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Preparation of Y1−xCaxBa2Cu3O7−δ thin films by the RF thermal plasma deposition method
PflYSICA ELSEVIER
PhysicaC 244 (1995) 256--262
Preparation of Y1 - x C a x B a 2 C u 3 0 7 - thin films by the RF thermal plasma deposition method H. Yakabe *, J.G. Wen, Y. Shiohara, N. Koshizuka Superconductivity Research Laboratory, ISTEC, 10-13Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan
Received 18 January 1995
Abstract Ca-doped YBa2Cu307_ ~ thin films were prepared on SrTiO3 substrates using the RF thermal plasma deposition method. Up to 50% Ca-concentration, single-phase Y~_~CaxBa2CuaO7_ ~thin films were obtained. When the doping level exceeded 50% Ca concentration, peaks corresponding to the YBa2Cu3OT_ ~structure became prominently broad and some impurity phases appeared in the XRD patterns. The resistivity and Tc o,~etof the as-grown samples decreased with increasing Ca content up to 50% Ca concentration. On the other hand, the T~ of samples with a Ca concentration higher than 50% increased with increasing Ca content, and CaBa2Cu307_ 8 thin film showed superconductivity below 90 KI Transport measurements revealed that the suppression of Tc by Ca doping is mainly due to the overdoping effect.
1. Introduction
Recently, C u - 1 2 ( n - l ) n superconductors have been synthesized by various techniques. Jin et al. [ 1 ] and Franco et al. [2] independently composed the n = 3, 4 and 5 members by the high pressure method. On the other hand, Adachi et al. reported (Ag, C, Cu)1201 thin film with a T~ of 50 K prepared by magnetron sputtering [ 3 ]. More recently, we reported that the Cu1212 phase exists in superconducting Ba-Ca-Cu-O thin films prepared by the RF thermal plasma deposition method [4]. The Ba--Ca-Cu-O thin films showed superconductivity below 90 K and we consider that the Cu-1212 phase shows superconductivity in the film. The Cu- 1212 phase is a member of the Cu- 12 (n - 1) n family, and its structure is the same structure as YBa2Cu307 _ ~ except that the Y site is replaced by Ca. In other words, the Cu-1212 compound is 100% Ca* Correspondingauthor. 0921-4534/ 95/ $09.50 © 1995ElsevierScienceB.V. All rights reserved SSDI0921-4534(95)00068-2
substituted YBCO. There have been many reports of studies for Ca doping in the YBaaCU3OT_~ system [ 5 11 ]. For bulk samples of Ca-doped YBCO, the solubility limit is about 30% Ca concentration [6-8]. On the other hand, the vapor-phase growth technique makes it possible to synthesize highly Ca-doped YBCO thin films [ 10]. In either case, T~ depression caused by Ca substitution is generally noticed [5-10]. However, the fact that the Tc .... , of our B a - C a - C u - O thin film remains nearly 90 K is contrary to the reported Tc depression with Ca substitution in YBCO. It is unclear whether Ca substitution depresses T~ in the YBCO system and why the Cu-1212 phase shows 90 K superconductivity. In this paper we report on the systematic preparation of Y~ _xCaxBa2Cu3OT_,~ thin films. The prepared samples are characterized by inductively coupled plasma atomic emission microscopy (ICP-AES), XRD, high-resolution transmission electron microscopy (HRTEM), and transport measurements.
257
H. Yakabe et al./ Physica C 244 (1995) 256--262
2. E x p e r i m e n t a l
x---0.7
Y1-xCaxBa2Cu307-8 thin films were prepared on SrTiO 3 single crystal substrates with (100) plane by the RF thermal plasma deposition method. The experimental apparatus used here is shown elsewhere [ 12]. Film growth conditions of the Yl-xCaxBa2Cu307-a thin films are listed in Table 1. For CaBa2Cu307_ ~ thin film, film growth conditions were different from those for YBa2Cu307-8 thin film [4]. Raw materials of Y203, CaO, BaCO3 and CuO powders were mixed and calcined at 910°C for 24 h. The nominal composition of Y~ _xCaxBa2Cu307_,~ starting materials were stoichiometric. After re-grinding, the powders were fed into an RF oxygen plasma environment with an Ar carrier gas. The substrates were spontaneously heated by a thermal plasma before feeding. Substrate temperature was monitored by a thermocouple, and thin films were prepared at various substrate temperatures of 6 0 0 ° C - 6 5 0 ° C . The fed powders were vaporized and codeposited onto the substrates in 1-3 min. The film thickness of the one-minute-deposited sample was usually about 300 ,~. After deposition, the samples were naturally quenched with a releasing vacuum to ambient pressure. Further oxygen treatment was performed in flowing 02 and Ar gas at 400°C for 1 h. The structures o f the prepared films were characterized by XRD and H R T E M measurements and overall composition was analysed by ICP-AES. DC electrical resistivity was measured by the standard four-probe technique.
3. R e s u l t s
Fig. 1 shows the XRD patterns of the Yl_xCaxBa2Cu307_ 8 (x=0-4).7) thin films with Table 1 Growth conditions of YI -xCa~Ba2Cu3Oa-8thin film RF input power
50 kW
Gas
7 l/min Ar+50 l/min 02 Y203, BaCO3,CaO and CuO (calcined at 910°C for 24 h) 200 Ton" 600-650 °C
Powder Pressure Substrate temperature Growth duration
1-3 min (film thickness= 300-1000 A)
x----05
A
ill
x=0.3
,[ l
x----0 (002) st'rio3(001)
(001 )
4
1. 10
(005)SrTiO3(002)
i
20
~ ' 40 20 (deg)
(o07)
50
60
Fig. 1. XRD patterns of Yt_xCa~Ba2Cu307-~ (0
258
H. Yakabe et al. / Physica C 244 (1995) 256-262
tallinity of all films is almost perfect and no second phase was observed. Electron diffraction patterns corresponding to the TEM images of Figs. 2(a), (b) and (c) are shown in Figs. 3(a), (b) and (c), respectively. All samples show fine diffraction patterns and neither
Fig. 2. Cross-sectional TEM images of thin films of (a) YBaaCu307_ ~,, (b) Yo.7Cao.3Ba2Cu307 _ ,s, and (c) Y~.sCao.~Ba,CusOT_ ~. For all films, a perfect YBCO structure grows epitaxially on the SrTiO3 substrates.
Fig. 3. Electron diffraction patterns of thin films of (a) YBa2Cu307 _ ,s, (b) Yo.7Cao.3Ba2Cu307 ~, and (c) Yo.sCao.sBaeCu307 _ ~. These patterns correspond to the TEM images of Figs. 2 ( a ) , (b) and (c), respectively.
H. Yakabe et al. / Physica C 244 (1995) 256-262
2
Fig. 4. Cross-sectional HRTEM image of the Ba-Ca--Cu-O thin film. The arrows indicate CuO chains and the indicated numbers correspond to the total numbers of CuO2 planes between the CuO chains.
