JP7402014B2 - Optical semiconductor elements, optical semiconductor devices - Google Patents

Description

The present invention relates to an optical semiconductor element and an optical semiconductor device.

The Internet continues to develop as an infrastructure of modern society. Optical communication, which has excellent high-speed and long-distance communication, makes up the majority of Internet communications, and with the continuous increase in Internet traffic, increasing the communication capacity has become an urgent issue. Optical transceivers are used in this optical communication, and are required to be faster, smaller, and more power efficient.

Optical transceivers have excellent high speed, small size, and power saving characteristics, and use semiconductor lasers that can output wavelengths compatible with optical fibers as internal light sources. In general, a direct modulation method is widely applied to a transmission light source for optical communications using a semiconductor laser, in which the injected current is largely modulated to digitally modulate the light intensity.

Japanese Patent Application Publication No. 2013-165133 JP2018-56212A Japanese Patent Application Publication No. 2018-93002

As the speed of optical transceivers has increased in recent years, there has been a demand for semiconductor lasers that can achieve even higher speeds. For this purpose, it is effective to increase the relaxation oscillation frequency. It is known that in a semiconductor laser having a quantum well, the relaxation oscillation frequency is proportional to the square root of the differential gain, and that the gain changes linearly with respect to the natural logarithm of the carrier density. Therefore, in order to increase the gain, it is necessary to increase the carrier density, but on the other hand, when the carrier density increases, the differential gain decreases and the relaxation oscillation frequency decreases.

Patent Documents 1 and 2 disclose buried-type lasers in which a part of a mesa stripe is buried in a semiconductor laser having an active layer of InGaAlAs. In the semiconductor lasers disclosed in Patent Documents 1 and 2, a mesa stripe is formed on an InP semiconductor substrate, and the mesa stripe has an InP cladding on top, a diffraction grating, an optical confinement layer (SCH layer), and a multiple quantum well below it. layer (MQW layer). In Patent Document 1, InP buried layers are arranged on both sides of a semiconductor multilayer below the upper InP cladding. Patent Document 2 discloses a structure in which the upper cladding layer and the diffraction grating layer are not buried, and the upper surface of the buried layer is disposed between the MQW layer and the diffraction grating layer. Patent Document 3 discloses an array semiconductor optical device having a plurality of unburied mesa structures. In light of the above-mentioned viewpoint of speeding up, it is necessary to improve optical confinement in order to achieve higher speed than the structures disclosed in the above-mentioned cited documents 1 to 3.

The present invention has been made in view of the above problems, and an object thereof is to provide an optical semiconductor element with a high relaxation oscillation frequency or an optical semiconductor element and an optical semiconductor device with excellent extinction characteristics.

(1) In order to solve the above problems, a semiconductor optical device that emits or absorbs light includes a lower structure including a multiple quantum well layer, an upper mesa structure disposed on the lower structure, and a semiconductor optical device that emits or absorbs light. a current injection structure disposed in the upper mesa structure, and when viewed from the optical axis of the emitted or absorbed light, the horizontal axis of the portion of the current injection structure that is in contact with the upper mesa structure is the upper mesa structure. , a portion of the current injection structure in contact with the upper mesa structure is made of InP, and the average refractive index of the upper mesa structure is higher than the refractive index of the InP of the current injection structure; The semiconductor optical device is characterized in that the semiconductor optical device further includes an insulating film that contacts and covers both side surfaces and a part of the upper surface of the upper mesa structure.

(2) The optical semiconductor device according to (1) above, which is configured to emit light, wherein the upper mesa structure has a diffraction grating layer, and the lower structure and the upper mesa structure are arranged to emit light. The method is characterized in that it further includes a buried semiconductor layer forming a mesa structure and burying both side surfaces of the lower structure.

(3) The optical semiconductor device according to (1) above, which is configured to absorb light, and wherein the lower structure is provided above the multiwell layer and has a bandgap larger than the bandgap of the multiwell layer. The upper mesa structure has a lower optical confinement layer having a large bandgap, and the upper mesa structure has an upper optical confinement layer provided above the lower optical confinement layer and has a bandgap larger than that of the multi-well layer. The method further includes a buried semiconductor layer that buries both side surfaces of the lower structure.

(4) The optical semiconductor device according to any one of (1) to (3) above, wherein the current injection structure is narrower than the width of the upper mesa structure by 0.05 μm or more.

(5) The optical semiconductor device according to (4) above, wherein the current injection structure has a width in a range of 0.1 μm or more and 0.7 μm or less.

(6) The optical semiconductor device according to (5) above, characterized in that the height of the current injection structure is less than 1 μm.

(7) The optical semiconductor device according to any one of (1) to (6) above, characterized by comprising at least two or more of the current injection layers.

(8) The optical semiconductor device according to (1) above, characterized in that the multiple quantum well layer is a layer made of multiple elements including Al element.

(9) The optical semiconductor device according to (2) above, wherein the upper mesa structure has an optical confinement layer having a higher refractive index than the current injection structure.

