JP2698364B2 - Superconducting contact and method of manufacturing the same - Google Patents

Description

The present invention relates to a superconducting contact and a method for manufacturing the same.

[Prior Art] Conventionally, in the field of superconducting microelectronics, Josephson devices have been greatly studied. The Josephson element has the following features: (1) tunnel type;
They are roughly classified into (2) point contact type, (3) bridge type and the like. For example, the tunnel type realizes a Josephson element by forming a sandwich structure of two superconducting thin films with a very thin (about 30 mm) insulating film, and superconducting electrons passing through the insulating film by a tunnel effect. .

Various superconducting materials have been used as materials for the Josephson device. Among them, Nb-based superconducting materials are mechanically strong and stable, and energetically studied. Among these Nb-based superconducting materials, Nb is selected as a material, and various oxides such as AlO _X and MgO and insulators are introduced between Nb electrodes as a tunnel barrier. I have.

Recently, the AC Josephson effect has been confirmed at the contact point between the oxide superconductor (Y-Ba-Cu-O) and the Nb needle. Also,
A fracture surface is formed in the oxide ceramic superconductor, and the fracture surface is re-contacted to obtain a Josephson junction operating near a critical temperature of 90K.

[Problems to be Solved by the Invention] However, when an oxide superconductor having a high critical temperature such as Y-Ba-Cu-O is brought into contact with a metal-based superconductor such as niobium or lead, The metal-based superconductor takes in oxygen on the surface of the oxide superconductor, and the surface of the metal-based superconductor is oxidized to form an insulator.

Furthermore, when oxygen is taken into the metal-based superconductor, the surface of the oxide superconductor becomes oxygen-deficient over a thickness of several hundred square meters, and the surface layer of the oxide superconductor becomes a semiconductor or insulator. The superconducting current formed does not flow and the superconducting contact cannot be made. An oxide superconductor having a high critical temperature is sensitive to oxygen vacancies and has a large effect on physical properties.

Accordingly, an object of the present invention is to solve the above-described problems and to provide a superconducting contact through which a superconducting current flows without forming an insulator or a semiconductor at an interface between an oxide superconductor and a superconductor. And a method for manufacturing the same.

[Means for Solving the Problems] To achieve the above object, a superconducting contact of the present invention comprises an oxide superconductor, a lower electrode whose surface is plasma-treated, and a plasma on the lower electrode. It has a contact layer made of a noble metal of 100 ° or more formed on the treated surface and a counter electrode made of a superconductor formed on the contact layer. Also,
The method for manufacturing a superconducting contact according to the present invention includes a step of plasma oxidizing a surface of an oxide superconductor serving as a lower electrode, and a step of forming a contact made of a noble metal on the surface of the plasma oxidized oxide superconductor. A step of forming a layer and a step of forming a superconductor layer as a counter electrode on the contact layer.

As the oxide superconductor used as the lower electrode, RE-
M-Cu-O-based (RE is a rare earth element, M is an alkaline earth metal excluding Ra) oxide superconductor can be exemplified.

The RE-M-Cu-O-based superconductor can be formed by dissolving a metal salt of an organic acid containing a component of the superconductor in an organic solvent, applying the metal salt to a substrate, and performing heat treatment.

Examples of a method for manufacturing the oxide superconductor include a thermal evaporation method such as a sputtering method and a molecular beam epitaxy method, and an electron beam evaporation method.

Since the oxide superconducting thin film manufactured by the above method is subjected to annealing treatment or taken out into the air, the surface layer of the oxide superconducting thin film is deteriorated by moisture or the like. Due to this moisture, the physical properties of the oxide superconductor change, and the superconductivity is greatly affected.

Plasma oxidation is performed for the purpose of improving superconductivity by preventing the entry of water. In order to reduce the damage of the oxide superconductor thin film due to this plasma oxidation and to uniformly modify the superconductivity of the surface, for example, under high pressure oxygen (1 Torr) and low power (11 W) conditions for a long time, (1 hour) It is necessary to perform plasma oxidation.

There are several methods for generating oxygen plasma, such as a method using a DC discharge and a method using a high-frequency or microwave oscillator.

In the superconducting contact of the present invention, a noble metal film that is hardly oxidized is interposed between the oxide superconductor and the superconductor. Examples of the noble metal include Au, Pt, and Ag. The purpose of this oxidation-resistant noble metal thin film is twofold.

The first purpose is to protect the surface layer of a plasma oxidized and modified oxide superconductor. Since a noble metal thin film that is not easily oxidized is used, there is no exchange of oxygen with the oxide superconductor. Therefore, it is possible to prevent the layer near the surface of the oxide superconductor from being in an oxygen deficiency state. it can. Further, it is possible to prevent the formation of a semiconductor or an insulator at the interface between the oxide superconductor and the hardly oxidizable noble metal thin film such as Au, Pt, or Ag.

A second object is to provide a superconductivity to a noble metal thin film such as Au having normal conductivity by a superconducting proximity effect. For this reason, a superconducting contact is made with the superconductor deposited on the noble metal thin film such as Au, and a superconducting current flows.

The thickness of the thin film of a noble metal such as Au plays a large role in the present invention. If the thickness of the noble metal thin film is 100 mm or less, crystal nuclei aggregate in the process of forming the noble metal thin film to form an island structure, and a uniform continuous film is not formed on the surface of the oxide superconductor. As a result, the oxide superconductor and the superconductor come into direct contact with each other, weakening the superconducting contact property.

