Microscopic structure, discommensurations, and tiling of Si(111)/Cu-‘‘5×5’’

We derive a detailed, microscopic description of the Si(111)/Cu-‘‘5×5’’ reconstruction. The key to understanding this structure is the x-ray standing-wave determination of the Cu registry with respect to the Si substrate. With Cu basically in ${\mathit{H}}_3$ and substitutional sites the buckled Si(111) surface bilayer converts to an almost planar, hexagonal ${\mathrm{Cu}}_2$Si layer. The straightened bond angles and the associated increase in the lateral lattice constant give rise to a hexagonal network of discommensurations of period ≊5.5${\mathit{a}}_{\mathrm{Si}}$. Complete tiling of the surface requires three types of twisted (±3°) domains, two of which are rotationally equivalent.