JPH0891985A - Electrooptical part and its production - Google Patents

Abstract

PURPOSE: To form an optical waveguide good in crystallinity according to a liquid-phase epitaxial method. CONSTITUTION: This electrooptical part is obtained by forming an electrooptical single crystal film constituted of the same kinds of elements as those of a substrate 9 comprising an electrooptical single crystal thereon according to a liquid-phase epitaxial method. A part 10 on the substrate side of the electrooptical single crystal film is formed by bringing the substrate 9 into contact with a melt at a prescribed temperature and the substrate 9 is then brought into contact with the melt at a higher temperature than the prescribed temperature to form a part 11 on the surface side of the electrooptical single crystal. Thereby, the refractive index of the part 11 on the surface side is higher than that of the part 10 on the substrate side. A liquid phase and a solid phase are preferably made to coexist in the melt and the temperature of the melt is subsequently lowered to keep the liquid phase in a supercooled state. The substrate is then brought into contact with the liquid phase to respectively epitaxially grow the part on the substrate side of electrooptical single crystal film and the part on the surface side thereof.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electro-optical product and a method for manufacturing the same.

[0002]

2. Description of the Related Art Lithium niobate (LiNbO ₃ ) single crystal and lithium tantalate (LiTaO ₃ ) single crystal are expected as materials for optoelectronics. It is known to form a lithium niobate thin film or a lithium tantalate thin film by a liquid phase epitaxial method on a substrate made of a lithium niobate single crystal or the like, and use this thin film as an optical waveguide.

In this case, in order to form the optical waveguide on the electro-optical single crystal substrate, it is necessary that the refractive index of the film is larger than that of the substrate. The following methods are known to realize such a combination of refractive indices.
For example, “Appl. Phys. Letters” Vol. 26. No.1 (1
975), pages 8-10, on a lithium tantalate single crystal substrate by a liquid phase epitaxial method,
A lithium niobate single crystal thin film having a substantially stoichiometric composition (Li / Nb = 1) is formed. "J. Appl. Phys." Vo
l. 70 No. 5 (1991), pages 2536 to 2541, a liquid phase epitaxial method was used to form a lithium niobate substrate doped with 5 mol% of magnesium oxide.
A single crystal thin film of lithium niobate is produced. `` J. Crys
t. Growth ”132 (1993) p. 48 to p. 60, niobium of almost stoichiometric composition was formed on a lithium niobate substrate doped with 5 mol% of magnesium oxide by a liquid phase epitaxial method. A lithium oxide single crystal thin film is formed.

[0004] Success in such a liquid phase epitaxial method
The film method will be described. Figure 6 shows the liquid phase epitaxial method.
In a graph that schematically shows the temperature schedule of the melt
And FIG. 7 shows, for example, LiNbO₃─ LiVO₃Pseudo binary
It is a graph which shows the solubility curve of a system. First, for example, Nio
Lithium butylate (solute) and LiVO₃(Melting medium)
Mix and mix. Saturation corresponding to the composition of this melt
Sum temperature to T ₀And Let the temperature of this melt be the saturation temperature T
₀Hotter than T₁Hold at Lithium Niobate and LiV
O₃And are evenly melted. In FIG. 6, “A” is
It corresponds to this molten state. Then the melt temperature is
Sum temperature T₀Lower temperature T _FourTo cool the melt
Let it cool. In FIG. 6, “C” indicates this supercooling.
Corresponds to the state. Substrate against melt in supercooled state
Contact.

[0005]

At present, lithium niobate and lithium tantalate single crystal substrates are produced by the pulling method. However, the lithium tantalate single crystal substrate obtained by the pulling method is inferior in crystallinity to the lithium niobate single crystal substrate, has not reached the optical grade, and has not been put to practical use in optical applications. Even when a thin film is formed on a substrate with poor crystallinity by the liquid phase epitaxial method, the crystallinity of this thin film is strongly influenced by the substrate, so it is difficult to form an optical waveguide that can withstand optical applications. is there. The lithium niobate single crystal substrate doped with magnesium oxide is also inferior in crystallinity similarly to the above, and even if a lithium niobate single crystal film is formed on this substrate, it is not possible to form an optical waveguide that withstands optical applications. It was difficult.

Further, when such a combination of a single crystal substrate and a single crystal film is adopted, the types of the constituent elements doped are different between the substrate and the film, so that the refractive indexes of the two must be made different. However, it is difficult to form a thin film with good crystallinity because lattice mismatch occurs at the interface between the single crystal substrate and the single crystal film at the same time.

At present, a lithium niobate single crystal substrate having excellent crystallinity is produced by a pulling method (CZ method) and is used for an optical device or the like. However, this lithium niobate single crystal is produced only with a harmonic melting composition. With the pulling method, it is difficult to produce a lithium niobate single crystal having a composition other than the harmonic melting composition, and an excellent crystalline substrate that can withstand optical applications has not been produced. Here, in the harmonic melting composition, the ratio of the number of atoms of lithium and niobium (Li / Nb) is 0.946, and the content ratio of lithium when the total number of atoms of lithium and niobium is 100%. Is 48.6%.

However, the compositions of the lithium niobate single crystal film which can be formed by the liquid phase epitaxial method are almost all stoichiometric compositions at present. Here, in the stoichiometric composition, the content ratio of lithium is 50.50 when the total number of atoms of lithium and niobium is 100%.
It is 0%. Meanwhile, “Appl. Phys. Letters” Vol. 1
2. According to the description on pages 92 to 94 of (1968), when the content ratio of lithium in the lithium niobate single crystal increases, the refractive index of the single crystal decreases. Therefore, even if a single crystal film is formed on a lithium niobate single crystal substrate having a harmonic melting composition, the single crystal film has a higher lithium content than the substrate, and therefore has a smaller refractive index. Light cannot be confined in this single crystal film.

An object of the present invention is to form an optical waveguide having good crystallinity by a liquid phase epitaxial method.

Another object of the present invention is to form an optical waveguide having a crystallinity equal to or higher than that of the electro-optical single crystal substrate by the liquid phase epitaxial method. Another object of the present invention is to form an optical waveguide having better crystallinity than this substrate on an optical grade electro-optic single crystal substrate. Another object of the present invention is to form an optical waveguide having better crystallinity than this substrate on an electro-optical single crystal substrate having an optical grade harmonically fused composition.

[0011]

An electro-optical article according to the present invention comprises a substrate made of an electro-optical single crystal and an electro-optical single crystal film formed on the substrate by a liquid phase epitaxial method. The electro-optic single crystal film is entirely composed of the same kind of element, and in the electro-optic single crystal film, the refractive index of the surface side portion is larger than the refractive index of the substrate side portion.

Further, in the manufacturing method according to the present invention,
When a liquid crystal epitaxial method is used to form an electro-optical single crystal film composed of the same kind of element as the substrate on a substrate composed of the electro-optical single crystal, a solute and a melting medium that compose the electro-optical single crystal are used. After forming the substrate side part of the electro-optic single crystal film by contacting the substrate at a predetermined temperature with the melt containing, the substrate is contacted with the melt at a temperature higher than the predetermined temperature. By forming the surface side portion of the optical single crystal film, the refractive index of the surface side portion is made larger than the refractive index of the substrate side portion.

[0013]

The process of research leading to the present invention will be described below. Conventionally, when producing a lithium niobate single crystal film by a liquid phase epitaxial method, only a film having a stoichiometric composition was produced. However, as a result of detailed study on the relationship between the composition of a single crystal film produced by the liquid phase epitaxial method and the film formation temperature, the present inventor has found that the film formation temperature can be precisely controlled by controlling the film formation temperature. ,
It was confirmed that lithium niobate single crystal films having various compositions could be produced.

The inventor first had to be able to control the film forming temperature precisely. This point will be described. Although the conventional liquid phase epitaxial method has been described with reference to FIGS. 6 and 7, the following problems occur here. As can be seen from FIG. 7, LiNbO ₃ (solute)
As the concentration of sucrose increases, the saturation temperature increases and approaches the growth temperature in the pulling method. Therefore, in order to form a film with good crystallinity, it is necessary to form the film at a temperature as low as possible. From this viewpoint, it is preferable to form the film at a low temperature of 1000 ° C. or less.

On the other hand, however, when the concentration of LiNbO ₃ becomes low, particularly when the saturation temperature becomes 1000 ° C. or less, the inclination of the liquidus line becomes very large this time. Therefore, even if the concentration of solute in the melt fluctuates slightly, the saturation temperature fluctuates greatly. In the liquid phase epitaxial method, first, the melt is held at a saturation temperature or higher, and then the film is formed in a supercooled state at a film forming temperature below the saturation temperature. The crystallinity of the film is determined by this supercooled state, and this supercooled state is determined by the saturation temperature and the film forming temperature. Therefore, if the concentration of the solute in the melt fluctuates slightly, it becomes impossible to form a film with good crystallinity. In particular, in the actual film forming process, when film formation on the substrate is repeated, the composition of the melt immediately changes, and it is not possible to maintain a constant solute concentration. Therefore, it is difficult to form a film with good reproducibility.

In particular, at a film forming temperature of 1000 ° C. or less, which should originally be able to form a single crystal film having good crystallinity, reproducibility is particularly poor, and the crystallinity is rather deteriorated. there were.

For these reasons, conventionally, it is difficult to precisely control the saturation temperature and the film formation temperature, and the film formation temperature is often set to about 900 to 950 ° C. Further, regarding the composition of the single crystal film, as described above, it has been considered that a film having a substantially stoichiometric composition is formed.

The present inventor first reexamined the process of the liquid phase epitaxial method in order to solve such a difficulty in research. Conventionally, a supercooled state was created by first sufficiently melting a solute and a melting medium at a sufficiently high temperature of 1000 to 1300 ° C. and then making the temperature lower than a saturation temperature corresponding to the charged composition. That is, it was a common sense that it was necessary to create a supercooled state from a sufficiently high temperature liquid phase.

The present inventor has paid attention to this point and has come up with a method that is essentially different from the conventional method. FIG. 1 is a graph schematically showing the temperature schedule of the melt in this liquid phase epitaxial method. 2A and 2B show the crucible 1
FIG. 3 is a cross-sectional view that schematically shows the state of the melt inside.

First, the solute and the melting medium are charged into the crucible 1 and mixed. The saturation temperature T ₀ of this melt corresponds to the concentration of solute in the melt, that is, the charged composition,
Set to a fixed value. This saturation temperature can be calculated, for example, from the liquidus line as shown in FIG.

First, the temperature of the melt is set to the saturation temperature T _0.
The temperature is maintained at a temperature T ₁ higher than that to uniformly melt the solute and the melting medium. In FIG. 1, “A” corresponds to this molten state. Moreover, as shown in FIG. 2A, all of the melt 2 is in a liquid phase.

Next, the temperature of the melt is set to the saturation temperature T₀Yo
Solid phase deposition temperature T ₂Cool down. In this state
The melt is initially supercooled, but at this temperature
If held for a long enough time, the solid phase will precipitate from the melt.
come. In FIG. 2, “B” is for solid phase deposition.
Corresponds to the holding state. At this time, as shown in Fig. 2 (b).
Thus, the melt 3 separates into a liquid phase 4 and a solid phase 5. this
The solid phase 5 is mainly deposited along the wall surface of the crucible 1.

Next, the temperature of the melt is lowered to bring the liquid phase 4 into a supercooled state. In FIG. 1, “C” corresponds to this supercooled state. The substrate 6 is lowered and brought into contact with the supercooled liquid phase 4 as shown by an arrow 7, and a single crystal film is epitaxially grown.

The following method was also examined. That is, first
The temperature of the melt was maintained at a high temperature T ₁ than the saturation temperature T _0, uniformly melting the solute and the melting medium. Then, the temperature of the melt is cooled to a holding temperature T ₂ higher than the saturation temperature T ₀ . Of course, the solid phase does not precipitate at this stage. Therefore, a predetermined amount of solute is newly added to the melt. Thus, when the solute is newly added, the saturation temperature of the melt rises to a temperature T ₄ higher than the initial saturation temperature T ₀ , and the saturation temperature T ₄ after the addition of the solute is the holding temperature. T
Higher than ₂ . Holding at this holding temperature T ₂ for a sufficiently long time stabilizes the state of the solid phase and the liquid phase.

Next, the temperature of the melt is lowered to T ₃ and the liquid phase 4 is brought into a supercooled state. The substrate 6 is lowered and brought into contact with the supercooled liquid phase 4 as shown by an arrow 7, and a single crystal film is epitaxially grown. Further, the melt in the liquid phase in a supercooled state, to the melt was kept at a temperature T _2, may be contacted with the substrate cooled to a lower temperature T ₃ than the temperature T _2. As a result, the melt near the surface of the substrate, as in the case where the temperature of the entire melt is cooled to T ₂ ,
The film is supercooled and a film is formed on the substrate.

As described above, in the present method, the starting point is the state B in which the solid phase and the liquid phase coexist stably, that is,
Starting from the temperature T ₂ and lowering the temperature from this state to the temperature T ₃ , the liquid phase is supercooled. Thus, in the state where the solid phase and the liquid phase coexist, the concentration of the solute in the liquid phase is maintained at the saturation concentration at the holding temperature T ₂ unless the saturation temperature of the entire system is exceeded.

For example, when the concentration of solute in the melt decreases, the amount of solid phase decreases by that amount at the holding temperature T ₂ , and when the concentration of solute increases, the amount of solid phase increases by that amount. Therefore, the temperature and concentration of the liquid phase are always kept constant. Since the film forming temperature T _{3 is} also set to a constant value, the difference (supercooling degree) between T ₂ and T ₃ is also kept constant, and the supercooled state is completely controlled.

As a result, even when the composition of the melt changes due to repeated film formation on the substrate in the actual film forming process, the supercooled state is maintained almost completely constant. . Therefore, a single crystal film with good crystallinity is
A film can be formed with good reproducibility.

Moreover, according to this film forming method, not only a single crystal film having a constant quality can be formed with good reproducibility, but also the crystallinity itself of the single crystal film is remarkably improved. In particular, under the conditions described below, we succeeded in forming a single-crystal film having a half-width of the X-ray rocking curve smaller than that of a single-crystal substrate, which could not be produced conventionally.

The reason for this is not clear, but probably
It seems that the reason is as follows. In the conventional method, when the substrate is brought into contact with the melt, the entire melt is in a uniform liquid phase. Therefore, at the moment when the substrate comes into contact with the melt, the solid phase is first deposited in the entire liquid phase on the surface of the substrate. Therefore, in order to start the growth of the single crystal film,
It can be estimated that a relatively large nucleation energy is required. Therefore, when the growth of the film starts at the interface between the substrate and the film, the nucleation energy is large, so that the crystallinity of the film is disturbed at this interface, and the crystallinity of the film deposited on the interface is It seems to reflect the disorder.

On the other hand, in the present invention, as shown in FIG. 2B, the solid phase 5 coexists in the melt 3 in advance before the substrate 6 contacts the melt 3. In this state,
Originally, at the interface between the solid phase 5 and the liquid phase 4, microscopically, melting and precipitation occur. Therefore, it is considered that even if the substrate 6 is brought into contact with the melt 3 again, the film growth is smoothly started and a single crystal film having excellent crystallinity can be produced.

The present inventor formed a single crystal film having a half width of the X-ray rocking curve smaller than that of the single crystal substrate by this method, formed an optical waveguide on the single crystal film, and measured its optical characteristics. As a result, it was confirmed that the optical damage resistance of the optical waveguide was significantly improved. As a result, the optical waveguide type component can be widely used as various optical components.

Moreover, this method makes it possible for the first time to precisely control the saturation temperature and the film formation temperature.
The present inventor has adopted this method to precisely control the film forming temperature and measure how the composition of the lithium niobate single crystal film changes corresponding to each film forming temperature.

In the LiNbO ₃ --LiVO ₃ pseudo-binary system, the composition of the melt was 40 mol% LiNbO ₃
-60 mol% LiVO _{3 was used} and the liquid phase epitaxial method was performed according to the temperature schedule of FIG.

Each melt 2 was heated to a sufficiently high temperature T ₁ (10
The mixture was stirred at 00 ° C. to 1300 ° C.) for 3 hours or more to obtain a sufficiently uniform liquid phase. Then, the melt is held at the holding temperature T ₂
After cooling to 12 hours or more, it was kept until supersaturated lithium niobate was nucleated and the solid phase 5 was precipitated. At this time, the liquid phase portion 4 of the melt is in a saturated state at the temperature T ₂ , and the inside of the melt 3 is a state in which the liquid phase 4 and the solid phase 5 of lithium niobate coexist. Then, the temperature of the melt 3 is cooled to a film forming temperature which is lower than T ₂ by a supercooling degree of 5 ° C., and immediately, the lithium niobate single crystal substrate 6
Was brought into contact with the melt to form a film. The composition of this single crystal film was measured by the following method. The results are shown in FIG. 3 and Table 1.

The composition of the formed film is determined by a method of measuring Curie point temperature by a differential thermal analysis method and a wavelength of 106.
It was performed by using a method of measuring the phase matching temperature of the second harmonic using a laser beam of 4 nm.

[0037]

[Table 1]

As can be seen from these results, unlike the conventional wisdom, when the film forming temperature was changed variously, the composition of the lithium niobate single crystal film was changed between the harmonic melting composition and the lithium content ratio of about 52.3 mol%. It turned out that I could change and control it.

Based on this finding, the inventor of the present invention formed a lithium niobate single crystal film at a predetermined temperature on a lithium niobate single crystal substrate having a harmonic melting composition by the above-mentioned liquid phase epitaxial method. As can be seen from FIG. 3, the content ratio of lithium in this film is less than 48.6% (harmonic melting composition), so that the refractive index of the film is smaller than that of the substrate. Then, when a lithium niobate single crystal film is formed at a film formation temperature higher than this predetermined temperature, the lithium content of this film becomes smaller than the lithium content of the portion below this, and therefore the refractive index Grows larger. Therefore, this surface side portion can be used as an optical waveguide.

Further, the present inventor has the same effect in the case of forming a film with the same kind of material as the electro-optical single crystal substrate, such as the case of forming the lithium tantalate single crystal film on the lithium tantalate single crystal substrate. It was confirmed. In addition,
It is also possible to add one or more elements selected from the group consisting of magnesium, neodymium, europium, zinc, titanium and vanadium to the single crystal film and the substrate.

As described above, according to the present invention, after the substrate side portion of the electro-optic single crystal film is formed by bringing the substrate into contact with the melt at a predetermined temperature, the temperature is higher than the predetermined temperature. By forming the surface side portion of the electro-optic single crystal film, the refractive index of the surface side portion can be made larger than that of the substrate side portion. Therefore, an optical waveguide could be formed for the first time when the substrate and the film were of the same type. Moreover, since the whole of the substrate and the electro-optic single crystal film are composed of the same kind of element, the lattice strain at the interface between them is reduced and the crystallinity of the film is improved, unlike the conventional case.

[0042]

EXAMPLES In the present invention, a liquid phase and a solid phase are allowed to coexist in a melt, the temperature of the melt is then lowered to bring the liquid phase into a supercooled state, and the electro-optic single crystal substrate is brought into contact with this liquid phase. It is preferable to epitaxially grow the electro-optic single crystal film, and as described above, it becomes possible to form a single crystal film having higher crystallinity than the substrate. In this case, it is particularly preferable to keep the temperature of the melt below its saturation temperature to precipitate the solid phase in the melt.

Further, the difference between the temperature when the liquid phase and the solid phase coexist and the temperature when the liquid phase is supercooled is 2
By setting the temperature to 0 ° C. or less, the crystallinity of the single crystal film was particularly improved.

Furthermore, the substrate is made of an optical grade electro-optic single crystal, and the full width at half maximum of the X-ray rocking curve of the electro-optic single crystal film can be made smaller than the full width at half maximum of the X-ray rocking curve of the substrate. Became. Here, the half width of the X-ray rocking curve will be described. The crystallinity of the single crystal substrate and the single crystal film can be evaluated by the half width of the X-ray rocking curve. In general, it can be judged that the smaller the half width is, the better the crystallinity of the single crystal is. This value itself varies depending on the reference crystal or the like used in the X-ray measuring apparatus, and therefore the absolute value cannot be specified.

However, the crystallinity of the single crystal thin film produced by the liquid phase epitaxial method is strongly influenced by the crystallinity of the single crystal substrate. Therefore, in order to judge the crystallinity of the produced single crystal film, the half width of the X-ray rocking curve of the substrate used must be used as a reference.

The electro-optic single crystal is a lithium niobate single crystal, and the surface side portion is formed by bringing the substrate into contact with the melt at a temperature higher than a predetermined temperature, whereby the lithium of the substrate side portion is formed. It is preferable to make the lithium content in the surface side portion smaller than the content. In this case, it is preferable that the substrate is made of a single crystal of lithium niobate having a harmonic melting composition, the substrate side portion is formed at a temperature of 700 ° C to 900 ° C, and then the surface side portion is formed at a temperature of 900 ° C to 1200 ° C.

Further, the front surface side portion and the substrate side portion are
It includes the case where they are clearly separated from each other and the case where the composition changes in a gradual manner and a boundary line cannot be clearly drawn. In the former case, the part on the substrate side becomes the base layer close to the substrate,
The surface side portion becomes an optical waveguide layer close to the surface of the film.

The electro-optic single crystal substrate is composed of lithium niobate (LiNbO ₃ ) single crystal, lithium tantalate (LiT).
aO ₃ ) single crystal, LiNb _x Ta _1-x O ₃ single crystal (0 <
It is preferably formed of one or more kinds of single crystals selected from the group consisting of x <1), and more preferably formed of lithium niobate single crystals.

The melting medium is preferably one or more kinds of melting medium selected from the group consisting of LiVO ₃ and LiBO ₂ . When this melting medium is adopted, the composition of the molten material is 10 mol% solute-90 mol% solvent.
Solute 60 mol% -solvent 40 mol% is preferable.

When the solute ratio is smaller than 10 mol%, the slope of the liquidus line becomes too steep in the phase diagram of the solute-melt medium pseudo binary system as shown in FIG.
The change in the concentration of the melt due to film growth becomes large, and it becomes difficult to maintain stable film forming conditions. Solute ratio is 60mo
If it is larger than 1%, the saturation temperature becomes high,
The film forming temperature becomes too high, which makes it difficult to form a single crystal film having good crystallinity.

The substrate side portion and the surface side portion can be formed by using different melts. In this case, the temperature schedule shown in FIG. 1 is carried out for each melt. However, it is preferable to form the substrate side portion and the surface side portion by using the same melt. For this purpose, a melt having a saturation temperature higher than the film forming temperature of the surface side portion is used, the melt is sufficiently melted at a high temperature, and then the temperature of the melt is made lower than the saturation temperature. , Coexisting a liquid phase and a solid phase in the melt, then lowering the temperature of the melt to the predetermined temperature to bring the liquid phase into a supercooled state, and bringing the substrate into contact with this liquid phase to form the substrate side portion . Next, the temperature of this melt is made higher than the film forming temperature of the surface side part and lower than the saturation temperature to maintain the coexistence state of the liquid phase and the solid phase, then lower the temperature of the melt and The phase is subcooled and the substrate is contacted with this liquid phase to form the front side portion.

That is, as shown in FIG. 4, a melt having a saturation temperature T ₀ higher than the film forming temperature t ₃ on the surface side portion is used. The temperature of the melt is higher than the saturation temperature T ₀ by T ₁
Hold to melt the solute and the melting medium uniformly.
"A" corresponds to this molten state. Then, the temperature of the melt is cooled to a solid phase deposition temperature T ₂ lower than the saturation temperature T ₀ . "B" corresponds to the retention state for this solid phase deposition. Then lowering the temperature of the melt to a predetermined temperature T _3, the liquid phase supercooled state. "C" corresponds to this supercooled state. At this predetermined temperature, the principal surface 9a of the substrate 9 schematically shown in FIG. 5A is brought into contact with the liquid phase, and the substrate-side portion 10 is formed on the principal surface 9a.

Then the temperature of the melt is raised to t ₂ ,
Hold at t ₂ . "B" corresponds to this holding state.
The holding temperature t ₂ is lower than the saturation temperature T ₀ and higher than the holding temperature T ₃ . The film forming temperature t ₃ is higher than the predetermined temperature T ₃ . Next, the temperature of the melt is lowered to the film forming temperature t ₃ to bring the liquid phase into a supercooled state. "C" corresponds to this supercooled state. At this film forming temperature, the main surface 9a side of the substrate 9 schematically shown in FIG. 5A is brought into contact with the liquid phase to form the surface side portion 11 on the surface 10a of the substrate side portion 10. Thereby, the optical waveguide substrate 8 which is an electro-optical product is obtained.

In the temperature schedule of FIG. 4, the substrate side portion 10 and the front surface side portion 11 were formed in two steps. However, it is also possible to divide these into three or more steps and sequentially form a film of three or more layers. In this case,
As shown in (b), the electro-optic single crystal film 13 composed of three or more layers is formed on the main surface 9a of the substrate 9, and the film 13 is formed.
Inside, the surface side portion closer to the surface 13a has a higher refractive index than the substrate side portion closer to the substrate side interface 13b.

Hereinafter, more specific experimental results will be described. According to the temperature schedule shown in FIG. 4, the optical waveguide substrate 8 shown in FIG. 5A was manufactured by the above method. The half-widths of the X-ray rocking curves of the optical-grade lithium niobate single crystal substrates used by the present inventors were all 6.8 to 6.9 [arc sec]. It was used as a reference for the crystallinity of the substrate. The full width at half maximum was measured by the double crystal method using reflection on the (001) plane. CuKα1 was used as the incident X-ray, and the monochromator (42
2) The surface was used.

In the LiNbO ₃ -LiVO ₂ quasi-binary system melt, the charged composition was 20 mol% LiNbO ₃
It was set to −80 mol% LiVO ₂ . In FIG. 4, the temperatures T ₀ , T ₁ , T ₂ , T ₃ , t ₂ and t ₃ are 96 respectively.
0 ° C, 1200 ° C, 805 ° C, 800 ° C, 905 ° C, 9
It was set to 00 ° C.

The extraordinary refractive index ne of the light having a wavelength of 633 nm was 2.205 for the substrate 9, 2.195 for the substrate side portion 10 and 2.204 for the front surface side portion 11. The ordinary light refractive indexes were almost the same. Further, the half-width of the X-ray rocking curve of the substrate side portion 10 thus created was 5.6 [arc sec], and the half-width of the X-ray rocking curve of the surface side portion 11 was 5.7 [arc sec]. . It can be seen that both are superior to the substrate.

Further, the surface side portion 11 was used as an optical waveguide, and light having a wavelength of 830 nm was allowed to pass therethrough. As a result, 2mW
No light damage occurred when using the output light of. On the other hand, the above-mentioned lithium niobate single crystal substrate 9 was used, and an optical waveguide was formed by a titanium diffusion method by a usual method. However, even when light with an output of 0.2 mW was used, optical damage occurred.

[0059]

As described above, according to the present invention, a good optical waveguide can be formed only when the electro-optical single crystal substrate and the electro-optical single crystal film are of the same kind. Since the entire substrate and the electro-optic single crystal film are composed of the same kind of element, unlike the conventional case, the lattice strain at the interface between them is reduced and the crystallinity of the film is improved. As a result, it was possible to prevent optical damage to the optical waveguide.

[Brief description of drawings]

FIG. 1 is a graph schematically showing a temperature schedule of a melt in a liquid phase epitaxial method developed by the present inventor.

FIG. 2 (a) is a cross-sectional view schematically showing a state in which a melt 2 is melted in the crucible 1, and FIG. 2 (b) is
It is a sectional view showing typically the state where solid phase 5 and liquid phase 4 coexist.

FIG. 3 is a graph showing the relationship between the composition of a lithium niobate single crystal film and the film formation temperature.

FIG. 4 is a graph schematically showing a temperature schedule that can be adopted in the present invention.

5A is a cross-sectional view schematically showing a state in which a substrate-side portion 10 and a surface-side portion 11 are formed on an electro-optical single crystal substrate 9, and FIG. 5B is an electro-optical single crystal. 3 is a cross-sectional view that schematically shows a state in which an electro-optic single crystal film 13 is formed on a substrate 9. FIG.

FIG. 6 is a graph showing a temperature schedule of a conventional liquid phase epitaxial method.

FIG. 7 is a graph showing an example of a liquidus line in a phase diagram of a solute-melting medium pseudo binary system.

[Explanation of symbols]

1 crucible 2 melted material that melts uniformly 3 melted material in which solid phase and liquid phase coexist 4 liquid phase 5 solid phase
6, 9 Electro-optic single crystal substrate 8, 12 Electro-optic product (optical waveguide substrate) 10 Substrate side part 11 Surface side part

Claims (12)

[Claims] 1. A substrate comprising an electro-optic single crystal and an electro-optic single crystal film formed on the substrate by a liquid phase epitaxial method, wherein the substrate and the electro-optic single crystal film are of the same kind. In the electro-optic single crystal film, the refractive index of the surface side portion is larger than the refractive index of the substrate side portion of the electro-optical single crystal film. 2. The electro-optic single crystal is a lithium niobate single crystal, and in the electro-optic single crystal film, the lithium content in the surface side is smaller than the lithium content in the substrate side. The electro-optical product according to claim 1. 3. The electro-optical article according to claim 2, wherein the electro-optical single crystal film includes an underlayer near the substrate and an optical waveguide layer near the surface. 4. The substrate is made of a lithium niobate single crystal having a harmonic melting composition, and the lithium content in the substrate-side portion of the electro-optic single crystal film is higher than the harmonic melting composition. The electro-optical product according to 2 or 3. 5. The substrate is made of an optical grade electro-optic single crystal, and the full width at half maximum of the X-ray rocking curve of the electro-optic single crystal film is smaller than the full width at half maximum of the X-ray rocking curve of the substrate. The electro-optical article according to any one of claims 1 to 4, which is characterized. 6. An electro-optical single crystal is formed on a substrate made of the electro-optical single crystal by forming a electro-optical single crystal film made of the same kind of element as the substrate by a liquid phase epitaxial method. After forming the substrate side portion of the electro-optical single crystal film by contacting the substrate at a predetermined temperature with respect to a melt containing a solute and a melting medium, the melt at a temperature higher than the predetermined temperature By forming the surface side portion of the electro-optic single crystal film by contacting the substrate, the refractive index of the surface side portion of the electro-optic single crystal film is made larger than that of the substrate side portion. A method for manufacturing an electro-optical product, comprising: 7. A liquid phase and a solid phase are allowed to coexist in the melt, the temperature of the melt is then lowered to bring the liquid phase into a supercooled state, and the electro-optic single crystal substrate is brought into contact with the liquid phase. 7. The method for manufacturing an electro-optical article according to claim 6, wherein the electro-optical single crystal film is epitaxially grown. 8. The electro-optical article according to claim 7, wherein a solid phase composed of the solute is deposited in the melt by maintaining the temperature of the melt at a temperature equal to or lower than its saturation temperature. Production method. 9. A difference between the temperature when the liquid phase and the solid phase coexist and the temperature when the liquid phase is supercooled is 20 ° C. or less. Claim 7 or 8
A method for manufacturing the electro-optical article described. 10. The substrate is made of an optical grade electro-optic single crystal, and the full width at half maximum of the X-ray rocking curve of the electro-optic single crystal film is smaller than the full width at half maximum of the X-ray rocking curve of the substrate. The method for manufacturing an electro-optical article according to claim 7, which is characterized in that. 11. The electro-optical single crystal is a lithium niobate single crystal, and the surface side portion of the electro-optical single crystal film is brought into contact by bringing the substrate into contact with the melt at a temperature higher than the predetermined temperature. By forming, the lithium content ratio of the surface side portion is made smaller than the lithium content ratio of the substrate side portion.
10. The method for manufacturing an electro-optical product according to any one of items 10. 12. The substrate is made of a lithium niobate single crystal having a harmonized composition, the substrate side portion is formed at a temperature of 700 ° C. to 900 ° C., and then the surface side portion is formed at a temperature of 900 ° C. to 1200 ° C. The method for manufacturing an electro-optical product according to claim 11, wherein: