JPH07309700A - Oxide thin film, its production and superconducting element using the same - Google Patents

Abstract

PURPOSE:To provide an oxide thin film with which the morphology of orientational properties, ruggedness, etc., of the interface between a substrate and an oxide superconductor layer or between an oxide superconductor layer and a barrier layer can be enhanced up to an atomic layer level. CONSTITUTION:The thin film has a composition substantially represented by the composition formula (RE1-xAEx)CuO3-d wherein: RE is Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm or Yb and AE is Ca, Sr or Ba, and RE and AE form a mixed crystal system; 0<=x<=1.0; and 0<=d<=1. This thin film has also a cubic or tetragonal perovskite or oxygen-deficient perovskite crystal structure, or a crystal structure which has a weaker third periodical structure in the direction of the (c) axis due to the formation of a solid solution consisting of an RE element and an AE element as compared with the crystal structure of REAE2Cu3Oy having superconducting characteristics and shows a symmetry close to that of the cubic or tetragonal perovskite crystal structure. This oxide thin film is used as a buffer layer of an oxide superconductor layer or a barrier layer between two oxide superconductor layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide thin film suitable for a buffer layer or a barrier layer of an oxide superconductor and a method for producing the same.
Furthermore, it relates to a superconducting element using them.

[0002]

2. Description of the Related Art The application of high-temperature oxide superconductors to Josephson devices, superconducting transistors, LSI wiring, etc. is under study. In any of these applications, it is essential to make the high temperature oxide superconductor into a thin film. But,
The conventional thin film technology using a high temperature oxide superconductor has the following problems.

For example, as oxide superconductors for devices, Y-Ba-Cu-O system, Bi-Sr-Ca-Cu-O system, Tl-Ba-Ca-Cu-O system.
The system etc. are used. As a substrate for forming these oxide superconductors, substrates such as SrTiO ₃ , MgO, and LaAlO ₃ are often used, but sputtering, laser ablation, CVD, vacuum deposition, etc. Regardless of the film formation method, there is a problem in that a layer with a disordered orientation or a layer with a large irregularity having a different orientation is likely to occur due to lattice mismatch at the interface between the substrate and the oxide superconductor thin film. As a result, the morphology is disturbed at the interface with the barrier layer and at the upper superconducting electrode, which leads to deterioration of superconducting characteristics and device operating characteristics.

Regarding the characteristics of the Josephson element having a three-layer structure of oxide superconductor layer / barrier layer / oxide superconductor layer, the characteristics of the Josephson device produced when the superconducting conductive electrode having the above-mentioned rough surface are used. , Leak current, flux flow characteristics, magnetic flux traps, etc. may be caused by the presence of impurities or differently oriented grains at the interface between the lower superconducting electrode and the barrier layer, uneven film thickness due to unevenness, and short circuit between the upper and lower electrodes. The phenomenon that seems to occur is very frequently observed, which causes deterioration of characteristics.

[0005]

As described above, the disorder of the morphology at the interface between the substrate and the oxide superconductor layer causes the orientation or unevenness at the interface between the oxide superconductor layer or the oxide superconductor layer and the barrier layer. It is a factor that deteriorates morphology such as. With the conventional thin film technology, it is not possible to sufficiently suppress the above-mentioned factors of deterioration of morphology, and therefore, in order to improve superconducting characteristics and device characteristics, generation of different phases or differently oriented grains at the substrate interface is generated. It is said that suppressing the above is the biggest issue. Further, due to such a problem, a superconducting element such as a laminated Josephson element has not yet achieved sufficient performance.

The present invention has been made in order to solve such a problem, and it is possible to obtain the morphology such as the orientation and unevenness of the interface between the substrate and the oxide superconductor layer, or the interface between the oxide superconductor layer and the barrier layer. It is an object of the present invention to provide an oxide thin film that can be increased to the layer level and a method for manufacturing the oxide thin film. Another object of the present invention is to provide a superconducting device whose performance is improved to a practical level by improving the morphology at the interface.

[0007]

Means and Actions for Solving the Problems In order to solve the above problems, an oxide material having excellent smoothness and orientation which can be a buffer layer or a barrier layer of an oxide superconductor layer is required. Structurally, in order to suppress the growth of crystal grains having anisotropy, an oxide whose crystal structure is as anisotropic as possible is preferable. Further, it is more preferable for interface control that the constituent elements are common to the oxide superconductor formed on the upper portion. In consideration of these matters, an oxide having a perovskite type crystal structure, which is a structural unit common to oxide superconductors, was found. The present invention has been made based on such findings, and uses the above oxide thin film as a buffer layer between the substrate and the oxide superconductor layer or as a barrier layer between the oxide superconductor layers.

That is, in the oxide thin film of the present invention,
The oxide thin film according to claim 1 has a composition formula: (RE _1-x AE _x ) CuO _3-d (1) (wherein RE is Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, H
at least one element selected from o, Er, Tm and Yb, AE represents at least one element selected from Ca, Sr and Ba, and RE and AE form a mixed crystal system,
x is a number satisfying 0 ≤ x ≤ 1, d is a number satisfying 0 ≤ d ≤ 1, and has a cubic or tetragonal perovskite type or oxygen deficiency. RE showing perovskite type crystal structure or superconducting property
AE ₂ Cu ₃ O _y (y is a number that satisfies 6.3 ≦ y ≦ 7.0)
The oxide thin film has a crystal structure with a symmetry similar to that of a cubic or tetragonal perovskite type, which has a weaker triple-periodic structure in the c-axis direction due to the solid solution of RE element and AE element than that of It is characterized by being used as a buffer layer of an oxide superconductor layer or as a barrier layer between two oxide superconductor layers.

The oxide thin film according to claim 2 has a composition formula: (Y _1-x Ba _x ) CuO _3-d (2) (where x is a number satisfying 0 ≦ x ≦ 1; d Is 0 ≤ d ≤ 1
Is a number that satisfies
And a cubic or tetragonal perovskite-type or oxygen-deficient perovskite-type crystal structure, or a superconducting YBa ₂ Cu ₃ O _y (y is a number satisfying 6.3 ≤ y ≤ 7.0) crystal structure. Due to the solid solution of Y element and Ba element, the triple periodic structure in the c-axis direction is weak, and it has a crystal structure with a symmetry close to that of a cubic or tetragonal perovskite type, and it also has a SrTiO ₃ (100) substrate or SrTiO ₃ (110)
An oxide thin film formed on a substrate, the SrTiO 3
_The lattice constant parallel to the (100) direction of the ₃ (100) substrate or the SrTiO ₃ (110) substrate is in the range of 0.3830 nm to 0.3890 nm.

In the method for producing an oxide thin film according to the present invention, the method for producing an oxide thin film according to claim 3 forms YBa ₂ Cu ₃ which exhibits superconducting properties when forming the oxide thin film according to claim 2. O _y is (100) orientation or (110) respectively
At a substrate temperature 20 to 100 K lower than the orientation temperature, the SrT
The oxide thin film is epitaxially grown on an iO ₃ (100) substrate or a SrTiO ₃ (110) substrate.

A method for manufacturing an oxide thin film according to claim 4 is
The method for producing an oxide thin film according to claim 3, wherein the oxide thin film is formed on the SrTiO ₃ (100) substrate to a film thickness of 1 to 100 nm, and the substrate temperature shows a superconducting property while the film formation is continued. YBa ₂ Cu ₃ O _y is the temperature is raised to the a-axis oriented orientation or c-axis, a of the YBa ₂ Cu ₃ O _y at that temperature
The feature is that an axial alignment film or a c-axis alignment film is formed.

Further, in the superconducting element of the present invention, the superconducting element according to claim 5 is formed by forming a lower oxide superconductor layer on the substrate and a barrier layer on the lower oxide superconductor layer. 3. An oxide thin film according to claim 1 or 2 in at least one of the buffer layer and the barrier layer between the substrate and the lower oxide superconductor layer in a superconducting device comprising the upper oxide superconductor layer. It is characterized by using it.

A superconducting device according to a sixth aspect of the present invention has a laminated portion in which a large number of the oxide thin film according to the first or second aspect and the oxide superconducting thin film are alternately laminated. The oxide superconductor thin films are characterized in that they are laminated to have a film thickness of one unit lattice or more.

