U.S. patent application number 10/652342 was filed with the patent office on 2004-03-04 for semiconductor light-emitting device.
This patent application is currently assigned to Mitsubishi Chemical Corporation. Invention is credited to Fujii, Katsushi, Goto, Hideki, Nagao, Satoru, Shimoyama, Kenji.
Application Number | 20040041162 10/652342 |
Document ID | / |
Family ID | 13626650 |
Filed Date | 2004-03-04 |
United States Patent
Application |
20040041162 |
Kind Code |
A1 |
Shimoyama, Kenji ; et
al. |
March 4, 2004 |
Semiconductor light-emitting device
Abstract
A semiconductor light-emitting device comprising a substrate
having a surface having an off-angle to a crystallographic plane of
low-degree surface orientation, the substrate having thereon:
compound semiconductor layers including an active layer; a
selective growth protective film formed on the compound
semiconductor layers and having an opening at the region
corresponding to a stripe region to which a current is injected;
and a ridge-shaped compound semiconductor layer fomred to cover the
opening. This semiconductor light-emitting device with stable laser
property can be manufactured in a simplified way.
Inventors: |
Shimoyama, Kenji;
(Ushiku-City, JP) ; Nagao, Satoru; (Ushiku-City,
JP) ; Fujii, Katsushi; (Ushiku-City, JP) ;
Goto, Hideki; (Ushiku-City, JP) |
Correspondence
Address: |
ARMSTRONG, KRATZ, QUINTOS, HANSON & BROOKS, LLP
1725 K STREET, NW
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
Mitsubishi Chemical
Corporation
Tokyo
JP
|
Family ID: |
13626650 |
Appl. No.: |
10/652342 |
Filed: |
September 2, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10652342 |
Sep 2, 2003 |
|
|
|
09274767 |
Mar 24, 1999 |
|
|
|
6639926 |
|
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Current U.S.
Class: |
257/91 |
Current CPC
Class: |
B82Y 20/00 20130101;
H01S 5/0658 20130101; H01S 5/34326 20130101; H01S 5/3206 20130101;
H01S 5/3202 20130101; H01S 5/2272 20130101; H01S 5/2231
20130101 |
Class at
Publication: |
257/091 |
International
Class: |
H01L 027/15 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 1998 |
JP |
77181/1998 |
Claims
What is claimed is:
1. A semiconductor light-emitting device comprising a substrate
having a surface having an off-angle to a crystallographic plane of
low-degree surface orientation, the substrate having thereon:
compound semiconductor layers including an active layer; a
selective growth protective film formed on the compound
semiconductor layers and having an opening at the region
corresponding to a stripe region to which a current is injected;
and a ridge-shaped compound semiconductor layer fomred to cover the
opening.
2. A semiconductor light-emitting device comprising a substrate
having a surface having an off-angle to a crystallographic plane of
low-degree surface orientation, the substrate having thereon:
compound semiconductor layers including an active layer; a
protective film fomred on the compound semiconductor layers and
having an opening at the region corresponding to a stripe region to
which a current is injected; and a ridge-shaped compound
semiconductor layer fomred to cover the opening, wherein at least a
portion of a side wall of the ridge-shaped compound semiconductor
layer has a forward mesa shape.
3. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein the compound semiconductor layers including an
active layer further include a first conductivity type cladding
layer and a second conductivity type first cladding layer.
4. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein ridge-shaped compound semiconductor layer includes
a second conductivity type second cladding layer.
5. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein no protective film is formed either on a top
portion or side surfaces of the ridge-shaped compound semiconductor
layer.
6. The semiconductor light-emitting device according to claim 5,
wherein further comprising a contact layer formed as to cover the
entire surface of the portion and the side surfaces of the
ridge-shaped compound semiconductor layer.
7. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein the ridge-shaped compound semiconductor layer is
formed as to cover a portion of the surface of the protective
film.
8. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein the crystallographic plane of the low-degree
surface orientation of the substrate is a (100) plane or a plane
crystallographically equivalent to a (100) plane.
9. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein the off-angle is 30 degrees or less.
10. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein a direction of the off-angle is within
.+-.30.degree. from a direction perpendicular to a longitudinal
direction of the stripe region.
11. The semiconductor light-emitting device according to claim 10,
wherein a longitudinal direction of the stripe region is within
.+-.30.degree. from a [0-11] direction or a direction
crystallographically equivalent to a [0-11] direction, and the
off-angle direction is within .+-.30.degree. from a [0-11]
direction or a direction crystallographically equivalent to a
[0-11] direction.
12. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein the active layer is an AlGaInP layer or a GaInP
layer.
13. The semiconductor light-emitting device according to claim 1 or
claim 2, wherein the substrate is made of a zinc-blende type
crystal.
14. The semiconductor light-emitting device according to claim 11,
wherein the substrate is made of GaAs.
15. The semiconductor light-emitting device according to claim 1 or
claim 2, further comprising an oxidation suppressive layer provided
between the protective film and the compound semiconductor layers
including an active layer so that the oxidation suppressive layer
covers the semiconductor layers including an active layer at the
opening of the protective film.
16. A method of manufacturing semiconductor light-emitting device
comprising the steps of: growing a compound semiconductor epitaxial
layer including an active layer on a substrate having a surface
having an off-angle to a crystallographic plane of low-degree
surface orientation; forming a protective film having an opening on
a surface of the compound semiconductor epitaxial layer; and
selectively growing a ridge-shaped compound semiconductor epitaxial
layer to cover the opening.
17. The method of manufacturing semiconductor light-emitting device
according to claim 16, wherein the compound semiconductor epitaxial
layers including an active layer further include a first
conductivity type cladding layer and a second conductivity type
first cladding layer.
