JPH0936446A - Josephson junction device and production thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a Josephson junction device comprising a Josephson junction element having SIS structure formed of an oxide superconductor, an insulator and an oxide superconductor and a suitable production method therefor. SOLUTION: In a Josephson junction device, a Josephson junction element having SIS structure is formed of an oxide superconductor 20 containing Tl or Y and O and an insulator 21 containing Tl or Y and O having composition similar to that of the oxide superconductor 20. The insulator 21 is formed by reforming the surface layer of oxide superconductor 20. The quality, thickness and the like of oxide superconductor 20 are controlled by controlling the lowering rate of temperature in heat treatment. The quality, thickness and the like of insulator 21 are controlled by controlling the concentration of Tl or Y in the heat treatment system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Josephson junction device having a Josephson junction element and a method for manufacturing the same. In particular, the present invention relates to a Josephson junction device including a Josephson junction element in which a high temperature oxide superconductor, an insulator and a high temperature oxide superconductor are sequentially joined, and a suitable Josephson junction for realizing the Josephson junction element. The present invention relates to a method of manufacturing a device.

[0002]

2. Description of the Related Art At present, Josephson junction devices are used for logic gates that are required to operate at the highest speed, and these Josephson junction devices are indispensable for realizing ultra-high speed devices. Josephson junction devices are superconducting
r), insulator (insulator), superconductor (superconducto
r) is composed of a SIS structure which is sequentially joined. As a method for forming a Josephson junction element, a formation method using a step edge junction and a formation method using a bicrystal (bicrystal) junction substrate are known.

In the method of forming a Josephson junction element using the step edge junction, a part of the substrate surface is etched, two surfaces having different heights are formed on the substrate surface, and A step is formed between them. A thin film of a superconductor is formed on the surface of the substrate including the step by an epitaxial growth method. A superconductor can be formed on two surfaces having different substrate heights, each having a crystal structure corresponding to the crystallinity of the substrate surface. On the other hand, a grain boundary is formed between one surface and the step, and between the other one surface and the step, that is, at the step edge, and the bond state between the composition elements of the superconductor is formed. Becomes unstable (weakly bonded portions or damaged regions are formed). Usually, the step has an inclined surface between step ends, and a superconductor can be formed on this inclined surface. That is, the Josephson junction device includes a superconductor formed on one surface, a weak coupling portion formed at the step end, a superconductor formed on the inclined surface of the step, a weak coupling portion formed at the step end, and other portions. SN (N: normal) having a superconductor formed on one surface
It is composed of an SNS structure.

In the forming method using the bicrystal bonded substrate, a bicrystal bonded substrate is produced by bonding two single crystal substrates having different crystal orientations by heat treatment. In the bicrystal bonded substrate, crystal grain boundaries can be formed at the bonded portions of the two substrates. When a thin film of a superconductor is formed on the surface of this bicrystal bonded substrate by epitaxial growth, the superconductor has a crystal structure corresponding to the crystallinity of the substrate surface on each surface of the two bonded substrates. Can be formed. On the other hand, a weakly-bonded region is formed in a portion where two substrates are bonded and a crystal grain boundary is formed, and this non-superconductor is used as an insulating film. That is, the Josephson junction element has an SNS structure having a superconductor formed on one of the substrates bonded together, a weak coupling region formed on a grain boundary, and a superconductor formed on the other substrate.

Recently, attention has been paid to researches in which a thallium (Tl) -based oxide superconductor is applied to a superconductor of a Josephson junction element. In the Tl-based oxide superconductor, the resistance (surface resistance: Rs) of the crystal surface through which the current flows is small,
Furthermore, there is a feature that it can be used at a high temperature with a critical temperature (transition temperature: Tc) exceeding 100K. As the Tl-based oxide superconductor, a TlBaCaCuO superconductor (thallium-barium-calcium-copper-oxygen-based superconductor: TBCCO-based superconductor) is generally used.

[0006]

[Problems to be Solved by the Invention]

(1) When an oxide high-temperature superconductor is used as the superconductor of the Josephson junction element, the coherence length of the oxide high-temperature superconductor is extremely short, about several Å, and the film thickness corresponding to this coherence length is good. I cannot make a thin film of a good insulator. Therefore, at the present time, a Josephson junction device having a SIS structure using an oxide high temperature superconductor cannot be realized, and there are no reports of actual trial manufacture.

In metal superconductors, Nb-based superconductors, etc., the coherence length is longer than that of oxide high temperature superconductors.
Since the insulator can be easily formed, the Josephson junction device having the SIS structure is realized.

(2) In both the method of forming a Josephson junction element using step edge junction and the method of forming a Josephson junction element using a bicrystal junction substrate, an insulator is formed by artificially formed crystal grain boundaries. it can. However, especially when an oxide high-temperature superconductor is used as the superconductor of the Josephson junction element, an insulator is not formed at the crystal grain boundaries artificially formed, but a non-superconductor and a conductive material are used. Is produced. That is, the Josephson junction element is not the SIS structure but the SNS.
It is not possible to realize a Josephson junction device having a SIS structure because of the structure.

The present invention has been made to solve the above problems.

Therefore, it is an object of the present invention to provide a Josephson junction device including a Josephson junction element having a SIS structure formed of an oxide superconductor, an insulator and an oxide superconductor.

Still another object of the present invention is to provide a preferable method for manufacturing the Josephson junction device.

[0012]

In order to solve the above problems, a Josephson junction device according to a first aspect of the present invention is an oxide superconductor containing thallium element or yttrium element and oxygen element, and the oxide superconductor. A Josephson junction element, comprising an insulator containing the same thallium element or yttrium element and oxygen element as the body composition substance, and the oxide superconductor, the insulator and the oxide superconductor are sequentially joined. It is characterized by having.

Tl-based oxide superconductor, specifically Tl _{α
Ba 2 Ca n-1 Cu n} O 2n + 4 superconductor (α = 0-3, n
Basic research was conducted by the present inventor on the correlation between the Tl concentration of = 1-4) and the crystallization temperature. As a result,
There is a correlation between the crystallization temperature and the Tl concentration, and the Tl ₂ Ba ₂ Ca ₁ Cu ₂ O _{2n +} is c-axis oriented within a specific crystallization temperature range and within a specific Tl concentration range. _Four
It has been found that superconductors can be formed. Further, in a temperature range lower than the specific crystallization temperature and in a range in which the Tl concentration is excessive, a TlBaCaO compound and CuO containing the same element as that composing the Tl ₂ Ba ₂ Ca ₁ Cu ₂ O _{2n + 4} superconductor. A region in which the compound was mixed was confirmed. Since the compound contains a plurality of the same composition elements as the Tl ₂ Ba ₂ Ca ₁ Cu ₂ O _{2n + 4} superconductor, Tl ₂ Ba ₂ C
It can be inferred that the surface layer of the a ₁ Cu ₂ O _{2n + 4} superconductor was a compound produced by modification. It is highly possible that these compounds have insulating properties, are of good quality without defects, and have an extremely thin film thickness corresponding to the coherence length. The inventor of the present application prototyped a Josephson junction device having a SIS structure using an insulator containing the compound, and measured the superconducting characteristics (critical current density Jc and current-voltage characteristics) of the Josephson junction device.
The SIS-like operation was confirmed.

Therefore, in the Josephson junction device according to the above-mentioned claim 1, an insulator containing the same composition element as the composition material of the oxide superconductor is formed, and the insulator is a good quality film corresponding to the coherence length. It can be formed thick.
Therefore, it is possible to realize the Josephson junction device having the SIS structure (indicating the SIS-like operation) in which the oxide superconductor, the insulator, and the oxide superconductor are sequentially joined.

A Josephson junction device according to a second aspect is the Josephson junction device according to the first aspect, wherein the oxide superconductor is formed along a substrate surface or a buffer layer surface on which a specific crystal structure can be formed. The oxide superconductor, the insulator and the oxide superconductor of the Josephson junction element are sequentially arranged, and the Josephson junction element has a planar structure.

A Josephson junction device according to a third aspect is the Josephson junction device according to the second aspect, wherein the oxide superconductor of the Josephson junction element is formed of particles, and the Josephson junction element of the Josephson junction element is formed. The insulator is formed at a grain boundary between the particles of the oxide superconductor.

A Josephson junction device according to a fourth aspect is the Josephson junction device according to the second aspect, wherein the oxide superconductor of the Josephson junction element is formed of particles, and the Josephson junction element of the Josephson junction element is formed. As the insulator, an insulator obtained by modifying the surface layer of the particles of the oxide superconductor is used.

A Josephson junction device according to a fifth aspect is the Josephson junction device according to the second aspect, wherein a step is formed on the substrate surface or the buffer layer surface, and on the substrate surface excluding the step or The oxide superconductor of the Josephson junction element is formed on the surface of the buffer layer, and the insulator of the Josephson junction element is formed on the step of the substrate surface or the surface of the buffer layer.

A Josephson junction device according to a sixth aspect is the Josephson junction device according to the second aspect, wherein the substrate or the buffer layer has two types of single crystal structures having different plane orientations, A bicrystal substrate or a bicrystal buffer layer having a crystal grain boundary between two types of single crystal structures is used, and the oxide of the Josephson junction device is on the substrate surface or the buffer layer surface excluding the crystal grain boundary. A superconductor is formed, and an insulator of the Josephson junction element is formed on a crystal grain boundary of the substrate surface or the buffer layer surface.

A Josephson junction device according to a seventh aspect is the Josephson junction device according to the second aspect, wherein the substrate is a MgO substrate, a LaAlO ₃ substrate, a SrTiO ₃ substrate, YSZ (yttrium-stabilized zirconia). One of a substrate and a CeO ₂ substrate is used, and the buffer layer is a MgO buffer layer or LaAl.
It is characterized in that any one of the O ₃ buffer layer, the SrTiO ₃ buffer layer, the YSZ buffer layer, and the CeO ₂ buffer layer is used.

The Josephson junction device according to claim 8 is the Josephson junction device according to claim 7, wherein the oxide superconductor is made of Tl ₂ Ba ₂ C.
a _n-1 Cu _n O _{2n + 4} superconductor (n = 1-4), Tl ₁ B
_{a 2 Ca n-1 Cu n} O 2n + 3 superconductor (n = 1-6), T
l ₂ Ba ₂ (Ca, Y) ₁ Cu ₂ O ₈ superconductor, Tl ₁
Sr ₂ Ca _n-1 Cu _n O _{2n + 3} superconductor (n = 2, 3),
_{(Tl, Pb) 1 Sr 2} Ca n-1 Cu n O 2n + 3 superconductor (n = 1,2,3), (Tl , Bi) 1 Sr 2 Ca n-1
Cu _n O _{2n + 3} superconductor (n = 2,3), Tl ₁ Sr
₂ (Ca, Ln) ₁ Cu ₂ O ₇ superconductor, (Tl, P
b) It is characterized in that a Tl-based oxide superconductor of any of the (Sr, Ln) ₂ Cu ₁ O ₅ superconductors is used.

A Josephson junction device according to a ninth aspect is the Josephson junction device according to the eighth aspect, wherein at least two kinds of composition elements of the thallium-based oxide superconductor are formed in the insulator. It is characterized in that an insulator in which the compound of (1) is mixed is used.

A Josephson junction device according to a tenth aspect of the present invention is the Josephson junction device according to the ninth aspect, wherein the insulator is a TlBaCaO compound generated from a composition element of the thallium-based oxide superconductor. It is characterized in that an insulator in which a CuO compound is mixed is used.

