JPS60169175A - Josephson junction element - Google Patents

Abstract

PURPOSE:To enhance the product of IcCRN of a critical current (IC) and a resistance (RN), by using an NbN thin film for a superconductive layer, using an MgO thin film for an insulating layer and constituting a quasi-plane type Josephson element (QPJJ). CONSTITUTION:An MgO film is formed on an Si substrate 1 by sputtering. A lower superconducting layer 2 of NbN is formed on the MgO film. An insulator layer 4 of MgO and an upper superconductive layer 3 of NbN are formed on the layer 2. A weak coupling part 5 is formed across the side surface of the thickness of the insulator layer of these laminated thin films. The upper and lower conducting layers are coupled. In this constitution, deterioration in superconductive characteristics of an interface layer between the superconductive layer and the insulator layer can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of a Josephson junction element, and more particularly to an improved quasi-planar Josephson junction element whose electrode material is NbN and whose insulating material is MgO.
ar Josephson Junetron: hereinafter referred to as l'QP J JJ).

As shown in FIG. 1 or 2, QPJJ is a lower part formed on the substrate l at least partially overlapping (partially facing each other in FIG. 1, and facing each other in FIG. 2). The superconductor layer 2 - the middle insulator layer 4 - the upper superconductor layer 3 are laminated thin films, and the upper and lower superconductor layers 1. It is a Josephson junction element consisting of a weak coupling part 5 that couples 2 (Tokuko Aki & -
77/2 issue.

Tokukai Shoj-7-7g/g7, Tokukai Sho! ;7-IO
(See '12g2).

QPJJ with is the length of the weak coupling part (Tsumaria)! It is extremely short, has a small capacitance, and has a critical current I of the element.
It has excellent characteristics such as a large resistance C and a large resistance RN ICRN, is resistant to mechanical vibration, and has many other features such as the geometrical shape of the element suitable for mass production.

By the way, in QPJJ with , deterioration of the superconducting properties of the interface layer between the upper and lower superconductor layers 2.3, which serve as electrodes, and the insulating layer 4, and changes in properties over time due to interdiffusion between the electrode material and the insulating material, etc. There is a problem. In particular, a decrease in Tc of the superconductor near the interface between the upper and lower superconductor layers and the insulator layer has a large impact on the device characteristics, and a solution to this problem is desired.

On the other hand, as shown in Fig. 3, a bridge-type Josephson junction device was fabricated using a NbN single crystal thin film 32° 33 as the upper electrode of the MgO single crystal facet 31, and good characteristics were obtained. It has been reported that

This is because MgO has the same crystal structure as NbN and has a similar lattice constant, so it is easy to grow epitaxially, and it is thought that making the NbN film into a single crystal solves the problem of the interface layer. It is difficult to fabricate a large number of devices consisting of on-plane KNbN single crystal thin films;
There are practical problems such as the cost of the board.

The present inventors formed an MgO thin film on an SI substrate by a high-frequency wave-to-edge method, formed an NbN thin film thereon, and examined the characteristics thereof. As a result, although none of the Mgo #NbN thin films were single crystal, it was found that N
It was found that an NbN thin film with a high critical temperature Tc was formed by epitaxial growth on the bNo. This is the X-ray diffraction pattern and turn when NbN/MgO is continuously formed by sputtering. Figure 3 shows St
The dependence of the critical temperature Tc on the film thickness is shown for an NbN thin film on an MgO and A/203 thin film formed with Si + ivy on a substrate, and an NbN thin film formed directly on a St substrate by sputtering. As is clear from the figure, (a) Nb
N / MgO / 5I (D jjJ combined K t;j,
A high Tc of about /lIK has been obtained even with a film as thick as 100X, and this value is much higher in the case of (bl, (e)).

This is due to the epitaxial formation between MgO and NbN.

This is related to the length. Isn't MgO7NbN a single crystal? At its interface, no transition layer is formed due to disorder of the crystal lattice, and the boundary is clearly separated. it is conceivable that.

Based on the above experimental results and their ideas, the present inventors have developed a Q
As a result of manufacturing PJJ, it was found that the above-mentioned problem of the interface layer was solved and a device with a high ICRN product could be obtained.

