Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal – Electromagnetic or particle radiation
Reexamination Certificate
1999-02-08
2002-05-28
Jackson, Jr., Jerome (Department: 2824)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
Electromagnetic or particle radiation
C257S435000, C257S184000, C385S014000, C385S131000
Reexamination Certificate
active
06396117
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor photodetector for converting light into electricity at high speed, which is suitable for use in optical communications, and more particularly, to a semiconductor photodetector which is capable of selective photoelectric conversion of light on the longer wavelength side in two-wavelength multiplex optical communications, thereby extracting the signal on this longer wavelength side, and to a method for manufacturing same and a photodetector module containing the photodetector.
2. Description of Related Art
One example of a conventional semiconductor photodetector (hereinafter, referred to as “photodetector” or “photo detecting element”) is described in the reference source (Hiker tsushin soshi kogaku, “Hakko/Juko soshi”, pp. 371-372, pp. 384, Kogaku Toshokan).
FIG. 6
is a general sectional view showing the composition of-the photodetector according to this source. This photodetector
100
is a planar surface-exposure-type photodetector using an InGaAs-type compound semiconductor. This photo detecting element
100
comprises an n-InP buffer layer
103
, an n-InGaAs light-absorbing layer
105
, an n-InP window layer
107
, and an insulating film
109
, laminated in this order onto an n
+
InP substrate
101
. A portion of the n-InP window layer
107
is constituted by a p
+
diffused region
111
. Furthermore, a p-side electrode
113
which is electrically connected to the p
+
diffused region
111
is provided on the p
+
diffused region
111
, and an n-side electrode
115
is provided on the rear face of the n
+
-InP substrate
101
. A short-wavelength shielding filter
117
formed by a dielectric film is provided on the light receiving surface of the photo detecting element
100
, on top of the insulating film
109
. The p-side electrode
113
is exposed outside the short-wavelength shielding filter
117
(FIG.
6
).
This photodetector
100
uses the photovoltaic effect of a pn junction diode. In two-waveleilgth multiplex optical communications, it performs photoelectric conversion of light on the longer wavelength side, selectively. Light on the shorter wavelength side is shut out by being reflected at the exposed surface of the aforementioned short-wavelength shielding filter
117
, so only light on the longer wavelength side reaches the light-absorbing layer
105
. Therefore, in the light-absorbing layer
105
, only this light on the longer wavelength side is photoelectrically converted, and hence it is possible to extract the electrical signal on the longer wavelength side alone.
Moreover,
FIG. 7
is a general compositional view of a conventional semiconductor photodetector module (hereinafter, referred to as “module”) provided with a short-wavelength shielding filter, and it shows a sectional view thereof. This module
200
comprises: an optical fiber
201
having a light emitting end face
201
a; a ferrule
203
for supporting the optical fiber
201
in the center portion thereof; a lens
205
for focusing light emitted from the optical fiber
201
; a semiconductor photodetector
207
onto which this focused light is irradiated; and a header
209
onto which the photodetector
207
is fixed. In this module
200
, a short-wavelength shielding filter
211
is provided between the lens
205
and the semiconductor photodetector
207
(FIG.
7
).
In this module
200
, if multiplexed light comprising two light beams of different wavelengths is emitted from the optical fiber
201
, then the light on the shorter wavelength side in the light focused by the lens
205
will be shut out by the short-wavelength shielding filter
211
. Therefore, light on the longer wavelength side will be irradiated onto the photodetector
207
. In this way, only light on the longer wavelength side is photoelectrically converted and so only the signal corresponding to this light can be extracted.
However, in a conventional photodetector as described above, a short-wavelength shielding filter made from a dielectric film is provided in order to shut out light on the shorter wavelength side from the light receiving surface of the element. The material used as the short-wavelength shielding filter for a semiconductor photodetector of this composition is a dielectric film. This dielectric film is formed from a different material to the dielectric films used in standard wafer processing, such as silicon oxide films, silicon nitride films, or the like. If a dielectric film forming a filter is provided on an underlying layer, then the difference in coefficient of expansivity between the filter and the underlying layer on which the filter is provided will be large, and therefore problems will arise in that the structural and operational reliability of the photodetector will deteriorate.
Moreover, processing problems also arise in that different etching methods are used for the filter and the other layers constituting the photodetector. Consequently, there is a possibility that the manufacturing equipment and processing involved will become more complex, thereby causing costs to rise also.
Furthermore, in a photodetector module having a structure wherein a short-wavelength shielding filter is provided between the semiconductor photodetector and the lens, increase in costs is unavoidable due to the need to provide a new, special filter in the conventional module structure.
There are also cases where light on the shorter wavelength side is reflected at the surface of the filter and is returned back to the transmitting side. This back-reflected light causes noise, and may degrade the reliability of the optical communications system.
Moreover, attenuation occurs in the light on the longer wavelength side transmitted by the filter, and therefore degradation of the signal may also occur.
SUMMARY OF THE INVENTION
This invention was devised with the foregoing in view. Therefore, it is an object of the invention to provide a semiconductor photodetector having a structure whereby light on the longer wavelength side can be photoelectrically converted and output reliably, whilst improving the reliability of the photodetector (or photo detecting element) in terms of its structure and operational performance, by means of a simple manufacturing process and inexpensive manufacturing costs.
It is a further object of this invention to provide a method for manufacturing a semiconductor photodetector (or photo detecting element) of this kind, inexpensively.
It is yet a further object of this invention to provide a photodetector (or photo detecting element) module for carrying out optical communications, whereby the reliability of an optical communications system is not degraded and no signal deterioration occurs.
According to a first aspect of the present invention, there is provided a semiconductor photodetector having the following composition. A first light-absorbing layer, a buffer layer of a second conductivity type, a second light-absorbing layer of a second conductivity type and a window layer of a second conductivity type are laminated successively onto a first principal surface of a substrate of a first conductivity type. The first light-absorbing layer comprises a region of a first conductivity type and a region of a second conductivity type provided in a laminated fashion in this order from the substrate. A diffused region of a first conductivity type having a depth extending from the upper face of the window layer to the interface between the window layer and the second light-absorbing layer is provided in a portion of the window layer. A main electrode of a first conductivity type, electrically connected to the diffused region, is provided on the diffused region; and a main electrode of a second conductivity type, electrically connected to the window region, is provided on the window region. In this semiconductor photodetector, the energy gap wavelength of the second light-absorbing layer is longer than the energy gap wavelength of the first light-absorbing layer, and the energy gap wavelength o
Furukawa Ryozo
Kato Masanobu
Burdett James R.
Jackson, Jr. Jerome
Oki Electric Industry Co,. LTD
Venable
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