JPH0569318B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser with low noise and stable transverse mode characteristics, and a method for manufacturing the semiconductor laser.

[Conventional technology]

When a semiconductor laser (hereinafter referred to as LD) is used as a light source for an optical information processing device, etc., noise (hereinafter referred to as return optical noise) generated when a part of the emitted light returns to the resonator due to reflection is generated. may occur, making it impossible to put it into practical use. As a means to reduce this return light noise, there are a method of laminating multiple layers of dielectric materials with different refractive indexes on the end face of the LD resonator to increase the reflectance of the end face, or a method of causing multi-axis oscillation of the longitudinal mode. As an example of the latter, JP-A-1988
-140774 and JP-A-60-150682.

On the other hand, the side surface of a ridge-shaped optical waveguide made of a - group compound semiconductor is embedded using the commonly used liquid layer growth method (hereinafter referred to as LPE method).
There is a method of forming a group compound semiconductor layer, or a method of forming a - group compound semiconductor layer by selective growth using a metal organic chemical vapor deposition method (hereinafter referred to as MOCVD method). An example of the latter is found in Japanese Journal of Applied Physics, Vol. 25, No. 6, pp. L498-L500, 1986.

[Problem that the invention seeks to solve]

However, the above-mentioned conventional technology has the following problems.

In the method of laminating multiple layers of dielectric materials with different refractive indexes on the end face of the LD resonator to increase the amount of end face reflection, thereby reducing the influence of returned light and reducing the return optical noise, the film thickness of each dielectric layer is adjusted to increase the reflectance. It is necessary to accurately control the output light wavelength of the LD. In addition, the high reflectance at the end face reduces the intensity of the emitted light. Therefore, when high output is required, such as when writing information onto a disk, it is necessary to further increase the LD drive current. The increase in LD drive current is
This increases power consumption and raises the temperature inside the element, leading to a reduction in the reliability and life of the element. Furthermore, the LD manufacturing process of multi-layer curtain lamination will be increased.

Next, in the method of reducing return optical noise by oscillating in longitudinal multi-axis mode, a gain waveguide type LD is used, which is formed by forming an electrode stripe on the top surface after forming a double heterojunction structure (hereinafter referred to as DH structure). It is possible that
A gain waveguide type LD is capable of longitudinal multi-axis mode oscillation and has reduced effects on noise caused by returned light and the like. However, in the gain waveguide type LD, the laser oscillation light in the direction parallel to the junction is guided inside the resonator due to the gain distribution formed by the injection current. Therefore, the equiphase front of the laser oscillation light is not a plane but has wavefront aberration, that is, astigmatism occurs, and a complicated optical system is required to focus the light on a minute spot. Furthermore, the near-field image changes due to fluctuations in the injected current or returned light, making the coupling with the optical system unstable, which poses a problem when applied to various devices.

FIG. 3 shows an embodiment (Japanese Unexamined Patent Publication No. 150682/1982) devised in consideration of the above problems. Figures 3b and 3c show cross-sectional views of this element. The shape of the groove formed under the active layer 302 is different in the central part of the resonator near the end face. A current blocking layer 301 as shown in FIG.
By forming a recess that is wider than the current injection groove width, the refractive index waveguide width becomes sufficiently wider than the current injection width, and as a result, a gain waveguide mechanism is formed in the center of the element, and longitudinal multi-axis mode oscillation occurs. can get. Therefore, the LD has a refractive index waveguide mechanism near the resonator end face and a gain waveguide mechanism in the center. However, in order to realize this structure, a complicated process of forming grooves of different depths after forming the current element layer 301 is required, and since each layer is formed by the LPE method, there is a problem in controllability of the film thickness. be. Variations in the groove shape and variations in the film thickness of each grown layer due to the formation of a substrate with a complicated shape lead to variations in the LD element. In the LPE method, the growth rate of each layer in the groove and flat areas differs greatly, making it difficult to control the film thickness and causing large variations in properties.

Furthermore, by stacking − group compound semiconductor layers,
In an LD in which a ridge-shaped waveguide having a DH structure is embedded with a - group compound semiconductor layer formed by the above-mentioned LPE method or MOCVD method, the influence of reactive current flowing outside the active region cannot be ignored. Therefore, the present invention is intended to solve these problems.The purpose of the present invention is to perform stable single transverse mode oscillation from low optical output operation to high optical output operation, and to reduce the return light by vertical multi-axis mode oscillation. It is an object of the present invention to provide a semiconductor laser that emits laser light with suppressed noise and small astigmatism difference, and oscillates with a low threshold current in which reactive current flowing outside the active region is suppressed as much as possible.

[Means for solving problems]

The semiconductor laser of the present invention has a ridge-shaped optical waveguide made of a - group compound semiconductor formed so as to leave a part of the cladding layer on the active layer, and a buried layer made of a semiconductor layer on the side surface of the optical waveguide. The semiconductor laser is characterized in that the width of the optical waveguide is narrower in the vicinity of at least one resonator end face than in the center, and the buried layer is the semiconductor layer made of a - group compound semiconductor.

[Effect]

First, it has a part that will become a constituent element of a ridge-shaped optical waveguide made of a - group compound semiconductor, etc., which is formed so as to leave a part of the cladding layer on the active layer, and the end face of the ridge is made of a - group compound semiconductor, etc. − such as ZnSe, which has a refractive index smaller than the −
By filling the optical waveguide with a group compound semiconductor, etc., it is possible to reduce the difference in refractive index between the ridge-shaped part and its side parts, and even if the width of the ridge-shaped optical waveguide is widened to some extent, single transverse mode oscillation is possible. becomes. Furthermore, by widening the width of this ridge-shaped optical waveguide, even if the current output is increased and the optical density is increased, the device characteristics do not deteriorate, so that a high optical output can be obtained. However, if the active layer is removed by etching to form a ridge-shaped optical waveguide, if there is a large difference in the refractive index of the optical waveguide and the refractive index of the part embedded in the side surface, the ridge-shaped optical waveguide will be formed. The problem is that single transverse mode oscillation cannot be achieved unless the width of the optical waveguide is made extremely narrow and difficult to control, and if the width of the ridge-shaped optical waveguide is made wide, higher-order transverse mode oscillation occurs. be.

Furthermore, according to the above configuration of the present invention, since the laser oscillation light is guided by the gain waveguide mechanism in the central part of the resonator, vertical multi-axis mode oscillation with less return light noise is performed, and in the vicinity of the output end face, the laser oscillation light is guided by the refractive index guide. Since the laser oscillation light is guided by a wave mechanism, stable single transverse mode oscillation with little astigmatism difference is possible, and since the structure is embedded with a high-resistance - group compound semiconductor, the reactive current is extremely small. This results in a semiconductor laser that oscillates at a low threshold.

〔Example〕

Embodiments of the present invention will be described below. Here, AlGaAs-based material, which is a representative of - group compound semiconductors, is used for the portions constituting the ribs, but the same applies to other compound semiconductors.

(Example 1) FIG. 1 shows an example of the present invention. 1st
Figure 1a shows a cross-sectional view near the end face of the resonator, and Figure 1b shows a cross-sectional view at the center. FIG. 2 shows a process for achieving the structure of an embodiment of the invention. 2nd below
The present invention will be explained using figures.

An n-type GaAs substrate 201, an n-type GaAs buffer layer 202, an n-type Al _x Ga _1-x AS first cladding layer 203, an Al _y Ga _1-y AS active layer (x>y) 204,
p-type Al _z Ga _1-z As second cladding layer (z>y) 2
05, consisting of p-type GaAs contact layer 206
Continuously forms DH structures. (Figure 2a) The above layers can be formed using LPE method, MOCVD method or
Any method such as the MBE method is possible. Next, a resist mask 207 for etching is formed by a normal photolithography process. (Second
Figure b) The shape of the resist mask 207 is as shown by diagonal lines 208 in Figure 2c. Next, the resist mask 207 is used as an etching mask for p-type etching.
GaAs contact layer 206 and p-type Al _z Ga _1-z
Etch a portion of As second cladding layer 205. At this time, a portion of the second cladding layer 205 on the active layer 204 remains on the active layer 204.
It is also characterized by etching in a ridge shape. After that, the resist mask 207 is removed. (Figure 2 d) Next, - group compound semiconductor ZnSe
A buried layer 209 is formed by MOCVD.
(FIG. 2e) It is also possible to use - group compound semiconductors other than ZnSe. Subsequently, a photolithography process and an etching process for the ZnSe buried layer 209 are performed. A top view of the device after etching is shown in FIG. 2g. The shaded area is the - group compound semiconductor layer, and the central stripe is the part where the p-type GaAs contact layer 206 is exposed by etching. Thereafter, a p-side electrode 210 is formed, a backside substrate cutting process is performed, and then an n-side electrode 211 is formed to complete the semiconductor laser of the present invention.

The refraction flow rate of the ZnSe buried layer 209 used in the present invention is smaller than that of the AlGaAs layer with any Al mixed crystal ratio, and the forbidden band width is smaller than that of the AlGaAs layer with any Al mixed crystal ratio.
It is a wider material than the AlGaAs layer. Therefore, in the waveguide formed according to the present invention, since absorption of laser oscillation light by the ZnSe layer does not occur, a refractive index difference formed by the real part of the complex refractive index occurs in the direction horizontal to the junction, and the refractive index guide It becomes a wave path. In addition, the remaining film thickness after etching of the second cladding layer, which is an important parameter that determines the refractive index difference in the direction horizontal to the junction, is thicker than that in the case of an embedded AlGaAs layer because the refractive index of the ZnSe layer is small. Also, a refractive index difference that enables single transverse mode oscillation can be obtained.

Near the end face of the resonator, the width of the refractive index waveguide and the current injection width are approximately the same, creating a refractive index waveguide mechanism, which enables stable single transverse mode oscillation and emits laser light with an extremely small astigmatism difference. be done.

On the other hand, in the center of the resonator, by making the width of the refractive index waveguide sufficiently wider than the current injection width, it becomes a gain waveguide mechanism, resulting in vertical multi-axis mode oscillation, and returning optical noise can be suppressed as much as possible.

Furthermore, since the ZnSe layer is a material with considerably higher resistivity than the AlGaAe layer, current confinement is effectively performed and reactive current flowing outside the active region can be suppressed as much as possible.

(Embodiment 2) FIG. 4 shows a structure showing another embodiment of the present invention. Figure 4a is a cross-sectional view near the resonator end face;
Figure b is a cross-sectional view at the center of the resonator.

In this example, etching was performed under the active layer 402 in forming the rib waveguide in the above-mentioned (Example 1).
This is a structure in which the ZnSe buried layer 401 is buried after advancing toward the GaAs substrate side.

The reason why vertical multi-axis mode oscillation, stable single transverse mode oscillation, and astigmatism difference are extremely small, as well as the reason why reactive current is extremely small (Example 1)
This is the same as the section.

[Effects of the Invention] As described above, according to the present invention, the following effects can be obtained.

1. In the center of the resonator, light is guided by a gain waveguide mechanism, so the LD of the present invention exhibits longitudinal multi-axis mode oscillation. Therefore, the return optical noise becomes an extremely small value.

2. For this reason, it can be widely applied as a light source for optical information processing.

3. In the vicinity of at least one resonator end face, light is guided by a refractive index waveguide mechanism, so the LD of the present invention
Also, stable single transverse mode oscillation can be obtained even with changes in injection current.

4 For the same reason as in 3, laser oscillation light with an extremely small astigmatism difference can be obtained.

5. For reasons 1, 3, and 4, when the LD of the present invention is incorporated into an optical head or the like, a complicated optical system required for output beam shaping, etc. is not required. Therefore, simplification and weight reduction can be achieved.

6. Since the current confinement layer is formed of a - group compound semiconductor that provides a layer with high resistivity, it is possible to suppress the reactive current flowing outside the active region as much as possible. Therefore, it is effective in reducing the threshold current.

7 The present invention has a structure that can be manufactured by two-step growth using the MOCVD method, which has excellent uniformity and reproducibility over a large area in film quality and thickness, so the characteristics of the LD of the present invention also have uniformity and reproducibility. It has excellent performance and reliability.

8 As mentioned above, the LD of the present invention has low noise, excellent transverse mode stability, and small astigmatism difference.
It is LD. Therefore, by forming a protective film on the resonator end face of the LD of the present invention to prevent end face deterioration, high output characteristics can be obtained in addition to the above characteristics.

9. Since the refractive index of the - group compound semiconductor in the buried tank is small, the spot size of the near-field image can be controlled by changing the remaining film thickness of the upper cladding layer, which is effective for increasing output.

10 Currently, the high-frequency weight method is considered effective for reducing noise, but the LD of the present invention can achieve lower noise without requiring such an additional method. Therefore, it is possible to realize miniaturization, weight reduction, and cost reduction without the need for additional circuits.

11 On the active layer made of a - group compound semiconductor or the like, a part of the cladding layer made of a - group compound semiconductor or the like is left and the cladding layer is processed into a ridge shape to form a component of an optical waveguide. Therefore, the side surfaces of the ridge-shaped optical waveguide components are made of −
Even if the ridge-shaped optical waveguide is buried with a group compound semiconductor, etc., the difference in refractive index between the ridge-shaped part and its side parts can be reduced, and single transverse mode oscillation can be achieved even if the width of the ridge-shaped optical waveguide components is widened to some extent. becomes possible. Furthermore, by increasing the width of the constituent elements of this ridge-shaped optical waveguide, even if the current output is increased and the optical density is increased, the device characteristics do not deteriorate, so that high optical output can be obtained.

[Brief explanation of the drawing]

FIGS. 1a and 1b are cross-sectional views showing one embodiment of the LD of the present invention. FIGS. 2a to 2g are manufacturing process diagrams for realizing the LD of the present invention. FIGS. 3a to 3c are diagrams showing a conventional example. FIGS. 4a to 4b are cross-sectional views showing one embodiment of the LD of the present invention. 201...n-type GaAs substrate, 202...n-type
GaAs buffer layer, 203...n-type Al _x Ga _1-x As
First cladding layer, 204... Al _y Ga _1-y As active layer, 205... p-type Al _z Ga _1-z As second cladding layer, 206... p-type GaAs contact layer, 20
7,208...Resist mask for etching, 2
09...ZnSe buried layer, 210...p side electrode,
211...n-side electrode, 301...current blocking layer, 3
02...Active layer, 401...ZnSe buried layer,
402...Active layer.

Claims (1)

[Claims] 1. (a) a first cladding layer of a first conductivity type formed on a substrate and having a - group compound semiconductor as a component; (b) a first cladding layer formed on the first cladding layer; , and an active layer comprising a - group compound semiconductor as a constituent element, (c) a thick ridge-shaped first portion formed on the active layer, and a thick ridge-shaped first portion formed on the active layer on both sides of the first portion; , a second cladding layer composed of a - group compound semiconductor of a second conductivity type, which is formed continuously with the first part and has a thickness thinner than the first part; d) a contact layer of a second conductivity type formed on the first portion of the second cladding layer and having a -group compound semiconductor as a component; (e) a contact layer having a refractive index lower than that of the second cladding layer; Embedded material has a small refractive index and is embedded so as to be in contact with the side surface of the first portion and the top surface of the second portion of the second cladding layer. (f) the ridge-shaped first layer of the second cladding layer;
the portion has a first width at a longitudinal end of the second cladding layer so as to constitute part of a refractive index waveguide, and to constitute part of a gain waveguide; A semiconductor laser characterized in that the second cladding layer has a second width wider than the first width at a central portion in the longitudinal direction. 2 (a) forming a first cladding layer of a first conductivity type having a - group compound semiconductor as a constituent element on the substrate; (b) forming a - group compound semiconductor as a constituent element on the first cladding layer; (c) forming a second cladding layer of a second conductivity type having a - group compound semiconductor as a component on the active layer; (d) forming a second cladding layer on the second cladding layer; (e) forming a contact layer of a second conductivity type comprising a di-group compound semiconductor; (e) selectively etching the second cladding layer and the contact layer; on a thick ridge-shaped first part and the active layer on both sides of the first part,
forming a second part continuous with the first part and having a thickness thinner than the first part; (f) having a refractive index smaller than the refractive index of the second cladding layer; and in contact with the side surface of the first portion and the top surface of the second portion of the second cladding layer, a buried layer comprising a - group compound semiconductor is selectively added. (g) selectively etching the second cladding layer to form a ridge-like structure on the second cladding layer; one portion has a first width at a longitudinal end of the second cladding layer so as to constitute a portion of a refractive index waveguide and constitute a portion of a gain waveguide. . A method of manufacturing a semiconductor laser, characterized in that the second cladding layer has a shape having a second width wider than the first width at a central portion in the longitudinal direction.