Thin Solid Films 520 (2012) 3651–3656

Contents lists available at SciVerse ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r .com/ locate / ts f

Tailoring in-plane crystallographic orientation of epitaxial YBa2Cu3O7− x thin filmshaving perpendicular c-axis texture

Lei Zheng a, Xiaodong Su a,c,⁎, Jun Fan a, Yajie Chen b,⁎⁎, Xiaoluan Yang a, Lixing You c, Vincent G. Harris b

a Jiangsu Key Laboratory of Thin Films, Department of Physics, Soochow University, Suzhou 215006, P. R. Chinab Department of Electrical and Computer Engineering, and the Center for Microwave Magnetic Materials and Integrated Circuits, Northeastern University, Boston, MA 02115, USAc State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road,Shanghai 200050, P. R. China

⁎ Correspondence to: X. Su, Jiangsu Key LaboratoryPhysics, Soochow University, Suzhou 215006, PR China.

E-mail addresses: xdsu@suda.edu.cn (X. Su), y.chen@

0040-6090/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.tsf.2011.12.084

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 June 2011Received in revised form 12 December 2011Accepted 30 December 2011Available online 8 January 2012

Keywords:YBCO filmIn-plane orientationPulsed laser depositionPole figureEpitaxial growth

Control of the in-plane crystallographic orientation of YBa2Cu3O7−x (YBCO) films on (100) MgO substrates isof significant application value due to the selective enhancement of superconducting properties. In the pre-sent work, the preparation, crystallographic and superconducting properties of YBCO films deposited on MgOsubstrates are reported. Crystallographic in-plane orientation was realized by means of tailoring the pulsedlaser deposition conditions and the use of interfacial buffering structures. Superconductiong propertieswere measured for films having different in-plane orienations. The results indicate that the 0° in-plane ori-ented films showed the highest current density of 1.62 MA/cm2 that was attributed to the elimination ofhigh-angle grain boudaries. Additionally, the growth mechanism of YBCO films was discussed in terms ofcrystallographic and thermodynamic theory.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

It has been known that YBa2Cu3O7− x (YBCO) crystallizes with adefected perovskite structure that is highly anisotropic and confinesconductivity within the CuO planes (i.e. a–b planes) where the criticalcurrent density is substantially larger than that along the c-axis [1].Such features are of great importance to the superconducting proper-ties of YBCO films deposited on various substrates, e.g., MgO, SrTiO3,LaAlO3, etc. MgO (100) crystals are attractive substrates for thegrowth of YBCO films since they are widely used in the electronicsand microwave industry to provide epitaxial growth of many oxidefilm systems [2–4]. More importantly, c-axis oriented YBCO thinfilms (i.e. out-of-plane orientation [00l] YBCO// [00l] MgO) can be easilygrown on (100) MgO substrates over a wide range of deposition con-ditions, leading to the potential to transfer technology to industry [5].

In addition to the crystallographic c-axis out-of-plane orientation,the orientation of the a–b, or basal plane, of YBCO crystallites withinthe film plane, strongly influences the superconducting propertiesof YBCO films due to the extremely small coherence length alongthe a–b planes (ξab≈1.5 nm [6], or the length of one YBCO unitcell). Since the weak-link behavior of grain boundaries (GBs) is relat-ed to the misorientation angle, θ, between adjacent crystallites, alarge misorientation angle dramatically decreases the critical current

of Thin Films, Department of

neu.edu (Y. Chen).

rights reserved.

density, Jc, as expressed: Jc(θ) ∝ exp(−θ/θ0) for θ0=0.4°–5° [7]. Pre-vious studies indicate that the in-plane misorientation, especially forhigh-angle GB's (θ>10°), results in Jc values of c-axis textured YBCOfilms much lower than that of YBCO single crystals, i.e. ~5 MA/cm2

[1,7]. As a result, the manipulation of the in-plane orientation forYBCO films having perpendicular c-axis texture is valuable for real ap-plications but challenging due to a large lattice parameter mismatchbetween MgO and YBCO.

Here, we define three types of in-plane crystallographic orientation ofc-axis textured YBCO films grown on MgO single crystal substrates, assketched in Fig. 1 [2,8]. For 0° orientation (Type A), all YBCO grains havea three-dimensional epitaxial growth on the MgO lattice (b100>YBCO//b100>MgO). For 45° orientation (Type B), YBCOfilms exhibit a b110>di-rection parallel to the b100> direction of the MgO crystal, i.e., the YBCOlattice has a 45° rotation relative to the MgO lattice of the substrateplane. The third classification (Type C) is a mixture of Type A and TypeB. Thus, a Type C film contains 0° and 45° orientation growth in the sub-strate plane.

It should be pointed out that due to the low density of high-angleGB's, Type A and Type B films possess greater potential for realizingpractical applications, such as YBCO devices where large transportcurrents or large microwave power is required to pass along thefilm plane. Additionally, this class of YBCO films can be used as seedlayers for the liquid phase epitaxial growth of YBCO thick films be-cause of its superheating nature and chemical stability [8–10].

For the pulsed laser deposition (PLD) technique, we note that noeffective approach has yet demonstrated in-plane oriented growth

Fig. 1. The schematic diagram of crystallographic relationship between (100) MgO andperpendicular c-axis oriented YBCO films. (a) Out-of-plane: perpendicular c-axisgrowth. And in-plane orientations: (b) 0° (Type A), (c) 45° (Type B), and (d) a mixtureof 0° and 45° (Type C).

3652 L. Zheng et al. / Thin Solid Films 520 (2012) 3651–3656

along either the 0° or 45° directions for YBCO films grown on (100)MgO single crystal substrates. Such films are anticipated to presenthigh critical currents. Preliminary experiments indicate c-axis orient-ed YBCO films deposited at low temperatures (i.e. 650–720 °C) expe-rience mixed in-plane orientations (or Type C) in which pole figuresdisplay eight-fold symmetry, i.e. 0° orientation dominates in thefilms. Obviously, those YBCO films contain copious high-angle GB's,which are not desirable for most applications, i.e. superconductingquantum interference devices, microwave devices, and coating con-ductors [11–15]. To this end, it has been a longstanding goal to dem-onstrate a single in-plane crystallographic orientation in single-crystal YBCO films grown on (100) MgO substrates.

In the present work, we present a systematic study of PLD growthconditions of YBCO films deposited on MgO substrates in which in-plane crystallographic orientation is tailored to achieve both Type Aand Type B structures.

2. Experimental details

YBCO filmswere deposited on (100)MgO single crystal substrates byPLD technique employing a 248 nm KrF excimer laser (Lambda Physik,Compex 205). The distance between target and substrate was 50 mm.The chamberwas pumped to a base pressure of 10−4 Pa and theworkinggas pressure, i.e. oxygen partial pressure (PO2), was varied among depo-sition runs from 0.02 to 5 Pa. The density of the laser energy was 2 J/cm2

with afixed pulse repetition rate of 3 Hz. A deposition ratewas estimatedto be 0.06 nm/s at a substrate temperature, Ts ranging from 720 to850 °C. As-deposited films underwent an in-situ thermal treatment atan oxygen pressure of 66.6 kPa at a temperature of 500 °C for a periodof 1 h in order to achieve near equilibrium oxygen stoichiometry. Thecrystalline structure of the as-prepared filmswas examined by X-ray dif-fraction (XRD) and pole figure analyses using a Rigaku D-MAX diffrac-tometer with Cu-Kα radiation. The thickness of the films was measuredby scanning electron microscopy (Hitachi S-4700) under an acceleratedvoltage of 15 kV. The surface morphologies of the films were examinedusing a NT-MDT Solver P47-PRO Atomic Force Microscope (AFM). Theaverage grain size and roughness (as root mean square, RMS)were eval-uated in a statisticsmode using theNOVA software. The superconductingtransition temperatures, Tc, and critical current density, Jc, of the filmswere measured using a standard four-point probe geometry by a physi-cal property measurement system (PPMS-9, Quantum Design).

