JPH013011A - Superconducting film and its manufacturing method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a superconducting film having a high critical temperature (Tc),
In particular, the present invention relates to a superelectric 4 film in which rare earth oxide films are laminated and a method for manufacturing the same.

[Conventional technology]

Conventionally, the LazCuOi system, which is a composite oxide of lanthanum (La) and copper (Cu), has been used as a material with a superconducting critical temperature (Tc) exceeding 30K, and was published in Journal of the Physical Society of Japan No. 42.
Vol. 2, p. 208 (1987). In addition, Ba-
Zeitschli, Futofuyu Physique B-Condensed Matter 64° 189 (1986) (Z, Phys,
B-CondensedMatter64,189 (
1986)).

[Problem to be solved by the invention]

The above-mentioned conventional technology does not give consideration to controlling the long-period ordered structure of the oxide material, and has a problem in that the Tc is low.

An object of the present invention is to provide a superconducting film having a high Tc by imparting long-period ordered structures to various rare earth oxides, improving the crystallinity of the oxides, and a method for manufacturing the same.

[Means to solve the problem]

The above purpose is to have a crystal structure similar to a perovskite crystal structure and a 1M element (M is Na, K).

Be, Mg, Ca, Sr, I3a, Sc, Y, Ti.

Zr, V, Nb, In, Sn, TI, Pb, Bi.

This is achieved by alternately laminating oxide films of Pa, Ir, and at least one of the rare earth elements with atomic numbers 57 to 71) and copper oxide films to give the oxide a long-period regular H structure. .

Figure 1 shows a unit cell of a perovskite structure.

KzNiF4 type, S r 3T i zOr type, 5ra
TiaOt.

This is the basic structure of the type and Bi+TiaO1z type structure. As can be seen from Figure 1, the perovskite structure is a stack of two layers: ff1 (A layer) consisting of a pair of 3 M atoms and 1 0 atom, and a layer (BU) consisting of a pair of 2 Cu atoms and 1 0 atom. It has 41 buildings. A layer and BWJ in Figure 1
By artificially stacking them, it is possible to obtain a material with a crystal structure similar to a perovskite crystal structure with a periodic structure. Furthermore, it is also possible to layer the A layer and BWj in FIG. 1 with an oxide film having a perovskite crystal structure in a blue color.

An example of this is an oxide in which AzCuOa and L2Cu04 are stacked. Here, A and L mean different types of rare earth elements. The rare earth oxide in which A2Cu0a and LzCuO4 are laminated has Tc due to the same periodic length of the vL layer.
is different, and when the period is less than 30 Å, Tc of more than 40
becomes. Further, the Tc of the rare earth element oxide film differs depending on the type of rare earth element occupying the oxide.

The Tc value of the superconducting film is determined by the crystallinity and periodicity of the oxide,
and depends on the amount and location of oxygen vacancies.

Tc of oxide layered with AzCuOa and BzCuOa
The shorter the period, the higher the Tc. This suggests that the interaction between electrons and the lattice involved in superconductivity greatly influences the value of Tc.

In addition, superconductivity is related to electrons near the interface and lattice strain, and when the period is short and the number of interfaces is large, lattice strain occurs near the interface and the electron (especially 4f electrons of rare earth elements)-lattice interaction changes. Tc increases.

When focusing on the Cu-0 atom pair, the value of Tc also strongly depends on the distance between the Cu-0 atom pair. Therefore, the long-period regular structure (
The value of Tc can be controlled by artificially creating the LRO (hereinafter abbreviated as LRO).

[Effect]

ArcuOa (A is a rare earth element) is known to have superconducting properties. In order to improve the crystallinity of A zc u Oa, Lx, which has a different lattice constant from A2Cu0a,
Cu Oa (L is a rare earth element) is laminated, and AzCu
Epitaxially grow Oi and LzCuO4. Ar
In the single-phase film of only cu04, the growth direction changes as the film thickness increases, and the crystal plane that grows parallel to the substrate surface changes with the film thickness. On the other hand, in a rare earth oxide film that is a laminated film of A2CuO4 and L2CuOt, the growth crystal plane does not vary depending on the film thickness, and the growth direction is almost constant. Therefore A
It is thought that the stacked film of zCuOi and LzCuO+ has fewer defects and less lattice disorder, and therefore scatters less electrons due to lattice vibration. In addition, AzCuO with high periodicity
The laminated film of t and L2Cu04 is A2CuO4 and L2C
Since lattice vibrations are relaxed near the stacked interface due to lattice strain at the uO+ interface, the amplitude of lattice vibrations is smaller than in a single-phase film. Furthermore, since rare earth oxides with LRO corresponding to the stacking period have a periodic potential in the stacking direction,
Electrons flow through the low potential section. This can be achieved by changing the distance between the films made of Cu-0 atom pairs and artificially creating a low potential region through which conduction electrons flow.

Rare earth light as a film between Cu-0 atom pairs? In addition to Na+ K, Be, Mg, Ca, Sr, Ba.

Examples include those consisting of Sc, Y, Ti, Zr, V, and Nb.

〔Example〕

The present invention will be explained in detail below. The KzNiFa type exists as a stable phase shown in FIG.

In order to produce perovskites such as 5raTi207 type, S riT i 3010 type, and BiaTia○12 type structures, or materials in which the A layer and BWI are artificially laminated, the A layer and B layer are mixed using the MBE method or the sputtering method.
There is a method of laminating WJs in an artificial period.

Table 3 shows the highest values of Tc when copper oxide and M atom oxide are laminated with a film thickness ratio of 1:1 and a copper oxide film thickness of 6 layers using a vapor deposition or sputtering method. The conditions and sputtering conditions are shown in Tables 1 and 2, and the oxygen gas pressure during vapor deposition was IX 10-3 Torr or less, but even if a single M atom was vaporized in an acid atmosphere, a periodically laminated film could not be formed. It can be made.

Table 1 Vapor deposition conditions Table 2 Sputtering conditions Table 3 What are the substrates used in vapor deposition or sputtering methods?

A Q 203. Whether it is MgO, Zroz, or Si, a single crystal substrate is more advantageous than a polycrystalline substrate in that films can be grown while being stacked in a specific direction.

Among the film growth conditions, particularly important conditions are the substrate temperature and oxygen gas pressure or oxygen partial pressure. When the substrate temperature is 300° C. or lower, the crystal structure of the film becomes close to amorphous, and a laminated film grown in a fixed direction is not formed.

Oxygen gas pressure during vapor deposition or acid V2 during sputtering
Partial pressure affects the Tc of the membrane. As an example, the 6th
There is a diagram. In Figure 6, if the amount of oxygen is too small, a large amount of oxygen defects will occur in the film, and the periodic structure will be disturbed, resulting in T
c becomes significantly lower. -However, a small amount of oxygen defects are Tc
This has the effect of increasing the critical current density (Jc) without reducing the current density. In Figure 6, the lower the oxygen partial pressure, the lower the electrical resistance, and the Jc of the film fabricated at 1 mTorr is 5000 A/ci.
On the other hand, the J c of the film produced at 10 mTorr was less than 1000 A/a.

The Tc values shown in Table 3 are calculated using the evaporation conditions shown in Table 1.
It shows the Tc of a film (total thickness of about 0.5 μm) in which oxides of O and M atoms are alternately stacked on an MgO substrate at a film thickness ratio of 1:1. The periodic structure was confirmed by small-angle X-ray diffraction experiments. Therefore, Cu is present in the KjJ film with the TC shown in Table 3.
It can be seen that the 1x consisting of the -〇 atom pair and the 1x consisting of the M-〇 atom pair grow alternately and periodically. Also, X in the high angle area
A peak due to a structure similar to a perovskite structure was observed in the line diffraction pattern. The Tc of the film sputtered (period: 12) using the oxide target shown in Table 3 and the Cu2O or CuO target alternately under the conditions shown in Table 2 is almost the same as the Tc shown in Table 3, and the difference is ± It was within the range of 5%.

