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Superconducting YBa2Cu3Ox films prepared by rf thermal plasma evaporation
PHYSIgA
Physica C 235-240 (I994)583-584 North-Holland
Superconducting YBa2Cu3Ox Films Prepared by rf Thermal Plasma E v a p o r a t i o n J.Tsujino, N.Tatsumi, and Y.Shiohara Superconducting Research Laboratory, ISTEC, 10-13 Shinonome, Koto-ku, Tokyo 135, Japan
Films prepared at high deposition rate greater 200 nm/min by this process showed c-axis orientation and inplane alignment. Jc values of the films~repared at deposition rates of 100 and 220 nm/min at zero applied field and 77K were 2. lxl05 and 5.4x104 A/cmL, respectively. 1.INTRODUCTION The rf thermal plasma evaporation technique is a vapor phase process using thermal 02 plasma with an extremely high temperature and active 02 atmosphere.I, 2 As reported in previous paper 3, asgrown YBa2Cu3Ox films prepared on (100)MgO by rf thermal plasma evaporation have advantages of a high deposition rate, a large area deposition, and high Jc( > 105 A/cm 2 at zero field and 77 K) values. AcS cordingly, this technique is considered to be one of the practical process to synthesize superconducting tape, wire, and magnetic shielding for engineering power applications. In this study, we prepared YBa2Cu3Ox films on (100)STO substrates which have more smooth surface and better lattice matching than (100)MgO substrates, and investigated surface morphologies, structure and superconducting properties of films prepared different deposition rates. Aim of this study is to understand influence of deposition rate on qualities of films. 2.EXPERIMENTAL PROCEDURES The 4 MHz rf Ar-O2 plasma was operated at 43 kW and 200 Torr as the input power of the plate power level and the total pressure, respectively. Ar[ 02 flowing gas ratio was 7/50. The YBa2Cu3Ox powder had an average particle size of approximately 3 ~m. The Y rich composition as raw material was fed axially into the plasma with Ar carrier gas.3 After the growth, the sample was quickly cooled to room temperature for approximately 10 min under 200 Torr 02 atmosphere. The holder and the substrate were heated by the thermal plasma. Temperature of the substrate was controlled by changing relative position between the holder and the plasma flame, and monitored by a thermocouple which was embedded in the holder and located 0.5 mm below the substrate The surface morphologies were examined by scanning electron microscopy (SEM). The crystal
structures of the grown films were measured by X-ray diffraction (XRD) using Cr K-or (2.291 A). The XRD was performed in the conventional 0-20 scanning and the 0 scanning for rocking curves. In-plane alignments were estimated by X-ray pole figure measurement using Schultz method. The Jc were measured by a conventional four-probe method. Electrical contact to the films were made by silver paste. The voltage contacts were separated by a distance of 2 mm. The criterion of the Jc was taken asl I.tV/cm. 3.RESULTS AND DISCUSSION Figurel shows SEM images for films prepared on (100)STO substrates at deposition rates of 100 and 220 nm/min and substrate temperature of 650 °C and 670 °C, respectively. Deposition time was 3 min. From XRD measurement, both films exhibited c-axis orientation. The surface of the film prepared with 100 nm/min is smooth, but impurity particles with diameter of appoloximetary l~m and a few pores exist. On the other hand, on the surface of the film prepared with 220 nm/min, not only impurity particles but
0921-4534/94/S07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00853-1
220
Figure 1
images, tilting 7 0 ° f r o m the the p l a n e o f f i l m s , p r e p a r e d at deposition rates o f 1 0 0 a n d 220 n m / m i n a n d substrate t e m p e r a t u r e o f 6 5 0 ° C a n d 6 7 0 ° C o n ( 1 0 0 ) S T O f o r deposition time o f 3 min. normal
SEM
to
,L T~ujino et al./Phvsica (7 235 240 (1994) 5,~'3 .%'4
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Temperature (K) Figure 2 Temperature dependences of Jc of films prepared at deposition rates of 100 and 220 nm/min and substrate temperature of 650 °C and 670 °C on (100) STO for deposition time of 3 min. large grains with size of 2-4 I.tm are observed, accordingly surface roughness of the film is larger. Since the result of XRD did not include peaks which exhibit other orientation such as (nO0) or (103), it indicative that c-axis of these large grains are almost perpen-
106~
I
I
O 100 nm/min • 220 nm/min
0 0 0 10s~.
I
0
ACKNOWLEDGMENT This work was performed under the management of the International Superconductivity Technology Center (ISTEC) as a part of the R&D of Industrial Science and Technology Frontier Program supported by the New Energy and Industrial Technology Development Organization.
0
0 v
0 10 4
10s 0
dicular to the film. Crystallinities of both films were estimated by FWHM of (005) rocking curves, and in result, film prepared with 100 nm/min was more highly crystallinity, respective values of FWHM were 0.5 ° and 0.6 °. It is considered that decrease of crystallinity was mainly caused by grains shown in Figure l(b). In-plane alignments of both films were seen from results of pole figure measurement, however, for further investigation of degrees or details of in-plane alignments, TEM observation may be needed. Figure 2 shows temperature dependences of Jc of films shown in Figurel. Jc values at zero field and 77K are 2.1z105 and 5.4x104 A/cm 2, respectively. Figure 3 shows magnetic field dependences of Jc of both films at 77K. Magnetic field direction was perpendicular to the films. Jc of the film prepared at 100 nm/min depend weakly on magnetic field and exhibits Jc values of 2x104 A/cm 2 at 4 T. On the other hand, Jc of the film prepared at 220 nm/min decrease rapidly at weak magnetic field. However, under B>IT, Jc is less dependence on B. These result show that the film at 220 nm/min contains weak links (large-angle grain boundaries), perhaps induced by grains shown in figurel (b). Accordingly, it is considered that Jc is particularly affected by outgrowth large grains due to increasing dcpositon rate in this deposition condition. In conclusion, films prepared at high deposition rate greater 220 nm/min by this process showed c-axis orientation and in-plane alignments. However, films became lower crystallinity and included large grains with increasing deposition rate. Jc values of films prepared with deposition rates of 100 and 220 nm/min at zero applied field and 77K were 2.1x105 and 5.4z104, respectively.
••
I
I
I
I
2
3
B (Tesla) Figure 3 Magnetic field dependences of Jc at 77K. The magnetic field was applied in B J.films.
REFERENCES 1. K.Terashima, K.Eguchi, T.Yoshida, and K.Akashi, Appl. phys. Lett. 52, 1274 (1988) 2. K.Terashima, H.Komaki, and T.Yoshida, IEEE Trans. Plasma Sci.18,980 (1990) 3. S.Yuhya, K.Kikuchi, Y.Shiohara, K.Terashima, and T.Yoshida, J. Mater. Res. 7, 2673 (1992)
Physica C 235-240 (I994)583-584 North-Holland
Superconducting YBa2Cu3Ox Films Prepared by rf Thermal Plasma E v a p o r a t i o n J.Tsujino, N.Tatsumi, and Y.Shiohara Superconducting Research Laboratory, ISTEC, 10-13 Shinonome, Koto-ku, Tokyo 135, Japan
Films prepared at high deposition rate greater 200 nm/min by this process showed c-axis orientation and inplane alignment. Jc values of the films~repared at deposition rates of 100 and 220 nm/min at zero applied field and 77K were 2. lxl05 and 5.4x104 A/cmL, respectively. 1.INTRODUCTION The rf thermal plasma evaporation technique is a vapor phase process using thermal 02 plasma with an extremely high temperature and active 02 atmosphere.I, 2 As reported in previous paper 3, asgrown YBa2Cu3Ox films prepared on (100)MgO by rf thermal plasma evaporation have advantages of a high deposition rate, a large area deposition, and high Jc( > 105 A/cm 2 at zero field and 77 K) values. AcS cordingly, this technique is considered to be one of the practical process to synthesize superconducting tape, wire, and magnetic shielding for engineering power applications. In this study, we prepared YBa2Cu3Ox films on (100)STO substrates which have more smooth surface and better lattice matching than (100)MgO substrates, and investigated surface morphologies, structure and superconducting properties of films prepared different deposition rates. Aim of this study is to understand influence of deposition rate on qualities of films. 2.EXPERIMENTAL PROCEDURES The 4 MHz rf Ar-O2 plasma was operated at 43 kW and 200 Torr as the input power of the plate power level and the total pressure, respectively. Ar[ 02 flowing gas ratio was 7/50. The YBa2Cu3Ox powder had an average particle size of approximately 3 ~m. The Y rich composition as raw material was fed axially into the plasma with Ar carrier gas.3 After the growth, the sample was quickly cooled to room temperature for approximately 10 min under 200 Torr 02 atmosphere. The holder and the substrate were heated by the thermal plasma. Temperature of the substrate was controlled by changing relative position between the holder and the plasma flame, and monitored by a thermocouple which was embedded in the holder and located 0.5 mm below the substrate The surface morphologies were examined by scanning electron microscopy (SEM). The crystal
structures of the grown films were measured by X-ray diffraction (XRD) using Cr K-or (2.291 A). The XRD was performed in the conventional 0-20 scanning and the 0 scanning for rocking curves. In-plane alignments were estimated by X-ray pole figure measurement using Schultz method. The Jc were measured by a conventional four-probe method. Electrical contact to the films were made by silver paste. The voltage contacts were separated by a distance of 2 mm. The criterion of the Jc was taken asl I.tV/cm. 3.RESULTS AND DISCUSSION Figurel shows SEM images for films prepared on (100)STO substrates at deposition rates of 100 and 220 nm/min and substrate temperature of 650 °C and 670 °C, respectively. Deposition time was 3 min. From XRD measurement, both films exhibited c-axis orientation. The surface of the film prepared with 100 nm/min is smooth, but impurity particles with diameter of appoloximetary l~m and a few pores exist. On the other hand, on the surface of the film prepared with 220 nm/min, not only impurity particles but
0921-4534/94/S07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)00853-1
220
Figure 1
images, tilting 7 0 ° f r o m the the p l a n e o f f i l m s , p r e p a r e d at deposition rates o f 1 0 0 a n d 220 n m / m i n a n d substrate t e m p e r a t u r e o f 6 5 0 ° C a n d 6 7 0 ° C o n ( 1 0 0 ) S T O f o r deposition time o f 3 min. normal
SEM
to
,L T~ujino et al./Phvsica (7 235 240 (1994) 5,~'3 .%'4
584
5.0xi 0"
©
O
O
0 •
4.0
100 nm/min 220 nm/min
O O
o
3.0
O 0
,,~
%o
2.0
@
•
1.0
• ,
0
I
I
20
30
,
I
I
40
50
,
I
I
60
70
,
~II
80
l
90
Temperature (K) Figure 2 Temperature dependences of Jc of films prepared at deposition rates of 100 and 220 nm/min and substrate temperature of 650 °C and 670 °C on (100) STO for deposition time of 3 min. large grains with size of 2-4 I.tm are observed, accordingly surface roughness of the film is larger. Since the result of XRD did not include peaks which exhibit other orientation such as (nO0) or (103), it indicative that c-axis of these large grains are almost perpen-
106~
I
I
O 100 nm/min • 220 nm/min
0 0 0 10s~.
I
0
ACKNOWLEDGMENT This work was performed under the management of the International Superconductivity Technology Center (ISTEC) as a part of the R&D of Industrial Science and Technology Frontier Program supported by the New Energy and Industrial Technology Development Organization.
0
0 v
0 10 4
10s 0
dicular to the film. Crystallinities of both films were estimated by FWHM of (005) rocking curves, and in result, film prepared with 100 nm/min was more highly crystallinity, respective values of FWHM were 0.5 ° and 0.6 °. It is considered that decrease of crystallinity was mainly caused by grains shown in Figure l(b). In-plane alignments of both films were seen from results of pole figure measurement, however, for further investigation of degrees or details of in-plane alignments, TEM observation may be needed. Figure 2 shows temperature dependences of Jc of films shown in Figurel. Jc values at zero field and 77K are 2.1z105 and 5.4x104 A/cm 2, respectively. Figure 3 shows magnetic field dependences of Jc of both films at 77K. Magnetic field direction was perpendicular to the films. Jc of the film prepared at 100 nm/min depend weakly on magnetic field and exhibits Jc values of 2x104 A/cm 2 at 4 T. On the other hand, Jc of the film prepared at 220 nm/min decrease rapidly at weak magnetic field. However, under B>IT, Jc is less dependence on B. These result show that the film at 220 nm/min contains weak links (large-angle grain boundaries), perhaps induced by grains shown in figurel (b). Accordingly, it is considered that Jc is particularly affected by outgrowth large grains due to increasing dcpositon rate in this deposition condition. In conclusion, films prepared at high deposition rate greater 220 nm/min by this process showed c-axis orientation and in-plane alignments. However, films became lower crystallinity and included large grains with increasing deposition rate. Jc values of films prepared with deposition rates of 100 and 220 nm/min at zero applied field and 77K were 2.1x105 and 5.4z104, respectively.
••
I
I
I
I
2
3
B (Tesla) Figure 3 Magnetic field dependences of Jc at 77K. The magnetic field was applied in B J.films.
REFERENCES 1. K.Terashima, K.Eguchi, T.Yoshida, and K.Akashi, Appl. phys. Lett. 52, 1274 (1988) 2. K.Terashima, H.Komaki, and T.Yoshida, IEEE Trans. Plasma Sci.18,980 (1990) 3. S.Yuhya, K.Kikuchi, Y.Shiohara, K.Terashima, and T.Yoshida, J. Mater. Res. 7, 2673 (1992)