Semiconductor optical device and method for manufacturing the same

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

[Technical field]

[0001] The present invention relates to a semiconductor optical device for use in a wavelength division multiplexing (WDM) optical communication system and a method for manufacturing the same. [Background technology]

[0002] In recent years, with the increase in communication capacity in data centers and other locations, wavelength division multiplexing (WDM) technology is becoming necessary for short-distance communications in data centers, etc. Since short-distance communications require a huge number of optical transceivers, WDM optical transceivers are required to be small, consume low power, and be able to be manufactured inexpensively.

[0003] Compound semiconductor optical devices that have a buried heterostructure formed in a stacked structure of multiple thin semiconductor layers have been developed as small, low-power-consumption heterogeneous integrated devices (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 small band gap is sandwiched vertically and horizontally between semiconductors with relatively low refractive index and large band gap. This structure makes it possible to improve the optical confinement coefficient in the active layer, which greatly contributes to various performance aspects of semiconductor devices.

[0004] In devices having a buried heterostructure within this thin film structure, a semiconductor multilayer structure with a thickness of about 250 nm to 500 nm is typically used, and power consumption can be reduced by reducing the volume of the semiconductor active layer.

[0005] In addition, by forming a diffraction grating with an arbitrary pitch on top of the active layer, it is possible to easily realize single mode operation at any wavelength required for WDM. In addition, a multiple quantum well (MQW) structure with excellent carrier coupling efficiency is used for the active layer. In order to apply an electric field and inject current into the device, a lateral pin structure is used in which the semiconductor layers on the left and right of the active layer are p-type and n-type.

[0006] Furthermore, to reduce the cost of transceivers, it is desirable to use a photonic integrated circuit (PIC) in which multiple devices are fabricated and integrated on a single wafer, rather than fabricating each device individually and then integrating them. By using a photonic integrated circuit, the cost of assembling the device can be significantly reduced.

[0007] Silicon photonics technology, which uses silicon (Si) microfabrication to create thin-line waveguides and passive devices, is widely used in the fabrication of optical integrated circuits. Silicon photonics applications are advantageous because the material is inexpensive and the microfabrication technology developed in the electronics field can be used. However, silicon is an indirect transition material, so highly efficient light-emitting devices have not yet been realized. Therefore, heterogeneous material integration with direct transition materials that can achieve highly efficient light emission is essential.

[0008] Therefore, according to the method of integrating a thin-film compound semiconductor device having the above-mentioned buried heterostructure on silicon, it is possible to fabricate a waveguide, a modulator, an arrayed waveguide grating (AWG), an optical switch, a photodetector, etc., fabricated by silicon photonics technology in the silicon layer below the active layer, thereby making it possible to fabricate an optical integrated circuit by silicon photonics integration.

[0009] To realize WDM technology using the above-mentioned optical integrated circuits, it is necessary to fabricate lasers that emit light at multiple wavelengths at the same time. In this case, it is required to precisely control the wavelength interval between the lasers and the absolute wavelength of the laser. For example, to comply with the 400GBASE-FR4 (CWDM grid) communication standard, the oscillation wavelengths required for the short-wave side and long-wave side lasers differ by 60 nm for four wavelengths and 140 nm for eight wavelengths, and the required accuracy of the absolute value of the oscillation wavelength of each laser 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 stricter, at ±1.0 nm. In addition, it is required to operate efficiently and at high speed over the entire wide wavelength range.

[0010] First, we will describe a method for fabricating a laser with a wide wavelength range. In order to obtain high-efficiency and high-speed operation in a wide wavelength range, it is necessary to match the material gain wavelength of the active layer with the oscillation wavelength. The full width at half maximum of the photoluminescence spectrum of the quantum well structure generally used as the active layer at room temperature is generally 30 to 40 meV or less, that is, 40 to 50 nm or less in the 1310 nm wavelength band. Therefore, when fabricating multiple directly modulated light sources with different operating wavelengths using a single active layer material, if the operating wavelength range is not sufficiently narrower than 40 to 50 nm, light sources with small material gain at the oscillation wavelength will be included, and high-efficiency and high-speed direct modulation operation will not be obtained with all light sources. In addition, if the operating wavelength range is significantly wider than 40 to 50 nm, it is difficult to obtain oscillation.

[0011] As a method for forming an active layer with multiple wavelengths in the range of 36 to 140 nm or more that conforms to the above-mentioned WDM standard, there is active layer selective growth. As shown in Fig. 14, when growing a III-V compound semiconductor by MOVPE, a III-V compound semiconductor 52 is grown on the surface of a substrate 53 by providing a III-V organic metal 51 such as trimethylindium (TMIn) in a vapor phase state toward a high-temperature semiconductor surface in an atmosphere of a V gas such as phosphine (PH3).

[0012] At this time, if a selective growth mask 54 made of SiO2, SiN or the like is formed in advance on the semiconductor surface, atoms are less likely to adhere to a dielectric surface than to a semiconductor surface, and therefore many of the group III atoms 51 supplied near the surface of mask 54 move horizontally across the mask surface relative to the substrate (surface migration and vapor phase diffusion), causing crystals to grow selectively on the semiconductor surface in the mask openings.

[0013] The probability (selectivity ratio) of atoms adhering to the semiconductor surface and the dielectric surface is extremely large, about 100 to 1000 times, and most of the Group III atoms supplied onto the mask do not adhere to the dielectric surface, but migrate to and adhere to the semiconductor surface.

