JP3155641B2 - Superconducting tunnel junction device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting tunnel junction device, and more particularly, to an SIS junction, a Josephson junction, a Josephson computer, a mixer, and a SQUA.
The present invention relates to a superconducting tunnel junction device that can be used not only for an ID, a sensor, a switching element, and the like but also for a system incorporating the device.

[0002]

2. Description of the Related Art In recent years, copper-containing composite oxide superconductors having an extremely high superconducting transition temperature have been discovered and attracted attention. As a typical material, YBa ₂ Cu ₃ O ₇ (Y
_{BCO), Bi 2 Sr 2 Ca} n Cu n + 1 O y (n = 0,1,
2 Bi-based) and _{Tl 2 Ba 2 Ca n Cu n} + 1 O y (n =
0, 1, 2 Tl system). With the discovery of these materials, the superconducting state can be obtained with inexpensive liquid nitrogen or a simple cooler without using expensive liquid helium as a cryogen. Is coming out.

[0003] Various superconducting tunnel junction devices using these copper-containing composite oxide superconductors have been proposed. Among them, the one known as a Josephson junction is almost entirely composed of crystal grain boundaries. This is a bonding of weak coupling using This grain boundary Josephson junction is a large number of naturally generated grain boundary junctions, and SQUIDs have been prototyped using these. In a stacked superconducting tunnel junction and a Josephson junction, a junction having a barrier layer between an upper electrode and a lower electrode is generally used, and various methods have been tried for this type. For example, a composite oxide superconductor containing copper is used for the lower electrode layer, a conventionally known metal-based superconductor for the upper electrode layer is used, and a lower layer is used for forming the upper electrode for the barrier layer. There is an oxide superconductor-degraded film-metal superconductivity type which attempts to utilize the deterioration of the interface between the electrode and the electrode.

In addition, an oxide superconductor-deteriorated film-metal SIN, in which the upper electrode of this type is made of a simple metal such as Au.
There are types. Further, a full-fledged SIS type of oxide superconductor-oxide barrier layer-oxide superconductor using an oxide for the barrier layer has been devised. As a material system which has been tried in this SIS type, a YBCO system is used for a superconducting layer and PrBa ₂ Cu ₃ O ₇ is used for an oxide barrier layer, and Bi ₂ Sr ₂ CaCu ₃ is used for a superconducting layer. Although there is an oxide barrier layer using Bi ₂ Sr ₂ CuO _y using O _y , none of them is a good SIS element or a junction type Josephson element.

[0005]

In the above-mentioned conventional Josephson junction using a superconductor crystal grain boundary, it is difficult to artificially make the junction, and the junction is eventually made up of a plurality of specially manufactured different types. The average of the junction was increased, and the noise was increased, which was not suitable for application. Moreover, since the conventional superconducting tunnel junction of the conventional example, the conventional metal-based superconductor is used for the upper electrode layer in the Josephson junction, the operating temperature eventually becomes lower than the transition temperature of the metal-based superconductor, Liquid helium was needed as a cryogen. In addition, SIN using metal for the upper electrode layer
The element type was inferior to the SIS type in element characteristics.

[0006] In the most preferred conventional example of the SIS type,
When PrB ₂ Cu ₃ O ₇ is used for the oxide barrier layer, B
Also when i ₂ Sr ₂ CuO _y is used, the barrier layer becomes a semiconductor or a metal, resulting in an SNS type junction, and a good SIS type junction having an insulating barrier layer has not yet been obtained. Absent. In addition, MgO is used for the barrier layer.
Although the use of other oxides such as these has been studied, when using these barrier layers, the difference in crystal structure between the composite oxide superconductor containing copper of the electrode layer and the barrier layer, and the film formation At present, good interfaces have not been obtained due to differences in temperature. Accordingly, it is an object of the present invention to provide a superconducting tunnel junction device that can utilize liquid nitrogen or a simple cooler. It is another object of the present invention to provide a superconducting tunnel junction device having a good junction interface.

[0007]

The above object is achieved by the present invention described below. That is, the present invention provides a superconducting device having two superconducting electrodes sandwiching the barrier layer, the superconducting electrode and the barrier layer has the general formula _{Ln 1-x Ca x Sr 2} Cu 3-x M y O 6 + z (Where Ln is Y or a lanthanoid element, and 0 ≦
x <0.6, 0.05 <y ≦ 1 and 0 <z <2,
M is Li, Al, Ti, V, Cr, Fe, Co, Ga,
Ge, seen including Mo, W, and a composite oxide having the composition represented by a is) any element Re, and, in the general formula
X of the barrier layer is larger than the value of x of the superconducting electrode part.
Is a superconducting tunnel junction device characterized in that it is also small . Further, a second aspect of the present invention is a method for manufacturing a semiconductor device, comprising the steps of:
In a superconducting device having two superconducting electrodes,
Conductive electrode and the barrier layer has the general formula _{Ln 1-x Ca x Sr 2} Cu 3-y M y O 6 + z ( where Ln is Y or lanthanide elements, 0 ≦
x <0.6, 0.05 <y ≦ 1 and 0 <z <2,
M is Li, Al, Ti, V, Cr, Fe, Co, Ga,
Ge, Mo, W, or Re)
Including a composite oxide having the composition shown, further comprising a superconducting electrode
The element of Ln of the barrier layer is different, and the Ln of the barrier layer is
Pr, and M of the superconducting electrode portion and the barrier layer is T
i, V, Fe, Co, Ga, Ge, Mo, W, and Re
A superconducting tunnel characterized by being one of the elements
Device.

More preferred embodiments of the superconducting tunnel junction device include the following embodiments. (1) The superconducting tunnel junction device according to the second aspect , wherein x in the barrier layer is smaller than x in the superconducting electrode portion. (2) The value of y in the barrier layer is larger than the value of y in the superconducting electrode portion, and M of the superconducting electrode portion is Ti, V, F
The above superconducting tunnel junction device, which is any one of e, Co, Ga, Ge, Mo, W, and Re. (3) The difference between the composition of the superconducting electrode and the barrier layer is only the oxygen amount z, the amount of oxygen in the barrier layer is smaller than the amount of oxygen in the superconducting material portion, and the difference between the superconducting electrode portion and the barrier layer. M
Wherein the element is any one of Ti, V, Fe, Co, Ga, Ge, Mo, W, and Re .

[0009]

[Action] In the superconducting tunnel junction device of the present invention, the superconducting electrode and the barrier layer _{Ln 1-x Ca x Sr 2} Cu 3-y M y O
_Since a very stable material having a _{6 + z} composition is used, a good interface can be obtained when forming a joint.
In addition, since a material having the same or slightly different crystal structure is used for the superconducting electrode and the barrier layer, a good interface can be obtained. Furthermore, since the composition of the superconducting electrode and the barrier layer is only slightly different, the film can be obtained at substantially the same substrate temperature, and thus comes from the thermal cycle seen when an insulating layer such as MgO is formed. Almost no deterioration of the film is observed.

The material used for the superconducting electrode of the present invention includes Ln _1-x Ca _x Sr ₂ Cu _{3- y My} O _{6 + z}
The smaller the substitution amount y with Cu and the larger the oxygen amount 6 + z, the better the superconducting properties. Ca may not be doped if the amount y of substitution with Cu is small,
When y is large, doping with Ca is superior in superconductivity. As the _{Ln 1-x Ca x Sr 2} Cu 3-y M y O 6 + z large substitution amount y of the Cu in the composition material used in the barrier layer of the present invention, it oxygen 6 + z is less insulation Excellent layer properties. In addition, when the barrier layer is not doped with Ca, the characteristics as an insulating layer are excellent. Also, a good insulating layer can be obtained by using Pr for Ln.

[0011] By selecting the material in this manner, a barrier layer having high insulating properties, which cannot be obtained conventionally, can be obtained extremely satisfactorily, and the bonding property and flatness of the interface are also improved. A thin insulating layer essential for the SIS of the conductor, particularly for the junction type Josephson device, is obtained. Further, since the material system used is less deteriorated by moisture than the YBCO system, the element can be obtained with good reproducibility. Further, a superconductor using a superconducting electrode also has a high superconducting transition temperature, so that it can be used not only with liquid nitrogen but also with a simple cooler. To create the superconducting tunnel junction device of the present invention, it is possible with a general film forming apparatus such as a physical vapor deposition method or a chemical vapor deposition method,
In order to obtain a device having particularly good characteristics, a method capable of epitaxial growth while controlling the composition, such as the MBE method, is useful.

