  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.