3. Results and discussion

3.1. Preparation of c-axis textured YBCO films

Fig. 2 shows XRD patterns of the YBCO films having thicknesses of~200 nm, which were grown at different oxygen pressures and sub-strate temperatures. Here, an oxygen pressure was fixed at 0.02, 0.2,and 2 Pa at substrate temperatures of 720, 750, 780 and 810 °C. It isapparent from Fig. 2 that perpendicular c-axis texture was readily

achieved in YBCO films over a wide range of deposition conditions.At temperatures ranging from 720 to 750 °C a perpendicular orienta-tion of films can be tailored from c-axis to a-axis by increasing the ox-ygen partial pressure. It is well known that the nucleation and growthof crystalline films are driven by supersaturation, Δμ=kTln(R/Re),where k is the Boltzmann constant, R is the actual deposition rate,and Re is the equilibrium deposition rate at temperature T [16]. In-creasing oxygen pressure increases the frequency of collisions withinthe plume among oxygen atoms/ions and YBCO atoms/ions, thus de-creasing the values of R and Δμ. Therefore, it is understandable thatthe critical supersaturation for a-axial nucleation should be lowerthan that for c-axial nucleation [17]. On the other hand, the crystallinefilm growth depends on the surface mobility of the adatoms that maydiffuse through several atomic distances before residing in a thermo-dynamically stable position within the newly formed film. The surfacetemperature of the substrate determines to a large extent the ada-tom's surface mobility and diffusion while high temperatures favorrapid and largely defect free crystal growth [18]. In this case, higherdeposition temperatures of 780 to 810 °C correspond to large Δμ,which is beneficial to the c-axis crystallographic orientation growth[6,19]. At Ts=810 °C and PO2=0.02 Pa, (103) preferential growthwas clearly observed, which has been attributed to domain epitaxialgrowth [5].

3.2. In-plane crystallographic orientations of c-axis textured YBCO films

Next, we focus on the analysis of in-plane crystallographic orien-tation for films deposited at 2 Pa and substrate temperatures of 750,780 and 810 °C. Note, all films show perpendicular c-axis orientationand similar on-set critical transient temperatures (Tc-onset) of approx-imately 90 K. However, (103) pole figures for the YBCO films pre-sented on the left column of Fig. 3 reveal pronounced differences insymmetry among the three samples. It is clear that the film grownat 750 °C displays an in-plane orientation of Type C, where the 0° ori-entated growth is dominant. With an increase of deposition temper-ature to 780 °C, in-plane 45° oriented growth gradually reduces. It isnoteworthy that Type A oriented growth becomes dominant as theMgO substrate is heated to 810 °C. It is found that increasing Ts fur-ther results in the deterioration of crystal quality of the YBCO filmdue to the partial decomposition of the Y123 superconductingphase. Most of YBCO grains for the film grown at 810 °C show strictcrystallographic registry with the MgO substrate, i.e., for out-of-plane (00l)YBCO∥(00l)MgO, and for in-plane, b100>YBCO∥b100>MgO.

AFM surface morphologies of the YBCO films are displayed on theright column in Fig. 3. The surface roughness (RMS) is estimated to be7.86, 4.41, and 3.99 nm for the films deposited at 750, 780 and 810 °C,respectively. From the AFM images, we attribute the bright spots inFig. 3a, b to those grains with an in-plane 45° orientation, which areembedded in a matrix with the in-plane 0° orientation. Althoughthe 45° orientation is minor, it still gives rise to a few high-angleGBs in the YBCO films, decreasing Jc disproportionately [20]. A highersubstrate temperature, i.e., 810 °C, improves the uniformity of thefilm, consequently leading to an in-plane 0° orientation. Actually,the in-plane 0° oriented films reflect 3-dimensional epitaxial growthmatching the (100) MgO substrate. However, at lower temperatures,i.e., 750 °C, a 7.8% lattice mismatch between the YBCO and MgO crys-tals leads to some in-plane 45° grains embedded in a 0° oriented crys-tal matrix. The in-plane 45° growth is assumed to be due to therelease of strain arising from the lattice mismatch during epitaxialgrowth. As temperatures increase to 780 °C, the thermal and kineticenergy is high enough to merge most of the 45° oriented grains intoa vicinal matrix consisting of 0° oriented grains, as presented inFig. 3b. This phenomenon is influenced by the chemical compatibilityand geometric lattice matching between the YBCO films and the MgOsubstrates as well as between the 45° oriented grains and the 0° ori-ented grains. Furthermore, at higher temperatures, i.e., 810 °C, a

Fig. 2. XRD patterns of YBCO films prepared by PLD under different deposition conditions. Red dot lines represent an a-axis orientation.

