Observation of an anti-phase domain structure in the Cu{100}/Sn surface alloy system

Low energy electron diffraction (LEED), scanning tunneling microscopy (STM) and the normal incidence X-ray standing wave (NIXSW) technique have been used to determine the structure of the first surface phase formed, on Cu{1 0 0} upon deposition of 0.21 monolayers (ML) of tin. At this coverage the LEED images exhibits a modified p(2 x 2) structure with split fractional order diffraction spots. A double scattering LEED pattern simulation consistent with both the Sn surface coverage and the complex split beam LEED pattern suggests an ordered phase based on a p(2x2) structure with light anti-phase domain walls. Possible adsorption sites are identified from NIXSW data by triangulation of the distances between the Sn atoms and the (2 0 0) and (1 1 1) planes. The results indicate that Sn atoms adsorb in a fourfold site but does not distinguish between overlayer and substitutional sites. The STM data reveal small domains of p(2 x 2) structure formed by the Sn atoms being adsorbed in substitutional sites. These domains typically contain 16 Sn atoms arranged in a square 4 x 4 array, however larger and smaller rectangular domains are also observed with sides containing three, five and six Sn atoms. The width of the domain wall is found to vary with rows of one, three or five copper atoms separating the domains in both the [0 1 1] and [0 11] directions. The STM images indicate that the Cu atoms in the domain walls are vertically displaced from their expected positions. This is attributed to elastic strain due to the large metallic diameter of Sn (0.162 nm) relative to Cu (0.127 nm). (C) 2003 Elsevier B.V. All rights reserved.