WO2023012925A1

WO2023012925A1 - Semiconductor optical device and method for producing same

Info

Publication number: WO2023012925A1
Application number: PCT/JP2021/028920
Authority: WO
Inventors: 拓郎藤井; 慎治松尾; 拓磨鶴谷
Original assignee: 日本電信電話株式会社
Priority date: 2021-08-04
Filing date: 2021-08-04
Publication date: 2023-02-09
Also published as: JPWO2023012925A1

Abstract

A semiconductor optical device (10) according to the present invention is provided with a plurality of waveguide structures (13_1, 13_2) on a second lower cladding layer (12) on a same substrate (11); the waveguide structures (13_1, 13_2) are sequentially provided with first lower cladding layers (141_1, 141_2), active layers (142_1, 142_2), first upper cladding layers (143_1, 143_2) and second upper cladding layers (15_1, 15_2); the waveguide structures respectively comprise diffraction gratings (18_1, 18_2); with respect to the waveguide structures (13_1, 13_2), the first lower cladding layers (141_1, 141_2), the active layers (142_1, 142_2) and the first upper cladding layers (143_1, 143_2) are different from each other in the thickness; and the coupling coefficients of the diffraction gratings (18_1, 18_2) are generally equal to each other.　Consequently, a semiconductor optical device according to the present invention is able to provide a multiwavelength laser operation in a single mode with high optical output and low threshold value.

Description

Semiconductor optical device and manufacturing method thereof

The present invention relates to a semiconductor optical device used in a wavelength division multiplexing (WDM) optical communication system and a manufacturing method thereof.

In recent years, with the increase in communication capacity in data centers and the like, wavelength division multiplexing (WDM) technology is required for short-distance communication in data centers and the like. Since the number of optical transceivers is enormous in short-distance communication, optical transceivers for WDM are required to be compact, consume little power, and be manufactured at low cost.

A compound semiconductor optical device, in which an embedded heterostructure is formed in a laminated structure in which multiple thin semiconductor layers are laminated, has been developed as a compact, low-power-consumption, heterogeneous-material integrated device (see Non-Patent Documents 1 to 5). A buried heterostructure is a structure in which a semiconductor (active layer) with a high refractive index and a small bandgap is sandwiched vertically and horizontally between semiconductors with a relatively low refractive index and a large bandgap. This structure makes it possible to improve the light confinement factor in the active layer, which greatly contributes to various performances of the semiconductor device.

A device with a buried heterostructure in this thin film structure typically uses a semiconductor multilayer structure with a thickness of about 250 nm to 500 nm, and power consumption can be reduced by reducing the volume of the semiconductor active layer.

Also, by forming a diffraction grating with an arbitrary pitch above the active layer, it is possible to easily achieve single-mode operation at an arbitrary wavelength required for WDM. In addition, the active layer employs a multiple quantum well (MQW) structure with excellent carrier coupling efficiency. In order to apply an electric field to the device and to inject current, a horizontal pin structure is employed in which the semiconductor layers on the left and right sides of the active layer are p-type and n-type.

In addition, in order to reduce the cost of the transceiver, instead of fabricating and integrating each device individually, a photonic integrated circuit in which multiple devices are collectively fabricated and integrated on a single wafer. , PIC). By using an optical integrated circuit, the device assembly cost can be greatly reduced.

In the fabrication of optical integrated circuits, silicon photonics technology is widely used to fabricate thin wire waveguides and passive devices by microfabrication of silicon (Si). Although silicon is an inexpensive material and microfabrication technology cultivated in the electronics field can be applied to photonics applications, silicon is an indirect transition material, so highly efficient light-emitting devices have not been realized. Therefore, it is essential to integrate heterogeneous materials with direct transition materials that can realize highly efficient light emission.

Therefore, according to the method of integrating a thin-film compound semiconductor device having the above-described buried heterostructure on silicon, a waveguide, a modulator, and an arrayed waveguide diffractometer fabricated by silicon photonics technology are formed in the silicon layer below the active layer. Gratings (AWG), optical switches, photodetectors, etc. can be produced, and optical integrated circuits can be produced by silicon photonics integration.

In order to realize WDM technology with the optical integrated circuit described above, it is necessary to collectively fabricate lasers that emit light at multiple wavelengths. At this time, it is required to precisely control the wavelength interval between the lasers and the absolute wavelength of the lasers. For example, when conforming to the 400GBASE-FR4 (CWDM grid) communication standard, the required oscillation wavelengths for short-wave and long-wave lasers differ by 60 nm for four wavelengths and by 140 nm for eight wavelengths. The required accuracy of the absolute value of the oscillation wavelength is ±6.5 nm. In the case of 400GBASE-FR8/LR8 (LAN-WDM), the required oscillation wavelength range is relaxed to 36 nm, but the required accuracy of the oscillation wavelength becomes strict to ±1.0 nm. It is also required to operate with high efficiency and high speed in all wide wavelength regions.

First, we will describe the method of fabricating a laser with a wide wavelength range. In order to obtain high-efficiency and high-speed operation over a wide wavelength range, matching between the material gain wavelength of the active layer and the oscillation wavelength is required. The full width at half maximum of the photoluminescence spectrum of a quantum well structure generally used as an active layer at room temperature is approximately 30 to 40 meV or less, that is, the full width at half maximum is 40 to 50 nm or less in the 1310 nm wavelength band. Therefore, when a single active layer material is used to fabricate a plurality of directly modulated light sources with different operating wavelengths, unless the operating wavelength range is sufficiently narrower than 40 to 50 nm, a light source with a small material gain is included at the oscillation wavelength. , high-efficiency and high-speed direct modulation operation cannot be obtained for all light sources. Also, if the operating wavelength range is much wider than 40-50 nm, it is difficult to obtain oscillation.

Active layer selective growth is a method for forming an active layer with multiple wavelengths in the range of 36 to 140 nm or more that conforms to the WDM standard described above. As shown in FIG. 14, when a III-V compound semiconductor is grown by the MOVPE method, in an atmosphere of a group V gas such as phosphine (PH3), a group III gas such as trimethylindium (TMIn) is directed toward the high-temperature semiconductor surface. A group III-V compound semiconductor 52 is grown on the surface of the substrate 53 by providing the organic metal 51 in a vapor phase state.

At this time, if a selective growth mask 54 made of SiO ₂ , SiN, or the like is formed in advance on the semiconductor surface, atoms are less likely to adhere to the dielectric surface than to the semiconductor surface. Most of the group III atoms 51 thus formed migrate (surface migration and vapor phase diffusion) across the mask surface in the horizontal direction with respect to the substrate, and crystals are selectively grown on the semiconductor surface in the mask openings.

The probability (selection ratio) of atoms adhering to the semiconductor surface and the dielectric surface is extremely high, about 100 to 1000 times. Migrate and adhere.

At this time, the migration length on the mask surface differs depending on the type of atoms. Using this, by changing the width and shape of the selection masks 54_1 and 54_2, it is possible to collectively grow a plurality of active layers 522_1 and 522_2 having different film thicknesses and mixed crystal compositions in one epitaxial growth. (Non-Patent Document 6). For example, as shown in FIG. 15, by changing the widths 54_1 and 54_2 of the selective growth masks, a thicker crystal structure (cladding layer 521_2, active layer 522_2) 52_2 can be obtained in the vicinity of the wider mask 54_2.

Next, a method of wavelength control in a semiconductor laser will be described. A distributed feedback (DFB) laser is generally used for precise oscillation wavelength control. In the DFB laser, by fabricating a diffraction grating structure near the active layer, it is possible to realize a single mode laser that oscillates near the Bragg wavelength λ determined by the diffraction grating spacing p and the equivalent refractive index _neq .

Specific single-mode laser configurations include a configuration that oscillates at the center of the Bragg wavelength by providing a 1/4 wavelength phase shift at the center of the diffraction grating (λ/4 shift DFB laser), There is also a configuration (distributed reflection laser/DR laser) in which regions with different Bragg wavelengths are provided in the region and used as a Bragg reflector to selectively oscillate one end of the stop band. One of the parameters that determine the characteristics of a laser using this diffraction grating is the product κL of the coupling coefficient κ of the diffraction grating and the length L of the diffraction grating.

FIG. 16 shows the relationship between κL in a DR laser with an active layer length of 100 mm and a DBR mirror length of 100 mm and the threshold gain of a laser with an active layer length of 200 μm. The detuning of the Bragg wavelengths of DFB and DBR was set to 2 nm. In order to stably oscillate the laser, it is important to reduce the threshold gain, and from this point of view it is effective to increase κ. However, increasing κL causes non-uniform light distribution in the cavity, which causes non-uniform carrier density and equivalent refractive index, thereby deteriorating the single-mode property (spatial hole burning).

Therefore, it is necessary to design the resonator so that κL achieves a low threshold gain within a range in which spatial hole burning does not occur. κ is determined by the equivalent refractive index difference of the regions forming the diffraction grating.

Therefore, as a method of changing the refractive index in the diffraction grating, there is a method of periodically changing the semiconductor structure itself by etching the semiconductor structure or the like, or a method of placing _SiO2 , SiN, SiON, _SiOx , etc. in the vicinity of the semiconductor structure depending on the operating wavelength. On the other hand, there is a method of periodically forming a transparent dielectric structure.

Regardless of which method is used, the semiconductor laser region fabricated on one wafer is generally designed to have a uniform structure and equivalent refractive index. Lasers of the same wavelength and κL can be made.

Further, even if lasers with different operating wavelengths are fabricated on one wafer, lasers with different oscillation wavelengths and substantially uniform κL can be fabricated by changing only the diffraction grating pitch (period). .

However, when fabricating a multi-wavelength laser for WDM using the semiconductor structure fabricated by the selective growth described above, each active layer structure is equivalent because it has an individual active layer structure optimized for each laser operating wavelength. Refractive index is very different.

For example, by selective growth, active layers with different peak wavelengths of about 150 nm can be grown all at once. In addition, in order to generate a wavelength change of about 40 nm, a film thickness change of about 35 nm is required, and when a wider wavelength change is required, it is necessary to increase the film thickness change amount (Non-Patent Document 5). ).

FIG. 17 shows the calculation result of the slab thickness dependence of the equivalent refractive index. Here, the slab thickness is the thickness of a layer (hereinafter referred to as "slab layer") consisting of a lower clad layer, an active layer, and an upper clad layer in the waveguide structure.

The calculation was performed using software (product name: "FIMMWAVE", distributor: Convenient Business Solutions). Here, the active layer was positioned in the center of the slab layer, and the layer thickness of the active layer was changed in proportion to the slab thickness. InP was used for the upper clad layer and the lower clad layer, and InGaAlAs multiple quantum well (MQW) was used for the active layer. The width of the active layer was 0.5 μm (black squares, dashed line in the figure), 0.7 μm (black triangles, dotted line in the figure), and 0.9 μm (black circles, solid line in the figure).

According to the calculation results, the equivalent refractive index change is about 4% when the film thickness changes from about 250 nm to 350 nm.

Accompanying this equivalent refractive index change, the diffraction grating coupling coefficient κ changes. FIG. 18 shows the calculation result of the dependence of κ on the thickness of the active layer. Calculations were performed in the same manner as described above (FIG. 17). Here, κ is shown when, for example, a diffraction grating with a depth of 20 nm is formed on the surface of the upper clad layer (InP) of the slab layer with different layer thicknesses. When forming the same diffraction grating for layer structures with slab layer thicknesses from 240 nm to 340 nm, κ varies from approximately 670 cm ⁻¹ to 400 cm ⁻¹ .

