Patent Application
20250129466
The present disclosure relates to a λ-Ti3O5 film forming substrate and a method for producing the λ-Ti3O5 film forming substrate.
Trititanium pentoxide (hereinafter, may be referred to as Ti3O5) is an oxide material having many crystal polymorphisms such as an α-phase, a β-phase, a γ-phase, a δ-phase, and a λ-phase. λ-type Ti3O5 (hereinafter, may be referred to as λ-Ti3O5) consisting of the λ-phase undergoes a phase transition into the α-phase or the β-phase by heat, light, or pressure. Particularly, the λ-Ti3O5 undergoes a reversible transition between the λ-phase and the β-phase by a nanosecond pulsed laser, and exhibits a photo-induced phase transition under visible light. Therefore, the λ-Ti3O5 is expected to be applied to an optical recording medium.
As a method for producing λ-Ti3O5, NPL 1 discloses a method for producing reduction-sintered SiO2-coated λ-Ti3O5 nanoparticles by using a reverse micelle method and a sol-gel method.
Since the λ-Ti3O5 is a metastable phase, the size of a crystal is limited to a nanometer order. In a large crystal, the β-phase is most stable, and it is difficult to obtain λ-Ti3O5. In the method described in NPL 1, enlarging of crystals is suppressed by coating with SiO2. However, in the method described in NPL 1, a process such as chemical etching with SiO2 is necessary.
As a method for producing the λ-Ti3O5 in which the chemical etching with SiO2 is not necessary, NPL 2 discloses a technology of forming MgTi2O5 as a seed layer on LaAlO3, and forming the λ-Ti3O5 film on the seed layer.
When the seed layer exists between the λ-Ti3O5 film and the substrate as in NPL 2, a component of the seed layer diffuses into the λ-Ti3O5 film, and the λ-phase is stabilized, and thus a phase transition into the β-phase is not exhibited. This has a problem when considering application to the optical recording medium.
The present invention has been made in consideration of such circumstances, and an object thereof is to provide a λ-Ti3O5 film forming substrate including a λ-Ti3O5 film that is formed directly on the substrate and is capable of stably transitioning from λ-Ti3O5 into another phase, and a method for producing the λ-Ti3O5 film forming substrate.
To solve the problem, the invention suggests the following means.
According to the aspect of the invention, it is possible to provide a λ-Ti3O5 film forming substrate including a λ-Ti3O5 film that is formed directly on the substrate and is capable of stably transitioning from λ-Ti3O5 into another phase, and a method for producing the λ-Ti3O5 film forming substrate.
In the producing method described in NPL 2, the λ-Ti3O5 film cannot be formed directly on the substrate, and the λ-Ti3O5 film is formed through a seed layer. The present inventors have made a thorough investigation on a method for forming the λ-Ti3O5 film directly on the substrate without through the seed layer. As a result, they obtained a finding that it is most important to control a temperature of the substrate during film formation for direct formation of the λ-Ti3O5 film. Specifically, they have found that the λ-Ti3O5 film can be formed directly on the substrate by controlling the temperature of the substrate to be higher than 1000° C. and lower than 1200° C. The invention is completed on the basis of the findings.
(λ-Ti3O5 Film Forming Substrate)
Hereinafter, description will be given of a λ-Ti3O5 film forming substrate according to an embodiment of the invention with reference to the accompanying drawings. Note that, for convenience, the drawings used in the following description may illustrate characteristic portions in an enlarged manner for easy understanding of characteristics, and it cannot be said that dimension ratios and the like of respective constituent elements are the same as actual dimension ratios and the like. In addition, materials, dimensions, and the like exemplified in the following description are illustrative only, the invention is not limited thereto, and the invention can be implemented with appropriate modifications within a range not departing from the gist of the invention.
