U.S. patent application number 16/315510 was filed with the patent office on 2019-08-01 for quantum confined stark effect electroabsorption modulator on a soi platform.
The applicant listed for this patent is POLITECNICO DI MILANO, ROCKLEY PHOTONICS LIMITED. Invention is credited to Andrea BALLABIO, Jacopo FRIGERIO, Giovanni ISELLA, Guomin YU.
Application Number | 20190235286 16/315510 |
Document ID | / |
Family ID | 58410409 |
Filed Date | 2019-08-01 |
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United States Patent
Application |
20190235286 |
Kind Code |
A1 |
YU; Guomin ; et al. |
August 1, 2019 |
QUANTUM CONFINED STARK EFFECT ELECTROABSORPTION MODULATOR ON A SOI
PLATFORM
Abstract
An electroabsorption modulator. The modulator comprising an SOI
waveguide; an active region, the active region comprising a
multiple quantum well (MQW) region; and a coupler for coupling the
SOI waveguide to the active region. The coupler comprising: a
transit waveguide coupling region; a buffer waveguide coupling
region; and a taper region; wherein, the transit waveguide coupling
region couples light between the SOI waveguide and the buffer
waveguide coupling region; and the buffer waveguide coupling region
couples light between the transit waveguide region and the active
region via the taper region.
Inventors: |
YU; Guomin; (Glendora,
CA) ; ISELLA; Giovanni; (Como, IT) ; FRIGERIO;
Jacopo; (Lecco, IT) ; BALLABIO; Andrea;
(Dolzago, IT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ROCKLEY PHOTONICS LIMITED
POLITECNICO DI MILANO |
London
Como |
|
GB
IT |
|
|
Family ID: |
58410409 |
Appl. No.: |
16/315510 |
Filed: |
January 16, 2017 |
PCT Filed: |
January 16, 2017 |
PCT NO: |
PCT/IT2017/000004 |
371 Date: |
January 4, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62359595 |
Jul 7, 2016 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/14 20130101; G02F
1/01708 20130101; G02F 1/01725 20130101; G02F 1/01716 20130101;
G02B 6/1228 20130101; B82Y 20/00 20130101; G02F 2001/0157
20130101 |
International
Class: |
G02F 1/017 20060101
G02F001/017 |
Claims
1. An electroabsorption modulator comprising: an SOI waveguide; an
active region, the active region comprising a multiple quantum well
(MQW) region; and a coupler for coupling the SOI waveguide to the
active region; the coupler comprising: a transit waveguide coupling
region; a buffer waveguide coupling region; and a taper region;
wherein, the transit waveguide coupling region couples light
between the SOI waveguide and the buffer waveguide coupling region;
and the buffer waveguide coupling region couples light between the
transit waveguide region and the active region via the taper
region.
2. The electroabsorption modulator of claim 1, wherein the taper
region comprises a multi-segment mode expander.
3. The electroabsorption modulator of claim 1, wherein the multiple
quantum well region is a Ge/SiGe multiple quantum well region.
4. The electroabsorption modulator of claim 1, wherein: the transit
waveguide coupling region comprises a first portion of a transit
waveguide; and the buffer waveguide coupling region comprises a
buffer waveguide located on top of a second portion of the transit
waveguide.
5. The electroabsorption modulator of claim 4, wherein: the transit
waveguide has a refractive index bigger than that of the SOI
waveguide but smaller than that of the buffer waveguide.
6. The electroabsorption modulator of claim 1, wherein: the SOI
waveguide is a 3 .mu.m waveguide; the transit waveguide has a
thickness of no more than 400 nm; and the buffer waveguide has a
thickness of no more than 400 nm.
7. The electroabsorption modulator of claim 6, wherein the transit
buffer waveguide has a thickness of no more than 600 nm.
8. The electroabsorption modulator of claim 6, wherein the transit
waveguide has a thickness of no more than 800 nm.
9. The electroabsorption modulator of claim 1, wherein each of the
buffer waveguide and transit waveguide are SiGe waveguides.
10. The electroabsorption modulator of claim 4; wherein the active
region comprises: a P-doped region between the buffer layer and the
lower surface of a spacer layer underneath a multiple quantum well;
and an N-doped region located at the upper surface of a spacer
layer on top of the multiple quantum well.
11. The electroabsorption modulator of claim 10, further comprising
multiple N-type doped layers with different germanium compositions
and doping concentrations.
