JPH04196183A - Superconducting element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

[Field of Industrial Application] The present invention relates to the field of superconducting electronics such as superconducting switching elements, as well as the field of low-temperature electronics elements using superconducting thin films, and is particularly applicable to the fields of high-speed digital, cycle circuits, and high-frequency analog circuits. This invention relates to oxide superconducting elements.

[Prior art J] When constructing superconducting electronic devices such as Josephson junctions and superconducting three-terminal devices using oxide superconducting 4A materials with high critical temperatures, device dimensions that correspond to the coherence length of the oxide superconducting materials are required. . The coherence length of Y-Ba-Cu oxide, which is a typical oxide superconducting material, is 0.3 to 1.4 nm. In order to realize device dimensions compatible with such short distances, for example, for Josephson junctions, there is an example of using crystal grain boundaries such as Y-Ba-Cu oxide in the junctions, as shown in Applied Physics Letters 51. Volume 200 (1987) (Applied P
hysics Lette “s, Vol, 51.p
, 200°+987). In this example, Y
The grain boundaries at the constricted portions of the -Ba-Cu oxide thin film are used as Josephson junctions, and such junctions are provided at two locations in the loop-shaped thin film to constitute a DC SQUID. [Problem to be Solved by the Invention I] When attempting to obtain a superconducting three-terminal device or a superconducting device such as a Josephson junction using an oxide-based superconducting material using the prior art, the following problems arise. occurs,
This makes device fabrication difficult and impedes future development of oxide-based electronic devices. That is, according to the prior art and knowledge, it is necessary to make the dimensions of the Josephson junction, etc., extremely short, on the order of Inm, which is the coherence length of the oxide superconducting material. Such a short=1 method was realized at the grain boundaries of superconducting oxides. Grain boundaries are the areas where polycrystalline crystals meet, and! Disturbances in atomic arrangement exist at the level of an atomic layer or two atomic layers. Inside the grain boundaries, the superconducting oxide has the original crystal lattice, atomic arrangement, and composition. Therefore, although the grain boundaries were already formed during the production of the oxide thin film, they made it possible to realize a nanometer-level normal conduction region. However, the above conventional technology has the following problems. That is, it is impossible to artificially form nanometer-level patterns in oxide superconducting materials even with current microfabrication technology. The lnm dimension according to the above-mentioned prior art was only realized with the help of natural phenomena. Therefore, it is impossible to form such minute dimensions and manufacture devices as designed in advance. This not only means that it is not possible to obtain a device whose characteristics are controlled exactly as set, but also that it is impossible to integrate a large number of devices with uniform characteristics on the same substrate. . Further, although it is possible to manufacture a two-terminal element like the conventional example, it is extremely difficult to manufacture a superconducting element such as a three-terminal element with a complicated structure using the conventional technology. Therefore, the object of the present invention is to provide a material for a normal conductive layer and a superconducting element that uses an oxide superconducting material having a high critical temperature as an electrode, which enables the design and manufacture in a size range that enables the fabrication of the element. The purpose is to provide the specifications. [Means for Solving the Problems] In order to solve the second problem, in the present invention, an oxide-based superconducting material is used as an electrode, and, for example, two of these superconducting electrodes are made of an oxide containing Cu as a component. The existence probability amplitude of the superconducting electron pair is determined by the clean limit of the distance between the superconducting electrodes in the region where the normal oxide part is sandwiched between the superconducting electrodes and the active region for connection via a normal conducting material, etc. be longer than the attenuation length and shorter than 10 times the attenuation length. The length determined by the clean limit is the attenuation length d for the existence probability amplitude of a superconducting electron pair in the normal layer given by the clean limit.
Corresponds to n. The distance between the superconducting electrodes is made equal to or larger than this attenuation length dn. The superconducting attenuation length dn at the Naori lean limit is given by hv/4π2kT. Here, 11 is Planck's constant, k is Boltzmann's constant, ■ is the Fermi velocity of the carrier, and the carrier mass is m, h
= h / 2π, where n is the free carrier density, given by 2(3×2n)+3. Further, T is the operating temperature. For example, this value can be calculated using oxide normal conductive material L a -B a -Cu
If applied to oxides, it will be 50 nm at 10 K. Therefore, this value is determined by the temperature at which the device is to be operated. As the oxide normal conducting part, it is preferable to use an oxide material that has the same basic crystal structure as the oxide superconducting material and has antiferromagnetic magnetic properties. Alternatively, an oxide material having the same basic crystal structure as the oxide superconducting material and having semiconductor-like electrical properties may be used as the oxide normal conducting portion. An example of such a material is Y-B', which is depleted in oxygen concentration.
a-Cu oxide, Pr-Ba-Cu oxide, La-Ba
-Cu oxide, Y-Ba-Cu oxide with Fe, Co, A
It includes magnetic elements such as I and Ga, and normal conductive oxides containing metal elements as impurities. However, the thickness of the normal conductive layer made of these oxide materials, that is, the distance between the superconducting electrodes, is set to be larger than the attenuation length given by the above equation, that is, the dimension individually determined by the Fermi velocity. The superconducting-normal-superconducting junction described above is the basic part of a superconducting three-terminal element. By providing an electrode for controlling the superconducting current flowing through this normal conducting layer, a three-terminal element can be obtained. One example of the control electrode is to stack it on a normal conductive layer and provide a gate electrode via an interlayer insulating film. By applying a voltage to this control electrode, the magnitude of the superconducting current is controlled.

