JP2831967B2 - Superconducting element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting element using an oxide superconductor, and more particularly to a superconducting element useful as a Josephson element in which superconducting electrodes are connected by a normal or insulating barrier layer.

[0002]

2. Description of the Related Art Conventionally, a tunnel type Josephson junction having a multilayer structure in which a superconducting integrated circuit is made of a metal superconductor such as Pb or Nb and an insulating layer thin enough to allow a superconducting pair to tunnel is sandwiched therebetween. It has been known. Such a conventional tunnel-type Josephson device is:
Cryogenic operation near liquid helium temperature is needed. In addition, since it exhibits current-voltage characteristics having hysteresis peculiar to a tunnel-type Josephson junction, it has a problem such as a complicated circuit configuration, and has not yet been widely used.

On the other hand, as a Josephson junction element using a metal superconductor and having no hysteresis characteristics, a so-called bridge-type junction in which a main electrode made of a metal superconductor is connected with a thin and thin metal laminated with the main electrode is developed. Is also underway. However, such a bridge-type junction requires cryogenic operation close to the liquid helium temperature, a low resistance of the bridge portion, and a superconducting gap of the metal superconductor, as in the case of the tunnel-type junction described above. Because of its small size, it is difficult to obtain a large output voltage, and it has not sufficiently contributed to the industry.

Under these circumstances, recently, oxide superconducting materials exhibiting superconducting properties at a high temperature equal to or higher than the temperature of liquid nitrogen have been discovered and have attracted much attention. Oxide superconductors have larger superconducting gaps than conventional metal superconductors.
It has the potential to be laminated with an oxide material which is about an order of magnitude larger and exhibits a wide range of characteristics from metallic conduction to insulator.
Using such an oxide superconductor, a tunnel type Josephson junction or an SNS (superconducting-normal-superconducting) type Josephson junction having a large output voltage and no hysteresis is manufactured to form an integrated circuit. If it is possible, there is no need to operate at least at cryogenic temperatures compared to conventional integrated circuits composed of Josephson junctions using metal superconductors, and because it can operate much faster due to the large output voltage Widespread application is expected.

[0005] Josephson junctions using oxide superconductors have been broadly divided into two directions. One is to use the fact that the crystal grain boundaries generated inside the oxide superconductor thin film function as a barrier.
One is a method in which a thin film of a normal conductor or an insulator is artificially inserted between two superconducting electrodes to form a Josephson junction, as in the case of a conventional metal superconductor. The former is relatively easy to fabricate, but has poor control of the junction characteristics due to the use of amorphous crystal grain boundaries, and is difficult to apply to integrated circuits. For this reason, the development of the latter junction using various barrier materials has been activated.

In bonding in which an oxide superconductor and a barrier material are laminated, attention must be paid to the relationship between the bonding surface and the orientation of the crystal axis of the oxide superconductor due to the anisotropy inherent in the oxide superconductor. You need to pay. In an oxide superconductor, the electrons in the superconducting state are confined in a Cu ₂ plane perpendicular to the c-axis of the crystal axis, so that the coherence length in the c-axis direction is extremely short. For this reason, in general, good Josephson characteristics cannot be obtained when the bonding surface is manufactured so as to be orthogonal to the c-axis. To avoid this problem, two types of joint structures are being studied.

One is a junction structure shown in FIG. 6, in which oxide superconductor thin films 1 and 2 are formed so that the c-axis direction thereof is parallel to the surface of substrate 3, and these oxide superconductor thin films 1 and 2 are formed.
In this case, a junction is produced by interposing a barrier layer 4 between the two. The joint having this structure is hereinafter referred to as a sandwich type.
The arrangement in which the c-axis of the oxide superconductor thin films 1 and 2 is parallel to the substrate surface includes a case where the a-axis or the b-axis of the crystal axis is orthogonal to the substrate surface and a case where the [110] axis is orthogonal to the substrate surface. And exists. In the figure, P shows a _dihedral CuΟ schematically.

Another structure is a bonding structure shown in FIG. 7, in which an oxide superconducting thin film 5 serving as a lower superconducting electrode is formed so that its c-axis is orthogonal to the surface of the substrate 3, and then a part thereof is formed. The end face 5a is etched so as to have an inclination angle with respect to the substrate surface, and a barrier layer 4 and an oxide superconductor thin film 6 serving as an upper superconducting electrode are laminated and formed on the inclined end face 5a to form a junction. This structure is hereinafter referred to as an inclined junction. Similarly, the oxide superconductor thin film 6 serving as the upper superconducting electrode is formed so that its c-axis is orthogonal to the substrate surface. In the figure, reference numeral 7 denotes an interlayer insulating film.

It has been confirmed that in the above-described joining of the two structures, Josephson characteristics can be obtained by selecting the constituent material of the barrier layer 4. In particular, in the case of the inclined junction, one dimension of the junction is determined by the thickness of the oxide superconductor thin film 5 used for the lower superconducting electrode and the inclination angle of the end face (etched surface) 5a. Since the oxide superconductor thin film 6 serving as an electrode can be continued up to the substrate surface exposed by etching,
The oxide superconductor thin film 6 can be used as it is as a wiring. These features are expected to be of great advantage in fabricating integrated circuits.

[0010]

However, a typical oxide superconductor, YBa ₂ , is used as the lower superconducting electrode.
Using a c-axis oriented film of Cu ₃ O ₇ (a thin film formed so that the c-axis of the crystal axis is perpendicular to the substrate surface; the same applies hereinafter), FIG.
When a graded junction as shown in Fig. 1 is actually fabricated, the output voltage is about twice (~ 30mV) the superconducting gap voltage expected for an ideal Josephson junction even in the cryogenic temperature range, although the Josephson characteristics are exhibited. However, it is becoming clear that the bonding characteristics vary greatly from wafer to wafer as well as from one wafer to another, even within the same wafer.

For these reasons, in the conventional inclined junction, it is necessary to stably obtain the originally expected output voltage and to suppress the variation and fluctuation of the junction characteristics. Was.

SUMMARY OF THE INVENTION The present invention has been made to address such a problem. For example, it is possible to obtain a large output voltage capable of coping with an integrated circuit with good reproducibility, and to reduce variations and fluctuations in junction characteristics. It is an object of the present invention to provide a superconducting element capable of suppressing as much as possible.

[0013]

SUMMARY OF THE INVENTION The inventors of the present invention have conducted a microscopic evaluation of the structure of an inclined junction and an investigation of the electrical characteristics in order to find the limiting factor of the output voltage and the cause of the characteristic variation in the conventional inclined junction. As a result of repeated characteristic evaluations, the following facts were first found out.

(1) In order to form an inclined end face on the lower superconducting electrode, for example, ion milling at oblique incidence is performed.
Since the structure of the oxide superconductor is destroyed in units of unit cells and the etching proceeds, as shown in FIG. 8A, the inclined end face 5a of the oxide superconductor thin film 5 when viewed microscopically.
Has a step-like form including a step portion S and a terrace portion T. The height h of the step portion S is not usually one unit cell, but rather several unit cells, and the level difference often reaches several nm. On the other hand, the width w of the terrace portion T is substantially determined by the height h of the step portion S and the average inclination of the entire inclined end face 5a, but varies depending on the height h of the front and rear step portions S.

(2) The barrier layer 4 on the inclined end face 5a of the lower superconducting electrode reflects the step shape of the base, and when a predetermined thickness is deposited, the thickness on the terrace portion T is almost equal to this. Matches. The same applies to the oxide superconductor thin film 6 as an upper superconducting electrode on the barrier layer 4. FIG. 8B schematically illustrates this state.

(3) As a result, when the inclination angle of the inclined end face 5a is generally used (<30 °), the c of the oxide superconductor thin film 5 is placed on the terrace portion T having a relatively large area.
Although a junction having the barrier layer 4 having a predetermined thickness is formed in the axial direction, in the a and b axis directions orthogonal to the c axis, the barrier layer 4 not only becomes thicker according to the inclination angle, A situation occurs in which the effective thickness of the barrier layer 4 is locally different due to the fluctuation of the width w of the terrace portion T.

Based on the above fact, the characteristics of a conventional inclined junction using a c-axis oriented film of an oxide superconductor as a superconducting electrode are analyzed as follows.

First, consider the case where the barrier material has the property of an insulator, and the mechanism for exhibiting the Josephson effect is a tunnel of a Cooper pair through a barrier layer. If there is no anisotropy in the height of the potential barrier of the barrier layer, electrons on the terrace surface tunnel in the thinnest direction of the barrier layer, that is, in the c-axis direction of the lower superconducting electrode. Electrons are expected to tunnel by selecting the direction with the highest tunnel probability from the position. Although the exact orientation depends on the shape of the barrier layer covering the step portion, it is clear that the probability in the direction perpendicular to the c-axis is limited.

That is, the direction of tunneling of electrons through the barrier layer is generally represented as shown in FIG. 9 (solid arrow A and broken arrow B). As described above, the coherence length in the c-axis direction is extremely short in an oxide superconductor due to its anisotropy. In such a case, the superconductivity is easily deteriorated from the junction interface toward the c-axis direction inside the oxide superconductor. That is, a tunnel of the Cooper pair does not effectively occur in the terrace portion T of the inclined junction (broken arrow B in the figure), and a quasiparticle tunnel that lowers the normal conduction resistance of the junction is dominant in this portion.

On the other hand, in the step portion S where an interface parallel to the c-axis is formed, tunneling of the Cooper pair is possible, but the effective barrier layer 4 becomes thicker and differs depending on the location. At the same time as the overall superconducting current is limited, its value varies. Although such a situation can be alleviated to some extent by using the barrier layer 4 having anisotropy in the height of the potential barrier, it is difficult to ensure uniformity of the covering of the step portion S. The problem of variations in junction characteristics cannot be avoided.

A similar situation occurs when a normal conductor is used as a barrier layer to obtain a Josephson junction by the proximity effect. FIG. 10 shows YBa ₂ Cu ₃ doped with impurities.
A normal conductor, such as an O ₇ material, having the same crystal structure and conduction anisotropy as the superconducting electrode, and having a significantly lower conductivity in the c-axis direction than in the direction orthogonal thereto is epitaxially grown as the barrier layer 4. FIG. 3 schematically shows a region (a hatched portion in the drawing) in which a Cooper pair extruded in the barrier layer 4 due to the proximity effect in the bonding.

Reflecting the conduction anisotropy of the barrier layer 4, the Cooper pair exudes from the step portion S in a direction perpendicular to the c-axis over a long distance, but from the terrace portion T in the c-axis direction. The oozing is extremely small. As a result, the area that can exist overlapping vertical Cooper pairs leaking from the superconducting electrodes is made to be restricted in the vicinity of the step portion S, the superconducting current i _s will flow only this portion. On the other hand, quasiparticles ( _iq ) also occur at the junction on the terrace T.
Flows, so that the normal conduction resistance of the junction is small. This means that the output voltage of the Josephson current decreases.

If the normal conductor forming the barrier layer does not have anisotropy in conduction, the output voltage of the Josephson junction drops by a slightly different mechanism. In other words, since the coherence length in the c-axis direction of the oxide superconductor is short near the interface on the terrace portion, if the Cooper pairs are extruded into the barrier layer by the proximity effect, the density of the Cooper pairs on the oxide superconductor side is reduced. It becomes significantly smaller. As a result, the junction on the terrace can carry only a small superconducting current. That is, the same result as when the anisotropic barrier layer is used is obtained.

As described above, in the conventional inclined junction using the c-axis oriented film of the oxide superconductor as an electrode, an integrated circuit using a Josephson junction is used due to miniaturization of a junction area and ease of wiring formation. Despite having many advantages in fabricating an oxide semiconductor, the output voltage decreases and the bonding characteristics vary due to the intrinsic anisotropy of the oxide superconductor and the microscopic shape formed by etching. Was found to have occurred.

In consideration of the above points, the structure of the superconducting electrode was examined repeatedly in order to improve and stabilize the characteristics of the inclined junction. As a result, the oxide superconductor formed so that the c-axis became parallel to the substrate surface was obtained. When a superconducting electrode made of a body thin film is used, even if the microscopic shape of the inclined end face is the same, the terrace surface having excellent uniformity of the thickness of the barrier layer is formed of CuO which performs superconductivity in the oxide superconductor. _Since the crystal planes are orthogonal to the _two planes, a tunnel of the Cooper pair effectively occurs from the terrace surface toward the barrier layer, or the area where the Cooper pair exudes due to the proximity effect increases, and the value of the superconducting current based on these increases. Was found to be able to be stabilized.

The present invention has been made on the basis of such findings, and the superconducting element of the present invention comprises a substrate, REBa ₂ Cu ₃ O _7-δ (where , RE are at least one element selected from rare earth elements other than Pr, and δ is a number from 0 to 0.5). so as to face substantially parallel, an oxide superconductor thin film formed on the substrate, forming on said oxide superconductor thin film
A lower superconducting electrode having an end face inclined with respect to the substrate surface, and an oxide material formed so as to cover at least the inclined end face of the lower superconducting electrode as a laminated film with the formed interlayer insulating film. A barrier layer, and an upper superconducting electrode made of an oxide superconducting thin film having a composition substantially represented by REBa ₂ Cu ₃ O _7-δ formed on the barrier layer. .

