JPH05319828A - Production of thin film superconductor - Google Patents

Abstract

PURPOSE:To produce a bismuth-contg. oxide superconducting thin film having a high critical superconduction temp. and a high critical current density. CONSTITUTION:A bismuth oxide (Bi2O3) block layer 21, a layer contg. CuO5 pyramids 23, a monoatomic layer 22 of one or more kinds of alkaline earth metals, a CuO2 plane 41, a monoatomic layer 22 of one or more kinds of alkaline earth metals and a layer contg. CuO5 pyramids 23 are successively laminated. Two layers laminated on each of both sides of the CuO2 plane 41, that is, the monoatomic layer 22 of one or more kinds of alkaline earth metals and the layer contg. CuO5 pyramids 23 form a perovskite type crystal structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a bismuth oxide superconducting thin film.

[0002]

2. Description of the Related Art A bismuth-based superconductor composed of at least bismuth element, one or more alkaline earth elements, copper and oxygen is a material having a superconducting critical temperature of 80 to 110 K and is expected to be used at liquid nitrogen temperature. ing. Further, it is known that when lead element is added to this, the critical temperature further rises by about 10K. It is considered that the reason is that the addition of lead facilitates the growth of the crystal phase having a high critical temperature. In order to apply this substance to an electronic device, it is essential to form a thin film, and a thin film of a bismuth superconductor has been formed by various methods.

For the bismuth-based superconducting thin film to which lead is not added, the maximum superconducting critical temperature is about 104K. [Kumiko Hirochi et al., Surface Science, Vol.9, 541-547
(1988)] In order to further raise the critical temperature, attempts have been made to add lead to the thin film. As this method, there is a method of obtaining a thin film containing lead from the beginning by heat treatment and a method of obtaining a thin film not containing lead by heat treatment in an atmosphere containing lead. In any case, since the formation of the superconducting phase is performed during the heat treatment involving lead, the thin film to be heat treated is preferably an amorphous, non-crystallized film formed at a low temperature. By this method, a bismuth-based superconducting thin film with a critical temperature of 105 K or higher has been obtained [Applied Physics Letters, Tanaka et al.
s), Vol.55, 1252-1254 (1989)].

[0004]

However, in the conventional method of this kind, the fine structure of the thin film is disturbed due to the growth of the crystal grains during the heat treatment, and the surface structure becomes rough. Therefore, although the critical temperature is high, the critical current is as low as tens of thousands of A / cm ² or less at the liquid nitrogen temperature (77K), which is a practical obstacle to the bismuth-based superconducting thin film.

An object of the present invention is to provide a method for producing a lead-added bismuth-based superconducting thin film having a high superconducting critical temperature and a large critical current density.

[0006]

According to a first aspect of the present invention, there is provided a perovskite including a block layer made of bismuth oxide and two layers of CuO ₅ pyramids facing each other through one atomic layer of at least one alkaline earth element. A type block layer is a method for producing a thin film superconductor in which a bismuth-based layered compound thin film having a main crystal structure alternately laminated is heat-treated in an atmosphere containing lead.

A second invention is a block layer made of bismuth oxide, a layer containing CuO ₅ pyramid-1 atomic layer of one or more alkaline earth elements-1 sheet of CuO ₂ face-1 or more alkaline species A thin film obtained by heat-treating a bismuth-based layered compound thin film having a main crystal structure in which a 1-atom layer of an earth element-a perovskite-type block layer of a layer containing a CuO ₅ pyramid are alternately stacked in this order in an atmosphere containing lead. It is a method of manufacturing a superconductor.

[0008]

According to the manufacturing method of the present invention, the thin film is crystallized into each structure from the time of formation, and only the electrical characteristics are improved during the heat treatment. In other words, the thin film crystallized from the beginning is
Almost no crystal growth occurs during heat treatment, and there is no deterioration of the microstructure due to the appearance of crystal grains. Therefore, the factor that impairs the critical current density is eliminated.

Although it has been conventionally considered that lead element is effective for the growth of a crystal phase of superconductivity, when a bismuth-based layered compound which has already been crystallized as in the present invention is heat-treated in an atmosphere containing lead, it has a The present inventors have confirmed that the conduction characteristics are improved. The reason for this is not clear, but it is thought that this is because the lead element replaced a part of the bismuth element during the heat treatment, which affected the electronic state of the superconductor.

[0010]

EXAMPLE In the manufacturing method of the present invention, a bismuth-based layered compound thin film is previously laminated on a substrate such as MgO single crystal which is used exclusively.

Examples of the method for forming the bismuth-based layered compound thin film include a high-frequency magnetron sputtering method, a molecular beam epitaxy method, an ordinary vapor-phase thin film forming method such as a resistance heating vapor deposition method, and the like. To control the layers layer by layer, the high frequency magnetron sputtering method is preferable.

