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PREPARATION OF SUPERCONDUCTING-OXIDE FILMS BY CVD AND THEIR PROPERTIES
H. Yamane, H. Kurosawa, T. Hirai
To cite this version:
H. Yamane, H. Kurosawa, T. Hirai. PREPARATION OF SUPERCONDUCTING-OXIDE FILMS BY CVD AND THEIR PROPERTIES. Journal de Physique Colloques, 1989, 50 (C5), pp.C5-131-C5-140.
�10.1051/jphyscol:1989519�. �jpa-00229542�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au no5, Tome 50, mai 1989
PREPARATION OF SUPERCONDUCTING-OXIDE FILMS BY CVD AND THEIR PROPERTIES H. YAMANE, H. KUROSAWA* and T. HIRAI
Institute for Materials Research, Tohoku University, ~ a t a h i r a 2-1-1, Sendai 980, Japan
* R I K E N Co. Kumagaya 810, Kumagaya 360, Japan
~ 6 s u m g - Des couches d'oxydes supraconducteurs constitugs de Y-Ba-Cu-0 (YBCO) et Ri-Sr-Ca-Cu-0 (BSCCO) ont 6t6 prgpar6es par le procgdg CVD 5 partir de chglates mgtalliques et d'alkoxyde de bismuth volatils.
a analyse thermogravim6trique a montr6 que les ch6lates pouvaient 6tre vaporis6s vitesse constante, pour une temp6rature donnge, pendant plusieurs heures sans se dgcomposer. Les couches YBCO sont gpitaxiges 2 8 0 0 - 9 0 0 ~ ~ sur SrTi03 monocristallin orient6 (100) avec l'axe c perpendiculaire au plan ( 1 00) du substrat. Les grains de forme rectangulaire sont align6s suivant 2 directions dans le plan des c o u c h e s . L e s c o u c h e s o b t e n u e s 2 8 5 0 o n t u n e t r a n s i t i o n ~ ~ supraconductrice avec une rgsistivit6 nulle S 9 3 K . La densits de courant maximum est sup6rieure 5 3 X 104 ~ / c m ~ B 77 K. Les couches BSCCO d8posGes B 770°c sur substrat MgO orient6 (100) ont subi un traitement th'ermique 5 890 OC pendant 5 h sous O2 a la pression atmosph6rique. Les couches ainsi traitges sont constituges d e diffgrentes phases appartenant au systgme BSCCO dont l'une a un paramstre CO = 3,7 nm. Ces couches ont une structure granulaire du type mica. La transition supraconductrice dgbute 5 115 K et la r6sistance commence diminuer vers 110 K et s'annule 2 77 K.
Abstract - The superconducting oxide films of Y-Ba-Cu-O(YBC0) and Bi- Sr-Ca-Cu-0 (BSCCO) systems were prepared by CVD using B-diketone m e t a l c h e l a t e s a n d b i s m u t h a l k o x i d e a s s t a r t i n g m a t e r i a l s . Thermogravimetric analysis revealed that the chelates vaporized at a constant rate at a fixed evaporation temperature for several hobrs without decompositions. YBCO films grew epitaxially along the direction of the c-axis which was perpendicular to the single crystal SrTi03(100) substrates at 8 0 0 - 9 0 0 ~ ~ . Rectangular grains aligned two- dimensionally in the planes of these films. The films prepared at 8 5 0 ~ ~ showed superconducting transition with zero resistivity at 93 K.
The maXimum current density was above 3x104~/cm2 at 77 K. BSCCO films were prepared on MgO(100) substrates at 7 7 0 ~ ~ and were annealed at 8 9 0 ~ ~ for 50 h under 1 atm of oxygen. The annealed films had a phase with c0=37 A as well as other phases in the BSCCO system. These films showed a mica-like grain structure. Superconducting transition began at 115 K and resistance started to drop at around 110 K attaining zero resistivity at 77 K.
1
-
INTRODUCTIONA number of physical and chemical film preparation techniques have been investigated so that the knowledge can be applied to produce high temperature (Tc) superconducting oxide films that can be used as electron devices and as energy transport systems. Chemical vapor deposition (CVD) is the most well known technique for the preparation of semiconductor films, insulator films and ceramic coatings. The discovery of YBa2Cu307-x(YBCO) which showed a superconducting transition above the boiling point of liquid nitrogen was r e p o r t e d i n 1 9 8 7 by W u e t al./l/. H o w e v e r , t h e p r e p a r a t i o n o f superconducting oxide films by CVD was not reported until a year after its discovery by the present author /2/ and Berry et a1./3/. The most important reason of this delay was that a suitable source material of the barium metal for CVD was not known.
The present authors and Berry et al. were the first to succeed in
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989519
C5-132 JOURNAL DE PHYSIQUE
preparing YBCO superconducting films by CVD using B-diketone metal chelates as source materials. They were previously used as a source material for CVD of binary oxides such as Zr02-Y303 and Mn02-Fe20 /4,5/.
The authors have prepared superconducting ~ B C O films by in-situ oxygen treatment of films obtained at 8 0 0 ~ ~ and 10 Torr on polycrystalline 8 mol%
yttria stabilized zirconia (YSZ) substrates/2,6/. YBCO films with c-axis orientation perpendicular to the substrate were prepared at 9 0 0 ~ ~ / 7 / . These films received in-situ oxygen treatment and showed Tc(resistivity zero:R=O) above the liquid nitrogen temperature of 77 ~ 1 8 1 . Recently, epitaxially grown YBCO films on the SrTi03 substrate showed T (R=O) = 84 K with a critical current density (Jc) of 2x10' ~ / c m ~ at 77
KI~F.
Berry et al. prepared superconducting YBCO films by heat treating amorphous films at 9 2 0 ~ ~ . The amorphous films were prepared by CVD at 4 0 0 ~ ~ and under ambient pressure/3/. Nakamori et a1./10/ and Abe et al. /l11 have also prepared YBCO films via CVD by use of the same source materials as in the present study. In the Nakamori et al. study, heat treatment was carried out on the amorphous films prepared at 6 0 0 ~ ~ . Abe et al. prepared YBCO film at 6 0 0 - 7 8 0 ~ ~ without heat treatment and showed TC(R=O) at 65 K.
Shinohara et a1./12/ used B-diketone metal chelates containing fluorine and attempted to obtain YBCO film at 6 0 0 ~ ~ . However, a heat treatment at 8 5 0 ~ ~ was necessary in order to form YBCO films.
Superconducting oxides in the system of Bi-Sr-Ca-Cu-O(BSCC0) were discovered by Maeda et a1.1131 early in 1988. Ihara et al. /l 4,151 first prepared BSCCO films by CVD using iodides a s source materials. The difficulty associated with the use of halides as the source materials is that the halogen gas generated by the decomposition of halide often corrodes the substrates and the reactors. We have successfully prepared BSCCO films by CVD using B-diketone metal chelates and bismuth alkoxide as halogen-free source materials11 6,171. The films showed Tc onset around 90 K and Tc(R=O) at 78 K. There was no evidence, at that time, of successfully preparing a film having Tc of as high as 110 K.
The present paper has a dual purpose: First, it reports the results of thermogravimetric analysis on the the stability of vaporization rate for B- diketone chelates of Y, Ba and Cu at a constant vaporization temperature.
Secondary, the effect of deposition temperature on microstructure, T and 3, of the superconducting YBCO films are described. These YBCO fifms were prepared by CVD on a SrTi03 (100) substrate in an attempt to obtakn a film with higher Tc and Jc. Jn addition, the results of heat treatment of BSCCO film prepared by CVD are reported.
