YBa2Cu3O7−x trilayered films on LaSrGaO4 substrates

PHYSICA ELSEVIER

Physica C 265 (1996) 328-334

Microstructural investigation of the growth of Y B a 2 C u 3 0 7 _ x / N d 2 C u O 4 / Y B a 2 C u 3 0 7 _ x trilayered films on LaSrGaO 4 substrates Y.H. Li a,*, A.E. Staton-Bevan a, Z. Trajanovic b, I. Takeuchi b, T. Venkatesan b a Department of Materials, Imperial College, London SW7 2BP, UK b Department of Physics, University of Maryland, Baltimore, MD 20742. USA

Received 19 February 1996; revisedmanuscriptreceived 29 April 1996

Abstract A HRTEM study of an YBCO/Nd2CuOJYBCO trilayered films on LaSrGaO4 substrate with PBCO as a template layer has shown that in-plane aligned a-axis oriented YBCO films may be grown on (100) LaSrGaO4 substrates by pulsed laser deposition, which contain some domains with the c-axis misaligned by 90° in plane of the films. A Nd2CuO 4 insulating layer, with a thickness of approximately 10 rim, may be grown epitaxially between a-axis oriented YBCO layers with its c-axis parallel to the c-axis of the YBCO layers. A 90°-misoriented a-axis YBCO grain in the lower YBCO layer can nucleate a 90°-misoriented Nd2CuO 4 grain in the a-axis oriented Nd2CuO 4 layer and this grain can further nucleate a 90°-misoriented YBCO grain in the top YBCO layer. Narrow vertical Nd20 3 plates were observed in 90°-misoriented Nd2CuO 4 grains formed epitaxially with the Nd2CuO4 grains. The interface between the PBCO template layer and the LaSrGaO 4 substrate is quite rough with some amorphous islands in the template layer.

1. Introduction Superconductor-insulator-superconductor( S / I / S ) trilayers represent the basic structure for superconducting quantum interference devices, including flux transformers [1,2], passive microwave devices [3] and tunnelling junctions [4,5]. Several groups have reported the growth of S / I / S structures using materials with the Perovskite structure, such as LaAlO 3 [6], NdGaO 3 [7], PrGaO 3 and NdA10 3 [9] as the insulator. a-axis oriented YBa2Cu3OT_, (YBCO) films have two advantages for multilayered devices based

* Corresponding author.

on the Josephson effect, namely a long superconducting coherence length ( ~ 3 nm) along the film normal and a very smooth surface on the atomic scale [10]. However, a-axis oriented YBCO films which are normally grown on MgO or LaAIO 3 substrates have a symmetry-induced domain structure resulting in many boundaries with a 90 ° misorientation of the c-axis across the boundary [11-15]. These boundaries have a detrimental effect on critical current densities. Therefore, in the present study, a (100) LaSrGaO 4 (LSGO) substrate was chosen in order to reduce the number of boundaries with a 90 ° misorientation, since LSGO has the K 2NiF4 structure with lattice constants ( a = 0.384 nm, c = 1.268 nm) which are close to those of YBCO. The insulating

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tion transmission electron microscopy (TEM) was used to examine the growth process of this multilayered system.

YBCO (200nm) NCO YBCO (200nm) PBCO LSGO (100) substrate

2. Experimental Trilayered films having the structure which is shown schematically in Fig. 1 were fabricated using pulsed laser deposition on (100) LSGO substrates, using a PrBa2Cu307_, (PBCO) template. The following film deposition sequence was used. The PBCO template was deposited at 5 Hz holding the temperature at 640°C for one minute and then ramp-

Fig. I. Schematic diagram showing the structure of the YBCO/NCO/YBCO trilayer films.

layer used in this study is Nd2Cu Q (NCO), which has the same structure as LSGO with lattice constants (a = 0.394 nm, c = 1.216 nm). High resolu-

a 010 •

000 I~

× LSGO 001m

I~ 100

o Aligned YBCO

•

13

•

®

•

•

• 90°.misaligned YBCO

C

Fig. 2. (a) A low magnification TEM micrograph of the YBCO/NCO/YBCO trilayer film and PBCO/LSGO substrate; (b) the corresponding diffraction pattern; and (c) the schematic diagram of the indexed diffraction pattern.

