JPH03171684A - Jusephson element - Google Patents

Abstract

PURPOSE:To obtain a Josephson element which is provided with a built-in superconductor and large in electrode effective area by a method wherein a diffusion preventing layer is formed on an insulating layer. CONSTITUTION:A YBCO thin film serving as a lower electrode 4 is formed on a substrate 5 of SrTiO3 through an r.f. magnetron sputtering method. Then, an insulating layer 1 is formed in such a manner that the YBCO superconductive layer is subjected to a plasma treatment in a CF4 atmosphere. A diffusion preventing layer 2 is formed of Au as thick as 60Angstrom using a metal mask through an electron beam evaporation method. In succession, an upper electrode 1 is formed of Nb through an electron beam evaporation method. When Au of the diffusion preventing layer 2 is joined to Nb, Cooper pairs becomes long in coherence length, so that the diffusion preventing layer 2 with no defect can be formed even if the surface of an RE-Ba-Cu-O oxide superconductive layer serving as the lower electrode 4 is rough, and Au of the layer 2 does not react with the fluorine injected YBCO insulating layer. A diffusion preventing layer prevents the component elements of an upper and a lower electrode from diffusing.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a Josephson device, and particularly to a Josephson device using an oxide superconductor. [Prior art] Elements that apply superconducting materials include the Sefson element and the superconducting FET. Superconducting transistors are known.
For example, tunnel-type Jimaro-Sefson elements are shown in Figures 7 and 8. Figure 7 is a cross-sectional view of a conventional element. 8th
The figure is a plan view. In this type of Josephson element, an insulating layer 7, k10I1, is sandwiched between an upper electrode 6 (Nb) and a lower electrode 8 (Nb), which are superconductors. 9
is the substrate. If an oxide superconductor is applied to the Josephson element shown in Figure 7, the element shown in Figure 9 will be obtained. Figure 9 is a cross-sectional view showing a different conventional di-storage Sefson element. This element uses YBa*CusOy-g (hereinafter referred to as YBC), which is an oxide superconductor, as the lower electrode 13.
A (abbreviated as O) is used, and A is used as the diffusion prevention layer 12.
Use u. This diffusion prevention layer is an A70 insulating layer l1.
This prevents the reaction between II and YBCO, which is the lower electrode 13. Figure 10 is a diagram showing the characteristics of different conventional Josephson elements. Josephson property is obtained. However, in this device, since there are irregularities on the YBCO surface, the uniform and defect-free insulating layer 11. Diffusion prevention layer 1
2 is not obtained, and as a result, the YBCO as the lower electrode 13 and the Nb, SI1 or Pb as the upper electrode 10 are partially short-circuited and repelled from each other, becoming insulators. As a result, the effective bonding area becomes smaller than the electrode area. The problem is that it is smaller compared to . The insulation [11] and the diffusion prevention layer 12 are splattered. YB is formed by vapor deposition, etc., and it is required to reduce its thickness.
Defects occur in the layer due to the influence of the surface condition of CO. In particular, regarding the Au of the diffusion prevention layer 12, the lower electrode 13
Reactivity was observed between YBCO and YBCO.
It is not possible to increase the thickness of the film due to the low diffusivity of the Couva electron pairs from CO. Figure 11 is a diagram showing the relationship between the critical current (Ie) and the magnetic field strength H (X 10''A/s) for different conventional diagonal Sefson elements. In Figure 11, the critical current value is zero. The strength of the magnetic field is related to the effective junction area. 10
This shows that it is 0. In order to solve this problem, a Josephson element as shown in FIG. 12 was devised. Fig. 12 shows a further different conventional Josephson element, Fig. 12 <<δ>> is a cross-sectional view,
Figure 12 (mountain) is a plan view. This Josephson element has an insulation 1111G obtained by treating YBCO, which is the lower electrode 17, with CF4 plasma. This insulating layer is YB
It is produced by injecting fluorine into CO. Substrate 18
uses SrTiOz. Since the insulation layer 6 is generated in a CPa atmosphere, a defect-free insulation layer is formed despite the unevenness of the YBC○ which is the lower electrode 17. [Problem II to be Solved by the Invention] However, when trying to reduce the thickness of the insulating layer 16 in order to obtain Josephson characteristics, YBCO, which is the lower electrode 17, and Wb, Sn, or Pb, which is the upper electrode 15, are It has been found that they diffuse through the insulating layer 16 and react with each other. Therefore, a diracefson characteristic cannot be obtained, but a nonlinear current-voltage characteristic as shown in FIG. 13 is obtained. This characteristic is thought to be due to the insulator formed by the reaction between YBCO and Nb. This invention has been made in view of the above-mentioned points, and its purpose is to form an upper electrode 15 of Nb, Sn or Pb through an insulating layer 16.
