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Formation of Tunnel Junctions from High Temperature Superconductors

IP.com Disclosure Number: IPCOM000107838D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 145K

Publishing Venue

IBM

Related People

Ronay, M: AUTHOR

Abstract

It is extremely difficult to make tunnel junctions from the new high temperature superconductors such as YBa2Cu3O7-x, where the yttrium can be replaced by other rare earth elements. The difficulties are so severe that no tunnel junctions have been successfully made. The most severe problem is that these materials have very short superconducting coherence lengths; it is estimated to be 5 angstroms in the crystallographic c direction and 36 angstroms in the a-b plane, giving a geometric mean coherence length of 20 angstroms. For tunnelling applications the films must be of excellent quality right up to the tunnel barrier since it is the material within the thickness of the coherence length adjacent to the tunnel barrier which determines the tunneling characteristics.

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Formation of Tunnel Junctions from High Temperature Superconductors

       It is extremely difficult to make tunnel junctions from
the new high temperature superconductors such as YBa2Cu3O7-x, where
the yttrium can be replaced by other rare earth elements.  The
difficulties are so severe that no tunnel junctions have been
successfully made.  The most severe problem is that these materials
have very short superconducting coherence lengths; it is estimated to
be 5 angstroms in the crystallographic c direction and 36 angstroms
in the a-b plane, giving a geometric mean coherence length of 20
angstroms.  For tunnelling applications the films must be of
excellent quality right up to the tunnel barrier since it is the
material within the thickness of the coherence length adjacent to the
tunnel barrier which determines the tunneling characteristics.

      The second problem is that these materials are very reactive
with most insulators which may be considered for a tunnel barrier.
This problem is aggravated by the high annealing temperature required
to form the superconducting films.  Related probably to the high
annealing temperature is that the films are too rough for good
tunneling.  Another extremely severe problem facing the application
of these high temperature superconductors for tunnel junctions is
that their surface becomes insulating upon exposure to air. The cause
of this is not yet well understood; both oxygen loss and
contamination with water vapor and carbon dioxide may contribute to
the effect.

      The problems listed above require that the tunnel junctions
must be produced in an ultrahigh or at least high vacuum
environment.  The process disclosed here consists of making, without
breaking, the vacuum three coplanar layers: a superconducting layer,
a tunnel barrier and a second superconducting layer.  The patterning
takes place subsequently by any of the known techniques such as ion
beam etching, laser ablation, etc.  The patterning is not the subject
of this disclosure, only the process by which the trilayer can be
made.

      Perhaps the single metal which forms an oxide and does not
interact with the high temperature superconductor is niobium and its
oxide Nb2O5 .  The problem is that niobium does not form Nb2O5 under
the low partial pressure regime applicable to an ultrahigh vacuum
system.  Actually the oxidation of niobium is so sluggish that even
in air it requires in the order of a day to produce an oxide about
20 angstroms thick.

      The proposed process in forming coplanar
superconductor-insulator-superconductor trilayers from which tunnel
junctions can be made is given in the following. First, the process
is described for producing Nb-Nb2O5 high Tc superconductor trilayers
which avoid the problem of roughness for the bottom superconductor.
Such a junction is already sufficient for the determination of the
superconducting gap of the new superconductors,  an extremely
important task for...