Electronic and atomic structure of Co/Ge nanoislands on the Ge(111) surface

Abstract

We report on a detailed investigation of the electronic and atomic structure of nanometer-size Co/Ge islands obtained by solid-state reactive deposition of Co atoms on a $\mathrm{Ge}(111)c(2\ifmmode\times\else\texttimes\fi{}8)$ surface. Relying on scanning tunneling microscopy (STM) and spectroscopy (STS) measurements combined with density functional theory based calculations, the atomic structure of the Co/Ge(111)$\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}\phantom{\rule{0.16em}{0ex}}R13.{9}^{\ensuremath{\circ}}$ surface reconstruction is determined. Real-space STM imaging combined with Fourier-transform analysis reveals the coexistence of two inequivalent phases of $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}\phantom{\rule{0.16em}{0ex}}R13.{9}^{\ensuremath{\circ}}$ reconstructed Co/Ge nanoislands that are rotated by $+13.{9}^{\ensuremath{\circ}}$ and $\ensuremath{-}13.{9}^{\ensuremath{\circ}}$ with respect to the $[2\overline{11}]$ direction. STS spectra probe a small band gap that varies within the $\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}\phantom{\rule{0.16em}{0ex}}R13.{9}^{\ensuremath{\circ}}$ surface unit cell between 10 and $250\phantom{\rule{4pt}{0ex}}\mathrm{meV}$, suggesting local metallic behavior. According to the proposed atomic-structure model, each Co/Ge(111)$\sqrt{13}\ifmmode\times\else\texttimes\fi{}\sqrt{13}\phantom{\rule{0.16em}{0ex}}R13.{9}^{\ensuremath{\circ}}$ surface unit cell contains one Ge adatom and six Co atoms that are located at hollow sites below the top surface Ge layer and that are stacked in the form of an equilateral triangle. The Ge adatom is located asymmetrically with respect to the Co triangle and occupies two different, yet physically equivalent, positions, giving rise to two chiral phases of Co/Ge nanoislands. The Co/Ge valence band is dominated by Co atom derived 3$d$ states, while states in the conduction band stem from Ge adatom and Ge rest-atom derived states. Analysis of the bonding properties confirms the stability of the proposed Co/Ge atomic structure and reveals significant charge transfer from Co atoms to Ge rest atoms, suggesting ionic or metallic-covalent interaction.