JPH07114297B2 - Oxide superconductor junction and method for forming the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconductor junction used in Josephson devices and a method for forming the same.

[0002]

2. Description of the Related Art Generally, a superconducting / normal conducting / superconducting (SNS) type device or a superconducting / insulating barrier / superconducting (SIS) type device is used as a superconducting device. First, the SNS type element uses weak coupling (proximity effect) of the superconducting wave function exuding in the normal conduction layer. In this case, the Josephson junction is formed by controlling the film thickness of the normal conductive layer to about the coherence length ζ _N in the normal conductive layer (about several tens nm in 2201 system). Moreover, in the SIS type element, the tunnel effect in the insulating barrier of the superconducting wave function is used. For example, the thickness of aluminum oxide used as a barrier in the conventionally used Nb-based Josephson element is about 2-3 nm.

In the formation of such a superconducting element, the wave function of the above Nb-based superconductor is isotropic and the coherence length ζ is about 50 nm. That is, SN
Or aside from the local superconductivity near the SI interface,
Whether the thin film is a polycrystal or a single crystal, if the superconducting property of Nb is secured at least in the range of 50 nm from the interface, it can operate as an element.

By the way, the critical temperature Tc of the Bi oxide superconductor is about 110K in the 2223 series, which is 20K higher than that of the Y series superconductor known as a high temperature superconductor. Therefore, it has a large temperature margin for the operation of the Josephson element at the liquid nitrogen temperature, and is expected to be put into practical use. However, in forming a superconducting element, there are difficulties in forming an element peculiar to an oxide superconductor. That is, the coherence length of the oxide superconductor is extremely short and anisotropic. In the case of the Bi-based oxide superconductor described above, the coherence length in the c-axis direction is about 0.2 nm, and the coherence length in the ab-axis direction is about 2 nm. Therefore, in order to obtain a good Josephson device using this superconductor, a device forming method that can maintain a good superconducting property up to the SN or SI interface is necessary. The necessity of synthesizing the entire device as a heteroepitaxially grown single crystal is suggested.

As a conventional example of joining using such a Bi-based oxide superconductor, there is an applied structure as shown in FIG.
Physics Letters (Applied Physi)
csLetters) 1990, 56, 1469-
There is a stacked SNS junction listed on page 1471.
In the case of this example, a c-axis oriented superconducting film and an intermediate layer of a normal conducting film are formed on the MgO substrate at 300, 60 and 200 nm, respectively.
The layers are stacked to form a 20 μm × 40 μm SNS type element. This device observes the AC Josephson effect (Shapiro step) for microwaves at 4.2K. Further, in this conventional example, it is described that the superconducting wave function is weakly coupled by the proximity effect via the intermediate layer of 60 nm. Such characteristics can be evaluated in terms of the possibility of applying the oxide superconductor to the device.

[0006]

However, this B
The DC Josephson effect on the magnetic field in the i-based oxide superconductor junction has not been reported. D for magnetic field
C Josephson effect is a magnetic quantum interference device (SQUID)
This effect depends on the homogeneity in the junction surface of the device, that is, the effective area that contributes to the Josephson coupling in the device, which is important for application to the device. However, if the film thickness of the intermediate layer cannot be uniform due to, for example, the surface flatness of the substrate or the phase difference in the film due to crystal disorder, etc., the intermediate layer (normal conductive layer)
A clear DC Josephson effect is not observed due to the Josephson coupling due to the pinhole leak.

Such incomplete crystal growth characteristically occurs when a Bi-based superconductor is directly grown on a substrate having four-fold symmetry such as a MgO, SrTiO ₃ substrate. This is because the Bi-based superconductor forms a domain structure in the plane, and grain boundaries are formed in the plane, which can be a pinhole path. Moreover, many different phases are likely to be contained in the film, which causes the interface flatness and the homogeneity of the intermediate phase to be disturbed. Therefore, in many cases, the operation as an SNS type element is incomplete, and it is not possible to observe the Fraunhofer type critical current interference pattern due to the DC Josephson effect. Even if the characteristics as an SNS type element are observed, the SI
Successful S-type tunnel junctions have not yet been reported.

Considering the anisotropy of the Bi-based oxide superconductor, it is considered advantageous to form the junction in the ab axis direction, which has a long coherence length. But especially Bi
The oxide superconductor has a strong anisotropy even in crystal growth, and the crystal growth rate is very fast in the ab plane. Therefore, when the normal (100) plane of MgO or SrTiO ₃ is used, the ab plane is exposed to form a c-axis oriented film. Also SrTiO ₃
When the (110) plane is used, epitaxial growth occurs in which the ab plane is inclined at an angle of 45 °, but + 45 ° in the film,
In addition to the formation of grain boundaries inclined at -45 °, the surface of the film is extremely uneven and it is difficult to form an element.

[0009]

As a result of intensive studies in view of the above problems, the present inventor has found that grain boundaries due to in-film domain structure are included by using a crystal growth method using steps on a single crystal substrate. Without the inclusion of heterogeneous phase in the film
It has been found that sufficient flatness of the interface can be ensured and a bond can be formed in the ab axis direction with a long coherence length. It was also found that providing a buffer layer on the substrate enables more reliable bonding formation. Thereby, SNS having a clear DC Josephson effect
Alternatively, it became possible to form a SIS junction.

That is, in the present invention, steps having a height of 0.38 nm or an integral multiple thereof are arranged in the [110] direction,
SrT having a terrace with a width of at least 0.54 nm
Using the (100) plane of the iO ₃ single crystal, Bi ₂ Sr ₂ CaCu ₂ O _x (2212) as the lower and upper superconducting layers,
Or Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x (2223)
And Bi ₂ Sr ₂ CuO _x (220
1), the crystal directions of the respective layers are uniquely aligned, and the lower layer and the intermediate layer, and the intermediate layer and the upper layer are formed due to the substrate shape. A favorable DC Josephson effect can be obtained by an oxide superconductor junction characterized by forming a laminated structure in which in-plane Cu-O layers are in contact with each other. Further, the reliability can be further improved by forming the Bi ₂ Sr ₂ CuO _x buffer layer before forming the superconducting layer. In this junction, Bi ₂ Sr _2-z M _z Cu _1-y N _y containing III a group element M and Va group element N instead of Bi ₂ Sr ₂ CuO _x of the intermediate layer is used.
It may be used O _x. Further, in these oxide superconductor junctions, an inclined SrTiO ₃ substrate in which the normal line of the substrate surface is inclined in the [111] direction is used. Alternatively, a substrate formed by arranging extremely shallow grooves in the [110] direction so as to form steps in the [110] direction on the (100) plane of the SrTiO ₃ substrate may be used.

Further, these junctions are formed by SrT before crystal growth.
When the iO ₃ substrate is heat-treated in an atomic oxygen atmosphere, or when the lower Bi ₂ Sr ₂ CaCu ₂ O _x layer or the Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x layer is formed, the composition of Bi at the initial growth stage Increasing the ratio results in more reliable bond formation.

[0012]

EXAMPLE 1 FIG. 1 is a sectional view of a Josephson device made of a Bi-based superconductor according to the present invention. In this embodiment, the ion beam sputtering method is used as the film forming method, and the substrate temperature during the formation of the superconducting thin film is 2212 and 2223.
The temperature is 700 ° C. when the superconducting layer is formed. During film formation, atomic oxygen is set to 2 × 10 ⁻³ Torr on the surface of the substrate, and a multilayer Bi-based superconducting thin film is formed in-situ. As the substrate 1, a (100) plane of SrTIO ₃ which was polished at about 4 ° C. in the [111] direction was used. In this case, steps having a height of 0.38 nm arranged in the [110] direction are fairly regularly formed at intervals of about 2.5 nm on the substrate surface. This is a high-resolution cross-sectional TEM and the RH of the substrate 1.
It was easily confirmed from the EED pattern. When such a substrate is normally exposed to the atmosphere, impurities easily adhere to the substrate surface. It is difficult to completely remove this impurity even if it is thoroughly washed. When the substrate 1 is placed in a vacuum chamber and heated, it firmly adheres to the surface of the substrate as a carbide, and when the superconducting thin film is formed, nuclei of different phases are formed. It becomes a formation site and causes the formation of foreign matter on the substrate surface. It is possible to oxidize and remove the carbide on the surface of the substrate by spraying atomic oxygen to the substrate 1 at about the superconducting thin film forming temperature with respect to this impurity. Such a substrate surface cleaning step using atomic oxygen is extremely effective in the element formation process, and its effect can be easily confirmed by SEM, AFM (atomic force microscope) or the like. This is also apparent from the observation result of the RHEED pattern of the substrate 1 that the reflection spots due to the carbide disappear before and after the cleaning step, and the intensity of the RHEED reflection streak increases. Furthermore, after this cleaning step, Sr
In the TiO ₃ substrate 1 or the like, a recurrence arrangement structure on the surface is observed, and surface cleaning at the atomic level is realized.

Next, two base superconductors on the substrate 1
The formation of the 212 or 2223-based superconducting layer will be described. Both the 2212 and 2223 series superconducting layers have an intergrowth crystal growth mechanism, but by increasing the Bi composition in the early stage of superconducting layer formation in order to promote this intergrowth, the repair of crystal defects is completed early and It is possible to form a good epitaxial single crystal thin film as a superconducting layer. The effect is easily confirmed by SEM, AFM and the like. In this embodiment, the chemical composition ratio of Bi (usually 2) was increased to 2.3 during the initial film growth of about 3 nm. Generally, a single layer thin film of perovskite (for example, BaTi
In the case of O ₃ ), the binding strain due to the substrate is usually about 200 nm from the substrate, but the 2212 series superconducting thin film of the present invention has a thickness of 20 nm.
Even the thin film of the above shows a good superconductivity of about 70K. From this, it is shown that the initial Bi composition adjustment of the buffer layer 3 and the superconducting layer is very effective in reducing the crystal defects in the superconducting thin film and, in turn, in obtaining good superconducting properties.

Then, an epitaxial single crystal thin film was grown to a thickness of 1.5 nm corresponding to the unit cell of the Bi-based superconductor.
Steps (for 2212 layers) are formed on the surface.
The 1.5 nm step structure is maintained on the surface of the film in order to basically perform step-flow crystal growth. After forming this base superconductor to a predetermined thickness, the intermediate layer 5,
The upper superconducting layer 6 is formed. When forming an SNS junction, it is the same as the buffer layer 3 as the normal conductive layer of the intermediate layer 22.
01 layers are used. The lower superconducting layer 4 and the intermediate layer 5 are in contact with each other via the step wall surface and the terrace surface. Similarly, the upper superconducting layer 6 and the intermediate layer 5 are also in contact with each other via the step wall surface and the terrace surface. Since the crystal growth mode of the Bi-based superconductor is the step flow as described above, the step wall surface and the terrace surface serve as the SN interface, and the crystal structure of the superconducting layer near the interface has few defects. The superconducting property near the step is good. Furthermore, the ab plane of the superconducting layer is exposed on the step wall surface (interface). That is, the structure is such that the upper and lower superconducting layers sandwiching the normal conducting layer form a junction in the ab axis direction having a long coherence length. When the (110) plane of the conventional SrTiO ₃ is used, the surface of the film is so uneven that it is almost difficult to form a bond in the ab axis direction. On the other hand, in the method using the inclined substrate according to the present invention. A bond is easily formed in the ab axis direction.

(Embodiment 2) FIG. 3 is a sectional view of a Josephson device made of a Bi-based superconductor according to the present invention. In this embodiment, a 2201 buffer layer 3 is provided on the substrate 1. In this embodiment, the ion beam sputtering method is used as the film forming method, and the substrate temperature during the formation of the superconducting thin film is 2201.
When the buffer layer 3 is formed at 550 ° C. and 2212/2223 superconducting layer, the temperature is 700 ° C. as in the first embodiment. After that, after forming the substrate 1 in the same manner as in Example 1, the buffer layer 3 is formed. On this inclined SrTiO ₃ substrate 1, 2
The 201 buffer layer 3 is constrained by the steps of the substrate 1, and the a, b, and c axes of the film have a regular crystal growth. That is, c
The axis is perpendicular to the terrace of the substrate 1, the a-axis of the film is parallel to the step edge 2, and the b-axis (direction including the matching mismatched modulation structure) is orthogonal to the step edge 2. Because of this, the film does not form in-plane domain structures. In forming the buffer layer 3, a substrate temperature of 500 to 600 ° C. is suitable. Below this temperature, the crystallinity of the 2201 layer deteriorates and many crystal defects are included. 220 above this temperature
Bi, Sr, and O, which are likely to be generated as an equilibrium phase with one layer, are generated as different phases. Therefore, the 2201 layer introduced for the purpose of reducing crystal defects and different phases in the film does not function as a buffer layer.

2201 buffer layer 3 formed here
2212 or 2223 which is continuously grown next
It suppresses the generation of different phases in the superconducting system layer and regulates the crystal orientation. However, the steps (0.3
Defects in the buffer layer 3 formed by 8 nm) remain in the buffer layer 3 as they are. The crystal defects remaining in the buffer layer 3 are automatically repaired by the intergrowth growth mechanism which is a crystal growth mode peculiar to the Bi-based superconductor when the next 2212 or 2223-based superconductor is formed. In order for the 2201 buffer layer 3 to function as a buffer, the minimum unit of the 2201 crystal lattice, about 1.2 nm, is sufficient. However, in actual film formation, a function as a buffer can be expected sufficiently by growing the film by about 5 nm in consideration of variations in migration on the substrate surface.

Next, the same junction as in Example 1 is formed on the 2201 buffer layer 3. While it was almost difficult to form a bond in the ab-axis direction, the method using the tilted substrate 1 according to the present invention easily forms a bond in the ab-axis direction, and the use of the buffer layer 3 improves reliability. It is possible to provide an element with high cost.

(Embodiment 3) Next, an embodiment of a method for forming a superconducting device having a wall surface of the step edge 2 as a joint surface according to the present invention will be described except that a tilted substrate 7 is used.

The substrate 7 shown in FIG. 4 is a normal (100) plane SrTiO ₃ substrate 7, but the junction formation portion is shallowly etched by using an ion milling method to form a valley on the substrate surface. Is. In the case of this example, a resist slit having a width of 20 μm was formed by lithography and was obliquely etched by ion milling to form a valley of about 500 nm in the center. In this case, the step interval formed on the substrate 7 is 7.5 nm,
Similar to a tilted substrate of about 3 degrees. In this way, the inclination is formed only in the element portion, and the lower superconducting electrode and the superconducting wiring portion are processed so as to use the a-plane having a sufficiently large critical current density. Can be given.

That is, the superconducting current flowing through the element is connected to the external element via the base superconducting electrode, but since the base superconducting layer is formed on the inclined substrate 1, the inclination direction is In the [111] direction, the current path in the [110] direction must always pass in the c-axis direction. The normal conduction resistance in the c-axis direction is 10,000 times higher than that in the ab-axis direction, and the superconducting current critical density is about one thousandth. The constraint on the device design increases because the maximum current density of a good superconducting device increases. However, FIG.
It becomes possible to solve this problem by the method shown in (1), and it becomes possible to integrate the superconducting elements.

(Fourth Embodiment) FIG. 5 shows an application example of the third embodiment. Using a resist stripe pattern with a width of 1 μm, ion milling treatment was performed so that the central portion had a depth of about 25 nm, and a plurality of shallow valleys were formed in the element formation portion of the substrate 7. In this case as well, the step interval on the substrate surface is substantially the same as that in the case of the inclination of 3 degrees. By forming such shallow valleys, the lithographic process is facilitated and further integration is possible.

(Embodiment 5) The stripe width of the substrate 7 is set to 50.
FIG. 6 shows an example in which the unevenness of the substrate 7 is about 0 nm and the unevenness of the substrate 7 is about 5 nm. By forming such unevenness 2201
Most of the unevenness of the substrate 7 is absorbed during the formation of the buffer layer 3. However, since the crystal growth orientation of the buffer layer 3 is restricted by the steps formed on the substrate 7, a flat single crystal thin film having a uniform in-plane crystal orientation is formed. Since the single crystal portion does not include the domain structure in the film described above, the element portion does not include grain boundaries. This has the same device structure as the conventional example described in FIG. 2, but is different in that the device does not include a grain boundary, and as a result of the buffer layer 3, there is no foreign phase contained in the film / interface. In the SNS element using this method, although the junction in the c-axis direction is used, the flatness of the SN interface is very excellent, and the complete suppression of the superconducting current Ic is observed even in the DC Josephson effect with respect to the magnetic field. It was

(Example 6) By substituting some elements in the intermediate layer 5 to make the thickness of the intermediate layer 5 much smaller, a SIS junction is formed. For example, a ₂ % of Bi ₂ Sr ₂ CuO _x
The specific resistance in the b-axis direction is about 1 milliohm at 4.2K, and in the c-axis direction is about 10 milliohms. Here, substitution of about 30% of IIIa group Y for Sr raises the specific resistance of the ab axis by about two digits, and substitution of Nb by about 10% of Cu similarly raises the specific resistance by about two digits. By performing these element substitutions, it becomes possible to increase the specific resistance of the intermediate layer 5 and to secure a sufficient barrier height with a sufficiently thin film thickness that can function as a tunnel barrier.

As described above, the effectiveness of the present invention is verified by using the ion beam sputtering method, but it is obvious that the thin film synthesizing method can be applied to any of the vapor deposition method, laser sputtering method, RF sputtering method and the like. Is.

[0025]

As described above, according to the present invention, an excellent SNS or SIS junction can be easily formed. Laminated superconducting layer The laminated structure required for device formation is epitaxially grown while reliably controlling the interface structure necessary for its operation, so that it can be reproduced well between lots or even within a single wafer. Sex is obtained. Than this,
It enables mass production and integration of devices.

[Brief description of drawings]

FIG. 1 is a diagram of a typical device structure of a stacked superconducting device according to the present invention.

FIG. 2 is a device structure diagram of a stacked superconducting device according to a conventional method.

FIG. 3 is a diagram of a typical device structure of a stacked superconducting device according to the present invention.

FIG. 4 is a diagram of an element structure showing an application example of the stacked superconducting element according to the present invention.

FIG. 5 is an element structure diagram showing an application example of the stacked superconducting element according to the present invention.

FIG. 6 is a diagram of an element structure showing an application example of the stacked superconducting element according to the present invention.

[Explanation of symbols]

1 Slanted SrTiO ₃ substrate 2 Step edge formed on a substrate 3 Bi ₂ Sr ₂ CuO _x buffer layer 4 Lower superconducting layer 5 Intermediate 2201 layer 6 Upper superconducting layer 7 Processed SrTiO ₃ substrate 8 MgO substrate

Claims (7)

[Claims] 1. A step having a height of 0.38 nm or an integral multiple thereof is arranged in the [110] direction and has a width of at least 0.
The (100) plane of SrTiO ₃ single crystal having a terrace of 54 nm or more is used, and Bi is used as the lower and upper superconducting layers.
₂ Sr ₂ CaCu ₂ O _x (2212) or Bi ₂
Sr ₂ Ca ₂ Cu ₃ O _x (2223) and Bi ₂ Sr ₂ CuO _x (2201) as an intermediate layer, the crystal directions of these layers are uniquely aligned, and the lower layer and the intermediate layer,
An oxide characterized in that the intermediate layer and the upper layer form a laminated structure such that the Cu-O layers in the a-b plane are in contact with each other through a step formed due to the shape of the substrate. Superconductor bonding. 2. The height is 0.38 nm or an integral multiple thereof.
Are aligned in the [110] direction and have a width of at least 0.
SrTiO having a terrace of 54 nm or more ₃ Single crystal
The (100) plane is used, and Bi is used as the lower and upper superconducting layers.
₂ Sr ₂ CaCu ₂ O _x (2212), or Bi ₂
Sr ₂ Ca ₂ Cu ₃ O _x (2223) as the intermediate layer
Containing IIIa group element M and Va group element N ₂ Sr
_2-z M _z Cu _1-y N _y O _x And the crystal directions of those layers
The orientations are uniquely aligned, and the lower and middle layers, and the middle and upper layers
The layer is formed through the steps due to the substrate shape,
Cu-O layers in each a-b plane contact each other
Oxide superconductivity characterized by forming such a laminated structure
Body joining. 3. Bi before the formation of the superconducting layer ₂ Sr ₂ CuO _x
A buffer layer is formed, The method of claim 1 or
2. The oxide superconductor junction according to 2. 4. The normal to the substrate surface is inclined in the [111] direction.
(100) SrTiO ₃ Characterized by using a substrate
The oxide superconductor junction according to claim 1 or 2. 5. SrTiO 3 ₃ On the (100) plane of the substrate
Extremely shallow so that steps are formed in the [110] direction
Using a substrate with grooves formed in the [110] direction
The oxide superconductor according to claim 1 or 2, characterized in that
Body joining. 6. SrTiO before crystal growth ₃ Atomic substrate
A contract characterized by heat treatment in an oxygen atmosphere
The method for forming an oxide superconductor junction according to claim 1 or 2. 7. The lower part Bi ₂ Sr ₂ CaCu ₂ O _x Layer if
Kuha Bi ₂ Sr ₂ Ca ₂ Cu ₃ O _x Dude when forming layers
And increasing the composition ratio of Bi at the initial stage of growth.
A method for forming an oxide superconductor junction according to claim 1 or 2.