JPH04146679A - Distributed feedback semiconductor laser device - Google Patents

Abstract

PURPOSE:To enable an InP long wavelength laser to easily manufactured by a method wherein a thin buffer layer is made to grow on the surface of a semiconductor layer rugged corresponding to a diffraction grating so as to partially fill the rugged surface, and an active layer is made to grow thereon. CONSTITUTION:A clad layer 3 and a semiconductor layer 4 are made to grow in crystal on a substrate 1 through a first epitaxial growth stage. Then, through an interference exposure method and a chemical etching method, a rugged form 5 corresponding to a diffraction grating is engraved on the semiconductor layer 4. On the second stage, an N-type InP buffer layer 6 is made to grow on the semiconductor layer 4, and a low impurity concentration In0.53Ga0.47 As active layer 7, a P-type InP clad layer 8, and a high concentration P-type In0.53Ga0.47As contact layer 9 are successively grown to constitute a double hetero-junction structure. In succession, an SiO2 insulating layer 12 is deposited on the upside of the contact layer 9, a stripe-like window is provided, and then electrode layers 11 and 12 are evaporated. Furthermore, the substrate 1 provided with the layers is cleaved into unit semiconductor laser elements. The buffer layer 6 and the active layer 7 are made to grow on the semiconductor layer 4 through an organic metal vapor growth method. At this point, the buffer layer 6 is so formed as not to make the rugged form 5 fully flat, and the left recess of the rugged form 5 is filled flat at the formation of the active layer 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor distributed feedback laser device used as an electro-optical conversion element.

The present invention relates to a long-distance large-capacity optical communication device, an optical information processing device,
Suitable for use as a light source for optical recording devices, optical measurement devices, and local electronic devices.

〔overview〕

The present invention provides a semiconductor distributed feedback laser device in which a diffraction grating is provided in the active layer and distributed light feedback is performed by periodic perturbation of the gain coefficient induced by the grating, in which a semiconductor layer is provided with an uneven shape corresponding to the diffraction grating. By growing a thin buffer layer on the surface so as to partially fill the uneven shape, and then growing an active layer on the surface, the growth conditions for the buffer layer are relaxed, making it particularly suitable for InP-based long wavelength lasers. It provides structure.

[Conventional technology]

Semiconductor distributed feedback laser devices, which generate stimulated emission light by performing distributed feedback of light to the active layer using a diffraction grating provided near the active layer, generally have a relatively simple configuration and generate stimulated emission with excellent oscillation spectrum characteristics. Since light can be obtained, a lot of research and development has been carried out in the past, and long-distance, high-capacity optical communication,
It is expected to be useful as a light source device suitable for use in optical information processing and recording, optical applied measurement, and the like.

Such a semiconductor distributed feedback laser device employs an optical waveguide structure in which the active layer is surrounded by a transparent heterojunction semiconductor layer or the like to efficiently generate stimulated emission light. In particular, a diffraction grating with a triangular wave cross section is formed on the interface of a transparent waveguide layer in close proximity to the active layer on the side far from the active layer, and light distribution is achieved by periodically changing the waveguide refractive index. Research and development toward repatriation is currently underway.

However, in such optical distribution feedback using refractive index coupling, appropriate feedback regarding the optical phase is not performed for light at the Bragg wavelength that is reflected in accordance with the period of layer thickness change of the optical waveguide layer. For this reason, stable laser oscillation cannot be obtained, and there is a high possibility that longitudinal mode oscillations of two wavelengths symmetrically spaced above and below the Bragg wavelength will occur simultaneously. Furthermore, even when only one of these two wavelengths of longitudinal mode oscillation occurs, it is difficult to select in advance which of the two wavelengths to cause longitudinal mode oscillation. However, the accuracy of setting the oscillation wavelength will be significantly impaired.

In other words, in optical distributed feedback using refractive index coupling based on periodic perturbation of the refractive index in the optical waveguide layer, in principle,
The problem of dual-wavelength longitudinal mode oscillation degeneracy arises, and it is difficult to avoid this problem.

Of course, various means for solving such difficulties have been studied in the past. However, in any case, such as a structure in which the phase is shifted by a quarter wavelength at approximately the center of the diffraction grating, the structure of the laser device becomes complicated and a manufacturing process that only requires a reduction in degeneracy is added. Moreover, it was necessary to form an antireflection film on the end face of the laser element.

On the other hand, if distributed optical feedback is performed using refractive index coupling as described above, an oscillation stop band will occur in the Bragg wavelength region, but if distributed optical feedback is performed using gain coupling based on periodic perturbations of the gain coefficient, then the oscillation stop band will be generated in the Bragg wavelength region. The fundamental theory that states that complete single-wavelength longitudinal mode oscillation should be obtained without the appearance of any 23rd
Pages 27 to 2335 (“Couplecl-Wa
veTheory of Distributed
Feedback La5ers.

Journal of Applied Physics
s, 1972 Vol, 43°pp 2327-23
35). This paper is just the result of a theoretical investigation, and there is no description of the structure of a semiconductor laser device or its manufacturing method for realizing the above-mentioned gain coupling.

Some of the inventors of the present invention invented a new semiconductor laser device applying the basic theory of Kogelnick et al. and filed the following patent application.

Japanese Patent Application No. 1-189593, filed on July 30, 1988, Japanese Patent Application No. 1-168729, filed on June 30, 1999, Japanese Patent Application No. 1-185001 to 185005, filed on July 18 of the same year.

Among these patent applications, in particular, the specification and drawings of Japanese Patent Application No. 1-168729 disclose that an uneven shape corresponding to a diffraction grating is imprinted on the surface of a semiconductor layer, which is a substrate on which an active layer is grown, and that the semiconductor layer is We have disclosed a technique in which a thin semiconductor buffer layer is first grown on the surface while preserving its uneven shape, and then an active layer is grown on the surface so as to fill in the concave portions of the uneven shape as much as possible.

[Problem to be solved by the invention]

However, especially in the case of an InP-based laser device, that is, a laser device with a lattice constant matched to InP, it is difficult to form a semiconductor buffer layer with a uniform thickness so that the uneven shapes on both sides are parallel. Ta.

SUMMARY OF THE INVENTION An object of the present invention is to solve these problems and provide a semiconductor distributed feedback laser device that is easy to manufacture and has a structure particularly suitable for InP-based long wavelength lasers.

[Means to solve the problem]

The semiconductor distributed feedback laser device of the present invention includes a semiconductor buffer layer in contact with the uneven shape formed as a diffraction grating in the active layer, and this buffer layer has the uneven shape recesses imprinted on the semiconductor layer serving as a base. It is characterized by being formed in a partially buried shape.

In order to manufacture such a semiconductor distributed feedback laser device, an uneven shape corresponding to a diffraction grating is imprinted on the surface of a semiconductor layer, which is a substrate on which an active layer is grown, and the surface of the semiconductor layer is A buffer layer is grown so that irregularities with a period corresponding to the irregular shape remain, and then an active layer is grown.

Although the structure of the present invention is particularly suitable for implementation in InP-based long wavelength lasers, it can equally be implemented in laser devices based on other materials, such as ALGaAs.

[For production]

When the active layer is provided with a concavo-convex shape along the direction of light propagation, the thickness of the active layer changes periodically according to the concavo-convex shape, so the gain based on the periodic perturbation of the gain coefficient in the theory by Kogelnitsu et al. The coupling provides light distribution feedback.

In order to provide an uneven shape in the active layer, the uneven shape is imprinted on the surface of a semiconductor layer serving as a substrate, a buffer layer is formed on the surface of the semiconductor layer, and then the active layer is grown. At this time, if the material and growth conditions are optimal, the thickness of the buffer layer can be made uniform. However, it is difficult to grow the buffer layer uniformly, especially in the case of InP-based materials.

Therefore, in the present invention, after the uneven shape on the surface of the semiconductor layer serving as the substrate is imprinted, a buffer layer is grown so that the influence of the uneven shape remains. At this time, the recess is filled, but
On the surface of the buffer layer, irregularities with a period corresponding to the irregular shape of the semiconductor layer remain. If an active layer is grown on this, an uneven shape will also be formed in this active layer.

〔Example〕

FIG. 1 is a perspective view of a semiconductor distributed feedback laser device according to a first embodiment of the present invention.

This semiconductor distributed feedback laser device includes an active layer 7 that generates stimulated emission light, and a diffraction grating that is provided on one surface of this active layer and that performs distributed light feedback to this active layer. An uneven shape is formed on one surface of the active layer 7.

More specifically, a cladding layer 3, a semiconductor layer 4, a buffer layer 6, an active layer 7, a cladding layer 8, and a contact layer 9 are laminated on the substrate 1, and an electrode layer 10 is formed on the back surface of the substrate 1.
is provided, and an electrode layer 11 is connected to the contact layer 9 via an insulating layer 12.

Here, the feature of this embodiment is that a semiconductor buffer layer 6 is provided in contact with the uneven shape of the active layer 7, and this buffer layer 6 partially covers the uneven shaped recesses stamped on the semiconductor layer 4 serving as a base. This is due to the fact that it was formed in a buried shape.

To manufacture this laser device, first, a high concentration n-type 1n
Each layer of a double heterojunction structure is epitaxially grown on a P substrate 1 in two stages. Each layer is lattice matched to the InP substrate 1.

In the first stage of epitaxial growth, for example, a 1 μm thick n-type 1nP cladding layer 3 and a 0.1 μm thick n-type cladding layer 3 are formed on the substrate 1.
Thick n-type 1no, t2Gao, 28ASO0s+P
o. 39 Semiconductor layer 4 is grown as a crystal. Next, the semiconductor layer 4 is etched with a period of 2 by interference exposure method and chemical etching.
An uneven shape 5 corresponding to a 56 nm diffraction grating is imprinted.

In the second step, an n-type InP buffer layer 6 having an average thickness of 3 Q nm is grown on the semiconductor layer 4 on which the uneven shape 5 is imprinted.

Further, on this buffer layer 6, a low-purity concentration Ino, 53Gao, aJS active layer 7 with an average thickness of 1100n, and 1
A μm-thick p-type 1nP cladding layer 8 and a 0.5 μm-thick high-concentration p-type Inn, s+Gao, <JS contact layer 9 are sequentially grown to complete a double heterojunction structure.

A 5102 insulating layer 12 is then deposited on the top surface of the contact layer 9 to form a striped window with a width of, for example, 10 μm, followed by the deposition of the electrode layers 11 and 10. moreover,
This is folded to complete individual semiconductor laser devices.

When growing the buffer layer 6 and the active layer 7 on the semiconductor layer 4 on which the concavo-convex shape 5 corresponding to a diffraction grating is imprinted, metal organic vapor phase epitaxy is used. At this time, the uneven shape of the buffer layer 6 is not completely flattened, and when the active layer 7 is grown, the remaining uneven recesses are filled. Thereby, a diffraction grating can be formed on the lower surface of the active layer 7.

The buffer layer 6 serves to prevent defects in the semiconductor crystal structure caused by the imprinting on the semiconductor layer 4 from affecting the active layer 7 . When growing the buffer layer 6, even under conditions where the recesses are easily filled, by making the thickness of the buffer layer 6 thinner than the uneven shape 5 imprinted on the semiconductor layer 4, the upper surface of the buffer layer 6,
In other words, unevenness can be left on the lower surface of the active layer 7.

The conductivity type and composition of each layer described above are summarized in Table 1.

Table 1 and FIG. 2 show scanning electron micrographs of a cross section along the direction of the active layer of an actually prototype semiconductor distributed feedback laser device. From this photograph, we can see that the semiconductor layer 4, the buffer layer 6 and the active layer 7
can be recognized. The growth conditions for the buffer layer 6 were not particularly changed to make it easier to preserve the uneven shape, and the concave portions grew at about five times the rate of the convex portions. Even under such growth conditions, a diffraction grating is formed on the lower surface of the active layer 7. The growth conditions for organometallic vapor phase epitaxy are as follows: [Raw materials] Phosphine PH3 Arsine AsH.

Triethyl indium (CJs) 3Jn triethyl gallium (C2H5), Ga dimethyl zinc (
CH3) 2Zn hydrogen sulfide H2S [Conditions] Pressure: 76 Torr Total flow rate: 5 slm Substrate temperature: 700°C (first time), 650°C (second time). Furthermore, it was confirmed by measuring the photoluminescence intensity that the active layer 7 was not affected by the defects caused by the marking on the semiconductor layer 4.

In this way, periodic changes in the gain coefficient can be obtained by the diffraction grating formed in the active layer 7, and the light distribution feedback based on the perturbation of the gain coefficient allows a simple signal to be generated at the Bragg wavelength corresponding to the period of change in the gain coefficient. Unimode oscillation was obtained.

In the above embodiment, the InP-based case was explained, but
A similar structure can also be implemented based on ALGaAs. Examples of the conductivity type and composition of each layer in that case are shown in Table 2.

(Hereinafter, the margin of this page) Table 3 is a perspective view of a semiconductor distributed feedback laser device according to a second embodiment of the present invention.

In this embodiment, an uneven shape 5 corresponding to the diffraction grating is imprinted directly on the substrate 1, which acts as a cladding layer, without providing another semiconductor layer between the substrate 1 and the buffer layer 6. A buffer layer 6 is grown on the substrate 1 on which the uneven shape 5 is imprinted. The structure other than this is the same as that of the first embodiment.

In the above explanation, an example has been described in which metal organic vapor phase epitaxy is used as the crystal growth method. However, the buffer layer 6
The present invention can be similarly implemented using other crystal growth methods, such as molecular beam epitaxial growth, as long as the ability to precisely control the film thickness and the ability to flatten the upper surface of the active layer 7 are satisfied.

〔Effect of the invention〕

As explained above, the semiconductor distributed feedback laser device of the present invention is different from the conventional index-coupled semiconductor distributed feedback laser device in that it performs longitudinal mode oscillation of a completely single wavelength. It is thought that such uncertainty in the oscillation wavelength is not observed.

However, although it is possible to achieve a completely single longitudinal mode with conventional semiconductor distributed feedback laser devices, in both cases the configuration of the semiconductor laser device becomes complicated, and in addition, it is necessary to form an anti-reflection film on the end face of the laser element, etc. In contrast to the increase in the number of manufacturing steps, the device of the present invention can easily realize a complete single longitudinal mode with almost no changes to the conventional manufacturing steps and without the need for anti-reflection measures.

Furthermore, since the semiconductor distributed feedback laser device of the present invention achieves optical distributed feedback through gain coupling, even if interference noise is induced by reflected return light from the near end or far end, It is expected that the size will be much smaller than in the case of conventional refractive index coupling.

Furthermore, in the semiconductor distributed feedback laser device of the present invention, since the resonator is caused by a periodic distribution of gain caused by current injection, it is possible to generate ultrashort pulses in high-speed current modulation, and the chirping of the oscillation wavelength is small. It is expected that

Furthermore, the semiconductor distributed feedback laser device of the present invention includes:
The structure is suitable for InP-based elements capable of long-wavelength oscillation, and it is not only promising as a high-performance light source required for long-distance optical communications and wavelength multiplexing communications, but also for optical information processing and recording, and optical applied measurement. The present invention is expected to be used as a high-performance, compact light source that can replace gas laser devices and solid-state laser devices that have been conventionally used in fields such as light sources for high-speed optical phenomena.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to a first embodiment of the present invention. Figure 2 is a scanning electron micrograph showing the crystal structure of the prototype. FIG. 3 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to a second embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Substrate, 3.8... Clad layer, 4... Semiconductor layer, 5... Uneven shape, 6... Buffer layer, 7... Active layer, 9... Contact layer, 10.11 River electrode layer, 1
2...Insulating layer. Patent applicant: Optical Measurement Technology Development Co., Ltd. Agent: Naotaka Ide, patent attorney

Claims (1)

[Claims] 1. An active layer that generates stimulated emission light, and a diffraction grating that is provided on one surface of this active layer and that performs light distribution feedback to this active layer, and this diffraction grating is provided in the active layer. In a semiconductor distributed feedback laser device having an uneven shape formed on one surface of the semiconductor layer, a semiconductor buffer layer is provided in contact with the uneven shape, and this buffer layer has an uneven shape recessed part imprinted on a semiconductor layer serving as a base. A semiconductor distributed feedback laser device characterized by being formed in a partially buried shape. 2. The semiconductor distributed feedback laser device according to claim 1, wherein the active layer is a layer of a material whose lattice constant substantially matches that of InP.