Resolving the Ga Ad-Dimer Location and Orientation on the Si(100) Surface

Quantitative x-ray standing wave measurements of the Ga ad-dimer bond length, orientation, and location for the Si(100)-(2 ×{} 2):Ga surface combined with first-principles molecular cluster calculations on four different ad-dimer models lead to the conclusion that Ga atoms form ad-dimers which are aligned parallel to the underlying Si dimers and lie in between Si dimer rows at the valley bridge site.