Thickness dependence of superconducting critical current densityin vicinal YBa 2 Cu 3O 7 thick filmsR. L. S. Emergo, J. Z. Wu, T. Aytug, and D. K. Christen

Citation: Applied Physics Letters 85, 618 (2004); doi: 10.1063/1.1775882 View online: http://dx.doi.org/10.1063/1.1775882 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/85/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Correlation between grain and grain-boundary critical current densities in ex situ coated conductors with variableYBa 2 Cu 3 O 7 layer thickness Appl. Phys. Lett. 88, 122502 (2006); 10.1063/1.2188044 Tuning porosity of Y Ba 2 Cu 3 O 7 vicinal films by insertion of Y 2 Ba Cu O 5 nanoparticles Appl. Phys. Lett. 87, 232503 (2005); 10.1063/1.2140467 Enhancement of flux pinning and critical currents in YBa 2 Cu 3 O 7 films by nanoscale iridium pretreatment ofsubstrate surfaces J. Appl. Phys. 98, 114309 (2005); 10.1063/1.2138370 Microcrack-free epitaxy of thick YBa 2 Cu 3 O 7 films on vicinal r-cut sapphire buffered with CeO 2 Appl. Phys. Lett. 86, 192507 (2005); 10.1063/1.1926415 Effect of varying material anisotropy on critical current anisotropy in vicinal YBa 2 Cu 3 O 7 thin films Appl. Phys. Lett. 83, 4999 (2003); 10.1063/1.1634687

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Thickness dependence of superconducting critical current densityin vicinal YBa 2Cu3O7−d thick films

R. L. S. Emergo and J. Z. Wua)

Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045

T. Aytug and D. K. ChristenOak Ridge National Laboratory, Oak Ridge, Tennessee 37831

(Received 15 March 2004; accepted 27 May 2004)

In YBa2Cu3O7−d (YBCO) thick films, the superconducting critical current densitysJcd decreaseswith increasing film thicknessstd. The mechanisms responsible for thisJc–t behavior remainunclear. To probe the correlation between the film microstructure andJc-thickness behavior, we havedeposited YBCO thick films up to 3.0mm in thickness on flat and surface-miscut(100) SrTiO3

substrates with 5°, 10°, and 15° vicinal angles. The microstructures of the YBCO films were foundto evolve differently on flat and miscut substrates, resulting in differentJc–t behaviors. Surprisinglythe small miscut angles of 5°–10° were favorable to obtain higherJc and smallerJc reduction atlarger film thickness. ©2004 American Institute of Physics. [DOI: 10.1063/1.1775882]

The 2nd generation high-Tc superconducting(HTS)wires are under development using the “coated conductor”approach, where long lengths of highly grain-oriented HTScoatings are epitaxially deposited on metal tapes.1,2 To carryhigh current, the HTS coatings must have thickness of a fewto several micrometers. Unfortunately, the critical currentdensitysJcd suffers a dramatic decrease with increasing filmthickness whether on single crystal oxide or on bi-axiallytextured metal substrates.3–10 An interesting question ariseson the mechanism of the observedJc vs. thicknesssJc–t inthe rest of the text) behavior. In thin YBa2Cu3O7 (YBCO)films, Jc can be sensitively affected by film microstructures,such as in-plane and out-of-plane misorientations, and vari-ous growth defects. It is generally agreed thatin situ fabri-cated,c-axis oriented YBCO films grow via an island-typespiral growth on flat single-crystal oxide substrates with rea-sonably small lattice mismatch.11,12 This island-type spiralgrowth dominates after the first few monolayers and contin-ues through moderate film thickness of submicrometers. Atlarger thickness, variation of the film microstructure may oc-cur due to formation of voids and misoriented YBCOgrains.7,9,13This microstructure variation depends sensitivelyon the substrates and processing conditions and results indegradation ofJc in the outer layer of thick YBCO films. Bycarefully controlling both substrate and processing character-istics, uniform microstructure can be achieved in YBCOfilms with thickness above 1.0mm.13,14Nevertheless, a simi-lar Jc–t behavior remains except for a slowerJc decreasewith increasing thickness.9,10 This suggests that although theoccurrence of microstructural nonuniformity contributes tothe decrease ofJc with increasing film thickness, there existsan “intrinsic” Jc–t behavior in “homogeneous” thick YBCOfilms. If a different “intrinsic” Jc–t behavior is associated toa different uniform microstructure, research to develop thickYBCO films of microstructure favorable for higherJc is nec-essary and important.

One way to alter the film microstructure is to modify thefilm nucleation mechanism. For example, the island-type

growth can be switched to a step-flow type of growth byemploying vicinal substrates.15–19The microstructure of vici-nal HTS films differs greatly from their nonvicinal counter-parts(“flat” in the rest of this letter). On thin vicinal YBCOfilms, columnar defects were reported near the normal of thesubstrates15 due to the development of antiphase grainboundaries across the substrate surface steps. The magneticflux pinning was observed to be improved in these thin vici-nal YBCO18 and HgBa2CaCu2O6

19 films. It should be no-ticed that most previous work on vicinal HTS films werelimited to small film thickness,0.5 mm. It is hence neces-sary to investigate to what extent the step-flow growth can beextended to large thickness and how this microstructuralmodification affects theJc–t behavior. Motivated by this, wehave fabricated thick vicinal YBCO films up to 3.0mm inthickness on SrTiO3 (STO) substrates and have carried out acomparative study of theirJc–t behaviors. In this letter, wereport our experimental results.

Pulsed laser deposition(PLD) was employed to fabricateYBCO thick films on flat, 5°, 10°, and 15° miscut STO sub-strates cut into squares of sides 2.5–5.0 mm, with ak100laxis aligned with one of the edges of the substrates. Thedepositions were made at 810°C in 400 mTorr O2 followedby annealing at 520°C and 350 Torr O2 for 45 min. MostYBCO films of the same thickness were made in the samerun to ensure a uniform deposition condition.

The YBCO films grown on flat STO substrates arec-axisoriented as confirmed in x-ray diffraction(XRD) u–2uscans. The full width at half maxima(FWHM) of the (005)peaks is shown as function of film thickness in Fig. 1(open)with minimal variations. At small thickness up to,1.0 mm,neither secondary phases nora-axis oriented grains were ob-servable. At larger thickness, a small volume portion ofa-axis oriented grains was observed, which is consistent withthat reported previously.9,13 When grown on the miscut sub-strates, thec axis of YBCO was tilted away from the normalby an angle close to the miscut angle of the substrates. Theinset of Fig. 1 depicts an XRD(005) pole figure of a 2.0-mm-thick YBCO film on a 10° miscut STO substrate. The tiltangle of,11.0° is clearly shown. The epitaxial growth ofthe film is evidenced from the four clear poles. However, the

a)Authors to whom correspondence should be addressed; electronic mail:jwu@ku.edu

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films experienced miscut-induced strain through their thick-ness as reported earlier by other groups.14–17 This is sup-ported by the overall larger FWHM of the(005) poles(squares in Fig. 1) as compared to that on flat films.

SEM studies of the film surface morphology have con-firmed the different microstructural evolution patterns on flatand vicinal YBCO films. Figure 2 compares surface mor-phology of flat, 5°, 10°, and 15° vicinal YBCO films at threethickness: 0.2, 1.5, and 3.0mm. At small thickness, such as0.2 mm (the top row of Fig. 2), the steps are clearly seen andthe density of the steps increases with increasing miscutangle from 5° to 15°. In addition, circular pores of typicallysub-mm dimension are visible on all vicinal films at small

thickness. This may directly relate to the dislocations due tostrain release at certain thickness. At small thickness,0.2 mm, particulates that are typical to the PLD YBCOfilms are visible on all films, whether flat or vicinal, whilethe density of the particulates is higher on flat YBCO films.With increasing film thickness, the surface morphology ofthe YBCO films experienced distinctly different evolution onflat and miscut substrates. On flat STO, the particulates grewfrom sub-micron size to micron size when the thickness wasincreased to,3.0 mm. Although the films look dense, thesurface morphology became extremely rough and misori-ented rectangular shaped(a-axis) grains are visible. On thickvicinal YBCO films, on the other hand, most particulatesdisappeared while the film become increasingly porous. At1.5 mm thickness, as shown in the middle row of Fig. 2,microsize rectangular shaped pores become a distinctive fea-ture on vicinal films in addition to the small circular poresalso observed on the thin vicinal YBCO films. The long sideof these rectangular pores is either perpendicular or parallelto the steps. Some of them are connected to form a right-angle pore. The density of these rectangular shaped pores ishigher on YBCO films grown on a larger miscut angle sub-strate, while that of small circular pores take an oppositetrend. Nevertheless, steps are still visible on most of the filmwhile the step width increased by 50% –100 %. When thethickness is further increased to 3.0mm, a “melted cheese”type of surface morphology was observed on vicinal YBCOfilms (see the bottom row of Fig. 22). Large sized pores ofirregular shapes are visible over the film surface and thegrain size decreases with increasing miscut angle. On lowangle miscut substrates, e.g., 5° miscut STO, there are stillmany small circular pores at the 3.0-mm-thick YBCO films.With increasing miscut angle, the density of the small circu-

FIG. 1. Thickness dependence of the full width at half maximum of YBCOfilms deposited on flat and 10° miscut STO at(005) peak. Inset: The XRD(005) pole figure of a 2.0-mm-thick YBCO film on 10° miscut ST0. Thesplit center pole is because of detector saturation and the extra four poles aredue to the YBCO(104) reflections having the samed-spacing as(005). Thefigure shows a tilt of about 11°.

FIG. 2. SEM pictures of YBCO films deposited on STO substrates with different miscut angles. The columns show the progression(from left to right) withrespect to miscut angle namely: flat, 5°, 10°, and 15° while the rows that with respect to film thickness. Top row: 2.0-mm-thick YBCO films on(a) flat, (b)5°, (c) 10°, and(d) 15° miscut STO. Middle row: 1.5-mm-thick YBCO films on(e) flat, (f) 5°, (g) 10°, and(h) 15° miscut STO. Bottom row: 3.0-mm-thickYBCO films on (i) flat, (j) 5°, (k) 10°, and(l) 15° miscut STO.

Appl. Phys. Lett., Vol. 85, No. 4, 26 July 2004 Emergo et al. 619

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lar pores reduces dramatically, and on 15° miscut substrates,few can be seen. Steps are barely visible on 3.0-mm-thickvicinal YBCO films. The only exception is 15° vicinalYBCO films, on which fat steps of width around 1.0mm canstill be observed.

Figure 3 shows a comparison of theJc–t curves mea-sured magnetically on flat and vicinal YBCO films using asuperconducting quantum interference device magnetometer.Bean model was applied for calculation of theJc.

20 Over thethickness range we have investigated, the 5° and 10° vicinalYBCO films carry higherJc’s than YBCO films on flat STO,while the 15° vicinal YBCO films have the lowestJc overall.At 0.2 mm thickness,Jc in the 5° vicinal YBCO film is,2.6 MA/cm2 at 77 K and self-field, about 20% higher thanthat of the flat YBCO film. At 1.0mm thickness, this differ-ence increased to the maximum value of 115%. This meansthat theJc’s on the 5° and 10° vicinal YBCO films not onlyare higher but also decrease more slowly with film thicknessfrom thin to 1.0mm thickness than that of the flat YBCOfilms. It should be realized thatJc’s are anisotropic in vicinalYBCO films along the direction parallel or perpendicular tothe steps.15–19 The anisotropy increases with vicinal anglesand the magnetically measuredJc’s shown in Fig. 3 representthe lowerJc along the direction perpendicular to the steps.The inset of Fig. 3 compares the magnetic and transportJc’smeasured on the same 15° vicinal YBCO film of 0.6mmthickness. After the film has been magnetically characterized,two perpendicular microbridgess203500 mm2d with onealong the longitudinal(parallel to the steps) and the otheralong the transverse(perpendicular to the steps) directionswere patterned on the film. 1mV/cm was applied to defineJc. Indeed, the transportJc’s are highly anisotropic and thetransverseJc agrees well with the magneticJc.

Generally speaking, vicinal YBCO thick films are highlyporous and it seems surprising that they carry much higherJc’s as compared to their flat counterparts of dense structure.One explanation is that the porous structure may facilitatemore complete oxidation of the YBCO films through theentire thickness. This argument seems inconsistent with theobserved uniformTc across the film thickness.21 It should bepointed out that neither misoriented grains nor large precipi-tates, which have been regarded as the major current ob-stacles in YBCO films with their thickness above,1.0 mm,were observed on thick vicinal films. This however cannot

account for the enhancedJc’s in thinner vicinal films. Onemay argue that the porous structure may be favorable struc-ture for high currents in YBCO films at large thickness. Thepresence of pores may help to locally divide a thick film intomany thin layers, where effectively each layer behaves as athin film.

In summary, YBCO thick films with thickness rangingfrom 0.2 to 3.0mm were deposited on flat as well as 5°, 10°,and 15° miscut SrTiO3 substrates. Switching from typicalisland-type growth to step-flow growth was confirmed,which was accompanied by a highly porous microstructurein vicinal, thick YBCO thick films. The thickness depen-dence of theJc has been investigated in these films and thedifferent evolution patterns of the film microstructures werefound to correlate strongly with theJc–t behavior. The sur-prising result of higherJc’s and smallerJc decrease with filmthickness on vicinal YBCO films suggests that the porousstructure may be favorable for HTS thick film coated con-ductors.

J.Z.W. acknowledges support from NSF and DOE forthis work. R.E. was supported by AFOSR. The research atORNL was sponsored by the U.S. Department of Energyunder Contract No. DE-AC05-00OR22725 with the OakRidge National Laboratory, managed by UT-Battelle, LLC.

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FIG. 3. Jc–t curves for YBCO films deposited on STO substrates withdifferent miscut angles. Inset shows a comparison of magnetic and transportJc of a 0.6mm YBCO film on 15° miscut STO substrate.

620 Appl. Phys. Lett., Vol. 85, No. 4, 26 July 2004 Emergo et al.

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