JPH0335571A - Tunnel element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to superconducting devices that require a tunnel barrier, such as Josephson devices, superconducting base transistors, and quasi-particle tunnel effect devices.

[Prior art] There are three types of Josephson elements: a point contact type, a grain boundary type, and a sandwich type. It is necessary to rub the type number to form a barrier. This sandwich type used Nb, which has a long coherence length, as the superconducting material and aluminum oxide as the tunnel barrier. Further, the tunnel barrier was formed by a vapor deposition method or a sputtering method.

[Problem to be solved by the invention] However, the sandwich type of oxide superconducting material has a critical temperature far exceeding that of liquid nitrogen, making it easier to use, and has a large energy gap, making it possible to integrate it with semiconductors and achieve high-speed switching. The device is not yet complete. The reason for this is that the coherence length of the oxide superconducting material is extremely short, 1/10 or less, compared to Nb. Because the coherence length is short, it is necessary to reduce the thickness of the tunnel barrier, and the method of forming an insulating film or the like using vapor deposition or sputtering to form a tunnel barrier requires advanced film formation technology even for Nb-based materials. In order to make the film even thinner, it becomes extremely difficult to control the film thickness and suppress pinholes.

It is thought that the critical temperature of superconducting materials will become higher in the future, but as the critical temperature increases, the coherence length will become shorter, so the above point will become an even bigger problem.

The present invention solves these problems and makes it possible to easily obtain a tunnel element that brings out the characteristics of superconducting materials, such as being easy to use at high critical temperatures, achieving high-speed switching, and being able to be integrated with semiconductors. That is.

[Means for Solving the Problems] In order to solve the above problems, the tunnel element of the present invention has the following features: 1)
It is characterized by subjecting the surface of a superconducting electrode to plasma treatment in an inert gas atmosphere to transform that surface into an insulating layer or a semiconductor layer, and then forming another superconducting or metal electrode thereon. do.

[Example] The present invention will be explained below with reference to Examples.

First, as shown in FIG. 1, a 250 nm thick Ho1Ba2Cu30Y film (hereinafter referred to as the lower electrode 2) is formed on a single crystal SrTiO3 substrate 1 by a reactive vapor deposition method.

Film formation uses Ho, Ba, and Cu metals as evaporation raw materials, with Ba being evaporated by electron beam and other metals being evaporated by Knuds.
This was done using an en cell. Conditions are initial vacuum level 2*10-
'Torr, degree of vacuum during film formation 3-9*10-'Torr,
Substrate temperature 560-650°C, film formation rate 20-35 nm/
The supply of oxygen is performed by irradiating the substrate with oxygen plasma activated by micro* (ECR) during film formation. The lower electrode 2 obtained in this step is subjected to X-ray diffraction, RHE
According to ED analysis and 4-terminal resistance measurement, it was an epitaxially grown superconducting film of 89 to 91.

Next, after masking a part of the lower electrode 2 with a resist, the surface of the lower electrode 2 is subjected to plasma treatment using a plasma reactor. During the plasma treatment, the portion exposed to the plasma is reduced and the structure is partially destroyed to become a semiconductor layer or an insulating layer 3. The thickness of this layer can be freely and accurately changed by setting the processing time when the plasma processing output is suppressed.

The gas used is an inert gas such as helium or argon. Gases with strong etching power, such as chlorine gas or fluorine-based gases, are not preferred because reduction and structural destruction of the superconducting portion cannot be controlled with high precision.

Next, after applying a metal mask, Au was deposited on the insulating layer 3 by electron beam evaporation to a thickness of 6 nm and a deposition rate of 6 nm/6 nm, respectively.
The intermediate layer 4 is formed by forming a film at a speed of min. This intermediate layer 4 is provided to prevent the insulating layer 3 from being oxidized and returning to a superconductor during the process of forming the Ho1Ba2Cu30Y film to be formed later. This intermediate layer 4 is not necessary when using a superconducting material that does not cause reversible changes in subsequent steps.

Next, as with the lower electrode 2, Ho1Ba2 was formed using the reactive vapor deposition method.
A Cu30Y film (hereinafter referred to as upper electrode 5) having a thickness of 1100 nm is formed.

The conditions were an initial vacuum level of 2*10-'Torr, a vacuum level during film formation of 7 to 9110-'Torr, a substrate temperature of 560°C, and a film formation rate of 20 to 25 nm/min. Oxygen was supplied by microwave (ECR). This was carried out by irradiating activated oxygen plasma during film formation, and after film formation, oxygen was introduced into the chamber to a pressure of 2 Torr, held at 560° C. for 30 minutes, and then slowly cooled.

The I-V characteristics and microwave irradiation characteristics of the device thus obtained were investigated.

As a result, the superconducting Josephson current in the IV characteristic is the Shapiro step (VSTEP=
n-f-h/2e was satisfied. where f is the microwave frequency, n=o, ±1. ±2゜±3...
h is a blank constant, and e is an elementary charge. ) was observed, confirming that it was a Josephson element.

Although the Josephson device has been explained here, any device that forms a tunnel barrier, such as a superconducting base transistor or a quasi-particle tunneling effect device, will have the same effect and there is no problem.

[Effects of the Invention] As described above, according to the present invention, a tunnel barrier can be formed with high precision, so a tunnel element can be easily obtained even if a superconducting material with a short coherence length is used. Therefore, it is possible to bring out the effective properties of superconducting materials, such as their high critical temperature, ease of use, high-speed switching, and compatibility with semiconductors.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of the film up to the formation of the insulating layer. FIG. 2 is a cross-sectional view of the tunnel element. 1... Board 2...Lower electrode 3...Insulating layer 4...middle class 5...Top electrode that's all

Claims (1)

[Claims] (1) The surface of a superconducting electrode is subjected to plasma treatment in an inert gas atmosphere to transform that surface into an insulating or semiconductor layer, and then another superconducting or metal electrode is formed on top of that. A tunnel element characterized by: