F436 Journal of The Electrochemical Society, 159 (8) F436-F441 (2012)0013-4651/2012/159(8)/F436/6/$28.00 © The Electrochemical Society

Structural Investigation of Perovskite Manganite and Ferrite Filmson Yttria-Stabilized Zirconia Substrates

C. R. Smith,a J. D. Sloppy,a H. Wu,a T. S. Santos,b,d E. Karapetrova,c J.-W. Kim,c P. J. Ryan,cM. L. Taheri,a and S. J. Maya,z

aDepartment of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USAbCenter for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439, USAcAdvanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA

Thin films of La0.7Sr0.3MnO3 and La0.7Sr0.3FeO3 have been deposited on single crystal yttria-stabilized zirconia (YSZ) substratesusing molecular beam epitaxy. Due to the large lattice mismatch, three-dimensional growth is observed with strain relaxationoccurring within 1-2 unit cells. The as-grown perovskite/YSZ interfaces are abrupt, despite the imperfections associated with strainrelaxation. The films exhibit a single crystallographic orientation along the growth direction, with the perovskite [011] parallel to theYSZ [001]. Synchrotron diffraction and plan view transmission electron microscopy reveal the presence of a columnar grain structurewith the in-plane [100] and [011̄] film directions preferentially orientated along the YSZ 〈100〉. The stabilization of perovskites onYSZ substrates with abrupt interfaces and a well-defined crystallographic orientation allows for the use of thin film model systemsto study cathode/electrolyte interfacial reactions and morphological changes relevant to solid oxide fuel cells.© 2012 The Electrochemical Society. [DOI: 10.1149/2.032208jes] All rights reserved.

Manuscript submitted March 22, 2012; revised manuscript received May 11, 2012. Published July 20, 2012.

Advances in synthesis techniques for oxide thin films have enabledatomically-controlled heteroepitaxial growth of a range of transitionmetal oxides. This ability to grow atomically abrupt heterostructuresof multifunctional oxide compounds has led to an intense researcheffort aimed at producing electronic devices and photovoltaics basedon complex oxides. An additional, yet less explored, research oppor-tunity that arises from the thin film growth of complex oxides is theformation of abrupt oxide-oxide interfaces to act as model systems forelectrochemical energy conversion and storage devices. For instance,this research strategy holds promise for disentangling the complex re-lationship between structural properties and electronic/ionic transportin solid oxide fuel cells.

Solid oxide fuel cells (SOFCs), in which perovskite oxides areused for cathodes and yttria-stabilized zirconia (YSZ) is the mostcommonly used electrolyte, are promising systems for energy conver-sion due to their high efficiency (and resulting low output of pollutants)in converting a wide range of fuels from hydrogen to hydrocarbonsinto electricity.1–4 Despite the potential of this technology for efficientenergy conversion, a number of materials-related issues remain asimpediments to the greater implementation of SOFCs. For example,changes in morphology at the cathode/electrolyte interface and the for-mation of detrimental interfacial phases lead to device degradation.5–15

However, the complex interfacial morphology in as-processed SOFCscomplicates the studies of chemical and morphological evolution atthe cathode/electrolyte junction. The growth of perovskite films onsingle crystal YSZ substrates provides a means to stabilize modelcathode/electrolyte structures with chemically abrupt interfaces. Suchstructures may prove useful in quantifying the kinetics of interfacialreactions at perovskite/zirconia interfaces and identifying the compo-sition of secondary phases. Additionally, perovskite thin films can actas model systems for exploring oxygen exchange kinetics at cathodesurfaces,16–20 cation migration,21,22 and the crystallographic depen-dence of oxygen transport.23

