Gain-coupled distributed feedback semiconductor laser device and production method therefor Number:7,016,391 from the United States Patent and Trademark Office (PTO) owispatent

Title: Gain-coupled distributed feedback semiconductor laser device and production method therefor

Abstract: The presence or absence and the intensity of refractive index distribution are easily controlled with high reproducibility without depending on the fabricating process accuracy. InGaAs well layers (14a) and (14b), which have a narrow bandgap and a high refractive index, are enclosed by a lower barrier layer (13), an intermediate barrier layer (15), an upper barrier layer (16) and a buried layer (18) of GaAsN-based materials of a wide bandgap. Then, by adjusting the nitrogen crystal mixture ratio of the GaAsN-based materials that constitute the barrier layers (13), (15) and (16) and the buried layer (18), the presence or absence and the intensity of the refractive index distribution are controlled. Thus, the refractive index distribution is easily controlled with high reproducibility without considering the configuration of a diffraction grating (17), a refractive index balance with respect to the buried layer (18) and so on, i.e., without depending on the fabricating process accuracy.

Patent Number: 7,016,391 Issued on 03/21/2006 to Takahashi

---

Inventors:	Takahashi; Koji (Tenri, JP)
Assignee:	Sharp Kabushiki Kaisha (Osaka, JP)
Appl. No.:	221363
Filed:	February 7, 2001
PCT Filed:	February 7, 2001
PCT NO:	PCT/JP01/00838
371 Date:	September 12, 2002
102(e) Date:	September 12, 2002
PCT PUB.NO.:	WO01/69735
PCT PUB. Date:	September 20, 2001

Foreign Application Priority Data

---

Mar 13, 2000[JP]

2000-068500

Current U.S. Class:	372/96; 372/45.01; 372/46.01
Current Intern'l Class:	H01S 3/08 (20060101); H01S 5/00 (20060101)
Field of Search:	372/45,46,50,96

---

References Cited [Referenced By]

---

U.S. Patent Documents

5093835	Mar., 1992	Takemoto et al.
5143864	Sep., 1992	Takemoto et al.
5276702	Jan., 1994	Meliga.
5452318	Sep., 1995	Makino et al.
5689123	Nov., 1997	Major et al.
5852625	Dec., 1998	Takahashi.
6151347	Nov., 2000	Noel et al.
6493369	Dec., 2002	Funabashi et al.
6519270	Feb., 2003	Kim et al.
6574256	Jun., 2003	Hofstetter et al.
Foreign Patent Documents
0 513 745	Nov., 1992	EP.
60-102788	Jun., 1985	JP.
5-29705	Feb., 1993	JP.
5-136527	Jun., 1993	JP.
5-145169	Jun., 1993	JP.
5-183236	Jul., 1993	JP.
6-7624	Jan., 1994	JP.
6-164051	Jun., 1994	JP.
7-45907	Feb., 1995	JP.
8-8394	Jan., 1996	JP.
8-37342	Feb., 1996	JP.

Other References

Kitatani et al; "Characterization of the Refractive Index of Strained GaInNAs Layers by Spectroscopic Ellipsometry"; Jpn, J. Appl.. Phys., vol. 37, 1998, pp. 753-757. Luo et al; "Purely Gain-Coupled Distributed Feedback Semiconductor Lasers"; Appl. Phys. Lett., vol. 56, No. 17, Apr. 23, 1990, pp. 1620-1622. Duling et al; "Time-Dependent Semiclassical Theory of Gain-Coupled Distributed Feedback Lasers"; IEEE Journal of Quantum Electronics, vol. QE-20, No. 10, Oct. 1984, pp. 1202-1207. "Effect of Misorientation on Characteristics of Gain/As/GaAs AQ Grown by MOVPE", Kikkawa et al. "First Order Gain-Coupled GaInAs/GaAs Distributed Feedback Laser Diodes Patterned by Focused Ion Beam Implantation", Orth et al., Appl. Phys. Lett., vol. 69, No. 13, Sep. 23, 1996, pp. 1906-1908. "Minimum Feature Sizes and Ion Beam Profile for a Focused Ion Beam System With Post-Objective Lens Retarding and Acceleration Mode", Kieslich et al., J. Vac. Sci. Technol., vol. 12, No. 6, Nov./Dec. 1994, pp. 3518-3522. "Gain-Coupled DFB Lasers for Spectroscopic Application", Tohmon et al., Proceedings of the SPIE, vol. 3537, Nov. 1998, pp. 96-105.

Primary Examiner: Epps; Georgia
Assistant Examiner: Monbleau; Davienne
Attorney, Agent or Firm: Nixon & Vanderhye P.C.

---

Claims

---

The invention claimed is:

1. A gain-coupled distributed-feedback type semiconductor laser device having a laminate structure and comprising:

a first semiconductor layer containing at least a Group V element(s) other than nitrogen, the first semiconductor layer having a periodic structure and emitting induced emission light; and

a second semiconductor layer comprising both nitrogen and a Group V element(s) other than nitrogen, wherein the nitrogen crystal mixture ratio in the second semiconductor layer is larger than the nitrogen crystal mixture ratio in the first semiconductor layer,

the second semiconductor layer having a refractive index approximately equal to a refractive index of the first semiconductor layer and a bandgap wider than a bandgap of the first semiconductor layer,

and wherein the periodic structure of the first semiconductor layer contacts the second semiconductor layer is buried flat in the second semiconductor layer.

2. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 1, wherein the first semiconductor layer contains no nitrogen.

3. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 1, wherein a laminate structure, which includes the first semiconductor layer and the second semiconductor layer, has a refractive index coupling coefficient k_i of not greater than 5 cm^-1.

4. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 1, wherein the second semiconductor layer contains indium and/or antimony at a prescribed crystal mixture ratio.

5. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 1, wherein the first semiconductor laser is formed while being crystallinically grown on a surface whose plane index is (100) or a surface crystallographically equivalent to (100) plane, and the periodic structure is formed in a [010] direction or a [00-1] direction or a direction crystallographically equivalent to the [010] and/or [00-1] direction.

6. A gain-coupled distributed-feedback type semiconductor laser device having a laminate structure and comprising:

a first semiconductor layer having a periodic structure;

a second semiconductor layer comprising both nitrogen and a Group V element(s) other than nitrogen, wherein the nitrogen crystal mixture ratio in the second semiconductor layer is larger than the nitrogen mixture ratio in the first semiconductor layer,

the second semiconductor layer having a refractive index approximately equal to a refractive index of the first semiconductor layer and a bandgap wider than a bandgap of the first semiconductor layer;

a third layer, in non-periodic form, for generating induced emission light,

wherein the first semiconductor layer is located in the vicinity of one surface of the third layer, contains at least a Group V element(s) other than nitrogen and absorbs induced emission light generated from the third layer, and the second semiconductor layer is provided closely to and contacts the first semiconductor layer.

7. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 6, wherein the first semiconductor layer contains no nitrogen.

8. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 6, wherein a laminate structure, which includes the first semiconductor layer and the second semiconductor layer, has a refractive index coupling coefficient k_i of not grater than 5 cm^-1.

9. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 6, wherein the second semiconductor layer contains indium and/or antimony at a prescribed crystal mixture ratio.

10. The gain-coupled distributed-feedback type semiconductor laser device as claimed in claim 6, wherein the first semiconductor layer is formed while being crystallinically grown on a surface whose plane index is (100) or a surface crystallographically equivalent to (100) plane, and the periodic structure is formed in a [010] direction or a [00-1] direction or a direction crystallographically equivalent to the [010] or [00-1] direction.

---

Description

---

This application is the US national phase of international application PCT/JP01/00838 filed Jul. 2, 2001, which designated the US.

TECHNICAL FIELD

The present invention relates to a gain-coupled distributed-feedback type semiconductor laser device (hereinafter referred to simply as a GC-DFB-LD (Gain-Coupled Distributed-FeedBack Laser Diode)) which uses distributed feedback by gain coupling and a fabricating method therefor.

BACKGROUND ART

The GC-DFB-LD has various excellent features such as a satisfactory single longitudinal mode property and a resistance to return light inductive noise.

In order to effectuate gain coupling, mainly two methods have been proposed and their characteristics have been reported. As disclosed in a plurality of reports including Japanese Patent Laid-Open Publication No. SHO 60-102788 (first prior art example), a first method is to periodically change the optical gain in an active layer by periodically arranging the active layer itself of a semiconductor laser or providing the active layer itself with a periodic structure (gain diffraction grating). Moreover, as disclosed in a plurality of reports including Japanese Patent Publication No. HEI 6-7624 (second prior art example), a second method is to periodically change a mode gain by periodically arranging an optical absorption layer (absorptive diffraction grating) in the vicinity of the active layer of a semiconductor laser.

The semiconductor laser structures disclosed in the first prior art example and the second prior art example, which are the basic structures for periodically changing the gain, also have a refractive index periodically changed with the gain. That is, in the aforementioned prior art examples, gain coupling and refractive index coupling are intermixed. Therefore, the structures cannot make the best use of the excellent original performance of the gain coupling.

Further, a plurality of reports including Japanese Patent Laid-Open Publication No. HEI 5-136527 (third prior art example) discloses a structure, in which the periodic change of refractive index is canceled in the gain diffraction grating represented by the first prior art example. The structure of the essential part of the GC-DFB-LD disclosed in the third prior art is as shown in a longitudinal cross section in FIG. 19.

In FIG. 19, the material and the layer thickness of each layer are as follows.

• Lower clad layer 1: n-type InP; 0.45 μm
• Semiconductor layer 2: n-type InGaAsP; 0.2 μm
• Buffer layer 3: n-type InP; 10 nm
• Active layer 4: i (intrinsic)-InGaAsP; 0.1 μm
• Guide layer 5: p-type InP; 1.2 μm
• Upper clad layer 6: p-type InP; 1.2 μm

This structure is obtained by laminating the lower clad layer 1 and the semiconductor layer 2 on an InP substrate through first-time crystal growth, thereafter forming a diffraction grating shaped corrugated configuration 7 on the surface of the semiconductor layer 2 by a two-beam interference exposure method and an etching technique and laminating the layers of the buffer layer 3 up to the upper clad layer 6 on the semiconductor layer 2 through second-time crystal growth.

In this case, the active layer 4 has a periodic structure under the influence of the corrugated configuration 7 of the semiconductor layer 2 of its groundwork, and this modulates the gain, causing gain coupling. On the other hand, refractive index distribution is increased in order of the guide layer 5, the semiconductor layer 2 and the active layer 4 by material selection. As a result, in a region A-A′ in FIG. 19, the active layer 4 of a large refractive index has a large volume, and the guide layer 5 of a small refractive index also has a large volume. Therefore, the large refractive index of the active layer 4 is canceled. On the other hand, in a region B-B′, the active layer 4 of a large refractive index has a small volume, and the guide layer 5 of a small refractive index accordingly has a small volume. Thus, by controlling the corrugated configuration 7 of the semiconductor layer 2, the post-burial configurations of the active layer 4 and the guide layer 5, which bury it, and the refractive indexes of the layers, it is enabled to achieve a balance so that an equivalent refractive index becomes constant not only in the regions A-A′ and B-B′ but also in arbitrary regions. Thus, a GC-DFB-LD, which substantially has no refractive index coupling, can be obtained.

