JPH04268718A - Bicrystal substrate - Google Patents

Abstract

PURPOSE:To improve reproducibility and stability of characteristics at low noise by providing a second phase on the surface of a bicrystal substrate consisting of at least two or more Si single crystals. CONSTITUTION:Using two single crystal blocks 1 and 2 as substrate material, a surface (001) is formed in parallel with the surface of paper. Square blocks are cut so that the axes (100) and (010) on the above-mentioned plane rotate at an angle of 24 deg. on the blocks 1 and 2, and the junction surface 3 of each block is flatly mirror-polished. Using a pyrania solution and cleaning fluid of acetone and ethyl alcohol, the blocks are connected in a hot-press furnace. Then, pressure is removed, the furnace is cooled down, and a bicrystal block is manufactured. This bicrystal block is cut into the prescribed shape, substrate surface is mirror-polished, and an Si bicrystal substrate is obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a substrate used in the synthesis of oxide superconducting thin films and a method for manufacturing the same.

0002

[Prior art] Y exhibiting superconducting properties above liquid nitrogen temperature
Oxide superconducting liquids such as oxide, Bi, Ti, etc. have been discovered, and research is being conducted on thin film production techniques using various methods such as CVD, sputtering, and vacuum evaporation. Furthermore, research on the practical application of superconducting applied products using these oxide superconducting thin films is also underway at many research institutions.

It is already known that oxide superconductors have anisotropic superconducting properties due to their two-dimensional structure, and that crystal grain boundaries affect superconducting properties. Therefore, in order to form a thin film with excellent superconducting properties, it is necessary to create a film in which the c-axis of the oxide superconductor is oriented in a direction perpendicular to the substrate surface, or even in a constant direction within the plane. Oriented epitaxially grown films are required, and single-crystal substrates that have good lattice constant matching with the lattice constant of oxide superconductors are being investigated. S
Single crystal substrates such as rTiO3 and LaAIO3, Si
Zirconia stabilized with yttria on a single-crystal substrate, or a substrate with yttria formed thereon, etc., are used. In addition, single crystal substrates such as MgO or LaAIO3 are often used for forming Ba-based oxide superconducting (hereinafter abbreviated as BSCCO) thin films, and MgO single crystal substrates are often used for forming TI-based oxide superconducting (hereinafter abbreviated as TBCCO) thin films. It is used.

[0004] Such an oxide superconducting thin film on a substrate is
90K for YBCO thin film, 105K for BSCCO thin film, T
The BCCO thin film shows zero resistance near 120K, and all films have a critical current density of 105 A/cm2 or more at 77K. Thin films with excellent superconducting properties are expected to be applied to Josephson devices such as SQUIDs, mixers, and Josephson FFTs, and research on Josephson junctions is being conducted. However, because the coherent length of oxide superconductors is shorter than that of metal-based superconductors, which are conventional materials, there are currently many problems with conventional junction fabrication techniques such as stacked tunnel barriers and thin-film weak junctions. be.

To date, in most of the Josephson junctions made of oxide superconducting thin films in which the Josephson effect has been observed, bridges are formed in the thin film to weaken the crystal grain boundaries of the thin film existing at the bridge portion. The bond is a grain boundary bonding type. Most grain boundary bonding types use naturally formed grain boundaries, and because the distribution of grain boundaries is random, it is difficult to form bridges on grain boundaries with good reproducibility. Furthermore, since the crystal grain size and grain boundary structure vary depending on the film forming conditions, there are problems with the reproducibility and stability of characteristics. Furthermore, since grain boundaries exist outside the bridge portion, magnetic flux is likely to be captured by the grain boundaries, and in 77K operation there is a problem in that characteristics such as energy resolution are degraded due to noise generated by the movement of magnetic flux.

