Series of layers and component containing such

Details Series of layers and component containing such Series of layers and component containing such

1999-02-27

2001-02-20

Kopec, Mark (Department: 1751)

Superconductor technology: apparatus, material, process

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

Superconductor next to layer containing nonsuperconducting...

C505S163000, C505S190000

Reexamination Certificate

active

06191073

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national stage of PCT/DE97/01857 filed Aug. 27, 1997 and based in turn upon German national applications 196 34 463.8 and 196.34 645.2 both of Aug. 27, 1996 under the International Convention.
FIELD OF THE INVENTION
Our present invention relates to a layer sequence of the type in which at least one high temperature superconductor material is bonded to a nonsuperconductive layer, to a cryogenic component and multiple layer sequence characterized by the same.
The basis for components in superconductivity electronics is an epitactic multilayer system with at least one layer sequence in which the superconductive material forms boundary interfaces with nonsuperconductive materials. If the component is to be grown on a desired substrate a buffer layer can optionally be required.
BACKGROUND OF THE INVENTION
The following are known as state of the art:
1. Components of superconductivity electronics
Epitactic layer sequences or multilayer systems for such components are comprised of one or more thin films of a superconductive material and one or more thin films of nonsuperconductive material. These nonsuperconductive materials are effective as barrier materials in Josephson contacts or junctions, for passivation or as a diffusion block. Based on the characteristics of the high-temperature superconductor, the following are the requirements for the nonsuperconductors:
The high temperature superconductive and the nonsuperconductive materials must be chemically compatible. This means that no chemical reactions should occur between the materials. The nonsuperconductive material should be able to grow epitaxially on the high-temperature super-conductive material and the high-temperature superconductive material should be correspondingly able to grow on the nonsuperconductive material with, indeed, the desired crystallographic orientation. The thus-resulting boundary interfaces should be atomically sharp and should not contain any defect-oriented regions or extraneous phases in their environs. Because of the relatively high fabrication temperatures of the layers, an interdiffusion of ions cannot be excluded and hence it must be ensured that any extraneous ions which are in the material affect the properties thereof to the smallest extent possible. It is, for example, known that above all small ions like those of Al, Ga, Ti, W, Fe, and Zn, or even Ce, Pr, reduce the superconductivity of the high-temperature superconductor REBa
2
CU
3
O
7-z
(RE=rare earth element).
This applies inter alia also to the oxygen content and the ordering of the oxygen atoms in the high-temperature superconductors in which the superconductivity is weakened by oxygen loss or oxygen disordering. A high degree of chemical compatibility is required when the nonsuperconductive material is used, for example, as thin barriers in Josephson junctions. Because of the reduced coherency lengths of high-temperature superconductors—typically in the range of 1 nm to 2 nm—it is required that the barrier material not have its coherency length, which is in the length range of the ordering parameter of the superconductive electrodes in the proximity of the boundary interface, reduced for example by ion diffusion or lattice dislocation. Up to now no material has been known which fulfills this requirement satisfactorily.
Materials research has been concentrated on two material classes. One class is oriented upon the structure of REBa
2
Cu
3
O
7-z
. The research here is in targeted replacement and doping with one or more ion types which reduce the superconductive properties or even completely suppress them. The second class encompasses Perovskite or Perovskite-like compounds. From each class at an appropriate location, one material will be discussed.
The nonsuperconductive material PrBa
2
Cu
3
O
7-z
differs chemically from YBa
2
Cu
3
O
7-z
only by the substitution of Y by Pr which effects the loss of suuperconductivity. The lattice-defect match with YBa
2
Cu
3
O
7-z
amounts to only 1%. By comparison to most of the other, hitherto researched, nonsuperconductive materials, PrBa
2
Cu
3
O
7-z
has the highest degree of chemical and structural compatibility with YBa
2
Cu
3
O
7-z
. For example, a monolayer of YBa
2
Cu
3
O
7-z
which serves as the intermediate layer in a PrBa
2
Cu
3
O
7-z
matrix, has a critical temperature {T
c
} of 30 K (T. Terashima et al,
Phys. Rev. Lett.
67, 1362 (1991)).
Similar experiments with other nonsuperconductive materials show that these values cannot be attained with any other material. Substantial disadvantages of PrBa
2
Cu
3
O
7-z
are, however, its relatively low specific resistance, which makes it less than satisfactory for insulation purposes, and the reduction of the ordering parameter at the boundary interface as a result of diffusion of Pr ions into the YBa
2
Cu
3
O
7-z
. If one replaces for example only 5% of the Y atoms in YBa
2
Cu
3
O
7-z
by Pr atoms, the critical temperature is already reduced from 92 K to 85 K (M. S. Hedge, et al.,
Phys. Rev.
B 48, 6465 (1993)).
A typical representative of the second class of Perovskite-like compounds is SrTiO
3
. This material has a cubic crystal structure whose lattice-defect matching to YBa
2
Cu
3
O
7-z
amounts to 1.2%. The specific resistance at 200 bMcm is clearly greater than that for PrBa
2
Cu
3
O
7-z
. It shows that with this material a heteroepitaxy is possible with YBa
2
Cu
3
O
7-z
. The chemical compatibility of the materials is, however, limited. The diffusion of Ti ions as well as their incorporation in the Cu sites of YBa
2
Cu
3
O
7-z
gives rise to a reduction of the ordering parameter in the vicinity of the boundary interface. Furthermore, the lattice distortion in the YBa
2
Cu
3
O
7-z
resulting from the boundary interface with the SrTiO
3
, reduces the ordering parameter in a noticeable manner.
2. Buffer layers
Application-oriented requirements can make it desirable to grow high-temperature superconductor thin layers or a component containing such a layer on a substrate which is not suitable, e.g. from the aspect of chemical compatibility. Examples of this are the materials silicon and sapphire. Both react in an undesired manner chemically with the high-temperature superconductor YBa
2
Cu
3
O
7-z
.
To form an epitaxy on these substrates, one or more so-called buffer layers are introduced which are disposed between the substrate and the thin layer/thin-layer system. Buffer layers are used to generate smoother surfaces of the high- temperature superconductor layer on certain substrates.
For SrTiO
3
buffer layers a so-called leveling effect is observed. That means that, when SrTiO
3
is grown on an atomic stage containing YBa
2
Cu
3
O
7-z
surfaces, it covers these surfaces and forms after several nm in thickness, a smooth [100] surface. This surface is then suitable for the c-axis-oriented growth of YBa
2
Cu
3
O
7-z
. A buffer layer serves in general the purpose of improving certain characteristics of a thin layer on a given substrate.
The requirements as to the quality of buffer layers are comparable to those of the nonsuperconductive layers for components. It is conceivable to take into consideration certain drawbacks of a buffer layer. For instance a local reduction of the ordering parameter at the boundary interface between substrate at high temperature superconductor can be expected to a certain extent when the layer thickness of the superconductor is greater than, for example, 30 nm.
For silicon, for example, yttrium-stabilized zircon (YSZ) can be used as a buffer layer. The lattice-defect matching of YSZ is relatively high at up to 6%. The chemical compatibility is only limited. It has been observed that at the boundary interface BaZrO
3
develops which, as an extraneous phase, reduces the ordering parameter of the YBa
2
Cu
3
O
7-z
. In addition, there is a diffusion of Zr to the Cu sites with the result that a reduced ordering parameter is communicated to the superconductor.
If YBa
2
Cu
3
O
7-z
is directly sputtered onto a sapphire substrate, BaA

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