JPH0621522A - Superconducting device - Google Patents

Abstract

PURPOSE:To provide a superconducting device, wherein electrons can be directly tunneled only in a superconductor layer and a good hysteresis curve is obtained. CONSTITUTION:In a superconducting device, a prescribed void, in which a tunneling phenomenon is generated, is provided only in a superconductor layer of an oxide superconductor 2 of a layer-shaped structure to arrange a normal conductor layer 4 and a tunnel junction is formed between the superconductor layer 2 and the layer 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to a microwave mixer,
The present invention relates to a tunnel-type junction structure superconducting device used for a superconducting transistor or the like.

[0002]

2. Description of the Related Art In recent years, the progress of superconducting electronics in Japan has been remarkable and the transition temperature Tc has been increased accordingly.
A high bismuth-based oxide superconducting material and a yttrium-based oxide superconducting material have been proposed.

By the way, when a superconducting device is produced by using the above oxide superconducting substance, for example, a tunnel junction having a constant operating voltage and excellent stability in circuit operation is used. This tunnel type junction is SIS or SI
It has a laminated structure composed of N (here, S is a superconducting thin film, I is an insulating layer, and N is a normal conductor).

Among oxide superconductors, Bi ₂ Sr ₂ Ca
Cu ₂ O _x (hereinafter referred to as BSCCO) or Y ₁ Ba ₂ Cu
A superconductor with a composition of ₃ Ox (hereinafter referred to as YBCO) is
It has a layered laminated structure in the direction perpendicular to the c-axis. For example, in BSCCO, BiO, SiO, CuO ₂
And Ca has a laminated structure, and CuO is contained in each layer.
_Two layers have superconducting layer (S) characteristics. This BSCCO
Has an N / S / S / N structure.

Conventionally, the tunnel type junction has a structure in which an insulating layer is provided on the (001) oriented surface of a layered oxide superconductor, and an S layer or an N layer is provided thereon as a counter electrode.
Therefore, when an oxide superconductor having a layered structure is used as the other electrode, a leak current is generated from the N layer closest to the surface of the oxide superconductor having no superconducting characteristics, and a current having an ideal hysteresis curve is generated. -Voltage (IV) characteristics were not obtained.

Therefore, the applicant of the present invention provides a superconducting layer or a normal conducting layer with an insulating film at a step portion located on a sectional wall parallel to the c-axis of an oxide superconductor single crystal having anisotropy. Has proposed a superconducting device (Japanese Patent Application No. 2-4134).
No. 80).

[0007]

In the structure described above, electrons can tunnel directly to the CuO ₂ surface of the superconductor from the step portion located on the cross section wall parallel to the c-axis of the oxide superconductor. .

However, even in the above structure,
Since the junction with the superconductor layer or the normal conductor layer is formed on the entire surface of the step portion through the insulating film, the leakage current is passed through the layer (N layer) in the oxide superconductor having the normal conductor characteristic. Occurs, and the IV characteristic having an ideal hysteresis curve is not obtained.

The present invention has been made to solve the above-mentioned conventional problems, and the purpose thereof is to allow electrons to be directly tunneled only to a superconductor layer,
An object of the present invention is to provide a superconducting device which can obtain an IV characteristic having a good hysteresis curve.

[0010]

In a superconducting device according to the present invention, a counter electrode is provided only for a superconducting layer of a layered structure oxide superconductor via a predetermined void or an insulating layer which causes a tunnel phenomenon, A tunnel junction is formed with the superconductor layer.

[0011]

According to the superconductor device of the present invention, the electrons can be tunneled only to the superconductor layer of the oxide superconductor having the layered structure, so that the nonlinear characteristic of the IV characteristic can be improved.

[0012]

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

1 and 2 are sectional views showing a superconducting device according to a first embodiment of the present invention in each manufacturing process.

This embodiment uses a Bi ₂ Sr ₂ CaCu ₂ Ox single crystal which is a layered oxide superconductor. Then, the needles are arranged at a distance of several nm only on the CuO ₂ surface having the superconducting layer characteristic in the boundary of the BSCCO single crystal. When a voltage is applied in that state, a tunnel effect occurs and the electrons of the superconductor are directly picked up from the CuO ₂ surface.

