Germanium Adsorption and Initial Growth on SrTiO₃(001) Surface: A First-Principles Investigation

Abstract

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).

Germanium Growth Orientation on SrTiO3(001) 2 × 1 Surface: Role of Surface Reduction

“…E slab , E Ge , and E total denote the calculated total energies per supercell calculated for the clean STO slab, isolated Ge atom, and the slab with Ge adsorbed on the surface, respectively. ΔE ads is negative if the adsorption of atomic Ge on the surface is favorable and exothermic, which is the opposite of the definition of adsorption energy in our previous work 27 to be in agreement with physicist habits. Thus, a lower adsoprtion energy is significant of a more stable configuration.…”

“…Former study shows that the Bader partial charges of Ti (2.17| e |) and O (−1.26| e |) in STO bulk are far from formal ionic charges (4| e | and −2| e |) because of the partly covalent nature of the Ti–O bond. The Ti–O bond covalency in TiO 2 surface layers is larger than that in the STO bulk because of the larger deviations of surface Ti and O partial charges from formal ionic charges.…”

“…STO(001) has alternating SrO and TiO 2 layers; therefore, it can have two nonpolar terminations: SrO and TiO 2 . The STO(001) 2 × 1 double-layered (DL) TiO 2 surface , adopted in our former and present work is different from other simple TiO 2 terminated surfaces, and one of its peculiarities is the occurrence of a floating oxygen atom. This kind of reconstruction was found to be stable in the presence of a substantial amount of oxygen vacancies after annealing the STO surface in UHV before growing semiconductor.…”

“…Unfortunately, detailed knowledge about this dependence is still lacking because of the difficulty to obtain the detailed surface reaction and semiconductor growth information experimentally. First-principles calculation is a tractable method to achieve information for the atom adsorption and layer growth process on surfaces and has been successfully applied to study the atom adsorption and film growth processes on perovskite surfaces. − …”

“…This kind of reconstruction was found to be stable in the presence of a substantial amount of oxygen vacancies after annealing the STO surface in UHV before growing semiconductor. In our recent work, 27 we studied the atom adsorption and the route for a (111)-oriented growth of Ge on the STO(001) 2 × 1 TiO 2 surface and we successfully compared theoretical results with a soft X-ray photoemission spectroscopy experiment. Our investigation proved that the floating oxygen atom structure is very critical for the Ge(111) growth.…”