Active solid-state devices (e.g. – transistors – solid-state diode – Incoherent light emitter structure – In combination with or also constituting light responsive...
Reexamination Certificate
2001-11-08
2004-01-27
Fahmy, Wael (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Incoherent light emitter structure
In combination with or also constituting light responsive...
C257S460000, C257S461000, C257S090000, C257S450000
Reexamination Certificate
active
06683326
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor photodiode mainly used for optical communications. (Hereafter a semiconductor photodiode is referred to as a “PD”.) Particularly the present invention relates to a surface-mounting type PD in which signal light enters from a bottom surface of the PD, and an optical receiver using such PD.
2. Definitions
In this specification and claims, the terms of “top surface”, “bottom surface” and “side surface” of a PD are defined as follows:
The “top surface” of PD means the top surface of a laminated layer farthest from a substrate.
The “bottom surface” of a PD usually means the bottom surface of substrate. Some semiconductor PDs have a few layers laminated on the bottom surface of its substrate. For example, in the case that a metallized layer for contact is formed on the bottom surface of a substrate, the bottom surface of a PD means the metallized surface, and not the bottom surface of the substrate itself.
The surface-mounting type PDs are classified into three types in accordance with the direction of light incident thereon: a top-incidence type, in which light enters from the top surface, a bottom-incidence type, in which light enters from the bottom surface, and a sidle-incidence type, in which light enters from the side surface.
The device in which the light enters from the top surface is referred to as an “Top-illuminated PD”. The device in which the light enters from the bottom surface is referred to as a “Bottom-illuminated PD”. The device in which the light enters from the side surface is referred to as a “Side-illuminated PD”.
3. Description of the Background Art
In order to meet the development of optical communications high-sensitivity and easy handling PDs have been required. As the first conventional example,
FIG. 1
shows a surface-mounting type Bottom-illuminated PD that was proposed in German Patent No. DE 35 43 558 C2 (Ref. No. 1). As a p-electrode is formed directly on the top surface of a Bottom-illuminated PD, it enables the diameter of the light receiving area to be small, and the shape of the p-electrode need not be a ring-type. While maintaining an area sufficient to receive light, the capacitance of a pn-junction can be made small, and thereby a high-sensitivity and high-speed-responsivity device can be obtained. In addition, such Bottom-illuminated PD is the most suitable to apply the surface-mount technologies because the light enters from a bottom surface. In other words, it is possible to structure such that is fixed facing upward at the end of a V-groove so as to receive the light from the bottom surface.
A Bottom-illuminated PD
1
includes a wide n-type portion
2
and a narrow p-type domain
3
. An interface between the n-type portion
2
and the p-type domain
3
is a pn-junction
4
. The n-type portion
2
includes an n-type substrate and an n-type epitaxial layer. A p-electrode having no aperture is formed on the p-type domain
3
. A ring-shaped n-electrode is formed on the bottom surface of the substrate. An Si-substrate
5
is a rectangular plate to be used for making a SM device. A V-groove
6
is formed along a center axial line of the substrate
5
. The V-groove
6
can be formed by the anisotropy-etching method. An optical fiber
7
is laid on the V-groove
6
, and then the fiber is fixed thereon. The ring-shaped n-electrode of the Bottom-illuminated PD is formed on the Si-substrate
5
. A p-electrode in a top surface
9
is connected to preamplifiers by wire bonding, that is not illustrated in FIG.
1
.
An incident light
8
that is emitted from the optical fiber
7
and travels along the V-groove
6
, is reflected at a mirror
11
, and after passing through the ring-shaped n-electrode, the light
8
enters from a bottom surface
10
in the n-type portion
2
and progresses to the pn-junction
4
, where the light
8
generates photocurrent at the pn-junction.
The second conventional example is shown in
FIG. 2
, that is a cross-sectional view of a p-i-n-PD, having an InGaAs photo-detecting layer, which has been frequently used in recent optical communications. (Ref. No. 2: U.S. Pat. No. 5,365,101) An n-InP buffer layer
13
, an n-InGaAs photo-detecting layer
14
and an n-InP window layer
15
are epitaxially grown in this order on an n-InP substrate
12
. A window layer used in this specification and claims is also called a cap layer in this field. P-type dopants are diffused from the top surface of the window layer
15
to the central and peripheral areas thereof to form a p-type domain
3
and a shield domain
16
. Interfaces between p-type domains and an n-type domain are pn-junctions
4
. A passivation film
17
is formed on a top surface in order to protect the pn-junctions
4
. A p-electrode
19
having no aperture is formed on the center of the p-type domain
3
. A ring-shaped n-electrode
18
is formed on a bottom surface of the n-InP substrate
12
. As this is the Bottom-illuminated PD, it has an aperture part for receiving the incident light
8
in the center of the bottom surface of the InP substrate
12
.
The conventional Bottom-illuminated PDs shown in
FIGS. 1 and 2
have an n-type InP substrate that is not always good for transmittance from the standpoint of light transmission through the substrate.
Higher transmittance of the n-InP substrate is necessary to improve the sensitivity of the Bottom-illuminated PD.
An InP substrate containing 3×10
18
cm
−3
to 10×10
18
cm
−3
of tin (Sn) or sulfur (S) has been generally used as an n-type substrate. However, such substrate absorbs from 10% to 20% light. Large amounts of these n-type dopants must be doped into the substrate to raise the resistivity of the n-type substrate. The increase in the absorption by n-type dopants results in the increased absorption in the substrate. If the sulfur density is lowered to 1×10
18
cm
−3
, the transmittance of the substrate can be considerably improved, but there are drawbacks such as an increase in the resistivity of the substrate, or an increase in the crystal-defect.
FIG. 3
is a diagram showing the relationship between transmittance and wavelength in the case of an S-doped n-InP and an iron (Fe)-doped semi-insulated (SI)-InP substrates, each having a thicknesses of 350 &mgr;m. The abscissa axis is the wavelength (nm), and the ordinate axis is the transmittance.
Sulfur is an n-type dopant in this case. The more dopants are contained, the more light is absorbed. This is because light absorption is caused mainly by the dopants. The longer the wavelength, the less the absorption becomes. However, when the sulfur density decreases, the minimum absorption wavelength moves to about 1.3 &mgr;m.
For example, in the case of carrier density of 6.5×10
18
cm
−3
, transmittance is about 0.75 at a 1.3 &mgr;m optical wavelength. In the case of carrier density of 3.3 ×10
18
cm
−3
, transmittance is about 0.87 at the same wavelength. In the case of carrier density of 1.0×10
8
cm
−3
, transmittance becomes 0.96. In other words, optical absorption still remains 0.04 in this case. On the other hand, in the case of carrier density of 1.0×10
18
cm
−3
Fe-doping, transmittance is 0.98 and absorption can be reduced to 0.02 at the same wavelength.
Conventionally low resistance n-type or p-type substrates have been used for PDs of the Si-series, GaAs-series or InP-series. As an electrode has been formed on a bottom surface of the substrate, the photocurrent has had to pass through the substrate. If the substrate has high resistance, photocurrent can not flow easily and response speed becomes slow. Therefore, the substrates have been made of low-resistivity p-type or n-type crystals.
On the other hand, Fe makes a deep energy level in a forbidden band in the InP crystal. As the deep energy level captures electrons, the movable electron density is decreased. So, an Fe-doped InP crystal becomes highly resistant. In other words, the Fe-doped substrate has insulating or semi-insulat
Iguchi Yasuhiro
Kuhara Yoshiki
Fahmy Wael
Farahani Dana
Smith , Gambrell & Russell, LLP
Sumitomo Electric Industries Ltd.
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