JPH0414881A - Tunnel junction element - Google Patents

Abstract

PURPOSE:To enable reduction in superconductivity in an interface state to be improved by laminating an oxide superconductive thin film which is orientated in C axis, a noble metal thin film, an MgO film which is orientated in C axis, and an oxide surperconductive thin film which is orientated in A axis in sequence. CONSTITUTION:SrTiO3 single-crystal substrates (110) (100) or a MgO single- crystal substrate (100) are used as an insulation substrate 1 and a Y-family superconductor Y1Ba2Cu3OX or Bi-family superconductor Bi2Sr2Ca2Cu3OY superconductive thin film 2 is produced by the sputter method and the reactive deposition method while introducing an oxygen gas within a vacuum bath. The superconductive film is turned into C-axis orientated film. Then, a noble metal film 3 consisting of Au is immediately mounted onto the superconductive thin film 2. An MgO film 4 is produced on the noble metal thin film 3. Finally, the oxide superconductive thin film 2 is mounted with the orientated axis in A-axis direction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electronic device using an oxide superconductor, and particularly to a tunnel junction device having a function similar to that of a Josephson device.

[Conventional technology]

In recent years, research has been conducted on the application of oxide superconductors with high critical temperatures to the electronics field, and research on Josephson devices has been actively conducted as an alternative to metal/alloy superconductors.

In other words, a Josephson device is a switching device that has a structure in which an insulating layer is sandwiched between two superconductors, and utilizes the fact that electron tunneling current (superconducting tunneling current) flows in the insulator. However, conventional Josephson devices using metal/alloy superconductors have a low critical temperature and must be cooled using expensive liquid helium, which is costly. Yttrium (Y) series, bismuth (Bi) whose temperature is higher than liquid nitrogen temperature.
)-based oxide superconductors, the superconductors constituting Josephson devices were replaced with these yttrium (Y)-based and bismuth (Bi)-based oxide superconductors, and Josephson devices with high critical temperatures were developed. Prototypes are being made.

[Problem to be solved by the invention]

However, it can be said that there are almost no reports on the detection of electron tunneling current in devices using oxide superconductors.

The reasons for this are that the coherence length of the oxide superconductor is short and that paired electrons are destroyed due to the presence of a normal conductive layer at the surface and interface.

There are also reports that tunnel currents have been detected, but in most cases they are quasi-particle tunneling currents, that is, those of normal phase tunneling currents found in metal-insulator-metal (MIM) junctions. As for metal/alloy superconductors, a Josephson element that operates at a liquid helium temperature of 4.2 has been developed, and has reached the level of Josephson computer development.

In order to improve the deterioration of superconducting properties due to the interface state, which is a problem when producing a tunnel junction element as described above, the present invention has devised the element configuration, and has developed a tunnel junction using an oxide superconductor. The purpose is to realize the device.

[Means and effects for solving the problem] In order to achieve the above object, a tunnel junction element according to the present invention includes a 0M oriented oxide superconducting thin film, a noble metal thin film, a C
Axially oriented Mg.

The film is characterized by being formed by sequentially stacking a-axis oriented oxide superconducting thin films.

Furthermore, in the above tunnel junction element, indium (Y)-based and bismuth (Bi)-based superconductors are used as the oxide superconductor, and gold (Au) and platinum (Pt) are used as the noble metals.
).

More specifically, the present invention relates to a tunnel junction element using an oxide superconductor, and the oxide superconductor includes YLBa, a Y-based superconductor.

Cu s O, (6, 5 < x < 7, 5),
Bi-based superconductor B 1 zS r zc a tc
u zoa, B i zS rzc a zCu s
Using MgO (
100) Substrate or 5rTiO, (110) (100
) After the C-axis is oriented on the substrate, a very thin noble metal thin film is placed on top of it, and then a C-axis oriented MgO film is placed on top.
A superconducting film with axial or C-axis orientation is fabricated and used as an element.

The details of the present invention will be explained below.

The superconductors used in the tunnel junction device according to the present invention include Y-based superconductors with a critical temperature of 90 K and Bi-based superconductors with a critical temperature of 80 or 100 K. There is anisotropy in critical current density, resistivity, etc., and the critical current density in the direction perpendicular to the C-axis is about 10 times different from the direction in the axial direction.

Furthermore, regarding crystallinity, there are characteristics such as the loss of superconductivity due to oxygen loss, especially in Y-based superconductors, and the short coherence length of about 10. If a high-resistance phase exists or the insulating layer is thick, there is a problem that electron tunneling current will not flow when a Josephson junction is fabricated.

Furthermore, the technology for epitaxially growing oxide superconductors on very thin insulating layers has not yet been established.