[Effect]

The basis for the fact that the above-mentioned means for solving the problems of the present invention is effective in producing an oxide-based superconducting three-terminal is based on new experimental facts supporting the present invention and the reasons described below. The new experimental facts will be explained using Figure 1. The present inventors used Y-Ba-Cu oxide thin film 3 as an electrode, and L a -
A planar type element was fabricated using the B a -Cu oxide thin film 2 as a normal conduction layer. The distance between Y--Ba--Cu oxide electrodes was made with different dimensions ranging from 0.1 μm to 0°811 m. FIG. 2 shows an example of voltage-current characteristics for a device in which the distance between the Y--Ba--Cu oxide electrodes is 0.2 .mu.m. As shown in the figure, superconducting current was obtained. Superconducting currents up to 70K were soon detected. Similar superconducting currents were obtained in other devices. The distance between the electrodes of the device fabricated here is the value considered as the coherence length of an oxide system, that is, about 1 nm, which is 10
It is 0 to 1000 times. From experimental facts, it has been found that the oxide normal conducting material has the same basic crystal structure as the oxide superconducting material and, in some cases, also has antiferromagnetic magnetic properties, or the basic form of the crystal structure of the oxide superconducting material. It has been found that it is better to use an oxide material that has the same characteristics and also has semiconductor-like electrical characteristics. At the interface between a superconductor and a normal conductor, superconducting electrons do not have a continuous density distribution due to discontinuous changes in atomic arrangement and constituent elements. Generally, at the interface, the superconducting electron density decays discontinuously toward the normal conducting layer. This attenuation rate increases as the types of superconducting and normal conducting materials differ. For example, this applies to the case where an oxide superconducting material and a metal normal conducting material are connected. In order to prevent the superconducting electron density from discontinuously attenuating at the interface, it is preferable to combine structurally similar superconducting materials and normal conducting materials. In particular, when a Cu-based oxide superconducting material is combined with a Cu-based oxide normal conducting material, the attenuation of the superconducting electron density can be sufficiently suppressed. Suitable Cu-based oxide normal-conducting materials are those that have antiferromagnetic properties or semiconductor-like electrical properties by reducing carriers in Cu-based oxide superconducting materials. In general, magnetism and superconductivity do not coexist in metallic superconducting materials, and superconducting properties are lost due to the presence of magnetic impurities. On the other hand, in oxide-based superconducting materials, the portion responsible for superconductivity is the CuO2 atomic plane. CuO is inherently an antiferromagnetic material. However, in the case of Cu-based oxide superconducting material A, C
When the valence of u becomes larger than 2, carriers, that is, holes are generated, and the localization of the wave function of the electronic system is weakened. Along with this, the antiferromagnetic properties become weaker. This means that in normal oxide materials, the magnetic properties do not suppress the development of superconductivity, but whether or not superconductivity occurs is determined by the presence or absence of carriers. Although the microscopic mechanism in normal oxide materials is not clear, phenomenologically, the CuO2 surface contains a mechanism of attractive interaction between carriers, which is the origin of superconductivity. For this reason, superconducting properties and antiferromagnetic properties coexist in oxide superconducting materials. By taking these into consideration, it can be seen that the distance between superconducting electron pairs leaking from the superconducting electrode to the normal conducting layer is not the coherence length determined by measurements such as the critical magnetic field, but is a larger method. From the Hall coefficient and resistivity, the attenuation length of a normal oxide material such as L a-13a -Cu oxide, divided by the dirty limit, is about 1 nm. This decay length apparently reflects the short mean free path of the oxide material. The cause of resistance in the oxide I1 material is not scattering due to lattice vibration, but is due to insufficient overlap of wave functions between adjacent atoms. As already mentioned, Cu-based oxides have superconducting properties even if they are normal conductive materials. Even though the superconducting electrons χj seeping into the normal conductive layer are subjected to scattering of 11·, they do not lose their superconducting properties. In this respect, the seepage distance of superconducting electron pairs is not the attenuation length determined by the dirty limit, but
Rather, it is defined by clean limits. The distance between the superconducting electrodes is attenuated exponentially by a constant determined by the clean limit, that is, the attenuation length dn listed above. Therefore, the distance between the electrodes suitable for producing Josephson coupling is in the range from the value of can to ten times the value of can. The reason for limiting the interelectrode distance to this range is that if it is smaller than the dn value, the phase of the superconducting wave function will not shift between the electrodes, and if it is at least 10 times larger than the dnO value, the superconducting current density of the device is practically necessary. This is because it becomes smaller than the value of 105 A/m2.

【Example】

Examples of the present invention will be described below. As shown in FIG. 3, a (110)-oriented single crystal of Sr'1riCh is used as a substrate 1 for a superconducting three-terminal device. A La-Ba-Cu oxide thin film is formed as a normal conductive layer 2 on a substrate 2. Film formation is performed by high frequency magnetron sputtering method. The atmospheric gas is Ar and oxygen gas with a concentration of 50%, and the total pressure is 0.4 Pa. The target material is a disc-shaped sintered body of La-Ba-Cu oxide. As a power source], 3, 56MI (using high frequency of z,
The input power is 100W. The substrate temperature during film formation was 55
The range is 0°C to 700°C. Under such film forming conditions, a La-BaCu oxide thin film having semiconductor-like electrical resistance characteristics is obtained. Next, a Y-T3 a-Cu oxide thin film is formed on the La-Ba-Cu oxide thin film 2 by a reactive vapor deposition method in an oxygen atmosphere using Y, Ba, and Cu metals as evaporation sources,
A superconducting electrode film 3 is used. The thickness of the Y-B a-Cu oxide thin film is 80 nm. ) The r-Ba-Cu oxide thin film has a superconducting critical temperature of 80°C. Next, an organic resist film with a width of 0.2
A groove pattern of 71 m is formed. A groove is formed by processing the Y--Ba--Cu oxide thin film from above using a reactive ion beam edge blowing method using Ar or a mixed gas of Ar and oxygen. Y-Ba-C due to processing process
The u oxide thin film is divided into two electrode parts, and the distance between the electrodes is maintained at 0.211 m. As a result, two superconducting electrode films 3 are obtained. Next, 3
A rTi03 thin film is formed to serve as a gate insulating film 4. The atmospheric gas is a mixed gas of Ar and oxygen at a concentration of 50%. Further, an Au thin film is formed directly above the Y--Ba--Cu oxide electrode groove to form the gate electrode film 5. As shown in Figure 4, in the oxide-based superconducting three-terminal device fabricated by the above method, a superconducting current flows between the Y-Ba-Cu oxide electrodes and a voltage is applied to the Au = 1 electrode. Accordingly, one shift of the superconducting current can be controlled. That is, by applying a positive voltage to Ga-1, the carrier concentration in the normal conduction layer decreases. As a result,
By shortening the effective attenuation length, the superconducting current can be reduced. This shows that the device operates as a superconducting three-terminal device. The distance between the electrodes of the Y-Ba-Cu oxide thin film is 0.2I.
By fabricating superconducting three-terminal devices having different dimensions in addition to Jm in the range from 0.11 Jm to 1.0 μm and measuring the device characteristics, it is shown that the superconducting current can be controlled by the gate voltage. When the distance between the electrodes is long, the superconducting current value is low under zero gate voltage, so by applying a negative gate voltage, an accumulation layer is formed as a normal conducting layer, the key layer concentration is increased, and the superconducting current is increased. . Furthermore, a superconducting three-terminal element using a Y-13a-Cu oxide as a superconducting electrode, a P r -B a -Cu oxide thin film as a normal oxide layer, a 5rTi03 thin film as a gate insulating layer, and an Au thin film as a Goo1 electrode. is also produced by a similar method. This superconducting three-terminal element is still Y
- A superconducting current flows between the -B a -Cu oxide electrodes,
By applying a voltage to the Au gate electrode, the level of superconducting current can be adjusted, indicating that the three-terminal operation is effective. 2 La-Ba-Cu oxide as an oxide normal conducting layer,
In addition to Pr-Ba-Cu oxide, L a -Cu, which is an oxide having a perososky type crystal structure and containing Cu,
oxide, B i -3r-Cu oxide, Y-Ba-Cu oxide with oxygen concentration depletion, Y-Ba-Cu oxide containing magnetic elements such as Fe, Co, AI, Ga, etc., and metal elements. A superconducting three-terminal element can be constructed in the same way by using a normal conductive oxide or the like contained as an impurity. Furthermore, in addition to Y-Ba-Cu oxide, oxide materials containing Cu such as B15r-Ca-Cu oxide and Tl-Ba-Ca-Cu oxide can be used as superconducting electrodes to produce a three-terminal element. Can be configured. Such oxide superconducting three-terminal devices are used as basic elements in high-speed digital logic circuits, memory circuits, analog circuits such as high-speed analog/digital converters, SQUIDs that detect magnetic field signals with high sensitivity, sensors that detect microwaves, etc. used.

