JPH02186681A - Superconductive junction device - Google Patents

Abstract

PURPOSE:To enable a junction device of this design to secure characteristics as a single Josephson junction and not to have such characteristics possessed of roundness peculiar to a grain boundary Josephson junction by a method wherein specified superconductive material containing oxygen as a component is formed into sandwich-type electrodes. CONSTITUTION:When a Y-Ba-Cu oxide thin film in which Y, Ba, and Cu are in the compositional ratio 1:2:3 is formed into an upper and a lower electrode, Y-Ba-Cu oxide thin films 2 and 3 are formed through a film forming method such as sputtering or the like at a high substrate temperature of 600 deg.C or more, whereby they can be made to have a superconductive property without being thermally treated after they have been formed. Furthermore, an amorphous thin film 1-10nm in thickness is formed of a Y-Ba-Cu oxide of the same composition as above through a method such as sputtering to serve as a tunnel barrier layer 4. A superconductive junction device constituted as mentioned above has not only a Josephson current but also hysteresis peculiar to a superconductive junction. Therefore, a junction device of this design does not have the roundness of a voltage-current characteristic near zero voltage peculiar to a grain boundary Josephson junction.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to the field of superconducting electronics such as superconducting switching devices that perform switching operations at high speed and with low power consumption, and particularly relates to superconducting bonding devices that can operate at liquid nitrogen temperatures. It is something.

[Conventional technology]

Y-Ba-Cu oxide or Bi-Sr-Ca-Cu
Oxide-based superconducting materials such as oxides have a critical temperature of 90 or higher and exhibit complete superconductivity at liquid nitrogen temperatures. In order to apply these superconducting materials such as Y-Ba-Cu oxide to the field of electronics, it is necessary to obtain a Josephson junction, which is a basic superconducting element, that is, a superconducting junction device.

A superconducting bonding device using Y-Ba-Cu oxide or Bi-SrCa-Cu oxide uses a single superconducting thin film made of polycrystals of these oxides, and a thin narrow constriction is formed in a part of the superconducting thin film. Form. The grain boundaries of polycrystalline superconducting thin films exhibit characteristics of superconducting weak bonds, ie Josephson junctions, when current is passed through them. Regarding the structure and properties of so-called grain boundary Josephson junctions, see Applied Physics Letters, Volume 51.

20 (1987), pp. 200-202 (Ap.
pljed Physics Letters, Vo
l, 51.

No. 20 (1987) pp. 200-202).

[Problem to be solved by the invention]

However, the above conventional technology had the following problems. That is, a grain boundary Josephson junction is an aggregate of minute Josephson junctions, and the minute Josephson junctions are connected in series and in parallel. Series connection of Josephson junctions particularly degrades the overall performance. If the critical current and voltage-current characteristics of the Josephson junctions are all the same, the form of the voltage-current characteristics of the entire Josephson junction is the same. When the critical current and voltage-current characteristics of Josephson junctions connected in series are different from each other, the voltage-current characteristics of the entire Josephson junction exhibit roundness near zero voltage. Such a rounded characteristic becomes clear when different voltage-current characteristics are added together.

The presence of roundness in voltage-current characteristics leads to a decrease in gain when a switching element is constructed or a decrease in sensitivity when a superconducting quantum interference device is constructed. In grain boundary Josephson junctions in which the elemental Josephson junctions cannot be artificially formed, it is inevitable that the overall joint characteristics will be rounded due to the mismatch in the elemental Josephson junction characteristics.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a superconducting bonding device that does not exhibit the rounded characteristics characteristic of grain boundary Josephson junctions, but has the characteristics of a single Josephson junction.

[Means to solve the problem]

The following technical means were adopted to achieve the above objectives.

That is, Y-Ba-Cu oxide or Bi5r-C
By forming a sandwich-type superconducting bonding device using a superconducting material containing oxygen as a component such as a-Cu oxide as an electrode, it is possible to eliminate the deteriorated characteristics due to series connection peculiar to grain boundary Josephson junctions. Obtain Josephson junction properties as a single junction. For the tunnel barrier layer sandwiched between two superconducting electrodes, its constituent elements are selected from among the elements constituting the superconducting electrode material, and by making this composition different from that of the superconducting electrode material, semiconductor or insulator properties can be improved. Form a membrane layer with For example, Y--Ba--Cu oxide exhibits insulating properties due to the lack of Y or Ba.