[0014] At this time, the migration length on the mask surface differs depending on the type of atoms. By utilizing this, it becomes possible to grow a plurality of active layers 522_1, 522_2 with different film thicknesses and mixed crystal compositions in one epitaxial growth by changing the width and shape of the selective masks 54_1, 54_2 (Non-Patent Document 6). For example, as shown in FIG. 15, by changing the widths 54_1, 54_2 of the selective growth masks, a thicker crystal structure (cladding layer 521_2, active layer 522_2) 52_2 can be obtained near the wider mask 54_2.

[0015] Next, we will discuss the wavelength control method for semiconductor lasers. When precise control of the oscillation wavelength is required, a distributed feedback (DFB) laser is generally used. In a DFB laser, a diffraction grating structure is fabricated near the active layer, and the diffraction grating spacing p and the equivalent refractive index n eq It is possible to realize a single mode laser that oscillates near the Bragg wavelength λ determined by

[0016] Specific single mode laser configurations include a configuration in which a phase shift of 1 / 4 wavelength is provided in the center of the diffraction grating to oscillate at the center of the Bragg wavelength (λ / 4 shifted DFB laser), and a configuration in which an area with a different Bragg wavelength is provided at the end of the diffraction grating to use it as a Bragg reflector and selectively oscillate one end of the stop band (distributed reflector laser / DR laser), etc. One of the parameters that determine the characteristics of lasers using this diffraction grating is the product κL of the coupling coefficient κ of the diffraction grating and the diffraction grating length L.

[0017] Figure 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 the DFB and DBR was set to 2 nm. To stably oscillate the laser, it is important to reduce the threshold gain, and from this perspective, it is effective to increase κ. However, increasing κL causes non-uniformity in the light distribution in the resonator, which causes non-uniformity in the carrier density and equivalent refractive index, deteriorating the single mode characteristics (spatial hole burning).

[0018] Therefore, it is necessary to design the resonator so that a low threshold gain is achieved with κL within a range where spatial hole burning does not occur. κ is determined by the equivalent refractive index difference of the region forming the diffraction grating.

[0019] Therefore, as a method for changing the refractive index in a diffraction grating, there is a method for periodically changing the semiconductor structure itself by etching the semiconductor structure, or a method for placing SiO2, SiN, SiON, SiO x There is a method for periodically forming a dielectric structure that is transparent to the operating wavelength.

[0020] 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, so that lasers with the same wavelength and the same κL can be fabricated by forming a uniform diffraction grating.

[0021] Furthermore, even if lasers with different operating wavelengths are fabricated on one wafer, by changing only the diffraction grating pitch (period), it is possible to fabricate lasers with different oscillation wavelengths and substantially uniform κL. [Prior art documents] [Non-patent literature]

[0022] [Non-Patent Document 1] “Directly modulated buried heterostructure DFB laser on SiO2 / Si substrate fabricated by regrowth of InP using bonded active layer,” Opt. Express, vol. 22, no. 10, p. 12139, 2014, doi:10.1364 / oe.22.012139.

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[0023] However, when a multi-wavelength laser for WDM is fabricated using the semiconductor structure fabricated by the above-mentioned selective growth, each active layer structure has an individual active layer structure optimized for each laser operating wavelength, and the equivalent refractive index differs greatly for each active layer structure.

[0024] For example, selective growth can grow active layers with peak wavelengths that differ by about 150 nm in one go. In addition, to produce a wavelength change of about 40 nm, a film thickness change of about 35 nm is required, and if a wider wavelength change is required, the film thickness change amount must be increased (Non-Patent Document 5).

[0025] Figure 17 shows the calculation results of the slab thickness dependency of the equivalent refractive index. Here, the slab thickness is the thickness of the layer consisting of the lower cladding layer, the active layer, and the upper cladding layer (hereinafter referred to as the "slab layer") in the waveguide structure.

[0026] The calculations were performed using software (product name: "FIMMWAVE", sold by Convenient Business Solutions). Here, the active layer was located in the center of the slab layer, and the thickness of the active layer was changed in proportion to the slab thickness. InP was used for the upper and lower cladding layers, and an InGaAlAs multiple quantum well (MQW) was used for the active layer. The active layer widths were 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).

[0027] According to the calculation results, the equivalent refractive index change when the film thickness changes from about 250 nm to 350 nm is about 4%.

[0028] The diffraction grating coupling coefficient κ changes with this change in the equivalent refractive index. Figure 18 shows the calculation results for the dependence of κ on the active layer thickness. The calculation was performed in the same way as described above (Figure 17). Here, κ is shown when a diffraction grating with a depth of, for example, 20 nm is formed on the surface of the upper cladding layer (InP) of slab layers with different layer thicknesses. When the same diffraction grating is formed on layer structures with slab layer thicknesses ranging from 240 nm to 340 nm, κ is approximately 670 cm -1 From 400cm -1 It changes up to.

[0029] In this way, in a multi-wavelength semiconductor laser having multiple 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, if diffraction gratings of the same configuration are formed in each region which has a different layer thickness, composition and equivalent refractive index, it is impossible to make κ uniform (constant), and it is difficult to achieve single-mode characteristics with high optical output and low threshold voltage.

[0030] This is particularly problematic in thin-film devices, where light is confined by dielectrics such as air and glass above and below the InP thin-film structure, and the equivalent refractive index is highly dependent on the film thickness.

[0031] Possible methods for changing κ in a semiconductor thin-film structure include (1) changing the material that makes up the diffraction grating, (2) changing the etching depth when the diffraction grating is formed by etching, and (3) changing the thickness of the dielectric when a dielectric diffraction grating is formed on a semiconductor. However, it is difficult to realize any of these methods in a single process on a single wafer.

[0032] Furthermore, when a plurality of processes are performed, it is possible to change κ by performing etching a plurality of times with different etching times, but this leads to an increase in the number of steps and costs. [Means for solving the problem]

[0033] In order to solve the above-mentioned problems, a semiconductor optical device according to the present invention includes a plurality of waveguide structures on a second lower cladding layer on the same substrate, the waveguide structures including, in order, a first lower cladding layer, an active layer, a first upper cladding layer, and a second upper cladding layer, the waveguide structures including a diffraction grating, and in each of the waveguide structures, the first lower cladding layer, the active layer, and the first upper cladding layer have different thicknesses, and the coupling coefficients of the diffraction gratings are approximately equal. The width of either the first upper cladding layer or the second upper cladding layer changes in the waveguiding direction at a predetermined period consisting of a first width and a second width, and in each of the waveguide structures, the first width is equal and the second width is different. It is characterized by the above. Effect of the Invention

[0035] According to the present invention, it is possible to provide a semiconductor optical device that operates as a single-mode, high-output, low-threshold, multi-wavelength laser, and a method for manufacturing the same. [Brief description of the drawings]

[0036] [Figure 1] FIG. 1 is a schematic bird's-eye view showing the configuration of a semiconductor optical device according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic front sectional view showing the configuration of a semiconductor optical device according to the first embodiment of the present invention. [Figure 3A] FIG. 3A is a schematic side cross-sectional view showing the configuration of a semiconductor optical device according to a first embodiment of the present invention. [Figure 3B] FIG. 3B is a schematic side cross-sectional view showing the configuration of the semiconductor optical device according to the first embodiment of the present invention. [Figure 4] FIG. 4 is a diagram for explaining the operation of the semiconductor optical device according to the first embodiment of the present invention. [Figure 5A] 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 present invention. [Figure 5B] FIG. 5B 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 present invention. [Figure 6] FIG. 6 is a diagram for explaining a method for manufacturing a semiconductor optical device according to the first embodiment of the present invention. [Figure 7A] FIG. 7A is a diagram for explaining the operation of the semiconductor optical device according to the first embodiment of the present invention. [Figure 7B] FIG. 7B is a diagram for explaining the operation of the semiconductor optical device according to the first embodiment of the present invention. [Figure 8A] FIG. 8A is a schematic top cross-sectional view showing an example of the configuration of the semiconductor optical device according to the first embodiment of the present invention. [Figure 8B]FIG. 8B is a schematic top cross-sectional view showing an example of the configuration of the semiconductor optical device according to the first embodiment of the present invention. [Figure 9] FIG. 9 is a diagram for explaining the effects of the semiconductor optical device according to the first embodiment of the present invention. [Figure 10] FIG. 10 is a diagram for explaining the effects of the semiconductor optical device according to the first embodiment of the present invention. [Figure 11A] FIG. 11A is a schematic top cross-sectional view showing the configuration of a semiconductor optical device according to a second embodiment of the present invention. [Figure 11B] FIG. 11B is a schematic top cross-sectional view showing the configuration of a semiconductor optical device according to the second embodiment of the present invention. [Figure 12] FIG. 12 is a schematic front sectional view showing the configuration of a semiconductor optical device according to the third embodiment of the present invention. [Figure 13] FIG. 13 is a schematic front sectional view showing the configuration of a semiconductor optical device according to the fourth embodiment of the present invention. [Figure 14] FIG. 14 is a diagram for explaining selective growth used for growing crystals in a conventional semiconductor optical device. [Figure 15] FIG. 15 is a diagram for explaining selective growth used for growing crystals in a conventional semiconductor optical device. [Figure 16] FIG. 16 is a diagram for explaining the operation of a conventional semiconductor optical device. [Figure 17] FIG. 17 is a diagram for explaining the operation of a conventional semiconductor optical device. [Figure 18] FIG. 18 is a diagram for explaining the operation of a conventional semiconductor optical device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] <First embodiment> A semiconductor optical device and a method for manufacturing the same according to a first embodiment of the present invention will be described with reference to FIGS.

[0038] <Configuration of semiconductor optical device> As shown in FIGS. 1 and 2, a semiconductor optical device 10 according to this embodiment includes, in order, a substrate 11, a common lower cladding layer (second lower cladding layer) 12, and a plurality of (two in this embodiment) waveguide structures 13_1 and 13_2.

[0039] The substrate 11 is made of Si, but may be made of other materials such as Si, SiO2, Al2O3, InP, GaAs, and SiC.

[0040] The common lower clad layer 12 is made of SiO2. x , SiN, SiON, Al2O3, or a structure combining these may be used, and it is sufficient that the refractive index is lower than that of the semiconductor layer and that the layer is transparent to the oscillation wavelength (for example, 300 nm to 1650 nm) of the laser to be produced.

[0041] The waveguide structures 13_1 and 13_2 each include a first lower clad layer 141_1, 141_2, an active layer 142_1, 142_2, and a first upper clad layer 143_1, 143_2 as a slab layer 14_1, 14_2, respectively, and further include a second upper clad layer 15_1, 15_2 on the slab layer 14_1, 14_2.

