Geometrical structure of the 1/2-ML (2X1) and 1/3-ML (2X3) Ba/Si(001) interfaces

The adsorption site of Ba deposited on a clean Si(001) surface was measured using x-ray standing-wave spectroscopy (XSW). Two interfaces were considered: half a monolayer with a (2 X 1) surface symmetry, and a third of a monolayer with a (2 X 3) symmetry. Two (111)-like Bragg reflections, ((1) over bar (1) over bar 1) and ((1) over bar (1) over bar 1), were utilized in the XSW experiments. Using the structural results obtained from these two reflections, as well as symmetry arguments, it was possible to establish, in three dimensions, the coordinates of the adsorption site for the two interfaces considered. In both cases, Ba lies near the center of the square formed by four Si surface atoms, at a height close to the projection of the next ideal Si(001) plane.