JPH01100021A - Superconducting thin film - Google Patents

Abstract

PURPOSE:To obtain a superconducting thin film having the high superconductivity critical temperature and critical current density and homogeneous composition, by constituting a thin film of a compound oxide based superconductor containing specific elements, Ba and Cu from a single crystal or polycrystal having c-axis orientation properties. CONSTITUTION:A superconducting thin film constituted of a single crystal or polycrystal having c-axis orienting properties in a compound oxide based superconductor thin film containing at least one of element Ln of Y, La, Gd, Dy, Ho, Er, Tm, Yb, Lu, Nd, Sm and Eu, Ba and Cu. This thin film is preferably constituted of the single crystal or polycrystal having c-axis orienting properties formed on a substrate having (100) face of a single crystal having a lattice constant close to that of the a- or b-axis of Ln1Ba2Cu3O7 crystals as a film-forming surface. This compound oxide based superconductor thin film can be normally formed by using a physical vapor deposition method, such as magnetron sputtering.

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION This invention relates to superconducting thin films. More specifically, the present invention relates to a superconducting thin film that not only has a high superconducting critical temperature but also has a high critical current density and has a uniform composition.

BACKGROUND OF THE INVENTION Superconductivity, which is said to be a phase transition of electrons, is a phenomenon in which the electrical resistance of a conductor becomes zero under certain conditions and exhibits complete diamagnetic properties.

In the field of electronics, which is a typical application field of superconductivity, various superconducting elements have been proposed and developed. A typical example is an element that utilizes the Josephson effect, in which a quantum effect appears macroscopically due to an applied current when superconducting materials are weakly bonded together. Further, tunnel junction type Josephson devices are expected to be extremely high-speed switching devices with low power consumption because the energy gap of the superconducting material is small. Furthermore, since the Josephson effect on electromagnetic waves and magnetic fields appears as a precise quantum phenomenon, it is expected that Josephson elements will be used as ultrasensitive sensors for magnetic fields, microwaves, radiation, etc. In ultra-high-speed electronic computers, the power consumption per unit area is reaching the limit of cooling capacity, so there is a demand for the development of superconducting elements.
Furthermore, as the degree of integration of electronic circuits increases, it is desired to use superconducting materials with no current loss as wiring materials.

However, despite various efforts, the superconducting critical temperature Tc of superconducting materials could not exceed 23K of Nb3Ge for a long period of time.

However, in 1986, a high T. With the discovery of complex oxide-based superconducting materials with 20% oxide, the possibility of high-temperature superconductivity has greatly opened up (Bednorz, Muller, "Z.

Phys, 864 (1986) 189”).

It has already been known that composite oxide-based ceramic materials exhibit superconducting properties; for example,
U.S. Pat. No. 3,932,315 describes Ba-Pb-B
It is stated that a Ba-B1 type composite oxide exhibits superconducting properties, and JP-A-60-173.885 states that a Ba-B1 type composite oxide exhibits superconducting properties. has been done. However, until now known complex oxide T. was below IOK, and the only way to cause superconductivity was to use liquid helium (boiling point 4.2'K).

The oxide superconductor discovered by Bednotes and Mineler et al. is (La, Ba) 2[:u O4, and this oxide superconductor is called the KJiFs type oxide, which was previously known as Although its crystal structure is similar to that of perovskite-type superconducting oxides, its Tc is about 30, which is significantly higher than that of conventional superconducting materials.

Furthermore, in February 1987, 90
A Ba-Y-based composite oxide has been discovered that exhibits a critical temperature of the same class. This Ba-Y based complex oxide called YBCO is a complex oxide represented by Y+BaaCuzOt-U.

Subsequently discovered Bi-3r-Ca-Cu and Tl-Ba-Ca-Cu composite oxides have a Tc of 100
Not only is it better than above, but it is also chemically stable.
There is little deterioration of superconducting properties over time, such as with YBCO.

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

Oxygen defects in the crystal play a major role in the superconducting properties of these composite oxide superconductors.

That is, if the oxygen defects in the crystal are not appropriate, Tc will be low and the difference between the onset temperature and the temperature at which the resistance becomes completely zero will also become large.

Conventionally, when producing the above composite oxide superconductor thin film,
After forming a film by physical vapor deposition such as a sputtering method using an oxide produced by sintering or the like as a deposition source, a treatment such as heat treatment in an oxygen atmosphere or exposure to oxygen plasma has been performed.

Problems to be Solved by the Invention The above-mentioned composite oxide superconductor material has a drawback in that its superconducting properties tend to deteriorate particularly when it is made into a thin film. In particular, in the superconducting thin films published so far, the critical current density (J
c) was so low as several hundred A/caf that it could not actually be used as a device.

Therefore, an object of the present invention is to solve the problems of the prior art described above, and to provide a thin film of a superconducting material having a high critical temperature Tc, a practical critical current density Jc, and a uniform composition and structure. It is about providing.

Means for Solving the Problems The present invention has been completed as a result of repeated various experiments and studies to solve the above-mentioned problems.According to the present invention, Y, La5Gd, Dy5flo, Br.

At least one element Ln selected from the group consisting of Tm5YbSLuSNd, Smjg and Eu, and B
There is provided a superconducting thin film of a composite oxide superconductor containing a and Cu, characterized in that the thin film is composed of a C-axis oriented single crystal or polycrystal.

The thin film of the present invention is a single crystal (100
) It is preferable that the film be formed of a C-axis oriented single crystal or polycrystal formed on a substrate with a film formation surface.

Furthermore, the superconducting thin film of the present invention is LnBazCusOt
The ratio of the reflection intensity INAX of the strongest reflection surface of the powder pattern of the oxide superconductor to the reflection intensity I OOn of the (00n) plane [where n is an integer] is I Oon / I NAX, and the strongest reflection surface of the powder pattern Reflection intensity JlIAX in the X-ray diffraction pattern of the superconducting thin film of crystal planes having the same index and the (00n) plane of the superconducting thin film [However,
n is an integer] reflection intensity J0°. The ratio of J'OOh /
JMAX and · have the following relationship: J 0OTh/ J ) IAX≧'2 (I oo-/
IMAX).

Specifically, in the X-ray diffraction pattern, the reflection intensity of the (002) plane, (003) plane, (005) plane, and (006) plane is twice or more that of the (111) plane and (112) plane. It is.

Note that the above-mentioned C-axis orientation is not limited to one in which the C-axis is oriented perpendicular to the film surface, but also includes one in which the C-axis is oriented at a predetermined angle.

Function The superconducting thin film according to the present invention has the following characteristics: y, t, a, Gd5D
y.

A thin film composed of a complex oxide superconductor containing an element Ln selected from the group consisting of HaSEr, Tm5Yb, LuSNd55m and Eu, Ba, and Cu, in which the thin film is a single crystal with C-axis orientation. Or, it is characterized by being composed of polycrystals.

The thin film made of the above composite oxide superconductor is generally LnlBa2Cu3C)y-x (where Ln is YSLaSGdSDy, Ho5Er
, Tm, Yb.

It represents an element selected from the group consisting of LuSNd55m and εU, and is a thin film of a composite oxide represented by Q<X<l.

Specifically, the following composite oxide thin films may be mentioned.

Y1Ba2Cu3o? -XSLa+Ba2Cua 0t
-x, Gd1Ba2Cu*Ov-x, Dy+
Ba2Cua O? -X, Ho1Ba2Cus 0t
-1% Or +Ba2Cua O? -x, TmtB
a2CusOt-1% Ytl+Ba*Cu5Ot-X
sLu! Ba2Cu30fu-xs NdIBazCu
30t-xSSm+Ba2Cua Ch-xsεulB
a2cu3C)v-xs (where X is a number satisfying Q<x<l) These composite oxide thin films are thin films having a perovskite crystal structure with oxygen vacancies.

The superconducting thin film of the present invention can be easily distinguished from conventional ones by having the following characteristics. First, the following terms will be defined.

I, lAX: LnBa2Cu=O? The reflection intensity of the strongest reflection surface of the powder pattern of the composite oxide crystal expressed by 100n: The reflection intensity of the (00n) plane of the above crystal [where n is an integer], JXAX: The reflection intensity of the composite oxide thin film according to the present invention 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, JOOn: Reflection intensity of the (OOn) plane of the thin film [where n is an integer].

A thin film composed of a composite oxide superconductor according to the present invention has an X-ray diffraction pattern having the following characteristics.

(1) Ratio ■ between the above respective reflection intensities. . 1%/I
NAX and ratio J. . , , /J, lAx have the following relationship: JOOr+ / JIIAX≧2(Ioo, /I
There is at least one (00n) plane that satisfies +ux ).

