- Email: [email protected]
Influence of substrate and precursor film composition on morphology and superconducting transition of Tl-2212 thin films characterized by microwaves
Physica C 354 (2001) 429±432
www.elsevier.nl/locate/physc
In¯uence of substrate and precursor ®lm composition on morphology and superconducting transition of Tl-2212 thin ®lms characterized by microwaves Chromik a,*, M. Jergel b, S. Gazi a, V. Strb õk a, F. Hanic c, C. Falcony b, S. Be M. Vasko a, S. nacka a a
b
Institute of Electrical Engineering, SAS, 842 39 Bratislava, Slovakia CINVESTAV-IPN, Departamento de Fõsica, Apdo. Postal 14-740, 07000 Mexico DF, Mexico c Institute of Measurement Sciences, SAS, 842 19 Bratislava, Slovakia
Abstract An in¯uence of various substrates (MgO, LaAlO3 , SrTiO3 , YSZ, CeO2 /r-sapphire) and Ba±Ca±Cu precursor chemical composition (212, 223) upon morphology and microwave quality of Tl2 Ba2 CaCu2 O8 (Tl-2212) thin ®lms has been studied. Under the same thalliation conditions, Tl-2212 phase has been synthesized in all ®lms independently of the initial precursor ®lm chemical composition (212, 223) with almost the same Tc values. However, the ®lm surface morphology and, thus, the microwave surface impedance, both were found to be strongly dependent from the type of substrate used as well as from the precursor chemical composition. Films with sharp transition to the superconducting state were prepared when 212 precursors were deposited on the MgO and/or LaAlO3 substrates. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Tl-based thin ®lms; Morphology; Microwaves
1. Introduction The type of substrate and precursor ®lm composition together with parameters of ex situ thalliation in¯uence ®nal properties of Tl2 Ba2 CaCu2 O8 (Tl-2212) superconducting ®lms [1±5]. However, some groups apply precursor with stoichiometry corresponding to Tl2 Ba2 Ca2 Cu3 Ox (Tl2223) [6,7]. Indeed, our experience con®rms, that X-ray spectra exhibit only 2212 phase and the
*
Corresponding author: Fax: +421-7-5477-5816. Chromik). E-mail address: [email protected] (S.
®lms have approximately the same dc measured critical temperature, in spite of dierent substrates and precursor ®lms when applying the same thalliation process. We decided therefore to study in this article more in detail the in¯uence of dierent substrates and two dierent compositions of precursor ®lms on some properties (morphology and superconducting transition characterized by the microwaves) of prepared Tl ®lms. Beside the study of phase composition and morphology, we applied a low magnetic ®eld microwave absorption (magnetic ®eld does not suppress superconducting properties) to characterize the transition of ®lms
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 1 1 0 - 1
430
Chromik et al. / Physica C 354 (2001) 429±432 S.
into the superconducting state, which is a more stringent test of the ®lm quality than the DC characterization [8]. 2. Experimental Tl-2212 thin ®lms 200 and 400 nm thick were synthesized on single-crystal MgO(1 0 0), LaAlO3 (1 0 0), SrTiO3 (1 0 0), YSZ(1 0 0) and r-sapphire buered by CeO2 , from precursor prepared by sequential thermal evaporation of BaF2 , Cu, and CaF2 components, to give a metallic ratios Ba:Ca:Cu 2:2:3 (223 precursor ®lm) or 2:1:2 (212 precursor ®lm) followed by vacuum annealing in dry oxygen to reduce F2 . Vacuum annealing in dry oxygen was performed at 700°C with gradually increasing partial pressure of oxygen [9]. Deposited precursors were thalliated at 850°C for 30 min in ¯owing oxygen with a crude Tl± Ba±Ca±Cu±O pellet serving as a source of partial pressure of thallous vapour. The ®lms were characterised by X-ray, SEM, resistance vs temperature (R±T) and microwave measurements. Standard four point resistive method with detection criterion 1 lV/cm was used to determine the Tco value. X-ray diraction patterns were examined by Philips powder diractometer, using CuKa radiation. The surface morphology of prepared ®lms was studied by Hitachi S-800 scanning electron microscope. The microwave absorption measurements of superconducting ®lms were performed in a cavity resonator ®lled with sapphire rod using TE011 mode. Resonant frequency was f0 10 GHz. Dissipated microwave power aects the quality factor Q of microwave cavity Q 1 f0 dP=W, where dP=W is a ratio of the dissipated power and energy stored in the resonator. As results from microwave analysis we determine the temperature dependence of parameter Q=QR where QR is a quality of empty resonator (2000). 3. Results and discussion X-ray diraction patterns of Tl±Ba±Ca-Cu±O ®lms prepared with a dierent composition of precursor ®lm (212 and 223), at the same vacuum
Fig. 1. X-ray diraction patterns of Tl-2212 ®lms on CeO2 /rsapphire substrates prepared from (a) 212 precursor, and (b) 223 precursor ®lms, s-sapphire substrate.
annealing and thalliation process, are compared for CeO2 /r-sapphire substrates in Fig. 1. We see that for both compositions practically only the peaks corresponding to preferentially c-axes oriented Tl-2212 phase appear. Small dierences in intensity of peaks can be a consequence of the bulk substrate angular accuracy setting. The DC measured Tco values of all prepared ®lms were in the range of 95±100 K. The measurements of critical temperature showed a tendency of lower Tco values by about 1±4 K in case of a single crystal YSZ and r-sapphire substrates buered by CeO2 against MgO, LaAlO3 and SrTiO3 . However, we did not observe any in¯uence of composition and thickness of precursor ®lm on critical temperature of Tl-2212 ®lm for the same substrate. On the base of above received results we suppose that Tl-2212 ®lm prepared from 223 precursor ®lm contains an excess material in an amorphous form and we expect a more complicated surface morphology and decrease of microwave Q-factor due to higher dissipation in non- or less-superconducting parts of the thin ®lm. Fig. 2 presents SEM images of Tl-2212 ®lms prepared by the thalliation of 212 and 223 precursor ®lms on CeO2 /r-sapphire substrate. The ®lm prepared using 212 precursor (Fig. 2a) evidently shows much better surface morphology with the diameter of the surface defects (voids) below 0.5 lm in comparison
Chromik et al. / Physica C 354 (2001) 429±432 S.
431
Fig. 4. SEM micrograph of Tl-2212 ®lm prepared from 223 precursor on YSZ substrate. Fig. 2. SEM micrographs of Tl-2212 ®lms of Fig. 1, prepared from (a) 212 precursor, and (b) 223 precursor ®lms.
to the ®lm having more granular structure prepared from 223 precursor. Fig. 3 compares SEM micrographs of two granular ®lms (typical for large lattice mismatch [1]) of dierent thickness on YSZ substrate (lattice mismatch 6%) for the 223 precursor composition. We observe in case of a double thickness more rough ®lm surface with larger voids between grains. It is evident that there is a correlation between the ®lm thickness and the size of grains and defects. We found out that the lattice matching of LaAlO3 and SrTiO3 with Tl2212 ®lm (mismatch 2%), prepared even from 223 precursor ®lm, gives a rather smooth ®lm with defects in the form of voids (Fig. 4). The welllattice matched substrates with the superconductor
Fig. 3. SEM micrographs of Tl-2212 ®lms on YSZ substrates prepared from 223 precursors, (a) 200 nm thick, (b) 400 nm thick.
unit cell enabled us to achieve biaxial grain alignment (a-axis of Tl-2212 aligned with a-axis of substrate), while the large lattice mismatch supports the tendency of creating the less oriented and more pronounced granular structure [1]. Fig. 5 shows typical temperature dependencies of microwave resonator normalized quality Q=QR . For MgO and CeO2 /r-sapphire substrates, we observe relatively similar behaviour. We observe higher quality values and sharper transition into the superconducting state in case of Tl-2212 ®lms
Fig. 5. Normalized quality (Q=QR ) vs. temperature dependence of the Tl-2212 ®lms prepared on MgO substrates from (a) 212 and (b) 223 precursors, and ®lms prepared on CeO2 /r-sapphire substrates from (c) 212 precursor and 223 precursor 200 nm thick (d) and 400 nm thick (e).
432
Chromik et al. / Physica C 354 (2001) 429±432 S.
prepared using 212-precursor ®lm (curves a,c) in comparison with ®lms prepared on 223 precursor (b,d,e). The lower Q=QR value of the resonator for Tl ®lms deposited on CeO2 buer layer (curve c) can be caused by structural defects in CeO2 which induce large-angle grain boundary weak links in the superconducting ®lm or possible consequence of a reaction between the CeO2 buer layer and Tl2212 ®lm, forming a layer of the BaCe(Tl)O3 at interface [10] and, thus, also an increase of microwave losses. The curve e con®rms an improving of microwave Q-factor in case of ®lms with double thickness where, in spite of a rough surface morphology, we observe the shift in superconducting transition curve to higher temperatures due to stronger coupling between larger grains, while the shape of Q±T dependence remains similar. This observations is in agreement with Martens et al. [11] conclusions that microwave losses decrease up to optimum ®lm thickness of about 0.6 lm. The above presented measurements show clearly that the presence of an excess material in Tl-2212 ®lms, as a consequence of using the 223 precursors, deteriorate the microwave properties of the superconducting ®lms. 4. Summary We examined the in¯uence of substrates and precursor ®lm composition on the surface morphology and superconducting transition of the Tl-2212 thin ®lms, characterized by means of a microwave method. The X-ray analyses indicate a presence of only Tl-2212 superconducting phase regardless of the used substrate, precursor composition (212 and 223) and the thickness (200 and/ or 400 nm) of the precursor ®lm, at the same annealing and thalliation processes. SEM investigations have shown clear in¯uence of precursor ®lm composition and a type of the substrate used on the surface morphology of ®lms.
The 223 precursor ®lm increases granularity and roughness of the prepared Tl-2212 ®lm. Microwave measurements of superconducting phase transition of Tl-2212 thin ®lms are sensitive to the composition of precursor ®lm and clearly show increase of microwave power dissipation in the ®lms of nonstoichiometric precursor. In case of single crystal substrates, the microwave measurements con®rm high quality of ®lms. Acknowledgements The authors appreciate the support of the Slovak grant agency (VEGA grant No. 6057/99) and I. Kostic, D. Machajdõk helping with the ®lm characterization. References [1] A.P. Bramley, J.D. OÕConnor, C.R.M. Grovenor, Supercond. Sci. Technol. 12 (1999) R57. [2] Y.Q. Tang, K.Y. Chen, S. Afonso, X.L. Xu, Q. Xiong, G. Salamo, F.T. Chan, R. Guo, A. Bhalla, J. Appl. Phys. 78 (1995) 6846. [3] W.L. Holstein, I.A. Parisi, D.W. Face, X.D. Wu, S.R. Foltyn, R.E. Muenchausen, Appl. Phys. Lett. 61 (1992) 982. [4] M. Nemoto, S. Yoshikawa, K. Shimaoka, K. Niki, I. Yoshida, Y. Yoshisato, IEEE Trans. Appl. Supercond. 7 (1997) 1895. [5] H.C. Lai, K.D. Vernon-Parry, J.D. Chern, C.R.M. Grovenor, Supercond. Sci. Technol. 4 (1991) 306. [6] A. Conde-Gallardo, G. Contreras-Puente, M. Jergel, C. Falcony, A. Escamilla, F. Hanic, Physica C 288 (1997) 64. [7] J. Chrzanowsky, J.C. Irwin, B. Heinrich, Physica C 182 (1991) 231. Chromik, S. Be [8] M. MahelÕ, R. Adam, M. Darula, S. nacka, Physica C 202 (1992) 243. Chromik, M. Jergel, F. Hanic, S. Be [9] S. nacka, J. Liday, H. Tollet, C. Creemers, Physica C 230 (1994) 82. [10] A.P. Bramley, S.M. Morley, C.R.M. Grovenor, B. Pecz, Appl. Phys. Lett. 66 (1995) 517. [11] J.S. Martens, V.M. Hietala, E.L. Venturini, W.Y. Lee, J. Appl. Phys. 73 (1993) 7571.
www.elsevier.nl/locate/physc
In¯uence of substrate and precursor ®lm composition on morphology and superconducting transition of Tl-2212 thin ®lms characterized by microwaves Chromik a,*, M. Jergel b, S. Gazi a, V. Strb õk a, F. Hanic c, C. Falcony b, S. Be M. Vasko a, S. nacka a a
b
Institute of Electrical Engineering, SAS, 842 39 Bratislava, Slovakia CINVESTAV-IPN, Departamento de Fõsica, Apdo. Postal 14-740, 07000 Mexico DF, Mexico c Institute of Measurement Sciences, SAS, 842 19 Bratislava, Slovakia
Abstract An in¯uence of various substrates (MgO, LaAlO3 , SrTiO3 , YSZ, CeO2 /r-sapphire) and Ba±Ca±Cu precursor chemical composition (212, 223) upon morphology and microwave quality of Tl2 Ba2 CaCu2 O8 (Tl-2212) thin ®lms has been studied. Under the same thalliation conditions, Tl-2212 phase has been synthesized in all ®lms independently of the initial precursor ®lm chemical composition (212, 223) with almost the same Tc values. However, the ®lm surface morphology and, thus, the microwave surface impedance, both were found to be strongly dependent from the type of substrate used as well as from the precursor chemical composition. Films with sharp transition to the superconducting state were prepared when 212 precursors were deposited on the MgO and/or LaAlO3 substrates. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Tl-based thin ®lms; Morphology; Microwaves
1. Introduction The type of substrate and precursor ®lm composition together with parameters of ex situ thalliation in¯uence ®nal properties of Tl2 Ba2 CaCu2 O8 (Tl-2212) superconducting ®lms [1±5]. However, some groups apply precursor with stoichiometry corresponding to Tl2 Ba2 Ca2 Cu3 Ox (Tl2223) [6,7]. Indeed, our experience con®rms, that X-ray spectra exhibit only 2212 phase and the
*
Corresponding author: Fax: +421-7-5477-5816. Chromik). E-mail address: [email protected] (S.
®lms have approximately the same dc measured critical temperature, in spite of dierent substrates and precursor ®lms when applying the same thalliation process. We decided therefore to study in this article more in detail the in¯uence of dierent substrates and two dierent compositions of precursor ®lms on some properties (morphology and superconducting transition characterized by the microwaves) of prepared Tl ®lms. Beside the study of phase composition and morphology, we applied a low magnetic ®eld microwave absorption (magnetic ®eld does not suppress superconducting properties) to characterize the transition of ®lms
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 1 1 0 - 1
430
Chromik et al. / Physica C 354 (2001) 429±432 S.
into the superconducting state, which is a more stringent test of the ®lm quality than the DC characterization [8]. 2. Experimental Tl-2212 thin ®lms 200 and 400 nm thick were synthesized on single-crystal MgO(1 0 0), LaAlO3 (1 0 0), SrTiO3 (1 0 0), YSZ(1 0 0) and r-sapphire buered by CeO2 , from precursor prepared by sequential thermal evaporation of BaF2 , Cu, and CaF2 components, to give a metallic ratios Ba:Ca:Cu 2:2:3 (223 precursor ®lm) or 2:1:2 (212 precursor ®lm) followed by vacuum annealing in dry oxygen to reduce F2 . Vacuum annealing in dry oxygen was performed at 700°C with gradually increasing partial pressure of oxygen [9]. Deposited precursors were thalliated at 850°C for 30 min in ¯owing oxygen with a crude Tl± Ba±Ca±Cu±O pellet serving as a source of partial pressure of thallous vapour. The ®lms were characterised by X-ray, SEM, resistance vs temperature (R±T) and microwave measurements. Standard four point resistive method with detection criterion 1 lV/cm was used to determine the Tco value. X-ray diraction patterns were examined by Philips powder diractometer, using CuKa radiation. The surface morphology of prepared ®lms was studied by Hitachi S-800 scanning electron microscope. The microwave absorption measurements of superconducting ®lms were performed in a cavity resonator ®lled with sapphire rod using TE011 mode. Resonant frequency was f0 10 GHz. Dissipated microwave power aects the quality factor Q of microwave cavity Q 1 f0 dP=W, where dP=W is a ratio of the dissipated power and energy stored in the resonator. As results from microwave analysis we determine the temperature dependence of parameter Q=QR where QR is a quality of empty resonator (2000). 3. Results and discussion X-ray diraction patterns of Tl±Ba±Ca-Cu±O ®lms prepared with a dierent composition of precursor ®lm (212 and 223), at the same vacuum
Fig. 1. X-ray diraction patterns of Tl-2212 ®lms on CeO2 /rsapphire substrates prepared from (a) 212 precursor, and (b) 223 precursor ®lms, s-sapphire substrate.
annealing and thalliation process, are compared for CeO2 /r-sapphire substrates in Fig. 1. We see that for both compositions practically only the peaks corresponding to preferentially c-axes oriented Tl-2212 phase appear. Small dierences in intensity of peaks can be a consequence of the bulk substrate angular accuracy setting. The DC measured Tco values of all prepared ®lms were in the range of 95±100 K. The measurements of critical temperature showed a tendency of lower Tco values by about 1±4 K in case of a single crystal YSZ and r-sapphire substrates buered by CeO2 against MgO, LaAlO3 and SrTiO3 . However, we did not observe any in¯uence of composition and thickness of precursor ®lm on critical temperature of Tl-2212 ®lm for the same substrate. On the base of above received results we suppose that Tl-2212 ®lm prepared from 223 precursor ®lm contains an excess material in an amorphous form and we expect a more complicated surface morphology and decrease of microwave Q-factor due to higher dissipation in non- or less-superconducting parts of the thin ®lm. Fig. 2 presents SEM images of Tl-2212 ®lms prepared by the thalliation of 212 and 223 precursor ®lms on CeO2 /r-sapphire substrate. The ®lm prepared using 212 precursor (Fig. 2a) evidently shows much better surface morphology with the diameter of the surface defects (voids) below 0.5 lm in comparison
Chromik et al. / Physica C 354 (2001) 429±432 S.
431
Fig. 4. SEM micrograph of Tl-2212 ®lm prepared from 223 precursor on YSZ substrate. Fig. 2. SEM micrographs of Tl-2212 ®lms of Fig. 1, prepared from (a) 212 precursor, and (b) 223 precursor ®lms.
to the ®lm having more granular structure prepared from 223 precursor. Fig. 3 compares SEM micrographs of two granular ®lms (typical for large lattice mismatch [1]) of dierent thickness on YSZ substrate (lattice mismatch 6%) for the 223 precursor composition. We observe in case of a double thickness more rough ®lm surface with larger voids between grains. It is evident that there is a correlation between the ®lm thickness and the size of grains and defects. We found out that the lattice matching of LaAlO3 and SrTiO3 with Tl2212 ®lm (mismatch 2%), prepared even from 223 precursor ®lm, gives a rather smooth ®lm with defects in the form of voids (Fig. 4). The welllattice matched substrates with the superconductor
Fig. 3. SEM micrographs of Tl-2212 ®lms on YSZ substrates prepared from 223 precursors, (a) 200 nm thick, (b) 400 nm thick.
unit cell enabled us to achieve biaxial grain alignment (a-axis of Tl-2212 aligned with a-axis of substrate), while the large lattice mismatch supports the tendency of creating the less oriented and more pronounced granular structure [1]. Fig. 5 shows typical temperature dependencies of microwave resonator normalized quality Q=QR . For MgO and CeO2 /r-sapphire substrates, we observe relatively similar behaviour. We observe higher quality values and sharper transition into the superconducting state in case of Tl-2212 ®lms
Fig. 5. Normalized quality (Q=QR ) vs. temperature dependence of the Tl-2212 ®lms prepared on MgO substrates from (a) 212 and (b) 223 precursors, and ®lms prepared on CeO2 /r-sapphire substrates from (c) 212 precursor and 223 precursor 200 nm thick (d) and 400 nm thick (e).
