Two-dimensional silicide 5×3 structure on Cu(001) as seen by scanning tunneling microscopy and helium-atom scattering

The incommensurate 5×3 structure, which is formed when silicon is deposited on copper(001) via a saturation exposure to silane at 420 K, has been examined with helium-atom scattering and scanning tunneling microscopy. The surface was found to be a two-dimensional copper-silicon alloy having large domains of a near perfect hexagonal structure with ${\mathrm{Cu}}_2$Si stoichiometry. The surface layer displays a rotation of one surface-lattice vector with respect to the substrate of 4.4°, and a high-order commensuration along the other surface-lattice vector, without rotation. The overlayer exhibited unequal densities of the four possible domains and it appears that the long-range order is influenced by the presence of steps on the copper(001) surface. The overall atomic density in the surface layer is 27.6% higher than on the unreconstructed surface. The driving force for the formation of the sheared hexagonal structure is believed to be the need to increase surface coordination and density together with a strongly directional in-plane bonding between Cu and Si atoms.