U.S. patent application number 14/682953 was filed with the patent office on 2016-01-21 for light receiving element.
This patent application is currently assigned to MITSUBISHI ELECTRIC CORPORATION. The applicant listed for this patent is Mitsubishi Electric Corporation. Invention is credited to Matobu KIKUCHI, Masaharu NAKAJI, Ryota TAKEMURA, Kazuki YAMAJI.
Application Number | 20160020339 14/682953 |
Document ID | / |
Family ID | 55075279 |
Filed Date | 2016-01-21 |
United States Patent
Application |
20160020339 |
Kind Code |
A1 |
KIKUCHI; Matobu ; et
al. |
January 21, 2016 |
LIGHT RECEIVING ELEMENT
Abstract
A light receiving element includes a substrate of a first
conduction type, a light absorbing layer of the first conduction
type formed on the substrate, a diffusion layer of a second
conduction type formed on a portion of the light absorbing layer, a
window layer of the first conduction type formed on the light
absorbing layer so as to surround the diffusion layer and having a
bandgap larger than that of the light absorbing layer, an anode
electrode formed on the diffusion layer, and a cathode electrode
provided on the substrate so as to contact the substrate without
contacting each of the window layer and the light absorbing layer,
wherein a groove is formed which surrounds a boundary between the
diffusion layer and the window layer as viewed in plan and extends
through the window layer and the light absorbing layer as viewed in
section.
Inventors: |
KIKUCHI; Matobu; (Tokyo,
JP) ; NAKAJI; Masaharu; (Tokyo, JP) ;
TAKEMURA; Ryota; (Tokyo, JP) ; YAMAJI; Kazuki;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mitsubishi Electric Corporation |
Tokyo |
|
JP |
|
|
Assignee: |
MITSUBISHI ELECTRIC
CORPORATION
Tokyo
JP
|
Family ID: |
55075279 |
Appl. No.: |
14/682953 |
Filed: |
April 9, 2015 |
Current U.S.
Class: |
257/435 ;
257/459 |
Current CPC
Class: |
H01L 31/02164 20130101;
H01L 31/02161 20130101; H01L 31/022408 20130101; H01L 31/109
20130101 |
International
Class: |
H01L 31/0224 20060101
H01L031/0224; H01L 31/0216 20060101 H01L031/0216 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2014 |
JP |
2014-145728 |
Claims
1. A light receiving element comprising: a substrate of a first
conduction type; a light absorbing layer of the first conduction
type formed on the substrate and having a bandgap smaller than that
of the substrate; a diffusion layer of a second conduction type
formed on a portion of the light absorbing layer; a window layer of
the first conduction type formed on the light absorbing layer so as
to surround the diffusion layer and having a bandgap larger than
that of the light absorbing layer; an anode electrode formed on the
diffusion layer; and a cathode electrode provided on the substrate
so as to contact the substrate without contacting each of the
window layer and the light absorbing layer, wherein a groove is
formed which surrounds a boundary between the diffusion layer and
the window layer as viewed in plan and extends through the window
layer and the light absorbing layer as viewed in section.
2. A light receiving element comprising: a substrate of a first
conduction type; a light absorbing layer of the first conduction
type formed on the substrate and having a bandgap smaller than that
of the substrate; a diffusion layer of a second conduction type
formed on a portion of the light absorbing layer; a window layer of
the first conduction type formed on the light absorbing layer so as
to surround the diffusion layer and having a bandgap larger than
that of the light absorbing layer; an anode electrode formed on the
diffusion layer; a cathode electrode provided on the substrate so
as to contact the substrate without contacting each of the window
layer and the light absorbing layer; and an ion-implanted portion
surrounding a boundary between the diffusion layer and the window
layer as viewed in plan, extending through the window layer and the
light absorbing layer as viewed in section, and shutting off a flow
of carriers.
3. The light receiving element according to claim 1, wherein the
whole of a lower surface of the substrate is a light receiving
surface.
4. The light receiving element according to claim 1, further
comprising a metal mask covering a portion of a lower surface of
the substrate.
5. The light receiving element according to claim 1, further
comprising an epitaxial layer covering a portion of a lower surface
of the substrate and capable of absorbing light.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a light receiving element
for use in optical communication for example.
[0003] 2. Background Art
[0004] Japanese Patent Laid-Open No. 2010-45417 discloses a light
receiving element having an anode electrode on the upper surface
side of a substrate, and a cathode electrode on a lower surface of
the substrate. Japanese Patent Laid-Open No. 2007-88496 discloses a
light receiving element having an anode electrode and a cathode
electrode on the upper surface side of a substrate.
