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Effects of La-doping on crystallinity and dielectric properties of SrAl0.5Ta0.5O3 thin films for high-Tc superconductor multilayer structure
Physica C 392–396 (2003) 1337–1341 www.elsevier.com/locate/physc
Effects of La-doping on crystallinity and dielectric properties of SrAl0:5Ta0:5O3 thin films for high-Tc superconductor multilayer structure Yoshihiro Takahashi *, Hironori Wakana, Akihiro Ogawa, Tadataka Morishita, Keiichi Tanabe Superconductivity Research Laboratory––ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan Received 11 November 2002; accepted 31 January 2003
Abstract Lax Sr1x Al0:5 Ta0:5 O3 (La–SAT) thin films were prepared to examine the effects of La-doping to SrAl0:5 Ta0:5 O3 (SAT) as intermediate insulating films for high-Tc devices. 300-nm-thick La–SAT films were grown on approximately 10-lmthick YBa2 Cu3 O7d (YBCO) films by metalorganic chemical vapor deposition with the La-doping ratio x of 0–0.2. The La–SAT films with x 6 0:1 exhibited good crystallinity and monotonic lattice contraction with increasing x. 300-nmthick La0:2 Y0:9 Ba1:9 Cu3 O7d (La–YBCO) films deposited on these La–SAT films had good Tc and Jc values comparable to those for the SAT films without La-doping. On the other hand, the La–SAT film with x ffi 0:2 changed to have random orientation and a La–YBCO film on the La–SAT film showed much poorer Tc and Jc values. These results suggest that the La solubility limit to SAT exists in the range of x ¼ 0:1–0.2, although a monotonic decrease in the dielectric constant with increasing x was observed for all the La–SAT films in the x range of 0–0.2 and low conductance less than 106 S. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.76.Bz; 68.55.Ln; 73.61.)r; 81.15.Gh; 85.25.)j Keywords: La-doping; Sr–Al–Ta–O; Dielectric properties; High-Tc superconductor; MOCVD
1. Introduction Insulator films are required as intermediate layers in high-Tc multilayer structures for electronic devices such as single flux quantum (SFQ) digital circuits. SrAl0:5 Ta0:5 O3 (SAT), a cubic perovskite compound, has been expected as a prom*
Corresponding author. Tel.: +81-3-3536-5709; fax: +81-33536-5714. E-mail address: [email protected] (Y. Takahashi).
ising intermediate insulator because of its lattice constant approximately twice of the a- and b-axes of YBa2 Cu3 O7d (YBCO) [1] and the low dielectric constant (e ¼ 23–30) [2]. In previous studies, we succeeded in reproducibly preparing insulating SAT thin films by metalorganic chemical vapor deposition (MOCVD) with high crystallinity [3] and optimizing the deposition conditions such as the oxygen partial pressure [4]. As a further examination of perovskite insulating films, it is interesting to dope another kind
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01191-2
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of cation such as La to the A-site of perovskite lattices. For example, in the studies of ferroelectric films such as Pb(Zr,Ti)O3 [5] and Bi4 Ti3 O12 [6] for ferroelectric random access memories (FeRAMs), La has been substituted to improve their fatigue resistance. For superconducting thin films and multilayers, use of (LaAlO3 )0:3 –(SrAl0:5 Ta0:5 O3 )0:7 (LSAT) as a substrate or an intermediate insulating layer has been extensively studied [7], because LSAT has a lower melting temperature than that of SAT and high-quality single crystals can be grown. In the present study, we tried substituting La for Sr in SAT. 300-nm-thick La-doped SAT thin films, that is, Lax Sr1x Al0:5 Ta0:5 O3 (La–SAT), were prepared on c-axis-oriented YBCO films by MOCVD. Their crystallinity and dielectric properties were examined to investigate the effects of a cation exchange and compared with the case of LSAT.
2. Experimental As lower substrates, c-axis-oriented, approximately 10-lm-thick YBCO films grown by liquid phase epitaxy (LPE) [8] on MgO single crystals were used. 300-nm-thick La–SAT films were prepared on the YBCO films by MOCVD with employing La(C11 H19 O2 )3 , Sr(C11 H19 O2 )2 , and TaAl(O-iC3 H7 )8 [9] as metalorganic sources. The source mixed ratio was controlled by the sublimation temperature and the carrier Ar gas flow rate of each source. After the preparation, the La– SAT composition was confirmed by X-ray fluorescence (XRF) analysis. The detailed preparation conditions of La–SAT films are shown in Table 1. The crystallinity of the La–SAT films were examined by X-ray diffraction (XRD) with varying the La-doping ratio x in a range of 0–0.2. The dielectric properties, the dielectric constant and the conductance, of the La–SAT films were measured using 300 lm£ planer capacitors. They had 600nm-thick sputtered Au electrodes and were patterned by standard photolithography and ion milling, after the lower thick YBCO films were oxygenated in 1-atm-oxygen atmosphere at 500 °C for 50 h [10].
Table 1 Preparation conditions of La–SAT films by MOCVD Source temperature/carrier Ar gas flow rate 198–200 °C/30 sccm 2 for Sr(C11 H19 O2 )2 for La(C11 H19 O2 )3 150–170 °C/30 sccm 70–80 °C/70–100 sccm for TaAl(O-iC3 H7 )8 O2 /total gas flow rate 200/1000 sccm Deposition pressure 13 hPa Substrate temperature 740 °C Deposition rate 200–230 nm/h
To examine the applicability of the La–SAT films as intermediate insulating layers in high-Tc multilayer structures, 300-nm-thick La0:2 –Y0:9 Ba1:9 Cu3 O7d (La–YBCO) films were grown as upper superconducting layers on the La–SAT films by an off-axis magnetron sputter method [11]. The superconducting properties of the upper La– YBCO films were investigated by four-probe resistivity and Jc measurements with bridge patterns fabricated by standard photolithography and ion milling.
3. Results and discussion Fig. 1 shows the XRD (h=2h) patterns for La– SAT films with the La-doping ratio x ¼ 0–0.2. With increasing the La-doping ratio to x ffi 0:1, the La–SAT(0 0 4) peak systematically shifts to the higher angle side and the lattice constant reduces by 0.16%. This indicates that La is actually incorporated into the perovskite lattice, since La3þ has a slightly smaller ionic radius than that of Sr2þ . However, for the case of x ffi 0:2, the La– SAT(0 0 4) peak almost disappears and another peak such as LaAlO3 (0 0 2) at 2h ¼ 48:0° is not observed. Fig. 2 shows the XRD (h=2h) patterns for the x ¼ 0 and 0.2 films fabricated with varying the (La + Sr)/(Al + Ta) composition ratio near stoichiometry, since the suitable A/B composition ratio might be changed by La-doping. Even for the optimized (La + Sr)/(Al + Ta) ratio, the La– SAT(0 0 4) intensity for x ¼ 0:2 drops below 10% of that for the films without La-doping (x ¼ 0), although the absolute (La + Sr)/(Al + Ta) values include some errors inevitable in the XRF analysis.
Y. Takahashi et al. / Physica C 392–396 (2003) 1337–1341 YBCO (006)
La-SAT (004)
400 Kα1 Kα 2
Intensity (kcps)
300
x =0 200
x = 0.03 x = 0.06
100
x = 0.11 0 45.8
x = 0.18 46.0
46.2
46.4
48.0
46.6
48.2
2 θ (deg) Fig. 1. XRD (h=2h) patterns of 300-nm-thick La–SAT films with the La-doping ratio x ¼ 0–0.2 deposited on YBCO thick films. Dotted lines show zero points of intensity for each x value. Each substrate was cut from the same YBCO/MgO sample.
