JP3207295B2 - Superconducting element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting element using a high-temperature oxide superconductor, and relates to a memory element, a logical operation element,
The present invention relates to a superconducting element suitable for a Josephson junction element used as a SQUID or the like.

[0002]

2. Description of the Related Art Josephson junctions using a high-temperature oxide superconductor include superconducting thin films / insulator thin films / superconducting thin film structures (S / I / S structures), superconducting thin films / normal conducting thin films /
A three-layer laminated structure of a superconducting thin film structure (S / N / S structure) has been studied. In order to use a Josephson junction for a switching element or a memory element, and to increase the output voltage of the Josephson junction element, a good S / I /
It is desired to obtain an S tunnel type junction. But,
Conventionally, it has been difficult to obtain an S / I / S tunnel junction using a high-temperature oxide superconductor with good reproducibility. In order to obtain an S / I / S tunnel junction with good reproducibility using a high-temperature oxide superconductor, first, it is necessary that the superconducting order parameter is well developed up to the interface between the superconducting layer and the insulating layer. It is. Second, it is necessary to form a thin insulating layer having a thickness of about 2 nm without causing a short circuit or a leak current.

For example, as a superconducting layer, Bi ₂ having c-axis orientation is used.
S / N using Sr ₂ Ca ₂ Cu ₃ O _y thin film and Bi ₂ Sr ₂ Cu O _y thin film which does not show superconductivity as a normal conductive layer
/ S junction is known. However, such a three-layer laminate type bond has a metamorphic layer at the laminate interface,
It has the disadvantage that its properties cannot be controlled. Further, there are disadvantages that a pinhole is easily generated in the intermediate N layer, and the N layer is easily made into a superconductor due to a change in carrier concentration or composition. For these reasons, the S / N / S junction as described above has a problem that the reproducibility of Josephson characteristics is poor.

Similarly, c-axis oriented Bi ₂ Sr ₂ CaCu ₂ O _y
An S / N / S junction sandwiching a Bi ₂ Sr ₂ Cu O _y thin film that does not exhibit superconductivity between the superconducting layers was formed by K. Mizuo et al.
Vol. 56, No. 15, pp. 1469 to 1471. In addition, c-axis oriented Bi ₂ Sr ₂ CaCu ₂ O _y
S / I / S in which a long-period layered copper-based oxide such as Bi ₂ Sr ₂ (Ca, Sr, Bi) ₇ Cu ₈ O ₂₀ is sandwiched between superconducting layers as an insulating layer
The bond was made by MEKlausme-ier-Brown et al. In Appl.
Hys. Lett. Vol. 60, No. 22, pp. 2806-2808. These are changed from the superconducting layer to the intermediate layer of the N layer or the I layer.
Ca and Sr diffuse, and a part of the intermediate layer locally becomes a superconductor, which has a disadvantage that a leak current is easily generated.

In addition, it has been attempted to form an S / I / S junction by forming a simple oxide thin film such as ZrO ₂ or MgO or a fluoride thin film as an insulating layer by a sputtering method or a vacuum evaporation method. . However, easy pinholes occur in the insulating layer, it is difficult to deposit good quality oxide superconductor layer thereon, there is a problem such that T _c decreases. For example, Japanese Patent Application Laid-Open No. 2-185077 describes a Josephson junction device using fluorite (CaF ₂ ) as an insulating layer.
This device does not consider growing the insulating layer into a layer in lattice matching with the oxide superconductor. Therefore, it has the disadvantage that the superconducting characteristics near the interface between the upper and lower oxide superconductors and the insulating layer are deteriorated, and furthermore, steps and pinholes are easily generated. In particular, it has the disadvantage that fluorine diffuses into the upper and lower oxide superconductor layers and deteriorates the superconducting characteristics. JP-A-3-296283 discloses a pair of REBa
_{2 Cu 3 O 7-x (} RE is a rare earth element) to the superconducting layers, PrO ₂
And Josephson junctions sandwiching an insulating layer represented by RE ₂ O _w such as Y ₂ O ₃ . This bonding has a drawback that a step or a pinhole is easily generated because no consideration is given to growing the insulating layer in layers.

Further, SrTiO ₃ , PrGaO ₃ , NdGaO
Attempts have been made to use perovskite oxides such as ₃ , LaSrGaO ₄ and BaTiO ₃ as insulating layers. When these simple insulating oxides are heteroepitaxially grown with a copper-based oxide superconductor, they essentially have the following disadvantages. When a copper-based layered oxide is grown as a thin film,
The growth unit is from a block layer that does not carry conduction to a block layer. For example, in the case of Bi ₂ Sr ₂ CaCu ₂ O _y , the growth unit is from the Bi-O plane to the Bi-O plane. Also, Y
In the case of Ba ₂ Cu ₃ O ₇ is, CuO _[delta] Chai from CuO _[delta] Chain
Up to n are growth units. The surface after growing the lower superconducting electrode made of an oxide superconductor becomes a block layer that does not have superconductivity. When an intermediate layer is laminated thereon and then the upper superconducting electrode is grown, the block layer directly above the intermediate layer is also a block layer which does not have superconductivity. Therefore,
At the interface between the CuO ₂ surface responsible for superconductivity and the insulating oxide (intermediate layer), a normal conductive layer that is neither an insulator nor a superconductor is formed. That is, even if an attempt is made to produce an S / I / S junction, there is a drawback that an S / N / I / N / S junction is essentially obtained. As a result, the superconducting order parameter is well developed up to the interface of the insulating layer and has good characteristics.
I / S junction cannot be obtained.

Further, as a Josephson junction having a three-layer structure by epitaxial growth, YBa ₂ Cu ₃ O ₇ / PrB
An a ₂ Cu ₃ O _7-δ / YBa ₂ Cu ₃ O ₇ structure junction has been prototyped. This example is provided by JBBarner et al in Appl. Phys.
Lett. Vol. 59, No. 6, pp. 742-744. However, this junction had a disadvantage that the intermediate layer could easily be made conductive or superconductive due to the interdiffusion of Y and Pr.

[0008] Further, Japanese Patent Application Laid-Open No. 4-105373 discloses that C was directed through a single fluorite type block (Ln ₂ O ₂ ).
A superconducting element using as a N layer or an I layer a substance in which a portion composed of an alkaline earth element adjacent to the apex of the uO ₅ pyramid and a pyramid and a portion composed of an oxide of Pb and Cu are alternately laminated is described. Have been. This superconducting element uses a material having a single fluorite type block.
Therefore, in order to function as an N layer or an I layer,
The thickness of the intermediate layer should be at least 2 unit cells thick (typically 50 nm
Thickness). A substance containing a single fluorite-type block easily becomes a superconductor due to composition fluctuation or element diffusion from the superconducting layer. The thickness of the single fluorite-type block, measured at the CuO ₂ plane spacing, is about 0.6 nm, which is not much longer than the coherence length in the c-axis direction. Therefore,
When this substance is made into a superconductor, superconducting coupling also occurs in the c-axis direction, resulting in a disadvantage that a leak current is easily generated.

On the other hand, it has been reported by D. Dimos, P. Chaudhari, J. Mannhart and others that an S / N / S Josephson junction having good reproducibility and characteristics can be formed by using grain boundaries of a high-temperature oxide superconductor. Review B, Vol.41, No.7, pp.4038〜404
9, (1990). S. by P. Gross et al.
upercond.Sci.Technol.Vol.4, pp.S253-255, (1991)
Reported a similar Josephson junction. These use grain boundaries of the YBa ₂ Cu ₃ O ₇ thin film formed on the twin boundaries of the substrate.

[0010] However, such a grain boundary Josephson junction has the following disadvantages. First, since a twin substrate is manufactured by a method such as bonding, the substrate manufacturing process becomes complicated. Second, a bond can be formed only on the twin interface of the substrate, and cannot be formed anywhere on the substrate. Third, only a planar junction can be performed, and a stacked junction in which a superconducting current flows in a direction perpendicular to the substrate surface cannot be manufactured. Fourth, because of the weakly-coupled Josephson junction, the I / S / I
_c R _n product cannot be increased.

[0011]

As described above, the conventional laminated Josephson junction has insufficient matching at the interface between the oxide superconductor layer and the intermediate layer, and has a step or a step on the intermediate layer itself. Since defects such as pinholes are easily generated, there is a disadvantage that a leak current is easily generated. Moreover,
There is a disadvantage that the intermediate layer is easily converted into a conductor or a superconductor due to a composition change or the like. For these reasons, the conventional stacked-type Josephson junction cannot obtain good Josephson characteristics with good reproducibility. On the other hand, although grain boundary Josephson junctions are superior in characteristics as compared to conventional multilayer junctions, they have disadvantages in that the fabrication process is complicated, and there are many restrictions on the form and location of formation, and they lack practicality. Was.

