U.S. patent application number 10/720918 was filed with the patent office on 2004-07-01 for semiconductor laser and production method thereof.
Invention is credited to Ukita, Masakazu.
Application Number | 20040125844 10/720918 |
Document ID | / |
Family ID | 18418523 |
Filed Date | 2004-07-01 |
United States Patent
Application |
20040125844 |
Kind Code |
A1 |
Ukita, Masakazu |
July 1, 2004 |
Semiconductor laser and production method thereof
Abstract
A semiconductor laser basically includes a first cladding layer;
an active layer; a second cladding layer; and a current
constriction means for defining a current injection region in the
active layer. The active layer has a gain region which acquires an
optical gain by current injection thereto; a saturable absorption
region in which current injection thereto little occurs and light
effusion thereto occurs; and an outside region., being in contact
with the saturable absorption region, in which current injection
thereto little occurs and light effusion thereto little occurs. In
this semiconductor laser, an effective band gap of the saturable
absorption region is set to be larger than that of the outside
region. With this configuration, carriers in the saturable
absorption region are efficiently migrated to the outside region,
so that the carrier lifetime in the saturable absorption region is
actually shortened. As a result, the semiconductor laser can
sustain the self pulsation at a high light output and a high
operational temperature, and further can be produced with a good
production yield.
Inventors: |
Ukita, Masakazu; (Kanagawa,
JP) |
Correspondence
Address: |
SONNENSCHEIN NATH & ROSENTHAL LLP
P.O. BOX 061080
WACKER DRIVE STATION, SEARS TOWER
CHICAGO
IL
60606-1080
US
|
Family ID: |
18418523 |
Appl. No.: |
10/720918 |
Filed: |
November 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10720918 |
Nov 24, 2003 |
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10191151 |
Jul 9, 2002 |
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6682949 |
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10191151 |
Jul 9, 2002 |
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09457133 |
Dec 9, 1999 |
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6470039 |
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Current U.S.
Class: |
372/45.013 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/0658 20130101; H01S 5/2231 20130101; H01S 5/2237 20130101;
H01S 5/2022 20130101; H01S 5/3415 20130101 |
Class at
Publication: |
372/046 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 10, 1998 |
JP |
P10-351623 |
Claims
What is claimed is:
1. A semiconductor laser comprising: a first cladding layer; an
active layer; a second cladding layer; and a current constriction
means for defining a current injection region in said active layer;
wherein said active layer has a gain region which acquires an
optical gain by current injection thereto; a saturable absorption
region in which current injection thereto little occurs and light
effusion thereto occurs; and an outside region, being in contact
with said saturable absorption region, in which current injection
thereto little occurs and light effusion thereto little occurs; and
an effective band gap of said saturable absorption region is larger
than that of said outside region.
2. A semiconductor laser comprising: a first cladding layer; an
active layer; a second cladding layer; and a current constriction
means for defining a current injection region in said active layer;
wherein said active layer has a gain region which acquires an
optical gain by current injection; and a saturable absorption layer
is provided in at least one of said first and second cladding
layers, said saturable absorption layer having a saturable
absorption region which has an effective band gap nearly equal to
or narrower than that of said active layer and in which light
effusion thereto occurs, and an outside region, being in contact
with said saturable absorption region, which has an effective band
gap smaller than that of said saturable absorption region and in
which light effusion thereto little occurs.
3. A semiconductor laser according to claim 1, wherein each of said
saturable absorption region and said outside region has a double
hetero structure; and an effective band gap of said saturable
absorption region is larger than that of said outside region.
4. A semiconductor laser according to claim 2, wherein each of said
saturable absorption region and said outside region has a double
hetero structure; and an effective band gap of said saturable
absorption region is larger than that of said outside region.
5. A semiconductor laser according to claim 1, wherein each of said
saturable absorption region and said outside region has a single or
multiple quantum well structure; and an effective band gap of a
quantum well layer in said saturable absorption region is larger
than that of an quantum well layer in said outside region.
6. A semiconductor laser according to claim 2, wherein each of said
saturable absorption region and said outside region has a single or
multiple quantum well structure; and an effective band gap of a
quantum well layer in said saturable absorption region is larger
than that of an quantum well layer in said outside region.
7. A semiconductor laser according to claim 1, wherein each of said
saturable absorption region and said outside region has a single or
multiple quantum well structure; and a thickness of a quantum well
layer in said saturable absorption region is smaller than that of
an quantum well layer in said outside region.
8. A semiconductor laser according to claim 2, wherein each of said
saturable absorption region and said outside region has a single or
multiple quantum well structure; and a thickness of a quantum well
layer in said saturable absorption region is smaller than that of
an quantum well layer in said outside region.
9. A semiconductor laser according to claim 1, wherein each of said
saturable absorption region and said outside region has a multiple
quantum well structure; and a thickness of each quantum well
barrier layer between two quantum well layers in said saturable
absorption region is larger than that of each quantum well barrier
layer between two quantum well layers in said outside region.
10. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
multiple quantum well structure; and a thickness of each quantum
well barrier layer between two quantum well layers in said
saturable absorption region is larger than that of each quantum
well barrier layer between two quantum well layers in said outside
region.
11. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
multiple quantum well structure; and the number of quantum well
layers in said saturable absorption region is smaller than that of
quantum well layers in said outside region.
12. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
multiple quantum well structure; and the number of quantum well
layers in said saturable absorption region is smaller than that of
quantum well layers in said outside region.
13. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and a band gap of a quantum wire in said
saturable absorption region is larger than that of a quantum wire
in said outside region.
14. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and a band gap of a quantum wire in said
saturable absorption region is larger than that of a quantum wire
in said outside region.
15. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and a thickness of a quantum wire in said
saturable absorption region is smaller than that of a quantum wire
in said outside region.
16. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and a thickness of a quantum wire in said
saturable absorption region is smaller than that of a quantum wire
in said outside region.
17. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and the number of quantum wires in said
saturable absorption region is smaller than that of quantum wires
in said outside region.
18. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and the number of quantum wires in said
saturable absorption region is smaller than that of quantum wires
in said outside region.
19. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and at least one of quantum wires is formed
in such a manner as to cross from said saturable absorption region
to said outside region.
20. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
quantum wire structure; and at least one of quantum wires is formed
in such a manner as to cross from said saturable absorption region
to said outside region.
21. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
plurality of quantum dot structures; and an average band gap of
quantum dots in said saturable absorption region is larger than
that of quantum dots in said outside region.
22. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
plurality of quantum dot structures; and an average band gap of
quantum dots in said saturable absorption region is larger than
that of quantum dots in said outside region.
23. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
plurality of quantum dot structures; and an average volume of
quantum dots in said saturable absorption region is smaller than
that of quantum dots in said outside region.
24. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
plurality of quantum dot structures; and an average volume of
quantum dots in said saturable absorption region is smaller than
that of quantum dots in said outside region.
25. A semiconductor laser according to claim 1, wherein each of
said saturable absorption region and said outside region has a
plurality of quantum dot structures; and a number density of
quantum dots in said saturable absorption region is smaller than
that of quantum dots in said outside region.
26. A semiconductor laser according to claim 2, wherein each of
said saturable absorption region and said outside region has a
plurality of quantum dot structures; and a number density of
quantum dots in said saturable absorption region is smaller than
that of quantum dots in said outside region.
27. A semiconductor laser according to claim 1, wherein said
outside region has a double hetero structure; and said saturable
absorption region has a quantum well structure, a quantum wire
structure, or a quantum dot structure.
28. A semiconductor laser according to claim 2, wherein said
outside region has a double hetero structure; and said saturable
absorption region has a quantum well structure, a quantum wire
structure, or a quantum dot structure.
29. A semiconductor laser according to claim 1, wherein said
outside region has a quantum well structure; and said saturable
absorption region has a quantum wire structure or a quantum dot
stricture.
30. A semiconductor laser according to claim 2, wherein said
outside region has a quantum well structure; and said saturable
absorption region has a quantum wire structure or a quantum dot
structure.
31. A semiconductor laser according to claim 1, wherein said
outside region has a quantum wire structure; and said saturable
absorption region has a quantum dot structure.
32. A semiconductor laser according to claim 2, wherein said
outside region has a quantum wire structure; and said saturable
absorption region has a quantum dot structure.
33. A method of producing a semiconductor laser, comprising: a
first growth step of sequentially growing, on a substrate, a first
cladding layer, a first active layer for forming a gain region, and
a second cladding layer, to form a stacked semiconductor layer; a
first groove formation step of forming stripe-like first grooves in
part of said stacked semiconductor layer with a specific gap kept
between the grooves, to form a stripe-like ridge between the
grooves and to expose the first cladding layer from the bottoms of
the first grooves; a second growth step of growing a second active
layer for forming a saturable absorption region on the first
cladding layer exposed in the first grooves, the second active
layer having a composition different from that of the first active
layer, and growing a current constriction layer for forming a
current constriction means; a second groove formation step of
forming second grooves on both sides of the ridge with specific
distances kept between the second grooves and the ridge, to expose
the first cladding layer from the bottoms of the second grooves;
and a third growth step of growing a third active layer for forming
an outside region on the first cladding layer exposed in the second
grooves, the third active layer having a composition different from
those of the first and second active layers for forming the gain
region and the saturable absorption region, and growing a current
constriction layer for forming the current constriction means;
wherein effective band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 of the
gain region, saturable absorption region and outside region,
respectively, are selected to satisfy an inequality of
Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3.
34. A method of producing a semiconductor laser, comprising: a
first growth step of sequentially growing, on a substrate, a first
cladding layer, an active layer having an effective band gap which
is uniform in a first direction and which becomes smaller from the
center to both sides of the active layer in a second direction
perpendicular to the first direction, and a second cladding layer,
to form a stacked semiconductor layer; a groove formation step of
forming stripe-like grooves spaced from each other at a specific
gap in the stacked semiconductor layer in such a manner as to leave
a portion, having a large effective band gap, of the active layer
in a stripe between the grooves and to leave a portion, having a
specific thickness, of the second cladding layer on the bottoms of
the grooves; and a second growth step of growing, in the grooves, a
current constriction layer, which forms a current constriction
means for defining a current injection region at the stripe
portion, having the large effective band gap, of the active
layer.
35. A method of producing a semiconductor laser, comprising: a
growth step of sequentially growing, on a substrate, a first
cladding layer, an active layer having an effective band gap which
is uniform in a first direction and which becomes smaller from the
center to both sides of the active layer in a second direction
perpendicular to the first direction, and a second cladding layer,
to form a stacked semiconductor layer; and a current constriction
layer formation step of implanting ions of an impurity in side
portions, each having a small effective band gap, on both sides of
a stripe portion, having a large effective band gap, of the active
layer, to form a current constriction layer which forms a current
constriction means for defining a current injection region at the
stripe portion, having the large effective band gap, of the active
layer.
36. A method of producing a semiconductor laser according to claim
34, further comprising: a step of forming, on the substrate,
stripe-like masks for selectively forming semiconductor films; and
a step of growing the stacked semiconductor layer after said mask
formation step
37. A method of producing a semiconductor laser according to claim
35, further comprising: a step of forming, on the substrate,
stripe-like masks for selectively forming semiconductor films; and
a step of growing the stacked semiconductor layer after said mask
formation step.
