JP2653003B2 - Oxide superconducting thin film synthesis method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for synthesizing an oxide superconducting thin film.

[0002]

2. Description of the Related Art Superconducting thin films are indispensable for applications to quantum magnetic interference devices based on Josephson junctions, superconducting LSI wiring, and superconducting active devices. In recent years, including a Y-type oxide superconductor with a critical temperature of 90 K class discovered by the University of Houston, Chu et al. In February 1987, a critical temperature of 11 by Maeda et al.
0K-class Bi-based oxide superconductors, and a critical temperature of 120 from ZZ Sheng et al.
K-class Tl-based oxide superconductors and oxide superconductors having a critical temperature exceeding liquid nitrogen temperature have been discovered one after another.
This means that superconducting applied devices which had to use liquid helium (He) in the past can be realized with liquid nitrogen. In particular, thinning of these oxide superconductors can be achieved by using a Josephson active device that operates at a temperature higher than the liquid nitrogen temperature, A superconducting LSI wiring is realized, and its application can be widely used. From the viewpoint of application of the oxide superconductor thin film device, what is called "in-situ" synthesis that does not require high-temperature heat treatment after film formation is important. In addition, from the viewpoint of device application, it is necessary to epitaxially grow a superconducting thin film on a substrate, and to continuously perform heteroepitaxial growth of a barrier layer and the like thereon. For that purpose, a technique for synthesizing an extremely flat thin film having no island-like growth or a different phase on the film surface is indispensable. In-s of Bi-based superconducting thin film
In the itu synthesis technology, various film formation methods such as a method using laser abrasion, a method using RF magnetron sputtering, a method using vacuum deposition, and a method using ion beam sputtering have been reported. In vacuum deposition or ion beam sputtering, which forms a film in an environment having a low oxygen partial pressure, island-like growth or generation of a heterogeneous phase on the surface of the thin film is a serious problem in device application.

[0003]

In the in-situ synthesis of an oxide high-temperature superconducting thin film, as-gro
It is important that the thin film surface has good flatness while having superconductivity at wn. as-grown
In order to obtain superconducting properties in the film, a film formation temperature at which surface migration is sufficient is required. However, Sr
When a TiO ₃ substrate is used, if a film is directly formed at this film forming temperature, a heterogeneous phase or an island-like growth occurs due to a reaction with the substrate during the initial film growth process at the substrate interface. Traces of growth and different phases remain as morphologies on the film surface to the end. An object of the present invention is to provide a method for in-situ synthesizing a Bi-based superconducting thin film having good flatness without occurrence of island-like growth grains and foreign phases on the film surface.

[0004]

SUMMARY OF THE INVENTION The present invention provides a strontium titanate (100) for a Bi-based oxide superconducting single crystal thin film.
In "in situ" (in-situ) synthesis of the substrate, the substrate temperature of film growth initially 550 to 650 ° _C., the Bi ₂ Sr ₂ CuO _x is hetero-epitaxially grown on the substrate as a buffer layer, then Substrate temperature B
A method for synthesizing an oxide superconducting thin film, which comprises growing a superconducting single crystal thin film by raising the temperature to a superconducting thin film synthesizing temperature of i ₂ Sr ₂ CaCu ₂ O _x . In the method of the present invention, first, SrTiO ₃ (100)
₃ is preferably homoepitaxially grown at a substrate temperature of 500 to 600 ° C., and then Bi ₂ Sr ₂ CaCu ₂ O _x is grown, and the SrTiO ₃ (100) single crystal substrate surface has a normal line ( It is preferable to use the substrate surface on which steps are formed in the <1-10> direction by polishing while tilting in the 111) direction as a substrate for synthesizing a Bi-based oxide superconducting thin film.