Specifically, the above (00n) plane is the (002) plane, (
003) plane, (005) plane, and (006) plane.

(2) In the X-ray diffraction pattern of the composite oxide thin film according to the present invention, the reflection intensity of the (002) plane, (003) plane, (005) plane, and (006) plane is the same as that of the (111) plane and (
112) It is more than twice the reflection intensity of the surface.

The thin film of this composite oxide superconductor can generally be formed using a physical vapor deposition method such as a sputtering method such as magnetron sputtering. In particular, thin films produced by magnetron sputtering have excellent superconducting properties in terms of crystal structure, oxygen deficiency state, etc. In the case of the above composite oxide thin film, the oxygen vacancy state in the crystal greatly affects its superconducting properties, so in order to properly control the amount of oxygen vacancies in the crystal, the formation of the thin film must be appropriately controlled. Preferably, this is carried out in an oxygen-containing atmosphere.

The above LnlBa2Cu30 made using physical vapor deposition method
The thin film of the oxide superconductor represented by 7-x exhibits a high Tc of about 90, but the thin film produced by the conventional method has a small superconducting critical current density Jc, which is a big problem in practice. One reason for this is that the value of the superconducting critical current density of the thin film of the above composite oxide superconductor has crystal anisotropy. That is, the oxide crystal has an extremely high superconducting critical current density in a direction parallel to the plane determined by the a-axis and b-axis of the crystal, but has a small superconducting critical current density in other directions. Conventional superconducting thin films did not take into account the crystal anisotropy of superconducting critical current density;
The crystals were not oriented in the same direction, and therefore the superconducting critical current density was small.

The present invention solves this problem by aligning the C-axis orientation of the crystals of the composite oxide superconductor forming the thin film. That is, in the superconducting thin film of the present invention, by uniformly aligning the C-axis orientation as described above, the superconducting critical current density in the direction parallel to the plane determined by the a-axis and b-axis of the crystal is extremely increased. be able to.

Therefore, if the direction in which the superconducting critical current density increases matches the direction in which the current flows, an extremely large superconducting critical current can be caused to flow in that direction.

The composite oxide constituting the thin film is preferably single crystal, but may be polycrystalline.

Furthermore, although the above C-axis is generally oriented in a direction perpendicular to the film surface, that is, the surface of the substrate, the present invention is not limited to those in which the C-axis is oriented perpendicularly; This also includes cases where it is oriented. In particular, the current density in the depth direction of the film can be increased by orienting the C-axis parallel to the film surface, that is, the surface of the substrate. The direction in which the C-axis is oriented is determined by the characteristics of the substrate, more precisely, the film-forming surface of the substrate.

When the C-axis is oriented in a direction perpendicular to the film surface, the substrate has an a-axis and/or b-axis close to the lattice constant of the a-axis and/or b-axis of the composite 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-mentioned Ln, Ba,
On any single crystal substrate having an a-axis or b-axis lattice constant close to the a-axis or b-axis lattice constant of the crystal represented by Cu or Ch, the (100) plane is used as the film formation surface. ,
By forming a thin film made 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.

Conversely, when the C axis is oriented in a direction parallel to the film surface, the (110) plane of the single crystal substrate may be used as the film forming surface.

Substrates that meet this condition include MgO, 5rTiO
s, AbO3, sapphire, Sin, quartz, YSZ (
Yztrium stabilized zirconia) and ZnO
etc. can be selected. In particular, in order to avoid applying unnecessary stress to the thin film that may destroy the thin film during sputtering and heat treatment, it is preferable to select MgO and 5rTi03 whose coefficient of thermal expansion is close to that of the thin film.

EXAMPLES The present invention will be explained in more detail with reference to examples below, but the following are merely examples, and it goes without saying that the technical scope of the present invention is not limited in any way by the following disclosure.

Implementation Example 1 Y2O3, BaO and CuO were weighed so that the atomic ratio of Y:Ba:Cu was 1:2.15:3.2 and fired at 900°C in the atmosphere.The obtained fired body was pulverized. A thin film was formed on a magnesium oxide single crystal substrate using high frequency magnetron sputtering using the obtained powder as a target.The film forming conditions were as follows.

Total pressure: 2 Xl0-2Torr 02 /Ar: 0.16 (pressure ratio) Substrate:
The (100) plane substrate temperature of MgO was 20° C. In this way, a thin film with a thickness of 1000 μm was obtained.

