JPH0621574A - Semiconductor laser and manufacturing method thereof - Google Patents

Abstract

PURPOSE:To obtain a DFB laser wherein diffraction efficiency is improved, threshold current is reduced, and output power is increased, and the manufacturing method of the DFB laser. CONSTITUTION:On an InP substrate 11, the following are formed in order via a buffer layer 12; a lower waveguide layer, an active layer 14, an upper waveguide layer, and a clad layer 16. The lower waveguide layer is constituted of an InGaAsP layer 13, (refractive index n1) and an InP layer 132 (refractive index n2<n1), and a lower diffraction grating 20 is formed on the interface. The upper waveguide layer is constituted of an InP layer 151 and an InGaAsP layer 152 having the same composition as the InGaAsP layer 131, and an upper diffraction grating 21 is formed on the interface. The active layer 14 is formed of an InGaAsP (refractive index n3>n1) whose forbidden bandwidth is smaller than that of the InGaAsP layer 131.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser and a manufacturing method thereof, and more particularly to a distributed feedback laser (DFB laser) effective for optical communication and optical measurement and a manufacturing method thereof.

[0002]

2. Description of the Related Art InP-based DFB lasers are indispensable as a light source for a 1.5 .mu.m long-wavelength optical fiber communication system. The DFB laser uses a diffraction grating as a reflecting mirror of a resonator of a semiconductor laser to enable single longitudinal mode oscillation. If a DFB laser is used as the light source and a silica single mode fiber with a minimum loss wavelength band of 1.5 μm is used to construct an optical communication system, chromatic dispersion can be suppressed to a small level, and long-distance, large-capacity optical communication is possible. Becomes

A conventional DFB laser is constructed as shown in FIGS. 5 (a) to 5 (c). FIG. 5A shows an n-type InP substrate 1 with an n-type InP buffer layer 2 interposed therebetween.
P waveguide layer 3a, InGaAsP active layer 4, p-type InP
The clad layer 5 is sequentially laminated, and the diffraction grating 6 is formed between the InP buffer layer 2 and the InGaAsP waveguide layer 3a. 5 (b), the p-type InGaAsP waveguide layer 3b is formed on the active layer 3 contrary to FIG. 5 (a), and the diffraction grating 6 is formed between the InGaAsP waveguide layer 3b and the cladding layer 5. Has been done. In FIG. 5C, InGaAsP waveguide layers 3 a and 3 b are formed below and above the active layer 4, and a diffraction grating 6 is formed between the upper InGaAsP waveguide layer 3 b and the cladding layer 5.

[0004]

In such a DFB laser, since stimulated emission is caused by distributed feedback of light utilizing diffraction, laser output light depends on feedback efficiency (that is, diffraction efficiency) by the diffraction grating. However, the conventional structure has a problem that it is difficult to obtain a sufficient feedback efficiency because the diffraction grating is formed on either the upper or lower side of the active layer. The present invention has been made in consideration of such circumstances, and it is possible to obtain a large laser light output by increasing the diffraction efficiency and decreasing the oscillation threshold current.
An object of the present invention is to provide a laser and a manufacturing method thereof.

[0005]

In the DFB laser according to the present invention, a lower waveguide layer, an active layer, an upper waveguide layer and a cladding layer are sequentially laminated on a semiconductor substrate via a buffer layer, and the lower waveguide layer is formed. The waveguide layer is composed of a first semiconductor layer having a larger refractive index than the buffer layer on the buffer layer side and a second semiconductor layer on the active layer side having a different refractive index from the buffer layer. A lower diffraction grating is formed at an interface between the first semiconductor layer and the second semiconductor layer, the active layer is formed of a third semiconductor layer having a bandgap smaller than those of the first and second semiconductor layers, and the upper waveguide layer is formed of the third semiconductor layer. A fourth semiconductor layer made of the same material as the second semiconductor layer on the active layer side and a fifth semiconductor layer made of the same material as the first semiconductor layer on the clad layer side are provided. Layers and the fifth half Wherein the upper diffraction grating at the interface of the body layer is formed. A method of manufacturing a DFB laser according to the present invention includes a buffer layer on a semiconductor substrate,
Forming a first semiconductor layer having a refractive index higher than that of the buffer layer and forming a part of the lower waveguide layer; patterning a mask material at predetermined intervals on the surface of the first semiconductor layer; The surface of the first semiconductor layer is etched by an anisotropic etching method using a mask,
Forming a grating groove to serve as a lower diffraction grating, and using the mask material as a mask for selective growth to fill the grating groove of the first semiconductor layer with a lower refractive index different from that of the first semiconductor layer. Growing a second semiconductor layer to be the rest of the waveguide layer, and forming on the second semiconductor layer a third semiconductor layer to be an active layer made of a material having a bandgap smaller than those of the first and second semiconductor layers. And the step of growing, using the mask material as a selective growth mask on the third semiconductor layer, made of the same material as the second semiconductor layer, and having a grating groove formed on the surface to serve as an upper diffraction grating. A step of growing a fourth semiconductor layer, which is a part of the layer, and a fifth step, which is made of the same material as the first semiconductor layer and is the rest of the upper waveguide layer so as to fill the lattice groove of the fourth semiconductor layer. Technology for growing semiconductor layers When, characterized by comprising a step of growing forming a clad layer on said fifth semiconductor layer.

[0006]

In the DFB laser according to the present invention, by providing the diffraction grating above and below the active layer, a high diffraction efficiency can be obtained, the oscillation threshold current can be lowered, and a large laser output light power can be obtained. Further, in the present invention, since the lower waveguide layer and the upper waveguide layer are each composed of two semiconductor layers having different refractive indices and the diffraction grating is formed at each interface, the light confining effect is large. , Excellent low threshold current characteristics can be obtained. Further, according to the manufacturing method of the present invention, the mask material used as the etching resistant mask when forming the lower diffraction grating is used as it is as a mask for the subsequent selective growth. The phase is exactly 1 and the phase is 1
It can be formed in a state shifted by 80 °. Thereby, excellent single longitudinal mode laser light can be obtained. Further, while leaving the mask material, the second semiconductor layer and the active layer of the lower waveguide layer are selectively grown and planarized, and the upper waveguide layer composed of the fourth and fifth semiconductor layers is selectively grown. As a result, the upper diffraction grating is formed in the upper waveguide without requiring an etching process.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. 1 (a) and 1 (b) show a D according to an embodiment of the present invention.
FIG. 3 is a perspective view showing a structure of an FB laser and a cross-sectional view of a main part along the optical axis thereof. An n-type InP buffer layer 12 is formed on an n-type InP substrate 11, and an n-type lower waveguide layer 1 is formed on the n-type InP buffer layer 12.
3, the i-type active layer 14, and the p-type upper waveguide layer 15 are sequentially stacked.

The lower waveguide layer 13 is the InGaAsP layer 1
The lower diffraction grating 20 is formed at the interface between them and the InP layer 132. The active layer 14 is InGa
It is an AsP layer. The upper waveguide layer 15 is composed of an InP layer 151 and an InGaAsP layer 152, and an upper diffraction grating 21 is formed at the interface between them. The composition ratio of the InGaAsP layer of the active layer 14 is set so that the band gap is smaller than that of the upper and lower waveguide layer materials.
InGaAsP layer 131 and InGaAsP layer 152
Have the same composition, their refractive index is n1, and the InP layer 1
If the refractive index of 32 and the InP layer 151 is n2, their compositions are selected so as to satisfy the relationship of n2 <n1 <n3 with respect to the refractive index n3 of the InGaAsP layer which is the active layer 14. . By selecting the composition ratio with such a refractive index distribution,
Carrier confinement in the active layer 14 and the lower waveguide 13
Also, light can be confined by the upper waveguide layer 15.

A p-type InP clad layer 18 is formed on the upper waveguide layer 15, and a high impurity concentration I is further formed thereon.
An nGaAs cap layer 19 is formed. These semiconductor layers are etched to a depth reaching the buffer layer and patterned in a stripe shape, around which p-type InP layer 16 and n-type In are formed as current blocking layers.
The P layer 17 is buried and formed. Electrodes are formed on the front surface of the cap layer 19 and the back surface of the substrate 11 although not shown in the drawing.

Incidentally, 23 in FIG. 1 (b) is the first InGaA.
It is an etching resistant mask for selectively etching the sP layer 131 to form the grating groove of the lower diffraction grating 20, and is also a mask material used for the subsequent selective growth. The mask material 23 is made of SiO, which is transparent at the laser oscillation wavelength.
An insulating film such as ₂ , Si ₃ N ₄ or a semiconductor film. When a semiconductor is used as the mask material 23, in order for the selective liquid phase growth to be effective, the deviation of the lattice constant from the material exposed in the mask opening may be about 0.2% or more. For example, InGaAs, InGaAsP or the like can be used as the mask material.

The manufacturing process of the DFB laser of this embodiment is as follows.
A description will be given below with reference to FIGS. 2 and 3. As shown in FIG. 2A, n-type InP is formed on the (100) InP substrate 11.
The buffer layer 12 is epitaxially grown, and further the InGaAsP layer 131 which becomes a part of the n-type lower waveguide layer 13 is grown, and then the mask material 23 is formed on the InGaAsP layer 131. Then, by using photolithography, the mask material 23 is patterned into a stripe lattice pattern as shown in FIG. Since the mask material 23 is left inside the element as it is, the material, the film thickness, the pattern width, etc. are selected so that the crystal lattice defects at the time of light absorption and regrowth do not become a problem. For example, using SiO ₂ , the film thickness,
Both widths are set to several tens nm or less. Since the pitch of the mask material 23 has a lattice constant, the required oscillation wavelength can be obtained in consideration of the difference in refractive index between the InGaAsP layer 131 (refractive index n1) and the InP layer 132 (refractive index n2) formed thereafter. To select.

Next, as shown in FIG. 2C, the mask material 23
Is used as an etching resistant mask, the InGaAsP layer 131 is mesa-etched by anisotropic etching,
The lower diffraction grating 20 is formed by a triangular groove whose (111) plane is exposed. Then, as shown in FIG. 2 (d), the mask material 23 is used as it is as a mask for selective growth, and by vapor phase epitaxial growth, In
The P layer 132 is selectively grown. Further, as shown in FIG. 3 (a), i-type InGaA was grown by vapor phase epitaxial growth method.
The sP active layer 14 (refractive index n3) is selectively grown to flatten the surface. Each of the above layers has a refractive index of n2 <n1 <n3
The composition ratio is selected so as to satisfy.

Next, as shown in FIG. 3B, the mask material 2
3 as a mask for selective growth, and further becomes a part of the upper waveguide layer 15 by the vapor phase epitaxial growth method.
The nP layer 151 is grown. At this time, InGaAsP
Since the active layer 14 is flattened and the (100) plane is exposed on the surface and growth does not occur on the mask material 23, the InP layer 151 with a triangular cross section is formed on the active layer 14, An upper diffraction grating 21 composed of a grating groove is automatically formed on the surface thereof. An InGaAsP layer 152 which is the rest of the upper waveguide layer 15 is further epitaxially grown on this. The InGaAsP layer 152 has the same composition as the InGaAsP layer 131 of the lower waveguide layer 13.

After that, as shown in FIG. 3C, p-type In
The P clad layer 18 and the InGaAs cap layer 19 are formed. Then, each of these semiconductor layers is etched to form a stripe pattern, and a p-type InP layer and an n-type InP layer to be a current blocking layer are epitaxially grown outside the stripe pattern. Finally, although not shown, electrodes are formed on both surfaces to complete the device.

In the DFB laser of this embodiment, a diffraction grating as a distributed reflector is formed above and below the active layer.
Moreover, the upper and lower diffraction gratings are formed as a 180 ° inversion pattern which is perfectly matched by the mask material, and contributes as feedback light for laser oscillation. The first-order diffracted light by each diffraction grating is perfectly aligned in phase. Will be things.
Therefore, the reflection efficiency is almost twice as high as in the conventional case where there is one diffraction grating. As a result, the threshold current of laser oscillation is reduced by about 30% as compared with the case of using only one diffraction grating, and as a result, the laser output light power at the same current density is improved. The lower waveguide layer and the upper waveguide layer are originally required to have one layer, but when the diffraction grating is formed in each one of the waveguide layers, there is a possibility that a sufficient optical confinement effect cannot be obtained. In this respect, in this embodiment, since the lower waveguide layer and the upper waveguide layer each have a two-layer structure in which a refractive index difference is provided and the diffraction grating is formed therein, a sufficient optical confinement effect can be obtained.

FIG. 4 shows a sectional structure of a main part of a DFB laser according to another embodiment of the present invention, corresponding to FIG. 1 (b). The basic structure and manufacturing process are similar to those of the previous embodiment, and therefore, parts corresponding to those of the previous embodiment are designated by the same reference numerals. The difference from the previous embodiment is that in this embodiment, the active layer 141 is already formed before the mask material 23 is formed. The lower diffraction grating 20 is formed by etching the active layer 141 and the InGaAsP layer 131 thereunder using the mask material 23. afterwards,
InP layer 132 and InGaAs are formed in the same manner as in the previous embodiment.
P active layer 142, InP layer 151 and InGaAsP
The upper waveguide layer 15 consisting of the layer 152 and the cladding layer 18 are formed.

In the structure of the previous embodiment, the active layer 14 is separated by the mask material 23 and becomes scattered, whereas in this embodiment, the active layers 141 and 142 are continuous and alternately arranged. Becomes Therefore, even if the mask material 23 absorbs light to some extent, its influence is reduced and higher efficiency can be obtained.