[0005]

In the heteroepitaxial growth of a Bi-based superconductor on a SrTiO ₃ substrate, <110> SrTiO ₃ ///
The <100> Bi-based superconductor is epitaxially grown in an orientation relationship, and its lattice mismatch is 0.1% or less, which is extremely effective for epitaxial growth. in-sit
The synthesis of Bi-based oxide superconductor at u is performed at 600 to 750 ° C.
Although it is possible within the range of the substrate temperature, good as-gro
In order to obtain the superconducting properties of the wn film, it is desirable that the substrate temperature be about 680 ° C. or higher. However, when film formation is started immediately at a substrate temperature at which such good superconducting properties can be obtained, the SrTiO ₃ substrate and the film react at the initial stage of film formation, and a heterogeneous phase or island-like growth occurs. Will remain. On the other hand, when the initial growth temperature of the film formation process is 550 to 650 ° C., for example, about 600 ° C.,
Bi ₂ Sr ₂ CuO _x layer, which is a homologous system of
By forming a buffer layer at the interface of the substrate as a buffer layer of Å or more, for example, about 60 Å, it is possible to prevent a reaction between the substrate and the film. After the formation of the buffer layer, Bi ₂ Sr ₂ 80K of CaCu ₂ O _x
By growing the superconducting layer at a substrate temperature of about 700 ° C., ultimately the flatness is extremely good and the as-gro
A Bi-based superconducting thin film exhibiting good superconducting properties at wn can be synthesized.

Although it is obvious that the surface flatness of a film is strongly affected by the occurrence of a different phase in the initial stage of film formation, the polished state of the substrate surface is also greatly related to the occurrence of the different phase. In particular, microscopically, a crystal defect on the substrate surface due to stress during polishing becomes a site for generating a different phase. As a pre-treatment before film formation, after a thermal oxidation treatment in a film formation chamber, a substrate temperature of 500 to
By performing homoepitaxial growth of SrTiO ₃ at 600 ° C., such crystal defects can be reduced and the film flatness of the surface of the final superconducting thin film can be improved.

[0007] Further, the Bi-based superconductor has an incommensurate modulation structure with a period about 5 times the period of the basic lattice in the b-axis direction. In film formation using a normal SrTiO ₃ (100) substrate surface, the film forms a domain structure, and the RHE
In the ED, this modulation structure is based on the <110> and <
1-10> Observed from both 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 shifted by 1 ° in the (111) direction.
When a film is grown on an inclined polished substrate inclined by about 7 °, epitaxial growth is performed so that 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. That is, a step structure is formed on the SrTiO ₃ substrate corresponding to the inclined angle, and the c axis of the film is epitaxially grown perpendicular to the terrace surface on the substrate surface. It is tilted in the angle <1 -10> direction. The film on the inclined substrate is substantially a, b, c of the film.
It becomes a so-called single crystal film in which the directions of the axes are all aligned, and is extremely important in producing a device utilizing the electrical conductivity anisotropy peculiar to a high-temperature oxide superconductor.

[0008]

EXAMPLES Specific examples are shown below. An ion beam sputtering apparatus was used for synthesizing the Bi-based oxide superconducting thin film. In this apparatus, atomic oxygen generated by microwave excitation is used as an oxidation source, and the oxygen partial pressure near the substrate is about 10 ⁻³ Torr.
Met. At this time, the amount of active oxygen reached near the substrate is 2
× 10 ¹⁵ / cm ² · s. The film growth rate is about 400 Å / H, and Bi ₂ O ₃ , Bi
_A target of ₂ Sr ₂ CuO _x and a target of Sr ₂ CaCu ₂ O _x is provided, and by controlling three sputter sources independently, control of a Bi composition during a film forming process and a buffer layer Bi ₂ Sr ₂ CuO _x and superconducting layer Bi ₂ S
Switching to r ₂ CaCu ₂ O _x is performed.

FIG. 2 shows Bi ₂ Sr ₂ CaC to which the present invention is applied.
It is a typical example of substrate temperature control at the time of synthesizing a u ₂ O _x film. (100) SrTiO ₃ was used as the substrate, and the initial film formation temperature was 600 ° C. Bi ₂ Sr ₂ CuO as buffer layer
_{The x} film is grown to about 60 angstroms, then the substrate temperature is raised to 700 ° C. After the substrate temperature reaches 700 ° C., Bi ₂ Sr ₂ CaCu ₂ O _x is grown on the buffer. According to the present invention Bi ₂ Sr ₂ CuO _x buffer 1 - has a layer ₂ (100) SrTiO ₃ Bi 2 on the substrate 1
The structure of the Sr ₂ CaCu ₂ O _x film 3 is shown. This buffer
By introducing the layer 2, the Bi-based superconducting layer performs epitaxial growth with very little flatness and good flatness. The thickness of this buffer layer should be at least 24 Å
The effect of suppressing the reaction with the substrate could be confirmed if it was not less than the threshold. The buffer layer is about 400 ° C or more and about 750 ° C.
Although grown below, the buffer layer grown at low temperature has poor crystallinity and affects the superconductivity crystallinity thereon. Therefore, a growth temperature of at least 550 ° C. is desirable. At a temperature of 650 ° C. or higher, the buffer layer itself reacts with the substrate, deviating from the originally intended place. Therefore, the growth condition of the buffer layer is preferably 550 ° C. or more and 650 ° C. or less.

The flatness of the thin film on the SrTiO ₃ substrate via the buffer layer is extremely good. For example, AF
Observation with M (Atom Force Microscopy) confirmed that the maximum surface step in the range of 10 μm ² was about 15 Å at the maximum. This corresponds to the step of the half unit cell of the Bi-based superconductor, and is a limit value that can be controlled as the film thickness, and the film flatness is extremely good. In addition, it is known that the RHEED of this film exhibits four-fold symmetry around the c-axis, and has a domain structure in which the incommensurate modulation directions are orthogonal to each other macroscopically within the film plane. The Bi ₂ Sr ₂ CaCu ₂ O _x thin film formed on this (100) SrTiO ₃ via a buffer exhibited good superconducting properties, for example, a superconducting transition temperature of 80 K at a thickness of 500 Å. Bi
The crystal structure of the system superconductor slightly extends in the direction of the modulation structure and is cubic. (100) plane SrTiO
_{3 Even} in a thin film synthesized on a single crystal substrate, this modulation structure is clearly observed by RHEED or the like, but in many cases, the modulation structure is observed four times symmetrically in the film plane. In a strict sense, the film is not completely epitaxially grown, but has a microscopic domain structure, and the orientation of the modulation structure in each domain is <110> or <1.
−10> can be taken separately.

On the other hand, the (100) SrTiO ₃ single crystal substrate surface polished while being tilted in the direction of 1 ° to 8 ° <111> from the <001> axis is used as a substrate for synthesizing a Bi-based oxide superconducting thin film. This allows the modulation structure to be <1
It was confirmed by RHEED pattern and X-ray diffraction that a Bi-based single crystal thin film aligned in the 10> direction could be synthesized. This film structure is shown in FIG. The membrane is made of Bi ₂
Using Sr ₂ CuO _x as a buffer, the reaction between the substrate and the superconducting layer is suppressed. Bi ₂ Sr ₂ CuO _x crystal is B
Like the i ₂ Sr ₂ CaCu ₂ O _x crystal, it has an incommensurate modulation structure in the b-axis direction. Steps are formed on the surface of the inclined substrate according to the inclination angle, which is <1.
It is formed at substantially equal intervals in the −10> direction. This step becomes a new creation site for crystal growth. When crystal growth proceeds along the step, the modulation structure is formed in a direction perpendicular to the step, so that the film (buffer) has an a-axis in the direction parallel to the step and a b-axis in the direction perpendicular to the step. Grow as a single crystal 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 with respect to the substrate surface. The Bi ₂ Sr ₂ CaCu ₂ O _x film on the buffer layer is made of a single crystal B
An i ₂ Sr ₂ CaCu ₂ O _x film grows. Of course this Bi
The c-axis of the ₂ Sr ₂ CaCu ₂ O _x film is also inclined from the original SrTiO ₃ substrate surface by the inclination angle.

The electrical characteristics of the tilted single crystal film are very specific. A-axis direction of the film (<110> SrTiO ₃
Direction) shows normal metallic electrical conductivity, and shows a good superconducting transition of about 80K. On the other hand, in the b-axis direction (<1−10> SrTiO ₃ direction), it exhibits semiconducting electric conduction characteristics, and similarly exhibits a superconducting transition of about 80K. This electric conductivity is similar to the electric conductivity in the c-axis direction of the bulk single crystal. This does not reflect the anisotropy in the b-axis direction of the film. Ab from conductivity measurement of bulk single crystal
The resistivity in the c-axis direction is higher than the in-plane resistivity by four digits or more. Since the c-axis is inclined in the electric conduction in the <1−10> direction by using the inclined substrate, the c-axis is included in the conduction path. For this reason, it can be interpreted that the anisotropy of c-axis electric conductivity was observed as the electric conductivity anisotropy in the substrate surface of the epitaxial film. The fact that such c-axis electric conductivity anisotropy appears in the plane of the epitaxial film on the inclined substrate is extremely useful for device application.

Next, the surface of the SrTiO ₃ substrate is rough even if it is mirror-polished. S on SrTiO ₃
The homoepitaxial growth of rTiO ₃ is performed at a substrate temperature of 400.
About 700 ° C., but especially about 500 ° C. to 600 ° C.
In the case of homoepitaxial growth of SrTiO _{3 of} about 100 Å or more, the flatness of the substrate surface is improved. For example, RHE
SrTiO with ED showing a somewhat diffuse pattern
₃ with respect to the substrate, substrate temperature 550 ° C., oxygen partial pressure 10 ⁻⁴
100 Å of SrTiO ₃ at Torr
By homo-epitaxial growth, it was possible to modify the substrate surface into a steep RHEED pattern. The surface flatness of the Bi-based superconducting thin film using this substrate is extremely good over a large area, and can be sufficiently applied to increase the film area, which is important for device application.

[0014]

As described above, by applying the present invention, a Bi-based epitaxial superconducting film having good flatness and superconducting characteristics can be easily synthesized in-situ. Also, a single crystal thin film on a tilted substrate is extremely useful for device application using its electrical anisotropy. Further, the application of the present invention is effective not only for the ion beam sputtering apparatus but also for the synthesis of the oxide superconducting thin film in a vacuum deposition apparatus.

[Brief description of the drawings]

FIG. 1 is a perspective sectional view of an example of a superconducting thin film obtained by a method of the present invention.

FIG. 2 is a diagram illustrating an example of substrate temperature control when a buffer layer is introduced.

FIG. 3 is a cross-sectional view of an example of a Bi-based superconducting thin film using an inclined substrate according to the present invention.

[Explanation of symbols]

Reference Signs List 1 SrTiO ₃ substrate 2 Bi ₂ Sr ₂ CuO _x buffer layer 3 Bi ₂ Sr ₂ CaCu ₂ O _x layer 4 step

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical indication location H01L 39/24 ZAA H01L 39/24 ZAAC (56) References JP-A-1-252534 (JP, A JP-A-1-145397 (JP, A) JP-A-64-27131 (JP, A) JP-A-64-100820 (JP, A) JP-A-1-167912 (JP, A)

Claims (3)

(57) [Claims] 1. An in-situ (i) film of a Bi-based oxide superconducting single crystal thin film on a strontium titanate (100) substrate
In the n-situ synthesis method, the substrate temperature at the initial stage of film growth was set at 550 to 650 ° C., and Bi ₂ Sr ₂ CuO _{x was used.}
Is heteroepitaxially grown on the substrate as a buffer layer, and then the substrate temperature is set to Bi ₂ Sr ₂ CaCu ₂ O _x
A method for synthesizing an oxide superconducting thin film, comprising raising the superconducting thin film synthesizing temperature to grow a superconducting single crystal thin film . 2. The oxide superconducting material according to claim 1, wherein SrTiO ₃ is first homoepitaxially grown on the SrTiO ₃ (100) substrate at a substrate temperature of 500 to 600 ° C., and then Bi ₂ Sr ₂ CaCu ₂ O _x is grown. Thin film synthesis method. 3. An SrTiO ₃ (100) single crystal substrate surface is polished while being tilted in the (111) direction from its normal line to form a substrate surface having steps formed in the <1-10> direction. 3. The method for synthesizing an oxide superconducting thin film according to claim 1, which is used as a substrate for synthesizing a superconducting thin film.