JPH04288885A - Tunnel-type josephson element - Google Patents

Abstract

PURPOSE:To achieve an improved junction at an interface between a superconductor and a non-superconductor layer and improved superconductive characteristics by using a conductive and tetragonal perovskite type oxide as a non-superconductor layer which is formed between a pair of superconductor layers. CONSTITUTION:A first superconductor layer 2, a non-superconductor layer 3, and a second superconductor layer 4 are laminated in sequence on a substrate 1. The non-superconductor layer 2 and the second superconductor layer 4 are partially eliminated, thus enabling the first superconductor layer 2 to be exposed partially. A first electrode 5a and a second electrode 5b are formed on the exposed surface of the first superconductor layer 2 and on the surface of the second superconductor layer 4, respectively. The superconductor and the non- superconductor layer are subjected to epitaxial growth mutually with the c axis of the superconductor layer in a direction which is vertical to the substrate. The non-superconductor layer has a lattice constant which is closer to a lattice constant of the superconductor layer and consists of a conductive or a tetragonal perovskite type oxide.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunnel-type Josephson device that takes advantage of the excellent properties of high-temperature superconducting materials.

0002

2. Description of the Related Art A Josephson element is constructed from weak coupling between superconductors. Several structures have been proposed as specific structures for realizing this weak coupling, and among them, a tunnel type Josephson element is known as having the most typical structure. This tunnel-type Josephson device has a structure in which an extremely thin non-superconductor layer is inserted between a pair of superconductor layers.

[0003] Tunnel-type Josephson devices that have actually been manufactured use Nb, NbN, Pb, etc. as superconductors.
As a non-superconductor, an oxide obtained by oxidizing the surface of Nb or Pb, or a vapor deposited film of MgO, α-Si, etc. is used. However, these metal-based superconductors generally have very low superconducting critical temperatures, and in fact do not exhibit effective properties unless they are cooled with extremely expensive liquid helium.

0004

[Problem to be solved by the invention] Meanwhile, in 1986,
It has been discovered that sintered oxides such as [La,Sn]2CuO4 are superconducting materials with high Tc, followed by oxide superconductors represented by Y1Ba2Cu3O7-X, It was confirmed that the material exhibits effective superconducting properties in a temperature range above the temperature of liquid nitrogen.

Materials that exhibit superconducting properties at such high temperatures can use inexpensive liquid nitrogen as a cooling medium, so practical applications of superconducting technology have been studied, and oxide high-temperature superconducting materials Various attempts have been made to realize Josephson devices using .

An example of a Josephson device prototyped using such an oxide high-temperature superconductor is a grain boundary junction type device. However, the formation of junctions in this type of Josephson element often relies on chance, and it is difficult to obtain Josephson elements with desired characteristics with good reproducibility. Therefore, the grain boundary junction type Josephson element lacks industrial practicality. In contrast, as an example of a tunnel-type Josephson device prototyped using an oxide high-temperature superconductor,
The following are known:

(1) As a superconductor, an oxide superconductor represented by Y1Ba2 Cu3 O7-X, whose c-axis is grown in a direction perpendicular to the substrate, is used, and as a non-superconductor, MgO,
(2) A combination of materials similar to (1), in which the a-axis of the superconductor is grown perpendicular to the substrate,

00
(3) As a superconductor, an oxide superconductor represented by Y1Ba2 Cu3 O7-X whose c-axis grew in a direction perpendicular to the substrate was used, and as a non-superconductor, Pr1 Ba2
A normal conductor represented by Cu3O7-X grown in the same direction as the superconductor, (4) A combination of materials similar to (3), where the a-axis of the superconductor and non-superconductor is However, these tunnel-type Josephson devices grown in the direction perpendicular to the substrate do not exhibit sufficient characteristics even at the liquid helium temperature of 4.2K.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a tunnel-type Josephson device that effectively exhibits the excellent superconducting properties of an oxide high-temperature superconductor.

0010

[Means for Solving the Problems] According to the present invention, a pair of superconductor layers made of an oxide high temperature superconductor, a non-superconductor layer made of a non-superconductor formed between the pair of superconductor layers, A tunnel-type Josephson device comprising: the c-axis of the superconductor layer being perpendicular to the substrate; the superconductor layer and the non-superconductor layer growing epitaxially with respect to each other; A tunnel Josephson device is provided, wherein the non-superconductor layer has a lattice constant close to that of the superconductor layer and is made of a conductive, tetragonal perovskite oxide.

The superconducting material used in the tunnel-type Josephson device of the present invention is Y1Ba2 Cu3O.
A composite oxide represented by 7-X, a composite oxide having a composition in which Y in this composite oxide is replaced with a lanthanoid element such as Ho or Er, Tl2 Ba2 Ca1 Cu2
Ox, Tl2 Ba2 Ca2 Cu3 O10-Y
, Bi2 Sr2 Ca1 Cu2 Ox , Bi2
Examples include Sr2 Ca2 Cu3 O10-Y, and composite oxides of these to which Pb is added.

[0012] Examples of non-superconducting materials include LaNiO3,
La0.5Sr0.5 VO3, SrCrO3, L
aTiO3, BaPbO3, SrFeO3, Ba
Examples include conductive and tetragonal ABO3 type perovskite oxides having a lattice constant close to that of superconducting materials, such as 1-X KX BiO3.

[0013]

[Operation] The present inventors have conducted various studies on various attempts that have been made in the past to obtain tunnel-type Josephson devices using oxide high-temperature superconductors, and have found that all of them have certain problems. We researched the reasons for this.

First, the method of forming an insulator by surface oxidation of a superconductor is not effective for oxide superconductors which are originally oxides. So, as already mentioned,
Another insulating material such as MgO, S is used between the pair of superconductor layers.
Attempts have been made to form an insulating layer made of rTiO3, Al2O3, etc., but no effective properties have been achieved. According to research by the present inventors, the causes are as follows.

(1) Oxide high temperature superconductors require a high temperature of 650 to 750° C. during film formation. In addition, since the coherence length of oxide high temperature superconductors is very short, when the c-axis grows in a direction perpendicular to the substrate and the non-superconducting layer is an insulator, the thickness of the non-superconducting layer The film must be extremely thin, with a thickness of 10 to 50 angstroms. Due to the high temperature and thin film thickness during film formation, diffusion occurs near the interface between the non-superconductor layer and the superconductor layer when the superconductor layer is formed. As a result, the non-superconductor layer changes from a flat surface to an island-like structure, and a pair of superconductor layers are directly bonded to each other without interposing the non-superconductor layer. (2) The lattice constants of the insulating layer and the oxide high-temperature superconductor often differ greatly, in which case distortion occurs at the interface between the two.

Next, it is known that the coherence length of an oxide high-temperature superconductor is longer in the a-axis direction than in the c-axis direction. The a-axis of the layer is grown in a direction perpendicular to the substrate,
Attempts have been made to increase the thickness of the insulating layer. However, the Josephson device manufactured in this manner did not exhibit effective characteristics. According to research by the present inventors, the causes are as follows.

(1) An oxide high temperature superconductor whose a-axis is grown perpendicular to the substrate has poor surface flatness. Therefore, when a non-superconductor is laminated on top of this, the non-superconductor becomes a discontinuous and non-flat layer due to the unevenness of the underlying layer. If an upper superconductor layer is laminated on top of this, the pair of superconductor layers will come into direct contact with each other.

(2) In order to obtain an oxide high temperature superconductor in which the a-axis is grown perpendicular to the substrate, the film-forming temperature must be lowered by 50 to 100° C. compared to the case where the a-axis is grown. When a film is formed at such a low temperature, the resulting layer has poor crystallinity and does not exhibit superconducting properties at liquid nitrogen temperatures. Therefore, post-annealing is performed to obtain superconducting properties, but at that time,
Diffusion occurs between the superconductor layer and the non-superconductor layer.

Furthermore, Y1 Ba2 Cu3 O7-X
Oxide high-temperature superconductors such as Pr1 and Pr1 Ba2 Cu3 O7-X lose their superconductivity and become non-superconductors due to substitution of constituent elements. Attempts have been made to form non-superconductor layers. However, the Josephson device fabricated in this manner also did not exhibit effective characteristics. According to research by the present inventors, the causes are as follows.

(1) A non-superconductor layer obtained by replacing the constituent elements of a superconductor has a strong anisotropy because the structure itself is the same as that of the superconductor. In the case of a tunnel type Josephson device in which the c-axis is grown perpendicular to the substrate, the thickness of the non-superconducting layer must be extremely thin, 10 to 50 angstroms, as described above. Therefore, the same phenomenon as in the case of the insulator described above occurs.

(2) Since the structure of the non-superconductor layer obtained by replacing constituent elements of the superconductor is the same as that of the superconductor, diffusion occurs between the substituted element and the substituted element at the interface. . In this attempt, when the a-axis was grown perpendicular to the substrate, problems similar to those described above occurred. The present invention was made based on the above study results.

In the tunnel-type Josephson device of the present invention, the distance between the pair of superconductor layers can be increased by increasing the carrier density of the non-superconductor layer sandwiched between the pair of superconductor layers, and the distance between the pair of superconductor layers can be increased. When the carrier density of the superconductor layer is similar to or higher than the carrier density of the superconductor layer, the proximity effect from the non-superconductor layer without anisotropy acts to alleviate the anisotropy of the superconductor layer. It is used to solve the above problems.

That is, in a tunnel Josephson device, a conductive tetragonal oxide is used as a non-superconductor layer provided between a pair of superconductor layers grown with the c-axis perpendicular to the substrate. Accordingly, the thickness of the non-superconductor layer can be increased, island-like formation can be prevented, and the influence of anisotropy can be reduced. Furthermore, as a non-superconductor layer, a tetragonal perovskite type (ABO) having a lattice constant close to that of the superconductor layer is used.
3) By using oxides, the difference in lattice constant, thermal expansion coefficient, etc. between the superconductor layer and the non-superconductor layer is reduced, allowing epitaxial growth to occur and suppressing property deterioration due to interface distortion. It is.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be explained in more detail by showing examples of the present invention with reference to the drawings. Note that the following examples are intended to illustrate the present invention, and do not limit the technical scope of the present invention in any way. FIG. 1 shows the structure of the tunnel-type Josephson device according to this embodiment.

As shown in FIG. 1, this tunnel-type Josephson device has a first superconductor layer 2, a non-superconductor layer 3, and a second superconductor layer 4 laminated in this order on a substrate 1. . Parts of the non-superconductor layer 3 and the second superconductor layer 4 are removed, and a part of the first superconductor layer 2 is exposed. A first electrode 5a is provided on this exposed surface of the first superconductor layer 2,
Second electrodes 5b are formed on the surface of the second superconductor layer 4, respectively. The tunnel type Josephson device of FIG. 1 configured as described above is manufactured as follows.

(100) of the substrate 1 made of MgO single crystal
The first superconductor layer 2 having a thickness of 1000 to 3000 angstroms was formed on this surface by RF sputtering using the surface as a film forming surface. Y1 as a target
An oxide sintered body having a composition of Ba2Cu3O7-X was used. The film forming conditions were: substrate temperature: 700°C, film forming rate: 1 angstrom/sec, RF power: 150 W,
Gas flow rate: Ar/O2 = 10SCCM/10SCCM
, Pressure: 400 mTorr.

Y1 Ba2 C thus obtained
According to the X-ray diffraction method, u3 O7-X thin film 2 has only a diffraction peak of (00l [L]) peculiar to the c-axis orientation, and this thin film is grown with the c-axis perpendicular to the substrate. It was confirmed that the film was a crystalline film.

Next, a film was formed under the same conditions as above, except that a BaPbO3 sintered body was used as the target.
BaPbO3 with a film thickness of 50-100 angstroms
A non-superconductor layer 3 made of a thin film was formed. On the non-superconductor layer 3 produced in this manner, a second superconductor layer 4 was further formed in the same manner as the first superconductor layer 2.

For the laminate thus obtained,
SF until some regions of the first superconductor layer 2 are exposed.
6. Etching was performed using RIBE (reactive ion beam etching) using gas. Next, Au is deposited on the exposed upper surfaces of the first superconductor layer 2 and the second superconductor layer 4, and the first and second electrodes 5a, 5
b was formed. When the voltage-current characteristics of the tunnel Josephson device obtained as described above were measured, the results shown in FIG. 2 were obtained. The measurements were carried out at a temperature of 77K.

As shown in FIG. 2, in the tunnel-type Josephson device according to this example, the current drops rapidly around 10 mV, which corresponds to the energy gap, indicating that a high-quality tunnel junction is formed. was confirmed.

According to the procedure explained above, 10 tunneling type Josephson element samples were prepared and the voltage corresponding to the energy gap was measured. All samples fell within the range of 5 mA to 12 mA, indicating good reproducibility. It was confirmed that

[0032]

[Effects of the Invention] As explained above, according to the present invention,
Since a conductive, tetragonal perovskite oxide is used as the non-superconductor layer formed between a pair of superconductor layers, high-quality bonding is achieved at the interface between the superconductor layer and the non-superconductor layer. has been realized, and a tunnel-type Josephson device that exhibits excellent superconducting properties can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of a tunnel-type Josephson device according to an embodiment of the present invention.

FIG. 2 is a voltage-current characteristic diagram of a tunneling Josephson device according to an embodiment of the present invention.

[Explanation of symbols]

1... Substrate, 2... First superconductor layer, 3... Non-superconductor layer,
4... Second superconductor layer, 5a... First electrode, 5b... Second
electrode.

Claims (1)

[Claims] 1. A tunnel-type Josephson device comprising a pair of superconductor layers made of an oxide high-temperature superconductor and a non-superconductor layer made of a non-superconductor formed between the pair of superconductor layers. The c-axis of the superconductor layer is perpendicular to the substrate, and the superconductor layer and the non-superconductor layer are grown epitaxially with respect to each other, and the non-superconductor layer is grown in a direction perpendicular to the substrate. A tunnel-type Josephson device characterized by having a lattice constant close to that of the layer and being made of a conductive, tetragonal perovskite-type oxide.