JPH10270794A - Manufacture of optical semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical semiconductor device manufacturing method by which bonding work can be made easier, by flattening the surface of an optical semiconductor element. SOLUTION: A mask 2 having an opening 3 is provided on the surface of a semiconductor substrate 1 having one conductivity and a semiconductor layer 4 containing a core layer is selectively grown in the opening 3. Then, after removing the mask 2, a semiconductor layer 6 having the opposite conductivity is grown on the entire surface of the substrate 1. After the semiconductor layer 6 is grown, a projecting section 8 which is held between two stripe-like grooves 7 is formed in the semiconductor layer 6 above the semiconductor layer 4 containing the core layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an optical semiconductor device, and more particularly to a method of manufacturing an optical semiconductor device such as a semiconductor laser in which a spot size converter used for optical fiber communication is integrated. .

[0002]

2. Description of the Related Art In recent years, since optical fiber communication can transmit a large amount of information with one optical fiber, its application range has been expanded from a conventional trunk system to a subscriber system or a network such as an optical LAN. It is requested to go.

[0003] In order to meet such demands, cost reduction is indispensable. In particular, optical coupling between an optical semiconductor element and an optical fiber in an optical module has become a major problem.

In order to solve such a problem, an optical semiconductor device in which a spot size converter having a structure in which the thickness of a core layer for guiding light is changed in a tapered shape toward an emission end face, in particular, is integrated, particularly an optical semiconductor device. , A semiconductor laser with a spot size converter (if necessary, H. Kobayashi et a.
l. , IEEE Photon. Tech. Let
t. , Vol. 6, 1994, p. 1080-108
1, Sugie et al., "1.3 μmLD with Butt-Joint type selective growth spot size conversion", Proc. Of the 1995 IEICE General Conference, p. 465, SC-4
-5 or Y. Tohmori et al. , E
electronics Letters, vol. 3
1,1995, pp. 1069-1070).

[0005] Such a semiconductor laser with a spot size converter can be used as a device that can perform optical coupling with high coupling efficiency without using a lens and has a large margin of positional accuracy, so that optical coupling can be simplified. Have been.

As such a semiconductor laser with a spot size converter, a semiconductor laser with a ridge-type tapered waveguide without using an embedded structure has been proposed. In this semiconductor laser with a ridge-type tapered waveguide, a core layer is used. Since there is no need for a process of regrowth of the buried layer after the mesa etching of the device, the device can be manufactured by a simpler process than an element having a buried structure.

Here, a schematic configuration of a conventional semiconductor laser having a ridge-type tapered waveguide will be described with reference to FIG. This semiconductor laser with a ridge-type tapered waveguide has an n-type I type.
On the nP substrate 31, an MQW core layer 32 having a constant thickness in the laser portion 35 while the thickness of the taper waveguide portion 36 decreases in a tapered shape toward the emission end face,
A stripe-shaped p-type InP ridge 34 is provided thereon with a thin p-type InP clad layer 33 interposed therebetween. In this case, the width of the p-type InP ridge 34 is constant in the laser section 35, and the tapered waveguide is formed. The portion 36 has a shape in which the film thickness expands in a flare shape toward the emission end face.

As a method of forming a film thickness tapered waveguide in such a semiconductor laser having a ridge-type tapered waveguide, a selective growth method using a dielectric mask (refer to Japanese Patent Laid-Open No.
-46295), a growth technique using a shadow mask (see Aoki et al., IEICE General Conference, C-372, 1996, if necessary), or etching (if necessary, T. Brenner) et
al. , Electronics Letters, v
ol. 31 pp. 1443-1445, 1995).

[0009] Of these three types of forming methods, the selective growth method using a dielectric mask, which is often used in the production of a tapered waveguide having a buried structure, has controllability of shape and shape. Since the reproducibility is excellent, a selective growth method using this dielectric mask will be described with reference to FIG.

Referring to FIG. 8 (a), first, in the surface of the n-type InP substrate 31, in a region where a tapered waveguide portion is formed, a tapered opening portion whose width gradually increases toward the emission end face is formed. In the region where the portion is to be formed, an SiO ₂ mask 37 having a stripe-shaped opening 38 which is a uniform and narrow opening is provided.

