JPH05121822A - Manufacture of semiconductor laser device - Google Patents

Abstract

PURPOSE:To control the thickness of an optical waveguide layer on an active layer in a non-ridge part and suppress the diffusion of a current into the active layer by a method wherein a mask layer in which a stripe-shaped window is formed is provided to make a second conductivity type cladding layer and a second conductivity type can layer grow. CONSTITUTION:A first conductivity type cladding layer 2 and an active layer 4 which is provided between optical waveguide layers 3 are successively built up on a (100) face GaAs substrate 1. Then a mask layer 7 in which a stripe- shaped window extended along the [011] direction of the crystal azimuth of the GaAs substrate 1 is provided to make a second conductivity type cladding layer 5 and a second conductivity type cap layer 6 grow. As a ridge is formed by crystal growth as described above, the thickness of the optical waveguide layer 3 on the active layer 4 in a non-ridge part can be controlled highly accurately with a crystal growth speed. Therefore, by suppressing the leakage of a current out of a light confining region, a low threshold stable lateral mode ridge waveguide type semiconductor laser can be obtain with high reproducibility.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser manufacturing method, and more particularly to a ridge waveguide type semiconductor laser having a low threshold current and capable of oscillating in a fundamental stable mode.

[0002]

2. Description of the Related Art Conventionally, a ridge waveguide of a ridge waveguide type semiconductor laser is formed by etching, and the etching of the non-ridge portion at that time reaches the optical waveguide layer, or
Alternatively, the depth is such that it can be close to this layer. FIG. 10 is a sectional view showing an example of a conventional ridge waveguide type semiconductor laser. In this figure, 1 is n-type GaA
a substrate made of s, 2 is a cladding layer made of n-type AlGaAs, 3 is an optical waveguide layer made of undoped AlGaAs,
4 is an active layer made of undoped AlGaAs, 5 is a cladding layer made of p-type AlGaAs, 8 is an insulator layer, 6 is a cap layer made of p-type GaAs, 10 is a p-side electrode, 1
Reference numeral 1 is an n-side electrode, 12 is a current injection region, and 13 is a light confinement region.

Next, the operation of this ridge waveguide type semiconductor laser will be described. When a voltage is applied between the p-side electrode 10 and the n-side electrode 11, a current flows in the cap layer 6 on the ridge.
Then, it is injected into the active layer through the ridge portion of the cladding layer. Therefore, in the active layer 4, current concentrates on the region under the ridge portion, and light corresponding to the forbidden band width of the active layer 4 is generated. On the other hand, since the effective refractive index of the optical waveguide layer 3 below the ridge portion for this light is higher than the effective refractive index of the optical waveguide layer 3 below the non-ridge portion,
The generated light is confined in the optical waveguide below the ridge.
As a result, the current injection region 12 and the light confinement region 13
Overlap with each other to increase the optical gain and achieve a low threshold current.

As described above, in the conventional ridge waveguide type semiconductor laser, in order to efficiently concentrate the current in the optical confinement region 13, the cladding layer 5 in the non-ridge portion reaches the optical waveguide layer 3 or in this region. It is necessary to perform etching until they can be close to each other to suppress the diffusion of current outside the light confinement region 13. However, in that case, a slight variation in the thickness of the cladding layer on the active layer in the non-ridge portion causes a large change in the effective refractive index of the optical waveguide layer in the non-ridge portion. In order to obtain good performance, it is necessary to accurately control the thickness of the cladding layer on the active layer in the non-ridge portion in units of 0.1 μm or less.

Next, a method of manufacturing this conventional semiconductor laser will be described. First, on a substrate made of an n-type GaAs substrate, a clad layer made of n-type AlGaAs and non-doped Al
Optical waveguide layer made of GaAs, active layer made of non-doped AlGaAs, optical waveguide layer made of non-doped AlGaAs, cladding layer made of p-type AlGaAs, p-type GaA
A cap layer made of s is sequentially formed by epitaxial growth. Next, a stripe-shaped mask made of a resist is formed on the cap layer by photolithography. Next, using the mask made of resist as an etching mask, the optical waveguide layer made of AlGaAs is left by an H ₂ SO ₄ + H ₂ O ₂ -based etching solution to leave the p-type G
The cap layer made of aAs and the clad layer made of p-type AlGaAs are etched to form a ridge portion. Since the cladding layer cannot be selectively etched with respect to the optical waveguide layer, the etching depth at this time is controlled only by the etching time. Next, after removing the masks made of resist, by removing the SiO ₂ of the ridge upper deposited an insulator made of SiO ₂ by photolithography, the non-ridge portion is buried with a current blocking layer made of SiO ₂ . Finally, a p-side electrode and an n-side electrode are formed to complete the conventional ridge waveguide type semiconductor laser.

[0006]

However, in such a conventional method of manufacturing a ridge waveguide type semiconductor laser, the thickness of the cladding layer on the active layer in the non-ridge portion is controlled only by the etching time. It was extremely difficult to accurately control the value in units of 0.1 μm. Also,
To obtain a stable fundamental mode, increase the thickness of the cladding layer on the optical waveguide layer in the non-ridge portion to reduce the effective refractive index difference between the optical waveguide layer in the ridge portion and the optical waveguide layer in the non-ridge portion. As a result of diffusion of the current into the active layer 4 in the non-ridge portion, a reactive current that does not contribute to laser oscillation is generated, and the threshold current is increased.

Therefore, the present invention has been made in order to solve such a problem, and the thickness of the clad layer on the active layer in the non-ridge portion can be accurately controlled in units of 0.1 μm. It is an object of the present invention to provide a method for manufacturing a semiconductor laser capable of suppressing the diffusion of current into the semiconductor laser and, as a result, obtaining desired characteristics with good reproducibility.

[0008]

Therefore, in the method of the present invention according to claim 1, an active layer sandwiched by a first conductivity type cladding layer and an optical waveguide layer is sequentially grown on a (100) plane GaAs substrate, and then, Providing a mask layer having a stripe-shaped window extending in the [011] direction with respect to the crystal orientation of the GaAs substrate and growing a second conductivity type clad layer and a second conductivity type cap layer. Solve the problem.

In the method of the invention according to claim 2, (1
An active layer sandwiched by a first conductivity type clad layer and an optical waveguide layer is sequentially grown on a (00) plane GaAs substrate, and then a striped window extending in the [011] direction with respect to the crystal orientation of the GaAs substrate is formed. The formed SiO ₂ layer is provided to grow the second conductivity type clad layer and the second conductivity type cap layer, and the active layer under the SiO ₂ layer is mixed with the optical waveguide layer to form a mixed crystal, which is forbidden. The problem is solved by including a step of performing heat treatment so that the band width is widened.

[0010]

According to the method of the first aspect of the present invention, since the ridge is formed by crystal growth, the thickness of the optical waveguide layer on the active layer in the non-ridge portion is controlled by the crystal growth rate. Since the crystal growth rate is usually 1 μm / h, it is possible to accurately control the thickness of the optical waveguide layer on the active layer in the non-ridge portion in units of 0.1 μm or less.

According to the method of the second aspect of the present invention, when the heat treatment is performed in the state where the SiO ₂ film is deposited on the optical waveguide layer which becomes the non-ridge portion, the activities adjacent to the SiO ₂ film having different compositions are activated. Interdiffusion is promoted between the layer and the optical waveguide layer to cause mixed crystal. Since the forbidden band width of this mixed crystal region is wider than that of the active region, the injected current is confined in the active region having a narrow forbidden band width, so that the diffusion of the current to the non-ridge portion is suppressed and the injection region is limited. ..

[0012]

Embodiments of the present invention will be described below with reference to the drawings. 1 is a sectional view showing a first embodiment of a semiconductor laser according to the present invention. 2 to 7 are cross-sectional views after each step of the method for manufacturing the semiconductor laser of this embodiment.

First, a 1 μm thick clad layer 2 made of Se-doped Al _X Ga _1-X As on the (100) plane of an n-type GaAs substrate 1 and a thickness of 0.1 μm made of non-doped Al _Y Ga _1-Y As.
1 μm optical waveguide layer 3, 0.01 μm thick active layer 4 made of non _- doped Al _Z Ga _1-Z As, non-doped Al _Y Ga
MOCV the optical waveguide layer 3 made of _1-Y As and having a thickness of 0.1 μm.
The layers are sequentially laminated by the D method (FIG. 2).

Next, SiO ₂ 7 is deposited to 1000 Å. After that, by photolithography, S
Opening the stripe-shaped window having a width 5μm to iO ₂ 7 (Fig. 3). Then, the Mg-doped Al _X Ga _1-X was formed by the MOCVD method.
1 μm thick clad layer 5 made of As, Mg-doped Ga
A cap layer 6 made of As and having a thickness of 0.1 μm is grown. In the MOCVD method, the growth rate is anisotropic and (10
The (0) plane is fast, and the (111) B plane hardly grows. Therefore, when the growth is selectively performed on the (100) plane of the striped window portion, a ridge having the (111) B plane as the side surface is formed (FIG. 4).

The ridge portion and the non-ridge portion are made of SiN.
_After covering with _X (8) (FIG. 5) and sealing the tube with arsenic in a quartz tube, heat treatment is performed at 850 ° C. for 1 hour in an electric furnace. Here, SiO _{2 is formed} on the AlGaAs multilayer thin film having different compositions.
When deposited and heat-treated, the mutual diffusion of the constituent elements of the multilayer thin films under the SiO ₂ is promoted. Therefore, in the non-ridge portion, the interdiffusion of Al and Ga between the layers is promoted by depositing SiO ₂ .
Mixed crystals occur between the active layer below SiO ₂ and the optical waveguide layer (FIG. 6). At this time, since the mutual diffusion is not promoted in the active layer under the ridge portion, this active layer is buried in the mixed crystal layer under the non-ridge portion. On the other hand, the boundary between the lower optical waveguide layer 3 and the lower cladding layer 2 is separated from SiO _2, and the diffusion distance in mutual diffusion is shorter (about 0.005 μm) than the thickness of the optical waveguide layer 3 (0.1 μm). Therefore, the composition change in the optical waveguide layer is small. Therefore, the effect of mutual diffusion on the optical waveguide can be ignored.

It should be noted that by performing heat treatment as a SiO ₂ cap, mutual constituent elements are exchanged between the multi-layered thin films thereunder and mutual diffusion is promoted.
ied Physics Letters. 56
(1), 1, 1990, pages 19-20, etc.

Next, SiN _x on the ridge is removed by photolithography (FIG. 7), and a p-side electrode metal is deposited. After polishing the substrate side to a thickness of 100 μm,
Evaporate n-side electrode metal.

In the above description, the case of using the n-type GaAs substrate has been described, but the present invention can be applied to the case of using the p-type GaAs substrate. However, in the case of a p-type GaAs substrate, the growth order by MOCVD is different, and a 1 μm thick cladding layer made of Mg-doped Al _X Ga _1-X As and a thickness made of non-doped Al _Y Ga _1-Y As are formed on the substrate. 0.1μ
m optical waveguide layer, a non-doped Al _Z Ga _1-Z As active layer having a thickness of 0.01 μm, and a non-doped Al _Y Ga _1-Y As optical waveguide layer having a thickness of 0.1 μm, which are laminated in this order. SiO ₂
Using the as a mask, a 1 μm thick clad layer made of Se-doped Al _x Ga _1-x As and a 0.1 μm thick cap layer made of Se-doped GaAs are grown to form a ridge. The procedure of the subsequent mutual diffusion is the same as that in the case of using the n-type GaAs substrate.

FIG. 8 is a cross-sectional view of another semiconductor laser to which the present invention is applied. In order to make the fundamental mode more stable, after laminating a thin clad layer on the optical waveguide layer in the non-ridge portion,
This is an example of forming a ridge.

FIG. 9 is a cross-sectional view of still another semiconductor laser to which the present invention is applied, which is an example in which the non-ridge portion is filled with an insulator or a high resistance layer so as to have a planar shape in order to enhance the heat radiation effect. ..

[0021]

According to the present invention, the thickness of the active layer in the non-ridge portion is determined by crystal growth, so that the thickness can be controlled with high accuracy, and the current leakage to the outside of the light confinement region can be suppressed to reduce the thickness. It is possible to obtain a ridge waveguide type semiconductor laser with a threshold stable lateral mode with good reproducibility.

Conventionally, mixed crystals were formed by thermal diffusion of impurities, ion implantation, and heat treatment. In that case, however, carriers are generated in the mixed crystal regions, so that light absorption by the carriers is caused. It was happening. However, such carriers are not generated in the mixed crystal formation using the insulating film, so that an optical waveguide with less optical loss is formed. Furthermore, since the insulator used as a mask when forming the ridge is used for mixed crystal formation, the manufacturing process is simple.

[Brief description of drawings]

FIG. 1 is a sectional view of a semiconductor laser to which a manufacturing method of the present invention is applied.

FIG. 2 is a method for manufacturing a semiconductor laser according to an embodiment of the present invention (stacking of optical waveguide layers).

FIG. 3 is a method for manufacturing a semiconductor laser according to an embodiment of the present invention (opening a stripe-shaped window).

FIG. 4 is a method for manufacturing a semiconductor laser according to an embodiment of the present invention (ridge formation).

FIG. 5 is a method for manufacturing a semiconductor laser according to an embodiment of the present invention (covering a non-ridge portion).

FIG. 6 is a method for manufacturing a semiconductor laser (mixed crystal) according to an embodiment of the present invention.

FIG. 7 is a method for manufacturing a semiconductor laser according to an embodiment of the present invention (removal of SiNx on a ridge).

FIG. 8 is a sectional view of another semiconductor laser to which the present invention is applied.

FIG. 9 is a sectional view of still another semiconductor laser to which the present invention is applied.

FIG. 10 is a sectional view of a conventional semiconductor laser.

[Explanation of symbols]

1 is an n-type semiconductor substrate, 2 is an n-type cladding layer, 5 is a p-type cladding layer, 3 is an optical waveguide layer, 4 is an active layer, 6 is a cap layer, 7 is SiO ₂ , 8 is SiN _x , and 9 is mixed. Crystallized region, 10
Is a p-side electrode, 11 is an n-side electrode, 12 is a current injection region, 1
Reference numeral 3 is a light confining region, and 14 is a buried layer.

Claims (2)

[Claims] 1. An active layer sandwiched by a first conductivity type clad layer and an optical waveguide layer is sequentially grown on a (100) plane GaAs substrate, and then extended in a [011] direction with respect to a crystal orientation of the GaAs substrate. And a step of growing a second conductivity type clad layer and a second conductivity type cap layer by providing a mask layer having a stripe-shaped window formed therein, and a method of manufacturing a semiconductor laser device. 2. An active layer sandwiched by a first conductivity type clad layer and an optical waveguide layer is sequentially grown on a (100) plane GaAs substrate, and then extended in a [011] direction with respect to a crystal orientation of the GaAs substrate. With a striped window
An iO ₂ layer is provided to grow a second conductivity type clad layer and a second conductivity type cap layer, and the active layer under the SiO ₂ layer is mixed with the optical waveguide layer to form a mixed crystal and a forbidden band width. And a step of performing a heat treatment so as to spread the semiconductor laser device.