Semiconductor laser element and light emitting device

Abstract

To provide a semiconductor laser element in which variation in characteristics among elements is reduced.SOLUTION: A distributed feedback semiconductor laser element 100 includes a current confinement structure and a diffraction grating 150. The diffraction grating 150 includes: a plurality of grooves 152; and a plurality of convex structures 170 sandwiched between the plurality of grooves 152. When the width of a top part 172 of a convex structure 170 is dTOP and the width of a bottom part 174 of the convex structure 170 of the diffraction grating is dBOTTOM, the relationship of dTOP<dBOTTOM is satisfied. The convex structure 170 has an inclined surface 176 continuing from its top part 172 to the adjacent groove 152.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a semiconductor laser device. [Background technology]

[0002] Semiconductor lasers made of gallium nitride (GaN)-based materials are widely used. Distributed feedback semiconductor laser diodes (DFB-LDs) are also known, in which a diffraction grating is formed on the side of a mesa structure (also called a ridge portion) for the purpose of controlling the oscillation mode of nitride-based semiconductor lasers (see Patent Documents 1 and 2, etc.). [Prior art documents] [Non-patent literature]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-039519 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-037495 [Patent Document 3] Japanese Patent Application Publication No. 11-40880 Summary of the Invention [Problem to be solved by the invention]

[0004] Various configurations have been proposed for DFB-LDs, including the laterally coupled DFB-LD. One type of laterally coupled DFB-LD has a layered structure of an N-type cladding layer, an active layer, and a P-type cladding layer, with a ridge (mesa structure) formed in the P-type cladding layer as a current confinement structure. A diffraction grating is then formed adjacent to the ridge in the P-type cladding layer. Furthermore, an insulating layer made of SiO2 or the like is formed on the same surface to suppress leakage current, etc.

[0005] The inventors have studied the horizontally coupled DFB-LD with the above structure and have come to recognize the following problem. In the horizontally coupled DFB-LD with the above structure, an insulating layer is formed so as to fill the grooves of the diffraction grating. When the insulating material is filled into the grooves of the diffraction grating, it is difficult for the insulating material to penetrate near the mesa portion, and voids are likely to occur. The voids in the grooves of the diffraction grating reduce the reproducibility of the refractive index distribution and cause characteristic variations between elements.

[0006] The present disclosure has been made in light of the above-mentioned circumstances, and an exemplary purpose of an embodiment thereof is to provide a semiconductor laser device with reduced variations in characteristics from device to device. [Means for solving the problem]

[0007] One aspect of the present disclosure relates to a distributed feedback semiconductor laser device. The semiconductor laser device has a laminated structure including a substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. The laminated structure is provided with a current confinement structure extending in a first direction perpendicular to the lamination direction, and a diffraction grating adjacent to the current confinement structure in a second direction perpendicular to the first direction. The width of the top of the convex structure of the diffraction grating is defined as d. TOP , the width of the bottom of the convex structure is d BOTTOM When d TOP <d BOTTOM The convex structure has an inclined surface that connects from its top to the adjacent groove.

[0008] Any combination of the above elements, or mutual substitution of elements or expressions between methods, devices, systems, etc., are also valid aspects of the present invention or the present disclosure. Furthermore, the description in this section (Means for Solving the Problems) does not explain all essential features of the present invention, and therefore, subcombinations of the described features may also constitute the present invention. [Effects of the Invention]

[0009] According to a semiconductor laser device according to an aspect of the present disclosure, it is possible to reduce variations in characteristics between devices. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a perspective view of a distributed feedback semiconductor laser device according to a first embodiment. [Figure 2] 2 is a cross-sectional view of the diffraction grating of the semiconductor laser device of FIG. 1 taken along the line BB'. FIG. [Figure 3] 1A to 1C are diagrams illustrating a method for manufacturing a diffraction grating. [Figure 4] FIG. 10 is a cross-sectional view of a diffraction grating of a semiconductor laser element according to a comparative technique. [Figure 5] FIG. 10 is a cross-sectional view of a diffraction grating of a semiconductor laser device according to Modification 1. [Figure 6] FIG. 10 is a cross-sectional view of a diffraction grating of a semiconductor laser device according to Modification 2. [Figure 7] FIG. 11 is a perspective view of a semiconductor laser device according to Modification 3. [Figure 8] 1A and 1B are diagrams illustrating a light emitting device according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] (Outline of the embodiment) A summary of some exemplary embodiments of the present disclosure is provided below. This summary is intended as a prelude to the more detailed description that follows, or to provide a basic understanding of the embodiments. This summary is intended to briefly explain some concepts of one or more embodiments and is not intended to limit the scope of the invention or disclosure. Furthermore, this summary is not an exhaustive overview of all possible embodiments, nor does it limit essential elements of the embodiments. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed herein.

[0012] (Outline of the embodiment) A distributed feedback semiconductor laser device according to one embodiment has a laminated structure including a substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer. The laminated structure is provided with a current confinement structure extending in a first direction perpendicular to the lamination direction, and a diffraction grating adjacent to the current confinement structure in a second direction perpendicular to the first direction. The width of the top of the convex structure of the diffraction grating is defined as d TOP , the width of the bottom of the convex structure is d BOTTOM When d TOP <d BOTTOM The convex structure has an inclined surface that connects from its top to the adjacent groove.

[0013] With this configuration, the inclined side surfaces of the convex structure of the diffraction grating make it difficult for voids to form in the grooves, thereby suppressing variations in the refractive index distribution caused by voids and reducing variations in the characteristics of the semiconductor laser.

[0014] In one embodiment, the width of the bottom of the groove may be greater than 10 nm.

[0015] In one embodiment, the angle between the inclined surface and the bottom surface of the groove may be 88° or less.

[0016] In one embodiment, the depth of the grooves may be 40 nm or more and 200 nm or less.

[0017] In one embodiment, the angled surface may include a curved surface.

[0018] In one embodiment, the angled surface may include a flat surface.

[0019] In one embodiment, the inclined surface may include multiple flat surfaces with different inclination angles.

[0020] In one embodiment, when the pitch of the diffraction grating is Λ, (d TOP +d BOTTOM ) / 2>0.5×Λ may be satisfied.

[0021] A light emitting device according to one embodiment includes any one of the distributed feedback semiconductor laser elements described above, a nonlinear optical element that generates a second harmonic of the emitted light from the distributed feedback semiconductor laser element, and a filter that cuts the emitted light.

[0022] (Embodiment) Preferred embodiments will be described below with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be designated by the same reference numerals, and redundant descriptions will be omitted where appropriate. Furthermore, the embodiments are merely examples and do not limit the disclosure or invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure or invention.

[0023] In addition, the dimensions (thickness, length, width, etc.) of each component shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple components do not necessarily represent their relative sizes, and even if a component A is depicted as being thicker than another component B in the drawings, it is possible that component A is thinner than component B.

[0024] (Embodiment 1) 1 is a perspective view of a distributed feedback semiconductor laser device 100 according to embodiment 1. The semiconductor laser device 100 includes an n-type substrate 110, an n-type semiconductor layer 120, an active layer 130, a p-type semiconductor layer 140, and a film made of an insulating material 160.

[0025] The substrate 110 is a nitride semiconductor, and x Al y Ga 1-x-y The substrate 110 may have a composition of GaN (0≦x≦1, 0≦y≦1, 0≦x+y≦1). To generate blue laser light at 444 nm, the material of the substrate 110 may be GaN (x=y=0). The substrate 110 is not limited to this, and any substrate that has a similar effect may be used, for example, a Si substrate or a sapphire substrate.

[0026] An n-type semiconductor layer 120, an active layer 130, and a p-type semiconductor layer 140 are formed in this order on an n-type substrate 110 by epitaxial growth, and form a stacked structure 102.

[0027] The n-type semiconductor layer 120 may include an n-type cladding layer and an n-type guide layer. For example, the material of the n-type cladding layer is n-Al 0.05 Ga 0.95 N, and the material of the n-type guide layer is n-GaN.

[0028] An active layer (light emitting layer) 130 having a quantum well structure is formed on the n-type semiconductor layer 120. When the oscillation wavelength is set to 444 nm, the material of the quantum well structure includes In as a barrier layer. 0.01 Ga 0.99 N as the well layer and In as the 0.15 Ga 0.85 You can choose N.

[0029] In order to suppress the diffusion of impurities from the n-type semiconductor layer 120 to the active layer 130, an undoped nitride guide layer (not shown) In is provided between them. 0.02 Ga 0.98 N can be inserted.

[0030] The p-type semiconductor layer 140 may include a carrier block (electron block EB) layer, a p-type guide layer, a p-type cladding layer, and a contact layer. For example, the material of the p-type guide layer is p-In 0.02 Ga 0.98 N, and the material of the p-type cladding layer is p-Al 0.04 Ga 0.96 The material of the contact layer is p-GaN.

