JPH05229897A - Lithium niobate single crystal thin film and its production - Google Patents

Abstract

PURPOSE:To establish a technique for producing a lithium niobate single crystal thin film having a moderate thickness, exhibiting same lattice constants even at room temperatures and high in crystallinity. CONSTITUTION:A mixture having a composition surrounded by a point (A) (44.49, 46.58, 8.93), a point (B) (40.30, 50.65, 9.05), a point (C) (39.61, 45.89, 14.50) and a point (D) (43.88, 43.06, 13.06) in an Li2O-V2O5-Nb2O5 three component triangular diagram (mol%) and containing Na2O and MgO each in an amount satisfying the molar ratio of (Na2O/Li2O)=(2.0/98.0) to (93.5/6.5) and (MgO/Nb2 O5)=(0.2/99.8) to (40.0/60.0) is used. The fused material of this mixture is kept at temperatures within a range of 900 to 1100 deg.C and a lithium tantalate substrate is brought into contact therewith, thus growing the objective lithium niobate single crystal thin film exhibiting an (a) axial lattice constant equal to that of the substrate at room temperatures.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium niobate single crystal thin film having a thickness suitable for use in various optical materials including a thin film waveguide type SHG element, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, research on shortening the wavelength of laser light has been actively conducted with the development of optical application technology. that is,
By shortening the wavelength of the laser light, the recording density and the photosensitivity can be remarkably improved when applied to optical devices such as optical disks and laser printers. In particular, research on a second harmonic generation (SHG) element capable of converting the wavelength of incident laser light into ½ is active.

As the second harmonic generation (SHG) element, a bulk single crystal of a non-linear optical crystal has been conventionally used with a high-output gas laser as a light source. However, there is a strong demand for downsizing devices such as optical disk devices and laser printers, and gas lasers require an external modulator for optical modulation. On the other hand, recently, a semiconductor laser, which can be directly modulated and is inexpensive, has been used in place of the gas laser. Along with this, since it is necessary to obtain high conversion efficiency with a low light source output of several mW to several tens of mW, a thin film waveguide type SHG element is in the spotlight.

As a non-linear optical material for such a thin film waveguide type SHG element, conventionally, a layer in which a refractive index is changed by diffusing Ti or the like into a lithium niobate bulk single crystal is used as a waveguide, or tantalum is used. It is known that a lithium niobate thin film formed by a high frequency sputtering method on a lithium oxide substrate is used as a waveguide. However, it has been difficult to form a lithium niobate thin film having excellent crystallinity and high conversion efficiency from these materials.

By the way, a single crystal thin film having excellent crystallinity is manufactured.
The liquid phase epitaxial method is suitable as the manufacturing method.
It is thought to be. And obtain a lithium niobate thin film
As a liquid phase epitaxial method for this, for example, “Appl
ied Physics Letters "Vol.26, No. 1, January p8
-10 (1975), Lithium tantalate substrate, Li
₂O, V₂O_FiveIs used as flux, and lithium niobate for optical waveguide is used.
Thin film formed by liquid phase epitaxial growth
An example is described in which the waveguide is guided.

In addition, Japanese Patent Publication No. 51-9720 discloses that
Lithium tantalate on the substrate, Li ₂O, V₂O_FiveFlack
For optical waveguide by liquid phase epitaxial growth method
A method for forming a lithium niobate thin film is described.
It

Further, in Japanese Patent Publication No. 56-47160, a lithium niobate / lithium tantalate solid solution thin film single crystal containing Mg is formed on a substrate in a liquid phase using Li ₂ O and V ₂ O ₅ as fluxes. A method of forming by an epitaxial growth method is described.

However, in each of the above techniques, not only is it impossible to form a lithium niobate single crystal having excellent crystallinity on a lithium tantalate substrate, but SH
There is a problem that it is difficult to obtain a lithium niobate single crystal thin film having a film thickness necessary for manufacturing the G element, and it is difficult to put it into practical use.

That is, the film thickness required for the thin film waveguide type SHG element is the fundamental wavelength light of the wavelength (λ) and the wavelength (λ /) in order to achieve phase matching between the incident laser light and the second harmonic. It is a film thickness sufficient to match the effective refractive index with the second harmonic wavelength light of 2). In particular, when an SHG element for a semiconductor laser made of a lithium tantalate substrate and a lithium niobate single crystal thin film is produced, in order to make the effective refractive indices consistent, in consideration of the amount removed by polishing, a thickness of 5 μm or more is taken into consideration. A single crystal thin film of lithium niobate is required.

As described above, a lithium niobate single crystal thin film having a film thickness necessary for producing an optical device such as an SHG element on a lithium tantalate substrate and having excellent optical characteristics has been practically used. The manufacturing technology has not been established. As a result of various researches on the problems that such conventional techniques have, such problems arise because, firstly, the lattice constant of the lithium niobate single crystal thin film is smaller than that of the lithium tantalate substrate. , It was found that the strain occurs in the crystal lattice during fabrication.

Therefore, in order to solve the problem, the present inventor once added to the lithium niobate single crystal Na or M.
By containing g, the lithium niobate single crystal is prevented from being damaged by light (the refractive index of the crystal is changed by irradiating strong light), and the lattice constant of the lithium niobate substrate is adjusted to adjust the lithium tantalate substrate. By practically producing a lithium niobate single crystal thin film having a film thickness necessary for producing an optical device such as an SHG element and having excellent optical characteristics by matching (lattice matching) with the lattice constant of , Which was previously proposed as Japanese Patent Application No. 1-2242285.

The technique of adding Na or Mg to lithium niobate has been used before that, for example, in "Journal of Crystal G".
rowth 54 ”(1981) 572-576, an example of forming a sodium-containing lithium niobate thin film single crystal with a film thickness of 20 μm on a Y-cut lithium niobate substrate was reported, and“ Journal of Crystal Growth ”. 84 ”(1987)
409-412 reports an example in which a sodium-containing lithium niobate single crystal thin film is formed on a Y-cut lithium tantalate substrate.

