JPH05251768A - Superconducting tunnel junction - Google Patents

Abstract

PURPOSE:To provide a niobium group Josephson junction with a magnetic tunnel barrier, in which there is no pinhole and no niobium lower oxide film is formed on a junction interface. CONSTITUTION:In a tunnel type Josephson junction, in which a lower electrode 11 and an upper electrode 15 are composed of niobium, a tunnel barrier 13 consisting of a nickel oxide film is held by aluminium 12, 14 having no magnetism. Consequently, the formation of a niobium lower oxide having an adverse effect on quasi-particle tunnel current characteristics on a junction interface by the reduction of a magnetic oxide can be prevented. Since aluminium is made easier to be oxidized than nickel, the thickness of the oxide of aluminium is thicker than that of niobium even when there is a pinhole. Accordingly, tunnel currents are made to flow only through the nickel oxide film.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a tunnel type Josephson junction used in a Josephson analog or digital circuit. More specifically, it relates to a Josephson junction that uses only quasiparticle tunneling current and does not use superconducting tunneling current.

[0002]

In some tunnel type Josephson junction application circuits such as low noise electromagnetic wave detectors, only the quasi-particle tunneling current of the Josephson junction is used. In this case, the superconducting tunnel current may cause deterioration of the characteristics. A tunnel junction having a tunnel barrier containing magnetism described in Technical Report SCE-84, page 55 of IEICE is suitable for such application. When magnetism exists in the tunnel barrier, the spin of electrons passing through it changes. Superconducting electrons must have opposite spins, and if the spin of one electron changes, the superconducting electron pair breaks. Therefore, superconducting electrons are less likely to pass through the tunnel barrier, and the superconducting tunnel current is reduced. In fact, it is easy to make the superconducting tunnel current completely zero as in the above-mentioned document. This change in spin is unlikely to occur when magnetism is in an ordered state, so in this case the tunnel barrier seems to be in a paramagnetic state. On the other hand, the quasiparticle tunneling current is not directly affected by magnetism.

In the above literature, the lower electrode material is tin, and the upper electrode also has a structure in which niobium is in point contact, but in order to apply to a wider range of applications, the upper and lower electrodes are made of niobium tunnel junction. Is desirable. FIG. 4 shows a structure in which the upper and lower electrodes are simply replaced by a niobium tunnel junction in the above-mentioned document. Figure 4
Then, the magnetic tunnel barrier 42 made of nickel oxide is sandwiched between the lower electrode 41 made of niobium and the upper electrode 43 made of niobium. Although there is a structure in which an oxide film is provided on both sides or one side of the magnetic tunnel barrier 42 in the above-mentioned document, such a structure increases the thickness of the tunnel barrier and reduces the quasi-particle tunnel current density. Depending on the application, a relatively high quasi-particle tunnel current density value is often required, and in such a case, a structure having an oxide film on both sides or one side of the magnetic tunnel barrier is not suitable.

[0004]

There are two problems in joining the structure shown in FIG. The first problem is the lower electrode 4
Since there are often undulations of several nm on one surface, 1
When a thin magnetic tunnel barrier 42 having a thickness of about 2 nm to 2 nm is formed on the lower electrode 41, the coating is insufficient and there is a high possibility that pinholes will be formed.

The second problem is the magnetic tunnel barrier 42.
There are nickel oxides, cobalt oxides, iron oxides, and the like as the oxides that make up Ni, but all of them are niobium monoxide (NbO) and niobium dioxide (NbO), which are lower oxides of niobium.
The heat of formation is smaller than that of O ₂ ), and when the upper electrode 43 niobium is deposited on the magnetic tunnel barrier 42, niobium deprives oxygen of the magnetic tunnel barrier and niobium lower oxide is generated at the junction interface. The niobium lower oxide has a metallic property, and if it is present at the junction interface, the quasi-particle tunnel current characteristic is significantly deteriorated.

It is an object of the present invention to eliminate such drawbacks of the prior art and to provide a niobium-based tunnel junction using a magnetic material having excellent quasi-particle tunnel current characteristics.

