JP2501035B2 - Superconducting thin film - Google Patents

Description

The present invention relates to a superconducting thin film. More specifically, the present invention relates to a superconducting thin film having not only a high superconducting critical temperature but also a high critical current density and a uniform composition.

2. Description of the Related Art The superconducting phenomenon, which is said to be a phase transition of electrons, is a phenomenon in which the electric resistance of a conductor becomes zero under certain conditions and shows perfect diamagnetism.

In the field of electronics, which is a typical application field of superconducting phenomena, various superconducting elements have been proposed and developed. A typical example is an element utilizing the Josephson effect in which quantum effects appear macroscopically by an applied current when superconducting materials are weakly joined. In addition, the tunnel junction type Josephson element is expected as an extremely high speed and low power consumption switching element because the energy gap of the superconducting material is small. Furthermore, since the Josephson effect with respect to electromagnetic waves and magnetic fields appears as an accurate quantum phenomenon, it is expected that the Josephson device will be used as an ultra-sensitive sensor for magnetic fields, microwaves, radiation, etc. In ultra-high-speed computers, the power consumption per unit area has reached the limit of cooling capacity, so the development of superconducting elements has been demanded, and as the degree of integration of electronic circuits increases, there is no current loss It is desired to use a superconducting material as a wiring material.

However, despite various efforts, the superconducting critical temperature Tc of the superconducting material could not exceed 23K of Nb ₃ Ge for a long time.

However, in 1986, when Bednotz and Neuler et al. Discovered a composite oxide superconducting material having a high Tc, the possibility of high temperature superconductivity opened up greatly (Bednorz, Muller, “Z, Phys. B64 (1986) 189 ").

So far, it is already known that the composite oxide-based ceramic material exhibits superconducting properties.For example, in U.S. Pat.No. 3,932,315, Ba-Pb-Bi-based composite oxides have superconducting properties. It is stated that it indicates,
Further, JP-A-60-173,885 describes that a Ba-Bi-based composite oxide exhibits superconducting properties. However, the Tc of the composite oxide known so far is 10 K or less, and the only way to cause superconductivity is to use liquid helium (boiling point 4.2 K).

The oxide superconductor discovered by Bednots and Mueller is (La, Ba) ₂ CuO ₄ , and this oxide superconductor is known as K ₂ NiF ₄ type oxide. Although its crystal structure is similar to that of superconducting oxide, its Tc is about 30K, which is dramatically higher than that of conventional superconducting materials.

Furthermore, in February 1987, Chu et al. Discovered a Ba—Y-based complex oxide having a critical temperature of 90K class. This Ba-Y type composite oxide called YBCO is Y ₁ Ba ₂ C
It is a composite oxide represented by u ₃ O _7-X .

The Bi-Sr-Ca-Cu-based and Tl-Ba-Ca-Cu-based composite oxides that were subsequently discovered not only have a Tc of 100 K or more, but are also chemically stable, and are similar to YBCO and the like. There is little deterioration with time of superconducting properties.

The discovery of these new composite oxide-based superconducting materials has dramatically increased the possibility of realizing high-temperature superconductors.

Oxygen defects in the crystal play a major role in the superconducting properties of these complex oxide superconductors. That is, if the oxidation defects in the crystal are not proper, Tc is low, and the difference between the onset temperature and the temperature at which the resistance is completely zero becomes large.

Conventionally, when the above-mentioned composite oxide superconductor thin film is produced, is it formed by physical vapor deposition such as a sputtering method using an oxide generated by sintering as a vapor deposition source, and then heat treatment is performed in an oxygen atmosphere? , The plasma was exposed to oxygen plasma.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention The above-mentioned composite oxide superconductor material has a drawback that its superconducting property is apt to be deteriorated particularly when it is thinned. In particular, the superconducting thin films that have been announced so far cannot be actually used as devices because the critical current density (Jc) is as low as several hundred A / cm ² .

Therefore, an object of the present invention is to solve the problems of the above-mentioned conventional techniques, have a high critical temperature Tc, and have a practical critical current density Jc, and a thin film of a superconducting material having a uniform composition and structure. To provide.

Means for Solving the Problems Various experiments for solving the above-mentioned problems, as a result of repeating examination, the present invention has been completed, and according to the present invention, Y, La, Gd, Dy, Ho, Er, Tm, Yb, Lu, Nd,
In a thin film of a complex oxide superconductor containing at least one element Ln selected from the group consisting of Sm and Eu, Ba, and Cu, the thin film is formed by a c-axis oriented single crystal or polycrystal. A superconducting thin film is provided which is characterized by being constituted.

The thin film of the present invention is a single crystal (100) having a lattice constant close to the lattice constant of the a-axis or the b-axis of Ln ₁ Ba ₂ Cu ₃ O ₇ (where Ln is an element as defined above) crystal. It is preferably composed of a c-axis oriented single crystal or polycrystal formed on a substrate whose surface is a film formation surface.

Furthermore, the superconducting thin film of the present invention is the reflection intensity of the strongest reflecting surface of the powder pattern of the oxide superconductor made of LnBa ₂ Cu ₃ O _7.
The ratio I _00n / I _MAX of the reflection intensity I _00n of I _MAX and the (00n) plane [where n is an integer], and the X-ray diffraction of the superconducting thin film of the crystal plane having the same index as the strongest reflection surface of the powder pattern. reflection intensity J _MAX and said superconducting thin film (00n) plane in the pattern [where, n is an integer] and the ratio J _00n / J _MAX the reflection intensity J _00n of, but following _{relations: J 00n / J MAX ≧ 2} ( I _00n / I _MAX ).

Specifically, in the X-ray diffraction pattern, (002)
The reflection intensities of the plane, the (003) plane, the (005) plane, and the (006) plane are more than twice the reflection intensity of the (111) plane and the (112) plane.

The above-mentioned c-axis orientation is not limited to the orientation in which the c-axis is perpendicular to the film surface, but also includes orientation with a predetermined angle.

Action The superconducting thin film according to the present invention is composed of Y, La, Gd, Dy, Ho and E.
In a thin film formed of a complex oxide superconductor containing Ba, Cu, and an element Ln selected from the group consisting of r, Tm, Yb, Lu, Nd, Sm, and Eu, this thin film is c It is characterized by being composed of an axially oriented single crystal or polycrystal.

A thin film made of the above complex oxide superconductor is generally Ln ₁ Ba ₂ Cu ₃ O _7-X (where Ln is Y, La, Gd, Dy, Ho, Er, Tm, Yb, Lu,
A thin film of a complex oxide represented by 0 <x <1, which represents an element selected from the group consisting of Nd, Sm, and Eu.

 Specifically, the following thin films of complex oxides may be mentioned.

Y ₁ Ba ₂ Cu ₃ O _7-X , La ₁ Ba ₂ Cu ₃ O _7-X , Gd ₁ Ba ₂ Cu ₃ O _7-X , Dy ₁ Ba ₂ Cu ₃ O _7-X , Ho ₁ Ba ₂ Cu ₃ O _7-X , Er ₁ Ba ₂ Cu ₃ O _7-X , Tm ₁ Ba ₂ Cu ₃ O _7-X , Yb ₁ Ba ₂ Cu ₃ O _7-X , Lu ₁ Ba ₂ Cu ₃ O _7-X , Nd ₁ Ba ₂ Cu ₃ O _7-X , Sm ₁ Ba ₂ Cu ₃ O _7-X , Eu ₁ Ba ₂ Cu ₃ O _7-X , where x is a number satisfying 0 <x <1 The oxide thin film is a thin film having a perovskite crystal structure having oxygen deficiency.

The superconducting thin film of the present invention has the following features,
It can be easily distinguished from the conventional one. First, the following terms are defined.

I _MAX : reflection intensity of the strongest reflection surface of the powder pattern of the complex oxide crystal represented by LuBa ₂ Cu ₃ O ₇ , I _00n : reflection intensity of the (00n) plane of the above crystal (where n is an integer), J _MAX : reflection intensity of a crystal plane having the same index as the strongest reflection plane of the powder pattern in the X-ray diffraction pattern of the complex oxide thin film according to the present invention, J _00n : (00n) plane of the thin film [where n is an integer ] The reflection intensity of.

The thin film composed of the complex oxide superconductor according to the present invention has an X-ray diffraction pattern having the following features.

(1) Ratio I _00n / I _MAX and ratio J between the above reflection intensities
_00n / J _MAX is the following relationship: J _00n / J _MAX ≧ 2 satisfies the _{(I 00n / I MAX) (} 00n) plane there is at least one.

Specifically, the (00n) planes above are (002) planes, (00
3) plane, (005) plane and (006) plane.

(2) The reflection intensities of the (002) plane, the (003) plane, the (005) plane, and the (006) plane in the X-ray diffraction pattern of the complex oxide thin film according to the present invention are the same as those of the (111) plane and the (112) plane. It is more than twice the reflection intensity.

The thin film of the complex oxide superconductor can be generally formed by using a physical vapor deposition method such as a sputtering method such as magnetron sputtering. In particular, a thin film produced by magnetron sputtering has excellent superconducting properties in terms of crystal structure, oxygen deficiency state and the like. In the case of several films of the above composite oxide, the oxygen deficiency state in the crystal has a great influence on its superconducting property, and therefore, in order to properly control the amount of oxygen deficiency in the crystal, it is necessary to form a thin film with an appropriate oxygen content. It is preferable to carry out in a containing atmosphere.

The thin film of the oxide superconductor represented by Ln ₁ Ba ₂ Cu ₃ O _7-X prepared by physical vapor deposition has a high Tc of about 90 K, but the thin film prepared by the conventional method is superconducting. Critical current density Jc
Was small and was a big problem in practical use. One of the reasons is that the value of the superconducting critical current density of the thin film of the complex oxide superconductor has crystal anisotropy. That is, in the crystal of the oxide, the superconducting critical current density in the direction parallel to the plane determined by the a-axis and the b-axis of the crystal is extremely large, but the superconducting critical current density in the other directions is small. In the conventional superconducting thin film, since the crystal anisotropy of the superconducting critical current density was not taken into consideration, the directions of the crystals were not aligned, and therefore the superconducting critical current density was small.

The present invention solves this problem by arranging the c-axis orientation of the crystals of the above-mentioned composite oxide superconductor that constitutes the thin film. That is, in the superconducting thin film of the present invention,
By making the c-axis orientation constant as described above,
The superconducting critical current density in the direction parallel to the plane defined by the a-axis and the b-axis of the crystal can be made extremely large. Therefore, if the direction in which the superconducting critical current density increases is made coincident with the direction in which the current flows, an extremely large superconducting critical current can flow in that direction.

The above-mentioned composite oxide forming the thin film is preferably a single crystal, but may be a polycrystal. Also, the above c
The axis is generally oriented in a direction perpendicular to the film surface, and thus the surface of the substrate, but the present invention is not limited to the one in which the c-axis is oriented perpendicularly, and is oriented at a predetermined angle. The case is also included. In particular, the c-axis can be oriented parallel to the film surface, and thus the surface of the substrate, to increase the current density in the depth direction of the film. In which direction the c-axis is oriented is determined by the substrate, more precisely, the characteristics of the film-forming surface of the substrate.

When the c-axis is oriented in a direction perpendicular to the film surface, the substrate should be a-axis and / or b-axis close to the lattice constant of a-axis and / or b-axis of the complex oxide crystal to be formed. A single crystal with a lattice constant of is used.

In a particularly preferred embodiment of the present invention, the above Ln ₁ Ba ₂ Cu ₃ O
_A close to the lattice constant of the a-axis or the b-axis of the crystal represented by ₇
On an arbitrary single crystal substrate with a lattice constant of axis or b axis,
By using the (100) plane as a film formation surface to form a thin film composed of the above c-axis oriented single crystal or polycrystal, the current density in the direction parallel to the film formation surface of the substrate is increased. I am trying to become.

On the contrary, when the c-axis is oriented in the direction parallel to the film surface, the (110) plane of the single crystal substrate may be used as the film formation surface.

Substrates satisfying this condition include MgO, SrTiO ₃ and Al.
₂ O ₃ , sapphire, SiO ₂ , quartz, YSZ (yttrium stabilized zirconia) and ZnO can be selected. In particular, it is preferable to select MgO and SrTiO ₃ whose coefficient of thermal expansion is close to that of the thin film in order to prevent unnecessary stress which may damage the thin film during sputtering and heat treatment.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following are merely Examples, and it goes without saying that the technical scope of the present invention is not limited by the following disclosure.

Example 1 Y ₂ O ₃ , BaO and CuO were mixed at an atomic ratio of Y: Ba: Cu of 1: 2.15: 3.2.
Were weighed and baked at 900 ° C. in the atmosphere. A thin film was formed on a magnesium oxide single crystal substrate by using high-frequency magnetron sputtering with a powder obtained by crushing the obtained fired body as a target. The film forming conditions are as follows.

Total pressure: 2 × 10 ^-2 Torr O ₂ / Ar: 0.16 (pressure ratio) Substrate: MgO (100) face Substrate temperature: 720 ° C Thus, a thin film with a thickness of 1000Å was obtained. This thin film was heated to 700 ° C in an oxygen stream and its temperature was raised to 2
It was kept for 4 hours and then cooled to room temperature at a cooling rate of 3 ° C / min.

The obtained thin curtain is Y ₁ Ba ₂ Cu ₃ O _7-X (where x is a number satisfying 0 <x <1 based on the following measurement results and having an orientation in which the c-axis is perpendicular to the MgO substrate. It was a polycrystalline film considered to be).

FIG. 1 is an X-ray diffraction pattern of the thin film produced as described above. The X-ray diffraction pattern was obtained using Cu thin film X-ray diffractometer manufactured by Rigaku Denki Co., Ltd. and Kα ray. On the other hand, FIG. 2 is an X-ray diffraction pattern of the oxide superconductor powder Y ₁ Ba ₂ Cu ₃ O ₇ .

In Fig. 2, the index of the crystal plane showing the strongest reflection intensity is (10
3), the (110) plane.

The reflection intensity of the strongest reflection surface of the powder pattern of the above oxide superconductor is I _MAX , and the reflection intensity of the (00n) surface [where n is an integer] is I _00n and has the same index as the strongest reflection surface of the powder pattern. The reflection intensity of the crystal plane in the X-ray diffraction pattern of the superconducting thin film of this embodiment is J _MAX , and the reflection intensity of the (00n) plane [where n is an integer] of the superconducting thin film is J _00n .

The relationship shown in Table 1 is established between these reflection intensities.

The results shown in Fig. 1 and Table 1 show that the thin film of the above complex oxide is (002) face, (003) face, (005) face and (006) face.
It is shown that the reflection intensity on the surface satisfies the above relationship: J _00n / J _MAX ≧ 2 (I _00n / I _MAX ).

Further, it was found by electron diffraction that the crystal structure of the thin film of the above composite oxide was c-axis oriented perpendicular to the film formation surface.

Next, a sample with a width of 1 mm was cut out from the above-mentioned thin film having a thickness of 1000 L, and the critical temperature Tc and the critical current density Jc were measured.
The four-terminal method was used to measure the critical temperature. The results obtained are shown below.

Tc: 85K Jc: 150,000A / cm ² (Liquid nitrogen temperature) These measurement results show that the in-plane critical current can be obtained by orienting the crystal structure of the thin film of the complex oxide superconductor in the c-axis direction perpendicular to the film formation surface. It shows that the density Jc is greatly improved.

Example 2 Y ₂ O ₃ , BaO and CuO were weighed so that the atomic ratio of Y: Ba: Cu was 1: 2.0: 3.1 and calcined at 900 ° C. in the atmosphere. A thin film was formed on a single crystal substrate of strontium titanate by using high frequency magnetron sputtering with a powder obtained by crushing the obtained fired body as a target. The film forming conditions are as follows.

Total pressure: 2 × 10 ^-2 Torr O ₂ / Ar: 0.15 (pressure ratio) Substrate: SrTiO ₃ (100) surface Substrate temperature: 720 ° C Thus, a thin film with a thickness of 1000Å was obtained. This thin film was heated to 710 ° C. in the atmosphere, kept at that temperature for 24 hours, and then cooled to room temperature at a cooling rate of 3 ° C./min.

The obtained thin curtain is considered to be Y ₁ Ba ₂ Cu ₃ O _7-X (where x is a number satisfying 0 <x <1) having an orientation in which the c-axis is perpendicular to the substrate. It was a single crystal.

 The relationship between the reflection intensities is shown in Table 2 as in Example 1.

A sample with a width of 1 mm was cut out from the thin film having a thickness of 1000 Å, and the critical temperature Tc and the critical current density Jc were measured. The four-terminal method was used to measure the critical temperature Tc. The results obtained are shown below.

Tc: 86K Jc: 160,000 A / cm ² Effect of the Invention As described above, according to the present invention, it becomes possible to obtain a superconducting oxide thin film having Jc much higher than that of the conventional superconductor. Therefore, the present invention is applied to a field in which a superconductor is applied as a thin film element, for example, a switching element of Matisoo called Josephson element, a memory element of Anacker, and a superconducting quantum interferometer (SQUID). Then it is effective.

[Brief description of drawings]

1 shows the X-ray diffraction pattern of the thin film according to the present invention, and FIG. 2 shows the X-ray diffraction pattern of the powder of the oxide superconductor Y ₁ Ba ₂ Cu ₃ O ₇ .

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tetsuji Ueshiro 1-1-1 Kunyokita, Itami City, Hyogo Prefecture, Sumitomo Electric Industries, Ltd. Itami Works (56) Reference JP-A-63-282152 (JP, A) ) JP-A-63-277555 (JP, A)

Claims (12)

(57) [Claims] 1. A general formula: Ln ₁ Ba ₂ Cu ₃ O _7-X (where Ln is Y, La, Gd, Ho, Er, Tm, Yb, Nd, S.
An element selected from m, Eu and Lu, 0 <X <1
The reflection intensity J _00n of the X-ray diffraction pattern on the (00n) plane of the crystal (where n is an integer) is expressed by the following equation. Superconducting thin film characterized by satisfying: J _00n / J _MAX ≧ 2 (I _00n / I _MAX ) (where I _MAX is X-ray diffraction of complex oxide crystal represented by Ln ₁ Ba ₂ Cu ₃ O ₇₎ I _00n is the reflection intensity of the (00n) plane (where n is an integer) in the X-ray diffraction pattern of the complex oxide crystal, and J _MAX is the superconducting thin film. In the X-ray diffraction pattern of
J _00n is the reflection intensity of the crystal plane having the same index as the strongest reflection plane of the X-ray diffraction pattern of the complex oxide crystal, where J _00n is the (00n) plane of the crystal in the X-ray diffraction pattern of the superconducting thin film (however, n Is an integer) is the reflection intensity). 2. The superconducting thin film according to claim 1, wherein the superconducting thin film is a thin film formed by physical vapor deposition on a single crystal substrate. 3. The (00n) plane of the crystal of the superconducting thin film is (002)
The superconducting thin film according to claim 1 or 2, which is a plane, a (003) plane, a (005) plane, or a (006) plane. 4. The superconducting thin film is composed of a composite oxide containing Y, Ba and Cu and exhibits the X-ray diffraction pattern shown in FIG. The superconducting thin film according to item. 5. The superconducting thin film according to any one of claims 1 to 4, wherein the superconducting thin film is a single crystal. 6. The a-axis and / or b-axis lattice constant of a single crystal of a substrate is represented by the general formula: Ln ₁ Ba ₂ Cu ₃ O ₇ (where Ln is Y, La, Gd, Ho, Er, Tm. , Yb, Nd, S
m, Eu, and Lu, which is an element selected from the following), which is a value close to the lattice constants of the a-axis and / or the b-axis of the crystal of the complex oxide represented by:
5. The superconducting thin film according to any one of 5 above. 7. The crystal plane on which the superconducting thin film of the single crystal substrate is formed is the (100) plane.
The superconducting thin film according to any one of 1. 8. The crystal plane on which the superconducting thin film of the single crystal substrate is formed is the (110) plane.
The superconducting thin film according to any one of 1. 9. The single crystal substrate is composed of any single crystal selected from the group consisting of MgO, SrTiO ₃ , Al ₂ O ₃ , sapphire, SiO ₂ , quartz, YSZ and ZnO. 9. The superconducting thin film according to any one of claims 2 to 8. 10. The (002) plane, (003) plane, (005) plane and (006) of the crystal in the X-ray diffraction pattern of the superconducting thin film.
The superconducting thin film according to any one of claims 1 to 9, wherein the reflection intensity of the plane is twice or more the reflection intensity of the (111) plane and the (112) plane. 11. The superconducting thin film according to claim 1, wherein the c-axis of the crystal of the composite oxide of the superconducting thin film is oriented perpendicular to the surface of the substrate. . 12. The superconducting thin film according to any one of claims 1 to 10, wherein the c-axis of the crystal of the complex oxide of the superconducting thin film is oriented horizontally with respect to the surface of the substrate. .