Interface of a Bi(001) film on Si(111)-7 x 7 imaged by surface x-ray diffraction

We show structural characteristics of an epitaxial Bi(001) film on the Si(111)-7 x 7 surface, including its atomic-layer-resolved interface, which were revealed by surface x-ray diffraction and direct methods of structure analysis. The lattice constants of a 6-nm-thick Bi(001) film at room temperature were determined to be a 4.50 +/- 0.02 angstrom and = c = 11.96 +/- 0.04 angstrom in the hexagonal coordination, which are compressed by 0.9% and are expanded by 0.8%, respectively, from the bulk structure. The coexistence of two rotational domains, related by a 180 degrees. rotation around the c axis to one another, was observed in all samples. The wetting layer of Bi, between the film and the substrate, was directly reconstructed by a holographic method, and the electron-density profile along the c axis was obtained by phase-retrieval methods. The spacing between the wetting layer and the film is wider than the spacing between the (001) bilayers by 36%. The film maintains its bulklike structure, except for small vertical relaxations of the top and bottom layers. These structural properties indicate that the film is nearly free standing without breaking the structural-inversion symmetry between the top and the bottom as expected from its electronic properties.