Aerosol assisted chemical vapor deposition of superconducting YBa2Cu3O7−χ

PHYSICA

Physica C 198 (1992) 303-308 North-Holland

Aerosol assisted chemical vapor deposition of superconducting YBaECuaO7_x K.V. Salazar, K.C. Ott, R.C. Dye, K.M. Hubbard, E.J. Peterson and J.Y. Coulter Los Alamos National Laboratory, Los Alamos, NM 87545, USA

T.T. Kodas Chemical and Nuclear Engineering Department and Center for Microengineered Ceramics, University of New Mexico, Albuquerque, NM 87131, USA

Received 31 March 1992 Revised manuscript received 8 June 1992

A hybrid process, aerosol-assisted chemical vapor deposition (AACVD), is described for reproducible preparation of superconducting thin films of YBa2Cu307_x. The process consists of atomizing a toluene solution of the Y, Ba, and Cu tetramethylheptanedionato complexes using an aerosol generator. The aerosol is transported into a CVD reactor where solvent and precursor evaporation and deposition occur at atmospheric pressure on heated suhstrates. The process provides stable evaporation rates for all three precursors, yielding constant film stoichiometry throughout the deposition period and from film to film. Superconducting films may be deposited in-situ at substrate heater temperatures above 825 °C, or may be formed at lower temperatures by deposition followed by post-deposition annealing at higher temperatures. The microstructure and quality of films are highly dependent on the conditions employed in deposition and in the case of films deposited below 825 °C, the post-deposition annealing conditions. Superconducting films prepared by the AACVD/post-annealing process have a metallic normal state resistivity signature with a zero resistance temperature typically above 88K, and are highly c-axis oriented. Transport critical current densities measured at 75 K on polycrystalline films prepared by the AACVD process are 220 000 A/cm 2 and 84 000 A/cm2 at self-field and 0. l T, respectively.

1. Introduction M a n y authors have r e p o r t e d the p r e p a r a t i o n o f superconducting thin films o f YBa2Cu3OT_x (Y- 123) by way o f chemical v a p o r d e p o s i t i o n ( C V D ) [1]. The potential for d e p o s i t i o n o f s m o o t h films coupled with the capability o f d e p o s i t i o n or infiltration into porous materials or onto n o n - p l a n a r substrates m a k e C V D an attractive d e p o s i t i o n technique for applic a t i o n to high t e m p e r a t u r e superconductors ( H T S ) . The d e v e l o p m e n t o f C V D processes for thin film deposition o f H T S has lagged far b e h i n d physical v a p o r d e p o s i t i o n techniques, were it has been shown that films m a y be routinely grown in-situ by laser ablation or sputtering [2 ]. A p p l y i n g C V D to H T S materials has been plagued by several difficulties such as the lack o f easily handled, stable precursors [ 3 ], particularly for the alkaline earth metals such as bar-

ium [4], the relatively high processing temperatures, a n d the large n u m b e r o f p a r a m e t e r s that must be controlled to m a i n t a i n stoichiometry during the p e r i o d o f deposition. Perhaps the most difficult p r o b l e m that has i m p e d e d the successful C V D o f H T S such as Y-123 has been the former, the lack o f easily handled, readily available volatile precursors. The precursors that have been used are m o d e r a t e l y volatile metal acetylacetonate derivatives such as the t e t r a m e t h y l h e p t a n e d i o n a t e s (the d e p r o t o n a t e d ligand is often referred to as t m h d ) or fluorinated derivatives such as the hexafluoroacetylacetonates (hfac) or heptafluorodimethyloctanedionates (fod). The t m h d complexes o f Y, Ba, a n d Cu have been most widely used precursors for C V D o f Y-123 films to date. The b a r i u m derivative, Ba (tmhd)2, has been found to have an irreproducible a n d variable transport rate because o f gas phase a n d solid state oli-