3653L. Zheng et al. / Thin Solid Films 520 (2012) 3651–3656

continuous epitaxial growth with an in-plane 0° orientation results,as seen in Fig. 3c. In this case, films exhibit grain size larger thanthose of the films deposited at 750 and 780 °C. The total critical cur-rent through GB's can be described as Ic=PλJJc, where P is the perim-eter of the YBCO grains, λJ is peripheral layer of thickness of YBCOgrains, and Jc is intragrain critical current density [7]. The large grainsnaturally accord with a large P and reduction of the number of weak-link GB's in the film, consequently increasing Ic. Note, the films grownat either 780 or 810 °C contain an internal strain, likely released withthe formation of dislocations inside the films as they cool down fromdeposition [21].

We conclude that high deposition temperatures (Ts~810 °C) trig-ger the crystal growth of Type A YBCO films. In particular, an in-planeorientation of YBCO films is shown to be effectively tailored by depo-sition temperature. We propose that Type A YBCO films have highersuperheating temperatures in an air atmosphere than those of TypeB and C films, which considerably impact applications such as theiruse as seed layers for liquid phase epitaxial growth of single-domain YBCO bulk crystals [22,23]. However, the growth of YBCOfilms with pure 45° in-plane orientation is more difficult than the 0°

oriented growth [14]. Our experiments indicated that a simple opti-mization of the deposition temperature and oxygen pressure failedto achieve Type B oriented YBCO films.

3.3. To realize in-plane 45° orientation of c-axis textured YBCO films

Yttria-stabilized zirconia (YSZ) is often used as buffer layer to pre-vent YBCO films from experiencing chemodiffusion with elementspresent in the substrate [9,24,25]. In the present work, a YSZ buffer

layer is employed to build a foundation in order to grow Type B in-plane oriented YBCO films on MgO substrates. It has been shownthat there are three main epitaxial relations (0°, 45° or 90°) betweenYSZ single crystal and YBCO film [26,27]. This allows for the possibil-ity to rotate the YBCO lattice by 45° with respect to the MgO substratelattice by means of a YSZ buffer layer.

Here, a YSZ buffer layer of ~20 nm was grown on a MgO (100)substrate prior to the growth of YBCO films. Both of YSZ and YBCOfilms were deposited at an oxygen pressure of 2 Pa and at a deposi-tion temperature of 810 °C. The YBCO films displayed highly texturedperpendicular c-axis orientation, verified by θ−2θ XRD spectrum, asshown in Fig. 4a. The YSZ (200) diffraction peak is also detected byXRD. Fig. 4b shows an AFM image of the YBCO/YSZ/MgO sample,and Fig. 4c presents the temperature dependence of resistivity forthe two films deposited at 810 °C, with and without a YSZ bufferlayer. Both show similar transition temperatures of Tc,onset=90 K,whereas the difference in the curve's slope is mostly assumed to bedue to the difference in microstructure between the two YBCOfilms. The pole figures illustrated in Figs. 4d, e, f correspond to MgO(220), YSZ (220) and YBCO (103), respectively. From these pole fig-ure analyses, we suggest that a YSZ buffered YBCO film displays anepitaxial relationship. That is, [00l]YBCO// [00l]YSZ // [00l]MgO andb100>YBCO//b110>YSZ//b110>MgO correspond to out-of-planeand in-plane epitaxial relationships, respectively. It is verified thatan epitaxial YSZ thin film has a- and b-axes parallel to a and b-axesof the MgO crystal, respectively. This means that a YSZ layer doesnot alter the epitaxial relationship in an out-of-plane direction be-tween YBCO and MgO crystals, but the in-plane orientation of theYBCO crystal rotates 45° relative to the MgO crystal. Thus, the YSZ

Fig. 3. (103) Pole figures and AFM morphologies for the perpendicular c-axis oriented YBCO films prepared at different temperatures, 750, 780 and 810 °C, corresponding to(a), (b) and (c), respectively.

3654 L. Zheng et al. / Thin Solid Films 520 (2012) 3651–3656

layer plays a crucial role in realizing Type B YBCO films on MgOsubstrates.