(10) The optical semiconductor device according to (9) above, wherein the optical confinement layer is made of InGaAsP.

(11) The optical semiconductor device according to (3) above is characterized in that the upper optical confinement layer and the lower optical confinement layer are made of InGaAsP.

(12) The optical semiconductor device according to (2) above, wherein the diffraction grating layer is made of InGaAsP.

(13) The optical semiconductor device of the present application is characterized by having the optical semiconductor element described in (3) above and a semiconductor laser integrally integrated with the optical semiconductor element.

According to the present invention, it is possible to provide an optical semiconductor device and an optical semiconductor element that have a high relaxation oscillation frequency and therefore a high modulation frequency band.

1 is a diagram showing a schematic configuration of an optical semiconductor device according to a first embodiment of the present invention. FIG. 2 is a sectional view taken along the section indicated by A in FIG. 1; 2 is a schematic vertical cross-sectional view parallel to the resonator direction of the semiconductor optical device shown in FIG. 1B. FIG. FIG. 1 is a cross-sectional view of an optical semiconductor device according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view of an optical semiconductor device according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view of an optical semiconductor device according to a first embodiment of the present invention. FIG. 3 is a cross-sectional view of an optical semiconductor device according to a second embodiment of the present invention. FIG. 3 is a cross-sectional view of an optical semiconductor device according to a second embodiment of the present invention. FIG. 2 is a schematic vertical cross-sectional view of the optical semiconductor device shown in B of FIG. 1 according to a third embodiment of the present invention. FIG. 3 is a cross-sectional view of an optical semiconductor device according to a third embodiment of the present invention. FIG. 3 is a cross-sectional view of an optical semiconductor device according to a third embodiment of the present invention. These are calculation results of Γ _QW /W _m, Γ _QW, and κ based on Patent Document 2 and the present invention.

Embodiments of the present invention will be described below with reference to the drawings.

[First embodiment]
FIG. 1 is a diagram showing a schematic configuration of an optical semiconductor device 1 according to a first embodiment of the present invention. The optical semiconductor device shown in FIG. 1 is a semiconductor laser, and oscillates laser light 3 from an oscillation region 2 by applying a voltage to two electrodes provided on opposing surfaces of a rectangular parallelepiped.

FIG. 2 is a sectional view taken along the section indicated by A in FIG. FIG. 2 is a schematic diagram of a cross section perpendicular to the optical axis of a semiconductor laser that oscillates in the 1.3 μm band of a transmission light source for optical fiber communication. The optical semiconductor device 1 includes a p-type InP semiconductor substrate 101, a p-type InP buffer layer 102 serving as a lower cladding, a p-type InGaAlAs-optical confinement (SCH) layer 103, a p-type InAlAs electron stop layer 104, and an InGaAlAs-multiple quantum well ( MQW) layer 105 and n-type InGaAlAs-SCH layer 106 are laminated in this order. The p-type InP buffer layer 102 to the n-type InGaAlAs-SCH layer 106 have structures such as doping and composition of a normal semiconductor laser. Furthermore, immediately above 106, an n-type InP layer 107 with a thickness of 2 nm, an n-type InGaAsP diffraction grating layer 108 with a thickness of 60 nm, an n-type InP layer 109 with a thickness of 40 nm, and an n-type InGaAsP layer 110 with a thickness of 80 nm are laminated in this order. has been done. The doping concentrations of the n-type semiconductor layers 107 to 110 are all 1×10 ¹⁸ cm ^-3 . The diffraction grating layer 108 has a diffraction grating structure in the direction perpendicular to the plane of the paper, and the cross section indicated by B in FIG. A lattice layer 108 is present periodically. Although FIG. 3 shows a completely periodic structure, in reality, a λ/4 shift structure is introduced to achieve single mode oscillation with a high yield. The optical semiconductor element 1 is a DFB type semiconductor laser. In FIG. 3, 120 is a dielectric low reflection coating film, and 121 is a dielectric high reflection coating film.

A part of 102 and 103 to 106 form a mesa structure (hereinafter referred to as a lower mesa structure) in which both sides are buried with InP buried layers 112. The InP buried layer has a multilayer structure of a semi-insulating InP layer formed by forming an n-type InP thin film on a layer in contact with the p-type InP buffer layer 102 and then doping Fe or Ru having high resistance. In addition, it may be a p-type semiconductor layer, a multilayer film consisting of a p-type semiconductor layer and an n-type semiconductor layer, or a multilayer film consisting of a high-resistance semiconductor layer, a p-type semiconductor layer, and an n-type semiconductor layer. Both sides of the elements 107 to 110 are covered with an SiO2 insulating film 114. 107 to 110 form an upper cladding layer 117, and inside the semiconductor laser, in the optical axis direction from which laser light is emitted, the upper cladding layer 117, the SCH layers 103 and 106, the MQW layer 105, and InP functioning as the lower cladding layer are formed. A waveguide is formed by the combination of buffer layers 102, and light is guided. On the n-type InGaAsP layer 110, an n-type InP current injection layer 111 having a width narrower than 110 is provided. Since the width is narrower than 110, some light seeps into 111, but 111 does not guide the wave. Therefore, it is usually sufficient for the height of the current injection layer to be 0.2 μm or more. Of course, the thickness may be higher than 0.2 μm depending on process issues and the like. However, if the thickness is increased by 1 μm or more, there is a concern that the element resistance will increase, so it is preferably less than 1 μm, particularly 0.6 μm or less. Since the left and right upper portions of the upper cladding layer 117 are covered with SiO2 having a low refractive index, light can be almost confined to the region below the upper cladding 117. An n-type InGaAsP contact layer 113 with a doping concentration of 1.5×10 ¹⁹ cm ⁻³ is placed directly above the current injection layer 111 and is in ohmic contact with the n-type electrode 115 . The electrode 115 may be a normal multilayer electrode for ohmic contact, and in this embodiment Ti/Pt/Au is used. A p-type electrode 116 having a multilayer structure of AuZn alloy and Ti/Pt/Au is provided under the p-type InP substrate. If 116 is also a normal p-type electrode, other configurations may be used.

The width Wa of the lower mesa structure is approximately 0.9 μm. Due to the process involved, the actual cross-sectional shape of the mesa is not a perfect rectangle, but has a slightly curved profile. The width of the upper cladding layer is also approximately 0.9 μm. The widths of these mesas may vary depending on the range in which light is guided. The width of the current injection layer 111 is approximately 0.5 μm, which is set narrower than the mesa width described above. Furthermore, since no light is guided, it is not necessary that the horizontal center of 111 coincides with the horizontal center of the mesas (102 to 106, 107 to 110) below.

Further, in this embodiment, the SiO ₂ insulating film 114 and the n-type electrode 115 are in contact with each other, but an insulating film such as polyimide may be inserted partially or completely between them to reduce the capacitance of the element.

Here, the structure of the optical semiconductor element according to the first embodiment shown in FIG. 2 and the structure of Patent Document 2 will be compared. In a semiconductor laser, there is a relationship between the intrinsic frequency band f _{3 dB} , which ignores parasitic capacitance, etc., and the relaxation oscillation frequency f _r , f _{3 dB} = 1.55 f _r , so it is important to improve the relaxation oscillation frequency f _r to increase speed. . In a semiconductor laser with a quantum well layer as an active layer, the relaxation oscillation frequency f _r is determined by the optical confinement coefficient Γ _QW per quantum well, the width W _a of the lower mesa structure, the differential gain dg/dn of the quantum well layer, and the drive current I _m The following relationship exists between (=operating current - threshold current).

In addition, semiconductor laser oscillation is the sum of the light generated from the quantum well layer, the optical loss α _i of the waveguide inside the semiconductor laser, and the mirror loss α _m that is a loss inside the laser by emitting the laser light to the outside, that is, g An oscillation state occurs when _m = α _i + α _m is balanced. Assuming that the optical gain within the semiconductor laser per unit length is g, the oscillation conditions are expressed as follows.

Here, N _W is the number of quantum wells. Further, the optical gain g is expressed by the following formula.

Here, n is the carrier density injected into the quantum well, n _tr is the carrier density at which the optical gain g becomes 0, and a is a constant. From equation (1), it is considered that a laser structure in which Γ _QW /W _a increases is desirable, and it is more desirable to decrease W _a . However, if W _a is decreased, Γ _QW is decreased, and it is necessary to increase the optical gain g in the oscillation condition equation (2). In order to increase g, it is necessary to inject more carriers n than in equation (3), but as can be seen from the equation, it is an upwardly convex function as a function of g and n, and its derivative, that is, the differential gain Since dg/dn decreases due to an increase in carrier density, f _r in equation (1) decreases. Therefore, a structure in which both Γ _QW /W _a and Γ _QW are large is required. Further, when the mirror loss α _m is large, it is also a factor that reduces the differential gain as described above, so it is desirable that α _m be low. In a DFB laser, the larger the optical coupling coefficient κ of the diffraction grating, the smaller the mirror loss α _m becomes.

