JP2012060158A - Optical semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical semiconductor device that comprises a linear waveguide linearly formed in a reversed-mesa direction and an oblique waveguide formed in a direction oblique to the reversed-mesa direction, which inhibits abnormal growth in fabricating an embedded waveguide and can reduce propagation loss by downsizing of the device itself.SOLUTION: The optical semiconductor device comprises a semiconductor laser 11 comprising an active layer 102, and a waveguide 12 comprising a waveguide layer 115 that is formed at the same height as the active layer 102 and guides light. The semiconductor laser 11 and waveguide 12 are formed as mesa structures, and the formed mesa structure of the waveguide 12 is lower than that of the semiconductor laser 11. On the side of the mesa structure of the semiconductor laser 11, a semiconductor layer 122 is laminated, which embeds the mesa structure of the waveguide 12.

Description

  The present invention relates to an optical semiconductor device that is a wavelength tunable semiconductor laser used as a light source for optical fiber communication and a light source for optical measurement, and more particularly, a light source for an optical wavelength (frequency) multiplexing system in optical communication, and light covering a wide wavelength band. It is suitable for a wavelength variable light source for measurement.

  While research on optical wavelength (frequency) multiplex communication systems has been conducted in response to an increase in communication information, a wide range of wavelength adjustment functions are required as a light source for transmission and a tunable light source for synchronous detection. In the field of optical measurement, it is desired to realize a wavelength tunable light source that covers a wide wavelength band.

  Various variable wavelength light sources have been studied so far, and they can be broadly divided into those that change continuously in one oscillation mode and those that change discontinuously with mode jumping. Can be divided. In consideration of application to an actual system, it is preferable that the wavelength changes continuously from the viewpoint of controllability. In order to control the wavelength change, two types are mainly used: one that controls the refractive index by changing the temperature and one that uses the refractive index change by current injection. Faster wavelength switching is possible by using the refractive index change by injection.

  Distributed lasers (DBR lasers) and double waveguide lasers (TTG lasers) have been studied as semiconductor lasers that can continuously change the oscillation wavelength using the refractive index change caused by current injection. As the continuous wavelength variable width, a value of 4.4 nm for DBR laser and 7 nm for TTG laser has been reported. In recent years, so-called short cavity DBR lasers in which the active layer region is shortened in order to suppress the mode jump of the DBR laser have been studied.

  As the discontinuous wavelength variable width with mode jump, a value of 10 nm is obtained in the DBR laser. Further, as a semiconductor laser that is discontinuous but has a wide wavelength tunable width, a Y-branch laser, a super-periodic structure diffraction grating laser, and the like have been prototyped, and a wavelength tunable width of 50 nm to 100 nm has been obtained.

However, the above prior art has the following problems.
In the TTG laser, a current is injected into an active waveguide layer that amplifies light to cause a laser oscillation operation, and a current is independently supplied to a wavelength control inactive waveguide layer formed in the immediate vicinity of the active waveguide layer. By injecting, the oscillation wavelength is changed. Here, if the period of the diffraction grating is Λ and the equivalent refractive index of the waveguide is n, the Bragg wavelength λb is expressed by the following equation (1).
λb = 2nΛ (1)

The laser oscillates in one resonance longitudinal mode near this Bragg wavelength. When current is injected into the inactive waveguide layer, the equivalent refractive index of the waveguide changes, and the Bragg wavelength also changes in proportion to the equation (1). Here, the change rate Δλb / λb of the Bragg wavelength is equal to the change rate Δn / n of the equivalent refractive index as shown in the following equation (2).
Δλb / λb = Δn / n (2)

In addition, the resonance longitudinal mode wavelength also changes as the equivalent refractive index changes due to current injection. In the case of a TTG laser, since the equivalent refractive index of the entire resonator changes uniformly, the change rate Δλr / λr of the resonant longitudinal mode wavelength becomes equal to the change rate Δn / n of the equivalent refractive index. That is, the following expression (3) is obtained.
Δλr / λr = Δn / n (3)

  From the formulas (2) and (3), in the TTG laser, the change in the Bragg wavelength is equal to the change in the resonance longitudinal mode, so that the oscillation wavelength continuously changes while the first oscillation mode is maintained. Has characteristics.

  However, in order to perform single transverse mode oscillation operation, the width of the double waveguide needs to be 1 μm to 2 μm, and the thickness of the n-type spacer layer formed between the active layer and the wavelength control layer Since it is necessary to reduce the thickness to 1 μm or less, the buried structure used in a normal semiconductor laser cannot be formed, and a structure for efficiently injecting current into each waveguide layer is manufactured. There was a problem that it was very difficult. Further, since it is different from a normal semiconductor laser structure, it is difficult to integrate with a semiconductor optical amplifier or the like, and there is a problem that a multifunctional integrated device cannot be configured.

In contrast, the DBR laser has a structure in which an active waveguide layer and an inactive waveguide layer for amplifying light are connected in series. Therefore, a buried stripe structure for current confinement as in a normal semiconductor laser is used. Further, independent current injection into each waveguide layer can be easily realized by separating the electrodes formed above each waveguide layer. The mechanism for changing the Bragg wavelength by changing the equivalent refractive index by injecting current into the inactive waveguide layer is the same as that of the TTG laser, but the region where the equivalent refractive index changes is limited to a part of the resonator. For this reason, the change amount of the Bragg wavelength does not match the change amount of the resonance longitudinal mode wavelength. The change ratio Δλr / λr of the resonant longitudinal mode wavelength is shorter than the change ratio Δn / n of the equivalent refractive index by the ratio of the effective length Le of the distributed reflector to the total resonator length Lt. ).
Δλr / λr = (Le / Lt) · (Δn / n) (4)

  Therefore, from the equations (2) and (4), the DBR laser has a drawback that the mode jump occurs because the Bragg wavelength and the resonance longitudinal mode wavelength are relatively separated as the wavelength control current is injected. It was. In order to prevent mode jumping, it is necessary to provide a phase adjustment region in which no diffraction grating is formed, and to match the amount of change in the resonant longitudinal mode and the amount of change in the Bragg wavelength by injecting current there.

