DOI: 10.1021/cg9009652
Supersaturation-Controlled Growth Orientation and Grain Boundary Transition in REBa2Cu3O7-δ (RE = Sm, Sm1-xYx) Liquid-Phase Epitaxial Films
2010, Vol. 10 575–579
Chen Y. Tang,† Yuan Y. Chen,† Wei Li,‡ Li J. Sun,† Xin Yao,*,† and Milos Jirsa# †
Department of Physics, State Key Laboratory for Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, P. R. China, ‡Instrumental Analysis Center, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, P. R. China, and # Institute of Physics ASCR, Na Slovance 2, CZ-182 21 Praha 8, Czech Republic Received August 14, 2009; Revised Manuscript Received December 27, 2009
ABSTRACT: Different kinds of REBa2Cu3O7-δ (RE = Sm, Sm-Y mixture) a/c grain boundaries are obtained by altering growth parameters such as temperature or oxygen partial pressure. We attribute the boundary structure variation to the a/c grain growth competition, which mainly depends on the supersaturation in the melt. A fine a/c boundary structure is obtained under certain growth conditions. The diffusion time, for which the growth atoms can stay on the growth interface, is considered as a unique kinetic limit in the growth orientation transition mechanism in the liquid-phase epitaxy (LPE) process. The growth mechanism is suggested as a combined effect of thermodynamic and kinetic factors.
Introduction REBa2Cu3O7-δ “REBCO” (RE = rare earth) crystals show considerable intrinsic anisotropy. The coherent length along the a and/or b-axis (ξa,b) is 1.5-2 nm, which is much longer than the coherent length along the c-axis (ξc ≈ 0.2-0.3 nm).1 This property makes control of the preferential orientation in these high-Tc superconducting oxides very important and attractive. On one hand, microelectronic devices based on the superconductor/insulator/superconductor (SIS) Josephson junctions require a/b-axis perpendicular to the substrate. There are numerous papers reporting growth of a-axis oriented films by different methods, such as pulsed laser deposition (PLD),2-4 sputtering,5,6 or liquid-phase epitaxy (LPE).7-10 On the other hand, coated conductors prefer c-oriented grains, and a great effort is made to avoid a-oriented phase during the preparation.11 Besides, an a/c mixed structure is also very attractive. The a/c grain boundary may serve as a junction instead of the conventional SIS structure. The a/c boundary also might be a source of flux pinning.12 Although many experimental results have shown an a/c mixed growth, only a few studies have focused on the details. The preferential crystallographic orientation of REBCO films is a very complicated issue because it sensitively depends on a number of growth parameters. In vapor phase deposition (VPD), the substrate temperature, the oxygen partial pressure, and the substrate itself could be the key factors. Shingai et al. fabricated an a/c grain boundary by choosing proper substrate types.12 In LPE, the growth temperature, atmosphere, and flux composition play a primary role. However, the a-c crystallographic axis transition mechanism, which is very helpful for a reliable control of the growth orientation of REBCO films, is still under discussion. LPE can easily produce high-quality films due to growth conditions close to the thermodynamic equilibrium. Moreover,
as a high-growth rate, low-cost, and nonvacuum method, it is very promising for large-scale production. In this work, we present a/c mixed growth obtained in SmBa2Cu3O7-δ (SmBCO) and (Sm-Y)Ba2Cu3O7-δ by means of liquid phase epitaxy. The relationship between supersaturation and the grain orientation is discussed. Experimental Procedures The vertical dipping method13 was used in our experiments. Powder of BaCO3 and CuO mixed in various molar ratios of Ba/ Cu = 3:5 (the 3:5 flux) or Ba/Cu = 3:7 (the 3:7 flux) were ground in an agate mortar. The ground powder was calcined at 900 °C for 48 h in air to form a Ba-Cu-O solvent. A Sm2O3 crucible was used to reduce contamination. Sm entered through the interaction between the molten Ba-Cu-O solvent and the Sm2O3 crucible. The growth temperature varied from 1035 to 1078 °C as the peritectic temperature of SmBCO changed with the flux composition and the ambient atmosphere. Table 1 shows the growth temperature range under different conditions. The oxygen partial pressure in the airtight system was controlled by a TORAY LC-750 Oxygen analyzer supervising the in situ process. The (110) NdGaO3 single-crystalline substrate, attached to a rotating rod, rotated with a speed of 80 rpm. The dipping time was about 10 s. Besides pure SmBCO and YBCO systems, we also studied mixed Y-Sm systems prepared from the 3:5 flux. For preparation of these binary films, a Y2O3-crucible was used, filled with Sm2O3 powder and Ba-Cu-O solvent. The growth temperature was 1037-1043 °C in oxygen atmosphere and 1020 °C in air. The film morphology was observed by an optical interference microscope (Olympus BX-51M). The composition of the grains was characterized by energy dispersive X-ray (EDX) analysis.
Results and Discussion
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Using various flux compositions and ambient atmospheres, we obtained different kinds of a/c grain boundaries. Figure 1a shows the morphology of an as-grown film prepared at 1050 °C in air. Except for a few small a-axis grains, sized about 2-3 μm along the film surface, the film was predominantly c-oriented. The grains were clear and free of flux rests. Figure 1b,c shows different details of the same image taken on
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Table 1. Growth Temperature Range As a Function of Growth Condition Ba/Cu ratio in the flux atmosphere growth temperature (°C)
3:5 air 1050-1070
O2 1075-1078
3:7 air 1035-1050
O2 1055-1060
Figure 1. Morphology of different kinds of a/c grain boundaries in SmBCO LPE films. The growth parameters (temperature, Ba/Cu ratio, atmosphere) were (a) 1050 °C, 3:5, air; (b, c) 1042 °C, 3:7, air; (d) 1060 °C, 3:5, air.
the film grown from the 3:7 flux in air. Figure 1b focuses on rod-like a-axis grains, about 20-30 μm long, while Figure 1c is focused on platelet-like c-axis grains just near the previous ones. Obviously, the distance between the levels of a- and cgrain tops is larger than the microscope’s depth of field and the microscope could not simultaneously focus on both grain types. This phenomenon appeared in all our experiments performed with the 3:7 flux in air. Noticeably, a thick flux layer is attached to the a-axis oriented grains. When the oxygen partial pressure rose to 1 atm, the a-axis grains joined together to form a continuous a-axis oriented film. Figure 1d presents morphology of a film prepared from 3:5 flux in air, at 1060 °C. It is similar to that from Figure 1b,c. On the basis of our experience, Tp of REBCO usually differs by about 15-20 deg when using the 3:5 and 3:7 fluxes. In the present case, the two optimal growth temperatures were 1042 °C with the 3:7 flux in air and 1060 °C with the 3:5 flux in air, corresponding to the temperature difference of 18 deg, coinciding with our expectations. This indicates that the two growth conditions might provide a similar level of supersaturation in the fluxes in the experiments. Some other growth conditions for fabricating films of this system were also studied in ref 17. Similar a/c grain boundaries were also obtained in the Y-Sm mixed system. In YBCO, the a-axis grains are usually small due to the low growth rate. It makes it difficult to observe a/c grain boundaries. In the SmBCO system, the growth rate is much higher. As a result, the surfaces of c-oriented films are not usually flat and clean (see Figure 1b,c), which disturbs the observation. In the Y-Sm mixed system, the growth rate can be adjusted to a proper level, and nice a/c grain
boundaries and a shiny surface can be obtained. Figure 2 shows a series of pictures with a mixed a-c axis growth similar to Figure 1 but with more characteristic details. Figure 2a shows a-oriented grains dispersed in the c-axis film. The growth spiral in the center and the typical c-axis twinning demonstrate the film orientation. The a-axis grains seem to be trapped by the c-oriented film. Grains marked as “1”, 2”, and “3” are partially covered, while grains “4”, “5”, and “6” are almost buried in the film. It directly proves that the c-oriented grains grow faster and cover the a-oriented grains, just opposite to the VPD films.15 As Shingai et al. mentioned,12 this kind of a/c boundary might be a potential source of flux pinning. Figure 2d shows an edge of the film. On the upper side, the characteristic square spiral indicates the c-axis is perpendicular to the substrate. On the bottom side, several half-covered elliptic spirals correspond to the a-axis growth. The c-axis oriented film is clearly thicker and has a tendency to cover the a-axis oriented film. We can see that the a-axis spiral cores, marked by a set of bluish arrows, lie on the a/c boundary. Perhaps it is because the core of a growth spiral always has the highest growth rate and stands on the top, which prevents the c-film from growing over. Compared to the three sides marked by the black arrows, the spiral line facing the a-axis film was obviously disturbed by the a-spirals, resulting in the wavy nonparallel lines marked by the white arrow. Our studies of the growth mode of the a/c transition in YBCO and SmBCO16,17 indicated that the supersaturation in the liquid played a very important role in the growth regime of REBCO LPE films. We supposed that various a/c grain boundaries in the (Y-Sm)BCO films could be also controlled via supersaturation. Figure 3 is a schematic illustration, which might be helpful to understand the scenario. Typical growth
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Figure 2. Morphology of different kinds of a-c grain boundaries on the (Y, Sm)BCO LPE film. The growth parameters (temperature, Ba/Cu ratio, atmosphere) were (a) 1037 °C, 3:5, O2; (b, c) 1043 °C, 3:5, O2; (d) 1020 °C, 3:5, air. The scale bar in (d) is different from the other three, because the picture was taken under a different magnification level.
