Elsevier

Acta Materialia

Volume 47, Issue 1, 11 December 1998, Pages 183-198
Acta Materialia

Atomistic structure of misfit dislocations in SrZrO3/SrTiO3 interfaces

https://doi.org/10.1016/S1359-6454(98)00334-6Get rights and content

Abstract

The atomistic structure of misfit dislocations at heterointerfaces between two ionic crystals with the perovskite structure, SrZrO3 and SrTiO3, is investigated. The interfaces were fabricated by metal–organic deposition of SrZrO3 layers on (001) SrTiO3 single crystal substrates. Under appropriate conditions the SrZrO3 layer grows epitaxially, with its crystal lattice parallel to the lattice of the substrate (“cube-on-cube” orientation relationship). In the layer/substrate interface a square network of dislocations accommodates the misfit between corresponding spacings in the two crystals. These misfit dislocations have edge character, 〈010〉 line directions, and 〈100〉 Burgers vectors parallel to the interface. The SrZrO3/SrTiO3 interface has been imaged with the misfit dislocations in end-on projection by high-resolution transmission electron microscopy and also by high-resolution scanning transmission electron microscopy. Quantitative image analysis has shown that a (002) layer of TiO2 terminates the SrTiO3 crystal, and the SrZrO3 crystal commences with a (002) layer of Sr–O. Concerning the {200} layers normal to the interface it was found that in interface regions of good match, between the misfit dislocations, the perovskite structure continues straight through the interface: Sr–O layers of SrTiO3 continue as Sr–O layers of SrZrO3, and TiO2 layers continue as ZrO2 layers. Where the misfit dislocations reside, however, in regions of poor match, a lateral offset exists between the two crystals and disrupts the perovskite structure: on the core plane (the symmetry plane) of the misfit dislocations a (200) TiO2 layer of SrTiO3 continues as a (200) SrO layer in SrZrO3.

Introduction

Langjahr et al.1, 2have shown that by means of MOD (metal–organic deposition[3]) one can grow epitaxial layers of SrZrO3 on (001) SrTiO3 substrates. Like SrTiO3[4], the SrZrO3 layer adopts the perovskite structure (Fig. 1), even though the room temperature modification only has pseudo-cubic Pnma symmetry5, 6. The SrZrO3 layer grows with its lattice parallel to the lattice of the SrTiO3 substrate. Thus, the misfit of corresponding spacings across the SrZrO3/SrTiO3 interface coincides with the misfit1aT−aZaZ=−4.9%between the lattice parameter aT=0.39 nm of SrTiO3 and aZ=0.41 nm, the arithmetic mean of the three SrZrO3 lattice parameters. At the SrZrO3/SrTiO3 interface, a square network of edge-type dislocations accommodates the misfit (1). These misfit dislocations have pure edge character, 〈100〉 Burgers vectors parallel to the interface, and 〈010〉 line directions (here and in the following, we refer to a cartesian reference frame parallel to the axes of the two crystal lattices, see Fig. 4). So far, however, details of the ion arrangement at the SrZrO3/SrTiO3 interface have not been studied[7]. In this paper we analyze the misfit dislocation core structures in MOD-grown SrZrO3/SrTiO3 interfaces from images we have obtained by HRTEM (high-resolution transmission electron microscopy) and by high-resolution STEM (scanning transmission electron microscopy).

Section snippets

High-resolution transmission electron microscopy

To image the SrZrO3/SrTiO3 interface in cross section, viewing down the [010] line direction of one set of misfit dislocations, we prepared TEM samples by applying the technique of Strecker et al.1, 8. For HRTEM we employed a JEM 4000 EX (JEOL) microscope, equipped with a top-entry objective lens (pole piece UHP-40H) and a LaB6 filament. Operating at an accelerating voltage of 400 kV, this microscope achieves a point resolution of 0.175 nm. At several different defocus settings we recorded images

High-resolution transmission electron microscopy

By means of HRTEM and high-resolution STEM we have recorded cross-sectional images of the SrZrO3/SrTiO3 interface in [010] projection. First, we discuss the results obtained by HRTEM. Fig. 2 presents two cross-sectional HRTEM images of the same interface region, recorded at two different defocus settings Δf. The upper image was recorded nearly at Gauß focus (Δf≈0), while for the lower image the defocus corresponds to the first reverse passband of the contrast transfer function (Δf≈−70 nm). The

Atomistic models of misfit dislocation cores

Without further analysis, the above experimental observations on misfit dislocation cores appear to agree with a variety of atomistic models, which we deduce in this section. Fig. 4 presents a schematic drawing of a misfit dislocation in the SrZrO3/SrTiO3 interface. Based on our experimental observations we assume that (i) the crystal lattices of SrZrO3 (top) and SrTiO3 (bottom) have the same orientation (“cube-on-cube orientation relationship”), (ii) the interface is sharp, flat, and lies

Processing of experimental HRTEM images

In order to compare our experimental HRTEM images with simulated images we have digitized the negatives and improved the signal-to-noise ratio of the images in the following three steps: First, we averaged over the intensity distributions of several dislocation images, which we have recorded under nearly identical HRTEM imaging conditions: in the same TEM specimen, at the same defocus, and in regions of similar specimen thickness. This procedure delivered best results (minimum level of

Discussion

Our quantitative evaluation of HRTEM images shows that in regions of good match the ion arrangement at the interface maintains the perovskite structure. This result corresponds to what one would expect: the perovskite structure constitutes the equilibrium structure of both SrZrO3 and SrTiO3, and the misfit between the corresponding interatomic spacings along the interface is rather small. Under the boundary condition of continuing the perovskite structure straight across the interface in

Conclusion

From our study on SrTiO3/SrZrO3 interfaces fabricated by MOD of SrTiO3 on (001) SrTiO3 single crystal substrates we draw the following conclusions: a layer of TiO2 terminates the SrTiO3 substrate crystal. In interface regions of good match the interface maintains the perovskite structure also adopted by the two individual crystals. In regions of poor match, misfit dislocations form with 〈100〉 Burgers vectors and 〈010〉 line directions. On the {200} core plane of these misfit dislocations a TiO2

Acknowledgements

The financial support of the MZT, project No. Z1-7964-0106-96 10083 (Slovenia), is acknowledged (A.R.). We thank F. F. Lange (University of California, Santa Barbara) and T. Wagner (Max-Planck-Institut für Metallforschung) for their collaboration on metal–organic deposition, S. J. Pennycook (Oak Ridge National Laboratory) for his assistance with the VG 603 DSTEM, and M. Wilson and M. Exner (Max-Planck-Institut für Metallforschung) for helpful discussions on the compressibility of ions in

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