However, although the reports disclosed in these publications describe that the lattice constant of the lithium niobate single crystal changes due to the inclusion of Na, the SAW (Su
rface Acoustic Wave) device technology,
There is no suggestion that "a film having excellent optical properties when lattice-matched" is obtained with the optical properties and the lithium tantalate substrate. Further, the lithium niobate single crystal thin film shown in these publications, the former one uses lithium niobate as a substrate, and the latter one is formed on a lithium tantalate substrate, but is a thin film. Neither of them can be used as an optical material for the purpose of the present application because of the lack of lattice matching between the substrate and the substrate.

Further, in US Pat. No. 4,093,781,
When a lithium ferrite film is formed on a substrate by a liquid phase epitaxial growth method, a method of forming a strain-free lithium ferrite film by replacing lithium with Na and matching the lattice constant with the substrate is described. However, this technique is a technique relating to lithium ferrite, and is not the lithium niobate single crystal thin film as the subject of the present invention, and cannot be used in the same field of optical materials as that of the present invention.

Further, Japanese Unexamined Patent Publication No. S52-142477 discloses a technique for obtaining a liquid phase epitaxial crystal having no lattice distortion by gradually growing the crystal by making the start of the crystal extremely gentle. However, this technique is a technique for manufacturing a semiconductor thin film, and is not a technique for forming a lithium niobate single crystal thin film on a lithium tantalate substrate.

[0016]

As described above,
In the prior art, there are known methods of changing the lattice constant of a thin film, matching the lattice constants of a substrate and a thin film, and producing a thin film with less strain by slow crystal epitaxial growth. Further, in the prior art proposed by the inventors of the present invention, a method of adding Mg or Na onto a lithium niobate substrate to make the lattice constants coincide with each other is proposed.

However, according to the research conducted by the present inventors, it was found that the above-mentioned respective technologies have common technical problems. It is not the case that the lattice constants of the substrate and the thin film epitaxially grown on it are still in perfect agreement even at room temperature, and most of them are in agreement at high temperature but not at room temperature. Therefore, not only the facets are generated, but also the crystallinity may be deteriorated.
That is, conventionally, the lattice constant of the lithium tantalate substrate and the lithium niobate single crystal thin film grown on it is,
The reality is that the technology for forming a lithium niobate thin film that completely matches even at room temperature has not been established. Therefore,
An object of the present invention is to establish a problem that requires the above-mentioned solution, that is, to establish a technique for manufacturing a thick lithium niobate single crystal thin film having a high crystallinity and a matching lattice constant even at room temperature.

[0018]

As a result of earnest research to realize the above-mentioned object of the present invention, as a result of eliminating facets,
In order to improve crystallinity, room temperature conditions (5 to 50
The inventors have found that it is effective to optimize the thin film composition and the growth temperature so that the lattice constants of the lithium tantalate substrate and the lithium niobate single crystal thin film are the same under (° C.) and completed the present invention.

That is, the present invention provides a method for epitaxially growing a single crystal thin film of lithium niobate on a lithium tantalate substrate by bringing the substrate into contact with a lithium niobate thin film growing melt, wherein the melt comprises: About Li ₂ O, V ₂ O ₅ , and Nb ₂ O ₅ mainly, point A in the Li ₂ O-V ₂ O ₅ -Nb ₂ O ₅ three-component triangular diagram (mol%) shown in FIG. 2 of the accompanying drawings. (44.49, 46.58, 8.93),
Point B (40.30, 50.65, 9.05), Point C (39.61, 45.89,
14.50), the composition within the range surrounded by point D (43.88, 43.06, 13.06), and for Na ₂ O and MgO, the molar ratio of Na ₂ O / Li ₂ O is 2.0 / 98.0 to 93.5 / 6.5. ,
The composition of MgO / Nb ₂ O _{5 in the} range of 0.2 / 99.8 to 40.0 / 60.0 is used.
By holding the substrate at 00 to 1100 ° C. and bringing them into contact with each other, the a-axis lattice constant of the single crystal thin film of lithium niobate and the a-axis lattice constant of the lithium tantalate substrate match at room temperature. We propose a method for producing a lithium niobate single crystal thin film, which comprises growing a lithium single crystal thin film.

The characteristics of the lithium niobate single crystal thin film produced by the above method are as follows. (However, LN in the formula is lithium niobate.). Incidentally, at least a part of the surface of the lithium tantalate substrate may be added with a different element described later,
The temperature for growing the lithium niobate single crystal thin film is preferably in the range of 935 to 945 ° C., and the lithium niobate single crystal thin film is preferably grown on the (0001) plane of the lithium tantalate substrate. Is.

[0021]

In the present invention, when a lithium niobate single crystal thin film is deposited on a lithium tantalate substrate by a liquid phase epitaxial growth method, it is mainly melted as Li ₂ O, V
₂ O ₅ , Nb ₂ O ₅ , Na ₂ O and MgO, and the composition range of Li ₂ O, V ₂ O ₅ and Nb ₂ O ₅ excluding Na ₂ O and MgO is shown in FIG.
_In the trigonometric diagram of the three-component system of ₂ O mol% -V ₂ O ₅ mol% -Nb ₂ O ₅ mol%, point A (44.49, 46.58, 8.93) and point B (4
0.30, 50.65, 9.05), point C (39.61, 45.89, 14.50)
), Point D (43.88, 43.06, 13.06) within the range surrounded by four composition points, and the composition range of Na ₂ O and MgO is 2.0 / 98.0 of Na ₂ O / Li ₂ O in molar ratios. ~ 93.5 /
6.5, MgO / Nb ₂ O ₅ is 0.2 in a molar ratio /99.8～40.0/60.0
It is necessary to use one having a composition range satisfying the above conditions.

The component composition of the melt is mainly Li ₂ O, V ₂
The reason why it is assumed to be composed of O ₅ , Nb ₂ O ₅ , Na ₂ O and MgO will be described below.

First, Li ₂ O and V ₂ O ₅ are compounds that act as a flux and contribute to the realization of liquid phase epitaxial growth of a lithium niobate single crystal.

When Na and Mg are contained in the lithium niobate single crystal thin film, they have the effect of increasing the lattice constant of the a-axis of the lithium niobate single crystal thin film. It is effectively used for matching the lattice constant of the axis with the lattice constant of the a-axis of the lithium tantalate substrate, and is also effective for obtaining a lithium niobate single crystal thin film having a large thickness.

