Melting of Oxygen Vacancy Order at OxideâHeterostructure Interface

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Melting of oxygen vacancy order at oxide-heterostructure interface Kei Hirai, Ryotaro Aso, Yusuke Ozaki, Daisuke Kan, Mitsutaka Haruta, Noriya Ichikawa, Hiroki Kurata, and Yuichi Shimakawa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08134 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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ACS Applied Materials & Interfaces

Melting of oxygen vacancy order at oxideheterostructure interface

Kei Hirai†, Ryotaro Aso†,§, Yusuke Ozaki†, Daisuke Kan†, Mitsutaka Haruta†, Noriya Ichikawa†, Hiroki Kurata†, and Yuichi Shimakawa†, ‡, *

†

Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

‡

Integrated Research Consortium on Chemical Sciences, Uji, Kyoto 611-0011, Japan .

KEYWORDS: oxide heterostructure, interface, oxygen vacancy, vacancy order, epitaxial thin film

ABSTRACT: Modifications of oxygen coordination environments in heterostructures consisting of dissimilar oxides often emerge and lead to unusual properties of the constituent materials. While lots of attention has been paid on slight modifications of rigid oxygen octahedra of perovskite-based heterointerfaces, revealing the modification behaviors of the oxygen coordination environments in the heterostructures containing oxides with oxygen vacancies has been challenging. Here we show that a significant modification of oxygen coordination

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environments —melting of oxygen vacancy order— is induced at the heterointerface between SrFeO2.5 and DyScO3. When an oxygen-deficient perovskite (brownmillerite structure) SrFeO2.5 film grows epitaxially on a perovskite DyScO3 substrate, both FeO6 octahedra and FeO4 tetrahedra in the (101)-oriented SrFeO2.5 thin film connect to the ScO6 octahedra in DyScO3. As a consequence of accommodating structural mismatch, the alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed in the 2-nm-thick heterointerface region. The stabilized heterointerface structure consists of Fe3+ octahedra with oxygen vacancy disorder. The melting of oxygen vacancy order, which in bulk SrFeO2.5 occurs at 1103 K, is induced at the present heterointerface at ambient temperatures.


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INTRODUCTION Oxygen coordination environments in heterostructures consisting of dissimilar oxides differ from those in the bulk materials. As in semiconductor heterostructures, the elastic strain energy imposed on the film by the substrate is often accommodated by changes in the lattice size, giving compressive or tensile metal-oxygen bonds. In perovskite-based oxide heterostructures, for instance, this type of structure modifications such as changes in size and shape of the constituent oxygen octahedra have been successful in tailoring piezoelectric, dielectric, and multiferroic properties.1-3 It has been demonstrated that the lattice-mismatch strain is accommodated not only by deforming the oxygen octahedra but also by changing the connectivity of the network of corner-sharing octahedra at the interface. Transferring the octahedron rotation pattern through the heterointerface induces additional displacements of oxygen atoms of the octahedra in the film. Such structural modifications at the heterointerface also influence the properties of the materials.4-8 Engineering the oxygen coordination environments at the heterostructure interfaces is thus considered a new way of tailoring functional properties.9-14 Because the modified oxygen coordination environments emerge as a consequence of accommodation of the structural mismatch at the interface, significant rearrangements in the oxygen coordination are expected at heterointerfaces containing oxygen polyhedra other than octahedra.15-17 Many oxides consisting of different types of polyhedra are utilized as important components in technological applications like solid oxide fuel cells, so revealing the oxygen coordination environments in heterointerfaces containing oxygen polyhedra is of particular interest. Here we focus on the oxide heterostructure consisting of oxygen-deficient and fully oxygenated perovskites.


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Perovskite-structure oxides, which consist of corner-sharing oxygen octahedra, readily accommodate oxygen vacancies, and the vacancies often show ordering due to strong electrostatic repulsive forces between them.18 For example, a series of oxygen-vacancy-ordered phases of Fe-containing perovskites, SrnFenO3n−1 (n = 1, 2, 4, 8, and ∞), is reported.19-21 The SrFeO2.5 (n = 2) phase containing Fe3+ is relatively stable and crystallizes in the brownmilleritetype structure (orthorhombic, a ≈ 5.67, b ≈ 15.58, and c ≈ 5.53 Å), in which the oxygen vacancy ordering stabilizes alternate stacking of the octahedral Fe3+O6 and tetrahedral Fe3+O4 layers.20, 22 Recent structure studies with such oxygen-deficient brownmillerite structure oxides confirm that the rearrangements in the oxygen coordination have strong influences on their functional properties.23-25 In this study we made the SrFeO2.5/DyScO3 heterostructure by growing the brownmillerite-structure SrFeO2.5 film on a fully oxygenated DyScO3 substrate. Our primary concern was how the oxygen-vacancy ordered SrFeO2.5 layer connects to a fully oxygenated perovskite at the interface. The (101)-oriented SrFeO2.5 thin film, in which the octahedral FeO6 and tetrahedral FeO4 layers are alternately ordered along the in-plane direction, grows epitaxially on the DyScO3 substrate.26 We have found, however, that this alternately ordered arrangement is significantly disturbed at the heterostructure interface, giving the oxygen-vacancy-disordered layer. This implies that, at the interface, melting of oxygen vacancy order occurs at ambient temperatures.

EXPERIMENTAL METHODS


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The SrFeO2.5/DyScO3 heterostructure was fabricated by epitaxially growing a 20-nmthick SrFeO2.5 film on a DyScO3 single crystal substrate by pulsed laser deposition. The SrFeO2.5 film layer was deposited by ablating a stoichiometric target using a KrF excimer laser (λ = 248 nm, COHERENT COMPex-Pro 205 F) at 2 Hz with a laser spot density of 1 J/cm2 on the target surface. During the deposition, the oxygen partial pressure and the substrate temperature were kept at 1 × 10–5 Torr and 650 ºC. The thickness of the deposited SrFeO2.5 layer was controlled by in-situ monitoring oscillations of reflection high energy electron diffraction (RHEED) spot intensity. Macroscopic structure of the deposited SrFeO2.5 film was identified by X-ray diffraction using a conventional four-circle diffractometer (PANalytical X’Pert MRD). Surface morphologies of the DyScO3 substrate and the deposited SrFeO2.5 film were observed using an atomic force microscope (HITACHI AFM5200S). Microscopic characterization of the cross section of the SrFeO2.5/DyScO3 heterostructure was carried out by using a scanning transmission electron microscopy (STEM). The specimen was thinned down to electron transparency by mechanical polishing and Ar-ion milling. The HAADF and ABF images of the heterostructures were acquired at room temperature in a spherical-aberration-corrected STEM (JEM-9980TKP1; accelerating voltage = 200 kV, Cs = –0.025 mm, C5 = 15 mm) equipped with a cold field emission gun. The annular detection angles for HAADF were 50–133 mrad and those for the ABF were 11–23 mrad. (The convergent semi-angle of the incident probe was 23 mrad.) To obtain high-resolution STEM images with a better signal-to-noise ratio and with minimized image distortions, we acquired 50 HAADF images and 50 ABF images from the same area with a short dwell time (≈4.2 µs/pixel). All HAADF and ABF images were then superimposed after correcting the relative drifts.27 The atom positions were determined from the sub-pixel resolution


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STEM images by using Bragg filtering and cubic interpolation techniques in the “Find Peaks” option (Peak Pairs Analysis software package (HREM Research).28, 29 The interatomic distances were evaluated by averaging them for over 18 unit cells along the in-plane direction (parallel to the [1–10]DSO direction). The dual EELS data for the 0-loss and Fe-L2,3 edge were corrected using an aberration-corrected JEM ARM-200F operating at 80 kV with a probe-forming aperture of 21.3 mrad. The inner and outer angles of the annular detector were 91.5 and 168.6 mrad, respectively, and the EELS collection angle was 91.5 mrad.

RESULTS AND DISCUSSION The SrFeO2.5/DyScO3 heterostructure was fabricated by growing the 20-nm-thick SrFeO2.5 film layer epitaxially on the (110) DyScO3 single-crystal substrate by pulsed laser deposition. As shown in Figure 1a, clear oscillations of RHEED spot intensity during the deposition indicate layer-by-layer growth of the SrFeO2.5. Note, however, the RHEED intensity oscillation for the first 30-40 seconds after the deposition starts is different from the constant oscillation, suggesting modified film growth at the interface. Surface morphology observation with AFM also confirms that the fabricated heterostructure has a step-and-terrace surface structure (Figure 1b) that is a replica of the original surface of the substrate. Figure 1c shows the 2θ/θ X-ray diffraction pattern for the heterostructure. Bragg reflections of the deposited film, which for the brownmillerite structure can be indexed as (h 0 h) of the brownmillerite structure, are seen but there are no superstructure reflections like (0 k 0) arising from the alternate stacking of the FeO4 and FeO6 layers. (e.g. (0 6 0) would appear at 2θ ~31º.30, 31) Reciprocal space mapping of X-ray diffraction intensity around the (620) reflection of DyScO3 is also shown in


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Figure 1d. The (484) reflection from the SrFeO2.5 layer appears at the same in-plane position as that the (620) reflection from of DyScO3 does, indicating that the in-plane lattice of SrFeO2.5 is fixed by the DyScO3 substrate lattice. The results indicate that the brownmillerite SrFeO2.5 film is epitaxially and coherently grown with the (101) orientation, where the octahedral FeO6 and tetrahedral FeO4 layers are alternately ordered along the in-plane direction of the film, as previously reported.26 The interface structure of the fabricated SrFeO2.5/DyScO3 heterostructure is closely investigated by making cross-sectional observations complementary using HAADF- and ABFSTEM combined with EELS.32 Figures 2a and 2b respectively show typical HAADF- and ABFSTEM images taken along the [001] direction of the DyScO3 crystal ([001]DSO). We can clearly see periodic modulation in the image contrast of the SrFeO2.5 layer along the in-plane direction, which corresponds to the alternate stacking of the FeO4 and FeO6 layers along the [1–10]DSO direction. The observation is consistent with the (101) orientation of a brownmillerite SrFeO2.5 film grown on a (110) DyScO3 substrate. Looking at the interface region closely, it should be noted that such periodic modulations are significantly disturbed in the SrFeO2.5 film layer but there are no apparent misfit dislocations. The ordered arrangement of the oxygen vacancies in the brownmillerite SrFeO2.5 structure thus seems to be modified at the interface. The structural modifications in the interface region are quantitatively evaluated by measuring interatomic distances in the heterostructure. The distances between cations (see structure model in Figure 3a) are obtained with sub-angstrom precision from an HAADF-STEM image (Figure 2a) and the oxygen-oxygen (O-O) distances were obtained from an ABF-STEM image (Figure 2b) taken from the same area. Figures 3b, 3c, and 3d respectively show the “inplane interatomic distances” of the B-site (Fe or Sc) cations, A-site (Sr or Dy) cations, and


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oxygen across the interface as a function of the atomic position along the out-of-plane direction of the film. The B-site interatomic distances (Fe-Fe and Sc-Sc) are constant at 3.95 Å in the entire heterostructure, which confirms the coherent growth of the SrFeO2.5 film on the DyScO3 substrate. In the region apart from the interface (atomic rows above 6th in the figures), both the A-site Sr-Sr interatomic distances and the O-O distances show two distinct values at ~3.5 and ~4.4 Å. This periodic modulation in the Sr-Sr and O-O distances correspond to the alternate stacking of the of the FeO6 and FeO4 layers in the brownmillerite structure. Importantly, these Sr-Sr and O-O distances gradually change within the thickness of about 5 atomic rows (corresponding to about 2 nm) above the interface, and eventually converge to the value of the pseudocubic lattice constant of DyScO3 (~3.95 Å).33, 34 Because the in-plane Fe positions are the same throughout the film, the observations indicate that oxygen coordination environments of Fe are modified in a way that results in a unique coordination environment at the interface. It had better mention that the observed modification of the oxygen coordination environments was not seen in a specific sample. Similar HAADF- and ABF-STEM images were obtained in other samples with different film thicknesses. Some typical observation examples are presented in Supporting Information Figure S-1. The reproducibility of the observed results is therefore confirmed. EELS analysis confirms that the ionic states of Fe centers in the modified oxygen coordinated environments are still 3+. Figure 4 shows spatial dependence of EELS spectra of FeL2,3 edge across the heterointerface. Atomic-level resolution is achieved along the out-of-plane direction, whereas the signals along the in-plane direction are averaged because each spectrum is acquired by scanning an electron probe horizontally. Those signals appear just above the interface, indicating the interface is sharp and clear without interdiffusion of the atoms. The


