JPH0648738A - Superconducting laminated thin film and its production - Google Patents

Abstract

PURPOSE:To reduce mutual diffusion or disturbance, etc., of orientation causing deterioration of supeconducting characteristics of a Bi based oxide superconductor and simultaneously facilitate formation of Josephson junction. CONSTITUTION:A buffer layer 2 obtained by subjecting Bi2Sr2CuO2 based oxide to heteroepitaxial growth is formed on a substrate 1 forming a step in <1-10> direction by polishing inclined in (111) direction from normal line to SrTiO3 (100) single crystal substrate face and Bi2(Sr1-XCaX)3Cu2Oy based superconductor layers 3 and 5 sandwiching the both sides of an intermediate layer of Bi2Sr2CuO2 based oxide layer 4 therebetween are formed on the buffer layer 2.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention has a high critical temperature (T _C ).
The present invention relates to a Bi-based superconducting layered thin film having the above and a manufacturing method thereof.

[0002]

Since the discovery of the Related Art Bi-based oxide having high T _C superconductor, its high results in T _C applied research in basic research and electronic devices such as for elucidating the superconductivity mechanism is actively conducted ing. Since the Bi-based oxide superconductor has a high T _C of 85K, inexpensive liquid nitrogen with a boiling point of 77K can be used as a refrigerant when using it, and the facility for maintaining its low temperature is simple. It has the advantage that it can be used. For this reason, it is a great industrial contribution to realize superconducting magnets and superconducting electronic devices that have been realized using conventional low T _C materials using Bi-based oxide superconductors with high T _C. Becomes

When manufacturing a superconducting electronic device using this Bi-based oxide superconductor, it is necessary to control the Josephson tunnel junction and the Josephson weak junction, which are important components of the device, with good controllability. . As a way to do this,
A general technique is to produce a laminated structure with a non-superconductor intermediate layer sandwiched between superconductors.

[0004]

As the non-superconducting material constituting the above-mentioned intermediate layer, the crystal structure is similar to that of the Bi-based oxide superconductor, and the interdiffusion with the Bi-based oxide superconductor. it is small, it is necessary to satisfy the conditions such that a sufficiently high specific resistance at a low temperature, an intermediate layer of Bi-based superconducting multilayer thin film, Al oxide used in the conventional low T _C superconducting multilayer structure If a non-superconducting substance such as a substance is used as it is, problems such as mutual diffusion and disorder of crystal orientation of the Bi-based superconducting laminated thin film occur, and it becomes impossible to manufacture the superconducting laminated thin film.

In order to solve such a problem, an intermediate layer suitable for a Bi-based oxide superconductor is Bi ₂ Sr ₂ CuO, which is a semiconductor oxide of the same family as the Bi-based oxide superconductor.
_x is selected. This oxide has the same structure as the Bi-based oxide superconductor except for the c-axis length, and because of the heteroepitaxial growth and no mutual reaction, the oxide of the Bi-based oxide superconductor is It was expected to be a promising middle class. Actually, on the MgO (100) single crystal substrate, the Bi-based oxide superconductor and the Bi ₂ Sr ₂ CuO were formed.
It has already been reported that a c-axis oriented laminated thin film composed of an _x intermediate layer can be produced with a very good interface.

However, the Bi-based oxide superconductor has a large anisotropy derived from its crystal structure.
Since the coherence length is short in the c-axis direction and a Josephson junction is difficult to form, the coherence length is long in the CuO _{2 in-} plane direction and a Josephson junction is easily formed. The Bi-based superconducting laminated thin film having the c-axis orientation prepared in 1) has a problem that it is disadvantageous in forming a Josephson junction.

On the other hand, for the purpose of solving this problem, by forming a Bi-based superconducting laminated thin film on a SrTiO ₃ (110) single crystal substrate, the c-axis is inclined 45 ° from the substrate surface, and CuO _{2 is formed.} Attempts have also been made to give the surface a vertical component of the substrate. In fact, in such an attempt, although the desired structure formation was successful, B
Since the i-based oxide superconductor is directly grown on the SrTiO ₃ single crystal substrate, a reaction occurs between the thin film and the substrate, and the superconducting property is deteriorated.
There was a problem in the purpose of device formation because the smoothness of the thin film surface, which is a feature of the 0) plane substrate, was extremely poor.

An object of the present invention is to provide a superconducting laminated thin film which has an intermediate layer that does not cause mutual diffusion and disorder of crystal orientation, and is advantageous for forming Josephson junctions, and a method for producing the same.

[0009]

The superconducting laminated thin film according to the first aspect of the present invention is <1-1 when polished by tilting in the (111) direction from the normal line of the SrTiO ₃ (100) single crystal substrate surface.
A single crystal substrate having steps formed in the 0> direction, a buffer layer made of Bi ₂ Sr ₂ CuO _z based oxide formed on the single crystal substrate, and B formed on the upper surface of the buffer layer.
i ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y- based superconductor first superconductor layer and Bi ₂ Sr ₂ CuO _z- based system formed on the upper surface of the first superconductor layer An intermediate layer made of an oxide and Bi ₂ (Sr _1-x C formed on the upper surface of the intermediate layer
It is characterized by having a second superconductor layer made of a ₁ ) ₃ Cu ₂ O _y based superconductor.

A second invention, in the first aspect, the intermediate layer is represented by _{Bi 2 Sr 2 (Cu 1-} a M a) O z consisting formula, M is Al, Fe, Zn, Ni, Pt And a is composed of a non-superconducting oxide having a composition of 0 <a ≦ 0.1.

According to a third invention, in the first invention, the intermediate layer is represented by the formula Bi ₂ (Sr _1-b L _b ) ₂ CuO _z , and L is Pr, Nd, Sm, Eu, Gd, Dy, Ho,
At least one of Er and Y, and b is 0 <b ≦
It is characterized by being composed of a non-superconducting oxide having a composition of 0.5.

A fourth invention is a method for manufacturing a superconducting laminated thin film according to the first to third inventions, wherein the film growth temperature is 550 to 6
Bi ₂ Sr ₂ as a buffer layer was set at 50 ° C.
After CuO _z was heteroepitaxially grown on the single crystal substrate and the buffer layer was formed, the substrate temperature was raised to 680 to 750 ° C., which is the superconducting thin film synthesis temperature, and Bi ₂ (Sr ₁₎ as the first superconductor layer was formed. _-x Ca _x ) ₃ Cu ₂ O _y
And BiSr ₂ CuO _y as an intermediate layer and Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y as a _second superconducting layer are successively heteroepitaxially grown.

[0013]

In the structure of the laminated thin film of the Bi-based oxide superconductor, not the usual SrTiO ₃ (100) single crystal substrate but the normal to the SrTiO ₃ (100) single crystal substrate surface (111) The tilted single crystal substrate in which the steps are formed in the <1-10> direction by polishing while tilting in the (1) direction is used for the Bi-based oxide superconductor and Bi ₂ S.
The r ₂ CuO _z- based oxide is (1) of the SrTiO ₃ single crystal substrate.
On the inclined substrate, the c-axis of the Bi-based oxide superconductor is inclined from the normal direction of the substrate surface by the inclination angle of the substrate and the CuO ₂ surface having a long coherence length is the normal direction on the inclined substrate. Since it will have ingredients,
This is because it is advantageous in manufacturing the Josephson junction.

In addition, a Bi-based oxide superconductor is usually used as Sr.
When formed on a TiO ₃ (100) single crystal substrate, the b-axis of the Bi-based oxide superconductor is observed in both <110> and <1-10> directions of the substrate, and the film has two types of domain structures orthogonal to each other. Strictly speaking, it does not become a single crystal thin film. On the other hand, when a Bi-based superconducting laminated thin film is formed on the above-mentioned graded SrTiO ₃ single crystal substrate,
The b-axis is perpendicular to the step on the substrate surface,
Since the a-axis is aligned in the direction parallel to the step, the thin film becomes a perfect single crystal. When a Josephson junction or the like is formed using such a single crystal thin film, it is not affected by crystal grain boundaries, which is advantageous in device fabrication.

