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Physica C 466 (2007) 111–114

Fabrication and characterization of superconducting(Cu,C)Ba2CuO4±d thin films

Shipra Singh a, Y. Tanaka b, A. Sundaresan a,*

a Chemistry and Physics of Materials Unit and Department of Science and Technology Unit on Nanoscience,

Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, Indiab Nanoelectronics Research Institute, National Institute of Advanced Science and Technology, 1-1-1 Umezono, Tsukuba 305 8568, Japan

Received 28 March 2007; received in revised form 4 June 2007; accepted 11 June 2007Available online 3 July 2007

Abstract

We have investigated the structural modulation and variation of superconducting transition temperature of (Cu,C)Ba2CuO4±d

((Cu,C)-1201) thin films prepared by rf magnetron sputtering on (100) SrTiO3 single crystal substrates in the off-axis configuration. Thinfilms were deposited in a gas mixture of Ar, O2 and CO2 where the Ar:O2 ratio was kept constant at 1:2 with a total pressure of 15 mTorrand CO2 gas pressure was varied between 1 and 3 mTorr. XRD patterns revealed the formation of high quality thin films with c-axisoriented perpendicular to the plane of the substrate. The value of the c-lattice parameter varied in the range, c = 8.29–8.33 A. These filmshad a superconducting resistive transition temperature (Tc onset) ranging between 25 and 40 K above which, almost all films showedsemiconductor like behavior. Superconductivity in (Cu,C)-1201 thin films was further confirmed by the diamagnetic response observedin ac susceptibility measurement.� 2007 Elsevier B.V. All rights reserved.

PACS: 81.15.Cd; 74.78.Bz; 74.62.�c

Keywords: Thin film; Sputtering; Cuprate superconductor

1. Introduction

The multilayer cuprates, (Cu,C)Ba2Can�1CunO2n+2±d,where n is the number of CuO2 planes per unit cell, isnot only interesting because of their high Tc (>116 K) butalso important since they exhibit many interesting physics[1–3]. These materials do not have any toxic elements andare isostructural with single Tl- and Hg-layer based multi-layer materials having tetragonal structure with the spacegroup P4/mmm. The chemical formula of the first memberof this series (n = 1) is (Cu,C)Ba2CuO4±d (Cu,C-1201). Inthis compound, Cu in the superconductive layer has octa-hedral coordination with oxygen. The higher members

0921-4534/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2007.06.008

* Corresponding author. Tel.: +91 80 22082824.E-mail address: sundaresan@jncasr.ac.in (A. Sundaresan).

can be considered as an intergrowth of (Cu,C)-1201, whichacts as charge reservoir layer, and the infinite layer (IL)CaCuO2. In the case of n = 2, which is formed from oneunit of each (Cu,C)-1201 and CaCuO2, the structure isanalogous to YBa2Cu3Oy except that the Cu–O chainsare replaced in the present case by a mixture of Cu and car-bonate group and Y3+ is replaced by Ca2+ ion. In this sys-tem, copper has pyramidal coordination with oxygenneighbors. For n = 3, in the multilayered materials, thereexist two different CuO2 layers which are characterizedby different coordination number of Cu with surroundingoxygen ions. There are two pyramidal CuO5 units whoseapical oxygen connects the Cu to the charge reservoir layerand these units are called outer planes (OP). The otherone is CuO2 square oxygen network which is sand-wiched between the OP that is termed inner planes (IP).It is rather remarkable to note that Tc remain high even

112 S. Singh et al. / Physica C 466 (2007) 111–114

in the overdoped region of these multilayered materials(n = 3) [4]. This is explained by the results of NMR mea-surements which reveal that the doping level in OP andIP are different [5]. Further, the analysis of Knight shiftand the nuclear spin-lattice relaxation rate (1/T1) of 63Cusuggest that the IP keeps high value of Tc by remainingnearly optimally doped while the OP is predominantlyoverdoped. This seems to be the origin of lowest supercon-ducting anisotropy reported in these multilayer cuprates.Electronic band structure calculations suggest that (Cu,C)-Ba2Can�1CunO2n+2±d can be considered as multibandsuperconductor [6]. Due to different carrier concentrationin IP and OP and weak interband interaction there are indi-cation for inherent lower Tc of OP that can not be seen inother conventional multiband superconductors. Moreover,a weak interband interactions could result in the formationof phase dislocation in the quantum condensation or inter-band phase difference soliton [7–10].

