JPH07202316A - Selectively grown waveguide optical control element - Google Patents

Abstract

PURPOSE:To obtain a waveguide optical control element with low capacitance and high speed by making such a structure of the waveguide optical control element that can utilize selective growth and by inserting a semiconductor buffer layer with low carrier concentration between a conductive semiconductor substrate and an inversely conductive semiconductor second clad layer. CONSTITUTION:A lightly doped n-InP buffer layer 102 with low carrier concentration is formed on an n-InP substrate 101 using organic metal vapor phase growth technique or the like. A SiO2 film is formed on this substrate to be patterned onto a pair of selectively growing SiO2 masks in a striped shape which are opposite to each other across a stripe-like gap. Then an n-InGaAsP optical waveguide layer 502, an n-InP spacer layer 503 or the like are sequentially stacked selectively only on the gap to form a mesa of a double hetero structure. As a result, no failure such as an increase in series resistance of an element occurs, coupling loss with a single mode fiber is small, speed is high and a yield in manufacture can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical control element which is an important element in an optical communication system and an optical information processing system. Further, the present invention relates to an optical control element such as an optical modulator using selective growth, which is excellent in uniformity of characteristics within a wafer and reproducibility of run-to-run characteristics.

[0002]

2. Description of the Related Art Optical modulators and distributed feedback semiconductor lasers (D
A waveguide type optical control element such as a modulator integrated light source in which an optical modulator is integrated in an FB laser) is considered to be one of the key elements of high-speed optical communication systems and optical information systems, and research and development has become active in various places. There is. For example, as the optical modulator, one using a dielectric such as LiNbO ₃ and one using an InP or GaAs semiconductor are considered.
In recent years, expectations are increasing for a semiconductor optical modulator that can be integrated with other optical elements such as an optical amplifier and electronic circuits such as FETs, and can be easily downsized and reduced in voltage. As a semiconductor modulator, a Franz-Kaldysh effect of a bulk semiconductor and a quantum confined Stark effect (Quantum Confined Stark E) in a multiple quantum well structure are used.
(FFect: QCSE), an absorption-type optical modulator that utilizes the effect of shifting the absorption edge to the longer wavelength side by applying an electric field, the electro-optic effect (Pockels effect) of a bulk semiconductor, and the quantum in a multiple quantum well structure. Confinement St
A Mach-Zehnder type modulator that utilizes the effect of changing the refractive index by applying an electric field like the ark effect is typical.

In recent years, the problems of the conventional semiconductor laser direct modulation system have become apparent as the speed and distance of optical fiber communication have increased. That is, in the semiconductor laser direct modulation method, wavelength chirping occurs at the time of modulation, which deteriorates the waveform after fiber transmission. This phenomenon becomes more remarkable as the signal transmission speed increases and the transmission distance increases. In particular, this problem is serious in a system using an existing 1.3 μm zero-dispersion fiber. Even if an attempt is made to extend the transmission distance by using a light source in the wavelength band of 1.55 μm, which has a small fiber propagation loss, it is caused by chirping. The transmission distance is limited due to the dispersion limitation. This problem can be solved by adopting an external modulation method in which the semiconductor laser is made to emit light at a constant light output and the emitted light of the semiconductor laser is modulated by an optical modulator different from the semiconductor laser. for that reason,
In recent years, the development of semiconductor optical modulators has become active.

Incidentally, the bulk semiconductor Franz-K
eldysh effect and quantum confinement Stark effect (Quantum Confine) in multiple quantum well structure
The absorption type optical modulator utilizing the effect of shifting the absorption edge to the long wavelength side by applying an electric field (e.g., dStark effect: QCSE effect) has an extinction ratio of 15 dB or more at an applied voltage of about 3 V even with an element length of about 300 μm. Is obtained, and it is attracting attention because of its small size and low operating voltage. Following the conventional semiconductor laser device structure and manufacturing process, an electro-absorption modulator using the Franz-Keldysh effect or the QCSE effect or a modulator integrated light source in which an electro-absorption modulator is integrated with a DFB laser is studied. ing. For example, an electro-absorption optical modulator having a structure in which bulk InGaAsP is used as an absorption layer and both ends of a double-hetero structure mesa stripe including the InGaAsP absorption layer are filled with Fe-doped high resistance InP is disclosed in H. By SODA and others ELECTRONICS LETTERS Vo
l. 23 (1987), PP. 123-1234.

However, in this optical modulator, the mesa stripe including the absorption layer is formed by the same wet etching as in the conventional semiconductor laser, but a narrow width mesa stripe is formed with good uniformity and reproducibility. Is difficult, and there is a problem in terms of device manufacturing yield. Further, in this example, both sides of the mesa stripe are filled with Fe-doped high resistance InP so that the capacitance of the element can be reduced and high-speed modulation can be performed. Is difficult to form.
If a good interface is not formed, when a reverse bias is applied to the mesa stripe including the absorption layer, a leakage current flows along the interface and a sufficient voltage is not applied to the absorption layer.

On the other hand, if selective crystal growth (hereinafter abbreviated as selective growth) is used, a semiconductor laser or an optical modulator can be manufactured only by patterning a thin dielectric mask and crystal growth without etching the semiconductor, Excellent uniformity of characteristics within a wafer and excellent reproducibility of run-to-run characteristics. A ridge-structure electro-absorption optical modulator using selective MOVPE growth is reported by Komatsu et al. In 1992 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, Volume 4-166, Volume 4. In this ridge structure electro-absorption optical modulator, first, a double hetero structure in which non-doped InGaAs is sandwiched between an n-InP cladding layer and a non-doped InP cladding layer is formed on the entire surface of the n-InP substrate. Thereafter, a pair of stripe-shaped SiO ₂ masks for forming a ridge portion by selective growth is formed on the non-doped InP clad layer, and the p-InP clad layer and the p-InGaAs cap layer are formed in the voids of the SiO ₂ mask. Form a ridge. By using this method, the optical modulator can be manufactured only by patterning a thin dielectric mask and crystal growth without etching the semiconductor.
The n-to-run characteristic reproducibility is excellent, and the manufacturing yield of the optical modulator is significantly improved as compared with the conventional one. However, when the ridge structure is used, the spot sizes of the incoming and outgoing light greatly differ in the horizontal direction and the vertical direction. Due to the asymmetry of the horizontal and vertical spot sizes, the coupling loss with the fiber becomes large in the ridge structure modulator. In the above example, the coupling loss with the single mode fiber is 4 dB per one end face. In order to solve this problem, it is effective to eliminate the asymmetry of the spot size by constructing the embedded modulator using selective growth. However, in the buried growth selective growth modulator, a homojunction is formed at the interface between the conductivity type substrate and the reverse conductivity type buried layer, and the carrier concentration is high in both the conductivity type substrate and the reverse conductivity type buried layer. Produces a larger capacitance. For this reason, the modulation frequency band is narrow, and there is a problem that it cannot be applied to a high-speed optical communication system.

[0007]

As described above, in the structure of the conventional semiconductor waveguide type optical control element, the mesa stripe including the absorption layer is formed by wet etching, and the width is narrow. Since it is difficult to form the GaN with good uniformity and reproducibility, the structure has a problem in terms of manufacturing yield of the device. In order to solve this problem, a structure that can utilize selective growth as a manufacturing method may be adopted. There is a problem that the coupling loss with the one-mode fiber is large. Further, in a waveguide type optical control element having a buried structure in which selective growth can be used as a manufacturing method, although the asymmetry of the spot size is improved, the homojunction at the interface between the conductivity type substrate and the reverse conductivity type buried layer is improved. Since the capacitance is large, the modulator frequency band is narrow,
There is a problem that it cannot be applied to high-speed optical communication systems.

The problem to be solved by the present invention is that the selective growth embedded structure waveguide type light which can use the selective growth capable of improving the manufacturing yield as a manufacturing method and has a small coupling loss with a single mode fiber. In the control element, there are no problems such as increase in series resistance of the element,
Another object of the present invention is to provide a structure of a waveguide type optical control element having a small capacitance and a high speed with almost no change in the element manufacturing process.

[0009]

In order to solve the above problems, according to the present invention, at least a conductive type semiconductor optical waveguide layer, a non-doped semiconductor absorption layer, a semiconductor active layer or a semiconductor is provided on a conductive type semiconductor substrate. A mesa stripe in which a waveguide layer and a semiconductor first cladding layer of the opposite conductivity type are sequentially laminated is partially formed by selective growth, and the bandgap energy of the non-doped semiconductor absorption layer, semiconductor active layer, or semiconductor waveguide layer is a stripe. The second conductivity type semiconductor second cladding layer and the second conductivity type semiconductor cap layer, which are partially different in the direction and cover the upper surface and the side surface of the mesa stripe and fill the mesa stripe, are also formed in the mesa stripe shape by selective growth. , The band gap energy is partially different in the stripe direction. An integrated structure semiconductor waveguide type optical control device comprising means for independently applying an electric field to portions having different band gaps in a semiconductor absorption layer, a semiconductor active layer or a semiconductor waveguide layer, wherein A structure of a selective growth waveguide type optical control element characterized in that a conductivity type low carrier concentration semiconductor buffer layer is inserted between the opposite conductivity type semiconductor second clad layer is adopted.

In order to solve the above-mentioned problems in the present invention, at least a conductive semiconductor first cladding layer, a non-doped semiconductor absorption layer, a semiconductor active layer, a semiconductor waveguide layer, or a reverse conductive type is provided on a conductive semiconductor substrate. A mesa stripe in which a semiconductor second clad layer is sequentially laminated is partially formed by selective growth, and a reverse conductivity type semiconductor third clad layer and a reverse conductivity type semiconductor that cover the upper surface and the side surface of the mesa stripe and fill the mesa stripe. A semiconductor waveguide type optical control element, wherein the cap layer is formed in a mesa stripe shape by selective growth, and is provided with a means for applying an electric field to the non-doped semiconductor absorption layer, semiconductor active layer or semiconductor waveguide layer. Structure of a selective growth waveguide type optical control device characterized in that the carrier type semiconductor substrate has a low carrier concentration , And a mesa stripe in which at least a conductive semiconductor optical wave layer, a non-doped semiconductor absorption layer, a semiconductor active layer, or a semiconductor waveguide layer, and a reverse conductive semiconductor first clad layer are sequentially grown on a conductive semiconductor substrate plate. The band gap energy of the non-doped semiconductor absorption layer, the semiconductor active layer, or the semiconductor waveguide layer is partially different in the stripe direction, and the mesa stripe is covered by the upper surface and the side surface of the mesa stripe. The reverse conductive semiconductor second clad layer and the reverse conductive type semiconductor cap layer to be embedded are also formed in a mesa stripe shape by selective growth,
Integrated structure semiconductor waveguide type light having means for independently applying an electric field to a portion having a different band gap in a non-doped semiconductor absorption layer, a semiconductor active layer, or a semiconductor waveguide layer having partially different band gap energies in the stripe direction A structure of a selective growth waveguide type optical control element, which is a control element and characterized in that the conductivity type semiconductor substrate has a low carrier concentration, is also adopted.

[0011]

In the present invention, since the structure of the waveguide type light control element is such that selective growth can be used, the waveguide type light control device can be formed only by patterning a thin dielectric film and crystal growth without etching the semiconductor. As compared with a device having a structure in which the device can be manufactured and the device is manufactured by using the conventional semiconductor etching, the uniformity of characteristics in the wafer and the run-
The reproducibility of to-run characteristics is significantly excellent.

Further, in the present invention, since the buried structure is adopted as the structure of the waveguide type light control element, the spot sizes of the incident and outgoing light are symmetrical in the horizontal and vertical directions,
The coupling loss with the single mode fiber can be reduced as compared with the ridge structure waveguide type optical control element.

Further, in the present invention, in contrast to the problem that the homojunction capacitance peculiar to the buried structure waveguide type optical control element is large, the conductivity type low carrier concentration is provided between the conductivity type substrate and the reverse conductivity type buried layer. The capacity of the homojunction is reduced by increasing the depletion layer thickness by inserting a semiconductor buffer layer or using a conductive type substrate with a low carrier concentration. It is possible to do with almost no.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a Franz-Keldysh as an example of the selective growth waveguide type optical control element according to the present invention.
FIG. 7 is a perspective view showing an embodiment of an InP-based electroabsorption modulator using the effect.

First, a method of manufacturing the InP-based electro-absorption modulator shown in FIG. 1 will be described with reference to FIG.

First, the carrier concentration is 2 × 10 ^-17 cm ^-3.
(100) orientation low-doped n-InP substrate 120
An SiO ₂ film having a thickness of 1000 Å serving as a dielectric mask for selective growth is formed on the entire surface (FIG. 2).
(A)). Thereafter, using conventional photolithography, the SiO ₂ film of SiO for selective growth of the stripe-shaped piece facing across the stripe gap portion ₂
Patterning is performed on the mask 201 (FIG. 2B). Here, the width W of the void 202 is 2 μm, and SIO ₂ for selective growth is used.
The width of the mask 201 is 10 μm. The void portion 20
The stripe direction of 2 is the [011] direction.

Next, using the MOVPE method, the n-InP cladding layer 1 is selectively formed only on the above-mentioned void 202.
03 (thickness 0.1 μm, carrier concentration 5 × 10 ¹⁷ c
m ^-3 ), i-InGaAsP absorption layer (composition wavelength: 1.45)
μm, thickness 0.25 μm) 104, i-InP clad layer (thickness 0.05 μm) 105, p-InP clad layer (thickness 0.1 μm, carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) are sequentially laminated. To form a mesa 203 having a double hetero structure (FIG. 2C). At this time, the side surface of the formed mesa 203 becomes the (111) B surface, and a smooth mesa side surface is obtained.

Next, the selective growth SiO ₂ mask 201 is removed from the side of the double hetero structure mesa 203 by a width of 1.5 μm to widen the gap 202 between the masks, that is,
A void having a width of 1.5 μm is formed again on both sides of the double hetero structure mesa 203 by using a normal photolithography process. Then, the p-InP cladding layer 107 and the p-InGaAs cap layer 108 are again formed by selective MOVPE growth on the newly formed void and on the double hetero structure mesa 203 (FIG. 2D). Here, the p-InP cladding layer 107 and the p-InGa
The thickness of the As cap layer 108 is 1 μm and
0.2 μm, carrier concentration is 5 × 10 ¹⁷ cm ⁻³ each
And 2 × 10 ¹⁸ cm ⁻³ . Next, SiO ₂ protective film 1
09 is formed on the entire upper surface of the substrate, a window for contacting the p-side electrode is formed in the SiO ₂ protective film 109, and then the p-side electrode 110 made of Cr and Au is formed (FIG. 2E). Finally, the low-doped n-InP substrate 120 is polished to a thickness of about 100 μm, and then Cr / A is formed on the low-doped n-InP substrate 120 side.
An n-side electrode 111 made of u is formed (FIG. 2D), the device is cleaved, and the input / output end faces are coated with irreversible ends to complete the device fabrication.

The operation of the InP-based electro-absorption modulator shown in FIG. 1 manufactured by the above manufacturing method will be described below.

The light wave having a wavelength of 1.55 μm incident from the incident end of the InP-based electro-absorption modulator shown in FIG.
Most of the energy is confined in the nGaAsP absorption layer 104 and propagates in the device. When the reverse bias voltage is not applied between the p-side electrode 101 and the n-side electrode 111, the i-InGaAsP absorption layer 104 having a composition wavelength of 1.45 μm has a low loss for propagating light having a wavelength of 1.55 μm. At this time, an optical output is obtained from the exit end of the modulator and the optical output is turned on. On the other hand, a reverse bias voltage is applied between the p-side electrode 101 and the n-side electrode 111 to apply Fr.
When the absorption edge is shifted to the long wavelength side by the anz-Kaldysh effect, the propagating light having a wavelength of 1.55 μm becomes i-In.
The GaAsP absorption layer 104 suffers a large absorption loss, and no optical output is obtained from the emission end, which is in the OFF state. In the case of the present invention, the reverse bias voltage OV is in an ON state, and 3.0 V
It can be realized in the FF state, and the extinction ratio is 1 with a 300 μm long element.
5 dB or more is obtained.

In the InP-based electroabsorption modulator shown in FIG. 1, the i-InGaAsP absorption layer 104 is used.
Since an embedded structure in which the periphery of the substrate is embedded with InP is adopted, the spot shape of the incident / emitted light is symmetrical in the vertical direction and the horizontal direction with respect to the substrate. Therefore, it can be coupled with a single mode fiber with low loss. The inventors of the present invention have conducted a Far Fi method using a selective growth modulator having a ridge structure and a buried structure modulator.
Experimentally compared coupling losses with eld Pattern and a single molded fiber. As a result, Far
Regarding the Field Pattern, the horizontal and vertical radiation angles of the ridge structure selective growth modulator are 11 °, respectively.
In contrast, the embedded structure modulator shown in Fig. 1 has large horizontal and vertical radiation angles of 40 ° and 47 °, respectively, while the asymmetry of radiation angle is significantly improved. It had been. Reflecting this, the aspherical lens 1
Regarding the coupling loss between the modulator and the single mode fiber, the coupling loss per one end face is 3.2 dB in the case of the ridge structure selective growth modulator.
On the other hand, in the case of the embedded structure modulator shown in FIG. 1, the coupling loss per end face is 2.2 dB, which is 1
An improvement in dB was observed. As a result, the insertion loss between single mode fibers when the single mode fibers were coupled to both ends of the modulator was 10.0 dB in the case of the ridge structure modulator, but in the embedded structure modulator shown in FIG. 6.
It was 2 dB, and a significant reduction in insertion loss was achieved. By the way, as described above, when the mesa 203 having the double hetero structure is formed by selective growth, the side surface of the mesa 203 becomes (11
1) It becomes a B crystal plane and a smooth mesa side surface is obtained. Therefore, the side surface of the mesa is much smoother than that in the case where the mesa is formed by etching, and the loss due to scattering of propagating light is significantly reduced. As described above, according to the present invention, it is possible to realize an InP-based electro-absorption modulator with significantly lower loss than conventional ones.

Next, the principle by which the capacitance of the device is reduced and a high-speed embedded structure optical modulator can be realized by the present invention will be described. FIG. 3 shows a cross-sectional structure of an InP-based embedded structure electro-absorption modulator according to the present invention (FIG. 3B) and a cross-sectional structure of a normal embedded structure electro-absorption modulator (FIG. 3).
(A)) is shown in comparison. In the case of a normal buried structure selective growth device, a homojunction portion 301 composed of high carrier concentration n-InP and high carrier concentration p-InP is formed on both sides of the double hetero structure mesa 203. Usually, as the n-InP substrate 101, one having a carrier concentration of about 2 × 10 ¹⁸ cm ⁻³ is used.
Since the carrier concentration of the clad layer 107 is set to 5 × 10 ¹⁷ cm ⁻³ or more in order to prevent the resistance component from increasing, the homojunctions 301 on both sides of the double hetero structure mesa 203 have high carrier concentration. It is formed by n-InP and p-InP having a high carrier concentration. At this time, a large capacitance is generated in the homojunction portion 301. As described above, the n-InP substrate 101
Has a carrier concentration of 2 × 10 ¹⁸ cm ⁻³ , the p-InP cladding layer 107 has a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ , and the homojunction portion 301 has a width of 2 μm on one side (thus 4 μm on both sides) and an element length. Homozygous portion 30 when the thickness is 300 μm
The capacitance at 1 is 2.0 pF. In the case of the InP-based buried structure electro-absorption modulator shown in FIG. 1, in addition to the above-mentioned capacitance at the homojunction portion, the capacitance at the heterojunction portion is 0.2.
Since pF and the capacitance of 0.8 pF at the p-side electrode pad 112 are added, the capacitance of the entire device becomes 3.0 pF. At this time, the modulation frequency determined by the RC time constant is 2.1 GHz in the normally used 50Ω impedance system,
It is also difficult to apply to a 2.5 Gb / s optical communication system. As a means for reducing the capacity of the homojunction portion 301, it may be possible to narrow the width of the p-InP cladding layer, that is, to narrow the width of the homojunction portion 301. However, p-I
When the width of the nP clad layer is narrowed, the horizontal light field feels air in the layer of the propagating light, and the single mode condition in the horizontal direction becomes severe in the layer, which is not desirable. Therefore, the width of the homojunction portion 301 needs to be 2 μm on each side. Further, it is possible to reduce the homojunction capacitance by making the device length shorter than 300 μm, but in that case, the extinction ratio is deteriorated, which is also undesirable.

On the other hand, in the present invention, FIG.
As shown in (b), a low-dope n-InP substrate ¹²⁰ having a low carrier concentration (about 2 × 10 ¹⁷ cm ⁻³ ) is used as the n-InP substrate. As a result, the capacity of the homojunction can be reduced, the capacity of the entire element can be reduced, and the modulation frequency band can be expanded. FIG. 4 shows the dependence of the homojunction capacitance on the carrier concentration of the n-InP substrate. When the carrier concentration of the n-InP substrate is set to 2 × 10 ¹⁷ cm ⁻³ , the capacitance at the homojunction portion 302 becomes 1.2 pF, which is half that in the case where the low-doped n-InP substrate is not used. At this time, the capacitance of the entire element is 2.2 pF and the modulation frequency band is 2.9 GH.
Expand to z. The carrier concentration of the n-InP substrate is 1
When it is reduced to × 10 ¹⁷ cm ^-3 , the capacitance at the homojunction portion 302 further decreases to 0.9 pF. At this time, the modulation frequency band is expanded to 3.4 GHz. In the example shown in FIG. 1, no means for reducing the capacitance of the p-side electrode pad 112 is used, so that the modulation frequency band is 3.4 GH even if the homojunction capacitance is reduced to 1 pF or less.
However, it is possible to expand the modulation frequency band to about 6 GHz by reducing the capacitance of the pad portion by forming a polyimide film directly under the p-side electrode. . On the other hand, in the conventional structure shown in FIG. 3A, even if the capacitance of the pad portion becomes zero, the homojunction capacitance is 2.0 pF and the heterojunction capacitance is 0.
Since 2 pF remains, the modulation frequency band remains at 3 GHz. Here, if a low-doped (100) plane orientation n-InP substrate is used as the substrate, there is a concern that the resistance in the substrate will increase, but if there is a carrier concentration of about 2 × 10 ¹⁷ cm ^{−3, the} substrate will have The resistance of is hardly increased. According to an experiment by the present inventors, the carrier concentration of the (100) plane direction low-doped n-InP substrate 120 in the configuration of FIG.
Even if it is × 10 ¹⁷ cm ^-3 , the carrier concentration of the substrate is 2 × 1.
Almost no increase in device resistance was observed as compared with the case of 0 ¹⁸ cm ⁻³ , and even if the (100) plane direction low-doped n-InP substrate 120 was made to have a low carrier concentration, it had no adverse effect on device characteristics such as extinction characteristics. It was confirmed not to give. Further, according to the homojunction capacitance reducing method of the present invention, since only a low-doped substrate is used as the substrate, it is possible to speed up the device without making any changes in the device manufacturing process.

FIG. 5 shows, as another embodiment of the selective growth waveguide type optical control element according to the present invention, InP using the QCSE effect.
FIG. 3 is a perspective view showing an embodiment of a modulator integrated light source in which a system multiple quantum well (MQW) modulator is integrated with a distributed feedback semiconductor laser (DFB laser).

First, a method of manufacturing the modulator integrated light source shown in FIG. 5 will be described. However, since the manufacturing method has many parts in common with the first embodiment, the manufacturing method is different from the manufacturing method of the first embodiment. Only the points will be explained in detail.

First, a low-doped n-InP buffer layer 1 is formed on an n-InP substrate 101 having a (100) plane orientation by a metal organic chemical vapor deposition method (hereinafter abbreviated as MOVPE method) or the like.
02 is formed on the entire surface of about 0.3 μm. Here, the carrier concentration of the low-doped n-InP buffer layer 102 is 2 × 10.
^{Keep the} concentration as low as ^-17 cm ^-3 . Then lightly doped n-
The period in the [011] direction is 2 on the InP buffer layer.
A diffraction grating 501 having a thickness of 40 nm and a depth of 30 nm is partially formed. A portion provided with the diffraction grating 501 is a DFB laser region 511, and a flat portion without the diffraction grating 501 is an optical modulator region 513. On this substrate, a SiO ₂ film having a thickness of 1000 Å which serves as a dielectric mask for selective growth is formed.
Form a film and use normal photolithography techniques
The SiO ₂ film is patterned into a pair of stripe-shaped SiO ₂ masks for selective growth that face each other across a stripe-shaped void. The shape of the selective growth SiO ₂ mask seen from the upper surface of the wafer is shown in FIG. Here, the void 20
The width W of 2 corresponds to the DFB laser region 511 and the optical modulator region 51.
3 is 2 μm, the width of the SiO ₂ mask 601 for selective growth is 17 μm in the DFB laser region 513, and the optical modulator region 51.
3 is 8 μm. The stripe direction of the void 202 is the [011] direction, and the stripe-shaped window region 514 having no void extends over 15 μm from the end face of the optical modulator region 513.

Next, using the MOVPE method, the n-InGaAsP optical waveguide layer 502 (having a thickness of 0.1 μm, having a composition wavelength of 1.15 μm) is selectively formed only on the above-mentioned void 202.
Carrier concentration 5 × 10 ¹⁷ cm ⁻³ ), n-InP spacer layer 503 (thickness 0.04 μm, carrier concentration 5 × 10 ¹⁷
cm ⁻³ ), composition growth of 1.3 μm i-InGaAsP barrier layer (thickness 10 nm) and i-InGaAs well layer (thickness 7 nm), 7-period undoped quantum well layer 50
4, i-InP clad layer (thickness 0.05 μm) 50
5. A p-InP clad layer (thickness: 0.1 μm, carrier concentration: 5 × 10 ¹⁷ cm ⁻³ ) 506 is sequentially laminated to form a medal having a budal hetero structure. At this time, the side surface of the formed mesa becomes the (111) B surface, and a smooth mesa side surface is obtained.