Another tunnel barrier layer structure material is obtained by forming a semiconductor and an M-edge body by a combination of elements selected from among the elements constituting the superconducting electrode material, and making the crystal structure different from that of the superconducting electrode material. . For example, Y
- In a Josephson junction using Ba-Cu oxide as a superconducting electrode, amorphous Y-Ba-Cu oxide or Ba
A Y--Ba--Cu oxide crystal in which the atomic positions of Y and Y are irregular is used as the tunnel barrier layer.

Furthermore, A u is added to both interfaces between the oxide 1-Hennel barrier layer obtained by the above method and the superconducting electrode material.

A superconducting bonding device is obtained by inserting a noble metal such as PT or Ag.

Examples of combinations of superconducting electrode materials and tunnel barrier layer materials in the above-mentioned superconducting bonding device and examples of their manufacturing methods are as follows.

First, when using Y-Ba-Cu oxide thin films with a stoichiometric composition ratio of Y, Ba, and Cu, that is, 1:2:3, as the upper and lower electrodes, the Y-Ba-Cu oxide thin films are formed by sputtering, etc. By forming the film at a high substrate temperature of 60° C. or higher using a film forming method, superconducting properties can be obtained without performing heat treatment after film formation. Furthermore, an extremely thin layer of Y--Ba--Cu oxide having the same composition, that is, an amorphous thin film with a thickness of 1 to 10 nm, is formed by a method such as sputtering at a substrate temperature of 300 DEG C. or less to form a tunnel barrier layer. Y
-Ba-Cu oxide thin film has a substrate temperature of 600°C to 700°C
A superconducting crystal can be obtained by depositing a thin film of Y-B a -Cu oxide in an amorphous state.
It cannot be crystallized by heat treatment below 50°C. Therefore, even by stacking Y--Ba--Cu oxide thin films, the tunnel barrier layer portion does not crystallize.

Y-B a where the composition ratio of Y, Ba and Cu is stoichiometric
-Cu oxide thin films are used as upper and lower electrodes, and Y-B with different compositions
In the case of a superconducting junction device using an ultra-thin a-Cu oxide film as a tunnel barrier layer, the process is as follows. That is,
The method for forming the upper and lower electrodes of Y--Ba--Cu oxide is the same as the method described above. Regarding the Y-Ba-Cu oxide tunnel barrier layer, for example, the composition ratio of Y:Ba:Cu is 2.
Y-Ba- where =1=1 and the film thickness is in the range of 1 to 10 nm
An ultra-thin Cu film is used. There is no problem whether the tunnel barrier layer is formed at room temperature or the same temperature as the electrode material formation temperature.

The tunnel barrier layer is made of Au, Y-Ba-Cu oxide and A
When making a sandwich shape of u, Y-B a -Cu
The method for forming the oxide electrode material is the same as the method described above. Regarding the tunnel barrier layer, the thickness of the Au film on both sides is 1 to 30 nm.
The film thickness of Y-Ba-Cu oxide is 1 to 10 nm.
The range is m.

The formation temperature of the Au film is room temperature, and the formation temperature and composition of the Y-Ba-Cu oxide thin film are arbitrary.

Even when Bi-Sr-Ca-Cu oxide is used as the superconducting electrode material, the same superconducting bonding device structure and manufacturing method are used.

Ag or pt instead of Au as tunnel barrier layer material
The same applies when using .

[Effect]

The superconducting junction device according to the present invention has the performance necessary to obtain superconducting tunneling characteristics in the following points.

In general, oxide-based superconducting materials such as Y--Ba--Cu oxides have a short coherence length, and their superconducting properties tend to be interrupted. For example, the coherence length of Y--Ba--Cu oxide is 1.4 nm, and this value is approximately equal to the lattice constant in the C-axis direction of an orthorhombic perovskite crystal. Furthermore, when a different material is connected to the Y-Ba-Cu oxide, contact resistance tends to increase. This is because when an oxide superconducting material such as Y-Ba-Cu oxide is brought into contact with a different substance, such as a metal or a semiconductor, the oxygen atoms of the Y-Ba-Cu oxide at the interface diffuse and the oxygen concentration decreases. At the same time, the superconducting properties deteriorate or the superconductivity of the interface disappears.