[0007]

According to the first invention of the present application, at least the lower part of the tunnel type Josephson junction using niobium or a niobium compound for the lower electrode and the upper electrode and including a magnetic material for the tunnel barrier is used. A non-magnetic first metal layer on the electrode, the tunnel barrier on the first metal layer, a non-magnetic second metal layer on the tunnel barrier, and the upper portion on the second metal layer. A superconducting tunnel junction characterized by arranging electrodes is obtained.

According to the second invention, in the tunnel type Josephson junction having niobium or niobium compound for the lower electrode and the upper electrode and having the magnetic material in the vicinity of the tunnel barrier, at least the lower electrode contains the magnetic material. A magnetic layer of a conductor, a metal layer on the magnetic layer that has a larger heat of formation of oxide than niobium oxide and does not have magnetism, the tunnel barrier formed of the oxide of the metal layer on the metal layer, and the tunnel barrier A superconducting tunnel junction is obtained which is characterized in that the upper electrode is arranged on top.

[0009]

In the first aspect of the present invention, the first metal layer containing no magnetism is provided between the magnetic tunnel barrier and the lower electrode.
This metal film covers the lower electrode surface without pinholes. Therefore, the tunnel barrier layer can be formed on the lower electrode surface without producing lower oxides of niobium by vapor-depositing the magnetic oxide and then performing thermal oxidation, or by depositing a magnetic material and then performing thermal oxidation. .. At this time, the film thickness of the magnetic oxide or the magnetic material to be vapor-deposited is only necessary for forming the magnetic tunnel barrier and is sufficiently thin. In this case, since the film thickness of the magnetic tunnel barrier is not sufficient to cover the first metal, there is a possibility that part of the surface of the first metal layer will be oxidized and become part of the tunnel barrier. However, also in this case, if a metal that is more easily oxidized than the magnetic material is selected as the first metal layer, the oxide film thickness on the exposed surface of the first metal layer can be made thicker than the magnetic oxide film. Since the tunnel current exponentially decreases with an increase in the tunnel barrier film thickness, the tunnel current can flow substantially only through the magnetic tunnel barrier. As described above, the first problem described above can be overcome.

In the first invention, niobium for the upper electrode is formed on the magnetic tunnel barrier via the second metal layer. Therefore, the magnetic oxide and niobium do not come into direct contact with each other and niobium lower oxide is not generated. When the heat of formation of the oxide of the second metal layer is larger than the heat of formation of the magnetic oxide, the second metal layer at the tunnel barrier interface is oxidized. However, if a material having a high insulating property such as an oxide such as aluminum is selected as the second metal layer, the oxide of the second metal layer does not adversely affect the quasi-particle tunnel current. As described above, the second problem described above can be overcome.

In the second aspect of the present invention, a conductive magnetic layer is formed at the interface with the lower electrode, and a layer having the same structure as an artificial barrier junction used in a normal niobium-based tunnel junction is formed thereon. .. Since the metal layer covers the surface of the lower electrode sufficiently, pinholes cannot be formed in the tunnel barrier and the first problem does not occur. Further, since the heat of formation of the insulating layer which is the tunnel barrier is larger than that of niobium oxide, niobium lower oxide is not generated during the film formation of the upper electrode niobium. Therefore, the second problem does not occur.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the first invention will be described with reference to FIG. A lower electrode 11 made of niobium and having a film thickness of 200 nm is formed on a substrate 10 made of silicon by a sputtering method. The surface of the lower electrode 11 has a texture of about 3 nm. When aluminum having a thickness of 6 nm is formed as the first metal layer 12 on the lower electrode 11 by the sputtering method, the surface of the lower electrode 11 is completely covered with the first metal layer 12. Next, nickel, which is a magnetic substance, is deposited by a 2 nm sputtering method and then thermally oxidized to form a tunnel barrier 13 made of nickel oxide. Nickel oxide usually exhibits antiferromagnetism, but in this case, nickel oxide is amorphous and in a paramagnetic state. Therefore, the effect of the magnetism of nickel oxide on the surrounding superconducting state is relatively small. Moreover, since the film thickness of nickel to be formed is 2 nm, the surface of the first metal layer 12 is not sufficiently covered, and there is a possibility that the surface of the first metal layer 12 is partially exposed during thermal oxidation. is there. If exposed, aluminum in the exposed portion is also oxidized during thermal oxidation and becomes part of the tunnel barrier. However, since aluminum is more easily oxidized than nickel, and nickel oxide has a semiconducting property and the barrier height is lower than that of aluminum oxide, which is an insulator, the product of the oxide film thickness and the square root of the barrier height is an exponential function. -Dependent tunneling current flows through aluminum oxide with little
It is thought to flow mainly through nickel oxide. On the other hand, since the first metal layer 12 completely covers the surface of the lower electrode 11, no niobium lower oxide that deteriorates the quasi-particle tunnel current characteristic is not generated.

Aluminum is deposited as a second metal layer 14 on the tunnel barrier by a 6 nm sputtering method to completely cover the surface of the tunnel barrier 13, and then niobium is deposited as an upper electrode 15 by a 200 nm sputtering method. Since nickel oxide has a lower heat of formation than a lower oxide of niobium, when the upper electrode 15 is directly formed on the tunnel barrier 13, the nickel oxide is partially reduced to form a lower niobium oxide. In this case, the quasi-particle tunnel current characteristic deteriorates, but this characteristic deterioration can be avoided by sandwiching the second metal layer 14 therebetween. On the other hand, by interposing the nickel oxide of the tunnel barrier 13 during the formation of the second metal layer 14, this magnetic deterioration can be avoided. On the other hand, the nickel oxide of the tunnel barrier 13 is partially reduced during the film formation of the second metal layer 14 to generate aluminum oxide. However, since aluminum oxide is a substance having a high insulating property, it adversely affects the quasi-particle tunnel current characteristics. It does not affect.

By using the structure shown in this embodiment, a tunnel type Josephson junction having a magnetic tunnel barrier using niobium as an electrode material can be formed without deteriorating the quasi-particle tunnel current characteristic. With this configuration, the film thickness of the magnetic material used is only the film thickness required for tunnel barrier formation, and can be made sufficiently thin. When a current is passed through this junction, the superconducting tunnel current is destroyed at the magnetic tunnel barrier 13 and does not flow, but only the quasi-particle tunnel current flows.

Although nickel is used as the magnetic material in this embodiment, other magnetic materials such as manganese, cobalt and iron can be used. Although the tunnel barrier 13 is formed by performing thermal oxidation after depositing a magnetic material, a magnetic oxide such as nickel oxide may be deposited directly. In this case, the pinholes of the tunnel barrier 13 can be closed with aluminum oxide by performing thermal oxidation after forming the magnetic oxide film. Although niobium was used as the electrode material, a niobium compound having superconductivity such as niobium nitride may be used. Aluminum is used as the first and second metals 12 and 14, but hafnium, zirconium, tantalum, or the like may be used.

A second embodiment of the present invention will be described with reference to FIG. A lower electrode 21 made of niobium having a film thickness of 200 nm is formed on a substrate 20 made of silicon by a sputtering method. The surface of the lower electrode 21 may have some undulations, but depending on the sputtering conditions, the undulations can be about 1 nm. When nickel is deposited as the magnetic layer 22 on the lower electrode 21 by sputtering to have a thickness of 2 nm, the surface of the lower electrode 21 can be completely covered with the magnetic layer 22. Nickel usually exhibits ferromagnetism, but in this case, nickel seems to be in an amorphous state and in a paramagnetic state, and the effect of the magnetism of nickel on the surrounding superconducting state is relatively small. Next, aluminum 4n is used as the metal layer 23.
After the film is formed by the m-sputtering method, thermal oxidation is performed to form the tunnel barrier 24 made of aluminum oxide. Since the surface of the lower electrode 21 is completely covered with the magnetic layer 22 and the metal layer 23, niobium lower oxide that deteriorates the quasi-particle tunnel current characteristic is not generated.

Niobium is formed as an upper electrode 25 on the tunnel barrier 24 by a 200 nm sputtering method. Since aluminum oxide has a higher heat of formation than the lower oxide of niobium, the lower oxide of niobium is not generated even when the upper electrode 25 is directly formed on the tunnel barrier 24.