In the above embodiment, the upper and lower waveguide layers have a two-layer structure in which InP is used on the active layer side and InGaAsP is used on the outer side, but these active layers and waveguide layers are all InG.
It can also be composed of an aAsP layer. For example, the active layer side of the upper and lower waveguide layers is the InGaAsP layer (refractive index n1), the outer side is the InGaAsP layer (refractive index n2), and the active layer is I.
For the nGaAsP layer (refractive index n3), these composition ratios can be selected so as to satisfy n1 <n2 <n3.

[0019]

As described above, the DFB according to the present invention
In the laser, by providing the diffraction grating above and below the active layer and in a state of being embedded in the waveguide layer having a difference in refractive index, high diffraction efficiency is obtained, and the oscillation threshold current is lowered, A large laser output light power can be obtained. Further, according to the method of the present invention, the mask material used as an etching resistant mask when forming the lower diffraction grating is used as a mask for the subsequent selective growth, and the upper and lower diffraction gratings have completely the same lattice constant. It is formed with the reversed pattern. Thereby, excellent single longitudinal mode laser light can be obtained. Further, by repeating the crystal growth with the mask material left, the upper diffraction grating is formed inside the upper waveguide layer without requiring an etching step.

[Brief description of drawings]

FIG. 1 is a perspective view and a sectional view of an essential part of a DFB laser according to an embodiment of the present invention.

FIG. 2 is a diagram showing a manufacturing process of the embodiment.

FIG. 3 is a diagram showing a manufacturing process of the embodiment.

FIG. 4 is a sectional view showing the structure of a main part of a DFB laser according to another embodiment of the present invention.

FIG. 5 is a cross-sectional structural diagram of a conventional DFB laser.

[Explanation of Codes] 11 ... n-type InP substrate, 12 ... n-type InP buffer layer,
13 ... N-type lower waveguide layer, 131 ... InGaAsP layer,
13 2 InP layer, 14 i-type InGaAsP active layer,
15 ... p-type upper waveguide layer, 151 ... InGaAsP layer,
152 ... InP layer, 16 ... p-type InP layer, 17 ... n-type I
nP layer, 18 ... p-type InP cladding layer, 19 ... cap layer, 20 ... lower diffraction grating, 21 ... upper diffraction grating, 23 ...
Mask material.

Claims (2)

[Claims] 1. A lower waveguide layer, an active layer, an upper waveguide layer and a clad layer are sequentially stacked on a semiconductor substrate with a buffer layer interposed therebetween, and the lower waveguide layer is on the buffer layer side. A first semiconductor layer having a refractive index larger than that of the layer, and a second semiconductor layer on the side of the active layer having a different refractive index from the first semiconductor layer, and a lower diffraction grating is formed at the interface between the first semiconductor layer and the second semiconductor layer. The active layer is formed of a third semiconductor layer having a forbidden band width smaller than that of the first and second semiconductor layers, and the upper waveguide layer includes the second semiconductor layer on the active layer side. A fourth semiconductor layer made of the same material and a fifth semiconductor layer made of the same material as the first semiconductor layer on the clad layer side are provided, and an upper diffraction layer is formed at the interface between the fourth semiconductor layer and the fifth semiconductor layer. The grid is formed A semiconductor laser characterized in that 2. A step of forming a first semiconductor layer, which has a refractive index higher than that of the buffer layer and is a part of a lower waveguide layer, on a semiconductor substrate via a buffer layer, and a predetermined surface on the surface of the first semiconductor layer. Patterning a mask material at intervals and etching the surface of the first semiconductor layer by an anisotropic etching method using the mask material as an etching resistant mask to form a grating groove to be a lower diffraction grating; Using the mask material as a mask for selective growth, a second semiconductor layer to be the rest of the lower waveguide layer having a refractive index different from that of the first semiconductor layer is grown so as to fill the lattice groove of the first semiconductor layer. A step of growing a third semiconductor layer, which is an active layer made of a material having a band gap smaller than that of the first and second semiconductor layers, on the second semiconductor layer; A fourth semiconductor layer made of the same material as the second semiconductor layer and having a grating groove serving as an upper diffraction grating is formed on the surface to form a part of the upper waveguide layer by using the mask material as a selective growth mask. And a step of filling the lattice groove of the fourth semiconductor layer with the first
A semiconductor comprising: a step of growing a fifth semiconductor layer made of the same material as the semiconductor layer and remaining of the upper waveguide layer; and a step of growing and forming a cladding layer on the fifth semiconductor layer. Laser manufacturing method.