Semiconductor photodetector

Details Semiconductor photodetector Semiconductor photodetector

1998-11-06

2001-06-12

Jackson, Jr., Jerome (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

Electromagnetic or particle radiation

C257S466000

Reexamination Certificate

active

06246097

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor photodetector (hereafter referred to in abbreviation as “PD”).
In recent years, side-entry type PDs with light to be detected entering in the horizontal direction relative to the substrate have been proposed as semiconductor photodetectors that are well suited for flat mounting modules. Generally speaking, the main objective of flat mounting modules is reduction of production cost, and implementation of non-alignment (passive alignment) mounting by using inexpensive optical elements.
In passive alignment mounting, precision alignment for perfectly matching the optical axes of the individual optical elements to be mounted is not implemented. As a result, a semiconductor photodetector to be employed in a flat mounting module must demonstrate outstanding tolerance characteristics so that sufficient sensitivity is assured even if misalignment of optical axes occurs within the mounting accuracy range of the passive alignment mounting method.
It is to be noted that tolerance characteristics in this context refer to characteristics representing the optical axis misalignment tolerance range over which a sufficient degree of photodetection sensitivity is achieved, and under normal circumstances, different characteristics manifest depending upon the direction in which the optical axis misalignment is measured. In the following explanation and in the attached drawings, a coordinate system is employed to explain the tolerance characteristics of semiconductor photodetectors. In the coordinate system, the X direction represents the direction of the width of the substrate, the Y direction represents the direction of the thickness of the substrate and the Z direction represents the direction of the depth of the substrate, relative to the direction in which light enters.
Generally speaking, a side-entry type PD is provided with a laminated body adopting a structure achieved by directly or indirectly sandwiching a light absorption layer with complementary type semiconductor layers from a vertical direction, with the laminated body functioning as a photodetection portion. In other words, in a side-entry type PD, light is detected by extracting to the outside an electrical charge generated by the entry of light into the light absorption layer of the laminated body via the complementary semiconductor layers.
Side-entry type PDs in the prior art include, for instance, (A) cleavage photodetection plane PDs and (B) light refraction type mesa photodetection surface PDs.
(A) A cleavage photodetection plane PD is a side-entry type PD that uses a cleavage plane formed at a side surface of the laminated body as a photodetection surface.
(1) Cleavage photodetection plane PDs in the prior art include a semiconductor photodetector
600
adopting a simple pin structure illustrated in FIG.
14
.
FIG. 14
is a cross section in the Y-Z direction that illustrates the schematic structure of the semiconductor photodetector
600
in the prior art. It is to be noted that the term simple pin structure is used to describe a laminated body with a pin junction in which the light absorption layer is directly sandwiched by complementary semiconductor layers.
As illustrated in
FIG. 14
, the semiconductor photodetector
600
in the prior art adopts a structure in which a laminated body
620
having a cleavage photodetection plane
620
a
formed at a side surface thereof is provided on, for instance, an n
+
-InP substrate
610
. In addition, the laminated body
620
is formed by sequentially laminating a buffer layer
630
, a light absorption layer
640
and a cap layer
650
on the substrate
610
.
During the process for manufacturing the semiconductor photodetector
600
in the prior art, the buffer layer
630
which is equivalent to a semiconductor layer on the substrate side is formed by, for instance, epitaxially growing an n-InP layer on the substrate
610
. In addition, the light absorption layer
640
is formed by, for instance, epitaxially growing an n
−
-InGaAsP layer on the buffer layer
630
. The cap layer
650
which is equivalent to a semiconductor layer is formed by, for instance, epitaxially growing a p
+
-InP layer on the light absorption layer
640
.
In the semiconductor photodetector
600
in the prior art adopting this structure, incoming light P
6
entering through the cleavage photodetection plane
620
a
is not wave-guided to the inside of the laminated body
620
. In other words, with the semiconductor photodetector
600
, in which the incoming light P
6
is completely absorbed at the light absorption layer
640
in the vicinity of the cleavage photodetection plane
620
a
, degradation of the cutoff frequency due to a local increase in the electrical charge density tends to occur.
(2) An example of cleavage photodetection plane PDs with cutoff frequency characteristics superior to those of the cleavage photodetection plane PDs having the simple pin structure described above is a side-entry type PD adopting a waveguide structure disclosed by M. Shishikura et al. In Electron. Lett. vol. 32, No. 20, p1882-1883, 1996. It is to be noted that the waveguide structure in this context refers to a structure having a light guide layer for guiding light into the inside of the laminated body between the light absorption layer and the semiconductor layer, i.e., a laminated structure in which the light absorption layer is indirectly sandwiched by semiconductor layers.
Now, a side-entry type PD adopting the waveguide structure in the prior art is explained in reference to a semiconductor photodetector
700
illustrated in FIG.
15
. It is to be noted that
FIG. 15
is a cross section in the Y-Z direction illustrating a schematic structure of the semiconductor photodetector
700
.
As illustrated in
FIG. 15
, the semiconductor photodetector
700
in the prior art adopts a structure achieved by forming a laminated body
720
having a cleavage photodetection plane
720
a
formed at a side surface thereof on, for instance, an n
+
-InP substrate
710
. The laminated body
720
is formed by sequentially laminating a buffer layer
730
, a first light guide layer
735
, a light absorption layer
740
, a second light guide layer
745
and a cap layer
750
on the substrate
710
.
During the process for manufacturing the semiconductor photodetector
700
, the buffer layer
730
which is equivalent to a semiconductor layer, is formed by, for instance, epitaxially growing an n-InP layer on the substrate
710
. In addition, the first light guide layer
735
is formed by, for instance, epitaxially growing an n-InGaAsP layer on the buffer layer
730
. The light absorption layer
740
is formed by, for instance, epitaxially growing an n
−
-InGaAsP layer on the first light guide layer
735
.
Furthermore, in the semiconductor photodetector
700
, the second light guide layer
745
is formed by, for instance, epitaxially growing an n-InGaAsP layer on the light absorption layer
740
. The cap layer
750
which is equivalent to a semiconductor layer is formed by, for instance, epitaxially growing a p
+
-InP layer on the second light guide layer
745
.
In the semiconductor photodetector
700
structured as described above, an incoming light P
7
is wave-guided to the inside of the laminated body
720
by the first light guide layer
735
and the second light guide layer
745
unlike in the semiconductor photodetector
600
adopting the pin structure illustrated in FIG.
14
. Therefore, degradation of the cutoff frequency due to a local increase in the electrical charge density does not occur so readily. As a result, the cleavage photodetection plane PD adopting the waveguide structure achieves outstanding characteristics with respect to the cutoff frequency, and is thus commonly employed as a side-entry type PD in the prior art together with the light refraction type mesa photodetection surface PD which is to be explained below.
Now, the tolerance characteristics in the direction of the X axis at a cleavage photodetection plane P

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