The superconducting element of the present invention is characterized in that the angle formed between the end face of the lower superconducting electrode and the substrate surface is not more than 45 degrees.

[0028]

Embodiments of the present invention will be described below.

FIG. 1 is a diagram schematically showing a configuration of a superconducting element according to one embodiment of the present invention. In the figure, reference numeral 11 denotes a substrate, and REBa ₂ Cu ₃ O _7-δ (where RE is at least one element selected from rare earth elements other than Pr;
Represents a number from 0 to 0.5), thereby forming an oxide superconductor thin film having a composition substantially represented by: RE among the elements constituting the oxide superconductor thin film may be at least one element selected from the broadly defined rare earth elements (including Y) excluding Pr, and typical examples include Y, Nd, and Gd. No. The reason why Pr is excluded from the rare earth elements is that a composition in which RE is Pr does not exhibit superconductivity.

The oxide superconductor thin film as the lower superconducting electrode 12 is formed such that the c-axis of its unit cell is parallel to the surface 11 a of the substrate 11. That is, it is an alignment film in which any one of the a-axis, the b-axis, and the [110] axis of the crystal axis is orthogonal to the substrate surface 11a. These orientations can be selected depending on the material and crystal plane of the substrate 11 to be used, and may be appropriately formed at least under the condition that the oxide superconductor thin film is favorably epitaxially grown.

The oxide superconducting thin film as the lower superconducting electrode 12 has an end surface (inclined end surface) 12a inclined at an angle α with respect to the substrate surface 11a. Such an inclined end surface 12a is formed by using a suitable mask such as a resist, for example.
The oxide superconducting thin film can be easily formed by selectively etching using a means such as ion milling so that the etched end face is exposed at an angle α with the substrate surface 11a. Actually, the interlayer insulating film 13 is formed on the oxide superconductor thin film, and etching is performed including the end face of the interlayer insulating film 13 to form the inclined end face 12a. When such etching is performed, as shown in an enlarged manner in FIG. 2, when viewed microscopically, the end face 12 a includes a step portion S composed of a surface orthogonal to the substrate surface 11 a and the substrate surface 1 a.
And a terrace portion T composed of a surface parallel to 1a. FIG. 2 shows a case in which an upper superconducting electrode 15 described later is also formed of an oxide superconducting thin film in which the c-axis is oriented parallel to the substrate surface 11a.

In the oxide superconductor, when etching is performed, removal of crystals proceeds in unit cell units, so that any one of the (100), (010), and (110) planes is formed on the terrace portion T. appear. These planes are orthogonal to the CuO ₂ plane P which is responsible for superconductivity in the oxide superconductor. On the other hand, in the step portion S, either a (001) plane orthogonal to the c-axis or a crystal plane similar to the terrace portion T appears. Substrate 1
If a material having a four-fold symmetry on the surface 11a is used as the material for forming the first material, there are two crystal grains having c-axis directions orthogonal to each other in the oxide superconductor thin film. The planes appear with equal probability and the lower symmetric substrate 11
If growth is performed with the c-axis orientation aligned using
Any one of the crystal planes appearing on the surface can be used.

The above-mentioned inclination angle α, that is, the angle formed between the end surface 12a of the lower superconducting electrode 12 and the substrate surface 11a represents the average angle of the step structure composed of the step portion S and the terrace portion T. This inclination angle α is preferably set to 45 degrees or less. By setting the inclination angle α to 45 degrees or less, regardless of the crystal plane of the step portion S, the step portion S
50% or more of the total joint area of
_The surface can be orthogonal to the _two surfaces P, and the superconducting current can flow efficiently. The lower limit value of the inclination angle α is not particularly limited, but is preferably 10 degrees or more in order to obtain a favorable effect of miniaturization of the bonding area by the inclined bonding.

A barrier layer 14 is formed on the oxide superconductor thin film as the lower superconducting electrode 12 so as to cover at least the inclined end face 12a. Specifically, the barrier layer 14 is formed from the surface 11a of the substrate 11 exposed by etching to the interlayer insulating film 13 via the oblique end surface 12a. This barrier layer 14 may be composed of either an insulator or a normal conductor.

Here, an oxide superconducting thin film as an upper superconducting electrode 15 which will be described in detail later is formed on the barrier layer 14, and it is preferable to epitaxially grow up to the upper superconducting electrode 15. It is important in obtaining a Josephson junction with characteristics. For this reason, as a material for forming the barrier layer 14, PrBa ₂ Cu ₃ O _7-δ , which is a layered perovskite oxide having no superconductivity, or C
o such REBa ₂ Cu ₃ O _7-δ in which weakened superconductivity by doping an impurity element, and further lower the superconducting electrode 12 and the pseudo such SrTiomikuron ₃ lattice-matched insulating oxide and CaRuO _3, etc. It is preferable to use a conductive oxide or the like.

[0036] Of _{these, REBa 2 Cu 3 O 7-} δ or the like doped with PrBa ₂ Cu ₃ O _7-δ and impurities having the same crystal structure and the lower superconducting electrode 12, an oxide as a lower superconducting electrode 12 It grows on the inclined end face 12a with the same orientation as the superconductor thin film. For this reason, the terrace section T
At the upper junction, due to the anisotropy of the barrier layer 14 itself,
If the barrier layer 14 is an insulator, the potential barrier is low and the tunneling probability is improved. If the barrier layer 14 is a normal conductor, a large superconducting current can be carried due to a long normal coherence length, and the junction characteristics are greatly reduced. improves. On the other hand, isotropic SrTiO ₃ or CaRuO ₃
Also when using such a method, unlike the conventional inclined junction, the terrace portion T having a large area can be used for manifesting the Josephson effect, so that the output voltage is improved.

The upper superconducting electrode 15 formed on the barrier layer 14 is made of REBa ₂ Cu like the lower superconducting electrode 12.
_It is composed of an oxide superconductor thin film having a composition substantially represented by ₃ O _7-δ . The oxide superconducting thin film as the upper superconducting electrode 15 is, like the lower superconducting electrode 12, generally a film oriented so that its c-axis is parallel to the substrate surface 11a. It is preferable from the viewpoint of easy flow. However, PrB
When a material other than a material having the same crystal structure as the lower superconducting electrode 12, such as a ₂ Cu ₃ O _7-δ or REBa ₂ Cu ₃ O _7-δ , is used, a barrier formed on the inclined end face 12a is used. In some cases, it is difficult to directly form a high-quality thin film having a c-axis parallel to the substrate surface 11a on the upper portion of the layer 14. This is because, on a material that is lattice-matched with the oxide superconductor in a quasi-lattice manner, the c-axis is set to
This is because a thin film parallel to the above does not necessarily show good characteristics unless an appropriate buffer layer is used.

In such a case, the upper superconducting electrode 15
Alternatively, an oxide superconductor thin film having a c-axis perpendicular to the substrate surface 11a may be used. In this case, although the characteristics are slightly reduced as compared with the case where the oxide superconductor film whose c-axis is parallel to the substrate surface 11a is used as the upper superconducting electrode 15, the characteristics still lower when compared with the conventional inclined junction. Can be expected to improve characteristics, and the effect of suppressing variations in characteristics can be obtained favorably. In particular, when a normal conductor is used as the barrier layer 14 to form a Josephson junction based on the proximity effect, the upper and lower superconducting electrodes 12 and 15 both have a c-axis that is parallel to the substrate surface 11a. By setting the thickness of the barrier layer 14 to about 1/2, similar characteristics can be obtained. In order to obtain the same characteristics in a conventional element in which the upper and lower superconducting electrodes are c-axis oriented, the barrier layer must be formed in accordance with the ratio of the coherence length in the c-axis direction of the oxide superconductor to the direction perpendicular thereto. It must be thin, making the device much easier to manufacture.

In the superconducting element according to the above-described embodiment,
The inclined end surface 12a of the lower superconducting electrode 12 made of an oxide superconducting thin film with the c-axis oriented parallel to the substrate surface 11a is:
As shown in FIG. 2 in an enlarged manner, a step portion S and a terrace portion T are formed, and the terrace portion T has Cu
One of (100), (010), and (110) planes orthogonal to the O ₂ plane P appears. That is, in the terrace portion T, the superconducting current easily flows to the upper superconducting electrode 15 via the barrier layer 14. Further, on the terrace portion T having a relatively large area, the barrier layer 14 having a thickness substantially equal to the deposition thickness is formed, and the thickness varies little. FIG. 2 shows a case where the upper superconducting electrode 15 is also formed of an oxide superconducting thin film in which the c-axis is oriented parallel to the substrate surface 11a.

For this reason, when an insulator is used as the barrier layer 14, for example, as shown in FIG. 2, the barrier layer 1 having a relatively large area and located above
4, the tunnel of the Cooper pair is effectively generated from the terrace portion T having a constant thickness. Therefore, it is possible to increase the superconducting current of the whole joint, it is possible to increase good reproducibility I _c · R _n product, which is the output voltage of the superconducting elements. In addition, since the thickness of the barrier layer 14 that affects the superconducting current flowing through the junction is substantially constant on the terrace T, it is possible to minimize variations and variations in the junction characteristics. That is, a superconducting element having a large output voltage can be obtained with good reproducibility, and for example, it is possible to suppress variations and fluctuations in the bonding characteristics of the superconducting elements between wafers or within the same wafer.

Similarly, when a normal conductor is used as the barrier layer 14, the barrier layer 14 has a relatively large area, and is located close to the terrace portion T where the thickness of the upper barrier layer 14 is constant. The oozing area of the Cooper pair due to the effect can be increased. Alternatively, the density of the Cooper pairs exuded by the proximity effect can be increased. Accordingly, since it is possible to flow from the terrace portion T of the superconducting current sufficiently, it is possible to increase good reproducibility I _c · R _n product, which is the output voltage of the superconducting elements. In addition, as in the case where an insulator is used as the barrier layer 14, it is possible to minimize variations and variations in bonding characteristics. That is, a superconducting element having a large output voltage can be obtained with good reproducibility, and for example, it is possible to suppress variations and fluctuations in the bonding characteristics of the superconducting elements between wafers or within the same wafer.

[0042]

Next, an embodiment of the present invention will be described.

Embodiment 1 FIG. 3 is a cross-sectional view schematically showing the structure of an inclined Josephson junction device according to an embodiment to which the superconducting device of the present invention is applied. In the figure, reference numeral 21 denotes a substrate made of a single crystal of SrTiO ₃ , and the surface thereof coincides with the (100) plane.
On this SrTiO ₃ (100) substrate 21, YBa ₂ Cu
A lower superconducting electrode 22 having a thickness of 250 nm and made of an oxide superconductor whose composition is substantially represented by ₃ O _7-δ is formed so that the [100] axis direction of its crystal structure is orthogonal to the substrate surface 21 a. ing.

In forming the lower superconducting electrode 22, a multi-source sputtering method is used. The thin film is grown at a substrate temperature of 833K for about 10 nm in the initial stage of growth, and then the growth is continued while the substrate temperature is continuously increased to 1033K. The growth temperature was fixed at 1033 K on the upper side 200 nm of the lower superconducting electrode 22. The reason for using such a growth method is as follows.

Generally, by setting the growth temperature on the SrTiO ₃ (100) substrate 21 to about 893 K or less, a YBa ₂ Cu ₃ O _7-δ thin film having the [100] axis perpendicular to the substrate surface can be obtained. At a higher temperature, a thin film whose [001] axis (c-axis) is perpendicular to the substrate surface grows. However, such a [100] alignment film grown at a relatively low single substrate temperature has site substitution of Y atoms and Ba atoms and lack of oxygen atoms.
It is difficult to obtain a superconducting thin film having a high critical temperature and a large critical current density.

On the other hand, a [100] -oriented YBa ₂ Cu ₃ O
_{On 7-δ} , [100] -oriented growth occurs due to epitaxial growth even at a high film formation temperature. The film forming method adopted in this example is based on this principle, and a [100] oriented film having good superconducting properties can be produced. Incidentally, the lower superconducting electrode 22 in this embodiment has a critical current density of 1.2 × 10 ⁶ A / cm at a critical temperature of 89K and 77K.
² shown.

After the lower superconducting conductive electrode 22 is formed, Sr is formed as an interlayer insulating film 23 at a substrate temperature of 953 K in the same vacuum vessel.
A TiO ₃ film was formed with a thickness of 200 nm. This SrTiO
_{The three} films were epitaxially grown with the lower superconducting electrode 22, and their surfaces were coincident with the (100) plane.

Next, the above-described laminated film is taken out of the vacuum container, a positive resist pattern is formed at a predetermined position by a usual optical exposure method, and the interlayer insulating film 23 and the lower superconducting film are formed by ion milling using this resist as a mask. The electrode 22 was continuously etched until the surface of the substrate 21 was exposed to form an inclined end face 22a at the edge. The inclination angle α of the inclined end surface 22a with respect to the substrate surface 21a was controlled by the irradiation angle of the ions to be milled. In the case of this embodiment, the inclined end face 22
The angle a was set to be 25 degrees. After removing the resist, Ar ions accelerated at a low voltage of 60 V are irradiated for several minutes to remove the strained region generated at the time of etching, and immediately mounted on a multi-source sputtering apparatus and heated in an oxygen atmosphere to thereby remove the edge end. The remaining strain was recovered.

Thereafter, PrBa ₂ C, which becomes the barrier layer 24,
10 nm of u ₃ O _7-δ thin film, YB to be the upper superconducting electrode 25
An a ₂ Cu ₃ O _7-δ thin film was laminated to a thickness of 200 nm. The substrate temperature at the time of stacking these layers was 923 K for the barrier layer 24 and 1033 K for the upper superconducting electrode 25. When manufactured at this substrate temperature, SrTiO as an interlayer insulating film 23 is formed on the inclined end surface 22a where the lower superconducting electrode 22 is exposed.
₃ SrTiO ₃ (1
The barrier layer 24 on any of the substrates 21
[010] The axis grows in an orientation state orthogonal to the substrate surface 21a,
The upper superconducting electrode 25 thereon is epitaxially grown on the barrier layer 24, and is grown with the [100] axis orthogonal to the substrate surface 21a.

On the other hand, when the barrier layer 24 is grown at a higher temperature, the barrier layer 24 and the upper superconducting electrode 25 are formed on the inclined end face 22a where the lower superconducting electrode 22 is exposed.
As a result of epitaxial growth of the barrier layer 2
[100] axis is the substrate surface 21 for both 4 and upper superconducting electrode 25
Although growth occurred perpendicular to a, the growth occurred on the surface of the substrate 21 exposed by etching and on the interlayer insulating film 23 in a direction in which the [001] axis was perpendicular to the substrate surface 21a. In this case, between the [001] -oriented portion of the upper superconducting conductive electrode 25 and the [100] -oriented portion on the inclined end face 22a, a crystal grain boundary where CuO ₂ planes are orthogonal to each other is formed. The bond was strong, and no phenomenon in which this portion operated as a parasitic Josephson junction was observed. Incidentally, the critical current density at the grain boundary reached 8 × 10 ⁵ A / cm ² at 77 K, and there was no practical problem.

For processing the upper superconducting electrode 25, a normal lithography step and etching with Ar ions were used to finally form a junction having a width of 5 μm on the inclined end face 22a. FIG. 4 is a current-voltage characteristic at a liquid helium temperature of the superconducting element according to this embodiment. Since PrBa ₂ Cu ₃ O _7-δ used as the barrier layer 24 has the property of a normal conductor, it exhibits characteristics as an SNS type Josephson junction without hysteresis.
It was confirmed that there was no leakage current due to micro-short between 2 and 25. The critical current due to the Josephson junction is 2 mA, and the output voltage of the device, Ι _c · R _n
About 7 mV was obtained as the product. This junction also operated at 77 K, and it was confirmed that the Ι _c · R _n product reached about 0.3 mV. These characteristics greatly surpass the characteristics of the conventional inclined junction.

Embodiment 2 Next, an embodiment in which the superconducting element of the present invention is applied to a Josephson junction using CaRuO ₃ , a normal conductor having a simple perovskite structure, as a barrier layer will be described with reference to FIG.

The substrate 31, lower superconducting electrode 32, and interlayer insulating film 33 used in this embodiment are the same as those in the first embodiment. After forming these layers, the inclined end face 32a was formed by using the same processing method as the process in the first embodiment.
The inclination angle of the end face 32a is 30 degrees. Thereafter, using a laser deposition method, a CaRuO ₃ having a thickness of 20 nm and a substrate temperature of 923 K is used.
A thin film is deposited as a barrier layer 34, and continuously at a substrate temperature 1
YBa ₂ Cu ₃ O _7-δ which becomes upper superconducting electrode 35 at 33K
The thin film was laminated at 200 nm.

The CaRuO used as the barrier layer 34
₃ is an oxide having a conductive perovskite structure having an orthorhombic crystal structure slightly distorted from cubic,
The lattice constant (integer multiple) of the [110] direction is YBa ₂ Cu ₃ O
[100], [010], and [001] directions of _7-δ , and SrT
substantially aligned with the [100] orientation lattice constant of the iΟ _3. For this reason, in the structure shown in FIG.
RTiomikuron ₃ thin film, on the inclined end face 32a, and CaRuO ₃ identical orientation in any on the surface 31a of the substrate 31 exposed is grown.

The upper superconducting electrode 35 made of YBa ₂ Cu ₃ O _{7-δ formed} on the barrier layer 34 made of CaRuO ₃ has a film thickness of [0098] at the film forming temperature used in this embodiment.
1] Grow with the axis orthogonal to the substrate surface 31a. This is a great difference from the first embodiment. By lowering the film forming temperature, the upper superconducting electrode 35 is moved to the
Although it is possible to fabricate the upper superconducting electrode 35 itself in such a case, it is difficult to obtain a Josephson junction having good characteristics because the superconducting characteristic of the upper superconducting electrode 35 itself deteriorates.

As described above, the proximity effect generated when a highly anisotropic oxide superconductor and a normal conductor are joined is weak in the [001] direction of the oxide superconductor, and is perpendicular to the direction. Then it appears strongly. In the case of this embodiment, the junction formed between the terrace of the lower superconducting electrode 32 and the barrier layer 34 corresponds to the latter, and the junction between the terrace of the barrier layer 34 and the upper superconducting electrode 35 corresponds to the former. Therefore, the Cooper pairs that seep into the normal-conductivity barrier layer 34 are supplied from the lower superconducting electrode 32 substantially. This means that the characteristics as a Josephson junction are inferior to the ideal case where the Cooper pair is supplied from both the upper and lower superconducting electrodes.

However, if only the lower superconducting electrode 32 is made so that the c-axis is parallel to the substrate surface 31a as in this embodiment, the terrace portion in the inclined end surface 32a can be used for developing Josephson characteristics. The characteristics could be greatly improved as compared with the inclined type junction of the conventional structure in which only the vicinity of the step portion contributed to the Josephson characteristics. Incidentally, the Josephson critical current density at 4.2 K obtained in this example was about 3 × 10 ⁵ A / cm ² , which was about one digit larger than that of the element having the conventional structure. In addition, the variation in the critical current of Josephson in many junctions fabricated on the same substrate is about ± 13%, and within ± 20% between different wafers.
It was confirmed that the reproducibility was excellent.

[0058]

As described above, according to the superconducting element of the present invention, a Josephson junction element having a large output voltage and a small area capable of operating even at a high temperature using an oxide superconductor is provided. It is possible to provide the information stably and with good reproducibility. Such a Josephson junction element is most suitable for application to, for example, a basic constituent element for realizing a single flux quantum transfer type ultra-high-speed logic circuit, an A / D converter, and the like, and a SQUID (superconducting flux quantum interferometer). The superconducting element of the present invention is expected to make a great contribution to industry.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a structure of a superconducting element according to one embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view showing a main part of the superconducting element shown in FIG.

FIG. 3 is a sectional view schematically showing the structure of the superconducting element according to the first embodiment of the present invention.

4 is a diagram showing current-voltage characteristics of the superconducting element shown in FIG. 3 at liquid helium temperature.

FIG. 5 is a sectional view schematically showing a structure of a superconducting element according to a second embodiment of the present invention.

FIG. 6 is a cross-sectional view schematically showing a conventional sandwich-type junction structure using a [100] -oriented oxide superconductor thin film as an electrode.

FIG. 7 is a cross-sectional view schematically showing a conventional inclined junction structure using a [001] -oriented oxide superconductor thin film as an electrode.

FIG. 8 is an enlarged sectional view showing a main part of the conventional inclined junction shown in FIG.

9 is a diagram schematically illustrating a direction in which electrons tunnel when the conventional inclined junction illustrated in FIG. 7 has a tunnel barrier.

10 is a diagram schematically showing a region where a Cooper pair exists in the normal barrier layer when the conventional inclined junction shown in FIG. 7 has a normal barrier layer.

[Explanation of symbols]

 11, 21, 31 ... substrate 12, 22, 32 ... lower superconducting electrode 12a, 22a, 32a ... inclined end face 14 ... barrier layer 15 ... upper superconducting electrode

Claims (2)

(57) [Claims] 1. A substrate and substantially REBA ₂ Cu ₃ O _7-δ (where RE is at least one element selected from rare earth elements other than Pr, and δ is a number from 0 to 0.5). It has a composition represented by the formula, composed of an oxide superconductor thin film formed on the substrate such that the c-axis of the unit cell is substantially parallel to the substrate surface, and is formed on the oxide superconductor thin film. Interlayer insulating film formed
A laminated film of a lower superconducting electrodes having inclined end faces to the substrate surface, a barrier layer made of an oxide material formed so as to cover at least the inclined end faces of the lower superconducting electrodes, wherein The REBa ₂ Cu ₃ O formed on the barrier layer;
_A superconducting electrode comprising an oxide superconducting thin film having a composition substantially represented by _7-δ . 2. The superconducting element according to claim 1, wherein an angle formed between an end surface of said lower superconducting electrode and said substrate surface is 45 degrees or less.