There have been few reports of highly crystalline perovskite filmsgrown on YSZ substrates despite the potential of such systems toprovide valuable insights to interfacial processes. Mori and coauthorsused pulsed laser deposition to grow La0.8Sr0.2CoO3 films on YSZsubstrates of various orientations.24 For films grown on YSZ (001),they reported an epitaxial orientation of [011] ‖ [001]; a similar crys-tallographic relationship was also observed for layered Nd2NiO4+δ

films on YSZ.25 It was found that the addition of a gadolinium-dopedceria film as a buffer layer between YSZ and La0.8Sr0.2CoO3 sta-bilizes a [110] ‖ [100] and [001] ‖ [001] relationship between the

dPresent address: HGST, A Western Digital Company, San Jose, California, USA.zE-mail: smay@coe.drexel.edu

perovskite and YSZ substrate.17 Studies of manganite and ferrite per-ovskites grown on YSZ have also been reported although these filmswere polycrystalline, lacking a unique out-of-plane crystallographicorientation with respect to the substrate.26,27

Here, we report the growth of La0.7Sr0.3MnO3 and La0.7Sr0.3FeO3

films on YSZ (001) substrates using molecular beam epitaxy. Partic-ular emphasis is placed on the structural properties of the film andinterface, which are of fundamental importance to all future studiesin which model thin film systems are used to investigate interfacialreactions and morphological changes that arise at high temperaturesor under polarization. Using a combination of X-ray diffraction andtransmission electron microscopy, we find a columnar microstructurewith a uniform crystallographic orientation present along the out-of-plane film direction and preferred in-plane orientations relative to thesubstrate.

Experimental

La0.7Sr0.3MnO3 (LSMO) and La0.7Sr0.3FeO3 (LSFO) films weredeposited on single crystal YSZ (001) substrates (MTI corporation)using ozone-assisted molecular beam epitaxy (MBE). Details of theMBE system, housed at the Center for Nanoscale Materials at ArgonneNational Laboratory, can be found in Ref. 28. Prior to growth, thesubstrates were cleaned with trichloroethylene and then annealed invacuum at 300◦C to remove organic contaminants. Films were grownunder flowing ozone at a chamber pressure of 2 × 10−6 Torr. Themetal cations were co-evaporated from Knudsen cells that containedelemental La, Sr, Mn, and Fe. Growth rates of ∼45 seconds per unitcell were used with an additional 25 second pause following eachunit cell. Reflective high energy electron diffraction (RHEED) wasperformed during growth to monitor the surface crystallinity.

X-ray diffraction and reflectivity were performed on a Bruker D8Discover system and at the Advanced Photon Source at Sector 33-BM.Surface morphology was imaged using a Veeco Multimode systemwith a NanoScope V Controller. Magnetic characterization was car-ried out in a Quantum Design SQUID system (MPMS). Specimensfor transmission electron microscopy (TEM) were prepared by grind-ing with SiC papers and then polishing with 1 μm diamond paste.Specimens were thinned to electron transparency using a Fischione1010 ion mill. TEM was performed on a JEOL 2100 LaB6 instrumentwith an accelerating voltage of 200 kV.

Experimental Results

Thin films (∼18 nm) of LSMO were grown at 600, 650 and 700◦Cto identify the optimal growth temperature. Temperatures higher than

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Figure 1. (Color online) Reflection high energy electron diffrac-tion (RHEED) results obtained during the growth of LSMO onYSZ at 700◦C (a-d). The pattern obtained from the bare YSZsubstrate is shown in (a). After a few unit cells, a pattern fromthe film is obtained (b) which persists to the end of growth. In(c), the RHEED pattern obtained from the film grown at 600◦Cis presented. The integrated RHEED intensity as a function ofgrowth time is given in (d) for the LSMO film grown at 700◦C.Line scans through the RHEED intensity are shown in (e); thelocation of the in-plane diffracted spots differ between the filmand substrate.

