Superconducting and non-superconducting oxide multilayers

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering B 32 ( 1995) 239 - 245

B

Superconducting and non-superconducting oxide multilayers M.S. Hegde a, K.M. Satyalakshmi b, S. S u n d a r M a n o h a r a n a, D h a n a n j a y K u m a r a aSolid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India bDepartment of Metallurgy, Indian Institute of Science, Bangalore 560 012, India

Abstract An overview of perovskite-related metallic oxides and their thin films is presented with an emphasis on the epitaxial growth of LaNiO 3 thin films on a variety of substrates. Using these films as metallic barrier layers, YBa2Cu307_6/LaNiO3/YBa2Cu307_6/ SrTiO 3 trilayer superconductor-normal metal (metallic oxide)-superconductor junctions have been fabricated. The results indicate that the coherence length of electrons in the normal metal LaNiO 3 region is of the order of 125 A. Besides, the growth of a-axisoriented LaBa2Cu2.sNio.207+6 metallic thin films is reported which can be used as a promising template to grow a-axis-oriented overlayers required preferably in the fabrication of multilayer devices.

Keywords:SNS junction; Perovskite oxides; Metallic oxides; Lanthanum nickelate

1. Introduction Following the discovery of superconductivity at high temperature in La2 xBaxCuO4 [1] and subsequently in Y B a 2 C u 3 0 7 - 6 (YBCO; Tc = 90 K)[2], extensive efforts have been made to fabricate thin films of these materials owing to the very high possibility of their use in electronic devices. Since then, several new series of superconducting oxides such as Bi2Sr2Ca n_ iCUnO2n+4 [3], T12Ba2Can_lCunO2n+4 [4] and Hg-based cuprates [5] have been discovered with Tc higher than that of the YBCO system. However, the major effort in the thin film area still continues on the growth of YBCO films, not only because of the relative ease of their formation with desired composition but also owing to their rather high critical current density (Jc) at 77 K. Chaudhari et al. [6] were the first to show that Jc as high as 106 A cm-2 can be achieved in epitaxially grown YBCO thin films on (100) SrTiO3 (STO) single-crystal substrates. Pulsed laser deposition (PLD) is the most successful technique among other commonly used techniques for fabricating good quality, high Tc thin films. The main advantages of PLD over other thin film deposition techniques are as follows: (a) the chemical composition of the multicomponent oxides in the target can be easily reproduced in the film; (b) in situ growth of 90 K YBCO film is achieved more conveniently by optimizing growth parameters such as substrate temperature and oxygen partial pressure; (c) film thickness can 0921-5107/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved

SSD10921-5107(95)03014-X

be controlled even up to unit cell dimensions; (d) the deposition rate is very high, which reduces the chances of formation of any intermediate phases. PLD has also been extended to grow a variety of epitaxial non-superconducting oxide thin films for superconductor-insulator-superconductor (SIS) and superconductor-normal metal (metallic oxide)-superconductor (SNS) trilayer heterostructures. These types of multilayer structures are required to realize Josephson junctions for superconducting quantum interference device (SQUID) fabrication. The fabrication of a high temperature trilayer junction using YBCO films as superconductors and P r B a 2 C u 3 0 7 (PBCO) as "an insulating barrier was reported for the first time by Rogers et al. [7], who showed that, owing to structural similarity, it was possible to grow c-axis-oriented PBCO and YBCO layers epitaxially on top of each other. The trilayer junction fabricated by them did show Josephson junction (JJ) behaviour, but the current (/)-voltage (V) characteristic of the junction was unlike that of an SIS junction fabricated from classical superconductors and an insulating barrier, such as Nb-A1203-Nb (Fig. l(a)). Subsequent attempts to make high temperature SIS junctions using other insulating layers such as NdGaO 3 and NdAIO 3 also yielded an I - V characteristic resembling that of an SNS junction [8,9]. The fact that the I - V characteristic of YBCO/PBCO/YBCO resembles that of an SNS junction (Fig. l(b)) [7] is thought to

M.S. Hegde et al.

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(a)

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Materials Science and Engineering B32 (1995) 239-245

I J

V

f ij

(b)

Cu 0 2- plane

w

BaO-plane

w

C u O-plane • -Cu,

V

Y- plane

@- Ba,

~]]) -- Y ,

O-oxygen

Fig. 2. Atomic )ositionsof various layers of YBa2Cu3O 7_ ~CUtin the [100] direction.

J Fig. l. Typical I - V characteristics of (a) a classical Nb-AI203-Nb SIS Josephson junction and (b) an SNS junction with YBa2Cu307_6 thin films. TiO 2- plane

be due to the absence of complete isolation of the two superconducting layers, since the thickness of the barrier layer was only 100-200 A. However, if the thickness of the insulating layer is increased in order to avoid shorting between the superconducting films, no supercurrent is seen owing to the shorter coherence length of the YBCO material. Therefore it is natural to consider the fabrication of SNS junctions using a compatible metal or metallic oxide film. The ease of fabrication of SNS junctions is provided by the larger normal metal coherence length, which in turn allows the growth of metal or metallic oxide films with relatively greater thickness. However, owing to the structural mismatch and/or the chemical reactivity of high temperature superconductors (HTSCs), metals are unsuitable for use in the fabrication of SNS-type Josephson junctions. Thus the investigation of the growth and characterization of metallic oxide thin films which form unreactive interfaces with HTSCs becomes an independent and important topic [10,11]. In this paper we present an overview of several perovskiterelated metallic oxides in connection with their use as normal metal barriers and demonstrate the fabrication of SNS junctions using LaNiO 3.

2. Structural criterion for HTSC-compatible metallic oxides It is well known that YBCO crystallizes in the orthorhombic perovskite structure with a = 3 . 8 3 A,

• -Ti ,

@-Sr,

SrO- plane O-Oxygen

Fig. 3. Atomic positions of various layers of SrTiO~ cut in the [100] direction.

b = 3.89 A and c = 11.65 A. The atomic positions in the CuO2, CuO, BaO and Y planes of YBCO are shown in Fig. 2. The atomic positions in the (100) plane of the cubic perovskite with space group P m 3 m are shown in Fig. 3. Thus, for ideal epitaxial growth of YBCO with the c axis perpendicular to the a - b plane, a or b of YBCO should match with the "a" parameter of the material used as a metallic barrier layer. Therefore perovskite-related metallic oxides are the righl kind of materials for superconductor-metallic oxide-superconductor trilayer structures. There are several metallic binary oxides such as TiO, VO and NbO (with rock salt structure) which can also support the epitaxial growth of high Tc films in the same way that YBCO film is grown on single-crystal (100) MgO substrates. However, the fabrication of TiO, VO and NbO thin films themselves is difficult, because all these metals can be oxidized to a higher valency, leading to the formation of TiO2, V205 and Nb205 respectively on the surface. Since the stability of these oxides is higher than that of CuO, oxygen can easily be leached away from YBCO, resulting in the formation of oxygen-deficient non-superconducting YBCO. Therefore TiO, VO and NbO are not suitable oxide materials for SNS-type trilayer junctions involving high Tc films.

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Materials Science and Engineering B32 (1995) 239-245

As mentioned earlier, metallic oxides with a perovskite-related structure will be structurally more compatible with HTSCs and a list of such oxides is presented in Table 1. The table summarizes the structural, electrical and magnetic properties of these oxide materials. It is clear from this table that ReO3, Na0.TWO 3 and La0.sSr0.sCoO 3 (LSCO) are quite promising as far as low resistivity and lattice match with YBCO are concerned. Their lattice parameter mismatches with YBCO are only 1.9%, 0.2% and 0.5% respectively, indicating the high possibility of oriented growth of YBCO on top of these materials. In fact, Char et al. [44] were successful not only in growing epitaxial La0.sSr0.sCoO3 thin films on (100) SrTiO 3

substrates

but

also

YBCO/LSCO/YBCO (LCMO) is a n o t h e r magnetic properties metal barrier for

in

241

fabricating

c-axis-oriented

SNS junctions. La0.TCa0.3MnO 3 cubic perovskite with ferrowhich has been used as a normal YBCO/LCMO/YBCO junctions

[45]. Metallic oxides such as LaRuO3, SrRuO 3 and CaRuO3 are also very important since they crystallize in the GdFeO 3 structure with an orthorhombic unit cell of approximate l e n g t h 21/2 a o a l o n g t h e a a n d b a x e s a n d 2 a o a l o n g t h e c a x i s , w h e r e a o is t h e l a t t i c e p a r a m e t e r o f t h e s m a l l p e r o v s k i t e u n i t cell. T h e a - b p l a n e of this class of compounds, i.e. w i t h t h e G d F e O 3 s t r u c t u r e , is s h o w n i n F i g . 4. T h e

a o parameter

of this cubic

Table 1 Perovskite-related metallic oxides System

Structure a

Space group

ReO 3 Na0.75WO 3 La0.sSr0.sCoO 3 ta0.67Ca0.33MnO 3 La0.67Ba0.33MnO 3

Cubic Cubic Cubic Cubic Rhomb.

Pm3m Pm3m Pm3m Pm3m R3c

La0.6Sr0.4MnO 3

Rhomb.

R3c

La0.62Pb0.38MnO3

Rhomb.

R3c

LaNiO3

Rhomb.

R3m

LaCuO 3

Rhomb.

R3 c

LaRuO 3

Ortho.

Pbnm

SrRuO 3

Ortho.

Pbnm

CaRuO 3

Ortho.

Pbnm

SrVO 3

Ortho.

Pbnm

CdVO 3

Ortho.

Pbnm

SrMoO 3 BaMoO 3 SrCrO 3 BaPbO 3 Lal.6Sr0.~CuO 4

Cubic Cubic Cubic Cubic Tetra.

Pm3m Pm3m Pm3m ---

LaBa+Cu2.sNio.2OT+~

Tetra.

P4/mmm

Resistivity (W cm)

Magnetic property b

Ref(s).

10 -5 10 -4 10 -3 10 -3 10 -3

PP PP FM FM FM

[12,13] [12,14,15] [16-18] [12,19,20] [19,21]

a o = 3.87

10 -3

FM

[12,19,22]

ao = 3.907

10 -3

FM

I23-25]

a o = 3.83

10- 4

pp

[23,26,27]

a o = 3.89

10- 3

pp

[23,28,29]

a o = 3.95

10 -3

AFM

[23,30]

a o = 3.92

10 -3

FM

[23,31,32]

ao = 3.84

10 _3

AFM

[23, 3 1 - 3 3 ]

--

10 -5

Weak FM

[23,34,35]

a o = 3.75

10 -5

PP

[36,37]

ao= ao = ao= ao = ao =

3.974 4.04 3.818 4.273 3.765

10 -4 10 -4 10 -5 10 -2 10 -3

PP PP PP NM AFM

[38,39] [38,39] [12,23,40] [41] [42]

ao=3.912

10 -3

--

[43]

Lattice parameter(s)

Pseudocell parameter

(A)

(A)

a a a a a a a c a a a a a c a b c a b c a b c a b c a b c a a a a a c a c

ao= ao = ao = ao= ao=

=3.748 = 3.74 =3.84 = 3.89 = 7.820 = 90 ° 12' = 5.47 = 13.36 =7.815 = 90 ° 24' = 7.676 = 90 ° 42' = 5.502 = 13.22 = 5.494 = 5.779 = 7.855 = 5.53 =5.57 = 7.85 = 5.36 = 5.53 = 7.66 = 5.41 =6.16 = 7.64 = 5.265 =5.35 =7.85 = 3.974 =4.04 =3.818 =4.273 = 3.765 = 13.27 =3.89 = 11.90

aRhomb., r h o m b o h e d r a l ; Ortho., o r t h o r h o m b i c ; Tetra, tetragonal; magnetic; N M , n o n - m a g n e t i c .

bpp,

3.748 3.740 3.89 3.89 3.91

Pauli paramagnetic; F M , ferromagnetic; A F M , antiferro-

M.S. Hegde et al.

242

/

ao

//

/

\

Materials Science and Engflneering B32 H 995) 239-245

\

©.

