UHV X-Ray Standing Wave Studies of Atomic Positions at Crystal Surfaces

In epitaxial growth the location of atoms in the first monolayer is of crucial importance for subsequent growth. Until recently, there has been no direct means of in situ atom location for crystal thickness of the order of a monolayer. Using a recently developed UHV x-ray standing wave facility, we have directly measured the position of atoms of Ga and As grown on a (111) (7x7) reconstructed silicon surface. Since there are two different sublattice sites on (111) planes of the diamond structure, the positions of the atoms on these sites is a question of some interest. As with previous studies on arsenic covered surfaces, we find that arsenic atoms are confined only to the topmost silicon layer which reconstructs to (1x1). With a high degree of coherence, Ga atoms occupy exclusively the other sublattice site on the (111) plane. A remarkable property of these monolayers is their stability with respect to surface contaminants. Neither 02 of H20 environments resulted in any degree in the coherent fraction of atoms located on the two sublattices.