Optical semiconductor device

Abstract

This optical semiconductor device (100) is provided with: a semiconductor laser unit (70) and an optical modulator unit (72) which are formed on a common semiconductor substrate (1); the semiconductor laser unit (70) is provided with: a lower cladding layer (2) of a first conductivity type, each of the lower cladding layer (2) comprising a group III-V semiconductor mixed crystal; an active layer (3) that emits a laser beam; and an upper cladding layer (4) of a second conductivity type provided with a primary diffraction grating (15). The optical modulator unit (72) is provided with: a light absorption layer (21) which is at least partially composed of a group III-V semiconductor mixed crystal containing Bi and which absorbs the laser beam incident from the active layer (3); and scattered light absorbing layers (20, 22) that face either the lower surface or the upper surface of the light absorbing layer (21), or a pair of scattered light absorbing layers (20, 22) that face the lower surface and the upper surface of the light absorbing layer (21), respectively.

Description

Technical Field

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

[0002] With the popularization of various information terminals and the cloudification of information in recent years, the amount of data communication is increasing rapidly. In order to cope with the demand for increasing data communication volume, the transmission speed and capacity within the base station of optical fiber communication are being improved.

[0003] As a light source for long-distance optical communication in optical fiber communication, a semiconductor laser device with an optical modulator (Electro-absorption Modulator Laser Diode: EML-LD) is used, which is a type of optical semiconductor device and in which a semiconductor laser unit and an optical modulator unit are monolithically integrated. The optical modulator unit is a type of external modulator, and compared to a direct modulation method that directly modulates the intensity of a laser beam, there is less degradation of the signal waveform, and high-speed and long-distance optical fiber transmission is possible.

[0004] In the EML-LD, in the joint (butt joint) between the semiconductor laser section composed of the distributed feedback laser (Distributed FeedBack Laser Diode: DFB-LD) and the optical modulator section composed of the electro-absorption optical modulator (Electro-absorption Modulator: EML), scattered light that is not waveguided to the light absorption layer of the optical modulator section in the laser beam incident from the semiconductor laser section is emitted to the outside from the emission end surface of the optical modulator section and becomes leakage light, which appears as a side peak of the output light, and becomes an obstacle to the optical axis adjustment of the EML-LD or a major factor in reducing the extinction ratio, that is, the ratio of light intensity in the on / off state of the light. The higher the output of the EML-LD, the greater the intensity of the leakage light emitted from the EML-LD to the outside, so that the extinction ratio is further reduced.

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 03-77386

[0006] In order to prevent the extinction ratio in the EML-LD from decreasing, for example, in the semiconductor light emitting device described in Patent Document 1, a shielding film having an opening on the end face of the light absorbing layer is provided on the output end face of the light modulator section.

[0007] The shielding film provided in the light modulator section of the semiconductor light emitting device described in Patent Document 1 functions to shield leakage light output from the output end face other than the light absorbing layer. The shielding film can reliably shield leakage light at the output end face of the light modulator section, thereby achieving the effect of increasing the extinction ratio during light modulation.

[0008] However, in order to provide a shielding film on the output end face of the optical modulator, after forming a metal film on the entire surface of the output end face, it is necessary to perform fine processing such as masking the portion other than the opening and removing it by ion etching, etc., but the processing on the output end face is very difficult and it is difficult to maintain the processing accuracy required for the shielding film. Therefore, there is a problem in manufacturing an optical semiconductor device with low leakage light, that is, a high extinction ratio, with good reproducibility. Summary of the invention

[0009] The present disclosure has been made to solve the above-mentioned problems, and an object of the present disclosure is to obtain an optical semiconductor device having a high extinction ratio even during high-output driving.

[0010] The optical semiconductor device disclosed in the present invention comprises a semiconductor laser unit and an optical modulator unit formed on a common semiconductor substrate, wherein:

[0011] The semiconductor laser unit includes the following layers:

[0012] The first conductive type lower cladding layer is composed of a III-V semiconductor mixed crystal, the active layer emitting a laser beam, and the second conductive type upper cladding layer is provided with a primary diffraction grating.

[0013] The optical modulator unit comprises:

[0014] a light absorbing layer at least partly composed of a III-V semiconductor mixed crystal containing Bi and absorbing the laser beam incident from the active layer; and

[0015] A scattered light absorbing layer facing one of the lower surface and the upper surface of the light absorbing layer, or a pair of scattered light absorbing layers facing the lower surface and the upper surface of the light absorbing layer, respectively.

[0016] According to the optical semiconductor device disclosed in the present invention, the scattered light absorption layer provided in the optical modulator portion absorbs scattered light other than the waveguide light waveguided to the optical absorption layer, thereby being able to prevent a decrease in the extinction ratio caused by the scattered light. Furthermore, by containing Bi in the optical absorption layer, it is also possible to simultaneously suppress a decrease in the extinction ratio caused by the accumulation of holes in the optical absorption layer due to heat generated by the absorption of scattered light by the scattered light absorption layer, thereby achieving the effect of obtaining an optical semiconductor device with a high extinction ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a cross-sectional view along the light waveguide direction in the optical semiconductor device according to Embodiment 1.

[0018] Figure 2 This is a cross-sectional view taken in a direction perpendicular to the waveguide direction of light in the optical semiconductor device according to the first embodiment.

[0019] Figure 3 This is a cross-sectional view showing the structure of a light absorbing layer of a light modulator section in the optical semiconductor device according to Embodiment 1.