As the oxygen concentration in the Y-Ba-Cu oxide decreases from its original value as an orthorhombic crystal, the superconducting critical temperature decreases from 93K, and at the point when the oxygen concentration corresponding to the concentration of Y atoms is lost. Superconductivity disappears. Especially Y-Ba
This phenomenon tends to occur because the oxygen atoms in the -Cu oxide have weak bonds and are unstable. It is desirable that the interface between the superconducting electrode and the tunnel barrier layer changes discontinuously from a superconducting state to a semiconductor or insulating state. As described above, the deterioration of superconducting properties at the interface makes it difficult to obtain superconducting tunneling properties, especially when an oxide superconducting material with a short coherence length is used.

In contrast, the superconducting junction device according to the present invention uses a material common to the oxide superconducting material for the tunnel barrier layer. Therefore, the oxygen concentration of the superconducting electrode material decreases at the interface between the superconducting electrode material and the tunnel barrier layer, and a layer with degraded superconducting properties does not occur. The reason for this is that the tunnel barrier layer material has a sufficient oxygen concentration so that there is no mixing due to oxygen atoms diffusion, that is, the composition ratio of Y:Ba:Cu is 1:
This is because oxygen having a composition ratio close to 7 to 2:3 is held in advance. Furthermore, insertion of noble metals such as Au, Pt or Ag prevents interdiffusion of oxygen atoms. Therefore, it is possible to discontinuously distribute the superconducting state to the insulating state from the superconducting electrode material toward the tunnel barrier layer.

〔Example〕

Examples of the present invention will be shown below.

[Example 1] A superconducting bonding device as shown in FIG. 1 made of Y--Ba--Cu oxide was manufactured by the steps shown below. Mg◯ single crystal 1 with a (100) crystal orientation was used as the substrate material. By performing discharge in a mixed gas atmosphere of Ar and oxygen using a high frequency magnetron sputtering device,
A Y-Ba-Cu oxide thin film 2 with a thickness of 400 nm was formed.

The substrate temperature during film formation was 700° C., the high frequency applied power was 100 W, and the film deposition rate was 3 nm/min.

The mixed gas pressure was 0.6 Pa, and the oxygen gas partial pressure was 50%. A sintered body of YBa-Cu oxide was used as the target material. The film was formed at a substrate temperature of 700°C, and the composition was Y,
By adjusting the ratio of Ba and Cu to be 1=2:3, a superconducting phase Y--Ba--Cu oxide thin film 2 was obtained.

Next, in the same sputtering apparatus, another sputtering target having a different composition was placed directly below the substrate material by moving the target electrode.

Under the same conditions as for obtaining the above superconducting phase oxide thin film,
A tunnel barrier layer material 4 was formed. The composition of the tunnel barrier layer material was adjusted so that the ratio of Y, Ba and Cu was 2:1:1, and the film thickness was 3 nm.

These lower electrode thin film 2 and tunnel barrier layer 4 are made of MgO
A thin line pattern of @0.511 m was formed by using a mask manufactured by Co., Ltd.

Furthermore, a two-part electrode 3 consisting of a Y-Ba-Cu oxide thin film
was similarly formed by high frequency magnetron sputtering method. The conditions for forming the thin film were the same as those for the lower electrode thin film 2, and the film thickness was 600 nm.

By using a mask made of MgO, a thin line pattern with a width of 0.5 mm and perpendicular to the tunnel barrier layer 4 and the lower electrode thin film 2 was obtained. Through the above steps, Y-Ba-C
A cross-shaped superconducting bonding device made of U oxide was obtained.

The critical temperature of the upper and lower electrode thin films 2.3 of Y--Ba--Cu oxide was above 70°C, and had an orthorhombic perovskite crystal structure. As shown in FIG. 4, the superconducting junction device has a Josephson current at 20K and has hysteresis peculiar to superconducting junctions.

[Example 2] A superconducting bonding device as shown in FIG. 2 made of Y-Ba-Cu oxide was manufactured by the steps shown below. Y-Ba
The -Cu oxide lower electrode 2 was formed using the same method and conditions as in Example 1. The tunnel barrier layer 5 was formed under the following conditions. That is, using the same target used to form the lower electrode thin film and keeping the substrate temperature below 300'C, a Y-Ba-Cu oxide thin film 5 with a thickness of 3 nm was formed by sputtering. Obtained. The composition ratio of Y, Ba and Cu is 1:2:3
It was adjusted so that The method and conditions for forming the upper electrode made of the Y--Ba--Cu oxide thin film 3 were the same as in Example 1.

The structure of the Y--Ba--Cu oxide thin film formed under the above conditions was found to be amorphous according to the results of examination using a separately formed thick film. The characteristics of the superconducting junction device formed as described above include a Josephson current and hysteresis peculiar to superconducting junctions, as in the first embodiment.

[Example 3] A superconducting bonding device as shown in FIG. 3 made of Y-Ba-Cu oxide was manufactured by the steps shown below. YBa-
The Cu oxide lower electrode 2 was formed using the same method and conditions as in Example 1. Tunnel barrier layer 5 was formed by the following method. That is, in the same sputtering apparatus in which the lower electrode thin film 2 was formed, Au
Using the plate as a target material, an Au thin film 6 is formed by sputtering. The film thickness is 3 nm. Next, using the same target used to form the lower electrode thin film 2 and keeping the substrate temperature below 300°C, a Y-B a -C film with a thickness of 3 nm was formed by sputtering.
A u oxide thin film 5 is obtained. Y.

The composition ratio of Ba and Cu was adjusted to be 1:2:3. Further, on top of this, a 3 nm thick Au film was formed by the above method.
A film 6 is formed. The method and conditions for forming the upper electrode 3 made of a Y--Ba--Cu oxide thin film are the same as in Example 1.

The characteristics of the superconducting bonding device formed as described above are shown in Example 1.
As in the case of , it has Josephson current and hysteresis characteristic of superconducting junctions. Furthermore, because of the hysteresis, the roundness of voltage-current characteristics near zero voltage that is characteristic of grain boundary Josephson junctions does not exist.

The manufacturing method, structure and characteristics of the superconducting bonding device as described above are not limited to Y-Ba-Cu oxide, but
-Sr-Ca-Cu oxide, TQ-B a -Ca -
The present invention can be similarly applied to the case where an oxide-based high critical temperature superconducting material such as Cu oxide is used.

〔Effect of the invention〕

According to the present invention, the superconducting bonding device has the following effects.

(1) It enables the formation of a sandwich-type superconducting bonding device using oxide-based superconducting materials. That is, the Josephson effect can be utilized at high temperatures of 4.2 or higher.

(2) Since it has hysteresis characteristics, the adjustment width of the junction current when a magnetic field signal is applied can be made larger than that of a conventional grain boundary Josephson junction. This leads to improved sensitivity when applied to superconducting quantum interference device magnetometers and improved gain when used in switching elements.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a superconducting bonding device according to a first embodiment of the present invention, FIG. 2 is a cross-sectional view of a superconducting bonding device according to a second embodiment of the present invention, and FIG. A sectional view of the superconducting bonding device, and FIG. 4 is a voltage-current characteristic diagram of the superconducting bonding device according to the first embodiment. 1-MgO substrate, 2-Y-Ba-Cu oxide lower electrode thin film, 3...Y-Ba-Cu oxide upper electrode thin film, 4=.Y-Ba-Cu (2: 1: 1 )
Oxide tunnel barrier layer, 5... Y-Ba-Cu amorphous tunnel barrier layer, 6... Au thin film.

Claims (1)

[Claims] 1. In a superconducting junction device having two superconducting electrodes made of a superconducting material containing oxygen as a component and a tunnel barrier layer sandwiched between the two superconducting electrodes, the tunnel barrier layer comprises: A superconducting bonding device characterized in that it is formed of a semiconductor or insulator material obtained from the elements constituting the superconducting electrode. 2. The superconducting material containing oxygen as a component is Y-Ba
2. The superconducting bonding device according to claim 1, wherein the superconducting bonding device is -Cu oxide or Bi-Sr-Ca-Cu oxide. 3. The superconducting junction device according to claim 1 or 2, wherein the material of the tunnel barrier layer has a composition different from that of the material of the superconducting electrode. 4. The superconducting junction device according to claim 1 or 2, wherein the crystal structure of the material of the tunnel barrier layer is different from the crystal structure of the material of the superconducting electrode. 5. The superconducting junction device according to any one of claims 1 to 4, characterized in that a noble metal is sandwiched between the interface between the tunnel barrier layer and the superconducting electrode. 6. The superconducting bonding device according to claim 5, wherein Au, Pt, or Ag is used as the noble metal.