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Phase formation and superconductivity in sputtered Pb-based cuprate thin films
PHYSICA
Physica C 185-189 (1991) 1987-1988 North-Holland
PHASE FORMATION AND SUPERCONDUCT(VITY IN SPUTTERED Pb-BASED CUPRATE THIN F[LMS H. Adachi, S. Adachi, Y. Ichikawa, K. Setsune and K. Wasa Central Research Laboratories, Matsushita Electric Industrial Co., Ltd. 3-15, Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan Phase formation in Pb-based cuprate thin films has been investigated for a search for new phases. Thin films of Pb-Sr-M-Ca-Cu-O (M= Ln, Bi, Sb, Ru, Sn, In) were prepared by rf-magnetron sputtering on MgO substrates. When M is rare-earth element (Ln) and Bi, c-axis oriented films of Pb-based cuprates were fabricated. The a t t e m p t to create a novel crystal structure in the sputtered film was carried out by increasing Ca and Cu contents, and new phases of Pb-cuprates with multiplied CuO 2 planes in the unit cell were fabricated as a intergrowth structure. 1. INTRODUCTION Thin-film processing sometimes creates a m e t a stable phase which is not obtained by solid-state reaction, and is attractive for the research of new superconducting phases. This is due to the none q u i l i b r i u m pro~.~ssing of f i l m growth, w h e r e m a t e r i a l s are directly transformed from vapor to solid phase. Although the most attractive thin-film processing seems to be artificially layer-by-layer depositing method, simple sputtering method also can prepare new phases with novel structuresl, 2,3. In this research, we adopt the sputtering processing for a search for new phases in Pb-based cuprate s y s t e m ( P b - S r - M - C a - C u - O ) w h e r e a n u m b e r of related phases have been discovered recently. The typical Pb-based cuprate is Pb2Sr2(Ln, Ca)Cu30 8, which is composed of triple PbO-Cu-PbO blocking layer and a perovskite-related layer with two CuO 2 p l a n e s (Pb-3212 phase). T h e most i n t e r e s t i n g s u b j e c t is to m u l t i p l y CuO 2 p lan es in t h e perovskite related layer. For this purpose, Ca and Cu contents were increased in the Pb-cuprate film and phase formation was investigated.
to be undesirable for the multiplication of CuO 2 planes because no Bi or Tl-based cuprates with more than three CuO 2 planes were realized in the case of containing Ln. Therefore, preparation of Pb-Sr-M-Ca-Cu-O films, where M= Bi, Sb, Ru, Sn and In, in addition to M=Ln was also tried. It was found that Pb-3212 phase was feasible when M was Bi. Using M=Ln and Bi, creation of new phases in the Pb-based cuprates was investigated. 3. RESULTS AND DISCUSSION 3.1. M= rare-earth element (Ln) When the film composition was near Pb2Sr 2 Ln0.5Ca0.5Cu3Ox, epitaxial thin films of standard Pb-3212 phase were grown with c-axis (c=1.58 nm) normal to the substrates as shown in Fig.l (a). |
i
'
|
o
A ,,m C
.d t~ L_
2. FILM PREPARATION Thin films of Pb-based cuprates were prepared by an r f - p i a n a r m a g n e t r o n s p u t t e r i n g using a c o m p o u n d oxide t a r g e t . Since the r e d u c i n g atmosphere is preferred in the preparation of Pb3212 phase, pure argon with no oxygen was used as a sputtering gas. Thin films were deposited on MgO (100) substrate heated at 550-630°C. For the multiplication of CuO 2 planes, Ca and Cu contents were increased in the Pb-Sr-Ln-Ca-CuO film. Meanwhile, rare-earth element (Ln) seems
,
r-
I pb-3zl.2 I I pb-32231 b)
I
I
I
r-
15
IO
15
!
m
20
25
-
!
30
20 (deg.) Fig.l. X-ray diffraction patterns of (a)Pb2Sr2Yo.5 Ca0.5Cu3Ox and (b)Pb2Sr2Yo.3Ca2Cu4.3Oy films.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.
H. Adachi et 02 / Sputtered Pb.based cuprate thin films
1988
The a s - g r o w n Pb-3212 film e x h i b i t e d s u p e r conducting transition at 75 K 4, When the Ca and Cu c o n t e n t s were i n c r e a s e d in the film, the diffraction angle of 001 peak got broader toward low angle as shown in fig.l (b), which may indicate the creation of new phases with longer periods along c - a x i s . From t h e c o n s i d e r a t i o n of peak broadening and the d i f f r a c t i o n angles, it is speculated that small amount of Pb-3223 phase with three CuO 2 planes was created among the major Pb-3212 structure. The T c of this film was similar to that of the single Pb-3212 film.
Pb2Sr2Bi0,3Ca2.2Cu4.5Oz film. Sharp p e a k at 20= 28 ° indicates that the diffraction p a t t e r n is due to Pb-cuprates, not Bi-cuprates, because in the case of Bi-cuprates sharp peak of 20=29 ° appears and shifts toward high angle with Pb doping 6. In the figure, new broad peaks are observed in addition to the Pb-3212 phase. These may be assigned by the longer c-axis period of 1.92 nm (corresponds to Pb3223 phase). In t h e M=Bi film, the appearance of new peaks was more conspicuous than the case of M=Ln. This film showed superconducting transition with a resistivity anomaly near 100 K.
3.2. M= bismuth e l e m e n t
3.3. C r o s s - S e c t i o n a l TEM a n a l y s e s
Thin films of Pb-3212 phase could be prepared in Pb-Sr°Bi-Ca-Cu-O system with no rare-earth e l e m e n t s 5. They showed s u p e r c o n d u c t i v i t y at around 70 K. In this s y s t e m , t h e a t t e m p t to i n c r e a s e Ca and Cu c o n t e n t s was also t r i e d . Figure 2 shows the x-ray diffraction pattern of the 11
|
w
|
O
i
,m
C
O i
O C .t=.
=I
I
20
2e
,,,
t
25
(deg.)
Fig.2, X-ray diffraction patterns of the Pb2Sr2Bi0. 3 Ca2.2Cu4.50 z thin film. ( o: Pb-3212 phase, *: c=l.92-nm phase)
Figure 3 shows t h e TEM i m a g e of the Pbc u p r a t e film w h e r e Ca and Cu c o n t e n t s w e r e increased (same sample as Fig.1 (b)). The image revealed that t h e r e were several intergrowths of new phases with the periods of 1.9 nm and 2.1 nm among the ordinary Pb-3212 phase with the period of 1.6 nm. The 1.9 nm structure corresponds to Pb-3223 phase with three CuO 2 planes and 2.1 nm structure corresponds to Pb-3234 phase with four CuO 2 planes in a unit cell. Such i n t e r g r o w t h structures were rarely observed in the m i c r o s t r u c t u r e of c e r a m i c s a m p l e s 7. The peak broadening in the x-ray diffraction was found to be caused by the intergrowth of new Pb-cuprates with multiplied numbers of CuO 2 planes. 4. SUI~MARY By the thin-film processing using magnetron sputtering, novel structures have been fabricated in Pb-Sr-M-Ca-Cu-O (M=Ln and Bi) system. Small amount of new phases which possessed more than three CuO 2 planes in a unit cell were observed as intergrowth structures. The present processing is e f f e c t i v e for t h e f o r m a t i o n of c u p r a t e superconductors with multiplied CuO 2 planes.
REFERENCES 1. A.F.Marshall, R.W.Barton, K.Char, A.Kapitulnik et al., Phys. Rev. B 37 (1988) 9353. 2, i2
m
3. 4. l Fig.3. TEM image of the Pb-cuprate film with increased Ca and Cu contents,
5. 6. 7.
I I A .J^_L.: Fiw/'~UdV~..,llip
x," t - ! . . : I . . . . . . ~ wJ:-_~t,; I * |¢k~llllrx~VV q~l 1~'~*i l l l I U ~ . , I I I ,
T ~/l.~e,,o~im,~ I *lYi~lbOblOIllliig.~
et al., Jpn. J. Appl. Phys. 29 (1990) L81. I.Yazawa, N.Terada et al., j"-pn. J. Appl. Phys. 29 (1990) L566. H.Adachi, S.Adachi, Y.Ichikawa, K.Setsune and K.Wasa, Jpn. J. Appl. Phys. 30 (1991) L39. R.Retoux, C.Michel, M.Hervie-u and B.Raveau, Mod. Phys. Lett. B 3 (1989) 591. Y.lwai et al., Physic~ C 170 (1990) 319. H.W.Zandbergen, K.Kadow-a~ et al., Physica C 158 (1989) 155.
Physica C 185-189 (1991) 1987-1988 North-Holland
PHASE FORMATION AND SUPERCONDUCT(VITY IN SPUTTERED Pb-BASED CUPRATE THIN F[LMS H. Adachi, S. Adachi, Y. Ichikawa, K. Setsune and K. Wasa Central Research Laboratories, Matsushita Electric Industrial Co., Ltd. 3-15, Yagumo-Nakamachi, Moriguchi, Osaka 570, Japan Phase formation in Pb-based cuprate thin films has been investigated for a search for new phases. Thin films of Pb-Sr-M-Ca-Cu-O (M= Ln, Bi, Sb, Ru, Sn, In) were prepared by rf-magnetron sputtering on MgO substrates. When M is rare-earth element (Ln) and Bi, c-axis oriented films of Pb-based cuprates were fabricated. The a t t e m p t to create a novel crystal structure in the sputtered film was carried out by increasing Ca and Cu contents, and new phases of Pb-cuprates with multiplied CuO 2 planes in the unit cell were fabricated as a intergrowth structure. 1. INTRODUCTION Thin-film processing sometimes creates a m e t a stable phase which is not obtained by solid-state reaction, and is attractive for the research of new superconducting phases. This is due to the none q u i l i b r i u m pro~.~ssing of f i l m growth, w h e r e m a t e r i a l s are directly transformed from vapor to solid phase. Although the most attractive thin-film processing seems to be artificially layer-by-layer depositing method, simple sputtering method also can prepare new phases with novel structuresl, 2,3. In this research, we adopt the sputtering processing for a search for new phases in Pb-based cuprate s y s t e m ( P b - S r - M - C a - C u - O ) w h e r e a n u m b e r of related phases have been discovered recently. The typical Pb-based cuprate is Pb2Sr2(Ln, Ca)Cu30 8, which is composed of triple PbO-Cu-PbO blocking layer and a perovskite-related layer with two CuO 2 p l a n e s (Pb-3212 phase). T h e most i n t e r e s t i n g s u b j e c t is to m u l t i p l y CuO 2 p lan es in t h e perovskite related layer. For this purpose, Ca and Cu contents were increased in the Pb-cuprate film and phase formation was investigated.
to be undesirable for the multiplication of CuO 2 planes because no Bi or Tl-based cuprates with more than three CuO 2 planes were realized in the case of containing Ln. Therefore, preparation of Pb-Sr-M-Ca-Cu-O films, where M= Bi, Sb, Ru, Sn and In, in addition to M=Ln was also tried. It was found that Pb-3212 phase was feasible when M was Bi. Using M=Ln and Bi, creation of new phases in the Pb-based cuprates was investigated. 3. RESULTS AND DISCUSSION 3.1. M= rare-earth element (Ln) When the film composition was near Pb2Sr 2 Ln0.5Ca0.5Cu3Ox, epitaxial thin films of standard Pb-3212 phase were grown with c-axis (c=1.58 nm) normal to the substrates as shown in Fig.l (a). |
i
'
|
o
A ,,m C
.d t~ L_
2. FILM PREPARATION Thin films of Pb-based cuprates were prepared by an r f - p i a n a r m a g n e t r o n s p u t t e r i n g using a c o m p o u n d oxide t a r g e t . Since the r e d u c i n g atmosphere is preferred in the preparation of Pb3212 phase, pure argon with no oxygen was used as a sputtering gas. Thin films were deposited on MgO (100) substrate heated at 550-630°C. For the multiplication of CuO 2 planes, Ca and Cu contents were increased in the Pb-Sr-Ln-Ca-CuO film. Meanwhile, rare-earth element (Ln) seems
,
r-
I pb-3zl.2 I I pb-32231 b)
I
I
I
r-
15
IO
15
!
m
20
25
-
!
30
20 (deg.) Fig.l. X-ray diffraction patterns of (a)Pb2Sr2Yo.5 Ca0.5Cu3Ox and (b)Pb2Sr2Yo.3Ca2Cu4.3Oy films.
0921-4534/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All fights reserved.
H. Adachi et 02 / Sputtered Pb.based cuprate thin films
1988
The a s - g r o w n Pb-3212 film e x h i b i t e d s u p e r conducting transition at 75 K 4, When the Ca and Cu c o n t e n t s were i n c r e a s e d in the film, the diffraction angle of 001 peak got broader toward low angle as shown in fig.l (b), which may indicate the creation of new phases with longer periods along c - a x i s . From t h e c o n s i d e r a t i o n of peak broadening and the d i f f r a c t i o n angles, it is speculated that small amount of Pb-3223 phase with three CuO 2 planes was created among the major Pb-3212 structure. The T c of this film was similar to that of the single Pb-3212 film.
Pb2Sr2Bi0,3Ca2.2Cu4.5Oz film. Sharp p e a k at 20= 28 ° indicates that the diffraction p a t t e r n is due to Pb-cuprates, not Bi-cuprates, because in the case of Bi-cuprates sharp peak of 20=29 ° appears and shifts toward high angle with Pb doping 6. In the figure, new broad peaks are observed in addition to the Pb-3212 phase. These may be assigned by the longer c-axis period of 1.92 nm (corresponds to Pb3223 phase). In t h e M=Bi film, the appearance of new peaks was more conspicuous than the case of M=Ln. This film showed superconducting transition with a resistivity anomaly near 100 K.
3.2. M= bismuth e l e m e n t
3.3. C r o s s - S e c t i o n a l TEM a n a l y s e s
Thin films of Pb-3212 phase could be prepared in Pb-Sr°Bi-Ca-Cu-O system with no rare-earth e l e m e n t s 5. They showed s u p e r c o n d u c t i v i t y at around 70 K. In this s y s t e m , t h e a t t e m p t to i n c r e a s e Ca and Cu c o n t e n t s was also t r i e d . Figure 2 shows the x-ray diffraction pattern of the 11
|
w
|
O
i
,m
C
O i
O C .t=.
=I
I
20
2e
,,,
t
25
(deg.)
Fig.2, X-ray diffraction patterns of the Pb2Sr2Bi0. 3 Ca2.2Cu4.50 z thin film. ( o: Pb-3212 phase, *: c=l.92-nm phase)
Figure 3 shows t h e TEM i m a g e of the Pbc u p r a t e film w h e r e Ca and Cu c o n t e n t s w e r e increased (same sample as Fig.1 (b)). The image revealed that t h e r e were several intergrowths of new phases with the periods of 1.9 nm and 2.1 nm among the ordinary Pb-3212 phase with the period of 1.6 nm. The 1.9 nm structure corresponds to Pb-3223 phase with three CuO 2 planes and 2.1 nm structure corresponds to Pb-3234 phase with four CuO 2 planes in a unit cell. Such i n t e r g r o w t h structures were rarely observed in the m i c r o s t r u c t u r e of c e r a m i c s a m p l e s 7. The peak broadening in the x-ray diffraction was found to be caused by the intergrowth of new Pb-cuprates with multiplied numbers of CuO 2 planes. 4. SUI~MARY By the thin-film processing using magnetron sputtering, novel structures have been fabricated in Pb-Sr-M-Ca-Cu-O (M=Ln and Bi) system. Small amount of new phases which possessed more than three CuO 2 planes in a unit cell were observed as intergrowth structures. The present processing is e f f e c t i v e for t h e f o r m a t i o n of c u p r a t e superconductors with multiplied CuO 2 planes.
REFERENCES 1. A.F.Marshall, R.W.Barton, K.Char, A.Kapitulnik et al., Phys. Rev. B 37 (1988) 9353. 2, i2
m
3. 4. l Fig.3. TEM image of the Pb-cuprate film with increased Ca and Cu contents,
5. 6. 7.
I I A .J^_L.: Fiw/'~UdV~..,llip
x," t - ! . . : I . . . . . . ~ wJ:-_~t,; I * |¢k~llllrx~VV q~l 1~'~*i l l l I U ~ . , I I I ,
T ~/l.~e,,o~im,~ I *lYi~lbOblOIllliig.~
et al., Jpn. J. Appl. Phys. 29 (1990) L81. I.Yazawa, N.Terada et al., j"-pn. J. Appl. Phys. 29 (1990) L566. H.Adachi, S.Adachi, Y.Ichikawa, K.Setsune and K.Wasa, Jpn. J. Appl. Phys. 30 (1991) L39. R.Retoux, C.Michel, M.Hervie-u and B.Raveau, Mod. Phys. Lett. B 3 (1989) 591. Y.lwai et al., Physic~ C 170 (1990) 319. H.W.Zandbergen, K.Kadow-a~ et al., Physica C 158 (1989) 155.