Stranski–Krastanow (SK) growth mode of layered γ-Na0.7CoO2 on (1 1 1) SrTiO3 substrate

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Journal of Crystal Growth 310 (2008) 3093– 3096

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Journal of Crystal Growth

0022-02

doi:10.1

� Corr

E-m

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

Stranski– Krastanow (SK) growth mode of layered g-Na0.7CoO2 on (111)SrTiO3 substrate

J.Y. Son a, J.H. Cho b,�

a Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Koreab RCDAMP and Department of Physics, Pusan National University, Pusan 609-735, Republic of Korea

a r t i c l e i n f o

Article history:

Received 21 January 2008

Received in revised form

26 February 2008

Accepted 28 February 2008

Communicated by M. Kawasakiunit cell. This large width of the terrace indicates that the surface diffusion of adatoms along the surface

Available online 4 March 2008

PACS:

61.05.cp

68.37.Ps

72.80.Ga

73.50.�h

Keywords:

A1.Surface structure

A1.X-ray diffraction

A3.Laser epitaxy

B1.Oxides

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

016/j.jcrysgro.2008.02.035

esponding author. Tel.: +82 51510 2968; fax:

ail address: jinhcho@pusan.ac.kr (J.H. Cho).

a b s t r a c t

We report the Stranski–Krastanow (SK) growth mode of layered g-Na0.7CoO2 on (111) SrTiO3 substrate

deposited by pulsed laser deposition method. The g-Na0.7CoO2 thin film shows SK growth mode with

multi-terraces, and nano-islands of �30 nm diameter were observed on the terraces. Large terraces with

long widths above 100 nm were observed and these terrace heights were close to the lattice constant of

is long enough and the kinetics of the step is widespread due to the low deposition rate inhibiting from

frequent nucleation of a adatoms. Nano-islands were almost formed on the plane of a terrace in order to

release the elastic strain energy due to the lattice misfit.

& 2008 Elsevier B.V. All rights reserved.

Recently, layered cobaltate Na0.7CoO2 has been attractivelystudied due to the discoveries of high thermoelectric power inmetallic Na0.7CoO2, superconductivity of Na0.3CoO2 �1.3H2O,and charge-ordered insulator of Na0.5CoO2, etc., [1–7]. NaxCoO2

has the triangular structure of the CoO2 plane and strongelectron correlation effect, which can provide a rich phasediagram of NaxCoO2 depending on the fraction of x [8,9].At x ¼ 0, x ¼ 0.5 and x ¼ 1, NaxCoO2 shows mott insulatorbehavior. For 1

4oxo13, superconductivity of NaxCoO2 �1.3H2O is

observed below 4 K [4–7]. Above x ¼ 34, NaxCoO2 exhibits an

unusually large thermopower and its susceptibility is Curie–Weisstype [10–12].

The crystal structure NaxCoO2 consists of two-dimensional(2D) triangular CoO2 layers of edge-sharing CoO6 octahedraseparated by an insulating layer of Na+ ions. There are four knownphases of NaxCoO2 with slightly different structures such as a-, a0-,b-, and g-phases of NaxCoO2 distinguished by stacking order ofCoO2 layers and Na–O environments [13–15]. Fig. 1 shows theschematic structure of g-NaxCoO2 in a–c plane. The g-phase has a

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+82 51515 2390.

hexagonal structure with a space group symmetry of P63/mmcand lattice constants of a ¼ 0.284 nm and c ¼ 1.081 nm.The in-plane direction of CoO6 octahedron in CoO2 layer isalternating with the nearest CoO2 layers. In the triangularstructure, since the Co ions get low-spin states of Co+3 and Co+4,geometrical frustration led by triangular structure at a lowtemperature can enrich the physical properties of NaxCoO2

system, where the Co+4 ions show spin of S ¼ 12 and the Co+3 ions

shows a low-spin state of S ¼ 0 [16–20]. In addition, CoO2 layersare 2D layers, and can exhibit a remarkable quantum effect suchas Mott insulator and spin entropy behavior due to the strongelectron correlation [4,8].

Polycrystalline and single-crystalline samples of NaxCoO2 havebeen widely researched [1–5] and, recently, a few studies onNaxCoO2 thin film were reported [21–23]. Moreover, the growthmechanism of heterogeneous thin films has been an importantsubject since these structures have many technically importantapplications. In the previous studies, reduction of deposition ratecould change the structure of b-NaxCoO2 film with island growthmode to that of a g-NaxCoO2 film with layer-by-layer growth modeon (0 0 1) sapphire substrate. The g-NaxCoO2 film exhibited spiralsurface growth with multi-terraced island and highly crystallizedtexture compared to that of the b-NaxCoO2 film [21]. Here,

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Fig. 1. Schematic structures of g-Na0.7CoO2. Rhombus and filled circle symbolizes a

CoO6 octahedron and Na ions, respectively. The in-plane direction of a CoO6

octahedron in a CoO2 layer has the opposite direction of the in-plane direction of a

CoO6 octahedron in the nearest CoO2 layers.

Fig. 3. The F-scans of the (10 4) peaks of the epitaxial g-Na0.7CoO2 thin film and

(110) peaks of the (111) SrTiO3 substrate.

Fig. 2. The X-ray diffraction pattern of the epitaxial g-Na0.7CoO2 thin film on (111)

SrTiO3 substrate.

J.Y. Son, J.H. Cho / Journal of Crystal Growth 310 (2008) 3093–30963094

we report the Stranski–Krastanow (SK) growth mode of layeredg-Na0.7CoO2 on (111) SrTiO3 substrate.

Na0.7CoO2 thin films have been grown by pulsed laserdeposition (PLD) method. Na0.8CoO2 target, which was used inPLD, was prepared by a conventional solid-state reaction method.Na2CO3 (99.995%) and Co3O4 (99.998%) were mixed in themolar ratio of Na:Co ¼ 0.8:1.0 and the mixed powder was pressedinto pellet and calcined at 750 1C for 12 h. The calcined pelletwas reground, pressed into pellet, and sintered at 850 1Cfor 24 h. (111) SrTiO3 was used for the thin film growthand the optimum substrate temperature for the g-Na0.7CoO2

thin films was 480 1C. The g-Na0.7CoO2 thin film was grown withlayer-by-layer growth mode by the low deposition rate of0.02 nm/pulse using the eclipse method on (0 0 1) sapphiresubstrates. When the deposition rate was increased, the mixedphase of b- and g-NaxCoO2 was observed. By applying thecondition of the high deposition rate of 0.2 nm/pulse, onlyb-NaxCoO2 thin film was grown with island growth mode. Afrequency-tripled (355 nm, �2 J/cm2) Nd:YAG laser was used forthe deposition and the distance between target and substratewas �4 cm. For the structural determination of g-NaxCoO2 thinfilm, X-ray diffraction data were obtained by conventionallaboratory X-ray as well as synchrotron X-ray sources at 5C2 inthe Pohang Light Source. The thickness of g-Na0.7CoO2 thinfilm was �100 nm as determined from the cross-sectional imagesof scanning electron microscope (SEM). The tentative compositionof g-Na0.7CoO2 was obtained by energy dispersive X-ray spectro-meter (EDS, errors of less than 3%). The surface topography ofg-Na0.7CoO2 thin film was observed by atomic force microscope(AFM).

Fig. 2 shows the X-ray diffraction pattern of the epitaxiallygrown g-Na0.7CoO2 thin film on (111) SrTiO3 substrate. The full-width at half-maximum (FWHM) of the rocking curve of (0 0 2)peak was about 1.21. This FWHM is relatively higher than that ofg-Na0.7CoO2 thin film on (111) sapphire substrate and this valueindicates that the g-Na0.7CoO2 thin film on (111) SrTiO3 substrateundergoes compressive stress due to the lattice misfit of 2.4%. Thec-lattice constant of 1.084 nm was obtained from the 2y value of

the (0 0 2) peak and this c-lattice constant is close to the bulklattice (c ¼ 1.081 nm). To check the in-plane orientation, weperformed X-ray diffraction with F-scan geometry.

Fig. 3 shows the F-scan of the (10 4) peak of the g-Na0.7CoO2

thin film and the F-scan of the (110) peak of the (111) SrTiO3

substrate. The six-fold symmetry represents the hexagonalstructure of g-Na0.7CoO2 thin film with twinning. The FWHM ofan in-plane (10 4) peak is about 0.651 and this FWHM value is alsolarger than that of the g-Na0.7CoO2 on the (111) sapphiresubstrate, which means the relative large strain of g-Na0.7CoO2

thin film on the (111) SrTiO3 substrate. From the 2y value of the(10 4) peak of g-Na0.7CoO2 thin film, a hexagonal a-axis constantof 0.283 nm was obtained, which is close to the bulk value ofa-lattice constant (a ¼ 0.284 nm). Based on the comparisonbetween the FWHM of the in-plane direction and the FWHM ofthe out-of-plane, it is inferred that the bulk modulus along the in-plane direction is small and the stress is an important parameterfor the growth of the epitaxial thin films.

Fig. 4 (a) shows the AFM images of the g-Na0.7CoO2 thin film on(111) SrTiO3 substrate. The spiral patterns with multi-terraces

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Fig. 5. The temperature dependences of the resistivities of g-Na0.7CoO2 thin film

on the (111) SrTiO3 substrates.

Fig. 4. (a) The AFM image of g-Na0.7CoO2 thin film on (111) SrTiO3 substrate.

Spiral patterns with multi-terraces and nano-islands of �30 nm diameter are

observed. (b) and (c) Sectional contour graphs of the ]1 and the line ]2 in

g-Na0.7CoO2 thin film.

J.Y. Son, J.H. Cho / Journal of Crystal Growth 310 (2008) 3093–3096 3095

and the nano-islands of �30 nm diameter are observed and theseterraces indicate that the thin film was grown with atomically flatsurface by monolayer steps. The root-mean-square (RMS) surfaceroughness of g-Na0.7CoO2 was about 1.1 nm. The nano-islandswould be formed with the release of the elastic strain energy dueto the compressive stress induced from the lattice misfitof 2.4%. This indicates that the growth mode changes from nearstep-flow growth mode to SK growth mode [24,25]. Fig. 4(c and d)shows the sectional contour graphs of the ]1-line and the ]2-line

on the AFM topography of the g-Na0.7CoO2 thin film on (111)SrTiO3 substrate. The terrace heights are close to the latticeconstant of unit cell along the out-of-plane direction. This largewidth of the terraces indicates that the surface diffusion ofadatoms along the surface is long enough and the kinetics of thestep is widespread due to the low deposition rate inhibiting fromfrequent nucleation of adatoms [26]. The terrace width of�100 nm is observed and this indicates the thin film has anatomically flat surface.

Fig. 5 shows the temperature dependences of the resistivitiesof g-Na0.7CoO2 thin film on the (111) SrTiO3 substrates. Theresistivity of Na0.7CoO2 thin film shows a metallic property similarto the resistivity of Na0.7CoO2 single crystal. Below 100 K, theNa0.7CoO2 thin film on the (111) SrTiO3 substrate shows that thetemperature dependence of the resistivity exhibits a linearvariation, indicating the transport behavior of a strongly corre-lated system, and this linear resistivity means the ‘‘Curie–Weiss’’metallic behavior of the Na0.7CoO2 thin film [3].

In conclusion, the layered g-Na0.7CoO2 on (111) SrTiO3

substrate was deposited by PLD method and the film showed SKgrowth mode. The g-Na0.7CoO2 thin film showed SK growth modewith multi-terraces, and nano-islands of �30 nm diameter on thehexagonal grains were observed. The large terraces with a widthof �100 nm were observed and these terrace heights were close tothe lattice constant of unit cell along the out-of-plane direction.This large width of the terrace indicates that the surface diffusionof adatoms along the surface is long enough and the kinetics ofthe step is widespread due to the low deposition rate inhibitingfrom frequent nucleation of adatoms. These nano-islands werechiefly formed on the plane of a terrace and this means that thesenano-islands are formed to release the elastic strain energy due tothe lattice misfit.

Acknowledgments

This work was supported by Grant no. KRF-2006-005-J02801from the Korea Research Foundation. J.Y. Son was supported bythe Brain Korea 21 Project 2006.

References

[1] Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685.[2] H. Zhang, W.D. Luo, M.L. Cohen, S.G. Louie, Phys. Rev. Lett. 93 (2004) 236402.

ARTICLE IN PRESS

J.Y. Son, J.H. Cho / Journal of Crystal Growth 310 (2008) 3093–30963096

[3] Y. Wang, N.S. Rogado, R.J. Cava, N.P. Ong, Nature 423 (2003) 425.[4] M.L. Foo, Y. Wang, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong,

Phys. Rev. Lett. 92 (2004) 247001.[5] R. Ray, A. Ghoshray, K. Ghoshray, S. Nakamura, Phys. Rev. B 59 (1999) 9454.[6] Y. Krockenberger, I. Fritsch, G. Christiani, H.-U. Habermeier, Li Yu, C. Bernhard,

B. Keimer, L. Alff, Appl. Phys. Lett. 88 (2006) 162501.[7] K. Sugiura, H. Ohta, K. Nomura, H. Yanagi, M. Hirano, H. Hosono, K. Koumoto,

Inorg. Chem. 45 (2006) 1894.[8] N.P. Ong, R.J. Cava, Science 305 (2004) 52.[9] J. Sugiyama, J.H. Brewer, E.J. Ansaldo, H. Itahara, T. Tani, M. Mikami, Y. Mori, T.

Sasaki, S. Hebert, A. Maignan, Phys. Rev. Lett. 92 (2004) 017602.[10] R. Ray, A. Ghoshray, K. Ghoshray, S. Nakamura, Phys. Rev. B 59 (1999)

9454.[11] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) 12685.[12] Mao Foo, Yayu Wang, Satoshi Watauchi, H.W. Zandbergen, Tao He, R.J. Cava,

N.P. Ong, Phys. Rev. Lett. 92 (2004) 247001.[13] R.J. Balsys Davis, Solid State Ion. 93 (1996) 279.[14] Yasuhiro Ono, Ryuji Ishikawa, Yuzuru Miyazaki, Yoshinobu Ishii, Yukio Morii,

Tsuyoshi Kajitani, J. Solid State Chem. 166 (2002) 177.

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

[16] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian,T. Sasaki, Nature 442 (2003) 53.

[17] R.E. Schaak, T. Klimczuk, M.L. Foo, R.J. Cava, Nature 424 (2003) 527.[18] R. Jin, B.C. Sales, P. Khalifah, D. Mandrus, Phys. Rev. Lett. 91 (2003)

217001.[19] F.C. Chou, J.H. Cho, P.A. Lee, E.T. Abel, K. Matan, Y.S. Lee, Phys. Rev. Lett. 92

(2004) 157004.[20] R. Jin, B.C. Sales, P. Khalifah, D. Mandrus, Phys. Rev. Lett. 91 (2003)

217001.[21] J.Y. Son, BogG. Kim, J.H. Cho, Appl. Phys. Lett. 86 (2005) 221918.[22] Y. Krockenberger, I. Fritsch, G. Cristiani, A. Matveev, L. Alff, H.-U. Habermeier,

B. Keimer, Appl. Phys. Lett. 86 (2005) 191913.[23] H. Ohta, S.-W. Kim, S. Ohta, K. Koumoto, M. Hirano, H. Hosono, Cryst. Growth

Des. 5 (2005) 25.[24] I.N. Stranki, L. Krastanow, S.W.S. Waa, -Nat. Kl. 2B 146 (1938) 797.[25] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943.[26] Alain Karma, Mathis. Plapp, Phys. Rev. Lett. 81 (1998) 4444.