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

We derive a detailed, microscopic description of the Si(111)/Cu-``5\ifmmode\times\else\texttimes\fi{}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 \ensuremath{\approxeq}5.5${\mathit{a}}_{\mathrm{Si}}$. Complete tiling of the surface requires three types of twisted (\ifmmode\pm\else\textpm\fi{}3\ifmmode^\circ\else\textdegree\fi{}) domains, two of which are rotationally equivalent.