JPH07111362A - Semiconductor laser and manufacture thereof - Google Patents

Abstract

PURPOSE:To enhance semiconductor in single mode oscillation yield without using an etching process by a method wherein a diffraction grating is so controlled in height by the growth of crystal as to be controlled in diffraction efficiency. CONSTITUTION:A distributed feedback diffraction grating 2 of certain period is formed on an N-InP substrate 1. An InP buffer layer 3, an InGaAsP (lambda=1.3mum) waveguide layer 4, an InGaAsP (lambda=1.4mum) well layer 5, an InGaAsP (lambda=1.1mum) barrier layer 6, a P-N-P InP buried layer 7, and a P-InGaAsP (lambda=1.3mum) cap layer 8 are formed thereon, and a P-side electrode 9 of Au/Zn and an N-side electrode of Au-Su are provided through evaporation. By this setup, a laser structure wherein the distributed feedback diffraction grating 2 is lessened in diffraction efficiency is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high performance semiconductor laser required for optical fiber communication and the like and a method for manufacturing the same.

[0002]

2. Description of the Related Art In a conventional semiconductor laser, a dynamic single-mode laser (Dymanic Single Mode-Laser Diode) that maintains single-mode operation even during high-speed direct modulation
Among them, the distributed feedback type (DFB) that can stably control the longitudinal mode and can stably obtain a high optical output has been proposed.
Lasers (eg Nakamura et al., IEE Journal of Quantum Electronics IEEE J.
Quantum Electron. QE-11, 436 (1975)) has been actively researched.

FIG. 11 shows the structure of a conventional DFB laser. Here, 1 is an n-InP substrate (semiconductor single crystal substrate), 2 is a distributed feedback diffraction grating formed on the n-InP substrate 1, and 4 is In grown by crystal growth on the distributed feedback diffraction grating 2.
GaAsP (λ = 1.3 μm) waveguide layer, 5 is InGa
InGaAsP (λ =
1.55 μm) active layer, 6 is p-InP / n-InP /
p-InP buried layer, 7 is p-InGaAsP (λ =
1.3 μm) cap layer, 8 is Au / Zn p-side electrode,
9 is an n-side electrode of Au-Sn.

As shown in FIG. 11, the distributed feedback diffraction grating 2 is formed on an n-InP substrate 1, and the InGaAsP waveguide layer 4 and InGaA are formed on the distributed feedback diffraction grating 2.
The sP active layer 5 is crystal-grown. In recent years, the DFB laser as shown in FIG. 11 has been attracting attention as a low noise and low distortion light source for analog transmission. This DFB for analog transmission
The laser is required to have a good linearity of the optical output current characteristics at the threshold current or more. If the linearity of the optical output current characteristic is poor, the transmission distortion will increase. As a cause of the deterioration of the linearity of the optical output current characteristic, hole burning caused by the nonuniform distribution of light in the cavity of the laser is considered.

The hole burning is a phenomenon in which the light is excessively fed back by the distributed feedback diffraction grating (grating) formed in the DFB laser, and the intensity of the light partially becomes too strong. Therefore, it is necessary to reduce the diffraction efficiency of the distributed feedback diffraction grating. To suppress this phenomenon,
The height of the distributed feedback diffraction grating is limited to reduce the diffraction efficiency.

On the other hand, by suppressing the Fabry-Perot mode,
In order to oscillate the semiconductor laser in a single mode, the height of the distributed feedback diffraction grating must be above a certain level. Therefore,
It is necessary to suppress the hole burning by controlling the height of the distributed feedback grating optimally and to realize stable single-mode oscillation to improve the laser yield. Since the intensity of the light returned by the distributed feedback diffraction grating changes not only in the height of the distributed feedback diffraction grating but also in the shape of the distributed feedback diffraction grating, the light diffracted by the distributed feedback diffraction grating The value of the diffraction efficiency is defined as the intensity of the incident light intensity ratio.

Conventionally, for controlling the height of the distributed feedback diffraction grating, a distributed feedback diffraction grating 75 having a sufficient height is formed by using a resist 72 on an InP substrate 71 as shown in FIG. After that, the resist 72 is removed, and a sulfuric acid-based etching solution (for example, H ₂ SO ₄ + H ₂ O ₂ + H
₂ O) to lower the height of the distributed feedback diffraction grating 75, or as shown in FIG. 12B, the InGaAsP layer 75
After patterning a resist 76 on the InP substrate 74 having grown thereon, InGaAsP with the resist 76 attached
Layer 75 may be a sulfuric acid based etching solution (eg, H ₂ SO ₄
+ H ₂ O ₂ + H ₂ O) to etch the resist 76
Was removed to form the distributed feedback diffraction grating 77. This is a distributed feedback diffraction grating 7 on a heterocrystal with a uniform film thickness.
There is a method in which the height of the distributed feedback type diffraction grating 77 is set to the film thickness of the heterocrystal (InGaAsP layer 75) by forming 7.

[0008]

However, as shown in FIG.
In the method of controlling the height of the distributed feedback diffraction grating 73 by etching as shown in (a), since the shape of the distributed feedback diffraction grating 73 is changed by etching, the initial distributed feedback diffraction grating 73 is changed. There is a problem that the amount of change in diffraction efficiency over time due to etching differs depending on the shape.

For example, before the etching, the variation of the diffraction efficiency within the InP substrate 71 forming the semiconductor laser is 1
InP substrate 71 after etching, even if it is about 0%
Diffraction efficiency may vary by about 50% within the substrate due to factors such as non-uniform etching within the substrate. Further, there is a problem that the etching solution is agitated by convection and the etching rate is different in the plane, the height of the distributed feedback diffraction grating 73 does not reach the target height, and the in-plane variation in height is large. There were variations in characteristics and the yield was low.

When the distributed feedback diffraction grating 77 is formed by etching a heterocrystal (InGaAsP layer 75) as shown in FIG. 12B, the height of the distributed feedback diffraction grating 77 is the same as that of the heterocrystal. It is possible to form the distributed feedback diffraction grating 77 at a uniform height within the plane by increasing the height of the hetero crystal and improving the in-plane uniformity of the film thickness of the heterocrystal. However, the distributed feedback diffraction grating 77 is formed. The control of the width of the heterocrystal is determined by the etching rate for side etching. The heterocrystal itself has a different etching rate in the plane, and the heterocrystal and the resist (photoresist) 7
Since the amount of side etching due to the adhesion with 6 also differs within the surface, the width of the distributed feedback diffraction grating 77 cannot be sufficiently controlled. As a result, the intensity of the returned light changes in the plane, and eventually the desired diffraction efficiency cannot be obtained. That is, since the width of the distributed feedback diffraction grating 77 varies within the crystal plane, the laser characteristics also vary.

Further, in the semiconductor laser of FIG.
The InGaAsP waveguide layer 4 also functions as a buffer layer for preventing the unevenness of the distributed feedback diffraction grating 2 from being transmitted to the InGaAsP active layer 5, and the InGaAsP waveguide layer 4 could not be made very thin. Therefore, there is also a problem that the half-value width of the light intensity of the emitted light, that is, the half-value width of the spread width of the emitted light is large and the coupling efficiency with respect to an optical fiber or the like is low.

An object of the present invention is to control the shape of the distributed feedback type diffraction grating by crystal growth instead of etching so that the height of the distributed feedback type diffraction grating is set to a desired height and the height of the distributed feedback type diffraction grating is increased in the plane. It is possible to suppress variations and realize a semiconductor laser with high yield. Another object of this semiconductor laser is to reduce the half-value width of the light intensity of the emitted light, that is, the half-value width of the spread width of the emitted light to realize a semiconductor laser having a high coupling efficiency with an optical fiber or the like.

[0013]

A semiconductor laser device according to claim 1 has a first crystal in which a diffraction grating is formed, and a second crystal having the same composition as the first crystal grown on the diffraction grating. , The degree of growth of the second crystal is controlled to adjust the diffraction efficiency of the diffraction grating. The semiconductor laser device according to claim 2 is refracted from a first crystal having a diffraction grating, a second crystal having a different composition from that of the first crystal grown on the diffraction grating, and a second crystal grown on the second crystal. The third crystal having a small ratio is used, and the film thickness of the second crystal is periodically changed by the diffraction grating.

A semiconductor laser device according to a third aspect is the semiconductor laser device according to the second aspect, wherein the diffraction efficiency of the diffraction grating is adjusted by selecting a combination of compositions of the first crystal, the second crystal and the third crystal. ing. A method of manufacturing a semiconductor laser device according to claim 4, wherein a diffraction grating forming step of forming a diffraction grating on the semiconductor single crystal substrate, an annealing step of reducing diffraction efficiency of the diffraction grating of the semiconductor single crystal substrate, and a semiconductor single crystal. A first crystal growth step of growing a crystal having the same composition as that of the semiconductor single crystal substrate as a buffer layer on the substrate, and thereafter forming a waveguide layer and an active layer made of crystals having a composition different from that of the semiconductor single crystal substrate on the buffer layer. A second crystal growth step of growing.

A method of manufacturing a semiconductor laser device according to a fifth aspect of the present invention comprises a diffraction grating forming step of forming a diffraction grating on a semiconductor single crystal substrate, an annealing step of reducing diffraction efficiency of the diffraction grating of the semiconductor single crystal substrate, A first crystal growth step of growing a crystal having a composition different from that of the semiconductor single crystal substrate as a first buffer layer on the semiconductor single crystal substrate; and a first buffer layer as a second buffer layer on the first buffer layer A second crystal growth step of growing a crystal having a smaller refractive index, and then a second crystal of growing a waveguide layer and an active layer made of a crystal having a different composition from the semiconductor single crystal substrate on the second buffer layer. And a growth step.

According to a sixth aspect of the present invention, there is provided a method of manufacturing a semiconductor laser device, which comprises a first crystal growth step of crystal-growing a first buffer layer having a composition different from that of the semiconductor single crystal substrate on the semiconductor single crystal substrate, and then a first crystal growth step. A diffraction grating forming step of forming a diffraction grating on one buffer layer, and a second crystal growing step of growing a crystal having the same composition as the semiconductor single crystal substrate as a second buffer layer on the first buffer layer; And a third crystal growth step of growing an active layer and a waveguide layer made of a crystal having a composition different from that of the semiconductor single crystal substrate on the rear second buffer layer.

[0017]

According to the present invention, by means of the above-mentioned means, it is possible to control the shape of the distributed feedback type diffraction grating to suppress the hole burning and improve the yield. That is, according to the configuration of the first aspect, by adjusting the growth of the second crystal grown on the diffraction grating, the height of the diffraction grating can be adjusted and the diffraction efficiency can be adjusted.