Without the magnetic layer 22 in FIG. 2, the metal layer 23
Can be regarded as substantially superconducting due to the proximity effect of superconductivity, and the junction has a superconducting / insulator / superconducting structure.
If there is, the superconducting electron pair is broken when passing through the magnetic layer 22, resulting in a superconducting / normal conducting / insulating film / superconducting structure. Moreover, it is considered that the distribution of the density of states changes discontinuously at the interface between the lower electrode and the magnetic layer. Therefore, the energy band of the structure shown in this example is schematically shown in FIG.
It seems to have the structure shown in. Vgb and Vg in the figure
c represents a half of the energy gap value of the lower electrode and that of the upper electrode, respectively. The quasi-particle tunneling current is usually composed of the metal layer 23 when the voltage between the tunnel barriers 24 exceeds the energy gap Vgc of the upper electrode 25. Although an attempt is made to flow into the conduction region, the current from the upper electrode 25 will pass through the metal layer 23 through the metal layer 23 while the voltage between the tunnel barriers 24 does not exceed the sum Vgb + Vgc of the energy gaps between the lower electrode 21 and the upper electrode 25. Since it cannot flow into the electrode 21, the quasi-particle tunnel current does not flow. For this reason, the quasi-particle tunneling current characteristics are basically the same as in the case without the magnetic layer 22. Although the influence of the magnetic layer 22 on the superconducting property of the lower electrode 21 is neglected here, some influence must be actually taken into consideration. However, the magnetic layer 2
2 is a paramagnetic material, which has a much smaller effect on superconducting properties than a ferromagnetic material, and when the magnetic layer 22 is a mixture of a magnetic material and a non-magnetic metal, the content of the magnetic material is This effect can be made sufficiently small because the strength of magnetism can be controlled arbitrarily by changing it.

By using the configuration shown in this embodiment, a tunnel type Josephson junction having a magnetic tunnel barrier can be formed by using niobium as an electrode material without deteriorating the quasi-particle tunnel current characteristic. Moreover, since different materials can be used for the magnetic layer 22 and the tunnel barrier 24, the degree of freedom in material selection is increased.

In this embodiment, nickel is used as the magnetic material, but other magnetic materials such as manganese, cobalt and iron can be used. Although niobium was used as the electrode material, a niobium compound having superconductivity such as niobium nitride may be used. Although aluminum is used as the metal layer 23, hafnium, zirconium, tantalum, or the like can be used.

[0021]

According to the first aspect of the present invention, it is possible to form a tunnel type Josephson junction having a magnetic tunnel barrier by using niobium or a niobium compound as an electrode material without degrading the quasi-particle tunnel current characteristics. Have. In addition, the thickness of the magnetic material used is only the thickness necessary for forming the tunnel barrier, and it has the effect that it can be made sufficiently thin.

The use of the second invention has an effect that a tunnel-type Josephson junction having a magnetic tunnel barrier can be formed by using niobium or a niobium compound as an electrode material without deteriorating the quasi-particle tunnel current characteristic. Further, since different materials can be used for the magnetic layer and the tunnel barrier, there is an effect that the degree of freedom in material selection is increased.

[Brief description of drawings]

FIG. 1 is a schematic view showing a cross section of a structure of the first invention.

FIG. 2 is a schematic view showing a cross section of the structure of the second invention.

FIG. 3 is an energy band diagram for explaining an energy state in the junction of the second invention.

FIG. 4 is a schematic cross-sectional view for explaining a conventional structure.

[Explanation of symbols]

 10, 20, 41 Substrate 11, 21, 42 Lower electrode 12 First metal layer 13, 24, 42 Tunnel barrier 14 Second metal layer 15, 25, 44 Upper electrode 22 Magnetic layer 23 Metal layer

Claims (2)

[Claims] 1. In a tunnel-type Josephson junction including niobium or a niobium compound for a lower electrode and an upper electrode and a magnetic material for a tunnel barrier, at least a first metal layer having no magnetism on the lower electrode, A superconducting tunnel junction, wherein the tunnel barrier is disposed on one metal layer, the second metal layer having no magnetism is disposed on the tunnel barrier, and the upper electrode is disposed on the second metal layer. 2. A tunnel type Josephson junction having niobium or a niobium compound for a lower electrode and an upper electrode and having a magnetic material in the vicinity of a tunnel barrier, wherein a magnetic layer of a conductor containing a magnetic material at least on the lower electrode, A metal layer having a larger heat of formation of oxide than niobium oxide and having no magnetism on the magnetic layer, the tunnel barrier made of the oxide of the metal layer on the metal layer, and the upper electrode on the tunnel barrier. A superconducting tunnel junction characterized by: