Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-08-22
2004-11-30
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C438S022000
Reexamination Certificate
active
06826216
ABSTRACT:
RELATED APPLICATION DATA
The present invention claims priority to Japanese Application No. P2000-258139, filed Aug. 23, 2000, which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser used in optical disk systems, optic-magnetic disk memory systems, laser beam printers, and other optical information apparatuses and in optical communications and a method of production thereof, more particularly relates to a semiconductor laser of an oscillation wavelength in the visible light region of the 0.6 to 1.5 &mgr;m band and a method of production thereof.
2. Description of the Related Art
The maximum output of a semiconductor laser is limited by the catastrophic optical damage (COD) occurring along with a sharp rise of the temperature of the emission surface of the laser light due to the absorption of the laser light. As a high output semiconductor laser able to suppress the absorption of laser light and prevent COD, there is known a semiconductor laser of a window structure. In this window structure type semiconductor laser, a light guide layer of a bandgap larger than an active layer is provided at the emission surface side to suppress the absorption of the laser light.
When fabricating such a window structure type semiconductor laser, usually two times of growth processes are needed, but there is also a window structure type semiconductor laser that can be fabricated by one time of growth process.
For example, in the example disclosed in the Japanese Unexamined Patent Publication (Kokai) No. 6-232309, a ridge extending along a [01-1] direction is provided in advance in a portion (center portion) along a [01-1] direction of a GaAs (100) substrate (tilt angle: ±0.1°). On the substrate, an active layer comprised of a GaInAs/AlGaAs multi-quantum well layer and other layers of the laser structure are deposited by a single growth process using molecular beam epitaxy. The active layer comprised of the GaInAs/AlGaAs multi-quantum well layer formed on the ridge has a high concentration of In in the GaInAs layer and is thick, so the bandgap of the active layer is small only on the ridge. The active layer at the emission surface sides where the ridge is not provided has a relatively large bandgap compared with the active layer on the ridge, so a semiconductor laser of a window structure is realized.
Below, a method for producing such a window structure type semiconductor laser will be depicted.
FIG. 1
is a perspective view of the structure of a substrate for fabricating the above semiconductor laser.
A ridge
112
(height: 3 &mgr;m, width: 5 &mgr;m, length: 500 &mgr;m) extending along the [01-1] direction is formed at a portion (center portion) along a direction [01-1] on an n-type (100) GaAs substrate
111
(tilt angle: ±0.1°). Because of the ridge
112
, the substrate
111
is separated into a ridge area
113
formed with the ridge
112
and two non-ridge areas
114
not formed with the ridge at the two sides of the ridge area
113
. The length of each non-ridge area is 20 &mgr;m. Note that the ridge
112
is able to be formed by photolithography and etching using an etchant including buffered HF:H
2
O
2
:H
2
O (=10:1:10). In this case, the side surfaces
112
a
of the ridge
112
are the (311)A plane and the (3-1-1)A plane as reported in
Applied Physics Letters,
54, pp. 433 (1989).
FIG.
2
and
FIG. 3
are respectively a cross-sectional view and perspective view of a window structure type semiconductor laser fabricated using a substrate shown in FIG.
1
. Below, the procedure for fabricating this semiconductor laser will be described.
First, an n-type Al
0.3
Ga
0.7
As cladding layer
115
(thickness: 1 &mgr;m) is formed on the substrate
111
by molecular beam epitaxy at a substrate temperature of 550° C.
Next, a multi-quantum well layer
116
including a GaInAs layer (active layer) and a Al
0.1
Ga
0.9
As layer (light guide layer) is formed by molecular beam epitaxy at a substrate temperature of 520° C.
Further, a p-type Al
0.3
Ga
0.7
As cladding layer
117
(thickness: 1 &mgr;m) and a p-type GaAs cap layer
118
(thickness: 500 nm) are successively formed by molecular beam epitaxy at a substrate temperature of 550° C.
Here, the multi-quantum well layer
116
has a 5-cycle multi-quantum well layer. One cycle worth of the configuration in a non-ridge area
114
includes Ga
0.85
In
0.15
As (thickness: 7 nm) and Al
0.1
Ga
0.9
As (thickness: 7 nm).
After the above layers are grown, an electrode
119
is formed. In the ridge area
113
, a ridge-shaped light guide having a width of 4 &mgr;m and extending along the [01-1] direction is formed as shown in FIG.
2
and
FIG. 3
by patterning so that the center of the light guide coincides with the center of the ridge structure on the substrate
111
. Then, this is cleaved at the center of the non-ridge areas
114
to produce a semiconductor laser having the (01-1) plane as the end surfaces of its resonator. By this, a window structure type semiconductor laser can be fabricated to have a 500 &mgr;m long active layer in the ridge area
113
and a 20 &mgr;m long light guide layer of an end surface of a resonator in each non-ridge area
114
.
In the above description and as shown in
Applied Physics Letters,
56, pp. 1939 (1990), when growing a multi-quantum well layer at a portion where a ridge is formed, the In atoms in a (311) plane and (3-1-1) plane diffuse into the (100) plane.
Therefore, compared with the multi-quantum well layer
116
grown in the non-ridge areas
114
, the multi-quantum well layer
116
grown in the ridge area
113
has a high In concentration, a large thickness, and a narrow bandgap.
Therefore, the active layer at the emission surface sides has a relatively larger bandgap than the active layer on the ridge, so a window structure type semiconductor laser can be formed.
In the aforethe example of the related art, however, there is a disadvantage that the technique cannot be applied to an AlGaInP laser.
If a laser including P such as an AlGaInP laser is grown on a ridge stripe in the [01-1] direction as mentioned above, abnormal growth occurs around the ridge.
Because the growth rate declines in the region in the vicinity of the abnormal growth area, the growth rate on the ridge stripe declines and the thickness becomes smaller. If the thickness becomes small, the width of the well of the multi-quantum well layer becomes narrower, so the quantum level rises and the bandgap large becomes larger. On the other hand, the In concentration on the ridge stripe becomes higher regardless of abnormal growth, so the bandgap becomes small. There is therefore the disadvantage that the effects of the growth rate and the In concentration offset each other making it difficult to fabricate a window structure using a ridge stripe.
In addition, since the growth rate on the ridge stripe declines compared with that in a non-ridge area (for example, declines by 40% in the case of a ridge 30 &mgr;m in width and 2.7 &mgr;m in height), there also arises the disadvantage that a large deviation occurs in the positions of the active layer on a ridge stripe and the light guide layer in the non-ridge areas.
Due to this deviation, there are the problems that the light guide is distorted, the wave surface shifts, and the far field pattern becomes asymmetric.
Further, there is the problem that if an AlGaInP laser is formed on a GaAs (100) substrate that is not tilted, the surface morphology degrades.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a window structure type semiconductor laser able to suppress abnormal growth in the vicinity of a ridge, having almost no decline in growth rate on a ridge stripe, and having a good surface morphology and a method of production thereof.
To attain the above object, according to a first aspect of the present invention, there is provided a semiconductor laser having a light guide
Mitomo Jugo
Narui Hironobu
Leung Quyen
Sonnenschein Nath & Rosenthal LLP
Sony Corporation
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