JP2577061B2 - Method for producing composite oxide superconductor thin film - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a composite oxide superconductor thin film. More specifically, the present invention relates to molecular beam epitaxy (MB
E) or atomic layer epitaxy (ALE) or molecular layer epitaxy (MLE)
The present invention relates to a novel method capable of producing a higher-quality composite oxide-based superconducting thin film.

2. Description of the Related Art The superconducting phenomenon, which is said to be a phase transition of electrons, is a phenomenon in which the electrical resistance of a conductor becomes zero under specific conditions, indicating complete diamagnetism. That is, under the superconductivity, even if a current flows through the superconductor, there is no power loss, and a high-density current continues to flow forever. Therefore, for example, if superconductivity is applied to the power transmission technology, about 7
% Inevitable transmission loss can be significantly reduced. In addition, the application as an electromagnet for generating a high magnetic field is MH in the field of power generation technology.
It is expected to be a technology that advantageously promotes the realization of nuclear fusion reactions, which are said to consume more power than the amount of power generated for startup, together with D power generation and electric motors. In addition, as power for magnetic levitation trains, electromagnetic propulsion ships, etc.
It describes its use in NMR, pion therapy, high energy physics experiments, etc.

Further, apart from the use in the large-sized apparatus as described above, application of the superconducting material to various electronic elements has been proposed. A typical example is an element using the Josephson effect in which a quantum effect appears macroscopically due to an applied current when weakly joining superconducting materials. Since the energy gap of the superconducting material is small, the tunnel junction type Josephson device is expected as a switching device with extremely high speed and low power consumption. In addition, since the Josephson effect on electromagnetic waves and magnetic fields appears as an accurate quantum phenomenon, it is expected that the Josephson element is used as an ultrasensitive sensor for magnetic fields, microwaves, radiation, and the like. Further, as the degree of integration of electronic circuits increases, the power consumption per unit area is expected to reach the limit of the cooling capacity. Therefore, the development of superconducting elements has been demanded for ultra-high-speed computers.

Conventionally, despite various efforts, the superconducting critical temperature Tc of a superconducting material has not been able to exceed 23 K of Nb ₃ Ge for a long time. On the other hand, in 1986, Bednotes and Mueller et al. Discovered that a composite oxide-based long-conducting material had a high Tc, greatly opening up the possibility of high-temperature superconductivity (Bednorz, Mller, “Z.
Phys. B64, 1986, 189 ").

It has been known that composite oxide-based ceramic materials exhibit superconducting properties. For example,
U.S. Pat.No. 3,932,315 describes that a Ba-Pb-Bi-based composite oxide exhibits superconducting properties, and JP-A-60-173,885 discloses a Ba-Bi-based composite oxide. It is stated that the material exhibits superconducting properties. However, the complex oxide superconducting materials
Tc is generally extremely low at 10 K or less, and the use of expensive and rare liquid helium (boiling point 4.2 K) was inevitable to obtain superconductivity.

The oxide superconductor discovered by Bednauts and Mueller has a composition of (La, Ba) ₂ CuO ₄ and K ₂ Ni
It is seen as having an F ₄ type crystal structure. This composite oxide superconducting material has a similar crystal structure to the conventionally known perovskite-type oxide-based superconducting material, but its Tc is about 30K, which is dramatically higher than that of the conventional superconducting material.
There was a value.

In February 1987, a Ba-Y-Cu-based composite oxide exhibiting a critical temperature of 90K class was discovered by Chu et al. It is considered that this composite oxide commonly called YBCO has a composition represented by Y ₁ Ba ₂ Cu ₃ O _7-X .

Furthermore, the subsequently discovered Bi-Sr-Ca-Cu system and Tl-
Ba-Ca-Cu-based composite oxides are suitable for practical use because Tc is not only higher than 100K but also chemically stable and there is little economical deterioration of superconducting characteristics such as YBCO. It is expected that.

With the discovery of these new composite oxide-based superconducting materials, the momentum for realizing high-temperature superconductors is rapidly increasing in recent years.

Problems to be Solved by the Invention The composite oxide-based superconducting material exhibiting a high critical temperature as described above was initially discovered in a crystalline form. Subsequently, various techniques for producing a composite oxide thin film by a sputtering method, an ion beam method, or the like using the composite oxide or a compound containing an element forming the composite oxide as a target have been proposed.

However, in the conventional composite oxide thin film,
In general, it is known that a sintered body having the same composition is inferior to, for example, a critical temperature and the like, and a method for producing a thin film exhibiting the inherently high superconductivity of the composite oxide-based superconducting material is being sought.

Furthermore, the composite oxide thin film produced by the conventional method is not uniform in film quality, and it has been extremely difficult to obtain a stable thin film capable of producing various devices using the thin film.

Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a novel method for producing a thin film that stably exhibits the original high superconducting characteristics of the composite oxide superconducting material as a whole. It is in.

Means for Solving the Problems That is, a method for forming a thin film of a composite oxide-based superconducting material having a layered crystal structure on a substrate, comprising an atomic beam of each element constituting the composite oxide-based superconducting material Alternatively, a molecular beam and an oxygen ion beam are irradiated simultaneously or alternately, and a monomolecular layer conforming to the crystal structure of the composite oxide-based superconducting material is sequentially laminated on the substrate, and a desired thickness is formed on the substrate. And a method for producing a composite oxide superconductor thin film characterized by depositing a single crystal thin film or a polycrystalline thin film.

The method for producing a composite oxide superconductor thin film according to the present invention comprises the steps of:
The main feature of the method is to form a high-quality superconducting thin film under extremely precise control by a method including a method.

That is, whereas the conventional method for producing a superconducting thin film merely controls the thin film formation only by the composition ratio and the film forming conditions, the method according to the present invention, specifically, as described later The formation of crystals is controlled by a process deeply related to the crystal structure itself, and a superconducting material is formed efficiently.
In addition, a monolayer thin film of a composite oxide can also be produced.

The so-called MBE (Molecular Beam Epitaxy) method or Atomic Layer Epitaxy (ALE) method is known to be applied in the field of semiconductor devices as a method of forming a thin film of GaAs, AlAs, etc. In addition, since the film can be formed at a low film formation rate in a high vacuum and the surface state of the film being formed can be inspected without inspecting the film state, extremely precise management of the film quality is possible. In addition, since the film formation can be performed at an extremely low film formation rate (1000 ° / hour to 10 μm / hour), the film formation can be controlled at the level of a monoatomic layer or a monolayer having a thickness of several mm.

The first major feature of the method for producing a thin film according to the present invention is as follows.
By utilizing the characteristics of the film forming method using a molecular beam or an atomic beam as described above, a characteristic crystal structure of a composite oxide-based superconducting material exhibiting effective superconducting characteristics is efficiently formed.

As a composite oxide-based superconducting material to which the method according to the present invention can be advantageously applied, a general formula: (α _1−x β _x ) γ _y O _z [where α is an element contained in Group IIIa of the periodic table, Represents, β represents an element included in Group IIa of the periodic table, γ represents at least one element selected from elements included in Groups Ib, IIb, IIIb, VIIIa, and IVa of the Periodic Table X is the atomic ratio of β to (α _1−x β _x ), a number satisfying 0.1 ≦ x ≦ 0.9, and y and z are
The atomic ratio of the element γ and oxygen (O) when (α _1−x β _x ) is 1, 0.3 ≦ y ≦ 3.0 and 1 ≦ z ≦ 5, respectively.
And a composite oxide having a composition represented by the following formula: This composite oxide-based superconducting material has oxygen vacancies in the crystal and has a so-called layered crystal structure. This crystal structure has a different composition and arrangement for each layer. On the other hand, in the conventional thin film manufacturing method, the formation of the crystal structure is controlled only by controlling the composition. It was not formed entirely, but it is considered that this was the cause of the deterioration of superconducting characteristics.

On the other hand, in the method according to the present invention, the atomic beam or the molecular beam containing the element α, the atomic beam or the molecular beam containing the element β, and the atom beam or the molecular beam containing the element γ, By sequentially irradiating in a predetermined order corresponding to the crystal structure of Therefore, in the method according to the present invention, a specific crystal structure exhibiting effective superconducting properties is formed with an extremely high probability. In the above operation, it is preferable that the operation of laminating each monolayer is repeated a plurality of times.

A second feature of the method for producing a thin film according to the present invention lies in a method for supplying oxygen to the thin film.

That is, MBE (Molecular Beam Epitaxy), which was conventionally performed
In a method such as an atomic layer epitaxy (ALE) method, an element is supplied to a substrate by evaporation of an evaporation source. However, oxygen essential for the formation of the composite oxide cannot be supplied alone by such a method. Therefore, the present inventors have devised the following various methods as a method of supplying oxygen to a thin film in forming a thin film using a molecular beam or an atomic beam. .

For each layer, a stoichiometrically adjusted source of oxide evaporation is used.

The film is formed using both the evaporation source of each element and the oxygen ion beam.

An organic substance is used as a means for supplying oxygen.

An oxide is used as a means for supplying oxygen.

Each layer is oxidized in an oxygen atmosphere.

An oxide evaporation source is used.

In this method, each layer having a target crystal structure is directly formed as an oxide by using an oxide as an evaporation source during film formation. Therefore, the supply amount of oxygen is controlled by the composition of the evaporation source. The ratio between each element and oxygen is
It is determined according to the ratio between each element and oxygen in each layer of the crystal.

According to one embodiment of the present invention, when an appropriate oxide evaporation source cannot be obtained, the supply amount of oxygen is adjusted by using both the evaporation source of each element alone and the oxide evaporation source. Can also.

 Oxygen ion beam is used together.

In this method, a film is formed by an MBE method using an evaporation source of each element alone, and oxygen is supplied to a thin film being formed using an oxygen ion beam. Further, according to one embodiment of the present invention, a method in which a beam irradiation of only oxygen may be performed between irradiation of a molecular beam of each element alone before and after each other.

 Organic substances are used as oxygen supply means.

The organic matter containing oxygen is adsorbed on the layer formed by the irradiation of the molecular beam of each element, and then the molecular chain of the organic matter is irradiated under excimer laser irradiation so that only oxygen in the organic matter remains on the layer. By performing the cutting operation, a so-called oxygen-only monomolecular film can be formed between the layers of the thin film being formed. Here, it is advantageous to adjust the wavelength and intensity of the excimer laser for each layer. In addition, the organic substances that can be used in such a method include aliphatic hydrocarbons containing oxygen such as aldehydes, alcohols, carboxylic acids, higher fatty acids, ketones, and esters, aromatic hydrocarbons, and alicyclic hydrocarbons. An oxygen derivative can be used.

Oxide is used as the O ₂ supply means.

Other oxides, such as antimony oxide (SbO), known to decompose leaving oxygen after being adsorbed on the substrate can be used.

Introduction of Oxygen Atmosphere Since oxygen has a small adsorption coefficient, the oxygen ratio can be adjusted by controlling the oxygen atmosphere in the chamber.

It is also conceivable to perform a film forming operation in the presence of oxygen gas. Furthermore, a method of performing heat treatment in an oxidizing atmosphere after each layer is formed may be considered. The processing temperature in this case is 200
It is preferable that the temperature is not in the range of -1000 ° C. Further, it is advantageous to adjust the oxygen partial pressure of each of the above layers in consideration of the adsorption coefficient of each element and / or its oxide or its excitable substance.

As described above, in the method for manufacturing a thin film according to the present invention, since each layer of the crystal is formed in accordance with the crystal structure, and composition control is appropriately performed for each layer, an effective superconducting substance is efficiently contained in the thin film. It is formed.

As the substrate that can be used in the method according to the present invention, any known substrate material can be used, but a thin film having the same constituent element as the target composite oxide produced by another method such as a sputtering method is used. It is advantageous to use those provided for Here, as the base layer of the substrate, a single crystal substrate such as magnesium oxide, strontium titanate, or silicon, which can be used as a substrate itself, or a ceramic substrate such as alumina, or a metal substrate may be used in some cases. Can be.

Here, when an MgO single crystal or SrTiO ₃ single crystal substrate is used as a substrate or a base substrate, it is preferable to use a substrate having a {001} plane or a {110} plane as a film formation surface. Further, the substrate temperature is preferably in the range of 200 to 1000 ° C.

Further, as the composite oxide-based superconducting material to which the present invention can be applied, in the above-mentioned general formula, the element α is Y, La, Gd,
Dy, Ho, Er, Tm, at least one element selected from the group consisting of Yb and Lu, wherein the element β is Ba or Sr
Which is particularly advantageous when the element γ is Cu.

Further, in addition to the above composite oxide, the following general formula: D ₄ (E _1−q , Ca _q ) _m Cu _n O _{p + r} [where D is Bi or Tl, and E is D Is Bi when Bi is Sr, when D is Tl it is Ba, m is a number satisfying 6 ≦ m ≦ 10, n is a number satisfying 4 ≦ n ≦ 8, and p is p = (6 + 2m + 2n) / 2, q is a number satisfying 0 <q <1, and r is a number satisfying −2 ≦ r ≦ 2.] It is known that a composite oxide-based superconducting material also has a layered crystal structure, and examples thereof can be given as examples to which the method of the present invention can be advantageously applied.

Hereinafter, the method according to the present invention will be described in more detail with reference to the drawings. However, the following disclosure is merely an example of the present invention, and does not limit the technical scope of the present invention.

Embodiment FIG. 1 shows an example of a crystal structure of a composite oxide superconducting material having a layered crystal structure to which the method for producing a thin film according to the present invention can be advantageously applied.

That is, this composite oxide-based superconducting material was bonded to oxygen.
IIIa element, IIa element and Ib, IIb, IIIb, VII
Group Ia or IVa group elements are layered with or without oxygen.

In the present invention, this crystal structure is formed by using the MBE method or by combining it with the ALM method.

That is, on the substrate, an atomic beam of each of the above-mentioned elements α, β and γ is applied under an oxygen atmosphere, or a molecular beam containing these elements and oxygen is applied to a monomolecular layer [I] in which the first layer is composed of the elements β and O. A second layer is a monolayer composed of elements γ and O [II], a third layer is a monolayer composed of elements β and O [III], and a fourth layer is a monomolecule composed of elements γ and O A layer [IV], a fifth layer is a monolayer composed of elements α and O [V], a sixth layer is a monolayer composed of elements γ and O [IV], and a seventh layer is elements β and O The eighth layer becomes a monolayer composed of the elements γ and O, and the ninth layer becomes a monolayer [I] composed of the elements β and O. Irradiation is performed in order, and these irradiation means are repeated to obtain a desired composite oxide-based superconducting thin film. The supply amount of each beam can be controlled by the energizing energy of the evaporation cell and the opening and closing of the shutter, as described later.

FIG. 2 (a) is a conceptual diagram of the device used in the present invention, and since the MBE device itself is well known, control devices such as a mass spectrometer of a detection system and an Auger analyzer are omitted in this figure. Is shown.

That is, this apparatus is mainly constituted by a vacuum chamber 1 provided with three evaporation cells 2, 2 ', 2 "and a substrate holder 4, and the vacuum chamber 1 is connected to a vacuum pump through an exhaust hole 6. The inside of the vacuum chamber can be evacuated to a high vacuum, and each cell 2, 2 ', 2 "is provided with a shutter 3 which can be controlled for each cell. Further, FIG. 2 (a) is provided with an excimer laser device 7 used in a second embodiment which will be described later, and an intake hole 5 for the vacuum chamber 1.

When the method according to the present invention is performed using the apparatus configured as described above, first, the substrate 10 is attached to the substrate holder 4 and the target is placed in the evaporation cells 2, 2 ', and 2 ". The vacuum chamber 1 is evacuated to a high vacuum by housing the element constituting the composite oxide or its oxide.
The evaporation cells 2, 2 ', 2 "are heated.
Since the molecular beam or the atomic beam from 2 ′ and 2 ″ can be controlled by opening and closing the respective shutters 3, as described above, the molecular beam or the atomic beam is sequentially adjusted according to the crystal structure of the target composite oxide. Atomic beam irradiation is performed.

Next, as a second embodiment of the present invention, an implementation of a method using the excimer laser device 7 shown in FIG. 2A will be described.

In this embodiment, a step of forming an oxygen molecular layer is added during the formation of the target composite oxide thin film.
The supply of oxygen is performed by irradiating the excimer laser device 7 with a laser beam after supplying the oxygen-containing organic gas into the chamber 1 through the supply hole 5 and adsorbing the gas onto the substrate or simultaneously with the supply. The oxygen organic matter is decomposed or dissociated so that only oxygen remains on the substrate.
In FIG. 2A, the organic gas is supplied from the supply hole 5, but in practice, it is preferable to use a supply device that directs the gas directly to the surface of the substrate 10. Also,
The wavelength of the excimer laser depends on the type of organic substance used, and the intensity is determined by the amount of oxygen attached to the substrate.

FIG. 2 (b) shows another configuration example of an apparatus which can be used in the case of forming a film while similarly supplying oxygen to the substrate.

That is, the apparatus shown in FIG. 2 (b) is a vacuum chamber 1 also provided with an exhaust hole 6 and three evaporation cells 2, 2 ', 2 ".
In addition, a pair of ion beam guns 11 is further provided. This ion beam gun 11 is used exclusively for supplying an oxygen ion beam. Therefore, when this apparatus is used for producing a thin film of the composite oxide-based superconducting material shown in FIG. 1, the procedure of beam irradiation is as follows, for example.

(1) The first layer [I] is formed.

(1-a) Irradiation with an atomic beam of the element β.

(1-b) Irradiation with an oxygen ion beam.

(2) Form a second layer [II].

(2-a) Irradiation with an atomic beam of the element γ.

(2-b) Irradiation with an oxygen ion beam.

(3) A third layer [III] is formed.

(3-a) Irradiation with an atomic beam of the element β.

(3-b) Irradiation with an oxygen ion beam.

(4) A fourth layer [IV] is formed.

(4-a) Irradiation with an atomic beam of the element γ.

(5-b) Irradiation with an oxygen ion beam.

(5) A fifth layer [V] is formed.

(5-a) Irradiation with an atomic beam of the element α.

(5-b) Irradiation with an oxygen ion beam.

(6) A sixth layer [IV] is formed.

(6-a) Irradiation with an atomic beam of the element γ.

(6-b) Irradiation with an oxygen ion beam.

(7) A seventh layer [III] is formed.

(7-a) Irradiation with an atomic beam of the element β.

(7-b) Irradiation with an oxygen ion beam.

(8) An eighth layer [II] is formed.

(8-a) Irradiation with an atomic beam of the element γ.

(8-b) Irradiation with an oxygen ion beam.

(9) The ninth layer [I] is formed.

(9-a) Irradiation with an atomic beam of the element β.

(9-b) Irradiation with an oxygen ion beam.

By repeating the above operations, a desired composite oxide-based superconducting thin film can be obtained.

3 (a) and 3 (b) are diagrams schematically showing the crystal structures of other composite oxides to which the method for producing a thin film according to the present invention can be advantageously applied.

