JPH0636440B2 - Superconducting switching element - Google Patents

Description

The present invention relates to the field of superconducting electronics, and more particularly to a field effect type superconducting switching element applied to the fields of digital circuits and analog circuits.

[Conventional technology]

The advantages of a field effect superconducting switching element compared to a Josephson junction element are that it has a three-terminal structure, has sufficient input / output separation, can be switched by a voltage signal, and can be driven by a DC power supply. Can be raised.

As the superconducting switching element using the electric field effect, an Nb-based superconducting material that requires liquid helium temperature operation is used, and the exuding effect of the superconducting conductor and CaAs or Si
A switching device using the electric field effect is known.
This example is Physical Review Letters, Vol. 54, 2449.
Page, 1985 (Physical Review Letters, Vol. 54, p. 24
49, 1985). This device has a structure in which a source and two superconducting films to be drain electrodes are arranged in close proximity on a semiconductor substrate, and a gate electrode film is inserted between them. That is, the structure is such that the source, gate and drain electrodes are arranged side by side on one side of the InAs semiconductor substrate. The superconducting current flows from the source through the semiconductor to the drain. The semiconductor part becomes a superconducting weak coupling part through which the superconducting current flows due to the exuding effect of the superconducting material.

[Problems to be Solved by the Invention]

The conventional field effect type superconducting switching element requires a very advanced technology for the following reasons when it is to be realized by using an oxide superconducting material having a high critical temperature.

When a superconducting film to serve as a source and a drain is placed close to each other on a semiconductor substrate, in order to allow the superconducting current to flow between the source and the drain, the length of the semiconductor portion where the superconducting current should flow, that is, the channel. It is necessary to make the length about the length of the superconducting coherence. When the channel length is longer than the coherence length, the resistance value between the source and drain electrodes is changed by the application of the gate voltage signal, but the superconducting current does not flow whether the gate voltage signal is on or off. A desirable switching operation mode of a field effect type superconducting three-terminal element is switching between a superconducting state of zero voltage and a normal conducting state of a finite voltage.

The coherence length is the carrier concentration and mobility of the semiconductor part,
Alternatively, although it depends on the mean free path, it is about 0.1 to 0.5 μm for a highly mobile semiconductor such as GaAs. However, when an oxide-based superconducting film is formed on a compound semiconductor such as CaAs, mutual diffusion or reaction occurs at the interface, contact resistance increases, and superconducting properties of the oxide deteriorate. In particular, it does not show superconductivity at the interface.

In order to eliminate the problem of high contact resistance at the interface and deterioration of superconducting properties of oxides, as a material of the semiconductor part,
It is desirable to use the oxide itself for the channel.
However, the mobility of the oxide-based semiconductor layer is low,
It is about m ² / Vs. Therefore, in the case of coupling with a semiconductor having such a low mobility, it is necessary to further shorten the channel length, that is, the distance between the source and the drain by one digit. When it is desired to operate the device at liquid nitrogen temperature instead of liquid helium, the coherence length is further shortened, and accordingly, the channel length needs to be further shortened. Even with the current processing technology or pattern formation technology, it is difficult to obtain a pattern of 0.05 μm or less. Further, in the conventional device structure, a gate electrode is inserted between such a short source and drain. Since it is necessary, it is more difficult to manufacture a fine element having such a structure.

An object of the present invention is to use an oxide-based (field effect type) that does not require fine processing of a superconducting electrode film, realizes a fine channel length, and can perform a switching operation by a voltage signal of a gate. ) To provide a superconducting switching element.

[Means for Solving the Problems]

In order to achieve the above object, a field-effect type superconducting switching element that causes a switching operation by a voltage signal is as follows.

First, the thickness of the oxide thin film is set to 10 nm or less, electrode films for applying a bias current or voltage are formed on both sides of the oxide thin film, and a gate electrode is formed on the oxide thin film via an insulating film. The electrode films are laminated. The gate electrode film is preferably a superconductor, but may be a normal conductor.