2
-
EXPERIMENTALThe source materials of B-diketone metal chelates (Y(thd)3, Ba(thd) C ~ ( t h d ) ~ and Ca(thd)2t (where (thd) represents 2,2,6,6,-tetramethyl-3,g:
heptanedione) were purchased from Toyo Stauffer Chemical Co. Ltd. The Sr(thd)2 was prepared based on the report of Hammond et a1./18/. Bismuth alkoxide (bismuth triethoxide) was purchased from TRI Chemical Laboratory.
Thermal analysis was carried out by means of a thermogravimetric analyzer (TG) with an infrared furnace(ULVAC DT-300 RH). The samples weighed about ten milligrams. Their weight changes were measured in the flow of Ar (
flow rate: 10 mllmin).
The CVD reactor was a hot-wall type and made of quartz glass. Figure 1 shows a schematic diagram of the CVD reactor for YBCO film preparation. The source materials of B-diketone and bismuth alkoxide were evaporated at various temperatures as listed in Table 1. The source materials were transported to the substrate by Ar gas. Oxygen gas was separately introduced into the reactor. The deposition conditions are also listed in Table 1.
After the deposition of YBCO films, the introduction of source gases to the furnace was halted with exception of the oxygen gas. The films were subjected to in-situ oxygen treatment in which the films were cooled to room temperature at the rate of 15Oc/min under 1 atm of oxygen.
The BSCCO films were cooled to room temperature at the rate of lsoc/min maintaining the deposition total pressure of 1 Torr. These BSCCO films were annealed at 8 9 0 ~ ~ for 5 h in an electric furnace under 1 atm of oxygen.
X-ray diffraction(XRD) patterns of the films o n the substrates were obtained by use of a diffractometer (RIGAKU RAD-B) with CuKa radiation.
Microstructure of the films w a s observed using a scanning electron
microscope(SEM)(Akashi ALPHA-30). Electrical resistivity and critical current were measured by a conventional DC four-probe method with indium electrode.
3
-
RESULTSAND
DISCUSSIONFigure 2 shows the results of the thermogravimetric analysis of Y(thd) Ba(thd)2 and Cu(thd) by TG at a heating rate of 10~C/min. The Y (thd ) 'exhibited two-step weig%t loss starting at around 70°c and then at near I ? ~ O ~ C , reaching total evaporation at 310 C. Similar two-step weight loss was also observed for Ba(thd)2 around 90°c and 2 6 0 ~ ~ . However, there remained approximately 30% of the residue at even 4 6 0 ~ ~ . Schwarberg et al.
/19/ also observed 1 1 % of residue in their TG analysis of Ba(l~hd)~. They assumed that this residue was oxides derived from the thermal decomposition of Ba(thd) Figure 2 also shows that C ~ ( t h d ) ~ continually lost weight beyond 100~$'and reached to total evaporation at around 2 9 0 ~ ~ .
The weight loss relationships of these chelates with heating time at a fixed evaporation temperature are shown in Figs. 3-5. Each sample was heated from room temperature to evaporation temperature for a duration of 10 minutes. As shown in Fig. 3(a) and (b), the weight of Y(thd) at 150 and 1 7 0 ~ ~ decreased about 10% from the initial weight during the Zirst several minutes and then decreased linearly with time. When the same sample was tested again after the experiment of (b), a linear weight loss was observed from the beginning and its slope was the same as that of (b), as shown in Fig.3(c). A similar behavior was also observed for Ba(thd)2. Approximately 5% of the weight evaporated during the first few minutes. (The weight decrease was observed even during the heating of the sample from room temperature to 290°c.) The same sample tested after the first experiment revealed a constant loss of weight with time as shown in Fig.4(b), having the same slope as observed for the first experiment (a) after the initial weight loss. The weight of C ~ ( t h d ) ~ decreased linearly with heating time from the beginning at 1 8 0 ~ ~ (Fig.5) without an initial sharp weight loss observed for Y(thd)3 or Ba(thd)2.