According to the structure of claim 2, by growing the second crystal having a composition different from that of the first crystal on the first crystal forming the diffraction grating, the diffraction grating can be preserved, Further, by adjusting the thickness of the third crystal, it is possible to adjust the distance between the diffraction grating and the active layer and thus the diffraction efficiency. According to the configuration of claim 3, the first crystal, the second crystal
The refractive index difference between the crystals can be adjusted by selecting the combination of the compositions of the crystals and the third crystal, and thus the diffraction efficiency of the diffraction grating provided between the crystals can be adjusted.

According to the structure of the fourth aspect, the height of the diffraction grating can be adjusted by growing a crystal having the same composition as that of the substrate on the diffraction grating of the semiconductor single crystal substrate and controlling the time. it can. According to the configuration of claim 5, the second
If the thickness of the second buffer layer is adjusted by adjusting the growth time of the buffer layer, the distance between the diffraction grating and the active layer can be adjusted.

According to the structure of claim 6, the difference in refractive index between the first buffer layer and the waveguide layer can be reduced,
Even when the diffraction grating having the same shape is manufactured, a small diffraction efficiency can be obtained, the thickness of the second buffer layer can be reduced, and reproducibility is improved.

[0021]

【Example】

(First Embodiment) FIG. 1 is a structural diagram of a semiconductor laser, that is, a DFB laser according to the first embodiment of the present invention. In FIG. 1, 1 is an n-InP substrate, 2 is a first distributed feedback diffraction grating with a period Λ1 = 200.0 nm,
Reference numeral 3 denotes In for adjusting the height of the distributed feedback diffraction grating 2.
P adjustment layer (InP buffer layer), 4 is InGaAsP
(Λ = 1.1 μm) Waveguide layer, 5 is 5 nm InGaA
sP (λ = 1.40 μm) well layer and 10 nm InGa
Quantum well type InGaAsP active layer consisting of 5 periods of AsP (λ = 1.10 μm) barrier layer, 6 is p-InP.
n-InP / p-InP buried layer, 7 is p-InGa
AsP (λ = 1.3 μm) cap layer, 8 is Au / Zn
Is a p-side electrode, and 9 is an Au-Sn n-side electrode.

In this embodiment, as shown in FIG.
-The distributed feedback diffraction grating 2 is formed directly on the InP substrate 1,
An InP adjusting layer 3 for adjusting the height of the diffraction grating is grown thereon, and an InGaAsP waveguide layer 4 and InG are further formed thereon.
and the aAsP active layer 5 is formed. Here, this semiconductor laser is different from the conventional semiconductor laser in that the InP adjusting layer 3 is formed on the distributed feedback diffraction grating 2, and the InP adjusting layer 3 causes the surface of the InP adjusting layer 3 to be different. The distributed feedback diffraction grating 2a having a low diffraction grating height reflecting the distributed feedback diffraction grating 2 is being formed.

Here, In is placed on the distributed feedback diffraction grating 2.
The effect of forming the P adjustment layer 3 will be described with reference to FIGS. 2 and 3. A distributed feedback diffraction grating is formed on the resist by the interference exposure method, and the distributed feedback diffraction grating is transferred to the crystal by etching the crystal using this resist. As shown in FIG. 2, the distributed feedback diffraction grating shape formed in the crystal has two patterns (a1),
(B1). That is, as shown in FIG. 2C0, when the resist 82 is formed on the InP substrate 81 and etched, the InP substrate 81 is slightly etched as shown in (c1) depending on the degree of etching. , (C2), the InP substrate 81 may be considerably etched. (C1) is (a1)
And (c2) corresponds to (b1).

The relationship between the crystal growth time and the diffraction efficiency after etching the InP substrate 81 is shown in FIG. A1, a2, b1, b2 in FIG. 3 are the patterns (a1),
These correspond to (b1), (a2), and (b2), respectively. In the case of the pattern (a1), when a crystal is grown on the distributed feedback diffraction grating 83, the distributed feedback diffraction grating 8 is used.
In the shape of No. 3, since the growth rate of the (111) plane is faster than that of the (100) plane, the height of the distributed feedback diffraction grating 83 decreases almost linearly with the growth of the crystal like the pattern (a2). However, the diffraction efficiency also decreases almost linearly. On the other hand, in the case of the pattern (b1), the height of the distributed feedback diffraction grating 83 once rises and then begins to decrease as the crystal grows, as in the pattern (b2), and the diffraction efficiency once rises. Similar to the pattern (a1), it decreases with the growth time.

Therefore, it is preferable to control the diffraction efficiency by reducing the etching amount when forming the distributed Bragg reflector type diffraction grating 83 to form the pattern (a1). The operation of the semiconductor laser of this embodiment constructed as above will be described below. The current is supplied from the p-side electrode 8 of Au / Zn, and p-InP.n-InP.p-I
After being sandwiched by the nP buried layer 6, it is injected into the InGaAsP active layer 5 composed of a well layer and a barrier layer.
The light generated in the InGaAsP active layer 5 is InGaAsP.
The light spills out into the waveguide layer 4 and oscillates at a wavelength on the long wavelength side of light having a DFB oscillation wavelength determined by the period of the distributed feedback diffraction grating 2.

