Local structure of Sn/Si(001) surface phases

The surface structures of the (6 x 2), c(8 x 4) and (5 x 1) phases of Sn/Si(001) were studied using the X-ray standing wave technique. Using the (004) and (022) Bragg reflections, we find that the (6 x 2) and c(8 x 4) phases are composed of highly buckled Sn-Sn ad-dimers located 1.58 Angstrom above the bulk-like Si(004) surface atomic plane. The Sn atoms occupy two distinct sites with a vertical separation of 0.68 Angstrom, resulting in a dimer buckling angle of approximately 14 degrees. Occupation of second-layer sites by Sn in the (5 x 1) phase, and even in the high-coverage region of the c(8 x 4) phase, changes the Sn spatial distribution normal to the surface, which we attribute to unbuckling and/or breaking of the dimers in the first layer.