Therefore, it is strongly desired to improve the Josephson characteristics and to improve the reproducibility thereof by solving problems in the structure and the material of the highly practical stacked Josephson junction. Was rare.

SUMMARY OF THE INVENTION The present invention has been made to address such a problem, and aims to stabilize the structure and characteristics of an insulating layer and to improve short-circuit current due to the insulating layer by improving the consistency of the lamination interface. Preventing the occurrence of leakage current,
It is another object of the present invention to provide a superconducting element in which the order parameter of superconductivity is well developed up to the interface between the superconducting layer and the insulating layer, and in which good Josephson characteristics can be obtained with good reproducibility.

[0014]

The superconducting element of the present invention comprises:
A lower superconducting layer composed of a copper-based oxide superconductor, an intermediate layer composed of a layered copper-based oxide laminated on the lower superconducting layer and including a multiple fluorite-type block in a crystal structure, and And an upper superconducting layer formed of a copper-based oxide superconductor.

The copper-based oxide superconductor used in the superconducting device of the present invention is not particularly limited as long as it can exhibit a superconducting state. However, a copper-based oxide superconductor that easily grows in layers is preferably used. Examples of such a copper-based oxide superconductor include a copper-based oxide superconductor containing Bi, a copper-based oxide superconductor containing Tl, a copper-based oxide superconductor containing Pb, and a copper-based oxide containing a rare earth element. A superconductor is exemplified. Specific examples of these copper-based oxide superconductors include those whose composition is substantially represented by the following chemical formula.

Chemical formula: Bi ₂ Sr ₂ Can _-1 Cu _n O _z (n = 1 to 4) (1): Tl ₂ Ba ₂ Can ₁ Cu _n O _z (n = 1 to 4) ……… (2): TlBa ₂ C a _n-1 Cu _n O _z (n = 1 to 4) ……… (3): (Pb ₂ Cu) Sr ₂ (Ca, RE) _n-1 C a _n O _z (n = 1 to 4) ……… (4): (Pb, Cu) Sr ₂ (Ca, RE) _n-1 Cu _n O _z (n = 1 to 4) ……… (5): REBa ₂ Cu ₃ O _{7 + δ} (6) (In the above formula, RE represents a rare earth element containing Y, and z represents the oxygen number.) Also, as a form of the copper-based oxide superconductor layer, the c-axis is Since the effects of the present invention cannot be sufficiently exerted in a form oriented in a random direction, it is preferable to orient the c-axis perpendicular to the substrate surface. Such an alignment state can be obtained by a well-controlled thin film formation method.

Here, in the copper-based oxide superconductor, the CuO ₂ plane commonly included in the crystal structure is responsible for electric conduction and, consequently, superconductivity. The part sandwiched between the CuO ₂ planes is extracted and is usually called a block layer (Tokura and Ar
ima, Jpn. J. Appl. Phys. Vol. 29, p. 2388- (1990)). Currently discovered in which T _c is 40K or more oxide superconductor, and both 1-2 kinds of blocking layer, during which
It can be regarded as a structure in which CuO ₂ faces are stacked. the above
The oxide superconductors represented by the formulas (1) to (6) each have, as a block layer, [Bi ₂ O ₂ ] ²⁺ , [Tl ₂ O ₂ ] ²⁺ ,
[TlO] ¹⁺ , [PbO-Cu-PbO] ¹⁺ , [(Pb, Cu) O] ¹⁺ , CuO _δ Chai
n etc. Such a block layer (hereinafter, [B]
), It can be said that the copper-based oxide superconductor easily grows in a layered manner. In particular, [Bi ₂ O ₂ ] ²⁺ and [PbO-Cu-Pb
O] ¹⁺ , Bi ³⁺ and Pb ²⁺ have a 6s ² lone pair,
Because the ions are strongly polarized, the bond becomes anisotropic,
A layered crystal structure is easily formed, and crystals are easily grown in layers. As another block layer, (Ce, RE) O ₂ (a typical fluorite type block, RE is a rare earth element) is known.

A block layer as described above generally comprises
It is not possible to stack multiple block layers without sandwiching the CuO ₂ surface. However, only the fluorite type block is an exception, and by controlling the film forming conditions and selecting the film forming source appropriately,
The present inventors have found that the fluorite-type block alone can be multi-layered (multiple fluorite-type block) and can be made to function as an insulating layer by selecting the number of layers.

The superconducting element of the present invention uses a layered copper-based oxide containing a multiple fluorite-type block as described above as an intermediate layer. By growing this multiple fluorite-type block in a layer with an arbitrary thickness, a good Josephson junction or the like can be obtained.

The layered copper-based oxide containing a portion in which the fluorite-type block is double or triple laminated in the crystal structure is as follows:
It has already been synthesized as a bulk sintered body. For example, T.
Wada et al., Physica C, Vol. 179, pp. 455-460 (1991)
In addition, (Pb, Cu) Sr
It is reported that polycrystalline bulk of ₂ (Ho, Ce) ₃ Cu ₂ O _{11+ δ} was synthesized. T. Wada et al., Physica C, Vo
l.175, pp.529-533 (1991), (Tl, Cu) Sr ₂ (RE, Ce)
We report that a polycrystalline bulk of ₃ Cu ₂ O _{11+ δ} was synthesized. A. Tokiwa et al., Physica C, Vol. 181,
pp. 311-319 (1991), Pb ₂ Sr ₂ (Y, Ce) _n Cu ₃ O
_The fact that a polycrystalline bulk of _{6 + 2n + δ} (n = 3,4) was synthesized, and these samples were observed with a transmission electron microscope, n = 5,
They report that there is intergrowth of 6,7. A. Tokiwa et al. Simultaneously reported that Pb (Ba, Sr) ₂ (Y, Ce) ₃ Cu ₃ O
It reports that polycrystalline bulk of _{11 + δ} was synthesized.

These are all reports on those synthesized as bulk. The present invention uses an oxide superconductor having a layered crystal structure or the above-described layered copper-based oxide, and makes it possible to create a crystal structure that is difficult to obtain in bulk by layered growth of a thin film. .

The layered copper oxide used as an intermediate layer in the present invention (interlayer material) has the _{formula: [(RE1 1-y RE2} y) O 2] m + 1 ......... (7) ( wherein, RE1 Is at least one element selected from a lanthanum group element and an actinoid element that form ions having a valence higher than trivalent, and RE2 is at least one selected from a lanthanum group element that forms a trivalent ion and yttrium. It represents one kind of element, and y is a number satisfying 0 ≦ y <1) in the crystal structure. That is, the intermediate layer in the superconducting element of the present invention basically has a thickness of 0.27 nm.
A multiple (m-fold) fluorite-type block is formed with a thickness of m times, and has a function as, for example, an insulating layer through which no ohmic current flows in the film thickness direction.

The other block layer [B] in the layered copper oxide as the intermediate layer material is the same as the block layer contained in the copper oxide superconductor layer (for example, [Bi]
₂ O ₂ ] ²⁺ , [Tl ₂ O ₂ ] ²⁺ , [TlO] ¹⁺ , [PbO-Cu-PbO]
¹⁺ , [(Pb, Cu) O] ¹⁺ , CuO _δ Chain, etc.). In particular, it is preferable to use the same block layer as the block layer contained in the used copper-based oxide superconductor layer. This facilitates epitaxial growth and reduces the number of deposition sources in the manufacturing process (for example, the number of Knudsen cells in the molecular beam epitaxy method).

A typical chemical formula of the layered copper oxide serving as the intermediate layer is as follows: Chemical formula: [B] AE ₂ (RE11 _-y RE2 _y ) _{m + 1} Cu ₂ O _z (8) Where [B] is a block layer, AE is an alkaline earth element, RE1 is at least one element selected from lanthanum elements and actinoid elements that form ions having a valence higher than 3, and RE2 is A valence of at least one element selected from lanthanum group elements and yttrium forming trivalent ions, m is a number satisfying m ≧ 2, y
Is a number satisfying 0 ≦ y <1, and z represents the amount of oxygen.)
Is exemplified. Since the compound of m = 1 becomes a superconductor depending on the carrier concentration and superconducting coupling occurs in the c-axis direction, m is set to 2 or more. As AE, all alkaline earth elements can be used,
Considering the ionic radius, Sr or Ba is preferable. FIG. 1 shows a typical example of the intermediate layer material for one unit (the crystal structure [B] to [B] is one unit). The intermediate layer material shown in FIG. 1 uses [PbO-Cu-PbO] ¹⁺ as [B] and contains an octuple-type block.