Therefore, in the present invention, in order to improve the influence of the interfacial state on the electron tunneling current in the above problem, the tunnel junction element is configured as described above, and a noble metal thin film is provided on the superconductor to form a superconducting thin film. In addition to preventing characteristic deterioration of
We aim to realize a tunnel junction device using an oxide superconductor by allowing a pair of electron current to flow due to the proximity effect between the metal and the superconductor.

〔Example〕

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic diagram of a tunnel junction element showing one embodiment of the present invention.

In FIG. 1, the tunnel junction device according to the present invention has an insulating substrate 1, a c(ft[I oriented oxide superconducting thin film 2, a noble metal thin film 3, a C-axis oriented MgO film 4, an a-axis oriented oxide It has a structure in which superconducting thin films 2 are sequentially laminated.

The manufacturing method will be explained below.

First, the insulating substrate l is a 5rTiOa single crystal substrate (110) (100) or an ltMgOjL crystal substrate (
100), and a Y-based superconductor Y! is placed on the substrate. B
azcuso□ or Bi-based superconductor B l as
The rtc a z Cu s○ superconducting thin film 2 is produced by a known sputtering method or reactive vapor deposition method while introducing oxygen gas in a vacuum chamber. Further, if necessary, the oxygen gas is brought into a plasma state and the substrate temperature is heated to 550° C. to 650° C. for fabrication.

This substrate temperature affects the orientation axis direction of the film.

For example, it has been reported that in a Y-based superconducting film, the a-axis orientation occurs at a substrate temperature of around 550° C., and the c-axis orientation occurs at higher temperatures. However, the critical temperature may become lower when the substrate temperature is lower. Also, it is desirable to keep the film thickness as thin as possible;
Usually ranges from 500 to 4000 people. Note that the superconducting film on this substrate is a C-axis oriented film.

Next, on this C-axis oriented oxide superconducting thin film 2,
Immediately, a noble metal film 3 made of Au (or Pt) is deposited by 10 to 1 person by electron beam evaporation or ion sputtering.
Can be worn with a film thickness of less than 0.00 people.

At this time, the deposition rate is made as slow as possible so that some amount of Au penetrates into the outermost surface of the superconducting film 2. Further, the substrate temperature is set to about 400° C., and a C-axis oriented film is formed.

Next, on this noble metal thin film 3, an Mg◯ film 4 is formed. This MgO film 4 is produced by a vacuum evaporation method, and the film thickness is made by several tens of people. Also, the substrate temperature at this time is 600℃~
The temperature is raised to 650° C. to form a C-axis oriented film.

Finally, the oxide superconducting thin film 2 is applied again, but here it is desirable that the orientation axis be in the a-axis direction.

Also, the film thickness is the same as above, but the substrate temperature is 6.
The temperature is set to oo°C or slightly lower to facilitate a-axis orientation.

Now, as a result of measuring the current-voltage characteristics of the tunnel junction device fabricated as described above using the well-known four-probe method,
The tunneling current is improved by the presence or absence of the Au (or Pt) film 3, and since the pair electrons exist over a long distance (several hundreds of people) at the ultra-low conductivity film interface due to the proximity effect due to the presence of Au, the insulating film Mg It has become clear that if an oriented film of ○ is formed, electron tunneling current will flow. In other words, in the case of devices using conventional oxide superconductors, due to the short coherence length, the presence of a high-resistance layer at the interface caused the problem that tunnel current immediately stopped flowing.
In the device of the present invention, due to the presence of Au, pair electrons are not destroyed, and a pair electron tunneling current flows.

When considering the direction of current (the direction of the arrow in FIG. 1), by making the orientation axes of the superconducting film different on the upper and lower sides, the current flows in the plane where the critical current density is high.

〔Effect of the invention〕

As explained above, according to the present invention, a noble metal thin film is provided on an oxide superconducting thin film constituting a tunnel junction element to prevent property deterioration of the superconducting thin film, and also to prevent deterioration of the characteristics of the superconducting thin film, and to reduce The problem of tunneling current being difficult to flow due to the state of the interface when using oxide superconductors has been solved by allowing electron current to flow, making it possible to realize tunnel junction devices using oxide superconductors. It becomes possible.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of a tunnel junction element showing one embodiment of the present invention. 1... Insulating substrate, 2... Oxide superconducting thin film, 3
...Precious metal thin film, 4...MgO film.

Claims (1)

[Claims] 1. A c-axis oriented oxide superconducting thin film, a noble metal thin film, a c-axis oriented MgO film, and an a-axis oriented oxide superconducting thin film are sequentially laminated on an insulating substrate. Characteristic tunnel junction device. 2. In the tunnel junction device according to claim 1, the oxide superconductor is yttrium (Y) based, bismuth (B
i) A tunnel junction element characterized in that it uses a superconductor and gold (Au) and platinum (Pt) are used as noble metals.