On the other hand, even when the thickness of the noble metal thin film such as Au is 1000 mm or more, the noble metal thin film exhibits the superconducting proximity effect, but its superconductivity is very weak, and the superconducting contact property is weakened.

Therefore, the thickness of the noble metal thin film is 100Å to 1000Å.
Preferably, the optimum thickness is considered to be several hundred degrees.

It is not preferable that the superconductor used as the counter electrode forms a solid solution with a hardly oxidizable noble metal thin film such as Au, Pt, or Ag because the superconducting contact property is reduced. For this purpose, Au, P
Nb, NbN, and MoN that do not form a solid solution with noble metals such as t and Ag are preferable. As the counter electrode, RE-M-
A Cu-O-based oxide superconductor can also be used.

Here, the high melting point metal such as Nb can be formed by, for example, a vacuum evaporation method such as an electron beam heating method.

NbN and MoN thin films are prepared by reactive sputtering, CVD, plasma C
It can be formed by a VD method or the like.

[Operation] According to the present invention, a lower electrode composed of an oxide superconductor, a contact layer composed of a noble metal formed on the lower electrode, and a counter electrode composed of a superconductor formed on the contact layer are provided. Because of the structure, oxygen of the oxide superconductor is not taken into the contact layer, the upper surface of the oxide superconductor does not form a semiconductor or insulator, and the counter electrode that is bonded to the contact layer is also a semiconductor. A superconducting current flows between the lower electrode and the counter electrode without forming an insulator.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

Embodiment FIG. 1 is a sectional view of a superconducting contact according to an embodiment of the present invention.

First, a Y-Ba-Cu-O superconductor thin film 2 is formed on a substrate 1 as a lower electrode by a conventional method. This Y-Ba-Cu
The surface of the -O superconductor thin film 2 is subjected to an oxygen pressure of 1 Torr and an applied power of 1
Oxygen plasma treatment was performed for 1 hour in high-frequency oxygen plasma under the conditions of 1 W and a DC bias voltage of 21 V. After the oxygen plasma treatment, a gold thin film 3 having a thickness of 300 mm was deposited as a contact layer. An Nb thin film 4 was deposited as a counter electrode on the evaporated gold thin film 3 by a vapor deposition method or a reactive sputtering method.

With respect to the superconductor contact thus formed, it was observed that a superconducting current flows between the Y—Ba—Cu—O superconductor thin film 2 and the Nb-based metal thin film 4. Even when an NbN thin film was used in place of the Nb thin film 4, it was observed that a superconducting current flows between the Y-Ba-Cu-O thin film and the NbN thin film.

In this embodiment, the Nb-based material thin film is used as the counter electrode. However, for example, a Y-Ba-Cu-O-based oxide superconductor thin film can be used.

Application Example FIG. 2 is a sectional view showing an application example of the present invention.

As shown in FIG. 2, Y-Ba-Cu-
An O superconducting thin film 2 is formed. On this Y—Ba—Cu—O superconducting thin film 2, a gold thin film 3 as a contact layer is applied for about several hundred Å.
Deposit. For example, MgO, Al ₂ O ₃
An artificial tunnel barrier 5 (barrier) as described above is deposited.
Further, a Nb thin film 4 is deposited as a counter electrode to form a Josephson junction device. Here, SiO is deposited on the gold thin film and the substrate other than the artificial tunnel barrier.

In this application example, the Nb thin film is used as the counter electrode. However, it goes without saying that an NbN thin film, a MoN thin film, or an oxide superconductor can be used.

[Effects of the Invention] As described above, according to the present invention, a lower electrode made of an oxide superconductor, a contact layer made of a noble metal formed on the lower electrode, and a super electrode formed on the contact layer Since it is composed of a counter electrode made of a conductor,
Oxygen of the oxide superconductor is not taken into the contact layer,
The upper surface of the oxide superconductor does not form a semiconductor or insulator, and the counter electrode bonded to the contact layer also forms a superconductor between the lower electrode and the counter electrode without forming a semiconductor or insulator. Electric current flows.

The superconducting contact of the present invention can be applied in many application fields requiring high speed, high sensitivity, and high accuracy. For example, it can be applied to a high-speed computer element that focuses on high speed and low power consumption. In addition, with the progress of element technology based on Nb-based materials, LSI integration becomes possible, and it can be applied as an integrated logic circuit.

Furthermore, superconducting contacts can be used in the field of microelectronics such as SQUID devices, high-precision quantum voltage devices, and mixer devices.

Further, it can be used as a contact between superconducting wires.

[Brief description of the drawings]

 FIG. 1 is a sectional view showing an embodiment of the present invention, and FIG. 2 is a sectional view showing an application example of the present invention. 1 ... substrate, 2 ... Y-Ba-Cu-O thin film, 3 ... gold thin film, 4 ... Nb thin film, 5 ... artificial tunnel barrier, 6 ... SiO.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-245688 (JP, A) JP-A-63-299283 (JP, A) Jpn. J. Appl. Phys. 26 [9] (1987) L1443-1444

Claims (2)

(57) [Claims] A lower electrode made of an oxide superconductor, the surface of which is plasma-treated; a contact layer made of a noble metal of 100 ° or more formed on the plasma-treated surface of the lower electrode; And a counter electrode made of a superconductor formed on the contact layer. 2. A step of plasma-oxidizing the surface of the oxide superconductor as the lower electrode, and a step of forming a contact layer made of a noble metal on the surface of the oxide superconductor subjected to the plasma oxidation. Forming a superconductor layer as a counter electrode on the contact layer.