Characterisation of Interfacial Phenomena in Multiferroic-based Thin Film Heterostructures via (Scanning) Transmission Electron Microscopy

Abstract

A range of transmission electron microscopy (TEM) and aberration-corrected (Cs-) scanning TEM (Cs-STEM) techniques were applied to FeV2O4 (FVO)-based and BiFeO3 (BFO)-based thin film heterostructures to investigate interfacial phenomena, such as lattice misfit strains, structural defects and chemical intermixing. FVO films were grown on various (001)-oriented substrates including SrTiO3 (STO), Nb-doped STO, MgAl2O4 (MAO), La0.67Sr0.33MnO3 (LSMO)-buffered STO and SrRuO3 (SRO)-buffered STO via pulsed laser deposition (PLD). Despite large substrate-film lattice mismatches, the as-grown FVO films were found to be both phasepure and epitaxial by X-ray diffraction (XRD) and TEM-based selected area diffraction pattern (SADP) analysis. High-resolution bright-field and Z-contrast images recorded down the [110] zone axis, revealed {111} stacking faults across the FVO films on both STO and MAO, together with other complex faults and point defects. These stacking faults in FVO/Nb:STO were suspected to be related to the presence of Fe3O4, as physical measurements revealed traces of Fe3+ as well as the non-stoichiometric nature of the FVO films. However, both energy-dispersive X-ray spectroscopy (EDS) maps and line scans showed Fe-deficiency and V-enrichment inside the {111} faults. EDS line scans across FVO-STO and FVO-MAO interfaces suggested interdiffusion in which Fe and V diffused into the substrate layer and elements from the substrates diffused into the FVO. Epitaxial BFO films (>50 nm) grown on LaAlO3(LAO) and LSMO (~3 nm)-buffered LAO were found by XRD, RSM and AFM measurements to exhibit only the tetragonallike (T-like) BFO phase. (S)TEM characterisation was applied to investigate the phase-stabilisation mechanism for single T-like BFO. However, T-like and rhombohedral-like (R-like) BFO phases were observed in (S)/TEM images from BFO specimens prepared by mechanical tripod polishing. It was later found that the formation of mixed-phases was induced by stresses applied during mechanical polishing. Defects with additional rows of extra atoms were observed throughout the BFO films. EDS analysis showed that the defected regions were Bi-rich. It was hypothesised that the β-Bi2O3phase formed during PLD growth was strained in the in-plane direction and tetragonally distorted in the out-of-plane direction, subsequently promoting the growth of surrounding T-like BFO.