Dominant Effect of Oxygen Enhancement on the Crystalline Orientation of YBa2Cu3Ox Film Prepared by Liquid-Phase Epitaxial Growth Yan Q. Cai,† Chen Y. Tang,† Li J. Sun,† Gang Jin,‡ Wei Li,§ Yi J. Lai,§ and Xin Yao*,†
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1469-1471
Department of Physics, State Key Laboratory for Metal Matrix Composites, Shanghai Jiao Tong UniVersity, 800 Dongchuan Road, Shanghai 200240, P. R. China, Department of Physics, Zhoukou Normal UniVersity, 21 Qiyi Road, Zhoukou 466000, P. R. China, and Instrumental Analysis Center, Shanghai Jiao Tong UniVersity, 1954 Huashan Road, Shanghai 200030, P. R. China ReceiVed January 26, 2007; ReVised Manuscript ReceiVed May 23, 2007
ABSTRACT: A pure oxygen atmosphere during liquid-phase epitaxial (LPE) growth of YBa2Cu3Ox (YBCO) significantly affected the crystalline orientation of the grown films. In the previous investigation, only the c-axis-oriented YBCO film can be obtained from the melt with a 3.0:5.0 Ba:Cu ratio under a typical air atmosphere. However, in our present study, it was found that highquality a-axis-oriented films readily grew from the flux with a 3.0:5.0 Ba:Cu ratio when the atmosphere was changed to pure oxygen. Furthermore, the growth “window” of a-axis films was obviously enlarged compared with experimental results in the air atmosphere. On the basis of the “surface migration” theory, we elucidate the origin of a-axis growth promotion by a suppression of the migration ability of the growth species in the melt with increasing content of oxygen. 1. Introduction The extensive studies of a-axis-oriented YBa2Cu3Ox (YBCO) films have led to a number of publications on various aspects, ranging from the deposition of superconducting films for HTS electronic devices to the investigation of anisotropic properties of YBCO.1-5 It is of fundamental importance to control the outof-plane orientation of the YBCO layer; for instance, the coherence length in the ab plane is considerably different with that along the c-axis,6 leading to the application of a-axis films in Josephson junction devices. Because high-quality a-axis YBCO films are indispensable in practical applications, the growth parameter control is of great importance both in the conventional vapor-phase deposition and the liquid-phase epitaxy (LPE) technique. Recently, growth regions with a-axis-oriented YBCO thick films grown by the LPE technique on (110) NdGaO3 (NGO) substrates were found. Similar achievement was also made in other systems, e.g., Y-Ca-Ba-La-Cu-O (YCBLCO) superconductors.7-9 However, the optimal growth “window” for a-axis films is usually narrow, characterized by high growth temperature (Tg) vicinal to the peritectic temperature (Tp) and relatively Cu rich fluxes (Ba:Cu ) 3.0:6.5). So far, nobody was able to obtain high-quality a-axis YBCO LPE films by using 3.0:5.0 Ba:Cu fluxes, the ratio that can be kept constant during the process of crystal growth.9 In our previous study, we pointed out that the growth of a-axis YBCO was enhanced by the rich oxygen content in the melt.11 Additionally, the oxygen partial pressure (Po) dependence of a-axis phase content in the vapordeposited YBCO thin film is also suggestive,12 in which the lower Po gave rise to the c-axis-oriented growth of YBCO film, whereas the higher Po is favorable to high-quality a-axis film. In this regard, it is plausible to broaden the growth “window” of a-axis YBCO merely by altering the oxygen environment. In this letter, we report on an oxygen-enhanced LPE growth * Corresponding author. Tel: 86-21-54745772. Fax: 86-21-54741040. E-mail:
[email protected]. † Department of Physics, Shanghai Jiao Tong University. ‡ Zhoukou Normal University. § Instrumental Analysis Center, Shanghai Jiao Tong University.
method, with which we, for the first time, obtained high-quality a-axis YBCO films from the 3.0:5.0 Ba:Cu fluxes under 1 atm oxygen pressure. Moreover, the region appropriate for a-axis growth was enlarged. 2. Experimental Section Our experiments were performed by a top-seeded solution growth (TSSG) method. The experimental apparatus comprised of a furnace containing the melt and a rod attached to the (110) NGO singlecrystalline substrate. The Y2O3 crucible was used to provide the Y nutrient and to prevent contamination. The 3BaO-5CuO powder as a solvent was filled into the crucible and heated to nearly 1030 °C to melt the precursor. An airtight system with gas circulation was utilized to be able to intentially alter the Po value; this was the growth environment. The Tp value at Po ) 1.00 atm and Ba:Cu)3.0:5.0 fluxes, 1022 ( 5° C,13 enabled us to vary the growth temperature from 1016 to 1001 °C. The substrate rotation rate was 100 rpm, the dipping time was 5 min, and the Po value was kept in the pure oxygen condition. In addition, pure nitrogen ambience was also applied in our LPE experiments to allow a comparison with pure oxygen condition. The crystallinity of the grown films was evaluated by X-ray diffraction (XRD). The microstructure of the as-grown films was observed by a Olympus interference optical microscope. The amplified surface morphology and cross-section of the films were observed by a FEI SIRION 200 scanning electron microscope (SEM).
3. Results and Discussion We succeeded in the increase of supersaturation from ∆T ) 6 K at Tg ) 1016 °C to ∆T ) 21 K at Tg) 1001 °C. By using a 3.0:5.0 Ba:Cu flux composition, we deposited the YBCO films epitaxially on (110) NGO substrates. The left part of Figure 1 shows the XRD data of the YBCO film obtained at 1016 °C, indicating a typical a-axis orientation but also including some weak peaks of impurity phases, identified as Ba2CuO3 and the (005) peak of YBCO. The signal of the impurity phases presumably came from the flux attached onto the film. The (005) peak indicates the presence of a few c-axis oriented grains helping to relax strain from interior grains to ambient media. In the right part of Figure 1, we show that the growth of LPE film was still in a pure a-axis mode, even under the undercooling over 20 K.
10.1021/cg070090k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/26/2007
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Figure 1. XRD patterns of YBCO films on (110) NGO substrates grown by using 3.0:5.0 Ba:Cu melt at (a) Tg ) 1016 °C and (b) Tg ) 1001 °C, under the pure oxygen environment. The asterisk in (a) represents an impurity phase identified to be Ba2CuO3.
Figure 3. SEM patterns of the as-grown a-axis YBCO films: (a) top view, and (b) cross-section.
Figure 2. Optical polarization micrographs of the as-grown a-axis YBCO film. The arrows indicate the crack and the typical spiral island on the a-axis LPE film, respectively.
We note that the data from literature8,14 and our previous experiments11 have not indicated any growth “window” for a-axis YBCO film growth with a 3.0:5.0 Ba:Cu ratio in the flux. Even if the Cu-rich melts (Ba:Cu ) 3.0:6.5 or 3.0:7.0) were applied, the a-axis growth region width was only about 10 K. On the contrary, we readily obtained high-quality a-axis YBCO LPE-films in 3.0:5.0 Ba:Cu fluxes by applying a pure oxygen atmosphere. The effects of enhanced oxygen partial pressure during LPE process manifested itself by promoting the a-axis growth, reflected both in the aspect of melt composition and the increased range of Tg. For comparison, we also attempted to grow a-axis-oriented films in pure nitrogen. The XRD analysis showed a pure c-axis growth for any growth temperature we chose. Figure 2 presents the optical polarization micrograph that reveals the surface morphology of the a-axis film. Evidently, a typical spiral island with concentric ellipse pattern is observed that is ascribed to the a-axis growth mode.15 The ratio of the major and minor radius is roughly 1.25:1. On the flat surface of a-axis film, nearly parallel cracks with a distance of 20-40 µm are visible. The presence of cracks is due to the high tensile stress caused by different thermal expansion coefficient of YBCO films and NGO substrates.16 SEM images depicting the surface and cross-section of the a-axis film are shown in panels a and b of Figure 3, respectively. The top view of the extremely flat surface with 0.5 µm wide crack reveals a “near-thermal equilibrium” growth. The cross-