38. A method of producing a semiconductor laser according to claim
34, further comprising: a step of disposing stripe-like masks for
selectively forming semiconductor films on the substrate or at
positions opposed to the substrate; and a step of growing the
stacked semiconductor layer through the masks.
39. A method of producing a semiconductor laser according to claim
35, further comprising: a step of disposing stripe-like masks for
selectively forming semiconductor films on the substrate or at
positions opposed to the substrate; and a step of growing the
stacked semiconductor layer through the masks.
40. A method of producing a semiconductor laser, comprising: a
first growth step of growing, on a substrate, a first cladding
layer, an active layer, a second cladding layer, and a saturable
absorption layer which is positioned at least one of the first and
second cladding layers and which forms a saturable absorption
region, to form a stacked semiconductor layer; a groove formation
step of forming, in part of the stacked semiconductor layer,
stripe-like grooves spaced from each other at a specific gap to a
depth crossing the saturable absorption layer for forming a
stripe-like ridge; and a second growth step of growing, in the
grooves, a saturable absorption layer for forming a current
constriction layer which forms, at least on both sides of the
ridge, a current constriction means for defining a current
injection region in the active layer, and for forming an outside
region which is in contact with the saturable absorption layer and
which has an effective band gap smaller than that of the saturable
absorption layer.
41. A method of producing a semiconductor laser, comprising: a
first growth step of growing, on a substrate, a first cladding
layer, an active layer, a second cladding layer, and a saturable
absorption layer positioned in at least one of the first and second
cladding layers and having an effective band gap which is uniform
in a first direction and which becomes smaller from the center to
both sides of the saturable absorption layer in a second direction
perpendicular to the first direction, to form a stacked
semiconductor layer; a groove formation step of forming stripe-like
grooves spaced from each other at a specific gap in the stacked
semiconductor layer to such a depth as to leave a portion, having a
large effective band gap, of the saturable absorption layer in a
stripe between the grooves and to leave part of the saturable
absorption layer and a portion, having a specific thickness, of the
second cladding layer, and a second growth step of growing, in the
grooves, a current constriction layer, which forms a current
constriction means for defining a current injection region in the
active layer, on both sides of the ridge.
42. A method of producing a semiconductor laser, comprising: a
growth step of growing, on a substrate, a first cladding layer, an
active layer, a second cladding layer, and a saturable absorption
layer positioned in at least one of the first and second cladding
layers and having an effective band gap which is uniform in a first
direction and which becomes smaller from the center to both sides
of the saturable absorption layer in a second direction
perpendicular to the first direction, to form a stacked
semiconductor layer; an impurity implantation step of implanting
ions of an impurity in a portion, having a small effective band
gap, of the saturable absorption layer of the stacked semiconductor
layer, to form a current constriction layer which forms a current
constriction means for defining a current injection region in the
active layer.
43. A method of producing a semiconductor laser according to claim
41, further comprising: a step of forming, on the substrate,
stripe-like masks for selectively forming semiconductor films; and
a step of growing the stacked semiconductor layer after said mask
formation step.
44. A method of producing a semiconductor laser according to claim
42, further comprising: a step of forming, on the substrate,
stripe-like masks for selectively forming semiconductor films; and
a step of growing the stacked semiconductor layer after said mask
formation step.
45. A method of producing a semiconductor laser according to claim
41, further comprising: a step of disposing stripe-like masks for
selectively forming semiconductor films on the substrate or at
positions opposed to the substrate; and a step of growing the
stacked semiconductor layer through the masks.
46. A method of producing a semiconductor laser according to claim
42, further comprising: a step of disposing stripe-like masks for
selectively forming semiconductor films on the substrate or at
positions opposed to the substrate; and a step of growing the
stacked semiconductor layer through the masks.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a semiconductor laser and a
production method thereof, and particularly to a self pulsation
type semiconductor laser and a production method thereof.
[0002] FIG. 16 is a schematic sectional view, seen along the
direction perpendicular to the resonator length direction, of a
related art inner stripe type semiconductor laser.
[0003] Layers are epitaxially grown in sequence on the entire
surface of a substrate 1 made from n-type GaAs: a first cladding
layer 2 made from n-type Al.sub.0.5Ga.sub.0.5As, an active layer 3
made from Al.sub.0.15Ga.sub.0.85As, a second cladding layer 4 made
from p-type Al.sub.0.5Ga.sub.0.5As, and a heavily doped contact
layer 5 made from p-type GaAs. The stacked layers are selectively
etched from the contact layer 5 side up to a depth reaching the
inner portion of the second cladding layer 4, to form two grooves
8, thereby forming a stripe-like ridge 7 extending in the direction
perpendicular to the paper plane of FIG. 16 between the grooves 8.
In this case, the depth of the groove 8 is selected such that the
second cladding layer 4 having a specific thickness "d" remains
under the grooves 8.
[0004] A current constriction layer 6 made from n-type GaAs is
grown in such a manner as to bury the grooves 8.
[0005] A first electrode 9 is formed on the contact layer 5 and the
current constriction layer 6 in such a manner as to be in
ohmic-contact therewith, and a second electrode 10 is formed on the
back surface of the substrate 1 in such a manner as to be in
ohmic-contact therewith.
[0006] In the semiconductor laser having such a configuration, the
active layer 3 is divided into a gain region 11, two saturable
absorption regions 12 on both sides of the gain region 11, and two
outside regions 13 on both sides of the saturable absorption
regions 12.
[0007] A current, which is restrictively supplied to the
stripe-like ridge 7 by the effect of the current constriction layer
6, is injected in the gain region 11 of the active layer, with a
result that a gain necessary for laser oscillation occurs only in
the gain region 11 of the active layer 3.
[0008] The saturable absorption region 12 does not undergo current
injection, and acts as a light saturable absorber which does not
absorb light when the light intensity increases to some extent and
becomes a transparent body.
[0009] The saturable absorber, therefore, acts as a Q switch, which
is capable of adjusting the ratio of light effused from the gain
region 11 to the saturable absorption region 12 by selecting the
width "W" of the gain region 11 and the thickness "d" of each of
portions, on both sides of the stripe-like ridge 7, of the second
cladding layer 4. The output of the laser light is periodically
changed by adjusting the ratio of light effused from the gain
region 11 to the saturable absorption region 12, to thus constitute
a self pulsation type semiconductor laser.
[0010] A light distribution region upon operation is schematically
shown by a chain line "a" in FIG. 16.
[0011] Such a self pulsation laser, which is low in coherence of
laser light and also low in a so-called optical feedback induced
noise due to an unstable laser oscillation state caused by return
of light, having been emitted from the laser, to the laser again,
is useful as an optical disk light source or a high-speed LAN
(Local Area Network) light source.
[0012] It is experientially known that the above-described self
pulsation laser is obtained by selecting the width "W" of the gain
region 11 at a narrow value (generally, 5 .mu.m or less) , and
setting a difference An in effective refractive index between the
gain region 11 and the saturable absorption region 12 at a small
value (generally, .DELTA.n.ltoreq.0.01) by adjusting the thickness
"d" of each of the portions, on both the sides of the stripe-like
ridge 7, of the second cladding layer 4. However, since the
allowable ranges of the width "W" and the thickness "d" are narrow,
it is difficult to adjust the width "W" and the thickness "d" at
the etching step for forming the grooves 8.
[0013] Accordingly, it is difficult to sustain the self pulsation
at a high light output, for example, 10 mW, and also it is
difficult to sustain the self pulsation at a high operational
temperature, for example, 70.degree. C. Further, it is difficult to
produce a self pulsation laser with a high production yield.
SUMMARY OF THE INVENTION
[0014] An object of the present invention is to provide a self
pulsation type semiconductor laser capable of sustaining the self
pulsation at a high light output and/or at a high operational
temperature, and to provide a method of producing the semiconductor
laser.
[0015] Another object of the present invention is to provide a self
pulsation type semiconductor laser which is produceable with a high
production yield, and to provide a method of producing the
semiconductor laser.
[0016] A semiconductor laser according to the present invention
basically includes a first cladding layer, an active layer, a
second cladding layer, and a current constriction layer.
[0017] The active layer may be formed in such a manner that it has
a gain region which is defined as a current injection region by the
current constriction means and which is capable of acquiring an
optical gain by current injection thereto; a saturable absorption
region in which there occurs light effusion thereto; and an outside
region in which there little occurs light effusion thereto. The
active layer may be also formed in such a manner that it has only a
gain region, and a saturable absorption layer, which has a
saturable absorption region disposed at such a position as to allow
the region to absorb light from the gain region and also has an
outside region disposed outside the saturable absorption region in
such a manner as to be in contact therewith, is provided,
separately from the active layer, in at least one of the first and
second cladding layers.
[0018] In each of these configurations, an effective band gap of
the saturable absorption region may be larger than that of the
outside region.
[0019] A method of producing a semiconductor laser according to the
present invention basically includes steps of sequentially growing
a first cladding layer, an active layer, and a second cladding
layer on a substrate, and forming a current constriction means.
[0020] The current constriction means can be formed in accordance
with a related art method.
[0021] In this method, the active layer may be formed in such a
manner that it has a gain region which is defined as a current
injection region by the current constriction means and which is
capable of acquiring an optical gain by current injection thereto;
a saturable absorption region in which there occurs light effusion
thereto; and an outside region in which there little occurs light
effusion thereto. The active layer may also be formed in such a
manner that it has only a gain region, and a saturable absorption
layer, having a saturable absorption region, disposed at such a
position as to allow the region to absorb light from the gain
region, in which there occurs light effusion thereto, and also
having an outside region, being in contact with the saturable
absorption region, in which there little occurs light effusion
thereto, is provided separately from the active layer.
[0022] In each of these methods, an effective band gap of the
saturable absorption region may be larger than that of the outside
region.
[0023] According to the semiconductor laser of the present
invention having the above-described configuration, it is possible
to sustain the self pulsation at a high light output and/or a high
operational temperature.
[0024] With respect to a self pulsation type semiconductor laser,
it is known that the function of a saturable absorption region can
be made higher by making the carrier lifetime in the saturable
absorption region shorter than the carrier lifetime in a gain
region and/or by making the differential gain in the saturable
absorption region larger than the differential gain in the gain
region (see H. Kawaguchi, Appl. Phys. Lett., 45(12)pp. 1264 (1984);
M. Ueno and R. Lang, J. Appl. Phys., 58(4)pp. 1689 (1985); and H.
Adachi, S. Kaminoyama, I. Kidoguchi, and T. Uenoyama, IEEE Photon.
Technol. Lett., 7(12)pp. 1406 (1995)).
[0025] By making higher the function of the saturable absorption
region as described above, it is possible to sustain the self
pulsation at a higher light output and/or a higher operational
temperature.
[0026] On the other hand, while one factor of defining the carrier
lifetime in a saturable absorption region is physical properties of
a semiconductor crystal forming the saturable absorption region,
the present inventor has found the fact that, in the case where
carriers can be migrated from the saturable absorption region to
the outside region being in contact therewith, the migration of the
carriers becomes a factor of defining an effective carrier lifetime
in the saturable absorption region.
