Journal of Crystal Growth 338 (2012) 280–282

Contents lists available at SciVerse ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jcrysgro

Microstructure and strain relaxation of orthorhombic TmMnO3 epitaxialthin films

Y. Yu a, X. Zhang a,n, J.J. Yang b, J.W. Wang b, Y.G. Zhao b

a Beijing National Center for Electron Microscopy, Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, Chinab Department of Physics and the Key Laboratory of Atomic and Nanosciences, Ministry of Education, Tsinghua University, Beijing 100084, China

a r t i c l e i n f o

Article history:

Received 5 October 2011

Received in revised form

7 November 2011

Accepted 14 November 2011

Communicated by A. Ohtomo/110S dislocations tend to dissociate into partial dislocations with Burgers vectors of type 1/2ap

Available online 19 November 2011

Keywords:

A1. Crystal structure

A1. Transmission electron microscopy

B1. Manganites

B1. Perovskites

B2. Ferroelectric materials

B2. Magnetic materials

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.jcrysgro.2011.11.031

esponding author. Tel.: þ86 10 62773999; fa

ail address: xzzhang@tsinghua.edu.cn (X. Zha

a b s t r a c t

Orthorhombic TmMnO3 (o-TMO) thin films have been epitaxially stabilized on (110) SrTiO3 substrates

by pulsed laser deposition (PLD) technique. The microstructure and strain relaxation mechanism of

o-TMO thin films are analyzed using transmission electron microscopy. It is shown that major defects

in the films are misfit dislocations with Burgers vectors of type ap/010S and ap/110S, whereas ap

/110S. Strain in o-TMO films is relaxed by misfit dislocations as well as surface fluctuations, which is

different from most of the previous studies of the perovskite thin films.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Nowadays, multiferroic materials have garnered tremendousresearch interests due to their unique physical properties and thepotential for future applications in multifunctional devices [1].Orthorhombic (o-) rare-earth manganites RMnO3 (R belonging tolanthanum series) represent an important class of ‘‘improper ferro-electrics’’ [2], where the ferroelectricity is not only coexisting with,but also intrinsically correlating with the magnetic order. A richvariety of subtle interplay among spin, charge, orbital, and latticedegrees of freedom are responsible for the distinctive features in thissystem. As the orthorhombic phase of RMnO3 with R¼Y, Ho-Lu is nota thermodynamically stable phase at the ambient conditions, obtainof o-RMnO3 (R¼Y, Ho-Lu) is therefore a prerequisite for the follow-upstudies. Epitaxial stabilization of this orthorhombic phase in thinfilms were achieved by metal–organic chemical vapor deposition(MOCVD) [3] and pulsed laser deposition (PLD) [4–6]. Meanwhile,recent studies have reported the strain-induced effects on ferroelec-tricity [7], magnetic ordering [8], magnetocapacitance [9], and mag-netoelectric coupling [10] in o-RMnO3 thin films. Large electricpolarization and magnetoelectric coupling have just been found ino-TmMnO3 (o-TMO) thin films [6]. In order to have a better under-standing of the physical properties of these materials and themechanisms involved, it is of vital importance to analyze the

ll rights reserved.

x: þ86 10 62771160.

ng).

microstructure of o-RMnO3 (R¼Y, Ho-Lu) thin films. However, anintensive microstructural study including defect structure and strainrelaxation of o-RMnO3 epitaxial thin films is still lacking. In thispaper, we report on the characterization of microstructure and strainrelaxation of o-TMO epitaxial thin films by transmission electronmicroscopy (TEM).

2. Experiment

o-TMO thin films were grown on (110)-oriented SrTiO3 (STO)single-crystal substrates using PLD with a 248 nm KrF excimerlaser. The films were all deposited under a 10 Pa atmosphere offlowing oxygen at 940 1C with a single-phased hexagonal TMOtarget, which was prepared by solid state sintering with stoichio-metric high-purity Tm2O3 and MnO2 powders as the startingmaterials. TEM cross-sectional specimens were then prepared bycutting, gluing, mechanical polishing, and dimpling, followed byargon ion milling in a stage cooled with liquid nitrogen. TEMstudies were carried out on a JEOL JEM-2011 and a JEOL JEM-2010F field-emission gun transmission electron microscope (FEG-TEM) operated at 200 kV.

3. Results and discussion

Fig. 1(a) shows the low-magnification cross-sectional image ofo-TMO/STO, where o-TMO is about 20 nm in thickness with rough

Fig. 1. (a) Low-magnification cross-sectional image of o-TMO/STO. (b) Typical [001] SAED pattern of o-TMO/STO. The subscript f and s in the indices denote diffraction

spots coming from the film and the substrate, respectively. Splitting of the spots due to lattice mismatch becomes obvious in high-order reflections, such as (600)f and

(330)s. (c) HRTEM image of the interface.