12. The electroabsorption modulator of claim 1, wherein the
waveguide slab of the P-type layer in the active region is P-doped
with ion implantation followed by an RTA process.
13. The electroabsorption modulator of claim 1, wherein the
electrodes are arranged in a ground-signal (GS) configuration,
where a ground electrode is located at an opposite side of the
active region from the signal electrode.
14. The electroabsorption modulator of claim 1, wherein the
electrodes are arranged in a ground-signal-ground (GSG)
configuration, where a first ground electrode and a second ground
electrode are located at the same side of the active region as the
signal electrode.
15. The electroabsorption modulator of claim 1, wherein the
multiple quantum well region includes at least 5 quantum wells.
16. The electroabsorption modulator of claim 1, wherein the
multiple quantum well region includes either 5, 7, or 10 quantum
wells.
17. The electroabsorption modulator of claim 1, wherein the
multiple quantum well region is no more than 240 nm thick, and is
preferably no more than 232 nm thick.
18. The electroabsorption modulator of claim 1, wherein a spacing
between respective pairs of the quantum wells is in the range of 10
nm to 20 nm.
19. The electroabsorption modulator of claim 1, wherein each of the
multiple quantum wells has a thickness in the range of 5 nm to 15
nm.
20. The electroabsorption modulator of claim 1, further comprising
a metal electrode in contact with a surface of the active region
opposite to the coupler, wherein the MQW region includes at least
one tapered portion of MQW material which extends into the taper
region; and wherein the metal electrode extends as far as the
tapered portion of MQW material.
21. The electroabsorption modulator of claim 20, wherein the
electrode has a length in the direction towards the taper region
which is greater than 2.5 .mu.m.
22. The electroabsorption modulator of claim 1, wherein the active
region includes an N-doped region located above the upper surface
of a spacer layer on top of the multiple quantum well region, and
wherein the N-doped region comprises Si.sub.0.9Ge.sub.0.1.
Description
FIELD
[0001] The present invention relates to a modulator and to optical
coupling within a modulator on an SOI platform and more
particularly to a SiGe quantum confined Stark effect (QCSE)
modulator.
BACKGROUND
[0002] It is desirable to make high speed SiGe quantum confined
Stark effect (QCSE) electroabsorption modulators (EAMs) operating
at O-band (1.3 .mu.m wavelength) and CMOS compatible on SOI
platform for data centre network applications. Due to the
limitation of germanium material properties (bandgap and
absorption), problems to make SiGe QCSE EAM operate at O-band on 3
.mu.m SOI platform include 1) the design of SiGe multiple quantum
well epitaxy (EPI) stack that can operate at 1.3 .mu.m wavelength;
2) the design of coupling structure that brings the light from 3
.mu.m SOI waveguide into the SiGe multiple quantum well (MQW)
waveguide with low loss based on the EPI structure due to large SOI
waveguide dimensions and big refractive index contrast between Si
and SiGe buffer layer for the SiGe MQW; and 3) the difficulty of
realizing 2V driving voltage with CMOS driver because of the
carrier screen effect in the MQW region. This invention aims to
overcome at least these three problems in making SiGe QCSE EAM on 3
.mu.m SOI platform.
[0003] Whilst this application focusses on coupling to 3 .mu.m SOI
waveguides, it should be understood that the physical structures
described herein could be scaled up or scaled down in size
accordingly to other sizes of waveguides.
SUMMARY
[0004] According to a first aspect, the invention provides an
electroabsorption modulator comprising: an SOI waveguide; an active
region, the active region comprising a multiple quantum well (MQW)
region; and a coupler for coupling the SOI waveguide to the active
region; the coupler comprising: a transit waveguide coupling
region; a buffer waveguide coupling region; and a taper region;
wherein, the transit waveguide coupling region couples light
between the SOI waveguide and the buffer waveguide coupling region;
and the buffer waveguide coupling region couples light between the
transit waveguide region and the active region via the taper
region.
