JPH05148084A - Superconductor - Google Patents

Abstract

PURPOSE:To obtain a superconductor having oxide superconducting thin films of a specific structure laminated in high quality on a substrate and anisotropic superconducting characteristics in the surface direction of the substrate and provide a superconductor in which the oxide superconducting thin films and the normal conducting thin films are selectively formed on the substrate. CONSTITUTION:The objective superconductor 2 is characterized by having an off substrate 1, composed of a single crystal and having a surface tilted at a prescribed angle to the crystal surface, oxide superconducting thin films (2a) and normal conducting thin films (2b), laminated and formed in desired selected positions on the surface of the off substrate 1. Joining defects caused by lattice constant misfit of the materials between the substrate surface and the thin films are reduced by using the off substrate 1. The oxide superconducting thin films and normal conducting thin films are selectively laminated and formed by controlling plural evaporation sources. In this example, Y-Ba-Cu-O-based oxide superconducting thin films and Pr-Ba-Cu-O-based normal conducting thin films are respectively used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconductor used in a superconducting device, and more particularly to a superconductor having an oxide superconducting thin film.

[0002]

2. Description of the Related Art In recent years, yttrium (Y), bismuth (Bi) and thallium (Tl) based oxide superconductors have been discovered which are in a superconducting state at a high critical temperature higher than liquid nitrogen temperature. Researches applied to superconducting devices are being actively conducted.

In order to manufacture such a superconducting device, an oxide superconducting thin film and a thin film of a different substance (for example, an insulator, a semiconductor, a normal conductor or a superconductor) are laminated on an insulator or a semiconductor substrate. Technology is important.

Conventionally, a jus having a surface parallel to the crystal plane
An oxide superconducting thin film is laminated on a t substrate by vapor deposition, sputtering, or the like. If the surface of this just substrate has irregularities, the oxide superconducting thin film formed on the just substrate may have pinholes or disorder of crystallinity. Defects occur and good bonding characteristics cannot be obtained. For this reason, the surface of the just substrate is sufficiently polished to obtain a surface as flat as possible, and an oxide superconducting thin film is formed there. As the insulator substrate, a SrTiO ₃ substrate, a MgO substrate, or the like is used,
An oxide superconducting thin film is laminated on these (100) planes.

Currently, SrTiO₃(100) just group
On the plate, the oxide superconducting thin film (Y₁Ba ₂Cu₃O_7-X) Is tied
It is known that each crystal unit grows with c-axis orientation.
(Reference: T. Terasima and Y. Bandou, Phys. Rev. Lett.
65. (1990) 2684). However, a just board like this
With the method used, the seed
Since the generation position cannot be specified, the desired position on the substrate surface
It is not possible to form an oxide superconducting thin film at selected locations.
Yes. Thus, in the conventional technology, the oxide superconducting thin film is used as the substrate.
It cannot be laminated with a predetermined structure in a high quality.

Further, in this case, the thin film obtained by laminating the oxide superconducting thin film on the entire surface of the substrate and having the Cu--O plane connected in parallel to the substrate surface has two-dimensional superconductivity on the substrate surface. Therefore, it is difficult to manufacture a film-structured superconductor having anisotropic superconductivity.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and a high-quality oxide superconducting thin film is laminated on a substrate in a predetermined structure and the surface direction of the substrate is The present invention provides a superconductor having anisotropic superconductivity.

[0008]

The present invention is directed to a substrate made of a single crystal and having a surface inclined at a predetermined angle with respect to the crystal plane, an oxide superconducting thin film and a normal conducting thin film formed on the surface of the substrate. There is provided a superconductor characterized by including:

[0009]

According to the present invention, a substrate (o) made of a single crystal and having a sawtooth surface inclined at a predetermined angle with respect to the crystal plane is used.
ff substrate) is used, so it can trigger crystal growth.
The generation position of seed can be specified, and the oxide superconducting thin film and the normal conducting thin film can be selectively formed.

[0010]

FIG. 1 is a block diagram of a superconductor according to an embodiment of the present invention, in which an oxide superconducting thin film 2a and a normal conducting thin film 2b are alternately deposited on an off substrate 1 and have a repeated laminated structure of both. A superconductor 2 is formed.

The off substrate 1 is manufactured by polishing with an inclination of a predetermined angle (4 °) with respect to the (100) plane of the SrTiO ₃ crystal. Therefore, a step surface 1a perpendicular to the crystal surface and a step surface 1b parallel to the crystal surface are formed on the surface. In this example, since the substrate 1 having an inclination angle of 4 ° is used, the length of the step surface 1a is A and the step surface 1b is
If the length of B is B, then A / B = tan 4 ° = 1/14.

During the deposition of the thin film, the thin film starts crystal growth from the step surface 1a on the step surface 1b of the substrate 1. The a-axis length and the b-axis length of the oxide superconducting thin film are 0.389 nm and 0.383 nm, respectively, and SrTiO ₃
Since the lattice constant of the (100) plane is 0.391 nm,
The mismatch (lattice mismatch) with the a-axis length is smaller, and the oxide superconducting thin film and the normal conducting thin film grow in the direction in which the a-axis is parallel to the Y-axis and form a pair of oxide superconducting thin films on one step surface. A normal conducting thin film is formed. As a result, Cu is formed in a plane direction (XY plane direction) parallel to the thin film growth surface of the substrate.
A c-axis oriented superconducting thin film having a -O plane grows, and a one-dimensional superconducting thin film having superconductivity only in the Y-axis direction can be formed.

The crystal of SrTiO ₃ in this example is a cubic system (lattice constant a = b = c = 0.3905 nm), and the length A of the step surface 1a on the side surface side is 0.3905 nm, and that on the upper surface side is 14 SrTiO ₃ on the step surface 1b
4 ° of which surface condition is uniform due to the arrangement of crystals
The f board 1 is obtained.

The superconductor 2 formed on the 4 ° off substrate 1 is an oxide superconductor Y-Ba-Cu-O (Y ₁ Ba ₂ C
_{u 3 O 7-X; 6.5} <7-δ <6.9) 2a and normal conductor Pr-Ba-Cu-O ( Pr 1 Ba 2 Cu 3 O 7-Y; 6.5
<7-δ <6.9, Pr; praseodymium) 2b, and Y-Ba-Cu-O is an orthorhombic system (lattice constant: a ≠ b ≠ c, a = 0.38815 nm, b =
0.38144 nm, c = 1.16749 nm).

Then, when the oxide superconducting thin film 2a (Y-Ba-Cu-O) and the normal conducting thin film 2b (Pr-Ba-Cu-O) are laminated on the 4 ° off substrate of SrTiO ₃ , Y-Ba is formed. The superconductor 2 is formed by a-b-c of -Cu-O and Pr-Ba-Cu-O and X, Y, Z in FIG.

A → Y, b → X, c → Z Therefore, as shown in FIG. 1, it is possible to obtain an oxide superconducting thin film having no crystal defects with aligned crystal axes (c-axis orientation). At this time, the material conditions for forming the oxide superconducting thin film and the normal conducting thin film are that the crystal matching between the crystal thin film interlayer contact surfaces and the crystal matching between them and the crystal plane of the 4 ° off substrate 1 are very high. It is expensive.

That is, if one of the a-axis length or the b-axis length of the oxide superconducting thin film and the normal conducting thin film is close to the lattice constant of the crystal plane of the substrate, the one of the a-axis lengths (for example, the a-axis length). The crystal matching becomes high, and the superconductor and the normal-conducting thin film grow, for example, with the a-axis parallel to the Y-axis and in the Z-axis direction. It is also possible to form a pair of oxide superconducting thin film and normal conducting thin film on one step surface. As a result, as shown in FIG. 1, the c-axis oriented superconducting thin film is selectively grown in the plane direction (XY plane direction) parallel to the thin film growth surface of the substrate, and the tilt direction of the substrate (X axis direction). It is possible to form an anisotropic superconducting thin film having superconductivity only in the direction (Y-axis direction) in the plane of the substrate perpendicular to.

Next, a method for manufacturing the superconductor 2 will be described. Figure 2 shows the molecular beam epitaxy method (MBE)
It is a schematic block diagram of the superconductor manufacturing apparatus by this, and the inside of the airtight container 3 is made into the growth chamber 4. The substrate 1 is mounted on the substrate holder 5 in the growth chamber 4. The heater 7 heats the substrate 1 to a predetermined temperature, and is formed of a platinum-rhodium alloy (Pt-Rh (20%)) to prevent its oxidation.

On the other hand, a plurality of evaporation sources 8a to 8d are provided at positions facing the substrate 1, from which Y, Pr, Ba and Cu necessary for producing the superconductor 2 are independently provided. It vaporizes in the atomic or molecular state and is irradiated onto the substrate. The evaporation source 8a is an electron beam evaporation source for Cu, and 8b, 8c and 8d are Y, Ba and Pr.
It is the crucible evaporation source.

An oxygen jet 9c connected to an oxygen source 9a is arranged in the vicinity of the substrate 1, and O ₂ or radicals O are supplied toward the substrate 1 from here. The valve 9b is for controlling the supply amount of oxygen.

On the front surface of each evaporation source 8, there is provided a shutter 10 that can be opened and closed to control the irradiation of evaporated material, and the opening and closing of this shutter 10 is performed by a thin film deposition controller 11. A film thickness detector 12 is connected to the thin film deposition controller 11, which detects the film thickness deposited on the substrate 1 and controls the opening and closing of the shutter 10 according to the film thickness. The thin film deposition controller 11 includes the evaporation sources 8a ...
The amount of evaporation is also controlled by the heating control of 8d.

Here, as the film thickness detector 12, for example, a crystal oscillator type is adopted. The crystal oscillator type film thickness detector 12 utilizes that the oscillation frequency changes depending on the amount of vapor deposition on the crystal plate which oscillates at 6 MHz. As the film thickness detector 12, a light emitting type or the like can be appropriately adopted.

A vacuum pump 13 is connected to the growth chamber 4 so that the pressure inside the growth chamber 4 can be adjusted to a predetermined value.

Further, in this apparatus, the growth chamber 4 is provided with a reflection type high-speed electron diffraction gun 14, a screen 15, and a quadrupole mass analyzer 16, and a sample preparation chamber 17 is connected thereto.

The reflection-type high-energy electron diffraction electron gun 14 and the screen 15 in the growth chamber 4 are RHEED based on the crystallinity of the surface of the substrate 1 before thin film formation and the crystallinity of the thin film surface during film formation.
(Reflection high-speed electron diffraction)
The quadrupole mass spectrometer 16 monitors the residual gas in the growth chamber 4 during film formation. By monitoring the film forming conditions by these and always maintaining the film forming conditions suitable, it is possible to form a high quality film. The sample preparation chamber 17 is intended to prevent the inside of the growth chamber 4 from being exposed to the atmosphere at the time of exchanging the sample, and to maintain the inside of the growth chamber 4 in a high vacuum.

In such an apparatus, the substrate 1 is set on the substrate holder 5 and controlled by the temperature heater 7 of the substrate 1. Then, Y, Pr, and Ba are evaporated by the evaporation source 8.
In addition to evaporating Cu and Cu, a predetermined amount of oxygen (O ₂ or radical O) is supplied into the growth chamber 4 through the oxygen jet port 9c.

Further, the thin film deposition controller 11 controls the opening and closing of the shutter 10 to simultaneously open the Y, Ba and Cu shutters (operation A). This corresponds to thin film production of Y ₁ Ba ₂ Cu ₃ O _7-X . On the other hand, in the case of producing a thin film of Pr ₁ Ba ₂ Cu ₃ O _7-X , the shutters of Pr, Ba and Cu are simultaneously opened (operation B). So very slowly (0.05 ~
(About 0.15 nm / sec) and the deposition of the film on the substrate 1 are repeated to form a thin film. At this time, since oxygen is supplied to the vicinity of the substrate 1 from the oxygen ejection port 9c, the thin film is formed while being oxidized, and Y-Ba-Cu-
Oxide superconducting thin film made of O (thickness = 10 to 100 nm
Or a normal conductor thin film made of Pr—Ba—Cu—O is formed on the substrate 1.

During the thin film growth, by repeating this operation A and operation B alternately, the superconducting thin film (S layer) made of Y-Ba-Cu-O and the normal conductive thin film (P-Ba-Cu-O) ( N layers) are alternately deposited and the SNS structure is repeated to obtain a superconductor 2 having a superlattice structure as shown in FIG.

In this way, the superconductor 2 in which the oxide superconducting thin films and the normal conductor are alternately laminated is supplied with O ₂ or radical O from the oxygen jet port 9c so that oxygen is not desorbed from the thin film. While receiving, the temperature of the substrate 1 is lowered and the substrate 1 is taken out from the airtight container 3.

Next, an example of actual vapor deposition will be described. The conditions at this time are as follows.

Evaporation amount: Y: 0.019 nm / sec Pr: 0.021 nm / sec Ba: 0.039 nm / sec Cu: 0.062 nm / sec O ^* : 1 × 10 ¹⁶ pieces / sec (*: O ₂ or Radical O) Film thickness: 10 to 100 nm Substrate temperature: 600 to 750 ° C. Substrate used: SrTiO ₃ (100) 4 ° off substrate

Under the above-mentioned production conditions, the superconducting thin film (Y ₁ Ba ₂ Cu
₃ O _7-X ) was added to the normal conductive thin film (Pr ₁ Ba ₂ Cu) by the operation B.
₃ O _{7 -Y} ) are alternately deposited.

First, when the operation A is started, the vibration intensity of the RHEED decreases. When the vibration intensity reaches the minimum value, the operation A is ended, and after 3 seconds, the operation B is started again.
Similarly, the operation B is ended at the maximum value of the vibration intensity, and the operation A is started again after 3 seconds have passed. At that time, the opening time of the shutter was 5 seconds in both the operation A and the operation B.