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ing it up to 780°C. A 200 nm thick layer of YBCO was then deposited at 780°C and 8 Hz followed by rapid cooling down to 640°C. The NCO insulating layer was deposited for 3.5 minutes at 2 Hz. Finally, the top 200 nm YBCO layer was deposited at 5 Hz holding the temperature at 640°C for the first 30 seconds and then ramping it up to 780°C. The whole deposition was done in 50 mTorr of 0 2. The final structure was cooled at 8 ° C / m i n rate in 200 Torr of oxygen to maximise the oxygen content of the YBCO. The superconducting critical temperature (T~,) for both YBCO layers was shown by AC susceptibility measurements to be approximately 87 K. Cross-section TEM samples were prepared using conventional techniques. Since the substrate is (100) LSGO with a tetragonal structure, two pieces of the sample, ~ 3 mm × 3 mm, misoriented by 90 °, were glued together face to face, so that observations of the sample along both [010] and [001] orientations of the substrate could be achieved on one cross-section sample. Slices of the 'sandwich' were mechanically thinned, then dimpled to a central region thickness of approximately ten microns and ion-milled to perforation. The samples were examined in a JEOL 2010 electron microscope.

3. Results and discussion

Fig. 3. A high resolution TEM micrograph of the NCO layer between the a-axis oriented YBCO layers and the image simulation of NCO along its [001] direction.

3.1. The complete structure A low magnification picture of an Y B C O / N C O / Y B C O trilayered film, grown with a PBCO template on a LSGO substrate, is shown in Fig. 2a. It may be seen that a NCO layer (indicated by an arrow), with a thickness of approximately 10 nm, was formed between the YBCO layers. Occasional gaps in the NCO layer were observed. An electron beam diffraction pattern of the film, shown in Fig. 2b, indicates that the YBCO layers are well aligned a-axis oriented single crystals with their c-axes parallel to the c-axis of the LSGO substrate (the electron beam direction is along the c-axis). The diffraction pattern of the aligned YBCO overlapped with that of LSGO. However, in the diffraction pattern, the appearance of some weak spots, e.g. as indicated by an arrow, located between the strong spots suggests that there are still some a-axis grains with a

90 ° misorientation of the c-axis even though an X-ray diffraction pattern indicated that there were less than 5% of misoriented grains. A schematic diagram of indexed diffraction pattern is shown in Fig. 2c, in which 001 m is the index for 90°-mis aligned YBCO.

3.2. The NCO insulating layer Fig. 3a shows a high resolution micrograph of the NCO layer between the a-axis oriented YBCO layers. It can be seen that the NCO layer has the following epitaxial relation with the YBCO layers: ( 100)NCO///(100)YBCO a n d [001]NCO//[001 ]YBCO. A higher magnification micrograph of the NCO layer and its image simulation are shown in Fig. 3b. The image simulation of NCO along its [001] direction

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by EMS software agrees with the experimental image for a sample thickness of 4 nm and defocus of 60 nm. It can be seen also that the roughness of the interface between the NCO layer and the YBCO layers is about 4 nm. The roughness of the top interface has resulted in the nucleation of some 90°-misoriented YBCO grains e.g. at A in Fig. 3a. The NCO layer is a single crystal but contains some stacking faults e.g. at S on the left side of the picture. A high resolution image of a 90°-misoriented a-axis YBCO grain in the lower YBCO layer and the neighbouring NCO layer is shown in Fig. 4a, in

a

which the top YBCO layer has been ion milled away. The electron beam direction is along the [001] direction of the YBCO layer. It can be seen in the figure that the part of the NCO layer on top of the 90 ° misoriented grain has a different orientation compared with the adjacent NCO layer. Lattice spacings of this image match the projection of NCO structure along the [100] direction, as shown in Fig. 4b indicating that a 90 ° misoriented a-axis YBCO grain in the lower YBCO layer has nucleated a 90°-misoriented a-axis NCO grain in the NCO layer. In addition, it should be noted that there are several narrow plates of different structure in the

10nm ,

Fig. 4. (a) A high resolution TEM micrograph of a 90°-misoriented a-axis YBCO grain in the lower YBCO layer and the neighbouring NCO layer; (b) a higher magnification picture of the NCO layer; and (c) a Nd203 plate with its image simulation (inset).

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misoriented NCO grain (indicated by arrows in Fig. 4a). The lattice spacings of this structure do not match those of the NCO structure so that these are not regions of NCO with different orientations. However, an image simulation of cubic Nd203 (NO) (a = 11.08) along the [110] direction, with a sample thickness of 5 nm and defocus of 40 nm gives the same structure, as shown in Fig. 4c. It is, therefore, concluded that these regions are Nd203 having the following epitaxial relationships with the NCO and YBCO: ( 0 0 I ) N o / / ( 0 0 1 ) N C O, [110]NO//[100]NCO and (110)NO//(010)YBCO, [110]NO//[100]yB¢O. This kind of epitaxial relationship has been observed both between cubic Y203 and YBCO in YBCO films [16] and between cubic Gd203 and GdBa2Cu3OT_ ~ in GdBa2Cu307_ ~ films [17]. This implies that it is more energetically favourable for oxide second phase to precipitate within a misoriented grain. This phenomenon has been observed both in GdBa2Cu307_ ~ and YBCO films [17,18]. The 90 ° c-axis misorientation may continue through all three layers of the trilayer. Fig. 5 shows a 90°-misoriented a-axis YBCO grain in the lower YBCO layer which has introduced a 90°-misoriented a-axis NCO grain into the NCO layer and this grain has further introduced a 90°-misoriented a-axis YBCO grain into the top YBCO layer. A second type of defect is seen at the right side of Fig. 5 at P. The lower YBCO layer has grown through a gap in the NCO layer. This is a so-called "pinhole" in the

