JPH05279192A - Synthesis of oxide superconductor thin film - Google Patents

Abstract

PURPOSE:To synthesize a Tl2Ba2CaCu2Ox single crystal thin film having excellent superconducting characteristics on a SrTiO3 substrate. CONSTITUTION:An extremely thin Tl2Ba2CuOx or Bi2Sr2CuOx layer is grown as a buffer layer 5 on an inclined SrTiO3 substrate 1 and an amorphous Tl2Ba2 CaCu2Ox film is deposited on the buffer layer at room temperature. The amorphous film is heat-treated in 1 atm oxygen atmosphere containing Tl vapor to effect the growth of a Tl2Ba2CaCu2Ox single crystal thin film 7. A Tl2Ba2 CaCu2Ox single crystal thin film having high flatness and uniform modulation structure can be synthesized by this process without causing the sporadic growth and the generation of hetero-phase on the film surface.

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 an oxide superconducting thin film.

[0002]

2. Description of the Related Art Superconducting thin films are indispensable for applications in quantum magnetic interference devices using Josephson junctions, superconducting LSI wiring, superconducting active devices and the like. Recently, 1987
February, 2004, including the Y-based oxide superconductor with a critical temperature of 90K discovered by the University of Houston, Chu, etc., and the Bi-based oxide superconductor with a critical temperature of 110K from Maeda et al. In addition, a Tl-based oxide superconductor having a critical temperature of 120K and an oxide superconductor having a critical temperature exceeding liquid nitrogen temperature were successively discovered by ZZ Sheng of the University of Arkansas, USA. This means that superconducting devices that had to use liquid helium in the past can be realized with liquid nitrogen. In particular, thinning these oxide superconductors into Josephson active devices and superconducting LSI wiring that operate at liquid nitrogen temperatures or higher. Also, it realizes a microwave device, and its application can be widely used.

From the viewpoint of device application, this oxide superconductor thin film is a so-called single crystal thin film in which a superconducting thin film is epitaxially grown on a substrate and the orientation of grains in the substrate plane is uniform.
In addition, a technique for synthesizing an extremely flat thin film without island growth or different phases on the film surface is essential. In Tl-based superconducting thin film
-In the situ synthesis technology, a method by laser ablation, a method by RF magnetron sputtering,
Although various film forming methods such as a method using vacuum evaporation have been reported, all of them are
In-plane orientation of each grain is disturbed due to island-like growth and heterogeneous phase that occur on the surface of the thin film. The grain boundaries in the plane of the thin film due to the island-like growth have become a serious problem in device application.

[0004]

In the synthesis of a Tl-based oxide high-temperature superconductor thin film, it is extremely difficult to directly synthesize it because the vapor pressure of oxidized Tl is very high. For this reason, conventionally, amorphous Tl ₂ B was formed on the substrate.
By laminating a ₂ CaCu ₂ O _x and heat-treating this in 1 atm oxygen and Tl vapor, Tl _{2 is formed} by solid-state reaction.
A method of synthesizing a Ba ₂ CaCu ₂ O _x thin film has been performed. However, (001) SrTi at 800-900 ° C
In the case of a solid-phase reaction on an O ₃ substrate, a heterogeneous phase is generated or island-shaped growth occurs in the film growth process, and the traces of the island-shaped growth and the heterogeneous phase remain as the morphology of the film surface until the end. An object of the present invention is to provide a method for synthesizing a single crystal Tl-based superconducting thin film which is free of island-shaped growth grains and foreign phases on the film surface and has good flatness. Further, the Tl-based superconductor has a characteristic incommensurate modulation structure, and an object thereof is to provide a method for obtaining a so-called high quality single crystal film in which the directions of this modulation structure are perfectly aligned.

