- Email: [email protected]
Formation of Ca-doped YBa2Cu4O8 phase in thin films
Physica C 170 ( 1990) 500-504 North-Holland
Formation of Ca-doped YBa2Cu408 phase in thin films R. Kita, S. Kawamoto, K. Ohata, H. Izumi, R. Itti, T. Morishita and S. Tanaka Superconductivity Research Laboratory, International Superconductivity Technology Center, I-10-13 Shinonome, Koto-ku, Tokyo 135, Japan Received 23 July 1990
Cadoped YBa,Cu,Os ( 124) thin films are prepared on ( 100) SrTiOs substrates by annealing the amorphous films deposited using a pulsed laser deposition technique. The X-ray diffraction measurements show that the Ca-doped YBazCu.,Os phase is formed by annealing below 800°C at a oxygen pressure of 1 atm. The 124 films have c-axis orientation normal to the substrates. As the Ca content increases, the proportion of the 123 impurity phase in the samples increases. The onset temperature of superconductivity of the Y ( Ca)Ba2Cu40s films increases from 79 K to 88 K with an increase Ca-substitution for 5 to 10% of Y.
The “ 124” superconductor YBazCu,Os containing double Cu-0 chains was first discovered as a planar defect in the “123” superconductor YBa$&O, with a single Cu-0 chain [ 11. The 124 superconductor has more stable oxygen content up to higher temperatures than the 123 superconductor [ 21. This feature is important for practical applications, but the lower critical temperature, T, (80 K), of the 124 compounds is not sufficient for practical use of this material at liquid-nitrogen temperature (77 K). Recently it was demonstrated by Miyatake et al. [ 31 that the T,of bulk 124 is increased up to 90 K by substituting Ca for 1O”lbof Y. For bulk samples, it takes a long time to complete the reaction to obtain the 124 phase by solid state reaction at an oxygen pressure of 1 atm [4] since it must be synthesized at low temperature. In contrast, 124 films are relatively easy to prepare by annealing [ 5-8 1. Cadoped 124 films, however, have not yet been successfully prepared. The optimal conditions for forming the 124 films also has not been established. In this letter, we report for the first time the preparation of Ca-doped thin films, discussing the thermodynamic stability of the 123 and 124 phases. The samples were deposited from sintered Y, _,Ca,Ba,Cu,O, targets by a laser deposition technique using a pulsed ArF eximer ( - 1 J/cm’). Four targets with nominal compositions of Y iBaZCu40y (x=0.05), (x=0), Yo.&ao.osBazCu&,
(x=0.10) and Yo.90Cao. 1&a2Cu40y Yo.ssCo.,sBa,Cu,O, (x=0.15) (labeled A, B, C and D, respectively) were made by calcining of Y203, CaCO,, BaCO, and CuO. Deposition was carried out on single-crystal ( 100) SrTi03 and ( 100) MgO substrates at room temperature under 0.0 1 Torr oxygen pressure, but the 124 phase was successfully formed only on SrTiOJ. As-deposited films of 1500 8, in thickness were translucent, amber colored and insulating. X-ray diffraction (XRD ) measurements showed them to be amorphous. The chemical composition of films without Ca was determined by the inductively coupled plasma technique (ICP) to be Y:Ba:Cu=1:2.0:4.0. The deposited films were subsequently annealed in flowing oxygen as follows: at the fist step, the films were kept at 650°C for 1 h, then at 750°C for 1 h and finally at various temperatures from 780°C to 805 “C for 1 h and cooled to room temperature. The heating rate and cooling rate were 5 “C/min and 2”C/min, respectively. The annealed films were black. The Ca content was examined for the annealed films using X-ray photoemission spectroscopy (XPS) The Ca 2p spectra of the annealed films are shown in fig. 1. Two kinds of peaks of Ca 2p,,, are observed and their binding energies are about 345 eV and 347 eV, which correspond to Ca in metallic compounds and in insulating ones, respectively. The existence of the Ca 2p,,, peak of 345 eV in samples
092 l-4534/90/%03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
R. Kita et al. / Ca-doped YBazCu,08 phase in thin films
-354
-350
-346
-342
Binding energy(eV)
Fig. 1. Ca 2p spectra of the Y(CA)-Ba-Cu-0 nealed at 800°C for 1 h.