In this way, in a multi-wavelength semiconductor laser having a plurality of waveguide structures, when the layer thickness and composition of the slab layer including the active layer of each waveguide structure are changed by selective growth, the layer thickness and composition are different and the equivalent refractive index If diffraction gratings with the same configuration are formed in regions with different λ, κ cannot be uniformed (constant), and it is difficult to achieve single-mode, high optical output, and low threshold characteristics.

This is particularly problematic in thin-film devices, because light is confined by dielectrics such as air and glass above and below the InP thin-film structure, and the equivalent refractive index greatly depends on the film thickness.

In order to change κ in the semiconductor thin film structure, (1) the material forming the diffraction grating is changed, (2) the etching depth is changed when the diffraction grating is formed by etching, and (3) the dielectric material is changed. A possible method is to change the thickness of a dielectric when forming a diffraction grating on a semiconductor, but it is difficult to realize all of them on a single wafer in a batch process.

Also, when performing multiple processes, it is possible to change κ by, for example, performing multiple etchings with different etching times, but this leads to an increase in man-hours and cost.

In order to solve the above-described problems, a semiconductor optical device according to the present invention includes a plurality of waveguide structures on a second lower clad layer on the same substrate, the waveguide structures sequentially comprising: A first lower cladding layer, an active layer, a first upper cladding layer, and a second upper cladding layer, the waveguide structures comprising a diffraction grating, each waveguide structure comprising: The thicknesses of the first lower clad layer, the active layer, and the first upper clad layer are different, and the coupling coefficients of the diffraction gratings are substantially the same.

Further, a method for manufacturing a semiconductor optical device according to the present invention includes the step of crystal-growing a semiconductor layer on a second lower clad layer on the same substrate; forming a mask layer for selective growth comprising: crystal-growing a selective growth layer having a first lower clad layer, an active layer and a first upper clad layer in the opening; forming a current injection layer on the side of the mesa structure; forming a diffraction grating on the upper surface of the first upper clad layer; and having the diffraction grating. forming a second clad layer on the upper surface of the first upper clad layer, wherein at least one of the area of the mask and the area of the opening changes in the mask layer for selective growth, The diffraction gratings formed on the top surface of the first upper cladding layer crystal-grown in the plurality of openings are substantially equal in coupling coefficient.

According to the present invention, it is possible to provide a single-mode, high-output, low-threshold, multi-wavelength laser-operating semiconductor optical device and a method for manufacturing the same.

FIG. 1 is a schematic bird's-eye view showing the configuration of a semiconductor optical device according to the first embodiment of the present invention. FIG. 2 is a schematic front sectional view showing the configuration of the semiconductor optical device according to the first embodiment of the present invention. FIG. 3A is a schematic side sectional view showing the configuration of the semiconductor optical device according to the first embodiment of the invention. FIG. 3B is a schematic side sectional view showing the configuration of the semiconductor optical device according to the first embodiment of the invention. FIG. 4 is a diagram for explaining the operation of the semiconductor optical device according to the first embodiment of the present invention. FIG. 5A is a schematic side cross-sectional view showing an example of the configuration of the semiconductor optical device according to the first embodiment of the invention. FIG. 5B is a schematic side sectional view showing an example of the configuration of the semiconductor optical device according to the first embodiment of the invention. FIG. 6 is a diagram for explaining the manufacturing method of the semiconductor optical device according to the first embodiment of the present invention. 7A is a diagram for explaining the operation of the semiconductor optical device according to the first embodiment of the present invention; FIG. 7B is a diagram for explaining the operation of the semiconductor optical device according to the first embodiment of the present invention; FIG. FIG. 8A is a schematic top sectional view showing an example of the configuration of the semiconductor optical device according to the first embodiment of the invention. 8B is a schematic top sectional view showing an example of the configuration of the semiconductor optical device according to the first embodiment of the present invention; FIG. FIG. 9 is a diagram for explaining the effect of the semiconductor optical device according to the first embodiment of the invention. FIG. 10 is a diagram for explaining the effect of the semiconductor optical device according to the first embodiment of the invention. FIG. 11A is a schematic top sectional view showing the configuration of a semiconductor optical device according to a second embodiment of the present invention. FIG. 11B is a schematic top sectional view showing the configuration of the semiconductor optical device according to the second embodiment of the present invention. FIG. 12 is a schematic front sectional view showing the configuration of a semiconductor optical device according to a third embodiment of the invention. FIG. 13 is a schematic front sectional view showing the configuration of a semiconductor optical device according to a fourth embodiment of the invention. FIG. 14 is a diagram for explaining selective growth used for crystal growth in a conventional semiconductor optical device. FIG. 15 is a diagram for explaining selective growth used for crystal growth in a conventional semiconductor optical device. FIG. 16 is a diagram for explaining the operation of a conventional semiconductor optical device. FIG. 17 is a diagram for explaining the operation of a conventional semiconductor optical device. FIG. 18 is a diagram for explaining the operation of a conventional semiconductor optical device.

<First Embodiment>
A semiconductor optical device and a manufacturing method thereof according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 10. FIG.

<Structure of semiconductor optical device>
As shown in FIGS. 1 and 2, the semiconductor optical device 10 according to the present embodiment includes, in order, a substrate 11, a common lower clad layer (second lower clad layer) 12, and a plurality (two in the present embodiment). (this) waveguide structures 13_1 and 13_2.

The substrate 11 is Si. Si, SiO ₂ , Al ₂ O ₃ , InP, GaAs, and SiC may also be used.

SiO ₂ is used for the common lower clad layer 12 . In addition, SiO _x , SiN, SiON, Al ₂ O ₃ , or a structure in which these are combined may be used, which has a lower refractive index than the semiconductor layer and is transparent to the oscillation wavelength (for example, 300 nm to 1650 nm) of the laser to be manufactured. I wish I had.

