JPH04155986A - Semiconductor distribution feedback type laser device - Google Patents

Abstract

PURPOSE:To form gain regions and absorptive regions alternately so as to get a large intermode gain by providing light absorbing layer at each apex of a semiconductor layer. CONSTITUTION:Each layer of double hetero junction structure is epitaxially grown separately in two stages. In the first stage, a clad layer 3, a semiconductor layer 4, a light absorbing layer 3 are grown on a substrate 1. Next, by interfering exposure method and chemical etching, irregularities equivalent to the diffraction grating 256nm in cycle are marked at the light absorbing layer 13 and the semiconductor layer 4. In the second stage, a shock absorbing layer 6 is grown on the light absorbing layer 13 and the semiconductor layer 4, and further an active layer 7, a clad layer 8, and a contact layer 9 are grown in order continuously to complete double hetero structure. Next, an SiO2 insulating layer 12 is stacked on the top of the contact layer 9 to form a stripe-shaped window, for example, 10mum in width, and then electrode layers 11 and 10 are deposited. Furthermore, this is cleaved to complete individual semiconductor laser elements.

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,
It is suitable for use as a light source for optical recording devices, optical application measurement devices, and other optoelectronic devices.

[Overview] The present invention provides a semiconductor distributed feedback laser device in which a buffer layer and an active layer are grown on a semiconductor layer provided with an uneven shape corresponding to a diffraction grating, and the active layer is provided with periodic thickness changes. By providing a light absorption layer on each top of the semiconductor layer, gain regions and absorption regions are alternately formed to obtain a large gain difference between modes.

[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 petrojunction semiconductor layer or the like to efficiently generate stimulated emission light. In particular, by forming a diffraction grating with, for example, a triangular wave cross-section on the interface of a transparent waveguide layer in close proximity to the active layer on the side far from the active layer, and periodically changing the waveguide refractive index, light distribution is achieved. 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 having a Bragg wavelength that is reflected in accordance with the cycle of layer thickness change of the leading 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. , 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, for example, 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 just to eliminate degeneracy needs to be added. First, 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 a completely single-wavelength longitudinal mode oscillation should be obtained without the appearance of any 23rd
Pages 27 to 2335 (“Coupled-Wav
eTheory of Distributed
Feedback La5ers”.

Journal of Applied Phys.
ics, 1972 Vol, 43°pp 2327-
2335). However, Kogelnick's paper is just the result of a theoretical study, and does not provide any concrete structure.

Some of the inventors of the present invention invented a new semiconductor laser device applying the above-mentioned basic theory of cogelnication, 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.

The structures shown in the specifications and drawings of each of these patent applications made it possible to realize distributed light feedback by gain coupling.

[Problem to be solved by the invention]

However, the structure shown in the above-mentioned patent application was based on Kogelnick's theory in which either the gain coefficient or the absorption coefficient was changed periodically.

Kogelnick's paper also discusses the case where the amount of perturbation of the gain coefficient is larger than the average value of the gain coefficient, that is, the case where gain regions and absorption regions exist alternately, and there is a large gain difference between modes. It has been shown that there are advantages such as the ability to obtain

An object of the present invention is to provide such a semiconductor distributed feedback laser device having a structure in which gain regions and absorption regions alternate.

[Means to solve the problem]

The semiconductor distributed feedback laser device of the present invention is formed by forming a semiconductor layer having a concavo-convex shape imprinted thereon corresponding to the period of a diffraction grating, and a periodic shape corresponding to the concave-convex shape remaining in contact with the semiconductor layer on the upper surface. In the semiconductor distributed feedback laser device, the semiconductor distributed feedback laser device is provided with a semiconductor buffer layer formed in the same manner as described above, and an active layer formed in contact with the buffer layer and provided with a thickness change corresponding to the periodic shape. It is characterized by having a light absorption layer.

In this specification, "upper" refers to the same direction as the direction of crystal growth during manufacturing. Moreover, "down" refers to the opposite direction.

To manufacture such a semiconductor distributed feedback laser device, an uneven shape corresponding to a diffraction grating is imprinted on the surface of a layer including a light absorption layer, and a buffer layer is formed while leaving the influence of the uneven shape on the surface of this layer. The active layer is grown so that the periodic unevenness appearing on the surface of the buffer layer becomes a diffraction grating, and fills the concave portions of the unevenness.

The thickness of the buffer layer is selected such that periodic irregularities remain on the top surface of the buffer layer. Depending on the growth conditions, this periodic unevenness can be made to be approximately congruent with the uneven shape of the underlying layer of the buffer layer, but they do not necessarily have to be congruent, and the growth conditions may be such that the concave portions are easily filled.

The active layer is grown so that the uneven shape becomes flat on its upper surface. By starting the growth of the next active layer while periodic irregularities remain on the upper surface of the buffer layer, a diffraction grating can be formed on the lower surface of the active layer.

[For production]

By periodically changing the thickness of the active layer, a periodic perturbation of the gain coefficient is obtained. At the same time, the thickness of the light absorption layer is also changed periodically to obtain periodic perturbations in the absorption coefficient. Strong gain coupling can be obtained by both of these effects, and by the distributed optical feedback, a laser device that oscillates in a single mode can be obtained.

Note that if there is a light absorption layer, there is a concern that the overall absorption will increase and the threshold current will increase, but the light absorption layer can be placed only at the nodes of the standing wave during oscillation. , it won't be a big problem.

〔Example〕

FIG. 1 shows the structure of a semiconductor distributed feedback laser device according to an embodiment of the present invention, and FIG. 2 shows the layer structure in the vicinity of the active layer.

This laser device consists of a semiconductor layer 4 on which a concavo-convex shape corresponding to the period of a diffraction grating is imprinted, and a semiconductor buffer formed in contact with this semiconductor layer 4 with a periodic shape corresponding to the concave-convex shape remaining on the top surface. The active layer 7 is formed in contact with the buffer layer 6 and has a thickness change corresponding to the periodic shape of the layer 6.

The feature of this embodiment is that a light absorption layer 13 is provided on each top of the uneven shape stamped on the semiconductor layer 4.

The structure of this laser device will be explained in more detail along with the manufacturing method. Here, an example of an InP system, that is, a case where each layer is lattice matched to InP, will be explained.

First, each layer of a double heterojunction structure is epitaxially grown in two stages on a highly doped n-type 1nP substrate 1.

Each layer is lattice matched to the InP substrate 1.

In the first stage of epitaxial growth, for example, an n-type 1nP cladding layer 3 with a thickness of 1 μm and a 0.1 μm thick n-type cladding layer 3 are formed on the substrate 1.
μm thick n-type Ino, t2Gao, 28AS0.6
IP0.3s semiconductor layer 4 and 0.05 μm thick n-type 1n
o, 53GaO, 47AS light absorption layer 13 is grown as a crystal. Next, an uneven shape 5 corresponding to a diffraction grating with a period of 256 nm is imprinted on the light absorption layer 13 and the semiconductor layer 4 by interference exposure method and chemical etching. Here, it is desirable that the islands and uneven shapes 5 that suppress excessive absorption penetrate the light absorption layer 13.

In the second stage of epitaxial growth, an average of 30n
An m-thick n-type InP buffer layer 6 is grown. Further, on this buffer layer 6, a low impurity concentration 1no.
, 5IGa0.4? an AS active layer 7, a p-type InP cladding layer 8 with a thickness of 1 tttn, and a highly doped p-type InP layer 8 with a thickness of 0.5 μm.
Shape 1no1%! JGao and 4JS contact layers 9 are successively grown in this order to complete a double heterojunction structure.

A 5lO7 insulating layer 12 is then deposited on the top surface of the contact layer 9, forming a striped window with a width of, for example, 10 μm, followed by the electrode layers 11 and 10 being vapor deposited. moreover,
This is folded to complete individual semiconductor laser devices.

A light absorption layer 13 with a concavo-convex shape 5 imprinted thereon corresponding to a diffraction grating
When growing the buffer layer 6 and the active layer 7 on the semiconductor layer 4, metal organic vapor phase epitaxy is used. At this time, the uneven shape of the buffer layer 6 is made not to be completely flattened, and the remaining uneven recesses are filled in when the active layer 7 is grown. Thereby, a diffraction grating can be formed on the lower surface of the active layer 7.

The buffer layer 6 serves to prevent the active layer 7 from being affected by defects in the semiconductor crystal structure caused by the markings on the light absorption layer 13 and the semiconductor layer 4 . When growing the buffer layer 6, even under conditions where the recesses are easily filled, the thickness of the buffer layer 6 is made thinner than the uneven shape 5 imprinted on the light absorption layer 13 and the semiconductor layer 4. Unevenness can be left on the upper surface, that is, on the lower surface of the active layer 7.

Growth conditions for organometallic vapor phase epitaxy include, for example, [Raw materials] Phosphine PH3 Arsine AsH3 Triethylindium (CJs) 31n Triethylgallium (C2H5) 3Ga Dimethylzinc (C
LLZn Hydrogen sulfide H2S [Conditions] Pressure: 76 Torr Total flow rate: 651 m Substrate temperature: 700°C (first time), 650°C (second time).

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

Table 1 In this way, a periodic change in the gain coefficient is obtained by the diffraction grating formed in the active layer 7, and a periodic change in the absorption coefficient is also obtained by the light absorption layer 13 cut by the uneven imprints. can get. By performing distributed optical feedback based on perturbations in these gain coefficients and absorption coefficients, a semiconductor distributed feedback laser device that causes single mode oscillation at a Bragg wavelength corresponding to the period of these changes can be obtained.

Although the case of InP type has been described here, a similar structure can also be realized with AlGaAs type. Examples of the conductivity type and composition of each layer in that case are shown in Table 2.

Table 2 In the explanation above, an example was explained in which metal organic vapor phase epitaxy was 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.

C. Effects of the Invention] As explained above, the semiconductor distributed feedback laser device of the present invention provides periodic thickness changes in both the active layer and the light absorption layer, and alternates the gain region and the absorption region. is forming. This structure has the effect of obtaining a large gain difference between modes and very stable single longitudinal mode oscillation.

In addition, since distributed optical feedback is achieved through gain coupling, unlike conventional index-coupled semiconductor distributed feedback laser devices, longitudinal mode oscillation of a single wavelength is performed, which is different from conventional devices. It is thought that there is no uncertainty in the oscillation wavelength. 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 optical distribution feedback using gain coupling is used, interference noise induced by reflected return light from the near end or the far end, even if it occurs, is much lower than when using conventional refractive index coupling. It is expected that it will become significantly smaller.

Therefore, the semiconductor distributed feedback laser device of the present invention has the following characteristics:
Gas lasers are not only promising as high-performance light sources necessary for long-distance optical communications and wavelength division multiplexing communications, but also have traditionally been used in fields such as optical information processing and recording, optical applied measurement, and light sources for high-speed optical phenomena. It is expected to be used as a high-performance, compact light source that can replace solid-state laser devices.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing the structure of a semiconductor distributed feedback laser device according to an embodiment of the present invention. FIG. 2 is an enlarged sectional view showing the layer structure near the active layer. 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... Electrode layer, 12... Insulating layer, 13... Light absorption layer. Patent applicant: Optical Measurement Technology Development Co., Ltd. Agent: Naotaka Ide, patent attorney

Claims (1)

[Claims] 1. A semiconductor layer in which an uneven shape corresponding to the period of a diffraction grating is formed, and a semiconductor formed in contact with this semiconductor layer with a periodic shape corresponding to the uneven shape remaining on the upper surface. In a semiconductor distributed feedback laser device comprising a buffer layer and an active layer formed in contact with the buffer layer and having a thickness varying corresponding to the periodic shape, each peak of the uneven shape has a light absorbing layer. A semiconductor distributed feedback laser device characterized by comprising a layer. 2. The semiconductor distributed feedback laser device according to claim 1, wherein the semiconductor layer, the light absorption layer, the buffer layer and the active layer are each made of a material that is substantially lattice matched to InP. 3. The semiconductor distributed feedback laser device according to claim 1, wherein the semiconductor layer, the light absorption layer, the buffer layer and the active layer are each made of a material that is substantially lattice matched to GaAs.