JP2746990B2 - Superconducting element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a superconducting element using an oxide superconductor.

(Prior Art) Conventionally, many superconducting materials whose electric resistance becomes zero below a certain temperature have been discovered, and various attempts to realize a superconducting wire or a superconducting device using them have been studied. ing. Recently, an oxide-based high-temperature superconductor having a transition temperature equal to or higher than the boiling point of liquid nitrogen, for example, a defect perovskite-type oxide superconductor having an oxygen defect represented by a Y-Ba-Cu-O system, High critical temperature Bi-Sr-Ca-Cu-O system and Tl-Ba-Ca-Cu
Various -O-based oxide superconductors have been discovered, and attempts to realize a superconducting element that does not require expensive liquid helium using these high-temperature superconductors have been attracting attention.

Many superconducting elements, as typified by a Josephson element, include, as a basic structure, a superconducting layer / insulating layer / superconducting layer or a junction of a superconducting layer / semiconductor layer / superconducting layer. Here, the insulating layer in the superconducting layer / insulating layer / superconducting layer junction must be formed sufficiently thin, such as 10 ° to 50 °, in order to obtain a sufficient tunnel current.

However, when it is intended to form a junction having the above three-layer structure using an oxide superconductor, an oxide film formed by oxidizing a metal thin film like a conventional alloy-based superconductor is used as an insulating layer. When a normal insulator is to be formed as a thin film, it is difficult to sufficiently improve the surface smoothness of the oxide superconductor layer itself. It has been very difficult to form a sufficiently thin insulating layer on the superconductor layer uniformly and with good reproducibility.

On the other hand, a superconducting element using a point contact type or a slit type joining method is also being studied in place of the above-mentioned three-layered tunnel junction. Is essential.

(Problems to be Solved by the Invention) As described above, in forming a Josephson junction of a superconducting layer / insulating layer / superconducting layer exhibiting good junction characteristics using an oxide superconductor having a high critical temperature, oxidation is performed. It is an essential condition that an insulating layer capable of obtaining a sufficient tunnel current is formed on the superconductor layer with good reproducibility. However, with the current thin film forming technology, a sufficiently thin insulating layer is formed on the oxide superconductor layer. It is extremely difficult to form a layer uniformly and with good reproducibility.

As described above, at present, a tunnel junction such as a stacked Josephson junction using an oxide superconductor and an insulator having good junction characteristics and excellent reproducibility has not yet been obtained. The same applies to a tunnel junction of a superconducting layer / insulating layer / semiconductor layer in a quasiparticle injection type three-terminal element or the like. In order to realize a specific superconducting element using an oxide superconductor, the junction characteristics and reproducibility of the stacked Josephson junction formed by interposing an insulating layer on the oxide superconductor layer have been improved. It is strongly desired to do so.

The present invention has been made to address such a problem, and regardless of the insulating layer forming technique, an insulating layer can be formed on an oxide superconductor layer that is excellent in reproducibility and provides good bonding characteristics. It is an object of the present invention to provide a superconducting element having a stacked Josephson junction formed by intervening.

[Means for Solving the Problems] That is, the present invention relates to a superconducting element having a laminated Josephson junction formed by interposing a thin-film insulating layer on an oxide superconducting layer. The insulating layer has an ionization energy in the range of 5 eV to 6.5 eV and functions as an insulator at the operating temperature of the superconducting element, and is selected from a group III-V compound semiconductor, a group II-VI compound semiconductor, and a group IV semiconductor. It is characterized by being constituted by at least one kind of semiconductor.

Many oxide superconductors are known, but in the present invention, oxide superconductors having a perovskite structure containing a rare earth element, Bi-Sr-Ca-Cu-O
An oxide-based superconductor, a Tl-Ba-Ca-Cu-O-based oxide superconductor, or the like is applied.

The oxide superconductor containing a rare earth element and having a perovskite structure may be any as long as it can realize a superconducting state. For example, an LnM ₂ Cu ₃ O _7-δ system (Ln is Y,
At least one element selected from rare earth elements such as La, Sc, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu is represented by M
Represents at least one element selected from Ba, Sr, and Ca,
Represents an oxygen vacancy, usually 1 or less, and part of Cu is Ti, V,
Can be replaced with Cr, Mn, Fe, Co, Ni, Zn, etc. )) And the like. Furthermore, Bi-Sr-Ca-Cu -O based oxide superconductor has the formula _{Bi 2 Sr 2 Ca 2 Cu 3} Ox ...... (I) Bi 2 (Sr, Ca) 3 Cu 3 Ox ...... (II) ( In the formula, a part of Bi can be replaced with Pb, Sb, or the like.), And the Tl—Ba—Ca—Cu—O-based oxide superconductor has a chemical formula of Tl ₂ Ba ₂ Ca ₂ Cu ₃ Ox ... (III) Tl ₂ (Ba, Ca) ₃ Cu ₃ Ox ... (IV)

The insulating layer in the superconducting element of the present invention has an ionization energy in the range of 5 eV to 6.5 eV and functions as an insulator at the operating temperature of the superconducting element (1 kΩ in resistance value).
The above is constituted by a semiconductor. For example, it is formed of a semiconductor that functions as an insulator (at a resistance value of 1 kΩ or more) at the operating temperature of an electric insulator or a superconducting element.

Such semiconductor materials include Ga _1-x Al _x As, GaAs
III-V group compound semiconductors such as _1-x P _{x and I} such as ZnSe _1-x S _x
At least one selected from group VI semiconductors such as I-VI group compound semiconductors, Se _1-x , Te _x , and S _1-x Se _{x is used,} and the ionization energy is adjusted to the above range by adjusting the doping amount of impurities. It is possible to control within.

Here, the ionization energy of the semiconductor to be the insulating layer is
The reason for limiting the range to 5 eV to 6.5 eV is as follows.

For example, when a metal-based superconductor is used, the height of the tunnel barrier formed by the superconducting layer / insulating layer / superconducting layer is substantially determined by the difference between the work function of the metal and the electron affinity of the insulator. When a superconductor is used, since the oxide superconductor is a p-type degenerate semiconductor, it is almost determined by the difference between the ionization energy of the oxide superconductor and the ionization energy of the insulating layer, and this value is made as small as possible. This makes it possible to ignore the thickness of the insulating layer. The ionization energy of the oxide superconductor is estimated to be in the range of 5 eV to 6.5 eV from the value of the conventional transition metal oxide. Therefore, the value of the ionization energy of a semiconductor that behaves as an insulator at a low temperature to be an insulating layer is 5 eV to 6.5
By setting the range of eV, even if the thickness of the insulating layer is made thick enough to form a good film, a Josephson junction with a low tunnel barrier can be obtained, and sufficient tunnel current can flow. .

In this case, the ionization energy of the insulating layer constituting material is ± 0% of the ionization energy of the oxide superconductor.
It is preferable to set to about 2 eV.

Josephson junctions in the superconducting element of the present invention include, for example, a Josephson element, a quasiparticle-injected three-terminal element, an SQ
Superconducting layer / insulating layer / superconducting layer in UID element, etc., or superconducting layer /
For example, an insulating layer / semiconductor layer is used.

The superconducting layer and the insulating layer in the superconducting element of the present invention may be formed by magnetron sputtering, ion beam sputtering, cluster ion beam, molecular beam epitaxy (MB
It can be obtained by various thin film forming methods such as E) method and plasma CVD method. Further, the thickness of the superconductor layer is preferably a thickness at which the oxide superconductor exhibits superconductivity, that is, approximately 1000 mm or more, and the thickness of the insulating layer is, as described above, the ionization energy of the material forming each layer. Depending on the height of the tunnel barrier determined by the difference, it is preferable that the thickness be such that a good film can be formed within a range that does not hinder the tunnel effect, for example, about 50 ° to 100 °.

(Operation) For example, in the case of a stacked Josephson junction using a superconducting layer / insulating layer / superconducting layer, if the thickness of the insulating layer is too large, a weak junction between the superconductors cannot be obtained, and a sufficient tunnel current can be obtained. Disappears. Therefore, in order to obtain a sufficient tunnel current, it is important to make the height of the tunnel barrier sufficiently low.

In the present invention, the height of the tunnel barrier formed by the interposition of the insulating layer on the oxide superconductor layer is substantially determined by the difference between the ionization energy of the oxide superconductor and the ionization energy of the insulating layer. The value of the ionization energy of the semiconductor that behaves as an insulator at a low temperature, which becomes the insulating layer, is set to 5 eV to 6.5 eV, which is equivalent to the ionization energy of the oxide superconductor. Can be reduced as much as possible, so that the thickness of the insulating layer can be made somewhat thick enough to form a good film. Therefore, for example, a stacked Josephson junction using an oxide superconductor layer / an insulating layer / an oxide superconductor layer can be obtained extremely easily and with good reproducibility.

(Example) Next, an example of the present invention will be described.

FIG. 1 is a view schematically showing a Josephson element according to an embodiment to which the present invention is applied. On a strontium titanate (SrTiO ₃ ) substrate 1, a YBa ₂ Cu ₃ O having a thickness of about 5000
First oxide superconductor layer 2 having a _7-δ composition (transition temperature = 90)
K) are formed in a stripe shape.

At a substantially central portion on the first oxide superconductor layer 2,
An insulating layer 3 having a thickness of about 50 ° is formed. This insulating layer 3
Is made of GaAs _0.4 P _0.6 , functions as an electrical insulator at about 50 K, and has an ionization energy of about 6 eV. On this insulating layer 3, a second oxide superconductor 4 (transition temperature = YBa ₂ Cu ₃ O _7-δ composition having a thickness of about 5000 °) is formed so as to be orthogonal to the first oxide superconductor layer 2. 75K) is formed in a stripe shape.