[0005] Here are presented a SiGe MQW EPI stack based on 3 .mu.m SOI
wafer and an electroabsorption modulator (EAM) based on the SiGe
EPI stack. The SiGe MQW EPI stack is designed such that it can: 1)
realize operating in O-band (1.3 .mu.m wavelength); 2) fulfil 2V
driving voltage for QCSE EAM that is compatible with CMOS drivers
by properly choosing the quantum well structure with low optical
loss; and 3) support the design of coupling structure to bring
light from 3 .mu.m SOI waveguide to the SiGe MQW active region. The
SiGe EPI stack comprises (from bottom to top): a transit buffer
layer that is for the first evanescent coupling structure to bring
light up from the 3 .mu.m SOI waveguide; a buffer layer that serves
as the virtual substrate to determine the strain in the well layer
and barrier layer in the SiGe MQW, as well as the second evanescent
coupling structure to bring light up from the transit buffer layer
to the SiGe MQW active region through a taper structure (mode
expander); a P-type (e.g. boron) doped layer that acts as the
P-side of the PIN junction in the SiGe EAM with a concentration of
1E18 cm.sup.-3; an intrinsic spacer layer that separates the P-type
doped layer and the SiGe quantum wells; a layer of five SiGe
quantum wells that comprise five germanium wells and six SiGe
barriers--the number of quantum wells should be chosen such that
with a 2V driving voltage an extinction ratio of 4 dB or bigger may
be achieved at 1.3 .mu.m; an intrinsic spacer layer to separate the
quantum wells and the N-type doped layers, a layer of N-type (e.g.
phosphorus) doped with a concentration of 1E18 cm.sup.-3 and the
same germanium composition as the P-type doped layer; a layer of
N-type doped with a concentration of 1E18 cm.sup.-3 but a lower
germanium composition; a layer of heavily N-type doped with a
concentration of 1E20 cm.sup.-3 and the same germanium composition
as the last N-type doped layer. The use of multiple N-type doped
layers with different germanium composition and different doping
concentrations is to realize both low optical loss and low series
resistance to reach high modulation speed.
[0006] The SiGe QCSE EAM comprises: an SOI waveguide; an active
region, the active region comprising a SiGe multiple quantum well
waveguide; and two coupling regions, the coupling region has at
least one evanescent coupling and one taper structure to couple
light between the SOI waveguide and the SiGe MQW waveguide active
region. The SOI waveguide has a typical thickness of 3 .mu.m and a
typical width of 2.6 .mu.m based on 3 .mu.m SOI wafers. The
evanescent coupling structures in the coupling region comprises a
transit buffer layer waveguide and a buffer layer waveguide on top
of the transit buffer layer waveguide. The light from the 3 .mu.m
SOI waveguide is coupled into the transit buffer layer waveguide
first; then the light is coupled into the buffer layer waveguide
which may comprise the buffer layer itself only or comprise the
buffer layer with the P-type doped layer together; then the light
is coupled into the SiGe MQW waveguide via a taper that comprises
the transit buffer layer, buffer layer, P-type doped layer, spacer
layer, SiGe MQW layers, spacer layer and N-type doped layers. The
taper structure is designed such that it expands the optical mode
of the buffer waveguide to the optical mode of the SiGe MQW
waveguide with low optical loss and minimizes the extra parasitical
capacitance to keep the EAM working at high speed. In the active
region, the SiGe waveguide is a rib waveguide with the P-type doped
layer on top of the slab, on which the metal electrode is
deposited. In order to reduce the contact resistance between the
metal electrode and the P-type doped layer, an ion implantation is
used to make it heavily P-type (e.g. boron) doped followed by a
rapid thermal annealing (RTA) process to activate the dopant. The
doped concentration is about 1E20 cm.sup.-3. The electrode on the
N-side of the PIN junction of the SiGe EAM contacts the heavily
doped N-type layer from the top of the waveguide with the bonding
pad on the waveguide slab. In order to reduce the parasitical
capacitance, the part of P-type doped layer on the waveguide slab
underneath the electrode and the bonding pad for the N-type layer
has to be removed. Two kinds of electrode pad arrangements have
been used. One arrangement is ground-signal (GS), and the other is
ground-signal-ground (GSG). In the GS configuration, a ground
electrode is located at an opposite side of the active region from
the signal electrode. In a ground-signal-ground (GSG)
configuration, a first ground electrode and a second ground
electrode are located at the same side of the active region as the
signal electrode. The active region may be an active region
waveguide. In embodiments of this invention, light is efficiently
coupled from 3 .mu.m SOI waveguide to SiGe multiple quantum well
(MQW) waveguide, where the light is modulated, then is coupled back
to the 3 .mu.m SOI waveguide. This overcomes the inherent problems
which arise due to an SOI waveguide having relatively large
dimensions and refractive index contrast between Si and the SiGe
buffer layer for the SiGe MQW waveguide.
[0007] The taper region may comprise a multi-segment mode
expander.
[0008] The multiple quantum well region may be a Ge/SiGe multiple
quantum well region
[0009] The transit waveguide coupling region may comprise a first
portion of a transit waveguide; and the buffer waveguide coupling
region comprises a buffer waveguide located on top of a second
portion of the transit waveguide.
[0010] Optionally, the transit buffer layer has a refractive index
bigger than that of the SOI waveguide but smaller than that of the
buffer layer.
[0011] Optionally, the SOI waveguide is a 3 .mu.m waveguide; the
transit buffer layer has a thickness of no more than 400 nm; and
the buffer layer has a thickness of no more than 400 nm.
[0012] Optionally, the transit buffer layer has a thickness of no
more than 600 nm.
[0013] Optionally, the transit buffer layer has a thickness of no
more than 800 nm.
[0014] Optionally, each of the buffer layer and transit buffer
layer are SiGe waveguides.
[0015] The active region may comprise: a P-doped region between the
buffer layer and the lower surface of a spacer layer underneath a
multiple quantum well; and an N-doped region located at the upper
surface of a spacer layer on top of the multiple quantum wells.
[0016] The modulator may further comprise multiple N-type doped
layers with different germanium compositions and doping
concentrations.
[0017] The waveguide slab of the P-type layer in the active region
may be P-doped with ion implantation followed by an RTA
process.
[0018] The electrodes may be arranged in a ground-signal (GS)
configuration, where a ground electrode is located at an opposite
side of the active region from the signal electrode.
[0019] The electrodes may be arranged in a ground-signal-ground
(GSG) configuration, where a first ground electrode and a second
ground electrode are located at the same side of the active region
as the signal electrode.
[0020] The multiple quantum well region may include at least 5
quantum wells. The multiple quantum well region may include either
5, 7, or 10 quantum wells.
[0021] The multiple quantum well region may be no more than 240 nm
thick, and is preferably no more than 232 nm thick.
[0022] The modulator may further comprise a metal electrode in
contact with a surface of the active region opposite to the
coupler, the metal electrode may extend beyond the active region in
a direction towards the taper region. The electrode may have a
length in the direction towards the taper region which is greater
than 2.5 .mu.m. Whilst the active region includes the MQW region,
the MQW region may extend beyond a length of the main body of the
active region i.e. the MQW region may be coterminous with the
spacer layer discussed above. In this way, the MQW region may
include at least one tapered portion of MQW material which extends
into the taper region; and wherein the metal electrode extends
beyond the active region in a direction towards the taper region as
far as the tapered portion of MQW material.
[0023] The active region may include an N-doped region located
above the upper surface of a spacer layer on top of the multiple
quantum well region, and the N-doped region may comprise
Si.sub.0.9Ge.sub.0.1.
[0024] The active region may include one or more angled interfaces,
which are angled relative to the wave-guiding direction of the SOI
waveguide. The angled interfaces may be formed by the interfaces
between the stack and an input and output waveguide. These
interfaces may be angled relative to a guiding direction of the SOI
waveguide i.e. angled relative to a length of the device. The angle
may be between 80.degree. and 89.degree.. The active region, as
viewed from above, may have a parallelogramal or trapezoidal
geometry. The angle may be chosen to match the angle of refraction
as dictated by Snell's law for light entering the active region
from the SOI waveguide or for light entering the SOI waveguide from
the active region. In more detail, at the interface between (for
example)_the SOI waveguide and the active region, the refractive
index of the material of the SOI waveguide and the refractive index
of the material of the active region are input into Snell's law to
determine the angle of refraction which occurs at that interface
due to the change in refractive index. The angle that the SOI
waveguide makes with the active region is then chosen so that the
SOI waveguide is orientated at a given angle of incidence so that
the active region is orientated at the corresponding angle of
refraction that has been calculated for the two materials and the
given angle of incidence (of course, the calculation may also be
carried out in reverse with the angle of refraction being the known
quantity). The same process can be carried out at an interface
formed between an opposite side of the active region and the SOI
waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] These and other features and advantages of the present
