Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-03-15
2003-09-09
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S2140RC, C257S431000
Reexamination Certificate
active
06617568
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a side-face incidence type photo detector in which incident light entering the photo detector from a side direction thereof is refracted and detected at the detection area. The present invention especially relates to a photo detector in which incident light is refracted at beveled face formed on at least one edge portion of a second face of a semiconductor substrate, whereby the light path of the incident light is diverted.
2. Description of the Related Art
For optical fiber communications, photo detectors which are sensitive to light of a long wavelength band of 1.3 &mgr;m to 1.55 &mgr;m are used. These photo detectors are pin photo diodes formed of a material selected from InGaAs/InP-type semiconductor materials. Such photo detectors are classified based on the direction from which the photo detector receives incident light. Front-face incidence type and rear-face incidence type photo detectors are generally used. In a front-face incidence type photo detector, incident light enters the semiconductor substrate through a face thereof on which the detection area is formed, i.e., a first principal face. In a rear-face incidence type photo detector, incident light enters the detector through a face which is opposite to the first principal face of the semiconductor substrate (i.e., through a second principal face). In such front-face incidence type and rear-face incidence type photo detectors, incident light enters from a direction substantially perpendicular to the semiconductor substrate.
On the other hand, side-face incidence type photo detectors also have been developed in which incident light enters through a side face of the photo detector. Such side-face incidence type photo detectors are especially advantageous over front-face incidence type and rear-face type photo detectors, in the cases where the photo detector is mounted on a base along with other elements. For example, in the case where the photo detector is provided with an optical fiber for supplying incident light, the photo detector is first bonded onto the base of a flat package, and then the optical fiber can be attached along a horizontal direction with regard to the photo detector. In the case where the light exiting from the rear of a semiconductor laser is monitored by a photo detector, the semiconductor laser and a principal face of the photo detector can be bonded onto the same base. In either case, the devices can be easily assembled.
Such side-face incidence type photo detectors have been implemented by employing so-called waveguide type pin photo diodes, or pin photo diodes having a refraction-based light path diversion means within the photo detector, so as to utilize their structural features to refract light entering the semiconductor substrate.
A pin photo diode having a refraction-based light path diversion means is described, for example, in Japanese Laid-open Publication No. 8-316506.
FIG. 6
shows a cross-sectional view of such a conventional photo detector
600
. With reference to
FIG. 6
, a conventional photo detector
600
includes on a first principal face
602
of an n-type InP semiconductor substrate
601
, an n-type Inp buffer layer
603
, a low concentration n-type InGaAs light absorbing layer
604
, and a low concentration n-type InP window layer
605
in this order. A first diffusion area
606
is formed in a portion of the window layer
605
, by means of diffusing a p-type impurity, e.g., Zn, in an island shape. A portion of the light absorbing layer
604
underlying the first diffusion area
606
functions as a photo detection area
607
. The photo detection area
607
is electrically connected to a negative electrode
608
through the first diffusion area
606
. The negative electrode
608
is formed on the first diffusion area
606
, and a positive electrode
609
is formed on a second diffusion area
610
. The second diffusion area
610
is provided to form the positive electrode
609
on the first principal face
602
. The second diffusion area
610
is simultaneously formed by the diffusion of p-type impurity when the first diffusion area
606
is formed. Beveled face
612
is formed on at least one edge portion of the second principal face
611
of the semiconductor substrate
601
by an etching process. In a conventional photo detector
600
having such a structure, incident light
613
from the side of the semiconductor substrate
601
is refracted at the beveled face
612
, and the refracted light
614
enters the photo detection area
607
.
The conventional photo detector
600
is characterized by refracting the incident light
613
at the beveled face
612
so as to divert the light path of the incident light. Japanese Laid-open Publication No. 8-316506 describes that the (
111
) surface having an angle of 54.7° with regard to the second principal face
611
is preferably used for the beveled face
612
. This is because the beveled face
612
is required to have a particular angle with respect to the second principal face
611
as well as to have a smooth surface. In order to ensure that the beveled face
612
on the semiconductor substrate
601
meets such requirements, it is the most convenient to use a wet etching, which exposes a certain crystal face orientation. For manufacturing the photo detector
600
, a semiconductor substrate whose principal face is the (
001
) surface is generally used as the semiconductor substrate
601
. When a wet etching for exposing a crystal face orientation is applied to such a semiconductor substrate
601
, the (
111
) surface is often exposed. The resultant beveled face
612
having the (
111
) surface form an angle of 54.7° with regard to the second principal face
611
. The incident light
614
entering the interior of the semiconductor substrate
601
is refracted at such a beveled surface
612
so as to form an angle of 25.7° with regard to the second principal face
611
.
As another conventional example, Japanese Laid-open Publication No. 11-307806 describes a pin photo diode having a refraction-based light path diversion means within the photo detector.
FIG. 7
shows a cross-sectional view of a second conventional photo detector
700
. With reference to
FIG. 7
, a conventional photo detector
700
includes on the first principal face
702
of an n-type InP semiconductor substrate
701
, a low concentration n-type InGaAs light absorbing layer
703
, and a low concentration n-type InP window layer
704
in this order. A diffusion area
705
is formed in the window layer
704
, by diffusing a p-type impurity, e.g., Zn, in an island shape. A portion of the light absorbing layer
703
underlying the diffusion area
705
functions as a photo detection area
706
. The photo detection area
706
is electrically connected to a negative electrode
707
through the diffusion area
705
. The negative electrode
707
is formed on the diffusion area
705
, and a positive electrode
709
is formed on a second principal face
708
of the semiconductor substrate
701
. Beveled face
710
is formed on at least one edge portion of the second principal face
708
of the semiconductor substrate
701
. In a conventional photo detector
700
having such a structure, the incident light
711
entering the semiconductor substrate
701
from the side thereof is refracted at the beveled face
710
, and the refracted light
712
enters the photo detection area
706
.
The conventional photo detector
700
is characterized by using the (
112
) surface as the beveled face
710
. When the beveled face
710
is the (
112
) surfaces, the beveled face
710
forms an angle of 35.3° with regard to the second principal face
708
. The (
112
) surface can be exposed by an etching using a mixed solution containing hydrochloric acid and nitric acid. Specifically, if the mixed solution for the etching is prepared so as to have an about 5:1 to about 3:1 hydrochloric acid-nitric acid volume ratio, the beveled face
710
may have an angle of exactly 35.3° with regard to the second prin
Pyo Kevin
Sohn Seung C.
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