JP3159914B2 - Selectively grown waveguide type optical control element and method of manufacturing the same - 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 light control element used for an optical communication system or an optical information processing system.
In particular, the present invention relates to a light control element such as a light modulator using selective growth.

[0002]

2. Description of the Related Art Conventionally, as this kind of light control element, for example, a selective growth waveguide type light control element disclosed in Japanese Patent Application Laid-Open No. 7-202316 is known. In this selective growth waveguide type optical control element, a buried structure is adopted as the structure of the waveguide type optical control element, and a semiconductor buffer layer having a conductive type low carrier concentration is provided between a conductive type substrate and a reverse conductive type buried layer. It has a structure in which a conductive type substrate having a low carrier concentration is inserted or inserted. By employing such a structure, a homojunction is formed at the interface between the substrate and the buried layer because the waveguide-type light control element structure is a buried structure, and the capacitance caused by the homojunction is reduced.

[0003]

However, in the selective growth waveguide type optical control device having the above structure, an optical waveguide region serving as an absorption layer or an active layer is grown using a selective growth mask having a width of 2 μm to 7 μm. However, if the width of the selective growth region is small, the growth speed is affected by the increase in the growth speed due to surface diffusion, so that the growth speed varies within the selective growth region, the crystallinity of the selective growth layer is deteriorated, and the characteristics of the device are deteriorated. There was a problem.

In the selective growth waveguide type optical control device having the above structure, a semiconductor buffer layer having a low carrier concentration is inserted between the conductive substrate and the buried layer of the opposite conductivity type. By using a conductive substrate,
Although the capacitance at the homojunction can be reduced to some extent, the homojunction at the interface between the substrate and the buried layer does not disappear, so the capacitance at the homojunction still exists. Therefore, there is a limit in increasing the operation speed of the device.

[0005]

In order to solve the above-mentioned problems, in a selective growth waveguide type optical control device according to the present invention, a second region is formed in a first region on a semiconductor substrate by selective growth and a second region is formed. An optical waveguide region formed over the entire surface of the optical waveguide region by whole-surface growth, and a first mesa-shaped cladding layer formed in a stripe shape along the longitudinal direction at a central portion in the width direction on the optical waveguide region; A second cladding layer formed of a medium having the same refractive index as the first cladding layer on both sides of the first cladding layer on the optical waveguide region to form a flat portion; And a means for applying

In the selective growth waveguide type optical control device having the above structure, the optical waveguide region partially formed by selective growth in the first region acts as an active layer, and is formed entirely in the second region. The optical waveguide region functions as an absorption layer. The mesa-shaped first cladding layer and the second cladding layers on both sides thereof form a ridge structure. By adopting this ridge structure, in the case of the buried structure, the homojunction formed at the interface between the substrate and the buried layer is eliminated, and the capacitance at the homojunction is also eliminated. In addition, the mesa-shaped first cladding layer is formed at the central portion in the width direction on the optical waveguide region, so that the central portion of the selective growth region (optical waveguide region) is not affected by an increase in growth rate due to surface diffusion. Alone can be used as the active layer and the absorbing layer.

In the method of manufacturing a selective growth waveguide type optical control element according to the present invention, a pair of stripe-shaped dielectric masks are formed at a predetermined interval in a first region on a semiconductor substrate, and the pair of dielectric masks is formed. Is used to selectively grow an optical waveguide region in the first region and the entire other region in the second region. Then, after removing the pair of dielectric masks, the entire region has the same refractive index. A first cladding layer and a second cladding layer of a medium are sequentially formed, and a stripe-shaped etching mask is formed thereon. Subsequently, the second cladding layer is etched into a mesa shape using the etching mask. Thereafter, a first electrode is vapor-deposited via an insulating film, and a contact is made with the second cladding layer, and a second electrode is formed on the lower surface side of the semiconductor substrate.

In the above manufacturing method, an optical waveguide region is selectively grown in the first region and entirely in the second region using a pair of dielectric masks, so that the first region and the second region are formed. The bandgap energy of the optical waveguide region is different from that of the second region, and a portion having a different bandgap energy can be formed in the same semiconductor substrate by one crystal growth. After the first cladding layer and the second cladding layer are sequentially formed, the second cladding layer is etched into a mesa shape, so that the first and second cladding layers made of a medium having the same refractive index form a ridge. A structure is formed.

[0009]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a perspective view showing an embodiment of a selective growth waveguide type optical control element according to the present invention.
nP electro-absorption type optical modulator integrated DFB (distributed fee)
dback (distribution feedback type) laser is shown.

In FIG. 1, a diffraction grating having a period of about 240 nm is partially formed on an n-type InP substrate 1. In this n-type InP substrate 1, a first region where a diffraction grating is formed is a DFB laser region, and a second region where no diffraction grating is formed is an optical modulator region 1.
It becomes 2. On the n-type InP substrate 1, a composition wavelength of 1.2
A μm InGaAsP optical waveguide layer 2 is formed.

