JPH0864879A - Metal oxide material superconducting junction element and substrate for superconducting element using the same material - Google Patents

Abstract

PURPOSE: To obtain metal oxide material useful as the composing material of a tunnel barrier layer, a buffer layer, a substrate, etc., of a superconductor mainly having a T' structure by using a metal oxide of CeCuPd containing a specific composition and metal. CONSTITUTION: A metal oxide material used for a superconducting junction element and a substrate for a superconducting element has a composition formula of Ln2a Cea Cu1b Pdb O4-c , 0<=a<=0.2, 0.25<=b<=0.5 and 0<=c<=0.1, and Ln is one or more types of elements or atomic groups selected from an element group consisting of La, Pr, Nd, Sm, Eu and Gd. When this copper oxide material is used for a tunnel barrier layer, a first superconducting layer 2, a barrier layer 3, and a second superconducting layer 4 are sequentially formed on an SrTiO6 board by a magnetron sputtering method. The laminated layer structure of the first superconducting layer 2, the barrier layer 3 and the second superconducting layer 4 formed in this manner forms a laminated Josephson element by lithography technology.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metal oxide material which is mainly used in combination with a copper oxide superconducting material, and more particularly, to a substrate for a superconducting element used for producing a superconducting electronic element,
Alternatively, the present invention relates to a metal oxide material useful as a buffer layer between a substrate and a superconductor or an insulating layer between a superconductor and a metal or a superconductor.

[0002]

2. Description of the Related Art In recent years, copper-containing oxide superconductors, which have been discovered one after another, have a Tc that greatly exceeds the conventionally known superconducting critical temperature (Tc) of niobium and the like. Applied research is underway. In the field of superconducting electronics, there are great expectations for its practical application.
Many research reports have already been published. A superconductor used in a superconducting electronic device is generally achieved by a thin film manufacturing technique. However, in order to form a device, not only a superconductor but also a tunnel barrier layer or a buffer used together with it. The materials of the layers, as well as the substrate, must be suitable.

When selecting these materials, it is an important point to obtain a good interface with the superconductor. For that purpose, the lattice constant of the superconductor and the tunnel barrier layer or the buffer layer that joins to it are important. It is desirable that the lattice constants of the substrate, etc. are the same. Further, it is desirable that the thermal expansion coefficients of both are similar. Furthermore, when a film is formed at a high temperature or when a process is performed at a high temperature, both elements are often diffused into each layer. It is necessary to select a material that has a small effect on the device.

As such an element, a superconductor YBa ₂
Cu ₃ in _{O y, PrBa 2 Cu 3 O} y having the same structure as this
Has been proposed as an SIS type element of oxide superconducting layer-oxide layer-oxide superconducting layer in which is combined as a tunnel barrier layer material. Also, as a superconductor, Ln _2-x Ce _x Cu
When a copper oxide such as O _y (Ln = Pr, Nd, Sm, Eu, Gd) whose carrier is n type is used, the material used for the tunnel barrier layer does not include Ce which is a semiconductor. Ln ₂ CuO ₄ (Ln = Pr, Nd,
Sm, Eu, Gd) and the like are listed as candidates.

[0005]

[Problems to be solved by the invention] However,
For example, the Nd ₂ CuO ₄ with respect to Nd _2-x Ce _x CuO _y when used in the tunnel barrier layer, or conditions such as synthesis temperature is substantially equal to both, or Nd ₂ CuO to _4/5 is for slightly lower However, there is a problem that Ce ions and the like are easily diffused from the superconducting layer and Nd ions and the like are easily diffused from the tunnel barrier layer. This is because the characteristics of the tunnel barrier layer become metallic when Ce ions diffuse into the tunnel barrier layer, and conversely, when Nd ions diffuse into the superconducting layer, the relative concentration of Ce. Is deteriorated, which causes deterioration of superconducting properties.

On the other hand, when La ₂ CuO ₄ having a higher synthesis temperature is used for the tunnel barrier layer in comparison with Nd _2-x Ce _x CuO _y , the lattice constants of both are a ( b) In the axial length, the former is tetragonal, that is, a = b
And 3.96Å, while the latter is orthorhombic,
To describe the azimuth in terms of the square root of the a and b axes,
They are 3.79Å and 3.82Å respectively, and the above Nd _2−x C
The lattice constant is 4 to 5% smaller than that of e _x CuO _y , and the internal stress becomes large for epitaxial crystal growth.
As a result, there is a problem in that the crystal becomes defective and does not function as an element.