FIG. 3 (a) is a diagram in which the crystal structure of the composite oxide having the composition shown by the formula shown above, respectively, is regarded as a layer structure particularly stacked along the c-axis. FIG. 3 (b) is a diagram three-dimensionally representing the crystal structure of the composite oxide-based superconducting material shown in FIG. 3 (a).

As shown in these drawings, Tl ₂ CaSr ₂ Cu ₂ O _y (Bi ₂ CaSr
₂ Cu ₂ O _y ) or Tl ₂ Ca ₂ Sr ₂ Cu ₂ O _y (Bi ₂ Ca ₂ Sr ₂ Cu ₂ O _y )
Have a remarkable layer structure, and the method for producing a thin film according to the present invention can be applied very advantageously.

That is, the procedure of atomic beam irradiation when these composite oxide-based superconducting thin films are manufactured according to the method of the present invention is as follows.

The first layer is a monolayer composed of Tl or Bi and O, the second layer is a monolayer composed of Ba and O, the third layer is a monolayer composed of Cu and O, and the fourth layer is Ca A fifth monolayer made of Cu and O, a sixth monolayer made of Ca and O, a seventh monolayer made of Cu and O, The eighth layer is a monolayer composed of Ba and O, the ninth layer is a monolayer composed of Tl or Bi and O, the tenth layer is a monolayer composed of Tl or Bi and O, and the eleventh layer Is a monolayer composed of Ba and O, the twelfth layer is a monolayer composed of Cu and O, the thirteenth layer is a monolayer composed of Ca and O, and the fourteenth layer is a monomolecule composed of Cu and O The 15th layer is a monolayer composed of Ca and O, the 16th layer is a monolayer composed of Cu and O, the 17th layer is a monolayer composed of Ba and O, (the 18th layer is (A monolayer composed of Tl or Bi and O) The 18th layer is formed on the next layer. It corresponds to the first layer of the crystal unit. Further, the first to ninth layers and the tenth to eighteenth layers have the same crystal structure and are horizontally shifted from each other.
Construct a unit crystal structure. Therefore, a desired composite oxide-based superconducting thin film can be obtained by repeating the above irradiation procedure.

Effects of the Invention As described in detail above, according to the method for producing a thin film according to the present invention, each crystal layer is formed in accordance with the crystal structure of the target composite oxide-based superconducting material, and composition control is performed for each layer. Therefore, an effective superconducting material is efficiently formed in the thin film.

Further, the method of the present invention is not limited to simple thin film production,
By further successively laminating layers of different substances,
It can be expanded and used as it is for the production of various devices.

[Main reference numbers]

 DESCRIPTION OF SYMBOLS 1 ... Vacuum chamber, 2, 2 ', 2 "... Evaporation cell, 3 ... Shutter, 4 ... Substrate holder, 5 ... Gas supply hole, 6 ... Exhaust hole, 7 ... Excimer laser, 10 ... ... substrate, 11 ... ion beam gun

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ Identification code Agency reference number FI Technical display location H01B 13/00 565 H01B 13/00 565Z H01L 39/24 ZAA H01L 39/24 ZAAAB // H01B 12 / 06 ZAA H01B 12/06 ZAA (72) Inventor Tetsuji Ueshiro 1-1-1, Kunyokita, Itami-shi, Hyogo Prefecture Itami Works, Sumitomo Electric Industries, Ltd. (56) References JP-A-60-112892 (JP, A ) Japanese Journal of Applied Physics Vol. 26 No. 5P. L709- L710

Claims (1)

(57) [Claims] 1. A method for forming a thin film of a composite oxide-based superconducting material having a layered crystal structure on a substrate, comprising the steps of: forming an atomic beam or a molecular beam of each element constituting the composite oxide-based superconducting material; Simultaneously or alternately irradiating with an oxygen ion beam, monolayers conforming to the crystal structure of the composite oxide superconducting material are sequentially laminated on the substrate, and a single crystal thin film of a desired thickness is formed on the substrate. Alternatively, a method for producing a composite oxide superconductor thin film, comprising depositing a polycrystalline thin film.