In the semiconductor laser according to this embodiment, (1) the height of the distributed feedback type diffraction grating 2 is adjusted by stacking the crystal of the InP adjusting layer 3 on the distributed feedback type diffraction grating 2 in a thickness of 300 nm. Lower as 2a,
The diffraction efficiency is lowered from 3.2 to 1.0. This reduces the appearance of hole burning.

(2) Since the height of the distributed feedback diffraction grating is lowered by growing the crystal using the MOCVD method which is excellent in film thickness controllability, the distributed feedback diffraction grating 2 formed on the n-InP substrate 1 The variation of the diffraction efficiency in the n-InP substrate 1 is significantly reduced as compared with the conventional one. (3) In the manufacturing process, variations in diffraction efficiency between wafers and batches are reduced as compared with the conventional case.

As described above, by controlling the shape of the distributed feedback diffraction grating by crystal growth instead of etching,
It is possible to set the height of the distributed feedback diffraction grating to a target height and suppress the height variation within the plane, and it is possible to realize a single-mode operation semiconductor laser with high yield. (Second Embodiment) FIG. 4 is a structural view of a semiconductor laser according to the second embodiment of the present invention. In FIG. 4, 1 is an n-InP substrate and 12 is an n-InP buffer layer, which is grown to a thickness of 3 μm in order to reduce the dopant concentration in a portion where the light intensity is strong. Carrier concentration is 1 × 10
^{It is 17} cm ^-3 . 2 is a distributed feedback diffraction grating with a period Λ1 = 200.0 nm, and 13 is n-InGaAsP (λ = 1.05) for adjusting the diffraction efficiency of the distributed feedback diffraction grating 2.
μm) adjustment layer (n-InGaAsP buffer layer), 14
Is an n-InP flattening layer for flattening the crystal, and 4 is n
InGaAsP (λ = 1.05 μm) waveguide layer, 5 is a quantum well type InGaAsP having 5 periods of a 5 nm InGaAsP (λ = 1.40 μm) well layer and a 10 nm InGaAsP (λ = 1.10 μm) barrier layer. An active layer, 6 is a p-InP.n-InP.p-InP buried layer, 7 is a p-InGaAsP (λ = 1.3 μm) cap layer, 8 is an Au / Zn p-side electrode, and 9 is Au-Sn. n
It is a side electrode.

In this embodiment, as shown in FIG.
The n-InP buffer element 12 is formed on the -InP substrate 1, the distributed feedback diffraction grating 2 is formed on the n-InP buffer layer 12, and the diffraction efficiency of the distributed feedback diffraction grating 2 is adjusted on the distributed feedback diffraction grating 2. An n-In layer for growing an n-InGaAsP control layer 13 and further planarizing a crystal on the n-InGaAsP control layer 13
A P flattening layer 14 is grown, and InGaAs is further formed thereon.
The P waveguide layer 4 and the InGaAsP active layer 5 are formed.

The operation of the semiconductor laser of this embodiment is almost the same as that of the first embodiment. Here, referring to FIG. 5 and FIG. 6, the operation of growing a crystal having a different composition from the crystal forming the distributed feedback diffraction grating on the distributed feedback diffraction grating as in the second embodiment will be described. Explain. FIG. 5 shows a case where a crystal having a composition different from that of the crystal in which the distributed feedback diffraction grating is formed is grown. In FIG. 5, 101 is I
The nP substrate 102 is a distributed feedback diffraction grating formed on the InP substrate 101. 103 is a distributed feedback diffraction grating 1
No. 02, which has a larger refractive index than the InP substrate 101. Since the growth rate of the (111) plane is high in the InP substrate 101, the film thickness of the crystal 103 grown in the recess of the distributed feedback diffraction grating 102 is large. When the crystal 103 having a large refractive index is grown on the distributed feedback diffraction grating 102, the refractive index of the concave portion of the distributed feedback diffraction grating 102 becomes larger than that of the convex portion, so that the crystal 103
Also acts as the distributed feedback diffraction grating 102.

Next, a crystal 104 having a smaller refractive index than the crystal 103 stacked on the distributed feedback diffraction grating 102, a waveguide layer 105 and an active layer 106 are continuously grown. The light intensity at the position of the distributed feedback diffraction grating 102 can be adjusted by adjusting the thickness of the crystal 104 having a small refractive index.
The effective diffraction efficiency of the distributed feedback diffraction grating 102 in the laser is the shape of the distributed feedback diffraction grating 102, the light intensity at the position where the distributed feedback diffraction grating 102 is present, and the crystal 101 forming the distributed feedback diffraction grating 102. , 103, and the effective diffraction efficiency is proportional to the light intensity.

Therefore, the distributed feedback diffraction grating 102, which is high in height and has high diffraction efficiency, has a large refractive index in the crystal 10.
The effective diffraction efficiency can be lowered by growing 3 to create a change in the refractive index, and by separating the distributed feedback diffraction grating 102 and the active layer 105 from each other to reduce the light intensity. FIG. 6 shows the relationship between the distance d between the active layer and the distributed feedback type diffraction grating and the diffraction efficiency. The variation in the diffraction efficiency of the distributed feedback diffraction grating increases when etching is performed to reduce the in-plane variation in the diffraction efficiency. By reducing the diffraction efficiency by utilizing the crystal growth with high in-plane uniformity without etching, the in-plane uniformity of the diffraction efficiency and the batch-to-batch uniformity are improved, and the yield is improved.

Further, in order to reduce the half width of the spread angle of the emitted light, it is necessary to make the waveguide layer thin. However, in the above-described laser shown in FIG. 1, the InGaAsP waveguide layer 4 has the distributed feedback diffraction grating 2
It also acts as a buffer layer for preventing the unevenness of a from being transmitted to the InGaAsP active layer 5, and there was a limit to the reduction of the thickness of the InGaAsP waveguide layer 4 in order to keep the InGaAsP active layer 5 good. However, in the semiconductor laser of FIG. 4, since a hetero crystal (n-InGaAsP adjusting layer 13) can be grown on the distributed feedback diffraction grating 2 formed in the InP buffer layer 12 to cause a change in the refractive index, As the buffer layer, a crystal having a small refractive index (InP flattening layer 14) is used instead of a crystal for a waveguide layer having a large refractive index (InGaAsP), and the distributed feedback diffraction grating 2 and the InGaAsP active layer 5 are separated from each other. be able to. As a result, the divergence angle of the emitted light can be reduced, the emitted light can be efficiently coupled to the optical fiber, and high output can be achieved.

Further, the composition of the first crystal (InP buffer layer 12) on which the distributed feedback diffraction grating is formed and the second crystal (n-InGaAsP adjusting layer 13) which changes the diffraction efficiency.
Of the third crystal (the InP flattening layer 1) when the distributed feedback diffraction grating remains after the growth of the second crystal.
By adjusting the composition of 4), the total diffraction efficiency can be made a target value. That is, the diffraction efficiency can be increased by increasing the difference in refractive index between the first crystal (InP buffer layer 12) and the second crystal (n-InGaAsP adjusting layer 13), and the second crystal (n-InGaAsP).
Refractive index of adjusting layer 13) and third crystal (InP flattening layer 1)
The diffraction efficiency can be reduced by increasing the refractive index of 4). As a result, the distributed feedback diffraction grating 2 and In
The diffraction efficiency can be adjusted without greatly changing the distance between the GaAsP active layers 5. This is effective when it is desired to reduce the film thickness of the third crystal (InP flattening layer 14) in order to improve carrier injection.

The above n-InGaAsP (λg = 1.1
5 μm) The adjustment layer 13 has grown to 50 nm. (11
1) 50n because the growth rate of the surface is higher than InP
At m, the crystal interface is almost flattened. Where InGaA
Although the composition of sP is set to λg = 1.15 μm, it is possible to increase the diffraction efficiency by increasing λg and decrease the diffraction efficiency by decreasing λg.
Further, the thickness of the n-InGaAsP adjusting layer 13 is set to 50 nm.
However, the film thickness is reduced to 20 nm, and n-InGa
The diffraction efficiency can also be reduced by allowing the distributed feedback diffraction grating to remain on the growth interface (surface) after the growth of the AsP control layer 13.

Further, in the n-InP flattening layer 14, in order to completely flatten the crystal interface between the n-InP flattening layer 14 and the InGaAsP active layer 5, the InGaAsP active layer 5 and the distributed feedback diffraction are formed. By reducing the light intensity by separating the grating 2 from each other, it is possible to reduce κL (effective diffraction efficiency) without reducing the diffraction efficiency of the distributed feedback diffraction grating 2. In other words, InGaAs
The distance between the P active layer 5 and the distributed feedback diffraction grating 2 is n-In.
InGaAs because it is separated by the P planarization layer 14
The light generated in the P active layer 5 does not feel the distributed feedback diffraction grating 2 much (the effective diffraction efficiency is small).

The thickness of the n-InP flattening layer 14 is set to 300 n.
By setting m, the distributed feedback diffraction grating 2 having the same diffraction efficiency
However, by sandwiching the n-InP flattening layer 14 between the InGaAsP active layer 5 and the distributed feedback diffraction grating 2, the effective diffraction efficiency can be reduced to ⅕, so that the diffraction of the distributed feedback diffraction grating 2 is reduced. Efficiency is set to 5 and κL (effective refractive index)
Can be the same as the diffraction efficiency of 1.0 in the first embodiment.

Further, the thickness of the n-InGaAsP adjusting layer 13 is set to 20 nm, and the InGaAsP layer 13 and the n-In are adjusted.
When a distributed feedback diffraction grating is stored between the P planarization layer 14 and the P planarization layer 14, the n-InP layer 14 is replaced by n-InGaAsP.
The target diffraction efficiency can be obtained by adjusting the refractive index difference using the layer 14a. That is, instead of the n-InP flattening layer 14, n-InGaAsP (λg = 1.05μ
m) By using the planarization 14a, n-InGaAs
P adjustment layer 13 (λg = 1.15 μm) and n-InGaA
Even if the distributed feedback diffraction grating is stored at the interface with the sP (λg = 1.05 μm) flattening layer 14a, it is possible to suppress a decrease in diffraction efficiency.

Furthermore, the variation in the diffraction efficiency between batches is also M.
By using the crystal growth by OCVD, it can be reduced more than before. When etching is performed as in the conventional case to reduce the diffraction efficiency, the variation in the diffraction efficiency of the distributed feedback diffraction grating increases as the etching is performed. Further, as shown in the first embodiment, even when a crystal is grown, if the initial diffraction efficiency is extremely high, it is necessary to increase the film thickness of the growing crystal. The in-plane variation of the diffraction efficiency will increase. In the case of this embodiment, the diffraction efficiency is reduced by the distributed feedback diffraction grating 2 and InG.
Since it is realized in relation to the position with the aAsP active layer 5, the uniformity of diffraction efficiency becomes extremely high when the MOVPE crystal growth method or the like, which has a high uniformity of film thickness, is used. As a result of the configuration of this example, the in-plane variation of the diffraction efficiency was extremely low as compared with the conventional one.

By using the above structure, the yield of the semiconductor laser in the single mode is improved, and the probability of hole burning is reduced by reducing the diffraction efficiency.
Further, the full width at half maximum of the emitted light can be reduced and the coupling efficiency with respect to the optical fiber or the like can be improved. (Third Embodiment) FIG. 7A shows the structure of a semiconductor laser according to the third embodiment of the present invention. Figure 1
In the first embodiment, the distributed feedback diffraction grating is used as the InP substrate 1
Although it was formed directly on top, here, InP is formed on the InP substrate 1.
N-InGaAsP buffer layer 1 having a composition different from that of the substrate 1
3a is grown and n-InGaAsP buffer layer 1
The distributed feedback diffraction grating 2 is formed on 3a, and an InP flattening layer 14 having the same composition as the InP substrate 1 for adjusting the diffraction efficiency is grown to adjust the diffraction efficiency and then n− for flattening. As a structure for growing the InGaAsP waveguide layer 4, the n-InGaAsP buffer layer 13a and the n-InG are used.
Even when a distributed feedback diffraction grating having the same shape was manufactured by reducing the difference in refractive index of the aAsP waveguide layer 4, a small diffraction efficiency was obtained, and the amount of the InP flattening layer 14 was reduced to improve reproducibility. .

In the embodiment shown in FIG. 7B, I
n-InGaAsP having a composition different from that of the substrate on the nP substrate 1
The buffer layer 13 is grown, the distributed feedback diffraction grating 2 is formed on the n-InGaAsP buffer layer 13, and n-
After the n-InGaAsP adjusting layer 15 which is a crystal having a composition different from that of the InGaAsP buffer layer 13 is grown to form a distributed feedback diffraction grating gently and the diffraction efficiency is controlled,
By adopting a structure for growing the n-InP flattening layer 14,
It is possible to adjust the diffraction efficiency of the distributed feedback type diffraction grating having a large diffraction efficiency to a target value and facilitate the production of the device.

(Fourth Embodiment) This fourth embodiment is shown in FIG.
Shown in. In FIG. 4 of the second embodiment, I is formed on the InP substrate 1.
The nP buffer layer 12 is formed.
In place of the nP buffer layer 12, n− having a different composition from the substrate
The InGaAsP buffer layer 12a is grown. Then, the distributed feedback diffraction grating 2 is formed on the n-InGaAsP buffer layer 12a, and the n-InGaAsP adjusting layer 13b, which is a crystal having a composition different from that of the n-InGaAsP buffer layer 12a, is grown to control the diffraction efficiency. n-In
The structure for growing the GaAsP flattening layer 14a made it easier to adjust the diffraction efficiency. That is, the diffraction efficiency formed on the first crystal is κ1,
Assuming that the diffraction efficiency of the distributed feedback diffraction grating remaining after the growth of the crystal is κ2, κ1 is proportional to the refractive index difference between the InGaAsP buffer layer 12a and the n-InGaAsP adjustment layer 13b. Further, κ2 is proportional to the refractive index difference between the n-InGaAsP adjusting layer 13 and the n-InGaAsP flattening layer 14a.

First, the n-InGaAsP buffer layer 12
From the viewpoint of carrier injection, a cannot be a crystal having a very large refractive index. Therefore, n-InGaA
sP (λg = 1.05 μm). Next, n-In
The GaAsP adjusting layer 13b needs to be a crystal having a slightly larger refractive index in order to eliminate the fluctuation of the diffraction efficiency due to the variation of the composition. Therefore, n-InGaAsP (λg
= 1.15 μm). In addition, the film thickness is set to 20 nm in order to improve carrier injection. As a result, the distributed feedback diffraction grating 2 cannot be sufficiently embedded, and the planarization layer 1
When the composition of 4a is InP and the difference in refractive index from the adjustment layer 13 is increased, the diffraction efficiency sharply decreases.
Therefore, the composition of the flattening layer 14a is set to n-InGaAsP.
As (λg = 0.95 μm), the value of the diffraction efficiency could be set to the target κL = 1 while ensuring the injection of carriers. Thus, it became clear that it is necessary to adjust the diffraction efficiency while changing the crystal composition in three steps by selecting the film thickness and the crystal composition for which the adjustment of the diffraction efficiency is stable.

(Fifth Embodiment) FIG. 9 shows a method for manufacturing a semiconductor laser according to a fifth embodiment of the present invention. In FIG. 9, first, FIG. 9A shows a diffraction grating manufacturing process for forming the distributed feedback diffraction grating 2 on the entire surface of the n-InP substrate 1 by the hologram exposure method. In this diffraction grating manufacturing process, a reduced pressure MOVP with a growth pressure of 60 torr
Annealing is performed in the PH ₃ atmosphere at 600 ° C. for 15 minutes using the E method to reduce the diffraction efficiency from 2.0% to 0.3%. In the case of annealing, unlike in the case of etching that is rate-controlled, the rate of reaction is rate-controlled, so that in-plane variations in diffraction efficiency are suppressed. Then, an n-InP buffer layer (carrier concentration n = 5 × 10 ¹⁷ ) is formed on the entire surface of the distributed feedback diffraction grating 2 having a reduced diffraction efficiency as a first epitaxial growth by using a low pressure MOVPE method with a growth pressure of 60 torr.
3 is 500 nm, and the n-InGaAsP waveguide layer 4 is 15 nm.
A quantum well type InGaAsP active layer 5 consisting of 5 periods of 0 nm, InGaAsP (λ = 1.40 μm) well layer and 10 nm InGaAsP (λ = 1.10 μm) barrier layer, and p-InP clad layer 16 of 0.5 μm. The first crystal growing step for growing is shown in FIG. 9 (b).

Next, a stripe 17 is formed by etching a part of the crystal surface from the crystal surface to the n-InP substrate 1 over a width of 1 μm in the <011> direction, and then p-I is formed.
nP.n-InP.p-InP buried layer 6, p-In
GaAsP cap layer 7 is grown by stripe embedding,
After that, the p-side electrode 8 of Au / Zn and the n-side electrode 9 of Au—Sn are formed by vapor deposition to obtain the structure of FIG.

By growing the n-InP buffer layer 3 on the InP substrate 1 by 500 nm, the height of the distributed feedback diffraction grating is reduced, and the diffraction efficiency of the distributed feedback diffraction grating is from 3.2 before the growth. It drops to 1.0. (Sixth Embodiment) FIG. 10 shows a method for manufacturing a semiconductor laser according to a sixth embodiment of the present invention. Figure 10
In FIG. 10A, a diffraction grating manufacturing process of crystal-growing the n-InP buffer layer 12 on the n-InP substrate 1 and then forming the distributed feedback diffraction grating 2 on the entire surface by a hologram exposure method is shown in FIG. .

An n-InGaAsP buffer layer (carrier concentration n = 5 ×) is formed on the entire surface of the distributed feedback diffraction grating 2 in which the diffraction efficiency is uniformly reduced by annealing in the MOVPE furnace by the MOVPE method as the second epitaxial growth.
10 ¹⁷ ) 13 to 50 nm, n-InP flattening layer (carrier concentration n = 5 × 10 ¹⁷ ) 14 300 nm, n-InG
The aAsP waveguide layer 4 is formed with 100 nm and 5 nm InGaA.
sP (λ = 1.40 μm) well layer and 10 nm InGa
FIG. 10B shows the second crystal growth step for growing the quantum well type InGaAsP active layer 5 consisting of 5 periods of the AsP (λ = 1.10 μm) barrier layer and the p-InP cladding layer 16 to 0.5 μm. Show.

Next, a stripe 18 is formed by etching a part of the crystal surface from the crystal surface to the n-InP substrate 1 over a width of 1 μm in the <011> direction.
nP.n-InP.p-InP buried layer 6, p-In
GaAsP cap layer 7 is grown by stripe embedding,
After that, the p-side electrode 8 of Au / Zn and the n-side electrode 9 of Au—Sn are formed by vapor deposition to obtain the structure of FIG.

Under the InGaAsP active layer 5, n-In
By growing the P buffer layer 14 to 300 nm, the distance between the distributed feedback diffraction grating 2 and the InGaAsP active layer 5 is increased, and the light intensity at the location of the distributed feedback diffraction grating is reduced to 1 /.
By reducing the number to 5, the effective diffraction efficiency of the distributed feedback diffraction grating becomes lower than that before the growth. In addition, n-InG
Since the growth rate of the aAsP buffer layer 13 on the concave surface is high, the surface becomes almost flat by stacking 40 nm.

Further, by stacking the n-InP flattening layer 14, the InGaAsP active layer 5 becomes extremely flat and the influence of the distributed feedback diffraction grating 2 is eliminated. In the first to sixth embodiments, the distributed feedback diffraction grating 2 is located on the substrate 1 or the n-InGaAsP buffer layer 12.
However, the distributed feedback diffraction grating 2 may be formed after the n-InGaAsP buffer layer 12 is grown on the substrate 1.

Further, in the second to fourth embodiments, the diffraction efficiency of the distributed feedback type diffraction grating 2 formed on the buffer layers 12, 13, 13a is set to the buffer layer 12 grown on the distributed feedback type diffraction grating 2. , 13, 13a may be adjusted with a crystal having the same composition, and the InP flattening layer 14 may be grown. Although the distributed feedback diffraction grating 2 is located under the InGaAsP active layer 5 in the first to sixth embodiments, it may be formed over the InGaAsP active layer 5. That is, the diffraction grating may be formed after crystal growth of the buffer layers 3, 12, 13 and the active layer 5 and the waveguide layer 4.

The buffer (planarization) layer 14 is made of InP.
Although a crystal is used, the composition does not matter as long as the crystal has a smaller refractive index than the n-InGaAsP buffer layer. Also,
Although the semiconductor crystal is InP, other semiconductor crystal substrates such as GaAs may be used. The position of the distributed Bragg reflector type diffraction grating may be located at the crystal interface having a different refractive index when the distributed Bragg reflector type diffraction grating is not formed in the active layer.

Further, although the laser structure is the DFB laser in the first to sixth embodiments, the oscillation wavelength is controlled with a distribution of the refractive index in the cavity length direction of a laser such as a DBR laser or a distributed feedback laser. Any laser can be applied, and the same effect as each embodiment can be obtained by changing the degree of change in the refractive index in the cavity length direction.

Further, good characteristics were confirmed regardless of the presence or absence of the n-InP buffer layer. Further, although the buffer layer, the waveguide layer and the InP layer are doped,
The use of undoped crystals is not limited to this.

[0055]

As described above, the semiconductor laser of the present invention has a large side mode suppression ratio, stable single mode oscillation can be obtained, and single mode characteristics and laser yield can be improved. At the same time, the full width at half maximum of the emitted light intensity can be reduced to realize a laser with high coupling efficiency, which has a very large influence in practical use.

[Brief description of drawings]

FIG. 1 is a structural diagram of a semiconductor laser according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a method of adjusting the distributed feedback diffraction grating shape according to the first embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between crystal growth time and diffraction efficiency in the first embodiment of the present invention.

FIG. 4 is a structural diagram of a semiconductor laser according to a second embodiment of the present invention.

FIG. 5 is a diagram showing a method of adjusting the distributed feedback diffraction grating shape according to the second embodiment of the present invention.

FIG. 6 is a diagram showing the relationship between the distance between the active layer and the distributed feedback type diffraction grating and the diffraction efficiency in the second embodiment of the present invention.

FIG. 7 is a structural diagram of a semiconductor laser according to a third embodiment of the present invention.

FIG. 8 is a structural diagram of a semiconductor laser according to a fourth embodiment of the present invention.

FIG. 9 is a diagram showing a method for manufacturing a semiconductor laser according to the fifth embodiment of the present invention.

FIG. 10 is a diagram showing a method for manufacturing a semiconductor laser according to the sixth embodiment of the present invention.

FIG. 11 is a structural diagram of a conventional DFB laser.

FIG. 12 is a diagram showing a conventional method of adjusting the shape of a distributed feedback diffraction grating.

[Explanation of symbols]

 1 n-InP Substrate 2 Distributed Feedback Diffraction Grating (Grating) 3 n-InP Control Layer (Buffer Layer) 4 InGaAsP Waveguide Layer 5 InGaAsP Active Layer 6 p-InP / n-InP / p-InP Embedded Layer 7 p- InGaAsP cap layer 8 Au / Zn p-side electrode 9 Au-Sn n-side electrode 12 n-InP buffer layer 13 n-InGaAsP adjusting layer (buffer layer) 14 InP flattening layer (buffer layer) 14a InGaAsP adjusting layer (buffer) Layer) 15 InGaAsP planarization layer (buffer layer) 16 p-InP clad layer 17 stripes 18 stripes

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasushi Matsui 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (6)

[Claims] 1. A first crystal having a diffraction grating formed therein and a second crystal having the same composition as the first crystal grown on the diffraction grating, and controlling the degree of growth of the second crystal. The semiconductor laser is characterized in that the diffraction efficiency of the diffraction grating is adjusted. 2. A refractive index of a first crystal having a diffraction grating, a second crystal having a different composition from that of the first crystal grown on the diffraction grating, and a refractive index of the second crystal grown on the second crystal. And a third crystal having a small thickness, and the film thickness of the second crystal is periodically changed by a diffraction grating. 3. The semiconductor laser according to claim 2, wherein the diffraction efficiency of the diffraction grating is adjusted by selecting a combination of compositions of the first crystal, the second crystal and the third crystal. 4. A diffraction grating forming step of forming a diffraction grating on a semiconductor single crystal substrate, an annealing step of reducing the diffraction efficiency of the diffraction grating of the semiconductor single crystal substrate, and a buffer layer as a buffer layer on the semiconductor single crystal substrate. A first crystal growth step of growing a crystal having the same composition as that of the semiconductor single crystal substrate;
Then, a waveguide layer and an active layer made of a crystal having a composition different from that of the semiconductor single crystal substrate are grown on the buffer layer.
And a method of manufacturing a semiconductor laser, the method including: 5. A diffraction grating forming step of forming a diffraction grating on a semiconductor single crystal substrate, an annealing step of reducing the diffraction efficiency of the diffraction grating of the semiconductor single crystal substrate, and a first step on the semiconductor single crystal substrate. A first crystal growing step of growing a crystal having a composition different from that of the semiconductor single crystal substrate as a buffer layer, and a crystal having a smaller refractive index than the first buffer layer as a second buffer layer on the first buffer layer And a second crystal growth step of growing an active layer and a waveguide layer made of a crystal having a composition different from that of the semiconductor single crystal substrate on the second buffer layer. Manufacturing method of semiconductor laser including. 6. A first crystal growth step of crystal-growing a first buffer layer having a composition different from that of the semiconductor single crystal substrate on the semiconductor single crystal substrate, and thereafter forming a diffraction grating in the first buffer layer. And a second crystal growth step of growing a crystal having the same composition as the semiconductor single crystal substrate as a second buffer layer on the first buffer layer, and then the second buffer layer. A method of manufacturing a semiconductor laser, comprising a waveguide layer made of a crystal having a composition different from that of the semiconductor single crystal substrate, and a third crystal growth step of growing an active layer thereon.