JP5721100B2 - Plate-like single crystal made of metal oxide, metal oxide thin film thereof, method for producing the same, and resistance variable element using them - Google Patents

Description

  The present invention relates to a plate-like single crystal made of a metal oxide, a metal oxide thin film thereof, a production method thereof, and a resistance variable element using the same.

  In recent years, an aqueous solution-based self-assembly technique performed under mild conditions has attracted attention. Research on building blocks or self-assembled functional thin films used in such technologies is thriving.

The present inventors have succeeded in synthesizing a layered hydroxide containing a transition metal that can be a building block (see, for example, Patent Document 1 and Non-Patent Documents 1 and 2). According to Patent Document 1 and Non-Patent Documents 1 and 2, by adding hexamethylenetetramine to an aqueous solution containing cobalt chloride (if necessary, iron chloride or nickel chloride) and refluxing, Co (OH) ₂ , High _- quality hexagonal plate-like layered crystals represented by Co _x Fe _1-x (OH) ₂ and Co _x Ni _1-x (OH) ₂ (0 <x <1) are obtained. Such a hexagonal plate-like layered crystal has a size of several micrometers in the plate-like direction and several tens of nanometers in the thickness direction, and has high two-dimensional anisotropy.

Furthermore, the present inventors have succeeded in synthesizing a functional thin film using a layered hydroxide containing a rare earth element (see, for example, Patent Document 2). According to Patent Document 2, the formula (R) (OH) _2.5 Z _{0.5 / m} · 0.125xH ₂ O ((R) is a functional site, Z is an anion, m is the valence of Z, 6 <X <8, (R) contains a RE element and an M element different from the RE element, and the RE element is one element selected from a trivalent rare earth element group. The element is an element selected from a trivalent rare earth element group and a trivalent metal element group), and a dispersion in which a layered rare earth hydroxide represented by water is dispersed in water is either hexane or toluene. A step of adding a first solvent to form an interface between the water in the dispersion and the first solvent, and further adding a second solvent that is an alcohol to trap the layered rare earth hydroxide at the interface; Adding a step, removing the first solvent, adding a second solvent, The solvent was immersed the substrate in dispersion has been removed by the steps of transferring the trapped layered earth hydroxides to a substrate, a functional thin film having high crystallinity and high orientation can be obtained.

  Development of applications of the hexagonal plate-like layered crystals described in Patent Document 1 and Non-Patent Documents 1 and 2 is desired. Further, development of a functional thin film made of a material other than the layered rare earth hydroxide of Patent Document 2 is expected.

On the other hand, a technique for synthesizing an oxide from a hydroxide is known (see, for example, Patent Document 3). According to Patent Document 3, an oriented hydroxide thin film is formed on a substrate, and an oxide thin film oriented in a specific direction can be synthesized by a dehydration reaction. Specifically, by heating the Ca (OH) ₂ film formed on the MgO substrate in air at 800 ° C. for 2 hours, a CaO film strongly oriented to (111) is obtained. In addition to the CaO film, (111) -oriented MgO, NiO, Mg (OH) ₂ , Ni (OH) ₂ , Co (OH) ₂ , and Cd (OH) ₂ by dehydrating the (001) -oriented film CoO and CdO films are obtained. However, according to Patent Document 3, it is difficult to obtain a completely (111) oriented oxide film, and improvement is desired.

  In recent years, a resistance switching effect (resistance change effect) has been confirmed in NiO and CoO oxide thin films formed by physical vapor deposition among the above-described oxides (see, for example, Non-Patent Document 3). According to Non-Patent Document 3, a resistance switching effect is confirmed in a three-layer structure of Pt / Ni—O / Pt and Pt / Co—O / Pt, and can function as a resistance variable element.

  However, the oxide thin film formed by physical vapor deposition in Non-Patent Document 3 is composed of an aggregate of particles, and there are many grain boundaries in the oxide thin film, which is not perfectly oriented. There is a problem that it cannot be obtained. In addition, for practical application of resistance variable elements, the rate of change in electrical resistance is 100 times or more (ratio between high resistance state and low resistance state), switching at a low current of about several tens of μA, and as fast as about 10 ns. Although an operation speed is required, the oxide thin film of Non-Patent Document 3 cannot meet such a request. Furthermore, the production of an oxide thin film requires an expensive large vacuum apparatus, and thus has problems such as high cost, complicated operation, and high environmental load.

  From the above, if a plate-like single crystal composed of a novel metal oxide is obtained using Patent Document 1 and Non-Patent Documents 1 and 2 as starting materials, it is extremely advantageous from the viewpoint of material design, and a new application can be expected.

  Accordingly, an object of the present invention is to provide a plate-like single crystal made of a metal oxide, a metal oxide thin film using the same, a manufacturing method thereof, and a resistance variable element using them.

In the plate-like single crystal comprising a metal oxide according to the present invention, the metal oxide is MO or M ₃ O ₄ (where M is at least one selected from the group consisting of Co, Fe, Ni, Zn and Cu). The shape of the plate-like single crystal made of the metal oxide is hexagonal, and the aspect ratio of the plate-like single crystal made of the metal oxide is 66 to is 500, the plane direction of the surface of the plate-like single crystal body composed of the metal oxide, the metal oxide (111) Ri crystal plane der, plate-like single crystal body composed of the metal oxide, the resistance It has a changing effect, thereby achieving the above-mentioned problem.
Electrical resistance in the high resistance state consisting previous SL metal oxide platy single crystal is in the range of 10 ⁹ Ω~10 ¹⁴ Ω, the electrical resistance in the low resistance state, a range of 10 ³ Ω~10 ⁷ Ω It may be.
The method for producing a plate-like single crystal made of the metal oxide is M (OH) ₂ (where M is a metal element selected from at least one selected from the group consisting of Co, Fe, Ni, Zn and Cu). The step of heating a plate-like single crystal made of a metal hydroxide represented by the formula (1) , wherein the plate-like single crystal made of the metal hydroxide has a hexagonal shape, and the metal hydroxide The aspect ratio of the plate-like single crystal made of a product is 66 to 500, and the plane surface of the plate-like single crystal made of the metal hydroxide is the (001) crystal face of the metal hydroxide. Yes, thereby achieving the above problem.
The heating step may heat the plate-like single crystal made of the metal hydroxide in a temperature range of 200 ° C. to 800 ° C. in a vacuum or an inert gas atmosphere.
The heating step may heat the plate-like single crystal made of the metal hydroxide in the temperature range of 400 ° C. to 700 ° C. in the atmosphere.
Metal oxide thin film composed of a plate-shaped single crystal body consisting of O Rukin genus oxide in the present invention, the metal oxide, MO or _M 3 _{O 4} (where, M is Co, Fe, Ni, Zn And the shape of the plate-shaped single crystal made of the metal oxide is hexagonal, and the plate-shaped single crystal made of the metal oxide is a hexagonal shape. The aspect ratio of the crystal is 10 to 500, and the plane surface of the plate-like single crystal made of the metal oxide is the (111) crystal plane of the metal oxide, and the plate-like single crystal Are arranged in a close-packed manner in the planar direction of the plate-like single crystal, and the metal oxide thin film is oriented in [111], thereby solving the above-mentioned problem.
The method of manufacturing a metal oxide thin film according to the present invention, the plate-like single crystal body composed of metallic oxide to the dispersion dispersed in water, was added to the first solvent is either hexane or toluene A step of forming an interface between the water in the dispersion and the first solvent, and a second solvent that is an alcohol are further added, and a plate-like single crystal made of the metal oxide is added to the interface. A step of trapping, a step of removing the first solvent, a step of applying a surface pressure to the plate-like single crystal made of the metal oxide trapped at the interface, and the first solvent being removed. Immersing the substrate in the dispersion and transferring the trapped plate-like single crystal made of the metal oxide to the substrate , wherein the metal oxide is MO or M ₃ O ₄ (Where M is Co, F e, Ni, Zn and Cu is a metal element selected from the group consisting of, and the shape of the plate-like single crystal made of the metal oxide is hexagonal, and the metal oxide An aspect ratio of the plate-like single crystal made of the metal oxide is 10 to 500, and a plane surface of the plate-like single crystal made of the metal oxide is a (111) crystal face of the metal oxide. The above-described problem is achieved.
Prior to the step of forming the interface, metal hydroxide represented by M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu). The method may further include the step of heating the plate-like single crystal made of a material to cause a phase transition to the plate-like single crystal made of the metal oxide.
The method for producing the metal oxide thin film according to the present invention includes M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn, and Cu). A first solvent that is either hexane or toluene is added to a dispersion in which a plate-like single crystal composed of a metal hydroxide represented by formula (1) is dispersed in water, and the water in the dispersion and the first A step of forming an interface with the first solvent, a step of further adding a second solvent that is an alcohol, and trapping the plate-like single crystal made of the metal hydroxide at the interface; Removing the solvent; applying a surface pressure to the plate-like single crystal made of the metal hydroxide trapped at the interface; and applying a substrate to the dispersion from which the first solvent has been removed. Soaked and trapped A step of transferring a plate-like single crystal made of the metal hydroxide to the substrate, and a step of heating the plate-like single crystal made of the metal hydroxide transferred to the substrate. The above-described problem is achieved.
The resistance variable element according to the present invention includes a first electrode, a resistor located on the first electrode, and a second electrode located on the resistor, wherein the resistor is It is a plate-like single crystal made of a metal oxide or a metal oxide thin film, thereby achieving the above object.
The reset current of the resistance variable element may be less than 1 mA.

The plate-like single crystal according to the present invention is represented by MO or M ₃ O ₄ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu). Since these metal oxides exhibit a resistance change effect, they can be applied to resistance change elements. Since these M elements are transition metal elements, they can also be applied to gas sensors that utilize changes in physical properties (electrical resistance, optical transmittance, color) due to the reaction between transition metal oxides and gases (CO, H _2, etc.). In addition, the shape of the plate-like single crystal of the present invention is a hexagonal shape having an aspect ratio of 10 to 500, and functions as a building block having high two-dimensional anisotropy. Enable design. In addition, the plate-like single crystal according to the present invention can be easily oriented on an arbitrary substrate because the plane in the plane direction is the (111) crystal plane of the metal oxide.

The method for producing the plate-like single crystal according to the present invention includes M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu). A step of heating a plate-like single crystal made of a metal hydroxide represented by the formula: By simply heating, M (OH) ₂ can be phase-changed into an oxide topologically while maintaining a single crystal state. As described in Patent Document 1 and Non-Patent Documents 1 and 2, the plate-like single crystal made of metal hydroxide has a size of several micrometers in the plate-like direction and several tens of nanometers in the thickness direction. Therefore, the obtained oxide can also have such a shape characteristic. Preferably, in the step of heating, if heating is performed in a temperature range of 200 ° C. to 800 ° C. in a vacuum, M (OH) ₂ is heated to MO, and if heated in a temperature range of 400 ° C. to 700 ° C. in the atmosphere, M Since (OH) ₂ can be phase-changed to M ₃ O ₄ , the production is easy.

  The metal oxide thin film according to the present invention is composed of a plate-like single crystal made of the above-described metal oxide, and the plate-like single crystals are arranged in a close-packed manner in the plane direction. As described above, a plate-like single crystal having a high two-dimensional anisotropy provides a metal oxide thin film that is perfectly oriented to [111] without being incompletely oriented like an aggregate of particles. be able to.

  In the method for producing a metal oxide thin film according to the present invention, a first solvent which is either hexane or toluene is added to a dispersion obtained by dispersing a plate-like single crystal composed of the above metal oxide in water, A step of forming an interface between the water in the dispersion and the first solvent, a step of further adding a second solvent that is an alcohol, and trapping a plate-like single crystal made of a metal oxide at the interface; Removing the first solvent, applying a surface pressure to the plate-like single crystal made of the metal oxide trapped at the interface, immersing the substrate in the dispersion, and from the trapped metal oxide And transferring the plate-like single crystal to the substrate. By the step of trapping and the step of applying a surface pressure, the plate-like single crystals made of metal oxide can be arranged in a close-packed manner. Moreover, according to the above-described manufacturing method, a special apparatus is not required, so that a metal oxide thin film can be provided at a low cost, and the area can be increased.

