Surface structure analysis of Eu Zintl template on Ge(001)

The surface reconstructions and surface core level shifts of 0.25 monolayer (ML), 0.5 ML and 1.0 ML of europium (Eu) on the Ge(001) surface are investigated experimentally and theoretically, using molecular beam epitaxy (MBE), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Starting with a clean Ge(001) surface we find that the surface reconstruction changes from the initial (2 × 1) to a (3 × 2), (8 × 1) and a final (3 × 1) pattern with increasing Eu coverage of 0.25 ML, 0.5 ML and 1.0 ML, respectively. XPS scans demonstrate that the Ge 3d core level shifts to higher binding energies with increasing Eu coverage. We find that Eu is in the 2+ oxidation state, transferring its charge to the Ge surface, resulting in the disappearance of the asymmetry of the dimer tilt at a coverage of 0.5 ML of Eu. From density functional calculations we find that the Ge dimers flatten out at 0.5 ML Eu coverage and partially break apart at 1.0 ML Eu.