ULTRAHIGH-VACUUM DIFFRACTOMETER FOR GRAZING-ANGLE X-RAY STANDING-WAVE EXPERIMENTS AT A VERTICAL-WIGGLER SOURCE

An ultrahigh vacuum (UHV) diffractometer has been designed for studies of surface structures using the grazing-angle x-ray standing-wave method. The design is featured by a horizontal plane of diffraction for use at a vertical-wiggler source of synchrotron radiation. A sample is horizontally mounted in an UHV chamber (4×10-7 Pa) placed on crossed swivels, which control the glancing-incidence angle of x rays on the sample surface with a 50-μrad accuracy. The chamber accepts a sample from a transportation vessel under high vacuum. A beryllium window allows x-ray fluorescence to reach a semiconductor detector at short access. The whole assembly sits on a high-precision rotary table, regulating the sample Δθ angle with a reproducibility of better than 0.5 μrad required for control of the x-ray field profile. The system has been successfully applied to an accurate determination of the in-plane ordering of As atoms on a Si(111) surface with a 1×1 structure. © 1995 American Institute of Physics.