Therefore, an object of the present invention is to use Ln _2-x Ce _x CuO _y (Ln = P) having a T'structure as a superconductor.
When r, Nd, Sm, Eu, Gd) is used, a metal oxide material useful as a constituent material of a tunnel barrier layer, a buffer layer, a substrate, or the like is provided.
Another object is to provide a superconducting junction element and a superconducting element substrate using the same.

[0008]

The above object can be achieved by the present invention described below. That is, in the present invention, the composition formula is Ln.
_In a metal oxide material represented by _2-a Ce _a Cu _1-b Pd _b O _4-c , 0 ≦ a ≦ 0.2, 0.025 ≦ b ≦ 0.5 and 0 ≦ c ≦ 0.1 And Ln is La, Pr, N
1 selected from the group of elements consisting of d, Sm, Eu and Gd
It is a metal oxide material characterized by being an element or atomic group of more than one kind, and a superconducting junction element and a substrate for a superconducting element using the metal oxide material.

[0009]

The elements constituting the metal oxide material of the present invention are Ln _2-x Ce _x CuO _{y which} is a conventionally known copper oxide superconductor.
(Ln = Pr, Nd, Sm, Eu, Gd) and the Pd element. In other words, the point different from the conventional one is that a part of Cu is partially replaced with Pd, and the composition ratio is almost the same as the conventional one. In the present invention, one of the effects of substituting a part of Cu with the Pd element is to raise the synthesis temperature of the metal oxide material and further eliminate the superconducting property. Here, the rate of increase in the synthesis temperature depends on the amount of Pd element,
For example, replacing half of Cu with the Pd element raises the synthesis temperature by about 100 ° C. That is, as described above, the fact that the synthesis temperature of the metal oxide material of the present invention can be made higher than that of the superconductor reduces diffusion of ions from the superconductor to the tunnel barrier layer or the like and reduces the superconductivity. This means that the characteristics are not significantly affected.

In addition, the lattice constant (a) of the metal oxide material of the present invention is Ln _2-x Ce _x CuO _y (Ln
= Pr, Nd, Sm, Eu, Gd) and other elements except Cu, including the composition ratio, are substantially constant, and change depending on the amount of Pd to be 3.9Å to 3.99Å. Since this value has almost the same lattice constant as that of the above-mentioned superconductor, the interface matching with the superconductor is improved, which is very advantageous as an element. That is, by combining the metal oxide material of the present invention with a superconducting material having a lattice constant similar to that of the metal oxide material and having little or no influence of diffusion, an element having a good junction can be obtained. Can be formed.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below with reference to preferred embodiments. The composition formula of the metal oxide material of the present invention is represented as Ln _2-a Ce _a Cu _1-b Pd _b O _4-c, and in the composition formula, 0 ≦ a ≦ 0.2, 0.025 ≦ b ≦
0.5 and 0 ≦ c ≦ 0.1, and Ln is La, P
It is characterized by being one or more kinds of elements or atomic groups selected from the group of elements consisting of r, Nd, Sm, Eu and Gd. That is, if it is a composition other than this, it is not possible to synthesize a single composition, that is, a single-phase sample, or it is not possible to obtain a semiconductor-like characteristic, and the excellent effect of the present invention is obtained. I can't.

The metal oxide material of the present invention may be any material as long as it has the above composition, but a material suitable in the present invention has a crystal structure of Ln ₂ CuO ₄ (Ln is Pr, N
It is a metal oxide material having the same T ′ structure as one or more elements or atomic groups selected from the group of elements d, Sm, Eu, and Gd.

As the method for producing the copper oxide material of the present invention as described above, any reaction and sintering method by heating from raw material powder, which is generally used for so-called ceramic materials, is used. It is possible. An example of such a method is the Material Research Bull
etin Vol.8, p.777 (1973), Solid
State Communication Volume 17 2
Page 7 (1975), Physical Review
Letters Vol. 58, No. 9, p. 908 (1987)
, Etc., and these methods are now qualitatively known as extremely general methods.