[0020] Figure 4 This is an energy band diagram of the optical modulator section in the optical semiconductor device according to Embodiment 1.

[0021] Figure 5 This is a cross-sectional view along the waveguide direction of light in the optical semiconductor device according to the second embodiment.

[0022] Figure 6 This is a cross-sectional view showing the structure of a light absorbing layer of a light modulator section in the optical semiconductor device according to the second embodiment.

[0023] Figure 7 This is an energy band diagram of the optical modulator section in the optical semiconductor device according to the second embodiment.

[0024] Figure 8 This is a cross-sectional view along the waveguide direction of light in the optical semiconductor device according to the third embodiment.

[0025] Fig. 9 This is a cross-sectional view of the optical semiconductor device according to the fourth embodiment, taken in a direction perpendicular to the waveguide direction of light. DETAILED DESCRIPTION

[0026] Implementation method 1.

[0027] <Structure of Optical Semiconductor Device of Embodiment 1>

[0028] Figure 1 It is a cross-sectional view along the waveguide direction of light in the optical semiconductor device 100 according to Embodiment 1. As an example of the optical semiconductor device 100 according to Embodiment 1, an EML-LD is given.

[0029] The optical semiconductor device 100 of the first embodiment is composed of the semiconductor laser section 70, the separation section 71, and the optical modulator section 72. In the following description, the up and down directions are defined as follows: in the direction perpendicular to the surface of the semiconductor substrate, with the active layer or the light absorbing layer as a reference, the direction toward the surface side of the crystal growth layer is defined as the up direction, and the direction toward the back side of the semiconductor substrate is defined as the down direction.

[0030] The semiconductor laser unit 70 is composed of various crystal growth layers formed in sequence on an n-type InP substrate 1 (semiconductor substrate 1), and each crystal growth layer is composed of an n-type InP lower cladding 2 (lower cladding 2 of the first conductivity type), an active layer 3, a p-type InP upper first cladding 4 (upper cladding 4 of the second conductivity type), a p-type InP upper second cladding 5, and a p-type InGaAs first contact layer 6a.

[0031] A primary diffraction grating 15 is formed in the p-type InP upper first cladding layer 4. In addition, the active layer 3 is typically composed of an InGaAsP multilayer quantum well structure.

[0032] A p-side first electrode 8a and a p-side second electrode 9a are formed on the p-type InGaAs first contact layer 6a through the opening of the surface protection insulating film 7a. An n-side first electrode 10 and an n-side second electrode 11 are formed on the back side of the n-type InP substrate 1.

[0033] The optical modulator section 72 is composed of crystal growth layers sequentially formed on the n-type InP substrate 1, and the crystal growth layers are composed of a lower scattered light absorption layer 20, an n-type InP lower cladding layer 2, a light absorption layer 21 composed of i-type InGaAsBi as a III-V semiconductor mixed crystal containing Bi (bismuth), a p-type InP upper first cladding layer 4, an upper scattered light absorption layer 22, a p-type InP upper second cladding layer 5, and a p-type InGaAs second contact layer 6b. In the following description, the lower scattered light absorption layer 20 and the upper scattered light absorption layer 22 are sometimes referred to as scattered light absorption layers. In addition, the lower scattered light absorption layer 20 and the upper scattered light absorption layer 22 are sometimes collectively referred to as a pair of scattered light absorption layers.

[0034] A p-side third electrode 8b and a p-side fourth electrode 9b are formed on the p-type InGaAs second contact layer 6b through the opening of the surface protection insulating film 7c. An n-side first electrode 10 and an n-side second electrode 11 are formed on the back side of the n-type InP substrate 1.

[0035] The semiconductor laser unit 70, the separator 71, and the optical modulator 72 are formed on a common n-type InP substrate 1. In addition, the n-side first electrode 10 and the n-side second electrode 11 are also formed integrally in the semiconductor laser unit 70, the separator 71, and the optical modulator 72.

[0036] The separation unit 71 has the same structure as the optical modulator unit 72 except that the p-type InGaAs second contact layer 6b is not provided, the surface is covered with a surface protection insulating film 7b, and the p-side third electrode 8b and the p-side fourth electrode 9b are not provided.

[0037] Figure 2 1 is a cross-sectional view of the optical modulator unit 72 in the direction perpendicular to the waveguide direction of light in the optical semiconductor device 100 according to the first embodiment.

[0038] The mesa strip 35 is formed by a pair of mesa grooves 35a and 35b provided on both sides. The bottom and side surfaces of the mesa grooves 35a and 35b are covered by a surface protection insulating film 7c. In the mesa strip 35, high-resistance InP buried layers 37a and 37b are formed on both side surfaces of the light absorption layer 21 composed of i-type InGaAsBi. As an example of a semiconductor material constituting high-resistance InP, semi-insulating InP doped with Fe (iron) can be cited.

[0039] Figure 3 This is a cross-sectional view of the MQW layer 31 composed of the multi-layer quantum well structure of the light absorption layer 21 composed of i-type InGaAsBi constituting the optical modulator section 72. The light absorption layer 21 composed of i-type InGaAsBi is composed of a lower SCH layer 30a, an MQW layer 31 composed of alternately stacked well layers 32 and barrier layers 33, and an upper SCH layer 30b, starting from the n-type InP substrate 1 side. In addition, MQW is the abbreviation of Multi Quantum Well, which means multi-layer quantum well. In addition, SCH is the abbreviation of Separate Confinement Heterostructure, which means a separated confinement layer.

[0040] The well layer 32 and the barrier layer 33, and the lower SCH layer 30a and the upper SCH layer 30b are all made of a III-V semiconductor mixed crystal containing Bi, typically i-type InGaAsBi.

[0041] <Method for Manufacturing Optical Semiconductor Device According to Embodiment 1>

[0042] Hereinafter, an outline of a method for manufacturing the optical semiconductor device 100 according to the first embodiment will be described.

[0043] First, the process of forming each crystal growth layer of the semiconductor laser portion 70 will be described.

[0044] An n-type InP lower cladding layer 2, an active layer 3, and a part of a p-type InP upper first cladding layer 4 are epitaxially grown in sequence in a region where a semiconductor laser unit 70 is to be formed on an n-type InP substrate 1. Examples of epitaxial crystal growth methods include metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).

[0045] After the above-mentioned epitaxial crystal growth, the primary diffraction grating 15 is formed on the surface of the p-type InP upper first cladding layer 4 using photolithography technology and etching technology. The remaining part of the p-type InP upper first cladding layer 4, the p-type InP upper second cladding layer 5, and the p-type InGaAs first contact layer 6a are sequentially epitaxially crystallized by MOCVD method or the like on the p-type InP upper first cladding layer 4 on which the diffraction grating 15 is formed. After the epitaxial crystal growth, the insulating film mask is patterned on the surface of the semiconductor laser unit 70 using photolithography technology and etching technology. As a material constituting the insulating film mask, for example, SiO2 is preferably used.

[0046] Next, the process of forming each crystal growth layer of the separation part 71 and the light modulator part 72 is described. On the n-type InP substrate 1, the region where the separation part 71 and the light modulator part 72 are to be formed is epitaxially crystal-grown in sequence by MOCVD or the like, such as the lower scattered light absorption layer 20, the n-type InP lower cladding layer 2, the light absorption layer 21 composed of i-type InGaAsBi, the p-type InP upper first cladding layer 4, the upper scattered light absorption layer 22, the p-type InP upper second cladding layer 5, and the p-type InGaAs second contact layer 6b.

[0047] After the formation of the crystal growth layers of the separation section 71 and the optical modulator section 72, the insulating film mask covering the semiconductor laser section 70 is removed. In addition, the insulating film mask is used to cover the portion except for the portion where the pair of mesa grooves 35a and 35b are to be formed. As a material constituting the insulating film mask, SiO2 is preferably used, for example.

[0048] Using the insulating film mask as an etching mask, a pair of mesa grooves 35a and 35b are formed through etching techniques such as dry etching and wet etching, which extend from the p-type InGaAs first contact layer 6a as the outermost crystalline growth layer to the n-type InP substrate 1 in the semiconductor laser section 70, from the p-type InP upper second cladding layer 5 as the outermost crystalline growth layer to the lower scattered light absorption layer 20 in the separation section 71, and from the p-type InGaAs second contact layer 6b as the outermost crystalline growth layer to the lower scattered light absorption layer 20 in the optical modulator section 72.

[0049] After forming a pair of mesa grooves 35a, 35b, high-resistance InP buried layers 37a, 37b are buried and grown by MOCVD or the like on the side portions of the side where the mesa stripes 35 are to be formed, with the insulating film mask remaining. After the buried growth, unnecessary portions are removed by etching or the like, thereby completing the mesa stripes 35.

[0050] An insulating film is formed in a manner that covers the entire crystal growth layer on the surface side of the EML-LD, and an opening is provided at a location where each electrode is to be formed using photolithography and etching techniques. The formed insulating film functions as a surface protection insulating film 7a, 7b, and 7c. In the semiconductor laser unit 70, a p-side first electrode 8a that contacts the p-type InGaAs first contact layer 6a via an opening of the surface protection insulating film 7a and a p-side second electrode 9a on the p-side first electrode 8a are formed by electron beam evaporation, etc., and then peeled off and patterned.

[0051] In the optical modulator section 72, the p-side third electrode 8b in contact with the p-type InGaAs second contact layer 6b via the opening of the surface protection insulating film 7c and the p-side fourth electrode 9b on the p-side third electrode 8b are formed by electron beam evaporation, etc., and then peeled off and patterned. In addition, the electrode formation of the semiconductor laser section 70 and the optical modulator section 72 can also be performed in the same process.

[0052] The n-side first electrode 10 and the n-side second electrode 11 are formed on the back side of the n-type InP substrate 1 by electron beam deposition, and then peeled off and patterned. The EML-LD is completed by separating the wafer into individual chips by cleavage.

[0053] The above is an overview of the method for manufacturing the EML-LD as an example of the optical semiconductor device 100 according to the first embodiment.

[0054] <Operation of Optical Semiconductor Device in Embodiment 1>

[0055] First, the basic operation of the EML-LD as an example of the optical semiconductor device 100 according to the first embodiment will be described below.

[0056] The semiconductor laser unit 70 is injected with current through the p-side first electrode 8a and the p-side second electrode 9a, thereby emitting the laser beam 25. Since the primary diffraction grating 15 is provided in the p-type InP upper first cladding layer 4 close to the active layer 3 of the semiconductor laser unit 70, the semiconductor laser unit 70 functions as a DFB-LD. Compared with a semiconductor laser without a diffraction grating, the DFB-LD has the advantage of being able to make the oscillation spectrum single longitudinal mode.