[0042] The first lower cladding layers 141_1 and 141_2 and the first upper cladding layers 143_1 and 143_2 are made of InP.

[0043] The active layers 142_1 and 142_2 are made of six layers of InAlGaAs in the 1310 nm wavelength band.

[0044] In the two waveguide structures, the slab layer 14_1 of one waveguide structure 13_1 may have a thickness of 315 nm, an active layer 142_1 may have 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 may have a thickness of 350 nm, an active layer 142_2 may have a composition wavelength of 1275 nm, and a laser oscillation wavelength of 1310 nm. The width of the active layers 142_1 and 142_2 of each waveguide structure is, for example, 0.7 μm.

[0045] Thus, in the semiconductor optical device 10, the active layers 142_1, 142_2 of the waveguide structures 13_1, 13_2 have different compositions and different thicknesses of the slab layers.

[0046] 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 bandgap wavelength of the active layer 142_2 of the other waveguide structure 13_2 is longer than the bandgap wavelength of the active layer 142_1 of the one waveguide structure 13_1.

[0047] 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 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, enabling electrical drive by pin junctions. When a current is injected through the electrodes, oscillation is obtained at a wavelength determined by the diffraction grating spacing and the equivalent refractive index of the buried heterostructure.

[0048] In each of the waveguide structures 13_1 and 13_2, as shown in Figures 3A and 3B, the diffraction gratings 18_1 and 18_2 have a periodic uneven structure formed on the surface of the first upper clad layer 143_1 and 143_2 in the light guiding direction. The second upper clad layers 15_1 and 15_2 are laminated on the diffraction gratings 18_1 and 18_2. Here, the light guiding direction is perpendicular to the end face of the waveguide structure.

[0049] Here, the second upper clad layers 15_1 and 15_2 are made of SiO2, and the other layers are made of SiO2, SiO x , SiN, SiON, and Al2O3 may also be used.

[0050] In this manner, a diffraction grating is formed at the boundary between the first upper cladding layers (InP) 143_1, 143_2 and the second upper cladding layers (SiO2) 15_1, 15_2.

[0051] In this embodiment, an example has been shown in which the diffraction grating is formed by etching the first upper cladding layer and then embedded with SiO2 or the like. However, the present invention is not limited to this, and a refractive index difference may be used by using an air layer as the second upper cladding layer without embedding the diffraction grating.

[0052] In each of the waveguide structures 13_1 and 13_2, the diffraction gratings 18_1 and 18_2 have a period 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 of the protrusions of the diffraction grating to the length T of one period of the diffraction grating, and takes a value between 0 and 1.

[0053] In one waveguide structure (waveguide structure with a thinner slab layer) 13_1, as shown in FIG. 3A, the length τ occupied by the convex portion of diffraction grating 18_1 is large, and the duty ratio is large.

[0054] In the other waveguide structure (waveguide structure having a thicker slab layer) 13_2, as shown in FIG. 3B, the length τ occupied by the convex portion of the diffraction grating 18_2 is small, and the duty ratio is small.

[0055] Here, the length T of one period of the diffraction grating is equal in each of the waveguide structures 13_1 and 13_2.

[0056] Figure 4 shows the calculation results of the duty ratio dependency of the coupling coefficient κ of the diffraction grating. The calculation was performed using the transfer matrix method. The figure also shows κ when a diffraction grating with a depth of, for example, 20 nm is formed on the surface of the upper cladding layer (InP) of the slab layer. The slab layer consists of a lower InP cladding layer (160 nm thick), an InGaAlAs-MQW (100 nm thick), and an upper InP cladding layer (70 nm thick).

[0057] In this way, even if the depth of the diffraction grating is the same, the κ can be changed by changing the duty ratio. Therefore, diffraction gratings with the same κ can be formed for multiple semiconductor waveguide structures with different equivalent refractive indices formed on the same substrate.

[0058] The duty ratio of the diffraction grating can be changed, for example, by changing the duty ratio of the mask used during etching of the diffraction grating. Since the shape of the mask can be determined arbitrarily by photolithography or electron beam lithography, diffraction gratings with different duty ratios can be easily formed simultaneously on the same substrate surface.

[0059] In this manner, 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 duty ratio of the other waveguide structure (the waveguide structure with the thicker slab layer) 13_2 is decreased, so that the coupling coefficients of the respective waveguide structures 13_1, 13_2 are set to be approximately equal.

[0060] Furthermore, as shown in Figures 5A and 5B, the diffraction grating of the semiconductor optical device may include rectangular structures 181_1 and 181_2 made of, for example, SiO2, which are periodically formed on the surfaces of first upper clad layers 143_1 and 143_2 in the light guide direction, and second upper clad layers (SiN) 15_1 and 15_2 on the first upper clad layers having the SiO2 periodic structure.

[0061] This diffraction grating (structure) is formed by laminating a layer (third upper cladding layer) made of, for example, SiO2 on the flat surface of the first upper cladding layer without processing the surface, and then periodically processing the SiO2 layer. After that, a second upper cladding layer (e.g., SiN) is laminated on the first upper cladding layer and the processed SiO2. In this case, too, it functions as a diffraction grating due to the difference in refractive index between, for example, SiN and SiO2.

[0062] The combination of the structure (third upper cladding layer) and the second upper cladding layer is not limited to SiO2 and SiN, but may be other SiO x Alternatively, a dielectric material such as SiON, Al2O3, etc. may be used. The structure (third upper cladding layer) and the second upper cladding layer may be made of materials having different refractive indices.

[0063] Alternatively, the structure (third upper cladding layer) may not be embedded, and the second upper cladding layer may be made an air layer, resulting in a refractive index difference.

[0064] <Method of manufacturing semiconductor optical devices> A method for manufacturing a semiconductor optical device according to this embodiment will be described with reference to FIGS. 6 to 7B.

[0065] First, a semiconductor layer 144 is formed on a substrate 11 on which a common lower cladding layer 12 is formed (step S1). Si is used as the substrate material. Other materials that may be used include SiO2, Al2O3, InP, GaAs, and SiC.

[0066] SiO2 is used for the common lower cladding layer 12. Alternatively, SiN, SiC, or a combination of these may be used, provided that the refractive index is lower than that of the semiconductor layer and that the layer is transparent to the oscillation wavelength (for example, 300 nm to 1650 nm) of the laser to be fabricated.

[0067] The semiconductor layer 144 is formed by direct wafer bonding or by crystal growth such as metal organic chemical vapor deposition (MOVPE) or molecular beam epitaxy (MBE).

[0068] Next, a selective growth mask layer 191 made of SiO2, SiN or the like is formed on the semiconductor layer 144 (step S2).

[0069] 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 selective growth layers 140_1 and 140_2 have, in order from the common lower cladding layer 12 side, a first lower cladding layer, an active layer, and a first upper cladding layer.

[0070] In selective growth, group III atoms supplied near the mask surface move horizontally across the mask surface relative 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 one to which atoms are less likely to adhere than the semiconductor surface.

[0071] In the selective growth mask layer 191, the width of the mask region in the mask varies from 0 to 55 μm, and the distance between the masks (opening width) is constant at 40 μm.

[0072] In this embodiment, an example has been shown in which the mask width is changed and the opening width is kept constant, but the opening width, or both the mask width and the opening width, may be changed.

[0073] Here, the wider the mask width, i.e., the area of ​​the mask, and the narrower the opening width, i.e., the area of ​​the opening, the more pronounced the change in crystal composition and the acceleration of the growth rate due to selective growth.

[0074] In this manner, in the selective growth mask layer, the selective growth film thickness can be changed by varying at least one of the area of ​​the mask and the area of ​​the opening within the mask layer, and a thicker crystal layer can be obtained with a mask having a larger area or an opening having a smaller area.

[0075] In the present embodiment, an example has been shown in which the mask region width is 0 to 55 μm and the mask-to-mask distance is 40 μm, but the present invention is not limited to this and may be about 500 nm to 500 μm, respectively, as long as it is equal to or greater than the surface migration length of Group III atoms.

[0076] The material and structure of the active layer formed by selective growth may be, for example, in the case of an InP system, a multiple quantum well structure using a mixed crystal made of InGaAs, InP, InGaAsP or InGaAlAs.

[0077] When a quantum well structure is selectively grown, in addition to the wavelength change due to the change in the mixed crystal composition, the emission wavelength change of the active layer caused by the change in the quantum well layer thickness can be used.

[0078] In addition, by adopting a quantum well structure in the active layer, it is possible to apply a very large strain (about 1.5%) to the well layer without generating crystal defects. This allows the gain coefficient to be increased, making it suitable for high efficiency and high speed operation of directly modulated lasers. Even when a bulk alloy is selectively grown instead of a multiple quantum well structure, it is possible to obtain an active layer that emits light at different wavelengths by changing the alloy composition.

[0079] In addition, by forming the semiconductor structure by selective growth as a thin film structure of about several hundred nm, it is possible to improve the optical confinement in the active layer. For example, Figures 7A and 7B show the calculation results of the dependence of the optical confinement in the active layer on the film thickness (slab thickness) of the entire InP-based thin film. The calculations were performed using software (product name: "FIMMWAVE", sold by Convenient Business Solutions).

[0080] For the calculations, a structure was used in which the slab layer was sandwiched between low-refractive index materials (SiO2, refractive index n = 1.47) from above and below, as shown in Figure 7A. The slab layer consisted of a multiple quantum well structure (MQW, n = 3.40) with a thickness of approximately 100 nm 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.

[0081] As shown in Figure 7B, the optical confinement coefficient in the active layer is maximum when the slab thickness (total thickness of the InP-based thin film) is about 150 nm, which is about three times higher than the optical confinement coefficient of a typical laser with a thickness of 3000 to 4000 nm. Furthermore, as the slab thickness increases, the film thickness dependency of the optical confinement coefficient decreases. Therefore, from the viewpoint of the optical confinement coefficient, it is effective to keep the slab thickness at about 500 nm or less.

[0082] After the selective growth is performed, a thin film (e.g., SiO2, SiN, etc.) to be used as a mask for buried growth is formed after or without removing the selective growth mask layer 191. The material of the thin film may be the same as or different from that of the selective growth mask layer.

[0083] Next, a mask pattern made of resist or the like is formed on the thin film used as a mask for burying growth above the active layer by lithography or the like, and a mask layer for burying growth 192 is formed by dry etching or the like.

[0084] Next, the slab layer is selectively etched using the buried growth mask layer 192. The etching may be dry etching, wet etching, or a combination of these. By this etching, the selective growth layers 140_1 and 140_2 are processed to the active layer width (mesa structure width, for example, 0.7 μm) in a semiconductor optical device (semiconductor laser). As a result, slab layers 14_1 and 14_2 including first lower cladding layers 141_1 and 141_2, active layers 142_1 and 142_2, and first upper cladding layers 143_1 and 143_2 are formed.

