MBE growth of compounds on the copper rich side of the (Sr,Ca)CuO system

Journel of

A ~ AND CO~OUNDS Journal of Alloys and Compounds 251 (1997) 240-242

ELSEVIER

MBE Growth of compounds on the copper rich side of the (Sr,Ca)CuO system B. Eustache ~'¢'*, F.C. Beuran ~, C. Deville Cavellin b, C.J. Hatterer ~, V. Mairet', C. Partiot ~, X.Z. Xu", E Germain ~, M. Lagui~s" ~Laboratoire Surfaces el Supraconducteurs CNRS UPRS. Ecole Supdrieure de Physique et Chimie Industrielles de la Ville tie Paris, IO Rue Vauquelin, 75005 Paris France hUniversitd Paris XII. 61 Avenue du Gdndml de Gaulle. 94010 Crdteil. Pari.~' Fram:e "Wintici S.A.. 17 Rue Jean Moulin. 94300 Paris France

Abstract Thins films belonging to the copper rich family in the (Sr,Ca)-Cu-O system have been synthesized using Molecular Beam Epitaxy (MBE) deposition. The growth conditions have been optimized on two different substrates, MgO (100) and SrTiO, (100). We discuss in this paper the variations of the electron diffraction pattern (RHEED) during the growth. The films obtained present a single orientation with good crystallization. The average composition measured by Rutherford Backscanering Spectroscopy (RBS) is (Sr,Ca),,,CuO.. This stoichiometry is close to the composition expected for the lirst members of the Spin Ladders homologous series Sr,,_ ,Cu,,+ ~O,,,. High resolution electron microcopy (HREM) and X-ray diffraction measurements on a four circles diffractometer are in progress. Preliminary results confirm that the structure is consistent with spin.ladders structures. Typical room temperature resistivity is of the order of 0.5 m,t2 cm. The search fof superconducting properties in these compounds is discussed.

K¢3,word~:Thin

tilm~

MBE

I, Im~tluetlon The search for high temperature superconductors has triggered the exploration of many capture oxide systems. Among them the Ca=Cu=O system is particularly im° portant as it contains the core structure of most superconducting phases, the so.called infinite layer phrase (ILP) [I I.

Table I Phases

a (A)

b (A)

c (A)

Ref,

S~ujO, StCu:O~ Cu~SrI ,sO~ ,, Sre~Cu:~O4, CaCu~O, Ca~Cu~O~

! 1,469 5.469 I 1.4751 I 1,466 9.942 2,807

13,426 5,469 13,4012 13.389 4.079 6,3~1

3,942 9,826 3,9496 3,94~8 3.459 10,~97

1131 ! 14l 1151 14,8,91 [ 161 l~ooVI

3.934 3,931

11,573 19,408

3,495 3.4t~)

[ l 0, I I I

Sr. ,Co..,O~. n~ 3 n~ 5

*Coffes~nding aulhor. Tel.: 40 79 47 03 or 40 79 47 01: Fax: 50 79 44 25; e-maih [email protected] 0925-8388/97/$17,00

@ 1~7 El~vier Science S,A. All rights reserved

Pll S0925-8388t96~02805.8

This system is cxlend~d Io (Sr~Ca)~Cu-O because it is difficult to stabilize the Sr fiee inlinite layer phase even with pressure techniques or thin tilms techniques. Various phases belonging to this system have been studied. We report here the growth of films of a copper rich hmily in the (Sr,Ca}CuO system with average composition (Sr,Ca)o~CuO c Some of tile well known phases which present a ratio A/Cu (A=Sr, Ca) smaller than I arc pre~nted in the Table I. Sr2CuO~ [2], SrCuO: 131. Srl~Cu:40~, [41, Ca~,CuTO~ 15~71 include structural planar .square CuO~ units but do not contain any CuO2 planes. (Sr,Ca)~4Cu240~t shows a complicated structure [8,91. It may be described as two interpenetrating substructures: Cu20 s sheets in alternation with Sr planes and CuO~ chains made of distorted CuO~ squares sharing edges. Another family includes double Cu-O rows as detects within the CuO: planes: the Sr,,~Cu,,,~O:. II0,111 homologous series which refers to the so-called spin ladders. In these compounds the ratio Sr/Cu is smaller than I. These phases a~¢ attractive because they contain a new type of Cu-O sheet including one dimensional Cu-O chains, units which could lead to superconductivity [12]. in this family, for n=3 the ratio Sr/Cu=0.5, close to the stoichiometry of the phases presented in this paper.

241

B. Eustache et ai. I Journal of Alloys and Compounds 251 (1997) 240-242

2. Experimental 25000

The films are grown by a Molecular Beam Epitaxy (MBE) deposition system on SrTiO 3 (100) and MgO (100) substrates (dimension 10×5×0.5 ram). An atomic oxygen plasma source is used to oxidize the film during the growth (global pressure in the chamber is around 10 -5 Ton). The temperature of the substrate is maintained at 550 °C. More details of the experimental process are described elsewhere [ 16,17], Reflection high-energy diffraction (RHEED) with an energetic beam of 35 kV is used for monitoring in situ the layer by layer sequential growth. The diffraction patterns recorded by a video camera are digitized in ~al time by a computer. The variation of the intensities of the diffraction's streaks are monitored during the growth. Strontium and calcium are evaporated from Knudsen cells while copper is evaporated using an electron gun. The shutter of each source is controlled by a computer. During the deposition of the first 50 ,~, the duration of the opening for each shutters is adjusted by operator in order to reach an extremum for the RHEED intensity. Then the process is computer controlled. The thicknesses of the films are in the range 200-400 A. The films are characterized by x-ray diffraction (0-20 geometry) using a CuK~ source. The composition of the films is measured by Rutherford Backscattering Spectroscopy (RBS). Resistivity measurements were performed by the standard four points technique.

3. Results The copper rich phase was grown both on SrTiO~ and on MgO. A diagram of the electron diffraction pattern variation during the growth on MgO substrate is presented on Fig. I. This pattern is clearly different from that of the RHEED of the infinite layer phase. A streak (denoted C) appears about 3 monolayers after the beginning of the growth, We have noted that when we evaporate strontium the intensity of the streak denoted A increases while the intensity of the streak denoted C decreases. When we deposit copper the intensity of the streak C increases while the intensity of the streak A decreases.

2OOO0

15000

10000

S000

dl

0

20

0

4O 2too

(5O

SO

Fig. 2. X-nly diffraction pattern of Sros,CuO.(d,). The subsln~: is

denoted by S.

Pure strontium, pure calcium and mixed strontium calcium phases have been synthesized. Fig. 2 shows a typical x-ray pattern of SrCaCuO: sample deposited on MgO substrate. The crystallization is excellent and it is well oriented: ~ scans (diffraction of x-ray with rotation of the film around the c axis of the substrate) confirm that this phase grows with a single orientation of the three axes. Fig. 3 shows the variation of the distance d4 versus the alkaline stoichiometry. This distance decreases linearly when the calcium concentration increases in agreement with the sizes of the Ca 2+ ion and Sr 2+ ion. Wc observe that on the SrTiO~ substrate there is a competition between the growth of the copper rich phase and the growth of the infinite layer phase: the infinite layer phase grows preferentially to this phase and we obtain frequently a mixing of both phases (Fig. 4). RBS measurements show two "families" in the pure strontium phase (i.e. pure calcium phase), the first one

Sr/Cu=0.5 (i.e.CalCu•O.5) and the second Sr/Cu~0.6 (Ca/Cu=0.6). These compositions coincide respectively with the

first and

1.68

the

second

members

of

the

11o

12

|.

I.¢K; 1~64

III

~" :~

1 C

:'

A

ll

A

C

I II

C

A

A

C

I I I .....

1.60 1.58

1

Q

Jt

1'~ ! 1.54 1.52

-0.2

0.0

0.2

o.~

o~6

o~a

Ca/(Sr-~a) Fig. I. Diagram of the variation of the electron diffraction patterns during the growth on MgO.

Fig. 3. Variation in the distance (001) d4 versus alkaline stoichiometry.

242

B. Eustache et al. I Journal c~' Alloys and Compounds 251 t 1997) 240-242

The resistivities of the films are strikingly low with a typical value of 1 m[/cm. The best crystallized samples exhibit a metallic behavior between 300 K and 150 K, and change to a semiconducting behavior for lower temperalures. A typical R(T) curve is shown in Fig. 5. The resistivity decreases slightly with decreasing temperature from 300 K to 150 K and then increases exponentially for lower temperature. No superconductive properties have been observed until now for these compounds.

m

S

1 1

l-

m

m

D

¢D

$

4. Conclusion m

to

m

SmfJ

Fig. 4. X-ray diffraction pattern with an intinite layer phase (°) and the copper rich phase (*). The substrate SrTiO~ is denoted by S.