Next, as shown in FIG. 8B, a reduced pressure MOVPE method (a reduced pressure metal organic chemical vapor deposition method).
In this case, the MQW core layer 32 and the p-type InP cladding layer 33 are sequentially grown. In this case, the MQW core layer 32 includes upper and lower optical guide layers each composed of an InGaAsP layer having a 1.1 μm wavelength composition, and 1.1 μm. InGa of wavelength composition
AsP barrier layer and 1.35 μm wavelength composition InGaAs
P well layers are constituted by quantum well layers alternately deposited so that the number of well layers becomes five.

At this time, the MQW core layer 3 depends on the shape of the stripe-shaped opening 38 provided in the SiO ₂ mask 37.
2 has a tapered shape in the tapered waveguide portion toward the emission end face and has a flat structure in the laser portion, but an abnormally grown portion 39 occurs at the end of the growth layer. .

Referring to FIG. 8C, a p-type InP ridge 34 surrounded by two stripe-shaped grooves 40 is formed by etching using a predetermined mask (not shown). The MQW core layer 32 located below the ridge 34 is an active region and an optical waveguide region.

Next, a semiconductor laser with a ridge-type tapered waveguide is mounted on a mount substrate 41 on which an optical fiber (not shown) is mounted without monitoring light, that is, by a passive alignment method. Then, junction-down bonding is performed using the solder layer 42 to complete the schematic configuration of the optical module.

[0015]

However, in a semiconductor laser with a ridge-type tapered waveguide using such a selective growth method using a dielectric mask, the surface becomes uneven due to the abnormal growth portion 39 at the end of the growth layer. Therefore, there is a problem that it is difficult to perform stable junction-down bonding.

At the same time, passive alignment for aligning the optical fiber with the semiconductor laser without actually monitoring the light becomes difficult, and a tapered waveguide portion, ie, a spot, is required to facilitate the assembly of the module. There is a problem that provision of the size converter disappears.

Therefore, an object of the present invention is to flatten the surface of a semiconductor laser having a ridge-type tapered waveguide and to facilitate bonding.

[0018]

FIG. 1 is an explanatory view of the principle configuration of the present invention. Referring to FIG. 1, means for solving the problems in the present invention will be described. 1 (a) to 1 (d) (1) The present invention relates to a method of manufacturing an optical semiconductor device, in which a mask 2 having an opening 3 on the surface of a semiconductor substrate 1 of one conductivity type.
A step of selectively growing a semiconductor layer 4 including a core layer only in the opening 3, a step of removing the mask 2, and growing a reverse conductivity type semiconductor layer 6 over the entire surface, Forming a convex portion 8 sandwiched between two stripe-shaped grooves 7 in a portion located on the semiconductor layer 4 including the core layer.

As described above, after the semiconductor layer 4 including the core layer is selectively grown, the opposite conductivity type semiconductor layer 6 is grown on the entire surface, so that the semiconductor layer to be selectively grown is limited to only the semiconductor layer 4 including the core layer. And the abnormally grown portion 5 formed at the end thereof can be reduced.
Further, the abnormally grown portion 5 is formed by the thick reverse conductivity type semiconductor layer 6.
The surface can be flattened without being greatly affected by the above, and bonding at the junction down required for passive alignment can be performed.

(2) Further, according to the present invention, in a method for manufacturing an optical semiconductor device, a mask 2 having an opening 3 is provided on the surface of a semiconductor substrate 1 of one conductivity type. Forming a groove in the opening 3 by etching, and selectively growing the semiconductor layer 4 including the core layer only in the groove formed in the opening 3 while leaving the mask 2; removing the mask 2 After that, a step of growing the opposite conductivity type semiconductor layer 6 on the entire surface, and a convex portion sandwiched between two stripe-shaped grooves 7 in a portion located on the semiconductor layer 4 including the core layer of the opposite conductivity type semiconductor layer 6 8 is formed.

As described above, the semiconductor layer 4 including the core layer in the groove is formed.
Is selectively grown, the abnormally grown portion 5 formed at the end of the semiconductor layer 4 including the core layer becomes extremely small, and the surface of the opposite conductivity type semiconductor layer 6 which is subsequently grown on the entire surface is flatter. Therefore, bonding at the junction down required for passive alignment becomes possible.

(3) In the present invention, in the above (1) or (2), the semiconductor layer 4 including the core layer selectively grown has a thickness gradually increasing toward the flat region and the light emitting end face. And a tapered region that becomes thinner.

As described above, the spot size converter can be formed by providing the tapered region in which the film thickness gradually decreases toward the light emitting end face, that is, by providing the tapered waveguide portion. It will be easier.

(4) Further, in the present invention, in any one of the above (1) to (3), the core layer is constituted by a multiple quantum well layer and an optical guide layer sandwiching the multiple quantum well layer. Features.

As described above, by forming the core layer with a multiple quantum well structure, the effective band gap due to the quantum level in the tapered waveguide section can be made larger than the effective band gap in the laser section. Light absorption loss in the wave path can be reduced.

(5) In the present invention, in any one of the above (1) to (4), as the uppermost layer of the semiconductor layer 4 including the core layer, the same composition as the reverse conductivity type semiconductor layer 6 provided on the entire surface. It is characterized in that a reverse conductivity type semiconductor layer is provided.

As described above, by providing a reverse conductivity type semiconductor layer having the same composition as that of the reverse conductivity type semiconductor layer 6 provided on the entire surface as the uppermost layer of the semiconductor layer 4 including the core layer, the mask 2 after the removal of the mask 2 is removed. The influence of the roughness of the growth interface accompanying the growth is not exerted on the core layer, and the heterojunction interface can be sharpened as compared with the case where the reverse conductivity type semiconductor layer 6 is directly grown on the core layer.

(6) The present invention provides the method according to any one of (1) to (5), wherein after removing the mask 2 and before growing the opposite conductivity type semiconductor layer 6 on the entire surface, the core is removed. The method is characterized by including a step of leaving the semiconductor layer 4 including the layer at a high temperature at which constituent atoms of the semiconductor layer 4 move.

As described above, before the reverse conductivity type semiconductor layer 6 is grown on the entire surface, the semiconductor layer 4 including the core layer is left at a high temperature at which constituent atoms of the semiconductor layer 4 move, thereby causing abnormal growth due to mass transport phenomenon. The protrusion of the portion 5 can be reduced, and the surface of the opposite conductivity type semiconductor layer 6 grown on the entire surface can be made flatter.

(7) The present invention is characterized in that in any one of the above (1) to (6), a step of forming a diffraction grating in at least a part of the opening 3 of the mask 2 is provided.

As described above, by forming a diffraction grating, a DFB (distributed feedback) semiconductor laser or a DB
An R (distributed Bragg reflection) type semiconductor laser can be configured, and the oscillating wavelength of the semiconductor laser with a ridge-type tapered waveguide can be unified.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Here, a first embodiment of the present invention will be described with reference to FIGS. Referring to FIG. 2A, first, a tapered waveguide portion is formed on the surface of an n-type InP substrate 11 having an electron concentration of 1.0 to 3.0 × 10 ¹⁸ cm ⁻³ , for example, 2.0 × 10 ¹⁸ cm ^−3. In a region having a length of 200 μm to be formed, a tapered opening having a width gradually increasing toward the emission end face is formed. On the other hand, in a region in which a laser portion having a length of 300 μm is formed, a uniform and narrow stripe-shaped opening is formed. Serving as a selective growth mask having an opening 13 made of Si
An O ₂ mask 12 is provided.

Next, referring to FIG. 2B, the MQW core layer 1 is formed by using the reduced pressure MOVPE method.
4, and a thickness of 50 to 300 nm, for example, 100 nm
Is selectively grown sequentially. Note that the p-type InP cladding layer 15 has a rough growth interface at the time of regrowth after removing the SiO ₂ mask 12 by MQW.
This is provided so as not to reach the core layer 14, that is, to sharpen the heterojunction interface.

At this time, the thicknesses of the upper and lower optical guide layers and the quantum well layers constituting the MQW core layer 14 are reduced to, for example, 1/3 over 200 μm in the tapered waveguide portion toward the emission end face. It is tapered and has a flat structure in the laser section.

In the first embodiment,
The thickness of the upper and lower optical guide layers is 50 to 150 nm, for example, 100 n.
m and an InGaAsP layer having a wavelength composition of 1.1 μm, and the quantum well layer is composed of six 10 nm-thick 1.
InGaAsP barrier layer having a wavelength composition of 1 μm and a thickness of 6 nm
The five InGaAsP well layers having a 1.35 μm wavelength composition are deposited.