[0057] The laser beam 25 of the semiconductor laser unit 70 is incident on the optical modulator unit 72 as the waveguide light 26 via the separator 71. If a reverse bias voltage is applied from the outside, in which the p-side third electrode 8b and the p-side fourth electrode 9b of the optical modulator unit 72 are negative and the n-side first electrode 10 and the n-side second electrode 11 are positive, the absorption spectrum of the light absorption layer 21 changes, and a light absorption phenomenon occurs. The laser beam 25 incident on the optical modulator unit 72 from the semiconductor laser unit 70 becomes the waveguide light 26, and the waveguide light 26 is absorbed in the light absorption layer 21 according to the magnitude of the reverse bias voltage, and electron and hole pairs are generated. When almost all of the waveguide light 26 is absorbed in the light absorption layer 21 due to the light absorption phenomenon, the waveguide light 26 is extinguished. That is, the waveguide light 26 is not emitted from the emission end face of the optical modulator unit 72. Based on the above operating principle, the intensity modulation of the laser beam 25 can be achieved in the optical modulator unit 72.

[0058] The above are the basic operations of EML-LD.

[0059] Next, a characteristic operation of the EML-LD as an example of the optical semiconductor device 100 according to the first embodiment will be described below.

[0060] As described above, in the conventional EML-LD, in the junction (separation) of the semiconductor laser section composed of the DFB-LD and the optical modulator section composed of the EML, the light that is not waveguided to the light absorption layer of the optical modulator section in the laser beam incident from the semiconductor laser section becomes scattered light and propagates in the optical modulator section, and is emitted from the emission end face of the optical modulator section to the outside as leakage light. As the light output emitted by the EML-LD increases, the intensity of the leakage light emitted from the emission end face of the optical modulator section to the outside also increases proportionally, so there is a problem of further reducing the extinction ratio.

[0061] In the EML-LD which is an example of the optical semiconductor device 100 according to the first embodiment, in order to prevent the extinction ratio from decreasing as the light output increases, a lower scattered light absorption layer 20 and an upper scattered light absorption layer 22 are respectively provided in the optical modulator section 72 so as to be opposite to the lower surface of the light absorption layer 21, i.e., the surface on the side of the n-type InP substrate 1, and the upper surface of the light absorption layer 21, i.e., the surface on the surface side of the crystal growth layer.

[0062] The lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22 function in a manner that the scattered light 27 that is not waveguided to the light absorbing layer 21 is absorbed in the light modulator section 72. That is, the scattered light 27 incident on the lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22 is absorbed as the absorbed light 28. Therefore, the leakage light emitted from the emission end face of the light modulator section 72 to the outside can be greatly reduced, thereby achieving an effect of achieving a high extinction ratio.

[0063] The lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22 are composed of, for example, a quaternary III-V semiconductor mixed crystal such as InGaAsP having a layer thickness of several 100 nm and having a band gap energy of the same degree as that of the active layer 3 of the semiconductor laser unit 70. The lower scattered light absorbing layer 20 may be doped with n-type impurities, and the upper scattered light absorbing layer 22 may be doped with p-type impurities.

[0064] Next, the function of the light absorbing layer 21 composed of i-type InGaAsBi will be described. Figure 4 is the energy band diagram of the optical modulator section 72. Figure 4 From the left side, the energy bands of each layer are shown, including the n-type InP substrate 1, the lower scattered light absorption layer 20, the n-type InP lower cladding 2, the light absorption layer 21 composed of i-type InGaAsBi, the p-type InP upper first cladding 4, the upper scattered light absorption layer 22, and the p-type InP upper second cladding 5.

[0065] The energy band of the light absorption layer 21 composed of i-type InGaAsBi is further composed of the energy bands of the following layers: the lower SCH layer 30a containing Bi, the MQW layer 31 composed of four barrier layers 33 containing Bi and three well layers 32 containing Bi stacked alternately, and the upper SCH layer 30b containing Bi. The well layer 32, the barrier layer 33, the lower SCH layer 30a, and the upper SCH layer 30b are each composed of i-type InGaAsBi. The band gap energy of the lower SCH layer 30a and the upper SCH layer 30b is set to be larger than that of the barrier layer 33, and the band gap energy of the barrier layer 33 is set to be larger than that of the well layer 32.

[0066] In a III-V semiconductor mixed crystal containing Bi, the temperature change of the band gap energy decreases with the Bi content. In particular, InGaAsBi has the property that the band gap (0.6 to 1.5 eV) becomes constant with respect to temperature changes. That is, the temperature dependence of the band gap energy of the III-V semiconductor mixed crystal containing Bi becomes smaller. Therefore, even if the temperature of the III-V semiconductor mixed crystal containing Bi rises, the degree to which the band gap energy decreases due to the temperature rise is significantly smaller than that of the III-V semiconductor mixed crystal not containing Bi.

[0067] In the EML-LD as an example of the optical semiconductor device 100 according to the first embodiment, as described above, the scattered light 27 incident on the lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22 is absorbed as the absorbed light 28. Due to the absorption of the scattered light 27, heat is generated in the lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22, and the heat is diffused in the crystal growth layers constituting the optical modulator section 72, so that the temperature of the optical modulator section 72 rises.

[0068] like Figure 4 As shown by the energy band represented by the dotted line in FIG, due to the heat generated by the absorption of the scattered light 27, the band gap energy of each of the n-type InP substrate 1, the lower scattered light absorption layer 20, the n-type InP lower cladding 2, the p-type InP upper first cladding 4, the upper scattered light absorption layer 22, and the p-type InP upper second cladding 5 is smaller than that in the case where no heat is generated, that is, Figure 4 The energy bands in are represented by solid lines.

[0069] On the other hand, since the light absorbing layer 21 composed of i-type InGaAsBi contains Bi as described above, the temperature dependence of the band gap energy is small, and therefore the energy band hardly changes even due to heat generated by absorption of the scattered light 27 .

[0070] Therefore, due to the generation of heat, the band discontinuity generated between the p-type InP upper first cladding layer 4 and the upper SCH layer 30b in the light absorption layer 21 composed of i-type InGaAsBi and in contact with the p-type InP upper first cladding layer 4, that is, the size of each barrier of the electron barrier ΔEc on the conduction band side and the hole barrier ΔEv on the valence band side is reduced. This is because the band gap energy of the p-type InP upper first cladding layer 4 is reduced by the influence of heat, while the band gap energy of the upper SCH layer 30b containing Bi is almost not changed by heat.

[0071] Here, the accumulation phenomenon of holes that has a great influence on the extinction ratio of EML-LD is explained. As the semiconductor material constituting EML-LD, III-V semiconductor mixed crystals such as InGaAsP grown by epitaxial crystallization on an InP substrate are generally used. In this semiconductor material system, the energy gap difference, that is, the band discontinuity (ΔEg) in the heterogeneous interface is distributed in the conduction band and the valence band at a ratio of 40:60. Therefore, there is a relatively large hole barrier ΔEv for the heavy hole 34. Therefore, in the case where high-intensity light is absorbed in the multi-layer quantum well structure constituting the light absorption layer to generate carriers, that is, electrons and holes 34, the heavy hole 34 is difficult to flow as a current through the relatively large hole barrier ΔEv on the valence band side compared to the small-mass electron. This phenomenon is called the accumulation phenomenon of holes. If the accumulation phenomenon of holes occurs, the carriers that are absorbed and accumulated become important factors that hinder high performance, such as the degradation of high-speed response characteristics and the reduction of extinction ratio caused by shielding the external electric field (screening effect).

[0072] In particular, when the MQW layer is directly sandwiched between the InP cladding layer with relatively large band gap energy, and when the SCH layer composed of InGaAsP is inserted between the MQW layer and the InP cladding layer, a large band discontinuity at the valence band end, i.e., a hole barrier ΔEv, exists at the heterojunction. As a result, the hole 34 crosses the barrier by thermal excitation and the residence time before being absorbed by the InP cladding layer becomes longer.

[0073] In the EML-LD which is an example of the optical semiconductor device 100 of embodiment 1, in order to prevent the decrease of the extinction ratio caused by the accumulation phenomenon of holes, it becomes a technical feature to utilize the heat generated by absorbing the absorption light 28 of the incident scattered light 27 by the lower scattered light absorption layer 20 and the upper scattered light absorption layer 22.

[0074] In the optical modulator section 72, the size of each barrier of the electron barrier ΔEc on the conduction band side and the hole barrier ΔEv on the valence band side generated between the p-type InP upper first cladding layer 4 and the upper SCH layer 30b in the light absorption layer 21 composed of i-type InGaAsBi and in contact with the p-type InP upper first cladding layer 4 due to the band discontinuity is reduced by the heat generated by the light generated by the absorption of the scattered light 27. In addition, InGaAsBi also has the same band discontinuity as InGaAsP.

[0075] As a result, even if light with high intensity is absorbed in the light absorption layer 21 composed of i-type InGaAsBi to generate electron and hole pairs, the hole barrier ΔEv on the valence band side is reduced compared to the case where no heat is generated, so that the holes 34 are more likely to flow through the hole barrier ΔEv as current. In other words, the influence of the hole accumulation phenomenon is reduced. As a result, the extinction ratio of the EML-LD is increased.

[0076] In addition, the advantages of using the light absorbing layer 21 composed of i-type InGaAsBi containing Be as the light absorbing layer of the EML-LD will be described below.

[0077] In the optical modulator section of the EML-LD formed on the InP substrate, the semiconductor material constituting the light absorption layer is generally InGaAsP or AlGaInAs, which are quaternary III-V semiconductor mixed crystals. The band gap energy of these varies with the ambient temperature, that is, the temperature dependence of the band gap energy is large. However, in order to obtain the desired characteristics in the optical modulator section, it is necessary to control the absorption spectrum on the order of several nanometers.

[0078] Therefore, in order to obtain the desired characteristics as the optical modulator part, a Peltier cooler as a temperature adjustment mechanism is usually installed, and it is used to control the temperature to a constant level. As other methods, a method of installing a mechanism for adjusting the bias voltage of the optical modulator part when the temperature changes is also listed. However, these additional mechanisms have problems such as an increase in power consumption, an increase in the complexity of the component structure, and an increase in manufacturing cost. Therefore, like the semiconductor laser part, if the optical modulator part can also perform an action under non-cooling, it is possible to achieve non-cooling action in the entire EML-LD.

[0079] In EML-LD, the light absorption layer of the optical modulator part is set to the light absorption layer 21 composed of i-type InGaAsBi containing Be, so that the band gap of InGaAsBi of the light absorption layer is almost constant with respect to temperature changes, thereby being able to suppress changes in light absorption characteristics at low and high temperatures. As a result, non-cooling operation of EML-LD can be performed.

[0080] In the above description, the following case is taken as an example: the MQW layer 31 composed of the alternately stacked well layers 32 and the barrier layers 33, and the lower SCH layer 30a and the upper SCH layer 30b constituting the light absorption layer 21 are all composed of a III-V semiconductor mixed crystal containing Bi. However, the same effect is achieved when only the lower SCH layer 30a and the upper SCH layer 30b are composed of a III-V semiconductor mixed crystal containing Bi. In addition, as other examples of a III-V semiconductor mixed crystal containing Bi, a quaternary III-V semiconductor mixed crystal composed of InGaPBi and a quinary III-V semiconductor mixed crystal composed of InGaPAsBi can be cited.

[0081] In the above description, the following structure is taken as an example: in the light modulator section 72, the lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22 are provided, that is, a pair of scattered light absorbing layers are provided so as to face the lower surface of the light absorbing layer 21, that is, the surface on the n-type InP substrate 1 side, and the upper surface of the light absorbing layer 21, that is, the surface on the surface side of the crystal growth layer. However, in the case of applying a structure in which only one of the lower scattered light absorbing layer 20 and the upper scattered light absorbing layer 22 is provided, that is, a structure in which the scattered light absorbing layer faces either the lower surface or the upper surface of the light absorbing layer 21, the effect of reducing the scattered light 27 is also achieved. In addition, if this structure is applied, the structure of the optical semiconductor device is simplified, and thus the effect of easy manufacturing is also achieved.

[0082] <Effects of Embodiment 1>

[0083] As described above, in the optical semiconductor device according to embodiment 1, a lower scattered light absorption layer opposite to the lower surface of the light absorption layer composed of i-type InGaAsBi and an upper scattered light absorption layer opposite to the upper surface are respectively provided, thereby absorbing and reducing scattered light that has a negative impact on the extinction ratio, and utilizing the heat generated by the absorption of the scattered light to reduce the accumulation phenomenon of holes, thereby also being able to simultaneously suppress the reduction in the extinction ratio caused by the accumulation phenomenon. Through such a synergistic effect, an optical semiconductor device (EML-LD) with a high extinction ratio is achieved.

[0084] Implementation method 2.

[0085] <Structure of optical semiconductor device according to Embodiment 2>

[0086] Figure 5 It is a cross-sectional view along the waveguide direction of light in the optical semiconductor device 110 according to Embodiment 2. As an example of the optical semiconductor device 110 according to Embodiment 2, EML-LD is given.

[0087] The optical semiconductor device 110 of the second embodiment is different in structure from the optical semiconductor device 100 of the first embodiment in that, in the light absorption layer 21a of the optical semiconductor device 110, only the well layer 32a of the MQW layer 31a contains Bi, and the barrier layer 33a of the MQW layer 31a, the lower SCH layer 30c, and the upper SCH layer 30d do not contain Bi. The other structures are the same as those of the optical semiconductor device 100 of the first embodiment.

[0088] <Operation of Optical Semiconductor Device in Embodiment 2>

[0089] Figure 7 : is an energy band diagram of the optical modulator section 72a in the optical semiconductor device 110 of Embodiment 2. Figure 7 As shown by the energy band represented by the dotted line in FIG, due to the heat generated by the absorption of the scattered light 27, the band gap energy of each of the p-type InP upper first cladding layer 4, the upper scattered light absorption layer 22, and the p-type InP upper second cladding layer 5 is greater than that when no heat is generated, that is, Figure 7 The energy band represented by the solid line is small.

[0090] In the light absorbing layer 21a, the band gap energies of the barrier layer 33a, the lower SCH layer 30c, and the upper SCH layer 30d of the MQW layer 31a not containing Bi are lower than those when no heat is generated. Figure 7 On the other hand, since the well layer 32a contains Bi as described above, the temperature dependence of the band gap energy is small, and therefore the energy band hardly changes even due to heat generated by absorption of the scattered light 27.

[0091] As a result, the energy band of the well layer 32a is almost unchanged by the heat generated by the absorption of the scattered light 27, but the energy band in the blocking layer 33a becomes smaller, so the hole barrier ΔEv between the well layer 32a and the blocking layer 33a also becomes smaller. Therefore, the holes 34 easily flow through the hole barrier ΔEv as current. That is, the influence of the hole accumulation phenomenon is reduced. As a result, the extinction ratio of the EML-LD is increased.

[0092] In the above description, the following case is taken as an example: in the MQW layer 31a composed of the well layers 32a and the barrier layers 33a stacked alternately constituting the light absorbing layer 21a, only the well layers 32a are composed of the III-V semiconductor mixed crystal containing Bi. However, the same effect is achieved even when only the barrier layers 33a, but not the well layers 32a, are composed of the III-V semiconductor mixed crystal containing Bi.

[0093] <Effects of Embodiment 2>

[0094] As described above, in the optical semiconductor device according to embodiment 1, a lower scattered light absorption layer is provided which is opposite to the lower surface of the light absorption layer which is only the well layer containing Bi and which constitutes the MQW layer, and an upper scattered light absorption layer is provided which is opposite to the upper surface of the light absorption layer, thereby absorbing and reducing the scattered light which has a negative impact on the extinction ratio, and the heat generated by the absorption of the scattered light is used to reduce the accumulation phenomenon of holes between the well layer and the barrier layer which constitute the MQW layer, thereby being able to simultaneously suppress the reduction in the extinction ratio caused by the accumulation phenomenon. Through such a synergistic effect, an optical semiconductor device (EML-LD) with a high extinction ratio is achieved.

[0095] Implementation method 3.

[0096] <Structure of optical semiconductor device according to Embodiment 3>

[0097] Figure 8 It is a cross-sectional view along the waveguide direction of light in the optical semiconductor device 120 according to Embodiment 3. As an example of the optical semiconductor device 120 according to Embodiment 3, EML-LD is given.

[0098] The optical semiconductor device 120 of the third embodiment differs from the optical semiconductor device 100 of the first embodiment in that a primary diffraction grating 16 is provided in the lower scattered light absorbing layer 20a of the optical modulator section 72b of the optical semiconductor device 120, and a primary diffraction grating 17 is provided in the upper scattered light absorbing layer 22a. The other structures are the same as those of the optical semiconductor device 100 of the first embodiment.

[0099] <Operation of Optical Semiconductor Device in Embodiment 3>

[0100] As described in Embodiment 1, in the EML-LD, generally speaking, in the junction (separation portion) of the semiconductor laser portion constituted by the DFB-LD and the optical modulator portion constituted by the EML, scattered light that is not waveguided to the light absorption layer of the optical modulator portion in the laser beam incident from the semiconductor laser portion is emitted from the emission end face of the optical modulator portion to the outside and becomes leakage light. In the optical semiconductor device 100 of Embodiment 1, the lower scattered light absorption layer 20 and the upper scattered light absorption layer 22 provided in the optical modulator portion 72 function in a manner of absorbing the scattered light 27 that is not waveguided to the light absorption layer 21 in the optical modulator portion 72. However, particularly in the high-output operation of the EML-LD, the lower scattered light absorption layer 20 and the upper scattered light absorption layer 22 may also produce a situation where the scattered light 27 is not completely absorbed.

[0101] In the optical semiconductor device 120 of the third embodiment, the primary diffraction grating 16 provided in the lower scattered light absorbing layer 20a and the primary diffraction grating 17 provided in the upper scattered light absorbing layer 22a diffract the scattered light 27 that is not completely absorbed as diffracted light 29 toward a direction other than the exit end surface side of the optical modulator section 72, thereby functioning to prevent the scattered light 27 from being emitted from the exit end surface of the optical modulator section 72 and becoming leakage light. Therefore, the optical semiconductor device 120 of the third embodiment has an effect of further reducing the leakage light emitted from the exit end surface of the optical modulator section 72 to the outside.

[0102] In the above description, the structure in which the diffraction gratings are provided in both the lower scattered light absorbing layer and the upper scattered light absorbing layer is used as an example, but even if the primary diffraction grating is provided in only one of the lower scattered light absorbing layer and the upper scattered light absorbing layer, the scattered light 27 can be reduced. If this structure is applied, the structure of the optical semiconductor device is simplified, and thus the manufacturing is also facilitated.

[0103] <Effects of Embodiment 3>

[0104] As described above, in the optical semiconductor device according to embodiment 3, since primary diffraction gratings are respectively provided in the lower scattered light absorption layer and the upper scattered light absorption layer of the optical modulator part, the leakage light emitted from the output end face of the optical modulator part to the outside can be further reduced compared with the optical semiconductor device of embodiment 1, thereby achieving the effect of obtaining an optical semiconductor device with a higher extinction ratio.

[0105] Implementation method 4.

[0106] <Structure of optical semiconductor device according to Embodiment 4>

[0107] Fig. 9It is a cross-sectional view in a direction perpendicular to the waveguide direction of light in the optical semiconductor device 130 according to Embodiment 4. As an example of the optical semiconductor device 130 according to Embodiment 4, EML-LD is given.

[0108] The optical semiconductor device 130 of the fourth embodiment differs from the optical semiconductor device 100 of the first embodiment in that side scattered light absorption layers 39a and 39b are provided on both side surfaces of the mesa strip 36 of the optical modulator section 72. The other structures are the same as those of the optical semiconductor device 100 of the first embodiment.

[0109] That is, the optical semiconductor device 130 of embodiment 4 is provided with the lower scattered light absorption layer 20 and the upper scattered light absorption layer 22 which are respectively opposite to the lower surface of the light absorption layer 21, that is, the surface on the side of the n-type InP substrate 1, and the upper surface of the light absorption layer 21, that is, the surface on the surface side of the crystal growth layer in the optical modulator part 72, and also has the side scattered light absorption layers 39a and 39b arranged on the side parts on both sides of the mesa strip 36 of the optical modulator part 72.

[0110] <Operation of Optical Semiconductor Device in Embodiment 4>

[0111] As described in Embodiment 1, in the EML-LD, generally speaking, in the junction (separation) between the semiconductor laser section constituted by the DFB-LD and the light modulator section constituted by the EML, light that is not waveguided to the light absorption layer of the light modulator section among the laser beam incident from the semiconductor laser section becomes scattered light 27. The scattered light 27 includes a component that travels not only in the vertical direction of the light absorption layer 21 but also in the lateral direction of the stripe-shaped light absorption layer 21 within the mesa stripes 36, that is, in the direction toward the side of the mesa stripes 36.

[0112] In the optical semiconductor device 130 of the fourth embodiment, the side scattered light absorption layers 39a and 39b are provided on the side surfaces of the mesa strips 36 of the optical modulator section 72 to absorb the scattered light 27 traveling in the lateral direction. As a result, the leakage light emitted from the emission end surface of the optical modulator section 72 to the outside can be further reduced.

[0113] <Effects of Embodiment 4>

[0114] As described above, according to the optical semiconductor device of embodiment 4, since side scattered light absorption layers are respectively provided on the side portions on both sides of the table strip of the optical modulator portion, the leakage light emitted from the output end face of the optical modulator portion to the outside can be further reduced compared with the optical semiconductor device of embodiment 1, thereby achieving the effect of obtaining an optical semiconductor device with a higher extinction ratio.

[0115] In Embodiments 1 to 4, as the semiconductor material constituting the light absorbing layers 21 and 21a, a III-V semiconductor mixed crystal containing Bi is exemplified, and InGaAsBi is cited as an example. However, the light absorbing layers 21 and 21a may also be constituted by a III-V semiconductor mixed crystal containing Sb (antimony). InGaAsSb is cited as an example of a III-V semiconductor mixed crystal containing Sb.

[0116] Although various exemplary embodiments and examples are described in the present disclosure, various features, forms, and functions described in one or more embodiments are not limited to application in specific embodiments, and may be applied to the embodiments alone or in various combinations.

[0117] Therefore, numerous modifications not shown in the examples are conceivable within the technical scope disclosed in the present specification, including, for example, the modification, addition or omission of at least one constituent element, and the extraction and combination of at least one constituent element with constituent elements of other embodiments.

[0118] Description of Reference Numerals

[0119] 1...n-type InP substrate (semiconductor substrate); 2...n-type InP lower cladding layer (lower cladding layer of the first conductivity type); 3...active layer; 4...p-type InP upper first cladding layer (upper cladding layer of the second conductivity type); 5...p-type InP upper second cladding layer; 6a...p-type InGaAs first contact layer; 6b...p-type InGaAs second contact layer; 7a, 7b, 7c...surface protection insulating film; 8a...p-side first electrode; 8b...p-side third electrode; 9a...p-side second electrode; 9b...p-side fourth electrode; 10...n-side first electrode; 11...n-side second electrode; 15, 16, 17...diffraction grating; 20, 20a...lower scattered light absorption layer; 21, 21a...light absorption layer; 2 2, 22a...upper scattered light absorption layer; 25...laser beam; 26...waveguide light; 27...scattered light; 28...absorbed light; 29...diffracted light; 30a, 30c...lower SCH layer; 30b, 30d...upper SCH layer; 31, 31a...MQW layer; 32, 32a...well layer; 33, 33a...barrier layer; 34...holes; 35, 36...mesa stripes; 35a, 35b...mesa grooves; 37a, 37b...high resistance InP buried layer; 39a, 39b...side scattered light absorption layer; 70...semiconductor laser unit; 71...separation unit; 72, 72a, 72b...optical modulator unit; 100, 110, 120, 130...optical semiconductor device.

Claims

1. An optical semiconductor device comprising a semiconductor laser unit and an optical modulator unit formed on a common semiconductor substrate, characterized in that: The semiconductor laser unit includes the following layers: The first conductive type lower cladding layer is composed of a III-V semiconductor mixed crystal, the active layer emitting a laser beam, and the second conductive type upper cladding layer is provided with a primary diffraction grating. The optical modulator unit comprises: A light absorbing layer at least partly composed of a III-V semiconductor mixed crystal containing Bi and absorbing the laser beam incident from the active layer; and A scattered light absorbing layer facing one of the lower surface and the upper surface of the light absorbing layer, or a pair of scattered light absorbing layers facing the lower surface and the upper surface of the light absorbing layer, respectively.

2. The optical semiconductor device according to claim 1, wherein When the scattered light absorption layer is arranged on the lower surface side of the light absorption layer, a lower cladding layer of the first conductive type is arranged between the light absorption layer and the scattered light absorption layer; when the scattered light absorption layer is arranged on the lower surface side of the light absorption layer, an upper cladding layer of the second conductive type is arranged between the light absorption layer and the scattered light absorption layer.

3. The optical semiconductor device according to claim 1 or 2, wherein: The light modulator section includes a mesa strip including at least the light absorbing layer and the scattered light absorbing layer, and a side scattered light absorbing layer is provided on a side surface of the mesa strip.

4. The optical semiconductor device according to any one of claims 1 to 3, wherein: The scattered light absorbing layer is provided with a primary diffraction grating.

5. The optical semiconductor device according to any one of claims 1 to 4, wherein: The light absorbing layer includes an MQW layer in which well layers and barrier layers are alternately stacked, a lower SCH layer formed on a lower surface of the MQW layer, and an upper SCH layer formed on an upper surface of the MQW layer.

6. The optical semiconductor device according to claim 5, wherein The MQW layer, the lower SCH layer, and the upper SCH layer are each composed of a III-V semiconductor mixed crystal containing Bi.

7. The optical semiconductor device according to claim 5, wherein Only the lower SCH layer and the upper SCH layer are composed of a III-V semiconductor mixed crystal containing Bi.

8. The optical semiconductor device according to claim 5, wherein Only the well layer is composed of a III-V group semiconductor mixed crystal containing Bi.

9. The optical semiconductor device according to claim 5, wherein Only the barrier layer is composed of a III-V semiconductor mixed crystal containing Bi.

10. The optical semiconductor device according to claim 7, wherein The lower SCH layer and the upper SCH layer are made of InGaAsBi.

11. The optical semiconductor device according to claim 8, wherein The well layer is composed of InGaAsBi.

12. The optical semiconductor device according to claim 9, wherein The barrier layer is composed of InGaAsBi.

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