  In another method for producing a metal oxide thin film according to the present invention, a first solvent that is either hexane or toluene is added to a dispersion obtained by dispersing a plate-like single crystal composed of the above metal hydroxide in water. A step of forming an interface between the water in the dispersion and the first solvent, and further adding a second solvent that is an alcohol, and trapping a plate-like single crystal made of a metal hydroxide at the interface A step of removing the first solvent, a step of applying a surface pressure to the plate-like single crystal made of a metal hydroxide trapped at the interface, and a step of immersing the substrate in the dispersion and trapping. The step of transferring the plate-like single crystal made of metal hydroxide to the substrate and the step of heating the plate-like single crystal made of the metal hydroxide transferred to the substrate are included. By the step of trapping and the step of applying surface pressure, the plate-like single crystals made of metal hydroxide are arranged in a close-packed manner, and this is heated to change the phase to metal oxide. Such a phase change occurs topologically, so that a metal oxide thin film having a [111] orientation can be obtained by arranging the metal oxides in a close-packed manner. Moreover, according to the above-described manufacturing method, a special apparatus is not required, so that a metal oxide thin film can be provided at a low cost, and the area can be increased.

  A resistance variable element according to the present invention includes a first electrode, a resistor positioned on the first electrode, and a second electrode positioned on the resistor, the resistor being the metal oxide described above. It is a plate-like single crystal made of or a metal oxide thin film. Unlike the metal oxide obtained by the existing physical vapor deposition method, the plate-like single crystal and the metal oxide thin film described above are a single single crystal having a high two-dimensional anisotropy, or an assembly thereof. Since it consists of a body, it has high resistance compared with the metal oxide obtained by the existing physical vapor deposition method, and can satisfy the requirements of a resistance variable element. Moreover, according to the resistance variable element using a single plate-like single crystal, it can be a micro or nanometer-sized small element, and if this is mounted, the entire memory can be miniaturized.

Schematic diagram of a plate-like single crystal comprising a metal oxide according to the present invention The figure which shows the crystal structure model of the plate-shaped single crystal body by this invention The flowchart which shows the manufacturing process of the plate-shaped single crystal by this invention The schematic diagram which shows the manufacturing process of the plate-shaped single crystal which consists of a metal oxide represented by MO by this invention Schematic diagram showing a manufacturing process of M ₃ made of a metal oxide represented by O ₄ platy single crystal according to the present invention Schematic diagram of a metal oxide thin film comprising a plate-like single crystal comprising a metal oxide according to the present invention The flowchart which shows the manufacturing process of the metal oxide thin film by this invention Schematic diagram showing the manufacturing process of metal oxide thin film according to the present invention The flowchart which shows another manufacturing process of the metal oxide thin film by this invention. Schematic diagram showing another manufacturing process of the metal oxide thin film according to the present invention. Schematic diagram showing a variable resistance element according to the present invention. Schematic diagram showing another variable resistance element according to the present invention. Schematic diagram showing the operation of the variable resistance element according to the present invention. Schematic diagram showing the memory operation of the resistance variable element of the present invention. The figure which shows a mode that a Co (OH) ₂ plate-like single crystal is manufactured. The figure which shows the external appearance (A) and SEM image (B) of Co (OH) ₂ plate-like single crystal by Example 1. The figure which shows the TEM image (A) and electron diffraction pattern (B) of Co (OH) ₂ plate-like single crystal by Example 1. The figure which shows the X-ray-diffraction pattern of Co (OH) ₂ plate-like single crystal by Example 1 The figure which shows the surface pressure-surface area ((pi) -A) curve of the trapped Co (OH) ₂ plate-shaped single crystal body The figure which shows the digital camera image (A) and SEM image (B) of the cross section of the film | membrane of Co (OH) ₂ film | membrane / Si film by Example 1. FIG. The figure which shows the SEM image of the surface of the film | membrane of Co (OH) ₂ film | membrane / Si by Example 1. The figure which shows the SEM image of the surface of another Co (OH) ₂ film | membrane / Si film | membrane by Example 1. The figure which shows the XRD pattern of the film | membrane of Co (OH) ₂ film | membrane / Pt (a), Co (OH) ₂ film | membrane / Si (b), and Co (OH) ₂ film | membrane / quartz (c) by Example 1. The figure which shows the XRD pattern of the film | membrane of Co (OH) ₂ film | membrane / Si (a) and CoO film | membrane / Si (b) by Example 1. The figure which shows the SEM image of the surface of the film | membrane of CoO film | membrane / Si by Example 1 The figure which shows the UV-vis absorption spectrum of CoO film | membrane / quartz by Example 1 The figure which shows the optical band gap of the CoO film | membrane / quartz by Example 1. Schematic diagram of variable resistance element according to Example 2 The figure which shows the SEM image of the resistance variable element by Example 2. The figure which shows the switching characteristic (IV) of the resistance variable element by Example 2. FIG. The figure which shows the result of the repetition test of the set / reset operation | movement of the resistance change element by Example 2 The figure which shows the AFM image of CoO film | membrane / Pt by Example 3. The figure which shows the TEM image (A) and electron diffraction pattern (B) of CoO plate-like single crystal / Pt by Example 3. Schematic diagram of variable resistance element according to Example 3 The figure which shows the switching characteristic (IV) of the resistance variable element by Example 3. It shows the XRD pattern of _Co 3 _{O 4} film / quartz according to Example 4 Shows a _Co 3 _{O 4} film / Co2p inner shell photoelectron spectra of quartz according to Example 4 It shows a TEM image of _Co 3 _{O 4} film / Si according to Example 4 The figure which shows the SEM image of the surface of the film | membrane of the (Co, Ni) (OH) ₂ film | membrane / Si film by Example 5. The figure which shows the XRD pattern of the film | membrane of (Co, Ni) (OH) ₂ film / Si (a) and Co ₂ NiO ₄ film / Si (b) by Example 5 It shows the XRD pattern of _Co 3 _{O 4} platy single crystal according to Example 6 It shows a SEM image of the surface of the _Co 3 _{O 4} film / quartz according to Example 6 It shows the XRD pattern of _Co 3 _{O 4} film / quartz according to Example 6

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, the same number is attached | subjected to the same element and the description is abbreviate | omitted.

(Embodiment 1)
In the first embodiment, a plate-like single crystal composed of the metal oxide of the present invention and a manufacturing method thereof will be described in detail.

  FIG. 1 is a schematic view of a plate-like single crystal made of a metal oxide according to the present invention.

The plate-like single crystal 100 according to the present invention is represented by MO or M ₃ O ₄ (wherein M is a metal element selected from the group consisting of Co, Fe, Ni, Zn, and Cu). Made of metal oxide. When Cu or Zn is selected as M in M ₃ O ₄ , Cu and Zn cannot be trivalent, and may be set so as to take charge neutrality in combination with Co, Fe, or Ni. . M may be, for example, a solid solution of Co and Ni, a solid solution of Co and Fe, or a solid solution of Co, Ni, and Fe. There is no limit. Whether or not it is a solid solution may be confirmed by checking whether or not the lattice constant of the metal oxide determined by X-ray diffraction or the like follows the Vegard law or by chemical composition analysis.

The specific metal oxide MO has a rock salt type structure, CoO, NiO, FeO, CuO, ZnO, (Co, Ni) O, (Co, Fe) O, (Co, Cu) O, (Co, Zn) O, (Ni, Fe) O, (Ni, Cu) O, (Ni, Zn) O, (Fe, Cu) O, (Fe, Zn) O, (Cu, Zn) O, (Co, Ni) , Fe) O, etc., but the combination is not limited to these. For example, in (Co, Ni) O, (Co, Ni) is synonymous with Co _x Ni _1-x O (0 <x <1), and the solid solution ratio of Co and Ni is arbitrary. Represents.

Specific metal oxides M ₃ O ₄ have a spinel structure, such as Co ₃ O ₄ , Ni ₃ O ₄ , Fe ₃ O ₄ , (Co, Ni) ₃ O ₄ , (Co, Fe) ₃ O. ₄ , (Co, Cu) ₃ O ₄ , (Co, Zn) ₃ O ₄ , (Ni, Fe) ₃ O ₄ , (Ni, Cu) ₃ O ₄ , (Ni, Zn) ₃ O ₄ , (Fe, Cu) ₃ O ₄ , (Fe, Zn) ₃ O ₄ , (Co, Ni, Fe) ₃ O _4, etc., but the combination is not limited to these. For example, (Co, _Ni) in 3 _{O 4,} (Co, Ni) and _has the same meaning as _{(Co x Ni 1-x)} 3 O 4 (0 <x <1), the solid solution of Co and Ni The ratio is arbitrary.

The plate-like single crystal body 100 according to the present invention has a hexagonal shape as shown in FIG. 1 and is a single crystal as a whole. Such a hexagonal shape does not depend on the crystal structures of MO and M ₃ O ₄ but reflects the hexagonal crystal structure of M (OH) ₂ which is a starting material described later. Furthermore, the aspect ratio of the plate-like single crystal 100 is 10 to 500, and has extremely high two-dimensional anisotropy. Specifically, the lateral size L of the plate-like single crystal body 100 is in the range of 200 nm to 10 μm, and the thickness size D is in the range of 20 nm to 100 nm. Furthermore, a plane surface (hereinafter, simply referred to as a plate crystal plane) 110 of the plate single crystal body 100 is a (111) crystal plane. As described above, the plate-like single crystal 100 of the present invention which is a complete single crystal having a high two-dimensional anisotropy rather than an aggregate of particles obtained by an existing physical vapor deposition method is a novel material. Can serve as a building block for

  FIG. 2 is a diagram showing a crystal structure model of a plate-like single crystal according to the present invention.

2A is a crystal model of a plate-like single crystal composed of a metal oxide represented by MO, and FIG. 2B is a plate model composed of a metal oxide represented by M ₃ O _4. It is a crystal model of a single crystal.

MO is an oxide having a rock salt structure and has a face-centered cubic lattice, but a plate-like single crystal 100 made of a metal oxide represented by MO according to the present invention is as shown in FIG. And a hexagonal shape reflecting a hexagonal system of M (OH) ₂ having a CdI ₂ type structure (see, for example, FIG. 4 described later). FIG. 2A is a crystal model of an octahedral unit arrangement when MO is viewed from the [111] direction, and an M atom is located at the center of the octahedral unit. Here, in the (111) plane of MO, M ions are surrounded by six oxygen atoms located at each vertex of the regular octahedron.

M ₃ O ₄ (for example, (Co ₃ O ₄ (Co ²⁺ Co ³⁺ ₂ O ₄ ), Ni ₃ O ₄ (Ni ²⁺ Ni ³⁺ ₂ O ₄ ), etc.) is an oxide having a spinel structure. The plate-like single crystal body 100 made of a metal oxide represented by M ₃ O ₄ according to the invention reflects the hexagonal system of M (OH) ₂ having a CdI ₂ type structure as shown in FIG. 2 (B). 2B is a crystal model of octahedral and tetrahedral unit arrangements when M ₃ O ₄ is viewed from the [111] direction, in which M atoms are located at the respective centers. In the (111) plane of M ₃ O ₄ , 1/2 of the octahedral unit array is occupied by M ³⁺ and 1/8 of the tetrahedral unit array is occupied by M ²⁺ in the M ^ions .

