JPH08228029A - Superconducting transistor - Google Patents

Abstract

PURPOSE: To enable the switching operation in superconductive mode having a sufficient gain to be used as a circuit element feasible by a method wherein an oxide containing tetragonal or orthogonal Cu is used for a channel region while the atomic surfaces in low conductivity as well as superconductivity out of crystalline ab surfaces are laminated in C axial direction. CONSTITUTION: Stepped parts are formed on a strontium titanate substrate 1 in (100) orientation so as to form a source electrode 2, a drain electrode 3, a channel layer and a pattern to be a wiring out of YBaCu oxide containing Co using an organic resist on the substrate 1 to be processed respectively and partially by etching step using Ar ion beams. Successively, a strontium titanate thin film to be a gate insulating film 5 is formed on the substrate 1 containing stepped parts further forming an Au film gate electrode 6 on the stepped parts. Through these procedures, the rapid switching operation in superconductive mode having a sufficient gain can be made feasible without enlarging the element dimension at all.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of superconducting electronics, such as high-speed digital circuits, analog data processing circuits, sensor circuit devices, and minute magnetic field signal detecting devices, which exhibit unique performance by using superconductivity. In particular, it relates to a superconducting transistor having high speed and low power consumption performance.

[0002]

2. Description of the Related Art A conventional superconducting transistor comprises superconducting source and drain electrodes, a non-superconducting channel layer made of a normal conductor or an insulator, a gate insulating film and a gate electrode. Due to the superconducting proximity effect, a superconducting current flows between the source electrode and the drain electrode through the non-superconducting channel layer. This superconducting current usually has a characteristic as a Josephson current and responds to microwaves and magnetic fields.

The amplitude of the superconducting current flowing by the superconducting proximity effect depends on the electrode spacing. That is, the superconducting current decreases exponentially with the superconducting decay length determined by the conduction characteristics of the channel layer as a basic unit. The amplitude of the superconducting current is determined by the ratio of the electrode spacing and the superconducting decay length.

The superconducting decay length depends on the carrier concentration in the channel layer. The higher the carrier concentration, the longer the superconducting decay length. When a voltage is applied to the gate to form the storage layer, the carrier concentration increases, the superconducting decay length increases, and the superconducting current increases. When the carrier is a hole, a storage layer is formed by applying a negative voltage. On the other hand, when a voltage is applied to the gate to form a depletion layer, the carrier concentration is lowered, the superconducting decay length is shortened, and the superconducting current is reduced. When carriers are holes, a depletion layer is formed by applying a positive voltage.

A metal such as Nb or an oxide such as YBa ₂ Cu ₃ O _{7-x is} used as a superconducting electrode material of such a superconducting transistor, and a semiconductor such as Si or a non-superconducting oxide is used as a channel layer material. Has been.

[0006]

The superconducting element having the conventional structure has the following problems and obstacles in obtaining the element having the control function and exhibiting the performance of the superconducting element.

The superconducting transistor is for switching between a zero voltage state and a voltage state. The voltage value of the voltage state is determined by the bias current and connection resistance,
Depending on the case, it can be set to an arbitrary voltage value.
However, the superconducting property, that is, the Josephson current component contributes at most several times the gap voltage of the superconducting electrode. When the applied voltage is higher than this, only the normal conduction current component is present, and the superconducting current component is not present. Therefore,
It is the normal conduction current component that changes depending on the gate voltage.

The gate voltage required to sufficiently modulate the superconducting properties is dictated by the properties of the channel layer. In order to increase the gain of the superconducting transistor, that is, the voltage between the source and drain that is the output signal with respect to the gate voltage that is the input signal, increase the bias current within the range where the superconducting current is maintained and increase the load resistance. It is to be. However, even if such measures are taken,
As described above, when the source / drain voltage becomes sufficiently higher than the gap voltage, the superconducting mode becomes ineffective. Increasing the load resistance further reduces the amount of current transfer from the transistor to the load resistance.

When the switching operation is performed in the superconducting mode, the superconducting current has an exponential dependence on the superconducting decay length as described above. The superconducting decay length is dependent on the gate voltage through the carrier concentration. That is, in the superconducting mode, the superconducting current has an exponential dependence on the carrier concentration. Correspondingly, the superconducting current is
It changes at an exponential rate. On the other hand, in the case of a field effect element using a normal semiconductor, the conductivity between the source and the drain changes in proportion to the carrier concentration.

From this point of view, the superiority of using the superconducting transistor in the superconducting mode is clear.
However, if the voltage value in the voltage state is sufficiently higher than the gap voltage of the superconducting electrode, such superiority cannot be utilized.

Therefore, an object of the present invention is to provide a superconducting transistor having source and drain electrodes made of a superconducting material, which has a gain sufficient for use as a circuit element, that is, the ratio of the output voltage to the input gate voltage is 1 or more. In addition to the characteristics of the above, the device structure in which the electric characteristics have a superconducting mode even in such a voltage state,
Or it is to give the manufacturing method.

[0012]

In order to obtain a field effect superconducting transistor for solving the above problems, the following device structure and manufacturing method are provided.

That is, the superconducting transistor is composed of a superconducting source electrode, a drain electrode, a channel layer sandwiched between superconducting electrodes, a gate insulating film and a gate electrode, and tetragonal or orthorhombic Cu is formed in the channel layer. An oxide that is a contained oxide and has a structure in which an atomic plane having low electrical conductivity and an atomic plane having superconductivity in the ab plane of the crystal are stacked in the c-axis direction is used.

In the channel layer portion, the oxide crystal has an azimuthally oriented structure in which the c-axis is not perpendicular to the substrate surface.

In the channel layer portion, superconducting bonds are formed between atomic planes having superconductivity via atomic planes having low electric conductivity, and a plurality of superconducting bonds each having a unit cell of such a crystal are connected in series. The source and drain electrode portions are arranged so that the atomic planes having superconductivity coincide with the direction of current flow.

As oxides used for the channel layer, Bi,
Oxide consisting of Sr, Ca and Cu, or T
An oxide composed of 1, Ba, Ca and Cu, in which Ca is partially replaced by a trivalent element.

Alternatively, the oxide used for the channel layer is an oxide of Y, Ba, and Cu in which Cu in the chain atom sequence of Cu and oxygen is replaced with Cr or Fe.

Further, in such a superconducting transistor, as a substrate on which the superconducting transistor is mounted, a step is formed between the source electrode and the drain electrode portion, the channel portion is inclined, and the channel layer is formed on the inclined substrate. To arrange.

[0019]

The structure and manufacturing method of the superconducting transistor described above have a gain sufficient for use as a circuit element and enable switching operation in the superconducting mode for the following reasons.

Compared with a gate voltage sufficient to modulate the conductance between the source and drain, the voltage value when switching to the voltage state is equal to or higher than this value, and even in the superconducting state, the superconducting mode is set. In order to hold the negative electrode, the source electrode 12, the channel layer 13 and the drain electrode 12 are connected in series as shown in FIG. But,
Such a series connection of superconducting transistors provides a function as a single element even though a large number of elements are arranged. Therefore, the area occupied by one element is increased, which is disadvantageous for integration. The superconducting transistor of the present invention has a superconducting mode in a voltage state and can obtain a voltage value corresponding to a gate voltage with a single element for the following reasons.

Oxide tetragonal or orthorhombic superconducting materials have extremely strong anisotropy, and their superconductivity in the c-axis direction is weaker than that in the ab plane. Looking at this in detail in a unit cell of a crystal, an ab plane atomic layer having high superconductivity and an ab plane atomic plane having weak superconductivity or non-superconductivity are alternately laminated in the c-axis direction. .

Therefore, as shown in FIG. 3, in one unit cell in the c-axis direction, there can be an atomic layer 23 having a strong superconducting property serving as a source or drain electrode and a non-superconducting layer 22 serving as a channel. Then, the output voltage having superconductivity can be increased in proportion to the number of unit cells connected in the c-axis direction. Oxide crystals may be arranged on the source electrode 2 and the drain electrode 3 of the entire device in a direction in which a superconducting current flows along an atomic plane having a strong superconducting property.