thin films an-
is consistent with the results reported in (Y 1_,Ca,)Ba,Cu_,O, bulk samples [ 91. XPS, however, does not have sufficient sensitivity for quantitative investigations of the Ca content in the samples because of the characteristically low X-ray photoelectron yield of Ca 2p. Considering the following XRD data, we are convinced that Ca is sub-
5+10
-f
501
stituted for Y by up to 10%. From now on, we use the terminology of “the films with x=0, 0.05, 0.10 and 0.15” instead of “the films deposited from the targets A, B, C and D”. Figure 2 shows the XRD patterns using Cu Ka radiation for the annealed films with x= 0, 0.05, 0.10 and 0.15. The X-ray pattern of the film without Ca indicates that the 124 compounds with the c-axis perpendicular to the substrate is formed by annealing at 800°C for 1 h. The arrows show the position of the peaks of the 123 phase. A trace of the 123 phase is present because weak (005) and (007) peaks of 123 are observed at the shoulder of the (0012) and (0016) peaks of 124, respectively. Besides the 123 phase, CuO should coexist in films but it is difficult to identify because the (005 ) peak of 123 is very close to the (111) peak of CuO. As the Ca content increases, the proportion of the 123 phase in the sample increases. The reason for this is not yet clear, but a possible reason is that the growth rate of the 124 phase is suppressed by Ca. The temperature dependence of the resistivity for these films is shown in fig. 3. The resistivity was measured using a conventional four-probe method. All the films show a metallic behavior in the normal
20
50
+
60
28 NW Fig. 2. X-ray diffraction of the 123 phase.
patterns
of Y (Ca)-Ba-Cu-0
thin films annealed
at 800°C for 1 h. The arrows indicate
the position
of the peaks
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R. Kita et al. / Ca-doped YBazCu,Os phase in thinfihzs
503
s
6 = P
(a)
N
29 (deg)
Fig. 5. X-ray diffraction patterns of Y,&a,Ba,Cu,Os thin thilmsannealed at 78O”C, 800°C and 805°C for I h. (a) x=0, (b) x=0.05, (c ) x= 0.10, (d) x= 0.15. The arrows indicate the position of the peaks of the 123 phase.
504
R. Kita et al. / Ca-doped YBa,Cu,Os
phase in thin/ilms
I \
As depo. 780/C
20 (dw) Fig. 5. Continued.
dustries supported by the New Energy and Industrial Technology Development Organization (NEDO). References [ I ] H.W. Zandbergen, R. Gronsky, Nature 33 1 ( 1988) 506.
[ 21 J. Karpinski,
K. Wang and G. Thomas,
E. Kaldis, E. ‘Jilek, S. Rusiecki and B. Bucker, Nature 336 (1988) 660. [ 31 T. Miyatake, S. Gotoh, N. Koshizuka and S. Tanaka, Nature 341 (1989) 1. [4] R.J. Cava, J.J. Krajewski, W.F. Peck Jr., B. Batlogg and L.W. Rupp Jr., Nature 338 (1989) 328. [ 5 ] A.F. Marshall, R.W. Barton, K. Char, A. Kapitulnik, B. Oh, R.H. Hammond and S.S. Ladermann, Phys. Rev. B37 (1988) 9353.
[6] K. Char, Mark Lee, R.W. Barton, A.F. Marshall, R.H. Hammond, M.R. Beasley, T.H. Geballe, A. Kapitulnik and S.S. Ladermann, Phys. Rev. B38 (1988) 834. [ 7 ] M.L. Mandich, A.M. DeSantolo, R.M. Sunshine, J. Kwo, M. Hong, T. Boone, T.Y. Komehani and L. MartinezMiranda, Phys. Rev. B38 ( 1988) 503 1. [8] R.C. Budhani, M.W. Ruckman, R.L. Sabatini, M. Suenaga and D.O. Welch, Solid State Commun. 7 ( 1990) 337. [ 91 R. Itti, T. Miyatake, K. Ikeda, Y. Yamaguchi, N. Koshizuka and S. Tanaka, Phys. Rev. B, to be published. [ lo] R.G. Buckley, J.L. Tallon, D.M. Pooke and M.R. Presland, PhysicaC 165 (1990) 391. [ 111 D.E. Morris, P. Narwankar, A.P.B. Shinha, K. Takano, B. Fayan and V.T. Shum, Phys. Rev. B41 (1989) 4118.
Formation of Ca-doped YBa2Cu408 phase in thin films R. Kita, S. Kawamoto, K. Ohata, H. Izumi, R. Itti, T. Morishita and S. Tanaka Superconductivity Research Laboratory, International Superconductivity Technology Center, I-10-13 Shinonome, Koto-ku, Tokyo 135, Japan Received 23 July 1990
Cadoped YBa,Cu,Os ( 124) thin films are prepared on ( 100) SrTiOs substrates by annealing the amorphous films deposited using a pulsed laser deposition technique. The X-ray diffraction measurements show that the Ca-doped YBazCu.,Os phase is formed by annealing below 800°C at a oxygen pressure of 1 atm. The 124 films have c-axis orientation normal to the substrates. As the Ca content increases, the proportion of the 123 impurity phase in the samples increases. The onset temperature of superconductivity of the Y ( Ca)Ba2Cu40s films increases from 79 K to 88 K with an increase Ca-substitution for 5 to 10% of Y.
The “ 124” superconductor YBazCu,Os containing double Cu-0 chains was first discovered as a planar defect in the “123” superconductor YBa$&O, with a single Cu-0 chain [ 11. The 124 superconductor has more stable oxygen content up to higher temperatures than the 123 superconductor [ 21. This feature is important for practical applications, but the lower critical temperature, T, (80 K), of the 124 compounds is not sufficient for practical use of this material at liquid-nitrogen temperature (77 K). Recently it was demonstrated by Miyatake et al. [ 31 that the T,of bulk 124 is increased up to 90 K by substituting Ca for 1O”lbof Y. For bulk samples, it takes a long time to complete the reaction to obtain the 124 phase by solid state reaction at an oxygen pressure of 1 atm [4] since it must be synthesized at low temperature. In contrast, 124 films are relatively easy to prepare by annealing [ 5-8 1. Cadoped 124 films, however, have not yet been successfully prepared. The optimal conditions for forming the 124 films also has not been established. In this letter, we report for the first time the preparation of Ca-doped thin films, discussing the thermodynamic stability of the 123 and 124 phases. The samples were deposited from sintered Y, _,Ca,Ba,Cu,O, targets by a laser deposition technique using a pulsed ArF eximer ( - 1 J/cm’). Four targets with nominal compositions of Y iBaZCu40y (x=0.05), (x=0), Yo.&ao.osBazCu&,
(x=0.10) and Yo.90Cao. 1&a2Cu40y Yo.ssCo.,sBa,Cu,O, (x=0.15) (labeled A, B, C and D, respectively) were made by calcining of Y203, CaCO,, BaCO, and CuO. Deposition was carried out on single-crystal ( 100) SrTi03 and ( 100) MgO substrates at room temperature under 0.0 1 Torr oxygen pressure, but the 124 phase was successfully formed only on SrTiOJ. As-deposited films of 1500 8, in thickness were translucent, amber colored and insulating. X-ray diffraction (XRD ) measurements showed them to be amorphous. The chemical composition of films without Ca was determined by the inductively coupled plasma technique (ICP) to be Y:Ba:Cu=1:2.0:4.0. The deposited films were subsequently annealed in flowing oxygen as follows: at the fist step, the films were kept at 650°C for 1 h, then at 750°C for 1 h and finally at various temperatures from 780°C to 805 “C for 1 h and cooled to room temperature. The heating rate and cooling rate were 5 “C/min and 2”C/min, respectively. The annealed films were black. The Ca content was examined for the annealed films using X-ray photoemission spectroscopy (XPS) The Ca 2p spectra of the annealed films are shown in fig. 1. Two kinds of peaks of Ca 2p,,, are observed and their binding energies are about 345 eV and 347 eV, which correspond to Ca in metallic compounds and in insulating ones, respectively. The existence of the Ca 2p,,, peak of 345 eV in samples
092 l-4534/90/%03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)
R. Kita et al. / Ca-doped YBazCu,08 phase in thin films
-354
-350
-346
-342
Binding energy(eV)
Fig. 1. Ca 2p spectra of the Y(CA)-Ba-Cu-0 nealed at 800°C for 1 h.
thin films an-
is consistent with the results reported in (Y 1_,Ca,)Ba,Cu_,O, bulk samples [ 91. XPS, however, does not have sufficient sensitivity for quantitative investigations of the Ca content in the samples because of the characteristically low X-ray photoelectron yield of Ca 2p. Considering the following XRD data, we are convinced that Ca is sub-
5+10
-f
501
stituted for Y by up to 10%. From now on, we use the terminology of “the films with x=0, 0.05, 0.10 and 0.15” instead of “the films deposited from the targets A, B, C and D”. Figure 2 shows the XRD patterns using Cu Ka radiation for the annealed films with x= 0, 0.05, 0.10 and 0.15. The X-ray pattern of the film without Ca indicates that the 124 compounds with the c-axis perpendicular to the substrate is formed by annealing at 800°C for 1 h. The arrows show the position of the peaks of the 123 phase. A trace of the 123 phase is present because weak (005) and (007) peaks of 123 are observed at the shoulder of the (0012) and (0016) peaks of 124, respectively. Besides the 123 phase, CuO should coexist in films but it is difficult to identify because the (005 ) peak of 123 is very close to the (111) peak of CuO. As the Ca content increases, the proportion of the 123 phase in the sample increases. The reason for this is not yet clear, but a possible reason is that the growth rate of the 124 phase is suppressed by Ca. The temperature dependence of the resistivity for these films is shown in fig. 3. The resistivity was measured using a conventional four-probe method. All the films show a metallic behavior in the normal
20
50
+
60
28 NW Fig. 2. X-ray diffraction of the 123 phase.
patterns
of Y (Ca)-Ba-Cu-0
thin films annealed
at 800°C for 1 h. The arrows indicate
the position
of the peaks
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R. Kita et al. / Ca-doped YBazCu,Os phase in thinfihzs
503
s
6 = P
(a)
N
29 (deg)
Fig. 5. X-ray diffraction patterns of Y,&a,Ba,Cu,Os thin thilmsannealed at 78O”C, 800°C and 805°C for I h. (a) x=0, (b) x=0.05, (c ) x= 0.10, (d) x= 0.15. The arrows indicate the position of the peaks of the 123 phase.
504
R. Kita et al. / Ca-doped YBa,Cu,Os
phase in thin/ilms
I \
As depo. 780/C
20 (dw) Fig. 5. Continued.
dustries supported by the New Energy and Industrial Technology Development Organization (NEDO). References [ I ] H.W. Zandbergen, R. Gronsky, Nature 33 1 ( 1988) 506.
[ 21 J. Karpinski,
K. Wang and G. Thomas,
E. Kaldis, E. ‘Jilek, S. Rusiecki and B. Bucker, Nature 336 (1988) 660. [ 31 T. Miyatake, S. Gotoh, N. Koshizuka and S. Tanaka, Nature 341 (1989) 1. [4] R.J. Cava, J.J. Krajewski, W.F. Peck Jr., B. Batlogg and L.W. Rupp Jr., Nature 338 (1989) 328. [ 5 ] A.F. Marshall, R.W. Barton, K. Char, A. Kapitulnik, B. Oh, R.H. Hammond and S.S. Ladermann, Phys. Rev. B37 (1988) 9353.
[6] K. Char, Mark Lee, R.W. Barton, A.F. Marshall, R.H. Hammond, M.R. Beasley, T.H. Geballe, A. Kapitulnik and S.S. Ladermann, Phys. Rev. B38 (1988) 834. [ 7 ] M.L. Mandich, A.M. DeSantolo, R.M. Sunshine, J. Kwo, M. Hong, T. Boone, T.Y. Komehani and L. MartinezMiranda, Phys. Rev. B38 ( 1988) 503 1. [8] R.C. Budhani, M.W. Ruckman, R.L. Sabatini, M. Suenaga and D.O. Welch, Solid State Commun. 7 ( 1990) 337. [ 91 R. Itti, T. Miyatake, K. Ikeda, Y. Yamaguchi, N. Koshizuka and S. Tanaka, Phys. Rev. B, to be published. [ lo] R.G. Buckley, J.L. Tallon, D.M. Pooke and M.R. Presland, PhysicaC 165 (1990) 391. [ 111 D.E. Morris, P. Narwankar, A.P.B. Shinha, K. Takano, B. Fayan and V.T. Shum, Phys. Rev. B41 (1989) 4118.