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Al2O3 thin films
Physica C 361 (2001) 121±129
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On the origin of surface outgrowths in pulsed-laser-deposited YBCO/CeO2/Al2O3 thin ®lms K.D. Develos *, H. Yamasaki, A. Sawa, Y. Nakagawa National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 22 January 2001; accepted 5 March 2001
Abstract Superconducting epitaxial YBa2 Cu3 O7 d (YBCO) ®lms were grown by pulsed laser deposition on CeO2 -buered sapphire (Al2 O3 ) substrates. Scanning transmission electron microscopy and energy dispersive X-ray spectrometry investigations of the outgrowths found in YBCO ®lms revealed these to be comprised of a multiphase microstructure, emanating from interfacial reactions of YBCO with the CeO2 ®lm and the Al2 O3 substrate. The surface outgrowths are mostly composed of CuO and YCuO2 phases which have segregated at the top of BaCeO3 , a product of the interfacial reaction of YBCO with CeO2 . Cross-sectional transmission electron microscopy observations further revealed a BaAl2 O4 phase formed beneath the BaCeO3 phase, including along the CeO2 ±Al2 O3 interface. Despite the presence of these outgrowths, high critical current density Jc values >2.0 106 A/cm2 were obtained. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 68.55. a; 81.15.Fg; 74.72.B Keywords: YBCO; CeO2 ; Sapphire; Interfacial reaction; Outgrowth
1. Introduction YBa2 Cu3 O7 d (YBCO) thin ®lms deposited on sapphire (Al2 O3 ) have generated much interest in research because of their reproducible high ®lm quality, high critical temperature Tc , high critical current density Jc and low microwave surface resistance Rs . Moreover, sapphire substrates are ideal candidates for device applications because of * Corresponding author. Address: Superconductor Technology Group, Energy Electronics Institute, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. Tel.: +81-298-615721; fax: +81-298-615726. E-mail address: [email protected] (K.D. Develos).
their stability at high temperatures, good mechanical strength, and availability in large wafer sizes at reasonably low cost. In particular, good thermal conducting characteristics of Al2 O3 substrates enable homogeneous quenching of YBCO ®lms, such as necessary for fast switching elements in fault current limiters [1]. Al2 O3 substrates are also ideal candidates for microwave device applications because of their low dielectric constant and dielectric loss tangent [2]. Successful deposition of superconducting YBCO thin ®lms has been possible through the use of buer layer materials like CeO2 to provide adequate lattice matching and prevent chemical reactions between Al2 O3 and YBCO [3,4]. However, YBCO ®lms, as fabricated
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by physical vapor deposition methods, are generally found to be characterized by numerous outgrowths which are distinct and protruding from the relatively smooth background matrix. It has been shown that the microstructure of the YBCO ®lm has a signi®cant eect on the Rs [5], thus the presence of outgrowths, defects, and impurity phases will in¯uence the performance of the ®lm in device applications. From our experimental observations, comparison of YBCO ®lms grown on CeO2 -buered sapphire revealed a higher density of outgrowths than those grown on other closely matched substrates such as LAO or STO [6]. To our knowledge, in-depth studies of the outgrowths have yet to be performed for pulsed-laser deposited YBCO ®lms on CeO2 -buered Al2 O3 : In this study we therefore seek to elucidate the nature and composition of the surface outgrowths which are observed to occur frequently in the YBCO/CeO2 / Al2 O3 system. 2. Experimental CeO2 and YBCO thin ®lms were deposited on R-cut (1 1 0 2) Al2 O3 substrates by pulsed-laser deposition utilizing a KrF excimer laser source (Compex 205, 248 nm) operated at 200 mJ. The deposition parameters are as listed in Tables 1 and 2. For the deposition of YBCO, the laser repetition rate was ®xed at 1 Hz (®rst step) for the ®rst ten minutes, then raised to 5 Hz (second step) for the remaining 26 min of the deposition time. The indicated value of the deposition temperature is the set value of the temperature controller. As measured by an optical pyrometer, the temperature of the substrate holder's surface is approximately 20°C higher than the set value. In Table 2, T1 and Table 1 List of deposition parameters and their values Deposition parameter
YBCO
CeO2
Laser energy (mJ) Laser repetition rate (Hz) Oxygen pressure (mTorr) Deposition temperature (°C) Film thickness (nm) Target-to-substrate distance (mm)
200 1; 5 300 720±800 260 68
200 1 100 740 30 68
Table 2 Values of the deposition temperatures used for the growth of the YBCO ®lms Film
T1 (°C)
T2 (°C)
Jc (106 A/cm2 )
Tconset (K)
A B C D E
720 760 740 760 800
720 740 760 760 800
2.2 2.1 2.3 2.2 ±
87.5 88.4 88.8 88.6 ±
Jc (77.3 K) and inductive Tconset values obtained for the YBCO ®lms are shown in the last two columns.
T2 refer to the deposition temperatures employed during the ®rst and second steps, respectively. After deposition, the ®lms are rapidly cooled to room temperature under 400 Torr ambient oxygen atmosphere. The structure of the ®lms was con®rmed by X-ray diraction (XRD) measurements. The Jc of the ®lm was measured by hysteretic magnetization measurements utilizing a vibrating sample magnetometer (VSM, Oxford Instruments) system. The critical temperature Tc was determined from ac magnetic susceptibility measurements. A selected ®lm was further observed in detail by transmission electron microscopy (TEM, Model JEM-2000EX) operated at 200 kV. Specimens for plan-view and cross-sectional TEM (XTEM) were prepared by standard techniques including Ar ion milling. The microscopic composition of the ®lm was analyzed by combined scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS, Oxford Instruments, ISIS).
3. Results and discussion 3.1. Film characterization Fig. 1 shows the h=2h XRD patterns of ®lms A± E. The XRD pattern of ®lm A (Fig. 1(a)) exhibits both (0 0 l) and (h 0 0) peaks of YBCO, including peaks of the CeO2 buer layer and Al2 O3 substrate. The presence of a-axis-oriented YBCO in this ®lm is due to the comparably low deposition temperature of only 720°C. Moreover, two additional low-intensity peaks at 2h 28:8° and 2h
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Fig. 1. h=2h XRD patterns of the ®lms A, B, C, D and E. The peaks are indexed to the (0 0 l) and (h 0 0) re¯ections of YBCO. Legend: ( ) BaCeO3 ; (j) BaAl2 O4 ; () YCuO2 ; (N) CuO; () unidenti®ed phase.
41:40° can be seen. These peaks can be identi®ed as the (1 1 0) and (2 0 0) planes of BaCeO3 , a cubic
structure with a lattice constant of a 0:4377 nm [7]. This conclusion is further corroborated by
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TEM and EDS results to be presented later in Section 3.2. The XRD patterns of ®lms B and C, which were deposited at higher temperatures of 760°C and 740°C (see Table 2), are shown in Fig. 1(b) and (c). In both ®lms, the (2 0 0) a-axis peak is still present but noticeably diminished in intensity compared to ®lm A. However, it can also be noticed that the peaks identi®ed with the BaCeO3 phase have also increased in intensities compared to those of ®lm A. In Fig. 1(d), we can see that ®lm D is almost completely c-axis oriented, and that the BaCeO3 phase is again present in this ®lm. Moreover, a rather weak peak indicated by ``j'' can be observed at 2h 19:78°. Finally, increasing the deposition temperature further to 800°C, we can observe in Fig. 1(e) that the resulting XRD pattern cannot even be indexed to either c-axis or a-axis-oriented YBCO. The BaCeO3 peaks have also evidently increased in intensity. In this ®lm, the peak occurring at 2h 19:78° is clearly distinct and can be identi®ed as the (1 0 0) re¯ection of BaAl2 O4 , a hexagonal structure with a and c lattice constants of 0.521 nm and 0.876 nm, respectively [8]. Aside from these, we can observe a number of
other visible peaks which can also be indexed to other re¯ections of BaAl2 O4 , YCuO2 [9] and CuO. The corresponding phases are indicated by symbols in the XRD pattern. Hence, the weak peak observed at 2h 19:78° in the XRD pattern of ®lm D is attributed to the same BaAl2 O4 (1 0 0) re¯ection. This conclusion will be corroborated by STEM and EDS results in Section 3.2. Fig. 2 shows the magnetization (M) vs. ®eld (H) hysteresis curves of the YBCO ®lms (A±D). To calculate the value of Jc , the following equation based on Bean's critical state model was utilized:
Fig. 2. Magnetic hysteresis curves of the YBCO ®lms at 77.3 K. Inset shows the temperature dependence of the ac magnetic susceptibility of the ®lms. The Jc and Tconset values are tabulated in Table 2.
Fig. 3. (a) Plan-view TEM image of the YBCO ®lm. The dark patches interspersed across the surface are identi®ed as the outgrowths. (b) STEM image and EDS elemental color mapping of the YBCO ®lm.
Jc
w 1
2DM w=3l
1
where DM is the width of the hysteresis loop for increasing and decreasing ®eld; w and l are the
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Fig. 3. (continued)
width and length of the ®lm, respectively. At zero ®eld and 77.3 K, all the ®lms have Jc values higher than 2:0 106 A/cm2 , and there is no signi®cant disparity among the obtained values of Jc . The inset in Fig. 2 shows the ac magnetic susceptibility
(v) curves versus temperature (T) for the dierent ®lms. The Jc and Tconset values are tabulated in Table 2. The ®lms have Tconset values greater than 88 K, except for ®lm A which has a comparatively low Tconset of 87.5 K.
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3.2. Microstructural and compositional analysis Among the ®lms, we have chosen to evaluate the morphological features of ®lm D in detail because it is almost completely c-axis oriented. In Fig. 3(a), detailed microstructure of the YBCO ®lm can be seen in the plan-view TEM image observed at a low magni®cation. Many dark patches interspersed across the YBCO background matrix
can be seen in the micrograph; these are identi®ed as the outgrowths on the ®lm surface. The sizes of these outgrowths range from 0.1 to 0:8 lm. SEM observations revealed no visible cracks on the surface of the YBCO ®lm. Fig. 3(b) shows the STEM image and elemental mapping by EDS of the YBCO ®lm measured at some area (not exactly the same location as shown in Fig. 3(a)). The central parts of the outgrowths
Fig. 4. (a) STEM and EDS elemental color mapping of the cross-section of the YBCO=CeO2 =Al2 O3 ensemble; the multiphase microstructure of BaCeO3 , BaAl2 O4 , CuO, and YCuO2 are indicated by symbols in the STEM image. (b) XTEM image of outgrowth A at a higher magni®cation. Arrows indicate areas where the thin BaAl2 O4 layer is found beneath the intact CeO2 ®lm.
