YBCO 113 orientation

Fig. 14.22. Schematic illustration of 103 YBCO on (110)SrTiO3 substrates where the four-fold symmetry of a (001)SrXiO3 substrate interface is avoided. In general, the (IIO)YBCO orientation, see Fig. 14.9(a), is the most desired type of film sinee 103 YBCO exhibits domain structures.

Figure 4. An XRD 6-26 scan, along with a schematic representation, of a YBCO-YSZ-Ce02-Ni multilayered structure. The XRD scan shows the out-of-plane (001) orientations of the YBCO and oxide buffer layers.

Fig. 11.2. Z-contrast image of an asymmetric section of a YBCO 30° [001] tilt boundary. For this boundary, grown on a similarly oriented SrXi03 substrate, an asymmetric boundary plane was the predominant feature.

In contrast to c-axis films, YBCO films synthesized with an a((>)-axis or (103) normal orientation exhibit a domain or grain structure [12.16-12.19]. This is defined by two symmetrically equivalent orientations of the c-axis at 90° to each other. In the case of a-axis films, the c-axis lies completely in the film plane in two orthogonal directions for (103) films the c-axis is at approximately 45° out of the plane. This results in an array of 90° [100] or... 

The vast majority of the reported work on YBCO thin films concerns c-axis films since the growth kinetics and surface energies usually promote this orientation [14.8-14.10]. This is illustrated in Fig. 14.3 where the surface is assumed not to interact with the YBCO film. The YBCO spontaneously forms platelets with the c-axis along the short axis of the plates. The a- and Z -axes are the rapid growth directions and the low-energy surfaces are (001), (100) and (010). The preferred orientation with respect to the substrate can be manipulated by the interaction between the film and the surface on which it is growing. 

Fig. 14.4. Schematic representation of the epitaxial orientation relationships for YBCO on (001) SrTiOs and (001) cubic LaAlOs substrates.

An epitaxial film is strained in the initial stages of film growth. The strain energy increases with film thickness and may eventually be relaxed by the introduction of misfit dislocations [14.32-14.35], see Fig. 14.7, or by formation of (110) twins in the YBCO [14.36]. The critical thickness at which the misfit dislocations form depends on the lattice mismatch and the elastic properties of the film. The misfit in epitaxial c-axis-oriented YBCO films is accommodated by the formation of twins and edge dislocations with Burgers vectors [100]ybco and [010]ybco [14.37],... 

An alternative stress relief mechanism is crack formation and propagation [14.42]. The phenomenon is, for example, observed in (110) oriented YBCO films on (110)SrTiO3 substrates [14.17] see Fig. 14.9. In this system, misfit dislocations are formed at the deposition temperature while the cracks are introduced during cooling to room temperature. The crack spacing, /, depends on the film thickness, h, according to the expression... 

Fig. 14.11. Schematic illustration of four techniques to introduce grain boundaries in YBCO thin films, (a) Bi-crystal substrate technique, (b) Bi-epitaxial technique where a template layer is used to change the epitaxial orientation of the YBCO film with respect to the substrate, (c) Step-edge on the substrate, (d) Surface modification.

Fig. 14.13. (a) The [001] YBCO aligns with the substrate surfaee normal on YSZ substrates, (b) The bi-erystal substrate is thermally etched during heat treatment and a groove develops at the substrate boundary. Its presence can locally change the orientation of the YBCO. 

There are also similarities between the YBCO boundaries obtained on Y— Zr02 and SrTiOs bi-crystal substrates. They both exhibit wavy YBCO boimd-ary morphologies with (100), (010) and (110) facets. Their formation mechanism is the same in both systems. It is thus likely that the interaction between the impinging species and the nucleated YBCO is stronger than that with the substrate. Otherwise, the bottom part of the YBCO grain boundary plane would coincide with the position of the substrate boundary. MgO substrates exhibit similar characteristics to Y— Zr02 in terms of the [OOIJybco orientation with respect to the substrate normal. There is, however, no chemical reaction observed at the MgO/YBCO interface. It is thus reasonable to assume that YBCO films grown on MgO bi-crystal substrates will have the same principal... 

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