The object of the present invention is to provide a Josephson device in which an oxide superconductor is incorporated and the effective area of the electrode is large by preventing the YBCO and the lower electrode 17 from reacting with each other. [Means for Solving the Problem] According to the present invention, the above-mentioned object has an upper electrode 1, a lower electrode 4ε, an insulating layer 3, and a diffusion prevention layer 2, and the lower electrode contains IIE as a rare earth element. RE-Ba when
-Cu-0 based oxide superconducting layer, the insulating layer is a layer obtained by plasma treatment of the surface of the lower electrode in a fluorine compound gas atmosphere, and the diffusion prevention layer is a layer made of Au. Yes, as mentioned above lol! i&lN
This is achieved by assuming that the upper electrode is a layer made of Nb, Sn or Pb and is formed on the diffusion prevention layer. [Function] When Au, which is the diffusion prevention layer 2, is bonded to Nb, the coherence length of Cooper electron pairs increases, so the thickness can be increased within that range. For this purpose, the lower electrode 4, RF! Despite the surface roughness of the -Ha-Cu-0 oxide superconducting layer, a defect-free diffusion prevention layer 2 can be formed. Au, which is the diffusion prevention layer 2, does not react with the insulating layer, which is YBCO doped with fluorine. The diffusion prevention layer is Au, and the upper electrode is Nb, Sn. P
b, and the constituent elements of the RE-Ba-Cu-0 series oxide, which is the lower electrode, are prevented from diffusing. [Example] Next, an example of this invention will be explained based on the drawings. FIG. 1 is a cross-sectional view of a Josephson element according to an embodiment of the present invention. This element is used as a substrate 5 and a lower electrode 4 (110).
YBCO oxide superconducting layer oriented in the direction and the insulating layer 3
and the diffusion prevention layer 2 made of ^U and the Nb which is the upper electrode 11.
It consists of Such a Jitasefuson element is manufactured as follows. FIG. 5 is a process diagram showing a method for preparing a Josephson device according to an embodiment of the present invention. S
YB, which is the lower electrode 4, is placed on the substrate 5 made of rTlOs.
CO thin film r. f. It is formed using magnetron sputtering method. *The formation conditions for M are shown in Table 1. Table 1: An impurity layer 19 is formed on the surface of the obtained YBCO superconducting layer. This layer is a layer where the group strength is shifted. This state is shown in Figure 5 1aJ. This impurity layer 19 is removed by oxygen plasma treatment. ai! The conditions for elementary plasma treatment are shown in Table 2. The state after the oxygen plasma treatment in Table 2 is shown in Figure 5 (bl) The oxygen plasma treatment also has the meaning of supplying oxygen to the YBCO superconducting layer. It is formed by plasma treatment of C.
F. The conditions for plasma treatment are shown in Table 3. The state of the insulating layer type 3*a in Table 3 is shown in FIG. 5(c). Further, using a metal mask 20, a diffusion prevention layer 2 of Au is formed to a thickness of 60 mm by electron beam evaporation. Table 4 shows the conditions for forming the diffusion prevention layer 2. Table 4 The state after formation of the diffusion prevention layer is shown in Figure 5+dl. Next, using a metal mask 20, an upper film is formed using Nb by electron beam evaporation. The upper electrode type and conditions are shown in Table 5. Figure 5 (11) shows the state after the formation of the upper electrode l in Table 5.
The characteristics of the Josephson element thus obtained (temperature, 4.2K) are shown in Figure 6. N which is the upper electrode 1
The reaction between YBCO and YBCO, which is the lower electrode 4, forms the diffusion prevention layer 2.
This can be prevented and good di-sefson characteristics can be obtained. When we investigated the dependence of the critical current on the magnetic field for this device, we found that the effective junction area was 80% of the electrode area. Figures 2 and 3. FIG. 4 is a diagram showing the element concentration distribution profile obtained by Auger electron spectroscopy of the Josephson device according to the embodiment of the present invention. The CF4 plasma treatment time is 0.2.15 min. It corresponds to the combination of
It can be seen that fluorine diffuses into YBCO due to plasma treatment. The insulating layer that is injected with fluorine and has a raw bottom is N.
b, Ba, Oetc. prevent the spread of If the CFa plasma treatment time is long, the superconductivity of YBCO is lost. When the thickness of the diffusion prevention layer 2 is less than 50A, a reaction occurs between the upper electrode and the lower electrode due to the unevenness of the YBCO surface. Also 5
Since superconducting current is not observed when the number of people exceeds 00, the thickness of the diffusion prevention layer is set between 50 and 500 people. In addition to Nb, Sn and Pb can also be used for the upper electrode. RE-Ba-Cu-0 can be used as the oxide superconducting layer. [Effects of the Invention] According to the invention, an upper electrode, a lower electrode, an insulating layer,
and a diffusion prevention layer, and the lower electrode is RE-Ba- when RE is a rare earth element.
A Cu-0 based oxide superconducting layer, the insulating layer is a layer obtained by plasma treatment of the surface of the lower electrode in a fluorine compound gas atmosphere, and the diffusion prevention layer is a layer made of AC+, The upper electrode is formed on the insulating layer, and the upper electrode is made of Nb, Sn, or Pb, and is formed on the diffusion prevention layer, so the Au, which is the diffusion prevention layer, is bonded to the Nb. In this state, the coherence length of Cooper electron pairs increases, and the thickness of the diffusion prevention layer can be increased within that range. Therefore, the lower electrode R
f! -Despite the surface roughness of the insulating layer formed on the surface of the Ba-Cu-0 based oxide superconducting layer or the oxide superconducting layer, a defect-free diffusion prevention layer can be formed, and the upper By preventing the reaction between the electrode and the lower electrode, it is possible to obtain a Cefson device incorporating an oxide superconductor with a large effective electrode area.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a Josephson device according to an embodiment of the present invention, and FIGS.
2.15min. Fig. 5 is a process diagram showing the method for manufacturing a Josephson device according to an embodiment of the present invention, and Fig. 6 is a diagram showing the profile of the element concentration distribution according to the optical electron spectroscopy when &l diagram showing the characteristics of the Josephson element according to the example, Fig. 7 is a sectional view showing the conventional Josephson element, Fig. 8 is a plan view showing the conventional Josephson element, and Fig. 9 is a different conventional Josephson element. 10 is a diagram showing current-voltage characteristics of different conventional Josephson elements; FIG. 11 is a diagram showing magnetic field dependence of critical current for different conventional Josephson elements; FIG. 12 12 shows a still different conventional Josephson element, FIG. 12 (al is a cross-sectional view, and the mountain in FIG. 12) is a plan view, and FIG. 13 is a diagram showing the current-voltage characteristics of a still different conventional Josephson element. ．． l: upper electrode, 2: diffusion prevention layer, 3: insulating layer, 4: lower electrode. Fig. 1 Fig. 2 Fig. 3 Fig. 7 Cell Guy Fig. 4 Fig. 5 Fig. 7 Fig. 9 Fig. 0. 2mV/div. Fig. 10 Fig. 11 Fig. 16 Upper % gutter (Nb) (216 Edge layer Fig. 12 Fig. 13

Claims (1)

[Claims] 1) It has an upper electrode, a lower electrode, an insulating layer, and a diffusion prevention layer, and the lower electrode has RE-Ba- when RE is a rare earth element.
The insulating layer is a layer obtained by plasma treatment of the surface of the lower electrode in a fluorine compound gas atmosphere, and the diffusion prevention layer is a layer made of Au. A Josephson device, characterized in that it is formed on an insulating layer, and the upper electrode is a layer made of Nb, Sn, or Pb and is formed on the diffusion prevention layer.