Particularly when the copper oxide material of the present invention is used for a tunnel barrier layer or a buffer layer, a sputtering method such as high frequency sputtering or magnetron sputtering using a target containing raw materials, electron beam evaporation,
MBE method, other vacuum deposition method or cluster ion beam method, CVD method using gas as a raw material, or plasma C
The material of the present invention can be formed into a thin film on a substrate or a superconducting thin film by using the VD method or the like.

The thus-obtained copper oxide material of the present invention exhibits semiconductor-like characteristics without showing superconducting transition or metallic behavior below the temperature of liquid nitrogen. In particular, since it has a sufficiently high electric resistivity at the temperature of liquid helium, it is effective as a tunnel barrier layer. In addition, since the raw materials used in the present invention are all inexpensive and the raw material cost can be kept low, the metal oxide material of the present invention can be provided at a low cost. In addition, the metal oxide material of the present invention is relatively stable in air and has little deterioration, and since it does not use a highly toxic material such as a heavy metal as a raw material, it is highly safe.

[0016]

EXAMPLES Next, the present invention will be described more specifically with reference to examples. Examples 1 to 10 and Comparative Examples 1 4 La ₂ O ₃ as raw _{materials, Pr 6 O 11, Nd 2} O 3, Sm
₂ O ₃ , Eu ₂ O ₃ , Gd ₂ O ₃ , Dy ₂ O ₃ , CeO ₂ and C
uO oxides were used, and these were weighed at the composition ratios shown in Tables 1 and 2 below, and then dry mixed. The obtained mixture was pressure-molded into pellets each having a diameter of 10 mm and a thickness of 1 mm, and the molded product was molded on an alumina boat at 100 mm.
Copper oxides of Examples 1 to 10 and Comparative Examples 1 to 4 of the present invention were prepared by reacting and sintering in air or nitrogen at 0 to 1350 ° C. or in a mixed gas atmosphere of the both.
The atmosphere gas used in this example is only the above-mentioned ones only because it is easily available, and does not need to be the above-mentioned gas.
There is no problem even if a gas such as oxygen or helium is used.

Table 1 below shows the composition ratios of the examples, and Table 2
The composition ratios of the comparative examples are shown in each. Here, the composition ratio is E
Since it is measured by PMA, there is an error of about 20% in the amount of oxygen.

Table 1 Compositions of metal oxide materials of Examples 1 to 10

Table 2 Compositions of metal oxide materials of Comparative Examples 1 to 4

FIG. 1 shows the X-ray diffraction pattern of the metal oxide material obtained in Example 2. From this figure, the sample of Example 2 has a = b = 3.961Å, c = 12.12Å
It has a tetragonal crystal structure with a lattice constant of. The value of this lattice constant is, for example, a copper oxide superconducting material which has excellent superconducting properties and in which the composition ratio of other elements except Cu and Pd is almost the same as the sample obtained in Example 2. Compared with Nd _1.85 Ce _0.15 CuO _4-y , the difference is less than 0.13% on the a (b) axis and less than 0.10% on the c axis. Therefore, it is understood that the metal oxide material obtained in Example 2 has good interface matching with such a superconductor and is effective as a tunnel barrier layer, a buffer layer or a substrate of the superconductor. . The mixture having the composition ratio of Example 2 was the same as the superconductor N
Even when synthesized with d _1.85 Ce _0.15 CuO _{4-y under the} same conditions, it does not exhibit superconducting properties but exhibits semiconductor-like behavior. In order to clarify this, a graph of temperature dependence of electric resistivity of the metal oxide material obtained in Example 2 is shown in FIG.

The metal oxide material obtained in Example 2 also has a decomposition temperature corresponding to the melting point of the superconductor Nd _1.85 Ce.
_Higher than _0.15 CuO _4-y , Nd _1.85 Ce _0.15 CuO
It was confirmed from the X-ray diffraction pattern that the crystal structure was maintained even at the temperature at which _4-y started to decompose by changing the synthesis temperature. This means that when the copper oxide material of the present invention is used as a tunnel barrier layer, it is possible to reduce the diffusion of ions or the reverse diffusion from the superconductor as described above, and the superconducting properties are not so affected. It is shown that there is an advantage in not giving

