ARTICLE IN PRESS

Journal of Crystal Growth 311 (2009) 1021–1024

Contents lists available at ScienceDirect

Journal of Crystal Growth

0022-02

doi:10.1

�Corr

E-m

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

Epitaxially strained Na0.7CoO2 thin films on SrTiO3 buffer layer

J.Y. Son, Y.-H. Shin, Hyungjun Kim �

Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea

a r t i c l e i n f o

Article history:

Received 20 October 2008

Received in revised form

17 November 2008

Accepted 20 November 2008

Communicated by M. Kawasakichange is caused by the structural symmetry break near the cubic-to-tetragonal transition temperature

Available online 25 November 2008

PACS:

61.05.cp

71.30.+h

72.15.�v

Keywords:

A1. Crystal structure

A1. X-ray diffration

A3. Laser epitaxy

B1. Oxides

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.jcrysgro.2008.11.072

esponding author. Tel.: +82 54 279 5409; fax:

ail address: hyungjun@postech.ac.kr (H. Kim)

a b s t r a c t

We prepared Na0.7CoO2 thin films on (0 0 1) Al2O3 substrates that showed a negligibly strained structure

and a metallic behavior similar to Na0.7CoO2 single crystal. In particular, a Na0.7CoO2 thin film on a (111)

SrTiO3/(0 0 1) Al2O3 substrate exhibited an elongated c-lattice constant as well as an anomaly in the

electric transporting behavior: a transition from an insulating state to a metallic state at 105 K. This

of SrTiO3. From spectroscopic ellipsometry analysis, we obtained optical conductivities for the

Na0.7CoO2 thin films on the Al2O3 and SrTiO3/Al2O3 substrates. The Na0.7CoO2 thin film on the SrTiO3/

Al2O3 substrate exhibited higher energy splitting between eg and a1g than the Na0.7CoO2 thin film on the

Al2O3 substrate, resulting from the c-lattice elongation.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Sodium cobalt oxide NaxCoO2 has been attractively researcheddue to its large thermoelectric power and low resistivity forvarious thermoelectric applications [1]. In NaxCoO2, large thermo-electric power is attributed to spin entropy in a low-spin state ofCo ion [2]. Many theoretical and experimental interests weretriggered by superconductivity of Na0.35CoO2 �1.3H2O, rich phasediagrams of NaxCoO2 with respect to x, and novel ground statesinduced by two-dimensional transition-metal oxide triangularlattice [3–5]. In addition, NaxCoO2 depending on the sodiumcontent x exhibited complicated metallic transport behaviors withvarious magnetic orderings of paramagnetic metal, ‘‘Cure–Weiss’’metal (anitiferromagnetic ordering), and spin-density wave(SDW) metal except a charge ordering state of anitiferromagneticordering [6,7]. Behind the rich physical properties, two-dimensionaltriangular lattice and mixed valence character led to puzzlingground states [6,7].

The crystal structure of NaxCoO2 consists of two-dimensionaltriangular CoO2 layers of edge-sharing CoO6 octahedrons sepa-rated by an insulating layer of Na+ ions. There are four knownphases of a-, a0-, b-, and g-distinguished by the stacking orders of

ll rights reserved.

+82 54 279 2399.

.

CoO2 layers and the concentrations of Na ions [8]. A b-phase has amonoclinic unit cell with a space group symmetry of C2/m andlattice constants of a=4.90, b=2.83, c=5.72 A, and b=105.961 and ag-phase has a hexagonal structure with a space group symmetryof P63/mmc and lattice constants of a=2.84 and c=10.81 A [9,10]. Ing-NaxCoO2, the in-plane directions of CoO6 octahedra along theout-of-plane direction of CoO2 layers are alternating with the typeof A–B, B–A, A–B, etc. where A and B are oxygen layers. Thesestructural varieties of NaxCoO2 imply that the bulk modulus alongthe in-plane direction is small and the stress generated by latticemisfit is an important parameter for the growth of epitaxial thinfilms because of various possible stacking structures of CoO2

layers and the possibility of the stacking fault.Physical properties obtained from NaxCoO2 single crystals

and powders have been widely researched but there have beena few reports about thin film studies [11–13]. From the stand-ing point of the strain effect, thin film growth can give anopportunity for a manipulation of physical properties by chang-ing stress using different substrates and growth parameters(substrate temperatures, oxygen partial pressures, etc.). Wecontrolled structures of NaxCoO2 thin films from a b-phase withan island growth mode to a g-phase with a layer-by-layer growthmode by varying a deposition rate [11]. In this study, we report thestrain effect on electrical transport properties of epitaxialNa0.7CoO2 thin films deposited on Al2O3 and (111) SrTiO3/Al2O3

substrates.

ARTICLE IN PRESS

J.Y. Son et al. / Journal of Crystal Growth 311 (2009) 1021–10241022

2. Experimental details

Na0.7CoO2 thin films were deposited on (0 0 1) Al2O3 substratesand (111) SrTiO3/Al2O3 substrates by an eclipse pulsed laserdeposition (PLD) method. In eclipse PLD, a deposition rate andenergies of adatoms simultaneously reduce because high-energyparticles are directly screened by a shadow mask. There is a latticemisfit of 2.9% between c-oriented Na0.7CoO2 and (0 0 1) Al2O3,which induces a compressive stress. A Na0.8CoO2 target wasprepared by a conventional solid-state reaction method [11].A commercially available 1 in SrTiO3 target was used for thefabrication of a SrTiO3 buffer layer on a (0 0 1) Al2O3 substrate.A frequency tripled (355 nm, �2 J/cm2) Nd:YAG laser was used forthe deposition, and the distance between the target and thesubstrate was �4 cm. For the deposition of SrTiO3 buffer layers, weused a substrate temperature of 800 1C and an oxygen partialpressure of 200 m Torr. For the deposition of Na0.7CoO2 thin films,the optimum substrate temperature was 480 1C and the optimumpartial oxygen pressure was 400 m Torr [11]. On (0 0 1) Al2O3

substrates, g-Na0.7CoO2 thin films with a layer-by-layer growthmode were grown by using a low deposition rate of 0.02 A/pulse.

The thickness of the Na0.7CoO2 thin films and the thickness ofthe SrTiO3 buffer layers were, respectively, �1000 and �500 Adetermined from cross-sectional scanning electron microscope(SEM) images. Tentative compositions of Na0.7CoO2 thin filmswere confirmed by energy dispersive X-ray spectrometer (EDS).For structural analysis, X-ray diffraction (XRD) data were obtainedby a conventional laboratory X-ray diffractometer (CuKa1 radia-tion, 1.540 A). Surface morphologies and topographies of Na0.7

CoO2 thin films were observed by scanning probe microscope(SEM) and atomic force microscope (AFM). Temperature depen-dence of resistivity was measured in the temperature range6–350 K. We analyzed spectroscopic ellipsometry data with anincident angle of 701 by a multi-wavelength variable-angleellipsometer (J.A. Woollam Co.). At the energy range of0.7–6.0 eV, ellipsometry parameters of C and D were measuredand these parameters are defined by Rr/Rs=tanC exp(iD), whereRr and Rs are complex reflection coefficients for the polarized lightin parallel and perpendicular to the plane of incidence, respec-tively. Based on a Levenberg–Marquardt Algorithm, we fit C0 andD0 for a model of Na0.7CoO2 thin films to experimental parametersof C and D.

3. Results and discussion

Fig. 1(a) shows the XRD pattern of the Na0.7CoO2 thin film onthe (0 0 1) Al2O3 substrate. For the (0 0 2) peak, the full-width at

Fig. 1. (a) The XRD pattern of the epitaxial Na0.7CoO2 thin film on the (0 0 1) Al2O3 subst

(10 4) peaks of the (0 0 1) Al2O3 substrate.

half-maximum (FWHM) of the rocking curve was about 0.81. Thec-lattice constant of 10.8 A was obtained from the 2y values of the(0 0 2) and (0 0 4) peaks, which agrees well with the experimentfor the variation of c-lattice constants depending on Naconcentration [4]. To check an in-plane crystalline structure, weperformed F-scans. Fig. 1(b) shows F-scans for the (10 4) peaks ofthe epitaxial Na0.7CoO2 thin film and the (10 4) peaks of the (0 0 1)Al2O3 substrate. The six-fold symmetry of the (10 4) peakrepresents a hexagonal structure of the Na0.7CoO2 thin film withtwinning. The FWHM of an in-plane (10 4) peak for a y�2y scanwas about 0.551. From the 2y value of the (10 4) peak, weobtained a hexagonal a-axis constant of 2.8 A.

In order to modify lattice misfit, we used a (111) SrTiO3 bufferlayer between a Na0.7CoO2 thin film and an Al2O3 substrate.A SrTiO3 buffer layer can give a lattice misfit in the range 2.4–2.9%.Fig. 2(a) shows the XRD pattern of the Na0.7CoO2 thin film on the(111) SrTiO3/Al2O3 substrate. For the (0 0 2) peak, the FWHM ofthe rocking curve was about 0.91 which is larger than 0.81 of theNa0.7CoO2 thin film on the (0 0 1) Al2O3 substrate. From the 2yvalue of the (0 0 2) peak, the c-lattice constant of 11.4 A wasobtained and this c-lattice constant is larger than that of theNa0.7CoO2 thin film on (0 0 1) Al2O3 substrate. This elongation ofthe c-lattice constant indicates that the Na0.7CoO2 thin filmadequately released a stress induced from the lattice misfit.

Fig. 2(b) shows F-scans of the (10 4) peaks of the epitaxialNa0.7CoO2 thin film, the (10 4) peaks of the SrTiO3 buffer layer,and the (10 4) peaks of the SrTiO3/Al2O3 substrate. The SrTiO3

buffer layer and the Na0.7CoO2 thin film have six-fold symmetrywith twinning. The FWHM of the in-plane (2 0 1) peak of theNa0.7CoO2 thin film is about 1.251 and this FWHM value is alsolager than that of the Na0.7CoO2 thin film on the (0 0 1) Al2O3

substrate. From the 2y value of the (10 4) peak, a hexagonal a-axisconstant of 2.8 A was obtained. Consequently, the Na0.7CoO2 thinfilm on the (111) SrTiO3/Al2O3 substrate has a larger strain thanthe Na0.7CoO2 thin film on the (0 0 1) Al2O3 substrate.

We observed spiral patterns with multi-terraces in the AFMimage of the Na0.7CoO2 thin film on the (111) SrTiO3/Al2O3

substrate (not shown). The terrace heights were close to thec-lattice constant. The terrace width of �100 nm was observed,indicating that the thin film had an atomically flat surface.Moreover, this large width of the terraces represents that surfacediffusion lengths of adatoms are quite long and kinetics of steps iswidespread [13]. The root mean square (RMS) surface roughnesswas about 1.0 nm.

Fig. 3 shows the temperature dependence of resistivities forthe Na0.7CoO2 thin films on the (0 0 1) Al2O3 and (111) SrTiO3/Al2O3 substrates. The resistivity of the Na0.7CoO2 thin film on the(0 0 1) Al2O3 substrate shows a metallic behavior similar to the

rate. (b) The F-scans of the (10 4) peaks of the epitaxial Na0.7CoO2 thin film and the

ARTICLE IN PRESS

Fig. 2. (a) The XRD pattern of the epitaxial Na0.7CoO2 thin film on the SrTiO3/Al2O3 substrate. (b) The F-scans of the (10 4) peaks of the epitaxial Na0.7CoO2 thin film, the

(10 4) peaks of the SrTiO3 buffer layer, and (10 4) peaks of the SrTiO3/Al2O3 substrate.

Fig. 3. The temperature dependence of resistivities of the Na0.7CoO2 thin films on

the (0 0 1) Al2O3 and (111) SrTiO3/Al2O3 substrates.

Fig. 4. The optical conductivities of the Na0.7CoO2 thin films on the (0 0 1) Al2O3

and (111) SrTiO3/Al2O3 substrates.

J.Y. Son et al. / Journal of Crystal Growth 311 (2009) 1021–1024 1023

Na0.7CoO2 single crystal [7]. Below 200 K, the Na0.7CoO2 thin filmon the (0 0 1) Al2O3 substrate shows a linear variation, indicatinga transport behavior for a strongly correlated system similarto the ‘‘Curie–Weiss’’ metallic behavior of the Na0.7CoO2 singlecrystal [7].

The Na0.7CoO2 thin film on the (111) SrTiO3/Al2O3 substrateshowed an anomaly in the electric transport behavior near 105 K(Fig. 4) and there is a cubic-to-tetragonal transition of SrTiO3 atthe same temperature. In the temperature range 105–300 K, an

insulating transport behavior is observed. It is suggested that thisbehavior is induced by a strained structure. Below 105 K,electronic transport changes from an insulating state to ametallic state. Since there is a cubic-to-tetragonal transition ofSrTiO3 near 105 K, which can distort the two-dimensionaltriangular lattice, it is inferred that this anomaly originates fromthe structural symmetry break of the SrTiO3 buffer layer [14,15].

