thickness and surface roughness, while in multilayered samples properties of the interface between layers can also be investigated. Due to the typically low number of atoms interacting with the beam, the high flux and very small vertical beam divergence of a synchrotron source are essential for this type of work.

By precision alignment of a flat sample such that it intercepts half the incident X-ray beam at extremely small angles of incidence, penetration of the X-rays into the sample is limited to the near surface region. Reflectivity measurements are performed on Station 2.3 at the SRS, which is located about 15m from a bending magnet. The station's monochromator provides X-rays in the range of approximately 0.5 to 2.5 Å. The X-ray beam delivered at the sample is defined by a set of 4-jaw centre-opening slits situated after the monochromator, giving adjustable vertical and horizontal aperture sizes of 10um to 2mm and 10um to 10mm respectively. For reflectivity work, a vertical beam size of 100um is routinely used. Samples are mounted on a dedicated goniometer stage with motorised vertical and rotational geometries to allow precision centring of the sample in the X-ray beam. The stage is located at the centre of a two-circle diffractometer, with detection of the reflected beam being via an enhanced dynamic range scintillation counter located on the 2-theta arm of the diffractometer. Collimation and resolution are provided by a pair of slits also mounted on the 2-theta arm in front of the detector. For layers with certain compositions, the wavelength tunability can be exploited to perform anomalous reflectivity.

station 2.3