The reason for containing Na and Mg together is M
This is because the LT substrate and the LN thin film described above can be lattice-matched by using only g, while the lattice matching of both can be performed by using Na alone, but optical damage cannot be prevented. It is known that this Mg has an effect of preventing optical damage.

As described above, according to the present invention, lithium niobate is used.
Single crystal thin film must contain both Na and Mg
Therefore, as described above, the composition of the melt is
Is Li ₂O, V₂O_Five, Nb₂O_FiveIn addition, make lattice matching easy.
To Na₂It is necessary to contain O and MgO.
It

By including Na and Mg in the lithium niobate single crystal thin film, the lattice constant of the a-axis of the lithium niobate single crystal thin film becomes large. This is because Na ions, Mg ions or these atoms are It is caused by being doped into the lithium niobate crystal lattice or being replaced with ions or atoms constituting the lithium niobate crystal lattice.

Next, Li ₂ O excluding Na ₂ O and MgO, V ₂ O ₅ and Nb
The composition range of ₂ O ₅ is Li ₂ O-V ₂ O ₅ -Nb expressed in mol%.
_In the trigonometric diagram of ₂ O ₅ ternary system, point A (44.49, 46.58
, 8.93), point B (40.30, 50.65, 9.05), point C (39.
61, 45.89, 14.50), point D (43.88, 43.06, 13.06)
Within the composition region surrounded by the four composition points of
The composition range of ₂ O and MgO is Na ₂ O / Li ₂ O in molar ratios.
2.0 / 98.0 to 93.5 / 6.5, and the molar ratio of MgO / Nb ₂ O ₅ is 0.
It is necessary to be within the range that satisfies the condition of 2 / 99.8 to 40.0 / 60.0, and it is also necessary to set the growing temperature to 900 to 1100 ° C.

The reason why the ternary system of Li ₂ O-V ₂ O ₅ -Nb ₂ O ₅ must be a composition surrounded by the above four points in the triangular diagram is as follows.
This is because if the composition is out of this range, facets are likely to occur, and a high-quality lithium niobate single crystal thin film having excellent optical characteristics cannot be obtained.

The reason why the composition range of Na ₂ O and MgO is limited to the above range is that Na ₂ O / Li ₂ O: 2.0 / 98 to 93.5 / 6.5 The ratio of Na ₂ O deviates from the above range. In this case, it is difficult to lattice-match the lithium tantalate substrate and the lithium niobate single crystal thin film. MgO / Nb ₂ O ₅ : 0.2 / 99.8 to 40.0 / 60.0 If the proportion of MgO is lower than the above range, the effect of preventing Mg from optical damage is insufficient, and if the proportion of MgO is higher than the above range, magnesium niobate-based This is because the crystals of (1) are deposited and a lithium niobate single crystal thin film cannot be obtained.

Further, the reason for limiting the growth temperature to 900 to 1100 ° C. is that lithium tantalate has a larger coefficient of thermal expansion than lithium niobate, so that if the temperature exceeds 1100 ° C., the melt according to the present invention is lattice-matched. This is because it is difficult to precipitate the separated crystals, and facets are generated below 900 ° C. The preferable growth temperature range is 935 to 945 ° C. Hereinafter, the present invention will be described in more detail.

(1) Substrate: In the present invention, the substrate is preferably lithium tantalate. The crystal system of the lithium tantalate substrate is similar to a lithium niobate single crystal and is easy to grow epitaxially. Furthermore, since the lithium tantalate substrate is commercially available, a good quality product can be stably obtained. Is.
It is desirable that the lithium tantalate substrate of the present invention contains a different element in at least a part of its surface.

The reason why the surface of the substrate contains a different element is that the refractive index of the substrate can be changed and the difference in refractive index between the substrate and the thin film waveguide layer can be increased. ..

The different element means an element different from the element constituting the substrate, and is preferably a metal element among them.

For example, the different elements are magnesium (Mg), titanium (Ti), vanadium (V), chromium (C
At least one selected from r), iron (Fe), nickel (Ni), neodymium (Nd) and the like is desirable.

In the present invention, a pattern having a relatively lower refractive index than a non-formed portion is formed by adding the different element to a specific portion (waveguide forming portion) of the lithium tantalate substrate. Thus, by simply forming a lithium niobate single crystal thin film (slab-like) on the substrate, the lithium niobate single crystal thin film formed in the pattern portion becomes a waveguide. Can be omitted.

As a method of making the substrate refractive index of the waveguide forming portion relatively lower than that of the non-forming portion, the substrate refractive index of the waveguide forming portion is lowered or the substrate refractive index of the non-forming portion is increased. Is desirable.

The different elements having the function of increasing the refractive index of the lithium tantalate substrate include Ti, Cr and N.
Mg, V, etc. are advantageous as d, Fe, Ni, etc., and as the different element having the action of lowering the refractive index.

With these elements, it is possible to change only the surface refractive index without substantially changing the characteristics that affect the thin film formation of the substrate, for example, the surface roughness, so that it has characteristics equivalent to those of ordinary substrates. The thin film can be manufactured under similar conditions.

The content of the different element added to the surface of the substrate is preferably in the composition range shown below. Ti; 0.2-30 mol% Cr; 0.02-20 mol% Fe; 0.02-20 mol% Ni; 0.02-20 mol% Nd; 0.02-10 mol% Mg; 0.1-20 mol% V; 0.05-30 mol% Above Content of different element / (LiTaO ₃ + different element) x 1
00 is calculated.

The reason why the above composition range is preferable is that when the composition ratio is higher than the above range, the crystallinity of the substrate is lowered,
Also, if the composition ratio is less than the above range, the refractive index does not change.

It is preferable that the content of the different element be within the range shown below. Ti; 1.0 to 15 mol% Cr; 0.2 to 10 mol% Fe; 0.2 to 10 mol% Ni; 0.2 to 10 mol% Nd; 0.5 to 5 mol% Mg; 2.0 to 10 mol% V; 1.0 to 15 mol%

As a method of adding the different element to the lithium tantalate substrate, it can be contained in various forms such as atoms, ions and oxides. For example, in addition to thermal diffusion, ion exchange, ion implantation, etc., a liquid phase epitaxial growth method, a method of preliminarily mixing different kinds of elements into the raw material of lithium tantalate bulk single crystal, and the like can be used.

When the thermal diffusion, ion exchange or ion implantation method is used, a diffusion layer of a different element is formed, and the thickness of this diffusion layer is preferably 0.01 to 20 μm. The reason is that the thickness of the diffusion layer is 0.01 μm.
If the ratio is less than the above, the proportion of guided light that spreads to the substrate portion where the different elements are not diffused increases, so that the refractive index required as the substrate cannot be satisfied, while if it exceeds 20 μm, the crystallinity of the substrate is increased. This is because it becomes worse and sufficient characteristics cannot be obtained as an optical waveguide.

In the present invention, it is desirable to form the lithium niobate single crystal thin film lattice-matched with the lithium tantalate substrate after adding the different element to at least a part of the surface of the lithium tantalate substrate.

For example, after heating the substrate for thermal diffusion of the different elements, it is desirable to bring the substrate into contact with a molten material for growing a lithium niobate thin film, that is, a molten material for liquid phase epitaxial growth in the heated state. This is because the crystallinity of the substrate decreases when the substrate is heated again for liquid phase epitaxial growth after being thermally diffused and then cooled. The thermal diffusion is preferably performed at the same time as preheating the substrate in the same furnace when heating the melt.

The thermal diffusion is preferably 850 ° C to 1000 ° C. The reason is that diffusion does not occur at a temperature lower than 850 ° C., and the crystallinity of the substrate decreases at a temperature higher than 1000 ° C., and Li outdiffusion occurs. The time required for the thermal diffusion is preferably 0.5 to 20 hours.

In the present invention, it is desirable to preheat the lithium tantalate substrate in advance before the substrate is immersed in the melt and brought into contact with it. The reason is that the lithium tantalate substrate is very sensitive to thermal shock. The preheating time is preferably 20 to 60 minutes. Preheating is preferably performed at a position 5 to 15 mm from the surface of the melt.

The feature of the present invention is that the lattice constant of the a-axis of the lithium tantalate substrate is adjusted by adding a different element to match the lattice constant of the a-axis of the lithium niobate crystal. To do.

Therefore, in order to make the above-mentioned lattice constants coincide with each other, firstly, there is a method of reducing the lattice constant of the a-axis of the lithium tantalate substrate. For this method,
It is desirable to contain Ti atoms or Ti ions in the substrate. This is because Ti atoms or Ti ions have the effect of reducing the a-axis lattice constant of the lithium tantalate substrate.

As described above, when Ti atoms or Ti ions are contained in the substrate, the content is preferably 0.2 to 30 mol% with respect to the lithium tantalate single crystal. The reason is that when the content of Ti atoms or ions is less than 0.2 mol%, the lattice constant does not become small enough to be lattice-matched with the lithium niobate single crystal. On the other hand, when it exceeds 30 mol%, conversely, This is because the lattice constant becomes too small, and in any case, lattice matching between the lithium tantalate substrate and lithium niobate cannot be obtained.

The CZ (Czochralski) method is preferable as the method for manufacturing the lithium tantalate substrate. Further, as the raw material thereof, for example, lithium carbonate, tantalum pentoxide, titanium oxide, vanadium pentoxide can be used.

In producing the substrate, it is advantageous to heat and dissolve the raw material in a platinum-rhodium crucible or an iridium crucible and pull up the lithium tantalate single crystal. The crucible made of iridium is preferable for producing an optical material because impurities are not mixed in the crystal.

Unlike the above-mentioned method, the a-axis lattice constant of the lithium tantalate substrate may be increased by adding a different element such as Na to achieve lattice matching.

In lattice matching, it is preferable to simultaneously change the a-axis lattice constants of the lithium niobate single crystal thin film and the lithium tantalate substrate. That is, if the lattice constant of the a-axis of the lithium niobate single crystal thin film is increased and at the same time the lattice constant of the a-axis of the lithium tantalate substrate is decreased, the amount of different elements contained in the substrate or the thin film can be reduced. This is because it is possible to improve the crystallinity.

The edges of the lithium tantalate substrate are preferably chamfered. This is because if the edges are not chamfered, minute scratches will be formed on the edges and cracks will occur due to thermal shock. The chamfer is the R surface,
It may be either C side.

The thickness of the lithium tantalate substrate is preferably 0.5 to 2.0 mm. Substrates thinner than 0.5 mm are more prone to cracks, and substrates thicker than 2.0 mm have a more pronounced pyroelectric effect (electric discharge effect due to heating).
This is because the particles are electrified by heating or polishing, so that polishing debris and the like are likely to adhere to the surface and scratches easily occur.

(2) Regarding the melt; In the present invention, the raw material composition for growing a lithium niobate single crystal thin film formed on the lithium tantalate substrate described above has a composition ratio as an oxide of the above-mentioned melt. Is selected within the composition range of. As this raw material component, an oxide or a compound that changes into an oxide by heating is desirable, and for example, in addition to a composition of Na ₂ CO ₃ , Nb ₂ O ₅ , Li ₂ CO ₃ , V ₂ O ₅ , MgO, etc. , NaNbO ₃ , NaVO ₃ , LiNbO ₃ and LiVO ₃ can also be used.

In the present invention, the above raw material composition for melts
As Li₂O, V₂O_Five, Nb₂O_Five, Na ₂In addition to O and MgO,
Ozimium (Nd), Rhodium (Rh), Zinc (Zn), Nickel
(Ni), cobalt (Co), titanium (Ti)
At least one oxide can be used. these
The elements of (Nd, Rh, Zn, Ni, Co, Ti, Cr, etc.) are niobium.
By including it in the lithium oxide single crystal thin film, the refractive index
It has the effect of changing the lattice constant and the lattice constant.

The Rh content is preferably 0.05 to 20 mol%. This is because if the content of Rh exceeds 20 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorates, while if it is less than 0.05 mol%, the refractive index hardly changes. The Rh content is preferably 0.1 to 10 mol%.

The Zn content is preferably 0.02 to 30 mol%. This is because if the Zn content exceeds 30 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorate, while if it is lower than 0.02 mol%, the refractive index hardly changes. The Zn content is preferably 0.5 to 15 mol%.

The Ni content is preferably 0.10 to 20 mol%. This is because if the Ni content exceeds 20 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorate, while if it is less than 0.10 mol%, the refractive index hardly changes. The Ni content is preferably 1.0 to 10 mol%.

The content of Co is preferably 0.05 to 20 mol%. This is because if the content of Co exceeds 20 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorates, while if it is less than 0.05 mol%, the refractive index hardly changes. The Co content is preferably 0.1 to 10 mol%.

The content of Cr is preferably 0.02 to 20 mol%. This is because if the Cr content exceeds 20 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorate, and if it is lower than 0.02 mol%, the refractive index hardly changes. The Cr content is preferably 0.2 to 10 mol%.

The Ti content is preferably 0.2 to 30 mol%. This is because if the Ti content exceeds 30 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorate, while if it is less than 0.2 mol%, the refractive index hardly changes. The content of Ti is preferably 1.0 to 15 mol%.

The Nd content is preferably 0.02 to 10 mol%. This is because if the Nd content exceeds 10 mol%, the optical characteristics of the lithium niobate single crystal thin film deteriorates, while if it is less than 0.02 mol%, the refractive index hardly changes. The Nd content is preferably 0.5 to 5 mol%.

The V content is preferably 0.05 to 30 mol%. This is because if the content of V exceeds 30 mol%, crystals having a different structure are deposited in the lithium niobate single crystal thin film and the optical characteristics are deteriorated.
This is because if it is lower than 05 mol%, the refractive index hardly changes. The content of V is preferably 1.0 to 15 mol%.

In the lithium niobate single crystal thin film waveguide layer, the Cr, Nd, Ti, V, Rh, Zn, Ni,
When a different element such as Co is contained, the lattice constant and the refractive index of the lithium niobate single crystal thin film change at the same time, so it is desirable to adjust the content of the different element as necessary.

(3) Growth of thin film-liquid phase epitaxial treatment: The raw material composition is melted by heating to be a melt maintained in a temperature range of 900 to 1100 ° C. The reason for holding the melt in this temperature range is 1100 ° C because lithium tantalate has a higher coefficient of thermal expansion than lithium niobate.
This is because it is difficult to precipitate a lattice-matched crystal in the melt according to the present invention when the temperature exceeds 90 ° C., while facets are generated when the temperature is lower than 900 ° C. It is desirable that the heating and melting be performed in an air atmosphere or an oxidizing atmosphere.

In the present invention, when the lithium niobate single crystal is epitaxially grown on the lithium tantalate substrate, after the melt is supercooled,
It is desirable to bring the lithium tantalate substrate into contact and grow.

The cooling rate for bringing the melt into a supercooled state is preferably 0.5 to 300 ° C./hour. Further, in the present invention, the cooling rate after the completion of the liquid phase epitaxial growth is preferably 0.5 to 300 ° C./hour. And
This cooling is preferably performed exponentially from 400 ° C.

At the temperature of the Curie point of the lithium tantalate substrate during the liquid phase epitaxial growth,
It is desirable to maintain the temperature for a certain period of time or to slowly cool it at a rate of 0.1 to 5 ° C / min. The reason for this is to prevent the occurrence of cracks due to the phase transition of crystals at the Curie point. The Curie point of this lithium tantalate substrate varies depending on the inclusion of a different element, but is generally 650 ° C.

The melt is preferably stirred for 6 to 48 hours before liquid phase epitaxial growth. If the stirring time is short, there are crystal nuclei that cannot be completely dissolved in the melt, and crystals grow around these crystal nuclei.Therefore, unevenness occurs on the surface of the lithium niobate single crystal thin film, and the crystallinity is Because it will decrease.

In the above liquid phase epitaxial treatment, it is desirable to use the (0001) plane of the lithium tantalate substrate as the growth plane of the lithium niobate single crystal thin film. (000 of the lithium tantalate substrate
The 1) plane means a plane perpendicular to the c-axis of lithium tantalate. The reason why it is desirable to use the (0001) plane of the lithium tantalate substrate as the growth surface of the lithium niobate single crystal thin film is because the lithium tantalate has a hexagonal crystal structure (see FIG. 1). Since the () plane is composed of only the a-axis, it is possible to relatively easily perform lattice matching by changing the lattice constant of the a-axis of the lithium niobate single crystal thin film by using the (0001) plane as a growth plane. ..

For lattice matching, it is desirable that the a-axis lattice constant of the lithium niobate single crystal thin film is 99.92 to 100.03% of that of the lithium tantalate substrate. If the lattice constant is out of the above range, the difference in the a-axis lattice between the lithium tantalate substrate and the lithium niobate single crystal thin film becomes large, and the crystal lattice of the lithium niobate single crystal thin film is distorted. It is impossible to form a lithium niobate single crystal thin film that can be used as a sufficiently thick film.

For example, when the a-axis lattice constant of the lithium tantalate substrate is 5.1538Å, the a-axis lattice constant of the lithium niobate single crystal is preferably in the range of 5.150 to 5.155Å.

In liquid phase epitaxial growth, it is desirable to rotate the lithium tantalate substrate during the growth. This is because by rotating the lithium tantalate substrate, crystals with uniform characteristics and film thickness can be formed. It is desirable that the rotation be performed in a horizontal state. The rotation speed is preferably 50 to 150 rpm.

It is desirable that at least one surface of the lithium tantalate substrate to be subjected to this treatment be optically polished or chemically etched. The surface roughness of the lithium niobate single crystal thin film forming surface of the lithium tantalate substrate is J
It is desirable that IS B0601 and Rmax = 80 to 1000A. The reason for this is that it is extremely difficult to reduce the value of Rmax to less than 80 A, and if the value of Rmax exceeds 1000 A, the crystallinity of the lithium niobate single crystal thin film decreases.

According to the above-mentioned method, the thickness of the lithium niobate single crystal thin film deposited on the lithium tantalate substrate can be controlled by appropriately selecting the contact time between the lithium tantalate substrate and the melt and the temperature of the melt. Can be controlled.

In the embodiment of the method of the present invention, the growth rate of the lithium niobate single crystal thin film is preferably 0.01 to 2.0 μm / min. This is because if the growth rate is higher than this, undulation occurs in the lithium niobate single crystal thin film, and if the growth rate is lower than this, it takes time to grow the thin film.

In the present invention, it is desirable to remove the flux from the surfaces of the lithium tantalate substrate and the lithium niobate single crystal thin film after the liquid phase epitaxial growth. That is, when the flux remains, in the cooling step of the lithium tantalate substrate and the lithium niobate thin film after liquid phase epitaxial growth, the lithium niobate thin film component dissolved in the flux is deposited, and only the film where the flux remains is deposited. This is because the thickness becomes thicker, and as a result, the film thickness in the film surface becomes non-uniform. This flux removal treatment can also be performed by rotating the lithium niobate single crystal thin film formed on the lithium tantalate substrate at 50 to 5000 rpm. The time required for rotation to remove the flux is preferably 5 to 60 minutes.

Lithium niobate (L) according to the present invention formed on the lithium tantalate substrate by the above-mentioned treatment.
N) For the single crystal thin film, A composition showing the composition is obtained.

The measurement of the lattice constant is carried out by usual powder X-ray diffraction. That is, the lattice constant is Cu-2.
15 of lithium niobate detected at θ = 45 to 90 °
It is calculated by the method of least squares using the value of 2θ of the peak of the book and its surface index. In the measurement, Si is used as an internal standard.

When the lithium niobate single crystal thin film of the present invention is used as an SHG element, the ordinary light refractive index no and the extraordinary light refractive index ne of the lithium niobate single crystal thin film have a wavelength of 0.83 μm. 2.25 ≦ n0 ≦ 2.40, 2.0 for each laser light source (fundamental wavelength)
<Ne <n0-0.01, and the extraordinary refractive index ne is the second value for the second high wavelength (0.415 μm) generated.
It is desirable that the refractive index is less than the ordinary refractive index no for harmonics.

By the way, in general, in order for lithium niobate or lithium tantalate single crystal to have various optically useful characteristics such as electro-optical effect and non-linear optical effect, the temperature is higher than the Curie point in the manufacturing process. It is necessary to heat and apply an electric field to poling (polarize) the crystal.
Further, it is known that single crystals such as lithium niobate and lithium tantalate containing different elements cannot be easily poled.

However, in the lithium niobate single crystal thin film obtained by the production method of the present invention, even when the lithium tantalate substrate, which is the substrate, is in the polarized state,
Even if it is electrically neutralized by polarization reversal, it is always in a polarized state and exhibits various characteristics such as an extremely excellent electro-optical effect and nonlinear optical effect.

Therefore, since the lithium niobate single crystal thin film and the lithium tantalate substrate of the present invention do not require a poling step, they can be easily manufactured, and the poling step is not required. It has an advantage that it is possible to use a lithium tantalate substrate containing a different element which was difficult.

As described above, in the lithium niobate single crystal thin film formed on the lithium tantalate substrate, the lattice constants of the lithium tantalate substrate and the lithium niobate thin film can be completely matched. Therefore, the thin film waveguide type S has a property suitable as an optical waveguide, a poling process is not required, and a thicker film than the conventional one can be obtained.
It is not only optimum as a constituent material of the HG element, but also can be used for an optical deflector, an optical modulator, and a multimode optical device.

[0089]

EXAMPLES Example 1: (1) Li ₂ CO ₃ : 19.4 g, V ₂ O ₅ : 53.3 g, Nb ₂ O ₅ : 15.4
g, Na ₂ CO ₃ : 11.8 g, MgO: 0.25 g Weighed (Li ₂ CO ₃ : 44.6
Mol%, V ₂ O ₅ : 46.5 mol%, Nb ₂ O ₅ : 8.9 mol%, Na ₂ C
O ₃ is added to Li ₂ CO ₃ at 29.8 mol%, MgO is added at 5 mol% to the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition {MgO / (LiNbO ₃ + MgO) * 100 = 5 mol% The mixture containing MgO 2 so as to satisfy the requirement}) was placed in a platinum crucible and heated to 1050 ° C. under an air atmosphere in an epitaxial growth and growth apparatus to dissolve the contents of the crucible and stirred for 20 hours.

(2) The above melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour, and further to 938 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane having an axial lattice constant of 5.1538) was immersed in the melt for 10 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt at a rotation speed of 1000 rpm for 30 seconds, then gradually cooled to room temperature, and Na having a thickness of about 16 μm was deposited on the substrate material. A Mg-containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 2.5 mol% and 1.6 mol%. The lattice constant (a-axis) of the thin film was 5.156, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.238 ± 0.001.

Example 2 (1) Li ₂ CO ₃ : 18.7 g, V ₂ O ₅ : 56.6 g, Nb ₂ O ₅ : 15.6
g, Na ₂ CO ₃ : 9.1 g, MgO: 0.25 g Weighing (Li ₂ CO ₃ : 40.6
Mol%, V ₂ O ₅ : 49.9 mol%, Nb ₂ O ₅ : 9.5 mol%, Na ₂ C
O ₃ is added to Li ₂ CO _{3 in} an amount of 25.4 mol%, and MgO is added in an amount of 5 mol% based on the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition {MgO / (LiNbO ₃ + MgO) * 100 = 5 mol% MgO is added so as to satisfy the above condition}), and the mixture is put into a platinum crucible,
1050 ° C in air atmosphere in epitaxial growth and growth system
The contents of the crucible were dissolved by heating until the mixture was stirred for 19 hours.

(2) The above melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour and further to 938 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane having an axial lattice constant of 5.1538) was immersed in the melt for 10 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and a Na film having a thickness of about 17 μm was formed on the substrate material. , Mg containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
2.0 mol% and 5.2 mol%. The lattice constant (a-axis) of the thin film was 5.153, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.230 ± 0.001.

Example 3 (1) Li ₂ CO ₃ : 19.3 g, V ₂ O ₅ : 53.5 g, Nb ₂ O ₅ : 22.4
g, Na ₂ CO ₃ : 4.9 g, MgO: 0.36 g Weighing (Li ₂ CO ₃ : 40.8
Mol%, V ₂ O ₅ : 46.0 mol%, Nb ₂ O ₅ : 13.2 mol%, Na ₂ C
O ₃ is added to Li ₂ CO ₃ at 14.9 mol% and MgO is added at 5 mol% based on the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition {MgO / (LiNbO ₃ + MgO) * 100 = 5 mol% MgO is added so as to satisfy the above condition}), and the mixture is put into a platinum crucible,
1050 ° C in air atmosphere in epitaxial growth and growth system
The contents of the crucible were dissolved by heating until the mixture was stirred for 21 hours.

(2) The above melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour, and further to 940 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane of the axial lattice constant 5.1538) was immersed in the melt for 10 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and Na having a thickness of about 9 μm was formed on the substrate material. A Mg-containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 2.2 mol% and 3.5 mol%. The lattice constant (a-axis) of the thin film was 5.154, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.234 ± 0.001.

Example 4 (1) Li ₂ CO ₃ : 20.3 g, V ₂ O ₅ : 50.4 g, Nb ₂ O ₅ : 21.9
g, Na ₂ CO ₃ : 8.3 g, MgO: 0.35 g Weighing (Li ₂ CO ₃ : 43.3
Mol%, V ₂ O ₅ : 43.7 mol%, Nb ₂ O ₅ : 13.0 mol%, Na ₂ C
O ₃ is added to Li ₂ CO ₃ at 22.1 mol%, MgO is added at 5 mol% to the theoretical amount of LiNbO _{3 which} can be precipitated from the melt composition {MgO / (LiNbO ₃ + MgO) * 100 = 5 mol% MgO is added so as to satisfy the above condition}), and the mixture is put into a platinum crucible,
1050 ° C in air atmosphere in epitaxial growth and growth system
The contents of the crucible were dissolved by heating until the mixture was stirred for 20 hours.

(2) The melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour, and further to 939 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane having an axial lattice constant of 5.1538) was immersed in the melt for 10 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and Na having a thickness of about 13 μm was formed on the substrate material. , Mg containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 2.1 mol% and 2.4 mol%. The lattice constant (a-axis) of the thin film was 5.154, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.236 ± 0.001.

Example 5 (1) Li ₂ CO ₃ : 19.6 g, V ₂ O ₅ : 51.9 g, Nb ₂ O ₅ : 18.9
g, Na ₂ CO ₃ : 9.7 g, MgO: 0.3 g Weighing (Li ₂ CO ₃ : 42.7
Mol%, V ₂ O ₅ : 45.8 mol%, Nb ₂ O ₅ : 11.5 mol%, Na ₂ C
O ₃ is 25.6 mol% with respect to Li ₂ CO ₃ , and MgO is 5 mol% with respect to the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition.
The added mixture (MgO / (LiNbO ₃ + MgO) * 100 = MgO added so as to satisfy 5 mol%}) was placed in a platinum crucible and placed in an epitaxial growth and growth apparatus under an air atmosphere for 10
The contents of the crucible were dissolved by heating to 50 ° C., and the mixture was stirred for 19.5 hours.

(2) The melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour, and further to 940 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane of the axial lattice constant 5.1538) was immersed in the melt for 10 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, and then gradually cooled to room temperature, and Na having a thickness of about 7 μm was formed on the substrate material. , Mg containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 1.7 mol% and 4.8 mol%. The lattice constant (a-axis) of the thin film was 5.153, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.232 ± 0.001.

Example 6 (1) Li ₂ CO ₃ : 20.5 g, V ₂ O ₅ : 52.1 g, Nb ₂ O ₅ : 19.0
g, Na ₂ CO ₃ : 8.5 g, MgO: 0.3 g Weighing (Li ₂ CO ₃ : 43.7
Mol%, V ₂ O ₅ : 45.0 mol%, Nb ₂ O ₅ : 11.3 mol%, Na ₂ C
22.4 mol% of O ₃ with respect to Li ₂ CO _{3 and} 5 mol% of MgO with respect to the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition {MgO / (LiNbO ₃ + MgO) * 100 = 5 mol% MgO is added so as to satisfy the above condition}), and the mixture is put into a platinum crucible,
1050 ° C in air atmosphere in epitaxial growth and growth system
The contents of the crucible were dissolved by heating to 1, and stirred for 18.5 hours.

(2) The above melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour and further to 940 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane of the axial lattice constant 5.1538) was immersed in the melt for 10 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and Na having a thickness of about 6 μm was formed on the substrate material. A Mg-containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 1.7 mol% and 4.9 mol%. The lattice constant (a-axis) of the thin film was 5.153, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.232 ± 0.001.

(5) FIG. 3 shows the results of measuring the half width of the X-ray rocking curve of the thin film thus obtained.

The thin films obtained in Examples 1 to 6 were all polarized without the polling treatment.

Comparative Example 1 (1) Li ₂ CO ₃ : 18.2 g, V ₂ O ₅ : 48.4 g, Nb ₂ O ₅ : 30.9
g, Na ₂ CO ₃ : 2.5 g, MgO: 0.5 g Weighing (Li ₂ CO ₃ : 39.2
Mol%, V ₂ O ₅ : 42.3 mol%, Nb ₂ O ₅ : 18.5 mol%, Na ₂ C
Addition of 8.6 mol% of O ₃ to Li ₂ CO _{3 and 5} mol% of MgO to the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition {MgO / (LiNbO ₃ + MgO) * 100 = 5 mol% MgO 2 was added thereto so as to satisfy the above condition}), and the mixture was put into a platinum crucible and heated to 1050 ° C. under an air atmosphere in an epitaxial growth and growth apparatus to dissolve the contents of the crucible and stirred for 19 hours.

(2) The above melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour and further to 938 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane of the axial lattice constant 5.1538) was immersed in the melt for 5 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and a Na film having a thickness of about 13 μm was formed on the substrate material. , Mg containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 0.6 mol% and 0.8 mol%. The lattice constant (a-axis) of the thin film was 5.150, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.240 ± 0.001.

Comparative Example 2 (1) Li ₂ CO ₃ : 21.4 g, V ₂ O ₅ : 59.6 g, Nb ₂ O ₅ : 15.1
g, Na ₂ CO ₃ : 4.0 g, MgO: 0.24 g Weighing (Li ₂ CO ₃ : 43.0
Mol%, V ₂ O ₅ : 48.6 mol%, Nb ₂ O ₅ : 8.4 mol%, Na ₂ C
O ₃ is 11.5 mol% with respect to LiNbO ₃ , and MgO is 5 mol% with respect to the theoretical amount of LiNbO _{3 that} can be precipitated from the melt composition.
The added mixture (MgO / (LiNbO ₃ + MgO) * 100 = MgO added so as to satisfy 5 mol%}) was put in a platinum crucible and placed in an epitaxial growth and growth apparatus under an air atmosphere for 10 minutes.
The contents of the crucible were dissolved by heating to 50 ° C., and the contents were stirred for 21 hours.

(2) The above melt was gradually cooled to 1000 ° C. at a cooling rate of 120 ° C. per hour, and further to 850 ° C. at a cooling rate of 60 ° C. per hour, and then lithium tantalate single crystal (a A substrate obtained by optically polishing the (0001) plane having an axial lattice constant of 5.1538) was immersed in the melt for 5 minutes while rotating at 100 rpm.

(3) The substrate material was pulled up from the above-mentioned melt, the melt was shaken off on the melt for 30 seconds at a rotation speed of 1000 rpm, then gradually cooled to room temperature, and Na having a thickness of about 17 μm was deposited on the substrate material. A Mg-containing lithium niobate single crystal thin film was obtained.

(4) The amounts of Na and Mg contained in the obtained lithium niobate single crystal thin film were
It was 2.5 mol% and 4.5 mol%. The lattice constant (a-axis) of the thin film was 5.157, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.234 ± 0.001.

The crystallinity of the lithium niobate single crystal thin films obtained in Examples 1 to 6 and Comparative Examples 1 and 2 was measured for half width by X-ray rocking curve method, and the results are shown in Table 1. It was After the thin film was grown, the surface was observed for the presence of facets by an electron microscope.

Example 7 This example is basically the same as Example 1, except that the substrate is T
A film of i, Cr, Fe, Ni, Nd, Mg, V is formed by photolithography and RF sputtering, and this is formed.
0.01 μm to 20 μm by thermal diffusion at 850 to 950 ℃
A diffusion layer having a thickness of 100 nm was formed. Furthermore, as in Example 1,
The full width at half maximum and the presence or absence of fassets were investigated. This is shown in Table 2.

Comparative Example 3 Basically the same as in Example 7, but a thin film was grown up to 850.degree. The results are shown in Table 3.

[0126]

As described above, according to the present invention, it is possible to easily form a facet-free lithium niobate single crystal thin film having good crystallinity on a lithium tantalate substrate. First, it is useful as a constituent material of an optical device.

[Brief description of drawings]

FIG. 1 is a schematic view showing a (0001) plane of a lithium tantalate substrate which is a growth surface of a lithium niobate single crystal.

FIG. 2 is a triangular diagram of a ternary system of Li ₂ O—V ₂ O ₅ —Nb ₂ O ₅ . Each composition point is (mol% of Li ₂ O, mol% of V ₂ O ₅ ,
Nb ₂ O ₅ mol%). (Li ₂ OV ₂ O ₅ Nb ₂ O ₅ ) A (44.49, 46.58, 8.93) B (40.30, 50.65, 9.05) C (39.61, 45.89, 14.50) D (43.88, 43.06, 13.06)

FIG. 3 is a graph showing the results of measuring the half width of the X-ray rocking curve of the lithium niobate single crystal thin film obtained in Example 6.

Claims (5)

[Claims] 1. Li ₃ O—V ₂ O ₅ —Nb ₂ O ₅ as a main component
In the triangular diagram (mol%) shown in Fig. 2 of the attached drawings, the components are point A (44.49, 46.58, 8.93) and point B (40.30, 5).
0.65, 9.05), point C (39.61, 45.89, 14.50), point D
(43.88, 43.06, 13.06) Included within the range,
Na ₂ O, MgO as a-axis lattice constant adjusting component of the single crystal thin film
In a molar ratio of Na ₂ O / Li ₂ O of 2.0 / 98.0 to 93.5 /
A niobate single crystal thin film obtained by liquid phase epitaxial growth of a melt containing 6.5 and MgO / Nb ₂ O ₅ satisfying the conditions of 0.2 / 99.8 to 40.0 / 60.0 on a LT substrate, This thin film (However, LN in the formula is lithium niobate.) A single crystal thin film of lithium niobate characterized by having a composition. 2. A method of epitaxially growing a single crystal thin film of lithium niobate on a lithium niobate thin film growing melt by bringing the lithium tantalate substrate into contact with the melt for growing a lithium niobate thin film.
Regarding ₂ O, V ₂ O ₅ , and Nb ₂ O ₅ , Li shown in FIG.
In _{2 O-V 2 O 5 -Nb} 2 O 5 3 -component trigonometric diagram (mol%),
Point A (44.49, 46.58, 8.93), Point B (40.30, 50.65)
, 9.05), point C (39.61, 45.89, 14.50), point D (4
3.88, 43.06, 13.06) and the composition within the range surrounded by Na ₂ O and MgO in molar ratios, respectively.
₂ O / Li ₂ O is 2.0 / 98.0 to 93.5 / 6.5, and MgO / Nb ₂ O ₅ is 0.
By using a material having a composition within the range of 2 / 99.8 to 40.0 / 60.0 and keeping the temperature of the melt at 900 to 1100 ° C. and bringing the substrates into contact with each other, a single crystal thin film of lithium niobate is formed. A method for producing a lithium niobate single crystal thin film, which comprises growing a lithium niobate single crystal thin film in which the a-axis lattice constant matches the a-axis lattice constant of a lithium tantalate substrate at room temperature. 3. The manufacturing method according to claim 1, wherein a different element is added to at least a part of the surface of the lithium tantalate substrate. 4. The manufacturing method according to claim 1, wherein the temperature for growing the lithium niobate single crystal thin film is in the range of 935 to 945 ° C. 5. The method according to claim 1, wherein the lithium niobate single crystal thin film is grown on the (0001) plane of a lithium tantalate substrate.