PhysicaC 175 (1991) 381-385

North-Holland

Crystal growth of YBa2Cu307_-x at the MgO ( 100) surface steps

Hiromi Takahashi, Yuji Aoki, Toshio Usui, Rainer Fromknecht, Tadataka Morishita and

Shoji Tanaka Superconductivity Research Laboratory, ISTEC, IO-13 Shinonome I-Chome, Koto-ku, Tokyo 135, Japan

Received 17 December 1990

We have investigated the crystal growth of YBazCu,07_, thin films on MgO( 100) with particular interest on the growth at

substrate surface steps. Films were deposited by the activated reactive co-evaporation method at a substrate temperature of 550°C.

Cross-sectional transmission electron microscopy was used to study the microstructure. The film is composed primarily of grains

oriented with the c-axis normal to the substrate surface and a small volume fraction of grains with the a-axis normal to the

substrate surface. This a-axis oriented grain grows at the substrate surface steps. Furthermore, a very small fraction of off-axis

grain grows at the substrate surface steps or microcracks. At both sides of the grain at the edge of an 8 A step, cationic stacking is

matched above the step. Many microstructure differences are observed at the substrate surface steps.

Since the discovery of superconductive Y-Ba-Cu- 0 and Bi-Sr-Ca-Cu-0 materials, many deposition techniques have been investigated to apply these ma- terials for device application. The quality of thin films will depend on the grain structure and orientation of the deposited superconducting material. Some de- vice applications require a surface roughness in the atomic scale of the epitaxial films. Therefore, the mi- crostructure at the substrate surface is important. The high resolution electron microscopy (HRTEM) is a powerful tool to study the microstructures in thin films. TEM observations were made in a microscope capable of achieving a 1.7 8, point to point resolu- tion operating at 400 kV. The sample preparation technique for HRTEM analysis was the normal

method. At YBa&u@_, thin films, several crystal prop-

erties were investigated by the high resolution trans-

mission electron microscopy (HRTEM). Its struc- tural defects were estimated as 248, 247, and 224, 245 compounds and edge dislocations and antiphase boundaries [ l-71. However, the interface of YBa2Cux0,_, and substrate have scarcely been in- vestigated. Recently, the epitaxy of a thin Y-Ba-Cu- 0 superconductor deposited by the pulse laser de- position on a single-crystal MgO( 100) was exam- ined by HRTEM [ 81. They observed a periodic ar-

ray of interfacial dislocation on the smooth

MgO ( 100 ) surface. Ordinary used MgO substrate is mechanically polished which include a lot of steps

and microcracks. To gain the understanding of crys- tal growth at the substrate surface steps and micro-

cracks, we investigated by the TEM. Films were deposited on the MgO( 100) substrate

at a set temperature of 550°C in oxygen plasma by

co-evaporation of Y, Ba and Cu. Oxygen gas was in-

troduced into a chamber through a stainless steel ring nozzle, below and 10 cm apart from the substrate, to

oxidize the metal source. The oxygen pressure dur-

ing deposition was measured as 2.0 x 1 0P4 Torr by an ionic gauge, being apart from the substrate. After

deposition, the films were in situ annealed at 550°C in oxygen plasma for 30 min, and cooled down to room temperature in oxygen plasma. The film thick- ness was about 1000 8, and the average growth rate was 0.2 A/s. The procedure results in an Tc,,,,t of

85 K and Tc,end of about 60 K. The X-ray diffraction

method, by a 0-20 method, was used and showed

a good c-axis orientation. Also the rocking curve at 006 reflection with 0.6 degree (FWHM) leads to the conclusion for a highly c-axis oriented film [ 91. This analysis method is widely used for the sample char- acterization and process improvements. In contrast to these conclusions, the HRTEM investigation

092 l-4534/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)

382

Fig. I. Low magnification TEM image of the c-axis oriented YBaZCuzO,_ I; film.

showed numerous differences in microstructures and posited primarily of grains oriented with the c-axis gives an insight to the interface between the sub- normal to the substrate surface and a small volume strate and YBa,Cu,O,_ ~ film. fraction of grains with the u-axis normal to the sub-

Figure 1 shows a low magnification cross-sectional strate surface. It is interesting to note that the surface

TEM image of the film. This image shows the MgO step on MgO surface is clearly visible (as marked by

substrate in the [ 0101 zone axis. The film is com- an arrow) in fig. 2(b) and is seen to be the starting

Fig. 2. (a) Selected area diffraction pattern. (b) High magnification TEM image of the a-axis oriented grain

Fig. 4. [OlO] atomic resolution image of the MgO substrate. Surface steps and microcracks are observed as marked by arrowheads

C and D.

point of an u-axis oriented grain. Its height is about 15 A. This grain exhibits a good crystal habit at the film 6 au-face and its width is about 500 A. The height of a-a lxis oriented grain is 1100 8, which is larger than that I of the c-axis oriented grain of 1000 A. This

H. Takahashi et al. / Crystal growth of YBa2CuJ07_, at the MgO(lO0) surface steps.

Fig. 3. Scanning electron micrographs of the c-axis oriented film surface.

growth rate difference is explained by the surfaa : en- ergy of the a-b and b-c plane [ 7 1. In this a-axis ori- ented grain, the b-axis is almost aligned to MgO( 010) direction as shown by the selected area diffraction pattern in fig. 2(a). The contrast be-

A, B,

384

Fig. 5. High magnification TEM image of the area as marked by arrowheads A and B.

tween the strain due to the formation of u-axis ori- ented grain and c-axis oriented grain is visible in fig. 2(b). Figure 3 shows the scanning electron micro- graphs of the same sample. The normal direction of the substrate surface is inclined 40” compared to the incident beam direction, and the cross section is a cleaved MgO substrate. In fig. 3, many grains which are aligned to MgO( 010) can be seen, and their width and aligned direction compared to the MgO substrate are closely matched with the u-axis ori- ented grain observed by the TEM. The details of the a-axis oriented grain growth mechanism will strongly depend on the substrate temperature (550”(Z), the deposition rate (0.2 A), in situ annealed time (30 min) and the deposition method (activated reactive co-evaporation ).

An u-axis oriented grain growth at the substrate step is clearly observed in the c-axis oriented film. Furthermore. small angle off-axis grains can be seen at the substrate surface steps and microcracks. Fig- ure 4 shows the [ 0101 atomic resolution image of the substrate and Y BazCu30, _ , film interface of the same sample as in fig. 1. The MgO surface include many steps and microcracks which are marked as A,

B, C and D. Figure 5 shows a high magnification TEM image of the areas marked as A and B. At the position A (arrowhead A) which has steps or cracks by polishing, the grain that is slightly inclined to the c-axis grows from the surface. At position B (arrow- head B), the height of step is 8 A. The mismatch of the lattice constant is reduced by the defect (as marked by arrowheads in fig. 5). Also, from a dis- tance of 8 A from the substrate surface, no defects can be seen. At the edge of the step, the Cu-0 chain is continued. Furthermore, at the position C and D (arrowheads C and D), the a-h plane of two grains beside this steps is curved in the lower substrate sur- face. At the center of these two steps, grain boundary (GB) exists as shown in fig. 4.

In summary. some microstructures in YBa,Cu? O,_, thin films on the MgO substrate were inves- tigated by the transmission electron microscopy (TEM ). YBazCuX07_, thin films were deposited at 550°C. The u-axis oriented symmetric grains grew from the MgO substrate surface step. And off-axis grains grew from the damaged area of the substrate surface. Furthermore. steps produce the grain boundary except the 8 A height step.

H. Takahashi et al. / Crystal growth of YBa2Cuj0,_ r at the MgO(lO0) surface steps. 385

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