【Effect of the invention】

The present invention has the following effects. (1) Conventionally, it was extremely difficult to obtain artificially.
ln is the superconducting coherence length of the oxide superconducting electrode
There is no need to form an element size of 0.171 m to 1
An element having superconducting properties can be obtained with a distance between superconducting electrodes of 11 m. (2) Such element dimensions do not require the adoption of crystal grain boundaries or a laminated structure, and are dimensions and shapes that can easily realize a planar structure that can be integrated, have three terminals, etc. (3) According to the present invention, element characteristics are artificially controlled = 1
6- It is possible to fabricate an oxide-based superconducting device including a three-terminal device in a complete form. This makes it extremely easy to manufacture switching circuits, microwave detection elements, and devices using oxide superconducting elements such as SQUID based on pre-designed specifications.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining the structure of the superconducting element that is the basis of the present invention, Figure 2 is a diagram for explaining the voltage-current characteristics of the superconducting element shown in Figure 1, and Figure 3 is for explaining the structure of the superconducting element shown in Figure 1. FIG. 4 is a diagram for explaining an oxide superconducting three-terminal element according to an embodiment of the invention, and is a diagram showing voltage-current characteristics of the oxide superconducting three-terminal element shown in FIG. 3. Explanation of symbols 1...Substrate, 2...Normal oxide layer, 3 Oxide superconducting electrode, 4...Gate insulating film, 5 Gate electrode film. Voltage (4 steps)

Claims (1)

[Claims] 1. A plurality of electrodes exhibiting superconductivity, and an active region for forming a channel for current flowing between the plurality of electrodes;
control means for controlling the current flowing through this active region, and the distance between the plurality of electrodes is hv/4π^2kT, where h is Planck's constant, v is Fermi velocity of carriers, k is Boltzmann's constant, and T is a superconducting element larger than the value given by the operating temperature. 2. The superconducting element according to claim 1, wherein the active layer is made of an oxide material containing as a Cu component. 3. The superconducting element according to claim 1, in which the distance between the plurality of electrodes is A superconducting element with a value smaller than 10 times the value given by 4π^2kT. 4. The superconducting device according to claim 1, wherein the active region has the same basic crystal structure as the electrode. 5. The superconducting device according to claim 1, wherein the active region has antiferromagnetic magnetic properties. 6. The superconducting device according to claim 1, wherein the active region has semiconductor-like electrical characteristics.