- Email: [email protected]
Visible influence of substrate-surface defects on YBa2Cu3O7–x film nucleation and growth on polycrystalline silver
Physica C 355 (2001) 194±202
www.elsevier.nl/locate/physc
Visible in¯uence of substrate-surface defects on YBa2Cu3O7±x ®lm nucleation and growth on polycrystalline silver D.X. Huang *, Y. Yamada, I. Hirabayashi Superconductivity Research Laboratory, ISTEC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan Received 9 November 2000; received in revised form 12 January 2001; accepted 22 January 2001
Abstract YBa2 Cu3 O7±x (YBCO) thin ®lms were grown on the polycrystalline-silver substrates by pulsed laser deposition. The in¯uences of various substrate-surface defects on the ®lm nucleation and growth have been systematically analyzed using transmission electron microscopy (TEM). The obtained results show that the YBCO ®lm usually nucleates and grows on the ¯at area of the substrate-surface with its a±b plane simply parallel to the substrate-surface plane, no matter the substrate-surface plane is a low-index or a high-index crystal plane. A small angle change of the substrate-surface plane will cause the same angle change of the c-axis orientation of the grown YBCO ®lm. On the defective surface with valleys, however, the YBCO ®lm cannot directly grow, instead, there is an intermediate layer with some other oxide phases previously grown on the substrate surface. Concerning the surface steps and hills, the TEM observation indicates that they can be simply covered by the lateral overgrowth of the YBCO crystals surrounding them. From the viewpoints of energy and kinetics, a detailed discussion has been given for the forming mechanism of various ®lm microstructures on the substrate-surface areas with dierent surface defects. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.25.Kx; 74.76.Bz; 68.55.-a Keywords: Surface defect; Superconducting ®lm; Microstructure
1. Introduction Superconducting wire and tape materials are most hopeful to be put into practical use in the near future. However, some technical diculties are still remained for how to grow good superconducting ®lms on metal substrates. Recently,
* Corresponding author. Address: Superconductivity Research Laboratory, ISTEC, Shinonome 1-10-13, Koto-ku, Tokyo 135-0062, Japan. Tel.: +81-3-3536-5711; fax: +81-33536-5717. E-mail address: [email protected] (D.X. Huang).
one of the promising substrates used for fabricating superconducting tapes and wires is polycrystalline silver due to its moderate cost, good ¯exibility [1,2], low-contact resistivity [3], as well as good chemical compatibility with high-Tc superconducting materials [4,5]. Many researchers already made eorts to grow high-quality YBa2 Cu3 O7±x (YBCO) ®lms on single crystal or polycrystalline silver substrates by using various thin-®lm-growth methods [6±14]. Also, using these methods highly c-axis-oriented YBCO ®lms could be obtained, but the Jc is much lower and the homogeneity is much worse than the ®lms grown on oxide single crystals [15].
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 1 4 6 - 0
D.X. Huang et al. / Physica C 355 (2001) 194±202
Many factors in¯uence the quality of the YBCO ®lms grown on Ag substrates [9]. One important factor is the substrate-surface roughness. It is known that a strict treatment to reduce the substrate-surface roughness is usually required before the YBCO ®lm deposition even for the oxide single crystal substrate such as SrTiO3 etc [16]. However, for the soft metal such as Ag, it is much more dicult to reduce the surface roughness and get a smooth surface, especially for polycrystalline silver. Therefore, the surface roughness in¯uence on the YBCO ®lm growth will be much more serious, which might be the main reason for the high inhomegeneity and the low Jc of the YBCO ®lms grown on Ag substrates. Even though the substrate-surface roughness has been considered to be an important in¯uence factor in the whole process of the ®lm nucleation and growth, however, for what kind of in¯uence it has and how large the in¯uence is, we still have not a good understanding. In this study, we aim at these points and systematically investigate the microstructural change with the substrate-surface morphology for YBCO ®lms grown on polycrystalline silver substrates using transmission electron microscopy (TEM). The local microstructure of the YBCO ®lm was found to be very sensitive to the substrate-surface morphology. The detailed discussion from the viewpoints of energy and kinetics may help us to better understand the forming mechanisms of various ®lm microstructures on ¯at surface, surface steps, surface hills, as well as surface valleys. 2. Experimental The growth of 200 nm YBCO thin ®lms on polycrystalline Ag substrates was carried out using single target pulsed-laser ablation (PLD). A Lambda-Physics KrF (248 nm) excimer laser was used and operated at 50 Hz with an energy density of 2±3 J/cm2 . Due to the absorption of Cu element by the silver substrates during the PLD process, the polycrystalline YBCO with some extra Cu was usually used as the target. The detailed composition of Y:Ba:Cu used in our experiment was 1:2:3.3. During deposition, the substrates were
195
heated to temperatures in the range of 680±780°C, and the oxygen partial pressure was held at about 200 mTorr. Cross-sectional TEM specimens which allowed examination of the substrate±®lm interface were prepared by the usual method. Small pieces were cut from the substrate side using a low-speed diamond saw. The ®lm sides of two pieces were then glued together, mechanically polished to about 20 lm and ®nally ion-thinned with a liquid-nitrogen cold stage. The TEM analyses for the substrate± ®lm interface were performed using a JEM 2010 high-resolution TEM operated at 200 keV. The composition analyses were carried out using the energy-dispersive X-ray spectroscopy (EDS) system attached on the TEM.
3. Results and discussion In this study, we deposited YBCO ®lms on polycrystalline silver substrates. The grain size of the used substrate is about 2±5 lm, which is much larger than the sizes of the surface defects, usually several or several tens of nanometers. The discussed surface defects, therefore, are usually on one single silver grain. In other words, the crystallographic orientation of the substrate is exactly the same at the dierent parts of the surface defects. The surface defects located at the grain boundaries of the silver substrate have not been discussed here. 3.1. On the ¯at substrate surface Fig. 1 shows some c-axis-oriented YBCO grains grown on a ¯at surface area on one single silver grain. This ¯at substrate-surface area is usually composed of many small ¯at surface planes with a size of about 100±300 nm. The normal directions of two neighboring small ¯at surface planes have a small angle dierence (usually within 5°). Correspondingly, a small angle dierence for the c-axis orientations of the YBCO ®lm grown on these two neighboring small ¯at surface planes can be also found. At the boundary of two neighboring small ¯at surface planes, a YBCO grain boundary in the
196
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fig. 1. Low-magni®cation image shows that the usual ¯at surface area on silver substrate actually consists of many small ¯at surface planes. The long arrows show the normal directions of the local substrate surface planes, as well as the c-axis of the YBCO grains. The black triangles indicate the positions of the YBCO grain boundaries. The insert is the diraction pattern for the central grain with the substrate. The smaller arrows show two diraction dots from the silver substrate and the larger arrows show the a and c directions of the grown YBCO grain.
®lm side can be usually observed. The insert is a diraction pattern showing the orientation relationship between the silver substrate and the YBCO grains grown on the central part of the ¯at surface area. The crystallographic orientation of the local substrate-surface plane is close to the ( 8 17 9) face of silver crystal, which is normal to the c-axis of the growth YBCO grain.
Fig. 2 is a high-resolution TEM image further showing that the angle between the c-axis of two neighboring YBCO grains is exact the same as that between the normal directions of two neighboring small ¯at surface planes on which the two YBCO grains grow. Microscopically, the c-axis of the deposited YBCO ®lm remains strictly parallel to the normal of substrate-surface plane and changes
Fig. 2. A high-resolution image further shows that, when the normal directions of the two neighboring small ¯at surface planes have a 3.5° angle dierence, the c-orientation of the grown YBCO ®lm will also change with the same angle, 3.5°.
D.X. Huang et al. / Physica C 355 (2001) 194±202
as the normal of substrate-surface plane changes. It should be mentioned that the substrate-surface plane here means not only some low-index plane as the cases of oxide single crystal substrates but any kind of low- or high-index planes. Furthermore, we ®nd that the number of the grain boundaries in the ®lm side is closely related to the number of the boundaries of the small ¯at planes on the substrate surface. Also, the size of the YBCO grain is usually limited by the size of the small ¯at surface plane and ranged approximately from 100 to 300 nm. 3.2. On the defective substrate surface 3.2.1. On the surface with steps Fig. 3 shows the YBCO ®lm growth on the Ag substrate surface with a step. The YBCO ®lm can grow to cover the surface step. Also, the same orientation relations between the YBCO grains and the substrate surface can be observed as in the case for the ®lm grown on the ¯at substrate-surface area. As shown in Fig. 3, for the two step terraces with a 4° angle dierence of the terrace normal directions, the same angle dierence can be also observed for the c-axes of the YBCO grains grown on these two step terraces. The two YBCO grains just grow with its a±b plane parallel to the local ¯at substrate surface and joint in the step area to form a grain boundary. No intermediate
197
phases and speci®c defects could be observed in ®lm side of the surface step area. 3.2.2. On the surface with hills Fig. 4 shows the YBCO ®lm growth on the substrate surface with a single hill surrounding by a large ¯at surface area. It seems that the surface hill does not in¯uence the YBCO ®lm growth so much. The YBCO ®lm can simply cover this surface hill by lateral over growth. The grain boundary starting from the peak of the hill is also formed due to the similar reason: the dierent normal orientations of the two small ¯at surface planes surrounding this surface hill. 3.2.3. On the surface with valleys As shown by Fig. 5, even though there is a single shallow valley (2±3 nm deep and 20 nm wide), it will cause an intermediate layer formed between the YBCO ®lm and the substrate. The local composition analysis by EDS indicates that this intermediate layer is not a superconducting phase and usually consists of some mixed oxide phases of Cu±O and Ba±Cu±O. Fig. 6 shows the situation for the ®lm deposition on a bit larger surface valley (10 nm deep and 30 nm wide), in which we can see the Cu oxide phase in the intermediate layer clearly. Interestingly, when the surface valley is small, e. g. less than 50 nm in width, it usually can be covered by the lateral over growth
Fig. 3. A high-resolution image to show the microstructure features of the YBCO ®lms grown on the surface step and on the grain boundary between the Ag substrate and the intermediate phase.
198
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fig. 4. A TEM image showing the microstructure feature for the YBCO ®lm grown on the surface hill. Two pairs of dashed line show that the YBCO grain grows with its a±b plane simply parallel to the substrate surface plane on the two sides of the surface hill. The c-orientations of the YBCO grains grown on the two sides of the surface hill diers with a small angle which just equals to the angle between the two surface planes surrounding the surface hill.
Fig. 5. A high-resolution image showing the microstructure of the YBCO ®lm on the substrate surface with a shallow valley. An intermediate layer exists in between the YBCO ®lm and the substrate. Some fringes diered from the YBCO crystal exist in the valley, indicating that the crystalline phases in this intermediate layer are not YBCO crystals but some other oxide phases.
of the YBCO grains surrounding it, with no in¯uence on the good YBCO ®lm growth. For the surface valley with a large size, e.g. several hundreds of nanometers in width, a very complicated structure of the intermediate layer are usually formed, which will strongly in¯uence the quality of the ®lm grown on it. For the detailed ®lm microstructure and the related forming mechanism, it will be described elsewhere [17].