Each waveguide structure 13_1, 13_2 includes, as slab layers 14_1, 14_2, first lower clad layers 141_1, 141_2, active layers 142_1, 142_2, and first upper clad layers 143_1, 143_2. Second upper clad layers 15_1 and 15_2 are provided on the slab layers 14_1 and 14_2.

InP is used for the first lower clad layers 141_1 and 141_2 and the first upper clad layers 143_1 and 143_2.

Six layers of InAlGaAs with a wavelength band of 1310 nm are used for the active layers 142_1 and 142_2.

In the two waveguide structures, the slab layer 14_1 of one waveguide structure 13_1 can have, for example, a layer thickness of 315 nm, an active layer 142_1 with a composition wavelength of 1240 nm, and a laser oscillation wavelength of 1275 nm. The slab layer 14_2 of the other waveguide structure 13_2 can have a layer thickness of 350 nm, a composition wavelength of the active layer 142_2 of 1275 nm, and a laser oscillation wavelength of 1310 nm. Also, the width of the active layers 142_1 and 142_2 of each waveguide structure is, for example, 0.7 μm.

Thus, in the semiconductor optical device 10, the compositions and slab layer thicknesses of the active layers 142_1 and 142_2 of the waveguide structures 13_1 and 13_2 are different.

The slab layer 14_2 of the other waveguide structure 13_2 is thicker than the slab layer 14_1 of the one waveguide structure 13_1. Also, the composition wavelength of the active layer 142_2 of the other waveguide structure 13_2 is longer than the composition wavelength of the active layer 142_1 of the waveguide structure 13_1.

In addition, p-type semiconductor layers 161_1 and 161_2 and n-type semiconductor layers 162_1 and 162_2 are provided as buried semiconductor layers so as to be in contact with the side surfaces of the slab layers 14_1 and 14_2. Electrodes 17_1 and 17_2 are formed on the p-type and n-type semiconductor layers so that they can be electrically driven by pin junctions. Injecting current through the electrodes results in oscillation at a wavelength determined by the grating spacing and the equivalent refractive index of the buried heterostructure.

In the respective waveguide structures 13_1 and 13_2, as shown in FIGS. 3A and 3B, the diffraction gratings 18_1 and 18_2 are formed on the surfaces of the first upper cladding layers 143_1 and 143_2, and have periodic concave-convex structures in the light guiding direction. is formed. Second upper clad layers 15_1 and 15_2 are laminated on the diffraction gratings 18_1 and 18_2. Here, the waveguide direction of light is the direction perpendicular to the end surface of the waveguide structure.

Here, the second upper cladding layers 15_1 and 15_2 are SiO ₂ , and SiO ₂ , SiO _x , SiN, SiON, and Al ₂ O ₃ may be used.

Thus, diffraction gratings are formed at the boundaries between the first upper clad layers (InP) 143_1, 143_2 and the second upper clad layers (SiO ₂ ) 15_1, 15_2.

In the present embodiment, an example is shown in which the diffraction grating is formed by etching the first upper clad layer and then embedded with SiO ₂ or the like. A refractive index difference due to an air layer may be used.

In the respective waveguide structures 13_1 and 13_2, the diffraction gratings 18_1 and 18_2 have periods corresponding to laser oscillation in the 1.3 μm wavelength band and have different duty ratios. The duty ratio of the diffraction grating is the ratio D of the length τ occupied by the projections of the diffraction grating to the length T of one period of the diffraction grating, and takes a value between 0 and 1.

In one waveguide structure (waveguide structure with a thinner slab layer) 13_1, as shown in FIG. 3A, the length τ occupied by the projections of the diffraction grating 18_1 is large and the duty ratio is large.

In the other waveguide structure (waveguide structure with a thicker slab layer) 13_2, as shown in FIG. 3B, the length τ occupied by the projections of the diffraction grating 18_2 is small and the duty ratio is small.

Here, in each of the waveguide structures 13_1 and 13_2, the length T of one period of the diffraction grating is the same.

FIG. 4 shows the calculation result of the duty ratio dependence of the coupling coefficient κ of the diffraction grating. Here, the calculation was performed by the transfer matrix method. Also, κ is shown when, for example, a diffraction grating with a depth of 20 nm is formed on the surface of the upper clad layer (InP) of the slab layer. The slab layer consists of a lower InP clad layer (160 nm thick), an InGaAlAs-MQW (100 nm thick), and an upper InP clad layer (70 nm thick).

Thus, even if the depth of the diffraction grating is the same, κ can be changed by changing the duty ratio. Therefore, diffraction gratings with the same κ can be formed for a plurality of semiconductor waveguide structures having different equivalent refractive indices formed on the same substrate.

As a method of changing the duty ratio of the diffraction grating, for example, the duty ratio of the mask used during diffraction grating etching may be changed. Since the shape of the mask can be arbitrarily determined by photolithography or electron beam lithography, diffraction gratings with different duty ratios can be easily formed collectively on the same substrate surface.

Thus, in the semiconductor optical device 10, the duty ratio of one waveguide structure (the waveguide structure with the thinner slab layer) 13_1 is increased, and the other waveguide structure (the waveguide structure with the thicker slab layer) is increased. ) 13_2, the coupling coefficients of the waveguide structures 13_1 and 13_2 are set substantially equal.

In addition, as shown in FIGS. 5A and 5B, the diffraction grating of the semiconductor optical device has structures 181_1 and 181_2 made of, for example, rectangular SiO ₂ on the surface of the first upper clad layers 143_1 and 143_2. A second upper cladding layer (SiN) 15_1, 15_2 may be provided on the first upper cladding layer periodically formed in the direction and having its SiO ₂ periodic structure.

This diffraction grating (structure) is formed by laminating a layer (third upper clad layer) made of, for example, SiO ₂ on the flat surface of the first upper clad layer without processing the surface, and then laminating the SiO ₂ layer. It is formed by periodic processing. After that, a second upper clad layer (eg, SiN) is laminated on the first upper clad layer and the processed SiO ₂ . Also in this case, the refractive index difference between, for example, SiN and SiO ₂ functions as a diffraction grating.