The support substrate 1 is not particularly limited as long as the λ-Ti3O5 film 2 can be formed on the support substrate 1. Examples of a material of the support substrate 1 include lanthanum aluminate (LaAlO3), neodymium aluminate (NdAlO3), yttrium aluminate (YAlO3), LSAT ((LaAlO3)0.3—(SrAl0.5Ta0.5O3)0.7), strontium titanate (SrTiO3), and the like. From the viewpoint of matching between an a-axis length, a b-axis length, and a c-axis length of the λ-Ti3O5 film, and surface lattice constants of the substrate, as the substrate, a (110) plane LaAlO3 substrate (LaAlO3 (110) substrate) is preferable.
(λ-Ti3O5 Film)
The λ-Ti3O5 film 2 is provided directly on the support substrate 1. The λ-Ti3O5 film 2 is a film consisting of a single crystal of λ-Ti3O5 in a λ-phase. As shown in
In thin film X-ray diffraction, when performing measurement at a position where a diffraction intensity A203 of a (203) plane of the λ-Ti3O5 film 2 becomes maximum, an intensity ratio A203/A20-3 between the diffraction intensity A203 of a (203) plane and a diffraction intensity A20-3 of a (20-3) plane of the λ-Ti3O5 film 2 is preferably 10 or more. More preferably, the intensity ratio A203/A20-3 is 50 or more. Still more preferably, the intensity ratio A203/A20-3 is 100 or more. When the intensity ratio A203/A20-3 is 10 or more, a rotational domain of the crystal is reduced, and a stable phase transition is observed.
In the thin film X-ray diffraction, when performing measurement at a position (hereinafter, may be referred to as ϕ180°) after the λ-Ti3O5 film forming substrate 10 is rotated by 180° from the position (hereinafter, may be referred to as ϕ0°) where the diffraction intensity A203 becomes maximum, an intensity ratio A20-3/A203 between the diffraction intensity A20-3 of the (20-3) plane and the diffraction intensity A203 of the (203) plane of the λ-Ti3O5 film 2 is preferably 10 or more. More preferably, the intensity ratio A20-3/A203 is 50 or more. Still more preferably, the intensity ratio A20-3/A203 is 100 or more. When the intensity ratio A20-3/A203 is 10 or more, a rotational domain of the crystal is reduced, and a stable phase transition is observed.
Next, description will be given of a method of measuring thin film X-ray diffraction.
In the thin film X-ray diffraction, an axis that is orthogonal to the surface of the λ-Ti3O5 film forming substrate 10 and passes through the center of the λ-Ti3O5 film forming substrate 10 becomes the ϕ-axis. A direction in which a direction connecting an X-ray incidence unit and an X-ray detection unit is projected to the λ-Ti3O5 film 2 is set as a first direction. The X-ray incidence unit is, for example, an X-ray generation unit and an incident optical system element. For example, the X-ray detection unit is a detector and a light-receiving optical element.
In the thin film X-ray diffraction, a [010] direction of the λ-Ti3O5 film 2 is arranged to be parallel to the first direction at ϕ0°. Note that, in a case where the λ-Ti3O5 film 2 is formed on an LaAlO3 (110) substrate, a [001] direction of the LaAlO3 substrate and a [010] direction of the λ-Ti3O5 film 2 may be arranged to be parallel to the first direction.
When performing the thin film X-ray diffraction measurement of the λ-Ti3O5 film forming substrate 10 with the [010] direction of the λ-Ti3O5 film 2 arranged to be parallel to the first direction, in an X-ray diffraction pattern (XRD pattern), a diffraction peak of a (203) plane or a diffraction peak of a (20-3) plane is shown. Here, a position where the intensity (diffraction intensity) of the diffraction peak of the (203) plane becomes maximum is set as ϕ0°. When measuring the thin film X-ray diffraction at a position of ϕ0°, an XRD pattern at the time of ϕ0° can be obtained. The intensity ratio A203/A20-3 can be obtained by analyzing the XRD pattern. In the obtained XRD pattern, the diffraction intensity A203 of the (203) plane is an intensity when 20 is 32.1°. The diffraction intensity A20-3 of the (20-3) plane is an intensity when 20 is 32.9° in the obtained XRD pattern. Note that, the position where the intensity of the diffraction peak of the (203) plane becomes maximum can be set, for example, by the following method. First, a sample is disposed at a position (initial position of 2θ, ω, χ, and ϕ) that is theoretically expected from a crystal structure and plane indexes of λ-Ti3O5. Then, the position can be set by adjusting 2θ, the ω-axis, the χ-axis, and the ϕ-axis so that the intensity of the diffraction peak of the (203) plane becomes a maximum value.
A position where the λ-Ti3O5 film forming substrate 10 is rotated by 180° around the axis ϕ from the position of ϕ0° is set as ϕ180°. 2θ, the ω-axis, the χ-axis, and the ϕ-axis are adjusted so that the intensity of the diffraction peak of the (20-3) plane becomes a maximum value at the position of ϕ180°. Then, the thin film X-ray diffraction is measured at the position of ϕ180° to obtain an XRD pattern at the time of ϕ180°. The intensity ratio A20-3/A203 can be obtained by analyzing the XRD pattern.
In a case where the support substrate 1 is an LaAlO3 (110) substrate, the λ-Ti3O5 film 2 preferably has a predetermined crystal orientation. Specifically, in a perpendicular-to-plane direction, [100] of the λ-Ti3O5 film 2 and [110] of LaAlO3 are preferably parallel to each other. In an in-plane direction, [010] of the λ-Ti3O5 film 2 and [001] of LaAlO3 are preferably parallel to each other. In a case where the λ-Ti3O5 film 2 has the orientation, the quality of the λ-Ti3O5 film 2 is improved, and thus this case is preferable. The orientation can be evaluated by the XRD pattern obtained in the thin film X-ray diffraction measurement of the λ-Ti3O5 film forming substrate 10.
The thickness of the λ-Ti3O5 film 2 is not particularly limited. For example, the thickness of the λ-Ti3O5 film 2 is from 1 nm to 1000 nm. The thickness of the λ-Ti3O5 film 2 is more preferably from 1 nm to 500 nm. The thickness of the λ-Ti3O5 film 2 is still more preferably from 1 nm to 200 nm.
In the λ-Ti3O5 film forming substrate 10 according to this embodiment, it is preferable that a component of the support substrate 1 substantially does not diffuse into the Ti3O5 film 2. When the component of the support substrate 1 substantially does not diffuse (the component of the support substrate 1 substantially does not diffuse into the λ-Ti3O5 film 2), in the λ-Ti3O5 film 2, a transition into another phase can be stably performed. Here, whether the component of the support substrate 1 substantially does not diffuse can be determined, for example, from an element mapping image. The element mapping image can be obtained by the following method. The λ-Ti3O5 film forming substrate 10 is processed by a focused ion beam (FIB) and the like, and an obtained cross-section is observed by a scanning transmission electron microscope (STEM). Next, the element mapping image can be obtained by performing element mapping by using an element analyzer (for example, an energy dispersive X-ray analyzer (EDS)) provided in the STEM.
A position spaced apart from an interface between the support substrate 1 and the λ-Ti3O5 film 2 toward a surface of the λ-Ti3O5 film 2 by 10 nm in a plate thickness direction is set as a position A, and a position of a surface of the λ-Ti3O5 film 2 is set as a position B. So as to stably perform a phase transition, in a region from the position A to the position B in the plate thickness direction, an element X contained in the support substrate 1 is preferably 0.1 atomic % or less of all elements in the λ-Ti3O5 film. In the region from the position A to the position B in the plate thickness direction, whether the element X contained in the support substrate 1 is 0.1 atomic % or less of all elements in the λ-Ti3O5 film can be confirmed, for example, by the following method.
