Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-08-31
2003-03-25
Davie, James (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013
Reexamination Certificate
active
06539040
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No. 2000-267634 filed on Sep. 4, 2000, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to laser diodes and more particularly to a laser diode of lateral-mode control type formed on a GaAs substrate.
Laser diodes that use AlInP or AlGaInP for the cladding layer have various advantageous features such as laser oscillation in a visible red wavelength band, capability of focusing the laser beam to a small spot, and the like, and are used extensively for the optical source of high-density optical disk recording apparatuses including a DVD apparatus.
AlInP or AlGaInP is a material that has the largest bandgap among the III-V materials that achieve a lattice matching with a GaAs substrate and are indispensable for the material of cladding layers of a laser diode that oscillates in a red wavelength band.
FIG. 1
is a diagram showing the construction of a typical conventional ridge-type laser diode
10
having an ordinary mesa structure that forms a refractive-index waveguide.
Referring to
FIG. 1
, the laser diode is constructed on an n-type GaAs substrate
11
and includes a buffer layer
12
of n-type GaAs formed on the substrate
11
, a cladding layer
13
of n-type AlGaInP formed on the buffer layer
12
with a composition of Al
0.35
Ga
0.15
In
0.5
P, and an active layer
14
of a strained multiple quantum well structure formed on the cladding layer
13
.
The active layer
14
may be formed of alternate and repetitive stacking of a quantum well layer of GaInP having a thickness of 6 nm and a barrier layer of AlGaInP having a thickness of 4 nm and a composition of Al
0.2
Ga
0.3
In
0.5
P, wherein the foregoing stacked structure forming the active layer
14
is vertically sandwiched by a pair of optical waveguide layers of AlGaInP having a thickness of 10 nm and a composition of Al
0.2
Ga
0.3
In
0.5
P.
On the active layer
14
, there is formed a cladding layer
15
of p-type AlGaInP having a composition represented as Al
0.35
Ga
0.15
In
0.5
P, and an etching stopper layer
16
of p-type GaInP is formed on the cladding layer
15
. Further, another cladding layer
17
of p-type AlGaInP having a composition of Al
0.35
Ga
0.15
In
0.5
P and an intermediate layer
18
of p-type GaInP are formed on the etching stopper layer
16
consecutively. Thereby, the cladding layer
17
and the intermediate layer
18
are patterned by a photolithographic process to form an ordinary mesa structure constituting a ridge structure extending axially through the laser diode, and a pair of current blocking regions
19
are formed at both lateral sides of the foregoing ridge structure.
On the current blocking regions
19
thus formed, there is formed a contact layer
20
of p-type GaAs such that the contact layer
20
makes a contact with the foregoing intermediate layer
18
on the top part of the foregoing mesa region.
The ridge-type laser diode of the foregoing construction is capable of realizing laser oscillation at a desired visible wavelength by using the strained multiple quantum well structure of GaInP/AlGaInP noted before for the active layer
14
. Further, the use of the current blocking regions
19
at both lateral sides of the ridge structure extending at the central part of the laser diode in the axial direction thereof enables confinement of the driving current to the foregoing ridge structure. Further, the use of the GaAs current confinement regions
19
in combination with the ridge structure is effective for confinement of the optical radiation formed in the active layer
14
in the ridge structure and for guiding therethrough.
In such a conventional ridge-type laser diode, on the other hand, it is required to conduct a photolithographic process for forming the mesa structure. Further, it is required to form the current blocking regions
19
by way of regrowth of a GaAs layer. Thus, the conventional ridge-type laser diode has a drawback of needing a complicated fabrication process. In addition, the ridge-type laser diode of
FIG. 1
has a drawback of increased threshold of laser oscillation caused as a result of optical absorption by the GaAs current blocking regions
19
. Thus, the conventional ridge-type laser diode has suffered from the problem of poor efficiency of laser oscillation.
It is also known to modify the ridge-type laser diode of
FIG. 1
by replacing the mesa structure with an inverse-mesa structure for reducing the device resistance. However, the foregoing problems cannot be avoided by such a modification of the conventional ridge-type laser diode.
Meanwhile, the inventor of the present invention has proposed, in the Japanese Laid-Open Patent Publication 06-045708, a so-called S
3
(self-aligned stepped substrate)-type laser diode
30
shown in FIG.
2
.
Referring to
FIG. 2
, the laser diode
30
is formed on a GaAs substrate
31
of n-type, wherein the GaAs substrate
31
is formed with a stripe region of an inclined surface, which may be a (311)A surface or a (411)A surface. The substrate
31
is covered with a buffer layer
32
of n-type GaAs, wherein the buffer layer
32
forms a stripe region defined by an inclined surface in correspondence to the stripe region on the GaAs substrate
31
. Further, the buffer layer
32
is covered by an intermediate layer
33
of n-type GaInP, wherein the intermediate layer
33
has an inclined stripe region formed in correspondence to the inclined stripe region on the buffer layer
32
and hence the inclined stripe region on the GaAs substrate
31
.
On the intermediate layer
33
, there is formed a cladding layer
34
of n-type AlGaInP in conformity with the underlying intermediate layer
33
, wherein the cladding layer
34
thus formed includes an inclined stripe region in correspondence to the inclined stripe region on the intermediate layer
33
.
On the cladding layer
34
, there is formed an active layer
35
of a strained multiple quantum well structure similar to the active layer
14
, in conformity with the underlying cladding layer
34
, wherein the active layer
35
includes an inclined stripe region corresponding to the inclined stripe region formed on the cladding layer
34
.
Further, a cladding layer
36
of p-type AlGaInP is formed on the active layer
35
in conformity with the underlying active layer
35
, wherein the cladding layer
36
includes an inclined stripe region corresponding to the inclined stripe region formed on the active layer
35
. The cladding layer
36
in turn is covered by a current confinement layer
37
of n-type AlGaInP formed in conformity with the underlying cladding layer
36
, wherein the current confinement layer
37
includes an inclined stripe region corresponding to the inclined stripe region formed in the cladding layer
36
.
Further, the current confinement layer
37
is covered by another cladding layer
38
of p-type AlGaInP in conformity with the current confinement layer
37
, wherein the cladding layer
38
includes an inclined stripe region in correspondence to the inclined stripe region formed in the underlying current confinement layer
37
. Further, the cladding layer
38
is covered with an intermediate layer
39
of p-type GaInP formed in conformity with the underlying cladding layer
38
, wherein the intermediate layer
39
includes an inclined stripe region formed in correspondence to the inclined stripe region of the cladding layer
38
. Further, the intermediate layer
39
is covered by a contact layer
40
of p-type GaAs formed in conformity with the underlying intermediate layer
39
, wherein the contact layer
40
includes an inclined stripe region formed in correspondence to the inclined stripe region of the intermediate layer
39
.
While not illustrated, the laser diode
30
of
FIG. 2
further includes an n-type electrode at a bottom principal surface of the GaAs substrate
31
and a p-type electrode is formed on the contact layer
40
.
The foregoing semiconductor layers
32
-
40
Davie James
Fujitsu Quantum Devices Limited
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