JP2635355B2 - Method for producing oxide superconductor film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for forming a thin film that becomes a superconductor at an absolute temperature of 110 K or higher which is higher than liquid nitrogen temperature (absolute temperature 77 K).

[Conventional technology]

Since it was known in 1987 that oxide compounds given by the chemical formula LnBa ₂ Cu ₃ O _x (Ln: Y, a lanthanoid element) have a superconducting transition temperature of 90 ° C. (−183 ° C.), The search for oxide superconducting materials with higher superconducting transition temperatures is active.

On January 22, 1988, it was reported that BiSrCaCu ₂ O _x , an oxide compound composed of Bi, Sr, Ca, and Cu, had a superconducting transition at absolute temperatures of 105K and 75K.

[Problems to be solved by the invention]

The above-mentioned substances have compounds (phases) having different transition temperatures coexisting (or coexisting), and identification of a compound having a high-temperature transition (105 K) phase has not been solved. Since this report, no information has been reported on the composition of compounds having an absolute temperature exceeding 105K.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Bi-Sr-Cu-O compound having a superconducting transition temperature exceeding an absolute temperature of 105 K, which is indispensable for electronics applications, and a method for producing a thin film having a high superconducting transition temperature.

[Means for solving the problem]

The method for producing an oxide superconductor film of the present invention is a method for producing an oxide superconductor film in which electric resistance becomes zero at a temperature of 100 K or more, wherein Cu, Bi, CaF ₂ , and SrF ₂ are formed on a substrate by vapor deposition. laminated on depositing a stack of a thickness of less than 2500 Å, represented in this composition the laminate was heat-treated in oxygen formula _{Bi u Sr v Ca w Cu x} O y,
It is characterized in that an oxide superconductor film having a ratio of u: v: w: x of 0.7 to 1.1: 1.4 to 3.1: 0.85 to 1.1: 2 is prepared.

Furthermore, the present invention production process is represented by a composition formula _{Bi u Sr v Ca w Cu x} O y, u: b: w: ratio of x 0.7 to 1.1: 1.4 to 3.1: 0.85 to 1.1: oxides than a 2 The step of depositing a conductor with a thickness of 2500 mm or less on a substrate and performing a heat treatment is repeated a plurality of times.

(Operation)

Bi-Sr-Ca-Cu-O quaternary (quinary including oxygen) -based oxide, Bi: Sr: Ca: Cu = 0.7-1.1: 1.4-3.1: 0.85-1.1:
A compound having a ratio of 2.0 is a new substance exhibiting a superconducting transition at an absolute temperature of about 110 K. A thin film having this transition temperature can be formed by setting the thickness of one deposition to 2500 ° or less.

〔Example〕

 Hereinafter, the present invention will be described in detail with reference to the drawings.

Example 1 Metal Cu, Bi and fluoride CaF by electron beam evaporation
_2, the substrate MgO (100) in order of SrF ₂ on the surface Cu: Bi: Ca: Sr = 2: 1:
1: about 1.5, Cu about 90Å, Bi about 130Å, CaF ₂ about 160
Å, SrF ₂ about 520F were laminated. However, this film thickness is an index by a film thickness meter during deposition, and the actual total film thickness was 775 °. This laminated film was subjected to a heat treatment in oxygen at about 870 ° C. for 1 hour to decompose and oxidize the fluoride to obtain an oxide compound thin film. When the composition of the oxide film after the heat treatment was analyzed by X-ray fluorescence analysis, the ratio of each element was Cu: Bi: Ca: Sr = 2.0: 1.0.
8: 1.08: 1.43. Analysis error ± 10 for each element
% Or less.

FIG. 1 shows the result of measuring the temperature change of the electrical resistance of this oxide film by the ordinary four-terminal method. It shows superconductivity where the electrical resistance drops sharply at about 110K and becomes zero at about 100K. The present inventor has found that this composition is an oxide compound having a transition temperature of 110K.

Example 2 Metal Cu, Bi, fluoride CaF ₂ , S by electron beam evaporation
Approximately 170 mm of Cu and approximately 240 Bi on the MgO (100) surface of the substrate in the order of rF ₂
Å, about 300 ₂ of CaF _{2 and} about 910Å of SrF ₂ were deposited. This thickness is a conversion based on the original film thickness, and the actual total thickness was 1993 mm. This laminated film was heat-treated in oxygen at about 870 ° C. for 1 hour to decompose the fluoride and oxidize it at the same time to obtain an oxide compound. The composition analysis of this oxide film by X-ray fluorescence analysis revealed that Cu: Bi: Ca: Sr = 2.0: 0.94: 0.88: 1.4. Analysis errors are within ± 10% for each element.

FIG. 2 shows the result of measuring the temperature change of the electrical resistance of the oxide film by the ordinary four-terminal method. It has a sharp drop in electrical resistance at about 110K and is a superconductor with a 110K transition temperature. However, it was found that the resistance was about 108K or less and the resistance became zero at 90K. Since the composition of the oxide film in Example 1 is almost the same within the analysis accuracy, it can be said that the setting of the electric resistance is a difference in the film thickness. That is, the total film thickness is about 2.6 times as large as that of Example 1. As a result of this difference in film thickness affecting the composition uniformity in the film, the electric resistance is settled. This is also clear from the third embodiment.

Example 3 Metal Cu, Bi, fluoride CaF ₂ , SrF by electron beam evaporation
Cu: 255 so that Cu: Bi: Ca: Sr = 2: 1.5: 1: 1 in the order of _2.
Å, Bi about 360Å, CaF ₂ about 450Å, SrF ₂ about 1350Å were deposited. This thickness was converted from a thickness gauge during deposition, and the actual total thickness was 2814 °. About 870 in this laminated film and oxygen
By heat treatment at 1 ° C. for 1 hour, the fluoride was decomposed and an oxide compound was formed at the same time. In the composition analysis by X-ray fluorescence analysis, Cu: Bi: Ca: Sr = 2.0: 0.93: 1.11: 1.48, and the analysis error is ± 10% or less for each element.

FIG. 3 shows the result of measuring the temperature change of the electrical resistance of this oxide film by the ordinary four-point method. Although the resistance drops sharply at about 110K, the resistance remains below about 107K and becomes zero at about 60K, indicating a two-step change.

Since the compound of this oxide film is the same as the compounds of Examples 1 and 2 within the accuracy of the composition analysis, the film showing the two-stage resistance change, in other words, the film showing a superconducting transition only at 110K is the initial deposited film thickness. It is nothing but the difference. That is, as compared with the case of Example 2, the Ca concentration was higher in the measurement accuracy (error ± 10
%), But the film thickness is 1.4 times thicker.

FIG. 4 shows X-ray diffraction images of the oxide films obtained in Example 1 and Example 3. Curve (a) is a film having a film thickness of 775 ° obtained in Example 1, and curve (b) is a film having 2814 ° obtained in Example 3.
3 is a diffraction pattern of the film of FIG. As shown in FIG. 4, each sample shows the same pattern, indicating that the oxide compound of BrSr _1.5 CaCu ₂ O _x (x is unknown) is oriented on the substrate, and almost the same structure is formed. . However, in the case of the thick film in the third embodiment, a slight peak at 2θ = 32 ° appears, which can be said to be the cause of the low-temperature electrical resistance. It is concluded that when the film thickness is less than 2500 ° C., the superconductor has a transition temperature of about 110 K, which is found by the present inventors together with the composition.

Example 4 After the lamination deposition (initial film thickness 1993 °) performed in Example 2, an oxidizing heat treatment was similarly performed, and after confirming a superconducting transition of about 110K, Bi _0.94 of about 2000 ° was again formed to have the same composition.
Deposit Sr _1.40 Ca _0.88 Cu _2.0 O _x , also in oxygen at 870 ℃
A time oxidation heat treatment was performed.

This oxide film has a thickness of about 4000 °. FIG. 5 shows the result of measuring the temperature change of the electric resistance by the ordinary four-terminal method.
In Example 3 having a film thickness of about 2800 mm, the temperature change of the electric resistance became two steps, and the resistance became zero at about 70 K. However, as shown in this example, the film thickness of about 4000 mm was repeated by repeatedly depositing and heat-treating a film having a thickness of 2000 mm. At about 110K, the resistance dropped sharply at about 110K, and a zero-resistance superconductor thick film was obtained at about 95K. That is, by repeating the deposition and heat treatment of a film of 2500 °
A film having a transition temperature is obtained. It is clear from the results of Examples 1 to 3 that the film thickness should be 2500 ° or less, and it can be easily judged from the results of Examples 1 to 3 that the thinner the film, the smaller the electrical resistance at the superconducting transition.

Example 5 Metal Cu, Bi and fluoride CuF ₂ ,
SrF ₂ was deposited on the MgO (100) surface of the substrate in this order. The film thickness is 2006
Was Å. This film was heat-treated in oxygen at about 870 ° C. for 1 hour to form an oxide thin film. FIG. 6 shows a change in electricity temperature of this film measured by the four probe method. At about 110K, the electric resistance sharply drops, and at about 108K, the electric resistance becomes zero, indicating superconductivity.

The oxide superconductor of the present invention has a center composition of Bi _1.0 Sr _1.5 Ca _1.0 Cu _2.0 O _y , and when the atomic ratio of Bi, Sr, Ca and Cu is 2, Cu is 0.7 to 1.1, Sr Is 1.4 to 3.1 and Ca is 0.85 to 1.1. If the composition is out of this range, good superconducting properties cannot be obtained.

In the example, CaF ₂ , S
Although rF ₂ was used, metallic Ca and Sr may be used, and the present invention is not limited or limited to the film deposition method. In the present invention, the oxide having the above-described composition has a thickness of 25%.
A high superconducting transition temperature can be obtained by forming an oxide superconductor film by depositing on a substrate at a temperature of 00 ° or less or by repeating deposition and heat treatment of a film having a thickness of 2500 ° or less.

〔The invention's effect〕

As described above, Bi-Sr-Ca-Cu-O quaternary (quinary when oxygen is included) -based oxide, Bi: Sr: Ca: Cu = 0.7 to 1.1:
Compounds having a ratio of 1.4 to 3.1: 0.85 to 1.1: 2.0 have an absolute temperature of about
It is a new substance that exhibits a superconducting transition at 110K. A thin film that maintains this transition temperature can be realized by reducing the thickness of one deposited film to 2500 ° or less. As a result, since the transition temperature is at least 30 degrees higher than the liquid nitrogen temperature 77, various electronic devices that can operate stably at 77K can be realized by the thin film having this composition ratio.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a temperature change of electric resistance of a Bi _1.08 Sr _1.43 Ca _1.08 Cu _2.0 O _x thin film having an initial film thickness of 775 mm. FIG. 2 is a Bi _0.94 Sr _1.40 Ca _0.88 Cu _2.0 O _x film having an initial film thickness of 1993 mm.
Characteristic diagram showing the temperature change of the electric resistance of the thin film, Bi _0.95 of Figure 3 is the initial film thickness _{2814Å Sr 1.48 Ca 1.10 Cu 2.0 O} x
FIG. 4 is a characteristic diagram showing a change in electric resistance of the thin film with temperature, and FIG.
X-ray diffraction diagram of the superconducting thin film, FIG. 5 is a characteristic diagram showing temperature change of electric resistance of a thin film having a thickness of about 4000 行 っ obtained by repeating deposition and heat treatment, and FIG. 6 is Bi _0.92 Sr _3.08 Ca of 2006 膜厚 thickness. FIG. 4 is a characteristic diagram showing a temperature change of electric resistance of a _1.04 Cu _2.0 O _x thin film.

Claims (2)

(57) [Claims] 1. A method for producing an oxide superconductor film having an electric resistance of zero at a temperature of 100 K or more, comprising the steps of: depositing Cu, Bi,
Laminate CaF ₂ and SrF ₂ on a substrate to deposit a laminate having a thickness of 2500 mm or less.
_u Sr _v Ca _w Cu _x O _y , where the ratio of u: v: w: x is 0.7-1.1: 1.4-
3.1: A method for producing an oxide superconductor film, comprising preparing an oxide superconductor film having a ratio of 0.85 to 1.1: 2. 2. The method for producing an oxide superconductor film according to claim 1, wherein the step of depositing the laminate with a thickness of 2500 ° or less on a substrate and performing a heat treatment is repeated a plurality of times.