The Bi ₂ Sr ₂ CuO _z type oxide was used as a buffer layer on the tilted SrTiO ₃ single crystal substrate because the buffer layer prevents the reaction between the substrate and the Bi type superconducting laminated thin film, and By relaxing the lattice misfit, the epitaxial property of the Bi-based superconducting laminated thin film can be enhanced, the imperfections of the crystal can be reduced, and the smoothness of the thin film surface can be enhanced.

Next, was used Bi ₂ Sr ₂ CuO _z based oxide as an intermediate layer, in this material is only different cognate lattice constant of the Bi-based oxide superconductor and a c-axis material, CuO ₂
Bi ₂ (Sr _1-x C
a _x ) ₃ Cu ₂ O _y based superconductor layer on top of Bi ₂ Sr ₂ C
Epitaxially growing a uO _z -based oxide layer, and conversely Bi on a Bi ₂ Sr ₂ CuO _z -based oxide layer
_The reason is that it is possible to epitaxially grow the ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y based superconductor layer. Another reason is that the mutual diffusion with the Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y based superconductor layer is small.
If Cu of the intermediate layer material is partially replaced by Al or the like or Sr is partially replaced by Pr or the like, the specific resistance of the Bi ₂ Sr ₂ CuO _z based oxide can be increased, and its application to the Josephson tunnel junction is considered. And more preferable.

In order to synthesize a Bi-based superconducting laminated thin film having good superconducting properties, it is desirable to form the film at a substrate temperature of 680 to 750 ° C. However, when film formation is suddenly started on the SrTiO ₃ single crystal substrate at the substrate temperature at which this excellent superconducting property is obtained, SrTiO ₃ is formed at the initial stage of film formation.
_{3 The} single crystal substrate and the thin film react with each other to cause heterogeneous phases and island-shaped growth, which causes a problem that the surface smoothness of the thin film is considerably deteriorated in the end. On the other hand, Bi ₂ Sr ₂ Cu was used as a buffer layer at the initial stage of film formation.
When the O _z -based oxide is formed at a substrate temperature of 550 to 650 ° C. for 24 Å or more, for example, about 72 Å, this layer serves as a buffer layer at the substrate interface, and it becomes possible to prevent the reaction between the substrate and the thin film. After the formation of the buffer layer, the substrate temperature is increased to a growth temperature of the Bi-based superconducting laminated thin film of 680 to 750 ° C. to form a film, so that B has excellent surface smoothness and good superconducting properties.
It is possible to obtain a laminated thin film of an i-based oxide superconductor.

[0018]

Embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 is a perspective sectional view of a Bi type superconducting laminated thin film of the present invention. In this example, the substrate 1, the buffer layer 2, and the non-superconductor intermediate layer Bi ₂ Sr ₂ CuO _{z were used.}
And a Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y -based superconductor layer 3 and 5 sandwiching both sides of the Bi ₂ Sr ₂ CuO _z -based oxide layer 4. ing.

In this example, an ion beam sputtering apparatus was used to manufacture a Bi type superconducting laminated thin film. In this apparatus, atomic oxygen excited by microwaves is used as an oxidation source of the thin film, and the oxygen partial pressure near the substrate 1 was about 10 ⁻³ Torr. Thin film growth temperature is about 40
It is 0 angstrom / H. In thin film manufacturing,
Bi ₂ O ₃ in the chamber, Bi ₂ Sr ₂ CuO _z, and provided with a target of _{Sr 2 CaCu 2 O x ',} 3
By controlling the two sputter sources independently, the buffer layer 2, Bi ₂ (Sr _1-x C
a _{x) 3} Cu ₂ O _y based superconductor layers 3,5, Bi ₂ Sr ₂
The CuO _z based oxide layer 4 is separately formed.