  However, this method requires an external circuit for controlling the wavelength control current to the two electrodes, and there is a problem that the device structure and control become complicated. Another method that does not cause mode jump is a short cavity DBR laser that shortens the cavity length and widens the longitudinal mode interval. However, it is necessary to shorten the active layer, and it is difficult to obtain a large output. There was a problem of being.

  The continuous wavelength tunable width in the TTG laser and the DBR laser is limited to the amount of change in the refractive index of the wavelength control layer, and the value remains at about 4 nm to 7 nm. In order to further widen the wavelength variable width, it is necessary to use a means that allows mode jumping and that the wavelength change amount of the wavelength filter is larger than the refractive index change amount. Both the Y-branch laser and the super-periodic structure diffraction grating laser use a means for increasing the filter wavelength change amount more than the refractive index change amount. In these lasers, in order to change the filter wavelength greatly and to obtain sufficient wavelength selectivity, it is necessary to control the current flowing through the two electrodes, and also to provide an electrode for controlling the resonance longitudinal mode wavelength. Become. As a result, in order to adjust the oscillation wavelength, it is necessary to control the injection current to the three electrodes, and there is a problem that the control becomes very complicated.

  In order to solve these problems, the oscillation wavelength can be continuously changed by about 4 nm to 7 nm by controlling the injection current to one electrode, and the current injection into the active waveguide layer and the inactive waveguide layer is also efficient. A semiconductor laser has been developed that can change the oscillation wavelength over a range of about 50 nm to 100 nm by obtaining a semiconductor laser that can be performed well and mode jumping, but with an injection current to two electrodes. Non-Patent Document 1 and Patent Document 1 disclose the structure of a distributed active DFB laser (TDA-DFB-LD). According to this structure, the active layer volume can be sufficiently secured, so that high output can be achieved.

  FIG. 7 shows a cross section of the structure of the distributed active DFB laser disclosed in Non-Patent Document 1.

  As shown in FIG. 7, the distributed active DFB laser has an active waveguide layer 402 and an inactive waveguide layer (wavelength control region) 403 alternately formed on the lower clad 401 with constant lengths La and Lt, respectively. It has a structure that is periodically coupled in series. An upper cladding 404 is formed on the active waveguide layer 402 and the inactive waveguide layer 403, and irregularities, that is, a diffraction grating 405, are formed between the active waveguide layer 402 and the inactive waveguide layer 403 and the upper cladding 404. Is formed. Furthermore, an active layer electrode 407 and a wavelength control electrode 408 are provided on the upper clad 404 corresponding to the active waveguide layer 402 and the inactive waveguide layer 403, respectively. A common electrode 410 is provided below the lower clad 401. In this distributed active DFB laser, gain is generated along with light emission by injection of the current Ia into the active waveguide layer 402, and it corresponds to the period of the diffraction grating 405 formed between the active waveguide layer 402 and the upper cladding 404. Only the wavelength is selectively reflected to cause laser oscillation.

On the other hand, when the current It is injected into the inactive waveguide layer 403, the refractive index of the inactive waveguide layer 403 changes due to the plasma effect generated according to the carrier density. And the optical period of the diffraction grating 405 formed between the upper cladding 404 and the upper cladding 404 change. Then, the equivalent refractive index of the inactive waveguide layer 403 changes, and the resonant longitudinal mode wavelength is shifted to the short wavelength side by the ratio of the length of the wavelength control region to the length of one cycle. If the length of one cycle of the repetitive structure is L and the wavelength control region length is Lt, the rate of change of the resonance longitudinal mode wavelength is expressed by the following equation (5).
Δλr / λr = (Lt / L) · (Δn / n) (5)

On the other hand, each wavelength of the plurality of reflection peaks is also shifted to the short wavelength side as a result of a change in equivalent refractive index due to current injection. Since the reflection peak wavelength is proportional to the average equivalent refractive index change within one period of the repetitive structure, the change ratio Δλs / λs of the reflection peak wavelength is expressed by the following equation (6).
Δλs / λs = (Lt / L) · (Δn / n) (6)

  From the equations (5) and (6), the reflection peak wavelength and the resonance longitudinal mode wavelength are shifted by the same amount. Therefore, in this laser, the wavelength continuously changes while maintaining the mode in which it oscillates first.

A cross section of the structure of the distributed active DFB laser disclosed in Patent Document 1 is shown in FIG.
Similarly to the distributed active DFB laser shown in FIG. 7, this distributed active DFB laser has an active waveguide layer 502 and an inactive waveguide layer 503 alternately formed on the lower clad 501 with constant lengths La and Lt, respectively. It has a structure that is periodically coupled in series. An upper clad 504 is formed on the active waveguide layer 502 and the inactive waveguide layer 503, and unevenness, that is, a diffraction grating 505 is formed between the active waveguide layer 502 and the upper clad 504. Furthermore, an active layer electrode 507 and a wavelength control electrode 508 are provided on the upper clad 504 corresponding to the active waveguide layer 502 and the inactive waveguide layer 503, respectively. A common electrode 510 is formed below the lower clad 501. In this distributed active DFB laser, the diffraction grating 505 is formed only partially, but the wavelength continuously changes in the same manner as the distributed active DFB laser of FIG.

  Further, in Patent Document 1, as shown in FIG. 9, it has the same structure as the distributed active DFB laser shown in FIG. 8, and the repetition periods of the active waveguide layer 502 and the inactive waveguide layer 503 are L1, A structure in which two different lasers L2 are coupled in series is also disclosed. Note that members that are substantially the same as those shown in FIG. 9 are given the same reference numerals, and descriptions thereof are omitted.

  On the other hand, a tunable array element that increases the tunable width by arraying tunable lasers has also been developed. Even if the wavelength tunable width of one wavelength tunable laser is about 7 nm, if this is integrated into a 6-element array, the wavelength tunable width 6 times is obtained. Here, FIG. 10 shows a plane of the laser array integrated element. As shown in FIG. 10, the laser array integrated element is provided on the same substrate 601, and includes a semiconductor laser (LD) portion 611, a waveguide portion 612, a coupler 613, a semiconductor amplifier (SOA). ) Part 614. The LD unit 611 includes six semiconductor lasers 611a to 611f. These six semiconductor lasers 611a to 611f are arranged in parallel. The waveguide section 612 includes waveguides 612a to 612f that connect the semiconductor lasers 611a to 611f and the coupler 613, respectively. Therefore, the output light from the LD unit 611 propagates through the waveguide unit 612 and is introduced into the coupler 613, and is guided to one waveguide by the coupler 613. Since the output is reduced by the coupler 613, the SOA 614 amplifies the output to compensate for this. In FIG. 10, a multimode interference (MMI) structure is used as the coupler 613, but a funnel type structure or the like is also proposed. The semiconductor lasers 611a to 611f need to be spaced to some extent in order to obtain thermal separation and electrical separation. However, since the semiconductor lasers 611a to 611f are combined into one waveguide by the coupler 613, the semiconductor lasers 611a to 611f are coupled to each other. The waveguide to the device 613 may be a curved waveguide or an oblique waveguide extending obliquely with respect to the extending direction of the semiconductor lasers 611a to 611f.

Japanese Patent No. 3237733 Ishii et al., “A Tunable Distributed Amplification DFB Laser (TDA-DFB-LD)”, IEEE Photonics Letters, vol.10, no.1, January 1998, p.30-32

  In general, when a waveguide of a semiconductor laser is manufactured with a buried heterostructure (BH), the waveguide is formed in a so-called reverse mesa direction in the <011> direction in the case of a (100) substrate. If the waveguide is formed in the forward mesa direction orthogonal to this, the crystal plane inappropriate for the subsequent process is generated at the time of regrowth. Therefore, it is difficult to manufacture. Here, in FIG. 11, after the mesa structure was fabricated, the insulating mask 709 at the time of mesa structure fabrication was used as it was as a selective growth mask, and both sides were buried and regrown with a semi-insulator 722 in which Fe was doped into InP. The rear cross section is shown schematically. 11A is a cross-sectional view orthogonal to the waveguide when the waveguide is manufactured in the reverse mesa direction, and FIG. 11B is orthogonal to the waveguide when the waveguide is manufactured in the forward mesa direction. FIG. As shown in FIG. 11B, when a waveguide is formed in the forward mesa direction, the crystal grows abnormally so that the semi-insulator 722 is raised on the mask (insulating film) 709, and the mask 709. A space 730 is formed on the top, making it difficult to manufacture the device. This is because the higher the height of the mesa structure, the greater the amount of semiconductor regrowth beside it, so the amount of abnormal growth also increases. In FIG. 11, reference numeral 701 indicates n-type InP, reference numeral 702 indicates an active layer, reference numeral 704 indicates p-type InP, and reference numeral 706 indicates a contact layer.

  In the LD array integrated element, as described above, it is necessary to form a waveguide at an angle shifted from the curved waveguide or the reverse mesa direction at the waveguide portion connecting the LD and the coupler. Here, FIG. 12A shows a plane schematically showing an array element in which LDs are arranged in the reverse mesa direction, FIG. 12B shows an xx ′ cross section in FIG. FIG. 12C shows a yy ′ cross section in FIG. The array element shown in FIG. 12A is the same as the array element shown in FIG. 10 described above, and the same members are denoted by the same reference numerals and the description thereof is omitted. In FIG. 12A, A11 indicates the reverse mesa direction, A12 indicates the extending direction of the waveguide 812f, and θ10 indicates the angle between the reverse mesa direction A11 and the extending direction A12 of the waveguide 812f. 12B and 12C, reference numeral 901 indicates n-type InP, reference numeral 902 indicates an active layer, reference numeral 904 indicates p-type InP, reference numeral 906 indicates a contact layer, and reference numeral 909 indicates insulation. Reference numeral 915 denotes a waveguide layer, and reference numeral 922 denotes semi-insulating InP.

  In the waveguide portion connecting the LD and the coupler, there is no problem even if the contact layer is buried because there is no need to inject current, and therefore the etching mask may be peeled off during regrowth and regrowth. However, when the amount of embedding is large, abnormal growth occurs and a space is formed (see FIG. 11B), or a eave 931 is formed as shown in FIG. 12C. This becomes more prominent as the angle from the reverse mesa direction increases. In addition, the higher the height of the mesa structure, the more prominent. Thereby, for example, when performing metal wiring on this, various problems, such as disconnection, may arise.

  In order to solve this problem (abnormal growth such as generation of space or eaves on the contact layer), the angle from the reverse mesa direction should be made as small as possible to prevent abnormal growth. If the angle is made shallower, the distance between the LD and the coupler becomes longer, leading to an increase in the element length and the size of the device itself. Furthermore, the propagation loss increases with the increase in size of the apparatus.

  Accordingly, the object of the present invention has been proposed in view of the above-described problems, and is a linear waveguide portion formed linearly in the reverse mesa direction and an inclined waveguide formed in a direction inclined with respect to the reverse mesa direction. An optical semiconductor device comprising: an optical semiconductor device capable of suppressing abnormal growth during the fabrication of a buried waveguide, miniaturizing the device itself, and reducing propagation loss.

An optical semiconductor device according to a first invention for solving the above-described problem is
An optical semiconductor device having a linear waveguide portion formed linearly in a reverse mesa direction and an inclined waveguide portion connected to the linear waveguide portion and formed in a direction inclined with respect to the reverse mesa direction. And
The linear waveguide portion comprises an active layer;
The inclined waveguide portion is formed at the same height as the active layer and includes a waveguide layer that guides light;
The linear waveguide portion is formed of a mesa structure;
The inclined waveguide portion is formed with a mesa structure, and the mesa structure of the inclined waveguide portion is formed lower than the mesa structure of the linear waveguide portion,
A semiconductor layer is stacked on a side portion of the mesa structure of the linear waveguide portion, and a mesa structure of the inclined waveguide portion is embedded with the semiconductor layer.

An optical semiconductor device according to a second invention for solving the above-described problem is an optical semiconductor device according to the first invention,
The mesa height of the linear waveguide portion is 2.5 μm or more,
The mesa height of the inclined waveguide portion is less than 2.5 μm.

An optical semiconductor device according to a third invention for solving the above-mentioned problem is an optical semiconductor device according to the second invention,
The height of the mesa structure of the inclined waveguide portion is equal to or greater than the thickness of the waveguide layer.

An optical semiconductor device according to a fourth invention for solving the above-described problem is an optical semiconductor device according to any one of the first to third inventions,
The linear waveguide portion and the inclined waveguide portion are formed on an n-type InP substrate or a semi-insulating InP substrate,
A p-InP layer is formed on the waveguide layer;
The p-InP layer has a thickness of 300 nm or less.

An optical semiconductor device according to a fifth invention for solving the above-described problem is an optical semiconductor device according to any one of the first to fourth inventions,
The semiconductor layer is a layer containing a semi-insulator doped with ruthenium.

An optical semiconductor device according to a sixth invention for solving the above-described problem is an optical semiconductor device according to any one of the first to fifth inventions,
The inclined waveguide portion may be an inclined waveguide extending linearly in a direction inclined by 5 degrees or more with respect to the reverse mesa direction, or a curved waveguide extending in a curved shape.

A semiconductor laser array according to a seventh invention for solving the above-described problem is characterized by including the optical semiconductor device according to any one of the first to sixth inventions.

A ring resonator according to an eighth invention for solving the above-described problem is characterized by including the optical semiconductor device according to any one of the first to sixth inventions.

A Mach-Zehnder interferometer according to a ninth invention for solving the above-described problem is characterized by including the optical semiconductor device according to any one of the first to sixth inventions.

  According to the optical semiconductor device of the present invention, in an optical semiconductor device having an inclined waveguide portion such as an LD array element, abnormal growth during the fabrication of a buried waveguide is suppressed, and the inclination angle from the reverse mesa direction is reduced at a short distance. An embedded waveguide connecting the straight waveguide portion and the inclined waveguide portion can be manufactured with a large waveguide, and the device itself can be miniaturized. Accordingly, propagation loss can be reduced.

1 is a plan view of a first embodiment of an optical semiconductor device according to the present invention. It is II-II 'sectional drawing shown in FIG. FIG. 3 is a cross-sectional view taken along the line III-III ′ shown in FIG. 1. It is a figure which shows the manufacturing process of 1st embodiment of the optical semiconductor device which concerns on this invention. It is a top view which shows typically 2nd embodiment of the optical semiconductor device which concerns on this invention. It is a top view which shows typically 3rd embodiment of the optical semiconductor device which concerns on this invention. It is explanatory drawing of the conventional distributed active DFB laser. It is sectional drawing of the other conventional distributed active DFB laser. It is sectional drawing of the other conventional distributed active DFB laser. It is a top view of the conventional wavelength variable array element. It is sectional drawing of the optical semiconductor device with which the conventional wavelength variable array element comprises, and a waveguide part. It is explanatory drawing of the conventional wavelength variable array element.

  Embodiments of an optical semiconductor device according to the present invention will be described in detail in the following embodiments.

[First embodiment]
A first embodiment of an optical semiconductor device according to the present invention will be described in detail with reference to FIGS.
In the present embodiment, a case where the present invention is applied to a semiconductor laser array will be described.
1 is a plan view schematically showing an optical semiconductor device, FIG. 2 is a cross-sectional view taken along the line II-II ′ shown in FIG. 1, and FIG. 3 is a cross-sectional view taken along the line III-III ′ shown in FIG. FIG. 4 is a diagram showing a manufacturing process of the optical semiconductor device, and FIGS. 4A to 4D show a manufacturing process of the II-II ′ cross-sectional structure shown in FIG. 1, and FIGS. 1 shows a manufacturing process of the III-III ′ cross-sectional structure shown in FIG.

  In the optical semiconductor device 10 according to the present embodiment, as shown in FIG. 1, an n-type InP (100) substrate is used as the substrate 1, and the waveguide direction of the LD is the reverse mesa direction A1 of the <011> direction. The optical semiconductor device 10 includes a semiconductor laser (LD) unit 11, a waveguide unit 12, a coupler 13, and a semiconductor amplifier (SOA) 14. The semiconductor laser unit 11 includes six semiconductor lasers 11a to 11f. These six semiconductor lasers 11a to 11f are arranged in parallel. The waveguide section 12 is composed of waveguides 12 a to 12 f that connect the semiconductor lasers 11 a to 11 f and the coupler 13. These waveguides 12a to 12f extend in a direction inclined with respect to the reverse mesa direction A1. For example, the waveguide 12f is formed to extend in an inclination direction A2 that is inclined by an angle θ1 with respect to the reverse mesa direction A1. And the output light from each semiconductor laser 11a-f is couple | bonded with the coupler 13 through each waveguide 12a-f. The combined light is amplified by the semiconductor amplifier 14 and output.

  As shown in FIG. 2, the semiconductor laser 11f has a structure in which an n-type InP101, an active layer 102, a p-type InP104, a contact layer 106, and an insulating film 109 are stacked in order from the lower layer side. A mesa structure 131 is formed of a part of the upper part of the n-type InP 101, the active layer 102, and the p-type InP 104. In addition, a semi-insulating InP (semi-insulator) 122 is laminated in contact with the upper part of the n-type InP 101, the active layer 102, and both sides of the contact layer 106. The semiconductor lasers 11a to 11e have the same layer structure as the semiconductor laser 11f, and the description thereof is omitted.

  As shown in FIG. 3, the waveguide 12f has a structure in which an n-type InP101, a waveguide layer 115, and a p-type InP104 are stacked in order from the lower layer side. A mesa structure 132 is formed of a part of the upper portion of the n-type InP 101, the waveguide layer 115, and the p-type InP104. The upper part of the n-type InP 101, the waveguide layer 115, and the p-type InP 104 are embedded with a semi-insulating InP (semi-insulator) 122. The height (mesa height) h2 of the mesa structure 132 in the waveguide 12f is formed lower than the height (mesa height) h1 of the mesa structure 131 in the semiconductor laser 11f. The waveguides 12a to 12e have the same layer structure as the waveguide 12f, and the description thereof is omitted.

  Here, a manufacturing method of the above-described semiconductor laser (straight waveguide portion) and waveguide (tilted waveguide portion) will be described with reference to FIG.

First, as shown in FIGS. 4A and 4E, a GaInAsP active layer 102 or a GaInAsP waveguide layer 115 is grown on an n-type InP substrate 101, and further a p-type InP cladding layer 104, a GaInAsP contact. Layer 106 is grown. An active layer may be grown in a region having a gain, such as LD or SOA, and a waveguide layer may be grown in a region where a waveguide or a coupler is manufactured, by a bad joint method using selective growth. In this embodiment, the metal organic chemical vapor deposition method (MOCVD method) is used. Further, after depositing a SiO ₂ insulating film, an etching mask (waveguide-shaped etching mask) 109 for forming a waveguide is formed on the surface of the GaInAsP contact layer 106 using a method such as lithography and reactive ion etching (RIE). (Etching mask forming step).

Subsequently, as shown in FIGS. 4B and 4F, the semiconductor is etched by a dry etching technique using the etching mask 109 made of the SiO ₂ insulating film as a mask (first etching step). Specifically, the GaInAsP contact layer 106 and the p-type InP cladding layer 104 are partially etched (removed). In this embodiment, inductively coupled plasma (ICP) RIE is used. In this embodiment, since the p-type InP clad 104 on the active layer 102 is 2.0 μm thick and the contact layer 106 is 0.2 μm thick, the etching depth is 2.0 μm.

  Subsequently, as shown in FIGS. 4C and 4G, while leaving the etching mask 109 of the semiconductor laser portion, only the etching mask 109 on the waveguide portion 12 is removed (etching mask removing step), Etching was again performed by ICP-RIE (second etching step). As a result, a two-step mesa structure is formed in the semiconductor laser portion by two-step etching as shown in the figure. On the other hand, since the p-type InP layer 104 and the contact layer 106 on the waveguide layer 115 are also etched in the waveguide portion, a mesa structure having a first height is formed. That is, in the waveguide portion, a mesa structure having a height corresponding to the depth of etching produced in the first etching step is formed. In the second etching, 1.5 μm was etched, so that a mesa structure having a total height of 3.5 μm was formed in the semiconductor laser part, and a mesa structure having a height of 2.0 μm was formed in the waveguide part. It was.

  Subsequently, as shown in FIGS. 4D and 4H, the mesa structure was buried and regrown with a semi-insulating InP doped with Ru (semi-insulator regrowth step). In FIG. 4D, since selective growth is performed with the etching mask left, a crystal grows only on the side of the mesa. The semiconductor laser portion has a high mesa structure of 3.5 μm, but abnormal growth does not occur because it is a waveguide in the reverse mesa direction. That is, in the semiconductor laser part 11, the semi-insulator 122 is laminated on the side part of the mesa structure. On the other hand, in FIG. 4H, since the etching mask is removed, the crystal grows as a whole. Since the mesa height is relatively low at 2.0 μm, the waveguide is inclined from the reverse mesa direction, but grows without abnormal growth. That is, in the waveguide portion 12, the mesa structure is embedded with the semi-insulator 122.

  Therefore, according to the optical semiconductor device 10 according to the present embodiment, the waveguide layer can be regrown without abnormal growth, so that the metal wiring formed on the device surface is not disconnected. Before applying this embodiment, the upper part of the abnormally grown portion has irregular irregularities, causing the wiring to become thin or thin, and the resistance value to increase, even if no disconnection occurs during device fabrication. However, the application of the present invention enables wiring as designed. From the current-voltage characteristics of the diode, resistance common to ordinary semiconductor lasers can be obtained. A value of 5Ω was obtained. In addition, a waveguide having a large inclination angle from the reverse mesa direction at a short distance can be formed, and an embedded waveguide connecting the straight waveguide portion and the inclined waveguide portion can be manufactured, and the device itself can be miniaturized. Accordingly, propagation loss can be reduced.

  In addition, according to the method for manufacturing the optical semiconductor device according to the present embodiment, the semiconductor laser is etched in the second etching step for the semiconductor from which the etching mask on the waveguide portion 12 has been removed in the etching mask removing step. The mesa structure 131 of the portion 11 and the mesa structure 132 of the waveguide portion 12 have different heights. Then, by performing a semi-insulator regrowth process after the second etching process, the semi-insulator 122 is stacked on the side of the mesa structure 131 of the semiconductor laser part 11, while the mesa structure 132 of the waveguide part 12. Is embedded with a semi-insulator 122. Therefore, it is possible to reliably manufacture the optical semiconductor device 10 that can suppress abnormal growth during the fabrication of the buried waveguide, reduce the size of the device itself, and reduce the propagation loss.

  In the present embodiment, the distributed active DFB laser is used as the semiconductor laser unit 11. However, other wavelength tunable lasers can also be used. Further, although the description has been made using the MMI as the coupler 13, other couplers such as a funnel type or a multistage Y branch may be used. The important point of the present invention is to reduce the height of the mesa structure of the waveguide having an angle inclined from the reverse mesa direction, not the type of each element constituting the optical semiconductor device which is an integrated element. It is.

  In the present embodiment, description has been made using a layer structure of a combination of InP and GaInAsP. However, not only this semiconductor material but also other semiconductor materials such as GaAs, AlGaAs, AlGaAsP, and GaInNAs can be used. The active layer and the waveguide layer can be not only bulk but also a quantum well structure or a low-dimensional quantum well structure such as a quantum wire or a quantum dot. In addition, an optical confinement layer having an intermediate refractive index between the core layer and the clad layer may be provided above and below the core layer of the active layer or the waveguide layer to form a separation heterostructure (SCH) or the like. .

  As a crystal growth method, not only the MOCVD method but also other growth methods such as a molecular beam epitaxial growth method (MBE) can be used.

As an etching mask and a mask for selective growth, not only SiO ₂ but also SiN, other films, or a mixed film thereof can be used.

  As the etching of the semiconductor, not only ICP-RIE but also other etching methods such as parallel plate RIE and ECR-RIBE can be used, as long as the method of manufacturing the shape of the optical semiconductor device according to the present embodiment is possible. It is possible to use.

  Also, depending on the etching method, the etching depth on the side (side) of the mesa structure may be different from the etching depth at a place away from the mesa structure, but the etching depth on the side of the mesa structure is desired. It is important to be deep.

  In this embodiment, the height of the mesa structure of the semiconductor laser part 11 is 3.5 μm and the height of the mesa structure of the waveguide part 12 is 2.0 μm. As a result of changing the height of the mesa structure, Abnormal growth can be suppressed by setting the height of the mesa structure of the waveguide portion to less than 2.5 μm. In this embodiment, the first-stage etching is 2.0 μm and the second-stage etching is 1.5 μm. However, the height of the mesa structure in the waveguide portion is less than 2.5 μm. The etching amount of the second and second stages may be changed. There is no restriction on the height of the mesa structure of the waveguide extending in the reverse mesa direction as in the semiconductor laser portion.

  Normally, in the case of a semiconductor laser having a wavelength band of 1.5 μm, the thickness of the waveguide layer is designed to be about 0.1 μm to 0.5 μm in consideration of single mode conditions and the like. In the present invention, the height of the mesa structure in the curved portion of the waveguide is lowered. However, if the waveguide layer disappears, light cannot be guided. Further, since it is necessary to perform optical confinement in the lateral direction of the waveguide, the height of the mesa structure of the waveguide portion is required to be at least the thickness of the waveguide layer.

  Further, in this embodiment, all the semiconductor lasers 11 that are linear waveguide portions have a high mesa structure, but at least an inclined waveguide portion, specifically, a direction inclined at a predetermined angle with respect to the reverse mesa direction. The height of the mesa structure of the inclined waveguide extending in a straight line or the curved waveguide extending in a curved line may be lowered, and the mesa structure in which a part of the straight waveguide part is low may be used. Is possible. That is, even in the straight waveguide portion, it is possible to make a part for injecting a current a high mesa structure and the other a low mesa structure.

  In the present embodiment, description has been made using Ru-doped InP as a semi-insulator that is a material for embedding a mesa structure, but Fe-doped InP can also be used. However, by doping with Ru, it is possible to suppress interdiffusion between Zn and Fe, which are dopants for forming the p-type, so that it is possible to obtain an insulating feature as designed. For this reason, a distributed active DFB laser or the like that can change the wavelength with a current as in this embodiment can operate even with a high-frequency signal for high-speed wavelength switching.

  In this embodiment, the (100) substrate is used and the LD direction is the <011> reverse mesa direction. However, when the substrate surface orientation is different, the LD direction is a crystal direction equivalent to the <011> direction. It is possible to think.

