Twin wall distortions through structural investigation of epitaxial BiFeO 3 thin films
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this work, epitaxial (001) BiFeO3 thin films were deposited on SrTiO3 and TbScO3 single-crystal substrates and analyzed with high-resolution x-ray diffraction—reciprocal space mapping. A basic method was developed to extract structural details of the as-grown BiFeO3 film, including (i) epitaxial strain, (ii) ferroelastic domains, and (iii) lattice rotations. After demonstrating the method, extrinsic distortions at vertical twin walls were determined for specific BiFeO3 heterostructures. A relatively large distortion (0.20° 6 0.08°) was measured in a multidomain (12) and incoherent film, while a nearly intrinsic distortion (0.04° 6 0.03°) was measured in a two-domain coherent film. This work offers insights into the structure of multiferroic BiFeO3 thin films with a general approach that is appropriate for low symmetry epitaxial heterostructures.
I. INTRODUCTION
The low rhombohedral symmetry of prototypical multiferroic BiFeO3 often causes complex domain structures, even in bulk crystals.1 A common approach to limit the complexity and open avenues for device realization is through epitaxy, where a well-suited technique to extract the structural details of these engineered epitaxial BiFeO3 thin films is high-resolution x-ray diffraction and reciprocal space mapping (HRXRD-RSM). For BiFeO3, HRXRDRSM is especially attractive because high crystalline quality epitaxial thin films can be deposited resulting in relatively sharp Bragg reflections.2–4 Additionally, in comparison with other rhombohedrally distorted perovskite based materials, including LaAlO3 and PbZr1xTixO3 (x 0.25), the bulk rhombohedral distortion (ar) in BiFeO3 is relatively large, ar 5 89.43°.5–7 This causes peaks from different structural domains to exhibit large splitting in reciprocal space increasing accessibility. Although these advantages exist, certain details of the epitaxial BiFeO3 film structure remain uncovered. In particular, ferroelastic twin walls are known to have unique properties in comparison with the bulk systems.8–10 However, understanding properties of twin walls in BiFeO3 thin films is still at its infancy.11 In this study, HRXRD-RSM was used to measure the extrinsic distortion of vertical twin walls in (001) BiFeO3. This was achieved by using a basic method to extract structural aspects of the film including (i) epitaxial strain, (ii) domain arrangements, and (iii) lattice rotations; from which, the twin wall distortion was calculated. Interestingly, although coherent twin walls are pre-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.327 2844
J. Mater. Res., Vol. 26, No. 22, Nov 28, 2011
http://journals.cambridge.org
Downloaded: 17 Mar 2015
ferred to minimize strain energy, we demonstrate that semicoherent walls can exist in certain BiFeO3 heterostructures, which exemplifies the importance of film growth kinetics on particular single-crystal substrate surfaces. The method developed to analyze the RSM data conveniently describes the BiFeO3 film and substrate with a subcell. The subcells used
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