JPS6341087A - Manufacture of superconductive thin film - Google Patents

Abstract

PURPOSE:To obtain a high Tc compound thin film having increased superconductivity by a method wherein, after an insulating single-crystal thin film whose lattice constant closely resembles the lattice constant of a superconductive thin film bas been deposited in advance on an insulating substrate whose coefficient of thermal expansion closely resembles the coefficient of thermal expansion of the superconductive thin film, a superconductive thin film of high critical temperature is grown epitaxially. CONSTITUTION:When a high critical temperature (Tc) compound superconductive thin film is manufactured for use as a superconductive device including a Josephson device, a substrate is selected on the basis of the coefficient of thermal expansion. An insulating thin film is selected on the basis of the crystal system and the a0, and is deposited on the substrate. Then, a high Tc compound thin film is formed. In order that the high Tc compound thin film can be grown epitaxially, the insulating thin film must be deposited when the lattice constant a0 is close to the lattice constant of a high Tc compound of the cubic crystallinity and its state is singlecrystalline. In this case, the deviation in the matching characteristic of the lattice constant must be kept within 10%.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for producing a compound superconducting thin film with a high critical temperature (hereinafter abbreviated as TC) for use in superconducting devices such as Josephson elements. .

[Conventional technology]

Superconducting devices are expected to be put to practical use as high-speed switching elements, high-sensitivity detection elements, and high-sensitivity magnetometers. These devices are constructed using superconducting thin films formed on insulating substrates, so the production of high-quality superconducting thin films is the key to realizing high-performance devices.

Conventionally, known methods for producing such superconducting thin films include using thermally oxidized silicon or sapphire (Atgos) as a substrate and depositing the desired superconducting thin film thereon by sputtering, electron beam evaporation, etc. ing.

However, when using a high Tc compound thin film made of Nt) and V compounds as a superconducting thin film, the following problems existed in the conventional method.

The first is that the superconducting thin film has non-uniformity in superconducting properties in the film thickness direction. The reason for this non-uniformity is that in the initial stage of deposition of these thin films, ao deviates from its original value due to differences in the crystal structure between the thin film and the substrate, mismatch in lattice constant a6, etc., and a layer with a low Tc grows. This is to do so. For example, when formed on an all-sapphire substrate with Nb3, which has the highest Tc (~22K) among known substances, if the film thickness is less than 1000A, a(1 is 1) (Tc is less than 15.Film thickness As it increases, ao gradually approaches its original value,
TC increases, and when the film thickness is 200OA, a6 becomes 5
．． 14A, Nb3Ge with a high Tc of 20 to 23. Therefore, in the high Tc N1)3()8 film, a low Tc layer exists from the substrate interface to a thickness of about 100 OA. Such non-uniformity of properties in the film thickness direction is a phenomenon observed not only in Nb3Ge but also in most high Tc compounds. Second, the superconducting properties are degraded due to internal stress resulting from the difference in thermal expansion coefficient between the substrate and the high Tc compound thin film. Generally, the internal stress σ derived from the difference in thermal expansion coefficients is given by the following equation.

σ = (α day α8) (T-Tm month 7/(1-νf month
(1) Here, α1 and α8 are the coefficients of thermal expansion of the thin film and the substrate, respectively, T is the temperature during thin film formation, Tm is the temperature at which the total stress is measured (usually room temperature), Ef1νf is the Young's modulus of the thin film, and the Boann ratio, respectively. It is.

For example, sapphire substrate (α81:5,7X10-'/
°C) and silicon substrate (αs2=2.3×10−
Nb5 ( ) fabricated at a growth temperature of 900 °C on
e Stress of the thin film (respectively σ1.σ2) beam / beam 1 = giant αf - α82) / (α day α81) 1. In other words, compared to sapphire, which has a small difference in thermal expansion coefficient from Nb3Ge, a silicon substrate has a large difference in thermal expansion coefficient.
Approximately 12 times as much stress is generated. In particular, in the case of a high Tc compound, a high temperature of 600 to 900° C. is required as a film forming temperature, so that the stress value becomes larger as can be seen from equation (1). Due to this difference in stress depending on the substrate, the Nb3Ge thin film (3000A) formed on the silicon substrate has a much lower Tc (<
20K) k. Such deterioration of superconducting properties due to stress is seen in most thin films of high Tc compounds.

[Problem that the invention seeks to solve]

Even if a high Tc compound thin film having non-uniform and deteriorated superconducting characteristics as described above is used as an electrode of a superconducting device, it is difficult to obtain a high-performance superconducting device.

The purpose of the present invention is to solve the drawback of the conventional technology that superconductivity and uniformity deteriorate in high Tc compound thin films used in superconducting devices, and to provide high Tc compound thin films with significantly improved superconductivity and uniformity. It is about providing.

[Means for solving problems]

To summarize the present invention, the present invention relates to Nb compounds or V
In a method for producing an A15 type structural superconducting thin film made of a compound and having a high Tci, an insulator having a thermal expansion coefficient close to that of the superconducting thin film is used as a substrate, and the superconducting thin film is deposited on the substrate. Improving the superconductivity of the p1 high Tc compound thin film by depositing in advance a single crystal thin film of an insulator having a lattice constant similar to the lattice constant, and then epitaxially growing a single crystal of the high Tc superconducting thin film. This is the most important feature.

In order to produce a high-Tc compound thin film with high quality and excellent uniformity, it is sufficient to use an insulating material as a substrate whose coefficient of thermal expansion, crystalline thread, and ao all match those of the high-Tc compound thin film.

However, as shown in Table 1, there is no commercially available substrate that satisfies all of these conditions. In the present invention, a substrate is selected based on the coefficient of thermal expansion. And crystal system,
On an ao t- basis, a thin film of insulator is selected and deposited on the substrate. Next, a desired high Tc compound thin film is formed. Here, in order for the high Tc compound thin film to grow epitaxially, the insulator thin film must be deposited in a cubic system, an aot- similar to that of the high Tc compound, and in a single crystal state. In this case, it is desirable that the matching of the lattice constants be as high as possible, and the deviation must be within 10 inches. Table 2 shows the major high Tc
The thermal expansion coefficient of a compound thin film (cubic system, A15 type structure), "sTQ'!" is shown. For example, as a high Tc compound thin film, Nb1Ge (α= lx, D
Tl:, ao = a14A), the substrate is Atzos (α = 5.7 x 1 o-6/r:
) is used as the insulator thin film deposited on the substrate.
= 5.14 A) Choose k. Then, since the Nb5Gθ thin film has a small difference in thermal expansion coefficient from the substrate (Attos), its internal stress becomes small. Also, Nb3Ge
Since the thin film is deposited on the insulating thin film EuO with the same crystal system and ao, epitaxial growth is possible.

Table 1 Insulator α (×10″″’/aθg) Crystal system ao
(A) At203 5.7 hexagonal -AtN 56 hexagonal
-BeO7,0 hexagonal crystal - Mgo 150 cubic crystal 4.2
0S1 2.2 Cubic crystal 4.
9Z r02 11L O cubic crystal 5.0
7EuO115 cubic crystal 5.14 Y203 9.3 cubic crystal 5
．． 29 Coating 2 High Tc compound α(Xf O-'/dθg)
a6 (A) TOVsSi
5 4.72 17. IV3()a
5 4.81
16.5Nb38n 6
a29 1a3Nb3At
6 5.18 1a9N'b3
Ga 6 5.17
2α2Nb3()e 6
5.14 21 If you make it like this, high T
This method differs from the conventional technology in that epitaxial growth or single crystallization of a c-compound thin film is possible.

〔Example〕

The present invention will be specifically celebrated below along with Examples, but the present invention is not limited to these Examples.

Example 1 A single crystal substrate (random orientation) of sapphire (1zos) was prepared, and a Y2O3 thin film was first sputtered at 200O
A thick layer was deposited. The deposition conditions are such that the sputtering gas composition is A.
r+20%0:, gas pressure is 5 Pa.

The power was low and the substrate temperature was room temperature. Next, N1
) 3Sn thin film k, Ntlo, yIsSmo, was formed with a film thickness of 200 to 4oooX by the DCff gnetron scattering method using an alloy target of 25. The formation conditions were a gas pressure (Ar) of 25 Pa and a discharge current of 1. OA
One substrate temperature was 900°C.

FIG. 1 shows the film thickness dependence of Tc of the Nb3Sn thin film produced by the present invention as a graph of the relationship between Tc (K, vertical axis) and film thickness (A% horizontal axis) together with the results of the conventional method.

From FIG. 1, it can be seen that the Nb3Sn produced according to the present invention
Even with a thin film thickness of A or less, Tc was as high as 15 or more, and it was found that the dependence of Tc on film thickness was smaller than that of the Nb3Sn thin film produced by the conventional method. This indicates that Nt)38n with good crystallinity was epitaxially grown from the initial stage of deposition.

Example 2 A Bθ0 substrate was prepared, and first, a 500A S1 thin film was deposited by vapor deposition at a substrate temperature of 900°C. Next, a v3s i, V3 Ga thin film-@2aaa film was formed by a vapor deposition method. The substrate temperature was 800°C.

In addition, a V3Ga thin film with a film thickness of 2000 Å was fabricated by direct deposition on a BeO substrate using a conventional method. The superconducting properties of the obtained thin film are shown in Table 3.

Here To. , Tc1, Tc, respectively at the start, middle, and end points of the superconducting transition, ΔTc+''j transition width (
”Tea T(!@).The nine films prepared by the conventional method have low TC, wide ΔTc, and poor film uniformity, whereas the films prepared by the present invention have low TC. It can be seen that ΔTc is high throughout, ΔTc is very narrow to α2, and the film has excellent uniformity.

Table 3 Compound Tco (FQ Tc+a QQ
Tco (K; l ΔTc (KJV3Si invention
17.1 17.0 16. ? CL2 standard product 1&8 1.0 14.8 2.0V3()
aThis invention 1&5 1/h4 16.3 [
L2 conventional 1/i, 4 15.8 1&52.
9 Example 3 A sapphire (At20s) single crystal substrate (random orientation) was prepared, and a EuO thin film was deposited to a thickness of 4000 Å by reactive RF magnetron sputtering using an Eu target. The deposition conditions are such that the sputtering gas composition is Ar+1.
0%02, gas pressure was 2 Pa, power was 200 W, and substrate temperature was 700°C.

Next, Nb5G! 3. Each thin film of Nb3Ga and N1)3SH was formed to a thickness of 3000 Å by a vapor deposition method. The substrate temperature was 900°C. In addition, according to the conventional method,
That is, Nb3 with a thickness of 3000 Å was deposited directly on a sapphire (kkos) single crystal substrate (random orientation).
Thin films of Ge, Nb3Ga, and Nb3Sn were formed.

The substrate temperature was 900°C. A tunnel-type Josephson device was fabricated using these thin films. The device structure has a lower electrode of Nb3Ge and Nb3Ga.

Nb3Sn,) The channel barrier layer was Nb2os, and the upper electrode was pb. Table 4 shows the characteristics of the device. In the table, va is the gap voltage (mV), Vl, l are the quality parameters (mV) (=tunnel resistance x maximum Josephson current at 2 mV). As is clear from Table 4,
In the device using the thin film made by the conventional method, Va is small and Va is 10 mV or less, which is low quality, whereas in the device using the thin film made by the present invention, vl is large and v
It can be seen that l is 40 mV or more, which indicates high quality.

Table 4 Compound ■, (mV) ■Ia (mV) Nb
3Ge Invention 5.0 45 Conventional product
&85 Nb3Ga This invention! i, 5 52 conventional product 3.15 Nb3 S n Invention Mu358 conventional product
2.88 As is clear from these results, the high T
The uniformity of superconducting properties of the c compound thin film was significantly improved compared to thin films produced by conventional methods.

〔Effect of the invention〕

As explained above, according to the present invention, the uniformity of the superconducting properties of a high Tc compound thin film can be greatly improved, so there is an advantage that a high quality superconducting element can be manufactured.

[Brief explanation of drawings]

Figure 1 shows Nb3Sn produced using the present invention and the conventional method.
0 is a graph showing the film thickness dependence of Tc of a thin film.

Claims (1)

[Claims] 1. In a method for producing an A15 type structured superconducting thin film having a high critical temperature made of a Nb compound or a V compound,
An insulator having a thermal expansion coefficient similar to that of the superconducting thin film is used as a substrate, and a single crystal thin film of an insulator having a lattice constant similar to that of the superconducting thin film is preliminarily deposited on the substrate. A method for producing a superconducting thin film, which comprises epitaxially growing the high critical temperature superconducting thin film after deposition.