Structural properties of the formation of yttrium germanides thin films on the Si(111) surface

We have performed first principles total energy calculations to investigate the deposit of yttrium digermanide on the Si(111) surface. We have used the periodic density functional theory as implemented in the Quantum-ESPRESSO package. For the adsorption of a monolayer of yttrium digermanide on the Si(111)-(1 × 1) surface, we have found that the most stable geometry corresponds to a configuration with Y atoms occupying the T4 site above a second layer Si atom, and with a Ge bilayer on top of the structure. The atomic structure of the Ge bilayer is similar to that of Si (Ge) in the bulk but rotated 180° with respect to the crystal. For the three dimensional growth of a few layers of yttrium digermanide on Si(111) we have considered a hexagonal structure with (√3 × √3) periodicity, similar to the one found in the growth of few layers of YSi2 on Si(111): graphite-like Ge planes (with vacancies) intercalated with yttrium planes. As in the case of a single layer of YGe2, there is a formation of a Ge bilayer on top of the structure. In this case, the Gedown atoms of the bilayer, which are on top the vacancies, move down towards the vacancy, while Ge atoms in the graphitic layer, which are below the Geup atoms of the bilayer, are displaced towards the vacancy.