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observed spectra from the SrFeO2.5 layer are similar to those observed for Fe3+-containing oxides such as CaFeO2.5 [35] and Fe2O3 hematite (see Supporting Information Figure S-2). An important point is that such Fe3+-featuring spectra are observed at exactly the same energy positions (no chemical shift) even from the region near the interface (atomic rows from 1th to 5th in the figure). This indicates that the valence states of Fe centers are 3+ throughout film, including the modified interfacial region. Besides, as shown in Figure 5, the in-plane B-site Fe and out-of-plane A-site Sr distances, which correspond to the pseudo-perovskite lattice parameters, are almost constant throughout the film. Because oxygen nonstoichiometry significantly affects the out-of-plane lattice parameters in perovskite-structure oxides,36 the results also support that the oxygen stoichiometry remains in the present heterostructure including the interface region. The results suggest that the average oxygen coordination numbers for Fe in the interfacial region of the film are the same as those in the brownmillerite SrFeO2.5. All the experimental results described above lead to a conclusion that the ordered arrangement of the oxygen vacancies, which make the octahedral FeO6 and tetrahedral FeO4 layers, in SrFeO2.5 are disturbed and reconstructed in the interface region in the heterostructure and that disordered oxygen vacancy arrangement is stabilized within the 5-unit-cell-thick SrFeO2.5 at the interface. When we look closely at the EELS spectra of regions only a few atoms away from the interface, we see a shoulder-like feature with subpeaks on the low energy side of the L3 peak, which are characteristic of the oxygen octahedral coordination (Supporting Information Figure S-2).35, 37 This feature is also consistent with the disordered oxygen vacancy arrangement in the octahedra. Note here that the order-disorder transition of oxygen vacancies in the bulk SrFeO2.5 occurs at 1103 K.38-40 In the interfacial region of the present heterostructure a structure like a high-temperature phase of SrFeO2.5 with randomly distributed oxygen vacancies


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is stabilized at ambient temperatures. Melting of oxygen vacancy order is therefore induced without elevating the temperature. Because this melting state of oxygen vacancy order is intrinsically stabilized by accommodating the structural mismatch at the interface not induced by thermodynamics, the melting transition would not be expected to occur below room temperature. The melting of oxygen vacancy order at the interface is a specific feature of the (101)oriented brownmillerite/perovskite heterostructure. It was shown that in the heterostructure of the (010)-oriented brownmillerite SrCoO2.5 film and the (100) perovskite SrTiO3 substrate, the tetrahedral CoO4 layer of the film connected to the octahedral TiO6 layer of the substrate.17 At the interface a tetrahedral CoO4 layer deformed slightly and accommodated the structural mismatch, while the oxygen coordinations of the Co centers remained essentially unchanged. Thus no oxygen vacancy disorder was observed. That heterostructure differs markedly from the present heterostructure containing (101)-oriented brownmillerite, where both tetrahedra and octahedra of the brownmillerite need to connect with the octahedra of the perovskite. The present result highlights that a key for the interfacial atomic reconstruction is the connection of oxygen polyhedra at the interface. Note also that the present result originated from the rearrangement of the oxygen vacancies with keeping the oxygen stoichiometry (see Supporting Information Figure S-3). This is in sharp contrast to the results obtained when the oxygen stoichiometry (oxygen vacancy concentration) in the epitaxial thin films was changed by changing the strain.41-43 In the (101)-oriented brownmillerite/perovskite heterointerface, interfacial lattice mismatch is accommodated by melting of oxygen vacancy order.

SUMMARY


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We have made a heterostructure consisting of a brownmillerite SrFeO2.5 film and a fully oxygenated perovskite DyScO3 substrate. XRD and STEM analyses reveal that the epitaxially grown SrFeO2.5 film has the (101) orientation, with which the octahedral FeO6 and tetrahedral FeO4 layers are alternately ordered along the in-plane direction. In the 2-nm-thick heterointerface region, however, this alternately ordered arrangement of oxygen vacancies is significantly disturbed and reconstructed. Even in the interfacial region, the valence states of Fe in the film remain to be 3+, indicating the average oxygen coordination numbers for Fe is the same as that expected from the chemical composition SrFeO2.5. The results indicate that the oxygen-vacancydisordered structure like the one seen in the high-temperature phase in SrFeO2.5 is stabilized at the interfacial region to accommodate the lattice mismatch by connecting both FeO6 octahedra and FeO4 tetrahedra in SrFeO2.5 to the ScO6 octahedra in DyScO3. This implies that melting of oxygen vacancy order is induced at the interface at ambient temperatures. The present results show that the oxygen coordination environments in the heterostructure can be controlled and highlight the possibility of stabilizing at the interface a structure, that otherwise is not stable at ambient temperatures.


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Figure 1. (a) Oscillations of RHEED spot intensity observed during the deposition of the SrFeO2.5 film. (b) AFM surface morphology of the fabricated SrFeO2.5/DyScO3 heterostructure. (c) 2θ/θ X-ray diffraction pattern for the fabricated SrFeO2.5/DyScO3 heterostructure. The inset shows the (202) SrFeO2.5 (SFO) and (220) DyScO3 (DSO) Bragg reflection profiles. (d) Reciprocal space mapping of X-ray diffraction intensity around the (620) reflection of DyScO3.


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(a) HAADF-STEM

(b) ABF-STEM

SrFeO2.5 DyScO3

2 nm

2 nm

Figure 2. Cross-sectional (a) HAADF- and (b) ABF-STEM images of the SrFeO2.5/DyScO3 heterostructure. The images were taken from the same area and along the [10−1] direction of SrFeO2.5 ([10−1]SFO (//[001]DSO)). The orange horizontal lines denote the heterointerface between SrFeO2.5 (SFO) and DyScO3 (DSO).


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Figure 3. (a) Projection of a crystal structure model of brownmillerite SrFeO2.5 along the [10–1] direction. In-plane interatomic distances are also shown in the figure. Obtained in-plane interatomic distances of (b) B-site (Fe or Sc) cations, (c) A-site (Sr or Dy) cations, and (d) oxygen across the interface as a function of the atomic position along the out-of-plane direction


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of the film. The orange horizontal lines denote the SrFeO2.5/DyScO3 heterointerface. The topmost Sc layer in DyScO3 is defined as the 0th atomic row. The atom positions were determined from sub-pixel-resolution images. See Supporting Information Figure S-4 for the 14 pm/pixel ABF-STEM image used for evaluating the distances between oxygen atoms.


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Figure 4. Spatial dependence of EELS spectra of Fe-L2,3 edge across the SrFeO2.5/DyScO3 heterointerface. Each spectrum was taken at the corresponding numbered atomic position in the HAADF-STEM image.


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(a) In-plane B-site cation

(b) Out-of-plane A-site cation

20 Atomic rows

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10

0 3.8

4.0

4.2

3.8

4.0

4.2

Interatomic distances (Å)

Figure 5. Interatomic distances of (a) in-plane B-site (Fe or Sc) cations and (b) out-of-plane Asite (Sr or Dy) cations as a function of the atomic position along the out-of-plane direction of the film. The orange horizontal lines denote the SrFeO2.5/DyScO3 heterointerface. The topmost Sc layer in DyScO3 is defined as the 0th atomic row.


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ASSOCIATED CONTENT Supporting Information. The supporting file is available free of charge. The supporting information consists of figures showing ABF-STEM images of some SrFeO2.5/DyScO3 heterostructures, Fe-L2,3 edge EELS spectra, O-K edge EELS spectra across the SrFeO2.5/DyScO3 heterointerface, and a high-resolution ABF-STEM image of the SrFeO2.5/DyScO3 heterostructure. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses §The Institute of Scientific and Industrial Research, Osaka University, Osaka 567-0047, Japan Author Contributions K.H., D.K. N.I., and Y.S. conceived and designed the study. K.H., Y.O., N.I., and D.K. fabricated the sample and measured the physical properties. R.A., M.H., and H.K. performed the STEM observation and EELS spectroscopy experiments. All of the authors contributed to the interpretation and discussion of the experimental results. K.H., R.A. M.H. D.K., H.K., and Y.S. wrote the manuscript. Funding Sources This work was partially supported by Grants-in-Aid for Scientific Research (Grants No. 15K13670, No. 16H02266, No. 16K13665, and No. 17K19177) and by a grant for the Integrated


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Research Consortium on Chemical Sciences from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The work was also supported by the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program (A) Advanced Research Networks and the Japan Science and Technology Agency (JST) CREST program. Support was also provided by the Nippon Sheet Glass Foundation for Materials Science and Engineering, the Sumitomo Foundation, and Okura.


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Melting of Oxygen Vacancy Order at OxideâHeterostructure Interface

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Melting of Oxygen Vacancy Order at OxideâHeterostructure Interface