[0031] In order to suppress the diffusion of impurities from the p-type semiconductor layer 140 to the active layer 130, an undoped nitride guide layer (not shown) can be inserted between them.

[0032] A ridge portion (also called a mesa portion) 142 is formed in the p-type semiconductor layer 140 (p-type cladding layer) as a current confinement structure 141. The height of the ridge portion 142 can be several hundred nm, and the width of the ridge portion 142 (mesa width) can be several microns. For example, the height may be 500 nm and the width may be 2 μm.

[0033] The coupling coefficient κ, which will be described later, also varies depending on the mesa width; the narrower the mesa width, the larger the coupling coefficient κ. However, when the mesa width is less than 1 μm, the lateral confinement weakens, and the laser characteristics begin to deteriorate. On the other hand, when the mesa width exceeds 3 μm, the transverse single mode changes to transverse multimode. Therefore, to obtain a high coupling coefficient κ in the transverse single mode, it is preferable that the mesa width Wm be 1 μm≦Wm≦3 μm.

[0034] The p-type cladding layer 140 having the ridge portion 142, together with the active layer 130 and the n-type cladding layer 120, forms a ridge-type waveguide 144. The ridge-type waveguide 144 extends in a first direction (the z-axis direction in the drawing).

[0035] The n-side electrode E1 is formed on the back surface of the substrate 110, and the p-side electrode E2 is formed on the top surface of the ridge portion 142.

[0036] Diffraction gratings 150_1 and 150_2 are formed on both sides of the mesa adjacent to the ridge waveguide 144 in the second direction (x-axis direction). Each of the diffraction gratings 150_1 and 150_2 has a plurality of grooves 152 formed in the p-type semiconductor layer 140 on the sides of the mesa. The plurality of grooves 152 are adjacent to each other in the first direction, and each groove extends in the second direction.

[0037] An insulating material 160 is formed on the p-type semiconductor layer 140. The insulating material 160 is formed so as to fill the grooves 152 of the diffraction grating 150.

[0038] 2 is a cross-sectional view taken along the line BB' of the diffraction grating 150 of the semiconductor laser device 100 of FIG. 1. Grooves (recessed structures) 152 are formed at equal intervals in the p-type semiconductor layer 140, and the remaining portions form protruding structures 170. The grooves 152 are filled with an insulating material 160. The refractive index n1 of the p-type semiconductor layer 140, which is the protruding structures 170, is higher than the refractive index n0 of the insulating material 160 filled in the grooves 152.

[0039] The period of the diffraction grating 150, that is, the pitch Λ of the grooves 152, is determined so as to obtain a desired oscillation wavelength.

[0040] Usually, a groove having a rectangular cross section is formed, and the cross section of the convex structure is also rectangular. In contrast, in this embodiment, the cross section of the groove 152 is an inverted trapezoid, and the cross section of the convex structure 170 is a trapezoid. In other words, the width of the top 172 of the convex structure 170 is d TOP , the width of the bottom 174 of the convex structure 170 of the diffraction grating 150 is d BOTTOM Then, the relation (1) holds. d TOP <d BOTTOM …(1)

[0041] Furthermore, the width d of the convex structure 170 TOP ,d BOTTOM It is preferable that the relationship between the pitch Λ and the thickness satisfies the relation (2). (d TOP +d BOTTOM ) / 2>0.5×Λ …(2)

[0042] d TOP / Λ is the top duty ratio r TOP , d BOTTOM / Λ is the bottom duty ratio r BOTTOM Then, equation (2) can be expressed as equation (3). (r TOP +r BOTTOM ) / 2>0.5 …(3)

[0043] The convex structure 170 has an inclined surface 176 that continues from its top 172 to the adjacent groove 152. In this embodiment, the inclined surface 176 includes a single flat surface. The cross section of the convex structure 170 can be said to have a tapered shape that narrows as it approaches the top 172.

[0044] 3 is a diagram illustrating a method for fabricating the diffraction grating 150. In a resist patterning step S1, an oxide film 161 made of an insulating material such as SiO2 and having a thickness of 100 nm is formed on the P-type semiconductor layer 140 on which the diffraction grating 150 is to be formed. Next, a photoresist film 300 having a thickness of 200 nm is applied onto the oxide film 161. Next, in an exposure step, a diffraction grating pattern 302 is formed in the photoresist film 162.

[0045] In the subsequent annealing step S2, the diffraction grating pattern 302 is deformed (sagged) and its corners are rounded by heat treatment (annealing) for 15 minutes in an N2 atmosphere at 120° C. This step S2 is not performed when rectangular grooves are formed.

[0046] In the subsequent hard mask patterning step S3, the diffraction grating pattern 302 is transferred onto the oxide film 161 by dry etching using, for example, CHF3 or the like as the main gas species, to form a hard mask 304. During this process, the photoresist film 300 is gradually etched while the oxide film 161 is being etched, so that the rounded bottom of the resist film 300 is removed faster than the center. As a result, the newly exposed oxide film 161 is etched, and the oxide film 161 is etched so that the width between patterns increases each time the bottom of the photoresist film 300 is etched. As a result, the shape of the hard mask 304 transferred onto the oxide film 161 becomes trapezoidal.

[0047] In the subsequent resist removal step S4, the photoresist film 300 remaining in the hard mask patterning step S3 is removed through a cleaning step using an organic solvent and an ashing step.

[0048] In the subsequent dry etching step S5, the diffraction grating pattern 302 is transferred to the P-type semiconductor layer 140 by dry etching using, for example, Cl2 or the like as the main gas species. At this time, the P-type semiconductor layer 140 is etched while the hard mask 304 is also etched, so that the bottom of the trapezoidal hard mask 304 is removed and lost faster than the center. As a result, the newly exposed P-type semiconductor layer 140 is also etched, and the width of the grooves 152 formed in the P-type semiconductor layer 140 increases as the dry etching progresses each time the bottom of the hard mask 304 is etched, resulting in the formation of the convex structure 170.

[0049] In the subsequent hard mask removal step S6, the hard mask 304 remaining after the dry etching step S5 is removed by wet etching using appropriately diluted buffered hydrofluoric acid or the like.

[0050] In the subsequent insulating material filling step S7, an insulating material 160 such as SiO2 is filled into the grooves 152 by an ALD (Atomic Layer Deposition) method or the like to form the diffraction grating 150. Since the grooves 150 are inverted trapezoidal, no voids are generated.

[0051] Here, the method for fabricating the diffraction grating is not limited to the above example. Also, although SiO2 is used as an example of the oxide film 161, it is not limited to this, and any material that performs the same function as SiN may be used. x A nitride film such as the above may also be used.

[0052] The above is the configuration of the semiconductor laser device 100. The advantages of the semiconductor laser device 100 become clear when compared with comparative techniques.

[0053] 4 is a cross-sectional view of a diffraction grating 150R of a semiconductor laser device according to a comparative example. In the comparative example, grooves 152R having a rectangular cross section are formed, and the cross section of the convex structure 170R is also rectangular. That is, d TOP =d BOTTOMIn the comparative technique, after forming the groove 152R, when forming a film made of the insulating material 160, the insulating material 160 does not easily penetrate into the bottom portion of the convex structure 170R, resulting in the formation of a void 180.

[0054] In particular, the average refractive index n AVG is the width of the void 180, d VOID , the refractive index of the void 180 is n VOID (For air, it can be set to 1), then it can be expressed as follows: n AVG ={d BOTTOM n1+(Λ-d BOTTOM )·n0-d VOID (n0-n VOID )} / Λ As a result, a void 180 is formed (d VOID >0) and the difference in size is n AVG This causes variations in the refractive index distribution of the diffraction grating, which reduces the reproducibility of the refractive index distribution and causes variations in the characteristics of each element.

[0055] Returning to Figure 2, in the semiconductor laser device 100 according to the embodiment, by forming the grooves 152 with an inverted trapezoidal cross section, it is possible to suppress the generation of voids when forming a film made of the insulating material 160. By suppressing the generation of voids, it is possible to improve the reproducibility of the refractive index distribution of the diffraction grating, reduce variations in the device, and obtain a stable optical output.

[0056] The width dg of the bottom surface 154 of the groove 152 is dg=Λ-d BOTTOM If this width dg is too narrow, voids may occur, so the width dg is preferably greater than 10 nm.

[0057] In order to prevent the occurrence of voids 180, the angle θ formed between the inclined surface 176 and the bottom surface 154 of the groove 152 is preferably 88° or less.

[0058] Furthermore, if the depth D of the grooves 152 is too deep, it becomes difficult for the insulating material to penetrate. Therefore, it is preferable that the depth D of the grooves 152 be 200 nm or less. Furthermore, if the depth of the grooves 152 is very shallow (for example, less than 40 nm), the risk of voids occurring decreases, so it is sufficient to form the convex structures 170 with rectangular cross sections, as in the conventional case. Therefore, the depth D of the grooves 152 is preferably in the range of 40 nm to 200 nm.

[0059] The cross-sectional shape of the groove 152 is not limited to a trapezoid.

[0060] 5 is a cross-sectional view of a diffraction grating 150A of a semiconductor laser device according to Modification 1. In Modification 1, an inclined surface 176 of a convex structure 170A includes a plurality of flat surfaces 176a, 176b. The angles (inclination angles) formed by the flat surfaces 176a, 176b with the horizontal plane are different, and the inclination angle θb of the upper flat surface 176b and the inclination angle θa of the lower flat surface 176a satisfy the relationship θb>θa. To suppress the generation of voids, it is preferable that θa<88°.

[0061] 6 is a cross-sectional view of a diffraction grating 150B of a semiconductor laser device according to Modification 2. In Modification 1, inclined surface 176c of convex structure 170B includes a curved surface. An angle θ formed between curved surface 176c and bottom 174 of convex structure 170B is preferably smaller than 88°.

[0062] 7 is a perspective view of a semiconductor laser device 100D according to Modification 3. In this modification, a diffraction grating 150 is also formed adjacent to a waveguide 146. This modification differs from the above-described structure in that the height of the upper surface of the waveguide 146 and the upper surface of the diffraction grating 150 are aligned.

[0063] Next, applications of the semiconductor laser device 100 will be described.

[0064] 8 is a diagram showing a light emitting device 200 according to an embodiment. The light emitting device 200 includes the above-described semiconductor laser device 100, a nonlinear optical element 210, and a filter 220.

[0065] The semiconductor laser element 100 oscillates in a single longitudinal mode at λ=444 nm. The nonlinear optical element 210 is a wavelength conversion element that generates the second harmonic λ / 2 (wavelength 222 nm) of the light emitted from the semiconductor laser element 100. The nonlinear optical element 210 may be made of, for example, β-BaB2O4 (BBO) or CsLiB6O 10 (CLBO) or the like is used. By irradiating this wavelength conversion element with a fundamental wave (444 nm) from an appropriate direction, a second harmonic wave (222 nm) can be generated. The filter 220 is a short-pass filter that removes the fundamental wave λ and transmits the second harmonic wave λ / 2.

[0066] The embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments as long as they do not deviate from the spirit of the present invention as defined in the claims. [Explanation of symbols]

[0067] 100 Semiconductor laser element 102 Laminated structure 110 Substrate 120 n-type semiconductor layer 130 Active layer 140 p-type semiconductor layer 141 Current confinement structure 142 Ridge 144 Ridge Waveguide 150 Diffraction Grating 152 Groove 160 Insulating Materials 170 Convex structure 172 Top 174 Bottom 176 Slope 180 Void 200 Light-emitting device 210 Nonlinear Optical Elements 220 filters

Claims

1. A distributed feedback semiconductor laser element, a laminated structure including a substrate, a first conductivity type semiconductor layer, an active layer, and a second conductivity type semiconductor layer; The laminated structure includes: a current confinement structure extending in a first direction perpendicular to the stacking direction; a diffraction grating adjacent to the current confinement structure in a second direction perpendicular to the first direction; is provided, The width of the top of the convex structure of the diffraction grating is d ＴＯＰ , the width of the bottom of the convex structure is d ＢＯＴＴＯＭ When d ＴＯＰ <d ＢＯＴＴＯＭ and the convex structure has an inclined surface continuing from its top to an adjacent groove.

2. 2. The semiconductor laser device according to claim 1, wherein the width of the bottom surface of the groove is greater than 10 nm.

3. 3. The semiconductor laser device according to claim 1, wherein the angle between the inclined surface and the bottom surface of the groove is 88 degrees or less.

4. 3. The semiconductor laser device according to claim 1, wherein the depth of the groove is 40 nm or more and 200 nm or less.

5. 3. The semiconductor laser device according to claim 1, wherein the inclined surface includes a curved surface.

6. 3. The semiconductor laser device according to claim 1, wherein the inclined surface includes a flat surface.

7. 7. The semiconductor laser device according to claim 6, wherein the inclined surface includes a plurality of flat surfaces with different inclination angles.

8. When the pitch of the diffraction grating is Λ, (d ＴＯＰ +d ＢＯＴＴＯＭ ) / 2>0.5×Λ 3. The semiconductor laser device according to claim 1, wherein the following relationship is satisfied:

9. a distributed feedback semiconductor laser element according to claim 1 or 2; a nonlinear optical element that generates a second harmonic of the light emitted from the distributed feedback semiconductor laser element; a filter that cuts the emitted light; A light emitting device comprising:

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