Figure 4. A schematic diagram indicating the origin of different growth modes. Figure 3. A schematic grain growth rate diagram, R vs supersaturation.
temperatures for SmBCO are (A) Tp = 1060 °C for 3:7 flux in O2; (B) Tp = 1042 °C for 3:7 flux in air or 1060 °C for 3:5 flux in air; (C) Tp = 1050 °C for 3:5 flux in air; and (D) Tp = 1048 °C for 3:5 flux in air. Basically, the supercooling of the system increases as the growth temperature decreases and supersaturation is positively correlated with supercooling. We do not know the exact supersaturation under each growth conditions; however, the tendency of δA > δB > δC > δD is clear. To grow pure a- or c-oriented LPE films, the supersaturation should be set into the respective area, A or D. When supersaturation lies in region B, the growth rate of a-oriented grains is higher than that of the c-axis grains. Consequently, the a-axis grains grow over the c-axis ones, just like in Figures 1b,c and 2b,c. Hereafter, we denote this kind of a growth mode as R-mode. As was pointed out above, the protuberant a-axis grains were surrounded by a thick layer of flux, and no clear a/c grain boundary was formed. The standing a-axis grains disturbed the solvent convection; when the growth front of the c-axis grains approached the a-axis grains, the nutrient
supplied by the liquid was constrained. As a result, the a/c boundary could not form, leaving the space filled with a trapped flux. If supersaturation lies in region C, the similar growth rates of a-axis and c-axis grains cause a quasi-flat film surface and the nutrient flow can easily reach the growth frontier. Under these circumstances, a higher growth rate of c-oriented grains leads to a quick lateral expansion of these grains. This in turn facilitates formation of a fine a/c mixed structure as presented in Figure 2a,d. We denote this growth mode as β-mode. We stress that the grains orientation is very sensitive to the growth conditions. A slight change in the parameters, for example, in the local flux composition or temperature, can cause a dramatic change in the film structure. The growth window for a fine a/c mixed structure is evidently very narrow, which requires more systematic and careful work. Figure 4 gives a direct insight into the estimation of R and β modes. The composition differences between a- and c-oriented grains were studied by means of EDX. The results are shown in Figure 5. The left part, marked as Ra ≈ Rc, refers to the β-mode, while the right part, marked as Ra > Rc, refers to the R-mode. Ra and Rc are the growth rates of a- and c-oriented
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Figure 6. A schematic kinetic model of the surface migration in LPE.
Figure 5. EDX results of RE element percentage in differently oriented grains.
grains, respectively. It turns out that if Ra and Rc are similar, like in the β-mode, the RE concentrations in a- and c-oriented grains are close; if the growth rates are different, like in the R-mode, a clear gap of RE concentration between a- and c-oriented grains appears. In our previous work, we suggested that the Sm-Ba mutual substitution might suppress the a-axis grain growth.17 On the basis of our analysis presented above, the R-mode appears in region B, where the supersaturation is relatively small. This coincides with the low RE concentration in a-oriented grains. Therefore, the a-axis growth is not restrained, which explains the higher a-axis growth rate compared to that of c-axis grains. In region C the higher supersaturation restricts the a-growth rate, which leads to the β-mode. The EDX results support this scenario. In vapor phase deposition, the presence of a-axis grains on the c-axis substrate has been commonly observed in thicker films.15 In the present study, we found that instead the c-axis grains cover the a-axis ones. This brings a new viewpoint into the discussion on the difference between a-c transition mechanisms in VPD and LPE. In VPD, the growth temperature is several hundred degrees below Tp of REBCO. At such a low temperature, the growth of both a- and c-axis grains is thermodynamically allowed. In such a case, kinetic energy of the migrating atoms or ions is the dominant factor determining the film orientation. The temperature range of the a-c transition is usually around 100 deg wide.3,18 In LPE, the growth condition is close to the thermodynamic equilibrium.19 The kinetic energy of the migrating atoms is no longer a limiting factor. The a-c transition happens dramatically, within 10 deg.9,10,20,21 This does not allow for enough kinetic energy difference necessary for the a-c transition. Hence, other factors have to stand behind the kinetic limit of a-c transition in LPE films. The time during which atoms can diffuse along the grain/flux interface plays a critical role in the difference between LPE and VPD. An atom returning to the environment from the interface requires an energy supply close to its sublimation energy in VPD. For LPE, the energy needed is close to its melting energy. At the same time, the growth temperature of vapor deposition is usually by several hundreds degrees lower than Tp, while that of LPE is nearly equal to it. Consequently, in VPD the atoms have practically no chance to go back to the environment, while in LPE the
atoms can do it quite easily. An illustration explaining the kinetic growth mechanism is in Figure 6. The scenario consists of four steps. First, an atom from the environment attaches the grain/flux interface (marked as “1”). Second, the atom diffuses on the interface (“4”). After a period of time, the atom either reaches a suitable site on the growth edge and becomes a part of the crystal matrix (marked as “3”) or it goes back to the environment (marked as “2”). In VPD, the atoms hardly go back to the environment; that is, the diffusion time of the atom can be practically infinite, making kinetic energy the only diffusion limit. In LPE the atoms can go back to the environment, so both the diffusion time and the diffusion length are finite and important. Suppose that tc is the average time during which an atom reaches a growth site in c-axis growth and ta is the mean time of atom diffusion in a-axis growth. According to the surface migration theory, tc is longer than ta. Let us define t as the mean time that atoms can stay on the interface. Consequently, if t > tc, LPE would produce a c-axis oriented film; if ta > t > tc, the LPE would result in an a-axis oriented film. In this case, the diffusion time becomes a controlling factor of the kinetic limit. A higher growth temperature can provide more migration energy; therefore, the atoms can more easily escape from the interface. A low supersaturation in the melt can reduce the slope of the RE element concentration curve (the solute concentration profile in the flux near the growth frontier of the crystal can be seen in ref 22), which makes it easier for atoms to return to the liquid and leads to an interfacial controlled growth mode.22 These two factors both reduce t as temperature increases. As the mean distance that atoms diffuse is in both cases the same, ta and tc depend only on the surface diffusion speed, which in turn relates to temperature. However, the temperature change in LPE is relatively small. Therefore, in LPE ta and tc remain almost the same. In summary, the interface diffusion time of the atoms becomes an important kinetic limit in the LPE growth orientation mechanism. Nucleation and growth driving force play also an important role as we described elsewhere.16,17 This model offers a new way in understanding the growth mechanisms, covering various controversial experimental results reviewed, for example, in the introduction of ref 16. Since the a-axis grain is a metastable phase, its thermal stability is weaker than that of the c-axis grains.23,24 If the growth temperature is very close to Tp, the a-axis phase may not exist due to its weak thermal stability. This can explain the c- to a-axis orientation transition in the work of Klemenz et al.,21 by the supercooling ΔT ranging from a few Kelvin to a little more than 10 K. The temperature is then too high to provide the a-axis grain growth at all. In Kitamura’s and our works,9,16 the supercooling ΔT ranged between 10 and 20 K. Within this range, the a-axis phase can stably exist and the kinetics plays a key role, as we illustrated above. In short, we suggest that the complex orientation transition
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mechanism in REBCO is a combined effect of both thermodynamic and kinetic factors. Conclusion We fabricated various kinds of REBCO a/c grain boundaries by means of LPE and found that the boundary structure strongly depends on the growth supersaturation. When the growth rate of the a-axis grain is close to that of the c-axis grain, it is possible to get a fine a/c mixed structure, which has good potential for application in flux pinning technologies. A new kinetic limit parameter, the diffusion time of growth atoms, is suggested to explain the different kinetics in the orientation transition mechanisms between VPD and LPE. Combining the thermal stability of grains and the kinetic growth modes, we obtained a complete explanation of the a/c orientation transition mechanism. Acknowledgment. Authors are grateful for financial support from National Science Foundation of China, the MOST of China (973 Project No. 2006CB601003, 863 project No. 2007AA03Z206) and Shanghai Science and Technology Committee of China (e.g., grant No. 08dj1400203). M.J. acknowledges the support of the Grant GACR No. 202/08/0722.
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