JP2973333B2 - Method for forming single crystal thin film on substrate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a lithium niobate single crystal thin film mainly used as a thin film waveguide type second harmonic generation element on a substrate by a liquid phase epitaxial growth method.

[0002]

2. Description of the Related Art With the progress of optical application technology in recent years,
Shortening of the wavelength of the light source is required. This is because recording density and photosensitivity can be improved by shortening the wavelength, and application to optical devices such as an optical disk and a laser printer can be considered.

For this reason, the wavelength of the incident laser light is reduced to 1 /
Research has been done on second harmonic generation (SHG) devices that can be converted to two. As such a second harmonic generation element, a bulk single crystal of a nonlinear optical crystal has conventionally been used with a high-power gas laser as a light source. However, there is a strong demand for miniaturization of devices such as optical disk devices and laser printers, and gas lasers require an external modulator for light modulation, whereas semiconductor lasers can directly modulate. Semiconductor lasers have been mainly used in place of gas lasers because of their low cost and low cost.

For this reason, a thin film waveguide type SHG element has been required from the viewpoint of obtaining high conversion efficiency with a low light source output of several mW to several tens mW. Such a thin film waveguide type SHG
As a nonlinear optical material for the element, a layer in which the refractive index is changed by diffusing Ti or the like into a conventional lithium niobate bulk single crystal as a waveguide, or a high-frequency sputtering method on a lithium tantalate substrate is used. Although a waveguide using the formed lithium niobate as a waveguide is known, none of them could obtain a high conversion efficiency.

[0005] By the way, the present inventor previously formed a waveguide made of a lithium niobate single crystal thin film on a substrate,
It has been found that a second harmonic generation device having high conversion efficiency can be obtained by adding a different element to the substrate and lattice-matching the substrate with lithium niobate.

[0006] As a method of manufacturing such a second harmonic generation element, a dissimilar element is added to a part of the substrate surface and thermally diffused, and then a lithium niobate single crystal thin film lattice-matched on this surface is provided. Liquid phase epitaxial growth is performed. Usually, in liquid phase epitaxial growth, a desired crystal plane is cut out from a single crystal bulk to form a substrate, and this crystal plane is optically polished to form a liquid phase epitaxial growth plane, which is held by a holder. Then, the substrate is immersed in a molten liquid of lithium niobate, and the substrate is gradually pulled up to form a lithium niobate single crystal thin film on the substrate surface.

However, in the conventional method, the edge of the plane parallel to the axis direction of the substrate, which is a point of contact between the substrate and the holder, is often minute on the edge of the plane parallel to the normal direction of the crystal growth surface of the substrate. Scratches were generated, cracked by thermal shock, and in severe cases, the substrate was broken.

FIG. 1 shows this state in the drawing. That is, when the substrate 1 is held by the holder 2 and immersed in the melt, fine scratches tend to occur at the contact point 3 between the substrate and the holder.

[0009]

Therefore, the present inventor has proposed:
As a result of various investigations to improve this point, it was found that the occurrence of scratches can be prevented by chamfering the edge seen on the substrate, and the present invention was completed, and the object of the present invention is to cause any damage to the substrate. It is an object of the present invention to provide a method for producing a lithium niobate single crystal thin film lattice-matched to a substrate without using the same.

[0010]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a method for liquid phase epitaxial growth of a single crystal thin film on a substrate, wherein at least the edge of the substrate and a portion of the substrate edge in contact with the substrate holder are chamfered. This is a method for forming a single-crystal thin film on a characteristic substrate. By chamfering, chipping caused by contact between the holder and the edge of the substrate and chipping at an edge portion caused by handling can be prevented.

That is, according to the present invention, when the edge of the substrate is chamfered and a fine scratch is generated on the edge, a large crack is generated from the scratch upon heating,
The substrate breaks. This prevents the generation of fine scratches that are likely to occur on the edge. This state is shown in FIG. In the drawing, an edge 3 of a surface parallel to the axial direction of the substrate, which is a contact point between the substrate 1 and the holder 2, is chamfered and mounted on the holder.

Hereinafter, the present invention will be described in detail. As the _{substrate, ZnO, Gd 3 Ga 5 0} 12, MgO, there are alumina, lithium tantalate is the best. The reason for using lithium tantalate is that the crystal system of the lithium tantalate substrate is similar to a lithium niobate single crystal and is easy to grow epitaxially.Because the lithium tantalate substrate is commercially available, a high-quality one is required. This is because it can be obtained stably.

The present invention is particularly advantageous for obtaining a lithium niobate single crystal thin film lattice-matched with a lithium tantalate substrate. As a method of manufacturing this substrate, when the substrate is made of lithium tantalate, CZ (Czochralski)
It is desirable to manufacture by the method. As a raw material, for example, tantalum pentoxide, lithium carbonate, titanium oxide, vanadium pentoxide, or the like is used. These raw materials are iridium crucibles,
Alternatively, it is advantageous to heat and melt in a platinum-rhodium crucible to pull up a lithium tantalate single crystal.

