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P-type carrier doping in ACuO2 infinite-layer thin film
PHYSICA Physica C 232 (1994) 371-378
ELSEVIER
p-type Carrier doping in ACuO: infinite-layer thin film H. Yakabe *, A. Kume, J.G. Wen, M. Kosuge, Y. Shiohara, N. Koshizuka Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan
Received 1 August 1994
Abstract
Thin films of ACuO2 (A: alkaline earth) with "infinite-layer" structure were prepared by the RF thermal plasma evaporation method on SrTiO3 (100) substrates. The lattice constant c increases systematically with the increase of A ion size. Electricresistivity measurements showed that all the as-grown samples were semiconducting. In order to dope holes into this system, two methods were tried. One is substitution of Na for the alkaline earth. The other one is exposing the samples to 02 plasma. 02 plasma annealing caused a structural change in the samples. The physical properties of the plasma-annealed samples were measured and a possible structural model is proposed. Neither the Na doped sample nor the O2 plasma-annealed sample showed superconductivity. The crystalline condition for the absence of superconductivity is discussed.
1. Introduction
The "infinite-layer" compound ACuO 2 (A: alkaline earth) is the simplest material containing CuO2 layers. Since the successful synthesis of the infinite-layer c o m p o u n d by Siegrist et al. [ 1 ], m a n y researchers have tried to make it a superconductor. The highpressure method made it possible to generate a superconductive c o m p o u n d with the infinite-layer structure [2,3 ]. Recently, Hiroi et al. reported that either p-type or n-type superconductors with the same crystal structure can be realized according to atmospheric conditions during preparation, and that defect-layer structures inserted parallel to the CuO2 plane are necessary for the emergence of superconductivity [ 4 ]. They commented that the infinite-layer c o m p o u n d is the first p-type superconductor that has no apical oxygen, and that the apical oxygen is not essential for p-type superconductivity. On the other hand, Adachi et al. claimed a new series of supercon* Corresponding author.
ducting cuprates 02 ( n - 1 )n with apical oxygen made by the same method as Hiroi [ 5 ]. For thin films, it is in general difficult to make the infinite-layer c o m p o u n d superconductive. Although (Sr,Nd) CuO2 thin films with the infinite-layer structure show n-type superconductivity [ 6 ], no groups have succeeded in synthesizing perfect p-type superconductive thin films with the infinite layer structure. Only a few anomalies of the resistivity or AC susceptibility were reported for S r - C a - C u - O thin films [ 7,8 ]. Recently, various kinds of carrier doping were tried. One was Li doping into CaCuO2 thin films [ 9 ]. Another was introducing apical oxygen by inserting BiO or BaO layers periodically [ 10,11 ]. While these reports showed the importance of the apical oxygen, whether the apical oxygen is necessary for the p-type superconductor is still under debate [ 12-17 ]. In this paper, we report on carrier doping in ACuO2 ( A : C a ~ Sr) thin films with the infinite-layer structure synthesized by a R F thermal plasma method. Na doped ACuO2 thin films with the infinite-layer structure were prepared to study the effects of hole dop-
0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0921-4534 ( 94 )00565-6
H. Yakabe et al. /Physica C 232 (1994) 371-378
372
ing. In addition to conventional annealing, the films were exposed to 02 plasma. After film exposure, the structures of the samples had changed. We also studied the properties of the plasma-annealed samples.
2. Experimental
We prepared the A C u O 2 thin films using the RF plasma-deposition technique. The preparation apparatus used here is shown elsewhere [ 18 ], and the film growth conditions are listed in Table 1. SrCO3, CaO, and CuO powders were used as starting materials. Stoichiometrically mixed powders were calcined at 1000°C for 10 min. After regrinding, the powders were fed into the RF O2 plasma with Ar carder gas. Vapored materials were codeposited onto (100) SrTiO3 single-crystal substrates for 1 min and the film thickness was usually ~ 300 A. The substrates were spontaneously heated by the thermal plasma up to 500°C before feeding. The substrate temperature was monitored by a thermocouple. Subsequently, the samples were naturally quenched down to 200°C on releasing vacuum. In order to dope Na, NaECO3 powders were added to the nominal composition before calcination. To oxidize, the as-grown samples were further exposed to the O2 plasma gas for 10 min. We investigated the crystalline properties of these films by X-ray diffraction (XRD) measurements with Cr Ka ( 2.291 A) radiation, inductively coupled plasma atomic emission microscopy (ICP-AES), Xray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The resistivity and Hall effect measurements were also done by using the standard four-probe technique.
3. Results and discussion
3.1. As-grownfilms We obtained nearly single-phase thin films with the infinite-layer structure ofACuO2 (A: Ca ~ Sr). Other phases like SrCuO2 [ 19 ], SrzCuO3 [20], and Srx4Cu24041 [21 ] coexisted occasionally. ICP measurements show that the average cation ratios in the thin films are almost equal to those of the nominal composition. Fig. 1 shows XRD patterns of the ACuO2 (A: Ca~ Sr) thin films and Fig. 2 shows the lattice constant c with Ca concentration x in Srl_xCaxCuO2 for as-grown and plasma-annealed films. With the increase of x, the lattice constant c decreases progressively for both films. The resistive measurements of the as-grown films
5
20
40
60 20 (deg)
80
100
Fig. 1. XRD patterns of Sr~_xCaxCuO2 thin films ( x = 0 - 1 ) . 4.0
o~
~
3.8
~-
3.6 ~
alasma annealed Table 1 Film growth conditions
.=
RF input power
46 kW
Gas
7 1/min A t + 50 1/min 02 ACO3 ( A: alkaline earth), CuO + Na2CO3 ( calcined at 1000°C ) 200 Torr 500-550"C
Powder Pressure Substrate temperature Growth duration
1 min ( 300 A thickness)
._~
3.4'
"~
3.2
3"~Io
as grown o.'2
I
o.,
o!6
o!a
,.o
x in Sr].xCaxCuO
Fig. 2. The variations of the lattice constant c for Sr~_xCaxCuO2 thin films ( x = 0 - 1 ) . Closed squares are for as-grown samples, closed triangles for plasma-annealed samples.
H. Yakabe et aL I Physica C 232 (1994) 371-378 show an insulating conductivity for CaCuO2 ( x = 1 ) and a semiconducting-like temperature dependence for Srl_xCaxCuO2 ( x < 0 . 8 ) . The typical temperature dependence of the resistivity for Sr0.7fao.aCuO 2 thin films is shown in Fig. 3. After 12 h annealing in flowing oxygen gas at 650°C, the annealed samples indicated no notable changes in structure and in resistive properties. Cross-sectional TEM images revealed that the main phase is the infinite-layer phase epitaxially grown along its c-axis as shown in Ref. [22 ]. Contrary to the result for the bulk sample [4], no defect layers parallel to the a-b plane existed in the infinite-layer phase. The same results were reported for SrCuO2 thin films made by a magnetron sputtering technique [23 ]. Although the film partially contains planar defects perpendicular to the a - b plane, almost all areas have perfect structural domains with an average size of 300 nm. Therefore it is considered that stoichiometrically composed samples have no extra carriers and this may be one reason for the absence of superconductivity.
373
constant c of the Na doped sample decreased with the increase of nominal Na content. Fig. 4 shows X R D patterns and Fig. 5 shows the variation of lattice constant c with nominal Na concentration x. With a higher than 30% Na concentration, we could not obtain single-phase thin films. The non-doped C a C u O 2 thin film is an insulator, but the resistivity of the samples decreased with increase of the Na content up to 30% Na to Ca as shown in Fig. 6. Fig. 7 shows the temperature dependence of the resistivity for the Na doped C a C u O 2 thin films. Although resistive data could not be measured in the low-temperature region due to the divergence of the resistance, the data for Na doped thin films can be expressed by the simple variable-range hopping formula, p =poexp{ (To/T) t/3} and conductivity may be caused by phonon-assisted hopping. While Na doping introduced holes into the infinite-layer thin films and caused a decrease of the reI Cr target
tjle°lSrTi [3
$rTi03
3.2. Na doping
(002)
Since the ionic radius of Na is close to that of Ca, the introduction of Na into the infinite-layer compound is considered to result in a substitution of Na for the Ca site, and thus it may result in hole doping to the infinite-layer thin films. For the Na doped samples, the content of Na in the sample has not been discovered. However, the lattice i
i
~
i
/,
x=0.3
=0.15 /
/ =0 5
40
60 20 (deg)
S0
100
Fig. 4. XRD patterns of Cal_xNaxCuO2thin films (x= 0-0.3 ).
o ~ 3.195 ~ annealed in flowing oxygen gas at 650°C
02 plasma annealed
~
i
~
~
I
0.~0
0.15'
03,0
0.;5
~a ~
"~ 10"l 10-2
,,
20
3.200
101 10o
|t
i
1o~ I ~
_
.190
3.185
10-3 Temperature (K)
Fig. 3. Temperature dependence of the resistivity for the Sre.TCao.3CuO2 thin film. Results of various annealings are simultaneously shown.
3"1s0%
0.~,5
0.~0
x in C a l . x N a x C u O 2
Fig. 5. The variation of the lattice constant c for Cal_xNaxCuO2 thin films (x= 0-0.3). The solid line is a guide to the eyes.
374
H. Yakabe et al. / Physica C 232 (1994) 371-378
i 14
i
¸
i
i
~
~
~
.
.
12
~0
pound is the existence of apical oxygen. The present results suggest that the apical oxygen plays an important role in superconductivity as previously reported [9,12-14].
8
"7. ~e
3.3. Plasma annealing
6 4 2 0
100
150
200
250
Temperature (K) Fig. 6. The temperature dependence of the resistivity for Cat_~Na~CuOz thin films ( x = 0 - 0 . 3 ) .
0,1
0.k
0.Y
0.;,
0.;0
T "1/3 ( K "1/3)
Fig. 7. The temperature dependence of the resistivity for Ca~_~Na.CuO2 thin films ( x = 0-0.3 ). The resistivities are shown on a logp--T -1/3 plot.
sistivity, the resistivity of the infinite-layer thin films showed only a semiconducting-like temperature dependence and no superconductivity was observed. These results are similar to those of Li doped CaCuO2 thin films measured by Kubo et al. [9 ]. They discussed the electronic structure of high-Tc superconductors in the double-band Hubbard model and considered the electronic state of the infinite-layer compound. They explained that the dz2 level of the Cu ion would be too low to realize the metallization for the infinite-layer compound that contains only square-coplanar four-fold-coordinated Cu ions. For our Na doped films, the amount of doped Na is less than that of doped Li. Based on their consideration, this is too little to metallize the infinite-layer compound. The marked difference between the usual ptype superconductors and the infinite-layer corn-
Exposure of the thin film to the 02 plasma gas introduces excess oxygen into the thin film and affects the film property. Although annealing in flowing oxygen gas caused no marked changes in the structure or resistive properties of the ACuO2 thin films, exposure of the samples to the 02 plasma gas induced structural and resistive changes on the samples with nominal composition (Sr,_xCax)CuO2 ( x < 0 . 7 ) . After a 10 min treatment under appropriate conditions on 02 plasma, (001) and (002) XRD peaks, corresponding to the infinite-layer structure, shifted to the lower-angle side as shown in Fig. 8. Although samples that contain the major Sr2CuO3 or Sr14Cu24041 phases showed no marked changes, the samples with the infinite-layer structure varied in structure, and the lattice constant c increased as shown in Fig. 2. The temperature dependence of the resistivity for the 02 plasma-annealed samples is also shown in Fig. 3. The samples tended to show a weak temperature dependence with variation of the structure. This resistive property is similar to those of Las_xSrxCusO2o_~ [ 24 ] or La2Sr6Cu8018-~ [ 25,26 ] which have three-dimensional perovskite-based structures with ordered oxygen vacant tunnels. SrTi03
Cr target
SrTiO 3
(002)
=
!
i(oo~) after plasma
_=
!i
annealing j i'~,...j ¸ ~ _ ~ , _ _ i~ ~'!
,i 5
20
40
60 20 (deg
_
( 80
IIH)
Fig. 8. The variation of the XRD pattern for Sro.7Cao.3CuO2 thin film after plasma annealing.
H. Yakabe et al. / Physica C 232 (1994) 371-378
Selected-area diffraction (SAD) patterns of the plasma-annealed sample show a 2x/~a × 2v/2a × c superstructure as shown elsewhere [ 22 ]. This superstructure is reported for SrsCu80,2 [4], (Sr, Nd)CuO2 [23], La2Sr6CusOls_a [25], and Las_xSrxCusO2o_a [27 ]. All these compounds contain oxygen-vacant CuO2 planes. Judging from the diffraction conditions obtained by the SAD patterns, we can propose two possible structural models for the plasma-annealed sample [22]. One is the structure belonging to the space group P4mbm such as Las_xSrxCu802o_~ [27] or La2Sr6CusO16 [28] and another is a new structure of the space group P4bm. Figs. 9(a) and (b) show the crystal structure of Las_xSrxCusO2o_~ and LaaSr6CusO16 , respectively and Fig. 10 shows the new structural model for the plasma-annealed sample. This model has an infinitelayer-based structure with apical oxygens at the corner and a center of 2x/~a × 2x/~a superstructure with the formula SrCuO2+a (d~ 0.25). Judging from the following properties of the plasma-annealed sample, we consider that the plasma-annealed sample has the SrCuO2 +a structure with partial apical oxygens. ( 1 ) ICP measurements showed that the ratio of total alkaline earth to Cu was nearly equal to 1 before and after plasma annealing. This means that the asgrown film had no defects of the alkaline earth cations and the alkaline each cations were not missing after plasma annealing. (2) Although La8_xSrxCusO2o_ ~and La2Sr6Cu8016 are stable, the plasma-annealed sample is very unstable and reacts easily with water. Moreover, the structure of the plasma-annealed sample is reversible and easily recomposed into the normal infinite-layer structure with annealing above 300°C. The ion-milling process and electron-beam illumination in the TEM can also change the structure of the plasma-annealed sample to the normal infinite-layer structure. Therefore, the structural change of the sample by plasma annealing is only connected with the entry and exit of oxygens. ( 3 ) Fig. 12 (b) shows the temperature dependence of the Hall coefficient of the plasma-annealed sample. Contrary to the result of the Hall coefficient measurement for Las_xSrxCusO2o_~ that has n-type carriers [ 24 ], the sign of the Hall coefficient is positive and carriers are of the hole type. This result suggests the existence of excess oxygens.
375
(a)
(b)
Fig. 9. (a) Crystal structure of Las_xSrxCusO2o_a. (b) Crystal structure of La2Sr6CusOl6.
(4) The XPS measurement shows that the ratio of the Cu 2p3/2 main intensity peak to the satellite intensity peak does not change after plasma annealing. This result reveals the absence of two-fold coordination which is typical for Cu ÷ in La2Sr6CusOl6 [28 ]. The above results are positive suggestions for the new structural model and we consider that the
376
H. Yakabe et al. / Physica C 232 (1994) 371-378 4.1 •
Cu
© Sr
(30
a)
I
I
I
I (Sr Ca0 ) Na CuO
4.0 ~
f
4.0
3.9
3.s ~'
~ 3.8
3.0 Y,"
~ 3.7 •"~
~ 2.s ~,
~ 3.6 "~
2.0
3.5
1.5
D
3.4 50
100
150
200
250
~l.0 300
Temperature(K)
2-¢~a
1.0
1.0 i i i i i . ~~S . ~aC0 )- . , •3)0.9Nao, ICuO2
0.8
0.8b
"= o.6
o.~
~o.4
04
" ~ ~ . ,
> 24~a Fig. 10. Structural model of the SrCuO2+, (c~~0.25) thin film. ~ 0.2
Cr target
SrTiOl 3
SrTiO3
oo
• (oo~)
after
. .
plasma annealing
as-grown 5
2O
40
60 20 (deg)
80
5'0 ~o ~o ~o 2~o ~oo
o.o, ~'o ~o ,~o ~
~o ~oo
Temperature(K)
Fig. 12. Temperature dependence of (a) the resistivity and (b) the Hall coefficient for (Sro.TCao.a)CuO2+6 and (Sro.TCao.3)o.9NaonCuO2+~thin films. Both films were annealed in the O2plasma gas for 10 rain.
a~
~'
" " ~ ' ~ Temperature(K)
(002)
"~
0.2
(bl
100
Fig. 11. The variation of the XRD pattern for the (Sro.TCao.3)o.9Nao.ICuO2 thin film after plasma annealing. plasma-annealed sample has the infinite-layer-based structure with apical oxygens at the corner and a center of 2v/2a × 2v/2a superstructure i.e. the new SrCuO2+~ (~~ 0.25) structure. The compounds with the three-dimensional structure like Las_xSrxCusO2o_6 show metallic but nonsuperconducting properties because of the imperfection of the CuO2 plane. On the contrary, as mentioned above, the CuO2 plane may be maintained in SrCuO2+6 and it may be possible to make SrCuO2+a superconductive by doping sufficient carriers into the CuO2 plane. In order to prepare SrCuO2+a with sufficient holes, we tried to expose the Na doped infi-
nite-layer thin films to the 0 2 plasma. The structures of the Na doped samples changed in the same way as the non-doped samples as shown in Fig. 11. Fig. 12 shows the resistivity and Hall coefficient comparison of Sro.7Cao.3CuO2 + 6 and ( Sro.7Cao.3) o.gNao,iCuO2 + 6. The temperature dependence of the Hall coefficient shows the same behavior as that of the (Lal_xSrx)2CuO4 system. The Hall coefficient of Sro.7Cao.3CuO2+6 exhibits a semieonductive temperature dependence like that of La2CuO4 and the carrier density is too little to achieve the superconducting state. On the other hand, the Hall coefficient of (Sro.7Cao.a)o.9Nao.lCuO2+,s shows a weak negative temperature dependence. The magnitude and the thermal behavior of the Hall coefficient are almost the same as those of (La~_xSr~)2CuO4 in the superconducting phase. The Hall number of (Sro.7Cao.3) o.9Nao.lCuO2 + ~ is nearly 0.4 per unit cell at 300 K and this is too high to achieve a superconducting state. We also prepared
H. Yakabe et al. / Physica C 232 (1994) 371-378 (Sro.7Cao.3)l_xYaxCu02+ ~ with less Na content x.
However,
the
resistive
property of all (Sro.7Cao.3) i _xNaxCuO2+6 was metallic-like and showed no superconductivity. The values of the resistivity of these samples are between those of Sro.7Cao.3CuO2+~ and (Sro.7Cao.3)o.9Nao.lCuO2+~. These results may imply, despite a sufficient carrier density, that the Sro.TCao.aCuO2+~system never shows superconductivity above 4.2 K. One reason for the absence of superconductivity in the SrCuO2+6 system could be the partial occupation of oxygens at the apical sites. The calculated average valence of copper is +2.25 in Sr0.7Cao.3CuO2+ ~ (~~ 0.25) and the value is sufficient to metallize the sample. However, the Sro.7Cao.3CuO2+ ~ (8~0.25) thin film shows a weak but semiconducting resistive property. This means that the doped carriers are not introduced into the CuO2 plane and the partial occupancy of the apical oxygen is insufficient to metallize the infinite-layer compound. Another reason could be a distortion of the CuO2 planes. Even though the CuO2 planes contain no oxygen defects in the SrCuO2+~ system, a distortion of the CuO2 plane may be caused by the partial and ordered apical oxygen as shown in Fig. 13. A similar deformation of the CuO5 pyramid in the CuO2 plane was reported for YBCO [29].
<
)
<
)
:L Fig. 13. Distorted CuO2 plane of SrCuO2+6 (~~ 0.25).
377
In conclusion, ACuO 2 (A: alkaline earth) thin films with the infinite-layer structure were synthesized by the RF thermal plasma method. Two types of hole doping were attempted. One was substitution of Na for the Ca site and another was annealing the A C u O 2 thin films in an oxygen plasma environment. In spite of Na doping, the thin films showed no metallic property, still less superconductivity. On the other hand, a structural change and metallization were induced by the exposure of the thin films to the O2 plasma. Two possible structure were considered. The experimental results support the infinite-layer-based structure with partial apical oxygens, i.e. the SrCuO2+~ structure. Although the combination of Na doping and the oxygen plasma annealing introduced a sufficient carrier density into the films, they did not bring about superconductivity.
Acknowledgements The authors wish to thank H. Yamauchi, N. Sugii, S. Adachi for their helpful discussions. They also wish to thank R. Itti for his help in the measurement of XPS. This work was partially supported by New Energy and Industrial Technology Development Organization for the R&D of Industrial Science and Technology Frontier Program.
References [1]T. Sicgrist, M. Zahurak, D.W. Murphy and R.S. Roth, Nature (London) 334 (1989) 231. [2] M.G. Smith, A. Manthiram, J. Zhou, J.B. Goodcnough and J.T. Markert, Nature (London) 351 ( 1991 ) 549. [ 3 ] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and T. Kusaka, Physica C 159 (1989) 375. [4 ] Z. Hiroi, M. Azuma and M. Takano, Physica C 208 ( 1993 ) 286. [ 5 ] S. Adachi, H. Yamauchi, S. Tanaka and N. Mori, Physica C 208 (1993) 226. [6] H. Adachi, T. Satoh, Y. Ichikawa, K. Setsunc and K. Wasa, Physica C 196 (1992) 14. [ 7 ] X. Li, M. Kanai, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 31 (1992) L217. [8] C. Niu and C.M. Lieber, J. Am. Chem. Soc. 115 (1993) 137. [9] K. Kubo and H. Yamauchi, Phys. Rev. B 49 (1994) 1289.
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[10] M. Lagues, X.M. Xie, H. Tebbji, X.Z. Xu, V. Mairet, C. Hatterer, C.F. Beuran and C.D. Cavellin, Science 262 (1993) 1850. [ 11 ] X. Li, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 33 (1994) L18. [ 12] W. Weber, Z. Phys. B 70 (1988) 323. [13] K.A. Muller, Z. Phys. B 80 (1990) 193. [14] H. Kamimura and M. Etoh, J. Phys. Soc. Jpn. 59 (1990) 3053. [ 15 ] Y. Ohta, T, Tohyama and S. Maekawa, Phys. Rev. B 43 (1991) 2968. [ 16 ] C. Di Castro, L.F. Feiner and M. Grilli, Phys. Rev. Lett. 66 ( 1991 ) 3209. [ 17 ] Kh. Eid, M. Matlak and J. Zielinski, Physica C 220 (1994) 61. [ 18 ] S. Yuhya, K. Kikuchi and Y. Shiohara, J. Mater. Res. 7 (1992) 2673. [ 19] V.C.L. Teske and H.M. Buschbaum, Z. Anorg. Allg. Chem. 379 (1970) 234.
[20] V.C.L. Teske and H.M. Buschbaum, Z. Anorg. Allg. Chem. 371 (1969) 325. [21]E.M. McCarron, M.A. Subramanian, J.C. Calabrese and R.L. Harlow, Mater. Res. Bull. 23 (1988) 1355. [ 22 ] J.G. Wen, H. Yakabe, A. Kume, Y. Shiohara, N. Koshizuka and S. Tanaka, Physica C 228 (1994) 279. [23] N. Sugii, M. Ichikawa, K. Hayashi, K. Yamamoto and H. Yamauchi, Physica C 213 (1993) 345. [ 24 ] T. Tamegai and Y. lye, Physica C 159 (1989) 181. [25 ] W.T. Fu, Q. Xu, A.A. Verheijen, J.M. van Ruitenbeek, H.W. Zandbergen and L.J. de Jongh, Solid State Commun. 73 (1990) 291. [26] H. Fujishita, M. Sera and M. Sato, Physica C 175 ( 1991 ) 165. [ 27 ] L. Er-Rakho and B. Raveau, J. Solid State Chem. 73 (1988 ) 514. [ 28 ] W.T. Fu, D.J.W. IJdo and R.B. Helmholdt, Mater. Res. Bull. 27 (1992) 287. [29] T. Krekels, T.S. Shi, J. Reyes-Gasga, G. Van Tendeloo, J. Van Landuyt and S. Amelinckx, Physica C 167 (1990) 677.
ELSEVIER
p-type Carrier doping in ACuO: infinite-layer thin film H. Yakabe *, A. Kume, J.G. Wen, M. Kosuge, Y. Shiohara, N. Koshizuka Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan
Received 1 August 1994
Abstract
Thin films of ACuO2 (A: alkaline earth) with "infinite-layer" structure were prepared by the RF thermal plasma evaporation method on SrTiO3 (100) substrates. The lattice constant c increases systematically with the increase of A ion size. Electricresistivity measurements showed that all the as-grown samples were semiconducting. In order to dope holes into this system, two methods were tried. One is substitution of Na for the alkaline earth. The other one is exposing the samples to 02 plasma. 02 plasma annealing caused a structural change in the samples. The physical properties of the plasma-annealed samples were measured and a possible structural model is proposed. Neither the Na doped sample nor the O2 plasma-annealed sample showed superconductivity. The crystalline condition for the absence of superconductivity is discussed.
1. Introduction
The "infinite-layer" compound ACuO 2 (A: alkaline earth) is the simplest material containing CuO2 layers. Since the successful synthesis of the infinite-layer c o m p o u n d by Siegrist et al. [ 1 ], m a n y researchers have tried to make it a superconductor. The highpressure method made it possible to generate a superconductive c o m p o u n d with the infinite-layer structure [2,3 ]. Recently, Hiroi et al. reported that either p-type or n-type superconductors with the same crystal structure can be realized according to atmospheric conditions during preparation, and that defect-layer structures inserted parallel to the CuO2 plane are necessary for the emergence of superconductivity [ 4 ]. They commented that the infinite-layer c o m p o u n d is the first p-type superconductor that has no apical oxygen, and that the apical oxygen is not essential for p-type superconductivity. On the other hand, Adachi et al. claimed a new series of supercon* Corresponding author.
ducting cuprates 02 ( n - 1 )n with apical oxygen made by the same method as Hiroi [ 5 ]. For thin films, it is in general difficult to make the infinite-layer c o m p o u n d superconductive. Although (Sr,Nd) CuO2 thin films with the infinite-layer structure show n-type superconductivity [ 6 ], no groups have succeeded in synthesizing perfect p-type superconductive thin films with the infinite layer structure. Only a few anomalies of the resistivity or AC susceptibility were reported for S r - C a - C u - O thin films [ 7,8 ]. Recently, various kinds of carrier doping were tried. One was Li doping into CaCuO2 thin films [ 9 ]. Another was introducing apical oxygen by inserting BiO or BaO layers periodically [ 10,11 ]. While these reports showed the importance of the apical oxygen, whether the apical oxygen is necessary for the p-type superconductor is still under debate [ 12-17 ]. In this paper, we report on carrier doping in ACuO2 ( A : C a ~ Sr) thin films with the infinite-layer structure synthesized by a R F thermal plasma method. Na doped ACuO2 thin films with the infinite-layer structure were prepared to study the effects of hole dop-
0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSDI 0921-4534 ( 94 )00565-6
H. Yakabe et al. /Physica C 232 (1994) 371-378
372
ing. In addition to conventional annealing, the films were exposed to 02 plasma. After film exposure, the structures of the samples had changed. We also studied the properties of the plasma-annealed samples.
2. Experimental
We prepared the A C u O 2 thin films using the RF plasma-deposition technique. The preparation apparatus used here is shown elsewhere [ 18 ], and the film growth conditions are listed in Table 1. SrCO3, CaO, and CuO powders were used as starting materials. Stoichiometrically mixed powders were calcined at 1000°C for 10 min. After regrinding, the powders were fed into the RF O2 plasma with Ar carder gas. Vapored materials were codeposited onto (100) SrTiO3 single-crystal substrates for 1 min and the film thickness was usually ~ 300 A. The substrates were spontaneously heated by the thermal plasma up to 500°C before feeding. The substrate temperature was monitored by a thermocouple. Subsequently, the samples were naturally quenched down to 200°C on releasing vacuum. In order to dope Na, NaECO3 powders were added to the nominal composition before calcination. To oxidize, the as-grown samples were further exposed to the O2 plasma gas for 10 min. We investigated the crystalline properties of these films by X-ray diffraction (XRD) measurements with Cr Ka ( 2.291 A) radiation, inductively coupled plasma atomic emission microscopy (ICP-AES), Xray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy (HRTEM). The resistivity and Hall effect measurements were also done by using the standard four-probe technique.
3. Results and discussion
3.1. As-grownfilms We obtained nearly single-phase thin films with the infinite-layer structure ofACuO2 (A: Ca ~ Sr). Other phases like SrCuO2 [ 19 ], SrzCuO3 [20], and Srx4Cu24041 [21 ] coexisted occasionally. ICP measurements show that the average cation ratios in the thin films are almost equal to those of the nominal composition. Fig. 1 shows XRD patterns of the ACuO2 (A: Ca~ Sr) thin films and Fig. 2 shows the lattice constant c with Ca concentration x in Srl_xCaxCuO2 for as-grown and plasma-annealed films. With the increase of x, the lattice constant c decreases progressively for both films. The resistive measurements of the as-grown films
5
20
40
60 20 (deg)
80
100
Fig. 1. XRD patterns of Sr~_xCaxCuO2 thin films ( x = 0 - 1 ) . 4.0
o~
~
3.8
~-
3.6 ~
alasma annealed Table 1 Film growth conditions
.=
RF input power
46 kW
Gas
7 1/min A t + 50 1/min 02 ACO3 ( A: alkaline earth), CuO + Na2CO3 ( calcined at 1000°C ) 200 Torr 500-550"C
Powder Pressure Substrate temperature Growth duration
1 min ( 300 A thickness)
._~
3.4'
"~
3.2
3"~Io
as grown o.'2
I
o.,
o!6
o!a
,.o
x in Sr].xCaxCuO
Fig. 2. The variations of the lattice constant c for Sr~_xCaxCuO2 thin films ( x = 0 - 1 ) . Closed squares are for as-grown samples, closed triangles for plasma-annealed samples.
H. Yakabe et aL I Physica C 232 (1994) 371-378 show an insulating conductivity for CaCuO2 ( x = 1 ) and a semiconducting-like temperature dependence for Srl_xCaxCuO2 ( x < 0 . 8 ) . The typical temperature dependence of the resistivity for Sr0.7fao.aCuO 2 thin films is shown in Fig. 3. After 12 h annealing in flowing oxygen gas at 650°C, the annealed samples indicated no notable changes in structure and in resistive properties. Cross-sectional TEM images revealed that the main phase is the infinite-layer phase epitaxially grown along its c-axis as shown in Ref. [22 ]. Contrary to the result for the bulk sample [4], no defect layers parallel to the a-b plane existed in the infinite-layer phase. The same results were reported for SrCuO2 thin films made by a magnetron sputtering technique [23 ]. Although the film partially contains planar defects perpendicular to the a - b plane, almost all areas have perfect structural domains with an average size of 300 nm. Therefore it is considered that stoichiometrically composed samples have no extra carriers and this may be one reason for the absence of superconductivity.
373
constant c of the Na doped sample decreased with the increase of nominal Na content. Fig. 4 shows X R D patterns and Fig. 5 shows the variation of lattice constant c with nominal Na concentration x. With a higher than 30% Na concentration, we could not obtain single-phase thin films. The non-doped C a C u O 2 thin film is an insulator, but the resistivity of the samples decreased with increase of the Na content up to 30% Na to Ca as shown in Fig. 6. Fig. 7 shows the temperature dependence of the resistivity for the Na doped C a C u O 2 thin films. Although resistive data could not be measured in the low-temperature region due to the divergence of the resistance, the data for Na doped thin films can be expressed by the simple variable-range hopping formula, p =poexp{ (To/T) t/3} and conductivity may be caused by phonon-assisted hopping. While Na doping introduced holes into the infinite-layer thin films and caused a decrease of the reI Cr target
tjle°lSrTi [3
$rTi03
3.2. Na doping
(002)
Since the ionic radius of Na is close to that of Ca, the introduction of Na into the infinite-layer compound is considered to result in a substitution of Na for the Ca site, and thus it may result in hole doping to the infinite-layer thin films. For the Na doped samples, the content of Na in the sample has not been discovered. However, the lattice i
i
~
i
/,
x=0.3
=0.15 /
/ =0 5
40
60 20 (deg)
S0
100
Fig. 4. XRD patterns of Cal_xNaxCuO2thin films (x= 0-0.3 ).
o ~ 3.195 ~ annealed in flowing oxygen gas at 650°C
02 plasma annealed
~
i
~
~
I
0.~0
0.15'
03,0
0.;5
~a ~
"~ 10"l 10-2
,,
20
3.200
101 10o
|t
i
1o~ I ~
_
.190
3.185
10-3 Temperature (K)
Fig. 3. Temperature dependence of the resistivity for the Sre.TCao.3CuO2 thin film. Results of various annealings are simultaneously shown.
3"1s0%
0.~,5
0.~0
x in C a l . x N a x C u O 2
Fig. 5. The variation of the lattice constant c for Cal_xNaxCuO2 thin films (x= 0-0.3). The solid line is a guide to the eyes.
374
H. Yakabe et al. / Physica C 232 (1994) 371-378
i 14
i
¸
i
i
~
~
~
.
.
12
~0
pound is the existence of apical oxygen. The present results suggest that the apical oxygen plays an important role in superconductivity as previously reported [9,12-14].
8
"7. ~e
3.3. Plasma annealing
6 4 2 0
100
150
200
250
Temperature (K) Fig. 6. The temperature dependence of the resistivity for Cat_~Na~CuOz thin films ( x = 0 - 0 . 3 ) .
0,1
0.k
0.Y
0.;,
0.;0
T "1/3 ( K "1/3)
Fig. 7. The temperature dependence of the resistivity for Ca~_~Na.CuO2 thin films ( x = 0-0.3 ). The resistivities are shown on a logp--T -1/3 plot.
sistivity, the resistivity of the infinite-layer thin films showed only a semiconducting-like temperature dependence and no superconductivity was observed. These results are similar to those of Li doped CaCuO2 thin films measured by Kubo et al. [9 ]. They discussed the electronic structure of high-Tc superconductors in the double-band Hubbard model and considered the electronic state of the infinite-layer compound. They explained that the dz2 level of the Cu ion would be too low to realize the metallization for the infinite-layer compound that contains only square-coplanar four-fold-coordinated Cu ions. For our Na doped films, the amount of doped Na is less than that of doped Li. Based on their consideration, this is too little to metallize the infinite-layer compound. The marked difference between the usual ptype superconductors and the infinite-layer corn-
Exposure of the thin film to the 02 plasma gas introduces excess oxygen into the thin film and affects the film property. Although annealing in flowing oxygen gas caused no marked changes in the structure or resistive properties of the ACuO2 thin films, exposure of the samples to the 02 plasma gas induced structural and resistive changes on the samples with nominal composition (Sr,_xCax)CuO2 ( x < 0 . 7 ) . After a 10 min treatment under appropriate conditions on 02 plasma, (001) and (002) XRD peaks, corresponding to the infinite-layer structure, shifted to the lower-angle side as shown in Fig. 8. Although samples that contain the major Sr2CuO3 or Sr14Cu24041 phases showed no marked changes, the samples with the infinite-layer structure varied in structure, and the lattice constant c increased as shown in Fig. 2. The temperature dependence of the resistivity for the 02 plasma-annealed samples is also shown in Fig. 3. The samples tended to show a weak temperature dependence with variation of the structure. This resistive property is similar to those of Las_xSrxCusO2o_~ [ 24 ] or La2Sr6Cu8018-~ [ 25,26 ] which have three-dimensional perovskite-based structures with ordered oxygen vacant tunnels. SrTi03
Cr target
SrTiO 3
(002)
=
!
i(oo~) after plasma
_=
!i
annealing j i'~,...j ¸ ~ _ ~ , _ _ i~ ~'!
,i 5
20
40
60 20 (deg
_
( 80
IIH)
Fig. 8. The variation of the XRD pattern for Sro.7Cao.3CuO2 thin film after plasma annealing.
H. Yakabe et al. / Physica C 232 (1994) 371-378
Selected-area diffraction (SAD) patterns of the plasma-annealed sample show a 2x/~a × 2v/2a × c superstructure as shown elsewhere [ 22 ]. This superstructure is reported for SrsCu80,2 [4], (Sr, Nd)CuO2 [23], La2Sr6CusOls_a [25], and Las_xSrxCusO2o_a [27 ]. All these compounds contain oxygen-vacant CuO2 planes. Judging from the diffraction conditions obtained by the SAD patterns, we can propose two possible structural models for the plasma-annealed sample [22]. One is the structure belonging to the space group P4mbm such as Las_xSrxCu802o_~ [27] or La2Sr6CusO16 [28] and another is a new structure of the space group P4bm. Figs. 9(a) and (b) show the crystal structure of Las_xSrxCusO2o_~ and LaaSr6CusO16 , respectively and Fig. 10 shows the new structural model for the plasma-annealed sample. This model has an infinitelayer-based structure with apical oxygens at the corner and a center of 2x/~a × 2x/~a superstructure with the formula SrCuO2+a (d~ 0.25). Judging from the following properties of the plasma-annealed sample, we consider that the plasma-annealed sample has the SrCuO2 +a structure with partial apical oxygens. ( 1 ) ICP measurements showed that the ratio of total alkaline earth to Cu was nearly equal to 1 before and after plasma annealing. This means that the asgrown film had no defects of the alkaline earth cations and the alkaline each cations were not missing after plasma annealing. (2) Although La8_xSrxCusO2o_ ~and La2Sr6Cu8016 are stable, the plasma-annealed sample is very unstable and reacts easily with water. Moreover, the structure of the plasma-annealed sample is reversible and easily recomposed into the normal infinite-layer structure with annealing above 300°C. The ion-milling process and electron-beam illumination in the TEM can also change the structure of the plasma-annealed sample to the normal infinite-layer structure. Therefore, the structural change of the sample by plasma annealing is only connected with the entry and exit of oxygens. ( 3 ) Fig. 12 (b) shows the temperature dependence of the Hall coefficient of the plasma-annealed sample. Contrary to the result of the Hall coefficient measurement for Las_xSrxCusO2o_~ that has n-type carriers [ 24 ], the sign of the Hall coefficient is positive and carriers are of the hole type. This result suggests the existence of excess oxygens.
375
(a)
(b)
Fig. 9. (a) Crystal structure of Las_xSrxCusO2o_a. (b) Crystal structure of La2Sr6CusOl6.
(4) The XPS measurement shows that the ratio of the Cu 2p3/2 main intensity peak to the satellite intensity peak does not change after plasma annealing. This result reveals the absence of two-fold coordination which is typical for Cu ÷ in La2Sr6CusOl6 [28 ]. The above results are positive suggestions for the new structural model and we consider that the
376
H. Yakabe et al. / Physica C 232 (1994) 371-378 4.1 •
Cu
© Sr
(30
a)
I
I
I
I (Sr Ca0 ) Na CuO
4.0 ~
f
4.0
3.9
3.s ~'
~ 3.8
3.0 Y,"
~ 3.7 •"~
~ 2.s ~,
~ 3.6 "~
2.0
3.5
1.5
D
3.4 50
100
150
200
250
~l.0 300
Temperature(K)
2-¢~a
1.0
1.0 i i i i i . ~~S . ~aC0 )- . , •3)0.9Nao, ICuO2
0.8
0.8b
"= o.6
o.~
~o.4
04
" ~ ~ . ,
> 24~a Fig. 10. Structural model of the SrCuO2+, (c~~0.25) thin film. ~ 0.2
Cr target
SrTiOl 3
SrTiO3
oo
• (oo~)
after
. .
plasma annealing
as-grown 5
2O
40
60 20 (deg)
80
5'0 ~o ~o ~o 2~o ~oo
o.o, ~'o ~o ,~o ~
~o ~oo
Temperature(K)
Fig. 12. Temperature dependence of (a) the resistivity and (b) the Hall coefficient for (Sro.TCao.a)CuO2+6 and (Sro.TCao.3)o.9NaonCuO2+~thin films. Both films were annealed in the O2plasma gas for 10 rain.
a~
~'
" " ~ ' ~ Temperature(K)
(002)
"~
0.2
(bl
100
Fig. 11. The variation of the XRD pattern for the (Sro.TCao.3)o.9Nao.ICuO2 thin film after plasma annealing. plasma-annealed sample has the infinite-layer-based structure with apical oxygens at the corner and a center of 2v/2a × 2v/2a superstructure i.e. the new SrCuO2+~ (~~ 0.25) structure. The compounds with the three-dimensional structure like Las_xSrxCusO2o_6 show metallic but nonsuperconducting properties because of the imperfection of the CuO2 plane. On the contrary, as mentioned above, the CuO2 plane may be maintained in SrCuO2+6 and it may be possible to make SrCuO2+a superconductive by doping sufficient carriers into the CuO2 plane. In order to prepare SrCuO2+a with sufficient holes, we tried to expose the Na doped infi-
nite-layer thin films to the 0 2 plasma. The structures of the Na doped samples changed in the same way as the non-doped samples as shown in Fig. 11. Fig. 12 shows the resistivity and Hall coefficient comparison of Sro.7Cao.3CuO2 + 6 and ( Sro.7Cao.3) o.gNao,iCuO2 + 6. The temperature dependence of the Hall coefficient shows the same behavior as that of the (Lal_xSrx)2CuO4 system. The Hall coefficient of Sro.7Cao.3CuO2+6 exhibits a semieonductive temperature dependence like that of La2CuO4 and the carrier density is too little to achieve the superconducting state. On the other hand, the Hall coefficient of (Sro.7Cao.a)o.9Nao.lCuO2+,s shows a weak negative temperature dependence. The magnitude and the thermal behavior of the Hall coefficient are almost the same as those of (La~_xSr~)2CuO4 in the superconducting phase. The Hall number of (Sro.7Cao.3) o.9Nao.lCuO2 + ~ is nearly 0.4 per unit cell at 300 K and this is too high to achieve a superconducting state. We also prepared
H. Yakabe et al. / Physica C 232 (1994) 371-378 (Sro.7Cao.3)l_xYaxCu02+ ~ with less Na content x.
However,
the
resistive
property of all (Sro.7Cao.3) i _xNaxCuO2+6 was metallic-like and showed no superconductivity. The values of the resistivity of these samples are between those of Sro.7Cao.3CuO2+~ and (Sro.7Cao.3)o.9Nao.lCuO2+~. These results may imply, despite a sufficient carrier density, that the Sro.TCao.aCuO2+~system never shows superconductivity above 4.2 K. One reason for the absence of superconductivity in the SrCuO2+6 system could be the partial occupation of oxygens at the apical sites. The calculated average valence of copper is +2.25 in Sr0.7Cao.3CuO2+ ~ (~~ 0.25) and the value is sufficient to metallize the sample. However, the Sro.7Cao.3CuO2+ ~ (8~0.25) thin film shows a weak but semiconducting resistive property. This means that the doped carriers are not introduced into the CuO2 plane and the partial occupancy of the apical oxygen is insufficient to metallize the infinite-layer compound. Another reason could be a distortion of the CuO2 planes. Even though the CuO2 planes contain no oxygen defects in the SrCuO2+~ system, a distortion of the CuO2 plane may be caused by the partial and ordered apical oxygen as shown in Fig. 13. A similar deformation of the CuO5 pyramid in the CuO2 plane was reported for YBCO [29].
<
)
<
)
:L Fig. 13. Distorted CuO2 plane of SrCuO2+6 (~~ 0.25).
377
In conclusion, ACuO 2 (A: alkaline earth) thin films with the infinite-layer structure were synthesized by the RF thermal plasma method. Two types of hole doping were attempted. One was substitution of Na for the Ca site and another was annealing the A C u O 2 thin films in an oxygen plasma environment. In spite of Na doping, the thin films showed no metallic property, still less superconductivity. On the other hand, a structural change and metallization were induced by the exposure of the thin films to the O2 plasma. Two possible structure were considered. The experimental results support the infinite-layer-based structure with partial apical oxygens, i.e. the SrCuO2+~ structure. Although the combination of Na doping and the oxygen plasma annealing introduced a sufficient carrier density into the films, they did not bring about superconductivity.
Acknowledgements The authors wish to thank H. Yamauchi, N. Sugii, S. Adachi for their helpful discussions. They also wish to thank R. Itti for his help in the measurement of XPS. This work was partially supported by New Energy and Industrial Technology Development Organization for the R&D of Industrial Science and Technology Frontier Program.
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