JPH05221800A - Ceramic superlattice - Google Patents

Abstract

PURPOSE:To provide a ceramic superlattice prepared by a technique capable of designing a nanometer-scale ceramic superlattice, i.e., accurately controlling the cycle of the superlattice in the molecular-layer order, and to produce a superlattice structural material capable of being applied to a new device capable of extending the physical properties of a thin ceramic film from the properties inherent in the material. CONSTITUTION:At least two kinds of oxide ceramics are laminated to form a ceramic superlattice. The oxide ceramics are a perovskite crystal and grown on a substrate while maintaining a fixed relation between their crystal orientations. The thickness of the grown perovskite crystal is integral multiples of the unit lattice length of the perovskite crystal as the growth unit. At least two kinds of oxide ceramics are successively laminated while counting the intensity frequency of a reflection high-speed electron beam diffraction (RHEED) to adjust the number of the oxide layers.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic superlattice formed by laminating oxide ceramic thin films and a low temperature varistor using the same. More specifically, the present invention relates to a ceramics superlattice produced while adjusting the laminated thickness and a low temperature varistor using the same.

[0002]

2. Description of the Related Art In the development of ceramic materials, artificially designing a nanometer-scale structure can provide a quantum size effect which is not expected in conventional ceramic materials. Such an effect is also obtained by manufacturing a tunnel type Josephson junction and
It can also be applied to forming an ultra thin film of an insulator or a dielectric on a semiconductor.

However, to date, ceramic superlattices have not been synthesized by the well-controlled methods found in the fabrication of semiconductor superlattices.

Attempts have already been made to apply the functions of the perovskite type crystal, such as ferroelectricity and superconductivity, to electronic devices by thinning the perovskite type crystal oxide ceramics. However, since the conventional perovskite type crystal oxide thin film is a polycrystal, the characteristics of the bulk crystal cannot be sufficiently exhibited due to the influence of grain boundaries and the like existing in the thin film.

Further, the conventional perovskite type crystal oxide thin film is often a single layer film, and in order to change the function of the thin film, the film is made into a solid solution. The function does not go beyond bulk crystals of comparable composition.

[0006]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a technique capable of designing a ceramic superlattice on a nanometer scale. It is another object of the present invention to provide a method for physicochemically stabilizing the structure of a ceramic superlattice and a ceramic superlattice obtained by the method. That is, it is an object of the present invention to provide a ceramics superlattice prepared by precisely controlling the period of the ceramics superlattice on the order of molecular layers and a method for producing the same. It is another object of the present invention to provide a superlattice structure material that can be applied to a novel device by expanding the physical properties of the oxide ceramic thin film from those specific to the substance.

[0007]

The present invention has been made to solve the above-mentioned technical problems, and at least two kinds of oxide ceramics are used, for example, the intensity of reflection high-energy electron diffraction (RHEED). Provided is a ceramics superlattice which is characterized by being sequentially laminated while adjusting the growth thickness of each oxide layer (the number of unit phases in the layer [unit cell]) by counting the frequency. And at least 2
In a ceramics superlattice in which oxide ceramics of different types are laminated, the oxide ceramics are perovskite type crystals, and they are grown on a substrate while keeping their crystal orientations in a constant relationship with each other. The ceramic superlattice is characterized in that the growth thickness of the layer is an integral multiple of the unit lattice length of the perovskite type crystal as a growth unit.

The perovskite type crystal oxide is S
rVO _3-X and SrTiO _3-Y (0 ≦ X, Y ≦ 0.5) are preferable. Also, the ceramic superlattice has a temperature of 1
It can also be used as a low temperature varistor operated at 00K or less. Further, the substrate is preferably a single crystal or single crystal thin film of perovskite type crystal oxide. The single crystal substrate of the perovskite type crystal oxide is preferably SrTiO ₃ .

[0009]

According to the present invention, a method is provided in which the function and characteristics of the oxide ceramic thin film can be made more than the characteristics peculiar to the substance. That is, by making each single crystal phase composed of a plurality of substances a superlattice structure, the physical properties and functions of the thin film could be made stable. Further, the physical properties of the oxide ceramics thin film can be expanded from those specific to the substance, and can be applied to a new device. According to the present invention, by laminating oxide ceramic crystals and forming a superlattice, the composite ceramic can be made into a single crystal body and can be physically and chemically stabilized, and its function and physical properties can be made stable. That is, in order to produce a superlattice composed of a single crystal body, a substance having the same crystal structure is selected as a material forming the superlattice to minimize the discontinuity of the crystal structure at the interface, that is, only the lattice constant is It is possible to reflect the characteristics such as ferroelectricity, ordinary superconductivity, and the like, which are characteristic of this crystal structure, in the superlattice by suppressing the difference between the above and particularly by selecting a substance having a perovskite type crystal structure.

Hereinafter, in the present specification, the present invention will be described as S
Examples of rTiO ₃ and SrVO ₃ will be described.
It goes without saying that the present invention is not limited to this, and can be similarly produced and applied to superlattices of other kinds of oxides and ceramic materials. In the description of the present invention, for the sake of convenience, it has been stated that the layer thickness is adjusted by counting the intensity frequency of reflection high-energy electron diffraction (RHEED). However, the present invention is not limited to the other film thickness measuring methods. It can also be implemented and is not bound by these methods.

In order to further add a quantum effect to the characteristics of the superlattice, the period of the superlattice, that is, the thickness of each layer is
For example, the unit cell length of a perovskite type crystal having a stable minimum size, that is, {100} plane, {110}
Fabrication is performed such that the surface spacing of the surfaces such as the surface and the {111} surface, which can be represented by the lowest Miller index, is an integral multiple thereof. That is, the period of the ceramic superlattice is
It must be precisely controlled and produced on the molecular layer order.

In order to accurately control the superlattice period by using the unit lattice length of the perovskite type crystal as a unit, the reflection high-energy electron diffraction observed in the growth of the single crystal thin film (hereinafter, in the present specification, , RHEED) of the pattern is utilized to vibrate with the unit lattice length of the growth direction as a period. That is, by counting the RHEED intensity frequency during growth of the thin film (that is, the number of peaks appearing in the profile in which the RHEED intensity changes with respect to the deposition time), it is possible to control the integral multiple of the unit lattice length (n ≧ 1) ..

For example, (SrVO _3−X ) (a-axis length = 0.385 nm) having lattice constants close to each other and layered epitaxial growth with each other (hereinafter referred to as SVO in the drawings and the specification).
And (SrTiO _{3 —Y} ) (a-axis length = 0.390)
nm) (hereinafter, abbreviated as STO in the drawings and specification)
(However, to make 0 ≦ X, Y <0.5) superlattice,
This can be achieved by a method of manufacturing an epitaxial artificial superlattice whose period is controlled in the unit lattice length order by the RHEED intensity frequency.

The (SrVO _3−X ) / (SrTiO _3−Y ) (where 0 ≦ X, Y <0.5) superlattice has a semiconducting property at a temperature lower than about 100 K because of its metallic conductive property. It is transformed into conductivity, and the resistivity at such a low temperature is varistor-like (that is, the IV characteristic is non-linear). This can be utilized as one of the low temperature electronic varistor.

As described above, for example, the laser molecular beam epitaxy (MBE) technique is used to monitor the intensity vibration of the RHEED during film deposition to digitally control the ceramic epitaxial layer thickness. , Proved that it can be laminated. Furthermore, this technology is extended to ceramic lattice engineering, that is, it enables systematic design and construction of ceramic superlattices.

The technique of the ceramic superlattice and the method for producing the same of the present invention is also useful for producing a low-temperature varistor, a superconductor, an oxide superconducting element such as a photodetector and a transistor. ..