The bismuth-based layered compound of the present invention has two crystal structures. One crystal structure is, for example (Fig. 2)
As shown in, the bismuth oxide (Bi ₂ O ₂ ) block layer 21 the layer containing CuO ₅ pyramid 23, the layer containing one or more alkaline earth element 1 atomic layer 22 CuO ₅ pyramid 23, stacked in this order. is doing. Of these, two Cu layers laminated on both sides of one or more alkaline earth element monoatomic layers 22
Three layers including the O ₅ pyramid 23 form a perovskite type crystal structure.

The other crystal structure is, for example, as shown in FIG. 4, a bismuth oxide (Bi ₂ O ₂ ) block layer 21, a layer containing a CuO ₅ pyramid 23, and one or more alkaline earth element monoatomic layers. 22 CuO ₂ surface 41 Layers containing one or more alkaline earth element monoatomic layers 22 CuO ₅ pyramids 23 are stacked in this order. Of these, CuO ₂ surface 41
Two layers, one layer of one or more alkaline earth element monoatomic layer 22 and one layer of CuO ₅ pyramid 23, each of which is laminated on both sides, form a perovskite type crystal structure.

Examples of alkaline earth elements applicable to such two crystal structures include Ca and Sr. Among them, it is preferable to use Ca and Sr in combination because good superconducting properties can be obtained. ..

The bismuth-based layered compound thin film thus formed in advance is heat-treated in an atmosphere containing lead.
The atmosphere containing lead is, for example, lead oxide (PbO) powder or lead-added bismuth-based ceramics (for example, Bi-Pb-
(Sr-Ca-Cu-O, etc.) powder on the bismuth-based layered compound thin film, or by supplying lead vapor onto the bismuth-based layered compound thin film, for example. Among these, the method of enclosing the powder, particularly the lead oxide powder and the bismuth-based layered compound thin film in the container has a simpler working process and is industrially superior.

The heat treatment temperature is preferably 700.degree. C. to 900.degree. C., and the zero resistance temperature is high especially when the temperature is 800.degree. C. to 870.degree. C. Specifically, for example, 90 in the crystal structure of FIG.
K or more, for example, 110 K or more in the crystal structure of FIG. 4, which is preferable. The heat treatment time depends on the film thickness of the bismuth-based layered compound thin film, the form of the atmosphere containing lead, and the like, but can be usually achieved in about 1 to 10 hours.

By heat-treating the thin film in the lead-containing atmosphere of the manufacturing method of the present invention, the superconducting critical temperature is increased by about 10 K as compared with the heat treatment in the lead-free atmosphere, and as a result, the critical temperature at the liquid nitrogen temperature is increased. The current density is also double to 10 times.

For a more detailed understanding of the present invention, the following description is given by way of detailed examples. Example 1 First, a crystallized bismuth-based layered compound thin film was formed. Sputtering was performed using a high-frequency magnetron sputtering device with a Bi-Sr-Ca-Cu-O baked oxide having a diameter of 100 mm as a target. The composition of the target was approximately Bi: Sr: Ca: Cu = 1.6: 1: 0.8: 1.5. 650
On a MgO single crystal (100) plane substrate heated to ℃, a sputtering discharge of 150 W was performed under a sputtering gas of a mixed gas of argon (50%) and oxygen (50%) of 0.5 Pa, and 2000 Å in about 30 minutes. A thin film of a certain degree was formed.
The X-ray diffraction pattern of the produced thin film is shown in FIG. As a result, it can be seen that a c-axis oriented bismuth-based layered structure having a c-axis length of 30.7Å was produced. The chemical composition of this thin film is approximately
Bi: Sr: Ca: Cu = 2: 2: 1: 2, and has a crystal structure shown in FIG. 2, that is, an oxide (Bi ₂ O ₂ ) whose main component is bismuth.
Block layer 21 consisting of and alkaline earth element 1 atomic layer 22
And a perovskite-type block layer 24 including two layers of CuO ₅ pyramids 23 facing each other via a layer structure alternately stacked.

This thin film was heat-treated in an atmosphere containing lead. A PbO powder and a thin film were placed in an alumina crucible, and the lid was closed and heated in air. When heated at 850 ° C. for 5 hours, a superconducting thin film having a zero resistance temperature of 95 K was obtained although no change in crystal structure was observed. The critical current density at 77K was 200,000 A / cm ² . Even if the same thin film is optimally heat-treated in a lead-free atmosphere, a zero resistance temperature of 83
K, because the critical current density is about 50,000 A / cm ² ,
It was confirmed that the method of the present invention is effective. The crystal structure of this thin film is not a structure having a high critical temperature in a bismuth-based superconductor, but it is effective for industrial production because it is easy to make a thin film with good reproducibility.

As a result of the examination of the heat treatment temperature,
In the case of heat treatment in the temperature range of 870 ° C, the zero resistance temperature is 90.
K or more was achieved and it was effective. Also, as a method for realizing an atmosphere containing lead, a bismuth-based superconductor containing lead is used.
A method in which Bi-Pb-Sr-Ca-Cu-O ceramics or powder itself is enclosed together with a thin film and heated is also effective. The simplest method was to enclose lead oxide and a thin film as in this example, and it was confirmed that the working process was simple and excellent in reproducibility.

(Example 2) Sputtering was performed using a high frequency magnetron sputtering device with a Bi-Sr-Ca-Cu-O calcined oxide having a diameter of 100 mm as a target. The composition of the target was approximately Bi: Sr: Ca: Cu = 1.6: 1: 1.6: 1.8. A thin film was formed on the MgO single crystal (100) plane substrate heated to 650 ° C. under the same conditions as in Example 1. The X-ray diffraction pattern of the produced thin film is shown in FIG. As a result, it is understood that a c-axis oriented bismuth-based layered structure having a c-axis length of 37.6Å was produced. This thin film has a chemical composition of approximately Bi: Sr: Ca: C.
u = 2: 2: 2: 3, and the crystal structure shown in FIG. 4, that is, the block layer 21 mainly composed of bismuth oxide (Bi ₂ O ₂ ) and one CuO ₂ surface 41 and Two layers of C facing each other through one atomic layer of alkaline earth element 22 on both sides
It has a structure in which the perovskite type block layers 42 including the uO ₅ pyramids 23 are alternately laminated.

This thin film was heat-treated in an atmosphere containing lead using PbO. When heated at 850 ° C for 5 hours,
No change in crystal structure, but zero resistance temperature 11
A superconducting thin film showing 5K was obtained. The critical current density at 77K was 1.5 million A / cm ² . Even if the same thin film is optimally heat-treated in a lead-free atmosphere, the zero resistance temperature 103
It was confirmed that the method of the present invention is very effective because the K and the critical current density are about 200,000 A / cm ² . The crystal structure of this thin film is a structure having a high critical temperature among bismuth-based superconductors, and is expected to be applied as a high-performance superconducting thin film. As a result of examining the heat treatment temperature, 8
In the case of heat treatment in the temperature range of 00 to 870 ° C., the zero resistance temperature of 110 K or higher was achieved, which was effective.

[0023]

As described above, the present invention comprises (1) a block layer made of bismuth oxide and two layers of CuO ₅ pyramids facing each other through one atomic layer of one or more alkaline earth elements. A perovskite-type block layer is formed by alternately heat-treating a bismuth-based layered compound thin film having a main crystal structure in a lead-containing atmosphere, or (2) a block layer made of bismuth oxide and a layer containing CuO ₅ pyramids. -1 or more and a perovskite type blocking layer of the layer including one atomic layer -CuO ₅ pyramid 1 atomic layer -1 single CuO ₂ planes -1 or more alkaline earth element of an alkaline earth element, this order Since the bismuth-based layered compound thin film having the main crystal structure laminated on is heat-treated in a lead-containing atmosphere,
The bismuth oxide superconducting thin film having a high superconducting critical temperature and a large critical current density was realized. In addition, the present invention has made it possible to synthesize a practically suitable superconducting thin film with good reproducibility and enables device application at a liquid nitrogen temperature, and the present invention has great industrial value.

[Brief description of drawings]

FIG. 1 is an X-ray diffraction pattern diagram of a Bi ₂ Sr ₂ CaCu ₂ O _x thin film in one example manufactured by the manufacturing method of the present invention.

FIG. 2 is an example of Bi ₂ produced by the production method of the present invention.
Crystal structure diagram of Sr ₂ CaCu ₂ O _x thin film

FIG. 3 is an X-ray diffraction pattern diagram of a Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _y thin film in one example manufactured by the manufacturing method of the present invention.

FIG. 4 is an example of Bi ₂ produced by the production method of the present invention.
Crystal structure diagram of Sr ₂ Ca ₂ Cu ₃ O _y thin film

[Explanation of symbols]

21 Bismuth oxide (Bi ₂ O ₂ ) block layer 22 Alkaline earth element 1 atom layer 23 CuO ₅ pyramid 24 Perovskite type block layer 41 CuO ₂ surface 42 Perovskite type block layer

Claims (4)

[Claims] 1. A block layer made of bismuth oxide, and 1.
A bismuth-based layered compound thin film having a main crystal structure in which two layers of CuO ₅ pyramid-containing perovskite-type block layers facing each other through one atomic layer of at least one alkaline earth element are alternately laminated, and containing lead. A method of manufacturing a thin film superconductor, characterized by performing heat treatment in an atmosphere. 2. A block layer made of bismuth oxide, and C
Layer containing uO ₅ pyramid-1 atomic layer of one or more alkaline earth elements-1 sheet of CuO ₂ face-1 atomic layer of alkaline earth element of 1 type or more-Perovskite type block of layer containing CuO ₅ pyramid And a bismuth-based layered compound thin film having a main crystal structure laminated in this order in a heat treatment in an atmosphere containing lead. 3. The method for producing a thin film superconductor according to claim 1, wherein the temperature of the heat treatment is 800 ° C. to 870 ° C. 4. The method for producing a thin film superconductor according to claim 1, wherein the alkaline earth element is a combination of strontium and calcium.