Oxide or B composed of Y, Ba and Cu
In the c-axis direction of the oxide composed of i, Sr, Ca, and Cu, such a layer having superconductivity and a layer having weak superconductivity are laminated in atomic layer units. However, even if a current is passed in the c-axis direction of these superconducting thin films such as YBaCu oxide, only bulk superconducting properties are detected.

On the other hand, an oxide composed of Bi, Sr, Ca, and Cu, an oxide composed of Tl, Ba, Ca, and Cu in which a part of Ca is replaced by a trivalent element, or Y, Ba. , And Cu, in which Cu in the chain atom sequence consisting of Cu and oxygen is replaced with Cr, Fe or the like, the superconducting property of a specific atomic layer in the c-axis direction becomes particularly weak. For example, Y, B
The oxides of a and Cu, in which Cu in the chain atom sequence consisting of Cu and oxygen is replaced with Cr or Fe, etc., have the same electrical properties as the oxygen atoms in the chain atom sequence are removed. The characteristics and superconductivity are shown. That is, by replacing Cu with Cr, Fe, or the like, the superconducting pair forming interaction does not change, and only the concentration of superconducting carriers decreases. As a result, the superconducting carriers are localized on the atomic plane composed of Cu and oxygen, which have stronger superconducting properties, and the distribution density of the superconducting carriers in the chain-like atomic sequence becomes low. Since the c-axis length of YBaCu oxide is 1.2 nm, a structure in which superconducting layers and non-superconducting layers are alternately laminated in unit cell units can be obtained.

Even if a structure in which superconducting layers and non-superconducting layers are alternately laminated in a unit cell unit is obtained, a superconducting transistor cannot be obtained only with such a modulation structure.
It is desirable that the source and drain electrodes themselves of the superconducting transistor do not have a modulation structure in the conduction direction. Therefore, as shown in FIG. 1, as a substrate on which the superconducting transistor is mounted, a step is formed between the source electrode 2 and the drain electrode portion 3, the channel portion 4 is inclined, and the channel layer is formed on the inclined substrate. In such a case, the c-axis of the oxide crystal is perpendicular to the substrate at the source electrode and drain electrode portions, and the current flows in the ab plane. On the other hand, in the channel layer 4, since the ab plane is not parallel to the substrate plane and forms an angle, the current path is a−
It is in the direction of passing over the plane b. Therefore, the current effectively flows in the c-axis direction in the channel layer.

[0026]

【Example】

(Example 1) As shown in FIG. 1, a superconducting transistor is manufactured by the following procedure. A step is formed on the strontium titanate substrate 1 having a (100) plane orientation. The steps are formed by forming a pattern on the organic resist film and etching using an Ar ion beam. The step angle is about 45 degrees, and the step height is 100 nm. Next, a YBaCu oxide thin film containing Co is formed by the laser deposition method. Laser deposition was performed in an oxygen atmosphere of about 0.2 Torr using a sintered body of YBaCu oxide containing Co as a target and an excimer laser having a wavelength of 248 nm as a light source, and the substrate was heated to 700 ° C. YBaCu containing Co
Deposit oxide. The thickness of the YBaCu oxide thin film containing Co is 20 nm or less.

Next, the source electrode 2, the drain electrode 3,
A pattern including the channel layer 4 and the wiring is formed in a YBaCu oxide thin film containing Co. The pattern is formed by using an organic resist and these source 2 and drain electrodes 3
And so on. Based on the organic resist pattern, by the etching method using Ar ion beam,
The YBaCu oxide thin film containing Co is processed. As a result, the pattern including the source and drain electrodes, the channel layer, and the wiring is transferred from the organic resist pattern to the YBaCu oxide thin film including Co. YBa containing Co
In the pattern of the Cu oxide thin film, the step portion is the channel layer 4, the flat portion in contact with the step is the source 2 and drain electrode 3, and the other portion is the wiring layer.

Further, a strontium titanate thin film is formed on the substrate including the step portion to form the gate insulating film 5. The strontium titanate thin film is formed by the laser vapor deposition method. Laser deposition was performed in an oxygen atmosphere of about 0.2 Torr using a strontium titanate single crystal as a target and an excimer laser with a wavelength of 248 nm as a light source.
A strontium titanate film is deposited on the substrate heated to 50 ° C. The thickness of the strontium titanate film is 300n
m. Further, an Au film to be the gate electrode 6 is formed on the step portion to obtain a superconducting transistor.

In the superconducting transistor formed by such a method, as shown in FIG. 4, a DC voltage step appears up to about 1 V with respect to microwave irradiation. This corresponds to showing superconductivity up to 1 V in the voltage state. This signal voltage is several tens to several hundreds times as large as the conventional superconducting transistor. This superconducting transistor is shown in Fig. 5.
As shown in, the superconducting current 32 becomes almost zero by applying a gate voltage of about 1V. By returning the gate voltage to zero, the superconducting current 31 is restored to its original value.

As described above, the superconducting transistor of the present invention has superconductivity even with a signal voltage at the same level as the gate voltage required to modulate the characteristics. The source and drain electrodes and the channel layer of such a superconducting transistor are not limited to YBaCu oxide containing Co. A similar superconducting transistor can be obtained by using another YBaCu oxide, such as using Fe as an element substituted for the Cu site. Further, a superconducting transistor is manufactured using an oxide composed of Bi, Sr, Ca, and Cu, or an oxide composed of Tl, Ba, Ca, and Cu in which a part of Ca is replaced by a trivalent element. Also has similar characteristics.

[0031]

The present invention has the following effects.

(1) The signal voltage in the superconducting mode can be increased to an arbitrary level without increasing the element size.

(2) The gain, that is, the ratio of the signal voltage to the gate voltage, in the state where the element is kept in the superconducting state can be made 1 or more.

(3) Since the superconducting mode is maintained even in the voltage state, the switching speed becomes faster than that of the conventional transistor.

[Brief description of drawings]

FIG. 1 is a sectional view of a superconducting transistor of Example 1.

FIG. 2 is a cross-sectional view of a series connection of superconducting transistors having a conventional structure.

FIG. 3 is a sectional view of the principle structure of the superconducting transistor of the present invention.

FIG. 4 is a characteristic diagram of microwave irradiation of the superconducting transistor of Example 1.

FIG. 5 is a switching characteristic diagram of an ON state and an OFF state of the superconducting transistor of Example 1.

[Explanation of symbols]

1 ... Substrate, 2 ... Source electrode, 3 ... Drain electrode, 4 ... Channel layer, 5 ... Gate insulating film, 6 ... Gate electrode.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Uki Kababun, Tokyo Kokubunji City Higashi Koikeku 1-chome 280, Central Research Laboratory, Hitachi, Ltd. Central Research Laboratory (72) Inventor Tokumi Fukasawa 1-280, Higashi Koikekubo, Kokubunji, Tokyo Hitachi Research Laboratory, Central Research Laboratory

Claims (6)

[Claims] 1. A superconducting transistor comprising a superconducting source electrode, a drain electrode, a channel layer sandwiched between superconducting electrodes, a gate insulating film and a gate electrode, wherein the channel layer contains tetragonal or orthorhombic Cu. A superconducting transistor, which is an oxide and is composed of an atomic plane having a low electrical conductivity and an atomic plane having superconductivity in the ab plane of a crystal. 2. The superconducting transistor according to claim 1, wherein a c-axis of a crystal is not perpendicular to a substrate surface in a portion corresponding to the channel layer. 3. The superconductivity in which a superconducting bond is formed between atomic planes having superconductivity via atomic planes having low electric conductivity in the channel layer, and a unit cell of such a crystal is used as a unit. A superconducting transistor in which a plurality of couplings are arranged in series in a channel layer. 4. The oxide forming the channel layer according to claim 1, wherein Bi, Sr, Ca, and Cu are oxides, or Tl, Ba, Ca, and Cu are some of the oxides. A superconducting transistor which is an oxide substituted with a trivalent element. 5. The oxide according to claim 1, wherein the oxide forming the channel layer is an oxide containing Y, Ba, and Cu.
A superconducting transistor which is an oxide in which Cu in a chain of atoms consisting of Cu and oxygen is replaced with Cr or Fe. 6. The substrate according to claim 1, wherein the substrate on which the superconducting transistor is mounted has a step between the source electrode and the drain electrode portion, is inclined in the channel portion, and is formed on the substrate in which the channel layer is inclined. A superconducting transistor that has been created.