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YBa2Cu3Oy films
Physica C 357±360 (2001) 1364±1367
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O2-annealing eects on dielectric properties of Sr2AlTaO6=YBa2Cu3Oy ®lms Yoshihiro Takahashi *, Hideaki Zama, Tadataka Morishita, Keiichi Tanabe SRL-ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan Received 16 October 2000; accepted 7 November 2000
Abstract Approximately 160-nm-thick Sr2 AlTaO6 (SAT) thin ®lms were prepared by metalorganic chemical vapor deposition on liquid phase epitaxy-grown 60-lm-thick YBa2 Cu3 O7 d (YBCO) ®lms. The eects of oxygen postannealing on the dielectric properties of SAT and the superconducting properties of YBCO were investigated. The c-axis length of and its Tc of 90 K was observed after oxidation for 250 h at 500°C in 1 atm O2 YBCO decreased from 11.82 to 11.70 A atmosphere, indicating that YBCO is almost fully oxygenated through SAT. Postannealed samples showed reasonably low dielectric constants for SAT of approximately 24 and low conductance of 10 8 S at 105 Hz which corresponds to a loss tangent of 5 10 4 . On the other hand, the SAT ®lms without postannealing exhibited an order of magnitude larger conductance. The improved dielectric properties of the annealed samples are probably attributed to compensation of oxygen defects at the SAT grain boundaries as well as oxidation of the lower YBCO ®lm. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 73.61. r; 77.55.f; 74.80.Dm; 74.76.Bz Keywords: Annealing; Sr±Al±Ta±O; Dielectric properties
1. Introduction Insulator ®lms are required as intermediate layers for superconducting electronic devices such as single ¯ux quantum (SFQ) digital circuits. However, because of a high temperature and low oxygen pressure during the growth of insulator ®lms, it is dicult to maintain oxygen in lower superconductor ®lms [1]. Thus insulator/superconductor ®lms have been postannealed after preparation of the insulator ®lms in some cases [2,3]. Sr2 AlTaO6 (SAT), a cubic perovskite compound, has been expected as a promising inter*
Corresponding author. Fax: +81-3-3536-5714. E-mail address: [email protected] (Y. Takahashi).
mediate insulator because of a lattice constant approximately twice of the a- and b-axes of YBa2 Cu3 O7 d (YBCO) [4] and a low dielectric constant (e 23±30) [5]. In the present study, we prepared an SAT thin ®lm on a YBCO thick ®lm and postannealed in O2 atmosphere to bring oxygen to the YBCO ®lm, which is considered as a groundplane. The eects of postannealing on dielectric properties of SAT/YBCO bilayers were then examined. 2. Experimental As a lower superconductor, a c-axis-oriented, 60-lm-thick YBCO ®lm prepared by liquid phase epitaxy (LPE) [6] on an MgO substrate was used.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 5 8 2 - 2
Y. Takahashi et al. / Physica C 357±360 (2001) 1364±1367
The as-grown YBCO was highly oxygen de®cient and not superconducting. The SAT thin ®lm was prepared by metalorganic chemical vapor deposition (MOCVD) because it has some advantages such as good ®lm coverage and large-area applications, compared with other deposition methods. Sr THD2 -2-tetraene (strontium bis-2,2,6,6-tetramethyl-3,5-heptanedione bis-tetraethylenepentamine adducts) [7, 8] was employed for a strontium precursor. For aluminum and tantalum precursors, we used a double-metal alkoxide, TaAl(O-iC3 H7 )8 (tantalum aluminum isopropoxide) [9,10] for easier control of the stoichiometry, Al:Ta 1:1. The detailed preparation conditions are summarized in Table 1. The stoichiometric ®lm was con®rmed in X-ray ¯uorescence (XRF) analysis and its thickness was approximately 160 nm by cross-sectional scanning electron microscopy (SEM). The SAT/YBCO bilayer sample was annealed in a conventional furnace at 500°C with a heating rate of 200°C/h and a cooling rate of 200°C/h or less. The oxygen pressure was 1 atm and the oxygen ¯ow rate was 1 slm. The oxygen amounts in the YBCO ®lm was estimated from the c-axis length determined by X-ray diraction (XRD). The superconductivity of the YBCO ®lm was evaluated by superconducting quantum interference device (SQUID) measurement in 3 Oe magnetic ®eld. For measurements of dielectric properties, parallel capacitors, as schematically shown in Fig. 1, were fabricated. After deposition of approximately 600-nm-thick Au layer on the SAT/YBCO ®lm by rf magnetron sputtering, the capacitor structures were fabricated by standard photoTable 1 Preparation conditions of SAT ®lms by MOCVD Vessel temperature Carrier Ar gas ¯ow rate O2 gas ¯ow rate Total gas ¯ow rate Reactor pressure Substrate temperature Deposition rate
Sr THD2 -2-tetraene TaAl(O-iC3 H7 )8 Sr THD2 -2-tetraene TaAl(O-iC3 H7 )8 100 sccm 1000 sccm 13 hPa 750°C(set value) 100±200 nm/h
100±130°C 100°C 300±500 sccm 5±10 sccm
1365
Fig. 1. Capacitor structure fabricated for the measurements of dielectric properties.
