Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
2000-12-08
2004-04-06
Font, Frank G. (Department: 2877)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
C385S129000
Reexamination Certificate
active
06718108
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor photodetector, and more particularly to a semiconductor photodetector exhibiting a high speed response and having a high external quantum efficiency.
FIG. 1
is a fragmentary cross sectional elevation view illustrative of a conventional semiconductor photodetector. The conventional semiconductor photodetector is provided over a semi-insulating InP substrate
20
. An n+-InGaAsP layer
21
having a thickness of 0.2 micrometers is selectively formed on a first region of an upper surface of the semi-insulating InP substrate
20
. A first polyimide insulating layer
27
a
is also selectively formed on a second region of the upper surface of the semi-insulating InP substrate
20
. A second polyimide insulating layer
27
b
is selectively formed on a first region of an upper surface of the n+-InGaAsP layer
21
. The second polyimide insulating layer
27
b
has a window through which the upper surface of the n+-InGaAsP layer
21
is shown. An n-contact
28
of AuGaNi is formed in the window and on the upper surface of the n+-InGaAsP layer
21
, so that the n-contact
28
is surrounded by the second polyimide insulating layer
27
b
. A multi-layered structure is provided selectively provided on a second region of the upper surface of the n+-InGaAsP layer
21
, so that the multi-layered structure is surrounded by the first and second polyimide insulating layers
27
a
and
27
b
. The multi-layered structure comprises the following five layers
22
,
23
,
24
,
25
and
26
. An undoped InGaAs optical absorption layer
22
having a thickness of 0.4 micrometers is selectively provided on the second region of the upper surface of the n+-InGaAsP layer
21
. A p+-InGaAs optical absorption layer
23
having a thickness of 0.2 micrometers is laminated on the undoped InGaAs optical absorption layer
22
. A p+-InGaAsP layer
24
having a thickness of 0.2 micrometers is laminated on the p+-InGaAs optical absorption layer
23
. A p+-InP layer
25
having a thickness of 0.5 micrometers is laminated on the p+-InGaAsP layer
24
. A p+-InGaAsP layer
26
having a thickness of 0.2 micrometers is laminated on the p+-InP layer
25
. An upper surface of the p+-InGaAsP layer
26
is leveled to the top surfaces of the first and second polyimide insulating layers
27
a
and
27
b
to form a palatalized surface. A p-contact
29
of AuZnNi is provided on the palatalized surface, wherein the p-contact
29
is in contact with the p+-InGaAsP layer
26
. The n-contact
28
and the p-contact
29
are electrically connected to each other through the n+-InGaAsP layer
21
, the undoped InGaAs optical absorption layer
22
, the p+-InGaAs optical absorption layer
23
, the p+-InGaAsP layer
24
, the p+-InP layer
25
and the p+-InGaAsP layer
26
. The optical waveguide comprises the n+-InGaAsP layer
21
, the undoped InGaAs optical absorption layer
22
, the p+-InGaAs optical absorption layer
23
, the p+-InGaAsP layer
24
, the p+-InP layer
25
and the p+-InGaAsP layer
26
. The n-contact
28
is bonded through a bonding wire to a pad formed on the top surface of the second polyimide insulating film
27
b
. The above undoped InGaAs optical absorption layer
22
further comprises three lamination layers of a p−-InGaAs layer, an i-InGaAs and a p−-InGaAs layer.
In order to realize the high speed response of the conventional semiconductor photodetector, a total thickness of the undoped InGaAs optical absorption layer
22
and the p+-InGaAs optical absorption layer
23
is thin, for example, 0.6 micrometers which is less than 1 micrometer. The reduction in the total thickness of the undoped InGaAs optical absorption layer
22
and the p+-InGaAs optical absorption layer
23
shortens a carrier traveling time to improve the high speed response of the conventional semiconductor photodetector. The reduction in the total thickness of the undoped InGaAs optical absorption layer
22
and the p+-InGaAs optical absorption layer
23
, however, raises a problem with a reduction in coupling efficiency to an incident light from an optical fiber. An optical absorption region of the above conventional semiconductor photodetector comprises the n+-InGaAsP layer
21
, the undoped InGaAs optical absorption layer
22
, the p+-InGaAs optical absorption layer
23
and the p+-InGaAsP layer
24
. A total thickness of the n+-InGaAsP layer
21
, the undoped InGaAs optical absorption layer
22
, the p+-InGaAs optical absorption layer
23
and the p+-InGaAsP layer
24
is 1 micrometer. Namely, the thickness of the optical absorption region of the above conventional semiconductor photodetector is 1 micrometer. A spot size of the incident light from the optical fiber is, however, about 9 micrometers. namely, the thickness of the optical absorption region of the above conventional semiconductor photodetector is much smaller than the spot size of the incident light from the optical fiber, for which reason an external quantum efficiency is low.
In the above circumstances, it had been required to develop a novel semiconductor photodetector free from the above problems.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a novel semiconductor photodetector free from the above problems.
It is a further object of the present invention to provide a novel semiconductor photodetector exhibiting a high speed response and having a high external quantum efficiency.
It is a still further object of the present invention to provide a novel semiconductor photodetector exhibiting a high speed response at about 20 GHz and having a high external quantum efficiency of not less than 90%.
It is yet a further object of the present invention to provide a novel semiconductor photodetector operable at a low voltage of not more than 1 volt and having a high reliability and a stable dark current characteristic.
The present invention provides an optical waveguide structure comprising plural periods of a multi-layered structure which comprises an InGaAs optical absorption layer of a first conductivity type, a pair of first and second InGaAsP cladding layers of the first conductivity type sandwiching the InGaAs optical absorption layer, and a pair of a first InP layer of the first conductivity type and a second InP layer of a second conductivity type, and the first and second InP layers sandwiching the first and second InGaAsP cladding layers.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.
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M. Zirngibl et al., “High-Speed Photodetectors on InGaAs/GaAs-on-GaAs superlattices,” Journal of Applied Physics, V. 69, 1991, pp. 8392-8398.
Font Frank G.
Kianni Kevin
NEC Compound Semiconductor Devices
Young & Thompson
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