JP4572386B2 - Fabrication method of high quality Bi-based oxide superconducting thin film - Google Patents

Description

In the present invention, the c-axis for obtaining a high-performance multilayer Josephson junction using an oxide superconductor, particularly a bismuth (hereinafter referred to as “Bi-based”) oxide superconductor, is parallel to the substrate surface. And an oxide superconductor in which the a-axis (or b-axis) is oriented perpendicular to the substrate surface, in particular a Bi-based oxide superconducting thin film, specifically, Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 ± X} (X is a positive number smaller than 1, hereinafter referred to as “Bi-2223”) or Bi ₂ Sr ₂ CuO _{6 ± Y} (Y is a positive number smaller than 1, hereinafter referred to as “Bi-2201”) The present invention relates to an oxide superconducting thin film and a manufacturing method thereof.

  The features of Josephson elements using superconductors are their high-speed operation and low power consumption. When applied to an integrated circuit, a high-speed switching operation can be performed with a small amount of electric power. Therefore, heat generated in a high-density integrated circuit which is a problem in a semiconductor is small, and high-speed computing performance can be expected compared with a semiconductor.

  Conventionally, Nb metal or NbN has been used as a superconductor used in the Josephson element. However, since the superconducting transition temperature is low, the operation of the Josephson element is normally performed at a liquid helium temperature of 4.2K. In comparison, oxide superconductors have a higher superconducting transition temperature, so Josephson devices using them can be operated at liquid nitrogen temperatures, which is promising from the viewpoint of resource and power savings. Can be expected.

  A superconducting element exhibiting the Josephson effect is called a Josephson junction. Josephson junctions suitable for integrated circuits using superconducting elements can be precisely controlled in dimensions and can produce a large number of junctions. As shown in FIG. 1, normal conductors and insulators are interposed between superconductor thin films. Laminated bonding with an ultra-thin barrier layer is promising. In fact, in a superconducting integrated circuit using Nb metal, a laminated junction is used as a Josephson junction.

  The problem to be broken through in order to fabricate a stacked Josephson junction using an oxide superconductor is closely related to the crystal structure of the oxide superconductor. Yttrium-based (hereinafter “Y-based”) oxide superconductors and Bi-based oxide superconductors have superconducting properties such as coherence length, magnetic flux penetration depth, and critical current density compared to conventional superconductors such as Nb. The anisotropy of is remarkable.

  These crystals are an orthorhombic lattice or a tetragonal lattice, but the strength of superconducting coupling in the c-axis direction is weaker than in-plane coupling perpendicular to the c-axis. In oxide superconductors, superconductivity is considered to occur on the CuO surface formed by copper (Cu) atoms and oxygen atoms (O).

  Therefore, the anisotropy of these superconducting couplings is due to the fact that the CuO plane is in the direction perpendicular to the c-axis (that is, the a or b-axis direction) and not in the c-axis direction. For this reason, the coherence length closely related to the Josephson junction (the distance between electrons that can form a superconductor pair) is significantly smaller in the c-axis direction than in the a-axis direction. This tendency is more remarkable in the Bi-based superconductor having a larger crystal structure anisotropy than the Y-based superconductor, and the coherence length in the c-axis direction is as extremely short as 0.2 nm.

  Thus, in an oxide superconductor, in particular, a Bi-based superconductor such as Bi-2223 or Bi-2201, the coherence length in the c-axis direction is extremely short. For this reason, in order to produce a laminated Josephson junction in the c-axis direction using the c-axis alignment film, it is indispensable to form a flat and extremely thin barrier layer. However, if the barrier layer is thin, unevenness due to precipitates or the like becomes a problem, and it is difficult to form a uniform ultrathin barrier layer, and current leakage occurs between the upper and lower superconductors sandwiching the barrier layer. Bonding has not been achieved. Even if a Josephson junction can be fabricated, the Josephson critical current density Jc and the Josephson characteristic parameter IcRn are small and good characteristics cannot be obtained.

  Therefore, in order to obtain a high-performance multilayer Josephson junction using a Bi oxide superconductor, it is indispensable to produce a junction in the non-c-axis direction having a coherence length longer than the c-axis direction. Among these, the direction with the longest coherence length is the a-axis (or b-axis) direction. Therefore, to obtain a high-performance multilayer Josephson junction using Bi-based oxide superconductors, the c-axis is parallel to the substrate surface and the a-axis (or b-axis) is perpendicular to the substrate surface. It is desired to produce a Bi-based oxide superconducting thin film oriented in the direction.

  As one method for achieving this, a step of supplying active oxygen and a part of metal components constituting the Bi-based oxide on the substrate to form a composition modulation film made of the oxide on the substrate; And a step of supplying an active oxygen and all metal components constituting the Bi-based oxide to form an oxide superconducting thin film on the composition-modulated film. A method is known (see Patent Document 1 below). However, in this method, depending on conditions, the ratio of the parallel c-axis to the substrate surface changes, and it cannot be said that a good Bi-based oxide superconducting thin film is obtained.

In addition, the performance of a Josephson element manufactured using a Bi-based oxide superconducting thin film with the c-axis parallel to the substrate surface and the a-axis (or b-axis) oriented perpendicular to the substrate surface is excellent. Although there is a document showing that (see Patent Document 2 below), specifically, the good c-axis is parallel to the substrate surface and the a-axis (or b-axis) is relative to the substrate surface. However, it is not shown whether a Bi-based oxide superconducting thin film that is vertically oriented can be obtained.
Japanese Patent Laid-Open No. 5-7027 Japanese Patent Laid-Open No. 9-246611

  Therefore, the present invention is intended to produce a high-quality a-axis (or b-axis) oriented Bi-based oxide superconducting thin film in order to obtain a high-performance multilayer Josephson junction using a Bi oxide superconductor. Objective.

Bi-2223 thin film is (110) plane LaSrAlO ₄ single crystal substrate, (110) plane LaSrGaO ₄ single crystal substrate, (10-10) plane (a plane) α-Al ₂ O ₃ single crystal substrate or ( An a-axis oriented Bi-2223 thin film grows in alignment with 3 units of the NdAlO ₃ single crystal substrate on the 10-10) plane (a plane).

Bi-2201 thin film is (110) plane LaSrAlO ₄ single crystal substrate, (110) plane LaSrGaO ₄ single crystal substrate, (10-10) plane (a plane) α-Al ₂ O ₃ single crystal substrate or ( An a-axis oriented Bi-2201 thin film grows in alignment with two units of the 10-10) plane (a plane) NdAlO ₃ single crystal substrate.

A high-quality a-axis oriented Bi-based oxide superconducting thin film is a tilted substrate cut at a finite angle θ in the [001] direction from the (110) plane of a single crystal such as LaSrAlO4 or LaSrGaO4, α-Al ₂ O ₃ or NdAlO ₃ single-crystal (10-10) plane (a-plane) using a tilted substrate cut with a finite angle θ in the [0001] direction, low deposition temperature T1 (500 ~ Bi-2223 thin film with a-axis orientation at 600 ° C) is heteroepitaxially grown, and then homoepitaxially grown on the grown film at a high deposition temperature T2 (650-750 ° C) (two temperature growth method) can do.

  When a Josephson device is manufactured using a high-quality a-axis-oriented Bi-based oxide superconducting thin film manufactured by the method of the present invention, a Josephson device with extremely excellent performance can be obtained.

  Below, the most preferable embodiment of this invention is shown.

FIG. 2 shows the consistency of the lattice constant of the (110) plane of a LaSrAlO4 single crystal substrate with respect to a-2 oriented Bi-2223. As shown in the figure, it can be seen that one unit cell of Bi-2223 is very well aligned with a three unit cell of LaSrAlO ₄ . Misfits of lattice constants of the a-axis length (or b-axis length) and c-axis length are −1.48% and 1.61%, which are extremely small.

Therefore, a Bi-2223 thin film with the c-axis parallel to the substrate and the a-axis (or b-axis) oriented perpendicular to the substrate can be epitaxially grown on the (110) LaSrAlO ₄ single crystal substrate. However, when a flat substrate is used, as shown in FIG. 3, two-dimensional nuclei grow on the substrate, a large number of grains are formed, and it is difficult to obtain a continuous and flat film. Deteriorate. Further, even if the two-temperature growth method alone is used, the size of each grain is increased, and it is difficult to obtain a continuous and uniform thin film, resulting in poor superconducting properties.

Therefore, a tilted substrate cut from the (110) plane of a single crystal such as LaSrAlO4 or LaSrGaO4 with a finite angle θ in the [001] direction, a (10-10) of a single crystal of α-Al ₂ O ₃ or NdAlO ₃ By using a tilted substrate cut at a finite angle θ in the [0001] direction from the plane (a-plane), and using the two-temperature growth method, the substrate step as shown in FIG. A thin film is formed on the substrate by step flow growth, and a high-quality a-axis oriented Bi-2223 thin film with good superconducting properties can be obtained.

A Bi-2223 superconducting thin film was fabricated by metal organic chemical vapor deposition (MOCVD) using a tilted substrate cut at a finite angle θ in the [001] direction from the (110) plane of LaSrAlO4 single crystal. An overview of the MOCVD system used this time is shown in FIG. Film formation conditions are Bi (C ₆ H ₅ ) ₃ , Sr (DPM) ₂ , Ca (DPM) ₂ and Cu (DPM) ₂ (DPM: dipivaloylmethan) as organic metal raw materials, 72 ° C, 176 ° C, Maintained at 161 ° C and 80 ° C, Ar carrier gas flow rate 100, 300, 300 and 70 sccm, total pressure 50 torr, oxygen partial pressure 23 torr, substrate temperature 553 ° C, Bi-2223 thin film with a-axis (or b-axis) orientation heterogeneous Epitaxial growth was performed, and then homoepitaxial growth was performed continuously at a high substrate temperature of 680 ° C. without changing the gas atmosphere. For this film formation, substrates with inclination angles θ = 5 °, 10 °, and 15 ° were used.

FIG. 6 shows an atomic force microscope (AFM) image of the surface of an a-axis oriented Bi-2223 thin film grown at two temperatures on a flat substrate and an inclined substrate.
FIG. 7 shows a cross-sectional view thereof. From these, it can be confirmed that the a-axis oriented Bi-2223 thin film grown on the inclined substrate at two temperatures is step-flow grown and is formed uniformly and continuously.

  FIG. 8 shows the temperature dependence of the resistance of the a-axis oriented Bi-2223 thin film grown at two temperatures on the flat substrate and the inclined substrate, respectively. The measurement was performed by the standard 4-terminal method. Among the films formed this time, the superconducting characteristics were the best when the inclination angle θ was 15 ° C.

  As described above, when the a-axis oriented Bi-2223 oxide superconducting thin film is formed, the thin film is formed by step flow growth with the inclined substrate and the two-temperature growth method. In addition, a high-quality a-axis oriented Bi-2223 thin film having a uniform surface, flat surface, and very good superconducting properties can be produced.

Example of a promising Josephson junction Diagram explaining the alignment status Two-dimensional nuclear growth model diagram Step flow growth model diagram Overview of MOCVD thin film production equipment Atomic force microscope (AFM) image of Bi-2223 thin film surface Cross section of Bi-2223 thin film atomic force microscope (AFM) image Temperature dependence of resistance of Bi-2223 thin film

Claims (8)

In the method for producing a Bi-based oxide superconducting thin film on a substrate, the lattice constant of the single crystal of the thin film is an integral multiple of the single crystal lattice constant of the substrate (excluding 1 time), When growing a Bi-based oxide superconducting thin film with the c-axis of the thin film parallel to the substrate surface and the a-axis or b-axis oriented perpendicular to the substrate surface, the thin film was cut from the substrate surface at a finite angle. A method for producing a Bi-based oxide superconducting thin film, comprising using an inclined substrate having steps .   The thin film is heteroepitaxially grown at a first temperature, and then the thin film is homoepitaxially grown on the thin film at a second temperature higher than the first temperature. Item 2. A method for producing a Bi-based oxide superconducting thin film according to Item 1. The Bi-based oxide superconducting thin film has Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 ± X} (X is a positive number smaller than 1) or Bi ₂ Sr ₂ CuO _{6 ± Y} (Y is a positive smaller than 1). The method for producing a Bi-based oxide superconducting thin film according to claim 1 or 2, wherein the method is a thin film. The single crystal _{substrate, LaSrAlO 4, LaSrGaO 4, α} -Al 2 O 3 or the method for manufacturing a Bi-based oxide superconducting thin film according to any one of claims 1 to 3, characterized in that NdAlO _3. A Bi-based oxide superconducting thin film deposited on a substrate, the lattice constant of the single crystal of the thin film being an integral multiple of the single crystal lattice constant of the substrate (excluding 1 time), The c-axis is parallel to the substrate surface, the a-axis or b-axis is oriented perpendicular to the substrate surface, and the substrate is an inclined substrate having steps cut from the substrate surface at a finite angle. A Bi-based oxide superconducting thin film characterized. The Bi-based oxide superconducting thin film has Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _{10 ± X} (X is a positive number smaller than 1) or Bi ₂ Sr ₂ CuO _{6 ± Y} (Y is a positive smaller than 1). The Bi-based oxide superconducting thin film according to claim 5, wherein the Bi-based oxide superconducting thin film is a thin film. 7. The Bi-based oxide superconducting thin film according to claim 5, wherein the single crystal substrate is LaSrAlO ₄ , LaSrGaO ₄ , α-Al ₂ O _3, or NdAlO ₃ .   A Josephson element, which is manufactured using the Bi-based oxide superconducting thin film according to any one of claims 5 to 7.