JP2012079990A - Integrated optical semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress reflection of light between a buried-type semiconductor laser and a high mesa ridge type modulator.SOLUTION: A buried-type semiconductor laser 12 and a high mesa ridge type modulator 14 are provided on an n-type InP substrate 10. The buried-type semiconductor laser 12 and the high mesa ridge type modulator 14 are coupled in the traveling direction of light. The buried-type semiconductor laser 12 includes a waveguide ridge 18 having an active layer 16 and buried layers 20 filling both sides of the waveguide ridge 18. The high mesa ridge type modulator 14 includes a high mesa ridge 24 having a modulation layer 22, silicon nitride films 26 contacting a side surface of the high mesa ridge 24, and silicon oxide films 28 provided on the silicon nitride films 26. The total film thickness of the silicon nitride film 26 and the silicon oxide film 28 is 1 μm or more. For this reason, the equivalent refractive index of the high mesa ridge type modulator 14 becomes 0.998 times to 1.0 times that of the buried-type semiconductor laser 12.

Description

  The present invention relates to an integrated optical semiconductor device used in the field of optical communications.

  There has been proposed an integrated optical semiconductor device in which a buried semiconductor laser and a high mesa ridge type modulator coupled in the light traveling direction are integrated on one substrate (see, for example, Patent Document 1). In the buried semiconductor laser, both sides of the waveguide ridge are buried with buried layers. On the other hand, in the high mesa ridge type modulator, both sides of the high mesa ridge are not embedded with a buried layer.

JP 2009-224626 A

  Since the structure of the buried semiconductor laser and the structure of the high mesa ridge type modulator are greatly different, the equivalent refractive indexes of the two are greatly different. For this reason, light is reflected between the two, and the reflected return light deteriorates the modulation waveform, and the laser oscillation characteristic is deteriorated.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain an integrated optical semiconductor device that can suppress reflection of light between the embedded semiconductor laser and the high-mesaridge modulator. Is.

  An integrated optical semiconductor device according to the present invention includes a substrate, an embedded semiconductor laser provided on the substrate, and a high mesa ridge type provided on the substrate and coupled to the embedded semiconductor laser in the light traveling direction. The embedded semiconductor laser has a waveguide ridge having an active layer and embedded layers that embed both sides of the waveguide ridge, and the high mesa ridge modulator has a high mesa ridge having a modulation layer. And a protective insulating film in contact with the side surface of the high mesa ridge, the protective insulating film having a thickness of 1 μm or more, and the equivalent refractive index of the high mesa ridge modulator is 0.998 of the equivalent refractive index of the semiconductor laser. It is characterized by being doubled to 1.0 times.

  According to the present invention, it is possible to suppress reflection of light between the embedded semiconductor laser and the high mesa ridge type modulator.

It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. FIG. 5 is a cross-sectional view taken along the line AA ′ of FIG. 4. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing in alignment with AA 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a top view for demonstrating the manufacturing method of the integrated optical semiconductor device which concerns on embodiment of this invention. It is sectional drawing along AA 'of FIG. It is sectional drawing along BB 'of FIG. It is a figure for demonstrating the equivalent refractive index method applied to the semiconductor laser or the modulator. It is a figure which shows the equivalent refractive index distribution of the horizontal direction (x-axis direction) of a semiconductor laser and a modulator. It is a figure which shows the calculation result of the reflectance between elements with respect to the film thickness of a protective insulating film.

  An integrated optical semiconductor device according to an embodiment of the present invention will be described with reference to the drawings. The same components are denoted by the same reference numerals, and repeated description may be omitted.

  FIG. 1 is a top view showing an integrated optical semiconductor device according to an embodiment of the present invention. 2 is a cross-sectional view taken along the line AA ′ of FIG. 1, and FIG. 3 is a cross-sectional view taken along the line BB ′ of FIG. An embedded semiconductor laser 12 and a high mesa ridge modulator 14 are provided on an n-type InP substrate 10 (substrate). The buried semiconductor laser 12 and the high mesa ridge type modulator 14 are coupled in the light traveling direction.

  The embedded semiconductor laser 12 includes a waveguide ridge 18 having an active layer 16 and an embedded layer 20 that embeds both sides of the waveguide ridge 18. The high mesa ridge type modulator 14 includes a high mesa ridge 24 having a modulation layer 22, a silicon nitride film 26 in contact with a side surface of the high mesa ridge 24, and a silicon oxide film 28 provided on the silicon nitride film 26. The active layer 16 of the embedded semiconductor laser 12 and the modulation layer 22 of the high mesa ridge type modulator 14 are made of InGaAsP, AlGaInAs, or the like, and have a well layer and a barrier layer.

  Openings are formed in the silicon nitride film 26 and the silicon oxide film 28 above the waveguide ridge 18 and the high mesa ridge 24. Through this opening, an electrode 30 is provided above the waveguide ridge 18, and an electrode 32 is provided above the high mesa ridge 24.

  As a feature of the present embodiment, the total film thickness of the silicon nitride film 26 and the silicon oxide film 28 is 1 μm or more. As a result, the equivalent refractive index of the high mesa ridge type modulator 14 becomes 0.998 times to 1.0 times the equivalent refractive index of the embedded semiconductor laser 12.

  Next, a method for manufacturing the integrated optical semiconductor device according to the embodiment will be described. First, as shown in FIGS. 4 and 5, the n-type InP cladding layer 34, the active layer 16, the p-type InP cladding layer 36, the diffraction grating layer 38, and the p-type InP cap are formed on the entire surface of the n-type InP substrate 10. Layer 40 is formed in sequence.

  Next, as shown in FIGS. 6 and 7, the diffraction grating layer 38 and the p-type InP cap layer 40 are removed while leaving only a part thereof, and are etched periodically to form a diffraction grating. The part where the diffraction grating is formed is a laser part that will later become a semiconductor laser, and the other part is a modulator part that later becomes an optical modulator.

  Next, as shown in FIGS. 8 and 9, a p-type InP cladding layer 42 is formed, and the diffraction grating layer 38 and the p-type InP cap layer 40 are embedded.

  Next, as shown in FIGS. 10 and 11, the laser part is masked with the insulating film 44, and the semiconductor layer of the modulator part is etched to the n-type InP cladding layer 34.

  Next, as shown in FIGS. 12 and 13, the modulation layer 22 and the p-type InP cladding layer 46 are selectively grown in order on the n-type InP cladding layer 34 in the modulator section. As a result, the semiconductor multilayer structure 48 of the laser part and the semiconductor multilayer structure 50 of the modulator part are formed side by side on the n-type InP substrate 10. Thereafter, the insulating film 44 is removed.

  Next, as shown in FIGS. 14 and 15, a p-type InP cladding layer 52 and a p-type InGaAs contact layer 54 are sequentially formed on the semiconductor stacked structures 48 and 50.

  Next, as shown in FIGS. 16 to 18, the p-type InGaAs contact layer 54 is patterned. As a result, the p-type InGaAs contact layer 54 which will later become the high mesa ridge of the optical modulator remains, and all the p-type InGaAs contact layer 54 on the semiconductor multilayer structure 48 is removed.

  Next, as shown in FIGS. 19 to 21, using the insulating film 56 as a mask, the semiconductor multilayer structure 48 is dry etched to form the waveguide ridge 18, and the p-type InGaAs contact layer 54 and the semiconductor multilayer structure 50 are dried. The high mesa ridge 24 is formed by etching. The high mesa ridge 24 is connected to the waveguide ridge 18.

  Next, as shown in FIGS. 22 to 24, both sides of the waveguide ridge 18 and the high mesa ridge 24 are embedded with a p-type InP layer 58, an n-type InP layer 60, and a p-type InP layer 62 (embedded layer). Thereafter, the insulating film 56 is removed.

  Next, as shown in FIGS. 25 to 27, a p-type InP cladding layer 64 and a p-type InGaAs contact layer 66 are sequentially formed on the entire surface.

  Next, as shown in FIGS. 28 to 30, the p-type InGaAs contact layer 66 is patterned. As a result, the p-type InGaAs contact layer 66 remains on the waveguide ridge 18 and the p-type InP layer 62 in the vicinity thereof and on the region that will later become the terrace of the semiconductor laser.

  Next, as shown in FIGS. 31 to 33, the p-type InP layer 58, the n-type InP layer 60, and the p-type InP layer 62 are wetted with a hydrochloric acid-based etchant using the p-type InGaAs contact layers 54 and 66 as a mask. Etch. Thus, a buried element having the waveguide ridge 18 and the p-type InP layer 58, the n-type InP layer 60, and the p-type InP layer 62 in the vicinity thereof and a high mesa ridge element having the high mesa ridge 24 are formed. Here, the embedded element is a semiconductor laser, and the high mesa ridge element is an optical modulator.

  Next, as shown in FIGS. 34 to 36, the p-type InGaAs contact layers 54 and 66 other than the portions where electrodes are formed later are selectively removed.

  Next, as shown in FIGS. 37 to 39, a silicon nitride film 26 and a silicon oxide film 28 are sequentially formed on the entire surface. Then, dry etching such as plasma ashing or reactive ion etching (RIE) is performed to form openings 68 in the silicon nitride film 26 and the silicon oxide film 28 at portions where electrodes are to be formed later.

