JPH05167111A - Tunnel type josephson junction element - Google Patents

Abstract

PURPOSE:To provide a high-quality tunnel type Josephson junction element which is free from mutual diffusion between its junction constituting layers and is provided with a tunnel barrier having no pin hole. CONSTITUTION:In the title Josephson junction element provided with a base electrode 12 made of an oxide superconductor, tunnel barrier 13 formed on part of the surface of the electrode 12, and counter electrode 14 formed on the surface of the barrier 13 on the opposite side of the electrode 12 by using a metallic superconductor, a calcium fluoride (CaF2) film is used for the barrier 13. Since the CaF2 film is thermally stable against the oxide superconductor constituting the base electrode 12 and has an excellent drape, a high-quality junction element which has a gap voltage originally required for the junction element and is small in sub-gap leak current can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tunnel type Josephson junction device using an oxide superconductor for a base electrode and a metal superconductor for a counter electrode.

[0002]

2. Description of the Related Art Tunnel-type Josephson junction devices are used as basic devices for digital circuits that operate at low power consumption and high speed, and various high-sensitivity detectors. Conventionally, a metal-based superconductor was used for the electrode of this element, but the oxide superconductor discovered in recent years has a superconducting transition temperature Tc that is one digit higher than that of a metal-based superconductor. It is of interest because it has a large energy gap Δ corresponding to For example, the energy gaps of Nb and NbN, which represent a metal-based superconductor, are 1.5 meV and 2.5 m, respectively.
eV, while YBa ₂ Cu ₃ O _{x has} a value of about 2
It is 0 meV. Therefore, it means that if a tunnel-type Josephson junction device using an oxide superconductor as an electrode is manufactured, a voltage-current characteristic having a cap voltage Vg that is an order of magnitude larger can be obtained.

A tunnel type Josephson junction device using an oxide superconductor for both the base electrode and the counter electrode is ideal, but a high quality device is manufactured by applying the oxide superconductor to the counter electrode. It's not easy. Since a high substrate temperature of 500 ° C. or higher is required to form an oxide superconductor crystal, the tunnel barrier is destroyed when the counter electrode is deposited. If the operating temperature of the device is limited to the liquid helium temperature (4.2K) which is the same as the conventional one, the gap voltage due to the use of the oxide superconductor for the base electrode even if the metal superconductor is used for the counter electrode. The effect of increase leads to improvement of device characteristics.

Conventionally, regarding a tunnel type Josephson junction device in which a base electrode is an oxide superconductor and a counter electrode is a metal superconductor, an Applied Physics Letters is used.
s Vol. 56 no. 8 (1990) 788-79
There are reports such as 0). The basic structure of the junction of the tunnel type Josephson junction device proposed here will be described with reference to FIG. This structure is based on magnesia (MgO) (10
0) A base electrode 32 made of a c-axis oriented YBa ₂ Cu ₃ O _x film (50 nm) epitaxially grown on a single crystal substrate 31, a tunnel barrier 33 made of a native layer formed on the surface of this base electrode, and this tunnel. Barrier 3
Lead (P) formed facing the base electrode 32 through
It consists of the counter electrode 34 which consists of b). This element is manufactured by the following steps. First, a 50 nm-thick c film was formed on the MgO (100) wall-opened substrate 31 using the MBE method.
An axially oriented YBa ₂ Cu ₃ O _x film is grown. After the film formation, the tunnel barrier 33 is formed by exposing the surface of the YBa ₂ Cu ₃ O _x film to the atmosphere or by performing heat treatment at 500 ° C. in an oxygen atmosphere. Next, after covering the area excluding the joint with polystyrene, Pb is vapor-deposited to form the counter electrode 34 and the device is completed.

In this device, the tunnel barrier is YBa ₂ C.
Since it is composed of oxides or hydroxides of Y, Ba, and Cu, which are the constituent elements of u ₃ O _x , it has poor chemical stability. Further, when forming the tunnel barrier, desorption of oxygen and deviation from the stoichiometric composition occur on the surface of the underlying YBa ₂ Cu ₃ O _x film, and the superconducting property is impaired. These phenomena are not preferable because the quality of the Josephson junction device is determined by the superconductor characteristics of the oxide superconductor surface having a coherence length (0.2 to 2 nm).

As a solution to this problem, an element using an insulator film that is artificially deposited on a tunnel barrier has been proposed by IEEE Transactions on Magnetics (IEEE Transactions).
on Magnetics vol. 27, no. 2 (1
991) 3102-3105) and the like.
As shown in FIG. 4, the structure is almost the same as that of FIG. 3, but the tunnel barrier 43 is made of AL ₂ O ₃ , MgO, Y ₂ O ₃ having a thickness of several nm instead of the native layer of FIG. It has a thin film. These films are formed on the YBa ₂ Cu ₃ O _x base electrode 42 at a substrate temperature of 50 to 350 ° C. by using a film forming method such as a reactive vapor deposition method.

[0007]

The tunnel barrier 43 has an A
When using the l ₂ O ₃ film or the MgO film, the base electrode 4
Yttrium (Y), which is a constituent element of the YBa ₂ Cu ₃ O _x film which is 2, is more stable as an oxide than aluminum (Al) or magnesium (Mg), and therefore Al ₂
O ₃ and MgO are easily decomposed at high temperature and the insulating property is easily damaged.
In addition, a transition region including a low Tc layer and a normal layer is formed at the junction interface due to the mutual diffusion of the base electrode material and the tunnel barrier material. As a result, problems such as a decrease in the gap voltage of the Josephson junction element and an increase in the subgap leak current occur. Further, the MgO film or the Y ₂ O ₃ film has a poor coverage with the base electrode, and the entire surface of the base electrode cannot be completely covered with a film thickness of several nm required as a tunnel barrier. Therefore, an electrical short circuit occurs between the base electrode 42 and the counter electrode 44, and a tunnel type Josephson junction device having the original characteristics cannot be obtained.

It is an object of the present invention to provide a tunnel type Josephson junction device which eliminates the above-mentioned conventional drawbacks.

[0009]

According to the present invention, a base electrode made of an oxide superconductor, a tunnel barrier formed on a part of the surface of the base electrode, and a base electrode facing the base electrode via the tunnel barrier are provided. In a tunnel-type Josephson junction device having a counter electrode made of a metal-based superconductor formed as described above, the tunnel barrier is calcium fluoride (Ca).
F ₂ ) film.

[0010]

In the present invention, the CaF ₂ film is used as the tunnel barrier. CaF ₂ has a large absolute value of energy generated per one fluorine atom and is thermally stable as compared with a fluoride for a constituent element of an oxide superconductor which functions as a base electrode. Therefore, even if the substrate is exposed to a high temperature during the film formation of the tunnel barrier and the counter electrode, the insulating property of the CaF ₂ film is impaired, and the low Tc layer is formed at the bonding interface due to the mutual diffusion with the base electrode material and the counter electrode material. And a normal layer is not formed. Further, the CaF ₂ film is superior in covering property to the oxide superconductor film as compared with the conventional tunnel barrier material, and the short circuit between electrodes hardly occurs even if the tunnel barrier film thickness is several nm. Further, even when the CaF ₂ film is epitaxially grown on the base electrode, this crystal has good lattice matching with the oxide superconductor, so that a high quality thin film can be formed at a relatively low substrate temperature. This means that a clean junction interface without interdiffusion can be obtained between the base electrode and the tunnel barrier. CaF above
The property of the _two films makes it possible to realize a tunnel type Josephson junction device having the original gap voltage of an oxide superconductor and a small subgap leakage current.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the tunnel type Josephson junction element of the present invention will be described with reference to the drawings. As shown in FIG. 1, the basic structure of the junction of this device is based on an oxide superconductor such as a YBa ₂ Cu ₃ O _x film formed on a substrate 11 made of strontium titanate (SrTiO ₃ ) single crystal or the like. Electrode 12 and Ca _{1 2} formed on the surface of this base electrode
And a counter electrode 14 made of Nb, NbN or the like formed facing the base electrode 12 via the tunnel barrier 13.

The base electrode 12 and the counter electrode 14 used in this embodiment both have a thickness of 200 nm, and the tunnel barrier 13 has a thickness of 200 nm.
Has a thickness of several nm. YBa used for the base electrode 12
_The fluorides for the respective constituent elements of the ₂ Cu ₃ O _x film and the energy of formation thereof per fluorine atom are YF ₃ (−
137 Kcal / mol), BaF ₂ (-144 Kca)
1 / mol) and CuF ₂ (−65 Kcal / mol). The absolute values of these generated energies are CaF ₂ (-
It is smaller than the absolute value of 146 Kcal / mol). This indicates that the CaF ₂ film on the YBa ₂ Cu ₃ O _x film is extremely stable against external heat. YB
Even if the laminated structure of a ₂ Cu ₃ O _x / CaF ₂ is formed and then exposed to high temperature, they do not mutually diffuse. Therefore,
As compared with the conventional structure, a low Tc layer or normal layer is less likely to be formed at the junction interface, and the tunnel type Josephson junction element having the original gap voltage of the oxide superconductor and the small subgap leakage current can be obtained.

In this embodiment, the case where the YBa ₂ Cu ₃ O _x film is used as the oxide superconductor forming the base electrode has been described, but Bi-Sr-Ca-Cu-O and Tl are used.
Similar effects can be obtained by using other oxide superconductors such as -Ba-Ca-Cu-O.

Next, a method of manufacturing the tunnel type Josephson junction element of the present invention will be described with reference to FIGS.

First, as shown in FIG. 2A, a 200 nm thick YBa ₂ Cu ₃ O _x (110) film is epitaxially grown on the SrTiO ₃ (110) single crystal substrate 21 by the reactive co-evaporation method. While irradiating the substrate 21 with an oxygen radical beam, yttrium (Y), barium (Ba), and copper (Cu) are simultaneously vapor-deposited at a constant rate ratio.
Subsequently, the substrate temperature is 400 ° C and the thickness is several nm in the same vacuum
Of CaF ₂ is vapor-deposited and the thickness of 200
A Nb film having a thickness of nm is vapor-deposited to complete the junction trilayer film. CaF
After ₂ layer deposition, CaF ₂ film by RHEED observation of the surface by in-situ that epitaxial growth, and the diffraction pattern of the YBa ₂ Cu ₃ O _x film underlying it was confirmed that not appear. From this fact CaF
_{It is shown that the two} films almost completely cover the surface of the YBa ₂ Cu ₃ O _x film. Next, a resist mask is formed on the three-layer film by using a normal lithography technique, and the Nb film is sequentially processed by the reactive ion etching method, and the YBa ₂ Cu ₃ O _x / CaF ₂ film is processed by the ion milling method. Then, a three-layer pattern of the base electrode 22, the tunnel barrier 23, and the counter electrode 24 is formed.

Next, as shown in FIG. 2B, the Nb film is etched using the resist mask 25 formed on a part of the surface of the counter electrode 24 to define the junction.

Next, as shown in FIG. 2C, the film thickness 35
After vapor-depositing 0 nm of silicon dioxide (SiO ₂ ), the resist mask 25 is lifted off and the portions other than the bonding portion are covered with the spacer 26. By lightly cleaning the surface of this sample with an argon (Ar) ion beam, the exposed oxide on the surface of the Nb counter electrode 24 is removed to a thickness of 30.
A 0 nm Nb film is deposited.

Finally, as shown in FIG. 2D, this Nb film is etched using a resist mask to form an upper wiring 27.

In this example, a YBa ₂ Cu ₃ O _x film deposited on the base electrode 22 by the reactive co-evaporation method was used.
However, other film forming techniques such as a sputtering method and a laser vapor deposition method may be used, and various oxide superconductor films such as Bi type and Tl type may be used instead of the Y type thin film. It is also clear that the method for producing the CaF ₂ film as the tunnel barrier is not limited to the vapor deposition method.

[0020]

According to the present invention, it is possible to obtain a high-quality tunnel-type Josephson junction device having a tunnel barrier which has no mutual diffusion between junction layers and has no pinhole.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a tunnel type Josephson junction element of the present invention.

2A to 2D are cross-sectional views showing a method of manufacturing a tunnel type Josephson junction element of the present invention in the order of steps.

FIG. 3 is a cross-sectional view showing a conventional tunnel type Josephson junction element.

FIG. 4 is a sectional view showing a conventional tunnel type Josephson junction element.

[Explanation of symbols]

11, 21, 31, 41 Substrate 12,22 Base electrode made of oxide superconductor 13,23 Tunnel barrier made of CaF ₂ 14,24 Counter electrode made of metal-based superconductor 25 Resist mask 26 Spacer 27 Upper wiring 32 , 42 Base electrodes 33, 43 Tunnel barriers 34, 44 Counter electrodes

Claims (1)

[Claims] 1. A base electrode made of an oxide superconductor, a tunnel barrier formed on a part of the surface of the base electrode, and a metal-based superconductor formed facing the base electrode via the tunnel barrier. A tunnel-type Josephson junction device having a counter electrode comprising a tunnel electrode made of calcium fluoride (CaF ₂ ) film.