[0012]

Next, the present invention will be described more specifically with reference to examples. Embodiment 1 FIG. 1 shows a cross-sectional view of an element according to an embodiment of the present invention.
In the figure, 11 is a lower superconducting electrode, 12 is an upper superconducting electrode, 1
3 is a barrier layer, 14 is a SrTiO ₃ substrate, 15 is a drive power supply, and 16 is a current monitor.

First, a composite oxide superconductor containing copper is subjected to magnetron sputtering to form an S superconductor having a (110) plane on its surface.
An rTiO ₃ substrate 14 having a film thickness of 100 at a substrate temperature of 700 ° C.
A lower superconducting electrode was formed by epitaxial growth. Next, the barrier layer 14 is epitaxially grown on the lower superconducting electrode 11 by 5 to 20 nm by changing the target or lowering the oxygen partial pressure during film formation without changing the substrate temperature. Without changing the upper superconducting electrode 1 in the same way as the lower superconducting electrode.
2 was epitaxially grown to produce an SIS device having a cross section as shown in FIG. Then, in order to evaluate the device characteristics of the device, a gold wire as shown in FIG. 1 was attached to the upper and lower superconducting electrodes, and a current-voltage characteristic was measured at a temperature of 20 to 80 K by a driving power supply 15 and a current monitor.

Table 1 shows the evaluation results of device characteristics when the material of the upper and lower superconducting electrodes and the material of the barrier layer are changed. No. 1 in Table 1. Reference numeral 1 denotes an example of the second embodiment in which the type of Ln of the material of the superconducting electrode and the material of the barrier layer, and the substitution element, substitution amount, and oxygen amount z to Cu are controlled.
o. No. 2 is a reference example in the case where the amount of the substitution element for Cu in the material of the superconducting electrode and the material of the barrier layer was controlled. 3
No. is a reference example when the oxygen amount z of the material of the superconducting electrode and the material of the barrier layer is controlled. 4 is an example of a case of controlling the Ca substitution amount to Ln of the materials of the barrier layer of the superconducting electrodes, No. Reference numeral 5 denotes an example of the second embodiment in the case where the material of the barrier layer was prepared by completely replacing Pr of Y in the superconducting electrode.
Is a facilities example, No. References 6 and 7 refer to cases where the type of substitution element for Cu in the material of the superconducting electrode and the material of the barrier layer was changed.
This is an example.

Further, in Table 1, No. 8 to 10 show comparative examples in which a combination of materials other than the present invention was used for comparison . The evaluation of the device characteristics in Table 1 was made by comprehensively judging the presence or absence of a Josephson current, the magnitude of a leak current, noise, and the like in the current-voltage characteristics of each device. FIG.
No. in Table 1 Current at 77K of No. 1 element-
4 shows a graph of voltage characteristics. No. 1 in Table 1. The devices of numbers 1 to 7 also obtained almost the same current-voltage characteristics. From this result, it can be seen that a good tunnel junction type Josephson device was obtained according to the present invention.

No. 1 in Table 1. FIG. 3 shows a graph of the current-voltage characteristics of the device having the material composition of No. 10. From this figure, it is conceivable that the barrier layer of this device was poor and the junction was connected between the superconducting electrodes. No. 1 in Table 1. The elements Nos. 8 and 9 also obtained almost the same current-voltage characteristics. From the above results, the combination of the materials of the present invention
Lever, it can be seen that good Josephson junction Ru obtained.

[0017]

[Table 1]

Reference Example An oxide superconductor YSr ₂ C containing copper on an MgO substrate 44
u _2.8 Mo _0.2 O _7.1 was formed to a thickness of 80 nm by RF sputtering, and then processed into the shape shown in FIG. Then, gold was vapor-deposited by using a mask only at the portion where the wires were joined, and then the surface of the lower superconducting electrode was treated with argon plasma. As a result, part of oxygen on the surface of the superconducting electrode was removed, and an insulating film was obtained. The insulating film is used as a barrier layer 43,
The same material as that of the lower superconducting electrode was formed thereon to form an upper superconducting electrode 42. And the cross section of the element as a whole
Processing was performed as shown in FIG. 4, and current-voltage characteristics were measured in liquid nitrogen. As a result, substantially the same current as in FIG. 2 of Example 1 - voltage characteristics obtained, it was found that good good tunnel junction is obtained.

