Physica C 201 (1992) 273-278 North-Holland PHYSICA

Crystal structures at the interface between a-axis oriented YBaECuaOT_x films and LaA103 (001) and SrTiO3 (001) substrates

H i r o m i Takahash i I K a t s u m i O h a t a a nd T a d a t a k a Mor i sh i t a Superconductivity Research Laboratory, ISTEC, 10-12, Shinonome 1-chome Koto-ku, Tokyo 135, Japan

Received 13 April 1992

Crystal structures between a-axis oriented YBa2CBaO7_x films and LaAIO3 (001 ), SrTiO3 (001 ) substrates have been investi- gated by cross-sectional transmission electron microscopy. As a whole, the a-axis oriented domains grow on the 3.4-4.5 nm width distorted structures which are formed on both substrates. At the substrate surface, partially hetero-epitaxially grown a-axis ori- ented YBa2Cu307_x is observed. The triclinic distortion of the a-axis oriented YBa2CuaOT_x structure is observed up to 1.2 nm and 0.6 nm from the LaAIO3(001 ) and SrTiO3(001 ) substrate surfaces, respectively. These triclinic distorted structures of YBa2Cu3OT_x would probably be caused by the structure transformation to reduce the lattice mismatch between the film and substrates (about 3.5% for LaAIOa and 0.7% for SrTiO3), particularly due to the structural transformation at 435°C for LaAIO3 under the cooling process.

1. Introduction

Since the discovery o f superconductive Y - B a - C u - O, many deposition techniques have been investi- gated to produce films suitable for various applica- tions. In particular, the a-axis oriented YBa2Cu307_~ film has somemer i t s for multilayered devices based on the Josephson effect: a longer superconducting coherence length (~ab ~ 10 ]~, ~ ~ 3 A) along the film normal and a very smooth surface on the atomic scale [l].

The quality of thin films is determined by the grain structure and the crystalline orientation, which crit- ically depend on an early stage o f crystal growth on a substrate surface [2] . High-resolution transmis- sion electron microscopy ( H R T E M ) is a powerful tool used to study the microstructure o f crystalline structures near the interface between film and sub- strate. Lisa et al. have discussed the first layer o f the c-axis oriented YBa2Cu307_x on MgO(001) sub- strates and have pointed out that the C u - O planes

Contact address: OKI Electric Industry Co., Ltd., Hybrid Mi- croelectronics Department, 550-5, Higashiasakawa-cho, Hachioji-shi, Tokyo 193, Japan Phone: (0426) 63-111 l, Fax: (0426) 65-9616.

adjacent to the Y plane provide the best match with the (001} planes o f MgO across the interface [3]. Ramesh et al. have reported that a periodic array of interfacial dislocations was observed at the interface, which was also due to the accommodat ion o f lattice mismatches [4]. In the case of a SrTiO3(001 ) sub- strate, Eihl et al. have reported that there is a 20-60

thick layer width defect, being the mismatch due to imperfect surfaces [ 5]. Furthermore, perfect ep- itaxial growth, an amorphous structure, and dis- torted and interdiffused regions, were observed at the interfaces o f the c-axis oriented YBa2Cu307_x [ 6-9 ]. As described above, the interface o f the c-axis oriented YBaECU307_x film and various substrates has been investigated thoroughly. However, there are few reports on the interface of a-axis oriented YBa2Cu307_ x films and substrates, including in our previous report [ 10 ].

In this study, LaAIO3(001 ) and SrTiO3(001 ) were chosen as substrates for epitaxial growth of the a-axis oriented YBa2CuaOT_x, because these crystals are cubic at 700°C and match well with the c /3 lattice constant o f YBaECU307_x. In this paper, we report on the examinations of the crystal structures at the interfaces between the a-axis oriented YBa2CuaO7_x

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274 H. Takahashi et aL / Crystal structures at interface between a-axis oriented YBCO films and substrates

film, the LaA103(001 ) and the SrTiO3(001 ) sub- strates by TEM observations.

2. Experimental

The details of the preparation procedure for YBa2Cu307_ x thin films have been previously re- ported [11], and are summarized here. Sr- TiO3(001 ) and LaA103(001 ) substrates were held at 700 ° C. The a-axis oriented films were deposited in an oxygen atmosphere at 50 mTorr on Sr- TiO3(001) and 100 mTorr on LaAIO3 (001), re- spectively. After deposition, the films were cooled in an oxygen gas pressure of 300 Torr. The target was a 30 mm-diameter cylinder of sintered Yla2Cu307_x. ArF excimer laser pulses of 193 nm wave length and 16 ns duration were used for ablation at a repetition rate of 5 Hz. The laser beam was focused on the tar- get down to an area of 1 × 3 mm 2 to produce a flu- ence of about 1.0 J / c m 2. The zero resistance tem- peratures (Too) of the films were determined by a DC four-probe method to be 62.0 and 81.8 K for LaA103 (001 ) and SrTiO3 (001 ) substrates, respectively.

