Title: Method of manufacturing a semiconductor laser
Abstract: An AlGaAs cladding layer is formed on a GaAs semiconductor substrate, and then an i-type GaAs layer is formed thereon. Then, stripe-like grooves are formed in the GaAs layer by the photolithography method, and then an InAs layer is formed by the MBE method. At this time, edges of convex portions are deformed to be gentle with a rise of a substrate temperature, and thus flat portions are almost eliminated. Then, InAs is hardly deposited on slant surfaces of the convex portions and the InAs is grown on the bottom portions of the grooves in the S-K mode, so that InAs islands that are separated mutually are formed. Then, quantum dots are formed by covering the InAs islands with i-type GaAs. In this manner, a periodic structure of the quantum dots is formed by using the grooves.
Patent Number: 6,867,057 Issued on 03/15/2005 to Hatori
| Inventors:
|
Hatori; Nobuaki (Meguro, JP)
|
| Assignee:
|
Fujitsu Limited (Kawasaki, JP)
|
| Appl. No.:
|
644853 |
| Filed:
|
August 21, 2003 |
Foreign Application Priority Data
| Aug 23, 2002[JP] | 2002-244090 |
| Current U.S. Class: |
438/29 |
| Intern'l Class: |
H01L 021//00 |
| Field of Search: |
438/22,29,31,46
|
References Cited [Referenced By]
U.S. Patent Documents
| 5113473 | May., 1992 | Yoshida et al. | 385/131.
|
| 6596555 | Jul., 2003 | Bensahel et al. | 438/22.
|
| 6815242 | Nov., 2004 | Mukai et al. | 438/47.
|
| Foreign Patent Documents |
| 5-251811 | Sep., 1993 | JP.
| |
| 9-326506 | Dec., 1997 | JP.
| |
| 2001-326421 | Nov., 2001 | JP.
| |
Primary Examiner: Zarneke; David
Assistant Examiner: Geyer; Scott B.
Attorney, Agent or Firm: Armstrong, Kratz, Quintos, Hanson & Brooks, LLP
Claims
What is claimed is:
1. A method of manufacturing a semiconductor laser comprising the steps of:
forming a semiconductor layer on a substrate;
forming periodic concave and convex portions on the semiconductor layer;
deforming flat surfaces of the convex portions in the semiconductor layer
into slant surfaces that are inclined toward bottom portions of the
concave portions; and
forming an active layer in which quantum dots are arranged
three-dimensionally only on insides of the concave portions.
2. A method of manufacturing a semiconductor laser, according to claim 1,
wherein the concave portions are formed to have a width that is wider than
the convex portions.
3. A method of manufacturing a semiconductor laser, according to claim 1,
wherein the concave and convex portions are formed periodically along a
light propagation direction at a period that is set to a same extent as a
positive-integral multiple of 1/2 of a wavelength of the quantum dots in
the waveguide.
4. A method of manufacturing a semiconductor laser, according to claim 1,
wherein the quantum dots are formed in a self-formation manner.
5. A method of manufacturing a semiconductor laser comprising the steps of:
forming a first layer made of a first semiconductor layer on a substrate;
forming periodic stripe-like grooves on the first layer;
deforming flat surfaces of the first layer between the grooves into slant
surfaces that are inclined toward bottom portions of the grooves, by
heating the first layer together with the substrate; and
forming quantum dots only on the bottom portions of the grooves by
utilizing a difference between lattice constants, by depositing a second
semiconductor on insides of the grooves.
6. A method of manufacturing a semiconductor laser, according to claim 5,
wherein deposition of the second semiconductor is executed by a molecular
beam epitaxy method.
7. A method of manufacturing a semiconductor laser, according to claim 5,
wherein a width of the grooves is set wider than a width of the flat
portions between the grooves.
8. A method of manufacturing a semiconductor laser, according to claim 5,
wherein the quantum dots are formed on insides of the grooves as a
layered-structure.
9. A method of manufacturing a semiconductor laser, according to claim 5,
wherein the substrate is formed of any one of a GaAs semiconductor
substrate, an InGaAs semiconductor substrate, and an InP semiconductor
substrate.
10. A method of manufacturing a semiconductor laser, according to claim 5,
wherein the quantum dots are made of any one of InAs, InGaAs, InGaAsP, and
GaInNAs.
11. A method of manufacturing a semiconductor laser, according to claim 5,
wherein the first layer is made of GaAs.
12. A method of manufacturing a semiconductor laser, according to claim 5,
wherein the grooves are formed at a period that is set to a same extent as
a positive-integral multiple of 1/2 of a wavelength in the waveguide.
13. A method of manufacturing a semiconductor laser, according to claim 5,
wherein a pitch of the grooves is set such that a wavelength of an output
light becomes 1.3 .mu.m.
14. A method of manufacturing a semiconductor laser, according to claim 5,
wherein a pitch of the grooves is set such that a wavelength of an output
light becomes 1.55 .mu.m.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims priority of Japanese Patent
Application No. 2002-244090, filed on Aug. 23, 2002, the contents being
incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor
laser having a quantum dot structure in an active layer and, more
particularly, a method of manufacturing a semiconductor laser suitable for
a light source of the high-speed long-distance optical fiber communication
or the access system optical fiber communication.
2. Description of the Prior Art
As the method of converting the electric signal into the optical signal,
there are the direct modulation system for modulating directly the light
being output from the semiconductor laser in response to change in the
modulation signal, and the external modulation system for applying the
modulation to the light being output from the semiconductor laser from the
outside.
Normally, the external modulation system is employed in the large-capacity
optical fiber communication system, and the transmitting device that
generates the optical signal consists of the quantum-well
distributed-feedback type semiconductor laser for generating the
continuous light and the external optical modulator for superposing the
signal onto the light that is output from the laser. Assume that the
quantum-well distributed-feedback type semiconductor laser is operated at
a high speed under the direct modulation, the refractive index of the
active layer is varied due to variation in the injection carrier density
since the structure used as the active layer of the semiconductor laser is
the quantum well. Thus, such a phenomenon called the chirping that shift
of the oscillation wavelength is caused to restrict the transmission
distance is generated.
Therefore, as described above, the external modulation system is employed
normally in the commercial large-capacity optical fiber communication
system. The experiment of the very high-speed/large-capacity optical fiber
communication, which is in excess of 10 Gbit/s, is being made by using
combination of the continuously oscillating single-mode semiconductor
laser and the high-speed optical modulator.
Meanwhile, as one of important parameters in the discussions of the
chirping, there is a linewidth enhancement factor. The linewidth
enhancement factor .alpha. is defined by a following expression (1).
##EQU1##
where .chi.(N) is a complex susceptibility of the active layer, Re and Im
are a real part and an imaginary part of .chi.(N) respectively, and N is a
carrier density. If it is employed that the real part and the imaginary
part of .chi.(N) are connected by the Kramers-Kronig relations and the
imaginary part of .chi.(N) is proportional to a gain g, the expression (1)
that defines the linewidth enhancement factor .alpha. can be expressed as
a following expression (2).
##EQU2##
where E' and E are the energy respectively, and P is Cauchy principal value
integral.
If the bulk structure is used as the active layer of the semiconductor
laser, the linewidth enhancement factor has a value of about 4 to 6 in
vicinity of the oscillation wavelength. The linewidth enhancement factor
can be reduced up to about 2 by employing the quantum well structure as
the active layer. It has been reported that, if the oscillation wavelength
in the DFB (distributed feedback type) structure is shifted to the peak
direction of the differential gain spectrum by adjusting material and
composition of the quantum well and the laser structure, the linewidth
enhancement factor can be reduced to a small value like 1.4 to 1.8.
However, it is difficult to reduce much more this factor by using the
quantum well structure. This is because, although such linewidth
enhancement factor becomes 0 at the peak of the differential gain spectrum
of the active layer, the peak position of the differential gain spectrum
is normally placed in the absorption range of the gain spectrum in the
quantum well structure and thus is displaced from the peak position of the
gain spectrum.
As the semiconductor laser that is expected to reduce the linewidth
enhancement factor much more than the quantum well structure semiconductor
laser, the quantum-dot distributed-feedback type semiconductor laser
having the quantum-dot structure in the active layer has been proposed.
FIG. 1 is a sectional view showing a structure of the quantum-dot
distributed-feedback type semiconductor laser in the prior art.
An n-type AlGaAs cladding layer 12 is formed on an n-type GaAs
semiconductor substrate 11, and then an i-type GaAs SCH (Separate
Confinement Heterostructure) layer 13 is formed on the n-type AlGaAs
cladding layer 12. An area in which quantum dots 14 are arranged in the
three-dimensional direction and an area in which the quantum dots 14 are
not formed are provided at a constant period alternatively in the GaAs SCH
layer 13. This i-type GaAs SCH layer 13 constitutes the active layer of
the semiconductor laser.
A p-type AlGaAs cladding layer 15 is formed on the GaAs SCH layer 13, and
then a p-type GaAs cap layer 16 is formed on the AlGaAs cladding layer 15.
Electrode 17a, 17b are formed under the GaAs semiconductor substrate 11 and
on the GaAs cap layer 16 respectively. Also, a high reflectance mirror 18
is formed on one end surface side of the GaAs SCH layer 13, and also a low
reflectance mirror 19 is formed on the other end surface side of the GaAs
SCH layer 13. The light is emitted through the low reflectance mirror 19.
Normally the quantum dots are formed in the strain system heteroepitaxial
structure such as InAs/GaAs, or the like by utilizing the S-K
(Stranski-Krastanov) mode growth that appears in the initial stage of the
heteroepitaxial growth (for example, see Patent Application Publication
(KOKAI) Hei 9-326506).
Next, a method of manufacturing the quantum-dot distributed-feedback type
semiconductor laser in the prior art will be explained with reference to
FIG. 1 and FIGS. 2A to 2G hereinafter.
First, as shown in FIG. 2A, the n-type AlGaAs cladding layer 12 of about
1400 nm thickness is formed on a (100) face of the n-type GaAs
semiconductor substrate 11 by the MOVPE (Metal Organic Vapor Phase
Epitaxy) method of the MBE (Molecular Beam Epitaxy) method. Then, an
i-type GaAs layer 21 of about 20 nm thickness is formed on the AlGaAs
cladding layer 12 by supplying TEGa (triethylgallium) and AsH.sub.3
(arsine) to the chamber. At this time, the substrate temperature is set to
620.degree. C., for example.
Then, as shown in FIG. 2B, supply of Ga is shut off and then the substrate
temperature is lowered to about 500.degree. C. Then, an i-type InAs layer
having a thickness that corresponds to one to several molecular layers is
deposited by introducing the molecular beam of In into the chamber. At
this time, the lattice constant of the InAs layer is slightly different
from the lattice constant of the GaAs layer 21. Therefore, as shown in
FIG. 2C, a large number of InAs islands 22 that are separated mutually are
generated on the GaAs layer 21 by the S-K mode growth.
After the first-layer InAs islands 22 are formed In this manner, as shown
in FIG. 2D, an intermediate layer 23 made of i-type GaAs and having a
thickness of 2 to 3 nm is deposited on the GaAs layer 21. Thus, the InAs
islands 22 are surrounded by the GaAs that has a large band gap, so that
quantum dots 14 that confine the carrier three-dimensionally are formed.
Then, the formation of the InAs islands 22 and the deposition of the i-type
GaAs intermediate layer 23 are repeated several times. Thus, as shown in
FIG. 2E, a layer 24 having a layered quantum-dot structure in which the
quantum dots 14 are arranged three-dimensionally is formed.
Then, as shown in FIG. 2F, stripe-like grooves 25 that reach the AlGaAs
cladding layer 12 are formed by etching the layer 24 by virtue of the
photolithography method. Then, as shown in FIG. 2G, an i-type GaAs layer
is deposited on the overall surface to bury the grooves 25, so that a
surface of the GaAs layer is planarized. In this manner, the GaAs SCH
layer 13 is formed. An area in which the quantum dots 14 are arranged in
the three-dimensional direction and an area in which the quantum dots 14
are not formed are provided at a constant period alternatively in the GaAs
SCH layer 13.
Then, as shown in FIG. 1, the p-type AlGaAs cladding layer 15 and the
p-type GaAs cap layer 16 are formed sequentially on the GaAs SCH layer 13.
Then, the electrodes 17a, 17b and the high reflectance mirror 18 and the
low reflectance mirror 19 are formed. As a result, the quantum-dot
distributed-feedback type semiconductor laser is completed.
As described above, in the large-capacity optical fiber communication
system, normally the transmitter is constructed by integrating the
semiconductor laser and the optical modulator. However, there is such a
drawback that a configuration of this system is complicated rather than
the direct modulation of the semiconductor laser single body, and thus a
production cost is increased.
The semiconductor laser having the quantum-dot structure has such
advantages that the linewidth enhancement factor is small rather than the
semiconductor laser having the quantum-well structure and also the
chirping is difficult to occur even when the direct modulation is applied.
However, in order to utilize the semiconductor laser in the very
high-speed/large-capacity optical fiber communication system, the
semiconductor laser in which the quantum dots can be formed uniformly at a
higher density and which can have a larger gain is desired.
SUMMARY OF THE INVENTION
From the above, it is an object of the present invention to provide a
method of manufacturing a semiconductor laser in which quantum dots can be
formed uniformly at a high density, which can have a large gain, and in
which a chirping is hard to occur even when the direct modulation is
applied thereto.
The above subject can be overcome by providing a method of manufacturing a
semiconductor laser that comprises the steps of forming a semiconductor
layer on a substrate; forming periodic concave and convex portions on the
semiconductor layer; deforming flat surfaces of the convex portions in the
semiconductor layer into slant surfaces that are inclined toward bottom
portions of the concave portions; and forming an active layer in which
quantum dots are arranged three-dimensionally only on insides of the
concave portions.
Also, the above subject can be overcome by providing a method of
manufacturing a semiconductor laser that comprises the steps of forming a
first layer made of a first semiconductor layer on a substrate; forming
periodic stripe-like grooves on the first layer; deforming flat surfaces
of the first layer between the grooves into slant surfaces that are
inclined toward bottom portions of the grooves, by heating the first layer
together with the substrate; and forming quantum dots only on the bottom
portions of the grooves by utilizing a difference between lattice
constants, by depositing a second semiconductor on insides of the grooves.
In the present invention, the periodic concave and convex portions (or the
grooves) are formed on the semiconductor layer made of the first
semiconductor. The period of the concave and convex portions is set to the
same extent as the positive-integral multiple of 1/2 of the wavelength of
quantum dots in the waveguide, for example.
Then, the flat surfaces in the convex portions are deformed into the slant
surfaces that are inclined toward the bottom portions of the concave
portions. For example, if a width of the convex portion is set
sufficiently small, a sectional shape of the convex portion becomes gentle
in the course of the substrate temperature rise, so that the flat surface
of the convex portion can be eliminated.
Then, the second semiconductor is deposited on the semiconductor layer by
the molecular beam epitaxy method, for example. In this case, the second
semiconductor is seldom deposited on the slant surfaces of the convex
portions and is deposited mainly on the bottom portions of the concave
portions. The second semiconductor is grown in the S-K mode on the bottom
portions of the concave portions, and thus the quantum dots are formed.
In this fashion, in the present invention, since the quantum dots are
self-formed only on the bottom portions of the concave portions (or the
grooves), the quantum dots can be formed uniformly at a high density. As a
result, it is possible to manufacture the semiconductor laser which can
have a large gain and in which the chirping is difficult to occur even
when the direct modulation is applied thereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view showing a structure of a quantum-dot
distributed-feedback type semiconductor laser in the prior art;
FIGS. 2A to 2G are views explaining a method of manufacturing a quantum-dot
distributed-feedback type semiconductor laser in the prior art;
FIGS. 3A to 3J are views showing a method of manufacturing a quantum-dot
distributed-feedback type semiconductor laser according to a first
embodiment of the present invention;
FIG. 4 is a sectional view showing a method of manufacturing a
semiconductor laser according to a second embodiment of the present
invention; and
FIG. 5 is a sectional view showing an example of a surface emitting
semiconductor laser.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention will be explained with reference to
the accompanying drawings hereinafter.
(First Embodiment)
FIGS. 3A to 3J are views showing a method of manufacturing a quantum-dot
distributed-feedback type semiconductor laser according to a first
embodiment of the present invention.
First, as shown in FIG. 3A, a cladding layer 52 made of n-type Al.sub.0.4
Ga.sub.0.6 As is formed on a (100) face of an n-type GaAs semiconductor
substrate 51 by the MBE method to have a thickness of about 1400 nm. At
this time, the substrate temperature is set to 620.degree. C., for
example. Here, an InGaAs semiconductor substrate or an InP semiconductor
substrate may be employed in place of the GaAs semiconductor substrate.
Then, as shown in FIG. 3B, an i-type GaAs layer 53 of about 600 nm
thickness is formed on the n-type AlGaAs cladding layer 52 by supplying
TEGa (triethylgallium) and AsH.sub.3 (arsine) to the chamber.
Then, the semiconductor substrate 51 is taken out of the chamber of the MBE
equipment. Then, photoresist is coated on a surface of the i-type GaAs
layer 53. Then, a diffraction pattern is drawn on the photoresist by the
electron beam exposure, and then the developing process is applied to the
photoresist. Thus, stripe-like resist masks (not shown) each having a
width of about 50 nm are formed to have a predetermined pitch.
Then, as shown in FIG. 3C, grooves 54 having a depth of about 400 nm are
formed by applying the dry etching to portions, which are not covered with
the resist masks, of the i-type GaAs layer 53. In this case, since the
etching proceeds in the direction perpendicular to the substrate 51 in the
dry etching, cross sections of the grooves 54 become almost a rectangle
and areas between the grooves (called "terraces" hereinafter) become flat,
as shown in FIG. 3C.
In this case, a pitch of the grooves 54 is set according to the oscillation
wavelength. In the present embodiment, since a periodic structure is
constructed by forming the quantum dots in each groove 54 as described
later, the pitch of the grooves 54 is decided such that the periodic
structure of the quantum dots is set to the same extent as the
positive-integral multiple of 1/2 of the wavelength in the waveguide. For
example, the pitch of the grooves 54 is decided as 200 nm if the
oscillation wavelength is set to 1.3 .mu.m, while the pitch of the grooves
54 is decided as 240 nm if the oscillation wavelength is set to 1.55
.mu.m. In this case, the oscillation wavelength depends on not only the
period of the quantum dots but also composition and size of the quantum
dots.
Then, the substrate 51 is loaded again into the chamber of the MBE
equipment, and then is heated up to about 500.degree. C. Thus, as shown in
FIG. 3D, edge portions of the grooves 54 become gentle curved surfaces and
thus flat portions of the terraces are almost eliminated.
Then, as shown in FIG. 3E, the i-type InAs layer is formed by introducing
the In and As molecular beams into the chamber. At this time, the InAs
molecules are hardly deposited on slant surfaces of convex portions and
are deposited on bottom portions of the grooves 54. Then, as shown in FIG.
3F, a large number of InAs islands 55 that are separated mutually appear
by the S-K mode growth when the i-type InAs layer is deposited on the
bottom portions of the grooves 54 to some extent.
After the first-layer InAs islands 55 are formed in this manner, as shown
in FIG. 3G, an i-type GaAs intermediate layer 56 is formed on the overall
surface to have a thickness of about 2 to 3 nm such that the InAs islands
55 are covered with the i-type GaAs intermediate layer 56. Accordingly,
the InAs islands 55 are surrounded by the GaAs that have a large band gap,
so that the quantum dots 57 that confine the carrier three-dimensionally
are formed.
In this case, the quantum dots 57 are formed of InAs in the present
embodiment. But such quantum dots 57 may be formed of InGaAs, InGaAsP,
GaInNAs, or the like.
Then, the formation of the InAs islands 55 and the deposition of the i-type
GaAs intermediate layer 56 are repeated several times. Thus, as shown in
FIG. 3H, the quantum dot structure in which the quantum dots 57 are
arranged on the inside of the grooves 54 three-dimensionally is formed.
Then, a surface of the intermediate layer 56 is planarized by depositing
the i-type GaAs until the grooves 54 are buried. Thus, as shown in FIG.
3I, an i-type GaAs SCH layer (active layer) 58 is formed. An area in which
quantum dots 57 are arranged in the three-dimensional direction and an
area in which the quantum dots 57 are not formed are provided periodically
alternatively in the GaAs SCH layer 58.
Then, as shown in FIG. 3J, a cladding layer 59 made of p-type Al.sub.0.4
Ga.sub.0.6 As is formed on the GaAs SCH layer 58 to have a thickness of
about 1400 nm. Then, a p-type GaAs cap layer 60 of about 400 nm thickness
is formed on the AlGaAs cladding layer 59. Then, a high reflectance mirror
62 is formed on one end surface side of the GaAs SCH layer 58, and also a
low reflectance mirror 63 is formed on the other end surface side of the
GaAs SCH layer 58. Then, an electrode 61a is formed on a bottom surface of
the substrate 51, and an electrode 61b is formed on the p-type GaAs cap
layer 60. As a result, the quantum-dot distributed-feedback type
semiconductor laser is completed.
According to the present embodiment, as shown in above FIG. 3E, the In
molecules and the As molecules, which come up to the slant surfaces of the
convex portions, are moved to the flat surfaces at the bottom portions of
the grooves 54 to form the quantum dots. Therefore, the quantum dots can
be formed uniformly at a high density rather than the case where the
quantum dots are formed on the flat plate-like substrate (see FIG. 1 and
FIGS. 2A to 2G). As a result, since the quantum dots exist locally at top
or bottom portions of the standing wave at the high density, an effect to
increase the gain can be attained. Accordingly, in the semiconductor laser
manufactured by the present embodiment, the chirping is hard to occur even
when the direct modulation is applied. Thus, such semiconductor laser can
be applied to the very high-speed/large-capacity optical fiber
communication system.
In this case, the inventors of this application have proposed the
semiconductor laser in which concave and convex portions are provided on
the semiconductor substrate periodically and then the quantum dots are
formed on bottom surfaces (root portions) of the concave portions and top
surfaces (peak portions) of the convex portions respectively (Patent
Application Publication (KOKAI) 2001-326421). In contrast to this
semiconductor laser, the quantum dots are formed only on the bottom
portions of the grooves in the semiconductor laser of the present
embodiment, and therefore such advantages can be achieved that the density
and the uniformity of the quantum dots are improved and the gain can be
increased much more.
(Second Embodiment)
FIG. 4 is a sectional view showing a method of manufacturing a
semiconductor laser according to a second embodiment of the present
invention. In this case, the present embodiment shows an example in which
the present invention is applied to manufacture of a ridge-mesa type
semiconductor laser.
First, an n-type AlGaAs cladding layer 72 is formed on a (100) face of an
n-type GaAs semiconductor substrate 71 by using the MBE method, and then
an i-type GaAs SCH layer 73 having a periodic structure of the quantum
dots therein is formed thereon. The i-type GaAs SCH layer 73 is formed by
the same method as the first embodiment (see FIGS. 3C to 3H).
Then, a p-type AlGaAs cladding layer 74 is formed on the i-type GaAs SCH
layer 73, and then a p-type GaAs cap layer 75 is formed thereon.
Then, a mask having a predetermined shape is formed on the p-type GaAs cap
layer 75. Then, a ridge-mesa structure is formed by etching respective
layers up to the middle of a thickness of the n-type AlGaAs cladding layer
72 using the mask. In this case, an n-type GaAs buffer layer may be formed
previously between the n-type GaAs semiconductor substrate 71 and the
n-type AlGaAs cladding layer 72, and then the etching may be carried out
until the GaAs buffer layer is exposed.
Then, an insulating film 76 made of SiN, or the like, for example, is
formed on an overall upper surface of the GaAs semiconductor substrate 71.
Then, an opening portion from which the n-type AlGaAs cladding layer 72 is
exposed and an opening portion from which the p-type GaAs cap layer 75 is
exposed are formed by etching selectively the insulating film 76 by virtue
of the photolithography method. Then, a metal film is formed on the
overall upper surface of the semiconductor substrate 71. Then, an
electrode 77a connected to the n-type AlGaAs cladding layer 72 and an
electrode 77b connected to the p-type GaAs cap layer 75 are formed by
patterning the metal film by virtue of the photolithography method.
Accordingly, the ridge-mesa type semiconductor laser is completed.
According to the present embodiment, like the first embodiment, the
stripe-like grooves are formed in the i-type GaAs layer, then the flat
portions of the terraces are almost eliminated by the heat, and then the
InAs quantum dots are formed by the MBE method. As a result, the
semiconductor laser in which the quantum dots can be formed uniformly at
the high density on the bottom portions of the grooves, which can have the
large gain, and in which the chirping is hard to occur even when the
direct modulation is applied thereto can be obtained.
(Other Embodiment)
In the above first and second embodiments, the case where the present
invention is applied to the manufacture of the edge emitting semiconductor
laser is explained. But the present invention can be applied to
manufacture of a surface emitting semiconductor laser. FIG. 5 is a
sectional view showing an example of the surface emitting semiconductor
laser.
First, an n-type AlGaAs cladding layer 82 is formed on an n-type GaAs
semiconductor substrate 81. This n-type AlGaAs cladding layer 82 is shaped
into the mesa type, and also a mirror layer 83 is formed on the mesa
portion of the n-type AlGaAs cladding layer 82. This mirror layer 83
consists of a GaAs layer and an AlGaAs layer, which are laminated
alternatively, for example.
Then, an i-type GaAs SCH layer 84 having a periodic structure of the
quantum dots is formed on the mirror layer 83. A mirror layer 85 is formed
on the i-type GaAs SCH layer 84. This mirror layer 85 also consists of the
GaAs layer and the AlGaAs layer, which are laminated alternatively, for
example. However, the number of laminated layers of the mirror layer 85 is
reduced smaller than that of the mirror layer 83 such that the light that
is amplified to some extent can pass through the mirror layer 85.
Then, a p-type AlGaAs cladding layer 86 is formed on the mirror layer 85.
Then, a p-type GaAs cap layer 87 is formed on the p-type AlGaAs cladding
layer 86.
Upper surfaces or side surfaces of the AlGaAs cladding layer 82, the mirror
layer 83, the i-type GaAs SCH layer 84, the mirror layer 85, the p-type
AlGaAs cladding layer 86, and the p-type GaAs cap layer 87 are covered
with an insulating film 88. One electrode 89a is connected to the AlGaAs
cladding layer 82 via an opening portion provided in the insulating film
88, and the other electrode 89b is connected to the p-type GaAs cap layer
87 via an opening portion provided in the insulating film 88 like a ring.
Then, a reflection preventing film 90 is formed on the inside of the
electrode 89b.
In the surface emitting semiconductor laser constructed in this manner, the
i-type GaAs SCH layer is formed by the method explained in the first
embodiment. That is, the grooves are formed like the stripe in the i-type
GaAs layer, then the flat portions of the terraces are almost eliminated
by the heat, and then the InAs quantum dots are formed by the MBE method.
In this case, in the surface emitting semiconductor laser, the period of
the quantum dots in the vertical direction is set to the same extent as
the positive-integral multiple of 1/2 of the wavelength in the waveguide.
*