[0005]

The first object of the present invention is to synthesize a Tl-based oxide superconducting thin film on a strontium titanate substrate by laminating amorphous Tl ₂ Ba ₂ CuO _x on the substrate. Then, heat treatment is performed in oxygen / Tl vapor, and ultra-thin Tl heteroepitaxially grown on the substrate.
_A method for synthesizing a single crystal Tl-based oxide superconducting thin film, which comprises forming a ₂ Ba ₂ CuO _x film and using the film as a buffer layer. A second aspect of the present invention is a method for synthesizing a Tl-based oxide superconducting thin film on a strontium titanate substrate, in which Ba ₂ CuO _x is heteroepitaxially grown on the substrate in an oxidizing atmosphere, and this is grown in 1 atm oxygen and Tl vapor. Ba ₂ CuO _x by solid-state reaction by heat treatment at
To form a very thin Tl ₂ Ba ₂ CuO _x film heteroepitaxially grown on the SrTiO ₃ substrate by diffusing Tl into
A method for synthesizing a single crystal Tl-based oxide superconducting thin film, which comprises using the film as a buffer layer. A third aspect of the present invention relates to a method for synthesizing a Tl-based oxide superconducting thin film on a strontium titanate substrate, wherein a single crystal Bi ₂ Sr ₂ Cu is used.
A method for synthesizing a single crystal Tl-based oxide superconducting thin film, which comprises growing O _x on a substrate and using it as a buffer layer.

Here, strontium titanate (00
1) A substrate surface on which a step is formed in the [1 -10] direction by inclining in a [111] direction from the normal to the single crystal substrate surface and used as a substrate for synthesizing a Tl-based oxide superconducting thin film Preferably. Further, according to the present invention,
Amorphous T on the ultra-thin single crystal Tl ₂ Ba ₂ CuO _x buffer layer or the ultra-thin single crystal Bi ₂ Sr ₂ CuO _x buffer layer
By stacking 1 ₂ Ba ₂ CaCu ₂ O _x and heat-treating the same in oxygen and Tl vapor at 1 atm, a single-crystal Tl ₂ Ba ₂ CaC was formed by a solid-phase reaction bound by a single-crystal buffer layer.
A method for synthesizing a single crystal Tl-based oxide superconducting thin film according to claim 4 for synthesizing a u ₂ O _x thin film is provided.

[0007]

In the heteroepitaxial growth of the Bi-based superconductor on the SrTiO ₃ substrate, [110] SrTiO ₃ [1
[00] Tl-based superconductor is epitaxially grown due to its orientation, and its lattice mismatch is about 2%, which is extremely effective for epitaxial growth. However, it is extremely difficult to synthesize a Tl-based oxide superconductor in-situ because of its high vapor pressure. For this reason, usually, amorphous Tl ₂ Ba ₂ CaCu ₂ O _x is laminated on a substrate, and this is subjected to a high temperature heat treatment at about 800-900 ° C. in 1 atm oxygen / Tl vapor. Occurs, and the in-plane orientation becomes random.

On the other hand, Tl ₂ Ba _{2 which} grows at a relatively low temperature
The amorphous ultrathin film of CuO _x layer is first laminated, and this is
Heteroepitaxially grown ultrathin Tl ₂ Ba ₂ bound to the substrate lattice by low temperature heat treatment at 00 ° C or lower
A CuO _x thin film can be formed. By using an ultrathin film, the solid-phase reaction sufficiently proceeds even at a low temperature of 800 ° C. or less, and heteroepitaxial growth strongly bound to the substrate surface becomes easy. This ultra-thin single crystal Tl ₂
When the conventional method for synthesizing a Tl-based superconducting thin film by using the Ba ₂ CuO _x layer as the buffer layer is performed, solid-phase reaction proceeds from the buffer layer to reduce island growth, and the in-plane orientation matches the buffer layer. The single crystal Tl-based superconducting thin film can be synthesized. As a method for growing this buffer layer, the ultrathin amorphous Tl ₂ Ba ₂ described above is used.
In addition to the method of low temperature heat treatment of the CuO _x film, in-situ
Heteroepitaxially grows very thin Ba ₂ CuO _x with
This is treated in oxygen / Tl vapor at 1 atm to form an ultrathin Tl ₂ Ba ₂ CuO _x buffer layer heteroepitaxially grown, and a Bi-based 220 having a similar crystal structure.
It is effective to grow one layer in-situ and use this as a buffer layer.

Furthermore, Tl using a tilted SrTiO ₃ substrate
The Tl-based superconductor thin film synthesized by the system-based superconductor thin film synthesis method is a single crystal thin film in which the a-axis and the b-axis are completely separated in the plane. Generally, a Tl-based superconductor has an incommensurate modulation structure with a period about 5 times that of the basic lattice in the b-axis direction. Normal SrTiO ₃ (00
1) In film formation using the substrate surface, the film forms a domain structure, and in RHEED of the film, this modulation structure is
10] and [1 -10] directions.
That is, the film is composed of two types of domains that are orthogonal to each other. On the other hand, the (100) SrTiO ₃ substrate was
When a film is grown on an inclined polished substrate inclined by 1 to 20 ° in the 11] direction, the b axis of the film is aligned with the [110] direction, and the c axis of the film is aligned with the [001] direction of the substrate. Epitaxial growth. That is, a step structure is formed on the SrTiO ₃ substrate corresponding to the tilted angle, and the c-axis of the film is epitaxially grown perpendicularly to the terrace surface on the substrate surface, so that the c-axis is tilted with respect to the substrate surface. Angle minutes [1
-10 0] direction. The film on this tilted substrate is a so-called single crystal film in which the directions of the a, b, and c axes of the film are all substantially aligned, and is useful for producing a device utilizing the electrical conduction anisotropy peculiar to a high-temperature oxide superconductor. Is extremely important.

[0010]

EXAMPLES Examples of the present invention will be described below.
An ion beam sputtering apparatus was used for synthesizing the Tl-based oxide superconductor thin film. This device uses atomic oxygen excited by microwaves as an oxidation source, and the oxygen partial pressure near the substrate is about 1
It was 0 ⁻³ Torr. At this time, the amount of active oxygen reaching the vicinity of the substrate is about 2 × 10 ¹⁵ pieces / cm ² s. The film growth rate is about 400 Å / H, and Bi
₂ Sr ₂ CuO _x , Tl ₂ O, Ba ₂ CuO _x and Ba ₂ C
By providing a target of aCu ₂ O _x and independently controlling four sputter sources, an ultra-thin single crystal Bi ₂ Sr ₂ CuO _x buffer layer can be synthesized in-situ and at the same time as the room temperature deposition process. The composition of the Tl composition and the composition of the buffer layer Tl ₂ Ba ₂ CuO _x and the superconducting layer Tl ₂ Ba ₂ CaCu ₂ O _x can be controlled.

FIG. 2 shows a schematic view of the surface state of the inclined SrTiO ₃ substrate according to the present invention. The substrate has a (001) plane tilted by 1 to 20 degrees in the [111] direction and is mechanochemically polished. On the substrate surface, steps of 3.9 angstroms corresponding to one unit cell of SrTiO ₃ perovskite are regularly formed.
For example, in the case of a tilt of 4 degrees, the terrace width forms one step every about 50 angstroms. This is the cross section T
It was confirmed by EM and RHEED, and especially in RHEED, when an electron beam is incident in the [1 -10] SrTiO ₃ direction,
The splitting of the basic reflection streak is seen corresponding to the terrace of about 50 angstrom width.

FIG. 3 shows an ultrathin Tl ₂ B according to the first invention.
It is a thin film synthesis process flowchart in the case of using the buffer layer by a ₂ CuO _x thin film. Amorphous Tl ₂ Ba ₂
In the buffer layer forming method using CuO _x , first, referring to FIG.
As shown in (a), Tl _{2 is formed} on the inclined SrTiO ₃ substrate 1 at room temperature.
Amorphous Tl ₂ Ba ₂ Cu using Ba ₂ CuO _x target
Laminate about 100 Å of O _x . Figure 3
By performing a heat treatment at about 800 ° C. for a short time in the electric furnace 3 in the atmosphere of 1 atmosphere of oxygen and Tl as shown in (b), the extremely thin Tl ₂ Ba ₂ CuO _x layer 5 is epitaxially grown on the graded SrTiO ₃ substrate. (FIG.3 (c)). At this time, the orientations of the a and b axes in the film plane are already bound to the steps of the substrate,
The b-axis of the film is a single crystal thin film aligned in the [1 -10] SiTiO ₃ direction. Amorphous Tl ₂ Ba ₂ CaCu ₂ O _x 6 was laminated on the buffer at room temperature as shown in FIG. 3D to a predetermined thickness, and again, as shown in FIG.
The entire film was heteroepitaxially grown on the substrate by heat treatment in an atmosphere of single crystal Tl ₂ Ba ₂ Ca.
The Cu ₂ O _x thin film 7 can be synthesized (see FIG. 3).
(F)).

The method of the second invention can also be used as a method of synthesizing the ultrathin Tl ₂ Ba ₂ CuO _x single crystal buffer layer formed by the processes of FIGS. 3 (a) to 3 (c). .. In the method according to the present invention, the Ba ₂ CuO _x epitaxial film is first formed in-situ on the inclined substrate. The epitaxial thin film of Ba ₂ CuO _x is 10 ⁻³
In-situ synthesis is easily performed at a substrate temperature of about 650 ° C. in an oxygen atmosphere of about Torr. This Ba ₂ Cu
The epitaxial thin film of O _x is subjected to the heat treatment process of FIG.
_{A 2} Ba ₂ CuO _x buffer can be synthesized.

Further, according to the third invention, it is possible to synthesize a single crystal Tl ₂ Ba ₂ CaCu ₂ O _x by using a Bi-based oxide superconductor (Bi ₂ Sr ₂ CuO _x ) as a buffer. Bi ₂ Sr ₂ CuO _x, which is a Bi-based oxide superconductor very similar to the Tl-based oxide superconductor, is easily synthesized under the conditions of a substrate temperature of 600 ° C. and an oxygen partial pressure of about 10 ⁻³ Torr. This Bi ₂ Sr ₂ CuO _x has an in-plane lattice constant substantially equal to that of the Tl-based oxide superconductor and is excellent in lattice matching, and also has an incommensurate rate modulation structure in the b-axis direction. It is very similar to the crystal structure.

Amorphous Tl ₂ B is formed on these buffer layers.
a ₂ CaCu ₂ O _x is laminated at room temperature, and this is shown in FIG.
1 atmosphere of oxygen at a temperature of about 800 ° C to 950 ° C.
When heat treatment is performed in a Tl atmosphere, the underlying Tl ₂ Ba ₂ Cu
O _x or Bi ₂ Sr ₂ CuO _x single crystal by the solid phase growth from here as a template Tl ₂ Ba ₂ CaCu ₂ O _x thin film grows.

By these methods, Tl ₂ Ba ₂ CaCu
It was confirmed by RHEED pattern and X-ray diffraction that the ₂ O _x single crystal thin film could be synthesized. This film structure is shown in FIG. As described above, the film is formed of Tl ₂ Ba ₂ CuO _x 5 or Bi ₂ Sr ₂ CuO _x as a buffer layer, and becomes a single crystal thin film 7 in which the reaction between the substrate and the superconducting layer is suppressed and the in-plane orientation is uniform. There is. In FIG. 1, first, step 8 is formed according to the inclination angle on the inclined substrate surface,
This is formed in the [1 -10] direction at substantially equal intervals. This step becomes the creation site during crystal growth of the buffer layer. Since the modulation structure is formed in the direction orthogonal to the step when the crystal growth proceeds along the step, the film (buffer) has the a-axis in the direction parallel to the step and the b-axis in the direction orthogonal to the step. It grows as a film. Further, since the c-axis of the film grows perpendicular to the terrace, the c-axis with respect to the substrate surface is inclined by the inclination of the substrate. Tl ₂ Ba ₂ CaC on this buffer layer
Since the u ₂ O _x film grows completely epitaxially in the buffer, a single crystal Tl ₂ Ba ₂ CaCu ₂ O _x film grows.
Of course, the c-axis of this Tl ₂ Ba ₂ CaCu ₂ O _x film is also tilted from the original SrTiO ₃ substrate surface by the tilt angle.

The electrical characteristics of this tilted single crystal film are extremely specific. A-axis direction of the film ([110] SrTiO ₃
(Direction) indicates a normal metallic electric conduction characteristic, and shows a good superconducting transition of about 110K. On the other hand, in the b-axis direction ([1 -10] SrTiO ₃ direction), semiconductor-like electric conduction characteristics are exhibited, and similarly, superconducting transition of about 110 K is exhibited.
This electric conduction characteristic is similar to the electric conduction characteristic of the bulk single crystal in the c-axis direction. This does not reflect the anisotropy of the film in the b-axis direction. From the conductivity measurement of bulk single crystal a,
The specific resistance in the c-axis direction is higher than the specific resistance in the b-plane by four digits or more. Since the c-axis is inclined in the electric conduction in the [1 -10] direction due to the use of the inclined substrate, the conduction path includes the c-axis. Therefore, it can be interpreted that the electric conduction anisotropy in the c-axis direction was observed as electric conduction anisotropy in the substrate surface of the epitaxial film. It is extremely useful for device application that such c-axis electric conduction anisotropy appears in the plane of the epitaxial film on the inclined substrate. In addition,
In this embodiment, an ion beam sputtering device was used,
Not limited to this, it is also effective for synthesizing an oxide superconducting thin film in a vacuum vapor deposition apparatus.

[0018]

As described above, by applying the present invention, a Tl-based single crystal epitaxial superconducting film having excellent superconducting properties can be easily synthesized. Further, the single crystal thin film on the inclined substrate is extremely useful for device application using its electrical anisotropy.

[Brief description of drawings]

FIG. 1 is a schematic view of T using an inclined substrate synthesized by the method of the present invention.
It is a figure which shows the cross-section of the 1 type superconducting thin film.

FIG. 2 is a diagram showing a surface step structure of a (001) SrTiO ₃ substrate tilted in the [111] direction used in the method of the present invention.

FIG. 3 is a flow chart diagram of a synthesis process of one embodiment of the present invention.

[Explanation of symbols]

1 SrTiO ₃ substrate 2 amorphous Tl ₂ Ba ₂ CuO _x layer 3 electric furnace 4 container enclosing Tl 5 Tl ₂ Ba ₂ CuO _x epitaxial layer (buffer layer) 6 amorphous Tl ₂ Ba ₂ CaCu ₂ O _x film 7 Single crystal Tl ₂ Ba ₂ CaCu ₂ O _x thin film 8 steps

Continued Front Page (72) Inventor Tetsuro Sato 5-7-1 Shiba, Minato-ku, Tokyo NEC Corporation

Claims (5)

[Claims] 1. A method for the synthesis of the Tl-based oxide superconducting thin film of strontium titanate substrate, amorphous Tl ₂
Forming an ultrathin Tl ₂ Ba ₂ CuO _x film heteroepitaxially grown on a substrate by laminating Ba ₂ CuO _x on a substrate and then performing heat treatment in oxygen / Tl vapor, and using the film as a buffer layer Single crystal T characterized by
Method for synthesizing 1-based oxide superconducting thin film. 2. A method for synthesizing a Tl-based oxide superconducting thin film on a strontium titanate substrate, wherein Ba ₂ CuO _x is heteroepitaxially grown on the substrate in an oxidizing atmosphere, and heat-treated in 1 atmosphere oxygen and Tl vapor. By diffusing Tl in Ba ₂ CuO _x by a solid-state reaction to form an ultrathin Tl ₂ Ba ₂ CuO _x film heteroepitaxially grown on a SrTiO ₃ substrate, and using the film as a buffer layer. A method for synthesizing a single crystal Tl-based oxide superconducting thin film. 3. A method for synthesizing a Tl-based oxide superconducting thin film on a strontium titanate substrate, comprising a single crystal Bi ₂
A method for synthesizing a single crystal Tl-based oxide superconducting thin film, which comprises growing Sr ₂ CuO _x on a substrate and using this as a buffer layer. 4. A strontium titanate (001) single crystal substrate surface is tilted in the [111] direction from its normal line and polished to form a step in the [1 -10] direction to form a Tl-based substrate surface. The method for synthesizing a single crystal Tl-based oxide superconducting thin film according to claim 1, which is used as a substrate for synthesizing an oxide superconducting thin film. 5. An amorphous Tl ₂ Ba ₂ CaCu ₂ O _x layered on an ultrathin single crystal Tl ₂ Ba ₂ CuO _x buffer layer or an ultrathin single crystal Bi ₂ Sr ₂ CuO _x buffer layer, By heat treatment in atmospheric pressure oxygen and Tl vapor, the single crystal T is bound by the solid-phase reaction bound to the single crystal buffer layer.
The method for synthesizing a single crystal Tl-based oxide superconducting thin film according to claim 4, wherein an l ₂ Ba ₂ CaCu ₂ O _x thin film is synthesized.