The above-described multiple fluorite type block functions stably as an insulating layer. Therefore, the effect of the intermediate layer according to the present invention can be obtained by depositing only one unit of the layered copper-based oxide containing multiple fluorite-type blocks. Further, the layered copper-based oxide can be used in a laminated state in the range of about 2 to 12 units.

As described above, the layered copper-based oxide containing multiple fluorite-type blocks basically has the multiple fluorite-type block functioning as an insulating layer. Unit) may be an insulating layer. In addition, only the multiple fluorite type block is used as the insulating layer, and the CuO ₂
The surface may be in an electrically active state, ie, a state having conductivity or superconductivity. When a Josephson junction device is produced with the superconducting device of the present invention, the intermediate layer is composed of one unit of the layered copper-based oxide, and only the multiple fluorite-type block is used as the insulating layer, in other words. The form in which the insulating layer does not include the CuO ₂ surface is optimal. Because the CuO ₂ surface is likely to have conductivity with slight composition fluctuation,
This is because there is a possibility that the function as an insulating layer may be reduced. In addition, as will be described in detail later, the superconducting order parameter is improved up to the interface between the superconducting layer and the insulating layer by using only the multiple fluorite-type block as the insulating layer and the upper and lower CuO ₂ surfaces exhibiting superconductivity. Develop. Also from this point, it can be said that an embodiment in which the intermediate layer is formed of one unit of the layered copper-based oxide is preferable in forming a Josephson junction.

In the case of a Josephson junction having an S / I / S structure, the thickness of the insulating layer (I layer) is preferably about 1 to 4 nm. Here, the thickness of the multiple fluorite type block is CuO ₂
0.595 + 0.27 (m-1) n when measured from the surface to the CuO ₂ surface
m. Therefore, one unit of the layered copper oxide has S /
When the insulating layer of the I / S junction is formed, the thickness of the multiple fluorite type block is preferably about 0.8 to 3.5 nm (m = 2 to 12). A more preferable value of m is about 4 to 10, and m =
A neighborhood of 7 is most preferred.

The above-described multiple fluorite type block can be regarded as a structure in which only cations and only oxygen are alternately stacked between CuO ₂ surfaces. As expressed by the above equation (7), the cation site (A1 site) in the multiple fluorite-type block is R
E1 and RE2 occupy. RE1 is a lanthanum element or actinoid element that forms ions having a valence higher than 3, such as Ce ⁴⁺ , Pr ⁴⁺ , Tb ⁴⁺ , Th ⁴⁺ , Pa ⁴⁺ , and U ⁴⁺ . Considering the ionic radius of each ion, Ce ⁴⁺
Is most desirable, and then U ⁴⁺ , Pr ⁴⁺ , and Th ⁴⁺ . Because the lattice matching between the multiple fluorite-type block and the CuO ₂ planes located above and below it is good, the intermediate layer compound is chemically stable, and carriers on the CuO ₂ plane included in the intermediate layer are localized. This is because it becomes difficult. RE1 is required to form multiple fluorite-type blocks chemically stable.

RE2 is an element that forms a trivalent ion, and all lanthanum elements (La, Pr, Nd, Sm,
Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and Y may be used. However, in the layered copper-based oxide, only La ³⁺ is 9 to 10
La ³⁺ is less preferred because coordination is preferred and other ions prefer 8-coordination. RE2 is not required to form multiple fluorite blocks. I.e. y
= 0 may be used. However, as will be described in detail later, it is necessary to develop the superconducting order parameter up to the interface between the superconducting layer and the insulating layer. In order to make the multiple fluorite-type block chemically stable, it is preferable that the value of y is 0.6 or less.

By the way, the fluorite-type crystal structure has an A1 site (8
Average ionic radius R (A1) is the ionic radius R (O) of oxygen
It is stable when it is 0.73 times or more and 0.84 times or less. R (A1) / R
When (O) is less than 0.73, a rutile structure in which oxygen is coordinated around a cation or a C-type rare earth structure (C-rare earth str
ucture: FSGalasso, ”Structure and Properties ofI
norganic Solids (Pergamon, Oxford, 1970)). Therefore, in the present invention, when focusing on the surface of only certain cations, the average ion radius R (A1) is 0.099
It is desirable to select the types of RE1 and RE2 and the composition ratio thereof so as to be in the range of 1 to 0.11592 nm (most preferably 0.10074 to 0.108 nm). However, Ce ⁴⁺ , Pr ⁴⁺ , Tb ⁴⁺ , Th ⁴⁺ ,
When Pa ⁴⁺ and U ⁴⁺ occupy 70% or more of the A1 site, these dioxides have a fluorite structure in the first place and are not limited to the above range. If RE2 (for example, Er ³⁺ ) having a relatively small ionic radius is used and its composition ratio exceeds twice that of RE1, oxygen above and below it is regularly lost, and Y ₂ O ₃ (C-type rare earth structure). The multiple fluorite type block in the intermediate layer material of the present invention may partially include the C-type rare earth structure.

In the formulas (7) and (8), the RE1 element and R
The range of the composition ratio y with the E2 element is preferably determined in consideration of the carrier concentration p. Here, the carrier concentration p 1 is an excess charge p per [CuO] ^{+ p} obtained from the composition of cations and anions under the condition of neutral charge. If the entire intermediate layer is made of a semiconductor or an insulator, it is necessary to select y so that p = 0. In addition, when it is desired to make the multiple fluorite type block thick enough to function stably as an insulating layer and to make the CuO ₂ surface included in the intermediate layer electrically active (that is, having superconductivity), p = 0.15 ~
It is preferable to choose y such that it is 0.25. For example, as a constituent material of the intermediate layer, (Pb ₂ Cu) Sr ₂ (Ce _1-y Eu _y )
_{When using m + 1} Cu ₂ O _{8 + 2m-δ} , if the whole intermediate layer is a semiconductor or insulator, set it near y = (1 + 2δ) / (m + 1) and include it in the intermediate layer. If it is desired to make the CuO ₂ surface electrically activated, the value is set to around y = (1.4 + 2δ) / (m + 1).

In order to obtain an S / I / S tunnel junction with a high-temperature oxide superconductor, it is necessary that the superconducting order parameter is well developed up to the interface between the superconducting layer and the insulating layer. For this purpose, it is desirable that the CuO ₂ surface included in the intermediate layer be in an electrically active state, that is, a conductive state or a superconductive state. That is, it is desirable to dope carriers on the CuO ₂ surface included in the intermediate layer. To do so, as mentioned above, p = 0.15-0.
After selecting y so as to be 25, it is desirable to localize trivalent RE2 in the vicinity of the CuO ₂ plane in the multiple fluorite-type block. Up to now, layered copper-based oxides containing multiple fluorite-type blocks have been synthesized as bulk polycrystals, but none of them have superconductivity and have the property that their electrical resistance increases with decreasing temperature. I have. A series of compounds containing multiple fluorite-type blocks in bulk polycrystalline samples, such as Pb ₂ Sr ₂ Y
₁ Ce _m Cu ₃ O _{8 + 2m-δ} : m = 1,2,3
It has been reported that the resistivity increases as m increases (A. Tokiwa et al., Physica C 181, 311 (1991)). The present inventors have found that this is because Ce ⁴⁺ and Y ³⁺ are randomly distributed in multiple fluorite-type blocks, and oxygen deficiency δ increases with an increase in m. Was.

To reduce lattice defects such as oxygen vacancies, and
The following method is effective to effectively carry out carrier doping on the CuO ₂ surface without increasing the value of y (in the range where the multiple fluorite-type block is chemically stable). In other words, it is effective to localize the trivalent RE2 at the A1 site adjacent to the CuO ₂ plane so that the RE1 having a higher valence than trivalent is located near the center of the multiple fluorite block as much as possible. . As a result, oxygen vacancies are less likely to be generated in the multiple fluorite-type block, and carrier doping can be effectively performed on the CuO ₂ surface. In order to make the CuO ₂ surface contained in the intermediate layer conductive or superconductive, CuO
Average ionic radius of A1 site adjacent to _two surfaces is 0.1007 ~
It is desirable to select the types of RE1 and RE2 and the composition ratio thereof so as to be in the range of 0.108 nm (most preferably in the range of 0.102 to 0.106 nm).

