Method for forming Josephson junction devices by radiation

A method for forming an insulating layer in an oxide high-temperature superconductor is described. The oxide high-temperature superconductor is exposed to radiation, whereby an interface showing superconducting characteristics and/or a weak link that is present at an interface in the said oxide high-temperature superconductor is transformed to a thin insulating layer.

FIELD OF THE INVENTION 
This invention relates to a method for forming an insulating layer in oxide 
high-temperature superconductors by irradiation. The insulating layer is 
suitable for the manufacturing electronic devices such as Josephson 
junctions that are composed of oxide high-temperature superconductors. 
BACKGROUND OF THE INVENTION 
Conventional metal or compound superconductors that are used as Josephson 
devices of a tunnel junction include those which have niobium pentoxide 
(Nb.sub.2 O.sub.5), silicon (Si) or aluminum oxide (Al.sub.2 O.sub.3) 
connected as an insulating layer between two niobium (Nb) layers, as well 
as those which have amorphous silicon or magnesium oxide (MgO) connected 
as an insulating layer between two niobium nitride (NbN) layers. 
Since the oxide high-temperature superconductor has a short coherence 
length, the fabrication of tunnel junction type Josephson devices requires 
providing a thin (&lt;100 nm) and uniform insulating layer. Such insulating 
layers can be prepared fairly easily by oxidizing the surface of metals, 
thus currently fabricated Josephson devices of the tunnel junction type 
have and insulating metal oxide layer sandwiched between an oxide 
high-temperature superconductor and a metal superconductor. 
Tunnel junction type Josephson devices having an insulating layer 
sandwiched between oxide high-temperature superconductors are also 
available and one example is a device that has a barrier PrBa.sub.2 
Cu.sub.3 O.sub.x sandwiched between two YBa.sub.2 Cu.sub.3 O.sub.x layers. 
In this device, PrBa.sub.2 Cu.sub.3 O.sub.x forming the barrier layer is 
in a rhombic system that is a different crystal system than the 
superconducting phase and, hence, nonuniformity of the critical current 
density along the barrier layer may result. To solve this problem, a 
Josephson device has been fabricated in which niobium (Nb) doped 
SrTiO.sub.3 is sandwiched between YBaCuO superconductors. 
The Josephson devices described above that use oxide high-temperature 
superconductors are fabricated by epitaxial growth, so the combinations of 
materials from which the devices can be manufactured are very limited from 
the viewpoint of matching between the superconductor and the barrier layer 
in terms of electrical and crystallographic characteristics. 
It is known that an irradiated part of an oxide high-temperature 
superconductor will turn into an insulator. S. Matsui et al. reported in 
J. Vac. Sci. Technol., B6 (3), 900 (1988) that an electronic circuit could 
be fabricated by applying a focused ion beam spot onto an oxide 
high-temperature superconductor so that the illuminated area would become 
an insulator in a width of no more than 1 .mu.m. However, as noted 
hereinabove, the thickness of the insulating layer must be controlled to 
less than 100 nm in order to fabricate a Josephson device of the tunnel 
junction type. In other words, it is very difficult to fabricate the 
desired tunnel junction type Josephson device by the FIB (focused ion 
beam) method. 
On the other hand, it is possible to fabricate a Josephson device of a 
bridge type by the FIB method. As a matter of fact, A. E. White et al. 
reported in Appl. Phys. Lett., 53 (11), 1010 (1988) that the Josephson 
effect was observed in a bridge type sample prepared by the FIB method. 
However, no one has ever reported the case of successful fabrication of a 
tunnel junction type Josephson device by irradiation of an oxide 
high-temperature superconductor. 
SUMMARY OF THE INVENTION 
An object, therefore, of the present invention is to provide a method by 
which a thin insulating layer can be formed in any kind of oxide 
high-temperature superconductors to fabricate Josephson junctions and 
other devices to be used in electronics. 
Another object of the present invention is to provide a process for 
fabricating an electronic device such as a Josephson junction that uses an 
oxide high-temperature superconductor as a component. 
The present inventor conducted intensive studies in order to solve the 
aforementioned problems of the prior art and, as a result, found that 
radiation effects on interfaces such as grain boundaries and stacking 
faults in the oxide high-temperature superconductor would be different 
from those on the superconducting phase in other areas and that such 
interfaces were microprocessable by irradiation. The present invention has 
been accomplished on the basis of these findings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with one aspect of the present invention, there is provided a 
method in which an oxide high-temperature superconductor is exposed to 
radiation, whereby an interface showing superconducting characteristics 
and/or a weak link layer that is present along an interface in the said 
oxide high-temperature superconductor is transformed to a thin insulating 
layer. 
In accordance with another aspect of the present invention, there is 
provided a process for producing a device such as a Josephson junction 
that is to be used in electronics, said process comprising the steps of: 
a) joining together two layers of similar or dissimilar oxide 
high-temperature superconductors; and 
b) exposing the assembly to a radiation so that the plane of junction is 
transformed to a thin insulating layer. 
This process is applicable to any kind of oxide high-temperature 
superconductors. 
The "insulating layer" as used herein means a layer that will no longer 
exhibit superconductivity by itself since the regularity of the 
crystalline structure of the superconductor at issue has been disordered. 
The term "thin" as used herein means a sufficient thickness for a tunnel 
current to flow by the Josephson effect. The term "interface" as used 
herein means an area of the oxide superconductor at issue that has a 
planar extent as exemplified by a plane of junction with another material, 
the grain boundary, the twin boundary, the stacking fault or modulated 
structural boundaries and that atomic arrangement in the matrix of the 
superconductor would be disordered at the interface. The "interface" 
includes both the planar extent that exhibits superconducting 
characteristics and a "weakly linked layer". 
Oxide high-temperature superconductors usually involve interfaces as 
defined above, which are generally referred to as "planar defects". 
Macroscopically, interfaces can be regarded to have the same morphology 
but, microscopically, they do not show a completely identical morphology. 
In other words, an interface exhibits essentially superconducting 
characteristics, another is that the superconducting crystals are weakly 
linked at the interface and that is generally referred to as a "weak 
link". Although the weak link exhibits superconducting characteristics 
under a certain condition, the weak link is readily decoupled by high 
current flow. In the present invention, an oxide high-temperature 
superconductor is irradiated, whereby an interface showing superconducting 
characteristics that is present in the superconductor and/or a weak link 
that is also present at an interface in said superconductor is transformed 
to a thin insulating layer. 
The principle behind this transformation is the observation that the 
interface is different in atomic arrangement from the perfect crystal. 
Hence, by exposing an oxide high-temperature superconductor to radiation 
that is insufficient to damage the crystal structure in areas other than 
the vicinity of an interface in the superconductor. microprocessing can be 
accomplished in such a way that only the interface is selectively 
transformed to a thin insulating layer. In a preferred embodiment of the 
present invention, the insulating layer has a thickness less than 100 nm. 
The thickness of the insulating layer can be varied by properly adjusting 
the exposure dose and/or dose rate of the radiation to be applied. If the 
thickness of the insulating layer is further increased by the irradiation, 
the interface will eventually become a literal "electric insulator". 
The formation of an insulating layer at an interface in the oxide 
high-temperature superconductor of interest can be checked by measuring an 
electrical characteristics of that superconductor. Stated more 
specifically, one may safely conclude that an interfacial insulating layer 
has formed when the transport critical current density measured after 
irradiation is lower than the initial value measured before the 
irradiation. This decrease in the transport critical current density by 
irradiation corresponds to a decrease in the superconducting transition 
temperature measured with high current density on account of irradiation. 
The mechanism behind the phenomenon in which an interface that exhibits 
superconducting characteristics and/or a weak link in an oxide 
high-temperature superconductor is transformed to an insulating layer upon 
irradiation has not been fully understood. In exposure to particle 
radiations such as heavy ions, electron beams and neutrons, atoms in the 
crystal lattice sites would be "knocked out" from their lattice position 
to cause an atomic disordering, eventually leading to the formation of an 
insulating layer at the interface. On the other hand, in exposure to light 
(electromagnetic waves) such as .gamma.-rays, X-rays and ultraviolet rays, 
the position of oxygen atoms would be changed by exciting lattice 
vibrations, thus forming an interfacial insulating layer. 
