PROPERTIES OF GRAZING-ANGLE X-RAY STANDING WAVES AND THEIR APPLICATION TO AN ARSENIC-DEPOSITED SI(111) 1X1 SURFACE

Grazing;angle diffraction of X-rays by crystal planes normal to a surf ace generates dynamical (lattice-modulated) standing waves, which are used in this paper to determine the in-plane structure of arsenic adat oms on an Si (111) surface of the 1x1 structure. The X-ray field, form ed by the interference of the incident, specular-reflected and Bragg-d iffracted beams above the surface, has two components with and without intensity modulation in the direction of the reciprocal-lattice vecto r parallel to the surface. The two components behave differently as a function of X-ray glancing incidence angle on the surface in the vicin ity of the critical angle for total external reflection. This property has been exploited to determine the ordering of the As atoms accurate ly using X-ray fluorescence signals observed from a sample in ultra-hi gh vacuum at a synchrotron source. The data show highly ordered As ato ms occupying the threefold-coordinated sites on the bulklike Si (111) surface. Displaced arsenic positions are not supported by the observat ion. The conclusion is fairly insensitive to the vertical height of th e overlayer atoms used in the analysis, in accordance with the slow va riation of the field profile along the surface normal. The grazing-ang le X-ray standing-wave method allows model-independent determination o f the registry of foreign atoms at a crystal surface with a positional accuracy of a few hundredths of an Angstrom.