In this embodiment, as shown in FIG. 6, the width of the selective growth SiO ₂ mask is changed between the DFB laser region 511 and the optical modulator region 513. As described above, when the SiO ₂ mask width is partially changed in the selective MOVPE growth, the diffusion of the growth raw material species (mainly the group III organometallic raw material) in the vapor phase depends on the SiO ₂ mask width. to solid phase composition of the grown layer due to the synergistic effect of resulting from the well layer thickness change that changes the growth rate by SiO ₂ mask width is dependent on the SiO ₂ mask width, the band gap energy is different within the same substrate The part can be formed by one MOVPE growth. Therefore, the DFB laser unit 511 and the optical modulator unit 5
The undoped quantum well layer 504 required to have different bandgap energies from 13 can be formed by one-time crystal growth and without interruption at the connection between both regions. This selective growth method is similar to a modulator integrated light source. It is ideal as a method for manufacturing various optical integrated devices. In this embodiment, the bandgap wavelength of the undoped quantum well layer 504 is DFB.
1.55 μm in laser region 511, optical modulator region 513
Is 1.48 μm.

Next, the width of the void 202 of the selective growth SiO ₂ mask 601 is 7 μm in the DFB laser region 511,
The width of the gap 202 between the SiO ₂ masks is widened so that the width becomes 5 μm in the optical modulator region 513. Then, the p-InP cladding layer 507 and the p-InGaA are formed on the newly formed void and the double hetero structure mesa.
The s cap layer 508 is formed again by selective MOVPE growth. Here, the p-InP clad layer 507 and p
-The thickness of each InGaAs cap layer 508 is 1 μm.
m and 0.2 μm, carrier concentration is 5 × 1 each
It is 0 ¹⁷ cm ^-3 and 2 × 10 ¹⁸ cm ^-3 . In order to electrically separate the DFB laser region 511 and the optical modulator region 513, a length of 25 μm from the boundary between the two regions to the optical modulator region side.
The p-InGaAs cap layer 508 is removed over m to form an isolation region 512. Then, the SiO ₂ protective film 10
9 is formed on the entire upper surface of the substrate, and p is formed on the SiO ₂ protective film 109.
Windows for side electrode contacts are formed in the DFB laser region 511 and the optical modulator region 513. After that, Cr and A
A p-side electrode made of u is vapor-deposited, the electrodes are patterned by using a photolithography technique, and a laser section p-side electrode 520, an optical modulator section p-side electrode 521, and an optical modulator section p are formed.
The side electrode pad 523 is formed. Finally, after polishing the n-InP substrate 101 to a thickness of about 100 μm, n-In
An n-side electrode 111 made of Cr / Au is formed on the P substrate 101 side, the element is cleaved, a non-reflective film is formed on the end surface of the optical modulator area side, and a high-reflection film is formed on the end surface of the DFB laser area side. To finish.

The operation of the optical modulator integrated light source shown in FIG. 5 manufactured by the above manufacturing method will be described below.

When a forward bias voltage is applied between the p-side electrode 520 and the n-side electrode 111 of the laser section of the DFB laser region 511 to inject a current into the DFB laser region 511,
Single-axis mode oscillation occurs at a wavelength (1.55 μm in this embodiment) determined by the pitch of the diffraction grating 501 and the effective refractive index determined by the cross-sectional structure of this region, and the output light of the DFB laser region 511 is the separation region 512. , And is coupled to the optical modulator region 513. Here, the optical modulator area 513
In the undoped quantum well layer 504, the bandgap wavelength is set to 1.48 μm so that light absorption at this wavelength without bias can be suppressed to such an extent that there is no practical problem. When a reverse bias voltage is applied to the undoped quantum well layer 504 in the optical modulator region 513, the absorption coefficient for the propagating light in the optical modulator region 513 increases due to the quantum confined Stark effect. Operates as a control light intensity modulator.

In this embodiment, the DFB laser region 511
Oscillated in a single axis mode with an oscillation wavelength of 1.55 μm and a threshold current of 10 mA. In addition, the coupling efficiency of light from the DFB laser region 511 to the optical modulator region 513 was almost 100%, and the emission light power of 10 mW was obtained from the end face on the optical modulator side. When a reverse bias voltage of 3V is applied between the p-side electrode pad 522 of the optical modulator section 513 and the n-side electrode 111 of the optical modulator area 513, the length of the optical modulator area 513 becomes 200.
An extinction ratio of about 15 dB was obtained at μm.

In this embodiment as well, as in the first embodiment, an embedded structure is used in the optical modulator region 513, which is the portion coupled to the single mode fiber. Therefore, similarly to the first embodiment, the spot shape of the emitted light is symmetrical with respect to the substrate in the vertical direction and the horizontal direction, and can be coupled to the single mode fiber with low loss. According to the measurement, the coupling loss between the optical modulator integrated light source shown in FIG. 5 and the single mode fiber is about 2.5 dB, which is a good value in consideration of the existence of the window region 514 between them. It was

Also, in the present embodiment, unlike the first embodiment, p-InP is used instead of using the low-doped substrate.
Clad layer 507 and (100) plane orientation n-InP substrate 1
01 and the low-doped n-InP buffer layer 102 is inserted. That is, as the substrate, an ordinary one having a high carrier concentration (2 × 10 ¹⁸ cm ⁻³ ) is used.
In order to reduce the capacity of the homojunction portion according to the same principle as in the first embodiment, a low-doped n-InP buffer layer is provided between the p-InP cladding layer 507 and the (100) plane orientation n-InP substrate 101 in this embodiment. 102 has been inserted. In the first embodiment, the substrate has a low carrier concentration,
In a homojunction formed by n-InP having a carrier concentration of 2 × 10 ¹⁷ cm ^-3 and p-InP having a carrier concentration of 5 × 10 ¹⁷ cm ^-3 , the depletion layer spreads toward the low-doped n-InP substrate 120 side in the opposite direction. 0.2 μm at a bias voltage of 3 V
It is a degree. Therefore, in order to reduce the capacity of the homojunction portion, it is not always necessary to make the entire substrate have a low carrier concentration, and the low-doped n-InP buffer layer 102 having a thickness of about 0.3 μm has a (100) plane orientation n-InP substrate. 101 and p
-If it is inserted between the InP clad layer 507,
It is possible to reduce the capacity in the same way as when using a low-doped substrate. If the thickness is about 0.3 μm, there is no concern that the resistance will increase even if the low-doped layer is inserted. As a result, the homojunction capacitance of the optical modulator region is reduced to 0.8 dB, and the capacitance of the entire optical modulator region including the electrode pad capacitance is reduced to 1.8 pF. Therefore, the modulation frequency band of 3.5 GHz is secured, and high-speed modulation can be sufficiently supported.

Further, in this embodiment, two regions having different bandgap energies are simultaneously formed by using selective growth. Moreover, what is necessary to form such regions having different band gap energies is Si
The width of the dielectric mask such as O ₂ is only partially changed. Therefore, as compared with the conventional method of forming regions having different bandgap energies by repeating the etching and crystal growth of the semiconductor layer, the manufacturing process is significantly simplified and the manufacturing yield is significantly improved. Not only that, but also the performance improvement such as the coupling efficiency of light in the DFB laser region and the optical modulator region becomes almost 100%, which also contributes to the improvement of the manufacturing yield.

[0037]

As described above, according to the present invention, there is no problem such as an increase in series resistance of the element, and the coupling loss with the single mode fiber is hardly changed in the element manufacturing process. It is possible to realize a semiconductor waveguide type optical control element having a small size, a high speed, and a high manufacturing yield.

The present invention is not limited to the above embodiment. Although the InP-based electroabsorption modulator and the modulator-integrated light source are taken as examples, the present invention is not limited to this, and other InP-based optical control elements such as a Mach-Zehnder modulator and a directional coupler type switch are used. The present invention can also be applied to it. In addition, a single semiconductor laser or DFB laser for direct modulation of an embedded structure using selective growth,
The present invention can be applied to a DBR laser and the like. Furthermore, the present invention is similarly applicable to a light control element using another semiconductor material such as GaAs. Needless to say, the present invention is not limited to the element shapes shown in the embodiments, that is, the thickness of each layer, the composition of each layer, the waveguide size, and the like.

[Brief description of drawings]

FIG. 1 is a perspective view showing the structure of an InP-based selective growth buried structure electroabsorption modulator that is an embodiment of the present invention.

FIG. 2 is a diagram illustrating a method for manufacturing an InP-based selective growth buried structure electroabsorption modulator according to the present invention.

FIG. 3 shows cross-sectional structures of a selective growth buried structure waveguide-type light control element (b) according to the present invention and a conventional selective growth buried structure waveguide-type light control element (a) for comparison. It is a thing.

FIG. 4 shows the dependence of the homojunction capacity on the carrier concentration of the buffer layer in order to explain the principle of the homojunction capacity reduction method according to the present invention.

FIG. 5 is a perspective view showing the structure of an optical modulator integrated light source that is an embodiment of the present invention.

FIG. 6 is a diagram for explaining a method of manufacturing an optical modulator integrated light source that is an embodiment of the present invention.

[Explanation of symbols]

101 (100) Orientation n-InP substrate 102 Low-doped n-InP buffer layer 103 n-InP clad layer 104 i-InGaAsP absorption layer 105,505 i-InP clad layer 106,506 p-InP clad layer 107,507 p- InP clad layer 108,508 p-InGaAsP cap layer 109 SiO ₂ protective film 110 p-side electrode 111 n-side electrode 112 p-side electrode pad 120 (100) orientation low-doped n-InP substrate 201,601 SiO ₂ mask for selective growth 202 Gap 203 Double hetero structure mesa 301 Homojunction 302 Homojunction 501 Diffraction grating 502 n-InGaAsP optical waveguide layer 503 n-InP spacer layer 504 Undoped quantum well layer 511 DFB laser region 512 Separation region 513 Light modulation Region 514 window region 520 laser unit p-side electrode 521 optical modulator section p-side electrode 522 optical modulator section p-side electrode pad

Claims (12)

[Claims] 1. A mesa stripe in which at least a conductivity type semiconductor optical waveguide layer, a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor waveguide layer, and a reverse conductivity type semiconductor first clad layer are sequentially laminated on a conductivity type semiconductor substrate. The band gap energy of the non-doped semiconductor absorption layer, the semiconductor active layer, or the semiconductor waveguide layer partially formed by selective growth is partially different in the stripe direction, and the mesa stripe is covered with the upper surface and the side surface of the mesa stripe. The non-conducting semiconductor absorption layer or semiconductor active layer in which the second conductivity type second semiconductor cladding layer and the opposite conductivity type semiconductor cap layer that fill the stripe are formed in the mesa stripe shape by selective growth, and the band gap energies are partially different in the stripe direction. Alternatively, the bandgap is applied to the semiconductor waveguide layer. An integrated structure semiconductor waveguide type optical control device comprising means for independently applying an electric field to different regions, wherein the conductivity type semiconductor waveguide is provided between the conductivity type semiconductor substrate and the reverse conductivity type semiconductor second cladding layer. A selective growth waveguide type optical control device, characterized in that a carrier concentration semiconductor buffer layer is inserted. 2. A mesa stripe in which at least a conductive semiconductor first clad layer, a non-doped semiconductor absorption layer, a semiconductor active layer or a semiconductor waveguide layer, and a reverse conductive semiconductor second clad layer are sequentially stacked on a conductive semiconductor substrate. The reverse conductivity type semiconductor third cladding layer and the reverse conductivity type semiconductor cap layer, which are partially formed by selective growth and cover the upper surface and the side surface of the mesa stripe and fill the mesa stripe, are also formed in the mesa stripe shape by selective growth. A semiconductor waveguide type optical control element comprising means for applying an electric field to the non-doped semiconductor absorption layer, semiconductor active layer or semiconductor waveguide layer, wherein the conductivity type semiconductor substrate has a low carrier concentration. Selective growth waveguide type optical control device. 3. A mesa stripe in which at least a conductive type semi-guided conductive wave layer, a non-doped semiconductor absorption layer, a semiconductor active layer or a semiconductor waveguide layer, and a reverse conductivity type semiconductor first clad layer are sequentially laminated on a conductive type semiconductor substrate. Are partially formed by selective growth, and the bandgap energies of the non-doped semiconductor absorption layer, the semiconductor active layer, or the semiconductor waveguide layer are partially different in the stripe direction, and cover the upper surface and the side surface of the mesa stripe. A second conductivity type second semiconductor cladding layer and a reverse conductivity type semiconductor cap layer filling the mesa stripe are also formed in a mesa stripe shape by selective growth, and the band gap energies are partially different in the stripe direction. The band gap in the layer or semiconductor waveguide layer. An integrated structure semiconductor waveguide type optical control device comprising means for independently applying an electric field to different regions, wherein the conductive type semiconductor substrate has a low carrier concentration. Light control element. 4. A conductive type semiconductor optical waveguide layer, a non-doped semiconductor absorption layer, a semiconductor active layer or a semiconductor waveguide layer,
And the mesa stripe in which at least the reverse conductivity type semiconductor first clad layer is laminated has a stripe direction of [011] on the (100) plane direction conductivity type semiconductor substrate or on the (100) plane direction conductivity type low carrier concentration buffer layer. 2. The selective growth waveguide type optical control element according to claim 1, wherein the selective growth waveguide type optical control element is formed by selective growth so as to be oriented, and the side surface of the mesa stripe is a (111) B plane. 5. A mesa stripe in which at least a conductivity type semiconductor first clad layer, a non-doped semiconductor absorption layer, and an opposite conductivity type semiconductor second clad layer are laminated, and a stripe direction is [100] oriented on a conductivity type semiconductor substrate. 011]
3. The selective growth waveguide type optical control element according to claim 2, which is formed by selective growth so as to be oriented in a direction, and the side surface of the mesa stripe is a (111) B plane. 6. A conductive semiconductor optical waveguide layer, a non-doped semiconductor absorption layer, a semiconductor active layer or a semiconductor waveguide layer,
And a mesa stripe in which at least a reverse-conductivity-type semiconductor first cladding layer is laminated is formed by selective growth on the (100) plane-oriented conductivity-type semiconductor substrate so that the stripe direction is the [011] direction. 4. The selective growth waveguide type optical control element according to claim 3, wherein the side surface is a (111) B plane. 7. A conductive type semiconductor substrate and a reverse conductive type semiconductor second
2. The selective growth waveguide type optical control element according to claim 1, wherein the carrier concentration of the conductive type low carrier concentration semiconductor buffer layer inserted between the clad layer and the clad layer is lower than the carrier concentration of the conductive type semiconductor substrate. . 8. A conductive type semiconductor substrate and a reverse conductive type semiconductor second.
The carrier concentration of the conductivity type low carrier concentration semiconductor buffer layer inserted between the cladding layer and the cladding layer is equal to
2. The selective growth waveguide type optical control element according to claim 1, wherein the carrier concentration is lower than that of the cladding layer. 9. A conductive type semiconductor substrate and a reverse conductive type semiconductor second.
2. The selective growth waveguide type optical control element according to claim 1, wherein the conductivity type low carrier concentration semiconductor buffer layer inserted between the cladding layer and the cladding layer has a carrier concentration of 5 × 10 ¹⁷ cm ⁻³ or less. . 10. The carrier concentration of the conductive type semiconductor substrate is:
3. The selective growth waveguide type optical control element according to claim 2, wherein the carrier concentration is lower than the carrier concentration of the reverse conductivity type semiconductor third cladding layer. 11. The carrier concentration of the conductive type semiconductor substrate is:
4. The selective growth waveguide type optical control device according to claim 3, wherein the carrier concentration is lower than the carrier concentration of the second conductivity type semiconductor second cladding layer. 12. The carrier concentration of the conductive semiconductor substrate is
3. It is 5 × 10 ¹⁷ cm ⁻³ or less.
Alternatively, the selective growth waveguide type optical control element according to claim 3.