Polar catastrophe in ultra-thin limit: A case of rare-earth perovskite LaNiO3

TL;DR

This study investigates the polar mismatch issue in ultra-thin LaNiO3 films on SrTiO3, revealing that ultra-thin films become insulating and undergo structural changes to compensate for polarity mismatch, with oxygen vacancies playing a key role.

Contribution

It demonstrates that ultra-thin LaNiO3 films require structural and chemical reconstruction, including oxygen vacancies, to address polar mismatch, which is a novel insight into interface physics.

Findings

Ultra-thin LaNiO3 films are insulating due to polarity mismatch.

Formation of La2Ni2O5 phase in ultra-thin films.

Oxygen vacancies reduce electric field at the interface.

Abstract

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.