Deposition of YBCO Thin Film by Aerosol Assisted ... - IEEE Xplore

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 21, NO. 3, JUNE 2011

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Deposition of YBCO Thin Film by Aerosol Assisted Spray Pyrolysis Using Nitrates Byeong-Joo Kim, Seok-Kwan Hong, Jae-Geun Kim, Jung-Ho Kim, S. X. Dou, L. Dunlop, A. Kursumovic, J. L. MacManus-Driscoll, Hee-Gyoun Lee, and Gye-Won Hong

Abstract—Coated conductor by MOCVD shows the best Ic L performance currently, but cost reduction is still ongoing issue. R&D effort for a process capable of utilizing cheap alternative precursors were tried by many research groups but few of them showed results having potential for replacing current MOCVD. Spray pyrolysis method adopting ultrasonic atomization was tried as one of the possible options. Y123 films have been deposited on LaAlO3 (100) single-crystal and IBAD substrates by aerosol assisted spray pyrolysis method. Ultrasonic atomization was used in order to generate fine droplets of precursor solution made of Y, Ba, Cu nitrate. A pre-heater was located between spraying nozzle and substrate for fast drying and enhancing decomposition of precursors. SEM and XRD observation revealed that deposited films have smooth and dense microstructure. The influence of operating parameters such as cation stoichiometry, oxygen partial pressure, substrate temperature on the microstructure, formation of superconducting 123 phases and superconducting properties of deposited films were tested. Ex-situ conversion was tried to decrease the possible reactions between precursor compounds and buffer layer materials of metal substrate showed the possibility of adopting this technique for epitaxial growth of 123 phase on metal substrate. Index Terms—CVD, nitrate, spray pyrolysis, YBCO.

I. INTRODUCTION OATED conductor is expected to be used as an electric wire for applications in electric power devices [1]. In recent years, long-length coated conductors have been successfully fabricated on RABiTS (rolling assisted biaxially textured substrates) and IBAD (ion beam assisted deposition) metallic substrates [2], [3]. High performance REBCO superconducting layers have been successfully fabricated by various techniques such as PLD (pulsed laser deposition) [4], MOD (metal organic deposition) [5], and MOCVD (metal-organic chemical vapor deposition) [6]–[8]. Among these processes, MOCVD is one of the most promising techniques for the mass production of

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Manuscript received August 03, 2010; accepted October 31, 2010. Date of publication December 23, 2010; date of current version May 27, 2011. This work was supported by the Australian Research Council Linkage International Project (LX0989591) and Manpower Development Program for Energy & Resources supported by the Korean Ministry of Knowledge and Economy (MKE). B.-J. Kim, S.-K. Hong, H.-G. Lee, and G.-W. Hong are with Korea Polytechnic University, Shiheung, Korea (e-mail: [email protected]). J.-G. Kim, J.-H. Kim, and S. X. Dou are with ISEM, University of Wollongong, Wollongong, NSW 2500, Australia. L. Dunlop, A. Kursumovic, and J. L. Driscoll are with University of Cambridge, UK. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2010.2091931

