Adsorbate structure and substrate relaxation for the Sb Si(001)-(2 × 1) surface

X-ray standing wave measurements were undertaken to study the bonding position of Sb adatoms on the Sb-saturated Si(001)-(2 × 1) surface. Using the (004) and (022) Bragg reflections, we find that the Sb atoms form dimers, and that the center of the Sb ad-dimers lies 1.64 Å above the bulk-like Si(004) surface atomic plane. When combined with a previous determination of the SbSi and SbSb bond lengths, our results show that the surface Si plane is contracted inward by 0.10 Å upon saturation with Sb. Our in-plane results are compared to two structural models consisting of dimers whose bonds are parallel to the surface plane and whose centers are either shifted or unshifted (parallel to the dimer bond direction) relative to the underlying substrate planes. We thus find two special cases consistent with our data: one with symmetric (unshifted) dimers having a dimer bond length of 2.81 Å, and the other with midpoint-shifted dimers, having a bond length of 2.88 Å and a lateral shift of 0.21 Å.