JPH0714079B2 - Oxide superconducting three-terminal device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a field-effect type oxide superconducting three-terminal element, and particularly to a superconducting electronics field as a superconducting switching element that performs switching operation with low power consumption. The present invention relates to an oxide superconducting three-terminal device applied to both digital circuits and analog circuits.

[Conventional technology]

The field-effect superconducting three-terminal element has a three-terminal structure as compared with the Josephson element, has sufficient input / output separation, and can perform switching with a voltage signal, and
It has the advantage that it can be driven by a DC power supply.

Conventionally, an Nb-based superconducting material that requires liquid helium temperature operation has been used as a superconducting three-terminal element that uses the electric field effect.Examples of using the exuding effect of a superconducting conductor and the electric field effect of GaAs or Si are Physical Review Redders, 54, 2449, 1985 (Physical Review Letters, Vo
l.54, p.2449, 1985). In this example, it should be the source and drain electrodes on the semiconductor substrate.
It has a structure in which a plurality of superconducting films are arranged close to each other and a gate electrode film is inserted therebetween. That is, the structure is such that the source, gate and drain electrodes are arranged side by side on one surface of the InAs semiconductor substrate. The superconducting current flows from the source through the semiconductor to the drain. The semiconductor portion becomes a superconducting weak coupling portion in which a superconducting current flows due to the seeping effect of the superconducting element.

[Problems to be Solved by the Invention]

The conventional field effect type three-terminal device is a semiconductor part in which a superconducting current should flow in order to allow a superconducting current to flow between a source and a drain when it is applied to an oxide superconducting material having a high critical temperature. The length, that is, the channel length, needs to be about the length of the superconducting coherence, so that a very advanced technique is required for manufacturing the device. When the channel length is longer than the coherence length, the resistance value between the source and drain electrodes changes due to 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 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 about 0.1 to 0.5 μm for a high mobility semiconductor such as GaAs, although it depends on the carrier concentration, mobility or mean free path of the semiconductor portion. However, when an oxide-based superconducting thin film is formed on a compound semiconductor such as GaAs, mutual diffusion or reaction occurs at the interface, the contact resistance increases, and the superconductivity of the oxide deteriorates. In particular, it does not show superconductivity at the interface.

In order to eliminate the problems of deterioration of superconducting properties of oxide and high contact resistance at the interface, it is desirable to use an oxide semiconductor layer. However, the mobility of the oxide-based semiconductor layer is low, about 0.01 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 attempted to operate the device at the liquid nitrogen temperature instead of the liquid helium temperature, the coherence length becomes shorter.

Correspondingly, it is necessary to further shorten the channel length. Even with the current processing technology or pattern forming technology, it is difficult to obtain a pattern of 0.05 μm or less. Furthermore, in the conventional device structure, it is necessary to insert a gate electrode between such a short source and drain. Such a structure makes the device more difficult to manufacture.

An object of the present invention is an oxide that does not require fine processing of 0.1 μm or less for a superconducting electrode film, realizes a minute channel length, and can perform a switching operation by a voltage signal to a gate electrode. It is to provide a superconducting three-terminal element of the system.

[Means for Solving the Problems]

In order to achieve the above object, in the present invention, a semiconductor layer constituting a superconducting three-terminal element, a gate insulating thin film, and a source electrode composed of a superconducting thin film, a drain electrode, a gate electrode is made of an oxide, and a source and As the superconducting thin film that constitutes the drain, a thin film in which crystal cracks are generated due to a difference in thermal expansion coefficient from the substrate when heat treatment is performed at high temperature is used. These cracks are
Be sufficiently electrically isolated. This electrical separation is realized by providing a gap in the cracked portion or by interposing an insulating substance. The peripheral region of the electrically conductive superconducting thin film serves as the source and drain electrodes, and the current flows between the crystal grain boundaries of the intermediate region through the semiconductor layer. That is, the structure is such that the source and drain electrodes are arranged across the semiconductor layer, and the gate electrode is arranged on the surface of the semiconductor layer via the gate insulating thin film.

The oxides forming the superconducting thin film, the semiconductor layer and the gate insulating thin film are Y-Ba-Cu oxide, Bi-Sr-Ca-Cu oxide,
La-Sr-Cu oxide, Tl-Ba-Ca-Cu oxide, Nd-Ce-Cu
An oxide based on a perovskite crystal structure containing Cu such as an oxide is used.

Regarding the structure of the superconducting three terminals, an insulating substrate, a source / drain electrode film, a semiconductor layer, a gate insulating film, and a gate electrode film are laminated in this order from the lower side of the device. Alternatively, a device structure in which the stacking order is reversed, such as an insulating substrate, a gate electrode film, a gate insulating film, a semiconductor layer and a source and drain electrode film, is also possible in this order from the lower side of the device.

The outline of the method for manufacturing the above-described field-effect type superconducting three-terminal element is as follows.

An epitaxy oxide thin film having a perovskite crystal structure such as Y-Ba-Cu oxide is formed by using a single crystal material having a perovskite crystal structure such as SrTiO ₃ as a substrate and performing film formation at a temperature of 500 ° C. or higher. To get By subjecting the Y-Ba-Cu oxide thin film thus formed to a heat treatment in vacuum, it is possible to obtain semiconductor-like or insulator-like electrical characteristics. On the contrary, by subjecting the Ya-Ba-Cu oxide thin film to a heat treatment at 500 ° C or higher in an atmosphere of oxygen at 1 atm, it is possible to obtain superconducting properties of 70K or higher. One method of forming crystal grain boundaries in a superconducting film is to use the difference in the coefficient of thermal expansion between a perovskite-based crystal and the substrate to bring it from a substrate temperature of 500 ° C or higher during film formation to a room temperature substrate temperature. This is a method that utilizes cracks that sometimes occur.

[Action]

The above-described structure and manufacturing method of the oxide-based superconducting three-terminal element enables switching between superconducting and normal conducting due to the electric field effect and provides an element structure which is easy to manufacture.

The characteristics required for a field-effect type superconducting three-terminal element are that when a gate voltage is applied, the source and drain become superconducting and a zero voltage current flows, and when no gate voltage is applied, a normal conducting state is applied. It becomes a voltage state. In order for the source and the drain to be in a superconducting state, it is necessary that the superconducting coherence length in the semiconductor layer when a gate voltage is applied be substantially equal to the channel length.

The coherence length in the semiconductor layer depends on the carrier concentration, mobility, and operating temperature of the semiconductor layer. The higher the carrier concentration and mobility, the longer the coherence length, and conversely, the higher the operating temperature. , The coherence length becomes shorter. When a voltage is applied to the gate, a storage layer is formed in the channel part of the semiconductor layer, and the carrier concentration increases. Therefore, when a sufficient gate voltage of several V or several tens of V is applied,
It is possible to have a superconducting state between the source and the drain. However, when the gate signal voltage is ten times or more higher than the superconducting gap voltage which is considered to be several tens of mV, the gain as an element cannot be obtained. When using a compound semiconductor with high mobility, the required channel length is 0.1
Is 0.5 μm. The mobility of oxide semiconductor is 0.01 m ² / Vs
It is the following. Since the mobility of the semiconductor layer is a value specific to the material, it cannot be increased significantly.

Considering the above points, it is necessary to set the channel length of the oxide superconducting three-terminal element to a value of 0.05 μm or less.
Especially when the superconducting three-terminal element is operated at the liquid nitrogen temperature instead of the conventional liquid helium temperature, such a short channel length is essential. However, it is impossible to set the distance between the source and the drain to 0.05 μm or less by starting from one superconducting thin film and processing it. On the other hand, in the device structure of the present invention, the crystal grain boundary peculiar to the oxide superconducting thin film can be used for the channel portion.