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Fig. 4. (continued)
labeled 2, 3 and 5 are clearly CuO impurities; outgrowths labeled 1 and 4 appear to be Ba and Cu rich, although not signi®cantly dierent in composition with respect to the YBCO matrix. In the Ce element mapping at the bottom right, a point near the center indicated by ``'' indicates that some Ce from the buer layer has also diffused to the YBCO surface. On the other hand, the extent of the outgrowths beneath the surface can be seen in the cross-sectional STEM image of the YBCO=CeO2 =Al2 O3 ensemble shown in Fig. 4(a). In the STEM image, the outgrowths can be seen emanating from interfacial reactions of the YBCO and CeO2 ®lms, including the Al2 O3 substrate. From the mapping of the Al and Ba elements, we can see that Al from the substrate material has diused through the damaged CeO2 buer layer and reacted with Ba of YBCO. This impurity phase, identi®ed as BaAl2 O4 [8], and also detected earlier in the XRD measurements, can be seen extending from the Al2 O3 substrate up to the YBCO ®lm as indicated in the outgrowth labeled ``A'' at the left of the STEM image. This phase can also be seen as a thin layer along the CeO2 ±Al2 O3
interface, beneath places where the CeO2 layer is still intact (see areas indicated by ``M''). The existence of the BaCeO3 phase is also con®rmed in this analysis, complimenting the XRD results presented in Section 3.1. In outgrowth A, the BaCeO3 phase is seen on top of the BaAl2 O4 phase while in outgrowth B this phase is seen as a rather long, bright strip above the CeO2 ®lm. Outgrowth B, which has jutted beyond the surface of the YBCO ®lm, is comprised mostly of CuO at the bottom with traces of YCuO2 near the top. On the other hand, the phase which has segregated at the top part of outgrowth A is identi®ed as YCuO2 . Analysis of various areas along the cross-section of the YBCO=CeO2 =Al2 O3 ensemble revealed that the multiphase structure of BaAl2 O4 , BaCeO3 , CuO, and YCuO2 is the most common microstructure comprising the outgrowths. It is therefore evident from these results that the outgrowths observed on the surface originate from interfacial reactions between the YBCO and CeO2 thin ®lms, including the Al2 O3 substrate. For clarity, outgrowth A is shown at a higher magni®cation in the XTEM image of Fig. 4(b).
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The YBCO layer is oriented with the c-axis perpendicular to the substrate surface and parallel to the [0 0 1] axis of CeO2 . The CeO2 buer layer has a columnar structure and the interface with the YBCO ®lm appears ¯at except for those areas where the interfacial reactions, seen in brighter contrast, have created breaks in the continuity of the interface. In conjunction with the EDS analysis in Fig. 4(a), the dierent phases are indicated as YCuO2 , BaCeO3 , and BaAl2 O4 . Furthermore, the thin layer within the vicinity of the Al2 O3 substrate, in bright contrast and appearing continuous beneath the intact CeO2 ®lm of 10±20 nm thickness, is also identi®ed as BaAl2 O4 . Among the areas investigated, most of the CuO and YCuO2 impurities are consistently segregated on top of the BaAl2 O4 and BaCeO3 reaction phases and protrude above the surface of the YBCO ®lm. The outgrowths' protruded parts range from 0.1 to 0:3 lm in height. 3.3. Surface outgrowth formation The occurrence of the BaCeO3 reaction phase in YBCO ®lms deposited on CeO2 at high temperatures has been reported in some earlier works [10,11], but has not been directly correlated to the surface outgrowth formation in the ®lms. This is also the ®rst time to identify the presence of a multiphase microstructure comprised by BaAl2 O4 , YCuO2 , and CuO phases which are located next to BaCeO3 . It is interesting to note, however, that the BaCeO3 interfacial reactions are localized. This may be attributed to the volume constraint imposed by the overlying YBCO ®lm which may act as a kinetic barrier, as explained in the work of Holesinger et al. [12]. EDS results suggest that a two-way diusion occurs to accommodate the chemical reaction at localized areas: Ba atoms diuse from the YBCO matrix to react with CeO2 and Al2 O3 ; and Ce atoms diuse into the YBCO matrix. It is logical to presume that the reaction forming BaCeO3 occurs ®rst before the reaction forming BaAl2 O4 , because the integrity of the CeO2 ®lm as a buer layer must be damaged ®rst before the Ba atoms can diuse to the Al2 O3 substrate. The formation of the multiphase mi-
crostructure, therefore, is limited by the reaction which forms the BaCeO3 phase. The growth of the BaCeO3 phase is further enhanced by the diusion of Ba and Ce ions along the grain boundaries of the growing YBCO ®lm [11]. As the BaCeO3 phase grows within a localized area, the CeO2 source diminishes in amount until ®nally the extent of the buer layer thickness is consumed. This will create a break in the continuity of the buer layer. The Ba atoms from YBCO can then freely diuse to the Al2 O3 substrate and react to form the BaAl2 O4 phase. In some areas, such as shown in outgrowth A of Fig. 4(a), Al can diuse upwards from the Al2 O3 substrate to the ®lm area and react with Ba to form BaAl2 O4 . However, close inspection of outgrowth A in Fig. 4(b) also shows that the BaAl2 O4 phase is actually surrounded by the BaCeO3 phase at the top, such that no side is in direct contact to YBCO. This observation supports the presumption of the formation of BaCeO3 before that of BaAl2 O4 . As indicated by arrows in Fig. 4(b), a continuous layer of BaAl2 O4 beneath the intact CeO2 ®lm is observed. This layer was also observed in other several TEM images. Hence the BaAl2 O4 phase seems to be found adjacent to either CeO2 or BaCeO3 , not directly with YBCO. These observations can be explained by a lower interface energy between BaAl2 O4 and the cerium-based phases than that between BaAl2 O4 and YBCO. The existence of the multiphase microstructure, suggests that the following chemical reaction occurs: 4CeO2 2YBa2 Cu3 O7 ! 4BaCeO3 2YCuO2 4CuO O2 2 such that BaCeO3 is formed together with the precipitates YCuO2 and CuO. Although we were not able to detect YCuO2 or CuO peaks in the XRD measurements of ®lm D, it is possible that the re¯ections may have been too weak to be detected by XRD. For example, the strongest peaks of CuO (1 1 1) occurring at 2h 38:9° and that of YCuO2 (1 0 2) occurring at 2h 33:2° overlap with the intense (0 0 5) YBCO peak and (2 0 0) CeO2 peak, respectively. Next, diusion of Ba across the formed BaCeO3 into the Al2 O3 substrate occurs,
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leading to the formation of BaAl2 O4 . Ba further diuses along the CeO2 ±Al2 O3 interface and forms a continuous BaAl2 O4 layer beneath the intact CeO2 ®lm. Regardless of the large amount of outgrowths in the YBCO ®lm, the Jc value of >2.0 106 A/cm2 is high enough to merit consideration in current-carrying applications such as fault current limiters. However, we expect that by reducing or eliminating the outgrowths and improving the microstructural homogeneity in the YBCO ®lm, further increase in the value and uniformity of Jc as well as its microwave property can be realized. Although we have not evaluated the ®lm's Rs value in this study, it is expected that the presence of outgrowths in the ®lm could introduce microwave losses and lead to an increase in the value of Rs as is known in literature [5,13,14]. With a better understanding of the origin of the outgrowth formation, we can then modify the deposition parameters in order to reduce the possibility of their occurrence. 4. Summary and conclusions We had investigated the nature and composition of the surface outgrowths found in YBCO ®lms grown by pulsed laser deposition on CeO2 buered Al2 O3 substrates. Cross-sectional microscopy observations and compositional analysis of the outgrowths revealed that these were comprised of a multiphase microstructure which included YCuO2 , CuO, BaCeO3 , and BaAl2 O4 . YCuO2 and CuO were segregated on top of the BaCeO3 phase; BaAl2 O4 phase was found beneath the BaCeO3 and along the CeO2 ±Al2 O3 interface. The bariumbased phases were products of the interfacial reactions of YBCO with the CeO2 buer layer and the Al2 O3 substrate. The formations of these
129
phases were enhanced by increasing the deposition temperature. The presence of these outgrowths in the YBCO ®lm, however, did not preclude the attainment of good superconducting properties such as a high Jc value (>2.0 106 A/cm2 ).
References [1] B. Gromoll, G. Ries, W. Schmidt, H.-P. Kraemer, B. Seebacher, B. Utz, R. Nies, H.-W. Neumeueller, E. Baltzer, S. Fischer, B. Heismann, IEEE Trans. Appl. Supercond. 9 (1999) 656. [2] M. Lorenz, H. Hochmuth, D. Natusch, G. Lippold, V.L. Svetchnikov, T. Kaiser, M.A. Hein, R. Schwab, R. Heidinger, IEEE Trans. Appl. Supercond. 9 (1999) 1936. [3] X.D. Wu, R.C. Dye, R.E. Muenchausen, S. Foltyn, M. Maley, A.D. Rollet, A.R. Garcia, N.S. Nogar, Appl. Phys. Lett. 56 (1990) 2165. [4] M.W. Denho, J.P. McCarey, J. Appl. Phys. 70 (1991) 3986. [5] G. Kastner, D. Hesse, R. Scholz, H. Koch, F. Ludwig, M. Lorenz, H. Kittel, Physica C 243 (1995) 281. [6] K.D. Develos, unpublished. [7] Powder Diraction File, Release 1998, JCPDS ± International Center for Diraction Data. [8] K. Dovidenko, S. Oktyabrsky, A. Ivanov, A. Michaltsov, D. Tokarchuk, L. Zherikhina, Physica C 185±189 (1991) 2131. [9] L.P. Guo, Y.J. Tian, J.Z. Liu, L. Li, Z.X. Zhao, S.F. Xu, H.B. Lu, Y.L. Zhou, Z.H. Chen, G.Z. Yang, Physica C 241 (1995) 30. [10] G.L. Skofronick, A.H. Carim, S.R. Foltyn, R.E. Muenchausen, J. Mater. Res. 8 (1993) 2785. [11] Yu.A. Boikov, T. Claeson, D. Erts, F. Bridges, Z. Kvitky, Phys. Rev. B 56 (1997) 11312. [12] T.G. Holesinger, S.R. Foltyn, P.N. Arendt, H. Kung, Q.X. Jia, R.M. Dickerson, P.C. Dowden, R.F. DePaula, J.R. Groves, J.Y. Coulter, J. Mater. Res. 15 (2000) 1110. [13] K.D. Develos, M. Kusunoki, M. Mukaida, S. Ohshima, Supercond. Sci. Technol. 12 (1999) 887. [14] Y.J. Tian, L. Li, L.P. Guo, J.Z. Liu, Z.X. Zhao, S.F. Xu, H.B. Lu, Y.L. Zhou, Z.H. Chen, D.F. Cui, G.Z. Yang, Appl. Phys. Lett. 65 (1994) 2356.
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On the origin of surface outgrowths in pulsed-laser-deposited YBCO/CeO2/Al2O3 thin ®lms K.D. Develos *, H. Yamasaki, A. Sawa, Y. Nakagawa National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan Received 22 January 2001; accepted 5 March 2001
Abstract Superconducting epitaxial YBa2 Cu3 O7 d (YBCO) ®lms were grown by pulsed laser deposition on CeO2 -buered sapphire (Al2 O3 ) substrates. Scanning transmission electron microscopy and energy dispersive X-ray spectrometry investigations of the outgrowths found in YBCO ®lms revealed these to be comprised of a multiphase microstructure, emanating from interfacial reactions of YBCO with the CeO2 ®lm and the Al2 O3 substrate. The surface outgrowths are mostly composed of CuO and YCuO2 phases which have segregated at the top of BaCeO3 , a product of the interfacial reaction of YBCO with CeO2 . Cross-sectional transmission electron microscopy observations further revealed a BaAl2 O4 phase formed beneath the BaCeO3 phase, including along the CeO2 ±Al2 O3 interface. Despite the presence of these outgrowths, high critical current density Jc values >2.0 106 A/cm2 were obtained. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 68.55. a; 81.15.Fg; 74.72.B Keywords: YBCO; CeO2 ; Sapphire; Interfacial reaction; Outgrowth
1. Introduction YBa2 Cu3 O7 d (YBCO) thin ®lms deposited on sapphire (Al2 O3 ) have generated much interest in research because of their reproducible high ®lm quality, high critical temperature Tc , high critical current density Jc and low microwave surface resistance Rs . Moreover, sapphire substrates are ideal candidates for device applications because of * Corresponding author. Address: Superconductor Technology Group, Energy Electronics Institute, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan. Tel.: +81-298-615721; fax: +81-298-615726. E-mail address: [email protected] (K.D. Develos).
their stability at high temperatures, good mechanical strength, and availability in large wafer sizes at reasonably low cost. In particular, good thermal conducting characteristics of Al2 O3 substrates enable homogeneous quenching of YBCO ®lms, such as necessary for fast switching elements in fault current limiters [1]. Al2 O3 substrates are also ideal candidates for microwave device applications because of their low dielectric constant and dielectric loss tangent [2]. Successful deposition of superconducting YBCO thin ®lms has been possible through the use of buer layer materials like CeO2 to provide adequate lattice matching and prevent chemical reactions between Al2 O3 and YBCO [3,4]. However, YBCO ®lms, as fabricated
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by physical vapor deposition methods, are generally found to be characterized by numerous outgrowths which are distinct and protruding from the relatively smooth background matrix. It has been shown that the microstructure of the YBCO ®lm has a signi®cant eect on the Rs [5], thus the presence of outgrowths, defects, and impurity phases will in¯uence the performance of the ®lm in device applications. From our experimental observations, comparison of YBCO ®lms grown on CeO2 -buered sapphire revealed a higher density of outgrowths than those grown on other closely matched substrates such as LAO or STO [6]. To our knowledge, in-depth studies of the outgrowths have yet to be performed for pulsed-laser deposited YBCO ®lms on CeO2 -buered Al2 O3 : In this study we therefore seek to elucidate the nature and composition of the surface outgrowths which are observed to occur frequently in the YBCO/CeO2 / Al2 O3 system. 2. Experimental CeO2 and YBCO thin ®lms were deposited on R-cut (1 1 0 2) Al2 O3 substrates by pulsed-laser deposition utilizing a KrF excimer laser source (Compex 205, 248 nm) operated at 200 mJ. The deposition parameters are as listed in Tables 1 and 2. For the deposition of YBCO, the laser repetition rate was ®xed at 1 Hz (®rst step) for the ®rst ten minutes, then raised to 5 Hz (second step) for the remaining 26 min of the deposition time. The indicated value of the deposition temperature is the set value of the temperature controller. As measured by an optical pyrometer, the temperature of the substrate holder's surface is approximately 20°C higher than the set value. In Table 2, T1 and Table 1 List of deposition parameters and their values Deposition parameter
YBCO
CeO2
Laser energy (mJ) Laser repetition rate (Hz) Oxygen pressure (mTorr) Deposition temperature (°C) Film thickness (nm) Target-to-substrate distance (mm)
200 1; 5 300 720±800 260 68
200 1 100 740 30 68
Table 2 Values of the deposition temperatures used for the growth of the YBCO ®lms Film
T1 (°C)
T2 (°C)
Jc (106 A/cm2 )
Tconset (K)
A B C D E
720 760 740 760 800
720 740 760 760 800
2.2 2.1 2.3 2.2 ±
87.5 88.4 88.8 88.6 ±
Jc (77.3 K) and inductive Tconset values obtained for the YBCO ®lms are shown in the last two columns.
T2 refer to the deposition temperatures employed during the ®rst and second steps, respectively. After deposition, the ®lms are rapidly cooled to room temperature under 400 Torr ambient oxygen atmosphere. The structure of the ®lms was con®rmed by X-ray diraction (XRD) measurements. The Jc of the ®lm was measured by hysteretic magnetization measurements utilizing a vibrating sample magnetometer (VSM, Oxford Instruments) system. The critical temperature Tc was determined from ac magnetic susceptibility measurements. A selected ®lm was further observed in detail by transmission electron microscopy (TEM, Model JEM-2000EX) operated at 200 kV. Specimens for plan-view and cross-sectional TEM (XTEM) were prepared by standard techniques including Ar ion milling. The microscopic composition of the ®lm was analyzed by combined scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS, Oxford Instruments, ISIS).
3. Results and discussion 3.1. Film characterization Fig. 1 shows the h=2h XRD patterns of ®lms A± E. The XRD pattern of ®lm A (Fig. 1(a)) exhibits both (0 0 l) and (h 0 0) peaks of YBCO, including peaks of the CeO2 buer layer and Al2 O3 substrate. The presence of a-axis-oriented YBCO in this ®lm is due to the comparably low deposition temperature of only 720°C. Moreover, two additional low-intensity peaks at 2h 28:8° and 2h
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Fig. 1. h=2h XRD patterns of the ®lms A, B, C, D and E. The peaks are indexed to the (0 0 l) and (h 0 0) re¯ections of YBCO. Legend: ( ) BaCeO3 ; (j) BaAl2 O4 ; () YCuO2 ; (N) CuO; () unidenti®ed phase.
41:40° can be seen. These peaks can be identi®ed as the (1 1 0) and (2 0 0) planes of BaCeO3 , a cubic
structure with a lattice constant of a 0:4377 nm [7]. This conclusion is further corroborated by
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TEM and EDS results to be presented later in Section 3.2. The XRD patterns of ®lms B and C, which were deposited at higher temperatures of 760°C and 740°C (see Table 2), are shown in Fig. 1(b) and (c). In both ®lms, the (2 0 0) a-axis peak is still present but noticeably diminished in intensity compared to ®lm A. However, it can also be noticed that the peaks identi®ed with the BaCeO3 phase have also increased in intensities compared to those of ®lm A. In Fig. 1(d), we can see that ®lm D is almost completely c-axis oriented, and that the BaCeO3 phase is again present in this ®lm. Moreover, a rather weak peak indicated by ``j'' can be observed at 2h 19:78°. Finally, increasing the deposition temperature further to 800°C, we can observe in Fig. 1(e) that the resulting XRD pattern cannot even be indexed to either c-axis or a-axis-oriented YBCO. The BaCeO3 peaks have also evidently increased in intensity. In this ®lm, the peak occurring at 2h 19:78° is clearly distinct and can be identi®ed as the (1 0 0) re¯ection of BaAl2 O4 , a hexagonal structure with a and c lattice constants of 0.521 nm and 0.876 nm, respectively [8]. Aside from these, we can observe a number of
other visible peaks which can also be indexed to other re¯ections of BaAl2 O4 , YCuO2 [9] and CuO. The corresponding phases are indicated by symbols in the XRD pattern. Hence, the weak peak observed at 2h 19:78° in the XRD pattern of ®lm D is attributed to the same BaAl2 O4 (1 0 0) re¯ection. This conclusion will be corroborated by STEM and EDS results in Section 3.2. Fig. 2 shows the magnetization (M) vs. ®eld (H) hysteresis curves of the YBCO ®lms (A±D). To calculate the value of Jc , the following equation based on Bean's critical state model was utilized:
Fig. 2. Magnetic hysteresis curves of the YBCO ®lms at 77.3 K. Inset shows the temperature dependence of the ac magnetic susceptibility of the ®lms. The Jc and Tconset values are tabulated in Table 2.
Fig. 3. (a) Plan-view TEM image of the YBCO ®lm. The dark patches interspersed across the surface are identi®ed as the outgrowths. (b) STEM image and EDS elemental color mapping of the YBCO ®lm.
Jc
w 1
2DM w=3l
1
where DM is the width of the hysteresis loop for increasing and decreasing ®eld; w and l are the
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Fig. 3. (continued)
width and length of the ®lm, respectively. At zero ®eld and 77.3 K, all the ®lms have Jc values higher than 2:0 106 A/cm2 , and there is no signi®cant disparity among the obtained values of Jc . The inset in Fig. 2 shows the ac magnetic susceptibility
(v) curves versus temperature (T) for the dierent ®lms. The Jc and Tconset values are tabulated in Table 2. The ®lms have Tconset values greater than 88 K, except for ®lm A which has a comparatively low Tconset of 87.5 K.
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3.2. Microstructural and compositional analysis Among the ®lms, we have chosen to evaluate the morphological features of ®lm D in detail because it is almost completely c-axis oriented. In Fig. 3(a), detailed microstructure of the YBCO ®lm can be seen in the plan-view TEM image observed at a low magni®cation. Many dark patches interspersed across the YBCO background matrix
can be seen in the micrograph; these are identi®ed as the outgrowths on the ®lm surface. The sizes of these outgrowths range from 0.1 to 0:8 lm. SEM observations revealed no visible cracks on the surface of the YBCO ®lm. Fig. 3(b) shows the STEM image and elemental mapping by EDS of the YBCO ®lm measured at some area (not exactly the same location as shown in Fig. 3(a)). The central parts of the outgrowths
Fig. 4. (a) STEM and EDS elemental color mapping of the cross-section of the YBCO=CeO2 =Al2 O3 ensemble; the multiphase microstructure of BaCeO3 , BaAl2 O4 , CuO, and YCuO2 are indicated by symbols in the STEM image. (b) XTEM image of outgrowth A at a higher magni®cation. Arrows indicate areas where the thin BaAl2 O4 layer is found beneath the intact CeO2 ®lm.
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Fig. 4. (continued)
labeled 2, 3 and 5 are clearly CuO impurities; outgrowths labeled 1 and 4 appear to be Ba and Cu rich, although not signi®cantly dierent in composition with respect to the YBCO matrix. In the Ce element mapping at the bottom right, a point near the center indicated by ``'' indicates that some Ce from the buer layer has also diffused to the YBCO surface. On the other hand, the extent of the outgrowths beneath the surface can be seen in the cross-sectional STEM image of the YBCO=CeO2 =Al2 O3 ensemble shown in Fig. 4(a). In the STEM image, the outgrowths can be seen emanating from interfacial reactions of the YBCO and CeO2 ®lms, including the Al2 O3 substrate. From the mapping of the Al and Ba elements, we can see that Al from the substrate material has diused through the damaged CeO2 buer layer and reacted with Ba of YBCO. This impurity phase, identi®ed as BaAl2 O4 [8], and also detected earlier in the XRD measurements, can be seen extending from the Al2 O3 substrate up to the YBCO ®lm as indicated in the outgrowth labeled ``A'' at the left of the STEM image. This phase can also be seen as a thin layer along the CeO2 ±Al2 O3
interface, beneath places where the CeO2 layer is still intact (see areas indicated by ``M''). The existence of the BaCeO3 phase is also con®rmed in this analysis, complimenting the XRD results presented in Section 3.1. In outgrowth A, the BaCeO3 phase is seen on top of the BaAl2 O4 phase while in outgrowth B this phase is seen as a rather long, bright strip above the CeO2 ®lm. Outgrowth B, which has jutted beyond the surface of the YBCO ®lm, is comprised mostly of CuO at the bottom with traces of YCuO2 near the top. On the other hand, the phase which has segregated at the top part of outgrowth A is identi®ed as YCuO2 . Analysis of various areas along the cross-section of the YBCO=CeO2 =Al2 O3 ensemble revealed that the multiphase structure of BaAl2 O4 , BaCeO3 , CuO, and YCuO2 is the most common microstructure comprising the outgrowths. It is therefore evident from these results that the outgrowths observed on the surface originate from interfacial reactions between the YBCO and CeO2 thin ®lms, including the Al2 O3 substrate. For clarity, outgrowth A is shown at a higher magni®cation in the XTEM image of Fig. 4(b).
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The YBCO layer is oriented with the c-axis perpendicular to the substrate surface and parallel to the [0 0 1] axis of CeO2 . The CeO2 buer layer has a columnar structure and the interface with the YBCO ®lm appears ¯at except for those areas where the interfacial reactions, seen in brighter contrast, have created breaks in the continuity of the interface. In conjunction with the EDS analysis in Fig. 4(a), the dierent phases are indicated as YCuO2 , BaCeO3 , and BaAl2 O4 . Furthermore, the thin layer within the vicinity of the Al2 O3 substrate, in bright contrast and appearing continuous beneath the intact CeO2 ®lm of 10±20 nm thickness, is also identi®ed as BaAl2 O4 . Among the areas investigated, most of the CuO and YCuO2 impurities are consistently segregated on top of the BaAl2 O4 and BaCeO3 reaction phases and protrude above the surface of the YBCO ®lm. The outgrowths' protruded parts range from 0.1 to 0:3 lm in height. 3.3. Surface outgrowth formation The occurrence of the BaCeO3 reaction phase in YBCO ®lms deposited on CeO2 at high temperatures has been reported in some earlier works [10,11], but has not been directly correlated to the surface outgrowth formation in the ®lms. This is also the ®rst time to identify the presence of a multiphase microstructure comprised by BaAl2 O4 , YCuO2 , and CuO phases which are located next to BaCeO3 . It is interesting to note, however, that the BaCeO3 interfacial reactions are localized. This may be attributed to the volume constraint imposed by the overlying YBCO ®lm which may act as a kinetic barrier, as explained in the work of Holesinger et al. [12]. EDS results suggest that a two-way diusion occurs to accommodate the chemical reaction at localized areas: Ba atoms diuse from the YBCO matrix to react with CeO2 and Al2 O3 ; and Ce atoms diuse into the YBCO matrix. It is logical to presume that the reaction forming BaCeO3 occurs ®rst before the reaction forming BaAl2 O4 , because the integrity of the CeO2 ®lm as a buer layer must be damaged ®rst before the Ba atoms can diuse to the Al2 O3 substrate. The formation of the multiphase mi-
crostructure, therefore, is limited by the reaction which forms the BaCeO3 phase. The growth of the BaCeO3 phase is further enhanced by the diusion of Ba and Ce ions along the grain boundaries of the growing YBCO ®lm [11]. As the BaCeO3 phase grows within a localized area, the CeO2 source diminishes in amount until ®nally the extent of the buer layer thickness is consumed. This will create a break in the continuity of the buer layer. The Ba atoms from YBCO can then freely diuse to the Al2 O3 substrate and react to form the BaAl2 O4 phase. In some areas, such as shown in outgrowth A of Fig. 4(a), Al can diuse upwards from the Al2 O3 substrate to the ®lm area and react with Ba to form BaAl2 O4 . However, close inspection of outgrowth A in Fig. 4(b) also shows that the BaAl2 O4 phase is actually surrounded by the BaCeO3 phase at the top, such that no side is in direct contact to YBCO. This observation supports the presumption of the formation of BaCeO3 before that of BaAl2 O4 . As indicated by arrows in Fig. 4(b), a continuous layer of BaAl2 O4 beneath the intact CeO2 ®lm is observed. This layer was also observed in other several TEM images. Hence the BaAl2 O4 phase seems to be found adjacent to either CeO2 or BaCeO3 , not directly with YBCO. These observations can be explained by a lower interface energy between BaAl2 O4 and the cerium-based phases than that between BaAl2 O4 and YBCO. The existence of the multiphase microstructure, suggests that the following chemical reaction occurs: 4CeO2 2YBa2 Cu3 O7 ! 4BaCeO3 2YCuO2 4CuO O2 2 such that BaCeO3 is formed together with the precipitates YCuO2 and CuO. Although we were not able to detect YCuO2 or CuO peaks in the XRD measurements of ®lm D, it is possible that the re¯ections may have been too weak to be detected by XRD. For example, the strongest peaks of CuO (1 1 1) occurring at 2h 38:9° and that of YCuO2 (1 0 2) occurring at 2h 33:2° overlap with the intense (0 0 5) YBCO peak and (2 0 0) CeO2 peak, respectively. Next, diusion of Ba across the formed BaCeO3 into the Al2 O3 substrate occurs,
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leading to the formation of BaAl2 O4 . Ba further diuses along the CeO2 ±Al2 O3 interface and forms a continuous BaAl2 O4 layer beneath the intact CeO2 ®lm. Regardless of the large amount of outgrowths in the YBCO ®lm, the Jc value of >2.0 106 A/cm2 is high enough to merit consideration in current-carrying applications such as fault current limiters. However, we expect that by reducing or eliminating the outgrowths and improving the microstructural homogeneity in the YBCO ®lm, further increase in the value and uniformity of Jc as well as its microwave property can be realized. Although we have not evaluated the ®lm's Rs value in this study, it is expected that the presence of outgrowths in the ®lm could introduce microwave losses and lead to an increase in the value of Rs as is known in literature [5,13,14]. With a better understanding of the origin of the outgrowth formation, we can then modify the deposition parameters in order to reduce the possibility of their occurrence. 4. Summary and conclusions We had investigated the nature and composition of the surface outgrowths found in YBCO ®lms grown by pulsed laser deposition on CeO2 buered Al2 O3 substrates. Cross-sectional microscopy observations and compositional analysis of the outgrowths revealed that these were comprised of a multiphase microstructure which included YCuO2 , CuO, BaCeO3 , and BaAl2 O4 . YCuO2 and CuO were segregated on top of the BaCeO3 phase; BaAl2 O4 phase was found beneath the BaCeO3 and along the CeO2 ±Al2 O3 interface. The bariumbased phases were products of the interfacial reactions of YBCO with the CeO2 buer layer and the Al2 O3 substrate. The formations of these
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phases were enhanced by increasing the deposition temperature. The presence of these outgrowths in the YBCO ®lm, however, did not preclude the attainment of good superconducting properties such as a high Jc value (>2.0 106 A/cm2 ).
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