3.4. Characterization of YBCO films

Some key properties for YBCO films prepared under different pro-cessing conditions are listed in Table 1, where the mean grain size isderived from the (005)YBCO diffraction peak by application of theScherrer's formula [28]. The ratio of 45° growth orientation was calcu-lated by the simple relationship: I45 /(I45+I0), where, I0 and I45 repre-sent the intensity of 0° and 45° orientation growth, respectively,which is derived from the pole figures presented in Figs. 3 and 4.Type C films, grown at 750 °C and 2 Pa and having high ratios of 45°

orientation, exhibit Tc, onset around 88 K, but Jc values are muchlower than those of either Type A films deposited at 810 °C and 2 Paor the YSZ buffered Type B films. The main reason for the low Jc inType C films is due to the high-angle GBs acting as weak-links origi-nating from 45° oriented YBCO grains of about 10.5% of the totalfilm volume. Furthermore, Type A or B films, having larger grains

due to preparation at higher Ts, foster an improvement in the super-conducting properties approaching that of single crystals.

3.5. Growth mechanism of YBCO films with YSZ buffer layer

In order to elucidate the growth mechanism for YBCO films withdifferent in-plane orientations, it was necessary to distinguish thecrystallographic structure of defected perovskite YBCO films fromcubic fluorite YSZ buffer layers and cubic halite MgO substrates.Fig. 5 is ball and stick model for an ideal epitaxial YBCO film on(100) MgOwith and without a YSZ buffer layer [29]. Fig. 5b illustratesan interface between the (010) plane of the MgO crystal and the(100) plane of YBCO. Based on a simplified theory for near coinci-dence site lattice for low-energy interfacial configurations [2], thecrystallographic relationship between YBCO and the MgO crystal iswell known, i.e. for out-of-plane: (001)YBCO∥(001)MgO; for in-plane:[100]YBCO∥ [100]MgO [30]. However, during actual growth of YBCOfilms on MgO substrates, we emphasize that the lattice mismatch isabout 8.8% (aYBCO=3.85 Å; aMgO=4.22 Å) on a basal plane. It hasalso been found that the 0° YBCO/MgO interface gives rise to a

Table 1Properties of YBCO films prepared at different deposition conditions.

750 °Cat 2 Pa

780 °Cat 2 Pa

810 °Cat 2 Pa

810 °C at 2 PaYSZ buffered

Type of in-planeorientation

Type C 0° and45°

Type C 0° and45°

Type A 0° Type B 45°

Ratio of 45°

orientation, %10.5 5.4 0.7 99.1

Grain size, nm 26 31 46 44Tc,Onset, K 88.4 89.2 90.1 89.8Jc, MA/cm2 0.61 0.83 1.62 0.95

Fig. 4. XRD Θ−2Θ pattern (a) and AFM image (b) for a YBCO film grown on a YSZ buffered (100) MgO substrate; (c) temperature dependence of resistivity for YBCO/MgO andYBCO/YSZ/MgO films; (d), (e) and (f) illustrate (220), (220) and (103) pole figures for MgO, YSZ and YBCO film, respectively.

3655L. Zheng et al. / Thin Solid Films 520 (2012) 3651–3656

terminal BaO layer, whereas a 45° interface between YBCO and MgOhas a terminal CuO chain layer [31]. Therefore, in a Type C film,there are two types of terminal layers, i.e. BaO and CuO chains.Since the 0° grains are dominant in the films, it is naturally assumedthat the BaO layer is of a lower energy and more stable than theCuO chain layer. With increasing Ts, an unstable CuO chain layer isreplaced by the BaO layer, ultimately resulting in a transformationfrom Type C to Type A. In other words, the 0° in-plane orientedgrowth is effectively tailored by a thermodynamic driving factor dur-ing an epitaxial growth.

