Active solid-state devices (e.g. – transistors – solid-state diode – Heterojunction device – Light responsive structure
Reexamination Certificate
2002-09-09
2004-09-14
Eckert, George (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Heterojunction device
Light responsive structure
C257S184000, C257S438000, C257S452000, C257S463000, C257S466000
Reexamination Certificate
active
06791124
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-284039, filed Sep. 18, 2001; and No. 2002-218311, filed Jul. 26, 2002, the entire contents of both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sequential mesa avalanche photodiode and a method of manufacturing the same, and in particular, to a sequential mesa avalanche photodiode having a sequential mesa structure in which, in an avalanche photodiode to be used as a light receiving element for converting a light signal to an electric signal in an optical communication network or the like, high sensitization can be realized and the fabrication costs of modularization can be greatly decreased, and to a method of manufacturing the same.
2. Description of the Related Art
As is well-known, recently, the signal speed of light signals used in optical communication networks has been made much more high-speed.
In accordance therewith, making the speed more high-speed has been required of light receiving elements built in optical communication equipment transmitting and receiving such light signals.
Further, in such light receiving elements, it is required that even low level light signals can be precisely received.
As such a light receiving element receiving high-speed and weak light signals, generally, an avalanche photodiode (hereinafter, abbreviated APD) has been put into practice.
In such an APD, in a state in which a depletion region is formed by applying reverse-bias voltage to a pn junction formed by a pair of semiconductor layers whose conductive types are different from one another, when an electromagnetic wave of a light signal or the like is incident from the exterior, a pair of an electron and a positive hole is generated.
Further, this pair of the electron and the positive hole is multiplied by the avalanche phenomenon in the APD, and taken out as voltage or electric current to the exterior.
There are various ways of classifying APDs. When classifying structurally, there are a planar type and a mesa type, and when classifying by main carrier, there are a positive hole type and an electron type.
Here, a sequential mesa structure used regardless of the type of the main carrier will be described.
Generally, in order to aim for making the APD high-speed, the mesa type, not the planar type, is generally used as the shape of the APD.
This is for decreasing the electric capacity of the APD element itself in order to make the APD high-speed.
In order to increase the permissible light-receiving current as an APD element, there is the need to remove the bias of the light-receiving current density flowing through the interior of the mesa portion.
Therefore, in a mesa type APD element, the shape of the mesa must be made to be isotropic, namely, as shown in
FIG. 9B
, formed conically as viewed from the top surface of a substrate.
Moreover, in a mesa type APD element, when the shape of the mesa is formed to be conical, attention must be paid such that the crystallinity of the cross-section of the mesa is not damaged.
Therefore, in a mesa type APD element, when the shape of the mesa is fabricated, diffusive wet-etching by an etchant which is not anisotropic is necessary.
By applying this diffusive wet-etching, the sequential mesa shape, which is a shape (generally, conical) in which the mesa diameter (cross-sectional area) widens as it approaches the substrate, can be obtained.
Accordingly, the sequential mesa type APD is generally used for making the APD high-speed.
Further, as APDs using positive holes as the main carrier, there are an APD in which the above-described pn junction is formed by epitaxial growth, and an APD in which the pn junction is formed by Zn diffusion.
FIGS. 9A and 9B
respectively show a cross-sectional view and an external perspective view of a sequential mesa type APD, in accordance with a prior art, which has a sequential mesa structure and in which positive holes are used as the main carrier and the pn junction is formed by epitaxial growth.
Hereinafter, on the basis of
FIGS. 9A and 9B
, the structure of the sequential mesa type APD according to the prior art will be described.
Namely, in the sequential mesa type APD according to the prior art, as shown in
FIGS. 9A and 9B
, an n-type buffer layer
2
a
, an n-type light absorbing layer
3
a
, an n-type electric field relaxation layer
4
a
, an n-type multiplying layer
5
a
, and a p-type contact layer
6
b
are successively formed by epitaxial growth by using an MOVPE (organometallic vapor phase epitaxial growth) method on an n-type semiconductor substrate
1
a
. Therefore, a conical sequential mesa portion
10
is formed by wet-etching from above.
Next, after a protective layer
7
is coated on the sequential mesa portion
10
, a p electrode
8
contacting the p-type contact layer
6
b
is formed.
Further, at the both sides of the sequential mesa portion
10
, an n electrode
9
is attached, via a protective layer
11
, to another mesa portion formed for attaching electrodes.
As shown by the arrow in
FIG. 9A
, light incident on the APD from the bottom surface of the semiconductor substrate
1
a
penetrates through the semiconductor substrate
1
a
and the buffer layer
2
a
and is absorbed at the light absorbing layer
3
a
, so that a pair of an electron and a positive hole is generated.
Among the pair of the electron and the positive hole generated in this way, the electron moves to the n electrode
9
via the semiconductor substrate
1
a
, and the positive hole is multiplied at the multiplying layer
5
a
, and moves to the p electrode
8
via the contact layer
6
b.
In order to make the positive hole be the main carrier among the pair of the electron and the positive hole, a great number of the carriers of the light absorbing layer
3
a
must be electrons.
Namely, the conductive type of the light absorbing layer
3
a
must be n type.
Such a sequential mesa type APD uses a so-called SAM (Separate Absorption and Multiplication) structure, in which the multiplying layer
5
a
and the light absorbing layer
3
a
are separated by the electric field relaxation layer
4
a
such that a low electric field intensity is applied to the light absorbing layer
3
a
while a high electric field intensity is applied to the multiplying layer
5
a.
In this case, because the electric field intensity of the n-type light absorbing layer
3
a
is suppressed by the electric field relaxation layer
4
a
, the conductive type of the electric field relaxation layer
4
a
is the same n type as that of the light absorbing layer
3
a.
Because such a sequential mesa type APD has a function avalanche-multiplying the light exciting carrier, the crystallinity of the above-described layers is considered to be extremely important.
Note that, in such a sequential mesa type APD, the epitaxial growth itself of each layer can be carried out, in theory, on a semiconductor substrate which is any of an n-type semiconductor substrate, a p-type semiconductor substrate, or a semi-isolated semiconductor substrate.
As described above, in the sequential mesa type APD, when considering the fact that light-receiving current flows via the semiconductor substrate, the semiconductor substrate which is used must be an n-type or a p-type semiconductor substrate.
However, as shown in
FIGS. 9A and 9B
, because a dopant such as Sn, S or the like included in the semiconductor substrate
1
a
does not diffuse during the epitaxial growth, the n-type semiconductor substrate
1
a
is suitable as a substrate for the epitaxial growth of each semiconductor layer.
On the other hand, in the p-type semiconductor substrate, there are problems such as the Zn included in the semiconductor substrate diffuses during the epitaxial growth, there is the need to form a thicker buffer layer by epitaxial growth in order to prevent the Zn from diffusing, and because the n-type semiconductor substrate layer is formed by epitaxial growth
Hiraoka Jun
Mizuno Kazuo
Sasaki Yuichi
Anritsu Corporation
Eckert George
Frishauf Holtz Goodman & Chick P.C.
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