Joined body, optical modulator, and method for manufacturing joined body

Abstract

According to the present invention, it is possible to provide a joined body in which a single crystal thin film having a high electro-optical coefficient is provided on a substrate. Furthermore, according to the present invention, it is possible to provide an optical modulator using the joined body and a method for manufacturing the joined body.　This joined body (1) comprises: a substrate (10) including a single crystal of a rock salt-type structure compound, a perovskite-type oxide, or a fluorite-type oxide on one main surface (10a); and a thin film (20) comprising a single crystal of barium titanate having a (100) orientation, the thin film (20) being provided on the main surface (10a) of the substrate (10). A ferroelectric domain (30) in a direction parallel to the main surface (10a) of the substrate (10) is present inside the thin film (20), and misfit dislocations (40) are present at an interface between the thin film (20) and the substrate (10).

Description

Joint, optical modulator, and method for manufacturing joint

[0001] The present invention relates to a bonded body, an optical modulator, and a method for manufacturing the bonded body.

[0002] Due to the increasing amount of data processing and power consumption in information and communications, the application of optical technology to the periphery of processors is expected. To achieve higher speeds and lower power consumption, it is essential to lay optical wiring close to the processor. In this case, the development of optical modulators, which are devices that convert the processor's electrical signals into optical signals, is important.

[0003] To achieve miniaturization, power saving, and improved frequency band (compatible with high frequencies), it is necessary to fabricate devices using thin films of materials with high electro-optic coefficients in a single crystal state. ３ ) has a high electro-optic coefficient (EO coefficient), and its thin films are expected to be used in next-generation electro-optic devices.

[0004] Non-Patent Document 1 discloses the electro-optical properties of a barium titanate thin film epitaxially grown on a silicon substrate.

[0005] S. Abel et al. , “A strong electro-optically active lead-free ferroelectric integrated on silicon”, Nature Communications, vol. 4, p. 1671, 2013.

[0006] The electro-optic coefficient is a tensor quantity, and its characteristics change depending on the electric field, the plane of polarization, and the crystal orientation (the domain orientation in the case of ferroelectrics). To maximize the inherent high electro-optic coefficient of barium titanate, a ferroelectric, it is necessary to precisely control the orientation of the ferroelectric domains. For example, to increase the electro-optic coefficient of a thin film made of a single crystal of barium titanate with a (100) orientation, it is known that an "a-domain," in which the polarization is parallel to the substrate plane, is preferable.

[0007] However, it is difficult to realize the a-domain in a thin film made of a single crystal of barium titanate having a (100) orientation.

[0008] To prepare a single crystal thin film of barium titanate having a perovskite structure and having a (100) orientation, it is necessary to use a single crystal substrate of a perovskite oxide having a lattice constant close to that of barium titanate, or a substrate having a single crystal thin film of a perovskite oxide having a lattice constant close to that of barium titanate disposed on its surface. However, single crystals of perovskite oxides are not readily available, for example, strontium titanate (SrTiO ３ In this case, when a typical dry process such as sputtering or pulsed laser deposition is used to form a film, the lattice of barium titanate is constrained by the substrate, causing compressive strain in the in-plane direction of the substrate, and only a single crystal thin film of c-axis-oriented barium titanate (i.e., a single crystal thin film of c-domain barium titanate) can be obtained.

[0009] To realize the a-domain of a thin film made of a single crystal of barium titanate with a (100) orientation, it is necessary to generate tensile strain in the in-plane direction of the substrate. For example, barium titanate (thermal expansion coefficient: 1.0 × 10 －５ It is effective to use a substrate with a thermal expansion coefficient smaller than 4×10 / K. －６ Single-crystal thin films of a-domain barium titanate have been fabricated by molecular beam epitaxy (MBE) using silicon substrates known to have a tensile stress of 1000 K. However, with MBE, silicon is the only substrate that can provide effective tensile stress to barium titanate, and there are no other substrate options available. Furthermore, the only reported examples of single-crystal thin films of a-domain barium titanate are those fabricated by MBE, and the fabrication methods are limited. Furthermore, the high cost of MBE makes it unsuitable for mass production of electro-optical devices, and the conditions under which a-domain single-crystal thin films can be fabricated are limited.

[0010] For these reasons, it has been extremely difficult to achieve a high electro-optic coefficient at low cost.

[0011] The present invention has been made to solve the above problems, and aims to provide a bonded structure in which a single crystal thin film having a high electro-optic coefficient is provided on a substrate. Another aim of the present invention is to provide an optical modulator using the bonded structure and a method for manufacturing the bonded structure.

[0012] The bonded structure of the present invention comprises a substrate having a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound on one of its main surfaces, and a thin film made of a single crystal of barium titanate having a (100) orientation and provided on the main surface of the substrate, wherein ferroelectric domains are present inside the thin film in a direction parallel to the main surface of the substrate, and misfit dislocations are present at the interface between the thin film and the substrate.

[0013] The optical modulator of the present invention is a Mach-Zehnder type optical modulator that includes the bonded structure of the present invention and utilizes a change in refractive index when an electric field is applied to the thin film of the bonded structure.

[0014] The method for producing a bonded body of the present invention includes a step of forming a film of a raw material solution, in which barium carboxylate and titanium alkoxide are dispersed in an organic solvent, on one main surface of a substrate containing a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound, in a state in which cluster formation in the solution is suppressed, and then performing a heat treatment in an oxygen-containing atmosphere to form a thin film made of a barium titanate single crystal having a (100) orientation, in which film formation from the raw material solution is performed one or more times, and the heat treatment is performed each time a film is formed from the raw material solution.

[0015] According to the present invention, it is possible to provide a bonded structure in which a single crystal thin film having a high electro-optic coefficient is provided on a substrate. Furthermore, according to the present invention, it is possible to provide an optical modulator using the bonded structure and a method for manufacturing the bonded structure.