The oxide thin film of the present invention basically has the above structure.
A cubic or tetragonal perovskite-type or oxygen-deficient perovskite-type crystal structure having a composition substantially represented by the formula (1) and having two lattice constants parallel to the substrate surface, or partially having a RE site REAE ₂ C exhibits superconducting properties as a result of site mixing of AE sites
Compared with the crystal structure of u ₃ O _y , it has a weaker triple-periodic structure in the c-axis direction, and has a crystal structure with symmetry close to that of cubic or tetragonal. X in equations (1) and (2) above
The value of can be selected from any number in the range of 0 to 1 (including 0 and 1), and can be selected according to the required lattice constant and the like. Further, as the value of d, any number in the range of 0 or more and 1 or less can be selected. When the value of d exceeds 1, it becomes difficult to maintain the perovskite type crystal structure.

Such an oxide thin film of the present invention has little structural anisotropy, is excellent in flatness, orientation, and crystallinity, and is SrTiO ₃ , MgO, LaAlO ₃ , NdGaO ₃ , sapphire,
It excels in lattice matching with substrate materials such as YSZ (Y-stabilized ZrO ₂ ), YAG, and Y ₂ O ₃ , and at the same time, it behaves electrically like a semiconductor.

For example, an oxide thin film in which the RE element is Y and the AE element is Ba in the above formula (1), that is, an oxide thin film having the composition represented by the above formula (2), is prepared by using SrTiO ₃ (100)
When formed on a substrate or SrTiO ₃ (110) substrate, the lattice constant of the above oxide thin film parallel to the substrate (100) direction is 0.3830.
The range is from nm to 0.3890 nm. The a-axis length of YBa ₂ Cu ₃ O _y , which is considered to have the highest utility value in the future, is 0.382 nm, and the above oxide thin film has excellent lattice matching with SrTiO ₃ (a-axis length = 0.3905 nm). Is.

The lattice constant of the oxide thin film of the present invention is
Since it can be changed by controlling the type and composition ratio of RE element and AE element in the above formula (1),
Not limited to rTiO ₃ substrate, MgO substrate, LaAlO ₃ substrate, NdGaO substrate
₃ substrates, sapphire substrate, YSZ substrate, YAG substrate, Y ₂ O
₃ It is possible to obtain an oxide thin film with improved lattice matching with a substrate or the like. For example, RE element is La, Pr, Nd, Sm, Eu, Gd, Tb, Dy,
The lattice constant can be increased as the La-side element is selected in the Y, Ho, Er, Tm, Yb array and the Ba-side element is selected as the AE element in the Ca, Sr, Ba array. Also,
It is also possible to change the electric conductivity by controlling the types and composition ratios of the RE element and the AE element, so that it is possible to provide an oxide material that can be widely used for electronic devices.

The oxide thin film of the present invention is used as a buffer layer between a substrate and an oxide superconductor layer or as a barrier layer between two oxide superconductor layers. By using the oxide thin film of the present invention as a buffer layer, different phases or differently oriented grains are formed in the interface with the substrate, the interface with the oxide superconductor layer formed on the oxide thin film, or the oxide superconductor layer. It can be prevented from occurring, and has crystallinity, orientation, surface flatness, compared to the oxide superconductor layer formed directly on the substrate.
An oxide superconductor layer having excellent electrical characteristics can be obtained. Also,
By using the oxide thin film of the present invention as a barrier layer, the flatness at the interface between the lower and upper oxide superconductors is improved, and the crystallinity, orientation, flatness, etc. of the upper oxide superconductor layer are improved. Is significantly improved.

In the oxide thin film of the present invention, an oxide thin film having a cubic or tetragonal perovskite-type or oxygen-deficient perovskite-type crystal structure and a three-fold periodic structure in the c-axis direction compared to the crystal structure of the superconducting phase are formed. The oxide thin film having a crystal structure having a symmetry close to that of a weak cubic crystal or a tetragonal crystal is suitable for the buffer layer and the barrier layer.

The oxide superconductor layer formed on the oxide thin film of the present invention is not limited to the YBa ₂ Cu ₃ O _y (y is a number satisfying 6.3 ≦ y ≦ 7.0) system and other REBa ₂ Cu ₃ O _y system (RE here is La, Nd, Pm, Sm, Bu, Gd, Tb, Dy, Ho,
Er, Tm, Yb and Lu form a mixed crystal system of at least one element, y is a number satisfying 6.3 ≤ y ≤ 7.0), Bi-Sr-Ca-Cu-O system, Tl-Ba-Ca -Cu-O system,
Hg-AE-Ca-Cu-O system (AE is a mixed crystal system of at least one element selected from Ca, Sr and Ba), (Ba,
AM) BiO ₃ system (AM is at least 1 selected from K and Rb)
It is also possible to apply other high temperature oxide superconductors, such as (which is a seed element). In particular, it is preferable to apply an oxide superconductor layer having the same constituent element as that of the oxide thin film of the present invention, from the viewpoint of controlling the interface and easiness of film formation. For example, in the oxide thin film according to the _{second aspect} , it is preferably used as a buffer layer or a barrier layer of the YBa ₂ Cu ₃ O _y based oxide superconductor layer.

The oxide thin film of the present invention as described above can be obtained by simply controlling the substrate temperature. That is, by depositing an oxide thin film whose composition is substantially represented by the above formula (1) at a substrate temperature lower than the deposition temperature of REAE ₂ Cu ₃ O _y exhibiting superconducting properties, cubic or tetragonal The perovskite-type or oxygen-deficient perovskite-type crystal structure or the crystal structure of REAE ₂ Cu ₃ O _y , which exhibits superconducting properties, has a weaker triple-periodic structure in the c-axis direction and a symmetry close to cubic or tetragonal. A crystal structure having is obtained. Any of these crystal structures can be obtained depending on the type of the substrate, the substrate temperature, etc., and if the substrate temperature during film formation is low, a perovskite type or oxygen deficient perovskite type crystal structure is easily obtained, and When the substrate temperature is set slightly higher than that, it is easy to obtain a crystal structure with a symmetry close to that of a cubic crystal or a tetragonal crystal in which the triple-periodic structure in the c-axis direction is weak.

In particular, the RE element in the above formula (1) is Y, and
When the AE element is Ba, this oxide thin film is YBa ₂ Cu ₃ O.
It can be fabricated at a substrate temperature 20 to 100 K lower than the temperature at which _y is a-axis oriented or c-axis oriented. If the substrate temperature during the formation of the oxide thin film is higher than the temperature 20 K lower than the temperature at which YBa ₂ Cu ₃ O _y is a-axis oriented or c-axis oriented, the crystal structure as described above cannot be obtained, and YBa ₂ Cu ₃ O _y is a
If the temperature is lower than 100 K lower than the temperature for axial or c-axis orientation, it will be difficult to crystallize sufficiently. Therefore,
After forming a film with a thickness of 1 to 100 nm using the above oxide thin film as a buffer layer, YBa ₂ Cu ₃ O _y was gradually formed while the film formation was continued.
heated to a temperature of a-axis oriented orientation or c-axis, YBa ₂
By forming a Cu ₃ O _y thin film, an a-axis oriented film or a c-axis oriented film of YBa ₂ Cu ₃ O _y having remarkably excellent crystallinity, orientation, and flatness can be easily obtained. The same applies when the oxide thin film is formed as a barrier layer. When forming an oxide thin film other than (Y, Ba) CuO _3-d , it is preferable to set the substrate temperature according to the composition of the oxide thin film and the like in addition to the above substrate temperature.

The method for forming the oxide thin film of the present invention and the oxide superconductor thin film formed on the oxide thin film is not particularly limited, and the sputtering method, the vacuum deposition method, the laser ablation method, the CVD method, etc. Various thin film forming methods can be applied.

The superconducting device according to claim 5 is composed of a lower oxide superconductor layer / barrier layer / upper oxide superconductor layer.
In an element having a three-layer laminated structure, the above-mentioned oxide thin film of the present invention is used as at least one of a buffer layer and a barrier layer between a substrate and a lower oxide superconductor layer.

By applying the oxide thin film of the present invention to the buffer layer, the crystallinity, orientation, flatness, electrical characteristics and the like of the oxide superconductor layer formed on the buffer layer can be improved. By using such an oxide superconductor layer as an electrode of a Josephson element, for example, RSJ (Re
High quality Josephson devices with sistively shunted junction) characteristics can be realized. The thickness of the buffer layer is not particularly limited, but it is preferably about 1 to 100 nm. If the thickness of the buffer layer is less than 1 nm, the above-mentioned effects such as flattening cannot be sufficiently obtained, and if the thickness exceeds 100 nm, no further effect can be obtained.

