JP3251191B2 - Semiconductor optical device used for optical communication - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor optical device used for optical communication and the like and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a semiconductor optical device such as a semiconductor laser or a wavelength filter having a distributed reflector using a diffraction grating,
Considering single-axis mode control of semiconductor lasers, improvement of high output characteristics and yield, improvement of wavelength control characteristics of wavelength filters, etc., optimally changing the optical coupling coefficient of the distributed reflector in the optical waveguide direction, There is a demand for a manufacturing method capable of manufacturing regions having different optical coupling coefficients at arbitrary positions in the optical waveguide direction. Conventional methods for manufacturing a diffraction grating include the following.

FIG. 6 is a schematic sectional view in the axial direction of a semiconductor optical device comprising two regions having different optical coupling coefficients κ.
To form a plurality of regions having different optical coupling coefficients κ in the optical waveguide direction in a conventional semiconductor optical device, it is necessary to use two
There were two ways.

One is that, in FIG. 6A, a diffraction grating 5 is formed on a semiconductor substrate 1, an optical waveguide layer 2 for embedding the diffraction grating 5 in a region 1 is grown, and then this optical waveguide layer 2 is formed. Is selectively removed by etching, and then the optical waveguide layer 3 for embedding the diffraction grating 5 in the region 2 is regrown. Furthermore, a method of obtaining a semiconductor optical device by growing an optical waveguide layer 4 including a gain generating layer and the like and a cladding layer 11 on the optical waveguide layers 2 and 3. In this manufacturing method, a plurality of regions having different optical coupling coefficients can be manufactured by selecting the crystal compositions of the optical waveguide layers 2 and 3. However, in this method, 1) crystal growth of the optical waveguide layer 2 → etching of the optical waveguide layer 2 → regrowth of the optical waveguide layer 3 → regrowth of the optical waveguide layers 4 and 11; 2) Since the diffraction grating in the region 2 passes through the temperature raising step twice, the shape of the diffraction grating is deteriorated when the temperature is raised. 3) Abnormal growth occurs at the boundary between the optical waveguide layers 2 and 3 and the light between the two regions. There is a problem that the coupling efficiency is reduced.

In a second method, a diffraction grating 5 constituting a distributed reflector in a region 1 is formed on a semiconductor substrate 1 in FIG. Diffraction gratings 6 having different sizes are formed, and a distributed reflector in region 2 is manufactured. Thereafter, the optical waveguide layers 2, 4 and the cladding layer 11 are grown to obtain a semiconductor optical device. Even by using this method, a plurality of regions having an arbitrary optical coupling coefficient can be manufactured by changing the depth of the diffraction gratings 5 and 6. However, according to this method, 1) etching must be performed a plurality of times when forming a diffraction grating, and the process is complicated. 2) The relative ratio between the etching depths of the diffraction grating 5 and the diffraction grating 6 is determined. It is difficult to control them. 3) When the optical waveguide layer 2 is grown, there is a problem that the quality of the crystal grown on the diffraction grating 6 having a large diffraction grating depth is deteriorated.

Further, as another new method, a method of controlling the optical coupling coefficient κ by using an electron beam exposure method has recently been developed.
Journal of Quantum Electronics vol.29, no.6, pp.2
081-2087, June 1993. FIG. 7 shows a method for manufacturing a diffraction grating using this method. The feature of this method is
The purpose is to form a comb-like diffraction grating 55 in an optical waveguide having a width L and change the optical coupling coefficient κ by changing the length l _{1 of the} comb. When (2l ₁ ) / L is 1, the largest optical coupling coefficient κmax is obtained, and as l ₁ becomes shorter, the optical coupling coefficient κ becomes smaller. However, in this method, 1) Since the diffraction grating 55 is formed in a comb shape, the diffraction grating is always located at the end of the optical waveguide, and the optical coupling coefficient is equal to the comb length l ₁ . On the other hand, it is difficult to control because it does not change linearly. 2) Since only one comb-like diffraction grating 55 is formed in a narrow optical waveguide having a width L = 1 to 2 μm, a There is a problem that it can be applied only to a structure in which the width L where the diffraction grating 55 is formed matches the width of the rib-type optical waveguide.

[0007]

As described above, in the conventional method of manufacturing a distributed reflector having different optical coupling coefficients, the manufacturing process is complicated, the structure of an applicable semiconductor optical device is limited, or the like. Further, it is difficult to form a distributed reflector having three or more arbitrary optical coupling coefficients or a distributed reflector whose optical coupling coefficient changes continuously on the same substrate.

The present invention has been made in view of such a situation, and an object of the present invention is to provide a semiconductor optical device having a distributed reflector using a diffraction grating at an arbitrary position in the optical waveguide direction. the size of the optical coupling coefficient of the reflector is to provide a semiconductor optical element can be controlled to an arbitrary value.

[0009]

The above object is achieved by the following means.

[0010] Namely, the present invention may have a diffraction grating composed of vertical stripes into the optical waveguide direction, of the stripe length
Changes at an arbitrary position in the optical waveguide direction in the optical waveguide.
Semiconductor optical device, wherein the length of the stripe is
Is less than the optical waveguide width, and the stripe is
Light guides in a direction perpendicular to the
Periodically formed over an area at least twice as wide as the waveguide width
It is intended to propose a semiconductor optical device characterized by the above.

Such a semiconductor optical device includes a step of forming a diffraction grating composed of stripes perpendicular to the optical waveguide direction on a semiconductor substrate and a step of forming an optical waveguide layer in contact with the diffraction grating. manufacturing methods odor Te, the E B exposure method, can be produced by fractionally drawn with a length of less than or equal to the optical waveguide width. The method of manufacturing a semiconductor optical element This step of forming the optical waveguide layer, on the semiconductor substrate, forming a pair of dielectric films that faces the change in width at any position of the optical waveguide direction, the Selectively crystal-growing an optical waveguide layer in an optical waveguide region sandwiched between dielectric thin films,
In the method for manufacturing a semiconductor optical device, the width of the optical waveguide region is 10 μm or less; in the method for manufacturing a semiconductor optical device, the semiconductor substrate is InP; , In or A
1 is an organic metal, and the group V raw material contains As, P, or N.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a semiconductor optical device and a method for manufacturing the same according to the present invention will be described. In the present invention, an object of the present invention is to provide a semiconductor optical device having a distributed reflector using a diffraction grating, in which the magnitude of the optical coupling coefficient κ of the distributed reflector in the optical waveguide direction can be arbitrarily controlled, and its manufacture In order to provide a method, control of an optical coupling coefficient κ by changing a stripe length in a striped diffraction grating will be described.

FIG. 1 shows a striped diffraction grating 5 of the present invention.
FIG. In the present invention, a semiconductor substrate such as InP is used.
Stripe diffraction using electron beam exposure method on the plate
Period of lattice 5₁ To draw. At this time, the drawing of the stripe
The picture is a stripe length l₁ From 0 to the optical waveguide width W
In the optical waveguide direction within the range described above. Further
The stripe has an optical waveguide width W of one cycle,
In the direction perpendicular to the wave direction, the area
Is drawn periodically. like this
When the striped diffraction grating 5 is formed in
Use a normal wet or dry etching
When transferred to an InP substrate, etc.
Even if the cutting depth is the same in all areas,
Length l ₁ To control the optical coupling coefficient κ.
Can be Further, the drawing area is larger than the optical waveguide width W.
Is large enough, so that the process
Optical waveguide pattern and striped diffraction grating
5 can be easily aligned.

FIG. 2 specifically shows an experimental result of the dependence of the diffraction grating on the normalized optical coupling coefficient κ / κ _{0 with} respect to the stripe length l ₁ normalized by the optical waveguide width W. It can be seen that the optical coupling coefficient κ changes almost linearly with the normalized stripe length l ₁ / W. When a diffraction grating of a 1.55 μm band distributed feedback (DFB) semiconductor laser was actually formed, the period of the diffraction grating Λ ₁ = 240 nm, the optical waveguide width W = 1.5 μm, and the normalized stripe length l ₁ / W =
When the optical coupling coefficient κ at the time of ¹ is set to 35 cm ⁻¹ , l
_{When 1 /} W = 0.5, an optical coupling coefficient κ = 16 cm ⁻¹ could be realized.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

Embodiment 1: Distributed reflection (DR) type semiconductor laser
FIG. 3C shows a distributed reflection (DR) type according to the first embodiment.
FIG. 3 is a diagram illustrating a structure of a semiconductor laser. Fig. 3 (c) smell
First, an electron beam exposure method is used on the n-InP substrate 1.
And draws a diffraction grating pattern as shown in FIG.
I do. At this time, the diffraction grating 7 in the region 1 has a stripe length.
l₁ And the diffraction grating 5 in the region 2 has a stripe length l._Two In
L₁ = L_Two / 2. Further, these diffraction gratings 5 and 7 serve as light guides.
In the direction perpendicular to the wave direction, the circumference of the optical waveguide width W = 1.5 μm
Period, six cycles, ie, W_EB= 9 μm diffraction grating drawing area
Area, and the direction of the optical waveguide is the period of the diffraction grating 7.
Huh₁ And the period of the diffraction grating 5 is Λ_Two Set as This
, The equivalent refraction of region 1 for an oscillation wavelength of 1.55 μm
Rate n_eq1 , The equivalent refractive index of region 2 to n_eq2 Then, n_eq1 Λ₁ = N_eq2 Λ_Two Are set so as to satisfy the relationship. After this, the diffraction grating
Using turns 5 and 7, normal wet etching etc.
In the n-InP substrate 1, both the region 1 and the region 2
A diffraction grating having a thickness of 60 nm is formed. Such a diffraction grating
By using the diffraction gratings in region 1 and region 2
The Bragg wavelength of the distributed reflector
Further, the optical coupling coefficient κ of the distributed reflector in region 1₁ Of area 2
Optical coupling coefficient κ of distributed reflector_Two Can be set to 1/2 of
it can. On the substrate on which the diffraction gratings 5 and 7 are formed,
As shown in FIG. 3B, in the <011> direction, SiO_Two growth
The blocking mask 9 is formed by setting the width Wm in the region 1 to 30 μm and in the region 2
An opening having a width of 1.5 μm so that the width Wm is 4 μm.
(Optical waveguide region). This
In the formation of the growth inhibition mask 9, the diffraction gratings 5, 7
Is the diffraction grating drawing area width W _EB= Formed in a wide area of 9 μm
The opening of 1.5 μm width can be easily
Could be placed on the child. Next, this mask is formed
N-InGaAsP semiconductor layer 2, n-I
nP spacer layer 8, n-InGaAsP light confinement layer
3. InGaAsP / InGaAsP five-layer multiple quantum
Well layer 4, InGaAsP light confinement layer 3, and p-I
The nP layer 10 is selectively grown. At this time, it grows in the opening
The composition of the InGaAsP quaternary semiconductor layers 2 and 3 is
Region 1 is different from region 2, and region 1 has a composition wavelength of 1.2 μm.
m, region 2 has a composition wavelength of 1.1 μm. At this time,
The optical coupling coefficient of the distributed reflector of the diffraction grating 7 in the region 1 is 50c.
m ^-1Is obtained, and the optical connection of the distributed reflector of the diffraction grating 5 in the region 2 is obtained.
Coefficient is 100cm^-1was gotten. In addition, multiple quantum wells
The layer 4 is composed of an InGaAsP quantum well (composition wave) in the region 1.
1.6 μm long, 7 nm thick) and InGaAsP barrier layer
(Composition wavelength: 1.2 μm, layer thickness: 10 nm).
The obtained peak wavelength could be set to 1.55 μm. Many in area 2
The quantum well layer 4 is composed of an InGaAsP quantum well (composition wavelength
1.25 μm, layer thickness 3 nm) and InGaAsP barrier layer
(Composition wavelength 1.2 μm, layer thickness 5 nm), and gain
The peak wavelength could be set to 1.2 μm. This
Thus, after forming each semiconductor layer, the growth inhibition mask 9 is formed.
Again, so that the mask opening width W becomes 6 μm in the entire region.
And the p-InP buried layer 11 is formed using this mask.
(Layer thickness 2.0 μm) is selectively grown by MOVPE. That
Later, SiO_Two A film 15 is formed, and an upper electrode 16 and a lower electrode
17 is formed by a normal sputtering method, etc.
Could be obtained. Optical coupling coefficient 50cm^-1Area 1 of
Optical coupling coefficient 100cm in active area^-1Area 2 of passive
Light can be emitted very easily from one end surface.
High-efficiency, high-performance, 1.55μm wavelength distribution
A reflector (DR) type semiconductor laser was realized.
A device with an active region length of 200 μm and a passive region length of 200 μm
When fabricated, at 25 ° C with both end faces cleaved,
Oscillation threshold current 3.5mA, one side optical output 35mW, one side
Good characteristics of 90% side differential quantum efficiency can be obtained.
Came.

Embodiment 2 Distributed Feedback (DFB) Semiconductor Laser Having Modulated Optical Coupling Coefficient FIG. 4C shows a distributed feedback (DFB) semiconductor laser having a modulated optical coupling coefficient according to a second embodiment. It is a figure showing a structure. In FIG. 4C, first, a diffraction grating pattern as shown in FIG. 4A is drawn on the n-InP substrate 1 by using an electron beam exposure method. At this time, the diffraction grating 5
The length of each stripe is varied depending on the position in the optical waveguide direction, and the optical coupling coefficient of the distributed reflector using the diffraction grating varies in the range of κ _{1 to} κ ₅ depending on the position in the optical waveguide direction. Set to Further, in the optical waveguide direction and a vertical direction, six cycles with a period of the optical waveguide width W = 1.5 [mu] m, i.e. drawn to the diffraction grating drawing area W _EB = 9 .mu.m, for an optical waveguide direction, the period lambda ₁ and Set. After that, n-In is performed using normal wet etching or the like.
A diffraction grating 5 having a depth of 50 nm is formed on a P substrate 1. Next, on the substrate on which the diffraction grating 5 is formed, FIG.
As shown in the figure, the mask width W _m = 15 μ in the <011> direction.
An SiO ₂ growth inhibition mask 9 is formed so as to face the opening (optical waveguide region) having a width of 1.5 μm.
Using this mask, the n-InGaAsP semiconductor layer 2, the n-InP spacer layer 8, the InGaAsP light confinement layer 3, the InGaAsP /
5 InGaAsP multiple quantum well layers 4, InGaAsP
The sP light confinement layer 3 and the p-InP layer 10 are selectively grown. At this time, the composition wavelength of the InGaAsP semiconductor layers 2 and 3 is 1.2 μm, and the multiple quantum well layer 4
aAsP quantum well (composition wavelength 1.6 μm, layer thickness 7 nm)
And an InGaAsP barrier layer (composition wavelength: 1.2 μm, layer thickness: 10 nm), and the gain peak wavelength of the quantum well could be set to 1.55 μm. As a result, the optical coupling coefficient κ of the distributed reflector using the diffraction grating 5 is κ ₁ = 5 cm ⁻¹ , κ ₂ = 10 cm ⁻¹ , κ ₃ = 15c at the position in the optical waveguide direction.
m ⁻¹ , κ ₄ = 20 cm ⁻¹ and κ ₅ = 25 cm ⁻¹ could be set. Next, the growth prevention mask 9 is formed again so that the mask width is 4 μm in all regions and the mask opening width is 6 μm in all regions.
The m-type p-InP buried layer 11 is selectively grown by MOVPE. Thereafter, the upper electrode 16 and the lower electrode 17 were formed by a normal sputtering method or the like, and a distributed feedback (DFB) type semiconductor laser having a modulated light coupling coefficient was obtained. The DFB laser having an element length of 300 μm manufactured in this manner has a small optical coupling coefficient at the center of the resonator and a large optical coupling coefficient at both ends.
The effect of axial spatial hole burning, which is a problem at the time of high output, can be reduced. Conventionally, in the state where both ends are cleaved, the longitudinal mode jumps at an optical output of about 15 mW at room temperature of 25 ° C., and the optical output is saturated. However, it was possible to achieve a characteristic without output saturation in the single axis mode up to an optical output of 30 mW.

Embodiment 3: Distributed Bragg reflection (DBR) mode-locked semiconductor laser FIG. 5C is a diagram showing the structure of a distributed Bragg reflection (DBR) mode-locked semiconductor laser according to a third embodiment. . The laser is composed of three regions. FIG.
5C, first, a diffraction grating pattern as shown in FIG. 5A is drawn on the n-InP substrate 1 only in the region 3 by using the electron beam exposure method. At this time, the diffraction grating 5
Of stripe length l ₁ is 1 of the optical waveguide width W = 1.5μm
In the direction perpendicular to the optical waveguide direction, drawing is performed in an area having a width of the optical waveguide width W of 6 periods, that is, a width of 9 μm. The period of the optical waveguide direction is lambda _1. After that, using this diffraction grating pattern as a mask, the n-InP substrate 1 is etched to a depth of 50 nm by ordinary wet etching or the like.
Etching is performed to form a diffraction grating 5 as shown in FIG. On the substrate on which the diffraction grating 5 is formed, FIG.
As shown in the <011> direction, the SiO ₂ growth blocking mask 9, such that the width Wm in the region 1 is 30 [mu] m, the width Wm in the regions 2 and 3 is 10 [mu] m, the opening of 1.5μm width (optical waveguide (Area). In the formation of the growth blocking mask 9, since the diffraction grating 5 was formed in a wide area having a width of 9 μm, an opening having a width of 1.5 μm could be easily arranged on the diffraction grating 5. Next, the SiO ₂ mask 9 n-InGaAsP semiconductor layer 2 on a substrate formed with, n-InP spacer layer 8, n-InGaAsP light confining layer 3, InGaAs
P / InGaAsP seven multiple quantum well layers 4, InG
The aAsP light confinement layer 3 and the p-InP layer 10 are selectively grown. At this time, the InGaAsP grown in the opening is formed.
The composition of the quaternary semiconductor layers 2 and 3 differs between the region 1 and the regions 2 and 3. In the region 1, the composition wavelength is 1.25 μm,
In Examples 3 and 3, the composition wavelength is 1.18 μm. At this time, the optical coupling coefficient κ of the distributed reflector of the diffraction grating 5 in the region 3 is 10c.
m ^-1 was obtained. Further, in the region 1, the multiple quantum well layer 4 has an InGaAsP quantum well (having a composition wavelength of 1.6 μm and a layer thickness of 7 nm) and an InGaAsP barrier layer (having a composition wavelength of 1.2 nm).
5 μm, layer thickness 10 nm), and the gain peak wavelength could be set to 1.55 μm. The multiple quantum well layers 4 in the regions 2 and 3 are made of InGaAsP quantum wells (composition wavelength 1.23
μm, a layer thickness of 3 nm) and an InGaAsP barrier layer (composition wavelength: 1.18 μm, layer thickness: 5 nm), and the gain peak wavelength could be set to 1.21 μm. After each semiconductor layer is formed in this manner, two more MOVPEs are performed.
Was used to form an Fe-InP current blocking layer, then an SiO ₂ film 15 was formed, and an upper electrode 16 and a lower electrode 17 were formed by a normal sputtering method or the like, whereby a semiconductor optical device could be obtained. . The region 1 is an active region, the region 2 is a passive region, and the region 3 is a DBR region.
A mode-locked semiconductor laser was realized. Active region length 700 μm, passive region length 4200 μm, DBR
When an element having a region length of 600 μm was fabricated, mode-locked at 8.5 GHz with an oscillation threshold current of 20 mA, and a width of 1
Good characteristics such as 9-ps pulse generation were obtained. Also, by using the diffraction grating pattern of the present invention (FIG. 5A), it is possible to easily realize a very weak optical coupling coefficient κ = 10 cm ⁻¹ with the diffraction grating 5 having a normal depth (50 nm). The reproducibility of a process for forming a diffraction grating having a weak optical coupling coefficient required for a distributed Bragg reflection (DBR) mode-locked semiconductor laser was remarkably improved, and a device yield of 90% was obtained.

In the second embodiment of the present invention, the optical coupling coefficient κ of the distributed reflector using the diffraction grating 5 is expressed as κ ₁ ,
_{κ 2, κ 3, κ 4} , κ 5, κ 4, κ 3, κ 2, κ 1 and 9
Although an example of a distributed feedback (DFB) type semiconductor laser modulated in stages has been described, the stripe length of the diffraction grating 5 of the second embodiment is more finely changed to modulate the optical coupling coefficient κ in more stages. The present invention is also effective for a semiconductor optical device that has been used.

The present invention can be applied not only to a diffraction grating whose refractive index is modulated in the optical waveguide direction but also to a diffraction grating whose gain and loss are modulated. For example, the present invention can be applied to a gain (loss) coupling type DFB laser or the like. Is also applicable. Also, the present invention
Materials other than InGaAsP / InP-based materials, for example, InGa
AsN / GaAs system, InGaAlAs / InGaAs
It is also effective in an optical integrated device using a P-based material or the like and in selective growth for realizing the same. In the present invention, S
The method of forming the optical waveguide by selective growth with the opening width of the iO ₂ mask being 1.5 μm has been described.
This is also effective for a selective growth method in which the opening width of the iO ₂ mask is made wider.

[0021]

A semiconductor optical element according to the present invention exhibits uniform period, using a diffraction grating depth, can be modulated in the light guide direction of the optical coupling coefficient kappa, lambda / 4 shift DFB laser, DR laser , DBR lasers, SSG-DBR lasers and the like. Conventionally, when fabricating an optical device structure having such a diffraction grating distributed reflector, a complicated fabrication process combining crystal growth and an etching process was required, but the present invention greatly simplified the fabrication process. There are advantages that can be done. Further, in the semiconductor optical device according to the present invention, even when the optical coupling coefficient is modulated, the semiconductor layers constituting the distributed reflector are connected by continuous layers, so that the optical coupling efficiency between the respective regions can be reduced. There is an advantage that can be made almost 100%.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a striped diffraction grating of the present invention.

FIG. 2 is a graph showing experimental results on the dependence of a normalized optical coupling coefficient of a diffraction grating on a normalized stripe length.

FIGS. 3A, 3B, and 3C are explanatory views of Embodiment 1 of the present invention.

FIGS. 4A, 4B, and 4C are explanatory diagrams of a second embodiment of the present invention.

FIGS. 5A, 5B, and 5C are explanatory diagrams of a third embodiment of the present invention.

6 (a) and 6 (b) are schematic sectional views in the axial direction showing a conventional method for manufacturing a semiconductor optical device.

FIGS. 7A and 7B are explanatory views showing a conventional method for manufacturing a diffraction grating.

[Explanation of symbols]

1 InP semiconductor substrate 2 waveguide layer 4 optical waveguide layers 5 and 7 a diffraction grating 8 including gain generating layer InP spacer layer 9 SiO ₂ mask 10 p-InP cladding layer 15 SiO ₂ 16 upper electrode 17 lower electrode 55 comb Diffraction grating

Claims (2)

(57) [Claims] [Claim 1] have a diffraction grating composed of vertical stripes into the optical waveguide direction, the length of the stripe, into the light guide
Light changing at an arbitrary position in the optical waveguide direction
An element, wherein a length of the stripe is equal to or less than an optical waveguide width.
Wherein the stripe is perpendicular to the optical waveguide direction.
In addition, with a period equal to or less than the width of the optical waveguide,
A semiconductor optical device formed periodically over an upper wide area . 2. The diffraction grating has a uniform depth.
The semiconductor optical device according to claim 1, wherein: