MOCVD Growth, Micro-Structural, and Superconducting Properties of

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Chem. Mater. 2004, 16, 608-613

MOCVD Growth, Micro-Structural, and Superconducting Properties of a-Axis Oriented TlBaCaCuO Thin Films Graziella Malandrino, Laura M. S. Perdicaro, and Ignazio L. Fragala`* Dipartimento di Scienze Chimiche, Universita di Catania, and I.N.S.T.M. UdR di Catania, Viale Andrea Doria 6, I-95125 Catania, Italy

Alberto M. Testa and Dino Fiorani I.S.M. CNR, Area della Ricerca di Roma, Via Salaria km 29, Roma, Italy Received October 9, 2003. Revised Manuscript Received December 18, 2003

The fabrication of pure a-axis oriented Tl2Ba2Ca1Cu2O8 (Tl-2212) superconducting thin films, prepared ex-situ by a combined approach of metal-organic chemical vapor deposition (MOCVD) and thallium vapor diffusion, is reported. The unusual orientation has been obtained by depositing the superconducting films on LaAlO3 (100) buffer layers grown on SrTiO3 (100) substrates. The micro-structural and superconducting properties of a-axis oriented films have been investigated and compared with those of the c-axis oriented samples simultaneously deposited.

Introduction Thallium-based superconductors such as the TlBaCaCuO compounds1 are of great scientific and technological interest because of their high transition temperature and low surface resistance at microwave frequencies.2-4 In the vast area of thin films applications, the preparation of Josephson’s junctions is also very interesting because it represents the starting point for the design of many devices.5 The fabrication of Josephson’s junctions can be highly simplified by adopting a-axis oriented high-temperature (HTc) superconducting films, as the required three-layer structure (superconducting-insulating-superconducting) can be fabricated more easily. In fact, a-axis oriented films are compatible with barrier in the nm range instead of the necessary few Å thickness required with c-axis oriented films, due to the anisotropic coherence length, which is longer along the a-axis. Moreover, a-axis oriented films may be used for preparing a particular type of Josephson’s junctions, the * Corresponding author. E-mail: [email protected]. (1) Hermann, A. M.; Yakhmi, J. V. Thallium Based High-Temperature Superconductors; Marcel Dekker: New York, 1994, and references therein. (2) (a) Speller, S. C. Mater. Sci. Technol. 2003, 19, 269. (b) Lauder, A.; Myers, K. E.; Face, D. W. Adv. Mater. 1998, 10, 1249. (3) Jenkins, A. P.; Dew-Hughes, D.; Edwards, D. J.; Hyland, D. M. C.; Grovenor, C. R. M. IEEE Trans. Appl. Supercond. 1999, 9, 2849. (4) (a) Schneidewind, H.; Manzel, M.; Stelmer, T. Physica C 2002, 372, 493. (b) Schneidewind, H.; Manzel, M.; Bruchlos, G.; Kirsch, K. Supercond. Sci. Technol. 2001, 14, 200. (c) Face, D. W.; Small, R. J.; Warrington, M. S.; Pellicone, F. M.; Martin, P. J. Physica C 2001, 357360, 1488. (5) (a) Warburton, P. A.; Kuzhakhmetov, A. R.; Chana, O. S.; Hyland, D. M. C.; Grovenor, C. R. M.; Burnell, G.; Blamire, M. G.; Schneidewind, H. Physica C 2002, 372-376, 322. (b) Chana, O. S.; Kuzhakhmetov, A. R.; Hyland, D. M. C.; Dew-Hughes, D.; Grovenor, C. R. M.; Koval, Y.; Kleiner, R.; Muller, P.; Warburton, P. A., Physica C 2001, 362, 265.

