Cs-Corrected Scanning Transmission Electron Microscopy Investigation of Dislocation Core Configurations at a SrTiO₃/MgO Heterogeneous Interface

Abstract

Heterostructures and interfacial defects in a 40-nm-thick SrTiO(3) (STO) film grown epitaxially on a single-crystal MgO (001) were investigated using aberration-corrected scanning transmission electron microscopy and geometric phase analysis. The interface of STO/MgO was found to be of the typical domain-matching epitaxy with a misfit dislocation network having a Burgers vector of ½ a(STO) <100>. Our studies also revealed that the misfit dislocation cores at the heterogeneous interface display various local cation arrangements in terms of the combination of the extra-half inserting plane and the initial film plane. The type of the inserting plane, either the SrO or the TiO(2) plane, alters with actual interfacial conditions. Contrary to previous theoretical calculations, the starting film planes were found to be dominated by the SrO layer, i.e., a SrO/MgO interface. In certain regions, the starting film planes change to the TiO(2)/MgO interface because of atomic steps at the MgO substrate surface. In particular, four basic misfit dislocation core configurations of the STO/MgO system have been identified and discussed in relation to the substrate surface terraces and possible interdiffusion. The interface structure of the system in reverse--MgO/STO--is also studied and presented for comparison.

Termination chemistry-driven dislocation structure at SrTiO3/MgO heterointerfaces

“…In addition, some experiments indicate that epitaxial growth can be achieved only for TiO 2 -terminated interface, whereas SrO-terminated interface leads to islanding or surface roughening 19 . However, the adhesion energies also reveal that either SrO or TiO 2 truncation is possible, which is in very good agreement with recent experiments that reveal mixed terminations due to the presence of steps at the interface 17,18 . Nevertheless, this is contrary to theoretical calculations that report only TiO 2 -terminated interfaces as being thermodynamically stable 21,22 .…”

“…Maintaining the same geometrical parameters and cube-on-cube orientation relationship, two different interfaces were constructed, corresponding to SrO-and TiO 2 -terminated STO. The rationale behind such a scheme, which is consistent with experimental observations [17][18][19] , is that, while rocksalt MgO has one neutral (100) plane, the neutral perovskite STO (100) planes alternate between SrO and TiO 2 . The size of the supercell in the interface normal direction was B4.3 nm.…”

“…To predict atomic-scale structure of the interface, atomic models of STO/MgO interfaces were constructed with the experimentally observed 17 17 (the misfit strain in this case is then 0.0012%). Minimizing misfit strain in the materials necessitates the formation of MDs at the interface.…”

“…In the remaining stackings, cation-cation or anion-anion nearest neighbours (or both) are present at the interface creating chemical frustration (unfavourable electrostatic interactions). On the basis of a similar electrostatic model, it was suggested that STO could not be grown on MgO unless the stacking sequence is initiated with TiO 2 planes 19 , although recent experiments have observed both interfaces 17,18 . The complex atomic arrangement at the STO/MgO interface within the coherent regions has been observed in the past, however, none of the studies consider different stackings in the same supercell, nor do they account for MDs at the interface due to system size limitations in first-principles calculations [20][21][22] .…”

Microstructure design for fast oxygen conduction

“…Kalabukhov et al 126 observed such dislocations at the LAOjSTO interface interspaced at every 15 nm. Similarly, Zhu et al 127 130 Since a high density of misfit dislocations is observed at oxide interfaces, they are another microstructural features that are being considered as possible fast-ion conduction pathways. Earlier experimental studies, such as those by Sillassen et al 67 have attributed high ionic conductivity partly to misfit dislocations.…”