invention will be appreciated and understood with reference to the
specification, claims, and appended drawings wherein:
[0026] FIG. 1 is a SiGe epitaxial layer structure including a
multiple QW structure, according to an embodiment of the present
invention;
[0027] FIG. 2A is a 3D view of a device design #1 based on the SiGe
epitaxial layer structure shown in FIG. 1 with GS electrode
structure;
[0028] FIG. 2B is a 3D view of a device design #1 based on the SiGe
epitaxial layer structure shown in FIG. 1 with GSG electrode
structure;
[0029] FIG. 3A is the top view of a device design #1 based on the
SiGe epitaxial layer structure shown in FIG. 1 with GS electrode
structure;
[0030] FIG. 3B is top view of a device design #1 based on the SiGe
epitaxial layer structure shown in FIG. 1 with GSG electrode
structure;
[0031] FIG. 4 shows the device design #1 top view with detailed
device structure for each section;
[0032] FIG. 5 is the section view along the middle line KK' of the
device design #1;
[0033] FIG. 6A is a 3D view of a device design #2 based on the SiGe
epitaxial layer structure shown in FIG. 1 with GS electrode
structure;
[0034] FIG. 6B is a 3D view of a device design #2 based on the SiGe
epitaxial layer structure shown in FIG. 1 with GSG electrode
structure;
[0035] FIG. 7A is the top view of a device design #2 based on the
SiGe epitaxial layer structure shown in FIG. 1 with GS electrode
structure;
[0036] FIG. 7B is the top view of a device design #2 based on the
SiGe epitaxial layer structure shown in FIG. 1 with GSG electrode
structure;
[0037] FIG. 8 shows the device design #2 top view with detailed
device structure for each section;
[0038] FIG. 9 is the section view along the middle line KK' of the
device design #2;
[0039] FIG. 10 is another SiGe epitaxial layer structure including
a multiple QW structure, according to an embodiment of the present
invention;
[0040] FIG. 11 shows the device design #3 top view with detailed
device structure for each section based on the SiGe epitaxial layer
structure shown in FIG. 10 with GS electrode structure;
[0041] FIG. 12 is the section view along the middle line KK' of the
device design #3;
[0042] FIG. 13 is another SiGe epitaxial layer structure including
a multiple QW structure, according to an embodiment of the present
invention;
[0043] FIG. 14 shows the device design #4 top view with detailed
device structure for each section based on the SiGe epitaxial layer
structure shown in FIG. 13 with GS electrode structure;
[0044] FIG. 15 is the section view along the middle line KK' of the
device design #4;
[0045] FIG. 16 is the input/output 3 .mu.m SOI waveguide for use
with all devices disclosed herein; and
[0046] FIG. 17 is typical optical transition from 3 .mu.m SOI
waveguide to SiGe MQW waveguide simulation result at 1.3 .mu.m
wavelength;
[0047] FIG. 18 is the top view of a device design #5 based on any
one of the SiGe epitaxial layer structures shown in FIG. 1, FIG.10
and FIG. 13, the device of FIG.18 having a GS electrode
structure;
[0048] FIG. 19 is the section view along the middle line KK' of the
device design #5
[0049] FIG.20 shows a device design #1B in a top view with detailed
device structure for each section;
[0050] FIG. 21 is the section view along the middle line KK' of the
device design #1B
[0051] FIG. 22 is another SiGe epitaxial layer structure including
a multiple QW structure, according to an embodiment of the present
invention;
[0052] FIG. 23 is another SiGe epitaxial layer structure including
a multiple QW structure, according to an embodiment of the present
invention;
[0053] FIG. 24 shows a device design #1C in a top view with
detailed device structure for each section; and
[0054] FIG. 25 is the section view along the middle line KK' of the
device design #1C.
DETAILED DESCRIPTION
[0055] The detailed description set forth below in connection with
the appended drawings is intended as a description of exemplary
embodiments of an electroabsorption modulator provided in
accordance with the present invention and is not intended to
represent the only forms in which the present invention may be
constructed or utilized. The description sets forth the features of
the present invention in connection with the illustrated
embodiments. It is to be understood, however, that the same or
equivalent functions and structures may be accomplished by
different embodiments that are also intended to be encompassed
within the spirit and scope of the invention. As denoted elsewhere
herein, like element numbers are intended to indicate like elements
or features.
[0056] A first embodiment ("EPI design #1) is shown in FIGS. 1 to
9.
[0057] FIG. 1 shows an example of a SiGe EPI structure in
accordance with the present invention in which a thin layer of
transit buffer SiGe is inserted between the 3 .mu.m SOI waveguide
and the SiGe buffer layer that is for the SiGe MQW.
