Microstructure and properties of YBa2Cu3O7−δ thin films with BaO precipitatesY. J. Tian, L. P. Guo, L. Li, Y. Q. Zhou, Y. Yang, Z. X. Zhao, S. F. Xu, H. B. Lu, Y. L. Zhou, Z. H. Chen, D.F. Cui, and G. Z. Yang Citation: Applied Physics Letters 65, 234 (1994); doi: 10.1063/1.112639 View online: http://dx.doi.org/10.1063/1.112639 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/65/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microstructural study of yttria stabilized zirconia buffered sapphire for YBa2Cu3O7−δ thin films J. Appl. Phys. 79, 940 (1996); 10.1063/1.360876 Processing dependence of the interfacial microstructure of Ag contacts to YBa2Cu3O7−δ thin films Appl. Phys. Lett. 63, 836 (1993); 10.1063/1.109870 Precipitates in YBa2Cu3O7−δ thin films annealed at low oxygen partial pressure Appl. Phys. Lett. 62, 3201 (1993); 10.1063/1.109624 Microstructure of superconducting YBa2Cu3O7−δ thin films on Si and alumina substrates with bufferlayers J. Appl. Phys. 66, 4886 (1989); 10.1063/1.343807 Preparation, patterning, and properties of thin YBa2Cu3O7−δ films Appl. Phys. Lett. 52, 1904 (1988); 10.1063/1.99739

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Microstructure and properties of YBa&u&+ thin films with BaO precipitates

Y. J. Tian, L. P. Guo, L. Li, Y. Q. Zhou, Y. Yang, and Z. X. Zhao The National Laboratory oj’ Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing (100080), China

S., F. Xu, H. 6. Lu, Y L. Zhou, Z. H. Chen, D. F. Cui, and G. Z. Yang Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China

(Received 24 January 1994; accepted for publication 23 April 1994)

BaO precipitates with sizes between lo-100 nm in laser ablated YBa&u,O,-, (YBCO) thin films on Y-stabilized Zirconia substrates have been identified by x-ray diffraction and transmission electron microscopy. The precipitates exhibit equiaxed shapes and grow epitaxially inside and on the surface of the YBCO films, with (001)BaO plane parallel to the a,b plane of YBCO. Some of these smaller precipitates and BaO/YBCO boundaries probably provide potential pinning sites for magnetic flux lines, which might contribute to the observed increase of critical current density with magnetic field B under B=~500 G in the case of B perpendicular to the G axis of the film.

A number of studies have been focused on the charac- terization of impurity phases that were incorporated in ‘YBa,CuaO,-s (YBCO) thin films prepared by various meth- ods. The results of the studies have shown that the formation of a second phase may strongly influence the growth mecha- nism and superconducting properties of YBCO films. The relation between impurity phases and film morphology has been revealed by scanning electron microscopy,’ which showed that the surface roughness increases with the de- crease of the CulY or Ba/Y, whereas increasing the Cu con- tent leads to the formation of small particles of CuO phase on the film surface. Recently, some groups have reported that Y203, CuO, Y,Cu,Os, and YzBaCuO, precipitates were present in laser-ablated films,’ Y,O, in sputtered films3 and CuYOz in films prepared by electron beam coevaporation.4 These second ‘phases may act as flux pinning sites, raising the critical current density. So the nonstoichiometric YBCO thin films, either Cu or Y rich in composition, can still ex- hibit excellent superconducting transition temperature T, and critical current density J, comparable to the stoichio- metric thin film~.~ However, the identification of BaO impure phase and its. effect on the superconducting properties of YBCO films have not been reported up to date.

In this letter, we identified BaO precipitates by determi- nation of its structure and relation of the epitaxial growth on the surface and inside of the YBCO films prepared by laser ablation. Its effects on the superconducting properties of the films have also been investigated.

A XeCl excimer pulsed laser with wavelength of 308 mn, pulsed width of 40 ns, repetition of 7 Hz was used to prepared the YBCO thin film. The YBCO films were depos- ited on (lOO)Y-stabilized zirconia (YSZ) substrates at tem- peratures of about 700-800 “C. The laser beam was focused onto a rotating sintered stoichiometric YBCO target (the sur- face of which contained no impurity particles) was placed in a stainless-steel vacuum chamber. Laser energy density was about 4 J/cmz. Substrate-target distance was 30 mm. Oxygen partial pressure was 200 mTorr during deposition, and after deposition, the films were cooled down to 480 “C in one

atmosphere of oxygen ambient for 20 min. A standard four- probe method was used to measure the superconducting properties, the results showed that T,=91 K and J,=2.5X 10’ A/cm2 at 77.5 K, zero field for the film depos- ited at temperature of 800 “C.

Transmission electron microscopy (TEM) specimens were prepared for planar view. The method for preparation of the specimen was by polishing the substrate side on emery papers to ~30 ,um in thickness, then ion milling the substrate side until perforation occurred, it was then put on a specimen grid and observed with a Hitachi 9OOONA electron micro- scope operating at 300 kV.

