JPH10186121A - Formation of absorption type diffraction grating and manufacture of gain coupling type dfb laser formed by utilizing this diffraction grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for forming absorption type diffraction gratings which are not continuously but discretely formed with absorption regions and a process for manufacture of a gain coupling type DFB (distributed feedback semiconductor) laser. SOLUTION: As this method for formation, a semiconductor single layer film 3 consisting of the compsn. different from the compsn. of first semiconductors 1, 2 or multilayered semiconductor films partly having the semiconductor layers consisting of the compsn. different from the compsn. of the first semiconductors 1, 2 are first formed on the semiconductors 1, 2. Next, the diffraction gratings 4 are manufactured by etching the single layer film 3 or the multilayered films. Semiconductor films 6 which constitute the absorption regions are thereafter formed on the diffraction gratings 4. The height 5 of the diffraction gratings 4 is then lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser suitable for a long-distance, large-capacity light source for optical communication and the like, and a method of manufacturing the same.

[0002]

2. Description of the Related Art A distributed feedback semiconductor laser (DFB laser) has been put to practical use as a light source for multi-channel image transmission such as large-capacity data transmission and CATV. This is because a normal Fabry-Perot type semiconductor laser oscillates in a multi-axis mode, whereas a single-axis mode oscillation is obtained, so that the noise level is low. Further, when the oscillation signal is transmitted through the transmission path, the oscillation signal is less affected by signal degradation due to dispersion.

[0003] Current DFB lasers are index-coupled DFBs.
Lasers are the mainstream. This index-coupled DFB laser has two oscillation modes on both sides (on both sides of the stop band) of the Bragg wavelength determined by the period of the diffraction grating and the refractive index of the resonator. Is selected, it is considered that the probability of obtaining a stable single oscillation mode is low. In addition, at the time of high injection of carriers, a phase change occurs due to a change in the refractive index due to the axial hole burning effect, a change in the oscillation mode is likely to occur, and a yield in which a single mode oscillation is stably obtained up to a high output is further increased. Become smaller. In addition, this index-coupled DFB laser is weak to return light, and when there is return light, a change in the oscillation state occurs, which increases noise and multi-mode operation. Become.

In recent years, gain-coupled DFB lasers have attracted attention as DFB lasers having a new structure that solve the problem of the index-coupled DFB laser. In the gain-coupled laser, the oscillation mode is basically defined by the Bragg wavelength mode, so that stable single-axis mode oscillation can be expected at a high yield without being affected by the end face phase. Further, since it is hardly affected by the phase fluctuation due to the axial hole burning, the single mode is maintained even at a high output, and the yield is improved. Further, it has been reported that the stability against the return light is also improved, and it can be expected as a low-cost single-axis mode light source.

FIG. 7 shows a typical configuration of a gain-coupled DFB laser. Here, n-InP 102 as a first cladding layer, n-InGaAs absorption type diffraction grating 103, n-InGaAsP lower optical guide layer 104, and multiple quantum well (MQW) activity of InGaAs / InGaAsP are formed on n-InP substrate 101. The layer 105, the p-InGaAsP upper light guide layer 106, the p-InP cladding layer 107, and finally the p-InGaAs cap layer 108 are formed, and the crystal growth is completed. n-InGaAsP lower light guide layer 1
P-InGaAs absorption type diffraction grating 1 embedded in the substrate 04
By the action of 03, a periodic change in the absorption coefficient is formed, a change in gain is obtained, and a gain-coupled oscillation is obtained. FIG.
A description will be given of an absorption-type diffraction grating that obtains good characteristics by using.

FIG. 8 shows an n-InGa layer 110 serving as an absorption layer.
This is the lateral thickness of As. Reference numeral 109 denotes a period of the diffraction grating 103. This duty ratio (thickness of the absorbing layer 110 /
It has been found that the diffraction grating period 109) is desirably about 0.1 to 0.2, and it is very important to control the shape of the diffraction grating with high reproducibility. However, the period 109 of the diffraction grating needs to be about 0.2 μm to 0.3 μm, and an ultra-fine pattern of 0.1 μm to 0.15 μm is required as a resist pattern at the time of manufacturing the same. Actually, electron beam (EB) exposure can be used to form such an ultrafine pattern with good reproducibility, but problems such as long time required for exposure and increase in apparatus cost arise. Not practical. Therefore, it is more practical to form the resist pattern using two-beam interference exposure and to use dry etching or wet etching for the semiconductor layer etching.