Since the oxide thin film of the present invention has an electrically semiconductive property, a so-called S / N / S type Josephson junction can be realized by using it as a barrier layer. The thickness of the barrier layer is set within a range in which bonding characteristics can be obtained, and is preferably about 1 to 100 nm, for example. Then, by using the oxide thin film of the present invention as a barrier layer, the flatness at the interface between the lower and upper oxide superconductor layers is improved, and the crystallinity and orientation of the upper oxide superconductor layer, The flatness etc. has improved remarkably,
It is possible to suppress deterioration of electrical characteristics such as T _c and J _c . Therefore, in device application, electronic state control at the interface can be performed with good reproducibility, and when used in a Josephson element or the like, a device with significantly improved characteristics can be manufactured. Further, the superconducting element according to claim 6 has a laminated portion in which a large number of the oxide thin film of the present invention and other oxide superconductor thin films are alternately laminated. Since the oxide thin film of the present invention has the functions of both a buffer layer and a barrier layer, when used in a multi-layered structure film, it is possible to achieve a uniform stacking over a thicker film than before. As a result, a multiple Josephson device or the like can be easily manufactured. The oxide thin film and the oxide superconductor thin film are formed to have a thickness of one unit lattice or more in order to obtain their respective functions.

[0027]

EXAMPLES Examples of the present invention will be described below.

Example 1 First, a (Y, Ba) CuO _3-d type cubic and tetragonal oxide thin film was formed by a sputtering method. An RF magnetron sputtering device equipped with a multi-source metal target was used as the sputtering device. As targets, metal Cu, metal Y, and Ba-Cu alloys were used, and RF power supplies were mounted on them. In addition, as the deposition substrate, SrTiO ₃ , MgO, L
aAlO ₃ , NdGaO ₃ , sapphire, YSZ, YAG, Y ₂ O ₃
Each of the substrates was used.

In particular, Table 1 and FIG. 1 show the film forming conditions when the film was formed on the SrTiO ₃ (100) substrate and the structure and physical properties of the obtained film. Note that FIG. 1 is a diagram schematically showing the crystal structure of tetragonal (Y, Ba) CuO _3-d when a film is formed at a substrate temperature of 843K. Structural analysis shows that each phase shown in Table 1 has a cubic or tetragonal perovskite type crystal structure in which Y and Ba are randomly solid-dissolved and oxygen deficiency exists, and behaves electrically and semiconductingly. Turned out to show. In this example,
The oxide thin film formed at 823K has a cubic oxygen-deficient perovskite crystal structure, and the oxide thin film formed at 843K is
The triple-periodic structure in the c-axis direction had a crystal structure with a symmetry similar to that of a weak tetragonal structure.

[0030]

[Table 1] It was found that the cubic crystal and tetragonal crystal formation conditions differ depending on the substrate type, substrate temperature, and the like.

Example 2 (Y, Ba) CuO was formed on a SrTiO ₃ (100) substrate under the conditions shown in FIG. 2 using the same sputtering apparatus as in Example 1.
_{A 3-d} type oxide thin film is formed, and YBa ₂ Cu ₃ O is further formed on it.
An a-axis oriented film of _y- based oxide superconductor was formed.

First, a tetragonal thin film of (Y ₁ Ba ₂ ) CuO _3-d composition was formed as a buffer layer at a substrate temperature of 843 K to a film thickness of about 50 nm, and then 3-4 K while continuing film formation. The substrate temperature is raised to 893K at 1 / min, and then YBa ₂ Cu ₃ at the substrate temperature of 893K.
An O _y a-axis alignment film was formed with a thickness of about 300 nm.

As a comparative example with the present invention, a SrTiO ₃ (100) substrate was prepared in the same manner as in the above example except that the (Y, Ba) CuO _3-d oxide thin film was not formed as the buffer layer. To
An a-axis oriented film of YBa ₂ Cu ₃ O _y was formed.

A of YBa ₂ Cu ₃ O _y obtained by the example
The axial alignment film has a half width of the (200) peak by XRD.
The film has an excellent crystallinity and orientation with 0.06 °, has a zero resistance of 90K, and shows a marked improvement in superconducting properties as compared with the film without a buffer layer. The results are shown in Table 2.

[0035]

[Table 2] Further, FIG. 3 shows the results of observing the surface irregularities by AFM. YBa ₂ according to Comparative Example 1 using no buffer layer
The Cu ₃ O _y film has irregularities of about 20 nm, while the YBa ₂ Cu ₃ O _y film according to Example 2 employing the buffer layer has a thickness of 1 nm.
There were only the following irregularities, and the surface properties were also markedly improved.

Example 3 (Y, Ba) CuO was formed on a SrTiO ₃ (100) substrate under the conditions shown in FIG. 4 using the same sputtering apparatus as in Example 1.
_{A 3-d} type oxide thin film is formed, and YBa ₂ Cu ₃ O is further formed on it.
A c-axis oriented film of _y- based oxide superconductor was formed.

First, a tetragonal thin film of (Y ₁ Ba ₂ ) CuO _3-d composition was formed as a buffer layer at a substrate temperature of 843 K to a film thickness of about 50 nm, and then the substrate was grown to 973 K while continuing film formation. After raising the temperature, a c-axis alignment film of YBa ₂ Cu ₃ O _y was formed with a film thickness of about 300 nm at a substrate temperature of 973K. The characteristics of the obtained film are shown in Table 3. In addition, Comparative Example 2 in Table 3 is a c-axis oriented film of YBa ₂ Cu ₃ O _y formed without forming a buffer layer.

[0038]

[Table 3] As is clear from Table 3, the c-axis oriented film of YBa ₂ Cu ₃ O _y can also be formed without using the buffer layer by forming the (Y, Ba) CuO _3-d oxide thin film as the buffer layer. The orientation, crystallinity, and electrical properties were markedly improved compared to those directly formed on the substrate. FIG. 5 shows the results of observing surface irregularities by AFM. The YBa ₂ Cu ₃ O _y film according to Comparative Example 2 which does not use the buffer layer has irregularities of about 20 nm, while the YBa ₂ Cu ₃ O _y film according to Example 3 which uses the buffer layer.
_{The 2} Cu ₃ O _y film had irregularities of about 1 nm, and the surface properties were significantly improved.

Example 4 Similar to Example 2, YBa ₂ was used as a lower superconducting electrode on a SrTiO ₃ (100) substrate on which a tetragonal thin film of (Y, Ba) CuO _3-d oxide was formed as a buffer layer. Cu ₃ O _y a-axis alignment film (film thickness = 200 nm) was formed, and a similar sputtering device was used as a barrier layer for PrBa ₂ Cu ₃ O _y a-axis alignment film (film thickness = 50 nm).
Was formed, and an a-axis oriented film of YBa ₂ Cu ₃ O _y was formed as an upper superconducting electrode to a film thickness of 100 nm to form a laminated structure film. As a result, the flatness at each interface and the surface of the upper superconducting electrode is significantly improved compared to the laminated structure film formed directly without using a buffer layer, and a laminated structure film containing almost no voids can be obtained. did it.

Next, the laminated structure film was processed by using a photolithography technique to manufacture a laminated Josephson element as shown in FIG. The stacked Josephson device shown in FIG. 6 is composed of (Y, Ba) C on the SrTiO ₃ (100) substrate 1.
A uO _3-d type tetragonal oxide thin film 2 is formed as a buffer layer, and on this buffer layer 2, an a-axis oriented film 3 of YBa ₂ Cu ₃ O _y is formed as a lower superconducting electrode, and PrBa is formed as a barrier layer.
₂ Cu ₃ O _y film 4, YBa ₂ Cu ₃ O _y as upper superconducting electrode
The a-axis alignment film 5 is sequentially laminated. This laminated structure film is patterned so that the junction area is 20 × 20 μm, and the wiring layer 7 is provided on the upper superconducting electrode 5 with the insulating film 6 interposed therebetween. It is connected to the layer 7 via a gold electrode 8.

Table 4 shows the evaluation results of the characteristics of the above-mentioned laminated Josephson device. By using a buffer layer composed of a (Y, Ba) CuO _3-d tetragonal oxide thin film during device structure fabrication, the interelectrode gap often seen in Josephson devices using conventional oxide superconductors A good quality Josephson device was obtained in which no short circuit or flux flow characteristics were observed.

[0042]

[Table 4] Example 5 As in Example 2, a YBa ₂ Cu ₃ O _y film formed on a SrTiO ₃ (100) substrate having a tetragonal thin film of (Y, Ba) CuO _3-d oxide as a buffer layer was used. An axial alignment film (film thickness = 250 nm) was used as a lower superconducting electrode, and a cubic or tetragonal (Y, Ba) CuO _{3 -d} oxide thin film similar to that of Example 1 was formed thereon as a barrier layer. Furthermore, a _Y- axis oriented film of YBa ₂ Cu ₃ O _y (film thickness =
A film having a thickness of 100 nm) was formed as an upper superconducting electrode to form a laminated Josephson device having the same structure as in Example 4. Table 5
And FIG. 7 shows some substrate temperature conditions when a tetragonal (Y, Ba) CuO _3-d oxide thin film is used as a barrier layer. Josephson characteristics were obtained under all the substrate temperature conditions shown in FIG. Also, Josephson characteristics were obtained when a cubic (Y, Ba) CuO _3-d oxide thin film was used as a barrier layer.

[0043]

[Table 5] The characteristics of the Josephson device manufactured under the temperature conditions shown in FIG. 7A are shown in FIGS. 8 and 9. The obtained Josephson element exhibits RSJ-like characteristics, and it can be seen from the magnetic field dependence of the critical current shown in FIG. 9 that a higher quality Josephson element is obtained. In addition, because of the good surface property, the thickness of the barrier layer can be reduced to about 1 nm, and the I _c · R _n value can be increased to about 10 times that of the conventional stacked Josephson element. did it.

Example 6 As shown in FIG. 10, on a SrTiO ₃ (100) substrate 11, a cubic or tetragonal (Y, Ba) CuO _{3 -d} oxide thin film 12 similar to that of Example 1 was formed. , A multi-layered film of YBa ₂ Cu ₃ O _{y and} the a-axis alignment film 13 was prepared using the same sputtering apparatus as in Example 1. Switching between the layers was performed by shuttering of a shutter located immediately above the target. Each layer 1
Nos. 2 and 13 maintained flatness on the order of a unit cell of about 0.39 nm, and exhibited characteristics of a multiple Josephson junction.

In each of the above embodiments, (Y, Ba) C
Although the example of using the uO _3-d oxide thin film as the buffer layer or the barrier layer has been described, the case where other (RE, AE) CuO _3-d oxide thin films are used as the buffer layer or the barrier layer is also described. A similar effect was obtained. The oxide superconductor layer is not limited to YBa ₂ Cu ₃ O _y -based oxide superconductor thin films, but other REBa ₂ Cu ₃ O _y -based, Bi-Sr-Ca-Cu-O-based, Tl-Ba -Ca -Cu
It was also possible to apply high-temperature oxide superconductors such as -O system, Hg-AE-Ca-Cu-O system, (Ba, AM) BiO ₃ system.

[0046]

As described above, according to the oxide thin film of the present invention, the orientation and flatness of the interface between the substrate and the oxide superconductor layer or the interface between the oxide superconductor layer and the barrier layer are at the atomic layer level. Can be increased up to. Therefore, according to the superconducting element of the present invention using such an oxide thin film of the present invention as a buffer layer or a barrier layer, the problem of the interface or the surface of the oxide superconductor thin film, which has been a problem in device application from the past, It is possible to provide an element having remarkably excellent characteristics. Further, the method for producing an oxide thin film of the present invention has a relatively low cost, and the oxide thin film can be produced by a simple operation. Therefore, it greatly contributes to the improvement of the utility value of the oxide superconductor thin film particularly for the purpose of electronics application.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing an example of a crystal structure of an oxide thin film according to Example 1 of the present invention.

FIG. 2 is a diagram showing production conditions of an oxide thin film and an oxide superconductor thin film according to Example 2 of the present invention.

FIG. 3 is a diagram showing a surface state of an oxide superconductor thin film obtained in Example 2 of the present invention in comparison with an oxide superconductor thin film formed without using a buffer layer.

FIG. 4 is a diagram showing production conditions of an oxide thin film and an oxide superconductor thin film according to Example 3 of the present invention.

FIG. 5 is a diagram showing a surface state of an oxide superconductor thin film obtained in Example 3 of the present invention in comparison with an oxide superconductor thin film formed without using a buffer layer.

FIG. 6 is a cross-sectional view showing the structure of a stacked Josephson device manufactured in Example 4 of the present invention.

FIG. 7 is a diagram showing manufacturing conditions of a laminated Josephson element manufactured in Example 5 of the present invention.

FIG. 8 is a diagram showing an example of current-voltage conditions of a laminated Josephson element manufactured in Example 5 of the present invention.

FIG. 9 is a diagram showing an example of the magnetic field dependence of the critical current of the stacked Josephson element manufactured in Example 5 of the present invention.

FIG. 10 is a cross-sectional view showing the structure of a multi-layer superconducting element manufactured in Example 6 of the present invention.

[Explanation of symbols]

1, 11 …… SrTiO ₃ (100) substrate 2 …… (Y, Ba) CuO _3-d type tetragonal oxide thin film buffer layer 3 …… Lower superconducting electrode 4 …… Barrier layer 5 …… Upper superconducting electrode 12 …… (Y, Ba) CuO _3-d oxide thin film 13 …… YBa ₂ Cu ₃ O _y a-axis oriented film

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location H01L 39/22 ZAA A H05K 1/09 ZAA A 7726-4E

Claims (6)

[Claims] 1. A composition formula: (RE _1-x AE _x ) CuO _3-d (where RE is Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, H).
at least one element selected from o, Er, Tm and Yb, AE represents at least one element selected from Ca, Sr and Ba, and RE and AE form a mixed crystal system,
x is a number satisfying 0 ≤ x ≤ 1, d is a number satisfying 0 ≤ d ≤ 1, and has a cubic or tetragonal perovskite type or oxygen deficiency. RE showing perovskite type crystal structure or superconducting property
AE ₂ Cu ₃ O _y (y is a number that satisfies 6.3 ≦ y ≦ 7.0)
The oxide thin film has a crystal structure with a symmetry similar to that of a cubic or tetragonal perovskite type, which has a weaker triple-periodic structure in the c-axis direction due to the solid solution of RE element and AE element than that of An oxide thin film characterized by being used as a buffer layer of an oxide superconductor layer or a barrier layer between two oxide superconductor layers. 2. A composition formula: (Y _1-x Ba _x ) CuO _3-d (where x is a number satisfying 0 ≦ x ≦ 1 and d is 0 ≦ d ≦ 1
Is a number that satisfies
And a cubic or tetragonal perovskite-type or oxygen-deficient perovskite-type crystal structure, or a superconducting YBa ₂ Cu ₃ O _y (y is a number satisfying 6.3 ≤ y ≤ 7.0) crystal structure. Y element and Ba element weakly three times periodic structure of c-axis direction by the solid solution of which has a crystal structure having a near symmetric cubic or tetragonal perovskite, SrTiO ₃ (100) substrate or SrTiO ₃ (110)
An oxide thin film formed on a substrate, the SrTiO 3
₃ An oxide thin film characterized in that the lattice constant parallel to the (100) direction of the (100) substrate or the SrTiO ₃ (110) substrate is in the range of 0.3830 nm to 0.3890 nm. 3. When forming the oxide thin film according to claim 2, at a substrate temperature 20 to 100 K lower than the temperature at which YBa ₂ Cu ₃ O _y exhibiting superconducting properties is (100) oriented or (110) oriented, respectively. A method for producing an oxide thin film, which comprises epitaxially growing the oxide thin film on the SrTiO ₃ (100) substrate or the SrTiO ₃ (110) substrate. 4. The method for producing an oxide thin film according to claim 3, wherein the oxide thin film is formed on the SrTiO ₃ (100) substrate in an amount of 1 to 100.
It was formed into a film having a thickness of nm, after the YBa ₂ Cu ₃ O _y where the substrate temperature indicating superconductivity was heated to a temperature of a-axis oriented orientation or c-axis while continuing the deposition, the at that temperature YBa ₂ Cu ₃ O
_A method for producing an oxide thin film, which comprises depositing an a-axis oriented film or a c-axis oriented film of _y . 5. A superconducting device comprising a lower oxide superconductor layer formed on a substrate and an upper oxide superconductor layer formed on the lower oxide superconductor layer with a barrier layer interposed therebetween. A superconducting element, wherein the oxide thin film according to claim 1 or 2 is used as at least one of a buffer layer and a barrier layer between the substrate and a lower oxide superconductor layer. 6. An oxide thin film according to claim 1 or 2, and a laminated portion in which a large number of oxide superconductor thin films are alternately laminated, and the oxide thin film and the oxide superconductor thin film are A superconducting element, each of which has a film thickness of one unit lattice or more.