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K.V. Salazar et aL / Aerosol assisted CVD of YBCO thin films

gomerization and/or hydrolysis reactions [ 3,4 ]. Because of the long residence times in the sublimation vessels and associated plumbing to which the metal precursors are subjected, the stability of the precursors is critical in maintaining deposition rate control and therefore stoichiometry control. The fod or hfac derivatives of Ba partially alleviate these problems, but their use requires the post-deposition hydrolysis of BaF2 to generate the oxide superconducting phase. Because of the required post-deposition hydrolysis step, the in-situ growth of Y-123 through fluorinated derivatives may be difficult. To alleviate some of the problems associated with low-vapor-pressure precursors that Ba(tmhd)2 appears to have, we have developed a variant to conventional CVD we refer to as "aerosol-assisted CVD" (AACVD). This technique has been used for singlecomponent systems [ 5 ] and has recently been extended to the Y - B a - C u - O system [6,7 ]. This technique minimizes the residence time at high temperature that any of the precursors must survive before deposition. The AACVD technique relies on a different mode of precursor introduction than conventional CVD. In a conventional CVD reactor, the precursors are placed into separate vessels that are heated to evaporate the materials, and then the vapors are swept with a carrier gas through heat-traced lines into a chamber where deposition occurs. In AACVD of Y-123, the volatile precursors are dissolved in a suitable solvent and the solution is then drawn into a nebulizer and atomized. The key difference between the approach in this work and earlier work by Driessen et al. [ 6 ] and Tang et al. [ 7 ] is that in this work the droplets first pass through a warm-wall system when the solvent and precursors evaporate before reaching the substrate to form the vapors of the individual metal tetramethylheptanedionate complexes. Deposition and decomposition of the metal tetramethylheptanedionates occur downstream on a heated substrate; see fig. 1 for a schematic of the AACVD reactor. Although the precursor introduction for AACVD is different from conventional CVD, once the solvent and precursors are evaporated, the reaction dynamics are the same for both methods. In principle, this hybrid technique of AACVD allows for the use of poorly volatile precursors because the aerosol may be transported into the preheated zone of the reaction chamber than can be held at a tem-

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Fig. 1. Schematicof the AACVDreactor used in this work. perature just sufficient to vaporize the compound. The aerosol containing the precursor(s) remains at no greater than ambient temperature until the aerosol is injected into the preheating zone of the reaction chamber and residence times are only about 0.5 second in the preheat zone before deposition on the heated substrate. Therefore in principle thermally sensitive precursors may be used. The solvent chosen to dissolve the precursors must be unreactive with the precursor, and must have a vapor pressure sufficient to be completely evaporated at the temperature of the preheating zone. If the precursor is highly soluble in the solvent, the mass transport of the precursor into the CVD chamber may be quite high, as aerosol generation rates of 0.5 g/ min of solvent are easily attained using ultrasonic nebulizers. The aerosol generated has a droplet size of the order of 0.1 to l micron. For most common organic solvents that have high vapor pressures at around room temperature, solvent evaporation rates of particles about 1 micron are high enough for the solvent to be completely evaporated in a few milliseconds [ 8 ], so the probability of droplets surviving long enough to contact the substrate (i.e., spray pyrolysis) is negligible. Another feature of AACVD as we have practiced it is the ability to operate at ambient pressure. Because of these features, AACVD reactors are inexpensive and simple to operate as compared to conventional CVD reactors.

2. Experimental The deposition of Y-123 by the AACVD process was accomplished by dissolving Y(tmhd)3, Ba(tmhd)2, and Cu(tmhd)2 (Strem Chemicals, Inc.)

K. V. Salazar et al. / Aerosolassisted CVD of YBCOthinfilms

in toluene in the molar ratio of 1 : 2.67: 2.78. The solution was passed through a commercial aerosol generator (TSI, Inc.). The carrier gas was 1% 02 in Ar. Caution: an oxygen concentration above 10% 02 with this concentration o f toluene may detonate. Check the explosive limits o f any solvent before attempting this process. The aerosol was passed at a flow rate of 4 L / min into a horizontal 30 m m i.d. quartz tube heated to a wall temperature of 200-300°C at 580 Torr (atmospheric pressure in Los Alamos). Deposition occurred on an electrically heated substrate. Substrate temperatures were measured with an optical pyrometer. The substrates ( 1 X 1 c m 2 ) chosen were Si (for determination of cation stoichiometry by Rutherford backscattering spectroscopy) or [ 100 ] LaA103 or MgO. The deposition rate at a substrate heater temperature of 725 °C on [ 100] LaA103 and the solution concentration stated above is 3 microns/h. The AACVD process is highly reproducible with regard to stoichiometry, deposition rate, and superconducting properties on [ 100 ] LaAIO3. The films as deposited at a substrate temperature of 725 °C by AACVD were amorphous as determined by X R D and had a stoichiometry within 5% of Y: 2Ba: 3Cu using the solution composition above. The films deposited on MgO using the same solution composition and substrate heater temperature were for an unknown reason of significantly variable stoichiometry, thickness, and quality. Because of these reasons, lanthanum aluminate became the substrate of choice and was the substrate used for the rest of the experiments discussed below. The amorphous films deposited on lanthanum aluminate exhibited columnar morphology and were poorly adherent. Post-deposition annealing was necessary to generate adherent, crystalline superconducting films. Annealing was carried out by heating the films in various partial pressures of oxygen to elevated temperatures, holding for 15 minutes to one hour, and then cooling in 100% oxygen. The success of post-deposition annealing to generate superconducting films was highly dependent on the partial pressure of 02 in the annealing furnace during the initial heating and also on the ultimate temperature of annealing. The surface morphology of the post-annealed films was also very dependent upon these parameters. The best superconducting films deposited by AACVD at a substrate temperature of 725°C were prepared by

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heating the film in less than 6 Torr 0 2 to a temperature of 880°C, switching to 100% 0 2 and then heating to 935 °C before cooling to room temperature. At higher oxygen pressures or lower temperatures, the films remained poorly crystalline or amorphous and the superconducting properties were poor. There was an abrupt increase in crystallinity of films annealed above 890 °C in 6 Torr 02. XRD of crystalline films indicated the films are well oriented with the c-axis perpendicular to the plane of the substrate. These films appeared to be polycrystalline with grain sizes of 1-10 microns as determined by SEM. Helium ion channeling yields of only 15% were consistent with the polycrystallinity of the films. The normal DC resistivity signature of the film was metallic. The zero resistance temperature was routinely above 88 K, with most films above 90 K with sharp transitions to the superconducting state. Dynamic impedance measurements as a function of temperature also showed sharp transitions with the absence of tailing to lower temperatures. Critical current densities of one of these films were measured and had a zero field J¢ of 80 000 A / c m 2 at 75 K. The field dependence of the critical current density is shown in fig. 2. Examination of SEM micrographs of the crystalline films showed smooth terraces suggesting that the films may have been formed in the presence of a liquid phase. Closer examination of a fractured surface of a crystalline film of 3 micron thickness indicated there was a dense layer of approximately 1 micron thickness adjacent to the substrate that may have evolved from a melt (fig. 3 ( a ) ) . On top of this dense layer was a less dense appearing layer of Y-123 that had the same columnar morphology as the as-deposited films. These features along with the sudden change of crystallinity and superconducting properties of the films annealed above 880-890°C in 6 Torr O2 suggested that a molten phase formed at around 880-900°C from which the dense layer o f Y 123 in contact with the substrate crystallized. We suspected that the dense material might have superior electrical properties compared to the columnar material, and that the dense layer might also form without the columnar material in thinner films. Indeed, films of 1.5 micron thickness lacked the top coating of columnar material (fig. 3 ( b ) ) , were more highly c-axis oriented, and had significantly higher critical current densities ( > 200 000 A / c m 2 versus

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K. V. Salazar et al. / Aerosol assisted CVD o f YBCO thin films 3x105

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Fig. 2. Field dependence Jc vs. Happlfor Y-123 films of 3 I.tmthickness (O) and 1.5 ~tm thickness (A). Happlis perpendicular to the plane of the film.

Fig. 3. SEM micrographs of fracture surfaces of 3 I.tm (A) and 1.5 ~tm (B) thick films. Arrows indicate the location of the dense layer adjacent the substrate. The micrograph A of the 3 lam thick film indicates the columnar, less dense material on top of the dense, thin layer adjacent to the substrate. The columnar overlayer is absent in the thinner film (B). 80 000 A / c m 2) (fig. 2) than the 3 micron thick films. The critical current densities of these films prepared by A A C V D is similar to polycrystaUine thin films o f Y- 123 prepared by P V D techniques [9 ]. Epitaxial thin films o f Y- 123 typically have Jcs exceeding 106 A / c m 2 at zero field and 77 K; polycrystalline films prepared under these conditions have Jcs o f about 104-105 A / c m 2 [9]. The lower value of J¢ for polycrystalline films has been attributed to high angle grain boundaries acting as weak links [ 10 ]. The mechanism o f deposition involved in AACVD is difficult to probe. Possibilities range from simple droplet contact with the substrate to CVD. There are

also plausible mechanisms intermediate to those extremes that are in diagram in fig. 4. It is possible that complete volatilization o f the precursors has not occurred, and deposition of precursor particles on the substrate occurs. It is also possible that in the presence o f oxygen in the carrier gas, particles a n d / o r vapors o f the tetramethylheptanedionato metal complexes react with oxygen to generate oxide particles that then deposit on the substrate. This latter mechanism may also occur in conventional C V D processes. U n d e r the conditions that the A A C V D films are deposited, it is highly unlikely that any particles of tmhd metal complexes remain unevaporated to

K. V. Salazar et aL / Aerosol assisted CVD of YBCO thin films

Solvent Evaporation

Evaporation +S(g) Intraparticle Reaction

307

+S(g) Precursor Evaporation

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Fig. 4. Summary of possible mechanisms for generation of films from aerosol deposition conditions. From left to right: spray pyrolysis, particle deposition arising from unevaporated or involatile reagent (s), chemical vapor deposition, and particle deposition arising from reagent (s) evaporated prior to deposition.

reach the substrate. As mentioned above, SEM micrographs of amorphous, unannealed films deposited at 725 °C indicate columnar growth in most areas suggesting that growth is most likely by vapor deposition while in some areas there is evidence of particles that have deposited. At the high operating pressures used in these studies, panicles should veer away from the relatively hot substrate because of thermophoresis [ 8 ], and the panicle deposition rate should drop dramatically with increasing substrate temperature. This experiment is complicated by the fact that if the substrate temperature is changed the film stoichiometry is also changed. Nevertheless, using a fixed solution composition and deposition time, the deposition rate decreased by a factor of 2.6 when the substrate temperature was increased from 425 °C to 725°C. This would suggest that panicle deposition may be an important contributor to the overall deposition mechanism. On the other hand, that the film composition differs markedly from the solution composition and the observation that the film composition is sensitive to the substrate material suggests that vapor deposition is operative. At this time, we must conclude that there is a mixed mechanism

of vapor and particle deposition operating for AACVD of Y-123 under these conditions, but that the major pathway is vapor deposition. There are parameters that can be altered to influence the relative efficacy of each mechanism, such as operating pressure and temperature. We have observed that films deposited at low substrate temperatures of between 400 ° and 460°C are shiny and smooth. This is indicative that vapor deposition may be the major contributing mechanism at lower substrate temperatures. Above 460°C, the films become rougher due to either the growth mechanism changing or as particle deposition begins to occur that subsequently nucleates growth from the panicles leading to rougher films. At substrate temperatures above 850 °C and in the presence of O2, Y-123 may be deposited in-situ that is highly c-axis oriented as indicated by X-ray diffraction. The in-situ grown AACVD films tend to be shiny and smoother than the post-annealed films, but to date the in-situ films have not been optimized and have much poorer electrical properties compared to the post-annealed films.

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3. Summary A h y b r i d process t e r m e d aerosol-assisted C V D has been described for preparing c-axis o r i e n t e d superconducting films o f Y-123 using t e t r a m e t h y l h e p t a n e d i o n a t o complexes o f Y, Ba, a n d Cu. The process involves forming an aerosol o f a solution to transport the thermally unstable B a ( t m h d ) 2 complex simultaneously with the Y a n d Cu t m h d complexes into a C V D reactor. F i l m s d e p o s i t e d at 725°C on single-crystal LaAIO3 a n d post-deposition a n n e a l e d have sharp transitions to the superconducting state with R = 0 typically above 88 K a n d critical current densities o f greater than 105 A / c m 2. These results are c o m p a r a b l e to those o b t a i n e d by other m e t h o d s to prepare polycrystalline films. The two-step post annealing process yields highly reproducible results. This reproducibility is due to the short residence time the volatile precursors are at high t e m p e r a t u r e p r i o r to deposition. This p r o v i d e s a stable d e p o s i t i o n rate versus time for each o f the precursors, a n d hence stable film stoichiometry throughout the p e r i o d o f deposition, a n d from film to film. F i l m s f o r m e d in-situ at higher t e m p e r a t u r e s are highly oriented, but have much p o o r e r electrical properties c o m p a r e d to postannealed films. This is perhaps due to changes in deposition m e c h a n i s m at the higher d e p o s i t i o n temperatures resulting in changes in stoichiometry that negatively affect the electrical properties.

Acknowledgements We would like to acknowledge the following people for their assistance: J. Tesmer, M. H o l l a n d e r a n d C. Evans o f the Ion Beam Materials Lab; P. A r e n d t

for patterning o f the films; and X.D. Wu and R.E. Muenchausen for useful discussions. This work was s u p p o r t e d by the D e p a r t m e n t o f Energy Superconductivity Pilot Center at LANL.

References [ 1] For CVD references,see: H. Yamane, T. Hirai, K. Watanabe, N. Kobayashi, Y. Muto, M. Hasei and H. Kurosawa, J. Appl. Phys. 69 ( 1991 ) 7948; C.S. Chern, J. Zhao, Y.Q. Li, P. Norris, B. Kear, B. Gallois and Z. Kalman, Appl. Phys. Lett. 58 ( 1991 ) 185; Y.Q. Li, J. Zhao, C.S. Chern, W. Huang, G.A. Kulesha, P. Lu, B. Gallois, P. Norris, B. Kear and F. Cosandey, Appl. Phys. Lett. 58 (1991) 648. [ 2 ] For PVD and laser ablation references, see: N. Newman, B.F. Cole, S.M. Garrison, K. Char and R.C. Taber, IEEE Trans. Magn. 27 ( 1991 ) 1276; J. Talvacchio, M.G. Forrester, J.R. Gavaler and T.T. Braggins, IEEE Trans. Magn. 27 ( 1991 ) 978; S.R. Foltyn et al., Appl. Phys. Len. 59 ( 1991 ) 1374. [3 ] G. Malandrino, D.S. Richeson, T.J. Marks, D.C. DeGroot, J.L. Schindler and C.R. Kannewurf, Appl. Phys. Lett. 58 (1991) 182. [4] R. Gardiner, D.W. Brown, P.S. Kirlin and A.L. Rheingold, Chem. Mater. 3 ( 1991 ) 1053. [ 5 ] J.C. Vignie and J. Spitz, J. Electro Chem. Soc. 122 ( 1975 ) 585. [ 6 ] A. Driessen, Q. Tang, L. Hilderink and Th.J.A. Popma, MRS Symp. Proc. 169 (1989) 601. [7] Q. Tang, A. Driessen, P. Hockstra, L.T. Hilderink, A. Van Silfhout and Th.J.A. Popma, J. Less-Common Met. 164165 (1990) 1587. [8] W. Hinds, Aerosol Technology (Wiley, NY, 1982) [9] D.P. Norton, D.H. Lowndes, D.K. Christen, E.C. Jones, J.D. Budai, T.D, Ketcham, D. St. Julien, K.W. Lay and J.E. Tkaczyk, Science and Technology of Thin Film Superconductors 2, eds. R.D. McConnell and R. Noufi (Plenum, New York, 1990 ) p. 157. [io] D. Dimos, P. Chaudhari, J. Mannhart and F.K. LeGoves, Phys. Rev. Lett. 61 (1988) 1653.