On the optical waveguide layer 2, an optical waveguide region 3 having a double hetero structure is formed. This optical waveguide region 3
n-InP spacer layer, InGa having a composition wavelength of 1.2 μm
It is composed of an undoped quantum well layer of five periods composed of an AsP barrier layer and an InGaAs well layer, and a p-type InP cladding layer. On the side of the DFB laser region 11, it is formed at the center of the substrate 1 in the width direction and acts as an active layer. Modulator area 12
On the side, it is formed over the entire surface and acts as an absorbing layer.

On the optical waveguide region 3, a trapezoidal (mesa-shaped) p-type InP is formed at the center in the width direction along the longitudinal direction.
A cladding layer 4 is formed in a stripe shape, and a p-type InP cladding layer 5 of a medium having the same refractive index as that of the cladding layer 4 is formed on both sides thereof as flat portions. A ridge structure is formed by the p-type InP cladding layers 4 and 5 of the medium having the same refractive index.

On the p-type InP cladding layers 4 and 5, S
An iO ₂ insulating film 6 is formed over the entire surface. SiO
_{2. On} the cladding layer 5 of the insulating film 6, a window 6a for p-side electrode contact is formed. Then, Cr / A is formed on the entire surface of the SiO ₂ insulating film 6 on the DFB region 11 side.
A p-side electrode 7 made of u is deposited and is in contact with the cladding layer 4 through the window 6a.

On the other hand, on the side of the optical modulator region 12, the p-side electrode 8 is formed only on the cladding layer 4, and is connected to the electrode pad 9 formed on the flat portion. On the lower surface side of the substrate 1, an n-side electrode 10 made of Cr / Au is formed over the entire bottom surface. The p-side electrodes 7 and 8 and the n-side electrode 10 constitute a means for applying an electric field to the optical waveguide region 3 partially (active layer, absorption layer).

Subsequently, In according to the present embodiment having the above-described structure,
The operation of the P-system electro-absorption type optical modulator integrated DFB laser will be described.

First, in the DFB laser region 11, when a forward voltage is applied between the p-side electrode 7 and the n-side electrode 10,
InGaAsP / InGaAs optical waveguide region (active layer) 3
Holes and electrons are injected into the substrate 1 to emit light, and the wavelength is selected by the diffraction grating formed on the substrate 1 to oscillate the DFB laser.

On the other hand, in the optical modulator region 12, the p-side electrode 8
When a reverse bias voltage is applied between the gate electrode and the n-side electrode 10,
The absorption coefficient of the InGaAsP / InGaAs optical waveguide region (absorption layer) 3 increases according to the applied voltage. This phenomenon of increasing the absorption coefficient is due to the quantum confined Stark effect (Stark eff
ect). Here, the Stark effect refers to the separation or shift of spectral lines or energy levels caused by an internal electric field due to atoms or ions existing in the vicinity of a solid or an externally applied electric field.

Also, when a bulk layer is used for the InGaAsP / InGaAs optical waveguide region (absorption layer) 3, a similar increase in the absorption coefficient is observed due to the Franz-Keldysh effect. Here, the Franz-Keldysh effect refers to an effect in which the wavelength of the light absorption edge of a semiconductor or an insulator appears to shift to a longer wavelength side when a strong electric field is applied.

As described above, in the waveguide type optical control element in which the optical modulator is integrated with the DFB laser, the ridge structure is adopted as the element structure. Since no homojunction is formed, the capacitance at the homojunction is also eliminated.
As a result, the frequency band of the modulator can be expanded and the operation of the element can be performed at a higher speed. Therefore, a waveguide type optical control element applicable to a high-speed optical communication system can be provided.

Incidentally, in the case of the ridge structure, the spot size of the incoming / outgoing light is largely different between the horizontal direction and the vertical direction. The asymmetry of the horizontal and vertical spot sizes is due to the fact that the active layer of the DFB laser region 11 and the optical modulator region 12
However, there is a problem that the coupling loss at the joint with the absorbing layer is increased. However, in the waveguide type optical control device according to the present invention, the active layer of the DFB laser region 11 and the optical modulator region 12
Form the same waveguide by the optical waveguide region 3, even if the ridge structure is adopted as the element structure,
In terms of coupling loss, the asymmetry of the spot size in the horizontal and vertical directions does not pose any problem.

Next, the InP according to this embodiment having the above-described structure is used.
A method for manufacturing a system-based electroabsorption type optical modulator integrated DFB laser will be described with reference to the process chart of FIG. Note that FIG.
Shows a cross section of a laser region portion of the present optical modulator integrated DFB laser. In FIG. 2, the same parts as those in FIG. 1 are denoted by the same reference numerals.

In the step (a), a diffraction grating having a period of about 240 nm is partially formed on the n-type InP substrate 1,
The portion where the diffraction grating is formed is referred to as a DFB laser region 11, and the portion where the diffraction grating is not formed is referred to as an optical modulator region 12.
On this substrate 1, metal organic chemical vapor deposition (MOVPE)
InGaAsP with a composition wavelength of 1.2 μm
The optical waveguide layer 2 is grown. Further thereon, an SiO ₂ film 21 having a thickness of about 1000 to 3000 Å, which serves as a dielectric mask for selective growth, is formed.

Subsequently, in the step (b), the SiO ₂ film 21 is patterned into a pair of stripe-shaped selective growth masks 22 and 23 using ordinary photolithography technology. The masks 22 and 23 are formed only in the DFB laser region 11, as shown in FIG. In the step (c), an n-InP spacer layer and a composition wavelength of 1.2 μm are used.
A five-period undoped quantum well layer composed of m InGaAsP barrier layers and InGaAs well layers, and a double heterostructure optical waveguide region 3 composed of a p-type InP cladding layer are formed epitaxially.

In this process, as is apparent from FIG.
Since the selective growth masks 22 and 23 do not exist in the optical modulator region 12, the optical waveguide region (absorbing layer) 3 of the optical modulator region 12 is grown over the entire surface. By forming the selective growth masks 22 and 23, the optical waveguide region (active layer) 3 of the DFB laser region 11 is selectively grown in the region between the masks 22 and 23. As a result, the bandgap energies of the optical waveguide region 3 in the DFB laser region 11 and the optical modulator region 12 are different, and portions having different bandgap energies can be formed in the same substrate by one crystal growth.

The reason why the selective growth is performed in this manner is that, during the growth, a concentration gradient of the raw material is generated between the regions on the selective growth masks 22 and 23 and the region to be grown, and the growth raw material is diffused by vapor phase. A change in composition and a change in growth rate occur, and the composition and thickness of the quantum well layer change. FIG.
FIG. 5 shows an example of the result of measuring the relationship between the distance from the SiO ₂ mask edge and the InP growth rate when InP is selectively grown.

The change in the growth rate R of InP caused by the gas phase diffusion is determined by the gas phase diffusion length L, and the relationship between the growth rate R and the distance x between the mask edges is expressed by the following equation.

(Equation 1) Here, A is a proportionality coefficient.

FIG. 4 shows an example in which the gas phase diffusion length L is 3
The calculated value and the actual measured value when the growth rate is calculated as 9 μm are shown. In the figure, the calculated value is indicated by a solid line, and the measured value is indicated by a circle. As is clear from FIG. 4, the measured value and the calculated value match well in the region where the distance from the mask edge is 5 μm or more, but the growth rate is higher than the calculated value in the region of 5 μm or less. This is presumably because the surface diffusion of the raw materials on the selective growth masks 22 and 23 increased the supply of the raw materials in the vicinity of the mask edges, and the growth rate was accelerated by the influence.

In a region affected by surface diffusion, the growth rate and composition rapidly change depending on the distance from the mask edge, and there is a large variation and it is difficult to control the composition. In order to obtain it, the selective growth masks 22 and 2 having the width of the selective growth region are required.
The interval of 3 needs to be about 10 μm to 40 μm.
When growing the optical waveguide region 3 by selective growth, DFB
Since the width of the InGaAsP / InGaAs optical waveguide region 3 in the laser region is determined by the interval between the selective growth masks 22 and 23, the width is about 10 μm to 40 μm.

In the step (d), the selective growth mask 22,
23 are removed, the p-type InP cladding layer 24 and the p-type InP cladding layer 25 of the medium having the same refractive index as a whole, a band gap wavelength of 1.3 μm, and an impurity concentration p = 5 ×
10 ¹⁸ cm ^-3 , p-type InGaAsP about 0.2 μm thick
The contact layers 26 are sequentially stacked. Next, in the step (e), a stripe-shaped SiO ₂ etching mask 27 having a width of about 3 μm is formed on the surface so as to cover the center of the selective growth region.

Then, in the step (f), the p-type InP cladding layer 25 is etched using the stripe-shaped SiO ₂ etching mask 27 to form the mesa-shaped cladding layer 4 in a stripe shape. As a result, a ridge structure is formed by the mesa-shaped cladding layer 4 and the cladding layers 5 serving as flat portions on both sides thereof.

Subsequently, in the step (g), the whole surface is made of SiO ₂
An insulating film 6 is formed, and a window for a p-type electrode contact is formed in the SiO ₂ insulating film 6. Then, the p of Cr / Au
The side electrode 7 (8) is vapor-deposited, and the electrode is patterned using a normal photolithography technique. Next, an n-side electrode 10 made of Cr / Au is formed on the lower side of the substrate 1, and an Al ₂ O ₃ film (refractive index 〜1.75) having a film thickness of about 2,000 Å is formed on the exit face of the modulator. AR coating is performed. As described above, the semiconductor optical modulator integrated D
An FB laser is configured.

As described above, when manufacturing a waveguide type optical control element in which an optical modulator is integrated with a DFB laser using selective growth which can improve the manufacturing yield, p-type InP of a medium having the same refractive index is used. Cladding layer 24 and p-type In
P cladding layers 25 are sequentially laminated, and striped SiO
_{(2) By} etching the p-type InP cladding layer 25 using the etching mask 27 and forming the mesa-shaped cladding layer 4 in a stripe shape, the homojunction formed at the interface between the substrate and the buried layer in the case of the buried structure. The ridge structure without the ridge can be easily formed.

The optical waveguide region 3 serving as an active layer or an absorption layer is grown by using a pair of selective growth masks having an interval of 10 μm to 40 μm, and the width of the optical waveguide region 3 is set relatively wide. By forming the cladding layer 4 in the central portion, the selective growth region which is not affected by the growth rate due to the surface diffusion, that is, only the central portion of the optical waveguide region 3 has the active layer or the active layer of the waveguide type optical control element. Since it can be used as an absorption layer, the crystallinity of the optical waveguide region 3 is good, and good device characteristics can be obtained.

In the above embodiment, the active layer of the DFB laser region 11 and the optical modulator region 1 are formed by the optical waveguide region 3.
2 is formed, but the DFB laser region 1
Since the optical waveguide layer exists at the joint between the active layer 1 and the absorption layer of the optical modulator region 12, the optical waveguide region 3 also includes this optical waveguide layer.

In the above embodiment, the first region of the optical waveguide region 3 is the laser region 11, the second region is the optical modulator region 12, and the laser light source and the optical modulator are integrated. Although the wave type light control element has been described, the present invention is not limited to this. The first region of the optical waveguide region 3 is an amplification region, and an optical amplifier for amplifying light introduced from the outside and an optical modulator are integrated. It is also possible to provide a waveguide-type light control element having the above configuration.

[0036]

As described in detail above, according to the present invention, a ridge structure is adopted as a device structure in a waveguide type light control device in which an optical modulator is integrated with a laser light source using selective growth. As a result, the homojunction formed at the interface between the substrate and the buried layer in the case of the buried structure is eliminated, and the capacitance at the homojunction is also eliminated, so that the modulator frequency band is expanded and the operation of the device is further accelerated. be able to. Also, since only the central portion of the selective growth region, which is not affected by the growth rate due to surface diffusion, can be used as the active layer and the absorption layer of the waveguide type optical control element, the crystallinity of the selective growth region is good and good. Element characteristics can be obtained.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of the present invention.

FIG. 2 is a process chart showing a procedure of a manufacturing method of the present invention.

FIG. 3 is a plan view in a step (b).

FIG. 4 is a characteristic diagram showing an example of a relationship between a distance from a mask edge and an InP growth rate.

[Explanation of symbols]

 Reference Signs List 1 n-type InP substrate 3 optical waveguide region (active layer, absorption layer) 4,5 clad layer 7,8 p-side electrode 10 n-side electrode

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-260727 (JP, A) JP-A-7-176827 (JP, A) JP-A-6-314657 (JP, A) JP-A 8- 220496 (JP, A) JP-A-8-162701 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (4)

(57) [Claims] An optical waveguide region partially formed by selective growth in a first region on a semiconductor substrate and entirely formed by overall growth in a second region; and a width direction on the optical waveguide region. A first cladding layer having a mesa shape formed in a stripe shape along a longitudinal direction at a central portion of the first cladding layer, and a refractive index of the first cladding layer on both sides of the first cladding layer on the optical waveguide region. A selective growth waveguide type optical control device, comprising: a second cladding layer formed of the same medium to form a flat portion; and a means for partially applying an electric field to the optical waveguide region. 2. The selective growth waveguide type optical control device according to claim 1, wherein the width of the selective growth region in the optical waveguide region is 10 μm to 40 μm. 3. A pair of stripe-shaped dielectric masks are formed at predetermined intervals in a first region on a semiconductor substrate, and the laser region is selectively formed using the pair of dielectric masks. After the optical waveguide region is entirely grown on the second region, and after removing the pair of dielectric masks, the first cladding layer and the second cladding layer of the medium having the same refractive index as a whole are sequentially arranged. Laminate, further form a stripe-shaped etching mask thereon, then use the etching mask to etch the second cladding layer into a mesa shape, and then deposit a first electrode via an insulating film And forming a second electrode on the lower surface of the semiconductor substrate while making contact with the second cladding layer. 4. The distance between said pair of dielectric masks is 10 μm.
4. The method according to claim 3, wherein the thickness is set to m to 40 [mu] m.