It was found that when the thickness of the CuzO film was kept constant for six people and the thickness of the oxide of M atoms was increased, Tc depended on the thickness of the oxide of M atoms. For example, I) b.

For Bi and TQ, TO with a cycle of 12 people and a film thickness ratio of 1:1.
<100K, whereas the film thickness of Cu2O was P by 6 people.
If the film thickness of b, Bi and TQ oxides is 25 Å or more, T
, c) 100K from FIG. 2. The reason for this increase in Tc is the long-period ordered structure (LRO)
It is thought that as the period becomes longer, the distance between the Cu-0 pairs becomes longer, so the potential of the conduction electrons flowing through the Cu-0 pairs becomes lower, and the electrons can move stably. On the other hand, even if the film thickness ratio is changed as shown in FIGS. 4 and 5, Tc does not exceed 100K. In FIGS. 4 and 5, the laminated films are AzCu04 and LzCuO4, so even if the film thickness ratio and periodicity are changed, the distance between the layers of Cu-○ atom pairs hardly changes. Therefore, it is considered that Tc does not increase. This will be explained with reference to FIGS. 10, 11, and 12. As shown in Figure 10, CuzO
When stacking CuzO and BizOa, CuzO (symbol 20) and BizC)a (symbol 30) are grown on an MgO substrate (symbol 10), and the periodic structure is confirmed by X-ray diffraction experiments. We confirmed that it has a long period regular structure (LRO) with units of .

Furthermore, as shown in FIG.
O4 (number 40) and LzCuO number (number 50) were laminated alternately and LRO was obtained in the X-ray diffraction experiment as in the case of Fig. 10.
It was confirmed. Considering the Cu-0 atom pair and the M-0 atom pair in FIGS. 10 and 11, in FIG.
Only Cu-0 atom pairs exist in u 20, and only Bi-○ atom pairs exist in BizOa, and Cu-0 atom pairs, B1-Cu atom pairs and B -i atom pairs exist at the interface between CuzO and BizOa.
There are three types of −0 atom pairs. In Figure 11, AzCu
Since the O4 film and the Lzcuo film are each based on the perodiskii 1-structure, if we focus on the Cu-0 atom pair, we can think of it as a stacked film as shown in Figure 12, which is a combination of AzCuOa (40) and L2CuOa (50). Then Cu-
It can be broken down into 0N (symbol 60), A-0 layer (symbol 70), Cu-0 layer (symbol 80), and L-0 layer (symbol 90). Therefore, Cu −0 atom pair and M
If we consider a layer of -○ atom pairs, the result will be as shown in Figures 10 and 12, and we can change the thickness of the layer of Cu-○ atom pairs and change the distance between the eyebrows of Cu-0 atom pairs. If you want to
It can be seen that the layers can be stacked as shown in Figure 0.

The phenomenon in which Tc increases by increasing the thickness of the oxide of M atoms as shown in FIG. 2 was also observed for the elements marked O in Table 3.

Figure 3 shows Cu2O and pb○z, Bi20a or T.
The relationship between the film period and Tc is shown when Q203 is laminated at a film thickness ratio of 1:1 using the deposition conditions shown in Table 2. The periodicity is the period of the long-period regular structure determined from the X-ray diffraction pattern, and is 11.4 Å or more. Figure 3 shows 11.4~
It showed Tc for 55 people's cycles, but Tc for 5 cycles of 100 people.
It was confirmed that c > 77K. The reason why Tc increases as the period becomes longer is considered to be due to the increase in the interlayer spacing between the Cu-- atomic pairs, as shown in FIG.

