STRUCTURE-ANALYSIS OF SI(111)-(SQUARE-ROOT-3 X SQUARE-ROOT-3)R 30-DEGREES/AG USING X-RAY STANDING WAVES

The x-ray standing-wave technique has been used to determine the positions of the Ag atoms in the Si(111)-(square-root 3 x square-root 3) R 30-degrees/Ag reconstruction. Using the (111) reflection, the Ag atoms are found to be located in one plane at a height of 3.44 +/- 0.02 angstrom above the center of the extrapolated bulk Si(111) bilayer. This rules out several models that have been previously proposed in the literature for the reconstructed surface. Data obtained by using the (111BAR) and (220) reflections show that the Ag atoms are reconstructed, i.e., are laterally displaced away from bulk Si positions. This directly rules out the Ag-honeycomb model. Taking into account the many other studies on this system, the most favored arrangement of the Ag atoms is in the form of triangles, with three Ag atoms per unit cell. The registry of this Ag triangle with respect to the substrate has also been determined.