Then, on both ends of the first and second oxide superconductor layers 2, 4, an Au electrode 5, which serves as a current-voltage terminal,
6, 7, 8 are formed to constitute a Josephson element. The first and second oxide superconductor layers 2, 4
The ionization energy of VBa ₂ Cu ₃ O _7-δ , which is a material for forming, is about 6.3 eV as estimated from UPS.

Such a Josephson element is manufactured, for example, as follows.

First, YBa ₂ Cu was deposited on a SrTiO ₃ substrate 1 by laser evaporation.
An oxide superconductor film of ₃ O _7-δ composition is formed. Next, GaAs _{0.4 is formed} on the oxide superconductor film by MBE at a predetermined width.
After the semiconductor film made of P _0.6 is formed, the first oxide superconductor layer 2 and the insulating layer 3 are formed by processing in a stripe shape by wet etching or the like so that the semiconductor film remains by a predetermined length. I do.

Next, masking is performed on the first oxide superconductor layer 2 and the insulating layer 3 formed in a stripe shape so that the second oxide superconductor layer 4 is formed orthogonally, and the laser vapor deposition method is performed again. Thereby, an oxide superconductor film having a composition of YBa ₂ Cu ₃ O _7-δ is deposited, and a second oxide superconductor layer 4 is formed.

Thereafter, Au electrodes 5, 6, 7, and 8 are formed on both ends of the first and second oxide superconductor layers 2 and 4 by a normal vapor deposition method to obtain a Josephson device.

When the voltage-current characteristics of the thus-produced Josephson device were measured at a liquid helium temperature, the results shown in FIG. 2 were obtained. As is clear from the figure, in the Josephson device of this example, a large superconducting gap (2Δ = 10 mV) was observed, and it was confirmed that a good Josephson junction was formed.

As described above, in the Josephson device of this embodiment, since the ionization energies of the first and second oxide superconductor layers 2 and 4 made of an oxide superconductor and the insulating layer 3 are close to each other, the insulation Despite the relatively thick layer 3 having a thickness of 50 °, a sufficient tunnel current is obtained, and as a result, a large superconducting gap is obtained.

Next, another embodiment of the present invention will be described. FIG. 3 is a view showing an embodiment in which the superconducting element of the present invention is applied to a quasiparticle injection type three-terminal element.

In the figure, reference numeral 11 denotes a semiconductor substrate made of GaAs,
On the semiconductor substrate 11, a YBa ₂ layer is formed via a GaAs _0.4 P _0.6 film 12 functioning as an insulator at the operating temperature of the three-terminal device.
A first oxide superconductor layer 13 having a composition of Cu ₃ O _7-δ is formed.

On the first oxide superconductor layer 13, similarly, GaAs
_A second oxide superconductor layer 15 having a composition of YBa ₂ Cu ₃ O _7-δ is formed via a _0.4 P _0.6 film 14 to constitute a quasiparticle-injection type three-terminal element.

In this quasiparticle injection type three-terminal element, GaAs _0.4 P
_In addition to the insulating layer that forms a tunnel junction between the first oxide superconductor layer 13 and the second oxide superconductor layer 15, the _0.6 A GaAs _0.4 P _0.6 film is also used as a tunnel barrier for particle injection, and each forms a good tunnel junction.

[Effects of the Invention] As described above, according to the superconducting element of the present invention, excellent junction characteristics in a stacked Josephson junction formed by interposing an insulating layer on an oxide superconductor layer are reproducibly obtained. Obtainable. Therefore, it greatly contributes to the realization of a superconducting element such as a Josephson element using an oxide superconductor.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a Josephson device according to one embodiment of the present invention, FIG. 2 is a diagram showing its current-voltage characteristics, and FIG. 3 is a quasi-particle injection type three-terminal according to another embodiment of the present invention. It is sectional drawing which shows the structure of an element typically. 2, 13 ... first oxide superconductor layer, 3 ... insulating layer made of a compound semiconductor, 4, 15 ... second oxide superconductor layer, 11 ... semiconductor substrate, 12, 14 ... GaAs _0.4 P _0.6 film.

Claims (1)

(57) [Claims] 1. A superconducting element having a laminated Josephson junction formed by interposing a thin insulating layer on an oxide superconductor layer, wherein the insulating layer has an ionization energy of 5 eV to 6.5 eV. The semiconductor device is in the range, and functions as an insulator at the operating temperature of the superconducting element, and is formed of at least one semiconductor selected from a group III-V compound semiconductor, a group II-VI compound semiconductor and a group VI semiconductor. Superconducting element.