section of the film exhibits a homogeneous dense structure throughout the thickness, reflecting the high quality of our LPE films. The average thickness of the grown films was about 11.5 µm, corresponding to a growth rate of 2.3 µm/min, which is much higher than that in conventional vapor-phase deposition. These characterization results imply that the a-axis growth in our experimental environment was stable and resulted in highquality films. In our previous investigation, the positive effect of enhancing the oxygen content in melt on the promotion of a-axis growth of YBCO films was proposed.11 In this regard, it occurred that for the stable and rapid growth of high-quality a-axis-oriented LPE YBCO films, the high oxygen pressure atmosphere is more effective than variation in the flux composition. The diagram in Figure 4 comprising data from the literature7,11 and results in this study shows the distinct tendency of crystalline orientation when taking the oxygen enhancement into consideration. The growth environments of Ba:Cu ) 3.0:5.0 in air, Ba:Cu ) 3.0:6.5 in air, and Ba:Cu ) 3.0:5.0 in pure oxygen are denoted as A, B, and C, respectively. Among these conditions, A could be considered as the lowest-oxygen-content environment. From condition A to B, a slight oxygen enhancement in the melt composition led to the appearance of growth “window” for a-axis films, even though this window was small and sensitive to supersaturation. Once the growth condition turned to C, the a-axis growth “window” was evidently broadened because of intensive oxygen enhancement in the atmosphere. As a result, the factor of oxygen gradually became dominant in comparison with supersaturation, which until now played the most important role in promoting the a-axis growth. In analogy with the formation of REBa2Cu3Ox (REBCO, RE ) rare earth) thin films,12,17 the concept of “surface migration” should be introduced in the LPE case as an essential determinant of the orientation of the YBCO films. The illustration in Figure 5 gives a simple explanation of the different surface migration lengths during a- and c-axis growth. In terms of the migration
Effect of O2 Enhancement on Crystalline Orientation of YBa2Cu3Ox
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vicinal to Tp. However, an enhanced undercooling induced assistance for surface migration, and the growth orientation therefore readily switched to the c-axis. An enhancement of oxygen concentration in the atmosphere (regime C) resulted in an intensive suppression of surface migration. It is thought to be crucial for broadening the a-axis growth “window” up to and over 20 K. 4. Conclusion
Figure 4. Schematic illustration showing the orientation transition of YBCO films dependent on the undercooling, the Ba:Cu melt ratio, and the oxygen partial pressure.
In summary, a pure oxygen environment has shown dominant influence on the growth of high-quality a-axis-oriented YBCO films. Analysis of surface migration was made to gain insight into the growth priority transition between a- and c-axis-oriented REBCO crystals. Besides, we would like to point out the advantages of the enhanced oxygen environment as a controlling thermodynamic parameter. The present work indicates feasibility of preparation of REBCO multilayer sandwiches with differently oriented layers by switching between two or more different oxygen partial pressure atmospheres during a single growth process. Acknowledgment. The authors are grateful for financial support from NSFC (Grant 50572065), the MOST of China (973 Project 2006CB601003), and the Shanghai Committee of Science and Technology (Grants 055207077 and 055211003). The authors thank Prof. M. Jirsa for helpful discussions. References
Figure 5. Surface migration model of the a- and c-axis YBCO films when different oxygen partial pressures are applied.
theory, growth of the c-axis YBCO is associated with a long surface migration of deposited species to reach the appropriate step edges of the existing grains. On the other hand, the species of incoming flux need much shorter migration to occupy the suitable crystal growth site in the a-axis growth mode (a twodimensional growth), because all atomic sites (Y, Ba, Cu) are located at the topmost surface of the growing film.12 Thus, if we aim to the pure a-axis growth, the surface migration should be significantly suppressed. Figure 5 also shows the influence of oxygen environment. Foremost, the chemical potential of oxygen in the melt should get into equilibrium with that in the surrounding oxygen gas. This directly effects the cation diffusion during the LPE deposition. Under insufficient oxygen supply, vacancies in the oxygen sublattice induce transformation of entire lattice units, facilitating cation migration.18 In terms of the migration theories, the corresponding high cation mobility or incentive of lattice jump is a benefit of the c-growth mode. On the contrary, the saturated oxygen state is thought to strongly reduce the cation mobility, up to a “frozen” state. As a consequence, the growth of a-axis-oriented films is significantly promoted. The above deductions are in good accordance with our experimental data on transition from c- to a-axis oriented growth. Under condition A or pure nitrogen gas, c-axis growth was dominant. Condition B provided weak oxygen enhancement, giving rise to the appearance of a-axis growth “window”
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