U.S. patent application number 10/229060 was filed with the patent office on 2003-09-04 for semiconductor laser diode and optical module.
This patent application is currently assigned to OpNext Japan, Inc.. Invention is credited to Nakahara, Kouji, Nomoto, Etsuko, Shimaoka, Makoto, Tsuji, Shinji.
Application Number | 20030165169 10/229060 |
Document ID | / |
Family ID | 27800060 |
Filed Date | 2003-09-04 |
United States Patent
Application |
20030165169 |
Kind Code |
A1 |
Nomoto, Etsuko ; et
al. |
September 4, 2003 |
Semiconductor laser diode and optical module
Abstract
The present invention provides a highly reliable ridge-waveguide
semiconductor laser diode and an optical module. The p-side
electrode of the ridge-waveguide laser diode has a first conductor
layer region and a second conductor layer region formed on the
first conductor layer region. At least one of facets of the second
conductor layer region is recessed inward from a reflection facet.
Thus, the ridge-waveguide semiconductor laser diode has a structure
in which strain which is caused by the electrode stress to be
applied on the diode facet is reduced and the saturable absorption
does not occur. The ridge-waveguide semiconductor laser diode thus
obtained is highly reliable, and the optical module using the same
is remarkably high in reliability.
Inventors: |
Nomoto, Etsuko; (Sagamihara,
JP) ; Nakahara, Kouji; (Kunitachi, JP) ;
Tsuji, Shinji; (Hidaka, JP) ; Shimaoka, Makoto;
(Ushiku, JP) |
Correspondence
Address: |
Stanley P. Fisher
Reed Smith LLP
Suite 1400
3110 Fairview Park Drive
Falls Church
VA
22042-4503
US
|
Assignee: |
OpNext Japan, Inc.
|
Family ID: |
27800060 |
Appl. No.: |
10/229060 |
Filed: |
August 28, 2002 |
Current U.S.
Class: |
372/46.01 |
Current CPC
Class: |
H01S 5/3201 20130101;
H01S 5/04254 20190801; H01S 5/22 20130101; H01S 5/04256 20190801;
H01S 5/16 20130101; H01S 2301/176 20130101; H01S 5/0021 20130101;
H01S 5/04252 20190801 |
Class at
Publication: |
372/46 |
International
Class: |
H01S 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2002 |
JP |
2002-055852 |
Claims
What is claimed is:
1. A semiconductor laser diode comprising: a semiconductor
substrate; a semiconductor layered body which is formed on the
semiconductor substrate and has at least an active region; a first
electrode provided on the semiconductor substrate at a side
opposite to a side on which the semiconductor layered body is
formed; and a second electrode provided at the side of the
semiconductor layered body; wherein: the semiconductor layered body
has a semiconductor layered portion on an upper region with respect
to the active region thereof, the semiconductor layered portion
being in the shape of a projection having its length in a
light-propagating direction; the second electrode on the
semiconductor layered body side contacts an upper face of the
projected semiconductor layered portion; and the second electrode
on the semiconductor layered body side includes a plurality of
conductor layers, at least one of the conductor layers or a partial
region which has a thickness larger than that of an edge of the
conductor layer having a facet position of the conductor layer or
an end position of the partial region which is thicker than the
edge of the conductor layer at a position inside at least one of
reflection facets constituting a cavity of the semiconductor
laser.
2. The semiconductor laser diode according to claim 1, wherein at
least one of the plurality of conductor layers has both of its
facets at positions recessed inward from both of the reflection
facets constituting the cavity of the semiconductor laser.
3. The semiconductor laser diode according to claim 1, wherein the
semiconductor layered body comprises a first conduction type first
cladding layer, the active region, a second conduction type second
cladding layer and a contact layer which are deposited in this
order, and the projected semiconductor layered portion is a
semiconductor layered portion region including the second cladding
layer and the contact layer.
4. The semiconductor laser diode according to claim 3, comprising a
semiconductor layer for carrier confinement in at least one of
regions between the first conduction type first cladding layer and
the active region and between the active region and the second
conduction type second cladding layer.
5. The semiconductor laser diode according to claim 1, wherein the
plurality of conductor layers constituting the second electrode on
the semiconductor layered body side include a first conductor layer
region which is close to the semiconductor layered body and a
second conductor layer region which is formed on the first
conductor layer region; an insulator film is formed on side faces
of the projected semiconductor layered portion, which are in
parallel with the length of the projected semiconductor layered
portion, and upper faces of the semiconductor layered body
extending outwardly from the side faces; the first conductor layer
region and the second conductor layer region cover the upper face
of the projected semiconductor layered portion and at least a part
of the insulator film; an edge of the first conductor layer region
at the side of at least one of the reflection facets of the laser
cavity is located at a position identical with that of the
reflection facet; and an edge of the second conductor layer region
at the side of at least one of the reflection facets of the laser
cavity is recessed inward from the edge position of the first
conductor layer region.
6. The semiconductor laser diode according to claim 1, wherein
edges of the first conductor layer region respectively at the sides
of both of the reflection facets of the laser cavity are located at
positions identical with those of the reflection facets; and edges
of the second conductor layer region respectively at the sides of
both of the reflection facets are recessed inward from the edge
positions of the first conductor layer region.
7. The semiconductor laser diode according to claim 1, wherein the
plurality of conductor layers constituting the electrode on the
semiconductor layered body side include a first conductor layer
region which is close to the semiconductor layered body and a
second conductor layer region which is formed on the first
conductor layer region; the second conductor layer region has a
portion having a thickness thinner than that of a central portion
thereof in the vicinity of a reflection facet of the laser cavity;
an edge of the thinner portion at the reflection facet side is
located at a position identical with that of an edge of the first
conductor layer region; and an edge of the thicker portion at the
reflection facet side is recessed inward from the edge position of
the first conductor layer region.
8. The semiconductor laser diode according to claim 7, wherein the
thinner portion of the second conductor layer region is provided at
each of the reflection facets of the laser cavity, and the edge of
the thinner portion of the second conductor layer region at the
reflection facet side is recessed inward from the edge position of
the first conductor layer region.
9. The semiconductor laser diode according to claim 1, wherein the
plurality of conductor layers constituting the first electrode on
the semiconductor layered body side include a first conductor layer
region which is close to the semiconductor layered body and a
second conductor layer region which is formed on the first
conductor layer region; an insulator film is formed on side faces
of the projected semiconductor layered portion, which are in
parallel with the length of the projected semiconductor layered
portion, and upper faces of the semiconductor layered body
extending outwardly from the side faces; the first conductor layer
region and the second conductor layer region cover the upper face
of the projected semiconductor layered portion and at least a part
of the insulator film; an edge of the first conductor layer region
at the side of at least one of the reflection facets of the laser
cavity is recessed inward from the reflection facet of the laser
cavity; and an edge of the second conductor layer region at the
side of at least one of the reflection facets of the laser cavity
is located at a position identical with that of the edge position
of the first conductor layer region.
10. The semiconductor laser diode according to claim 9, wherein
edges of the first conductor layer region and the second conductor
layer region are recessed inward from the reflection facets of the
laser cavity at both sides of the reflection facets of the laser
cavity.
11. The semiconductor laser diode according to claim 1, wherein the
plurality of conductor layers constituting the electrode on the
semiconductor layered body side include a first conductor layer
region which is close to the semiconductor layered body and a
second conductor layer region which is formed on the first
conductor layer region; an insulator film is formed on side faces
of the projected semiconductor layered portion, which are in
parallel with the length of the projected semiconductor layered
portion, and upper faces of the semiconductor layered body
extending outwardly from the side faces; the first conductor layer
region covers the upper face of the projected semiconductor layered
portion and at least a part of the insulator film; an edge of the
first conductor layer region at the side of at least one of the
reflection facets of the laser cavity is recessed inward from the
reflection facet of the laser cavity; and an edge of the second
conductor layer region at the side of at least one of the
reflection facets of the laser cavity is recessed inward from the
edge position of the first conductor layer region.
12. The semiconductor laser diode according to claim 11, wherein
edges of the first conductor layer region at the sides of both of
the reflection facets of the laser cavity are recessed inward from
the reflection facets of the laser cavity; and edges of the second
conductor layer region at the sides of both of the reflection
facets of the laser cavity are recessed inward from the edge
positions of the first conductor layer region.
13. A semiconductor laser diode comprising: a semiconductor
substrate; a semiconductor layered body which is formed on the
semiconductor substrate and has at least an active region; a first
electrode provided on the semiconductor substrate at a side
opposite to a side on which the semiconductor layered body is
formed; and a second electrode provided at the side of the
semiconductor layered body; wherein: the semiconductor layered body
has a semiconductor layered portion on an upper region with respect
to the active region thereof, the semiconductor layered portion
being in the shape of a projection having its length in a
light-propagating direction; the second electrode on the
semiconductor layered body side contacts at least an upper face of
the projected semiconductor layered portion; the second electrode
on the semiconductor layered body side includes a plurality of
conductor layers, the plurality of conductor layers comprising a
first conductor layer region which is close to the semiconductor
layered body and a second conductor layer region which is formed on
the first conductor layer region; an insulator film is formed on a
portion of at least one of the reflection facets of the laser
cavity on the upper face of the projected semiconductor layered
portion, side faces of the projected semiconductor layered portion,
which are in parallel with the length of the projected
semiconductor layered portion, and upper faces of the semiconductor
layered body extending outwardly from the side faces; the first
conductor layer region covers the upper face of the projected
semiconductor layered portion and at least a part of the insulator
film; an edge of the first conductor layer region at the side of at
least one of the reflection facets of the laser cavity is recessed
inward from the reflection facet of the laser cavity; an edge of
the second conductor layer region at the side of at least one of
the reflection facets of the laser cavity is located at a position
identical with the edge position of the first conductor layer
region; one facet of the insulator film formed on a part of the
upper face of the projected semiconductor layered portion is
located at a position identical with that of the reflection facet
of the laser cavity; and other facet of the insulator film formed
on a part of the upper face of the projected semiconductor layered
portion is located at a position identical with or recessed inward
from the edge position of the first conductor layer region.
14. The semiconductor laser diode according to claim 13, wherein
edges of the first conductor layer region and the second conductor
layer region are recessed inward from the reflection facets of the
laser cavity at both sides of the reflection facets of the laser
cavity; and the insulator film is formed on a portion of each of
the reflection facets of the laser cavity on the upper face of the
projected semiconductor layered body.
15. The semiconductor laser diode according to claim 13, wherein
the plurality of conductor layers include a first conductor layer
region which is close to the semiconductor layered body and a
second conductor layer region which is formed on the first
conductor layer region; an insulator film is formed on a portion of
at least one of the reflection facets of the laser cavity formed on
the upper face of the projected semiconductor layered portion, side
faces of the projected semiconductor layered portion, which are in
parallel with the length of the projected semiconductor layered
portion, and upper faces of the semiconductor layered body
extending outwardly from the side faces; the first conductor layer
region covers the upper face of the projected semiconductor layered
portion and at least a part of the insulator film; an edge of the
first conductor layer region at the side of at least one of the
reflection facets of the laser cavity is recessed inward from the
reflection facet of the laser cavity; an edge of the second
conductor layer region at the side of at least one of the
reflection facets of the laser cavity is recessed inward from the
edge position of the first conductor layer region; one facet of the
insulator film formed on the upper face of the projected
semiconductor layered body is located at a position identical with
that of the reflection facet of the laser cavity; and other facet
of the insulator film formed on the upper face of the projected
semiconductor layered body is located at a position identical with
or recessed inward from the edge position of the first conductor
layer region.
16. The semiconductor laser diode according to claim 15, wherein
edges of the first conductor layer region at the sides of both of
the reflection facets of the laser cavity are recessed inward from
the reflection facets of the laser cavity; edges of the second
conductor layer region at the sides of both of the reflection
facets of the laser cavity are recessed inward from the edge
positions of the first conductor layer region; and the insulator
film is formed on a portion of each of the reflection facets of the
laser cavity on the upper face of the projected semiconductor
layered portion.
17. The semiconductor laser diode according to claim 1, wherein the
plurality of conductor layers constituting the electrode on the
semiconductor layered body side include a first conductor layer
region and a second conductor layer region which is formed on the
first conductor layer region, the first conductor layer region
having a plurality of layers of a titanium layer and a platinum
layer or a plurality of layers of a titanium layer and a nickel
layer and the second conductor layer region being a gold layer.
18. The semiconductor laser diode according to claim 1, wherein the
second electrode on the semiconductor layered body side includes a
tungsten silicide layer.
19. The semiconductor laser diode according to claim 1, wherein the
plurality of conductor layers constituting the electrode on the
semiconductor layered body side include a first conductor layer
region which is close to the semiconductor layered body and a
second conductor layer region which is formed on the first
conductor layer region; and the product of a distance from the edge
of the first conductor layer region at the side of the reflection
facet of the laser cavity to the edge of the second conductor layer
region at the side of the reflection facet of the laser cavity and
a sheet resistance of the first conductor layer region is 2
.OMEGA..multidot.mm or less.
20. An optical module comprising: a substrate; a semiconductor
laser diode mounted on the substrate; and a package having in its
interior the substrate on which at least the semiconductor laser
diode is mounted; wherein: the semiconductor laser diode comprises
a semiconductor substrate; a semiconductor layered body which is
formed on the semiconductor substrate and has at least an active
region; a first electrode provided on the semiconductor substrate
at a side opposite to a side on which the semiconductor layered
body is formed; and a second electrode provided at the side of the
semiconductor layered body; the semiconductor layered body having a
semiconductor layered portion on an upper region with respect to
the active region thereof, the semiconductor layered portion being
in the shape of a projection having its length in a
light-propagating direction; the second electrode on the
semiconductor layered body side contacting an upper face of the
projected semiconductor layered portion; and the second electrode
on the semiconductor layered body side including a plurality of
conductor layers, at least one of the conductor layers or a partial
region which has a thickness larger than that of an edge of the
conductor layer having a facet position of the conductor layer or
an end position of the partial region which is thicker than the
edge of the conductor layer at a position inside at least one of
reflection facets constituting a cavity of the semiconductor laser.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a semiconductor laser diode
and an optical module on which the semiconductor laser diode is
mounted. Examples of the optical module include an optical
transmitter, an optical transceiver and so forth.
[0002] Conventionally, an InGaAsP-based material which is lattice
matched on an InP substrate has been used for a 1.3 .mu.m-1.55
.mu.m-band semiconductor laser diode which is used as a light
source in the optical communication. Such semiconductor laser diode
has been mounted on an optical transmission module together with a
thermoelectric cooler in the prior art. One of the conventional
semiconductor laser diodes will be described by way of example.
FIG. 13 shows a ridge-waveguide semiconductor laser diode having an
electrode structure which extends to a reflection face of a laser
cavity. In this example, an n-type InP cladding layer 41, an
InGaAsP active layer 42 and a p-type InP cladding layer 5 are
deposited on an n-type InP substrate 1. A p-type InGaAs contact
layer 6 is formed on an upper face of the P-InP cladding layer 5
which is formed in the shape of a projection for an emission
region. A p-side electrode 8, which is an ohmic electrode, is
formed on the p-type InGaAs contact layer 6. In general, the p-side
electrode 8 comprises a plurality of conductor layers. A bonding
pad 11 is formed on the p-side electrode 8, in a manner extending
from the p-side electrode 8. An n-side electrode 10 is provided on
a backside of the n-type InP substrate 1.
[0003] In turn, an InGaAlAs-based semiconductor laser diode, which
operates in a wider temperature range to replace the InGaAsP based
ones, is reported by C. E. Zah et al. in "IEEE Journal of Quantum
Electronics, Vol. 30, No. 2, P. 511 (1994)". The InGaAlAs-based
semiconductor laser diode does not require the thermoelectric
cooler during operation at high temperatures. Since a lower cost is
desired for the short range Datacom network, developments in a
direct modulation type InGaAlAs-based semiconductor laser diode and
an optical transmission module including the laser diode is in
progress.
[0004] Further, in known structures of a semiconductor laser diode
using nitride semiconductor material and a buried heterostructure
semiconductor laser diode, an electrode metal layer facet in the
vicinity of the reflection face is recessed to avoid troubles in
the diode fabrication process. For example, Japanese Patent
Laid-open No. 2000-277846 discloses a structure of the
semiconductor laser diode using nitride semiconductor material
wherein a p-side electrode is formed on a contact portion as being
extended to a facet of a cavity and a main p-side electrode having
its facet at a portion recessed inward from the cavity facet is
formed on the p-side electrode. However, since the substrate does
not have cleavage properties, the effect of the structure is
nothing but a prevention of peeling of the electrodes due to impact
accompanying cleavage which occurs at the time of forming the
cavity facets and sagging of the main p-side electrode toward the
cavity facet. Japanese Patent Laid-open No. 11-340573 discloses a
structure wherein no electrode is provided in the vicinity of a
reflection face for the purpose of self-sustained pulsation of the
gallium nitride-based laser diode, while Japanese Patent Laid-open
No. 10-27939 discloses a similar structure for the purpose of
preventing electrodes from peeling off due to impact caused by
separation at the time of forming cavity facets of a nitride
semiconductor laser diode.
[0005] Further, Japanese Patent Laid-open No. 3-206678 discloses
the conventional buried heterostructure semiconductor laser diode
as shown in FIG. 14; however, an effect achieved by a shape of an
electrode on a facet is not defined therein. In FIG. 14, reference
numeral 1 denotes an n-type InP substrate, 7 denotes a passivation
film, 8 denotes a p-side electrode, 9 denotes a first conductor
layer of p-side electrode, 10 denotes an n-side electrode, 41
denotes an n-type InP cladding layer, 42 denotes an InGaAsP active
region, 43 denotes a lasing region, 44 denotes a p-type InP
cladding layer, 45 denotes a p-type InP buried layer, 46 denotes an
i-type InP buried layer, 47 denotes an n-type InP buried layer, 48
denotes a p-type InP buried layer, 49 denotes a p-type InGaAsP
buried layer, 50 denotes a mesa channel and 51 denotes a buried
mesa
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to provide a highly
reliable ridge-waveguide semiconductor laser diode and an optical
module using the same.
[0007] The inventors of the present invention have found that, if
the above-described structure shown in FIG. 13 is applied to a
ridge-waveguide semiconductor laser diode which is fabricated by
using the newly introduced materials such as an InGaAlAs-based
material or an InGaAsP-based material, reliability of
characteristics of such ridge-waveguide semiconductor laser diode
is deteriorated. The deterioration is due to a sudden degradation
in emission properties during operation or heavy current
injection.
[0008] In view of the above-mentioned situation, a primary object
of the present invention is to provide a highly reliable
ridge-waveguide semiconductor laser diode. Further, another object
of the present invention is to provide a highly reliable optical
transmission module using the semiconductor laser diode of the
present invention.
[0009] Technical aspects for achievement of the above objects are
as follows. A first technical aspect is a reduction in strain which
is caused by an electrode stress applied to a facet of a
ridge-waveguide semiconductor laser diode. A second technical
aspect is a structure capable of preventing saturable
absorption.
[0010] A so-called upper electrode of a ridge-waveguide laser
diode, which is an ohmic electrode, is formed on a contact layer of
a semiconductor layered body for semiconductor laser. In the
present invention, the following characteristics are added to the
ohmic electrode. More specifically, the upper electrode, i.e., an
electrode on the semiconductor layered body is so formed that the
electrode has a facet of its electrode layer at a position recessed
inward from at least one of reflection facets constituting a cavity
of the semiconductor laser. Alternatively, the electrode has an
edge portion which is reduced in thickness, and an edge portion of
the electrode which is continuous with and thicker than the thinner
edge portion is located at a position recessed inward from at least
one of the reflection facets constituting the cavity of the
semiconductor laser.
[0011] In general, the ohmic electrode comprises a plurality of
conductor layers. Hereinafter, the present invention will be
described using an example of the electrode comprised of a
plurality of conductor layers. For brevity, a layer region of the
upper electrode at the side of the semiconductor layered body will
be referred to as a first conductor layer region and a layer region
formed on the first conductor layer region is referred to as a
second conductor layer region. A representative mode of the present
invention is such that at least one of facets of the second
conductor layer region, which is formed on the first conductor
layer region, is located at a position recessed inward from a facet
of the first conductor layer region. Thus, it is possible to reduce
a thickness of the conductor layers for the electrode in the
vicinity of a reflection facet of an optical cavity or to recess
the conductor layers for the electrode inward from the vicinity of
the reflection facet of the optical cavity.
[0012] The ridge-waveguide is a type of a semiconductor laser diode
wherein a semiconductor layered body in the shape of a projection
having a width substantially corresponding to an emission region
and a length in a light-propagating direction is provided on a
portion upper from an active region of the semiconductor laser
diode with respect to a semiconductor substrate. In general, the
semiconductor laser diode which is provided with the projected
semiconductor layered body having a width corresponding to an
emission region is called the ridge-waveguide laser. In many cases,
a semiconductor layered body which is upper part of a cladding
layer formed on an active region is formed as the projected
semiconductor layered body as described above. Of course, it is
possible to vary the structure of the projected semiconductor
layered body when so required.
[0013] There are variations of the mode of locating at least one of
the facets of the second conductor layer region at a position
recessed inward from the facet of the first conductor layer region.
Of course, the effect of the present invention is achieved by
locating one of the facets in the above-described manner; however,
it is preferred to apply the mode to both of the facets of the
optical cavity. The reason therefor will be understood in terms of
factors of the effect.
[0014] The following modes are representative of the mode of
locating at least one of the facets of the second conductor layer
region at a position recessed inward from the facet of the first
conductor layer region.
[0015] (1) The facet of the first conductor layer region and the
facet of the optical cavity are located at substantially identical
positions, and the facet of the second conductor layer region is
recessed inward from the first conductor layer region.
[0016] (2) Both of the facets of the first and the second conductor
layer region are recessed inward from the facet of the optical
cavity.
[0017] (3) Both of the facets of the first and the second conductor
layer region are recessed inward from the facet of the optical
cavity, and the facet of the second conductor layer region is
recessed inward from the first conductor layer region.
[0018] (4) The names "first conductor layer region" and "second
conductor layer region" are used for brevity in the above
description; however, if the first conductor layer region comprises
a plurality of conductor layers, it is possible to recess a part of
the layers inward from the facet of the optical cavity and to
locate the second conductor layer region, the first conductor layer
region and the facet of the optical cavity at substantially
identical positions. Owing to the partially recessed conductor
layer, it is possible to reduce the thickness of the electrode in
the vicinity of the facet of the optical cavity. Alternatively, the
same effect can be achieved by reducing a thickness in the vicinity
of a relevant edge portion of a layer in place of removing the
vicinity of an edge of the part of the layers. In addition, it is
practical to perform such layer processing on the uppermost layer
of the conductor layers. The use of a gold layer for an uppermost
layer is considerably effective.
[0019] (5) In general, layers other than the contact layer, with
which at least the first conductor layer region contacts, are
covered with an insulator film. It is possible to cover the
vicinity of at least one of facets of the contact layer with the
insulator film and to form the first conductor layer region of the
electrode on the contact layer so as to cover at least a part of
the insulator film with the facet thereof being located at a
position substantially identical with the facet of the optical
cavity or at a position recessed inward from the optical cavity
facet.
[0020] In addition, the above structures of the electrode in the
vicinity of one of the facets may be used in combination for
opposite facets.
[0021] Thus, remarkably high reliability is achieved by mounting
any one of the ridge-waveguide semiconductor laser diodes of the
present invention on an optical module.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a perspective view showing a semiconductor laser
diode according to a first embodiment of the present invention;
[0023] FIG. 2 is a graph showing an example of a relationship
between an upper electrode stress of the semiconductor laser diode
and a rate of increase in threshold current after starting
operation;
[0024] FIG. 3 is a graph showing an example of a relationship
between an upper electrode stress of the semiconductor laser diode
and a rate of increase in threshold current after starting
operation;
[0025] FIG. 4 is a graph showing an example of calculation results
of a stress applied on an active region by the upper electrode of a
ridge-waveguide semiconductor laser diode of the conventional
structure;
[0026] FIG. 5 is a graph showing an example of calculation results
of a stress applied on an active region by the upper electrode of
the ridge-waveguide semiconductor laser diode of the present
invention;
[0027] FIG. 6 is a diagram showing a mounting method of the
semiconductor laser diode according to the first embodiment of the
present invention;
[0028] FIG. 7 is a perspective view showing a semiconductor laser
diode according to a second embodiment of the present
invention;
[0029] FIG. 8 is a diagram showing a mounting method of the
semiconductor laser diode according to the second embodiment of the
present invention;
[0030] FIG. 9 is a perspective view showing a semiconductor laser
diode according to a third embodiment of the present invention;
[0031] FIG. 10 is a perspective view showing a semiconductor laser
diode according to a fifth embodiment of the present invention;
[0032] FIG. 11 is a perspective view showing an optical
transmission module using the semiconductor laser diode of the
present invention;
[0033] FIG. 12 is a perspective view showing an example of the
structure of an optical transmission module;
[0034] FIG. 13 is a perspective view showing a conventional
semiconductor laser diode provided with an electrode which extends
to reflection facets;
[0035] FIG. 14 is a perspective view showing a conventional buried
heterostructure semiconductor laser diode wherein an electrode
metal is recessed inward in the vicinity of reflection facets;
[0036] FIG. 15 is a diagram showing the semiconductor laser diode
according to the first embodiment of the present invention as
viewed from a cavity facet;
[0037] FIG. 16 is a diagram showing the conventional buried
heterostructure semiconductor laser diode as viewed from a cavity
facet;
[0038] FIG. 17 is a sectional view showing a section parallel with
a waveguide of the conventional buried heterostructure
semiconductor laser diode wherein an-side electrode metal is
recessed in the vicinity of a reflection facet; and
[0039] FIG. 18 is a sectional view showing a section parallel with
a waveguide of the semiconductor laser diode according to the fifth
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Basic aspects (general technical aspects) of the present
invention will be described, followed by descriptions of respective
modes of embodiments of the present invention.
[0041] To be brief, an upper electrode is firstly formed on a
contact layer and then an electrode in the vicinity of a facet is
reduced in thickness in the present invention. Therefore, if a
substrate is an n-type semiconductor substrate, the p-side
electrode on the contact layer is formed by depositing titanium,
platinum and gold in this order, and a thickness of gold on the
diode facet is reduced while the thicknesses of titanium and
platinum, which are barrier metals, are left as they are. In order
to diminish the stress to be caused by the whole electrodes, the
thicknesses of titanium and platinum, which serve as an ohmic
electrode, may be reduced within the extent to which effects of the
barrier metals are achieved.
[0042] More specifically, each of the side faces of the
ridge-waveguide is covered with an insulator film, and the
electrode comprises the ohmic electrode which is formed on the
contact layer to cover the outside of the insulator film on the
side faces of the ridge and the electrode metal which is formed to
cover the ohmic electrode with at least one of its facets being
located at a position recessed inward from the ohmic electrode
facet. It is practical that the ohmic electrode includes titanium
and platinum layers in this order on the contact layer and that an
uppermost layer of the electrode metal is formed of gold. Also, it
is preferable that the ohmic electrode includes titanium and nickel
layers in this order on the contact layer and that the uppermost
layer of the electrode metal is formed of gold or tungsten
silicide.
[0043] In a representative method of fabricating the electrode, for
example, materials respectively containing titanium and platinum
for forming the ohmic electrode and gold for forming the uppermost
layer of the electrode metal are deposited continuously, and then
part or whole of the thickness of the gold layer is reduced or
eliminated such that a facet of the gold layer is located at a
position recessed inward from a facet of the ohmic electrode.
[0044] In this case, a resistance of the thus obtained ohmic
electrode is increased to thereby reduce current diffusion in a
direction vertical to the thickness of the cladding layer even if
InP, which tends to cause the current diffusion of the ridge, is
used for the ridge. Therefore, the current is prevented from
flowing to the facets to thereby cause saturable absorption in some
cases. For example, in the case where a height of the ridge is 1.7
.mu.m and the removed thickness of the uppermost layer of the
electrode metal is 50 .mu.m, the saturable absorption occurs if a
sheet resistance of the ohmic electrode is 60 .OMEGA., but not if
the sheet resistance of the ohmic electrode is 40 .OMEGA.. Also,
the saturable absorption does not occur if the removed thickness of
the uppermost layer of the electrode metal is 25 .mu.m even when
the sheet resistance of the ohmic electrode is 60 .OMEGA..
Practical values should be set in view of the characteristics such
as the current diffusion and saturable absorption as described
above.
[0045] It is also possible to recess the electrode facet from the
diode facet to such an extent that the recession does not cause the
saturable absorption. In this case, an insulator film is firstly
attached on an upper face of the contact layer at a portion close
to the diode facet to form the p-side electrode on the insulator
film, and a degree of the recession of the electrode facet from the
diode facet can sufficiently be defined depending on the insulator
film. Here, the p-side electrode facet or the upper electrode is
recessed inward from the diode facet to reduce a stress applied on
the diode facet. For example, in the case where the p-side
electrode facet is recessed inward from the diode facet, the
saturable absorption occurs if the height of the ridge is 1.7 .mu.m
and the insulator film on the contact layer upper face is recessed
inward from the facet by 10 .mu.m, but not if the recession is 7
.mu.m or less. Thus, if the insulator film is recessed inward from
the facet by 7 .mu.m, the recession of the p-side electrode facet
may be from about 2 .mu.m to about 6 .mu.m, for example.
[0046] Since it is necessary to secure an area for conduction, the
range of elimination of the upper electrode metal may preferably be
small as possible.
[0047] In the junction-down mounting process, the upper electrode
metal is degraded in heat dissipation in the case of insufficient
solder wetting. Therefore, an area for the solder wetting must be
secured. On the other hand, a facet of the upper electrode metal is
recessed inward from the diode facet by about 2 .mu.m, for example,
in order to prevent the solder stress in fusing the semiconductor
laser diode and a heat sink from affecting the diode facet.
[0048] Specific values of the elimination of the electrode at the
facet are decided depending on factors such as the resistance and
cleavage of the ohmic electrode. Under the above-described
conditions, it is preferable that the product of the distance from
the ohmic electrode facet to the electrode metal facet and the
sheet resistance of the ohmic electrode is 2 .OMEGA..multidot.mm or
less. As mentioned above, the saturable absorption does not occur
when the distance from the ohmic electrode facet to the electrode
metal facet is 50 .mu.m (i.e. 0.05 mm) and the sheet resistance of
the ohmic electrode is 40 .OMEGA.. In this case, the product of the
distance and the sheet resistance is 2 .OMEGA..multidot.mm. Also,
the saturable absorption does not occur when the distance from the
ohmic electrode facet to the electrode metal facet is 25 .mu.m
(i.e. 0.025 mm) and the sheet resistance of the ohmic electrode is
60 .OMEGA.. In this case, the product of the distance and the sheet
resistance is 1.5 .OMEGA..multidot.mm. In turn, the saturable
absorption occurs when the distance is 50 .mu.m (i.e. 0.05 mm) and
the sheet resistance in the ohmic electrode is 60 .OMEGA. as
mentioned above. In this case, the product of the distance and the
sheet resistance is 3 .OMEGA..multidot.mm. As a result of the study
conducted under the above-described conditions, the saturable
absorption does not occur if the product of the distance from the
ohmic electrode facet to the electrode metal facet and the sheet
resistance in the ohmic electrode is 2 .OMEGA..multidot.mm or
less.
[0049] The insulator film is provided on the projected
semiconductor layered body constituting the ridge-waveguide of the
semiconductor laser diode except for the region on which the
conductor layer for electrode is provided. In practice, an
electrode pad portion which extends from the conductor layer for
electrode is formed.
[0050] <Comparison Between Experiments based on the Conventional
Structure and the Present Invention>
[0051] Causes for deterioration in characteristics of a
semiconductor laser diode during ordinary operations are estimated
as follows. In a ridge-waveguide semiconductor laser diode, strain
caused by a stress of an electrode near an active region affects a
ridge joint portion due to a form with a level difference on a
surface near the active region. A thermal expansion coefficient of
metals used for the electrode is twice that of a semiconductor
substrate. This is because a tensile stress remains during heating
and then cooling the whole semiconductor laser diode in mounting
the semiconductor laser diode. In the structure where the electrode
is extended to the reflection facet as shown in FIG. 13, the stress
applied on the ridge joint portion is so heavy that the ridge
portion tends to be bent.
[0052] Further, as is apparent from the characteristics shown in
FIGS. 2 and 3, the stress influences largely on the reliability of
the semiconductor laser diode. FIG. 2 shows characteristics of a
diode when an electrode stress is 73 MPa, and FIG. 3 shows
characteristics of a diode when an electrode stress is 76 MPa. Each
of the graphs of FIGS. 2 and 3 shows a rate of increase in
threshold current with respect to operating time when the
semiconductor laser diode is driven at a constant current. In
either cases, the operating current was 4.3.times.10.sup.8
A/M.sup.2, and the diode was operated in a nitrogen atmosphere of
100.degree. C. The graphs are for the comparison of characteristics
of diodes fabricated under the same conditions; however, quite a
few variations in the rate of increase in threshold current are
observed in FIG. 3, wherein the electrode stress is larger and such
electrode stress leads to a generation of a large amount of diodes
which are deteriorated in characteristics. In turn, the rate of
increase in threshold current and the deterioration in
characteristics are smaller in FIG. 2 in which the electrode stress
is smaller.
[0053] In addition, in each of the diodes used for the above
measurements, the stress was changed by changing the thickness of
the electrode near the active region. As shown in FIG. 3, the
threshold current rapidly increased during the ordinary operation
to deteriorate the semiconductor laser diode when the electrode
stress is large. The above results are sufficient for estimating
that the laser diode having a low electrode stress is highly
reliable, while the stress of the electrode near the active region
causes the strain and the strain affecting the active region and
the ridge-waveguide leads the laser diode to the degradation.
[0054] FIG. 4 is a graph showing results of calculation of a stress
applied to an active region of a ridge-waveguide semiconductor
laser diode, the stress being caused by an electrode near the
active region. The horizontal axis is a distance from a reflection
facet on the active region to a position at the interior of an
optical cavity, and the vertical axis is a stress at the position.
As seen from the calculation results of FIG. 4, the tensile stress
caused by a metal film which is used for the upper electrode on the
active region of the ridge-waveguide semiconductor laser is
particularly high in the vicinity of the reflection facet. Strain
caused by the stress is large on the reflection facet on the active
region. Therefore, dislocation occurs on the active region, which
is apt to be increased. The rate of degradation of semiconductor
laser diode will be increased due to such dislocation and its
propagation. At a portion of the active region at which the tensile
strain occurs, a band gap of the semiconductor is smaller than that
of other portions. Therefore, temperature rise is caused by light
absorption at the time of semiconductor lasing, which leads to a
further reduction in the band gap to cause the light absorption.
Due to such positive feedback, the laser diode ultimately causes a
light power failure in some cases. An experiment was conducted by
driving the conventional semiconductor laser diode shown in FIG. 13
under an automatic power control for light power of 10 mW in a
nitrogen atmosphere of 85.degree. C. As a result, the light power
of 10 mW was not maintained due to deterioration in differential
quantum efficiency.
[0055] FIG. 5 is a graph showing results of calculation of a stress
in the active region which is caused by the upper electrode in the
above-described embodiment of the present invention. In the
structure of the laser diode, a portion of about 2 .mu.m of the
metal on the upper face of the ridge of the ridge-waveguide laser
diode was eliminated. The horizontal axis of the graph is a
distance from a reflection facet on the active region to a position
at the interior of an optical cavity, and the vertical axis is a
stress at the position. As seen from the calculation results of
FIG. 5, the tensile stress caused by a metal film which is used for
the p-side electrode on the active region of the ridge-waveguide
semiconductor laser is reduced by 20% when the portion of about 2
.mu.m of the metal at the upper face of the ridge was eliminated as
compared with the case wherein gold is deposited on the whole of
the ridge upper face. In this example, only the ridge upper face is
subjected to the elimination of gold; however, a lower stress can
be achieved by eliminating gold deposited on both side faces of the
ridge.
[0056] In turn, causes of the degradation due to the heavy current
injection are estimated as follows. Current-light power
characteristics of two structures were compared with stress caused
by an electrode in the vicinity of a reflection facet being kept at
a substantially equal level. A sudden deterioration occurred when a
heavy current of 2500A/mm.sup.2 was injected to the ridge-waveguide
semiconductor laser diode having the structure shown in FIG. 13. In
turn, in a modified structure of the one shown in FIG. 13, wherein
an ohmic electrode 9 and a p-side electrode 8 were attached on a
p-type InAlAs contact layer 6 which is covered with a silicon oxide
insulator film up to a portion of 5 .mu.m from the edge, light
power was saturated when the heavy current of 2500A/mm.sup.2 was
injected to a ridge-waveguide semiconductor laser diode without the
degradation. In the latter diode, the current was not injected in
the vicinity of the reflection facet, and carriers are supplied to
the facet region by the current diffusion in the p-type InP
cladding layer. That is to say, the sudden degradation hardly
occurs in a diode having a low current density if the current
density is uniform in the central area of the diode, and thus such
diode is highly reliable. Thus, it is estimated that, if the
current density is large in the vicinity of the reflection facet,
heat is generated due to a nonradiative recombination through an
interfacial state between the native oxide and the facet coating
film on the active region facet, and such heat leads to the
degradation.
[0057] Since the tensile strain occurs in a region which is distant
by about 2 .mu.m from the reflection facet on the active region as
can be seen from the calculation results of FIG. 4, it is also
necessary to reduce the light absorption by reducing the current
density in the region to be smaller than that of other regions in
order to prevent the carrier from being used for the nonradiative
recombination. For that purpose, the current density in the
vicinity of the active region in the reflection facet must be made
smaller than that in the cavity. However, if the current density on
the reflection facet on the active region is too low for the
current to flow to the facet, the saturable absorption occurs to
cause the degradation of diode. Therefore, in order to reduce the
light absorption, it is necessary either to reduce the thickness of
the electrode on the facet to such a degree as not to cause the
saturable absorption or to reduce the area of the electrode.
[0058] The calculation of FIG. 4 does not allow for the strain on
the semiconductor layer since the semiconductor layer is assumed to
be made of InP; however, if compressive strain occurs on the active
region, the tensile strain occurs on the reflection facet due to
release of the strain. Therefore, it can be estimated that the
actual strain may be larger as being added up with the strain of
the electrode metal film.
[0059] The above description is for the comparison and examination
of characteristics based on the basic aspects of the present
invention. Hereinafter, specific modes of embodiment of the present
invention will be described.
[0060] First Embodiment
[0061] A conductor layered body constituting a p-side electrode,
i.e., an upper conductor layer of a first conductor layer region
and a second conductor layer region, more specifically, an area in
the vicinity of a reflection facet on the second conductor layer
region, is removed in this embodiment. Thus, a part of an electrode
near an active region is recessed inward from the reflection
facet.
[0062] FIG. 1 is a perspective view showing a semiconductor laser
diode of the present embodiment. On an n-type InP substrate 1, a
layered body of a compound semiconductor having a typical double
heterostructure is formed by the metal organic vapor phase epitaxy.
More specifically, an n-type InAlAs cladding layer 2, an InGaAlAs
SCH (Separate Confinement Heterostructure) layer (not shown since
this is an additional layer), an InGaAlAs active region 3, an
InGaAlAs SCH layer (not shown since this is an additional layer), a
p-type InAlAs first cladding layer 4, a p-type InP second cladding
layer 5 and a p-type InGaAs contact layer 6 are formed in this
order. Here, the SCH layer is a type of layer for separating a
carrier confinement region from an optical confinement region,
which is known.
[0063] The p-type InP second cladding layer 5 and the p-type InGaAs
contact layer 6 are formed by the photolithography so as to provide
a ridge-waveguide structure.
[0064] A silicon oxide film 7 is formed on a region other than a
surface of the p-type InGaAs contact layer 6 which is a
ridge-waveguide region as a passivation film by the plasma CVD
(Chemical Vapor Deposition).
[0065] As a p-side electrode 8 for constituting an ohmic electrode,
a first and second conductor layer regions 9-1 and 9-2 of the
p-side electrode are formed on the silicon oxide film 7. More
specifically, a titanium layer having a thickness of 100 nm, a
platinum layer having a thickness of 100 nm and a gold layer having
a thickness of 600 nm are deposited in this order on the silicon
oxide film 7 by vapor deposition. Then, the conductor layers of the
first and second conductor layer regions 9-1 and 9-2 are processed
to leave an upper face of the ridge and a bonding pad 11.
[0066] A portion of the gold layer which is an uppermost layer
among the layers forming the electrode in the vicinity of the
reflection facet is removed by the photolithography. The titanium
layer, platinum layer and part of the gold layer are remained as
they are. The titanium layer and platinum layer constitute the
first conductor layer region 9-1, and the gold layer is the second
conductor layer region 9-2. The portion to be removed of the gold
layer in the vicinity of the reflection facet may be small as
possible. For example, the removed portion may be 10 .mu.m.
Preferable example of an etching solution to be used may be one
prepared by dissolving ammonium iodine (NH.sub.4I) and iodine
(I.sub.2) in pure water.
[0067] After thinning a bottom face of the n-type InP substrate 1
by grinding to achieve a thickness of 120 .mu.m, an n-side
electrode 10 is formed by depositing gold-germanium, nickel and
gold on the bottom face by vapor deposition. Then, the thus
obtained semiconductor laser wafer is cleaved to form a pair of
laser cavity facets 13 and 14. An ordinary insulator film is formed
on each of the cavity facets for the purpose of protection thereof
and adjustment of reflectance. The insulator film is not shown in
FIG. 1.
[0068] As shown in FIG. 6, the thus obtained semiconductor laser
diode 21 is connected to a silicon submount 22 by the junction-up
method using a gold-tin based soldering material, and a diode
electrode and a package electrode lead are connected to each other
using a gold wire (not shown). The junction-up method is a type of
mounting method for placing an active region on an upper position
with respect to the submount 22.
[0069] The semiconductor laser diode of the present invention
operates stably for 5,000 hours in a nitrogen atmosphere of
85.degree. C. under a constant driving condition of 10 mW of light
power. Estimated life of the diode of the present embodiment is
100,000 hours. Further, in the present embodiment, it is possible
to deposit the p-side electrode 8 and the ohmic electrode 9
continuously in one process step. Therefore, a diode resistance can
readily be regulated to a value which is substantially the same as
that of the conventional diode. Thus, the fabrication process of
the present embodiment is advantageously simple.
[0070] <Second Embodiment>
[0071] In the present embodiment, an edge of a p-side electrode at
the side of an emission face is recessed inward from a reflection
facet. Further, the first conductor layer region which constitutes
the p-electrode is left as it is, while a reflection facet side of
a second conductor layer region which is formed on a first
conductor layer region is recessed more inward from a reflection
facet side of the first conductor layer region.
[0072] FIG. 7 is a perspective view showing a semiconductor laser
diode according to the present embodiment. On an n-type InP
substrate 1, an n-type InAlAs cladding layer 2, an InGaAlAs SCH
(Separate Confinement Heterostructure) layer (not shown), an
InGaAlAs active region 3, an InGaAlAs SCH layer (not shown), a
p-type InAlAs first cladding layer 4, a p-type InP second cladding
layer 5 and a p-type InGaAs contact layer 6 are formed in this
order by the metal organic vapor phase epitaxy. Then, in the same
manner as in the foregoing embodiment, the p-type InP second
cladding layer 5 and the p-type InGaAs contact layer 6 are
processed so as to form a so-called ridge waveguide by the
photolithography. Regions other than a surface of the p-type InGaAs
contact layer 6 and a portion having a width of 7 .mu.m in the
vicinity of a facet of the p-type InGaAs contact layer 6 are
covered with a silicon oxide film 7 which is formed by the plasma
CVD.
[0073] On the silicon oxide film 7, a first conductor layer region
9-1 and a second conductor layer region 9-2 are formed continuously
as a p-side electrode 8 by depositing a titanium layer having a
thickness of 200 nm, a platinum layer having a thickness of 150 nm
and a gold layer having a thickness of 600 nm in this order. Thus,
a layered body for an ohmic electrode is formed as the p-side
electrode 8.
[0074] The layered body except for the portion of 7 .mu.m at its
facet is processed so as to leave an upper face of the ridge and a
bonding pad 11. In this case, a portion of the second conductor
layer region, which is an uppermost face of the first conductor
layer region and the second conductor layer region in the vicinity
of the reflection facet, is removed by the photolithography to
leave the first conductor layer region and other portion of the
second conductor layer region as the ohmic electrode. In this
embodiment, the first conductor layer region includes the titanium
layer and the platinum layer, and the second conductor layer region
is the gold layer. The removed portion on the gold layer in the
vicinity of the reflection facet is, for example, 5 .mu.m from the
facet of the first conductor layer. An etching solution to be used
is prepared by dissolving ammonium iodide (NH.sub.4I) and iodine
(I.sub.2) in pure water.
[0075] Then, after thinning a bottom face of the n-type InP
substrate 1 by grinding to achieve a thickness of 120 .mu.m, an
n-side electrode 10 is formed by depositing gold-germanium, nickel,
titanium, platinum and gold on the bottom face by vapor
deposition.
[0076] The thus obtained semiconductor laser wafer is cleaved to
form a pair of laser cavity facets. An insulator film is formed on
each of the cavity facets for the purpose of protection thereof and
adjustment of reflectance. The insulator film is not shown in FIG.
7.
[0077] FIG. 8 is a perspective view showing a state in which a
semiconductor laser diode 21 is mounted on a submount 22. The
semiconductor laser diode 21 is mounted on the silicon submount 22
on which a gold-tin based solder material is patterned in the same
manner as that of the p-side electrode 8 of the semiconductor laser
diode by the junction-down method. In the mounting, the p-side
electrode 8 and the patterned solder are connected by transmitting
infrared ray to confirm the positions of positioning markers 12,
and a semiconductor laser diode electrode and a package electrode
lead are connected using a gold wire (not shown).
[0078] The semiconductor laser diode of the present invention
operates stably for 5,000 hours in a nitrogen atmosphere of
85.degree. C. under a constant driving condition of 10 mW of light
power. Estimated life of the diode of the present embodiment is
100,000 hours.
[0079] <Third Embodiment>
[0080] The third embodiment is similar to the first and second
embodiments, but different in fabrication process. In this
embodiment, a second conductor layer region is formed after forming
a first conductor layer region, not continuously.
[0081] FIG. 9 is a perspective view showing a semiconductor laser
diode according to the present embodiment. In the same manner as in
the foregoing embodiments, an n-type InAlAs cladding layer 2, an
InGaAlAs SCH (Separate Confinement Heterostructure) layer (not
shown), an InGaAlAs active region 3, an InGaAlAs SCH layer (not
shown), a p-type InAlAs first cladding layer 4, a p-type InP second
cladding layer 5 and a p-type InGaAs contact layer 6 are formed on
an n-type InP substrate 1 in this order by the metal organic vapor
phase epitaxy. The p-type InP second cladding layer 5 and the
p-type InGaAs contact layer 6 are processed so as to form a ridge
waveguide by the photolithography. Regions other than a surface of
the p-type InGaAs contact layer 6 are covered with a silicon oxide
film 7 which is formed by the thermal CVD.
[0082] On the silicon oxide film 7, a titanium layer having a
thickness of 100 nm, a platinum layer having a thickness of 100 nm
and a gold layer having a thickness of 50 nm are deposited in this
order by vapor deposition to form a first conductor layer region
9-1 of a p-side electrode 8. Then, the first conductor layer region
is processed so as to leave an upper face of the ridge and a
bonding pad 11. Regions other than the first conductor layer region
9-1 and a portion of 10 .mu.m from a reflection facet on the ridge
upper face are protected by photoresist. On the photoresist layer,
a second conductor layer 9-2 of the p-side electrode is formed by
depositing a titanium layer having a thickness of 100 nm and a gold
layer having a thickness of 500 nm in this order by vapor
deposition. Then, the second conductor layer region 9-2 is
processed using the photoresist by the lift-off method. Thus, the
second conductor layer which is recessed inward from the reflection
facet by 10 .mu.m is formed.
[0083] Then, after thinning a bottom face of the n-type InP
substrate 1 by grinding to achieve a thickness of 120 .mu.m, an
n-side electrode 10 is formed by depositing gold-germanium, nickel
and gold on the bottom face by vapor deposition. The thus obtained
semiconductor laser wafer is cleaved to form a pair of laser cavity
facets 13 and 14. An insulator film is formed on each of the cavity
facets by an ordinary method for the purpose of protection thereof
and adjustment of reflectance. The insulator film is not shown in
FIG. 9.
[0084] As shown in FIG. 6, the thus obtained semiconductor laser
diode 21 is connected to a silicon submount 22 using a gold-tin
based solder material by the junction-up method, and a diode
electrode and a package electrode lead are connected to each other
using a gold wire.
[0085] The semiconductor laser diode of the present embodiment
operates stably for 5,000 hours in a nitrogen atmosphere of
60.degree. C. under a constant driving condition of 10 mW of light
power, and an estimated life of the diode is 200,000 hours.
[0086] <Fourth Embodiment>
[0087] The structure of the present embodiment is the same as that
of the first embodiment, except for changing a part of materials of
the p-side electrode.
[0088] A perspective view of the structure according to the present
embodiment is the same as that shown in FIG. 1. A crystal structure
and a method of forming a ridge of the present embodiment are the
same as those of the first embodiment. More specifically, regions
other than a surface of a p-type InGaAs contact layer 6 are covered
with a silicon oxide film which is formed by the thermal CVD. On
the silicon oxide film, an ohmic electrode, i.e., a first conductor
layer region 9-1 of a p-side electrode 8 is formed by depositing a
titanium layer having a thickness of 100 nm, a nickel layer having
a thickness of 300 nm and a gold layer having a thickness of 50 nm
in this order by vapor deposition. Then, the first conductor layer
region 9-1 is processed so as to leave an upper face of the ridge
and a bonding pad 11.
[0089] Regions other than the first conductor layer region 9-1 and
a portion of 10 .mu.m from an edge of the ridge upper face are
protected by photoresist. On the first conductor layer region 9-1,
a second conductor layer region 9-2 is formed by depositing a
titanium layer having a thickness of 100 nm and a gold layer having
a thickness of 500 nm in this order by vapor deposition. Then, the
second conductor layer region 9-2 is processed using the
photoresist by the lift-off method. Thus, the second conductor
layer which is recessed inward from the reflection facet by 10
.mu.m is formed.
[0090] After thinning a bottom face of an n-type InP substrate 1 by
grinding to achieve a thickness of 120 .mu.m, an n-side electrode
10 is formed by depositing gold-germanium, nickel and gold on the
bottom face by vapor deposition. The thus obtained semiconductor
laser wafer is cleaved to form a pair of laser cavity facets 13 and
14. An insulator film is formed on each of the cavity facets for
the purpose of protection thereof and adjustment of reflectance.
The insulator film is not shown in FIG. 1.
[0091] As shown in FIG. 8, a semiconductor laser diode 21 is
connected, by the junction-down method, to a silicon submount 22 on
which a gold-tin based solder material is patterned in the same
manner as that of the p-side electrode 8 of the semiconductor laser
diode, and a diode electrode and a package electrode lead are
connected to each other using a gold wire (not shown).
[0092] The semiconductor laser diode of the present invention
operates stably for 5,000 hours in a nitrogen atmosphere of
60.degree. C. under a constant driving condition of 10 mW of light
power, and an estimated life thereof is 100,000 hours.
[0093] <Fifth Embodiment>
[0094] In the present embodiment, a p-side electrode 8 itself,
i.e., both of a first and second conductor layer regions of the
p-side electrode 8 are recessed inward from an edge of the emission
region.
[0095] FIG. 10 is a perspective view showing a semiconductor laser
diode according to the present embodiment. In the same manner as in
the foregoing embodiments, an n-type InAlAs cladding layer 2, an
InGaAlAs SCH (Separate Confinement Heterostructure) layer (not
shown), an InGaAlAs active region 3, an InGaAlAs SCH layer (not
shown), a p-type InAlAs first cladding layer 4, a p-type InP second
cladding layer 5 and a p-type InGaAs contact layer 6 are formed on
an n-type InP substrate 1 in this order by the metal organic vapor
phase epitaxy. The p-type InP second cladding layer 5 and the
p-type InGaAs contact layer 6 are processed so as to form a ridge
waveguide by the photolithography.
[0096] Regions other than a surface of the p-type InGaAs contact
layer 6 and a portion of 7 .mu.m from a reflection facet on the
p-type InGaAs contact layer 6 are covered with a silicon oxide film
7 which is formed by the plasma CVD. On the silicon oxide film 7, a
first conductor layer region 9-1 and a second conductor layer
region 9-2 are formed continuously as the p-side electrode 8 by
depositing a titanium layer having a thickness of 200 nm, a
platinum layer having a thickness of 50 nm and a gold layer having
a thickness of 600 nm in this order by vapor deposition. The first
and second conductor layer regions except for a portion of 5 .mu.m
from the reflection facet are processed so as to leave a ridge
upper face and a bonding pad 11. More preferably, a length of the
silicon oxide film 7 covering the vicinity of the facet on the
p-type InGaAs contact layer may be from 2.5 .mu.m to 9.5 .mu.m, and
a recession thereof from the facet of the p-side electrode 8 may be
from 0.1 .mu.m to 7.5 .mu.m.
[0097] After thinning a bottom face of an n-type InP substrate 1 by
grinding to achieve a thickness of 120 .mu.m, an n-side electrode
10 is formed by depositing gold-germanium, nickel, titanium,
platinum and gold on the bottom face by vapor deposition. The thus
obtained semiconductor laser wafer is cleaved to form a pair of
laser cavity facets 13 and 14. An insulator film is formed on each
of the cavity facets for the purpose of protection thereof and
adjustment of reflectance. The insulator film is not shown in FIG.
10.
[0098] As shown in FIG. 6, a semiconductor laser diode 21 is
connected, by the junction-up method, to a silicon submount 22
using a gold-tin based solder material, and a diode electrode and a
package electrode lead are connected using a gold wire (not
shown).
[0099] The semiconductor laser diode of the present embodiment
operates stably for 5,000 hours in a nitrogen atmosphere of
85.degree. C. under a constant driving condition of 10 mW of light
power, and it is estimated that a life thereof is 100,000 hours by
extrapolation.
[0100] <Sixth Embodiment>
[0101] A tungsten silicide layer may be used for an electrode at
the side of a semiconductor layered body of any one of the laser
diodes of the present invention. One embodiment thereof will be
briefly described below. A basic structure of the present
embodiment is the same as that of the first embodiment, for
example. Descriptions of a crystal structure and a method of
forming a ridge shape are omitted since they are the same as those
of the foregoing embodiments, and only a method of fabricating the
electrode will be described.
[0102] In this embodiment, the electrode on the semiconductor
layered body side is formed by depositing a tungsten silicide
layer. A contact resistance of the tungsten silicide layer is
easily maintained at a constant value, and a degree of mutual
diffusion of the tungsten silicide layer with a compound
semiconductor material used for the base does not cause troubles in
maintaining the characteristics.
[0103] The process of fabricating the p-side electrode will be
described with reference to FIG. 1. A tungsten silicide layer
having a thickness of 300 nm is formed by the argon ion sputtering.
Then, the tungsten silicide layer is processed so as to leave an
upper face of a ridge and a bonding pad by the reactive ion
etching, to thereby achieve a desired shape. A portion having a
width of 5 .mu.m from a facet at the side of a reflection facet of
a cavity of the ridge upper face on the tungsten silicide layer is
reduced in thickness to achieve a thickness of 50 nm. In turn,
after thinning a backface of an n-type InP substrate 1 by grinding
to achieve a thickness of 120 .mu.m, an n-side electrode 10 is
formed by depositing gold-germanium, nickel and gold in this order
on the backface by vapor deposition. Other processes are the same
as those of the first embodiment. In addition, the cavity facets
may be formed by the reactive ion etching. As shown in FIG. 6, a
diode is connected to a silicon submount 22 by the junction-up
method. In this embodiment, too, the semiconductor laser diode
operates stably for 5,000 hours in a nitrogen atmosphere of
80.degree. C. under a constant driving condition of 10 mW of light
power, and it is estimated that a life thereof is 100,000 hours by
extrapolation.
[0104] <Seventh Embodiment>
[0105] An optical transmission module according to the present
embodiment will be described below.
[0106] FIG. 11 is a perspective view showing an optical
transmission module using a semiconductor laser diode according to
the embodiments of the present invention. The semiconductor laser
diode of the present invention is mounted on a silicon substrate in
such a manner as to achieve optical coupling with an optical fiber
31 together with a photo diode for monitoring and a thermistor, and
is sealed in a ceramic package 32. The semiconductor laser diode
used in the present embodiment may be any one of the diodes
according to the first, second, third, fourth, fifth and sixth
embodiments.
[0107] FIG. 12 is a perspective view showing an example of the
module, i.e., the interior of the ceramic package 32 shown in FIG.
11. In FIG. 12, a semiconductor diode portion is shown while
omitting a lens system and wiring. A submount 61 is mounted on a
heat sink 62 in the ceramic package 32, and a semiconductor laser
diode 64 is mounted on the submount 61. One electrode on the diode
is connected to a lead 68 via a pad 65. The other electrode is also
connected to a lead 67 via a pad 63. In this example, a photo diode
66 and a thermistor 71 are also sealed in the interior of the
ceramic package 32. The semiconductor laser diode 64 is
electrically connected with the pad 65 and the heat sink 62 by
wires 69 and 70.
[0108] The semiconductor laser diode of the present invention can
be used without a thermoelectric cooler even in the case of
operation at a high temperature. When the optical transmission
module according to the present embodiment is directly modulated by
external driving current pulse signals of 2.5 Gb/s at 25.degree. C.
and 60.degree. C., it is confirmed that optical pulse waveforms are
uniform since a clear eye opening is obtained in the eye pattern,
which is a pattern obtained by overlapping optical output pulse
signals respectively generated with respect to current pulse
signals.
[0109] In another embodiment of the optical transmission module, an
integrated circuit element for driving is sealed in the same
package together with a photo diode for monitoring and a
thermistor. More specifically, the semiconductor laser diode of the
present invention is mounted on a silicon substrate in such a
manner as to achieve optical coupling with an optical fiber 31
together with a light reception element for monitoring, a
thermistor and an integrated circuit element for driving, and is
sealed in a ceramic package 32. The semiconductor laser diode used
in the present embodiment may be any one of the diodes according to
the first, second, third, fourth, fifth and sixth embodiments.
[0110] The optical transmission module according to the present
embodiment achieves clear eye openings of operational waveforms of
10 Gb/s at 25.degree. C. and 85.degree. C.
[0111] The embodiments of the present invention are described
hereinbefore. In the ridge-waveguide semiconductor laser diode, an
electrode in the vicinity of a reflection facet of the
semiconductor laser diode is removed or reduced in its thickness
while leaving an electrode required for conduction. Therefore, a
tensile strain towards a diode facet due to the electrode is
reduced. Owing to the reduction in the tensile strain, bending of a
neck portion of the ridge and a generation of dislocation which is
one of causes for the degradation are prevented. Since the current
density in the vicinity of the reflection facet of the diode is
smaller than that of the central area of the diode, carriers on the
diode edge portion are reduced in number, to thereby reduce
nonradiative recombination. Therefore, generation of heat and light
absorption are prevented. The deterioration in emission
characteristics of the semiconductor laser diode is prevented owing
to the reduction in the current densities on the diode facets and
the reduction in the electrode stress.
[0112] [Comparison with Known Techniques]
[0113] Comparisons between the above-mentioned structures wherein
an electrode metal layer facet is recessed inward in the vicinity
of the reflection facet and the structure of the present invention
will be described. The known techniques are different from the
present invention in type of problem to be solved or the structure
of the semiconductor laser diode.
[0114] The primary object of the present invention is to reduce
strain caused by an electrode stress towards a reflection facet of
a ridge-waveguide laser diode, to thereby enhance reliability of
the diode. Such object is to solve a problem specific to the
ridge-waveguide semiconductor laser diode. The object and effect of
the present invention are different from those of the known
techniques.
[0115] For example, the invention disclosed in Japanese Patent
Laid-open No. 2000-277846 is limited to a semiconductor laser diode
using a nitride semiconductor material, and a substrate used for
such semiconductor laser diode does not have the cleavage
properties. Therefore, the object of the invention is to prevent
peeling of an electrode which is caused by impact accompanying a
cleavage in the fabrication of cavity facets and sagging of a main
p-side electrode. This invention discloses nothing but the above
effect. Japanese Patent Laid-open No. 11-340573 relates to a
gallium nitride-based semiconductor laser diode, and Japanese
Patent Laid-open No. 10-27939 relates to a nitride semiconductor
laser diode. The object mentioned in each of the above publications
is the prevention of the peeling of electrode, which is the same as
that of the foregoing publication, and does not suggest the effect
of the present invention.
[0116] Further, Japanese Patent Laid-open No. 3-206678 discloses a
conventional buried heterostructure semiconductor laser structure.
For comparison, a ridge-waveguide semiconductor diode of the
present invention as viewed from a cavity facet is shown in FIG.
15, and a buried heterostructure semiconductor laser diode as
viewed from a cavity facet is shown in FIG. 16. A width A of an
active region 3 of FIG. 15 is a width sufficient for confining
light and carrier, which is, for example, from about 1 .mu.m to
about 2 .mu.m. A height D of the ridge is from about 1 .mu.m to
about 2 .mu.m, and a thickness of a p-side cladding layer 4 is 1
.mu.m or less, for example. The width and the height of the ridge
are almost identical to each other, and a distance from an
electrode to an emission region on the active region is 1 .mu.m or
less and, therefore, it is necessary to allow for stress which is
caused by the electrode on upper and side faces of the ridge to
affect the active region. In turn, a width A of a lasing region 43
of FIG. 16 is sufficient for confining light and carrier, which is,
for example, from about 1 .mu.m to about 2 .mu.m. A width B of a
buried mesa channel 51 for confining the carriers in the lasing
region is about 7 .mu.m, which is disclosed in "ELECTRONICS LETTER,
Vol. 18, No. 22, P. 953 (1982)", for example. Also, as disclosed in
the abovementioned Japanese Patent Laid-open No. 3-206678, a width
C of a mesa top which is sandwiched between mesa channels 50 is
about 30 .mu.m at the maximum, and a depth D of each of the mesa
channels 50 is about 9 .mu.m. Therefore, a width of a mesa
structure itself is about three times that of its height to be
mechanically stable as compared with the ridge-waveguide, and,
since a distance from an electrode to an emission region on an
active region is about 7 .mu.m and the distance is the same as the
width B, it is unnecessary to allow for stress which is caused by
the electrode on an upper and side faces of the mesa structure to
affect the active region. Since the distance from the electrode to
the emission region on the active region is relatively long, the
effects of the stress can substantially be ignored, and, thus,
problems of the buried heterostructure semiconductor laser diode
are different from those of the ridge-waveguide semiconductor laser
diode in principle. FIG. 17 is a sectional view showing a section
parallel with a waveguide of the conventional buried
heterostructure semiconductor laser diode wherein an electrode
metal is recessed inward in the vicinity of a reflection facet, and
FIG. 18 is a sectional view showing a section parallel with a
waveguide of the semiconductor laser diode according to the fifth
embodiment of the present invention. A current flows during
operation in the manner indicated by arrows in each of the
drawings. The broken line arrow indicates that the current is lower
in the current density than that of the continuous line arrows. In
the structure of FIG. 17, the current flows downwards in the
vicinity of a cavity facet 13, and the same applies to the central
area of the cavity. In turn, in the structure of FIG. 18, the
current flows from a contact portion of an electrode and a
semiconductor in a vertical direction due to an insulator film 7 on
a cavity facet 13, and the current density is reduced as
approaching to the cavity facet 13. In the structure of FIG. 17, a
reduction in the current density on the cavity facet is controlled
by an electrode thickness achieved by a first conductor layer 9-1
and an electrode facet position set by a second conductor layer
9-2. In the structure of FIG. 18, since it is possible to control
the reduction in the current density in the vicinity of the cavity
facet 13 by changing an area of the insulator film 7, the saturable
absorption does not occur and conditions for suppressing
temperature rise on an active region is readily set, to thereby
obtain a highly reliable semiconductor laser.
[0117] Note:
[0118] 1. A semiconductor laser diode comprising:
[0119] a semiconductor substrate;
[0120] a semiconductor layered body which is formed on the
semiconductor substrate and has at least an active region;
[0121] a first electrode provided on the semiconductor substrate at
a side opposite to a side on which the semiconductor layered body
is formed; and
[0122] a second electrode provided at the side of the semiconductor
layered body;
[0123] wherein:
[0124] the semiconductor layered body has a semiconductor layered
portion on an upper region with respect to the active region
thereof, the semiconductor layered portion being in the shape of a
projection having its length in a light-propagating direction;
[0125] the second electrode on the semiconductor layered body side
contacts an upper face of the projected semiconductor layered
portion; and
[0126] the second electrode on the semiconductor layered body side
includes a facet position of the conductor layer or an end position
of the partial region of the electrode which is thicker at a
position inside at least one of reflection facets constituting a
cavity of the semiconductor laser.
[0127] 2. The semiconductor laser diode according to the above,
wherein the second electrode on the semiconductor layered body side
is a tungsten silicide layer.
[0128] Reference Numerals
[0129] 1: n-type InP substrate, 2: n-type InAlAS cladding layer, 3:
InGaAlAs active region, 4: p-type InAlAs cladding layer, 5; p-type
InP cladding layer, 6: p-type InGaAs contact layer, 7: silicon
oxide film, 8: p-side electrode, 9-1: first conductor layer of
p-side electrode, 9-2: second conductor layer of p-side electrode,
10: n-side electrode, 11: boding pad, 12: marker for positioning,
13: cavity facet, 14: cavity facet, 21: semiconductor laser diode,
22: silicon submount, 31: optical fiber, 32: ceramic package, 41:
n-type InP cladding layer, 42: InGaAsP active layer, 43: lasing
region, 44: p-type InP cladding layer, 45: p-type InP buried layer,
46: i-type InP buried layer, 47: n-type InP buried layer, 48:
p-type InP buried layer, 49: p-type InGaAsP buried layer, 50: mesa
channel, 51: buried mesa channel, 61: submount, 62: heat sink, 63:
pad, 64: laser diode, 65: pad, 66: photo diode, 67: lead, 68: lead,
69: wire, 70: wire, 71: thermistor.
* * * * *