JP2959290B2 - Superconducting laminated thin film and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high critical temperature (T _C)
And a method of manufacturing the same.

[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. Bi-based oxide superconductors, due to its high T _C of 85K, when its use is inexpensive liquid nitrogen having a boiling point of 77K can be used as a refrigerant, and easy installation for the low-temperature maintenance It has the advantage that it can be used. Therefore, realizing a conventional superconducting magnet or superconducting electronic element using a substance having a low T _C by using a Bi-based oxide superconductor having a high T _C will greatly contribute to industry. Becomes

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

[0004]

The non-superconducting material constituting the above-mentioned intermediate layer has a crystal structure similar to that of the Bi-based oxide superconductor, and 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 interdiffusion and disorder in the crystal orientation of the Bi-based superconducting laminated thin film occur, and it becomes impossible to produce the superconducting laminated thin film.

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

[0006] However, Bi-based oxide superconductors have large anisotropy derived from their crystal structure.
Since the coherence length is short in the c-axis direction and a Josephson junction is difficult to form, while the coherence length is long in the CuO ₂ plane direction and the Josephson junction is easy to form. The Bi-based superconducting laminated thin film exhibiting the c-axis orientation produced in the above method had a problem that it was disadvantageous for forming a Josephson junction.

On the other hand, in order to solve this problem, by forming a Bi-based superconducting laminated thin film on a SrTiO ₃ (110) single-crystal substrate, the c-axis is tilted by 45 ° from the substrate surface and CuO ₂ Attempts have been made to make the surface have a component perpendicular to the substrate. In fact, in such an attempt, although the intended structure was successfully formed, B
Since the i-based oxide superconductor is grown directly on the SrTiO ₃ single crystal substrate, a reaction occurs between the thin film and the substrate, deteriorating the superconductivity, and (11)
0) There was a problem in the purpose of device formation because the smoothness of the surface of the thin film, which is a feature on the surface substrate, was extremely poor.

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

[0009]

The superconducting laminated thin film of the first invention is polished by being tilted in the (111) direction from the normal line of the SrTiO ₃ (100) single crystal substrate surface to <1-1.
0> and the single crystal substrate formed with steps in the direction, and the buffer layer made of Bi ₂ Sr ₂ CuO _z-based oxide formed on a single crystal substrate, which is formed on the upper surface of the buffer layer B
_{i 2 (Sr 1-x Ca} x) 3 Cu 2 O y based superconductor and a first superconductor layer of the first Bi ₂ Sr ₂ CuO _z system formed on the upper surface of the superconductor layer An intermediate layer made of an oxide, and Bi ₂ (Sr ₁ -xC) formed on the upper surface of the intermediate layer.
It is characterized by having a _{1) 3} Cu ₂ O _y system second superconductor layer made of a superconductor.

According to a second aspect, in the first aspect, the intermediate layer is represented by a formula of Bi ₂ Sr ₂ (Cu _{1 -a} M _a ) O _z , wherein M is Al, Fe, Zn, Ni, Pt. Wherein a is a non-superconducting oxide having a composition of 0 <a ≦ 0.1.

According to a third aspect, in the first aspect, the intermediate layer is represented by a formula of Bi ₂ (Sr ₁ -bL _b ) ₂ CuO _z , wherein L is Pr, Nd, Sm, Eu, Gd, Dy, Ho,
At least one of Er and Y, and b is 0 <b ≦
It is made of a non-superconducting oxide having a composition of 0.5.

A fourth invention is a method for producing a superconducting laminated thin film according to any one of 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.
The CuO _z grown heteroepitaxially on a single crystal substrate, after the formation of the buffer layer, increasing the substrate temperature to six hundred and eighty to seven hundred fifty ° C. a superconducting thin film synthesis temperature, Bi ₂ (Sr ₁ as a first superconductor layer _{-x Ca x) 3 Cu 2 O} y
When the BiSr ₂ CuO _y as an intermediate layer, and wherein _{Bi 2 (Sr 1-x Ca} x) 3 Cu 2 O y and turn it to heteroepitaxial growth of a second superconductive layer.

[0013]

[Action] In the structure of the laminated thin films of Bi-based oxide superconductor, rather than the usual SrTiO ₃ (100) single crystal substrate as a substrate, to SrTiO ₃ (100) single crystal substrate surface, from the normal (111 The inclined single crystal substrate having steps formed in the <1-10> direction by being polished in the inclined direction is used for Bi-based oxide superconductors 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 oriented in the normal direction on the inclined substrate. Since it comes to have components,
This is because it is advantageous in producing a Josephson junction.

Usually, the Bi-based oxide superconductor is made of 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 perpendicular 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 inclined 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, the device is not affected by crystal grain boundaries, which is advantageous in device fabrication.

[0015] as a buffer layer on the inclined SrTiO ₃ single crystal substrate, it was used Bi ₂ Sr ₂ CuO _z based oxide, together with the buffer layer prevents the reaction of the substrate and the Bi-based superconductors laminated thin film, both This is because by relaxing the lattice misfit, the epitaxial property of the Bi-based superconducting laminated thin film can be enhanced, crystal imperfections 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 Bi ₂ Sr ₂ C on the Cu ₂ O _y -based superconductor layer
epitaxial growth of a uO _z -based oxide layer, and conversely, Bi on the Bi ₂ Sr ₂ CuO _z -based oxide layer
Things _{2 (Sr 1-x Ca x} ) 3 Cu 2 O y based superconductor layer can be epitaxially grown is the reason. Moreover, the reason that the mutual diffusion is small between the _{Bi 2 (Sr 1-x Ca} x) 3 Cu 2 O y based superconductor layer.
The Cu in the intermediate layer material with Al or the like, or if partially substituted with Sr in Pr or the like, can increase the resistivity of Bi ₂ Sr ₂ CuO _z based oxide, consider the application to Josephson tunnel junctions Is more preferable.

In order to synthesize a Bi-based superconducting laminated thin film having good superconducting characteristics, it is desirable to form a film at a substrate temperature of 680 to 750 ° C. However, when the film formation is started on the SrTiO ₃ single crystal substrate at the substrate temperature at which the good superconductivity is obtained, the SrTiO
_{(3) The} single-crystal substrate and the thin film react with each other to generate a different phase or island-like growth, which causes a problem that the surface smoothness of the thin film eventually deteriorates considerably. In contrast, Bi ₂ Sr ₂ Cu
O _z based oxide substrate temperature 550 to 650 ° C. at 24 angstroms, for example, formed of about 72 angstroms, the layer is a buffer layer in the substrate interface, it is possible to prevent the reaction between the substrate and the thin film. After the formation of the buffer layer, the substrate temperature is raised to a growth temperature of 680 to 750 ° C. for the Bi-based superconducting laminated thin film to form a film, so that B having excellent surface smoothness and excellent superconductivity is obtained.
A laminated thin film of the i-based oxide superconductor can be obtained.

[0018]

Next, embodiments of the present invention will be described with reference to the drawings.

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

In the present embodiment, an ion beam sputtering apparatus was used to manufacture a Bi-based superconducting laminated thin film. In this apparatus, atomic oxygen generated by microwave excitation was used as a thin film oxidation source, and the oxygen partial pressure near the substrate 1 was about 10 ^-3 Torr. Thin film growth temperature is about 40
0 Å / H. In thin film manufacturing,
A target of Bi ₂ O ₃ , Bi ₂ Sr ₂ CuO _z , and Sr ₂ CaCu ₂ O _{x ′} was provided in the chamber, and 3
By independently controlling the two sputter sources, 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
It is doing a separate formation of CuO _z-based oxide layer 4.

The substrate 1 is polished with respect to the SrTiO ₃ (100) single crystal substrate surface at an angle of 8 ° from the normal line to the (111) direction, thereby forming 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 0.5 mm thick. On the substrate 1, first as a buffer layer 2, 2-3 units layer Bi ₂ Sr ₂ CuO _z based oxide (approximately 48-72 Å) is grown. The buffer layer 2 may have a thickness of at least one unit layer. When the buffer layer 2 is formed, the substrate is formed at a slightly lower temperature of 550 ° C. to 650 ° C. This is intended to suppress the reaction between the substrate 1 and the thin film. If there is no buffer layer 2, Bi ₂ (Sr _1-x C _ax ) ₃ formed thereafter
While the Cu ₂ O _y -based superconductor layers 3 and 5 react with the substrate 1 to deteriorate the crystallinity, the surface smoothness of the thin film, and the superconducting characteristics, the formation of the buffer layer 2 allows A thin film having both excellent surface smoothness and excellent superconductivity can be obtained.

After forming the buffer layer 2, the substrate temperature is reduced to about 7
To 00 ° C. is raised _{Bi 2 (Sr 1-x Ca} x) 3 Cu
₂ O _y based superconductor layer 3 is deposited, Bi ₂ Sr thereon
₂ CuO _z based oxide layer 4 is deposited, finally, that on the _{Bi 2 (Sr 1-x Ca} x) 3 Cu 2 O y based superconductor layer 5
Is deposited to produce a laminated thin film.

When the Bi-based superconducting laminated thin film thus manufactured is analyzed by a secondary ion mass spectrometer (SIMS), it is confirmed that each layer is deposited without interdiffusion, and a high quality laminated thin film is produced. Was.

Further, when evaluating the structure of the multilayer thin film by X-ray _{diffractometry, Bi 2 (Sr 1-x} Ca x) 3 Cu 2
Diffraction peaks of only the O _y -based oxide phase and the Bi ₂ Sr ₂ CuO _z -based oxide phase were observed, and no hetero phase was observed.

Further, 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 the laminated thin film and the tilted SrTiO ₃ single crystal substrate is shown by RHEED (reflection).
high energy electron dif
Inspection by F. fraction and transmission electron microscopy shows that the c-axis of each layer is (001) of the SrTiO ₃ single crystal substrate as shown in the detailed view of the sectional structure of the laminated thin film in FIG.
The growth was perpendicular to the surface, and it was confirmed that the substrate was shifted from the normal direction of the substrate surface by 8 °, which is the inclination angle of the substrate. Therefore, in the Bi-based oxide superconductor, the CuO ₂ plane having a long coherence length also has an angle of 8 ° with the substrate surface. As a result, as shown in FIG.
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
CuO ₂ surface is continuously connected across the Bi ₂ Sr ₂ CuO _z -based oxide layer 4 which is an intermediate layer. Compared to the conventional Bi-based superconducting laminated thin film which was located in the c-axis direction having a short coherence length but not in the c-axis direction, the laminated thin film according to the present invention was characterized in that the upper superconductor and the lower superconductor had a CuO ₂ surface having a long coherence length. It was found that the structure was very advantageous for the purpose of making a 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 will necessarily be 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 a good quality laminated thin film was obtained without the influence of the crystal 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
The upper and lower portions of Bi ₂ (Sr ₁ -xC) on the Sr ₂ CuO _z -based oxide layer 4.
a _{x) 3} Cu ₂ O _y system resistance of the superconductor layers is sufficiently high, since it is close to the value expected from the resistance value of c-axis direction, Bi ₂ Sr ₂ CuO _z based oxide layer 4 is Bi ₂ (Sr
It was confirmed that the layer was formed uniformly between the _1-x Ca _x ) ₃ Cu ₂ O _y -based superconductor layers. Further, Bi ₂ Sr ₂ C
The c-axis direction of the specific resistance of uO _z based oxide 4, by a part replacement of Sr by partially substituted and Pr, etc. Cu of Al or the like, could be enhanced more than twice as compared with the case without the substitution .

[0029]

As described in detail above, the present invention provides
High-quality Bi-based superconducting laminate that has an intermediate layer that does not cause interdiffusion and disordered crystal orientation, and that has a long coherence length CuO ₂ surface has a normal component, which is advantageous for Josephson junction fabrication The present invention provides a thin film, and has a great effect in applying a Bi-based superconductor to a superconducting electronic device.

[Brief description of the drawings]

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

FIG. 2 is a cross-sectional view showing the 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 system step b Bi superconductor layer 4 Bi ₂ Sr ₂ CuO _z based oxide layer 6 SrTiO ₃ single crystal substrate surface B-axis direction of Bi-based superconducting laminated thin film c-axis direction of Bi-based superconducting laminated thin film

──────────────────────────────────────────────────続 き Continuation of front page (56) References Fujita et al. , "In-plane anisotropic transport properties obsessed in epitaxial Bi2 (Sr, Ca) 3Cu2Ox films growth on tiled (001) SrTiO3. Phys. Lett. Vol. 19, 4 Nov. 1991, p. 2445-2447 T.C. Satoh et al. , "Microstructure of Bi2 (Sr, Ca) 3Cu2Ox / Bi2Sr2CuOy / Bi2 (Sr, Ca) 3Cu2Ox trilayer films fabricated by steaming proofs. 183, 1991, pp. 286-292 Satoh et al. , "M icrostructure of B i2 ( Sr, Ca) 3Cu2Ox th in films on vicina l SrTiO3 substrate s, Physica C, Vol.191, 1992, pp.359-362 (58) investigated the field (Int.Cl. ^6, DB name) C01G 29/00 C01G 1/00 C30B 1/00-35/00 H01B 12/06 H01B 13/00 H01L 39/24 CA (STN)

Claims (4)

(57) [Claims] (1) polishing by tilting in the (111) direction from the normal line of the surface of the SrTiO ₃ (100) single crystal substrate to <1
-10> and a single crystal substrate formed with steps in the direction, and the buffer layer made of Bi ₂ Sr ₂ CuO _z-based oxide formed on a single crystal substrate, Bi ₂ formed on the upper surface of the buffer layer (Sr _{1 -x} Ca
_{x) 3} Cu ₂ O _y based superconductor and a first superconductor layer of the first upper surface formed Bi ₂ Sr ₂ Cu 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
Superconducting multilayer thin film characterized by having a second superconductor layer of Cu ₂ O _y based superconductors. 2. The superconducting laminated thin film according to claim 1,
The intermediate layer is represented by a formula of Bi ₂ Sr ₂ (Cu _{1 -a} M _a ) O _z , where M is at least one of Al, Fe, Zn, Ni, and Pt, and 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 , where 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, wherein the film growth temperature is from 550 to 650 ° C.
Is set to Bi ₂ Sr ₂ CuO as a buffer layer.
_z is heteroepitaxially grown on a single crystal substrate, and after the formation of the 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₋ and _{x Ca x) 3 Cu 2 O} y, and BiSr ₂ CuO _y as the intermediate layer, thereby sequentially heteroepitaxial growth and _{Bi 2 (Sr 1-x Ca} x) 3 Cu 2 O y as a second superconducting layer A method for producing a superconducting laminated thin film, comprising: