JP2958645B2 - Oxide superconductor element and method for producing oxide superconductor thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an oxide superconducting element and an oxide superconducting thin film used for a SQUID, a Josephson element, a superconducting transistor, an electromagnetic wave sensor, an element wiring, an electrode and the like. It relates to a manufacturing method.

[Prior Art] The oxide superconducting material of the present topic has a strong anisotropy due to the crystal structure.
It is 1/5 to 1/10 in the b-axis direction. Therefore, it can be said that epitaxial growth is indispensable for applying the oxide superconducting material to a thin film device which requires a high critical current density or needs to suppress a potential barrier. For epitaxial growth, it is necessary to match the lattice of the substrate and the superconducting material. Generally, PHYSICAL REVIE
WB VOL.38 No.1 (1988) 765-767 and APPLIED FHYSICS L
As described in ETTERS VOL. 53 NO. 17 (1988) 165-1656, a single crystal substrate such as MgO has been used.

[Problems to be Solved by the Invention] However, conventional oxide single crystal substrates used for forming oxide superconducting thin films are limited to those having a crystal diameter of about 5 cmφ or less even with MgO, which can have a relatively large diameter. Had been.
Also, its production took a long time. For this reason, it is impossible to increase the diameter, and there are problems that only small-sized elements can be applied and the application is limited, mass productivity is not good like a semiconductor, and the price is high.

In addition, when a single crystal silicon wafer with a large diameter up to about 20 cmφ is used and an oxide superconducting thin film is directly applied, not only does it react with the silicon wafer and does not grow epitaxially but also becomes a low critical temperature phase or a severe one is a superconducting phase. Was broken and became a semiconductor phase.

The present invention is intended to solve such a problem. The purpose of the present invention is to obtain an oxide superconducting thin film having a large diameter, a high critical current density and excellent mass productivity without limitation of application at a low cost. Is what you do.

[Means for Solving the Problems] The present invention is characterized by the following 1 and 2.

1. a single-crystal silicon wafer substrate and a composition formed on the single-crystal silicon wafer substrate and having a composition of A _x Ga _y O
_z (where A represents a rare earth element) is 0.95
≤ x ≤ 1.05, 0.9 ≤ y ≤ 1.1, and a Bi-M-Cu-O-based or Bi-Pb formed on the oxide layer
-M-Cu-O-based oxide superconducting thin film (here, M represents an alkaline earth).

2. On a single-crystal silicon wafer substrate, set the composition to A _x Ga _y O _z
(Where A represents a rare earth element) 0.95 ≦
An oxide layer satisfying x ≦ 1.05, 0.9 ≦ y ≦ 1.1 is formed, and a Bi-M-Cu-O-based or Bi-Pb-M-Cu- is formed on the oxide layer.
It is characterized by forming an O-based oxide superconducting thin film (here, M represents an alkaline earth).

When the values of x and y are out of the above composition ranges, the oxide layer does not have a stable crystal structure. This means an increase in lattice mismatch, which causes the epitaxial growth of the oxide superconducting thin film to be hindered. It is more preferable that both values are closer to 1. Although z cannot be confirmed because it is difficult to measure a thin film, z is 3 in an optimum composition in a bulk.

[Examples] Hereinafter, the present invention will be described according to examples.

First, an oxide film having a composition shown in Table 1 is formed on a single crystal silicon wafer substrate by a reactive evaporation method at 450 to 500 nm. (Composition analysis was performed by ICP, X-ray fluorescence analysis, etc.) Film formation conditions were rare earth and Ga metal as evaporation source, substrate temperature 500.
° C to 650 ° C, degree of vacuum 4 to 6 * 10 ^-4 Torr, deposition rate 18 to 2
3 nm / min. Oxygen is supplied to the film by spraying oxygen around the substrate (the periphery of the substrate is estimated to be in the order of 10 ⁻² Torr), and the evaporation source is activated by RF plasma before reaching the substrate. The obtained oxide film was analyzed by X-ray diffraction and RHEED.
As a result, the film was epitaxially grown. The oxide used here can be said to be a substance that is relatively easy to grow epitaxially on a silicon wafer (conspicuously as compared with an oxide superconducting material).

Next, a Bi1.8Pb0.2Sr2Ca2Cu3Oδ (this value is a target value and slightly varied) superconducting film having a thickness of 150 nm was formed on the oxide film by MBE (molecular beam epitaxy). The film formation conditions used Bi-Pb alloy, Sr, Ca, and Cu metal as the evaporation source. (Evaporation is performed by electron beam for Bi-Pb alloy and Knudsen for other metal
Cell was performed), the degree of vacuum was 3 to 6 * 10 ^-5 Torr, the substrate temperature was 600 to 680 ° C., and the film formation rate was 20 to 35 nm / min. Irradiation during film formation.

Next, annealing is performed in an oxygen atmosphere at 500 ° C. for 15 hours to obtain an oxide superconducting thin film. However, an as-grown superconducting film may be obtained, and annealing is performed as necessary.

When the obtained oxide superconducting thin film was analyzed by X-ray diffraction and RHEED, it was a film grown epitaxially.

Example-2 A Pr1.0Ga1.0Oz oxide layer was formed to a thickness of 500 nm on a single crystal silicon wafer substrate under the same conditions as in Example-1, and then Bi1.97S
An r2.0Ca1.1Cu2.1Oδ thin film was formed to a thickness of 100 nm to obtain an oxide superconducting thin film.

Next, the critical temperature and critical current density of the obtained oxide superconducting thin film were measured by a four-terminal method. The measurement temperature is Example-1:
77K, Example-2: 60K, and the measurement atmosphere was helium gas. For cooling, Daikin Industries cryogenic refrigerator UV20
4SR was used.

The results are shown in Table 2 (Example-1) and Table 3 (Example-2).
Are shown together with Comparative Examples. In the comparative example, a Bi-Sr-Ca-Cu-O film was directly formed on a single-crystal silicon wafer substrate (100 nm and 700 nm in thickness of F and G, respectively), and a MgO single crystal was used for the substrate (H and I). ).

As can be seen from the table, the oxide superconducting thin film according to the present invention can be used even when a silicon wafer capable of increasing the diameter is used as a substrate.
It becomes a high critical current density close to that when using the MgO single crystal substrate. Comparative Example F did not become superconducting because the deposited material reacted with the silicon wafer over the entire film and took the crystal structure of the superconducting material. It is not. In the present invention, since this reaction can be suppressed, it is possible to obtain superconducting characteristics that can be formed as thin as 100 nm. The critical temperature of Comparative Example G (thickness: 700 nm) is a relatively good value of 99 K, but the critical current density is extremely low. This is because the superconducting thin film is not epitaxially grown.

Also, in the examples, B has a higher critical current density than A and C because the oxide layer is close to the optimum composition and has an optimal crystal structure, lattice matching is improved, and epitaxial growth of the oxide superconducting film is improved. This is because it has a good effect. That is, there is a range in the composition of the oxide, and when expressed as AxGayOz (where A represents a rare earth element), 0.95 ≦ x ≦ 1.
05, 0.9 ≦ y ≦ 1.1 must be satisfied, and the critical current density sharply decreases when the value is out of the range. La-Ga-O system, Nd-Ga-O
This is because the Pr-Ga-O system has the highest critical current density among the Pr-Ga-O systems, so that the lattice matching is the best.

Table 3 shows an example of a superconducting thin film having a Bi2Sr2Ca1Cu2Oδ system composition, that is, a low critical temperature phase film having one less perovskite layer per unit cell than the so-called Example-1.
Similarly, the critical current density is almost the same as when the MgO single crystal substrate is used. This film can be easily made into a single phase and has good epitaxial growth properties as compared with the high critical temperature phase film of Example 1.

Table 4 shows the price of one single crystal silicon wafer substrate and one MgO single crystal substrate that has been used in the past. Single-crystal silicon wafer substrate is 4 inches (about 10cmφ) and MgO
The price is about 1/20 in spite of the large diameter of about twice the single crystal substrate. By employing a single crystal silicon wafer substrate in this manner, not only the diameter can be increased but also the cost can be significantly reduced.

According to the first aspect of the present invention, the oxide layer contains a rare earth element, and the oxide superconducting thin film on the oxide layer also contains the rare earth element. Even if there is mixing of components at the interface of the conductive thin film, the effect is small. As a result, the oxide superconducting thin film has good crystallinity, and has a high critical current density and a high critical temperature. In addition, since a single crystal silicon wafer substrate that can be easily obtained is used as a substrate, there is a remarkable effect that low cost and excellent mass productivity are achieved.

According to the second aspect of the present invention, since the oxide superconducting thin film containing the rare earth element is formed on the oxide layer containing the rare earth element, the composition at the interface between the oxide layer and the oxide superconducting thin film is formed. Even if there is mixing, the effect can be kept low. As a result, a remarkable effect that an oxide superconducting thin film having very good crystallinity can be formed is obtained.

Claims (2)

(57) [Claims] 1. A single crystal silicon wafer substrate, formed on the single crystal silicon wafer substrate,
When the composition is represented by A _x Ga _y O _z (where A represents a rare earth element), an oxide layer satisfying 0.95 ≦ x ≦ 1.05, 0.9 ≦ y ≦ 1.1, and an oxide layer formed on the oxide layer Bi-M-Cu-O-based or Bi-Pb-M-Cu-O-based oxide superconducting thin film (here,
M represents an alkaline earth), and an oxide superconducting element comprising: 2. The composition on a single crystal silicon wafer substrate.
A _x Ga _y O _z (wherein, A is shows the rare-earth element) to form an oxide layer is 0.95 ≦ x ≦ 1.05,0.9 ≦ y ≦ 1.1 when expressed as, Bi-M on the oxide layer -Cu-O-based or Bi-Pb-
A method for producing an oxide superconducting thin film, comprising forming an M-Cu-O-based oxide superconducting thin film (here, M represents an alkaline earth).