JPS6386485A - Tunnel type josephson junction device - Google Patents

Abstract

PURPOSE:To have a gap voltage proper to NbN and reduce the sub-gap leakage current without interposing a low-Tc layer or normal layer by providing auxiliary layers made of Si or Ge and having a film thickness not greater than the coherence length between the super-conducting layers made of NbN and the tunnel barrier layer. CONSTITUTION:Title device is constructed by a first superconducting layer 12 made of NbN and formed on an insulating substrate or a substrate 11 having an insulator on the surface thereof, a first auxiliary layer 13 made of Su or Ge and formed on the first superconducting layer 12, a tunnel barrier layer 14 formed on the first auxiliary layer 13, a second auxiliary layer 15 made of Si or Ge and formed so as to be opposed to the first auxiliary layer 13 through the tunnel barrier layer 14, and a second superconducting layer 16 made of NbN and formed on the second auxiliary layer 15. With this, the reactions between the first and second superconducting layers 12, 16 and the tunnel barrier layer 14 are prevented by the first and second auxiliary layers 13, 15. Further, since the film thicknesses of the auxiliary layers 13, 15 are made about 50Angstrom which is not greater than the penetration depth of cooper pain, a junction device can be obtained which maintains a gap voltage proper to a NbN/ NbN junction device and has a small sub-gap leakage current.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a tunnel type Josephson junction device.

(Prior Art) Logic circuits and memory circuits composed of Josephson junction elements have the great advantage of consuming less power and operating at high speed compared to semiconductor circuits.

As circuits become more integrated, there is a need to develop Josephson junction devices that can take full advantage of these advantages. The main characteristics required of Josephson junction devices for high-speed operation are high gap voltage, low junction capacitance, and low sub-gap leakage current. These characteristics can be achieved by using a superconductor layer with a large energy gap, that is, a superconductor layer with a large superconducting transition temperature Tc, and a tunnel barrier layer with a small dielectric constant, and by using a superconductor layer with a large energy gap, that is, a tunnel barrier layer with a small dielectric constant, This is achieved by making the transition region exhibiting low Tc or normal properties as thin as possible. A Josephson junction element that uses an NbN film as a superconductor layer and a metal or semiconductor oxide film as a tunnel barrier layer is attracting attention as a Josephson junction element most suitable for this purpose.

In the case of NbN films, films with high Tc (
Tc-15K) can be obtained at a relatively low substrate temperature of 100°C or less, so even when depositing the superconductor layer after forming the tunnel barrier layer, the heat between the superconductor layer and the tunnel barrier layer can be reduced. The reaction progresses more slowly than with other high Tc superconductor films, and good bonding properties can be expected.

As a conventional example, there is a paper published in 1985 by Ueda et al. in the technical report of the Institute of Electronics and Communication Engineers, pages 85-27 to 12 of CPM. As shown in FIG. 2, the Josephson junction device proposed here includes a first superconductor layer 22 made of NbN formed on a substrate 21, and a superconductor layer 22 formed on the first superconductor layer 22. A metal layer was deposited on the formed tunnel barrier layer 23 and a second superconductor layer 22 made of NbN formed opposite to the first superconductor layer 22 via the tunnel barrier layer 23. The method of thermally oxidizing or plasma oxidizing the surface in a gas atmosphere containing oxidation to form the tunnel barrier layer 23 makes it possible to achieve a uniform oxide film thickness within the substrate surface compared to a method of depositing the oxide film itself. It is commonly used because of its excellent properties and controllability.

(Problems to be Solved by the Invention) However, in the structure of the Josephson junction element described above,
First superconductor layer 22 made of NbN and tunnel barrier layer 2
The A1 films 3 and 3 are in direct contact with each other. Therefore, a reaction occurs in which AI, which has a low nitriding energy, deprives N atoms from NbN, and a low Tc layer or a normal layer containing fewer N atoms than the stoichiometric composition is formed on the surface of the first superconductor layer 22. .

In this way, the transition region between the first superconductor layer 22 and the tunnel barrier layer 23 widens, resulting in the problem that the gap voltage of the Josephson junction device decreases and the subgap leakage current increases. occurs. Also,
The same problem as in the case of AI arises because the tunnel barrier layer 23 and the second supergear are smaller than the corner values.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Josephson junction device that eliminates these conventional drawbacks.

(Means for Solving the Problems) According to the present invention, the first
a superconductor layer, a tunnel barrier layer formed on this first superconductor layer, and NbN formed opposite to the first superconductor layer via this tunnel barrier layer. In a Josephson junction element having a second superconductor layer as a basic component, there is a layer between the first superconductor layer and the tunnel barrier layer, or between the tunnel barrier layer and the second superconductor layer. A Josephson junction element is obtained, which is characterized in that an auxiliary layer made of Si or Ge and having a thickness equal to or less than the penetration depth of the Cooper pair is interposed between at least one of the layers.

(Function) In the structure according to the present invention, between the first superconductor layer made of NbN and the tunnel barrier layer, or between the tunnel barrier layer and the NbN
Even if the formation energy of nitridation per nitrogen atom is lower in the semiconductor than in the tooth with the second superconductor layer made of N, the metal or semiconductor is the first and second superconductor. Reactions that deprive the layer of N atoms are prevented by the auxiliary layer. In addition, since the nitridation energy per nitrogen atom of Si or Ge used in the auxiliary layer is equal to or higher than that of Nb, the reaction between the first and second superconductor layers and the auxiliary layer is Difficult to progress. Furthermore, since the thickness of the auxiliary layer is less than the penetration depth of the Cooper pair, the superconducting properties of NbN, which is the first J second superconductor material, are maintained due to the proximity effect, and the sub-gap leakage current is reduced. Small Josephson junction elements can be formed.

After depositing the metal or semiconductor on the first superconductor layer,
A tunnel barrier layer formed by a chemical reaction such as oxidation, nitridation, or fluorination has excellent film thickness uniformity and controllability within the substrate surface. In addition, the production energy per atom of oxygen, nitrogen, and fluorine in the above chemical reaction is
Alternatively, the reaction between the auxiliary layer and the tunnel barrier layer can be significantly reduced by using a metal or semiconductor compound having a smaller reaction value than the same reaction value in Ge.

(Example) Embodiments of the present invention will be described in detail with reference to the drawings.

This Josephson junction element, as shown in FIG.
a first auxiliary layer 13 made of Si or Ge formed on the superconductor layer 12; a tunnel barrier layer 14 formed on this first auxiliary layer 13; A second auxiliary layer 15 made of Si or Ge formed opposite to the first auxiliary layer 13;
5 and a second superconductor layer 16 made of NbN formed on top of the superconductor layer 16. First and second superconductor layers 12.16
is an NbN film with a thickness of 2000 nm obtained by sputtering an Nb target in an argon-nitrogen atmosphere. The Tc of this film is 15-16. First and second auxiliary R13
, 15 is an electron beam evaporated film having a film thickness of approximately 50 mm. For the tunnel barrier layer 14, an AI oxide film is used, which is formed by first sputtering several tens of Al films and then thermally oxidizing the surface thereof in a pure oxygen atmosphere.

According to this embodiment, the first and second superconductor layers 12°16
Since the first and second auxiliary layers 13.15 are provided between the first and second superconductor layers 12.16 and the tunnel barrier layer 14, the reaction between the first and second superconductor layers 12.16 and the tunnel barrier layer 14 is as follows. 1,
This is prevented by the second auxiliary layer 13.15. In addition, the production energy per nitrogen atom in the nitriding reaction of SL and Ge used for the auxiliary layer is t'L-59, -3, and 8, respectively.
In kcal/mol, the value of Nb in the same reaction -5
It is equivalent to or more than 6kcal/mol. Therefore, even if Si or Ge is in contact with NbN of the superconductor layer, the reaction in which Si or Ge deprives NbN of N atoms is difficult to proceed. Furthermore, the thickness of the auxiliary layer is approximately 50 people,
NbN/
A Josephson junction element that maintains the original gap voltage of the NbN junction element and has a small sub-gap leakage current can be obtained.

The production energy per oxygen atom in the oxidation reaction of AI used to form the tunnel barrier layer 14 in this example is -130 kcal/mol, which is higher than the values of -108 and -63 kcal/mol in the same reaction of Si and Ge. Since it has a large absolute value, no reaction occurs between the AI oxide film and the superconductor layer. From the above, a good Josephson junction element with a thin transition region at each layer interface can be formed.

In this example, the auxiliary layer was provided both between the first superconductor layer 12 and the tunnel barrier layer 14 and between the tunnel barrier layer 14 and the second superconductor layer 16. Either one can have a great effect depending on the conditions. In particular, when the tunnel barrier layer 14 is formed by oxidation, nitriding, or fluoridation after depositing a metal or semiconductor, the first superconductor layer 12 and the tunnel barrier layer 14 = AI oxide film are used. Similar effects can be obtained with insulating films formed by oxidizing, nitriding, or fluoriding other metals or semiconductors, or with deposited insulating films or semiconductor films. In particular, the tunnel barrier layer 14 is more stable when the metal or semiconductor atoms that constitute the tunnel barrier layer 14 have lower production energy for each of the above reactions than Si or Ge.

(Effects of the Invention) As explained above, according to the present invention, an auxiliary layer made of Si or Ge having a thickness equal to or less than the coherence length is provided between the superconductor layer made of NbN and the tunnel barrier layer. As a result, a Jose7son junction device with the original gap voltage of NbN and a small sub-gap leakage current can be obtained without intervening a low Tc layer or a normal layer generated by the reaction between the superconductor layer and the tunnel barrier layer. . Also, Si, G
By using a compound formed by oxidizing, nitriding, or fluoridating a metal or semiconductor whose reaction energy is lower than that for the tunnel barrier layer, chemical stability, film thickness uniformity, and control can be achieved. A Josephson junction element with further improved properties is obtained.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing a Josephson junction element of the present invention;
FIG. 2 is a sectional view showing a conventional Josephson junction element. In the figure, 11.21 is the substrate, 12.22 is the first superconductor layer made of NbN, 13 is the first auxiliary layer made of Si or Ge,
14.23 is a tunnel barrier layer, 15 is a second auxiliary layer made of Si or Ge, and 16.24 is a second superconductor layer made of NbN.

Claims (1)

[Claims] 1. A first superconductor layer made of NbN formed on a substrate, a tunnel barrier layer formed on this first superconductor layer, and In the Josephson junction element, the basic component is a second superconductor layer made of NbN formed opposite to the first superconductor layer, wherein the first superconductor layer and the tunnel An auxiliary layer made of Si or Ge having a thickness equal to or less than the penetration depth of the Cooper pair is interposed between the barrier layer and at least one of the tunnel barrier layer and the second superconductor layer. Characteristic Josephson junction device. 2. The tunnel barrier layer is a compound formed by a chemical reaction of oxidation, nitridation, or fluorination of a metal or semiconductor, and the production energy per atom of oxygen, nitrogen, or fluorine in the chemical reaction is higher than that of Si or Ge. The Josephson junction device according to claim 1, characterized in that the response is smaller than that of the same reaction.