JP2001220300A - Oxide superlattice and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide superlattice in which thin film characteristics by formation of superlattice are highly functionalized and a method for producing the superlattice. SOLUTION: A ratio of film thickness of BaTiO3 to film thickness of SrTiO3 is differentiated from 1:1 and unbalanced. Thereby, lattice strain is unevenly distributed to enhance residual polarization value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an oxide superlattice formed by laminating oxide thin films and a method for producing the same.

[0002]

2. Description of the Related Art Hitherto, in order to realize a highly functional thin film structure, studies have been made on a thin film having a super lattice structure.

[0003] A superlattice is a thin film in which epitaxial thin films or single crystal thin films of different materials are alternately stacked at a molecular layer level. This superlattice may exhibit some excellent properties even in a material system in which bulk properties cannot be sufficiently reproduced only by thinning a solid solution.

[0004] The superlattice can be applied to various materials, and in particular, there are many applications to magnetic materials. Recently, Japanese Patent Application Laid-Open Nos. Hei 7-82097 and Hei 5 No. 221800.

FIG. 4 shows the structure of such a conventional oxide superlattice. In FIG. 4, da 'and db' are different types of oxide thin films, and these two types of oxide thin films are alternately laminated.

In the above-mentioned application, two types of perovskite type dielectric materials are laminated, and the thickness per layer is reduced to a monomolecular layer, thereby exhibiting excellent dielectric properties. For the above two types of materials, dielectric materials having similar crystal structures but slightly different lattice constants are selected, and utilizing the fact that slight lattice mismatch exerts strong stress on the lamination interface, Lattice strain is introduced into the film. Such a superlattice is called a “strained superlattice” and is effective for increasing the relative dielectric constant.

Further, the above-mentioned application proposes a process for forming a superlattice by controlling the film thickness in units of molecular layers while observing a dielectric thin film by electron beam diffraction.

[0008]

As described above, by superlatticing two kinds of oxides, characteristics that cannot be obtained with a solid solution thin film are exhibited, but they are sufficient compared to bulk characteristics. However, further improvement in characteristics was desired.

SUMMARY OF THE INVENTION An object of the present invention is to provide an oxide superlattice which solves the above-mentioned problems and has improved characteristics (especially remanent polarization) by introducing lattice strain by forming a superlattice of a thin film, and a method of manufacturing the same. It is in.

[0010]

According to the oxide superlattice of the present invention, when at least a first type and a second type of oxide are laminated on a substrate, the thickness of the first type of oxide is reduced. The thickness is different from the thickness of the two oxides. With this configuration, desired characteristics, particularly remanent polarization characteristics, are enhanced.

Further, the oxide superlattice of the present invention comprises Ca,
When at least one element of Sr, Ba, Pb, and La is A, and at least one element of Ti, Zr, Nb, and Ta is B, the first and second oxides are: Each is a perovskite oxide represented by ABO ₃ . This perovskite oxide has an element A at each apex of the cube and oxygen at the center of the plane of the cube,
Although the structure is such that the element B is contained in the octahedron surrounded by the six oxygen atoms, polarization is likely to occur because a small element B between oxygen ions occupies a place shifted from the center of the crystal. Therefore, excellent dielectric properties such as relative permittivity and remanent polarization can be easily exhibited by forming the first and second types of oxide dielectrics into strained superlattices.

Further, in the oxide superlattice of the present invention, the thickness of the first type and second type oxides is an integral multiple of the unit cell of the perovskite oxide. With this structure, lattice distortion occurs at the interface between the first type and the second type, and characteristics due to the strained superlattice structure are exhibited.

The method for producing an oxide superlattice according to the present invention comprises the steps of:
When at least one element of a, Sr, Ba, Pb, and La is A, and at least one element of Ti, Zr, Nb, and Ta is B, the first element represented by ABO ₃ A second type of perovskite oxide is prepared as first and second sintered bodies, and a plurality of sintered bodies including at least the first and second sintered bodies are formed on a substrate by a laser ablation method. When a thin film is formed on the substrate, the film thickness of the first type oxide is set to a film formation condition different from the film thickness of the second type oxide. By this manufacturing method, each oxide thin film having a small composition deviation from the target and a high-accuracy film thickness is formed, and the characteristics of the strained superlattice are easily expressed.

[0014]

FIG. 1 is a sectional view of an oxide superlattice according to an embodiment of the present invention. Here, the substrate is a single crystal substrate of strontium titanate SrTiO ₃ having a plane orientation of (100). On this substrate, barium titanate BaTiO ₃ having a thickness of da and strontium titanate SrTiO ₃ having a thickness of db are alternately laminated to form BaTiO ₃ / SrT.
It constitutes an iO ₃ superlattice.

FIG. 2 is a view showing an apparatus for producing the above-mentioned oxide superlattice. In FIG. 2, a film forming container 1 is a vacuum chamber, in which a table 2 capable of selectively moving two types of targets (target X and target Y) to a predetermined position, and a substrate facing the predetermined position of the table. 4 are arranged. A laser device 3 that irradiates a laser beam to a target disposed at a predetermined position inside the container is provided outside the film forming container 1. The RHEED gun 5 causes an electron beam having an energy of 10 keV to several tens keV to enter. The monitor 6 observes the state of diffraction from a thin film (surface) formed on the substrate 4.

As the target X, BaTiO ₃ ,
A sintered body of SrTiO ₃ is used as the target Y. The substrate is a single crystal substrate of SrTiO ₃ , which is maintained at a high temperature of 500 ° C. or higher in a film forming container and is formed by a laser ablation method in an oxidizing atmosphere. That is, by condensing and irradiating a predetermined target with laser light, the target surface of the condensing part is explosively vaporized and evaporated,
Gaseous particles such as excited atoms, excited molecules, ions, etc. are emitted in columns, and the columnar particles (plume) are diffused.
A thin film is formed by adhering and depositing on the substrate surface.

The laser ablation method is characterized in that a film forming speed is high and a film forming container and an energy source can be separately provided, so that various parameters in manufacturing can be controlled in various ways.

The thickness of the SrTiO ₃ thin film or BaTiO ₃ thin film formed on the substrate is controlled by the number of times of emission of the pulse laser output from the laser device 3 or the irradiation time of the laser light. The film thickness is detected by observing the intensity oscillation of reflection high-energy electron beam diffraction (RHEED) on the monitor 6. In a perovskite-type oxide (containing BaTiO ₃ , SrTiO ₃ ) that grows in a layered manner, the RHEED intensity oscillation corresponds to the growth of one molecular layer. Accordingly, the number of molecular layers having a grown thickness of the oxide (SrTiO ₃ or BaTiO ₃ ) layer is controlled by counting the intensity frequency of the reflection high-energy electron beam diffraction.

The oxide superlattice formed as described above is a triaxially oriented film in which (001) planes are arranged in the normal direction of the substrate and in-plane oriented in the c-plane. Diffraction (X
RD) and reflection high-energy electron diffraction (RHEED).

Next, the film thickness of SrTiO ₃ is kept constant,
The remanent polarization of the oxide superlattice when the film thickness of aTiO ₃ was changed in a plurality of steps was examined. The result is shown in FIG.

Here, the thickness of SrTiO ₃ is set to 1.2 nm.
(Three molecular layers), and the thickness of BaTiO ₃ is 1.2 nm.
(3 molecular layers), 4 nm (10 molecular layers), and 6 nm (15 molecular layers). At this time, since the total thickness of the entire oxide superlattice was 100 nm, the total number of layers differs depending on the thickness of BaTiO ₃ . That is, the total number of layers is
84 layers when the thickness of BaTiO ₃ is 1.2 nm, 4 nm
In this case, there are 25 layers, and in the case of 6 nm, there are 17 layers.

Here, the remanent polarization is SrTiO ₃ <(S
r, Ba) TiO ₃ <BaTiO _3. As is clear from the results of FIG. 3, SrTiO ₃ and BaTiO ₃
The remanent polarization when the film thickness of ₃ is 1.2 nm is:
2Pr = 18 [μC / cm ² ], and the residual polarization 2Pr = 1 of the single-layer film of BaTiO _{3 formed} by the same film forming apparatus.
The value is larger than 4 [μC / cm ² ]. However, the difference is small. On the other hand, when the thickness of BaTiO ₃ is larger than that of SrTiO ₃ , the remanent polarization value 2Pr increases, and when the thickness of BaTiO ₃ is 6 nm (15 molecular layers),
2Pr = 46 [μC / cm ² ], which was a very large value.

This is because the thickness of BaTiO ₃ is changed to SrTiO.
₃ and SrTiO ₃ and BaTi
In addition to the effect of increasing remanent polarization due to lattice distortion caused by lamination with O ₃ , the composition of the A site (Sr and Ba) is increased by making the film thickness of the ferroelectric BaTiO ₃ thicker than that of the paraelectric SrTiO _3. This is considered to be the result of the ratio approaching the Ba-rich side.

Thus, BaTiO ₃ / SrTi
Excellent ferroelectric characteristics could be obtained by utilizing the effect of the strained superlattice of O ₃ .

In the embodiment described above, the site A has Sr
Alternatively, Ba was used and Ti was used for the B site. However, at least one of Ca, Pb, and La may be used as the A site, and at least one of Zr, Nb, and Ta may be used as the B site.

In the embodiment, two kinds of oxide thin films are stacked to form an oxide superlattice. However, two or more kinds of the above perovskite-type oxides are used, and these oxide thin films are sequentially stacked. Good.

[0027]

According to the first aspect of the present invention, the characteristics of the strained superlattice can be expressed, and the characteristics can be determined according to the composition ratio of the first type and second type oxides. Excellent dielectric properties such as large polarization can be obtained.

According to the second aspect of the present invention, excellent dielectric properties such as remanent polarization can be easily exhibited by superlattices of the first and second types of oxide dielectrics.

According to the third aspect of the present invention, a larger lattice strain is generated at the interface between the first type and the second type of oxide dielectric, and characteristics due to the strained superlattice structure are exhibited.

According to the fourth aspect of the present invention, each oxide thin film can be formed with a small thickness and a high precision with a small composition deviation from the target, and the characteristics of the strained superlattice can be easily exhibited. be able to.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of an oxide superlattice according to an embodiment.

FIG. 2 is a diagram showing a configuration of an apparatus for manufacturing the same oxide superlattice.

FIG. 3. In the SrTiO ₃ / BaTiO ₃ superlattice,
The thickness of the SrTiO ₃ layer is fixed, shows a residual change in polarization value when changing the thickness of the BaTiO ₃ layer

FIG. 4 is a cross-sectional view of a conventional oxide superlattice.

Claims (4)

[Claims] An oxide superlattice comprising at least a first oxide and a second oxide stacked on a substrate, wherein the first oxide has a thickness of at least 2 An oxide superlattice that is different from the seed oxide thickness. 2. The method according to claim 1, wherein at least one of Ca, Sr, Ba, Pb and La is A, and at least one of Ti, Zr, Nb and Ta is B. oxides of 2 species, oxide superlattice according to claim 1 wherein each perovskite oxide represented by ABO _3. 3. The film thickness of the first type and second type oxides is
The oxide superlattice according to claim 2, wherein the oxide superlattice is an integral multiple of a unit cell of the perovskite oxide. 4. When at least one of Ca, Sr, Ba, Pb and La is A, and at least one of Ti, Zr, Nb and Ta is B, AB
First and second types of perovskite-type oxides represented by O ₃ are prepared as first and second sintered bodies, and at least
When a thin film is formed on a substrate by a laser ablation method using a plurality of sintered bodies including the second sintered body as a target, the thickness of the first type oxide is reduced by the second type oxide. A method for manufacturing an oxide superlattice in which a film formation condition different from a film thickness is set.