FIG. 4 is a temperature (T)-electrical resistance (ρ) curve of a rare earth oxide film in which LazCuOt and LuzCuO4 are alternately laminated at the lamination period. A rare earth oxide film having the characteristics shown in FIG. 4 was produced by the MBE (molecular beam epitaxial) method. The manufacturing conditions are shown in Table 1.

Maintain the temperature at approximately 100°C or more higher than the substrate temperature during vapor deposition (30°C
min) and the degree of vacuum after slow cooling was defined as the ultimate degree of vacuum. 1st
Under the vapor deposition conditions shown in the table, LazCuOa and L u Cu Oa
are laminated alternately with a film thickness ratio of 1:1, the electrical resistance ρ is
As shown in FIG. 4, the shorter the 5th cycle, the higher the Tc, which was 78 for D=25 people. Furthermore, when D becomes short, the change in ρ near Tc becomes rapid, and no increase in ρ is observed at temperatures just above Tc. The reason why the relationship between Tc and D is as described above is related to the crystallinity of the oxide.

- When the film thickness of the layer increases, the film grows in a crystal orientation that deviates from the preferential growth direction, so defects (dislocations and vacancies) due to the difference in growth orientation become present in the film. These defects scatter electrons involved in superconductivity, causing a decrease in Tc. FIG. 4 shows a laminated film of La2CuO+ and LuzCuOa, and when various rare earth oxides were laminated on LazCuOa, the Tc results were as shown in Table 4. Tc in Table 4 is based on the oxide film thickness ratio of 1:1. D=5
These are the results obtained by the electrical resistance method for a film manufactured by 0 people using the manufacturing conditions shown in Table 1.

Table 4 From the results shown in Table 4, Tc exceeded 70K for Ce, Pr, and Ha. L a 2Cu prepared in Table 1
04 and LuzCuOa laminated film, LazCuO4 is 12
．． When changing the film thickness of Lu2CuO+ as 5 people,
ρ is as shown in Figure 5. In Figure 5, d LazCu
O4:d The value of LuzCuO4 (d: single phase film FA) is shown. As the film thickness of Lu2CuO< increases, Tc decreases to 20 for dLuzcuo+=125 people.

From this, it can be seen that as the thickness of one of the rare earth oxides increases, Tc decreases.

FIG. 6 shows ρ when changing the oxygen gas pressure (PO2) during vapor deposition. When increasing the oxygen gas pressure.

Tc also increases, and at P ox = 10 mTorr Tc
When e Po z = 95 is low, there is a shortage of oxygen atoms in the rare earth oxide, and the number of oxygen atoms arranged in an octahedron is reduced, causing disturbance in the lattice vibration, and when Tc becomes low, Conceivable. Figure 7 shows L a 2Cu 0
4 and LuzCuOt product P7I film with Cu and Ag.

ρ of a laminated film substituted with Au, Pd, and Pt is shown.

When Cu is replaced with Ag, Tc increases, and Tc = 85
Reached K. In addition, rare earth oxides substituted with Au, Pd, and Pt showed an increase in ρ and a decrease in Tc compared to Cu.

Figure 8 shows (L a X) ZCu 02 and La2CuO
ρ of the rare earth oxide layered with i is shown. The film J ratio is 1:
1, and the stacking period is 25 people. The amount of element X added was La:x=9:L. In addition, the 8th (・4) shows the ρ- of LazCuO4 and LLIZCIJO4 for comparison.
A T curve was shown. The Tc of the rare earth oxide in this system is 75 or more, and (Luo, 45Bao, oa)2cuo<
The Tc of the L a 2Cu 04 rare earth oxide is approximately 1.
It became OOK.

Figure 9 shows the ρ of a rare earth oxide layered with Cu and La stacked at a film thickness ratio of 1:1 in an atmosphere with an oxygen gas pressure (Pot) of 10 mTorr.The shorter the period around 0, the higher the Tc. Then, D=25 people and Tc=60. Further, the value of Tc depends on PO2, and as shown in FIG. 6, even in a rare earth oxide film in which Cu and La are laminated, Tc tends to decrease as PO2 is decreased.

