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Superconductivity in sputtered thin films of Tl-Ba-Ca-Cu-O
Volume 131, number 3
PHYSICS LETTERS A
15 August 1988
SUPERCONDUCTIVITY IN SPUTTERED THIN FILMS OF T I - B a - C a - C u - O J.H. K A N G , R.T. K A M P W I R T H and K.E. G R A Y Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
Received 27 April 1988; revised manuscript received 1 July 1988; accepted for publication 1 July 1988 Communicated by J.l. Budnick
Films of T1-Ba-Ca-Cu-O have been made by multi-target magnetron sputtering. The best films show an onset of superconductivity at ~ 110 K and zero resistance at 96 K. Preliminary X-ray diffraction analysis suggests the films to be predominantly oriented with the c-axis perpendicular to the film surface with the lowest multiple of lattice spacing along the c-axis being ~ 2.94 nm, consistent with the TI2Ba2CaCu20~(2212 ) phase.
A new high-temperature superconductor has recently been described by Sheng et al. [ 1 ], who reported superconductivity in the T I - B a - C u - O system. The addition o f Ca has increased the transition temperatures from [ 1 ] ~ 81-107 K to as high as [ 2,3 ] 120-125 K. This system has many similarities to the B i - C a - S r - C u - O system, but has the advantage that the higher Tc phases seem to be easier to stabilize. For example, Ginley et al. [2 ] report a tetragonal T1BaCaCu2Ox phase (a = b = 0.3841 nm and c = 1.977 n m ) with a Tc of 120 K, while Parkin et al. [ 3 ] find To= 125 K in a body-centered-tetragonal phase ( a = b = 0 . 3 8 2 2 n m and c = 3.626 n m ) containing trilayers o f Cu perovskite-like units separated by bilayers of T1-O units. Parkin et al. [ 3] also found a T12Ba2CaCu20~ structure ( a = b = 0 . 3 8 5 7 nm and c = 2.939 n m ) containing only bilayer perovskite-like unit cells and exhibiting To= 108 K. Hazen et al. [4] had previously isolated two phases with Tc ~ 100 K which have pseudotetragonal unit cells: T12Ba2Ca2Cu30~ with a = b = 0.540 nm and c = 3.625 nm and Tl2Ba2CaCu2Ox with a = b = 0 . 5 4 4 n m and c = 2 . 9 5 5 n m and claimed that the 2223 phase contains extra Ca and Cu layers than the 2122 phase. Morosin et al. [ 5 ] have reported a single crystal of T15BasCa2Cu60~ exhibiting an orthorhombic unit cell ( a = 0 . 5 4 5 8 nm, b = 0 . 5 4 6 7 nm and c = 2 . 9 3 0 6 n m ) which has Tc = 106 K. We present preliminary results showing that superconducting films of the T 1 - B a - C a - C u - O system 208
can be prepared by magnetron sputtering. The best films show an onset o f superconductivity at ~ 110 K and zero resistance at T o ( 0 ) = 9 6 K. Because o f the high degree of c-axis orientation normal to the film surface, only information about the lattice spacing along the c-axis ( ~ 2 . 9 4 n m ) could be determined from X-ray diffraction. The composition was measured in a SEM on the interconnected backbone strcuture o f the film using energy dispersive X-rays. The result is T12Ba2Cal.3Cu2Ox, which is in reasonable agreement with the 2212 structure reported by Parkin et al. [ 3], who also found the c-axis spacing to be ~ 2 . 9 4 nm. Critical current densities are ~ 2 × 10 4 A / c m 2 in zero field and at 4.2 K. We have prepared these thin films using three dc magnetron sputtering guns aimed at a c o m m o n point in space, which provides compositional uniformity to + 1% over a 2 cm 2 area. Targets o f T1, Cu and a 1:1 BaCa mixture were simultaneously sputtered in a 20 mTorr argon atmosphere with an oxygen partial pressure o f 2 4 × 10 -5 Torr being introduced directly adjacent to the substrates. This is similar to the method used to make [ 6 ] films of B i - C a - S r - C u O. The best films were deposited onto polycrystalline ZrO~-9%Y203 substrates, but superconducting films were also made on (100) oriented single-crystal MgO. The substrates were kept at ambient temperature during the deposition. In order to avoid the loss o f the highly-volatile T1, the films were put in a closed Au crucible surrounded by flowing oxygen
