Au-induced superstructure formation on vicinal Si(001) studied by low-energy electron diffraction and reflectance anisotropy spectroscopy

We present a low-energy electron diffraction (LEED) and reflectance anisotropy spectroscopy (RAS) study of the ordering of monolayer Au coverages on clean $1°$ and $4°$ offcut $\mathrm{Si}\mathrm{}\left(001\right)-(1\mathrm{}\times 2)$ vicinal surfaces. On depositing $2.5\pm 0.1\mathrm{ML}$ of Au on the $4°$ offcut sample followed by annealing up to $900°\mathrm{C},$ a well-ordered single-domain $\mathrm{Si}\mathrm{}\left(001\right)-(\sqrt{26}\times 3)-\mathrm{Au}$ superstructure is formed, as revealed by LEED, indicating a highly ordered anisotropic surface. Changes in splitting of the LEED spots indicates a larger step separation on the Si(001)-Au surface than on the clean surface. RAS is used to follow the formation of this structure with increased annealing temperature after room-temperature deposition of Au. Significant changes in optical anisotropy are observed. Comparison of the $\mathrm{Si}\mathrm{}\left(001\right)-(\sqrt{26}\times 3)-\mathrm{Au}$ superstructure grown on $1°$ offcut Si(001) shows a similar RA line shape, although reduced in intensity. Scanning tunneling microscopy (STM) images of the double-domain $(\sqrt{26}\times 3)-\mathrm{Au}$ superstructure, together with the LEED data presented here, indicates that the single-domain superstructure will be terminated by long Au row structures aligned parallel to the surface steps. We fit the RA spectrum for the $4°$ offcut $(\sqrt{26}\times 3)-\mathrm{Au}$ structure using a Kramers-Heisenberg dielectric function expression and identify two surface induced optical anisotropic features at 2.8 and 3.9 eV that are stronger for light polarized perpendicular to these chains. Similarities with the RA response from the $\mathrm{Si}\left(111\right)-(5\mathrm{}\times 2)-\mathrm{A}\mathrm{u}$ superstructure are discussed.