  Next, as shown in FIGS. 1 to 3, electrodes 30 and 32 are formed. Thereafter, the back surface of the n-type InP substrate 10 is polished until the total thickness becomes about 100 μm, and a back electrode (not shown) is formed on the back surface of the n-type InP substrate 10. The integrated optical semiconductor device according to the embodiment is manufactured through the above steps.

  Then, the effect of this Embodiment is demonstrated. As described above, in the present embodiment, the thickness of the protective insulating film in contact with the side surface of the high mesa ridge 24 is set to 1 μm or more, and the equivalent refractive index of the high mesa ridge type modulator 14 is increased to increase the equivalent refractive index of the embedded semiconductor laser 12. 0.998 times to 1.0 times.

  Here, the equivalent refractive index method and the equivalent refractive index will be described. When the distribution in the y-axis direction is steeper than the distribution in the x-axis direction in a two-dimensional waveguide, it is known that an approximate analysis can be performed as a one-dimensional model, which is called an equivalent refractive index method. FIG. 40 is a diagram for explaining an equivalent refractive index method applied to a semiconductor laser or a modulator. First, as shown in FIG. 40A, the horizontal direction is the x-axis and the vertical direction is the y-axis, and the regions are divided according to the refractive index. For the sake of simplicity, a three-layer structure is used as an example. However, even if there are more layers, the same analysis can be performed.

  From the design conditions of the thickness and width of each layer, the distribution in the y-axis direction changes more rapidly than the distribution in the x-axis direction. Therefore, as shown in FIG. 40B, a one-dimensional light distribution is calculated for each region having the same refractive index distribution in the y-axis direction, and equivalent refractive indexes n1 'and n3' of the respective regions are obtained. The equivalent refractive index is an average refractive index felt by light propagating through a one-dimensional waveguide.

  Next, as shown in FIG. 40C, a one-dimensional light distribution in the x-axis direction is calculated based on the equivalent refractive indexes n1 'and n3' to obtain an equivalent refractive index nt. The light distribution as the two-dimensional waveguide is a product of the one-dimensional light distribution in the y-axis direction and the one-dimensional light distribution in the x-axis direction. This equivalent refractive index nt is an equivalent refractive index as a two-dimensional waveguide, and this nt is called an equivalent refractive index of the high-mesa ridge type modulator 14 or the embedded semiconductor laser 12.

  FIG. 41 is a diagram showing an equivalent refractive index distribution in the horizontal direction (x-axis direction) of the semiconductor laser and the modulator. The equivalent refractive index of the active layer and modulation layer regions is 3.419, the equivalent refractive index of the buried layer region is 3.17, the equivalent refractive index of the insulating film region is 1.6, and the refractive index of the groove region (air) is 1. .0. The refractive indexes of the p, n and i layers of the buried layer were the same. Since no light is distributed outside the groove of the modulator, the refractive index distribution up to the groove is used. Based on this equivalent refractive index distribution, the equivalent refractive index in the horizontal direction (equivalent refractive index of the high mesa ridge type modulator 14 or the embedded semiconductor laser 12) was calculated. The embedded semiconductor laser 12 has a mesa width of 10 μm, the active layer 16 has a width of 2 μm, and the high mesa ridge 24 has a width of 2 μm.

  When the silicon nitride film 26 having a thickness of 160 nm and the silicon oxide film 28 having a thickness of 1.6 μm are formed on the side surface of the high mesa ridge 24, the equivalent refractive index of the buried semiconductor laser 12 is 3.182 and the equivalent of the high mesa ridge type modulator 14. The refractive index was 3.177, and the inter-element reflectance was 6.2E-5%. In this case, the equivalent refractive index of the high mesa ridge type modulator 14 is 0.998 times the equivalent refractive index of the buried semiconductor laser 12 (3.177 / 3.182 = 0.998).

  On the other hand, when only the 160 nm-thick silicon nitride film 26 is formed on the side surface of the high mesa ridge 24, the equivalent refractive index of the buried semiconductor laser 12 is 3.182, the equivalent refractive index of the high mesa ridge type modulator 14 is 3.155, and the element The inter-reflectance was 1.8E-3%. Since the protective insulating film on the side surface of the ridge of the embedded semiconductor laser 12 is separated from the active layer 16 by 5 μm or more and light is not affected by the protective insulating film, the equivalent refractive index of the embedded semiconductor laser 12 is the protective insulating film. It does not depend on the film thickness.

  As a result, when the protective insulating film on the side surface of the high mesa ridge 24 is thickened, the equivalent refractive index of the high mesa ridge type modulator 14 is increased and the difference from the equivalent refractive index of the embedded semiconductor laser 12 is reduced, so that the inter-element reflectance can be reduced. I understood.

  FIG. 42 is a diagram illustrating a calculation result of the inter-element reflectance with respect to the thickness of the protective insulating film. In the calculation, the thickness of the silicon nitride film 26 of the first layer was set to 160 nm, and the thickness of the silicon oxide film 28 of the second layer was changed. As a result, it was found that when the thickness of the protective insulating film was 1 μm or more, the inter-element reflectance could be 3.0E-4% or less. Here, it has been experimentally confirmed that when the inter-element reflectance is larger than 3.0E-4%, the oscillation stability of the semiconductor laser is rapidly deteriorated.

  Therefore, in this embodiment, the thickness of the protective insulating film is set to 1 μm or more. In this case, the equivalent refractive index of the high mesa ridge type modulator 14 is 0.998 times to 1.0 times the equivalent refractive index of the embedded semiconductor laser 12. Thereby, since reflection of the light between both can be suppressed, the deterioration of the modulation waveform and the laser oscillation characteristic due to the reflected return light can be prevented.

  In the present embodiment, a silicon nitride film 26 in contact with the side surface of the high mesa ridge 24 and a silicon oxide film 28 provided on the silicon nitride film 26 are used as the protective insulating film. Here, the top width of the high mesa ridge 24 is as narrow as 3 μm or less, and the silicon nitride film 26 has a high etching rate. Therefore, when the protective insulating film is only the silicon nitride film 26, the silicon nitride film 26 is peeled off at the side of the ridge during dry etching for forming the electrode contact opening 68 at the top of the high mesa ridge 24. As a result, the semiconductor on the side of the ridge and the electrode come into contact with each other without the contact layer, and a reactive current is generated. On the other hand, when the protective insulating film is only the silicon oxide film 28, such a problem hardly occurs because the etching rate is slow. However, since the silicon oxide film 28 is inferior in quality to the silicon nitride film 26, a leak current is likely to occur at the contact interface between the silicon oxide film 28 and the semiconductor. On the other hand, these problems can be avoided by using the protective insulating film of this embodiment.

  In order to prevent the silicon nitride film 26 from disappearing on the side surface of the ridge, the film thickness of the silicon oxide film 28 is desirably three times or more the film thickness of the silicon nitride film 26.

In this embodiment, two layers of SiN / SiO ₂ are used as the insulating film covering the side surface of the ridge. However, for convenience of the process, three layers of SiN / SiO ₂ / SiN or more multilayer films are used. Also good. In this embodiment, the optical modulator has a Mach-Zehnder (MZ) structure. However, the present invention is not limited to this, and a single linear waveguide structure may be used.

  In the present embodiment, the width of the active layer 16 of the buried semiconductor laser 12 is 2 μm and the mesa width is 10 μm, and the width of the modulation layer 22 (high mesa ridge width) of the high mesa ridge modulator 14 is 2 μm. However, the present invention is not limited to this, and the width of the active layer 16 and the modulation layer 22 can be set to 1 μm or more due to processing limitations, and can be set to 3 μm or less where only the fundamental mode light distribution is allowed. The mesa width of the embedded semiconductor laser 12 can be set to 8 μm or more so that the light distribution is not affected by the groove, and can be set to 12 μm or less determined by the capacitance. If it is this range, the reflectance between elements can be represented by the curve of FIG.

10 n-type InP substrate (substrate)
12 buried semiconductor laser 14 high mesa ridge type modulator 16 active layer 18 waveguide ridge 20 buried layer 22 modulation layer 24 high mesa ridge 26 silicon nitride film (protective insulating film)
28 Silicon oxide film (protective insulating film)

Claims (3)

A substrate,
An embedded semiconductor laser provided on the substrate;
A high mesa ridge type modulator provided on the substrate and coupled with the buried semiconductor laser in the light traveling direction;
The embedded semiconductor laser has a waveguide ridge having an active layer, and an embedded layer that embeds both sides of the waveguide ridge,
The high mesa ridge type modulator has a high mesa ridge having a modulation layer, and a protective insulating film in contact with a side surface of the high mesa ridge,
The integrated light, wherein the protective insulating film has a thickness of 1 μm or more, and the equivalent refractive index of the high mesa ridge modulator is 0.998 times to 1.0 times the equivalent refractive index of the semiconductor laser. Semiconductor device.   2. The integrated optical semiconductor device according to claim 1, wherein the protective insulating film includes a silicon nitride film in contact with a side surface of the high mesa ridge and a silicon oxide film provided on the silicon nitride film.   3. The integrated optical semiconductor device according to claim 2, wherein the thickness of the silicon oxide film is at least three times the thickness of the silicon nitride film.

Images