Semiconductor laser

Details Semiconductor laser Semiconductor laser

2001-06-22

2003-09-09

Lee, Eddie (Department: 2815)

Coherent light generators

Particular active media

Semiconductor

C372S043010, C372S044010, C372S045013

Reexamination Certificate

active

06618416

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a semiconductor laser device, and in particular to a semiconductor laser device formed of InGaAlN-based materials.
BACKGROUND ART
InGaAlN-based materials are used for emitting blue to green light. Specifically, there have been attempts to apply a refractive index waveguide-type InGaAlN-based semiconductor laser device for an optical disk pickup since this type of semiconductor laser device emits light which is close to a plane wave and thus can reduce an astigmatic difference.
FIG. 4
shows a structure of a conventional refractive index waveguide-type blue semiconductor laser device
450
. The conventional device
450
includes a sapphire substrate
400
and the following layers sequentially formed on the sapphire substrate 400: an n-type GaN contact layer
401
, an n-type A
0.1
Gag
0.9
N cladding layer
402
having a thickness of 0.5 &mgr;m, an n-type GaN optical guide layer
403
having a thickness of 100 nm, a multi-layer quantum well active layer
404
which includes three In
0.2
Ga
0.8
N quantum well layers each having a thickness of 2.5 nm and four In
0.05
Ga
0.95
N barrier layers each having a thickness of 3 nm, an Al
0.1
Ga
0.9
N protective layer
405
having a thickness of 25 nm, a p-type GaN optical guide layer
406
having a thickness of 50 nm, a p-type Al
0.1
Ga
0.9
N cladding layer
407
having a total thickness of 0.6 &mgr;m which includes a lower flat region
410
having a thickness of 0.1 &mgr;m and a ridge stripe portion
411
having a width of 2 &mgr;m and a height of 0.5 &mgr;m, and a p-type GaN contact layer
408
having a width of 2 &mgr;m and a thickness of 0.2 &mgr;m. The p-type GaN contact layer
408
is formed on the ridge stripe portion
411
of the p-type cladding layer
407
. An SiO
2
insulating layer
409
having a thickness of 0.3 &mgr;m and a smaller refractive index than that of the ridge stripe portion
411
is formed so as to cover a top surface of the lower flat region
410
of the p-type cladding layer
407
and side surfaces of the ridge stripe portion
411
. An n-type electrode
413
is formed on an exposed surface of the n-type GaN contact layer
401
, and a p-type electrode
414
is formed on a surface of the p-type contact layer
408
on the ridge stripe portion
411
. In the figure, reference numeral
412
represents ridge corners.
In the conventional semiconductor laser device
450
, the refractive index of the SiO
2
insulating layer
409
is smaller than the refractive index of the InGaAlN materials, and therefore the effective refractive index of the outside of the ridge is reduced, so that laser light is guided to the ridge region and the vicinity thereof.
The conventional InGaAlN-based semiconductor Ilaser device
450
was subjected to a reliability test under the conditions of 60° C. and a constant output of 5 mW. The value of the operating current increased to 1.2 times or more the initial value, and the device
450
malfunctioned within 100 hours. Accordingly, the conventional semiconductor laser device is considered to have a life of about 100 hours. It was found that a life of 5000 hours or more, which is necessary for a semiconductor laser device used for an optical disk apparatus, cannot be realized.
The laser device before the reliability test and the laser device which malfunctioned during the reliability test were compared and analyzed. As a result, the post-malfunction laser device was observed to have crystal defects increased in the ridge corner areas
412
(both ends of the base of the ridge stripe portion
411
and the vicinity thereof). The present inventors found that in accordance with the increase in the crystal defects, the emission efficiency drastically declines in areas of the multi-layer quantum well active layer
404
which are contained in the ridge corner areas
412
, and this is the main cause of reduction In the life of the conventional device
450
. In the conventional device having the above-described structure, the crystal defect density of the post-malfunction ridge corner areas
412
was 6×10
11
to 8×10
11
cm
−2
, which was higher than that of the defect density of the remaining area (3 to 7×10
10
cm
−2
) by one order of magnitude.
The decline in the reliability due to the increase in the crystal defects is considered to be caused by the strong inter-atom bond of the InGaAlN-based crystal itself and by a large difference between the thermal expansion coefficient of the material forming the ridge stripe portion
411
(p-type Al
0.1
Ga
0.9
N cladding layer
407
; 5.6×10
−6
/° C.) and the thermal expansion coefficient of the.material surrounding the bottom portion of the ridge stripe portion
411
(SiO
2
insulating layer
409
; 0.5×10
−6
/° C.), the difference being as large as +5.1×10
−6
/° C. In other words, when the conventional device
450
which has such a large difference in the thermal expansion coefficient between the two materials is made conductive, heat is generated In the ridge stripe portion
411
and the vicinity thereof where the current is concentrated, and thus the temperature is raised locally. It Is considered that the crystal defeat and crystal breakage occurred for the following reason. When the local temperature rise occurs in the InGaAlN-based crystal of the above-described structure, thermal distortion is induced at the ridge corner areas
412
by the difference in the thermal expansion coefficient between the p-type Al
0.1
Ga
0.9
N cladding layer
407
forming the ridge stripe portion
411
and the SiO
2
insulating layer
409
surrounding the ridge stripe portion
411
. The crystal defeat and crystal breakage started from the ridge corner areas
412
. Such a phenomenon, which was not observed with InGaAlAs-based materials or InGaAlP-based materials which were conventionally laser used for ridge stripe-type laser devices, was specific to an InGaAlN-based (nitride-based) semiconductor device having a strong bond between a group III atom and a nitrogen atom.
DISCLOSURE OF THE INVENTION
A semiconductor laser device according to the present invention Is an InGaAlN-based semiconductor laser device including a first layer of a first conductivity type, an active layer having a smaller forbidden band than that of the first layer, and a second layer of a second conductivity type having a larger forbidden band than that of the active layer, wherein the second layer includes a flat region and a stripe-shaped projecting structure; a stripe-shaped optical waveguide forming layer of the second conductivity type having a larger refractive index than that of the second layer is formed on the stripe-shaped projecting structure; a current-constricting layer of the first conductivity type or of a high resistance is formed for covering a top surface of the flat region of the second layer, a side surface of the projecting structure of the second layer, and a side surface of the optical waveguide forming layer; and a difference between a thermal expansion coefficient of the current-constricting layer and a thermal expansion coefficient of the second layer is in the range of −4×10
−9
/° C. to +4×10
−
/° C.
Due to such a structure, even when the temperature of the device is locally increased in the stripe-shaped projecting structure and the vicinity thereof in which the current is injected in a concentrated manner, the thermal distortion of the device is suppressed owing to the small difference in the thermal expansion coefficient between the current-constricting layer and the second conductive layer. Therefore, a local crystal defect or crystal breakage is avoided, thus extending the life of the laser device.
In one embodiment of the invention, the second layer and the current-constricting layer are formed of an InGaAlN-based semiconductor material of the same composition. For example, the second layer may be formed of Al
x
Ga
1−x
N, and the current-constricting layer may be formed of Al
y
Ga
1−y
N, (−0.08≦x−y&lE

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