That is, in an oxide superconducting thin film having a perovskite-based polycrystalline structure, the superconducting property is weakened at the crystal grain boundaries, causing a decrease in the critical current. When a film is formed in a state in which stress is applied to the superconducting thin film, cracks occur at the crystal grain boundaries. In order to enable the formation of such cracks, the difference in the coefficient of thermal expansion between the oxide superconducting thin film and the substrate material is used. For example, the coefficient of thermal expansion of the Y-Ba-Cu-O film is 1.5 × 10 ^-5 / deg and that of the SrTiO ₃ film is 1.1.
Since it is × 10 ^-5 / deg, there is a difference of 4 × 10 ^-6 per 1 deg.
A crack can be formed by utilizing this difference and the weak bond in the C-axis direction of the oxide thin film. To give a numerical example,
In the Y-Ba-Cu-O film formed on (110) of the SrTiO ₃ substrate and heat-treated at 850 ° C., cracks with a width of 30 nm occur at intervals of 10 μm on average. The intervals, widths, and positions of cracks can be detected by a scanning electron microscope. The crystal grain spacing in the crack portion can be arbitrarily adjusted by, for example, the heat treatment temperature condition after the film formation, the selection of the substrate material, and the like. As the heat treatment temperature becomes higher, the gap becomes narrower and the width becomes wider. In such a film structure, the crystal grain boundaries are not electrically connected. Therefore, a current flows through the semiconductor layer formed in contact with the superconducting film. The electrical insulation between the crystal grains can be further sufficiently separated by exposing the oxide thin film to fluorine plasma to cause impurities such as fluorine to enter the crystal grain boundary portion.

In the present invention, the source and drain electrodes are further provided on both sides of the semiconductor layer, and the gate electrode is provided on the semiconductor layer through the gate insulating thin film, thereby removing the limitation on the gate electrode film width and It enables a channel length of μm or less. As described above, the present invention realizes a field-effect type superconducting three-terminal element with an extremely fine size that can be operated at a high temperature near the liquid nitrogen temperature.

〔Example〕

An embodiment of the present invention will be described below with reference to the drawings.
FIG. 1 is a top view of this embodiment, and FIG. 2 is a sectional view taken along line XX thereof. A SrTiO ₃ (110) -oriented single crystal substrate is used as the substrate 1, and a Y-Ba-Cu oxide superconducting thin film 2 is formed on this face.
To a thickness of 70 nm. The film was formed by a high frequency magnetron sputtering method. The atmosphere gas is a mixed gas of 50% each of argon and oxygen, and the total pressure is 30 mTorr. The target material is a disk-shaped sintered body of Y-Ba-Cu oxide having an outer diameter of 90 mm. Powered at a frequency of 13.56MHz as a power supply
A high frequency of 100 W was used. The substrate temperature at the time of film formation is 700 ° C., and after formation, heat treatment is performed in an oxygen atmosphere at 850 ° C.
-The Ba-Cu oxide superconducting film 2 is obtained. The superconducting critical temperature of this film is 82K. The thin film 2 is epitaxially grown on the (110) plane of the SrTiO ₃ substrate, and the C axis is orthogonal to the (110) plane. By performing the heat treatment at 850 ° C., cracks having a width of 30 μm are formed on the SrTiO ₃ (110) at intervals of 10 μm along the plane. After that, the film surface is exposed to a plasma atmosphere using CF ₄ gas. This creates a fluorinated state at the cracked interface. CF ₄ gas plasma is 100mT
It is generated by applying a high frequency of 100 W to the electrode in the CF ₄ gas atmosphere of orr. When exposed to CF ₄ gas plasma, the substrate temperature is heated to 100 ° C or higher.

Patterns for the source electrode 6 and the drain electrode 7 are formed on the Y-Ba-Cu oxide superconducting thin film that has been subjected to the above-mentioned treatment, by an ion beam etching method using O ₂ . Next, to be the semiconductor layer 3 is formed by PrBa ₂ Cu ₃ O _7-X oxide thin film RF magnetron sputtering method. The film formation temperature is 650 ° C and the film thickness is 200 nm. This PrBa
After coating a resist film on the ₂ Cu ₃ O _7-X oxide thin film, a pattern as a semiconductor layer 3 is processed and formed by an ion beam etching method using an Ar beam, as shown in FIG. Next, the gate insulating film 4 is formed by a high frequency macnetron sputtering method. The film formation temperature is 600 ° C. or lower, and the film thickness is 20 nm. Further, a Y-Ba-Cu oxide thin film is formed as the gate electrode 5 to a thickness of 100 nm by a high frequency magnetron sputtering method. The atmosphere gas is a mixed gas of 50% each of Ar and oxygen, and the total pressure is 50 mTorr.
And The substrate temperature during film formation is 650 ° C. After forming the film, no heat treatment is performed for the purpose of preventing diffusion between the stacked layers. By obtaining a film of stoichiometric composition, the supercritical temperature is 72K.
Is. An oxide superconducting three-terminal element is obtained by the above manufacturing process.

The characteristics of the superconducting three-terminal element produced by the above method are
When the gate voltage was not applied, the superconducting current did not flow and the state was high resistance. On the other hand, when a gate voltage of 200 mV or higher was applied, a superconducting current of about 50 μA flowed and the resistance in the voltage state also decreased. Such element characteristics have characteristics as switching elements of digital circuits and analog circuits, and are applied to logic circuits, storage circuits, digital-analog conversion circuits, and the like.

〔The invention's effect〕

The superconducting three-terminal element according to the present invention has the following effects.

(1) An element structure that enables a channel length of 0.05 μm or less, which is required when using an oxide semiconductor having low mobility as a semiconductor layer.

(2) This enables switching between superconductivity and normal conduction, or between zero-voltage state and high-low state, not only at liquid helium temperature but also at high temperature of several tens of K, and is necessary for constructing a circuit. It is possible to obtain a value of gain 1 or more, which is a simple condition.

(3) The above element characteristics have characteristics as a switching element of a digital circuit or an analog circuit, and thus can be used as an active element of a logic circuit, a storage circuit, a digital-analog conversion circuit, or the like.

[Brief description of drawings]

FIG. 1 is a top view of an embodiment of the present invention, and FIG.
It is an X sectional view. Explanation of symbols 1 ... Substrate, 2 ... Oxide superconducting thin film, 3 ... Semiconductor layer, 4 ... Gate insulating film, 5 ... Gate electrode, 6 ... Source electrode, 7 ... Drain electrode.

Claims (4)

[Claims] 1. A field effect type superconducting three-terminal element comprising a source / drain electrode and a gate electrode formed of a superconducting thin film, a semiconductor layer, and a gate insulating thin film, wherein the superconducting thin film, the semiconductor layer, and the gate insulating thin film. Is composed of an oxide, and as a superconducting thin film that constitutes the source / drain electrodes, there is a crystal crack that occurs due to the difference in the coefficient of thermal expansion from the substrate during heat treatment, and is electrically separated at this crystal crack portion. A thin film is used, the peripheral region of this electrically separated superconducting thin film is used as a source / drain electrode, and a current flows through the intermediate region of the semiconductor layer, and a gate insulating thin film is formed on the surface of the semiconductor layer. An oxide superconducting three-terminal element, characterized in that a gate electrode is formed via the. 2. The superconducting thin film constituting the source / drain electrodes according to claim 1, is strontium titanate (SrTiO ₃ ).
An oxide superconducting three-terminal device characterized in that a film is formed on the (110) direction of, and a film whose C axis is in-plane at right angles to [110]. 3. An oxide superconducting three-terminal element, wherein the oxide constituting the superconducting thin film, the semiconductor layer and the gate insulating thin film according to claim 1 is an oxide having a perovskite crystal structure. 4. The superconducting thin film which constitutes the source / drain electrodes according to claim 1, is formed by using a (110) substrate of SrTiO ₃ single crystal material at a temperature of 600 ° C. or higher, and further at 800 ° C. or higher. An oxide superconducting three-terminal element characterized by being heat-treated.