JP2007272062A - Wavelength conversion element and optical module - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength conversion element provided with a pseudo-phase matching structure for outputting higher light density than the conventional, and to provide an optical module. <P>SOLUTION: A first region, having a first crystal axis oriented in a first direction to a substrate 30 and a second region, having a second crystal axis orientated in a second direction different from the first direction are formed on a semiconductor core layer 37, alternately repeated cyclically with respect to a light wave guide direction and the substrate 30. The wavelength conversion element has semiconductor clad layers 36, 38 of refractive indexes lower than that of the semiconductor core layer 37 by being formed vertically of the semiconductor core layer 37. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength conversion element and an optical module, and more particularly to a wavelength conversion element having a quasi-phase matching structure made of a compound semiconductor and an optical module including the wavelength conversion element.

  Non-patent document 1 below describes quasi-phase matching (QPM: Quasi-phase Matching) in which quasi-phase matching is performed by periodically reversing the dielectric polarization direction of a ferroelectric nonlinear optical crystal by 180 degrees. As announced by Armstrong et al. In 1962.

According to the QPM, it is possible to efficiently extract the nonlinear optical effect even for a material in which phase adjustment such as angle matching is difficult. As ferroelectric crystals that enable such quasi-phase matching, lithium niobate (LN: LiNbO ₃ ) and lithium tantalate (LT: LiTaO ₃ ) are known.

  Non-Patent Document 2 discloses that the polarization reversal forcibly reversing the polarization of the ferroelectric is performed by, for example, partially lowering the Curie temperature by periodically diffusing Ti on the surface of the LN substrate and performing heat treatment. Are listed.

    Also, in the following Patent Document 1 and Patent Document 2, a ferroelectric layer in which a polarization inversion layer is periodically formed is used as a wavelength conversion element for second harmonic generation (SHG). Is described.

    Patent Document 2 describes the use of electron beam irradiation and electric field application as a method of forming a periodic domain-inverted layer.

  However, it is difficult to form a fine polarization phase inversion layer on the ferroelectric layer with high precision by electron beam irradiation, electric field application, or Ti diffusion.

  In addition, the ferroelectric crystal is easily deteriorated with respect to the input of a fundamental wave having a high intensity. This is because LN and LT have an ilmenite structure, and the domain is determined by the difference in the direction of the electric field created by the space charge due to the movement of charges in the crystal. Because it is easy to change. In order to suppress such optical damage, it is known to dope MgN, ZnO or the like into LN and LT.

  For the ferroelectric layer as described above, Non-Patent Document 3 describes a front-illuminated SHG wavelength conversion element that utilizes the difference in the nonlinear optical constant of GaAs / AlAs. This optical element has a structure in which a polarization inversion layer and a polarization non-inversion layer made of GaAs / AlAs are formed vertically on the (311) B surface of a GaAs substrate, and a fundamental wave is incident in the vertical direction. Yes.

  However, with this structure, optical connection becomes difficult when an SHG wavelength conversion element and an edge-emitting semiconductor laser are to be formed on the same substrate.

  On the other hand, Non-Patent Document 4 describes that a nonlinear optical element is formed by forming periodically poled GaN (PPGaN) in the horizontal direction on the substrate surface, and the SHG effect is confirmed. .

As a method of forming the periodically inverted GaN, first, a GaN layer is formed on the sapphire substrate via an AlN buffer, and then the AlN / GaN and sapphire substrate are etched to a predetermined depth to form periodic stripes. Thereafter, a method of growing GaN at a high temperature of 720 ° C. by MBE on a striped AlN / GaN and a concave sapphire substrate is employed. According to this, Ga-polar GaN grows on the striped AlN / GaN, and N-polar GaN grows on the sapphire substrate therebetween.
JAArmstrong, N. Bloembergen, J. Ducuing and PsPershan, Phys. Rev. 127 (1962), 1918 Electronics Letter (Electron. Lett., 1989, 25, 731) NS.Nakagawa, N. Yamada, N. Mikosiba, DEMars, Applied Physics Letters, 66 (1995), 2159 A.Chowdhury, Hock M.Ng, M. Bhardwaj, NG Weimann, Applied Physics Letters, 82 (2003), 1326 JP-A-4-276725 JP 2001-337355 A

  However, an element in which Ga-polar GaN and N-polar GaN formed on a sapphire substrate are only periodically formed in the horizontal direction of the substrate is difficult to design an appropriate mode and cannot output a high light density.

  An object of the present invention is to provide a wavelength conversion element and an optical module having a quasi-phase matching structure that can output a higher light density than before.

  According to a first aspect of the present invention for solving the above-described problems, a first region formed on a substrate and having a first crystal axis oriented in a first direction with respect to the substrate, and the first region A second region having a second crystal axis oriented in a second direction different from the first direction is formed on the substrate, and the semiconductor core layer is alternately and periodically repeated in the optical waveguide direction. A wavelength conversion element comprising: a semiconductor clad layer formed above and below the semiconductor core layer and having a lower refractive index than the semiconductor core layer.

  According to a second aspect of the present invention, in the wavelength conversion element according to the first aspect, the semiconductor core layer and the semiconductor clad layer have a region having the first crystal axis and the second crystal axis. The region is formed on a flat surface of a semiconductor layer that is alternately and periodically repeated in the optical waveguide direction.

  According to a third aspect of the present invention, in the wavelength conversion element according to the first or second aspect, a ridge stripe structure is formed at least in the upper semiconductor clad layer.

  According to a fourth aspect of the present invention, in the wavelength conversion element according to any one of the first to third aspects, the combination of the semiconductor core layer and the semiconductor clad layer includes GaN and AlGaN, InGaN and GaN, and AlGaN. It is one of AlN.

  According to a fifth aspect of the present invention, in the wavelength conversion element according to any one of the first to fourth aspects, the fundamental wave incident from one end of the semiconductor core layer is at least perpendicular to the surface of the substrate. It has a structure that propagates as a single transverse mode.

  According to a sixth aspect of the present invention, in the wavelength conversion element according to any one of the first to fifth aspects, the first region and the second region of the semiconductor core layer have different crystal strains. Features.

  According to a seventh aspect of the present invention, in the wavelength conversion element according to any one of the first to sixth aspects, the first crystal axis is oriented in the + c axis with respect to the substrate, and the second crystal The axis is oriented in the −c axis with respect to the substrate.

  According to an eighth aspect of the present invention, in the wavelength conversion element according to any one of the first to seventh aspects, at least one of an incident end face on the light incident side and an outgoing end face on the light exit side of the semiconductor core layer. Is characterized by having a single-layer or composite optical coating.

According to a ninth aspect of the present invention, in the wavelength conversion element according to any one of the first to eighth aspects, the substrate is at least one of sapphire, silicon, gallium nitride, aluminum nitride, and silicon carbide. ,
The semiconductor core layer is a group III-V compound semiconductor containing nitrogen.

  According to a tenth aspect of the present invention, there is provided the wavelength converter according to any one of the first to ninth aspects, a light emitting element that outputs fundamental light, and an optical waveguide for guiding the fundamental to the wavelength converter. And a light emitting device.

  According to an eleventh aspect of the present invention, there is provided a plurality of wavelength conversion elements according to any one of the first to ninth aspects, and an optical waveguide means for separately guiding light to each of the plurality of wavelength conversion elements. A light emitting device comprising: a plurality of light emitting elements that irradiate each of the plurality of wavelength conversion elements with light having different wavelengths via the optical waveguide means.

  According to the present invention, the high refractive index core layer made of a semiconductor having a periodically poled structure and the lower refractive index cladding layer are formed on the substrate, so that the SHG conversion efficiency can be increased at high density. it can. In addition, according to such a multilayer structure, it is possible to increase the magnitude of polarization by causing lattice distortion in the core layer, and to further increase the SHG conversion efficiency.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
1-3 is sectional drawing which shows the manufacturing process of the wavelength conversion element which concerns on 1st Embodiment of this invention.

  First, as shown in FIG. 1A, a single crystal GaN substrate 1 having a diameter of 2 inches is prepared. The GaN substrate 1 is grown on a base substrate (not shown) to a thickness of 300 μm by, for example, a hydride VPE method, and is peeled off from the base substrate after growth. As the base substrate, for example, a sapphire substrate, a silicon substrate, a SiC substrate, or a substrate in which a GaN layer or an AlN layer is formed on the main surface of the substrate is used.

  The first surface 1a of such a GaN substrate 1 is a Ga (gallium) surface and the second surface 1b is an N (nitrogen) surface. The first surface 1a is a cation surface oriented in the + c axis direction, and the second surface 1b is an anion surface oriented in the −c axis direction. The plane orientation of the first plane 1a is (1000).

Next, as shown in FIG. 1B, an SiO ₂ film 2 is formed on the second surface 1b of the GaN substrate 1 by a CVD method or the like. Then, a resist (not shown) is applied on the SiO ₂ film 2, and this is exposed and developed to form a line & space pattern having a period length Λ. The direction of the period is, for example, a direction orthogonal to the cleavage plane of the GaN substrate 1.

After that, the portion of the SiO ₂ film 2 not covered with the resist pattern is etched by a reactive ion etching method or the like, thereby forming an opening 2a as shown in FIG. The pattern is transferred to the SiO ₂ film 2. Then, the patterned SiO ₂ film 2 is used as a mask M. The material of the mask M is not limited to the SiO ₂ film, and for example, a metal film such as titanium (Ti) may be used.

  Next, as shown in FIG. 1D, the GaN substrate 1 is etched to a depth of about 10 μm through the opening 2a of the mask M by a technique such as high-speed dry etching using a chlorine-based reaction gas. As a result, the pattern of the mask M is transferred to the second surface 1b of the GaN substrate 1 to form the recesses 3 with the periodic length Λ.

  Thereafter, as shown in FIG. 1 (e), the mask M is removed with buffered hydrofluoric acid (BHF: Buffered HF).

  Further, as shown in FIG. 2A, a GaN layer 4 is grown on the entire surface at a substrate temperature of about 1000 ° C. by a metal organic chemical vapor deposition (MOCVD) method using a reactive gas such as trimethylgallium or ammonia. Before the GaN layer 4 is grown, a buffer layer made of GaN or AlN is grown at a substrate temperature of 800 ° C. or higher by molecular beam epitaxy (MBE), and then Ga-polar GaN is grown at a substrate temperature of 800 ° C. or higher. The layer may be grown.