  In the present embodiment, the inclination of the waveguide portion 12 from the reverse mesa direction is approximately 40 degrees. When the height of the mesa structure is about 3.5 μm, abnormal growth occurs even if the inclination from the reverse mesa direction is about 10 degrees. However, according to the present invention, a waveguide having a large inclination from the reverse mesa direction is used. Even if there is, it is possible to regrow the growth without any problems. As a result, the distance between the semiconductor laser unit and the coupler can be shortened, the element length can be shortened, and the device itself can be miniaturized. Accordingly, propagation loss can be reduced.

  Further, the inclined waveguide portion can be applied to an optical semiconductor device having an inclined waveguide inclined at 5 degrees or more with respect to the reverse mesa direction. By applying it to an optical semiconductor device having a waveguide having such a shape, an eaves is formed above the mask, or the upper part of the mask is covered with a semiconductor, and a space is formed between the mask and the semiconductor. And the electrode can be easily formed above the mask. Therefore, an increase in manufacturing time can be suppressed and an increase in cost can be suppressed.

  Here, the thickness of the InP layer left on the waveguide layer by the second-stage etching will be considered. In general, the interface of regrowth is likely to cause crystal defects, and unintentional recrystallization will occur. Therefore, in the case of a device that uses current injection such as a semiconductor laser, the regrowth interface should be as much as possible. The improvement of the characteristics can be expected by separating from the active layer. On the other hand, it is known that p-type InP has a large light absorption loss. In the case of this embodiment, since two-step etching is applied to the curved portion of the waveguide where current is not injected, in order to reduce the absorption loss of p-type InP rather than the influence of crystal defects at the regrowth interface. In addition, it is better to reduce the thickness of the p-InP layer on the waveguide layer as much as possible and reduce the proportion of the optical field applied to the p-InP layer. The ratio of the guided light field to p-InP on the waveguide layer is 80% at 300 nm, 63% at 200 nm, and 100% when the thickness of p-InP is 1 μm. To 41%. Therefore, the effect of reducing the absorption loss due to the p-type InP layer can be obtained by setting the p-InP layer on the waveguide layer to 300 nm or less.

  In the present embodiment, the substrate 1 on which each element such as the semiconductor laser unit 11 and the waveguide unit 12 is formed is n-type InP, but this substrate is p-type InP or semi-insulating InP, for example, Fe It is also possible to use InP doped with. When the substrate is a p-type InP and an optical semiconductor device in which each of the above elements is formed on this substrate, the p-type InP 104 is formed above the active layer in the semiconductor laser portion and above the waveguide layer in the waveguide portion. Instead, n-type InP may be stacked. When the substrate is made of semi-insulating InP, and an optical semiconductor device in which each element is formed on the substrate, an n-type InP layer is laminated on the semi-insulating substrate, and a layer such as an active layer is laminated. A type InP layer may be stacked. Even in such an optical semiconductor device, an effect of reducing absorption loss due to the p-type InP layer can be obtained by setting the p-InP layer on the active layer or the like to 300 nm or less. In the case of an optical semiconductor device using a semi-insulating substrate, it is also possible to stack a p-type InP layer on the semi-insulating substrate, and then stack an n-type InP layer after stacking layers such as an active layer. is there.

[Second Embodiment]
A second embodiment of the optical semiconductor device according to the present invention will be described in detail with reference to FIG.
FIG. 5 is a plan view schematically showing the optical semiconductor device.
The optical semiconductor device according to the present embodiment is an apparatus when the optical semiconductor device according to the first embodiment described above is applied to a ring resonator.

  The optical semiconductor device 200 according to the present embodiment is a device including a ring resonator 250 and an input / output waveguide 260 as shown in FIG. The ring resonator 250 and the input / output waveguide 260 are optically coupled by a directional coupler. Note that the ring resonator 250 and the input / output waveguide 260 are formed on the same substrate 201.

  Specifically, the ring resonator 250 is a device including a first curved waveguide section 210, a second curved waveguide section 220, and a straight waveguide section 230. The first curved waveguide section 210 is formed of a first curved waveguide 211 that is convex toward the left side in the drawing and extends in a curved shape. The second curved waveguide section 220 is formed of a second curved waveguide 221 that is convex toward the right side in the drawing and extends in a curved shape. The linear waveguide section 230 includes a first linear waveguide 231 that connects one input / output section of the first curved waveguide 211 and one input / output section of the second curved waveguide 221, A second linear waveguide 232 connecting the other input / output part of one curved waveguide 211 and the other input / output part of the second curved waveguide 221 is provided.

  In order to make the ring resonator 250 have a gain, it is necessary to provide an active layer and inject current. Therefore, an active layer is provided in a straight portion, and this portion is formed with a high mesa structure. On the other hand, the curved portion is formed with a low mesa structure so that abnormal growth does not occur during buried regrowth.

  The layer structure used in this embodiment is the same as that of the first embodiment, the straight line portion of this embodiment corresponds to the laser unit 11 of the first embodiment, and the curved portion is the guide of the first embodiment. It corresponds to the waveguide section 12.

  In this embodiment, using the two-stage etching described in the first embodiment, the first-stage etching is 1.5 μm, the second-stage etching is 1.5 μm, and the height of the high mesa structure is 3 The height of the low mesa structure was set to 1.5 μm.

  According to the optical semiconductor device 200 according to the present embodiment, the waveguide portion having a larger angle from the reverse mesa direction is regrown as compared with the optical semiconductor device 10 according to the first embodiment. By reducing the height of the mesa structure lower than in the case of, regrowth without abnormal growth could be performed.

[Third embodiment]
A third embodiment of the optical semiconductor device according to the present invention will be described in detail with reference to FIG.
FIG. 6 is a plan view schematically showing the optical semiconductor device.
The optical semiconductor device according to the present embodiment is an apparatus when the optical semiconductor device according to the first embodiment described above is applied to a Mach-Zender interferometer.

  As shown in FIG. 6, the optical semiconductor device 300 according to the present embodiment is a device including a first curved waveguide section 310, a second curved waveguide section 320, and a straight waveguide section 330. .

  Specifically, the first curved waveguide section 310 includes a first 2 × 2 MMI coupler (multiplexer / demultiplexer) 315 having two input sections 311 and 312 and two output sections 313 and 314. To do. The second curved waveguide section 320 includes a second 2 × 2 MMI coupler (multiplexer / demultiplexer) 325 having two input sections 321 and 322 and two output sections 323 and 324.

  The straight waveguide section 330 connects the first optical waveguide 331 that connects one output section 313 of the first 2 × 2 MMI coupler 315 and one input section 321 of the second 2 × 2 MMI coupler 325. And a second optical waveguide 332 that connects the other output 314 of the first 2 × 2 MMI coupler 315 and the other input 322 of the second 2 × 2 MMI coupler 325. It is.

  However, the optical semiconductor device 300 according to the present embodiment is a device having a layer structure having no active layer and any region having a waveguide layer. In the straight waveguide portion 330 that is a straight portion, it is necessary to electrically change the refractive index, so that the original layer structure remains as a high mesa structure. The curved waveguide portions 310 and 320 including the multiplexer / demultiplexer have a low mesa structure by two-stage etching. That is, when the two-step etching is applied, the etching mask on the curved waveguide portions 310 and 320 is removed after the first-stage etching, while the etching mask on the straight waveguide portion 330 is left, In this state, the second-stage etching was performed to make the straight waveguide portion 330 a higher mesa structure than the curved waveguide portions 310 and 320.

  In the optical semiconductor device 300 according to the present embodiment, since the p-type InP clad layer above the waveguide layer is 2.5 μm, the first-stage etching is 2.5 μm and the second-stage etching is 2.0 μm. It was. As a result, the height (mesa height) of the high mesa structure was 4.5 μm, and the height (mesa height) of the low mesa structure was 2.5 μm. As described above, in this embodiment, since the mesa height is as high as 4.5 μm, abnormal growth occurs even when the mesa direction is inclined about 5 degrees from the reverse mesa direction. However, by applying the present invention, abnormal growth occurs. Re-growth without burying became possible.

  As described above, the present invention can be applied not only to the semiconductor laser but also to other optical elements.

  Therefore, according to the optical semiconductor device 300 according to the present embodiment, like the optical semiconductor device 10 according to the first embodiment described above, a waveguide having a large inclination angle from the reverse mesa direction is formed at a short distance, and a straight waveguide. An embedded waveguide connecting the portion and the inclined waveguide portion can be manufactured, and the device itself can be downsized. Accordingly, propagation loss can be reduced.

  The optical semiconductor device according to the present invention can be used for a wavelength tunable semiconductor laser used as a light source for optical fiber communication and a light source for optical measurement. For example, a light source for optical wavelength (frequency) multiplexing system in optical communication, a broadband wavelength band Can be used as a light source for optical measurement.

10, 200, 300 Optical semiconductor device 11 Semiconductor laser unit 12 Inclined waveguide unit 101 Lower cladding 102 Active layer 104 Upper cladding layer 106 Contact layer 109 Insulating film 115 Waveguide layer 122 Semi-insulator 250 Ring resonator 315, 325 2 × 2 MMI coupler h1 Mesa height h2 of semiconductor laser Mesa height of waveguide section

Claims (9)

An optical semiconductor device having a linear waveguide portion formed linearly in a reverse mesa direction and an inclined waveguide portion connected to the linear waveguide portion and formed in a direction inclined with respect to the reverse mesa direction. And
The linear waveguide portion comprises an active layer;
The inclined waveguide portion is formed at the same height as the active layer and includes a waveguide layer that guides light;
The linear waveguide portion is formed of a mesa structure;
The inclined waveguide portion is formed with a mesa structure, and the mesa structure of the inclined waveguide portion is formed lower than the mesa structure of the linear waveguide portion,
An optical semiconductor device, wherein a semiconductor layer is stacked on a side portion of a mesa structure of the straight waveguide portion, and a mesa structure of the inclined waveguide portion is embedded with a semiconductor layer. An optical semiconductor device according to claim 1,
The height of the mesa structure of the straight waveguide portion is 2.5 μm or more,
The height of the mesa structure of the inclined waveguide portion is less than 2.5 μm. An optical semiconductor device according to claim 2,
The height of the mesa structure of the inclined waveguide portion is equal to or greater than the thickness of the waveguide layer. An optical semiconductor device according to any one of claims 1 to 3,
The linear waveguide portion and the inclined waveguide portion are formed on an n-type InP substrate or a semi-insulating InP substrate,
A p-InP layer is formed on the waveguide layer;
The optical semiconductor device, wherein the p-InP layer has a thickness of 300 nm or less. An optical semiconductor device according to any one of claims 1 to 4,
An optical semiconductor device, wherein the semiconductor layer is a layer containing a semi-insulator doped with ruthenium. An optical semiconductor device according to any one of claims 1 to 5,
The light characterized in that the inclined waveguide portion is an inclined waveguide extending linearly in a direction inclined by 5 degrees or more with respect to the reverse mesa direction, or a curved waveguide extending in a curved shape. Semiconductor device. A semiconductor laser array comprising the optical semiconductor device according to any one of claims 1 to 6. A ring resonator comprising the optical semiconductor device according to any one of claims 1 to 6. A Mach-Zehnder interferometer comprising the optical semiconductor device according to any one of claims 1 to 6.

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