so-called bi-epitaxial junctions.6 In this case, the tunneling effect is observed between two different epitaxially grown films deposited on the same monocrystalline substrate. The different epitaxial growth can be driven by using an intermediate buffer layer that alters the pristine epitaxial relation between the superconducting film and the substrate.7 In this general context, it should be noted that the large majority of current studies report on the preparation of TlBaCaCuO c-axis oriented films.8-10 The c-axis growth comes in handy for many applications, as films having high critical currents and low surface resistance result from the growth of the superconducting CuO2 sheets parallel to the substrate surface. c-Axis films have been obtained on a variety of commercially available substrates such as SrTiO3, LaAlO3, NdGaO3, YSZ (yttria stabilized zirconia), and MgO, or on buffer layers such as CeO2, SrTiO3, and Sr2AlTaO6. The best combination of HTS material and substrate is usually strongly dependent on the specific end-use, thus the substrate choice is actually driven by the final application requirements. To date, all the reported TlBaCaCuO films are c-axis oriented, independently of substrate nature and of operating conditions used during the annealing step. (6) Char, K.; Colclough, M. S.; Garrison, S. M.; Newman, N.; Zaharchuk, G. Appl. Phys. Lett. 1991, 59, 733. (7) (a) Scotti di Uccio, U.; Lombardi, F.; Miletto Granozio, F. Physica C 1999, 323, 51. (b) Linehan, D. S.; Kvam, E. P.; Hou, L.; McElfresh, M. Supercond. Sci. Technol. 1996, 9, 739. (8) Bramley, A. P.; O’Connor, J. D.; Grovenor, C. R. M. Supercond. Sci. Technol. 1999, 12, R57. (9) Chromik, Sˇ .; Jergel, M.; Gazˇi, Sˇ .; Sˇ trbı`k, V.; Hanic, F.; Falcony, C.; Vasˇko, M.; Benˇacˇka, Sˇ . Physica C 2001, 354, 429. (10) Zhang, G.; Luo, S.; Shen, Q.; Li, C.; Lu, R.; Yan, S.; Huang, S.; He, Y. Physica C 2003, 388-389, 755.

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Properties of a-Axis Oriented TlBaCaCuO Thin Films

a-Axis oriented crystals have been occasionally observed as unexpected outgrowths of minor or major extension versus c-axis orientation. In this context, our early reported example of TlBaCaCuO a-axis oriented films11 remains still unique to our knowledge. It is worth mention that, because of the similarity of a-axis values shown by the different TlBaCaCuO phases it was not possible to attribute the phase from the θ-2θ X-ray diffraction pattern. Metal organic chemical vapor deposition (MOCVD) has been widely applied to film deposition of TlBaCaCuO phases containing double TlO layer phases, such as Tl-2212 and Tl-2223,12 single TlO layer phases, such as Tl-1212, Tl-1223, and Tl-1234,13 and intergrowth phases.14 In addition, this technique has the potential advantage of being a very reliable and reproducible method for the fast production of films with high uniformity degree in both thickness and composition over large areas. The present paper reports on the fabrication and complete structural characterization of Tl2Ba2Ca1Cu2O8 (Tl-2212) a-axis oriented films on LaAlO3 (100) buffer layer grown on SrTiO3(100) substrates. MOCVD has been successfully applied for the deposition of both the LaAlO3 buffer layer and of the Tl-2212 films. In addition, magnetic studies have been carried out both on a-axis and c-axis oriented films simultaneously fabricated. Experimental Section The BaCaCuO(F) matrixes were deposited in a low-pressure cold wall reactor from Ba(hfa)2tetraglyme, Ca(hfa)2tetraglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, tetraglyme ) 2,5,8,11,14-pentaoxatetradecane), synthesized as reported previously,15 and Cu(tmhd)2 (Htmhd ) 2,2,6,6-tetramethyl3,5-heptandione) precursors, purchased from Aldrich. In particular, the MOCVD reactor is fitted with individual inlet tubes for the carrier (Ar 80 sccm) and reaction (O2 500 sccm) gases. Each precursor reservoir, an Al2O3 boat placed inside the reactor in the sublimation zone, was resistively heated in a suited temperature range to control the growth rate (Figure 1). Depositions were carried out for 2 h at 3-4 Torr total pressure. The BaCaCuO(F) matrixes were then heated in Al2O3 boats closed with a gold foil containing a thallium source formed by an appropriate mixture of oxides (Tl2O3, BaO, CaO, and CuO), that was finely ground for 30 min in an agate mortar. The (11) (a) Malandrino, G.; Frassica, A. R.; Condorelli, G. G.; Lanza, G.; Fragala`, I. L. J. Alloys Compd. 1997, 251, 314. (b) Malandrino, G.; Fragala`, I. L.; Scotti di Uccio, U.; Valentino, M. Nuovo Cimento 1997, 19 D, 1053. (12) (a) Malandrino, G.; Richeson, D. S.; Marks, T. J.; DeGroot, D. C.; Schindler, J. L.; Kannewurf, C. R. Appl. Phys. Lett. 1991, 58, 182. (b) Zhang, X. F.; Sung, Y. S.; Miller, D. J.; Hinds, B. J.; McNeely, R. J.; Studebaker, D. B.; Marks, T. J. Physica C 1997, 274, 146. (c) Hinds, B. J.; McNeely, R. J.; Studebaker, D. B.; Marks, T. J.; Hogan, T. P.; Schindler, J. L.; Kannewurf, C. R.; Zhang, X. F.; Miller, D. J. J. Mater. Res. 1997, 12, 1214. (d) McNeely, R. J.; Belot, J. A.; Marks, T. J.; Wang, Y.; Dravid, V. P.; Chudzik, M. P.; Kannewurf, C. R. J. Mater. Res. 2000, 15, 1083. (13) (a) Hamaguchi, N.; Gardiner, R.; Kirlin, P. S.; Dye, R.; Hubbard, K. M.; Muenchausen, R. E. Appl. Phys. Lett. 1990, 57, 2136. (b) Ladd, J. A.; Collins, B. T.; Matey, J. R.; Zhao, J.; Norris, P. Appl. Phys. Lett. 1991, 59, 1368. (c) Richeson, D. S.; Tonge, L. M.; Zhao, J.; Zhang, J.; Marcy, H. O.; Marks, T. J.; Wessels, B. W.; Kannewurf, C. R. Appl. Phys. Lett. 1989, 54, 2154. (14) Malandrino, G.; Condorelli, G. G.; Fragala` , I. L.; Miletto Granozio, F.; Scotti di Uccio, U.; Valentino, M. Supercond. Sci. Technol. 1996, 9, 570. (15) Malandrino, G.; Castelli, F.; Fragala`, I. L. Inorg. Chim. Acta 1994, 224, 203.

