JPH05167118A - Superconducting oxide transistor and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain a superconducting oxide transistor having excellent electric characteristics by providing a superconducting oxide thin film formed on a substrate, a first and second main electrodes which are brought into ohmic- contact with the thin film, and a control electrode which is formed on the top and controls a superconducting current flowing through the superconducting thin film. CONSTITUTION:A BiBa1-xRbxO3 thin film 14 is formed on an MgO substrate 10 with a buffer layer 12 in between and a first and second main electrodes respectively provided as a source and drain electrodes 18 and 20 are brought into ohmic-contact with the thin film 14. Then a gate electrode 22 is provided between the electrodes 18 and 20 on the surface of the thin film 14 with a BiBaO3 film 16 corresponding to a gate insulating film in between. Therefore, a superconducting transistor which can be easily manufactured at a high yield and has excellent electric characteristics can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide superconducting transistor.

[0002]

2. Description of the Related Art Semiconductor transistors have problems such as high power consumption, and the advent of superconducting transistors is desired. Regarding a superconducting transistor corresponding to an FET (field effect transistor), Nishino et al. And Takayanagi et al. Have already disclosed a model structure in the following documents.

Torr. Nishino et a
l. : IEEE Electron Device L
ett. , EDL-6 (1985) 297H. Takayanagi and T.M. Kawaka
mi: Phys, Rev. Lett. , 54 (198
5) 2449 Although the structures disclosed in these documents form the source and drain regions with a superconductor material, they are not structures using an oxide superconductor material. If an oxide high-temperature superconductor material can be used for the superconducting transistor, it is expected that a superconducting transistor having a high cutoff frequency can be obtained because the critical temperature Tc is high and the energy gap is large. As typical materials for this oxide high temperature superconductor, at present, Y (yttrium) -Ba (barium) -Cu (copper) -O (oxygen) and Bi (bismuth) -S are used.
r (strontium) -Ca (calcium) -Cu
(Copper) -O (oxygen) and Tl (tellurium) -Ba (barium) -Ca (calcium) -Cu (copper) -O (oxygen)
Has been introduced.

[0004]

However, since each of the above materials has a large anisotropy in the crystal structure (the superconducting parameter varies depending on the orientation of the crystal structure), the anisotropy also appears in the electrical characteristics. Will end up. Therefore, during the process of manufacturing an oxide superconducting transistor, when forming a thin film using these materials, the crystal orientation of the film must be controlled so that excellent electrical characteristics can be obtained. Furthermore, in order to form a good thin film with these materials, a treatment step of 600 ° C. or higher is usually required, so that the compositional components of these materials, especially oxygen, are thermally diffused to the base, so that oxides Oxygen was lost in the superconductor thin film, and as a result, an oxide superconducting transistor having excellent characteristics could not be obtained.

Therefore, an object of the present invention is to provide an oxide superconducting transistor which is easy to manufacture and has excellent electric characteristics, and a manufacturing method thereof.

[0006]

In order to achieve this object, an oxide superconducting transistor of the present invention comprises an oxide superconducting thin film provided on a base and an ohmic contact with the oxide superconducting thin film. A first and a second main electrode, and a control electrode located above the oxide superconductor thin film between the first and second main electrodes and controlling a superconducting current flowing in the superconductor thin film. It is characterized by

In carrying out the present invention, the oxide superconductor thin film is formed of BiBa _{1 -X} Rb _X O ₃ (however, provided as a basic material of BiBaO ₃ having a perovskite structure.
X is a value that gives the composition ratio, and 0.25 <X <0.5.
The value is within the range. ) Is preferable.

According to a preferred embodiment of the present invention, the control electrode is preferably provided on the superconductor thin film via an insulating film made of BiBaO ₃ .

According to the method for manufacturing an oxide superconducting transistor of the present invention, (a) a step of depositing a BiBaO ₃ film on the underlayer, and (b) a BiBa _1-X oxide superconducting thin film on the BiBaO ₃ film. Rb _X O ₃ (where X is a value that gives the composition ratio and is a value within the range of 0.25 <X <0.5.), A step of laminating thin films, and (c) the oxide superconductivity A step of forming an insulating film pattern of a BiBaO ₃ film on a part of the body thin film, and (d) a control electrode on the insulating film pattern and ohmic contact with the control electrode on both sides of the control electrode on the oxide superconductor thin film. Forming the first and second main electrodes in contact with each other.