18. The method of manufacturing semiconductor light-emitting device
according to claim 16, wherein the ridge-shaped compound
semiconductor epitaxial layer includes a second conductivity type
second cladding layer.
19. The method for manufacturing semiconductor light-emitting
device according to claim 18, wherein the second conductivity type
second cladding layer is grown as to cover a portion of a surface
of the protective film.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a semiconductor light-emitting
device and, more particularly, to a semiconductor light-emitting
device having a ridge waveguide type stripe structure, which is
suitable for a semiconductor laser device, and a manufacturing
method for this semiconductor light-emitting device.
DESCRIPTION OF RELATED ART
[0002] A structure so-called as a ridge waveguide type is
frequently used to produce semiconductor light-emitting devices
without difficulties. FIG. 2 shows a manufacturing method for such
a structure. First, a first conductivity type cladding layer 202,
an active layer 203, a second conductivity type cladding layer 204,
and a second conductivity type contact layer 205 are grown on a
substrate 201. A photoresist 211 having stripe openings as a
pattern made by photolithography is formed on a wafer surface to
form a stripe-shaped ridge 209 by a wet etching process using the
photoresist 211 as a mask so that the second conductivity type
cladding layer remains with a prescribed thickness. An insulating
protective film 206 such as SiNx is subsequently formed on the
whole surface on an epitaxial side, and only the protective film at
the top of the ridge is removed by etching using a photoresist
having stripe openings as a pattern made by photolithography. This
structure prevents a current from flowing through portions other
than the top of the ridge. Another layer of protective film may
further be formed on the ridge side surface. Then, an epitaxial
side electrode 207 and a substrate side electrode 208 are
formed.
[0003] According to this structure, currents are injected into the
active layer 203 after injected through the ridge portion 209 of
the cladding layer. Currents are thus concentrated into the active
layer region under the ridge portion 209, thereby generating light
having a wavelength corresponding to the band gap of the active
layer. At that time, the band gap of the active layer is ordinarily
smaller than those of the upper and lower cladding layers, and the
refractive index of the active layer is larger than those of the
upper and lower cladding layers, so that carriers and light can be
confined effectively in the active layer. Because the protective
film 206 having a smaller refractive index than the semiconductor
portions is formed at a non-ridge portion 210, the effective
refractive index of the active layer region under the non-ridge
portion 210 becomes smaller than that of the ridge portion 209.
Consequently, the generated light is confined in the active layer
region under the ridge portion 209. This structure thus can
stabilize the transverse mode for laser oscillation and can reduce
the threshold currents.
[0004] With such a conventional manufacturing method for ridge
waveguide type semiconductor light-emitting device, the ridge
portion is formed by the etching process, so that it is difficult
to accurately control the thickness of the cladding layer at the
non-ridge portion 210. As a result, the effective refractive index
at that portion largely varies due to slight differences of the
thickness of the cladding layer at the non-ridge portion, thereby
deviating the laser characteristics of the semiconductor
light-emitting device, and rendering product yields hardly improve.
Where a laser device of a single transverse mode is produced, a
very highly accurate alignment technique is required, because it is
difficult to use process simplifying techniques such as a
self-alignment in the conventional manufacturing method, though the
top width of the ridge portion is at most about several microns.
Such a complicated, fine photolithographic technology makes device
production steps complicated and device production yields reduced.
If a SiNx film is formed on the ridge side wall, a deletion layer
of about 0.1 micron may be formed on the surface side of the ridge
side wall to narrow the effective current channel width, thereby
raising a problem that the pass resistance becomes larger.
[0005] Meanwhile, as a light source for information processing to
improve the recording density, visible laser devices (ordinarily
630 to 690 nm) using AlGaInP basis in lieu of conventional AlGaAs
basis (wavelength about 780 nm) are put to practical use, but the
following researches have been made to realize shorter wavelength,
lower threshold, and high temperature operation.
[0006] In a production of an AlGaInP/GaInP based visible laser
device, use of a substrate having an off-angle from the (100) plane
toward the [011] direction (or [0-1-1] direction) allows the band
gap from narrowing due to formation (ordering) of natural super
lattices, thereby rendering the wavelength shorter readily,
facilitating high concentration doping of p-type dopants (e.g., Zn,
Be, and Mg), and improving the oscillation threshold current of the
device by enhancement of the hetero-barrier and temperature
characteristics. If the off-angle is too small, step bunching
appears outstandingly, and large undulations are formed at the
hetero-boundaries, so that a shift amount in which the PL
wavelength (or oscillation wavelength) is shortened by quantum
effects to the bulk active layer may be smaller than the designed
amount where a quantum well structure (GaInP well layer of about 10
nm or less) is manufactured. If the off-angle is made lager, the
step bunching is reduced, and the hetero-boundaries become flat,
thereby making the wavelength shorter by the quantum effect as
designed. Thus, a substrate having an off-angle of 6 to 16 degrees
from the (100) plane toward the [011] direction (or [0-1-1]
direction) is generally used to suppress formation of natural super
lattices and generation of step bunching, which impede the
wavelength from becoming shorter, as well as to suppress the
oscillation threshold current from increasing due to shortened
wavelength from p-type high concentration doping and impairment of
temperature characteristics. A proper off-angel should be selected
in consideration of thickness of the GaInP well layer and the
stress amount depending on the targeted wavelength such as 650 nm
or 635 nm.
[0007] When natural super lattice is formed in the active layer, it
is deformed to be mixed crystal during current injection whereby
problems may be raised such that oscillation wavelength or emission
wavelength is changed and devise properties are impaired. Natural
super lattice is easily formed in an active layer made of materials
including In and Ga as constituent elements such as GaInAs,
AlGaInAs, InGaAsP as well as the above-mentioned GaInP and AlGaInP.