coated conductors because of its advantages such as high deposition rate, good film uniformity, scalability and large area deposition. However, the use of expensive organic precursors such as tetramethylheptanes-dionate (TMHD) as starting materials is the main disadvantage of the MOCVD technique, and it is necessary to reduce the cost of precursors for MOCVD to give it practical competitiveness with other processes such as MOD. Therefore, it is important to find less expensive organic precursors or to develop a CVD process that can use less expensive inorganic precursors such as nitrate, chloride and sulfate. Metal inorganic precursors have extremely low vapor pressure and it is hard to obtain the vapor of metal-inorganic precursors. Therefore aerosol assisted spray pyrolysis deposition is adopted to accelerate the decomposition of inorganic precursors and enhance the film deposition. Many research works carried out by others [9]–[15] and our previous studies [16]–[18] showed a possibility for fabricating high quality YBCO superconducting film on LaAlO (100) single crystal by aerosol assisted spray pyrolysis method using an inexpensive nitrate precursor. However, the deposition on metal substrate didn’t show good results due to the interaction between buffer layer and superconducting YBCO layer, possibly because of higher deposition temperature and more corrosive atmosphere reasoned to decompose nitrate-based aqueous precursor solution. To overcome those issues, two approaches were tested in this study. One is a preheating of the precursor mist prior to deposition at high temperature and another is two step process consisted of depositing films at low temperature followed by conversion at high temperature. II. EXPERIMENTAL As a source for the aerosol, precursor solutions were prepared H O (99.9%, Aldrich), Ba NO by dissolving Y NO (99.999%, Aldrich) and Cu NO H O (77–80%, Shinyo Pure Chemical Co.) in distilled water separately. The solutions were then mixed together and diluted to the desired composition and concentration. A starting solution with a cation ratio of was used for deposition of films. Fig. 1 Y Ba Cu shows the schematic diagram of spray pyrolysis system using ultrasonic nebulizer. An ultrasonic transducer used in household humidifier was used to generate fine droplets of precursor solution which are carried into the reactor by Ar/O mixture gas. A quartz nozzle with pipet shape was used to connect the mist generating chamber and deposition chamber. A pre-heater was inserted between spraying nozzle and substrate to accelerate drying and decomposition of precursors. The distance between substrate and nozzle is controlled between 15–20 cm. The preheater and substrate heater can be heated to 1000 C. LaAlO

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Fig. 1. Schematic diagram of spray pyrolysis system using ultrasonic nebulizer.

(100) single crystal and IBAD template prepared by SuNAM (LaMnO Epi-MgO/IBAD-MgO/Y O Al O Hastelloy, from here after IBAD substrate) was used as substrate. The base pressure of the deposition chamber and oxygen partial pressure was maintained at 10–15 torr and 1–5 torr respectively. Texture and second phases of YBCO films were analyzed by scan. Microstructure and X-ray diffraction (XRD) of thickness of the films were characterized using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The critical current was measured by a four-probe method with the criterion of 1 V/cm at 77 K in a self-magnetic field.

Fig. 2. The XRD patterns of YBCO films deposited (a) on LAO (100) and (b) on IBAD substrates.

III. RESULTS AND DISCUSSION We experienced high reactivity problem between buffer layer and YBCO layer in our previous studies using concentric nebulizer to generate aerosol [16]–[18], and stability of the process was not enough for long continuous operation. To solve the problems, ultrasonic transducer was used to generate aerosol of precursor solution. As a result, we achieved better control of mist size by changing the transducer input power, and ease of carrying the mist into the deposition chamber by using quartz nozzle, and better flow stability. The insertion of cylindrical pre-heater brought the complete decomposition of nitrate precursor and less reactivity and better buffer stability was anticipated. But excessive preheating resulted in the rough surface morphology caused by homogeneous 123 phase formation within preheating zone and so the preheating temperature was controlled below 800 C. Figs. 2(a) and 2(b) show the X-ray diffraction patterns of YBCO films deposited on LAO(100) single crystal substrate and on IBAD substrate, respectively. The temperatures of substrate and pre-heater were 710–750 C and 500 C, respectively. aligned YBCO phase In Fig. 2(a), we can see very good in the temperature range of 730–750 C. The intensity of (005) peaks increase with the deposition temperature. Fig. 3 shows the SEM surface microstructure and result of Ic measurement. Well connected YBCO grains with smooth surface morphology were observed, and the thickness of the film was 0.3–0.6 m. The Ic of deposited film was measured as 2–3 A for 4 mm width

Fig. 3. (a) SEM image of YBCO film deposited on LAO (100) single crystal at 745 C with oxygen partial pressure of 1.5 torr and preheating of 500 C. (b) I-V curves for Ic measurement. Inserted SEM image shows the cross section image.

samples, showing intensive optimization works of the process parameters such as temperature, pressure and compositions are required. Fig. 2(b) showed the effect of substrate moving speed on XRD patterns for the film deposited at 770 C on IBAD substrate with preheating of 450 C and oxygen partial pressure of 1 torr. The film deposited with high moving speed showed the c-axis growth of YBCO, but film deposited with low moving speed showed that the c-axis growth of YBCO was not observed. Also, the texture of the buffer layer was ruined by the reaction with the YBCO compound because of long exposure of metal substrate at highly reactive deposition atmosphere. This results lead us to the conclusion that the deposition of high quality thick YBCO film is very difficult because two operating parameter

KIM et al.: DEPOSITION OF YBCO THIN FILM BY AEROSOL ASSISTED SPRAY PYROLYSIS USING NITRATES

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Fig. 5. SEM images of YBCO film on IBAD substrate (a) as deposited at 500 C and (b) converted at the various temperatures in Ar atmosphere with 1000 ppm oxygen.

Fig. 4. The X-ray diffraction patterns of YBCO films deposited at 500 C (a) on LAO single crystal and (b) on IBAD substrate. Conversion was done at described temperatures in Ar atmosphere with 1000 ppm oxygen.

conflict each other, such as the long deposition at high temperature for high quality thick film and short deposition at low temperature for buffer stability to maintain texture. It seems that we need to find more stable buffer layer to overcome this difficulties or modify the process. To solve the confliction, ex-situ conversion route of the YBCO after deposition at low temperature is introduced. A precursor YBCO film was deposited at 500 C and heat treated at higher temperature in various oxygen partial pressure for conversion to Y123 phase. Figs. 4(a) and 4(b) are the XRD patterns for the film deposited at 500 C followed by conversion heat treatment at 700–900 C in flowing O Ar mixture gas atmosphere on LAO single crystal and IBAD substrate, respectively. As deposited films on both substrates showed no 123 phase peaks except for weak CuYO . And we can assume that precursor film is consisted of decomposed amorphous oxides of Y, Ba and Cu. The film after conversion heat treatment showed strong c-axis peaks on both substrates. The conversion seems peaks increases to begin at around 800 C and intensity of with the increase of conversion temperature. Fig. 5 shows the SEM microstructure of as deposited films on IBAD substrate and after conversion treatment at various temperatures. As deposited film showed good surface coverage. The separated randomly oriented columnar grains became connected as the

Fig. 6. (a) XRD patterns and (b) SEM images of YBCO film converted at the different temperatures from 800 C to 930 C in Ar atmosphere with 500 ppm oxygen.

conversion temperature increased. As the conversion temperature increases, the fraction of aligned grain increases with the decrease of the columns and mostly well connected aligned grains are observed in the sample converted at 900 C. Fig. 6 shows the XRD patterns and SEM microstructure with EDS composition analysis for the films deposited at 500 C and converted at the temperature range of 880 C–930 C in 500 ppm O Ar atmosphere. All sample showed good c-axis oriented peaks with small (103) peaks in XRD and well connected dense surface morphology by SEM observation. Also, The films were thicker than 1 m in most cases, and the film composition analysis by EDS noted in the figures matches well with the starting composition of 1:2:3 except little shortage of Cu. This result suggested that the material yield of this process may be very high, and which can be a great advantage in process economy and process control. The reduction of oxygen partial pressure from 1000 ppm to 500 ppm enhance the aligned c-axis

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growth of the 123 grains as the amount of liquid phase maybe increased at lower oxygen partial pressure. This work is still in progress and the effect of conversion conditions and film stoichiometry are scheduled.

IV. CONCLUSION The possibility of aerosol assisted spray pyrolysis method using metal nitrate precursor for epitaxial deposition of YBCO superconducting layer was proved. Epitaxial YBCO film was successfully prepared on LAO(100) single crystal substrate by moving in-situ deposition. The results of in-situ deposition on metal substrates showed that thermal stability of buffer layer was not enough for resisting high temperature and reactive atmosphere when substrate stayed in deposition zone for a long time. Ex-situ conYBCO layer could version route showed highly aligned be deposited on metal substrate. The resulting stoichiometry of ex-situ converted YBCO layer coincides well with that of starting precursor solution.

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