The combination of the structure (third upper clad layer) and the second upper clad layer _is not limited to _SiO2 and SiN, and other dielectrics such as _SiOx , SiON, and _Al2O3 may be used. . The structure (third upper clad layer) and the second upper clad layer may be made of materials having different refractive indices.

Alternatively, the refractive index difference may be used by using an air layer as the second upper clad layer without embedding the structure (the third upper clad layer).

<Method for Manufacturing Semiconductor Optical Device>
A method for manufacturing a semiconductor optical device according to this embodiment will be described with reference to FIGS. 6 to 7B.

First, the semiconductor layer 144 is formed on the substrate 11 on which the common lower clad layer 12 is formed (step S1). Si is used as the substrate material. In addition, SiO ₂ , Al ₂ O ₃ , InP, GaAs, SiC, etc. may be used.

SiO ₂ is used for the common lower clad layer 12 . In addition, SiN, SiC, or a structure combining these may be used as long as it has a lower refractive index than the semiconductor layer and is transparent to the oscillation wavelength (for example, 300 nm to 1650 nm) of the laser to be manufactured.

The film formation of the semiconductor layer 144 is performed by wafer direct bonding or by crystal growth such as metalorganic chemical vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE).

Next, a selective growth mask layer 191 made of SiO ₂ , SiN, or the like is formed on the semiconductor layer 144 (step S2).

Next, selective growth layers 140_1 and 140_2 having active layers of multiple wavelengths are selectively grown by MOVPE or the like (step S3). Here, the selectively grown layers 140_1 and 140_2 have a first lower clad layer, an active layer, and a first upper clad layer in order from the common lower clad layer 12 side.

In the selective growth, the group III atoms supplied near the mask surface move horizontally on the mask surface with respect to the substrate 11, and the group III atoms selectively adhere to the semiconductor surface in the mask openings. Therefore, the material of the mask layer for selective growth should be such that atoms are less likely to adhere to the surface of the semiconductor.

In addition, in the selective growth mask layer 191, the mask region width in the mask varies from 0 to 55 μm, and the interval (opening width) between the masks is constant at 40 μm.

Although the mask width is changed and the opening width is kept constant in this embodiment, the opening width or both the mask width and the opening width may be changed.

Here, the wider the width of the mask, ie, the area of the mask, and the narrower the width of the opening, ie, the area of the opening, the more pronounced the crystal composition change and growth rate acceleration due to selective growth.

Thus, in the mask layer for selective growth, by changing at least one of the mask area and the opening area within the mask layer, the selective growth film thickness can be changed. Narrower openings allow for thicker crystal layers.

In this embodiment, an example was shown in which the width of the mask region was 0 to 55 μm, and the gap between the masks was 40 μm. It is sufficient if it is equal to or longer than the surface migration length of group III atoms.

For the material and structure of the active layer formed by selective growth, for example, in the case of an InP system, a multiple quantum well structure using a mixed crystal of InGaAs, InP, InGaAsP, or InGaAlAs may be used.

When selectively growing a quantum well structure, it is possible to use the emission wavelength change of the active layer due to the quantum well layer thickness change in addition to the wavelength change due to the mixed crystal composition change.

Also, by adopting a quantum well structure for the active layer, it is possible to apply a very large strain (about 1.5%) to the well layer without causing crystal defects. This makes it possible to increase the gain coefficient, which is suitable for achieving high efficiency and high-speed operation of a directly modulated laser. Even if a bulk mixed crystal is selectively grown instead of the multiple quantum well structure, an active layer that emits light at different wavelengths can be obtained by changing the composition of the mixed crystal.

In addition, light confinement in the active layer can be enhanced by making the semiconductor structure formed by selective growth into a thin film structure of about several hundred nm. For example, FIGS. 7A and 7B show calculation results of the dependence of light confinement in the active layer on the thickness of the entire InP thin film (slab thickness). Here, the calculation was performed using software (product name: "FIMMWAVE", distributor: Convenient Business Solutions).

For the calculation, as shown in FIG. 7A, a structure in which a slab layer is sandwiched between low refractive materials (SiO ₂ , refractive index n=1.47) from above and below was used. The slab layer consists of a multiple quantum well structure (MQW, n=3.40) about 100 nm thick and InP (n=3.17) surrounding the MQW. The slab thickness was changed by changing the InP film thickness above and below the multiple quantum well layer.

As shown in FIG. 7B, the light confinement factor in the active layer reaches a maximum when the slab thickness (thickness of the entire InP-based thin film) is about 150 nm, which is higher than that of a general laser with a thickness of 3000 to 4000 nm. about three times higher. Also, the dependence of the optical confinement coefficient on the film thickness decreases as the slab thickness increases. Therefore, from the viewpoint of the light confinement factor, it is effective to set the slab thickness to about 500 nm or less.

After performing the selective growth, a thin film (for example, SiO ₂ , SiN, etc.) to be used as a mask for buried growth is formed after removing the mask layer 191 for selective growth or without removing it. The material of the thin film may be the same as or different from that of the selective growth mask layer.

Next, a mask pattern made of resist or the like is formed on the active layer by lithography or the like on the thin film used as the buried growth mask, and the buried growth mask layer 192 is formed by dry etching or the like.

Next, selective etching of the slab layer is performed using the buried growth mask layer 192 . The etching may be dry etching, wet etching, or a combination thereof. By this etching, the selectively grown layers 140_1 and 140_2 are processed to have the active layer width (mesa structure width, eg, 0.7 μm) in a semiconductor optical device (semiconductor laser). As a result, slab layers 14_1 and 14_2 including first lower clad layers 141_1 and 141_2, active layers 142_1 and 142_2, and first upper clad layers 143_1 and 143_2 are formed.

Next, buried semiconductors 160_1 and 160_2 are formed on the sides of the active layer by MOCVD, MBE, or the like, and planarized (step S4).

The embedded growth mask layer 192 is removed. If the semiconductor surface is slightly uneven, it may be further planarized by slightly regrowing the semiconductor layer after removing the buried growth mask layer 192 .

Next, some of the semiconductor layers 162_1 and 162_2 are made n-type by, for example, Si ion implantation, and some of the semiconductor layers 161_1 and 161_2 are made to be p-type by, for example, Zn thermal diffusion. Thus, current injection layers 161_1, 161_2, 162_1, and 162_2 are formed. Furthermore, by forming electrodes 17_1 and 17_2 so as to be in contact with the p-type semiconductor layers 161_1 and 161_2 and the n-type semiconductor layers 162_1 and 162_2, electrical driving is enabled (step S5). A contact semiconductor layer for obtaining ohmic current-voltage characteristics may be formed between the p-type semiconductor layer and the electrode or between the n-type semiconductor layer and the electrode.

Next, by partially etching the upper surfaces of the first upper clad layers 143_1 and 143_2, diffraction gratings 18_1 and 18_2 are formed in the light guiding direction (the direction perpendicular to the end surface of the waveguide structure) (step S6). Alternatively, after forming a third upper cladding layer made of SiO ₂ , SiO _x , SiN, SiON, Al ₂ O ₃ or the like on top of the first upper cladding layer, by partially etching them, diffraction Gratings (structures) 181_1 and 181_2 are formed.

Here, in the diffraction gratings 18_1 and 18_2 (or 181_1 and 181_2), the duty ratio of one slab layer (thin slab layer) 14_1 is increased, and the other waveguide structure (the waveguide structure with the thicker slab layer) By reducing the duty ratio of 14_2, the coupling coefficients of 14_1 and 14_2 of the respective slab layers are set substantially equal.

Finally, second upper clad layers 15_1 and 15_2 having a refractive index smaller than that of the semiconductor are formed (step S7).

In this embodiment, an example is shown in which two waveguide structures each having an active layer and a slab layer with different film thicknesses are provided on the same substrate. You may prepare. For example, in the case of application to 8-wavelength WDM, a waveguide structure having 8 active layers and slab layers with different film thicknesses and diffraction gratings with appropriate duty ratios corresponding to each should be provided. Further, there is no problem even if the arrangement of the p-type semiconductor layer and the n-type semiconductor layer is horizontally reversed, and it is not necessary that the directions of all the channels are the same.

Also, in this embodiment, an example in which the diffraction grating is formed above the slab layer is shown, but the diffraction grating may be formed in another region as long as the waveguide mode exists. For example, as shown in FIGS. 8A and 8B, diffraction gratings may be formed on the side portions (side surfaces) of waveguide structures 13_3 and 13_4. FIG. 8A shows a diffraction grating with a large duty ratio, and FIG. 8B shows a diffraction grating with a small duty ratio.

In this case, the diffraction grating may be formed on the side portion of the waveguide structure in the step immediately before the step of forming the buried semiconductor (step S4). By using photolithography or electron beam lithography, the mask for forming the diffraction grating of the active layer can be easily formed in any number of different shapes on the same substrate, forming diffraction gratings with different duty ratios in a single etching. can. Here, the diffraction grating forming mask is formed on the first upper clad layer, and the diffraction gratings can be formed on the side portions (side surfaces) of the waveguide structures 13_3 and 13_4 by etching using this mask. After that, the mask for forming the diffraction grating may be used as it is to carry out burying growth.

Moreover, the diffraction grating may be formed on a part of the side portion (side surface) of the waveguide structure, and includes the first lower clad layer, the active layer, the first upper clad layer, and the second upper clad layer. It may be formed on a part of the side portion (side surface) with the layer.

<effect>
Effects of the semiconductor optical device according to this embodiment will be described with reference to FIGS.

Fig. 9 shows the calculation results of the slab thickness dependence of κ. Here, as an example, κ is shown when a diffraction grating having a depth of 20 nm and duty ratios of 0.5, 0.4, 0.3, and 0.25 is formed.

Calculations were performed by the transfer matrix method. The calculations were also performed for a waveguide layer structure comprising a slab layer consisting of an upper InP cladding layer, an InGaAlAs-MQW active layer and a lower InP cladding layer, and SiO ₂ layers above and below the slab layer. The grating was assumed to be rectangular in shape and sufficiently wide for the cross-sectional mode field of the light. Also, the slab thickness was varied from 240 nm to 340 nm. At this time, the layer thickness of the active layer was changed in proportion to the slab thickness.

When forming a diffraction grating with a duty ratio of 0.5, κ changes from about 670 cm ⁻¹ to 400 cm ⁻¹ (solid line with white circles in the figure). On the other hand, when a diffraction grating with a duty ratio of 0.25 is formed, κ changes from approximately 370 cm ⁻¹ to 220 cm ⁻¹ (in the figure, dashed-dotted black circle line).

Therefore, for example, a diffraction grating with a κ of about 400 cm ⁻¹ for all film thicknesses (slab thickness), and a duty ratio of the diffraction grating in the range of about 0.25 to 0.5 for the film thickness (slab thickness). It can be easily formed by setting appropriate values. Specifically, the coupling coefficient κ of the diffraction grating can be made constant by increasing the duty ratio of the diffraction grating as the film thickness (slab thickness) increases.

Next, the effect of the threshold gain of the semiconductor optical device according to this embodiment will be described.

FIG. 10 shows the film thickness dependence of the threshold gain in the semiconductor optical device according to this embodiment. Here, the diffraction grating shape is changed according to the slab thickness so that the diffraction grating coupling coefficient κ is constant.

For comparison, the film thickness dependence of the threshold gain in a conventional semiconductor optical device (conventional structure) is also shown. In the conventional structure, the same diffraction grating shape is set regardless of the slab thickness.

Calculations were performed using the transfer matrix method.

The waveguide structure used for calculation is the same as described above (calculation in FIG. 17). The slab thickness was varied from 240 nm to 340 nm. The cavity is assumed to be a DR laser consisting of a DFB region with an active layer length of 100 μm and a DBR mirror with a length of 200 μm, and the Bragg wavelength detuning of each is set to 2 nm.

It was assumed that the diffraction grating depth in the conventional structure was 20 nm, the duty ratio was 0.5, and the width of the diffraction grating was sufficiently wide relative to the width of the cross-sectional mode field.

On the other hand, in the semiconductor optical device according to the present embodiment, the calculation was performed assuming that κ=400 cm ⁻¹ is constant in each waveguide structure.

As shown in FIG. 10, the threshold gain is about 14.4 cm ⁻¹ to 30.1 cm ⁻¹ when forming a diffraction grating with a duty ratio of 0.5 for slab layers with different film thicknesses in the conventional structure. up to about twice or more (broken black square line in the figure).

On the other hand, in the semiconductor optical device according to the present embodiment, when forming the diffraction grating so that κ=400 cm ⁻¹ is constant for slab layers with different film thicknesses, it is almost constant regardless of the film thickness. A threshold gain can be obtained (solid line with white circles in the figure).

Here, the diffraction gratings should be formed so that κ is approximately the same for slab layers with different film thicknesses. The term "substantially equivalent" includes a complete equivalent, a range including a difference of about ±30%, and a range in which a laser operates in a single mode with a high optical output and a low threshold in each waveguide structure.

As described above, according to the semiconductor optical device and the manufacturing method thereof according to the present embodiment, in a plurality of waveguide structures having different slab thicknesses and active layer compositions produced by selective growth, the coupling coefficient κ of the diffraction grating is approximately By setting the duty ratios of the diffraction gratings to be equivalent, laser oscillation with uniform characteristics (low threshold) can be realized at different wavelengths that are precisely controlled in each waveguide structure.

In addition, it is possible to realize a semiconductor optical device that operates as a multi-wavelength laser with a single mode, high optical output and low threshold in a plurality of wide wavelength ranges, and can be applied to multi-wavelength lasers in WDM optical communication systems.

<Second Embodiment>
A semiconductor optical device according to a second embodiment of the present invention will be described with reference to FIGS. 11A and 11B.

<Structure of semiconductor optical device>
The semiconductor optical device 20 according to the present embodiment differs from the first embodiment in the mode of change of the diffraction grating coupling coefficient κ of the diffraction grating. Other configurations are substantially the same as those of the first embodiment.

In the first embodiment, the diffraction grating coupling coefficient κ is appropriately set by changing the duty ratio of the diffraction grating according to changes in the slab layer thickness and active layer composition in each waveguide structure. rice field.

In this embodiment, a similar effect can be obtained by changing the width of the diffraction grating while keeping the duty ratio constant. Here, the width of the diffraction grating is the length in the direction perpendicular to the waveguide direction of light on the horizontal plane of the waveguide structure.

Here, as in the first embodiment, the slab layer 24_2 of the other waveguide structure 23_2 is thicker than the slab layer 24_1 of the one waveguide structure 23_1. Also, the composition wavelength of the active layer 242_2 of the other waveguide structure 23_2 is longer than the composition wavelength of the active layer 242_1 of the one waveguide structure 23_1.

In the semiconductor optical device 20, as shown in FIGS. 11A and 11B, the width of the second upper clad layers 25_1 and 25_2 formed above the first upper clad layers 243_1 and 243_2 formed above the active layer is set to the optical is changed periodically in the waveguide direction.

Specifically, the widths of the first upper clad layers 243_1 and 243_2 are constant (dotted lines in FIGS. 11A and 11B).

On the other hand, the widths of the second upper cladding layers 25_1 and 25_2 change in a cycle consisting of the first widths W1_1 and W1_2 and the second widths 2_1 and W2_2.

The first widths W1_1 and W1_2 are equivalent to the widths of the first upper clad layers 243_1 and 243_2. The second widths W2_1, W2_2 are wider than the widths of the first upper cladding layers 243_1, 243_2 and are different for the second upper cladding layers 25_1, 25_2.

The second width W2_1 of the second upper clad layer 25_1 in one waveguide 23_1 is wider than the second width W2_2 of the second upper clad layer 25_2 in the other waveguide 23_2. Further, the duty ratio is constant in each of the waveguide structures 23_1 and 23_2.

Here, the first widths W1_1, W1_2 and the second widths W2_1, W2_2 may be set within the range of 500 nm to 3 μm.

With this configuration, in each waveguide structure, the coupling coefficient of the diffraction grating can be changed according to changes in the slab layer thickness and the composition of the active layer.

In the present embodiment, an example is shown in which the width of the uneven structure of the first upper clad layer is constant and the width of the second upper clad layer thereon is varied, but the present invention is not limited to this. The width of the concave-convex structure of the first upper clad layer may be changed, and the width of the second upper clad layer thereon may be kept constant.

Also, the first width may not be the same as that of the first upper clad layer.

<Third Embodiment>
A semiconductor optical device according to a third embodiment of the present invention will be described with reference to FIG.

<Structure of semiconductor optical device>
The semiconductor optical device according to this embodiment differs from the horizontal pin structure in the first and second embodiments in that it has a vertical pin structure. Other configurations are substantially the same as those of the first and second embodiments.

In the semiconductor optical device, as shown in FIG. 12, the first upper clad layers 343_1 and 343_2 are p-type InP and extend to the top of the buried layers 361_1 and 361_2. As a result, p-type InPs 343_1 and 343_2 are arranged on the buried layers 361_1 and 361_2, and electrodes 37_1 and 37_2 are arranged on the p-type InPs 343_1 and 343_2. Here, the buried layers 361_1 and 361_2 are semi-insulating InP.

On the other hand, the first lower clad layers 341_1 and 341_2 are n-type InP and extend to the bottom of the buried layers 362_1 and 362_2. As a result, n-type InPs 341_1 and 341_2 are arranged under the buried layers 362_1 and 362_2, and electrodes 37_1 and 37_2 are arranged on the n-type InPs 341_1 and 341_2. Here, the buried layers 362_1 and 362_2 are semi-insulating InP.

Thus, in the semiconductor optical device 30, the p-type InP (first upper clad layer) above the active layer, the active layer, and the n-type InP (first lower clad layer) below the active layer A pin structure is formed, current is injected from p-type InP to n-type InP, and a so-called vertical current injection laser is formed.

Also, the diffraction gratings 18_1 and 18_2 are formed on the surfaces of the first upper clad layers 343_1 and 343_2 and are embedded with the second upper clad layers 15_1 and 15_2.

According to the semiconductor optical device according to this embodiment, the same effects as those of the first and second embodiments are obtained.

Also, the present embodiment may be combined with the first and second embodiments. For example, after forming a rectangular periodic structure made of a dielectric (e.g., SiO ₂ ) on the surface of the first upper clad layer (p-type InP), a layer having a refractive index different from that of the first upper clad layer (eg, SiN) may be formed.

In addition, in the present embodiment, a structure in which p-type, i-type, and n-type semiconductors are formed in order from the upper clad layer to the lower clad layer is shown. A structure that becomes

<Fourth Embodiment>
A semiconductor optical device according to a fourth embodiment of the present invention will be described with reference to FIG.

<Structure of semiconductor optical device>
The semiconductor optical device 40 according to the present embodiment includes a core 49 below the slab layers 14_1 and 14_2 in the common lower clad layer (second lower clad layer) 12, as shown in FIG. Si is used for the core 49 . In addition, a semiconductor such as InP may be used.

According to the semiconductor optical device of the present embodiment, the same effects as those of the first or second embodiment are obtained, and the laser light emitted (oscillated) in the active layers 142_1 and 142_2 is guided by the core 49. be able to.

The core 49 may be arranged in the second upper clad layers 15_1 and 15_2. The core 49 may be arranged within a range where the laser light emitted (oscillated) in the active layers 142_1 and 142_2 can be coupled.

Also, in this embodiment, an example of using a channel-type waveguide in which the outer periphery of an isolated core is embedded is shown, but a rib-type waveguide may be used.

Also, in combination with the third embodiment, the second core may be arranged near the active layer having the vertical pin structure.

Also, in this embodiment, an example in which the diffraction grating is formed on the upper portion of the slab layer is shown, but the diffraction grating may be formed on the second core. Here, the diffraction grating may be formed on any of the core upper portion, the core bottom portion, and the core side portion of the second core.

In the embodiment of the present invention, an example of the structure, dimensions, materials, etc. of each component is shown in the structure, manufacturing method, etc. of the semiconductor optical device, but the present invention is not limited to this. Any material may be used as long as it exhibits the function of a semiconductor optical device and produces an effect.

The present invention can be applied to multi-wavelength lasers in wavelength division multiplexing (WDM) optical communication systems.

10 semiconductor optical device 11 substrate 12 second lower clad layers 13_1, 13_2 waveguide structures 141_1, 141_2 first lower clad layers 142_1, 142_2 active layers 143_1, 143_2 first upper clad layers 15_1, 15_2 second upper clad Layers 18_1, 18_2 diffraction grating

Claims

a plurality of waveguide structures over a second lower cladding layer on the same substrate;
The waveguide structure, in turn,
a first lower cladding layer;
an active layer;
a first upper cladding layer;
a second upper cladding layer;
the waveguide structure comprises a diffraction grating,
In each of the waveguide structures, the thicknesses of the first lower clad layer, the active layer, and the first upper clad layer are different,
A semiconductor optical device, wherein the coupling coefficients of the diffraction gratings are substantially the same.
2. The semiconductor optical device according to claim 1, wherein the diffraction grating is a concave-convex structure that is periodic in the waveguide direction on the surface of the first upper clad layer on the side of the second upper clad layer.
2. The diffraction grating according to claim 1, wherein the diffraction grating comprises structures arranged periodically in the waveguide direction between the first upper clad layer and the second upper clad layer. semiconductor optical devices.
The diffraction grating has periodic irregularities in the waveguide direction on part of side surfaces of the first lower clad layer, the active layer, the first upper clad layer, and the second upper clad layer. 2. The semiconductor optical device of claim 1, wherein the structure is a structure.
5. The semiconductor optical device according to claim 1, wherein the diffraction grating has a different duty ratio in each waveguide structure.
the width of either the first upper cladding layer or the second upper cladding layer changes in the waveguide direction at a predetermined period consisting of a first width and a second width;
4. The semiconductor optical device according to any one of claims 1 to 3, wherein the first widths are the same and the second widths are different in each of the waveguide structures.
7. A core is provided in at least one of the second lower cladding layer below the active layer and the second upper cladding layer above the active layer. The semiconductor optical device according to any one of Claims 1 to 3.
crystal-growing a semiconductor layer on the second lower clad layer on the same substrate;
forming a selective growth mask layer having a plurality of masks and a plurality of openings on the semiconductor layer;
crystal-growing a selective growth layer having a first lower clad layer, an active layer and a first upper clad layer in the opening;
processing the selectively grown layer into a mesa structure;
forming a current injection layer on the side of the mesa structure;
forming a diffraction grating on the top surface of the first upper cladding layer;
forming a second clad layer on the upper surface of the first upper clad layer having the diffraction grating;
In the selective growth mask layer, at least one of the area of the mask and the area of the opening changes,
A method of manufacturing a semiconductor optical device, wherein coupling coefficients of diffraction gratings formed on the upper surface of the first upper clad layer crystal-grown in the plurality of openings are substantially equal to each other.