Materials Science and Engineering B 118 (2005) 60–65

Microstructure of (1 1 0)-oriented epitaxial SrRuO3 thin filmsgrown on off-cut single crystal YSZ(1 0 0) substrates

Xinhua Zhua,∗, Sung Kyun Leea, Ho Nyung Leeb, Dietrich Hessea

a Max-Planck Institut f¨ur Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germanyb Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA

Abstract

The microstructure of (1 1 0)pc-oriented epitaxial SrRuO3 (SRO) thin films grown by pulsed laser deposition on (1 0 0)YSZ (YSZ: yttria-stabilized zirconia) single crystal substrates with a miscut angle of 5◦ has been investigated by X-ray diffraction (XRD) and transmissionelectron microscopy (TEM). The films grow epitaxially with their pseudocubic (1 1 0) plane parallel to the (1 0 0) surface of the YSZ singlecrystal substrate, and with an in-plane orientation relationship of [1 1 1]SRO//[0 1 1]YSZ. Cross-sectional TEM investigations show that thefilms have a rough, facetted surface. Generally, four different azimuthal domains are present in (1 1 0)SRO films on (1 0 0)YSZ. Their numbercan be significantly reduced using annealed offcut YSZ substrates before SRO deposition, and this reduction effect is shown to be muchs and theird EM.©

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tronger on [0 1 1]-miscut (1 0 0)YSZ than on [0 0 1]-miscut ones. Size and morphology of the azimuthal pseudocubic domainsomain boundaries, as well as of anti-phase domains and their domain boundaries are studied by plan-view and cross-section T2004 Elsevier B.V. All rights reserved.

eywords:Microstructure; Epitaxial growth; SrRuO3 thin films; Azimuthal domains

. Introduction

In recent years much attention has been paid to the epitax-al growth of SrRuO3 (SRO) thin films since they can be useds excellent electrodes for ferroelectric random access mem-ries, superconductor-normal metal-superconductor junc-

ions, and dielectric capacitors[1–6]. SRO has a GdFeO3-ype orthorhombic structure with the space groupPbnm(No.2) at room temperature, and with the lattice parameters= 0.5538,b= 0.5573 andc= 0.7856 nm. Its lattice can alsoe considered as a pseudocubic perovskite structure with= 0.3928 nm[7]. (In this paper, pseudocubic indexing issed throughout.) A number of investigations on epitaxialrowth, physical properties, and microstructure of epitaxialRO films have been carried out. It was reported that SROlms grown on exact (0 0 1)SrTiO3 substrates show a verymooth surface and a very sharp film–substrate interface.hey consist of two azimuthal 90◦ type domains[8,9]. How-ver, the SRO films grown on vicinal (0 0 1)SrTiO3 substrates

∗ Corresponding author. Tel.: +49 345 5582 668; fax: +49 345 5511 223.

had a nearly single-domain structure[8,10]. These results reveal that a miscut of the (0 0 1)SrTiO3 substrate along th[1 0 0] direction has a strong effect on the microstructurthe SRO films, which is ascribed to the specific step-growth mode of the film due to the miscut. SRO filmsposited on (0 0 1)LaAlO3 substrates were reported to havrough surface and three different orientation domains[11,12],different from the SRO films deposited on the (0 0 1)SrT3substrates that have an atomically flat surface and only90◦-type domains. These differences were attributed toeffect of the film–substrate lattice mismatch. The strucevolution in epitaxial SRO films grown on (0 0 1)SrTiO3 bypulsed laser deposition during the early stages was itigated by Kim et al.[13]. Their most important findingthat below a critical thickness (≤11 nm), only one type odomains exists, with a relatively poor atomic orderingthe in-plane direction. As the film grows further, 90◦ ro-tated domains evolve, resulting in a significant enhanceof atomic ordering in that direction. Such domain evoluis likely due to the relaxation of stress accumulated infilm. While on (0 0 1)SrTiO3 substrates, SRO films grow w

E-mail address:xhzhu@mpi-halle.de (X. Zhu). their pseudocubic (0 0 1) plane parallel to the substrate plane,

921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2004.12.048

X. Zhu et al. / Materials Science and Engineering B 118 (2005) 60–65 61

SRO films on (1 0 0)YSZ substrates (YSZ: yttria-stabilizedzirconia) grow with their pseudocubic (1 1 0) plane paral-lel to the substrate surface, see e.g.[14,15,20]. The growthof SRO on (1 0 0)YSZ is significant in view of the impor-tance of YSZ buffer layers on Si(1 0 0) for high-TC super-conducting[16] and ferroelectric epitaxial thin films[6,17].The specific “rectangle-on-cube” epitaxial orientation rela-tionship of SRO(1 1 0) on YSZ(1 0 0) gives rise to a fourfoldazimuthal domain structure involving difference angles of theorder of 20◦ between corresponding lattice directions of dif-ferent azimuthal pseudocubic domain variants[6,14,15,17].Although the latter papers describe the application of (1 1 0)-oriented SRO films as conducting epitaxial templates for vari-ous perovskite-related ferroelectric thin films, the microstruc-

ture of (1 1 0)SRO films has to our knowledge so far not beendescribed in detail. In addition, we have recently appliedoff-cut (1 0 0)YSZ substrates to reduce the number of az-imuthal pseudocubic domain variants in (1 1 0)-oriented SROfilms and in (1 0 0)/(1 1 8)-oriented Bi3.25La0.75Ti3O12 filmsgrown thereon[18]. In this paper a microstructural study ofthe domain-reduced (1 1 0)-oriented SRO films is presented.

2. Experimental procedure

SrRuO3 films were grown by pulsed laser deposition oncommercial vicinal (1 0 0)YSZ single crystal substrates withmiscut angle of 5◦ and miscut direction along [0 1 1] and

Fp

ig. 1. Cross-sectional TEM images of SRO films (a) taken in the [0 1 0]YSZ direcattern taken along the [0 1 0]YSZ direction. The inset in (b) is the SAED pattern

tion and (b) taken in the [0 11]YSZ direction. The inset in (a) is the SAEDtaken along the [0 11]YSZ direction.

62 X. Zhu et al. / Materials Science and Engineering B 118 (2005) 60–65

[1 1 0], respectively. Before SRO film deposition, the off-cut YSZ single crystal substrates were thermally annealedat 1200◦C for 10 min in air to improve their surface qual-ity, and finally to enhance the effect of the reduction in thenumber of the azimuthal domains of SRO films. The pulsedlaser radiation of a KrF excimer laser (λ= 248 nm, 5 Hz) wasfocused onto a stoichiometric SrRuO3 ceramic target at anenergy fluence of 1.7 J/cm2 to deposit SRO films onto vic-inal YSZ substrates. During deposition, the substrate tem-perature was kept at 775◦C and the oxygen pressure wasmaintained at 14 Pa, respectively[15]. The thickness of thefilms investigated in this work has been chosen 30–80 nm.The crystallographic orientations and epitaxial relations werecharacterized by X-ray diffraction (XRD)φ scans and polefigure measurements using a Philips X’Pert MRD four-circlediffractometer (Cu K� radiation). Both cross-sectional andplan-view specimens were used for microstructural investiga-tion. Cross-sectional specimens were prepared by cutting thesample into slices along (0 0 1)YSZ and (0 1 1)YSZ planes, re-spectively. Two slices were glued together face-to-face join-ing the film-covered surface. After the glue cured, disks witha diameter of 3 mm were obtained by cutting away redundantepoxy. These disks were then ground, dimpled, and polished,followed by Ar-ion milling in a Gatan Precision Ion Polish-ing System (PIPS, Model 691) at 4 keV with an incident an-g ◦ isksw -p ound,d ion-m ac-t hilipsC igh-r edi ateda

Fig. 2. XRDφ scan patterns of the SRO films deposited at 775◦C onto theas-received (1 0 0) YSZ single crystals with different miscut directions. (a)Exactly cut; (b) [0 0 1]-miscut; and (c) [0 1 1]-miscut.

3. Results and discussion

In a preliminary study on the reduction of the azimuthaldomains in SrRuO3 thin films[19], the miscut angleα of the(1 0 0)YSZ single crystal substrates was varied from 0◦ to5◦ in steps of 1◦, and miscut directions along the [0 0 1] and[0 1 1] directions of the (1 0 0)YSZ substrate, respectively,were used. As a result of this study, in the present work themiscut angleα is fixed at 5◦. The epitaxial growth of the SROfilms on the [0 1 1]-miscut (1 0 0)YSZ crystal substrates was

F eudocu -o eomet of fourd rate su tioned in tt

le of 6 . Plan-view samples were prepared by cutting dith a diameter of 3 mm from the SrRuO3/YSZ(1 0 0) samles using an ultrasonic cutter. Then these disks were grimpled, and finally thinned to perforation by Gatan dualilling (Model 600) from the substrate side. Electron diffr

ion patterns and dark-field images were recorded in a PM20T electron microscope operated at 200 kV, and h

esolution TEM (HRTEM) investigations were performn a JEOL JEM-4010 high-resolution microscope opert 400 kV.

ig. 3. (a) Schematic top-view diagram showing the four azimuthal psn-cube relationship of (1 1 0) SrRuO3 and (1 0 0)YSZ. (b) Qualitative gomains due to the asymmetry of the [0 1 1]-miscut (1 0 0)YSZ subst

ext. (Angles and differences betweend-values are shown not to scale.).

bic domains and the peak separation of∼20◦ and∼70◦ in the epitaxial rectanglerical model (top view) describing the predominant growth of two outrface. The double-ended arrows indicate the unit cell diagonals menhe

X. Zhu et al. / Materials Science and Engineering B 118 (2005) 60–65 63

Fig. 4. XRDφ scans and pole figures of SRO films deposited on an annealed [0 0 1]-miscut YSZ substrate [(a) and (c)], and on an annealed [0 1 1]-miscut YSZsubstrate [(b) and (d)].

confirmed by selected area electron diffraction (SAED). Across-sectional image of a SRO film taken in the [0 1 0]YSZdirection is shown inFig. 1a, demonstrating the morphol-ogy of the SRO film. From the image, it can be seen that thesurface of the SRO film is facetted and that the thickness ofthe SRO film is about 75 nm. The inset is the SAED patterntaken at the interface between the SRO film and the substratealong the [0 1 0]YSZ direction. Separation of diffraction spots

between SRO film and YSZ substrate along the [1 0 0]YSZand [0 0 1]YSZ directions can be clearly observed. Based onthis and similar SAED patterns, the epitaxial relationshipbetween the SRO film and the substrate can be describedas (1 1 0)SRO||(1 0 0)YSZ, [1 1 1]SRO||[0 1 1]YSZ. A cross-sectional image taken along the [0 11]YSZ direction is shownin Fig. 1b. Domain boundaries appear straight and nearly par-allel to the substrate normal in these cross-section images.

Fig. 5. Bright-field TEM image taken from a planar (1 1 0)-oriented SR

◦ O film deposited on an annealed 5[0 1 1]-miscut (1 0 0)YSZ substrate.

64 X. Zhu et al. / Materials Science and Engineering B 118 (2005) 60–65

XRD φ scan patterns of SRO films deposited onto as-received (1 0 0)YSZ single crystals with different miscut di-rections ((a) exactly cut, (b) [0 0 1]-miscut, and (c) [0 1 1]-miscut) are shown inFig. 2. Eight diffraction peaks from the(1 0 0) reflection of SRO are present, which indicates that allthe SRO films consist of four kinds of azimuthal pseudocubicdomains. Each of the fours sets of diffraction peaks has twopeaks separated by an offset�φ of ∼20◦. This phenomenonoriginates from the diagonal-type rectangle-on-cube epitaxyrelation of (1 1 0)SrRuO3 on (1 0 0)YSZ[14,15], as schemat-ically shown inFig. 3a. It can also be observed inFig. 2a andb that the intensities of the eight diffraction peaks from theSRO films deposited on exactly cut and [0 0 1]-miscut of YSZsingle crystals are almost equal. That means the number of thefour different azimuthal domains in these two types of SROfilms is nearly equal. However, the intensity asymmetry ofthe two sets of diffraction peaks shown inFig. 2c implies thatthe number of two azimuthal domains is larger than that of theother two azimuthal domains in the SRO film deposited on[0 1 1]-miscut (1 0 0)YSZ. (Note that the sample tilt had beenprecisely adjusted by optimizing theψ angles.) The origin ofthis phenomenon can be explained by a qualitative geomet-rical model related to the lattice mismatch between the SROfilm and the YSZ substrate, as shown inFig. 3b. This schemeshows the asymmetry of the substrate surface resulting fromt ngt rowsi ado inst ainv nismo lita-t ostp y tot er ZSs 00f msd YSZs ly,t g then y, ana ainsh ealed[ byT

ar(( ,o sizeo ninga aighta icald so nti-p es of

Fig. 6. (a) Selected area electron diffraction pattern of SRO film (same sam-ple as inFig. 5) taken along the [1 1 0]SRO direction. Reflections numbered1–4 and 1′–4′ are of type{0 0 1}, those numbered 5, 5′, 6, and 6′ are oftype {1 1 0}. (b) Dark-field image using the strong reflection spot (No. 1,non-dashed), and (c) dark-field image using the weak reflection spot (No.1′, dashed). The letters A, B, and C mark identical positions for convenientcomparison.

predominant azimuthal domains in the SRO film. This wasfurther demonstrated by dark-field images taken from a pla-nar SRO sample.Fig. 6a shows a SAED pattern taken alongthe [0 0 1]YSZ direction from a plan-view specimen preparedfrom the same film as the one studied by cross-sectional TEM.It shows pairs of equivalent dashed- and non-dashed diffrac-tion spots with clearly different diffraction intensity. Thisasymmetry of the dashed- and non-dashed diffraction spotsimplies that there are two kinds of predominant azimuthal do-mains and two kinds of minority domains. The related dark

he offcut, in terms of two different misfit values arising alohe two diagonals of the unit cell (see double-ended arn Fig. 3b). In additionFig. 3b demonstrates that now instef difference angles of 20◦ between next azimuthal doma

hose of 70◦ are predominant, because now only two domariants are present. In terms of the atomistic mechane has however, to keep in mind that this is only a qua

ive model; the growth of two predominant domains is mrobably realized via a step-flow mechanism in analog

he one on offcut SrTiO3 substrates[8,10]. To enhance theduction effect of azimuthal domains, the miscut (1 0 0)Yingle crystal substrates were thermally annealed at 12◦Cor 10 min in air. XRDφ scans and pole figures of SRO fileposited on the annealed [0 0 1]- and [0 1 1]-miscutubstrate are shown inFig. 4a to d, respectively. Obvioushe thermal annealing has a significant effect on reducinumber of azimuthal pseudocubic domains. Particularllmost 50% reduction of the number of azimuthal domas been achieved in the SRO film deposited on the ann

0 1 1]-miscut YSZ substrate. This was also confirmedEM investigations as described in the following.

A bright-field TEM image taken from the plan1 1 0)-oriented SRO film deposited on a 5◦ [0 1 1]-miscut1 0 0)YSZ substrate is shown inFig. 5. From this imagene can recognize the azimuthal domains with lateralf 50–100 nm having rounded boundaries, in turn containti-phase domains of 10–50 nm lateral size having strnti-phase boundaries (APBs). For structural and chemetails of APBs in SRO films, see[5]. There are two kindf predominant azimuthal orientations of the straight ahase boundaries, indicating that there are only two typ

X. Zhu et al. / Materials Science and Engineering B 118 (2005) 60–65 65

Fig. 7. HRTEM image of a place where four azimuthal domains (marked“1” to “4”) meet. Two anti-phase domains are marked “A” and “B”. The SROfilm was grown on a thermally treated 5◦ [0 1 1]-miscut (1 0 0)YSZ substrate(same sample as inFig. 5).

field images taken with the strong reflection spot (No. 1, non-dashed) and the weak one (No. 1′, dashed), are shown inFig. 6b and c, respectively. The high density of the bright re-gions inFig. 6b and the low density inFig. 6c clearly prove theasymmetry of distribution of the azimuthal domains.Fig. 7shows a high-resolution TEM (HRTEM) plan-view image ofa place where four azimuthal domains (marked “1” to “4”)with curved boundaries meet. Two anti-phase domains aremarked “A” (in the azimuthal domain 1) and “B” (in theazimuthal domain 3), respectively, both of which are lim-ited by straight, long anti-phase boundaries. These includean angle of∼70◦ (see white lines) demonstrating that theazimuthal domains 1 and 3 include an azimuthal differenceangle of∼70◦, in accordance with the scheme inFig. 3b.More HRTEM investigations are in progress.

4. Conclusions

The microstructure of (1 1 0)-oriented epitaxial SrRuO3thin films grown by pulsed laser deposition on vicinal(1 0 0)YSZ single crystal substrates with miscut angle of5◦ and miscut direction along the [0 1 1] direction has beencharacterized. X-ray diffractionφ scans, pole figures, andselected area electron diffraction patterns demonstrate thatt noo theS SZs nt on

[0 1 1]-miscut (1 0 0)YSZ than on [0 0 1]-miscut ones. Cross-sectional TEM investigations show that the (1 1 0)-orientedSrRuO3 films have rough, facetted surfaces. The azimuthaldomain boundaries appear straight and nearly parallel to thesubstrate normal in cross-section images, whereas plan-viewimages reveal their curved habit. In addition, each azimuthaldomain contains anti-phase domains, separated by straightanti-phase boundaries.

Acknowledgements

Work supported by DFG via the Group of Researches FOR404 at Martin Luther University Halle-Wittenberg. One of theauthors (X.H. Zhu) acknowledges the financial support fromthe Alexander von Humboldt Foundation.

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