In order to effectively carry out carrier doping on the CuO ₂ surface of the intermediate layer, there is a desirable condition in the other block layer [B]. That is, it is desirable that the charge of [B] be as small as possible. [B] includes [Bi ₂ O ₂ ] ²⁺ and [Tl ₂ O
_2] than ^{2+, [TlO] 1+, [} PbO-Cu-PbO] 1+, [(Pb, Cu) O]
¹⁺ , [(Tl _0.5 Cu _0.5 ) O] ^0.5+ , [(Tl, Pb) O] ^{0.5+ to 1.5+}
Is more desirable.

By satisfying the above conditions, the order parameter of superconductivity is well developed up to the interface between the superconducting layer and the insulating layer. As a result, an S / I / S tunnel type junction excellent in characteristics and reproducibility can be obtained. Here, the order parameter (ψ) of superconductivity indicates the degree of superconductivity of the electron locally there, and is expressed by the following equation.

Order parameter (ψ) = (superconducting density) ² = (amplitude of macroscopic wave relation of superconducting) Order parameter of superconductivity is well developed up to the interface between superconducting layer (S) and insulating layer (I) Fig. 2
(A). FIG. 2B shows a state in which the order parameter of superconductivity is not well developed. FIG.
As shown in (a), the superconducting order parameter is well developed up to the S / I interface, so that a good superconducting gap △ can be obtained.

According to the present invention, good characteristics are obtained firstly because the superconducting order parameter is well developed up to the interface between the superconducting layer and the insulating layer, and secondly, short-circuiting is less likely to occur in the insulating layer. The obtained S / I / S tunnel type Josephson junction is obtained. Also, the Josephson junction according to the present invention can be used for a superconducting three-terminal element.
As one of the superconducting three-terminal elements, an S / I / S / I / S type element has been proposed. This is a stack of two S / I / S junctions. Such a three-terminal device can be realized by stacking two S / I / S tunnel-type Josephson junctions in which three layers of the thin film of the present invention are stacked.
Further, a superconducting base transistor can be realized by laminating a semiconductor layer on (or below) an S / I / S tunnel type Josephson junction in which three thin films of the present invention are laminated. In addition, an S layer in which three thin films of the present invention are laminated
Above (or below) / I / S tunnel type Josephson junction
The S / N / S junction is laminated on the S / I / S
A quasiparticle-injection three-terminal element having a / N / S structure can be realized. Further, the superconducting element of the present invention can be applied to a DC transformer or the like. Further, the intermediate layer material in the present invention can be used as an insulating layer for element isolation by setting m to 100 or more.

[0038]

In the superconducting device of the present invention, the multiple (m-fold) fluorite type block in the layered copper oxide used as the intermediate layer is
By setting the number of m to 2 or more, it is possible to function stably as an insulating layer. Further, the multiple (m-fold) fluorite block does not easily become a conductor or a superconductor due to interdiffusion, a change in carrier concentration or composition, or the like.
That is, the multiple fluorite-type block not including the CuO ₂ surface does not become conductive even when Cu, Pb, Bi, AE, etc. are partially mixed by diffusion, or when carriers are injected. Therefore, an S / I / S junction excellent in stability can be obtained.

Further, since the layered copper-based oxide containing the multiple fluorite-type block layer is excellent in lattice matching with the copper-based oxide superconductor, the intermediate layer and the upper and lower oxide superconductor layers are each epitaxially grown. be able to. Here, in particular, the same block layer as the copper-based oxide superconductor layer [B]
By using an intermediate layer containing, the epitaxial growth can be performed with good consistency with the upper and lower superconducting layers. As a result, a metamorphic layer is less likely to be generated near the interface between the superconducting layer and the intermediate layer, and the superconducting order parameter can be favorably maintained up to the interface with the insulating layer. Furthermore, to optimize the ionic radius and composition ratio of RE1 and RE2 in multiple fluorite-type blocks, to localize the position occupied by trivalent RE2 near the CuO ₂ plane, By selecting a small one, the order parameter of superconductivity can be favorably maintained up to the CuO ₂ plane in the intermediate layer at the superconducting layer / insulating layer interface.

Further, since the intermediate layer includes the block layer [B], the intermediate layer is easily grown in a layered manner. Multiple fluorite-type block layers
Since it has a crystal structure in which the cation layers of (RE1, RE2) and the O ²⁻ layers are alternately stacked, layer growth is easy. By epitaxially growing such a substance which is easy to grow in layers,
A stacked film with few steps and defects can be obtained, and a Josephson junction in which short-circuit current hardly occurs even if the insulating layer is thin can be obtained. Further, since the multiple fluorite-type block can be manufactured to an arbitrary thickness in units of 0.27 nm, the thickness of the insulating barrier can be changed according to the application.

[0041]

Embodiments of the present invention will be described below.

FIG. 3 is a cross-sectional view showing the structure of a main part of a superconducting element according to one embodiment of the present invention. In the figure, 1 is
This is a substrate made of SrTiO ₃ single crystal or the like. On this substrate 1, as a lower superconducting layer 2, a copper-based oxide superconducting layer in which the c-axis of the crystal is aligned perpendicular to the substrate surface, for example, as described above (1)
An oxide superconductor layer substantially represented by the formulas (6) to (6) is provided. On the lower oxide superconductor layer 2, as the intermediate layer 3, a layered copper-based oxide layer containing multiple fluorite-type blocks, substantially represented by the above-mentioned formula (8), is formed. ing. Further, a similar copper-based oxide superconductor layer is formed thereon as an upper superconducting layer 4. These layers 2, 3, and 4 form a three-layer laminated structure.

FIG. 4 is a diagram showing an example of the three-layer laminated structure of the superconducting element from the viewpoint of the crystal structure. In FIG. 4, the oxygen ions are connected by solid lines in order to easily understand the coordination of the oxygen ions. FIG.
(Pb _0.65 Cu _0.35 ) Sr ₂ (Ca
_0.5 Dy _0.5 ) Cu ₂ O _7-δ superconductor (superconducting transition temperature T _c
= 80K), and as a middle layer 3 thereon, a hexafluorite-type block containing (Pb _0.65 Cu _0.35 ) Sr ₂ (Ce, Eu) ₇
After growing Cu ₂ O _z and further forming the upper superconducting layer 4,
(Pb _0.65 Cu _0.35 ) Sr ₂ (Ca _0.76 Dy _0.24 ) ₂ Cu ₃ O _9-δ Superconductor (T _c = 90K) is shown grown. As shown in the figure, the multiple fluorite-type block contained in the layered copper-based oxide as the intermediate layer 3 is represented by (RE1, RE2) (specifically,
(Ce, Eu)) has a crystal structure in which cation layers and O ²⁻ layers are alternately stacked. The upper superconducting layer 4 is an oxide superconductor that cannot be obtained in bulk and contains three CuO ₂ planes in a unit cell. This is an oxide superconductor obtained for the first time by a molecular beam epitaxy technique or the like. In the three-layer structure shown in FIG. 4, the lattice matching of the three layers is good, and the layers can be epitaxially grown.

Next, a specific example of forming a thin film for manufacturing a superconducting element having the above-described configuration and an evaluation result thereof will be described.

FIG. 5 is a diagram schematically showing the configuration of a film forming apparatus by molecular beam epitaxy used for forming a thin film in this embodiment. In the figure, reference numeral 11 denotes a film forming chamber,
Evaporation sources such as Pb, Sr,
Knudsen cells 12, 1 containing Ca, RE, Cu, etc.
3, 14, 15, and 16 are arranged. Further, a gas source nozzle 17 is also provided. Each of the Knudsen cells 12, 13, 14, 15, 16 and the gas source nozzle 17 are provided with a heater (not shown). These are arranged so that the metal element or the gas containing the metal element evaporates toward the substrate 1 set on the substrate holder 18. Each Knudsen cell 12,
13, 14, 15, 16 and nozzle 1 for gas source
7, shutters 19, 20, 21, 2 respectively.
2, 23 and 24 are provided so that the supply of each element to the substrate 1 can be stopped. Substrate holder 18
Is heated by a substrate heater 25 provided behind the substrate.

In the film forming chamber 11, an oxygen gas introducing pipe 26 made of a quartz tube is provided. A coil 27 is wound around the oxygen gas introduction pipe 26. Then, oxygen plasma is generated in the oxygen gas introduction pipe 26 by the power supplied from the high frequency power supply 28, and the active oxygen is supplied to the substrate 1 set in the substrate holder 18. Reference numeral 29 in the drawing denotes a shutter used when the active oxygen is not directly irradiated on the substrate 1, and is normally opened.