Furthermore, a plurality of reports including Japanese Patent Publication No. HEI 8-8394 (fourth prior art example) and Japanese Patent Laid-Open Publication No. HEI 5-29705 (fifth prior art example) disclose structures for canceling the periodic change of refractive index in the absorptive layer in an absorptive diffraction grating represented by the second prior art example.

In the aforementioned third through fifth prior art examples, a refractive index perturbation caused by the provision of the corrugated configuration in the active layer or the absorptive layer is canceled by providing an anti-phase refractive index perturbation in the neighborhood. The GC-DFB-LD, which substantially contains no refractive index coupling component, is called the intrinsic GC-DFB-LD.

However, the aforementioned third through fifth prior art examples have the following problems. That is, in the aforementioned third through fifth prior art examples, the refractive index distribution is canceled by the well-balanced provision of the anti-phase refractive index distribution for the active layer or the absorptive layer having the corrugated configuration. This theoretically requires an extremely high processing accuracy in controlling the corrugated configurations of the gain diffraction grating and the absorptive diffraction grating as well as the burial configuration of the buried layer. That is, the perturbation of the equivalent refractive index disadvantageously largely changes even with a little change in the diffraction grating and burial configurations, and this disadvantageously puts the refractive index distribution canceling balance into disorder.

Moreover, a plurality of devices are normally collectively fabricated in a wafer. In the above case, it is difficult to avoid the influences of the variations in shape of the diffraction gratings in the wafer and the variations in shape occurring every production lot, and a thorough process management is indispensable in order to obtain the intrinsic GC-DFB-LD.

DISCLOSURE OF THE INVENTION

Accordingly, the object of this invention is to provide a GC-DFB-LD capable of easily controlling the presence or absence of and the intensity of a refractive index distribution with high reproducibility without depending on the fabricating process accuracy, and a fabricating method therefor.

In order to achieve the above object, there is provided a gain-coupled distributed-feedback type semiconductor laser device having a prescribed refractive index and a prescribed bandgap, the device comprising:

a first layer having a periodic structure; and

a second layer, which has a refractive index approximately equal to a refractive index of the first layer and a bandgap wider than a bandgap of the first layer and in which the periodic structure of the first layer is buried flat.

According to the above-mentioned construction, the periodic structure is formed in the first layer that has a bandgap narrower than that of the second layer, and therefore, gain coupling is provided when the first layer is made to function as a light-emitting layer or an absorptive layer. On the other hand, the first layer and the second layer, in which the periodic structure of this first layer is buried flat, have approximately equal refractive indexes, and therefore, no refractive index coupling component is owned. That is, the GC-DFB-LD of this invention functions as an intrinsic GC-DFB-LD. Furthermore, the periodic structure of the first layer, which is merely buried flat in the second layer, is therefore formed without utterly depending on the fabricating process accuracy.

In one embodiment of the present invention, a laminate structure, which includes the first layer and the second layer, has a refractive index coupling coefficient κ_i of not greater than 5 cm^-1.

According to the above-mentioned construction, the refractive index coupling coefficient κ_i of the laminate structure that includes the first layer and the second layer, in which the periodic structure of this first layer is buried flat, is not greater than 5 cm^-1. Therefore, the influence of the refractive index coupling component is sufficiently small, and the refractive indexes of both the layers are considered to be approximately equal to each other.

In one embodiment of the present invention, the first layer emits induced emission light, and

the second layer is provided closely to the first layer and contains nitrogen.

According to the above-mentioned construction, the second layer, which has a bandgap wider than that of the first layer and is transparent with respect to the induced emission light from the first layer, is provided closely around the periphery of the first layer, which has the periodic structure and emits the induced emission light. Thus, the confinement of carriers in the first layer is efficiently achieved. Furthermore, the periodic structure of the second layer is formed closely to the first layer. Therefore, by adjusting the nitrogen crystal mixture ratio of the second layer, the intensity of the periodic change of the equivalent refractive index in the laminate structure that includes the first and second layers can be controlled. Therefore, in the gain diffraction grating type GC-DFB-LD, the intensity of the periodic change of the refractive index can easily be controlled with high reproducibility without considering the shape and so on of the first and second layers, i.e., without depending on the fabricating process accuracy.

One embodiment of the present invention further comprises:

a third layer for generating induced emission light,

the first layer being located in the vicinity of one surface of the third layer, absorbing the induced emission light generated from the third layer, and

the second layer being provided closely to the first layer and containing nitrogen.

According to the above-mentioned construction, the second layer, which has a bandgap wider than that of the first layer and is transparent with respect to the induced emission light from the third layer, is provided closely around the periphery of the first layer, which has the periodic structure and emits the induced emission light. Thus, the periodic structure of the second layer is formed. Therefore, by adjusting the nitrogen crystal mixture ratio of the second layer, the intensity of the periodic change of the equivalent refractive index in the laminate structure that include the first and second layers can be controlled. Therefore, in the absorptive diffraction grating type GC-DFB-LD, the intensity of the periodic change of the refractive index can easily be controlled with high reproducibility without considering the shape and so on of the first and second layers, i.e., without depending on the fabricating process accuracy.

In one embodiment of the present invention, the multi-layer structure constructed of the first layer and the second layer has a flat surface.

According to the above-mentioned construction, the periodic structure of the first layer is buried in the second layer, and the surface of the multi-layer structure constructed of the first layer and the second layer is a flat surface. Therefore, the intensity of the periodic change of the refractive index set by adjusting the nitrogen crystal mixture ratio is not changed by the surface configuration of the multi-layer structure.

In one embodiment of the present invention, the second layer contains at least one of indium and antimony at a prescribed crystal mixture ratio. According to the above-mentioned construction, the second layer contains at least one of indium and antimony at a prescribed crystal mixture ratio. Therefore, a change in the lattice constant occurring when the refractive index is adjusted by mixing the second layer crystallinically with an appropriate amount of nitrogen is canceled by the crystallinic mixture of In or Sb. Thus, an intrinsic GC-DF B-LD, which has more excellent characteristics, is obtained.

