High-resolution core-level photoemission study of Eu-induced(3×2)∕(3×4)reconstruction on Ge(111)

Abstract

We have investigated Eu-induced $\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$ reconstruction by high-resolution core-level photoelectron spectroscopy using synchrotron radiation and low-energy electron diffraction. Recent scanning tunneling microscopy (STM) observations [Phys. Rev. B 73, 125332 (2006)] revealed that the Ge arrangement of this reconstruction can be well described in terms of the honeycomb chain-channel (HCC) geometry proposed earlier for metal/$\mathrm{Si}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}1)$ and $\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)$ surfaces; the Eu atoms, however, were found to reside at two different adsorption sites in the $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$ reconstruction, in contrast to the equivalent adsorption sites (e.g., $T4$) occupied in the case of Si. The present photoemission results provide further information about the atomic arrangement of $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$. In particular, we show that the Ge $3d$ core-level data cannot be interpreted by the HCC structure with the Eu atoms adsorbed only on $T4$ sites, giving a spectroscopic support for the suggestions based on the earlier STM data. We consider here a modified HCC-based configuration for the $\mathrm{Eu}∕\mathrm{Ge}(111)\text{\ensuremath{-}}(3\ifmmode\times\else\texttimes\fi{}2)∕(3\ifmmode\times\else\texttimes\fi{}4)$ surface where the Eu atoms occupy two different sites in the empty channel between the neighboring Ge honeycomb chains. The atomic models are discussed in the context of the Ge $3d$ and Eu $4f$ data as well as the previous results available in the literature. Finally, we propose a structural model that allows us to account for the present photoemission and earlier STM findings.