In spite of the large misfit between YSZ and MgO (aYSZ=5.16 Å;18.01% mismatch on the basal plane), we have also found that theYSZ buffer layer indeed demonstrates a 3-dimensional growth (i.e.cube-on-cube epitaxial growth) on a (100) MgO substrate [32]. How-ever, due to very large lattice misfit between YSZ and MgO, it cannotbe solely compensated by elastic strain during crystal growth andconsequently forms a spontaneous semicoherent YSZ/MgO phaseboundary that acts to release microstrain in the heterostructure

[29]. Fig. 5a indicates the cross sections through a (110) plane ofYSZ buffered MgO and the (100) plane of YBCO. Here, the lattice con-stant of (110)YSZ (

ffiffiffi

2p

aYSZ=2=3.68 Å) is very close to that of the(001)YBCO (i.e., 3.85 Å), and it results in a 4.6% mismatch on a basalplane: A more manageable interfacial strain. In this case, an in-planeorientation of the YBCO film rotates by 45° in the (a–b) plane to

Fig. 5. Ball and stick model for ideal epitaxial YBCO films (after D.B. Fenner et al. [29]).The sketches illustrate the cross sections (a) through an (110) plane of YSZ bufferedMgO crystal and (b) through an (010) plane of MgO crystal. Suppose that an originalfree surface (001) of MgO crystal is upward.

3656 L. Zheng et al. / Thin Solid Films 520 (2012) 3651–3656

minimize the mismatch with the (110)YSZ lattice, as depicted inFig. 5a. Therefore, the YSZ layer functions as a structural link betweenthe MgO and YBCO lattice, while the YBCO lattice rotates 45° in planerelative to the MgO lattice. In brief, such buffered structures can leadto Type B YBCO films under certain deposition conditions, which stemfrom unique crystallographic relationships among the three crystallo-graphic structures.

4. Conclusion

In summary, in-plane crystallographic orientation of perpendicu-lar c-axis textured YBCO films grown on (100) MgO substrates is tai-lored by optimization of PLD deposition conditions in combinationwith the use of a YSZ buffer layer. An in-plane 0° oriented YBCOfilm can be achieved by elevating substrate temperature to 810 °C,deposited directly on a (100) MgO substrate, whereas an in-plane45° oriented YBCO film is produced by employing a 20 nm thick YSZbuffer layer. A 45° in-plane rotation for the YBCO film is verified byXRD pole figure analyses. Either of 0° or 45° in-plane oriented YBCOfilms with perpendicular c-axes is of great application value, such astransporting large current in many electronic devices, and servicingas seed layers that tolerate high superheating temperatures enablingthe growth of large diameter YBCO bulk crystals. This work provides apathway to realizing YBCO films with controllable in-plane crystallo-graphic orientations.

Acknowledgments

This work was supported by the Priority Academic Program Devel-opment of Jiangsu Higher Education Institutions (PAPD), and by the

Key Project in Natural Science Foundation of Jiangsu Education Com-mittee of China under the Grant No. 10KJA140044.

References

[1] T.R. Dinger, T.K. Worthington, W.J. Gallagher, R.L. Sand-Strom, Phys. Rev. Lett. 58(1987) 2687.

[2] D.M. Hwang, T.S. Ravi, R. Ramesh, S.W. Chan, C.Y. Chen, L. Nazar, X.D. Wu, A. Inam,T. Venkatesan, Appl. Phys. Lett. 57 (1990) 1690.

[3] E.E. Mitchell, S. Gnanarajan, K.L. Green, C.P. Foley, Thin. Solid. Film 437 (2003)101.

[4] D.L. Zhu, J. Huang, H. Li, M.R. Shen, X.D. Su, Phys. C 469 (2009) 1977.[5] J. Chen, X.D. Su, D.L. Zhu, J. Fan, H. Yang, Y. Chen, V.G. Harris, Scr. Mater. 64 (2011)

450.[6] R.K. Singh, D. Kumar, Mater. Sci. Eng. R22 (1998) 113.[7] A. Gurevich, E.A. Pashitskii, Phys. Rev. B 57 (1998) 13878.[8] L. Cheng, X. Wang, X. Yao, W. Wan, F.H. Li, J. Xiong, B.W. Tao, M.J. Jirsa, Phys.

Chem. 114 (2010) 7543.[9] D.P. Norton, A. Goyal, J.D. Budai, D.K. Christen, D.M. Kroeger, E.D. Specht, Q. He,

B. Saffian, M. Paranthaman, C.E. Klabunde, D.F. Lee, B.C. Sales, F.A. List, Science274 (1996) 755.

[10] C.Y. Tang, Y.Y. Chen, W. Li, L.J. Sun, X. Yao, M. Jirsa, Cryst. Growth Des. 10 (2010)575.

