JP2955407B2 - 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.

[0002]

2. Description of the Related Art Conventionally, as a superconducting element, a tunnel-type Josephson junction having a structure in which a thin superconducting element can be tunneled by a metal superconductor such as Pb or Nb is sandwiched. Such a conventional tunnel-type Josephson device needs to operate at a very low temperature close to the temperature of liquid helium. In addition, since the current-voltage characteristic having hysteresis characteristic of a tunnel-type Josephson junction is exhibited, there is a problem that a circuit configuration is complicated and the like, and it 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 main electrodes made of a metal superconductor are connected with a thin and thin metal has also been developed. It has been advanced. If such a bridge-type junction has a cross-sectional area of the bridge portion sufficiently smaller than that of the electrode portion and the length of the bridge portion can be made substantially equal to the coherence length, current-voltage characteristics without hysteresis can be obtained. It has been expected that there is a possibility that a Josephson junction having the following characteristics will be obtained, and an output voltage as high as a tunnel junction may be obtained.
However, in order to construct such a bridge type junction on a plane using a conventional metal superconductor, ultra-fine processing of about 0.1 μm is required, and since the material used for the bridge part is metal, However, it has difficulties such as not being able to sufficiently increase the junction resistance, and thus has not been put to practical use. In addition, as in the case of the above-described tunnel junction, there is a disadvantage that an extremely low temperature operation close to the temperature of liquid helium is required.

Under these circumstances, recently, oxide superconductor materials exhibiting superconductivity at a temperature equal to or higher than the temperature of liquid nitrogen have been discovered and have attracted much attention. If it becomes possible to produce a good Josephson junction using this oxide superconductor, there is no need for at least cryogenic operation as compared with the above-mentioned conventional Josephson junction using a metal superconductor. Therefore, a wide range of applications are expected.

[0005]

However, the oxide superconductor material has the essential problems that its surface is easily degraded in the atmosphere and its coherence length is small, so that a clear Josephson material is required. At present, good bonding showing characteristics has not been obtained. In other words, the development of Josephson junctions using oxide superconductors with high critical temperatures
Since it is expected to greatly contribute to the industry, development of a joining structure for making this practical and a method of manufacturing the same has been a challenge.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has a superconductivity using an oxide superconductor which exhibits good Josephson characteristics, is excellent in controllability, and can obtain a large output voltage. It is intended to provide an element.

[0007]

That is, the superconducting element of the present invention comprises two main electrode portions made of an oxide superconductor;
A superconducting element interposed between these main electrode portions and having a connection portion for electrically connecting the main electrode portions, wherein the connection portion has a non-superconducting structure similar in crystal structure to the oxide superconductor. It has a plurality of superconducting current paths scattered in the oxide , and these superconducting current paths are non-conductive
Both are formed along the grain boundaries of the superconducting oxide.
To its respective cross-sectional area is characterized by the go sufficiently small compared to the cross-sectional area of the main electrode portion.

[0008] As described above, the connecting portion in the superconducting element of the present invention has a plurality of superconducting current paths embedded and scattered in a non-superconductor such as an insulator or a normal conductor. Such a superconducting current path can be obtained with good reproducibility by being formed along the crystal grain boundaries of the non-superconductor constituting the connection portion.

For example, when an oxide superconductor and a non-superconductor having a similar crystal structure to the oxide superconductor are continuously laminated,
High-speed diffusion of elements occurs along the grain boundaries existing in the non-superconductor, and the conductive properties in the immediate vicinity of the grain boundaries change significantly. As an example, a YBa ₂ Cu ₃ O ₇ film (hereinafter, Y-Ba-
Cu-O film) and PrBa ₂ Cu ₃ O ₇ film (hereinafter Pr-Ba-Cu
-O film), Y diffuses only along the grain boundaries, and the low-temperature Pr-Ba-Cu-O film is an insulator.
A portion that performs metallic conduction can be locally generated. On the other hand, the crystal structure of the Y-Ba-Cu-O film and that of the Pr-Ba-Cu-O film are the same, and the lattice constants are extremely close. It is continuously connected at the interface with the Pr-Ba-Cu-O film. As a result, Y-Ba-Cu-O / Pr-Ba-Cu-O / Y-Ba-Cu-O
When a three-layer structure is fabricated, Pr-Ba-
A metallic conduction path is formed along the almost linearly connected grain boundaries through the Cu-O film to the upper Y-Ba-Cu-O film. This conduction path is the superconducting current path in the present invention.

A superconducting current path utilizing such a grain boundary is composed of a Y-Ba-Cu-O-based oxide superconductor and a Pr-Ba-Cu
The present invention is not limited to the case where the insulator is combined with an -O-based insulator, and can be formed by combining various oxide superconductors and a non-superconductor having a similar crystal structure to the oxide superconductor.

[0011] Thus a superconducting current path obtained, the cross-sectional area than that of the main electrode portion can be made sufficiently small, and Ri magnetic field penetration depth Saya the distance between the superconducting current path cross in the connecting portion It can be made sufficiently small. Therefore, it is possible to form a bridge-type junction by using a connection portion having such a superconducting current flow path. The superconducting element of the present invention can be regarded as a variable thickness bridge (Variable Thickness Bridge: VTB) junction in a broad sense.

The density of superconducting current paths to be formed and the distance between them are set in accordance with the area of the junction. The density and the distance between the superconducting current paths can be set to desired values by controlling the size of the crystal grains of the non-superconductor constituting the connection portion. Various values can be obtained for the size of the crystal grains by controlling the conditions for forming the thin film. In addition, the length of the superconducting current path is set according to the coherence length of the oxide superconductor to be used, and by controlling the thickness of the non-superconductor constituting the connection portion, the desired value can be easily obtained. can do.

[0013]

In the superconducting element of the present invention, a junction having a plurality of superconducting current paths embedded and scattered in a non-superconductor forms a Josephson junction. The superconducting current path can be formed by using, for example, a crystal grain boundary between an oxide superconductor serving as a main electrode portion and a non-superconducting oxide forming a connection portion. it is possible to sufficiently smaller than the area to the cross-sectional area of the main electrode portion, it is possible to sufficiently reduce Ri magnetic field penetration depth Sayo at the connecting portion the distance between the superconducting current path other. Further, the density of the superconducting current paths and the distance between the superconducting current paths can be set to a desired value by controlling the size of the crystal grains of the non-superconducting oxide constituting the connection portion. Can be produced. Furthermore, by setting the thickness of the non-superconducting oxide constituting the connection portion to a desired value, it is possible to satisfy the condition that the superconducting current flowing through the superconducting current path performs a coherent Josephson response. With these, it is possible to obtain good characteristics using an oxide superconductor only by controlling the conditions of the laminated structure without performing the ultra-fine processing required in the case of a VTB junction manufactured on a conventional flat surface. A Josephson junction can be obtained.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the superconducting element of the present invention will be described below.

FIG. 1 shows that the present invention is applied to Y—Ba—Cu—O / P
It is a figure showing the section structure of the example applied to the superconducting element which has r-Ba-Cu-O / Y-Ba-Cu-O laminated structure. In FIG. 1, reference numeral 1 denotes an insulating substrate such as a SrTiO ₃ (100) substrate, and on the SrTiO ₃ (100) substrate 1, a Y—Ba—Cu—O-based oxide superconductor serving as a lower main electrode portion is provided. A body film 2 is formed. This Y-Ba-
On the Cu-O-based oxide superconductor film 2, a Pr-Ba-Cu-O-based insulator film 3 is formed as a non-superconductor layer constituting a connection portion. In the Pr-Ba-Cu-O-based insulator film 3, a plurality of superconducting current paths (not shown) are formed along the grain boundaries of the Pr-Ba-Cu-O crystal. These superconducting current paths have respective cross-sectional areas sufficiently smaller than the cross-sectional area of the main electrode portion, and the distance between the superconducting current paths is set sufficiently smaller than the magnetic field penetration depth at the connection portion. ing.

On such a Pr-Ba-Cu-O-based insulator film 3, a Y-Ba-Cu-O-based oxide superconductor film 4 serving as an upper main electrode portion is formed. The VTB junction is formed by the laminated structure of the Y-Ba-Cu-O-based oxide superconductor film 2 / Pr-Ba-Cu-O-based insulator film 3 / Y-Ba-Cu-O-based oxide superconductor film 4. It is configured. On the upper Y-Ba-Cu-O-based oxide superconductor film 4, a wiring 6 to the upper Y-Ba-Cu-O-based oxide superconductor film 4 is provided via an insulating film 5. ing.

The superconducting element having such a configuration is manufactured, for example, as follows. FIG. 2 shows a manufacturing process of the superconducting element. First, on the SrTiO ₃ (100) substrate 1, the lower Y-Ba-Cu-
An O-based oxide superconductor film 2, a Pr-Ba-Cu-O-based insulator film 3, and an upper Y-Ba-Cu-O-based oxide superconductor film 4 are sequentially laminated (FIG. 2A). The thickness of each layer is 300 in this example.
nm, 20 nm, and 200 nm.

In this case, it is important that the laminated structure is formed continuously in the same apparatus and is not exposed to the atmosphere, and that the lower Y-Ba-Cu-O-based oxide superconductor film 2 is formed at the time of film formation. The purpose is to set the substrate temperature such that a predetermined superconducting transition temperature is maintained and crystal grains have a predetermined size. The former is Y
This is because the crystal arrangement is not disordered at the interface between the -Ba-Cu-O film and the Pr-Ba-Cu-O film, and the latter is indispensable for obtaining the above-mentioned predetermined superconducting Josephson current value. It is. That is, the size of the crystal grains in the Pr-Ba-Cu-O-based insulator film 3 is determined by the size of the crystal grains of the lower Y-Ba-Cu-O-based oxide superconductor film 2. is there. And this Pr-Ba-Cu-O
The density of the superconducting current paths and their mutual intervals are determined by the size of the crystal grains in the system insulator film 3. The density of these superconducting current paths and the distance between them are set in accordance with the area of the junction as described above.

In this embodiment, the film was formed with the substrate temperature set at 680 ° C. In this case, the lower Y-Ba-Cu-O-based oxide superconductor film 2 has an a-axis orientation, and the size of crystal grains is on average.
It was about 50 nm. The superconducting transition temperature (critical temperature) of the Y-Ba-Cu-O films 2 and 4 was 83K. The reason why the critical temperature is lower than the ideal temperature of 92K of the Y-Ba-Cu-O film is that the substrate temperature is set lower so that the crystal grains have the set size. In order to improve the critical current without changing the crystal grain size, for example, on a SrTiO ₃ (100) substrate 1,
In advance, a Pr-Ba-Cu-O film is formed at a substrate temperature at which the required crystal grain size can be obtained, and then at a substrate temperature at which a high critical temperature can be obtained, the lower Y-Ba-Cu-O-based oxide superconductor is formed. The body film 2 may be formed. In this case, the size of the crystal grain is Pr-Ba-Cu-O
This is because it is determined by the film, and the critical temperature is determined by the temperature at which the Y-Ba-Cu-O film is formed.

Under these conditions, the Y-Ba-Cu-O-based oxide superconductor film 2 / Pr-Ba-Cu-O-based insulator film 3 / Y-Ba-Cu-O-based oxide superconductor By forming the laminated structure of the body film 4 by continuous film formation, Y diffuses along the grain boundaries of the Pr—Ba—Cu—O based insulator film 3 in the process of this film formation, and is essentially an insulator. Pr-B
The conduction properties near the grain boundaries of a-Cu-O crystals change significantly. Thereby, as shown in FIG. 3, a plurality of current paths 7 for flowing a superconducting current are formed along the crystal grain boundaries of the Pr—Ba—Cu—O-based insulator film 3. FIG. 3 is a diagram schematically showing an enlarged view of the joint, in which 8 is a crystal grain of a Pr—Ba—Cu—O-based insulator, and 9 is a Y—Ba—Cu—O-based oxide. 2 shows crystal grains of a superconductor.

The superconducting current path 7 formed along the crystal grain boundary in this manner has a cross-sectional area of
4 and sufficiently smaller than the cross-sectional area of
Mutual distance of the magnetic field penetration depth Saya Ri sufficiently small at the connecting portion. Due to the existence of such a superconducting current path 7, the Y-Ba-Cu-O-based oxide superconductor film 2 / Pr-
The laminated structure of the Ba-Cu-O-based insulator film 3 / Y-Ba-Cu-O-based oxide superconductor film 4 functions as a VTB junction.

The processing steps after the formation of the laminated film are the same as those used for manufacturing a normal semiconductor device. First, the pattern of the bonding portion is transferred to the resist film by an optical exposure method, and then the Y-Ba-Cu-O-based oxide superconductor film 2 and Pr are adjusted to the bonding area by ion milling using the resist as a mask. -Ba-Cu-O-based insulator film 3 is etched (Fig. 2-
b). In this embodiment, the bonding area was 10 μm × 10 μm.

Next, an insulating film 5 for insulating the wirings to the lower and upper Y-Ba-Cu-O-based oxide superconductor films 2 and 4 from each other is formed in layers, and this insulating film 5 is Only the opening is made (FIG. 2-c). As the insulating film 5, various materials can be used. In this embodiment, a negative resist is used for the purpose of simplifying the process. Thereafter, the wiring 6 to the upper Y-Ba-Cu-O-based oxide superconductor film 4 is formed of, for example, Au (FIG. 2D), thereby completing the superconducting element.

As described herein, the superconducting element of the present invention can be manufactured only by a usual optical exposure step, and does not require extremely fine processing. This is important in considering the actual industrial application of the present invention.

FIG. 4 shows a current-voltage characteristic at a liquid helium temperature of the superconducting element obtained in the above embodiment. 1.2mA as critical current, 4 mV was obtained as I _c · R _n product, which is the output voltage of the device. Compared to the expected output voltage of 20mV for oxide superconductors, Pr-Ba-Cu-O
This is because the thickness of the system-based insulator film 3, that is, the length of the superconducting current path has not been optimized, and even better characteristics can be obtained by making the Pr-Ba-Cu-O-based insulator film 3 thinner. be able to.

FIG. 5 shows the dependence of the critical current on the applied magnetic field, which was performed to confirm that the superconducting element of this embodiment actually showed uniform Josephson characteristics. As can be seen from FIG. 5, the produced superconducting element exhibited an ideal Fraunhofer pattern. Such Josephson characteristics were confirmed even at the temperature of liquid nitrogen, and it was verified that the device of the present invention can operate at a high temperature.

In the above embodiment, Y-Ba-Cu-O
Although the example in which the superconducting element of the present invention is applied to a laminated structure of a film / Pr-Ba-Cu-O film / Y-Ba-Cu-O film has been described, the present invention is not limited to this. It is also possible to apply the present invention to a combination of an oxide superconductor material and a non-superconductor having a similar crystal structure to the above oxide superconductor material. However, according to the combination of materials according to the above embodiments, two points, that is, control of the density of crystal grain boundaries of the intermediate layer and change in conductivity due to element diffusion along the grain boundaries can be provided in a reliable and controlled manner. Therefore, the present invention can be particularly effectively used.

[0028]

As described above, according to the present invention,
A Josephson junction device that can operate at a high temperature using an oxide high-temperature superconductor can be easily obtained with good reproducibility and without using an extremely fine processing technique. The superconducting element according to the present invention has ideal Josephson characteristics, shows no hysteresis in current-voltage characteristics, and has a high output voltage. It is suitable as a basic component for realizing a non-latch type Josephson integrated circuit. Thus,
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 sectional view showing a configuration of a superconducting element according to one embodiment of the present invention.

FIG. 2 is a sectional view showing a manufacturing process of the superconducting element shown in FIG.

FIG. 3 is an enlarged schematic view of a junction of the superconducting element shown in FIG. 1;

FIG. 4 is a diagram showing current-voltage characteristics of a superconducting element obtained according to one embodiment of the present invention.

FIG. 5 is a diagram showing the applied magnetic field dependence of the critical current of the superconducting device obtained according to one embodiment of the present invention.

[Explanation of symbols]

1. SrTiO ₃ (100) substrate 2. Lower Y-Ba-Cu-O-based oxide superconductor film 3. Pr-Ba-Cu-O-based insulator film 4. Upper Y-Ba-Cu- O-based oxide superconductor film 5: insulating film 6: wiring 7: superconducting current flow path

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Shinji Inoue 1 Koga Toshiba-cho, Saisaki-ku, Kawasaki-shi, Kanagawa Prefecture Toshiba Research Institute Co., Ltd. No. 1 Toshiba Research Institute, Inc. (56) References JP-A-2-39476 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01L 39/22 H01L 39/24 H01L 39 / 00

Claims (1)

(57) [Claims] 1. A superconducting element comprising: two main electrode portions made of an oxide superconductor; and a connection portion interposed between the main electrode portions and electrically connecting the main electrode portions, The connection portion has a crystal structure similar to that of the oxide superconductor.
A plurality of superconducting current paths scattered in the non-superconducting oxide , and these superconducting current paths constitute the connection portion.
Formed along the grain boundaries of the non-superconducting oxide
Both superconductive elements each of its cross-sectional area and wherein the go sufficiently small compared to the cross-sectional area of the main electrode portion.