[0010]

According to the structure of the oxide superconducting transistor of the present invention, when the oxide superconductor is cooled to a temperature at which it exhibits superconducting characteristics and a bias voltage is applied to the control electrode, the transistor Below the threshold voltage of, the depletion layer existing in the oxide superconductor thin film under the control electrode does not reach the base, and therefore,
A current path exists in the oxide superconductor thin film between the depletion layer and the base. Therefore, if a potential difference is given between the first and second main electrodes that are in ohmic contact with the oxide superconductor thin film, the first and second main electrodes will pass through this current path,
A superconducting current flows through this, and this transistor is turned on.

However, when the bias voltage exceeds the threshold voltage of this transistor, the depletion layer reaches the base beyond the thickness of the oxide superconductor thin film, and the oxide superconductivity is provided between the depletion layer and the base. There is no current path in the body membrane. Therefore, the first and second main electrodes are blocked by the depletion layer, and the superconducting current does not flow between the two main electrodes, so that the transistor is turned off.

As described above, the transistor having this structure can achieve a switching effect as a field effect type oxide superconducting transistor.

According to the method of manufacturing an oxide superconducting transistor of the present invention, the oxide superconductor thin film is formed of BiBa.
Formed with _1-X Rb _X O ₃ . Therefore, in this invention,
First, BiBa having a typical perovskite structure is used as a base material of BiBa _1-X Rb _X O ₃ on the lower ground.
Deposit O ₃ . Then, BiB is deposited on the BiBaO ₃ film.
a _1-X Rb _X O ₃ is laminated, and this BiBa _1-x Rb _x O is laminated.
_A BiBaO ₃ film is formed on ₃ as an insulating film for a control electrode. Therefore, BiBa _1-X Rb _X O ₃ at a temperature as low as 400 ° C. or lower, which does not cause a thermal diffusion obstacle,
It can be formed as a high quality thin film without any difficulty.

[0014]

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

Incidentally, the figures are such that the present invention can be understood.
The shape, size, and positional relationship of each component are shown only schematically.

The structure of the oxide superconducting transistor of the present invention will be described together with its manufacturing method.

FIG. 1 is a sectional view showing an embodiment of the oxide superconducting transistor of the present invention.

First, the oxide superconductor thin film 14 is formed on the underlayer 10. Therefore, in this embodiment, an oxide substrate or a semiconductor substrate can be used as the base 10.

When an oxide substrate is used, magnesium oxide (MgO), strontium titanate (SrTi)
O ₃ ) is preferably used, and when a semiconductor substrate is used, silicon (Si) or gallium-arsenic (GaAs) is preferably used. In this embodiment, the MgO substrate is used as the base 10.

A BiBaO ₃ film is formed on the MgO substrate 10 by a suitable method selected from MBE (Molecular Beam Epitaxial Growth), CVD (Chemical Vapor Deposition), sputtering, laser ablation, etc. until the film surface becomes flat. Are stacked as the buffer layer 12. Whichever method is used, a high-quality thin crystal film 12 can be formed in a substrate temperature range of 400 ° C to 500 ° C. Here, B
A case of forming the iBaO ₃ buffer layer 12 by the MBE method will be described.

Using MgO (100) substrate 10 and keeping the substrate temperature at 400 ° C., for example, metal barium (Ba) and metal bismuth (Bi) are heated and vaporized by a Knutsen cell to obtain a temperature of 10 ¹⁴ to 10. These metals were vapor-deposited by supplying to the surface of the substrate 10 at an appropriate rate within the range of 10 ¹⁵ atoms / cm ² · sec. Next, 10 ¹⁴ to 10 ¹⁸ ozone /
Supply in an appropriate amount within the range of cm ² · sec. By doing so, B having a (100) orientation and a flat surface is obtained.
The iBaO ₃ film 12 is formed on the substrate 10.

The thickness of the buffer layer 12 is, eg, 1,000-10,000 A °.

Further, the BiBa formed in this way
O ₃ has a typical perovskite structure as represented by the chemical formula of ABO ₃ (where A and B represent elements).

Next, as the oxide superconductor thin film 14 on the buffer layer 12, in this embodiment, BiBa _1-X Rb _X O is used.
Form a thin film of ₃ . Here, X is a value that determines the composition ratio, and is preferably a value within the range of 0.25 <X <0.5. This is because a thin film of good quality can be obtained by setting the value of X within such a range.

First, the substrate temperature is lowered to 350 ° C. to 400 ° C., and bismuth (Bi), rubidium (Rb), and barium (Ba) are vapor-deposited using the Knudsen cell as described above. However, rubidium (Rb) and barium (Ba)
The molecular beam intensity of Rb: Ba is 0.3 to 0.45: 1.
The evaporation rate of rubidium (Rb) is adjusted so that it becomes 00.