\

o?,l.

/

/~\

(~)/

"~

/

-",Q ° .0,-" ao~

liD" a/,/2-~- b/,/"2"-

0 - F'e ~ O - O × y g ~ n

Fig. 4. Atomic positions in the (100) plane of GdFeO3-type

oxides. pseudocell is close to the a or b parameter of YBCO as well as to the lattice parameter of substrates commonly used for the growth of YBCO films. Therefore the mating of the pseudocell lattice of these oxides with the lattice of the substrate (e.g. LaA103) produces a 45 ° rotation [46]. This happens because such growth is a relatively low energy phenomenon owing to the two adjoining crystal lattices having a relatively dense set of coincident lattice sites as observed in the growth of YBCO films on MgO and yttria-stabilized zirconia (YSZ) substrates. Thus these oxide materials appear to be good candidates for supporting epitaxial growth of overlayers and thereby acquire the potential for use in the construction of multilayer devices. In fact, heterolayer devices have been fabricated using CaRuO 3 [47] and SrRuO 3 [48]. LaNiO3 (LNO), LaCuO3, LavxBaxMnO3, Lal ~PbxMnO3 and La~ _xSr~MnO3 constitute an interesting class of perovskite-related metallic oxides with rhombohedcral structure. The interest in these compounds for use as a compatible material with HTSCs arises because their (100) plane belongs to the pseudocell and is cubic. The atomic positions in the (100) plane are similar to those of a cubic perovskite. Thus these oxides are also good lattice-matched metallic oxides for epitaxial growth of multilayers with YBCO. Among them, LaCuO 3 (bulk solid) cannot be prepared at ambient pressure. However, metallic LaCuO3_ ~ has been grown by the pulsed laser deposition technique [49]. SrMoO3 and BaMoO3 are cubic perovskites and metallic in nature and are prepared by reducing SrMoO 4 and BaMoO 4 respectively in a hydrogen atmosphere around 800-900°C. Therefore in-situ growth of YBCO on SrMoO3 is likely to be difficult, because YBCO is grown under oxidizing conditions. However, SrMoO 3 is likely to be a good metallic oxide barrier layer for n-type Ndz_xCexCuO 4 superconductor, since thin films of this superconductor are grown either in a mildly reducing ambient or in a very low oxygen partial pressure [50].

BaPbO 3 is a nearly metallic or semimetallic oxide with a = 4.27 A, which is too large compared with a or b of YBCO. However, this is an important material because it is the only perovskite-related oxide which does not contain any transition metal ions and is nonmagnetic. In addition to these metallic perovskites suitable for SNS junctions, cation-substituted superconducting oxides themselves can also act as metallic oxide barriers. Lal.sSr0.sCuO4 6 is a metallic oxide with a good lattice match with BYCO in the a - b plane [42]. For x > 0.6, Y1-xPrxBa2Cu307 is non-superconducting and for x < 0.6 it is superconducting (Tc far less than 90 K). Above T c the material shows metallic behaviour as function of temperature and hence can be used as a metallic barrier in SNS devices as demonstrated by Wu et al. [51]. LaBa2Cu2.8Ni0.207+ ~ (LBCNO) is another cation-substituted metallic oxide synthesized recently by Sundar Manoharan et al. [43] and its thin films may be useful in making SNS devices. Epitaxial films of SrRuO3, CaRuO3, La0.sSr0.sCoO 3 and La 1 xMxMnO3 ( M - C a , Ba and Pb) have been grown and studied in detail by several groups [10,11,45,52,53]. Recently we have been successful in growing epitaxial LNO and a-axis-oriented LBCNO films on substrates routinely used for the growth of high Tc films and hence will be concentrating hereafter only on these two systems.

3. Experimental details The laser ablation experiments for the growth of YBCO, LNO and LBNCO films were carried out using a KrF 248 nm excimer laser and a 300 mm focal length quartz lens for focusing the laser beam. The excimer pulse had a maximum energy of 1200 mJ with a pulse width of 25 ns and 1-10 Hz variable repetition rate. The laser fluence was varied from 1.5 to 2.5 J c m - : by varying the laser output energy. The substrates were held on the heater faceplate by Ag paste and the temperature on the surface of the heater was measured using a thermocouple fixed to it by Ag paste. The temperature difference between the heater faceplate and the substrate top surface was 30-40 °C, which was calibrated by fixing another thermocouple on the top surface of the substrate. The oxygen pressure during film deposition of all three materials was 350-450 mTorr. The temperature of the heater faceplate maintained during growth of YBCO, LNO and LNBCO films was 790, 700 and 780 °C respectively. The growth and structural characterization of LNO films have been reported in detail in earlier publications [54,55]. The YBCO/LNO/YBCO trilayer structure was fabricated by first depositing a YBCO film over the

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Materials Science and Engineering B32 (1995) 239-245

entire surface of an STO substrate and subsequently depositing an LNO film on the previously deposited YBCO film by masking some portion of it with silver foil. This was followed by the deposition of a YBCO film on the central region of the LNO film by masking the remaining portion with silver foil. It is important to mention here that during the second (LNO film) and third (YBCO film) depositions the oxygen pressure was maintained at 400 mTorr from the beginning of heating the films to the optimum growth temperature in order to avoid out-diffusion of oxygen from previously deposited films. Structural characterization of all the individual films as well as of the trilayer was carried out using 0-20 X-ray diffraction (XRD), J-scan XRD, scanning electron microscopy (SEM) and energydispersive X-ray analysis (EDX). The films were also characterized by the four-probe method and the I - V characteristic of the trilayer was measured using a laser-patterned microbridge.

4. Results and discussion 4.1. Metallic L a N i O 3 thin f i l m s

The quality of LNO films grown on various substrates was first evaluated by recording the temperature dependence of the resistance of the films by the standard four-probe method. Shown in Fig. 5 are resistance (R) vs. temperature (T) plots for LNO films grown on (100) MgO, (100) LaAIO3 (LAO) and SiO2/ (100) Si substrates. It is clear from these plots that the quality of the LNO film grown on the Si substrate is inferior to that of the films on LAO and MgO, as revealed by the poor slope of the R - T plot. However, the realization of metallic LNO films on SiO2/Si substrates is an encouraging first result and is worthy of investigation in more detail owing to the technological importance of SiO2/Si substrates.

The microstructure of LNO films grown on LAO and MgO was analysed by XRD studies. Displayed in Fig. 6 are XRD patterns of LNO films on these substrates. Only (h00) peaks are observed for the LNO film on the LAO substrate, while in the case of the LNO film on the MgO substrate the (110) line also appears. 4. 2. L a B a 2 Cu 3_ ~ N i

x

0 7+6

thin f l m s

A recent study carried out by Sundar Manoharan et al. [43] has shown that Ni substitution for copper in LaBa2Cu307_ ~ (LBCO) yields a metallic phase for the Ni composition x in the range 0.06 < x < 0.3. The compounds are conspicuous among rare-earth-based cuprates in being metallic but not superconducting. Owing to their structural similarity and metallic property, Ni-doped LBCO films in the aforesaid range can be used as a compatible metallic oxide barrier layer in the fabrication of SNS devices. The LBCNO films were found to possess a room temperature resistivity of the order of a few microohm centimetres, which did not vary with temperature down to 50 K. Shown in Fig. 7 is the XRD pattern of an LBCNO film 3000 A thick which was grown on a (100) LAO substrate at 780 °C. The appearance of only (h00; h = 1,2,3) lines corresponding to LBCNO indicates the growth of an LBCNO film with its a axis oriented normal to the substrate surface. The formation of the a-axis-oriented LBCNO film can be explained via the occurrence of a thermodynamically more favourable situation, since its lattice mismatches with LAO along both the a amd c axes are almost the same. The area of the basal plane in contact with the substrate surface is three times greater when the film grows a axis oriented than when it grows with

LNO/LAO (100i

A

.~,

.A

0 o t.t t-

LNO/MgO (100)

>,, LoNiO3/LoAIO

,0

z

,

3 c

z

z

al, o

1

tn,"

0

I

I

100

200

300

Temp (K)

Fig. 5. R vs. T plots for LNO thin films grown on LAO, MgO and SiO2/Si.

20

25

30

35 40 20(deg} CuK~

45

50

Fig. 6. Cu K a 0 - 2 0 scans of L N O thin films grown on M g O and LAO.

244

M.S. Hegde et al.

Materials Science and Engineering B32 (1995) 239-245 ]

J o <

E :D

el

>,

u O

C

O flD

C

i1°

10

_

o

II

cA o

v~ o o

I

I

20

30

410

510

60

7~0

'E 20

the c axis oriented normal to the substrate surface. The former is thermodynamically more stable and hence responsible for the a-axis-oriented growth of LBCNO films. The realization of a-axis-oriented LBCNO films at temperatures comparable with the processing temperature of YBCO films is very advantageous, since such films can act as a template for the growth of a-axisoriented YBCO films. The advantage arises because of the fact that the coherence length in the copper oxide plane is more than an order of magnitude larger than that in the plane normal to the copper oxide plane. The ability to grow a-axis-oriented YBCO films using an LBCNO metallic oxide template is thus expected to be of great technological importance in fabricating high Tc Josephson devices.

~..

n'*, Z I I _.] >.- ..J

O

OO

o

O

5

II

Ooollo

lllll

~.

2 O ( d e g ) Cu g,~

Fig. 7. Cu Ka 0-20 scan of an LBCNO thin filmgrown on LAO.

o

n

~ Jo

O

I

o

..&

Z~

35 50 28 (deg) Cu K~

~ o

65

¢~-

80

Fig. 8. Cu Ka 0-20 scan of a YBCO/LNO/YBCO trilayer structure grown on LAO. 25 J

20 15 10 5 -~ *'~ 0 ~ -5

Y B CO S aNiO3 N YBCO S

-10 -15 -2 0

-25 4.3. Y B C O / L N O / Y B C O heterostructures

YBCO/LNO/YBCO trilayer structures were fabricated in planar form with an LNO barrier thickness of 1000A. Fig. 8 shows the X-ray 0-20 scan of a YBCO/LNO/YBCO/STO trilayer structure. Only the (001) lines of YBCO and the (h00) lines of LNO are seen in this figure. There are no peaks of any other orientation, which confirms the textured growth of the YBCO and LNO layers on each other. In order to measure the I - V characteristic of the above structure, a microbridge 100/~m wide and 300 p m long was patterned by laser irradiation. Fig. 9 shows the I - V characteristic of the junction at 77 K. Tc of the YBCO film across the LNO barrier was about 86 K and Jc of the junction at 77 K was 300 A cm -2. The coherence length ~n in the normal metal (LNO) region was found to be 125 A, which was calculated using the formula [47] Jc(d)=Jco exp

-~)

where d is the barrier thickness.

-2.5

-1.5

-0.5

0

0.5

1.5

2.5

V (mV)

Fig. 9. I - V characteristic of a YBCO/LNO/YBCO trilayer junction.

The figure of merit of the Josephson junction, IcR ~ is found to be of the order of 120 pV, which is comparable with the values reported for SNS junctions fabricated using YBCO superconducting films and various metallic oxide films as barrier layers [44].

5. Conclusions

The paper presents an overview of several perovskite-related materials which can support the epitaxial growth of YBCO films and hence can be used as metallic barrier layers required in the fabrication of SNS junctions. Trilayer YBCO/LNO/YBCO Josephson junctions fabricated using epitaxial LNO films were found to have a figure of merit R n A of the order of 10 9 ~-~ cm 2. The formation of a-axis-oriented

M.S. Hegde et al.

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Materials Science and Engineering B32 (1995) 239-245

L B C N O thin films on L A O substrates has also been reported, which can be used to bring about a-axisoriented growth of Y B C O films required preferentially in multilayer device fabrication.

Acknowledgments T h e authors thank the D e p a r t m e n t of Science and Technology, G o v e r n m e n t of India for financial support. K.M.S. thanks the Council of Scientific and Industrial Research, N e w Delhi for the award of a fellowship.

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