In this embodiment, the RE1 element indispensable for the formation of the multiple fluorite type block, for example, Ce, is supplied from an organic metal gas source. As the Ce source, a method of sublimating Ce (DPM) ₃ or Ce (DPM) ₄ from a Knudsen cell may be used, but the organometallic gas source has higher stability. In the figure, reference numeral 30 denotes a raw material heating oven. An organic metal material containing Ce, for example, a β-diketone metal complex (Ce (DPM) ₃
And Ce (DPM) ₄ ) have been introduced. The piping 32 connected to the raw material container 31 and the gas source nozzle 17 is maintained at a reduced pressure below the atmospheric pressure. The pressure in the pipe 32 is monitored by a pressure gauge 33, and the raw material container 31 and the pipe 32 are monitored by two variable leak valves 34 and 35.
The pressure inside is controlled. These are also arranged in the heating oven 30 so that the organometallic raw materials do not aggregate on the pipes, pressure gauges, valves and the like. The supply amount of the Ce-containing organometallic gas supplied from the gas source nozzle 17 toward the substrate 1 is controlled by the pressure in the pipe 32 and the temperature of the raw material heating oven 31. For example, when Ce (DPM) ₃ is used as a raw material, the temperature of the raw material heating oven 31 is set to 130 ° C. to 2 ° C.
At 00 ° C., the pressure in the pipe 31 is set to a condition of 0.01 Torr to 10 Torr.

For forming the multiple fluorite-type block layer, R such as Ce
The element E1 is indispensable, and molecular beam epitaxy is effective for producing the superconducting device of the present invention. When a multiple fluorite-type block is formed by molecular beam epitaxy, it is effective to supply an organic metal gas containing Ce as a Ce source toward the substrate. Thereby, Ce can be supplied stably. Since Ce has a low vapor pressure even at a high temperature, it is difficult to evaporate it in a Knudsen cell, and the amount of evaporation is not stable even by electron beam evaporation. The use of an organometallic gas source can overcome these disadvantages. Further, in the case of forming a multiple fluorite-type block layer by using a molecular beam epitaxy method, the layer growth becomes easier by separately supplying (RE1, RE2) and active oxygen separately.

First, a single-layer Pb—Sr—Ca—RE—Ca—O oxide superconductor thin film used as an upper and lower superconducting layer by using a film forming apparatus (molecular beam epitaxy apparatus) having the above-described configuration. An example in which is deposited will be described. The substrate 1
The oxide superconductor thin film was formed by the following procedure using the (100) plane of SrTiO ₃ .

First, after the inside of the film forming chamber 11 is evacuated to less than 10 ^-8 Torr, each Knudsen cell 12, 13, 14, 1
5 and 16 are heated to a temperature corresponding to the required evaporation rate of each evaporation source. The source temperatures of Pb, Sr, Ca, RE2 (Eu or Dy), and Cu are, for example, 600 ° C, 480 ° C, 526 ° C,
32 ° C (or 1033 ° C) and 1060 ° C. The substrate 1 was heated to 800 ° C. while supplying active oxygen. Thereafter, deposition of the metal component was started while supplying active oxygen. The introduction flow rate of oxygen is about 0.3 to 0.6 SCCM, and the oxygen partial pressure in the film forming chamber 11 is about
The operation was performed at 1.1 to 2 × 10 ⁻⁵ Torr and input high frequency power of 40 to 100 W. After the film formation, the shutters 19, 20, 21, 22, and 23 immediately above each Knudsen cell are closed to stop deposition of the metal component, and the substrate temperature is reduced while the supply of active oxygen is maintained.
Reduced to 600-300 ° C. Thereafter, the high-frequency discharge was stopped, and the temperature of the substrate was lowered to 280 ° C. or lower, and then the sample was taken out.

By using such a method,
(Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O _8-δ superconducting thin film or (Pb _0.65 Cu _0.35 ) Sr ₂ (Ca _0.5 Dy _0.5 ) Cu ₂ O _7-δ
A superconducting thin film can be formed. These two types of oxide superconductors can be made separately by changing the composition and the irradiation conditions of active oxygen. Furthermore, a thin film having excellent crystallinity and superconducting characteristics can be obtained by automatically controlling the shutter and stacking components separately according to the crystal structure. Furthermore, the number of CuO ₂ planes in the unit cell increased to 3, (Pb _0.65 Cu _0.35 ) Sr ₂ (Ca _0.76 Eu _0.24 ) ₂ Cu
₃ O _9-δ superconducting thin film, (Pb ₂ Cu) Sr ₂ (Ca _0.8 E
u _0.2), such as ₂ Cu ₃ O _{10- δ} superconducting thin film, it can be obtained not obtained oxide superconductor in bulk.

As a specific example, (Pb ₂ Cu) Sr ₂ (Ca
An example of producing a _0.5 Eu _0.5 ) Cu ₂ O _8-δ superconducting thin film will be described. Oxygen partial pressure 2 × 10 ^-5 Torr, input high frequency power 100W
Shutters 19, 20, 21, 2
Components 2 and 23 were automatically controlled, and the components were laminated according to the crystal structure. The stacking order is Pb + Sr (40sec) / Pb + Sr + Cu (40sec)
/ Pb + Sr (40sec) / Ca + Eu + Cu (40sec) / Ca + Eu (40sec) / Ca + Eu + C
u (40 sec) is a unit cell, and this was repeated 30 cycles. As a result, a 47 nm-thick (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu
_0.5 ) A Cu ₂ O _8-δ superconducting thin film was obtained. FIG. 6 shows the result of examining this sample with an X-ray diffractometer. c-axis length 1.
Only the (00n) peak corresponding to 58 nm was observed, confirming that the c-axis was strongly oriented perpendicular to the substrate surface. Superconducting transition temperature T _c of the thin film was about 70K. The growth process of the thin film was monitored by reflection high-energy electron diffraction (RHEED). As a result, a streak pattern and the oscillation of its reflection intensity were observed from the 6th cycle to the 20th cycle, and (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O _8-δ grew in layers in each unit cell. I knew it was.

Here, the most important thing for growing a flat layer to a film thickness of 35 nm or more is not to form an island during the initial Pb deposition. If the growth conditions deviate from the optimum values, the RHEED pattern becomes spot-like within a few seconds after opening the shutter of the Pb source during the first Pb oxide deposition, which hinders subsequent layer growth. This spot pattern is considered to be a result of the island-like growth of the SrPbO ₃ -based compound while maintaining an epitaxial relationship with the substrate. In the worst case, the crystal structure of the oxide superconductor cannot be formed, resulting in a b-axis oriented film of PbO _1.57 (JCPDS card 26-577). On the other hand, during the first Pb oxide deposition, the streak pattern became weak after opening the shutter of the Pb source, and almost no diffraction pattern became visible. When the streak pattern comes back after passing
Thereafter, layer growth is easy.

As described above, in order to realize a layered initial growth, it is effective to satisfy any of the following growth conditions. First, the composition of Sr in the thin film sample must be equal to or higher than the stoichiometric composition. If Sr is deficient by 20%, layer growth becomes difficult, and if Sr is excessive by 10%, layer growth becomes easy. However, when Sr is excessive by 30% or more, (Pb, Cu) (Sr, Ca, RE) ₂ CuO _5-δ is mixed as an impurity phase. Second, a SrO layer is first deposited on the substrate as a buffer layer in excess of one atomic layer and not more than three atomic layers, and then a Pb-based oxide superconductor is deposited. As another buffer layer, S
A layer deposited in the order of r / Cu / (Eu _0.5 Ca _0.5 ) / Cu / Sr by one atomic layer (however, with an excess of Sr by about 10%) is also effective. Third, the oxygen partial pressure is reduced only during the initial growth. For example, only during the growth of the first 1-3 unit vesicles, reduce the oxygen partial pressure to about 1/2. Fourth, the substrate temperature is raised by about 20 ° C. only during the initial growth. If the substrate temperature is lowered by about 20 ° C. during the initial growth, on the contrary, layer growth becomes difficult.

Of the above initial growth conditions, the first and second
Using growth conditions of (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 )
The Cu ₂ O _8-δ superconducting thin film is reduced to a partial pressure of oxygen in the film forming chamber 11.
It was manufactured at 1.1 × 10 ^-5 Torr and input high frequency power of 100W. (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O _8-δ can be synthesized at a lower oxygen partial pressure than Bi ₂ Sr ₂ CaCu ₂ O _8-δ described later. Sr was made about 10% excess as the composition of the whole film. The buffer layer is made of Sr / Cu / (Ca _0.5 Eu _0.5 ) / Cu / Sr / Pb / C
The layers were laminated in the order of u / Pb / Sr / Cu / (Ca _0.5 Eu _0.5 ) / Cu / Sr / Pb / Cu / Pb / Sr. Thereafter, (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu) for 25 unit cells was obtained by the shutter sequence shown in the lower part of FIG.
_0.5 ) Cu ₂ O _8-δ was deposited. The growth process was monitored by RHEED. As a result, a streak pattern was observed from the start of the growth to the end of the growth, and it was confirmed that the growth was flat and layered.