In one embodiment of the present invention, the first layer is formed while being crystallinically grown on a surface whose plane direction extends in a (100) plane or a surface crystallographically equivalent to the (100) plane, and

• the periodic structure is formed in a [010] direction or a [00-1] direction or a direction crystallographically equivalent to the [010] or [00-1] direction.

According to the above-mentioned construction, when the second layer mixed crystallinically with nitrogen is crystallinically grown on the periodic structure of the first layer, the crystallinic mixture of nitrogen in the growing layer becomes uniform without receiving the influence of the corrugation of the groundwork, and the control of the refractive index coupling coefficient is performed more accurately. Thus, an intrinsic GC-DFB-LD, which has more excellent characteristics, is obtained.

There is also provided a gain-coupled distributed-feedback type semiconductor laser device fabricating method comprising the steps of:

forming a first layer having a periodic structure of a group III-V compound semiconductor; and

forming a second layer of a group III-V compound semiconductor, which has a bandgap wider than that of the first layer and contains nitrogen, so that the periodic structure of the first layer is buried flat.

According to the above-mentioned construction, the first layer, which has the periodic structure, is formed of the group III-V compound semiconductor, therefore functions as a light-emitting layer when placed between the p/n reverse conducting type clad layers or functions as an absorptive layer when buried in a p-type or n-type clad layer. Then, the second layer, which has a wider bandgap and is constructed of the group III-V compound semiconductor, is formed closely to the light-emitting layer or the absorptive layer that has this periodic structure. Therefore, by forming the periodic structure in the second layer and adjusting the nitrogen crystal mixture ratio of the second layer, the intensity of the periodic change of the equivalent refractive index in the laminate structure including the first and second layers can be controlled. Thus, the intensity of the periodic change of the refractive index can easily be controlled with high reproducibility without depending on the fabricating process accuracy.

In one embodiment of the present invention, the second layer is formed by being crystallinically grown at a growth rate of not higher than 1 μm/hour.

According to the above-mentioned construction, the second layer, in which the periodic structure of the first layer is buried flat, is crystallinically grown at a growth rate of not higher than 1 μm/hour. With this arrangement, the surface diffusion of the raw material seed that includes nitrogen is sufficiently performed in the second layer, and the periodic distribution of the nitrogen crystal mixture ratio attributed to the periodic structure disappears. Therefore, the refractive index distribution in the second layer becomes more uniform. Furthermore, the flattening of the surface of the second layer is promoted, and the periodic structure of the first layer is buried more flatly.

In one embodiment of the present invention, the second layer has a refractive index approximately equal to a refractive index of the first layer.

According to the above-mentioned construction, the periodic structure is formed in the first layer, which functions as the light-emitting layer or the absorptive layer, providing gain coupling. On the other hand, the first layer and the second layer located closely to the first layer have refractive indexes approximately equal to each other, and therefore, no refractive index coupling component is owned. That is, an intrinsic GC-DFB-LD is easily formed with high reproducibility without depending on the fabricating process accuracy.

In one embodiment of the present invention, a laminate structure, which include the first layer and the second layer, has a refractive index coupling coefficient κ_i of not greater than 5 cm^-1.

According to the above-mentioned construction, the periodic structure is formed in the first layer, which functions as the light-emitting layer or the absorptive layer, providing gain coupling. On the other hand, the refractive index coupling coefficient κ_i of the laminate structure that includes the first and second layers is not greater than 5 cm^-1. Therefore, the refractive indexes of both the layers are considered to be approximately equal to each other, and the influence of the refractive index coupling component becomes sufficiently small. That is, an intrinsic GC-DFB-LD is easily formed with high reproducibility without depending on the fabricating process accuracy.

In one embodiment of the present invention, the second layer has a refractive index set by adjusting the crystal mixture ratio of nitrogen.

According to the above-mentioned construction, the refractive index of the second layer is easily controlled with high reproducibility merely by adjusting the nitrogen crystal mixture ratio of the second layer and is allowed to be set approximately equal to the refractive index of the first layer.

In one embodiment of the present invention, the first layer is formed by being crystallinically grown on a surface whose plane direction extends in a (100) plane or a surface crystallographically equivalent to the (100) plane, and forming the periodic structure in a [010] direction or a [00-1] direction or a direction crystallographically equivalent to the [010] or [00-1] direction.

According to the above-mentioned construction, when the second layer mixed crystallinically with nitrogen is crystallinically grown on the periodic structure of the first layer, the nitrogen crystal mixture ratio in the growing layer becomes uniform without receiving the influence of the corrugation of the groundwork, and the control of the refractive index coupling coefficient is performed more accurately. That is, the intensity of the periodic change of the refractive index can easily be controlled with high reproducibility and controllability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a GC-DFB-LD of this invention;

FIGS. 2A, 2B and 2C are perspective views of a laminate structure during the forming processes of the GC-DFB-LD shown in FIG. 1;

FIG. 3 is a graph showing the temperature dependency of the oscillation wavelength of the GC-DFB-LD shown in FIG. 1;

FIG. 4 is graph showing the temperature dependency of the oscillation wavelength of the GC-DFB-LD of a first comparative example;

FIGS. 5A and 5B are graphs showing changes in refractive index and bandgap when GaAs is mixed crystallinically with In or N;

FIGS. 6A and 6B are graphs showing a band diagram and an equivalent refractive index distribution in the resonator direction of the GC-DFB-LD of FIG. 1 or the first comparative example;

FIG. 7 is a graph showing a correlation between the nitrogen crystal mixture ratio and Δn_eq in the resonator of the GC-DFB-LD shown in FIG. 1;

FIGS. 8A, 8B and 8C are longitudinal sectional views of an essential part located in the vicinity of the active layers of the GC-DFB-LD's of the first through third modification examples;