Fig. 3 shows the scanning electron microscope (SEM) images of La–SAT surfaces. For the films with x ¼ 0 and 0.11, square-shape grains are observed and aligned in the same direction. Because the lower LPE YBCO is a quasi-single-crystal and has an atomically flat surface with terraces and
YBCO (006)
steps with the height equal to the c-axis length, the square-shape grains come from the SAT cubic structure and indicate good in-plane alignment of the La–SAT grains. However, for x ¼ 0:18, square grains are hardly observed but rounded grains are formed, suggesting that the grains of La–SAT or other oxides have random orientations in this film, which seems consistent with the X-ray results. From these results, we speculate that the La solubility limit in the SAT lattice exists in the x range between 0.11 and 0.18, and thus formation of Labased second phases would disturb cube-on-cube arrangement of the La–SAT grains on the YBCO, although such second phases were not clearly observed in the SEM picture possibly because of their small grain size. Fig. 4 shows the dielectric properties of La– SAT films with the La-doping ratio x ¼ 0, 0.12, and 0.20. The dielectric constant is approximately 19–27 and slightly reduced with increasing the x value. The conductance is low enough, below 106 S, for all x values, indicating that the La-doping does not deteriorate the SAT dielectric properties. However, the dielectric constant less than 20 for the x ¼ 0:2 film may be due to inclusion of other oxides such as La2 O3 . Fig. 5 shows the superconducting properties of 300-nm-thick upper La–YBCO films on the La– SAT films with x ¼ 0–0.2. For x ¼ 0:1, the Jc value is approximately 107 A/cm2 at 4 K and slightly
YBCO (006)
SAT (004)
400
La-SAT (004)
300
Kα 2
Sr/(Al+Ta) 0.94 0.97
200
0.99 100
1.02
x
=0 1.03 0 45.8 46.0 46.2 46.4 46.6 46.8 47.0 2θ (deg)
Intensity (kcps)
40 Kα 1
Intensity (kcps)
1339
(La+Sr)/(Al+Ta) 0.91
30
0.94 20
0.98 10
1.01
x
= 0.2 1.03 0 45.8 46.0 46.2 46.4 46.6 46.8 47.0 2θ (deg)
Fig. 2. XRD (h=2h) patterns for La–SAT films with the La-doping ratio x ¼ 0 and 0.2 which were deposited with varying the (La + Sr)/ (Al + Ta) composition ratio near stoichiometry. The intensity scale for x ¼ 0:2 is expanded by 10 times of that for x ¼ 0 and the dotted lines show the zero point of the intensity for each x value.
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Fig. 3. SEM images of La–SAT surfaces with the La-doping ratio x ¼ 0, 0.11, and 0.18.
0
50
10
-2
40
30
10
-4
20
10
10
10
Dielectric constant
x =0
40
0 3 10
4
10
0
50
10
-2
40
30
10
-4
-6
20
10
-8
10
10
10-10
5
10 10 10 Frequency (Hz)
x = 0.12
0 3 10
6
4
5
10 10 10 Frequency (Hz)
10
0
10
-2
30
10
-4
-6
20
10
-6
-8
10
10
-8
10-10 0 3 10
6
x = 0.20
4
5
Conductance (S)
10
50
10-10
10 10 10 Frequency (Hz)
6
8
10
7
10
6
800 x=0 x = 0.11
105 104 10 10
Resistivity ( µΩ cm)
10
x = 0.18
3
(a)
Resistivity (µΩ cm)
2
Jc (A/cm )
Fig. 4. Dielectric properties of 300-nm-thick La–SAT films with the La-doping ratio x ¼ 0, 0.12, and 0.20, which were evaluated using 300 lm£ planer capacitors. The results of four capacitors are displayed for each La-doping ratio.
2
0
20
40
60
80
Temperature (K)
(b) 600 x = 0.18
400
40
x=0
30 20
x = 0.11
10 0 82
84
86
88
Temperature (K)
200
x=0 x = 0.11
0
0
100 200 Temperature (K)
300
Fig. 5. Superconducting properties, (a) Jc value for 5-lm-wide and 50-lm-long bridges (b) resistivity for 50-lm-wide and 150-lm-long bridges, of 300-nm-thick La–YBCO films on the La–SAT films with the La-doping ratio x ¼ 0–0.2. The Jc values were determined with a criterion of 20 lV/mm.
lower than that for x ¼ 0, while the Tc value is almost the same, approximately 84 K. In contrast, the Tc and Jc values for the x ¼ 0:18 film become significantly lower, below 50 K and 104 A/cm2 at 4 K, respectively. These results are consistent with the change in the film orientation, as shown in
Figs. 1–3, which reflects the La solubility limit in the SAT lattice. In the case of LSAT, two individual compounds, LaAlO3 and SAT, are mixed with the La/ (La + Sr) ratio of 0.3. In contrast, in La–SAT, because La3þ is supposed to be substituted for
Y. Takahashi et al. / Physica C 392–396 (2003) 1337–1341
Sr2þ , the average A-site ionic radius becomes smaller and the cation charge may also change. From the XRF analysis, the mole ratio of the Bsite elements in the La–SAT films was confirmed to be Al=Ta ¼ 0:99–1.01, suggesting that no sole compound with Ta partially substituted by Al such as La0:1 Sr0:9 Al0:55 Ta0:45 O3 and La0:2 Sr0:8 Al0:6 Ta0:4 O3 exists to keep the charge balance. If they were generated, tantalum-based compounds such as Ta2 O5 would be simultaneously generated to satisfy the total mole ratio of Al=Ta ¼ 1:0. A more plausible way of keeping the charge balance is that the valence of Ta ions decreases to, for example, +3, or a slight amount of excess oxygen is incorporated in the lattice, that is, Lax Sr1x Al0:5 Ta0:5 O3þd (d x=2) is formed. In these cases, the La solubility limit in La–SAT is supposed to be dominated by the tolerance limit in the variation of the Ta valence or the oxygen content.
4. Conclusion La–SAT films on c-axis-oriented YBCO films show lattice contraction and a reduction of its dielectric constant with increasing the La-doping ratio to x ffi 0:1. The La solubility limit exists in the x range between 0.1 and 0.2, and the La–SAT changes to have random orientation at x ffi 0:2, although its dielectric properties become still better. It was also confirmed that the superconducting properties of upper La–YBCO thin films grown on La–SAT with x 6 0:1 are almost comparable to those for the case without La-doping.
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Acknowledgements The authors would like to thank K. Nomura, S. Hoshi, T. Izumi, and Y. Shiohara of SRL-ISTEC for providing YBCO thick films grown by liquid phase epitaxy. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development for Superconductivity Applications.
References [1] C.D. Brandle, V.J. Fratello, J. Mater. Res. 5 (1990) 2160. [2] A.T. Findikoglu, C. Doughty, S. Bhattacharya, Q. Li, X.X. Xi, T. Venkatesan, R.E. Fahey, A.J. Strauss, J.M. Phillips, Appl. Phys. Lett. 61 (1992) 1718. [3] Y. Takahashi, H. Zama, Y. Ishimaru, N. Inoue, Y. Wu, T. Morishita, K. Tanabe, Jpn. J. Appl. Phys. 41 (2002) 590. [4] Y. Takahashi, Y. Nakajima, T. Morishita, K. Tanabe, Physica C 378–381 (2002) 1357. [5] K. Miura, M. Tanaka, Jpn. J. Appl. Phys. 35 (1996) 3488. [6] B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee, W. Jo, Nature 401 (1999) 682. [7] Y. Li, X.J. Wu, J.G. Wen, Y. Takahashi, K. Tanabe, J. Appl. Phys. 87 (2000) 3707. [8] K. Nomura, S. Hoshi, X. Yao, K. Kakimoto, T. Izumi, Y. Shiohara, in: Advances in Superconductivity, XII, Springer-Verlag, Tokyo, 2000, p. 891. [9] H. Zama, Y. Takahashi, K. Tanabe, T. Morishita, Jpn. J. Appl. Phys. 40 (2001) L167. [10] Y. Takahashi, H. Zama, T. Morishita, K. Tanabe, Physica C 357–360 (2001) 1364. [11] H. Wakana, S. Adachi, M. Horibe, Y. Ishimaru, O. Horibe, Y. Tarutani, K. Tanabe, Jpn. J. Appl. Phys. 41 (2002) L239.
Effects of La-doping on crystallinity and dielectric properties of SrAl0:5Ta0:5O3 thin films for high-Tc superconductor multilayer structure Yoshihiro Takahashi *, Hironori Wakana, Akihiro Ogawa, Tadataka Morishita, Keiichi Tanabe Superconductivity Research Laboratory––ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan Received 11 November 2002; accepted 31 January 2003
Abstract Lax Sr1x Al0:5 Ta0:5 O3 (La–SAT) thin films were prepared to examine the effects of La-doping to SrAl0:5 Ta0:5 O3 (SAT) as intermediate insulating films for high-Tc devices. 300-nm-thick La–SAT films were grown on approximately 10-lmthick YBa2 Cu3 O7d (YBCO) films by metalorganic chemical vapor deposition with the La-doping ratio x of 0–0.2. The La–SAT films with x 6 0:1 exhibited good crystallinity and monotonic lattice contraction with increasing x. 300-nmthick La0:2 Y0:9 Ba1:9 Cu3 O7d (La–YBCO) films deposited on these La–SAT films had good Tc and Jc values comparable to those for the SAT films without La-doping. On the other hand, the La–SAT film with x ffi 0:2 changed to have random orientation and a La–YBCO film on the La–SAT film showed much poorer Tc and Jc values. These results suggest that the La solubility limit to SAT exists in the range of x ¼ 0:1–0.2, although a monotonic decrease in the dielectric constant with increasing x was observed for all the La–SAT films in the x range of 0–0.2 and low conductance less than 106 S. Ó 2003 Elsevier B.V. All rights reserved. PACS: 74.76.Bz; 68.55.Ln; 73.61.)r; 81.15.Gh; 85.25.)j Keywords: La-doping; Sr–Al–Ta–O; Dielectric properties; High-Tc superconductor; MOCVD
1. Introduction Insulator films are required as intermediate layers in high-Tc multilayer structures for electronic devices such as single flux quantum (SFQ) digital circuits. SrAl0:5 Ta0:5 O3 (SAT), a cubic perovskite compound, has been expected as a prom*
Corresponding author. Tel.: +81-3-3536-5709; fax: +81-33536-5714. E-mail address: [email protected] (Y. Takahashi).
ising intermediate insulator because of its lattice constant approximately twice of the a- and b-axes of YBa2 Cu3 O7d (YBCO) [1] and the low dielectric constant (e ¼ 23–30) [2]. In previous studies, we succeeded in reproducibly preparing insulating SAT thin films by metalorganic chemical vapor deposition (MOCVD) with high crystallinity [3] and optimizing the deposition conditions such as the oxygen partial pressure [4]. As a further examination of perovskite insulating films, it is interesting to dope another kind
0921-4534/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0921-4534(03)01191-2
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Y. Takahashi et al. / Physica C 392–396 (2003) 1337–1341
of cation such as La to the A-site of perovskite lattices. For example, in the studies of ferroelectric films such as Pb(Zr,Ti)O3 [5] and Bi4 Ti3 O12 [6] for ferroelectric random access memories (FeRAMs), La has been substituted to improve their fatigue resistance. For superconducting thin films and multilayers, use of (LaAlO3 )0:3 –(SrAl0:5 Ta0:5 O3 )0:7 (LSAT) as a substrate or an intermediate insulating layer has been extensively studied [7], because LSAT has a lower melting temperature than that of SAT and high-quality single crystals can be grown. In the present study, we tried substituting La for Sr in SAT. 300-nm-thick La-doped SAT thin films, that is, Lax Sr1x Al0:5 Ta0:5 O3 (La–SAT), were prepared on c-axis-oriented YBCO films by MOCVD. Their crystallinity and dielectric properties were examined to investigate the effects of a cation exchange and compared with the case of LSAT.
2. Experimental As lower substrates, c-axis-oriented, approximately 10-lm-thick YBCO films grown by liquid phase epitaxy (LPE) [8] on MgO single crystals were used. 300-nm-thick La–SAT films were prepared on the YBCO films by MOCVD with employing La(C11 H19 O2 )3 , Sr(C11 H19 O2 )2 , and TaAl(O-iC3 H7 )8 [9] as metalorganic sources. The source mixed ratio was controlled by the sublimation temperature and the carrier Ar gas flow rate of each source. After the preparation, the La– SAT composition was confirmed by X-ray fluorescence (XRF) analysis. The detailed preparation conditions of La–SAT films are shown in Table 1. The crystallinity of the La–SAT films were examined by X-ray diffraction (XRD) with varying the La-doping ratio x in a range of 0–0.2. The dielectric properties, the dielectric constant and the conductance, of the La–SAT films were measured using 300 lm£ planer capacitors. They had 600nm-thick sputtered Au electrodes and were patterned by standard photolithography and ion milling, after the lower thick YBCO films were oxygenated in 1-atm-oxygen atmosphere at 500 °C for 50 h [10].
Table 1 Preparation conditions of La–SAT films by MOCVD Source temperature/carrier Ar gas flow rate 198–200 °C/30 sccm 2 for Sr(C11 H19 O2 )2 for La(C11 H19 O2 )3 150–170 °C/30 sccm 70–80 °C/70–100 sccm for TaAl(O-iC3 H7 )8 O2 /total gas flow rate 200/1000 sccm Deposition pressure 13 hPa Substrate temperature 740 °C Deposition rate 200–230 nm/h
To examine the applicability of the La–SAT films as intermediate insulating layers in high-Tc multilayer structures, 300-nm-thick La0:2 –Y0:9 Ba1:9 Cu3 O7d (La–YBCO) films were grown as upper superconducting layers on the La–SAT films by an off-axis magnetron sputter method [11]. The superconducting properties of the upper La– YBCO films were investigated by four-probe resistivity and Jc measurements with bridge patterns fabricated by standard photolithography and ion milling.
3. Results and discussion Fig. 1 shows the XRD (h=2h) patterns for La– SAT films with the La-doping ratio x ¼ 0–0.2. With increasing the La-doping ratio to x ffi 0:1, the La–SAT(0 0 4) peak systematically shifts to the higher angle side and the lattice constant reduces by 0.16%. This indicates that La is actually incorporated into the perovskite lattice, since La3þ has a slightly smaller ionic radius than that of Sr2þ . However, for the case of x ffi 0:2, the La– SAT(0 0 4) peak almost disappears and another peak such as LaAlO3 (0 0 2) at 2h ¼ 48:0° is not observed. Fig. 2 shows the XRD (h=2h) patterns for the x ¼ 0 and 0.2 films fabricated with varying the (La + Sr)/(Al + Ta) composition ratio near stoichiometry, since the suitable A/B composition ratio might be changed by La-doping. Even for the optimized (La + Sr)/(Al + Ta) ratio, the La– SAT(0 0 4) intensity for x ¼ 0:2 drops below 10% of that for the films without La-doping (x ¼ 0), although the absolute (La + Sr)/(Al + Ta) values include some errors inevitable in the XRF analysis.
Y. Takahashi et al. / Physica C 392–396 (2003) 1337–1341 YBCO (006)
La-SAT (004)
400 Kα1 Kα 2
Intensity (kcps)
300
x =0 200
x = 0.03 x = 0.06
100
x = 0.11 0 45.8
x = 0.18 46.0
46.2
46.4
48.0
46.6
48.2
2 θ (deg) Fig. 1. XRD (h=2h) patterns of 300-nm-thick La–SAT films with the La-doping ratio x ¼ 0–0.2 deposited on YBCO thick films. Dotted lines show zero points of intensity for each x value. Each substrate was cut from the same YBCO/MgO sample.
Fig. 3 shows the scanning electron microscope (SEM) images of La–SAT surfaces. For the films with x ¼ 0 and 0.11, square-shape grains are observed and aligned in the same direction. Because the lower LPE YBCO is a quasi-single-crystal and has an atomically flat surface with terraces and
YBCO (006)
steps with the height equal to the c-axis length, the square-shape grains come from the SAT cubic structure and indicate good in-plane alignment of the La–SAT grains. However, for x ¼ 0:18, square grains are hardly observed but rounded grains are formed, suggesting that the grains of La–SAT or other oxides have random orientations in this film, which seems consistent with the X-ray results. From these results, we speculate that the La solubility limit in the SAT lattice exists in the x range between 0.11 and 0.18, and thus formation of Labased second phases would disturb cube-on-cube arrangement of the La–SAT grains on the YBCO, although such second phases were not clearly observed in the SEM picture possibly because of their small grain size. Fig. 4 shows the dielectric properties of La– SAT films with the La-doping ratio x ¼ 0, 0.12, and 0.20. The dielectric constant is approximately 19–27 and slightly reduced with increasing the x value. The conductance is low enough, below 106 S, for all x values, indicating that the La-doping does not deteriorate the SAT dielectric properties. However, the dielectric constant less than 20 for the x ¼ 0:2 film may be due to inclusion of other oxides such as La2 O3 . Fig. 5 shows the superconducting properties of 300-nm-thick upper La–YBCO films on the La– SAT films with x ¼ 0–0.2. For x ¼ 0:1, the Jc value is approximately 107 A/cm2 at 4 K and slightly
YBCO (006)
SAT (004)
400
La-SAT (004)
300
Kα 2
Sr/(Al+Ta) 0.94 0.97
200
0.99 100
1.02
x
=0 1.03 0 45.8 46.0 46.2 46.4 46.6 46.8 47.0 2θ (deg)
Intensity (kcps)
40 Kα 1
Intensity (kcps)
1339
(La+Sr)/(Al+Ta) 0.91
30
0.94 20
0.98 10
1.01
x
= 0.2 1.03 0 45.8 46.0 46.2 46.4 46.6 46.8 47.0 2θ (deg)
Fig. 2. XRD (h=2h) patterns for La–SAT films with the La-doping ratio x ¼ 0 and 0.2 which were deposited with varying the (La + Sr)/ (Al + Ta) composition ratio near stoichiometry. The intensity scale for x ¼ 0:2 is expanded by 10 times of that for x ¼ 0 and the dotted lines show the zero point of the intensity for each x value.
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Fig. 3. SEM images of La–SAT surfaces with the La-doping ratio x ¼ 0, 0.11, and 0.18.
0
50
10
-2
40
30
10
-4
20
10
10
10
Dielectric constant
x =0
40
0 3 10
4
10
0
50
10
-2
40
30
10
-4
-6
20
10
-8
10
10
10-10
5
10 10 10 Frequency (Hz)
x = 0.12
0 3 10
6
4
5
10 10 10 Frequency (Hz)
10
0
10
-2
30
10
-4
-6
20
10
-6
-8
10
10
-8
10-10 0 3 10
6
x = 0.20
4
5
Conductance (S)
10
50
10-10
10 10 10 Frequency (Hz)
6
8
10
7
10
6
800 x=0 x = 0.11
105 104 10 10
Resistivity ( µΩ cm)
10
x = 0.18
3
(a)
Resistivity (µΩ cm)
2
Jc (A/cm )
Fig. 4. Dielectric properties of 300-nm-thick La–SAT films with the La-doping ratio x ¼ 0, 0.12, and 0.20, which were evaluated using 300 lm£ planer capacitors. The results of four capacitors are displayed for each La-doping ratio.
2
0
20
40
60
80
Temperature (K)
(b) 600 x = 0.18
400
40
x=0
30 20
x = 0.11
10 0 82
84
86
88
Temperature (K)
200
x=0 x = 0.11
0
0
100 200 Temperature (K)
300
Fig. 5. Superconducting properties, (a) Jc value for 5-lm-wide and 50-lm-long bridges (b) resistivity for 50-lm-wide and 150-lm-long bridges, of 300-nm-thick La–YBCO films on the La–SAT films with the La-doping ratio x ¼ 0–0.2. The Jc values were determined with a criterion of 20 lV/mm.
lower than that for x ¼ 0, while the Tc value is almost the same, approximately 84 K. In contrast, the Tc and Jc values for the x ¼ 0:18 film become significantly lower, below 50 K and 104 A/cm2 at 4 K, respectively. These results are consistent with the change in the film orientation, as shown in
Figs. 1–3, which reflects the La solubility limit in the SAT lattice. In the case of LSAT, two individual compounds, LaAlO3 and SAT, are mixed with the La/ (La + Sr) ratio of 0.3. In contrast, in La–SAT, because La3þ is supposed to be substituted for
Y. Takahashi et al. / Physica C 392–396 (2003) 1337–1341
Sr2þ , the average A-site ionic radius becomes smaller and the cation charge may also change. From the XRF analysis, the mole ratio of the Bsite elements in the La–SAT films was confirmed to be Al=Ta ¼ 0:99–1.01, suggesting that no sole compound with Ta partially substituted by Al such as La0:1 Sr0:9 Al0:55 Ta0:45 O3 and La0:2 Sr0:8 Al0:6 Ta0:4 O3 exists to keep the charge balance. If they were generated, tantalum-based compounds such as Ta2 O5 would be simultaneously generated to satisfy the total mole ratio of Al=Ta ¼ 1:0. A more plausible way of keeping the charge balance is that the valence of Ta ions decreases to, for example, +3, or a slight amount of excess oxygen is incorporated in the lattice, that is, Lax Sr1x Al0:5 Ta0:5 O3þd (d x=2) is formed. In these cases, the La solubility limit in La–SAT is supposed to be dominated by the tolerance limit in the variation of the Ta valence or the oxygen content.
4. Conclusion La–SAT films on c-axis-oriented YBCO films show lattice contraction and a reduction of its dielectric constant with increasing the La-doping ratio to x ffi 0:1. The La solubility limit exists in the x range between 0.1 and 0.2, and the La–SAT changes to have random orientation at x ffi 0:2, although its dielectric properties become still better. It was also confirmed that the superconducting properties of upper La–YBCO thin films grown on La–SAT with x 6 0:1 are almost comparable to those for the case without La-doping.
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Acknowledgements The authors would like to thank K. Nomura, S. Hoshi, T. Izumi, and Y. Shiohara of SRL-ISTEC for providing YBCO thick films grown by liquid phase epitaxy. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development for Superconductivity Applications.
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