U.S. patent application number 13/748469 was filed with the patent office on 2013-06-13 for optical semiconductor device and pumping light source for optical fiber amplifier.
This patent application is currently assigned to Furukawa Electric Co., Ltd.. The applicant listed for this patent is Furukawa Electric Co., Ltd.. Invention is credited to Harunobu Ikeda, Satosho Irino, Hirokazu Itoh, Akihiko Kasukawa, Masako Kobayakawa, Hidehiro Taniguchi, Naoki Tsukiji, Junji Yoshida.
Application Number | 20130149804 13/748469 |
Document ID | / |
Family ID | 44224669 |
Filed Date | 2013-06-13 |
United States Patent
Application |
20130149804 |
Kind Code |
A1 |
Yoshida; Junji ; et
al. |
June 13, 2013 |
OPTICAL SEMICONDUCTOR DEVICE AND PUMPING LIGHT SOURCE FOR OPTICAL
FIBER AMPLIFIER
Abstract
A semiconductor device of the invention is formed so that n-type
InP current blocking layers enter the inside of p-type InP cladding
layers, i.e., the n-type current blocking layers ride over the
upper part of the p-type InP cladding layers, so that a distance
between the n-type InP current block layers composing a current
blocking region is narrower than a width of the p-type cladding
layers contacting with the n-type InP current blocking layers.
Thereby, the semiconductor device whose leak current in the current
blocking region may be reduced which permits high-output and
high-temperature operations may be readily fabricated.
Inventors: |
Yoshida; Junji; (Tokyo,
JP) ; Tsukiji; Naoki; (Tokyo, JP) ; Taniguchi;
Hidehiro; (Tokyo, JP) ; Irino; Satosho;
(Tokyo, JP) ; Itoh; Hirokazu; (Tokyo, JP) ;
Ikeda; Harunobu; (Tokyo, JP) ; Kobayakawa;
Masako; (Tokyo, JP) ; Kasukawa; Akihiko;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Furukawa Electric Co., Ltd.; |
Tokyo |
|
JP |
|
|
Assignee: |
Furukawa Electric Co., Ltd.
Tokyo
JP
|
Family ID: |
44224669 |
Appl. No.: |
13/748469 |
Filed: |
January 23, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12683676 |
Jan 7, 2010 |
8385379 |
|
|
13748469 |
|
|
|
|
Current U.S.
Class: |
438/29 |
Current CPC
Class: |
H01S 5/3434 20130101;
H01L 2924/0002 20130101; H01L 2924/0002 20130101; H01S 5/3406
20130101; H01L 33/60 20130101; H01S 5/2275 20130101; B82Y 20/00
20130101; H01S 5/02415 20130101; H01L 2924/00 20130101; H01S 5/2206
20130101; H01S 5/02438 20130101; H01S 5/2222 20130101; H01S 5/02284
20130101 |
Class at
Publication: |
438/29 |
International
Class: |
H01L 33/60 20060101
H01L033/60 |
Claims
1. A method of producing an optical semiconductor device,
comprising the steps of: preparing a semiconductor substrate,
forming a first cladding layer on one flat surface of said
semiconductor substrate, forming an active layer on said first
cladding layer, forming a second cladding layer on said active
layer; forming said first cladding layer, said active layer and
said second cladding layer as a mesa-shape structure, forming first
current blocking layers of a first conductive type on said first
cladding layer, said active layer and a part of said second
cladding layer at both side surfaces of said mesa-shape structure,
forming second current blocking layers of a second conductive type
as covering said first current blocking layers; wherein in the step
of forming said first current blocking layers and the step of
forming said second current blocking layers, said first current
blocking layers and said second current blocking layers are formed
by metal-organic vapor phase epitaxy, and in the step of forming
said second current blocking layers, said second current blocking
layers are formed so that upper surfaces of said second current
blocking layers are same or substantially same as an upper surface
of said second cladding layer and said second current blocking
layers have a structure wherein end portions of said upper surfaces
of said second current blocking layers extend over said first
current blocking layer, and ride over said second cladding layer so
as to enter the inside of said second cladding layer by the
mass-transport phenomenon.
2. The method of producing an optical semiconductor device
according to claim 1, wherein said second current blocking layers
are formed as films deposited at a deposition temperature T
satisfying that the following formula is negative regarding said
deposition temperature T of said second current blocking layer
W1-W2(.mu.m)=-0.0092.times.T+6.18, where, W1 is a distance between
said end portions of said second current blocking layers and W2 is
a width between said first current blocking layers contacting said
second cladding layer.
3. The method of producing an optical semiconductor device
according to claim 2, wherein in the step of forming said first
current blocking layers and the step of forming said second current
blocking layers, said first current blocking layers and said second
current blocking layers are formed so that the (W1-W2) satisfies
-0.5 .mu.m.ltoreq.(W1-W2)<0 .mu.m.
4. The method of producing an optical semiconductor device
according to claim 1, wherein in the step of forming said second
current blocking layers, said second current blocking layers are
formed so that concentration of said first conductive type impurity
of said first current blocking layers is in a range from
0.8.times.10.sup.18 cm.sup.-3 to 1.4.times.10.sup.18 cm.sup.-3.
5. The method of producing an optical semiconductor device
according to claim 1, wherein in the step of preparing said
semiconductor substrate, a semiconductor substrate comprising InP
is prepared. 12/8003-FDK-US (FRKWP147USA)
6. The method of producing an optical semiconductor device
according to claim 5, wherein in the step of forming said active
layer, said active layer is formed so as to have a quantum well
structure or a strain super-lattice quantum well structure.
7. The method of producing an optical semiconductor device
according to claim 6, wherein in the step of forming said active
layer, said active layer is formed so that a width of said active
layer is selected so as to meet a single-transverse mode operation
across a light waveguide direction.
8. The method of producing an optical semiconductor device
according to claim 1, further comprising a step of forming films
having different reflectivity on the both end surfaces in a
resonator direction of said optical semiconductor device.
9. The method of producing an optical semiconductor device
according to claim 6, wherein in the step of forming said active
layer, said active layer is formed so as to comprise GaInAsP.