These interesting properties and the nontoxic nature ofthese multilayer cuprates are very promising. However,unlike other multilayer cuprates these materials requirehigh pressure to stabilize the phase [1–3]. We thought thatit would be interesting from both fundamental and applica-tion point of view to fabricate thin films of such multilayercuprates. There are few reports on the preparation of thefirst member (n = 1,2) of the series (Cu,C)Ba2Can�1Cun-O2n+2±d using rf-magnetron sputtering and pulsed laserablation (PLA) techniques using a ceramic target [11–15].It requires stringent growth conditions and therefore onlyfew groups have succeeded in achieving superconductivity[11–14]. The higher members (n = 3) have not beenreported by the conventional thin film fabrication tech-nique i.e. using a single target with the composition(Cu,C)Ba2Ca2Cu3Oy. An artificial superlattice method,involving deposition of charge reservoir layer BaCuO2 or(Cu,C)-1201 and CaCuO2 layers alternatively, was appliedto grow thin films of such higher members using PLA andMolecular Beam Epitaxy techniques [16–21]. However, themaximum Tc was observed to be around 80 K. The low Tc

has been attributed to a possible disorder among the layers[24]. By adapting the superlattice method, our goal is toachieve higher members with Tc higher than 100 K usingrf magnetron sputtering. The advantage of this techniqueis, one can prepare large area films. In order to fabricatesuch higher members, at first it is important to grow ametallic or superconductive charge reservoir layer,(Cu,C)-1201. By combining the (Cu,C)-1201 unit anddesired number of CaCuO2 one should be able to preparesuperlattice thin films corresponding to n = 3, 4 or higher.In this article, we report the preparation and characteriza-tion of good quality superconducting (Cu,C)-1201 thinfilms.

2. Experimental

Ceramic targets of the composition BaCu0.75Oy andBaCu0.85Oy were prepared by solid state reaction route

from the starting materials, BaCO3 and CuO. The startingmixture was calcined at temperatures 870 �C, 880 �C forand 890 �C for 36 h with intermittent grindings. The resul-tant powder was pressed into a disc of 50 mm diameter andthen sintered at 900 �C. X-ray Diffraction (XRD) patternof the product indicated no stable compound formationbut a mixture of BaCuO2 and CuO with relative intensitiesroughly proportional to the expected ones based on stoichi-ometry. Thin films were deposited by rf magnetron sputter-ing on (100) SrTiO3 single crystal substrate in the off-axisconfiguration using KE702-6 type sputtering machine (K-Science, Japan). The substrates were glued to a silver–nickel substrate holder with silver paste and heated to atemperature of 540 �C which was measured by an opticalpyrometer. The distance between the target surface andsubstrate was fixed at 75 mm. The deposition was carriedout in a gas mixture of Ar, O2 and CO2 where the Ar:O2

ratio was kept constant at 1:2 with a total pressure of15 mTorr and CO2 gas pressure was varied between 1and 3 mTorr. After deposition, films were cooled to roomtemperature in 1 atm O2 pressure with different coolingrates; 1�, 3� and 5� celsium per minute. The rf power tothe target was kept constant at 25 W. Structural character-ization of these films was carried out using Rigaku andBruker AXS D8 Discover X-ray diffractometers with CuKa radiation. DC resistance measurement was made bythe standard four probe method using resistivity optionin the Physical Property Measurement System (PPMS),Quantum Design, USA. AC magnetic measurements werealso performed with the PPMS.

3. Results and discussion

In the case of bulk materials, it is well known that car-bon is essential to stabilize the compounds (Cu,C)Ba2-Can�1CunO2n+2±d and if the amount of carbon substitutingcopper in the charge reservoir layer is high we would expectthat the superconducting properties will be deteriorated asreported earlier [22,23]. Therefore, it is important to useminimum amount of carbon which can stabilize thesephases. In the present work, the CO2 pressure was variedto find minimum amount required to stabilize (Cu,C)-1201 phase and it was found that 1 mTorr of CO2 gas pres-sure is sufficient to stabilize this phase and therefore it hasbeen fixed. The XRD pattern of a typical (Cu,C)-1201 filmcooled to room temperature at 1 �C/min is shown inFig. 1a. It can be seen from this figure that the c-axis ofthe film is oriented perpendicular to the plane of the sub-strate with the lattice parameter c � 8 A which is almosttwice the lattice parameter of infinite layer (IL) BaCuO2

(c � 4 A). The doubling of the c-lattice parameter indicatesthe growth of the carbon incorporated modulated struc-ture. It should be noticed that the films grown withoutintroducing CO2 gas always resulted in IL compound asshown in Fig. 1b. The importance of (Cu,C)-1201 phasefor the fabrication of multilayer cuprates is that it can take

10 20 30 40 50 60

(b)

(a)

(001

) B

aCuO

2

*

*

(001

) ST

O

(002

) S

TO

(002

) B

aCuO

2

(005

) (C

u,C

) - 1

201

(004

)(C

u,C

)- 1

201

(003

) (C

u,C

) -

1201

(002

) (C

u,C

) -

1201

(001

) (C

u,C

) -

1201

Inte

nsity

(a.

u.)

