X-Ray Standing Wave and Tunneling Microscope Location of Gallium Atoms on a Silicon Surface.

The position of gallium atoms on a silicon (111) surface has been completely determined using the tunneling microscope and X-ray standing wave methods. The (sqrt 3 times sqrt 3) R30 degree electron diffraction pattern observed with 1/3 monolayer coverages is shown to result from a simple adatom gallium lattice with the adatoms 1.49angstrom above the filled three fold silicon surface sites. Comparison with L.D.A. density function calculations shows the theory to be in agreement with the lowest energy three fold site for gallium with some discrepancy in bond lengths.