JP2760276B2 - Selectively grown waveguide type optical control device - Google Patents

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, and in particular, has a small coupling loss with a single mode fiber and enables high-speed modulation operation. Also, the present invention relates to an optical control element such as an optical modulator using selective growth which is excellent in uniformity of characteristics in 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 light control element such as a modulator-integrated light source in which an optical modulator is integrated with an FB laser is considered to be one of the key elements of a high-speed optical communication system and an optical information system. I have. For example, optical modulators using a dielectric such as LiNbO ₃ and those using a semiconductor such as InP or GaAs are considered.
In recent years, expectations for a semiconductor optical modulator that can be integrated with other optical elements such as an optical amplifier and an electronic circuit such as an FET, and that can be easily reduced in size and reduced in voltage have been increasing. Examples of the semiconductor modulator include a Franz-Kaldysh effect of a bulk semiconductor and a quantum confined Stark effect (Quantum Confined Stark E) in a multiple quantum well structure.
absorption type optical modulator utilizing the effect that the absorption edge shifts to the longer wavelength side when an electric field is applied, such as Ffect: QCSE, an electro-optic effect (Pockels effect) of a bulk semiconductor, and quantum in a multiple quantum well structure. Confinement St
A typical example is a Mach-Zehnder modulator utilizing an effect of changing a refractive index by applying an electric field such as an ark effect.

[0003] In recent years, with the increase in speed and distance of optical fiber communication, the problems of the conventional semiconductor laser direct modulation system are becoming apparent. That is, in the semiconductor laser direct modulation method, wavelength chirping occurs at the time of modulation, thereby deteriorating the waveform after fiber transmission. This phenomenon becomes more conspicuous as the signal transmission speed increases and the transmission distance increases. Particularly, in a system using an existing 1.3 μm zero-dispersion fiber, this problem is serious. Even if an attempt is made to extend the transmission distance using a 1.55 μm wavelength light source having a small propagation loss of the fiber, the problem is caused by chirping. The transmission distance is limited by the dispersion restriction. This problem can be solved by causing the semiconductor laser to emit light with a constant light output and employing an external modulation method in which the light emitted from the semiconductor laser is modulated by an optical modulator different from the semiconductor laser. for that reason,
In recent years, semiconductor optical modulators have been actively developed.

Meanwhile, the bulk semiconductor Franz-K
The eldysh effect and the quantum confined Stark effect in a multiple quantum well structure (Quantum Confine
An absorption type optical modulator utilizing the effect that the absorption edge shifts to the longer wavelength side by applying an electric field such as dStar 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. Are obtained, and are noted because of their small size and low operating voltage. Following the element structure and manufacturing process of a conventional semiconductor laser, a modulator integrated light source in which an electroabsorption modulator utilizing the Franz-Keldysh effect or QCSE effect or an electroabsorption modulator integrated with a DFB laser is being studied. ing. For example, an electroabsorption optical modulator having a structure in which bulk InGaAsP is used as an absorption layer and both ends of a mesa stripe having a double hetero structure including the InGaAsP absorption layer are filled with Fe-doped high-resistance InP is mixed. ELECTRONICS LETTERS Vo by SODA and others
l. 23 (1987), PP. 1233-1234.

However, in this optical modulator, the mesa stripe including the absorption layer is formed by the same wet etching as the conventional semiconductor laser, but the narrow mesa stripe is formed with good uniformity and reproducibility. Is difficult and there is a problem in the production yield of the device. Furthermore, in this example, both sides of the mesa stripe are buried with Fe-doped high-resistance InP to reduce the capacitance of the element and enable high-speed modulation. It is difficult to form a good interface.
If a good interface is not formed, when a reverse bias is applied to the mesa stripe including the absorption layer, a leak current flows along the interface, causing a problem that a sufficient voltage is not applied to the absorption layer.

On the other hand, when 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 in uniformity of characteristics within a wafer and reproducibility of run-to-run characteristics. A ridge-structured electroabsorption optical modulator using selective MOVPE growth is reported by Komatsu et al. In the 1992 IEICE Autumn Conference Proceedings, Volume 4-166. In this ridge structure electroabsorption optical modulator, first, a non-doped InGaAs is formed over the entire surface of an n-InP substrate to form a double hetero structure in which the n-InP clad layer and the non-doped InP clad layer are interposed. Thereafter, a pair of stripe-shaped SiO ₂ masks for forming a ridge portion by selective growth are formed on the non-doped InP cladding layer, and a p-InP cladding layer and a p-InGaAs cap layer are formed in gaps of the SiO ₂ mask. Form a ridge. Using this method, an optical modulator can be manufactured only by patterning and crystal growth of a thin dielectric mask without etching a semiconductor, and can achieve uniformity of characteristics within a wafer and ru.
The reproducibility of n-to-run characteristics is excellent, and the manufacturing yield of the optical modulator is greatly improved as compared with the related art. However, when the ridge structure is used, the spot size of the incoming / outgoing light greatly differs between the horizontal direction and the vertical direction. Due to the asymmetry of the horizontal and vertical spot sizes, the coupling loss with the fiber in the ridge structure modulator increases. 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 configure the modulator having the buried structure by using selective growth to eliminate the asymmetry of the spot size. However, in a selective growth modulator having a buried structure, a homojunction is formed at the interface between the conductive type substrate and the reverse conductive type buried layer, and the carrier concentration is usually high in both the conductive type substrate and the reverse conductive type buried layer. Causes a larger capacitance. Therefore, there is a problem that the modulation frequency band is narrow and cannot be applied to a high-speed optical communication system.

[0007]

As described above, in the structure of the conventional semiconductor waveguide type light control element, the mesa stripe including the absorption layer is formed by wet etching, and the width of the mesa stripe is narrow. Since it is difficult to form the device with good uniformity and reproducibility, the structure has a problem in terms of the production yield of the device. In order to solve this problem, a structure that can use selective growth as a manufacturing method may be adopted. However, in a selective growth waveguide type optical control element having a ridge structure, the asymmetry of the spot size of incoming and outgoing light is large, and There is a problem that the coupling loss with the one-mode fiber is large. Furthermore, 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 conductive type substrate and the opposite conductive type buried layer is improved. Modulator frequency band is narrow due to large capacitance,
There is a problem that it cannot be applied to a high-speed optical communication system.

The problem to be solved by the present invention is that a selective growth buried structure waveguide type optical system which can use a selective growth method capable of improving the production yield as a manufacturing method and has a small coupling loss with a single mode fiber. In the control element, there is no defect such as an increase in the series resistance of the element,
It is another object of the present invention to provide a structure of a high-speed guided light control element with small capacitance and little change in the element manufacturing process.

[0009]

According to the present invention, at least a conductive semiconductor optical waveguide layer, a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor active layer is provided on a conductive semiconductor substrate. A mesa stripe in which a waveguide layer and a reverse conductive semiconductor first cladding layer are sequentially laminated is partially formed by selective growth, and the band gap energy of the non-doped semiconductor absorption layer, the semiconductor active layer, or the semiconductor waveguide layer is reduced by a stripe. The second conductive layer and the second conductive semiconductor cap layer, which cover the top and side surfaces of the mesa stripe and bury the mesa stripe, are also formed in a mesa stripe shape by selective growth. A band gap energy partially different in the stripe direction. An integrated structure semiconductor waveguide type optical control device which includes means bandgap flop semiconductor absorption layer or a semiconductor active layer or the semiconductor waveguide layer to apply an electric field independently to different sites, said conductive semiconductor substrate The structure of the selective growth waveguide type light control element is characterized in that a conductive low carrier concentration semiconductor buffer layer is inserted between the mesa stripe and the opposite conductive type semiconductor second cladding layer.

In the present invention, in order to solve the above-mentioned problems, at least a conductive type semiconductor first cladding layer, a non-doped semiconductor absorbing layer or a semiconductor active layer or a semiconductor waveguide layer, a reverse conductive type A mesa stripe in which a semiconductor second clad layer is sequentially laminated is partially formed by selective growth, and a third clad layer of a reverse conductivity type semiconductor and a reverse conductivity type semiconductor which bury the mesa stripe covering the upper surface and side surfaces of the mesa stripe. A semiconductor waveguide type light control device, wherein a cap layer is also formed in a mesa stripe shape by selective growth, and comprising a means for applying an electric field to the non-doped semiconductor absorption layer or semiconductor active layer or semiconductor waveguide layer, Of selective growth waveguide type optical control device characterized in that the semiconductor substrate has a low carrier concentration. , And at least conductive semiconductor optical waveguide layer to the conductive type semiconductor substrate, a non-doped semiconductor absorption layer or a semiconductor active layer or the semiconductor waveguide layer, and the opposite conductivity type semiconductor first mesa stripe cladding layer are sequentially stacked by selective growth The band gap energy of the partially doped non-doped semiconductor absorbing layer or semiconductor active layer or semiconductor waveguide layer is partially different in the stripe direction, and covers the upper and side surfaces of the mesa stripe and embeds the mesa stripe. The opposite conductive type semiconductor second clad layer and the opposite conductive type semiconductor cap layer are also formed in a mesa stripe shape by selective growth, and the non-doped semiconductor absorption layer or the semiconductor active layer or the semiconductor conductive layer having a band gap energy partially different in the stripe direction. Band gap in wave layer An integrated structure semiconductor waveguide type optical control device which includes means for applying an electric field independently to different sites selectively grown waveguide type optical control, wherein said conductive semiconductor substrate has a low carrier concentration The structure of the element was also adopted.

[0011]

According to the present invention, since the structure of the waveguide type optical control element is a structure that can be used for selective growth, the waveguide type optical control element can be formed only by patterning a thin dielectric film and growing a crystal without etching a semiconductor. A device can be manufactured. Compared with a device having a structure in which manufacturing of a device using conventional semiconductor etching is considered, uniformity of characteristics within a wafer and run-
The reproducibility of to-run characteristics is remarkably excellent.

Further, in the present invention, since a buried structure is employed as the structure of the waveguide type light control element, the spot sizes of the incoming and outgoing light are symmetric 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 order to solve the problem of a large homojunction capacitance peculiar to the buried-structure waveguide type optical control element, a conductive type low carrier concentration is provided between the conductive type substrate and the opposite conductive type buried layer. By inserting a semiconductor buffer layer or using a conductive type substrate with a low carrier concentration, the thickness of the depletion layer is increased to reduce the homojunction capacitance. Is possible with little to no.

[0014]

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

FIG. 1 shows a Franz-Keldysh as an example of a selective growth waveguide type optical control element according to the present invention.
FIG. 4 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) plane low-doped n-InP substrate 120
A 1000 Å thick SiO ₂ film serving as a dielectric mask for selective growth is formed on the entire surface (FIG. 2).
(A)). Thereafter, using a normal photolithography technique, the above-mentioned SiO ₂ film is formed as an integrated stripe-shaped selective growth SiO ₂ opposed to each other with a stripe-shaped gap therebetween.
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 gap 20
The stripe direction of No. 2 is the [011] direction.

Next, using the MOVPE method, the n-InP cladding layer 1 is selectively formed only on the above-mentioned gap portion 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 cladding layer (thickness 0.05 μm) 105, and p-InP cladding layer (thickness 0.1 μm, carrier concentration 5 × 10 ¹⁷ cm ⁻³ ) in this order. Thus, a mesa 203 having a double hetero structure is formed (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 SiO ₂ mask 201 for selective growth is removed from the side of the double hetero structure mesa 203 by 1.5 μm to increase the width of the gap 202 between the masks.
A gap 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 formed again by selective MOVPE growth on the newly formed void and on the double heterostructure 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 5 × 10 ¹⁷ cm ^-3
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 p-side electrode contact is formed in the SiO ₂ protective film 109, and a 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 the Cr / A
An n-side electrode 111 made of u is formed (FIG. 2 (d)), the device is cleaved, and a non-reversible coating is applied to the input / output end surface, thereby completing the device.

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

The light wave of 1.55 μm wavelength incident from the incident end of the InP electroabsorption modulator shown in FIG.
Most of the energy is confined in the nGaAsP absorption layer 104 and propagates in the device. When a 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 low loss with respect to propagation light having a wavelength of 1.55 μm. At this time, a light output is obtained from the emission end of the modulator, and the modulator is turned on. On the other hand, by applying a reverse bias voltage between the p-side electrode 101 and the n-side electrode 111, Fr
When the absorption edge is shifted to the longer wavelength side by the anz-Kaldysh effect, the propagating light having a wavelength of 1.55 μm is i-In.
The GaAsP absorption layer 104 receives a large absorption loss, and no light output is obtained from the emission end, so that the GaAsP absorption layer 104 is turned off. In the case of the present invention, the ON state occurs at the reverse bias voltage OV, and the O state occurs at 3.0 V.
It can be realized in the FF state and has an extinction ratio of 1 with a 300 μm long element.
5 dB or more is obtained.

By the way, in the InP-based electroabsorption modulator shown in FIG. 1, the i-InGaAsP absorption layer 104
, The spot shape of the incoming / outgoing light is symmetrical in the vertical and horizontal directions with respect to the substrate. Therefore, it can be coupled with a single mode fiber with low loss. The present inventors have developed a Far Fi using a selective growth modulator having a ridge structure and a buried structure modulator.
The coupling loss between eld Pattern and single-mold fiber was compared experimentally. As a result, Far
Regarding the Field Pattern, the horizontal and vertical radiation angles of the ridge structure selective growth modulator are each 11 °.
In contrast, the embedded structure modulator shown in Fig. 1 has horizontal and vertical radiation angles of 40 ° and 47 °, respectively, and the asymmetry of the radiation angle is greatly improved. It had been. Reflecting this, aspheric lens 1
Regarding the coupling loss between the fiber and the single-mode fiber, the coupling loss was 3.2 dB in the case of the ridge structure selective growth modulator.
In contrast, in the case of the embedded structure modulator shown in FIG. 1, the coupling loss per end face is 2.2 dB, and 1
An improvement in dB was observed. As a result, the insertion loss between single mode fibers when a single mode fiber is coupled to both ends of the modulator was 10.0 dB in the case of the ridge structure modulator, but in the case of the embedded structure modulator shown in FIG. 6.
2 dB, and a great reduction in insertion loss was achieved. 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 mesa side surface is much smoother than the case where the mesa is formed by etching, and the loss due to scattering of the propagating light is greatly reduced. As described above, according to the present invention, it is possible to realize an InP-based electro-absorption modulator with much lower loss than the conventional one.

Next, the principle by which the capacitance of the element is reduced by the present invention and a high-speed embedded structure light modulator can be realized will be described. FIG. 3 shows a cross-sectional structure (FIG. 3B) of an InP-based buried structure electroabsorption modulator according to the present invention, and a cross-sectional structure of a normal buried structure electroabsorption modulator (FIG. 3).
(A)) is shown in comparison. In the case of a normal buried structure selective growth device, a homojunction portion 301 made of n-InP having a high carrier concentration and p-InP having a high carrier concentration is formed on both sides of the double hetero structure mesa 203. Normally, an n-InP substrate 101 having a carrier concentration of about 2 × 10 ¹⁸ cm ⁻³ is used.
The carrier concentration of the cladding layer 107 is set to 5 × 10 ¹⁷ cm ⁻³ or more in order to prevent the resistance component from becoming large. Therefore, the homojunction 301 on both sides of the double hetero structure mesa 203 has a high carrier concentration. It is formed of n-InP and p-InP having a high carrier concentration. At this time, a large capacitance is generated in the homo junction 301. As described above, the n-InP substrate 101
Has a carrier concentration of 2 × 10 ¹⁸ cm ⁻³ , a carrier concentration of the p-InP cladding layer 107 of 5 × 10 ¹⁷ cm ⁻³ , a homojunction 301 having a width of 2 μm on one side (therefore, 4 μm on both sides), and an element length. Homojunction 30 when is 300 μm
The capacitance at 1 is 2.0 pF. In the case of the InP-based buried structure electroabsorption modulator shown in FIG. 1, in addition to the above-described capacitance at the homojunction, the capacitance at the heterojunction 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 is 3.0 pF. At this time, the modulation frequency determined by the RC time constant is 2.1 GHz in a commonly used 50Ω impedance system.
It is difficult to apply to a 2.5 Gb / s optical communication system. As a means for reducing the capacitance at the homo-junction 301, it is conceivable to reduce the width of the p-InP cladding layer, that is, to reduce the width of the homo-junction 301. However, pI
When the width of the nP cladding layer is reduced, the horizontal light field feels air in the layer of the propagating light, and the horizontal single mode condition is undesirably increased in the layer. For this reason, 2 μm on one side is required as the width of the homo-joining portion 301. It is also possible to reduce the homojunction capacitance by making the element length shorter than 300 μm, but in this 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-dove 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, and the capacity of the entire device can be reduced and the modulation frequency band can be expanded. FIG. 4 shows the n-InP substrate carrier concentration dependence of the homojunction capacitance. When the carrier concentration of the n-InP substrate is set to 2 × 10 ¹⁷ cm ⁻³ , the capacitance at the homojunction 302 becomes 1.2 pF, which is half that in the case where the lightly doped n-InP substrate is not used. At this time, the capacitance of the entire device is 2.2 pF, and the modulation frequency band is 2.9 GHz.
Expand to z. The carrier concentration of the n-InP substrate is set to 1
When it is reduced to × 10 ¹⁷ cm ⁻³ , the capacitance at the homojunction 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, since no means for reducing the capacitance at the p-side electrode pad 112 is employed, even if the homojunction capacitance is reduced to 1 pF or less, the modulation frequency band is 3.4 GHz.
Although it remains at z, the modulation frequency band can be extended to about 6 GHz by reducing the capacitance of the pad portion by forming a polyimide film directly below the p-side electrode, for example. . 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 0.2.
Since 2 pF remains, the modulation frequency band remains at 3 GHz. Here, when a low-doped (100) plane n-InP substrate is used as the substrate, there is a concern that the resistance of the substrate may increase. However, if the carrier concentration is about 2 × 10 ¹⁷ cm ^{−3, the} substrate may not be used. Resistance hardly increases. According to an experiment by the present inventors, the carrier concentration of the (100) plane low-doped n-InP substrate 120 was 1 in the configuration of FIG.
Even when the carrier concentration is 2 × 1 even when the carrier concentration is × 10 ¹⁷ cm ^−3.
In comparison with the case of 0 ¹⁸ cm ^-3 , an increase in the device resistance is hardly observed, and even if the (100) plane low-doped n-InP substrate 120 has a low carrier concentration, the device characteristics such as the extinction characteristics are completely adversely affected. Is not given. Further, according to the method of reducing the homojunction capacitance according to the present invention, it is only necessary to use a low-doped substrate as the substrate, so that the speed of the device can be increased without any change in the device manufacturing process.

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

First, the method of manufacturing the modulator integrated light source shown in FIG. 5 will be described. However, since the manufacturing method has many parts common to the first embodiment, it differs from the manufacturing method of the first embodiment. I will only explain the points in detail.

First, a low-doped n-InP buffer layer 1 is formed on an (100) -oriented n-InP substrate 101 by using 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 . Next, lightly doped n-
The period in the [011] direction is 2 on the InP buffer layer.
A diffraction grating 501 having a depth of 40 nm and a depth of 30 nm is partially formed. A portion provided with the diffraction grating 501 is referred to as a DFB laser region 511, and a flat portion having no diffraction grating 501 is referred to as an optical modulator region 513. 1000 Å thick SiO _{2 serving} as a dielectric mask for selective growth is formed on this substrate.
Form a film, using normal photolithography technology,
The SiO ₂ film is patterned into a pair of stripe-shaped selective growth SiO ₂ masks opposed to each other with a stripe-shaped gap therebetween. FIG. 6 shows the shape of the selective growth SiO ₂ mask viewed from the upper surface of the wafer. Here, the gap 20
2, the width W of the DFB laser region 511 and the light modulator region 51
3 were 2 μm, the width of the selective growth SiO ₂ mask 601 was 17 μm in the DFB laser region 513, and the optical modulator region 51.
3 is 8 μm. Note that the stripe direction of the void 202 is the [011] direction, and the window region 514 having no void is formed in a stripe shape over 15 μm from the end face of the optical modulator region 513.

Next, an n-InGaAsP optical waveguide layer 502 having a composition wavelength of 1.15 μm (having a thickness of 0.1 μm and a thickness of 0.1 μm) is selectively formed only on the above-mentioned gap portion 202 using the MOVPE method.
Carrier concentration 5 × 10 ¹⁷ cm ⁻³ ), n-InP spacer layer 503 (0.04 μm in thickness, carrier concentration 5 × 10 ¹⁷⁾
cm ⁻³ ), a seven-period undoped quantum well layer 50 composed of an i-InGaAsP barrier layer (thickness: 10 nm) and a i-InGaAs well layer (thickness: 7 nm) having a compositional growth of 1.3 μm.
4. i-InP cladding layer (thickness 0.05 μm) 50
5. A p-InP cladding layer (thickness: 0.1 μm, carrier concentration: 5 × 10 ¹⁷ cm ⁻³ ) 506 is sequentially laminated to form a mesa having a Budar heterostructure. 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 SiO ₂ mask for selective growth is changed between the DFB laser region 511 and the optical modulator region 513. As described above, in the selective MOVPE growth, when the SiO ₂ mask width is partially changed, the diffusion of the growth material species (mainly, a group III organometallic material) in the gas phase is dependent 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 portion can be formed by one MOVPE growth. Therefore, the DFB laser section 511 and the optical modulator section 5
13, the undoped quantum well layer 504 requiring a different band gap energy can be formed by a single crystal growth and without interruption at the connection between the two regions. This selective growth method is similar to a modulator integrated light source. It is ideal as a method for manufacturing a simple optical integrated device. In this embodiment, the band gap wavelength of the undoped quantum well layer 504 is DFB.
1.55 μm in laser region 511, light modulator region 513
Becomes 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 increased so as to be 5 μm in the optical modulator region 513. The p-InP cladding layer 507 and p-InGaAs are formed on the newly formed void and on the double hetero structure mesa.
The s cap layer 508 is formed again by selective MOVPE growth. Here, the p-InP cladding layer 507 and the p-InP
-The thickness of each of the InGaAs cap layers 508 is 1 μm.
m and 0.2 μm, the carrier concentration is 5 × 1
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 both regions to the optical modulator region side.
The p-InGaAs cap layer 508 is removed over the area m to form an isolation region 512. Subsequently, the SiO ₂ protective film 10
9 was formed on the entire top surface of the substrate, p the SiO ₂ protective film 109
A window for side electrode contact is formed in the DFB laser region 511 and the optical modulator region 513. Then, Cr and A
A p-side electrode made of u is vapor-deposited, and the electrode is patterned by using a photolithography technique. The laser-part p-side electrode 520, the light modulator part p-side electrode 521, and the light modulator part p
The side electrode pad 523 is formed. Finally, after polishing the n-InP substrate 101 to a thickness of about 100 μm,
An n-side electrode 111 made of Cr / Au is formed on the P substrate 101 side, the device is cleaved, and a non-reflection film is formed on an end surface on a side of an optical modulator region and a high reflection film is formed on an end surface on a DFB laser region side. To end.

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 portion of the DFB laser region 511 and a current is injected into the DFB laser region 511,
Single-axis mode oscillation occurs at a wavelength determined by the pitch of the diffraction grating 501 and the effective refractive index determined by the cross-sectional structure of this region (1.55 μm in this embodiment), and the output light of the DFB laser region 511 is separated from 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 band gap wavelength is set to 1.48 μm so that light absorption at this wavelength when there is no bias is suppressed to a practically acceptable level. 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 light propagating in the optical modulator region 513 increases due to the quantum confined Stark effect. Operates as a controlling light intensity modulator.

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

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

Also, in this embodiment, unlike the first embodiment, p-InP
Cladding layer 507 and (100) plane orientation n-InP substrate 1
01, a lightly doped n-InP buffer layer 102 is inserted. That is, a substrate having a normal high carrier concentration (2 × 10 ¹⁸ cm ⁻³ ) is used as the substrate.
In order to reduce the capacity of the homojunction according to the same principle as in the first embodiment, in this embodiment, a lightly doped n-InP buffer layer is provided between the p-InP cladding layer 507 and the (100) plane n-InP substrate 101. 102 is inserted. In the first embodiment, the entire 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 expansion of the depletion layer toward the lightly doped n-InP substrate 120 is reversed. 0.2 μm at 3V bias voltage
It is about. Therefore, it is not always necessary to make the entire substrate have a low carrier concentration in order to reduce the capacity of the homojunction, and the low-doped n-InP buffer layer 102 having a thickness of about 0.3 μm is formed on the (100) plane n-InP substrate. 101 and p
-If it is inserted between the InP cladding layer 507,
Capacitance reduction similar to using a low-doped substrate is possible. If the thickness is about 0.3 μm, there is no concern that the resistance will increase even if a lightly 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, a modulation frequency band of 3.5 GHz is secured, and it is possible to sufficiently cope with high-speed modulation.

Further, in this embodiment, two regions having different band gap energies are simultaneously formed using selective growth. What is necessary to form such regions having different band gap energies is that Si
It is only necessary to partially change the width of a dielectric mask such as O ₂ . Therefore, as compared with the conventional method of forming regions having different band gap energies by repeating etching and crystal growth of the semiconductor layer, the manufacturing process is greatly simplified and the manufacturing yield is greatly improved. In addition, since the light coupling efficiency between the DFB laser region and the optical modulator region becomes almost 100%, the performance can be improved, 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 the series resistance of the device, and the coupling loss with the single mode fiber can be achieved without changing the device manufacturing process. It is possible to realize a semiconductor waveguide type optical control element which is small in size, high in speed and high in production yield.

The present invention is not limited to the above embodiment. As an example, an InP-based electro-absorption modulator and a modulator integrated light source have been described. However, the present invention is not limited to this, and the present invention is applicable to other InP-based light control elements such as a Mach-Zehnder modulator and a directional coupler type switch. The present invention can be applied to this. Also, 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 or the like. Further, the present invention can be similarly applied to a light control element using another semiconductor material such as GaAs. In addition, it goes without saying that the present invention is not limited to the element shapes shown in the examples, that is, the thickness of each layer, the composition of each layer, the waveguide dimensions, and the like.

[Brief description of the drawings]

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

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

FIG. 3 shows a cross-sectional structure of a selectively grown buried structure waveguide type optical control element (b) according to the present invention and a conventional selectively grown buried structure waveguide type optical control element (a). It is a thing.

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

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

FIG. 6 is a diagram illustrating a method of manufacturing an optical modulator integrated light source according to one embodiment of the present invention.

[Explanation of symbols]

101 (100) orientation n-InP substrate 102 lightly doped n-InP buffer layer 103 n-InP cladding layer 104 i-InGaAsP absorption layer 105,505 i-InP cladding layer 106,506 p-InP cladding layer 107,507 p- InP cladding 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) plane orientation lightly doped n-InP substrate 201, 601 for selective growth SiO ₂ mask 202 Air gap 203 Double hetero structure mesa 301 Homo junction 302 Homo junction 501 Diffraction grating 502 n-InGaAsP optical waveguide layer 503 n-InP spacer layer 504 Undoped quantum well layer 511 DFB laser area 512 Separation area 513 Optical 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

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-110186 (JP, A) 1991 IEICE Autumn Conference C-133 4-163 1991 IEICE Autumn Conference C-131 4-161 (58) Field surveyed (Int. Cl. ⁶ , DB name) H01S 3/18 G02F 1/025

Claims (12)

(57) [Claims] 1. A mesa stripe in which at least a conductive semiconductor optical waveguide layer, a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor waveguide layer, and a reverse conductive semiconductor first cladding layer are sequentially laminated on a conductive semiconductor substrate. The band gap energy of the undoped semiconductor absorption layer, the semiconductor active layer, or the semiconductor waveguide layer is partially different in the stripe direction, and is formed partially by selective growth. A reverse conductive semiconductor second cladding layer and a reverse conductive semiconductor cap layer for embedding the stripe are also formed in a mesa stripe shape by selective growth, and a non-dove semiconductor absorption layer or a semiconductor active layer having a band gap energy partially different in the stripe direction. Alternatively, a band gap Flops an integrated structure semiconductor waveguide type optical control device which includes means for applying an electric field independently to a different site, the said conductive semiconductor substrate Mesasuto
Ripe and opposite conductivity type semiconductor second selective growth waveguide type optical control device conductivity type low carrier concentration semiconductor buffer layer is characterized in that it is inserted between the cladding layer. 2. A mesa stripe in which at least a first conductive semiconductor clad layer, a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor waveguide layer, and a reverse conductive semiconductor second clad layer are sequentially laminated on a conductive semiconductor substrate. A reverse conductive semiconductor third cladding layer and a reverse conductive semiconductor cap layer which are partially formed by selective growth and bury the mesa stripe covering the top and side surfaces of the mesa stripe are also formed in a mesa stripe by selective growth. A semiconductor waveguide type light control element comprising means for applying an electric field to the non-doped semiconductor absorption layer or semiconductor active layer or semiconductor waveguide layer, wherein the conductive semiconductor substrate has a low carrier concentration. Selective growth waveguide type optical control element. 3. The semiconductor device according to claim 1, wherein the conductive type semiconductor substrate has at least a conductive type half.
A mesa stripe in which a conductive optical waveguide layer , a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor waveguide layer, and a reverse conductive semiconductor first cladding layer are sequentially laminated is partially formed by selective growth, and the non-doped semiconductor absorption layer is formed. Alternatively, the bandgap energy of the semiconductor active layer or the semiconductor waveguide layer is partially different in the stripe direction, and the reverse conductivity type semiconductor second cladding layer which covers the upper surface and the side surface of the mesa stripe and embeds the mesa stripe and the reverse conductivity. The semiconductor cap layer is also formed in a mesa stripe shape by selective growth, and is independent of a non-doped semiconductor absorption layer, a semiconductor active layer, or a semiconductor waveguide layer having a different band gap in the stripe direction. Means to apply an electric field to A Bei the integrated structure semiconductor waveguide type optical control element, the selective growth waveguide type optical control element, wherein said conductive semiconductor substrate has a low carrier concentration. 4. A conductive semiconductor optical waveguide layer, a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor waveguide layer,
And a mesa stripe on which at least the reverse conductive semiconductor first cladding layer is laminated is formed on a (100) plane conductive semiconductor substrate or on a (100) plane low conductive buffer layer with a stripe direction of [011]. 2. The selective growth waveguide type optical control device according to claim 1, wherein said mesa stripe is formed by selective growth so as to have a direction, and a side surface of said mesa stripe is a (111) B plane. 5. A mesa stripe in which at least a conductive type semiconductor first cladding layer, a non-doped semiconductor absorbing layer, and a reverse conductive type semiconductor second cladding layer are laminated, a stripe direction is [100] plane direction conductive semiconductor substrate. 011]
3. The selective growth waveguide type optical control device according to claim 2, wherein the mesa stripe is formed by selective growth so as to be oriented in a direction, and a side surface of the mesa stripe is a (111) B plane. 6. A conductive semiconductor optical waveguide layer, a non-doped semiconductor absorption layer or a semiconductor active layer or a semiconductor waveguide layer,
And a mesa stripe on which at least the reverse conductive semiconductor first cladding layer is laminated is formed by selective growth on the (100) plane conductive semiconductor substrate so that the stripe direction is the [011] direction. 4. The selective growth guided light control device according to claim 3, wherein the side surface is a (111) B surface. 7. A conductive semiconductor substrate and a reverse conductive semiconductor second substrate.
2. The selective growth waveguide type optical control device according to claim 1, wherein the carrier concentration of the conductive type low carrier concentration semiconductor buffer layer inserted between the cladding layer and the conductive type semiconductor substrate is lower than the carrier concentration of the conductive type semiconductor substrate. . 8. A conductive semiconductor substrate and a reverse conductive semiconductor second substrate.
The carrier concentration of the low-conductivity semiconductor buffer layer inserted between the semiconductor layer and the cladding layer is lower than that of the second conductivity-type semiconductor buffer layer.
2. The selective growth guided light control device according to claim 1, wherein the carrier concentration is lower than the carrier concentration of the cladding layer. 9. A conductive semiconductor substrate and a reverse conductive semiconductor second substrate.
2. The selective growth waveguide type optical control device according to claim 1, wherein the carrier concentration of the conductive type low carrier concentration semiconductor buffer layer inserted between the cladding layer and the cladding layer is 5 × 10 ¹⁷ cm ⁻³ or less. . 10. The conductive type semiconductor substrate has a carrier concentration of:
3. The selective growth waveguide type optical control device 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 conductive semiconductor substrate has a carrier concentration of:
4. The selective growth waveguide type light control device according to claim 3, wherein the carrier concentration is lower than the carrier concentration of the reverse conductivity type semiconductor second cladding layer. 12. The conductive type semiconductor substrate has a carrier concentration of:
3. The structure according to claim 2, wherein the size is 5 × 10 ¹⁷ cm ⁻³ or less.
4. A selective growth waveguide type optical control device according to claim 3.