[0085] Next, buried semiconductors 160_1 and 160_2 are formed beside the active layer by MOCVD, MBE or the like, and are then planarized (step S4).

[0086] The burying growth mask layer 192 is removed. If the semiconductor surface is slightly uneven, the surface may be further planarized by regrowing a small amount of semiconductor layer after removing the burying growth mask layer 192.

[0087] Next, a part of the semiconductor layers 162_1, 162_2 is made n-type by, for example, Si ion implantation, and another part of the semiconductor layers 161_1, 161_2 is made p-type by, for example, Zn thermal diffusion. In this manner, the current injection layers 161_1, 161_2, 162_1, 162_2 are formed. Furthermore, electrodes 17_1, 17_2 are formed so as to contact the p-type semiconductor layers 161_1, 161_2 and the n-type semiconductor layers 162_1, 162_2, thereby enabling electrical drive (step S5). Note that a contact semiconductor layer may be formed between the p-type semiconductor layer and the electrode or between the n-type semiconductor layer and the electrode to obtain ohmic current-voltage characteristics.

[0088] Next, the upper surfaces of the first upper cladding layers 143_1 and 143_2 are partially etched to form diffraction gratings 18_1 and 18_2 in the light guide direction (direction perpendicular to the end face of the waveguide structure) (step S6). x After forming a third upper cladding layer made of SiN, SiON, Al2O3 or the like, the third upper cladding layer is partially etched to form diffraction gratings (structures) 181_1 and 181_2.

[0089] Here, in the diffraction gratings 18_1, 18_2 (or 181_1, 181_2), the duty ratio of one slab layer (thin slab layer) 14_1 is increased and the duty ratio of the other waveguide structure (waveguide structure with a thicker slab layer) 14_2 is decreased, so that the coupling coefficients of the respective slab layers 14_1, 14_2 are set to be approximately equal.

[0090] Finally, second upper cladding layers 15_1 and 15_2 having a refractive index smaller than that of the semiconductor are formed (step S7).

[0091] In this embodiment, an example is shown in which a waveguide structure having two active layers and slab layers with different thicknesses is provided on the same substrate, but this is not limiting, and three or more waveguide structures may be provided. For example, when applying to an eight-wavelength WDM, a waveguide structure having eight active layers and slab layers with different thicknesses and diffraction gratings with appropriate duty ratios corresponding to each layer may be provided. In addition, there is no problem even if the arrangement of the p-type semiconductor layer and the n-type semiconductor layer is reversed left to right, and it is not necessary for all channels to be oriented in the same direction.

[0092] In this embodiment, an example in which the diffraction grating is formed on the upper part of the slab layer has been shown, but the diffraction grating may be formed in another region within the range in which the guided mode exists. For example, as shown in Figures 8A and 8B, the diffraction grating may be formed on the side (side surface) of the waveguide structures 13_3 and 13_4. Figure 8A shows a diffraction grating with a large duty ratio, and Figure 8B shows a diffraction grating with a small duty ratio.

[0093] In this case, a diffraction grating may be formed on the side of the waveguide structure in a step immediately prior to the step (step S4) of forming the buried semiconductor. The mask for forming the diffraction grating in the active layer can be easily formed in any and a plurality of different shapes on the same substrate by using photolithography or electron beam lithography, and diffraction gratings with different duty ratios can be formed by a single etching. Here, the mask for forming the diffraction grating is formed on the first upper cladding layer, and the diffraction gratings can be formed on the side (side surface) of the waveguide structures 13_3 and 13_4 by etching using this mask. Thereafter, the mask for forming the diffraction grating may be used as is for buried growth.

[0094] Furthermore, the diffraction grating may be formed on a part of the side (side surface) of the waveguide structure, and may be formed on a part of the side (side surface) of the first lower cladding layer, the active layer, the first upper cladding layer, and the second upper cladding layer.

[0095] <Effects> The effects of the semiconductor optical device according to this embodiment will be described with reference to FIGS.

[0096] The calculation results of the slab thickness dependency of κ are shown in Figure 9. As an example, κ is shown when a diffraction grating with a depth of 20 nm and duty ratios of 0.5, 0.4, 0.3, and 0.25 is formed.

[0097] The calculations were performed using the transfer matrix method. The calculations were performed for a waveguide layer structure that includes a slab layer consisting of an upper InP cladding layer, an InGaAlAs-MQW active layer, and a lower InP cladding layer, and SiO2 layers above and below the slab layer. The diffraction grating was assumed to have a rectangular shape and a width sufficiently wide for the cross-sectional mode field of the light. The slab thickness was changed from 240 nm to 340 nm. The layer thickness of the active layer was changed in proportion to the slab thickness.

[0098] When a diffraction grating with a duty ratio of 0.5 is formed, κ is about 670 cm -1 From 400cm -1On the other hand, when a diffraction grating with a duty ratio of 0.25 is formed, κ changes to about 370 cm -1 From 220cm -1 (The black circle with a dashed line in the figure.)

[0099] Therefore, for example, 400 cm for all film thicknesses (slab thicknesses) -1 A diffraction grating with a κ of about 100 nm can be easily formed by setting the duty ratio of the diffraction grating to a value in the range of about 0.25 to 0.5 according to the film thickness (slab thickness). In detail, the coupling coefficient κ of the diffraction grating can be kept constant by increasing the duty ratio of the diffraction grating as the film thickness (slab thickness) increases.