[0058] This transit buffer SiGe layer: [0059] a) has a refractive
index larger than that of Si and smaller than that of SiGe buffer
layer, therefore, light can be evanescently coupled from the SOI
waveguide to the transit buffer SiGe waveguide; and [0060] b)
serves as an extra-buffer layer for the SiGe buffer layer of the
MQW waveguide to ease the stress due to the crystal lattice
mismatch between Si and SiGe MQW, which is critical for the SiGe
MQW EPI quality.
[0061] The transit buffer SiGe layer shown has a germanium content
of 20% (Si.sub.0.8Ge.sub.0.2). Optionally, this transit buffer SiGe
layer may have a germanium content ranging from 5%
(Si.sub.0.95Ge.sub.0.05) to 50% (Si.sub.0.5Ge.sub.0.5) and a
thickness ranging from 400 nm to 1000 nm.
[0062] Based on the proposed SiGe EPI structure, the waveguide
evanescent coupling structure brings light from the SOI waveguide
(which may be a 3 .mu.m SOI waveguide) to a SiGe MQW waveguide in
the following steps: [0063] a) From a SOI waveguide (which may be a
3 .mu.m SOI waveguide) to a transit buffer SiGe waveguide (which
may be a 400 nm transit buffer SiGe waveguide) [0064] b) From the
transit buffer SiGe waveguide (which may be a 400 nm transit buffer
SiGe waveguide) to a buffer SiGe waveguide (which may be a 400 nm a
buffer SiGe waveguide). The buffer SiGe layer shown has a germanium
content of 79% (Si.sub.0.21Ge.sub.0.79). Optionally, this buffer
SiGe layer may have a germanium content ranging from 70%
(Si.sub.0.3Ge.sub.0.7) to 95% (Si.sub.0.05Ge.sub.0.95), and a
thickness ranging from 400 nm to 1000 nm. [0065] c) From the buffer
SiGe waveguide (which may be a 400 nm a buffer SiGe waveguide) to a
SiGe MQW waveguide via a taper structure. The taper structure
expends the optical mode of the buffer waveguide to the optical
mode of the SiGe MQW waveguide. Wherein the taper structure and the
SiGe MQW waveguide may consist of: the transit buffer SiGe, 400 nm
buffer SiGe, 200 nm P-layer, 50 nm spacer, 140 nm quantum well
layer (5 QW) which has 15 nm spacers between respective quantum
wells which are 10 nm thick, 50 nm spacer, 300 nm N-layer, 200 nm
N-doped cover layer and 100 nm heavily N-doped cover layer as shown
in FIG. 1 . The P-layer, spacer layers and N-layer may have the
same germanium content, (79%, Si.sub.0.21Ge.sub.0.79), as that of
buffer layer. Optionally, the P-layer, spacer layers and N-layer
may have a germanium content ranging from 70%
(Si.sub.0.3Ge.sub.0.7) to 90% (Si.sub.0.1Ge.sub.0.9). The N-doped
cover layer and heavily N-doped cover layer may have a germanium
content less than that of buffer layer, P-layer, spacer layers and
N-layer. The germanium content of the N-doped cover layer and
heavily doped N-doped cover layer may be 20%
(Si.sub.0.8Ge.sub.0.2). Optionally, the germanium content of the
N-doped cover layer and heavily doped N doped cover layer may range
from 5% (Si.sub.0.95Ge.sub.0.05) to 50% (Si.sub.0.5Ge.sub.0.5). The
germanium content of the barrier layer in the quantum well is 65%
(Si.sub.0.35Ge.sub.0.65). Optionally, the germanium content of the
barrier layer in the quantum well may range from 60%
(Si.sub.0.4Ge.sub.0.6) to 85% (Si.sub.0.15Ge.sub.0.85) with a
general rule that the average germanium content in the Ge/Si
quantum well is the same or substantially the same as the germanium
content of the buffer layer. The number of quantum wells is five in
this EPI structure. Optionally, the number of the quantum wells may
range from 5 to 15.
[0066] The QCSE EAM consists of two coupling regions, which have
two waveguide evanescent coupling structures and one taper
structure, and one active region between the two coupling
regions
[0067] The active region preferably has the same waveguide
structure as the SiGe MQW waveguide.
[0068] Light from the Si waveguide (which may be a 3 .mu.m Si
waveguide) travels through the first coupling region to reach the
active region.
[0069] In the active region, light is absorbed and modulated
according to the external bias voltage.
[0070] After modulation, the light goes through the second coupling
region back to a/the Si waveguide (which may be a 3 .mu.m Si
waveguide).
[0071] Three examples of devices which incorporate the
electroabsorption modulator of the present invention are now
described.
[0072] The first example (device design #1 based on EPI design #1)
can be seen in the 3D views shown in FIG. 2A and FIG. 2B and also
in the top views as shown in FIG. 3A (GS electrodes) and FIG. 3B
(GSG electrodes). In this device design #1, the taper structure
comprises 3 segments to expand the optical mode of buffer waveguide
to the optical mode of SiGe MQW waveguide. An example of
measurements for the entire device is shown in FIG. 4 and a section
view of the device is shown in FIG. 5. The simulation results for
device design #1 at 1.3 .mu.m wavelength for TE mode are below:
insertion loss 4.87 dB, extinction ratio 4.16 dB and link penalty
9.97 dB.
[0073] The second example (device design #2 based on EPI design #1)
can be seen in the 3D views shown in FIG. 6A and FIG. 6B and also
in the top views as shown in FIG. 7A (GS electrodes) and FIG. 7B
(GSG electrodes). In this device design #2, the taper structure
comprises 4 segments to expand the optical mode of buffer waveguide
to the optical mode of SiGe MQW waveguide. An example of
measurements for the entire device of the second example (device
design #2 based on EPI design #1) is shown in FIG. 8 and a section
view of the device is shown in FIG. 9. The simulation results for
device design #2 at 1.3 .mu.m wavelength for TE mode are below:
insertion loss 4.43 dB, extinction ratio 4.16 dB and link penalty
9.53 dB.
[0074] An example (EPI design #2) and an associated device design
#3 is shown in FIGS. 10-12. this embodiment differs from that of
FIG. 1 (EPI design #1) in that: [0075] a) Transit buffer layer: 600
nm, Si.sub.0.9Ge.sub.0.1 [0076] b) cover N-doped layer:
Si.sub.0.9Ge.sub.0.1
[0077] As with the devices described above, devices including the
EPI design of the second embodiment may be fabricated with: [0078]
a) GS electrodes; or [0079] b) GSG electrodes
[0080] In a third example, device design #3, the taper structure
comprises 4 segments to expand the optical mode of buffer waveguide
to the optical mode of SiGe MQW waveguide. The simulation results
for device design #3 at 1.3 .mu.m wavelength for TE mode are below:
insertion loss 4.87 dB, extinction ratio 4.16 dB and link penalty
9.97 dB.
[0081] A third embodiment (EPI design #3) of the present invention
and an associated device design #4 is shown in FIGS. 13-15. This
embodiment differs from that of FIG. 1 (EPI design #1) in that:
[0082] a) Transit buffer layer: 800 nm. Si.sub.0.9Ge.sub.0.1 [0083]
b) cover N-doped layer: Si.sub.0.9Ge.sub.0.1
[0084] As with the devices described above, devices including the
EPI design of the third embodiment may be fabricated with: [0085]
a) GS electrodes; or [0086] b) GSG electrodes.
[0087] In this device design #4, the taper structure comprises 4
segments to expand the optical mode of buffer waveguide to the
optical mode of SiGe MQW waveguide. The simulation results for
device design #4 at 1.3 .mu.m wavelength for TE mode are below:
insertion loss 4.66 dB, extinction ratio 4.16 dB and link penalty
9.76 dB.
[0088] An example of in input (and/or output waveguide) for
coupling to any one of the EPI regions described herein is shown in
FIG. 16. In some embodiments, this may take the form of a 3 .mu.m
SOI waveguide.
[0089] Typical optical transition from 3 .mu.m SOI waveguide to
SiGe MQW waveguide simulation result at 1.3 .mu.m wavelength is
shown in FIG. 17 for a half device structure. The device is
symmetric, so the half structure simulation enabled relevant
information to be achieved whilst conserving computer space.
[0090] FIG. 18 shows another example of a device design; "device
design #5". This device design is could contain any one of the EPI
structures of the embodiments (EPI design #1, EPI design #2 and EPI
design #3) shown in FIG. 1, FIG. 10 and FIG. 13. The taper
structure of device design #5 comprises 2 segments which act to
expand the optical mode of buffer waveguide to the optical mode of
SiGe MQW waveguide. An advantage of using such a taper structure
with 2 segments is the ease of device fabrication process with
fewer processing steps. FIG. 19 shows the section view of design #5
along the middle line KK'.
[0091] A fourth embodiment (EPI design #1B) of the present
invention and associated device design is shown in FIG. 20-21.
[0092] An example of measurements for the entire device is shown in
FIG. 20, and a section view of the device is shown in FIG. 21. This
design differs from that shown in, for example, FIG. 14, in at
least that the electrodes of FIG. 20 which are in contact with a
surface of the active region opposite the coupler extend beyond the
main body of the active region in a direction towards their
respective taper regions. In this embodiment, the MQW region
comprises a main section in the main body of the active region, but
also includes at least one tapered portion of MQW material which
extends from the main body of the active material forming active
tapered portions overlaying the tapered layers underneath. As can
be seen in FIG. 20, the tapered portions of the MQW material
therefore extend outwards from the main body of the active region
(marked "E") into at least the active mode expander region (marked
"D").
[0093] A fifth embodiment (EPI design #1B (Modified)) of the
present invention is shown in FIG. 22. in which a thin layer of
transit buffer SiGe is inserted between the 3 .mu.m SOI waveguide
and the SiGe buffer layer that is for the SiGe MQE. Broadly it is a
modified version of the EPI structure corresponding with the device
in FIGS. 20 and 21.
[0094] This embodiment differs from that of FIG. 1 in that:
[0095] a) the lower N layer comprises Si.sub.0.24Ge.sub.0.76 and is
100 nm thick;
[0096] b) the spacer layers comprise Si.sub.0.21Ge.sub.0.79 and are
20 nm thick;
[0097] c) the MQW region comprises 10 quantum wells, is 232 nm
thick, has 12 nm spaces between quantum wells which are 10 nm
thick, and comprises Ge/Si.sub.0.44Ge.sub.0.56; and
[0098] d) there is no discrete `P-layer`, instead the buffer layer
is doped with P-type dopants to a concentration of
1.times.10.sup.18 cm.sup.-3 and comprises
Si.sub.0.24Ge.sub.0.76.
[0099] A sixth embodiment (EPI design #1C (Modified)) of the
present invention and associated device design is shown in FIG.
23-25.
[0100] An example of measurements of the entire device is shown in
FIG. 24, and a section view of the device is shown in FIG. 25. The
active region could contain an EPI structure as shown in FIG. 22 or
FIG. 23.
[0101] FIG. 23 shows an example of a SiGe EPI structure in
accordance with the present invention in which a thin layer of
transit buffer SiGe is inserted between the 3 .mu.m SOI waveguide
and the SiGe buffer layer that is for the SiGe MQW.
[0102] This embodiment differs from that of FIG. 1 in that:
[0103] a) the N layer adjacent to the spacer layer comprises
Si.sub.0.24Ge.sub.0.76 and is 100 nm thick;
[0104] b) the spacer layers both comprise
Si.sub.0.24Ge.sub.0.76;
[0105] c) the MQW region comprises 7 quantum wells, is 166 nm
thick, has 12 nm spaces between respective quantum wells which are
10 nm thick, and comprises Ge/Si.sub.0.44Ge.sub.0.56; and
[0106] d) there is no discrete `P-layer` below the multiple quantum
well region, instead the buffer layer is doped with P-type dopants
to a concentration of 1.times.10.sup.18 cm.sup.-3 and comprises
Si.sub.0.24Ge.sub.0.76.
[0107] A further difference between the device shown in FIG. 4 and
those shown in FIGS. 20 and 24 is that the electrodes of FIGS. 20
and 24 which are in contact with a surface of the active region
opposite the coupler extend beyond the active region in a direction
towards their respective taper regions. For example, the electrodes
in FIGS. 20 and 24 have a length in the direction towards the taper
region which is greater than 2.5 .mu.m.
[0108] Although exemplary embodiments of an electroabsorption
modulator have been specifically described and illustrated herein,
many modifications and variations will be apparent to those skilled
in the art. Accordingly, it is to be understood that an
electroabsorption modulator constructed according to principles of
this invention may be embodied other than as specifically described
herein. The invention is also defined in the following claims, and
equivalents thereof.
* * * * *