Figure 1 shows the x-ray diffraction pattern of the thin film sample. It can be seen that two peaks corresponding to interplanar spacings of 0.2761 and 0.1381 nm exist in the pattern (denoted by asterisks) besides the c-axis oriented (OOL) peaks of YBCO film and the (200) peak of YSZ sub- strate. The phase fitting to the ASTM x-ray index card best is BaO, with (002) and (004) reflections, respectively. BaO is fee in structure (u=O.552 nm) and belongs to the space group Fm3m. So the dominant (001) planes of BaO would be parallel to the (001) YBCO planes. In addition, we have found that when the substrate-target distance is kept at 30 mm during deposition at temperatures between 700 and

20K -

$ 1OK

40 50 60 70 -W&r.)

FIG. 1. X-ray diffraction pattern of YBCO thin film with BaO precipitates.

234 Appl. Phys. Lett. 65 (2), 11 July 1994 0003-6951/94/65(2)/234/3/$6.00 Q 1994 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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800 “C, the two BaO peaks always appeared on the x-ray diffraction pattern, but the relative intensities varied with the temperature.

Systematic TEM examination was carried out on the same film sample shown in Fig. 1. A low magnification TEM micrograph (not shown here) shows equiaxed precipitates having sizes between 10 and 100 nm, with a density of the order of lO’/cmz were present inside and on the surface of the c-axis oriented YBCO film. The observed preferential sites for the nucleation of the precipitates are at the domain boundaries, Fig. 2(a), the c-axis orientation grain and a-axis orientation grain boundaries (due to the existence of some a-axis orientation grams in the c-axis orientation film) Fig. 2(b) and inside the c-axis domains, Fig. 2(c). The selected area electron diffraction pattern (WED) of precipitate corre- sponding to Fig, 2(a) is shown in Fig. 3, which show the (001) oriented precipitate of BaO. Energy dispersive x-ray spectroscopy (EDAX) of the precipitates conmmed that they significantly contained more Ba than the 123 matrix phase. The dominant epitaxial relationship between BaO and

FIG. 2. (a) TETM tiicrograph of BaO precipitate at the domain boundary of YBCO film. (b) TEM micrograph of a BaO precipitate at the boundary of the u-axis grain and c-axis grain. (c) TEM micrograph of the BaO precipi- tate within the domain of YBCO film.

c-axis oriented YBCO film was (OOl)BaO~~(OOl)YBCO, [OOl]BaO~~[OOl]YBCO.

From the SAED result it also verified the presence of the BaO precipitates in our laser ablated YBCO thin film, no BaCO, precipitates were detected. However, the formation of BaO persists in contact with YBCO seems to be unex- pected from the equilibrium phase diagram. The phase rela- tions near YBazCu306,, indicate that CuO and BaCuO, are stable in contact with YBCO at high oxygen pressure and BaCu,Oz and YBa,Cu,O,,, are stable in contact with YBCO at low oxygen pressure,5 which is close to the oxygen pressure used in preparation of our films. First, the existence of BaO/YBCO interfaces in the film may be due to the ki- netic hindrance of the nucleation and growth of the expected phases, or to lower the interfacial energies between YBCO and BaO than between YBCO and YBa3Cu206+,. Second, the pulsed laser ablation process is a nonequilibrium process and the structures observed here may not correspond to the equilibrium state of the films, since the formation of the non- equilibrium phase is controlled by kinetics, the metal-stable

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a 0 *

0 l *

* 8 l

*

0 0 *

a 0

(4

e l 0

* *

* 0 *

*

e e 0 *

* 0 e e

-k *

6 * 0

m YBCO<OOl> * BaO<OOl> L BaO,double diffraction

FIG. 3. (a) SAED of the BaQ precipitate in Fiis. 2(a) and Z(b) its schematic illustration showing (001) BaO superimposed on the (001) YBCO pattern.

structures tend to be either simple structures or polymorphic structures,4 therefore the occurrence of BaO precipitates in contact with YBCO is reasonable.

The existence of BaO precipitates in c-oriented YBCO thin films did not deteriorate the superconducting properties

. .

0.5 ,- .--...I 0.0 0. I 0.2 0.3 0.4 0.5

B 0

FIG. 4. J, vs B when B is perpendicular to the c axis of the film.

of the film, the T,, are still quite high ~90 K. Figure 4 shows Jc versus B under low magnetic field at 77 K. The experi- mental result indicate that the maximum of J,(B) exceeds the value of J, at zero magnetic field in the case of magnetic field perpendicular to the c axis of the film, the position of the maximum is at about 0.05 T, which implies that some of the small BaO precipitates or the boundaries between the YBCO/BaO may act as pinning centers for the flux lines.

In summary, BaO precipitates in laser ablated YBCO tilms on YSZ substrates have been identified by x-ray dif- fraction and EM. They are characterized by equiaxed shapes and grow epitaxially inside and on the surface of the YBCO films. The orientation relationship with respect to the c-axis film matrix was (OOl)BaO]](OOl)YBCO, [OOl]BaO~~[OOl}YBCO. Some of the smaller BaO precipi- tates and/or the boundaries between YBCO/BaO may act as pinning centers and hence affect the critical current density of the film.

This work was supported by the National Center for R & D on Superconductivity of China.

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