The Josephson junction device according to claim 11 is the Josephson junction device according to claim 7, wherein the oxide superconductor is Y ₁ Ba ₂ C.
u ₃ O ₇ superconductor, Y ₁ Ba ₂ Cu ₄ O ₈ superconductor, Y
₁ Ba ₂ Cu _3.5 O _7.5 superconductor, (Y, Ca) ₁ Ba
₂ Cu ₄ O ₈ superconductor, (Ln, Ce) ₂ (Ba, L
n) Any of the Y-based oxide superconductors of ₂ Cu ₃ O ₁₀ superconductor is used.

A Josephson junction device according to a twelfth aspect is the Josephson junction device according to the eleventh aspect, wherein at least two kinds of composition elements of the yttrium oxide superconductor are formed in the insulator. It is characterized in that an insulator in which the compound of (1) is mixed is used.

In the method for manufacturing the Josephson junction device according to the thirteenth aspect, the first heat treatment is performed within the range of the crystallization temperature of the oxide superconductor, and the oxide containing thallium element or yttrium element and oxygen element. A step of forming a superconductor, and a second heat treatment is performed within a range from a temperature at which crystallization of a compound generated by a composition element of the oxide superconductor is started to a temperature lower than a crystallization temperature of the oxide superconductor. Forming an insulator containing the same thallium element or yttrium element and oxygen element as the composition material of the oxide superconductor on the surface of the oxide superconductor. In the method for manufacturing a Josephson junction device according to the thirteenth aspect, the oxide superconductor can be formed by the first heat treatment, and the insulator containing the same composition element as the oxide superconductor can be formed by the second heat treatment. That is, the oxide superconductor and the insulator can be easily formed by controlling the temperatures of the first and second heat treatments.

A method for manufacturing a Josephson junction device according to claim 14 is the method for manufacturing a Josephson junction device according to claim 13, wherein after the oxide superconductor is formed by the first heat treatment, Subsequently, a second heat treatment is performed on the oxide superconductor at a temperature lowering rate at which the temperature drops every tens of degrees per minute, and the surface layer of the oxide superconductor is modified into an insulator. .

In the method for manufacturing the Josephson junction device according to the fourteenth aspect, after the oxide superconductor is formed by the first heat treatment, continuous treatment is performed and the temperature range of the first heat treatment is changed to the temperature range of the second heat treatment. The surface layer of the oxide superconductor can be modified only by cooling to form an insulator. In the second heat treatment, the temperature lowering rate for lowering the temperature every tens of degrees per minute corresponds to the temperature lowering rate of furnace cooling, the surface layer of the oxide superconductor is sufficiently modified, and it has a good quality and an appropriate film thickness. An insulator can be formed.

A method for manufacturing a Josephson junction device according to a fifteenth aspect is the method for manufacturing a Josephson junction device according to the thirteenth aspect, wherein after the oxide superconductor is formed by the first heat treatment, Subsequently, the oxide superconductor is subjected to a second heat treatment in which the temperature lowering rate for lowering the temperature per unit time is set in at least two stages, and the surface layer of the oxide superconductor is modified into an insulator. Is characterized by.

In the method for manufacturing the Josephson junction device according to the fifteenth aspect, the degree of modification of the surface layer of the oxide superconductor can be controlled and the film thickness can be controlled to form an optimal insulator according to the purpose. it can.

A method for manufacturing a Josephson junction device according to a sixteenth aspect is the method for manufacturing a Josephson junction device according to the fifteenth aspect, wherein the second heat treatment is performed at a particular temperature lowering rate at a particular temperature. It is characterized in that the state is maintained for a certain period of time. In the method for manufacturing the Josephson junction device according to the sixteenth aspect, the degree of modification of the surface layer of the oxide superconductor can be increased, the film thickness can be increased, and an optimum insulator suitable for the purpose can be formed.

A method for manufacturing a Josephson junction device according to claim 17 is the method for manufacturing a Josephson junction device according to claim 13, wherein a thallium-based oxide superconductor is used as the oxide superconductor. The first heat treatment of the thallium-based oxide superconductor is 800 ° C. or higher 9
The temperature range is set to 00 ° C or less, and the second heat treatment is performed at 4
It is characterized in that it is set in a temperature range of 50 ° C. or higher and lower than 800 ° C.

[0033]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below.

<Josephson Junction Device 1> FIG. 1
(B) is a cross-sectional view of a Josephson junction device according to the present invention. As shown in the figure, the Josephson junction device has the Josephson junction element 2 mounted on the surface of the substrate 1. The Josephson junction element 2 is an oxide superconductor (S: superconductor) 20, an insulator (I: insulator)
21 and an oxide superconductor (S: superconductor) 20 are sequentially joined to each other to form an SIS structure.

When the oxide superconductor 20 of the Josephson junction element 2 is directly formed on the surface of the substrate 1, a substrate material that can form the oxide superconductor 20 on the substrate 1 with a specific crystal structure. Is used. In this embodiment, since the Tl-based oxide superconductor having the perovskite crystal structure is used for the oxide superconductor 20, M is used for the substrate 1.
gO substrate, LaAlO ₃ substrate, SrTiO ₃ substrate, YS
Either a Z (yttria-stabilized zirconia) substrate or a CeO ₂ substrate is used. Especially MgO substrate, LaAlO
Each of the _three substrates is characterized in that high frequency characteristics are enhanced in terms of dielectric constant and dielectric loss (propagation loss: tan δ).

When the oxide superconductor 20 is indirectly formed by interposing a buffer layer on the surface of the substrate 1, the same material as the substrate material can be used for the buffer layer. That is, the buffer layer is a MgO buffer layer, L
aAlO ₃ buffer layer, SrTiO ₃ buffer layer, YS
Either a Z buffer layer or a CeO ₂ buffer layer can be used.