A typical method of forming an absorption layer using an interference exposure method will be described with reference to FIGS. 9 (a), 9 (b) and 9 (c).
First, a grating 112 is formed on an n-InP 111 serving as a substrate. On this grating 112, as shown in FIG. 9B, n-InGaAs 1 serving as an absorption layer is formed.
13 is formed as a whole. FIG. 9C shows an enlarged area of the absorption layer 113. n-InGaAs
Numeral 113 is a grating valley and has a high growth rate, and therefore easily accumulates in the valley. Therefore, the thickness of the valley 115 and the top 1
14 have different thicknesses and a periodic structure is formed. here,
At least the film thickness of the top portion 114 is set to the extent that the quantum effect is generated (an energy gap is widened and absorption does not occur if the thickness is reduced to this extent), and light of the active layer is not absorbed. As a result, the valley 115 absorbs light from the active layer, and the top 114 does not absorb light from the active layer, so that an absorption type DFB laser structure can be realized. This phenomenon is intended to realize the absorption type diffraction grating shown in FIG.

[0008] However, this method also has problems. Since the absorbing InGaAs 113 is also formed on the top of the grating 112, it is continuously connected to the groove or the valley. For this reason, the region acting as the absorption layer changes depending on the growth state and the in-plane distribution at the time of manufacturing the grating, and it is difficult to manufacture a laser having desired characteristics with high yield. In a severe case, it is considered that the film thickness of the top portion 114 increases and absorption occurs in this portion as well, and apparently no diffraction grating is easily formed.

[0009]

As described above, in the conventional method using two-beam interference exposure, a film having a narrow band gap remains at the top of the grating, and the width of the absorption layer changes depending on the growth conditions. The characteristics varied. Further, in some cases, a problem arises in that the top portion also functions as an absorption layer, does not form a diffraction grating, and does not function as a DFB laser.

Accordingly, an object of the present invention is to provide a method of forming an absorption type diffraction grating in which a film having a narrow band gap does not remain on the top as in the prior art using a two-beam interference exposure method excellent in mass productivity and the like. An object of the present invention is to provide a method of manufacturing a gain-coupled DFB laser. Specifically, the present invention provides a method of forming a light absorbing layer on a grating and then partially removing the convex portion of the grating. As a result, an absorption-type diffraction grating or a gain-coupled DFB laser in which the absorption region is formed discretely instead of continuously can be realized. In addition, a configuration with a small step at the time of regrowth after manufacturing the absorption type diffraction grating was realized.

[0011]

A first aspect of the present invention for achieving the above object is as follows.
According to the invention (corresponding to the invention according to claim 1), a semiconductor single-layer film having a different composition from the first semiconductor or a semiconductor layer having a different composition from the first semiconductor is partially provided on the first semiconductor. A step of forming a semiconductor multilayer film, a step of etching the single-layer film or the multilayer film to form a diffraction grating (typically using a two-beam interference exposure method, but not limited thereto); Forming a semiconductor film to be an absorption region (hereinafter, as described below, there is a case where the semiconductor film is formed uniformly and a case where the semiconductor film is formed in a groove portion) and a step of reducing the height of the diffraction grating (There are several methods for this, as described below.) This is a method for forming an absorption-type diffraction grating.

Second invention (corresponding to the invention according to claim 2)
According to the first aspect, the step of forming a semiconductor film to be an absorption region on the diffraction grating is a step of uniformly forming a semiconductor film to be an absorption region on the diffraction grating.

Third invention (corresponding to the invention according to claim 3)
Then, the step of reducing the height of the diffraction grating in the second invention includes a step of forming an etching protection layer for protecting a semiconductor film serving as an absorption region in a groove of the diffraction grating and a step of performing selective etching. Features.

Fourth invention (corresponding to the invention according to claim 4)
Then, the step of reducing the height of the diffraction grating in the second invention includes a step of performing light etching for forming a semiconductor film which is an absorption region uniformly formed on the diffraction grating and a step of performing selective etching. It is characterized by.

In the fifth invention (corresponding to the invention according to claim 5), the step of forming a semiconductor film serving as an absorption region on the diffraction grating according to the first invention comprises the step of forming an absorption region in a groove of the diffraction grating. Forming a semiconductor film to be formed.

Sixth invention (corresponding to the invention according to claim 6)
Then, the step of reducing the height of the diffraction grating in the first or fifth invention is a step of performing selective etching.

Seventh invention (corresponding to the invention according to claim 7)
Then, the step of reducing the height of the diffraction grating in the first, second or fifth aspect of the present invention includes a step of filling and flattening a groove of the diffraction grating and a step of performing uniform etching. .

According to the above-described method of forming an absorption type diffraction grating,
A method of manufacturing an absorption type diffraction grating that does not leave an absorption film in a region corresponding to the top of the grating can be realized using two-beam interference exposure with high productivity and a normal etching method.

In the eighth and eleventh to sixteenth inventions (corresponding to the eighth and eleventh to sixteenth inventions),
Gain-coupling type DF forming absorption type diffraction grating by the above forming method
It is a method for manufacturing a B laser.

For example, a step of forming a multilayer film on a substrate;
Forming a grating including the multilayer film;
A method of manufacturing a semiconductor laser that does not leave a film having a band gap narrower than the active layer in a region corresponding to the top of the grating by forming an absorption layer on the grating and etching a part of the multilayer film during regrowth. Can be realized using two-beam interference exposure with high productivity and a normal etching method.

Ninth invention (corresponding to the ninth invention)
Is characterized in that the absorption type diffraction grating is formed in a light guide layer.

A tenth invention (corresponding to the tenth invention) is characterized in that the absorption type diffraction grating is formed above an active layer.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment A first embodiment of the present invention will be described with reference to FIG. 1 is n-InP which is a substrate. On this, n-InP shown in 2 is formed to a thickness of 0.1 μm. Subsequently, InGaAsP shown in FIG.
μm is formed, and the growth is once terminated.

Thereafter, a diffraction grating 4 is formed on the grown films 2 and 3 by using a resist and a two-beam interference exposure method. The semiconductor films 2 and 3 are etched by dry etching of chlorine to form a diffraction grating 4 having a depth of 0.15 μm as shown in FIG. Here, the substrate 1 and n-In
The presence of the etching auxiliary film 3 having a different composition from the first semiconductor of the P layer 2 allows the grating 4 to be formed deeply, and the groove 16 of the grating 4 to be formed thin.

Subsequently, a second growth is performed. FIG. 1 (c)
The n-InGaAs 6 serving as an absorption layer is formed as shown in FIG. Here, the film is grown so as to have a thickness of 30 nm in the groove 16. At this time, a slight n-InGaAs film also grows on the top portion 14 of the diffraction grating 4. Subsequently, an etching protection layer shown by 7 is formed. This layer 7 has a role of protecting the InGaAs absorption layer 6 in the subsequent etching. The composition of the film shown in 7 is made of n-InP, and the film thickness is 20 nm at the valley. In order to form the protective layer 7 only at the valleys, etching may be performed with hydrochloric acid after the film formation.

Subsequently, I at the top of the diffraction grating shown in FIG.
The nGaAsP film 6 is etched. The etching solution uses a sulfuric acid system, and sulfuric acid: water: hydrogen peroxide solution = 1: 1: 10
0 ratio. The shape after etching is shown in FIG. The absorption layer InGaAs film 6 becomes only the groove protected by the protection layer 7, and the InGa film on the top of the diffraction grating shown in FIG.
The substrate becomes substantially flat by the etching of the AsP film.
The n-InP film 7 shown in FIG. 1D does not function as an absorption layer and may be left, but may be removed if unnecessary.

The important point here is that by etching the top 14 (convex portion) of the grating 4, the n-I
The point is that the nGaAs absorption layer 6 is not continuous but discrete and the grating 4 having a large height indicated by 5 can be realized by the etching auxiliary film 3 formed first. As a result of this latter, the tip of the groove 16 of the grating 4 becomes narrower, and as shown at 15 in FIG.
Has become narrower. Thus, an absorption grating having a duty ratio of 0.3 was realized.

FIG. 2 shows the subsequent manufacturing process of the semiconductor laser. On the substrate 1 on which the absorption layer 6 is formed, 8 non-doped InGaAsPs serving as optical guide layers are grown to a thickness of 0.1 μm, and an active layer 9 is laminated thereon. In the configuration of the active layer 9, a pair of InGaAs (thickness: 6 nm) as a quantum well and InGaAsP (thickness: 10 nm) as a barrier layer is repeated five times. Further, the thickness of the non-doped InGaAsP10 as the upper light guide layer is 0.1 μm, the thickness of the Be-doped InP11 as the upper cladding layer is 1.5 μm,
Be-doped InGaAs 12 serving as a cap layer is formed to a thickness of 0.2 μm.

The etching depth 1 shown in FIG.
The variation of 3 is not so important. This variation has an effect as a step at the time of regrowth shown in FIG. 1D, but even a slight deviation does not hinder growth.

As described above, according to the present manufacturing method, the absorption layer 6 can be formed discretely, and variations in the semiconductor laser characteristics are reduced. Further, the groove 16 of the grating 4 became narrow, and the duty ratio became small. With this configuration,
A stable gain-coupled DFB laser was manufactured.

Second Embodiment A second embodiment of the present invention will be described with reference to FIG. This embodiment shows an example in which an absorption type diffraction grating is formed in a light guide layer.

In FIG. 3A, reference numeral 21 denotes an n-InP film serving as a substrate. On top of this, n shown in FIG.
-Forming an InGaAsP film having a thickness of 0.1 μm; Reference numeral 23 denotes an n-InP film assisting film having a thickness of 0.1 μm.
m. As shown in the first embodiment, the auxiliary film 23 narrows the end of the groove of the grating and improves (reduces) the duty ratio of the absorption region formed at the end. After this growth is completed, a grating 25 is formed as shown in FIG. The depth 26 of the grating 25 is 0.15 μm. After this, FIG.
As shown in (c), the n-InGaAs layer 27 is formed so that the thickness of the groove becomes 25 nm. n-InGaAs
The film 27 is also formed on the slope and the top of the grating 25. Thereafter, the n-InGaAs 27 formed on the slope and the top is removed with a sulfuric acid-based etchant to obtain the shape shown in FIG. An n-InGaAs 28 serving as an absorption coupling type grating is formed in the groove. Finally, the etching auxiliary layer 23 is removed, and the formation of the absorption layer 28 is completed.

The manufacturing process of the semiconductor laser on the absorption substrate will be described with reference to FIG. Absorption type diffraction grating 2
A non-doped InGaAsP, which is a light guide layer shown in FIG. An active layer 30 is formed thereon. In the configuration of the active layer 30, pairs of InGaAs (thickness 10 nm) as a quantum well and InGaAsP (thickness 10 nm) as a barrier layer are repeated six times. Reference numeral 31 denotes a non-doped InGaAsP serving as an upper light guide layer, and has a thickness of 0.05 μm. 32 is Be doped InP which is an upper cladding layer.
And a thickness of 1.3 μm is formed. Reference numeral 33 denotes Be-doped InGaAs, which is a cap layer, and is formed to a thickness of 0.2 μm. Thus, the growth is completed.

Here, the absorption type diffraction grating 28 is made of n-InG
It is formed in the aAsP film 22. Therefore, 22 functions as a light guide layer, and light is drawn to the light guide layer 22. That is, the light absorption ratio increases, and the coupling can be strengthened.

Also in this embodiment, the variation of the etching depth 34 in FIG. 4 is not so important. Although this variation affects as a step at the time of regrowth shown in FIG. 3E, even a slight deviation does not hinder growth.
If the growth conditions are optimized, the state shown in FIG. 3B can be directly changed to the state shown in FIG. 3D.

As described above, the absorption type diffraction grating having the absorption region 28 only in the groove portion can be formed by the two-beam interference exposure, the wet etching, and the dry etching by the steps shown in this embodiment. Also, the duty ratio of the absorption region 28 could be improved.

Third Embodiment A third embodiment of the present invention will be described with reference to FIG. In this embodiment, the absorption type diffraction grating of the present invention is applied to a substrate having an active layer.

In FIG. 5, reference numeral 41 denotes a p-InP film serving as a substrate. On this, a non-doped InGaAsP of a light guide layer indicated by reference numeral 42 is formed to a thickness of 0.2 μm. On this, an active layer shown by 43 is formed. Active layer 4
3 is composed of InGaAs (having a thickness of 6 n) as a quantum well.
m) and InGaAsP (12 nm thick) as a barrier layer
Is repeated four times. Reference numeral 44 denotes non-doped InGaAsP as an upper light guide layer. Further, 49 is an absorption-type diffraction grating, 50 is Si-doped InP as an upper cladding layer, and has a thickness of 1.3 μm. Reference numeral 51 denotes a cap layer having a thickness of 0.2 μm with Si-doped InGaAs.
Has formed. This is the overall configuration.

A method for forming the absorption type diffraction grating 49 will be described with reference to FIG. In FIG. 6A, reference numeral 40 indicates the layers from the layer 41 to the layer 43 described in FIG. On top of this, 44 non-doped InGa to be the upper light guide layer
AsP is formed to a thickness of 0.1 μm. Reference numeral 45 denotes an InP film which is an etching auxiliary layer, and has a thickness of 0.08 μm. FIG. 6B shows the structure after the etching.
The groove depth 47 of the grating 46 is 0.13 μm. After this etching, the n-InG
aAs is formed to a thickness of 20 nm (FIG. 6C).

Subsequently, n formed on the top and the slope
The InGaAs 48 is removed by light etching with a sulfuric acid-based etchant (FIG. 6D). Thereafter, when the etching auxiliary layer 45 is removed, the structure shown in FIG. Subsequent growth follows the procedure described with reference to FIG.

This example shows that the present invention can be applied to a substrate having an active layer. In the third embodiment shown in FIG. 5, the upper cladding 50 is formed immediately on the absorption type diffraction grating 49, but an optical guide layer may be inserted in this portion. With this light guide layer, the coupling coefficient can be improved.

[0042]

As described above, according to the present invention, a diffraction grating having an isolated absorption region can be formed by using an interference exposure method excellent in mass productivity, and variations in semiconductor laser characteristics are reduced. Further, the groove portion of the grating was narrowed, and the duty ratio was reduced. With this configuration, a stable gain-coupled DFB laser was obtained.

Further, an absorption layer can be formed in the light guide layer,
The absorption coefficient was increased, and stable operation was confirmed. Further, even when the underlying layer has an active layer, a gain-coupled semiconductor laser can be manufactured with good reproducibility.

[Brief description of the drawings]

FIG. 1 is a view showing a manufacturing process of a first embodiment of the present invention.

FIG. 2 is a sectional view of the overall configuration of the device manufactured in the first embodiment of the present invention.

FIG. 3 is a view showing a manufacturing process of a second embodiment of the present invention.

FIG. 4 is a sectional view of the entire configuration of a device manufactured in a second embodiment of the present invention.

FIG. 5 is a cross-sectional view of the overall configuration of a device manufactured in a third embodiment of the present invention.

FIG. 6 is a diagram showing a manufacturing process according to a third embodiment of the present invention.

FIG. 7 is a sectional view of the entire configuration of a conventional example.

FIG. 8 is a view for explaining a conventional absorption type diffraction grating.

FIG. 9 is a diagram for explaining a method of manufacturing a conventional diffraction grating and problems.

[Explanation of symbols]

 1, 21, 41 Substrate 2 InP layer 3, 23, 45 Etching auxiliary layer 4, 25, 46 Grating 6, 27, 28, 48, 49 Absorbing layer (absorbing region) 7 Etching protective layer 8, 10, 29, 31, 42,44 Light guide layer 9,30,43 Active layer 11,32,50 Cladding layer 12,33,51 Cap layer 14 Top of grating 16 Grating groove 22 InGaAsP layer

Claims (16)

[Claims] A step of forming a semiconductor single-layer film having a composition different from that of the first semiconductor or a semiconductor multilayer film partially including a semiconductor layer having a composition different from that of the first semiconductor on the first semiconductor; A step of forming a diffraction grating by etching the single-layer film or the multilayer film; a step of forming a semiconductor film serving as an absorption region on the diffraction grating; and a step of reducing the height of the diffraction grating. Of forming an absorption type diffraction grating. 2. The method according to claim 1, wherein the step of forming the semiconductor film serving as the absorption region on the diffraction grating is a step of uniformly forming the semiconductor film serving as the absorption region on the diffraction grating. Of forming an absorption type diffraction grating. 3. The step of reducing the height of the diffraction grating,
3. The method of forming an absorption type diffraction grating according to claim 2, further comprising a step of forming an etching protection layer for protecting a semiconductor film serving as an absorption region in a groove of the diffraction grating and a step of performing selective etching. 4. The step of reducing the height of the diffraction grating,
3. The method for forming an absorption type diffraction grating according to claim 2, further comprising a step of performing light etching and a step of performing selective etching of a semiconductor film which becomes an absorption region uniformly formed on the diffraction grating. 5. The absorption according to claim 1, wherein the step of forming the semiconductor film serving as an absorption region on the diffraction grating is a step of forming a semiconductor film serving as an absorption region in a groove of the diffraction grating. Method of forming a diffraction grating. 6. The step of reducing the height of the diffraction grating,
6. The method for forming an absorption type diffraction grating according to claim 1, wherein the step of performing selective etching is performed. 7. The step of reducing the height of the diffraction grating,
2. The method according to claim 1, further comprising a step of filling and flattening the groove of the diffraction grating and a step of performing uniform etching.
6. The method for forming an absorption type diffraction grating according to 2 or 5. 8. In a method for manufacturing a gain-coupled DFB laser, a semiconductor single-layer film having a different composition from the first semiconductor or a semiconductor layer having a different composition from the first semiconductor is partially formed on the first semiconductor. Forming a semiconductor multilayer film having: a step of etching the single-layer film or the multilayer film to form a diffraction grating; a step of forming a semiconductor film serving as an absorption region on the diffraction grating; A method of manufacturing a gain-coupled DFB laser, wherein an absorption-type diffraction grating is formed by a formation method having a step of reducing the size. 9. The gain coupling type D according to claim 8, wherein said absorption type diffraction grating is formed in a light guide layer.
Fabrication method of FB laser. 10. The gain coupling type D according to claim 8, wherein said absorption type diffraction grating is formed on an active layer.
Fabrication method of FB laser. 11. The method according to claim 8, wherein the step of forming a semiconductor film to be an absorption region on the diffraction grating is a step of uniformly forming a semiconductor film to be an absorption region on the diffraction grating. 11. The method for producing a gain-coupled DFB laser according to 9 or 10. 12. The step of reducing the height of the diffraction grating includes a step of forming an etching protection layer for protecting a semiconductor film serving as an absorption region in a groove of the diffraction grating, and a step of performing selective etching. A method of manufacturing a gain-coupled DFB laser according to claim 11. 13. The step of reducing the height of the diffraction grating includes a step of performing light etching for forming a semiconductor film which becomes an absorption region uniformly formed on the diffraction grating and a step of performing selective etching. The method of manufacturing a gain-coupled DFB laser according to claim 11, wherein 14. The method according to claim 8, wherein the step of forming a semiconductor film to be an absorption region on the diffraction grating is a step of forming a semiconductor film to be an absorption region in a groove of the diffraction grating. 11. The method for manufacturing a gain-coupled DFB laser according to 10. 15. The gain coupling DF according to claim 8, wherein the step of reducing the height of the diffraction grating is a step of performing selective etching.
Method for manufacturing B laser. 16. The method according to claim 8, wherein the step of reducing the height of the diffraction grating includes a step of filling and flattening a groove of the diffraction grating and a step of performing uniform etching. A gain-coupled DFB according to item 11 or 14.
Laser fabrication method.