Volume 131, number 3
PHYSICS LETTERSA
during the post-anneal, in which the loaded crucible was placed in a tube furnace, set to ~850°C, for about 4 minutes. Using the usual 0/20 scan, the X-ray diffraction pattern, shown in fig. l, indicates a high degree of caxis orientation perpendicular to the film surface in which only multiples of the (00n) reflections are found. Since the crystal structure of the films could not be ascertained, and several have been reported for the T1 system [2-5 ], only information about the lowest multiple of lattice spacing along the c-axis is reported here. The value of 2.94 nm is found to be in agreement with ref. [ 3 ]. Scanning electron micrographs appear to indicate a smoother and more dense surface than our films of YBa2CuaO7_y. There is an interconnected backbone structure, which looks topologically similar to that found in epitaxial YBaECU307_yfilms on SrTiO3 substrates [ 7 ], except for the lack of epitaxial orientation for our T 1 - B a - C a - C u - O films. The composition was measured on the interconnected backbone structure using energy dispersive X-rays, and the result is T12Ba2Cal.aCUEOx, which is in reasonable agreement with the 2212 structure reported by Parkin et al. [3]. Occasionally, star-like objects are seen which have a composition of Tl2.7Ba2.4CaCuE.9Ox,i.e., very Ca deficient. This may be evidence for the original T I - B a - C u - O material synthesized by Sheng et al. [ 1 ]. Clearly more work is needed to optimize the composition near the stoichiometry of the highest Tc phase. The resistance was measured versus temperature on samples which were ~ 0.4 lxm thick, 2.5 m m wide 90
g == 60
o) 30 E
5
30
45
60
26 Fig. l. X-ray diffraction pattern showing only the multiples of the (00n), i.e., c-axis, reflections.
15 August 1988 i
E
i
i
i
3
? E
2 o o
1
o o
0
I
50
o lOO
i
i
i
150 T (K)
200
250
00
Fig. 2. The resistivityof one of our films plotted against temperature using a sample current density of ~ 1 A/cm2. A single superconducting transition is observed which begins at about 110 K and shows zero resistanceat 96 K. and 5 m m long using a standard four-probe method and a current density o f ~ 1 A / c m 2. Electrical contacts were made with silver paint. Fig. 2 shows resistivity versus temperature for one of our samples prepared on a polycrystalline ZrO2-9%Y203 substrate. The film shows good metallic behavior, as evidenced by a factor of 2.5 decrease in resistivity from room temperature to just above the onset, although the magnitude of resistivity is almost an order of magnitude higher than the best bulk, single crystals of YBa2Cu307_y.A single superconducting transition is observed which begins at about 110 K and shows zero resistance at T c ( 0 ) = 9 6 K. Some samples prepared under similar sputtering conditions exhibited onsets at ~ 120 K, but with broader transitions and lower To(0) values. Preliminary measurements of the upper critical field, both parallel and perpendicular to the c-axis, indicate narrow transitions with larger critical field slopes at Tc than found in other high-temperature superconductors [ 8 ]. Details will be reported in a future publication. The authors thank Richard Lee for EDAX measurements and Sonya Marsh and S.J. Stein for technical assistance. This work is supported by the U.S. Department of Energy, BES-Materials Sciences, under contract No. W-3 I- 109-ENG-38.
Note added. Recently prepared samples of the 2212 209
Volume 131, number 3
PHYSICS LETTERS A
phase h a v e s h o w n Tc ( 0 ) = 101 K a n d a critical curr e n t d e n s i t y o f 5 0 0 0 0 A / c m 2 at 77 K a n d z e r o field. All films h a v e c o n t i n u e d to s h o w a high degree o f caxis o r i e n t a t i o n .
References [ 1 ] Z.Z. Sheng, A.M. Hermann, A. El All, C. Almasan, J. Estrada, T. Datta and R.J. Matson, Phys. Rev. Lett. 60 (1988) 937; A.M. Hermann, Z.Z. Sheng, D.C. Vier, S. Shultz and S. Osheroff, unpublished. [2] D.S. Ginley, E.L. Venturini, J.F. Kwak, R.J. Baughman, M.L. Carr, P.F. Hlava, J.E. Schirber and B. Morosin, Physica C 152 (1988) 217.
210
15 August 1988
[ 3 ] S.S.P. Parkin, V.Y. Lee, E.M. Engler, A.I. Nazzal, T.C. Huang, G. Go rman, R. Savoy and R. Beyers, Phys. Rev. Lett. 60 (1988) 2539. [4] R.M. Hazen, L.W. Finger, R.J. Angel, C.T. Prewitt, N.L. Ross, C.G. Hadidiacos, P.J. Heaney, D.R. Veblen, Z.Z. Sheng, A. E1Ali and A.M. Herman, Phys. Rev. Lett. 60 (1988) 1657. [5] B. Morosin, D.S. Ginley, E.L. Venturini, P.F. Hlava, R.J. Baughman, J.F. Kwak and J.E. Schirber, Physica C 152 (1988) 223. [6] J.H. Kang, R.T. Kampwirth, K.E. Gray, S. Marsh and E.A. Huff, Phys. Lett. A 128 (1988) 102. [7] S.H. Liou, M. Hong, B.A. Davidson, R.C. Farrow, J. Kwo, T.C. Hsieh, R.M. Fleming, H.S. Chen, L.C. Feldman, A.R. Kortan and R.J. Felder, in: Proc. Amer. Vac. Soc. Topical Conf. on Thin film processing and characterization of high temperature superconductors (Am. Institute of Phys., New York, 1988). [8] J.H. Kang, R.T. Kampwirth and K.E. Gray, Appl. Phys. Lett. 52 (1988) 2080.
PHYSICS LETTERS A
15 August 1988
SUPERCONDUCTIVITY IN SPUTTERED THIN FILMS OF T I - B a - C a - C u - O J.H. K A N G , R.T. K A M P W I R T H and K.E. G R A Y Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, USA
Received 27 April 1988; revised manuscript received 1 July 1988; accepted for publication 1 July 1988 Communicated by J.l. Budnick
Films of T1-Ba-Ca-Cu-O have been made by multi-target magnetron sputtering. The best films show an onset of superconductivity at ~ 110 K and zero resistance at 96 K. Preliminary X-ray diffraction analysis suggests the films to be predominantly oriented with the c-axis perpendicular to the film surface with the lowest multiple of lattice spacing along the c-axis being ~ 2.94 nm, consistent with the TI2Ba2CaCu20~(2212 ) phase.
A new high-temperature superconductor has recently been described by Sheng et al. [ 1 ], who reported superconductivity in the T I - B a - C u - O system. The addition o f Ca has increased the transition temperatures from [ 1 ] ~ 81-107 K to as high as [ 2,3 ] 120-125 K. This system has many similarities to the B i - C a - S r - C u - O system, but has the advantage that the higher Tc phases seem to be easier to stabilize. For example, Ginley et al. [2 ] report a tetragonal T1BaCaCu2Ox phase (a = b = 0.3841 nm and c = 1.977 n m ) with a Tc of 120 K, while Parkin et al. [ 3 ] find To= 125 K in a body-centered-tetragonal phase ( a = b = 0 . 3 8 2 2 n m and c = 3.626 n m ) containing trilayers o f Cu perovskite-like units separated by bilayers of T1-O units. Parkin et al. [ 3] also found a T12Ba2CaCu20~ structure ( a = b = 0 . 3 8 5 7 nm and c = 2.939 n m ) containing only bilayer perovskite-like unit cells and exhibiting To= 108 K. Hazen et al. [4] had previously isolated two phases with Tc ~ 100 K which have pseudotetragonal unit cells: T12Ba2Ca2Cu30~ with a = b = 0.540 nm and c = 3.625 nm and Tl2Ba2CaCu2Ox with a = b = 0 . 5 4 4 n m and c = 2 . 9 5 5 n m and claimed that the 2223 phase contains extra Ca and Cu layers than the 2122 phase. Morosin et al. [ 5 ] have reported a single crystal of T15BasCa2Cu60~ exhibiting an orthorhombic unit cell ( a = 0 . 5 4 5 8 nm, b = 0 . 5 4 6 7 nm and c = 2 . 9 3 0 6 n m ) which has Tc = 106 K. We present preliminary results showing that superconducting films of the T 1 - B a - C a - C u - O system 208
can be prepared by magnetron sputtering. The best films show an onset o f superconductivity at ~ 110 K and zero resistance at T o ( 0 ) = 9 6 K. Because o f the high degree of c-axis orientation normal to the film surface, only information about the lattice spacing along the c-axis ( ~ 2 . 9 4 n m ) could be determined from X-ray diffraction. The composition was measured in a SEM on the interconnected backbone strcuture o f the film using energy dispersive X-rays. The result is T12Ba2Cal.3Cu2Ox, which is in reasonable agreement with the 2212 structure reported by Parkin et al. [ 3], who also found the c-axis spacing to be ~ 2 . 9 4 nm. Critical current densities are ~ 2 × 10 4 A / c m 2 in zero field and at 4.2 K. We have prepared these thin films using three dc magnetron sputtering guns aimed at a c o m m o n point in space, which provides compositional uniformity to + 1% over a 2 cm 2 area. Targets o f T1, Cu and a 1:1 BaCa mixture were simultaneously sputtered in a 20 mTorr argon atmosphere with an oxygen partial pressure o f 2 4 × 10 -5 Torr being introduced directly adjacent to the substrates. This is similar to the method used to make [ 6 ] films of B i - C a - S r - C u O. The best films were deposited onto polycrystalline ZrO~-9%Y203 substrates, but superconducting films were also made on (100) oriented single-crystal MgO. The substrates were kept at ambient temperature during the deposition. In order to avoid the loss o f the highly-volatile T1, the films were put in a closed Au crucible surrounded by flowing oxygen
Volume 131, number 3
PHYSICS LETTERSA
during the post-anneal, in which the loaded crucible was placed in a tube furnace, set to ~850°C, for about 4 minutes. Using the usual 0/20 scan, the X-ray diffraction pattern, shown in fig. l, indicates a high degree of caxis orientation perpendicular to the film surface in which only multiples of the (00n) reflections are found. Since the crystal structure of the films could not be ascertained, and several have been reported for the T1 system [2-5 ], only information about the lowest multiple of lattice spacing along the c-axis is reported here. The value of 2.94 nm is found to be in agreement with ref. [ 3 ]. Scanning electron micrographs appear to indicate a smoother and more dense surface than our films of YBa2CuaO7_y. There is an interconnected backbone structure, which looks topologically similar to that found in epitaxial YBaECU307_yfilms on SrTiO3 substrates [ 7 ], except for the lack of epitaxial orientation for our T 1 - B a - C a - C u - O films. The composition was measured on the interconnected backbone structure using energy dispersive X-rays, and the result is T12Ba2Cal.aCUEOx, which is in reasonable agreement with the 2212 structure reported by Parkin et al. [3]. Occasionally, star-like objects are seen which have a composition of Tl2.7Ba2.4CaCuE.9Ox,i.e., very Ca deficient. This may be evidence for the original T I - B a - C u - O material synthesized by Sheng et al. [ 1 ]. Clearly more work is needed to optimize the composition near the stoichiometry of the highest Tc phase. The resistance was measured versus temperature on samples which were ~ 0.4 lxm thick, 2.5 m m wide 90
g == 60
o) 30 E
5
30
45
60
26 Fig. l. X-ray diffraction pattern showing only the multiples of the (00n), i.e., c-axis, reflections.
15 August 1988 i
E
i
i
i
3
? E
2 o o
1
o o
0
I
50
o lOO
i
i
i
150 T (K)
200
250
00
Fig. 2. The resistivityof one of our films plotted against temperature using a sample current density of ~ 1 A/cm2. A single superconducting transition is observed which begins at about 110 K and shows zero resistanceat 96 K. and 5 m m long using a standard four-probe method and a current density o f ~ 1 A / c m 2. Electrical contacts were made with silver paint. Fig. 2 shows resistivity versus temperature for one of our samples prepared on a polycrystalline ZrO2-9%Y203 substrate. The film shows good metallic behavior, as evidenced by a factor of 2.5 decrease in resistivity from room temperature to just above the onset, although the magnitude of resistivity is almost an order of magnitude higher than the best bulk, single crystals of YBa2Cu307_y.A single superconducting transition is observed which begins at about 110 K and shows zero resistance at T c ( 0 ) = 9 6 K. Some samples prepared under similar sputtering conditions exhibited onsets at ~ 120 K, but with broader transitions and lower To(0) values. Preliminary measurements of the upper critical field, both parallel and perpendicular to the c-axis, indicate narrow transitions with larger critical field slopes at Tc than found in other high-temperature superconductors [ 8 ]. Details will be reported in a future publication. The authors thank Richard Lee for EDAX measurements and Sonya Marsh and S.J. Stein for technical assistance. This work is supported by the U.S. Department of Energy, BES-Materials Sciences, under contract No. W-3 I- 109-ENG-38.
Note added. Recently prepared samples of the 2212 209
Volume 131, number 3
PHYSICS LETTERS A
phase h a v e s h o w n Tc ( 0 ) = 101 K a n d a critical curr e n t d e n s i t y o f 5 0 0 0 0 A / c m 2 at 77 K a n d z e r o field. All films h a v e c o n t i n u e d to s h o w a high degree o f caxis o r i e n t a t i o n .
References [ 1 ] Z.Z. Sheng, A.M. Hermann, A. El All, C. Almasan, J. Estrada, T. Datta and R.J. Matson, Phys. Rev. Lett. 60 (1988) 937; A.M. Hermann, Z.Z. Sheng, D.C. Vier, S. Shultz and S. Osheroff, unpublished. [2] D.S. Ginley, E.L. Venturini, J.F. Kwak, R.J. Baughman, M.L. Carr, P.F. Hlava, J.E. Schirber and B. Morosin, Physica C 152 (1988) 217.
210
15 August 1988
[ 3 ] S.S.P. Parkin, V.Y. Lee, E.M. Engler, A.I. Nazzal, T.C. Huang, G. Go rman, R. Savoy and R. Beyers, Phys. Rev. Lett. 60 (1988) 2539. [4] R.M. Hazen, L.W. Finger, R.J. Angel, C.T. Prewitt, N.L. Ross, C.G. Hadidiacos, P.J. Heaney, D.R. Veblen, Z.Z. Sheng, A. E1Ali and A.M. Herman, Phys. Rev. Lett. 60 (1988) 1657. [5] B. Morosin, D.S. Ginley, E.L. Venturini, P.F. Hlava, R.J. Baughman, J.F. Kwak and J.E. Schirber, Physica C 152 (1988) 223. [6] J.H. Kang, R.T. Kampwirth, K.E. Gray, S. Marsh and E.A. Huff, Phys. Lett. A 128 (1988) 102. [7] S.H. Liou, M. Hong, B.A. Davidson, R.C. Farrow, J. Kwo, T.C. Hsieh, R.M. Fleming, H.S. Chen, L.C. Feldman, A.R. Kortan and R.J. Felder, in: Proc. Amer. Vac. Soc. Topical Conf. on Thin film processing and characterization of high temperature superconductors (Am. Institute of Phys., New York, 1988). [8] J.H. Kang, R.T. Kampwirth and K.E. Gray, Appl. Phys. Lett. 52 (1988) 2080.