Next, after the SiO ₂ mask 12 is removed, a thickness of 2000 to 6000 nm is again obtained by the reduced pressure MOVPE method, as shown in FIG.
For example, a 4000 nm (4 μm) p-type InP cladding layer 16 and a thickness of 200 to 1000 nm, for example, 500
A nm p-type InGaAs contact layer 17 is deposited.

Next, as shown in FIG. 3D, using a resist pattern (not shown) as a mask, dry etching is performed using an ethane-based gas to a width of 10 μm and a distance of 5 μm from the surface of the MQW core layer 14.
P-type In having a thickness of 0 to 300 nm, for example, 100 nm
Two stripe-shaped grooves 18 having a depth at which the P clad layer 15 or the p-type InP clad layer 16 remain are formed, and a ridge 19 between the grooves is formed.

In this case, the resist pattern is formed so that the width of the ridge 19 is, for example, 2 μm in the laser portion, flares toward the emission end surface in the tapered waveguide portion, and becomes 5 μm in the emission end surface.

Next, after removing the p-type InGaAs contact layer 17 in a region of 150 μm from the tip of the tapered waveguide portion 21 by etching, although not shown, the entire surface is made of SiO _2.
An opening for contact is provided only to the p-type InGaAs contact layer 17 on the top of the ridge 19, a p-side electrode is provided so as to cover the opening, and n-type By providing the side electrodes, the basic structure of the semiconductor laser with a ridge-type tapered waveguide is completed.

In the first embodiment, the thickness of the semiconductor layer to be selectively grown is about 600 nm only with the MQW core layer 14 of 290 nm and the p-type InP cladding layer 15 of 300 nm. The thickness can be significantly smaller than that of the semiconductor layer, and thus the protrusion of the abnormally grown portion can be significantly reduced.

In this case, since the thick p-type InP cladding layer 16 and the p-type InGaAs contact layer 17 are grown on the entire surface, the surface can be made flatter. Therefore, the junction is formed after the ridge 19 is formed. Even if bonding is performed down, accurate and stable passive alignment can be performed.

In order to make the abnormally grown portion smaller, the MO is grown before the p-type InP cladding layer 16 is grown.
By maintaining a high temperature of about 620 ° C. in a PH ₃ atmosphere in a VPE apparatus, the shape of the abnormally grown portion may be smoothed by a mass transport phenomenon, thereby further improving the flatness of the surface.

Next, a second embodiment of the present invention will be described with reference to FIGS. Referring to FIG. 4A, first, a tapered waveguide portion is formed on the surface of an n-type InP substrate 11 having an electron concentration of 1.0 to 3.0 × 10 ¹⁸ cm ⁻³ , for example, 2.0 × 10 ¹⁸ cm ^−3. In a region having a length of 200 μm to be formed, a tapered opening having a width gradually increasing toward the emission end face is formed. On the other hand, in a region in which a laser portion having a length of 300 μm is formed, a uniform and narrow stripe-shaped opening is formed. Serving as a selective growth mask having an opening 13 made of Si
An O ₂ mask 12 is provided, and a dry
A groove 2 having a depth corresponding to a film thickness to be selectively grown on the n-type InP substrate 11 by etching, for example, a depth of 400 nm.
Form 2

Next, referring to FIG. 4B, M is formed inside the groove 22 by using a reduced pressure MOVPE method.
A QW core layer 14 and a p-type InP clad layer 15 having a thickness of 50 to 300 nm, for example, 100 nm, are sequentially selectively grown. In this case, the p-type InP cladding layer 1
Reference numeral 5 is also provided in order to prevent the roughness of the growth interface from reaching the MQW core layer 14 at the time of regrowth after the removal of the SiO ₂ mask 12.

At this time, the thicknesses of the upper and lower optical guide layers and quantum well layers constituting the MQW core layer 14 are reduced to, for example, 1/3 over 200 μm toward the emission end face in the tapered waveguide portion. It is tapered and has a flat structure in the laser section.

Incidentally, also in the second embodiment,
The thickness of the upper and lower optical guide layers is 50 to 150 nm, for example, 100 n.
m and an InGaAsP layer having a wavelength composition of 1.1 μm, and the quantum well layer is composed of six 10 nm-thick 1.
InGaAsP barrier layer having a wavelength composition of 1 μm and a thickness of 6 nm
The five InGaAsP well layers having a 1.35 μm wavelength composition are deposited.

Next, after the SiO ₂ mask 12 is removed, a thickness of 2000 to 6000 nm is again applied by the reduced pressure MOVPE method, as shown in FIG.
For example, a 4000 nm (4 μm) p-type InP cladding layer 16 and a thickness of 200 to 1000 nm, for example, 500
A nm-type p-type InGaAs contact layer 17 is sequentially deposited.

Next, as shown in FIG. 5D, using a resist pattern (not shown) as a mask, dry etching is performed using an ethane-based gas to obtain a 10 μm-wide pattern from the surface of the MQW core layer 14.
P-type In having a thickness of 0 to 300 nm, for example, 100 nm
Two stripe-shaped grooves 18 having a depth at which the P clad layer 15 or the p-type InP clad layer 16 remain are formed, and a ridge 19 between the grooves is formed.

In this case, the resist pattern is formed so that the width of the ridge 19 is, for example, 2 μm at the laser portion, flares toward the emission end face at the tapered waveguide portion, and becomes 5 μm at the emission end face.

[0050] FIG. 5 (e) see Then, after the p-type InGaAs contact layer 17 from the tip of 150μm regions of the tapered waveguide portion 21 is removed by etching, but thereafter not shown, SiO on the entire surface ₂
An opening for contact is provided only to the p-type InGaAs contact layer 17 on the top of the ridge 19, a p-side electrode is provided so as to cover the opening, and n-type By providing the side electrodes, the basic structure of the semiconductor laser with a ridge-type tapered waveguide is completed.

In the second embodiment, the grooves 22
MQW core layer 14 and p-type InP clad layer 15 inside
Is grown, so that the projection of the abnormally grown portion is
Can be made much smaller than in the embodiment described above, whereby the surface flatness is further improved.

Also, in the second embodiment,
If the thickness of the growth layer formed by the selective growth is not constant, a step is inevitably generated, and if the thickness of the growth layer of the laser unit 20 is larger than the depth of the groove 22, the first embodiment will be described. However, although abnormally grown portions are produced, the size becomes smaller than in the case of the first embodiment.

In order to reduce the abnormally grown portion, p
Before growing the type InP cladding layer 16, MOVPE
By maintaining a high temperature of about 620 ° C. in a PH ₃ atmosphere in the apparatus, the shape of the abnormally grown portion may be smoothed by the mass transport phenomenon.

Next, a third embodiment of the present invention will be described with reference to FIG. See FIG. 6. The semiconductor laser with a ridge-type tapered waveguide shown in FIG.
This is an FB type semiconductor laser, and is a process similar to that of the first embodiment. However, after forming the SiO ₂ mask shown in FIG. After the lattice 23 is provided, the same steps as the steps after FIG. 2B may be performed. After forming the diffraction grating 23 partially, an SiO ₂ mask may be formed first.

In this case, an AR coating (anti-reflection coating) 24 is applied to the emission end face side, and an HR coating (high reflection coating) 25 is applied to the back side. Note that such an HR coating 25 is provided also in the first and second embodiments.

Further, such a diffraction grating 23 is provided in the second
May be provided in the semiconductor laser with a ridge-type tapered waveguide according to the embodiment described above. In that case, FIG.
After forming a groove having a width of 20 μm in a region where a laser portion is to be formed as shown in (1), a diffraction grating may be formed in a central portion having a width of about 5 μm.

In such a DFB type semiconductor laser, a single oscillation wavelength can be obtained together with the effect of increasing the spot size by the tapered waveguide.

Although the embodiments of the present invention have been described above, in the above-described first and second embodiments, a quantum well layer having a thickness of 10 nm and a wavelength of 1.1 μm having a wavelength composition of 1.1 μm is used.
The nGaAsP barrier layer and the InGaAsP well layers each having a thickness of 1.35 μm and having a thickness of 6 nm are alternately deposited so as to have five well layers. However, the present invention is not limited to such a structure. Required wavelength, for example, 1.5μ
The composition, thickness, and number of layers of each layer may be arbitrarily selected according to the m band and the output.

Further, in each of the above embodiments, I
Although the present invention has been described as an nGaAsP / InP-based optical semiconductor device, the present invention is not limited to the InGaAsP / InP-based optical semiconductor device, but may be an InAlGaAs-based or InGaAs / Ga-based semiconductor device.
As / AlGaAs system or InGaP / AlIn
The present invention can also be applied to a GaP system or the like.

[0060]

According to the present invention, since the cladding layer for forming the ridge of the semiconductor laser having a ridge-type tapered waveguide is formed by whole-surface growth, the surface can be flattened, thereby making it possible to form a passive layer. Since bonding at the junction down which is indispensable for alignment becomes possible, assembly of the optical module can be simplified, which greatly contributes to the development of optical fiber communication.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of the present invention.

FIG. 2 is an explanatory diagram of a manufacturing process partway through the first embodiment of the present invention.

FIG. 3 is an explanatory view of a manufacturing process of the first embodiment of the present invention after FIG. 2;

FIG. 4 is an explanatory diagram of a manufacturing process partway through a second embodiment of the present invention.

FIG. 5 is an explanatory diagram of a manufacturing process after FIG. 4 according to the second embodiment of the present invention.

FIG. 6 is an explanatory diagram of a third embodiment of the present invention.

FIG. 7 is a perspective view of a main part of a conventional semiconductor laser with a ridge-type tapered waveguide.

FIG. 8 is an explanatory diagram of a manufacturing process of a conventional semiconductor laser with a ridge-type tapered waveguide.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 one conductivity type semiconductor substrate 2 mask 3 opening 4 semiconductor layer including core layer 5 abnormal growth portion 6 reverse conductivity type semiconductor layer 7 stripe-shaped groove 8 convex portion 11 n-type InP substrate 12 SiO ₂ mask 13 opening 14 MQW core Layer 15 p-type InP cladding layer 16 p-type InP cladding layer 17 p-type InGaAs contact layer 18 stripe-shaped groove 19 ridge 20 laser part 21 taper waveguide part 22 groove 23 diffraction grating 24 HR coating 25 AR coating 31 n-type InP substrate 32 MQW core layer 33 p-type InP cladding layer 34 p-type InP ridge 35 laser part 36 taper waveguide part 37 SiO ₂ mask 38 opening 39 abnormal growth part 40 stripe-shaped groove 41 mount substrate 42 solder layer

Claims (7)

[Claims] 1. A step of providing a mask having an opening on the surface of a semiconductor substrate of one conductivity type, selectively growing a semiconductor layer including a core layer only in the opening, and removing the mask and then conducting reverse conductivity on the entire surface. Growing a type semiconductor layer; and
Forming a convex portion sandwiched between two stripe-shaped grooves in a portion of the opposite conductivity type semiconductor layer located on the semiconductor layer including the core layer. 2. A mask having an opening is provided on the surface of the one-conductivity-type semiconductor substrate, and a groove is formed in the opening by etching the one-conductivity-type semiconductor substrate using the mask, and the mask is left. Selectively growing a semiconductor layer including a core layer only in a groove formed in the opening while leaving the mask, removing the mask and growing a reverse conductivity type semiconductor layer over the entire surface; and 2. The method of manufacturing an optical semiconductor device according to claim 1, further comprising the step of forming a projection sandwiched between two stripe-shaped grooves at a portion of the semiconductor layer including the core layer of the semiconductor layer. Method. 3. The semiconductor layer including the core layer selectively grown includes a flat region and a tapered region whose film thickness gradually decreases toward the light emitting end face. A method for manufacturing an optical semiconductor device according to claim 1. 4. The optical semiconductor device according to claim 1, wherein said core layer comprises a multiple quantum well layer and an optical guide layer sandwiching said multiple quantum well layer. Manufacturing method. 5. The semiconductor device according to claim 1, wherein a reverse conductive semiconductor layer having the same composition as the reverse conductive semiconductor layer provided on the entire surface is provided as an uppermost layer of the semiconductor layer including the core layer. 2. The method for manufacturing an optical semiconductor device according to claim 1. 6. A step of leaving the semiconductor layer including the core layer at a high temperature such that atoms constituting the semiconductor layer including the core layer move after removing the mask and before growing a reverse conductivity type semiconductor layer on the entire surface. The method of manufacturing an optical semiconductor device according to claim 1, wherein the method includes: 7. The method for manufacturing an optical semiconductor device according to claim 1, further comprising a step of forming a diffraction grating in at least a part of the opening of the mask.