Since MO and M ₃ O ₄ constituting the plate-like single crystal body 100 according to the present invention both exhibit a resistance change effect, they can be applied to a resistance variable element. Furthermore, the plate-like single crystal 100 according to the present invention has a high electric resistance in the range of 10 ⁹ Ω to 10 ¹⁴ Ω in the initial state (immediately after production) and the high resistance state. This is because the plate-like single crystal body 100 according to the present invention is a single crystal having the above-mentioned aspect ratio in addition to the above-mentioned metal oxide, so that the metal composed of particles obtained by the existing physical vapor deposition method. This is because there is no grain boundary path in the oxide thin film. Moreover, the plate-like single crystal 100 according to the present invention has an electric resistance in the range of 10 ³ Ω to 10 ⁷ Ω in the low resistance state. When the plate-like single crystal body 100 of the present invention having such a high electric resistance is applied to a resistance variable element, switching at an electric resistance change rate of 100 times or more and a low current of about several tens of μA can be realized. obtain. Moreover, since the window in the resistance between the high resistance state and the low resistance state is 10 ³ or more, the information can be reliably recorded.

Further, since MO and M ₃ O ₄ constituting the plate-like single crystal body 100 according to the present invention are both transition metal oxides, they easily react with gas (CO, H _2, etc.) and change physical properties (electricity). Resistance, optical transmittance, color). The plate-like single crystal body 100 according to the present invention can also be applied to a gas sensor using such physical property changes.

  Next, a method for producing the plate-like single crystal body 100 will be described.

FIG. 3 is a flowchart showing a manufacturing process of a plate-like single crystal according to the present invention.
FIG. 4 is a schematic view showing a production process of a plate-like single crystal made of a metal oxide represented by MO according to the present invention.

Step S310: A plate made of a metal hydroxide represented by M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu). The single crystal is heated.

Here, the plate-like single crystal 400 made of a metal hydroxide is a hexagonal single crystal. The aspect ratio of the plate-like single crystal is 10 to 500, and the plate-like crystal surface 410 in the plane direction satisfies the (001) crystal surface of M (OH) ₂ .

Here, the plate-like single crystal body 400 made of a metal oxide represented by M (OH) ₂ can be manufactured by, for example, the methods described in Patent Document 1 and Non-Patent Documents 1 and 2. Specifically, the M salt selected from the above, HMT or urea in an aqueous solution containing hexamethylenetetramine (HMT) or urea may be decomposed to hydrolyze the M salt. Such decomposition and hydrolysis can be achieved by refluxing the aqueous solution. The plate-like single crystal 400 made of the metal hydroxide thus obtained surely has the shape and size described above.

In the heating step S310, the plate-like single crystal 400 made of a metal hydroxide is preferably heated in a temperature range of 200 ° C. to 800 ° C. in a vacuum or an inert gas atmosphere. Thereby, as shown in FIG. 4, M (OH) ₂ has a phase change to MO having a (111) plane and a hexagonal shape. That is, M (OH) ₂ is phase-changed topologically to MO. Since this step is a dehydration reaction, dehydration can be reliably performed at 200 ° C. or higher. Further, if the heating temperature exceeds 800 ° C., a high temperature furnace is required and costs are not preferred. The degree of vacuum is preferably 10 ⁻² Pa or less. The inert gas atmosphere can typically be Ar, N ₂ . Moreover, the time of a heating is 5 minutes-1 hour, for example. By heating in a vacuum or an inert gas atmosphere, water in M (OH) ₂ is efficiently dehydrated by heating at a temperature of 200 ° C. to 800 ° C. without oxidizing M atoms, and M (OH ) ₂ can be phase-changed to MO in a topotactic manner.

In this dehydration reaction, referring to FIG. 4, each M ion in M (OH) ₂ and MO is surrounded by six oxygen atoms located at the apex of the octahedral unit (FIG. 4). In addition, the arrangement and coordination of metal atoms as seen from the [111] direction of MO are very similar to those as seen from the [001] direction of M (OH) ₂ . That is, the chemical bonding state and coordination in MO already exist in M (OH) ₂ . As a result, the phase transition is topological as described above, and the morphology of M (OH) ₂ is maintained.

FIG. 5 is a schematic view showing a production process of a plate-like single crystal made of a metal oxide represented by M ₃ O ₄ according to the present invention.

  Again, the manufacturing process will be described in detail with reference to FIGS.

Step S310: A plate made of a metal hydroxide represented by M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu). The single crystal is heated. Here, since the plate-like single crystal 400 made of a metal hydroxide is as described above, the description thereof is omitted.

In step S310 of heating, the plate-like single crystal made of a metal hydroxide is preferably heated in the temperature range of 400 ° C. to 700 ° C. in the atmosphere. Thereby, as shown in FIG. 5, a dehydration reaction and an oxidation-reduction reaction are caused, and M (OH) ₂ changes to M ₃ O ₄ having a (111) plane and having a hexagonal shape. That is, M (OH) ₂ can be changed topically to M ₃ O ₄ . Specifically, some of the M atoms are M ²⁺ without being oxidized, and some are oxidized to M ³⁺ . The heating time is, for example, 30 minutes to 2 hours.

In this dehydration / redox reaction, referring to FIG. 5, each M ion in M (OH) ₂ is located in the center of an octahedral unit surrounded by six oxygen atoms, but in M ₃ O ₄ Part of the M ions moves to the center of the tetrahedral unit with four oxygen atoms. However, the arrangement and coordination of metal atoms as seen from the [111] direction of M ₃ O ₄ is very similar to that seen from the [001] direction of M (OH) ₂ . Therefore, the phase transition from M (OH) ₂ to Co ₃ O ₄ requires chemical and crystallographic reconfiguration, but if the method of the present invention is employed, the morphology of M (OH) ₂ is maintained. Enables topotactic phase transition.

As described in detail with reference to FIGS. 3 to 5, according to the present invention, a plate-like single crystal composed of a metal hydroxide represented by M (OH) ₂ is used as a starting material, and predetermined conditions are satisfied. A plate-like single crystal composed of a metal oxide represented by MO or M ₃ O ₄ can be obtained simply by heating at.

(Embodiment 2)
Embodiment 2 details the metal oxide thin film which consists of a plate-shaped single crystal demonstrated in Embodiment 1, and its manufacturing method.

  FIG. 6 is a schematic view of a metal oxide thin film made of a plate-like single crystal made of a metal oxide according to the present invention.

  In FIG. 6, a single-layer metal oxide thin film 600 located on the substrate 610 is shown. Thus, if the metal oxide thin film 600 exists on the base material 610, since handling is easy, it is advantageous. Here, the base material 610 may be a base material of any material such as a semiconductor substrate such as Si or GaAs, a quartz substrate, a glass substrate, an organic substrate such as plastic, or a metal substrate. Moreover, the base material 610 is not limited to a flat plate, and may have a curvature or unevenness on the surface.

The metal oxide thin film 600 is composed of the plate-like single crystal 100 made of the metal oxide represented by MO or M ₃ O ₄ described in detail in the first embodiment. Since the plate-like single crystal body 100 is as described above, the description thereof is omitted.

  As shown in FIG. 6, the plate-like single crystals 100 are arranged in a close-packed manner in the planar direction to form a metal oxide thin film 600. As described above, the metal oxide thin film 600 according to the present invention is not a film made of an aggregate of particles as seen in an existing physical vapor deposition method, but a film made of an aggregate of plate-like single crystals 100. Therefore, the influence of the grain boundary can be ignored and the characteristics of the single crystal can be fully exhibited. In the present specification, “close-packing” means complete close-packing as well as partial close-packing substantially but not close-packing, and does not deteriorate the characteristics when applied to a device. The filling of the plate-like single crystal body 100 to the extent is intended.

  The metal oxide thin film 600 of the present invention has high crystallinity and orientation due to the plate-like crystal face 110 of the plate-like single crystal body 100 constituting the metal oxide thin film 600. Specifically, since the plate-like crystal face 110 of the plate-like single crystal body 100 is the (111) face, the metal oxide thin film 600 oriented in [111] is obtained by the close-packed arrangement. As described above, the metal oxide thin film 600 according to the present invention is not a film made of an aggregate of particles as seen in an existing physical vapor deposition method, but a film made of an aggregate of plate-like single crystals 100. Therefore, complete [111] orientation can be achieved.

  Since the thickness of the plate-like single crystal body 100 is in the range of 20 nm to 100 nm, the thickness of the single-layer metal oxide thin film 600 is also 20 nm to 100 nm reflecting the thickness of the plate-like single crystal body 100. It is a range. Note that the film thickness of the metal oxide thin film 600 of the present invention can be controlled according to the number of laminated single crystal bodies 100.

Since the metal oxide thin film 600 according to the present invention also has the same effect as the plate-like single crystal body 100, it is applied to a resistance variable element, a gas sensor and the like. For example, unlike the existing physical vapor deposition method, the metal oxide thin film 600 according to the present invention does not have a grain boundary path or the like in the film, and thus has a high electrical resistance (10 ⁹ Ω to 10 ¹⁴ Ω) can also be exhibited in the film, so that it can be replaced with a thin film by an existing physical vapor deposition method.

  Next, a method for manufacturing the metal oxide thin film 600 will be described.

FIG. 7 is a flowchart showing a manufacturing process of a metal oxide thin film according to the present invention.
FIG. 8 is a schematic view showing a process for producing a metal oxide thin film according to the present invention.

  Step S710: The first solvent 810 is added to the dispersion (state (A) in FIG. 8) in which the plate-like single crystal body 100 described with reference to FIG. An interface 820 between 800 and the first solvent 810 is formed (state (B) in FIG. 8). Exemplary first solvent 810 is hexane, toluene. Since the specific gravity of these first solvents 810 is lighter than that of water, the first solvent 810 is separated so as to become water 800 in the lower layer and the first solvent 810 in the upper layer. As a result, an interface 820 is formed. Is done.

  Step S720: The second solvent 830 is further added. Thereby, the plate-like single crystal body 100 is trapped at the interface 820 formed in step S710 (state (C) in FIG. 8). The second solvent 830 is optional as long as it is an alcohol, but ethanol, propanol, and butanol are preferable from the viewpoint of versatility and ease of handling.

  The plate-like single crystal body 100 is not randomly oriented with respect to the interface 820 but is trapped in a self-organized manner so that the plate-like crystal surface 110 is parallel to the interface 820. This is because it is most stable that the plate-like single crystal body 100 has the two-dimensional anisotropy so that the plate-like crystal plane is arranged in parallel to the interface 820, that is, [111] orientation with respect to the interface 820. is there. In addition, the plate-like single crystals do not overlap each other due to surface tension, charge repulsion on the crystal surface, etc., and are arranged as a monolayer in a self-organized manner.

  Step S730: The first solvent 810 is removed. When the first solvent 810 is removed in the state (C) of FIG. 8, the state (D) of FIG. 8 is obtained, and the plate-like single crystal body 100 made of a metal oxide trapped at the interface 820 remains as a film.

Step S740: A surface pressure 840 is applied to the plate-like single crystal body 100 made of the trapped metal oxide (state (E) in FIG. 8). Thereby, the close-packed-like arrangement of the plate-like single crystal body 100 can be ensured. The applied surface pressure is preferably in the range of 5 mNm ^{−1 to} 25 mNm ⁻¹ . When the applied surface pressure is less than 5 mNm ⁻¹ , the arrangement of the plate-like single crystals 100 is disturbed and does not reach the close-packed arrangement, and the resulting metal oxide thin film 600 has a gap between the plate-like single crystals 100. There may be gaps. When the applied surface pressure exceeds 25 mNm ⁻¹ , the plate-like single crystal body 100 partially overlaps due to the surface pressure being too high, and the crystallinity of the resulting metal oxide thin film 600 may be lowered.

  Step S750: Substrate 610 (FIG. 6) is immersed in the dispersion, and plate-like single crystal body 100 trapped at interface 820 is transferred (transferred) to substrate 610 (state (E) in FIG. 8). The trapped plate-like single crystal body 100 is [111] preferentially oriented with respect to the interface 820, but the plate-like single crystal body 100 has priority over the base material 610 when the base material 610 is pulled up. It is transferred to the substrate 610 while maintaining the orientation. In this way, a metal oxide thin film 600 composed of the plate-like single crystal bodies 100 which are [111] -oriented on the substrate 610 and arranged in a close-packed manner is obtained. Note that steps S740 and S750 can be performed simultaneously.

  Note that it is easily understood that a metal oxide thin film (not shown) made of the plate-like single crystal body 100 having a multilayer structure can be obtained by repeating step S750 a plurality of times. It is optional depending on the desired film thickness of the thin film.

Prior to step S710, a step (not shown) of heating plate-like single crystal 400 (FIGS. 4 and 5) made of a metal hydroxide represented by M (OH) ₂ is performed, and MO Alternatively, a phase transition may be made topotropically to the plate-like single crystal 100 (FIG. 1) made of a metal oxide represented by M ₃ O ₄ . The heating conditions here are as described in detail with reference to FIGS.

As described above with reference to FIGS. 7 to 8, the metal oxide thin film 600 of the present invention is represented by MO or M ₃ O _{4 in} which the metal hydroxide 400 is phase-changed topologically. It is manufactured using the two-dimensional anisotropy of the plate-like single crystal 100 of metal oxide. Since the method of the present invention does not require a complicated, large-scale and expensive apparatus represented by physical vapor deposition or chemical vapor deposition, a functional thin film having excellent crystallinity and orientation easily and inexpensively. Can provide. Further, according to the production method of the present invention, if a large container is obtained, the area can be easily increased, which is suitable for practical use.

FIG. 9 is a flowchart showing another manufacturing process of the metal oxide thin film according to the present invention.
FIG. 10 is a schematic view showing another manufacturing process of the metal oxide thin film according to the present invention.

Step S910: Dispersion in which a plate-like single crystal 400 (FIGS. 4 and 5) made of a metal hydroxide represented by M (OH) ₂ described with reference to FIGS. 3 to 5 is dispersed in water 800 The first solvent 810 is added to the liquid (state (A) in FIG. 10), thereby forming an interface 820 between the water 800 in the aqueous solution and the first solvent 810 (state (B) in FIG. 10). . Exemplary first solvent 810 is hexane, toluene. Since the specific gravity of these first solvents 810 is lighter than that of water 800, the first solvent 810 is separated so as to become water 800 in the lower layer and the first solvent 810 in the upper layer, thereby forming an interface 820. Is done.

  Step S920: The second solvent 830 is further added. Thereby, the plate-like single crystal 400 is trapped at the interface 820 formed in step S910 (state (C) in FIG. 10). The second solvent 830 is optional as long as it is an alcohol, but ethanol, propanol, and butanol are preferable from the viewpoint of versatility and ease of handling. The plate-like single crystal 400 is not randomly oriented with respect to the interface 820 but is trapped in a self-organized manner so that the plate-like crystal surface 410 is parallel to the interface 820. This is because it is most stable that the plate-like single crystal body 400 has the two-dimensional anisotropy so that the plate-like crystal face 410 is arranged in parallel to the interface 820, that is, [001] orientation with respect to the interface 820. It is. In addition, the plate-like single crystals do not overlap each other due to surface tension, charge repulsion on the crystal surface, etc., and are arranged as a monolayer in a self-organized manner.

  Step S930: The first solvent 810 is removed. When the first solvent 810 in the upper layer portion is removed in the state (C) of FIG. 10, the state (D) is obtained.

Step S940: A surface pressure 840 is applied to the plate-like single crystal 400 made of the trapped metal hydroxide. Thereby, the close-packed-like arrangement of the plate-like single crystal bodies 400 can be ensured. The applied surface pressure is preferably in the range of 5 mNm ^{−1 to} 25 mNm ⁻¹ . When the applied surface pressure is less than 5 mNm ⁻¹ , the plate-like single crystal 400 does not reach the close-packed arrangement, and a gap is formed between the plate single crystals 100 in the finally obtained metal oxide thin film 600. It can happen. When the applied surface pressure exceeds 25 mNm ⁻¹ , the surface pressure is too high, and the plate-like single crystal 400 partially overlaps, which may reduce the crystallinity of the resulting metal oxide thin film 600.

  Step S950: Substrate 610 (FIG. 6) is immersed in the dispersion obtained in Step S940, and plate-like single crystal layer 400 made of metal hydroxide trapped at interface 820 is transferred to substrate 610 (transfer) (State (E) in FIG. 10). Although the trapped plate-like single crystal 400 is [001] oriented with respect to the interface 820, the plate-like single crystal 400 is [001] with respect to the substrate 610 when the substrate 610 is pulled up. It is transferred to the substrate 610 while maintaining the orientation. In this way, the metal hydroxide film 1000 made of the plate-like single crystal 400 made of the metal hydroxide having [001] orientation and arranged in a close-packed manner is obtained on the base material 610. Here, in the metal hydroxide film 1000, as schematically shown in FIG. 10F, each of the plate-like single crystals 400 is [001] -oriented, [001] orientation.

  Step S960: The metal hydroxide film 1000 (state (F) in FIG. 15) made of the plate-like single crystal 400 obtained in step S950 is heated. As a result, each of the plate-like single crystals 400 made of a metal hydroxide undergoes a topotropic phase transition to the plate-like single crystal 100 made of a metal oxide, and the metal hydroxide film 1000 becomes a plate-like single crystal. A metal oxide thin film 600 made of the crystal 100 is obtained (state (G) in FIG. 15).

The heating conditions here are the same as the heating conditions described in detail with reference to FIGS. That is, the heating conditions for obtaining a plate-like single crystal made of a metal oxide represented by MO from a plate-like single crystal made of a metal hydroxide represented by M (OH) ₂ are vacuum or insoluble. In an active gas atmosphere, a temperature range of 200 ° C. to 800 ° C. is preferable, and a plate-like single crystal made of a metal hydroxide represented by M (OH) ₂ is made of a metal oxide represented by M ₃ O ₄ The heating conditions for obtaining the plate-like single crystal are preferably in the temperature range of 400 ° C. to 700 ° C. in the air.

  As described with reference to FIGS. 4 and 5, the topological phase transition from the plate-like single crystal 400 made of metal hydroxide to the plate-like single crystal 100 made of metal oxide is caused by M atoms. Proceeds in such a way that the sequence of is basically maintained. Specifically, the plate-like single crystal body 400 has [001] orientation, but becomes a [111] -oriented plate-like single crystal body 100 due to a topotactic phase transition caused by heating. As a result, the metal hydroxide film 1000 becomes a metal oxide thin film 600 composed of the plate-like single crystal body 100, and the metal oxide thin film 600 is completely oriented to [111] as a whole.

  In addition, in the topological phase transition by heating in step S960, the morphology of the plate-like single crystal 400 and the arrangement on the base material 610 are maintained also in the plate-like single crystal 100, so that the plate-like single crystal The metal oxide thin film 600 in which the bodies 100 are arranged in a close-packed manner without overlapping each other is obtained.

  Since step S960 is heated together with the base material 610, in addition to the topological phase transition, there is an effect of improving the adhesion between the base material 610 and the metal oxide thin film 600. According to the manufacturing method of FIG. 9 and FIG. 10, further annealing or the like for bringing the base material 610 and the metal oxide thin film 600 into close contact with each other is not necessary.

  Note that by repeating step S950 a plurality of times, a metal hydroxide film (not shown) in which the plate-like single crystal body 400 is laminated is obtained. By performing step S960 on such a metal hydroxide film, a metal oxide thin film (not shown) having a multilayer structure can be obtained. The frequency | count of step S950 is arbitrary according to the desired film thickness of a metal oxide thin film.

As described above with reference to FIGS. 9 and 10, the metal oxide thin film 600 of the present invention is a two-dimensional structure of the plate-like single crystal 400 made of a metal hydroxide represented by M (OH) _2. Manufactured using anisotropy. Since the method of the present invention does not require a complicated, large-scale and expensive apparatus represented by physical vapor deposition or chemical vapor deposition, a functional thin film having excellent crystallinity and orientation easily and inexpensively. Can provide. Further, according to the production method of the present invention, if a large container is obtained, the area can be easily increased, which is suitable for practical use. In addition, according to the method of the present invention, the adhesion between the metal oxide thin film 600 and the base material 610 is improved by heating for the topological phase transition from M (OH) ₂ to MO or M ₃ O ₄ . In order to improve, further annealing is unnecessary, which is preferable.

In the second embodiment, a metal oxide thin film using a plate-like single crystal composed of a metal oxide represented by MO and M ₃ O ₄ alone has been described in detail, but MO and M ₃ O ₄ may be used in combination.

(Embodiment 3)
In the third embodiment, the use of the plate-like single crystal 100 (FIG. 1) and the metal oxide thin film 600 (FIG. 6) described in the first and second embodiments will be described in detail.

  FIG. 11 is a schematic diagram showing a resistance variable element according to the present invention.

  The resistance variable element 1100 includes a first electrode 1110, a resistor 1120 located on the first electrode 1110, and a second electrode 1130 located on the resistor 1120. FIG. 11 shows a state where a power source 1140 such as a pulse voltage application device is connected to the first electrode 1110 and the second electrode 1130.

In FIG. 11, the first electrode 1110 includes a base material 1150 and an electrode material 1160 located on the base material 1150. The base material 1150 can be, for example, a silicon substrate, a glass substrate, a quartz substrate, a plastic substrate, or the like. The electrode material 1160 may be Ag, Au, Pt, Ru, RuO ₂ , Ir, IrO ₂ , TiO, TiN, TiAlN, Ta, TaN, a conductive polymer, or the like. As shown in FIG. 11, if the first electrode 1110 includes the base material 1150 and the electrode material 1160, the resistance variable element 1100 can be easily handled. Note that when the base material 1150 is a metal substrate made of a conductive metal material such as Ag, Au, or Pt, or an organic substrate made of a conductive polymer, the electrode material 1160 is unnecessary.

The resistor 1120 is the plate-like single crystal 100 (FIG. 1) made of the metal oxide represented by MO or M ₃ O ₄ described in Embodiment 1. A description of the plate-like single crystal body 100 is omitted.

Similarly to the electrode material 1160, the second electrode 1130 may be Ag, Au, Pt, Ru, RuO ₂ , Ir, IrO ₂ , TiO, TiN, TiAlN, Ta, TaN, a conductive polymer, or the like. In FIG. 11, the second electrode 1130 is shown to be provided on the resistor 1120, but a needle-like electrode material may be brought into contact with the resistor 1120 as the second electrode 1130.

  For example, in the manufacturing method of the resistance variable element 1100 including the single plate-like single crystal 100 as the resistor 1120, the application of the surface pressure in step S740 or step S940 is omitted in the manufacturing method of FIGS. That's fine. Thereby, since the plate-like single crystal bodies 100 are randomly arranged on the first electrode 1100 without being arranged in a close-packed manner, the resistance variable element 1100 can be obtained by appropriately cutting. .

  FIG. 12 is a schematic diagram showing another variable resistance element according to the present invention.

  Similar to the resistance variable element 1100 of FIG. 11, the resistance variable element 1200 includes a first electrode 1110, a resistor 1210 positioned on the first electrode 1110, and a second electrode positioned on the resistor 1210. An electrode 1130. FIG. 12 shows a state where a power source 1140 such as a pulse voltage application device is connected to the first electrode 1110 and the second electrode 1130.

A variable resistance element 1200 is obtained from the plate-like single crystal 100 (FIG. 1) made of the metal oxide represented by MO or M ₃ O ₄ described in Embodiment 2 instead of the resistor 1120 of FIG. This is the same as the resistance variable element 1100 except that the metal oxide thin film 600 (FIG. 6) is a resistor 1210.

  Next, the operation of the resistance variable elements 1100 and 1200 having such a configuration will be described.

  FIG. 13 is a schematic diagram showing the operation of the resistance variable element according to the present invention.

