JPH03205882A - Tunnel junction elements using composite oxide superconductivity materials - Google Patents

Abstract

PURPOSE:To make it possible to produce effectively, enhance the quality of each superconductivity layer and hence exert excellent performances as a tunnel junction element by equalizing the composition of the superconductivity layers and non-superconductivity layers except for the content of oxygen. CONSTITUTION:With regards to Bi2O powder, SrCO3 powder, SrCO3 powder, and CaCO3 powder, which are available on the market, each powder Bi, Sr, Ca, and Cu are mixed so that the atomic ratio of Bi:Sr:Ca:Cu may be specified. The mixed powder is sintered and results in a composite oxide sintered body with Bi-Sr-Ca-Cu-O, which is used as a target. A film is formed on a substrate 1. High frequency electric powder is reduced when the thus formed film thickness of a superconductivity layer 2 has reached a specified value and the formation of the film is continued. When the thickness of a non-superconductivity layer 3 has reached a specified value, the high frequency electric power is returned to the original value again and the film formation of a superconductivity layer 4 is further continued.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a tunnel junction element substrate using a composite oxide superconducting material. More specifically, the present invention relates to the configuration of a novel base material for forming an element including a tunnel junction formed using a composite oxide superconducting material or a circuit including the tunnel junction element.

BACKGROUND OF THE INVENTION In the field of superconductivity, there is a technique that utilizes a tunnel junction formed by a pair of superconducting layers sandwiching a very thin non-superconducting layer. This tunnel junction exhibits nonlinear current/voltage characteristics that reflect the superconducting energy gap of the superconducting material, and is used in sensors and switching as SQUIDs and Josephson devices.

FIG. 1 is a cross-sectional view schematically showing the structure of the tunnel junction of the device as described above. That is, as shown in the figure,
The tunnel junction is constructed by sequentially laminating a first superconducting layer 2, a non-superconducting layer 3, and a second superconducting layer 4 on a substrate 1.

In this superconducting tunnel junction, the first and second superconducting layers 2, 4 are formed of any superconducting material, and the non-superconducting layer 2 is usually formed of other materials. In the case of superconducting tonless junctions, the non-superconducting layer can also be formed of a normal-conducting material, and in fact, in consideration of the effect on the adjacent superconducting layer, conventionally the non-superconducting layer was formed of an Au layer, etc. There were many.

However, even if a chemically stable material such as Au is used, it cannot be said that there is no effect on superconducting materials having delicate structures such as oxide superconducting materials.

That is, the superconductivity of the surrounding superconducting layer may be impaired due to diffusion of the non-superconducting layer material into the adjacent superconducting layer, or the superconductivity of the surrounding superconducting layer may be impaired due to a mismatch between the underlying non-superconducting layer and the second superconducting layer during a certain film process. 2. The characteristics of the superconducting layer may deteriorate.

Therefore, if the superconducting layer is made of a material that contains the same elements as the superconducting layer but does not have superconductivity, for example, YBa2Cu307, a composite oxide with a different composition ratio of Y, Ba, and Cu such as 2:2:3 may be used. proposed a method for forming a non-superconducting layer. However, when producing a tunnel junction using an oxide containing the same element as a non-superconducting layer as in the above example, a certain film operation becomes actually complicated. That is, for example, if a film is to be formed by vapor deposition using a certain target, it becomes necessary to replace the target in order to form a non-superconducting layer following a superconducting layer, and it is also necessary to form a second superconducting layer. In order to do this, you need to change the target again. Therefore, not only the number of man-hours for the coating process increases, but also the time required for manufacturing.

Problems to be Solved by the Invention Therefore, an object of the present invention is to solve the problems of the above-mentioned prior art and to provide a new structure for a tunnel junction element that can be manufactured by a simpler method.

According to a means for solving the problem, that is, according to the present invention, a substrate, a first superconducting layer formed on the substrate using an oxide, and a first superconducting layer having the same composition ratio as the first superconducting layer except for the composition ratio of oxygen. A non-superconducting layer formed on the first superconducting layer using an oxide containing the same element as the first superconducting layer, and a second superconducting layer formed on the non-superconducting layer having the same composition as the first superconducting layer. Provided is a tunnel junction element comprising:

Function The main feature of the tunnel junction device according to the present invention is that the set of superconducting layers and the set of non-superconducting layers are the same except for the oxygen composition ratio.

That is, when manufacturing a conventional tunnel junction device using a composite oxide superconducting material, the composition of the non-superconducting layer was different from that of the superconducting layer, so the first superconducting layer was deposited on the substrate. After that, it was necessary to perform operations such as replacing the Yuichi target and depositing a non-superconducting layer, and then replacing the target and depositing a second superconducting layer.

On the other hand, the tunnel junction device according to the present invention constructs the non-superconducting layer by changing only the oxygen composition ratio, so by only partially changing the film conditions, the superconducting layer and the non-superconducting layer can be changed. The layers can be applied in succession. Furthermore, since only the oxygen composition ratio changes between each layer, there is little mismatch at the interface.

Note that the following film-forming conditions can be used to control the oxygen content.

(2) Changing the RF power applied at certain membrane times.

■Change oxygen supply.

■Changing the oxygen partial pressure in a certain membrane atmosphere.

■Change the substrate temperature.

Since all of these controls can be performed without interrupting a certain film ablation, the number of man-hours and processing time for forming the non-superconducting layer of the tunnel junction device do not increase. Furthermore, in the tunnel junction device according to the present invention, simply changing certain film conditions of the non-superconducting layer does not necessarily mean replacing the target or the two evaporation sources. has been done. Therefore, the mismatch between the non-superconducting layer and the superconducting layer is also very small.

In addition, as a material which can form such a tunnel junction element based on this invention, the following are mentioned as a preferable example.

