Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-10-26
2004-10-26
Wong, Don (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010
Reexamination Certificate
active
06810055
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device which is used for an optical disk and so on and to a manufacturing method thereof. More particularly, the present invention relates to a window structure semiconductor laser device which is especially excellent in characteristics of high output operation and to a manufacturing method thereof.
Recently, various kinds of semiconductor lasers have been widely used as a light source for an optical disk device. In Particular, a high output semiconductor laser has been used as a light source for write access to a disk such as a MD (Mini Disc) player and a CD-R (Compact Disc Recordable) drive, and is strenuously required to have higher output.
One of the factors which do not allow a semiconductor laser to have a higher output is catastrophic optical damage (COD). COD is generated when an optical output density increases in an active layer region in the vicinity of a laser resonator end surface which is served as a light emitting end surface.
Generation of the COD is attributed to the fact that the active region in the vicinity of the laser resonator end surface is a region for absorbing laser beams. In the laser resonator end surface, there are a number of non-radiative recombination centers of a surface level or an interface level. Carriers injected into the active layer in the vicinity of the laser resonator end surface are dissipated due to the non-radiative recombination, which makes density of the carriers injected in the active layer in the vicinity of the laser resonator end surface lower than that in a center portion thereof. As a result, the active layer region in the vicinity of the laser resonator end surface acts as an absorbing region for a wavelength of the laser beam formed with high injected carrier density in the central portion.
With higher optical output density, heat generation becomes larger locally in the absorbing region, which increases a temperature and thereby reduces bandgap energy. As a result, there is established positive feedback in which an absorption coefficient further increments and the temperature increases, which causes the temperature of the absorbing region in the vicinity of the laser resonator end surface to reach a melting point of the absorbing region, thereby generating COD.
As one of methods for allowing a semiconductor laser to have a higher output in order to improve the COD level, there has been employed a method for utilizing a window structure through disordering a multiquantum well structure active layer as disclosed in Japanese Patent Laid-Open Publication HEI No. 9-23037.
For showing a prior art example of semiconductor lasers having the window structure, structural drawings of a semiconductor laser device described in Japanese Patent Laid-Open Publication No. H9-23037 are provided in
FIGS. 12A
to
12
C.
FIG. 12A
is a perspective view including a laser resonator end surface.
FIG. 12B
is a cross sectional view taken along a line Ia-Ia′ of
FIG. 12A
showing a wave guide.
FIG. 12C
is a cross sectional view in layer thickness direction taken along a line Ib-Ib′ of FIG.
12
A. In
FIGS. 12A
to
12
C, there are shown a GaAs substrate
1001
, an n-type AlGaAs lower cladding layer
1002
, a quantum well active layer
1003
, a p-type AlGaAs first upper cladding layer
1004
a
, a p-type AlGas second upper cladding layer
1004
b
, a p-type Gas contact layer
1005
, a vacancy diffusion region
1006
(shaded portion), a proton injection region
1007
(shaded portion), a negative electrode
1008
, a positive electrode
1009
, a laser resonator end surface
1020
, a region
1003
a
of the quantum well active layer
1003
contributing to laser emission, and a window structure region
1003
b
of the quantum well active layer
1003
formed in the vicinity of the laser resonator end surface
1020
.
Next, description will be given of a method for manufacturing conventional semiconductor laser devices with reference to a flowchart shown in FIG.
13
.
On an n-type GaAs substrate
1001
, there are epitaxially grown an n type AlGaAs lower cladding layer
1002
, a quantum well active layer
1003
, and a p-type AlGaAs first upper cladding layer
1004
a
in sequence (FIG.
13
A). Next, an SiO
2
film
1010
is formed on the surface of the p-type AlGaAs first upper cladding layer
1004
a
. Further, a stripe-shaped opening
1010
a
is formed in the SiO
2
film
1010
in such a way to extend in laser resonator direction but not to reach a laser resonator end surface (
FIG. 133
) A thus-prepared wafer is annealed at a temperature of 800° C. or more in an As atmosphere. During annealing, the SiO
2
film
1010
absorbs Ga atoms from the surface of the adjacent p-type AlGaAs first upper cladding layer
1004
a
, so that Ga vacancies are formed within the p-type AlGaAs first upper cladding layer
1004
a
. Some of the Ga vacancies diffuse till they reach the quantum well active layer
1003
inside the crystal, thereby causing disorder of the quantum well structure. In a disordered active layer region, an effective forbidden bandwidth extends, and therefore the disordered active layer region functions as a transparent window for emitted laser beans.
Then, the SiO
2
film
1010
is removed, and there are epitaxially grown a p-type AlGaAs second upper cladding layer
1004
b
and a p-type GaAs contact layer
1005
in sequence on the p-type AlGaAs first upper cladding layer
1004
a
(FIG.
13
C). Next, a resist film is formed on the p-type GaAs contact layer
1005
a
. Then, a stripe-shaped resist
1011
is formed by photolithographic technique above the same region as the stripe-shaped opening
1010
a
of the SiO
2
film
1010
. Then, with the stripe-shaped resist
1011
as a mask, protons are injected thorough the surface of the p-type GaAs contact layer
1005
into the p-type AlGaAs second upper cladding layer
1004
b
so as to form a high resisting region
1007
which serves as a current blocking layer (FIG.
13
D). Finally, a negative electrode
1008
is formed on the GaAs substrate
1001
, and a positive electrode
1009
is formed on the p-type GaAs contact layer
1005
. The thus-completed wafer is cleaved to provide the semiconductor laser device of FIG.
12
.
In the case of the conventional window structure semiconductor laser device, an SiO
2
film
1010
is formed on the surface of the p-type AlGaAs first upper cladding layer
1004
a
so that bandgap energy In the disordered region formed in the vicinity of the laser resonator end surface may become larger than bandgap energy corresponding to a laser emission wavelength. By formation of the SiO, film
1010
, Ga vacancies are created in the p-type AlGas first upper cladding layer
1004
a
adjacent to the Sio
2
film
1010
, and some of the Ga vacancies are diffused into the quantum well active layer
1003
.
Creation and diffusion of the Ga vacancies are conducted in the region covered with the SiO
2
film
1010
. However, annealing at a temperature of 800° C. or more causes creation of Ga vacancies, though small in amount due to re-evaporation of Ga atoms, in the uppermost surface of the region not covered with he SiO
2
film
1010
(region inside the laser resonator). As a result, the Ga vacancies are diffused into the quantum well active layer
1003
.
Consequently, the bandgap of the quantum well active layer
1003
fluctuates inside the laser resonator, and the long-term reliability of the quantum well active layer
1003
is degraded due to deterioration of crystallinity.
If annealing temperature is lower or if annealing time is shorter as an countermeasure against the above problem, diffusion of Ga vacancies to the quantum well active layer
1003
may be inhibited inside the laser resonator. However, it becomes insufficient to create vacancies in the region covered with the SiO
2
film
1010
and to diffuse the vacancies to the quantum well active layer
1003
in the region covered with the SiO
2
film
1010
. This results in adsorption of laser beams in the vicinity of the laser resonator end surface
Al-Nazer Leith A
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Wong Don
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