JPH0369596A - Substrate material for superconducting thin film - Google Patents

Abstract

PURPOSE:To improve quality by consisting the above substrate material of the single crystal of the K2MnF4 structure constituted of 4 components of the elements shown by a specific stoichiometric ratio. CONSTITUTION:The substrate material for the superconductive thin film of the defect-free tetragonal crystal is obtd. by using the single crystal of the compd. having the K2MnF4 structure which is constituted of >=4 components of the elements expressed by the stoichiometric ratio ReABX4 (X is >= one kind of anions; Re is >= one kind of rare earth element ions; A and B are respectively cations which may be partly substd.) and consisting further preferably of alkaline earth element ions, such as Sr ions, such as the cation A, element ions of group VIa, VIIa and VIII transition metals, and Ga ions as the cation B and is expressed by the table.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a substrate crystal material for realizing a single crystalline thin film of an oxide high temperature superconductor such as YBa2Cu or Ox. Here, the term "single-crystalline thin film" refers to a thin film that is not a complete single crystal but includes a film that is crystallographically oriented with respect to the substrate surface.

[Prior art] Since it was discovered that (La,, Ba,) 2Cub4 becomes a superconductor at about 30°C, the superconducting transition temperature (To)
ReBa2Cu30. (Re: rare earth element), B1-3r-Ca-Cu- with Tc of about 1108
0 series, Tl-Ba-CaCu-0& with Tc of about 120
have been discovered one after another, and as a result, the formation of thin films of these oxide high-temperature superconductors has been actively researched and used for device applications.
being developed. A substrate is required to form this thin film, but up until now, oxide crystal JO, 5rTi(h, YsZ
(Y-added zro2) is exclusively used. The properties of a superconductor thin film greatly depend on its crystallinity, and therefore an epitaxial single crystal film that is lattice-matched to the substrate is desirable, but the lattice constant of MgO is
That of SZ is a-b-3,64 people, c-5,27A, and aχb, which is the lattice constant of the so-called oxide high temperature superconductor.
χ38 people, JTi (=f
T5 ) x 5 , which is significantly different from the 4-input method, cannot obtain a high-quality single crystal film. The lattice constant of SrTiO3 is a
-b-c-3,905 people, which is promising because it is close to that of oxide high-temperature superconductors, but its crystallinity is not necessarily satisfactory due to the large number of sub-grain boundaries caused by the crystal manufacturing method (Hernoulli Io). It cannot be called a board. Very recently, the so-called pseudo-belouskite structure, which has a crystal structure based on oxygen octahedrons, has been developed.08. Although LaGa0+ has been attracting attention, since they are basically rhombohedral crystals, they are greatly distorted from the basic perowskite structure, and contain twin crystals (tWin), which are crystal defects. It is not possible to remove it due to the nature of the wood.

[Problem to be solved by the invention]

As mentioned above, the crystal quality of oxide high temperature superconductor thin films using conventional substrates is not good, and generally results in oriented polycrystalline I-like films.

An object of the present invention is to provide a new substrate material that has very few lattice constant mismatches and crystal defects (twins), which are disadvantages of conventionally used oxide substrates.

[Means for Solving the Problems] The present invention provides a stoichiometric ratio Re8 to Each of B is a cation that may be partially substituted.

[Function] The new material of the present invention belongs to a tetragonal system with an oxygen octahedron as its basic structure, has a crystalline structure of 2MnF4, and has a chemical formula of Re8 to X4. (X mainly represents oxygen, but is not limited to this.)R
e is one or more rare earth ions, and A and B are cations, each of which may be partially substituted with another cation. The component ratio and crystal structure are completely different from conventional materials.

First, the concept of selecting a substrate material related to the present invention will be explained. The three major factors for obtaining high-quality epitaxial single crystal thin films are: ■ Matching of lattice constants, ■ Similarity of crystal structure,
■ Matching of thermal expansion coefficients.

■Regarding the lattice constant, as the oxide high-temperature superconductor that is the thin film to be formed, (La+-
The lattice constant of YBa2Cu3Ox is orthorhombic and the lattice constant of YBa2Cu3Ox is orthorhombic.
．． 82 entries, BO 43.88 people, Tc about 80 and about 1
The lattice constant of the 3i-5r-Ca-Cu -0 system from which the phase No. 10 is obtained is tetragonal and a. Yu 5, 4 people, the best Tc1
The Tl-Ba-5r-Cu-0 system in which the phase 20 appears is also a tetragonal system. χ3.86 people are generally accepted. Considering the interval line lattice of the unit cell treated here in crystallography, the lattice constant ao of the Bi system is
' χao/JTz3. a There will be two people. That is, the lattice constants of high-temperature superconductors are all within the range of 378 to 3.86 and do not depend greatly on the material system.

