JP2014106397A - Electro-optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electro-optical element formed as a thin film on a Si substrate by epitaxially growing a cladding layer and an electro-optical layer having an ilmenite type structure such as LiNbO, with favorable crystallinity on a single crystal Si substrate.SOLUTION: An electro-optical element comprises: a single crystal Si substrate; a buffer layer of ZrOepitaxially grown on the substrate; a cladding layer of YOepitaxially grown on the buffer layer; and an electro-optical layer having an ilmenite type structure epitaxially grown on the cladding layer.

Description

  The present invention relates to an electro-optic element using an epitaxial thin film on a single crystal silicon substrate.

Ilmenite-type lithium niobate (LiNbO ₃ ) has a large electro-optic constant, and has been applied to devices such as optical modulators, optical gyros, optical switches, and electric field sensors.

Currently, LiNbO ₃ is mainly used to produce devices using a bulk single crystal, but a technique for forming a LiNbO ₃ thin film on a sapphire substrate or a silicon (Si) substrate has also been disclosed. In particular, the Si substrate is cheaper than the sapphire substrate and has a larger diameter, and if a good thin film can be formed on the Si substrate, it is very advantageous in mass production of devices.

Since the refractive index of the LiNbO ₃ is smaller than the refractive index of Si, to the LiNbO ₃ film and the optical waveguide layer, forming a small clad layer refractive index than LiNbO ₃ on Si, LiNbO ₃ thereon It is necessary to form a film. Further, if the LiNbO ₃ film is not an epitaxial film that is as close to a single crystal as possible, the expected electro-optical characteristics cannot be obtained.

In Patent Document 1, an (001) -oriented Al ₂ O ₃ epitaxial film serving as a cladding layer is formed on a (111) -oriented Si substrate, and an (001) -oriented LiNbO ₃ epitaxial film is formed thereon. Is formed. However, the orientation of the Al ₂ O ₃ film and the LiNbO ₃ film by X-ray diffraction is not disclosed, and it is difficult to actually produce an epitaxial film with good crystallinity.

Patent Document 2 discloses a (111) -oriented Si substrate in which a 3C-SiC epitaxial film and then an MgO epitaxial buffer layer are formed, and an LiNbO ₃ epitaxial film is formed thereon. Yes. However, the orientation of the 3C—SiC film, the MgO film, and the LiNbO ₃ film by X-ray diffraction is not disclosed, and it is difficult to actually form an epitaxial film with good crystallinity.

The above two documents are epitaxial laminated thin films formed on a Si substrate, but electro-optical characteristics and light propagation characteristics are not evaluated, and it is unclear whether they can actually be used as optical devices. In addition, there have been no known reports of measuring electro-optical characteristics of LiNbO ₃ epitaxially grown on a Si substrate.

JP-A-5-72428 JP 2007-182335 A

The present invention epitaxially grows a clad layer having a good crystallinity on a single-crystal Si substrate and having a refractive index smaller than that of the electro-optic layer, and an electro-optic layer having an ilmenite structure such as LiNbO _3. An electro-optic element formed of a thin film is provided.

The present invention includes a single crystal Si substrate, a ZrO ₂ buffer layer epitaxially grown on the single crystal Si substrate, a Y ₂ O ₃ cladding layer epitaxially grown on the buffer layer, and an ilmenite structure epitaxially grown on the cladding layer. An electro-optic element having an electro-optic layer.

The electro-optic layer having an ilmenite structure is preferably LiNbO ₃ .

It is preferable that the crystal orientation of the Si substrate and the laminated film has a laminated structure that satisfies the conditions defined in the following directions.
Si (111) / ZrO ₂ (111) / Y ₂ O ₃ (111) / Ilmenite type (001)

The ZrO ₂ film thickness is preferably ₂ nm to 75 nm. With this film thickness, the crystallinity of the cladding layer is improved.

The Y ₂ O ₃ film thickness is preferably 500 nm to 2000 nm. With this thickness, the electro-optic layer is excellent in crystallinity, the average surface roughness can be kept small, and light can be confined in the electro-optic layer.

The Y ₂ O ₃ film thickness is more preferably 800 nm to 1600 nm. In the film thickness, the electro-optic layer is excellent in crystallinity, and the variation in average surface roughness can be reduced.

  A Si oxide layer is preferably present on the surface of the Si substrate.

