Polar catastrophe in ultra-thin limit: A case of rare-earth perovskite LaNiO3
We address the fundamental issue of growth of perovskite ultra-thin films under the condition of a strong polar mismatch at the heterointerface exemplified by the growth of a correlated metal LaNiO$_3$ on the band insulator SrTiO$_3$ along the pseudo cubic [111] direction. While in general the metallic LaNiO$_3$ film can effectively screen this polarity mismatch, we establish that in the ultra-thin limit, films are insulating in nature and require additional chemical and structural reconstruction to compensate for such mismatch. A combination of in-situ reflection high-energy electron diffraction recorded during the growth, X-ray diffraction, and synchrotron based resonant X-ray spectroscopy reveal the formation of a chemical phase La$_2$Ni$_2$O$_5$ (Ni$^{2+}$) for a few unit-cell thick films. First-principles layer-resolved calculations of the potential energy across the nominal LaNiO$_3$/SrTiO$_3$ interface confirm that the oxygen vacancies can efficiently reduce the electric field at the interface.