U.S. patent application number 10/768922 was filed with the patent office on 2004-09-30 for sequential mesa avalanche photodiode capable of realizing high sensitization and method of manufacturing the same.
This patent application is currently assigned to ANRITSU CORPORATION. Invention is credited to Hiraoka, Jun, Mizuno, Kazuo, Sasaki, Yuichi.
Application Number | 20040188807 10/768922 |
Document ID | / |
Family ID | 26622450 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040188807 |
Kind Code |
A1 |
Hiraoka, Jun ; et
al. |
September 30, 2004 |
Sequential mesa avalanche photodiode capable of realizing high
sensitization and method of manufacturing the same
Abstract
A sequential mesa type avalanche photodiode (APD) comprises a
semiconductor substrate and a sequential mesa portion formed on the
substrate. In the sequential mesa portion, a plurality of
semiconductor layers, including a light absorbing layer and a
multiplying layer, are laminated by epitaxial growth. In the
plurality of semiconductor layers, a pair of semiconductor layers
forming a pn junction is included. The carrier density of a
semiconductor layer which is near to the substrate among the pair
of semiconductor layers is larger than the carrier density of a
semiconductor layer which is far from the substrate among the pair
of semiconductor layers. In the APD, light-receiving current based
on movement of electrons and positive holes generated in the
sequential mesa portion when light is incident from the substrate
toward the light absorbing layer is larger at a central portion
than at a peripheral portion of the sequential mesa portion.
Inventors: |
Hiraoka, Jun; (Atsugi-shi,
JP) ; Mizuno, Kazuo; (Zama-shi, JP) ; Sasaki,
Yuichi; (Atsugi-shi, JP) |
Correspondence
Address: |
FRISHAUF, HOLTZ, GOODMAN & CHICK, PC
767 THIRD AVENUE
25TH FLOOR
NEW YORK
NY
10017-2023
US
|
Assignee: |
ANRITSU CORPORATION
TOKYO
JP
|
Family ID: |
26622450 |
Appl. No.: |
10/768922 |
Filed: |
January 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10768922 |
Jan 30, 2004 |
|
|
|
10238362 |
Sep 9, 2002 |
|
|
|
Current U.S.
Class: |
257/623 ;
257/E31.021; 257/E31.038; 257/E31.064 |
Current CPC
Class: |
H01L 31/03042 20130101;
Y02E 10/544 20130101; H01L 31/1075 20130101; H01L 31/035281
20130101 |
Class at
Publication: |
257/623 |
International
Class: |
H01L 029/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 18, 2001 |
JP |
2001-284039 |
Jul 26, 2002 |
JP |
2002-218311 |
Claims
What is claimed is:
1. A sequential mesa type avalanche photodiode comprising: an
n-type semiconductor substrate; and a sequential mesa portion
formed on an upper part of the n-type semiconductor substrate, said
sequential mesa portion comprising a plurality of semiconductor
layers which include: an n-type light absorbing layer, an n-type
electric field relaxation layer formed on an upper part of the
n-type light absorbing layer, a p-type electric field concentration
layer formed on the n-type electric field relaxation layer, and a
p-type multiplying layer formed on an upper part of the p-type
electric field concentration layer and laminated by epitaxial
growth, wherein the p-type electric field concentration layer and
the n-type electric field relaxation layer form a pn junction,
wherein a carrier density of the n-type electric field relaxation
layer is larger than a carrier density of the p-type electric field
concentration layer, and wherein when light is incident from the
n-type semiconductor substrate toward the n-type light absorbing
layer, electrons and positive holes are generated in the sequential
mesa portion and positive holes are a main carrier, and there is a
single-peaked characteristic in which light-receiving current based
on movement of the electrons and the positive holes is larger at a
central portion of the sequential mesa portion than at a peripheral
portion of the sequential mesa portion.
2. The sequential mesa type avalanche photodiode according to claim
1, wherein the plurality of semiconductor layers include an n-type
buffer layer formed between the n-type semiconductor substrate and
the n-type light absorbing layer.
3. The sequential mesa type avalanche photodiode according to claim
1, wherein the n-type light absorbing layer comprises an
n.sup.--type InGaAs.
4. The sequential mesa type avalanche photodiode according to claim
1, wherein the plurality of semiconductor layers include a p-type
contact layer formed on the p-type multiplying layer.
5. The sequential mesa type avalanche photodiode according to claim
4, wherein the p-type contact layer comprises a p.sup.+-type
InGaAs.
6. The sequential mesa type avalanche photodiode according to claim
1, wherein the n-type semiconductor substrate comprises an
n.sup.+-type InP.
7. The sequential mesa type avalanche photodiode according to claim
6, wherein the n-type electric field relaxation layer comprises an
n.sup.+-type InP, the p-type electric field concentration layer
comprises a p.sup.--type InP, and the p-type multiplying layer
comprises a p.sup.--type InP.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a Divisional of U.S. application
Ser. No. 10/238,362 filed Sep. 9, 2002, which is based upon and
claims the benefit of priority from the prior Japanese Patent
Applications No. 2001-284039, filed Sep. 18, 2001; and No.
2002-218311, filed Jul. 26, 2002, the entire contents of both of
which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a sequential mesa avalanche
photodiode and a method of manufacturing the same, and in
particular, to a sequential mesa avalanche photodiode having a
sequential mesa structure in which, in an avalanche photodiode to
be used as a light receiving element for converting a light signal
to an electric signal in an optical communication network or the
like, high sensitization can be realized and the fabrication costs
of modularization can be greatly decreased, and to a method of
manufacturing the same.
[0004] 2. Description of the Related Art
[0005] As is well-known, recently, the signal speed of light
signals used in optical communication networks has been made much
more high-speed.
[0006] In accordance therewith, making the speed more high-speed
has been required of light receiving elements built in optical
communication equipment transmitting and receiving such light
signals.
[0007] Further, in such light receiving elements, it is required
that even low level light signals can be precisely received.
[0008] As such a light receiving element receiving high-speed and
weak light signals, generally, an avalanche photodiode
(hereinafter, abbreviated APD) has been put into practice.
[0009] In such an APD, in a state in which a depletion region is
formed by applying reverse-bias voltage to a pn junction formed by
a pair of semiconductor layers whose conductive types are different
from one another, when an electromagnetic wave of a light signal or
the like is incident from the exterior, a pair of an electron and a
positive hole is generated.
[0010] Further, this pair of the electron and the positive hole is
multiplied by the avalanche phenomenon in the APD, and taken out as
voltage or electric current to the exterior.
[0011] There are various ways of classifying APDs. When classifying
structurally, there are a planar type and a mesa type, and when
classifying by main carrier, there are a positive hole type and an
electron type.
[0012] Here, a sequential mesa structure used regardless of the
type of the main carrier will be described.
[0013] Generally, in order to aim for making the APD high-speed,
the mesa type, not the planar type, is generally used as the shape
of the APD.
[0014] This is for decreasing the electric capacity of the APD
element itself in order to make the APD high-speed.
[0015] In order to increase the permissible light-receiving current
as an APD element, there is the need to remove the bias of the
light-receiving current density flowing through the interior of the
mesa portion.
[0016] Therefore, in a mesa type APD element, the shape of the mesa
must be made to be isotropic, namely, as shown in FIG. 9B, formed
conically as viewed from the top surface of a substrate.
[0017] Moreover, in a mesa type APD element, when the shape of the
mesa is formed to be conical, attention must be paid such that the
crystallinity of the cross-section of the mesa is not damaged.
[0018] Therefore, in a mesa type APD element, when the shape of the
mesa is fabricated, diffusive wet-etching by an etchant which is
not anisotropic is necessary.
[0019] By applying this diffusive wet-etching, the sequential mesa
shape, which is a shape (generally, conical) in which the mesa
diameter (cross-sectional area) widens as it approaches the
substrate, can be obtained.
[0020] Accordingly, the sequential mesa type APD is generally used
for making the APD high-speed.
[0021] Further, as APDs using positive holes as the main carrier,
there are an APD in which the above-described pn junction is formed
by epitaxial growth, and an APD in which the pn junction is formed
by Zn diffusion.
[0022] FIGS. 9A and 9B respectively show a cross-sectional view and
an external perspective view of a sequential mesa type APD, in
accordance with a prior art, which has a sequential mesa structure
and in which positive holes are used as the main carrier and the pn
junction is formed by epitaxial growth.
[0023] Hereinafter, on the basis of FIGS. 9A and 9B, the structure
of the sequential mesa type APD according to the prior art will be
described.
[0024] Namely, in the sequential mesa type APD according to the
prior art, as shown in FIGS. 9A and 9B, an n-type buffer layer 2a,
an n-type light absorbing layer 3a, an n-type electric field
relaxation layer 4a, an n-type multiplying layer 5a, and a p-type
contact layer 6b are successively formed by epitaxial growth by
using an MOVPE (organometallic vapor phase epitaxial growth) method
on an n-type semiconductor substrate 1a. Therefore, a conical
sequential mesa portion 10 is formed by wet-etching from above.
[0025] Next, after a protective layer 7 is coated on the sequential
mesa portion 10, a p electrode 8 contacting the p-type contact
layer 6b is formed.
[0026] Further, at the both sides of the sequential mesa portion
10, an n electrode 9 is attached, via a protective layer 11, to
another mesa portion formed for attaching electrodes.
[0027] As shown by the arrow in FIG. 9A, light incident on the APD
from the bottom surface of the semiconductor substrate 1a
penetrates through the semiconductor substrate 1a and the buffer
layer 2a and is absorbed at the light absorbing layer 3a, so that a
pair of an electron and a positive hole is generated.
[0028] Among the pair of the electron and the positive hole
generated in this way, the electron moves to the n electrode 9 via
the semiconductor substrate 1a, and the positive hole is multiplied
at the multiplying layer 5a, and moves to the p electrode 8 via the
contact layer 6b.
[0029] In order to make the positive hole be the main carrier among
the pair of the electron and the positive hole, a great number of
the carriers of the light absorbing layer 3a must be electrons.
[0030] Namely, the conductive type of the light absorbing layer 3a
must be n type.
[0031] Such a sequential mesa type APD uses a so-called SAM
(Separate Absorption and Multiplication) structure, in which the
multiplying layer 5a and the light absorbing layer 3a are separated
by the electric field relaxation layer 4a such that a low electric
field intensity is applied to the light absorbing layer 3a while a
high electric field intensity is applied to the multiplying layer
5a.
[0032] In this case, because the electric field intensity of the
n-type light absorbing layer 3a is suppressed by the electric field
relaxation layer 4a, the conductive type of the electric field
relaxation layer 4a is the same n type as that of the light
absorbing layer 3a.
[0033] Because such a sequential mesa type APD has a function
avalanche-multiplying the light exciting carrier, the crystallinity
of the above-described layers is considered to be extremely
important.
[0034] Note that, in such a sequential mesa type APD, the epitaxial
growth itself of each layer can be carried out, in theory, on a
semiconductor substrate which is any of an n-type semiconductor
substrate, a p-type semiconductor substrate, or a semi-isolated
semiconductor substrate.
[0035] As described above, in the sequential mesa type APD, when
considering the fact that light-receiving current flows via the
semiconductor substrate, the semiconductor substrate which is used
must be an n-type or a p-type semiconductor substrate.
[0036] However, as shown in FIGS. 9A and 9B, because a dopant such
as Sn, S or the like included in the semiconductor substrate 1a
does not diffuse during the epitaxial growth, the n-type
semiconductor substrate 1a is suitable as a substrate for the
epitaxial growth of each semiconductor layer.
[0037] On the other hand, in the p-type semiconductor substrate,
there are problems such as the Zn included in the semiconductor
substrate diffuses during the epitaxial growth, there is the need
to form a thicker buffer layer by epitaxial growth in order to
prevent the Zn from diffusing, and because the n-type semiconductor
substrate layer is formed by epitaxial growth after the p-type
semiconductor substrate is formed by epitaxial growth, the time
after the epitaxial growth of the p-type semiconductor layer
becomes longer. Thus, diffusion of the Zn which is the dopant in
the p-type semiconductor layer formed by the epitaxial growth
easily arises.
[0038] Namely, the p-type semiconductor substrate having such
problems is not generally suitable for a sequential mesa type APD
in which crystallinity is considered to be extremely important.
[0039] Accordingly, it is preferable that the n-type semiconductor
substrate 1a is used as the sequential mesa type APD in order to
epitaxially grow a semiconductor layer having good quality
crystallinity.
[0040] In this way, in order to obtain a good light-receiving
characteristic in a sequential mesa type APD in which the positive
holes are the main carrier and the pn junction is formed by
epitaxial growth, the n-type light absorbing layer 3a and the n
type field relaxation layer 4a are necessary, and the semiconductor
substrate which is used is preferably the n-type semiconductor
substrate 1a.
[0041] Further, as described above, in a sequential mesa type APD
in which the positive holes are the main carrier and the pn
junction is formed by the epitaxial growth, as shown in FIGS. 9A
and 9B, the p-type contact layer 6b is used in order to ensure an
ohmic electrode in the p electrode 8.
[0042] At the time of epitaxial growth of the contact layer 6b, the
contact layer 6b is doped to p type by using a p-type dopant such
as Zn or the like.
[0043] Note that, in order to obtain the ohmic electrode, the
p-type carrier density of the contact layer 6b is preferably set to
be as high as possible, for example, about 5.times.10.sup.18
(cm.sup.-3) or more.
[0044] Note that the above-described MOVPE method or the like is
mainly used as a growth method (manufacturing method) of the
contact layer 6b.
[0045] Further, due to the Zn which is the dopant of the contact
layer 6b being diffused in the n-type electric field relaxation
layer 4a, the conductive type of the multiplying layer 5a is made
to be n type so that the appropriate internal electric field
intensity distribution in the direction perpendicular to the n-type
semiconductor substrate 1a is not destroyed.
[0046] Accordingly, the pn junction in the sequential mesa type APD
is formed by the p-type contact layer 6b and the n-type multiplying
layer 5a.
[0047] Note that, in this case, the carrier density of the p-type
contact layer 6b is particularly high as compared with the carrier
density of the n-type multiplying layer 5a.
[0048] Therefore, it is ideal that the sequential mesa type APD, in
which the positive holes are used as the main carrier and the pn
junction is formed by epitaxial growth, has the structure shown in
FIGS. 9A and 9B.
[0049] Namely, because the sequential mesa type APD basically does
not use a Zn diffusing process to be described later, there is the
advantage that the manufacturing process (the process steps) can be
simplified.
[0050] Further, because the sequential mesa type APD uses an n-type
semiconductor in the electric field relaxation layer 4a which is
difficult to be manufactured by a p-type semiconductor, there is
the advantage that MOVPE, which can epitaxially grow at the wafer a
semiconductor layer having high crystallinity, can be used as the
method of manufacturing the sequential mesa type APD.
[0051] Next, a sequential mesa type APD, which has a sequential
mesa structure and in which positive holes are used as the main
carrier and the pn junction is formed by Zn diffusion, will be
described.
[0052] The structure itself of such a sequential mesa type APD is
the same as the structure of the sequential mesa type APD shown in
FIGS. 9A and 9B.
[0053] As described above, in order to acquire excellent
characteristics at the sequential mesa type APD in which the
positive holes are the main carrier, the n-type light absorbing
layer 3a and the n-type electric field relaxation layer 4a are
necessary, and it is preferable to use the n-type semiconductor
substrate 1a. This is also true in the case of a sequential mesa
type APD in which the pn junction is formed by Zn diffusion, and in
the case of the above-described sequential mesa type APD, in which
the pn junction is formed by epitaxial growth.
[0054] Further, the contact layer 6b is made to be p type by
diffusing Zn therein by a Zn diffusion method in order to ensure an
ohmic electrode in the p electrode 8.
[0055] Note that, in order to obtain the ohmic electrode, the
p-type carrier density of the contact layer 6b is preferably set to
be as high as possible, for example, about 5.times.10.sup.18
(cm.sup.-3) or more.
[0056] Further, in the Zn diffusing method, by heating the Zn raw
material and the wafer contained in a container filled with an
inert gas atmosphere, the Zn is diffused from the surface of the
wafer to the interior of the wafer.
[0057] At this time, in order to carry out sufficient Zn diffusion,
there is the need to control the gas pressure of the inert gas
atmosphere so as to maintain a relatively high value by using an
exclusively-used controller, and there is the problem that the
manufacturing process (process steps) is complicated.
[0058] The Zn diffused in this way remains in the contact layer 6b,
and the p-type carrier density is enhanced to a degree at which an
ohmic electrode can be obtained, for example, to 5.times.10.sup.18
(cm.sup.-3) or more.
[0059] Note that, at this time, because the Zn is not diffused in
the multiplying layer 5a, the conductive type of the multiplying
layer 5a is maintained as n type.
[0060] In accordance therewith, the pn junction is formed by the
p-type contact layer 6b, in which the p-type carrier density is
increased by Zn diffusion, and the n-type multiplying layer 5a.
[0061] As a result, also in the case of a sequential mesa type APD
in which positive holes are used the main carrier and the pn
junction is formed by Zn diffusion, the structure shown in FIGS. 9A
and 9B is ideal.
[0062] Further, the sequential mesa type APD in which the pn
junction is formed by Zn diffusion has the advantage that the
desired pn junction can be formed by appropriately setting the
diffusing conditions of the Zn.
[0063] Further, the sequential mesa type APD in which the pn
junction is formed by Zn diffusion also has the advantage that,
because an n-type semiconductor is used as the electric field
relaxation layer 4a which is difficult to fabricate by a p-type
semiconductor, the MOVPE method, by which a highly crystalline
semiconductor layer can be epitaxially grown on the wafer, can be
used as the manufacturing method.
[0064] On the other hand, because the sequential mesa type APD uses
a Zn diffusing process, the sequential mesa type APD has the
drawback that the manufacturing process (process steps) is
complicated due to the above-described reasons.
[0065] Next, the sequential mesa type APD, which has a sequential
mesa structure and in which electrons are used as the main carrier
and the pn junction is formed by epitaxial growth, will be
described.
[0066] FIG. 10 shows a cross-sectional view of the sequential mesa
type APD which has a sequential mesa structure and in which
electrons are used as the main carrier and the pn junction is
formed by epitaxial growth.
[0067] Note that, in this FIG. 10, portions which are the same as
those of the sequential mesa type APD shown in FIG. 9A are denoted
by the same reference numerals.
[0068] Further, an external perspective view of the sequential mesa
type APD, which is shown in FIG. 10 and in which electrons are used
as the main carrier and the pn junction is formed by epitaxial
growth, is the same as in FIG. 9B, and thus, illustration is
omitted.
[0069] Namely, as shown in FIG. 10, in the sequential mesa type APD
in which electrons are used as the main carrier and the pn junction
is formed by epitaxial growth, after the n-type buffer layer 2a,
the n-type multiplying layer 5a, the p-type electric field
relaxation layer 4b, the p type light absorbing layer 3b, a p-type
window layer 13b, and the p-type contact layer 6b are successively
formed by epitaxial growth on the n-type semiconductor substrate 1a
by using an epitaxial growth method, the conical sequential mesa
portion 10 is formed by wet-etching from above.
[0070] Further, after the protective layer 7 is coated on the
sequential mesa portion 10, the p electrode 8 contacting the p-type
contact layer 6b is formed.
[0071] Further, on the both sides of the sequential mesa portion
10, the n electrodes 9 are attached, via the protective layer 11,
to another mesa portion formed for attaching electrodes.
[0072] In such a sequential mesa type APD in which electrons are
the main carrier, as shown by the arrow in FIG. 10, light incident
from the bottom surface of the semiconductor substrate 1a
penetrates through the semiconductor substrate 1a, the buffer layer
2a, the multiplying layer 5a, and the electric field relaxation
layer 4b and is absorbed at the light absorbing layer 3b, so that a
pair of an electron and a positive hole is generated.
[0073] Among the pair of the electron and the positive hole
generated in this way, the electron is multiplied at the
multiplying layer 5a and moves to the n electrode 9 via the n-type
semiconductor substrate 1a, and the positive hole moves to the p
electrode 8 via the contact layer 6b.
[0074] In order to make the electron be the main carrier among the
pair of the electron and the positive hole, a great number of
carriers of the light absorbing layer 3b must be positive
holes.
[0075] Namely, in this case, the conductive type of the light
absorbing layer 3b must be p type.
[0076] In such a sequential mesa type APD in which electrons are
the main carrier, the above-described SAM structure, in which the
multiplying layer 5a and the light absorbing layer 3b are separated
by the electric field relaxation layer 4b such that a low electric
field intensity is applied to the light absorbing layer 3b while a
high electric field intensity is applied to the multiplying layer
5a, is used.
[0077] In this case, because the electric field intensity of the p
type light absorbing layer 3b is suppressed by the electric field
relaxation layer 4b, the conductive type of the electric field
relaxation layer 4b is p type which is the same as that of the
light absorbing layer 3b.
[0078] Further, because such a sequential mesa type APD in which
electrons are the main carrier has a function avalanche-multiplying
the light exciting carrier, the crystallinity of the
above-described layers is considered to be extremely important.
[0079] In order to obtain excellent crystallinity of each
semiconductor layer, for the same reasons as in the case of the
sequential mesa type APD described in FIGS. 9A and 9B in which
positive holes are the main carrier, the semiconductor substrate
which is used is preferably the n-type semiconductor substrate
1a.
[0080] Moreover, in order to improve the accuracy of the electric
field intensity distribution in the sequential mesa portion 10 in
the direction perpendicular to the semiconductor substrate 1a,
because the pn junction is preferably formed between the p-type
electric field relaxation layer 4b and the multiplying layer 5a,
the multiplying layer 5a is n type.
[0081] Such a formed position of the pn junction is also preferable
for making estimation of the amount of decrease in the electric
field intensity in the multiplying layer 5a be unnecessary.
[0082] Accordingly, in the sequential mesa type APD in which
electrons are the main carrier, the pn junction is formed by the
p-type electric field relaxation layer 4b and the multiplying layer
5a.
[0083] In this way, in order to obtain excellent light-receiving
characteristics in a sequential mesa type APD in which the
electrons are the main carrier and the pn junction is formed by
epitaxial growth, the p type light absorbing layer 3b, the p-type
electric field relaxation layer 4b, and the n-type multiplying
layer 5a are necessary, and the semiconductor substrate which is
used is preferably the n-type semiconductor substrate 1a.
[0084] In such a sequential mesa-type APD, the window layer 13b
also is necessary in order to prevent the electrons which are a
light exciting carrier from diffusing/moving to the contact layer
6b.
[0085] Note that GS-MBE (gas-molecule beam epitaxy), MBE (molecule
beam epitaxy), and the like are mainly used as the epitaxial growth
method.
[0086] Further, in order to ensure the ohmic electrode of the p
electrode 8, the conductive type of the contact layer 6b is p
type.
[0087] Moreover, at the time of epitaxial growth, the contact layer
6b is doped to a p type by using a p-type dopant such as Be or the
like.
[0088] Note that, in order to obtain the ohmic electrode, the
p-type carrier density of the contact layer 6b is preferably set to
be as high as possible, for example, about 5.times.10.sup.18
(cm.sup.-3) or more.
[0089] Accordingly, in a sequential mesa type APD in which
electrons are the main carrier, the structure shown in FIG. 10 is
ideal, and because electrons having a light effective mass are the
main carrier, there is the feature that it is advantageous with
respect to the point of high-speed performance.
[0090] However, in the APDs having the sequential mesa structures
shown in FIGS. 9A, 9B and FIG. 10, there are still the following
problems which must be improved.
[0091] Firstly, in the sequential mesa type APD in which positive
holes are the main carrier, or also in the sequential mesa type APD
in which electrons are the main carrier, there is the problem that,
in each semiconductor layer forming the sequential mesa portion 10,
except for the case of selectively diffusing Zn at a specific
portion in the surface parallel to the semiconductor substrate, it
is difficult for the in-surface distribution of electric field
intensity in a surface parallel to the semiconductor substrate to
concentrate at the central portion of the mesa by only the
epitaxial growth process.
[0092] FIG. 3 shows measured results of the light-receiving
sensitivity distribution characteristic of a sequential mesa type
APD whose light-receiving diameter is 40 .mu.m.
[0093] Concretely, FIG. 3 shows measured values of light-receiving
current (.mu.A) obtained between the p electrode 6 and the n
electrode 9 at each position (.mu.m) in a case in which the
irradiating position of an extremely thin light beam is
successively moved within the aforementioned range of 40 .mu.m.
[0094] In FIG. 3, characteristic B shows the light-receiving
sensitivity distribution characteristic of the sequential mesa type
APD as shown in FIGS. 9A and 9B.
[0095] As illustrated, characteristic B is a double-peaked
characteristic in which the light-receiving current at the
peripheral portion of the mesa shown by the positions -20 .mu.m,
+20 .mu.m from the central position (0) is larger than the
light-receiving current at the central portion of the mesa.
[0096] A sequential mesa type APD whose light-receiving
characteristic is a double-peaked characteristic in this way has
the problem that it is difficult to align the optical axes at the
time of actual use when made into a module, and the yield of the
modularization deteriorates. Because alignment of the optical axes
must be carried out at the central portion of the mesa at which the
light-receiving current is smaller than that of the peripheral
portion of the mesa, a sufficient light-receiving characteristic
cannot be exhibited. In addition, it is difficult to realize high
sensitization by keeping to a minimum the effects of the dark
current and noise contained in the light-receiving signal relating
to the problem of crystallinity described later, and to decrease
the fabricating costs of modularization.
[0097] Hereinafter, reasons why these problems arise will be
described.
[0098] Because the APD shown in FIGS. 9A and 9B is a sequential
mesa type structure, the more the electric field intensity
increases, the more the carrier of the positive holes or the
electrons is multiplied.
[0099] Accordingly, the magnitude of the light-receiving current
shows the magnitude of the electric field intensity at the pn
junction.
[0100] It can be said that the electric field intensity at the
periphery of the mesa is higher and the electric field intensity at
the central portion of the mesa is low in the sequential mesa type
APD shown in FIGS. 9A and 9B.
[0101] FIG. 11 shows the way of broadening (width) of the depletion
region (depletion layer) by built-in potential from the pn junction
in the sequential mesa type APD shown in FIGS. 9A and 9B in which
positive holes are used as the main carrier.
[0102] Note that, as described above, because the carrier density
of the p-type contact layer 6b forming the pn junction is higher
than the carrier density of the multiplying layer 5a, the majority
of the depletion region (depletion layer) is formed at the
semiconductor substrate 1a side of the pn junction.
[0103] As shown in FIG. 11, because this APD has a sequential mesa
structure, the ratio of the cross-sectional area showing the
depletion region of the p-type contact layer 6b structuring the pn
junction and the cross-sectional area showing the depletion region
of the n-type multiplying layer 5a greatly differs at the central
portion of the mesa and at the peripheral portion of the mesa.
[0104] Here, considering from the standpoint of depleting the pn
junction portion, because the APD has a sequential mesa structure,
at the vicinity of the periphery of the mesa, there is a state in
which the carrier density of the multiplying layer 5a is
substantially higher than at the central portion of the mesa.
[0105] In contrast, at the contact layer 6b, conversely, there is a
state in which the carrier density is weak. However, because the
carrier density is originally high at the contact layer 6b, even if
it is in a state in which the carrier density is substantially
weak, the effect is small.
[0106] As a result, in the sequential mesa type APD, the way of
broadening (width) of the depletion region is shorter (narrower)
than the way of broadening (width) of the central portion.
[0107] Namely, it can be understood that the electric field
intensity at the peripheral portion of the mesa is higher than that
at the central portion of the mesa in the sequential mesa type
APD.
[0108] FIG. 12 shows the way of broadening (width) of the depletion
region (depletion layer) by built-in potential from the pn junction
portion in the sequential mesa type APD as shown in FIG. 10 in
which electrons are used as the main carrier.
[0109] In this sequential mesa type APD, the carrier density of the
p-type electric field relaxation layer 4b forming the pn junction
is higher than the carrier density of the multiplying layer 5a.
Thus, as shown in FIG. 12, in accordance with the principles of
charge neutrality, the way of broadening (width) of the depletion
region at the vicinity of the periphery of the mesa is shorter
(narrower) than the way of broadening (width) of the central
portion.
[0110] Namely, in the sequential mesa type APD, the electric field
intensity at the peripheral portion of the mesa is higher than that
at the central portion of the mesa.
[0111] The reason for this is that, in the sequential mesa type
APD, it is difficult for the in-surface distribution of field
intensity in a surface parallel to the semiconductor substrate to
concentrate at the central portion of the mesa by only the
epitaxial growth process, so that there is a double-peaked
characteristic in which the light-receiving current at the
peripheral portion of the mesa is greater than the light-receiving
current at the central portion of the mesa.
[0112] In this way, in the sequential mesa type APD in which
positive holes or electrons are used as the main carrier and the pn
junction is formed by epitaxial growth, the way of broadening
(width) of the depletion region at the vicinity of the periphery of
the mesa is shorter (narrower) that at the central portion, and the
electric field intensity at the peripheral portion of the mesa is
higher than at the central portion of the mesa.
[0113] Here, the relationship between the crystallinity and the
light-receiving characteristic of the sequential mesa type APD will
be described.
[0114] As described above, a sequential mesa type APD of this type,
the light-receiving current is multiplied by an avalanche
multiplying function.
[0115] Further, the noise at the time of the avalanche multiplying
function greatly depends on the crystallinity of the sequential
mesa type APD.
[0116] Accordingly, even among light-receiving elements in which
crystallinity is considered to be important, in particular, the
crystallinity of a sequential mesa type APD is important.
[0117] In a sequential mesa type APD, a mesa side surface 10a
formed by mesa-etching is provided at the peripheral portion of the
mesa of a sequential mesa portion 10.
[0118] Generally, the mesa side surface 10a has a great number of
crystal defects as compared with the interior portion of the
mesa.
[0119] Further, the crystal defects adversely affect the
consideration of solutions for decreasing dark current in the
sequential mesa type APD, decreasing noise, high sensitization, and
modularization.
[0120] Namely, in a sequential mesa type APD in which positive
holes or electrons are used as the main carrier and the pn junction
is formed by epitaxial growth, as shown by characteristic B of FIG.
3, when the light-receiving characteristic of the sequential mesa
type APD is dominant at the peripheral portion of the mesa, the
good crystallinity which the central portion of the mesa has is not
reflected in the light-receiving characteristic of the entire
sequential mesa type APD. As a result, it is a cause for the
light-receiving characteristic of the entire sequential mesa type
APD to deteriorate, and for it to be difficult to align optical
axes at the time of making the APD a module, and for the yield of
modularization to be poor, and for the fabricating costs of
modularization to increase.
BRIEF SUMMARY OF THE INVENTION
[0121] An object of the present invention is to provide a
sequential mesa type avalanche photodiode which is achieved on the
basis of the above-described circumstances, and in which, in a
sequential mesa type APD in which positive holes or electrons are
used as the main carrier and a pn junction is formed by epitaxial
growth, by making the distribution of the electric field
concentrate at the central portion of the mesa, the effects of dark
current and noise contained in a light-receiving signal can be kept
to a minimum, and high sensitization can be realized, and the
fabricating costs at the time of modularization of the APD can be
greatly decreased.
[0122] Another object of the present invention is to provide a
method of manufacturing a sequential mesa type avalanche photodiode
which is achieved on the basis of the above-described
circumstances, and in which, in a sequential mesa type APD in which
positive holes or electrons are used as the main carrier and a pn
junction is formed by epitaxial growth, by making the distribution
of the electric field concentrate at the central portion of the
mesa, the effects of the dark current and noise contained in a
light-receiving signal can be kept to a minimum, and high
sensitization can be realized, and the fabricating costs at the
time of modularization of the APD can be greatly decreased.
[0123] First, the point of interest of the present invention will
be described.
[0124] As described above, in a sequential mesa type APD in which
the pn junction is formed by only an epitaxial growth process, it
is difficult to concentrate, at the central portion of the mesa,
the in-surface distribution of the electric field intensity in a
surface parallel to the semiconductor substrate.
[0125] Therefore, conventionally, regardless of the fact that there
is the difficulty that the manufacturing process (process steps) is
complicated, the sequential mesa type APD, in which the pn junction
is formed by using the Zn diffusion process which can make the
distribution of the electric field concentrate at the central
portion of the mesa by selectively diffusing the Zn at a specific
portion in a surface parallel to the semiconductor substrate, is
exclusively used.
[0126] The present inventor has used in combination contrivances
for concentrating the distribution of the electric field at the
central portion of the mesa which has not been carried out in the
prior art, in a sequential mesa type APD in which positive holes or
electrons are used as the main carrier and the pn junction is
formed by only an epitaxial growth process.
[0127] From the concept opposite that of a sequential mesa type APD
in which the pn junction is formed by using the conventional Zn
diffusion process, the present inventor, as a result of diligently
searching for such contrivances, has found that it suffices that
the carrier density of a semiconductor layer which is near to the
semiconductor substrate among a pair of semiconductor layers
forming the pn junction is larger than the carrier density of a
semiconductor layer which is far from the semiconductor substrate
among the pair of semiconductor layers.
[0128] In a sequential mesa type APD structured by satisfying such
a relationship, as described above, the ratio of the
cross-sectional areas of the pair of semiconductor layers
structuring the pn junction formed within the mesa portion is
constant at the central portion of the mesa, and is different at
the central portion of the mesa and at the peripheral portion of
the mesa.
[0129] Here, considering from the standpoint of depleting the pn
junction portion, because the APD has a sequential mesa structure,
among the pair of semiconductor layers structuring the pn junction,
at the vicinity of the periphery of the mesa, there is a state in
which the carrier density of the semiconductor layer which is far
from the semiconductor substrate is substantially weaker than at
the central portion of the mesa.
[0130] In contrast, at the semiconductor layer which is near to the
semiconductor substrate, conversely, there is a state in which the
carrier density is high. However, because the carrier density is
originally high at the semiconductor layer which is near to the
semiconductor substrate, even if it is in a state in which the
carrier density is substantially high, the effect is small.
[0131] Namely, the way of broadening (width) of the depletion
region at the peripheral portion of the mesa is greater than the
way of broadening (width) of the depletion region at the central
portion of the mesa, and the electric field intensity at the
central portion of the mesa is greater than the electric field
intensity at the peripheral portion of the mesa.
[0132] Thus, in such a sequential mesa type APD according to the
present invention, the component at the central portion of the mesa
contained in the overall light-receiving characteristic of the APD
can be increased, and the component at the peripheral portion of
the mesa can be decreased.
[0133] Accordingly, in such a sequential mesa type APD according to
the present invention, the effects of the dark current and noise
caused due to crystal defects which are many at the peripheral
portion of the mesa can be kept to a minimum, and decreasing of
dark current, decreasing of noise, and high sensitization in the
overall light-receiving characteristic of the APD can be attempted.
Further, since the yield of modularizing is improved by making the
alignment of the optical axes at the time of modularizing be easy
and exact, the fabricating costs of modularizing can be greatly
decreased.
[0134] In such a sequential mesa type APD, the relationship of the
magnitude of the carrier densities of the pair of semiconductor
layers forming the pn junction is a relationship opposite to the
relationship of the magnitude of the carrier densities of the pair
of semiconductor layers forming the pn junction in a conventional
sequential mesa type APD in which the pn junction is formed by a Zn
diffusion process.
[0135] Next, the background of the difficulty of the idea of the
relationship of the magnitude of the carrier densities of the pair
of semiconductor layers forming the pn junction will be
described.
[0136] Namely, in a conventional sequential mesa type APD in which
the pn junction is formed by using a Zn diffusion process, as
described above, the pn junction is formed by the p-type contact
layer 6b, in which the p-type carrier density is made high by Zn
diffusion, and the n-type multiplying layer 5a.
[0137] At this time, it is preferable, for the contact layer 6b as
well, that the p-type carrier density of the contact layer 6b is
set to be as high as possible, for example, about 5.times.10.sup.18
(cm.sup.-3) or more. The gas pressure of the inert gas atmosphere
used for carrying out sufficient Zn diffusion is controlled so as
to maintain a relatively high value by using an exclusively-used
controller.
[0138] Namely, in the conventional sequential mesa type APD in
which the pn junction is formed by using a Zn diffusion process,
there is a relationship in which the carrier density of the p-type
contact layer 6b (the semiconductor layer which is far from the
semiconductor substrate) forming the pn junction is larger than the
carrier density of the n-type multiplying layer 5a (the
semiconductor layer which is near to the semiconductor
substrate).
[0139] Accordingly, in the sequential mesa type APD of the present
invention, whose relationship is opposite to this relationship and
in which the pn junction is formed by epitaxial growth, it would
not be generally thought to make the carrier density of the
semiconductor layer which is near to the semiconductor substrate
among the pair of semiconductor layers forming the pn junction,
larger than the carrier density of the semiconductor layer which is
far from the semiconductor substrate among the aforementioned pair
of semiconductor layers.
[0140] Further, in the sequential mesa type APD of the present
invention in which the pn junction is formed by epitaxial growth,
the semiconductor layer which is far from the semiconductor
substrate among the pair of semiconductor layers forming the pn
junction, is formed separate from and at the lower layer of the
p-type contact layer, and the gas pressure of the inert gas
atmosphere used for Zn diffusion for obtaining an ohmic electrode
of the contact layer 6b may be maintained at a relatively low
value.
[0141] Accordingly, in accordance therewith, in the sequential mesa
type APD of the present invention in which the pn junction is
formed by epitaxial growth, it would not be generally thought to
make the carrier density of the semiconductor layer which is near
to the semiconductor substrate among the pair of semiconductor
layers forming the pn junction, larger than the carrier density of
the semiconductor layer which is far from the semiconductor
substrate among the aforementioned pair of semiconductor
layers.
[0142] In order to achieve the above object, there is provided a
sequential mesa type avalanche photodiode comprising:
[0143] a semiconductor substrate; and
[0144] a sequential mesa portion formed on the semiconductor
substrate, a plurality of semiconductor layers which include a
light absorbing layer and a multiplying layer being laminated by
epitaxial growth, in the sequential mesa portion, and a pair of
semiconductor layers which form a pn junction being included in the
plurality of semiconductor layers, wherein
[0145] the carrier density of a semiconductor layer which is near
to the semiconductor substrate among the pair of semiconductor
layers is larger than the carrier density of a semiconductor layer
which is far from the semiconductor substrate among the pair of
semiconductor layers, and
[0146] in accordance therewith, in the sequential mesa type
avalanche photodiode, light-receiving current based on movement of
electrons and positive holes generated in the sequential mesa
portion when light is incident from the semiconductor substrate
toward the light absorbing layer is larger at a central portion
than at a peripheral portion of the sequential mesa portion.
[0147] According to a second aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the first aspect, wherein the semiconductor substrate is
structured from an n-type semiconductor substrate, and any of the
electrons or the positive holes are the main carrier.
[0148] According to a third aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the second aspect, wherein the n-type semiconductor substrate is
a semiconductor substrate formed from n.sup.+-type InP.
[0149] According to a fourth aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the second aspect, wherein the semiconductor layer which is near
to the n-type semiconductor substrate among the pair of
semiconductor layers is an n-type semiconductor layer, and the
semiconductor layer which is far from the n-type semiconductor
substrate among the pair of semiconductor layers is a p-type
semiconductor layer.
[0150] According to a fifth aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the fourth aspect, wherein the n-type semiconductor layer (4a,
14a, 15a) is an n-type electric field relaxation layer (4a), and
the p-type semiconductor layer (5b, 14b, 12b) is a p-type
multiplying layer (5b), and the positive holes are the main
carrier.
[0151] According to a sixth aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the fifth aspect, wherein the n-type electric field relaxation
layer is an electric field relaxation layer formed from
n.sup.+-type InP, and the p-type multiplying layer is a multiplying
layer formed from p.sup.--type InP.
[0152] According to a seventh aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the fourth aspect, wherein the n-type semiconductor
layer is an n-type electric field relaxation layer, and the p-type
semiconductor layer is a p-type electric field concentration layer,
and the positive holes are the main carrier.
[0153] According to an eighth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the seventh aspect, wherein the n-type electric field
relaxation layer is an electric field relaxation layer formed from
n.sup.+-type InP, and the p-type electric field concentration layer
is a electric field concentration layer formed from p.sup.--type
InP.
[0154] According to a ninth aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the fourth aspect, wherein the n-type semiconductor layer is an
n-type multiplying layer, and the p-type semiconductor layer is a
p.sup.--type electric field concentration layer, and the electrons
are the main carrier.
[0155] According to a tenth aspect of the present invention, there
is provided a sequential mesa type avalanche photodiode according
to the ninth aspect, wherein the n-type multiplying layer is a
multiplying layer formed from n type InP, and the p-type electric
field concentration layer is an electric field concentration layer
formed from p.sup.--type InP.
[0156] According to an eleventh aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the fourth aspect, wherein the n-type semiconductor
layer is an n-type electric field concentration layer, and the
p-type semiconductor layer is a p-type electric field concentration
layer, and the electrons are the main carrier.
[0157] According to a twelfth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the eleventh aspect, wherein the n-type electric field
concentration layer is a first electric field concentration layer
formed from n.sup.+-type InP, and the p-type electric field
concentration layer is a second electric field concentration layer
formed from p.sup.--type InP.
[0158] According to a thirteenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the first aspect, wherein the semiconductor substrate
is formed from a p-type semiconductor substrate, and any of the
electrons and the positive holes are the main carrier.
[0159] According to a fourteenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the thirteenth aspect, wherein the p-type
semiconductor substrate is a semiconductor substrate formed from
p.sup.+-type InP.
[0160] According to a fifteenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the thirteenth aspect, wherein a semiconductor layer
which is near to the p-type semiconductor substrate among the pair
of semiconductor layers is a p-type semiconductor layer, and a
semiconductor layer which is far from the p-type semiconductor
substrate among the pair of semiconductor layers is an n-type
semiconductor layer.
[0161] According to a sixteenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the fifteenth aspect, wherein the p-type semiconductor
layer is a p-type contact layer, and the n-type semiconductor layer
is an n-type multiplying layer, and the positive holes are the main
carrier.
[0162] According to a seventeenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the sixteenth aspect, wherein the p-type contact layer
is a contact layer formed from p.sup.+-type InGaAs, and the n-type
multiplying layer is a multiplying layer formed from n.sup.--type
InP.
[0163] According to an eighteenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the fifteenth aspect, wherein the p-type semiconductor
layer is a p-type electric field relaxation layer, and the n-type
semiconductor layer is an n-type multiplying layer, and the
electrons are the main carrier.
[0164] According to a nineteenth aspect of the present invention,
there is provided a sequential mesa type avalanche photodiode
according to the eighteenth aspect, wherein the p-type electric
field relaxation layer is an electric field relaxation layer formed
from p.sup.+-type InP, and the n-type multiplying layer is a
multiplying layer formed from n.sup.--type InP.
[0165] In order achieve the above object, according to a twentieth
aspect of the present invention, there is provided a method of
manufacturing a sequential mesa type avalanche photodiode,
comprising the steps of:
[0166] preparing a semiconductor substrate;
[0167] laminating a plurality of semiconductor layers, including a
light absorbing layer and a multiplying layer, on the semiconductor
substrate by epitaxial growth, a pair of semiconductor layers which
form a pn junction being included in the plurality of semiconductor
layers; and
[0168] forming a sequential mesa portion having a sequential mesa
portion structure including therein the plurality of semiconductor
layers, wherein
[0169] the carrier density of a semiconductor layer which is near
to the semiconductor substrate among the pair of semiconductor
layers is larger than the carrier density of a semiconductor layer
which is far from the semiconductor substrate among the pair of
semiconductor layers, and
[0170] in accordance therewith, in the sequential mesa type
avalanche photodiode, light-receiving current based on movement of
electrons and positive holes generated in the sequential mesa
portion when light is incident from the semiconductor substrate
toward the light absorbing layer is larger at a central portion
than at a peripheral portion of the sequential mesa portion.
[0171] According to a twenty-first aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the twentieth aspect, wherein the
semiconductor substrate is formed from an n-type semiconductor
substrate, and any of the electrons or the positive holes are the
main carrier.
[0172] According to a twenty-second aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the twenty-first
aspect, wherein the n-type semiconductor substrate is a
semiconductor substrate formed from n.sup.+-type InP.
[0173] According to a twenty-third aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the twenty-first aspect, wherein
the semiconductor layer which is near to the n-type semiconductor
substrate among the pair of semiconductor layers is formed from an
n-type semiconductor layer, and the semiconductor layer which is
far from the n-type semiconductor substrate among the pair of
semiconductor layers is formed from a p-type semiconductor
layer.
[0174] According to a twenty-fourth aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the twenty-third
aspect, wherein the n-type semiconductor layer is formed from an
n-type electric field relaxation layer, and the p-type
semiconductor layer is formed from a p-type multiplying layer, and
the positive holes are the main carrier.
[0175] According to a twenty-fifth aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the twenty-fourth aspect, wherein
the n-type electric field relaxation layer is an electric field
relaxation layer formed from n.sup.+-type InP, and the p-type
multiplying layer is a multiplying layer formed from p.sup.--type
InP.
[0176] According to a twenty-sixth aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the twenty-third aspect, wherein
the n-type semiconductor layer is formed from an n-type electric
field relaxation layer, and the p-type semiconductor layer is
formed from a p-type electric field concentration layer, and the
positive holes are the main carrier.
[0177] According to a twenty-seventh aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the twenty-sixth
aspect, wherein the n-type electric field relaxation layer is an
electric field relaxation layer formed from n.sup.+-type InP, and
the p-type electric field concentration layer is an electric field
concentration layer formed from p.sup.--type InP.
[0178] According to a twenty-eighth aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the twenty-third
aspect, wherein the n-type semiconductor layer is formed from an
n-type multiplying layer, and the p-type semiconductor layer is
formed from a p-type electric field concentration layer, and the
electrons are the main carrier.
[0179] According to a twenty-ninth aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the twenty-eighth aspect, wherein
the n-type multiplying layer is a multiplying layer formed from n
type InP, and the p-type electric field concentration layer is an
electric field concentration layer formed from p.sup.--type
InP.
[0180] According to a thirtieth aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the twenty-third aspect, wherein
the n-type semiconductor layer is formed from an n-type electric
field concentration layer, and the p-type semiconductor layer is
formed from a p-type electric field concentration layer, and the
electrons are the main carrier.
[0181] According to a thirty-first aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the thirtieth aspect, wherein the
n-type electric field concentration layer is a first electric field
concentration layer formed from n.sup.+-type InP, and the p-type
electric field concentration layer is a second electric field
concentration layer formed from p.sup.--type InP.
[0182] According to a thirty-second aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the twentieth aspect,
wherein the semiconductor substrate is formed from a p-type
semiconductor substrate, and any of the electrons and the positive
holes are the main carrier.
[0183] According to a thirty-third aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the thirty-second aspect, wherein
the p-type semiconductor substrate (1b) is a semiconductor
substrate formed from p.sup.+-type InP.
[0184] According to a thirty-fourth aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the thirty-second
aspect, wherein a semiconductor layer which is near to the p-type
semiconductor substrate among the pair of semiconductor layers is
formed from a p-type semiconductor layer, and a semiconductor layer
which is far from the p-type semiconductor substrate among the pair
of semiconductor layers is formed from an n-type semiconductor
layer.
[0185] According to a thirty-fifth aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the thirty-fourth aspect, wherein
the p-type semiconductor layer is formed from a p-type contact
layer, and the n-type semiconductor layer is formed from an n-type
multiplying layer, and the positive holes are the main carrier.
[0186] According to a thirty-sixth aspect of the present invention,
there is provided a method of manufacturing a sequential mesa type
avalanche photodiode according to the thirty-fifth aspect, wherein
the p-type contact layer is a contact layer formed from
p.sup.+-type InGaAs, and the n-type multiplying layer is a
multiplying layer formed from n.sup.--type InP.
[0187] According to a thirty-seventh aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the thirty-fourth
aspect, wherein the p-type semiconductor layer is formed from a
p-type electric field relaxation layer, and the n-type
semiconductor layer is formed from an n-type multiplying layer, and
the electrons are the main carrier.
[0188] According to a thirty-eighth aspect of the present
invention, there is provided a method of manufacturing a sequential
mesa type avalanche photodiode according to the thirty-seventh
aspect, wherein the p-type electric field relaxation layer is an
electric field relaxation layer formed from p.sup.+-type InP, and
the n-type multiplying layer is a multiplying layer formed from
n.sup.--type InP.
[0189] Additional objects and advantages of the invention will be
set forth in the description which follows, and in part will be
obvious from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0190] The accompanying drawings, which are incorporated in and
constitute a part of the specification, illustrate presently
preferred embodiment of the invention, and together with the
general description given above and the detailed description of the
preferred embodiment given below, serve to explain the principles
of the invention.
[0191] FIGS. 1A and 1B are a cross-sectional view and an external
perspective view showing a schematic structure of a sequential mesa
type avalanche photodiode according to a first embodiment of the
present invention;
[0192] FIG. 2 is a view showing the way of broadening (width) of a
depletion region by a built-in potential of the sequential mesa
type avalanche photodiode according to the first embodiment;
[0193] FIG. 3 is a graph showing light-receiving distribution
characteristics of the sequential mesa type avalanche photodiode
according to the first embodiment and a conventional avalanche
photodiode;
[0194] FIG. 4 is a cross-sectional view showing a schematic
structure of a sequential mesa type avalanche photodiode according
to a second embodiment of the present invention;
[0195] FIG. 5 is a cross-sectional view showing a schematic
structure of a sequential mesa type avalanche photodiode according
to a third embodiment of the present invention;
[0196] FIG. 6 is a cross-sectional view showing a schematic
structure of a sequential mesa type avalanche photodiode according
to a fourth embodiment of the present invention;
[0197] FIG. 7 is a cross-sectional view showing a schematic
structure of a sequential mesa type avalanche photodiode according
to a fifth embodiment of the present invention;
[0198] FIG. 8 is a cross-sectional view showing a schematic
structure of a sequential mesa type avalanche photodiode according
to a sixth embodiment of the present invention;
[0199] FIGS. 9A and 9B are a cross-sectional view and an external
perspective view showing a schematic structure of the conventional
sequential mesa type avalanche photodiode;
[0200] FIG. 10 is a cross-sectional view showing a schematic
structure of another conventional sequential mesa type avalanche
photodiode;
[0201] FIG. 11 is a view showing the way of broadening (width) of a
depletion region by a built-in potential of the conventional
sequential mesa type avalanche photodiode; and
[0202] FIG. 12 is a view showing the way of broadening (width) of a
depletion region by a built-in potential of the other sequential
mesa type avalanche photodiode.
DETAILED DESCRIPTION OF THE INVENTION
[0203] Reference will now be made in detail to the presently
preferred embodiments of the invention as illustrated in the
accompanying drawings, in which like reference numerals designate
like or corresponding parts.
[0204] Hereinafter, embodiments of the present invention will be
described with reference to the figures.
[0205] (First Embodiment)
[0206] FIG. 1A is a cross-sectional view of a sequential mesa type
avalanche photodiode (APD) according to a first embodiment of the
present invention.
[0207] FIG. 1B is an external perspective view of the sequential
mesa type APD according to the first embodiment of the present
invention.
[0208] In FIGS. 1A and 1B, portions which are the same as those of
the conventional sequential mesa type APD shown in FIGS. 9A and 9B
are denoted by the same reference numerals, and detailed
description of the repeated portions is omitted.
[0209] In the sequential mesa type APD of the first embodiment, a
positive hole is used as the main carrier, and a pn junction is
formed by epitaxial growth.
[0210] Namely, as shown in FIGS. 1A and 1B, in the sequential mesa
type APD of the first embodiment, after a buffer layer 2a formed
from n.sup.+-type InP, a light absorbing layer 3a formed from
n.sup.--type InGaAs, an electric field relaxation layer 4a formed
from n.sup.+-type InP, a multiplying layer 5b formed from
p.sup.--type InP, and a contact layer 6b formed from p.sup.+-type
InGaAs are successively formed by epitaxial growth on a
semiconductor substrate 1a formed from n.sup.+-type InP by using
the MOVPE (organometallic vapor phase epitaxial growth) method, for
example, a conical sequential mesa portion 10 is formed by
wet-etching from above.
[0211] After a protective layer 7 is coated on the sequential mesa
portion 10, a p electrode 8 contacting the p-type contact layer 6b
is formed.
[0212] Further, on the both sides of the sequential mesa portion
10, n electrodes 9 are attached, via the protective layer 11, to
another mesa portion formed for attaching electrodes.
[0213] Accordingly, in the sequential mesa type APD of the first
embodiment, the pn junction is formed by the electric field
relaxation layer 4a formed from n.sup.+-type InP and the
multiplying layer 5b formed from p.sup.--type InP.
[0214] Further, the carrier density of the electric field
relaxation layer 4a, which is formed from n.sup.+-type InP and
which is near to the n-type semiconductor substrate 1a, is set to,
for example, 1.times.10.sup.18 (cm.sup.-3), which is larger than
the carrier density, for example, 5.times.10.sup.16 (cm.sup.-3), of
the p-type multiplying layer 5b far from the n-type semiconductor
substrate 1a.
[0215] In the sequential mesa type APD structured in this way, as
shown by the arrow in FIG. 1A, light incident from the bottom
surface of the semiconductor substrate 1a penetrates through the
semiconductor substrate 1a and the buffer layer 2a and is absorbed
at the light absorbing layer 3a, thereby a pair of an electron and
an positive hole is generated.
[0216] Among the pair of the electron and the positive hole
generated in this way, the electron moves to the n electrode 9 via
the n-type semiconductor substrate 1a, and the positive hole is
multiplied at the multiplying layer 5b and moves to the p electrode
8 via the contact layer 6b.
[0217] Moreover, in the sequential mesa type APD of the first
embodiment which is structured in this way, in the sequential mesa
portion 10, as described above, the carrier density of the n-type
electric field relaxation layer 4a near to the n-type semiconductor
substrate 1a is set to, for example, 1.times.10.sup.18 (cm.sup.-3),
which is larger than the carrier density, for example,
5.times.10.sup.16 (cm.sup.-3), of the p-type multiplying layer 5b
far from the n-type semiconductor substrate 1a.
[0218] Therefore, the in-surface distribution of field intensity in
a surface parallel to the semiconductor substrate 1a concentrates
at the central portion of the mesa.
[0219] Next, the reason why the in-plane distribution of the
electric field intensity concentrates at the central portion of the
mesa will be described.
[0220] FIG. 2 shows the way of broadening (width) of the depletion
region (depletion layer) by built-in potential from the pn junction
in the sequential mesa type APD of the first embodiment in which
positive holes are used as the main carrier as shown in FIGS. 1A
and 1B.
[0221] Note that, as described above, the ratio of the
cross-sectional areas of the n-type electric field relaxation layer
4a and the p-type multiplying layer 5b forming the pn junction is
constant at the central portion of the mesa. However, the ratio at
the central portion of the mesa is different from that at the
vicinity of the periphery of the mesa.
[0222] Here, considering from the standpoint of depleting the pn
junction portion, because the APD has a sequential mesa structure,
at the vicinity of the periphery of the mesa, there is a state in
which the carrier density of the multiplying layer 5b is
substantially weaker than at the central portion of the mesa.
[0223] In contrast, at the n-type electric field relaxation layer
4a, conversely, there is a state in which the carrier density is
high. However, because the carrier density is originally high at
the n-type electric field relaxation layer 4a, even if it is in a
state in which the carrier density is substantially high, the
effect is small.
[0224] Namely, as shown in FIG. 2, the way of broadening (width) of
the depletion region at the peripheral portion of the mesa is
larger than the way of broadening (width) of the depletion region
at the central portion of the mesa. Thus, the electric field
intensity at the central portion of the mesa is higher than the
electric field intensity at the peripheral portion of the mesa.
[0225] Characteristic A in FIG. 3 shows an actually-measured
light-receiving sensitivity distribution characteristic of the
sequential mesa type APD according to the first embodiment.
[0226] Note that the light-receiving diameter of the sequential
mesa type APD of the first embodiment is 30 .mu.m.
[0227] As can be understood from the actually-measured
characteristic A, the sequential mesa type APD of the first
embodiment has a single-peaked characteristic in which the
light-receiving current at the central portion of the mesa is
larger than the light-receiving current at the peripheral portion
of the mesa.
[0228] In the sequential mesa type APD whose light-receiving
characteristic is a single-peaked characteristic, as described
later, alignment of the optical axes is easy when the APD is
modularized and used in actuality, and alignment of the optical
axes may be carried out at the central portion of the mesa at which
the light-receiving current is larger than that of the peripheral
portion of the mesa. Therefore, there are the advantages that a
sufficient light-receiving characteristic can be exhibited, and
high sensitization is realized by keeping to a minimum the effects
of the dark current and noise contained in the light-receiving
signal relating to the above-described problem of
crystallinity.
[0229] Therefore, the component of the mesa central portion, which
component is contained in the overall light-receiving
characteristic of the sequential mesa type APD of the first
embodiment, can be increased, and the component of the peripheral
portion of the mesa can be decreased.
[0230] Accordingly, the sequential mesa type APD according to the
first embodiment can keep to a minimum the effects of the dark
current and noise caused due to crystal defects which are many at
the peripheral portion of the mesa including a mesa side surface
10a, and decreasing of dark current, decreasing of noise, and high
sensitization in the overall light-receiving characteristic of the
sequential mesa type APD can be attempted.
[0231] (Second Embodiment)
[0232] FIG. 4 is a cross-sectional view of a sequential mesa type
avalanche photodiode (APD) according to a second embodiment of the
present invention.
[0233] In FIG. 4, portions which are the same as those of the
sequential mesa type APD as shown in FIG. 1A according to the first
embodiment are denoted by the same reference numerals, and detailed
description of the repeated portions is omitted.
[0234] In the sequential mesa type APD of the second embodiment, in
the same way as in the above-described sequential mesa type APD of
the first embodiment, positive holes are used as the main carrier,
and the pn junction is formed by epitaxial growth.
[0235] Namely, as shown in FIG. 4, in the sequential mesa type APD
of the second embodiment, an electric field concentration layer
14b, which is formed from p.sup.--type InP and which is a layer for
concentrating electric fields, is provided between the electric
field relaxation layer 4a formed from n.sup.+-type InP and the
multiplying layer 5b formed from p.sup.--type InP in the sequential
mesa portion 10.
[0236] Further, the pn junction is formed by the electric field
relaxation layer 4a formed from n.sup.+-type InP and the electric
field concentration layer 14b formed from p.sup.--type InP.
[0237] In the sequential mesa type APD of the second embodiment as
well, with respect to the relationship of the magnitude of the
carrier densities of the n-type electric field relaxation layer 4a
and the p-type electric field concentration layer 14b, the carrier
density of the n-type electric field relaxation layer 4a which is
near to the semiconductor substrate 1a is set to, for example,
1.times.10.sup.18 (cm.sup.-3), which is larger than the carrier
density, for example, 5.times.10.sup.16 (cm.sup.-3), of the p-type
electric field concentration layer 14b which is far from the
semiconductor substrate 1a.
[0238] Therefore, in the sequential mesa type APD of the second
embodiment, the distribution of the electric field intensity within
the mesa surface concentrates at the central portion of the
mesa.
[0239] Accordingly, in the sequential mesa type APD of the second
embodiment as well, substantially the same effects as those of the
sequential mesa type APD of the previously-described first
embodiment can be obtained.
[0240] Note that, in the sequential mesa type APD of the second
embodiment, with respect to the relationship of the magnitude of
the carrier densities of the electric field concentration layer 14b
formed from p.sup.--type InP and the multiplying layer 5b formed
from p-type InP, the setting of these carrier densities can be
arbitrarily carried out regardless of the convergence of electric
fields.
[0241] Therefore, in the sequential mesa type APD of the second
embodiment, there is no problem even if the multiplying layer 5b
formed from InP is p.sup.+-type.
[0242] (Third Embodiment)
[0243] FIG. 5 is a cross-sectional view of a sequential mesa type
avalanche photodiode (APD) according to a third embodiment of the
present invention.
[0244] In FIG. 5, portions which are the same as those of the
conventional sequential mesa type APD shown in FIG. 10 are denoted
by the same reference numerals, and detailed description of the
repeated portions is omitted.
[0245] In the sequential mesa type APD of the third embodiment, in
the same way as in the conventional sequential mesa type APD shown
in FIG. 10, electrons are used as the main carrier, and the pn
junction is formed by epitaxial growth.
[0246] Namely, as shown in FIG. 5, in the sequential mesa type APD
of the third embodiment, the buffer layer 2a formed from
n.sup.+-type InP, the multiplying layer 5a formed from n.sup.--type
InP, a first electric field concentration layer 15a formed from
n.sup.+-type InP, a second electric field concentration layer 12b
formed from p.sup.--type InP, the electric field relaxation layer
4b formed from p.sup.+-type InP, a light absorbing layer 3b formed
from p.sup.--type InGaAs, a window layer 13b formed from p-type
InP, and the contact layer 6b formed from p.sup.+-type InGaAs are
successively formed by epitaxial growth on the semiconductor
substrate 1a formed from n.sup.+-type InP by using the
above-described MBE (molecular beam epitaxy) method. Therefore, the
sequential mesa portion 10 is formed by wet-etching.
[0247] After the protective layer 7 is coated on the sequential
mesa portion 10, the p electrode 8 contacting the p-type contact
layer 6b is formed.
[0248] Further, on the both sides of the sequential mesa portion
10, the n electrodes 9 are attached, via the protective layer 11,
to another mesa portion formed for attaching electrodes.
[0249] Accordingly, in the sequential mesa type APD of the third
embodiment, the pn junction is formed between the first electric
field concentration layer 15a formed from n.sup.+-type InP and the
second electric field concentration layer 12b formed from
p.sup.--type InP.
[0250] Further, in the sequential mesa type APD of the third
embodiment as well, the carrier density of the first electric field
concentration layer 15a which is formed from n.sup.+-type InP which
is near to the n-type semiconductor substrate 1a is set to, for
example, b 1.times.10.sup.18 (cm.sup.-3), which is larger than the
carrier density, for example, 5.times.10.sup.16 (cm.sup.-3), of the
second electric field concentration layer 12b which is formed from
p-type InP and which is far from the n-type semiconductor substrate
1a.
[0251] Therefore, in the sequential mesa type APD of the third
embodiment, the distribution of the electric field intensity in the
mesa surface concentrates at the central portion of the mesa.
[0252] Accordingly, in the sequential mesa type APD of the third
embodiment as well, substantially the same effects as in the
respective sequential mesa type APDs of the first and second
embodiments can be obtained.
[0253] (Fourth Embodiment)
[0254] FIG. 6 is a cross-sectional view of a sequential mesa type
avalanche photodiode (APD) according to a fourth embodiment of the
present invention.
[0255] In FIG. 6, portions which are the same as those of the
conventional sequential mesa type APD shown in FIG. 10 are denoted
by the same reference numerals, and detailed description of the
repeated portions is omitted.
[0256] In the sequential mesa type APD of the fourth embodiment, in
the same way as in the conventional sequential mesa type APD shown
in FIG. 10, electrons are used as the main carrier, and the pn
junction is formed by epitaxial growth.
[0257] Namely, as shown in FIG. 6, in the sequential mesa type APD
of the fourth embodiment, the buffer layer 2a formed from
n.sup.+-type InP, the multiplying layer 5a formed from n.sup.+-type
InP, the electric field concentration layer 14b formed from
p.sup.--type InP, the electric field relaxation layer 4b formed
from p.sup.+-type InP, the light absorbing layer 3b formed from
p.sup.--type InGaAs, the window layer 13b formed from p-type InP,
and the contact layer 6b formed from p.sup.+-type InGaAs are
successively formed by epitaxial growth on the semiconductor
substrate 1a formed from n.sup.+-type InP by using the
above-described MBE (molecular beam epitaxy) method. Therefore, the
sequential mesa portion 10 is formed by wet-etching.
[0258] After the protective layer 7 is coated on the sequential
mesa portion 10, the p electrode 8 contacting the p-type contact
layer 6b is formed.
[0259] Further, on the both sides of the sequential mesa portion
10, the electrodes 9 are attached, via the protective layer 11, to
another mesa portion formed for attaching electrodes.
[0260] Accordingly, in the sequential mesa type APD of the fourth
embodiment, the pn junction is formed between the multiplying layer
5a formed from n.sup.+-type InP and the electric field
concentration layer 14b formed from p.sup.--type InP.
[0261] Further, in the sequential mesa type APD of the fourth
embodiment as well, the carrier density of the multiplying layer
5a, which is formed from n.sup.+-type InP and which is near to the
n-type semiconductor substrate 1a, is set to, for example,
5.times.10.sup.17 (cm.sup.-3), which is larger than the carrier
density, for example, 5.times.10.sup.16 (cm.sup.-3), of the
electric field concentration layer 14b which is formed from
p.sup.--type InP and which is far from the n-type semiconductor
substrate 1a.
[0262] Therefore, in the sequential mesa type APD of the fourth
embodiment, the distribution of the electric field intensity within
the mesa surface concentrates at the central portion of the
mesa.
[0263] Accordingly, in the sequential mesa type APD of the fourth
embodiment as well, substantially the same effects as in the
respective sequential mesa type APDs of the first, second, and
third embodiments can be obtained.
[0264] (Fifth Embodiment)
[0265] FIG. 7 is a cross-sectional view of a sequential mesa type
avalanche photodiode (APD) according to a fifth embodiment of the
present invention.
[0266] In FIG. 7, portions which are the same as those of the
sequential mesa type APD shown in FIG. 10 and relating to the first
embodiment are denoted by the same reference numerals, and detailed
description of the repeated portions is omitted.
[0267] In the sequential mesa type APD of the fifth embodiment, the
p-type semiconductor substrate 16 is used as the semiconductor
substrate, positive holes are used as the main carrier, and the pn
junction is formed by epitaxial growth.
[0268] Namely, as shown in FIG. 7, in the sequential mesa type APD
of the fifth embodiment, the buffer layer 2a formed from
p.sup.+-type InP, the contact layer 6b formed from p.sup.+-type
InGaAs, the multiplying layer 5a formed from n.sup.--type InP, the
electric field relaxation layer 4a formed from n.sup.+-type InP,
the light absorbing layer 3a formed from n.sup.--type InGaAs, the
window layer 13a formed from n.sup.+-type InP, and a contact layer
16a formed from n.sup.+-type InGaAs are successively formed by
epitaxial growth on the semiconductor substrate 1b formed from
p.sup.+-type InP by using the above-described MBE (molecular beam
epitaxy) method. Thereafter, the sequential mesa portion 10 is
formed by wet-etching.
[0269] After the protective layer 7 is coated on the sequential
mesa portion 10, the n electrode 9 contacting the n type contact
layer 16a is formed.
[0270] Further, on the both sides of the sequential mesa portion
10, the p electrodes 8 are attached, via the protective layer 11,
to another mesa portion formed for attaching electrodes.
[0271] Accordingly, in the sequential mesa type APD of the fifth
embodiment, the pn junction is formed by the contact layer 6b
formed from p.sup.+-type InGaAs and the multiplying layer 5a formed
from n.sup.--type InP formed within the sequential mesa portion
10.
[0272] Further, in the sequential mesa type APD of the fifth
embodiment, the carrier density of the contact layer 6b, which is
formed from p.sup.+-type InGaAs and which is near to the p-type
semiconductor substrate 1b, is set to, for example,
5.times.10.sup.17 (cm.sup.-3), which is larger than the carrier
density, for example, 5.times.10.sup.16 (cm.sup.-3), of the
multiplying layer 5a which is formed from n-type InP and which is
far from the p-type semiconductor substrate 1b.
[0273] Therefore, in the sequential mesa type APD of the fifth
embodiment, the distribution of the electric field intensity within
the mesa surface concentrates at the central portion of the
mesa.
[0274] Accordingly, in the sequential mesa type APD of the fifth
embodiment as well, substantially the same effects as in the
respective sequential mesa type APDs of the first through fourth
embodiments can be obtained.
[0275] Note that, in the sequential mesa type APD of the fifth
embodiment, it is possible to eliminate the contact layer 6b formed
from p.sup.+-type InGaAs, and to form the p electrode 8 directly
from the semiconductor substrate 1b formed from p.sup.+-type
InP.
[0276] Further, in the sequential mesa type APD of the fifth
embodiment, it is possible to eliminate the contact layer 16a
formed from n.sup.+-type InGaAs, and to form the n electrode 9
directly from the window layer 13a formed from n.sup.+-type
InP.
[0277] (Sixth Embodiment)
[0278] FIG. 8 is a cross-sectional view of a sequential mesa type
avalanche photodiode (APD) according to a sixth embodiment of the
present invention.
[0279] In FIG. 8, portions which are the same as those of the
sequential mesa type APD shown in FIG. 7 and relating to the fifth
embodiment are denoted by the same reference numerals, and detailed
description of the repeated portions is omitted.
[0280] In the sequential mesa type APD of the sixth embodiment, the
p-type semiconductor substrate 1b is used as a semiconductor
substrate, and electrons are used as the main carrier, and the pn
junction is formed by epitaxial growth.
[0281] Namely, as shown in FIG. 8, in the sequential mesa type APD
of the sixth embodiment, the buffer layer 2b formed from
p.sup.+-type InP, the contact layer 6b formed from p.sup.+-type
InGaAs, the window layer 13b formed from p.sup.+-type InP, the
light absorbing layer 3b formed from p.sup.--type InGaAs, the
electric field relaxation layer 4b formed from p.sup.+-type InP,
the multiplying layer 5a formed from n.sup.--type InP, and the
contact layer 16a formed from n.sup.+-type InGaAs are successively
formed by epitaxial growth on the semiconductor substrate 1b formed
from p.sup.+-type InP, and thereafter, the sequential mesa portion
10 is formed by wet-etching.
[0282] After the protective layer 7 is coated on the sequential
mesa portion 10, the n electrode 9 contacting the n-type contact
layer 16a is formed.
[0283] On the both sides of the sequential mesa portion 10, the p
electrodes 8 are attached, via the protective layer 11, to another
mesa portion formed for attaching electrodes.
[0284] Further, in the sequential mesa type APD of the sixth
embodiment, the pn junction is formed between the electric field
relaxation layer 4b formed from p.sup.+-type InP and the
multiplying layer 5a formed from n.sup.--type InP, within the
sequential mesa portion 10.
[0285] Moreover, the carrier density of the electric field
relaxation layer 4b, which is formed from p.sup.+-type InP and
which is near to the p-type semiconductor substrate 1b, is set to,
for example, 5.times.10.sup.17 (cm.sup.-3), which is larger than
the carrier density, for example, 5.times.10.sup.16 (cm.sup.-3), of
the multiplying layer 5a which is formed from n.sup.--type InP and
which is far from the p-type semiconductor substrate 1b.
[0286] Therefore, in the sequential mesa type APD of the sixth
embodiment, the distribution of the electric field intensity within
the mesa surface concentrates at the central portion of the
mesa.
[0287] Accordingly, in the sequential mesa type APD of the sixth
embodiment as well, substantially the same effects as in the
respective sequential mesa type APDs of the first through fifth
embodiments can be obtained.
[0288] Note that, in the sequential mesa type APD of the sixth
embodiment, it is possible to eliminate the contact layer 6b formed
from p.sup.+-type InGaAs, and to form the p electrode 8 directly
from the semiconductor substrate 1b formed from p.sup.+-type
InP.
[0289] Further, it is possible to eliminate the contact layer 16a
formed from n.sup.+-type InGaAs, and to form the n electrode 9
directly from the window layer 13a formed from n.sup.+-type
InP.
[0290] As described above, in all of the first through sixth
embodiments, it is important that, among a pair of semiconductor
layers forming the pn junction formed by epitaxial growth in the
sequential mesa portion 10 at the sequential mesa type APD, the
carrier density of the semiconductor layer which is near to the
semiconductor substrates 1a, 1b is larger than the carrier density
of the semiconductor layer which is far from the semiconductor
substrates 1a, 1b, and in accordance therewith, the distribution of
the electric field intensity in a surface of the mesa concentrates
at the central portion of the mesa.
[0291] Accordingly, in the present invention, except for the
relationship of the magnitude of the carrier densities of the pair
of semiconductor layers forming the pn junction of the sequential
mesa type APD by epitaxial growth, any semiconductor layer
structure can be arbitrarily set.
[0292] As described above, in the sequential mesa type avalanche
photodiode of the present invention, the carrier density of a
semiconductor layer which is near to the semiconductor substrate is
larger than the carrier density of a semiconductor layer which is
far from the semiconductor substrate in a pair of semiconductor
layers structuring the pn junction formed by epitaxial growth in
the sequential mesa portion of the avalanche photodiode. Therefore,
the light-receiving current based on the movement of the electrons
and the positive holes generated in the sequential mesa portion
when light is incident from the aforementioned semiconductor
substrate toward the aforementioned light absorbing layer, is
larger at the peripheral portion of the aforementioned mesa portion
than at the central portion.
[0293] Accordingly, in accordance with the sequential mesa type
avalanche photodiode of the present invention, the distribution of
the electric field intensity in a surface of the mesa concentrates
at the central portion of the mesa. Therefore, the effects of the
dark current and noise caused due to crystal defects which are many
at the peripheral portion of the mesa including a mesa side surface
can be kept to a minimum, and decreasing of dark current,
decreasing of noise, and high sensitization in the overall
light-receiving characteristic of the sequential mesa type
avalanche photodiode can be attempted.
[0294] Further, because the electric field concentrates at the
central portion of the mesa, the following great effects can be
obtained with respect to the points of mounting/evaluation of the
APD as well.
[0295] First, as shown by characteristic A in FIG. 3, because the
APD according to the present invention has a single-peaked
characteristic in which the light-receiving current at the central
portion of the mesa is larger than the light-receiving current at
the peripheral portion of the mesa, there is only one peak of the
photoelectric current. Thus, the center-adjusting work (the
above-described alignment of the optical axes), which sets a
micromotion platform such that the light from a fiber is irradiated
onto a light-receiving portion of the APD and the photoelectric
current of the APD is made to be a maximum, can be easily carried
out.
[0296] As a result, the time required for the center-adjusting work
can be greatly shortened as compared with the APD according to the
prior art in which a plurality of peaks of photoelectric current
exist circumferentially as viewed from above the mesa. Therefore,
making the work more efficient can be attempted.
[0297] Further, in the center-adjusting work according to the prior
art, there is little photoelectric current at the central portion
of the mesa at which the crystallinity is good and there is low
noise, and the photoelectric current is large at the peripheral
portion of the mesa at which the crystallinity deteriorates and
there is much noise. Therefore, it is unclear at which portion of
the mesa the sensitivity as a module on the communication measured
after the APD is modularized, will be a maximum when the light is
incident. However, in the APD according to the present invention,
because the maximum photoelectric current can be obtained at the
central portion of the mesa at which the crystallinity is good, at
the time of carrying out the center-adjusting work, the alignment
position at which the maximum sensitivity as a module on
communication is obtained can be already known.
[0298] Namely, in the APD according to the prior art, it is easy
for errors in the center-adjusting work, by which it is determined
to be a defective good as a result after modularizing, to arise.
However, in the APD according to the present invention, errors in
the center-adjusting work discovered after modularizing do not
arise, and yield is improved over the APD according to the prior
art.
[0299] The improvement in the yield can resolve the uncertainty
that modularizing progresses while it is unclear whether the item
is a good item or a defective item which is the problem so far, and
can greatly decrease the fabricating costs of modularizing an APD,
because the work of modularizing the APD through many processes is
made to be reliable.
[0300] As described above in detail, in accordance with the present
invention, there is provided a sequential mesa type avalanche
photodiode in which, in a sequential mesa type APD in which
positive holes or electrons are used as the main carrier and a pn
junction is formed by epitaxial growth, by making the distribution
of the electric field concentrate at the central portion of the
mesa, the effects of the dark current and noise contained in the
light-receiving signal can be kept to a minimum, and high
sensitization can be realized, and the fabricating costs at the
time of modularization of the APD can be greatly decreased.
[0301] Further, in accordance with the present invention, there is
provided a method of manufacturing a sequential mesa type avalanche
photodiode in which, in a sequential mesa type APD in which
positive holes or electrons are used as the main carrier and a pn
junction is formed by epitaxial growth, by making the distribution
of the electric field concentrate at the central portion of the
mesa, the effects of the dark current and noise contained in the
light-receiving signal can be kept to a minimum, and high
sensitization can be realized, and the fabricating costs at the
time of modularization of the APD can be greatly decreased.
[0302] Additional advantages and modifications will readily occur
to those skilled in the art. Therefore, the invention in its
broader aspects is not limited to the specific details and
representative embodiments shown and described herein. Accordingly,
various modifications may be made without departing from the spirit
or scope of the general inventive concept as defined by the
appended claims and their equivalents.
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