Next, a manufacturing example of this superconducting device is shown in FIG.
And it demonstrates in detail according to FIG. The substrate 1 is La
An AlO ₃ (100) oriented substrate, a SrTiO ₃ (100) oriented substrate or a MgO (100) oriented substrate is used. In this example, a SrTiO ₃ (100) oriented substrate is used as the substrate 1. Then, the substrate 1 is set to an ultra high vacuum (10 ⁻⁸
Bi), S as a vapor deposition source by the MBE method under
After 1 unit cell vapor deposition using r, Ca and Cu, the substrate temperature was set to 300 to 350 ° C. and NO ₂ (10 ⁻⁵ Pa)
Oxidation and crystallization are performed in the atmosphere. The above operation is repeated to form a layered BSCCO superconducting thin film 2 (see FIG. 1A). This BSCCO superconducting thin film 2 is used as a base layer.

BSCCO formed by this MBE method
The oxide superconducting thin film has a Tc of 85K.

The BSCCO film 2 thus formed is etched by, for example, the IBE (ION BEAM CHING) method to expose a cross section parallel to the c-axis. That is, the CuO ₂ surface of the BSCCO film 2 oriented along the c-axis is exposed (see FIG. 1B).

Other etching methods include RIE.
A (reactive ion beam etching) method, an ECR plasma etching method, a mixed aqueous solution of HCLO ₄ , NaClO ₄ , or a chemical etching method using an ethanol solution of Br may be used.

Next, the substrate 1 including the BSCCO film 2
A semiconductor film 3 of SnOx or the like, which is a collector material, is laminated on the entire surface (see FIG. 1C).

Then, the semiconductor film 3 is removed by etching so that CuO ₂ on the cross section parallel to the c-axis of one side of the BSCCO film 2 is exposed (see FIG. 1D).

Then, Cu of the exposed BSCCO film 2
The needle 4 is placed close to about 1 nm on the O ₂ surface (see FIG. 2A). When voltage is applied in this state, a phenomenon called tunnel effect occurs, and Cu
Electrons can be tunneled directly from the O ₂ surface. As the needle 4, the same needle as that used in the STM (scanning microscope) may be used. That is, C
An extremely fine needle may be formed of platinum (Pt) or the like so as to face only the uO ₂ surface. Then, the needle 4 is used as an emitter, an electron extraction electrode 5 is provided on one surface of the BSCCO film 2, a single electron tunnel element is formed between the needle 4 (emitter layer) and the BSCCO film 2, and a collector is further provided. A superconducting three-terminal element using the semiconductor layer 3 such as SnOx can be obtained (see FIG. 2B).

By thus forming the tunnel junction only on the CuO ₂ surface, the leak current does not flow, and the tunnel current having an ideal hysteresis curve can be obtained.

FIG. 3 is an IV characteristic diagram of the superconducting device according to the present invention and the conventional superconducting device. Figure 3
FIG. 3 (a) is a superconducting device according to the present invention, and FIG. 3 (b) is an insulating film and a normal conductor layer bonded to a step portion located on a sectional wall parallel to the c-axis of an oxide superconductor single crystal having anisotropy. The IV characteristics of the conventional superconducting device are shown. As is apparent from FIG. 3, according to the conventional superconducting device in which the normal conductor layer is provided on the entire surface of the stepped portion with the insulating film interposed therebetween, the I-
V characteristics cannot be obtained. On the other hand, according to the present invention, the leakage current from the normal conductor (N layer) is eliminated, and the superconducting tunnel occurs only from the superconducting layer CuO ₂ of the superconducting device, so that the ideal hysteresis curve I- V characteristics are obtained.

In the above embodiment, the BSCC
Although the O film 2 was formed by the MBE method, the BSCCO film 2 can be formed by using an ion beam sputtering method other than the MBE method. When the ion beam sputtering method is used, a MgO (100) oriented substrate is used as the substrate 1,
Using BiO and Sr ₂ Ca ₂ Cu ₃ Ox as the target,
Under an oxygen (O ₂ ) atmosphere of 10 ¹⁵ atoms / cm ² , the substrate temperature is 600 to 650 ° C., and the sputtering rate is 0.01 n.
m / sec, BiO 0.3 nm, Sr ₂ Ca ₂ Cu
₃ Ox is alternately laminated to 0.6 nm. Then, by cooling in an O ₂ atmosphere, a BSCCO oxide film superconducting thin film can be obtained. With this BSCCO oxide superconducting thin film, Tc = 76K is obtained by as-deposition.

Further, in the above-mentioned embodiment, the BSCCO film is provided on the substrate. However, using a BSCCO single crystal substrate, this substrate is subjected to step processing to form a cross-section wall surface parallel to the c-axis. Good.

Next, a second embodiment of the present invention will be described with reference to FIGS. 4 and 5 are sectional views showing the superconducting device of the second embodiment of the present invention in each manufacturing process.

In this embodiment, a Bi ₂ Sr ₂ Cu ₁ Ox single crystal substrate 10 is prepared (see FIG. 4A), and an insulating film 11 of SrTiO ₃ , MgO or the like is formed on the surface of the single crystal substrate 10.
Are formed by a sputtering method or the like (see FIG. 4B). The thickness of the insulating film 11 is controlled to be equal to the distance between the lowermost BiO surface of the layered oxide superconductor Bi ₂ Sr ₂ CaCu ₂ Ox and the arbitrary CuO ₂ . For example, a half unit cell Bi ₂ Sr ₂ CaC
In the case of u ₂ Ox, the film thickness is set to 0.46 or 1.04 nm.

The insulating film 11 has a film thickness of 100 to 200.
The emitter layer 12 made of gold (Au) or platinum (Pt) of nm is formed by vapor deposition or sputtering (FIG. 4).
(See (c)).

Subsequently, after mechanical polishing, the substrate 10, the insulating film 11 and the emitter layer 12 are polished by ion milling, and as shown in FIG. 4 (d), a part of the surface of the substrate 10 has an angle θ from the surface. The emitter layer 12 is formed in a tapered shape of 5 to 30 °. The insulating film 11 is still interposed between the emitter layer 12 and the substrate 10. The tip 12a of the emitter layer 12 is used as a needle.

Then, as shown in FIG. 4 (e), etching is performed with a 7% hydrofluoric acid (HF) solution for about 1 to 10 seconds,
A part of the insulating film 11 is removed.

Then, as shown in FIG. 5A, MBE was performed on Bi ₂ Sr ₂ CaCu ₂ Ox which is a layered oxide superconductor.
A film is formed on the substrate 10 by a method or the like. By this film formation,
Epitaxial Bi ₂ Sr ₂ CaCu ₂ Ox on the substrate 10
A base layer 13 made of the thin film 1 is formed. On the other hand, since the emitter layer 12 is made of gold or platinum, the amorphous thin film 14 is formed thereon. Further, the film thickness of the insulating layer 11 is Bi ₂ Sr ₂ CaCu ₂ which is a layered oxide superconductor.
Since the distance between the BiO surface of the lowermost layer of Ox and an arbitrary CuO ₂ is controlled to be equal, the tip portion 12a of the emitter layer 12 is disposed so as to face only the CuO ₂ surface having superconducting layer characteristics. To be done. Further, a gap of about 1 nm is provided between the CuO ₂ surface and the tip portion 12a of the emitter layer 12, and a tunnel effect is generated by applying a voltage.

After that, the base layer 13 is patterned by using HNO ₃ and H ₂ PO ₄ , and the amorphous thin film 14 is formed.
Are removed (see FIG. 5B).

Next, by providing a collector region 15 made of a semiconductor such as SnOx, a single electron tunnel element is used as an emitter, an oxide superconductor is used as a base, and a semiconductor layer 3 such as SnOx is used as a collector. A superconducting three-terminal element is obtained (see FIG. 5 (c)).

Next, a third embodiment of the present invention will be described with reference to FIGS. 6 and 7. 6 and 7 are cross-sectional views showing the superconducting device of the third embodiment of the present invention in each manufacturing process. The third embodiment differs from the above-described second embodiment mainly in the step of forming the emitter layer 12 in a tapered shape.

This embodiment is similar to the above-mentioned embodiment in Bi.
_{A 2} Sr ₂ Cu ₁ Ox single crystal substrate 10 was prepared (FIG. 6A).
Then, an insulating film 11 made of amorphous MgO is formed on the surface of the single crystal substrate 10 by a sputtering method or the like (see FIG. 6B). The film thickness of this insulating film 11 is Bi ₂ Sr ₂ CaCu which is a layered oxide superconductor to be formed later.
_It is controlled to be equal to the distance between the BiO surface of the lowermost layer of ₂ Ox and arbitrary CuO ₂ . For example, half unit cell B
In the case of i ₂ Sr ₂ CaCu ₂ Ox, 1.04 + 30.36
The film thickness is set to × n (n: 2 to 3Å).

Then, an emitter layer 12 made of gold (Au) having a film thickness of 200 nm is formed on the insulating film 11 by vapor deposition or sputtering (see FIG. 6C).

Then, a resist film 21 is formed on the emitter layer 12.
After coating and patterning (see FIG. 6D), a part of the insulating film 11 and the emitter layer 12 are polished by ion milling using the resist film 21 as a mask at an angle (θ) of 30 to 45 °. As shown in FIG. 6 (e), the emitter layer 12 is formed so that the angle (θ) of the tip is tapered from the surface by 30 to 45 °. The insulating film 11 is still interposed between the emitter layer 12 and the substrate 10.
The tip 12a of the emitter layer 12 is used as a needle.

As this ion milling, for example, ion beam milling using argon ions is used. In this process, the gas pressure is 2 × 10 ⁻⁴ Torr and the acceleration voltage is 3
It is possible to mill an Au layer of 200 nm with a milling time of 10 minutes by setting the ion current density to 00 V and the ion current density to 0.32 A / cm ² .

After that, as shown in FIG. 7A, etching is performed with an aqueous solution of 7% hydrofluoric acid (HF) for about 1 to 10 seconds,
A part of the insulating film 11 is removed.

After that, as shown in FIG. 7B, MBE was performed on the layered oxide superconductor Bi ₂ Sr ₂ CaCu ₂ Ox.
A film is formed on the substrate 10 by a method or the like. By this film formation,
Epitaxial Bi ₂ Sr ₂ CaCu ₂ Ox on the substrate 10
A base layer 13 made of the thin film 1 is formed. On the other hand, since the emitter layer 12 is made of gold, the amorphous thin film 14 is formed thereon. Further, the thickness of the insulating layer 11 is controlled so as to be equal to the distance between the lowermost BiO surface of the layered oxide superconductor Bi ₂ Sr ₂ CaCu ₂ Ox and an arbitrary CuO _2. 12 tips 1
2a is arranged so as to face only the CuO ₂ surface having superconductor layer characteristics. Further, as in the above-described embodiment, a gap of about 1 nm is provided on the CuO ₂ surface and the tip portion 12a of the emitter layer 12, and a tunnel effect is generated by applying a voltage.

After that, the base layer 13 is patterned using HNO ₃ and H ₂ PO ₄ , and the amorphous thin film 14 is formed.
Are removed (see FIG. 7C).

Next, by providing a collector region 15 made of a semiconductor such as SnOx, a single electron tunnel element is used as an emitter, an oxide superconductor is used as a base, and a semiconductor layer 3 such as SnOx is used as a collector. A superconducting three-terminal element is obtained (see FIG. 7 (d)).

Next, a fourth embodiment of the present invention will be described with reference to FIGS. 8 and 9. In the first to third embodiments described above, the distance between the base and the emitter layer is 1 nm.
In the fourth embodiment, the insulating layer 22 made of an amorphous MgO film having a film thickness of about 1 to 7 nm is formed on the base and emitter layers. Is formed by a vacuum deposition method or the like to form a base layer 1
3, the insulating layer 22 is interposed between the emitter 12.