Further, as the oxide thin film, Y-Ba-Cu oxide, Bi-sr-Ca-Cu oxide, La-Sr-Cu oxide, Nd-Ce-Cu oxide, or Tl-Ba having high critical temperature characteristics is used. An oxide-based superconducting thin film based on a perovskite-based crystal structure containing Cu, including a —Ca—Cu oxide.

Regarding the method for manufacturing a superconducting switching element of the present invention, a single crystal material having a perovskite crystal structure such as SrTiO ₃ is used as a substrate, and a film is formed at a temperature of 500 ° C. or higher to obtain a Y-Ba-Cu oxide or the like. To obtain a continuous film having a perovskite crystal structure of 10 nm or less in film thickness. Further, an interlayer insulating film, a gate electric field film, and an electrode film connected to the ultrathin superconducting film are formed on the oxide superconducting film to obtain a device. As the interlayer insulating film, CaF ₂ or Mg
Fluorides such as F ₂ and oxides such as MgO and Al ₂ O ₃ are suitable. These interlayer insulating films can be formed by a sputtering method or a reactive vapor deposition method.

[Action]

Generally, when the superconductor is maintained at a temperature within 5% of the critical temperature, or because the number of superconducting conductors is small due to the shape effect or the original properties of the material, the superconducting state is apparently mixed even in the superconducting state. To do. When a current is passed through such a superconductor, a voltage is generated despite the superconductor. Such a phenomenon is said to be a fluctuation effect, but the present invention uses an oxide-based superconducting thin film having the fluctuation effect.

Generally, in order to generate superconductivity, the superconducting pair must exist within the dimension region of the superconducting coherence length. For example, such a state may occur where the superconductor density is low, the superconductor has a thin film shape, and the film thickness is very thin. At a certain moment, a portion which is a normal conducting region and a portion which is a superconducting region where superconducting conductors exist are mixed.
Therefore, even if the material is considered to be in a superconducting state in bulk, the superconducting region is not connected in the ultrathin film shape, and a voltage is generated when a current is applied to both ends of the sample.

The conditions for obtaining a superconducting sample in which such a situation occurs are that the superconducting electron density and the film thickness are mutually related. According to our study, in order to produce such a fluctuation effect in an oxide-based superconducting thin film based on a perovskite-based crystal structure containing Cu, the oxide thin film has a thickness of 10 n
It has been clarified that the thickness is preferably m or less, more preferably 8 nm or less.

The carrier concentration of an oxide-based superconductor containing Cu such as a Y-Ba-Ca-based oxide is about 10 ²¹ cm ^-3 . The carrier concentration of this oxide is a low value for a superconducting material, and it can be considered to be the ratio of one carrier existing in a volume of 1 nm ³ . Even if the carrier concentration is such a low value, if the volume of the superconductor is large, the number of carriers also becomes a macro value, and the behavior as a normal superconductor is exhibited, and no peculiar phenomenon occurs.

However, if the oxide thin film is made thinner,
For example, when the film thickness is set to 10 nm or less, a portion where no carriers are present locally occurs when viewed from the film thickness direction.
Since the superconducting coherence length of the oxide superconductor is as short as about 1 nm, it is in the normal conducting state in the portion where no carrier exists. That is, the entire ultrathin superconducting thin film is composed of a superconducting state region and a normal conducting state region.

When an electric field is applied to the region in the normal conducting state, the electric field penetrates into the inside instead of completely entering the inside unlike the superconductor. That is, when an electric field is applied to the ultrathin oxide superconducting film, a portion where no electric field is generated in a superconducting state and a portion where an electric field is generated in a normal conducting state are locally formed.

The bending of the energy band occurs in the part where the electric field is generated in the normal conducting state. The energy edge of the band can be raised to the Fermi surface in response to the bending of the energy band. As a result, carriers can be generated in this portion. Due to the generation of carriers, this portion becomes superconducting and no voltage difference occurs. By such a phenomenon, it is possible to increase the carrier concentration by applying an electric field, enhance the superconductivity of the thin film, and reduce the electric resistance of the entire thin film. At this time, the superconducting thin film remains in the state where superconducting and normal conducting are mixed,
As shown in FIG. 5, since the proportion of the superconducting region is large, the whole is connected in the superconducting region and no voltage difference occurs. By applying an electric field in this way, a switching operation can be caused.
The above is the operation principle and operation method of the superconducting switching device according to the present invention.

In the ultrathin oxide film used in the present invention, it is important to obtain a uniform continuous film without pinholes.
For this purpose, it is desirable to grow the oxide thin film by selecting the substrate material as the base. A preferable substrate material for epitaxially growing the oxide thin film is, for example, single crystal SrTiO ₃ .

〔Example〕

 An embodiment of the present invention will be described below.

As shown in FIG. 1 and FIG. 2, SrTiO ₃ (1
10) Using the plane-oriented single crystal as the substrate 1, the Y-Ba-Cu oxide thin film 2 is formed to a thickness of 8 nm. The film is formed by a high frequency magnetron sputtering method. The atmosphere gas is a mixed gas of Ar and 50% of oxygen, and the total pressure is 5 mTorr. The target material is a Y-Ba-Cu oxide disc-shaped sintered body. Frequency of 13.5 as power source
A high frequency power of 110 W at 6 MHz is used. The substrate temperature during film formation is 600 ° C. This Y-Ba-Cu oxide thin film 2 has a crystal orientation in which the c-axis of the orthorhombic crystal exists in the plane of the substrate. The superconducting critical temperature of the Y-Ba-Cu oxide thin film 2 is 40K.

A MgO thin film 3 is formed on the Y-Ba-Cu oxide film 2 by a reactive vapor deposition method in an oxygen gas atmosphere. Y-
From the Ba—Cu oxide film 2 to the formation of the MgO thin film 3,
It is performed in the same apparatus without exposing the substrate surface to the atmosphere. The thickness of the MgO thin film 3 is 10 nm. This MgO
The thin film 3 is a layer for protecting the Y-Ba-Cu oxide film 2 from the pattern forming process.

An organic resist mask pattern is formed in the state of the Y-Ba-Cu oxide-MgO bilayer film, and a pattern as an ultrathin oxide superconducting film is formed. The dimension of the ultrathin oxide superconducting film to which an electric field is applied is 10 μm in both length and width. The length and width correspond to the channel length and the channel width, respectively. The pattern of the bilayer film is formed by an ion beam etching method using an Ar beam. Next, a resist pattern for the MgO interlayer insulating film is formed. This pattern is MgO
Before forming the film 4, a reverse pattern for lift-off is formed in advance. M on top of this with a film thickness of 100 nm
By forming the gO thin film 4 and removing the resist film, a pattern as an interlayer insulating film is obtained. The resist film is removed by dissolving it in an organic solvent such as acetone.

Further, the Au electrode film 5 connected to the Y-Ba-Cu oxide thin film 2 is formed. For this purpose, Y-Ba-
The protective MgO film 3 formed on the Cu oxide thin film 2 is removed by the ion beam etching method. The pattern of the Au electrode film 5 is also obtained by forming the film on the resist pattern formed in advance by the lift-off method. The Au electrode film 5 is formed by a vacuum vapor deposition method.

Further, the pattern and film formation of the Au gate electrode film 6 for applying an electric field are performed. The Au gate electrode film 6 is formed by using a vacuum evaporation method, and the pattern is formed by using a lift-off method. A superconducting switching device is obtained by the above manufacturing process.

The characteristics of the superconducting switching device manufactured by the above method are as shown in FIG. That is, when the gate voltage is not applied, the high resistance state is set. In the oxide thin film of the channel portion at this time, as shown in FIG. 4, the superconducting regions 1 are discretely present in the normal conducting region in the channel direction (for example, the AA ′ direction in FIG. 4A). To do. Therefore, when looking at the cross section along the channel direction as shown in FIG. 4 (b), the superconducting region 1 is separated by the normal conducting region 2, so that the channel is not connected only by the superconducting region 1 and a high resistance is obtained. It becomes a state. On the contrary,
When a gate voltage of 100 mV or higher is applied, approximately 30μ
The superconducting current of A flows. In the oxide thin film of the channel portion at this time, the superconducting region 1 is enlarged as compared with the case of FIG. 4 as shown in FIG. Therefore, for example, when looking at the BB ′ cross section of FIG. 5A, the superconducting region 1 is separated by the normal conducting region 2, but for example, in the CC ′ cross section, the channel is connected only by the superconducting region 1. . Therefore, a superconducting current flows through the channel. Such a device characteristic is 40 K which is the critical temperature of the Y-Ba-Cu oxide thin film.
It is observed up to the neighborhood. Since such a switching operation is a phenomenon performed in a two-dimensional thin film, it is performed in a very short time on the order of picoseconds. In addition, the switching delay due to heat dissipation when the transition from the high resistance state (high heat generation state) to the superconducting state (low heat generation state) can be ignored, as is apparent from the element shape.

Such a superconducting switching device has characteristics as a switching element of a digital circuit or an analog circuit, and can be applied to a logic circuit, a storage circuit, a digital / analog conversion circuit, or the like. Further, it is possible to configure a high speed processing system using these various circuits such as a computer unit and a signal processing device.

The superconducting switching device according to the present invention can be implemented not only in the element structure described above, but also in a structure in which the vertical relationship between the ultrathin superconducting oxide thin film and the gate electrode film is reversed. Further, as an oxide thin film, not only the above Y-Ba-Cu oxide but also B-Sr-Ca-Cu oxide, Tl-Ba
-Ca-Cu oxide, La-Sr-Cu oxide, Nd-
A device can be similarly constructed by using Ce-Cu oxide or the like,
In addition, similar element characteristics and element performance can be exhibited. Despite the use of these different element structures or superconducting materials, the operating principle of switching is still based on the fluctuation of the superconducting carrier as described in the section of action.

〔The invention's effect〕

The superconducting switching device according to the present invention has the following effects.

(1) It is possible to operate without forming a submicron channel length like a conventional field effect type superconducting transistor.

(2) It is possible to switch between superconductivity and normal conduction, or between zero voltage state and high resistance state, not only at liquid helium temperature but also at high temperature of several tens of K.

(3) The above element characteristics have characteristics as a switching element of a digital circuit or an analog circuit. Therefore, it can be used as an active element of a logic circuit, a memory circuit, a digital / analog conversion circuit, or the like.

[Brief description of drawings]

FIG. 1 is a sectional view of an embodiment of a superconducting switching element according to the present invention, FIG. 2 is a plan view of an embodiment of a superconducting switching element according to the present invention, and FIG. 3 is a view of a superconducting switching element according to the present invention. FIG. 4 is a diagram showing the voltage-current characteristics in one embodiment, and FIGS. 4 and 5 are diagrams for explaining the operation of the element of one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1 ... SrTiO ₃ substrate, 2 ... Y-Ba-Cu oxide thin film, 3 ... MgO thin film, 4 ... MgO interlayer insulating film, 5
...... Au electrode film, 6 ... Au gate electrode film.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-58-87884 (JP, A) JP-A-2-194667 (JP, A) JP-A-3-104177 (JP, A)

Claims (2)

[Claims] 1. An oxide-based superconducting thin film containing Cu, a pair of electrode films formed in contact with and separated from the superconducting thin film, and an insulating film for the oxide-based superconducting thin film. A superconducting switching device comprising a gate electrode film disposed via a gate electrode film, wherein the oxide-based superconducting thin film has a thickness of 10 nm or less, and the resistance between the pair of electrode films is equal to that of the gate electrode film. A superconducting switching element characterized by being controlled by an applied voltage. 2. The superconducting switching element according to claim 1, wherein the oxide-based superconducting thin film is made of any one of YBaCu oxide, BiSrCaCu oxide and TBaCaCu oxide.