FIG. 7 shows the change over time in the reflection intensity of the central streak. It can be seen that the reflection intensity oscillates for each unit cell until the end of the growth, and the unit cell having a c-axis length of 1.58 nm grows in a layered manner. In particular, the reflection intensity becomes maximum at the end of the growth of the Ca-Eu-Cu-O perovskite portion, and a flat surface is formed. When the growth of the [PbO-Cu-PbO] block layer is started, the reflection intensity decreases. FIG. 8 shows an X-ray diffraction pattern of this sample. As shown in FIG. 6, the diffraction intensity is higher and a sharp peak is obtained, which indicates that an oxide superconductor with good orientation and good crystallinity is obtained. By the way, RHE
When observing the growth process with ED, not only the reflection intensity but also the diffraction pattern may periodically change. For example, Pb-
The spot-like pattern is superimposed on the streak during the Sr-O deposition, but only the streak is formed later.

For comparison, Bi ₂ Sr ₂ CaCu ₂ O _8-δ
RHEED observation was performed while growing the superconducting thin film by stack control with the shutter control by separating the components according to the crystal structure. The film was formed at an oxygen partial pressure of about 2 × 10 ⁻⁵ Torr and an applied high frequency power of 100 W. (Pb ₂ Cu) Sr ₂ (Ca _0.5 E
u _0.5 ) Unlike Cu ₂ O _8-δ , in this system [Bi ₂ O ₂ ]
^The reflection intensity is maximized at the midpoint during ²⁺ block layer deposition.
That is, a half unit cell of BiO from BiO is a growth unit. Compared to Bi ₂ Sr ₂ CaCu ₂ O _8-δ , (Pb ₂ Cu) Sr ₂ (Ca
_{In 0.5} Eu _0.5 ) Cu ₂ O _8-δ , the streak pattern and the periodic fluctuation of its reflection intensity were clearly observed, and it was found that layer growth was easier. Therefore, the [PbO-Cu-PbO] ¹⁺ block layer is more suitable for the superconducting element of the present invention than the [Bi ₂ O ₂ ] ²⁺ block layer. The cause is considered as follows.

That is, at a high temperature of about the substrate temperature (about 800 ° C.), the vapor pressure of Pb and Pb oxide is high, so that even if Pb adheres to the substrate, re-evaporation is severe. Then, (Pb ₂ Cu)
When producing Sr ₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O _8-δ , the evaporation amount of Pb from the Knudsen cell is Bi ₂ Sr ₂ CaCu ₂ O _8-δ
It is necessary to set about 10 to 20 times as large as the evaporation amount of Bi in the case of manufacturing, and the crystal grows in an atmosphere containing excess Pb. It is considered that Pb or Pb oxide is frequently adsorbed and re-evaporated on the film growth surface, but once Pb enters the site where the crystal should enter, it becomes difficult to re-evaporate. In other words, only Pb necessary for the formation of the crystal structure is incorporated into the film, and other adsorbed lead is re-evaporated, so that the crystal is formed without waiting for the migration or diffusion of heavy elements (Pb or Bi). Is done. As a result, it is thought that flat layer growth is likely to occur.

(Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O
_{An example in which an 8-δ} superconducting thin film is manufactured by another lamination method will be described. Oxygen partial pressure is 2 × 10 ^-5 Torr, input high frequency power is 100W
Was formed. Sr / Pb / C by shutter control
It was prepared by stacking 30 unit cells for each element in the order of u / Pb / Sr / Cu / (Ca _0.5 Eu _0.5 ) / Cu. After the film formation, the supply of the active oxygen to the substrate was stopped when the substrate temperature dropped to 600 ° C. The temperature at which the supply of this active oxygen was stopped was Bi ₂ Sr ₂ CaCu ₂ O
Higher than the temperature at which the _8-δ superconductor is obtained. A superconducting thin film grown flat and layered was obtained even if the layers were stacked for each element. However,
When only Pb is supplied to the substrate, as shown in the first example, Sr
Compared to the case of Pb + Sr / Pb + Sr + Cu / Pb + Sr supplied simultaneously with Cu and Cu, the adhesion rate of Pb to the substrate is reduced to about 1/3.
As a result of chemical analysis of the composition of the prepared film, Pb _0.56 Sr _1.47 Ca
_0.55 Eu _0.45 Cu _2.83 O _y . In order to investigate the superconducting transition temperature of this sample, the temperature dependence of the electrical resistivity was measured. FIG. 9 shows the result. About 70K when the temperature is lowered
Then the resistance began to decrease, and at about 9K the resistance became zero. It was confirmed that superconductivity was obtained despite the lack of Pb.
In addition, the number of CuO ₂ surfaces in a unit cell was increased to three, (Pb
₂ Cu) Sr ₂ (Ca _0.8 Eu _0.2 ) ₂ Cu ₃ O _{10- δ} Superconducting, by automatically controlling the shutter, Pb + Sr / Pb + Sr + Cu / Pb + Sr / Ca + as a unit cell Eu + Ca / Ca + Eu / Ca + Eu + Ca / Ca +
The stacking of Eu / Ca + Eu + Ca was repeated 25 cycles.
As a result of examining this sample with an X-ray diffractometer, a (00n) peak corresponding to a c-axis length of 1.9 nm to 2.0 nm was observed.

Next, (Pb ₂ Cu) Sr ₂ (Ce _1-y Eu _y ) _{m + 1} Cu ₂ O is used as an intermediate layer by using the above-mentioned film forming apparatus.
An example in which _{8 + 2m-δ} is deposited will be described. When using molecular beam epitaxy, the fluorite type block
It is desirable to use Eu, Sm, Dy or the like having a high vapor pressure at a high temperature as the RE2 element.

First, Pb, Cu, Sr, and Eu were evaporated from the Knudsen cell, and a Ce (DPM) ₃ organometallic gas was supplied. At the same time, the substrate was irradiated with active oxygen under the same conditions as in the above example. Ce (DPM) ₃ decomposes on the heated substrate, and Ce is deposited on the substrate. In the conventional bulk, compounds with m ≧ 3 could not be synthesized in a single phase (A. Tokiwa et al., Physica C, Vol. 181, pp. 311).
319 (1991) etc.). On the other hand, in the present invention, shutters 19, 20, 21, 22, 23, and 24 are automatically controlled to separate components according to the crystal structure and to stack the components. It is possible to synthesize as a single phase. Further, it is possible to epitaxially grow a layer on the lower superconducting layer.

First, as a preliminary experiment, by repeating an experiment in which only the compound layer used as the intermediate layer is stacked for 30 unit cells, a single phase is obtained, and film forming conditions (substrate temperature, active oxygen irradiation Conditions, etc.) (Pb ₂ Cu) Sr ₂ (Ce _1-y Eu _y ) containing hexafluorite-type blocks
₇ The stacking order when growing Cu ₂ O _{20- δ} is Pb + Sr
(40sec) / Pb + Sr + Cu (40sec) / Pb + Sr (40sec) / Cu (40sec) / Ce +
The unit cell was Eu (18sec) / Ce (300sec) / Ce + Eu (18sec) / Cu (40sec), and this was repeated 30 cycles. Here, in order to make the CuO ₂ plane included in the intermediate layer electrically active, y = 0.2 is set assuming oxygen deficiency δ = 0, and Eu is CuO
_The A1 site adjacent to the _two surfaces was occupied. Also,
The average ionic radius R (A1) at the A1 site adjacent to the CuO ₂ plane is
0.1037 nm. As a result of monitoring the growth process by RHEED, it was confirmed that layer growth occurred for each unit cell as in the case of FIG. In addition, the multiple fluorite type block [(Ce, Eu) O
₂ ] A streak pattern was observed during _{m + 1} deposition,
It was confirmed that the layer was grown flat in a layer. This is because the lattice constant of CeO ₂ (a =
This is probably because the lattice mismatch between the copper-based layered oxide and 0.5409 nm) is 1% or less.

Further, by alternately supplying (Ce, RE2) and active oxygen separately, the streak pattern becomes clearer and the layer grows more easily. In such a case, for example, with the active oxygen shutter 29 closed, the shutter 22 on the Eu Knudsen cell and the Ce gas source shutter 24 are operated for a time equivalent to one atomic layer deposition (about 60 seconds). Only after opening and closing it, the active oxygen shutter 29 is opened, for example.
You can repeat the operation of opening for 10 to 60 seconds.

On the above-mentioned layered copper-based oxide layer as an intermediate layer, a copper-based oxide superconductor film as an upper superconducting layer is grown under the same conditions as those described above, whereby the layer is epitaxially grown. did it. As a result, a three-layer stacked structure excellent in lattice matching and epitaxially grown in layers can be obtained.

In the above-described embodiment, an example in which a molecular beam epitaxy method is used to fabricate a three-layered Josephson junction according to an embodiment of the present invention has been described.
The superconducting element of the present invention can be manufactured by another film forming method such as a sputtering method, a cluster ion beam method, or a CVD method, if the components are laminated according to the crystal structure. .

Next, a specific example in which a three-layered Josephson junction according to one embodiment of the present invention is manufactured by using the above-described film forming method and film forming conditions will be described.

Example 1 A (100) plane of SrTiO ₃ single crystal was used as a substrate 1 and (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 ) C
The u ₂ O _8-δ superconducting thin film was deposited for 50 unit cells (79 nm). While maintaining the substrate temperature at a high temperature, (Pb ₂ Cu) Sr ₂ (Ce _1-y Eu _y ) _{m + 1} Cu ₂ O _{8 + 2m- δ}
(m = 7) was deposited for one unit. The thickness of the multi-fluorite-block which acts as an insulating layer in this case, the CuO ₂ plane
When measured from the distance to the CuO ₂ plane, it is about 2.2 nm. Middle layer 3
When depositing, the film formation conditions (substrate temperature,
Active oxygen irradiation conditions, stacking order, etc.) were used. Further, as the upper superconducting layer 4, (Pb ₂ Cu) Sr ₂ (Ca _0.5
Eu _0.5 ) Cu ₂ O _8-δ superconducting thin film was deposited for 50 unit cells (79 nm). All thin film depositions were performed while confirming that they were grown in layers by RHEED. Thereafter, the laminated film was patterned by a metal mask and ion milling, and electrodes were taken out from the lower superconducting layer 2 and the upper superconducting layer 4. The current-voltage characteristics of the Josephson junction thus obtained were measured at a temperature of 40K. The result is shown in FIG. As is clear from FIG. 10, a clear Josephson effect was observed. Example 2 A (100) plane of SrTiO ₃ single crystal was used as a substrate 1, and (Pb _0.65 Cu _0.35 ) Sr ₂ (Ca _0.5 Dy) was used as a lower superconducting layer 2 first.
_0.5 ) A Cu ₂ O _7-δ superconducting thin film (T _c = 80K) was grown. On the intermediate layer 3, (Pb _0.65 Cu _0.35 ) Sr ₂ (Eu, Ce) ₇ Cu ₂ O _z containing a hexafluorite-type block was grown for one unit. Furthermore, as the upper superconducting layer 4 thereon, (Pb _0.65 Cu _0.35 ) Sr ₂ (Ca _0.76 Dy _0.24 ) ₂ Cu ₃ O
_{A 9-δ} superconducting thin film (T _c = 90K) was formed. FIG. 4 shows this laminated structure from the viewpoint of the crystal structure. This laminated film was patterned by a metal mask and ion milling, and electrodes were taken out from the lower superconducting layer 2 and the upper superconducting layer 4. As a result of measuring the current-voltage characteristics of the Josephson junction thus obtained, a clear Josephson effect was observed.

Example 3 A (100) plane of SrTiO ₃ single crystal was used as a substrate 1, and a Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _10-δ superconductor was first grown as a lower superconducting layer 2. As an intermediate layer 3, Bi ₂ Sr ₂ (Sm, Ce) ₉ Cu ₂ O _z containing an octuple-fluorite type block was grown for one unit. Further, a Bi ₂ Sr ₂ CaCu ₂ O _8-δ superconducting thin film was formed thereon as the upper superconducting layer 4. This laminated film was patterned by a metal mask and ion milling, electrodes were taken out from the lower superconducting layer 2 and the upper superconducting layer 4, and the current-voltage characteristics of the Josephson junction were measured. As a result, the Josephson effect was observed.

In each of the embodiments described above, the example in which the superconducting element of the present invention is applied to an S / I / S tunnel type junction element has been described. However, the superconducting element of the present invention may be applied to a superconducting three-terminal element. it can. For example, S / I / S /
The I / S type superconducting three-terminal element is formed by stacking two S / I / S junctions. A current is caused to flow while one S / I / S junction is in a voltage generating state, and the tunneling quasiparticles accumulate in the middle S layer, so that the current-voltage characteristic of the other S / I / S junction changes to 3 Operates as a terminal element. Here, it is necessary to change the thickness of the I layer of the two junctions so that each junction has an optimum current-voltage characteristic. Such a three-terminal device can be realized by stacking two S / I / S tunnel-type Josephson junctions in which three layers of the thin film of the present invention are stacked. Also, by changing the thickness of the multiple fluorite type block of the two junctions, it is possible to produce a superconducting element having different characteristics from each other in the two junctions and optimal current-voltage characteristics for each junction. it can.

In particular, in the present invention, for example, as shown in FIG. 11, an intermediate layer (intermediate layer material A and intermediate layer substance B) containing multiple fluorite-type blocks of two unit cells is laminated to form a middle layer. 3 terminal element (S / I ₁
/ S / I ₂ / S). Further, it is possible the thickness of the respective insulating layers (the thickness of the I ₁ layer and I ₂ layer) is easily prepared different. And since the middle S layer can be made very thin, the density of quasi-particles accumulated therein increases and the operation efficiency increases. In addition, since the middle S layer is very thin, the uppermost S layer is shifted from the lowermost S layer.
Quasiparticle current and superconducting current that tunnel to the layer can also be used for device operation. For example, in a three-terminal element called a gap tunnel transistor, the tunneling probability is changed by changing the potential of a current tunneling from the uppermost superconducting electrode to the lowermost superconducting electrode by changing the potential of the S layer (base) sandwiched in the middle. Things.
This device has the advantage that the middle S layer is very thin.

The S / I in which three layers of the thin film of the present invention are laminated
By stacking a semiconductor layer on (or below) the / S tunnel type Josephson junction, a superconducting base transistor can be realized. The S / I / S junction in this case has a role of generating quasiparticles for injection into the semiconductor layer. Furthermore, the S / I /
Above (or below) the S tunnel type Josephson junction, S
By laminating / N / S junctions, S / I / S / N /
A quasiparticle-injection type three-terminal element having an S structure can be realized. The S / I / S junction in this case has a role of generating quasiparticles to be injected into the S / N / S junction.

Next, an example in which the superconducting element of the present invention is applied to a DC transformer will be described.

As shown in FIG. 12, SrTiO ₃ was used as a substrate.
First, (Pb ₂ Cu) Sr ₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O _8-δ superconductor was deposited as a lower superconducting layer 41 for 50 units (79 nm) using the (100) plane of the single crystal. Continue with the middle layer 42
(Pb ₂ Cu) Sr ₂ (Ce _1-y Eu _y ) _{m + 1} Cu ₂ O
_{8 + 2m-δ} (m = 70) was deposited for one unit. In this case, the thickness of the multiple fluorite-type block acting as an insulating layer is CuO ₂
The distance from the surface to the CuO ₂ surface is about 19.5 nm. Further, as the upper superconducting layer 43, (Pb ₂ Cu) Sr
₂ (Ca _0.5 Eu _0.5 ) Cu ₂ O _8-δ superconductor for 50 units
(79 nm) deposited. Lower superconducting layer 41 and upper superconducting layer 43
The electrode was taken out from the sample, and current-voltage characteristics were measured. As a result, no Josephson effect was observed. That is, the tunnel phenomenon of the Cooper pair did not occur from the upper superconducting layer 43 to the lower superconducting layer 41, and the superconducting current did not flow.

Next, the laminated film sample was placed on the upper superconducting layer 4
5K lower than the temperature at which the resistance of 3 or lower superconducting layer 41 becomes zero
The temperature was kept low, and a magnetic field of 0.003 T was applied perpendicular to the film surface. This sample, as shown in FIG. 12, electric current I to the lower superconducting layer 41 were measured voltage V ₁ of the voltage V ₀ and the upper superconducting layer 43 of the lower superconducting layer 41. Lower superconducting layer 4
In No. 1, a voltage V ₀ due to flux creep was generated. The voltage V ₁ was generated in the same order as V ₀ although no current was flowing through the upper superconducting layer 43. In other words, it was confirmed that it worked as a DC transformer. The vortex penetrating through the lower superconducting layer 41 is the current I
It is considered that the vortex penetrating through the upper superconducting layer 43 moved as it received Lorentzka from, and as a result, a voltage was generated in the upper superconducting layer 43.