The dose rate of an exposured radiation is a critical factor in insuring 
that only an interface in the oxide high-temperature superconductor is 
transformed to an insulating layer having a suitable thickness. Stated 
more specifically, with a given exposure dose, a short irradiation will 
suffice if the dose rate is high but, although difficulty is sometimes 
involved in controlling the thickness of the insulating layer being 
formed. If the dose rate is low, it is easy to control the thickness of 
the insulating layer but, on the other hand, it takes an unduly long time 
to complete the irradiation. It should also be noted here that if the dose 
rate is low, it is comparatively easy to form an insulating layer only at 
an interface in the superconductor without causing any change in areas 
other than the interface. 
In a preferred embodiment of the present invention, (Bi,Pb)Sr.sub.2 
Ca.sub.2 Cu.sub.3 O.sub.10 is irradiated with .sup.60 Co .gamma.-rays and, 
in this case, satisfactory results are obtained with the dose rate being 
in the range of ca. 1-5 Mrad/h. If the radiation is an electron beam of 3 
MeV, the dose rate may be in the range of ca. 1-100.times.10.sup.16 
m.sup.-2 s.sup.-1 ; if the radiation is He.sup.+ of 200 keV, the dose rate 
may be in the range of 1-10.times.10.sup.13 m.sup.-2 s.sup.-1 ; and if the 
radiation is N.sup.+ of 200 keV, the dose rate may be in the range of ca. 
1.times.10.sup.12 m.sup.-2 s.sup.-1. 
In another preferred embodiment of the present invention, YBa.sub.2 
Cu.sub.3 O.sub.7 is irradiated with .sup.60 Co .gamma.-rays and, in this 
case, satisfactory results are obtained with the dose rate being in the 
range of ca. 1-10 Mrad/h. If the radiation is an electron beam of 3 MeV, 
the dose rate may be in the range of ca. 1-100.times.10.sup.17 m.sup.-2 
s.sup.-1 ; if the radiation is He.sup.+ of 200 keV, the dose rate may be 
in the range 1-10.times.10.sup.14 m.sup.-2 s.sup.-1 ; and if the radiation 
is N.sup.+ of 200 keV, the dose rate may be in the range of ca. 
1-10.times.10.sup.12 m.sup.-2 s.sup.-1. 
When oxide high-temperature superconductors are irradiated, the thickness 
of the insulating layer being formed will usually increase with the 
exposure dose. As already mentioned hereinabove, the primary purpose of 
the present invention is to form an insulating layer at an interface in an 
oxide high-temperature superconductor that is thin enough to permit the 
flow of a tunnel current under the Josephson effect. A desired thin 
insulating layer cannot be obtained if excessive radiations are applied. 
Guide figures for the exposure dose of radiations that is appropriate for 
attaining the objects of the present invention are as follows. In the case 
of irradiating (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 with 
.sup.60 Co .gamma.-rays, the total dose is preferably in the range of ca. 
1-25 Mrad; if the radiation is an electron beam of 3 MeV, the total dose 
is preferably in the range of ca. 1-100.times.10.sup.19 m.sup.-2 ; if the 
radiation is He.sup.+, the total dose is preferably in the range of 
1-10.times.10.sup.13 m.sup.-2 ; and if the radiation is N.sup.+ of 200 
keV, the total dose is preferably in the range of ca. 1-10.times.10.sup.11 
m.sup.-2. In the case of irradiating YBa.sub.2 Cu.sub.3 O.sub.7 with 
.sup.60 Co .gamma.-rays, the total dose is preferably in the range of ca. 
1-100 Mrad; if the radiation is an electron beam of 3 MeV, the total dose 
is preferably in the range of ca. 2-5.times.10.sup.22 m.sup.-2 ; if the 
radiation is He.sup.+ of 200 keV, the total dose is preferably in the 
range of 1-50.times.10.sup.15 m.sup.-2 ; and if the radiation is N.sup.+ 
of 200 keV, the total dose is preferably in the range of ca. 
1-50.times.10.sup. 13 m.sup.-2. 
The oxide high-temperature superconductor that can be used in the present 
invention include Y-system which contain yttrium as an essential 
constituent element, Bi-system which contain bismuth as an essential 
constituent element, T1-system which contain thallium as an essential 
constituent element, and any other compounds that have those chemical 
compositions which exhibit superconductivity. 
The superconductor to be used in the present invention may be prepared by 
sintering, melting, deposition on substrates and any other methods. These 
superconductors may be of any shape or form, as exemplified by a 
polycrystalline form, a single-crystal form, a thin film or a thick film. 
In short, the oxide high-temperature superconductors to be used in the 
present invention are not limited in any way as regards the production 
process, shape or form. 
The radiations to be used in the present invention include, but are not 
limited to, electromagnetic waves such as .gamma.-rays, X-rays and laser 
light, and particle radiations such as electrons, positrons, 
.alpha.-particles and heavy ions. Such radiations may be generated by any 
methods including, for example, the use of a radiation generator, an 
accelerator or a nuclear reactor, and radioactive decay. 
As described above, the radiations to be applied in the present invention 
are not limited to any particular type. However, particularly good results 
were obtained when .gamma.-rays, electron beams or heavy ions were used. 
In the most preferred embodiment of the present invention, a sintered 
(Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 superconductor is 
irradiated with .sup.60 Co .gamma.-rays for a total dose of ca. 1-25 Mrad 
at a dose rate of ca. 1-5 Mrad/hr, thereby forming a thin insulating 
layer. 
In accordance with the other aspect of the present invention, there is 
provided a process for fabricating an electronic device such as a 
Josephson junction that uses an oxide high-temperature superconductor as a 
component. 
According to this process, two layers of similar or dissimilar oxide 
high-temperature superconductors are joined together. The two layers can 
be joined together by heat treating the two superposed single crystals of 
oxide high-temperature superconductor at an appropriate temperature. 
However, this is not the sole case of the present invention and an 
interface may be sandwiched between two evaporated layers of a 
superconductor; this method has conventionally been adopted in the 
fabrication of Josephson junctions. The plane of junction of the two 
superconductive layers provides an interface of the type described above 
and a thin insulating layer is formed at this interface by applying a 
radiation to the assembly. As a result, a Josephson junction is produced 
as an assembly of two layers of oxide high-temperature superconductor 
having a thin insulating layer sandwiched therebetween. In a particularly 
preferred embodiment of the present invention, two layers of a sintered 
(Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 are joined together and 
.sup.60 Co .gamma.-rays are applied to the resulting interface for a total 
dose of ca. 1-25 Mrad at a dose rate of ca. 1-10 Mrad/hr, thereby forming 
a thin insulating layer at that interface. 
The present inventors made a close study on the radiation effects to the 
superconducting transition temperature. As a result, they found that in 
the early period of exposure (i.e., with low-dose exposure), the 
superconducting transition temperature measured with high current density 
became higher than before the exposure. The transition temperature 
decreased upon further exposure to the radiation. This phenomenon was 
noticeable when exposure was made to particle radiations such as electron 
beams or heavy ions. As the result of further studies, it became clear 
that the phenomenon under consideration occurred because in the early 
period of exposure to radiation, the weak link present at an interface in 
an oxide high-temperature superconductor was transformed, partly to be 
superconducting phase. Upon further exposure to the radiation, the 
interface would be transformed to the insulating layer described above. 
Therefore, if utmost care is taken to prepare an ideal oxide 
superconductor which does not involve a weak link, the above-described 
increase in the transition temperature will not be observed in the early 
period of exposure to a radiation. 
The oxide high-temperature superconductor having the weak link and the high 
quality superconductor having no such weak link will behave differently in 
the early period of exposure to radiation but the technical rationale of 
the present invention will not be lost by this fact. In other words, a 
thin insulating layer will eventually be formed at an interface in oxide 
high-temperature superconductors regardless of their type if they are 
subjected to sufficient exposure to radiation. 
As described on the foregoing pages, the method of the present invention 
which is characterized by forming an insulating layer in the oxide 
high-temperature superconductor through exposure to a radiation is 
applicable to any type of oxide high-temperature superconductors. Hence, 
the present invention will open a wide door to the electronics application 
of oxide high-temperature superconductors as exemplified by the 
fabrication of Josephson junctions which will work at the temperature of 
liquid nitrogen. 
The following examples are provided for the purpose of further illustrating 
the present invention but are in no way to be taken as limiting. 
