Optical semiconductor device

Abstract

The present invention addresses the problem of providing an optical semiconductor device in which a decrease in product yield and an increase in product cost are suppressed. An optical semiconductor device (100) is provided with: an n-type cladding (120); a p-type cladding layer (140); and an active layer (130) interposed between the n-type cladding (120) and the p-type cladding (140), the n-type cladding (120) comprising: a base layer (121); and an electric field distribution adjustment structure (122) comprising a plurality of first layers (122a) having the same refractive index as the base layer (121) and a plurality of second layers (122b) having a higher refractive index than the first layers (122a), the electric field distribution adjustment structure (122) being configured by alternately and periodically laminating the first layers (122a) and the second layers (122b). The plurality of second layers (122b) include a relaxation layer (122c) that is present at a position of the electric field distribution adjustment structure (122) that is closer to at least one end from the center in the lamination direction of the first layer (122a) and the second layer (122b), and that has a smaller thickness than the other second layer (122b).

Description

Technical Field

[0001] The present invention relates to an optical semiconductor device. Background Art

[0002] The following technology is disclosed: In optical semiconductor devices with active layers, such as semiconductor laser elements and semiconductor optical amplifiers, a layer with a higher refractive index is provided on the n-type cladding layer to bias the electric field distribution of laser light propagating through the active layer toward the n-type cladding. This suppresses inter-valence electron band absorption in the p-type cladding layer, improves the kink level, and adjusts the far-field pattern in the vertical direction (Patent Documents 1-6). Such a layer with a higher refractive index is also called an electric field distribution adjustment layer.

[0003] For such an electric field distribution adjustment layer, a stacked structure in which a plurality of layers and layers with a low refractive index are alternately and periodically stacked is preferred over a thick single-layer structure because crystal growth with fewer defects can be achieved.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2000-174394

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2001-210910

[0008] Patent Document 3: Japanese Patent No. 3525257

[0009] Patent Document 4: Japanese Patent Application Laid-Open No. 2004-356608

[0010] Patent Document 5: Japanese Patent Application Laid-Open No. 2013-120893

[0011] Patent Document 6: International Publication No. 2013 / 151145 Summary of the Invention

[0012] Problems to be solved by the invention

[0013] During the manufacturing process of an optical semiconductor device, as an intermediate inspection of the characteristics of the active layer of the optical semiconductor device, light is irradiated onto the active layer from the side opposite to the substrate to emit light, the emission spectrum is measured, and the characteristics of the active layer are inspected based on the peak wavelength of the emission spectrum.

[0014] However, the inventors' research has revealed that in optical semiconductor devices having an electric field distribution adjustment layer such as the one described above, the active layer characteristics obtained from the emission spectrum during intermediate inspection may differ from those of the finished active layer. This difference can lead to misjudgment of the active layer characteristics during intermediate inspection, potentially reducing the yield of optical semiconductor devices and increasing manufacturing costs.

[0015] The present invention has been made in view of the above, and an object of the present invention is to provide an optical semiconductor device that suppresses a decrease in manufacturing yield and an increase in manufacturing cost.

[0016] Solutions to Problems

[0017] One embodiment of the present invention is an optical semiconductor device comprising: an n-type cladding layer; a p-type cladding layer; and an active layer, which is sandwiched between the n-type cladding layer and the p-type cladding layer, the n-type cladding layer including: a base layer; and an electric field distribution adjustment structure, which includes a plurality of first layers having the same refractive index as the base layer and a plurality of second layers having a refractive index higher than that of the first layer, and is constituted by alternating and periodically stacking the first layers and the second layers, the plurality of second layers including a buffer layer that is located in the electric field distribution adjustment structure from the center of the stacking direction of the first layers and the second layers toward at least one end and has a thickness smaller than that of the other second layers.

[0018] The refractive index of the relaxation layer may be lower than the refractive index of the other second layers.

[0019] The plurality of second layers may include a first relaxation layer as the relaxation layer located closer to the first end from the center and a second relaxation layer as the relaxation layer located closer to the second end from the center.

[0020] The refractive index of at least one of the first and second relaxation layers may be lower than the refractive index of the other second layer.