432
Chromik et al. / Physica C 354 (2001) 429±432 S.
prepared using 212-precursor ®lm (curves a,c) in comparison with ®lms prepared on 223 precursor (b,d,e). The lower Q=QR value of the resonator for Tl ®lms deposited on CeO2 buer layer (curve c) can be caused by structural defects in CeO2 which induce large-angle grain boundary weak links in the superconducting ®lm or possible consequence of a reaction between the CeO2 buer layer and Tl2212 ®lm, forming a layer of the BaCe(Tl)O3 at interface [10] and, thus, also an increase of microwave losses. The curve e con®rms an improving of microwave Q-factor in case of ®lms with double thickness where, in spite of a rough surface morphology, we observe the shift in superconducting transition curve to higher temperatures due to stronger coupling between larger grains, while the shape of Q±T dependence remains similar. This observations is in agreement with Martens et al. [11] conclusions that microwave losses decrease up to optimum ®lm thickness of about 0.6 lm. The above presented measurements show clearly that the presence of an excess material in Tl-2212 ®lms, as a consequence of using the 223 precursors, deteriorate the microwave properties of the superconducting ®lms. 4. Summary We examined the in¯uence of substrates and precursor ®lm composition on the surface morphology and superconducting transition of the Tl-2212 thin ®lms, characterized by means of a microwave method. The X-ray analyses indicate a presence of only Tl-2212 superconducting phase regardless of the used substrate, precursor composition (212 and 223) and the thickness (200 and/ or 400 nm) of the precursor ®lm, at the same annealing and thalliation processes. SEM investigations have shown clear in¯uence of precursor ®lm composition and a type of the substrate used on the surface morphology of ®lms.
The 223 precursor ®lm increases granularity and roughness of the prepared Tl-2212 ®lm. Microwave measurements of superconducting phase transition of Tl-2212 thin ®lms are sensitive to the composition of precursor ®lm and clearly show increase of microwave power dissipation in the ®lms of nonstoichiometric precursor. In case of single crystal substrates, the microwave measurements con®rm high quality of ®lms. Acknowledgements The authors appreciate the support of the Slovak grant agency (VEGA grant No. 6057/99) and I. Kostic, D. Machajdõk helping with the ®lm characterization. References [1] A.P. Bramley, J.D. OÕConnor, C.R.M. Grovenor, Supercond. Sci. Technol. 12 (1999) R57. [2] Y.Q. Tang, K.Y. Chen, S. Afonso, X.L. Xu, Q. Xiong, G. Salamo, F.T. Chan, R. Guo, A. Bhalla, J. Appl. Phys. 78 (1995) 6846. [3] W.L. Holstein, I.A. Parisi, D.W. Face, X.D. Wu, S.R. Foltyn, R.E. Muenchausen, Appl. Phys. Lett. 61 (1992) 982. [4] M. Nemoto, S. Yoshikawa, K. Shimaoka, K. Niki, I. Yoshida, Y. Yoshisato, IEEE Trans. Appl. Supercond. 7 (1997) 1895. [5] H.C. Lai, K.D. Vernon-Parry, J.D. Chern, C.R.M. Grovenor, Supercond. Sci. Technol. 4 (1991) 306. [6] A. Conde-Gallardo, G. Contreras-Puente, M. Jergel, C. Falcony, A. Escamilla, F. Hanic, Physica C 288 (1997) 64. [7] J. Chrzanowsky, J.C. Irwin, B. Heinrich, Physica C 182 (1991) 231. Chromik, S. Be [8] M. MahelÕ, R. Adam, M. Darula, S. nacka, Physica C 202 (1992) 243. Chromik, M. Jergel, F. Hanic, S. Be [9] S. nacka, J. Liday, H. Tollet, C. Creemers, Physica C 230 (1994) 82. [10] A.P. Bramley, S.M. Morley, C.R.M. Grovenor, B. Pecz, Appl. Phys. Lett. 66 (1995) 517. [11] J.S. Martens, V.M. Hietala, E.L. Venturini, W.Y. Lee, J. Appl. Phys. 73 (1993) 7571.