2 Theta (deg.)

Fig. 1. (a) XRD pattern of (Cu,C)Ba2CuO4±d thin film grown on (100)SrTiO3 substrate with c-parameter twice (c � 8.0) that of BaCuO2 infinitelayer. (b) XRD pattern of BaCuO2 thin film with c � 4 A. A small peakmarked with an asterisks comes from the substrate.

0 50 100 150 200 250 3000.0

1.0x10-3

2.0x10-3

3.0x10-3

4.0x10-3

Res

istiv

ity (

Ω·c

m)

Temperature (K)

c = 8.29 Å (5oC/min)

c = 8.32 Å (1oC/min)

c = 8.33 Å (3oC/min)

Fig. 3. Temperature dependence of resistivity for (Cu,C)Ba2CuO4±d thinfilm prepared from BaCu0.75Oy target. It can be seen that superconductingtransition temperature varies with lattice parameter and/or cooling rate.

S. Singh et al. / Physica C 466 (2007) 111–114 113

apical oxygen as discussed in the introduction and providecharge carries to the CuO2 layers.

Depending on the cooling rate, the lattice parameter ofthe films varied between 8.29 A and 8.33 A. We believe thatthe incorporation of apical oxygen is mainly responsiblefor the variation of c-axis lattice parameter as the variationof CO2 pressure does not have any significant effect on thec-axis lattice parameter as reported earlier [12]. Formationof high quality epitaxial film was also confirmed by thehigh resolution XRD pattern of (004) reflection withaccompanying Laue oscillations (Fig. 2). Film thicknesscalculated by matching experimental and simulated datawas found to be around 80 nm which agrees with the valueobtained from Dektak stylus profilometer.

Fig. 3 shows the variation of resistivity with temperaturefor three films deposited from the BaCu0.75Oy target with

42.4 42.8 43.2 43.6 44.0

1E-5

1E-4

1E-3

Inte

nsity

(a.

u.)

2θ (deg.)

(004)

Raw data Sim data

Fig. 2. High resolution XRD pattern showing Laue oscillations aroundthe (004) peak of an epitaxial (Cu,C)Ba2CuO4±d thin film and itscomparison with the simulated pattern.

same growth condition but with three different coolingrates. It can be seen that the film cooled at 1 �C/min havingthe c-parameter value c = 8.32 A seems to have a maxi-mum Tc onset. The other two films show lower Tc. Itshould be emphasized that the films cooled without 1 atmoxygen showed semiconducting behavior. This demon-strates that cooling in oxygen atmosphere is essential tohave superconductivity. Although temperature variationof resistivity for (Cu,C)-1201 thin films grown under vari-ous partial pressure of CO2 gas has been reported [13], wecould not find any role of CO2 pressure variation upon thestructure of the thin film. The semiconductor behavior ofthe q–T curve above superconducting onset temperature(25–40 K) as shown in Fig. 3 suggest the under-doped stateof the samples. Film prepared from the target compositionBaCu0.85Oy showed a metallic behavior (Fig. 4) down to75 K which is different from the films prepared fromBaCu0.75Oy target which indicates the importance of high

0 50 100 150 200 250 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Cu0.85-1201

R(T

)/R

(300

K)

Temperature (K)

Cu0.75-1201

Fig. 4. Normalized resistance versus temperature graph showing thedifferent behavior of films prepared from the BaCu0.75Oy (Cu0.75-1201)and BaCu0.85Oy (Cu0.85-1201) targets.

0 12 15

-12

-9

-6

-3

0

f = 9997 Hzhac = 1 Oe

(Cu0.75

,C) - 1201

χ ac

' (

emu/

cm3 )

Temperature (K)

(Cu0.85

,C) - 1201

3 6 9

Fig. 5. Diamagnetic response of (Cu0.85,C)-1201 thin film showing acritical temperature at around 7 K. For (Cu0.75,C)-1201, the Tc is ataround 5 K.

114 S. Singh et al. / Physica C 466 (2007) 111–114

copper content for metallicity. Superconductivity in thesefilms was further confirmed by the AC susceptibility mea-surements. The critical temperature of the film withc = 8.31 A deposited from the BaCu0.85Oy target was mea-sured to be 7 K as shown in Fig. 5.

4. Conclusions

High quality (Cu,C)Ba2CuO4±d thin films have beenprepared with superconducting onset between 25 K and40 K. Incorporation of carbon as carbonate group stabilizethe (Cu,C)-1201 phase. Our experiments suggest that postannealing is important to induce superconductivity in thissystem. The high quality of (Cu,C)-1201 films with super-conductive nature demonstrate that the (Cu,C)-1201 maybe combined with infinite layer CaCuO2 to make multilayercuprates and this work is under progress.

