Structural analysis ofBa-induced surface reconstruction onSi(111)by means of core-level photoemission

The surface structures of the $\mathrm{Ba}$-induced $3\times 1$, $5\times 1$, and $2\times 8$ reconstructions on the $\mathrm{Si}\left(111\right)$ surface were investigated by means of $\mathrm{Si}2p$ and $\mathrm{Ba}4d$ core-level photoemission and the coverage was estimated by x-ray photoemission (XPS). In the $\mathrm{Si}2p$ core-level spectrum of every reconstructed surface, three prominent surface components that are shifted to lower and higher binding energies from the bulk peak have been observed. In the case of the $3\times 1$ surface, these surface components are well explained by the so-called honeycomb-chain-channel (HCC) model which is considered to be the most plausible structure for alkali-metal-induced $3\times 1$ surfaces. The similarity of the core-level spectra between the $2\times 8$ surface and the $\mathrm{Ca}$- or $\mathrm{Yb}$-induced $2\times 1$ surface suggests that both the $2\times 8$ and $2\times 1$ surfaces have similar short range $\mathrm{Si}$ structures. The observed double component in the $\mathrm{Ba}4d$ core-level spectra of the $5\times 1$ and $2\times 8$ surfaces implies the existence of more than one $\mathrm{Ba}$ adsorption configuration on these surfaces. Our estimated metal coverages by XPS for the $5\times 1$ and $2\times 8$ surfaces are $\sim 0.2$ and $\sim 0.3$ monolayers which are different from those of previous studies for the $\mathrm{Ca}∕\mathrm{Si}\left(111\right)$ system. On the basis of these results we proposed possible surface structural models for the $5\times 1$ and $2\times 8\text{−}\mathrm{Ba}$ surfaces based on a combination model of the HCC and Seiwatz chain structures. The intensity change of each $\mathrm{Si}2p$ surface component and the other experimental results are qualitatively explained with the models.