Next, the structure of the ceramic superlattice formed on the substrate according to the present invention and the method for producing the same will be specifically described by way of examples, but the present invention is not limited thereto.

[0018]

Example 1 SrTiO _{3 having a} perovskite crystal structure
And SrVO ₃ crystal are RHEE under the same thin film formation conditions.
It grows in layers while oscillating at D intensity, and it is easy to form a superlattice. Figure 1 shows [(SrVO
_3-X ) ₂ / (SrTiO _3-Y ) ₁ ] _{9 When} a superlattice was created,
FIG. 6 is a graph showing monitored RHEED intensity oscillations,
This corresponds to the stage in which the 1st to 1.5th cycles are created on the buffer layer.

1 and 2 were observed when a [(SrVO _3-X ) ₂ (2 unit lattice) / (SrTiO _3-Y ) (1 unit) superlattice (9 periods) was prepared. An example of RHEED intensity vibration is shown. RHEED intensity oscillations were observed on a 00 reciprocal lattice rod (center strike) containing specular reflection points.
That is, in FIG. 1, the substrate temperature is 650 ° C. and the degree of vacuum is 1 × 10.
^{At -8} Torr or more, first, a SrTiO _{3 —Y} thin film as a buffer layer is laminated by a laser MBE method in a 15 unit lattice layer on a SrTiO ₃ (001) substrate.

By this treatment, the streak of the RHEED pattern of the substrate changes into a high-intensity sharp strike,
It was judged that the SrTiO _{3 —Y} layer (about 6 nm thick) worked as a buffer layer for improving the surface condition of the substrate.
Next, after the RHEED intensity of the SrTiO _{3 -Y} buffer layer is sufficiently recovered, (SrVO _{3 -X} ) ₂ (2 unit cell),
(SrTiO _{3 —Y} ) ₁ (1 unit lattice) was laminated in this order while counting the RHEED intensity frequency. As shown in FIG.
It shows a change in the RHEED intensity vibration corresponding to the stacking in the fifth period from. The lamination was completed with the SrTiO _{3 —Y} layer in the 9th cycle. The RHEED pattern during fabrication of the superlattice was always streak-like, and the Kikuchi line was also clearly observed. Hereinafter, SrVO _3-X will be referred to as SVO, and SrTiO _3-Y will be referred to as STO.

FIG. 3 shows the prepared [(SrVO _3-X ) ₂ / (Sr
TiO _3-Y ) ₁ ] ₉ superlattice film [(SrVO _3-X ) ₂ is used as a two-unit lattice.
X-ray diffraction of a combination of 1 unit lattice of (SrTiO _{3 —Y} ) and 9 unit lattices laminated] is shown. Diffraction peaks from SIVO _3-X, in order SrTiO ₃ and the lattice constants are close, and appears as a high angle side of the shoulder of the SrTiO ₃ (002) peak. Further, since the superlattice period is as short as 9 periods, the sub-maximum peak of the Laue diffraction function is detected at the position shown by the arrow in FIG. (Arrow peak)

The total film thickness of the superlattice calculated from the angular position of the valley portion of the Laue function peak was 10.3 nm, which was in agreement with the design value 10.4 nm. In addition, the total film thickness of the superlattice including the buffer layer measured by the stylus method was 15 nm, which almost coincided with the design value of 16.3 nm. based on the above results,
It was confirmed that the [(SrVO _3-X ) ₂ / (SrTiO _3-Y ) ₁ ] ₉ superlattice was constructed in a structure almost as designed.

[0023]

Example 2 The following is the result of an experiment in which one period of the ceramic superlattice is thickened. Figure 4 shows SrT
Fabricated on an iO ₃ (001) substrate at a temperature of 700 ° C. [S
₃ is a graph showing the RHEED intensity vibration of the second period of rVO _3-X (4 nm) / SrTiO _3-Y (4 nm)] ₅ superlattice.

FIG. 5 shows X of the ceramic superlattice prepared.
It is a graph which shows a RD pattern. (2, 3, in Figure 5
The peaks shown as 4 are secondary due to the superlattice structure,
It is the satellite peak of the 3rd and 4th order,
The three peaks between the peaks are (5-2 = 3) Laue function peaks due to the small superlattice period of 5. ) The thickness of one cycle calculated from the observed angular positions of satellite peaks was 8.6 nm, which was in good agreement with the design value of 8.0 nm. The total film thickness of the superlattice calculated from the angular positions of the adjacent valleys of the sub-maximum peak was 34 nm, and the value measured by the stylus method was 35 nm, which was almost the same as the design value of 40 nm. From these results, it was confirmed that a superlattice structure almost as designed was produced.

[0025]

[Embodiment 3] Under the superlattice formation conditions performed in Embodiments 1 and 2, SrVO _3-X is an n-type conductor, SrTiO _3-Y.
Is also an oxygen-deficient n-type conductor. Substrate temperature 650 ° C
Each epitaxial single thin film produced by: SrTiO _3-Y
The results of measuring the low temperature resistivity of SrVO ₃ -X and SrVO _{3 -X} are shown in FIG. The thickness of the prepared thin film was about 15 nm, and the result was the same even when the measurement current was changed from 1 μA to 10 mA.

Each of the thin films exhibits a metallic behavior in which the resistivity decreases with a decrease in temperature, and the measured current is 0.0
The profile was exactly the same when changing from 01 mA to 10 mA. The resistivity of each epitaxial film was measured by cutting the film into a square of 4 × 4 mm and measuring by a four-terminal method using a vapor deposition gold electrode.

FIG. 7 shows that [(SrVO _3-X ) ₂ / (SrTiO 3
_3-Y ) ₁ ] ₉ Superlattice low temperature resistivity. Due to superlattice formation, the temperature dependence of the resistivity changed from [metallic] to [semiconductor] at about 100K. Further, at a temperature of about 100 K or less, the resistivity after transferring to the semiconductor changed greatly depending on the current at the time of measurement. After a series of measurements,
The sample taken out into the air was measured again, and the same result was obtained.

FIG. 8 is a graph in which the measurement results of FIG. 7 are arranged in the relationship between conductivity and applied electric field. ● indicates metallic behavior at a temperature of 100 K or higher, and ○ indicates semiconductor behavior at low temperature. Each curve corresponds to the relationship between conductivity and applied electric field at each temperature. [(SrVO _3-X ) (4nm) / (SrTi
Even in the case of O _3−Y ) (4 nm)] ₅ superlattice, as shown in FIG.
[Metal-semiconductor transition] could be observed at a temperature of about 100K. In addition, in FIG. 9, [(SrVO _3-X ) (4 nm) /
(SrTiO _{3 —Y} ) (4 nm)] ₉ Same condition as superlattice (ie 700
The resistivity of each single film produced at a substrate temperature of ° C) is shown. This [metal-semiconductor transition] is for each single crystal and Sr.
It has not been confirmed in the (V, Ti) O ₃ solid solution,
This is considered to be a characteristic peculiar to the superlattice of [(SrVO _3-X ) / (SrTiO _3-Y )].

From the above results, [(SrVO _3-X ) / (SrVO
TiO _{3 −Y} )] superlattice has a current-voltage (IV) characteristic of 1
It means that it is non-linear at temperatures below 00K,
It is shown that the superlattice can be applied as a low temperature varistor for operating at a low temperature of 100K or less.

[0030]

As described above, the ceramic superlattice of the present invention has the following remarkable technical effects. That is, the synthesis of ceramics superlattice
This will be possible in a well-controlled manner, opening up a new field of ceramics lattice engineering, and expecting quantum effects in ceramics superlattices. Second, it was possible to provide a physicochemically stable perovskite type crystal superlattice.

Thirdly, by the laser MBE processing film forming technique, a superlattice can be obtained, a unique function can be imparted to a thin film, and a technique capable of applying this function to an electronic device can be provided. It was Fourth, (SrVO
It was possible to provide a low temperature operation varistor composed of a _3-X ) / (SrTiO _{3 -Y} ) superlattice.

[Brief description of drawings]

FIG. 1 is a graph showing RHEED intensity oscillation monitored when a [(SrVO _3-X ) ₂ / (SrTiO _3-Y ) ₁ ] ₉ superlattice was prepared.

FIG. 2 [(SrVO _3-X ) ₂ / (SrTiO _3-Y ) ₁ ] ₉ superlattice monitored RH when prepared in the 3rd to 5th periods
It is a graph which shows EED intensity vibration.

FIG. 3 shows [(SrVO _3-X ) ₂ / (SrTiO _3-Y ) in FIGS.
₁ ] ₉ is a graph showing an X-ray diffraction (XRD) pattern when a superlattice is formed.

FIG. 4 [(SrVO _3-X ) (4 nm) / (SrTiO _3-Y ) (4 nm)]
₅ is a graph showing an example (substrate temperature 700 ° C.) of change in RHEED intensity monitored when a ₅ superlattice was manufactured.

FIG. 5 [(SrVO _3-X ) (4 nm) / (SrTiO _3-Y ) (4 nm)]
It is an XRD pattern of the superlattice designed and produced in ₅ .

FIG. 6 was prepared at a substrate temperature of 650 ° C. (SrVO _3-X ),
The low temperature resistivity of the (SrTiO _{3 —Y} ) epitaxial thin film is shown.

FIG. 7: [(SrVO _3-X ) fabricated at a substrate temperature of 650 ° C.
₂ shows the low temperature electrical resistivity of a ₂ / (SrTiO _{3 —Y} ) ₁ ] ₉ superlattice.

8] [(SrVO _3-X ) ₂ / (SrTi obtained from FIG. 7]
The relationship between the conductivity of the O _3-Y ) ₁ ] ₉ superlattice and the applied electric field is shown.

FIG. 9 shows [(SrVO _3-X ) (4 nm) / (SrTi shown in FIG.
O _{3 −Y} ) (4 nm)] ₅ superlattice and low temperature resistivity of each single thin film prepared under the same conditions (700 ° C.).

Claims (7)

[Claims] 1. A ceramics superlattice in which at least two kinds of oxide ceramics are laminated, wherein the oxide ceramics are perovskite type crystals and are grown on a substrate while maintaining a constant crystal orientation relationship with each other. The ceramic superlattice is characterized in that the growth thickness of the layer of the perovskite type crystal is an integral multiple of the unit lattice length of the perovskite type crystal as a growth unit. 2. The oxide of the perovskite type crystal is
SrVO _3-X and SrTiO _3-Y (0 ≦ X, Y ≦ 0.5)
The ceramic superlattice according to claim 1, wherein 3. The ceramic superlattice according to claim 1 or 2, wherein the substrate is a single crystal or a single crystal thin film of a perovskite type crystal oxide. 4. The ceramic superlattice according to claim 3, wherein the single crystal substrate of the perovskite type crystal oxide is SrTiO ₃ . 5. The ceramic superlattice according to any one of claims 1 to 4, wherein the growth thickness of the perovskite type crystal layer is the reflection high energy electron diffraction (RHEED) observed during the growth of the crystal film. ) Is adjusted by counting the intensity frequency of the pattern).
4. The ceramic superlattice according to any one of 4 above. 6. A low-temperature varistor using the ceramic superlattice according to claim 1, which is operated at a temperature of 100 K or less. 7. At least two kinds of oxide ceramics are sequentially laminated by adjusting the number of oxide layers while counting the intensity frequency of reflection high energy electron diffraction (RHEED). Characterized by making a superlattice structure 2
A method for manufacturing a ceramics superlattice comprising at least one kind of oxide ceramics thin film.