(001) Bi2Sr2Ca2Cu3O10 Superconducting Thin Films on Substrates with Large Film-Substrate Lattice Mismatch and Different Film-Substrate Lattice Mismatch Anisotropy K. Endo† and P. Badica*,‡
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 391–394
Research Laboratory for Integrated Technological Systems, Kanazawa Institute of Technology (KIT), 3-1, Yatsukaho, Hakusan, Ishikawa 924-0838, Japan, and National Institute of Materials Physics, Bucharest-Magurele, POB MG-7, 077125, Romania ReceiVed June 10, 2008; ReVised Manuscript ReceiVed September 23, 2008
ABSTRACT: Thin films of (001) Bi2Sr2Ca2Cu3O10 with high zero-resistance critical temperature Tc0 ) 75-95.1 K and low roughness up to three half-c-axis unit cells were grown by metal-organic chemical vapor deposition (MOCVD) on substrates with large film-substrate lattice mismatch and different film-substrate mismatch anisotropy. Comparative analysis of the films on (001) and (110) MgO and NdGaO3 suggests that Bi-2223 films can easily accommodate large mismatch film-substrate differences, while mismatch on different directions, that is, mismatch anisotropy, has a strong influence on the quality of the film. The highest quality (low roughness, high uniformity, and high Tc0(R)0)) is obtained when mismatch anisotropy, taken as the mismatch ratio (r), is given only by compressive or only by tensile mismatch stress, and it is around 1, that is, it is as for the (001) MgO substrate. Mismatch anisotropy (value and sign) is an important parameter and can be used to tune superconducting properties of the film. 1. Introduction Superconducting c-axis oriented thin films of the phase Bi2Sr2Ca2Cu3O10 (Bi-2223) were grown by different methods1-6 and on different substrates. Difficulties encountered in the growth of epitaxial high-quality single-phase Bi-2223 thin films were discussed from several points of view such as narrow growth window and phase stability, difficulties in the precise control of the stoichiometry (oxygen, cations, and internal substitutions) and hence of the crystal structure, specific growth effects that occur when growth is performed by certain methods (e.g., pulsed-laser-deposition), and the influence of the substrate through such elements as the type of the terminal plane and surface or substrate-film interdiffusion or mismatch. For the last parameter, growth modes were studied, and in some cases it was concluded that in-plane orientation relationships observed experimentally are not explained by the classic near coincidence site lattice model between superconducting phase and the substrate.7,8 Presented aspects are pointing toward complexity of the growth processes as well as on the insufficient understanding of the growth of these thin films resulting in poor property control with severe negative implications on their use in applications. On the other hand, high Tc superconducting c-axis, in-plane oriented Bi2Sr2Ca2Cu3O10 (Bi-2223) thin films were prepared by metal-organic chemical vapor deposition (MOCVD) on LaA1O3.9 These films can be considered single-crystal thin films without weak links because of high critical current density values Jc at 77 K observed up to high magnetic fields. If an appropriate substrate is selected, investigation of these Bi2Sr2Ca2Cu3O10 thin films as a potential candidate for the microwave applications (band-pass filters, resonators, etc.) or as coated conductors is of high interest. In this article, we report a few new details of the growth of Bi-2223 thin films by MOCVD on different substrates toward * To whom correspondence should be addressed. Address for correspondence: Mainz University, Institute of Physics, Staudingerweg 9, D-55099, Germany. Tel: +49-6131-39-26788. Fax: +49-6131-39-24076. E-mail: badica2003@ yahoo.com. † Kanazawa Institute of Technology. ‡ National Institute of Materials Physics.
a better property control of the films. Substrates were of (001) and (110) MgO and NdGaO3, and they were selected so that there is a certain film-substrate (F-S) relationship (Figure 1) reflected by a different degree (Table 1) of lattice mismatch (LM) as well as by a different lattice mismatch anisotropy (LMA given by mismatch ratio, r). Comparative analysis between the Bi-2223 thin films grown on (001) and (110) MgO with large F-S lattice mismatch and the films on (001) and (110) NdGaO3 with low F-S lattice mismatch (about 1 order of magnitude lower than for MgO) reveals that LM has a small influence on the film quality, and in fact, a key parameter is the LMA. Films were characterized from structural, microstructural, and electrical resistivity points of view. Also, MgO substrate is an excellent substrate for microwave applications. However, the detailed investigations of the films by surface resistance measurements will be addressed in a future work.
2. Experimental Section Films were grown in a specially designed cold-wall type MOCVD apparatus presented elsewhere10 using as source materials Bi(C6H5)3 and M(DPM)2 with M ) Sr, Ca, and Cu and DPM ) dipivaloylmethane. Argon flow was used to transport vapors to the reaction tube and oxygen was introduced directly into the reactor. Substrates of (001) and (110) MgO or NdGaO3 (NGO) were placed on an Inconel susceptor and were inductively heated to 800 °C. Inductively coupled plasma spectroscopy (ICP, SPS 7700, Seiko Instruments Inc.) showed that as-prepared thin films have approximately constant composition with average cation ratio Bi:Sr:Ca:Cu ) (1-1.1): 1:1:(1.5-1.7), and the films thickness is around 500 A. X-ray diffraction patterns (XRD) were taken with a D500, Siemens diffractometer, CuKR radiation. The microstructure was investigated by atomic force microscopy (AFM, SPA 300, Seiko Instruments Inc.). Electrical resistivity curves versus temperature, R(T), were measured by the standard fourprobe method.
3. Results and Discussion X-ray diffraction patterns of the films on MgO and NGO with different orientations are presented in Figure 2. The profile of the XRD pattern is approximately the same for all the films. Peaks can be indexed as (001) of Bi2Sr2Ca2Cu3O10 phase. This result clearly shows the c-axis growth of the Bi2Sr2Ca2Cu3O10
10.1021/cg800604b CCC: $40.75 2009 American Chemical Society Published on Web 12/04/2008
392 Crystal Growth & Design, Vol. 9, No. 1, 2009
Endo and Badica
Figure 1. Epitaxial relationship between Bi2Sr2Ca2Cu3O10 and (a) (001) MgO, (b) (110) MgO, (c) (001) NdGaO3, and (d) (110) NdGaO3 (see also Table 1). Table 1. Lattice Mismatch (A and B) on Different Indicated Directions of the Substrates (S), Lattice Mismatch Anisotropy (LMA) on the Considered Directions Given by the Ratio ra, Zero-Resistance Critical Temperature Tc0 of the Bi-2223 Superconducting Films, Room-Temperature Resistivity R300K, Ratio R300K/RTc,onset, Tc,onset,bMean Square Roughness (rms), and Ratio of the (002) and (0012) Peak Intensities from the XRD Patterns S (001) (110) (001) (110)
NGO NGO MgO MgO
A mismatch on
B mismatch on
mismatch ratio, r ) A/B
Tc0 K
R300K (10-6 Ωm)
R300K/RTc,onset
Tc,onset K
rms nm
I002/I0012
[001] ) -0.55% [110] ) -1.31% [001] ) -9.38% [001] ) -28.6%
[010] ) -1.81% [001] ) -1.95% [010] ) -9.38% [110] ) +8.47
-0.55/-1.81 ) +0.3 -1.31/-1.95 ) +0.67 -9.38/-9.38 ) +1 -28.6/+8.47 ) -3.37
87.6 92.6 95.1 75
4.5 3.8 4 15
2.19 2.63 1.42 1.42
147 129 135 123
2.39 2.64 1.83 4.33
0.85 0.56 0.97 0.63
a Ratio r ) A/B is taken with |A| < |B| when A and B are both compressive or both tensile and with |A| > |B| when stresses are opposite, i.e., one is compressive and the other one is tensile or tensile and compressive. b Defined as the temperature where resistivity deviates from the linear metallic behavior.
Figure 2. X-ray diffraction patterns for the Bi2Sr2Ca2Cu3O10 thin films on (a) (001) MgO, (b) (110) MgO, (c) (001) NdGaO3, and (d) (110) NdGaO3.
films on substrates with high value of lattice mismatch and with different orientations. The level of the impurity phases/orientations is relatively low. However, the position of the (002) peak is shifting to higher 2θ angles suggesting the occurrence of Bi2Sr2CaCu2O8 (Bi-2212) intergrowth in the film grown on (110) MgO. Another observation is that the relative intensity
of the (001) peaks is slightly changing from sample to sample; for example, in Table 1 is presented the ratio between the intensities I for the (002) and (0012) peaks, and the values of this parameter are between 0.56 and 0.97. The full width at half-maximum (fwhm) of the (002) line takes values from 0.58 to 0.87 depending on the substrate type. Responsible for the
Superconducting Thin Films
Figure 3. AFM images (2 µm × 2 µm) for the thin films grown on (a) (001) MgO, (b) (110) MgO, (c) (001) NdGaO3, and (d) (110) NdGaO3.
variation of these parameters could be the already mentioned Bi-2212 intergrowth or the regions with the defect/distorted structure formed as a consequence of the residual strain induced by the lattice mismatch between the film and substrate. AFM images of the films are similar for all investigated substrates (Figure 3). Films, from the AFM point of view, show in-plane alignment; rectangular grains of large area (up to 0.5 µm × 2 µm) are parallel to each other without evidence for twinning (apparently less probable for the film from Fig 3d) or the grains with different in-plane orientation. Detailed further structural measurements are required to confirm the AFM information on the in-plane alignment. These images also indicate the two-dimensional nucleation growth mechanism of
Crystal Growth & Design, Vol. 9, No. 1, 2009 393
the films. Roughness of the presented microstructure of the films on (001) and (110) substrates is approximately 1-3 times the half c-axis unit cell of the Bi2Sr2Ca2Cu3O10 superconductor. The lowest root-mean-square roughness (rms) is obtained for the film on (001) MgO, while the highest is for the one on (110) MgO (Table 1). rms values of the films on NGO are less scattered and are closer to the values for the film on (001) MgO with the lowest roughness than to the film on (110) MgO with the maximum roughness. Electrical resistivity curves R(T) are shown in Figure 4. Zeroresistance critical temperature of the films Tc0, room-temperature resistivity R300K, ratio R300K/RTc,onset, and Tc,onset are presented in Table 1. Films with higher Tc0 have shown lower values of R300K (around 4 × 10-6 Ωm) and higher values for the ratio R300K/RTc,onset. If more precise, for the films on MgO, the ratio R300K/RTc,onset is constant at 1.42, and it is lower than the values for the films on NGO (2.19 and 2.63 for the (001) and (110) orientations). Curve R(T) of the film on (110) MgO with the lowest Tc0 ) 75 K, highest R300K ) 15 × 10-6 Ωm, lowest ratio R300K/RTc,onset ) 1.42, and highest roughness rms ) 4.33nm has two steps. This relatively low quality is explained by the presence of two superconducting phases: Bi-2223 and intergrowth of Bi-2212 as indicated by the XRD results (see above paragraphs). On the other side, the film with the highest quality has maximum Tc0 ) 95.1 K, is apparently single phase, has minimum roughness rms ) 1.8 nm, and has been grown on (001) MgO. Growth of likely in-plane epitaxial c-axis thin films on MgO of relatively high quality is surprising considering the unusually large lattice mismatch between the substrate and superconductor. It is reported for the 3C-SiC MOCVD film on Si substrate11 and for the GaN MOCVD film on GaAs substrate12 that a strain resulting from such a large mismatch is relaxed by the formation of stacking faults or misfit-dislocation. Currently, an explanation for the mechanism of the strain accommodation or relaxation induced by the mismatch in BiSrCaCuO system is missing, and further investigations are required. Two factors might influence strain relaxation behavior: (1) layered nature of the Bi based superconducting phases in which the bonding between layers and especially between the two neighboring Bi
Figure 4. Electrical resistivity curves vs temperature for films grown on (a) (001) MgO, (b) (110) MgO, (c) (001) NdGaO3, and (d) (110) NdGaO3.
394 Crystal Growth & Design, Vol. 9, No. 1, 2009
ones (van der Waals) is weak so that layers can easily slide and hence can release strain (somehow similar to the formation of stacking faults or misfit-dislocation) and (2) specific features of the MOCVD process, which is closer to the equilibrium than for the other deposition techniques. For the BSCCO superconducting phases, formation of structural disordering such as stacking faults, bilayer delineation effects, ab-plane misorientation, and others is well-known,13 and it is related to structural modulation. Modulation does not influence superconducting properties, and cation concentration and oxygen content control Tc. These results were usually reported for bulks. In the case of thin films, the situation is more complex because crystallization and growth take place under strain formed as a consequence of the substrate’s influence. It is evident that crystallization conditions play a significant role in structural and microstructural characteristics and therefore in the final properties and quality of the film. Our films were prepared under identical growth regime, and the lattice mismatch is about 1 order of magnitude higher for the MgO than for the NGO substrates, but this has low influence on the quality of the film (Table 1). Moreover, even if we accept that there is a mechanism by which strain is released, it is difficult to understand our results, that is, the best quality is obtained for the film on (100) MgO and the worse is obtained on (110) MgO. Films on NGO with low mismatch have Tc0 lower than for (100) MgO with high