JP2835235B2 - Method of forming oxide superconductor thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is suitable for high-frequency devices such as resonators, strip lines, and filters.
The present invention relates to a method for forming an oxide superconductor thin film having a small surface resistance.

[0002]

2. Description of the Related Art In recent years, the critical temperature (Tc) of superconductivity has exceeded the temperature of liquid nitrogen (77 K).
CuO system, BiSrCaCuO system, TlBaCaCuO
Oxide superconductors have been discovered one after another, and research on their practical application in various fields such as power, electromagnetic fields, and electronics has been greatly advanced, along with elucidation of the superconducting mechanism. In order to apply the above-mentioned oxide superconductor to the field of electronics, a technique for thinning the oxide superconductor is indispensable. So far, sputtering, laser vapor deposition, various physical vapor deposition methods, etc. Application studies of a vapor phase growth method such as a (PVD method) or a chemical vapor deposition method (CVD method) have been made. Thus, the method for forming an oxide superconductor thin film by the above-described vapor phase growth method includes:
MgO or LaA with good lattice matching with oxide superconductor
The constituent elements of the oxide superconductor are deposited in an oxidizing atmosphere on the (100) plane of a single-crystal substrate such as lO ₃ to react with the oxide superconductor, and the crystal is formed of C having excellent superconducting properties.
This is a method of axially orienting.

[0003]

However, as shown in the side sectional view of FIG. 2, the oxide superconductor thin film formed by the vapor In the superconductor crystal 11, CuO crystal grains 12 grow in a columnar shape perpendicular to the substrate surface, and have a structure exposed to the surface of the oxide superconductor thin film. There is almost no problem when using the superconductor thin film for direct current transmission, but when using it for high frequency application devices, etc., the CuO crystal grains exposed on the surface of the oxide superconductor thin film and the CuO crystal grains and the oxide superconductor crystal The boundary portion increases the loss of millimeter waves and microwaves in the oxide superconductor thin film, and as a result, the surface resistance is significantly increased, and high frequency characteristics cannot be obtained.

[0004]

SUMMARY OF THE INVENTION In view of such circumstances, the present invention has conducted intensive studies and has found that C (C) is formed on a (100) plane of a MgO single crystal substrate or the like.
It was found that when a composite oxide consisting of constituent elements other than Cu of an oxide superconductor containing u was deposited, and when each constituent element of the oxide superconductor was deposited thereon, CuO crystal grains would not be generated. The present invention has been completed through further studies. That is, the present invention provides a method for depositing each constituent element of an oxide superconductor containing Cu as a constituent element in an oxidizing atmosphere on a (100) single crystal substrate, and forming the oxide superconductor in a film form on the substrate. In the method for forming an oxide superconductor thin film formed on the substrate, the start of vapor deposition of the Cu element is delayed more than the other elements, and a composite oxide layer composed of constituent elements other than the Cu element of the oxide superconductor is formed on the substrate by several atomic layers. Then, each constituent element of the oxide superconductor containing Cu is vapor-deposited on the composite oxide layer.

[0005] The method of the present invention comprises the steps of YBa ₂ Cu ₃ O _Y , Bi
₂ Sr ₂ Ca ₂ Cu ₃ O _Y , Bi ₂ Sr ₂ Ca ₁ Cu ₂
O _Y , Tl ₂ Ba ₂ Ca ₂ Cu ₃ O _Y , Tl ₂ Ba ₂ C
a ₁ Cu ₂ O _Y or a general formula LnBa ₂ Cu ₃ O
_7-x (where Ln is one of Y and the lanthanoid metal element)
The present invention is applied to the formation of a thin film of any oxide superconductor containing Cu having a composition such as element or plural elements) as a constituent element.
In the method of the present invention, the reason why the composite oxide layer made of the constituent elements other than the Cu element of the oxide superconductor containing Cu as a constituent element is provided immediately above the substrate is that (100) single crystal of MgO or the like in which CuO easily grows. This is for preventing Cu from being directly vapor-deposited on the substrate to grow crystal grains of CuO. The reason why the thickness of the composite oxide layer is limited to a thickness of several atomic layers is that when the composite oxide layer is thick, the lattice constant of the (100) single crystal substrate is not sufficiently inherited, and as a result, This is because the C-axis orientation of the oxide superconductor thin film formed on the oxide layer is reduced. The thickness of the composite oxide layer,
The thickness is several atomic layers, that is, about 10 mm, and the deposition time is about several seconds.

Hereinafter, a method for forming an oxide superconductor thin film of the present invention will be specifically described with reference to the drawings. FIG. 1 is an explanatory view of a main part showing an example of an embodiment of a molecular beam epitaxy (MBE) apparatus used in the method of the present invention. This MBE apparatus includes a plurality of cells a, b, c,
d and a substrate 2 for depositing a gas generated from the evaporation source are arranged to face each other. The cell a
To d, an electron gun 3 for heating the evaporation source, a cell shutter for controlling the amount of vapor deposition on the substrate 2, and a quartz vibrating film thickness meter 4 for monitoring the evaporation rate. Are provided respectively. A substrate shutter 5 is disposed below the substrate 2. The measurement result of the film thickness meter 4 is fed back to the electron gun 3 to automatically control the deposition rate. The substrate 2 is controlled to a predetermined temperature by the heater 10,
The crystal structure of the oxide superconductor layer deposited on the substrate 2 is adjusted. The inside of the chamber 1 is evacuated to a predetermined pressure by an exhaust pump 6. The vicinity of the cells a to d is kept at a high vacuum of 5 × 10 ⁻⁷ Torr to prevent oxidation of the evaporation source. In the vicinity of the substrate 2, an oxygen gas or an active gas such as O ₃ or N ₂ O capable of releasing oxygen atoms is supplied in a gas cylinder so that oxygen is sufficiently supplied to the oxide superconductor layer formed on the substrate 2. 7 passes through a gas flow meter 8 and is sprayed from a nozzle 9.
A differential exhaust plate (not shown) is arranged between the substrate 2 and the cells a to d in order to maintain a difference between the two degrees of vacuum.

[0007]

According to the method of the present invention, Cu is deposited on a (100) single crystal substrate.
In a method for forming an oxide superconductor thin film in which each constituent element of an oxide superconductor containing as a constituent element is deposited in an oxidizing atmosphere and the oxide superconductor is formed into a film on the substrate, Delay the start of vapor deposition compared to other elements,
Since the composite oxide layer made of the constituent elements other than the Cu element of the oxide superconductor is provided immediately above the substrate, the Cu element is not directly deposited on the substrate surface, and thus the generation of CuO crystal grains is prevented. Since the composite oxide layer is provided with a thickness of several atomic layers, the oxide superconductor layer formed on the composite oxide layer takes over the lattice constant of the (100) plane of the substrate, and the C axis is It grows with a crystal orientation excellent in superconducting properties oriented in a direction perpendicular to the plane.

[0008]

The present invention will be described below in detail with reference to examples. Example 1 Using the MBE apparatus shown in FIG.
A Y-based oxide superconductor thin film was formed on the (100) plane. The cells a, b, and c are filled with metals of Y, Ba, and Cu, respectively, as the evaporation sources.
Y: Ba: Cu was heated and evaporated independently so that the atomic ratio of Y: Ba: Cu on the substrate was 1: 2: 3. The evaporation rate of each element was individually measured by a quartz vibrating film thickness meter 4 and fed back to each electron gun 3 for automatic control. After confirming that the evaporation rates of Y, Ba, and Cu reached a predetermined rate, the shutters B, B and 5 were opened, and Y and Ba were deposited on the substrate 2. 2 seconds after the start of vapor deposition (approximately 10
Equivalent to Å. 2) The shutter immediately above the Cu cell c was also opened. The MgO single crystal substrate 2 is heated by the heater 10 for 670 minutes.
Heated and maintained at ° C. Oxygen gas was blown from the nozzle 9 to the vicinity of the substrate 2 so that oxygen was sufficiently taken in the deposition layer. The oxygen pressure was adjusted to 5 × 10 ⁻⁴ Torr near the substrate 2. Thus, an oxide superconductor thin film having a composition of YBa ₂ Cu ₃ O _7-x was formed on the substrate 2 to a thickness of 3000 mm.

Embodiment 2 Using the MBE apparatus shown in FIG.
A Bi-based oxide superconductor thin film was formed on the (100) plane.
As the evaporation source, cells a, b, c, to d Bi ₂ O _3, S
An oxide or metal material of r, Ca, Cu is filled and each is independently heated by the electron gun 3 so that Bi: Sr: Ca: Cu has an atomic ratio of 2: 2: 2: 3 on the substrate. Evaporated. The evaporation rate of each element was measured and controlled individually by a quartz vibrating film thickness meter 4. After confirming that the evaporation rate of each element of Bi, Sr, Ca, and Cu has reached a predetermined rate, the shutters I, B, C, and 5 are opened and Bi, S
Each element of r and Ca was deposited on the substrate. Two seconds after the start of the vapor deposition (corresponding to a thickness of the vapor deposition layer of about 10 °), the shutter immediately above the cell d for Cu was also opened. The MgO single crystal substrate 2 was heated and maintained at 800 ° C. by the heater 10. Oxygen gas was blown from the nozzle 9 to the vicinity of the substrate 2 so that oxygen was sufficiently taken into the deposition layer. Oxygen pressure is 5 near substrate 2
It was adjusted to be × 10 ^-4 Torr. Thus, an oxide superconductor thin film having a composition of Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _7-x was formed on the substrate 2 to a thickness of 3000 mm.

Comparative Example 1 A Y-based oxide superconductor thin film was formed in the same manner as in Example 1 except that the shutter immediately above the Cu cell c was opened 10 seconds after the start of vapor deposition. Comparative Example 2 A Y-based oxide superconductor thin film was formed in the same manner as in Example 1, except that the shutters I, B, C, and 5 were simultaneously opened to start vapor deposition. Comparative Example 3 A Bi-based oxide superconductor thin film was formed in the same manner as in Example 2, except that the shutters I, B, C, D, and 5 were simultaneously opened to start vapor deposition. For each of the thus obtained oxide superconductor thin films,
Tc and critical current density (Jc) were measured by a DC four-terminal method with a gold terminal attached to a predetermined location. Surface resistance (R
s) was measured in a state irradiated with a microwave of 10 GHz. All measurements were performed in liquid nitrogen (77K).
The results are shown in Table 1.

[0011]

[Table 1]

As is clear from Table 1, the sample of the present invention (No.
1, 2) have high Tc and Jc and have a high surface resistance (R
s) was small and had excellent characteristics for use in millimeter-wave / microwave devices. As a result of performing X-ray diffraction for each oxide superconductor thin film, each was found to be C-axis oriented. When the microstructure was observed with a transmission electron microscope,
No CuO grains were observed. On the other hand, in Comparative Example No. 3, the composite oxide layer containing no Cu was too thick because the start of deposition of Cu was too slow, and the oxide superconductor layer formed on the composite oxide layer was The lattice constant of the (100) plane could not be sufficiently inherited, and as a result, the C-axis orientation decreased and all the superconducting properties deteriorated. In No. 4 and No. 5, columnar crystal grains of CuO grew in the direction perpendicular to the substrate because the Cu element was vapor-deposited at the same time as the other elements, and this was exposed on the surface of the oxide superconductor thin film. )increased.

[0013]

As described above, according to the method of the present invention, the generation of CuO crystal grains that increase the surface resistance is prevented, so that an oxide superconductor thin film suitable for high-frequency devices can be obtained. Has a remarkable effect.

[Brief description of the drawings]

FIG. 1 is a main part explanatory view showing an example of an embodiment of a molecular beam epitaxy apparatus used in the present invention.

FIG. 2 is an explanatory side sectional view of an oxide superconductor thin film in which columnar crystal grains of CuO are mixed.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Chamber 2 Substrate 3 Electron gun 4 Quartz vibrating film thickness meter 5 Substrate shutter 6 Exhaust pump 7 Gas cylinder 8 Mass flow meter 9 Nozzle 10 Heater 11 Oxide superconducting crystal 12 CuO crystal grains a, b, c, d cells Shutters for i, b, c, nicelle

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 39/02 ZAA H01L 39/02 ZAAD 39/24 ZAA 39/24 ZAAB

Claims (1)

(57) [Claims] 1. An oxide superconductor containing Cu as a constituent element is vapor-deposited on an (100) single crystal substrate in an oxidizing atmosphere, and the oxide superconductor is formed into a film on the substrate. In the method for forming an oxide superconductor thin film to be formed,
By delaying the start of vapor deposition of the Cu element relative to other elements, a composite oxide layer composed of constituent elements other than the Cu element of the oxide superconductor is provided with a thickness of several atomic layers immediately above the substrate, and then the composite oxide A method for forming an oxide superconductor thin film, comprising depositing each constituent element of an oxide superconductor containing Cu on a layer.