A plane (0001) perpendicular to the c-axis is cut out from the obtained lithium tantalate bulk single crystal to obtain a substrate. The lithium tantalate single crystal is hexagonal,
The (0001) plane is shown in FIG. As substrate (000
The reason for using the 1) plane is that the (0001) plane is composed of only the a-axis, and as described later, it is easy to adjust the lattice matching with the lithium niobate single crystal.

In the present invention, it is preferable to chamfer the edge of the (0001) plane. A preferred range of the chamfered R surface is JIS B0701 R0.1 to R2.0,
The preferred range of the C plane is JIS B0701 C0.1
~ C2.0. These R surface and C surface are JIS B070
The preferred range of the R plane is a value represented by a radius mm of the R plane assuming a circle, and the preferred range of the C plane is cut at an angle of 45 °, and the cut length m
This is the value indicated by m.

According to the present invention, a lithium niobate single crystal thin film is formed on the substrate. At this time, the substrate and the lithium niobate single crystal thin film must be lattice-matched. Hereinafter, the lattice matching will be described. Lattice matching means that the lattice constant of the thin film is 99.81 to 100.07% of that of the substrate, preferably 9
9.93 to 100.03%.

Means for lattice-matching the lithium niobate single crystal thin film with the lithium tantalate substrate is not particularly limited, but the lattice constant of the a-axis of lithium tantalate is usually the lattice constant of the a-axis of lithium niobate. If it is larger than (5.148 °), the lattice constant of lithium niobate formed on lithium tantalate is increased. As the means, it is preferable to mix a different element in lithium niobate, and it is advantageous to include Na and Mg in the lithium niobate single crystal thin film. The reason for this is that Na or Mg ions or atoms are substituted for lithium niobate crystal lattice,
Alternatively, the lattice constant of lithium niobate (a
Axis). By adjusting the composition of Na and Mg, the lattice matching between the lithium tantalate substrate and the lithium niobate single crystal can be easily obtained. In particular, Na can greatly increase the lattice constant of lithium niobate. Mg can also increase the lattice constant, but is not as effective as Na. However, it has an important effect of preventing optical damage. When Na and Mg are contained, the content is 0.1 to 14.3 mol%, preferably 0.3 mol%, based on each lithium niobate single crystal.
~ 4.8 mol%, the amount of Mg is 0.8-10.8 mol%, preferably 3.5
It is desirably about 8.6 mol%. The reason is that when the content of Na is less than 0.1 mol%, the lattice constant does not become large enough to be able to have a lattice constant with the lithium tantalate substrate regardless of the addition amount of Mg, and 14.3 mol% When the value exceeds, the lattice constant becomes too large, and in any case, the lattice matching between the lithium tantalate substrate and the lithium niobate single crystal cannot be obtained.

FIG. 6 shows the relationship between the contents of Na and Mg in the lithium niobate single crystal and the lattice constant of the a-axis of the lithium niobate single crystal. Another method for increasing the lattice constant of lithium niobate is to change the ratio of Li / Nb in lithium niobate. The state of the change is shown in FIG. From this figure, the molar ratio of Li / Nb is preferably 41/59 to 56/44.

As another means, when lithium niobate is lattice-matched with a single crystal substrate having an a-axis lattice constant smaller than 5.148 °, a different element is mixed with lithium niobate to reduce the lattice constant of lithium niobate. Lattice matching. When Ti is added as a different element, the lattice constant of the a-axis of lithium niobate changes as shown in FIG. An appropriate amount of Ti to be added is 0.2 to 30 mol.

As a means of lattice matching, a different element can be added to a lithium tantalate substrate to match the lattice constant of the lithium niobate on the a-axis. in this case,
As the adding means, there is a diffusion means or a method in which, in addition to the raw material, the material is pulled up by the CZ method and is then formed into a substrate.

In the present invention, lithium tantalate used as a substrate can be used as long as it has a hexagonal crystal structure and a lattice constant of a-axis of 5.128 to 5.173 °. The shape of the substrate is not limited to a flat plate. , Fiber or bulk. In the present invention, 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.

The (0001) plane of the lithium tantalate substrate refers to 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 that the lithium tantalate has a hexagonal crystal structure (third crystal structure).
This is because the (0001) plane is composed of only the a-axis, and thus can be lattice-matched to the lithium niobate single crystal thin film only by changing the lattice constant of the a-axis.

The surface roughness of the thin film forming surface of the lithium tantalate substrate greatly affects the crystallinity of the lithium niobate single crystal. The surface roughness of the lithium niobate single crystal thin film forming surface of the lithium tantalate substrate is JIS B0601, Rmax = 10 to 1000
Å is desirable. This is because the value of Rmax is 10Å
This is because it is extremely difficult to make the thickness smaller, and when the value of Rmax is larger than 1000 °, the crystallinity of the lithium niobate single crystal thin film is reduced.

In the present invention, the measurement of the lattice constant is performed by ordinary powder X-ray diffraction. The lattice constant is Cu-2
15 peaks of lithium niobate detected at θ = 45 to 90 °
It is calculated by the least squares method using the value of 2θ of the peak and its surface index. In the measurement, Si is used as an internal standard. As a specific method of lattice matching, a liquid phase epitaxial growth method is used, and a lithium tantalate substrate is brought into contact with a melt composed of Li ₂ O, Nb ₂ O ₅ , V ₂ O ₅ , Na ₂ O, MgO, etc.
The lattice constant of the a-axis of the lithium niobate single crystal thin film is matched with the lattice constant of the a-axis of the lithium tantalate substrate.

In the present invention, Li excluding Na ₂ O and MgO is used.
The composition range of ₂ O, V ₂ O ₅ and Nb ₂ O ₅ is Li ₂ OV ₂ O ₅ -Nb ₂ O ₅
In the three-component triangular diagram, A (88.90, 2.22, 8.88), B (55.
00,43.00,2.00), C (46.50,51.50,2.00), D (11.11,80.0
0, 8.89) and E (37.50, 5.00, 57.50) are advantageously in the composition range indicated by the region surrounded by the five composition points. The composition point is represented by (mol% of Li ₂ O, mol% of V ₂ O ₅ , Nb ₂ O
₅ mol%).

Further, Li ₂ O, V excluding Na ₂ O and MgO
_As the composition range of ₂ O ₅ and Nb ₂ O ₅ , Li ₂ O−V ₂ O ₅ −Nb ₂ O ₅
In the triangular diagram of the component system, F (49.49, 45.46, 5.05), G (1
1.11, 80.00, 8.89), preferably a composition ratio surrounded by three composition points of H (42.81, 22.94, 34.25), and
The composition range of Li ₂ O-V ₂ O ₅ -Nb ₂ O ₅ is I (47.64, 46.12, 6.24), J (27.01, 64.69, 8.30), K
(36.71, 37.97, 25.32), a range surrounded by four composition points of L (44.05, 32.97, 22.98) is preferable, and M (45.36, 46.45,
8.19), N (32.89,57.05,10.06), O (36.71,44.30,18.99), P
The range surrounded by the four composition points (44.95, 40.54, 14.51) is optimal (see FIG. 7).

The reason why such a composition range is advantageous is that lattice matching between the lithium niobate single crystal thin film and the lithium tantalate substrate by Na and Mg becomes easy, and the obtained lithium niobate single crystal thin film This is because a high quality lithium niobate single crystal thin film having excellent optical characteristics, particularly low light propagation loss, can be obtained. Further, as the composition ratio of Na ₂ O in the present invention, the molar ratio of Na ₂ O / Li ₂ O
Is preferably in the range satisfying 2.0 / 98.0 to 93.5 / 6.5.

The reason is that when the ratio of Na ₂ O is out of the above range, it is difficult to lattice match the lithium tantalate substrate and the lithium niobate single crystal thin film. It is advantageous to add at least K ₂ O and V ₂ O ₅ as flux to the melt by the epitaxial growth method in the present invention.

The reason will be described below. As a melting agent
By using K ₂ O and V ₂ O ₅ , the supply of Li from the melting agent can be prevented, so by changing the composition ratio of Li ₂ O and Nb ₂ O _{5 in} the raw material, niobium that precipitates The ratio of Li / Nb in the lithium oxide single crystal thin film can be changed. The Li /
When the value of Nb changes, the lattice constant of the a-axis also changes. Therefore, the lattice constant of the a-axis of the lithium niobate single crystal thin film is set to L in the raw material.
It can be controlled by controlling the composition ratio of i ₂ O and Nb ₂ O ₅ , and the lithium niobate single crystal thin film and the lithium tantalate substrate can be lattice-matched.

Further, Na ₂ O or MgO may be added to the melt comprising K ₂ O, V ₂ O ₅ , Li ₂ O, and Nb ₂ O ₅ . This is because the addition of Na ₂ O or MgO can increase the a-axis lattice constant of the lithium niobate single crystal thin film. MgO can also prevent light damage. In the melt composition, the molar ratio of Li ₂ O / Nb ₂ O is desirably 43/57 to 56/44, preferably 43/57 to 50/50. The reason for this is that, when the ratio is outside the above range, the crystal structure of the precipitated lithium niobate single crystal changes, and the optical characteristics deteriorate. Further, the MgO has a theoretical value of a single crystal of lithium niobate that can be precipitated from the MgO / raw material composition, and the value of the theoretical amount is 30 in a molar ratio.
It is desirable not to exceed the range satisfying / 100. The reason for this is that if the above ratio is exceeded, Mg niobate-based crystals will precipitate.

Further, K ₂ O, the composition of V ₂ O _{5 is, K 2 O 2, V 2} O 5 made of the molten material (KVO ₃ conversion) / feed stoichiometric amount of precipitation can be lithium niobate single crystal of a composition Is 25/75 to 7 in molar ratio
It is desirable that the range satisfy 5/25. The reason for this is
If the ratio is out of the above range, the crystal structure of the precipitated lithium niobate single crystal changes, and the optical characteristics are deteriorated. Further, the composition ratio of K ₂ O and V ₂ O ₅ is 1 to 1 in molar ratio.
Advantageously it is one.