The Si oxide layer is formed by thermal oxidation of a Si substrate, and the thickness is preferably 0.5 nm or more and 10 nm or less.

  In the ilmenite structure film, it is preferable that the half width of the rocking curve of (006) reflection measured by X-ray diffraction is 0.58 ° or less.

  The average surface roughness of the surface of the ilmenite structure film is preferably 9.2 nm or less.

According to the present invention, it is possible to provide an electro-optic element formed by epitaxially growing a clad layer having good crystallinity on a single-crystal Si substrate and an electro-optic layer having an ilmenite structure such as LiNbO ₃ and forming a thin film on the Si substrate.

1 is a side sectional view showing an electro-optical element according to an embodiment. It is a schematic diagram of an electric force line at the time of arranging an electrode only in the upper part. It is a schematic diagram of electric lines of force when one electrode is connected to a conductive substrate. 1 is a side sectional view showing an electro-optical element according to an embodiment. In different _{Si (111) / ZrO 2 /} Y 2 O 3 40nm film of ZrO ₂ film _thickness, a graph of the half width and the surface average surface roughness of the X-ray diffraction rocking curve of Y 2 _{O 3} film. 1 is an X-ray diffraction 2θ-θ scan diagram of Example 1. FIG. ₂ is an X-ray diffraction rocking curve diagram of Y ₂ O ₃ in Example 1. FIG. ₂ is an X-ray diffraction rocking curve diagram of LiNbO ₃ of Example 1. FIG. ₃ is a pole figure of X-ray diffraction LiNbO ₃ (014) of Example 1. FIG. It is a schematic diagram of the propagation loss measurement by a scattering detection method. 3 is a light propagation intensity graph in the scattering detection method of Example 1. It is a schematic diagram of the electro-optic measurement by the prism coupler method. 3 is a graph showing a change in refractive index in the prism coupler method of Example 1. It is a graph of the half value width and surface average surface roughness of the X-ray diffraction rocking curve of the LiNbO ₃ film when the Y ₂ O ₃ film thickness of Example 2 is changed. It is a graph of the half value width and surface average surface roughness of the X-ray diffraction rocking curve of the LiNbO ₃ film when the Y ₂ O ₃ film thickness is changed in Comparative Example 2.

  Hereinafter, preferred embodiments of the present invention will be described. The subject of the present invention is not limited to the following embodiment. In addition, the constituent elements described below include those that can be easily assumed by those skilled in the art and substantially the same elements, and the constituent elements can be appropriately combined. Further, the explanatory diagram is schematic, and for convenience of explanation, the relationship between the thickness and the planar dimension may be different from the actual structure within a range in which the effect of the present embodiment can be obtained.

  Note that the epitaxial film in this embodiment is such that the crystal is aligned along the X-axis, Y-axis, and Z-axis directions when the film plane is the XY plane and the film thickness direction is the Z-axis. Is. In order to prove this, first, the peak intensity at the orientation position by 2θ-θ X-ray diffraction is confirmed, and second, the pole is confirmed.

Specifically, first, when measurement by 2θ-θ X-ray diffraction is performed, all peak intensities other than the target surface are 10% or less, preferably 5% or less of the maximum peak intensity of the target surface. There must be. For example, in the c-axis oriented epitaxial film of LiNbO ₃ , the peak intensity other than the (00L) plane is 10% or less, preferably 5% or less of the maximum peak intensity of the (00L) plane. (00L) is a display collectively referring to equivalent surfaces such as (001) and (002).

Second, in pole measurement, it is necessary to see the pole. The condition for confirming the peak intensity at the first orientation position described above shows only the orientation in one direction. Even if the first condition is obtained, the crystal orientation is aligned in the plane. If not, the intensity of the X-ray does not increase at a specific angular position, and no pole is seen. Since LiNbO ₃ has a trigonal crystal structure, there are three pole points of LiNbO ₃ (014) in the single crystal.

  By obtaining both the first and second conditions described above, it proves that the crystals are aligned along the X-axis, Y-axis, and Z-axis directions, and can be said to be an epitaxial film. .

In the case of LiNbO ₃ , it is known that epitaxial growth occurs in a so-called twin state in which crystals rotated by 180 ° about the c-axis are symmetrically coupled. In this case, since three poles are symmetrically coupled, the number of poles is six.

The crystal structure of LiNbO ₃ is pseudo ilmenite, ilmenite similar type, variant ilmenite, as such LiNbO ₃ type, in the notation, there is a case to be distinguished from ilmenite, in this embodiment, collectively referred to And defined as the ilmenite type.

(Embodiment 1)
The structure of the electro-optic element of Embodiment 1 is shown using FIG. The electro-optic element according to the first embodiment includes a single crystal Si substrate 1, a buffer layer 2, a cladding layer 3, an electro-optic layer 4, and an electrode 6.

  The single crystal Si substrate 1 in Embodiment 1 preferably uses a (111) plane. The ilmenite type structure is a trigonal system, and the crystal orientation of the buffer layer and the cladding layer formed on Si can be optimized.

  Single crystal Si substrates having various electrical resistances are commercially available, but the electrical resistance of the substrate is not limited from the viewpoint of epitaxial growth of the laminated film. When the present invention is used as a high-frequency device, a high-resistance device is preferable, and when used at a low frequency or DC, a low-resistance Si substrate may be used.

  The crystal orientation axis in the single crystal Si substrate 1 is perpendicular to the substrate surface or tilted within several degrees.

The buffer layer 2 formed on the surface of the Si substrate 1 is a ZrO ₂ epitaxial film. Compared with the case without the buffer layer 2, the crystallinity of the cladding layer 3 and the electro-optic layer 4 is improved. Since the surface of the Si substrate at a high temperature is very reactive, an unnecessary compound is formed before the epitaxial growth, thereby inhibiting the epitaxial growth on the Si substrate. However, ZrO ₂ can be formed on a Si substrate as a thin film having high crystallinity and excellent surface flatness. By forming this base film, epitaxial layers of higher quality cladding layers and electro-optic layers can be formed. A membrane can be obtained.

ZrO ₂ of the buffer layer 2 in Embodiment 1 has a thickness of ₂ nm to 75 nm, and more preferably 5 nm to 50 nm. If it is too thin, sufficient crystalline ZrO ₂ cannot be obtained, and if it is too thick, it causes deterioration of crystallinity and surface flatness, cracks due to increased stress, and the like.

FIG. 5 shows a rocking half-value width of Y ₂ O ₃ (222) in X-ray diffraction for a sample in which ZrO ₂ having a different thickness is formed on a Si (111) substrate and Y ₂ O ₃ is formed to a thickness of 40 nm. shows the average surface roughness of Y ₂ O ₃ surface by AFM (atomic force microscope). The graph plots points where the thickness of ZrO ₂ is 0 nm, 2 nm, 5 nm, 15 nm, 50 nm, 75 nm, and 100 nm. The average surface roughness (Ra) was 4.70 nm, 2.09 nm, 0.82 nm, 0.32 nm, 0.52 nm, 0.94 nm, and 1.68 nm, respectively.

The rocking half-widths were 3.69 degrees, 1.77 degrees, 1.67 degrees, 1.58 degrees, 1.38 degrees, 1.90 degrees, and 3.07 degrees, respectively. When the thickness of ZrO ₂ is ₂ nm to 75 nm, the locking half-value width is reduced to 1.38 degrees or more and 1.90 degrees or less, and the crystallinity of the Y ₂ O ₃ film is improved. Moreover, in the measurement by AFM (atomic force microscope) when the thickness of ZrO ₂ is 5 nm to 50 nm, the variation of the average surface roughness (Ra) of the Y ₂ O ₃ surface is in the range of 0.32 nm to 0.82 nm. It can be made smaller. In addition, the rocking half-value width is also 1.38 degrees or more and 1.67 degrees or less, and the fluctuation is small. Therefore, it is possible to stably maintain good crystallinity.

This ZrO ₂ can be formed, for example, by depositing metal Zr under a high temperature condition of a substrate temperature of 600 ° C. to 1200 ° C. in an oxidizing atmosphere. When a Si (111) substrate is used, the crystal growth surface is ZrO ₂ (111).

In order to obtain good crystallinity and surface properties, ZrO ₂ in Embodiment 1 is preferably such that the ratio of Zr in the constituent elements excluding oxygen in the zirconium oxide-based layer is 98 mol% or more, more preferably 99.5 mol. % Or more. Further, since the difficult separation of ZrO ₂ and HfO ₂ with the current high purification technique, the purity of ZrO ₂ is usually refers to purity with Zr + Hf, the zirconium oxide based layer in HfO ₂ is present invention Since it functions in exactly the same way as ZrO ₂ , there is no problem. Compared to a stabilized zirconia, which is added with a rare earth such as yttrium, the high-purity zirconia has higher internal stress but better crystallinity and surface properties.

The clad layer 3 is necessary for confining light in the electro-optic layer 4, and the refractive index must be smaller than that of the electro-optic layer 4. The cladding layer 3 in this embodiment is a Y ₂ O ₃ epitaxial film. An oxide having a refractive index smaller than that of LiNbO ₃ is limited to Al ₂ O ₃ , MgO, Y ₂ O ₃ , SiO ₂ , La ₂ O ₃ , Ga ₂ O ₃ , etc., and Y ₂ O ₃ is Good crystallinity can be obtained with a small mismatch in lattice constant with the underlying ZrO ₂ .

Y ₂ O ₃ in Embodiment 1 has a thickness of 500 nm to 2000 nm. Although depending on the wavelength of the light used and the thickness of the electro-optic layer 4, the Y ₂ O ₃ film thickness is preferably 500 nm or more in order to confine light in the electro-optic layer. In order to prevent deterioration of crystallinity and surface flatness, the film thickness is preferably 2000 nm or less. In order to reduce the variation of the average surface roughness (Ra) of the electro-optic layer 4, the film thickness is more preferably 800 nm or more and 1600 nm or less.

This Y ₂ O ₃ can be formed, for example, by vapor-depositing metal yttrium (Y) under a high temperature condition of a substrate temperature of 600 ° C. to 1200 ° C. in an oxidizing atmosphere. The crystal growth surface when using the Si (111) substrate is Y ₂ O ₃ (111).

An additive may be introduced into Y ₂ O ₃ for improving characteristics. By adding an additive, the resistivity, dielectric constant, and lattice constant of the film can be controlled. For example, Al and Si have an effect of improving the resistivity of the film. Transition metal elements such as Mn, Fe, Co, and Ni can form a level (trap level) due to impurities in the film, and conductivity can be controlled by using this level.

The electro-optical layer 4 in Embodiment 1 is a dielectric film having an ilmenite structure. The dielectric film having an ilmenite structure is preferably LiNbO ₃ . LiTaO ₃ having the same ilmenite _structure, or a solid solution of LiNbO ₃ and LiTaO _3, i.e. _{LiNb 1-x Ta x O 3} (0 <x <1) may also be used. Moreover, a small amount of an additive of another element may be replaced with Li, Nb, or Ta. When Li is A, Nb or Ta is B and expressed by the chemical formula of the general formula ABOx, the molar ratio of A / B ions is 0.9 to 1.1, and further 0.95 to 1.05. It is preferable.

  X is not limited to 3, but may be an oxygen defect or an oxygen excess state. The value of X is usually 2.8 or more and 3.2 or less. The value of X can vary depending on the type, amount and valence of the element to be added. As an example of replacement of the A and B ions, examples of the A ions include K, Na, Rb, Cs, Be, Mg, Ca, Sr, and Ba. B ions include Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, and the like. It is also possible to replace part of X with one or more of these listed elements.

  The electro-optic layer 4 in Embodiment 1 is 500 nm to 2000 nm. A certain amount of thickness is required to confine light, but if it is too thick, it may cause cracks due to increased stress. The substrate temperature during film formation is 400 ° C. to 700 ° C., and the crystal growth surface in the case of using a Si (111) substrate is the c-axis orientation of ilmenite (001).

  The electrode 6 may be disposed only on the upper portion as shown in FIG. 2 or may be connected to the Si substrate 1 as shown in FIG. In the latter case, a substrate having a low resistance can be used for the Si substrate 1 and the substrate itself can be used as the lower electrode. By applying an electric field α to the electro-optic layer 4, the refractive index changes and can be used as an electro-optic element. In addition, in the portion 11 in which light is guided in the drawing, a three-dimensional optical waveguide that confines light in the lateral direction may be formed by ion diffusion or ridge formation.

  The material of the electrode 6 may be anything such as gold or platinum. Of course, a multilayer structure may be formed by combining with a layer for improving adhesion. Moreover, there is no limitation also in thickness.

When the orientation of the film is Si (111) / ZrO ₂ (111) / Y ₂ O ₃ (111) / ilmenite type (001), one of the electrodes is directly above the portion 11 where light is guided. To place. This is because applying the electric field 10 in the c-axis direction maximizes the electro-optic effect.

In the first embodiment, the buffer layer 2 and the clad layer 3 are produced by a vacuum deposition method, and the electro-optic layer 4 is produced by a sputtering method, but the film formation method can be any method such as a laser deposition method, a chemical vapor deposition method (CVD), or a sol-gel method. May be used. In addition, the oxidizing gas may be any of oxygen, ozone, atomic oxygen, NO _x , radical oxygen, and the like.

(Embodiment 2)
In the case of Embodiment 1 shown in FIG. 1, light propagation loss occurs due to the electrodes. By inserting the buffer layer 5 between the electro-optic layer 4 and the electrode 6 as shown in FIG. 4, light can be confined in the electro-optic layer 4 and propagation loss can be reduced. The buffer layer 5 is made of a material having a refractive index lower than that of the electro-optic layer 4, such as silica or alumina. The thickness is 500 nm to 5000 nm. If the buffer layer 5 is thin, the light propagation loss increases, and if the buffer layer 5 is thick, the voltage applied to the electro-optic layer is reduced. Therefore, it is necessary to set the thickness appropriately. The buffer layer 5 may be formed by any method such as sputtering, laser deposition, chemical vapor deposition (CVD), and sol-gel method.

  In the following, an electro-optic element was actually fabricated and evaluated, which will be described in detail.

Example 1
As a substrate on which an oxide thin film is epitaxially grown, a Si single crystal substrate having a resistivity of 0.05 ohm · cm or less and being mirror-polished by cutting so that the substrate surface becomes a (111) plane was used.

The Si substrate is fixed to a holder equipped with a heating mechanism installed in a vacuum chamber, the chamber is evacuated to 1 × 10 ⁻⁴ [Pa] or less by a vacuum pump, and then the substrate is introduced while introducing oxygen gas. The temperature was heated to 600 ° C. to 1200 ° C., and a native oxide of about 5 nm was formed on the Si substrate surface.

Thereafter, while introducing the oxygen gas while maintaining the substrate temperature, the metal Zr having a purity of 99.8% is evaporated toward the substrate surface, whereby the ZrO ₂ epitaxial base film 15 nm as a buffer layer is formed on the Si substrate. Formed.

Next, while maintaining the substrate temperature and the introduction of oxygen gas in the chamber in which the ZrO ₂ film is formed, the metal Y having a purity of 99.9% is evaporated toward the substrate surface, thereby the Y ₂ O ₃ epitaxial film. 1200 nm was formed.

Here, when the refractive index of the Y ₂ O ₃ film was measured by the prism coupler method, it was 1.906 at a wavelength of 632.8 nm.

LiNbO ₃ was formed on a Si substrate on which an epitaxial film of ZrO ₂ and Y ₂ O ₃ was formed using a sputtering apparatus equipped with a substrate heating apparatus. A sputtering gas in which 20% to 50% O ₂ and Ar are mixed is introduced into a vacuum chamber evacuated to 1 × 10 ⁻⁴ [Pa] or less, the gas pressure is set to 0.1 Pa to 1.0 Pa, and the substrate temperature is set. Formed a 1000 nm film under the conditions of 400 ° C. to 700 ° C.

The sputter target used in Example 1 is a LiNbO ₃ sintered body manufactured with a Li / Nb molar ratio of 1.0. Although the molar ratio of Li / Nb in the film varies depending on the film forming conditions, the crystallinity was generally good when the molar ratio was about 0.9 to 1.1.

The structure of film formation in Example 1 is Si (111) / ZrO ₂ (15 nm) / Y ₂ O ₃ (1200 nm) / LiNbO ₃ (1000 nm) from the substrate side.

Regarding the LiNbO ₃ film, the refractive index at a laser wavelength of 632.8 nm was measured by the prism coupler method, and found to be 2.289 in the TE mode and 2.205 in the TM mode. This value is close to the literature value of single crystal LiNbO ₃ .

Further, when the surface of the LiNbO ₃ film was measured with an atomic force microscope (AFM), the average surface roughness (Ra) was 6.6 nm, which was good flatness.

  The average surface roughness was measured using AFM: DIMENSION 3100 manufactured by Digital Instruments. The probe used was UCHV-A manufactured by BRUKER and measured in a tapping mode with a scan rate of 0.5 Hz. The measurement area is 10 μm square.

