JPH0764680B2 - Substrate for oxide superconductor thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a substrate for forming an oxide superconductor thin film, and particularly to a high temperature oxide superconductor represented by YBa ₂ Cu ₃ O _x . The present invention relates to a substrate for obtaining an orthorhombic a-axis oriented epitaxial single crystal film or an orthorhombic b-axis oriented epitaxial single crystal film.

[Prior Art] Since the discovery of oxide superconductor YBa ₂ Cu ₃ O _x (Tc> 90K) whose superconducting transition temperature (Tc) exceeds liquid nitrogen temperature of 77K, such superconducting materials for electronic applications have been developed. Research and development on thin film formation of conductors have become active. As a substrate crystal that has been used for thin film formation, Al of corundum structure has been used.
₂ O ₃ (lattice constant a ＝ 4.78Å, c ＝ 12.95Å), NgO with NaCl structure
(A ＝ 4.215Å), Y stabilized ZrO ₂ (a ＝ 3.64Å, c ＝ 5.27
Å), MgAl ₂ O _{4 with} spinel structure (a = 8.08 Å), SrTiO _{4 with} perovskite structure (a = 3.91 Å), Si with diamond structure (a = 5.43 Å), LiNbO with pseudo-ilmenite structure
₃ (a = 5.148Å, c = 12Å), etc. The thin film forming method is also performed by a sputtering method, a vacuum co-evaporation method, a laser vapor deposition method, a metal organic thermal decomposition method, or the like. Among them, SrTiO ₃ and MgO are generally used as substrates. Currently, superconducting thin films with a high Tc are obtained at a substrate temperature of about 650 ° C or higher, regardless of the film formation method. However, the thin film may be a polycrystalline oriented film, and even if it is a single crystalline thin film, the c-axis of YBa ₂ Cu ₃ O _x is perpendicular to the substrate surface.
This is a so-called c-axis alignment film. On the other hand, considering the application of oxide superconductors to electronics, the need for a-axis oriented or b-axis oriented films other than c-axis oriented films is also expected. Recently, it has been reported that when the substrate temperature during film formation is set to about 590 ° C or lower, an a-axis orientation axis is formed on the substrate SrTiO ₃ (H. Asano et.
al.Japanese Journal Applied Physics., Vol.28, No.6
(1989), L981).

[Problems to be Solved by the Invention] However, the crystallinity of the substrate SrTiO ₃ is extremely poor and the large aperture ratio is difficult. The relative permittivity is 200 to 300,77K at room temperature.
It is as large as 1000, and is a substrate material unsuitable for future devices. On the other hand, by making the surface of the substrate SrTiO ₃ a (110) surface, a
The formation of an axial alignment film has been discussed, but the above-mentioned drawbacks cannot be excluded in this case as well. In addition, lattice constant mismatch | a _sub −a _o | /
(A _sub + a _o ) × 0.5 (where a _sub and a _o are the lattice constants of the substrate and the grown thin film, respectively) is 0.77%, which is larger than the so-called heteroepitaxial lattice matching of 0.1 to 0.5%. Has poor crystallinity.

An object of the present invention is to provide a substrate having good crystallinity for obtaining an a-axis oriented and an axially oriented epitaxial film of a YBa ₂ Cu ₃ O _x superconductor.

[Means for Solving the Problems] In order to achieve such an object, the present invention provides a substrate for forming an orthorhombic a-axis oriented epitaxial single crystal film of an oxide high temperature superconductor, Formula REVO ₃ or RECrO
It is characterized by being a single crystal composed of ₃ (RE: rare earth element).

Furthermore, the present invention is a substrate for forming an orthorhombic b-axis oriented film of an oxide high temperature superconductor, which is characterized by being a single crystal having a chemical formula of LaC ₀ O ₃ .

[Operation] First, a guideline for material selection that has led to the present invention will be described below.

In order to obtain a high-quality epitaxial thin film, the three factors of the lattice constant matching, the thermal expansion coefficient matching, and the crystal structure similarity between the substrate crystal and the thin film crystal are important. Paid attention. At room temperature, the lattice constant of the oxide superconductor YBa ₂ Cu ₃ O _x with a superconducting transition temperature of 90 K is orthorhombic and is a ₀ = 3.82Å, b ₀ = 3.884Å, C ₀ = 11.684Å (value Has a difference of ± 0.002Å depending on the reporter), the coefficient of thermal expansion is approximately 8 to 11 × 10 ^-6 / K in each axial direction, and the crystal structure is an oxygen-deficient perovskite structure. FIG. 1 shows temperature changes of lattice constants a ₀ and b ₀ . This measurement crystal is JDJorgense
n et al. (Physical Review B, Vol. 36, No. 7 (1987), 3608-3
616), but it should be noted here that as the temperature rises, it undergoes a phase transition from the orthorhombic system to the tetragonal system, and the phase transition temperature is between about 600 ℃ and 700 ℃ depending on the amount of oxygen. Is to be. a _T is the lattice constant of the tetragonal a-axis. For conventional devices, low-temperature film formation in which the substrate temperature is made lower is indispensable. At that time, in order to grow a good-quality epitaxial single crystal film, it is necessary to obtain lattice matching at the film formation temperature.

The present invention provides a YBa ₂ Cu ₃ O _x a-axis oriented film at a temperature of about 550.
In order to obtain a b-axis oriented film at a temperature of about 450 ° C or lower,
Each of the substrates presents a lattice matching and a thermal expansion coefficient matching and a crystal structure similar to each other.

[Examples] Hereinafter, the present invention will be described in detail with reference to Examples.

Among the above three elements for heteroepitaxial growth, Gd containing oxygen octahedron as a unit as a compound similar in crystal structure
A complex oxide ABO ₃ with FeO ₃ structure was selected. This is because the above-mentioned oxide superconductor has an oxygen octahedral structure centered on Cu. The thermal expansion coefficient of the above composite oxide is 6
Since it has been arranged for a long time that it is within the range of up to 11 × 10 ⁻⁶ / K, the matching condition of the thermal expansion coefficient of the three elements is satisfied. Matched to the a ₀ axis length of the oxide superconductor from the complex oxides limited to these two elements (mismatch ratio 0.5%
Below: Table 1 shows the results of careful selection of compounds that: | a _sub −a _o | / (a _sub + a _o ) × 0.5) and compounds that match the b ₀ axis.
In addition to ABO _3, Table 1 also shows ABO ₄ compounds with good lattice matching.

Among them, LaC ₀ O ₃ is a rhombohedron with a _p = 5.436, α _p = 60 ° 48 ′, but if it has a pseudo-rhombohedral structure, a _r = 7.64Å, α _r = 90
At 42 ', it is almost the same as pseudo-tetragonal a = a _r / 2 = 3.82Å.
On the other hand, REVO ₃ and RECrO ₃ (RE: La, Ce, Rr, Nd, Sm) are said to be almost cubic, and the oxide superconductor YBa ₂ Cu ₃ O _{x has} a C ₀
As shown in the table, there is a mismatch of less than 0.67% with the axis. Also Sr ₂ T
iO ₄ , Sr ₂ IrO ₄ , LaSrMnO ₄ and LaSrGaO ₄ are YBa because they are tetragonal.
₂ Cu ₃ O _x is almost aligned with the a ₀ and b ₀ axes, but has a drawback that it is largely inconsistent with the c ₀ axis.

Example 1 On a LaC ₀ O ₃ (110) crystal substrate at a temperature of about 400 ° C. YBa ₂ Cu ₃ O
As a result of forming the _x film by laser vapor deposition, YBa ₂ C
A b-axis oriented film in which the b ₀ axis of u ₃ O _x was oriented was confirmed by X-ray diffraction.

Example 2 YBa ₂ Cu ₃ O _x was deposited on a (110) crystal substrate of REVO ₃ and RECrO ₃ by laser deposition at a substrate temperature of about 500 ° C., and as a result, a ₀ axis was oriented perpendicular to the substrate surface. The alignment film was confirmed by x-ray diffraction.

It was confirmed by X-ray diffraction that the grown YBa ₂ Cu ₃ O _x films were in an epitaxial relationship because the coefficients of thermal expansion were almost the same (8 to 10 × 10 ⁻⁶ / K) in all the examples. did it.

Example 3 One of ABO ₄ Sr ₂ TiO _{4 on} the (001) plane substrate was also 400-500.
The film was formed at 0 ° C., but the film was a c-axis oriented epitaxial film, and an a or b oriented film could not be obtained.

As described above in detail, we have presented and confirmed a substrate crystal suitable for the superconductor YBa ₂ Cu ₃ O _x based on the three elements of epitaxial growth. However, superconductivity in which a rare earth element is replaced in place of YBa ₂ Cu ₃ O _x The lattice constant of the body is a ₀ = 3.799 to 3.854Å, b ₀ = 3.874 to 3.91
The substrate crystal of the present invention is not necessarily optimum because it changes depending on the element up to 4Å, but the optimum substrate for each oxide superconductor can be selected according to the three principles of material selection that led to the present invention. It is clear that it is limited by REVO ₃ , RECrO ₃ and LaC ₀ O ₃ .

[Effects of the Invention] As described above, according to the present invention, REC ₀ O ₃ , REVO ₃ , REC
By using rO ₃ as a substrate, the lattice matching at the temperature during film formation is 0.5% or less with the a ₀ axis or b ₀ axis of the superconductor to be the film. And b
There is an advantage that an axially oriented epitaxial film can be obtained.

The lattice constant temperature of YBa ₂ Cu ₃ O _x and LaC ₀ O ₃ , REVO ₃ (RE ＝ Pr,
Nd, Sm), RECrO ₃ (RE = Ce, Pr, Nd) lattice temperature changes are also shown in Fig. 1, but the difference in lattice constant at the temperature during film formation is extremely small (around 0.2%). Understand that
By selecting this substrate material, extremely good epitaxial YBa ₂
A Cu ₃ O _x a- or b-axis oriented film can be obtained.

[Brief description of drawings]

 FIG. 1 is a characteristic diagram showing changes in lattice constant with temperature.

Claims (2)

[Claims] 1. A substrate for forming an orthorhombic a-axis oriented epitaxial single crystal film of an oxide high temperature superconductor, the single crystal having a chemical formula of REVO ₃ or RECrO ₃ (RE: a rare earth element). A substrate for an oxide superconductor thin film, characterized in that 2. A substrate for forming an orthorhombic b-axis oriented film of an oxide high temperature superconductor, which is a single crystal having a chemical formula of LaC ₀ O _3. Substrate for thin film.