FIG. 13A schematically shows changes in the resistance state of the plate-like single crystal body 100 and the metal oxide thin film 600 according to the present invention. FIG. 13A shows a state where the plate-like single crystal body 100 and the metal oxide thin film 600 are sandwiched between electrodes. As shown in the left diagram of FIG. 13A, for example, in the initial state, the electric resistance (10 ⁹ Ω to 10 ¹⁴ Ω) of the plate-like single crystal body 100 and the metal oxide thin film 600 is extremely high, It is in such a state. On the other hand, when a predetermined voltage is applied to the plate-like single crystal body 100 and the metal oxide thin film 600 as shown in the right diagram of FIG. A linear conductive path 1310 is generated, and the electrical resistance is reduced.

  FIG. 13B schematically shows a relationship between current and voltage in the plate-like single crystal body 100 and the metal oxide thin film 600 according to the present invention.

As shown in the profile (1), in the initial state of the plate-like single crystal body 100 and the metal oxide thin film 600, even when a voltage is applied, a current does not substantially flow, and the state is like an insulator. However, when the voltage reaches a predetermined voltage (V _F ), a conduction path 1310 is generated, and a current rapidly flows through the plate-like single crystal body 100 and the metal oxide thin film 600, resulting in a low resistance state. This operation of profile (1) is called forming operation.

As shown in the profile (2), when a voltage is applied to the plate-like single crystal body 100 and the metal oxide thin film 600 that are in a low resistance state after the forming operation, when the voltage reaches a predetermined voltage (V _R ), the plate The conductive path 1310 in the single crystal body 100 and the metal oxide thin film 600 is broken, the current suddenly stops flowing, and the high resistance state is restored. Therefore, when the high resistance state is information “0”, information “0” is recorded in the plate-like single crystal body 100 and the metal oxide thin film 600 by the profile (2). Once the plate-like single crystal body 100 and the metal oxide thin film 600 are in a high resistance state, the high resistance state is maintained even if the voltage is removed. That is, the written information is non-volatile.

The operation of the profile (2) is called a reset operation, and the current that flows in the reset operation is called a reset current (I _R ). Here the reset current I _R of the resistance variable element using the plate-shaped single crystal body 100 and the metal oxide thin film 600 of the present invention is less than 1 mA, not only improves the operation speed, the resistance variable element This is advantageous because heat generation can be suppressed.

As shown in profile (3), when a voltage is applied to the plate-like single crystal body 100 and the metal oxide thin film 600 that are in a high resistance state after the reset operation, when the voltage reaches a predetermined voltage (V _S ), the plate A conductive path 1310 is generated again in the single crystal body 100 and the metal oxide thin film 600, and a current starts to flow suddenly, resulting in a low resistance state. Therefore, when the low resistance state is information “1”, information “1” is recorded in the plate-like single crystal body 100 and the metal oxide thin film 600 by the profile (3). Once the plate-like single crystal body 100 and the metal oxide thin film 600 are in a low resistance state, the low resistance state is maintained even if the voltage is removed. That is, the written information is non-volatile. This operation of profile (3) is called a set operation.

  Thus, the plate-like single crystal body 100 and the metal oxide thin film 600 have a resistance change effect, and realize a memory operation by repeating a high resistance state and a low resistance state.

  FIG. 14 is a schematic diagram showing the memory operation of the resistance variable element of the present invention.

  After the forming operation, the resistance variable elements 1100 and 1200 are in the low resistance state. The high resistance state is information “0”, and the low resistance state is information “1”. The assignment of the information “0” and “1” is arbitrary.

If you resistance variable element 1100 and 1200 record information "0", a voltage pulse is applied _P R (voltage value of _{P R} is equal to _{V R)} by the power supply 1140. Thereby, the resistors 1120 and 1210 of the resistance variable elements 1100 and 1200 change from the low resistance state to the high resistance state. As a result, information “0” is recorded in the resistance variable elements 1100 and 1200 (reset operation).

If you resistance variable element 1100 and 1200 record information "1", a voltage pulse is applied _P S (voltage value of _{P S} is equal to _{V S)} by the power supply 1140. Thereby, the resistors 1120 and 1210 of the resistance variable elements 1100 and 1200 change from the high resistance state to the low resistance state. As a result, information “1” is recorded in the resistance variable elements 1100 and 1200 (set operation).

Thereafter, the memory operation is performed by repeatedly performing the reset operation and the set operation. Here, the voltage pulse P _R and P _S, a direction different voltages, the resistance variable element 1100 and 1200 of the present invention has been operated at bipolar, it may be operated in a unipolar type.

Next, when reading the information recorded in the resistance variable elements 1100 and 1200, voltages (<< V _S , V _R ) whose resistance does not change via the power source 1140 are changed to the resistance variable elements 1100, It may be applied to 1200. At this time, the current flowing through the resistance variable elements 1100 and 1200 varies depending on the state of the resistors 1120 and 1210 (high resistance state or low resistance state), and thus information can be easily read from these current values. .

  Furthermore, the resistance variable elements 1100 and 1200 of the present invention are nonvolatile because the electrical resistance of the resistors 1120 and 1210 does not change even when the power is turned off.

  The present invention will now be described in detail using specific examples, but it should be noted that the present invention is not limited to these examples.

In Example 1, a plate-like single crystal composed of a metal hydroxide represented by Co (OH) _{2 in} which M is Co (hereinafter simply referred to as a Co (OH) ₂ plate-like single crystal) is used as a starting material. 9 and 10 were used to manufacture a metal oxide thin film (hereinafter simply referred to as a CoO film) made of a plate-like single crystal made of a metal oxide represented by CoO.

Prior to the production of the CoO film according to Example 1, a Co (OH) ₂ plate-like single crystal was produced.

FIG. 15 is a diagram showing how a Co (OH) ₂ plate-like single crystal is manufactured.

As shown in FIG. 15, referring to Non-Patent Document 1, an aqueous solution containing CoCl ₂ · 6H ₂ O and hexamethylenetetramine (HMT) (CoCl ₂ · 6H ₂ O concentration was 5 mM and 7.5 mM). Co (OH) ₂ plate-like single crystal was produced by a uniform precipitation method in which the solution was refluxed.

It was confirmed by observation and structural analysis that the product thus obtained was a Co (OH) ₂ plate-like single crystal. Specifically, the appearance and the like of the product were observed using a digital camera, a scanning electron microscope SEM (Keyence VE8800), and a transmission electron microscope TEM (JEOL JEM-3100F). Moreover, the product was obtained by an electron diffraction pattern using TEM and an X-ray diffraction pattern using a powder X-ray diffractometer (Rigaku RINT-2200, CuKα ray (λ = 0.15405 nm) monochromatized by a monochromator). The structural analysis of These results are shown in FIGS. FIGS. 16-17 show the results for products made from aqueous solutions with a CoCl ₂ .6H ₂ O concentration of 5 mM.

FIG. 16 is a diagram showing an appearance (A) and an SEM image (B) of the Co (OH) ₂ plate-like single crystal according to Example 1.

  According to FIG. 16 (A), the product obtained by the homogeneous precipitation method was milky pink. According to FIG. 16 (B), it was confirmed that the product was hexagonal and the lateral size L was about 4 μm. The thickness size was about 30 nm and the aspect ratio was about 111.

FIG. 17 is a diagram showing a TEM image (A) and an electron diffraction pattern (B) of a Co (OH) ₂ plate-like single crystal according to Example 1.

  According to FIG. 17A, it was found that the product had a hexagonal shape as in FIG. FIG. 17B shows perfect hexagonal diffraction spots that can be indexed as (100), (110) and (010) as two-dimensional in-plane diffraction, and are obtained along the [001] direction. It was found to be an electron diffraction pattern.

18 is a diagram showing an X-ray diffraction pattern of the Co (OH) ₂ plate-like single crystal according to Example 1. FIG.

All diffraction peaks were indexed to brucite-type Co (OH) ₂ (space group P 3 m1) (JCPDS card No. 74-1057), and no other diffraction peaks were observed.

Although not shown, the Co (OH) ₂ plate single crystal produced from an aqueous solution having a CoCl ₂ .6H ₂ O concentration of 7.5 mM is the same as FIGS. 16 to 18 except that the lateral size is 2 μm. Met.

As described above, it was confirmed from FIGS. 16 to 18 that the obtained milky white pink product was a plate-like single crystal composed of a metal hydroxide represented by Co (OH) ₂ .

The Co (OH) ₂ plate-like single crystal thus obtained (horizontal size 4 μm, 2 μm) was subjected to gravity sedimentation and centrifugation (500 rpm, 10 minutes) 7 to 10 times, and the horizontal size monodispersity Improved.

Next, the CoO film | membrane of this invention was manufactured on the base material using Co (OH) ₂ plate-like single crystal. As the base material, a (100) Si substrate, a quartz substrate, and a conductive Pt substrate were used. The Si substrate and the quartz substrate were each immersed in a HCl / CH ₃ OH solution (1: 1) and then concentrated H ₂ SO ₄ for 30 minutes and washed. The Pt substrate was obtained by evaporating Pt on a thermally oxidized Si substrate provided with an adhesive Ti layer. The Pt substrate was photochemically cleaned by irradiating UV light in an ozone atmosphere.

In Langmuir trough (Filgen, LB40-KBC) containing water (Milli-Q water, 80 cm ³ ) as a sewage phase, Co (OH) ₂ plate-like single crystal (80 mg) with improved monodispersibility is dispersed, A dispersion was obtained (FIG. 10A). Langmuir Traf is coated with Teflon (registered trademark) and has a Wilhelmy balance for measuring surface pressure. The effective trough surface area and trough volume of Langmuir Traf were 24.3 × 5 cm ² and 100 cm ³ , respectively.

Next, hexane (8 cm ³ ) was added as a first solvent to the dispersion to form an interface between water and hexane (first solvent) (step S910 in FIG. 9 and FIG. 10B).

Using a Hamilton syringe operated with a syringe pump (Harvar Apparatus, MA 170-228), butanol (1.5 cm ³ ) as a second solvent was slowly added to the interface at a rate of 0.1 cm ³ / min (FIG. 9 step S920 and FIG. 10 (C)). The Co (OH) ₂ plate-like single crystal in the dispersion before the butanol addition had a contact angle of less than 90 ° with respect to the interface, but after the butanol addition, the Co (OH) ₂ plate-like single crystal The body had a contact angle of 90 ° to the interface due to changes in surface energy. As a result, the Co (OH) ₂ plate-like single crystal dispersed in the sewage phase is immediately trapped at the interface between water and hexane, and a continuous film (trapped Co (OH) ₂ plate-like single crystal is obtained. Or a simple Co (OH) ₂ film).

Hexane (first solvent) was removed by evaporation (step S930 in FIG. 9 and FIG. 10D). Subsequently, a surface pressure was applied to the trapped Co (OH) ₂ plate-like single crystal (Co (OH) ₂ film) (step S940 in FIG. 9 and FIG. 10E). The Co (OH) ₂ film was applied at 3 mm / min until the surface pressure of the film was 15 mNm ⁻¹ .

FIG. 19 is a diagram showing a surface pressure-surface area (π-A) curve of a trapped Co (OH) ₂ plate-like single crystal.

FIG. 19 shows the behavior of a film transferred to a substrate under a constant surface pressure of 15 mNm- ¹ . The horizontal axis of FIG. 19 shows the ratio of the surface to the area before the surface pressure is applied and before the film is transferred to the substrate.

Next, a base material (Si substrate, quartz substrate, conductive Pt substrate) is immersed in the dispersion, and trapped Co (OH) ₂ plate-like single crystal by a vertical pulling method (pulling speed 3 mm / min) (Co (OH) ₂ film) was transferred to the substrate (step S950 in FIG. 9 and FIG. 10E). At this time, the surface pressure was maintained at 15 mNm- ¹ . Thus, a Co (OH) ₂ plate-like single crystal (Co (OH) ₂ film) trapped on each substrate was obtained. For simplicity, in the following, Co _{(OH) 2} film on the Si substrate, Co _{(OH) 2} film on the quartz substrate, and a Co _{(OH) 2} film on Pt substrate, respectively, Co _{(OH) 2} Film / Si, Co (OH) ₂ film / quartz, and Co (OH) ₂ film / Pt.

The cross section and surface of the Co (OH) ₂ film / Si were observed with a digital camera and SEM. The observation results are shown in FIGS. X-ray diffraction patterns were measured for Co (OH) ₂ film / Si, Co (OH) ₂ film / quartz, and Co (OH) ₂ film / Pt. The measurement results are shown in FIG.

Co (OH) ₂ film / Si, Co (OH) ₂ film / quartz, and Co (OH) ₂ film / Pt are heated in vacuum at 600 ° C. for 10 minutes using an infrared image furnace (MILA-3000). Then, Co (OH) ₂ was phase-shifted to CoO (step S960 in FIG. 9 and step S310 in FIG. 3).

The X-ray diffraction pattern was measured for the Co (OH) ₂ film / Si after heating, and the surface was observed by SEM. The results are shown in FIGS. 24 and 25. The UV-vis absorption spectrum of the Co (OH) ₂ film / quartz after heating was measured using a spectrophotometer (Hitachi, U-4100). The measurement results are shown in FIG. Further, the optical band gap was calculated from the UV-vis absorption spectrum. The results are shown in FIG.

  The results of FIGS. 20 to 27 will be described in detail.

20 is a diagram showing a digital camera image (A) and an SEM image (B) of a cross section of a Co (OH) ₂ film / Si film according to Example 1. FIG.

Samples of FIG. 20 is a Co _{(OH) 2} film using Co _{(OH) 2} platelike single crystal of horizontal size 4 [mu] m. 20A and 20B, it was found that a continuous film made of a Co (OH) ₂ plate-like single crystal was obtained on a Si substrate. The film thickness was about 30 nm, reflecting the thickness size of the Co (OH) ₂ plate-like single crystal. Although not shown, even when using the Co (OH) ₂ platelike single crystal of lateral size 2.mu. m with similar results.

FIG. 21 is a diagram showing an SEM image of the surface of the Co (OH) ₂ film / Si film according to Example 1. FIG.
22 is a view showing an SEM image of the surface of another Co (OH) ₂ film / Si film according to Example 1. FIG.

The samples of FIGS. 21 and 22 are Co (OH) ₂ films using Co (OH) ₂ plate-like single crystals with lateral sizes of 4 μm and 2 μm, respectively. According to FIGS. 21 and 22, either, regardless of the horizontal size of the Co _{(OH) 2} platelike single crystals, hexagonal Co _{(OH) 2} platelike single crystal, the plane of the Si substrate It was located parallel to the surface. Specifically, the Co (OH) ₂ plate-like single crystals may be arranged so as to be in contact with each other, that is, in a close packing manner in the plane direction of the Co (OH) ₂ plate-like single crystals. I understood.

FIG. 23 shows XRD patterns of Co (OH) ₂ film / Pt (a), Co (OH) ₂ film / Si (b), and Co (OH) ₂ film / quartz (c) film according to Example 1. FIG.

In all of FIGS. 23A to 23C, all the diffraction peaks except the diffraction peaks from the substrate (Pt (111) and Si (400)) are in Brusite Co (OH) ₂ (JCPDS74-1057). Consistent with the indexed bottom reflection series 00l (l = 1, 2, 3). The face spacing was 4.6 mm. In-plane reflection (for example, (100), (110)) seen in the powder X-ray diffraction pattern of the Co (OH) ₂ plate-like single crystal shown in FIG. 17 is shown in FIGS. No XRD pattern was found.

  In a brucite-type hydroxide, octahedral units composed of Co ions coordinated with six hydroxyl groups share a ridge and form an infinitely spread charge neutral layer.

From these FIGS. 20 23, regardless of the horizontal size or substrates of the Co _{(OH) 2} platelike single crystal, the Co _{(OH) 2} platelike single crystal body is close-packed like in the planar direction A Co (OH) ₂ film (the film is completely oriented in [001]) in which each of the Co (OH) ₂ plate-like single crystals was aligned in the [001] direction was obtained. It was confirmed.

FIG. 24 is a diagram showing XRD patterns of Co (OH) ₂ film / Si (a) and CoO film / Si (b) films according to Example 1. FIG.

  FIG. 24A is the same as FIG. Comparing FIGS. 24A and 24B, the XRD pattern changed dramatically by heating at 600 ° C. for 10 minutes in a vacuum. According to the XRD pattern of FIG. 24B, all the diffraction peaks of FIG. 24A disappeared, and only two diffraction peaks were shown at different positions.

These two diffraction peaks were found to be indexed to the (111) and (222) reflections of rock salt type cobalt oxide I (CoO, JCPDS 75-0418). This result indicates that the [001] oriented Co (OH) ₂ film / Si has undergone a phase transition to the [111] fully oriented CoO film / Si by the heating. Although not shown, similar XRD patterns were obtained for the heated Co (OH) ₂ film / quartz and Co (OH) ₂ film / Pt.

  25 is a view showing an SEM image of the surface of the CoO film / Si film according to Example 1. FIG.

Samples of FIG. 25 is a Co _{(OH) 2} film was heated sample using Co _{(OH) 2} platelike single crystal of horizontal size 2 [mu] m. According to FIG. 25, the plane of the hexagonal plate-shaped single crystal having a lateral size of 2 μm and a thickness of 30 nm was positioned so as to be parallel to the surface of the Si substrate. Since it is known from FIG. 24 that the entire sample is CoO, this hexagonal plate-like single crystal is called a plate-like single crystal made of a metal oxide represented by CoO (called a CoO plate-like single crystal). ). According to FIG. 25, the heated Co (OH) ₂ film / Si is such that the edges of the CoO plate-like single crystal are in contact with each other on the Si substrate, that is, the CoO plate-like single crystal is in the plane direction. It was found that they were arranged in a close packed manner.

When FIG. 21 is compared with FIG. 25, it can be seen that the size, morphology, and close-packed arrangement of the Co (OH) ₂ plate-like single crystal used as the starting material are maintained by the heating. confirmed. Although not shown, the same applies to the heated Co (OH) ₂ film / quartz and Co (OH) ₂ film / Pt.

For the sake of simplicity, Co (OH) ₂ film / Si, Co (OH) ₂ film / quartz, and Co (OH) ₂ film / Pt are heated and phase transition samples are respectively obtained as CoO film / Si and CoO. Film / quartz and CoO film / Pt.

24 and 25, it was shown that M (OH) ₂ is phase-changed topologically to MO by heating in step S310 in FIG. 3 and step S960 in FIG. Further, if the manufacturing method of FIG. 9 is used, a plate-like single crystal made of a metal oxide represented by MO is arranged in a close-packed manner in the plane direction, and a [111] fully oriented metal oxide thin film. Was shown to be obtained.

FIG. 26 is a view showing a UV-vis absorption spectrum of the CoO film / quartz according to Example 1.
FIG. 27 is a diagram showing the optical band gap of the CoO film / quartz according to Example 1. FIG.

The band gap Eg can be calculated from the following equation.
αhν = A (hν−Eg) ⁿ
Here, α is an absorption coefficient, hν is photon energy, A is a material-specific constant, and n is 2 (in the case of indirect transition) or 1/2 (in the case of allowable direct transition). When (αhν) ^{1 / n} is plotted against hν (eV), a straight line portion is obtained as shown in FIG.

As shown in FIG. 27, when n = 1/2, the straight line portion was fitted best. Therefore, it was found that the CoO film is an allowable direct transition. By sweeping the straight line portion to (αhν) ² = 0, the optical band gap of the CoO film was calculated to be 2.6 eV. This value agreed well with that of CoO reported as a Mott insulator (about 2.5 ± 0.3 eV).

  As described above, FIG. 26 and FIG. 27 show that the MO obtained by heating in step S310 in FIG. 3 and step S960 in FIG. 9 is an insulator in the initial state. Further, if the manufacturing method of FIG. 9 is used, the plate-like single crystals made of the metal oxide represented by MO are arranged in a close-packed manner in the plane direction, and [111] is perfectly oriented, making it insulative. It was shown that an excellent metal oxide thin film can be obtained.

  In Example 2, the variable resistance element was manufactured using the CoO film / Pt substrate obtained in Example 1. A schematic diagram of the resistance variable element is shown in FIG.

  FIG. 28 is a schematic diagram of a resistance variable element according to Example 2.

The resistance variable element 2800 includes a first electrode 1110, a resistor 1210 placed on the first electrode 1110, and a second electrode 1130 located on the resistor 1210. Here, the first electrode 1110 is the above-described Pt substrate obtained by vapor-depositing Pt on a thermally oxidized Si substrate provided with an adhesive Ti layer. The resistor 1210 is a CoO film manufactured on the Pt substrate using the manufacturing method of FIG. 9 (the lateral size of the Co (OH) ₂ plate-like single crystal used as the starting material was 2 μm). Ta or Pt metal was deposited on the resistor 1210 as the second electrode 1130 using a shadow mask (Φ = 100 μm). Such a resistance variable element 2800 was connected as a power source 1140 to a pulse voltage application device.

  The resistance variable element 2800 was observed with an SEM, and electrical characteristics were measured using a semiconductor parameter analyzer (Keithley, 4200-SCS) equipped with a pulse generator. The results are shown in FIGS.

  FIG. 29 is a view showing an SEM image of the resistance variable element according to Example 2. FIG.

  In FIG. 29, it was confirmed that a circular portion with a bright contrast was Ta (Pt) metal deposited as the second electrode 1130. Since the diameter Φ of the second electrode 1130 is 100 μm, the second electrode 1130 covers several hundreds of CoO plate-like single crystals in the CoO film.

  FIG. 30 is a diagram illustrating the switching characteristics (IV) of the resistance variable element according to Example 2.

  A power source 1140 was connected between the first electrode 1110 and the second electrode 1130, and electrical characteristics (IV) were measured using a W probe. A voltage was applied from 0 V to 5 V to the resistance variable element 2800 (profile a), then a voltage was applied from 0 V to 1.5 V (profile b), and a voltage was applied again from 0 V to 3 V (profile c). The current change at the time was examined.

In FIG. 30, the CoO film was found to have a high resistance of 10 ¹³ Ω or more in the initial state (the low voltage side of profile a). Since the CoO film has such a high resistance, it has been found that the CoO film does not have a clear leak path due to the arrangement of the CoO plate-like single crystals, and has uniform characteristics throughout the film. According to profile a, when the voltage was increased from 0 V to about 4.5 V, the resistance variable element 2800 showed a sudden resistance change from the high resistance state to the low resistance state. The electrical resistance of the CoO film in the low resistance state was about 1 × 10 ⁷ Ω. From this behavior, it was confirmed that profile a was a forming operation.

According to the profile b, when the voltage was increased from 0 V to 1.0 V, the resistance variable element 2800 again showed a resistance change from the low resistance state to the high resistance state. From this behavior, it was confirmed that profile b was a reset operation. Here, the reset current was several 10 ⁻⁴ A and less than 1 mA. Such a low reset current is advantageous because it not only improves the operation speed of the resistance variable element but also suppresses heat generation of the resistance variable element.

  According to the profile c, when the voltage was increased from 0 V to about 2 V, the resistance variable element 2800 again showed a resistance change from the high resistance state to the low resistance state. From this behavior, it was confirmed that the profile c was a set operation.

The resistance window between the high resistance state and the low resistance state (R _HRS / R _LRS ) is about 1 × 10 ³ , which exceeds that using CoO films manufactured by existing physical vapor deposition methods. Or it was found to have comparable performance. Therefore, when the MO film obtained by the manufacturing method of the present invention is applied to a resistance variable element, information (“1” or “0”) can be reliably distinguished by a wide window.