second phase nor superstructure can be seen. However, as Ca concentration increases, the intensity of the diffraction spots corresponding to large g values becomes rapidly weaker. This means that the crystallinity of the Yo.sCao.sBa2Cu307_~ thin film is inferior to that of Y B a 2 C u 3 0 7 _ ~ thin film, and is consistent with the result of the XRD measurement and may arise from the randomness of Ca substitution and oxygen vacancies in CuO chains. For the sample with 100% Ca concentration in the starting powder, prepared under the same film growth conditions as those for the YBCO film, the main phase was BaCuO2 infinite-layer phase and the film showed a semiconducting transport property. Only when the input plasma power was lower than that for the YBCO film and the substrate temperature was about 500°C was a superconducting film obtained. Although no remarkable peaks corresponding to the (00l) peaks of YBCO structure can be seen in the XRD pattern of superconducting Ba-Ca~Cu-O thin film, TEM measurement revealed that the Cu-1212 structure (i.e., CaBa2Cu3OT_~ structure) exists in almost the entire area of the superconducting B a - C a - C u - O film, as shown in Fig. 4. The bright lines, as indicated by arrows, correspond to the CuO chains. The total numbers of CuO2 planes between the arrows are indicated
259
in the figure. The one unit structure between the upper two arrows is similar to that of YBCO, and this structure is the Cu-1212 structure. Another phase corresponding to the n = 3 member of Cu- 12 (n - 1) n can also be seen in the TEM image. Although the Cu-1212 phase exists in almost all areas of the superconducting B a - C a - C u O film, the Cu-1223 phase exists only in some small parts of the film. Therefore, we consider that the Cu1212 phase shows superconductivity in the Ba--CaCu-O thin film. Fig. 5 shows the variation of lattice parameter c with nominal Ca concentration x for the as-grown Y l - x f a x B a 2 C u 3 O T - , S thin films. Although the length of the lattice parameter c is very sensitive to oxygen deficiency 6 [ 13,14], it increases monotonically as x increases. This tendency is in agreement with previous works which reported the increase of the lattice parameter c with increasing Ca concentration [ 15,16]. Temperature dependence of the resistivity for the Ba--Ca-Cu-O and the as-grown Y,_ ~CaxBa2Cu307 _ (0_
,
,
,
,
,
,
o< ¢d
11.85 11.80 11.75
'~ 11.70 11.65
I
I
I
I
I
I
I
10
20
30
40
50
60
70
Ca content (%) Fig. 5. The variations of the lattice constant c of Y i _ xCa,BazCu307 ( 0 < x < 0 . 7 ) thin films.
H. Yakabe et al. /Physica C 244 (1995) 256-262
260
1•41
'
'
!
!
x=l.0
~1.2
~
x=0.9
1.0
"'0.8
~o.~ x--0.7
~0.4i
0A1
-/,"% 50
,
100
,
150
Temperature 0.5
u
u
|
,
200
,
250
-I
300
(K)
!
!
x=0 ~0.3
..~o.1 ~q 0.11
~ ' - x = O . S I r
50
dn," I
100
I
I
I
150
200
250
Temperarure 0
~ 300
(K)
Fig. 6. Temperature dependence of DC electrical resistivity for Yi-,Ca~Ba2Cu307-,~ ( 0 _ < x ~ 1 ) thin films. The points of Tco,set on the resistivity curve are indicated by the arrows.
4. Discussion
We may separately consider the effects of Ca substitution under Ca doping levels above and below 50% Ca concentration. The following is a summary of the results of Ca doping for the Y~_xCaxBa2Cu307 ,~ (x<0.5) thin films. ( 1) For the non-doped YBCO thin film, judging from the values of the lattice parameter c and To oxygen deficiency 8 is smaller than 0.1. On the other hand, the lattice parameter c of the 50% Ca-doped sample is longer than the previously reported value [ 15]. This means that the as-grown 50% Ca-doped thin film is a more oxygen-deficient sample. The oxygen deficiency in the as-grown sample increases with Ca substitution. (2) It has already been reported that Ca substitution at the Y site causes oxygen deficiencies, and the increase of the oxygen deficiencies gives rise to an increase of the resistivity p of YBCO [8]. However, the resistivity
of our as-grown sample decreased with increasing Ca content up to 50% Ca concentration. This means that the extra charge introduced by the substitution of Ca at the Y site is not compensated by oxygen deficiencies, and the Y t - x C a x l a 2 C u 3 0 7 - 8 ( 0 . 1 _
ann
~
0.15
~
0.10
L A;,as ed in
0.05
¢o 0.~
50
•°
100
150
200
250
Temperature (K) Fig. 7. The effects of thermal treatment on the DC resistivity. The 50% Ca-doped film was annealed in flowing 02 and Ar gas at 400 ° C for I h. The points of Tc o~t are indicated by the arrows on the resistivity curve.
H. Yakabe et al. / Physica C 244 (1995) 256-262
Judging from the above results, it is concluded that the depression of T~ in the present Ca-doped sample is mainly due to the overdoping effect induced by the replacement of y3 + by Ca 2 +. For the samples with a higher Ca concentration than 70%, transport properties were different from those of the samples with a Ca concentration lower than 50%. Curiously, T~.... t increases with increasing Ca concentration. Moreover, annealing in flowing oxygen gas increases the T~ of the samples. There is a possibility that a phase separation occurred and the samples contained two phases of Ca-rich YBCO phase and Ca-poor YBCO phase, since the (00l) peaks of these samples in the XRD patterns were broadened. The Ca-poor YBCO phase may result in relatively higher T~.... t than that of the 50% Ca-doped sample and because of the presence of the Ca-rich YBCO phase the transition may be broadened. On the contrary, the B a - C a - C u - O thin film shows the onset of resistivity drop at 90 K whereas it contains no yttrium. Magnetoresistance measurements under pulsed magnetic fields up to 20 T confirmed that the B a - C a - C u - O thin film shows superconductivity below 90 K [4]. Therefore, we may consider that the increase of Z c . . . . . t with increasing Ca concentration is not due to the above two phase separation, but an intrinsic property for the samples with a higher Ca concentration than 50%, and the Cu-1212 phase brings about superconductivity below 90 K in the B a - C a 4 2 u - O thin film. At a low Ca doping level, the sample is in the overdoped state caused by Ca doping and Tc decreases with increasing Ca content. When the Ca concentration exceeds 50%, it may become difficult to construct a perfect YBCO structure, resulting in the further increase of the oxygen vacancies in the CuO chains. The oxygen vacancies may come to counterbalance the excess holes and the Tc goes up again as Ca concentration increases. In conclusion, Yl_xCaxBa2Cu307_ ~ ( x = 0 - 1 . 0 ) films were prepared by the RF thermal plasma deposition method. The XRD and TEM measurements showed that single-phase Yl_xCaxBa2Cu307_~ was obtained up to 50% Ca substitution. Although the B a C a - C u - O thin film showed no remarkable peaks corresponding to the YBCO structure in the XRD pattern, the TEM measurement revealed that a Cu- 1212 structure (i.e., Ca123 structure) exists in the film. The results of thermal treatment and the resistivity meas-
261
urements showed that the resistivity and Tc onset of the sample decrease with increasing Ca content up to 50% Ca concentration because of the overdoping effect. For the samples with a Ca concentration higher than 70%, Tc increases with increasing Ca content while the resistivity increases. The B a - C a - C u - O thin film showed a resistivity drop below 90 K and it is concluded that the Cu- 1212 phase is the origin of superconductivity below 90 K in the Ba42a--Cu-O thin films.
Acknowledgements We are grateful to J. Tsujino and N. Tatsumi for their help in sample preparation, and to T. Machi and M. Kosuge for many helpful discussions. This work was partially supported by the New Energy and Industrial Technology Development Organization for the R&D of Industrial Science and Technology Frontier Program.
References [1 | C.Q. Jin, S. Adachi, X.J. Wu, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 238. [21 M.A.A. Franco, C. Chaillout, J.J. Capponi, J.L. Tholence and B. Souletie, Physica C 222 (1994) 52. [3] H. Adachi, M. Sakai, K. Mizuno and K. Setsune, in: Abstracts of the Meeting of the Physical Society of Japan, Shizuoka, 1994, Sectional meeting, part 3, p. 279. 141 H. Yakabe, J.G. Wen, A. Kume, Y. Shiohara, N. Koshizuka and S. Tanaka, Physica C 231 (1994) 330. [5] Y. Tokura, J.B. Torrance, T.C. Huang and A.I. Nazzal, Phys. Rev. B 38 (1988) 7156. 16] A. Tokiwa, Y. Syono, M. Kikuchi, R. Suzuki, K. Kajitani, N. Kobayashi, T. Sasaki, O. Nakatsu and Y. Muto, Jpn. J. Appl. Phys. 27 (1989) LI009. 17] T. Kawashima, Y. Matsui and E. Takayama-Muromachi, Physica C 224 (1994) 69. 181 B. Fisher, J. Genossar, C.G. Kuper, L. Patlagan, G.M. Reisner and A. Knizhink, Phys. Rev. B 47 (1993) 6053. [ 91 C.L. Gledel, J.F. Marucco, E. Vincent, D. Favrot, B. Poumellec, B. Touzelin, M. Gupta and H. Alloul, Physica C 175 (1991) 279. [ 10 ] Y. Sun, G. Strasser and E. Gronik, Physica C 206 (1993) 291. [ 11 ] V.P.S. Awana and A.V. Narlikar, Phys. Rev. B 49 (1994) 6353. [ 12] S. Yuhya, K. Kikuchi and Y. Shiohara, J. Mater. Res. 7 (1992) 2673. [ 13] J.D. Jorgensen, B.W. Veal, A.P. Paulikas, H. Claus and W.K. Kwok, Phys. Rev. B 41 (1990) 1863.