[11] T. Izumi, K. Kakimoto, K. Nomura, Y.J. Shiohara, J. Cryst. Growth 219 (2000) 228.[12] R. Ramesh, D. Hwan, T.S. Ravi, A. Inam, J.B. Barner, L. Nazar, S.W. Chan, C.Y. Chen,

B. Dutla, D.T. Venkatesan, X.D. Wu, Appl. Phys. Lett. 56 (1990) 2243.[13] D. Dimos, P. Chaudhari, J. Mannhart, F.K. LeGoues, Phys. Rev. Lett. 60 (1988) 1653.[14] Y.Q. Cai, W.Wan, F.H. Li, X. Wang, S.B. Yan, C.Y. Tang, X. Yao, M. Jirsa, J. Xiong, B.W.

Tao, Cryst. Growth Des. 9 (2009) 3218.[15] S.R. Foltyn, P.N. Arendt, Q.X. Jia, H. Wang, J.L. MacManus-Driscoll, S. Kreiskott, R.F.

DePaula, L. Stan, J.R. Groves, P.C. Dowden, Appl. Phys. Lett. 82 (2003) 4519.[16] J.D. Suh, G.Y. Sung, Phys. C 282 (1997) 579.[17] Y.Q. Cai, X. Yao, Y.J. Lai, J. Appl. Phys. 99 (2006) 113309.[18] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, J. Wiley, New

York, 1994.[19] C.B. Eom, A.F. Marshall, S.S. Laderman, R.D. Jacowitz, T.H. Geballe, Science 249

(1990) 1549.[20] D.T. Verebelyi, D.K. Christen, R. Feenstra, C. Cantoni, A. Goyal, D.F. Lee,M. Paranthaman,

P.N. Arendt, R.F. DePaula, J.R. Groves, C. Prouteau, Appl. Phys. Lett. 76 (2000) 1755.[21] D.B. Buchholz, J.S. Lei, S. Mahajan, P.R. Markworth, R. Chang, B. Hinds, T.J. Marks,

J.L. Schindler, C.R. Kannewurf, Y. Huang, K.L. Merkle, Appl. Phys. Lett. 68 (1996)3037.

[22] C.Y. Tang, X. Yao, J. Hu, Q. Rao, Y.R. Li, B.W. Tao, Supercond. Sci. Technol. 18 (2005)L31.

[23] X. Wang, Y.Q. Cai, X. Yao, W. Wan, F.H. Li, J. Xiong, B.W. Tao, J. Phys. D: Appl. Phys.41 (2008) 165405.

[24] K. Kawagishi, K. Komori, M. Fukutomi, K. Togano, Supercond. Sci. Technol. 13(2000) 1553.

[25] M.Y. Li, H.L. Kao, W.J. Chang, C.L. Lin, C.C. Chi, W.Y. Guan, M.K. Wu, J. Appl. Phys. 77(1995) 4584.

[26] J.G. Wen, C. Traeholt, H.W. Zandbergen, K. Joosse, E.M. Reuvekamp, H. Rogalla,Phys. C 218 (1993) 29.

[27] Y.M. Lai, P.A. Lin, C.C. Chi, J. Phys. D: Appl. Phys. 40 (2007) 3670.[28] H. Li, J. Huang, Q.F. Li, X.D. Su, J. Sol-Gel Sci. Technol. 52 (2009) 309.[29] D.B. Fenner, D.K. Fork, G.A.N. Connell, J.B. Boyce, A.M. Viano, T.H. Geballe, IEEE

Trans. Magnetics 27 (1991) 958.[30] R.W. Balluffi, A. Brokman, A.H. King, Acta Metall. 30 (1982) 1453.[31] J.S. Matsuda, F. Oba, T. Murata, T. Yamamoto, Y. Ikuhara, M. Mizuno, K. Nomura,

T. Izumi, Y.J. Shiohara, Mater. Res. 19 (2004) 2674.[32] X.D. Wu, L. Luo, R.E. Muenchauseq, K.N. Springet, S. Foltyn, Appl. Phys. Lett. 60

(1992) 1381.