  From the above profiles a to c, the metal oxide thin film represented by MO according to the present invention was formed by a reset operation in which local conductive filaments were formed in the metal oxide thin film by a forming operation or a set operation. It has been found that the conductive filament has a resistance change effect with the same mechanism as the existing binary oxide, in which the conductive filament is broken. However, since the metal oxide thin film of the present invention is a thin film in which a plate-like single crystal of metal oxide represented by MO is arranged in a predetermined arrangement, it is a binary oxidation obtained by an existing physical vapor deposition method. Unlike products, it was confirmed that it can be reset with an extremely low reset current (1 <mA) and exhibits completely new characteristics.

  FIG. 31 is a diagram illustrating a result of a repetition test of a set / reset operation of a resistance variable element according to Example 2.

  A voltage of 2.8 V / 20 ns for the set operation and a voltage of −3.0 V / 20 ns for the reset operation were repeatedly applied to the resistance variable element 2800 up to 50 times. The resistance state of the resistance variable element 2800 during the repetition was read using a minute voltage (0.1 V) that does not cause a reset operation.

  As shown in FIG. 31, it was found that the high resistance state and the low resistance state were stably shown even when the set operation and the reset operation were repeated. From this, it was shown that the resistance variable element of the present invention is excellent in operational stability and can be a low-voltage driven nonvolatile memory.

In Example 3, resistance change using the single CoO plate-like single crystal obtained in Example 1 (the lateral size of the Co (OH) ₂ plate-like single crystal used as the starting material was 4 μm) A mold element was manufactured. Example 3 was the same as Example 1 except that the same Pt substrate as in Example 1 was used and Step S940 (application of surface pressure) in FIG. 9 was omitted.

In Example 3, the Co (OH) ₂ plate-like single crystal trapped at the interface and transferred to the Pt substrate without applying surface pressure has a plate-like crystal plane parallel to the plane of the Pt substrate. However, it is different from the first embodiment in that it is not arranged in a close-packed manner in the plane direction, but is arranged with random and gaps. The surface of the CoO film on the Pt substrate thus obtained was observed with an atomic force microscope AFM (SII nanotechnology, E-Sweep). For observation, a Si cantilever coated with conductive Pt / Ir was used. The observation results are shown in FIG.

  FIG. 32 is a diagram showing an AFM image of the CoO film / Pt according to Example 3.

  According to FIG. 32, it was found that the CoO film was not a close-packed arrangement of CoO plate single crystals, but a random arrangement having gaps between the CoO plate single crystals. From this, it was shown that the application of the surface pressure (step S740 in FIG. 7 or step S940 in FIG. 9) is suitable for arranging the plate-like single crystals in the closest packing manner.

  Next, the obtained CoO film / Pt was appropriately cut to manufacture resistance change elements using individual CoO plate-like single crystals. The CoO plate-like single crystal on the Pt substrate after cutting was observed with TEM. The observation results are shown in FIG.

  FIG. 33 is a diagram showing a TEM image (A) and an electron diffraction pattern (B) of CoO plate-like single crystal / Pt according to Example 3.

  According to FIG. 33 (A), it was found that the CoO plate-like single crystal had a hexagonal shape as in FIG. 17 (A). FIG. 33B shows perfect hexagonal diffraction spots, which can be indexed as (101 二), (21￣1￣) and (11￣0) as two-dimensional in-plane diffraction, [111] It was found to be an electron diffraction pattern obtained along the direction. In addition, the symbol “￣” means that it is added on the immediately preceding number.

  The electrical characteristics of the variable resistance element using a single CoO plate-like single crystal were measured. AFM was used for the measurement. The configuration of the resistance variable element is shown in FIG. 34, and the electrical characteristics are shown in FIG.

  FIG. 34 is a schematic diagram of a resistance variable element according to Example 3.

  The resistance variable element 3400 includes a first electrode 1110, a resistor 1120 placed on the first electrode 1110, and a second electrode 1130 located on the resistor 1120. Here, the first electrode 1110 is the above-described Pt substrate obtained by vapor-depositing Pt on a thermally oxidized Si substrate provided with an adhesive Ti layer. The resistor 1120 is a CoO plate-like single crystal manufactured on a Pt substrate using the manufacturing method of FIG. 9 (except step S940). A conductive Pt / Ir-coated Si cantilever was used as the second electrode 1130 on the resistor 1120. Such a resistance variable element 3400 was connected to a power source (not shown).

  FIG. 35 is a diagram illustrating the switching characteristics (IV) of the resistance variable element according to Example 3.

  A DC voltage sweeping device (not shown) was connected between the first electrode 1110 and the second electrode 1130, and the electrical characteristics (IV) were measured. A voltage is applied from 0 V to 5 V to the resistance variable element 3400 (profile a), then a voltage is applied from 0 V to 1.0 V (profile b), and a voltage is applied from 0 V to -3 V (profile c). The change in current when voltage was applied from −3 V to 3 V (profile d) was examined.

In FIG. 35, the CoO plate-like single crystal was found to have a high resistance of 10 ¹² Ω in the initial state (low voltage side of profile a). Since the CoO plate-like single crystal has such a high resistance, it matches well with the properties of the Mott type insulator. According to profile a, when the voltage was increased from 0 V to about 3 V, the resistance variable element 3400 suddenly changed in resistance from the high resistance state to the low resistance state. In the low resistance state, the resistance variable element 3400 shows 100 nA, and this value is the compliance current. The resistance of the CoO film in the low resistance state was about 1 × 10 ⁷ Ω. From this behavior, it was confirmed that profile a was a forming operation.

  According to the profile b, the resistance variable element 3400 maintained the low resistance state even when the voltage was increased from 0V to 1.0V. That is, if the voltage is 1.0 V or less after the forming operation, a constant compliance current is achieved, so that information can be stored in a nonvolatile manner.

  According to the profile c, when the voltage was increased from 0 V to −3.0 V, the resistance variable element 3400 again showed a resistance change from the low resistance state to the high resistance state when the applied voltage was −2.8 V. From this behavior, it was confirmed that profile c was a reset operation. Here, the reset current was 100 nA and less than 1 mA. Such a low reset current is advantageous because it not only improves the operation speed of the resistance variable element but also suppresses heat generation of the resistance variable element.

  According to the profile d, when the voltage was increased from −3.0 V to about 3.0 V, the resistance variable element 3400 again showed a resistance change from the high resistance state to the low resistance state. From this behavior, it was confirmed that the profile d was a set operation.

  Thus, according to Example 3, even in a single CoO plate-like single crystal, a high resistance state (that is, information “0” or off) and a low resistance state (information “1” or on) It was confirmed that the transition between and can be induced by an external electric field. As described above, the single MO plate single crystal of the present invention can be applied to a resistance variable element and can function as a nonvolatile memory.

In Example 4, a Co (OH) ₂ plate-like single crystal having M of Co (the lateral size was 4 μm) was used as a starting material, and the production method of FIGS. 9 and 10 was used to produce Co ₃ O ₄ A metal oxide thin film (hereinafter simply referred to as a Co ₃ O ₄ film) made of a plate-like single crystal made of a metal oxide represented by

Example 4 is the same as Example 1 except that the heating conditions (in vacuum, 600 ° C., 10 minutes) using the infrared image furnace in Example 1 are changed to 800 ° C. for 2 hours in the atmosphere using a muffle furnace. It was. The X-ray diffraction pattern of the Co (OH) ₂ film / quartz after heating was measured and observed by TEM. The results are shown in FIGS. 36 and 38. The Co (OH) ₂ film / quartz after heating was subjected to hard X-ray photoelectron spectroscopy at SPring-8 (BL15XU, hν = 5.95 eV @ 300K). The binding energy was calculated using the Fermi level of the gold thin film.

FIG. 36 is a diagram showing an XRD pattern of Co ₃ O ₄ film / quartz according to Example 4.

The XRD pattern of FIG. 36A is an XRD pattern of Co (OH) ₂ film / quartz before heating, and the XRD pattern of FIG. 36B is an XRD pattern of Co (OH) ₂ film / quartz after heating. It is a pattern. The XRD pattern shown in FIG. 36A is a (001) -oriented Co (OH) ₂ film showing diffraction peaks of (001), (002), and (003), similar to that shown in FIG. 23 (a). It was confirmed.

According to FIG. 36 (b), three diffraction peaks are shown. These three diffraction peaks are (111), (222) and (222) of spinel-type cobalt oxide (Co ₃ O ₄ , JCPDS 74-2120). 333) It was found that the reflection is indexed. From this, it was confirmed that Co (OH) ₂ after heating is Co ₃ O ₄ , and [111] perfectly oriented Co ₃ O ₄ film / quartz is obtained.

FIG. 37 is a diagram showing a Co ₂ p inner-shell photoelectron spectrum of Co ₃ O ₄ film / quartz according to Example 4.

FIGS. 37A and 37B are the inner-shell photoelectron spectra of the CoO film / quartz according to Example 1 and the Co ₃ O ₄ film / quartz according to Example 4, respectively. According to FIG. 37A, the binding energy of the main peak is 780.1 eV, indicating that Co is in a divalent state. On the other hand, according to FIG. 37 (b), the main peak was shifted to a lower energy side as compared with that of FIG. 37 (a), and had a binding energy of 778.6 eV. This also indicates that a part of Co is in a trivalent state, and Co (OH) ₂ was changed to Co ₃ O ₄ by the heating.

FIG. 38 is a view showing a TEM image of Co ₃ O ₄ film / Si according to Example 4.

According to FIG. 38, it was found that the Co ₃ O ₄ film obtained in Example 4 was also composed of a Co ₃ O ₄ plate-like single crystal having a hexagonal shape. Although not shown, it was confirmed by observation by SEM that the Co ₃ O ₄ film obtained in Example 4 also had a hexagonal shape of Co ₃ O ₄ plate-like single crystals arranged in a close-packed manner. . However, unlike the CoO plate-like single crystal of Example 1, the Co ₃ O ₄ plate-like single crystal was rough and porous. As described with reference to FIG. 2 and FIG. 5, Co ₃ O ₄ needs to be partially oxidized from Co ²⁺ to Co ³⁺ , and the phase from Co (OH) ₂ to Co ₃ O ₄ This is because the transition involves chemical and crystallographic reconstruction. However, in spite of the occurrence of such reconstruction, if the heating conditions of the present invention (FIGS. 3 and 5) are employed, the hexagonal shape of the Co (OH) ₂ plate-like single crystal is maintained, It has been shown that a Co ₃ O ₄ plate-like single crystal can be obtained.

In Example 5, a plate-like single crystal composed of a metal hydroxide represented by Co _0.67 Ni _0.33 (OH) _{2 in} which M is Co and Ni (hereinafter simply referred to as (Co, Ni) (OH ) as ₂ is referred to as a plate-like single crystal) the starting material, by using the manufacturing method of FIGS. 9 and 10, a metal oxide comprising a plate-like single crystal body consisting of a metal oxide represented by Co ₂ NiO ₄ A thin film (hereinafter simply referred to as a Co ₂ NiO ₄ film) was produced.

A plate-like single crystal of metal hydroxide ((Co, Ni) (OH) ₂ ) represented by Co _0.67 Ni _0.33 (OH) ₂ is added to CoCl ₂ .6H ₂ O in addition to NiCl 2. except plus ₂ · 6H _{2 O,} it was prepared as in example 1. Using the (Co, Ni) (OH) ₂ plate-like single crystal thus obtained (lateral size was 2 μm) as a starting material, a Co ₂ NiO ₄ film was formed on a Si substrate in the same procedure as in Example 4. And produced on a glass substrate.

A sample ((Co, Ni) (OH) ₂ film / Si) transferred to a Si substrate using a (Co, Ni) (OH) ₂ plate-like single crystal trapped at the interface as a base material is observed by SEM, and X The line diffraction pattern was measured. The results are shown in FIGS. 39 and 40.

An X-ray diffraction pattern of a sample (Co ₂ NiO ₄ film / Si) obtained by heating (Co, Ni) (OH) ₂ film / Si in the atmosphere at 800 ° C. for 2 hours using a muffle furnace was measured. The results are shown in FIG.