This thin film was heated to 700° C. in an oxygen stream, maintained at that temperature for 24 hours, and then cooled to room temperature at a cooling rate of 3° C./min.

The obtained thin film is Y, Ba2CuaOv-x, with an orientation in which the C axis is perpendicular to the MgO substrate, based on the following measurement results.
(However, X is a number satisfying Q<x<l.) It was a polycrystalline film.

FIG. 1 is an X-ray diffraction pattern of the thin film produced as described above. Note that this X-ray diffraction pattern was obtained using a thin-film X-ray diffraction apparatus manufactured by Rigaku Corporation using α-rays for Cu. On the other hand, FIG. 2 shows an X-ray diffraction pattern of an oxide superconductor powder of Yl Ba2Cu30t.

In Figure 2, the index of the crystal plane showing the strongest reflection intensity is (10
3), it was a (110) plane.

The reflection intensity of the strongest reflection surface of the powder pattern of the above oxide superconductor is IMAX%.The reflection intensity of the (OOn) surface [where n is an integer] is ■. . , and the reflection intensity in the X-ray diffraction pattern of the superconducting thin film of this example of the crystal plane having the same index as the strongest reflection surface of the powder pattern is JMAxs
The reflection intensity of the (00n) plane (where n is an integer) of the superconducting thin film is J. Let it be oo.

The relationship shown in Table 1 holds between these reflection intensities.

Table 1 In addition, in Table 1, - represents the following ratio.

I Ioo-/I NAX The results shown in Figure 1 and Table 1 show that the thin film of the above composite oxide has a reflection intensity of (002), (003), (005), and (006) planes. The above relationship: J OOn / J WAX≧2 (I oo,, /
Ixax).

Furthermore, electron beam diffraction revealed that the crystal structure of the thin film of the above composite oxide had a C-axis orientation perpendicular to the film-forming surface.

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

Tc: 85K Jc: 150,0OOA/cfll (liquid nitrogen temperature) These measurement results show that by aligning the crystal structure of the composite oxide superconductor thin film with the C-axis perpendicular to the film formation surface, the in-plane critical current can be increased. This shows that the density Jc is greatly improved.

Example 2 Y2O2, BaO and CuO were weighed so that the atomic ratio of Y:Ba:Cu was 1:2.0:3.1, and fired at 900°C in the atmosphere. A thin film was formed on a single crystal substrate of strontium titanate using high frequency magnetron sputtering using a powder obtained by pulverizing the obtained fired body as a target. The film forming conditions are as follows.

Total pressure: 2 Xl0-'Torr 02 /Ar: 0.15 (pressure ratio) Substrate:
Temperature of the (100) original substrate of 5rTiOa=720° C. In this way, a thin film with a thickness of 1000 μm was obtained.

This thin film was heated to 710° C. in the atmosphere, maintained at that temperature for 24 hours, and then cooled to room temperature at a cooling rate of 3° C./min.

The obtained thin film is Y1Ba2Cu3O7-X (where X is 0 and x
It was a single crystal that is considered to be a number that satisfies <l).

Similar to Example 1, the relationship between reflection intensities is shown in Table 2.

Table 2 A sample with a width of 1 mm was cut from this 1,000-thickness thin film, and the critical temperature Tc and critical current density Jc were measured. A four-terminal method was used to measure the critical temperature Tc. The results obtained are shown below.

Tc: 86K Jc: 160,0OOA/cd m] As explained above, the present invention makes it possible to obtain a superconducting oxide thin film having a much higher Jc than conventional superconductors. Therefore, the present invention is applicable to fields where superconductors are applied as thin film elements, such as Matisoo's switching elements called Josephson elements, Anacker's memory elements, and superconducting quantum interferometers (SQUIDs). It is effective when used for.

[Brief explanation of the drawing]

FIG. 1 shows the X-ray diffraction patterns of thin films according to the invention, and FIG. 2 shows Y, Ba2Cu30. This is an X-ray diffraction pattern of oxide superconductor powder. Patent applicant: Sumitomo Electric Industries, Ltd.

Claims (1)

[Claims] Y, La, Gd, Dy, HO, Er, Tm, Tb, L
In a thin film of a composite oxide superconductor containing at least one element Ln selected from the group consisting of u, Nd, Sm and Eu, Ba, and Cu, the thin film is a c-axis oriented single crystal. Or a superconducting thin film characterized by being composed of polycrystals.