FIGS. 9A, 9B and 9C are longitudinal sectional views of an essential part located in the vicinity of the active layer of the GC-DFB-LD of a fourth modification example;

FIG. 10 is a longitudinal sectional view of a GC-DFB-LD different from that of FIG. 1;

FIGS. 11A, 11B and 11C are perspective views of a laminate structure during the forming processes of the GC-DFB-LD shown in FIG. 10;

FIG. 12 is a longitudinal sectional view of an essential part located in the vicinity of the diffraction grating of a GC-DFB-LD different from those of FIG. 1 and FIG. 10;

FIG. 13 is a longitudinal sectional view of an essential part located in the vicinity of the diffraction grating of a GC-DFB-LD different from those of FIG. 1, FIG. 10 and FIG. 12;

FIG. 14 is a longitudinal sectional view of an essential part located in the vicinity of the diffraction grating of a GC-DFB-LD different from those of FIG. 1, FIG. 10, FIG. 12 and FIG. 13;

FIG. 15 is a longitudinal sectional view of an essential part located in the vicinity of the diffraction grating of a GC-DFB-LD different from those of FIG. 1, FIG. 10, and FIG. 12 through FIG. 14;

FIG. 16 is a longitudinal sectional view of an essential part located in the vicinity of the diffraction grating of a GC-DFB-LD different from those of FIG. 1, FIG. 10, and FIG. 12 through FIG. 15;

FIG. 17 is a longitudinal sectional view of an essential part located in the vicinity of the diffraction grating of a GC-DFB-LD different from those of FIG. 1, FIG. 10, and FIG. 12 through FIG. 16;

FIG. 18 is a perspective view of a laminate structure obtained after a diffraction grating is engraved during the forming process of the GC-DFB-LD shown in FIG. 17; and

FIG. 19 is a longitudinal sectional view of an essential part of a prior art GC-DFB-LD.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail below on the basis of the embodiments shown in the drawings.

First Embodiment

The present embodiment is related to a gain diffraction grating type GC-DFB-LD characterized in that an intrinsic GC-DFB-LD is obtained by forming a transparent layer, which is located adjacent to well layers (light-emitting layers) formed periodically and mixed crystallinically with a small amount of nitrogen.

FIG. 1 schematically shows the cross-sectional structure of the gain diffraction grating type GC-DFB-LD of the present embodiment. The structure, the material and the layer thickness of each portion are as follows.

• Substrate 11:
  • n-type GaAs; 100 μm
• Lower clad layer 12:
  • n-type Al_0.3Ga_0.7As; 1.0 μm
• Lower barrier layer 13:
  • i-GaAs_0.9952N_0.0048; 70 nm (thickest portion)
• Well layer 14:
  • i-In_0.2Ga_0.8As; 10 nm
• Intermediate barrier layer 15:
  • i-GaAs_0.9952N_0.0048; 20 nm
• Upper barrier layer 16:
  • i-GaAs_0.9952NO_0.0048; 20 nm
• Buried layer 18:
  • i-GaAs_0.9952N_0.0048; 50 nm (thinnest portion)
• Upper clad layer 19:
  • p-type Al_0.3Ga_0.7As; 1.0 μm
• Contact layer 20:
  • p⁺-GaAs; 0.5 μm
• P-electrode metal 21:
  • AuZn
• N-electrode metal 22:
  • AuGe/Ni

The GC-DFB-LD 10, which has the above-mentioned structure, is formed as follows. FIGS. 2A-2C are perspective views of a laminate structure during the forming process of the GC-DFB-LD 10 shown in FIG. 1. The method for fabricating the GC-DFB-LD 10 shown in FIG. 1 will be described below with reference to FIG. 2.

First of all, as shown in FIG. 2A, the layers of the lower clad layer 12 to the upper barrier layer 16 are successively laminated on an n-type GaAs substrate 11 through first-time crystal growth using the metal-organic vapor deposition method. In the above case, the (100) plane of the n-type GaAs substrate 11 is used. According to the metal-organic vapor deposition method, there were used trimethylaluminum, trimethylgallium, trimethylindium, arsine and dimethylhydrazine as the raw materials of Al, Ga, In, As and N, respectively. The laminate structure, which has thus undergone the first-time crystal growth, is taken out of the crystal growth chamber, and a grating-shaped photoresist mask (not shown) having a cycle of 0.28 μm and a duty ratio of 0.5 is formed on its surface by the two-beam interference exposure method.

Next, by an etchant obtained by mixing hydrochloric acid with a hydrogen peroxide aqueous solution at a ratio of 1:50 and diluting the mixture with five parts of pure water, regions where the photoresist mask is not formed are etched by a thickness of 75 nm from the surface. In the above case, the upper barrier layer 16 has a film thickness of 20 nm, the well layers 14a and 14b have a film thickness of 10 nm and the intermediate barrier layer 15 has a film thickness of 20 nm. Therefore, a total film thickness of the upper barrier layer 16 to the lower well layer 14a is 60 nm. Therefore, when the photoresist mask is removed, as shown in FIG. 2B, a diffraction grating 17, in which the two layers of the well layers 14a and 14b are periodically divided in the direction in which the n-type GaAs substrate 11 extends, is obtained.

Next, the laminate structure shown in FIG. 2B is put again into the crystal growth chamber, and the layers of the buried layer 18 to the contact layer 20 are grown on the diffraction grating 17 through second-time crystal growth as shown in FIG. 2C. In the above case, the buried layer 18 is required to undergo the crystal growth by selecting the crystal growth conditions so that the nitrogen distribution in the buried layer 18 becomes uniform and an interface between the buried layer 18 and the upper clad layer 19 becomes flat. The laminate structure, which has thus undergone the second-time crystal growth, is taken out of the crystal growth chamber, and a current constriction layer 23 of silicon nitride is formed on its surface as shown in FIG. 2C. A stripe-shaped aperture 24 of a width W (=5 μm) is formed in a direction orthogonal to the direction in which the diffraction grating 17 extends by the normal photolithography and wet etching. Finally, as shown in FIG. 2C, the p-electrode 21 is formed on the upper surface of the laminate structure, and the n-electrode 22 is formed on the lower surface. Then, by cleaving the laser light-emitting end surface, the gain diffraction grating type GC-DFB-LD 10 is obtained.

As the result of flowing a current through the p-electrode 21 and the n-electrode 22 of the gain diffraction grating type GC-DFB-LD 10 obtained as described above, oscillation occurred at a single wavelength of 980 nm at a threshold current density of 0.5 kA/cm². FIG. 3 shows the temperature dependency of the oscillation wavelength of this GC-DFB-LD 10. In FIG. 3, it can be understood that the oscillation occurs at a complete single wavelength at a device temperature of -20° C. to +80° C. at a sub-mode suppression ratio of not lower than 20 dB in an identical longitudinal mode (m(0)). Moreover, it was discovered that no stop band was observed in the oscillation spectrum and the refractive index coupling component was zero.

Moreover, as the result of examining the oscillation spectrum of a plurality of gain diffraction grating type GC-DFB-LD's 10 obtained from one wafer, it was discovered that the single wavelength oscillation occurred with a probability of 98% despite the fact that no antireflection coating was provided on the laser light-emitting end surface, and this showed that the single wavelength laser fabrication yield was very high. These features are the characteristics peculiar to the intrinsic GC-DFB-LD that includes no refractive index coupling component.

FIRST COMPARATIVE EXAMPLE

There was fabricated a gain diffraction grating type GC-DFB-LD 10, in which the materials of only the lower barrier layer 13, the intermediate barrier layer 15, the upper barrier layer 16 and the buried layer 18 were changed from i-GaAsN to i-GaAs and the materials of other layers were made quite same on the basis of the structure of the GC-DFB-LD of the first embodiment.

The GC-DFB-LD of this first comparative example also oscillated at a single wavelength of 980 nm at a threshold current density of 0.5 kA/cm². FIG. 4 shows the typical example of the temperature dependency of the oscillation wavelength of this GC-DFB-LD. In FIG. 4, oscillation occurs in the single longitudinal mode (m(0)) at a device temperature of +10° C. to +50° C. However, when the device temperature was set lower than +10° C. or higher than +50° C., there were observed a mode hop that the oscillation shifts to oscillation in another adjacent longitudinal mode (m(+1), m(-1)) and oscillation in a Fabry-Perot mode (f-p), and instability of the oscillation wavelength occurred. Moreover, it was discovered that a stop band existed in the oscillation spectrum, and the gain coupling and the refractive index coupling coexisted to cause the instability of the oscillation wavelength.

Moreover, as the result of examining the oscillation spectrum of a plurality of GC-DFB-LD's for comparison use obtained from one wafer, it was discovered that the single wavelength oscillation occurred with a probability of 65% at a room temperature in a state in which no antireflection coating was provided on the laser light-emitting end surface. It was discovered that the single wavelength laser fabrication yield was low in comparison with the case of the first embodiment where the transparent layer with which a small amount of nitrogen was crystallinically mixed was formed adjacently to the well layer.

The operation and effects of the first embodiment will be described in detail below according to the first embodiment and the first comparative example.

The present first embodiment is characterized in that the barrier layer, which has a wide bandgap and is located adjacent to the InGaAs well layer 14, which is divided periodically into parts like the diffraction grating 17, is constructed while being mixed crystallinically with a minute amount (0.48% in the first embodiment) of nitrogen. That is, by mixing the barrier layer crystallinically with a minute amount of nitrogen, the refractive index is adjusted to a prescribed value without largely changing the bandgap. This fact will be described with reference to FIGS. 5A, 5B and FIGS. 6A, 6B.

FIG. 5A shows a state of change in the refractive index and the bandgap when GaAs is mixed crystallinically with In. FIG. 5B shows a state of change in the refractive index and the bandgap when GaAs is mixed crystallinically with N. FIG. 5A and FIG. 5B show that In_0.2Ga_0.8As (point "a" in FIG. 5A) employed for the well layer 14 and GaAs_0.9952N_0.0048 (point "b" in FIG. 5B) employed for the barrier layers 13, 15 and 16 and the buried layer 18 located adjacent to the well layer 14 have same refractive index in the first embodiment. In contrast to this, with regard to the bandgap, it can be understood that the GaAs_0.9952N_0.0048 employed for the barrier layers 13, 15 and 16 and the buried layer 18 has a bandgap of about 1.35 eV from FIG. 5B and is transparent with respect to the emission wavelength of 980 nm from the well layer 14. That is, as is apparent from FIG. 6A that shows the band diagram and the distribution of the refractive index n_eq in the direction of the resonator (in the direction of X-X′ in FIG. 1), the active region in the first embodiment has no periodic structure in the equivalent refractive index n_eq despite the fact that it has a periodic structure in the bandgap.

That is, the well layer 14, in which the bandgap is narrow, is enclosed by the barrier layers 13, 15 and 16 and the buried layer 18, which are transparent and in which the bandgap is wide. Therefore, the barrier layers 13, 15 and 16 and the buried layer 18 all have the same refractive index in spite of the gain diffraction grating whose gain regions are periodically formed, and the perturbation of equivalent refractive index does not occur at all.

Therefore, in the first embodiment, there is no need for considering the configuration of the diffraction grating 17, the balance of refractive index with respect to the buried layer 18 and so on, dissimilarly to the case of the conventional structure in which the refractive index perturbation of the active layer is canceled by the anti-phase refractive index perturbation provided in the neighborhood. No refractive index coupling substantially occurs so long as the diffraction grating 17 is buried flat, and the intrinsic GC-DEB-LD can easily be obtained.

In contrast to this, in the first comparative example, a layer 18′, which is located around a well layer 14′ of a narrow bandgap, is not mixed crystallinically with nitrogen. Therefore, as shown in FIG. 6B, the equivalent refractive index n_eq also periodically changes in synchronization with the periodic change of the band structure in the active region.

Normally, the group III-V compound semiconductor has such a relation that the refractive index becomes high when the bandgap becomes narrow. Moreover, the rate of change in the above case is on the same level substantially not depending on the material system. For example, in InGaAs-based mixed crystals, the rate of change of the refractive index due to a change in the bandgap when the crystal mixture ratio of In is changed is about 0.4 [per eV]. Also, in the case of AlGaAs-based mixed crystals, the rate of change of the refractive index when the crystal mixture ratio of Al is changed is about 0.4 [per eV].

Therefore, when a gain diffraction grating is produced with these material systems and the heterojunction of them as in the case of the first comparative example, the refractive index of the active layer (well layer) portion, in which the bandgap is the narrowest, comes to have the highest refractive index, and the refractive index becomes low in the surrounding barrier layer portion, in which the bandgap is comparatively wide, meaning that the refractive index perturbation cannot be avoided.

However, in the first embodiment, the barrier layers 13, 15 and 16 and the buried layer 18 are slightly mixed crystallinically with nitrogen. In this GaAsN mixed crystal system, the rate of change of the refractive index when the nitrogen crystal mixture ratio is changed is about 1.4 [per eV], and this means that the rate of change of the refractive index is several times greater than that of the other material systems. That is, by slightly mixing GaAs crystallinically with N, as shown in FIG. 5B, the GaAsN mixed crystal system can be regarded as a special mixed crystal system, which can obtain a material of a very high refractive index while maintaining the bandgap comparatively wide since the influence of an increase in the refractive index is large although the bandgap is slightly reduced. The first embodiment positively utilizes the rate of change of the refractive index peculiar to the material with which a small amount of nitrogen is crystallinically mixed. By enclosing the InGaAs well layer that has a narrow bandgap and a high refractive index with the GaAsN-based material that has a wide bandgap and adjusting the nitrogen crystal mixture ratio of the GaAsN-based material, the structure of a diffraction grating in which no refractive index distribution occurs is obtained.

If crystal growth is performed under inappropriate crystal growth conditions when the buried layer 18 mixed crystallinically with nitrogen is crystallinically grown on the diffraction grating 17 in the structure of a gain diffraction grating type GC-DFB-LD 10 as shown in FIG. 1, then a nitrogen crystal mixture ratio distribution sometimes occurs in accordance with the corrugated configuration of the diffraction grating 17. This is attributed to the fact that the plane direction dependency of the groundwork crystals is large with respect to the incorporation rate of nitrogen into the crystalline film during the crystal growth. That is, the crystallographic plane direction differs between the flat portions of the ridge and groove and the inclined portions exposed by etching of the corrugated configuration of the diffraction grating 17 shown in FIG. 1. If the layer mixed crystallinically with nitrogen is crystallinically grown on the groundwork that has different plane directions as described above, then a periodic distribution occurs in the nitrogen crystal mixture ratio depending on the plane directions of the groundwork.

In more concrete, nitrogen is easily taken into the surface inclined from the (100) plane in the [011] direction and hardly taken into the surface inclined in the [01-1] direction. Therefore, the nitrogen crystal mixture ratio distribution periodically occurs in the buried layer 18 located on the diffraction grating 17 where the plurality of surfaces periodically appear repetitively. As described above, if a nitrogen crystal mixture ratio distribution occurs in the buried layer 18, then a refractive index distribution, which coincides with the cycle of the corrugated configuration of the diffraction grating 17 that is the groundwork, occurs inside the buried layer 18. In the above case, the object of the present embodiment to obtain a diffraction grating structure in which no refractive index distribution occurs by adjusting the nitrogen crystal mixture ratio cannot be achieved.

The present inventor discovered that setting the growth rate during the crystal growth to a sufficiently slow rate of not higher than 1 μm/hour was effective in order to eliminate the nitrogen crystal mixture ratio distribution during the grown crystal according to the corrugated configuration of the groundwork crystals as described above. This is presumably ascribed to the fact that the surface diffusion of the raw material seed supplied during the crystal growth sufficiently occurs when the growth rate is sufficiently slow and the atoms that constitute the crystals are sufficiently mixed randomly.

As described above, to sufficiently cause the surface diffusion of the raw material seed by sufficiently slowing down the growth rate produces an effect also in promoting the flattening of the upper surface of the buried layer 18. That is, the crystal growth progresses in turn before the surface diffusion of the raw material seed sufficiently occurs if the growth rate is too fast, and therefore, the flattening is not promoted with the initial corrugation maintained also after the crystal growth. In order to promote this flattening, the sufficiently slow growth rate of not higher than 1 μm/hour is essential.

The present embodiment can be achieved by developing a technique for growing crystals, which have a uniform nitrogen crystal mixture ratio distribution on the groundwork that has a corrugated configuration and have sufficiently flattened surfaces. The above-mentioned fact holds not only in the case of the GaAsN material of the present embodiment but also in other material systems described in connection with the following embodiments. Moreover, the same thing can be said for other crystal growth methods, other kinds of raw materials and so on.

It is often the case where GaAs is employed as a barrier layer for the InGaAs well layer. In the first embodiment, GaAsN, which has a slightly narrower bandgap, is employed for the barrier layers 13, 15 and 16 and the buried layer 18 in place of GaAs. Accordingly, there is concern about a reduction in the band offset with respect to the well layer. However, since a conduction band discontinuity (ΔE_c) is about 160 meV when GaAs_0.9952N_0.0048 is employed as the barrier layers 13, 15 and 16 and the buried layer 18 for the In_0.2Ga_0.8As well layer 14, and the valence band discontinuity (ΔE_v) is about 50 meV. Therefore, it can be said that the carrier confinement in the well layer 14 is sufficiently achieved. Even in the actual GC-DFB-LD 10, the characteristic temperature indicated a sufficient value of about 110 K.

Moreover, since GaAsN is employed as the barrier layers 13, 15 and 16 and the buried layer 18 in place of GaAs, there appears concern about the occurrence of lattice defect as a consequence of the deviation of the lattice constant of the barrier layer from the lattice constant of the GaAs substrate 11 due to the above fact. However, the deviation of GaAs_0.9952N_0.0048 from GaAs in terms of lattice constant has a small value Δa of about 0.1% at most, and it can be said that the lattice defect does not occur even when GaAsN is employed for the barrier layers 13, 15 and 16 and the buried layer 18 in place of GaAs. The actual GC-DFB-LD 10 had a sufficient device operating life of not shorter than 5000 hours at a device temperature of 80° C. with an output of 10 mW.

The present embodiment is described taking the intrinsic GC-DFB-LD, in which the periodic fluctuation of the equivalent refractive index is zero, as an example. In spite of the description that "the periodic fluctuation of the equivalent refractive index is zero" provided herein, it is a matter of course that the periodic fluctuation of the equivalent refractive index substantially has no problem even if it is not completely zero so long as the periodic fluctuation is small to the extent that the influence of the refractive index coupling on the gain coupling becomes sufficiently small. According to the results of examination conducted by the present inventor, it was discovered that the influence of the refractive index coupling was able to be reduced in the case of the periodic fluctuation of the equivalent refractive index such that the refractive index coupling coefficient κ_i, which represents the degree of refractive index coupling caused by the periodic fluctuation of the refractive index, becomes equal to or smaller than 5 cm^-1. Therefore, as described above, when the same refractive index as that of the well layer 14 is obtained by adjusting the nitrogen crystal mixture ratio of the buried layer 18, it can be said enough to adjust the nitrogen crystal mixture ratio so that the refractive index coupling coefficient κ_i becomes equal to or smaller than 5 cm^-1.

On the other hand, depending on the use of the GC-DFB-LD or the characteristics expected for the GC-DFB-LD, it is sometimes possible to require both of a specified quantity of refractive index coupling and gain coupling instead of the intrinsic GC-DEB-LD that has no refractive index distribution. The first embodiment can also satisfy the above-mentioned requirement. FIG. 7 shows a correlation between the nitrogen crystal mixture ratio (horizontal axis of FIG. 7) in the portions that employ the GaAsN mixed crystal (lower barrier layer 13, intermediate barrier layer 15, upper barrier layer 16, and buried layer 18) and Δn_eq (vertical axis of FIG. 7) that represents the intensity of the periodic change of the equivalent refractive index n_eq inside the resonator in the structure of the gain diffraction grating type GC-DFB-LD 10 shown in FIG. 1. In this case, Δn_eq was defined as a difference (Δn_eq=n_eq(Y)-n_eq(Z)) between the equivalent refractive index n_eq(Y) in the cross section of the protruding portion indicated by Y-Y′ of the diffraction grating 17 and the equivalent refractive index n_eq(Z) in the cross section of the recess portion indicated by Z-Z′ of the diffraction grating 17 in FIG. 1.

Then, the magnitude of the refractive index coupling coefficient κ_i has a strong correlation to the absolute value of this Δn_eq. When the nitrogen crystal mixture ratio is "0.0048", corresponding to the first embodiment, the periodic change Δn_eq of the equivalent refractive index n_eq inside the resonator is "0" and is not accompanied by the refractive index coupling, as is apparent from FIG. 7. In contrast to this, when the nitrogen crystal mixture ratio is "0", corresponding to the first comparative example, the periodic change Δn_eq of the equivalent refractive index n_eq inside the resonator exists while being accompanied by the refractive index coupling.

If the nitrogen crystal mixture ratio of the GaAsN mixed crystals is herein set at an arbitrary value between "0" to "0.0048", then a GC-DFB-LD, the degree of refractive index coupling of which is set at an arbitrary value, can be obtained. In this case, there can be obtained an in-phase type gain diffraction grating, in which the portion of a high equivalent refractive index n_eq coincides with the portion of a high gain. Moreover, if the nitrogen crystal mixture ratio is set at a value greater than "0.0048", then there can be obtained an anti-phase type gain diffraction grating, in which the portion of a high equivalent refractive index n_eq coincides with the portion of a low gain.

As described above, by providing the layer that emits the induced emission light or the layer that absorbs the induced emission light with the periodic structure, providing the layer crystallinically mixed with nitrogen adjacently to the layer and variously setting the nitrogen crystal mixture ratio, a variety of characteristic gain diffraction gratings can be obtained. That is, the characteristics of the GC-DFB-LD can easily be controlled.

In the first embodiment, there has been described the structure in which the InGaAs well layer 14 is periodically divided into parts and thereafter peripherally enclosed by GaAsN as one example of the structure of the gain diffraction grating type GC-DFB-LD in which the barrier layers 13, 15 and 16 and the buried layer 18 are formed of GaAsN and the well layer 14 is formed of InGaAs. However, as the structure of the gain diffraction grating of this invention, the structures described in connection with the following first through fourth modification examples can be provided.

FIRST MODIFICATION EXAMPLE

FIG. 8A shows a longitudinal cross-section of an essential part located in the vicinity of the active layer placed between the upper and lower clad layers in the gain diffraction grating type GC-DFB-LD of the first modification example. In this case, the construction, the material and the film thickness of each portion are as follows.