FIG. 6 shows an X-ray diffraction 2θ-θ scan of the laminated film of Example 1. In the figure, the vertical axis is logarithmic so that all peaks are visible. The peaks of LiNbO ₃ (006) and LiNbO ₃ (0012) that are c-axis oriented are observed, and the peaks of LiNbO ₃ of other orientations are not seen. 7a and 7b show the rocking curves measured for the peaks of Y ₂ O ₃ (222) and LiNbO ₃ (006), respectively. Y ₂ O ₃ (222) had a full width at half maximum of 0.65 degrees, and LiNbO ₃ (006) had a full width at half maximum of 0.50 degrees. The crystallinity was good.

  X-ray diffraction was measured with ATX-E manufactured by Rigaku Corporation. The optical system used was a Ge (220) channel cut monochromator, and 0.2 mm slits were inserted before and after the Ge single crystal on the incident side, and 1 mm slits were inserted before the detector. The X-ray tube voltage / current was 40 kV / 40 mA, the scan was in steps of 0.02 degrees, and the speed was 2 degrees per minute.

FIG. 8 is a diagram illustrating the pole 12 of LiNbO ₃ (014) in Example 1. Originally, the trigonal LiNbO ₃ single crystal shows three poles in this pole measurement, but in the case of Example 1, it shows six poles, indicating that it is in a twin crystal state. This is a twin epitaxial film in which crystals rotated by 180 ° around the c-axis are combined in a symmetric state.

With respect to the laminated thin film, the propagation loss of light β was measured by the scattering detection method schematically shown in FIG. Using a semiconductor laser with a wavelength of 1550 nm and a prism 7 of a rutile (TiO ₂ ) single crystal, changes in the amount of scattered light γ in the TE mode and TM mode were measured by changing the distance of the probe 8 (the moving distance is Zmm). The results are shown in FIG. The horizontal axis is the distance Z from the reference point, and the vertical axis is the change in the amount of scattered light γ from the reference point. The propagation loss was obtained from the slope of the graph. The propagation loss was 1.1 dB / cm in the TE mode and 0.9 dB / cm in the TM mode.

Further, the refractive index was measured with an electric field applied by the prism coupler method schematically shown in FIG. The upper electrode for applying an electric field was formed by depositing Pt 10 nm on LiNbO 3 by vapor deposition, and the lower electrode was a low-resistance Si substrate. The amount of change in the refractive index in the TM mode when a voltage of −10 V to +10 V was applied was measured. For the measurement, a semiconductor laser having a wavelength of 1550 nm and a rutile (TiO ₂ ) single crystal prism 7 were used. The results are shown in FIG. The horizontal axis represents the electric field E applied to LiNbO ₃ , and the vertical axis represents the refractive index change. The refractive index is linearly changed with respect to the applied voltage (electric field), and the electro-optic coefficient r is 15 pm / V as derived from Equation 1 shown below. Here, n represents the refractive index before the electric field E is applied, and Δn represents the change in the refractive index when the electric field E is applied. The electric field used was obtained by calculation in consideration of the voltage applied to ZrO ₂ and Y ₂ O ₃ .

(Example 2)
The Y ₂ O ₃ film thickness of Example 1 was changed from 40 nm to 2400 nm, and the X-ray diffraction rocking half-width and average surface roughness (Ra) of the LiNbO ₃ film were measured. The result was as shown in FIG. The graph plots the Y ₂ O ₃ film thickness at points of 40 nm, 100 nm, 400 nm, 800 nm, 1200 nm, 1600 nm, 2000 nm, and 2400 nm. The average surface roughness (Ra) was 2.6 nm, 3.3 nm, 4.2 nm, 6.0 nm, 6.6 nm, 7.2 nm, 9.2 nm, and 15.3 nm, respectively. Moreover, the half widths of rocking are 0.72, 0.56, 0.53, 0.51, 0.50, 0.53, 0.58, and 0.92 degrees, respectively. there were.

The average surface roughness (Ra) deteriorates as the film thickness of Y ₂ O ₃ increases, but when Y ₂ O ₃ is 100 nm or more and 2000 nm or less, the average surface roughness (Ra) is 3.3 nm or more, 9 The half width (degree) of rocking is 0.50 degree or more and 0.58 degree or less at 2 nm or less, and both surface properties and crystallinity are good. When the film thickness of Y ₂ O ₃ exceeds 2000 nm, the full width at half maximum of rocking increases rapidly, so that the crystallinity of the LiNbO ₃ film according to the fluctuation of the film thickness of Y ₂ O _{3 is} easily affected.

Also, when the film thickness of Y ₂ O ₃ is less than 100 nm, the full width at half maximum of rocking increases rapidly, so that the crystallinity of the LiNbO ₃ film according to the variation in the film thickness of Y ₂ O ₃ is susceptible to fluctuations. . Furthermore, when Y ₂ O ₃ is 800 nm or more and 1600 nm or less, the average surface roughness (Ra) according to the variation in the film thickness of Y ₂ O ₃ is in the range of 6.0 nm or more and 7.2 nm or less. It is possible to reduce the variation of the roughness (Ra). Therefore, even if the film thickness of Y ₂ O ₃ varies, it is considered that the output ratio of the emitted light with respect to the incident light, that is, the fluctuation of the light propagation loss can be reduced when used as an optical modulation element.

(Comparative Example 1)
Alumina (Al ₂ O ₃ ) was tried instead of the cladding layer 3 of the example in place of Y ₂ O ₃ , but Al ₂ O ₃ became an amorphous layer. Even if the temperature, film forming speed, etc. were changed, the epitaxial growth could not be performed. The LiNbO _{3 of the} electro-optic layer 4 was not oriented.

(Comparative Example 2)
This is a configuration in which ZrO _{2 is} omitted from the buffer layer 2 of the example. The structure of film formation is Si (111) / Y ₂ O ₃ (1200 nm) / LiNbO ₃ (1000 nm) from the substrate side. In measurement by X-ray diffraction, the rocking half width of Y ₂ O ₃ (222) was 2.6 degrees, and the rocking half width of LiNbO ₃ (006) was 2.1 degrees. The average surface roughness by an atomic force microscope was 18.3 nm, and the refractive index measured by the prism coupler method was 2.276 in the TE mode and 2.216 in the TM mode. The electro-optic coefficient (r33) by the prism coupler method was 1 pm / V. Further, the refractive index difference between the TE mode and the TM mode was smaller than that in Example 1. Since the electro-optic coefficient (r33) is considered to increase as the difference in refractive index between the TE mode and TM mode increases, the difference in refractive index between the TE mode and TM mode is an index of the electro-optic coefficient (r33). It is effective as

Also in Comparative Example 2, when the Y ₂ O ₃ film thickness was changed as in Example 2, it was as shown in FIG. Compared to the examples, both the average surface roughness and the rocking half width were bad. When the average surface roughness increases, the light propagation loss increases.

  Table 1 summarizes the examples and comparative examples. This example had better crystallinity and surface properties than the comparative example, and had the only large electro-optic coefficient.

  The present invention can be applied to devices such as an optical modulator, an optical fiber gyro element, an optical switch, and an electric field sensor.

DESCRIPTION OF SYMBOLS 1 Single-crystal Si substrate 2 Buffer layer 3 Cladding layer 4 Electro-optic layer 5 Buffer layer 6 Electrode 7 Prism 8 Probe 9 Detector 11 Light guide part 12 Pole

Claims (9)

A single crystal Si substrate, a ZrO ₂ buffer layer epitaxially grown on the substrate, a Y ₂ O ₃ cladding layer epitaxially grown on the buffer layer, and an electro-optic layer having an ilmenite structure epitaxially grown on the cladding layer An electro-optic element comprising: The electro-optic element according to claim 1, wherein the electro-optic layer having the ilmenite structure is LiNbO ₃ . The electro-optical element according to claim 1, wherein the crystal orientation of the Si substrate and the laminated film satisfies a condition defined in the following direction.
Si (111) / ZrO ₂ (111) / Y ₂ O ₃ (111) / Ilmenite type (001) The electro-optic element according to claim 1, wherein the ZrO ₂ film thickness is ₂ nm to 75 nm. The electro-optic element according to claim 1, wherein the Y ₂ O ₃ film thickness is 500 nm to 2000 nm. The electro-optical element according to claim 5, wherein the Y ₂ O ₃ film thickness is 800 nm to 1600 nm.   The electro-optic element according to claim 1, wherein a Si oxide layer is present on the surface of the Si substrate.   The electro-optic element according to any one of claims 1 to 7, wherein in the ilmenite structure film, a half width of a rocking curve of (006) reflection measured by X-ray diffraction is 0.58 ° or less. . The electro-optic element according to claim 1, wherein the ilmenite structure film has an average surface roughness of the electro-optic layer of 9.2 nm or less.

Images