- Email: [email protected]
Superconducting (Sr, Nd) CuOy thin films with infinite-layer structure
PHYSICA
Physica C 196 (1992) 14-16 North-Holland
Superconducting (Sr, Nd ) CuO.,, thin films with infinite-layer structure H. Adachi, T. Satoh, Y. Ichikawa, K. Setsune and K. Wasa Central Research Laboratories. Matsushita Electric Industrial Co.. Ltd.. Moriguchi, Osaka 570. Japan
Received 16 March 1992
Thin films with infinite-layer structure have been prepared using a magnetron sputtering systcm and the substitution of neodymium for strontium in SrCuO2 has been successfully carried out, which is similar to the case of the high-pressure synthesis. ( St, Nd)CuO r films were grown in a reducing atmosphere on SrTiO3 single crystals and epitaxial growth with [001 ] orientation normal to the substrate was realized at the substrate temperature of 550-600°C. Superconducting transition was observed tbr some as-grown films, and the Sro.8sNd0~2CuOvfilm showed the highest T~.of 16 K.
The recent discovery of superconductivity in SrCuO2-related c o m p o u n d s with infinite-layer structure [ 1 ] has attracted much attention since the crystal structure is quite simple and an elucidation o f the m e c h a n i s m o f cuprate superconductors is expected to follow [2]. Takano et al. have reported the high transition temperature of 80-100 K in (Sr,Ba)CuO2. and they considered that the superconducting phase is the infinite-layer structure with a modulation along the c-axis [ 3 ]. Smith et al. have substituted some trivalent lanthanoid ( L n ) elements for divalent Sr in the infinite-layer SrCuO2 and derived superconductivity in the (Sr,Ln)CuO2 system [4]. These materials have so far only been m a d e by a high-pressure sintering process and this fact has restrained the worldwide research of infinite-layer superconductors. Meanwhile, thin-film processing sometimes creates metastable phases that are not o b t a i n e d by a standard solid-state reaction, and can also artificially control crystal structures. As for the infinite-layer structure, Yazawa et al. have successfully obtained CaCuO2 thin films by R F sputtering [5]. Epitaxial (Ca,Sr)CuO2 and SrCuO2 thin films with infinitelayer structure have also been p r e p a r e d by the laser ablation m e t h o d [6,7]. Recently, we have prepared infinite-layer-type SrCuO2 films by R F - m a g n e t r o n sputtering and have studied the effect o f n e o d y m i u m substitution for strontium. Here, we report the preparation and superconducting properties of (Sr,
N d ) C u O 2 films with the infinite-layer structure. Thin films were prepared by a R F - p l a n a r magnetron sputtering system. The substrates used were SrTiO3 single crystals with ( 1 0 0 ) plane. The targets were complex c o m p o u n d s of S r - N d - C u - O . where the content of n e o d y m i u m was varied, i.e., SG ~Nd~Cu~_l.30, ( x = 0 - 0 , 1 3 ) . Cu contents in the targets were c o m p e n s a t e d to keep the film composition near (Sr, N d ) C u O , . The film composition was precisely measured by the inductively coupled plasma-emission spectrometry analyses for the S r N d - C u - O films grown on MgO ( 1 0 0) substrates. The n e o d y m i u m contents ( N d / ( S r + N d ) ) in the films were almost equal to those in the targets. So. we regarded the N d content of the films grown on SrTiO3 substrate as that of the target, although the evaluation of Sr contents in the film on SrTiO3 cannot be identified. Since a reducing atmosphere is preferred for the growth of superconducting (Sr. N d ) C u O 2 [4], only argon was used as a sputtering gas and gas pressure during the sputtering was as low as 0.5 Pa. The input R F power was 100 W for the target d i a m e t e r of 80 mm. Thin films about 3000 ,& thick were obtained by 30 min deposition. After that. films were cooled down to room t e m p e r a t u r e for about 1 h in the same atmosphere as the deposition. The epitaxial thin films with infinite-layer structure were grown with [001 ] orientation normal to (1 0 0 ) SrTiO3 at a substrate temperature ranging
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserwed.
