Au enrichment and vertical relaxation of theCu3Au(111)surface studied by normal-incidence x-ray standing waves

Abstract

We have investigated the ${\mathrm{Cu}}_{3}\mathrm{Au}(111)$ surface, prepared under ultrahigh vacuum conditions by sputtering and annealing, by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and normal incidence x-ray standing waves (NIXSW). We find the surface to be depleted with Cu and enriched with Au at the same time, yielding a nominal Cu:Au ratio of 0.61:0.39 in the topmost layer. The STM images reveal that the first layer is nearly closely filled with atoms and contains a small amount of vacancies with an area concentration of about $5%$. Together with the Au enrichment, these cause local short-range disorder of the Au $p(2\ifmmode\times\else\texttimes\fi{}2)$ reconstruction. From this data, the average stoichiometry of the $p(2\ifmmode\times\else\texttimes\fi{}2)$ surface unit cell is estimated at $\mathrm{C}{\mathrm{u}}_{2.22}\mathrm{A}{\mathrm{u}}_{1.44}{\ensuremath{\square}}_{0.20}$ (instead of $\mathrm{C}{\mathrm{u}}_{3.00}\mathrm{A}{\mathrm{u}}_{1.00}{\ensuremath{\square}}_{0.00}$ of the ideal surface; \ensuremath{\square} denotes an atomic vacancy site). From NIXSW we find a significant outward relaxation of both the Cu and Au atoms of the topmost layer by 0.28 \AA{} and 0.33 \AA{}, which corresponds to $13%$ and $15%$ of the (111) bulk layer spacing of $\mathrm{C}{\mathrm{u}}_{3}\mathrm{Au}$. We suggest that this originates from a widening of the first/second layer spacing, by $6.8%$ and $8.8%$ for the Cu and Au atoms, respectively, plus an additional rigid increase in the second/third layer spacing by $6.2%$. We explain this by steric repulsions between Au atoms of the topmost layer, replacing smaller Cu atoms, and Au atoms in the second layer in combination with disorder. Finally, a lateral reconstruction, similar to that on the Au(111) surface, but with a much larger periodicity of 290 \AA{}, is identified from LEED.