In this embodiment, after the collector region 15 is formed in the above-mentioned second and third embodiments,
The deposition temperature is room temperature, the deposition rate is 0.1 nm / sec, the degree of vacuum during deposition is set to 1 × 10 ⁻⁶ Torr, and the amorphous MgO film 22 having a thickness of 1 to 7 nm is formed as the emitter layer 12 by vacuum deposition. It is formed on the base layer 13 (see FIG. 8). By providing the insulating layer 22 made of the amorphous MgO film in this manner, as shown in the enlarged view of FIG. 9, the insulating layer 22 made of the amorphous MgO film is provided between the base layer 13 and the emitter layer 12. To be

In the above embodiment, the emitter layer 12
The gap between the tip portion 12a and the base layer 13 is set to be slightly larger than that in the second and third embodiments, that is, selected in the range of 1 to 7 nm so that the MgO film surely enters between them. Better.

With this structure, it is possible to prevent the tip of the emitter 12 from being displaced from the surface having the superconducting property due to the temperature change and to increase the mechanical strength.

In each of the above embodiments, the substrate 1
A Bi ₂ Sr ₂ CuOx single crystal substrate was used as 0.
It may be used a thin film formed on a substrate of rTiO _3.
Further, its chemical composition is Bi ₂ Sr ₂ CaCu ₂ Ox, Bi.
₂ Sr ₂ Ca ₂ Cu ₃ Ox or the like may be used. Further, the composition of the base layer is Bi ₂ Sr ₂ CuOx, Bi ₂ Sr ₂ Ca ₂ C
u ₃ Ox or the like may be used.

In each of the above embodiments, Bi ₂ Sr ₂ CaCu ₂ O _x is used as the layered oxide superconductor, but other layered oxide superconductors such as Y ₁ Ba ₂ Cu ₃ Ox composition superconductors are used. The body can also be used.

[0049]

As described above, according to the superconductor device of the present invention, electrons can be tunneled only to the superconductor layer of the oxide superconductor having the layered structure, so that the nonlinear characteristic of the IV characteristic is obtained. Can be improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing a superconducting device of a first embodiment of the present invention in each manufacturing process.

FIG. 2 is a cross-sectional view showing the superconducting device of the first embodiment of the present invention for each manufacturing process.

FIG. 3 is an IV characteristic diagram of a superconducting device according to the present invention and a conventional superconducting device.

FIG. 4 is a cross-sectional view showing a superconducting device of a second embodiment of the present invention for each manufacturing process.

FIG. 5 is a cross-sectional view showing a superconducting device of a second embodiment of the present invention in each manufacturing process.

FIG. 6 is a cross-sectional view showing a superconducting device of a third embodiment of the present invention in each manufacturing process.

FIG. 7 is a cross-sectional view showing a superconducting device of a third embodiment of the present invention for each manufacturing process.

FIG. 8 is a sectional view showing a superconducting device of a fourth embodiment of the present invention.

FIG. 9 is a sectional view showing a main part of a fourth embodiment of the present invention.

[Explanation of symbols]

1 Substrate (SrTiO ₃ ) 2 BSCCO thin film (base layer) 3 Semiconductor film (collector layer) 4 Needle (emitter layer) 10 BSCCO single crystal substrate 11 Insulating film 12 Emitter layer 13 BSCCO thin film (base layer) 14 Semiconductor film (collector layer) ) 22 Insulating layer (amorphous MgO film)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junnobu Zenzato 2-18, Keihanhondori, Moriguchi City, Osaka Prefecture Sanyo Electric Co., Ltd.

Claims (2)

[Claims] 1. A tunnel junction is formed between the superconducting layer and the superconducting layer of the layered structure superconducting layer, the counter electrode is provided with a predetermined gap in which a tunnel phenomenon occurs. Is a superconducting device. 2. A superconducting device, wherein a counter electrode is provided only through a superconducting layer of a layered structure oxide superconductor with an insulating layer interposed therebetween to form a tunnel junction with the superconducting layer. device.