Layer structure with an epitaxial, non-c-axis oriented HTSC thin

Details Layer structure with an epitaxial, non-c-axis oriented HTSC thin Layer structure with an epitaxial, non-c-axis oriented HTSC thin

1998-06-22

2000-12-05

King, Roy V.

Superconductor technology: apparatus, material, process

High temperature devices, systems, apparatus, com- ponents,...

Substrate for supporting superconductor

505701, 428930, B32B 1800

Patent

active

06156706&

DESCRIPTION:

BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage of PCT/DE 96/02475 filed Dec. 18, 1996 and based in turn on German National Applications 195 49 291.9 of Dec. 22, 1995 and 196 48 234.8 of Nov. 21, 1996 under the International Convention.


FIELD OF THE INVENTION

The invention relates to a layer sequence with at least one epitaxial non c-axis oriented high temperature superconductor thin film, especially one which is approximately oriented to the a-axis or b-axis, or a structure provided with a layer crystallographically comparable to a high temperature superconductor.
The invention relates, more particularly, to a layer sequence for components used in the field of superconductive electronics and to the production of epitaxial thin films of oxide ceramic high temperature superconductors as well as to materials with similar crystallographic structures which can be used together with high temperature superconductors in epitaxial multilayer systems. These materials have a layer structure with strongly anisotropic characteristics.


BACKGROUND OF THE INVENTION

In high temperature superconductors, the layers contain planar structural units which contain tightly bonded CuO.sub.2 planes. These planes also determine substantially the lattice constants a and b within the layer planes (for high temperature superconductors a.apprxeq.b.apprxeq.3.9 Angstroms). The layer structure ensures, for example, a high critical current density and a relatively large coherence length in the layer planes as well as a small critical current perpendicular to the layer planes along the crystallographic c-axis.
In order to produce, for example, components with planar high temperature superconductor (HTSC) Josephson contacts it has been desirable to produce very many simple layer sequences as required and to provide an access to the film surfaces of the well-conducting a, b planes of the Josephson contact electrodes (or to the later internal boundary surfaces of the Josephson contacts).
Furthermore it is important to prepare the epitaxial thin films as unidirectionally oriented domains which avoid grain boundaries. Grain boundaries between differently oriented domains give rise to undesirably weakened superconductive regions which disrupt the function of the component.
The epitaxial HTSC thin films are as a rule grown on (preferably perovskite-like) substrates with cubic or pseudocubic (slightly distorted from the cubic) crystal structures with the greatest possible matching of the lattice constants (a.sub.substrate .apprxeq.a.sub.film .apprxeq.b.sub.film).
Typically the lattice defect match ##EQU1## is less than several percent.
On (001) oriented substrates, up to now, epitaxial HTSC thin films have been deposited with layer planes perpendicular to normals to the substrate surface as is shown in FIG. 1a and which are described as c-axis oriented films, the deposition being effected at relatively high substrate temperatures which has its basis in the fact that, as a rule, only such films have the characteristics necessary for applications which require a relatively high current carrying capacity.
An orientation with the conducting planes perpendicular to the surface can be achieved only for relatively low substrate temperatures. These so-called a-axis oriented films are comprised of domains with opposite c-axes oriented perpendicularly to one another in the thin film plane. Both the low substrate temperature and the resulting grain boundaries between the domains give rise to a diminution of the superconductive characteristics of these films as is known for example from Journal of Appl. Phys., 70, (1991), 7167 or Physica C 194, (1992), 430.
Phys. Rev. B. 46, (1992), 11902 or Proceedings of European Conf. on Appl. Superconductivity (EUCAS '93), Oct. 4-8, 1993 , Gottingen, describe the growth of thin films for (101)-oriented substrates at high substrate temperatures with (103)-oriented or (103)-oriented domains, whose CuO.sub.2 planes are tilted by 45.degree. to the substrate surface. These so-called (103)-fil

REFERENCES:
Kromann et al, J. Appl. Phys. 71(7) pp. 3419-3426, Apr. 1992.
Sung et al, Appl. Phys. Lett. 67(8), pp. 1145-1147, Aug. 1995.
Homma et al, Appl. Phys. Lett. 59(11), pp. 1383-1385, Sep. 1991.
"Transition from (110) to (103)/(013) growth in Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x thin filmns on (110) SrTiO.sub.3 substrates" by Poelders et al. (Physica C 247(1995) 309-318).
"Growth mechanisms and properties of 90.degree.C. grain boundaries in Yba.sub.2 Cu.sub.3 O.sub.7 thin films" By Eom et al. (1992 The American Physical Society) vol. 46, No. 18, pp. 11902-11913.

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