10. The method of producing an optical semiconductor device
according to claim 7, wherein in the step of forming said active
layer, said active layer is formed so as to contain an active layer
in which thickness of a well layer is 6 nm or more.
11. The method of producing an optical semiconductor device
according to claim 6, wherein in the step of forming said active
layer, said active layer is formed so as to comprise AlGaInAs.
12/8003-FDK-US (FRKWP147USA)
12. The method of producing an optical semiconductor device
according to claim 1, wherein in the step of forming said first
current blocking layers and the step of forming said second current
blocking layers, said first current blocking layers and said second
current blocking layers are formed so as to establish pn junction
or np junction.
Description
RELATED APPLICATION
[0001] The present application is a divisional application from,
and claiming priority to, U.S. patent application Ser. No.
12/683,676, filed Jan. 7, 2010, the entirety of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an optical semiconductor
device having a buried hetero-structure and to a pumping light
source for optical fiber amplifier using the same.
[0004] 2. Description of the Related Art
[0005] A current injection type semiconductor device adopts a
structure having a current confinement function to efficiently
inject current to an active layer, i.e., an emitting region. One of
the current confinement structures uses a structure having buried
current blocking regions on the both sides of a mesa-stripe
containing the active layer. A structure including inversely biased
pn junction is used in general as the current blocking region. It
is noted that a difference of refractive indices is generated
between the current blocking region and the active layer in the
semiconductor device having such buried current blocking region, so
that the semiconductor device has a horizontal optical waveguide
function for confining generated light within the active layer. Due
to this optical waveguide function, the shape of the generated
light (laser beam) becomes substantially circular and the
semiconductor device has an advantage that it may be coupled with
an optical fiber with high coupling efficiency through an optical
part such as a lens.
[0006] FIG. 5 is a diagrammatic view of a section of a known
semiconductor device perpendicular to a direction of a resonator
having a so-called buried hetero-structure (BH) having a current
device blocking layer grown by metal-organic vapor phase epitaxy
(MOVPE). In the semiconductor device shown in FIG. 5, a mesa-stripe
is formed by an n-type InP cladding layer 2, a GaInAsP active layer
3 and a p-type InP cladding layer 4 on an n-type InP substrate 1
and the current blocking regions composed of a p-type InP current
blocking layer 5 and an n-type InP current blocking layer 6 are
formed on the both side surfaces of the mesa-stripe by means of
buried growth. Further, a p-type InP cladding layer 7 and a p-type
GaInAsP contact layer 8 are formed at predetermined position on the
mesa-stripe and the current blocking regions. Then, an n-type
electrode 9 is formed under the n-type InP substrate 1 and a p-type
electrode 10 is formed on the contact layer 8 to inject electric
current.
[0007] In the semiconductor device in FIG. 5, similarly to the pn
double hetero-junction including the GaInAsP active layer 3,
forward bias is applied to the pn junction between the p-type InP
current blocking layer 5 and the n-type InP cladding layer 2 when
the semiconductor device is operative. Further, the p-type InP
cladding layer 7, the n-type InP current blocking layer 6, the
p-type InP current blocking layer 5 and the n-type InP substrate 1
compose a pnpn-type thyristor structure 11.
[0008] When plus voltage is applied to the p-side electrode 10 and
minus voltage to the n-side electrode 9 in the semiconductor device
having such structure, the most of the injected current is injected
to the active layer 3 due to a difference of built-in voltage of
the pn junction. However, because forward bias is applied in a path
I-II in FIG. 5, a leak current is generated through the pn junction
between the p-type InP current blocking layer 5 and the n-type InP
cladding layer 2. This leak current turns out a gate current of the
thyristor structure 11, so that a leak current flows through a path
III-IV corresponding to an anode current. Heretofore, technologies
aiming at reducing the leak current are disclosed in Patent
Documents 1 through 5.
[0009] Patent Document 1: Japanese Patent Application Laid-open No.
2008-198662
[0010] Patent Document 2: Japanese Patent Application Laid-open No.
Hei. 8-127251
[0011] Patent Document 3: Japanese Patent Application Laid-open No.
Hei. 5-13869
[0012] Patent Document 4: Japanese Patent Application Laid-open No.
Hei. 5-129723
[0013] Patent Document 5: Japanese Patent Application Laid-open No.
Hei. 5-75209
[0014] However, the leak current generated in the current blocking
region of the semiconductor device having the buried hetero (BH)
structure may become a cause that induces an increase of threshold
current, thermal saturation of light output and discontinuous point
(kink) in the light-current output characteristics. The
discontinuous point (kink) is problematic in particular because it
remarkably drops high output and high temperature operations of the
semiconductor device.
[0015] In view of the problems described above, the present
invention aims at providing a semiconductor device whose leak
current in a current blocking region is small and which is capable
of performing high-output and high-temperature operations, without
causing any trouble in a process for fabricating the semiconductor
device.
SUMMARY OF THE INVENTION
[0016] In order to solve the problems described above and to
achieve the purposes described above, an optical semiconductor
device of the invention is characterized in that it includes: a
mesa-stripe having a double hetero-structure containing an active
layer interposed between a n-type cladding layer and a p-type
cladding layer formed on a n-type substrate; and a current blocking
region having pn junction and composed of one material on side
surface portions of the mesa of the double hetero-structure; and is
arranged such that a distance W1 between the n-type current
blocking layers composing the current blocking region is narrower
than a width W2 of the p-type cladding layer contacting with the
n-type current blocking layer.
[0017] The semiconductor device of the invention is also
characterized in that a difference W1-W2 between the distance W1
between the n-type current blocking layers and the width W2 of the
p-type cladding layer contacting with the n-type current blocking
layer is more than -0.5 .mu.m and less than 0 .mu.m.
[0018] The semiconductor device of the invention is also
characterized in that concentration of p-type impurity of the
p-type current blocking layer is in a range from 1.times.10.sup.18
cm.sup.-3 to 1.2.times.10.sup.18 cm.sup.-3.
[0019] The semiconductor device of the invention is also
characterized in that a material composing the n-type substrate is
InP.
[0020] The semiconductor device of the invention is also
characterized in that the active layer has a quantum well structure
or a strain super-lattice quantum well structure.
[0021] The semiconductor device of the invention is also
characterized in that a width of the active layer is selected so as
to meet a single-transverse mode operation across a light waveguide
direction.
[0022] The semiconductor device of the invention is also
characterized in that films having different reflectivity are
formed on the both end surfaces in a resonator direction of the
semiconductor device.
[0023] The semiconductor device of the invention is also
characterized in that the active layer is composed of GaInAsP.
[0024] The semiconductor device of the invention is also
characterized in that the active layer contains an active layer in
which thickness of a well layer is 6 nm or more.
[0025] The semiconductor device of the invention is also
characterized in that the active layer is composed of AlGaInAs.
[0026] A pumping light source for fiber amplifier whose fiber end
light output is 400 mW or more is characterized in that it
includes: the optical semiconductor device composed of the
semiconductor device of the invention; a temperature controlling
module for controlling temperature of the optical semiconductor
device; an optical fiber for guiding laser light outputted out of
the optical semiconductor device to the outside; and an optical
coupling lens system for optically coupling the optical
semiconductor device and the optical fiber.
[0027] The pumping light source for fiber amplifier is
characterized in that it further includes an optical detector for
measuring the light output of the optical semiconductor device and
an isolator.
Advantages of the Invention
[0028] Because the optical semiconductor device of the invention is
arranged such that the distance W1 between the n-type current
blocking layers composing the current blocking region is narrower
than the width W2 of the p-type cladding layer contacting with the
n-type current blocking layer, the semiconductor device in which a
contact region of the p-type cladding layer and the p-type current
blocking layer is reduced and the anode current in the thyristor
structure is reduced may be realized. Still more, the semiconductor
device having the current blocking region by which current
constrictivity is improved and by which turn-on current is hardly
generated may be realized by reducing the anode current.
[0029] Further, because the semiconductor device of the invention
is arranged such that the difference W1-W2 between the distance W1
between the n-type current blocking layers and the width W2 of the
p-type cladding layer contacting with the n-type current blocking
layer is more than -0.5 .mu.m and less than 0 .mu.m, it is possible
to realize the highly reliable and high-output operable optical
semiconductor device that is not influenced by thermal saturation
of light output otherwise caused by an increase of device
resistance due to diffusion of impurity in the p-type cladding
layer generated in the fabrication process.
[0030] Further, because the semiconductor device of the invention
is arranged such that concentration of p-type impurity of the
p-type current blocking layer is in a range from 1.times.10.sup.18
cm .sup.-3 to 1.2.times.10.sup.18 cm.sup.-3, the anode current in
the current device layer may be cut and absorption of light by the
p-type impurity is cut. Therefore, it is possible to realize the
optical semiconductor device capable of performing the high-output
operations even when driven with high-current.
[0031] Further, because the semiconductor device of the invention
is arranged such that the material composing the n-type substrate
is InP, the optical semiconductor device most suitable for
communication wavelength band may be realized with low threshold
current and high efficiency.
[0032] Further, because the semiconductor device of the invention
is arranged such that the active layer has the quantum well
structure, the semiconductor device that highly efficiently
operates with the low threshold current may be realized. Further,
the active layer has the strain super-lattice quantum well
structure, the semiconductor device that operates more highly
efficiently with the low threshold current may be realized.
[0033] Further, because the semiconductor device of the invention
is arranged such that the width of the active layer is selected so
as to meet the single-transverse mode operation across the light
waveguide direction, the optical semiconductor device capable of
performing the high-output operation with the low threshold current
that is most suitable for an optical communication system may be
realized.
[0034] Further, because the semiconductor device of the invention
is arranged such that the films having different reflectivity are
formed on the both end surfaces in the resonator direction of the
semiconductor device, light may be taken out efficiently from the
end surface whose reflectivity is low. Still more, light from the
end surface whose reflectivity is high may be received by a light
receiving device to utilize the light as monitor light for
controlling an output of the semiconductor device.
[0035] Further, because the semiconductor device of the invention
is arranged such that the active layer is composed of GaInAsP, the
optical semiconductor device capable of performing the high-output
operation with the low threshold current that is most suitable for
the optical communication system may be realized.
[0036] Further, because the semiconductor device of the invention
is arranged such that the active layer contains an active layer in
which thickness of a well layer is 6 nm or more, it is possible to
realize the highly efficient optical semiconductor device which
receives less influence of the short wavelength shift caused by a
thermal process and whose threshold value is low.
[0037] Further, because the semiconductor device of the invention
is arranged such that the active layer is composed of AlGaInAs, it
is possible to realize the highly efficient optical semiconductor
device in which the short wavelength shift caused by the thermal
process is barely generated and whose threshold value is low.
[0038] Further, because a pumping light source for fiber amplifier
whose fiber end light output is 400 mW or more may be realized by
including: the semiconductor laser device composed of the
semiconductor device of the invention described above; the
temperature controlling module for controlling temperature of the
optical semiconductor device; the optical fiber for guiding laser
light output of the optical semiconductor device to the outside;
and the optical coupling lens system for optically coupling the
optical semiconductor device and the optical fiber, a fiber
amplifier that supports a more large capacity optical communication
system may be realized.
[0039] Further, because the fiber amplifier further including the
optical detector for measuring the light output of the optical
semiconductor device and the isolator may be realized, the large
capacity communication system using the erbium doped fiber
amplifier may be realized.
[0040] The above and other objects, features, advantages and
technical and industrial significance of this invention will be
better understood by reading the following detailed description of
presently preferred embodiments of the invention, when considered
in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIGS. 1A through 1C are diagrammatic section views showing
processing steps for fabricating a semiconductor device according
an embodiment of the invention;
[0042] FIG. 2 is a graph showing light output-current
characteristics of the semiconductor devices of the invention and
of a comparative example;
[0043] FIG. 3 is a graph showing histograms of short-wavelength
shift amounts of the semiconductor devices of the embodiments of
the invention and of the comparative example;
[0044] FIG. 4 is a side section view showing a laser module
structure using the semiconductor device of the embodiment of the
invention; and
[0045] FIG. 5 is a diagrammatic section view showing a
semiconductor device having a prior art structure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0046] An embodiment of a semiconductor device of the present
invention will be explained below with reference to the drawings.
It is noted that the present invention is not limited by this
embodiment. Still more, the same or corresponding components will
be denoted by the same reference numerals throughout the
drawings.
[0047] FIGS. 1A through 1C show processing steps for fabricating
the semiconductor device in which current blocking layers composing
current blocking regions are buried and grown on a n-type InP
substrate by metal-organic vapor phase epitaxy according to the
embodiment of the invention. It is noted that an n-type InP
substrate in which sulfur (S) is doped as impurities and whose
density of dislocation is small may be used to grow high-quality
crystals on the n-type InP substrate.
[0048] As shown in FIG. 1A, an n-type InP cladding layer 2, a
GaInAsP active layer 3, a p-type InP cladding layer 4 and a
p-GaInAsP cap layer 12 are grown sequentially on the n-type InP
substrate 1 and then an etching mask 13 composed of a SiN.sub.x
dielectric film is formed on the p-GaInAsP cap layer 12 in stripe
to form a mesa-stripe by means of wet-etching. A mesa having a
curved surface by which no specific plane orientation appears may
be formed in forming the mesa-stripe. Thereby, it becomes possible
to realize a width of the active layer that satisfies a
single-transverse mode with high control by controlling a width of
the mesa right under the SiN.sub.x dielectric film while confirming
by an optical microscope. It is noted that there is no problem even
if the mesa-stripe is formed by combining the wet etching with dry
etching.
[0049] Here, as a crystal growing method, molecular beam epitaxy
(MBE) and chemical beam epitaxy (CBE) may be used beside the
metal-organic vapor phase epitaxy. Still more, SiO.sub.2 and the
like may be used for the etching mask.
[0050] The GaInAsP active layer 3 may be composed of a multiple
quantum well structure composed of five layers of GaInAsP
compressive strain quantum well layer (thickness: 4 nm) of around
1% of lattice mismatching degree and GaInAsP barrier layer
(thickness: 10 nm) whose composition wavelength is 1.2 .mu.m and
six layers of separate-confinement hetero-structure (SCH) layer
composed of GaInAsP whose composition wavelength varies from 0.95
.mu.m to 1.2 .mu.m and whose each layer is 40 nm thick. Still more,
when the lattice mismatching degree of the active layer is to be
increased, a strain compensated structure in which net amount of
strain of the quantum well layer is reduced by using GaInAsP having
the lattice mismatching degree that causes tensile strain in the
barrier layer may be used.
[0051] Next, by utilizing the characteristics of the metal-organic
vapor phase epitaxy that no crystal is grown on the etching mask 13
composed of dielectrics, the current blocking regions composed of
the p-type InP current blocking layer 5 and the n-type InP current
blocking layer 6 are formed in the regions other than the upper
surface of the mesa-stripe and the current blocking regions on the
side surface of the mesa-stripe are grown so that they are almost
leveled with the etching mask 13.
[0052] In the p-type InP current blocking layer 5, preferably zinc
(Zn) is added as the p-type dopant by more than 0.8.times.10.sup.18
cm.sup.-3 and less than 1.4.times.10.sup.18 cm .sup.-3. When the
concentration of zinc of the p-type InP current blocking layer 5 is
less than 0.8.times.10.sup.18 cm.sup.-3, reactive current flowing
from III to IV shown in FIG. 1C is generated during driven with
high-current, reducing the light output. When the concentration of
zinc exceeds 1.4.times.10.sup.18 cm.sup.-3 on the other hand,
absorption of light distributed in the current blocking region
increases, thus increasing oscillation threshold value and dropping
the external quantum efficiency. It is essential to reduce the
absorption of light in the current blocking region and is required
to lower the p-type impurity concentration as much as possible to
realize high-output operation like a semiconductor pumped laser.
Accordingly, from an aspect of achieving the both reduction of the
leak current in the current blocking region and the reduction of
the absorption of light, the concentration of zinc in the p-type
InP current blocking layer is preferable to be more than
1.times.10.sup.18 cm .sup.-3 and less than 1.2.times.10.sup.18 cm
.sup.-3.
[0053] The n-type InP current blocking layer 6 enters the inside of
the p-type InP cladding layer 4, i.e., the n-type InP current
blocking layer 6 rides over the p-type InP cladding layer 4, so
that a distance W1 between the n-type InP current blocking layers 6
composing the current blocking region is narrower than a width W2
between the p-type cladding layers 4 contacting with the n-type InP
current blocking layer 6.
[0054] As a result of ardent study, the inventor et al. of the
present invention found that the structure of the n-type InP
current blocking layer 6 described above may be obtained by
optimizing growth temperature of the current blocking region. This
structure is considered to be obtained because the shape of part of
the side surface of the mesa-stripe of the p-type InP cladding
layer 4 and the p-GaInAsP cap layer 12 transforms due to a
mass-transport phenomenon by which group III atoms are diffused in
burying and growing the p-type InP current blocking layer 5 and the
n-type InP current blocking layer 6 composing the current blocking
region.
[0055] More specifically, the inventor et al. confirmed by
experiments that this may be achieved with the growth temperature
of 672.degree. C. or more. This may be approximated by a linear
function of temperature, as follows;
W1-W2(.mu.m)=-0.0092.times.T+6.18 (1)
[0056] It is noted that T denotes the growth temperature and is
temperature of a tray for mounting a wafer measured by a radiation
thermometer. Accordingly, the higher the growth temperature T, the
narrower the distance W1 than the distance W2 may be.
[0057] Although the smaller the difference W1-W2 between the
distance W1 between the n-type InP current blocking layers 6 and
the width W2 of the p-type InP cladding layer 4 contacting with the
n-type InP current blocking layer 6, the lower the leak current may
be, it is desirable to be -0.5 .mu.m or more because the light
output drops if it is too small. This is considered to occur
because zinc (Zn) that is the impurity within the p-type cladding
layer contacting with the n-type InP current blocking layer 6 is
diffused due to heat in growing the current blocking layer, the Zn
concentration within the p-type cladding layer is reduced, electric
resistance of the device is increased and thermal saturation of the
light output becomes dominant. Accordingly, the difference W1-W2 is
desirable to be more than -0.2 .mu.m and less than 0 .mu.m in order
to suppress the saturation of light output by the thermal
saturation and to realize high reliability. Still more another
problem occurs as the distance W1 becomes narrow that a gain
spectrum determining a laser oscillation state is shifted to the
short wavelength side and oscillation wavelength is shortened.
[0058] Here, the shortwave shift of the emission wavelength means
that there is a problem that it is difficult to realize a desirable
wavelength, leading to a drop of manufacturing yield, when the
growth temperature varies per manufacturing batch and when there is
in-plane distribution in the substrate depending on the growth
temperature in a process for fabricating a semiconductor laser
device for example. Specifically, a difference between a gain
wavelength and diffractive grating wavelength varies in a DFB laser
having a diffractive grating near the active layer and a DBR laser
having a diffractive grating in a light waveguide around the active
layer, causing also another problem of characteristic degradation
of an increase of a laser oscillation threshold value and a drop of
efficiency. The shortwave shift is considered to be caused by
different atoms diffused at the interface of a quantum well and a
barrier layer composing the active layer due to the heat of the
growth temperature in growing again on after the second growth when
a plurality of times of crystal growing process is included in
fabricating a laser and the like having the buried hetero
structure. It is known that the short wavelength shift is generated
when a GaInAsP material is used in particular by mutual diffusion
of the different group V atoms, i.e., As atoms and P atoms.
[0059] Then, in order to have the effect of reducing the leak
current flowing through the current blocking region and to reduce
the influence of the short wavelength shift by the growth
temperature of the current blocking layer, preferably the thickness
of the well layer of the GaInAsP active layer 3 is set to be 6 nm
or more. It is noted that the thicker the thickness of the well
layer, the less the influence of the mutual diffusion of the group
V becomes at the interface between the well layer and the barrier
layer in principle. Then, while the inventor et al. verified this
in the invention, it was experimentally verified that there is no
difference in the amount of short wavelength shift when the
thickness of the well layer is 6 nm or more. Still more, when a
quantum well layer composed of around 1% of compressed strain
superlattice is applied to the active layer, deterioration of
crystal quality caused by faults due to lattice mismatching is
concerned as the well layer is thickened. Accordingly, the
thickness of the well layer is desirable to be 10 nm or less. It is
noted that no short wavelength shift occurs when AlGaInAs is used
as a material composing the active layer, instead of GaInAsP. This
is considered because the mutual diffusion of As atoms and P atoms
that is the cause the short wavelength shift occurs at the
interface between the quantum well layer and the barrier layer
since the group V atom composing the active layer is only As
atom.
[0060] After the process shown in FIG. 1B, the etching mask 13 and
the p-GaInAsP cap layer 12 are removed by etching and a p-type InP
cladding layer 7 and a p-type GaInAsP contact layer 8 whose
composition wavelength is 1.2 .mu.m are grown by using the
metal-organic vapor phase epitaxy as shown in FIG. 1C. Then, in
order to adjust the thickness of the substrate, the under surface
of the n-type InP substrate 1 (the surface on the opposite side
from the surface on which the respective semiconductor layers are
formed as described above) is thinned by polishing around to 130
.mu.m of thickness and an n-side electrode 9 is formed under the
n-type InP substrate 1 and a p-side electrode 10 is formed on the
p-type GaInAsP contact layer 8 respectively as shown in FIG. 1C. It
is noted that molecular beam epitaxy and chemical beam epitaxy may
be used for the method for growing crystals in this process.
[0061] The semiconductor device shown in FIG. 1C of the present
embodiment fabricated as described above reduces the contact of the
p-type InP current blocking layer 5 with the p-type InP cladding
layer 7 in the mesa region as compared to the prior art
semiconductor device shown in FIG. 5 by entering the part of the
n-type InP current blocking layer 6 to the inside of 7. Due to
that, the leak current otherwise flowing through the path I-II may
be reduced.
[0062] Next, a laser module assembled as a batter-fly package by
using the semiconductor device of the invention will be explained.
FIG. 4 is a side elevational section view showing a structure of
the semiconductor laser module. A semiconductor laser device 29
constructed as junction-down structure in which a p-type electrode
is junctioned to a laser mount 25. A temperature controlling module
27 is disposed as a temperature controller on the bottom of the
inside of a package 28 formed of ceramics or the like as a case of
the semiconductor laser module.
[0063] A base 24 is disposed on the temperature controlling module
27 and the laser mount 25 is disposed on the base 24. While
electric current not shown is applied to the temperature
controlling module 27 to cool or heat depending on its polarity,
the temperature controlling module 27 functions mainly as a cooler
to prevent oscillation wavelength drift caused by an increase of
temperature of the semiconductor laser device 29. That is, when
laser light has a wavelength longer than a desirable wavelength,
the temperature controlling module 27 controls the temperature of
the laser mount by cooling it down and when the laser light has a
wavelength shorter than the desirable wavelength, controls the
temperature by heating.
[0064] Specifically, this temperature is controlled based on a
detected value of a thermistor 26 disposed in the vicinity of the
semiconductor laser device 29 on the laser mount 25 and a
controller not shown controls the temperature controlling module 27
so that the temperature of the laser mount 25 is kept constant.
Further, the controller not shown controls the temperature
controlling module 27 so that the temperature of the laser mount 25
drops as driving current of the semiconductor laser device 29
increases. The output stability of the semiconductor laser device
29 may be improved and the yield may be improved by performing the
temperature control as described above. It is noted that the laser
mount 25 is desirable to be formed by a material having a thermal
conductivity such as diamond for example. It is because heat
developed when high electrical current is applied may be suppressed
when the laser mount 25 is formed of diamond.
[0065] Disposed on the base 24 are the laser mount 25 on which the
semiconductor laser device 29 and the thermistor 26 are disposed, a
first lens 30 and a light monitoring light receiving device 23. The
laser light outputted out of the semiconductor laser device 29 is
guided to an optical fiber 22 through the first lens 21, an
isolator 31 and a second lens 21. The second lens 21 is provided on
an optical axis of the laser light and on a package 28 and is
optically coupled with the optical fiber 22. It is noted that the
light monitoring light receiving device 23 monitors and detects
light leaked from the high reflection film side of the
semiconductor laser device 29.
[0066] The isolator 31 is interposed between the semiconductor
laser device 29 and the optical fiber 22 so that no reflected
return light reflected by other optical parts and the like returns
to the resonator in this semiconductor laser module. It is noted
that although the isolator whose extinction ratio is -20 dB is used
here, it is preferable to use an isolator whose extinction ratio is
less than -20 dB to suppress reflection from parts composing the
system.
First Embodiment
[0067] An embodiment of the invention will be explained next. After
forming the n-type InP cladding layer 2 of lum thick, the GaInAsP
active layer 3 and the p-type InP cladding layer 4 of 0.5 .mu.m
thick and the p-GaInAsP cap layer 12 of 0.05 .mu.m thick whose
compositional wavelength is 1.2 .mu.m thick sequentially on the
n-type InP substrate 1 in which sulfur (S) is doped as impurity at
670.degree. C., the etching mask 13 composed of the SiNx dielectric
film is formed on the p-GaInAsP cap layer 12 of the stripe to form
the mesa-stripe by using wet etching. In forming the mesa-stripe,
the mesa is formed so as to have the curved shape by which no
specific plane orientation appears. The GaInAsP active layer 3 is
composed of the multiple quantum well structure composed of five
layers of GaAsP compressed strain quantum well layer of 4 nm thick
whose lattice mismatching degree is around 1% and a GaInAsP bather
layer of lOnm thick whose compositional wavelength is 1.2 .mu.m and
six layers of multistage optical confinement layer composed of
GaInAsP in which each layer is 40 nm thick whose compositional
wavelength varies from 0.95 .mu.m to 1.2 82 m. The composition of
the quantum well layer is determined so that the oscillation
wavelength turns out 1480 nm
[0068] Next, the current blocking regions composed of the p-type
InP current blocking layer 5 of 0.9 .mu.m thick to which
1.times.10.sup.18 cm.sup.-3 of zinc (Z) is doped and the n-type InP
current blocking layer 6 to which 1.times.10.sup.18 cm.sup.-3 of
sulfur (S) is doped are formed in the regions other than the upper
surface of the mesa-stripe by using the etching mask 13 and the
current blocking regions on the side surface portions of the
mesa-stripe are grown so that they are substantially leveled with
the etching mask 13. The growth temperature at this time was
694.degree. C.
[0069] After that, the etching mask 13 and the p-GaInAsP cap layer
12 are removed by means of etching and the p-type InP cladding
layer 7 and the p-type GaInAsP contact layer 8 whose composional
wavelength is 1.2 .mu.m are grown by about 3 .mu.m by using the
metal-organic vapor phase epitaxy under 670.degree. C. of growth
temperature. Then, the semiconductor device of the present
embodiment is achieved by thinning the under surface of the n-type
InP substrate 1 by polishing around to 130 .mu.m and forming an
n-side electrode 9 under the substrate 1 and an p-side electrode 10
on the upper surface of the p-type GaInAsP contact layer 8,
respectively, as shown in FIG. 1C. As a result of observation of
the distance W1 between the n-type InP current blocking layers 6 of
this semiconductor and the width W2 of the p-type InP cladding
layer 4 contacting with the n-type InP current blocking layer 6 by
means of sectional SEM (scanning electron microscope), the distance
W1 was narrower than the width W2 by 0.2 .mu.m in the present
embodiment.
[0070] In order to evaluate laser characteristics of the
semiconductor device of the embodiment fabricated as described
above, the wafer after forming electrodes is cleaved into a bar of
1500 .mu.m of resonator length. Then, a dielectric reflection film
of 1.5% of reflectivity was formed on one end surface thereof and a
dielectric reflection film of 98% of reflectivity was formed on the
other end surface. After that, the device was isolated into
individual devices. It is noted that the resonator length here is
1500 .mu.m, it is desirable to have resonator length of 1500 .mu.m
in order to suppress increases of radiation and of temperature of
the active layer and to assure the reliability of the device.
However, because the longer the resonator length, the lower the
slope efficiency is, the both high output and high reliability may
be realized by reducing the internal loss of the laser and by
optimizing the reflectivity of the output end surface of the laser
light.
[0071] In order to realize higher output characteristics, it is
effective to minimize a difference of between the refractive index
of InP and equivalent refractive index of the laminate structure
composing the laser and to widen a cutoff width of
single-transverse mode. In order to realize this, the cutoff width
may be widened to 5 .mu.m or more by adopting the n-type cladding
layer composed of GaInAsP and having a thickness of 4 .mu.m or more
instead of the n-InP cladding layer adopted in the present
embodiment.
[0072] It is noted that while the reflectivity of the reflection
end surface of the laser light is 98%, the laser light from this
end surface is utilized as a monitor light for stabilizing the
output. Accordingly, while the reflectivity is determined
corresponding to a system to be applied, there will be no problem
in terms of characteristics if the reflectivity is 90% or more as a
high-output laser device.
[0073] Light output-current characteristics were measured as laser
characteristics of the semiconductor device of the present
embodiment. FIG. 2 shows its result. Still more, threshold current,
slope efficiency in the vicinity of the threshold value and light
output when driven by 1.5 A were measured. Table 1 shows its
results.
FIRST COMPARATIVE EXAMPLE
[0074] In order to compare the laser characteristics of the
semiconductor device of the invention, a semiconductor device
having the prior art structure shown in FIG. 5 was fabricated as a
first comparative example. It is noted that in order to compare the
laser characteristics caused by the difference of the buried
structure, the structures other than the buried shape, i.e., the
laminate structure, resonator length, the reflectivity of the
dielectric reflection film provided at the end face and others of
the laser were made to be the same with those of the embodiment. As
a result of sectional SEM observation of the distance W1 between
the n-type InP current blocking layers 6 of the semiconductor
device of the comparative example and the width W2 of the p-type
InP cladding layer 4 contacting with the n-type InP current
blocking layer 6, the distance W1 was wider than the width W2 by
0.4 .mu.m. Similarly to the semiconductor device of the embodiment,
light output-current characteristics, threshold current, slope
efficiency in the vicinity of the threshold value and light output
when driven by 1500 mA were measured as the laser characteristics
also for the semiconductor device of the comparative example. Table
1 also shows its results.
[0075] From FIG. 2 and Table 1, it can be seen that the
semiconductor device of the first embodiment realizes low threshold
current, high slope efficiency and high-output operation as
compared to the semiconductor device of the first comparative
example.
TABLE-US-00001 TABLE 1 FIRST COMPARATIVE EMBODIMENT EXAMPLE
THRESHOLD CURRENT (mA) 29.3 32.5 SLOPE EFFICIENCY (mW/mA) 0.450
0.415 LIGHT OUTPUT 527.2 484.3 (when driven by 1.5 A)
[0076] It is noted that when a semiconductor device in which only a
width of the mesa-stripe is changed from the first embodiment was
fabricated, it was confirmed that no discontinuous point (kink) is
generated in the optical-current output characteristics in which
the single-transverse mode is kept until the width of the
mesa-stripe, i.e., the width of the active layer is 3 .mu.m. It is
noted that although the present embodiment adopts the straight
mesa-stripe structure in which the width of the active layer is
equal with respect to the light propagation direction, there is no
problem even if a tapered stripe structure that keeps the
single-transverse mode in which the width of the stripe varies with
respect to the light propagation direction is adopted.
[0077] Still more, a semiconductor device of a second embodiment in
which the distance W1 between the n-type InP current blocking
layers 6 is narrower than the width W2 of the p-type InP cladding
layer 4 contacting with the n-type InP current blocking layer 6 by
0.5 .mu.m was obtained by forming the semiconductor device
similarly to the semiconductor device of the first embodiment by
raising the growth temperature in forming the current blocking
layers to 730.degree. C. Light output of the semiconductor device
of the second embodiment was equal with that of the semiconductor
device of the first embodiment. However, while a current value
saturating the light output was 1700 mA in the first embodiment, it
was 1500 mA in the second embodiment. It was considered to have
occurred due to the diffusion of zinc in the p-type cladding layer
increased in the second embodiment more than that of the first
embodiment and due to the increased influence of thermal saturation
of the light output due to the increase of resistance of the device
by growing the current blocking layers at 730.degree. C. of growth
temperature.
[0078] As a result of mutual comparison of the characteristics of
the modules composed of the semiconductor devices of the first
embodiment and of the first comparative device, while the
semiconductor device of the comparative example outputs 368 mW when
operated with 1.5 A of direct current, the semiconductor device of
the first embodiment outputs 400 mW. Accordingly, it was confirmed
that the semiconductor device of the first embodiment is effective
in the high-output operation also in the module. It was also
confirmed that no kink is generated in the light output-current
characteristics in a wide output range up to 400 mW after
oscillating laser and that the semiconductor device of the first
embodiment is effective as a pumping light source capable of
amplifying from a less number of signals to a large number of
signals. It was confirmed that the semiconductor device of the
first embodiment operates in high-output as compared to the first
comparative example also in a laser module with a fiber grading in
which a diffractive grating is formed in part of the optical fiber
in a structure not using the isolator as a pumping light source for
Raman amplifier. The stability of wavelength was confirmed in the
output up to 400 mW after oscillation of laser.
[0079] Semiconductor devices of third and fourth embodiments having
the same growth conditions and structure with those of the first
embodiment except of that layer thickness and a number of quantum
well are set as shown in Table 2 were fabricated. A second
comparative example shown in Table 2 has the active layer having
the same structure with that of the first embodiment. It is noted
that although the diffractive grating layer was composed of GaInAsP
and was disposed between the GaInAsP active layer 3 and the p-type
InP cladding layer 4 described above in the first embodiment, it
may be formed also in the GaInAsP active layer 3.
TABLE-US-00002 TABLE 2 LAYER THICKNESS NUMBER OF WAVELENGTH OF
RESONANCE OF QUANTUM QUANTUM THE DIFFRACTIVE WAVELENGTH WELL (nm)
WELL GRATING (nm (.mu.m) THIRD 6 6 1550 300 EMBODIMENT FOURTH 7 5
EMBODIMENT SECOND 4 8 COMPARATIVE EXAMPLE
[0080] FIG. 3 shows results of histograms of a short-wavelength
shift amount (=before forming current blocking layer--after forming
current blocking layer) compared in the respective structures by
means of photoluminescence measurement before and after forming the
current blocking layer. It is noted that measuring spots were
sampled from a 2-inch wafer and the short-wavelength shift amount
was denoted by energy (meV). It is noted that Table 3 below
indicates a number of measurements, median values and standard
deviation of the respective structures.
TABLE-US-00003 TABLE 3 MEDIAN STANDARD NUMBER OF VALUE DEVIATION
MEASUREMENTS (meV) (meV) THIRD 76 6.2 5.5 EMBODIMENT FOURTH 84 6.7
2.2 EMBODIMENT SECOND 71 15.5 9.2 COMPARATIVE EXAMPLE
[0081] It can be seen from FIG. 3 that the short-wavelength shift
amount of the both third and fourth embodiments is reduced and its
distribution is small as compared to those of the second
comparative example. From this point, it can be also seen that it
is effective to thicken the layer thickness of the quantum well
like the third and fourth embodiments to achieve the both reduction
of the leak current of the current blocking layer and of the
short-wavelength shift. Meanwhile, it can be also seen from Table 3
that the short-wavelength shift amount of the both structures A and
B of the embodiments is reduced by about 9 meV and the standard
deviation of the third and fourth embodiments is remarkably reduced
as compared to the second comparative example as the thickness of
the well layer is increased to 6 nm and 7 nm
[0082] Still more, a dielectric multi-layer film having 0.1% of
reflectivity was formed on the laser output end surface of the
device and that having 98% of reflectivity was formed on the
reflection end surface of the laser to implement characteristic
evaluation of the laser (DFB laser). As a result, effects of
reducing the threshold current and of improving the slope
efficiency were confirmed in the third and fourth embodiments.