Sr,_tCu,,+tO2, , homologous series: n=3 (SrCu203. Sr/ Cu=0.5) and n=5 (Sr2Cu30 s, Sr/Cu=0.6). These phases have been discovered by Hiroi et al. [10, I 1] who report for the phase n=3 and the phase n=5 an orthorhombic structure with the parameters presented in Table I. These phases are referred as spin ladders because the Cud4 tetragonal units are distributed along the stripes as ladders. The magnetic properties of these compounds [10,1 I] are supposed to be very interesting due to the frustration of the antiferromagnetic order for special values of n (3, 7 .... ). It was suggested by Rice [12] that this frustration could lead to superconductivity with a high critical temperature. The compound n ~3 presents a zero magnetic su~eptibility at very low temperature but no superconductivity was observed until now. To improve the structural description of these phases, high resolution electron microscopy (HREM) and xoray diffraction measurements on a tbur circles diffractometer are in progress. HREM already provides clear evidence of the presence of spin ladders liB1,

1AxI0~, o e,

1 1,2x10~1.

%

%

\

t

1,0xlO~ o

1

i

e % o

O,Ox104 o

We have optimized the growth of thins films with stoichiometry (Sr, Ca)o.6CuO :. These films are well crystallized and their resistivities are low. H R E M already provides clear evidence of the presence of spin ladders.

Acknowledgments We thank J.P. Lauriat, LURE Universit~ d'Orsay, for X-ray measurements on the four cities diffractometer, and J.Y. Laval, ESPC! Paris. for the study by HREM.

References I I I M. Lagu~s, C.F. Beuran. C. Hatterer. V. Mairet, E Laffcz, X.M. Xie. X.Z. Xu, C. Deville Cavellin, B. Eustache and C. Coussot, to be published in Coherence in high Tc Sulwrconductors. A. Revcolevski (ed.), G. Deut~cher World Scientific, 1996. (21 C.L~ T~ske and H. MtilleroBuschbaum, Z. Anorg. AIIg0Chem., 371 (1069) 325. 131 C.L. Te~ke atld H. M01letoOuschbaum. Z. Andre. AIIg. Chem.. 379 (1970) 234. [41 T. Siegri~t, L.F. Schneemeher, S.A. Sund~ine and J.V. Was~,.¢zak, Mat. Res. Bull., 23 11988) 345. 151 T.G.N. Babu and G. Greaves, Mat. Res. Bull., 26 (1991) 499. 161 O. Milat, G. Van Tendeloo, S. Amelinckx, T.G.N. Babu and C. Greaves, J. Solid State Chem., 97 (1992) 405. 171 O. Milat, G. Van Tendeloo. S. Amelinckx. T.G.N. Babu and C. Greaves J. Solid State Chem., IOI (1992) 92. 181 R.S. Roth, J. Rawn, CJJ. Rifler and B.P. Burton. J. Am. Chem. Sin:., 72 (1989) 1545. 191 O. Miiat. G. Van Tendeh~, S Amelinckx. M. Mehbod and R. Deltour, Acta C~.sr. A, 48 (1992) 618. I IOI Z. Hiroi, M. Azuma. M. Takano and Y. Bando, J. Solid State Chem.. 95 (1991) 230. II II Z. Hiroi, M. Takano, M, Azuma and Y, Takeda. Nat.re. 356 (1992) 775. ll21 T,M, Rice Emw~hy,s, Lett,, 23 ~1~3) 44,q, 1131 ASTM 39~250,

6,0x104 ,o

6O Fig, 5. Variation of

100

180 T ~

20O (K)

25O

3OO

the resistivity versus temperaturefor Ca,, ~CuO..

[14l ASTM 38-1178. JiS] ASTM 39~489. 116j X.Z. Xu, Ph.D. Thesis Paris Vl University (1993). [17l V. Mairet Ph,D Thesis Paris VII University (1995), [18l J.Y. Loyal et al., personal communic:ation,