It was confirmed that the copper oxide materials of Examples other than Example 2 also showed a similar crystal structure and a similar electrical resistivity. Here, Pd occupies a position crystallographically equal to Cu with a certain existence probability, but the lattice constant gradually increases as the amount of Pd increases. It was also confirmed that the temperature at which decomposition started also increased with an increase in the amount of Pd, and for example, the copper oxide material obtained in Example 8 maintained its crystal structure even at 1350 ° C. On the other hand, Table 2
With the copper oxide materials having the composition ratios of Comparative Examples 1 to 4 of the present invention shown in FIG. 2, a single composition, that is, a single-phase sample cannot be synthesized, or semiconductor characteristics cannot be obtained. Therefore, the characteristics were poor.

Example 11 Copper of the present invention obtained in the same manner as in Examples 1 to 10 using Nd _1.85 Ce _0.15 CuO _4-y , which is an oxide superconductor, as the first and second superconducting layers. Nd _1.85 which is an oxide material
Ce _0.15 Cu _0.9 Pd _0.1 O ₄ was used as the tunnel barrier layer. FIG. 3 shows a schematic view of the laminated Josephson device of this embodiment. In the figure, 1 is a SrTiO ₃ substrate, 2 is a first superconducting layer, 3 is a barrier layer, and 4 is a second superconducting layer.
First, the magnetron sputtering method was used to form the first superconducting layer 2 with a film thickness of 4000 Å, the barrier layer 3 with a film thickness of 25 Å, and the second superconducting layer with a film thickness of 3000 Å. Next, the laminated thin film of the first superconducting layer 2 / the barrier layer 3 / the second superconducting layer 4 formed in this way is processed by using an ordinary photolithography technique. A laminated Josephson device as shown was created.

The element manufactured as described above is 15K
Then, the current-voltage characteristics as shown in FIG. 4 were exhibited, and it was confirmed that the Josephson element operates well. As can be seen from this, the copper oxide material of the present invention has a lattice constant close to that of a superconductor, good compatibility with the superconductor, and little diffusion of ions from the superconductor. It can be seen that it is a very good non-superconductor metal oxide material with little influence on the characteristics. In addition to the combination of the superconducting layer and the barrier layer combined in this example, similar results were obtained by using other metal oxide materials of the present invention as the barrier layer.

Example 12 SrTiO ₃ was used as a substrate, and the metal oxide material of the present invention had a lattice constant (a) of 3.90Å and Gd.
_A film having a composition of ₂ Cu _0.9 Pd _0.1 O ₄ and a film thickness of 500
Å formed, and on top of it is a copper oxide superconductor
NdBa ₂ Cu ₃ O _y having a 23 structure has a film thickness of 4000 Å
Formed. This NdBa ₂ Cu ₃ O _y is an orthorhombic crystal,
The lattice constants a and b are 3.89Å and 3.91Å, respectively. The magnetron sputtering method was used for forming the respective films, as in Example 11.

The superconducting film thus prepared was evaluated for superconducting properties by changing the electric resistivity with temperature.
The electric resistance disappeared at about 90 K, and a sharp superconducting transition was observed. Also, when observed by an X-ray diffraction method, it was found that on the metal oxide material of the present invention, Gd ₂ Cu _0.9 Pd _0.1 O ₄ ,
It was confirmed that NdBa ₂ Cu ₃ O _y, which is a copper oxide superconductor, was formed oriented in the c-axis direction. From the above results, the metal oxide material of the present invention was used as a substrate for a superconducting device. It can be seen that it is excellent as a buffer layer. Similar results were obtained even when Al ₂ O ₃ or the like was used as the substrate. Further, when other metal oxide materials of the present invention different from the above are used, similar results can be obtained by appropriately selecting the copper oxide superconductor to be joined with the metal oxide material in consideration of its lattice constant and melting point. Was obtained.

Example 13 The metal oxide material of the present invention has a lattice constant (a) of 3.9.
A material having a composition of Gd ₂ Cu _0.9 Pd _0.1 O ₄ at 0Å was pressure-molded by a uniaxial pressure device and sintered to form a polycrystal oriented in the c-axis direction, which was used as a substrate. Got ready. Then, in the same manner as in Example 12, NdBa ₂ Cu ₃ O _y , which is a copper oxide superconductor, was deposited on this substrate to a thickness of 4000 Å.
Were laminated. When the superconducting film of the superconducting film prepared as described above was evaluated for superconducting properties by changing the electric resistivity with temperature, the electric resistance disappeared at about 89 K and a sharp superconducting transition was observed. Also, when evaluated by X-ray diffraction, N
It was confirmed that dBa ₂ Cu ₃ O _y was formed in the c-axis direction. From the above results, it can be seen that the metal oxide material of this example is excellent as a substrate for a superconducting element. Further, when another metal oxide material of the present invention different from the above is used, the same result can be obtained by appropriately selecting the copper oxide superconductor to be joined to the copper oxide superconductor in consideration of its lattice constant and melting point. Was obtained.

[0028]

As described above, according to the present invention, the following excellent effects can be obtained. (1) The metal oxide material of the present invention has a novel composition ratio and has a lattice constant of superconductor Ln _2−x Ce _x CuO.
_y (Ln = Pr, Nd, Sm, Eu, Gd) is almost the same, and the electric conduction characteristics are semiconductor characteristics below the liquid nitrogen temperature, and have sufficiently high electric low efficiency at the liquid helium temperature. It is particularly suitable as a tunnel barrier layer. (2) The element used in the metal oxide material of the present invention is a superconductor Ln _2−x Ce _x CuO _y (Ln = Pr, N
d, Sm, Eu, Gd) and the decomposition temperature is higher than that of the superconductor, the diffusion of elements into the respective layers is small when the two are joined, and the influence on the superconducting properties Is extremely small. (3) The metal oxide material of the present invention and Ln that is a superconductor
_2-x Ce _x CuO _y (Ln = Pr, Nd, Sm, Eu, G
The bondability of d) is good, which makes it possible to provide a superconducting junction element such as a Josephson element having excellent characteristics. (4) By using the metal oxide material of the present invention as a buffer layer of a substrate for a superconducting device or a substrate, it becomes possible to provide a superconducting device having excellent superconducting properties.

[Brief description of drawings]

FIG. 1 Nd _1.85 Ce _0.15 Cu _0.9 Pd _0.1 O of Example 2
₄ shows the X-ray diffraction pattern of ₄ . The X-ray source used for the measurement is CuKα ray.

FIG. 2 Nd _1.85 Ce _0.15 Cu _0.9 Pd _0.1 O of Example 2
It is a graph of the temperature dependence of the electrical resistivity of _4.

FIG. 3 is a schematic view showing a laminated Josephson element showing an example of the superconducting junction element of the present invention obtained in Example 11.

4 is an example of current-voltage characteristics of a laminated Josephson element showing an example of the superconducting junction element of the present invention obtained in Example 11. FIG.

[Explanation of symbols]

1: SrTiO ₃ substrate 2: first superconducting layer 3: barrier layer 4: second superconducting layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location C30B 29/22 501 M 9261-4G H01B 12/06 ZAA 13/00 565 D H01L 39/22 ZAA A

Claims (5)

[Claims] 1. The composition formula is Ln _2-a Ce _a Cu _1-b Pd _b O.
_{In the} metal oxide material represented by _4-C , 0 ≦ a ≦ 0.
2, 0.025 ≦ b ≦ 0.5 and 0 ≦ c ≦ 0.1, and Ln is La, Pr, Nd, Sm, Eu and Gd.
A metal oxide material comprising one or more elements or atomic groups selected from the group consisting of 2. A crystal structure of Ln ₂ CuO ₄ (Ln is Pr,
The metal oxide material according to claim 1, which has the same T ′ structure as one or more elements or atomic groups selected from the group of elements consisting of Nd, Sm, Eu, and Gd. 3. A superconducting junction obtained by joining a superconductor having a T ′ structure and a metal oxide material according to claim 1 or 2, which is a non-superconductor. element. 4. Forming the metal oxide material according to claim 1 or 2 on an arbitrary substrate, and forming a superconducting element using a copper oxide superconductor on the metal oxide material. A substrate for a superconducting device, which has been made possible. 5. A superconducting device using the single crystal or molded body of the metal oxide material according to claim 1 or 2 in a desired shape and using a copper oxide superconducting material on the metal oxide material. A substrate for a superconducting device, which is capable of forming a.