As the oxide superconductor 20 of the Josephson junction element 2, a Tl-based oxide superconductor is used. Specifically, as a Tl-based oxide superconductor, Tl ₂ Ba ₂ C an _-1 Cu _n which can obtain superconducting properties at a high temperature of 100 K or higher.
O _{2n + 4} superconductors (n = 1-4) are used. Especially as a standard TlBaCaCuO superconductor, Tl ₂ Ba ₂
Ca ₀ Cu ₁ O ₆ superconductor, Tl ₂ Ba ₂ Ca ₁ Cu ₂
Either an O ₈ superconductor or a Tl ₂ Ba ₂ Ca ₂ Cu ₃ O ₁₀ superconductor is used.

Further, in the present invention, TlBaCaCu
_{Tl 1 Ba 2 Ca n-1} Cu n O 2n + 3 superconductor as O superconductor (n = 1-6) can be used. In the present invention, as a Tl-based oxide superconductor other than the above, Tl ₂ Ba is used.
₂ (Ca, Y) ₁ Cu ₂ O ₈ superconductor, Tl ₁ Sr ₂ C
a _n-1 Cu _n O _{2n + 3} superconductor (n = 2, 3), (Tl,
Pb) ₁ Sr ₂ Ca _n-1 Cu _n O _{2n + 3} superconductor (n =
1, 2, 3), (Tl, Bi) ₁ Sr ₂ Can _n-1 Cu _n
O _{2n + 3} superconductor (n = 2,3), Tl ₁ Sr ₂ (Ca,
Ln) ₁ Cu ₂ O ₇ superconductor, (Tl, Pb) (Sr,
Any of the Ln) ₂ Cu ₁ O ₅ superconductors can be used.

The Tl-based oxide superconductor 20 is formed in a particle state in this embodiment, and a plurality of oxide superconductors 20 are arranged along the surface of the substrate 1. That is, the oxide superconductor 20 of the Josephson junction element 2, the insulator 21,
The oxide superconductors 20 are basically arranged in the lateral direction, and the Josephson junction element 2 has a planar structure. As shown in FIG. 1B, in this embodiment, the oxide superconductor 2 of the Josephson junction element 2 is formed on the surface of the substrate 1.
0, the insulator 21, and the oxide superconductor 20 are arranged in the vertical direction, and the Josephson junction element 2 is also formed in the vertical direction.

Insulator 21 of the Josephson junction element 2
Contains a plurality of composition elements that are the same as the composition elements of the oxide superconductor 20, that is, the Tl-based oxide superconductor. Specifically, the insulator 21 is formed by mixing the TlBaCaCuO compound and the CuO compound. The insulator 21 contains the same composition element as that of the Tl-based oxide superconductor, but the insulator 21 is a non-oxide superconductor and has an insulating property.

The insulator 21 is formed between the oxide superconductors 20. In reality, since the surface layer of the oxide superconductor 20 formed in the shape of particles is modified (superconducting property is lost) and the insulator 21 is formed, the insulator 21 is almost all of the oxide superconductor 20. Formed on the surface and at the grain boundaries between the oxide superconductors 20.

<Josephson Junction Device Manufacturing Method 1
> Next, a method for manufacturing the above-mentioned Josephson junction device will be described. 1A and 1B are cross-sectional views showing the above-mentioned Josephson junction device in each manufacturing process.

First, in the first step, a precursor of a Tl-based oxide superconductor is formed on the surface of the substrate 1 (or the buffer layer), and this precursor is subjected to a heat treatment at a crystallization temperature, as shown in FIG. ) As shown in FIG.
Can be formed.

A MgO substrate is used as the substrate 1.
A precursor of a Tl-based oxide superconductor is, for example, a high frequency (R
F) The magnetron sputtering method is used, and the film forming conditions of the precursor are set as follows, for example.

(1) Target composition: Tl ₂ Ba ₂ C
a ₁ Cu ₂ O _x (2) Target-substrate 1 distance: 40 mm (3) Substrate temperature: room temperature (4) Sputtering gas: Ar (5) Gas pressure: 30 mtorr (6) High frequency power: 90 W (7) Deposition rate 1.6 〓 / sec The precursor is formed in an amorphous structure under the above film forming conditions. After the precursor is formed, a heat treatment is performed within a crystallization temperature range for forming a Tl-based oxide superconductor, and the Tl-based oxide superconductor has a perovskite crystal structure and is set in the c-axis orientation, that is, for example, Tl ₂ Ba ₂
A Ca ₁ Cu ₂ O _x superconductor is formed.

FIG. 2 is a phase diagram showing the correlation between the Tl concentration and the heat treatment temperature of the Tl-based oxide superconductor. In FIG. 2, the vertical axis represents the heat treatment temperature (° C.) performed on the precursor. The horizontal axis _{Tl y Ba 2 Ca 2 Cu 3} O δ compound pellets T
1 shows composition (y = 0-3). The compound pellets are installed in the same heat treatment system as the precursor, and the Tl concentration in the heat treatment system can be adjusted. That is, the Tl concentration in the heat treatment system is adjusted by the Tl contained in the compound pellets, and the crystallization heat treatment of the precursor can be performed with the Tl composition ratio of the precursor appropriately maintained. Usually, Tl easily evaporates, so that the crystallization heat treatment of the precursor is performed with the vapor pressure of Tl increased. As shown in FIG.
Although the temperature range of the crystallization heat treatment of the precursor changes depending on the composition (Tl concentration in the heat treatment system),
When the composition is 1, the crystallization heat treatment is 800-900.
Set to ° C. When the Tl composition of the compound pellets is 1.5, the crystallization heat treatment is set to 810-900 ° C, and when the Tl composition of the compound pellets is 2, the crystallization heat treatment is set to 850-900 ° C. To be done. In this embodiment, the crystallization heat treatment is set to 865 ° C. In the crystallization heat treatment, the precursor itself can be selectively heated without affecting the surroundings.
Either Annealing) or FA (Flash Annealing) is used.