mismatch. Apart from the lattice mismatch, the legitimate idea would be that the type of substrate or orientation can control the final quality. However, this cannot explain our results either because Tc0 for the film on (001) NGO with lower mismatch is smaller than for the film on (110) NGO with higher mismatch. The behavior of Tc0 for the films on NGO can be probably understood within the results of Locquet et al.14 and Bozovic et al.15 Namely, they showed that Tc doubled by compressive epitaxial strain induced by the substrate in La2-xSrxCuO4 thin films and that films under strain conditions are extremely sensitive to the oxygen intake. To some extent, this mechanism might work also for the films on MgO in the sense that the release of strain might not be total. If so, there is a high probability that it will depend on strain distribution. To easily see such influence, we have introduced in our analysis lattice mismatch anisotropy (LMA) defined here by the mismatch ratio between mismatch directions, r. Indeed, variation of this parameter versus Tc0 suggests that the higher the deviation from r ) 1 is, the lower the Tc0 is (Table 1). In other words, strain distribution should be as uniform and symmetric as possible along a and b axes of the superconducting phase for an efficient relaxation and growth of high-quality material. The optimum value of r ) 1 is justified considering that geometrically the ab-plane is a square, a ) b. However, from the chemical symmetry point of view, a and b directions are not equivalent and, for example, the growth rate on the a-direction is higher than that on the b-one. In these circumstances, optimum r might take a slightly different value than the indicated one of r ) 1. Also, strain on a- or b-direction should be of the same type: only compressive or only tensile and not mixed, that is, r > 0. Our results indicate that mismatch anisotropy (LMA) is an important parameter for growth, and it can provide a new tool for growth and properties control opening new opportunities
Endo and Badica
for application of BSCCO superconducting thin films. Mismatch anisotropy effects might be essential also for other materials, but they overlap on the effects induced by the type and state of the substrate, film or deposition technique, and growth conditions (temperature, pressure) so that depending on the investigated property and the indicated parameters, the degree of influence may or may not be significant. For our films, there is no clear correlation between r and some of the films’ characteristics (see Table 1).
4. Conclusion In summary, we have grown by MOCVD (001) superconducting Bi-2223 thin films on (001) and (110) MgO or NGO. It was found that the magnitude of the lattice film-substrate mismatch has low influence on the quality of the film, and the strongest influence is given by the mismatch anisotropy (value and sign). High quality of the films on both substrates and especially on (001) MgO makes them promising for different applications particularly for microwave ones. Further research is needed to understand how the large F-S lattice mismatch is accommodated. Acknowledgment. PB acknowledges financial support from CNCSIS IDEI PCCE (project “Magnetic anisotropy in complex units, supramolecular systems and at nano-scale”, Romania). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Ohkubo, M.; Geerk, J.; Linker, G.; Meyer, O. Appl. Phys. Lett. 1995, 67, 2403. (2) Hatano, T.; Nakamura, K.; Narita, H.; Sato, J.; Ikeda, S; Ishii, A. J. Appl. Phys. 1994, 75, 2141. (3) Zakharov, N. D.; Hesse, D.; Auge, J.; Roskos, H. G.; Kurz, H.; Hoffschulz, H.; Dresen, J.; Stahl, H.; Guntherodt, G. J. Mater. Res. 1996, 11, 2416. (4) Lorentz, M.; Becker, S.; Dietze, H. J.; Schmitz, W.; Brunner, B.; Renk, K. F. Physica C 1991, 182, 114. (5) Gruber, H.; Krautz, E.; Fritzer, H. P.; Popitsch, A. Phys. Status Solidi A: Appl. Res. 1991, 126, 229. (6) Bozovic, I. IEEE Trans. Appl. Supercond. 2001, 11, 2686 (part 3). (7) Ohkubo, M.; Brecht, E.; Linker, G.; Geerk, J.; Meyer, O. Appl. Phys. Lett. 1996, 69, 574. (8) Marechal, C.; Lacaze, E.; Seiler, W.; Perriere, J. Physica C 1998, 294, 23. (9) Endo, K.; Yamasaki, H.; Misawa, S.; Yoshida, S.; Kajimura, K. Nature 1992, 355, 327. (10) Endo, K.; Moriyasu, Y.; Misawa, S.; Yamasaki, H.; Yoshida, S. Thin Solid Films 1991, 206, 125. (11) Yoshida, S.; Misawa, S.; Okumura, H.; Sakuma, E. Bull. Electrotech. Lab. 1992, 58, 3. (12) Trampert, A.; Bradt, O.; Yang, H.; Ploog, K. H. Appl. Phys. Lett. 1997, 70, 583. (13) Horiuchi, S.; Takayama-Muromachi, E. Crystal structure. In Bismuthbased high-temperature superconductors; Maeda, H., Togano, K., Eds.; Marcel Dekker Inc.: New York, 1996; pp 7-32; ISBN: 0-8247-9690X. (14) Locquet, J.-P.; Perret, J.; Fompeirine, J.; Maechler, E.; Seo, J. W.; Van Tandeloo, G. Nature 1998, 394, 453. (15) Bozovic, I.; Logvenov, G.; Belca, I.; Narimbetov, B.; Sveklo, I. Phys. ReV. Lett. 2002, 89, 107001.
CG800604B