As described above, when the copper-based oxide superconductor is used as the upper layer and the lower layer and a magnetic field is applied perpendicularly to the CuO ₂ plane, the pinning force of the magnetic flux is weak.
An efficient DC transformer can be realized. In order to realize a DC transformer, it is desirable that the thickness of the intermediate layer be about 10 nm to 20 nm. In this case, the layered copper-based oxide layer of the intermediate layer is not limited to one unit but may be a layered layer of about 2 to 12 units.

[0076]

As described above, according to the present invention, a multi-layer fluorescent material which is excellent in structural and characteristic stability, lattice matching with a copper-based oxide superconductor, easily grows in layers, and functions as an insulating layer. Since the layered copper-based oxide containing the stone block in the crystal structure is used as the intermediate layer, good Josephson characteristics can be obtained and a superconducting element excellent in its reproducibility can be provided. In particular, it is possible to provide a Josephson junction in which a short current or the like is unlikely to be generated even when the insulating layer is thin.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a crystal structure of an example of an intermediate layer material in a superconducting element of the present invention.

FIG. 2 is a diagram for explaining an order parameter of superconductivity in an S / I / S junction.

FIG. 3 is a cross-sectional view schematically illustrating a configuration of a main part of a superconducting element according to an embodiment of the present invention.

FIG. 4 is a diagram schematically showing a three-layer laminated structure of a superconducting element according to an embodiment of the present invention from the viewpoint of a crystal structure.

FIG. 5 is a diagram schematically showing a configuration of a film forming apparatus used in an example of the present invention.

FIG. 6 is a diagram showing an X-ray diffraction pattern of an example of an oxide superconductor thin film manufactured in an example of the present invention.

FIG. 7 is a diagram showing the time dependence of the reflected high-speed electron beam diffraction intensity during the growth process of the oxide superconductor thin film manufactured in Example of the present invention.

FIG. 8 is a diagram showing an X-ray diffraction pattern of an oxide superconductor thin film manufactured in another example of the present invention.

FIG. 9 is a diagram showing the temperature dependence of the electrical resistivity of an oxide superconductor thin film manufactured in another example of the present invention.

FIG. 10 is a diagram showing current-voltage characteristics of a stacked Josephson device according to one embodiment of the present invention.

FIG. 11 shows that the superconducting element of the present invention is S / I / S / I / S
It is a schematic diagram which shows the example applied to the superconducting three-terminal element which has a junction from a viewpoint of a crystal structure.

FIG. 12 is a diagram schematically showing a configuration of a DC transformer according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Lower superconducting layer 3 ... Intermediate layer 4 ... Upper superconducting layer

Continuation of the front page (56) References JP-A-4-105373 (JP, A) JP-A-3-173184 (JP, A) Ikegawa et. al. "Hall effect in oxi de superconductors having fluorite-type layers" Adv. Supercond. vol. 2, p. 517-520 (1990) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01L 39/22-39/24 H01L 39/00 JICST file (JOIS)

Claims (7)

(57) [Claims] 1. A lower superconducting layer made of a copper-based oxide superconductor, and an intermediate layer laminated on the lower superconducting layer ,
Interposed between the CuO ₂ surface and the CuO ₂ surface, [(RE1 _1-y R
E2 _y ) O ₂ ] _{m + 1} (However, RE1 has higher valence than trivalent
Lanthanide elements and actinide sources forming ions
RE2 has at least one element selected from
Lanthanum elements and yttrium forming trivalent ions
Represents at least one element selected from the group consisting of
≦ y <1 , m is a number that satisfies 2 ≦ m ≦ 100)
And at the same time,
The crystal structure in which the ON layer and the O2 ^- anion layer are alternately stacked
Having multiple fluorite-type blocks and laminated with the CuO ₂ surface
[Bi ₂ O ₂ ] ²⁺ , [Tl ₂ O ₂ ] ²⁺ , [TlO] ¹⁺ , [PbO-
Selected from Cu-PbO] ¹⁺ , [(Pb, Cu) O] ¹⁺ , and CuO _δ chains
Containing at least one kind of alkaline earth element
A superconducting element comprising: an intermediate layer made of a layered copper-based oxide having a layer; and an upper superconducting layer formed on the intermediate layer and formed of a copper-based oxide superconductor. 2. A layered copper-based oxide constituting said intermediate layer.
In the above, the RE2 element is a positive element in the multiple fluorite-type block.
The ion site is located at the site adjacent to the CuO ₂ surface.
And the RE1 element is the multiple fluorite-type block.
Occupy the cation site near the center of the
The superconducting element according to claim 1, which is a feature . 3. The cation collector adjacent to the CuO ₂ surface.
The average ionic radius of the site is in the range of 0.1007 to 0.108 nm
3. The superconducting element according to claim 2, wherein: 4. A process for a superconductor / insulator / superconductor structure.
2. The method according to claim 1, wherein the element has a Sefson junction.
Superconducting element. (5)(Pb _Two Cu) Sr _Two (Ca, RE) _n-1 Cu _n O
_z (However, RE indicates a rare earth element containing Y, and n is 1 to
4 is the number, z is the number of oxygen)
HavingA lower superconducting layer made of a copper-based oxide superconductor; and a laminate formed on the lower superconducting layer., (Pb _Two Cu)AE_Two(R
E1_1-y RE2_y)_{m + 1}Cu_TwoO_z(However,AE is alkaline earth
RE1 forms ions with valences higher than three
Selected from lanthanum elements and actinoid elements
At least one element, RE2 is a trivalent ion
Selected from lanthanum elements and yttrium
At least one element, where m is 2 ≦ m≤100Full
Y is a number that satisfies 0 ≦ y <1, and z is an acid
Represents the elementary quantity)Having a substantially represented composition, Multiple fluorite
Consists of a layered copper-based oxide containing a mold block in the crystal structure
An intermediate layer, which is laminated on the intermediate layer,(Pb _Two Cu) Sr _Two (Ca, RE)
_n-1 Cu _n O _z (However,,RE indicates rare earth elements including Y
Where n is a number from 1 to 4 and z is the number of oxygen)
Having the composition representedUpper part made of copper-based oxide superconductor
A superconducting element comprising a superconducting layer. 6. REBa ₂ Cu ₃ O _7-δ (where RE includes Y
Represents a rare earth element, and δ represents oxygen deficiency.)
A copper oxide superconductor or Rana Ru lower superconducting layer having a composition expressed, it is laminated on the lower superconductive _{layer, Cu AE 2 (RE1 1-} y R
E2 _y ) _{m + 1} Cu ₂ O _z ( where AE is an alkaline earth element, and RE1 is at least one element selected from lanthanide elements and actinoid elements that form ions having a valence higher than trivalent.) And RE2 represent at least one element selected from lanthanum group elements and yttrium that form trivalent ions, m is a number satisfying 2 ≦ m ≦ 100 , and y is a number satisfying 0 ≦ y <1 is a number which, z is a tier in at an oxygen amount) has a substantially composition represented, ing a layered copper oxide containing multiple fluorite-block in the crystal structure, the intermediate Layered on top of the REBA ₂ Cu ₃ O _7-δ (only
RE indicates a rare earth element containing Y, and δ indicates oxygen deficiency.
And an upper superconducting layer made of a copper-based oxide superconductor having a composition substantially represented by the following formula: 7. Bi ₂ Sr ₂ Can _-1 Cu _n O _z (where n is
The number from 1 to 4, z represents the number of oxygen)
Lower superconducting layer composed of copper oxide superconductor having composition
And laminated on the lower superconducting layer, and Bi ₂ Sr ₂ (RE1 _1-y
RE2 _y ) _{m + 1} Cu ₂ O _z (However, RE1 has a valence of more than 3
Lanthanum and Actino Form Highest Ions
RE2 is at least one element selected from the group consisting of
Lanthanum group elements and trivalent ions forming trivalent ions
At least one element selected from thorium,
m is a number that satisfies 2 ≦ m ≦ 100, y is 0 ≦ y <1
And z represents the amount of oxygen).
It has a composition, and is represented by _{[(RE1 1-y RE2 y} ) O 2] m + 1
Rutotomoni, RE1 cation layer elements or RE2 element and O ^2-
Multiple fluorescents having a crystal structure in which anion layers are alternately stacked
An intermediate layer consisting of a layered copper oxide containing stone type block, are stacked on the intermediate _{layer, Bi 2 Sr 2 Ca n-} 1 Cu n O z
(Where n is a number from 1 to 4 and z is an oxygen number)
From a copper-based oxide superconductor having a qualitatively expressed composition.
Superconducting element, characterized in that it comprises an upper superconducting layer that
Child.