Use of an off-angle substrate suppresses formation of natural super
lattice and effectively solves the problems.
[0008] To reduce waveguide loss and mirror loss, a resonator is
formed in extending in a striped shape as much as vertical to the
off-angled direction of the substrate. FIGS. 3(a) and 3(b) show
cross sections of conventional ridge and groove type inner stripe
structures made of a semiconductor using a current block layer. In
FIGS. 3(a), 3(b), numeral 301 is a substrate; numeral 302 is a
first conductivity type cladding layer; numeral 303 is an active
layer; numeral 304 is a second conductivity type cladding layer;
numeral 305 is a first conductivity type current block layer;
numeral 306 is a second conductivity type contact layer; numeral
307 is an epitaxial side electrode; numeral 308 is a substrate side
electrode; numeral 311 is a substrate; numeral 312 is a first
conductivity type cladding layer; numeral 313 is an active layer;
numeral 314 is a second conductivity type first cladding layer;
numeral 315 is a first conductivity type current block layer;
numeral 316 is a second conductivity type second cladding layer;
numeral 317 is a second conductivity type contact layer; numeral
318 is an epitaxial side electrode; and numeral 319 is a substrate
side electrode. In this situation, because the shape of the ridge
or groove may become horizontally asymmetric or the optical density
profile may become horizontally asymmetric due to the off-angle of
the substrate, problems may be raised such that a stable
fundamental transverse mode required for a laser diode for
information processing such as for optical discs may not be easily
obtained, that kink level may be lowered, and that the horizontal
asymmetry of the beam profile may increase. Particularly, in the
case of real refractive index guide in which ends of the optical
profile come out to the block layer, this problem may become
apparent.
[0009] In consideration of those problems in the conventional art,
it is an object of the invention to provide a semiconductor
light-emitting device having a ridge waveguide type stripe
structure which can be manufactured in a simple way with stable
laser property. It is also another object of the invention to
provide a semiconductor light-emitting device having a stable
fundamental transverse mode at a high power operation stage where
the horizontal symmetry of the ridge shape of the ridge waveguide
type laser is almost not affected by the horizontal asymmetry of
the optical intensity profile even where a substrate having a large
off-angle for shortening the wavelength as for the AlGaInP/GaInP
based visible laser diode is used. It is yet another object of the
invention to provide a method for manufacturing semiconductor
light-emitting device with good production yield in a simplified
step for producing such a device without requiring any complicated,
fine photolithographic technology.
SUMMARY OF THE INVENTION
[0010] The inventors have discovered, upon extensive researches to
accomplish the above objects, that covering both sides of a stripe
region with a protective film makes a complicated, fine
photolithographic technology unnecessary, simplifies a
manufacturing process for the device, and greatly improves
production yield of the device. The inventors also found that the
semiconductor light-emitting device having such a structure can be
manufactured easily by a selective growth using the protective
film, and reached the invention upon finding that a semiconductor
light-emitting device can obtain a stable fundamental transverse
mode at a high power operation stage where the horizontal symmetry
of the ridge shape of the ridge waveguide type laser is almost not
affected by the horizontal asymmetry of the optical intensity
profile even where a substrate having a large off-angle for
shortening the wavelength as for the AlGaInP/GaInP based visible
laser diode is used.
[0011] That is, this invention is to provide a semiconductor
light-emitting device comprising a substrate having a surface
having an off-angle to a crystallographic plane of low-degree
surface orientation, the substrate having thereon: compound
semiconductor layers including an active layer; a selective growth
protective film formed on the compound semiconductor layers and
having an opening at the region corresponding to a stripe region to
which a current is injected; and a ridge-shaped compound
semiconductor layer fomred to cover the opening.
[0012] In another aspect of the invention, a semiconductor
light-emitting device comprises a substrate having a surface having
an off-angle to a crystallographic plane of low-degree surface
orientation, the substrate having thereon: compound semiconductor
layers including an active layer; a protective film fomred on the
compound semiconductor layers and having an opening at the region
corresponding to a stripe region to which a current is injected;
and a ridge-shaped compound semiconductor layer fomred to cover the
opening, wherein at least a portion of a side wall of the
ridge-shaped compound semiconductor layer has a forward mesa
shape.
[0013] In the semiconductor light-emitting device according to the
invention, preferably, the compound semiconductor layers includes
an active layer further include a first conductivity type cladding
layer and a second conductivity type first cladding layer, and
ridge-shaped compound semiconductor layer includes a second
conductivity type second cladding layer. No protective film is
preferably formed either on a top portion or side surfaces of the
ridge-shaped compound semiconductor layer, and a contact layer may
be formed as to cover the entire surface of the top portion and the
side surfaces of the ridge-shaped compound semiconductor layer. The
ridge-shaped compound semiconductor layer is preferably formed as
to cover a portion of the surface of the protective film. The
crystallographic plane of the low-degree surface orientation of the
substrate may be a (100) plane or a plane crystallographically
equivalent to a (100) plane; the off-angle may be 30 degrees or
less; and a direction of the off-angle is preferably within .+-.30
.degree. from a direction perpendicular to a longitudinal direction
of the stripe region. A longitudinal direction of the stripe region
may be within .+-.30.degree. from a [0-11] direction or a direction
crystallographically equivalent to a [0-11] direction, and the
off-angle direction may be preferably within .+-.30 .degree. from a
[0-11] direction or a direction crystallographically equivalent to
a [0-11] direction. The active layer is preferably, an AlGaInP
layer or a GaInP layer, and the substrate is preferably made of a
zinc-blende type crystal such as GaAs. An oxidation suppressing
layer may be preferably provided between the protective film and
the compound semiconductor layers including an active layer so that
the oxidation suppressive layer covers the semiconductor layers
including an active layer at the opening of the protective
layer.
[0014] In yet another aspect of the invention, a method of
manufacturing semiconductor light-emitting device comprises the
steps of: growing a compound semiconductor epitaxial layer
including an active layer on a substrate having a surface having an
off-angle to a crystallographic plane of low-degree surface
orientation; forming a protective film having an opening on a
surface of the compound semiconductor epitaxial layer; and
selectively growing a ridge-shaped compound semiconductor epitaxial
layer to cover the opening.
[0015] In the method of manufacturing semiconductor light-emitting
device according to the invention, preferably, the compound
semiconductor layers includes an active layer further include a
first conductivity type cladding layer and a second conductivity
type first cladding layer, and ridge-shaped compound semiconductor
layer includes a second conductivity type second cladding layer.
The second conductivity type second cladding layer is preferably
grown as to cover a portion of a surface of the protective
film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1(a) to FIG. 1(c) are cross-sectional views
illustrating a semiconductor light-emitting device according to the
invention and a manufacturing method for this semiconductor
light-emitting device.
[0017] FIG. 2(a) to FIG. 2(c) are cross-sectional views
illustrating a conventional semiconductor light-emitting device in
which a ridge portion is formed by etching process and a
manufacturing method for this semiconductor light-emitting
device.
[0018] FIG. 3(a) and FIG. 3(b) are cross-sectional views
illustrating a conventional semiconductor light-emitting device
having a ridge type or groove type inner stripe structure in which
a current block layer made of semiconductor is used, and a
manufacturing method for this semiconductor light-emitting
device.
DETAILED DESCRIPTION OF THE INVENTION
[0019] Hereinafter, in regard with the semiconductor light-emitting
device of the invention, details of respective layers and an
example of a manufacturing process are described specifically.
[0020] The semiconductor light-emitting device of the invention at
least includes on a substrate compound semiconductor layers
including an active layer, a protective film formed on the compound
semiconductor layers and having an opening at the region
corresponding to a stripe region to which a current is injected,
and a ridge-shaped compound semiconductor layer formed to cover the
opening.
[0021] On a substrate, a buffer layer can be formed. The first
conductivity type cladding layer may have a double layer structure
made of a first conductivity type first cladding layer and a first
conductivity type second cladding layer. The cladding layers
sandwiching the active layer has a smaller refractive index than
that of the active layer. A compound semiconductor made of those
cladding layers and active layer may include a layer functioning as
an optical guide layer.
[0022] The protective film having an opening is formed on the
second conductivity type first cladding layer. An oxidation
suppressive layer may be formed between the second conductivity
type first cladding layer and the protective film. A stripe region
is defined by the opening in the protective film, and a ridge type
compound semiconductor layer having a smaller refractive index than
that of the active layer is formed on the stripe region. A major
portion of the compound semiconductor layer is ordinarily made of a
second conductivity type second cladding layer. The compound
semiconductor layer may include a layer functioning, e.g., an
optical guide layer in addition to the second conductivity type
second cladding layer. A low resistance contact layer may
preferably cover substantially the whole surfaces of the top and
the side surfaces of the ridge.
[0023] The substrate used for the semiconductor light-emitting
device according to the invention is not specifically limited as
far as allowing a double heterostructure crystal to grow on the
substrate. What is preferable is a conductive material, and
desirably, the substrate is a crystal substrate made of, e.g.,
GaAs, InP, GaP, ZnSe, ZnO, Si, and Al.sub.2O.sub.3 suitable for
growth of a crystal thin film on the substrate, more preferably, a
crystal substrate having a zinc-blende structure. The crystal
growth surface on the substrate is a plane with a low degree
orientation or its crystallographically equivalent plane, more
preferably a (100) plane and its crystallograplically equivalent
planes such as (100) plane and (111) plane.
[0024] In this specification, "(100) plane" is not necessary to be
strictly a just (100) plane and can encompass cases that the
substrate has an off-angle of 300 at most. In regard with the scale
of the off-angle, the upper limit is preferably 30.degree. or less,
more preferably 16.degree. or less, whereas the lower limit is
preferably 0.5.degree. or greater, more preferably 2.degree. or
greater, further preferably 7.sup.0 or greater, and most preferably
10.degree. or greater. The direction of the off-angle is preferably
within .+-.30 .degree. from a direction perpendicular to a
longitudinal direction of the stripe region, more preferably,
within .+-.7.degree., and most preferably, within .+-.2.degree..
The direction of the stripe region is preferably, a [0-11]
direction or a direction crystallographically equivalent to a
[0-11] direction in the case where the crystallographical plane of
the substrate is (100), and the off-angle direction is preferably
within .+-.30.degree. from a [0-11] direction or a direction
crystallographically equivalent to a [0-11] direction. The
substrate may be a hexagonal system substrate such as wurtzite
structure substrate, and in such a case, Al.sub.2O.sub.3, 6H-SiC,
etc. can be used.
[0025] The material and structure of the cladding layer, the active
layer, and the contact layer are not specifically limited. It is
preferable to use a general group III-V or II-VI semiconductor such
as AlGaAs, AlGaInAs, AlGaInP, GaInAsP, AlGaInN, BeMgZnSe, MgznSSe,
and CdZnSeTe. As a cladding layer, a material having a smaller
refractive index than that of the active layer is selected, and as
a contact layer, a material having a narrower band gap than that of
the cladding layer is selected. As a proper carrier density of a
low resistance to gain an ohmic contact with electrodes, the lower
limit is preferably 1.times.10.sup.18 cm.sup.-3 or greater, more
preferably, 3.times.10.sup.18 cm.sup.-3 or greater, most
preferably, 5.times.10.sup.18 cm.sup.-1 or greater. The upper limit
is preferably 2.times.10.sup.20 cm.sup.-3 or less, more preferably,
5.times.10.sup.19 cm.sup.-3 or less, most preferably,
3.times.10.sup.19 cm.sup.3 or less.
[0026] The active layer is preferably made of compound
semiconductor materials including In and Ga as constituent elements
such as GaInP, AlGaInP, GaInAs, AlGaInAs, InGaAsP. An off-angle
substrate could effectively suppress formation of natural super
lattice, which easily occurs to these active layer materials.
[0027] The active layer is not limited to a single layer and can be
a single quantum well structure (SQW) composed of a quantum well
layer and optical guide layers vertically sandwiching the quantum
well layer or a multiple quantum well structure (MQW) composed of
plural quantum well layers, barrier layers sandwiched between the
quantum well layers, and optical guide layers respectively formed
on the uppermost quantum well layer and under the lowermost quantum
well layer.
[0028] With the semiconductor light-emitting device of the
invention, an oxidation suppressive layer can be formed on the
compound semiconductor layer. The oxidation suppressive layer can
easily prevent a high resistance layer, which increases passing
resistance on a re-growth boundary where the clad is formed in a
ridge shape by re-growth, from occurring.
[0029] For this purpose, the oxidation suppressive layer should be
formed between the compound semiconductor layer including an active
layer and the protective film mentioned below. As a result, the
oxidation suppressive layer covers the surface of the compound
semiconductor layer including an active layer at the opening of the
protective film.
[0030] As the oxidation suppressive layer, there is no special
limitation on selection of the material as far as it is hardly
oxidized or it is cleaned up easily. More specifically, a compound
semiconductor layer of III-V group having a low containing rate of
readily oxidized elements such as Al (about 0.3 or less) is
exemplified. It is preferable that the oxidation suppressive layer
does not absorb light from the active layer by selecting the
material or thickness of the oxidation suppressive layer. The
material of the oxidation suppressive layer can be ordinarily
selected from materials having a wider band gap than that of the
active layer, but a material, even where its band gap is narrow,
can be used where the thickness is 50 nm or less, preferably, 30 nm
or less, more preferably, 10 nm or less because light absorbing can
be substantially neglected.
[0031] The protective film used in the semiconductor light-emitting
device of the invention is also not particularly limited, but it is
necessary to perform current injections only to a region of the
active layer located below the ridge portion, which is formed as a
stripe region. That is, to confine currents by the protective film
on both sides of the stripe shaped opening, the protective film has
to be insulation. The refractive index of the protective film is
preferably smaller than that of the ridge-shaped semiconductor
layer, namely the second conductivity type second cladding layer,
to give effective refractive index difference between the ridge
portion and the non-ridge portion in a horizontal direction in the
active layer and to stabilize the transverse mode of the laser
oscillation. However, as a practical matter, if the refractive
index difference is too large between the protective film and the
cladding layer, the second conductivity type first cladding layer
below the ridge has to be thicker because the effective refractive
index step in the transverse direction tends to be larger in the
active layer, thereby increasing leak currents in the transverse
direction. To the contrary, if the refractive index difference is
too small between the protective film and the cladding layer, the
protective film has to be formed thicker to some extent since the
light easily leaks outside the protective film, but this tends to
impair the cleavage property. In consideration of those, together,
the lower limit of the refractive index difference between the
protective film and the cladding layer is 0.2 or greater, more
preferably, 0.3 or greater, and most preferably, 0.5 or greater.
The upper limit is 3.0 or less, more preferably, 2.5 or less, and
most preferably, 1.8 or less. There would be no problem, in regard
with the thickness of the protective film, as far as the protective
film can show a sufficient insulation property and has a thickness
such that light does not come outside the protective film. The
lower limit of the protective film is preferably 10 nm or greater,
more preferably, 30 nm or greater, and most preferably, 50 nm or
greater. The upper limit is preferably 500 nm or less, more
preferably, 300 nm or less, and most preferably, 200 nm or
less.
[0032] The protective film is preferably a dielectric, and more
specifically, can be selected preferably from a group of SiN.sub.x
film, SiO.sub.2 film, SiON film, Al.sub.2O.sub.3 film, ZnO film,
SiC film, and amorphous Si film. Particularly, SiN.sub.x film,
SiO.sub.2 film, and Al.sub.sO.sub.3 film are preferable since
suitable for the selective growth at the ridge portion. The
protective film is used as a mask for formation of the ridge
portion through a re-growth using MOCVD and is also used for the
purpose of current squeezing. For simplifying the process, it is
preferable to use a film having the same composition commonly for
current squeezing and for selective growth, but layers having
different compositions may be formed as a multilayer when
necessay.
[0033] In the case where the substrate has a zinc blende structure
and the surface of the substrate is the (100) plane or a plane
which is crystallographically equivalent to the (100) plane, in
order to cause the second conductivity type second claadding layer
to be easily grown on the protective film and the contact layer
(which is described later) to be easily grown on the side surfaces
of the ridge (the second conductivity type second cladding layer),
a stripe region which is defined by an opening of the protective
film preferably elongates in the [01-1]B direction or a direction
which is crystallographically equivalent to the direction. In this
case, it is often that most of the side surfaces of the ridge are
configured by the {311}A planes such as (311)A plane and (3-1-1)A
plane, and it is possible to grow the contact layer on a
substantially whole area of the growing surface of the second
conductivity type second cladding layer which constitutes the
ridge. For the same reason, in the case where the substrate has
wurtzite sutrcture, a stripe region preferably elongates in a
[11-20] or [1-100] direction on a (0001) surface, for example. When
the epitaxial layers are grown by HVPE (Hydride Vapor Phase
Epitaxy), both [11-20] and [1-100] provide the same result. When
the epitaxial growth method is MOCVD, [11-20] is preferable. This
tendency is particularly noticeable when the second conductivity
type second cladding layer is made of AlGaAs, particularly, having
an Al composition of 0.2 to 0.9, preferably, 0.3 to 0.7. In the
specification, the term "[01-1]B direction" defines the [01-1]B
direction so that, in a usual group III-V or II-VI semiconductor,
the (11-1) plane existing between the (100) plane and the (01-1)
plane is a plane in which the element of group V or VI appears.
Furthermore, the term is not limited to a direction which is
strictly just as the [01-1]B direction and includes directions
which are deviated from the [01-1]B direction by about
.+-.10.degree.. The embodiment of the invention is not limited to
the case where the stripe region elongates in the [01-1]B
direction. Hereinafter, other embodiments are described.
[0034] In the MOCVD, for example, the growth conditions are
suitably selected, i.e., when the stripe region which is defined by
the opening of the protective film is set to elongate in the [011]A
direction, the growth rate can be provided with anisotropy. Namely,
the growth rate can be set so that the growth is rapid in the (100)
plane and is hardly conducted in the {111}B planes such as (1-11)B
plane and (11-1)B plane. When selective growth is performed in the
(100) plane of the stripe-like window under such anisotropic
conditions, a ridge in which a side face is the {111}B planes such
as (1-11)B plane and (11-1)B plane is formed. In this case, when
conditions of the MOCVD are selected so that growth of higher
isotropy occurs, the contact layer can be formed on the top of the
ridge which is the (100) plane, and also on the ridge side face
composed of the {111}B planes such as (1-11)B plane and (11-1)B
plane.
[0035] The method of producing the semiconductor light-emitting
device of the first aspect of the invention is not particularly
limited. Generally, the double heterostructure is formed on the
substrate, and the second conductivity type second cladding layer
and the contact layer which constituted a ridge shape are then
selectively grown, by using the protective film on the stripe
region into which a current is injected. At this time, in order to
enable a part of the second conductivity type second cladding layer
to be formed on the protective film, conditions are set in which
the second conductivity type second cladding layer easily grows in
a direction vertical to the direction along which the stripe region
elongates within the substrate surface plane, or, in other words, a
condition where a lateral growth suitably occurs. Specifically,
when the surface of the substrate is the (100) plane, the direction
along which the stripe region defined by the opening of the
protective film is set to be the [01-l]B direction, and the
temperature, the supply amount of the source material, and the like
are appropriately adjusted. When the second conductivity type
second cladding layer is made of a III-V compound semiconductor, a
lateral growth occurs more easily as the temperature is lower and
the V/III ratio is larger. When a composition containing Al is
used, a lateral growth occurs more easily as the Al content is
higher.
[0036] As shown in FIG. 1(b), the protective film is in contact
with the second conductivity type cladding layer only on the side
surfaces and the top surface which defines the stripe region, and
it is preferable that no protective film is formed on the top and
sides of the ridge type second conductivity type second cladding
layer. Such a structure is preferable because the device can be
manufactured easily and because the characteristics in the passing
resistance or the like are improved. The width of the stripe region
is set preferably to 2.2 to 1000 microns. With the width in this
range, the laser can realize a high power operation.
[0037] The height (thickness) of the second conductivity type
second cladding layer is preferably set to 0.25 to 2.0 times of the
stripe region width. If within this range, it is preferable because
the second conductivity type second cladding layer would not be
projected so much in comparison with the current block layer or
ridge dummy layer as described below, because the device life would
not be affected due to stresses exerted to the ridge portion when
the device is used in a manner of the junction down, and because
post processes such as a forming process for electrodes are done
easily since it is very low in comparison with its vicinity.
[0038] A contact layer is preferably formed on the top and/or sides
of the ridge of the second conductivity type second cladding layer.
A preferable configuration is to form the contact layer on the
whole surface of the top and sides of the ridge. By holding a
sufficient contact area between the contact layer and an electrode
formed on the contact layer or the second conductivity type second
cladding layer, the resistance of the entire apparatus can be
maintained at a low level. A portion of the top and sides of the
ridge on which the contact layer is formed can also be covered with
a protective film for the purpose of prevention of oxidation. In
this embodiment, the resistance of the entire apparatus can be
suppressed to be smaller than that of the apparatus in which a
protective film is formed on ridge side surfaces without forming
any contact layer. Particularly, with respect to a material having
a high specific resistance (especially, p-type) such as AlGaInP
basis or AlGaInN basis, it is effective to reduce the resistance of
the entire device.
[0039] When the semiconductor light-emitting device of the
invention is manufactured, methods conventionally used can be
selected properly. The method for growing the crystal is not
specifically limited, and for the crystal growth of the double
hetero-structure or for selective growth of the ridge portion,
known growing methods such as metal organic chemical vapor
deposition (MOCVD), molecular beam epitaxy (MBE method), hydride or
hallide vapor phase epitaxy (VPE method), and liquid phase epitaxy
(LPE method) can be used upon proper selection.
[0040] The semiconductor light-emitting device according to the
invention can be manufactured by the steps of: growing an epitaxial
layer including a first conductivity type cladding layer, an active
layer, and a second conductivity type first cladding layer on a
substrate having a major surface having an off-angle to a low
degree crystallographic plane; forming a protective film having an
opening on a surface of the epitaxial layer; and selectively
growing a second conductivity type second cladding layer at the
opening. It is preferable to form electrodes on the top and sides
of the ridge without forming any protective film on the ridge side
surface. The specific conditions for growing the respective layer
may vary depending on the layer's composition, growing method,
shape of the apparatus, etc., and in a case that a compound
semiconductor of group III-V is grown by the MOCVD, preferably, the
double hetero-structure is formed at a growing temperature of about
650 to 750.degree. C. with a V/III ratio of about 20 to 60 (in the
case of AlGaAs) or about 350 to 550 (in the case of AlGaInP),
whereas the ridge portion is formed at a growing temperature of 600
to 700.degree. C. with V/III ratio of about 40 to 60 (in the case
of AlGaAs) or about 350 to 550 (in the case of AlGaInP).
[0041] Where the ridge portion selectively grown in use of the
protective film contains, particularly, Al such as in AlGaAs and
AlGaInP, it is very preferable if a very small amount of an HCl gas
is introduced during the growth, because the gas prevents
polycrystals from depositing. However, as the Al is contained much
more in the composition, or as the ratio of the mask portion to the
opening is higher, a necessary introduction amount of HCl increases
for making a selective growth only on the opening (selective mode)
in preventing polycrystals from depositing where other growing
conditions are unchanged. On the other hand, if the HCl gas is
introduced too much, the AlGaAs layer may not be grown, and
conversely, although the semiconductor layer is etched (etching
mode), a necessary introduction amount of HCl increases for
entering to the etching mode as the Al is contained much more in
the composition, even where other growing conditions are unchanged.
The optimum introduction amount of HCl greatly depends on a
molecular number of the group III source supply including Al such
as trimethylaluminum or the like. More specifically, the ratio of
the supply molecular number of HCl to group III source supply
molecular number including Al (HCl/Group III) is equal to or
greater than 0.01 and equal to or less than 50, more preferably,
equal to or greater than 0.05 and equal to or less than 10, and
most preferably, equal to or greater than 0.1 and equal to or less
than 5. It is to be noted that the ridge composition may not be
controlled easily where a chemical compound including In in the
ridge is selectively grown (particularly, HCl introduced).
[0042] The second conductivity type second cladding layer is
preferably grown as to cover the top surface of the protective film
serving as an insulator to properly control the light profile
otherwise coming out the vicinity of the protective film and the
ridge. A contact layer may be grown on substantially the entire
surface on which crystals can be grown on the second conductivity
type second cladding layer, thereby suppressing the side of the
cladding layer from oxidizing, increasing the contact area with the
electrode on the epitaxial surface side to reduce the contact
resistance with the electrode. Growth of the cladding layer of the
re-growth portion and the contact layer as to cover the top of the
insulation film can be done individually or in a combination of
those.
[0043] Furthermore, where the ridge is formed by re-growth, a ridge
dummy layer, to which a current is not injected, having a larger
area than the ridge portion into which a current is injected can be
formed to improve the controllability of the composition of ridge
portion, the carrier concentration, and the growth rate. In this
situation, an insulation covering layer such as an oxide layer or a
thyristor structure is formed at a portion of the ridge dummy layer
to prevent the current from passing. Where the current injection
stripes are formed on the off-angled substrate in a perpendicular
direction as much as possible to the off direction, although the
ridge of the re-growth becomes horizontally asymmetric, the light
profile that comes out the vicinity of the protective film and the
ridge has a good symmetry, because the refractive index difference
between the protective film and the cladding layer of the ridge
portion is easily made larger than the conventional block layer
made of a semiconductor layer as shown in FIG. 2, and because the
cladding layer of the re-growth portion can be grown as to cover
the top surface of the protective film by selecting the stripe
direction properly, and therefore, this device can obtain a
fundamental transverse mode oscillation which is stable even at a
high output stage. Thus, this invention is applicable to various
ridge stripe type waveguide structure semiconductor light-emitting
devices.
[0044] As the optimum figure to which this invention is applied, a
protective film made of an insulator is formed on a substrate
having an off-angle to the low degree crystallographic plane; where
an oxidation suppressive layer is formed on the epitaxial surface
side of the double hetero-structure, a cladding layer in the ridge
shape and a contact layer on the cladding layer is re-grown at the
stripe region to which a current is injected as to cover the top of
the protective film made of an insulator; in this situation, a
ridge dummy layer to which no current is injected is formed having
a larger area than the ridge portion; the electrodes are formed on
the top and sides of the ridge where no protective film made of an
insulator is formed on the ridge side surface.
[0045] A laser chip cut out by cleavage from the wafer to which the
electrodes are attached is sealed in a CAN package in a nitrogen
atmosphere ordinarily together with a heat sink and a photodiode
for optical output monitoring and is assembled. Recently, the laser
chip may be assembled as an integrated optical circuit unifying
other optical parts for rendering the device smaller and further
inexpensive. This invention can serve for wide uses including
this.
[0046] In the above, light sources for information processing
(ordinarily AlGaAs basis, wavelength about 780 nm; AlGaInP basis,
wavelength about 600 nm; and InGaN basis, wavelength about 400 nm)
are described as examples of semiconductor laser devices utilizing
the present invention. It should be understood that the present
invention can be applied to a wide variety of devices such as a
light source for communication signal laser (ordinarily InGaAsP or
InGaAs based active layer, wavelength about 1.3 .mu.m, 1.5 .mu.m),
a light source for exciting fiber laser (InGaAs stressed quantum
well active layer and GaAs based substrate, wavelength about 980
nm; InGaAsP stressed quantum well active layer and InP based
substrate, wavelength about 1480 nm), particularly devices of a
high power operation. The present invention can be applied to
light-emitting diode as well as semiconductor laser.