Next, 10 ¹⁴ to 10 ¹⁸ ozone / cm 3 is added.
BiBaO ₃ is supplied in an appropriate amount within the range of ² sec.
BiBa that becomes an oxide superconductor thin film on the buffer film 12
Laminate _1-X Rb _X O ₃ . Although the film thickness will be described in the element operation, it should be about 30 A °, and the film is formed while precisely controlling the number of atoms in the single digit.

Therefore, in this embodiment, a method of accurately growing each lattice by monitoring the vibration of the surface reflection electron beam intensity with a reflection high-energy electron diffraction device (RHEED) is adopted (for example, Japanese Patent Laid-Open No. HEI-1). -303770).

Superconducting layer BiBa thus formed
As a result of the analysis, the chemical formula of _1-X Rb _X O ₃ is such that X giving the composition ratio is in the range of 0.25 <X <0.5, where x = 0.35, and BiBa _0.65 Rb _0.35 O ₃
It was a thin film of. It is known that BiBa _0.65 Rb _0.35 O ₃ is an oxide superconductor material, but the resistance-temperature characteristic was examined for confirmation. In addition, this BiBa
_The film thickness of the _0.65 Rb _0.35 O ₃ film was set to about 30 A °.

FIG. 2 is a resistance-temperature characteristic curve in which the vertical axis represents resistance (Ω) and the horizontal axis represents absolute temperature (K). As can be understood from this experimental data, the critical temperature indicating superconductivity is shown. Was about 30K.

Further, on the BiBa _1-X Rb _X O ₃ layer 14, a gate insulating film 16 for disposing a gate electrode as a control electrode 22 in this embodiment is provided at a substrate temperature of 400 ° C.
Film thickness 20 to 30 A ° by BiBaO ₃ at ˜500 ° C.
To stack.

Next, the formation of the first and second main electrodes and the control electrode will be described. In this embodiment, these first and second main electrodes are the source electrode and the drain electrode. A positive resist is applied over the entire surface of the substrate on the BiBaO ₃ film of the gate insulating film 16, and the resist portions required for the first and second main electrodes are stripped by photolithography to partially remove the BiBaO ₃ film 16. After exposure, the exposed portion of the BiBaO ₃ film is etched with oxygen plasma so as to reach a depth of about 30 to 40 A ° from the surface thereof.

A metal electrode layer is provided on the portion of BiBa _0.65 Rb _0.35 O ₃ of the oxide superconductor exposed by this etching to form the source electrode 18 and the drain electrode 20. As the electrode material, for example, aluminum (Al) or silver (Ag) is preferably used. To form this electrode, first, an electrode material is vapor-deposited as a source / drain electrode at about 1000 A ° on the entire surface of the substrate. Next, the resist is removed with an organic solvent, that is, a so-called lift-off process, and the electrode is left only in a desired portion.

BiBaO ₃ film 16 remaining by the same procedure
The gate electrode 22 is formed of aluminum (Al).

The source electrode 18 formed by such a process
When the contact resistance between the drain electrode 20 and the superconducting active layer, that is, the oxide superconductor thin film 14 forming the channel layer was measured, a good resistance value of 10 ⁻⁶ Ωcm ⁻² or less was always secured.

Therefore, it can be seen that a very good ohmic contact is established between the electrodes 18 and 20 and the oxide superconductor thin film 14.

In the oxide superconducting transistor of the present invention thus obtained, the BiBa _1-X Rb _X O ₃ thin film 14 as an oxide superconducting thin film is an underlying MgO substrate through the buffer layer 12. It ’s located on top of 10,
The first and second main electrodes serving as the source electrode 18 and the drain electrode 20 are in ohmic contact with the oxide superconductor thin film 14. A gate electrode 22 as a control electrode is provided between the source electrode 18 and the drain electrode 20 and above the superconductor thin film 14 with a BiBaO ₃ film 16 corresponding to a gate insulating film interposed. ..

Next, the operation principle of the superconducting transistor thus obtained will be described with reference to FIGS. 3A and 3B.
Will be described. In these figures, the same components as those shown in FIG. 1 are designated by the same reference numerals.

The transistor is cooled to a temperature at which the oxide superconductor thin film exhibits superconducting properties.

Then, the source electrode 18 and the gate electrode 2
A constant potential difference is given between 0 and 0. Now, the gate electrode 2
The bias voltage (VG) is gradually increased to 2.
Then, until the bias voltage reaches the threshold voltage, the depletion layer 30 under the gate electrode 22 has not yet reached the base substrate 10, and the depletion layer 30 and BiBaO ₃ are not yet reached.
A gap between the layer 12 and the layer 12 (a portion of the BiBa _1-X Rb _X O ₃ thin film 14 shown by 24 in the figure) serves as a current path. Therefore, the current path 24 is formed between the source electrode 18 and the drain electrode 20.
A superconducting current flows through the transistor and the transistor is in the ON state.

On the other hand, FIG. 3B shows a state in which the bias voltage exceeds the threshold voltage and the depletion layer 30 reaches the underlying substrate 10. In this state, BiBa under the depletion layer 30
_{The portion of the 1-X} b _X O ₃ thin film layer 14 is not the current path 24, so that the superconducting current does not flow between the source electrode 18 and the drain electrode 20 and the transistor is turned off.

Next, the control voltage required for switching, here, the value of the gate voltage will be concretely obtained.

The thickness W of the depletion layer is

[Equation 1]

[0043]

Substituting the vacuum dielectric constant ε ₀ and the amount of electronic charge q into, the carrier density N _B when BiBa _1-X Rb _X O ₃ exhibits superconductivity is 10 ²¹ / cm ^3.

[Equation 2]

[0045]

It becomes Here, BiBa _1-X Rb _X O ₃
If the permittivity ε _S of _S is 20, the gate voltage V _G is 4.0.
When the voltage is volt, the thickness W of the depletion layer is about 30 A °.

In FIG. 4, the vertical axis represents the superconducting current I _DS flowing between the source and drain electrodes, the horizontal axis represents the bias voltage V _G applied to the gate electrode, and V _Gth is the threshold voltage. It is experimental data showing superconducting current characteristics. As can be understood from FIG. 4, the threshold voltage V
Below _Gth , the superconducting current I _DS is flowing in the on state, but above the threshold voltage V _Gth , it is in the off state and I
_DS becomes zero.

In the transistor of this example, the threshold voltage V _Gth was 4 volts.

[0049]

As is apparent from the above description, according to the oxide superconducting transistor and the method for manufacturing the same of the present invention, typical materials for oxide high-temperature superconductors conventionally used for superconducting transistors. Was Y-Ba
Instead of -Cu-O and Bi-Sr-Ca-Cu-O, Bi-Ba-Rb-O having a small anisotropy of electric characteristics is used. Therefore, it is possible to solve the problems in the conventional manufacturing process such as controlling the crystal orientation, and thus to provide a superconducting transistor that is easy to manufacture, has a good manufacturing yield, and has excellent electrical characteristics. Became possible.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of an oxide superconducting transistor according to the present invention.

FIG. 2 is a resistance-temperature characteristic curve showing the critical temperature of the oxide superconductor thin film used in the present invention.

3A and 3B are diagrams for explaining the operation of the oxide superconducting transistor according to the present invention.

FIG. 4 is a graph showing the relationship between the gate voltage and the superconducting current between the source and drain of the oxide superconducting transistor according to the present invention.

[Explanation of symbols]

10: Base substrate (MgO) 12: Buffer film (BiBaO ₃ film) 14: Oxide superconductor thin film (BiBa _1-X Rb _X O ₃ thin film) 16: Insulating film (BiBaO ₃ film) 18: First main electrode ( Source electrode) 20: Second main electrode (drain electrode) 22: Control electrode (gate electrode) 24: Current path 30: Depletion layer

Claims (4)

[Claims] 1. An oxide superconductor thin film provided on a lower surface, first and second main electrodes in ohmic contact with the oxide superconductor thin film, and these first and second main electrodes.
An oxide superconducting transistor, comprising: a control electrode located between the main electrodes and above the oxide superconducting thin film, the control electrode controlling a superconducting conductive flow flowing in the superconducting thin film. 2. The oxide superconductor thin film according to claim 1, wherein BiBa _1-X Rb _X O ₃ (provided that X is _X is formed by using BiBaO ₃ having a perovskite structure as a basic material).
Is a value that gives the composition ratio, and is a value within the range of 0.25 <X <0.5. ), The oxide superconducting transistor characterized by being formed by. 3. The control electrode according to claim 1, wherein the control electrode is BiBa.
An oxide superconducting transistor which is provided on the superconductor thin film via an insulating film made of O ₃ . 4. A process of (a) depositing a BiBaO ₃ film on a base, and (b) a B oxide film as an oxide superconductor thin film on the BiBaO ₃ film.
iBa _1-x Rb _X O ₃ (where X is a value that gives the composition ratio, and is a value within the range of 0.25 <X <0.5.)
A step of laminating thin films, and (c) BiBaO ₃ on a part of the oxide superconductor thin film.
Forming an insulating film pattern of a film, and (d) a control electrode on the insulating film pattern and first and second main electrodes on both sides of the control electrode and in ohmic contact with the oxide superconductor thin film. A method of manufacturing an oxide superconducting transistor, comprising the step of forming an electrode.