[0021] One embodiment of the present invention is an optical semiconductor device comprising: an n-type cladding layer; a p-type cladding layer; and an active layer, which is sandwiched between the n-type cladding layer and the p-type cladding layer, the n-type cladding layer including: a base layer; and an electric field distribution adjustment structure, which includes a plurality of first layers having the same refractive index as the base layer and a plurality of second layers having a refractive index higher than that of the first layer and is constituted by alternating and periodically stacking the first layers and the second layers, the plurality of second layers including a first buffer layer located at a position close to the first end from the center in the stacking direction of the first and second layers of the electric field distribution adjustment structure and a second buffer layer located at a position close to the second end from the center, the refractive index of the first buffer layer and the second buffer layer being lower than the refractive index of the other second layers.

[0022] The base layer and the first layer may be made of InP, and the second layer may be made of a Group III-V compound semiconductor containing As and P as components.

[0023] Effects of the Invention

[0024] According to the present invention, there is an effect of realizing an optical semiconductor device in which a decrease in manufacturing yield and an increase in manufacturing cost are suppressed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic cross-sectional view of the optical semiconductor device according to the first embodiment.

[0026] Figure 2 It shows Figure 1 Graph showing the relationship between the stacked structure of semiconductor layers and the refractive index.

[0027] Figure 3A is a schematic diagram showing a situation where ripples are suppressed.

[0028] Figure 3B is a schematic diagram showing a situation where ripples are suppressed.

[0029] Figure 4 This is a diagram showing the relationship between the stacked structure of semiconductor layers and the refractive index in the optical semiconductor device according to the second embodiment.

[0030] Figure 5 This is a diagram showing the relationship between the stacked structure of semiconductor layers and the refractive index in the optical semiconductor device according to the third embodiment. DETAILED DESCRIPTION

[0031] Hereinafter, the embodiment will be described with reference to the accompanying drawings. It should be noted that the present invention is not limited to this embodiment. In addition, in the description of the accompanying drawings, the same or corresponding elements are appropriately marked with the same figure marks, and repeated descriptions are appropriately omitted. In addition, the accompanying drawings are schematic diagrams, and it is necessary to pay attention to the situation where the relationship between the dimensions of each element, the ratio of each element, etc. are different from the actual situation. There are also cases where the relationship between the dimensions and the ratio of each element are different between the drawings.

[0032] The inventors investigated the reasons why the characteristics of the active layer obtained during intermediate testing differed from those of the finished product. They confirmed that the emission spectrum obtained during intermediate testing contained a component of reflected light from the electric field distribution adjustment layer. Furthermore, they found that the wavelength spectrum of this reflected light was uneven in intensity and contained ripples, which sometimes made it difficult to accurately measure the emission spectrum, particularly the peak wavelength. The inventors then devised a method to suppress the ripples in the wavelength spectrum of the reflected light by providing a relaxation layer within the structure including the electric field distribution adjustment layer, leading to the present invention.

[0033] (Implementation Method 1)

[0034] [Overall Structure of Optical Semiconductor Device]

[0035] Figure 1 This is a schematic cross-sectional view of an optical semiconductor device according to Embodiment 1. Optical semiconductor device 100 is configured as a semiconductor laser element. Optical semiconductor device 100 includes an n-type cladding layer 120 having an n-side electrode 110 formed on the back surface, an active layer 130, a p-type cladding layer 140, a current blocking layer 150, a contact layer 160, and a p-side electrode 170. Optical semiconductor device 100 outputs laser light from active layer 130 in a direction perpendicular to the paper. The wavelength of the laser light is, for example, in the 1.55 μm band. Indium phosphide (InP)-based materials are known as semiconductor materials for achieving a laser light wavelength in the 1.55 μm band.

[0036] The n-type cladding layer 120 is a semiconductor layer having n-type conductivity. The p-type cladding layer 140 is a semiconductor layer having p-type conductivity. The active layer 130 is interposed between the n-type cladding layer 120 and the p-type cladding layer 140 .

[0037] The n-type cladding layer 120 includes base layers 121 and 123 and an electric field distribution adjusting structure 122 interposed between the base layers 121 and 123 .

[0038] The base layer 121 has a structure in which a buffer layer composed of n-type InP (hereinafter referred to as n-InP) is stacked on a substrate composed of n-InP by epitaxial growth or the like. The base layer 123 is composed of n-InP. The electric field distribution adjustment structure 122 will be described in detail later.

[0039] The n-type semiconductor layer in this specification includes, for example, silicon (Si), sulfur (S), and selenium (Se) as n-type impurities, but the n-type impurities are not particularly limited.

[0040] The active layer 130 has an MQW-SCH structure, comprising a multi-quantum well (MQW) layer composed of multiple barrier layers and multiple well layers, and two separate confinement heterostructure (SCH) layers sandwiching the MQW layer. The active layer 130 is composed, for example, of n-type GaInAsP, an InP-based quaternary semiconductor material. The composition ratio of the semiconductor material constituting the well layer of the active layer 130 is set to emit light at the desired laser excitation wavelength λc. The composition ratio of the semiconductor material constituting the barrier layer and the SCH layer is set to meet their respective functions. It should be noted that the active layer 130 may also have a single quantum well structure.

[0041] The p-type cladding layer 140 has a stacked structure of semiconductor layers 141 and 142 composed of p-type InP (hereinafter referred to as p-InP as appropriate).

[0042] The p-type semiconductor layer in this specification contains zinc (Zn) as a p-type impurity, for example, but the p-type impurity is not particularly limited.

[0043] Part of the n-type cladding layer 120, the active layer 130, and part of the p-type cladding layer 140 form a striped mesa structure. The striped mesa structure is etched, for example, to a width suitable for single-mode optical waveguide of light in the 1.55 μm band (e.g., 2 μm). The two ribs of the striped mesa structure (left and right in the drawing) are buried in a current blocking layer 150, which consists of a stack of a p-InP current blocking layer 151 and an n-InP current blocking layer 152. Furthermore, a semiconductor layer 142 is formed to cover both semiconductor layer 141 and current blocking layer 150.

[0044] The contact layer 160 is made of, for example, p-type GaInAsP, and is in ohmic contact with the p-side electrode 170. The p-side electrode 170 is made of, for example, titanium, platinum, gold, or the like.

[0045] The n-side electrode 110 is provided so as to be in ohmic contact with the substrate of the n-type cladding layer 120. The n-side electrode 110 is made of, for example, gold, nickel, or the like.

[0046] The two end faces of the optical semiconductor device 100, parallel to the drawing, are formed by cleavage. A high-reflection (HR) film with a relatively high reflectivity is formed on one end face, while an anti-reflection (AR) film is formed on the other end face. The HR and AR films form a laser resonator. Laser light from the optical semiconductor device 100 is primarily emitted from the end face with the AR film.

[0047] [Structure and Refractive Index of the Electric Field Distribution Adjustment Structure]

[0048] Figure 2 It shows Figure 1 The relationship between the stacked structure of semiconductor layers and the refractive index. Figure 2 , the refractive indices of the base layers 121 and 123, the electric field distribution adjusting structure 122, the active layer 130, and the p-type cladding layer 140 are shown. The base layers 121 and 123 and the p-type cladding layer 140 are all composed of InP and therefore have equal refractive indices. Furthermore, the active layer 130 is composed of n-GaInAsP and has a higher refractive index than the base layers 121 and 123 and the p-type cladding layer 140. It should be noted that region P in the refractive index of the active layer 130 represents the refractive index of the portion where the well layers and barrier layers are alternately stacked. However, within this region P, portions with a higher refractive index alternate with portions with a relatively lower refractive index.

[0049] Next, refer to Figure 2 The structure and refractive index of the electric field distribution adjustment structure 122 are described in detail. The electric field distribution adjustment structure 122 includes a plurality of first layers 122a and a plurality of second layers 122b. The electric field distribution adjustment structure 122 is formed by alternating and periodically stacking the first layers 122a and the second layers 122b. Figure 2 The number of the second layers 122 b is five, but the number of the second layers 122 b is not limited to five.

[0050] The first layer 122a is made of a semiconductor having the same refractive index as the base layers 121 and 123. For example, the first layer 122a is made of n-InP. Furthermore, the thickness of all the first layers 122a is the same. The thickness of the first layer 122a is, for example, 120 nm, but is not limited thereto.

[0051] The second layer 122b has a higher refractive index than the first layer 122a. Furthermore, the refractive indices of the second layer 122b are all equal. That is, the second layer 122b has a higher refractive index than the base layers 121 and 123. For example, the second layer 122b is made of n-GaInAsP, and its composition is adjusted to achieve a desired refractive index. For example, the composition of GaInAsP is adjusted so that the composition wavelength is 1.2 μm. Here, the composition wavelength is the wavelength of light equivalent to the band gap energy of the semiconductor material. Thus, when the composition of the semiconductor layer is changed, the refractive index, band gap energy, and composition wavelength of the semiconductor layer change.

[0052] Note that GaInAsP is an example of a Group III-V compound semiconductor containing As and P. The second layer 122 b is also referred to as an electric field distribution adjustment layer.

[0053] Here, the plurality of second layers 122b include a relaxation layer 122c. The relaxation layer 122c exists at a position close to one end from the center C in the stacking direction of the first layer 122a and the second layer 122b, specifically, exists at the end of the electric field distribution adjustment structure 122 on the side away from the active layer 130. The relaxation layer 122c has a smaller thickness than the other second layers 122b. For example, the thickness of the other second layers 122b is 20nm, but the thickness of the relaxation layer 122c is 10nm. Thus, since the thickness of the relaxation layer 122c is smaller than the thickness of the other second layers 122b, the equivalent refractive index of the relaxation layer 122c is also smaller than the equivalent refractive index of the other second layers 122b. Specifically, the equivalent refractive index of the relaxation layer 122c is closer to the refractive index of the first layer 122a composed of n-InP than the equivalent refractive index of the other second layers 122b.

[0054] Assuming that the multiple second layers 122b, including the relaxation layer 122c, all have the same layer thickness and the same refractive index, as described above, when the luminescence spectrum of the active layer 130 is measured, the reflection spectrum with ripples overlaps with the luminescence spectrum, so the original luminescence spectrum of the active layer 130 cannot be measured, and it is difficult to accurately know the characteristics of the active layer 130.

[0055] In contrast, in the optical semiconductor device 100 of Embodiment 1, the presence of the relaxation layer 122c, which has a smaller thickness than the second layer 122b, in the electric field distribution adjustment structure 122 suppresses ripples in the reflection spectrum. Consequently, the optical semiconductor device 100 can undergo more accurate in-process inspection, thereby minimizing a decrease in manufacturing yield and an increase in manufacturing costs.

[0056] Figure 3A 、 3BSchematic diagram showing how ripples are suppressed in the reflectivity spectrum. The wavelength range shown is the light emission band of the active layer 130 . Figure 3A The following shows a case where all the second layers 122b, including the relaxation layer 122c, have the same thickness and the same refractive index. In this case, ripples appear in the reflectivity spectrum. Figure 3B This shows the case where the electric field distribution adjustment structure 122 includes a relaxation layer 122c having a smaller thickness than the other second layer 122b. Figure 3B It can be seen that in this case the reflectivity spectrum is Figure 3A The spectrum is suppressed and becomes flat compared to the ripple.

[0057] Furthermore, in the optical semiconductor device 100 , the equivalent refractive index of the relaxation layer 122 c is made smaller than that of the other second layer 122 b by adjusting the thickness of the relaxation layer 122 c . This facilitates adjustment of the equivalent refractive index in the crystal growth process, and consequently, ripple suppression.

[0058] Furthermore, in the optical semiconductor device 100 , the electric field distribution adjusting structure 122 can suppress the valence electron interband light absorption in the p-type cladding layer 140 , improve the kink level, and adjust the far-field pattern in the vertical direction.

[0059] (Implementation Method 2)

[0060] Next, an optical semiconductor device according to Embodiment 2 will be described. Figure 4 This diagram illustrates the relationship between the stacked structure of semiconductor layers and the refractive index in optical semiconductor device 100A according to Embodiment 2. Optical semiconductor device 100A has a structure in which electric field distribution adjusting structure 122 in optical semiconductor device 100 according to Embodiment 1 is replaced with electric field distribution adjusting structure 122A. Therefore, the following description focuses primarily on electric field distribution adjusting structure 122A.

[0061] like Figure 4 As shown, the electric field distribution adjusting structure 122A has a structure in which the second layer 122b located at the end of the electric field distribution adjusting structure 122 on the side closer to the active layer 130 is replaced with a relaxation layer 122c. The relaxation layer 122c has the same thickness and refractive index as the relaxation layer 122c located at the end of the electric field distribution adjusting structure 122A on the side farther from the active layer 130.

[0062] The relaxation layer 122c located at the end of the electric field distribution adjusting structure 122A closer to the active layer 130 is an example of a second relaxation layer located closer to the second end of the electric field distribution adjusting structure 122A from the center C. Furthermore, the relaxation layer 122c located at the end of the electric field distribution adjusting structure 122A farther from the active layer 130 is an example of a first relaxation layer located closer to the first end from the center C.

[0063] Similar to the optical semiconductor device 100, the optical semiconductor device 100A can suppress inter-valence electron band light absorption in the p-type cladding layer 140, improve the kink level, and adjust the far-field pattern in the vertical direction. Furthermore, in the optical semiconductor device 100A, the relaxation layer 122c is provided with both an end distal from the active layer 130 and an end proximal to the active layer 130, further suppressing ripple in the reflection spectrum. As a result, the optical semiconductor device 100A further minimizes reductions in manufacturing yield and increases in manufacturing costs.

[0064] (Implementation Method 3)

[0065] Next, an optical semiconductor device according to a third embodiment will be described. Figure 5 This diagram illustrates the relationship between the stacked structure of semiconductor layers and the refractive index in optical semiconductor device 100B according to Embodiment 3. Optical semiconductor device 100B has a structure in which electric field distribution adjusting structure 122A in optical semiconductor device 100A according to Embodiment 2 is replaced with electric field distribution adjusting structure 122B. Therefore, the following description focuses primarily on electric field distribution adjusting structure 122B.

[0066] like Figure 5 As shown, the electric field distribution adjusting structure 122B has a structure in which the second layer 122b in the structure of the electric field distribution adjusting structure 122A is replaced by a second layer 122Bb, and the relaxation layer 122c is replaced by a relaxation layer 122Bc.

[0067] That is, the electric field distribution adjustment structure 122B has a plurality of first layers 122a and a plurality of second layers 122Bb. The electric field distribution adjustment structure 122 is formed by alternating and periodically stacking the first layers 122a and the second layers 122Bb. Figure 5 The number of the second layers 122Bb is five, but the number of the second layers 122Bb is not limited to five.

[0068] The second layer 122Bb has a higher refractive index than the first layer 122a. For example, the second layer 122Bb is made of n-type GaInAsP, and its composition is adjusted to achieve a desired refractive index. Furthermore, the thickness of the second layer 122Bb is uniform across the entire layer.

[0069] The plurality of second layers 122Bb include two relaxation layers 122Bc. The two relaxation layers 122Bc are located closer to one end from the center C in the stacking direction of the first layer 122a and the second layer 122b. Specifically, they are located at the end of the electric field distribution adjustment structure 122B that is closer to the active layer 130 or farther from the active layer 130. The relaxation layer 122Bc located at the end farther from the active layer 130 is an example of a first relaxation layer located closer to the first end from the center C, while the relaxation layer 122Bc located at the end closer to the active layer 130 is an example of a second relaxation layer located closer to the second end from the center C.

[0070] Here, the relaxation layer 122Bc has a lower refractive index than the other second layers 122Bb. For example, the composition of GaInAsP in the other second layers 122Bb is adjusted so that the wavelength is 1.2 μm. In contrast, the composition of GaInAsP in the relaxation layer 122Bc is adjusted so that the wavelength is 1.05 μm, which is shorter than 1.2 μm, resulting in a lower refractive index. Thus, the refractive index of the relaxation layer 122Bc is lower than that of the other second layers 122Bb, and therefore the equivalent refractive index of the relaxation layer 122Bc is also lower than that of the other second layers 122Bb.

[0071] In the optical semiconductor device 100B thus configured, the presence of the relaxation layer 122Bc, which has a lower refractive index than the other second layer 122Bb, in the electric field distribution adjusting structure 122B suppresses ripples in the reflection spectrum. Consequently, the optical semiconductor device 100B can undergo more accurate in-process inspection, thereby minimizing a decrease in manufacturing yield and an increase in manufacturing costs.

[0072] Furthermore, when adjusting the composition to create a difference in the equivalent refractive index between the relaxation layer 122Bc and the other second layer 122Bb, it may be difficult to create a sufficient difference, and the effect of suppressing reflection ripples may not be fully achieved. In contrast, in the optical semiconductor device 100B, two relaxation layers 122Bc are provided. Therefore, the reflection ripple suppression effects provided by each relaxation layer 122Bc are combined, resulting in a sufficient effect.

[0073] Furthermore, in the optical semiconductor device 100B, similarly to the optical semiconductor device 100A, it is possible to suppress the inter-valence electron band light absorption in the p-type cladding layer 140 , improve the kink level, and adjust the far-field pattern in the vertical direction.

[0074] It should be noted that in the above-mentioned embodiments 1 and 2, the refractive index of the relaxation layer 122c is equal to the refractive index of the other second layers 122b, but the refractive index of the relaxation layer 122c can also be lower than the refractive index of the other second layers 122b. In this way, the effect of suppressing reflection ripples can be further achieved.

[0075] In addition, in the above-mentioned embodiments 1 to 3, the relaxation layer is located at the end of the electric field distribution adjusting structure in the stacking direction. However, the location of the relaxation layer is not limited to this. The relaxation layer is effective as long as it is located closer to the end from the center of the electric field distribution adjusting structure in the stacking direction. It is particularly effective if it is located within 1 / 3 of the thickness of the electric field distribution adjusting structure from the end in the stacking direction.

[0076] In Embodiments 1 to 3 above, the optical semiconductor device is configured as a semiconductor laser element. However, the optical semiconductor device may also be configured as a semiconductor optical amplifier. When configured as a semiconductor optical amplifier, the optical semiconductor device is configured without a laser resonator. Furthermore, when the optical semiconductor device is configured as a distributed feedback (DFB) laser element, a diffraction grating layer is provided near the active layer.

[0077] In addition, the present invention is not limited to the above-mentioned embodiment. The mode of appropriately combining the above-mentioned various components and forming is included in the present invention. In addition, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader scheme of the present invention is not limited to the above-mentioned embodiment, and various changes can be made.

[0078] Industrial applicability

[0079] The present invention can be utilized in an optical semiconductor device having an active layer.

[0080] Description of Reference Numerals

[0081] 100, 100A, 100B: Optical semiconductor device

[0082] 110: n-side electrode

[0083] 120: n-type cladding

[0084] 121, 123: Grassroots

[0085] 122, 122A, 122B: Electric field distribution adjustment structure

[0086] 122a: First floor

[0087] 122b, 122Bb: Second layer

[0088] 122c, 122Bc: Mitigation layer

[0089] 130: Active layer

[0090] 140: p-type cladding

[0091] 141, 142: Semiconductor layer

[0092] 150, 151, 152: Current blocking layer

[0093] 160: Contact layer

[0094] 170: p-side electrode

[0095] C: Central

[0096] P: Region.

Claims

1. An optical semiconductor device, wherein: The optical semiconductor device includes: n-type cladding; p-type cladding; as well as an active layer sandwiched between the n-type cladding layer and the p-type cladding layer, The n-type cladding layer includes: a base layer; and an electric field distribution adjustment structure comprising a plurality of first layers having the same refractive index as the base layer and a plurality of second layers having a higher refractive index than the first layers, wherein the first layers and the second layers are alternately and periodically stacked. The plurality of second layers include a relaxation layer that is located from the center of the first and second layers in the stacking direction toward at least one end of the electric field distribution adjustment structure and has a thickness smaller than that of the other second layers.

2. The optical semiconductor device according to claim 1, wherein The refractive index of the relaxation layer is lower than the refractive index of the other second layers.

3. The optical semiconductor device according to claim 1, wherein The plurality of second layers include a first relaxation layer as the relaxation layer located at a position closer to the first end from the center and a second relaxation layer as the relaxation layer located at a position closer to the second end from the center. The optical semiconductor device according to claim 3 , wherein The refractive index of at least one of the first and second relaxation layers is lower than the refractive index of the other second layer.

5. An optical semiconductor device, wherein: The optical semiconductor device includes: n-type cladding; p-type cladding; as well as an active layer sandwiched between the n-type cladding layer and the p-type cladding layer, The n-type cladding layer includes: a base layer; and an electric field distribution adjustment structure comprising a plurality of first layers having the same refractive index as the base layer and a plurality of second layers having a higher refractive index than the first layers, wherein the first layers and the second layers are alternately and periodically stacked. The plurality of second layers include a first relaxation layer located near a first end from a center in a stacking direction of the first layer and the second layer of the electric field distribution adjustment structure, and a second relaxation layer located near a second end from the center. The refractive index of the first and second relaxation layers is lower than the refractive index of the other second layers.

6. The optical semiconductor device according to any one of claims 1 to 5, wherein The base layer and the first layer are composed of InP, The second layer is composed of a Group III-V compound semiconductor containing As and P as components.

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