[0047] Hereinafter, an example is described to illustrate the
invention in detail. The material, concentration, thickness,
manipulation order, and the like are properly changeable as far as
not goes beyond the spirit of the invention. Accordingly, the scope
of the invention is not limited to the details shown in the
following example.
EXAMPLE
[0048] This example is shown in FIG. 1. On a GaAs substrate 101
having a thickness of 350 microns and an off-angle of about
10.degree. to 15.degree. in a [0-1-1]A direction from (100) plane,
first, a Si doped n-type GaAs buffer layer (n=1.times.10.sup.18
cm.sup.-3), not shown, having a thickness of 0.5 micron, a Si doped
Al.sub.0.75Ga.sub.0.25As cladding layer 102 (n=1.times.10.sup.18
cm.sup.-3) having a thickness of 1.5 microns, a Si doped n-type
(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5p cladding layer 103
(n=1.times.10.sup.18 cm.sup.-3) having a thickness of 0.2 micron, a
multiple quantum well (MQW) active layer 107 made of non-doped
Ga.sub.0.44In.sub.0.56P well layer 106 (four layers) having a
thickness of 5 to 6 nm sandwiched by non-doped
(Al.sub.0.7Ga.sub.0.3).sub- .0.5In.sub.0.5P optical guide layers
104 having a thickness of 30 nm or non-doped
(Al.sub.0.7Ga.sub.0.3).sub.0.5In.sub.0.5P barrier layers 105 having
a thickness of 5 nm, a Zn doped p-type (Al.sub.0.7Ga.sub.0.3).sub.-
0.5In.sub.0.5P cladding layer 108 (p=7.times.10.sup.17 cm.sup.-3)
having a thickness of 0.2 micron, a Zn doped p-type
Ga.sub.0.5Ino.sub.0.5P oxidation suppressive layer 109
(p=1.times.10.sup.18 cm.sup.-3) having a thickness of 5 nm were
accumulated orderly to form a double hetero-structure by MOCVD
(FIG. 1(a)). At that time, the oxidation suppresive layer
preferable has a selected composition so as not to absorb light
generated by re-combinations in the active layer in order to reduce
the threshold current, but can be used as an saturable absorbing
layer upon absorbing light intentionally to do self-pulsation. It
is further effective to change the composition of the GaxP
oxidation suppresive layer with Ga rich side (X=0.5 to 1) or to add
Al in a small amount ((Al.sub.xGa.sub.1-x).sub.0.5In.sub.0.5P,
X=approximately 0.1 to 0.2) to prevent the light from being
absorbed. Subsequently, a SiN.sub.x protective film 110 as an
insulator (having a refractive index 1.9 at wavelength 650 nm) was
deposited by 200 nm on the surface of the double heteroepitaxial
substrate. Many stripe shaped windows 107 were opened in the
SiN.sub.x film by photolithography in having a width of 2.2 microns
in a [01-1] B direction, which is perpendicular to the off-angle
direction. A ridge made of a Zn doped p-type
Al.sub.0.77Ga.sub.0.23As cladding layer 112 (p=1.5.times.10.sup.18
cm.sup.-3, refractive index 3.3, wavelength 650 nm) having a
thickness of 1.2 microns and a Zn doped GaAs contact layer 113
having a thickness of 0.3 micron, was formed by selective growth
using MOCVD (FIG. 1(b)). At that time most of the side surfaces of
the ridge was frequently (311) A plane or close plane, and the
cladding layer of the re-growth portion was grown as to cover the
top surface of the protective film serving as an insulator, thereby
allowing the contact layer to grow on substantially the entire
surface on which a crystal can grow on the cladding layer of the
re-growth portion. Therefore, the device can make better the
controllability of the optical profile which comes out the vicinity
of the protective film and the ridge, can suppress the side surface
of the cladding layer from oxidizing, and can reduce the contact
resistance with the electrode by increasing the contact area in
contact with the electrode on the epitaxial surface side. This
tendency is remarkable where the re-growth ridge is AlGaAs,
particularly where the Al composition of AlGaAs alloy semiconductor
is set 0.2 to 0.9, preferably 0.3 to 0.8. In a general III-V group
chemical compound semiconductor, a [01-1]B direction is defined so
that the (11-1) plane located between the (100) plane and the
(01-1) plane is a plane where the V group element appears.
[0049] In the above MOCVD, trimethyl gallium (TMG), trimethyl
aluminum (TMA), and trimethyl indium (TMI) were used for source
materials for III group source, and arsine and phosphine were used
for source materials for V group, and hydrogen was used for carrier
gas. Dimethyl zinc was used for the p-type dopant, and disilane was
used for the n-type dopant. Moreover, when the ridge is grown, the
HCl gas is introduced at a molecular ratio of HCl/group III of 0.2,
particularly, 0.3 as a molecular ratio of HCl/TMA.
[0050] Subsequently, a p-type electrode 114 is evaporated on the
side of the epitaxial surface. After the substrate 101 is made
thinner to 100 microns, an n-type electrode 115 is evaporated on
the substrate and is alloyed (FIG. 1(c)). A laser resonator
structure was formed by cutting chips by cleavage from the wafer
thus produced. The characteristics of the laser chip thus produced
turned out that the laser oscillated at around a wavelength of 650
nm when the distribution was measured in the batch or among the
batches, that the characteristics of the threshold current and
slope effectiveness are uniform, and that very high reliability can
be obtained. In addition, no kink was observed until at least a
high output of about 50 mW, and it turned out that the laser could
oscillate with a stable transverse mode.
* * * * *