We obtained optical conductivities of the Na0.5CoO2/Al2O3 andNa0.7CoO2/SrTiO3/Al2O3 calculated from ellipsometer data (Fig. 4).Three peaks of optical conductivities are observed. We denotethree transitions to A, B, and C transitions which reflect thetransitions of 3d electrons in a low-spin state of Co ions [16,17].Based on the local density approximation (LDA) calculations onNaxCoO2, the O 2p bands with the band width of 5 eV exist underthe Fermi energy at a distance of 2 eV and the hybridization of Co3d and O 2p is weak [16]. There are suitable three transitions froman initial state to a final state in the a1g (3z2

�r2) and the e0g states[17]. The A and B transitions have the initial state of the a1g

(3z2�r2) and the e0g states in the Co+4 of spin S ¼ 1

2 and the Ctransition has the initial state of the a1g (3z2

�r2) and the e0g statesin the Co+3 of spin S=0 [17]. For the Na0.7CoO2/SrTiO3/Al2O3, thephoton energies for the A, B, and C transitions are higher thanthose of the Na0.5CoO2/Al2O3. This also means that the energysplitting between eg and a1g increased resulting from the strainedstructure in the Na0.7CoO2/SrTiO3/Al2O3 that causes a distortion ofa CoO6 octahedron.

4. Conclusions

We observed a step flow growth of Na0.7CoO2 thin films withhexagonal grains. The resistivity of the Na0.7CoO2 thin film on the(0 0 1) Al2O3 substrate shows a metallic property similar to theNa0.7CoO2 single crystal. The Na0.7CoO2 thin film on the SrTiO3/Al2O3 (0 0 1) substrate showed the anomaly near the structuraltransition temperature of SrTiO3. Above 105 K, an insulatingtransport of the Na0.7CoO2 thin film on the (111) SrTiO3/Al2O3

was observed. Below 105 K, the electron transport property waschanged from an insulating state to a metallic state which causedby the structural symmetric break near the cubic-to-tetragonaltransition of SrTiO3. The Na0.7CoO2/SrTiO3/Al2O3 exhibited themore energy splitting between eg and a1g than the Na0.7CoO2/Al2O3 due to its strained structure.

Acknowledgement

This work was supported by the Brain Korea 21 Project 2006.

ARTICLE IN PRESS

J.Y. Son et al. / Journal of Crystal Growth 311 (2009) 1021–10241024

References

[1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) 12685.[2] Y. Wang, Nyriss S. Rogado, R.J. Cava, N.P. Ong, Nature 423 (2003) 425.[3] Kazunori Takada, Hiroya Sakurai, Eiji Takayama-Muromachi, Fujio Izumi,

Ruben A. Dilanian, Takayoshi Sasaki, Nature 422 (2003) 53.[4] Y. Krockenberger, I. Fritsch, G. Christiani, H.-U. Habermeier, Li. Yu, C. Bernhard,

B. Keimer, L. Alff, Appl. Phys. Lett. 88 (2006) 162501.[5] K. Sugiura, H. Ohta, K. Nomura, H. Yanagi, M. Hirano, H. Hosono, K. Koumoto,

Inorg. Chem. 45 (2006) 1894.[6] Yayu Wang, Nyrissa S. Rogado, R.J. Cava, N.P. Ong, Nature 423 (2003) 425.[7] M.L. Foo, Y.Y. Wang, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong,

Phys. Rev. Lett. 92 (2004) 247001.[8] C. Fouassier, G. Matejka, J.-M. Reau, P. Hagenmuller, J. Solid State Chem. 6

(1973) 532.

[9] Yasuhiro Ono, Ryuji Ishikawa, Yuzuru Miyazaki, Yoshinobu Ishii, Yukio Morii,Tsuyoshi Kajitani, J. Solid State Chem. 166 (2002) 177.

[10] C. Thinaharan, D.K. Aswal, A. Singh, S. Bhattacharya, N. Joshi, S.K. Gupta,J.V. Yakhmi, Cryst. Res. Technol. 39 (2004) 572.

[11] J.Y. Son, Bog G. Kim, J.H. Cho, Appl. Phys. Lett. 86 (2005) 221918.[12] H. Zhou, X.P. Zhang, B.T. Xie, Y.S. Xiao, C.X. Yang, Y.J. He, Y.G. Zhao, Thin Solid

Films 479 (2006) 338.[13] W.J. Chang, C.C. Hsieh, T.Y. Chung, S.Y. Hsu, K.H. Wu, T.M. Uen, J.-Y. Lin, J.J. Lin,

C.-H. Hsu, Y.K. Kuo, H.L. Liu, M.H. Hsu, Y.S. Gou, J.Y. Juang, Appl. Phys. Lett. 90(2007) 061917.

[14] Alain Karma, Mathis Plapp, Phys. Rev. Lett. 81 (1998) 4444.[15] Eric Courtens, Phys. Rev. Lett. 29 (1972) 1380.[16] D.J. Singh, Phys. Rev. B. 61 (2000) 13397.[17] W.B. Wu, D.J. Huang, J. Okamoto, A. Tanaka, H.J. Lin, F.C. Chou, A. Fujimori,

C.T. Chen, Phys. Rev. Lett. 94 (2005) 146402.