[0005] Increasing the response speed of a light receiving element
requires eliminating a "current component delayed in response"
which is a current component generated after a lapse of a certain
time period from a time at which light enters the light receiving
element. With the light receiving element disclosed in Japanese
Patent Laid-Open No. 2010-45417, there is a problem that if the
thickness of the substrate is increased, the distance through which
carriers travel is increased and the response speed is reduced. In
the light receiving element disclosed in Japanese Patent Laid-Open
No. 2007-88496, the anode electrode and the cathode electrode are
adjacent to each other and there is a possibility of a short
circuit between the anode electrode and the cathode electrode.
SUMMARY OF THE INVENTION
[0006] The present invention has been achieved to solve the
above-described problems, and an object of the present invention is
to provide a light receiving element capable of having an increased
response speed preventing a short circuit between the anode
electrode and the cathode electrode.
[0007] The features and advantages of the present invention may be
summarized as follows.
[0008] According to one aspect of the present invention, a light
receiving element includes a substrate of a first conduction type,
a light absorbing layer of the first conduction type formed on the
substrate and having a bandgap smaller than that of the substrate,
a diffusion layer of a second conduction type formed on a portion
of the light absorbing layer, a window layer of the first
conduction type formed on the light absorbing layer so as to
surround the diffusion layer and having a bandgap larger than that
of the light absorbing layer, an anode electrode formed on the
diffusion layer, and a cathode electrode provided on the substrate
so as to contact the substrate without contacting each of the
window layer and the light absorbing layer, wherein a groove is
formed which surrounds a boundary between the diffusion layer and
the window layer as viewed in plan and extends through the window
layer and the light absorbing layer as viewed in section.
[0009] According to another aspect of the present invention, a
light receiving element includes a substrate of a first conduction
type, a light absorbing layer of the first conduction type formed
on the substrate and having a bandgap smaller than that of the
substrate, a diffusion layer of a second conduction type formed on
a portion of the light absorbing layer, a window layer of the first
conduction type formed on the light absorbing layer so as to
surround the diffusion layer and having a bandgap larger than that
of the light absorbing layer, an anode electrode formed on the
diffusion layer, a cathode electrode provided on the substrate so
as to contact the substrate without contacting each of the window
layer and the light absorbing layer, and an ion-implanted portion
surrounding a boundary between the diffusion layer and the window
layer as viewed in plan, extending through the window layer and the
light absorbing layer as viewed in section, and shutting off a flow
of carriers.
[0010] Other and further objects, features and advantages of the
invention will appear more fully from the following
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a sectional view of a light receiving element
according to a first embodiment;
[0012] FIG. 2 is a plan view of the light receiving element;
[0013] FIG. 3 is a sectional view of a light receiving element
according to a second embodiment;
[0014] FIG. 4 is a sectional view of a light receiving element
according to a third embodiment; and
[0015] FIG. 5 is a plan view of the light receiving element shown
in FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0016] A light receiving element according to embodiments of the
present invention will be described with reference to the drawings.
The same or corresponding components are given the same numerals
and repetition of the description may be omitted.
First Embodiment
[0017] FIG. 1 is a sectional view of a light receiving element 10
according to a first embodiment of the present invention. The light
receiving element 10 has a substrate 12 of a first conduction type.
The substrate 12 is, for example, n-type InP. A light absorbing
layer 14 of the first conduction type is formed on the substrate
12. The light absorbing layer 14 has a bandgap smaller than that of
the substrate 12. The light absorbing layer 14 is, for example,
n-type InGaAs.
[0018] A diffusion layer 16 of a second conduction type is formed
on a portion of the light absorbing layer 14. The diffusion layer
16 is, for example, p-type InP. A window layer 18 of the first
conduction type is formed on the light absorbing layer 14. The
window layer 18 is formed so as to surround the diffusion layer 16
as viewed in plan. The window layer 18 has a bandgap larger than
that of the light absorbing layer 14. The window layer 18 is, for
example, n-type InP.
[0019] Grooves 20 and 22 are formed on the upper surface side of
the substrate 12. The groove 20 surrounds a boundary 23 between the
diffusion layer 16 and the window layer 18 as viewed in plan, and
extends through the window layer 18 and the light absorbing layer
14 as viewed in section. The groove 22 is formed outside the groove
20. The groove 22 extends through the window layer 18 and the light
absorbing layer 14 as viewed in section.
[0020] An insulating film 24 is formed on the upper surface side of
the light receiving element 10. The insulating film 24 is formed on
the upper surface of the window layer 18 and in the grooves 20 and
22. The insulating film 24 in the groove 20 abuts the window layer
18, the light absorbing layer 14 and the substrate 12. The material
of the insulating film 24 is, for example, SiN.
[0021] The insulating film 24 has an opening right above the
diffusion layer 16. An anode electrode 26 is formed on the
diffusion layer 16. The insulating film 24 in the groove 22 has an
opening in its lower end portion. A cathode electrode 28 is
provided in this opening so as to contact the substrate 12. The
cathode electrode 28 is provided on (above) the substrate 12. The
cathode electrode 28 abuts the insulating film 24 in the groove 22
and does not contact the window layer 18 and the light absorbing
layer 14. The anode electrode 26 and the cathode electrode 28 are
each formed, for example, of a multilayer structure including Ti
and Au, i.e., Ti/Au (the material on the left-hand side of the
symbol "/" is closer to the substrate, materials shown in the same
way below), Ti/Pt/Au, Pt/Ti/Pt/Au, or the like.
[0022] A portion of a lower surface of the substrate 12 is covered
with a metal mask 30. The metal mask 30 is made of the material
which does not transmits light. However, the metal mask 30 is
formed, for example, of a multilayer structure including Ti and Au,
i.e., AuGe/Ni/Ti/Pt/Au, AuZn/Ti/Pt/Au, or the like. The metal mask
30 is not formed right below the diffusion layer 16. That is, the
metal mask 30 has an opening right below the diffusion layer 16. A
low-reflection film 32 is formed in this opening so as to contact
the lower surface of the substrate 12. The low-reflection film 32
allows light to pass therethrough without reflecting light. The
low-reflection film 32 is not particularly specified in other
respects. However, the low-reflection film 32 is formed, for
example, an Si-SiO.sub.2 multilayer structure, i.e., Si/SiO.sub.2,
Si/SiO.sub.2/Si/SiO.sub.2, or the like.
[0023] FIG. 2 is a plan view of the light receiving element 10. The
groove 20 surrounds the boundary 23 between the diffusion layer 16
and the window layer 18 as viewed in plan. The boundary 23 is
indicated by a broken line. In FIG. 2, the outer circumference of
the low-reflection film 32 is also indicated by a broken line. The
outer circumference of the low- reflection film 32 and the inner
circumference of the groove 20 are superposed on each other. FIG. 1
is a sectional view taken along a line indicated by a dot-dash line
in FIG. 2.
[0024] The operation of the light receiving element 10 will be
described. A reverse bias is applied to the pn junction between the
diffusion layer 16 and the window layer 18 and the pn junction
between the diffusion layer 16 and the light absorbing layer 14 to
form a depletion layer. In this state, light enters the light
receiving element 10 from a direction indicated by an arrow in FIG.
1. The light having entered generates carriers (electrons and
holes) in the light absorbing layer 14. The generated carriers are
accelerated by the depletion layers to flow to the anode electrode
26 or the cathode electrode 28. As a result, a current is
detected.
[0025] It is preferable that when carriers are generated by light,
they are rapidly detected as a current. In ordinary light receiving
elements, however, the distance through which carriers generated in
a place remote from the depletion layer (diffusion layer) move is
long and the time taken for the carriers to move before being
detected as a current is considerably long. Carriers move through a
long distance and become a current component after a lapse of a
certain time period from the time at which light enters the light
receiving element, which means degradation in response of the light
receiving element. That is, if carriers having moved through a long
distance contribute to the detected current, a current component
delayed in response will be generated.
[0026] In the light receiving element 10 according to the first
embodiment of the present invention, however, carriers generated by
light entering the element outside the region surrounded by the
groove 20 cannot reach the depletion layer because of the existence
of the groove 20. Therefore, even when light enters outside the
region surrounded by the groove 20, carriers generated by the light
do not contribute to the detected current. Thus, the generation of
a current component delayed in response can be prevented.