Fig. 6. A high resolution micrograph showing a faulted misoriented a-axis NCO grain in the NCO layer on a 90°-misoriented a-axis YBCO grain in the lower layer.

NCO layer. Such breaks in the insulating layer are undesirable. c-axis misorientations other than 90 ° were also sometimes observed. An example is shown in Fig. 6, which shows a faulted, misoriented a-axis NCO grain in the NCO layer at N, on a 90 ° misoriented a-axis YBCO grain in the lower layer at A1. The fault has nucleated a misoriented YBCO grain in the top YBCO layer at A2 with its c-axis neither parallel nor perpendicular to the NCO layer. It should be noted that within the misoriented a-axis NCO grain, there is another NCO grain (indicated by an arrow) with its c-axis parallel to the c-axis Of the YBCO layers. 3.3. The LSGO substrate and PBCO template

Fig. 5. A high resolution micrograph showing the 90 ° c-axis misorientation continuing through all three layers of the trilayer.

In order to prevent reaction between LSGO substrate and YBCO layer, a PBCO template layer was grown between the substrate and the YBCO layer. Fig. 7 shows a 90°-misoriented a-axis YBCO grain nucleated from the c-axis oriented PBCO templatelayer between the lower YBCO layer and the LSGO substrate. It can be seen from the lattice fringes that, as expected, the c-axis of the substrate is parallel to the c-axis of the YBCO film. The interface between the PBCO layer and the substrate is quite rough ( ~ 4 nm) and there are visible amorphous areas in the PBCO layer, e.g. at A, probably due to reactions

Y.H. Li et al./Physica C 265 (1996) 328-334

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4. Conclusions

Fig. 7. A high resolution micrograph showing a 90°-misoriented a-axis YBCO grain nucleated from the c-axis oriented PBCO template layer between the lower YBCO layer and the LSGO substrate.

between the LSGO and the PBCO which suggests that the reactions were confined within the template layer. Fig. 8 shows an image of the interface between the PBCO layer and the LSGO substrate with the electron beam along the a-axis of the substrate. It can be seen again that the interface between the PBCO layer and the substrate is not very flat with roughness similar to that seen in Fig. 7 and there are some amorphous areas (,-, 20 nm), which are larger than those observed in Fig. 7.

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A HRTEM study of an Y B C O / N C O / Y B C O multilayer thin film on P B C O / L S G O substrates has shown that: (i) In-plane aligned a-axis oriented YBCO films may be grown on (100) LSGO substrates by pulsed laser deposition. The majority of the YBCO film is single crystal with the c-axis parallel to the c-axis of the substrate. However, the YBCO crystal contains domains with the c-axis misaligned by 90 ° in plane of the film. Some non-90 ° misaligned domains are also observed. (ii) A NCO insulating layer, with a thickness of approximately 10 nm, may be grown epitaxially between a-axis oriented YBCO layers with its c-axis parallel to the c-axis of the YBCO layers. The layer contains some 90°-misaligned grains and "pinholes" containing YBCO. The interface roughness between NCO and YBCO is about 4 nm. Misoriented YBCO grains may be nucleated at rough top interfaces. (iii) Narrow vertical Nd203 plates (2-5 nm thick) were observed in 90 ° misoriented NCO grains, formed epitaxially within the NCO grains. (iv) A 90°-misoriented a-axis YBCO grain in the lower YBCO layer can nucleate a 90°-misoriented NCO grain in the a-axis oriented NCO layer and this grain can further nucleate a 90°-misoriented YBCO grain in the top YBCO layer. (v) The interface between the PBCO template layer and the LSGO substrate has a roughness of ~ 3 nm and there are some amorphous islands in the template layer which suggests that the reactions were confined within the template layer. A few 90°-miso riented a-axis YBCO grains may nucleate from the template layer.

Acknowledgements This work has been supported by UK Engineering and Physical Science Research Council.

References Fig. 8. A high resolution micrograph showing the interface between the PBCO layer and the LSGO substrate with the electron beam direction along the a-axis of the substrate.

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