262
H. Yakabe et al. / Physica C 244 (1995) 256-262
1141 R.J. Cava, A.W. Hewat, B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419.
115] Y. Sun, G. Strasser, E. Gornik and X.Z. Wang, Physica C 223 (1994) 14. 116] V.P.S. Awana, Ashwin Tulapurkarand S.K. Malik, Phys. Rev. B 50 (1994) 594.
PhysicaC 244 (1995) 256--262
Preparation of Y1 - x C a x B a 2 C u 3 0 7 - thin films by the RF thermal plasma deposition method H. Yakabe *, J.G. Wen, Y. Shiohara, N. Koshizuka Superconductivity Research Laboratory, ISTEC, 10-13Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan
Received 18 January 1995
Abstract Ca-doped YBa2Cu307_ ~ thin films were prepared on SrTiO3 substrates using the RF thermal plasma deposition method. Up to 50% Ca-concentration, single-phase Y~_~CaxBa2CuaO7_ ~thin films were obtained. When the doping level exceeded 50% Ca concentration, peaks corresponding to the YBa2Cu3OT_ ~structure became prominently broad and some impurity phases appeared in the XRD patterns. The resistivity and Tc o,~etof the as-grown samples decreased with increasing Ca content up to 50% Ca concentration. On the other hand, the T~ of samples with a Ca concentration higher than 50% increased with increasing Ca content, and CaBa2Cu307_ 8 thin film showed superconductivity below 90 KI Transport measurements revealed that the suppression of Tc by Ca doping is mainly due to the overdoping effect.
1. Introduction
Recently, C u - 1 2 ( n - l ) n superconductors have been synthesized by various techniques. Jin et al. [ 1 ] and Franco et al. [2] independently composed the n = 3, 4 and 5 members by the high pressure method. On the other hand, Adachi et al. reported (Ag, C, Cu)1201 thin film with a T~ of 50 K prepared by magnetron sputtering [ 3 ]. More recently, we reported that the Cu1212 phase exists in superconducting Ba-Ca-Cu-O thin films prepared by the RF thermal plasma deposition method [4]. The Ba--Ca-Cu-O thin films showed superconductivity below 90 K and we consider that the Cu-1212 phase shows superconductivity in the film. The Cu- 1212 phase is a member of the Cu- 12 (n - 1) n family, and its structure is the same structure as YBa2Cu307 _ ~ except that the Y site is replaced by Ca. In other words, the Cu-1212 compound is 100% Ca* Correspondingauthor. 0921-4534/ 95/ $09.50 © 1995ElsevierScienceB.V. All rights reserved SSDI0921-4534(95)00068-2
substituted YBCO. There have been many reports of studies for Ca doping in the YBaaCU3OT_~ system [ 5 11 ]. For bulk samples of Ca-doped YBCO, the solubility limit is about 30% Ca concentration [6-8]. On the other hand, the vapor-phase growth technique makes it possible to synthesize highly Ca-doped YBCO thin films [ 10]. In either case, T~ depression caused by Ca substitution is generally noticed [5-10]. However, the fact that the Tc .... , of our B a - C a - C u - O thin film remains nearly 90 K is contrary to the reported Tc depression with Ca substitution in YBCO. It is unclear whether Ca substitution depresses T~ in the YBCO system and why the Cu-1212 phase shows 90 K superconductivity. In this paper we report on the systematic preparation of Y~ _xCaxBa2Cu3OT_,~ thin films. The prepared samples are characterized by inductively coupled plasma atomic emission microscopy (ICP-AES), XRD, high-resolution transmission electron microscopy (HRTEM), and transport measurements.
257
H. Yakabe et al./ Physica C 244 (1995) 256--262
2. E x p e r i m e n t a l
x---0.7
Y1-xCaxBa2Cu307-8 thin films were prepared on SrTiO 3 single crystal substrates with (100) plane by the RF thermal plasma deposition method. The experimental apparatus used here is shown elsewhere [ 12]. Film growth conditions of the Yl-xCaxBa2Cu307-a thin films are listed in Table 1. For CaBa2Cu307_ ~ thin film, film growth conditions were different from those for YBa2Cu307-8 thin film [4]. Raw materials of Y203, CaO, BaCO3 and CuO powders were mixed and calcined at 910°C for 24 h. The nominal composition of Y~ _xCaxBa2Cu307_,~ starting materials were stoichiometric. After re-grinding, the powders were fed into an RF oxygen plasma environment with an Ar carrier gas. The substrates were spontaneously heated by a thermal plasma before feeding. Substrate temperature was monitored by a thermocouple, and thin films were prepared at various substrate temperatures of 6 0 0 ° C - 6 5 0 ° C . The fed powders were vaporized and codeposited onto the substrates in 1-3 min. The film thickness of the one-minute-deposited sample was usually about 300 ,~. After deposition, the samples were naturally quenched with a releasing vacuum to ambient pressure. Further oxygen treatment was performed in flowing 02 and Ar gas at 400°C for 1 h. The structures o f the prepared films were characterized by XRD and H R T E M measurements and overall composition was analysed by ICP-AES. DC electrical resistivity was measured by the standard four-probe technique.
3. R e s u l t s
Fig. 1 shows the XRD patterns of the Yl_xCaxBa2Cu307_ 8 (x=0-4).7) thin films with Table 1 Growth conditions of YI -xCa~Ba2Cu3Oa-8thin film RF input power
50 kW
Gas
7 l/min Ar+50 l/min 02 Y203, BaCO3,CaO and CuO (calcined at 910°C for 24 h) 200 Ton" 600-650 °C
Powder Pressure Substrate temperature Growth duration
1-3 min (film thickness= 300-1000 A)
x----05
A
ill
x=0.3
,[ l
x----0 (002) st'rio3(001)
(001 )
4
1. 10
(005)SrTiO3(002)
i
20
~ ' 40 20 (deg)
(o07)
50
60
Fig. 1. XRD patterns of Yt_xCa~Ba2Cu307-~ (0
258
H. Yakabe et al. / Physica C 244 (1995) 256-262
tallinity of all films is almost perfect and no second phase was observed. Electron diffraction patterns corresponding to the TEM images of Figs. 2(a), (b) and (c) are shown in Figs. 3(a), (b) and (c), respectively. All samples show fine diffraction patterns and neither
Fig. 2. Cross-sectional TEM images of thin films of (a) YBaaCu307_ ~,, (b) Yo.7Cao.3Ba2Cu307 _ ,s, and (c) Y~.sCao.~Ba,CusOT_ ~. For all films, a perfect YBCO structure grows epitaxially on the SrTiO3 substrates.
Fig. 3. Electron diffraction patterns of thin films of (a) YBa2Cu307 _ ,s, (b) Yo.7Cao.3Ba2Cu307 ~, and (c) Yo.sCao.sBaeCu307 _ ~. These patterns correspond to the TEM images of Figs. 2 ( a ) , (b) and (c), respectively.
H. Yakabe et al. / Physica C 244 (1995) 256-262
2
Fig. 4. Cross-sectional HRTEM image of the Ba-Ca--Cu-O thin film. The arrows indicate CuO chains and the indicated numbers correspond to the total numbers of CuO2 planes between the CuO chains.
second phase nor superstructure can be seen. However, as Ca concentration increases, the intensity of the diffraction spots corresponding to large g values becomes rapidly weaker. This means that the crystallinity of the Yo.sCao.sBa2Cu307_~ thin film is inferior to that of Y B a 2 C u 3 0 7 _ ~ thin film, and is consistent with the result of the XRD measurement and may arise from the randomness of Ca substitution and oxygen vacancies in CuO chains. For the sample with 100% Ca concentration in the starting powder, prepared under the same film growth conditions as those for the YBCO film, the main phase was BaCuO2 infinite-layer phase and the film showed a semiconducting transport property. Only when the input plasma power was lower than that for the YBCO film and the substrate temperature was about 500°C was a superconducting film obtained. Although no remarkable peaks corresponding to the (00l) peaks of YBCO structure can be seen in the XRD pattern of superconducting Ba-Ca~Cu-O thin film, TEM measurement revealed that the Cu-1212 structure (i.e., CaBa2Cu3OT_~ structure) exists in almost the entire area of the superconducting B a - C a - C u - O film, as shown in Fig. 4. The bright lines, as indicated by arrows, correspond to the CuO chains. The total numbers of CuO2 planes between the arrows are indicated
259
in the figure. The one unit structure between the upper two arrows is similar to that of YBCO, and this structure is the Cu-1212 structure. Another phase corresponding to the n = 3 member of Cu- 12 (n - 1) n can also be seen in the TEM image. Although the Cu-1212 phase exists in almost all areas of the superconducting B a - C a - C u O film, the Cu-1223 phase exists only in some small parts of the film. Therefore, we consider that the Cu1212 phase shows superconductivity in the Ba--CaCu-O thin film. Fig. 5 shows the variation of lattice parameter c with nominal Ca concentration x for the as-grown Y l - x f a x B a 2 C u 3 O T - , S thin films. Although the length of the lattice parameter c is very sensitive to oxygen deficiency 6 [ 13,14], it increases monotonically as x increases. This tendency is in agreement with previous works which reported the increase of the lattice parameter c with increasing Ca concentration [ 15,16]. Temperature dependence of the resistivity for the Ba--Ca-Cu-O and the as-grown Y,_ ~CaxBa2Cu307 _ (0_
,
,
,
,
,
,
o< ¢d
11.85 11.80 11.75
'~ 11.70 11.65
I
I
I
I
I
I
I
10
20
30
40
50
60
70
Ca content (%) Fig. 5. The variations of the lattice constant c of Y i _ xCa,BazCu307 ( 0 < x < 0 . 7 ) thin films.
H. Yakabe et al. /Physica C 244 (1995) 256-262
260
1•41
'
'
!
!
x=l.0
~1.2
~
x=0.9
1.0
"'0.8
~o.~ x--0.7
~0.4i
0A1
-/,"% 50
,
100
,
150
Temperature 0.5
u
u
|
,
200
,
250
-I
300
(K)
!
!
x=0 ~0.3
..~o.1 ~q 0.11
~ ' - x = O . S I r
50
dn," I
100
I
I
I
150
200
250
Temperarure 0
~ 300
(K)
Fig. 6. Temperature dependence of DC electrical resistivity for Yi-,Ca~Ba2Cu307-,~ ( 0 _ < x ~ 1 ) thin films. The points of Tco,set on the resistivity curve are indicated by the arrows.
4. Discussion
We may separately consider the effects of Ca substitution under Ca doping levels above and below 50% Ca concentration. The following is a summary of the results of Ca doping for the Y~_xCaxBa2Cu307 ,~ (x<0.5) thin films. ( 1) For the non-doped YBCO thin film, judging from the values of the lattice parameter c and To oxygen deficiency 8 is smaller than 0.1. On the other hand, the lattice parameter c of the 50% Ca-doped sample is longer than the previously reported value [ 15]. This means that the as-grown 50% Ca-doped thin film is a more oxygen-deficient sample. The oxygen deficiency in the as-grown sample increases with Ca substitution. (2) It has already been reported that Ca substitution at the Y site causes oxygen deficiencies, and the increase of the oxygen deficiencies gives rise to an increase of the resistivity p of YBCO [8]. However, the resistivity
of our as-grown sample decreased with increasing Ca content up to 50% Ca concentration. This means that the extra charge introduced by the substitution of Ca at the Y site is not compensated by oxygen deficiencies, and the Y t - x C a x l a 2 C u 3 0 7 - 8 ( 0 . 1 _
ann
~
0.15
~
0.10
L A;,as ed in
0.05
¢o 0.~
50
•°
100
150
200
250
Temperature (K) Fig. 7. The effects of thermal treatment on the DC resistivity. The 50% Ca-doped film was annealed in flowing 02 and Ar gas at 400 ° C for I h. The points of Tc o~t are indicated by the arrows on the resistivity curve.
H. Yakabe et al. / Physica C 244 (1995) 256-262
Judging from the above results, it is concluded that the depression of T~ in the present Ca-doped sample is mainly due to the overdoping effect induced by the replacement of y3 + by Ca 2 +. For the samples with a higher Ca concentration than 70%, transport properties were different from those of the samples with a Ca concentration lower than 50%. Curiously, T~.... t increases with increasing Ca concentration. Moreover, annealing in flowing oxygen gas increases the T~ of the samples. There is a possibility that a phase separation occurred and the samples contained two phases of Ca-rich YBCO phase and Ca-poor YBCO phase, since the (00l) peaks of these samples in the XRD patterns were broadened. The Ca-poor YBCO phase may result in relatively higher T~.... t than that of the 50% Ca-doped sample and because of the presence of the Ca-rich YBCO phase the transition may be broadened. On the contrary, the B a - C a - C u - O thin film shows the onset of resistivity drop at 90 K whereas it contains no yttrium. Magnetoresistance measurements under pulsed magnetic fields up to 20 T confirmed that the B a - C a - C u - O thin film shows superconductivity below 90 K [4]. Therefore, we may consider that the increase of Z c . . . . . t with increasing Ca concentration is not due to the above two phase separation, but an intrinsic property for the samples with a higher Ca concentration than 50%, and the Cu-1212 phase brings about superconductivity below 90 K in the B a - C a 4 2 u - O thin film. At a low Ca doping level, the sample is in the overdoped state caused by Ca doping and Tc decreases with increasing Ca content. When the Ca concentration exceeds 50%, it may become difficult to construct a perfect YBCO structure, resulting in the further increase of the oxygen vacancies in the CuO chains. The oxygen vacancies may come to counterbalance the excess holes and the Tc goes up again as Ca concentration increases. In conclusion, Yl_xCaxBa2Cu307_ ~ ( x = 0 - 1 . 0 ) films were prepared by the RF thermal plasma deposition method. The XRD and TEM measurements showed that single-phase Yl_xCaxBa2Cu307_~ was obtained up to 50% Ca substitution. Although the B a C a - C u - O thin film showed no remarkable peaks corresponding to the YBCO structure in the XRD pattern, the TEM measurement revealed that a Cu- 1212 structure (i.e., Ca123 structure) exists in the film. The results of thermal treatment and the resistivity meas-
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urements showed that the resistivity and Tc onset of the sample decrease with increasing Ca content up to 50% Ca concentration because of the overdoping effect. For the samples with a Ca concentration higher than 70%, Tc increases with increasing Ca content while the resistivity increases. The B a - C a - C u - O thin film showed a resistivity drop below 90 K and it is concluded that the Cu- 1212 phase is the origin of superconductivity below 90 K in the Ba42a--Cu-O thin films.
Acknowledgements We are grateful to J. Tsujino and N. Tatsumi for their help in sample preparation, and to T. Machi and M. Kosuge for many helpful discussions. This work was partially supported by the New Energy and Industrial Technology Development Organization for the R&D of Industrial Science and Technology Frontier Program.
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