The substrate 1 is an SrTiO ₃ (100) single crystal substrate surface which is inclined by 8 ° in the (111) direction from its normal line and polished to form steps in the <1-10> direction. A crystal substrate was used. The size of the substrate 1 is 1
It is 5 mm square and has a thickness of 0.5 mm. First, as the buffer layer 2, a Bi ₂ Sr ₂ CuO _z- based oxide is grown on the substrate 1 for 2 to 3 unit layers (about 48 to 72 Å). The buffer layer 2 may have a thickness of 1 unit layer or more. When the buffer layer 2 is formed, the substrate temperature is formed at a temperature as low as 550 ° C to 650 ° C. This is intended to suppress the reaction between the substrate 1 and the thin film. If this buffer layer 2 does not exist, Bi ₂ (Sr _1-x Ca _x ) ₃ formed thereafter is formed.
The Cu ₂ O _y -based superconductor layers 3 and 5 react with the substrate 1 to deteriorate crystallinity, thin film surface smoothness, and superconducting properties, while forming the buffer layer 2 results in crystallinity. A thin film having excellent surface smoothness and excellent superconducting properties can be obtained.

After forming the buffer layer 2, the substrate temperature is set to about 7
Bi ₂ (Sr _1-x Ca _x ) ₃ Cu by raising the temperature to 00 ° C
₂ O _y- based superconductor layer 3 is deposited, and Bi ₂ Sr is deposited thereon.
₂ CuO _z based oxide layer 4 is deposited, and finally, Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y based superconductor layer 5 is deposited thereon.
Are deposited to form a laminated thin film.

When the Bi-based superconducting laminated thin film thus produced was analyzed by a secondary ion mass spectrometer (SIMS), it was confirmed that each layer was deposited without mutual diffusion, and a good laminated thin film was produced. It was

When the structure of this laminated thin film was evaluated by an X-ray diffraction method, Bi ₂ (Sr _1-x Ca _x ) ₃ Cu _{2 was obtained.}
Peaks of diffraction O _y-bearing oxide and Bi ₂ Sr ₂ CuO _z based oxide phase alone is observed, heterogeneous phase was observed at all.

Furthermore, when the surface of this laminated thin film and the surface of each single-layer thin film produced under the same conditions were observed with a scanning electron microscope, the surface was flat with an accuracy of 50 A or less, and a very good laminated thin film was obtained. It was confirmed that it was formed.

The crystal orientation relationship between this laminated thin film and the tilted SrTiO ₃ single crystal substrate is shown by RHEED (reflection).
high energy electron dif
Fraction) and a transmission electron microscope, the c-axis of each layer is (001) of the SrTiO ₃ single crystal substrate as shown in the detailed view of the cross-sectional structure of the laminated thin film in FIG.
It has been confirmed that the growth is perpendicular to the surface, and for this reason, it deviates from the normal direction of the substrate surface by 8 ° which is the inclination angle of the substrate. Therefore, the CuO ₂ surface having a long coherence length in the Bi-based oxide superconductor also has an angle of 8 ° with the substrate surface. As a result, as shown in FIG. 2, B
i ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y- based superconductor layer 5 and Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y- based superconductor layer 3
The CuO ₂ surface of is continuously connected with the Bi ₂ Sr ₂ CuO _z based oxide layer 4, which is an intermediate layer, interposed therebetween. In contrast to the conventional Bi-based superconducting laminated thin film having a short coherence length in the c-axis direction, the laminated thin film according to the present invention has a CuO ₂ surface with a long coherence length in the upper superconductor and the lower superconductor. It was found that the structure is very advantageous for the purpose of making the Josephson junction.

Further, in the in-plane epitaxial relationship, the modulation structure in the b-axis direction peculiar to the Bi-based oxide superconductor is always perpendicular to the steps formed on the substrate. The a-axis inevitably becomes parallel to the step. Therefore, the laminated thin film according to the present invention is
It is a complete single crystal thin film without domain structure,
It was confirmed that the film was a good quality thin film without the influence of grain boundaries.

Bi ₂ (Sr _1-x
Ca _{x) 3} Cu ₂ O _y system was measured temperature change of the electric resistance of the superconductor layers 3 and 5, was confirmed to have a T _C of about any of the layers 85K, excellent quality superconducting properties of _{Bi 2 (Sr 1-x Ca} x) 3 Cu 2 O y based superconductor layers 3 and 5 were found to be produced. Also, Bi ₂
Sr ₂ CuO _z based oxide layer 4 upper and lower Bi ₂ (Sr _1-x C
Since the resistance value between the a _x ) ₃ Cu ₂ O _y based superconductor layers is sufficiently high and is close to the value expected from the resistance value in the c-axis direction, the Bi ₂ Sr ₂ CuO _z based oxide layer 4 is Bi ₂ (Sr
It was confirmed that _1-x Ca _x ) ₃ Cu ₂ O _y- based superconductor layers were uniformly formed. Furthermore, Bi ₂ Sr ₂ C
The specific resistance in the c-axis direction of the uO _z -based oxide 4 could be increased by a factor of two or more as compared with the case without no replacement, by partially substituting Cu with Al or the like and partially substituting Sr with Pr or the like. .

[0029]

As described in detail above, the present invention is
A high-quality Bi-based superconducting stack, which has an intermediate layer that does not cause mutual diffusion and disorder of crystal orientation, and has a long coherence length CuO ₂ surface that has a normal direction component, which is advantageous in manufacturing Josephson junctions. A thin film is provided, and the effect of applying a Bi-based superconductor to a superconducting electronic device is great.

[Brief description of drawings]

FIG. 1 is a perspective sectional view of a Bi-based superconducting laminated thin film of the present invention.

FIG. 2 is a sectional view showing a structure of a Bi-based superconducting laminated thin film of the present invention.

[Explanation of symbols]

1 substrate 2 buffer layer 3,5 Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y based superconductor layer 4 Bi ₂ Sr ₂ CuO _z based oxide layer 6 SrTiO ₃ step of single crystal substrate surface b Bi -Axis direction of system-based superconducting laminated thin film c-axis direction of Bi-system superconducting laminated thin film

Claims (4)

[Claims] 1. SrTiO ₃ (100) single crystal substrate is tilted in the (111) direction from the normal to the surface and polished to <1.
A single crystal substrate having a step formed in the −10> direction, a buffer layer made of Bi ₂ Sr ₂ CuO _z based oxide formed on the single crystal substrate, and Bi ₂ (Sr ₁ formed on the upper surface of the buffer layer. _-x Ca
_x ) ₃ Cu ₂ O _y based superconductor, a first superconductor layer, and Bi ₂ Sr ₂ Cu formed on the upper surface of the first superconductor layer.
An intermediate layer made of an O _z -based oxide and Bi ₂ (Sr _1-x Ca ₁ ) ₃ formed on the upper surface of the intermediate layer
A superconducting laminated thin film having a second superconductor layer made of a Cu ₂ O _y based superconductor. 2. The superconducting laminated thin film according to claim 1,
Intermediate layer is represented by _{Bi 2 Sr 2 (Cu 1-} a M a) O z consisting formula, M is Al, Fe, Zn, Ni, and at least one of Pt, a is 0 <a ≦ A superconducting laminated thin film comprising a non-superconducting oxide having a composition of 0.1. 3. The superconducting laminated thin film according to claim 1,
The intermediate layer is represented by the formula Bi ₂ (Sr _1-b L _b ) ₂ CuO _z , and L is Pr, Nd, Sm, Eu, Gd, Dy, H.
at least one of o, Er, and Y, and b is 0 <b
A superconducting laminated thin film comprising a non-superconducting oxide having a composition of ≦ 0.5. 4. The method for producing a superconducting laminated thin film according to claim 1, 2, or 3, wherein the film growth temperature is 550 to 650 ° C.
And Bi ₂ Sr ₂ CuO as a buffer layer
_After heteroepitaxially growing _z on a single crystal substrate and forming a buffer layer, the substrate temperature is raised to 680 to 750 ° C. which is the superconducting thin film synthesis temperature, and Bi ₂ (Sr _{1- x} Ca _x ) ₃ Cu ₂ O _y , BiSr ₂ CuO _y as an intermediate layer, and Bi ₂ (Sr _1-x Ca _x ) ₃ Cu ₂ O _y as a _second superconducting layer are sequentially heteroepitaxially grown. A method of manufacturing a superconducting laminated thin film, comprising: