Structure and width of Au-adsorbed narrow Si(111) terraces

The structure and width of Au-adsorbed narrow terraces on a vicinal Si(111) surface misoriented toward the $\left[1¯1¯2\right]$ direction are studied by scanning tunneling microscopy. We find that, at a coverage of 0.15 ML, narrow Au-adsorbed bunched steps are separated by wide $7\times 7$ terraces. The Au-adsorbed terraces are classified into three types according to the structures on the neighboring terraces. Where the Au-adsorbed terrace is next to the $7\times 7$ terrace, the boundary facing the $7\times 7$ terrace has a characteristic structure depending on whether it faces the faulted or the unfaulted halves. On the other hand, where the Au-adsorbed structure is sandwiched by the other Au-adsorbed terraces, the width of the Au-adsorbed terrace along the $\left[1¯1¯2\right]$ direction can be expressed by $\mathrm{}(5N+5+2/3)\mathrm{}\sqrt{3}/2\mathrm{}$, where the length is represented in units of the lattice constant of the $1\times 1$ surface, and the $N$ is the number of the $5\times 2$ unit cells along the $\left[1¯1¯2\right]$ direction. For $N=0\mathrm{}$, the terrace width is equal to that of a (755) facet. On this terrace, $5\times 2$ reconstruction cannot be observed although the size of the terrace is sufficient to form $5\times 2$ reconstruction. We propose a model for the boundaries between the $7\times 7$ terrace and the Au adsorbed structure on the narrow (111) terraces.