Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2002-03-12
2003-02-25
Flynn, Nathan J. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S186000, C257S189000, C257S436000, C257S438000, C257S452000, C257S457000, C257S459000, C257S461000
Reexamination Certificate
active
06525347
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photodetector and a unit mounted with a photodetector, and in particular relates to a photodetector that selectively receives signal light of a long wavelength (on the long wavelength side) when there is a plurality of incident signal lights of different wavelengths.
Currently there are broad applications for PIN photodiodes of a compound semiconductor material as photodetectors for optical fiber communications.
These PIN photodiodes employ a window structure to increase light-receiving sensitivity. In these PIN photodiodes, a light absorbing layer with a small forbidden bandwidth (long absorption edge wavelength) is formed on a semiconductor substrate, and a filter layer (window layer) with a large forbidden bandwidth (short absorption edge wavelength) is formed thereon. This structure makes it possible for light of the absorption edge wavelengths of the light absorption layer and the filter layer to be efficiently received.
A typical PIN photodiode is made of InGaAs/InP, where InGaAs is the material for the light absorption layer and InP is the material for the filter layer. In this case, the signal light in the wavelength range of 0.9 &mgr;m to 1.65 &mgr;m, which are the absorption edge wavelengths of InP and InGaAs, respectively, can be received with high sensitivity. An example of such a PIN photodiode structure is disclosed in Japanese Laid-Open Patent Publication No.1-238070.
Hereinafter, a conventional PIN photodiode will be described with reference to FIG.
10
.
FIG. 10
is a cross-sectional view of a conventional PIN photodiode.
As shown in
FIG. 10
, an n
−
-InP carrier blocking layer (buffer layer)
102
, an n
−
-InGaAs light absorption layer
103
, an n
−
-AlAsSb carrier blocking layer
104
, and an n
−
-InP filter layer
105
are sequentially laminated onto an n+-InP substrate
101
. Here, the forbidden bandwidth of the AlAsSb is larger than that of the InP, and has an absorption edge wavelength of 0.67 &mgr;m.
A p+ diffusion region
106
made by the diffusion of Zn is formed in the filter layer
105
and the carrier blocking layer
104
, and a surface protection film
107
is formed thereon. Additionally, a ringed anode electrode
108
is formed on the diffusion region
106
, and a cathode electrode
109
is formed on the back face of the substrate
101
.
It should be noted that since the PIN photodiode shown in
FIG. 10
includes the carrier blocking layer
104
, it has an improved sensitivity with respect to signal light of wavelengths of 0.9 &mgr;m or less over a PIN photodiode in which the carrier blocking layer
104
is not formed.
To explain the reason for this, signal light of wavelengths of 0.9 &mgr;m or less are absorbed by the filter layer
105
and generate electron-hole pairs, and in the case of PIN photodiodes in which the carrier blocking layer
104
is not formed, some of the electrons generated in the filter layer
105
flow into the light absorption layer
103
to become photoelectric current. In contrast, when there is the carrier blocking layer
104
, the electrons are blocked by the hetero barrier at the interface between the filter layer
105
and the carrier blocking layer
104
, and therefore do not become photoelectric current.
Passband type PIN photodiodes that are sensitive only to light of a narrower wavelength range have also been developed. For example, when performing wavelength multiplex communication with signal light of a 1.3 &mgr;m wavelength and signal light of a 1.55 &mgr;m wavelength, a passband type PIN photodiode sensitive only to those wavelengths is useful.
Here a PIN photodiode with short wavelength passband properties that is adequately sensitive to signal light of a 1.3 &mgr;m wavelength but has almost no sensitivity to signal light of a 1.55 &mgr;m wavelength can be readily achieved by forming the light absorption layer of the above PIN photodiode made of InGaAs, using an InGaAsP with an absorption edge wavelength of 1.4 &mgr;m.
On the other hand, long wavelength passband properties where there is sufficient sensitivity to signal light of a 1.55 &mgr;m wavelength but almost no sensitivity to 1.3 &mgr;m wavelength signal light can be attained by taking InP as the material for the carrier blocking layer
104
and InGaAsP with an absorption edge wavelength of 1.4 &mgr;m as the material for the filter layer
105
. The relationship of the forbidden bandwidth of the carrier blocking layer
104
being larger than that of the filter layer
105
is maintained.
However, in conventional PIN photodiodes having passband properties, the filter layer
105
had to be made thick to increase the sensitivity difference with respect to long wavelength side signal light and short wavelength side signal light.
Accordingly, to achieve a PIN photodiode with long wavelength passband properties the material for the filter layer
105
should be InGaAsP, as described above. However, there is the problem that epitaxially growing a thick InGaAsP easily causes fluctuations in the composition of the InGaAsP.
On the other hand, in a planar PIN photodiode, in which impurities such as Zn are diffused in an island in the filter layer
105
to form a diffusion region, the thickness of the carrier blocking layer
104
is 1 to 2 &mgr;m, and typically the diffusion of impurities is carried out to this depth.
This means that in conventional PIN photodiodes, even taking InGaAsP as the material for the filter layer
105
and being able to epitaxially grow the filter layer
105
thickly leaves the problem that it is difficult to diffuse impurities deeply and with good control. It should be noted that if the photodiode is a mesa PIN photodiode, in which the carrier blocking layer
104
and the filter layer
105
are doped so as to be p-type in advance and the element is separated by etching, it is no longer necessary to partially diffuse the impurities in an island-shape, but mesa PIN photodiodes, particularly those that are InGaAs/InP, have a large dark current and are less reliable than planar PIN photodiodes fabricated by impurity diffusion.
SUMMARY OF THE INVENTION
It is a main object of the present invention to provide a photodetector with long wavelength passband properties in which a high sensitivity ratio is obtained even with a thin filter layer and the depth of impurity diffusion in the light-receiving portion is equivalent to that of conventional PIN photodiodes.
A first photodetector according to the present invention includes a semiconductor substrate, a filter layer formed on a first principal face of the semiconductor substrate, a light absorption layer formed in an island on the filter layer, and a light incidence portion formed on the filter layer at a portion where the light absorption layer has not been formed; wherein the absorption edge wavelength of the filter layer is longer than the absorption edge wavelength of the semiconductor substrate and shorter than the absorption edge wavelength of the light absorption layer.
With the first photodetector, incident light that enters obliquely from the light incidence portion formed on the first principal face side of the semiconductor substrate is reflected at the second principal face so that the light passes through the filter layer, which is formed between the semiconductor substrate and the light absorption layer, twice before it is incident to the light absorption layer. This means that compared to photodetector with a structure in which the incident light passes through the filter layer a single time, the thickness of the filter layer can be made substantially double.
Additionally, it is preferable that in the first photodetector, a reflective film is formed on a second principal face of the semiconductor substrate.
Further, it is preferable that the first photodetector includes a carrier blocking layer formed between the filter layer and the light absorption layer, and of a material with a larger forbidden bandwidth than that of the filter layer; and a wide band gap layer formed on the light ab
Cole Thomas W.
Flynn Nathan J.
Greene Pershelle
Matsushita Electric - Industrial Co., Ltd.
Nixon & Peabody LLP
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