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Figure 1. Experimental setup: 1, carrier gas; 2, precursor boats; 3, reaction gas; 4, susceptor, 5, heaters.

Figure 2. Schematics showing the Ψ angles of the (107) plane in c-axis and a-axis grains. reaction atmosphere was controlled by mixing different flows of oxygen and argon gases. The oxygen partial pressure is calculated through the following relation:

PO2 ) XO2Ptot ) [FO2/(FO2 + FAr)]Ptot where Ptot ) 1 atm and XO2 is the mole fraction of O2. The PTl2O was maintained constant from run to run and carried out under identical annealing parameters (temperature, time, and PO2) using the same quantity (500 mg) of the Tl2O3, BaO, CaO, and CuO oxide mixtures. The film chemical composition was analyzed by energydispersive X-ray (EDX) analysis, using an IXRF solid-state detector. Morphologies were examined through scanning electron microscopy (SEM), using a LEO Iridium 1450 microscope. Structures of crystalline phases and in-plane and outof-plane alignments were studied through accurate X-ray diffraction (XRD) measurements. θ-2θ XRD patterns were recorded using a θ-θ Bruker D5005 diffractometer, and pole figures were recorded on a θ-2θ Bruker D5005 diffractometer equipped with an Eulerian cradle. The macroscopic superconducting properties were investigated by means of magnetization vs temperature and magnetic field carried out by a commercial SQUID magnetometer (Hmax ) 55 kOe).

Results and Discussion Heteroepitaxial LaAlO3 (100) buffer layers have been grown on SrTiO3 (100) using MOCVD procedures. Details of deposition conditions and film characterization have been reported elsewhere.16 The a-axis oriented TlBaCaCuO films have been grown by an ex-situ process combining MOCVD and thallium vapor diffusion. Superconducting films have been simultaneously grown on both commercial 〈100〉 LaAlO3 substrates (hereafter C-LaAlO3) and on LaAlO3 (100)/SrTiO3 (100) buffer layer (hereafter B-LaAlO3) to compare the properties of a-axis film with those of c-axis oriented films processed under identical conditions. A schematic sketch of a-axis and c-axis oriented cells is reported in Figure 2. Poorly crystalline BaCaCuO(F) matrixes have been deposited using MOCVD from Ca(hfa)2tetraglyme, Ba(16) Malandrino, G.; Fragala`, I. L.; Scardi, P. Chem. Mater. 1998, 10, 3765.

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Figure 4. θ-2θ XRD patterns of (a) a c-axis oriented Tl-2212 film grown on C-LaAlO3 and (b) an a-axis oriented Tl-2212 film grown on B-LaAlO3.

Figure 3. Arrhenius diagrams for vaporization of (a) Ba(hfa)2‚ tetraglyme, (b) Ca(hfa)2‚tetraglyme, and (c) Cu(tmhd)2.