Y. Yu et al. / Journal of Crystal Growth 338 (2012) 280–282 281

surface. The substrate STO has a cubic perovskite structure (spacegroup Pm-3m) with lattice constant of 3.905 A. The space groupof bulk o-TMO is determined to be Pbnm with lattice constantsao-TMO¼ap

ffiffiffi

2p¼5.228 A, bo-TMO¼ap

ffiffiffi

2p¼5.809 A, and co-TMO¼

2ap¼7.318 A [11], where ap denotes the pseudocubic perovskiteunit-cell lattice parameter. A selected area electron diffraction(SAED) pattern, obtained from the interface area including boththe TMO thin film and the STO substrate, is shown in Fig. 1(b).This result reveals that the a-axis oriented o-TMO epitaxial thinfilm was successfully grown on the STO substrate without any secondphase. The epitaxial relationship is TMO(100)[010]99STO(110)[110],showing the o-TMO thin film suffers a tensile strain of 6.3% in [001]direction and a compressive strain of 5.2% in [110] direction. High-resolution TEM (HRTEM) image of the interface is shown in Fig. 1(c),which further confirms the ‘‘cube-on-cube’’ epitaxy of o-TMO on STOand the existence of a sharp and clean interface.

In heteroepitaxy, the strain in the thin film is released if thefilm is thicker than the critical thickness. In our case, the criticalthickness for the o-TMO thin film with large lattice misfit (�5%)is estimated to be less than several atomic layers according to theMatthews–Blakeslee model [12]. Therefore, misfit dislocations(MDs) are assumed to form at the interface to release the epitaxialstrain in our 20 nm-thick TMO films. Hereafter, for the conve-nience of defect studies, pseudocubic notation is used for o-TMOso that the crystalline orientations of the film and the substrateare formally equivalent. To investigate the MDs, two-beam brightfield TEM were carried out under the conditions of g¼[110] andg¼[020], respectively, as shown in Fig. 2(a) and (b). MDs, denotedby arrows, are visible in both situations. According to the extinc-tion criterion g�b¼0, the Burgers vectors of MDs should havetheir edge components parallel to [110] or [020]. As the perfectdislocations in the perovskite system are of the type ap/110S andap/010S [13], it is very likely that MDs in our films are exactly ofthese two types. This conjecture can be confirmed by furtheranalyses of the MDs. We carefully analyzed the structural detailsof several MDs along the interface by drawing Burgers circuitssurrounding the dislocation cores in the HRTEM images, only Burgersvector ap/110S or ap/010S is found. Fig. 2(c) and (d) show twotypical HRTEM images of MDs with b¼ap[110] and b¼ap[010],respectively. The dislocation cores are observed to tend to locate in

the film side very close to the interface with the extra plane on thesubstrate side. In Fig. 2(d), a Burgers circuit drawn around thedislocation core yields a closure failure with a displacement vectorap[010]. In contrast to the ap/010S MDs, the ap/110S MDs areprone to dissociate into two partial dislocations with Burgers vectorsof the type 1/2ap/110S, as indicated in Fig. 2(c). The inset is theenlarged image of one of the 1/2ap[110] partial dislocations. Thedissociation can be written as

ap½110� ¼1

2ap½110�þ

1

2ap½110�,

which is energetically favorable as the sum of b2 gets smaller afterthis reaction, that is, the dislocation energy can be reduced. These twotypes of MDs both have their projected Burgers vector componentsparallel to the interface and, therefore, participate in misfit relaxation.Besides, if the thin film is fully relaxed by MDs, periodicallydistributed MDs should be formed with the theoretical equilibriumspacing D¼bo-TMO�2dSTO/110S/(bo-TMO�2dSTO/110S)¼11 nm. How-ever, the spacing of MDs in our o-TMO/STO system is found non-uniform (Fig. 2(a) and (b)). Meanwhile, the spacing value averagedover a long distance along the interface is also lager than 11 nm. Thus,only part of the strain is released by MDs.

Apart from the contribution of MDs to the strain relaxation,other factors should be active in order to release the residualstrain. These factors were considered to be the formation ofthreading dislocations (TDs) [13,14] or crystallographic domains[15,16] in previous studies of perovskite epitaxial thin films. Asour o-TMO thin films are observed to be single domain withoutTDs, the residual strain may be released in an unusual way. It isnoteworthy that the surface of our TMO thin films is not flat, asseen in low-magnification TEM images and also in Fig. 3. There-fore, we speculate that the residual strain in our TMO thin films isreleased by surface fluctuations [17]. Fig. 3 illustrates themechanism of surface fluctuations. When the MDs form atthe interface to partially relax the strain, the local strain statenear the dislocation cores is changed. Thus the strain distributionin the film becomes non-uniform. Consequently, the chemicalpotential develops a gradient along the surface. Then this gradientprovides a driving force for diffusive mass transport along thesurface, which attempts to search for an equilibrium surface

Fig. 2. Bright field images recorded under two-beam conditions (a) g¼[110] and (b) g¼[020]. MDs are denoted by arrows. (c) and (d) are two typical HRTEM images of

MDs with Burgers vectors b¼ap[110] and b¼ap[010], respectively. The Burgers circuits around the dislocation cores are indicated. The inset in (c) is the enlarged image of

one of the 1/2ap[110] partial dislocations.