The λ-Ti3O5 film forming substrate 10 is processed by the FIB and the like, and an obtained cross-section is observed by the STEM. In an obtained observation image, an EDS spectrum is measured from the support substrate 1 to the λ-Ti3O5 film 2 so that an interface between the support substrate 1 and the λ-Ti3O5 film 2 is included in a plate thickness direction. From the obtained EDS spectrum, a plot in which a position where the interface between the support substrate 1 and the λ-Ti3O5 film 2 is set to 0 nm is set as the horizontal axis, and a spectral intensity of each constituent element is set as the vertical axis is created. Next, the spectral intensity of the constituent element is differentiated at the position by using the obtained plot. Through the differentiation, a differential intensity profile of each element in which the intensity becomes maximum at the interface between the support substrate 1 and the λ-Ti3O5 film 2 is obtained. Fitting is performed on the profile by using a Gaussian function of Expression (1). Here, σ2 represents a variance, and x represents a position (nm).
A ratio of element diffusion in the vicinity of the interface between the support substrate 1 and the λ-Ti3O5 film 2 can be estimated on the basis of σ obtained by the above-described method and the Gaussian function in the following Expression (1). For example, 99.7% of entirety of elements fall within a range of positions of −3σ to 3σ. On the basis of this idea, in a case where σ obtained in a peak of any element is 2.6 nm or less, in a region spaced apart from the interface between the support substrate 1 and the λ-Ti3O5 film by 10 nm or more, a ratio of the element can be estimated to be 0.1% or less of the entirety of elements. By the above-described method, it can be confirmed that the element X contained in the support substrate 1 is 0.1 atomic % or less of all elements in the λ-Ti3O5 film. Note that, a value of σ of a peak of each element is preferably 2.6 nm or less. The value of σ of each element is more preferably 2.0 nm or less. The value of σ is still more preferably 1.0 nm or less. Since as the value of σ is smaller, diffusion less occurs, the value of σ is preferably smaller.
(Method for Producing λ-Ti3O5 Film Forming Substrate)
Next, an example of the method for producing the λ-Ti3O5 film forming substrate will be described. In the method for producing the λ-Ti3O5 film forming substrate according to the present disclosure, the λ-Ti3O5 film 2 is formed on the support substrate 1 by a physical vapor deposition method while controlling the temperature of the support substrate 1 to higher than 1000° C. and lower than 1200° C. Hereinafter, the method for producing the λ-Ti3O5 film forming substrate of the present disclosure will be described.
Examples of the physical vapor deposition method used in the method for producing the λ-Ti3O5 film forming substrate of the present disclosure include a pulsed laser deposition method (PLD method), a thermal vapor deposition method, a sputtering method, a molecular beam epitaxy method, and the like. As the physical vapor deposition method, particularly, the PLD method is preferable. Hereinafter, description will be made with reference to the PLD method, but the invention is not limited to the PLD method.
The PLD method is a method in which a substrate and a target that is a material of a thin film are installed in a vacuum chamber, a target surface is irradiated with a high-output pulsed laser to generate a plume (aggregates of atoms, molecules, clusters, and the like) of a high-energy body, and the plume is deposited on an opposing substrate to form a thin film.
The support substrate 1 is installed on the sample stage 49. Evacuation is performed from the vacuum exhaust port 50 by using a vacuum pump to create a vacuum state (for example, a pressure of 1×10−6 Pa or less). A higher degree of vacuum is preferable. When oxygen and the like remain, reduction may not be possible, and the λ-Ti3O5 film 2 may not be formed.
Next, the support substrate 1 is heated to a temperature region of higher than 1000° C. and lower than 1200° C. by using the heating unit 48. Note that, the temperature is assumed as a temperature of a surface of the support substrate 1. When the temperature of the support substrate 1 is 1000° C. or lower, a Magneli phase such as Ti4O7 is formed, and the λ-Ti3O5 film 2 may not be obtained. In a case where the temperature of the support substrate 1 is 1200° C. or higher, a polycrystal is formed, and the single-phase λ-Ti3O5 film 2 may not be obtained. The temperature of the support substrate 1 is more preferably from 1050° C. to 1150° C.
Typically, as a method of heating the support substrate, a halogen lamp, heating by an infrared laser, and a method using a heat-generating body such as SiC heater can be exemplified. However, an upper limit of a typical substrate temperature is 800° C. at the halogen lamp, and an upper limit temperature of the infrared laser or the SiC heater is 1000° C. In the heating unit 48 of the present disclosure, a temperature of higher than 1000° C. is realized by condensing a heating laser (for example, an infrared laser), but the producing method of the present disclosure is not limited thereto.
Next, a gas is introduced into the vacuum chamber 60 from the gas inlet port 46. In a case where the target is TiO2, it is preferable to flow an Ar gas. A pressure of the Ar gas is preferably from 1×10−5 Torr to 1×10−3 Torr. Particularly, the pressure of the Ar gas is preferably from 1×10−4 Torr to 9×10−4 Torr. When the pressure of the Ar gas is lower than 1×10−5 Torr, a Magneli phase such as Ti4O7 may be formed. When the pressure of the Ar gas is higher than 1×10−3 Torr, Ti2O3 reduced from Ti3O5 may be formed.
In a case where the target is TiO, the gas is preferably O2. A pressure of the O2 gas is, for example, 1×10−6 Torr. In a case of the O2 gas, a relationship between the gas pressure and a crystal is opposite to the case of Ar. When the pressure of the O2 gas is raised, an oxidized phase is obtained, and when the gas pressure is lowered, a reduced phase is obtained.
Next, the target 42 is irradiated with the laser 40 to generate plume. The generated plume reaches the support substrate 1 and the λ-Ti3O5 film is formed. According to this, a λ-Ti3O5 film forming substrate is obtained. The target 42 is, for example, TiO or TiO2. TiO may be a polycrystalline sintered body or a single crystal. TiO2 may be a polycrystalline sintered body or a single crystal. The laser 40 is not particularly limited as long as the plume is efficiently generated. For example, the laser 40 is a KrF excimer laser (wavelength: 248 nm). Laser irradiation conditions are not particularly limited. For example, an energy density is 1.1 J/cm2, and a frequency is 5 Hz. A film formation rate is not particularly limited. The film formation rate is, for example, 100 nm/hour.
Hereinbefore, the λ-Ti3O5 film forming substrate of the present disclosure has been described in detail. In addition, the constituent elements in the above-described embodiment can be approximately substituted with known constituent elements within a range not departing from the gist of the invention, and the modification examples may be appropriately combined.
Next, examples of the invention will be described, but conditions in the examples are conditional examples employed to confirm executability and an effect of the invention, and the invention is not limited to the conditional examples. The invention can employ various conditions as long as the object of the invention is accomplished without departing from the gist of the invention.
Film formation of the λ-Ti3O5 film was carried out by a pulsed laser deposition method. An LaAlO3 (110) substrate was used as the substrate, and a single crystal of TiO2 was used as the target. The substrate and the target were installed in a vacuum chamber, and evacuation (5×10−7 Pa) was performed. The surface temperature of the LaAlO3 (110) substrate was heated to 1100° C. by condensing an infrared laser. The Ar gas pressure was set to 1×10−4 Torr, and the target was irradiated with a KrF excimer laser (wavelength: 248 nm). As the irradiation conditions, an energy density was set to 1.1 J/cm2 and a frequency was set to 5 Hz. The λ-Ti3O5 film was formed on the substrate with a film formation rate set to 100 nm/hour, thereby obtaining a substrate (film thickness: 100 nm) of Example 1.
A LaAlO3 (110) substrate was used as the substrate, and a single crystal of TiO2 was used as the target. The substrate and the target were installed in the vacuum chamber, and evacuation (5×10−7 Pa) was performed. The surface temperature of the LaAlO3 (110) substrate was heated to 1200° C. by condensing an infrared laser. The Ar gas pressure was set to 1×10−4 Torr, and the target was irradiated with a KrF excimer laser (wavelength: 248 nm). As the irradiation conditions, an energy density was set to 1.1 J/cm2 and a frequency was set to 5 Hz. A film was formed on the substrate with a film formation rate set to 100 nm/hour, thereby obtaining a substrate (film thickness: 100 nm) of Comparative Example 1.
In the thin film X-ray diffraction measurement, SmartLab (manufactured by Rigaku Corporation) was used. A CuKα1 (wavelength: 1.5406 Å) was used as a radiation source, and measurement was performed at room temperature. Note that, the room temperature was set to from 23° C. to 30° C. The substrate of Example 1 was disposed so that a [010] direction of the λ-Ti3O5 film and the LaAlO3 [001], and a direction in which a direction connecting an X-ray incidence unit and an X-ray detection unit is projected to the λ-Ti3O5 film become parallel to each other. A position where a diffraction intensity of a (203) plane becomes highest was set as ω0°, a position after rotation around a rotational axis passing through the center of the substrate from the position of ϕ0° by 180° was set as ϕ180°, and the thin film X-ray diffraction measurement was performed at the position of ϕ0° and the position of ϕ180°. Obtained XRD patterns are shown in
Raman measurement on the substrate of Example 1 was performed by using LabRAM HR-800 (manufactured by HORIBA, Ltd.). The laser was an He—Ne laser (wavelength: 633 nm), and measurement was performed at room temperature. Disposition was performed so that an electric field vector of laser light becomes orthogonal to the direction of the λ-Ti3O5 film. Obtained results are shown in
The substrate of Example 1 was processed with FIB. Observation was performed by using a scanning transmission electron microscope, JAM-ARM200F (manufactured by JEOL Ltd.) so as to project a (010) plane and a (001) plane of the λ-Ti3O5 film of Example 1. Element mapping was performed on an obtained observation image by using an EDS attached to the scanning transmission electron microscope. Obtained results are shown in
A temperature variation of resistivity of the substrate of Example 1 was measured. Measurement was performed by four-terminal method, and was performed in a temperature range of 10 K to 650 K. Obtained results are shown in
An EDS spectrum was measured from the support substrate of Example 1 to the λ-Ti3O5 film so that an interface between the support substrate and the λ-Ti3O5 film is included (a region surrounded by a solid line in
A λ-Ti3O5 film was formed on the LaAlO3 (110) substrate by the same method as in Example 1 to obtain a substrate of Example 2 (film thickness: 100 nm).
The thin film X-ray diffraction measurement was performed with respect to the substrate of Example 2 by changing a temperature. The thin film X-ray diffraction measurement was performed at an accurate single crystal X-ray diffraction station (BL4C) of synchrotron radiation experimental facility of high energy accelerator research organization (KEK-PF) that is inter-university research institute corporation. Measurement was performed at a temperature from 300 K to 520 K for every 10 K by using a radiation source with energy of 8 keV (wavelength: 1.5498 Å). The substrate of Example 2 was disposed so that a direction of the λ-Ti3O5 film and LaAlO3 [1-10], and a direction in which a direction connecting the X-ray incidence unit and the X-ray detection unit is projected to the λ-Ti3O5 film are parallel to each other. Diffraction peak position of a (600) plane, a (601) plane, and a (60-1) plane were investigated.
From X-ray diffraction data of respective diffraction planes at respective temperatures, an angle β of a unit lattice constituting the Ti3O5 film was calculated. In the calculation of the angle β, the following Expression (2) was used. In Expression (2), dhkl represents a plane interval of an (hkl) plane, and a, b, and c represent lengths of respective axes of the unit lattice.
As illustrated in
According to the λ-Ti3O5 film forming substrate and the method for producing the λ-Ti3O5 film forming substrate according to this embodiment, since direct formation on the substrate is possible, and phase transition from λ-Ti3O5 to another phase can occur stably, the invention has high industrial applicability.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-144696 | Sep 2021 | JP | national |
This application is the U.S. National Stage entry of International Application No. PCT/JP2022/033363, filed on Sep. 6, 2022, which, in turn, claims priority to Japanese Patent Application No. 2021-144696, filed on Sep. 6, 2021, both of which are hereby incorporated herein by reference in their entireties for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2022/033363 | 9/6/2022 | WO |
