Semiconductor laser and fabricating method therefor

Details Semiconductor laser and fabricating method therefor Semiconductor laser and fabricating method therefor

2002-09-05

2003-12-30

Whitehead, Jr., Carl (Department: 2813)

Semiconductor device manufacturing: process

Electron emitter manufacture

C438S022000, C438S032000, C438S047000

Reexamination Certificate

active

06670202

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor lasers and more particularly to a semiconductor laser that has a window region of a small quantity of laser light absorption at its light-emitting end surfaces. A semiconductor laser of this type is applied to an optical disk drive and the like that requires a high output.
The present invention also relates to a semiconductor laser fabricating method capable of fabricating the semiconductor laser of the above-mentioned type with high accuracy.
In the high-output semiconductor laser for use in an optical disk drive or the like, the light-emitting end surface sometimes deteriorates due to high density of light, possibly causing damage called COD (Catastrophic Optical Damage). As a measure against this, it has been proposed to provide the light-emitting end surfaces with a window region that absorbs less laser light than the inside of the active layer does.
As a conventional high-output semiconductor laser that has a window region at its light-emitting end surfaces, there is one as shown in
FIG. 20
(see WO96/11503). This semiconductor laser has on an n-type GaAs substrate
1
an n-conductivity type buffer layer
11
, an n-conductivity type first cladding layer
2
′, a first separate confinement layer
2
″, an active layer
3
, a second separate confinement layer
4
″, a p-conductivity type second cladding layer
4
′ and an etching stopper layer (having a thickness of 0.01 &mgr;m)
5
. A p-conductivity type second cladding layer
4
0
, a p-conductivity type intermediate layer
9
and a p-conductivity type first contact layer
10
are provided on this etching stopper layer
5
so as to constitute a mesa
12
that extends in a striped shape in the direction of line XXI—XXI in FIG.
20
. Regions at both sides of the mesa
12
are filled with an n-type current blocking layer
13
. A second contact layer
6
and an electrode (connection conductor)
7
are provided over the mesa
12
and the n-type current blocking layer
13
. On the other hand, an electrode (connection conductor)
8
is formed over the rear surface of the n-type GaAs substrate
1
.
As shown in
FIG. 21
(showing a cross section taken along the line XXI—XXI of FIG.
20
), the active layer
3
is constructed of a laminate of two quantum well layers
3
′ and a barrier layer
3
″ therebetween. Portions, which belong to the active layer
3
and are located near light-emitting end surfaces (exit surfaces)
50
and
51
, serve as window regions (passive regions)
3
B where the laser light absorption is less than in the active layer inside
3
A.
This semiconductor laser is fabricated as follows. As shown in
FIG. 22
, the layers of the n-conductivity type buffer layer
11
through the contact layer
10
are first grown on the n-type GaAs substrate
1
by OMVPE (organometallic vapor phase epitaxy). Next, a masking layer
30
made of silicon oxide is formed so as to have opening portions
31
and
32
along the light-emitting end surfaces
50
and
51
. The wafer in this state is introduced in a closed capsule together with zinc arsenide and the capsule is heated to a temperature of 600° C., so that Zn atoms
59
diffuse from the upper surface side of the contact layer
10
beyond the active layer
3
. Through these processes, local intermixing of the active layer
3
(namely making a part of the active layer
3
a mixed crystal) takes place at the portions near the light-emitting end surfaces
50
and
51
, which serve as the window regions
3
B where the energy bandgap is greater and accordingly the laser light absorption is less than in the active layer inside
3
A. After the mask
30
is removed, a strip-shaped mask
40
is formed, which extends perpendicularly to the light-emitting end surfaces
50
and
51
, as shown in FIG.
23
. Next, the mesa
12
is formed just under the mask
40
by etching the semiconductor layers
10
,
9
, and
4
0
at portions on both sides of the mask
40
until the etching stopper layer
5
is reached. Subsequently, as shown in
FIG. 20
, the blocking layer
13
is formed on both sides of the mesa
12
by OMVPE. After planarizing the blocking layer and removing the mask
40
, the second contact layer
6
is formed by using the OMVPE method again. Then, the electrodes
7
and
8
are formed over the upper surface of the contact layer
6
and the lower surface of the substrate
1
, respectively (the fabrication completed).
According to the aforementioned fabricating method, during the step of forming the window regions (passive regions)
3
B through intermixing of the active layer
3
by diffusion of impurity, intermixing of the etching stopper layer
5
may also take place. Then, there will be a problem that the etching stopper layer
5
and the second cladding layer (lower portion)
4
′ are etched in the process of forming the mesa
12
, which leads to reduction of the processing accuracy of the mesa
12
. If the etching progresses extremely, there may arise a further problem that the current blocking layer
13
and the n-type cladding layer
11
are disadvantageously electrically short-circuited. On the other hand, if the annealing temperature and time are reduced to avoid these problems related to the fabricating process, then there may conversely arise a problem that sufficient intermixing does not take place in the window region
3
B, resulting in difficulties in obtaining the effect of restraining photoabsorption.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a semiconductor laser which has a window region in its light-emitting end surfaces and is able to be easily fabricated with high accuracy.
Another object of this invention is to provide a method for easily fabricating a semiconductor laser having a window region in its light-emitting end surfaces, with high accuracy.
In order to accomplish the first object, there is provided, according to an aspect of the present invention, a semiconductor laser, which emits laser light through a light-emitting end surface, comprising:
a lower cladding layer, an active layer for generating laser light, a first upper cladding layer and an etching stopper layer stacked in this order on a substrate;
a second upper cladding layer formed in a shape of a ridge on the etching stopper layer, the ridge extending perpendicularly to the light-emitting end surface;
a current blocking layer disposed in regions on both sides of the second upper cladding layer; and
an impurity diffused in a portion extending along the light-emitting end surface from the etching stopper layer to the active layer and located at least under the ridge for local intermixing in this portion to restrain laser light absorption, wherein
in a region along the light-emitting end surface, the etching stopper layer has a bandgap smaller in portions thereof disposed in positions corresponding to both sides of the ridge than in a portion thereof located just under the ridge.
In the semiconductor laser of the present invention, in the region along the light-emitting end surface, the energy bandgap of the portions, of the etching stopper layer, that correspond to both sides of the ridge is smaller than the energy bandgap of the portion, of the etching stopper layer, that is located just under the ridge. Therefore, in the region along the light-emitting end surface, the portions corresponding to both sides of the ridge of the etching stopper layer can effectively fulfill the function to stop the etching when the second upper cladding layer is formed in a ridge shape on the etching stopper layer. Therefore, this semiconductor laser is easily fabricated with high accuracy.
In one embodiment, in the region along the light-emitting end surface, the active layer has a bandgap larger in a portion thereof located just under the ridge than in portions thereof disposed in positions corresponding to both sides of the ridge.
Accordingly, the portion, which belongs to the active layer and is located just under the ridge in the region along the light

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