Misfit strain driven cation inter-diffusion across an epitaxial multiferroic thin film interface

Abstract

Cation intermixing at functional oxide interfaces remains a highly controversial area directly relevant to interface-driven nanoelectronic device properties. Here, we systematically explore the cation intermixing in epitaxial (001) oriented multiferroic bismuth ferrite (BFO) grown on a (001) lanthanum aluminate (LAO) substrate. Aberration corrected dedicated scanning transmission electron microscopy and electron energy loss spectroscopy reveal that the interface is not chemically sharp, but with an intermixing of ∼2 nm. The driving force for this process is identified as misfit-driven elastic strain. Landau-Ginzburg-Devonshire-based phenomenological theory was combined with the Sheldon and Shenoy formula in order to understand the influence of boundary conditions and depolarizing fields arising from misfit strain between the LAO substrate and BFO film. The theory predicts the presence of a strong potential gradient at the interface, which decays on moving into the bulk of the film. This potential gradient is significant enough to drive the cation migration across the interface, thereby mitigating the misfit strain. Our results offer new insights on how chemical roughening at oxide interfaces can be effective in stabilizing the structural integrity of the interface without the need for misfit dislocations. These findings offer a general formalism for understanding cation intermixing at highly strained oxide interfaces that are used in nanoelectronic devices.

Atomic structure and microstructures of supertetragonal multiferroicBiFeO3thin films

“…It is worth noting that this width is about two times narrower than the one reported in Ref. [22] and estimated from the Fe-L 23 and La-M 45 integrated intensity; using the same procedure (not shown), we have measured an interface width of 1.5 nm, confirming that cationic interdiffusion has not exceed three pseudocubic unit cells. This narrower interface width compare to the one reported in Refs.…”

“…A cation interdiffusion has been evidenced in Ref. [22], and a continuous expansion of the BFO lattice associated with FeO 6 distortions and correlated to a polarization relaxation has been reported in Ref. [21].…”

“…[20]. Very recently, the structure of BFO//LAO interfaces has been investigated by spherical aberration (Cs)-corrected annular bright field-scanning transmission electron microscopy (ABF-STEM) [21] and by a combination of high-angle annular dark field (HAADF)-STEM and electron energy loss spectroscopy (EELS) experiments [22]. A cation interdiffusion has been evidenced in Ref.…”

Measurement Error in Atomic-Scale Scanning Transmission Electron Microscopy—Energy-Dispersive X-Ray Spectroscopy (STEM-EDS) Mapping of a Model Oxide Interface

“…While great strides have been made in the precision synthesis of atomically-sharp thin film heterostructures , interface charge (Nakagawa et al, 2006), strain (Sankara Rama Krishnan et al, 2014), and entropy can all drive film-substrate intermixing that can greatly affect properties. With the development of commercial spherical aberration (C s ) corrected microscopes, analytical scanning transmission electron microscopy (STEM) is now increasingly used to characterize nanoscale interfaces .…”

Surface and finite size effects impact on the phase diagrams, polar, and dielectric properties of (Sr,Bi)Ta2O9 ferroelectric nanoparticles

“…The characteristic thickness of the layer enriched by defects is determined by the screening length, and their maximal concentration is limited by steric effect [27,28].…”