FIG. 12 shows the calculation results of Examples 1 and 3 of Patent Document 2 and the present example of the present invention regarding the dependence of Γ _QW and κ on Γ _QW /W _a , and the calculation results are obtained by changing the active layer width W _a . are doing. In FIG. 12, the white marker is Γ _QW on the left axis, the black marker is the value of κ, the square marker is Example 1 of Patent Document 2, the triangular marker is Example 3 of Patent Document 2, and the round marker is the bookmark. These are calculation results based on this example of the patent. As can be seen from FIG. 12, in Example 1 of Patent Document 2, Γ _QW and Γ _QW /W _a are relatively high values, but κ is small. Conversely, in Example 3 of Patent Document 2, κ is large, but Γ _QW and Γ _QW /W _a are low. In contrast, in this embodiment of the present patent, Γ _QW , Γ _QW /W _a , and κ all have high values. This is due to the following reason. That is, in this configuration, since light hardly spreads to the current injection layer 111 disposed above the upper cladding layer 117, the light is confined in a limited area. Therefore, both Γ _QW and Γ _QW /W _a become large. Furthermore, since the diffraction grating 108 is present in the upper cladding layer 117 in which light is confined, κ also becomes large. In Patent Document 2, an InP cladding layer having the same width as the diffraction grating layer is arranged above the diffraction grating layer that is not embedded with InP. Therefore, the light spreads to this InP cladding, and the light cannot be sufficiently confined in the layer below the diffraction grating layer. On the other hand, in this embodiment, the InP layer on the upper cladding layer including the diffraction grating layer is a layer provided only for current injection, and its function as a cladding layer for confining light is very small. This is due to the effect of narrowing the mesa width compared to the upper cladding layer. This structure can sufficiently confine light within the structure below the upper cladding layer and improve the optical confinement rate at the same mesa width. Also contributing to this effect is that the average refractive index of the upper cladding layer 117 is higher than the refractive index of InP, which is the main element of the current injection layer 111. In particular, by including the n-type InGaAsP layer 110 that functions as a light confinement layer in the upper cladding layer 117, light can be more effectively focused on the upper cladding layer 117 side. As a result, a semiconductor laser with excellent high-speed response can be realized. Furthermore, by burying the sides of the MQW layer with semiconductors, it is possible to reduce leakage current on the sides of the mesa and to isolate the MQW layer from the outside air, resulting in excellent reliability.

Reflecting the excellent leakage current blocking characteristics according to the first embodiment, the optical semiconductor device 1 has a resonator length of 150 μm and has an antireflection coating of 0.3% or less on the front end surface and a high reflection film on 95% of the rear end surface. The slope of the relaxation oscillation frequency fr with respect to the square root of the drive current was 6.6 GHz/mA ^1/2 and 4.8 GHz/mA ^1/2 at 25° C. and 85° C., respectively, and excellent characteristics were obtained. Furthermore, the estimated life time at 85° C. was 3.2×10 ⁵ hours, indicating high reliability.

In this embodiment, the width W _a of the lower mesa structure is set to 0.9 μm, but it is sufficient as long as Γ _QW and Γ _QW /W _a are in a high range, and preferably between 0.4 and 1.4 μm. The upper cladding layer is desirably 0.4 μm or more and Wa-0.2 μm to Wa+0.05 μm, taking into account process variations and improvement in the values of Γ _QW and κ. From the viewpoint of confining light in the upper cladding layer, the current injection layer is preferably at least 0.05 μm narrower than the upper cladding layer width, and preferably 0.1 μm or more and less than 0.7 μm.

Similar results can be obtained even if the upper cladding layer 117 of this embodiment is composed of an n-type InP layer 107 and an n-type InGaAsP diffraction grating layer 108 as shown in FIG. Also in this configuration, the average refractive index of the upper cladding layer 117 is higher than that of InP of the current injection layer 111. In FIG. 4, a polyimide organic insulating film is inserted between the n-electrode 115 and the SiO ₂ insulating film 114, reducing the capacitance of the entire device. In addition, as shown in FIG. 5, the upper cladding layer 117 is formed of an n-type InP layer 107, an n-type InGaAsP layer 118, an n-type InP layer 119, an n-type InGaAsP diffraction grating layer 108, an n-type InP layer 109, and an n-type InGaAsP layer 110. A similar effect can be obtained even with a multilayer structure.

Furthermore, since the current injection layer on top of the upper cladding layer 117 makes almost no contribution to the waveguide of light, the current injection layer 111 may be divided into two as shown in FIG. Similarly, the same effect can be obtained by using three or more current injection layers. This configuration makes it possible to reduce element resistance.

In this embodiment, we have described a semiconductor laser that is formed on a p-type InP substrate and in which the semiconductor layer above the MQW layer is composed of an n-type semiconductor. A semiconductor laser structure in which all n-type semiconductors are reversed also works. In this case, the current injection layer 111 is made of a p-type semiconductor with high resistivity, but since light is not guided, the height can be reduced, and an increase in resistance can be suppressed.

Furthermore, a similar effect can be obtained in a semiconductor laser in which a semi-insulating substrate in which an InP substrate is doped with Fe or the like is used and the InP buffer layer 102 is doped with p-type or n-type. Furthermore, although the embodiment describes a single semiconductor laser element, similar effects can be obtained even when applied to an array-type optical semiconductor device in which a plurality of optical semiconductor elements 1 are arranged side by side on an InP semiconductor substrate. .

Although not shown in the drawings of this embodiment, an isolation mesa groove may be provided in a region 0.5 μm or more away from the end of the lower mesa structure in order to reduce capacitance caused by the buried layer.