700◦C were not explored due to concerns about intermixing at thefilm/substrate interface and the possible formation of second phases.All films exhibited a three-dimensional growth mode as determinedfrom in situ RHEED measurements. Strain relaxation was observedto occur before the completion of two unit cells for all films. Figure 1shows the RHEED patterns obtained for the film grown at 700◦C,which are representative of the films grown at 650 and 700◦C. TheRHEED pattern obtained at 700◦C from the YSZ substrate consistsof sharp spots and Kikuchi lines (Fig. 1a). After the deposition ofa few monolayers of LSMO, diffraction spots are no longer visiblehaving been replaced by diffuse intensity. As more layers of LSMOare deposited, a new RHEED pattern emerges (Fig. 1b) consistingof both spots from three-dimensional islands (the intensity of whichare relatively constant upon rotation of the film) and the symmetryof the perovskite film. Figure 1d shows the evolution of the RHEEDintensity as a function of growth time for this film. The in-planediffracted spots for the film and substrate are observed at differentlocations in reciprocal space (Fig. 1e), consistent with film relaxationduring the first few unit cells of growth. The film grown at 600◦Cexhibited a similar transition from sharp substrate peaks to diffuseintensity; however, the final RHEED pattern consists of a specularspot with circular rings indicative of poor crystallinity (Fig. 1c).The island growth observed by RHEED measurements is to be ex-pected due to the large lattice mismatch between the perovskite film(a = 3.87 and 3.945 Å for bulk LSMO and LSFO, respectively) andthe zirconia substrate (a = 5.148 Å).

The island morphology is clearly visible in atomic force mi-croscopy (AFM) images of the films, shown in Figure 2a for thefilm grown at 700◦C. For this film, peak-to-valley heights on the or-der of 2.5 nm are observed. To quantify the surface roughness andLSMO/YSZ interfacial roughness over macroscopic length scales,X-ray reflectivity measurements were performed on the three films.The reflectivity data was fit using the GenX software,29 which utilizesa recursive algorithm based on Parratt’s formalism. Figure 2b presentsthe measured data and fit for the film grown at 700◦C; the fit is rep-resentative of the three films. For the fit, the film was split into twolayers, a bulk layer and a surface layer. The bulk density was fixedto the calculated value for La0.7Sr0.3MnO3 with the measured latticeparameters. The density of the surface layer was found to be reduceddensity by up to 15% compared to the bulk layer, with the largestdensity reduction measured in the film grown at 600◦C. The thicknessof the surface layer was 4 nm or less for all three films. As can be seenin Fig. 2c, the surface roughness is found to decrease with increasinggrowth temperature, a likely result of the increased thermal energyallowing for greater coalescence of islands during growth. We notethat the roughness values obtained from reflectivity are larger than theRMS roughness obtained from the AFM scans. For instance, rough-ness values of 1.2 and 0.6 nm were obtained from reflectivity andAFM, respectively, for the film grown at 700◦C. The models obtained

Figure 2. (Color online) Surface characterization performed by atomic forcemicroscopy and X-ray reflectivity. The surface morphology, measured byatomic force microscopy, is shown in (a) for the 18 nm LSMO film grownat 700◦C. The scale bar height is 5 nm and the scan area is 500 × 500 nm2.X-ray reflectivity is presented in (b) for the same film. The open circles are themeasured data points, while the solid line shows the fit. The surface roughnessvalues obtained from the reflectivity fits are shown in (c) as a function ofgrowth temperature for the 18 nm LSMO films.

F438 Journal of The Electrochemical Society, 159 (8) F436-F441 (2012)

Figure 3. (Color online) X-ray diffraction of LSMO films (t ∼ 18 nm) deposited at different temperatures (a). The films grown at 650 and 700◦C exhibit anout-of-plane orientation of (011)LSMO ‖ (001)YSZ, which is shown in an idealized schematic in (b). In (b), the green spheres represent Zr (Y); the red spheresrepresent O; the yellow spheres represent La (Sr); and the Mn cation sits at the center of the blue octahedra. Wide angle diffractions scans of LSMO (c) and LSFO(d), both t ∼ 30 nm, confirm the absence of secondary phases.

from the reflectivity fits also yield an abrupt LSMO/YSZ interface(roughness < 0.5 nm), although it should be noted that the differencein scattering length density between the two materials is rather smalland thus the fit is not overly sensitive to the interfacial width.

Despite the rapid strain relaxation and island growth mode, thefilms deposited at 650 and 700◦C exhibit excellent out-of-planecrystallinity, as indicated by standard 2θ-θ X-ray diffraction scans(Figure 3a). The correlation length obtained from the width of the

Figure 4. (Color online) Magnetization of the ∼30 nm thick LSMO film as afunction of temperature, measured with a 1 kOe field applied along the in-planedirection.

(011) Bragg peak is equal to the film thickness, confirming thefilms are coherent crystals along the growth direction. The diffrac-tion measurements also reveal the out-of-plane epitaxial relationshipto be (011)LSMO ‖ (001)YSZ. This orientation, shown schematically inFigure 3b, is consistent with previous reports of cobaltite films growndirectly on YSZ.24 For simplicity’s sake, we use the pseudocubic nota-tion to represent the perovskite unit cell, even though bulk LSMO andLSFO exhibit rhombohedral (R3̄c) and mixed rhombohedral (R3̄c)and orthorhombic (Pnma) structures, respectively.30,31

Thicker films (∼30 nm) of LSMO and LSFO were grown to fa-cilitate additional characterization measurements. The growth tem-perature for both films was 700◦C. Diffraction measurements takenover a wide angular range (Figure 3c, 3d) confirm that both films aresingle phase and exhibit a single crystallographic growth direction. Inferromagnetic manganite films, the Curie temperature is commonlyused to assess film quality. Figure 4 shows the film magnetization as afunction of temperature measured with a 1 kOe field applied in-plane.A Curie temperature of ∼345 K is observed. This value is comparableto the highest values reported for LSMO grown on perovskite oxidesubstrates,32–37 confirming that the film is high quality with the correctstoichiometry.

Synchrotron diffraction was performed on the 30 nm LSMO andLSFO films to further study the crystallographic relation between thefilm and substrate. Figure 5a, 5b shows the out-of-plane diffractionresults, similar to those in Fig. 3a, albeit with much greater resolutionand intensity due to the synchrotron source. Thickness fringes arepresent in both samples arising from interference between the surfaceand film/substrate interface.

To determine the in-plane orientation, a HK scan with fixed L, sim-ilar to a phi scan, was performed about a (031) peak. The scan reveals

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Figure 5. Out-of-plane synchrotron diffractionmeasurements for the 30 nm LSMO (a) and LSFO (b)films. In-plane measurements reveal the presence of(031) LSMO peaks along all both the [100] and [010]directions of the YSZ substrate (c). This indicatesa preferred in-plane crystallographic orientation (d)with either the perovskite [100] or [011̄] laying alongthe YSZ [100]. The nucleation energies of these twoorientations are energetically equivalent, resulting inapproximately equal volume fractions, as shown in(c). In (c), L is fixed at 3.76 r.l.u. (of YSZ) while thepeaks occur at (±1.87 0 L) and (0 ±1.87 L) where Hand K are indexed to the YSZ substrate.

the presence of four peaks from the film parallel to the YSZ [100],[010], [1̄00], and [01̄0] directions (Fig. 5c). The presence of four broadpeaks, instead of two, indicates that the films are not single crystalsbut instead exhibit a columnar structure with coherent crystallinityout-of-plane but a granular in-plane structure. If the films were singlecrystal only two (031) peaks would be present, appearing 180◦ apart.These peaks would be parallel to the [0K0] direction of the YSZ, whileonly (H00) film peaks would be present along the [H00] direction ofthe YSZ. The presence of (031) peaks with roughly equal intensityalong both the H and K directions of the substrate indicates that thefilm has roughly equal volume fractions of the two symmetrically-equivalent orientations (Fig. 5d). This columnar structure is a naturalproduct of the film nucleating at multiple locations on the substrate.The two crystallographic orientations result in the same interfacialand strain energies and thus occur with the same frequency. There arealso weak (031) peaks separated by 45◦ from the four intense filmspeaks. These peaks could arise from grains in which the film [100]direction is along the YSZ [110] direction.