In the field of semiconductor engineering, which is the most technologically advanced in terms of heteroepitaxy, lattice mismatches of 1% or less are tolerated, but in recent III-V/Si heteroepitaxy, 4% is allowed. ing. Therefore, here, the maximum allowable mismatch is set to 4%. Now, let the lattice constant of the thin film be a. , if the lattice constant of the substrate is bo, then the lattice mismatch Mf is crystallographically defined as Mf−Shanagaku2LLxtoo (%) Kagamiueb, so the above a. = 3.78~
It corresponds to 3.86λ. is (3
,6318~3.9343A)~(3,7086~4
．． 0176 people). That is, a substrate having a lattice constant of 3.6318 to 4.0178 is suitable for the superconductor thin film.

■Regarding the crystal structure It is accepted that the basic crystal structure of oxide superconductors is that oxygen and octahedral BO6 (B: cation) form the skeleton, forming a so-called ring bellowskite structure. is included. however,
Mineralogically, the bellowsky 1-structure oxide is CaTi(L
It is a compound with the chemical formula B03 as represented by +, and its lattice constant is generally larger than that of the superconductor mentioned above, and depending on the combination of the he element and the B element, the lattice constant is larger than that of the superconductor mentioned above. However, because it is easily distorted and becomes a structure with low crystal symmetry below the lateral crystal system (for example, a monoclinic system), twin crystals, which are crystal defects, are generated. Therefore, a tetragonal system is preferable as a crystal structure in which twins do not occur.

■Regarding the coefficient of thermal expansion: The coefficient of thermal expansion of tetragonal perowskite oxides has been determined experimentally, and ranges from 10 to 10.
Based on a lot of literature data, it has been found that the thermal expansion coefficient is 12 ppm/''C, which is extremely close to the above-mentioned superconductor's thermal expansion coefficient of 11 to 12 ppm/l.

11eAB04 was discovered. Here, Re is a rare earth element of 1f111 or more, and each of A and B is a cation that may be partially substituted with another ion. Table 1 shows typical R
eABO, a compound, its lattice constant, and the most suitable superconductor for growing these as a substrate are shown. It can be seen that these are included in the values of the lattice constants mentioned in each section.

Among the ions of ReABX<, cations A are alkaline earth element ions, especially Sr ions, and B ions are transition element ions, especially Vla group, ■a group, and group ■ transition metal elements. ions and Ga ions are preferred.

(Left below) [Example] Based on numerical and crystallographic considerations regarding the above three elements, the present inventors developed various types of oxide superconductor YBa2Cu3Ox having a 2MnF4 structure as a substrate material by simultaneous vapor deposition using RF sputtering method. About 300 Å was deposited on the substrate at a substrate temperature of 650-780°C. As a result, La5rGaO+ , l, asr
For cr04LaSrFeO4 substrate, the transition temperature is 85
A good single crystal film exhibiting an IK of ~9 was obtained.

Example 2 (La,,Sr,)2Cub4 was formed by RF sputtering L
a S r kl n As a result of deposition on the Os substrate, T
A thin film of approximately 32 c was obtained, and the surface morphology was 1, a
It was a little rough compared to the srco04 and La5rGa04 substrates. This is considered to be because the lattice mismatch Mf is about 2.6%, which is larger than epitaxy in a narrow sense (Mf about 1%).

The lattice constant value of the substrate is appropriately selected to be Mfχ1% depending on the type of superconductor to be formed. That is, it goes without saying that it is desirable to select the substrate material of the present invention from among the substrate materials of the present invention exemplified in Table 1 depending on the type of superconductor. The basis of the present invention is to present Re8 and O4 compounds based on the three elements for epitaxial growth mentioned above. This is not intended to limit the invention.

〔Effect of the invention〕

As explained above, the substrate according to the present invention is a tetragonal crystal without crystal defects, has a crystal structure having an oxygen octahedral skeleton common to superconductors, and has a crystal structure similar to an oxygen octahedral perouskite compound. Since it has characteristics such as a high coefficient of thermal expansion, the use of the substrate of the present invention has the advantage that an extremely high-quality single-crystalline epitaxial superconductor thin film can be obtained.

Claims (1)

[Claims] The four components indicated by the stoichiometric ratio ReABX_4 (however, X
is one or more types of anions, Re is one or more types of rare earth element ions, and A and B are each cations that may be partially substituted. Substrate material for superconductor thin films.