4. Mechanism discussion for the substrate-surface morphology related ®lm growth Generally, the ®lm deposition by the PLD can be divided in two stages. One is the ®lm nucleation and the second is the ®lm growth. The ®rst stage consists in the formation of critical nuclei from the supersaturated vapor and the adsorbed phases. The second stage consists in the growth of the
D.X. Huang et al. / Physica C 355 (2001) 194±202
199
Fig. 6. A high-resolution image shows the microstructure feature of the YBCO ®lm on the substrate surface with a relative large valley. The intermediate layer can be seen clearly in the place between YBCO ®lm and the substrate.
supercritical nuclei, further expending in the substrate-surface plane, and ®nally covering the whole substrate surface. Due to the anisotropic surface free energy of YBCO crystal nucleus, its shape is usually a thin disc and its surface usually keeps (0 0 1) face: the face with lowest free energy [18]. In the early stage of the ®lm deposition, then, the 2D nucleation and growth mode is predominated [19±25]. For the later ®lm growth in the second stage, it is mainly controlled by kinetic factors, such as diusion coecients, growth velocities, and so on [26]. 4.1. The preferred nucleation and growth on ¯at surface area The nature of the thin disc shape of the YBCO nucleus decides that YBCO ®lm nucleation will be very sensitive to the substrate-surface morphology. As we know, the total free energy of the nucleus is composed of four parts: bulk Gibbs free energy (Gv ), substrate±nucleus interfacial energy (rs±n ), substrate±vapor surface free energy (rs±v ), and nucleus±vapor surface free energy (rn±v ). For the thin disc nucleus, it has a very large substrate± nucleus interface. Therefore, the rs±n becomes speci®cally important to decide the total free en-
ergy of the nucleus. Except for the intrinsic factor: the chemical bonding energy between the substrate and the nucleus, another main factor to in¯uence rs±n is the substrate-surface geometry and morphology. For a thin disc shape YBCO nucleus on the ¯at substrate-surface area, it always remains its lowerenergy (0 0 1) face contacted with the substrate surface and the rs±n will be much lower. Therefore the YBCO nucleus can easily grows to over the critical size and further grows to be larger and larger. However when the YBCO crystal nucleates on the rough substrate surface, the nucleus has to contact with the substrate surface with some other high-free-energy faces, which will largely increase the rs±n The YBCO nucleus will be unstable and dicult to grow up. This phenomenon has already been con®rmed indirectly by AFM observations [22] and directly con®rmed by HREM observations [27±29]. The TEM observation results by Wen et al. [29] somehow give us the size of the required ¯at surface area for YBCO nucleation and growth is larger than 30 nm and less than 100 nm on MgO substrates. If the ¯at surface area is less than this size, it is dicult to ®nd YBCO ®lms directly grow. For silver substrate, this required size is similar from our TEM observation in this study.
200
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fig. 7. A schematic showing the ®lm forming processes (a) on the ¯at surface, (b) on the defective surfaces with a surface step, (c) a surface hill and (d) a surface valley.
From the above experimental facts, we tend to think that the ¯at substrate surface with a certain large area is the preferential site for the YBCO nucleation and growth. The surface diusion length in the ®lm growth processes will decide the size of this certain large ¯at surface area [28]. Due to the high-surface diusion rate in PLD processes, therefore, the required size of the ¯at surface area for the successful nucleation and growth of YBCO ®lms will be quite large. 4.2. On the usual ¯at surface area As described above, the usual ¯at surface area on silver substrates is composed of many small ¯at surface planes, the normal directions of which are dierent one another with a small angle (<5°). On this kind of surface, since the size of the small ¯at surface plane (100±300 nm) is usually larger than the surface diusion length (<100 nm). Reasonably, the YBCO ®lm nucleation will independently take place on each small ¯at surface, and then the nuclei on dierent surface planes grow up rapidly and merge at the surface junctions where two neighboring small ¯at surface planes connect with each other. Finally we can observe the high-quality
c-oriented YBCO ®lms on dierent small ¯at surface planes and many grain boundaries formed in the merging areas. Fig. 7a gives the schematic showing the forming process of this kind of microstructure feature. 4.3. On the surface with steps and hills Since there are no ¯at areas on the surface steps and hills, the nucleation of YBCO crystals on the rough surface area will remarkably increase the sunbstrate/nucleus interfacial energy. Therefore, these nuclei are very unstable and will be decomposed or vaporized rapidly. The rapid growth of the surrounding YBCO grains will ®nally cover these surface steps and hills without leaving any large defects in the YBCO ®lms. Fig. 7b and c schematically show the formation of the microstructure feature of the YBCO ®lm on the substrate surface with surface steps and surface hills. 4.4. On the surface with valleys For the similar reason, the nucleation for the YBCO crystal on the rough substrate valley will introduce much higher-interfacial energy and the
D.X. Huang et al. / Physica C 355 (2001) 194±202
YBCO nucleus will be unstable and dicult to grow up. Meanwhile, the YBCO grains nucleated on the surrounding ¯at surface areas cannot grow downward to ®ll the surface valley and further to cover the surface valleys. In these cases, the other oxide phases such as Ba±Cu±O, Cu±O or their mixture can nucleate in the surface valleys since their crystal have not so strong anisotropy of the free energy as YBCO. Their substrate/nucleus interfacial energy will not be in¯uenced by the surface roughness so much. In other words, the rough surface will not cause the obvious increase of the free energy for the nucleation of other oxide phases. Therefore, on the rough bottom plane of the surface valley, it is possible that the free energy for the nucleation of other oxide phases will be lower than the free energy for the nucleation of YBCO crystal. As a result, instead of YBCO crystal, Ba±Cu±O and/or Cu±O can preferentially nucleate and grow in the surface valleys. Due to the speci®c environment in PLD processes, the congruent growth rate of YBCO ®lms is much faster than that of other oxide phases. After the other oxide phases grow, the surface valleys are ®lled and leveled up, then, these other oxide phases are ®nally covered by the lateral over growth of the surrounding YBCO grains. Fig. 7d gives a schematic description for the ®lm microstructure formation on the substrate surface with valleys. However, when the size of the surface valley is much larger than the surface diusion length in the PLD process, only the lateral growth mechanism cannot explain the formation of the complex microstructures in this area. In some case, the other phase can be found to nucleate even on the ¯at surface area [30], but the incongruent nucleation and growth will make it dicult to grow up. Therefore, on the ¯at surface area the YBCO nucleation and growth is dominated. 5. Conclusions In this study, we did a systematic investigation for the microstructure features related to the surface defect morphology in the YBCO ®lms deposited by the PLD method on polycrystalline silver substrates. The obtained results indicate that
201
the substrate-surface morphology is a very sensitive factor to in¯uence the ®lm forming process strongly. It was con®rmed that the ¯at surface plane is the preferential site for the YBCO ®lm nucleation. On silver substrates, the ¯at surface area is usually composed of many small ¯at surface planes, the normal directions of which are dierent from one another with a small angle. The small angle dierence of the normal direction between the small ¯at surface planes can cause the c-axis orientation of the grown YBCO ®lm changed with the same angle and further results in the grain boundaries formed at the junctions of these small ¯at surface planes. In the surface valleys, the nucleation of the other oxide phases, such as Cu±O and/or Ba±Cu±O, has a lower free energy, so that the formation of other oxide phases is dominated in these regions. The substrate-surface steps and hills are not the suitable places for the nucleation and growth of any crystals, but they can be simply covered by the rapid growth of YBCO grains surrounding them and do not in¯uence the ®nal quality of the ®nal-formed YBCO ®lms so much. The crystallographic orientation of the substrate-surface plane will strongly in¯uence the orientations of a, b axes of YBCO grains but not in¯uence the c-axis orientation of the ®lm, which will be reported elsewhere [31]. The large surface valleys ( several hundreds of nanometers in size) are the fatal defects to in¯uence the ®lm microstructure strongly. For depositing good YBCO ®lms on polycrystalline silver, we have to eliminate the large-size surface valleys on the substrates, as well as to control the crystallographic orientations of the substrate±crystal grains. Acknowledgements One of the authors (D.X. Huang) would like to thank Dr. Hirayama and all the TEM group members in Japan Fine Ceramics Center (JFCC) for their kind help in the TEM experiments. Also many thanks due to the always support of Prof. Y. Ikuhara in Tokyo University. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of
202
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fundamental Technologies for Superconductivity Applications. References [1] J.P. Singh, D. Shi, D.W. Capone, Appl. Phys. Lett. 53 (1988) 237. [2] S. Jin, G.W. Kammlott, T.H. Tiefel, S.K. Chen, Physica C 198 (1992) 333. [3] M.E. Tidjiani, R. Gronsky, Physica C 191 (1992) 260. [4] C.T. Cheung, E. Ruckenstein, J. Mater. Res. 4 (1989) 1. [5] R. Bohnenkamp-Weiss, R. Schmid-Fetzer, Physica C 220 (1994) 396. [6] J. Zhao, Y.Q. Li, C.S. Chern, P. Norris, B. Kear, B.W. Wessels, Appl. Phys. Lett. 58 (1991) 89. [7] Y. Masuda, K. Matsubara, R. Ogawa, Y. Kawate, Jpn. J. Appl. Phys. 31 (1992) 2709. [8] J.D. Budai, R.T. Young, B.S. Chao, Appl. Phys. Lett. 62 (1993) 1836. [9] L. Chen, T.W. Piazza, B.E. Schmidt, J.E. Kelsey, A.E. Kaloyeros, D.W. Hazelton, M.S. Walker, L. Luo, R.C. Dye, C.J. Maggiore, D.J. Wilkins, D.B. Knorr, J. Appl. Phys. 73 (1993) 7563. [10] F.C. Yuan, Y.Y. Xie, J.P. Chen, G.W. Yang, B.J. Cheng, Supercond. Sci. Technol. 9 (1996) 991. [11] R. Eggenhoner, A. Tuccio, R. Masini, A. Diaspro, S. Leporatti, R. Rolandi, Supercond. Sci. Technol. 10 (1997) 142. [12] N. Otake, H. Matsumoto, T. Asai, K. Kato, Jpn. J. Appl. Phys. 36 (1997) 7162. [13] Y. Niiori, Y. Yamada, I. Hirabayashi, Physica C 296 (1998) 65. [14] D.B. Studebaker, J. Zhang, T.J. Marks, Y.Y. Wang, V.P. Dravid, J.L. Schindler, C.R. Kannewurf, Appl. Phys. Lett. 72 (1998) 1253.
[15] E. Narumi, L.W. Song, F. Yang, S. Patel, Y.H. Kao, D.T. Shaw, Appl. Phys. Lett. 58 (1991) 1202. [16] J.P. Contour, D. Ravelosona, C. Sant, C. Fretigny, C. Dolin, J. Rioux, P. Auvray, J. Caulet, J. Cryst. Growth 141 (1994) 141. [17] D. Huang, Y. Yamada, I. Hirabayashi, in preparation. [18] B. Dam, J.H. Rector, J.M. Huijbregtse, R. Griessen, Physica C 296 (1998) 179. [19] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki, H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [20] D.P. Norton, D.H. Lowndes, X.Y. Zheng, S. Zhu, R.J. Warmack, Phys. Rev. B 44 (1991) 9760. [21] M.A. Harmer, C.R. Fincher, B.A. Parkinson, J. Appl. Phys. 70 (1996) 2760. [22] B. Dam, N.J. Koeman, J.H. Rector, B. Stauble-P umpim, U. Poppe, R. Griessen, Physica C 261 (1996) 1. [23] C. Marechal, E. Lacaze, W. Seiler, J. Perriere, Physica C 294 (1998) 23. [24] D. Chambonnet, D.K. Belouet, Physica C 302 (1998) 198. [25] F.M. Granozio, U. Scotti di Uccio, J. Alloys Comp. 251 (1/2) (1997) 56. [26] V. Agrawal, N. Chandrasekhar, V.S. Achutaraman, A.M. Goldman, Physica C 205 (1993) 161. [27] A. Gervais, D. Keller, Physica C 246 (1995) 29. [28] S.J. Pennycook, M.F. Chisholm, D.E. Jesson, R. Feenstra, S. Zhu, X.Y. Zheng, D.J. Lowndes, Physica C 202 (1992) 1. [29] J.G. Wen, S. Tokunaga, Y. Ishimaru, K. Suzuki, Y. Enomoto, N. Koshizuka, in: H. Hayakawa, Y. Enomoto (Eds.), Advances in Superconductivity VIII, Proceedings of the 8th International Symposium on Superconductivity (ISSÕ95), Springer, Tokyo, 1995, p. 1097. [30] X.F. Zhang, H.H. Kung, S.R. Foltyn, Q.X. Jia, E.J. Peterson, D.E. Peterson, J. Mater. Res. 14 (1999) 1204. [31] D. Huang, Y. Yamada, I. Hirabayashi, in preparation.
www.elsevier.nl/locate/physc
Visible in¯uence of substrate-surface defects on YBa2Cu3O7±x ®lm nucleation and growth on polycrystalline silver D.X. Huang *, Y. Yamada, I. Hirabayashi Superconductivity Research Laboratory, ISTEC, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587, Japan Received 9 November 2000; received in revised form 12 January 2001; accepted 22 January 2001
Abstract YBa2 Cu3 O7±x (YBCO) thin ®lms were grown on the polycrystalline-silver substrates by pulsed laser deposition. The in¯uences of various substrate-surface defects on the ®lm nucleation and growth have been systematically analyzed using transmission electron microscopy (TEM). The obtained results show that the YBCO ®lm usually nucleates and grows on the ¯at area of the substrate-surface with its a±b plane simply parallel to the substrate-surface plane, no matter the substrate-surface plane is a low-index or a high-index crystal plane. A small angle change of the substrate-surface plane will cause the same angle change of the c-axis orientation of the grown YBCO ®lm. On the defective surface with valleys, however, the YBCO ®lm cannot directly grow, instead, there is an intermediate layer with some other oxide phases previously grown on the substrate surface. Concerning the surface steps and hills, the TEM observation indicates that they can be simply covered by the lateral overgrowth of the YBCO crystals surrounding them. From the viewpoints of energy and kinetics, a detailed discussion has been given for the forming mechanism of various ®lm microstructures on the substrate-surface areas with dierent surface defects. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 85.25.Kx; 74.76.Bz; 68.55.-a Keywords: Surface defect; Superconducting ®lm; Microstructure
1. Introduction Superconducting wire and tape materials are most hopeful to be put into practical use in the near future. However, some technical diculties are still remained for how to grow good superconducting ®lms on metal substrates. Recently,
* Corresponding author. Address: Superconductivity Research Laboratory, ISTEC, Shinonome 1-10-13, Koto-ku, Tokyo 135-0062, Japan. Tel.: +81-3-3536-5711; fax: +81-33536-5717. E-mail address: [email protected] (D.X. Huang).
one of the promising substrates used for fabricating superconducting tapes and wires is polycrystalline silver due to its moderate cost, good ¯exibility [1,2], low-contact resistivity [3], as well as good chemical compatibility with high-Tc superconducting materials [4,5]. Many researchers already made eorts to grow high-quality YBa2 Cu3 O7±x (YBCO) ®lms on single crystal or polycrystalline silver substrates by using various thin-®lm-growth methods [6±14]. Also, using these methods highly c-axis-oriented YBCO ®lms could be obtained, but the Jc is much lower and the homogeneity is much worse than the ®lms grown on oxide single crystals [15].
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 1 4 6 - 0
D.X. Huang et al. / Physica C 355 (2001) 194±202
Many factors in¯uence the quality of the YBCO ®lms grown on Ag substrates [9]. One important factor is the substrate-surface roughness. It is known that a strict treatment to reduce the substrate-surface roughness is usually required before the YBCO ®lm deposition even for the oxide single crystal substrate such as SrTiO3 etc [16]. However, for the soft metal such as Ag, it is much more dicult to reduce the surface roughness and get a smooth surface, especially for polycrystalline silver. Therefore, the surface roughness in¯uence on the YBCO ®lm growth will be much more serious, which might be the main reason for the high inhomegeneity and the low Jc of the YBCO ®lms grown on Ag substrates. Even though the substrate-surface roughness has been considered to be an important in¯uence factor in the whole process of the ®lm nucleation and growth, however, for what kind of in¯uence it has and how large the in¯uence is, we still have not a good understanding. In this study, we aim at these points and systematically investigate the microstructural change with the substrate-surface morphology for YBCO ®lms grown on polycrystalline silver substrates using transmission electron microscopy (TEM). The local microstructure of the YBCO ®lm was found to be very sensitive to the substrate-surface morphology. The detailed discussion from the viewpoints of energy and kinetics may help us to better understand the forming mechanisms of various ®lm microstructures on ¯at surface, surface steps, surface hills, as well as surface valleys. 2. Experimental The growth of 200 nm YBCO thin ®lms on polycrystalline Ag substrates was carried out using single target pulsed-laser ablation (PLD). A Lambda-Physics KrF (248 nm) excimer laser was used and operated at 50 Hz with an energy density of 2±3 J/cm2 . Due to the absorption of Cu element by the silver substrates during the PLD process, the polycrystalline YBCO with some extra Cu was usually used as the target. The detailed composition of Y:Ba:Cu used in our experiment was 1:2:3.3. During deposition, the substrates were
195
heated to temperatures in the range of 680±780°C, and the oxygen partial pressure was held at about 200 mTorr. Cross-sectional TEM specimens which allowed examination of the substrate±®lm interface were prepared by the usual method. Small pieces were cut from the substrate side using a low-speed diamond saw. The ®lm sides of two pieces were then glued together, mechanically polished to about 20 lm and ®nally ion-thinned with a liquid-nitrogen cold stage. The TEM analyses for the substrate± ®lm interface were performed using a JEM 2010 high-resolution TEM operated at 200 keV. The composition analyses were carried out using the energy-dispersive X-ray spectroscopy (EDS) system attached on the TEM.
3. Results and discussion In this study, we deposited YBCO ®lms on polycrystalline silver substrates. The grain size of the used substrate is about 2±5 lm, which is much larger than the sizes of the surface defects, usually several or several tens of nanometers. The discussed surface defects, therefore, are usually on one single silver grain. In other words, the crystallographic orientation of the substrate is exactly the same at the dierent parts of the surface defects. The surface defects located at the grain boundaries of the silver substrate have not been discussed here. 3.1. On the ¯at substrate surface Fig. 1 shows some c-axis-oriented YBCO grains grown on a ¯at surface area on one single silver grain. This ¯at substrate-surface area is usually composed of many small ¯at surface planes with a size of about 100±300 nm. The normal directions of two neighboring small ¯at surface planes have a small angle dierence (usually within 5°). Correspondingly, a small angle dierence for the c-axis orientations of the YBCO ®lm grown on these two neighboring small ¯at surface planes can be also found. At the boundary of two neighboring small ¯at surface planes, a YBCO grain boundary in the
196
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fig. 1. Low-magni®cation image shows that the usual ¯at surface area on silver substrate actually consists of many small ¯at surface planes. The long arrows show the normal directions of the local substrate surface planes, as well as the c-axis of the YBCO grains. The black triangles indicate the positions of the YBCO grain boundaries. The insert is the diraction pattern for the central grain with the substrate. The smaller arrows show two diraction dots from the silver substrate and the larger arrows show the a and c directions of the grown YBCO grain.
®lm side can be usually observed. The insert is a diraction pattern showing the orientation relationship between the silver substrate and the YBCO grains grown on the central part of the ¯at surface area. The crystallographic orientation of the local substrate-surface plane is close to the ( 8 17 9) face of silver crystal, which is normal to the c-axis of the growth YBCO grain.
Fig. 2 is a high-resolution TEM image further showing that the angle between the c-axis of two neighboring YBCO grains is exact the same as that between the normal directions of two neighboring small ¯at surface planes on which the two YBCO grains grow. Microscopically, the c-axis of the deposited YBCO ®lm remains strictly parallel to the normal of substrate-surface plane and changes
Fig. 2. A high-resolution image further shows that, when the normal directions of the two neighboring small ¯at surface planes have a 3.5° angle dierence, the c-orientation of the grown YBCO ®lm will also change with the same angle, 3.5°.
D.X. Huang et al. / Physica C 355 (2001) 194±202
as the normal of substrate-surface plane changes. It should be mentioned that the substrate-surface plane here means not only some low-index plane as the cases of oxide single crystal substrates but any kind of low- or high-index planes. Furthermore, we ®nd that the number of the grain boundaries in the ®lm side is closely related to the number of the boundaries of the small ¯at planes on the substrate surface. Also, the size of the YBCO grain is usually limited by the size of the small ¯at surface plane and ranged approximately from 100 to 300 nm. 3.2. On the defective substrate surface 3.2.1. On the surface with steps Fig. 3 shows the YBCO ®lm growth on the Ag substrate surface with a step. The YBCO ®lm can grow to cover the surface step. Also, the same orientation relations between the YBCO grains and the substrate surface can be observed as in the case for the ®lm grown on the ¯at substrate-surface area. As shown in Fig. 3, for the two step terraces with a 4° angle dierence of the terrace normal directions, the same angle dierence can be also observed for the c-axes of the YBCO grains grown on these two step terraces. The two YBCO grains just grow with its a±b plane parallel to the local ¯at substrate surface and joint in the step area to form a grain boundary. No intermediate
197
phases and speci®c defects could be observed in ®lm side of the surface step area. 3.2.2. On the surface with hills Fig. 4 shows the YBCO ®lm growth on the substrate surface with a single hill surrounding by a large ¯at surface area. It seems that the surface hill does not in¯uence the YBCO ®lm growth so much. The YBCO ®lm can simply cover this surface hill by lateral over growth. The grain boundary starting from the peak of the hill is also formed due to the similar reason: the dierent normal orientations of the two small ¯at surface planes surrounding this surface hill. 3.2.3. On the surface with valleys As shown by Fig. 5, even though there is a single shallow valley (2±3 nm deep and 20 nm wide), it will cause an intermediate layer formed between the YBCO ®lm and the substrate. The local composition analysis by EDS indicates that this intermediate layer is not a superconducting phase and usually consists of some mixed oxide phases of Cu±O and Ba±Cu±O. Fig. 6 shows the situation for the ®lm deposition on a bit larger surface valley (10 nm deep and 30 nm wide), in which we can see the Cu oxide phase in the intermediate layer clearly. Interestingly, when the surface valley is small, e. g. less than 50 nm in width, it usually can be covered by the lateral over growth
Fig. 3. A high-resolution image to show the microstructure features of the YBCO ®lms grown on the surface step and on the grain boundary between the Ag substrate and the intermediate phase.
198
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fig. 4. A TEM image showing the microstructure feature for the YBCO ®lm grown on the surface hill. Two pairs of dashed line show that the YBCO grain grows with its a±b plane simply parallel to the substrate surface plane on the two sides of the surface hill. The c-orientations of the YBCO grains grown on the two sides of the surface hill diers with a small angle which just equals to the angle between the two surface planes surrounding the surface hill.
Fig. 5. A high-resolution image showing the microstructure of the YBCO ®lm on the substrate surface with a shallow valley. An intermediate layer exists in between the YBCO ®lm and the substrate. Some fringes diered from the YBCO crystal exist in the valley, indicating that the crystalline phases in this intermediate layer are not YBCO crystals but some other oxide phases.
of the YBCO grains surrounding it, with no in¯uence on the good YBCO ®lm growth. For the surface valley with a large size, e.g. several hundreds of nanometers in width, a very complicated structure of the intermediate layer are usually formed, which will strongly in¯uence the quality of the ®lm grown on it. For the detailed ®lm microstructure and the related forming mechanism, it will be described elsewhere [17].
4. Mechanism discussion for the substrate-surface morphology related ®lm growth Generally, the ®lm deposition by the PLD can be divided in two stages. One is the ®lm nucleation and the second is the ®lm growth. The ®rst stage consists in the formation of critical nuclei from the supersaturated vapor and the adsorbed phases. The second stage consists in the growth of the
D.X. Huang et al. / Physica C 355 (2001) 194±202
199
Fig. 6. A high-resolution image shows the microstructure feature of the YBCO ®lm on the substrate surface with a relative large valley. The intermediate layer can be seen clearly in the place between YBCO ®lm and the substrate.
supercritical nuclei, further expending in the substrate-surface plane, and ®nally covering the whole substrate surface. Due to the anisotropic surface free energy of YBCO crystal nucleus, its shape is usually a thin disc and its surface usually keeps (0 0 1) face: the face with lowest free energy [18]. In the early stage of the ®lm deposition, then, the 2D nucleation and growth mode is predominated [19±25]. For the later ®lm growth in the second stage, it is mainly controlled by kinetic factors, such as diusion coecients, growth velocities, and so on [26]. 4.1. The preferred nucleation and growth on ¯at surface area The nature of the thin disc shape of the YBCO nucleus decides that YBCO ®lm nucleation will be very sensitive to the substrate-surface morphology. As we know, the total free energy of the nucleus is composed of four parts: bulk Gibbs free energy (Gv ), substrate±nucleus interfacial energy (rs±n ), substrate±vapor surface free energy (rs±v ), and nucleus±vapor surface free energy (rn±v ). For the thin disc nucleus, it has a very large substrate± nucleus interface. Therefore, the rs±n becomes speci®cally important to decide the total free en-
ergy of the nucleus. Except for the intrinsic factor: the chemical bonding energy between the substrate and the nucleus, another main factor to in¯uence rs±n is the substrate-surface geometry and morphology. For a thin disc shape YBCO nucleus on the ¯at substrate-surface area, it always remains its lowerenergy (0 0 1) face contacted with the substrate surface and the rs±n will be much lower. Therefore the YBCO nucleus can easily grows to over the critical size and further grows to be larger and larger. However when the YBCO crystal nucleates on the rough substrate surface, the nucleus has to contact with the substrate surface with some other high-free-energy faces, which will largely increase the rs±n The YBCO nucleus will be unstable and dicult to grow up. This phenomenon has already been con®rmed indirectly by AFM observations [22] and directly con®rmed by HREM observations [27±29]. The TEM observation results by Wen et al. [29] somehow give us the size of the required ¯at surface area for YBCO nucleation and growth is larger than 30 nm and less than 100 nm on MgO substrates. If the ¯at surface area is less than this size, it is dicult to ®nd YBCO ®lms directly grow. For silver substrate, this required size is similar from our TEM observation in this study.
200
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fig. 7. A schematic showing the ®lm forming processes (a) on the ¯at surface, (b) on the defective surfaces with a surface step, (c) a surface hill and (d) a surface valley.
From the above experimental facts, we tend to think that the ¯at substrate surface with a certain large area is the preferential site for the YBCO nucleation and growth. The surface diusion length in the ®lm growth processes will decide the size of this certain large ¯at surface area [28]. Due to the high-surface diusion rate in PLD processes, therefore, the required size of the ¯at surface area for the successful nucleation and growth of YBCO ®lms will be quite large. 4.2. On the usual ¯at surface area As described above, the usual ¯at surface area on silver substrates is composed of many small ¯at surface planes, the normal directions of which are dierent one another with a small angle (<5°). On this kind of surface, since the size of the small ¯at surface plane (100±300 nm) is usually larger than the surface diusion length (<100 nm). Reasonably, the YBCO ®lm nucleation will independently take place on each small ¯at surface, and then the nuclei on dierent surface planes grow up rapidly and merge at the surface junctions where two neighboring small ¯at surface planes connect with each other. Finally we can observe the high-quality
c-oriented YBCO ®lms on dierent small ¯at surface planes and many grain boundaries formed in the merging areas. Fig. 7a gives the schematic showing the forming process of this kind of microstructure feature. 4.3. On the surface with steps and hills Since there are no ¯at areas on the surface steps and hills, the nucleation of YBCO crystals on the rough surface area will remarkably increase the sunbstrate/nucleus interfacial energy. Therefore, these nuclei are very unstable and will be decomposed or vaporized rapidly. The rapid growth of the surrounding YBCO grains will ®nally cover these surface steps and hills without leaving any large defects in the YBCO ®lms. Fig. 7b and c schematically show the formation of the microstructure feature of the YBCO ®lm on the substrate surface with surface steps and surface hills. 4.4. On the surface with valleys For the similar reason, the nucleation for the YBCO crystal on the rough substrate valley will introduce much higher-interfacial energy and the
D.X. Huang et al. / Physica C 355 (2001) 194±202
YBCO nucleus will be unstable and dicult to grow up. Meanwhile, the YBCO grains nucleated on the surrounding ¯at surface areas cannot grow downward to ®ll the surface valley and further to cover the surface valleys. In these cases, the other oxide phases such as Ba±Cu±O, Cu±O or their mixture can nucleate in the surface valleys since their crystal have not so strong anisotropy of the free energy as YBCO. Their substrate/nucleus interfacial energy will not be in¯uenced by the surface roughness so much. In other words, the rough surface will not cause the obvious increase of the free energy for the nucleation of other oxide phases. Therefore, on the rough bottom plane of the surface valley, it is possible that the free energy for the nucleation of other oxide phases will be lower than the free energy for the nucleation of YBCO crystal. As a result, instead of YBCO crystal, Ba±Cu±O and/or Cu±O can preferentially nucleate and grow in the surface valleys. Due to the speci®c environment in PLD processes, the congruent growth rate of YBCO ®lms is much faster than that of other oxide phases. After the other oxide phases grow, the surface valleys are ®lled and leveled up, then, these other oxide phases are ®nally covered by the lateral over growth of the surrounding YBCO grains. Fig. 7d gives a schematic description for the ®lm microstructure formation on the substrate surface with valleys. However, when the size of the surface valley is much larger than the surface diusion length in the PLD process, only the lateral growth mechanism cannot explain the formation of the complex microstructures in this area. In some case, the other phase can be found to nucleate even on the ¯at surface area [30], but the incongruent nucleation and growth will make it dicult to grow up. Therefore, on the ¯at surface area the YBCO nucleation and growth is dominated. 5. Conclusions In this study, we did a systematic investigation for the microstructure features related to the surface defect morphology in the YBCO ®lms deposited by the PLD method on polycrystalline silver substrates. The obtained results indicate that
201
the substrate-surface morphology is a very sensitive factor to in¯uence the ®lm forming process strongly. It was con®rmed that the ¯at surface plane is the preferential site for the YBCO ®lm nucleation. On silver substrates, the ¯at surface area is usually composed of many small ¯at surface planes, the normal directions of which are dierent from one another with a small angle. The small angle dierence of the normal direction between the small ¯at surface planes can cause the c-axis orientation of the grown YBCO ®lm changed with the same angle and further results in the grain boundaries formed at the junctions of these small ¯at surface planes. In the surface valleys, the nucleation of the other oxide phases, such as Cu±O and/or Ba±Cu±O, has a lower free energy, so that the formation of other oxide phases is dominated in these regions. The substrate-surface steps and hills are not the suitable places for the nucleation and growth of any crystals, but they can be simply covered by the rapid growth of YBCO grains surrounding them and do not in¯uence the ®nal quality of the ®nal-formed YBCO ®lms so much. The crystallographic orientation of the substrate-surface plane will strongly in¯uence the orientations of a, b axes of YBCO grains but not in¯uence the c-axis orientation of the ®lm, which will be reported elsewhere [31]. The large surface valleys ( several hundreds of nanometers in size) are the fatal defects to in¯uence the ®lm microstructure strongly. For depositing good YBCO ®lms on polycrystalline silver, we have to eliminate the large-size surface valleys on the substrates, as well as to control the crystallographic orientations of the substrate±crystal grains. Acknowledgements One of the authors (D.X. Huang) would like to thank Dr. Hirayama and all the TEM group members in Japan Fine Ceramics Center (JFCC) for their kind help in the TEM experiments. Also many thanks due to the always support of Prof. Y. Ikuhara in Tokyo University. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of
202
D.X. Huang et al. / Physica C 355 (2001) 194±202
Fundamental Technologies for Superconductivity Applications. References [1] J.P. Singh, D. Shi, D.W. Capone, Appl. Phys. Lett. 53 (1988) 237. [2] S. Jin, G.W. Kammlott, T.H. Tiefel, S.K. Chen, Physica C 198 (1992) 333. [3] M.E. Tidjiani, R. Gronsky, Physica C 191 (1992) 260. [4] C.T. Cheung, E. Ruckenstein, J. Mater. Res. 4 (1989) 1. [5] R. Bohnenkamp-Weiss, R. Schmid-Fetzer, Physica C 220 (1994) 396. [6] J. Zhao, Y.Q. Li, C.S. Chern, P. Norris, B. Kear, B.W. Wessels, Appl. Phys. Lett. 58 (1991) 89. [7] Y. Masuda, K. Matsubara, R. Ogawa, Y. Kawate, Jpn. J. Appl. Phys. 31 (1992) 2709. [8] J.D. Budai, R.T. Young, B.S. Chao, Appl. Phys. Lett. 62 (1993) 1836. [9] L. Chen, T.W. Piazza, B.E. Schmidt, J.E. Kelsey, A.E. Kaloyeros, D.W. Hazelton, M.S. Walker, L. Luo, R.C. Dye, C.J. Maggiore, D.J. Wilkins, D.B. Knorr, J. Appl. Phys. 73 (1993) 7563. [10] F.C. Yuan, Y.Y. Xie, J.P. Chen, G.W. Yang, B.J. Cheng, Supercond. Sci. Technol. 9 (1996) 991. [11] R. Eggenhoner, A. Tuccio, R. Masini, A. Diaspro, S. Leporatti, R. Rolandi, Supercond. Sci. Technol. 10 (1997) 142. [12] N. Otake, H. Matsumoto, T. Asai, K. Kato, Jpn. J. Appl. Phys. 36 (1997) 7162. [13] Y. Niiori, Y. Yamada, I. Hirabayashi, Physica C 296 (1998) 65. [14] D.B. Studebaker, J. Zhang, T.J. Marks, Y.Y. Wang, V.P. Dravid, J.L. Schindler, C.R. Kannewurf, Appl. Phys. Lett. 72 (1998) 1253.
[15] E. Narumi, L.W. Song, F. Yang, S. Patel, Y.H. Kao, D.T. Shaw, Appl. Phys. Lett. 58 (1991) 1202. [16] J.P. Contour, D. Ravelosona, C. Sant, C. Fretigny, C. Dolin, J. Rioux, P. Auvray, J. Caulet, J. Cryst. Growth 141 (1994) 141. [17] D. Huang, Y. Yamada, I. Hirabayashi, in preparation. [18] B. Dam, J.H. Rector, J.M. Huijbregtse, R. Griessen, Physica C 296 (1998) 179. [19] T. Terashima, Y. Bando, K. Iijima, K. Yamamoto, K. Hirata, K. Hayashi, K. Kamigaki, H. Terauchi, Phys. Rev. Lett. 65 (1990) 2684. [20] D.P. Norton, D.H. Lowndes, X.Y. Zheng, S. Zhu, R.J. Warmack, Phys. Rev. B 44 (1991) 9760. [21] M.A. Harmer, C.R. Fincher, B.A. Parkinson, J. Appl. Phys. 70 (1996) 2760. [22] B. Dam, N.J. Koeman, J.H. Rector, B. Stauble-P umpim, U. Poppe, R. Griessen, Physica C 261 (1996) 1. [23] C. Marechal, E. Lacaze, W. Seiler, J. Perriere, Physica C 294 (1998) 23. [24] D. Chambonnet, D.K. Belouet, Physica C 302 (1998) 198. [25] F.M. Granozio, U. Scotti di Uccio, J. Alloys Comp. 251 (1/2) (1997) 56. [26] V. Agrawal, N. Chandrasekhar, V.S. Achutaraman, A.M. Goldman, Physica C 205 (1993) 161. [27] A. Gervais, D. Keller, Physica C 246 (1995) 29. [28] S.J. Pennycook, M.F. Chisholm, D.E. Jesson, R. Feenstra, S. Zhu, X.Y. Zheng, D.J. Lowndes, Physica C 202 (1992) 1. [29] J.G. Wen, S. Tokunaga, Y. Ishimaru, K. Suzuki, Y. Enomoto, N. Koshizuka, in: H. Hayakawa, Y. Enomoto (Eds.), Advances in Superconductivity VIII, Proceedings of the 8th International Symposium on Superconductivity (ISSÕ95), Springer, Tokyo, 1995, p. 1097. [30] X.F. Zhang, H.H. Kung, S.R. Foltyn, Q.X. Jia, E.J. Peterson, D.E. Peterson, J. Mater. Res. 14 (1999) 1204. [31] D. Huang, Y. Yamada, I. Hirabayashi, in preparation.