EXAMPLE 1 
A sintered Bi.sub.1.5 Pb.sub.0.5 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 pellet 
measuring ca. 10 mm.times.1.5 mm.times.0.5 mm was prepared and exposed to 
.sup.60 Co .gamma.-rays at a dose rate of 1.5 Mrad/hr. After exposure to 
.gamma.-rays, the electric resistance of the pellet was measured with high 
current density to investigate the radiation effects on the transition 
temperature. The current density was 115 kA/m.sup.2. The transition 
temperature of 97.5K before exposure to .gamma.-rays was decreased to 
97.0K upon exposure of .gamma.-rays to a total dose of 2 Mrad. Upon 
further exposure of .gamma.-rays to a total dose of 7.5 Mrad, the 
transition temperature further decreased to 92.1K. This verified the 
formation of a thin insulating layer along the interface in the pellet. 
A pellet was prepared in the same manner as described above and the 
transport critical current density was measured over the temperature range 
from 20K to 90K. As a result, the transport critical current density was 
found to decrease at temperatures below 65K after exposure of .gamma.-rays 
to a dose of 2 Mrad. Upon further exposure of .gamma.-rays up to a total 
dose of 24 Mrad. Upon further exposure of .gamma.-rays up to a total dose 
of 24 Mrad, the transport critical current density in the low temperature 
regime decreased with an increase in the dose and, at the same time, the 
upper limit of temperatures where the radiation effect was observed became 
higher with the increase in exposure of .gamma.-rays. This was another 
evidence that verified the formation of a thin insulating layer at the 
interface in the pellet by the exposure to .gamma.-rays. 
EXAMPLE 2 
A sintered (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10 pellet 
measuring ca. 10 mm.times.1.5 mm.times.0.5 mm was prepared and exposed to 
electron beams of 3 MeV for a total dose of 0.5-50.times.10.sup.19 
m.sup.-2. The dose rate of electron beams was 0.8-30.times.10.sup.15 
m.sup.-2 s.sup.-1. After exposure to electron beams, the electric 
resistance of the pellet was measured with high current density to 
determine the superconducting transition. The current density was 85.5 
kA/m.sup.2. The transition temperature of 103.1K before exposure to 
electron beams increased to 105.3K upon exposure of electron beams to a 
total exposure of 1.0.times.10.sup.19 m.sup.-2. Upon further exposure of 
electron beams to a dose of 4.0.times.10.sup.19 m.sup.-2, the transition 
temperature decreased to 100K. This verified the formation of a thin 
insulating layer at the interface in the crystal of the pellet. 
EXAMPLE 3 
A sintered YBa.sub.2 Cu.sub.3 O.sub.7 pellet measuring ca. 10 mm.times.1.5 
mm.times.0.5 mm was prepared and exposed to electron beams of 3 MeV for a 
total dose of 0.5-3.times.10.sup.22 m.sup.-2. The dose rate of electron 
beams was 4.4.times.10.sup.17 m.sup.-2 s.sup.-1. After exposure to 
electron beams, the electric resistance of the pellet was measured with 
high current density to determine the superconducting transition 
temperature. The current density was 100 kA/m.sup.2. The transition 
temperature of 88.6K before exposure to electron beams increased to 90.4K 
upon exposure of electron beams to a total exposure of 1.5.times.10.sup.22 
m.sup.-2. Upon further exposure of electron beams to a total dose of 
3.times.10.sup.22 m.sup.-2, the critical temperature decreased to 89K. 
This verified the formation of a thin insulating layer at the interface in 
the pellet. 
EXAMPLE 4 
A sintered YBa.sub.2 Cu.sub.3 O.sub.7 pellet measuring ca. 10 mm.times.1.5 
mm.times.0.5 mm was prepared and exposed to nitrogen ions (N.sup.+) of 200 
keV. The nitrogen ions had been accelerated with an ion accelerator. The 
dose rate of nitrogen ions was 0.3-6.0.times.10.sup.13 m.sup.-2 s.sup.-1. 
Before and after the exposure to nitrogen ions, the electric resistance of 
the pellet was measured with high current density to determine the 
superconducting transition temperature. The current density was 100 
kA/m.sup.2. The transition temperature of 87.9K before exposure to 
nitrogen ions increased to 90.9K upon exposure to nitrogen ions for a 
total exposure of 8.4.times.10.sup.-3 m.sup.-2. Upon further exposure of 
nitrogen ions to a total dose of 1.times.10.sup.15 m.sup.-2, the critical 
temperature decreased to 89K. This verified the formation of a thin 
insulating layer at the interface in the pellet. 
The insulating layers formed in Examples 1-4 by irradiation were stable to 
heat cycles between room temperature and 20K, as well as under prolonged 
standing at room temperature.