JPH03131075A - Josephson element - Google Patents

Abstract

PURPOSE:To obtain a tunnel-type Josephson junction element which can get such excellent properties as to operate at a relatively high temperature and solve the problem of nonuniformity of the thickness of a junction layer and the deterioration of the crystal property of a superconductor layer by making first and second superconductive layers out of metallic oxide superconductors expressed by the specified composition formulas, and making the junction layer out of a metallic oxide expressed by the specified composition formula. CONSTITUTION:This is one which generates oxygen gas ion beams from a high frequency plasma generation source with the constituent element of a ceramic superconductor as an evaporation source and at the same time performs deposition and oxidation. Steams of Ba, Pr, Y and Cu are generated from crucibles 3, 4, 5 and 6, respectively, and a film grows while intercepting the steam of Pr by a shutter 7, thus a superconductor layer is made. Next, by shifting the shutter 7 to intercept the steam of Y, a tunnel barrier layer is made, and further the shutter is returned to the former position to form a superconductive layer again. In case of getting tunnel type Josephson junction by a second method, the film of three-layer structure is made by changing the temperatures of crucibles as occasion demands, using three of the crucibles and without using the shutter.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a tunnel-type Josephson junction device that uses a yttrium-based oxide superconductor and can be operated at relatively high temperatures.

[Conventional technology]

A tunnel type Josephson junction element is basically composed of first and second superconducting layers and a junction layer (tunnel barrier layer) sandwiched between the two layers, and is a 5QUID.

It is widely used in electronic devices such as photodetection and switching. Conventionally, niobium (Nb) is used in the first and second superconducting layers of this tunnel-type Josephson junction device.
), lead (Pb), Nb compounds, pb compounds, etc., but in recent years, ceramic materials that exhibit superconductivity at temperatures above liquid nitrogen temperature have been discovered, such as Y-Ba-Cu.
-0 series metal oxides, etc.

Tunnel-type Josephson junction devices using such Y-Ba-Cu-0 based oxide superconductors have already been manufactured.

[Problem that the invention is trying to solve] However,
In tunnel type Josephson junction devices using ceramic materials such as Y-Ba-Cu-0, it is necessary to form a thin (and uniformly thick) junction layer and to improve the crystallinity of the superconducting layer. It is very difficult to have a good face contact,
There was a drawback that it was not possible to obtain an element with excellent characteristics.

In view of the above-mentioned drawbacks, for example, JP-A-63-3O6
676, JP-A 63-3O6678, JP-A 64-12585, and JP-A 64-51680 disclose Y-Ba- which forms the superconducting layer in a tunnel type Josephson device. Cu-0 based oxide superconductor is Y-Ba-C with different oxygen content or copper content.
It is disclosed that a u-0 type oxide is used as a bonding layer.

However, in the above prior art, the oxygen content of the bonding layer is changed (the oxygen content is increased (or decreased)).
However, it is difficult to control the film formation conditions, and changing the copper content (increasing the copper content will result in excessive copper segregation in the layer, which will result in excess copper on the bonding layer). This leads to deterioration of the crystallinity of the conductive layer.

Furthermore, in Japanese Patent Application Laid-Open No. 64-13777, the bonding layer is made of Ln.
-Ba-Cu-0 based oxide (Ln=Ce or P
(r), but since the above oxide does not exist stably in bulk, the manufacturing conditions are difficult, and it is also insufficient in terms of improving the crystallinity of the superconducting layer on the bonding layer. there were.

Therefore, an object of the present invention is to use a tunnel-type Josephson junction that operates at a relatively high temperature (e.g., above liquid nitrogen temperature) using a high-temperature superconductor (ceramic superconductor) as the first and second superconducting layers. The purpose is to provide an element.

Furthermore, it is an object of the present invention to solve the problems of non-uniformity in the thickness of the bonding layer sandwiched between the first and second superconducting layers and deterioration of the crystallinity of the superconductor layer, and to solve Another object of the present invention is to provide a tunnel-type Josephson junction device that has even better surface contact and provides excellent characteristics.

Still another object of the present invention is to provide a tunnel type Josephson junction device that is easy to manufacture.

[Means for solving problems]

The above object is achieved by the present invention as described below.

That is, the present invention consists of two inventions. 2
The superconducting layer is formed of a metal oxide superconductor having the compositional formula LnBa2Cu3O7-6, and the bonding layer has the compositional formula Ln0.5-aPrO,5+aBa
This is a Josephson element characterized by being formed of a metal oxide represented by 2Cu3O7-6.

(However, Ln is La, Nd, Sm, Eu, Gd
, Dy, Ha, Er.

At least one element selected from Tm, Yb, Lu and Y, O≦δ<0.5, 0.1≦a<0.5. )
Furthermore, the second invention provides a first superconducting layer on the substrate surface.

In a Josephson device formed by sequentially laminating a bonding layer and a second superconducting layer, the first and second superconducting layers are formed of a metal oxide superconductor represented by the composition formula LnBa 2 Cu 3 O 7-δ. and the bonding layer has a composition formula L n'
This is a Josephson element characterized in that it is formed of a metal oxide represented by +, z+b Ba +, s-b Cu3O7-a. (However, Ln is La, Nd
, Sm, Eu.

Gd, Dy, H0. At least one element selected from Er, Tm, Yb, Lu and Y; Ln is La, N
d.

At least one element selected from Eu and Gd, O≦
δ<0, 5.0≦b<1.8. ) In the present invention, as a means of forming a junction layer (hereinafter referred to as a tunnel barrier layer) of a tunnel-type Josephson junction element, instead of using an oxygen-deficient layer of a compound forming the first and second superconducting layers,
By replacing some of the constituent elements other than oxygen with other elements or by changing the composition ratio of constituent elements other than oxygen, the 1-2-3 type structure of LnBa 2 Cu 3Of-15 can be obtained. A tunnel barrier layer was formed by imparting insulation properties while maintaining the .

The first specific method is to use the L as a tunnel barrier layer.
This is a method using a compound in which part of Ln in nBa2Cu3Or-a is substituted with Pr.

1-2-3 type compound LnBa2 Cu3O7-a is Ln
Rare earth elements other than Ce and Tb and Y are possible, but in compounds containing elements other than Pr, by sufficiently containing oxygen through appropriate treatment, both Tc and Y can be used.
-90 class superconductor. However, it is known that Pr compounds act as insulators regardless of the oxygen content.

Also, it is not necessary that all Ln sites be occupied by Pr (,
For example, when Y and Pr occupy Ln sites in a ratio of 426, an insulator is formed. (References Shin Niyamagata et al., Physical Society of Japan 43rd Annual Meeting 3a-PS-46, Proceedings Volume 3 p3
10) As a result of intensive research, the present inventors found that LnBa 2 which forms the first and second superconducting layers as tunnel barrier layers
60% or more and less than 100% of Ln in Cu3O7-a is P
Compounds substituted with r, that is, the compositional formula is L n 0.
5-aPr0. The inventors have discovered that it is preferable to use a compound represented by s+aBa2Cu3Or-a in order to provide a tunnel-type Josephson junction device that has good surface bonding and provides good characteristics, and has arrived at the present invention. (
However, Ln is La, Nd, Sm, Eu, Gd.

Dy, H0. Er, Tm, Yb, Lu and Y
At least one element selected from 0≦δ<0.5.
0.1≦a<0.5. ) The second specific method is to vary the ratio of the rare earth element and Ba content from 1:2, increasing the rare earth element and decreasing the amount of Ba (a method to obtain insulation by doing so). It is.

This method is effective when a light rare earth element is used as the rare earth element. For example, in Nd compounds, compounds containing equal amounts of Nd and Ba, that is, N d +, s Ba +, s Cu 3
Even in the case of O y-a, it maintains a 1-2-3 type structure and is electrically an insulator. (References Shungi Ta
kekawa et.

al: JJAP 26 (1987) L207
6-L2079) As a result of intensive research, the present inventors found that Ba of Ln' Ba2 Cu3Oy-a was used as a tunnel barrier layer.
A compound in which 10% to 100% of is substituted with Ln', that is, the compositional formula is Ln' +, z+b B a +, a-
The inventors have discovered that it is preferable to use a compound represented by Cu 3 O 7-15 in order to provide a tunnel-type Josephson junction device that has good surface bonding and provides good characteristics, leading to the present invention. (However, Ln is La,
At least one element selected from Nd, Eu and Gd, 0≦δ<0.5.0≦b≦1.8. ) The Josephson element of the present invention can be produced by switching the evaporation sources of Pr and other rare earths or by changing the evaporation rates of rare earths and Ba in the film formation process including ion beam evaporation and plasma oxidation. are formed by sequentially laminating them.

This method is possible by switching the evaporation source using a shutter, etc., or by adjusting the power input to the evaporation source, and it is also simple in terms of the manufacturing process since there is no need to change the plasma oxidation conditions during film formation. be.

Preferred embodiments of the invention will now be described in more detail with reference to the drawings, in which preferred embodiments of the invention are illustrated.

FIG. 1 is a schematic diagram of a vapor deposition apparatus using the method of the present invention.

In the figure, 1 is a substrate, 2 is a thin film with a three-layer structure of superconductor layer/tunnel barrier film/superconductor layer, 3 to 6 are crucibles containing evaporated substances, and 7 is crucible 4 or 5. A shutter for blocking vapor from either side; 8 an ionization unit for ionizing the vapor of the evaporated substance coming out of the crucible; 9 an accelerating electrode for accelerating the ionized vapor; It is an ion beam. 1
1 is an acceleration power source for accelerating steam, and is capable of accelerating up to about 10 KV. 12 is a vacuum vessel, 13 is a high frequency plasma generation source, and 14 is a gas ion beam. 15
is a shutter to block the beam incident on the substrate. Note that the temperature of the substrate 1, the temperature of the crucibles 3 to 6, the ionization current in the ionization unit 9, the gas flow rate in the high-frequency plasma generation source 13, the input power, and the acceleration voltage can be independently controlled by a device not shown.

In the present invention, the above-described apparatus is used, the constituent elements of the ceramic superconductor are used as evaporation sources independently from the crucibles 3 to 6, and further,
An oxygen gas ion beam is generated from a high-frequency plasma source, and evaporation and oxidation are performed simultaneously. By performing vapor deposition while generating high-frequency plasma in this manner, oxidation of the thin film is further promoted. In this way, a superconducting thin film with sufficiently good characteristics can be obtained without heat treatment.

In order to obtain a tunnel-type Josephson junction using the first method, for example, vapors of Ba, Pr, Y, and Cu are generated from the crucibles 3, 4, 5, and 6, respectively, and the Pr vapors are blocked by the shutter 7. A superconductor layer is formed by depositing the superconductor layer. Next, by moving the shutter 7 to block the vapor of Y, a tunnel barrier layer is formed.
Furthermore, the shutter is returned to its original position and a superconductor layer is formed again.

When obtaining a tunnel Josephson junction using the second method, three of the crucibles of the present device are used, and no shutter is used. A three-layer thin film is formed by changing the temperature of the crucible as necessary.

Next, the present invention will be specifically explained with reference to Examples.

[Example 1] YBa2 cu3O7-6 (δ
~0.2), P r O as the material of the tunnel barrier layer,
9Y 0. +Ba2Cu3Oy was selected.

Using the apparatus shown in FIG. 1, Ba, Pr, Y, and Cu were placed in crucibles 3 to 6 as evaporation sources, 5rTiO3 was used as a substrate, and the substrate temperature was set at 600°C. The degree of vacuum in the vacuum vessel 18 was adjusted to 2×10 Torr, and oxygen gas from a high-frequency plasma source was introduced at a rate of 9 ml/min from an inlet not shown in FIG. The ionization current of each component was set to 50 mA, the accelerating voltage was set to 0.5 KV, the high frequency power of the high frequency plasma source was set to 300 W, and the accelerating voltage was set to 0.3 KV.

Adjust the temperature of crucibles 3 to 6, and apply Y: Pr on the substrate.
:Ba:Cu=1:1:2:3.

After doing this, the vapor of Pr was blocked by the shutter 7, and the deposition of the superconductor layer was started. The film forming rate at this time is 2 to 3 people/second.

After that, the shutter 7 is opened and closed appropriately to block the steam of Y and allow the steam of Pr to pass through.P r O, 9Y 0. +
A barrier layer is formed by depositing B a 2Cu3O7.

After forming 30 layers, the shutter 7 was returned to its original position to form a superconductor layer.

The three-layered film thus formed was shaped into the shape shown in FIG. 2 in order to examine its characteristics. 16 is S r
T r 03 , the substrate 17 is a superconductor layer, 18 is a tunnel barrier layer, 19 is a superconductor layer, and 20 is a gold electrode provided for measurement.

This element exhibited current/voltage characteristics as shown in FIG. 3 when measured at 4.2K. It is recognized that a better tunnel-type Josephson junction is formed than this.

[Example 2] Ba, La, Cu were placed in crucible 3.5.6 using the above-mentioned apparatus.
to form a superconductor layer in the same manner as in Example 1.

Thereafter, the shutter 15 is closed once and the atomic ratio on the substrate is changed by the temperature of the crucible so that it becomes La:Ba:Cu=1+1:2, and then the shutter 15 is opened to form a tunnel barrier layer of 30 atoms. . Thereafter, the shutter 15 is closed, the temperature of the crucible is returned to the original temperature, and then the shutter 15 is opened again to form a superconductor layer.

The bond formed in this way also had the same characteristics as in Example 1.

[Other Examples]

Note that the embodiments of the present invention are not limited to the above embodiments. That is, in addition to the method of using a mechanical shutter as described above and the method of changing the temperature of the crucible as a means of changing the evaporation substance and the vapor deposition rate,
For example, means such as reversing the polarity of the voltage applied to the accelerating electrode may be used. Further, the substrate is not limited to 5rTiO3, but other substrates such as MgO can also be used.

[Effect] The Josephson element of the present invention described above operates at a relatively high temperature (e.g., higher than the liquid nitrogen temperature), and the first and second
Since the thickness of the bonding layer sandwiched between the superconducting layers is uniform, and the crystallinity of the superconducting layer is also good, excellent properties can be obtained with good surface bonding.

Furthermore, it has the advantage of simplifying the manufacturing process.

[Brief explanation of the drawing]

FIG. 1 is a diagram schematically showing a vapor deposition apparatus used in the present invention. FIG. 2 is a schematic diagram showing a cross section of the element of the present invention. FIG. 3 shows the current-voltage characteristics at 4.2 K of the device of the present invention. 1: 5rTiO3 substrate 2: Three-layer structure film 3 to 6: Crucible 7: Shutter 8; Ionization unit 9: Acceleration electrode 10: Ion beam 11: Acceleration power source 12: Vacuum vessel 13: High frequency plasma source 14: 15: 16: 17: 18: 19: 20: Gas ion beam shutter 5rTiO3 substrate First superconductor layer Tunnel barrier layer Second superconductor layer Electrode ? two mouths

Claims (2)

[Claims] (1) In a Josephson device in which a first superconducting layer, a bonding layer, and a second superconducting layer are sequentially laminated on a substrate surface, the first and second superconducting layers have a composition formula of LnBa_2Cu_
The bonding layer is formed of a metal oxide superconductor represented by 3O_7_-_δ, and the bonding layer has a composition formula Ln_0_. ＿５＿－＿a
Pr_0_. _5_+_aBa_2Cu_3O_7_-
A Josephson element characterized by being formed of a metal oxide represented by _δ. (However, Ln is La, Nd, Sm, Eu, Gd, Dy,
At least one element selected from Ho, Er, Tm, Yb, Lu and Y, 0≦δ<0.5, 0.1≦a<0.
It is 5. ) (2) In a Josephson device in which a first superconducting layer, a bonding layer, and a second superconducting layer are sequentially laminated on a substrate surface, the first and second superconducting layers have a composition formula of LnBa_2Cu_
The bonding layer is formed of a metal oxide superconductor represented by 3O_7_-_δ, and the bonding layer has a composition formula Ln'_1_. ＿２＿＋＿
bBa_1_. A Josephson element formed of a metal oxide represented by _8_-_bCu_3O_7_-_δ. (However, Ln is La, Nd, Sm, Eu, Gd, Dy,
At least one element selected from Ho, Er, Tm, Yb, Lu and Y, Ln' is La, Nd, Eu and Gd
At least one element selected from 0≦δ<0.5,
0.1≦b<1.8. )