lithography and ion milling. The admittance was measured using an HP4194A impedance analyzer in the frequency range of 103 ±106 Hz. The sample temperature was varied from 4 to 300 K in a cryostat. Moreover, another SAT/YBCO sample without postannealing was prepared to investigate the dierence of dielectric properties between before and after postannealing, where the YBCO thickness was thinner (6 lm) to shorten the annealing time. It was postannealed after estimation of dielectric properties, then its dielectric properties were estimated again.
3. Results and discussion Fig. 2 shows the XRD (h=2h) peaks of SAT(0 0 8) and YBCO(0 0 1 2) before and after postannealing in 1 atm O2 atmosphere at 500°C for 250 h. After postannealing for 250 h, the YBCO(0 0 1 2) peak shifted to a higher angle position which corresponds to the c-axis length of while the SAT(0 0 8) peak did not shift. 11.70 A, Fig. 3 shows the c-axis length of the YBCO ®lm calculated from the YBCO(0 0 1 2) peak at each annealing time. Because the YBCO ®lm was very thick (60 lm), it needs more than 50 h to reach the Fig. 4 shows the temperature length of 11.70 A. dependence of magnetic moment by SQUID measurement in a zero-®eld cooled condition for the 250-h-postannealed SAT/YBCO ®lm. A sharp evolution of diamagnetic moment is observed at 90 K. The decrease of the c-axis length and the relatively high Tc reveal almost full oxidation of the
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Y. Takahashi et al. / Physica C 357±360 (2001) 1364±1367
Fig. 4. Magnetic moment of the SAT/YBCO ®lm after postannealing at 500°C for 250 h by SQUID measurement. Fig. 2. XRD pattern near SAT(0 0 8) and YBCO(0 0 1 2) peaks before and after postannealing of the SAT/YBCO ®lm.
Fig. 5. Dielectric constant and conductance of the postannealed SAT/YBCO ®lm in a frequency range of 103 ±106 Hz evaluated at dierent temperatures. Fig. 3. c-axis length of YBCO calculated from the XRD peak at each postannealing time.
YBCO ®lm through the upper SAT ®lm by postannealing, at least within the X-ray penetration depth (10±20 lm) from the interface. Fig. 5 shows the dielectric constant and conductance of the SAT ®lm on YBCO after postan-
nealing at dierent temperatures of 4±300 K. The dielectric constant is approximately 24 and the conductance is quite low, 10 8 S at 105 Hz which corresponds to a loss tangent of 5 10 4 . Fig. 6 shows the dielectric properties in another SAT/ YBCO sample before and after postannealing in 1 atm O2 atmosphere at 500°C for 250 h. Conductance before postannealing is about ten times lar-
Y. Takahashi et al. / Physica C 357±360 (2001) 1364±1367
1367
electric properties of the SAT ®lm. The SAT ®lm after postannealing exhibits almost temperature independent dielectric constant of approximately 24 and low conductance corresponding to a loss tangent of 5 10 4 which are considered to be brought about by compensation of oxygen defects at the SAT grain boundaries.
Acknowledgements
Fig. 6. Dielectric constant and conductance of the SAT/YBCO ®lm at 4 K before and after postannealing.
ger than that of the postannealed sample. Though it is not clear whether oxygen could pass through the Au layer, it might pass through the SAT in the area without Au, and diuse horizontally. These results demonstrate that postannealing is eective to improve not only the superconductivity of lower YBCO but also the dielectric properties of SAT. The as-prepared SAT ®lm consists of many small grains and there may be substantial oxygen defects at the grain boundaries that cause leakage current. These defects are probably compensated in oxygen postannealing.
4. Conclusion The LPE-grown 60-lm-thick YBCO ®lm can be almost fully oxygenated through the 160-nm-thick upper SAT ®lm by oxygen postannealing at 500°C for 250 h. The oxygen postannealing improves not only the superconductivity of YBCO but also di-
The authors would like to thank K. Nomura, S. Hoshi, T. Izumi, and Y. Shiohara of SRL-ISTEC for providing YBCO thick ®lms grown by LPE. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development for Superconductivity Applications.
References [1] R.H. Hammond, R. Bormann, Physica C 162/164 (1989) 703. [2] J. Talvacchio, R.M. Young, M.G. Forrester, B.D. Hunt, IEEE Trans. Appl. Supercond. 9 (1999) 1990. [3] Y. Li, J.G. Wen, K. Tanabe, Physica C 333 (2000) 53. [4] C.D. Brandle, V.J. Fratello, J. Mater. Res. 5 (1990) 2160. [5] A.T. Findikoglu, C. Doughty, S. Bhattacharya, Q. Li, X.X. Xi, T. Venkatesan, R.E. Fahey, A.J. Trauss, J.M. Phillips, Appl. Phys. Lett. 61 (1992) 1718. [6] K. Nomura, S. Hoshi, X. Yao, K. Kakimoto, T. Izumi, Y. Shiohara, Advances in Superconductivity XII, Springer, Tokyo, 2000, p. 891. [7] T. Kimura, H. Yamauchi, H. Machida, H. Kokubun, M. Yamada, Jpn. J. Appl. Phys. 33 (1994) 5119. [8] H. Zama, T. Morishita, Jpn. J. Appl. Phys. 35 (1996) L770. [9] S. Govil, P.N. Kapoor, R.C. Mehrotra, Inorg. Chim. Acta. 15 (1975) 43. [10] D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides, Academic Press, San Diego, CA, 1978, p. 319.
www.elsevier.com/locate/physc
O2-annealing eects on dielectric properties of Sr2AlTaO6=YBa2Cu3Oy ®lms Yoshihiro Takahashi *, Hideaki Zama, Tadataka Morishita, Keiichi Tanabe SRL-ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japan Received 16 October 2000; accepted 7 November 2000
Abstract Approximately 160-nm-thick Sr2 AlTaO6 (SAT) thin ®lms were prepared by metalorganic chemical vapor deposition on liquid phase epitaxy-grown 60-lm-thick YBa2 Cu3 O7 d (YBCO) ®lms. The eects of oxygen postannealing on the dielectric properties of SAT and the superconducting properties of YBCO were investigated. The c-axis length of and its Tc of 90 K was observed after oxidation for 250 h at 500°C in 1 atm O2 YBCO decreased from 11.82 to 11.70 A atmosphere, indicating that YBCO is almost fully oxygenated through SAT. Postannealed samples showed reasonably low dielectric constants for SAT of approximately 24 and low conductance of 10 8 S at 105 Hz which corresponds to a loss tangent of 5 10 4 . On the other hand, the SAT ®lms without postannealing exhibited an order of magnitude larger conductance. The improved dielectric properties of the annealed samples are probably attributed to compensation of oxygen defects at the SAT grain boundaries as well as oxidation of the lower YBCO ®lm. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 73.61. r; 77.55.f; 74.80.Dm; 74.76.Bz Keywords: Annealing; Sr±Al±Ta±O; Dielectric properties
1. Introduction Insulator ®lms are required as intermediate layers for superconducting electronic devices such as single ¯ux quantum (SFQ) digital circuits. However, because of a high temperature and low oxygen pressure during the growth of insulator ®lms, it is dicult to maintain oxygen in lower superconductor ®lms [1]. Thus insulator/superconductor ®lms have been postannealed after preparation of the insulator ®lms in some cases [2,3]. Sr2 AlTaO6 (SAT), a cubic perovskite compound, has been expected as a promising inter*
Corresponding author. Fax: +81-3-3536-5714. E-mail address: [email protected] (Y. Takahashi).
mediate insulator because of a lattice constant approximately twice of the a- and b-axes of YBa2 Cu3 O7 d (YBCO) [4] and a low dielectric constant (e 23±30) [5]. In the present study, we prepared an SAT thin ®lm on a YBCO thick ®lm and postannealed in O2 atmosphere to bring oxygen to the YBCO ®lm, which is considered as a groundplane. The eects of postannealing on dielectric properties of SAT/YBCO bilayers were then examined. 2. Experimental As a lower superconductor, a c-axis-oriented, 60-lm-thick YBCO ®lm prepared by liquid phase epitaxy (LPE) [6] on an MgO substrate was used.
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 5 8 2 - 2
Y. Takahashi et al. / Physica C 357±360 (2001) 1364±1367
The as-grown YBCO was highly oxygen de®cient and not superconducting. The SAT thin ®lm was prepared by metalorganic chemical vapor deposition (MOCVD) because it has some advantages such as good ®lm coverage and large-area applications, compared with other deposition methods. Sr THD2 -2-tetraene (strontium bis-2,2,6,6-tetramethyl-3,5-heptanedione bis-tetraethylenepentamine adducts) [7, 8] was employed for a strontium precursor. For aluminum and tantalum precursors, we used a double-metal alkoxide, TaAl(O-iC3 H7 )8 (tantalum aluminum isopropoxide) [9,10] for easier control of the stoichiometry, Al:Ta 1:1. The detailed preparation conditions are summarized in Table 1. The stoichiometric ®lm was con®rmed in X-ray ¯uorescence (XRF) analysis and its thickness was approximately 160 nm by cross-sectional scanning electron microscopy (SEM). The SAT/YBCO bilayer sample was annealed in a conventional furnace at 500°C with a heating rate of 200°C/h and a cooling rate of 200°C/h or less. The oxygen pressure was 1 atm and the oxygen ¯ow rate was 1 slm. The oxygen amounts in the YBCO ®lm was estimated from the c-axis length determined by X-ray diraction (XRD). The superconductivity of the YBCO ®lm was evaluated by superconducting quantum interference device (SQUID) measurement in 3 Oe magnetic ®eld. For measurements of dielectric properties, parallel capacitors, as schematically shown in Fig. 1, were fabricated. After deposition of approximately 600-nm-thick Au layer on the SAT/YBCO ®lm by rf magnetron sputtering, the capacitor structures were fabricated by standard photoTable 1 Preparation conditions of SAT ®lms by MOCVD Vessel temperature Carrier Ar gas ¯ow rate O2 gas ¯ow rate Total gas ¯ow rate Reactor pressure Substrate temperature Deposition rate
Sr THD2 -2-tetraene TaAl(O-iC3 H7 )8 Sr THD2 -2-tetraene TaAl(O-iC3 H7 )8 100 sccm 1000 sccm 13 hPa 750°C(set value) 100±200 nm/h
100±130°C 100°C 300±500 sccm 5±10 sccm
1365
Fig. 1. Capacitor structure fabricated for the measurements of dielectric properties.
lithography and ion milling. The admittance was measured using an HP4194A impedance analyzer in the frequency range of 103 ±106 Hz. The sample temperature was varied from 4 to 300 K in a cryostat. Moreover, another SAT/YBCO sample without postannealing was prepared to investigate the dierence of dielectric properties between before and after postannealing, where the YBCO thickness was thinner (6 lm) to shorten the annealing time. It was postannealed after estimation of dielectric properties, then its dielectric properties were estimated again.
3. Results and discussion Fig. 2 shows the XRD (h=2h) peaks of SAT(0 0 8) and YBCO(0 0 1 2) before and after postannealing in 1 atm O2 atmosphere at 500°C for 250 h. After postannealing for 250 h, the YBCO(0 0 1 2) peak shifted to a higher angle position which corresponds to the c-axis length of while the SAT(0 0 8) peak did not shift. 11.70 A, Fig. 3 shows the c-axis length of the YBCO ®lm calculated from the YBCO(0 0 1 2) peak at each annealing time. Because the YBCO ®lm was very thick (60 lm), it needs more than 50 h to reach the Fig. 4 shows the temperature length of 11.70 A. dependence of magnetic moment by SQUID measurement in a zero-®eld cooled condition for the 250-h-postannealed SAT/YBCO ®lm. A sharp evolution of diamagnetic moment is observed at 90 K. The decrease of the c-axis length and the relatively high Tc reveal almost full oxidation of the
1366
Y. Takahashi et al. / Physica C 357±360 (2001) 1364±1367
Fig. 4. Magnetic moment of the SAT/YBCO ®lm after postannealing at 500°C for 250 h by SQUID measurement. Fig. 2. XRD pattern near SAT(0 0 8) and YBCO(0 0 1 2) peaks before and after postannealing of the SAT/YBCO ®lm.
Fig. 5. Dielectric constant and conductance of the postannealed SAT/YBCO ®lm in a frequency range of 103 ±106 Hz evaluated at dierent temperatures. Fig. 3. c-axis length of YBCO calculated from the XRD peak at each postannealing time.
YBCO ®lm through the upper SAT ®lm by postannealing, at least within the X-ray penetration depth (10±20 lm) from the interface. Fig. 5 shows the dielectric constant and conductance of the SAT ®lm on YBCO after postan-
nealing at dierent temperatures of 4±300 K. The dielectric constant is approximately 24 and the conductance is quite low, 10 8 S at 105 Hz which corresponds to a loss tangent of 5 10 4 . Fig. 6 shows the dielectric properties in another SAT/ YBCO sample before and after postannealing in 1 atm O2 atmosphere at 500°C for 250 h. Conductance before postannealing is about ten times lar-
Y. Takahashi et al. / Physica C 357±360 (2001) 1364±1367
1367
electric properties of the SAT ®lm. The SAT ®lm after postannealing exhibits almost temperature independent dielectric constant of approximately 24 and low conductance corresponding to a loss tangent of 5 10 4 which are considered to be brought about by compensation of oxygen defects at the SAT grain boundaries.
Acknowledgements
Fig. 6. Dielectric constant and conductance of the SAT/YBCO ®lm at 4 K before and after postannealing.
ger than that of the postannealed sample. Though it is not clear whether oxygen could pass through the Au layer, it might pass through the SAT in the area without Au, and diuse horizontally. These results demonstrate that postannealing is eective to improve not only the superconductivity of lower YBCO but also the dielectric properties of SAT. The as-prepared SAT ®lm consists of many small grains and there may be substantial oxygen defects at the grain boundaries that cause leakage current. These defects are probably compensated in oxygen postannealing.
4. Conclusion The LPE-grown 60-lm-thick YBCO ®lm can be almost fully oxygenated through the 160-nm-thick upper SAT ®lm by oxygen postannealing at 500°C for 250 h. The oxygen postannealing improves not only the superconductivity of YBCO but also di-
The authors would like to thank K. Nomura, S. Hoshi, T. Izumi, and Y. Shiohara of SRL-ISTEC for providing YBCO thick ®lms grown by LPE. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development for Superconductivity Applications.
References [1] R.H. Hammond, R. Bormann, Physica C 162/164 (1989) 703. [2] J. Talvacchio, R.M. Young, M.G. Forrester, B.D. Hunt, IEEE Trans. Appl. Supercond. 9 (1999) 1990. [3] Y. Li, J.G. Wen, K. Tanabe, Physica C 333 (2000) 53. [4] C.D. Brandle, V.J. Fratello, J. Mater. Res. 5 (1990) 2160. [5] A.T. Findikoglu, C. Doughty, S. Bhattacharya, Q. Li, X.X. Xi, T. Venkatesan, R.E. Fahey, A.J. Trauss, J.M. Phillips, Appl. Phys. Lett. 61 (1992) 1718. [6] K. Nomura, S. Hoshi, X. Yao, K. Kakimoto, T. Izumi, Y. Shiohara, Advances in Superconductivity XII, Springer, Tokyo, 2000, p. 891. [7] T. Kimura, H. Yamauchi, H. Machida, H. Kokubun, M. Yamada, Jpn. J. Appl. Phys. 33 (1994) 5119. [8] H. Zama, T. Morishita, Jpn. J. Appl. Phys. 35 (1996) L770. [9] S. Govil, P.N. Kapoor, R.C. Mehrotra, Inorg. Chim. Acta. 15 (1975) 43. [10] D.C. Bradley, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides, Academic Press, San Diego, CA, 1978, p. 319.