The first stage weight loss in the two-step weight loss observed in the TG curves in Fig.2 for Y(thd)2 and Ba(thd)2 corresponds to the weight drops observed in the first few minutes as shown in Figs. 3 and 4. The weight losses started around 1 0 0 ~ ~ in Fig.2 were probably caused by evaporation of adsorbed water, residual solvent used for chelate preparation, and the excess amount of 8-diketone not composing chelate with metal. The results presented in Figs. 3-5 suggest that the metal chelates of 8-diketone evaporate c o n s t a n t l y f o r a t l e a s t 2 h w i t h o u t g o i n g t h r o u g h a n y c h a n g e a n d decomposition.
Figure 6 shows the relation between evaporation temperature and evaporation rates which were calculated from the weight losses of the source materials during CVD at 10 Torr. The preparation of films of YBCO single phase was realized at the molar ratio of Y(thd)3 : Ba(thd)2 : C ~ ( t h d ) ~ nearly equal to 1 : 2 : 3.
XRD patterns of YBCO films deposited on various substrates at 9 0 0 ~ ~ are shown in Fig. 7. Observed high relative intensities of (001) peaks indicate that the c axis oriented perpendicular to'the substrate. As reported in the previous study/7/, BaA1203 was formed by the reaction between the films and A1203 polycrystal and sapphire substrates. The level of crystalline orientation for the films on YSZ substrate diminished as the substrate temperature was lowered. At 800°C and lower temperature, randomly oriented YBCO films were formed on the YSZ substrates/Z/. In the present study, even at 800°~, YBCO films with c-axis orientation have been prepared on SrTi03(100) single-crystal substrates. The epitaxial growth of the films from the SrTi03 single-crystal substrates must have contributed to the formation of c-axis oriented films even at this temperature.
Figure 8 shows the scanning electron micrographs of the YBCO films deposited on the substrates of 8 mol% YSZ and A1203. The films on the YSZ substrates are composed of plate-like grains densely connected and overlapping each other(Fig. 8(a,b)). For the deposits o n the A1203 substrates, each grain grew separately with crystal facets on the top.
Reaction between the film and substrate can be observed(Fig. 8(c,d)) from cross-sectional view at substrate temperature of 9 0 0 ~ ~ . The growth rate of
C5-134 JOURNAL DE PHYSIQUE
films was estimated to be about 1-4 pm/h based on the SEM photographs of fracture cross section.
For the films formed on the SrTi03 substrates, rectangular grains of the size of 1-2 pm were aligned in a two-dimensional manner in the film plane (Fig. 9(a)). Some narrow cavities are also observed between some of the grains. Figure 9(b) shows that the films deposited at 8 0 0 ~ ~ are composed of small rectangular grains with the size of less than 0.5 pm. The grains are aligned in two-dimensions without any cavities.
At above 40 K a semiconducting behavior was observed for the sample de osited on YSZ substrate at 9 0 0 ~ ~ and subsequently cooled at a rate of 15 C/min under 10 Torr(Ar:02
1:
= 2:l). Resistivity dropped significantly below 40 K but did not reach zero resistance at 4 K. The films subjected to the in-situ oxygen treatment exhibited metallic behavior. The onset was found at 90 K and the completion of the superconducting transition occurred at 87 K.According to the relation reported among the c-axis length, oxygen vacancy and superconducting transition temperature of sintered YBa2Cu 07-x /20/, the length of c axis measured from XRDopattern of the films not subjected to in-situ oxygen treatment was 11.74 A which corresponded to X = 0.6 an< T = 40 K. The films subjected to in-situ oxygen treatment had c = 1 1 -68 A wgich corresponded to X = 0.1 and Tc = 90 K.
The films deposited at 900, 850 and 8 0 0 ~ ~ on SrTi03 substrates showed Tc(R=O) at 84, 93 and 91 K, respectively. The temperature dependence of the resistivity of these samples was metallic above Tc but the resistivity of the films deposited at 9 0 0 ~ ~ was higher than those deposited at 850 and 8 0 0 ~ ~ .
An interdiffusion of SrTi03 and YBCO elements was observed for the films (thickness:0.5-1.0 pm) prepared by sputtering and then heat treated at high temperatures/21/. These films generally exhibit a higher normal resistivity and lower T
.
These characteristics may be derived by enhanced interdiffusion due to %he higher deposition temperature which introduces impurities and deviations from stoichiometry of YBCO composition into the films. The authors believe that lowering deposition temperature from 9 0 0 ~ ~ to 8 5 0 ~ ~ reduced the interdiffusion and resulted in higher Tc of the present CVD films.Critical current density (Jc) at 77 K and zero magnetic field was 4x10'
~ / c m ~ for the films (thickness: 1 pm) deposited on YSZ substrates. This value was almost equal to the values of sintered samples. In contrast, the Jc of the films (thickness: 1 pm) prepared on SrTi03 single crystal substrates was above 3x10" ~ / c m ~ , which is one to two orders of magnitude higher than the former value. It was not possible to obtain an accurate measurement of Jc for the films above 3x10' ~ / c m ~ by the present DC four-probe method due to the heat generation between the films and electrodes. For these cases, the measurement of Jc must be accomplished by use of patterned films and pulsed current.
YBCO has a layered perovskite structure and exhibits anisotropic properties. Dimons et a1./22/ revealed that in addition to the c-axis orientation, a proper alignment of a- and b-axis directions is also very important in obtaining a high Jc. The films prepared on the YSZ and SrTi03 substrates have c-axis orientation with the a-b plane parallel to the substrate planes. However, the films obtained on the YSZ substrate exhibit randomly oriented a and b axes in the plane of the films(Fig. 8). While on SrTi03 substrates each YBCO grain grew epitaxially and aligned in a two- dimensional pattern as shown in Fig. 9. This difference in grain orientation may be contributing to the difference in the value of Jc between the films on YSZ and SrTi03(100).
Figure 10 shows XRD patterns of annealed BSCCO films. The film was c o m p o s e d o f p h a s e s w i t h c 0 = 3 0
A
( i d e a l c h e m i c a l c o m p o s i t i o n : Bi2(Sr,Ca)3Cu20x, Tc = 85 K), c. = 374
(ideal chemical composition:Bi2(Sr,Ca)4Cu30x, Tc = 110 K), CO = 24 A (ideal chemical composition:
Bi (Sr,Ca)2CuOx, Tc = 22 K) and (Sr,Ca)3Cu50x. High relative intensities of (061) peaks for the BSCCO phases suggest that the orientation of c axis is perpendicular to the substrates(Fig.10).
The SEM micrographs of the surface and fracture cross section of the annealed films are shown in Fig. 11. Flat and mica-like flakes overlap in the films. The thickness of the films grown for 1 h was about 2
urn.
The annealed films showed Tc (onset) at 1 15 K and the resistance decreased largely at 110 K. However, T (R=O) was relatively low 78
k.
This result may have occurred because the films were mixture of the high-T~ andlow Tc phases of BSCCO and a semiconducting phase in Sr-Ca-Cu-0 system.
4 - SUMMARY
B-diketone metal chelates of Y, Ba and Cu are used as CVD source materials to prepare superconducting YBCO films. The present paper reports the stability of B-diketone metal chelate supply as measured by the use of thermogravimetric analyses. The analyses showed that at a fixed evaporation temperature these chelates evaporate at a constant rate for the duration of several hours. In our previous study, the YBCO films prepared on the single crystal SrTiO substrates at 9 0 0 ~ ~ showed Tc(R=O) at 8 4 K. The present study observe2 an increase of T ( R = O ) at 93 K for epitaxially grown YBCO films on single crystal s ~ T ~ o ~ ( 180) substrates by lowering the deposition temperature from 900 to 8 5 0 ~ ~ . The critical current density (Jc) of the films was above 3x10' ~ / c m ~ at 77 K. BSCCO superconducting oxide films prepared by CVD at 7 7 0 ~ ~ were annealed at 8 9 0 ~ ~ at 1 atm of oxygen. The films contained a phase with c. = 37
8
and showed Tc(onset) at 115 K and T,(R=O) at 77 K.Acknowledgment
The authors thank Professor Y. Muto, Dr. N. Kobayashi, Dr. H. Iwasaki and Dr. K. Watanabe of IMR, Tohoku University for measurement of Tc and Jc.
We also thank Mr. A. Suhara for his help of TG measurements.
REFERENCES
/l/ Wu, M.K., Ashburn, J.R., Torng, C.J., Hor, P.H., Meng, R.L., Gao, L., Huang, Z.J., Wang, Y.Q. and Chu, C.W., Phys. Rev. Lett. 58 (1987) 908.
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2
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(1 982) 46./5/ Itoh, H., Yamaguchi, H. and Naka, S., Yogyo-Kyokai-Shi 95 (1987) 50.
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Kobayashi, N., Muto, Y. and Kurosawa, H., Appl. Phys. Lett.
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(1988) 219.JOURNAL DE PHYSIQUE
Table 1. Deposition Conditions.
YBCO BSCCO
Vaporizer Temperature
Deposition Temperature
Total Gas Pressure Carrier Gas(Ar)
Flow Rate 02 Gas Flow Rate Deposition Time Subs trate
I 0 Torr 200 ml/min 100 ml/min
I h YSZ, A1203, sapphire, Mg0( 100 )
SrTi03(1 00)
1 Torr 200 ml/min
YSZ : 8 mol% Y203-Zr02
Fig. 1
-
Schematic diagram of CVD apparatus.l:Y(thd)g boat, 2:Ba(I~hd)~ boat, 3:C~(thd)~ boat, 4:substrate, 5:electrlc furnace.
Fig. 3
-
Remaining versus heating Fig. 4-
Remaining versus heating temperature;(a) Y(thd) at 150°c, temperature at 290°c;(a) Ba(thd) ( b ) at 7 7 0 ~ ~ and (c) alter the and (b) after the experiment of fa) experiment of (b).1 0 0 . .P
8 0 . 0 z
-
z 6 0 --
QI
W a 4 0 .
W -I
4
2 0 -in
t l m i n
Fig. 2
-
ThermogravimetricFig. 5
-
Remaining versus heating temperature at 1 8 0 ~ ~ for C ~ ( t h d ) ~ .-71 a I
2.0 2.2 2.4 2.6 2.8
1000 K I T
OO 100 200 300 400
5b0
curves of Y(thdf3, Ba(thdj2TEMPERATURE /'C and C ~ ( t h d ) ~ .
Fig. 6
-
Relationships between the evaporation rate and temperature for Y(thdI3, Ba(thdI2 and C~(thd)~.Bo 290°C ( a )
-
*
Z
2
0 30 60 90 120 0 30 60 90 120
t lrnin t l m i n
z
a
6 0 .I
g
2
4 0 .V)
3
2 0 .
0 ,
Z
2 6 0 -
5
2
IL 4 0 - V)2
2 0
0
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Fig. 7
-
XRD patterns of films deposited at 9 0 0 ~ ~ o n substrates of Ysz(a), ~ 1 ~ 0 ~ ( b ) , sapphire(c), Mg0(100)(d) and SrTi03(100)(e).Fig. 3 - Scanning electron nicrogra2hs of surfaces and fracture cross sections of Y2CO films deposited at 9 0 0 ~ ~ on YSZ(a,S) and. A1203(c,d) substrates.
Fig. 9 - Scanning electron micrographs of YRCO films deposited on SrTi03 (100) substrates at 900°c(a) and 800°~(b).
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Fig. 10 - X-ray diffraction pattern of a BSCCO film deposited at 7 7 0 ~ ~ and annealed at 8 9 0 ~ ~ .
Fig. 1 1 - Scanning electron micrographs of a BSCCO film deposited at 7 7 0 ~ ~ and annealed at 890°c;(a) surface and ( b ) fracture cross section.