In the present invention, the lattice constant of the a-axis of the lithium tantalate substrate is adjusted by adding a different element to the a-axis lattice constant of the lithium niobate crystal, thereby achieving lattice matching. Can do it. Next, after the melt having the above composition is supercooled, the lithium tantalate substrate is brought into contact with the melt. The cooling rate for bringing the melt into a supercooled state is 0.5 to 300 ° C.
/ Hr is desirable.

The temperature for epitaxial growth is 60
Desirably, the temperature is 0 to 1250 ° C. The reason is that the melting point of lithium niobate is 1250 ° C, crystals do not crystallize at temperatures higher than that, and 600 ° C is the melting point of the melting agent, so the raw material is a melt at lower temperatures. This is because they cannot do it. During the epitaxial growth,
It is desirable to rotate the lithium tantalate substrate.
This is because by rotating the lithium tantalate substrate, crystals having uniform characteristics and thickness can be formed. Preferably, the rotation of the substrate is performed in a horizontal state. The rotation speed is desirably 50 to 150 rpm.

Preferably, at least the growth surface of the lithium tantalate substrate is optically polished and then subjected to a chemical etching treatment. The thickness of the lithium tantalate substrate is desirably 0.5 to 2.0 mm. The reason for this is that a substrate thinner than 0.5 mm is prone to cracking, and a substrate thicker than 2.0 mm has a problem with the pyroelectric effect (discharge effect by heating), and is charged by heating and polishing.
This is because scratches are likely to occur due to the attachment of polishing dust and the like. The thickness of the lithium niobate single crystal thin film of the present invention crystallized 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. it can.

The growth rate of the lithium niobate single crystal thin film of the present invention is desirably 0.01 to 1.0 μm / min. If it is faster than this, undulations will occur in the lithium niobate single crystal thin film,
On the other hand, if it is slower than this, it takes too much time to grow the thin film. In the present invention, after the liquid phase epitaxial growth, it is desirable to remove the flux from the surfaces of the lithium tantalate substrate and the lithium niobate single crystal thin film. If the flux remains, the film thickness becomes uneven. It is desirable that the flux be removed by rotating the lithium niobate single crystal thin film formed on the lithium tantalate substrate at 100 to 10,000 rpm.

The time required for the rotation is desirably 5 to 60 minutes. The stirring time of the melt is desirably 6 to 48 hours in an appropriate stirring operation. The reason for this is that if the stirring time is short, crystal nuclei that cannot be completely dissolved are generated in the melt, and crystal growth occurs around the crystal nuclei, so that irregularities are generated on the surface of the lithium niobate single crystal thin film. is there. The surface roughness of lithium niobate single crystal thin film is
JIS BO601 Rmax = 0.001 to 0.1 µm is preferred.

As a method for manufacturing the lithium tantalate substrate, a CZ (Czochralski) method is preferable. Examples of the raw material include lithium carbonate, tantalum pentoxide, titanium oxide, and vanadium pentoxide. The lattice constant of the a-axis of the lithium tantalate substrate can be increased by adding a different element such as sodium. By the way, in order to use a lithium niobate single crystal as a constituent material of an optical device such as an SHG element, the lithium niobate or lithium tantalate single crystal is required to have optically useful properties such as an electro-optic effect and a nonlinear optical effect. It is necessary to have properties.

Generally, in order for a lithium niobate or lithium tantalate single crystal to have various optically useful properties such as an electro-optic effect and a non-linear optical effect, it is necessary to perform the following steps in the manufacturing process.
Heat to a temperature above the Curie point and apply an electric field to polish the crystal.
It is necessary to ring (polarize). It is also known that single crystals such as lithium niobate and lithium tantalate containing different elements cannot be easily polled. However, the lithium niobate single crystal thin film of the present invention has a structure in which even if lithium tantalate as a substrate is in a polarized state or is electrically neutralized by polarization inversion,
It is always in a polarized state and exhibits various characteristics such as extremely excellent electro-optic effect and nonlinear optical effect.

Therefore, the lithium niobate single crystal thin film and the lithium tantalate substrate of the present invention do not require a polling step, so that the manufacturing process is simple, and the dissimilar elements which have been conventionally difficult to use are removed. This has the advantage that a lithium tantalate substrate can be used. As a different element,
Mg, Ti, V, and Cr are preferably at least one selected from Fe, Ni, Nd, and the like. As a method of increasing the difference in refractive index between the thin film waveguide layer and the substrate, it is preferable to reduce the refractive index of the lithium tantalate substrate or increase the refractive index of the lithium niobate thin film.

For this purpose, Mg is applied to a lithium tantalate substrate.
Or, V, Ti, Cr, Fe, Ni, Nd on lithium niobate thin film
It is good to contain different elements such as, respectively. In particular, SH
When performing G oscillation, it is necessary to increase the difference in the refractive index between the lithium tantalate substrate and the lithium niobate thin film. When the different element is contained in the lithium tantalate substrate, the different element may not be uniformly present in the entire substrate. In the present invention, a different element is added to a specific portion of the lithium tantalate substrate to form a pattern in which the refractive index of the waveguide forming portion is relatively lower than that of the non-forming portion, thereby forming niobium on the substrate. Just by forming the lithium oxide single crystal thin film in a slab shape, the lithium niobate single crystal thin film formed in the pattern portion becomes a waveguide,
A processing step for forming a waveguide can be omitted.

As a method of lowering the refractive index of the substrate in the portion where the waveguide is formed as compared with the portion where the waveguide is not formed, the refractive index of the substrate in the portion where the waveguide is formed is reduced or the refractive index of the substrate in the portion where the waveguide is not formed is increased. It is desirable. When the heterogeneous elements are non-uniformly present, a portion that meets the conditions of the waveguide becomes a waveguide.

Further, the content of the different element on the surface portion of the lithium tantalate substrate is desirably in the following composition range. Ti; 0.2 to 30 mol% Cr; 0.02 to 20 mol% Fe; 0.02 to 20 mol% Ni; 0.02 to 20 mol% Nd; 0.02 to 10 mol% Mg; 0.1 to 20 mol% V; 0.05 to 30 mol% Is calculated by the formula: heterogeneous element / (LiTaO ₃ + heterogeneous element) × 100.

The lithium niobate single crystal thin film of the present invention has chromium (Cr), neodymium (Nd), rhodium (Rh), zinc (Zn) in order to change optical characteristics such as refractive index as required. ), Nickel (Ni), cobalt (Co), titanium (Ti), and vanadium (V). The content of each of these elements is: Rh; 0.05 to 20 mol% Zn; 0.02 to 30 mol% Ni; 0.1 to 20 mol% Co; 0.05 to 20 mol% Cr; 0.02 to 20 mol% Ti; 0.2 to 30 mol % Nd; 0.02 to 10 mol% V; 0.05 to 30 mol% is desirable.

Next, the present invention will be described in more detail with reference to examples.

EXAMPLE _{1 (1) Na 2 CO 3} 20 mol%, Li ₂ CO ₃ 30 mol%, V ₂ O ₅ 39 mole%, N
b ₂ O ₅ 11 mole%, the mixture was placed was added 2 mol% of MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition in a platinum crucible, under an air atmosphere in an epitaxial growth apparatus,
The contents of the crucible were dissolved by heating to 1100 ° C. Further, the melt was stirred with a propeller at a rotation speed of 100 rpm for 12 hours.

(2) The (0001) plane of the lithium tantalate single crystal having a thickness of 2 mm was optically polished and then chemically etched. Subsequently, chamfering (R 0.5) was performed using water-resistant abrasive paper (# 2000). The surface roughness of this substrate was JIS B0601 Rmax = 100 °. After the melt was gradually cooled to 915 ° C. at a cooling rate of 60 ° C. per hour, the substrate was preheated at 915 ° C. for 30 minutes, and immersed in the melt for 8 minutes while rotating at 100 rpm. The growth rate of lithium niobate was 1 μm / min.

(3) The substrate material is pulled up from the melt, and the melt is shaken off at a rotational speed of 1000 rpm for 30 seconds.
The resultant was gradually cooled to room temperature at a rate of ° C./min to obtain a sodium-magnesium-containing lithium niobate single crystal thin film having a thickness of about 8 μm on a substrate material. (4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 3 mol% and 2 mol%, respectively. 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 2.235 ± 0.001. The surface roughness of lithium niobate was JIS BO601 Rmax = 1000 °.

[0047] Example _{2 (1) Li 2 CO 3} 51 mol%, V ₂ O ₅ 39 mole%, Nb ₂ O ₅ 10 mole%, Na
The ₂ CO ₃ with respect to the theoretical amount of precipitation can be LiNbO ₃ from the melt composition, which can be precipitated added 45 mol%, the MgO from the melt composition LiNb
A mixture containing 6 mol% of the theoretical amount of O ₃ was placed in a platinum crucible and heated to 1100 ° C. in an air atmosphere in an epitaxial growth and growth apparatus to dissolve the contents of the crucible. Next, the melt was mixed with a propeller at a rotation speed of 100 rpm for 6 minutes.
Allowed to stir for hours.

(2) After optically polishing the (0001) plane of a 1.5 mm-thick lithium tantalate single crystal, chamfering (C 1.0) was performed using a water-resistant abrasive paper (# 2000) and a polishing cloth coated with colloidal silica. ). The surface roughness of the substrate is JIS
BO601, Rmax = 170 °. 60 melts per hour
After slowly cooling to 922 ° C at a cooling rate of
For 10 minutes, and immersed in the melt for 23 minutes while rotating at 100 rpm. The growth rate of lithium niobate was 1.65 μm / min.

(3) The substrate material is pulled up from the melt, and the melt is shaken off at a rotation speed of 1000 rpm for 30 seconds, and then the lithium tantalate single crystal is cured at a cooling rate of 300 ° C. per hour. After slowly cooling to a temperature and maintaining at that temperature for 1 hour, gradually cool to room temperature at a cooling rate of 60 ° C. per hour, and place sodium and magnesium-containing lithium niobate monoliths having a thickness of about 38 μm on the substrate material. A crystalline thin film was obtained.

(4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were as follows:
They were 1.5 mol% and 5.5 mol%, respectively. Also, the lattice constant (a
Axis) was 5.155 °, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.231 ± 0.001. The surface roughness of the lithium niobate single crystal thin film was JIS BO601 Rmax = 1000 °.

[0051] Example _{3 (1) Na 2 CO 3} 12.0 mol%, V ₂ O ₅ 39 mole%, Nb ₂ O ₅ 11 mole%,
Li ₂ CO ₃ 38.0 mol%, MgO can be precipitated from the melt composition Li
A mixture containing 5 mol% of the theoretical amount of NbO ₃ was placed in a platinum crucible and heated to 1100 ° C. in an air atmosphere in an epitaxial growth and growth apparatus to dissolve the contents of the crucible.
Next, the melt was rotated using a propeller at a rotation speed of 150 rpm.
The mixture was stirred for 20 hours.

(2) Using a water-resistant abrasive paper (# 2000), a thickness of 1 mm
After chamfering (R1.0) the lithium tantalate substrate of
A 250Å MgO layer was formed by sputtering and thermally diffused at 940940C to form a 250Å thick diffusion layer. The refractive index was 0.015 lower than when MgO was not diffused. The surface roughness of the substrate surface was JIS BO601 Rmax = 170 °. Mg
The diffusion amount of O was 10 mol%.

(3) The melt is cooled at a cooling rate of 60 ° C. per hour to 93
After slowly cooling to 8 ° C., the substrate was preheated at 938 ° C. for 50 minutes and immersed in the melt for 20 minutes while rotating at 100 rpm. The growth rate of lithium niobate was 0.7 μm / min. (4) Pull up the substrate material from the melt and rotate at 100 rpm for 30 minutes.
After shaking off the melt on the melt for 2 seconds, it was slowly cooled to room temperature at 1 ° C./min to obtain a sodium and magnesium-containing lithium niobate single crystal thin film having a thickness of about 14 μm on the substrate material.

(5) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were as follows:
They were 1 mol% and 6 mol%, respectively. The lattice constant (a-axis) is
5.153Å, the refractive index measured at the incident light wavelength 1.15 μm is 2.23
It was 1 ± 0.001. The surface roughness of the lithium niobate single crystal thin film was JIS BO601 Rmax = 1400 °. When the Bockels constant of this thin film was measured with a Mach-Zehnder interferometer, r = 22.0 × 10 ⁻¹² m / V and r ₁₃ = 6.6 × 10 ⁻¹² m / V were obtained. According to AppliedPhysics Letters, Vol. 24, No. 9, 1974, r ₃₃ = 12 × 10 ⁻¹² m / V, r ₁₃ = 2.3 × 10 ⁻¹² m / V, a known 2-3 times value. It is.

Example 4 (1) Li ₂ CO ₃ 50 mol%, V ₂ O ₅ 40 mol%, Nb ₂ O ₅ 10 mol%, Na ₂ C
With respect to the theoretical amount of LiNbO ₃ capable of precipitating O ₃ from the melt composition, 43 mol% is added, and LiNb capable of precipitating MgO from the melt composition.
The mixture to which 7 mol% was added to the theoretical amount of O ₃ was placed in an iridium crucible and heated to 1100 ° C. in an air atmosphere in an epitaxial growth apparatus to dissolve the contents of the crucible. Next, the melt was stirred with a propeller at a rotation speed of 200 rpm for 12 hours.

(2) The melt is cooled at a cooling rate of 60 ° C./hour to 91
The mixture was gradually cooled to 5 ° C. (0001) of lithium tantalate single crystal
The surface was optically polished to a thickness of 1.8 mm. Then, chamfering (C0.5) was performed using a water-resistant abrasive paper (# 2000) and a wrapping film (abrasive grains 1 μm). Photolithography and R
After forming an MgO film having a thickness of 1000 μm and a width of 5 μm by the F sputtering method, it was thermally diffused at 920 ° C., and a substrate having an MgO diffusion channel of a width of 1000 μm and a width of 5 μm was used as a substrate material. The thickness of the diffusion layer was 1000 mm. In this channel portion, the ordinary light refractive index was reduced by 15 × 10 ^-3 as compared with the portion where MgO was not diffused. The surface roughness was JIS BO601 Rmax = 300 °. After preheating this substrate material at 915 ° C. for 30 minutes, it was immersed in the melt for 17 minutes while rotating at 100 rpm. The growth rate was 1.94 μm / min.

(3) The substrate material is pulled up from the melt, the melt is shaken off at a rotational speed of 1000 rpm for 30 seconds, and the curie temperature of the lithium tantalate single crystal at a cooling rate of 300 ° C. per hour. (650 ° C), hold at that temperature for 1 hour, then slowly cool down to room temperature at a cooling rate of 60 ° C per hour, and place a 33 μm thick sodium and magnesium-containing niobate on the substrate material. A lithium single crystal thin film was obtained.

(4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were as follows:
They were 2 mol% and 6 mol%, respectively. The lattice constant (a-axis) is
5.155 °, the refractive index measured at the incident light wavelength 1.15 μm is 2.23
It was 1 ± 0.001. (5) The obtained lithium niobate single crystal thin film was
Polish the end face perpendicular to the gO diffusion channel to
When the laser beam was incident on the end face and the near-field pattern of the emitted beam was observed, it was confirmed that the laser beam was well confined on the MgO diffusion channel with a width of 5 μm.
The surface roughness of the surface of the obtained lithium niobate single crystal thin film was JIS BO601 Rmax = 1000 °.

Example 5 (1) 26.4 mol% of Li ₂ CO _3, 28.6 mol% of Nb ₂ O _5, 22.5 mol% of K ₂ CO _3, 22.5 mol% of V ₂ O ₅ and MgO can be precipitated from the above melt composition A mixture containing 5 mol% of the theoretical amount of LiNbO ₃ was placed in a platinum crucible and heated to 1100 ° C. in an air atmosphere in an epitaxial growth apparatus to dissolve the contents of the crucible.

(2) The melt was cooled at a cooling rate of 60 ° C./hour to 89
The solution was gradually cooled to 3 ° C. (0001) of lithium tantalate single crystal
After the surface is optically polished to a thickness of 1.5 mm, Mg of 800 μm in thickness and 5 μm in width is formed by photolithography and RF sputtering.
O film and 400 μm thick Cu on the part other than the 5 μm wide MgO film.
After forming the film, it was thermally diffused at 1000 ° C. The diffusion layer is Mg
O and CuO were both 500Å. Then, a substrate having an MgO diffusion channel having a width of 5 μm was chemically etched to obtain a substrate material. Chamfering was performed using a 1 μm diamond slurry. (R 0.5)

The channel portion in which MgO is diffused and the portion in which Cu is diffused other than the channel portion have an ordinary light refractive index of 10 × 10 5 compared to a substrate material in which nothing is diffused.
^-3 decreased and ^{1x10 -3} increased. Surface roughness is JIS BO6
01 Rmax = 400Å. 15mm on the melt
At 893 ° C and then 100
It was immersed for 11 minutes while rotating at rpm. The growth rate of the lithium niobate single crystal thin film was 0.54 μm / min.

(3) The substrate material is pulled up from the melt, and the melt is shaken off at a rotational speed of 1000 rpm for 30 seconds.
Cool slowly to room temperature at a cooling rate of ° C / min.
A MgO-containing lithium niobate single crystal thin film having a thickness of μm was obtained. The lattice constant (a-axis) was 5.153 °, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.230 ± 0.001. The surface roughness of the surface of the obtained lithium niobate single crystal thin film is JIS BO601
Rmax = 1400 °. The thickness of this thin film is referred to as the phase matching thickness.
(2.50 ± 0.05 μm) by ion beam etching to produce an SHG device. A laser having a wavelength of 830 nm was incident on this SHG element, and the second harmonic (415 nm) was confirmed.

[0063] Example _{6 (1) Li 2 CO 3} 48 mol%, V ₂ O ₅ 42 mole%, Nb ₂ O ₅ 10 mole%, Na
₂ CO ₃ can be precipitated from the melt composition, based on the theoretical amount of LiNbO ₃ can be added 43 mol%, MgO can be precipitated from the melt composition LiNb
The mixture added in an amount of 7 mol% based on the theoretical amount of O ₃ was placed in an iridium crucible and heated to 1100 ° C. in an air atmosphere in an epitaxial growth and growth apparatus to dissolve the contents of the crucible. Next, the melt was stirred with a propeller at a rotation speed of 200 rpm for 12 hours.

(2) The melt is cooled at a cooling rate of 60 ° C. per hour,
It was gradually cooled to 915 ° C. Lithium tantalate single crystal (000
1) The surface was optically polished to a thickness of 1.8 mm. Subsequently, chamfering (C 0.2) was performed using a water-resistant abrasive paper (# 2000) and a 1 μm diamond slurry. An MgO film having a thickness of 1000 mm and a width of 5 μm was formed by photolithography and RF sputtering. Then, the substrate was brought close to the melt in the epitaxial growth and growth apparatus at 2.67 cm / min, and preheating and thermal diffusion were performed simultaneously. The thickness of the diffusion layer was 1000 mm.

This substrate material was immersed in the melt for 17 minutes while rotating at 100 rpm. The growth rate was 1.94 μm / min. (3) Pull up the substrate material from the melt, and rotate at 1000 rpm for 30 minutes.
After shaking off the melt on the melt for 2 seconds, 300
Cool slowly to the Curie temperature of lithium tantalate single crystal at a cooling rate of ℃, keep at that temperature for 1 hour, then 60 ℃ for 1 hour
Slowly cool to room temperature at a cooling rate of about 37μm on the substrate material
A sodium- and magnesium-containing lithium niobate single-crystal thin film having a thickness of 3 mm was obtained. (4) The amounts of sodium and magnesium contained in the obtained lithium niobate single crystal thin film were 2 mol% and 6 mol%, respectively. The lattice constant (a-axis) was 5.155 °, and the refractive index measured at an incident light wavelength of 1.15 μm was 2.231 ± 0.001.

(5) The obtained lithium niobate single crystal thin film is vertically polished with respect to an MgO diffusion channel having a width of 5 μm, laser light is incident on the end surface, and a near-field pattern of emitted light is obtained. When the laser beam was observed, the laser beam was
It was confirmed that the gas was well confined on the diffusion channel of gO. The surface roughness of the surface of the obtained lithium niobate single crystal thin film was JIS BO601 Rmax = 900 °.

[0067] Comparative Example _{1 (1) Na 2 CO 3} 20 mol%, Li ₂ CO ₃ 30 mol%, V ₂ O ₅ 39 mole%, N
b ₂ O ₅ 11 mole%, the mixture was placed was added 2 mol% of MgO based on the theoretical amount of precipitation can be LiNbO ₃ from the melt composition in a platinum crucible, under an air atmosphere in an epitaxial growth apparatus,
The contents of the crucible were dissolved by heating to 1100 ° C. Further, the melt was stirred with a propeller at a rotation speed of 100 rpm for 12 hours.

(2) Lithium tantalate single crystal having a thickness of 2 mm
After the (0001) plane was optically polished, it was chemically etched. The surface roughness of this substrate was JIS BO601 Rmax = 100 °. After gradually cooling the melt to 915 ° C. at a cooling rate of 60 ° C. per hour, the substrate is preheated at 915 ° C. for 30 minutes, and the melt is shaken off at 100 rpm for 30 seconds in the melt. After
After slowly cooling to room temperature at a rate of ° C./min, a large crack was generated on the substrate.

The results of Examples 1 to 6 and Comparative Example 1 are described in Table 1 with respect to the state of the substrate and the propagation loss of the thin film.

[0070]

[Table 1]

Embodiment 7 This embodiment is basically the same as Embodiment 1, except that Ti, Cr, Ni, Nd, V, Fe, Rh, Co and Zn are diffused into thin films and substrates. (Thickness of 500 mm), and the propagation loss was measured. The results are shown in Table 2. All showed good propagation loss.

[0072]

[Table 2]

The numbers in the table are propagation loss (dB / cm), and the contents of different elements are average values near the surface.

[0074]

As described above, in the present invention,
In a method of liquid phase epitaxial growth of a lithium niobate single crystal thin film lattice-matched with a substrate on a surface obtained by adding a heterogeneous element to at least a part of the substrate and thermally diffusing the same, parallel to the a-axis direction of the substrate. By chamfering the edge of the proper surface (0001), it was possible to prevent the occurrence of flaws, which are likely to occur at the contact point between the substrate and the holder.

[0075]

[Brief description of the drawings]

FIG. 1 is an explanatory view of a state in which a conventional substrate is held by a holder.

FIG. 2 is an explanatory view of a state in which a chamfered substrate according to the present invention is held by a holder.

FIG. 3 is a schematic view showing a (0001) plane of a lithium tantalate substrate on which a niobium lithium single crystal thin film is grown.

FIG. 4 is a graph showing the relationship between the li / Nb molar ratio of lithium niobate and the lattice constant of the a-axis of lithium niobate single crystal.

FIG. 5 is a diagram showing the relationship between the Ti content and the lattice constant of the a-axis of a lithium niobate single crystal.

FIG. 6: Content of Na and Mg and a of lithium niobate single crystal
Diagram of relationship between axis and lattice constant

Fig. 7 Triangular composition diagram of three components LiO-VO-Nb

[Explanation of symbols]

 1 substrate 2 holder 3 chamfered surface

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-4-12095 (JP, A) JP-A-61-146799 (JP, A) JP-A-62-148394 (JP, A) JP-A-50- 92300 (JP, A) JP-A-3-69586 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C30B 1/00-35/00

Claims (2)

(57) [Claims] To 1. A substrate, a single crystal thin film in a method of liquid phase epitaxial growth, at least the edge of the substrate and the substrate holder - a single-crystal thin film on a substrate, which comprises chamfering a substrate edge of the contact portion between the How to form. 2. The method for forming a lithium niobate single crystal thin film on a substrate according to claim 1, wherein the lithium niobate single crystal thin film is formed on a substrate.