[0019]

(1) By combining a novel superconducting electrode material and a barrier layer material, a superconducting tunnel junction device having a good junction interface can be obtained. (2) A tunnel junction device using a high-temperature oxide superconductor that can be used with liquid nitrogen or a simple cooler is obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 6 is a graph showing device current-voltage characteristics of No. 1.

FIG. 10 is a graph showing device current-voltage characteristics of Comparative Example 10;

FIG. 4 is a cross-sectional view of a device according to another embodiment of the present invention.

[Explanation of symbols]

11: Lower superconducting electrode 12: Upper superconducting electrode 13: Barrier layer 14: SrTiO ₃ substrate 15: Drive power supply 16: Current monitor 41: Lower superconducting electrode 42: Upper superconducting electrode 43: Barrier layer 44: MgO substrate

Continuation of the front page (56) References JP-A-3-205882 (JP, A) JP-A-3-131075 (JP, A) JP-A-4-175224 (JP, A) JP-A-4-275918 (JP) , A) Physica C, vol. 196, no. 1 & 2, p. 141-152 (1992) (58) Field surveyed (Int. Cl. ⁷ , DB name) H01L 39/22-39/24 H01L 39/00

Claims (7)

(57) [Claims] 1. A superconducting device having two superconducting electrodes sandwiching a barrier layer, wherein the superconducting electrode and the barrier layer are formed by a general formula Ln _1-x Ca _x Sr ₂ Cu _{3- y My} O _{6 + z} ( However, Ln is Y or a lanthanoid element, and 0 ≦
x <0.6, 0.05 <y ≦ 1 and 0 <z <2,
M is Li, Al, Ti, V, Cr, Fe, Co, Ga,
Ge, seen including Mo, W, and a composite oxide having the composition represented by a is) any element Re, and, in the general formula
X of the barrier layer is larger than the value of x of the superconducting electrode part.
A superconducting tunnel junction device characterized in that it is also small . 2. The method according to claim 1, wherein y in said general formula is the same as y in the barrier layer.
Is larger than the value of y in the superconducting electrode part, and M in the general formula of the superconducting electrode part is Ti, V, Fe, Co, Ga,
The superconducting tunnel junction device according to claim 1, wherein the superconducting tunnel junction device is any one of Ge, Mo, W, and Re. 3. The difference in composition between the superconducting electrode and the barrier layer is as described above.
Only the oxygen amount z in the general formula, the oxygen amount z of the barrier layer is smaller than the oxygen amount of the superconducting material part, and M of the superconducting electrode part and the barrier layer is Ti, V, Fe, Co. , G
The superconducting tunnel junction device according to claim 1, wherein the superconducting tunnel junction device is any one of a, Ge, Mo, W, and Re. 4. It has two superconducting electrodes sandwiching a barrier layer.
Electrode and barrier layer in a superconducting device
But the general formula _{Ln 1-x Ca x Sr 2} Cu 3-y M y O 6 + z ( where Ln is Y or lanthanide elements, 0 ≦
x <0.6, 0.05 <y ≦ 1 and 0 <z <2,
M is Li, Al, Ti, V, Cr, Fe, Co, Ga,
Ge, Mo, W, or Re)
And the superconducting electrode and the barrier layer have different elements of Ln, Ln of the barrier layer is Pr, and M of the superconducting electrode portion and the barrier layer is T
i, V, Fe, Co, Ga, Ge, Mo, W, and Re
A superconducting tunnel junction device , characterized in that it is any one of the above elements. 5. The method according to claim 1, wherein x in said general formula is x in said barrier layer.
The superconducting tunnel junction device according to claim 4 , wherein is smaller than the value of x of the superconducting electrode portion. 6. In the general formula, y is the direction of y of the barrier layer.
Is larger than the value of y in the superconducting electrode portion, and the superconducting electrode
Where M is Ti, V, Fe, Co, Ga, Ge, Mo,
The element according to claim 4, which is any one of W and Re.
Superconducting tunnel junction device. 7. The difference in composition between the superconducting electrode and the barrier layer is as described above.
Only the oxygen amount z in the general formula, and the oxygen amount of the barrier layer
z is smaller than the oxygen content of the superconducting material and
M of the conductive pole portion and the barrier layer is Ti, V, Fe, Co, G
a, Ge, Mo, W, or Re
A superconducting tunnel junction device according to claim 4.