Fig. 3. HRTEM image of the wavy surface of o-TMO. The mechanism of surface

fluctuations is shown schematically.

Y. Yu et al. / Journal of Crystal Growth 338 (2012) 280–282282

shape with minimum free energy and a uniform distribution ofchemical potential. As the high temperature in thin film deposi-tion facilitates the diffusive mass transport, surface fluctuationscan be achieved and the strain can be partly released.

4. Conclusion

a-axis oriented o-TMO thin films are successfully epitaxialstabilized on (110) STO substrates by PLD. MDs with Burgersvectors of type ap/010S and ap/110S are observed at theinterface, and the ap/110S dislocations tend to dissociate intopartial dislocations with Burgers vectors of type 1/2ap/110S.Strain relaxation mechanism of o-TMO films is determined to bethe formation of MDs as well as the surface fluctuations. Thisfinding may contribute to paving the way towards high-qualityo-RMnO3 epitaxial thin film growth and further research intostrain-induced effects in these materials.

Acknowledgment

We would like to acknowledge the financial support given by theNational Science Foundation of China (U0734001, 11074141) and theMinistry of Science and Technology of China (2009CB929202). Thiswork made use of the resources of the Beijing National Center forElectron Microscopy.

References

[1] W. Eerenstein, N.D. Mathur, J.F. Scott, Nature (London) 442 (2006) 759.[2] S.-W. Cheong, M. Mostovoy, Nature Materials 6 (2007) 13.[3] A.A. Bosak, A.A. Kamenev, I.E. Graboy, S.V. Antonov, O.Y. Gorbenko, A.R. Kaul,

C. Dubourdieu, J.P. Senateur, V.L. Svechnikov, H.W. Zandbergen, B. Hollander,Thin Solid Films 400 (2001) 149.

[4] J.G. Lin, T.C. Han, C.T. Wu, M.-W. Chu, C.H. Chen, Journal of Crystal Growth310 (2008) 3878.

[5] T.C. Han, J.G. Lin, Applied Physics Letters 94 (2009) 082502.[6] T.C. Han, H.H. Chao, Applied Physics Letters 97 (2010) 232902.[7] J. Fontcuberta, I. Fina, L. F�abrega, F. Sanchez, X. Martı, V. Skumryev, Phase

Transitions 84 (2011) 555.[8] X. Marti, V. Skumryev, V. Laukhin, R. Bachelet, C. Ferrater, M.V. Garcıa-

Cuenca, M. Varela, F. Sanchez, J. Fontcuberta, Journal of Applied Physics 108(2010) 123917.

[9] T.H. Lin, H.C. Shih, C.C. Hsieh, C.W. Luo, J.-Y. Lin, J.L. Her, H.D. Yang, C.-H. Hsu,K.H. Wu, T.M. Uen, J.Y. Juang, Journal of Physics: Condensed Matter 21 (2009)026013.

[10] X. Marti, I. Fina, V. Skumryev, C. Ferrater, M. Varela, L. Fabrega, F. Sanchez,J. Fontcuberta, Applied Physics Letters 95 (2009) 142903.

[11] V.Yu. Pomjakushin, M. Kenzelmann, A. Donni, A.B. Harris, T. Nakajima,S. Mitsuda, M. Tachibana, L. Keller, J. Mesot, H. Kitazawa, E. Takayama-Muromachi, New Journal of Physics 11 (2009) 043019.

[12] J.W. Matthews, A.E. Blakeslee, Journal of Crystal Growth 27 (1974) 118.[13] Y.L. Qin, C.L. Jia, K. Urban, J.H. Hao, X.X. Xi, Journal of Materials Research 17

(2002) 3117.[14] T. Suzuki, Y. Nishi, M. Fujimoto, Philosophical Magazine A 79 (1999) 2461.[15] Y.B. Chen, M.B. Katz, X.Q. Pan, C.M. Folkman, R.R. Das, C.B. Eom, Applied

Physics Letters 91 (2007) 031902.[16] O.Y. Gorbenko, S.V. Samoilenkov, I.E. Graboy, A.R. Kaul, Chemistry of

Materials 14 (2002) 4026.[17] F. Jonsdottir, Modelling and Simulations in Materials Science and Engineering

3 (1995) 503.