[0100] Next, the effect of the semiconductor optical device according to this embodiment on the threshold gain will be described.

[0101] 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.

[0102] For comparison, the film thickness dependence of 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.

[0103] The calculations were performed using the transfer matrix method.

[0104] The waveguide structure used in the calculation is the same as that described above (calculation in FIG. 17). The slab thickness was changed from 240 nm to 340 nm. The resonator was 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 was set to 2 nm.

[0105] It was assumed that the grating depth in the conventional structure was 20 nm, the duty ratio was 0.5, and the grating width was sufficiently wide relative to the width of the cross-sectional mode field.

[0106] On the other hand, in the semiconductor optical device according to the present embodiment, in each waveguide structure, κ=400 cm -1 The calculations were carried out assuming that it was constant.

[0107] As shown in FIG. 10, in the conventional structure, when a diffraction grating with a duty ratio of 0.5 is formed for slab layers of different thicknesses, the threshold gain is about 14.4 cm -1 From 30.1 cm -1 The change is approximately two-fold or more (black dashed square in the figure).

[0108] On the other hand, in the semiconductor optical device according to the present embodiment, for the slab layers having different film thicknesses, κ=400 cm -1 When the diffraction grating is formed so that the threshold gain is constant, a substantially constant threshold gain can be obtained regardless of the film thickness (white circle solid line in the figure).

[0109] Here, the diffraction gratings should be formed so that κ is approximately equal for slab layers with different thicknesses. Approximately equal includes perfect equality, a range including a difference of about ±30%, and a range in which each waveguide structure operates as a single mode laser with high optical output and low threshold.

[0110] 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 fabricated by selective growth, by setting the duty ratios of the diffraction gratings so that the coupling coefficients κ of the diffraction gratings are substantially equal, it is possible to realize laser oscillation having uniform characteristics (low threshold value) at different wavelengths precisely controlled in each waveguide structure.

[0111] It is also possible to realize a semiconductor optical device that operates as a multi-wavelength laser in a single mode with high optical output and low threshold value over a wide range of wavelengths, and this device can be applied to multi-wavelength lasers in WDM optical communication systems.

[0112] <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.

[0113] <Configuration of semiconductor optical device> The semiconductor optical device 20 according to this embodiment differs from the first embodiment in the manner in which the diffraction grating coupling coefficient κ of the diffraction grating changes, but the other configurations are substantially similar to those of the first embodiment.

[0114] In the first embodiment, the diffraction grating coupling coefficient κ is appropriately set by changing the duty ratio of the diffraction grating in response to changes in the slab layer thickness and the active layer composition in each waveguide structure.

[0115] In this embodiment, a similar effect can be obtained by varying the width of the diffraction grating while keeping the duty ratio constant, where the width of the diffraction grating is the length in the horizontal plane of the waveguide structure perpendicular to the guiding direction of light.

[0116] Here, similarly to the first embodiment, the slab layer 24_2 of the other waveguide structure 23_2 is thicker than the slab layer 24_1 of one waveguide structure 23_1. Also, the bandgap wavelength of the active layer 242_2 of the other waveguide structure 23_2 is longer than the bandgap wavelength of the active layer 242_1 of the one waveguide structure 23_1.

[0117] As shown in FIGS. 11A and 11B, in the semiconductor optical device 20, the widths of second upper cladding layers 25_1 and 25_2 formed on first upper cladding layers 243_1 and 243_2 formed on the active layer are periodically changed in the light guide direction.

[0118] In detail, the widths of the first upper cladding layers 243_1 and 243_2 are constant (dotted lines in FIGS. 11A and 11B).

[0119] 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 W2_1 and W2_2.

[0120] The first widths W1_1 and W1_2 are equal to the widths of the first upper cladding layers 243_1 and 243_2. The second widths W2_1 and W2_2 are greater than the widths of the first upper cladding layers 243_1 and 243_2 and are different for the second upper cladding layers 25_1 and 25_2.

[0121] The second width W2_1 of the second upper cladding layer 25_1 in one waveguide 23_1 is wider than the second width W2_2 of the second upper cladding layer 25_2 in the other waveguide 23_2. Moreover, the duty ratio is constant in each of the waveguide structures 23_1 and 23_2.

[0122] Here, the first widths W1_1, W1_2 and the second widths W2_1, W2_2 may be set in the range of 500 nm to 3 μm.

[0123] This configuration allows the coupling coefficient of the diffraction grating to be changed in each waveguide structure by varying the slab layer thickness and the active layer composition.

[0124] In the present embodiment, an example has been shown in which the width of the relief structure of the first upper cladding layer is constant and the width of the second upper cladding layer thereon is variable, but this is not limiting. The width of the relief structure of the first upper cladding layer may be varied and the width of the second upper cladding layer thereon may be constant.

[0125] Also, the first width does not have to be equal to the first upper cladding layer.

[0126] <Third embodiment> A semiconductor optical device according to a third embodiment of the present invention will be described with reference to FIG.

[0127] <Configuration of semiconductor optical device> The semiconductor optical device according to the present embodiment differs from the horizontal pin structure of the first and second embodiments in that it has a vertical pin structure, while the other configurations are substantially similar to those of the first and second embodiments.

[0128] In the semiconductor optical device, as shown in Fig. 12, the first upper cladding layers 343_1, 343_2 are p-type InP and extend to the tops of the buried layers 361_1, 361_2. As a result, the p-type InP 343_1, 343_2 are disposed on the buried layers 361_1, 361_2, and the electrodes 37_1, 37_2 are disposed on the p-type InP 343_1, 343_2. Here, the buried layers 361_1, 361_2 are semi-insulating InP.

[0129] On the other hand, the first lower cladding 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, the n-type InP 341_1 and 341_2 are disposed under the buried layers 362_1 and 362_2, and the electrodes 37_1 and 37_2 are disposed on the n-type InP 341_1 and 341_2. Here, the buried layers 362_1 and 362_2 are semi-insulating InP.

[0130] Thus, in the semiconductor optical device 30, a pin structure is formed by the p-type InP (first upper cladding layer) above the active layer, the active layer, and the n-type InP (first lower cladding layer) below the active layer, and current is injected from the p-type InP to the n-type InP, forming a so-called vertical current injection laser.

[0131] Moreover, the diffraction gratings 18_1 and 18_2 are formed on the surfaces of the first upper cladding layers 343_1 and 343_2 and are buried in the second upper cladding layers 15_1 and 15_2.

[0132] The semiconductor optical device according to the present embodiment provides the same effects as the first and second embodiments.

[0133] This embodiment may be combined with the first and second embodiments. For example, after forming a rectangular periodic structure made of a dielectric material (e.g., SiO2) on the surface of the first upper cladding layer (p-type InP), a layer (e.g., SiN) having a refractive index different from that of the first upper cladding layer may be formed.

[0134] In addition, in this embodiment, a structure in which the layers are p-type, i-type, and n-type semiconductors from the upper cladding layer to the lower cladding layer is shown, but the structure may be reversed, that is, n-type, i-type, and p-type semiconductors.

[0135] <Fourth embodiment> A semiconductor optical device according to a fourth embodiment of the present invention will be described with reference to FIG.

[0136] <Configuration of semiconductor optical device> 13, a semiconductor optical device 40 according to this embodiment includes a core 49 below the slab layers 14_1 and 14_2 in a common lower cladding layer (second lower cladding layer) 12. The core 49 is made of Si. Alternatively, it may be made of a semiconductor such as InP.

[0137] The semiconductor optical device according to the present embodiment has the same effects as those of the first or second embodiment, and can guide the laser light emitted (oscillated) in the active layers 142_1 and 142_2 in the core 49.

[0138] The core 49 may be disposed in the second upper cladding layers 15_1 and 15_2. The core 49 may be disposed within a range in which the laser light emitted (oscillated) in the active layers 142_1 and 142_2 can be coupled.

[0139] In addition, in this embodiment, an example has been shown in which a channel-type waveguide in which the outer periphery of an isolated core is embedded is used, but a rib-type waveguide may also be used.

[0140] In addition, in combination with the third embodiment, a second core may be disposed in the vicinity of an active layer having a vertical pin structure.

[0141] In the present embodiment, an example in which the diffraction grating is formed on the top of the slab layer has been shown, but the diffraction grating may be formed on the second core. Here, the diffraction grating may be formed on any of the top, bottom, and side of the second core.

[0142] In the embodiments of the present invention, examples of the structure, dimensions, materials, etc. of each component in the configuration and manufacturing method of the semiconductor optical device are shown, but the present invention is not limited to these. Anything that can exhibit the functions and effects of the semiconductor optical device can be used. [Industrial Applicability]

[0143] The present invention can be applied to a multi-wavelength laser in a wavelength division multiplexing (WDM) optical communication system. [Explanation of symbols]

[0144] 10 Semiconductor optical devices 11 Substrate 12 Second Lower Cladding Layer 13_1, 13_2 Waveguide structure 141_1, 141_2 First lower cladding layer 142_1, 142_2 active layer 143_1, 143_2 First upper cladding layer 15_1, 15_2 Second upper cladding layer 18_1, 18_2 Diffraction grating

Claims

1. a plurality of waveguide structures on 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; Equipped with the waveguide structure comprises a diffraction grating; In each of the waveguide structures, the first lower cladding layer, the active layer, and the first upper cladding layer have different thicknesses, The coupling coefficients of the diffraction gratings are approximately equal, a width of either the first upper cladding layer or the second upper cladding layer changes in a waveguiding direction at a predetermined period consisting of a first width and a second width; In each of the waveguide structures, the first width is equal and the second width is different. A semiconductor optical device comprising:

2. The diffraction grating has a periodic uneven structure in a waveguiding direction on a surface of the first upper cladding layer facing the second upper cladding layer.

2. The semiconductor optical device according to claim 1 .

3. The diffraction grating includes structures that are periodically arranged in the waveguiding direction between the first upper cladding layer and the second upper cladding layer.

2. The semiconductor optical device according to claim 1 .

4. The diffraction grating has a periodic uneven structure in a waveguiding direction on part of the side surfaces of the first lower cladding layer, the active layer, the first upper cladding layer, and the second upper cladding layer.

2. The semiconductor optical device according to claim 1 .

5. The duty ratio of the diffraction grating is different in each of the waveguide structures.

5. The semiconductor optical device according to claim 1, wherein the first and second electrodes are arranged parallel to each other.

6. A core is provided below the active layer in the second lower cladding layer and / or above the active layer in the second upper cladding layer.

6. The semiconductor optical device according to claim 1, wherein the first and second electrodes are arranged parallel to each other.

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