The samples for cross-sectional TEM studies were prepared by a standard mechanical thinning tech- nique, followed by Ar ion milling with liquid nitro- gen cooling. TEM observations were made by a JEM- 4000EX microscope with 0.17 nm point-to-point resolution. The cross-sectional TEM images were taken along the LaA103[010] and SrTiO3[010] directions.

3. Results and discussion

Figure 1 (a) shows the cross-sectional TEM image of the a-axis oriented YBa2Cu3OT_x film on a LaAIO3(001) substrate. In this area, the orienta- tional relationship is YBa2Cu3OT_ x [ 001 ] II LaA103 [ 100 ], which is recog- nized from the diffraction pattern in fig. 1 (a). Fig- ure 1 (b) is an enlarged image of the domain A in fig. 1 (a). And fig. 3(a) shows a schematic drawing of the crystal structure at the substrate surface in fig. l (b ) . The stacking fault in a-axis oriented YBa2Cu307_x structures is observed around 1.2 nm

away from the substrate surface, as marked by I in fig. l (b ) . At this part, the (100) planes of the YBa2Cu307_x are shifted along the [001 ] direction of the a-axis oriented YBa2Cu307_x. Below this stacking fault, the triclinic distortion of the a-axis oriented YBa2Cu307_x structures are observed. Above this layer, the direction of the a-axis is aligned to the LaA103 [001 ] direction. As a whole, the a-axis oriented domains grow on the 4.5 nm width triclinic distorted structures which are formed on the sub- strate surface, as is seen in fig. 1 (c). And fig. 3(b) shows a schematic drawing of the crystal structure at the substrate surface in fig. 1 (c). The [ 100] direc- tion of a-axis oriented YBa2Cu307_x deviates from the LamlO 3 [001 ] direction, up to about 4.5 nm away from the surface. That is, the direction of the Cu-O chain deviates from the LaA103 [ 001 ] direction. The crystal structure of LamlO3 is also distorted over 3.0 nm below the surface.

Figure 2 (a) shows a cross-sectional image of the a-axis oriented YBa2Cu3OT_x on a SrTiO3(001) substrate. The orientational relationship is YBa2Cu307-x [ 001 ] 11SrTiO3 [ 100 ]. At the substrate surface, partially heteroepitaxially grown a-axis ori- ented YBa2Cu307_x is observed in fig. 2 (b), where it is an enlarged image of the domain C in fig. 2 (a). And fig. 3 (c) shows a schematic drawing of the crys- tal structure at the substrate surface in fig. 2 (b). At this domain, the thickness of the triclinic distorted structure is 0.6 nm. The (100) planes of YBa2CuaO7_x are shifted along the [001 ] direction of the a-axis oriented YBa2Cu307_x. Above this layer, the direction of the a-axis of YBa2Cu3OT_x is aligned to the LaA103 [ 001 ] direction. As a whole, the a-axis oriented domains grow on the 3.4 nm width distor- tion of the a-axis oriented YBa2Cu307_x structures, as is seen in fig. 2(c) . The (100) planes of YBa2Cu307_x are shifted along the [001 ] direction of the a-axis oriented YBa2Cu307_x, as shown in fig. 3(d) . Above this distorted domain, the a-axis ori- ented YBa2Cu307_~ grows perfectly, where the a-axis is well aligned to the SrTiO3 [001 ] direction. No de- viation of the YBa2CuaO7_x [ 100 ] direction from SrTiO3 [001 ] is observed.

LaAIO3 has a cubic symmetry of (a=0.3792 nm) above 435°C and shows a rhombohedral distortion (a=0.5357 nm, c=1.311 nm, and a = 6 0 ° 6 ' ) at room temperature [ 12,13 ]. SrTiO3 is cubic

H. Takahashi et al. / Crystal structures at interface between a-axis oriented YBCO films and substrates 275

Fig. 1. (a) Cross-sectional TEM image of the interface between the a-axis oriented YBa2Cu307_x and the LaA103(001 ) substrate. The orientational relationship is YBa2Cu307 _x [ 001 ] II LaAIOa [ 100 ]. ( b ) Enlarged image of domain A in fig. 1 (a). The interface between the a-axis oriented YBa2Cu307_ x and the distorted YBa2Cu3OT_~ is marked as I and the distance from the substrate surface is 1.2 nm. The interface between the film and the substrate is marked by an arrowhead. (c) Enlarged image of domain B in fig. 1 (a). The widths of the distorted domains of YBa2Cu3OT_~ and LaAIO3 are about 4.5 and 3.0 nm, respectively.

( a = 0 . 3 9 0 n m ) and exhibits no phase transforma- t ion above room temperature. At a deposit ion tem-

perature of 700 ° C, YBa2Cu3OT_x is tetragonal with c / 3 = 0.3965 nm. The lattice mismatches between c / 3 of the a-axis oriented YBa2CuaOT_x and the LaA103 and SrTiO3 substrates are about 3.5% and 0.7%, re- spectively. The thermal expansion of YBa2Cu307_x along the c-axis ( ~ 25 X 1 0 - 6 / K ) is larger than that

of LaAIO3 and SrTiO3 ( ~ 10× 1 0 - 6 / K and ~ 11 × 10 - 6 / K ) [ 13]. The film is under tensile stress

at the interface at room temperature. For the case of the LaA103 substrate, the triclinic distorted struc- tures of YBa2Cu307_x would probably be caused by the structure t ransformat ion to reduce the lattice mismatch and tensile stress at the interface, partic- ularly the structural t ransformation for LaAIO3 un-

276 H. Takahashi et al. / Crystal structures at interface between a-axis oriented YBCO films and substrates

Fig. 2. (a) Cross-sectional TEM image of the interface between the a-axis oriented YBa2CtI307_ x and the SrTiO3 (001 ) substrate. The orientational relationship is YBa2Cu3OT_x [ 001 ] IlSrTiO3 [ 100]. (b) Enlarged image of domain C in fig. 2 (a). The interface between the a-axis oriented YBa2Cu307_x and the distorted YBa2Cu307_x is marked as I and the distance from the substrate surface is 0.6 nm. The interface between the film and the substrate is marked by an arrowhead. (c) Enlarged image of domain D in fig. 1 (a). The width of the distorted domain of YBa2Cu3OT_~ is 3.4 rim.

der the cooling process after the deposi t ion. And the stress at the interface would be reduced by the stack- ing fault, as shown in fig. 1 (b ) . However , in the do- ma in in fig. 1 (c ) , the YBa2CuaOT_x and LaA103 crystals are probably d is tor ted by the stress at the in- terface which is not reduced by the stacking fault, as shown in fig. 1 (b ) . Therefore, the di rect ion of the C u - O chain deviates f rom the LaA103 [001 ] direc- t ion up to 4.5 nm away from the substrate surface, and the crystal structure of LaAIO3 is also d is tor ted

3.0 nm away from the substrate surface. For the case o f the SrTiO3 substrate, the triclinic dis tor ted struc- tures of YBa2Cu3OT_x from the SrTiO3 substrate surface would probably be caused by the structure t ransformat ion only, to reduce the tensile stress at the interface. And the stress at the interface would be reduced at the stacking fault, as is seen in fig. 2 (b ) . However , in the domain in fig. 2 (c ) , the dis tor t ion o f YBa2Cu307_x crystals up to 3.4 nm away from the substrate surface is probably due to the tr icl inic dis-

H. Takahashi et al. / Crystal structures at interface between a-axis oriented YBCO films and substrates 277

YBa2Cu307-x

I I t I [ I I YBa2Cu307-x I 1 I I I I I

I I I 1 1 1 1 1

Stacking f a u l t ( l ) ~ I I I [ I I I I I l l l I l l

' " ' " ' I / " ; ; / : I I / / / I / . . 1.2nm Subs t ra tesur face-9~[ ( ( ( ( ( (" . . . . . . . . . . . . . . . . . . . . . . . . . ~ ~ x t ~ ~ X, ~.

l J

I I

(a) (b)

4 . 5 n m

L 0 n m

YBa2Cu3OT-x

ak "

d, Stacking fault (I) --------~- [ i i • Substratesurface ~ " I I I (..~:.6nmO

i i i

- -SrTiO3 - - I I I

(c)

YBa2Cu307-x I

a~ I 1 I

11 ll\) [ [ I I 3 . 4 n m I I I I I I [

--SrTiO3 - I I I

(d) Fig. 3. (a), (b), and (c), (d), schematic drawings of the crystal structure of a-axis oriented YBa2Cu3OT_x and LaAIO3, SrTiO3 substrates in figs. 1 (b) and (c), and 2(b) and (c), respectively.

tortion at the interface which is not reduced by the stacking fault, as shown in fig. 2 (b ) .

4. C o n c l u s i o n

The distorted structures at the interfaces between the a-axis oriented YBa2Cu307_x film and two kinds o f substrates, LaAIO3 (001 ) and SrTiO 3 (001 ), have been investigated by cross-sectional transmission electron microscopy. SrTiO3 (001) and La- A103(001 ) substrates were held at 700°C. The a-axis oriented films were deposited in oxygen atmosphere at 50 mTorr on SrTiO3(001 ) and 100 mTorr on LaAIO3(001 ), respectively. As a whole, the a-axis

oriented domains grow on the 3.4-4.5 nm width dis- toned structures which are formed on both the sub- strates. At the substrate surface, partially heteroe- pitaxially grown a-axis oriented YBa2Cu307_ x is observed. In the case of LaA103 (001) substrates, thc crystal structure of films is improved in regions 1.2 nm from the substrate surface, while in the case o f SrTiO3(001 ) substrates, the thickness o f the dis- tot ted structure is reduced to 0.6 nm. These triclinic distorted structures o f YBa2Cu307_x would proba- bly be caused by the structure transformation to re- duce the lattice mismatch between the film and sub- strates at 700°C (about 3.5% for LaA103 and 0.7% for SrTiO3) and the stress at the interface, particu- larly due to the structural transformation at 435 °C of LaA10~.

278 H. Takahashi et al. /Crystal structures at interface between a-axis oriented YBCO films and substrates

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