Germanium adsorption and initial growth on SrTiO₃ (001) surface: A first-principles investigation

The initial stages of germanium (Ge) adsorption and heteroepitaxial growth on the SrTiO₃ (001) surface have been investigated with first-principles calculations. Our results reveal that the 2 × 1 DL TiO₂ reconstruction plays a major role in the Ge chemisorption. This surface reconstruction involves a charge transfer between Ti and O atoms at the surface that locally increases the attractiveness for Ge nucleation. The calculated charge transfer is in good agreement with the core level shift observed by soft X-ray photoemission spectroscopy. Moreover, our potential-energy surface calculations for Ge adatoms on the SrTiO₃ (001) 2 × 1 DL TiO₂ surface show that the surface diffusions of Ge atoms are highly anisotropic. Ge adatoms are easy to jump from other sites to the most stable site nearby the floating oxygen with energy barriers <0.45 eV. Therefore, the Ge adsorption configuration around surface floating oxygen atom, a particularity of this surface, is identified as the nucleation center of the further Ge (111) growth. A route for a bidimensional (111)-oriented growth of Ge on STO(001) surface has been identified and confirmed by the X-ray photoemission spectroscopy experiment results. These findings help us to understand how the 2 × 1 DL TiO₂ reconstruction can be the starting point for the growth of (111)-oriented Ge on STO(001).