In recent years, IBM's Gross
(to be published Appl.P
hys. Lett). Gross
S et al. fabricated a bicrystal substrate using SrTiO3 single crystal, in which the plane orientation of each substrate surface was (001), and the [100] axis direction of the two crystals sandwiching the bonding part of the substrates differed by 20 degrees, and this A thin film is formed on the substrate. In the YBCO thin film on each single-crystal substrate, no grain boundaries are formed due to epitaxial growth, but grain boundaries (angle grain boundaries) with in-plane rotation at the same angle as the substrate are formed on the bonding interface of the substrates. Ru. This artificially tilted grain boundary is used as a Josephson junction as a dc-
By fabricating a SQUID, we have obtained improved reproducibility of characteristics and improved noise characteristics.

However, when SrTiO3 is used as a material for a bicrystal substrate, SrTiO3's high dielectric constant (
1875 at 80K) and loss tangent (1 at 80K, 1GHz)
00), which makes it impractical due to the large dielectric loss, and furthermore, the substrate is limited. Therefore, using semiconductor technology, which is currently very highly developed, we have developed Josephson junctions using oxide superconductors and semiconductors. The problem was that it was difficult to construct a device that integrated the two.

[0008]

[Problems to be Solved by the Invention] The present invention has been made by focusing on the problems of the prior art described above, and it is possible to fabricate a Josephson junction with low noise and excellent reproducibility and stability of characteristics. The present invention aims to provide a substrate and a method for manufacturing the same, which can be applied to microwave application devices such as mixers and voltage standards, and which can also construct devices that integrate semiconductors and Josephson junctions.

[0009]

[Means for Solving the Problems] In order to solve the above problems, the present invention provides a bicrystal substrate made of at least two or more Si single crystals, and further has a second phase on the surface of the substrate. A bicrystal substrate and a method for manufacturing the same.

More specifically, the bicrystal substrate of the present invention employs a Si single crystal as the substrate material, has at least two Si single crystals bonded together, and has a bonding boundary on either of the substrate surfaces. Use a bicrystal substrate. The orientation of each single crystal in the bicrystal substrate aligns one crystal axis, and the other crystal axes are rotated at arbitrary angles. The crystal axes of each single crystal to be matched can be adjusted according to the crystal orientation of the superconducting thin film required for each Si single crystal and the crystal orientation required for the semiconductor device formed on the Si single crystal. It is the crystal axis. In order to form a c-axis oriented oxide superconducting thin film or epitaxially grown oxide superconducting thin film on a bicrystal substrate, the substrate surface should be a (001) plane, or
Alternatively, it is preferable that the deviation in the orientation of the [001] axis perpendicular to the (001) plane be within several degrees, and the orientations of the [100] and [010] axes be at arbitrary angles. Si, which is the substrate material, has a dielectric constant of about 12 at room temperature and 1 MHz, which is about the same value as MgO, so that the dielectric loss caused by the substrate can be reduced.

[0011] Furthermore, the bicrystal substrate has a second phase on its surface, and the second phase is at least an oxide. In a Josephson junction made of an oxide superconducting thin film on a bicrystal substrate, this second phase is generated between the junction and Si because microwave applications utilize the high voltage side characteristics of the junction.
There are no particular limitations on the portion where semiconductor devices are formed on the bicrystal substrate. In addition, in order to make the grain boundaries of the oxide superconducting thin film along the substrate joint part the joint, the second phase grain boundaries are formed along the joint interface of each Si single crystal. . Furthermore, a second phase is formed on a bicrystal substrate by sequentially laminating at least one or two or more of zirconia, yttria, macnesia, and lanthanum aluminate stabilized with zirconia, yttria, calcia, etc. .

[0012] By providing these second phases, mutual diffusion between Si and the oxide superconducting thin film caused by substrate heating during formation of the oxide superconducting thin film is prevented, and the bonding between the substrate and the oxide superconducting thin film is prevented. Relieves stress caused by differences in thermal expansion coefficients. It also has the function of insulating between Si and the oxide superconducting thin film. Furthermore, it has the function of improving the lattice constant matching between the Si single crystal and the oxide superconducting thin film, and promoting the epitaxial growth of the oxide superconducting thin film on each Si single crystal substrate. The combination of two or more layers is good as long as it can at least prevent mutual diffusion, relieve stress, promote epitaxial growth, and promote insulation.

A more specific method of manufacturing a bicrystal substrate according to the present invention involves polishing at least the bonding surfaces of each Si single crystal block to a flat and mirror surface, and polishing the joint surface with a mixed solution of hydrogen peroxide and sulfuric acid (a piranha solution). ) Alternatively, remove organic matter from the surface using an organic solvent such as methanol, ethanol, or acetone, and then remove oxides from the surface using a hydrogen fluoride solution. Next, each Si single crystal block is pasted together in the above-mentioned orientation and placed in a heating furnace, and oxides are formed on the surface of the Si single crystal block in a vacuum, reducing atmosphere, or inert gas atmosphere. Heating in a non-forming atmosphere. The heating temperature is at least below the melting point of Si, preferably 1000°C.
to 1300°C. Further, pressure is applied in a direction perpendicular to the joint surfaces of the respective Si single crystal blocks. A bicrystalline substrate is manufactured by cutting and polishing the Si single crystal block bonded by the method described above so as to have the desired surface orientation and size of the substrate.

[0014] The second phase may be formed on the bicrystal substrate by any method that forms the second phase on the bicrystal substrate, such as sputtering, vacuum evaporation, or chemical vapor deposition. That's fine. Furthermore, in order to minimize the grain boundaries of the oxide superconducting thin film on each Si single crystal, a method that allows epitaxial growth is preferred.

[0015] A bicrystal substrate bonded with Si single crystals allows the formation of a Josephson junction using an oxide superconductor on the substrate, and it is also possible to form a Josephson bond stably with good reproducibility. . Furthermore, by forming a second phase on this substrate that provides insulation, stress relaxation, interdiffusion prevention, and promotes epitaxial growth, the substrate becomes a substrate on which a better oxide superconducting thin film can be epitaxially grown.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to FIGS. 1 to 8. 1 to 3 show the manufacturing process of a bicrystal substrate. Two Si single crystal blocks were used as the substrate material. As shown in Fig. 1, the two Si single crystal blocks have a (001) plane parallel to the plane of the paper, and the [1
For the 00] and [010] axes, block 1 and 2 are each cut into square blocks so that they rotate at an angle of 24°, and the joint surfaces 3 of each block are mirror-polished to a flat surface, and polished with piranha solution, acetone, and ethyl alcohol. After washing,
Bonding was carried out in a hot press furnace. Bonding is performed at a heating temperature of 12
In an atmosphere of 00°C and vacuum level of 10-5 Torr,
The pressure was maintained at 1 kg/mm2 for 1 hour. Thereafter, the pressure was removed and the mixture was cooled in a furnace to produce the bicrystal block shown in FIG. 2. This bicrystal block was cut into a substrate having the shape shown in FIG. 3, and the surface 4 of the substrate was mirror polished to produce a bicrystal substrate made of Si.

[0017] On the above bicrystal substrate, 9mo I
% of yttria-stabilized zirconia (YSZ) phase was deposited using electron beam evaporation at a substrate temperature of 800°C and 5 × 10
It was formed to a thickness of 80 nm under a pressure of -5 Torr. Further, an yttria phase was formed to a thickness of 20 nm on the YSZ phase by the same method as the YSZ phase. Figures 4, 5, and 6 show the (111) diffraction peak of Si and YS, respectively.
The X-ray diffraction patterns measured by the Schulz reflection method are shown for the (111) diffraction peak of Z and the (404) diffraction peak of yttria. Since the YSZ phase and the yttria phase are formed in the same direction as the in-plane orientation of the bicrystal substrate, it can be seen that the same crystal grain boundaries as the substrate are formed in each phase.

A YBCO thin film with a thickness of 100 nm was formed on the substrate according to this example by RF-sputtering. FIG. 7 shows an X-ray diffraction pattern of the YBCO thin film on the substrate of this example measured by the 2θ-θ method. It can be seen that the c-axes of the YSZ phase, the yttria phase, and the YBCO thin film are all oriented perpendicular to the substrate surface. YBC formed
FIG. 8 shows the results of measuring the temperature dependence of the resistance of the O thin film using the four-terminal method. The thin film showed zero resistance at 89K. In addition, the grain boundary current density of the thin film was measured with the bonding interface of the substrate existing between the voltage terminals. The thin film is 5 at 77K.
The value was 104 A/cm2.

[0019]

Effects of the Invention According to the bicrystal substrate and the manufacturing method thereof according to the present invention, the substrate is made of Si single crystal with low dielectric loss, so it can be applied to the microwave field. This is a substrate that allows the construction of a device that integrates a semiconductor device formed in the above-mentioned method and a Josephson device made of an oxide superconducting thin film. Furthermore, since it is possible to form thin films other than oxide superconducting thin films, this substrate can also be applied to tunnel diodes with crystal grain boundaries as barriers by forming a Si thin film.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing two Si single crystal blocks.

FIG. 2 is a diagram showing a bicrystal block.

FIG. 3 is a diagram showing a bicrystal substrate.

FIG. 4 is an X-ray diffraction diagram of a bicrystal substrate obtained by Schulz reflection method.

FIG. 5 is a diagram of an X-ray diffraction pattern of the YSZ phase obtained by the Schulz reflection method.

[Figure 6] X by Schulz reflection method of yttria phase
It is a diagram of a line diffraction pattern.

FIG. 7 is a diagram of an X-ray diffraction pattern of a YBCO thin film on a bicrystal substrate according to an example.

FIG. 8 is a diagram showing the temperature dependence of the resistance of a YBCO thin film on a bicrystal substrate.

[Explanation of symbols]

1 Si single crystal 2 Si single crystal 3 Joint surface 4 Substrate surface

Claims (8)

[Claims] Claim 1: A substrate consisting of at least two or more Si single crystals, each single crystal having one crystal axis aligned, and other crystal axes rotating at an arbitrary angle around the aligned crystal axis. A bicrystal substrate characterized by: 2. The bicrystal substrate according to claim 1, wherein one crystal axis of each single crystal to be aligned is the [001] axis. 3. The bicrystal substrate according to claim 1, having a second phase on the surface of the substrate, the second phase being made of an oxide and functioning as an insulating layer. 4. The method according to claim 1, wherein in the second phase formed on the substrate surface, grain boundaries of the second phase are formed along the bonding interface of each Si single crystal. bicrystal substrate. 5. Claim 1, wherein the second phase formed on the substrate surface is composed of at least one phase of zirconia, stabilized zirconia, yttria, magnesia, and lanthanum aluminate. Bicrystal substrate as described. 6. Heating to a temperature below the melting point of Si in a vacuum, a reducing atmosphere, or an inert atmosphere,
The bicrystal substrate according to claim 1, comprising a single crystal of Si bonded by applying pressure. 7. The bicrystal substrate according to claim 1, wherein no bonding phase is present at the bonding interface of each Si single crystal. 8. Polishing at least the bonding surface of each Si single crystal block to a flat and mirror surface, removing surface organic substances and surface oxides, and then bonding and heating each Si single crystal block. Place the block in a furnace and heat it in an atmosphere where no oxide is formed on the surface of the Si single crystal block, such as in a vacuum, in a reducing atmosphere, or in an inert gas atmosphere. Manufacture of a bicrystal substrate, which consists of applying pressure in a direction perpendicular to the bonding surface, cutting and polishing the bonded Si single crystal block to the desired plane orientation and dimensions of the substrate. Method.