First, preferred examples of the substrate material include a Mg◯ substrate, an AI203 single crystal substrate, a SrTiO+ single crystal substrate, a LaAI◯3 single crystal substrate, and a zr02 single crystal substrate.

In addition, as the superconducting layer, La-Ba-Cu, La
-4 which is a composite oxide such as SrCu○, Y-Ba-Cu, etc.
In addition to elemental oxide superconducting materials, Bi-Sr-Ca-
Five-element complex oxides such as Cu and TIBa-Ca-Cu can be exemplified as oxide superconducting materials that have a particularly high superconducting critical temperature and become superconducting when cooled with liquid nitrogen, but are not limited thereto. isn't it.

These composite oxide-based superconducting materials form a specific crystal structure that exhibits effective superconducting properties when they have the above-mentioned specific composition and under specific manufacturing conditions.

On the other hand, if the oxygen content deviates from the above composition ratio, the superconducting properties will be lost or the superconducting critical temperature will drop significantly. Therefore, such a composite oxide with deteriorated superconducting properties can be used as a material for a non-superconducting layer in a tunnel junction element.

The superconducting material that can be applied to the tunnel junction element according to the present invention is not limited to these composite oxides, but any oxide superconductor whose superconducting properties change depending on the oxygen content can be used. It can also be used with anything.

Hereinafter, the present invention will be described in more detail by giving specific examples, but the following disclosure is only one example of the present invention, and does not limit the technical scope of the present invention in any way.

EXAMPLE By RF magnetron sputtering method, BISr
A tunnel junction device using a -Ca-Cu-based composite oxide superconductor was fabricated.

Commercially available Bi203 powder, SrC○3 powder, CaC○3 powder, and Cu○ powder were used as B1, and each powder of Sr, Ca, and Cu was used in an atomic ratio of B+:Sr:Ca:Cu of 2.6.
:2. Bi-Sr obtained by sintering mixed powder at 820°C for 8 hours in a ratio of O:2.0:3.0.
-Ci-Cu10 composite oxide sintered body was used as a target.

The film conditions for the superconducting layer are as follows:
: MgO single crystal {110} surface spasotering gas: Mixed gas of Ar and 02 [02/(Ar-!-0
2) F C0.2 (volume ratio) Sputtering pressure: 2 x 10-2Torr substrate temperature
:750℃ High frequency power: 50'vV (0.64'v
V/aIIi) Film speed: 0.3C
Under the above-mentioned film forming conditions, a film was formed on the substrate for 60 minutes, and when the thickness of the superconducting layer reached about 1000 nanometers, the high frequency power was reduced to IQW. Filming was continued for 10 minutes in this state, and the thickness of the non-superconducting layer was approximately 100%.
When it reached A, the high frequency power was returned to 50W,
Furthermore, one film was continued until the thickness of the superconducting layer reached 1,000 layers. The thin film thus obtained was a complex oxide with an atomic ratio of B+:Sr:Ca:Cu of 2:2:2:3 near the surface.

When the sample with the oxide multilayer film prepared as described above was cooled to 40K and the current-voltage characteristics were measured by applying 9 GHz microwave, a clear Nyapirotemperature was observed as shown in Figure 2. was observed. Therefore, it is considered that an effective Josephson junction is formed in this sample.

Example 2 A tunnel junction element was fabricated using a Bi-Sr-Ca-Cu based composite oxide superconductor by simultaneous vacuum evaporation.

Using commercially available metal B1, metal Ca, metal Cu and SrF2 as evaporation sources, metal B1, metal Ca and metal Cu
was evaporated using a K cell, and SrF2 was evaporated using an electron gun. Other film conditions are as follows: Substrate: Mg○ single crystal {110} plane Oxygen partial pressure: 2 x 10-6 Torr Substrate temperature: 750°C Film speed: 5 people/sec Film as above After forming a film on the substrate for 10 minutes under the following conditions, the heating temperature was once lowered to 600°C, and the substrate temperature was 600°C.
Immediately after reaching 00°C, the substrate was heated to the original temperature, and from when the substrate temperature reached 750°C, a certain film was continued for an additional 3 minutes. During these operations, one membrane was run in succession, with other membrane conditions constant.

In addition, the composition near the surface of the obtained thin film or the atomic ratio Bi
It was a composite oxide in which the ratio of :Sr:Ca:Cu was 2:2:2:3.

The sample of the oxide multilayer film prepared as described above was cooled to 50K, and as in Example 1, 9GHz microwave was applied to measure the current-voltage characteristics. steps were observed. Therefore, it is considered that an effective Josephson junction is formed in this sample.

Effects of the Invention As explained above, in the tunnel junction device according to the present invention, the composition of the superconducting layer and the non-superconducting layer is the same except for the oxygen content, and each layer can be formed into a film in succession during manufacturing. other,
There is very little mismatch between each layer. Therefore, it is expected that efficient production can be achieved, that each superconducting layer has high quality, and that it exhibits excellent performance as a tunnel junction element.

The tunnel junction device according to the present invention can be used by itself as a tunnel effect device, and can also be used as a base material for manufacturing other devices or integrated circuits by processing it by a known method. You can also use

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view schematically showing the basic structure of a tunnel junction element, and FIG. 2 is a graph showing the current-voltage characteristics of the tunnel junction element manufactured according to the present invention. [Main reference numbers] 1...Substrate, 2.4...Superconducting layer, 3...
・Non-superconducting layer

Claims (1)

[Claims] a first superconducting layer formed on the substrate by an oxide; 1
A tunnel junction element comprising: a non-superconducting layer formed on a superconducting layer; and a second superconducting layer formed on the non-superconducting layer having the same composition as the first superconducting layer.