  The thickness of the GaN layer 4 is 20 μm in order to completely fill the recess 3 of the GaN substrate 1. Note that, after the GaN layer 4 is grown by 1 μm by the MOCVD method, it may be regrown by a hydride vapor phase epitaxy method (HVPE), thereby improving the embedding efficiency.

  Since the growth temperature of the GaN layer 4 is as high as about 1000 ° C., the GaN layer 4 is grown starting from the N face among the elements constituting the GaN, but changes to a more stable Ga face under these conditions in the initial stage of growth. The GaN layer 4 which is the majority of the regrowth layer has + c-axis oriented Ga polarity.

  Although the mask M is removed before the GaN layer 4 is grown, the GaN layer 4 may be grown with this mask remaining. In this case, since the growth of the GaN layer 4 proceeds centering on the opening 2a of the mask M, the embedding efficiency is better, but abnormal growth tends to occur at the edge portion of the opening 2a of the mask M.

  In this state, irregularities are generated on the surface of the GaN layer 4 regardless of the presence or absence of the mask M. Therefore, as shown in FIG. The GaN layer 4 other than is removed. Polishing is performed by polishing cloth in an aqueous alkali solution, for example, KOH solution, using abrasive grains such as colloidal silica having a particle size of 0.1 μm or less.

  When the GaN layer 4 is grown without removing the mask M, the mask M is removed by etching with BHF before polishing.

  As a result of investigating the flatness improvement results when the amount of grinding by polishing was 2 μm, according to the atomic microscope (AFM) measurement, the surface roughness (roughness) immediately after the growth of the GaN layer 4 was 0.1 μm or more. The surface roughness of the GaN layer 4 and the GaN substrate 1 after polishing was 10 nm or less, which was an optically flat surface. Optical polishing for removing the cause of such scattering loss is hereinafter referred to as optical polishing.

Subsequently, as shown in FIG. 2C, after a SiO ₂ film 5 is formed on the polished surface of the GaN layer 4 and the GaN substrate 1 by a CVD method, a photoresist (not shown) is applied thereon. By exposing and developing this, a stripe pattern having a width of 3 μm extending in a direction perpendicular to the cleavage plane of the GaN substrate 1 is formed. Thereafter, the SiO ₂ film 5 is dry-etched using the striped photoresist as a mask to transfer the resist pattern to the SiO ₂ film 5.

Further, after the photoresist is removed, the GaN layer 4 and the GaN substrate 1 are etched to a depth of 3 μm, for example, using the pattern of the SiO ₂ film 5 as a mask, as shown in FIG. The pattern of the SiO ₂ film 5 is transferred to form a ridge stripe structure 6 that is long in the direction orthogonal to the cleavage direction. The ridge stripe structure 6 has a width and a height of 3 μm.

Here, the SiO ₂ film 5 is used as a protective film of the ridge stripe structure 6 without removing it. It is possible to embed both sides of the ridge stripe structure 6 with polyimide, which is a low refractive index material. However, in a high-power optical element, polyimide may be deteriorated, so that burying is avoided.

  Next, as shown in FIG. 2 (d), a metal film made of a combination of Ti and Ni, or a combination of Cr and AuSn, and other metals is formed on the first surface 1a of the GaN substrate 1, Is a fusion film 7 to the heat sink. In addition, a reflective metal film may be formed on the surface of the ridge stripe structure 6, but there is a risk of optical loss depending on the processing form of the waveguide, the transverse mode design, and the like.

  The wafer process as described above is performed on the entire surface of the GaN substrate 1 having a circular planar shape as shown in FIG. 4, for example, and the ridge stripe structure 6 is formed in parallel at a predetermined interval in a plurality of regions.

  Therefore, it is necessary to divide the wavelength conversion element into a predetermined size, and the length of the wavelength conversion element, for example, 1000 μm to 5000 μm, is applied to the GaN substrate 1 along the cleavage plane perpendicular to the extending direction of the ridge stripe structure 6. A plurality of rod-shaped bodies as shown in FIG.

  Thereafter, end face films (coating) are formed on both end faces of the rod-shaped body.

  A first end face film 8 having a high reflectivity with respect to the second harmonic is formed on the first end face which is the fundamental wave incident face of the wavelength conversion element. That is, in the three layers of ridge stripe structure 6 and GaN existing around the ridge stripe structure 6, optical coating material, and air, the reflectivity with respect to the wavelength of the lowest second harmonic as the material of the first end face coating 8. Select a refractive index material that maximizes.

  For example, when the fundamental wave has a wavelength of 1.06 μm, silicon nitride (SiN) having a thickness of 266 nm is formed by catalytic CVD. The refractive index of the SiN film is adjusted to 2.0 for the second harmonic wave having a wavelength of 0.53 μm, and this film functions as a low-reflectance film for the fundamental wave having a wavelength of 1.06 μm, while having a wavelength of 0 It functions as a high reflectivity coating for the second harmonic of .53 μm.

  A second end face film 9 that reflects the fundamental wave and has a lower reflectivity than the first end face film 8 at the second end face that is the second harmonic output end of the wavelength conversion element. Form. As the second end face film 9, for example, a desired reflectivity performance could be obtained by forming a SiN film to a thickness of 560 nm. The SiN film has a refractive index of about 1.9 with respect to a fundamental wave wavelength of 1.06 μm.

  After the first and second end face films 8 and 9 are formed, the wavelength conversion elements are individually separated by cutting each side portion of the ridge stripe structure 6 present in parallel in the rod-like body with a dicer.

  Thus, the SHG wavelength conversion element 10 as shown in FIG. 6 is completed. The wavelength conversion element 10 is employed as a part of a light emitting device having a configuration as shown in FIG.

  In FIG. 7, the SHG wavelength conversion element 10 is die-bonded to a heat sink 11. Further, a Peltier element (not shown) is mounted on the heat sink 11 as a temperature control element. This is because in order to obtain stable wavelength conversion by the SHG wavelength conversion element 10, it is necessary to control the element temperature.

  Further, first and second optical fibers 12 and 13 having wedged tips are prepared, the wedge-shaped tips of the first optical fiber 12 are butted against the first end face of the SHG wavelength conversion element 10, and the second optical fiber is prepared. 13 wedge-shaped tips are butted against the second end face of the SHG wavelength conversion element 10. In this case, by using optical fibers on which DFB (fiber Bragg grating) is formed as the first and second optical fibers 12 and 13, the requirements required for the first and second end face coatings 8 and 9 are satisfied. Can be reinforced to enhance performance.

  The grating formed on the first optical fiber 12 has a pitch for selecting the wavelength of the fundamental wave, and the grating formed on the second optical fiber has a pitch for selecting the wavelength of the second harmonic. . According to this, it becomes possible to make said 1st, 2nd end surface coatings 8 and 9 different from said conditions.

  In addition, as the first and second optical fibers 12 and 13, polarization maintaining fibers that make the polarization state of incident light input to the SHG wavelength conversion element constant may be used.

  A lens may be connected to the tip of the first and second optical fibers 12 and 13 instead of a wedge, or an etalon or an external grating is connected to a normal optical fiber instead of the DFB. Also good. Further, instead of using a polarization maintaining fiber as the first and second optical fibers 12 and 13, a polarizing plate may be connected to the optical fiber.

  The fundamental wave light incident on the first end face of the SHG wavelength conversion element 10 is output from the semiconductor laser module 14 via the first optical fiber 12, for example. The semiconductor laser module 14 has a thermistor, a Peltier element, and the like so that the temperature is controlled and the oscillation wavelength is constant. Further, since the upper surface of the ridge stripe structure 6 is flat, high output can be obtained with little optical loss.

  Moreover, since the upper surface of the ridge stripe structure 6 is flattened by polishing, the conversion efficiency to SHG is good.

  When the width of the ridge stripe structure 6 of the SHG wavelength conversion element 10 is 3 μm as described above, the SHG wavelength conversion element 10 has a single transverse mode in which the incident fundamental wave is at least perpendicular to the substrate surface. Used as a propagating element. When the width is about 100 μm, the fundamental wave can be used as an element that propagates in multiple transverse modes.

  The SHG wavelength conversion element 10 having a ridge stripe structure 6 having a width of 3 μm becomes a medium output element having an emitted light intensity of about 0.2 W when the incident light intensity is about 1 W. However, a field close to a circle can be obtained. Excellent emission quality of second harmonic.

  On the other hand, the SHG nonlinear optical element 10 having the ridge stripe structure 6 having a width of 100 μm can be made to have an incident light intensity of 10 W or more by coupling with the multimode fiber, and if the incident light with a wavelength of 1060 nm is set to 30 W, The emitted light having a wavelength of 530 nm can be about 6 W.

  FIG. 8 shows a light-emitting device that includes three SHG wavelength conversion elements 10R, 10G, and 10B, and the semiconductor laser modules 14R, 14G, and 14B having different oscillation wavelengths in the SHG wavelength conversion elements 10R, 10G, and 10B are light beams. They are connected via fibers 12R, 12G, and 12B.

  For example, the oscillation wavelength of the first semiconductor laser module 14B is 900 nm, the oscillation wavelength of the second semiconductor laser module 14R is 1060 nm, and the oscillation wavelength of the third semiconductor laser module 14G is 1300 nm.

  Further, the polarization inversion period Λ in the crystal axis direction of the SHG wavelength conversion elements 10R, 10G, and 10B that receive light from the semiconductor laser modules 14B, 14R, and 14G is the wavelength and refractive index of the wurtzite GaN shown in FIG. It should be different based on the characteristics.

  That is, when outputting 450 nm blue light from the SHG wavelength conversion element 10B optically connected to the first semiconductor laser module 14B via the optical fiber 12R, the polarization inversion period of the crystal axis in the ridge stripe structure 6 The length is 2.9 μm. When red light of 650 nm is output from the SHG wavelength conversion element 10R optically connected to the second semiconductor laser module 14R via the optical fiber 12R, the polarization inversion period length of the crystal axis of the ridge stripe structure 6 is set to 8. 1 μm. When green light of 530 nm is output from the SHG wavelength conversion element 10G optically connected to the third semiconductor laser module 14G via the optical fiber 12G, the polarization inversion period length of the ridge stripe structure 6 is 4.84 μm. And

  Such SHG wavelength conversion elements 10B, 10R, 10G, optical fibers 12B, 12R, 12G and semiconductor laser modules 14B, 14R, 14G can constitute a high output RGB light source. According to this RGB light source, a total power of 10 W or more can be obtained. 1 W of light corresponds to a light beam of 680 lumen when the wavelength is 550 nm. Of the RGB light sources, blue and red have lower visibility than green at around 550 nm, so it is necessary to generate a larger light power than green, but a luminous flux of 2000 lumens is obtained with almost three colors. Therefore, even under room light illuminance, it is possible to obtain a sufficient amount of light for projecting an image on a screen of about 1 m × 2 m.

  Therefore, by arranging them in multiple areas and irradiating them in an area of several meters square, sufficient screen brightness can be obtained and the ultimate color reproducibility and resolution can be expected, so it can be applied to large screen and optical resolution displays. Therefore, an unprecedented presence can be obtained, which is suitable for a virtual experience system or the like.

  In such a display device, light enters the SHG wavelength conversion element 10 from the semiconductor laser modules 14R, 14G, and 14B with a high light density.

  Table 1 shows the polarization inversion periods of GaN and LN which is a ferroelectric when the fundamental wave is 1060 nm. Thereby, the SHG wavelength conversion elements 10B, 10R, and 10G having the waveguide of the GaN ridge stripe structure 6 can shorten the polarization inversion period as compared with the SHG wavelength conversion element using the ferroelectric LN. It can be seen that compactization is possible.

  As shown in FIG. 10A, the periodic polarization inversion of GaN having a wurtzite structure constituting the SHG wavelength conversion element 10 is caused by the Ga polarity of the GaN layer having + c axis orientation with respect to the substrate surface. As shown in FIG. 10 (b), since the N-polarity of the −c-axis oriented GaN layer with respect to the substrate surface is used, the direction of polarization does not change due to heat or an electric field and does not deteriorate due to high-density light incidence. .

  Therefore, since driving with a large light density is realized, the conversion efficiency per unit waveguide length of the element can be increased. The length of the element for obtaining the same conversion efficiency can be reduced.

  In addition, the ferroelectrics such as LN and LT having an ilmenite structure determine the domain depending on the direction of the electric field generated by the space charge, so that the domain is easily changed by high-density light irradiation.

The wavelength conversion element having the ridge stripe structure according to this embodiment can be used for a light emitting device that generates a sum frequency or a difference frequency, or for parametric oscillation.
(Second Embodiment)
FIG. 11 and FIG. 12 are cross-sectional views illustrating manufacturing steps of the wavelength conversion element according to the second embodiment of the present invention.

  First, as shown in FIG. 11A, a single crystal GaN substrate 1 having a diameter of 2 inches is prepared as in the first embodiment. The first surface 1a of the GaN substrate 1 is a Ga (gallium) surface, and the second surface 1b is an N (nitrogen) surface. The first surface 1a is a cation surface with the + c axis oriented, and the second surface 1b is an anion surface with the -c axis oriented.

Next, as shown in FIG. 11B, an SiO ₂ film 16 is formed on the first surface 1a of the GaN substrate 1 by a CVD method or the like. Then, a resist (not shown) is applied on the SiO ₂ film 16, and this is exposed and developed to form a line & space pattern with a period length Λ. The direction of the period is, for example, a direction orthogonal to the cleavage plane of the GaN substrate 1.

Thereafter, the portion of the SiO ₂ film 16 not covered with the resist pattern is etched by a reactive ion etching method or the like to form an opening 16a as shown in FIG. The pattern is transferred to the SiO ₂ film 16. Then, the patterned SiO ₂ film 16 is used as a mask M. The material of the mask M is not limited to the SiO ₂ film 12, and a metal film such as titanium (Ti) may be used, for example.

  Next, as shown in FIG. 11D, the GaN substrate 1 is etched to a depth of about 10 μm through the opening 16a of the mask M by a technique such as high-speed dry etching using a chlorine-based reaction gas. As a result, the pattern of the mask M is transferred to the first surface 1a of the GaN substrate 1 to form the recesses 17 with the periodic length Λ.

  Thereafter, as shown in FIG. 11E, the mask M is removed with a BHF solution.

  Further, as shown in FIG. 12A, the GaN layer 18 is grown on the entire surface at a substrate temperature of about 600 ° C. or less by the MBE method. The thickness of the GaN layer 15 is 20 μm in order to completely fill the concave portion 17 of the GaN substrate 1.

  Since the growth temperature of the GaN layer 18 is as low as 600 ° C. or lower, the surface of the GaN layer before starting the growth starts from the Ga polar face, but changes to a stable N polarity at a low temperature. Therefore, the GaN layer 15 has an N polarity of approximately −c axis orientation. At this time, in order to obtain an N-polar growth film, it is necessary to supply a sufficient N radical concentration. Since the substrate temperature is low, direct supply of ammonia is not sufficient, and it is necessary to adopt a plasma gun or a catalyst decomposition method.

  Note that the mask M is removed before the GaN layer 18 is grown, but the GaN layer 18 may be grown with the mask M remaining. In this case, the growth proceeds around the opening 16a of the mask M.

  By the way, regardless of the presence or absence of the mask M, the surface of the GaN layer 18 is uneven, so that the surface is polished and flattened as shown in FIG. The GaN layer 18 is removed. As in the first embodiment, the polishing method is performed by polishing cloth in an alkaline aqueous solution, for example, a KOH solution, using abrasive grains such as colloidal silica having a particle size of 0.1 μm or less.

  When the GaN layer 18 is grown without removing the mask M, the mask M is removed with a BHF solution before this polishing.

  The surface roughness of the GaN layer 18 and the GaN substrate 1 subjected to optical polishing was 10 nm or less, which was an optically flat surface as in the first embodiment.

Subsequently, after the SiO ₂ film 19 is formed on the polished surface of the GaN layer 18 and the GaN substrate 1 by the CVD method, the SiO ₂ film 19 is patterned in the same manner as in the first embodiment to cleave the GaN substrate 1. A stripe pattern having a predetermined width extending in a direction orthogonal to the width, for example, 3 μm to 100 μm is formed.

Further, when the GaN layer 18 and the GaN substrate 1 are etched to a depth of 3 μm using the pattern of the SiO ₂ film 19 as a mask, as shown in FIG. 13, the GaN layer 18 and the GaN substrate 1 are the same as in the first embodiment. The ridge stripe structure 6 of the GaN substrate 1 is obtained. The ridge stripe structure 6 has a height of 3 μm and a width of 3 μm to 100 μm.

Here, the SiO ₂ film 19 is left as a protective film for protecting the ridge stripe structure 6.

  Next, as shown in FIG. 12 (d), a metal film made of a combination of Ti and Ni as in the first embodiment is formed on the lower surface of the GaN substrate, and this is used as a fusion film 20.

  The wafer process as described above is performed on the entire surface of a circular GaN substrate 1 as shown in FIG. 4, for example, and the ridge stripe structure 6 is formed in parallel at predetermined intervals in a plurality of regions.

  Such a GaN substrate 1 is cleaved like a rod as shown in FIG. 5, and further, similarly to the first embodiment, the first end face film 8 is formed on the first end face of the stick-like body, and the second end face is formed. Then, the second end face film 9 is formed. Thereafter, when the elements are separated using a dicer, a wavelength conversion element 21 as shown in FIG. 13 is obtained.

  According to the wavelength conversion element 21 of the present embodiment, since the GaN substrate 1 has the Ga polarity and the GaN layer 18 has the N polarity, the GaN substrate 1 and the GaN layer 18 have different polar directions from the first embodiment. However, Ga polarity and N polarity are alternately and periodically formed in the length direction of the ridge stripe structure 6 as in the first embodiment.

Accordingly, in the wavelength conversion element 21, the transverse mode is controlled by the ridge stripe structure 6 as in the first embodiment, and the upper surface of the ridge stripe structure 6 is flattened by polishing, so that there is little optical loss and high output. The function and application are the same as those of the wavelength conversion element 10 of the first embodiment.
(Third embodiment)
14 and 15 are cross-sectional views illustrating the steps for manufacturing the wavelength conversion element according to the third embodiment of the present invention.

  First, as shown in FIG. 14A, the surface of the sapphire substrate 30 with the plane orientation (0001) is the main surface, and a buffer layer 31 made of AlN is formed thereon by MOCVD at a low temperature of 600 ° C. or less and a thickness of 10 nm. Grow to a degree. Instead of the sapphire substrate 30, a SiC substrate, Si substrate, AlN substrate, or other substrate may be used.

  Subsequently, the first GaN layer 32 is grown to a thickness of about 20 μm by MOCVD. During the growth of the first GaN layer 32, the substrate temperature is set to 1000 ° C., and the first atomic growth on the buffer layer 31 is assumed to be nitrogen (N). The first GaN layer 32 has a crystal structure as shown in FIG. 10A and is a Ga-polar layer that is + c-axis oriented with respect to the main surface of the sapphire substrate 30.

Next, as shown in FIG. 14B, an SiO ₂ film 33 is formed on the first GaN layer 32 by a CVD method or the like. Then, a resist (not shown) is applied on the SiO ₂ film 33, and this is exposed and developed to form a line & space pattern with a period length Λ. The direction of the period is, for example, a direction orthogonal to the cleavage plane of the sapphire substrate 21.

After that, by etching the portion of the SiO ₂ film 33 not covered with the resist pattern by a reactive ion etching method or the like, an opening 33a is formed as shown in FIG. The pattern is transferred to the SiO ₂ film 33. Then, the patterned SiO ₂ film 33 is used as a mask M. The material of the mask M is not limited to the SiO ₂ film, and for example, a metal film such as titanium Ti may be used.

  Next, as shown in FIG. 14D, the first GaN layer 32 is made shallow through the opening 33a of the mask M by a technique such as high-speed dry etching using a chlorine-based reaction gas, for example, a depth of about 10 μm. Etch. As a result, the pattern of the mask M is transferred to the first GaN layer 32, and the recesses 34 are formed with the periodic length Λ.

  After removing the mask M with BHF, as shown in FIG. 15A, the second GaN layer 35 is formed at a substrate temperature of 600 ° C. or lower by a gas source MBE method using a reactive gas such as trimethylgallium or ammonia. It grows on the entire surface of one GaN layer 32. The thickness of the second GaN layer 35 is 20 μm in order to completely fill the concave portion 34 of the first GaN layer 32.

  Since the growth temperature of the second GaN layer 35 is set to a low value of 600 ° C. or lower, the growth of GaN begins to grow from the Ga plane, and thus the second GaN layer 35 has an N polarity of −c axis orientation. .

  The second GaN layer 35 may be grown while leaving the first GaN layer 32 mask M. When the second GaN layer 35 is grown without removing the mask M, the mask M is removed by etching with BHF after the growth.

  After the growth of the second GaN layer 35, the surface is uneven, so that the surface is optically polished and flattened as shown in FIG. The second GaN layer 35 is removed. As in the first embodiment, the polishing method is performed by polishing cloth with an alkaline aqueous solution using abrasive grains having a particle diameter of 0.1 μm or less.

    Thereby, on the sapphire substrate 30, a polarization inversion layer in which the Ga polarity and the N polarity are repeated in a predetermined period in a direction parallel to the substrate surface is formed.

  Next, as shown in FIG. 15C, a first cladding made of AlGaN having a thickness of 2 to 5 μm is formed on the polished surfaces of the first and second GaN layers 32 and 35 by a gas source MBE method. A layer 36, a waveguide layer 37 made of GaN having a thickness of 0.1 μm to 1.0 μm, and a second cladding layer 38 made of AlGaN having a thickness of 2 μm to 5 μm are grown in this order.

  Trimethylaluminum, trimethylgallium, and ammonia are used as source gases for AlGaN growth, and trimethylgallium and ammonia are used as source gases for GaN growth. The substrate temperature during growth is set to a value higher than 600 ° C. and lower than 1000 ° C., that is, a medium temperature.

  When the Al composition of AlGaN is 20%, the thickness of the cladding layers 36 and 38 is preferably about 2 μm in consideration of lattice matching and the like. Moreover, in order to suppress the reduction of the polarization effect due to the loss of the waveguide layer 37, about 0.1 μm is preferable.

  For the waveguide layer 37, a material having a higher refractive index than that of the cladding layers 36 and 38 is selected. For example, as a combination of the waveguide layer 37 and the cladding layers 36 and 38, any one of InGaN and GaN, AlGaN and AlN, for example. May be adopted.

  The first clad layer 36, the waveguide layer (core layer) 37, and the second clad layer 38 grown under such conditions are respectively the first GaN layer 32 and the second GaN layer 35 that are the base layers. Since the crystallinity is inherited as it is, the polarity becomes Ga polarity on the first GaN layer 32 and becomes N polarity on the second GaN layer 35, whereby a long-period QPM whose polarity is periodically inverted becomes the waveguide layer 47. In addition, the clad layers 36 and 38 are formed.

  Depending on the growth conditions of the first GaN layer 32 and the second GaN layer 35, the strain is changed between the region on the first GaN layer 32 and the region on the second GaN layer 35 in the waveguide layer 37. It becomes possible.

Next, as shown in FIG. 16A, a SiO ₂ film 39 is formed on the second cladding layer 38 by a CVD method, and a photoresist (not shown) is applied thereon. Then, by exposing and developing the photoresist, a stripe pattern having a width of 3 μm to 100 μm extending in a direction orthogonal to the cleavage plane of the sapphire substrate 30 is formed. Thereafter, the SiO ₂ film 39 is dry-etched using the photoresist as a mask to transfer the photoresist pattern to the SiO ₂ film 39.

After removing the photoresist, the ridge stripe structure 40 as shown in FIG. 16 is formed by etching at least the second cladding layer 38 to a predetermined depth using the pattern of the SiO ₂ film 39 as a mask. The etching may be performed to a depth that reaches the first cladding layer 36.

Here, the SiO ₂ film 39 is not removed and used as a protective film for the ridge stripe structure 40.

  Next, as shown in FIG. 16B, a metal film 41 made of a combination of Ti and Ni, or a combination of Cr, AuSn, and other metals is formed on the lower surface of the sapphire substrate 30, and this is melted into a heat sink. Apply film.

  The wafer process as described above is performed on the entire surface of the substrate as illustrated in FIG. 4 of the first embodiment, and the ridge stripe structure 40 is formed in parallel at predetermined intervals in a plurality of regions.

  Thereafter, as in the first embodiment, the sapphire substrate 30 is formed into a rod shape by cleaving in the extending direction of the ridge stripe structure 40 for each length of, for example, 1000 μm to 5000 μm.

  Further, as shown in FIG. 16C, the first end surface on the fundamental wave incident side of the wavelength conversion element has a high reflectivity with respect to the second harmonic, as in the first embodiment. The end face film 42 is formed.

  On the other hand, similar to the first embodiment, the second end face serving as the emission end of the second harmonic of the wavelength conversion element reflects the fundamental wave, and the second harmonic is at least from the first end face coating 42. The second end face film 43 having a low reflectance is also formed.

  After the first and second end face films 42 and 43 are formed, the side regions of the plurality of ridge stripe structures 40 formed in parallel to the rod-shaped body are cut with a dicer to separate the wavelength conversion elements.

  Thus, the wavelength conversion element 50 as shown in FIG. 17 is completed. According to the wavelength conversion element, since the waveguide layer 37 and the cladding layers 36 and 38 are epitaxially grown on the optically polished surfaces of the GaN layers 32 and 35, the waveguide design in the stacking direction becomes possible, and the higher A high density optical waveguide can be formed, and SHG wavelength conversion efficiency can be increased.

  In addition, a lattice strain layer may be formed as a multilayer structure, thereby increasing the magnitude of polarization and further increasing the SHG conversion efficiency.

  Also, the wavelength conversion element 50 according to the present embodiment can obtain a high output with little optical loss, and the use is the same as the wavelength conversion element 10 of the first embodiment.

FIG. 1 is a sectional view (No. 1) showing a manufacturing process of a wavelength conversion element according to the first embodiment of the present invention. FIG. 2 is a sectional view (No. 2) showing the manufacturing process of the wavelength conversion element according to the first embodiment of the present invention. FIG. 3 is a perspective view showing a manufacturing process of the wavelength conversion element according to the first embodiment of the present invention. FIG. 4 is a plan view showing a substrate on which the wavelength conversion element according to the embodiment of the present invention is formed. FIG. 5 is a plan view showing a rod-like body obtained by cleaving the substrate on which the wavelength conversion element according to the embodiment of the present invention is formed. FIG. 6 is a perspective view showing the wavelength conversion element according to the first embodiment of the present invention. FIG. 7 is a cross-sectional view showing a first example of a light emitting device using the wavelength conversion element according to the first embodiment of the present invention. FIG. 8 is a cross-sectional view showing a second example of the light emitting device using the wavelength conversion element according to the first embodiment of the present invention. FIG. 9 is a diagram showing the relationship between the wavelength and refractive index of GaN used in the wavelength conversion element according to the embodiment of the present invention. FIG. 10 is a diagram showing a crystal structure according to a difference in polarization direction of GaN used in the wavelength conversion element according to the embodiment of the present invention. FIG. 11: is sectional drawing (the 1) which shows the manufacturing process of the wavelength conversion element which concerns on 2nd Embodiment of this invention. FIG. 12 is a cross-sectional view (part 2) illustrating the manufacturing process of the wavelength conversion element according to the second embodiment of the present invention. FIG. 13 is a perspective view showing a wavelength conversion element according to the second embodiment of the present invention. FIG. 14 is a cross-sectional view (No. 1) showing the manufacturing process of the wavelength conversion element according to the third embodiment of the invention. FIG. 15: is sectional drawing (the 2) which shows the manufacturing process of the wavelength conversion element which concerns on 3rd Embodiment of this invention. FIG. 16: is sectional drawing (the 3) which shows the manufacturing process of the wavelength conversion element which concerns on 3rd Embodiment of this invention. FIG. 17 is a perspective view showing a wavelength conversion element according to the third embodiment of the present invention.

Explanation of symbols

1: GaN substrate 2, 5: SiO ₂ film 3: concave portion 4: GaN layer 6: ridge stripe structure 7: fusion film 8, 9: end face coating 10, 10R, 10G, 10B: wavelength conversion element 11: heat sink 12, 12R, 12G, 12B, 13: optical fibers 14, 14R, 14G, 14B: semiconductor laser module 16, 19: SiO ₂ film 17: recess 18: GaN layer 20: fusion film 30: sapphire substrate 31: AlN layer 32; GaN layer 33: SiO ₂ film 34: recess 35, 39: GaN layer 36, 38: cladding layer 40: ridge stripe structure 41: fusion film

Claims (11)

A first region formed on the substrate and having a first crystal axis oriented in a first direction relative to the substrate, and a second direction oriented in a second direction different from the first direction; A semiconductor core layer in which second regions having crystal axes are alternately and periodically repeated in the optical waveguide direction;
A wavelength conversion element comprising: a semiconductor clad layer formed on the substrate and formed above and below the semiconductor core layer and having a refractive index lower than that of the semiconductor core layer. The semiconductor core layer and the semiconductor clad layer are flat surfaces of a semiconductor layer in which a region having the first crystal axis and a region having the second crystal axis are alternately and periodically repeated in the optical waveguide direction. The wavelength conversion element according to claim 1, wherein the wavelength conversion element is formed on the substrate. 3. The wavelength conversion element according to claim 1, wherein a ridge stripe structure is formed at least in the semiconductor clad layer on the upper side. The combination of the semiconductor core layer and the semiconductor clad layer is any one of GaN and AlGaN, InGaN and GaN, and AlGaN and AlN. Wavelength conversion element. 5. The semiconductor core layer according to claim 1, wherein the semiconductor core layer has a structure in which a fundamental wave incident from one end thereof propagates as a single transverse mode at least in a direction perpendicular to the surface of the substrate. The wavelength conversion element as described in any one. The wavelength conversion element according to any one of claims 1 to 5, wherein the first region and the second region of the semiconductor core layer have different crystal strains. 7. The first crystal axis is oriented in a + c axis with respect to the substrate, and the second crystal axis is oriented in a -c axis with respect to the substrate. The wavelength conversion element as described in one. 8. The semiconductor core layer according to claim 1, further comprising an optical coating of a single layer or a composite layer on at least one of an incident end face on a light incident side and an exit end face on a light exit side of the semiconductor core layer. The wavelength change element as described in one. The substrate is at least one of sapphire, silicon, gallium nitride, aluminum nitride, silicon carbide,
The wavelength conversion element according to any one of claims 1 to 8, wherein the semiconductor core layer is a group III-V compound semiconductor containing nitrogen. The wavelength conversion element according to any one of claims 1 to 9,
A light emitting device that outputs fundamental light;
And a light guide device for guiding the fundamental wave to the wavelength conversion element. A plurality of wavelength conversion elements according to any one of claims 1 to 9,
Furthermore, an optical waveguide means for separately guiding light to each of the plurality of wavelength conversion elements,
A light emitting apparatus comprising: a plurality of light emitting elements that irradiate light having different wavelengths to each of the plurality of wavelength conversion elements via the optical waveguide means.

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