FIG. 39 is a view showing an SEM image of the surface of the (Co, Ni) (OH) ₂ film / Si film according to Example 5.

According to FIG. 39, the hexagonal (Co, Ni) (OH) ₂ plate-like single crystal having a lateral size of 2 μm and a thickness of 30 nm is positioned so that its plane is parallel to the surface of the Si substrate. Was. Specifically, the edges of the (Co, Ni) (OH) ₂ plate-like single crystal are in contact with each other, that is, the (Co, Ni) (OH) ₂ plate-like single crystal is most in the plane direction. It was found that they were arranged like a close packing.

FIG. 40 is a diagram showing XRD patterns of the (Co, Ni) (OH) ₂ film / Si (a) and Co ₂ NiO ₄ film / Si (b) films according to Example 5.

According to FIG. 40 (a), all diffraction peaks coincided with the bottom reflection series 00l (l = 1, 2, 3) indexed to brucite Co (OH) ₂ (JCPDS74-1057). The position of the diffraction peak was slightly shifted from the position of the diffraction peak shown in FIG. 23B (Co (OH) ₂ film / Si). This is because Ni was dissolved in the Co site of Co (OH) ₂ . Although not shown, the lattice constant obtained from the (Co, Ni) (OH) ₂ film is the lattice constant obtained from the XRD pattern of the Ni (OH) ₂ film and the lattice constant obtained from the XRD pattern of FIG. It was confirmed that Vegard's law was followed.

According to FIG. 40 (b), three diffraction peaks are shown. These three diffraction peaks are (111), (222) and (222) of spinel-type cobalt oxide (Co ₃ O ₄ , JCPDS 74-2120). 333) It was found that the reflection is indexed. The position of the diffraction peak was slightly shifted from the position of the diffraction peak shown in FIG. 36B (Co ₃ O ₄ film / quartz). This is because Ni was dissolved in the Co site of Co ₃ O ₄ . Although not shown, the lattice constant obtained from the (Co, Ni) (OH) ₂ film after heating is obtained from the lattice constant obtained from the XRD pattern of the Ni ₃ O ₄ film and the XRD pattern shown in FIG. It was confirmed that the (Co, Ni) (OH) ₂ film after heating was a Co ₂ NiO ₄ film in accordance with Vegard's law based on the lattice constant.

As described above, according to FIGS. 39 and 40, the plate-like single crystal composed of a metal hydroxide represented by M (OH) _{2 in} which M is a solid solution system of two or more types is used as a starting material. with 10 manufacturing method, by employing a predetermined heating condition, the metal oxide comprising M comprises a metal oxide represented by M ₃ O ₄ is more solid-solution plate-like single crystal It was confirmed that a thin film could be manufactured.

In Example 6, a Co ₃ O ₄ film was produced using the production method of FIGS. 7 and 8 using a Co (OH) ₂ plate-like single crystal (lateral size 2 μm) as a starting material, as in Example 1. did.

A Co (OH) ₂ plate-like single crystal produced by the same procedure as in Example 1 was heated in the atmosphere at 800 ° C. for 2 hours using a muffle furnace. The X-ray diffraction pattern of the heated sample (Co ₃ O ₄ plate-like single crystal) was measured. The results are shown in FIG.

FIG. 41 is a view showing an XRD pattern of the Co ₃ O ₄ plate-like single crystal according to Example 6.

According to FIG. 41, all diffraction peaks are represented by (111), (220), (211), (222), (400), (422) of spinel type cobalt oxide (Co ₃ O ₄ , JCPDS 74-2120). ), (511), and (440) reflections were found to be indexed. Although not shown, a plate-like single crystal having a lateral size of 2 μm and a thickness of 30 nm and having a hexagonal shape was confirmed by SEM observation of the Co ₃ O ₄ plate-like single crystal after heating. Therefore, it was shown that the Co (OH) ₂ plate-like single crystal phase-changed into a Co ₃ O ₄ plate-like single crystal by the heating described above.

Next, Using the thus obtained Co ₃ O ₄ platy single crystal body, a metal oxide thin film of the present invention was produced on the substrate (quartz substrate). The quartz substrate was immersed in a HCl / CH ₃ OH solution (1: 1) and then concentrated H ₂ SO ₄ for 30 minutes and washed.

A Langmuir trough containing water (Milli-Q water, 80 cm ³ ) as a sewage phase was dispersed with a Co ₃ O ₄ plate-like single crystal (80 mg) with improved monodispersibility to obtain a dispersion (FIG. 8). (A)).

Next, hexane (8 cm ³ ) was added as a first solvent to the dispersion to form an interface between water and hexane (first solvent) (step S710 in FIG. 7 and FIG. 9B).

Using a Hamilton syringe operated with a syringe pump, butanol (1.5 cm ³ ) as a second solvent was slowly added to the interface at a rate of 0.1 cm ³ / min (step S720 in FIG. 7 and FIG. 9 (FIG. 9)). C)). The Co ₃ O ₄ plate single crystal in the dispersion before the butanol addition had a contact angle of less than 90 ° with respect to the interface, but after the butanol addition, the Co ₃ O ₄ plate single crystal was The contact angle was 90 ° with respect to the interface due to changes in surface energy. As a result, the Co ₃ O ₄ plate-like single crystal dispersed in the sewage phase is immediately trapped at the interface between water and hexane, and a continuous film (the trapped Co ₃ O ₄ plate-like single crystal or It was confirmed that a (Co ₃ O ₄ film) was formed.

Hexane (first solvent) was removed by evaporation (step S730 in FIG. 7 and FIG. 8D). Subsequently, a surface pressure was applied to the trapped Co ₃ O ₄ plate single crystal (Co ₃ O ₄ film) (step S740 in FIG. 7 and FIG. 8E). The surface pressure of the Co ₃ O ₄ film was applied at 3 mm / min until 15 mNm ⁻¹ was reached.

Next, the quartz substrate was immersed in the dispersion, and the trapped Co ₃ O ₄ plate single crystal (Co ₃ O ₄ film) was transferred to the quartz substrate by the vertical pulling method (pulling speed 3 mm / min). (Step S750 in FIG. 7 and FIG. 8E). At this time, the surface pressure was maintained at 15 mNm- ¹ . Thus, a Co ₃ O ₄ plate-like single crystal (Co ₃ O ₄ film) (Co ₃ O ₄ film / quartz) trapped on the quartz substrate was obtained. The Co ₃ O ₄ film / quartz was observed with an SEM, and an X-ray diffraction pattern was measured. The results are shown in FIGS. 42 and 43.

42 is a view showing an SEM image of the surface of the Co ₃ O ₄ film / quartz according to Example 6. FIG.

According to FIG. 42, the hexagonal Co ₃ O ₄ plate-like single crystal having a lateral size of 2 μm was positioned so that its plane was parallel to the surface of the quartz substrate. In addition, although some disorder of the arrangement is observed, the edges of the Co ₃ O ₄ plate single crystal are in contact with each other, that is, close packed in the planar direction of the Co ₃ O ₄ plate single crystal. I found out that it was arranged.

FIG. 43 is a view showing an XRD pattern of a Co ₃ O ₄ film / quartz according to Example 6.

The XRD pattern of FIG. 43A is the same as the XRD pattern of FIG. 41, and the XRD pattern of FIG. 43B is a Co ₃ O ₄ film / quartz XRD pattern. The XRD pattern in FIG. 43 (b) is different from the XRD pattern in FIG. 43 (a) and shows only two diffraction peaks. These two diffraction peaks are indexed to the (111) and (222) reflections of spinel-type cobalt oxide (Co ₃ O ₄ , JCPDS 74-2120), and [111] fully oriented Co ₃ O ₄ film / quartz. I found out.

As described above, according to FIGS. 42 and 43, the M ₃ O made of a metal hydroxide represented by ₄ platy single crystal as a starting material, by using the manufacturing method of FIGS. 7 and 8, M ₃ It was confirmed that a metal oxide thin film made of a plate-like single crystal made of a metal oxide represented by O ₄ can be produced.

  The plate-like single crystal according to the present invention is applied to building blocks, ultra-small resistance variable elements, and sensors. The metal oxide thin film of the present invention is applied to a resistance variable element and a sensor.

DESCRIPTION OF SYMBOLS 100 Plate-like single crystal consisting of metal oxide 110, 410 Plate-like crystal plane 400 Plate-like single crystal consisting of metal hydroxide 600 Metal oxide thin film 610 Base material 800 Water 810 First solvent 820 Interface 830 Second Solvent 840 Surface pressure 1000 Metal hydroxide film 1100, 1200, 2800, 3400 Resistance variable element 1110 First electrode 1120, 1210 Resistor 1130 Second electrode 1140 Power supply 1310 Conduction path

JP 2008-290913 A JP 2010-229007 A JP-A-6-256959

Liu et al. Am. Chem. Soc. 2005, 127, 13869 Liang et al., Chem. Mater. 2010, 22, 371 Shima et al., Appl. Phys. Lett. 2008, 93, 113504

Claims (6)

A metal oxide thin film made of a plate-like single crystal made of a metal oxide,
The metal oxide is represented by MO or M ₃ O ₄ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu),
The shape of the plate-like single crystal made of the metal oxide is a hexagonal shape,
The aspect ratio of the plate-like single crystal made of the metal oxide is 10 to 500,
The plane surface of the plate-shaped single crystal made of the metal oxide is a (111) crystal plane of the metal oxide,
The plate-like single crystals are arranged in a close-packed manner in the planar direction of the plate-like single crystals,
The metal oxide thin film is a metal oxide thin film oriented in [111]. It is a manufacturing method of the metal oxide thin film according to claim 1 ,
A first solvent that is either hexane or toluene is added to a dispersion in which a plate-like single crystal made of a metal oxide is dispersed in water, and the water in the dispersion and the first solvent Forming an interface;
Further adding a second solvent that is an alcohol, and trapping the plate-like single crystal made of the metal oxide at the interface;
Removing the first solvent;
Applying a surface pressure to the plate-like single crystal made of the metal oxide trapped at the interface;
Dipping a base material in the dispersion from which the first solvent has been removed, and transferring the trapped plate-like single crystal made of the metal oxide to the base material,
The metal oxide is represented by MO or M ₃ O ₄ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn, and Cu), and the metal oxide The shape of the plate-like single crystal made of a product is hexagonal, and the aspect ratio of the plate-like single crystal made of the metal oxide is 10 to 500, and the plate-like single crystal made of the metal oxide The plane in the plane direction is a (111) crystal plane of the metal oxide. Prior to the step of forming the interface,
A plate-like single crystal made of a metal hydroxide represented by M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu) is heated. The method according to claim 2, further comprising a phase transition to a plate-like single crystal made of the metal oxide. It is a manufacturing method of the metal oxide thin film according to claim 1 ,
A plate-like single crystal made of a metal hydroxide represented by M (OH) ₂ (where M is a metal element selected from the group consisting of Co, Fe, Ni, Zn and Cu) Adding a first solvent that is either hexane or toluene to a dispersion in which the body is dispersed in water to form an interface between the water in the dispersion and the first solvent;
Further adding a second solvent that is an alcohol, and trapping the plate-like single crystal made of the metal hydroxide at the interface;
Removing the first solvent;
Applying a surface pressure to the plate-like single crystal made of the metal hydroxide trapped at the interface;
Immersing a substrate in the dispersion from which the first solvent has been removed, and transferring the trapped plate-like single crystal made of the metal hydroxide to the substrate;
Heating the plate-like single crystal composed of the metal hydroxide transferred to the substrate. A first electrode;
A resistor positioned on the first electrode;
A second electrode located on the resistor;
The resistance variable element according to claim 1 , wherein the resistor is the metal oxide thin film according to claim 1 . The resistance variable element according to claim 5, wherein a reset current of the resistance variable element is less than 1 mA.