Effect of polar discontinuity on the growth of LaNiO3/LaAlO3 superlattices (original) (raw)
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Effect of polar discontinuity on the growth of LaNiO[sub 3]/LaAlO[sub 3] superlattices
Applied Physics Letters, 2010
We have conducted a detailed microscopic investigation of [LaNiO3(1 u.c.)/LaAlO3(1 u.c.)]N superlattices grown on (001) SrTiO3 and LaAlO3 to explore the influence of polar mismatch on the resulting electronic and structural properties. Our data demonstrate that the initial growth on the non-polar SrTiO3 surface leads to a rough morphology and unusual 2+ valence of Ni in the first LaNiO3 layer, which is not observed after growth on the polar surface of LaAlO3. A newly devised model suggests that the polar mismatch can be resolved if the perovskite layers grow with an excess of LaO, which also accounts for the observed electronic, chemical, and structural effects.
Epitaxial growth of (111)-oriented LaAlO 3/LaNiO 3 ultra-thin superlattices
2012
Abstract The epitaxial stabilization of a single layer or superlattice structures composed of complex oxide materials on polar (111) surfaces is severely burdened by the reconstructions at the interface that commonly arise to neutralize the polarity. We report on the synthesis of high quality LaNiO 3/mLaAlO 3 pseudo cubic (111) superlattices on polar (111)-oriented LaAlO 3, the proposed complex oxide candidate for a topological insulating behavior.
Confinement-induced metal-to-insulator transition in strained LaNiO_{3}/LaAlO_{3} superlattices
Physical Review B, 2011
Using density functional theory calculations including a Hubbard U term we explore the effect of strain and confinement on the electronic ground state of superlattices containing the band insulator LaAlO3 and the correlated metal LaNiO3. Besides a suppression of holes at the apical oxygen, a central feature is the asymmetric response to strain in single unit cell superlattices: For tensile strain a band gap opens due to charge disproportionation at the Ni sites with two distinct magnetic moments of 1.45µB and 0.71µB. Under compressive stain, charge disproportionation is nearly quenched and the band gap collapses due to overlap of d 3z 2 −r 2 bands through a semimetallic state. This asymmetry in the electronic behavior is associated with the difference in octahedral distortions and rotations under tensile and compressive strain. The ligand hole density and the metallic state are quickly restored with increasing thickness of the (LaAlO3)n/(LaNiO3)n superlattice from n = 1 to n = 3.
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 an emergent 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. KEYWORDS: Interfacial polarity mismatch, ultra--thin film of perovskite oxides, chemical and structural reconstruction, oxygen vacancy.
ACS Applied Materials & Interfaces, 2011
Transition metal oxides exhibit a huge variety of intrinsic functionalities such as magnetism, superconductivity, thermoelectricity, ferroelectricity or multiferroic behavior. 1À26 Experimentally, there is also growing interest in the creation of "oxide heterostructures" consisting of alternating layers of different transitionÀmetal oxides. 2,3 OxideÀoxide interfaces often give rise to novel physical phenomena not exhibited in the bulk constituents, due to the rearrangement of charge, spins, orbitals, lattice and the resulting rebalancing of their mutual interactions. Thus interface can be exploited to modify electronic structure and to tailor novel properties and behaviors.
Conductivity enhancement of ultrathin LaNiO3 films in superlattices
Applied Physics Letters, 2010
The resistance of superlattices composed of bilayers of ultrathin ͑ϳ4 unit cells͒ of LaNiO 3 and ϳ3 unit cells of insulating SrTiO 3 is explored as a function of temperature and the number of bilayers. All superlattices with more than one bilayer are metallic, whereas a single bilayer is insulating. Two possible interpretations of the electrical characteristics of the superlattices are discussed. The first model involves conduction in parallel-connected layers, whereas the second model assumes coupling of layers, each of which is near the percolation threshold for a metal-insulator transition.
Interface-Induced Polarization in SrTiO3-LaCrO3Superlattices
Advanced Materials Interfaces, 2016
Epitaxial interfaces and superlattices comprised of polar and non-polar perovskite oxides have generated considerable interest because they possess a range of desirable properties for functional devices. In this work, emergent polarization in superlattices of SrTiO 3 (STO) and LaCrO 3 (LCO) is demonstrated. By controlling the interfaces between polar LCO and nonpolar STO, polarization is induced throughout the STO layers of the superlattice. Using x-ray absorption near-edge spectroscopy and aberration-corrected scanning transmission electron microscopy displacements of the Ti cations off-center within TiO 6 octahedra along the superlattice growth direction are measured. This distortion gives rise to built-in potential gradients within the STO and LCO layers, as measured by in situ x-ray photoelectron spectroscopy. Density functional theory models explain the mechanisms underlying this behavior, revealing the existence of both an intrinsic polar distortion and a built-in electric 2 field, which are due to alternately positively and negatively charged interfaces in the superlattice. This study paves the way for controllable polarization for carrier separation in multilayer materials and highlights the crucial role that interface structure plays in governing such behavior.
Physical Review B
The interfacial atomic structure of a metallic LaNiO3/LaAlO3 superlattice grown on a LaSrAlO4 substrate was investigated using a combination of atomically resolved electron energy loss spectroscopy (EELS) at the Al K, Al L2,3, Sr L2,3, Ni L2,3, La M4,5, and O K edges as well as hybridization mapping of selected features of the O K-edge fine structure. We observe an additional La1−xSrxAl1−yNiyO3 layer at the substrate-superlattice interface, possibly linked to diffusion of Al and Sr into the growing film or a surface reconstruction due to Sr segregation. The roughness of the LaNiO3/LaAlO3 interfaces is found to be on average around one pseudocubic unit cell. The O K-edge EELS spectra revealed reduced spectral weight of the prepeak derived from Ni-O hybridized states in the LaNiO3 layers. We rule out oxygen nonstoichiometry of the LaNiO3 layers and discuss changes in the Ni-O hybridization due to heterostructuring as possible origin.