In the epitaxial orientation that we observe, the film’s in-plane crys-tallographic relationship with the substrate is LSMO[100] ‖ YSZ[100]

and LSMO[011̄] ‖ YSZ[010]. This gives a lattice mismatch of −5.9%along the LSMO [011̄] direction and ∼33% along the LSMO [100] di-rection. A lattice mismatch of 33% is extremely large, but we note thatin this epitaxial arrangement 4 times the LSMO a-axis parameter isnearly equal to the 3 times the YSZ a-axis parameter. Similar domain-matching epitaxial arrangements have been observed in a wide rangeof thin films.38 We speculate that a domain matching scenario may bethe origin of the epitaxial arrangement in the films.

Transmission electron microscopy was performed to provide di-rect visualization of the microstructure of the films. Figure 6 showscross-sectional images obtained from the 30 nm thick LSMO film.The columnar structures can be seen in Fig. 6a as contrasting re-gions within the film. In Fig. 6b, a higher resolution image of thefilm/substrate interface is presented. The abrupt nature of the inter-face and the crystalline quality of the film along the growth directionis evident. The columnar structure of the films is best visualized us-ing plan view TEM. A series of such images from both the LSMOand LSFO films are given in Figs. 7 and 8, respectively. Analysisof the low magnification images reveals an average column width of15 ± 1.8 nm in the LSMO. In the LSFO, an average grain width of

Figure 6. Cross-sectional transmission electron micrographs obtained fromthe 30 nm LSMO film on YSZ. As can be seen in (a), the columnar microstruc-ture produces contrasting regions in the film. A higher resolution image of thesame film is shown in (b).

F440 Journal of The Electrochemical Society, 159 (8) F436-F441 (2012)

Figure 7. Plan view transmission electron micrographs obtained from the30 nm LSMO film showing the in-plane granularity of the film.

Figure 8. Plan view transmission electron micrographs obtained from the30 nm LSFO film at two different magnifications.

62.1 ± 15.6 nm is measured. The presence of smaller grain sizes inthe LSMO suggests this material has a greater nucleation rate thanLSFO when grown on YSZ (001).

The stabilization of manganite and ferrite films, with compositionsthat are employed as cathodes in SOFCs, with a uniform crystallo-graphic growth direction on single crystal YSZ substrates is a criticalfirst step toward the use of model thin film systems to better un-derstand the degradation processes that occur at cathode/electrolyteinterfaces. For instance, experiments on perovskite nanoparticles de-posited on YSZ substrates have provided new insights into high tem-perature morphological stability of manganite/zirconia interfaces.39,40

The films reported here enable similar studies with additional controlover the crystallographic orientation of the interface.

Conclusions

The structural properties of La0.7Sr0.3MnO3 and La0.7Sr0.3FeO3

thin films deposited on YSZ (001) substrates have been characterizedusing a variety of real and reciprocal space techniques. A growthtemperature of 700◦C was observed to produce films with excellentout-of-plane crystallinity and reduced surface roughness as comparedto films grown at lower temperatures. The films exhibit a columnarmicrostructure with a single out-of-plane growth direction in whichthe perovskite [011] is parallel to the YSZ [001], while multipleperovskite directions are found to lay along the YSZ [100]. Planview TEM has been used to visualize the columnar structure; grainsizes on order of 10-60 nm were observed. We anticipate that theseresults will serve as a scientific foundation for future structural studiesof perovskite/zirconia interfaces with the aim of understanding theprocesses that occur at cathode/electrolyte interfaces in solid oxidefuel cells.

Acknowledgments

Acknowledgement is made to the Donors of the American Chem-ical Society Petroleum Research Fund for partial support of this re-search. C. R. S. was supported by the US Department of Educationunder the GAANN program (#P200A100134). Use of the Center forNanoscale Materials and the Advanced Photon Source was supportedby the US Department of Energy, Office of Science, Office of Ba-sic Energy Science under Contract No. DE-AC02-06CH11357. TEMwas performed using the Centralized Research Facilities of the Col-lege of Engineering at Drexel University. We thank Craig Johnson forassistance with the instrument.

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