Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2003-07-31
2004-10-26
Cuneo, Kamand (Department: 2829)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S962000
Reexamination Certificate
active
06808957
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for improving the optical coupling of the edge-coupled photodetector, especially a high-speed semiconductor edge coupled photodetectors which usually have a coupling aperture much smaller than the output spot-size (mode-field diameter) of the coupler.
2. Description of the Prior Art
Semiconductor photodetectors are indispensable in fiberoptic communications for receiving transmitted information and, as required data capacity increases, it is essential for the detectors to achieve high-speed data operation with high receiving efficiency. As shown in FIG.
1
(
a
), a conventional semiconductor photodetector has a layer structure wherein P+ type and N+ type semiconductor layers (
111
and
113
) sandwich therebetween a low- or un-doped optical absorption layer
112
. This photodetector is generally applied with a reverse bias between the P+ and N+-type semiconductor layers to deplete the optical absorption layer of carriers, and uses the high electric field generated in the depleted region to collect the photo-generated carriers.
Conventional surface-coupled photodetectors, which receive the incoming light
114
from the top of the device, have their optical absorption path parallel to and overlapping with carrier transit path. Reduction of the carrier transit path, though enhances the operating speed (if the detector bandwidth is transit-time-limited), results in lower absorption efficiency and thus degrades detector sensitivity. In other words, the surface-coupled photodetectors can not have the maximum bandwidth and maximum quantum efficiency simultaneously. More specifically, there exists a maximum value for the bandwidth-efficiency product of surface-coupled photodetectors. For instance, 30 GHz is a typical bandwidth-efficiency product value for InP-based long-wavelength surface-coupled photodetectors.
On the other hand, the edge-coupled photodetectors, which receive the incoming light
115
from the edge of the device, have their optical absorption path &Dgr;Z and carrier transit path &Dgr;X perpendicular to each other, therefore the two path lengths can be independently tuned. Ideally, the edge-coupled photodetectors can have their highest bandwidth along with the highest quantum efficiency. However, in practical case, as the absorption layer thickness &Dgr;X decreases for shortening the carrier transit time, the optical coupling using conventional waveguide device, such as the optical fiber, gets harder since the epi-structure-defined coupling aperture shrinks accordingly. For instance, &Dgr;X must be smaller than 1 &mgr;M for 30 GHz detector bandwidth, while typically the single-mode fiber has a beam diameter larger than 5 &mgr;m. It is therefore more difficult to adapt the coupling aperture of the photodetector to the mode field diameter of an optical fiber, thereby causing a problem of coupling loss therebetween.
As shown in FIG.
1
(
a
), the coupling aperture of edge-coupled photodetectors locates at the edge side of the device; therefore its effective size is determined by the cross section of the semiconductor layer structure and can be approximately represented by &Dgr;X&Dgr;Y Using an optical fiber as a coupler for direct coupling the light
115
into the effective aperture of a high-speed photodetector for optical absorption in the thin absorption layer
112
, one can achieve the high coupling efficiency only if the optical spot size is adequately small and the optical alignment, especially in the direction perpendicular to the device substrate, is accurate. For this coupling issue, a photodetector with a waveguide structure, which enlarges the effective coupling aperture, has been disclosed both in U.S. Pat. No. 5,991,473 and 5,998,851, and is schematically shown in FIG.
1
(
b
). Through this waveguide-forming layer structure, the effective coupling aperture (mode-field diameter) can be increased approximately from, for example, 0.41 &mgr;m to 3 &mgr;m, and thus the detector efficiency can be significantly promoted. However, the total layer thickness of the epi-structure is typically larger than 7 &mgr;m, which requires the epitaxial growth time at least double than the conventional detector structure (FIG.
1
(
a
)) for additional thick cladding layers
114
and
115
. The photodetector with tapered absorption layer proposed in U.S. Pat. No. 5,998,851 even requires a re-growth process, which introduces almost doubled epitaxy cost, and reproducibility and reliability issues. These additional cost and issues also happen in the waveguide-integrated photodetectors disclosed in U.S. Pat. Nos. 4,835,575, 5,285,514, 5,521,994, and 6,498,337. Besides, in addition to direct coupling, these waveguide-type or waveguide-integrated photodetector utilizes indirect evanescent coupling, which requires at least 20-30 &mgr;m detector length for the propagating light to be absorbed and therefore inevitably introduces additional junction capacitance. Larger junction capacitance results in lower device bandwidth.
According to the definition of the active pn junction region, the edge-coupled photodetectors can be divided into two categories: junction-mesa type as shown in FIG.
1
(
a
) and selective-area-diffused (SAD) mesa type as shown in FIG.
1
(
c
). The junction mesa type has a pn junction formed during the layer epitaxy and has a junction area defined by photolithography and etching process. The SAD-mesa type has a pn junction formed by localized diffusion process, which in the meantime defines the junction area. Due to that the depleted absorption region is sealed inside, the SAD-mesa type photodetector is generally considered more reliable than the junction-mesa type photodetector. Let us consider an exclusive problem encountered by the SAD-mesa type photodetector. As shown in FIG.
1
(
c
), the photodetector is composed by a highly n-type doped wide-bandgap semiconductor layer
113
, a low- or un-doped narrow-bandgap semiconductor absorption layer
112
, and a highly p-type doped wide-bandgap semiconductor region
111
a
, which defines the active region of the photodetector and is formed by the diffusion process, surrounded by a low- or un-doped wide-bandgap semiconductor region
111
b
. The low- or un-doped region
111
c
with an appropriate width &Dgr;t left in front of the active region effectively protects the active region from the outer environment. The regions outside the borders defined by the diffusion area with &Dgr;t outward extensions (i.e., the border defined by the thick dash lines) are out of the reach of the biasing field, and therefore are regarded as the inactive regions. Such regions exist exclusively only in the SAD-mesa type photodetectors. During optical coupling, the misalignment results in the coupling loss, which is no exception to the junction-mesa type photodetectors. However, besides coupling loss, lateral misalignment in Y direction can result in optical absorption in the inactive absorption regions of the SAD-mesa type photodetectors. Those photons being absorbed give up the energy which excite the electrons in the valence band and consequently generate electron/hole pairs. The electron/hole pairs generated in the inactive absorption regions either recombine in a short periods of time or slowly diffuse into the biased active region, at which they are then accelerated by the biasing field toward the respective electrodes. These “slow” carriers, relative to the “fast” carriers generated and drifting in the biased active region, result in signal tailing and deteriorate the device bandwidth. In summary, besides the coupling loss, there exist other issues relating to the misalignment, such as bandwidth reduction and linearity degradation caused by the slow carriers.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a light funnel integrated right in front of the coupling aperture of the edge-coupled photodetector for enlarging the effective coupling aperture and thereby enhancing the optical coupling effi
Chen Yao-Shing
Ho Chong-Long
Ho Wen-Jeng
Liaw Jy-Wang
Lin Chia-Ju
Chunghwa Telecom Co. Ltd.
Cuneo Kamand
Kilday Lisa
Rabin & Berdo P.C.
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