〔Effect of the invention〕

According to the present invention, a high Tc film can be obtained by stacking oxide films having a long-period regular structure whose basic structure is similar to a perovskite structure. Further, by adding a long-period structure to the rare earth oxide and controlling crystallinity and lattice defects, a material having a Tc of 30 or more can be obtained.

In particular, the type of rare earth element with two periods, the metal element in the M element hehedral structure, and the point defects of oxygen atoms greatly affect Tc, but as shown in the example, Tc of 77K (L-N2 temperature) or higher Since it can be obtained, it can be applied as a superconducting film using liquid nitrogen as a cooling medium. As an application of this, L −
A Josephson device using NZ as a coolant, a superconducting transistor, etc. can be considered.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing the unit cell of perovskite structure,
Figure 2 is a characteristic diagram showing the relationship between Tc and the film thickness of Pb0z+Bi20a+TQOs, Figure 3 is a characteristic diagram showing the relationship between Tc and period, and Figures 4 to 9 are characteristic diagrams showing the relationship between Tc and the film thickness of Pb0z+Bi20a+TQOs. The electrical resistance-temperature characteristic diagrams and FIGS. 10 to 12 are structural diagrams of the laminated film. 1...0 atom, 2...Cu atom, 3...M atom,
10...MgO substrate, 20...=Cu20,3O-Bi
203.40"・AzCu○4.50- LzCu 0
4.60...Cu-0 layer, 70-A-0 layer, 8O
-Cu-0 layer. 90...L-0 layer. 1st Mouth J...-/''I original) 20th PbOz, 8izOJ, Ti1tθjsl!!4 3rd m Period D (A recommendation 40: Counterfeit T (K) 5 A Mild T (K) Half b Prison star 1fT ( Arrival chant 'I's 51st sho T (ni) 8th Kaon Hiroshi 7/Ni no prison 9 Figure LI: i T (K) 10th sentry 11 country +z O

Claims (10)

[Claims] 1. A superconducting film characterized in that layers made of Cu--O atomic pairs and layers made of other oxides are alternately laminated. 2. 2. The superconducting film according to claim 1, wherein the layer consisting of Cu--O atom pairs and the other oxide layer have a long-period structure with a period longer than 11.3 Å. 3. In claim 1, the film made of an oxide other than the film made of Cu-O atom pairs includes Na, K, Be, Mg.
, Ca, Sr, Ba, Sc, Y, Ti, Zr, V, Nb
, In, Sn, Tl, Pb, Bi, Po, Ir, and rare earth elements with atomic numbers from 57 to 71. 4. In claim 1, at least a part of Cu is fcc
A superconducting film characterized by being substituted with an element and laminated with other oxides. 5. A superconducting film characterized in that A_2CuO_4 films and L_2CuO_4 films (A and L are different rare earth elements) are alternately laminated. 6. In claim 5, at least a part of Cu is Ag,
A superconducting film characterized by being replaced with at least one of Au, Pd, and Pt. 7. 6. The superconducting film according to claim 5, wherein at least a portion of the rare earth elements A and L are replaced with at least one of Sr, Ca, Ba, and Mg. 8. 1. A method for producing a superconducting film, which comprises producing a superconducting film composed of a layer made of Cu--O atomic pairs and a layer made of other oxides by molecular beam epitaxial method (MBE method). 9. A superconductor characterized by fabricating a superconducting film with a long-period structure by alternately stacking superconducting films composed of Cu-O atomic pairs and layers of other oxides by sputtering. Membrane manufacturing method. 10. A method for producing a superconducting film, characterized in that when A_2CuO_4 layers and L_2CuO_4 layers (A, L are different rare earth elements) are alternately laminated, they are grown in an oxygen atmosphere using a molecular beam epitaxial method (MBE method).