[0027] To be more specific, if carriers are readily migrated from
the saturable absorption region to the outside region, the carrier
lifetime in the saturable absorption region is actually shorter
than the carrier lifetime defined by the physical properties of the
semiconductor crystal forming the saturable absorption region. On
the contrary, if the carriers are not migrated or slowly migrated
from the saturable absorption region to the outside region, the
carrier lifetime in the saturable absorption region is actually
equal to or only slightly shorter than the carrier lifetime defined
by the physical properties of the semiconductor crystal forming the
saturable absorption region.
[0028] The semiconductor laser of the present invention has been
made on the basis of the above-described knowledge, and is
characterized in that an effective band gap of a saturable
absorption region is set to be larger than that of an outside
region. With this configuration, because of a property of carriers
easy to be migrated from a region having a large effective band gap
to a region having a small effective band gap, carriers generated
in the saturable absorption region due to optical absorption are
readily migrated to the outside region having a small effective
band gap, so that the carrier lifetime in the saturable absorption
region is actually shortened, to thereby enhance the function of
the saturable absorption region.
[0029] The semiconductor laser of the present invention thus
obtained can sustain the self pulsation at a high light output
and/or a high operational temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a schematic sectional view of a semiconductor
laser according to one embodiment of the present invention;
[0031] FIGS. 2 to 6 are views showing sequential steps of producing
a semiconductor laser according to one embodiment of the present
invention;
[0032] FIG. 7 is a schematic sectional view of a semiconductor
laser according to another embodiment of the present invention;
[0033] FIGS. 8A to 8C are diagrams showing band models of the
semiconductor laser shown in FIG. 7;
[0034] FIG. 9 is a schematic sectional view of a semiconductor
laser according to a further embodiment of the present
invention;
[0035] FIGS. 10 and 11 are views showing sequential steps of
producing a semiconductor laser according to another embodiment of
the present invention;
[0036] FIG. 12 is a schematic sectional view of a semiconductor
laser according to a further embodiment of the present
invention;
[0037] FIGS. 13A and 13B are schematic sectional views showing a
method of producing a semiconductor laser according to a further
embodiment of the present invention;
[0038] FIG. 14 is a schematic sectional view showing a method of
producing a semiconductor laser according to a further embodiment
of the present invention;
[0039] FIGS. 15A and 15B are schematic sectional views showing a
method of producing a semiconductor laser according to a further
embodiment of the present invention, and
[0040] FIG. 16 is a schematic sectional view showing a related art
semiconductor laser.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIEMNTS
[0041] A first semiconductor laser of the present invention
basically includes: a first cladding layer; an active layer; a
second cladding layer; and a current constriction means for
defining a current injection region in the active layer. The active
layer has a gain region which acquires an optical gain by current
injection thereto; a saturable absorption region in which current
injection thereto little occurs and light effusion thereto occurs;
and an outside region, being in contact with the saturable
absorption region, in which current injection thereto little occurs
and light effusion thereto little occurs. In this semiconductor
laser, an effective band gap of the saturable absorption region is
set to be larger than that of the outside region.
[0042] A second semiconductor laser of the present invention
basically includes: a first cladding layer; an active layer; a
second cladding layer; and a current constriction means for
defining a current injection region in the active layer. The active
layer has a gain region which acquires an optical gain by current
injection. A saturable absorption layer is provided in at least one
of the first and second cladding layers. The saturable absorption
layer has a saturable absorption region which has an effective band
gap nearly equal to or narrower than that of the active layer and
in which light effusion thereto occurs, and an outside region,
being in contact with the saturable absorption region, which has an
effective band gap smaller than that of the saturable absorption
region and in which light effusion thereto little occurs. The
saturable absorption region is disposed at a position near the
active layer for effectively absorbing light from the gain
region.
[0043] A first method of producing a semiconductor laser according
to the present invention, includes: a first growth step of
sequentially growing, on a substrate, a first cladding layer, a
first active layer for forming a gain region, and a second cladding
layer, to form a stacked semiconductor layer; a first groove
formation step of forming stripe-like first grooves in part of the
stacked semiconductor layer with a specific gap kept between the
grooves, to form a stripe-like ridge between the grooves and to
expose the first cladding layer from the bottoms of the first
grooves; a second growth step of growing a second active layer for
forming a saturable absorption region on the first cladding layer
exposed in the first grooves, the second active layer having a
composition different from that of the first active layer, and
growing a current constriction layer for forming a current
constriction means; a second groove formation step of forming
second grooves on both sides of the ridge with specific distances
kept between the second grooves and the ridge, to expose the first
cladding layer from the bottoms of the second grooves; and a third
growth step of growing a third active layer for forming an outside
region on the first cladding layer exposed in the second grooves,
the third active layer having a composition different from those of
the first and second active layers for forming the gain region and
the saturable absorption region, and growing a current constriction
layer for forming the current constriction means. In this method,
effective band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 of the gain
region, saturable absorption region and outside region,
respectively, are selected to satisfy an inequality of
Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3- .
[0044] A second method of producing a semiconductor laser according
to the present invention, includes: a first growth step of
sequentially growing, on a substrate, a first cladding layer, an
active layer having an effective band gap which is uniform in a
first direction and which becomes smaller from the center to both
sides of the active layer in a second direction perpendicular to
the first direction, and a second cladding layer, to form a stacked
semiconductor layer; a groove formation step of forming stripe-like
grooves spaced from each other at a specific gap in the stacked
semiconductor layer in such a manner as to leave a portion, having
a large effective band gap, of the active layer in a stripe between
the grooves and to leave a portion, having a specific thickness, of
the second cladding layer on the bottoms of the grooves; and a
second growth step of growing, in the grooves, a current
constriction layer, which forms a current constriction means for
defining a current injection region at the stripe portion, having
the large effective band gap, of the active layer.
[0045] A third method of producing a semiconductor laser according
to the present invention, includes: a growth step of sequentially
growing, on a substrate, a first cladding layer, an active layer
having an effective band gap which is uniform in a first direction
and which becomes smaller from the center to both sides of the
active layer in a second direction perpendicular to the first
direction, and a second cladding layer, to form a stacked
semiconductor layer; and a current constriction layer formation
step of implanting ions of an impurity in side portions, each
having a small effective band gap, on both sides of a stripe
portion, having a large effective band gap, of the active layer, to
form a current constriction layer which forms a current
constriction means for defining a current injection region at the
stripe portion, having the large effective band gap, of the active
layer.
[0046] A fourth method of producing a semiconductor laser according
to the present invention, includes: a first growth step of growing,
on a substrate, a first cladding layer, an active layer, a second
cladding layer, and a saturable absorption layer which is
positioned at least one of the first and second cladding layers and
which forms a saturable absorption region, to form a stacked
semiconductor layer; a groove formation step of forming, in part of
the stacked semiconductor layer, stripe-like grooves spaced from
each other at a specific gap to a depth crossing the saturable
absorption layer for forming a stripe-like ridge; and a second
growth step of growing, in the grooves, a saturable absorption
layer for forming a current constriction layer which forms, at
least on both sides of the ridge, a current constriction means for
defining a current injection region in the active layer, and for
forming an outside region which is in contact with the saturable
absorption layer and which has an effective band gap smaller than
that of the saturable absorption layer.
[0047] A fifth method of producing a semiconductor laser according
to the present invention, includes: a first growth step of growing,
on a substrate, a first cladding layer, an active layer, a second
cladding layer, and a saturable absorption layer positioned in at
least one of the first and second cladding layers and having an
effective band gap which is uniform in a first direction and which
becomes smaller from the center to both sides of the saturable
absorption layer in a second direction perpendicular to the first
direction, to form a stacked semiconductor layer; a groove
formation step of forming stripe-like grooves spaced from each
other at a specific gap in the stacked semiconductor layer to such
a depth as to leave a portion, having a large effective band gap,
of the saturable absorption layer in a stripe between the grooves
and to leave part of the saturable absorption layer and a portion,
having a specific thickness, of the second cladding layer; and a
second growth step of growing, in the grooves, a current
constriction layer, which forms a current constriction means for
defining a current injection region in the active layer, on both
sides of the ridge.
[0048] A sixth method of producing a semiconductor laser according
to the present invention, includes: a growth step of growing, on a
substrate, a first cladding layer, an active layer, a second
cladding layer, and a saturable absorption layer positioned in at
least one of the first and second cladding layers and having an
effective band gap which is uniform in a first direction and which
becomes smaller from the center to both sides of the saturable
absorption layer in a second direction perpendicular to the first
direction, to form a stacked semiconductor layer; an impurity
implantation step of implanting ions of an impurity in a portion,
having a small effective band gap, of the saturable absorption
layer of the stacked semiconductor layer, to form a current
constriction layer which forms a current constriction means for
defining a current injection region in the active layer.
[0049] First Embodiment
[0050] FIG. 1 is a schematic sectional view taken on a plane
perpendicular to a resonator length direction, showing a
semiconductor laser according to a first embodiment of the present
invention. The semiconductor laser in this embodiment has an active
layer of a DH (Double Hetero) structure.
[0051] Referring to FIG. 1, the semiconductor laser in this
embodiment includes a substrate 21 of a first conduction type
(n-type in this embodiment), and layers sequentially formed on the
substrate 21: a first cladding layer 22 having the same conduction
type as that of the substrate 21, an active layer 23, and a second
cladding layer 24 having a second conduction type (p-type in this
embodiment). Grooves 28 are provided in the second cladding layer
24, so that a stripe-like ridge 27 extending in the direction
perpendicular to the paper plane of FIG. 1 is formed as part of the
second cladding layer 24 in such a manner as to be held between the
grooves 28. A current constriction layer 26 constituting a current
constriction means for defining a current injection region in the
active layer 23 is buried in each groove 28. A p-type contact layer
25 is formed over the entire surface to cover the upper surface of
the ridge 27 as part of the cladding layer 24.
[0052] A first electrode 29 is formed on the contact layer 25 in
such a manner as to be in ohmic-contact therewith, and a second
electrode 30 is formed on the back surface of the substrate 21 in
such a manner as to be in ohmic-contact therewith.
[0053] The substrate 21 is configured as an n-type GaAs
substrate.
[0054] The first cladding layer 22 is configured as an n-type
Al.sub.0.5Ga.sub.0.5As layer, and the second cladding layer 24 is
configured as a p-type A.sub.0.5Ga.sub.0.5As layer.
[0055] The thickness of the first cladding layer 22 is in a range
of 0.5 .mu.m to 3 .mu.m, and the dose of an impurity doped in the
first cladding layer 22 is in a range of 2.times.10.sup.16
pieces/cm.sup.3 to 3.times.10.sup.18 pieces/cm.sup.3.
[0056] With respect to the second cladding layer 24, the thickness
of the portion inside the ridge 27 is in a range of 0.5 .mu.m to 3
.mu.m, and the thickness "d" of the portion outside the ridge 27 is
in a range of 0.1 .mu.m to 1 .mu.m; and the dose of an impurity
doped in the second cladding layer 24 is in a range of
2.times.10.sup.16 pieces/cm.sup.3 to 3.times.10.sup.18
pieces/cm.sup.3.
[0057] The contact layer 25 is configured as a p-type GaAs layer.
The thickness of the contact layer 25 is in a range of 0.01 .mu.m
to 1 .mu.m, and the dose of an impurity doped in the contact layer
25 is in a range of 5.times.10.sup.17 pieces/cm.sup.3 to
3.times.10.sup.18 pieces/cm.sup.3.
[0058] The current constriction layer 26 is configured as an n-type
GaAs layer. The thickness of the current constriction layer 26 is
in a range of 0.3 .mu.m to 3 .mu.m, and the dose of an impurity
doped in the current constriction layer 26 is selected at a such a
value as to be 1.times.10.sup.16 pieces/cm.sup.3 or more and to
allow the current constriction layer 26 to sufficiently achieve the
current constriction effect to the ridge 27.
[0059] The active layer 23 of the DH structure is configured as an
AlGaAs layer. The thickness of the active layer 23 is set at 0.1
.mu.m. To be more specific, with respect to the active layer 23, a
portion forming a gain region 231 under the ridge 27 is configured
as an Al.sub.X1Ga.sub.1-X1As layer; portions forming saturable
absorption regions 232 outside the gain region 231 are configured
as an Al.sub.X2Ga.sub.1-X2As layer; and portions forming outside
regions 233 outside the saturable absorption regions 232 are
configured as an Al.sub.X3Ga.sub.1-X3As layer, where x.sub.1,
x.sub.2 and X.sub.3 each representing the component ratio (atomic
ratio) of Al satisfy an inequality of
x.sub.1.gtoreq.x.sub.2>x.sub.3, preferably,
x.sub.1>x.sub.2>x.sub.3.
[0060] Here, the Al component ratio x.sub.1 in the gain region 231
substantially determines the oscillation wavelength of the
semiconductor laser. Assuming x.sub.1=0.15, the oscillation
wavelength of the semiconductor laser becomes about 770 nm.
[0061] In this case, the Al component ratio x.sub.2 in the
saturable absorption region 232 and the Al component ratio x.sub.3
in the outside region 233 are selected to satisfy an inequality of
0.15.gtoreq.x.sub.2>x.sub.3.gtoreq.0, preferably,
0.15>x.sub.2>x.sub.3>0, for example, selected at
x.sub.2=0.1 and x.sub.3=0.
[0062] The width W of the gain region 231 is specified depending on
the width of the ridge 27, and selected in a range of 5 .mu.m or
less, preferably, 1 .mu.m to 3 .mu.m.
[0063] The width Ws of the saturable absorption region 232 is
specified depending on a width in which a laser light distribution
shown by a chain line "a" is present. The above width, in which the
laser light distribution is present, is determined by the width W
of the ridge 27, the thickness "d" of the cladding layer 24 under
the current constriction layer 26, the Al component ratio x.sub.1
and the thickness of the gain region 231, and the Al component
ratio x.sub.2 and the thickness of the saturable absorption region
232. The width Ws is selected at a value in a range of about 1
.mu.m to about 3 .mu.m.
[0064] A method of producing the semiconductor laser having the
structure shown in FIG. 1 will be described below.
[0065] The above semiconductor laser can be produced, for example,
in accordance with the above-described first production method of
the present invention. Here, one embodiment of the first production
method applied to production of the above semiconductor laser will
be described.
[0066] At a first epitaxial growth step shown in FIG. 2, a first
cladding layer 22 made from n-type AlGaAs, an active layer 231L
made from Al.sub.X1Ga.sub.1-X1As for forming a gain region, a
second cladding layer 24 made from p-type AlGaAs, and a contact
layer 25 made from P-type GaAs are sequentially formed on a
substrate 21 made from n-type GaAs by a MOCVD (Metalorganic
Chemical Vapor Deposition) method, MBE (Molecular Beam Epitaxy)
method, or LPE (Liquid Phase Epitaxy) method.
[0067] At a first groove formation step shown in FIG. 3, the
contact layer 25, the second layer 24, and the active layer 231L
are selectively etched from the contact layer 25 side to such a
depth as to expose the first cladding layer 22, to form a pair of
opposed first grooves 281 and also form, between the grooves 281, a
stripe-like ridge 27 extending in the direction perpendicular to
the paper plane of FIG. 3. The gap between the grooves 281 is
selected at a value corresponding to the above-described width W.
The remaining portion of the active layer 231L constitutes a gain
region 231.
[0068] At a second epitaxial growth step shown in FIG. 4, a
saturable absorption layer 232L made from Al.sub.X2Ga.sub.1-X2As
for forming saturable absorption regions and having the same
thickness as that of the active layer 231, a second cladding layer
24 made from p-type AlGaAs and having a thickness "d", a current
constriction layer 26 made from n-type GaAs, and a contact layer 25
made from p-type GaAs are epitaxially grown in sequence on the
portion of the first cladding layer 22 exposed in the first grooves
281 by the MOCVD method, MBE method-or LPE method.
[0069] At a second groove formation step shown in FIG. 5, the
stacked layers 25, 26, 24 and 232L are selectively etched from the
contact layer 25 side to such a depth as to expose the first
cladding layer 22, to form second grooves 282 for forming outside
regions 233 shown in FIG. 1. The remaining portions of the
saturable absorption layer 232L constitute saturable absorption
regions 232.
[0070] At a third epitaxial growth step shown in FIG. 6, an active
layer 233L made from Al.sub.X3Ga.sub.1-X3As for forming the outside
regions 233, a second cladding layer 24 made from AlGaAs and having
the thickness "d", a current constriction layer 26 made from n-type
GaAs, and a contact layer 25 made from p-type GaAs are epitaxially
grown in sequence on the portion of the first cladding layer 22
exposed in the second grooves 282 by the MOCVD method, MBE methods
or LPE method.
[0071] Finally, a first electrode 29 formed of a Cr layer or TiPt
layer is deposited as an ohmic electrode on the contact layer 25,
and a second electrode 30 formed of an Au layer is deposited as an
ohmic electrode on the back surface of the substrate 21. In this
way, the semiconductor laser of the present invention having the
configuration shown in FIG. 1 is obtained.
[0072] The semiconductor laser configured as described above has
the DH structure, in which the Al component ratio x.sub.2 in the
saturable absorption region 232 is larger than the Al component
ratio x.sub.3 in the outside region 233 (x.sub.2>x.sub.3), so
that the band gap in the saturable absorption region 232 is
selected to be larger than the band gap of the outside region 233.
With this configuration, because of a property of carriers easy to
migrate from a region having a large band gap to a region having a
small band gap, carriers generated in the saturable absorption
region 232 due to optical absorption readily migrate to the outside
region 233, with a result that the effective carrier lifetime in
the saturable absorption region 232 is shortened. Accordingly, the
ability of the saturable absorption region 232 is enhanced, to
allow self pulsation to be sustained at a high optical output
and/or at a high operational temperature. As a result, the
allowable range of the width W of the gain region 231 or the
thickness "d" of the second cladding layer 24 under the current
constriction layer 26 can be made large. This makes it possible to
facilitate the production of the semiconductor laser and hence to
improve the production yield of the semiconductor laser.
[0073] Further, in the case where the Al component ratio x.sub.1 in
the gain region 231 is set to be larger than the Al component ratio
x.sub.2 in the saturable absorption region 232
(x.sub.1>x.sub.2), the differential gain of the saturable
absorption region 232 can be made larger than the differential gain
of the gain region 231. As a result, self pulsation can be
sustained at a higher optical output and/or a higher operational
temperature, so that the allowable range of the width W of the gain
region 231 and the thickness "d" of the second cladding layer 24
under the current constriction layer 26 can be made larger.
[0074] In the semiconductor laser and the production method thereof
according to the present invention, described with reference to
FIG. 1 and FIGS. 2 to 6, the active layer is configured as that
having the DH structure; however, the active layer may be
configured as that having a quantum well structure, for example, a
single quantum well structure or a multiple quantum well
structure.
[0075] In this case, with respect to an Al component ratio x of an
Al.sub.xGa.sub.1-xAs quantum well layer for forming a quantum well
structure, assuming that the Al component ratio x in a gain region
231 is taken as x.sub.1; the Al component ratio x in a saturable
absorption region 232 is taken as x.sub.2; and the Al component
ratio x in an outside region 233 is taken as x.sub.3, by specifying
the ratios x.sub.1, x.sub.2 and x.sub.3 such that x.sub.1, x.sub.2
and x.sub.3 satisfy an inequality of
x.sub.1.gtoreq.x.sub.2>x.sub.3, preferably,
x.sub.1>x.sub.2>x.sub.3, the same effect as that of the
semiconductor laser shown in FIG. 1 can be obtained.
[0076] The active layer 23 can be also configured as that having a
quantum wire or quantum dot structure of a single or a plurality of
quantum wires or quantum dots.
[0077] In this case, with respect to an Al component ratio x of the
quantum wire or quantum dot structure, assuming that the Al
component ratio x in a gain region 231 of the quantum wire or
quantum dot structure is taken as x.sub.1; the Al component ratio x
in a saturable absorption region 232 thereof is taken as x.sub.2;
and the Al component ratio x in an outside region 233 thereof is
taken as x.sub.3, by specifying the ratios x.sub.1, x.sub.2 and
x.sub.3 such that x.sub.1, x.sub.2 and x.sub.3 satisfy an
inequality of x.sub.1.gtoreq.x.sub.2>x.sub.3, preferably,
x.sub.1>x.sub.2>x.sub.3, the same effect as that of the
semiconductor laser shown in FIG. 1 can be obtained.
[0078] In the active layer 23 having the quantum wire structure, if
at least one or all of the quantum wires constituting the quantum
wire structure are formed in such a manner as to cross the
saturable absorption region 232 and the outside region 233,
carriers generated in the saturable absorption region 232 due to
optical absorption are allowed to more readily migrate to the
outside region 233. With this configuration, the above-described
effect can be more enhanced.
[0079] The semiconductor laser having the quantum well structure,
quantum wire structure or quantum dot structure according to the
present invention can be produced in accordance-with a method
corresponding to that shown in FIGS. 2 to 6. To be more specific,
each of the active layers 231L to 233L may be formed in such a
manner as to have the quantum well structure, quantum wire
structure, or quantum dot structure by the MOCVD method, MBE
method, or LPE method in accordance with the production method
shown in FIGS. 2 to 6.
[0080] Second Embodiment
[0081] FIG. 7 is a schematic sectional view showing a second
embodiment having a multiple quantum well (MQW) structure. In this
figure, parts corresponding to those shown in FIG. 1 are designated
by the same reference numerals and the overlapped explanation is
omitted. In this embodiment, the composition and thickness of each
of layers other than the active layer may be the same as those in
the first embodiment described with reference to FIG. 1. The active
layer 23 in this embodiment has a SCH (Separate Confinement
Heterostructure)-MQW structure in which a first light confinement
layer 41 is disposed on a first cladding layer 22 side and a second
light confinement layer 42 is disposed on a second cladding layer
24 side.
[0082] To be more specific, the active layer has the SCH-MQW
structure in which the active layer is composed of a plurality of
quantum well layers 23w, between two adjacent layers of which a
quantum barrier layer 23b is disposed, and the first light
confinement layer 41 is disposed adjacent to the first cladding
layer 22 and the second light confinement layer 42 is disposed
adjacent to the second cladding layer 24. Each of the first and
second light confinement layers 41 and 42 and the quantum barrier
layer 23b is made from Al.sub.0.35Ga.sub.0.65As, and the quantum
well layer 23w is made from Al.sub.0.1Ga.sub.0.9As.
[0083] FIGS. 8A, 8B and 8C show band model diagrams of forbidden
bands on the conduction band side in the gain region 231, saturable
absorption regions 232 outside the gain region 231, and the outside
regions 233 outside the saturable absorption regions 232,
respectively.
[0084] Assuming that the thickness of each of semiconductor layers
411 and 421 constituting the first and second light confinement
layers 41 and 42 in a gain region 231 is taken as d.sub.c1; the
thickness of each of semiconductor layers 412 and 422 constituting
the first and second light confinement layers 41 and 42 in a
saturable absorption region 232 is taken as d.sub.c2; the thickness
of each of semiconductor layers 413 and 423 constituting the first
and second light confinement layers 41 and 42 in an outside region
233 is taken as d.sub.c3; the thicknesses of quantum well layers
231w, 232w and 233w in the regions 231, 232 and 233 are taken as
d.sub.w1, d.sub.w2 and d.sub.w3, respectively; and the thicknesses
of quantum barrier layers 231b, 232b and 233b in the regions 231,
232 and 233 are taken as d.sub.b1, d.sub.b2 and d.sub.b3,
respectively, the thicknesses d.sub.w1, d.sub.w2 and d.sub.w3 are
selected to satisfy an inequality of
d.sub.w1.ltoreq.d.sub.w2<d.sub.w3, preferably,
d.sub.w1<d.sub.w2<d.sub.w3, for example d.sub.w1=100 .ANG.,
d.sub.w2=110 .ANG., d.sub.w3=130 .ANG.; the thicknesses d.sub.b1,
d.sub.b2 and d.sub.b3 are selected to satisfy an inequality of
d.sub.b1 .gtoreq.d.sub.b2.gtoreq.d.sub.b3, preferably,
d.sub.b1.gtoreq.d.sub.b2>- ;db.sub.3, for example,
d.sub.=d.sub.b2=d.sub.b3=about 80 .ANG.; and the thicknesses
d.sub.c1, d.sub.c2, and d.sub.c3 are selected at about 500
.ANG..
[0085] The number of the quantum wells is selected to be in a range
of 2 to about 10.
[0086] The effective band gap in the MQW structure is specified
depending on the band gap determined by the composition of the
quantum well layer and the thickness of the quantum well layer, and
the band gap determined by the composition of the quantum barrier
layer and the thickness of the quantum barrier layer. In the case
of changing the thickness of the quantum well layer, if the
thickness becomes larger, the energy of the quantum level formed in
the quantum well becomes lower, with a result that the effective
band gap becomes smaller.
[0087] Accordingly, in the active layer having the SCH-MQW
structure in the second embodiment, letting the effective band gaps
in the gain region 231, saturable absorption region 232 and outside
region 233 be Eg.sub.1, Eg.sub.2, and Eg.sub.3, respectively, there
is given an inequality of Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3 or
Eg.sub.1>Eg.sub.2>Eg.sub.3.
[0088] The semiconductor laser including the active layer having
the SCH-MQW structure can be produced in accordance with a method
basically similar to that described with reference to FIGS. 2 to 6.
To be more specific, in formation of films by epitaxial growth for
forming the gain region 231, saturable absorption region 232 and
the outside region 233 of the active layer 23 in accordance with
the method shown in FIGS. 2 to 6, each film is configured to have
the SCH-MQW structure.
[0089] In the semiconductor laser according to this embodiment,
since the thickness d.sub.w2 of the quantum well layer 232w having
the SCH-MQW structure in the saturable absorption region 232 is set
to be thinner than the thickness of d.sub.w3 of the quantum well
layer 233w having the SCH-MQW structure in the outside region 233,
the effective band gap of the MQW structure in the saturable
absorption region 232 becomes larger than the effective band gap in
the outside region 233. Accordingly, because of a property of
carriers easy to migrate from a region having a large effective
band gap to a region having a small effective band gap, carriers
generated in the saturable absorption region 232 due to optical
absorption readily migrate to the outside region 233, so that the
effective carrier lifetime in the saturable absorption region 232
is shortened. As a result, the ability of the saturable absorption
region 232 is enhanced, to thereby allow self pulsation to be
sustained at a high optical output and/or at a high operational
temperature. Accordingly, the allowable range of the width W of the
gain region 231 or the thickness "d" of the second cladding layer
24 under the current constriction layer 26 can be made large.
[0090] Further, in the case where the thickness d.sub.w1 of the
quantum well layer 231w having the SCH-MQW structure in the gain
region 231 is set to be smaller than the thickness d.sub.w2 of the
quantum well layer 232w having the SCH-MQW structure in the
saturable absorption region 232 (d.sub.w1<d.sub.w2), the
differential gain of the saturable absorption region 232 can be
made larger than the differential gain of the gain region 231. As a
result, self pulsation can be sustained at a higher optical output
and/or a higher operational temperature, so that the allowable
range of the width W of the gain region 231 and the thickness "d"
of the second cladding layer 24 under the current constriction
layer 26 can be made larger.
[0091] In the second embodiment, the present invention can be
applied to the semiconductor laser including the active layer
having a so-called SQW (Single Quantum Well) structure, in addition
to the above-described semiconductor laser including the active
layer having the MQW structure.
[0092] In this case, the active layer 23 has the SQW structure that
the active layer 23 is composed of a single quantum well 23w with
no quantum barrier layer, unlike the MQW structure, and the first
light confinement layer 41 is disposed adjacent to the first
cladding layer 22 and the second light confinement layer 42 is
disposed adjacent to the second cladding layer 24.
[0093] Like the MQW structure, letting the thickness of the quantum
well layer 231w in the gain region 231 be d.sub.w1 the thickness of
the quantum well layer 232w in the saturable absorption region 232
be d.sub.w2, and the thickness of the quantum well layer 233w in
the outside region 233 be d.sub.w3, the thicknesses d.sub.w1,
d.sub.w2 and d.sub.w3 are specified to satisfy an inequality of
d.sub.w1.ltoreq.d.sub.w2<d.s- ub.w3, preferably,
d.sub.w1<d.sub.w2<d.sub.w3, for example, d.sub.w1=100 .ANG.,
d.sub.w2=110 .ANG., and d.sub.w3=130 .ANG..
[0094] The semiconductor laser having the SQW structure can be
produced basically in accordance with the above-described
production method for the semiconductor laser having the MQW
structure. To be more specific, in formation of films for forming
the gain region 231, saturable absorption region 232 and the
outside region 233 of the active layer 23, each film is formed to
have the SQW structure in place of the MQW structure. That is to
say, information of the films for forming the gain region 231,
saturable absorption region 232 and the outside region 233, each
film may be formed of the single quantum well layer 23w with no
barrier layer.
[0095] The effective band gap in the SQW structure is specified
depending on the band gap determined by the composition of the
quantum well layer and the thickness of the quantum well layer. In
the case of changing the thickness of the quantum well layer, if
the thickness becomes larger, the energy of the quantum level
formed in the quantum well becomes lower, with a result that the
effective band gap becomes smaller.
[0096] Accordingly, in the active layer having the SCH-SQW
structure, like the SCH-MQW structure, the effective band gaps
Eg.sub.1, Eg.sub.2 and Eg.sub.3 in the gain region 231, saturable
absorption region 232 and outside region 233 satisfy an inequality
of Eg.sub.1.gtoreq.Eg.sub.2>E- g.sub.3 or
Eg.sub.1>Eg.sub.2>Eg.sub.3, so that the same effect as that
obtained by the SCH-MQW structure can be obtained.
[0097] In this embodiment, the above-described quantum well
structure may be configured as a quantum wire or quantum dot
structure.
[0098] In this case, the active layer 23 may be formed of
semiconductor layers having the quantum wire or quantum dot
structure, which are formed by burying quantum wires or quantum
dots in place of the quantum well layers 23w having the SCH-MQW
structure. The other configuration may be the same as that of the
SCH-MQW structure.
[0099] The active layer 23 may be also configured as a single
semiconductor layer having the quantum wire or quantum dot
structure, in place of the quantum well layer having the SQW
structure.
[0100] Letting the thicknesses of the quantum wires or the average
sizes (volumes) of the quantum dots in the gain region 231,
saturable absorption region 232 and the outside region 233 be
d.sub.w1, d.sub.w2 and d.sub.w3, respectively, there is given an
inequality of d.sub.w1.ltoreq.d.sub.w2<d.sub.w3, preferably,
d.sub.w1<d.sub.w2<- ;d.sub.w3.
[0101] The semiconductor laser having the quantum wire structure or
the quantum do-. structure can be produced in accordance with a
method substantially similar to the production method for the
semiconductor laser having the SCH-MQW structure. To be more
specific, in formation of films by epitaxial growth for forming the
gain region 231, saturable absorption region 232 and the outside
region 233 of the active layer 23, each film may be formed of the
semiconductor layers having the quantum wire or the quantum dot
structure in place of the quantum well layers having the SCH-MQW
structure.
[0102] In the semiconductor laser having the quantum wire or
quantum dot structure, as the thickness of the quantum wire or the
size (volume) of the quantum dot becomes larger, the energy of the
quantum level of the quantum wire or the quantum dot becomes lower,
with a result that the effective band gap in the quantum wire or
quantum dot structure becomes smaller. Accordingly, in the case
where the thicknesses d.sub.w1, d.sub.w2 and d.sub.w3 of the
quantum wires or the average sizes (volumes) d.sub.w1, d.sub.w2 and
d.sub.w3 of the quantum dots in the gain region 231, saturable
absorption region 232 and the outside region 233, respectively are
specified to satisfy an inequality of
d.sub.w1.ltoreq.d.sub.w2<d.sub.w3, preferably,
d.sub.w1<d.sub.w2<- ;d.sub.w3 as described above, the
effective band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 in the gain
region 231, saturable absorption region 232 and the outside region
233 satisfy, like the SCH-MQW structure, an inequality of
Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3 or
Eg.sub.1>Eg.sub.2>Eg.sub- .3.
[0103] As a result, even in the case of the semiconductor laser
including the active layer having the quantum wire or quantum dot
structure, the same effect as that described above can be
obtained.
[0104] In the active layer having the quantum wire structure, if at
least one or all of the quantum wires constituting the quantum wire
structure are formed in such a manner as to cross the saturable
absorption region 232 and the outside region 233, carriers
generated in the saturable absorption region 232 due to optical
absorption are allowed to more readily migrate to the outside
region 233. With this configuration, the above-described effect can
be more enhanced.
[0105] Third Embodiment
[0106] In this embodiment, the present invention is applied, like
the second embodiment, to a semiconductor laser having a SCH-MQW
structure substantially similar to the SCH-MQW structure described
with reference to FIGS. 7 and 8A to 8C. In this embodiment,
however, assuming that the thicknesses of quantum well layers 231w,
232w and 233w in a gain region 231, a saturable absorption region
232 and an outside region 233 are taken as d.sub.w1, d.sub.w2 and
d.sub.w3, respectively; the thicknesses of quantum barrier layers
231b, 232b and 233b in the regions 231, 232 and 233 are taken as
d.sub.b1, d.sub.b2 and d.sub.b3, respectively; and the thickness of
each of first and second light confinement layers 411 and 421 in
the gain region 231 is taken as d.sub.c1, the thickness of each of
the first and second light confinement layers 412 and 422 in the
saturable absorption region 232 is taken as d.sub.c2, and the
thickness of each of first and second light confinement layers 413
and 423 in the outside region 233 is taken as d.sub.c3, the
thicknesses d.sub.w1, d.sub.w2 and d.sub.w3 are selected at
d.sub.w1=d.sub.w2=d.sub.w3=about 100 .ANG.; the thicknesses
d.sub.b1, d.sub.b2 and d.sub.b3 are selected to satisfy an
inequality of d.sub.b1.gtoreq.d.sub.b2>d.sub.b3, preferably,
d.sub.b1>d.sub.b2>d.sub.b3, for example, d.sub.b1=80 .ANG.,
d.sub.b2=70 .ANG., and d.sub.b3=50 .ANG.; and the thicknesses
d.sub.c1, d.sub.c2, and d.sub.c3 are selected at about 500
.ANG..
[0107] In the MQW structure, as the thickness of the quantum
barrier layer for separating the quantum well layers from each
other becomes thinner, the connection between the quantum well
layers becomes stronger, with a result that the energy of the
quantum level becomes lower and thereby the effective band gap of
the MQW structure becomes smaller. Accordingly, in the case where
the thicknesses d.sub.b1, d.sub.b2 and d.sub.b3 of the quantum
barrier layers 231b, 232b and 233b in the gain region 231,
saturable absorption region 232 and the outside region 233 are
specified to satisfy the inequality of
d.sub.b1.gtoreq.d.sub.b2>d.sub.b3, preferably,
d.sub.b1>d.sub.b2>d.sub.b3 as described above, the effective
band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 in the gain region 231,
saturable absorption region 232 and the outside region 233 satisfy
an inequality of Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3 or
Eg.sub.1>Eg.sub.2>Eg.sub.3. As a result, the same effect as
that described in the second embodiment can be obtained.
[0108] The semiconductor laser in this embodiment can be produced
in accordance with the same method as that described in the second
embodiment.
[0109] Fourth Embodiment
[0110] In this embodiment, the present invention is applied, like
the second embodiment, to a semiconductor laser having a SCH-MQW
structure. With respect to the active layer having the SCH-MQW
structure in this embodiment, letting the number of quantum well
layers 231w in the gain region 231 be Nw.sub.1; the number of
quantum well layers 232w in the saturable absorption region 232 be
Nw.sub.2; and the number of quantum well layers 233w in the outside
region 233 be Nw.sub.3, the values Nw.sub.1, Nw.sub.2 and Nw.sub.3
are specified to satisfy an inequality of
Nw.sub.1.ltoreq.Nw.sub.2<Nw.sub.3, preferably,
Nw.sub.1<Nw.sub.2<- ;Nw.sub.3.
[0111] In the MQW structure, as the number of the quantum well
layers becomes larger, the two-dimensional characteristic of the
MQW structure becomes weaker, with a result that the effective band
gap becomes smaller. Accordingly, in the case where the numbers
Nw.sub.1, Nw.sub.2 and Nw.sub.3 of the quantum well layers in the
gain region 231, saturable absorption region 232 and the outside
region. 233 are specified to satisfy the inequality of
Nw.sub.1.ltoreq.Nw.sub.2<Nw.sub.3, preferably,
Nw.sub.1<Nw.sub.2<Nw.sub.3 as described above, the effective
band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 in the gain region 231,
saturable absorption region 232 and the outside region 233 satisfy
an inequality of Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3 or
Eg.sub.1>Eg.sub.2>Eg.sub.3.
[0112] As a result, even in this embodiment, the same effect as
that described in the first, second, and third embodiments can be
obtained.
[0113] The semiconductor laser having the above-described structure
can be produced basically in accordance with the method shown in
FIGS. 2 to 6. To be more specific, the active layer composed of
quantum well layers and quantum barrier layers, and light
confinement layers are epitaxially grown in each of the gain region
231, saturable absorption region 232 and the outside region 233 in
such a manner that the number of the quantum well layers are
selected to satisfy the above-described relationship.
[0114] With respect to the active layer having the SCH-MQW
structure in this embodiment, the multiple quantum well (MQW)
structure may be replaced with the quantum wire or quantum dot
structure.
[0115] In this case, the active layer 23 may be formed of
semiconductor layers having the quantum wire or quantum dot
structure, which are formed by burying quantum wires or quantum
dots in place of the quantum well layers 23w having the SCH-MQW
structure. The other configuration may be the same as that of the
SCH-MQW structure.
[0116] The active layer 23 may be also configured as a single
semiconductor layer with no quantum barrier layer, having the
quantum wire or quantum dot structure.
[0117] Assuming that the numbers of the quantum well wires or the
number densities of the quantum dots in the gain region 231,
saturable absorption region 232 and the outside region 233 are
taken as Nw.sub.1, Nw.sub.2 and NW.sub.3, respectively, the values
Nw.sub.1, Nw.sub.2 and Nw.sub.3 are specified to satisfy an
inequality of Nw.sub.1.ltoreq.Nw.sub.2<Nw.sub.3, preferably,
Nw.sub.1<Nw.sub.2<- ;Nw.sub.3.
[0118] The semiconductor laser having the quantum wire or quantum
dot structure can be produced in accordance with a method
substantially similar to the method for producing the semiconductor
laser having the SCH-MQW structure. To be more specific, in
formation of films by expitaxial growth for forming the gain region
231, saturable absorption region 232 and the outside region 233 of
the active layer 23, each film is formed of the semiconductor
layers having the quantum wire or quantum dot structure in place of
the quantum well layers having the SCH-MQW structure.
[0119] In the semiconductor laser having the quantum wire or
quantum dot structure, as the number of quantum wires or the number
density of the quantum dots becomes larger, the connection between
the quantum wires or between the quantum dots becomes stronger,
with a result that the one-dimensional characteristic of the
quantum wire structure or the zero-dimensional characteristic of
the quantum dot structure becomes weaker and thereby the effective
band gap of the quantum wire or quantum dot structure becomes
smaller. Accordingly, in the case where the numbers Nw.sub.1,
Nw.sub.2 and Nw.sub.3 of the quantum wires or the number densities
Nw.sub.1, Nw.sub.2 and Nw.sub.3 of the quantum dots in the gain
region 231, saturable absorption region 232 and the outside region
233, respectively are specified to satisfy an inequality of
Nw.sub.1.ltoreq.Nw.sub.2<Nw.sub.3, preferably,
Nw.sub.1<Nw.sub.2<- ;NW.sub.3 as described above, the
effective band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 in the gain
region 231, saturable absorption region 232 and the outside region
233 satisfy, like the active layer having the SCH-MQW structure, an
inequality of Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3 or
Eg.sub.1>Eg.sub.2>Eg.sub.3.
[0120] As a result, even in the semiconductor laser including the
active layer having the quantum wire or quantum dot structure, the
same effect as that described above can be obtained.
[0121] In the active layer 23 having the quantum wire structure, if
at least one, preferably, the number Nw.sub.2 of the quantum wires
are formed in such a manner as to cross the saturable absorption
region 232 and the outside region 233, carriers generated in the
saturable absorption region 232 due to optical absorption are
allowed to more readily migrate to the outside region 233. With
this configuration, the above-described effect can be more
enhanced.
[0122] In the semiconductor laser in each of the above-described
embodiments, the active layer is divided into the gain region 231,
saturable absorption region 232 and the outside region 233, and the
band gaps Eg.sub.1, Eg.sub.2 and Eg.sub.3 in these regions are
specified to satisfy the inequality of
Eg.sub.1.gtoreq.Eg.sub.2>Eg.sub.3, preferably,
Eg.sub.1>Eg.sub.2>Eg.sub.3; however, a saturable absorption
layer including the saturable absorption region and the outside
region may be formed separately from the active layer including the
gain region.
[0123] The embodiment having such a configuration will be described
below.
[0124] Fifth Embodiment
[0125] FIG. 9 is a schematic sectional view of a semiconductor
laser having a DH structure. Referring to FIG. 9, the semiconductor
laser in this embodiment includes, like the first embodiment shown
in FIG. 1, a first cladding layer made from n-type AlGaAs on a
substrate 21 made from an n-type GaAs. In this embodiment, an
active layer 23 made from Al.sub.X1Ga.sub.1-X1, which includes a
gain region 231; a second cladding layer 24 made from a p-type
AlGaAs; and a contact layer 25 made from p-type GaAs are formed on
the first cladding layer 22. Grooves 28 are provided in the second
cladding layer 24, so that a stripe-like ridge 27 having a width W
is formed as part of the second cladding layer 24 in such a manner
as to be held between the grooves 28. A current constriction layer
26 made from n-type Al.sub.0.5Ga.sub.0.5As for defining a current
injection region, that is, a gain region 231 in the active layer 23
is buried in each groove 28.
[0126] A saturable absorption layer 43 having a saturable
absorption region 232 and outside regions 233 is formed at a
position separated from the active layer 23. The saturable
absorption region 232 made from Al.sub.X2Ga.sub.1-X.sub.2As is
located in the stripe-like ridge 27 in such a manner as to be
vertically separated from the active layer 23 with a lower side
second cladding layer 241 put therebetween. The outside regions 233
made from Al.sub.X3Ga.sub.1-X3As are located on both sides of
stripe-like ridge 27, that is, adjacent to both sides of the
saturable absorption region 232 in such a manner as to be
vertically separated from the active layer 23 with the lower side
second cladding layer 241 and a lower side current constriction
layer 261 put therebetween.
[0127] Each of the active layer 23 and the saturable absorption
layer 43 is set at 0.1 .mu.m and has a DH structure.
[0128] With respect to the active layer 23 and the saturable
absorption layer 43, Al component ratios x.sub.1, x.sub.2 and
x.sub.3 in the gain region 231, saturable absorption region 232 and
the outside region 233 are specified to satisfy an inequality of
x.sub.1.gtoreq.x.sub.2>x.sub- .3, preferably,
x.sub.1>x.sub.2>x.sub.3, for example, x.sub.1=0.15,
x.sub.2=0.1, and x.sub.3=0.
[0129] The semiconductor laser having this structure can be
produced in accordance with the above-described fourth production
method. One example of producing the semiconductor laser in
accordance with the fourth production method will be described
below.
[0130] Referring to FIG. 10, layers are epitaxially grown in
sequence on a substrate 21 made from n-type GaAs: a first cladding
layer 22 made from n-type Al.sub.0.5Ga.sub.0.5As; an active layer
23 made from Al.sub.XGa.sub.1-XAs (x=x.sub.1) for forming a gain
region 231; a lower side second cladding layer 241 made from p-type
Al.sub.0.5Ga.sub.0.5As; a saturable absorption layer 43 made from
Al.sub.XGa.sub.1-XAs (x=x.sub.2) for forming a saturable absorption
region 232; an upper side second cladding layer 242 made from
p-type Al.sub.0.5Ga.sub.0.5As, and a contact layer 25 made from
p-type GaAs. These layers constitute a stacked semiconductor layer
31.
[0131] Referring to FIG. 11, the contact layer 25, upper side
second cladding layer 242, saturable absorption layer 43, and the
lower side second cladding layer 241 are selectively etched from
the contact layer 25 side to such a depth that the lower side
second cladding layer 241 having a thickness "d" remains, to form a
pair of opposed grooves 28 and also form a stripe-like ridge 27
extending in the direction perpendicular to the paper plane of FIG.
11 between the grooves 28. At this time, the saturable absorption
region 232 as part of the saturable absorption layer 43 is formed
in the ridge 27.
[0132] A gap between the grooves 28 is selected at a value
corresponding to the above-described width W.
[0133] Then, as shown in FIG. 9, a lower side current constriction
layer 261 made from n-type Al.sub.0.5Ga.sub.0.5As is epitaxially
grown on the lower side second cladding layer 241 exposed in the
grooves 28. A saturable absorption layer 43 made from
Al.sub.XGa.sub.1-XAs (x=x.sub.3) for forming an outside region 233
is expitaxilly grown on the lower side current constriction layer
261 in such a manner as to be located on both sides of the
saturable absorption region 232 and to be at the same level as that
of the saturable absorption region 232. An upper side current
constriction layer 262 made from n-type Al.sub.0.5Ga.sub.0.5As is
epitaxially grown on the saturable absorption layer 43.
[0134] The expitaxial growth of each of the above-described
semiconductor layers can be performed by the MOCVD method, MBE
method or LPE method.
[0135] A first electrode 29 made from Cr or TiPt is deposited over
the contact layer 25 and the upper side current constriction layer
262 in such a manner as to be in ohmic-contact therewith, and a
second electrode 30 made from Au is deposited on the back surface
of the substrate 21 in such a manner as to be in ohmic-contact
therewith.
[0136] Even in the semiconductor laser in this embodiment, since
the Al component x.sub.2 in the saturable absorption region 232 is
set to be larger than the Al component x.sub.3 in the outside
region 233 (x.sub.2>x.sub.3), the band gap in the saturable
absorption region 232 is set to be larger than the band gap in the
outside region 233. With this configuration, since carriers are
easy to migrate from a region having a large band gap to a region
having a small band gap as described above, carriers generated in
the saturable absorption region 232 due to optical absorption
readily migrate to the outside region 233, with a result that the
effective carrier lifetime in the saturable absorption region 232
is shortened. Accordingly, the ability of the saturable absorption
region 232 is enhanced, to allow self pulsation to be sustained at
a high optical output and/or at a high operational temperature.
[0137] As a result, the allowable range of the width W of the gain
region 231 or the thickness "d" of the lower side second cladding
layer 241 under the lower side current constriction layer 261 can
be made large. This makes it possible to facilitate the production
of the semiconductor laser and hence to improve the production
yield of the semiconductor laser.
[0138] Further, in the case where the Al component ratio x.sub.1 in
the gain region 231 is set to be larger than the Al component ratio
x.sub.2 in the saturable absorption region 232
(x.sub.1>x.sub.2), the differential gain of the saturable
absorption region 232 can be made larger than the differential gain
of the gain region 231. As a result, self pulsation can be
sustained at a higher optical output and/or a higher operational
temperature. This makes it possible to further facilitate the
production of the semiconductor laser and hence to further improve
the production yield of the semiconductor laser.
[0139] In the fifth embodiment, as shown in FIGS. 10 and 11, the
saturable absorption layer 43 including the saturable absorption
region 232 is formed between the lower side second cladding layer
241 and the upper side cladding layer 242; however, the saturable
absorption layer 43 may be formed in the first cladding layer 22 or
the second cladding layer 24 in such a manner as to be separated
from the active layer 23. Even in this case, portions, at which
light effusion occurs due to light emission in the gain region 231,
of the saturable absorption layer 43 are taken as the saturable
absorption regions 232 and portions, at which light effusion little
occurs, located outside the saturable absorption regions 232 are
taken as the outside regions 233.
[0140] The semiconductor laser having the above configuration can
be produced basically in accordance with the method described
above. Even in this case, all semiconductor layers are epitaxially
grown once in a specific order, and thereafter, grooves 28 are
formed to a position crossing the saturable absorption layer and a
saturable absorption layer for forming outside regions 233 is
formed again.
[0141] The gain region 231, saturable absorption region 232 and the
outside region 233 are each configured as that having the DH
structure in the fifth embodiment; however, either or each of them
may be configured as that having a quantum well structure. In this
case, with respect to the structure and its production method shown
in FIGS. 9 to 11, either or each of the active layer 23 and the
saturable absorption layer 43 having the DH structure may be
replaced with that having the quantum well structure.
[0142] Further, the quantum well structure may be replaced with a
quantum wire or quantum dot structure.
[0143] In the case of forming the saturable absorption layer 43
having the quantum well structure, Al component ratios x.sub.2 and
x.sub.3 of quantum well layers in the saturable absorption region
232 and the outside region 233 are specified as x.sub.2>x.sub.3
like the fifth embodiment, or thicknesses d.sub.w2 and d.sub.w3 of
the quantum well layers in both the regions are specified as
d.sub.w2<d.sub.w3. In the case of forming the saturable
absorption region 43 having the MQW structure, thickness d.sub.b2
and d.sub.b3 of quantum barrier layers in both the regions are
specified as d.sub.b2<d.sub.b3 like the third embodiment, or the
numbers Nw.sub.2 and Nw.sub.3 of the quantum well layers in both
the regions are specified as Nw.sub.2<Nw.sub.3. The above
configurations may be suitably combined with each other. In each
case, the effective band gaps Eg.sub.2 and Eg.sub.3 of the quantum
well structures of the saturable absorption region 232 and outside
region 233 satisfy an inequality of Eg.sub.2>Eg.sub.3. As a
result, the same effect as that described in each of the
above-described embodiment can be obtained.
[0144] In the case of replacing the quantum well structure with the
quantum wire or quantum dot structure, Al component ratios x.sub.2
and x.sub.3 of quantum wires or quantum dots constituting the
quantum wire or quantum dot structures in the saturable absorption
region 232 and outside region 233 are specified as
x.sub.2>x.sub.3 like the fifth embodiment; thicknesses d.sub.w2
and d.sub.w3 of the quantum wires or sizes d.sub.w2 and d.sub.w3 of
quantum dots in both the regions are specified as
d.sub.w2<d.sub.w3 like the second embodiment; or the number
Nw.sub.2 and Nw.sub.3 of the quantum wires or the number densities
Nw.sub.2 and Nw.sub.3 of the quantum dots in both the regions are
specified as Nw.sub.2<Nw.sub.3 like the fourth embodiment. The
above configurations may be suitably combined with each other. In
each case, the effective band gaps Eg.sub.2 and Eg.sub.3 of the
quantum wire or quantum dot structures of the saturable absorption
region 232 and outside region 233 satisfy an inequality of
Eg.sub.2>Eg.sub.3. As a result, the same effect as that
described in each of the above-described embodiment can be
obtained.
[0145] Sixth Embodiment
[0146] In this embodiment, the present invention is applied, like
the second embodiment shown in FIG. 7, to a self pulsation laser
having a SCH-MQW structure. An active layer 23 has a gain region
231, a saturable absorption region 232, and an outside region 233.
As shown by the band diagrams in FIGS. 8A to 8C, the gain region
231 has quantum well layers 231w, each barrier layer 231b, and
first and second light confinement layers 411 and 421 disposed with
the quantum well layers 231w and barrier layer 231b put
therebetween; the saturable absorption region 232 has quantum well
layers 232w, each barrier layer 232b, and first and second light
confinement layers 412 and 422 disposed with the quantum well
layers 232w and barrier layer 232b put therebetween; and the
outside region 233 has quantum well layers 233w, each barrier layer
233b, and first and second light confinement layers 413 and 423
disposed with the quantum well layers 233w and barrier layer 233b
put therebetween. The semiconductor laser in this embodiment is
characterized in that the thickness of the active layer 23 is
smoothly increased from the gain region 231 to the saturable
absorption region 232 and is further increased from the saturable
absorption region 232 to the outside region 233.
[0147] FIG. 12 is a schematic sectional view of the semiconductor
laser according to the sixth embodiment. In the figure, parts
corresponding to those shown in FIG. 7 are designated by the same
characters. In this embodiment, referring to FIGS. 8A to 8C,
letting average thicknesses of quantum well layers 231w, 232w and
233w in the gain region 231, saturable absorption region 232 and
outside region 233 be d.sub.w1, d.sub.w2 and d.sub.w3,
respectively; average thicknesses of quantum barrier layers 231b,
232b and 233b in the regions 231, 232 and 233 be d.sub.b1, d.sub.b2
and d.sub.b3, respectively; and average thicknesses of each of
light confinement layers 411 and 421, each of light confinement
layers 412 and 422, and each of light confinement layers 413 and
423 be d.sub.c1, d.sub.c2 and d.sub.c3, the above average
thicknesses are specified to satisfy inequalities of
d.sub.w1<d.sub.w2<d.sub.w3, d.sub.b1<d.sub.b2<d.sub.b3,
and d.sub.c1<d.sub.c2<d.sub.c3 depending on a change in
thickness of the active layer 23, for example, d.sub.w1=100 .ANG.,
d.sub.w2=110 .ANG., d.sub.w3=130 .ANG., d.sub.b1=50 .ANG.,
d.sub.b2=55 .ANG., d.sub.b3=65 .ANG., d.sub.c1=400 .ANG.,
d.sub.c2=440 .ANG. and d.sub.c3=520 .ANG..
[0148] The semiconductor laser can be produced in accordance with
the above-described second production method. One example of
producing the semiconductor laser in accordance with the second
production method will be described below.
[0149] In this example, as shown in FIG. 13A, stripe-like masks 61
spaced at specific intervals for performing stripe-like selective
epitaxy are formed on one principal plane of a substrate 21 made
from n-type GaAs in such a manner as to extend in the direction
perpendicular to the paper plane of FIG. 13A. The masks 61, made
from SiO.sub.2 on which epitaxial growth of a semiconductor layer
does not occur, are formed by forming an SiO.sub.2 layer over the
entire surface by a CVD method, and selectively etching the
SiO.sub.2 layer into a specific stripe pattern by
photolithography.
[0150] Layers are sequentially formed on the principal plane of the
substrate 21 through gaps between the selective expitaxial growth
masks 61: a first cladding layer 22 made from n-type
Al.sub.0.5Ga.sub.0.5As; an active layer 23 made from
Al.sub.0.15Ga.sub.0.85As; a second cladding layer 24 made from
p-type Al.sub.0.5Ga.sub.0.5As, and a contact layer 25 made from
p-type GaAs, to thereby form a stacked semiconductor layer 31
having a stripe pattern corresponding to the arrangement pattern of
the masks 61.
[0151] While not shown in FIGS. 13A and 13B, the active layer 23
has a SCH-MQW structure in which first and second- light
confinement layers are formed on lower and upper sides respectively
and quantum well layers and quantum barrier layers are stacked
between the first and second light confinement layers.
[0152] As shown in FIG. 13B, the stripe-patterned stacked
semiconductor layer 31 is selectively etched from the contact layer
25 side to a depth reaching the second cladding layer 24, to form a
pair of grooves on both sides of each stripe portion of the stacked
semiconductor layer 31 along the extending direction of the stripe
portion of the semiconductor layer 31 and also to form a
stripe-like ridge 27 between the pair of the grooves 28.
[0153] As shown in FIG. 14, a current constriction layer 26 made
from n-type GaAs is buried on the second cladding layer 24 exposed
in the grooves 28.
[0154] While not shown, a first electrode 29 made from Cr or TiPt
is deposited on the contact layer 25 and the current constriction
layer 26 in such a manner as to be in ohmic-contact therewith, and
a second electrode 30 made from Au is deposited on the back surface
of the substrate 21 in such a manner as to be in ohmic-contact
therewith.
[0155] The expitaxial growth of the above-described semiconductor
layers can be performed by the MOCVD method, MBE method or LPE
method.
[0156] In this way, a plurality of semiconductor laser elements can
be simultaneously formed on the common substrate 21. The
semiconductor laser elements can be divided to be used for
single-beam laser devices or can be configured as a multi-beam
laser device as it is.
[0157] In the case of forming the masks 61 on the substrate 21 and
forming the stacked semiconductor layer 31 by epitaxial growth
through the gaps between the masks 61 as described above, a source
gas supplied to the mask 61 for epitaxial growth is not deposited
thereon, being moved to the gap between the masks 61, and is
deposited and epitaxially grown on the portion of the substrate 21
exposed through the gap between the masks 61. As a result, the
growth rate of the film at the positions of the substrate 21
located at the edges of the gap between the masks 61 becomes
higher, and thereby the thickness of the growth film thereat
becomes larger.
[0158] The thickness of the active layer 23 is uniform along the
direction (called the first direction) perpendicular to the paper
planes of FIGS. 13A, 13B and 14 but is non-uniform in the direction
(called second direction) perpendicular to the above first
direction. To be more specific, in the direction perpendicular to
the above first direction, the thickness of the active layer 23 is
thinnest at the position of the substrate 21 corresponding to the
center of the gap between the masks 61 and is gradually increased
toward the positions of the substrate 21 corresponding to both side
ends of the gap between the masks 61. As a result, in the case of
forming the ridge 27 at the position of the substrate 21
corresponding to the center of the gap between the masks 61, the
thickness of the active layer 23 is thinnest at the gain region 231
formed under the ridge 27 at which current is constricted by the
current constriction layer 26, and is gradually increased toward
the saturable absorption region 232 located outside the gain region
231 and further toward the outside region 233 located outside the
saturable absorption region 232. That is to say, the thicknesses of
the first and second light confinement layers, quantum well layers,
and quantum barrier layers constituting the active layer having the
SCH-MQW structure are all gradually increased from the center gain
region 231 to the outside region 233.
[0159] In the above method shown in FIGS. 13A, 13B and 14, the
selective epitaxial growth masks 61 are deposited on the substrate
21 to form the semiconductor layer with its thickness increased
from the center to both the sides of each stripe in the width
direction; however, the masks 61 may be mounted on the substrate
21. Alternatively, there may be adopted a method shown in FIG. 15A,
in which stripe-like metal masks 71 spaced at specific intervals
are disposed over the surface of the substrate 21 in such a manner
as to face thereto, and the above-described semiconductor layers,
that is, the first cladding layer 22 made from n-type
Al.sub.0.5Ga.sub.0.5As, active layer 23, second cladding layer 24
and contact layer 25 are sequentially grown on the substrate 21 via
the masks 71, to form the stacked semiconductor layer 31 having a
stripe pattern corresponding to the arrangement pattern of the
masks 71.
[0160] While not shown, the active layer 23 has a SCH-MQW structure
in which first and second light confinement layers are formed on
the lower and upper sides respectively and quantum well layers and
quantum barrier layers are stacked therebetween.
[0161] Then, as shown in FIG. 15B, the stacked semiconductor layer
31 is selectively etched from the contact layer 25 side to a depth
reaching the second cladding layer 24, to thereby form a plurality
of stripe-like grooves 28 in such a manner as to form ridges 27
along the thinned portions formed directly under the stripe-like
masks 71. A current constriction layer 26 made from n-type GaAs is
buried on the second cladding layer 24 exposed in the grooves
28.
[0162] While not shown, a first electrode 29 made from Cr or TiPt
is deposited on the contact layer 25 and the current constriction
layer 26 in such a manner as to be in ohmic-contact therewith, and
a second electrode 30 made from Au is deposited on the back surface
of the substrate 21 in such a manner as to be ohmic-contact
therewith.
[0163] According to this method, since the source gas for epitaxial
growth flows even to a portion of the substrate 21 directly under
the mask 71, a film is epitaxially grown on such a portion in
co-operation of the migration effect by selecting the distance
between the mask 71 and the substrate 21, the width of the mask 71,
and the gap between the masks 71. In this case, the supplied amount
of the source gas to the portion directly under the mask 71 can be
set to be smaller than the supplied amount of the source gas to a
portion located at the gap between the masks 71. With this
configuration, the growth rate of the film, that is, the film
thickness can be gradually increased from the center to both the
sides of the portion directly under the mask 71.
[0164] Even in this case, a plurality of semiconductor laser
elements can be simultaneously formed on the common substrate 21.
The semiconductor laser elements can be divided to be used for
single-beam laser devices or can be configured as a multi-beam
laser device as it is.
[0165] The epitaxial growth of the above-described semiconductor
layers can be also performed by the MOCVD method, MBE method or LPE
method.
[0166] In the semiconductor laser according to the sixth
embodiment, the thicknesses of the quantum well layer and the
quantum barrier layer in the MQW structure of the active layer are
gradually increased from the gain region 231 toward the saturable
absorption region 232 and further toward the outside region 233.
With this configuration, letting average effective band gaps in the
regions 231, 232 and 233 be Eg.sub.1, Eg.sub.2 and Eg.sub.3, there
simultaneously occur an effect of Eg.sub.1>Eg.sub.2>Eg.sub.3
due to the change in thickness of the quantum well layer and an
effect of Eg.sub.1<Eg.sub.2<Eg.sub.3 due to the change in
thickness of the quantum barrier layer. However, since the
thickness of the quantum barrier layer is smaller than that of the
quantum well layer, the effect of Eg.sub.1>Eg.sub.2>Eg.sub.3
due to the change in thickness of the quantum well layer becomes
dominant.
[0167] Accordingly, even in the semiconductor laser in the sixth
embodiment, because of a property of carriers easy to migrate from
a region having a large band gap to a region having a small band
gap, carriers generated in the saturable absorption region 232 due
to optical absorption readily migrate to the outside region 233,
with a result that the effective carrier lifetime in the saturable
absorption region 232 is shortened. Accordingly, the ability of the
saturable absorption region 232 is enhanced, to allow self
pulsation to be sustained at a high optical output and/or at a high
operational temperature. As a result, the allowable range of the
width W of the gain region 231 or the thickness "d" of the second
cladding layer 24 under the current constriction layer 26 can be
made large.
[0168] In the method of producing the semiconductor laser using the
masks 61 shown in FIGS. 13 and 14 and in the method of producing
the semiconductor laser using the masks 71 shown in FIGS. 15A and
15B, the epitaxial growth can be performed only by the first
expitaxial growth step for forming the stacked semiconductor layer
31 by continuous epitaxial growth and the second epitaxial growth
step for forming the current constriction layer 26 by epitaxial
growth, so that the number of the epitaxial growth steps is not
increased and thereby the mass-productivity is not lowered.
[0169] In the sixth embodiment, the active layer is configured as
that having the SCH-MQW structure; however, it may be configured as
that having a single quantum wire structure or a multiple quantum
wire structure, or a single quantum dot structure or a multiple
quantum dot structure. In these cases, by hanging the thickness or
average thickness of the quantum wires, or the size (volume),
average volume or number density of the quantum dots in the width
direction of the stripe portion of the active layer, the same
effect as that described in this embodiment can be obtained.
[0170] The semiconductor laser including the active layer having
the quantum wire or quantum dot structure can be produced in a
method substantially similar to the method of producing the
semiconductor laser using the masks 61 or 71 in this embodiment. To
be more specific, in place of the active layer having the SCH-MQW
structure, the active layer having the quantum wire or quantum dot
structure is produced in accordance with the first production
method described with reference to FIGS. 13A, 13B and 14 in such a
manner that the thickness of the quantum wires of the quantum wire
structure or the volume or number density of the quantum dots of
the quantum dot structure is largest at a position of the substrate
21 corresponding to the center of the stripe-like mask 61 and
becomes smaller toward both the sides of the stripe-like mask
61.
[0171] The semiconductor laser including the active layer having
the quantum wire or quantum dot structure can be also produced in
the second production method described with reference to FIGS. 15A
and 15B in such a manner that the thickness of the quantum wires of
the quantum wire structure or the volume or lumber density of the
quantum dots of the quantum dot structure is largest at a position
of the substrate 21 corresponding to the center of the stripe-like
mask 71 and becomes smaller toward both the sides of the
stripe-like mask 71.
[0172] In each of the above-described production methods, the
stacked semiconductor layer is formed in such a manner that the
grooves for forming the ridge is formed and the current
constriction layer 26 is formed by epitaxial growth in the grooves;
however, it may be formed in accordance with the above-described
third production method in which ions of an impurity, for example,
an n-type impurity are implanted or diffused in the stacked
semiconductor layer to form a current constriction layer without
formation of the grooves.
[0173] According to the present invention, in the method shown in
FIGS. 13A to 15B, the active layer is formed in such a manner that
the effective band gap is changed in the above-described second
direction; however, the saturable absorption layer may be formed in
accordance with the above-described fifth or sixth method in such a
manner that the band gap is uniform in the first direction and
becomes smaller to the outside in the second direction
perpendicular to the first direction.
[0174] The semiconductor laser and its production method according
to the present invention are not limited to the above-described
embodiments but may be produced in accordance with a method
obtained by combination of some of the above-described embodiments.
In this case, it is expected to obtain a larger effect.
[0175] The conduction type of the substrate 21 or each of the
semiconductor layers formed thereon may be opposed to, that
described above.
[0176] In the above-described embodiments, the current constriction
means is configured as a current constriction layer made from a
semiconductor; however, it may be configured as a space or an
insulator.
[0177] The specific terms such as materials for forming the
substrate 21 and the semiconductor layers, composition of each
material, film thickness, and concentration of carriers in the
above-described embodiments can be variously changed or modified
without departing from the scope of the present invention.
[0178] The epitaxial growth of the semiconductor layers performed
by the MOCVD method, MBE method or LPE method in the
above-described embodiments may be performed by a gas source MBE
method, ALE method or the like.
[0179] The semiconductor laser according to the present invention
is not limited to the above-described stripe structure but may be
applied to other structures insofar as the structure has a gain
region, a saturable absorption region and an outside region.
* * * * *