[Second embodiment]
FIG. 7 is a cross-sectional view of an optical semiconductor device according to a second embodiment of the present invention. FIG. 8 is a schematic diagram of a cross section perpendicular to the optical axis of an optical semiconductor element that oscillates in the 1.3 μm band of a transmission light source for optical fiber communication. The optical semiconductor device according to the second embodiment has an n-type InGaAlAs-SCH layer 203, a p-type InAlAs layer 204, an InGaAlAs-MQW layer 205, a p-type InGaAlAs-SCH layer 206, and a p-type InGaAlAs layer on an n-type InP semiconductor substrate 201. An etch stop layer 202, a p-type InP layer 207, a p-type InGaAsP diffraction grating layer 208, a p-type InP layer 209, and a p-type InGaAsP layer 210 are laminated in this order. The n-type InGaAlAs-SCH layer 203 to the p-type diffraction grating layer 208 have the doping, composition, etc. of a normal semiconductor laser. The diffraction grating layer 208 has a diffraction grating structure in a direction perpendicular to the paper plane of FIG.

An upper cladding layer 217 is formed from the p-type InP layer 207 to the p-type InGaAsP layer 210, and the layers below 207 constitute a lower structure that spreads laterally around the upper cladding layer. The lower structure and the upper cladding layer 217 constitute a ridge structure. A p-type InP current injection layer 211 having a width narrower than the upper cladding layer 217 is provided on the p-type InGaAsP layer 210 . Although the width is narrower than the upper cladding layer 217 and some light seeps out, it does not guide waves. Therefore, it is usually sufficient for the height of the current injection layer to be 0.2 μm or more. It may be higher than this value depending on the process and other issues. In this embodiment, the height of 211 is set to 0.5 μm. Since the left and right upper portions of the upper cladding layer 217 are covered with a SiO2 insulating film 214 having a low refractive index, the light guided within the laser structure can be confined in the upper cladding layer 217. A p-type InGaAs contact layer 213 with a doping concentration of 2×10 ¹⁹ cm ⁻³ is provided directly above the current injection layer 211 and is in ohmic contact with the p-type electrode 215 . The p-type electrode 215 may be an ordinary ohmic contact electrode, and in this embodiment Ti/Pt/Au is used. Electrical connection is made using an AuGe-based ohmic contact electrode under the n-type InP substrate.

The width of the upper cladding layer 217 is 1.0 μm. The width of the current injection layer 211 is narrower than the upper cladding layer, and is 0.5 μm in this embodiment. Since light is not guided, the center of the upper cladding layer 217 and the center of the current injection layer 211 do not necessarily have to coincide. In this embodiment, the width of the current injection layer 211 is 0.5 μm, but from the viewpoint of confining light in the upper cladding layer, the current injection layer is narrower than the width of the upper cladding layer by at least 0.05 μm and 0.1 μm or more and 0.7 μm. A range below is desirable.

In this embodiment, the upper cladding layer has a p-type InGaAsP layer 210 laminated above the diffraction grating. The InGaAsP layer 210 has a higher refractive index than the p-type InP current injection layer 211, and has a high light confinement rate. That is, the average refractive index of the upper cladding layer 217 is greater than the refractive index of the p-type InP current injection layer 211. Therefore, leakage of the guided light to the p-type InP current injection layer 211 is sufficiently suppressed, and with the added effect that the upper cladding layer is covered with the SiO2 insulating film 214, the light is strongly confined to the upper cladding layer side. becomes possible. As a result, a semiconductor laser with excellent high frequency characteristics can be realized.

In this embodiment, κ and Γ _QW are the same compared to the conventional ridge type laser, but Γ _QW /W _a increases by 10% to 20%, and the relaxation oscillation frequency fr improves by that amount, and the frequency band also increases. It increased. Although not shown in FIG. 7, an isolation groove for reducing the capacitance may be provided at a position 2 μm or more away from the edge of the upper cladding layer 217.

In the optical semiconductor device according to the second embodiment, a 0.1% or less anti-reflection coating was applied to the front end face, and a 95% high reflection coating was applied to the rear end face. Further, the resonator length is 140 μm, and the diffraction grating structure has an equivalent λ/4 shift at a position 40 μm from the rear end surface. The optical semiconductor device according to the second embodiment had threshold currents of 7.3 mA and 15.1 mA at 25° C. and 85° C., respectively, which were low threshold currents for a ridge type laser. The characteristic temperature of the threshold current was 82K, which was a good value.

The slope efficiency was good at 0.28 W/A and 0.21 W/A at 25° C. and 85° C., respectively. Further, the slopes of the relaxation oscillation frequency fr with respect to the square root of the drive current were 5.2 GHz/mA ^1/2 and 3.9 GHz/mA ^1/2 at 25° C. and 85° C., respectively, and excellent characteristics were obtained. Furthermore, the estimated life time at 85° C. was 1.9×10 ⁵ hours, indicating high reliability.

Similar results can be obtained even if the upper cladding layer 217 of this embodiment is composed of a p-type InP layer 207 and a p-type InGaAsP diffraction grating layer 208 as shown in FIG.

[Third embodiment]
9, 10, and 11 are cross-sectional views of an optical semiconductor device according to a third embodiment of the present invention. While the first and second embodiments described above employ a direct modulation method (that is, a current injection structure with a narrower width than the underlying mesa structure is employed in the semiconductor laser), this embodiment The current injection structure described above is also applied to a semiconductor electroabsorption field absorption modulator. More specifically, this embodiment is an optical semiconductor device in which a semiconductor laser that oscillates in the 1.3 μm band of a transmission light source for optical fiber communication and an electroabsorption modulator are integrated. FIG. 9 is a schematic diagram of a cross section parallel to the optical axis. In FIG. 9, the multilayer on the left side of the drawing is a semiconductor laser region, the multilayer on the right side of the drawing is an electroabsorption modulator, and the area near the center is a waveguide layer that optically connects the semiconductor laser and the electroabsorption modulator. FIG. 10 shows a cross-sectional view of the semiconductor laser region perpendicular to the optical axis. In FIG. 10, the semiconductor laser is formed on an n-type InP substrate 301, with an n-type InGaAsP-SCH layer 303, an InGaAsP-MQW layer 305, a p-type InGaAsP-SCH layer 306, a p-type InP layer 307, a p-type InGaAsP diffraction grating layer 308, and a A type InP layer 309 and a p-type InGaAsP layer 310 are sequentially laminated. The n-type InGaAsP-SCH layer 303 to the p-type InGaAsP diffraction grating layer 308 have the doping, composition, film thickness, diffraction grating structure, etc. of a normal semiconductor laser. A part of the n-type InP substrate 301, the n-type InGaAsP-SCH layer 303, the InGaAsP-MQW layer 305, and the p-type InGaAsP-SCH layer 306 form a mesa structure with a width of 1.0 μm, and the left and right sides are made of high-resistance Fe or It is buried with an InP layer 312 doped with Ru. The p-type InGaAsP layer 310 has a doping concentration of 1×10 18 cm −3 and a thickness of 70 nm. As shown in FIG. 9, the diffraction grating layer 308 is periodically present in the p-type InP layers 307 and 309. Although FIG. 9 shows a schematically uniform periodic structure, single mode oscillation is actually achieved with a high yield by introducing a λ/4 shift structure or the like.

The p-type InP layer 307 to the p-type InGaAsP layer 310 form an upper cladding layer 317 having a mesa structure with a width of 1.0 μm, and the left, right, and upper portions are covered with an SiO ₂ insulating film 314 . A current injection layer 311 having a width of 0.5 μm, which is narrower than the upper cladding layer 317, is formed on the p-type InGaAsP layer 310. Since the width of the current injection layer 311 is narrower than the upper cladding layer 317, some light leaks into the current injection layer 311, but the light is not guided in the direction of the resonator in the current injection layer 311. Light is guided in the direction of the cavity by the combination of the upper cladding layer 317, the SCH layers 306 and 303, the MQW layer 305, and the n-type InP substrate 301 functioning as the lower cladding layer. A p-type InGaAs contact layer 313 is provided on the current injection layer 311 and is ohmically connected to the p-type electrode 315. Since the current injection layer 311 does not guide light, its height may be low, and a height of 0.2 μm or more is sufficient. Depending on process issues, etc., the thickness may be higher than 0.2 μm. Furthermore, since light is not guided, the center of the current injection layer 311 does not need to coincide with the center of the mesa structure 317 below.

FIG. 11 shows a cross-sectional view perpendicular to the optical axis of the electro-absorption modulator having the multilayer structure on the right side of FIG. In FIG. 11, on an n-type InP substrate 301, an EA part n-type InGaAsP-SCH layer 303', an EA part InGaAsP-MQW layer 305', an EA part p-type InGaAsP-SCH layer (lower optical confinement layer) 306', a p-type InP A layer 307 and a p-type InGaAsP layer (upper optical confinement layer) 310 are laminated in this order. The EA part p-type InGaAsP-SCH layer 306' and the p-type InGaAsP layer 310 have a larger band gap than the EA part InGaAsP-MQW layer 305'. That is, the EA part p-type InGaAsP-SCH layer 306' and the p-type InGaAsP layer 310 have a smaller refractive index than the EA part InGaAsP-MQW layer 305'. The EA part n-type InGaAsP-SCH layer 303' to the EA part p-type InGaAsP-SCH layer 306' have the doping, composition, and film thickness configuration of a normal electroabsorption modulator. The p-type InP layer 307 and the p-type InGaAsP layer 310 have the same structure as the semiconductor laser region on the left side of FIG. A part of the n-type Inp substrate 301, the EA part n-type InGaAsP-SCH layer 303', the EA part InGaAsP-MQW layer 305', and the EA part p-type InGaAsP-SCH layer 306' form a mesa structure with a width of 1.0 μm. The left and right sides are filled with InP layers 312 doped with high resistance Fe or Ru. This InP layer 312 has the same structure as the semiconductor laser region on the left side of FIG.

Similar to the semiconductor laser region, the p-type InP layer 307 to the p-type InGaAsP layer 310 form an upper cladding layer 317 with a mesa structure with a width of 1.0 μm, and the left, right, and upper portions are covered with an SiO ₂ insulating film 314. It is being said. A current injection layer 311' having a width of 0.5 μm and narrower than the upper cladding layer 317 is formed on the p-type InGaAsP layer 310. Since the width of the current injection layer 311' is narrower than the upper cladding layer 317, some light leaks into the current injection layer 311', but no light is guided in the direction of the resonator in the current injection layer 311'. A waveguide is formed by the combination of the upper cladding layer 317, the SCH layers 303' and 306', the MQW layer 305', and the n-type InP substrate 301 functioning as the lower cladding layer, and the light is directed in the optical axis direction within the electroabsorption modulator. waveguide. A p-type InGaAs contact layer 313' is provided on the current injection layer 311' and is ohmically connected to the p-type electrode 315'. Since the current injection layer 311' does not guide light, its height may be low, and a height of 0.2 μm or more is sufficient. In the region of the electroabsorption modulator, a polyimide film 322 is inserted between the SiO2 insulating film 314 and the p-type electrode 315' to reduce the capacitance.

Near the center in FIG. 9, a waveguide layer (WG section) that optically connects the semiconductor laser and the electro-absorption modulator has a structure similar to that of the electro-absorption modulator. A cross-sectional structure perpendicular to the optical axis is omitted. It has a mesa structure with a width of 1.0 μm consisting of a lower WG InGaAsP-SCH layer 323, a WG part InGaAsP core layer 324, and an upper WG part InGaAsP-SCH layer 325, and the left and right sides of this mesa structure are doped with Fe or Ru. It is filled with an InP layer 312. The upper cladding layer is composed of a p-type InP layer 307 and a p-type InGaAsP layer 310 similarly to the electro-absorption modulator region, and has a mesa structure with a width of 1.0 μm. A SiO2 insulating film is provided on the side of the mesa structure of the upper cladding. There is no contact layer or electrode in the waveguide layer because there is no need to pass current through it. Further, in order to increase the electrical resistance of the semiconductor laser region and the electroabsorption modulator, the thickness of the current injection layer 311' is set as low as 0.1 μm.

In this embodiment, the optical confinement coefficient of the MQW 305' is about 5% to about 15% higher than that of an ordinary electroabsorption modulator with a buried structure, so that light can be modulated with a lower modulation voltage amplitude. Alternatively, a high extinction ratio can be obtained with a shorter modulator length. This is because the upper cladding layer 317 includes the InGaAsP upper optical confinement layer 310 and has a higher average refractive index than the current injection layer 311', and the width of the current injection layer 311' is narrower than the width of the upper cladding layer 317. This is due to this. Furthermore, since the optical confinement coefficient of a semiconductor laser increases, laser oscillation becomes possible with a low threshold current.

The optical semiconductor device in which the semiconductor laser and the electro-absorption modulator are integrated according to the third embodiment has an anti-reflection coating of 0.1% or less applied to the front end surface on the side where the electro-absorption modulator is installed. A 90% high reflection coating was applied to the rear end surface on the side where the semiconductor laser was installed. The resonator length of the semiconductor laser is 300 μm, and the diffraction grating structure is shifted by λ/4 at a position 35 μm from the rear end face. The semiconductor laser region according to the third embodiment had low threshold currents of 6.7 mA and 14.2 mA at 25° C. and 85° C., respectively.

The modulator length of the electroabsorption modulator is 70 μm, which can be shortened by about 20% to about 50% compared to the conventional technology. , the capacitance of the pin diode made up of the EA part p-type InGaAsP-SCH layer 306' can be reduced. In this embodiment, the overall capacitance of the electroabsorption modulator including the electrodes could be reduced to 0.13 pF. Reflecting this low capacitance, an optical semiconductor device with an integrated electro-absorption modulator was able to obtain a high band of 64 GHz. Furthermore, the estimated life time at 85° C. for both the semiconductor laser region and the modulator region was 2.8×10 ⁵ hours, indicating high reliability.

In this embodiment, the MQW layer and the SCH layer in the semiconductor laser region and the electroabsorption modulator region are used as the InGaAsP semiconductor layer, but it goes without saying that the same effect can be obtained by using an InGaAlAs semiconductor layer.

In the present invention, a single semiconductor laser and an optical device that integrates a semiconductor laser and an electro-absorption modulator have been described. It goes without saying that an integrated MZ modulator integrated semiconductor laser can be made in a similar manner.

1 semiconductor laser, 2 light emitting region, 3 laser light, 101 p-type InP substrate, 102 p-type InP buffer layer (lower cladding layer), 103 p-type InGaAlAs-SCH layer, 104 p-type InAlAs electron stop layer, 105 InGaAlAs-MQW layer, 106 n-type InGaAlAs-SCH layer, 107 n-type InP layer, 108 n-type InGaAsP grating layer, 109 n-type InP layer, 110 n-type InGaAsP layer, 111 n-type InP current injection layer, 112 high-resistance InP buried layer , 113 n-type InGaAsP contact layer, 114 SiO ₂ insulating film, 115 n-type electrode, 116 p-type electrode, 117 upper cladding layer, 118 n-type InGaAsP layer, 119 n-type InP layer, 120 dielectric anti-reflection coating film, 121 Dielectric high reflection coating film, 122 polyimide film, 201 n-type InP substrate, 202 p-type InGaAlAs etch stop layer, 203 n-type InGaAlAs-SCH layer, 204 p-type InAlAs layer, 205 InGaAlAs-MQW layer, 206 p-type InGaAlAs-SCH layer, 207 p-type InP layer, 208 p-type InGaAsP grating layer, 209 p-type InP layer, 210 p-type InGaAsP layer, 211 p-type InP current injection layer, 213 p-type InGaAs contact layer, 215 p-type electrode, 216 n type electrode, 217 upper cladding layer, 301 n-type InP substrate, 303 n-type InGaAsP-SCH layer, 305 InGaAsP-MQW layer, 306 p-type InGaAsP-SCH layer, 307 p-type InP layer, 308 p-type InGaAsP grating layer, 309 p-type InP layer, 310 p-type InGaAsP layer, 311 p-type current injection layer, 313 p-type InGaAs contact layer, 315 p-type electrode, 303' EA part n-type InGaAsP-SCH layer, 305' EA part InGaAsP-MQW layer , 306' EA part p-type InGaAsP-SCH layer, 323 WG lower InGaAsP-SCH layer, 324 WG part InGaAsP core layer, 325 WG part upper InGaAsP-SCH layer, 312 high resistance (semi-insulating) InP layer, 317 upper part cladding layer.

Claims (13)

A semiconductor optical device that emits or absorbs light,
a substructure including a multi-quantum well layer;
an upper mesa structure disposed on the lower structure and serving as an upper cladding layer ;
a current injection structure disposed on the upper mesa structure,
When viewed from the optical axis of the emitted or absorbed light, the width of a portion of the current injection structure in contact with the upper mesa structure is smaller than the width of the upper mesa structure,
A portion of the current injection structure in contact with the upper mesa structure is made of InP,
The average refractive index of the upper mesa structure is higher than the refractive index of the InP of the current injection structure,
The semiconductor optical device further includes an insulating film that contacts and covers both side surfaces and a part of the top surface of the upper mesa structure.
An optical semiconductor device characterized by: The optical semiconductor device according to claim 1,
configured to emit light;
the upper mesa structure has a diffraction grating layer;
The lower structure constitutes one mesa structure together with the upper mesa structure,
further comprising a buried semiconductor layer burying both sides of the lower structure;
An optical semiconductor device characterized by: The optical semiconductor device according to claim 1,
configured to absorb light;
The lower structure has a lower optical confinement layer provided above the multiple quantum well layer and having a band gap larger than the band gap of the multiple quantum well layer,
The upper mesa structure has an upper optical confinement layer provided above the lower optical confinement layer and having a band gap larger than that of the multiple quantum well layer,
further comprising a buried semiconductor layer burying both sides of the lower structure;
An optical semiconductor device characterized by: The optical semiconductor device according to any one of claims 1 to 3,
the current injection structure is narrower than the width of the upper mesa structure by 0.05 μm or more;
An optical semiconductor device characterized by: The optical semiconductor device according to claim 4,
The current injection structure has a width in a range of 0.1 μm or more and 0.7 μm or less,
An optical semiconductor device characterized by: The optical semiconductor device according to claim 5,
The height of the current injection structure is less than 1 μm.
An optical semiconductor device characterized by: The optical semiconductor device according to any one of claims 1 to 6,
comprising at least two or more of the current injection structures ;
An optical semiconductor device characterized by: The optical semiconductor device according to claim 1,
The multi-quantum well layer is a layer made of multiple elements including Al element,
An optical semiconductor device characterized by: The optical semiconductor device according to claim 2,
The upper mesa structure further includes an optical confinement layer having a higher refractive index than the current injection structure.
An optical semiconductor device characterized by: The optical semiconductor device according to claim 9,
The optical confinement layer is made of InGaAsP.
An optical semiconductor device characterized by: The optical semiconductor device according to claim 3,
The upper optical confinement layer and the lower optical confinement layer are made of InGaAsP.
An optical semiconductor device characterized by: The light according to claim 2 is a conductive element,
The diffraction grating layer is made of InGaAsP.
An optical semiconductor device characterized by: The optical semiconductor device according to claim 3;
comprising a semiconductor laser integrally integrated with the optical semiconductor element;
Optical semiconductor device.

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