(hfa)2tetraglyme, and Cu(tmhd)2 precursors. Figure 3 shows the Arrhenius relationships for the Ba(hfa)2tetraglyme, Ca(hfa)2tetraglyme, and Cu(tmhd)2 precursors obtained by plotting the ln vaporization rate as a function of reciprocal temperature in the range 100-150 °C. On the basis of experimental curves, the apparent sublimation or evaporation enthalpies have been estimated to be 94 ( 3 kJ‚mol-1, 69 ( 2 kJ‚mol-1, and 92 ( 3 kJ‚mol-1 for Ba(hfa)2tetraglyme, Ca(hfa)2tetraglyme, and Cu(tmhd)2, respectively. The great difference in the values of ∆H associated with the vaporization processes of Ba(hfa)2tetraglyme and Cu(tmhd)2 with respect to the Ca(hfa)2tetraglyme precursor, is due to sublimation processes of the solid Ba(hfa)2tetraglyme and Cu(tmhd)2, which need more energy than the evaporation occurring from the molten Ca(hfa)2tetraglyme adduct. Particular attention has been devoted to the optimization of precursor sublimation temperatures to obtain

the correct matrix stoichiometry. In particular, each precursor was maintained at a constant temperature in the range of 88-95 °C for Ca(hfa)2‚tetraglyme, 110125 °C for the Cu(tmhd)2, and 122-130 °C for the Ba(hfa)2‚tetraglyme, whereas the substrates were heated to 500 °C. EDX analyses of the matrix indicates a 2:1:2 stoichiometry over the whole 10 × 10 mm2 surface. The as-deposited matrixes have been directly annealed under Tl2O vapors to incorporate Tl and yield the TlBaCaCuO phases. The annealing adopted the following process parameters: (i) 760-820°C annealing temperatures, (ii) 40-80 min annealing time, (iii) PO2 ) 0.67 atm, and (iv) a 2:1:2:3 Tl2O3/BaO/CaO/CuO oxide mixture. The superconducting TlBaCaCuO films are about 1 µm thick and have a mirror-like dark gray surface. The film thickness has been evaluated through profilometer measurements. A 2:2:1:2 stoichiometry has been observed through energy-dispersive X-ray analyses over the whole 10 × 10 mm2. No C contamination has been found within the detectability limit of the EDX technique, neither has it been found through X-ray photoelectron spectroscopy depth profiling. Figure 4a shows a typical X-ray diffraction pattern of samples deposited on C-LaAlO3 substrates. The XRD pattern shows all the (00λ) reflections of the Tl-2212 phase in addition to the peaks at 2θ ) 23.45° and 2θ ) 48.15° associated with the C-LaAlO3 (100) and (200) reflections, respectively. This indicates that the film is oriented with the c-axis perpendicular to the substrate surface. The onset temperature of diamagnetism, determined by low-field (Ha ) 1 Oe) magnetization vs temperature measurements (Figure 5) has been found

Properties of a-Axis Oriented TlBaCaCuO Thin Films

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Figure 6. 3D graph of an a-axis oriented Tl-2212 film (2θ angle has been varied in the range 25-35° and Ψ angle has been varied in the range 25° to 65°). Figure 5. Magnetization as a function of temperature at Ha ) 1 Oe perpendicular to the film plane for a c-axis oriented Tl-2212 film grown on C-LaAlO3. Inset: magnetization vs temperature at Ha ) 1 Oe perpendicular to the film plane for an a-axis oriented Tl-2212 film grown on B-LaAlO3.

at 100-103K, comparable to literature results for the Tl-2212 phase.4b The XRD pattern of the TlBaCaCuO film deposited on B-LaAlO3 is reported in Figure 4b. It shows a unique peak at 47.20° associated with the (200) reflection of a thallium-based phase. In fact, peaks observed at 22.75° and 46.50° can be assigned to (100) and (200) reflections, respectively, of the SrTiO3 substrate, whereas peaks at 23.50° and 48.10° can be attributed to the LaAlO3 buffer layer (100) and (200) reflections, respectively. This indicates that the film grains are oriented with their a-axes perpendicular to the substrate surface. For a-axis oriented films the onset temperature of diamagnetism has been found at 96-98 K (inset of Figure 5). The close similarities of a-axis lattice constants for all the TlBaCaCuO phases preclude any reliable identification of the superconducting phase from the θ-2θ X-ray diffraction (XRD) pattern. It, therefore, becomes mandatory to refer to reflections related also to the c-axis parameter. This information can be obtained by recording a series of θ-2θ scans, in a suited 2θ range, upon varying the Ψ angle to detect the most intense (h0λ) reflections. Ψ represents the angle between the normal to the film surface and the plane of the X-ray beam. The Ψ angle between a chosen plane and the growth plane (i.e., {100} for the a-axis films and {001} for the c-axis films) can be calculated through equation 1 as follows:

cosΨ(h1k1l1)-(h2k2l2) ) (h1h2 + k1k2)/a2 + l1l2/c2

x((h12 + k12)/a2 + l12/c2)((h22 + k22)/a2 + l22/c2)

(1)

Figure 6 shows a 3D graph plotting θ-2θ patterns (in the 25-35° range) vs the Ψ angle whose values range from 25° to 65°. Three peaks have been detected in the 3D plot at 2θ 27.60°, 31.45°, and 32.90°. The peak at 32.90° and Ψ ) 45° is not indicative of any particular phase (being associated with the (110) reflection), and

also the peak at 2θ ) 31.45° and Ψ ) 47° can be attributed to two different reflections, the (107) or the (103) of the Tl-2212 and Tl-1212 phases, respectively. In fact, the Tl-2212 (107) and Tl-1212 (103) reflections are positioned at 2θ ) 31.46° and 2θ ) 31.47°, respectively, as reported in the ICDD database.17 In addition, also the Ψ angles, calculated through eq 1, of 42.55° for the Tl-2212 (107) and 42.32° for the Tl-1212 (103) reflections, do not allow any distinction between the two different phases. The peak at 2θ ) 27.60° and Ψ ) 33° can be univocally attributed to the (105) reflection of the Tl-2212 phase, because the (102) reflection of Tl1212 phase should have been observed at 2θ ) 27.10° and Ψ ) 31°. This undoubtedly indicates that the a-axis oriented films consist of the Tl-2212 phase, analogously to the c-axis films simultaneously produced. To confirm the out-of-plane alignment, rocking curves have been recorded in both cases. Small full width halfmaximum values (fwhm) indicate a good alignment of the axis perpendicular to the substrate. The rocking curve of the (00,12) reflection of the c-axis oriented Tl2212 film (Figure 7a) shows a fwhm of 0.72°, which indicates that grains have a low dispersion. The rocking curve of the (200) reflection of an a-axis oriented Tl2212 film (Figure 7b) has a fwhm of 0.98°. This is indicative of a similar grain dispersion of a-axis vs c-axis oriented films. The in-plane alignment has been studied by recording pole figures of the a-axis and c-axis oriented films. Figure 8a shows the pole figure of the c-axis oriented Tl-2212 (107) reflection. Four poles are observed at Ψ ) 47° every 90 degrees of φ, as expected for a tetragonal symmetry. This is indicative of an in-plane alignment of superconducting grains vs the substrate. Analogously, also the pole figure of the a-axis oriented Tl-2212 (107) reflection (Figure 8b) shows four poles at Ψ ) 42°, every 90° of φ. This indicates that also the a-axis oriented samples are epitaxially grown. Note that the Ψ values, calculated through eq 1, between the {107} planes with {001} and {100} growth planes are 47.6° and 42.4°, respectively, as indicated in Figure 2. The epitaxial relationship between the a-axis and c-axis oriented Tl-2212 films and the (100) B-LaAlO3 and (100) C-LaAlO3 substrates, respectively, has been (17) ICDD 39-1482 (Tl-2212); ICDD 41-123 (Tl-1212).

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Figure 7. Rocking curves of (a) the (00,12) reflection of the c-axis oriented Tl-2212 film grown on C-LaAlO3 and (b) the (200) a-axis oriented Tl-2212 film grown on B-LaAlO3.