The present invention is based on the above findings, and provides an NbN superconductor layer formed on a substrate at least partially overlapping the Mg
The laminated thin film of O insulator layer-NbN superconductor layer and this fj
This is a Josephson junction device characterized by comprising a weak coupling portion that connects upper and lower superconductor layers across the thickness side of a TtN thin film insulator layer.

Hereinafter, the present invention will be explained in detail with reference to Examples.

A QPJJ having the structure shown in FIG. 1 was fabricated using a high-frequency snowflake device, photolithography technology, and electron beam phosphorography technology. Approximately 10mm thick on SS board 1
A lower superconductor layer 2 of NbN is deposited on the sputter-deposited MgO film of 00 A to a thickness of approximately /g θoA @ -0 μm, M
The thickness of the gO insulator layer 4 is about 00A.

The upper superconductor layer 3 of NbN has a thickness of about θooh and a width of -0 μm, a weak coupling part 5′fr of NbN: a thickness of about 1 OoA and a width of λ μm, and the length of the overlapping portion of the upper and lower superconductor layers b.
= 0 μm. Note that the film forming conditions are Mg
In the case of O; discharge Ar gas pressure! ;OmTorr.

Film formation rate: Approximately Jnm/m1ns Substrate temperature: 3θOoC, Nb
In the case of N; discharge gas pressure P (Ar + N2): s
J Om Torr.

Film formation rate/Q nm/ml n%Substrate temperature 30
It is 0°C.

The ICRN& and microphone four-wave characteristics of QPJJ obtained above were investigated. Typical C-8QUI in 0.2K
The IV characteristics of D are shown in FIG. 6, and the characteristics when irradiated with /OGHz microwaves are shown in FIG.

The ICRN of this device was: /mV, and a clear Shapiro step structure was observed up to this value. Also,
The insulation resistance of the NbN/MgO/NbN laminated thin film is 41.
Even at 2 K, the resistance was several tens of Ω, and this was measured for all elements before forming the weak coupling portion 5.

Table 1 shows the IC, RN and I of the 70 devices obtained.
Shows the CRN product.

Table 7 It can be seen that although there is some variation between devices, a high IcRN product of approximately /mV is obtained. This value is ΔN
bN (F,,2K): ,2. The value rn V (T
C: The value for a weakly coupled tunnel junction calculated from 16 K) is lower than J, gmV, but I
cRN: '1. If gmV is obtained, this value Dl
rty Llmjt value (value when the mean free path of NbN is shorter than the length of the weak coupling part or h),! r, close to -OmV, and is understood to be an extremely excellent device.

[Brief explanation of drawings]

Figures 1 and 2 are enlarged perspective views showing the structure of QPJJ, Figure 3 is an enlarged plan view of a conventional bridging element formed of a bN single crystal thin film on a open plane of a single MgO single crystal, and Figure 4 (a
) and (b) are MgO obtained in experimental examples of the present invention. A graph showing the X-ray diffraction pattern of a NbN thin film. FIG. 5 is a graph showing the film thickness dependence of the critical temperature Tc of the NbN thin film obtained in an experimental example of the present invention. Fig. s6 and Fig. 7 are graphs respectively showing the IV characteristics and microwave characteristics of the elements obtained in the examples of the present invention. (Symbols in the figure) 1...Substrate 2.3...Superconductor layer 4...Insulator layer 5...・・・・・・Weak coupling part!・・・・・・・・・Length of the weak coupling part b ・・・・・・・・・Length of the overlapping part of the superconductor layer Patent applicant RIKEN Child 4 Figure Prefecture 5 figure

Claims (1)

[Scope of Claims] l) A laminated thin film of superconductor-insulator-superconductor formed at least partially overlappingly on a substrate; In the Josephson junction device, the superconductor layer is composed of the insulator layer or the MgO thin film, and the superconductor layer is the N-type superconductor layer.
A Josephson junction device comprising a bN thin film. h) M in which the lower superconductor layer is formed on a St substrate.
The Josephson junction device according to claim 1, which is formed on a gO thin film.