Still more, while a yield of the semiconductor device whose
threshold current is 9 mA or less and whose slope efficiency is 0.3
W/A or more was around 40% in the second comparative example, it
was around 60% and was remarkably improved in the third and fourth
embodiments. It can be seen from what described above that the
embodiments of the invention not only suppress the short-wavelength
shift amount but also are effective in improving the in-plane
uniformity and bring about effects in improving the yield and
lowering cost of the laser device.
[0083] It is noted that when a semiconductor device having the same
structure and growth condition with those of the first embodiment
(second comparative example) described above except of that the
material composing the active layer is AlGaInAs was fabricated and
when the short-wavelength shift amount was evaluated in the same
manner with the third and fourth embodiments and the second
comparative example by means of the photoluminescence measurement
before and after forming the current blocking layer, a remarkable
effect of suppressing the short-wavelength shift of around 1 meV
was obtained in the semiconductor device as compared to the second
comparative example of the second embodiment whose shift amount was
15.5 meV. Further, when characteristic evaluation of the laser (DFB
laser) was carried out, while the characteristics were the same
with the equal detuning amount, the same characteristics with the
second comparative example was realized even in 95.degree. C.,
while the second comparative example was operated only up to
85.degree. C. It is noted that the detuning amount is defined by a
difference between a wavelength of the diffractive grating and a
peak wavelength of laser gain spectrum. Accordingly, it can be seen
that introducing the AlGaInAs material into the active layer brings
about the effects of not only suppressing the short-wavelength
shift but also improving the high-temperature characteristics. It
is noted that although the explanation was made by exemplifying the
DFB laser here, it is needless to say that the same effect is
brought about even in the pumping light source for optical fiber
amplifier using AlGaInAs as the material of the active layer.
Specifically, because it becomes possible to operate in
high-temperature by using AlGaInAs as the material of the active
layer more than the prior art active layer using the GaInAsP
material, it becomes possible to obtain advantages that power
consumption of the electronic cooler using a Peltier device may be
cut by operating the device in high-temperature and that a highly
reliable uncooled high-output laser may be realized in a
communication wavelength band of 1200 to 1650 nm
[0084] As described above, the semiconductor device of the
invention has the advantages that it reduces the leak current
flowing through the current blocking region and that enables one to
realize the device that is less influence by the short wavelength
shift due to the growth temperature of the current blocking layer.
Therefore, the semiconductor device of the invention can realize
the low threshold current and highly efficient and high-output
operation. Still more, the invention has such advantages that it
enables to realize the structure of the invention readily by the
growth temperature of the current blocking region without
complicated process in forming the mesa in growing the current
blocking region.
[0085] Specifically, although the semiconductor device in which the
material composing the active layer is GaInAsP has had a problem
that the longer the oscillation wavelength generated from the
semiconductor device, the harder the high-output operation becomes
due to the Auger effect, the high-output semiconductor device of
the invention is effective as a semiconductor device for the
pumping light source for erbium doped fiber amplifier (EDFA) of
1460 nm or more and for the pumping light source for Raman
amplifier that is required to output high-output of 300 mW or more.
When compared with the equal light output in particular, the
semiconductor device of the invention is advantageous in terms of
high-reliability because it suppresses the increase of the
temperature of the active layer by reducing the driving current.
Further, the invention may realize the high-performance
semiconductor device without increasing a number of processing
steps as compared to that of the process for fabricating the
semiconductor device having the prior art structure.
[0086] Still more, the pumping light source for the fiber amplifier
described above has the structure in which the semiconductor
device, the optical system such as a lens, the isolator and the
output controlling monitor light receiving device are disposed on
the electronic cooling system disposed within the n-type InP
substrate 14-pin butterfly package and a light beam is coupled to
the optical fiber through the optical system. Because it becomes
possible to operate efficiently with the low threshold value as
compared to the semiconductor device having the prior art structure
by using the semiconductor device of the invention, the power
consumption may be cut and that of the electronic cooling system
may be cut. Still more, because the temperature of the active layer
may be reduced, the semiconductor device of the invention has such
an advantage that the long-term reliability of the laser module may
be improved. Further, although intensity of excitation light to the
fiber amplifier must be changed corresponding to a number of
signals to be amplified when the semiconductor device is used in an
optical communication system, the semiconductor device of the
invention has such an effect that because it permits the
high-output operation as compared to the prior art one, a number of
signals to be amplified may be increased.
[0087] The dielectric reflection film of the invention may be
formed by using chemical vapor deposition (CVD) and sputtering. It
is noted that the dielectric film having 1.5% of reflectivity has
been used as the dielectric reflection film having low reflectivity
in the embodiments described above, it is preferable to use a
dielectric multi-layered film having low reflectivity of 0.1% or
less in a case of a semiconductor device having a complex resonator
combined with an outside reflecting mirror to suppress Fabry-Perot
resonance in the semiconductor device.
[0088] The material composing the active layer of the semiconductor
device of the invention may be AlGaInAsP, GaInNAs and AlGaInN,
instead of GaInAsP, and may be appropriately selected corresponding
its use and purpose. Still more, the substrate (InP substrate, GaAs
substrate, GaInAs substrate and the like) suitable for the
respective materials may be appropriately used.
[0089] The active layer in the semiconductor device of the
invention has the strain quantum well structure and may be
accompanied by quantum dot, quantum wire and quantum dash
structure. Still more, the optical confinement structure may be a
single-stage SCH layer or a GRIN-SCH (Grated-Index
Separate-Confinement Hetero-structure).
[0090] There is no problem even if the configuration of the
semiconductor device of the invention to an optical function
integrated device such as a modulator integrated laser and
wavelength variable laser in which a plurality of functions are
integrated on one and same substrate.
[0091] Although the invention has been described with respect to
specific embodiments for a complete and clear disclosure, appended
claims are not to be thus limited but are to be construed as
embodying all modifications and alternative constructions that may
occur to one skilled in the art that fairly fall within the basic
teaching herein set forth.
* * * * *