Acknowledgements

A.S. thank Japan Science and Technology, Japan andCouncil of Scientific and Industrial Research, India forexperimental and financial support, respectively. He alsothanks N. Terada and K. Tokiwa for helpful discussion.This paper is dedicated to H. Ihara.

References

[1] H. Ihara, K. Tokiwa, H. Ozawa, M. Hirabayashi, H. Matuhata, A.Negishi, Y.S. Song, Jpn. J. Appl. Phys. 33 (1994) L300.

[2] H. Ihara, K. Tokiwa, H. Ozawa, M. Hirabayashi, A. Negishi, H.Matuhata, Y.S. Song, Jpn. J. Appl. Phys. 33 (1994) L503.

[3] T. Kawashima, Y. Matsui, E. Takayama-Muromachi, Physica C 224(1994) 69.

[4] M. Ogino, T. Watanabe, H. Tokiwa, A. Iyo, H. Ihara, Physica C 258(1996) 384.

[5] Y. Tokunaga, K. Ishida, Y. Itaoka, K. Asayam, K. Tokiwa, A. Iyo,H. Ihara, Phys. Rev. B 61 (2000) 9707.

[6] N. Hamada, H. Ihara, Physica B 284 (2000) 1073.[7] K. Kotegawa, Y. Tokunaga, K. Ishida, G.-q. Zheng, Y. Kitaoka, H.

Kito, A. Iyo, K. Tokiwa, T. Watanabe, H. Ihara, Phys. Rev. B 64(2001) 064515.

[8] H. Suhl, B.T. Matthias, L.R. Walker, Phys. Rev. Lett. 3 (1959) 552.[9] Y. Tanaka, A. Iyo, N. Shirakawa, M. Ariyama, M. Tokumoto, S.I.

Ikeda, H. Ihara, Physica C 357 (2001) 222.[10] Y. Tanaka, J. Phys. Soc. Jpn. 70 (2001) 2844.[11] H. Adachi, M. Sakai, T. Satoh, K. Setsune, Physica C 244 (1995) 282.[12] E. Koller, L. Fabrega, J.-M. Triscone, M. Decroux, Ø. Fisher, J. Low

Temp. Phys. 105 (1996) 1325.[13] H. Wakamatsu, T. Ogata, N. Yamaguchi, S. Mikusu, K. Tokiwa, T.

Watanabe, K. Ohki, K. Kikunaga, N. Terada, N. Kikuchi, Y.Tanaka, A. Iyo, J. Phys.: Conf. Series 43 (2006) 289.

[14] K. Kikunga, T. Yamamoto, K. Takeshita, K. Oki, T. Okuda, K.Obara, K. Tokiwa, H. Wakamatsu, T. Watanabe, N. Kikuchi, Y.Tanaka, N. Terada, J. Phys.: Conf. Series 43 (2006) 247.

[15] N. Kikuchi, A. Sundaresan, N. Terada, M. Hirai, K. Tokiwa, T.Watanabe, Y. Kodama, A. Iyo, Y. Tanaka, Vacuum 74 (2004) 585.

[16] D.P. Norton, B.C. Chakoumakos, J.D. Budai, D.H. Lowdnes, B.C.Sales, J.R. Thomson, D.K. Christen, Science 265 (1994) 2074.

[17] A. Tebano, C. Aruta, N.G. Boggio, P.G. Medaglia, G. Balestrinio,Supercond. Sci. Technol. 19 (2006) S45.

[18] M. Kanai, T. Kawai, Physica C 235–240 (1994) 174.[19] S. Colonna, F. Arciprete, A. Balzarotti, G. Balestrino, P.G. Meda-

glia, G. Petrocelli, Physica C 334 (2000) 64.[20] H. Shibata, S. Karimoto, A. Tsukada, T. Makimoto, Physica C 445

(2006) 862.[21] A. Sundaresan, M. Hirai, J.C. Nie, K. Hayashi, Y. Ishiura, H. Ihara,

Physica C 357 (2001) 1403.[22] Y. Masuda, R. Ogawa, Y. Kawate, K. Matsubara, T. Tateishi, S.

Sakka, J. Mater. Res. 8 (1993) 696.[23] M. Uehara, M. Uoshima, S. Ishiyama, H. Nakata, J. Akimitsu, Y.

Matsui, T. Arima, Y. Tokura, N. Mori, Physica C 229 (1994) 310.[24] G. Balestrino, A. Crisan, S. Lavanga, P.G. Medaglia, G. Petrocelli,

A.A. Varlamov, Phys. Rev. B 60 (1999) 10504.