JPH05343755A - Superconductive element - Google Patents

Abstract

PURPOSE:To provide a superconductive element which can get an oxide superconductor film, whose c axis becomes parallel with the face of a substrate, and its stack structure excellently in reproducibility by the prevention of crack occurrence, etc., and is excellent in controllability and further which can get large output voltage. CONSTITUTION:A middle layer 2, which has composition expressed by RE (Ba2-xREx) Cu3O7 (RE shows at least one kind of element being selected from among rare earth elements, and x is 0<X<1) is provided on a substrate 1 consisting of an oxide material. An oxide superconductive film 3, which has composition being substantially expressed by REBa2Cu3O7 and is oriented so that the c axis of its unit lattice may be parallel with the film formation face of a board, is formed on this middle layer 2. This is a stacked type superconductive element making use of this oxide superconductor film 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting device using an oxide superconducting thin film, and more particularly to a laminated superconducting device in which a direction in which a large superconducting current can flow is set in a direction perpendicular to a substrate.

[0002]

2. Description of the Related Art Conventionally, as a superconducting element, a tunnel-type Josephson junction element or the like having a laminated structure in which a metal superconductor such as Pb or Nb is used and a thin insulating layer capable of tunneling a superconducting conductor pair is sandwiched is used. Are known. Such a conventional tunnel type Josephson element is required to operate at an extremely low temperature close to the temperature of liquid helium. Further, since it exhibits a current-voltage characteristic having a hysteresis peculiar to the tunnel type Josephson junction, there is a problem that the circuit configuration becomes complicated, and it has not been widely put into practical use.

On the other hand, as a Josephson junction element using a metal superconductor and having no hysteresis characteristic, a so-called bridge-type junction in which main electrodes made of a metal superconductor are connected by a thin metal laminated on the main electrodes is also developed. It is being advanced. However, such a bridge-type junction needs to operate at an extremely low temperature close to the temperature of liquid helium, has a low resistance in the bridge portion, and has a superconducting property of a metal superconductor, as in the case of the tunnel-type junction described above. Since the gap itself is small, it is difficult to obtain a large output voltage.

Under these circumstances, an oxide superconducting material which exhibits superconducting properties at a temperature higher than the liquid nitrogen temperature has recently been discovered and has been attracting a great deal of attention. If it becomes possible to fabricate a stacked tunnel Josephson junction or a bridge junction using this oxide superconductor, at least as compared with the Josephson junction configured using the conventional metal superconductor described above. A wide range of applications is expected because there is no need for cryogenic operation.

[0005]

However, the oxide superconductor has a large anisotropy in its characteristics, and in the laminated structure in which the c-axis direction having a short coherence length is perpendicular to the substrate,
There is an essential problem that a superconducting current that is practically sufficient cannot flow through the laminated interface.

On the other hand, by making the c-axis of the oxide superconductor parallel to the substrate and producing a laminated structure having an interface parallel to this, it becomes possible to flow a practical superconducting current through the laminated interface. Due to the unusually large coefficient of thermal expansion in the c-axis direction specific to the oxide superconductor, a large strain is generated between the oxide superconductor and the substrate crystal at the time of cooling after film formation at high temperature, causing cracks in the oxide superconductor thin film. There was a problem that it was easy.

Thus, the development of a superconducting element such as a Josephson junction using an oxide superconductor having a high critical temperature is
It is expected to bring great effects to the industry,
At present, the superconducting current that can be flowed is insufficient, or the oxide superconductor thin film cannot be stably formed. Therefore, it has not been sufficiently put to practical use. Therefore, there is an urgent need to develop a laminated structure and a manufacturing method thereof that can be put to practical use for a superconducting element using an oxide superconductor.

The present invention has been made in order to solve such a problem, and reproduces an oxide superconductor thin film and its laminated structure in which the c-axis is parallel to the substrate surface while preventing the occurrence of cracks and the like. It is an object of the present invention to provide a superconducting element that can be obtained with good performance, is excellent in controllability, and can obtain a large output voltage.

[0009]

The superconducting element of the present invention comprises:
It has a composition substantially represented by a substrate made of an oxide material and REBa ₂ Cu ₃ O ₇ (RE represents at least one element selected from rare earth elements), and the c-axis of its unit cell is In a superconducting element comprising an oxide superconductor thin film oriented so as to be parallel to the film forming surface of the substrate, between the substrate and the oxide superconductor thin film, RE (Ba _2-x RE _x ) Cu ₃ O
It is characterized in that an intermediate layer having a composition represented by ₇ (0 <x <1) is provided.

The oxide superconductor thin film in the superconducting element of the present invention has a chemical formula: REBa ₂ Cu ₃ O ₇ (1) (where RE is Y, La, Sc, Nd, Sm, Eu, Gd, Dy, Ho, E
At least selected from rare earth elements such as r, Tm, Yb, and Lu
(Indicating one kind of element). However, the molar ratio of each element does not have to strictly satisfy the above ratio, and some variation is allowed as long as the superconducting characteristics can be obtained. The oxide superconductor thin film of the present invention is a thin film oriented so that the c-axis of its unit cell is parallel to the film forming surface of the substrate, and is oriented in the direction perpendicular to the substrate surface (the film stacking direction). It is a so-called a-axis alignment film capable of flowing a large superconducting current.

In the superconducting device of the present invention, the chemical formula: RE (Ba _{2 -x} RE _x ) Cu ₃ O ₇ ..... (2) is provided between the oxide substrate and the a-axis oriented oxide superconductor thin film. ) (Where RE is Y, La, Sc, Pr, Nd, Sm, Eu, Gd, Dy, H
an at least one element selected from rare earth elements such as o, Er, Tm, Yb, and Lu, where x is a number satisfying 0 <x <1) and an intermediate layer having a composition represented by ing. The constituent compound of this intermediate layer has the same crystal structure as that of the oxide superconductor, but part of Ba in the crystal lattice is replaced with a rare earth atom, and the lattice constant in the c-axis direction of the oxide superconductor is the same as that of the oxide superconductor. In addition to being small, the thermal expansion coefficient at room temperature or higher is also small, and the thermal expansion coefficient is intermediate between that of the oxide substrate and that of the oxide superconductor. These lattice constants and coefficients of thermal expansion are based on the substitution amount (value of x) of Ba with rare earth atoms.
However, if the value of x is 1 or more, the crystal structure changes, so the value of x in the above formula (2) is set to less than 1.

By interposing such an intermediate layer, the strain caused by the difference in thermal expansion coefficient between the oxide substrate and the oxide superconductor thin film can be relaxed. In addition,
The rare earth element (RE) in the compound to be the intermediate layer may be the same as or different from the rare earth element of the applied oxide superconductor, and in consideration of the coverage of the substrate surface and the growth uniformity. And decide. In some cases, the effect may be more remarkable when two or more kinds of rare earth elements are mixed and used instead of using one kind of rare earth element.

The specific structure of the intermediate layer as described above is as follows: (1) From the interface side with the oxide substrate to the interface side with the oxide superconductor thin film, the Ba substitution rate ( A compound layer in which the value of x is continuously changed by gradually decreasing the value of x.

(2) A plurality of compound layers having different Ba substitution rates (values of x) are laminated, and the value of x of the plurality of compound layers is changed from the interface side with the oxide substrate to the oxide superconductor thin film. Set to decrease toward the interface side of.

Etc. are preferred. Thus, Ba in the middle layer
By increasing the substitution rate on the substrate side and making it close to zero on the oxide superconductor thin film side, the matching of the crystal lattices of both the oxide substrate and the oxide superconductor thin film during film formation at high temperature is improved. It Therefore, the accumulation of strain in the cooling process after film formation is alleviated, and it is possible to prevent the occurrence of cracks and the like. However, the present invention does not exclude the intermediate layer composed of the compound of the formula (2) having a single Ba substitution ratio.

Further, the large coefficient of thermal expansion in the c-axis direction of the oxide superconductor is linked to the orthorhombic structure, which is a condition for manifestation of superconductivity, so that it is tetragonal at the interface with the oxide substrate. It is preferable to set the Ba substitution ratio in the formula (2) so that an orthorhombic crystal is formed at the interface with the oxide superconductor thin film, and a larger effect can be expected.

[0017]

[Function] Generally, in order to form a good oxide superconductor film, it is made of a material that does not react with the oxide superconductor at a temperature of about 700 ° C. necessary for thin film formation and has a crystal lattice size close to that of the material. It is necessary to use a substrate. As such a substrate, SrTiO ₃ or a similar oxide having a perovskite structure is widely used. As shown in FIG. 2, these substrates show good lattice matching with the c-axis of the oxide superconductor at room temperature, but have poor matching at high temperatures where film formation is performed. This is because the coefficient of thermal expansion of the oxide superconductor in the c-axis direction exhibits a unique temperature dependence. This peculiar temperature dependence corresponds to the phenomenon that the copper and oxygen atoms in the oxide superconductor form a one-dimensional ordered structure, and the crystal system changes from tetragonal at high temperature to orthorhombic at low temperature. ing. Therefore, when a film is formed by orienting so that the c-axis of the oxide superconductor is parallel to the substrate surface, the oxide superconductor film on the substrate should grow while receiving a compressive force when the film is formed at a high temperature. become.
Further, when the substrate is cooled after the film formation is completed, the oxide superconductor film contracts, and the compressive force changes to the tensile force.
As a result, cracks occur in the oxide superconductor film during the process of cooling to room temperature. Since the oxide superconductor has a strong cleavage property along the c-plane, it is difficult to prevent this crack even if the oxide superconductor film is thin.

On the other hand, in the RE-Ba-Cu-O system compound, when the RE molar ratio is larger than 1 and the Ba molar ratio is smaller than 2, some of the rare earth atoms enter the barium atom positions and the crystal structure The crystal grows without significant change.
In this case, since the size of the rare earth atom is smaller than that of barium, the lattice constant is small. Further, the position occupied by oxygen atoms also partly changes, and the one-dimensional chain structure of copper and oxygen, which was the cause of the large coefficient of thermal expansion in the c-axis direction of the oxide superconductor, is not formed. As a result, the coefficient of thermal expansion does not take a large value as seen in FIG. Such changes depend on the amount of rare earth atoms occupying the barium position.
Further, by setting the molar ratio of rare earth atoms to 1.3 or more (that is, the molar ratio of barium atoms to 1.7 or less), it is possible to maintain a tetragonal crystal even at room temperature.

The superconducting element of the present invention utilizes the above-mentioned phenomenon, and basically, an intermediate lattice constant and thermal expansion between the oxide substrate and the oxide superconductor thin film are provided between them. By interposing an intermediate layer having a coefficient, strain caused by a difference in thermal expansion or the like is relaxed.
Further, for example, in the step of growing the intermediate layer in contact with the oxide substrate, as compared with the case of obtaining the oxide superconductor film, the rare earth atoms are excessively supplied, the barium atoms are supplied in a small amount, and the film is gradually formed. By changing the ratio to a superconducting composition, the lattice constant and the coefficient of thermal expansion of the thin film can be changed,
This makes it possible to further reduce the strain energy accumulated in the oxide superconductor film formed above. Therefore, no large stress remains in the oxide superconductor thin film of REBa ₂ Cu ₃ O ₇ composition oriented on the oxide substrate so that the c-axis is parallel to the substrate surface. Further, it is possible to prevent the occurrence of cracks, the reduction of the superconducting transition temperature, and the like, and thus it becomes possible to form the laminated element having good characteristics on the oxide superconductor thin film.

The composition of the intermediate layer may be changed continuously, but in view of easiness of control, the intermediate layer of which the composition is changed has a multi-layer structure, and the composition between the layers is slightly different. A method of gradually changing while maintaining continuity is preferable. At the interface of the intermediate layer on the substrate side, controlling the composition ratio so that the crystal structure is kept tetragonal even at room temperature is effective for reducing the strain between the substrate and the intermediate layer. Is clear from.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the structure of a Josephson junction element of an embodiment to which the superconducting element of the present invention is applied. In the figure, 1 is a substrate made of a SrTiO ₃ (100) single crystal, and a compound film represented by the above formula (2), for example, an intermediate layer 2 on the SrTiO ₃ (100) substrate 1,
Pr (Ba _2-x Pr _x ) Cu ₃ O ₇ film (hereinafter Pr- (Ba, Pr) -Cu-O
(Hereinafter referred to as a film) is formed. The Pr- (Ba, Pr) -Cu-O film 2 has a large Pr excess on the substrate 1 side (for example, x
Is 1.5), and on the YBa ₂ Cu ₃ O ₇ film 3 side described later,
In order to approximate the Pr molar ratio to 1, Pr and Pr
It is the one that changes the molar ratio of Ba (that is, the value of x).

Further, the Pr- (Ba, Pr) -Cu-O film 2 uses the SrTiO ₃ (100) single crystal as the substrate 1, and
By controlling the film forming conditions, the film is oriented so that the c-axis of the unit lattice is parallel to the film forming surface of the substrate 1.

On the Pr-Ba-Cu-O layer 2 having a thickness of 300 nm,
Lower electrode 3 consisting of YBa ₂ Cu ₃ O ₇ film (hereinafter referred to as Y-Ba-Cu-O film), PrBa ₂ Cu ₃ O ₇ film with a thickness of 25 nm (hereinafter Pr
-Ba-Cu-O film), a barrier layer 4 having a thickness of 100 nm
An upper electrode 5 made of a Y-Ba-Cu-O film is sequentially laminated. Y-Ba-Cu forming the lower electrode 3 and the upper electrode 5
Each of the -O films is a film oriented such that the c-axis of its unit cell is parallel to the film-forming surface of the substrate 1, that is, an a-axis oriented film. In addition, the thickness of 1
A surface protection layer 6 made of a 00 nm gold vapor deposition film is provided.

The barrier layer 4, the upper electrode 5 and the gold protective layer 6 are patterned so that the bonding area is 10 μm square. The laminated portion and the lower electrode 3 formed by them are covered with the insulating film 7, and the Au wiring layer 8 is connected to the upper electrode 5 through the contact hole 7a provided in the insulating film 7. A stacked Josephson junction element is constructed.

The Josephson junction device of the above embodiment is a
Since a layered junction is formed by a healthy oxide superconductor thin film that is axially oriented, a practical superconducting current can be passed through the layer interface, and a large output voltage can be obtained.

The Josephson junction device according to the above embodiment is manufactured, for example, as follows. The manufacturing process of the Josephson junction device of this embodiment will be described with reference to FIG.

First, as shown in FIG. 1 (a), SrTiO ₃
An Pr- (Ba, Pr) -Cu-O layer 2 which is an intermediate layer is formed on the (100) substrate 1 by a multi-reactive sputtering method. In this film formation, Pr, Cu, and Ba ₂ CuO ₃ targets were used, and the composition ratio of Pr and Ba was gradually changed by gradually changing the high-frequency power applied to the Pr target and Ba ₂ CuO ₃ target during the thin film formation process. Was changed. In this example, the molar ratios of Pr and Ba at the interface on the substrate 1 side were each set to 1.5 and gradually changed to Pr: Ba = 1: 2 within the thickness range of 100 nm. Then, on this intermediate layer 2, in the same vacuum container, Y, Cu, Ba ₂
Using each target of CuO ₃ , thickness as the lower electrode 3
300nm Y-Ba-Cu-O film, 25nm Pr as barrier layer 4
-Ba-Cu-O film, Y-Ba-Cu-O with a thickness of 100 nm as the upper electrode 5
The film was grown sequentially. The temperature during film formation was set at 680 ° C., and it was confirmed by X-ray diffraction that the c-axis of each of the prepared thin films was a-axis oriented parallel to the substrate surface.

The superconducting transition temperature of the obtained laminated film was 83K. This transition temperature is ideal for Y-Ba-Cu-O films.
It is slightly lower than K because the substrate temperature during film formation is low. In order to maintain the a-axis orientation and raise the transition temperature, the above temperature is set when the intermediate layer 2 whose orientation is determined is produced,
The substrate temperature may be raised during the production of the oxide superconductor film.
The surface of the produced laminated film was extremely smooth, and cracks and the like were not observed even by a high-resolution electron microscope.

Further, as a comparison with the present invention, a laminated film not using the intermediate layer 2 and a laminated film using a simple intermediate layer having a composition of PrBa ₂ Cu ₃ O ₇ were prepared. A crack structure was observed and the superconducting transition temperature was around 70K. This clearly shows the effect of the present invention.

The processing steps after forming the above-mentioned laminated film are the same as those used in the manufacture of ordinary semiconductor elements.
That is, as shown in FIG. 3 (b), after forming a 100 nm-thick gold vapor deposition film 6 to protect the surface, the pattern of the bonding portion is transferred to the resist film by an optical exposure method, and then the resist is masked. As a result of the ion milling method, the Y-Ba-Cu-O film 5 serving as the upper electrode and the intermediate Pr-Ba-Cu-O serving as the barrier layer
The film 4 is etched. The joint area was 10 μm square.

Next, as shown in FIG. 3 (c), an insulating film 7 for insulating the wirings to the upper electrode 5 and the lower electrode 3 from each other is laminated, and a hole (contact hole 7a) is formed only in the upper portion of the junction. It is processed into a shape with a blank. Various materials can be used as the insulating film 7, but in this embodiment, a negative resist is used for the purpose of simplifying the process. After that, the wiring 8 to the upper Y-Ba-Cu-O film 5 is formed of Au to complete the stacked Josephson element.

FIG. 4 shows current-voltage characteristics of the superconducting element obtained in this example at the temperature of liquid helium. The critical current is 1.2 mA, and the output voltage of the device is I _c · R _n
A product of 4 mV was obtained. The output voltage lower than 20 mV expected for the oxide superconductor is because the thickness of the Pr-Ba-Cu-O film 4 is not optimized. By making the thickness thinner, better characteristics can be obtained.

FIG. 5 is a diagram showing the dependence of the critical current on the applied magnetic field, performed to confirm that the superconducting element of this example actually exhibits uniform Josephson characteristics. As can be seen from this figure, the fabricated device showed an ideal Fraunhofer pattern. Such Josephson characteristics were confirmed even at the liquid nitrogen temperature, and it was verified that the superconducting device according to the present invention could operate at a high temperature.

In the Josephson junction device of the above embodiment, an example in which a Pr- (Ba, Pr) -Cu-O film is used as the intermediate layer 2 has been described, but the present invention is not limited to this. However, if the composition controllability of the intermediate layer and the prevention of diffusion of the rare earth element between the intermediate layer and the oxide superconductor film are satisfied, the same effect can be expected when other rare earth elements are used. However, since the Pr- (Ba, Pr) -Cu-O film has excellent wettability with the oxide substrate, it can be said to be a material suitable for the present invention.

[0036]

As described above, according to the present invention,
The stability and reproducibility of various superconducting devices with a laminated structure, including Josephson junction devices that can operate at high temperatures using oxide high-temperature superconductors, can be greatly improved, making a great contribution to the industry. It is expected.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a superconducting element according to an embodiment of the present invention.

FIG. 2 is a diagram showing the temperature dependence of the thermal expansion coefficient of a Y-Ba-Cu-O-based oxide superconductor and an oxide for a substrate.

FIG. 3 is a cross-sectional view showing a manufacturing process of a superconducting element according to an embodiment of the present invention.

FIG. 4 is a diagram showing current-voltage characteristics of a superconducting element obtained according to an example of the present invention.

FIG. 5 is a diagram showing applied magnetic field dependence of a critical current of a superconducting device obtained according to an example of the present invention.

[Explanation of symbols]

1 …… SrTiO ₃ (100) substrate 2 …… Intermediate layer consisting of Pr- (Ba, Pr) -Cu-O film 3 …… Lower electrode consisting of Y-Ba-Cu-O film 4 …… Pr-Ba- Barrier layer composed of Cu-O film 5 ... Upper electrode composed of Y-Ba-Cu-O film

Claims (1)

[Claims] 1. A substrate made of an oxide material, and REBa ₂ Cu ₃
It has a composition substantially represented by O ₇ (RE represents at least one element selected from rare earth elements), and the c-axis of its unit cell is parallel to the film-forming surface of the substrate. In a superconducting device comprising an oriented oxide superconductor thin film, RE (Ba _2-x RE) is provided between the substrate and the oxide superconductor thin film.
_x ) A superconducting device provided with an intermediate layer having a composition represented by Cu ₃ O ₇ (0 <x <1).