- Email: [email protected]
Microstructure of RE2O3 layers on cube textured Ni substrates
Physica C 384 (2003) 54–60 www.elsevier.com/locate/physc
Microstructure of RE2O3 layers on cube textured Ni substrates J.H. Je a,b, H. You a,*, W.G. Cullen a, V.A. Maroni a, B. Ma a, R.E. Koritala a, M.W. Rupich c, C.L.H. Thieme c b
a Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Department of Materials Science and Engineering, POSTECH, Pohang 790-784, South Korea c American Superconductor, Westborough, MA 01581, USA
Received 24 June 2002; accepted 26 July 2002
Abstract We investigated the structure and alignment of epitaxial Y2 O3 and Gd2 O3 films on cube textured Ni(0 0 1) substrates and used the findings to develop a general interpretation of the morphology of RE2 O3 films (RE ¼ Y or a rare earth element) on cube textured nickel and nickel-based alloys. The [1 0 0] axis of p RE oxides mostly prefers aligning to the ffiffiffi 2 O3 p ffiffiffi Ni[1 1 0] axis, while the [0 0 1] axis is aligned with the Ni[0 0 1] axis. This ð2 2 2 2ÞR45° extended domain matching (EDM) seems to be favored by a high coincidence site density (CSD) with the Ni(0 0 1) surface atoms, in spite of the existence of large lattice mismatches to Ni(0 0 1). Meanwhile a 3 3 EDM is not favored on Ni(0 0 1), in spite of the relatively small lattice mismatch to Ni(0 0 1), because of the existence of a low CSD. The broad mosaic distributions of RE2 O3 (0 0 l) grains (6–9° full-width at half-maximum) indicate that RE2 O3 layers prefer growing along the Ni(0 0 1) planes rather than along the Ni surface, a condition presumably induced by the favorable energetics for EDM. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 58.55.)a; 74.76.Bz; 68.35.Bs; 68.55.J Keywords: RE2 O3 /Ni; Coincidence site density; Extended domain matching; Cube textured Ni substrates
1. Introduction During the fabrication of coated-conductortype superconducting tapes, the growth of a Y1 Ba2 Cu3 O7 (YBCO) layer with a high degree of biaxial texture (required for a high critical-current density) is typically achieved through the use of a nickel or Ni-base alloy substrate and the deposition of a buffer layer with a sharp, biaxial texture [1,2]. In one approach, a Ni or Ni alloy foil is made *
Corresponding author. Tel.: +1-630-252-3429; fax: +1-630252-7777. E-mail address: [email protected] (H. You).
with a sharp (0 0 1) [1 0 0] cube texture [2]. Buffer layers are deposited onto this substrate in an epitaxial manner. The buffer layer prevents diffusion of Ni into the YBCO layer, reduces interfacial strain due to lattice mismatch, and in addition, transmits the cube texture of the substrate to the YBCO layer. Various types of buffer layers have been tested [2–12]. RE2 O3 (RE ¼ Y or a rare earth element) films have the potential to simplify the conductor manufacturing process because the requirements for the buffer layer can be met with a single layer [8,9]. We present the results of an investigation of the structure and alignment of epitaxial Y2 O3 and
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 9 8 2 - 2
J.H. Je et al. / Physica C 384 (2003) 54–60
Gd2 O3 films on cube textured Ni(0 0 1) substrates using synchrotron X-ray scattering techniques. The findings of this study have allowed us to develop a general interpretation of the morphology of RE2 O3 films on cube textured nickel and nickelcontaining alloys. Particular p emphasis ffiffiffi pffiffiffi is placed on the extent to which the ð2 2 2 2ÞR45° extended domain matching (EDM) observed for most RE2 O3 buffer layers on Ni(0 0 1) is due to high coincidence site density (CSD) with the Ni(0 0 1) surface atoms.
55
conventional h–2h scattering profiles in the specular direction (along the surface normal, i.e., [0 0 1] direction), (2) additional Bragg reflections in offspecular directions, and (3) the rocking curves of the Bragg reflections. Measurements of surface roughnesses of the as-annealed nickel substrate and the coated substrates were made using a Digital Instruments Dimension 3100 scanning probe microscope, operated in the tapping mode. A scanning electron microscope (Hitachi S-4700-II ) was used to measure the average grain size. Results of these measurements are reported in Table 1.
2. Experimental section A (0 0 1) [1 0 0] cube textured nickel ribbon (99.98þ%) was produced by cold-rolling to over 95% deformation, followed by recrystallization at temperatures ranging from 900 to 1100 °C [13]. The thickness of the textured Ni substrate was 50 lm. The texture-annealed Ni substrates were heated resistively in an e-beam system to 550 °C to remove surface oxides. Y2 O3 films (20 nm thick) were deposited by electron beam evaporation [9], while Gd2 O3 films (20 nm thick) were deposited from a solution containing Gd-alkoxide by metal organic deposition (MOD) carried out at 1000– 1150 °C and ambient pressure under reducing conditions. The X-ray scattering measurements were performed using the BESSRC beamline (12BM) at the Advanced Photon Source. The incident X-rays were focused vertically and horizontally by a toroidal mirror and monochromatized to a wave using a double-bounce length of 1.03314 A Si(2 2 0) monochromator. Two sets of slits in front of the detector provided sufficient detector resolution to achieve Dð2hÞ=2h ¼ 1 104 . The experiments were carried out by measuring (1) the
3. Results and discussion The conventional h–2h scan patterns of the Y2 O3 /Ni and Gd2 O3 /Ni structures consist only of the Y2 O3 (0 0 l) and Gd2 O3 (0 0 l) reflections, respectively, together with substrate reflections, indicating that Y2 O3 and Gd2 O3 films grow with an (0 0 l) preferred orientation. Fig. 1(a) shows a /-scan of the off-specular Y2 O3 (2 2 2) reflection (54.7° off from the specular (0 0 1) direction) obtained for the 20 nm thick Y2 O3 film on Ni(0 0 1). The four well-defined Y2 O3 (2 2 2) reflections at 90° intervals indicate that the Y2 O3 layer grows epitaxially on Ni(0 0 1). Comparison with the /-scan of Ni(1 1 1), Fig. 1(c), shows that the Y2 O3 (2 2 2) reflections are rotated 45°. This indicates that the [1 0 0] axis of the Y2 O3 domains is aligned to the Ni[1 1 0] axis in the in-plane direction. The epitaxial relationship can be summarized as Y2 O3 (0 0 1)kNi(0 0 1) and Y2 O3 [1 0 0]kNi[1 1 0]. The /-scan of the Gd2 O3 (2 2 2) reflection from the 20 nm thick Gd2 O3 film on Ni(0 0 1), Fig. 1(b), also shows four well-defined Gd2 O3 (2 2 2) reflections at 90° intervals, rotated 45° from the
Table 1 Roughness parameters and grain sizes for the cube textured Ni substrate and the RE2 O3 layers Constituent
Deposition method
Nickel Y2 O3 /Ni Gd2 O3 /Ni
– E-beam evaporation Chemical (MOD)
Roughness (nm)
Grain size
RMS
Average
0.56 0.9 10.4
0.46 0.7 8.3
10–50 lm
20 nm
20 nm
56
J.H. Je et al. / Physica C 384 (2003) 54–60 Y2O3 (Gd2O3)[110]
(a)
Ni[100]
Y2O3 (Gd2O3)[100]
Ni[100]
(b)
pffiffiffi pffiffiffi Fig. 2. Pictorial representation of (a) the ð2 2 2 2ÞR45° EDM of RE2 O3 [1 0 0] on Ni[1 1 0] and (b) the (3 3) EDM of RE2 O3 [1 0 0] on Ni[1 0 0]. The large open circles represent Y(Ga) metal ions, while the small solid circles represent Ni atoms. The unit cells of Y2 O3 (Gd2 O3 ) are illustrated by the large squares drawn with thick lines.
Fig. 1. The /-scans for the (a) Y2 O3 (2 2 2), (b) Gd2 O3 (2 2 2), and (c) the Ni(1 1 1) off-specular reflections.
Ni(1 1 1) reflections. The majority of the Gd2 O3 layer grows epitaxially on Ni(0 0 1) with the same 45° rotation as for the Y2 O3 /Ni film. However, we also observe minor domains (at a level typically obscured in measurements with laboratory X-ray sources) where a small fraction (a few percent) of the Gd2 O3 layer has a cube-on-cube orientation without the 45° rotation. It is well known that most RE2 O3 buffer layers (Y2 O3 , Gd2 O3 , Yb2 O3 , Eu2 O3 , etc.) grow epitaxially on Ni(0 0 1) surfaces with the same type of inplane epitaxial relationship as that of the Y2 O3 /Ni film [8,9]. This epitaxial relationship pffiffiffi pffiffiffi can be understood in terms of a ð2 2 2 2ÞR45° EDM. An EDM that uses multiple unit cells for lattice matching can explain the epitaxial growth of grains with a large lattice mismatch [4]. The crystal structure of Y2 O3 (Gd2 O3 ) is analogous to the flu-
orite (CaF2 ) structure, with one-fourth of the oxygen atoms removed in order to maintain charge neutrality between Y3þ (Gd3þ ) and O2 [9]. The lattice parameter for Y2 O3 (Gd2 O3 ), 10.604 A ), is about three times that of Ni (3.520 (10.813 A pffiffiffi ). Fig. 2(a) presents a schematic of a ð2 2 A pffiffiffi 2 2ÞR45° EDM on a Ni(0 0 1) surface. The large open circles represent Y3þ (Gd3þ ) ions, while the small solid circles represent Ni atoms. For the unit cell of Y2 O3 (Gd2 O3 ), illustrated by the large squares drawn with thick lines, the Y2 O3 (Gd2 O3 ) [1 0 0] axis is aligned to the Ni[1 1 0] axis in the inplane direction. Note also for this alignment that two Y(Gd) atomic distances in the Y2 O3 (Gd2 O3 ) [1 0 0] direction match to four Ni atomic distances in pthe ffiffiffi Ni[1 pffiffiffi 1 0] direction. This depiction of a ð2 2 2 2ÞR45° EDM is consistent with the observation that the off-specular Y2 O3 (2 2 2) and Gd2 O3 (2 2 2) Bragg reflections (Fig. 1(a) and (b), respectively) are rotated 45° from the off-specular Ni(1 1 1) reflections in the in-plane direction (Fig. 1(c)). In the 3 3 EDM, on the other hand, two Y(Ga) atomic distances in the Y2 O3 (Gd2 O3 ) [1 0 0] direction match to three Ni atomic distances in the Ni[1 0 0] direction, as is illustrated in Fig. 2(b). The lattice mismatch between the Y2 O3 or Gd2 O3 unit cell and the Ni super unit cell in the 3 3 EDM (0.4% and 2.4%, respectively) ispffiffiffisubstantially pffiffiffi smaller than that in the ð2 2 2 2ÞR45° EDM
J.H. Je et al. / Physica C 384 (2003) 54–60
(6.5% and 8.6%, respectively). The degree of CSD matching [14] serves as an explanation for why the 3 3 EDM is not preferred for either Y2 O3 /Ni or Gd2 O3 /Ni, in spite of their smaller lattice mismatch. For an array of 8 Y3þ (or Gd3þ ) metal ions lying in a two-dimensional 3 3 superlattice on Ni(0 0 1) planes, only one-fourth are coincident with underlying Ni atoms, while three-fourths are not coincident with underlying Ni atoms as shown in Fig. 2(b). This higher degree of non-coincidence naturally raises the interfacial energy. It is therefore less likely that a Y2 O3 or Gd2 O3 layer would adopt a 3 p 3 ffiffiffiEDM pffiffiffion Ni(0 0 1). On the other hand, the ð2 2 2 2ÞR45° EDM has 100% coincidence with Ni(0 0 1) in terms of the Y3þ (or Gd3þ ) metal ions in the Y2 O3 (or Gd2 O3 ) layer (Fig. 2(a)). This very high site coincidence drastically the interfacial energy, and the pffiffiffi reduces pffiffiffi ð2 2 2 2ÞR45° EDM is the preferred orientation for the Y2 O3 or Gd2 O3 on Ni(0 0 1) despite the larger lattice mismatch. The 3 3 EDM observed for a small fraction of the Gd2 O3 domains (Fig. 1(b)) could be due to the formation of some monoclinic Gd2 O3 at high temperatures, which convertspback toffiffifficubic at lower temperature. ffiffiffi p A ð2 2 2 2ÞR45° EDM is observed for most RE2 O3 layers grown on cube textured Ni(0 0 1) substrates [8,9]. The lattice mismatch offfiffiffi RE2 pffiffiffi p O3 (0 0 1) on Ni(0 0 1) in the ð2 2 2 2ÞR45° EDM (Table 2), exceeds in most cases that in the 3 3 EDM. Exceptions are In2 O3 and Sc2 O3 , which, to our knowledge, have not been used as buffer layers for YBCO coated conductors. Again, pffiffiffi p ffiffiffi the prevalence of the ð2 2 2 2ÞR45° EDM for most RE2 O3 /Ni systems is attributed to the high (100%) CSD of RE3þ cations with Ni(0 0 1) surface atoms. This result suggests that the decrease in the interfacial energy caused by the high CSD dominates, while the increase in the strain energy caused by the concomitant larger lattice mismatch is less important. The mosaic distributions of Y2 O3 (0 0 2) and Gd2 O3 (0 0 2) grains, as illustrated in Fig. 3(a) and (b), exhibit a 6° full-width at half-maxima (FWHM), comparable to other RE2 O3 layers on Ni (6–9° FWHM [8,9]). The Ni(0 0 2) grains produce a non-Gaussian distribution with a total spread of about 12°, as shown in Fig. 3(c). The
57
Table 2 Lattice mismatches (D) pffiffiffi p ffiffiffi of RE2 O3 (0 0 1) with Ni(0 0 1) for (3 3) and ð2 2 2 2ÞR45° EDMs ) RE2 O3 a (A D (%) for Dp (%) ffiffiffi forpffiffiffi (3 3) EDM ð2 2 2 2ÞR45° EDM La2 O3 Pr2 O3 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Tb2 O3 Dy2 O3 Ho2 O3 Er2 O3 Tm2 O3 Yb2 O3 Y2 O3 Lu2 O3 In2 O3 Cm2 O3 Pu2 O3 Sc2 O3 Tl2 O3
11.380 11.136 11.048 10.932 10.866 10.813 10.728 10.667 10.607 10.547 10.488 10.439 10.604 10.391 10.118 11.000 11.040 9.845 10.543
7.765 5.456 4.621 3.523 2.898 2.396 1.591 1.013 0.445 )0.123 )0.682 )1.146 0.417 )1.600 )4.186 4.167 4.545 )6.771 )0.161
14.303 11.852 10.968 9.803 9.140 8.608 7.754 7.141 6.539 5.936 5.343 4.851 6.509 4.369 1.627 10.486 10.888 )1.115 5.896
observation of this irregular distribution pattern is facilitated by using a high brilliance/high-resolution X-ray source/detector combination. The inset in Fig. 3(c) shows the typical rocking curve of sharp peaks on top of a Gaussian distribution measured on the same sample using a 12 kW rotating anode X-ray source with a graphite monochromator. The Y2 O3 and Gd2 O3 layers, like most other RE2 O3 layers [8,9], do not inherit the Ni pattern but instead produce a smooth distribution, which is most probably a consequence of the large difference in grain size (see Table 1). The small RE2 O3 domains that grow on the much larger Ni grains are likely to be somewhat tilted or twisted due to the strain created by the lattice mismatch, thus producing a smoother distribution. However, the overall orientation of RE2 O3 (0 0 2) domains approximates the spread in orientation of the Ni grains. Fig. 4 contains a representation showing why the mosaic distributions for RE2 O3 (0 0 2) domains are so broad, i.e., comparable to that for the span of the spiky Ni(0 0 2) distribution. The broad mosaic distributions for the RE2 O3 (0 0 2) domains in the out-of-plane direction (6–9° FWHM)
58
J.H. Je et al. / Physica C 384 (2003) 54–60
Fig. 3. Mosaic distributions for (a) Y2 O3 (0 0 4), (b) Gd2 O3 (0 0 4), and (c) Ni(0 0 2) grains.
suggest that RE2 O3 grains prefer growing along the Ni(0 0 2) planes rather than along the Ni surface, as shown in Fig. 4. The solid circles represent the Ni atoms in the (0 0 1) stacking, while the open (hatched) circles represent the metal (oxygen) ions in the RE2 O3 layers. The large neighboring Ni(0 0 2) grains that are typically 10–50 lm in size can be misoriented from one another by up to 5° [15]. The localized roughness of the Ni surface, which is controlled mainly by the rolling and heat treatment conditions used to texture the Ni substrate, was measured to be approximately 0.5 nm by AFM, as shown in Table 1. While this indicates the presence of a relatively smooth surface on a microscopic scale, the misorientation distribution of the Ni(0 0 2) grains can be significant. For instance, we note that adjacent Ni grains of 50 lm dimension with 2° misorientation (as shown in Fig. 4) can induce a large sunken depression ( 900 nm) at the side of each Ni grain, assuming the absence of any steps on the Ni(0 0 1) planes. However, the relatively smooth Ni surface ( 0.5 nm) indicates the existence of many steps or terraces on the Ni(0 0 1) planes. The growth of small RE2 O3 grains on the Ni(0 0 1) terraces, somewhat tilted or twisted due to the strain coming from the lattice
Fig. 4. Representation of two neighboring Ni(0 0 2) grains with 2° misorientation that have a small-grained RE2 O3 layer on top of the Ni(0 0 2) planes. The solid circles represent the Ni atoms in the (1 1 1) stacking array, while the open (hatched) circles represent the metal (oxygen) ions in the RE2 O3 layer.
J.H. Je et al. / Physica C 384 (2003) 54–60
59
mismatch, is then likely to produce a smooth mosaic distribution. Assuming that RE2 O3 grains grow on Ni(0 0 1) planes rather than on the Ni surface, these grains should unavoidably follow the large angle spread of the Ni(0 0 1) grains in macroscale due to the large misorientation distribution of the latter. If the RE2 O3 grains grew on the Ni surface as reported previously for the Pd/ Ni system [16], the mosaic distribution in the out of plane direction should have improved significantly. We suppose from this that the aligning of RE2 O3 (0 0 2) grains on the Ni(0 0 2) lattice planes pffiffiffi rather pffiffiffi than on the Ni surface favors the ð2 2 2 2ÞR45° EDM.
face, an effect that is quite possibly induced by the EDM.
4. Conclusions
References
The structure and alignment of epitaxial Y2 O3 and Gd2 O3 films on cube textured Ni(0 0 1) substrates were investigated by X-ray scattering techniques employed at a third generation synchrotron X-ray source and the results were used to develop a general interpretation of the morphology of RE2 O3 films on cube textured nickel and nickel-based alloys. The [1 0 0] axis of RE2 O3 layers mostly prefers aligning to Ni[1 1 0], while the [0 0 1] axis is aligned 0 1]. This can be pffiffiffi withpffiffiNi[0 ffi explained by the ð2 2 2 2ÞR45° EDM, wherein two-metal atomic distances in the RE2 O3 [1 0 0] direction match to four-Ni atomic distances in the pffiffiffi p ffiffiffi Ni[1 1 0] direction. Such ð2 2 2 2ÞR45° EDM seems to be favored by satisfying the conditions of cube-on-cube and high (100%) CSD with the Ni(0 0 1) surface atoms, in spite of larger lattice mismatches to Ni(0 0 1). Meanwhile the 3 3 EDM is not favored on Ni(0 0 1)––in spite of smaller mismatches to Ni(0 0 1)––because of the existence of a low (25%) CSD. This result suggests that the decrease in the interfacial energy created by the high CSD of RE2 O3 to Ni(0 0 1) is more influential in epitaxy development than the increase in the strain energypcaused ffiffiffi pffiffiby ffi the larger lattice mismatch in the ð2 2 2 2ÞR45° EDM. The broad mosaic distributions for RE2 O3 oxides in the out-of-plane direction (6–9° FWHM) indicate that the RE2 O3 oxides prefer growing along the Ni(0 0 1) planes rather than along the Ni sur-
Acknowledgements Work performed at the Argonne National Laboratory was sponsored by the US Department of Energy, Office of Basic Energy Sciences and Office of Energy Efficiency and Renewable Energy, as part of a DOE program to develop electric power technology, under Contract W-31-109-ENG-38. One of the authors (J.H. Je) acknowledges the support of BK21 Korea, NRL(KISTEP), and the LG Yonam Foundation (2001).
[1] S.R. Foltyn, Q.X. Jia, P.N. Arendt, L. Kinder, Y. Fan, J.F. Smith, Appl. Phys. Lett. 75 (1999) 3692. [2] 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, Science 274 (1996) 755. [3] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hartfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [4] C.L.H. Thieme, E. Thompson, S. Annavarapu, Q. Li, W. Zhang, M.W. Rupich, M. Paranthaman, A. Goyal, D.F. Lee, E.D. Specht, F.A. List, Progress in TFA solutionprocessed YBCO coated conductors, 2001 ISTEC/MRS International Workshop on Superconductivity, Hawaii, 24–27 June 2001. [5] C.L.H. Thieme, S. Annavarapu, W. Zhang, V. Prunier, L. Fritzemeier, Q. Li, U. Schoop, M.W. Rupich, M. Gopal, S. Foltyn, T. Holesinger, Non-magnetic substrates for low cost YBCO coated conductors, IEEE Trans. Appl. Supercond. 11 (2001) 3329. [6] S. Annavarapu, L. Fritzemeier, Q. Li, A. Malozemoff, V. Prunier, M.W. Rupich, C. Thieme, W. Zhang, M. Gopal, I. Seleznev, M.J. Cima, M. Paranthaman, A. Goyal, D.F. Lee, Physica C 341–348 (2000) 2319. [7] M. Paranthaman, A. Goyal, F.A. List, E.D. Specht, D.F. Lee, P.M. Martin, Q. He, D.K. Christen, D.P. Norton, J.D. Budai, D.M. Kroeger, Physica C 275 (1997) 266. [8] Q. He, D.K. Christen, J.D. Budai, E.D. Specht, D.F. Lee, A. Goyal, D.P. Norton, M. Paranthaman, F.A. List, D.M. Kroeger, Physica C 275 (1997) 155. [9] M. Paranthaman, T.G. Chirayil, S. Sathyamurthy, D.B. Beach, A. Goyal, F.A. List, D.F. Lee, X. Cui, S.W. Lu, B. Kang, E.D. Specht, P.M. Martin, D.M. Kroeger, R. Feenstra, C. Cantoni, D.K. Christen, IEEE Trans. Appl. Supercond. 11 (2001) 3146.
60
J.H. Je et al. / Physica C 384 (2003) 54–60
[10] Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Appl. Phys. Lett. 60 (1992) 769. [11] K. Masumoto, S.B. Kim, K. Yamagiwa, Y. Koike, I. Hirabashi, T. Watanabe, N. Uno, M. Ikeda, Physica C 335 (2000) 39. [12] C. Cantoni, D.K. Christen, L. Heatherly, M.M. Kowalewski, F.A. List, A. Goyal, G.W. Ownby, D.M. Zehner, B.W. Kang, D.M. Kroeger, J. Mater. Res. 17 (10) (2002).
[13] M. Paranthaman, D.F. Lee, A. Goyal, E.D. Specht, P.M. Martin, X. Cu, J.E. Mathis, R. Feenstra, D.K. Christen, D.M. Kroeger, Supercond. Sci. Technol. 12 (1999) 319. [14] A. Zur, T.C. McGill, J. Appl. Phys. 55 (1984) 378. [15] G. Friedel, Lecons de Cristallographie, Berger Levrault, Paris, 1926. [16] J.H. Je, H. You, W.G. Cullen, V.A. Maroni, B. Ma, R.E. Koritala, C. Thieme, Physica C 383 (2002) 241.
Microstructure of RE2O3 layers on cube textured Ni substrates J.H. Je a,b, H. You a,*, W.G. Cullen a, V.A. Maroni a, B. Ma a, R.E. Koritala a, M.W. Rupich c, C.L.H. Thieme c b
a Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Department of Materials Science and Engineering, POSTECH, Pohang 790-784, South Korea c American Superconductor, Westborough, MA 01581, USA
Received 24 June 2002; accepted 26 July 2002
Abstract We investigated the structure and alignment of epitaxial Y2 O3 and Gd2 O3 films on cube textured Ni(0 0 1) substrates and used the findings to develop a general interpretation of the morphology of RE2 O3 films (RE ¼ Y or a rare earth element) on cube textured nickel and nickel-based alloys. The [1 0 0] axis of p RE oxides mostly prefers aligning to the ffiffiffi 2 O3 p ffiffiffi Ni[1 1 0] axis, while the [0 0 1] axis is aligned with the Ni[0 0 1] axis. This ð2 2 2 2ÞR45° extended domain matching (EDM) seems to be favored by a high coincidence site density (CSD) with the Ni(0 0 1) surface atoms, in spite of the existence of large lattice mismatches to Ni(0 0 1). Meanwhile a 3 3 EDM is not favored on Ni(0 0 1), in spite of the relatively small lattice mismatch to Ni(0 0 1), because of the existence of a low CSD. The broad mosaic distributions of RE2 O3 (0 0 l) grains (6–9° full-width at half-maximum) indicate that RE2 O3 layers prefer growing along the Ni(0 0 1) planes rather than along the Ni surface, a condition presumably induced by the favorable energetics for EDM. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 58.55.)a; 74.76.Bz; 68.35.Bs; 68.55.J Keywords: RE2 O3 /Ni; Coincidence site density; Extended domain matching; Cube textured Ni substrates
1. Introduction During the fabrication of coated-conductortype superconducting tapes, the growth of a Y1 Ba2 Cu3 O7 (YBCO) layer with a high degree of biaxial texture (required for a high critical-current density) is typically achieved through the use of a nickel or Ni-base alloy substrate and the deposition of a buffer layer with a sharp, biaxial texture [1,2]. In one approach, a Ni or Ni alloy foil is made *
Corresponding author. Tel.: +1-630-252-3429; fax: +1-630252-7777. E-mail address: [email protected] (H. You).
with a sharp (0 0 1) [1 0 0] cube texture [2]. Buffer layers are deposited onto this substrate in an epitaxial manner. The buffer layer prevents diffusion of Ni into the YBCO layer, reduces interfacial strain due to lattice mismatch, and in addition, transmits the cube texture of the substrate to the YBCO layer. Various types of buffer layers have been tested [2–12]. RE2 O3 (RE ¼ Y or a rare earth element) films have the potential to simplify the conductor manufacturing process because the requirements for the buffer layer can be met with a single layer [8,9]. We present the results of an investigation of the structure and alignment of epitaxial Y2 O3 and
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 9 8 2 - 2
J.H. Je et al. / Physica C 384 (2003) 54–60
Gd2 O3 films on cube textured Ni(0 0 1) substrates using synchrotron X-ray scattering techniques. The findings of this study have allowed us to develop a general interpretation of the morphology of RE2 O3 films on cube textured nickel and nickelcontaining alloys. Particular p emphasis ffiffiffi pffiffiffi is placed on the extent to which the ð2 2 2 2ÞR45° extended domain matching (EDM) observed for most RE2 O3 buffer layers on Ni(0 0 1) is due to high coincidence site density (CSD) with the Ni(0 0 1) surface atoms.
55
conventional h–2h scattering profiles in the specular direction (along the surface normal, i.e., [0 0 1] direction), (2) additional Bragg reflections in offspecular directions, and (3) the rocking curves of the Bragg reflections. Measurements of surface roughnesses of the as-annealed nickel substrate and the coated substrates were made using a Digital Instruments Dimension 3100 scanning probe microscope, operated in the tapping mode. A scanning electron microscope (Hitachi S-4700-II ) was used to measure the average grain size. Results of these measurements are reported in Table 1.
2. Experimental section A (0 0 1) [1 0 0] cube textured nickel ribbon (99.98þ%) was produced by cold-rolling to over 95% deformation, followed by recrystallization at temperatures ranging from 900 to 1100 °C [13]. The thickness of the textured Ni substrate was 50 lm. The texture-annealed Ni substrates were heated resistively in an e-beam system to 550 °C to remove surface oxides. Y2 O3 films (20 nm thick) were deposited by electron beam evaporation [9], while Gd2 O3 films (20 nm thick) were deposited from a solution containing Gd-alkoxide by metal organic deposition (MOD) carried out at 1000– 1150 °C and ambient pressure under reducing conditions. The X-ray scattering measurements were performed using the BESSRC beamline (12BM) at the Advanced Photon Source. The incident X-rays were focused vertically and horizontally by a toroidal mirror and monochromatized to a wave using a double-bounce length of 1.03314 A Si(2 2 0) monochromator. Two sets of slits in front of the detector provided sufficient detector resolution to achieve Dð2hÞ=2h ¼ 1 104 . The experiments were carried out by measuring (1) the
3. Results and discussion The conventional h–2h scan patterns of the Y2 O3 /Ni and Gd2 O3 /Ni structures consist only of the Y2 O3 (0 0 l) and Gd2 O3 (0 0 l) reflections, respectively, together with substrate reflections, indicating that Y2 O3 and Gd2 O3 films grow with an (0 0 l) preferred orientation. Fig. 1(a) shows a /-scan of the off-specular Y2 O3 (2 2 2) reflection (54.7° off from the specular (0 0 1) direction) obtained for the 20 nm thick Y2 O3 film on Ni(0 0 1). The four well-defined Y2 O3 (2 2 2) reflections at 90° intervals indicate that the Y2 O3 layer grows epitaxially on Ni(0 0 1). Comparison with the /-scan of Ni(1 1 1), Fig. 1(c), shows that the Y2 O3 (2 2 2) reflections are rotated 45°. This indicates that the [1 0 0] axis of the Y2 O3 domains is aligned to the Ni[1 1 0] axis in the in-plane direction. The epitaxial relationship can be summarized as Y2 O3 (0 0 1)kNi(0 0 1) and Y2 O3 [1 0 0]kNi[1 1 0]. The /-scan of the Gd2 O3 (2 2 2) reflection from the 20 nm thick Gd2 O3 film on Ni(0 0 1), Fig. 1(b), also shows four well-defined Gd2 O3 (2 2 2) reflections at 90° intervals, rotated 45° from the
Table 1 Roughness parameters and grain sizes for the cube textured Ni substrate and the RE2 O3 layers Constituent
Deposition method
Nickel Y2 O3 /Ni Gd2 O3 /Ni
– E-beam evaporation Chemical (MOD)
Roughness (nm)
Grain size
RMS
Average
0.56 0.9 10.4
0.46 0.7 8.3
10–50 lm
20 nm
20 nm
56
J.H. Je et al. / Physica C 384 (2003) 54–60 Y2O3 (Gd2O3)[110]
(a)
Ni[100]
Y2O3 (Gd2O3)[100]
Ni[100]
(b)
pffiffiffi pffiffiffi Fig. 2. Pictorial representation of (a) the ð2 2 2 2ÞR45° EDM of RE2 O3 [1 0 0] on Ni[1 1 0] and (b) the (3 3) EDM of RE2 O3 [1 0 0] on Ni[1 0 0]. The large open circles represent Y(Ga) metal ions, while the small solid circles represent Ni atoms. The unit cells of Y2 O3 (Gd2 O3 ) are illustrated by the large squares drawn with thick lines.
Fig. 1. The /-scans for the (a) Y2 O3 (2 2 2), (b) Gd2 O3 (2 2 2), and (c) the Ni(1 1 1) off-specular reflections.
Ni(1 1 1) reflections. The majority of the Gd2 O3 layer grows epitaxially on Ni(0 0 1) with the same 45° rotation as for the Y2 O3 /Ni film. However, we also observe minor domains (at a level typically obscured in measurements with laboratory X-ray sources) where a small fraction (a few percent) of the Gd2 O3 layer has a cube-on-cube orientation without the 45° rotation. It is well known that most RE2 O3 buffer layers (Y2 O3 , Gd2 O3 , Yb2 O3 , Eu2 O3 , etc.) grow epitaxially on Ni(0 0 1) surfaces with the same type of inplane epitaxial relationship as that of the Y2 O3 /Ni film [8,9]. This epitaxial relationship pffiffiffi pffiffiffi can be understood in terms of a ð2 2 2 2ÞR45° EDM. An EDM that uses multiple unit cells for lattice matching can explain the epitaxial growth of grains with a large lattice mismatch [4]. The crystal structure of Y2 O3 (Gd2 O3 ) is analogous to the flu-
orite (CaF2 ) structure, with one-fourth of the oxygen atoms removed in order to maintain charge neutrality between Y3þ (Gd3þ ) and O2 [9]. The lattice parameter for Y2 O3 (Gd2 O3 ), 10.604 A ), is about three times that of Ni (3.520 (10.813 A pffiffiffi ). Fig. 2(a) presents a schematic of a ð2 2 A pffiffiffi 2 2ÞR45° EDM on a Ni(0 0 1) surface. The large open circles represent Y3þ (Gd3þ ) ions, while the small solid circles represent Ni atoms. For the unit cell of Y2 O3 (Gd2 O3 ), illustrated by the large squares drawn with thick lines, the Y2 O3 (Gd2 O3 ) [1 0 0] axis is aligned to the Ni[1 1 0] axis in the inplane direction. Note also for this alignment that two Y(Gd) atomic distances in the Y2 O3 (Gd2 O3 ) [1 0 0] direction match to four Ni atomic distances in pthe ffiffiffi Ni[1 pffiffiffi 1 0] direction. This depiction of a ð2 2 2 2ÞR45° EDM is consistent with the observation that the off-specular Y2 O3 (2 2 2) and Gd2 O3 (2 2 2) Bragg reflections (Fig. 1(a) and (b), respectively) are rotated 45° from the off-specular Ni(1 1 1) reflections in the in-plane direction (Fig. 1(c)). In the 3 3 EDM, on the other hand, two Y(Ga) atomic distances in the Y2 O3 (Gd2 O3 ) [1 0 0] direction match to three Ni atomic distances in the Ni[1 0 0] direction, as is illustrated in Fig. 2(b). The lattice mismatch between the Y2 O3 or Gd2 O3 unit cell and the Ni super unit cell in the 3 3 EDM (0.4% and 2.4%, respectively) ispffiffiffisubstantially pffiffiffi smaller than that in the ð2 2 2 2ÞR45° EDM
J.H. Je et al. / Physica C 384 (2003) 54–60
(6.5% and 8.6%, respectively). The degree of CSD matching [14] serves as an explanation for why the 3 3 EDM is not preferred for either Y2 O3 /Ni or Gd2 O3 /Ni, in spite of their smaller lattice mismatch. For an array of 8 Y3þ (or Gd3þ ) metal ions lying in a two-dimensional 3 3 superlattice on Ni(0 0 1) planes, only one-fourth are coincident with underlying Ni atoms, while three-fourths are not coincident with underlying Ni atoms as shown in Fig. 2(b). This higher degree of non-coincidence naturally raises the interfacial energy. It is therefore less likely that a Y2 O3 or Gd2 O3 layer would adopt a 3 p 3 ffiffiffiEDM pffiffiffion Ni(0 0 1). On the other hand, the ð2 2 2 2ÞR45° EDM has 100% coincidence with Ni(0 0 1) in terms of the Y3þ (or Gd3þ ) metal ions in the Y2 O3 (or Gd2 O3 ) layer (Fig. 2(a)). This very high site coincidence drastically the interfacial energy, and the pffiffiffi reduces pffiffiffi ð2 2 2 2ÞR45° EDM is the preferred orientation for the Y2 O3 or Gd2 O3 on Ni(0 0 1) despite the larger lattice mismatch. The 3 3 EDM observed for a small fraction of the Gd2 O3 domains (Fig. 1(b)) could be due to the formation of some monoclinic Gd2 O3 at high temperatures, which convertspback toffiffifficubic at lower temperature. ffiffiffi p A ð2 2 2 2ÞR45° EDM is observed for most RE2 O3 layers grown on cube textured Ni(0 0 1) substrates [8,9]. The lattice mismatch offfiffiffi RE2 pffiffiffi p O3 (0 0 1) on Ni(0 0 1) in the ð2 2 2 2ÞR45° EDM (Table 2), exceeds in most cases that in the 3 3 EDM. Exceptions are In2 O3 and Sc2 O3 , which, to our knowledge, have not been used as buffer layers for YBCO coated conductors. Again, pffiffiffi p ffiffiffi the prevalence of the ð2 2 2 2ÞR45° EDM for most RE2 O3 /Ni systems is attributed to the high (100%) CSD of RE3þ cations with Ni(0 0 1) surface atoms. This result suggests that the decrease in the interfacial energy caused by the high CSD dominates, while the increase in the strain energy caused by the concomitant larger lattice mismatch is less important. The mosaic distributions of Y2 O3 (0 0 2) and Gd2 O3 (0 0 2) grains, as illustrated in Fig. 3(a) and (b), exhibit a 6° full-width at half-maxima (FWHM), comparable to other RE2 O3 layers on Ni (6–9° FWHM [8,9]). The Ni(0 0 2) grains produce a non-Gaussian distribution with a total spread of about 12°, as shown in Fig. 3(c). The
57
Table 2 Lattice mismatches (D) pffiffiffi p ffiffiffi of RE2 O3 (0 0 1) with Ni(0 0 1) for (3 3) and ð2 2 2 2ÞR45° EDMs ) RE2 O3 a (A D (%) for Dp (%) ffiffiffi forpffiffiffi (3 3) EDM ð2 2 2 2ÞR45° EDM La2 O3 Pr2 O3 Nd2 O3 Sm2 O3 Eu2 O3 Gd2 O3 Tb2 O3 Dy2 O3 Ho2 O3 Er2 O3 Tm2 O3 Yb2 O3 Y2 O3 Lu2 O3 In2 O3 Cm2 O3 Pu2 O3 Sc2 O3 Tl2 O3
11.380 11.136 11.048 10.932 10.866 10.813 10.728 10.667 10.607 10.547 10.488 10.439 10.604 10.391 10.118 11.000 11.040 9.845 10.543
7.765 5.456 4.621 3.523 2.898 2.396 1.591 1.013 0.445 )0.123 )0.682 )1.146 0.417 )1.600 )4.186 4.167 4.545 )6.771 )0.161
14.303 11.852 10.968 9.803 9.140 8.608 7.754 7.141 6.539 5.936 5.343 4.851 6.509 4.369 1.627 10.486 10.888 )1.115 5.896
observation of this irregular distribution pattern is facilitated by using a high brilliance/high-resolution X-ray source/detector combination. The inset in Fig. 3(c) shows the typical rocking curve of sharp peaks on top of a Gaussian distribution measured on the same sample using a 12 kW rotating anode X-ray source with a graphite monochromator. The Y2 O3 and Gd2 O3 layers, like most other RE2 O3 layers [8,9], do not inherit the Ni pattern but instead produce a smooth distribution, which is most probably a consequence of the large difference in grain size (see Table 1). The small RE2 O3 domains that grow on the much larger Ni grains are likely to be somewhat tilted or twisted due to the strain created by the lattice mismatch, thus producing a smoother distribution. However, the overall orientation of RE2 O3 (0 0 2) domains approximates the spread in orientation of the Ni grains. Fig. 4 contains a representation showing why the mosaic distributions for RE2 O3 (0 0 2) domains are so broad, i.e., comparable to that for the span of the spiky Ni(0 0 2) distribution. The broad mosaic distributions for the RE2 O3 (0 0 2) domains in the out-of-plane direction (6–9° FWHM)
58
J.H. Je et al. / Physica C 384 (2003) 54–60
Fig. 3. Mosaic distributions for (a) Y2 O3 (0 0 4), (b) Gd2 O3 (0 0 4), and (c) Ni(0 0 2) grains.
suggest that RE2 O3 grains prefer growing along the Ni(0 0 2) planes rather than along the Ni surface, as shown in Fig. 4. The solid circles represent the Ni atoms in the (0 0 1) stacking, while the open (hatched) circles represent the metal (oxygen) ions in the RE2 O3 layers. The large neighboring Ni(0 0 2) grains that are typically 10–50 lm in size can be misoriented from one another by up to 5° [15]. The localized roughness of the Ni surface, which is controlled mainly by the rolling and heat treatment conditions used to texture the Ni substrate, was measured to be approximately 0.5 nm by AFM, as shown in Table 1. While this indicates the presence of a relatively smooth surface on a microscopic scale, the misorientation distribution of the Ni(0 0 2) grains can be significant. For instance, we note that adjacent Ni grains of 50 lm dimension with 2° misorientation (as shown in Fig. 4) can induce a large sunken depression ( 900 nm) at the side of each Ni grain, assuming the absence of any steps on the Ni(0 0 1) planes. However, the relatively smooth Ni surface ( 0.5 nm) indicates the existence of many steps or terraces on the Ni(0 0 1) planes. The growth of small RE2 O3 grains on the Ni(0 0 1) terraces, somewhat tilted or twisted due to the strain coming from the lattice
Fig. 4. Representation of two neighboring Ni(0 0 2) grains with 2° misorientation that have a small-grained RE2 O3 layer on top of the Ni(0 0 2) planes. The solid circles represent the Ni atoms in the (1 1 1) stacking array, while the open (hatched) circles represent the metal (oxygen) ions in the RE2 O3 layer.
J.H. Je et al. / Physica C 384 (2003) 54–60
59
mismatch, is then likely to produce a smooth mosaic distribution. Assuming that RE2 O3 grains grow on Ni(0 0 1) planes rather than on the Ni surface, these grains should unavoidably follow the large angle spread of the Ni(0 0 1) grains in macroscale due to the large misorientation distribution of the latter. If the RE2 O3 grains grew on the Ni surface as reported previously for the Pd/ Ni system [16], the mosaic distribution in the out of plane direction should have improved significantly. We suppose from this that the aligning of RE2 O3 (0 0 2) grains on the Ni(0 0 2) lattice planes pffiffiffi rather pffiffiffi than on the Ni surface favors the ð2 2 2 2ÞR45° EDM.
face, an effect that is quite possibly induced by the EDM.
4. Conclusions
References
The structure and alignment of epitaxial Y2 O3 and Gd2 O3 films on cube textured Ni(0 0 1) substrates were investigated by X-ray scattering techniques employed at a third generation synchrotron X-ray source and the results were used to develop a general interpretation of the morphology of RE2 O3 films on cube textured nickel and nickel-based alloys. The [1 0 0] axis of RE2 O3 layers mostly prefers aligning to Ni[1 1 0], while the [0 0 1] axis is aligned 0 1]. This can be pffiffiffi withpffiffiNi[0 ffi explained by the ð2 2 2 2ÞR45° EDM, wherein two-metal atomic distances in the RE2 O3 [1 0 0] direction match to four-Ni atomic distances in the pffiffiffi p ffiffiffi Ni[1 1 0] direction. Such ð2 2 2 2ÞR45° EDM seems to be favored by satisfying the conditions of cube-on-cube and high (100%) CSD with the Ni(0 0 1) surface atoms, in spite of larger lattice mismatches to Ni(0 0 1). Meanwhile the 3 3 EDM is not favored on Ni(0 0 1)––in spite of smaller mismatches to Ni(0 0 1)––because of the existence of a low (25%) CSD. This result suggests that the decrease in the interfacial energy created by the high CSD of RE2 O3 to Ni(0 0 1) is more influential in epitaxy development than the increase in the strain energypcaused ffiffiffi pffiffiby ffi the larger lattice mismatch in the ð2 2 2 2ÞR45° EDM. The broad mosaic distributions for RE2 O3 oxides in the out-of-plane direction (6–9° FWHM) indicate that the RE2 O3 oxides prefer growing along the Ni(0 0 1) planes rather than along the Ni sur-
Acknowledgements Work performed at the Argonne National Laboratory was sponsored by the US Department of Energy, Office of Basic Energy Sciences and Office of Energy Efficiency and Renewable Energy, as part of a DOE program to develop electric power technology, under Contract W-31-109-ENG-38. One of the authors (J.H. Je) acknowledges the support of BK21 Korea, NRL(KISTEP), and the LG Yonam Foundation (2001).
[1] S.R. Foltyn, Q.X. Jia, P.N. Arendt, L. Kinder, Y. Fan, J.F. Smith, Appl. Phys. Lett. 75 (1999) 3692. [2] 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, Science 274 (1996) 755. [3] A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hartfield, V.K. Sikka, Appl. Phys. Lett. 69 (1996) 1795. [4] C.L.H. Thieme, E. Thompson, S. Annavarapu, Q. Li, W. Zhang, M.W. Rupich, M. Paranthaman, A. Goyal, D.F. Lee, E.D. Specht, F.A. List, Progress in TFA solutionprocessed YBCO coated conductors, 2001 ISTEC/MRS International Workshop on Superconductivity, Hawaii, 24–27 June 2001. [5] C.L.H. Thieme, S. Annavarapu, W. Zhang, V. Prunier, L. Fritzemeier, Q. Li, U. Schoop, M.W. Rupich, M. Gopal, S. Foltyn, T. Holesinger, Non-magnetic substrates for low cost YBCO coated conductors, IEEE Trans. Appl. Supercond. 11 (2001) 3329. [6] S. Annavarapu, L. Fritzemeier, Q. Li, A. Malozemoff, V. Prunier, M.W. Rupich, C. Thieme, W. Zhang, M. Gopal, I. Seleznev, M.J. Cima, M. Paranthaman, A. Goyal, D.F. Lee, Physica C 341–348 (2000) 2319. [7] M. Paranthaman, A. Goyal, F.A. List, E.D. Specht, D.F. Lee, P.M. Martin, Q. He, D.K. Christen, D.P. Norton, J.D. Budai, D.M. Kroeger, Physica C 275 (1997) 266. [8] Q. He, D.K. Christen, J.D. Budai, E.D. Specht, D.F. Lee, A. Goyal, D.P. Norton, M. Paranthaman, F.A. List, D.M. Kroeger, Physica C 275 (1997) 155. [9] M. Paranthaman, T.G. Chirayil, S. Sathyamurthy, D.B. Beach, A. Goyal, F.A. List, D.F. Lee, X. Cui, S.W. Lu, B. Kang, E.D. Specht, P.M. Martin, D.M. Kroeger, R. Feenstra, C. Cantoni, D.K. Christen, IEEE Trans. Appl. Supercond. 11 (2001) 3146.
60
J.H. Je et al. / Physica C 384 (2003) 54–60
[10] Y. Iijima, N. Tanabe, O. Kohno, Y. Ikeno, Appl. Phys. Lett. 60 (1992) 769. [11] K. Masumoto, S.B. Kim, K. Yamagiwa, Y. Koike, I. Hirabashi, T. Watanabe, N. Uno, M. Ikeda, Physica C 335 (2000) 39. [12] C. Cantoni, D.K. Christen, L. Heatherly, M.M. Kowalewski, F.A. List, A. Goyal, G.W. Ownby, D.M. Zehner, B.W. Kang, D.M. Kroeger, J. Mater. Res. 17 (10) (2002).
[13] M. Paranthaman, D.F. Lee, A. Goyal, E.D. Specht, P.M. Martin, X. Cu, J.E. Mathis, R. Feenstra, D.K. Christen, D.M. Kroeger, Supercond. Sci. Technol. 12 (1999) 319. [14] A. Zur, T.C. McGill, J. Appl. Phys. 55 (1984) 378. [15] G. Friedel, Lecons de Cristallographie, Berger Levrault, Paris, 1926. [16] J.H. Je, H. You, W.G. Cullen, V.A. Maroni, B. Ma, R.E. Koritala, C. Thieme, Physica C 383 (2002) 241.