In FIG. 2, region B has Tl ₂ Ba ₂ Ca ₁ Cu ₂ having a perovskite crystal structure and c-axis oriented.
This is a region where the O _x superconductor is formed. Therefore, the precursor is subjected to the crystallization heat treatment in the region B to form the oxide superconductor 20. Area A is Tl ₂ Ba ₂ Ca
_This is a non-oriented region in which ₁ Cu ₂ O _x superconductors are randomly mixed. Region C is a region in which the TlBaCaCuO compound and the CuO compound are mixed. This region C is a region for forming the insulator 21, which will be described later.

FIG. 3 is a diagram showing the correlation between the heat treatment temperature and the heat treatment time of the Tl-based oxide superconductor. In FIG. 3, the vertical axis represents the heat treatment temperature (° C.) performed on the precursor. The horizontal axis indicates the heat treatment time. As shown in FIG. 3, the crystallization heat treatment is performed at 865 ° C. for about 60 minutes.

Next, in the second step, from the crystallization heat treatment temperature at which the oxide superconductor 20 is formed, the oxide superconductor 20 is subsequently cooled (removed) at a specific temperature lowering rate, and as shown in FIG. Insulators 21 are formed at the grain boundaries between the superconductors 20. When the insulator 21 is formed, the oxide superconductor 2
0, the insulator 21, and the oxide superconductor 20 are sequentially arranged to form a junction with each other to form the Josephson junction element 2 of the SIS structure, and the Josephson junction device is completed.

The insulator 21 is the area C shown in FIG.
Is formed within the range. As a result, the insulator 21 is formed of an insulator in which a TlBaCaO compound and a CuO compound which are the same as the composition elements of the oxide superconductor 20 and contain most of the composition elements are mixed.

As shown in FIG. 2 described above, in the formation of the insulator 21 according to this embodiment, the Tl of the compound pellet is
The composition ratio is set within the range of about 0.8-2.2. Further, in forming the insulator 21, the TlBaCaO
The heat treatment is performed within a range from the temperature at which the crystallization of the compound or CuO compound is started to the temperature below the crystallization temperature of the oxide superconductor 20. For example, as shown in FIG. 2, when the Tl composition of the compound pellet is 1, the heat treatment for forming the insulator 21 is set within the range of 450 ° C. to less than 800 ° C. When the Tl composition of the compound pellet is 1.5, the heat treatment is set within the range of 450 ° C to less than 810 ° C,
If the Tl composition of the compound pellet is 2, the heat treatment is 4
It is set within the range of 50 ° C to less than 850 ° C.

The critical current density J of the Josephson junction device (sample) depends on whether the film quality and the film thickness of the insulator 21 are optimal for the SIS-like operation characteristics of the Josephson junction device.
It can be considered in c. If the constant K, the sample temperature T, and the superconducting critical temperature Tc are set, the critical current density Jc is expressed by the following equation.

Jc = K (1-T / Tc) n In the above-mentioned FIG. 3, the crystallization temperature, specifically 865 ° C.
3 shows the case where three kinds of insulators 21 are formed at three kinds of temperature decreasing rates. The temperature decrease curve a is set to a temperature decrease rate at which the temperature decreases by several hundreds of degrees Celsius per minute. The cooling curve a shows a tendency corresponding to rapid cooling. Specifically, since there is a temperature drop of 200 ° C. in 30 seconds, the temperature drop rate indicated by the temperature drop curve a is −40.
This corresponds to 0 ° C / min. Temperature drop curve b is several tens of degrees Celsius per minute
It is set to the cooling rate that lowers the temperature. Temperature drop curve b
Indicates a tendency corresponding to furnace cooling. In detail, 20 minutes in 20
Since there is a temperature drop of 0 ° C., the temperature drop rate indicated by the temperature drop curve a corresponds to −10 ° C./min. The temperature decrease curve c is set to two stages of temperature decrease rates having different slopes, and the front side of the temperature decrease curve c that contributes to the final composition of the insulator 21 is set to a temperature decrease rate of decreasing the temperature by several degrees Celsius per minute. . The temperature decrease curve c shows a tendency corresponding to super-cooling. In detail, 60 minutes 1
Since there is a temperature drop of 00 ° C., the cooling rate indicated by the front side of the cooling curve a is equivalent to −1.7 ° C./min.

The critical current density Jc of the Josephson junction element 2 using the insulator 21 formed on the basis of the temperature drop curve a is 8.9 × 10 ³ A / cm ² (Tc: 101K).
Since the index n of the critical current density Jc is close to 2, the Josephson junction device 2 has SNS-like operation characteristics. The critical current density Jc of the Josephson junction element 2 using the insulator 21 formed based on the temperature drop curve b is 4.5 × 1.
Since the index n of the critical current density Jc is close to 1 at 0 ⁵ A / cm ² (Tc: 101K), the Josephson junction element 2 has SIS-like operating characteristics. Whether or not the Josephson junction element 2 has an SIS-like operation characteristic cannot be generally judged only by the temperature dependence of the critical current density Jc, and finally the current-voltage characteristic (I- When the V characteristic) shows a hysteresis characteristic, it can be determined to be an SIS-like operation characteristic. Josephson junction element 2 using insulator 21 formed based on temperature drop curve c
Has a critical current density Jc of 4.6 × 10 ³ A / cm ² (T
c: 101K), the index n of the critical current density Jc is 1
Therefore, the Josephson junction element 2 has SIS-like operation characteristics.

That is, as shown by the temperature lowering curves a, b, and c in FIG. 3 described above, basically, when the temperature lowering rate is gently controlled, the Josephson junction element 2 forms the insulator 21 exhibiting SIS-like operation characteristics. it can. It should be noted that in the region C shown in FIG. 2, a good quality insulator 21 can be formed even if the temperature is kept stable at a specific temperature for a certain period of time during cooling. For example, from 865 ° C. to 76 ° C. along the front side of the cooling curve b shown in FIG.
Even if the temperature is lowered to 5 ° C. and kept stable at 765 ° C. for about 60 minutes and then further lowered from 765 ° C. along the rear stage side of the temperature decreasing curve c, the good-quality insulator 21 can be formed.

Further, the film quality and the film thickness of the insulator 21, which are optimum for the SIS-like operation characteristics of the Josephson junction element, can be controlled by the Tl composition ratio of the compound pellet in the heat treatment system. As shown in FIG. 2, when the Tl composition ratio of the compound pellet is high, both the TlBaCaO compound and the CuO compound are likely to be produced. That is, if Tl is excessively supplied to the surface of the oxide superconductor 20, the reforming of the surface layer of the oxide superconductor 20 is promoted, and a thin film having good quality and coherence length is formed on the surface of the oxide superconductor 20. The insulator 21 having a thickness is easily generated.

FIG. 4 is a diagram showing the X-ray diffraction results of the surface layer of the oxide superconductor 20 for each specific temperature. In FIG. 4, the horizontal axis represents the X-ray diffraction angle 2θ (deg). The vertical axis represents the X-ray intensity (au). The sample for X-ray analysis is TlBaC
An aCuO superconductor precursor (thin film) is used.
The precursor was heat treated at a crystallization temperature of 880 ° C.
The X-ray diffraction result at the stage where the BaCaCuO superconductor is formed is shown on the upper side. In the crystallization heat treatment, Tl _1.8 Ba ₂ Ca ₂ Cu ₃ O was used in the same heat treatment system as the precursor.
_{The δ} compound pellets are arranged, and the Tl concentration (vapor pressure) and the oxygen concentration (vapor pressure) are adjusted respectively. Hereinafter, the results of the X-ray diffraction performed while the temperature was sequentially lowered are shown on the lower side. That is, 865 ° C, 850 ° C, 830 ° C, 8
X-ray diffraction results are shown at each temperature drop stage of 00 ° C.

In FIG. 4, the steps of 880 ° C., 865 ° C., 850 ° C. and 830 ° C. are basically within the crystallization temperature of the precursor, and as a result of X-ray diffraction, Tl ₂ Ba ₂ CaCu ₂ O _δ superconductor (00
1) The peak of the crystal plane is seen. Moreover, as the temperature decreased from 880 ° C to 830 ° C, Tl ₂ Ba ₂ C
The peak of the (001) crystal plane of the aCu ₂ O _δ superconductor becomes smaller.

Since the Tl concentration in the heat treatment system was adjusted with the Tl _1.8 Ba ₂ Ca ₂ Cu ₃ O _δ compound pellets, and the composition of Tl of the compound pellets was set to _1.8 , the above-mentioned FIG. 2 was used. Area B- near 820-830 ° C
There are boundaries between regions C. Therefore, 800 shown in FIG.
The ℃ stage is in the category of region C, and as a result of X-ray diffraction, Tl
Each peak of the BaCaO compound and the CuO compound is seen.

That is, as a result of the X-ray diffraction, when the temperature is lowered within the range from the region B to the region C shown in FIG. 2, the surface layer of the oxide superconductor 20 is modified, and the modified surface is obtained. It can be considered that an insulator 21 is formed in the layer. Moreover, since the surface layer of the oxide superconductor 20 is modified, the insulator 21 can be basically formed of a compound containing the same composition element as the composition element of the oxide superconductor 20. The thickness of the insulator 21 corresponds to the thickness of the surface layer of the oxide superconductor 20 to be modified, and the thickness of the insulator 21 can be increased by slowing the temperature lowering rate in the region C.

<Josephson Junction Device 2> Next, a Josephson junction device provided with a Josephson junction element utilizing step edge junction will be described.

FIG. 5C is a sectional view of the Josephson junction device according to the present invention. As shown in the figure, the Josephson junction device has the Josephson junction element 2 mounted on the surface of the substrate 1. The Josephson junction element 2 is
Basically, the oxide superconductor 2 has the same structure.
0, the insulator 21, and the oxide superconductor 20 are sequentially joined to form a SIS structure.

A part of the surface of the substrate 1 is etched, two surfaces having different heights are formed on the surface of the substrate 1, and a step 1S is formed between the two surfaces. The oxide superconductor 20 is formed on each of the two surfaces of the substrate 1, and a Tl-based oxide superconductor is used for the oxide superconductor 20 as described above. The insulator 21 is formed on the step 1S of the substrate 1 between the two oxide superconductors 20. Similarly to the above, the insulator 21 is formed of an insulator in which the surface layer of the oxide superconductor 20 is modified and the same composition element as that of the oxide superconductor 20 is contained. In particular, in the Josephson junction element 2 using the step edge junction, the weakly bonded portion or the damaged region generated between the oxide superconductors 20 on the step 1S of the substrate 1 is modified as the insulator 21, The generation rate of the insulator 21 can be increased.

<Josephson Junction Device Manufacturing Method 2
> Next, a method for manufacturing the Josephson junction device shown in FIG. 5C will be described. 5A to 5C are cross-sectional views showing the above-mentioned Josephson junction device in each manufacturing process.

First, in the first step, a part of the surface of the substrate 1 (or the buffer layer) is etched, and as shown in FIG.
As shown in (A), two surfaces having different heights are formed on the substrate 1, and a step 1S is formed between the two surfaces.

In the second step, a precursor of a Tl-based oxide superconductor is formed on the two surfaces of the substrate 1 having different heights and on the step 1S. The precursor is formed by an epitaxial growth method or a sputtering method. Then, this precursor is subjected to a heat treatment at a crystallization temperature, and the height of the substrate 1 is different as shown in FIG.
An oxide superconductor 20 can be formed on each surface. A weakly coupled portion (or damaged region) 20A containing the same composition element as that of the oxide superconductor 20 is formed from the precursor on the step 1S.

In the third step, the oxide superconductor 20 is used.
From the crystallization temperature at which the film was formed, cooling is subsequently carried out at a specific cooling rate, and as shown in FIG. 5C, the insulator 21 is formed on the step 1S, particularly between the oxide superconductors 20. That is, the insulator 21 that functions as the tunnel insulating film of the Josephson junction element 2 is formed by modifying the weakly coupled portion 20A. When this insulator 21 is formed, the Josephson junction element 20 of the SIS structure in which the oxide superconductor 20, the insulator 21, and the oxide superconductor 20 are sequentially arranged and joined to each other is formed, and the Josephson junction device is formed. Complete.

FIG. 6A shows the SI shown in FIG. 5C.
FIG. 6B is a current-voltage characteristic diagram of the Josephson junction device 2 having the S structure, and FIG. 6B is a current-voltage characteristic diagram of the Josephson junction device 2 having the SNS structure. FIG. 6 (A)
6B, the horizontal axis represents voltage and the vertical axis represents current.

As shown in FIG. 6A, in the Josephson junction element 2 shown in FIG. 5C, a superconducting phenomenon in which a current flows at a voltage value of 0 can be confirmed, and a hysteresis characteristic can be confirmed. The SIS-like operation characteristics can be confirmed. On the other hand, in the Josephson junction element 2N having the SNS structure, which is conveniently created by short-circuiting the oxide superconductors 20 of the Josephson junction element 2 with the normal conductor 22, the superconducting phenomenon can be confirmed, but the hysteresis occurs. Characteristics cannot be confirmed.

<Application Example 1> In the present invention, an yttrium (Y) -based oxide superconductor can be used as the oxide superconductor 20 of the Josephson junction element 2 described above. As the Y-based oxide superconductor, Y ₁ Ba ₂ Cu ₃ O ₇ superconductor, Y
₁ Ba ₂ Cu ₄ O ₈ superconductor, Y ₁ Ba ₂ Cu _3.5 O
_{Any of 7.5} superconductor, (Y, Ca) ₁ Ba ₂ Cu ₄ O ₈ superconductor, and (Ln, Ce) ₂ (Ba, Ln) ₂ Cu ₃ O ₁₀ superconductor can be used.

When a Y-based oxide superconductor is used as the oxide superconductor 20, a compound containing the same composition element as that of the Y-based oxide superconductor, specifically, YBa.
Insulator 21 in which CuO compound and CuO compound are mixed
Is formed.

The most effective method of forming the insulator 21 is to set the Y concentration to a high value during cooling after forming the oxide superconductor 20 at the crystallization temperature so that the surface of the oxide superconductor 20 is The layer is modified and the insulator 21 can be easily formed.

<Application Example 2> In the present invention, the above Josephson junction element 2 can be formed by utilizing a bicrystal junction substrate. That is, the oxide superconductors 20 of the Josephson junction element 2 are formed on the surfaces of the bicrystal junction substrate having different plane orientations. The insulator 21 of the Josephson junction element 2 is the oxide superconductor 2
1 is formed by the weakly coupled portion 20A on the crystal grain boundary artificially provided on the bicrystal junction substrate.

<Application Example 3> In the above-described method for manufacturing the Josephson junction device, the oxide superconductor 20 is cooled at a specific temperature-decreasing rate from the crystallization temperature, and is continuously insulated from the oxide superconductor 20. A body 21 is formed. In the present invention, the oxide superconductor 20 is rapidly cooled from the crystallization temperature to a temperature (for example, 400 ° C.) at which the insulator 21 is not formed, and the oxide is again heated within the range C shown in FIG. By performing heat treatment on the superconductor 20, the insulator 21 can be formed discontinuously from the oxide superconductor 20.

[0075]

According to the present invention, it is possible to provide a Josephson junction device including a Josephson junction element having a SIS structure formed of an oxide superconductor, an insulator and an oxide superconductor.

Furthermore, the present invention can provide a preferable method for manufacturing the Josephson junction device.

[Brief description of drawings]

1A and 1B are cross-sectional views showing a Josephson junction device according to the present invention in each manufacturing process.

FIG. 2 is a phase diagram showing a correlation between a heat treatment temperature of a Tl-based oxide superconductor and a Tl concentration.

FIG. 3 is a diagram showing a correlation between a heat treatment temperature and a heat treatment time of a Tl-based oxide superconductor.

FIG. 4 is a diagram showing an X-ray diffraction result of a surface layer of an oxide superconductor for each specific temperature.

5A to 5C are cross-sectional views showing the Josephson junction device in each manufacturing process.

FIG. 6A is a current-voltage characteristic diagram of a Josephson junction device having a SIS structure, and FIG. 6B is a current-voltage characteristic diagram of a Josephson junction device having an SNS structure.

[Explanation of symbols]

1 substrate, 2,2N Josephson junction element, 20 oxide superconductor, 21 insulator, 1S step, 20A weak coupling part, 22 normal conductor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junnobu Zenzato 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd.

Claims (17)

[Claims] 1. An oxide superconductor containing thallium element or yttrium element and oxygen element, and an insulator containing the same thallium element or yttrium element and oxygen element as the composition material of the oxide superconductor,
And a Josephson junction element in which the oxide superconductor, the insulator, and the oxide superconductor are sequentially joined. 2. The Josephson junction device according to claim 1, wherein the oxide superconductor of the Josephson junction element is provided along a substrate surface or a buffer layer surface on which the oxide superconductor can be formed with a specific crystal structure. A Josephson junction device, wherein a body, an insulator, and an oxide superconductor are sequentially arranged, and the Josephson junction element has a planar structure. 3. The Josephson junction device according to claim 2, wherein the oxide superconductor of the Josephson junction element is formed of particles, and the insulator of the Josephson junction element is of the oxide superconductor. A Josephson junction device characterized by being formed at grain boundaries between grains. 4. The Josephson junction device according to claim 2, wherein the oxide superconductor of the Josephson junction element is formed of particles, and the oxide superconductor is formed in an insulator of the Josephson junction element. Josephson junction device, characterized in that an insulating material obtained by modifying the surface layer of the particles is used. 5. The Josephson junction device according to claim 2, wherein a step is formed on the substrate surface or the buffer layer surface, and the Josephson junction is formed on the substrate surface or the buffer layer surface excluding the step. A Josephson junction device, wherein an oxide superconductor of the element is formed and an insulator of the Josephson junction element is formed on a step on the surface of the substrate or the surface of the buffer layer. 6. The Josephson junction device according to claim 2, wherein the substrate or the buffer layer has two different plane orientations.
A bicrystal substrate or a bicrystal buffer layer having two kinds of single crystal structures and having a crystal grain boundary between two kinds of single crystal structures is used, and the substrate surface or the buffer layer surface excluding the crystal grain boundaries is used. The oxide superconductor of the Josephson junction element is formed on the substrate, and the insulator of the Josephson junction element is formed on the crystal grain boundary of the substrate surface or the buffer layer surface. device. 7. The Josephson junction device according to claim 2, wherein the substrate is a MgO substrate, a LaAlO ₃ substrate, or SrTi.
Any one of an O ₃ substrate, a YSZ substrate, and a CeO ₂ substrate is used, and the buffer layer is any one of a MgO buffer layer, a LaAlO ₃ buffer layer, a SrTiO ₃ buffer layer, a YSZ buffer layer, and a CeO ₂ buffer layer. Josephson junction device characterized in that a buffer layer is used. 8. The Josephson junction device according to claim 7, wherein the oxide superconductor is Tl ₂ Ba ₂ C an _-1 Cu _n.
O _{2n + 4} superconductor (n = 1-4), Tl ₁ Ba ₂ Ca _n-1
Cu _n O _{2n + 3} superconductor (n = 1-6), Tl ₂ Ba
₂ (Ca, Y) ₁ Cu ₂ O ₈ superconductor, Tl ₁ Sr ₂ C
a _n-1 Cu _n O _{2n + 3} superconductor (n = 2, 3), (Tl,
Pb) ₁ Sr ₂ Ca _n-1 Cu _n O _{2n + 3} superconductor (n =
1, 2, 3), (Tl, Bi) ₁ Sr ₂ Can _n-1 Cu _n
O _{2n + 3} superconductor (n = 2,3), Tl ₁ Sr ₂ (Ca,
Ln) ₁ Cu ₂ O ₇ superconductor, (Tl, Pb) (Sr,
A Josephson junction device, characterized in that a thallium-based oxide superconductor of any one of Ln) ₂ Cu ₁ O ₅ superconductor is used. 9. The Josephson junction device according to claim 8, wherein the insulator is an insulator in which at least two kinds of compounds generated by the composition element of the thallium-based oxide superconductor are mixed. Josephson junction device characterized by being used. 10. The Josephson junction device according to claim 9, wherein the insulator is a mixture of a TlBaCaO compound and a CuO compound generated by a composition element of the thallium-based oxide superconductor. A Josephson junction device characterized in that 11. The Josephson junction device according to claim 7, wherein the oxide superconductor is Y ₁ Ba ₂ Cu ₃ O ₇ superconductor, Y ₁ Ba ₂ Cu ₄ O ₈ superconductor, Y. ₁ Ba ₂ Cu
_3.5 O _7.5 Superconductor, (Y, Ca) ₁ Ba ₂ Cu ₄ O ₈
Superconductor, (Ln, Ce) ₂ (Ba, Ln) ₂ Cu ₃ O
_A Josephson junction device, characterized in that an yttrium-based oxide superconductor is used as one of the ₁₀ superconductors. 12. The Josephson junction device according to claim 11, wherein the insulator is an insulator in which at least two kinds of compounds generated by the composition element of the yttrium oxide superconductor are mixed. Josephson junction device characterized by being used. 13. A method of manufacturing a Josephson junction device, comprising the following steps (1) and (2). (1) A step of performing first heat treatment within a range of crystallization temperature of the oxide superconductor to form an oxide superconductor containing thallium element or yttrium element and oxygen element. (2) The second heat treatment is performed within a range from a temperature at which crystallization of a compound generated by the composition element of the oxide superconductor is started to a temperature lower than a crystallization temperature of the oxide superconductor, and A step of forming an insulator containing the same thallium element or yttrium element and oxygen element as the composition material of the oxide superconductor on the surface of the body. 14. The method for manufacturing a Josephson junction device according to claim 13, wherein after the oxide superconductor is formed by the first heat treatment, the temperature is continuously lowered every tens of degrees per minute. A second heat treatment is performed on the oxide superconductor at a temperature lowering rate to change the surface layer of the oxide superconductor to an insulator, thereby manufacturing a Josephson junction device. 15. The method for manufacturing a Josephson junction device according to claim 13, wherein after the oxide superconductor is formed by the first heat treatment, the oxide superconductor is continuously formed on a unit time basis. A method for manufacturing a Josephson junction device, characterized in that the surface layer of the oxide superconductor is modified into an insulator by performing a second heat treatment in which a temperature lowering rate for lowering the temperature is set to at least two stages or more. . 16. The method for manufacturing a Josephson junction device according to claim 15, wherein the second heat treatment holds a specific temperature state for a certain time while the second heat treatment is decreasing at a specific temperature decreasing rate. Method for manufacturing Josephson junction device. 17. The method for manufacturing a Josephson junction device according to claim 13, wherein a thallium-based oxide superconductor is used as the oxide superconductor, and the first heat treatment of the thallium-based oxide superconductor is performed. Is 800 ° C
The method for manufacturing a Josephson junction device is characterized in that the temperature is set to a temperature range of 900 ° C. or higher and the second heat treatment is set to a temperature range of 450 ° C. or higher and lower than 800 ° C.