[0016] Fig. 1 is a schematic diagram showing an example of a bonded structure of the present invention. Fig. 2 is a schematic diagram showing an example of a substrate constituting the bonded structure of the present invention. Fig. 3 is a schematic diagram showing another example of a substrate constituting the bonded structure of the present invention. Fig. 4 is a schematic diagram showing an example of a Mach-Zehnder type optical modulator. Fig. 5 is a schematic diagram showing a BaTiO ３6 is an X-ray diffraction pattern of the thin film of BaTiO in Example 1. ３ 7 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the in-plane direction of the thin film. ３ 8 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the direction perpendicular to the surface of the thin film. ３ 9 is a piezoelectric response microscope image (phase image) of the thin film. ３ Thin films and SrTiO ３ 10 is a transmission electron microscope image (dark field image) of the interface with the substrate. ３ 11 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the in-plane direction of the thin film. ３ 12 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the in-plane direction of the thin film. ３ 1 is an X-ray diffraction pattern of a thin film.

[0017] The bonded structure of the present invention will be described below. Note that the present invention is not limited to the following configurations, and may be modified as appropriate within the scope of the present invention. In addition, a combination of multiple individual preferred configurations described below also falls within the scope of the present invention.

[0018] In this specification, terms indicating the relationship between elements (e.g., "perpendicular," "parallel," "orthogonal," etc.) and terms indicating the shape of elements are not expressions that only express a strict meaning, but are expressions that also include a range of substantial equivalence, for example, a difference of about a few percent.

[0019] The drawings shown below are schematic diagrams, and the dimensions, aspect ratio, and other scales may differ from those of the actual product. In the drawings, the same or equivalent parts will be designated by the same reference numerals. In addition, the same elements will be designated by the same reference numerals in each drawing, and duplicate explanations will be omitted.

[0020] FIG. 1 is a schematic diagram showing an example of the bonded body of the present invention.

[0021] The bonded structure 1 shown in FIG. 1 includes a substrate 10 and a thin film 20 .

[0022] The substrate 10 includes a single crystal of a perovskite oxide, a fluorite oxide, or a rock-salt structure compound on one main surface 10a. That is, the substrate 10 includes a single crystal of a perovskite oxide, a single crystal of a fluorite oxide, or a single crystal of a rock-salt structure compound on one main surface 10a.

[0023] The thin film 20 is made of a single crystal of barium titanate having a (100) orientation, and is provided on the main surface 10 a of the substrate 10 .

[0024] In order to epitaxially grow barium titanate having a perovskite structure as a thin film 20, it is necessary to arrange a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound on the main surface 10a of the substrate 10.

[0025] Examples of perovskite oxides contained in the substrate 10 include strontium titanate, potassium tantalate, barium zirconate, lanthanum strontium aluminum tantalate, strontium ruthenate, lanthanum strontium manganate, and lanthanum nickelate. Examples of fluorite oxides contained in the substrate 10 include zirconium oxide, cerium oxide, and yttrium-doped zirconium oxide. Examples of rock salt structure compounds contained in the substrate 10 include magnesium oxide and titanium nitride.

[0026] In this specification, barium titanate is BaTiO ３ or an oxide represented by BaTiO ３ The main component may be an oxide in which a part of the Ba site and / or Ti site of the above is substituted. For example, a part of the Ba site may be substituted with an alkaline earth metal element such as Pb, Sr, or Ca, or a part of the Ti site may be substituted with a tetravalent element such as Zr. ３ As long as ３ The same effect can be obtained for other perovskite oxides. For example, strontium titanate is SrTiO ３or an oxide represented by SrTiO ３ The Sr site and / or Ti site of the above may be partially substituted with an oxide.

[0027] The thickness of the substrate 10 constituting the bonded body 1 is not particularly limited, and may be the same as, greater than, or smaller than the thickness of the thin film 20. The thickness of the substrate 10 may be the thickness of a very thin single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock-salt structure compound. For example, the substrate 10 may include a state in which even a single atomic layer of a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock-salt structure compound exists on the surface on the thin film 20 side.

[0028] Fig. 2 is a schematic diagram showing an example of a substrate constituting the bonded structure of the present invention, and Fig. 3 is a schematic diagram showing another example of a substrate constituting the bonded structure of the present invention.

[0029] The substrate 10 constituting the bonded body 1 may be a substrate 10A entirely made of a single crystal of a perovskite oxide, a fluorite oxide, or a rock salt structure compound, as shown in Fig. 2, or may be a substrate 10B in which a substrate thin film 11 made of a single crystal of a perovskite oxide, a fluorite oxide, or a rock salt structure compound is provided on the surface of a base substrate 12, as shown in Fig. 3. As long as the outermost thin film of the substrate thin film 11 is a single crystal of a perovskite oxide, a fluorite oxide, or a rock salt structure compound, the substrate thin film 11 may have a structure in which a plurality of thin films are stacked.

[0030] The substrate 10A may be, for example, a substrate entirely made of a single crystal of strontium titanate, a substrate entirely made of a single crystal of lanthanum strontium aluminum tantalate (LSAT), or a substrate entirely made of a single crystal of potassium tantalate (KTaO ３ Examples of the substrate 10B include a substrate made of a single crystal of magnesium oxide (MgO), or a substrate made entirely of a single crystal of magnesium oxide (MgO). Examples of the substrate 10B include a silicon substrate having, on its main surface, a thin film for a substrate made of a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound.

[0031] The thickness of the thin film 20 constituting the bonded body 1 is not particularly limited and is, for example, 7 nm or more. On the other hand, from the viewpoint of easily obtaining a single crystal state, the thickness of the thin film 20 is preferably 1000 nm or less.

[0032] 1, in the bonded body 1, ferroelectric domains 30 parallel to the main surface 10a of the substrate 10 are present inside the thin film 20. Specifically, after heating to a temperature equal to or higher than the phase change temperature (Curie temperature) of barium titanate from tetragonal to cubic and cooling to room temperature, the ferroelectric domains 30 parallel to the main surface 10a of the substrate 10 should be present inside the thin film 20.

[0033] Furthermore, misfit dislocations 40 are present at the interface between the thin film 20 and the substrate 10. In the example shown in Fig. 1, edge dislocations are present on the main surface 10a side of the substrate 10, but depending on the type of substrate 10, edge dislocations may be present on the main surface 10a side of the substrate 10 or on the thin film 20 side.

[0034] When a thin film 20 is formed on one main surface 10a of a substrate 10 containing a single crystal of a perovskite oxide, as described above, the lattice of barium titanate is constrained by the substrate 10, causing compressive strain in the in-plane direction of the substrate 10, and therefore only the c-domain is normally obtained. In contrast, in the bonded body 1 shown in FIG. 1, the presence of misfit dislocations 40 at the interface between the thin film 20 and the substrate 10 causes lattice relaxation in the in-plane direction of the thin film 20, and therefore thermal contraction of the thin film 20 during film formation and firing occurs preferentially in the direction perpendicular to the surface. As a result, the lattice constant of barium titanate in the direction perpendicular to the main surface 10a of the substrate 10 is d ＶＴ , the lattice constant of barium titanate in the direction parallel to the main surface 10a of the substrate 10 is d ＬＴ When this is the case, d ＬＴ / d ＶＴ This makes it possible to realize a state in which the lattice constant ratio of the thin film 20, expressed as: ∑ i = 1 / 2 n ∑ ...

[0035] Unlike Non-Patent Document 1, the thermal expansion coefficient of the substrate 10 is not particularly limited, and is, for example, 4.5×10 －６ On the other hand, the thermal expansion coefficient of the substrate 10 may be, for example, 2.5×10 －５ / K or less.

[0036] The thermal expansion coefficient of substrate 10A shown in FIG. 2 refers to the thermal expansion coefficient of the entire substrate, and the thermal expansion coefficient of substrate 10B shown in FIG. 3 refers to the thermal expansion coefficient of the base substrate 12. Specifically, the linear thermal expansion coefficient of the material constituting the substrate portion can be used. The linear thermal expansion coefficient is given by (dl / dT) / l, where l is the length of the material and T is the absolute temperature. Here, dl / dT is the temperature derivative of the length of the material. The temperature used for differentiation can be any value, but room temperature (298 K) is often used.

[0037] d ＬＴ / d ＶＴ The lattice constant ratio of the thin film 20 expressed by d is preferably larger than 1.0030. ＬＴ / d ＶＴ The lattice constant ratio of the thin film 20 expressed by is, for example, 1.0087 or less.

[0038] In the bonded structure 1, it is preferable that the phase of the polarization of the thin film 20, the phase of the change in strain when an AC electric field is applied, or the phase of the change in refractive index when an AC electric field is applied is not inverted by applying a DC electric field in the direction perpendicular to the surface of the thin film 20, but is inverted by applying a DC electric field in the in-plane direction of the thin film 20.

[0039] The electro-optic coefficient of the thin film 20 is preferably 150 pm / V or more. On the other hand, the electro-optic coefficient of the thin film 20 is, for example, BaTiO ３ Electro-optic tensor component r of bulk single crystal ４２ When this is adopted, the value is 730 pm / V or less.

[0040] The coercive electric field E when the phase of the refractive index change of the thin film 20 is reversed by applying a DC electric field in the in-plane direction of the thin film 20 when an AC electric field is applied to the thin film 20 ｃ is 5.5 x 10 ５ In this case, the coercive electric field E ｃSince the polarization coefficient is small, it can be easily polarized by applying a DC electric field in the in-plane direction of the thin film 20, and a high electro-optic coefficient can be obtained.

[0041] The coercive electric field when the phase of the refractive index change of the thin film 20 is reversed by applying a DC electric field in the in-plane direction of the thin film 20 when an AC electric field is applied to the thin film 20 is defined as E ｃ When E>E ｃ When a DC electric field E in the range of is applied to the thin film 20, it is preferable that the intensity of the change in refractive index of the thin film 20 when an AC electric field is applied does not saturate with an increase in the DC electric field E. This allows a high electro-optic coefficient to be obtained even under a high bias.

[0042] The bonded structure of the present invention is used, for example, as a Mach-Zehnder optical modulator. Such an optical modulator also constitutes the present invention. The optical modulator of the present invention utilizes a change in refractive index when an electric field is applied to a thin film of the bonded structure.

[0043] FIG. 4 is a schematic diagram showing an example of a Mach-Zehnder type optical modulator.

[0044] As shown in FIG. 4, a Mach-Zehnder optical modulator 50 uses an optical waveguide (Mach-Zehnder optical waveguide) having a Mach-Zehnder interferometer structure in which light emitted from a single light source is split into two beams, which are passed through different paths and then superimposed again to cause interference.

[0045] The bonded body of the present invention can be produced, for example, by the following method.

[0046] First, a raw material solution is prepared by dispersing barium carboxylate and titanium alkoxide in an organic solvent.

[0047] The concentration of barium titanate in the raw material solution is, for example, 0.07 mol / L or more and 0.30 mol / L or less.

[0048] The prepared raw material solution is formed into a film on one main surface of a substrate containing a single crystal of a perovskite oxide, a fluorite oxide, or a rock-salt structure compound, with cluster formation suppressed in the solution. To suppress cluster formation in the solution, for example, it is preferable that the average diameter of the clusters present in the raw material solution is 1 nm or less. In other words, it is preferable that the prepared raw material solution is formed into a film on one main surface of a substrate containing a single crystal of a perovskite oxide, a fluorite oxide, or a rock-salt structure compound, with the average diameter of the clusters present in the raw material solution being 1 nm or less.

[0049] In the step of forming a film from the raw material solution, the method for applying the raw material solution to the main surface of the substrate is not particularly limited, but from the viewpoint of easily forming a uniform film, a spin coating method is preferred.

[0050] From the viewpoint of facilitating the formation of misfit dislocations at the interface between the thin film and the substrate, it is preferable to clean the main surface of the substrate before forming a film from the raw material solution. Methods for cleaning the surface of the substrate include, for example, a method of heat-treating the substrate in an oxygen-containing atmosphere and a method of irradiating the surface of the substrate with ultraviolet light.

[0051] After forming the film from the raw material solution, heat treatment is performed in an oxygen-containing atmosphere, thereby forming a thin film made of single crystal barium titanate with a (100) orientation.

[0052] The film formation from the raw material solution is carried out one or more times, and a heat treatment is carried out for each film formation from the raw material solution. The film formation from the raw material solution may be carried out once or multiple times. This makes it easier to introduce misfit dislocations at the interface between the thin film and the substrate. ＬＴ / d ＶＴ This results in a state in which the lattice constant ratio of the thin film, expressed as: ##EQU1## is greater than 1, and ferroelectric domains parallel to the main surface of the substrate are easily formed inside the thin film.

[0053] In this way, the bonded body of the present invention is produced.

[0054] The above method allows for the formation of a single-crystal thin film of barium titanate with a high electro-optic coefficient by simply applying a raw material solution containing barium and titanium to a substrate and then firing the applied solution, which is therefore suitable for mass production at low cost.

[0055] The present specification discloses the following:

[0056] <1> A joined body comprising: a substrate including, on one main surface, a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound; and a thin film made of a single crystal of barium titanate having a (100) orientation and provided on the main surface of the substrate, wherein ferroelectric domains are present inside the thin film in a direction parallel to the main surface of the substrate; and misfit dislocations are present at the interface between the thin film and the substrate.

[0057] <2> The lattice constant of the barium titanate in the direction perpendicular to the main surface of the substrate is d ＶＴ , the lattice constant of the barium titanate in a direction parallel to the main surface of the substrate is d ＬＴ When this is the case, d ＬＴ / d ＶＴ The bonded structure according to <1>, wherein the lattice constant ratio of the thin film represented by the formula (1) is greater than 1.

[0058] <3> The bonded structure according to <2>, wherein the thin film has a lattice constant ratio of greater than 1.0030.

[0059] <4> The thermal expansion coefficient of the substrate is 4.5×10 －６ / K or more.

[0060] <5> The joined body according to any one of <1> to <3>, wherein the substrate is a substrate entirely made of a single crystal of strontium titanate, a substrate entirely made of a single crystal of lanthanum strontium aluminum tantalate, a substrate entirely made of a single crystal of potassium tantalate, or a substrate entirely made of a single crystal of magnesium oxide.

[0061] <6> The bonded body according to any one of <1> to <3>, wherein the substrate is a silicon substrate having, on the main surface thereof, a thin film for substrate formed of a single crystal of the perovskite-type oxide, the fluorite-type oxide, or the rock salt-type structure compound.

[0062] <7> The bonded structure according to any one of <1> to <6>, wherein the thin film has a thickness of 7 nm or more.

[0063] <8> The joined body according to any one of <1> to <7>, wherein the polarization of the thin film, the phase of the change in strain when an AC electric field is applied, or the phase of the change in refractive index when an AC electric field is applied is not inverted by applying a DC electric field in a direction perpendicular to the surface of the thin film, but is inverted by applying a DC electric field in an in-plane direction of the thin film.

[0064] <9> The bonded structure according to <8>, wherein the thin film has an electro-optic coefficient of 150 pm / V or more.

[0065] <10> A coercive electric field E when a DC electric field is applied in the in-plane direction of the thin film to invert the phase of the refractive index change when an AC electric field is applied to the thin film ｃ is 5.5 x 10 ５ The bonded structure according to <8> or <9>, wherein the surface tension is V / m or less.

[0066] <11> When a DC electric field is applied in the in-plane direction of the thin film, the coercive electric field when the phase of the refractive index change of the thin film when an AC electric field is applied to the thin film is reversed is defined as E ｃ When E>E ｃ <11> The joined body according to any one of <8> to <10>, wherein, when a DC electric field E in the range of is applied to the thin film, the intensity of the change in refractive index of the thin film when an AC electric field is applied does not saturate with an increase in the DC electric field E.

[0067] <12> A Mach-Zehnder optical modulator comprising the bonded structure according to any one of <1> to <11>, wherein a change in refractive index when an electric field is applied to the thin film of the bonded structure is utilized.

[0068] <13> A method for producing a bonded body, comprising: forming a film of a raw material solution, in which barium carboxylate and titanium alkoxide are dispersed in an organic solvent, on one main surface of a substrate including a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock-salt-type structure compound, while suppressing the formation of clusters in the solution; and then performing a heat treatment in an oxygen-containing atmosphere to form a thin film made of a barium titanate single crystal having a (100) orientation; wherein film formation of the raw material solution is performed one or more times, and the heat treatment is performed each time a film of the raw material solution is formed.

[0069] <14> The method for producing a bonded body according to <13>, wherein the clusters present in the raw material solution have an average diameter of 1 nm or less.

[0070] <15> The method for producing a joined body according to <13> or <14>, wherein the concentration of barium titanate in the raw material solution is 0.07 mol / L or more and 0.30 mol / L or less.

[0071] <16> The method for producing a bonded body according to any one of <13> to <15>, further comprising the step of cleaning the main surface of the substrate before forming a film from the raw material solution.

[0072] <17> The method for producing a bonded body according to <16>, wherein in the step of cleaning the main surface of the substrate, the substrate is heat-treated in an atmosphere containing oxygen.

[0073] Examples that more specifically disclose the bonded body of the present invention are given below, but the present invention is not limited to these examples.

[0074] [Example 1] <Sample Preparation> Barium acetate was dissolved in acetic acid, and titanium isopropoxide was dissolved in 2-methoxyethanol, and the concentration was then adjusted to 0.15 mol / L (0.15 M). At this time, it is preferable to adjust the average cluster diameter in the solution to 1 nm or less. The raw material solution is sensitive to moisture, and hydrolysis occurs immediately, forming micro-sized sol (clusters) in the solution. If a thin film is prepared in this state, triaxially oriented growth is inhibited, and a-domain BaTiO ３The moisture contamination is caused by residual moisture in the solvent or moisture in the air, and the source solution is coated on the substrate after removing these to prevent the growth of sol clusters. In Example 1, SrTiO with (100) orientation was used. ３ The substrate used was made of a single crystal of SrTiO ３ The thermal expansion coefficient of －６ / K.

[0075] Examples of methods for applying the raw material solution include spin coating, drop casting, and dip coating. Among these, spin coating is preferred because it is easy to form a uniform film. It is important to clean the surface of the substrate before application. For example, by heat treating the substrate in an oxygen atmosphere, surface contaminants can be burned and removed. Alternatively, it is also effective to remove surface contaminants using ozone gas generated by ultraviolet irradiation. In this example, SrTiO ３ The single crystal substrate was heated at 450°C to clean the surface. ３ Since the atomic plane of BaTiO is exposed, ３ Misfit dislocations are more likely to form at the interface with the thin film. It is also effective to further increase the heating temperature. For example, heating at around 1000°C will cause atomic steps to form on the surface, which is more advantageous for triaxial orientation and the formation of misfit dislocations.

[0076] Surface-treated SrTiO ３ The raw material solution was spin-coated onto the single crystal substrate at a speed of 4000 rpm for 1 minute, and the solvent was evaporated by heating on a hot plate at 150°C for 3 minutes. After that, the substrate was heat-treated by firing at 900°C for 10 minutes in a high-speed firing furnace in an oxygen atmosphere, resulting in a BaTiO ３ A thin film of a desired thickness can be obtained by changing the number of times the process from film formation to baking is repeated. In this example, the process from film formation to baking was repeated three times to obtain a thin film with a thickness of 49 nm.

[0077] FIG. 5 shows the BaTiO ３ 1 is an X-ray diffraction pattern of a thin film.

[0078] From FIG. 5, BaTiO ３ A thin film consisting of single crystals was obtained.

[0079] In addition, reciprocal lattice mapping of X-ray diffraction obtained around the reciprocal lattice point (203) revealed that d ＬＴ / d ＶＴ was calculated to be 1.0045. Specifically, BaTiO ３ The X-ray diffraction intensity was obtained around the diffraction peak (203) by varying 2θ from 86.5° to 91.5° in 0.004° steps and ω from −0.5° to +0.8° in 0.01° steps relative to the peak position.

[0080] <Electro-optical Measurement> Platinum (Pt) in-plane pattern electrodes were formed on the obtained thin film by electron beam evaporation. The gap between the electrodes was 100 μm. The electrodes were made of BaTiO ３ The thin film was placed so that an electric field was applied in the <110> direction within the film plane. Electro-optical measurements were performed using the field modulation ellipsometry method. ３ A HeNe laser (continuous light) with a wavelength of 632.8 nm was incident on the thin film from the normal direction of the film surface. The laser was linearly polarized, and the polarization plane was adjusted with a half-wave plate so that it was at a 45° angle with the electric field direction. ３ The laser light that penetrated the thin film and the substrate was BaTiO ３ The birefringence of the polarized light causes a phase change, resulting in an elliptically polarized state. The elliptically polarized light was then converted back to linearly polarized using a quarter-wave plate, and the light was then incident on the analyzer. The light that passed through the analyzer was converted into a voltage signal by a semiconductor detector.

[0081] BaTiO ３ The voltage signal was measured in two ways: when an electric field was applied to the in-plane electrodes on the thin film and when it was not. When an electric field was applied, the electric field strength was 1.0 × 10 ５ An AC electric field of 1000 V / m was applied. The voltage signal was acquired by rotating the analyzer around the laser incident axis. In this case, the voltage signal becomes a sine wave with respect to the rotation angle, and the rotation angle (δ) of the polarization plane can be derived from the amplitude with and without voltage application. The effective electro-optic (EO) coefficient r ｅｆｆ is given by the following formula: r ｅｆｆ= (2δλ) / (πn ３ E ａｃ dν) where λ is the wavelength of the laser light (632.8 nm) and n is BaTiO ３ Refractive index (2.389), E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m) or the effective value of AC field strength (0.707 × 10 ５ V / m), d is BaTiO ３ The film thickness is 49 nm, and ν is the relative volume fraction of the ferroelectric domain parallel to the electric field (1.0). The effective value of the AC electric field strength is calculated by doubling the amplitude of the AC electric field strength. １／２ The value is divided by the square root of 2. ３ After applying a DC electric field equal to or greater than the coercive electric field of E to polarize the a-domain, the above measurement was carried out. ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), the effective EO coefficient r ｅｆｆ 198pm / V,E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), the effective EO coefficient r ｅｆｆ A high value of 280 pm / V was obtained. ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m) ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m) ｅｆｆ 2 １／２ This is the value multiplied by (the square root of 2).

[0082] Next, with the rotation angle of the analyzer fixed, an AC electric field (1.0 × 10 ５ V / m) and a DC electric field (maximum value 2.0 × 10 ６ The bias voltage dependence of the electro-optic coefficient was measured by applying a bias voltage (V / m).

[0083] FIG. 6 shows the BaTiO ３ 10 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the in-plane direction of the thin film.

[0084] From Figure 6, it can be seen that domain inversion occurs in the in-plane direction of the thin film. The coercive field is 5.5 × 10 ５ V / m, and the a-domain BaTiO ３ The obtained value was nearly half that of the thin film. Furthermore, it was observed that the EO signal increased without saturating as the DC electric field increased. Thus, a high EO coefficient was obtained even when used under a DC electric field.

[0085] On the other hand, the position of the electrode was changed to BaTiO ３ The bias voltage dependence of the electro-optic coefficient was measured by applying an electric field perpendicular to the film surface.

[0086] FIG. 7 shows the BaTiO ３ 10 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in a direction perpendicular to the surface of the thin film.

[0087] From FIG. 7, BaTiO ３ Even when an electric field was applied perpendicular to the surface of the thin film, no domain inversion was observed.

[0088] Therefore, in Example 1, BaTiO ３ It is clear that the thin film is highly susceptible to domain inversion in the in-plane direction.

[0089] Also, BaTiO ３ Immediately after deposition of the thin film (heated above the tetragonal to cubic phase change temperature (Curie temperature) and then cooled to room temperature), a piezoelectric response microscope image (phase image) was taken. In this image, an AC electric field was applied to the probe, and the torsional signal of the cantilever synchronized with this was detected, allowing the visualization of the in-plane ferroelectric domains.

[0090] FIG. 8 shows the BaTiO ３ This is a piezoelectric response microscope image (phase image) of a thin film.

[0091] It can be seen from FIG. 8 that ferroelectric domains are formed in a direction parallel to the main surface of the substrate.

[0092] In Example 1, the thermal expansion coefficient is 9.4 × 10 －６ / K ３However, the thermal expansion coefficient of the substrate is not particularly limited. For example, a substrate with a larger thermal expansion coefficient (for example, a thermal expansion coefficient of 13.1×10 －６ The same effect as above can be obtained by using a substrate having a low thermal expansion coefficient (magnesium oxide: MgO) with a thermal expansion coefficient of 0.1 / K.

[0093] In Example 1, BaTiO was prepared using a solution with a raw material concentration of 0.15 mol / L. ３ The BaTiO thin film was prepared in the same manner as in Example 1, except that the raw material concentration of the solution was changed to 0.06 mol / L. ３ When a thin film was produced, the film quality was poor because the raw material solution did not wet and spread well on the substrate. In addition, BaTiO ３ When a thin film was prepared, pores were generated in the film, and although a-domains were obtained, the crystallinity was reduced.

[0094] <Effects> Conventionally, a single crystal thin film of barium titanate with an a-domain (100) orientation could only be obtained by using the MBE method and a silicon substrate, but the results of Example 1 show that a single crystal thin film of barium titanate with an a-domain (100) orientation can be obtained using any substrate in the present invention. This makes it possible to achieve an EO coefficient equivalent to the world's highest level reported previously, at a low cost.

[0095] <Principle> Both barium titanate and strontium titanate have perovskite-type crystal structures, but barium titanate has a larger lattice constant, resulting in a lattice mismatch of 2.3%. When a single-crystal thin film of barium titanate with a (100) orientation is formed on a single-crystal substrate of strontium titanate with a (100) orientation using methods such as pulsed laser deposition, sputtering, or MBE, atomic fragments undergo heterogeneous nucleation at the kink sites of the substrate, and atoms are deposited in the normal direction of the substrate surface, resulting in the growth of the thin film. At this time, the crystal lattice of barium titanate formed in a lattice-matched state is constrained by the strontium titanate substrate, resulting in compressive stress in the in-plane direction of the barium titanate thin film, and d ＬＴ / d ＶＴ When the thickness of the grown thin film exceeds the critical thickness that allows for lattice mismatch (approximately 7 nm in the case of the interface between barium titanate and strontium titanate), edge dislocations are generated in the barium titanate thin film, and the lattice distortion is alleviated. Even after dislocations are generated, the growth of barium titanate continues in a lattice-matched state, but d ＬＴ / d ＶＴ In Non-Patent Document 1, by using a silicon substrate with a smaller thermal expansion coefficient than barium titanate, tensile strain is applied to the barium titanate during the cooling process after film formation, and the d ＬＴ / d ＶＴ >1 state is realized.

[0096] On the other hand, in the film formation method using a solution as in Example 1, heat treatment is performed after each film formation of the solution. If the formed barium titanate layer exceeds the critical film thickness, misfit dislocations are introduced between the barium titanate thin film and the strontium titanate substrate by the heat treatment, thereby alleviating the lattice strain caused by lattice mismatch. Furthermore, during the heat treatment, the solvent and the organic ligands of the metal cations are burned, causing the volume of the film to shrink. At this time, since the lattice strain in the in-plane direction of the thin film is relaxed, film shrinkage in the thickness direction occurs preferentially. As a result, d ＬＴ / d ＶＴ >1 state is easily obtained, resulting in the a-domain state.

[0097] FIG. 9 shows the BaTiO ３ Thin films and SrTiO ３ This is a transmission electron microscope image (dark field image) of the interface with the substrate.

[0098] From FIG. 9, BaTiO ３ Thin films and SrTiO ３ At the interface with the substrate, dark spots (roughly indicated by arrows) are observed at intervals of just under 20 nm. The calculated dislocation spacing when the lattice mismatch is 2.3% is 17.3 nm, which is close to the spacing of the dark spots in Figure 9 of just under 20 nm, indicating that the dark spots in Figure 9 are misfit dislocations. In other words, in the present invention, a bonded structure is obtained in which misfit dislocations exist at the interface between the thin film and the substrate.

[0099] On the other hand, when a barium titanate film is formed without misfit dislocations, lattice distortion resulting from lattice mismatch remains. In this case, the volume shrinkage of the film caused by the combustion of the solvent and organic ligands of the metal cation occurs isotropically, so d ＬＴ / d ＶＴ The state of >1 is difficult to form, and therefore the a domain is difficult to obtain.

[0100] It is known that ferroelectric domains tend to grow from lattice defects such as dislocations. In the present invention, misfit dislocations exist at the interface between the thin film and the substrate, and domains tend to grow from these as starting points. Therefore, in the present invention, the coercive field for domain inversion is smaller than in Non-Patent Document 1, which does not have misfit dislocations, and a-domains can be obtained with a low DC bias voltage. Because the field-induced polarization is likely to grow, the EO coefficient increases without saturating when a DC current is applied.

[0101] Example 2: BaTiO was formed in the same manner as in Example 1, except that the number of film formation times was eight. ３ When a thin film was prepared, the lattice constant ratio d ＬＴ / d ＶＴ A thin film with an effective EO coefficient r = 1.0038 was obtained. ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 209 pm / V, and E ａｃis the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 296 pm / V. ＬＴ / d ＶＴ By increasing the value, it became easier to obtain the a domain, and thus a high EO coefficient was obtained.

[0102] Example 3: BaTiO was formed in the same manner as in Example 1, except that the number of film formation times was 10. ３ When a thin film was prepared, the lattice constant ratio d ＬＴ / d ＶＴ A thin film with an effective EO coefficient r = 1.0061 was obtained. ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 195 pm / V, and E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 276 pm / V. ＬＴ / d ＶＴ By increasing the value, it became easier to obtain the a domain, and thus a high EO coefficient was obtained.

[0103] Example 4: BaTiO was formed in the same manner as in Example 1, except that the number of film formation times was six. ３ When a thin film was prepared, the lattice constant ratio d ＬＴ / d ＶＴ A thin film with an effective EO coefficient r = 1.0031 was obtained. ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 156 pm / V, and E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 221 pm / V. ＬＴ / d ＶＴ By increasing the value, it became easier to obtain the a domain, and thus a high EO coefficient was obtained.

[0104] From the reciprocal lattice mapping, d ＬＴ The average values ​​of (010) and (001) (a-axis lattice constant: a and c-axis lattice constant: c, respectively) are obtained as follows. ＶＴ If = a, then d ＬＴ / dＶＴ = (c / a+1) / 2. Bulk BaTiO ３ Since c / a is 1.011, d ＬＴ / d ＶＴ The theoretical upper limit of is 1.0055. However, if a strong tension state occurs, the upper limit may be exceeded.

[0105] From the results of Examples 2 to 4, d ＬＴ / d ＶＴ By controlling this, it is possible to achieve an EO coefficient that exceeds the world's highest level reported previously.

[0106] Example 5 An LSAT substrate made of a single crystal of lanthanum strontium tantalate (LSAT) was used, and BaTiO ３ BaTiO was formed in the same manner as in Example 1, except that the number of times of thin film formation was eight. ３ When a thin film was prepared, d ＬＴ / d ＶＴ The LSAT substrate was (LaAlO ３ ) ０．３ -(SrAl ０．５ Ta ０．５ O ３ ) ０．７ The thermal expansion coefficient of LSAT is 8.2 × 10 －６ / K.

[0107] FIG. 10 shows the BaTiO ３ 10 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the in-plane direction of the thin film.

[0108] From Figure 10, it can be seen that domain inversion occurs in the in-plane direction of the thin film. That is, even when the LSAT substrate is used, a domain is stably formed. The effective EO coefficient r ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 172 pm / V, and E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 243 pm / V. ＬＴ / dＶＴ By increasing the value, it became easier to obtain the a domain, and thus a high EO coefficient was obtained.

[0109] [Example 6] Potassium tantalate (KTaO ３ ) single crystal of KTaO ３ The substrate was BaTiO ３ BaTiO was formed in the same manner as in Example 1, except that the number of times the thin film was formed was 10. ３ When a thin film was prepared, d ＬＴ / d ＶＴ A thin film with a KTaO ３ The thermal expansion coefficient of －６ / K. When confirmed in the same manner as in Example 5, it was found that a domain was formed. ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 204 pm / V, and E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 288 pm / V. ＬＴ / d ＶＴ By increasing the value, it became easier to obtain the a domain, and thus a high EO coefficient was obtained.

[0110] [Example 7] In contrast to Example 1, the substrate was changed to an MgO substrate made of a single crystal of magnesium oxide (MgO). MgO is a rock salt structure compound. ３ The thin film was prepared by depositing BaTiO ３ Single crystals can be directly formed as a film, or a thin film for substrate made of a single crystal of perovskite oxide can be formed on an MgO substrate, and then BaTiO ３ It is also possible to form a single crystal film. In Example 7, barium zirconate (BaZrO ３ ) single crystal BaZrO ３ After the substrate thin film is formed on the MgO substrate, BaZrO ３ BaTiO on the thin film for substrate ３ BaTiO consisting of a single crystal ３A thin film was formed. BaZrO ３ For the substrate thin film, the same as BaTiO was used except that titanium isopropoxide was replaced with zirconium isopropoxide. ３ The film can be formed under the same conditions as those for forming the thin film for the substrate. ３ When forming the thin film for the substrate, the number of film formations was one, and the firing temperature was 900°C. ３ BaZrO was formed in the same manner as in Example 1, except that the number of times the thin film was formed was 10. ３ BaTiO on the thin film for substrate ３ When a thin film was prepared, d ＬＴ / d ＶＴ = 1.0051. The thermal expansion coefficient of MgO is 13.1 × 10 －６ / K.

[0111] FIG. 11 shows the BaTiO ３ 10 is a graph showing the bias voltage dependence of the electro-optic coefficient when an electric field is applied in the in-plane direction of the thin film.

[0112] From Figure 11, it can be seen that domain inversion occurs in the in-plane direction of the thin film. That is, even when an MgO substrate is used, a domain is stably formed. The effective EO coefficient r ｅｆｆ Is E ａｃ is the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 209 pm / V, and E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 296 pm / V. ＬＴ / d ＶＴ By increasing the value, it became easier to obtain the a domain, and thus a high EO coefficient was obtained.

[0113] Example 8 BaTiO ３ BaTiO was prepared in the same manner as in Example 7, except that the number of times the thin film was formed was four. ３ When a thin film was prepared, the lattice constant ratio d ＬＴ / d ＶＴ A thin film with an effective EO coefficient r = 1.0002 was obtained. ｅｆｆ Is E ａｃis the amplitude of the AC electric field strength (1.0 × 10 ５ V / m), it is calculated as 82 pm / V, and E ａｃ is the effective value of the AC field strength (0.707 × 10 ５ V / m), it was calculated to be 116 pm / V. ＬＴ / d ＶＴ By increasing the value, it became easier to obtain the a domain, and therefore a high EO coefficient was obtained, although it was lower than in other examples.

[0114] From the results of Examples 1 to 8, it is clear that a-domains can be stably obtained even when substrates with various thermal expansion coefficients are used, and that this leads to the formation of BaTiO ３ Thin films were successfully prepared.

[0115] Comparative Example 1 BaTiO was prepared in the same manner as in Example 1, except that the heat treatment temperature was 800°C. ３ A thin film was prepared.

[0116] FIG. 12 shows the BaTiO ３ 1 is an X-ray diffraction pattern of a thin film.

[0117] From FIG. 12, BaTiO ３ Peaks other than (100) were observed, and a single crystal thin film was not obtained.

[0118] REFERENCE SIGNS LIST 1 Bonded body 10, 10A, 10B Substrate 10a Main surface 11 Substrate thin film 12 Underlying substrate 20 Thin film 30 Ferroelectric domain 40 Misfit dislocation 50 Optical modulator

Claims

a substrate having a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound on one main surface; a thin film made of a single crystal of barium titanate having a (100) orientation, the thin film being provided on the main surface of the substrate; a ferroelectric domain parallel to a main surface of the substrate is present inside the thin film; A bonded structure in which misfit dislocations exist at the interface between the thin film and the substrate. 　 The lattice constant of the barium titanate in the direction perpendicular to the main surface of the substrate is d ＶＴ , the lattice constant of the barium titanate in a direction parallel to the main surface of the substrate is d ＬＴ When this is the case, d ＬＴ / d ＶＴ 2. The bonded structure according to claim 1, wherein the lattice constant ratio of the thin film represented by the formula: 　 The bonded body according to claim 2 , wherein the lattice constant ratio of the thin film is greater than 1.0030. 　 The thermal expansion coefficient of the substrate is 4.5×10 －６ The bonded body according to any one of claims 1 to 3, wherein the bonded body has a viscosity of 1 / K or more. 　 The joined body according to any one of claims 1 to 3, wherein the substrate is a substrate entirely made of a single crystal of strontium titanate, a substrate entirely made of a single crystal of lanthanum strontium aluminum tantalate, a substrate entirely made of a single crystal of potassium tantalate, or a substrate entirely made of a single crystal of magnesium oxide. 　 The bonded body according to any one of claims 1 to 3, wherein the substrate is a silicon substrate having a substrate thin film formed on the main surface thereof and made of a single crystal of the perovskite-type oxide, the fluorite-type oxide, or the rock salt-type structure compound. 　 The bonded structure according to any one of claims 1 to 6, wherein the thin film has a thickness of 7 nm or more.

8. The joined body according to claim 1, wherein a phase of polarization of the thin film, a phase of a change in strain when an AC electric field is applied, or a phase of a change in refractive index when an AC electric field is applied is not inverted by applying a DC electric field in a direction perpendicular to the surface of the thin film, but is inverted by applying a DC electric field in an in-plane direction of the thin film.

9. The bonded structure according to claim 8, wherein the thin film has an electro-optic coefficient of 150 pm / V or more. 　 The coercive electric field E when the phase of the refractive index change when an AC electric field is applied to the thin film is reversed by applying a DC electric field in the in-plane direction of the thin film ｃ is 5.5 x 10 ５ The bonded body according to claim 8 or 9, wherein the surface tension is 0.05 V / m or less. 　 The coercive electric field when the phase of the refractive index change of the thin film when an AC electric field is applied to the thin film is reversed by applying a DC electric field in the in-plane direction of the thin film is defined as E ｃ When E>E ｃ 11. The joined body according to claim 8, wherein, when a DC electric field E in the range of is applied to the thin film, the intensity of the change in refractive index of the thin film when an AC electric field is applied does not saturate with an increase in the DC electric field E. 　 A bonded body according to any one of claims 1 to 11, A Mach-Zehnder type optical modulator that utilizes a change in refractive index when an electric field is applied to the thin film of the junction. 　 The method comprises a step of forming a thin film of a single crystal of barium titanate having a (100) orientation by forming a raw material solution in which barium carboxylate and titanium alkoxide are dispersed in an organic solvent on one main surface of a substrate containing a single crystal of a perovskite-type oxide, a fluorite-type oxide, or a rock salt-type structure compound in a state in which cluster formation in the solution is suppressed, and then performing a heat treatment in an oxygen-containing atmosphere; The method for manufacturing a bonded body, wherein the film formation from the raw material solution is carried out one or more times, and the heat treatment is carried out every time a film is formed from the raw material solution. 　 The method for producing a bonded body according to claim 13 , wherein the clusters present in the raw material solution have an average diameter of 1 nm or less.

15. The method for producing a bonded body according to claim 13, wherein the concentration of barium titanate in the raw material solution is 0.07 mol / L or more and 0.30 mol / L or less. 　 The method for manufacturing a bonded body according to any one of claims 13 to 15, further comprising the step of cleaning the main surface of the substrate before forming a film from the raw material solution. 　 The method for manufacturing a bonded body according to claim 16 , wherein the step of cleaning the main surface of the substrate comprises heat treating the substrate in an atmosphere containing oxygen.

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