15
H. Adachi et al. /(Sr, Nd)CuOy thin films
from 550 to 600°C. Although the infinite-layer phase tended to disappear in neodymium-doped films strong reduction in the sputtering conditions stabilized the infinite-layer structure in the deposited films. Figure l ( a ) shows the X-ray diffraction ( X R D ) pattern of the Sro.98Ndo.o2CuOyfilm with the almost single infinite-layer structure. When the Nd content was increased to more than 0.05, small amounts of impurity phases (SrCu203, Sr2CuO 3 or unknown phases) appeared in addition to the infinite-layer phase. Figure 2 shows the variation of the lattice parameter c with the Nd content x. When the Nd was not doped, i.e., SrCuOy film, c length was 3.475 A, which is longer than that of SrCuO2 ceramics (3.43 A) made by high-pressure sintering. This may be attributable to the difference in synthetic technique. As the Nd was substituted for Sr, c length decreased, similar to the case of (Sr, Nd)CuO2 ceramics [4]. However, when the Nd content exceeded 0.1, another phase having a longer lattice parameter c (3.6 /k) began to coexist. This phase may be the infinitelayer-related structure with 2x//2a × 2x/2a × c lattice cell (c ~ 3.7/k) which shows metallic behavior [8]. Figure l ( b ) shows the XRD pattern of (Sro 88Ndo ~2)CuOy film with a mixture of the infi'
Ca)
'
'o
•
~
'
'
J
(~
~
""2 -=
(c)
5
1'0
I
' 2'0
30 20
40
50
60
(deg,)
Fig. 1. X-ray diffraction patterns of the (Sr~_xNdx)CuOr thin films with (a) x=0.02, (b) x=0.12 and (c) x=0.13. Stars are peaks from the infinite-layer-relatedphase with longer c length. Arrows indicate the impurity phases.
3.7 0 major phase minor phase A v
/XA 0 3.6
E t~ tl.
3.5
O
O ..-I
O O
O
O
3.4 I
0
0.05
O
I
0.1
0.15
x in (Srl_xNdx)CUOy
Fig. 2. Variation of the lattice parameter c as a function of Nd content x. nite-layer phase and the infinite-layer-related phase with longer c length. Further increase of Nd content in the film resulted in the disappearance of the infinite-layer phase and only the longer-c-length phase was grown. Figure 1 (c) shows the XRD pattern of the (Sro.s7Nd0. ~3) CuOy film with the longer-c-length phase, where no trace of the infinite-layer phase is seen. As for a-axis length, we could not measure it accurately since the c-axis of the infinite-layer film is oriented normal to the substrate. From the reflection high-energy electron diffraction analyses, a-axis length was found to be around 3.85-3.95 A for each film. The transport properties of the films were measured along the in-plane direction. The resistivity of the films tended to decrease with Nd doping, suggesting electron doping through the substitution of trivalent Nd for divalent Sr. For the films with 0.07 < x, the resistivity at room temperature was less than 10 -2 ohm cm. Figure 3 shows the temperature dependence of the resistivity for as-grown (Sr~ _xNdx)CuOy films with x = 0.09, 0.11, 0.12 and 0.13. A few films showed the resistivity drops which came from the superconducting transition. In the films with the infinite-layer phase, Tc became higher with the decrease of c length, i.e., Nd doping. This dependence is different from the (Sr,Ln)CuO2 ce-
16
H. Adachi et al. / (Sr, Nd)CuO,. thin films
6 {I
0
?
E o
"
[] ~j
4 •
×
OO9
x
Oil
x:O
12
x=O
13
• e• e []u
ee•
0
3
:~:~
% • • • • e m e o e e o e e e ~ • e o e e e e o o o n o•oeeeeooee o
>
if_
2
o
1
"~o
0
'sz:,':,-~. / , .
[ ][J[ ~
lh
L
0
100
]1 ][J J[ ][ i ]
i
,
_ _ _ 1
Temperature
t
200
__
_
300
(K)
Fig. 3. Temperature dependence of resistivity lor (Sr~ ,Nd~)CuO,, thin films with various neodymium content x. ramics where T,. is not significantly affected by the lanthanoid content [4,9]. The highest superconducting transition o f 16 K ( o n s e t ) was o b t a i n e d in the Sro.ssNd0 ~2CuOv film. F o r the x = 0.1 3 film with the single longer-c-length phase, metallic temperature d e p e n d e n c e o f the resistivity is seen. The T~ in the (Sr, N d ) C u O v film is lower than that in the highpressure-synthesized ceramic samples. This difference m a y be due to the phase segregation into the longer-c-length phase in high N d - d o p e d films. Thus, the doping o f N d in the infinite-layer phase by thinfilm processing is at present less efficient than that by high-pressure sintering. The suppression o f phase segregation into the longer-c-length phase would improve T~, in (Sr, Nd)CuO.v thin films. Further post-annealing in a v a c u u m is effective for some films in i m p r o v i n g the superconductivity; however, annealing in the atmosphere containing oxygen certainly degraded the superconductivity. This is considered to be one p r o o f that the superconducting (Sr, N d ) C u O 2 system is of the electrondoped type. The Hall coefficients of (Sq _xNdx)CuOy films with x = O . 1 1 and 0.12 were measured at r o o m
t e m p e r a t u r e and found to be positive. This result does not i m m e d i a t e l y lead to the negation o f electron doping because of the existence of impurity phases in these films. In summary, the infinite-layer superconductor, for whose synthesis high-pressure sintering has so far been required, has been realized by thin-film processing. The present sputtering process enabled the fabrication of SrCuO2 and electron doping in it, which resulted in the superconducting (Sr, N d ) CuO~ thin films. This matter resembles the synthesis of d i a m o n d which is usually m a d e by a high-pressure heating and can also be fabricated from a vapor phase using thin-film processing. The realization of infinite-layer-type superconductors by a simple sputtering m e t h o d will widen the opportunities for research on this kind o f material.
Acknowledgements The authors would like to thank T. Nitta and K, Kanai for their continuous encouragement and also thank S. Hatta and T. W a d a for useful discussions.
References [I]T. Siegrist, S.M. Zahurak, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231. I2] R.J. Cava, Nature 351 ( 1991 ) 518. [ 3 ] M. Takano, M. Azuma, Z. Hiroi, Y. Band• and Y. Takeda, Physica C 176 (1991) 441. [4] M.G. Smith, A. Manthiram, J. Zhou, J.B. Go•den•ugh and J.T. Markert, Nature 351 ( 1991 ) 549. [ 5 ] 1. Yazawa, N. Terada, K. Matsutani, R. Sugise, M. J• and H. Ihara, Jpn. J. Appl. Phys. 29 (1990) L566. [61 M. Kanai, T. Kawai and S. Kawai, Appl. Phys. Left. 5,~ (1991) 771. [7] H. Koinuma, H. Nagata, T. Hashimoto, T. Tsukahara, S. Gonda and M. Yoshimoto, Proc. 3rd Int. Symp. on Superconductivity, (Springer, Berlin, 1991 ) 1135. [ 8 ] M. Takano, Kotaibuturi (Solid State Physics) 25 (1992) 251. [9] G. Er. Y. Miyamolo, F. Kanamaru and S. Kikkawa, Physica C181 (1991)206.
Physica C 196 (1992) 14-16 North-Holland
Superconducting (Sr, Nd ) CuO.,, thin films with infinite-layer structure H. Adachi, T. Satoh, Y. Ichikawa, K. Setsune and K. Wasa Central Research Laboratories. Matsushita Electric Industrial Co.. Ltd.. Moriguchi, Osaka 570. Japan
Received 16 March 1992
Thin films with infinite-layer structure have been prepared using a magnetron sputtering systcm and the substitution of neodymium for strontium in SrCuO2 has been successfully carried out, which is similar to the case of the high-pressure synthesis. ( St, Nd)CuO r films were grown in a reducing atmosphere on SrTiO3 single crystals and epitaxial growth with [001 ] orientation normal to the substrate was realized at the substrate temperature of 550-600°C. Superconducting transition was observed tbr some as-grown films, and the Sro.8sNd0~2CuOvfilm showed the highest T~.of 16 K.
The recent discovery of superconductivity in SrCuO2-related c o m p o u n d s with infinite-layer structure [ 1 ] has attracted much attention since the crystal structure is quite simple and an elucidation o f the m e c h a n i s m o f cuprate superconductors is expected to follow [2]. Takano et al. have reported the high transition temperature of 80-100 K in (Sr,Ba)CuO2. and they considered that the superconducting phase is the infinite-layer structure with a modulation along the c-axis [ 3 ]. Smith et al. have substituted some trivalent lanthanoid ( L n ) elements for divalent Sr in the infinite-layer SrCuO2 and derived superconductivity in the (Sr,Ln)CuO2 system [4]. These materials have so far only been m a d e by a high-pressure sintering process and this fact has restrained the worldwide research of infinite-layer superconductors. Meanwhile, thin-film processing sometimes creates metastable phases that are not o b t a i n e d by a standard solid-state reaction, and can also artificially control crystal structures. As for the infinite-layer structure, Yazawa et al. have successfully obtained CaCuO2 thin films by R F sputtering [5]. Epitaxial (Ca,Sr)CuO2 and SrCuO2 thin films with infinitelayer structure have also been p r e p a r e d by the laser ablation m e t h o d [6,7]. Recently, we have prepared infinite-layer-type SrCuO2 films by R F - m a g n e t r o n sputtering and have studied the effect o f n e o d y m i u m substitution for strontium. Here, we report the preparation and superconducting properties of (Sr,
N d ) C u O 2 films with the infinite-layer structure. Thin films were prepared by a R F - p l a n a r magnetron sputtering system. The substrates used were SrTiO3 single crystals with ( 1 0 0 ) plane. The targets were complex c o m p o u n d s of S r - N d - C u - O . where the content of n e o d y m i u m was varied, i.e., SG ~Nd~Cu~_l.30, ( x = 0 - 0 , 1 3 ) . Cu contents in the targets were c o m p e n s a t e d to keep the film composition near (Sr, N d ) C u O , . The film composition was precisely measured by the inductively coupled plasma-emission spectrometry analyses for the S r N d - C u - O films grown on MgO ( 1 0 0) substrates. The n e o d y m i u m contents ( N d / ( S r + N d ) ) in the films were almost equal to those in the targets. So. we regarded the N d content of the films grown on SrTiO3 substrate as that of the target, although the evaluation of Sr contents in the film on SrTiO3 cannot be identified. Since a reducing atmosphere is preferred for the growth of superconducting (Sr. N d ) C u O 2 [4], only argon was used as a sputtering gas and gas pressure during the sputtering was as low as 0.5 Pa. The input R F power was 100 W for the target d i a m e t e r of 80 mm. Thin films about 3000 ,& thick were obtained by 30 min deposition. After that. films were cooled down to room t e m p e r a t u r e for about 1 h in the same atmosphere as the deposition. The epitaxial thin films with infinite-layer structure were grown with [001 ] orientation normal to (1 0 0 ) SrTiO3 at a substrate temperature ranging
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserwed.
15
H. Adachi et al. /(Sr, Nd)CuOy thin films
from 550 to 600°C. Although the infinite-layer phase tended to disappear in neodymium-doped films strong reduction in the sputtering conditions stabilized the infinite-layer structure in the deposited films. Figure l ( a ) shows the X-ray diffraction ( X R D ) pattern of the Sro.98Ndo.o2CuOyfilm with the almost single infinite-layer structure. When the Nd content was increased to more than 0.05, small amounts of impurity phases (SrCu203, Sr2CuO 3 or unknown phases) appeared in addition to the infinite-layer phase. Figure 2 shows the variation of the lattice parameter c with the Nd content x. When the Nd was not doped, i.e., SrCuOy film, c length was 3.475 A, which is longer than that of SrCuO2 ceramics (3.43 A) made by high-pressure sintering. This may be attributable to the difference in synthetic technique. As the Nd was substituted for Sr, c length decreased, similar to the case of (Sr, Nd)CuO2 ceramics [4]. However, when the Nd content exceeded 0.1, another phase having a longer lattice parameter c (3.6 /k) began to coexist. This phase may be the infinitelayer-related structure with 2x//2a × 2x/2a × c lattice cell (c ~ 3.7/k) which shows metallic behavior [8]. Figure l ( b ) shows the XRD pattern of (Sro 88Ndo ~2)CuOy film with a mixture of the infi'
Ca)
'
'o
•
~
'
'
J
(~
~
""2 -=
(c)
5
1'0
I
' 2'0
30 20
40
50
60
(deg,)
Fig. 1. X-ray diffraction patterns of the (Sr~_xNdx)CuOr thin films with (a) x=0.02, (b) x=0.12 and (c) x=0.13. Stars are peaks from the infinite-layer-relatedphase with longer c length. Arrows indicate the impurity phases.
3.7 0 major phase minor phase A v
/XA 0 3.6
E t~ tl.
3.5
O
O ..-I
O O
O
O
3.4 I
0
0.05
O
I
0.1
0.15
x in (Srl_xNdx)CUOy
Fig. 2. Variation of the lattice parameter c as a function of Nd content x. nite-layer phase and the infinite-layer-related phase with longer c length. Further increase of Nd content in the film resulted in the disappearance of the infinite-layer phase and only the longer-c-length phase was grown. Figure 1 (c) shows the XRD pattern of the (Sro.s7Nd0. ~3) CuOy film with the longer-c-length phase, where no trace of the infinite-layer phase is seen. As for a-axis length, we could not measure it accurately since the c-axis of the infinite-layer film is oriented normal to the substrate. From the reflection high-energy electron diffraction analyses, a-axis length was found to be around 3.85-3.95 A for each film. The transport properties of the films were measured along the in-plane direction. The resistivity of the films tended to decrease with Nd doping, suggesting electron doping through the substitution of trivalent Nd for divalent Sr. For the films with 0.07 < x, the resistivity at room temperature was less than 10 -2 ohm cm. Figure 3 shows the temperature dependence of the resistivity for as-grown (Sr~ _xNdx)CuOy films with x = 0.09, 0.11, 0.12 and 0.13. A few films showed the resistivity drops which came from the superconducting transition. In the films with the infinite-layer phase, Tc became higher with the decrease of c length, i.e., Nd doping. This dependence is different from the (Sr,Ln)CuO2 ce-
16
H. Adachi et al. / (Sr, Nd)CuO,. thin films
6 {I
0
?
E o
"
[] ~j
4 •
×
OO9
x
Oil
x:O
12
x=O
13
• e• e []u
ee•
0
3
:~:~
% • • • • e m e o e e o e e e ~ • e o e e e e o o o n o•oeeeeooee o
>
if_
2
o
1
"~o
0
'sz:,':,-~. / , .
[ ][J[ ~
lh
L
0
100
]1 ][J J[ ][ i ]
i
,
_ _ _ 1
Temperature
t
200
__
_
300
(K)
Fig. 3. Temperature dependence of resistivity lor (Sr~ ,Nd~)CuO,, thin films with various neodymium content x. ramics where T,. is not significantly affected by the lanthanoid content [4,9]. The highest superconducting transition o f 16 K ( o n s e t ) was o b t a i n e d in the Sro.ssNd0 ~2CuOv film. F o r the x = 0.1 3 film with the single longer-c-length phase, metallic temperature d e p e n d e n c e o f the resistivity is seen. The T~ in the (Sr, N d ) C u O v film is lower than that in the highpressure-synthesized ceramic samples. This difference m a y be due to the phase segregation into the longer-c-length phase in high N d - d o p e d films. Thus, the doping o f N d in the infinite-layer phase by thinfilm processing is at present less efficient than that by high-pressure sintering. The suppression o f phase segregation into the longer-c-length phase would improve T~, in (Sr, Nd)CuO.v thin films. Further post-annealing in a v a c u u m is effective for some films in i m p r o v i n g the superconductivity; however, annealing in the atmosphere containing oxygen certainly degraded the superconductivity. This is considered to be one p r o o f that the superconducting (Sr, N d ) C u O 2 system is of the electrondoped type. The Hall coefficients of (Sq _xNdx)CuOy films with x = O . 1 1 and 0.12 were measured at r o o m
t e m p e r a t u r e and found to be positive. This result does not i m m e d i a t e l y lead to the negation o f electron doping because of the existence of impurity phases in these films. In summary, the infinite-layer superconductor, for whose synthesis high-pressure sintering has so far been required, has been realized by thin-film processing. The present sputtering process enabled the fabrication of SrCuO2 and electron doping in it, which resulted in the superconducting (Sr, N d ) CuO~ thin films. This matter resembles the synthesis of d i a m o n d which is usually m a d e by a high-pressure heating and can also be fabricated from a vapor phase using thin-film processing. The realization of infinite-layer-type superconductors by a simple sputtering m e t h o d will widen the opportunities for research on this kind o f material.
Acknowledgements The authors would like to thank T. Nitta and K, Kanai for their continuous encouragement and also thank S. Hatta and T. W a d a for useful discussions.
References [I]T. Siegrist, S.M. Zahurak, D.W. Murphy and R.S. Roth, Nature 334 (1988) 231. I2] R.J. Cava, Nature 351 ( 1991 ) 518. [ 3 ] M. Takano, M. Azuma, Z. Hiroi, Y. Band• and Y. Takeda, Physica C 176 (1991) 441. [4] M.G. Smith, A. Manthiram, J. Zhou, J.B. Go•den•ugh and J.T. Markert, Nature 351 ( 1991 ) 549. [ 5 ] 1. Yazawa, N. Terada, K. Matsutani, R. Sugise, M. J• and H. Ihara, Jpn. J. Appl. Phys. 29 (1990) L566. [61 M. Kanai, T. Kawai and S. Kawai, Appl. Phys. Left. 5,~ (1991) 771. [7] H. Koinuma, H. Nagata, T. Hashimoto, T. Tsukahara, S. Gonda and M. Yoshimoto, Proc. 3rd Int. Symp. on Superconductivity, (Springer, Berlin, 1991 ) 1135. [ 8 ] M. Takano, Kotaibuturi (Solid State Physics) 25 (1992) 251. [9] G. Er. Y. Miyamolo, F. Kanamaru and S. Kikkawa, Physica C181 (1991)206.