studied as well. In particular, the in-plane relationship of Tl-2212 films relative to the underlying LaAlO3 substrate can be obtained from Φ-scans of the Tl-2212/ C-LaAlO3 and the Tl-2212/B-LaAlO3/SrTiO3 systems (Figure 9a). Φ-scans of c-axis oriented Tl-2212 films on commercial (100) LaAlO3 substrate have been recorded using as poles the Tl-2212 (107) reflection (2θ ) 31.45°) and the LaAlO3 (111) reflection (2θ ) 41.16°) at ψ tilt angles of 47° and of 54°, respectively. The four peaks, 90° spaced, of the Tl-2212 {107} planes are shifted 45° with respect to the {111} LaAlO3 Φ positions. This clearly demonstrates that the in-plane directions [100] and [010] of the c-axis Tl-2212 grains are aligned to the substrate axis. Φ-scans of a-axis oriented Tl-2212 films grown on the (100) LaAlO3/SrTiO3 system have been recorded using as poles the Tl-2212 (107) reflection (2θ ) 31.45°), the LaAlO3 (111) reflection (2θ ) 41.16°), and the SrTiO3 (111) reflection (2θ ) 39.93°) at ψ tilt angles of 42°, 54°, and 54°, respectively. Analogously, the Φ-scan (Figure 9b) of the a-axis oriented Tl-2212 film shows four peaks, 90° spaced, shifted of 45° with respect to the MOCVD grown LaAlO3 film, whose Φ positions are instead coincident with the SrTiO3 peaks. In this case, there are two possible isoenergetic alignments involving the in-plane directions [100] and [001] or [010] and [001] of the a-axis Tl-2212 grains with the substrate axis. The coincidence of the {111} LaAlO3 and the {111} SrTiO3 Φ positions points to a cube on cube growth of the buffer layer.16 The morphology has been investigated by scanning electron microscopy. SEM images of the two films are reported in Figure 10. The c-axis oriented films (Figure 10a) show a typical platelike morphology with grain size

Figure 8. Pole figures (107) of (a) the c-axis oriented Tl-2212 film grown on C-LaAlO3 and (b) the a-axis oriented Tl-2212 film grown on B-LaAlO3. Ψ angles are 47° and 42° for the c-axis and a-axis oriented samples, respectively.

of ≈5 µm, whereas the a-axis oriented films show a particular texture typical of a-axis grown grains (Figure 10b). Note that the SEM image shows the two distinct domains of a-axis grains 90° rotated in-the-plane. Nevertheless, each domain type gives rise to four poles coincident with those due to the 90° rotated domains, and therefore only four poles are observed in the (107) pole figure. Present results together with the previously reported data11 on film annealed at 900 °C for 5 min confirm that the formation of a-axis oriented films is independent from the annealing temperature. Therefore, a completely different mechanism is likely to govern the growth of a- or c-axis TlBaCaCuO oriented films as compared to those reported for YBaCuO18 and BiSrCaCuO19 films. In those latter cases, the a-axis growth depends on temperature and deposition rates. It should also be noted that the present synthetic route involves (18) Miletto Granozio, F.; Salluzzo, M.; Scotti di Uccio, U.; MaggioAprile, I.; Fischer, O. Phys. Rev. B 2000, 61, 756. (19) Sugimoto, T.; Nakagawa, M.; Shiohara, Y.; Tanaka, S. Physica C 1992, 192, 108.

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Figure 10. SEM images of (a) a c-axis oriented Tl-2212 film grown on C-LaAlO3 and (b) an a-axis oriented Tl-2212 film grown on B-LaAlO3.

orientations have been obtained simultaneously on substrates with the same chemical composition. Conclusions

Figure 9. φ scans of the (a) Tl-2212/C-LaAlO3 and (b) Tl2212/B-LaAlO3/SrTiO3 (100) systems: the Tl-2212 (107) (2θ ) 31.45°), the LaAlO3 (111) (2θ ) 41.16°), and the SrTiO3 (111) (2θ ) 39.93°) reflections have been used as poles.

an ex situ process at variance to the YBaCuO and BiSrCaCuO in situ growths. In the case of TlBaCaCuO, the driving force of c-axis orientation is generally due to a kinetic factor, as the growth rate along the 〈001〉 direction is much faster than that along the 〈100〉 direction, although in this latter case other factors are responsible of the unusual orientation. It is likely that the a-axis Tl-2212 growth is affected by the morphological nature of the B-LaAlO3 substrate. Indeed, the chemical nature can be ruled out because different

A full MOCVD route has been applied to reproducibly and selectively fabricate a-axis oriented TlBaCaCuO films on LaAlO3(100)/SrTiO3(100) substrates. The obtained results seem to lend support to the main role played by the substrate (i.e., its morphological characteristics) in promoting the a-axis orientation of films. Our findings represent interesting issues in the perspective of fabrication of bi-epitaxial Josephson junctions. Further work is in progress in order to study the buffer-layer/superconducting film interface and to assess the role of the LaAlO3 buffer layer roughness on the growth of the a-axis oriented TlBaCaCuO films. Acknowledgment. We thank the Consiglio Nazionale delle Ricerche (CNR, under Contributo 5%) and the Ministero dell’Istruzione, dell’Universita` e della Ricerca (MIUR, Rome, under Progetto Cluster 14 P.E. 2) for financial support. CM034979+