[0027] In some cases of optical system design, the thickness of the
substrate 12 is increased to a thickness of about several hundred
microns for example. In such a case, if the cathode electrode is
provided on the lower surface of the substrate 12, it is necessary
for carriers to travel through the thick substrate, resulting in
generation of a current component delayed in response. In contrast,
in the light receiving element 10 according to the embodiment of
the present invention, the cathode electrode 28 is provided on the
upper surface side of the substrate 12, so that the carrier travel
distance is not increased even when the thickness of the substrate
12 is increased, thus enabling prevention of a current component
delayed in response.
[0028] Since the anode electrode 26 and the cathode electrode 28
are provided on the upper surface side of the substrate 12,
mounting of the light receiving element 10 is completed by only
die-bonding the element to a submount. In the case where the
cathode electrode is formed on the lower surface of the substrate
while the anode electrode is formed on the upper surface of the
substrate, it is necessary to fix the anode electrode on the
submount before wire bonding on the cathode electrode.
[0029] In the case where the cathode electrode is formed in the
groove 20, there is a risk of a short circuit between the anode
electrode and the cathode electrode. According to the first
embodiment of the present invention, however, the problem of a
short circuit between the anode electrode 26 and the cathode
electrode 28 can be avoided since the groove 20 exists between
these electrodes.
[0030] The light receiving element 10 according to the first
embodiment of the present invention can be variously modified. For
example, the outer circumference of the low-reflection film 32 (the
opening of the metal mask 30) may be set larger than the inner
circumference of the groove 20 since only light entering the region
surrounded by the groove 20 contributes to the detected current.
Thus, the entire pn junction can be used for current generation.
Further, the entire lower surface of the substrate 12 may be used
as a light receiving surface by omitting the metal mask 30. Thus,
the amount of light blocked by the metal mask 30 can be reduced by
increasing the opening of the metal mask 30 or omitting the metal
mask 30, resulting in reduction of the light-electricity conversion
loss.
[0031] While the first conduction type and the second conduction
type are assumed to be the n-type and the p-type, respectively, the
conduction types may be reversed. A buffer layer and the like may
be provided between the layers. These modifications can be applied
as desired to light receiving elements according to embodiments
described below. The light receiving elements according to the
embodiments described below have a number of commonalities with the
first embodiment and will therefore be described mainly with
respect to points of difference from the first embodiment.
Second Embodiment
[0032] FIG. 3 is a sectional view of a light receiving element
according to a second embodiment of the present invention. A
portion of the lower surface of the substrate 12 is covered with an
epitaxial layer 60. The material of the epitaxial layer 60 is not
particularly limited as long as it is capable of absorbing light.
However, the material of the epitaxial layer 60 is, for example,
n-type InGaAs. The epitaxial layer 60 has an opening right below
the diffusion layer 16. A low-reflection film 62 is formed in this
opening.
[0033] The metal mask 30 in the first embodiment reflects light and
there is a possibility of the reflected light interfering with the
incident light. However, in the second embodiment of the present
invention, such reflection of light can be prevented since the
epitaxial layer 60 capable of absorbing light is formed.
Third Embodiment
[0034] FIG. 4 is a sectional view of a light receiving element
according to a third embodiment of the present invention. This
light receiving element is provided with an ion-implanted portion
70 formed, for example, by implantation with Ti and the like. The
ion-implanted portion 70 surrounds the boundary between the
diffusion layer 16 and the window layer 18 as viewed in plan, and
extends through the window layer 18 and the light absorbing layer
14 as viewed in section. When light enters the ion-implanted
portion 70, no carriers are generated. Also, the ion-implanted
portion 70 shuts off the flow of carriers. FIG. 5 is a plan view of
the light receiving element shown in FIG. 4. The ion-implanted
portion 70 indicated by a broken line surrounds the boundary 23
between the diffusion layer and the window layer as viewed in
plan.
[0035] The ion-implanted portion 70 has the same function as that
of the groove 20 in the first embodiment. In addition, the
ion-implanted portion 70, can easily be made by only performing ion
implantation after the window layer 18 is formed. A suitable
combination of the features of the light receiving elements
according to the embodiments described above may be made and
used.
[0036] According to the present invention, the anode electrode and
the cathode electrode are formed on the upper surface side of the
substrate and a groove or an ion-implanted portion is formed
between the anode electrode and the cathode electrode, thereby
enabling increasing the response speed and preventing a short
circuit between the anode electrode and the cathode electrode.
[0037] Obviously many modifications and variations of the present
invention are possible in the light of the above teachings. It is
therefore to be understood that within the scope of the appended
claims the invention may be practiced otherwise than as
specifically described.
* * * * *