Germanium growth orientation on SrTiO₃ (001) 2 × 1 surface: Role of surface reduction

We present a theoretical study of germanium growth orientation dependence on SrTiO₃ surface structure using first-principles calculations. Contrary to much work on the SrTiO₃ surface which considers the simple TiO₂ termination, we work on the now well known 2 × 1 reconstructed surface, which is terminated with a TiO₂ double layer.(29) We study the oxygen reduction at this surface maintaining the 2 × 1 reconstruction. Analysis of its electronic structure shows that the reduction leads to a metallic surface and changes of the surface bond characteristics. Therefore, the growth routes for Ge epitaxy are studied. It is demonstrated that the reduction favors the creation of the Ge-O bonds by weakening the strength of the Ti-O bonds. It gives the possibility of Ge[001] growth direction, which is not possible on a nonreduced surface.