FIG. 3 shows the time change of the RHEED vibration at this time.
Shown in. In this case, since the length B of one step surface in the Z-axis direction is 6 nm, as shown in FIG. 1, a pair of superconducting thin film and normal conducting thin film each have 3 nm in the Z-axis direction on one step surface. It can be seen that they are formed in an array.

FIGS. 4A to 4D and 5A to
(C) shows a crystal growth process of the oxide superconducting thin film and the normal conducting thin film formed by the above method on the off substrate, and a forming process of the superconductor 2.

First, the molecular beam S (Y ₁ Ba ₂ C
u ₃ O _7-X ) is irradiated on the substrate 1, and Y ₁ Ba ₂ Cu ₃ O _7-X crystals are formed on the step surface 1b of the crystal plane of the substrate 1 as shown in FIGS. The growth is started from the step surface 1a side, and the first layer 2a ₁ of the oxide superconducting thin film is deposited (FIG. 5 (A)). Here, s _A represents _one crystal unit of Y ₁ Ba ₂ Cu ₃ O _7-X .

Subsequently, when the operation A is finished and the operation B is started, the molecular beam N (Pr ₁ Ba ₂ Cu ₃ O _7-Y ) is irradiated,
As shown in FIG. 4 (C), crystals of Pr ₁ Ba ₂ Cu ₃ O _{7 -Y} start to grow from the first layer 2a ₁ side of the superconducting thin film on the step surface 1b. Then, when the operation B is completed at the point where the adjacent step surface is reached, the crystal growth is stopped, the first layer 2b ₁ of the normal conductor thin film is deposited, and the first layer 2 ₁ of the superconductor is formed.
Are formed (FIG. 5 (B)). Where n _B is Pr ₁ Ba
₂ shows one unit of Cu ₃ O _{7 -Y} crystal. After the second layer, 2a ₂ and 2 are formed on the first layer in the same manner as the first layer.
b ₂ , 2a ₃ , 2b ₃ .... are deposited (Fig. 4
(D)) and FIG. 5 (C)).

In this embodiment, the operation A and the operation B are performed as described above.
Was repeated 10 cycles to alternately deposit the superconducting thin film and the normal conducting thin film to produce a superconductor having a superlattice structure.

In the present example, the constituent materials of the superconducting thin film and the normal conducting thin film were Y ₁ Ba ₂ Cu ₃ O _7-X and Pr.
_{Although 1} Ba ₂ Cu ₃ O _7-Y is used, a superconducting thin film and a normal conducting thin film made of another substance may be used as long as the conditions such as the above-mentioned lattice constant are met. Further, although the 4 ° -off substrate is used in the present embodiment, it is possible to use an off substrate having a slope other than this depending on the types of constituent materials of the superconducting thin film and the normal conducting thin film to be laminated thereon. .. Further, in forming the oxide superconducting thin film,
Other methods such as RF magnetron sputtering may be used.

FIG. 6 shows the electric resistance-temperature characteristic of a superconductor having a superlattice structure manufactured according to an embodiment of the present invention. A current was made to flow in the Z-axis direction and the Y-axis direction parallel to the substrate surface of the superconductor, the current value was set to I _C / 2, and the temperature characteristic of the electric resistance was measured. Here, I _C is a superconducting critical current.

In FIG. 6, almost semiconductor behavior is shown in the Z-axis direction, and superconductivity behavior at the superconducting critical temperature T _C ≈80 K is shown in the Y-axis direction. Therefore, this superconductor
It can be seen that this is an anisotropic superconducting thin film having superconductivity only in the Y-axis direction.

[0042]

According to the present invention, a substrate (of which is made of a single crystal and has a surface inclined at a predetermined angle with respect to the crystal plane of the substrate (of
It is possible to selectively form the superconducting thin film and the normal conducting thin film on the (f substrate), and thereby obtain a superconductor having anisotropic superconducting properties in the plane direction of the substrate.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a superconductor according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a configuration of a thin film forming apparatus according to an embodiment of the present invention.

FIG. 3 is a diagram showing a time change of RHEED vibration in a thin film manufacturing process of an example of the present invention.

FIG. 4 is a diagram showing a growth process of a superconductor thin film according to an embodiment of the present invention.

FIG. 5 is a diagram showing a process of forming a superconductor according to an embodiment of the present invention.

FIG. 6 is a diagram showing temperature characteristics of electric resistance of a superconductor according to an example of the present invention.

Claims (1)

[Claims] 1. A substrate comprising a single crystal and having a surface inclined at a predetermined angle with respect to the crystal plane, an oxide superconducting thin film and a normal conducting thin film formed on the surface of the substrate. And a superconductor.